Combined acceleration of electrons by whistler‐mode and compressional ULF turbulences near the geosynchronous orbit

Abstract

In the quasi‐linear approximation, we study electron acceleration process generated by whistler‐mode and compressional ULF (fast mode waves) turbulences near the Earth's synchronous orbit. The results show that the whistler‐mode turbulence (0.1 f ce ≤ f ≤ 0.75 f ce ) can accelerate substorm injection electrons with several hundreds of keV through wave‐particle gyroresonant interaction and hence may play an important role in the electron acceleration during substorms. The compressional ULF turbulence (2–15 mHz) can accelerate both lower‐energy background electrons (<30 keV) and substorm injection electrons (∼30–300 keV) through the transit‐time damping mechanism. So the compressional ULF turbulence acceleration mechanism is important during both substorms and quiet times. The compressional ULF turbulence accelerates substorm injection electrons more effectively than whistler‐mode turbulence. The combined electron acceleration by whistler‐mode and ULF turbulences is most effective and can cause the number density of the relativistic electrons increase largely within about 8 hours. Substorms can offer both substorm injection electrons and strong turbulences, and therefore large flux enhancement events of relativistic electrons (≥1 MeV) always occur during substorm time. For magnetic storms that are composed of a series of substorms, extremely large flux enhancement events of the relativistic electrons can thus occur.