Abnormal magnetospheric magnetic gradient direction reverse around the indented magnetopause

Time History of Events and Macroscale Interactions during Substorms (THEMIS) data are used to investigate the magnetic field structures in the vicinity of the magnetopause. Generally, the tendency that the farther away from the Earth, the weaker the detected magnetic field is expected inside the dayside magnetopause. Here we show two cases which conflict with the expectation that the magnetic field gradient direction reverses from inward to outward in a short time interval. After a further analysis, it is found that the THEMIS probes encountered a magnetopause indentation moving along the magnetopause towards the dawn in one case, and for the other case, they crossed an evolutive indented magnetopause that was produced locally and then recovered to its normal state. These two magnetopause indentation may be related with the fast magnetosheath flow. Accordingly, we suppose that the fast magnetic gradient direction reverse is caused by the abnormal magnetic field distribution adjacent to the deformed magnetopause.