Part III ... by Bob Brown .. NM7M

Friends in Radio Land -

Having spent some time with the ionosphere, now we have to be more
practical, speaking of propagation modes and the things that can
go wrong when DXing. But modes are the first order of business.
In that regard, everyone knows about HF hops from the various
regions - in the range of 1,500-1,750 km from the E-region and
about 3,000-3,500 km from the F-region. Of course, it depends on
frequency and the radiation angle at which signals are launched.

The electron distribution, having greater density at the higher
altitudes, always refracts signals downward. That may seem a bit
strange but that is the case; rays which are ascending are bent
back toward the earth and the same is true of rays which are going
down. The rate of bending is greater at the higher altitudes,
when rays are close to the greatest concentration of electrons,
but it is always AWAY from the region of higher ionization. And
as I indicated earlier, how far rays proceed in the ionosphere
depends on the effective vertical frequency (EVF) when they were
launched, just like the baseball. Remember?

Let's take the case of some rays where the EVF is very close to
the critical frequency at the peak of the F-layer. In the figure
below, Ray A is one where the EVF is less that foF2 and it is bent
back toward ground while Ray B is one where the EVF is greater
than foF2 and it penetrates the F-peak and goes on to Infinity.

But notice that both rays A and B are bent or refracted AWAY from
the region where the ionization is the greatest, the F-layer peak.
That's a general feature of refraction in the upper range of the
HF spectrum. Now one other thing; it seems rays can be reversed
in electromagnetic theory so Ray B could be the path for galactic
radio noise which penetrates the F-region below. OK?
/
/
/ B
EVF > foF2 /
/
/
/
/
. - -> - - - -> - - - . F-peak
/ \ \
/ \ \
/ \ \
/ EVF < foF2 \ \ EVF = foF2
/ \ \
/ A A \ C \
/ \ \
/ \ \
------------------------------------------------------------------
Xmtr Ground RX #1 RX #2

Now we come to Ray C, one where the EVF is very, very close to
the critical frequency of the F-layer. That type of ray, moving
almost parallel the the earth's surface is called a Pedersen Ray.
Those rays can give very long hops but they are essentially
unstable in the sense that any little increase or decrease in the
electron density and they diverge, going back to ground like Ray A
or off through the F-peak to Infinity like Ray B.

Just in case you missed the idea, Pedersen Rays at the peak of the
F-region involve the upper portion of the HF spectrum as the
oblique path must reach those altitudes; that is not possible for
the bottom of the HF spectrum (3 MHz) as even vertical rays can't
penetrate that far up in the ionosphere as foF2 is just too high.

But that is not to say that Pedersen Rays are impossible at the
bottom of the HF spectrum; it's just that type of refraction
takes place down around the E-region where the electron density
levels off for a short range of altitude. So let's look at some
ray paths there, for 80 and 160 meter signals with EVF close to
the value of foE, especially at night:
F-region


-
B / \ B
/ \
/ \
. - -> - - - -> - - . E-region
/ \ \
/ \ \
/ \ \
/ A \ A B \
/ \ \
/ \ \
------------------------------------------------------------------
Xmtr Ground RX #1 RX #2

Ray path A corresponds to a E-hop where EVF < foE and covers only
a short distance to a receiver. But Ray B is one where the signal
has an EVF that's very, very close to foE. But it penetrates the
E-layer and ascends into the F-region; however, its EVF is still
too low to reach the higher portions of the F-region and so it is
refracted back down. If the down-going angle of the ray has not
been affected, it will continue for a distance along the level of
the E-region and then be returned to ground. In a sense, the path
resembles that followed by a Pedersen Ray but there is that short
excursion into the F-region making it an E-F path.

Whether at the level of the E-region or the F-peak, paths which
have Pedersen-like refraction cover greater distances than the
simple E-or F-hops. As such, they would contribute to paths with
few hops and stronger signals; however, as noted earlier, they may
be unstable and only have brief existences. With the varied paths
that amateurs use, such situtions are not readily identified;
however, for fixed paths in commercial use, it is a different
story. In that regard, it is pointed out in Davies' book that HF
Pedersen rays tend occur around local noon on fixed paths across
the North Atlantic, when the density gradients along the path are
at a minimum.

So the above examples cover the simple, single hops that can
occur, from short E-hops to long E-F hops, then F-hops and even
long Pedersen hops. After that, we get into multiple hops; those
are more complicated, of course, but there is some simplicity in
the second and third hops in that reflections involve equal angles
of incidence and reflection from a surface. But even then, there
is the odd chance of complexity if the surface is not flat or not
smooth. The former would, in effect, change the next launching
angle of a ray, adding or subtracting the tilt of the surface to
its original angle relative to the horizontal direction.

As for rough surfaces, they can give a diffuse reflection and that
serves to reduce the power carried forward in the original
direction. At surface reflections, there can be some signal loss,
depending on the signal polarization, surface material and the
frequency. As you know, we distinguish between horizontally and
vertically polarized waves, meaning the electric field of the wave
is either parallel to the earth's surface or perpendicular to it,
as for radiation from a horizontal dipole or a vertical antenna.

While there may be signal loss (in dB) on reflection, the process
is discussed first in terms of reflection coefficients, meaning
the amplitude of the reflected wave compared to the incident wave.
The graphic below illustrates the case for good ground material
and 14 MHz signals; clearly, the small reflection coefficient for
vertical polarization around 25 degrees means there would be a
large signal loss for waves incident at that radiation angle. But
horizontal polarization is much better in that regard and is the
reason why most DXers prefer horizontally polarized antennas.

Reflection Coefficient
1.0 HV
| Good ground, 14 MHz
| H H - horizontal polarization
0.8 + V H V - Vertical polarization
| H
| V H
0.6 + H
| V H
| H
0.4 + V H H HV HV
| V
| V V
0.4 + V V
| V V
| V V
+-----+-----+-----+-----+-----+-----+-----+-----+-----+ 0
10 20 30 40 50 60 70 80 90 Radiation
Angle (degrees)

Of course, once signals leave an antenna, their progress is part
of the discussion of propagation. Everyone knows that salt water
is the best reflecting surface for RF and fortunately 78% of the
earth is covered by oceans. That really helps DXing. But a
significant fraction of ground (and amateur population) lies in
the northern hemisphere and the rest of the earth involves ice and
snow in the polar caps so the distribution of surface material
shown below is of some interest to the propagation of signals:

      0E             180E              360E
North *************************************
      .........GGGGG...............G.....**   * = snow/ice
      .GGGGGGGGGGGGGGGGGG*GGGGGGGG*G..*....
      .GGGGGGGGGGGGGG.........GGGG.GG.....G   G = ground
      GGGGGGGGGGGGGGG.........GGGGGGG.....G
      G..GGGGGGGGGGGGGG........GGGG.......G   . = salt water
      GGGGGGGGGGGG..............G........GG
      GGGGG..G..G.................G......GG   for
      GGGGG......G................GGGG....G
      .GGG........................GGGGG....   10 deg x 10 deg
      .GGG..........G..............GGGG....
      .GG.........GGGG..............GG.....   areas over the
      ..............GG...............G.....
      .....................................   earth.
      .....................................
      .....................................
      *************************************
South *************************************
      0E              180E             360E

We'll do more with reflection loss later on but for the moment, it
is important to know it is there and extracts signal strength with
every bounce. But there is one more point to bear in mind; the
angle of reflection can be as important as the polarization, the
surface or frequency. Thus, losses off of water at low angles are
about 1 dB, about 3 dB off of the various forms of ground and in
excess of 6 dB off of snow/ice. The situation gets progressively
worse at higher radiation angles so low radiation angles should be
the order of the day. But you knew that, just because the hops
are longer at low angles.

Finally, it should be noted that we've pretty well assumed the
ionosphere to be concentric with the spherical earth. That is a
simplifiction, of course, and we have to expect tilts in the
ionosphere and those will have effects on waves returned from the
higher altitudes. For one thing, a tilt ALONG the path will
change the angle of return to the ground; for another, a tilt
ACROSS the direction of a path will affect the polarization in the
sense that what was a horizontally polarized wave may now have a
vertical component to it. So the next ground reflection becomes a
bit more complicated, the signal loss now depends on how the two
polarizations are reflected. And then there are phase changes on
reflection. But nobody said radio was simple, did they?.

Friends in Radio Land -

Let's go on with multiple hops, putting in more of the details.
One matter of interest is the radiation angle throughout a path.
Thus, one might pick one angle, say at the peak of the antenna
radiation pattern, and try to follow it along a path. But while
the Laws of Optics apply, with angles equal for incidence and
reflection from a surface, the angle may change due to a tilt of
the ionosphere on one hop or change of inclination or slope of
ground at a reflection point.

