Local Time Differences in the Ionospheric Electromagnetic Ion Cyclotron Waves during Storm Time

Abstract

By using high-resolution (50 Hz) magnetic field observations from Swarm constellation, the temporal and spatial characteristics of Electromagnetic Ion Cyclotron (EMIC) waves in different local time sectors at mid-latitudes are investigated during the magnetic storm period of 16—25 March 2015. There are comparable number of EMIC wave events in the dawn and dusk sectors, while there are more EMIC wave events in the pre-noon sector than in the pre-midnight sector. The duskside preference is related to the plasmaspheric plume, while the dawnside preference is related to the enhanced solar wind dynamic pressure and dense cold plasma. The wave frequency in the dawn and pre-noon sectors is higher than that in the dusk and pre-midnight sectors, implying the local time difference in the source location and the ion composition. Most of the duskside events occur in the early recovery phase, while the dawnside events occur in the late recovery phase. The time difference comes from the time required for the westward drift of energetic ions and local time difference in the plasmapause position. H⁺ band waves are mainly found in the 06:00 MLT—10:00 MLT sector, while He⁺ band waves are mainly confined in the 18:00 MLT—22:00 MLT sector. There is no H⁺ band but a two band (He⁺ and O⁺) EMIC wave during the storm main phase, indicating the role of the dense oxygen ion in inhibiting the H⁺ band in the inner magnetosphere.