Spatial Distribution of Fine‐Structured and Unstructured EMIC Waves Observed by the Arase Satellite. Issue 21 (5th November 2018) - Matsuda, S.
Abstract: We surveyed 378 electromagnetic ion cyclotron (EMIC) wave events using Arase during the first 12 months. We discriminated the observed EMIC waves into two types ( fine‐structured EMIC waves and unstructured EMIC waves ), according to whether or not they exhibited clear periodic amplitude modulations. We found a clear afternoon peak in the occurrence probability of unstructured EMIC waves in the region of L = 5–9. Fine‐structured EMIC waves were mainly distributed in the off‐equatorial region, and their occurrence probability was clearly high around the noon sector. Such noon sector EMIC wave peaks had not been found in the THEMIS, AMPTE, or Cluster measurements, due to their limited L shell coverages. The Van Allen probes, too, were not able to measure the major region of fine‐structured EMIC waves because of their limited latitudinal coverage. Our unique statistical results demonstrate the importance of discriminating the wave structures as well as of performing observations over a wide range of latitudes. Plain Language Summary: The spatial distribution of electromagnetic ion cyclotron (EMIC) waves is important and fundamental information from the point of view of wave‐particle interaction process. Several studies have determined the distributions of EMIC waves. However, they have been controversial, because there are several possible mechanisms for generating EMIC waves. We discriminated 378 events observed by Arase into two types, according to whether or not they exhibited clear periodic amplitude modulations. We found a clear afternoon peak in the occurrence probability of unstructured EMIC waves in the region of L = 5–9. This is consistent with previous observations. Fine‐structured EMIC waves were mainly distributed in the off‐equatorial region, and their occurrence probability was clearly high around noon. Such noon peaks had not been found in the THEMIS, AMPTE, or Cluster measurements, due to their limited L shell coverages. The Van Allen probes measured the region of L = 2–6, but they also did not report such a noon sector peak. Our statistical results suggest that the Van Allen probes, too, were not able to measure the major region of fine‐structured EMIC waves because of their limited latitudinal coverage. Our unique statistical results demonstrate the importance of discriminating the wave structures as well as of performing observations over a wide range of latitudes. Key Points: We clarified the comprehensive spatial distribution of EMIC waves in the region of L < 9 We found a clear occurrence peak of fine‐structured EMIC waves in the noon, off‐equatorial region The occurrence peak of unstructured EMIC waves in the afternoon is consistent with past reports