So there could be some variability in the radiation angle. And,
of course, the height of the ionosphere is not constant along a
path, changing if the path goes from being in sunlight to being in
darkness. All those aspects of the path serve to change the
distance per hop or, for that matter, how close the path for a
given radiation angle comes to the target QTH.

Leaving aside the variations which result from surface reflections
and the like, one can illustrate path structures by making various
combinations of hops. Without citing any particular type of the
ionospheric circumstances, some common paths are shown below:

F-hop
-
/ \
/ \
E-hop / \
_ / \
/ \ / \
/ \ / \
/ \ / \
/ \ / \
-----------------------------------------------------------------

- - F-region
/ \ / \
/ \ / \
/ \ / \
/ -- \
/ Sporadic E \
/ \
/ \
/ \
-----------------------------------------------------------------

and various other combinations are possible. The modes shown
above are specified as as E-F and F-Es-F. For longer paths, the
number of E- and F-hops may be larger, depending on how the path
is located relative to the terminator. As for desirability, the
rule is that E-hops on a path are where most losses occur, with
ionospheric absorption on the sunlit legs and ground losses, while
F-hops in darkness have less loss, with fewer ground reflections
for a given distance from point A to B.

The presence of a sporadic E reflection, without any intermediate
ground reflection between reflections from the F-layer, brings up
another type of path that contributes to long-path propagation.
Here, the idea is the same as with the Es reflection except that
the ground reflection is missing because of ionospheric tilts,
shown as dotted lines, between the two portions of the F-region:

. .
. .
F-tilt . - - - > - - - . F-tilt
. / \ .
. / \ .
/ \
/ \
/ \
/ \
/ \
/ \
/ \
-----------------------------------------------------------------
Xmtr Rcvr

The figure above is "Flat Earth Physics" but in reality, the ray
reflected off the first part of the F-region did bend downward but
it didn't go down far and the curved earth fell away from it so it
missed the earth and went on to the F-region again. Make a curved
sketch to see what I mean. OK?

While the tilts shown above are exaggerated, such circumstances
are found regularly on paths going across the geomagnetic equator
in the afternoon/evening hours and give rise to long, chordal hops
with correspondingly stronger signals. But it should be noted
that "tilts" really are another way of representing the changes in
the electron density distribution along a path. Thus, an upward
tilt, one that gives a longer hop, really is the same as the case
where the electron density DECREASES along a path direction and
results in less downward refraction. That is called a negative
gradient and, of course, a positive gradient is just the opposite.

Finally, there is another interesting variation on path structure
that results from a negative gradient along a path, ducting. In
that case, the situation is like the E-F hop discussed last time
but the excursions into the F-region are repeated several times:

- - - -
/ \ / \ / \ / \
/ \ / \ / \ / \
- - - - - - - -
/ E-layer
/ Ducting
/
/ A
------------------------------------------------------------------
Xmtr Ground
Again, the above representation is "Flat Earth Physics" and
involves a negative gradient, just like the chordal hop mentioned
earlier. But those long hops are more characteristic of the upper
end of the HF spectrum, 14 MHz and above, and require almost the
full height of the ionosphere for their completion. That is the
case as even a reduction in electron density along a path does not
reduce refraction at the higher frequencies to a great extent.

The ducting shown above is for the low end of the HF spectrum and
involves smaller vertical excursions of ray paths than the case
for chordal hops. That is the case as refraction varies with the
inverse-square of the frequency; thus, for the same gradient or
reduction in electron density along the path, the change in the
downward refraction is much greater at the low end of the HF
spectrum and less of the ionosphere is required for the same type
of effects.

Now, having gone through a wide range of mode structures that are
possible, one can use those ideas in dealing with propagation.
But, face it, the RF from one's antenna pattern goes off into all
the possible modes, be they E-, E-F or F-hops and, depending on
the operating frequency, some of the exotic modes, like chordal
hops or chordal ducting are possible too. But the mode that gets
through for your DX contact is something of a "survivor", giving
signals where the others have died out due to absorption or have
the wrong radiation angles for the path or receiving antenna.

But at this point, about all we're prepared to think about are the
more common modes and those would be in relatively calm, stable
conditions. In short, we'd be looking at the indicators, SSN and
the like, perhaps a map with great-circle paths on it and pointed
our beams in the right directions. But the "when, why and how"
have yet to be discussed, to say nothing of circumstances that are
out of the ordinary.

Myself, I consider "when, why and how" to be the "propagation
imperatives", the ideas that every DXer should have in mind before
turning on the rig in pursuit of a "New One". In short, those
ideas should be "Second Nature", the sort of thing you'd have in
mind if shipwrecked on a desert island with nothing but the
makings of a ham station at your disposal. You should be able to
think of the DX QTH, have a feeling for what could be done on a
given date and think of when to get on the band of your choice.
Sometimes the answers are not to one's liking but an answer should
be forthcoming without too much head-scratching.

So let's see what we can do to get that right, at least for normal
conditions, and then deal with disturbances and see what they'd
mean for us. That won't be too burdensome as once the broad
outlines are established, you'll have a propagation program to
fill in the quantitative details, case by case.

Friends in Radio Land -

Now we've discussed some of the general ideas behind propagation
in the HF part of the spectrum and you should have a good grasp of
what it all depends on - enough ionization overhead to refract
signals downward, keeping them in the F-region, and signals
getting through the ionization down in the D-region with enough
strength to overcome the local noise.

With that in mind, let's explore propagation with a practical
case, say making a contact between a central location in the USA
and Togo, West Africa in the upcoming CQ WW CW contest in late
November. That'd be a good test to see just how far we can go in
predicting propagation using the simple ideas developed so far.
That done, we can look at how computer programs do it and see what
other details they offer.

So let's use Omaha, NE as our QTH in the USA; that's at 41N, 96W.
Togo is a bit harder so we have to go to the ARRL Operating Manual
to find that it's located in the Horn of Africa, at 6N, 1E, close
to the Greenwich Meridian. Looking at those coordinates, one
thing is immediately clear - it's quite a ways from Omaha to Togo,
more than 90 degrees difference in longitude and more than 35
degrees difference in latitude.

Considering that the distance around the earth is about 40,000 km,
one can conclude immediately that the distance to Togo from Omaha
is better than 10,000 km, a quarter the way around the world.
That's confirmed by going to the azimuthal equidistant map for
Central USA in the ARRL Operating Manual; Togo is half way to the
antipodal circle, making it quite a haul. But it's not all that
hard if you're on the right band at the right time.

Now we're talking about late November this year so we can take
the effective sunspot number as around 80, judging by recent
reports from NOAA. The chances of making a contact on the higher
bands are pretty good when you consider that Togo is at a low
latitude, where the the electron distribution of the F-region is
quite robust. So we only have to worry about launching the high
band RF from Omaha.

As a first approximation, let's think of trying for a contact on
28 MHz. For that, ionization and the MUF are the important things
and tell us that the contact should be tried during the time the
path is well illuminated. So with a longitude difference of about
97 degrees, we'd like to have the sun at least midway between the
two QTHs, say at about 47 west longitude. With the sun advancing
westward at 15 degrees of longitude per hour, that means the time
should be about 3 hours after 1200 GMT or 1500 GMT.

But remember Togo is at a low latitude so the critical frequency
of the F-region there is less of a problem than at Omaha. That
being the case, it would be better to choose a later hour, one
when the sun is closer to the longitude of Omaha, raising the
critical frequency near there. But the time should not be so late
as to have the sun set anywhere on the path. That means we have
to look into the sunrise/sunset tables in the ARRL Operating
Manual and see when the sun would set at Togo.

In that regard, the Operating Manual gives SR/SS data for November
21 and we can use that as an approximation, taking the ground
sunset at Togo as 1736 GMT. That would suggest, as a first
correction, that the 28 MHz band be tried between 1530 UTC and
1730 UTC. The same would apply for 21 MHz too, knowing that less
ionization is needed for propagation on that band, so an operating
window might be better if widened to start earlier and end later,
say from 1500 UTC to 1800 UTC.

As an aside, I should say that last idea has some generality to
it, at least for the bands where MUF are important. So from a
given QTH, the lowest bands open the earliest, the highest bands
the latest, and band closing is in reverse order. Of course, that
is just the availability of the path; the signal/noise situation
still has to be looked at for the best times of operation.

