Abstract
Magnetic pressure inside the magnetopause is usually balanced with a sum of thermal plasma and magnetic pressures on the magnetosheath side. However, observations reveal that the magnetosheath magnetic field can be frequently larger than that in the magnetosphere (inverse magnetic field gradient across the magnetopause), and thus, the enhanced pressure from the magnetosheath side seems to be uncompensated. Such events are rare in the subsolar region, but their occurrence rate increases toward flanks. The analysis, based on statistical processing of about 35,000 THEMIS magnetopause crossings collected in the course of the years 2007–2017, shows that these events are more frequently observed under enhanced geomagnetic activity that is connected with a strong southward IMF. Case studies reveal that such a state of the magnetopause boundary layers can persist for several hours. This study discusses conditions and mechanisms keeping the pressure balance across the magnetopause under these conditions.
Highlights
The magnetopause is a current sheet forming the boundary between the magnetic pressure of theEarth’s dipole on the one side and the shocked supersonic solar wind with an embedded interplanetary magnetic field (IMF) on the other side
We present a statistical study of the magnetic field gradient across the magnetopause with an emphasis on the flank magnetopause and events when the magnetospheric magnetic field was lower than that in the adjacent magnetosheath
Based on the above discussion, we can believe that we understand the formation of magnetopause layers that can lead to an inverse magnetic gradient across the magnetopause for southward IMF
Summary
The magnetopause is a current sheet forming the boundary between the magnetic pressure of the. Lukin et al (2020) compared the characteristics of magnetic field and plasma populations during simultaneous magnetopause crossings, which are separated by about 50 RE (dayside vs night sides), and found that the magnetosheath current sheet profiles are similar at these two locations. They have suggested that this dense plasma can load the near-Earth plasma sheet and get driven into the inner magnetosphere when the IMF turns southward, creating a strong ring current This would imply that at least at the beginning of a storm, the ring current is formed by solar wind plasma (Kistler, 2020). In order to account for the uncertainty of the identification of the exact time of magnetopause crossings by our automated routine, we have skipped 1 min on both sides of a particular crossing
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