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1037the only negative ion which has been identified in HC1 by previous workers is Cl~. Charge transfer would not be expected to play a significant role if this were the ion in our experiment. The observed value of 0.71 ±0.02 cm 2 /v-sec is to be contrasted with that of 0.88 expected if Eq. (14) is applicable. This suggests that an orientation-dependent R~* interaction is important in this case, so that the mobility should vary as T 1/6 . If, however, these forces averaged out, so that only the INTRODUCTION R ADIATIVE capture of neutrons by Li 7 leads to an unstable isotope, Li 8 , which decays with about a 0.9 sec half-life by emitting electrons with a maximum energy of 13 Mev. 1 The residual nucleus, Be 8 , further decays into two alpha particles in about 10~1 5 second. Since the spin and parity of the ground state of Li 8 are 2, even, 2 and the spin and parity of the ground state of Li 7 are f, odd, the excited state of Li 8 formed by s-wave neutron capture in Li 7 will decay to the ground state by electric dipole emission. Calculations of the cross section for this process have been made by Thomas 3 and his results predict a cross section which decreases smoothly with increasing neutron energy. However, a maximum in the capture cross section might be expected near 250 kev because of a resonance in the total cross section of Li 7 at this energy. 4 The 2.28-Mev state in Li 8 , which is responsible for this resonance, has spin 3 and even parity and may decay to the ground state by 1 W. F. Hornyak and T.

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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. 13,829 13,830 IMHOF ET AL.' RELATIVISTIC ELECTRON MICROBURSTS

An investigation has been made of electron and associated ion precipitation spikes near the plasmapause that are narrow in L shell and in which relativistic electrons are favored. The electron energy spectra during the spikes sometimes had equivalent e fold energies in excess of 500 keV. In approximately 31% of these spike events observed from the low‐altitude polar orbiting satellites P72‐1, P78‐1, and S81‐1, nearly simultaneous precipitation was measured in energetic ions above ∼30 keV at about the same L value. Several of the precipitation spikes occurred primarily in the drift loss cone, but in some cases, significant precipitation (∼10−2 ergs/cm² s) was also observed in the bounce loss cone. The electron spikes occurred preferentially in the evening sector, and all of the associated narrow ion spikes were in that local time interval. Narrow relativistic electron spikes were observed on less than 1% of the crossings of the plasmapause. From the set of S81‐1 events a search was made for those also observed on the NOAA 6 spacecraft. On rare occasions, nearly simultaneous (<2000 s) narrow spikes with hard electron spectra were found at approximately the same L value from both spacecraft and at longitudes differing by 8°–47°. These findings suggest a patchy profile, sometimes with an arc structure which may extend over longitude intervals as great as 25° and time intervals as long as 2000 s. From consideration of the AE index for 17 events, 12 were found to occur close to the times of substorms. The spike precipitation is interpreted in terms of cyclotron resonance wave‐particle interactions involving radiation belt particles, the narrow widths being associated with fine structure in the cold plasma density profiles near the plasmapause and the energy selectivity associated with an upper frequency cutoff in the waves.

Temporal and spectral signatures of a lightning‐induced electron precipitation (LEP) burst observed on the S81‐1 (SEEP) satellite are analyzed and compared with the predictions of a test particle model of the gyroresonant whistler‐particle interaction in the magnetosphere. The flux to be detected by specific detectors on the low altitude (∼220 km) satellite at L ≃ 2.24 is calculated in terms of the integral counting rate as a function of time and in terms of the dynamic energy spectra during the initial ∼300‐ms precipitation pulse. For a whistler wave packet with frequency range 500 Hz to 6 kHz the dynamic energy spectra are found to depend sensitively on the electron angular distribution in the vicinity of the loss cone. In the case of a whistler wave originating in northern hemisphere lightning the maximum whistler‐induced pitch angle scattering of electrons occurs near ∼10°S geomagnetic latitude. However, scattering occurring over the latitude range of ∼20°N to ∼20°S is found to be significant and contributes to the observed LEP pulse. The dynamic energy spectra of the LEP pulse and the temporal profile of the integral counting rate are consistent with the predictions of a test particle model of the gyroresonant scattering of the electrons by a whistler wave having an equatorial intensity at 6 kHz of ∼200 pT. The measured LEP pulse pitch angle distribution is wider than that estimated on the basis of the test particle model.

Radiation belt electrons precipitated by controlled injection of VLF signals from a ground based transmitter have been directly observed for the first time. These observations were part of the SEEP (Stimulated Emission of Energetic Particles) experiment conducted during May ‐ December 1982. Key elements of SEEP were the controlled modulation of VLF transmitters and a sensitive low altitude satellite payload to detect the precipitation. An outstanding example of time‐correlated wave and particle data occurred from 8680 to 8740 seconds U.T. on 17 August 1982 when the satellite passed near the VLF transmitter at Cutler, Maine (NAA) as it was being modulated with a repeated ON (3‐s)/OFF (2‐s) pattern. During each of twelve consecutive pulses from the transmitter the electron counting rate increased significantly after start of the ON period and reached a maximum about 2 seconds later. The measured energy spectra revealed that approximately 15 to 50 percent of the enhanced electron flux was concentrated near the resonant energies for first order cyclotron interactions occurring close to the magnetic equator with the nearly monochromatic waves emitted from the transmitter.

The temporal and spectral shape and the absolute flux level of particle pulses precipitated by a VLF transmitter are examined from a theoretical point of view. A test‐particle model of the gyroresonant wave‐particle interaction is applied to the parameters of the observed cases for calculating the precipitation characteristics. The temporal shapes of the precipitation pulses are found to be controlled (1) by the pitch angle dependence of the particle distribution near the edge of the loss cone and (2) by the multiple interaction of the particles with the waves due to significant atmospheric backscatter.