Dependence of Electromagnetic Ion Cyclotron Wave Occurrence on North‐South Orientation of Interplanetary Magnetic Field: THEMIS Observations

We investigate the L‐MLT (i.e., L‐shall versus magnetic local time) distributions of electromagnetic ion cyclotron (EMIC) waves, using the Time History of Events and Macroscale Interactions during Substorms data from 2009 to 2014, under prolonged (more than 4 h) intervals of northward and southward interplanetary magnetic fields (IMFs). H‐band EMIC waves show high occurrence in the dawn‐to‐postnoon sector (0400–1600 MLT) at L > ~7 for northward IMFs. On the other hand, for southward IMFs, H‐band EMIC waves occur in the morning‐to‐afternoon sector (0700–1500 MLT), and no strong wave activity is found in the dawn sector (0400–0700 MLT). H‐band EMIC waves are frequently observed in the region where the ambient temperature anisotropy for ~1–25 keV ions is high. He‐band EMIC waves show high occurrence in the noon‐to‐dusk sector (1100–2000 MLT) for both IMF BZ conditions. In the dusk sector, however, the high occurrence region is shifted into the inner L‐shell for southward IMFs (L