Statistical study of magnetic holes in the upstream region of Mercury’s bow shock

Magnetic holes are magnetic depression structures that exist widely in many plasma environments. The magnetic holes with durations of >1 s in the solar wind at Mercury’s orbit have drawn much attention, but the properties of the magnetic holes with shorter durations are still unclear. Here, we investigate the magnetic holes with durations of 0.1–100 s in the upstream region of Mercury’s bow shock based on observations by the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft. They can be divided into two groups according to the distribution of their duration: small-duration magnetic holes (SDMHs, <0.6 s) and large-duration magnetic holes (LDMHs, >0.6 s). The duration of each group approximately obeys a log-normal distribution with a median of ~0.25 s and 3 s, respectively. Approximately 1.7% (32.6%) of the SDMHs (LDMHs) reduce the magnetic field strength by more than 50%. For both groups, some structures have a linear or quasi-linear polarization, whereas others have an elliptical polarization. The magnetic hole events in both groups tend to have a higher rate of occurrence when the interplanetary magnetic field strength is weaker. Their occurrence rates are also affected by Mercury’s foreshock, which can increase (decrease) the occurrence rate of the SDMHs (LDMHs). This finding suggests that Mercury’s foreshock might be one source of the SDMHs and that the foreshock can destroy some LDMHs. These observations suggest that a new group of magnetic holes with durations of <0.6 s exist in the upstream region of Mercury’s bow shock.