Extreme Relativistic Electron Fluxes in the Earth's Outer Radiation Belt: Analysis of INTEGRAL IREM Data

Abstract:

The flux of relativistic electrons (E > 500 keV) in the Earth's outer radiation belt is highly dynamic with fluxes changing by orders of magnitude on timescales ranging from minutes to weeks. These so-called "killer" electrons cause internal charging and are an important space weather hazard. An important question is "How large can the fluxes become during an extreme event ?". Modern satellites in medium Earth orbit and at geosynchronous orbit have life expectancies of 10-20 years. Satellite operators and engineers thus need realistic estimates of the flux levels that may be reached on these and longer timescales in order to assess the likely impact of an extreme event on the satellite fleet and to improve the resilience of future satellites by better design of satellite components. In this study we perform an extreme value analysis of the relativistic electron fluxes in the Earth's outer radiation belt as a function of energy and L*. We use data from the Radiation Environment Monitor (IREM) on board the International Gamma Ray Astrophysical Laboratory (INTEGRAL) spacecraft from 17 October 2002 to 31 December 2016. The 1 in 10 year flux at L*=4.5, representative of equatorial medium Earth orbit, decreases with increasing energy ranging from 1.36x10⁷ cm^-2s^-1sr^-1MeV^-1 at E=0.69 MeV to 5.34x10⁵ cm^-2s^-1sr^-1MeV^-1 at E=2.05 MeV. The 1 in 100 year flux at L*=4.5 is generally a factor of 1.1 to 1.2 larger than the corresponding 1 in 10 year flux. The 1 in 10 year flux at L*=6.0, representative of geosynchronous orbit, decreases with increasing energy ranging from 4.35x10⁶ cm^-2s^-1sr^-1MeV^-1 at E=0.69 MeV to 1.16x10⁵ cm^-2s^-1sr^-1MeV^-1. The 1 in 100 year flux at L*=6.0 is generally a factor of 1.1 to 1.4 larger than the corresponding 1 in 10 year flux. The ratio of the 1 in 10 year flux at L*=4.5 to that at L*=6.0 increases with increasing energy ranging from 3.1 at E=0.69 MeV to 4.6 at E=2.05 MeV. The analysis shows that the electron fluxes generally tend to limiting values, which are up to factors of 1.9 and 1.4 times larger than the 1 in 100 year events at L* = 4.5 and 6.0 respectively. The extreme event flux levels computed here can be used as benchmarks for comparison with any short time constant event (1 hour, 1 day or otherwise) and can be compared with theoretical maximum fluxes inferred from radiation belt models.