Van Allen Probes observations of electromagnetic ion cyclotron waves triggered by enhanced solar wind dynamic pressure

Abstract

Magnetospheric compression due to impact of enhanced solar wind dynamic pressure Pdyn has long been considered as one of the generation mechanisms of electromagnetic ion cyclotron (EMIC) waves. With the Van Allen Probe‐A observations, we identify three EMIC wave events that are triggered by Pdyn enhancements under prolonged northward interplanetary magnetic field (IMF) quiet time preconditions. They are in contrast to one another in a few aspects. Event 1 occurs in the middle of continuously increasing Pdyn while Van Allen Probe‐A is located outside the plasmapause at postmidnight and near the equator (magnetic latitude (MLAT) ~ −3°). Event 2 occurs by a sharp Pdyn pulse impact while Van Allen Probe‐A is located inside the plasmapause in the dawn sector and rather away from the equator (MLAT ~ 12°). Event 3 is characterized by amplification of a preexisting EMIC wave by a sharp Pdyn pulse impact while Van Allen Probe‐A is located outside the plasmapause at noon and rather away from the equator (MLAT ~ −15°). These three events represent various situations where EMIC waves can be triggered by Pdyn increases. Several common features are also found among the three events. (i) The strongest wave is found just above the He+ gyrofrequency. (ii) The waves are nearly linearly polarized with a rather oblique propagation direction (~28° to ~39° on average). (iii) The proton fluxes increase in immediate response to the Pdyn impact, most significantly in tens of keV energy, corresponding to the proton resonant energy. (iv) The temperature anisotropy with T⊥ > T|| is seen in the resonant energy for all the events, although its increase by the Pdyn impact is not necessarily always significant. The last two points (iii) and (iv) may imply that in addition to the temperature anisotropy, the increase of the resonant protons must have played a critical role in triggering the EMIC waves by the enhanced Pdyn impact.

Simultaneous Observations of Electromagnetic Ion Cyclotron (EMIC) Waves and Pitch Angle Scattering During a Van Allen Probes Conjunction

“…This may partially explain why the EMIC wave activity consists of longer bursts during the inbound interval. The range of solar wind dynamic pressures and background magnetic field perturbations observed during both the outbound and inbound passes through the regions of EMIC wave activity is similar to those reported in other studies of EMIC waves associated with magnetospheric compressions (e.g., Cho et al, 2016; McCollough et al, 2010; Usanova et al, 2008).…”

Test of Ion Cyclotron Resonance Instability Using Proton Distributions Obtained From Van Allen Probe‐A Observations

“…It should be emphasized that the analysis performed in section 4 was applied to the times 20 s before and after EMIC wave onsets as well as the non‐EMIC wave times, but not to the entire period of each EMIC wave event. The anisotropy can evolve during EMIC wave periods in accordance with changes in EMIC wave activity: Examples can be found in the previous work in Cho et al (). In particular, the proton anisotropy can decrease during EMIC wave duration as noted by Saikin et al ().…”

Location of intense electromagnetic ion cyclotron (EMIC) wave events relative to the plasmapause: Van Allen Probes observations

“…As has been found in many other studies, the six example Van Allen Probes passes presented in ssection 3 showed that the conditions for EMIC wave occurrence required sufficiently high densities of ring current ions [e.g., Cho et al ., ]. In many but not all the examples these waves were also associated with increased densities of either or both warm (plasma sheet) ions and thermal (cold) ions.…”