[PEM19-07] Statistical characteristics and forming mechanisms of trunk structure of ring current ions: Arase ion observations
Keywords:ring current, Arase satellite, structure on E-t diagram, Trunk structure, simulation, Electric field
We analyze geomagnetic activity, local time, latitude and L-value dependences of the trunk. The minimum L-shell point of the trunk is distributed mostly around L = 2.0 – 2.5 off the equator, extending from dusk to pre-midnight. Geomagnetic storm and substorm dependences are similar between the event and the non-event periods. We also perform particle backward tracing simulations using a dipole magnetic field and a SAPS electric field model in addition to some standard empirical models of large-scale convection electric field. As a result, we show that the trunk structure can be formed when the Alfven layer is deformed due to a local and temporally enhanced electric field.
The previous study suggested that an impulsively enhanced electric field or a temporary gap of injection from the tail region combined with the charge exchange process could cause the formation of the trunk. However, our result suggests a different generation mechanism: Temporary and local electric field enhancements causes the transportation of ions to low L-shell region and the formation of trunk structure.
Beside the trunk structures, “inverse trunk” in which the energy of peak flux increases in the lower L-shell are also found from the Arase observations. We show statistical characteristics of trunk and inverse trunk structures on the basis of the Arase observations and the forming mechanism of Trunk shown by simulations.