As for the transition bands, 10 MHz to 18 MHz, absorption plays a
role there and good sense indicates the effect can be minimized by
avoiding times when the path is well illuminated, with the sun
around its midpoint. In addition, we know that ionization lingers
after sunset, thanks to the role of the geomagnetic field and the
slow recombination rate of electrons and positive ions up there in
the F-region. As a result, propagation on those bands would be
supported around sunset and on into the evening hours.

In addition, the rising sun on the path near Omaha would open up
propagation, at least until absorption became too great. That
being the case, we can expect the bands to open shortly after the
sunrise at Omaha, roughly 1350 UTC according to the Operating
Manual. And with sunset around 1730 UTC at Togo, another two or
three hours could be added to the operating time.

Things are shaping up, at least for the bands where F-region
ionization and D-region absorption are important. That would give
a starting point as sunrise at Omaha, about 1400 UTC, and a
closing time of about 2030 UTC for the transition bands. The
higher bands would start later, of course, and end sooner, the
general principle mentioned earlier.

The lower bands, 160 meters - 40 meters, where D-region absorption
dominates, would be open from sunset at Omaha til sunrise at Togo.
Going to the Operating Manual, we find low-band operations could
start at Omaha around 2300 UTC and end around 0545 UTC.

But there is the question of noise, man-made or atmospheric in
origin, to compete with signals. Here, experience shows that man-
made noise is less as the hour goes past the end of the working
day. And atmospheric noise, say at Togo, would be the lowest at
times close to dawn. So low-band operation probably would be more
productive in the later hours of the operating window. But in
view of the high level of ionospheric absorption and distance
involved, it could be much more difficult to make a contact on the
lower bands than the higher ones. In addition, antennas and power
play a greater role in that part of the spectrum. Those resources
are developed over time by DXers and related to their operating
experience in that part of the amateur spectrum. Put another way,
DXing on the lower bands, 80 and 160 meters, is tough and not
always rewarding for casual operators.

Now, to add a realistic twist to this discussion, let me say that
I worked 5V7A on 20 CW last year at 2312 UTC on November 29. If
you look into it, you will see that was over five hours AFTER
ground level sunset at Togo!. (See? Ionization does linger on in
the dark, especially at low latitudes!) I would hope you could do
the same this year. At least, the above example shows how you can
"sharpshoot" for a New One, even with only primitive tools at
one's disposal. Give it a try. OK?

If you want to learn more about the 5V7A operation, propagation
from Togo to the far corners of the world, and when you might be
able to work them, you should their website:



and see all that is offered. My good friend, Carl/K9LA, did the
propagation forecasting for them and you can see how you might be
able to work them too. If you do, I'd like to hear about it, by
e-mail, and would appreciate getting an analysis of your QSO. OK?

Now I didn't work out all the aspects of contest propagation for
the 5V7A group; you'll see what their own propagation guru came up
with but I'm sure it was based on the principles I outlined above.
I have done that sort of thing before, for the recent 8Q7AA and
3B7RF DXpeditions. In that sort of circumstance, the idea is to
forecast so they can "Work the World". So every time interval has
to be looked and in every direction to find the best way for them
to operate in the contest.

The first one for the 8Q7AA group went very well, operations going
essentially as predicted. But the second one for 3B7RF got into a
bit of trouble; that was interesting in itself as it will lead us
into the matter of ionospheric disturbances of geophysical origin.
Leaving that to later, let's go beyond slow, mechanical methods,
how "The Ancients" handled the propagation problem, and look at
how it's done by computers.

As you know, they do everything practically at the speed of light.
But how well do they do it? That's a good question. As a matter
of fact, given what you know now, you might wonder if they just do
the old-fashioned calculations faster and not add much to the
problem. So we'll go with that for a while, looking at how
computers handle these questions and then look at a few new ideas.

Reference Notes:

If DX contesting is the sort of thing that interests you, let me
say that the 5V7A crew were kind enough to provide me with their
'96 and '97 contest logs for analysis. I was more interested in
them for the aspects of 160 meter propagation but you might look
at my article in the March/April '98 issue of The DX Magazine. It
also shows how demographics overpowers propagation.

Friends in Radio Land -

Now the past little exercise used old-fashioned tools to do the
5V7A propagation prediction but at a miserably slow pace. Those
really drew on three fundamental ideas - the presence of F-region
ionization, D-region absorption limiting signal strengths and the
geomagnetic field organizing the ionosphere. So using nothing
more than the times of sunrise and sunset, those concepts gave a
qualitative view of propagation. But without hard numbers, MUFs
and signal/noise ratios, that would never meet the needs of the
tough decision-making for a DXpedition or a DX contest operation.

With computers brought into the matter, the times of sunrise and
sunset can be calculated with astronomical precision and DX
windows found for working 5V7A on the low bands. The next big
problem would be finding the sort of signal strength that could be
expected. So a knowledge of the operating modes or hop structures
is required, primarily a problem in two dimensions, in the plane
of the great-circle path. That sort of thing is done very well by
the ray-tracing in the PropLab Pro program.

On the higher bands, where MUFs, absorption and E-cutoffs are a
concern, computer programs can do a decent job of finding how the
ordinary modes would change in the course of a day, say E-hops
during the day and F-hops at night as well as mixed modes across
sunrise and sunset. But those programs cannot deal with the
ionospheric effects from electron density gradients near the
terminator or geomagnetic equator so certain modes, like chordal
hops and ducting, would not included in their analysis. That's
leaves a gap when it comes to having a complete prediction and so
computers are fast but will not be as fully quantitative as hoped
for in replacing the qualitative efforts used earlier.

As you might expect, the earliest computer program in amateur use,
MINIMUF, resembled the scheme with ionospheric maps from the Dept.
of Commerce and just used the control point method for MUFs, via
F-region propagation. Neither signal strength nor noise were
considered so the method worked best at the top of the amateur
spectrum and for very high levels of solar activity. That was
unfortunate as amateurs used the same methods at low levels of
solar activity, often with misleading or disappointing results.

But MINIMUF fired the imagination of many amateurs and various
accessories, including E-layer cutoff calculations, were added to
the original code. For example, MINIPROP Version 1 used the F-
layer model in MINIMUF and had calculations for E-cutoff and
signal strength as well. The early work of Raymond Fricker,
MICROMUF 2+ published by Radio Netherlands, was similar but the
E-cutoff was regarded as giving values for the LUF, the lowest
useable frequency. That's not right as LUF is a D-region matter.

But there was a basic difference between Fricker's MICROMUF 2+ and
MINIMUF, how the critical frequency information was obtained.
Fricker's F-region algorithm used 13 mathematical functions to
simulate the database for critical frequencies from vertical
sounding while MINIMUF relied on just one function, adjusted to
represent the results of a limited set of oblique soundings.

In another program, IONPRED, Fricker introduced a novel scheme of
hop-testing. Essentially, the program looked at each hop in
detail, at the points where the E-layer was crossed and at the
highest point where the critical frequency of the F-region was
important. So the hop-testing involved determining whether the
mode was reliable by seeing if operating frequency was above or
below the E-cutoff frequency by 5% and less than the critical
frequency for F-region propagation by 5%.

With an initial choice of radiation angle, the path structure
could be sorted according to E- and F-hops, depending on the
outcome of the tests along the way. Fricker also adjusted the
height of the F-region according to local time so hop lengths were
not constant along a path. As a result, the path could over- or
under-shoot the target QTH. If the error was more than 25 km,
another radiation angle was chosen and the process started again.
In IONPRED, Fricker also calculated the ionospheric absorption, in
dB, and added that to the signal loss due to spatial spreading or
attenuation and ground reflections.

Another innovative feature of IONPRED was the use of availability
of the path, the number of days of the month it would be open for
reliable communiction. That was something like the FOT-MUF-HPF
idea discussed earlier but in the case of IONPRED, the number of
days was treated as a continuous variable in contrast to the upper
or lower decile approach with the FOT-MUF-HPF method.

The IONCAP program has many other methods beside FOT-MUF-HPF and
some give long-term availability figures, the fraction of a month
the path would be open, as well as reliability values, the
fraction of time the signal/noise ratio would exceed some minimum
value. Thus, in contrast to Fricker's method which is based only
on F-region considerations, IONCAP deals with fluctuations of
signal strength, a D-region factor, as well as man-made noise.

Nowadays, the method used by Fricker in IONPRED has been improved
upon by the use of mode-searching in the MINIPROP PLUS program.
There, the idea is to work up a number of successful modes and
then find the one with the greatest signal strength. With
computer speeds in the '80s, Fricker's method was extremely time-
consuming, to say the least, but nowadays computer speeds are such
that the whole process of mode-searching takes a second or two!

