Eclipses and the D-layer absorption
We all know that at daytime, the level of absorption of shortwaves by the D-layer of the ionosphere is high, reaching at noon 25 dB on the 80-meter band, and up to 33 dB at short distances on the 180-meter band. Amateurs can experiment a relatively simple method to estimate this absorption level of the D-layer (and any other ionospheric layer).
It consists in measuring the echoes of an omnidirectional beacon. There are well some low band beacons available but their power is far to weak (often < 1 W) to be of some use. The solution consists thus in setup a transmitter connected to vertical antenna or a dipole in the field emitting powerful tunes during a ten of seconds. The receiver will be constituted either of a digisonde (mostly used by professionals) or simpler by a team of amateurs spread in several near countries, and measuring the strenght of the beacon signal thanks to simple antennas like long wires.
The constraint of this solution is that amateurs have to make the measurements not only at daytime but also at night to determine the level of absorption, leaving the emitter working on the air non-stop during a few days to get accurate data. It is thus better to use this method during holidays or to entrust it to retired or unemployed amateurs who are not afraid to work late in the evening or in the first hour of the morning several consecutive days.
But there is another solution : it is to take advantage of a total eclipse of the Sun. Occuring at daytime, we can observe directly the change in the level of absorption in contacting amateurs before and during the eclipse. This is this experiment that we are going to analyze.
Effect of the MUF
These measurement are however proned to a bias factor, the critical frequency, MUF. Knowing that the D-layer disappears at night, the reflection is then realized at the level of the F-layer, thus at about 250 km aloft instead of 50 or 90 km, but only below certain angles and frequencies as we explained in the referenced link. However, as the frequency increases, at night at mid and high latitudes, we rapidly exceed the MUF; in other words there is no more propagation, even for kW-class transmitters. This effect applies mainly to short distance contacts (less than 250-500 km) where stations need to use the high angle radiation of their antenna to get a reflection at short distance. It is thus recommended to make such experiments at frequencies below or close to the MUF but not over, when possible. This problem concerns all measurements made during the night, but also at eve or at dawn.
Knowing the critical frequency for the F-layer, the antenna radiation pattern for different distances from the beacon, and the height of the D- and F-layers, we can estimate the "boost factor" in absence of the Sun.
How to proceed in the field ? One of the most amazing experience easy to set up is using the scare times during which you can assist to a total eclipse of the Sun within 3000 km away or so. Indeed, not only observing a total eclipse of the Sun is one of the most exciting natural phenomenon at which you can assist, but this event is also very interesting to work remote stations on low bands, whether you are equipped with a dedicated low band antenna or a simple dipole and a bare foot transceiver
Beside the marvelous "black Sun" crowned with its corona and prominences, at the instant of the totality, not only you will assist to a high drop of temperature (15°), an increasing of the relative moisture level (40 to 80%), and a drop of the light intensity (88000 to 350 Lux), but the propagation conditions will change also drastically. During the phase of totality, we observe that the D-layer disappears over a large area, much wider that the physical occulted area projected on the ground (274 km wide maximum). Thanks to the reflection on the F-layer signals reach distances over 3000 km in only one hop (on 160 meters), and free of most absorption, they are also much stronger than during the hour preceeding the partial phase.
The Amateur radio experiment
Here is an example of amateur radio experiment conducted by J.C. Jodogne from the belgian Royal Meteorological Institute (RMI) in collaboration with ON4UBA, the amateur radio station of the UBA, the belgian IARU society under the direction of John Devoldere, ON4UN. They asked to UBA members to listen a beacon of 600 W on 160, 80 and 40-meter bands before and during the August 11, 1999 eclipse of the Sun that occured over south Belgium and Luxembourg among other countries (best site over Turkey), and to report their results for further processing.
The beacons were installed at Ghent, in the North-Western part of Belgium, and near Arlon, in the southern part of Belgium, close to the line of centrality but not inside. These beacons sent a 16-seconds tune as well as their respective callsign ON4UBA/B in CW at 25 WPM, then left a short break of 6 seconds before recycling. The beacons were active between August 7 to 10 between 03:00-19:00 UTC, and between 03:30 UTC and 13:00 UTC the day of the eclipse.
