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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 30, issue 11
Ann. Geophys., 30, 1567–1573, 2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 30, 1567–1573, 2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.

Regular paper 06 Nov 2012

Regular paper | 06 Nov 2012

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

J. J. Lee1, G. K. Parks2, E. Lee3, B. T. Tsurutani4, J. Hwang1, K. S. Cho1, K.-H. Kim3, Y. D. Park1, K. W. Min5, and M. P. McCarthy6 J. J. Lee et al.
  • 1Department of Space Science Research, Astronomy and Space Science Institute, Daejeon, Korea
  • 2Space Sciences Laboratory, University of California, CA, USA
  • 3School of Space Research, Kyung Hee University, Gyeonggi, Korea
  • 4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
  • 5Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon, Korea
  • 6Department of Planetary and Space Sciences, University of Washington, Seattle, WA, USA

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.

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