H. D. Voss

Satellite observations of lightning-induced electron precipitation

Lightning-induced electron precipitation (LEP) from the Earths 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 September 9, 1982, and eight events on October 20, 1982, are correlated on a one-to-one basis with one-hop whistlers at Palmer, Antarctica. The temporal profile within a LEP burst has a remarkable fine structure. It is shown to be associated with bunches of magnetically guided and focused 100-to-200 keV electrons that 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 ∼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 nonducted 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.

Relativistic electron microbursts

We report the first satellite observations of relativistic (>1 MeV) electron precipitation in microbursts with measured durations of less than 1 s. Microbursts of lower-energy electrons (10–100 keV) have been found to occur preferentially in the early daylight hours and to be closely associated with VLF chorus emissions. In contrast, the relativistic electron microbursts occurred more frequently near 2230 LT than 1030 LT, and no association was found with ELF/VLF chorus, consistent with the fact that resonant interactions with ∼ 1-MeV electrons require significantly lower frequencies. The available data on these relativistic microbursts thus appear to indicate that many of the bursts may be due to wave-particle interaction not with whistler mode chorus but possibly with other waveforms. The locations of many of the relativistic microbursts are concentrated at the outer edge of the trapped radiation belt, where the gyroradii of the electrons are comparable to the curvature of the magnetic field lines and stable trapping may therefore not occur. The preferred location of the microbursts, which may be primarily spatial in character, implies the possible importance of irregularities in the magnetic field lines near the trapping boundary as the responsible mechanism.

Association of waves with narrow particle dropouts in the outer radiation belt

An investigation has been made of the association of plasma waves with narrow dropouts (<30-min observation time) in the fluxes of trapped energetic electrons and protons within the radiation belts. The experiment, conducted with instruments on the CRRES satellite, indicates that 5 Hz < frequency < 1 kHz waves are nearly always produced at the times and positions of narrow energetic electron flux dropouts. The waves may be produced at the positions of steep particle flux gradients. This phenomenon is best studied during short-duration events that provide measurements on both sides of a boundary in a relatively quiescent state. In addition to an investigation of the waves associated with narrow dropouts the data presented here show the general correlations between waves and particle fluxes, particularly after new injections or plasmapause crossings. In some cases, energetic proton dropouts also occur and may contribute to the generation of waves.

Diffusion of 155 to 430 keV protons in the Earth's radiation belt

Pitch angle distributions and energy spectra of 155–430 keV trapped protons between L=4 and L=5.5 are examined to study pitch angle and radial diffusion rates during a period of low magnetic activity. To improve the sensitivity to pitch angle diffusion, the measurements were made near the loss cone where angular gradients in the distribution function are large. In this region the phase space density is such that both radial diffusion and pitch angle diffusion lead to flux increases while charge exchange leads to proton loss. However, the conventional prescriptions for the diffusion coefficients predict too much diffusion at L=4, implying growth in the proton fluxes while observations show loss. At L=5, theory and observation both show slight growth, and at low pitch angles diffusion in pitch angle is necessary to compensate for charge-exchange losses. Although Dst was low during the 13 days preceding the measurements, the proton belt had not reached a steady state with diffusion sources matching charge-exchange losses.

Kilometer scale structures in auroral X ray arcs detected by combining satellite X ray images with in situ energetic electron flux profiles

This paper reports on analysis of combined x-ray and electron energy measurements by satellite of auroral arcs, to reveal more information on the spatial scale of the arcs. Bremsstrahlung radiation from precipitating electrons of auroral arcs stopping in the earths atmosphere provide x-ray images of the regions of the arcs. They have the advantage that a satellite can sense them from a distance, and thus can sample a larger area than just the path of its orbit. The x-ray images also reveal narrower features than seen from optical emission. Here x-ray images from the XRIS imager on the S81-1 polar orbiting satellite are analyzed. 4 to 40 keV x-rays were observed, with the 40 km resolution of the XRIS imager. However, by combining this with high time resolution measurements of electrons with energies above 16 keV, which could be measured along the satellite track, one can see 2 km spatial features. These measurements showed km scale structure within the auroral arcs.