The earth's electric field and the sun
Anthony Hopwood investigates the effectof solar radio emission on the atmosepheric electric field.
Having spent some ten years monitoring the atmospheric electric field, which is sustained by the avalanche of charged- particles and ionizing radiation from the sun. I wanted to find, out how closely changes in solar radio emission are reflected in the electric field at ground level.
This meant building a simple radiotelescope to monitor solar radio emission and detect fluctuations of solar output and then comparing the radio signal level with fluctuations in the ambient electric field recorded by a sensitive antenna electrometer.
The sun exercises a direct influence on the upper atmosphere by its ultra violet, X-ray and cosmic ray and particle emissions. These increase enormously during a solar flare, so a frequency of around 200 MHz was selected because there is a very short delay between the appearance of an optical flare and the corresponding change in radio emission at this frequency.
There were two other good reasons for choosing 200 MHz. The first was the unpopulated nature of this part of the radio spectrum (for how long?) since. Band III television transmitters closed down and the second was the possibility of a relatively compact antenna. A 10-element fishbone antenna was constructed to resonate around 210 MHz and fitted on a tracking equatorial mount.
To take advantage of the unpopulated nature of band III, I decided to use a TRF receiver rather than the more usual RR converter. The advantage of a broadband 'straight' receiver is the large gain in solar signal input from it simple antenna when compared with the tim signal at a 10kHz bandwidth into an RF converter feeding a communication receiver. The line-up contprises a 210MHz tuned. 15dB gain RS560C RF pre-amplifier chip feedimg another 20dB of TRF amplification. The amplified RF signal is detected and fed to a CA30140 mosfet-input temperature stabilized 25dB gain DC op-amp with negative feedback operating from a stabilized 9V supply tapped at 3V for the preamplifier. The overall system gain can be set at the detector and output stages, as well as by detuning the inputs.
The system is very stable and detects little RF interference from other sources. To monitor any sustained interference, the DC output is split. An audio signal is extracted, amplified and fed to a small loudspeaker which is on whenever the system is recording. The other output to the pen recorder is damped by a large capacitor to clean up the trace, which is usually run at a nominal 50mm per hour with one or six minute timing markers.
The record is made by two Hewlett and Packard 680M pen recorders side by side in a rack mounting, but electrically independent, with a common event marker signalled from the antenna rotator, which closes a contact for four seconds every six minutes. Both recorders run at the same speed, the second logging the output of a fixed DC electrometer connected to a highly insulated vertical whip antenna fitted with a radioactive atmosphere coupling plate.
Over the last few weeks. synchronized recordings have shown that there is a very close link between the atmospheric electric field and solar radio emission. Getting clear evidence of that link has not been so easy, since the atmospheric potential varies widely with the weather. and often totally masks any minor solar effects. The best conditions are when the weather is quiet and bright and when the atmospheric baseline holds steady with a clear sky and little convective cloud.
The sun's radio emission at 210MHz is effectively a continuous analogue of the nuclear turmoil on its visible surface. In practice most of the radio noise comes from the highly disturbed regions round sunspots. When a flare occurs, it is immediately visible, but the intense burst of ultraviolet and X-radiation it releases blocks local radio emission for a short while. This drop in emission is often the first radio signal of a flare, shown in Fig. 1.
Fig. 1. UV and X-radiation from a flare blocks terrestrial radio fora time.
On earth, the arrival of a burst of ionizing radiation or charged particles alters the atmospheric electric field. The recordings show that the lag between a radio transient and a change in the ground electric field is from less than one to about eight minutes. This suggests that there are several modes of short-term atmospheric excitation. the fastest being the ultra violet and X-ray component which arrives with the visible component of the flare at the speed of light. This is followed by various particle emissions travelling at different speeds down to about half the speed of light.
At this stage it has not been possible to detail the individual arrivals without more sophisticated equipment. This would require a multichannel system with separate sensors logging radio, UV. cosmic rays and electric field.
Although very basic, the recordings do show the surprising fidelity of the ionospheric response to the thousands of daily small nuclear events on the visible solar disc. The synchronized recordings were made at a nominal 50 mm/h with increasing output positive-going. The ionizing component of a typical small flare causes a decrease in atmospheric voltage as the ionosphere is momentarily `lowered'. The negative spikes associated with changes in solar emission during a sequence of disturbances starting at 103013ST on 26 April 1989 are well shown in Fig. 2.
Fig. 2. Negative spikes in atmospheric voltage caused by a change in solar emission.
The effect of an isolated event is nicely shown on linked traces taken around midday on 2 May. 1989. Fig. 3.
Fig. 3. Linked traces show the interactive effect of an isolated event.
A comparison between Figs. 2 & 3 shows that atmospheric response to solar emissions varies widely - sometimes the electrometer signal change is disproportionately large compared with the radio signature, and vice versa; this is largely due to the condition of the ionosphere at the time of arrival. Towards noon, ionization is nearing the daily maximum, so a small increase in solar emission may trigger a large alteration, possibly in the form of a transient E layer.
These notes are intended to stimulate further research into the fine grain of the interaction between the atmospheric electric field and solar emission. A great deal of this research is open to amateurs, because the equipment needed is modest. More sophisticated equipment will show greater detail, but whatever the installation, if a continuous log is kept, the chances are that at least one major solar flare will be recorded as we approach sunspot maximum. In addition, experience will be gained on the electrometer's reaction to the initial flare and the later ionospheric and auroral effects, so that the electrometer can be used as a simple early-warning system of major ionospheric disturbances.
The ideal set-up would be a multichannel system recording the electric field, magnetic deviation, solar radio, UV, and cosmic ray emission, ionospheric sounding and 50MHz lift - this really would give a fascinating profile of the whole gamut of solar effects on our planet.
The neglected field of electrometry also offers the prospect of a simple and accurate log of the health of the beleaguered ozone layer, because a properly co-ordinated network of electrometer stations will show long-term alterations in the average electric field at ground level, which will be directly proportional to solar UV and ionizing radiation penetration.
Further reading
- The fishbone antenna is described in Radio Engineering by F.E. Terman, McGraw-Hill, 1937, chapter XV.
- Observational radio astronomy, by J.P. Wild, volume VII of Electronics and Electron Physics, 1955. Academic Press.
- Radio Astronomy, F. Graham Smith, Penguin. 1960.
- The Sun as never seen before, Edward Gibson, National Geographic, vol. 146, no 4. Skylab report.