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

Magnetospheric compression due to impact of enhanced solar wind dynamic pressure P_dyn 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 P_dyn 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 P_dyn 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 P_dyn 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 P_dyn 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 P_dyn 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 P_dyn 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 P_dyn 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 P_dyn impact.