Source and seed populations for relativistic electrons: their roles in radiation belt changes

Thursday, 17 December 2015
Poster Hall (Moscone South)
Allison N Jaynes1, Daniel N. Baker2, Howard J Singer3, Juan V Rodriguez2, Paul T M Loto'aniu2, Ashar Ali2, Scot Richard Elkington4, Xinlin Li2, Shrikanth G Kanekal5, Seth G Claudepierre6, J. F. Fennell7, Wen Li8, Richard M Thorne8, Craig Kletzing9, Harlan E. Spence10 and Geoffrey D Reeves11, (1)University of Colorado at Boulder, LASP, Boulder, CO, United States, (2)University of Colorado at Boulder, Boulder, CO, United States, (3)NOAA Boulder, Boulder, CO, United States, (4)Laboratory for Atmospheric and Space Physics, Boulder, CO, United States, (5)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (6)Aerospace Corporation Santa Monica, Santa Monica, CA, United States, (7)Aerospace Corporation Los Angeles, Los Angeles, CA, United States, (8)University of California Los Angeles, Los Angeles, CA, United States, (9)University of Iowa, Iowa City, IA, United States, (10)University of New Hampshire Main Campus, Space Science Center, Durham, NH, United States, (11)Los Alamos National Laboratory, Los Alamos, NM, United States
Strong enhancements of outer Van Allen belt electrons have been shown to have a clear dependence on solar wind speed and on the duration of southward interplanetary magnetic field. However, individual case study analyses also have demonstrated that many geomagnetic storms produce little in the way of outer belt enhancements and, in fact, may produce substantial losses of relativistic electrons. In this study, focused upon a key period in August-September 2014, we use GOES geostationary orbit electron flux data and Van Allen Probes particle and fields data to study the process of radiation belt electron acceleration. One particular interval, 13-22 September, initiated by a short-lived geomagnetic storm and characterized by a long period of primarily northward IMF, showed strong depletion of relativistic electrons (including an unprecedented observation of long-lasting depletion at geostationary orbit) while an immediately preceding, and another immediately subsequent, storm showed strong radiation belt enhancement. We demonstrate with these data that two distinct electron populations resulting from magnetospheric substorm activity are crucial elements in the ultimate acceleration of highly relativistic electrons in the outer belt: the source population (tens of keV) that give rise to VLF wave growth; and the seed population (hundreds of keV) that are, in turn, accelerated through VLF wave interactions to much higher energies. ULF waves may also play a role by either inhibiting or enhancing this process through radial diffusion effects. If any components of the inner magnetospheric accelerator happen to be absent, the relativistic radiation belt enhancement fails to materialize. This comparative study of two distinct types of storms demonstrates the conditions under which a strong MeV outer radiation belt can be formed and maintained.