During the four days preceeding the event, UBA asked amateurs to make reference recordings on 80 and 40 meters in order to calibrate the system and check the system for any artifact or unattended event. As the team expected a strong attenuation on 40 meters and focused on this band, the 160 meters was not much sollicited as there is only ground waves propagation on this band at day time, and no propagation via ionospheric reflection, most of the long distance traffic being made at night. To avoid a lack of propagation at short distance two dipoles cut for 40 and 80 meters were setup in Arlon, one place 12m high the other 20m high to preserve their radiation pattern. On 160 meters a vertical was used showing a main take-off angle of 25-30°.
To log data, Jacques Debouche, ON5OO, provided to all observers (there were 150 participants) a logging software recording automatically the beacon strenght all through the few days that last the experiment (before the eclipse too to get a reference base). ON6BG provided the team with a pen recorder that delivered high resolution fading measurement too on the three frequencies.
The team received nearly 150 reports, most on 40 and 80 meters, The team received however enough report on 160 meters to draw some trends, including a handful of VLF reports from CCRM, the equivalent of 600 MB of data to process.
What did this experiment learn us about effects of an eclipse on the D-layer absorption level ? The Moon moving at about 2850 km/h in front of the Sun, and being not at its closest distance to the Earth, the instant of totality last only 2m27s (the maximum length is 7m58s but it has never happened in ten thousands years), and occured at 10:26 UTC near Arlon. The absorption by the D-layer at daytime was easy to record with a low speed pen recorder. Just before the sunrise, the beacon signal reached -15 dB on 80 meters, and was thus easy readable all across Europe, including in the USA. From dawn, on 160- or 80-meter band, each hour the beacon signal decreased of about 10 dB to reach the background hash by 12:30 UTC, when the Sun was no more occulted and at its maximum heigth in the sky over the beacon. At night, on the 80-meter band the background hash reached approximatively -43 dB and even -47 dB two hours after the totality, once the eclipse was over.
But what happened on low bands during the eclipse ? On 160 meters (1831.5 kHz), at the instant of the totality, all stations located a few hundreds kilometers (50-1000 km) both sides of the eclipse path (perpendical to the path) saw a boost of the beacon signal between 30-60 dB during 30 minutes to more than two hours, the boost showing a bell shape around the instant of the totality. The signals were subject to very fast fading with a period about 30 seconds, including periods of deep fading lasting 5 minutes or so and periods less disturbed.
As expected, the more spectacular effect occured on the 80-meter band (3522.5 kHz). As the MUF for the F-layer was at 4.7 MHz, there were always more than one signal path and up to four hops depending on the distance to travel. In the field, as displayed above, during 45 minutes preceeding and following the totality, all stations experimented a strong signal over S9 with a boost up to 26 dB during the totality. On the contrary, all stations located at short distance from the beacon (less than 250 km) experimented a short-term fading of typically 10-15 dB instead of the regular sinusoidal signal experimented in presence of the Sun. This fast fading was of totally random nature, probably causes by multipath propagation. All observers noticed also that the beacon signal peak was observed not at the instant of the totality but between 5 to 15 minutes before the event.
On the 40-meter band (7012.5 kHz), as the MUF was at 6325 kHz at the instant of totality, at short distances it was interesting to note that all stations located less than 250 km experimented a complete blackout during 90 minutes around the totality. This confirmed the close relationship between the critical frequency and the signal dropout. This is the same phenomenon that boosted the propagation around 5h30-6h UTC. Some stations however, located a bit farther and using probably a lower angle of radiation received the beacon with 5 dB gain more than before or after the eclipse, the signal peaking at the instant of the totality.
By way of conclusion
One more time this experiment shows that amateurs can provide a great amount of useful information to professionals who can so study further the various effects of a total solar eclipse. As there are between 2 and 7 eclipses (partial, annular or total) of the Sun each year, we have a lot of opportunities to develop this fascinating study, all the more that it involves amateurs from various countries simultaneously and does not require sophisticated and cumbersome low band antennas.
For your information, the next total eclipse of the Sun will occur in Europe on August 12, 2026 where the North of Spain will be gratify of 2m18s of totality. For all Western European that will be an excellent opportunity to see how far and with what power you will reach stations in North and Central Africa on 160 and 80 meters, and how deep will be the blackout on 40 meters.
You will find a complete review of this amateur radio experiment in the January 2000 issue of QST as well as in CQ-QSO and various local newspapers.
Good luck !