Characterisation of the Number Density and Temperature of Energetic Electrons in the Inner Magnetosphere

Abstract

The Earth’s inner magnetosphere contains multiple electron populations influenced by different factors. The cold electrons of the plasmasphere, warm plasma that contributes to the ring current, and the relativistic plasma of the radiation belt often seem to behave independently. Using omnidirectional flux and energy measurements from the HOPE and MagEIS instruments on board the Van Allen Probes, we provide a detailed density and temperature description of the inner magnetosphere, offering a comprehensive statistical analysis of the entire Van Allen Probe era. While number density and temperature data at geosynchronous orbit are available, this study focuses on the inner magnetosphere (2 < L∗ < 6). Values of density and temperature are extracted by fitting energy and phase space density to obtain the distribution function. The fitted distributions are related to the zeroth and second moments to estimate the number density and temperature. Analysis has indicated that a two Maxwellian fit is sufficient over a wide range of L∗ and that there are two independent plasma populations which have been interpreted as the substorm-injected source and seed population, where the source population typically has energies of 10s of keV and the seed population typically has energies of 100s of keV. The more energetic population has a median number density of approximately 1.2 × 104 m−3 and a temperature of around 130 keV, with a temperature peak observed between L* = 4 and L* = 4.5. This population is relatively uniform in MLT. In contrast, the less energetic warm electron population has a median number density of about 2.5×104 m−3 and a temperature of 7.4 keV. The values of number density and temperature are then sorted by substorm phase as identified by the SOPHIE substorm list. The evolution of how the number density and temperature of the source and seed populations with substorm phase is presented, showing that typically substorms only inject the source population and sometimes inject the source and seed populations simultaneously.

Date of Award	19 Feb 2026
Original language	English
Awarding Institution	Northumbria University
Supervisor	Clare Watt (Supervisor), Sarah Bentley (Supervisor) & Jasmine Mann (Supervisor)