Magnetotail response to prolonged southward IMF Bz intervals: Loading, unloading, and continuous magnetospheric dissipation

Abstract

The response of the Earth's magnetotail to prolonged southward interplanetary magnetic field (IMF) has been determined for the three Geotail magnetotail seasons from November to April, 1999-2002. We examine the total magnetotail pressure PT,tail = B2/2μ0 + NikTi because variations should be similar in this parameter in the lobes and in the plasma sheet. We found 13 events when IMF Bz remained southward for 8 hours or longer and Geotail was located within the magnetotail farther than 10 RE downstream. All 13 events were subdivided into separate intervals characterized as (1) loading, if the tail total pressure increased more than 100%; (2) unloading, if the total pressure decreased by more than 50%; and (3) what we term here continuous magnetospheric dissipation (CMD), if the tail total pressure increased by less than 100% and/or decreased less than 50% during the entire mode interval. In total, 37 loading, 37 unloading, and 28 CMD events were found. The plasma sheet magnetic flux transfer rate, φEearth ≈ vx · Bz, and plasma bulk velocity has been analyzed to determine the steadiness of the plasma sheet convection. Plasma sheet convection was found to be highly disturbed and intense plasma flows (BBFs and FBs) were observed during all convection states. However, the occurence rate and amplitude of plasma flows distinguish loading-unloading and continuous dissipation periods from each other. BBFs seem to be more numerous (135) but weaker (about 500 km/s) during continuous dissipation intervals compared with BBFs existing during unloading mode (61 and 660 km/s). Finally, it was found that CMD-type convection is more likely when the mean southward IMF Bz > -5 nT, while loading-unloading is more likely when IMF Bz < -5 nT. Copyright 2005 by the American Geophysical Union.