Plasmasphere Control of ULF Wave Distribution at Different Geomagnetic Conditions

Magnetic storms and substorms cause global disturbances in the Earth's magnetosphere. Plasma clouds injected from the magnetotail during storm or substorm drift around the Earth and generate ultra-low frequency (ULF) waves via various mechanisms. At the same time, the inner part of the magnetosphere called plasmasphere is filled with cold particles and its characteristics are sensitive to the geomagnetic activity level. Previous theoretical and some observational studies suggested plasmasphere and its boundary, plasmapause, are special regions for ULF waves to interact with charged particles. We present a statistical analysis of ULF waves during different geomagnetic conditions. We utilized Arase satellite magnetic field and electron density measurements from March 2017 to December 2020 to investigate spatial distribution of ULF waves and its dependence on the plasmapause location. A 1-2 RE gap between the plasmapause and a region of high transverse waves occurrence rate was found. This gap keeps during quiet geomagnetic conditions when plasmasphere expands, and we concluded that the plasmapause controls the ULF wave distribution in the magnetosphere. ULF wave occurrence rate significantly decreases at quiet time, but dayside and dawnside maxima still occur for poloidal and compressional, and toroidal waves, respectively. Thus, we can distinguish internally and externally excited waves. Average wave frequency distribution revealed field-line resonance character of toroidal waves as frequency increases toward the Earth. Poloidal and compressional waves distributions clearly distinguish low frequency externally excited waves and high frequency storm-time pulsations. Plasma environment and magnetic field inside the magnetosphere are very dynamic and strongly depend on geomagnetic activity level. Magnetic storms and substorms affect all the processes and systems, including the plasmasphere formation and ultra-low frequency (ULF) wave generation. The plasmasphere is a dense inner part of the magnetosphere filled with cold protons and electrons, while ULF waves are responsible for energy transfer on a large distance and intensively interact with charged particles. We used 46 months statistics of Arase satellite measurements and found a clear dependence between ULF waves observations and the plasmasphere boundary location. ULF waves are mostly observed in 1-2 Earth radii from the plasmasphere boundary even when it moves outward during long period of quiet geomagnetic conditions. Although ULF waves rare occur at quiet geomagnetic conditions, they still appear at the dayside and dawnside magnetosphere. We think, these waves are excited by external sources from the solar wind or magnetosphere boundary-magnetopause. This conclusion is supported by wave frequency analysis showing a clear difference between lower frequency dayside waves and higher frequency nightside waves, which are associated with external and internal sources, respectively. Thus, we found distinct signatures of plasmasphere control of ULF wave spatial distribution. Poloidal and toroidal waves are detected mostly outside the plasmasphere with 1-2 Earth radii from itPlasmasphere expansion and shrinkage impact the ultra-low frequency wave distribution at the dusk and night sectors of the magnetosphereExternally excited dayside waves usually have lower frequencies than nightside waves excited by internal instabilities