Small‐Scale Magnetic Holes in the Solar Wind Observed by Parker Solar Probe

The small-scale magnetic hole (SSMH), characterized by magnetic field depression, is a structure with the size in the order of proton gyro-radius. SSMHs near the Earth or other planets have been widely observed in recent years. However, SSMHs in the solar wind near the Sun are rarely investigated due to mission constraints. In the present study, SSMHs in the pristine solar wind within a wide heliocentric distance range are analyzed based on the Parker Solar Probe (PSP) Mission measurements. A total of 2,416 SSMHs are successfully identified during the orbits of PSP from 2nd October, 2018, to 31st December, 2020, with an average occurrence rate of similar to 5.8 events/day. The occurrence rate of SSMHs decreases from similar to 29.5 to similar to 0.6 events/day as the heliocentric distance R increases. The spatial scale of these SSMHs obeys a bi-log-normal distribution, with the median scale L similar to 137 km (similar to 6 rho(p), proton gyro-radius). As interplanetary magnetic field B-ave increases or R decreases, the upper limit of the spatial scale L corresponding to each bin extends to a larger value. The L corresponding to the maximum occurrence rate also increases when B-ave increases and R decreases. Besides, the SSMHs tend to occur more frequently in the solar wind environment with weak B-ave and high thermal pressure P-t. Our results shed light on the characteristics and the origin of SSMHs in the pristine solar wind.