It is well known HF radio propagation is affected by many factors. The
most well known is the 11-year solar cycle. As sunspots increase and decrease over this period,
so does propagation. Coronal mass ejections, the result of solar flares, prominences and
coronal holes, have short-term dramatic affects and are less predictable. They induce
geomagnetic storms that often attenuate signals for days. Those concerned with HF
communication are very familiar with the solar cycle and geomagnetic activity. Unfortunately,
even with a good understanding of these phenomena, there remains a great deal of uncertainty in
predicting HF conditions. The critical frequency often referred to as the MUF (maximum usable
frequency) is ever changing. In addition to the two factors mentioned, the critical frequency
also depends on geographical latitude, localized magnetic anomalies, time of day, temporary
ionospheric conditions like sporadic-E or meteor showers, etc.
Most of us throw up our hands at this point. There are too many factors to
consider. Predicting band conditions appears to be nothing more than a guessing game. This is
true to some degree, especially on a daily basis. Long term prediction is somewhat easier. As
I mentioned above, we all know about the 11 year cycle and can predict propagation trends with
reasonable certainty over a span of years.
Superimposed on this 11 year cycle is yet another relatively stable and
predictable cycle. This is yearly propagation variability. As I will explain, research has
shown a repeating trend that is remarkably consistent.
The "Lack of Activity" Factor
Amateurs who frequent the HF bands know activity drops off during local
summer. There are several reasons for this. Perhaps the biggest reason is that people enjoy
the fine weather. They spend more of their free time on other activities. It's hard to justify
sitting in front of the rig when the sun is shinning and it's 25 to 30 degrees outside. Those
who remain dedicated to amateur radio during this time may still not be on the air as often.
Antenna projects and similar outdoor station enhancements are typically scheduled for the summer.
Another reason is the increase in the QRN, especially below 20 meters. 20 db over S-9 static
crashes are normal on the lower bands during the summer. While propagation may be good, it is
difficult to carry on a QSO under these circumstances.
It is important to remember the cycle I will be describing is independent
of activity. Often we tune our favourite band, hear little or nothing, and assume conditions
are poor. We may notice this more in the summer and incorrectly conclude the bands are in bad
shape. As we will see, April, May and June can be good months for DX from a propagation
standpoint. They are also good months for fishing, swimming and barbecues.
Seasonal Variation of the Critical Frequency
If we remove the lack of activity and low band QRN factors, given
comparable solar flux, shouldn't propagation be the same year round? It isn't. Even when the
flux is high (>200 SFU) we often lose 10, 12 and sometimes 15 meters for days at a time during
certain periods of the year. This happens even in the absence of upset magnetic conditions.
We all know the most common form of HF propagation is ionospheric F2
refraction of radio waves. We also know it is the ultraviolet radiation from the Sun that
ionizes the upper atmosphere. The MUF, or critical frequency, is directly proportional to the
amount of F2 ionization. As the level of F2 ionization increases so does the critical frequency
, often exceeding 50 MHz in the three years around the solar maximum. It has been proven that
the 10.7 cm solar flux is directly proportional to the sunspot number and thus to the amount of
ultraviolet radiation produced by the Sun. It has also long been thought that the degree of
planetary F2 ionization is proportional to the amount of ultraviolet radiation reaching earth.
Research is now showing that when we analyze the degree of planetary F2
ionization over a long period of time, we find the same level of solar activity generates
significantly different amounts of global ionization. This depends on the position of the Earth
during its yearly revolution around the Sun. The ionization density, and thus the critical
frequency, exhibits a pronounced maximum around the equinoxes. Conversely, there is a strong
minimum around the solstices. Ten years of historical data from 1953 to 1963 was analyzed by
Chaman Lal(a). This analysis covered solar cycle 19 that began in 1953 and peaked in the winter
of 1957. Chaman Lal's work clearly shows that "a remarkable feature of the planetary ionization
. . . is the appearance of semi-annual maxima that occur regularly and persistently during the
months of April and October and a minimum which appears in July every year." His work shows
that, given equal solar flux, the level of F2 ionization during the equinoxes is about 50%
greater than in July.
This phenomenon is independent of the 11-year solar cycle. The yearly
average of the critical frequency follows the solar cycle very closely. However, we see the
equinoctial/solstitial maximum/minimum ratio every year regardless of the point in the cycle.
Seasonal Variation in Geomagnetic Activity
As with F2 ionization, a great deal of research has been done on the
statistical nature of geomagnetic activity. It has been recognized for over fifty years that
these activities exhibit a marked semi-annual variation(c). A study of the Ap magnetic index
for the years 1932 through 1989 clearly shows this trend. Even before good statistical records
were kept, it was obvious that auroras were more common during the equinoctial periods.
Geomagnetic activity usually reaches its minimum during the solstitial months of June and
December and a maximum around March and September.
It has also been noted that major geomagnetic storms occur most often in
the spring and fall. There were forty-two major magnetic storms from 1940 to 1990. None
occurred during the months of June or December. 40% occurred during March and September(b).
Why the Seasonal Variability?
This research explains what a lot of DXers already know by experience and
intuition. We usually work more DX in October-November and April-May than we do in
January-February and July-August. We now have a scientific explanation for a trend that we knew
was there. However, some questions remain. Why should the time of year have any affect on how
much a given quantity of ultraviolet light ionizes the F2 layer? Why should it have any bearing
on how upset the geomagnetic field becomes? Are not both results of happenings on the Sun, not
Earth?
It is clear that coronal mass ejections (which upset Earth's magnetic field
) do not occur with any pattern that can be correlated to the seasons on Earth. A fairly good
explanation of this variation in magnetic activity has been presented by researchers at various
sites(b). The physics involved are quite complex. Essentially it is the orientation of Earth's
magnetic field with respect to the interplanetary magnetic field within the solar wind. When
solar material and shock waves reach Earth their effects may be enhanced or damped depending on
the angle at which they arrive.
The question of the cyclic F2 ionization remains unanswered. Its similar
behaviour to the seasonal geomagnetic variations suggests they may be related. Further research
is needed before this conclusion can be made with any degree of certainty.
Summary
I have presented an explanation of why we work more DX in the spring and
fall than in the winter and summer. It is obvious that historical data shows a seasonal trend
that cannot be explained as random chance. F2 layer HF radio propagation is enhanced during the
equinox and attenuated during the solstitial months.
References:
(a) Chaman Lal, Global F2 Ionization and Geomagnetic Activity, Journal of Geophysical Research (Space Physics), Vol. 97, No. A8, pages 12,153-12,159, August 1, 1992
(b) Crooker N.U., Cliver E.W., and Tsurutani B.T., The Semiannual Variation of Great Geomagnetic Storms and the Postshock Russell-McPherron Effect Preceding Coronal Mass Ejecta, Geophysical Research Letters, Vol. 19, No. 5, pages 429-432, March 3, 1992
(c) Cortie, A.L., Sunspots and Terrestrial Magnetic Phenomena, 1893-1911, Mon. Not. R. Astron. Soc., 73, 52-60, 1912