In a sense, the ray-tracing in PropLab Pro is like hop-testing as
it just goes forward for a given choice of radiation angle and the
calculation stops if the trace is lost to Infinity or stops in the
vicinity of the target QTH. As you might expect, the main problem
with that approach is that the hops may either fall short or go
beyond the target, making it a slow, iterative process to get the
path for RF from point A with point B. Beside that, the user
would have to evaluate the suitability of the path, whether the
number of E-hops would make it too lossy or otherwise. For that
reason, I admire how PropLab Pro goes about a problem but it's too
slow for an impatient person like me.

But we can use the ray-tracing in the PropLab Pro program to see
paths in both two or three dimensions. It should be said the 2-D
case comes fairly close to dealing with the problem in a proper
sense by putting in the appropriate ionosphere for each hop on the
path, considering date, time and SSN. But it does not take into
account terrain, such as the slope of the ground nor the nature of
the reflecting surface. Taking one hop at a time, the calculation
does takes into account the change in height of the ionosphere but
not any tilts or gradients. That is left for the 3-D case.

The three-dimensional ray-tracing is based on solving equations of
motion for the ray path, just like Newtonian Mechanics finds the
paths of satellites and spacecraft. There are equations for the
path advance along and upward in the great-circle as well as the
motion perpendicular to that plane. The skewing of paths is small
in the HF range and thus, it is usually neglected in ray-tracing.
That is because refraction goes inversely as the square of the
frequency and electron density gradients across paths that occur
in the quiet ionosphere are relatively small. The exception to
that statement is the auroral zones where large gradients occur.

But at lower frequencies, like 1.8 Mhz in the 160 meter band, the
refraction or bending of paths becomes larger because of the lower
frequency and other effects become important. In particular, the
gyration of ionospheric electrons around the geomagnetic field
occurs at a rate which is comparable to the signal frequency. So
the entire approach to the ionosphere has to be redone, put in
more general terms without any approximations. That complete
theory was due to Appleton, is called magneto-ionic theory and has
been around for about 60 years.

Among the results of the more general theory are that propagation
now depends on the angle between a ray path and the local magnetic
field; further, the waves which are propagated in the medium are
elliptically polarized, another way of saying they consist of two
components at right angles to each other and which have a phase
difference between them. Beyond that, there are two modes, with
opposite senses of rotation of the electric field vector, the
ordinary and extra-ordinary waves.

The simple, linearly polarized waves that are so familiar in the
discussion of HF signals are just a limiting case of elliptical
polarization, when one of the two components at right angles has a
very small amplitude compared to the other one. In magneto-ionic
theory, that limiting type of polarization results when signals
are sent perpendicular to the magnetic field. The other case is
circular polarization, when signals are sent along the magnetic
field direction. Then, the two components at right angles are
equal in amplitude and out of phase by 90 degrees.

Those features of propagation were evident in the early days of
ionospheric sounding as two echoes were returned for each signal
sent upward, the ordinary and extra-ordinary waves, and you will
see them on any ionograms that you may inspect. So magneto-ionic
theory is a part of the reality of radio propagation. But, for
DXers, there is something of a happy simplification as over long
distances, the extra-ordinary wave is heavily absorbed and only
the ordinary wave needs to be considered.

There is another interesting aspect to propagation down on the 160
meter band, the coupling of RF into the ionosphere. As you know,
there is a polarization to the waves emitted by an antenna and on
160 meters, vertical antennas are used most often. That is due to
the wavelength being so long that most horizontal dipoles cannot
be placed very high, in terms of wavelengths, and thus suffer from
high radiation angles, being the so-called "cloud warmers".

Now in magneto-ionic theory, the polarization of a wave changes
continously in the ionosphere as it is propagated through the
geomagnetic field. But there are two limiting polarizations,
typically at altitudes around 60 km, where the wave enters the
ionosphere near point A and where it leaves the ionosphere near
point B. When worked out in detail, the theory says that there
will be a signal loss, in dB, at entry because of any mismatch
between the wave polarization from the antenna and the limiting
(elliptical) polarization at entry point A.

For example, signals going in the E-W direction from a vertical
antenna at the equator are poorly coupled into the ionosphere
because of the polarization mismatch, with vertically polarized
waves going against the horizontal field lines. Similarly, there
may be signal loss at the exit point B due to any mismatch between
the limiting polarization on exit from the ionosphere and the
polarization of the antenna at point B.

As indicated, magneto-ionic theory is quite complicated, with
elliptically polarized waves and all that, but for signals going
from point A to point B, we need not concern ourselves about what
goes on high up in the ionosphere between those two points, only
the antenna types and the limiting polarizations at the endpoints
of the path. That makes life a lot simpler.

Another point about this frequency range; signals can become
trapped in the electron density valley above the E-region at
night. Thus, if they enter the region, they may be reflected back
and forth between the bottom of the F-region and the lower limit
at the top of the E-region. That means they'll rattle back and
forth between those altitude limits like a ball sliding down a
smooth trough. Only if the walls of the trough change in height
can the ball get out or, equivalently, can signals get out of the
duct if the lower ionosphere changes. In that regard, ducting is
undoubtedly responsible for the long-haul DXing done on 160 meters
as it avoids repeated ground reflections and traversals of the
lower ionosphere which absorb signals at a very high rate.

Reference Notes: \

A review of various propagation programs can be found in the QST
issues for September and October '96.

The above discussion gives a very brief summary of the principal
aspects of magneto-ionic theory, as it applies to propagation. An
analytical summary of the theory is given in Davies' recent book,
Ionospheric Radio; however, it really requires a strong background
in electromagnetic theory at the level found in university courses
in physics and engineering. It should be noted that the method of
the theory has a broader application as it represents the first
steps toward the study of plasmas in the solar system and in out
space.

A discussion and some quantitative aspects of polarization loss on
160 meters are given in my article in the March/April '98 issue of
The DX Magazine. In addition, a fuller discussion of magneto-
ionic theory and 160 meter DXing is given in Top Band Anthology,
published recently by the Western Washington DX Club. You can
contact me for details.

Friends in Radio Land -

We turn now to other aspects of propagation, from predictions to
those circumstances which may disrupt propagation and make
predictions go awry. But in doing that, a bit of history would
help chart the course.

First, radio is almost 100 years old now and the course of events
has been onward and upward, in frequency and into the ionosphere.
Thus, the earliest signals were down in the kHz region and now
technology has advanced to the point where amateurs are operating
in the GHz part of the spectrum. But it has been a steady advance
in frequency and as we know now, that means signals going higher
and higher into the ionosphere as their effective vertical
frequency increased.

Amateur operations start in the medium frequency (MF) range with
the 160 meter band, around 1.8-2.0 MHz. If one looks into the
ray-traces for that band, it is clear that signals in normal
communications circumstances stay below the 200 km level most of the
time. Of course, ionospheric absorption on that band is so great
that DX operations are attempted only on paths in full darkness.

Going to the high frequency (HF) range, 3 - 30 MHz, signals go
higher toward the F-region peak around 300-400 km and darkness
becomes less of a necessity near the top part of the spectrum. In
fact, solar radiation is needed to bring the level of ionization
up to the level required for propagation.

Historically, in the time that operating frequencies rose, the
range of DX contacts increased and it became apparent that the
solar cycle played a role in propagation. Moreover, various
disturbances became apparent. So the early '20s had amateurs
opening up trans-Atlantic operations and that was commercialized
in the late '20s with the advent of radiotelephone circuits to
Europe. In that time, it was found that the communication links
failed during geomagnetic storms. Those could last for days but
there were also strange blackouts that lasted anywhere from just a
few minutes up to an hour. In 1937, those short wave fadeouts
(SWF) were found to be associated with solar flares. Moreover, it
was becoming apparent that the disruptions to magnetic storming
came a day or so AFTER solar flares.

From all that, it became clear that the sun was a major player in
the field of radio propagation and scientists began looking into
the details. The SWF problem was fairly simple, just being the
release of electrons in the ionosphere from the photoelectric
effect of solar Xrays. The magnetic storm effect was a more
subtle problem as it implied some slower process, not Xrays moving
across the solar system at the speed of light. In that regard,
those geophysicists who studied the earth's magnetic field
proposed that there was a stream of matter sent out from the sun
and then its encounter with the geomagnetic field was the
triggering mechanism. From the time delays between flares and
storms, first estimates were made of the speed of the solar
matter. More than that, they could not say at the time.

