SM21B-2518
The Cause of the Hardest Electron Precipitation Events Seen by SAMPEX: a Statistical Survey of Circumstantial Evidence

Tuesday, 15 December 2015
Poster Hall (Moscone South)
David Miles Smith1, Eric Pun Casavant2, Max D Comess3,4, Richard Selesnick5, Robyn M Millan6, John Glen Sample7, Xinqing Liang1, Gregory S Bowers1, Jacob Bortnik8 and Lasse Boy Novock Clausen9, (1)University of California Santa Cruz, Santa Cruz, CA, United States, (2)University of California, Irvine, Irvine, CA, United States, (3)UCSC, Santa Cruz, CA, United States, (4)SpaceX Corp., Hawthorne, CA, United States, (5)Kirtland Air Force Base, Kirtland AFB, NM, United States, (6)Dartmouth College, Hanover, NH, United States, (7)University of California Berkeley, Berkeley, CA, United States, (8)University of California Los Angeles, Los Angeles, CA, United States, (9)University of Oslo, Oslo, Norway
Abstract:
We used spectral information from SAMPEX/PET during the period 1992-2004 to find the spectrally hardest sub-population of MeV electron precipitation events (e-folding energy > 400 keV). Contrary to our expectations based on the model of this class of events as precipitation by electromagnetic cyclotron (EMIC) waves, we find no enhanced plasma density during these events using IMAGE/EUV, we find the peak in magnetic local time (MLT) shifted several hours from the peak of EMIC waves at geosynchronous orbit, and we find that the hardest precipitation events do not correlate with high solar wind proton density as EMIC waves do. We will present first results of a comparison of the hardest SAMPEX events with a data-driven model of EMIC wave occurrence. We will also discuss an alternate model for these hard events: current-sheet scattering (loss of the first adiabatic invariant at field inhomogeneities near midnight). The MLT distribution of the hard precipitation events already provides some circumstantial evidence for this model: as the e-folding energy softens from 700 keV to below 400 keV, the average MLT moves smoothly from about 20h to midnight (see Figure). This is consistent with a picture of electrons lost to current-sheet scattering, which would be expected to scatter the highest-energy electrons (with the largest cyclotron radii) first as the population drifts from dusk to midnight, where the inhomogeneities are thought to be most effective and can reach to lower energies.
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