Significant loss of energetic electrons at the heart of the outer radiation belt during weak magnetic storms

Abstract

[1] For various reasons, the Earth's outer radiation belt often exhibits dramatic and sudden increases or decreases in the observed particle flux. In this paper, we report three dropout events of energetic electrons observed by multiple spacecraft while traveling across the outer radiation belt. The three events were first identified based on observations of a significant dropout in the >2 MeV electron flux at geosynchronous orbit. Subsequently, for each event, we analyzed the energetic electron data obtained near the magnetic equator by THEMIS spacecraft to determine the responses of the entire outer radiation belt. Our analysis is mainly based on the electron fluxes measured at energies of 52 keV, 203 keV, and 719 keV, and on the phase space densities estimated for the first adiabatic invariant μ values of 100 MeV/G, 200 MeV/G, and 300 MeV/G. The main shared feature among the three events is that while, for the lowest energy, sources from the convection and/or particle injections of plasma sheet electrons dominate over losses, the higher energies exhibit a dramatic dropout effect that penetrates deeply into L~4.5 -5. In terms of the phase space density, a similar dropout effect is clearly seen for the μ values of 200 MeV/G and 300 MeV/ G, while the convection effect and/or injections dominates for μ = 100 MeV/G. What is astonishing about this dropout phenomenon is that the three events are all associated with only very weak magnetic storms with a SYM-H minimum of -40 nT or larger. This implies that a significant loss of electrons deep inside the outer radiation belt can occur even during a very weak magnetic storm. Low-altitude observations of electrons by NOAA POES satellites indicate no significant atmospheric precipitation due to strong diffusion. Our simulations with various conditions suggest that radial diffusion effect in combination with the magnetopause shadowing are responsible for the observed dropouts to a large extent for all of the three events, although the contribution by the weak atmospheric precipitation that might have been missed by the NOAA POES observations can be non-negligible.

Comprehensive analysis of the flux dropout during 7–8 November 2008 storm using multisatellite observations and RBE model

“…Here we used the all available POES satellites: NOAA‐15, NOAA‐16, NOAA‐17, NOAA‐18, and NOAA‐19. Since POES MEPED can only observe precipitation into the narrow field of view angle of 30° [ Hwang et al , , and references there in], we also examined data from the energy channel of >2 MeV electrons by GOES 11.…”

“…These observations revealed the interesting nature of this dropout and provided strong evidence of the dominant loss mechanisms responsible for the drastic loss of energetic particles throughout Earth's inner magnetosphere. More detailed and quantitative analysis on this event based on the PSD calculation was performed in the previous study in Hwang et al []. Figure shows the overall solar wind conditions during the dropout: solar wind speed, dynamic pressure, density, interplanetary magnetic field (IMF) B z , geomagnetic indices of Dst and AE , and electron flux of >2 MeV by GOES 11 from top to bottom.…”

“…Dropouts usually involve the drastic decrease in electron flux over a broad range in energy, equatorial pitch angle, and radial distance in only a few hours, as observed by spacecraft, and the dominant mechanism(s) responsible for these events remains a topic of debate [ Turner et al , , , and reference therein]. It is known that this kind of ambiguity on the cause of the outer belt loss will be better understood using the distributions of electron phase space density (PSD) in adiabatic invariant coordinates during the dropout [ Turner et al , ; Hwang et al , ; Reeves et al ., ].…”

“…Dropouts usually involve the drastic decrease in electron flux over a broad range in energy, equatorial pitch angle, and radial distance in only a few hours, as observed by spacecraft, and the dominant mechanism(s) responsible for these events remains a topic of debate [Turner et al, 2014a, 2014b, andreference therein]. It is known that this kind of ambiguity on the cause of the outer belt loss will be better understood using the distributions of electron phase space density (PSD) in adiabatic invariant coordinates during the dropout Hwang et al, 2013; few hundreds keV electrons and electromagnetic ion cyclotron (EMIC) waves for~MeV electrons [Summers and Thorne, 2003]. Until now, many particle kinetic models have been established to simulate and to provide interpretation for observed various features in the Earth's radiation belt dynamics.…”

Radial diffusion simulations of the 20 September 2007 radiation belt dropout

“…Comparing observations to simulations with constant boundary conditions, Shprits et al (2005) obtained τ = 3/Kp (in days) outside the plasmasphere as a reasonable, purely empirical fit, and τ = 5/Kp using variable boundary conditions. As a compromise, Hwang et al (2013) used τ = 4/Kp, while Li et al (2014) used τ = 6/Kp. More physically, Shprits et al (2007) and Gu et al (2012) presented fits in L, E, and Kp to simple estimates of the timescale of pitch angle diffusion by chorus waves.…”