Pressure balance at the dayside equatorial magnetopause
Abstract
The magnetic pressure inbound the magnetopause would be balanced with a sum of thermal plasma and magnetic pressures on the magnetosheath side because the magnetospheric plasma pressure is negligible. However, the THEMIS spacecraft observed a number of magnetopause crossings characterized by the magnetosheath magnetic field larger than that in the magnetosphere, thus the pressure inside the boundary layer seems to be uncompensated. A formation of the boundary layers adjacent to the dayside magnetopause is strongly influenced by a direction of the interplanetary magnetic field (IMF) in general and by the sign of its vertical (Bz) component in particular. The northward pointing IMF leads to a creation of the plasma depletion layer or magnetic barrier characterized by the enhanced magnetic field and depressed plasma density just in front of the magnetopause. Reconnection of interplanetary and lobe field lines behind the cusps results in a thick outer part of the low-latitude boundary layer inbound the dayside magnetopause that is occupied by a magnetosheath-like plasma. These features are generally absent during intervals of southward IMF due to effectiveness of dayside reconnection. A presence of an enhanced magnetic field magnitude at the magnetopause under the southward IMF was also reported but it was treated as a remnant of the previous northward IMF interval. However, THEMIS observations reveal that a region of the enhanced magnetic field (larger magnetic field on the magnetosheath than on magnetospheric side of the magnetopause) can be created even under strong southward IMF and that such structure of the boundary layer can persist for several hours. We present case studies as well as a small statistics showing that such a structure of magnetopause layers is predominantly connected with an enhanced geomagnetic activity.
- Publication:
-
43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E1132G