SM22A-01:
No Gain, Know Drain: Quantifying Radiation Belt Precipitation Loss
Tuesday, 16 December 2014: 10:20 AM
Harlan E. Spence1, Alexander J Boyd2, Chia-Lin Huang3, Sonya S Smith3, Michael G Henderson4, Reiner H W Friedel4, Brian Larsen5, Geoffrey D Reeves4, J Bernard Blake6, Seth G Claudepierre7, Joseph Fennell8, Daniel N. Baker9 and Shri Kanekal10, (1)University of New Hampshire Main Campus, Space Science Center, Durham, NH, United States, (2)University of New Hampshire Main Campus, Durham, NH, United States, (3)University of New Hampshire, Durham, NH, United States, (4)Los Alamos National Laboratory, Los Alamos, NM, United States, (5)The New Mexico Consortium, Los Alamos, NM, United States, (6)The Aerospace Corp, Los Angeles, CA, United States, (7)Aerospace Corporation Santa Monica, Santa Monica, CA, United States, (8)Aerospace Corporation, Los Angeles, CA, United States, (9)University of Colorado, Laboratory for Atmospheric and Space Physics, Boulder, CO, United States, (10)NASA Goddard Space Flight Center, Greenbelt, MD, United States
Abstract:
We quantify the loss of radiation belt electrons into Earth’s atmosphere with measurements from the Radiation Belt Storm Probes – Energetic Particle, Composition, and Thermal plasma (RBSP-ECT) instrument suite onboard NASA’s Van Allen Probes mission. Understanding the relative importance of radiation belt enhancements and decreases relies on a quantitative assessment of the state of the belts. For instance, estimates of electron loss through precipitation into the atmosphere only become meaningful when compared to the total belt population. Previous studies used observations to quantify the total number of energetic electrons in the Van Allen belts. Baker et al. (2004) used measurements from the low altitude SAMPEX mission to estimate the radiation belt content, while Selesnick and Kanekal (2009) used high altitude NASA/Polar measurements. Our work extends these pioneering studies by taking advantage of measurements from RBSP-ECT, providing complete coverage of electron energy and pitch angle, near the magnetic equator, and with sufficiently high time cadence. We integrate the electrons measured by RBSP-ECT over energy and pitch angle throughout the belts every half orbit on each spacecraft to establish a quantitative estimate of the total radiation belt electron content (TRBEC) and its time variation. We identify a several week period in March 2013 during which TRBEC slowly decayed following an impulsive enhancement. Over that interval, electron phase space density profiles with L* are consistent with inward radial diffusion conditions, at all first and second invariants, with no indications of significant local acceleration or impulsive injections. From this slowly-evolving, quiet interval, we may thus constrain the total amount that was lost through precipitation to the atmosphere in the global system. In the absence of new sources, and assuming only the one sink in such a state, we effectively quantify the efficiency of the “drain” (precipitation loss to the atmosphere) by watching the TRBEC level decrease with time from the radiation belt electron reservoir. We compare this loss estimate with previous estimates and, finally, discuss the implications of this energetic electron precipitation on atmospheric chemistry.
