Long-term variations of quasi-trapped and trapped electrons in the inner radiation belt observed by DEMETER and SAMPEX

TL;DR

This study analyzes 17 years of satellite and ground data to understand long-term variations of quasi-trapped and trapped electrons in Earth's inner radiation belt, revealing their dependence on solar activity and geomagnetic conditions.

Contribution

It provides new insights into the production mechanisms and long-term behavior of electrons in the inner belt, highlighting the role of cosmic rays and geomagnetic storms.

Findings

Electron flux at L ≤ 1.14 is anti-correlated with sunspot number.

Electron flux at L ≤ 1.14 is proportional to cosmic ray intensity, indicating CRAND origin.

Electron enhancements during geomagnetic storms at L ≥ 1.2.

Abstract

Electrons in the Earth's radiation belts can be categorized into three populations: precipitating, quasi-trapped and trapped. We use data from the DEMETER and SAMPEX missions and from ground-based neutron monitors (NM) and sunspot observations to investigate the long-term variation of quasi-trapped and trapped sub-MeV electrons on different L shells in the inner belt. DEMETER and SAMPEX measurements span over 17 years and show that at $L \leq 1.14$ the electron flux is anti-correlated with sunspot number, but proportional to the cosmic ray intensity represented by NM count rates, which suggests that electrons at the inner edge of the inner belt are produced by Cosmic Ray Albedo Neutron Decay (CRAND). The solar cycle variation of cosmic rays increased the electron flux at $L \leq 1.14$ by a factor of two from solar maximum at 2001 to solar minimum at 2009. At $L \ge 1.2$, both quasi-trapped and trapped electrons are enhanced during geomagnetic storms and decay to a background level during extended quiet times. At $L>2$, quasi-trapped electrons resemble trapped electrons, with correlation coefficients as high as 0.97, indicating that pitch angle scattering is the dominant process in this region.