Interpretation of the Northward B sub Z (NBZ) Birkeland Current System and Polar Cap Convection Patterns in Terms of the Impulsive Penetration Model

摘要

Impulsive penetration theory is used to show that in the Northern Hemisphere, the region of preferred penetration of solar wind plasmoids into the magnetotail is shifted toward dusk for a positive By, and toward dawn for By less than 0. The direction of these shifts is reversed in the Southern Hemisphere. The polar cap convection pattern associated with impulsive penetration of solar wind plasma irregularities is therefore also shifted toward dusk or dawn, according to the value of By. The northward Bz (NBZ) Birkeland currents driven at the interfaces between the penetrating plasmoids and the ambiant geomagnetic field are downward (upwards) in the dusk (dawn) side for both hemispheres. The amplitudes of NBZ currents are expected to be larger when Bz is larger (and positive). The region of reversal of the NBZ currents is also shifted toward dawn or dusk depending on the sign of By. These transient field aligned currents, driven upwards and downwards, are associated with the nonzero field aligned component of curl B sub bar (i.e., with magnetic shears) in the vicinity of the penetrating diamagnetic plasmoids.