Now that brings up the question of just how far out geomagnetic
field lines extend from the earth. Of course, that goes to the
model of the geomagnetic field in use at the time. That was, in
simple terms, the sort of thing you get if you stuff a bar magnet
into the earth and look at how the field lines extend past the
surface of the earth. In short, the model back in the 40's and
50's was that for a centered dipole field that was tipped with
respect to geographic coordinates, the dipole axis piercing the
earth's surface at 79.3N, 71.8W at the north pole and the south
pole through the corresponding antipodal point.

That was the field used when the first PIONEER space shots took
place after the IGY, an experiment looking at the strength and
orientation of the earth's field as the spacecraft moved out, away
from the earth. That flight produced a REAL surprise, with data
showing the earth's field varying slowly and in an orderly fashion
as the spacecraft moved outward but then suddenly, when it reached
something like 8 earth radii, the field became weaker and less
organized, almost random in its orientation. Clearly, the orderly
dipole field no longer described the situation at those distances,
giving way to the presence of an interplanetary magnetic field.
And what was previously considered as empty space, except for
meteoritic dust and debris, was also found to contain of plasma
(protons and electrons) that was streaming away from the sun.

Now, before exploring that extreme, we should look at the dipole
field and see what could be expected from it. As you know, say
from your high school physics course, the field lines pass out of
the southern hemisphere and then after going out some distance,
they return and enter the northern hemisphere of the earth. That
was the classical picture; so let's see what it says, at least
until we get into trouble with the Pioneer data.

Now the magnetic dipole has a system of coordinates of its own,
related to the direction of its axis relative to the geographic
axis and equatorial plane. With the dipole orientation given
above, one can work out the magnetic coordinates of any point on
the earth. For example, my location at 48.5N and 122.6W is one
that corresponds to 54.4N, 62.1W in the dipole coordinates. OK?

But let's look at the dipole and its field lines. They go out
from the southern hemisphere and come back down into the northern
hemisphere. But how far do they go out? That would be important
when it comes to thinking about the collision of solar plasma and
the dipole field, suggested by the geomagneticians. It's not hard
to work out where the magnetic field lines cross the plane of the
geomagnetic equator and there is a simple relation between that
distance and the magnetic latitude where the field lines start:

SQRT(L)=1/COS(PHI)

with PHI as the magnetic latitude and L is the distance, measured
in earth radii (Re). Now if you conjure up the image of a dipole,
surrounded by its magnetic lines of force, you can see that low-
latitude field lines do not go out very far from the surface of
the earth. But it's a different story for high latitude field
lines and if worked out, we obtain the following:

Mag Lat (degs) Distance (L in Re)

10 1.03
20 1.13
30 1.33
40 1.70
50 2.42
60 4.00
70 8.55
80 33.2

So the high latitude field lines are the ones in harm's way when
it comes to the collision between the plasma coming from the sun
and the earth's field. And, by the same token, the low-latitude
field lines that go out only short distances from the center of
the earth are pretty well protected from the direct effects of
the collision between solar plasma and the geomagnetic field. Of
course, that fits with your operating experience, paths going
across the polar cap are far more subject to disruption than those
going to low latitudes.

Before getting to the nature of the various propagation effects
that originate on the sun, we should note briefly that the view of
the earth's field that I gave earlier in #4 of Prop. 101 is not
quite the full story. In particular, it was suggested that the
solar wind blowing by the obstacle of the geomagnetic field is
like the flow problem of a bullet in air, but now with the bullet
(geomagnetic field) fixed and the air (solar wind) in relative
motion. So it was suggested (and verified) that a bow shock in
the solar wind was out there in front of the magnetosphere:

*
* BOW SHOCK
*
Magneto-tail * * * * * <----
* . . . * * * *
* . . * * . * * <----
* . . * * . . * *
* . . * * . . * * <----
* . Magnetosphere (Earth) . * * SUN
* . . * * . . * * <----
* . . * * . . * *
* . . * * . * * <----
* . . . * * * *
* * * * * <----
*
* SOLAR WIND

Now, to carry the aerodynamics a bit further, it was suggested
that the position of the bow shock would vary, moving closer to
the earth at higher speeds of the solar wind. And that proved to
be the case, obtained by satellite observations after the original
work with Pioneer I. But the geomagnetic field is a bit different
than a hard obstacle and it was expected that the field could be
compressed at times, particularly if the solar wind came at it as
a sudden blast. And, as you guessed, that is the case as shown by
magnetic sensors on geostationary satellites. During some severe
magnetic storms, those satellites report conditions which put them
right in the interplanetary magnetic field, showing that the
magnetosphere has been compressed by the solar wind and that the
magnetopause was temporarily inside 6.6 Re. Absolutely amazing!

Now, having told you about the troubles of geomagnetic field
lines, think back a bit to what I said earlier: they are the
things which hold your precious ionospheric electrons in place!
So maybe all those disruptions in propagation during magnetic
storms are not all that surprising, with field lines being pushed
around by the solar wind.

There's more to magnetic storm effects than just compressing the
field lines in front of the earth. As I suggested way back in the
4th session of Prop. 101, field lines on the front of the magneto-
sphere can be dragged into the magnetotail. In that process, the
ionospheric electrons of the F-region on those field lines are
removed from the front of the magnetosphere and, in essence, are
distributed on much longer field lines on the rear of the magneto-
sphere. On both counts, the high-latitude F-region suffers a loss
in ionization and critical frequencies in the affected regions are
reduced. Of course, the sun shines, day in and day out, so with
some magnetic quiet, solar illumination will restore the regions
and communications across those high latitudes returns to normal.

Those words of explanation will have to suffice as the problems of
the magnetosphere are quite complicated, with unfamiliar or non-
classical ideas, and are best left for the magnetospheric physics-
types to wrestle with. We need not get enmeshed in the details,
only be able to recognize when there's a problem and consequences
that will follow. In that regard, the records of magnetometers at
high latitudes are our best bet as they give vivid portrayals of
the storms that develop, thanks to simultaneous, yet secondary
effects which result. There, I am thinking of the aurora, both
optical and radio, as well as the current systems which build up
during a disturbance initiated by the solar wind.

Again, the details need not concern us but the main features are
what we note: optical emissions coming from above the 100 km
layer, VHF reflections off of auroral displays, ionospheric
absorption of signals going across an active auroral zone and
strong magnetic disturbances observed on the ground from the
current systems which develop along the ionized region. More on
this next time.

Research Notes:

A good historical account of the early days of radio can be found
in the first chapter of McNamara's book, "Radio Amateurs Guide to
the Ionosphere". And it's a good book too. Get a copy if you are
serious about radio propagation.

And the end of the second volume of the book, "Geomagnetism" by
Chapman and Bartels, has an interesting account dealing with the
first days of magnetic observations in Sweden by Celsius and one
of his graduate students. Knowing what we do now, I consider that
as "Day One" of the Space Age. But I have to marvel that it took
75 years until Oersted came up with the idea of a current (like an
ionospheric electrojet) giving rise to magnetic deflections (on
the ground below an aurora) of a compass. Compare that time with
the five years it took the French mathematicians to come to grips
with the Biot-Savart Law for magnetic effects of currents.
Interesting!

Finally, an excellent discussion of early auroral observations in
Norway can be found in the last chapter of Brekke's book, "Physics
of the Upper Polar Atmosphere" published by Wiley & Sons in 1997.
Brekke, being a Norwegian, pays homage to the works and tradition
of good auroral physics established by Stoermer. It's worth a bit
of reading time, believe me.

Friends in Radio Land -

At the end of the last session we made note that magnetic storms
give rise to auroral disturbances, with optical emissions coming
from above the 100 km layer, VHF reflections off the ionization in
auroral displays, ionospheric absorption of signals going across
an active auroral zone and strong magnetic disturbances observed
on the ground from the current systems which develop along the
ionized region. All that from an enhancement in the solar wind,
perhaps coming at a greater speed, with a greater particle density
or with the interplanetary magnetic field pointing south with
respect to the earth's field.

Nowadays, we can read about all those changes on the Internet.
But the most important one for magnetic storming has to do with
the interplanetary field and its orientation. With the field
pointing south, conditions when Bz is negative, the interplanetary
field can merge with the terrestrial field (a non-classical
concept) and field lines on the front of the magnetosphere then
transferred to the tail region as the solar plasma sweeps by.

These ideas came forward in the '50s, thanks to the efforts of J.
Dungey of the U.K. and others. As I said earlier, they go beyond
the elementary considerations we get in classical courses on
electromagnetic theory and are best left for the theorists to
discuss. We only need to know what happens to the ionosphere and
there, the news is BAD as the F-region loses ionization with
the development of a magnetic storm.

