A possible way of Forecasting good Radio Propagation conditions on 136kHz

Alan Melia G3NYK

As I described in my earlier articles and presentations to the HF Conventions, it is possible to forecast the collapse of propagation on LF by means of the Planetary K geomagnetic Index (Kp).. A value of 0 or 1 means quiet geomagnetic conditions and values up to 9 indicate disturbances up to severe storm. It has been observed that night-time conditions decline about two days after a minor storm (Kp=5) or more vigourous disturbance. The problem for radio amateurs, seeking favourable conditions for long distance experiments, has been that the path does not recover when the Kp index drops back into the "quiet" range ( 1 to 3) again. My observations, confirmed by reference to professional papers on the subject, suggest that there is an excess of absorbing ionisation in the D-layer, through which night-time signals must pass before being returned earthwards by refraction in the lower boundary of the E-layer. This absorbing D-layer ionisation seems to remain for some time after the geomagnetic conditions recover. I had postulated that "hot" electrons ( high energy electrons ~100Kev ) might survive for a considerable time in the D-layer. This idea is not very tenable because the pressure at 50 to 80kms is such that there would be many collisions with neutral atoms and ions, and even allowing for the mass differences, the electrons would soon loose energy and become liable to capture. The effect of a severe geomagnetic storm (Kp=9) can often be seen in excess absorption of a radio signal at night for periods of up to 30 days after the event. The end result of this is that whilst it was possible to forecast the collapse of "good conditions", it was only possible to estimate roughly the delay before recovery by reference to the severity of the causing storm. This is often further complicated by smaller events occurring during the "recovery period".

I have recently been made aware of some current papers discussing the contribution of the "Equatorial Ring Current" to auroral activity. There still seems to be some controversy about some of the theories, but the supporting evidence is growing. The equatorial ring current is a stream of charged particles, both electrons and ions, that circulate under the influence of the Earth's magnetic field at a distance of between 2 and 7 Earth radii above the equator. It would seem that ions and electrons from the Coronal Mass Ejections are trapped in the Magnetosphere "tail", which is on the "leeward" side of the Earth, and there are fed into the "ring current". There is an interaction with the ionsphere on the sunward side of the earth where the geomagnetic field lines are distorted by the radiation pressure and solar wind effects. The exact details are being hotly contested by geophysicists, but the basic idea of the ring current as a "reservoir" of charged particles which may be injected into the ionosphere could have some interesting results for radio propagation.

The particle densities in the ring current have been measured by satellites, but the "intensity" of the ring current can be deduced from the variations in the horizonatal component of the geomagnetic field at the Equator. An index, refered to as Dst, which I believe refers to Disturbance Storm Time, is calculated by reference to a number of variables. A reliable figure is only available several weeks in arrears, but some University research establishments are attempting to predict a "real time value" for Dst. In simple terms I believe we may consider the index Dst as a measure of the number of charged particles in the ring current, and so the decay of the Dst index after a geomagnetic storm could be related to the ablity of the ring current to precipitate absorbing charge into the D-layer. One encouraging feature is that the Dst index decays much more slowly than the Kp index, and shows the "topping-up" effect of minor geomagnetic events during this "recovery period"

The chart below is a re-plotting of my previously published data on the received strength of the CFH transmission on 137.00kHz against the published values for Dst (blue) from Kyoto University for that year. The graph line (red) for CFH signal strength has been moved forward 3 days to allow for the delay in precipitated electrons reaching the D-layer at the appropriate latitude. The ordinate of the red curve is the number of minutes in the night when the CFH signal exceeded 30dBuV at my location. It can be seen that in general the peak values occur when the Dst is below 40nT. Geomagnetic storms produce a large negative excursions in Dst and it can be seen that in these cases the red trace drops to zero. The cumulative effect of small negative excursions in Dst during the signal recovery phase can be seen at several places.

The definition of "good conditions" is somewhat subjective and very dependent upon the mode of transmission and the message length being used. The highest signal levels recorded are usually before the conditions have become totally "quiet". At this time there is sufficient ionisation to support multiple paths which can "interfere" constructively and produce to produce signal levels up to 6dB ( for two paths of approximately the same path loss ) in excess of those from a single propagation path. Likewise at some locations the multiple paths may interfer destructively leading to reduced signal levels.

I have begun to include a table of the last 30days Dst index in my Propagation Reports. During the period of observations so far, CFH in Halifac Nova Scotia has been off-air and I have been unable to obtain any DCF39 measurements from Brian CT1DRP due to a Computer and station rebuild. Daily real time plots have been available from Steve Dove W3EEE and observations have been made of Joe Craig's (VO1NA) signals in the UK. A recent expedition by Ed RU6LA to Vladivostok, allowed monitoring of the UA0 to ZL path (approximately 10300 kms) over a number of nights. All the subjective observations from this period suggest that best conditions seem to be when the Dst index has "dropped" close to zero, below -20nT. ( In fact this is really "rising" because a geomagnetic storm depresses the Dst index negative in the -100 to -400 range. ) Even small excursions in the index (rising to -30) caused by changes in the Solar wind, have been noted to coincide with slightly degraded conditions.

In conclusion it would seem to be possible to provide reasonably accurate propagation forecasts for long distance night-time paths at 136khz by refernece to the Estimated Dst index figures produced in near real time by Kyoto University.

References
Daglis, et al, The terrestrial ring current: Origin, formation, and decay, Rev. Geophys., 37(4), 407-438, 1999
http://www.ifsi.rm.cnt.it/ Lecture from Alpback Summer School

http://swdcdb.kugi.kyoto-u.ac.jp/dstdir/dst1/q/Dstqthism.html a table of estimated hourly Dst figures for the current month

http://spidr.ngdc.noaa.gov/spidr/ has only retrospectively calculated values, about 3 months in arrears

(c) G3NYK Alan Melia, 2004