Energetic Electron and Proton Interactions with Pc5 Ultra Low Frequency (ULF) Waves during the Great Geomagnetic Storm of 15–16 July 2000

Abstract

The dynamics of the outer zone radiation belt has received a lot of attention mainly due to the correlation between the occurrence of enhancing relativistic electron flux and spacecraft operation anomalies or even failures (e.g., Baker et al. 1994). Relativistic electron events are often observed during great storms associated with ultra low frequency (ULF) waves. For example, a large buildup of relativistic electrons was observed during the great storm of March 24, 1991 (e.g., Li et al. 1993; Hudson et al. 1995; Mann et al. 2013). However, the dominant processes which accelerate magnetospheric radiation belt electrons to MeV energies are not well understood. In this paper, we present observations of Pc5 ULF waves in the recovery phase of the Bastille day storm of July 16, 2000 and electron and proton flux simultaneously oscillating with the same frequencies as the waves. The mechanism for the observed electron and proton flux modulations is examined using groundbased and satellite observations. During this storm time, multiple packets of discrete frequency Pc5 ULF waves appeared associated with energetic particle flux oscillations. We model the drift paths of electrons and protons to determine if the particles drift through the ULF wave to understand why some particle fluxes are modulated by the ULF waves and others are not. We also analyze the flux oscillations of electrons and protons as a function of energy to determine if the particle modulations are caused by a ULF wave drift resonance or advection of a particle density gradient. We suggest that the energetic electron and proton modulations by Pc5 ULF waves provide further evidence in support of the important role that ULF waves play in outer radiation belt dyanamics during storm times.

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“…Additionally, it is recognized that a major component propelling this process are plasma waves, specifically ULF waves. The interactions between ULF wave and particle were noted by Lee et al [8] during the geomagnetic storm's recovery phase on July 16, 2000. The global ground-based magnetometer arrays were used to collect magnetic field data.…”

“…Consequently, they put forth two theories to account for the flux modulation that was seen: (1) Energy-dependent particle density gradients advecting (protons and electrons show the similar amplitude and lack of phase change with energy); (2) Energetic electronic drift resonance (distinct flux oscillations varying in amplitude and phase over various energy channels). Given the obvious connection between Pc5 waves and particle flux oscillations, Pc5 ULF waves could be crucial for electron acceleration in radiation belts [8].…”