But the E-region can gain ionization, with the penetration of
auroral electrons. Those particles are from here inside the
magnetosphere itself, not directly from the solar wind, and are
accelerated locally, going from a fraction of an electron-Volt up
to tens of kilovolts energy. And their flux can be quite large,
resulting in electron densities of a million or more per cc from
electron collisions with atmospheric constituents in the tens of
kilometers above the 100 km level. The colors of the aurora are
testimony to the collisions with the neutral constituents and the
electron densities that result can give rise to signal absorption.

That last point may seem strange if you go back to the curves that
were given in the first session of Prop. 201. There, the relative
absorption efficiency per electron was dropping off quite rapidly
above 100 km. But in the case of aurora, there are millions of
electrons per cc up there and even if electron-neutral collisions
are less frequent above 100 km, losses result just from the sheer
amount of ionization that goes with an aurora.

But to give some numbers, auroral absorption of up to 5 dB or so
is found in the riometer records of 30 MHz galactic radio noise
coming in vertically. But that is just for one pass through the
ionosphere. For amateur communications, say on 28 MHz, that
should be doubled for a complete hop, increased even further by a
factor of 3-4 for the oblique angle of the path and adjusted for
the inverse-square frequency variation. At lower frequencies,
that last adjustment shows even greater losses on those bands. So
it should be no real surprise that auroral absorption represents
an adverse factor for amateur communications.

Those remarks dealt with the electron density; one should also
note the geometry and activity of the aurora. In regard to
geometry, auroral activity at any given time is restricted to a
narrow latitude range. (See Research Notes) But it can extend
over a wide range of longitude and the type of activity varies
from west to east. In evening hours, aurora tend to be quiet and
not involve a lot of energetic particles (and ionization). Around
midnight, the activity may increase dramatically, with displays
flashing wildly overhead and in considerable motion. It is even
possible to note from the distinct ray structures that the
electron influx comes down the inclined magnetic field lines.
Then in the morning hours, the aurora becomes more diffuse, shows
some pulsating patches and more ionospheric absorption, slowly
varying compared to that around midnight and much greater than
before midnight.

HF signals that go across an auroral region will show effects
characteristic of the activity - steady signals going across in
local evening, considerable rapid absorption and flutter from the
moving regions of ionization around local midnight and just strong
absorption for local morning. Of course, all those ideas have to
be tempered by the frequency involved, with devastating absorption
on 160 meters and possible auroral reflections above the HF range.

The magnetic disturbances at high latitudes which accompany aurora
give qualitative measures of the energy input to the magnetosphere
from the impact of the solar wind. Nowadays, one can go to NOAA
satellite data and obtain numerical values for the power input
from observations of the influx of auroral electrons with energies
up to about 25 keV. The numbers can be quite large, from 1 to 500
Gigawatts over one hemisphere. Such inputs can have profound
influences, auroral heating and magnetic activity, but our concern
is only with communications so we have to look at how frequently
these events occur and if they can be anticipated.

Recent data published by NOAA gives a summary of magnetic storm
activity over Solar Cycles 17-22 to suggest how the levels of
magnetic activity might vary, year by year, in Cycle 23. Now when
it comes to magnetic activity, indices are used to characterize
what level of disturbance (from quiet conditions) is in effect, say
in a 3-hour period or averaged over a day. In that regard, a
number of magnetic observatories have been selected to provide
data for use in making planetary averages. The actual data sets
are normalized to common scales, 0 to 9 for the 3-hour Kp-index
and 0 to 400 for the daily Ap-index.

One can obtain those data from the Internet and keep records to
see if there is any recurrence tendencies. Indeed, there are and
logging Ap indices is one way to anticipate possible disturbances
that come from long-lived solar streams sweeping past the earth or
stable active regions which are the source of increased levels of
ionizing radiation.

Magnetic storminess is categorized in terms of Ap values and minor
storms correspond to elevated levels of Ap while actual storms
correspond to Ap greater than 40 and severe storms are when Ap is
greater than 100. In that regard, the storm of May 3, 1998 had an
Ap level of 112 while the greatest storm ever recorded was in
September 1941 and had an Ap value of 312! Like the March '89
storm which put the Province of Quebec in the dark for a day, that
one affected the power grid in the Northeast. Nowadays, the power
industry is keenly aware of the magnetic storm problem and tries
to anticipate problems by getting solar wind data from satellites,
out there ahead of the earth and in the solar wind.

Anyway, both minor and major storms affect HF propagation for
hours at a time or a day by their adverse effects on F-region
ionization but severe storms reduce the bands to barren wastelands
for days at a time. Propagation doesn't return until slow photo-
ionization processes replace the F-region electrons.

The propagation aspects of magnetic activity are found in the
"hfprop" report prepared by the U.S. Air Force and re-transmitted
daily by the Space Environment Center of NOAA. That report is
limited to observations within the northern hemisphere and gives
both a summary and forecast in 6-hour periods in four longitude
ranges - 0 to 90W, 90W to 180, 180 to 90E and 90E to 0 - and five
latitude ranges - polar, auroral, middle, low and equatorial. The
conditions are for single hops within those regions and given as
N, U, and W, with numerical modifiers from 0 (low) to 9 (high).
The N stands for normal but, in my view, the U stands for "Ugly"
and W for "Wretched". I think you get the idea. There are
earadditional notations during poor conditions, essentially the
percentage decrease in MUFs on the paths.

The effects of magnetic storming are the greatest, as you might
suspect, at the higher latitudes and on the higher frequencies.
For communications over any distance, differences in longitude
mean that great-circle paths usually swing north and thus are at
risk during magnetic activity. This is not too bad for short-path
communications as the windows of opportunity can be rather wide.
But that is not the case for long-path propagation; there, the
path opens with the rise in F-region critical frequency with
sunrise on the path and closes shortly thereafter as D-region
absorption increases at lower altitudes. In short, if an
opportunity is lost on a given day, one must wait for another day
and try again. But having spent many happy hours in pursuit of
long-path contacts, I can say it is worth it.

Turning to longer ranges in forecasts, the recent NOAA prediction
for magnetic storminess during Cycle 23 is shown below:


Cycle 23 Magnetic Storms
Minor Major Severe

1997 Year 1 12 4 1
1998 Year 2 15 7 2
1999 Year 3 24 17 4
2000 Year 4 29 18 3
2001 Year 5 26 11 3
2002 Year 6 30 23 5
2003 Year 7 33 16 3
2004 Year 8 34 12 2
2005 Year 9 42 17 2
2006 Year 10 34 6 1
2007 Year 11 15 4 1

Given that forecast, we can look forward to major storm activity
rising to about 2 per month by Year 6 (2002) in Cycle 23. That is
not a good prospect but there are uncertainties in forecasts so
one can hope for less and see what happens.

The 10.7 cm solar flux is an indication of active regions on the
solar disk and that is a quantity that warrants logging. Early in
a cycle, new active regions begin to appear but later, some
regions are quite stable, particularly around solar maximum, and
knowing when the flux may peak again is quite helpful to DXers.

The origins of the magnetic activity differ throughout a solar
cycle, however, with early part of the cycle giving more of the
sporadic coronal mass ejections responsible for solar wind blasts
hitting the magnetosphere. On the other hand, the latter part of
a cycle is one characterized by fast streams from coronal holes
sweeping past the earth. Those can be long-lasting so logging
magnetic activity, with the A-index from Boulder for several solar
rotations is a good idea, enabling one to avoid times of strong
magnetic activity.

One aspect of strong magnetic activity is equatorward expansion of
auroral displays, associated with the loss of magnetic field lines
from the front of the magnetosphere to the magneto-tail. From the
standpoint of propagation, that results in very low MUFs in the
polar cap. But it is accompanied by an expansion of the polar cap
that can bring on heavy, long-duration ionospheric absorption.
That is the case with solar proton events, so-called polar cap
absorption (PCA) events. Those events differ in striking ways
with auroral absorption (AA) events but both can be present at the
same time. Those events will be our next topic of discussion.

Research Notes:

I have already given some words of praise for the book,"Physics of
the Upper Polar Atmosphere", by A. Brekke. To that I would like
to add that the front cover has an ABSOLUTELY FANTISTIC photo of
an aurora taken from a satellite. There is a catch, however; the
photo was made in Antarctica and the book must be turned upside
down to get the aurora positioned OVER the polar cap. But like
Confucius said, "A graphic is worth many kilobytes of text."

Friends in Radio Land -

We are now into disturbances of propagation, those nasty things
that can plague us, sometimes without our even knowing it. The
last topic was magnetic storms and aurora. Those represent
disturbances of the F- and E-regions, respectively.

