Wave storms in the Earth's magnetosphere

Abstract

Ultra-low frequency (ULF) waves permeate near-Earth space and play a key role in the transfer of electromagnetic energy from the solar wind to the magnetosphere. These waves contribute to magnetosphere-ionosphere coupling, and are central to the dynamics of the inner magnetosphere, in particular through particle acceleration and transport in the radiation belts. ULF waves in the Pc5 frequency band (2-7 mHz) are especially important for radiation belt dynamics, as they cause radial diffusion of electrons and, when their amplitude is large enough, nonlinear interactions. Extensive statistical surveys have shown that the ULF wave power in the Pc5 range tends to increase during geomagnetic storms. However, we note that widely-used geomagnetic indices such as Kp, AE and Dst do not intrinsically quantify the level of ULF wave activity inside the magnetosphere. The goals of this study are (1) to define wave storms in Earth's magnetosphere, that is, intervals of elevated ULF wave power, irrespective of the occurrence of geomagnetic storms (as defined by the Dst or SYM-H index), (2) to identify the drivers of the most intense wave storms and (3) to compare the occurrence of wave storms with that of geomagnetic storms. The ULF wave activity inside the magnetosphere is quantified based on the ground ULF wave power index described by Pilipenko et al. [2017]. Similarly as storms being classified as moderate or intense based on the minimum Dst value reached during the event, we define two levels for wave storms, moderate and intense wave storms. We analyse the occurrence of wave storms as a function of large-scale solar wind drivers (interplanetary coronal mass ejections – ICMEs – and high-speed solar wind streams – HSSs). We find that about half of the wave storms are driven by HSSs, possibly due to the long duration of these structures, while ICMEs tend to drive the most intense wave storms. We also compare electron fluxes at geostationary orbit during wave storms and quiet times, and find significantly enhanced fluxes during wave storms, as expected. Finally, we compare the occurrence of wave storms with that of geomagnetic storms, and discuss possible applications of this new wave storm definition.