Observations of relativistic electron microbursts in association with VLF chorus

Abstract

Abstract.The Solar, Anomalous and Magnetospheric Particle Explorer (SAMPEX) satellite frequently observes relativistic (> 1 MeV) electron precipitation in the radiation belts at oe shells of 4-6 with bursty temporal structure lasting < 1 s. This phenomenon can occur at all local times but is most often seen between 0200 and 1000 magnetic local time. VLF chorus is also observed to occur preferentially at these same local times. Using electron observations from the SAMPEX satellite Heavy Ion Large Telescope and data from the Polar satellite plasma wave instrument, we show correlation between observations of relativistic electron microbursts and VLF chorus with frequencies <2 kHz. In addition, the duration of the individual rising frequency chorus elements is comparable to the duration of the relativistic electron microbursts. It has been speculated that relativistic electron microbursts are caused by wave-particle interactions, which strongly scatter electrons into the loss cone for a short period. Lower-energy electron microbursts in the range from tens to hundreds of keV have long been associated with chorus waves, since these lower-energy electrons can resonate at the equator with whistler-mode waves at chorus frequencies. Electrons of MeV energies do not satisfy the first-order cyclotron resonance condition with chorus wave frequencies at the equator. However, MeV electrons may interact with chorus through higher-order resonances or off-equatorial interactions.

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

“…In addition, it should be noted that the parallel e-folding energy and fluxes are smaller than the perpendicular ones even during the storm. While Lorentzen et al (2001) and O'Brien et al (2004) calculated microburst loss rate by assuming isotropic pitch angle distributions, our observations indicate energetic electron loss might be smaller than their estimation.…”

“…Some researchers have calculated electron precipitation rates of relativistic microbursts and showed wave particle interaction is a dominant electron loss process of radiation belt-relativistic electrons (Lorentzen et al, 2001;O'Brien et al, 2004;Thorne et al, 2005). They estimated the electron loss rate using SAMPEX data assuming downward isotropic pitch angle distribution.…”

Pc5 geomagnetic pulsations, pulsating particle precipitation, and VLF chorus: Case study on 24 November 2006

“…The pitch-angle scattering of the radiation belt energetic electrons by chorus can lead to significant precipitation into the atmosphere [Lorentzen et al, 2001;Meredith et al, 2001;Summers, 2005]. Tsurutani and Smith [1974] and Anderson and Maeda [1977] found that the onset of the chorus emissions coincided with the injection of substorm electrons with energies of ∼10-100 keV.…”

The Energetic Electron Response to Magnetic Storms: HEO Satellite Observations

“…The bottom panel shows a line plot of the estimated minimum daily plasmapause position [O'Brien and Moldwin, 2003]. Superimposed on the line plot are colored bars indicating where electron precipitation microbursts [Lorentzen et al, 2001a and2001b;] were observed at L < 4 (red bars) and L > 4 (blue bars).…”