Inner Magnetospheric ULF Waves: The Occurrence and Distribution of Broadband and Discrete Wave Activity

Murphy, K. R., Inglis, A. R., Sibeck, D. G., Watt, Clare and Rae, I. J. (2020) Inner Magnetospheric ULF Waves: The Occurrence and Distribution of Broadband and Discrete Wave Activity. Journal of Geophysical Research: Space Physics, 125 (9). e2020JA027887. ISSN 2169-9380

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Abstract

Ultralow frequency (ULF) waves are electromagnetic pulsations observed throughout the magnetosphere driven by processes both external and internal to the magnetosphere. Within the magnetosphere, discrete and broadband ULF wave activity can couple to the local plasma via coherent or stochastic wave-particle interactions. These wave-particle interactions can lead to dynamic changes in local plasma including rapid acceleration and transport of radiation belt electrons. Using observations from GOES-15 and the Automated Flare Inference of Oscillations algorithm we investigate the distribution and occurrence of broadband and discrete ULF waves to help understand the relative importance of coherent and stochastic wave-particle interactions. We find that intervals of discrete ULF waves are more commonly identified during slow and low-density solar wind and when Bz is near zero. Broadband waves are more commonly identified during periods of active solar wind, including periods of high solar wind speeds and large density perturbations, and large negative Bz. We also find that under all solar wind conditions the number of intervals of broadband ULF wave power exceeds that of discrete wave power; for example, ULF wave activity is more likely to be broadband. These results suggest that radial diffusion due to incoherent broadband waves is an important driver of wave-particle interactions, especially during active solar wind conditions. However, the presence of discrete waves during both active and quiet solar wind conditions suggests that these waves and the corresponding wave-particle interactions cannot be ignored, especially since discrete wave-particle interactions tend to be more efficient than radial diffusion.

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