Temporal-spatial Distribution And Propagation Characteristics Of Ionospheric Electromagnetic Ion Cyclotron Waves

Electromagnetic ion cyclotron waves are closely related to the earth's space environment,playing an important role in magnetospheric dynamics.Based on high-resolution magnetic field observations and plasma density data,the temporal-spatial distribution and propagation characteristics of ionospheric EMIC waves are studied systematically and the corresponding physical mechanisms are discussed.The main conclusions are as follow:1.Using high-resolution magnetic field observations from Swarm satellites,the temporal-spatial characteristics of EMIC waves in different local time sectors at mid-latitudes are investigated during the magnetic storm period of March 17-25,2015.The research includes the distribution of EMIC waves with magnetic local time and invariant latitude,the distribution of EMIC waves with magnetic storm phase and the distribution characteristics of EMIC waves in different bands.The main results are as follow:a)There are comparable number of EMIC wave events in the dawn and dusk sectors.The duskside events is related to the plasmaspheric plumes and the energetic particles injected during substorm,while the dawnside events is related to the enhanced solar wind dynamic pressure and the dense cold plasma.b)Most EMIC waves occur in the recovery phase of the magnetic storm.The majority of duskside waves occurs in the early recovery phase,while the majority of dawnside waves occurs in the late recovery phase.The dawn-dusk time difference comes from the time required for the westward drift from duskside to dawnside of energetic ions injected from nightside and the plasmaspheric plumes in duskside.c)The frequency of EMIC waves in the dawn and pre-noon sectors is higher than that in the dusk and pre-midnight sectors,indicating that there are local time differences in the response of plasma and magnetic field environment to solar wind dynamic pressure,the position of plasmapause and the composition of cold-heavy particles in source region.d)H⁺band and He⁺band waves are dominate during the magnetic storm.H⁺ waves are mainly distributed in the dusk and pre-noon sectors,which are related to magnetospheric compression.He⁺band waves are mainly confined in the dawn and pre-midnight sectors,which are related to the plasmaspheric plumes.e)There is no H⁺band but a two-band(He⁺and O⁺)EMIC wave during the storm main phase,indicating that the dense oxygen ions can inhibit the generation and propagation to the ionosphere of H⁺band waves in the inner magnetosphere.2.The magnetic local time and latitude(MLT and MLat)distributions of ionospheric large-spatial-scale(>20°MLat)EMIC waves are investigated statistically using high-resolution 50 Hz geomagnetic field data and 16 Hz plasma density data from 2015 to 2018 of Swarm A and B satellites.Both parallel and transverse waves are studied in a comparative manner for different geomagnetic activities and seasons.The statistical research on correlation between parallel waves and electron density oscillation are conducted for the first time.The new conclusions are as follow:a)Frequent occurrences of large-spatial-scale waves in the South Atlantic Anomaly and North America,where the parallel waves propagate over the longest distance,are observed.In these two regions,drift shell spitting is conducive to the generation of EMIC waves,while the weak magnetic field intensity and high electron density are conducive to the propagation of waves in ionospheric waveguides.There are similar local time and longitudinal variation trend between the electron density of F region and ionospheric large-spatial-scale EMIC waves,indicating that high electron density in the waveguides is conducive to the long-distance propagation of EMIC waves.b)Waves appear mostly in the 02–10 MLT sectors.Parallel waves propagate the farthest in latitudinal range in pre-noon sectors while transverse waves propagate the farthest in post-midnight sectors.With the enhancement of substorm,both transverse and parallel waves increase in occurrence.The dayside occurrence rate is higher during weak substorm,whereas the situation is reversed on the nightside.c)The dayside waves are located outside the mid-latitude trough,while the nightside waves are located near(inside)the equatorward boundary of the mid-latitude trough.d)Large-spatial-scale waves tend to occur at the equinoxes when the absolute value of the dipole tilt angle is less than 10°,while the long-distance propagation in the waveguide occurs in the pre-noon sectors in summer.e)Parallel waves occur mostly when the electron density is about 10^5.5cm^-3, while transverse waves occur mostly when the electron density is about 10~5 cm^-3,indicating that in the region with high electron density,EMIC waves tent to propagate in parallel form.Both parallel waves and electron density oscillation have two main peaks at 03-06 MLT and 09-12 MLT,and a secondary peak at 18-21 MLT.The high correlation between the two curves shows that there is a close relationship between EMIC waves and electron density oscillation.