Multipoint Measurement of Fine-Structured EMIC Waves by Arase, Van Allen Probe A, and Ground Stations

We examined the growth and propagation of fine-structured electromagnetic ion cyclotron (EMIC) waves related to time-varying density irregularities using multipoint measurement data observed by Arase, Van Allen Probe A, and two ground-based induction magnetometers (Gakona and Dawson) during a field line conjunction event on April 18, 2019. We analyzed the wave data obtained by the aforementioned spacecraft and stations, and found that the appearance of fine structures in the observed EMIC waves clearly coincided with the ambient electron density irregularities in the magnetosphere, which can cause periodic wave growth and waveguiding on their propagation. Furthermore, we found that the latitudinal widths of the EMIC wave activity region and the wave propagation duct were similar to 185 km and less than 80 km at an auroral altitude of 100 km, respectively. We also found thermal ion heating (<200 $< 200$ eV/q) during the EMIC wave activity.