Highly relativistic magnetospheric electrons: A role in coupling to the middle atmosphere?

Abstract

Long‐term (1979‐present) observations of relativistic electrons (2–15 MeV) at geostationary orbit show a strong solar cycle dependence. Such electrons were largely absent near the last solar maximum (1979–80), while they were prominent during the approach to solar minimum (1983–85). This population now is dwindling as solar minimum has been reached. The strong magnetospheric presence of this population closely parallels the presence of high‐speed solar wind streams which results from solar coronal hole structures during the approach to solar activity (sunspot) minimum. We clearly observe 27‐day periodic enhancements of the relativistic electrons in association with concurrently measured solar wind streams (VSW ≳ 600 km/s). We have used a numerical transport code to study the coupling of these high‐energy electrons to earth's upper and middle atmosphere. We calculate using the observed energy spectra of the electrons that, when precipitated, these electrons show a large (maximum of ∼100 keV/cm³‐s) energy deposition at 40–60 km altitude, which is 3–4 orders of magnitude greater than the galactic cosmic ray or solar EUV energy deposition at these altitudes. We also find that the global energy deposition in the mid‐latitudes totals nearly 1021 ergs for a typical 2–3 day event period. We conclude that this previously unrecognized electron population could play an important role in coupling solar wind and magnetospheric variability (on 27‐day and 11‐year cycles) to the middle atmosphere through a modulating effect on lower D‐region ionization and, possibly, on upper level ozone chemistry. These electrons also may contribute to the recent Antarctic polar ozone depletion phenomenon.