A statistical study of field-aligned currents in the Earth's inner magnetosphere.

摘要

Field-aligned currents (FACs) play a major role in the solar wind-magnetospheric interaction by electrodynamically coupling magnetospheric and ionospheric plasmas, so that stresses applied to the outer magnetospheric plasma are transmitted to the ionosphere and ultimately to the upper atmosphere. FACs have been studied extensively since 1967 and all of our knowledge and understanding of this coupling mechanism is based on data from low altitude polar orbiting satellites. The purpose of this dissertation is to address some of the questions related to the persistence of magnetospheric FACs during low levels of geomagnetic activity, their motion throughout the magnetosphere and their possible source mechanisms. We detrend and rotate ISEE-1 and 2 magnetometer data into a field-aligned coordinate system using the Tsyganenko (1987) magnetic field model in order to identify current sheet crossings. We calculate the properties of the FACs to include their current intensity, density, velocity and thickness and discuss the technique used to extrapolate those properties to the ionosphere. We also determine the location and orientation of the FACs as well as a measure of how closely they resemble sheets of current. We identify two types of FACs in the magnetosphere; one that is quasi-stationary and another that is moving quickly so that it is observed simultaneously by the spacecraft. The quasi-stationary FACs are magnetospheric extensions of the low altitude region 1 and 2 currents while the fast moving FACs are possibly current carrying waves and/or transient phenomena. Both currents are highly responsive to geomagnetic activity with the fast moving currents responding strongly to high activity. During periods of quiet activity, magnetospheric FACs are infrequently observed except for a region on the dusk night side from mid to high L-values suggesting that global convection does drive the dusk FACs but that the three dimensional current system is non existent on the dawn side. We also examine the night side quasi-stationary FACs with respect to substorm phase and find that when classified as a region 1 or 2 sense current, the night side region 2 currents are responsive to internal magnetospheric stresses due to night side reconnection whereas the night side region 1 currents are responsive to variations in the stress associated with both day side and night side reconnection.