Whistler-mode waves and their effects on the radiation belts

Thorne reported modelling work with Abel which showed that Coulomb scattering was an effective loss mechanism only for L<1.2. Except for pitch-angles very close to 90°, where Landau resonance was important, pitch angle scattering by cyclotron resonance was the dominant process in determining electron lifetimes over essentially all of the radiation belts. Using estimates of whistler mode wave intensity distributions and spectra in different regions, it was estimated that signals from VLF transmitters were most important in the inner region, hiss in the outer region, and lightning whistlers in the intermediate region.

Vampola reviewed the satellite evidence on the scattering of resonant electrons into the drift loss cone by powerful VLF transmitters. The frequency, modulation format and particularly longitude were all important factors. One of the most effective transmitters had been the Soviet UMS transmitter. The precipitation patterns around each transmitter and its conjugate, formed by scattering into the bounce loss cone, were also discussed. The "natural" radiation belts in the absence of VLF transmitters have never been observed and it is an intriguing question what the effect will be if/when all the VLF transmitters are turned off (this trend has already begun with the recent closure of the NSS transmitter in Maryland and the planned shutdown of the Omega system on 30 September 1997).

Walt reviewed LEP as seen by low altitude satellites such as SEEP. Calculations of trapped electron lifetimes due to interactions with ducted whistlers (which Trimpi event evidence suggests are more important than unducted whistlers) were critically dependent on poorly known quantities such as the size and distribution of whistler ducts. He pointed out that the spatial extent of LEP as seen by low-altitude satellites could be used to infer the size of ducts; the limited cases so far studied suggested duct sizes of order 100 km, leading to an estimate of electron lifetime of only 17 days (changing the size to 25 km would increase this to 270 days).

Sonwalkar reported a study relating cloud-ground lightning seen by the SUNY network, and whistlers observed on the DE-1 satellite. There were some interesting conclusions, e.g. that one-to-one correlations were seen only on 6 days out of three years examined. On those days, more than half the cloud-ground (C-G) lightning discharges resulted in an observable whistler, but there were more whistlers seen than C-G lightning, suggesting that cloud-cloud and intra-cloud discharges also make a contribution.

Inan discussed the importance of nonducted vs. ducted whistlers using a test-particle model. Although the interaction with trapped electrons is much less efficient for the non-ducted waves, the magnetospheric energy density of non-ducted whistlers is believed to be higher than for ducted whistlers, and thus their effect may therefore need to be considered.

Horne discussed the guiding and focussing effect of the plasmapause, estimated using the HOTRAY code. In particular this can lead to large wave damping (and thus electron heating) at the higher frequencies, and to upper cutoffs well below half the gyrofrequency.

Sulic reported ray-tracing results of ducted whistler propagation in field-aligned troughs, rather than the enhancements normally considered. The results, for L=2.7, showed that although guiding by troughs is theoretically possible, in practice for a dipole geometry and realistic density models, the waves were always guided by the inner edge of the trough, equivalent to guiding by a one-sided ledge. The study was 2D; it remains to be seen whether the same conclusions would be reached for a more realistic 3D field model.

Strangeways gave a good review which is now published in Review of Radio Science 1993-1996 (Oxford, 1996) as Lightning, Trimpis, and Sprites. It contains a lengthy bibliography and should serve as a useful reference source even though parts will soon become dated in this rapidly developing subject area.

Inan and Dowden (in separate papers) discussed the various mechanisms proposed for sprite formation: EMP (electromagnetic pulse), QE (quasi-electrostatic) fields, and runaway electrons from cosmic ray secondaries under the action of large QE fields. Hayakawa presented a new finite-element 2D model for VLF propagation perturbed by arbitrarily sharp discontinuities, while Yeo reported preliminary results from his LEP scattering model.

Steinbach reported on a trimpi receiver network now set up in Hungary, though no results were shown. Smith described Trimpi observations from Rothera and Faraday (Antarctica) in 1995; the dramatic decrease in event occurrence between the NPM-FA and NPM-RO paths, and the small percentage of simultaneous events, were interpreted, respectively, in terms of the poleward edge of the LEP region and the spatial size of the individual LEP patches compared to the RO-FA separation.

Llanwyn Jones discussed the detection and source location of "Q-bursts", which are rare but very intense signatures of certain lightning discharges. Hayakawa reviewed electromagnetic phenomena associated with earthquakes. Parrot reviewed power line harmonic radiation; spectral lines spaced 50-60 Hz apart in the magnetosphere and generally observed to drift in frequency. The "Sunday effect" had been used confirm an anthropogenic source (no natural 7-day period). He speculated on an atmospheric/ionospheric link between PLHR and the greenhouse effect (in addition to the usually quoted increase in CO₂ emissions associated with power generation), which could increase in importance as electrical power consumption continued to increase.

Chairman's introduction and report

Future of the working group

URSI Co-chairmanship

Symposia for future assemblies

USA

Yearby said that he had been employed on the CLUSTER team at Sheffield, but had also spent some time recently working with BAS VLF data on whistler direction finding and on pulsations in whistler mode signals from VLF transmitters.

Finland

France

* Unfortunately the launch of MARS-96 on 16 November 1996 was unsuccessful.

Japan

Localised ionospheric perturbations related to lightning and VLF transmitters

Other forthcoming meetings

EGS, Vienna, 1997

URSI, Toronto, 1999

News from Finland

Almost every VLF researcher has observed some HF radio broadcasts in their recordings. Usually the phenomenon has been interpreted as due to demodulation by non-linearities in the receiver electronics. Identification of the detected broadcasts was done by an experienced radio amateur. Three different RF receivers were used, spanning the frequency range 30 kHz to 30 MHz; although these could receive all kind of transmissions (CW, SSB, FM, AM, etc.), only AM broadcasts seemed to be detected in the VLF frequency range. A spectrum analyser was used to investigate the power distribution across a broad frequency band (~1-10 Mhz). The available transmitter identifications showed that the distance to the transmitter was often quite long. Only one station at a time has usually been heard, showing that the process is very selective. It is also common that the event shows clear periodicity. Usually demodulated signals seem to favour speech and "classical" music.