The effects of magnetic storms can be world-wide in the sense that
ionospheric electrons are removed from field lines, lowering the
MUFs on paths across great distances. The part of the ionosphere
which is disturbed the most is in the polar cap as that is the
region whose field lines are most at risk. And recovery from
magnetic storms is a slow process, requiring the electrons in the
F-region be re-supplied by sunlight, a slow, tedious process which
can take days after a severe storm.

The effects of an aurora, by itself, are much more localized in
the sense that the increased ionization is confined to the field
lines that guided auroral electrons downward. Short of being
in a full-blown magnetic storm, the effects tend to be brief,
measured in minutes or hours, and when the aurora ends, it is a
fairly rapid process. Essentially, the problem is to have the
electrons in the ionization recombine with the positive ions which
were generated by the influx of energetic auroral electrons.

But now we come to solar proton events. Those will affect the D-
region and originate on the sun, with protons and other particles
accelerated up to energies of millions, sometimes even billions,
of electron-Volts (MeV or BeV). So solar proton energies, from
acceleration on the sun, are high in contrast to those of auroral
electrons which are accelerated locally, within the magnetosphere,
up to tens of kiloelectron-Volts. The protons are accelerated in
connection with some solar flares and then can leave the scene,
passing through both the solar and the interplanetary field.

The interplanetary field generally points toward or away from the
sun and the outward progress of protons depends on the degree to
which they go along the field lines or perpendicular to them as
they leave the sun. But the interplanetary field is not well-
ordered like the geomagnetic field close to the earth so protons
will diffuse through the region and their progress will depend on
their momentum or the radius of curvature of their path. The more
energetic protons will have radii of curvature which are large
compared to the scale-size of field variations so those protons
will follow more rectilinear paths. On the other hand, less
energetic protons will have smaller radii of curvature in the
field and their progress will be more like diffusion, scattered by
the small-scale, organized portions of the interplanetary field.

All that is a way of saying that the high energy-protons will
leave the region close to the sun faster and make their effects
felt more promptly, albeit briefly. On the other hand, the low-
energy protons will diffuse slowly through the field and their
effects will be of longer duration. It should not be forgotten,
however, that the duration of the acceleration process is of
interest too. Generally, it is considered to be the same as the
actual flare process but those can be brief, in minutes, or
longer, measured in hours.

Another way of saying the same thing is if the flare region is off
the to the east of the solar disk, solar protons heading toward
the earth will have to stagger through the field lines which are
more or less perpendicular to their paths. That is a slower
process and protons can be held in the magnetic field region for
times which are long compared to the acceleration process that
started them. As an example, I had experience with one east limb
event in August '79 where the solar protons finally reached the
ionosphere 18 hours after the flare! Staggering, diffusion? Yep!

On the other hand, flare sites toward the west limb of the sun
send protons out into the field which generally trails behind the
rotating sun and we get "sprayed", as it were, by protons going
along the field lines. That is called the "garden hose" effect.
The Great Solar Flare Event of February 23 1956 was a case in
point, a west limb flare where the travel time was measured in
minutes. Those were relativistic particles and had so much energy
(over 10 BeV) that they penetrated to ground level, even at the
magnetic equator! Been there, seen that!

But what are their effects? Given the remarks in the last
paragraph, one can expect that the duration of the proton
bombardment of the earth will depend on the location of the flare
site. That is one propagation clue that NOAA provides with every
announcement of a solar flare, the solar longitude involved. So
that is one item of interest, east or west of central meridian.

But as to the effects of the protons, those depend on their flux
or number per square-cm per second and proton energy. The low-
flux, low-energy solar proton events were only conjecture until
the Space Age but are detected nowadays by satellites and one can
see the data in the Tiger Plots on a NOAA website. But events
with higher fluxes and greater energies can penetrate the earth's
field and get reach into the ionosphere, the atmosphere and, on
rare occasions, they can reach ground level.

Our interest, of course, is with ionospheric effects and being
energetic charged particles, the protons will leave a wake of
ionization as they plow through the atmosphere. The extent of the
wake will depend on the relative numbers of protons in the various
energy ranges - around 1 MeV, around 10 MeV, near 100 MeV and
beyond. But generally, being both energetic and massive particles
as compared to puny auroral electrons, protons penetrate deeper
into the ionosphere (if they get that far through the geomagnetic
field) and the heavy ionization near the end of their physical
ranges can cause huge ionospheric absorption of signals because of
the greater electron-neutral collision rate deep in the D-region.

For solar protons to get down to the ionosphere, they must first
enter the geomagnetic field out at the magnetopause and then
follow field lines, according on their momentum. The present
view of these matters is in sharp contrast with the early days of
ionospheric radio. Then, the dipole model of the earth's field
was taken as the standard and all discussions about the effects of
solar protons were based on work done by the Carl Stoermer, the
Norwegian auroral physicist. So the idea was that protons were
sorted out according to momentum (or energy) by the field and
there was a sharp cut-off energy which varied with latitude. But
with the IGY, things changed; the use of riometers, looking at
ionospheric absorption due to the protons, showed that the cut-off
idea was all wrong and the polar cap was wide open, full of low-
energy protons, all the way down to the auroral zones where the
cut-off energy was supposed to be 100 MeV. That was one of the
first clues that the earth's field was not that of a dipole; then
measurements made by satellite-borne magnetometers gave the final
story, with the field configuration I've sketched earlier.

The coverage of the large polar cap area with solar protons is in
sharp contrast with the narrow latitudinal coverage of the auroral
zones by energetic electrons; beyond that, there is the difference
in levels of absorption, tens of dB on 30 MHz for solar protons as
compared to a few dB for the auroral electrons. So all in all,
solar proton events that reach the ionosphere, so-called polar cap
absorption (PCA) events, can be devastating when it comes to
propagation across the high latitudes.

But there are few more aspects to PCAs to think about. For
example, the access for solar protons to the polar cap is one
thing but it has been found that solar protons can get into the
magnetosphere via the magnetotail. And the access to the two
polar caps is not always equal for solar protons, judging by
satellite data. So there can be different ionospheric reports
from the two polar caps, depending on sunlight on each and the
access of the protons. All this makes propagation interesting and
confusing!

When it comes to ham radio propagation, there is a propagation
effect that can mask the access to the polar caps. Here, I refer
to the fact that there is a reduction in ionospheric absorption in
darkness, the number of dB in absorption going down by a factor
the order of 5 or so. This is due to the fact that the electrons
created by solar protons may attach themselves to oxygen molecules
and form negative ions. Negative ions are so massive that they do
not participate in the absorption process. So absorption in a
darkened polar cap, at night or in winter, is less and might be
interpreted as a low proton flux without satellite data to clarify
the situation.

The electrons bound in negative ions are released when sunlight is
restored to the D-region. That is the case for proton events but
not for auroral electron events where the ionization is at much
higher altitudes and electron detachment results from collisions
with atomic oxygen, abundant above 100 km. So auroral absorption
(AA) events do not show any day/night effect like PCA events.

To summarize now and put things in perspective: auroral absorption
events are limited in time and space, found during magnetic
disturbances, large or small. Polar cap absorption covers a wide
range of latitudes, the whole polar cap, and can last for days at
a time after some solar flares. And the ionospheric absorption is
large, making PCAs a real threat to ham radio communications. And
if the polar cap expands in size in the late phase of a magnetic
storm, solar protons can then reach down to much lower latitudes
and have even greater effects of our HF propagation.

The beauty of PCAs, if one would call it that, is that they are
relatively infrequent. The real threat to ham radio communication
is the effects of the solar wind, so I would say that magnetic
storming is the thing to watch out for, by logging K-and A-indices
to identify any possible repititions and then by checking each day
by whatever means are available. Magnetic storming is THE threat
to our peace and quiet; what the sun provides in the way of higher
critical frequencies by UV radiation can be taken away in a jiffy
by a blast of the solar wind triggering a magnetic storm, minor or
major.

So monitor/log the magnetic indices; they hold the key to success
in high latitude DXing on the bands! But when the high latitudes
are disrupted, try the other directions, say across the equator.
That is pretty safe, the field lines there being shielded from the
ravages of the solar wind. And there's a lot of rare DX there to
make things interesting.

Friends in Radio Land -

This is the end of the line and time to wrap up the discussion.
It should be in two parts, the theoretical side which we compare
with the experimental part. In regard to theory, the most general
discussion would be one which uses ray-tracing with the best
available model for the ionosphere and geomagnetic field. That is
simple to say but as you know, words come easy. But let's look at
how it's done and what it means to us. Then we can go to the
experimental part.

Now it may sound strange but the magneto-ionic theory that I
mentioned earlier is all cast in terms of frequencies. Obviously,
the operating frequency is of utmost importance. But then there
are three other frequencies; how they compare with the operating
frequency (QRG) determines features of propagation.

