Storm‐Time Magnetopause: Pressure Balance

• Published in: Journal of geophysical research. Space physics 2022-11, Vol.127 (11), p.n/a
• Main Authors: Grygorov, K., Němeček, Z., Šafránková, J., Šimůnek, J., Gutynska, O.
• Format: Article
• Language: English
• Subjects: Analyzers; boundary layer; Cold; Cold plasmas; Cold storage; Density; Geomagnetic field; Geomagnetic storms; Geomagnetism; Interplanetary magnetic field; Magnetic fields; Magnetic flux; Magnetic storms; Magnetism; Magnetopause; Magnetosheath; Magnetospheres; Plasma; Plasma pressure; Plasmasphere; pressure balance; Solar wind; Spectrometers; Statistical analysis
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2022JA030803

The magnetopause is treated as a boundary where the pressure of the incoming solar wind is balanced by the pressure of the geomagnetic field and the plasma pressure inside the magnetopause is often neglected. However, published studies of pressure balance at the magnetopause reveal an excess of the magnetosheath pressure. Moreover, our survey of about 50,000 THEMIS magnetopause crossings shows that about 1% of them exhibits even larger magnetic field in the magnetosheath than in the magnetosphere. A subsequent analysis of crossings observed in the subsolar region under a southward interplanetary magnetic field shows the pressure of the dense cold ion population as an important component of the total magnetospheric pressure. This component is too cold to be registered by standard ion analyzers but its density can reach 300 cm−3. The second effect connected with a presence of cold plasma population is a reduction of the reconnection rate that slows down the transport of the magnetic flux down the tail and leads to magnetic pile‐up in the subsolar magnetosheath. Plain Language Summary The magnetopause is usually treated as a boundary where the pressure of the incoming solar wind is balanced by the pressure of the geomagnetic field and the plasma pressure inside the magnetopause is often neglected. However, we have found a large number of subsolar magnetopause crossings exhibiting larger magnetic field in the magnetosheath than in the adjacent magnetospheric region. Taking into account the contribution of the magnetosheath plasma, the pressure balance at the magnetopause seems to be violated. Statistical analysis revealed that these events are observed predominantly during geomagnetic storms when the cold plasma from the plasmasphere comes toward the magnetopause. This plasma is too cold to be registered by the ion spectrometers but its density is sufficient to keep the pressure balance across the magnetopause as we demonstrate in two case studies. Our analysis explains contradiction that can be found in several already published papers pointing out a lack of the magnetospheric pressure at the magnetopause. Key Points About 1% of subsolar magnetopause crossing exhibits larger magnetic field in the magnetosheath than in the magnetosphere Such magnetopause configuration is typical for periods of geomagnetic storms The cold dense plasma of plasmaspheric origin was identified as a component keeping the pressure balance across the magnetopause