Variations of Mid‐Magnetotail Magnetic Pressure During Substorms

Magnetic reconnection is commonly observed in the mid‐tail plasma sheet during substorms, the mechanism behind this spatial preference, however, remains unsolved. This study focuses on magnetic pressure evolution within this critical region during substorms. We demonstrate that during the substorm growth phase, both the mid‐tail lobes and plasma sheet undergo continuous compression. The observations can be interpreted as a result of magnetopause reshaping. During the growth phase, the dayside magnetopause is eroded by magnetic reconnection, whereas the magnetotail expands as the newly reconnected fluxes from dayside are progressively deposited into the lobes. The transition region between the flared and flat magnetopause thus shifts tailward. The local mid‐tail plasma sheet that was initially beneath the flat magnetopause is now under the flared magnetopause, bears additional solar wind dynamic pressure and is compressed further. The compressed current sheet thereby provides proper conditions for initiation of magnetic reconnection.