Occurrence of EMIC Waves in the Magnetosphere According to Their Distance to the Magnetopause. Issue 3 (1st February 2021) - Grison, B.
Abstract: Wave growth of electromagnetic ion cyclotron (EMIC) emissions observed in the outer magnetosphere is mainly controlled by compression events resulting from solar wind dynamic pressure pulses. During such events wave growth is expected to be maximum close to the magnetopause. In previous studies, distribution of EMIC waves was analyzed according to their distance from the Earth, which is inadequate for studying the magnetopause region. We map a data set of EMIC waves observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft according to their distance from case‐by‐case modeled magnetopause. EMIC occurrence rate is found to be maximum within two Earth radii from the magnetopause and then it linearly decreases with an increasing distance, especially close to the local noon. Asymmetries between the morning and evening magnetic sectors are explained by asymmetries in the upstream conditions and by the presence of another EMIC population of a different origin. Plain Language Summary: The magnetopause is the boundary of the Earth's magnetosphere cavity in the heliosphere. When the magnetosphere is compressed by particles of solar origin (solar wind), the magnetopause is moving toward the Earth and specific waves, called electromagnetic ion cyclotron (EMIC), are expected to be strongly amplified just inside the magnetopause. Several studies found that EMIC waves are more frequently observed at a larger distance from the Earth. The influence of the distance from magnetopause was thus never directly investigated because the magnetopause is not observed at a fixed distance from the Earth. To address this problem, we use a magnetopause model whose position depends on solar wind properties. We then compute the distance from the magnetopause for a large data set of observed EMIC waves. Our results prove that EMIC waves in the outer magnetosphere are most frequently observed in the vicinity of the magnetopause, at distances less than two Earth radii. We also note and discuss a local time asymmetry in EMIC occurrence rates. Key Points: Distance to the magnetopause organizes compression‐driven electromagnetic ion cyclotron (EMIC) wave observations better than L value EMIC waves in the magnetosphere are the most frequently observed close to the magnetopause model In this region EMIC waves are more frequent in the noon and dawn sectors than in the afternoon