State studies of Earth's plasmasphere

As an important part of the inner magnetosphere, the Earth's plasmasphere plays a vital role in linking the occurrence and development of space weather processes. The Earth's plasmasphere is a torus-shaped cold (< 10 eV) and dense (10~10⁴ cm⁻³) plasma region of ionospheric origin co-rotating with the planet. The plasmasphere contains several populations of particles such as electrons, H⁺, He⁺, and O⁺ ions. The outer boundary of the plasmasphere is defined by a sharp gradient of density called the plasmapause, in which the density decreases by 1~ 2 orders of magnitude within 0.5 R_E (the Earth radius). Additionally, large scale plasmaspheric features have been observed, including shoulders, plumes, notches, bulges, and refiling. The Earth's plasmasphere is dynamic, and the abundance of particles in it changes substantially with interplanetary and geomagnetic activity. The large-scale structure evolution of the plasmasphere during geomagnetic storms controls the generation and propagation of waves in the plasmasphere, thus affecting the wave-particles interaction, resulting in a change in the spatial distribution of electrons and ions in the plasmasphere, and then affecting other magnetospheric and ionospheric processes. During periods of geomagnetic storms, the plasmaspheric material eroded is transported sunward and observed near the dayside magnetopause regularly. This local plasmaspheric density enhancement greatly impacts global large-scale convection. Therefore, the plasmaspheric density is undoubtedly an important parameter in space weather. The structural dynamic change of the Earth's plasmasphere is an indicator of the disturbance state of the space weather environment. Its structural form and dynamic process are controlled by the geomagnetic activity, and the short-term changes in the geomagnetic field originate from the sun-terrestrial disturbance caused by solar activity. The high-speed plasma ejected from the solar corona impacts the Earth and induces large-scale convective motion in the magnetosphere. Part of the energy enters the magnetosphere, causing disturbances in its inner regions, including the plasmasphere. Further research on the Earth’s plasmasphere is of great significance to reveal the mass transport and energy transfer in the solar wind-magnetosphere-ionosphere coupling process, and space weather forecast. Here, we introduce the research progress in the response of the plasmasphere to geomagnetic activities, the waves in the plasmasphere, variation of the electron content in the topside ionosphere and plasmasphere, and plasmasphere models. Finally, we also list some important issues for future studies.