When the interplanetary magnetic field (IMF) clock angle θC ≡ atan2(BY, BZ) lies in the range 0° < |θC| < 30°, there only persist IMF-lobe and lobe-closed reconnection (both of which we call interchange-type), with Dungey-type reconnection (IMF-closed and lobe-lobe) being absent. Reconnection for IMF BY > 0 (BY < 0) occurs in the northern dusk (dawn) quadrant and in the southern dawn (dusk) quadrant. The IMF-lobe reconnection in one quadrant is coupled to the lobe-closed reconnection in the other quadrant by magnetic flux reciprocation, and in the ionosphere, that reconnection pair produces an "interchange-type merging cell" in the hemisphere of IMF-lobe reconnection and a reciprocal cell in the other hemisphere. When the polar cap is completely divided into two parts by the transpolar plasma sheet (the theta aurora configuration), the interchange-type merging cell transports open magnetic flux from one part of the polar cap to the other without affecting the theta aurora configuration, causing the dawn-dusk motion of the transpolar plasma sheet. This type of flux transport is destroyed if any Dungey-type reconnection occurs. Thus a necessary condition for the theta aurora configuration to be maintained during the dawn-dusk travel of the transpolar plasma sheet is the absence of Dungey-type reconnection. This condition is reasonably satisfied for 0° < |θC| < 30°. The absence of Dungey-type reconnection in the presence of IMF B Y can be understood as a transitional state of the magnetic flux circulation when the IMF rotates from due northward to dawnward or duskward.
All Science Journal Classification (ASJC) codes
- Space and Planetary Science