M. Walt

Introduction to geomagnetically trapped radiation

Preface 1. The Earths radiation belts 2. Charged particle motion in magnetic and electric fields 3. The geomagnetic field 4. Adiabatic invariants 5. Particle fluxes, distribution functions and radiation belt measurements 6. Particle diffusion and transport 7. Diffusion in pitch angle 8. Diffusion in the L coordinate or radial diffusion 9. Summary and comments References Appendices Index.

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.

γ-Ray emission produced by a relativistic beam of runaway electrons accelerated by quasi-electrostatic thundercloud fields

In an experiment described by Fishman et al. [1994], high energy photons of atmospheric origin were detected by the Burst and Transient Source Experiment (BATSE) detectors, located on the Compton Gamma Ray Observatory (CGRO). In this paper we assess the possibility that the bursts may be bremsstrahlung produced by relativistic (>1 MeV) runaway electron beams accelerated in an avalanche process by quasi-electrostatic thundercloud fields. We consider the height-dependent density profile of the relativistic electrons specified as a function of time in the context of a previously reported runaway model [Bell et al., 1995]. The electron beam is modeled as a vertical cylinder with radius 10 km, and numerical estimates are provided of γ-ray fluxes which would be observed at the satellite. The predicted fluxes at the satellite altitude and at horizontal distances of up to 500 km from the source are found to be comparable to the experimental data.

Lightning-induced energetic electron flux enhancements in the drift loss cone

With its relatively low altitude (520 670 km) orbit, SAMPEX is mostly below the stable trapping region where charged particles repeatedly drift around the Earth, especially at midlatitudes. Recent analyses of SAMPEX data have revealed a surprisingly common set of observations of enhanced energetic (150 keV) electron fluxes at L 3, during times when SAMPEX was located such that any electrons that it observed were in the drift loss cone (and were thus destined to be precipitated upon reaching the longitude of the South Atlantic Magnetic Anomaly in the course of their eastward drift). The data were acquired with the Heavy Ion Large Telescope on board SAMPEX, which provides high time resolution measurements (30 ms sample rate) of electrons above energy thresholds of 150 keV and 1 MeV. Preliminary examination of 1995 SAMPEX data (when the 150 keV detector was available) revealed hundreds of cases of newly enhanced drift loss cone fluxes in localized L shell regions often associated with individual thunderstorms. In one case, SAMPEX data from three consecutive days (95/196, 95/197, and 95/198) were analyzed as the satellite proceeded northbound over the same ground track from west of New Zealand and Hawaii toward Alaska, with the underlying lightning activity documented by the Optical Transient Detector on board the OrbView-1 satellite (750 km, 70 inclination circular orbit). Enhanced fluxes observed on SAMPEX during day 95/197 were directly associated with an oceanic storm just to the west of the SAMPEX ground track, which was well placed to generate the observed drift loss cone flux enhancements. The drift loss cone electron flux enhancements were also observed 20 min later as SAMPEX crossed the same L shells in the north and in subsequent orbits, indicating that lightning-induced precipitation of electrons into the drift loss cone persisted at least for a few hours. Data from UARS satellite, passing through the same region within the same hour, also confirmed the presence of L-dependent structure and further allowed the determination of the electron energy spectra, which exhibited a general shape and range strikingly similar to previously documented spectral characteristics of lightning-induced electron precipitation (LEP) events in the bounce loss cone (Voss et al., 1998). This similarity lends support to the argument that the observed drift loss cone features are produced by the LEP process. In summary, SAMPEX data indicate that globally distributed thunderstorms may continually precipitate energetic electrons from the radiation belts, producing transient enhancements in the drift loss cone that are detected within the few hour periods as they drift around the Earth and precipitate in the South Atlantic Magnetic Anomaly.

