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Trapped Electron Energy Inferred From Cosmic Noise Absorption Signals Through Drift‐Time Analysis in Empirical Electric and Semi‐Empirical Magnetic Fields

Riometers are ground‐based passive radio receivers that detect cosmic noise emanating from distant stars and galaxies. Some of the radiowave energy from these galactic sources is absorbed in the lower ionosphere by free electrons, precipitated from magnetospheric sources. The eastward motion of magnetospheric electrons, due to the combined effects of convection and gradient and curvature drift, results in a delay in signal between ground stations separated by 100s and 1,000s of kilometers. In this study, the signal delay is used to infer the energy of trapped electrons by employing a full bounce‐averaged drift velocity computation using two semi‐empirical magnetic field models, an empirical electric field, and 13 Riometer stations in the NORSTAR ground observation network. It is shown statistically, that riometers respond to 20–80 keV electrons, and that the inferred energy rises with an increase in absorption. The inferred energy is also shown to depend on magnetic local time, and the choice of the magnetic field model. This study quantifies the capability to infer magnetically trapped electron energies from riometer signals. This important population of 10s of keV electrons acts as a source of electromagnetic wave energy, necessary for acceleration of trapped electrons to higher energies where they may be hazardous to space hardware.