Empirically Estimated Electron Lifetimes in the Earth's Radiation Belts: Van Allen Probe Observations.

S G Claudepierre,T P O'Brien,J Bortnik,J B Blake,Q Ma,J F Fennell

doi:10.1029/2019gl086053

Abstract

We use measurements from NASA's Van Allen Probes to calculate the decay time constants for electrons over a wide range of energies (30 keV to 4 MeV) and L values ( L = 1.3–6.0) in the Earth's radiation belts. Using an automated routine to identify flux decay events, we construct a large database of lifetimes for near‐equatorially mirroring electrons over a 5‐year interval. We provide the first accurate estimates of the long decay timescales in the inner zone ( ∼100 days), which are highly resolved in energy and free from proton contamination. In the slot region and outer zone, we compare our lifetime calculations with prior empirical estimates and find good quantitative agreement (lifetimes ∼1–20 days). The comparisons suggest that some prior estimates may overestimate electron lifetimes between L≈ 2.5–4.5 due to instrumental effects and/or background contamination. Previously reported two‐stage decays are explicitly demonstrated to be a consequence of using integral fluxes.

Highlights

The Earth's electron radiation belts rarely, if ever, reach a state of equilibrium and exist in a constant state of flux, the result of competition between various source and loss processes
Many authors have calculated these electron decay time constants, or “lifetimes,” from observations at various energies and locations throughout the belts to help elucidate the relevant physics (e.g., Albert, 2000; Baker et al, 2007; Benck et al, 2010; Fennell et al, 2012; Meredith et al, 2006, 2009; Ripoll et al, 2015; Roberts, 1969; Seki et al, 2005; Su et al, 2012; Vampola, 1971; West Jr et al, 1981). Such lifetime estimates are useful for radiation belt modeling, whereby the complexity of the problem can be reduced by incorporating all of the loss processes and loss physics into a single model parameter
The calculated lifetimes could potentially be influenced by a source and may not always be representative of the true, underlying decay timescale

Summary

Introduction

The Earth's electron radiation belts rarely, if ever, reach a state of equilibrium and exist in a constant state of flux, the result of competition between various source and loss processes. Many authors have calculated these electron decay time constants, or “lifetimes,” from observations at various energies and locations throughout the belts to help elucidate the relevant physics (e.g., Albert, 2000; Baker et al, 2007; Benck et al, 2010; Fennell et al, 2012; Meredith et al, 2006, 2009; Ripoll et al, 2015; Roberts, 1969; Seki et al, 2005; Su et al, 2012; Vampola, 1971; West Jr et al, 1981) Such lifetime estimates are useful for radiation belt modeling, whereby the complexity of the problem can be reduced by incorporating all of the loss processes and loss physics into a single model parameter. A companion paper uses the lifetime estimates to constrain and inform our understanding of the relevant physical processes that contribute to the loss of electrons from the radiation belts

Results

Discussion

Conclusion