The first frequency is the plasma frequency; for a given position
in the ionosphere, it is another way of specifying the electron
density. Plasma frequencies in the lower ionosphere increase with
height, up to the F-region peak, and decrease with latitude toward
the poles. And, in a complicated way, they depend on the earth's
magnetic field and sunlight. But for signals to be contained, not
penetrating into the topside of the ionosphere, their effective
vertical frequency (EVF) must be less than the plasma frequency at
the peak of the F-region.

The second frequency is the collision frequency between electrons
and the neutral constituents which surround them. As you know,
collision frequencies Fc determine ionospheric absorption and are
greatest (<2 MHz region) in the lower ionosphere. The comparison
of interest is the operating frequency QRG and Fc. If QRG >> Fc,
then ionospheric absorption is not of great importance. And a
good example of that would be up on the 10 meter band. But the
plasma frequency is still of great importance as well as sunlight 
on a path.

The third frequency is the electron gyro-frequency Fg, the number
of times per second an electron goes around the local field lines.
For the geomagnetic field, that ranges from 0.6 to about 1.6 MHz,
in going from low latitudes to polar regions. And the comparison
between QRG and Fg becomes very important down on the 160 meter
band as 1.8 MHz is comparable to values of Fg along a path. The
consequences of including the geomagnetic field in ionospheric
theory are very important and should not be overlooked in thinking
about propagation.

Before getting to them, we should recognize that geomagnetic
effects have been neglected in almost all the discussion so far.
True, it was pointed out that the earth's field serves to keep
ionospheric electrons from running away, once released, but that
was about it. So for most amateurs, theory is quite simple: some
ionospheric absorption on the lower bands but otherwise, RF is
linearly polarized, depending on the transmitting antenna.
But all that changed when Appleton embarked on formulating a more
general theory which included the geomagnetic field. The results
are not to difficult to obtain but hard to comprehend, given that
the earlier theory is so deeply ingrained in our thinking. But
let's take a look at a few of them and see how things go.

First, the strength and direction of the local magnetic field is
important and propagation depends on the direction of wave travel
relative to the magnetic field. That is a new idea to most hams
but is the case as in the more general theory, RF waves are now
elliptically polarized, depending on the direction of propagation.
That may be hard to picture so think of a wave moving along with
its E-field vector going around the direction of propagation but
with varying amplitude as its tip traces out an ellipse.

Not only are waves elliptically polarized but there are two types,
depending on the direction of rotation of the electric field -
ordinary and extra-ordinary waves. The two waves propagate with
different speeds and, oddly enough, are absorbed in the ionosphere
(remember the collision frequency?) at different rates.

Rather than leaving things as they stand at this point, it should
be noted that the wave polarizations go over to simpler cases when
propagation is along or perpendicular to the field direction.
To use modern advertising parlance, there are also cases in the
"not exactly" category, quasi-longitudinal and quasi-transverse
propagation where the waves are close to, but "not exactly", the
strict limits mentioned above. That makes magneto-ionic theory
less stern and forbidding as the elliptically polarized waves are
close to circular or linear in those cases.

That is a brief summary of what happens to RF when the QRG is
comparable to the electron gyro-frequency, say around 1.8 MHz.
Added to that is the idea of limiting polarizations where RF
enters or leaves the lower ionosphere. So there could be a
mis-match between wave polarization at launch and the limiting
polarization at the bottom of the D-region. In that case, the
mis-match between the two polarizations means the coupling of RF
into the ionosphere is less than 100% That is part of the "bad news"
at the low end of the amateur spectrum. Of course, there is also
the question of the how the polarization of the emerging wave
matches that of the receiving antenna. And the other "bad news"
is one mode, the extra-ordinary polarization, is heavily absorbed
over distance, meaning that more power could be lost from that
effect.

All this emerged when Appleton worked through the more general
theory of how ionospheric electrons respond to RF in the presence
of the geomagnetic field. Once that is done, the next step is to
incorporate the results into the "equations of motion" for waves
and do ray-tracing with the best field model available. The
consequences are interesting, as you can imagine, with the
important result that ducting is possible just with the typical
electron density gradients present in the ionosphere.

All this is probably more than you wanted to read about but you
should know that the simple ideas that are abroad are not the
final story. But one idea from magneto-ionic theory that applies
at frequencies way beyond the electron gyro-frequency is the
rotation of the plane of wave polarization. Ordinarily, changes
in HF polarization are attributed to ionospheric tilts, not an
effect from the magnetic field. But it is real, seen with
satellites on VHF.

The idea comes from sending linearly-polarized signals along the
field direction. If you think about it, a linearly-polarized wave
is the same as the sum of two circularly polarized-waves of equal
amplitude but rotating in opposite directions. The rest is
straight-forward as the two circular polarized waves travel with
different speeds, meaning that one gets ahead of the other, and
the polarization of the resultant linearly-polarized wave is
rotated as it travels along. That is Faraday Rotation and is an
important part of work on VHF where two circular polarizations can
be present with essentially equal amplitudes.

But a problem with Faraday Rotation comes up on the lower bands
as the extra-ordinary wave is heavily absorbed and over any great
distance, the ordinary wave is the only one that survives. So it
is not so much a question of Faraday Rotation on 1.8 MHz but one
of the remaining ordinary polarization and how it compares with
the limiting polarizations at the bottom of the ionosphere and
antenna polarizations.

As for the experimental side, that really deals with what we know
about our surroundings. Starting from the ground and going up -
the geomagnetic field, the neutral atmosphere, how solar radiation
affects the atmosphere and creates the ionosphere, the solar wind
and its effects on (or in) the earth's field, the solar magnetic
field and solar activity. There's a lot to know and more to the
point, it's important to appreciate that we're dealing with a
coupled system. So any effect that is dealt with in isolation may
not be well understood.

The present situation as far as propagation is concerned depends
on the use of computers and that brings up the question about the
programs that are available. For the geomagnetic field, there is
the International Geomagnetic Reference Field (IGRF) while the
models of the ionosphere are found in the Internation Reference
Ionosphere (IRI 90). Those two serve as research sources but also
find their way into software such as PropLab Pro.

Then there are also the various propagation programs that are
available at present. Viewed by themselves, they are efforts done
in isolation with quiet-day representations of the ionosphere. So
additional consideration must be given to the details of the
critical frequencies all along a path and also the geomagnetic
circumstances and any unusual ionization, say from solar protons.
That's where mapping programs and the NOAA websites on the
Internet prove their value. Without using that information, it is
hardly possible to make a realistic prediction of anything.

As an example, the week of Nov. 8-14 was characterized as one of
considerable magnetic activity and solar activity. Thus, the
following A-indices were reported from the Boulder magnetometer:

Sun: 68, Mon: 78, Tues: 6, Wed: 4, Thurs: 4, Fri: 60, Sat: 38

Without that knowledge, the results for propagation conditions
from a computer program, using only input with regard to sunspot
counts, would make you think you live on a different planet as
they would have little bearing on actual conditions.

But that is not the whole story as the coronal mass ejection
that was responsible for the magnetic activity also produced a
solar proton event on November 14. Then, 10 MeV protons, which
are capable of reaching the ionosphere, appeared at satellite
altitudes around 0600 GMT. The the proton flux peaked at 300
p.f.u. (proton flux units or protons/sq-cm/sec/ster) around 1245
GMT and continued coming out of the interplanetary field for more
than a day. Also, there was a weak flux (6 p.f.u.) of 100 MeV
protons, capable of reaching balloon altitudes (about 30 km), was
present. In addition, there was a strong increase in 1-8 A Xray
background on the 13th.

As I said, these are coupled systems and we have to look at more
than one limited aspect if propagation is really our interest. Of
course, as we go toward solar maximum, this will be the case more
and more often. But on the cheery side, the week of Nov. 8-14 has
to be an exception. For example, in the year that I spent in my
long-path study around the maximum in Cycle 22 , something like
80% of the days were free of any significant disturbance and even
with minor or major disturbances on the rest of the days, I was
able to make a long-path contact on over 90% of the days.

That suggests a cautious but optimistic approach is called for,
watching all the disturbance indicators on a regular basis, "going
for it" when propagation looks good and even "looking around" when
conditions may not be the most promising. I like to say "DXing is
an intellectual pursuit" so it's worth a bit of study; that makes
the rewards all the more enjoyable.

Conclusion -

I think I've said all I wanted to so let me close with words of 
a great man that I'm sure you'll recognize: "That's all folks!"

73,

Bob, NM7M