Lightning-induced electron precipitation (LEP) from the earth's radiation belt has been observed on numerous occasions with detectors on the low-altitude S81-1/SEEP satellite. A sequence of seven LEP events on 9 September 1982 and eight events on 20 October 1982 are correlated on a one-to-one basis with one-hop whistlers at Palmer, Antarctica. The temporal profile within a LEP burst has remarkable fine structure. It is shown to be associated with bunches of magnetically guided and focused 100-to-200 keV electrons which are repeatedly scattered by the atmosphere and bounce between the northern and southern hemispheres. The delay time between the lightning sferic and the arrival of the first electron bunch increases with increasing L as predicted by the first order gyroresonance theory. The global distribution of strong LEP events observed with the SEEP payload correlates with lightning activity and shows a preferred distribution at 2 < L < 3. This L shell range corresponds to the slot region in the electron radiation belt. A single LEP burst (10-3 erg s-1 cm-2) in the slot region is estimated to deplete about 0.001% of the particles in the region covered by the burst magnetic field lines. The evidence supports the production of structured LEP by ducted rather than non-ducted whistlers. It is found that ducted whistlers can be an important pitch angle diffusion mechanism for 100-250 kev electrons in the 2 < L < 3 range although a number of uncertainties in the various parameters remain to be resolved. It is suggested that observations of LEP can be a new tool to measure the presence and transverse dimensions of plasmaspheric whistler mode ducts.
Lightning-induced electron precipitation (LEP) from the earth's radiation belts, caused by whistler wave-particle interactions, is a known troposphere-to-magnetosphere coupling mechanism. Experimental evidence for this phenomena include ELF, VLF, and MF wave propagation anomalies or Trimpi events (Helliwell et al., 1973, Carpenter et al., 1984, Burgess and Inan, 1993); rocket measurements (Rycroft, 1973, Goldberg 1987); and satellite measurements (Voss et al., 1984, Imhof et al., 1986). Electron-cyclotron (whistler) waves, which are believed to scatter the electrons, are electromagnetic plasma waves which propagate with right-circular polarization in field-aligned ducts of enhanced ionization. Pitch angle scattering of energetic radiation belt electrons by whistler-mode waves can result in the precipitation of these electrons into the atmosphere (Dungey, 1963, Cornwall, 1964, Roberts, 1969, Inan et al., 1978, and Chang and Inan, 1985). While the evidence for such direct precipitation is now overwhelming and the causes of Trimpi events are well understood, the significance of this effect for major geophysical processes, such as trapped electron loss rates and ionospheric perturbations, is not clear. Additional data on the LEP effect are needed both to clarify the physical processes involved and to evaluate the impact of these sporadic precipitation events on the magnetosphere and ionosphere.
Electron spectroscopy measurements from a low-altitude (~220 km) satellite at L ~2.2 have on occasion shown a one-to-one correlation of precipitation bursts with ducted whistlers (Voss et al., 1984; Imhof et al., l986). These measurements were obtained with the Stimulated Emissions of Energetic Particles experiment (SEEP) on the three-axis stabilized, low-altitude (170-280 km), polar orbiting S81-1 satellite. The mission operation time for this satellite was May 1982 to December 1982. The satellite location relative to the radiation belt and the wave-particle interaction region is schematically illustrated in Figure 1. In this figure lightning flashes in the Northern hemisphere produce whistlers which propagate along field lines towards the Southern hemisphere, deflecting some of the north-bound electrons into trajectories which carry them to the S81-1 satellite. Because the SEEP particle detectors had relatively large geometric factors and the satellite was below the interfering background of trapped radiation for much of its orbit, the SEEP sensors had a high signal-to-background ratio for measuring low intensity fluxes of electrons in the bounce and drift loss-cones. During most of the satellite lifetime VLF wave data were recorded at ground stations in Antartica for comparison with the precipitating particle events.
The emphasis of this paper is on the experimental measurements made with the SEEP detectors over a 6 month period in which numerous cases of LEP were observed. A previous brief report (Voss et al., 1984) describes several cases of LEP detected on 9 September, 1982, and a detailed simulation of the whistler wave-electron interaction was completed for one event (Inan et al., 1989). Other examples of LEP were described briefly in studies of short duration electron precipitation bursts (Imhof et al., 1986, 1989) However, during the life of S81-1 over one hundred events with electron signatures identical to those of the previously reported LEP were recorded. This unique data set allows a more complete study of the LEP process and an assessment of its overall global significance. These data also contain information bearing on the presence and sizes of plasmaspheric ducts which appear to guide the whistler waves responsible for LEP. Our primary purposes in this paper are to: report on the energy and time structure of several lightning-induced electron precipitation bursts, show the global distribution of LEP bursts, estimate the induced loss rate of electrons from the radiation belt, and infer the characteristics of magnetospheric ducts from the LEP signatures. The possibility of using satellite traces of LEP to assess the transverse dimensions of plasmaspheric ducts has not been considered previously.
The detailed spectroscopic measurements of lightning-induced electron precipitation bursts afford a powerful technique to: 1) clarify the complex physics of the wave-particle interaction, 2) evaluate the magnitude of radiation belt loss processes associated with terrestrial lightning, 3) study the atmospheric scattering and energy loss processes for electrons near the edge of the loss cone, 4) determine the spatial and temporal conductivity enhancements in the lower ionosphere produced by LEP events (associated with Trimpi events), 5) investigate the modes of wave propagation in the magnetosphere that are conducive to LEP electron precipitation, and 6) identify the magnetospheric duct structures which guide VLF waves.
SEEP Instrumentation table