Losses of ring current ions by strong pitch angle scattering

High angular resolution measurements of 155 keV ions in the ring current during the magnetic storm of August 6, 1998 show filled loss cones indicating that very rapid pitch angle scattering is taking place above the satellite location. The measurements were made with the SEPS detector on the Polar satellite during its passages through the ring current regions, usually at magnetic latitudes near ±45° and at magnetic local times of about 04:00 and 16:00 hrs. The observed strong pitch angle scattering implies a trapping lifetime of less than an hour and may explain the early rapid recovery of Dst during magnetic storms.

Properties of the magnetospheric hot plasma distribution deduced from whistler mode wave injection at 2400 Hz: Ground-based detection of azimuthal structure in magnetospheric hot plasmas

Siple station VLF wave injection experiments aimed at finding the properties of the magnetospheric hot plasma were conducted for a 9-hour period between 1705 and 0210 UT on January 23–24, 1988. A special frequency versus time format, lasting l min and transmitted every 5 min, consisted of a sequence of pulses, frequency ramps, and parabolas, all in a 1-kHz range centered on 2400 Hz. The transmitted signals, after propagating along a geomagnetic field-aligned duct, were recorded at Lake Mistissini, Canada. At various times during the 9-hour interval the Siple signals showed features characteristic of wave-particle interactions, including wave growth, sidebands, and triggered emissions. Our observations, primarily at 2400 Hz, show that (1) there were no correlations between the initial levels, the growth rates, and the saturation levels of constant-frequency pulses; (2) in general, the values of growth rate and saturation level of two pulses injected within 30 s were nearly the same; (3) the initial level, growth rate, and saturation level showed temporal variations over 5–15 min and 1–2 hour timescales; (4) the leading edges of constant-frequency signals underwent spatial amplification; and (5) under conditions of saturation the received signal bandwidth (∼ 20 Hz) remained constant over a 1-hour period, although the saturation level and growth rate varied during the same period. On the assumption that gyroresonant interactions were responsible for the observed wave growth and saturation, the timescales over which those phenomena varied provide constraints on the possible energetic electron population within the duct. In the reference frame of the duct (L ∼ 5.1, Ne ∼ 280 cm−3) the particle fluxes showed no variation over a 30-s timescale but varied over 5–15 min and 1–2 hour timescales. The 5–15 min timescale variations indicate longitudinal structures ranging from ∼ 0.2° or ∼100 km (in the equatorial plane) for electrons with energy E = 0.6 keV and pitch angle α = 40°, to ∼ 5° or ∼2800 km for electrons with energy E = 11 keV and pitch angle α = 80°. The hour-long time variations indicate longitudinal structures ranging from ∼ 2° or ∼1100 km (in the equatorial plane) for electrons with energy E = 0.6 keV and pitch angle α = 40°, to ∼ 45° or ∼25,000 km for electrons with energy E = 11 keV and pitch angle α = 80°. We conclude that ground-based active and passive wave experiments have substantial potential for investigating properties of the hot plasma of the magnetosphere.

The correlation of AKR waves with precipitating electrons as determined by plasma wave and X‐ray image data from the POLAR spacecraft

Simultaneous measurements are presented of the intensities of AKR waves in the 60 kHz to 800 kHz range and the local time distributions of the fluxes of electrons precipitating into the atmosphere using wave and bremsstrahlung x-ray data acquired on the POLAR spacecraft. The images of 2 to 12 keV x-ray emission measured with the PIXIE spectrometer were used to obtain the positions and intensities of the electron precipitation regions. Electric field wave measurements of the AKR were obtained with the Plasma Wave Instrument (PWI). Data are presented from a satellite pass on 19 April 1996 in which there were several short term enhancements in the intensities of both waves and x-rays. The AKR electric field strengths were correlated with x-rays emitted over a six hour local time range in the pre-midnight sector with a correlation coefficient of 0.51. At the times of some of the rapid increases in the x-ray fluxes the AKR intensities exhibited similar sharp onsets.

