Storm Time EMIC Waves Observed by Swarm and Van Allen Probe Satellites

The temporal and spatial evolution of electromagnetic ion cyclotron (EMIC) waves during
the magnetic storm of 21–29 June 2015 was investigated using high-resolution magnetic field observations
from Swarm constellation in the ionosphere and Van Allen Probes in the magnetosphere. Magnetospheric
EMIC waves had a maximum occurrence frequency in the afternoon sector and shifted equatorward during
the expansion phase and poleward during the recovery phase. However, ionospheric waves in subauroral
regions occurred more frequently in the nighttime than during the day and exhibited less obvious
latitudinal movements. During the main phase, dayside EMIC waves occurred in both the ionosphere
and magnetosphere in response to the dramatic increase in the solar wind dynamic pressure. Waves were
absent in the magnetosphere and ionosphere around the minimum SYM-H. During the early recovery
phase, He⁺ band EMIC waves were observed in the ionosphere and magnetosphere. During the late
recovery phase, H⁺ band EMIC waves emerged in response to enhanced earthward convection during
substorms in the premidnight sector. The occurrence of EMIC waves in the noon sector was affected by
the intensity of substorm activity. Both ionospheric wave frequency and power were higher in the summer
hemisphere than in the winter hemisphere. Waves were confined to an MLT interval of less than 5 hr with a
duration of less than 186 min from coordinated observations. The results could provide additional insights
into the spatial characteristics and propagation features of EMIC waves during storm periods