New high temporal and spatial resolution measurements by SAMPEX of the precipitation of relativistic electrons

Parks²,

et al. 2012

Ann. Geophys.

Abstract. Electron microburst energy spectra in the range of 170 keV to 360 keV have been measured using two solid-state detectors onboard the low-altitude (680 km), polar-orbiting Korean STSAT-1 (Science and Technology SATellite-1). Applying a unique capability of the spacecraft attitude control system, microburst energy spectra have been accurately resolved into two components: perpendicular to and parallel to the geomagnetic field direction. The former measures trapped electrons and the latter those electrons with pitch angles in the loss cone and precipitating into atmosphere. It is found that the perpendicular component energy spectra are harder than the parallel component and the loss cone is not completely filled by the electrons in the energy range of 170 keV to 360 keV. These results have been modeled assuming a wave-particle cyclotron resonance mechanism, where higher energy electrons travelling within a magnetic flux tube interact with whistler mode waves at higher latitudes (lower altitudes). Our results suggest that because higher energy (relativistic) microbursts do not fill the loss cone completely, only a small portion of electrons is able to reach low altitude (∼100 km) atmosphere. Thus assuming that low energy microbursts and relativistic microbursts are created by cyclotron resonance with chorus elements (but at different locations), the low energy portion of the microburst spectrum will dominate at low altitudes. This explains why relativistic microbursts have not been observed by balloon experiments, which typically float at altitudes of ∼30 km and measure only X-ray flux produced by collisions between neutral atmospheric particles and precipitating electrons.

Section: Observationmentioning

confidence: 90%

Section: Introductionmentioning

confidence: 99%

Anisotropic pitch angle distribution of ~100 keV microburst electrons in the loss cone: measurements from STSAT-1

Parks²,

et al. 2012

Ann. Geophys.

“…They suggested that breakdown in adiabatic motion also could be involved to produce the microburst. However, observations from the SAMPEX satellite unequivocally showed that microbursts occur not only near the trapping boundary but often well inside [Nakamura et al, 1995;Blake et al, 1996]. Thus a breakdown in adiabatic motion of the relativistic electrons cannot be the single cause of the microbursts [Blake et al, 1996].…”

Section: Introductionmentioning

confidence: 99%

“…However, observations from the SAMPEX satellite unequivocally showed that microbursts occur not only near the trapping boundary but often well inside [Nakamura et al, 1995;Blake et al, 1996]. Thus a breakdown in adiabatic motion of the relativistic electrons cannot be the single cause of the microbursts [Blake et al, 1996]. During magnetic storms, dynamical changes of relativistic electrons in the radiation belt have been observed [e.g., Baker et al, 1986].…”

Section: Introductionmentioning

confidence: 99%

SAMPEX observations of precipitation bursts in the outer radiation belt

Nakamura¹,

Isowa²,

Kamide³

et al. 2000

J. Geophys. Res.

Abstract. The occurrence frequency of precipitation bursts of > 1 MeV electrons in the outer radiation belt is examined using data from the SAMPEX satellite. Electron burst characteristics shown in this paper include the dependence of the precipitation on magnetic local time, radial distance and geomagnetic activity. Precipitation bursts with timescales < 1 s, i.e., microbursts, are studied in detail, including their dependence on the phases of geomagnetic storms. It is found that precipitation bursts occur typically in the region between L = 4 and L = 6. Microbursts tend to occur at L lower than the bursts with timescales of several tens of seconds. The number of observed microbursts significantly increases during storms, appearing mainly in the morning sector early in the recovery phase of storms. These findings suggest that the microbursts may be due to interactions with electron whistler waves, which take place near the dawnside plasmapause in the density irregularities that are perhaps created in the "recovering" plasmasphere. The prevalence of bursty precipitation indicates that this enhanced loss component of the relativistic electron flux should be taken into account in any quantitative model of relativistic electron acceleration processes.

Dynamics of the Earth's Radiation Belts and Inner Magnetosphere

“…For more than twenty years, observations of 'microbursts,' short bursts of relativistic electrons, have been seen by balloon detectors or low altitude satellites imaging the loss cone [Blake et al, 1996;Millan andThorne, 2007, Millan et al, 2011 for review]. Although electromagnetic ion cyclotron waves [Thorne and Kennel, 1971;Alpert and Bortnik, 2009] are often proposed to provide the scattering, no good explanation has yet been presented that can account for all the observed phenomenology.…”

Section: Waves Are Associated With Microburstsmentioning

confidence: 99%

Large-Amplitude Whistler Waves and Electron Acceleration in the Earth's Radiation Belts: A Review of Stereo and Wind Observations

Cattell¹,

Breneman²,

Goetz³

et al. 2013

One of the critical problems for understanding the dynamics of Earth's radiation belts is determining the physical processes that energize and scatter relativistic electrons. We review measurements from the Wind/Waves and STEREO S/Waves waveform capture instruments of large amplitude whistler-mode waves. These observations have provided strong evidence that large amplitude (100s mV/m) whistlermode waves are common during magnetically active periods. The large amplitude whistlers have characteristics that are different from typical chorus. They are usually nondispersive and obliquely propagating, with a large longitudinal electric field and significant parallel electric field. We will also review comparisons of STEREO and Wind wave observations with SAMPEX observations of electron microbursts. Simulations show that the waves can result in energization by many MeV and/or scattering by large angles during a single wave packet encounter due to coherent, nonlinear processes including trapping. The experimental observations combined with simulations suggest that quasilinear theoretical models of electron energization and scattering via smallhttps://ntrs.nasa.gov/search.jsp?R=20120016644 2018-05-13T02:45:38+00:00Z amplitude waves, with timescales of hours to days, may be inadequate for understanding radiation belt dynamics.