Probing properties of the magnetospheric hot plasma distribution by whistler mode wave injection at multiple frequencies: Evidence of spatial as well as temporal wave growth

This is the second of two papers on the use of whistler mode wave injection to investigate properties of the magnetospheric hot plasma. Paper 1, [Sonwalkar et al., this issue] emphasized the use of signals at a single frequency to identify longitudinal structures ranging from 100 to 25,000 km in extent in ∼1–10 keV electrons drifting azimuthally through whistler ducts. This short paper discusses and illustrates the use of wave injection at multiple discrete frequencies to study temporal changes in magnetospheric hot electrons with parallel (gyroresonant) velocities in various nonoverlapping ranges. As in paper 1, the data studied were acquired during a special 9-hour period of 1.9 – 2.9 kHz VLF transmissions from Siple Station, Antarctica, to Lake Mistissini, Canada, on January 23–24, 1988. The amplitudes of the leading edges of constant frequency pulses at 1900, 2150, and 2400 Hz were found to vary independently with time. This is interpreted as evidence of a spatial amplification process that accompanied the well known and more readily identifiable phenomena of exponential temporal growth to a saturation level. Evidence of wave-hot plasma interactions showed a dependence on df/dt of the input signal frequency versus time format; in general, the slow frequency ramps showed the highest amplitudes and the fast ramps and parabolas the lowest, in agreement with past work.

Association of electron precipitation with auroral kilometric radiation

An examination is made of a large body of auroral X ray and auroral kilometric radiation (AKR) data acquired on the Polar satellite in order to investigate the relationship between these two phenomena on a global scale. A detailed study was made of 14 cases selected with strong X ray and AKR emissions and appreciable time variations. In nearly all cases a maximum in the cross-correlation coefficient of auroral X rays and AKR occurred for X rays emitted slightly before local midnight, indicating that the AKR sources were concentrated in this magnetic local time (MLT) sector. The correlation is quite strong with coefficients between 0.29 and 0.82. The enhancement in the correlation coefficient generally extends over an MLT interval of 6 hours or less, even though electron precipitation above 2 keV extends over a longer MLT interval. This behavior is consistent with the importance of another factor, such as the presence of plasma cavities, being necessary for AKR production. Owing to statistical uncertainties, it was not possible from apogee data to establish whether the X rays or AKR occurred first during the short enhancements. Higher time resolution was achievable when Polar was near perigee, but AKR propagation to Polar was not assured at this location. During a perigee pass on January 13, 1997, Polar observed an auroral X-ray enhancement in time coincidence with inverted-V electrons, upward streaming ions, and enhanced AKR. In this event the inverted-V potential structure extended over at least 2 hours of MLT and lasted for only l min.

Relationship between X‐Ray, ultraviolet, and kilometric radiation in the auroral region

An investigation is made of the simultaneous emission of X-ray and UV photons as a function of time and location in the auroral oval, and comparisons are made of their time variations with the intensity variations of Auroral Kilometric Radiation (AKR). The X rays were measured with the Polar Ionospheric X-ray Imaging Experiment (PIXIE) instrument on the Polar satellite, and ultraviolet emissions were acquired with the UVI instrument also on the Polar satellite. AKR was measured using the Plasma Wave Instrument (PWI) carried on the Geotail satellite. For the events studied the time profiles of AKR were compared with those of electron precipitation into the auroral atmosphere as measured by the emission of X rays and ultraviolet radiation. The latter emissions exhibited fluctuations which on a scale of minutes to tens of minutes were coherent over several hours of magnetic local time (MLT). The polar region images of X-ray and UV emissions were usually very similar. The cross-correlation coefficients between X-ray and ultraviolet emissions were as high as 0.8 over a wide range of MLT, indicating that the time variations of these different emissions were similar, although the UV observations were limited to the nightside of the Earth and to the field-of-view of the UVI. The correlation coefficient between AKR and precipitation as a function of the MLT of the precipitation often exceeded 0.8, usually near midnight, but was >0.5 over several hours of MLT. This widespread correspondence of AKR and precipitation fluctuations is traced to the large-scale coherence of precipitation fluctuations around the auroral oval.