Determining the Mode, Frequency, and Azimuthal Wave Number of ULF Waves During a HSS and Moderate Geomagnetic Storm

Abstract

Ultralow frequency (ULF) waves play a fundamental role in the dynamics of the inner magnetosphere and outer radiation belt during geomagnetic storms. Broadband ULF wave power can transport energetic electrons via radial diffusion, and discrete ULF wave power can energize electrons through a resonant interaction. Using observations from the Magnetospheric Multiscale mission, we characterize the evolution of ULF waves during a high-speed solar wind stream (HSS) and moderate geomagnetic storm while there is an enhancement of the outer radiation belt. The Automated Flare Inference of Oscillations code is used to distinguish discrete ULF wave power from broadband wave power during the HSS. During periods of discrete wave power and utilizing the close separation of the Magnetospheric Multiscale spacecraft, we estimate the toroidal mode ULF azimuthal wave number throughout the geomagnetic storm. We concentrate on the toroidal mode as the HSS compresses the dayside magnetosphere resulting in an asymmetric magnetic field topology where toroidal mode waves can interact with energetic electrons. Analysis of the mode structure and wave numbers demonstrates that the generation of the observed ULF waves is a combination of externally driven waves, via the Kelvin-Helmholtz instability, and internally driven waves, via unstable ion distributions. Further analysis of the periods and toroidal azimuthal wave numbers suggests that these waves can couple with the core electron radiation belt population via the drift resonance during the storm. The azimuthal wave number and structure of ULF wave power (broadband or discrete) have important implications for the inner magnetospheric and radiation belt dynamics.

Cold Plasmaspheric Electrons Affected by ULF Waves in the Inner Magnetosphere: A Van Allen Probes Statistical Study

“…For the event observed by Probe B, there are lower limit and peak period but without upper limit, which is referred as Type II. Even though ULF waves are successively observed by Probes B and A in the event of Figure , the wave power spectra exhibit different intensities, wave peak periods, and spread width in Figure a, indicating the dynamical evolution of wave power spectra under different conditions (e.g., Murphy et al, ; Ren, Zong, Miyoshi, et al, ; Ren et al, ). Besides, if there is only lower limit (not shown here), the event will be referred as Type III.…”

“…It should be noted that the event is not counted if the PSD is large enough but does not exhibit a peak in Pc3–5 bands. In other words, events with ULF wave activities in Figure do not contain those with broadband ULF wave power (Murphy et al, ). In Figures a and b, the average of the ratio is 95.6% and 80.8%, respectively.…”

“…Besides, if there is only lower limit (not shown here), the event will be referred as Type III. In both Types I and II, ULF waves have discrete wave power; but in Type III, they have broadband wave power (Brautigam et al, ; Murphy et al, ).…”

“…Type I means that there are lower limit, peak period, and upper limit of wave period range where the PSD satisfies log 10 (PSD) ≥ 0.5 nT 2 /Hz (or (mV/m) 2 /Hz); Type II means that there are lower limit and peak period but without upper limit; Type III means that there is only lower limit. The green horizontal dashed line represent the lower threshold (15 eV waves have discrete wave power; but in Type III, they have broadband wave power (Brautigam et al, 2005;Murphy et al, 2018). Figure 6b shows the resonant energy range of electrons estimated based on electron observations from HOPE instrument.…”

Selective Acceleration of O+ by Drift‐Bounce Resonance in the Earth's Magnetosphere: MMS Observations

“…This suggests the selective acceleration of O + due to the drift-bounce resonance. High-speed solar wind easily causes the K-H instability on the magnetopause and excites toroidal mode ULF waves (e.g., Claudepierre et al, 2008;Liu et al, 2010;Murphy et al, 2018;Nosé et al, 1995;Rae et al, 2005;Takahashi & Ukhorskiy, 2007). Oimatsu, Nosé, Teramoto, et al (2018) proposed that the solar wind driving Pc5 wave fed energy to the O + at <~20 keV through the N = 2 drift-bounce resonance.…”