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 ∼5.8 events/day. The occurrence rate of SSMHs decreases from ∼29.5 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 ∼ 137 km (∼6 ρp, proton gyro‐radius). As interplanetary magnetic field Bave 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 Bave increases and R decreases. Besides, the SSMHs tend to occur more frequently in the solar wind environment with weak Bave and high thermal pressure Pt. Our results shed light on the characteristics and the origin of SSMHs in the pristine solar wind. Plain Language Summary: Small‐Scale Magnetic Hole (SSMH) is a common structure in the universe, characterized by the magnetic field depression in a short time period, with the size in the order of proton gyro‐radius. The possible source region of SSMH has been debated a lot but cannot be researched carefully due to the limitation of the satellite mission in the solar wind. Using the in‐situ data from the Parker Solar Probe mission, which covers the region near the Sun previously unexplored, this statistical study focuses on SSMHs in the pristine solar wind within a wide range of heliocentric distances. The results show that the occurrence rate decreases with the increasing distance. SSMHs prefer to occur in a weak magnetic field with high thermal pressure in the solar wind. The maximum size of SSMHs increases as the magnetic field increases and the heliocentric distance decreases. The results on the properties of SSMHs and their occurrence rate will shed light on the origin of SSMHs in the pristine solar wind. Key Points: The occurrence rate of SSMHs decreases as moving away from the Sun, from ∼29.5 events/day to ∼0.6 events/dayUpper limit of L extends a little to larger value when Bave increases and R decreases, as well as L corresponding to the max occurrence rateSSMHs prefer to occur in the solar wind environment with a weak interplanetary magnetic field and high thermal pressure [ABSTRACT FROM AUTHOR]