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 B_Z conditions. In the dusk sector, however, the high occurrence region is shifted into the inner L-shell for southward IMFs (L < 8) in contrast to that observed for northward IMFs (L < 10). He-band EMIC waves are shown to be generated in the region where the total plasma density is high (especially near the plasmapause). The occurrence rates of EMIC waves are lower overall for southward IMFs than for northward IMFs for both H-band and He-band waves. We suggest that an increase of heavy ion (i.e., He⁺ and O⁺) concentration is likely associated with low occurrence rates in both frequency bands for southward IMFs.