Charged Particle Behavior in the Growth and Damping Stages of Ultralow Frequency Waves: Theory and Van Allen Probes Observations

Abstract:

Ultralow frequency (ULF) electromagnetic oscillations in the magnetosphere can accelerate electrons via a process called drift resonance. In the conventional drift-resonance theory [Southwood & Kivelson, 1981], a default assumption is that the wave growth rate is time-independent, positive, and extremely small. However, this may not be always the case in the magnetosphere. The ULF waves should have experienced a growth stage when their energy was taken from external and/or internal sources, and as time progresses the waves have to be damped with a negative growth rate. Therefore, a more generalized theory on particle behavior during different stages of ULF waves is required. In this paper, we introduce a time-dependent imaginary wave frequency to accommodate the growth and damping of the waves in the conventional drift-resonance theory, to study the particle interactions with the waves during the entire wave lifespan. We then predict from the generalized theory the particle signatures for different stages of the waves, which agree very well with Van Allen Probe observations. The more generalized theory, therefore, provides a new understanding of wave-particle interactions and ULF wave evolution in the magnetosphere.