Time constants of Earth's magnetotail for responding to a southward excursion of IMF: The super‐high‐pressure magnetotail event of 10 December 1996

Abstract

Geomagnetic and plasma conditions in the nightside magnetosphere and the northern polar region are examined in detail for the first 6 hours of 10 December 1996. In association with a southward turning of the interplanetary magnetic field, the polar cap potential drop began to increase and maximized within several minutes after the peak of the energy inflow from the solar wind into the magnetosphere. The total pressure observed by Geotail in the midmagnetotail, however, continued increasing for ∼50 min after the peak of the polar cap potential drop, resulting in an exceptionally high total pressure in the midmagnetotail. Auroral images from Polar and magnetograms from ground stations show that there was no major breakup of substorms during the period concerned. This unique event provides us with an opportunity to study the solar wind–magnetosphere coupling without any intense disturbances associated with major substorms. Using an equivalent circuit model, it is attempted to reproduce variations in the polar cap potential drop and the midtail total pressure. The time lag between them is particularly focused on in this study. It is found that the characteristic time constants of the equivalent circuit model for this event are significantly longer than those estimated in previous papers.