Magnetospheric balance of solar wind dynamic pressure

The magnetopause is the boundary established by pressure balance between the solar wind flow in the magnetosheath and the magnetosphere. Generally, this pressure balance is represented to be between the solar wind, the dynamic pressure, and the magnetic pressure of Earth's dipole field. The plasma actually in contact with the magnetosphere is the slowed, compressed, and heated solar wind downstream of the shock. The force exerted on the magnetosheath plasma is the J × B force produced by the Chapman‐Ferraro current that flows on the magnetopause. Under typical solar wind conditions of relatively high magnetosonic Mach number flow (>6), this simple picture is a reasonable description of the situation. However, under conditions of low solar wind magnetosonic Mach number flow (~2) the force on the solar wind plasma is not exerted at the magnetopause and must be exerted at the bow shock by currents that connect to the Region 1 currents. In this paper we present observations from two magnetopause crossings observed by the Time History of Events and Macroscale Interactions during Substorms spacecraft to compare and contrast the force balance with the solar wind for two situations with very different solar wind magnetosonic Mach numbers. Key Points Under low solar wind Mach number conditions, the Chapman‐Ferraro current does not exert a force on the solar wind The bow shock always exerts a force on the solar wind, and sometimes, this force is the dominant force in the system Plain Language Summary When the solar wind hits Earth's magnetic field, the magnetic field pushes back on the solar wind. Typically, this force is exerted at the boundary between Earth's magnetic field and the solar wind. However, when the supersonic solar wind has a low Mach number (about 2, as opposed to the typical value of >6), the place where the force is exerted on the solar wind is at the bow shock wave in front of Earth. This force is produced by an electric current that flows on the bow shock and which connects to currents that flow directly into Earth's ionosphere in the polar regions. This paper presents spacecraft observations documenting this unusual situation.