Magnetic activity in the polar caps: Relation to sudden changes in the solar wind dynamic pressure

Abstract

Response of the polar cap (PC) magnetic activity to sudden changes in solar wind dynamic pressure is analyzed with use of the northern and southern polar cap indices for 1998–2002 and corresponding solar wind parameters: geoeffective interplanetary electric field (Em) and the solar wind dynamic pressure (PSW), reduced to the magnetopause. Only sudden pressure pulses starting against the background of steady quiet pressure levels were taken for the analysis. To separate effects produced by Em and PSW, we examined the behavior of the average characteristics PSW, Em, and PC under the different restrictions imposed in turn on (1) electric field Em, (2) magnitude of the pressure PSW after the jump, and (3) rate of the pressure increase (dPSW/dt), the other two quantities being kept invariant in so doing. The following results have been obtained. The electric field Em determines behavior of the PC index, the value of which is directly influenced by the Em increase with a delay time of about 15–30 min. The solar wind pressure growth rate (i.e., jump power dPSW/dt) appears to be the second most important factor for the PC index increase, the pressure gradient dPSW = 1 nPa being approximately equivalent to the action of dEm = 0.33 mV/m, with a delay time shortened up to a few minutes. The growth rate of the pressure gradient seems to be the sole factor for the PC index increase under conditions of the northward interplanetary magnetic field BZ and for the PC index decrease after the dynamic pressure negative jump.