Magnetic Field Gradient Across the Flank Magnetopause

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.

Storm‐Time Magnetopause: Pressure Balance

“…By contrast to flank events investigated in Němeček et al. (2021), this study reveals a principal role of the low‐energy plasma of a plasmaspheric origin in the total pressure at the magnetospheric side of the MP.…”

“…These features would be absent during intervals of southward IMF due to the effective process of dayside reconnection (Němeček et al, 2015) but the reconnection rate is decreased by a presence of the plasmaspheric plasma in such events. By contrast to flank events investigated in Němeček et al (2021), this study reveals a principal role of the low-energy plasma of a plasmaspheric origin in the total pressure at the magnetospheric side of the MP.…”

“…Figure 6g shows that the ESA energetic range is not sufficient and a part of this magnetospheric population has energies above its range. It could result in underestimation of the density and temperature of the hot population in onboard moments and to underestimation of the plasma pressure computed from these moments (Němeček et al., 2021).…”

“…We call such events as the inverse magnetic field gradient across the MP. A similar way was applied on the magnetopause flanks by Němeček et al (2021) and the authors have shown that the pressure balance through the MP in events exhibiting inverse magnetic gradient is supported by the enhanced density and temperature of energetic ions on the magnetospheric side of the MP. A majority of these ions are out of the measuring range of the THEMIS ion spectrometer and thus the moments calculated from the measured distributions are underestimated.…”

On the Role of Kinetic Alfven Waves in the Magnetosheath Ion Thermalization Around the Night‐Side Magnetopause

“…Panel 1(c) shows that the sum of ion kinetic energy (energy of ion flows) and thermal energy (ion temperature) is almost constant across the magnetopause, that is, fast magnetosheath plasma should be thermalized (with the kinetic energy transferred to the thermal energy) during their crossing the magnetopause and penetrating from the magnetosheath to the magnetosphere. Note that this energy conservation does not directly relate to the pressure balance, because (a) a significant portion of the pressure balance is provided by the magnetic field (H. Hasegawa, 2012; Němeček et al., 2021), (b) plasma dynamical pressure does not really contribute to the pressure balance at the flank magnetopause (Artemyev, Angelopoulos, Runov, et al., 2017; H. Hasegawa, 2012).…”