Statistical Properties of Sub-Ion Magnetic Holes in the Solar Wind at 1 AU

Sub-ion magnetic holes are rich in the terrestrial plasma sheet and magnetosheath. Here, we statistically investigate 60 sub-ion magnetic holes in the solar wind at 1 AU using the high-resolution data measured by the Magnetospheric Multiscale mission. We find that they are observed with a duration of 0.1 -0.5 s, and the lengths of their cross-section are similar to 0.1 -0.6 ion gyroradius. These structures prefer to occur in the slow solar wind with a weak ambient magnetic field strength. They also prefer to occur in the marginally mirror stable or unstable environments. Electron vortices as well as an enhancement of the electron perpendicular temperature and electron fluxes at similar to 90 degrees pitch angle tend to be observed inside some magnetic holes with a large ambient magnetic field strength. By contrast, there are no clearly observational electron vortices as well as the electron fluxes at similar to 90 degrees pitch angle inside some magnetic holes with a weak ambient magnetic strength. The current density with a value of similar to 10 -50 nA/m(2) reveals that the corresponding maximum electron velocity is <10 km/s inside some magnetic holes, lower than the level of the observational electron velocity noise, which prevents the detection of the weak electron vortex. We suggest that electron vortices exist inside all the sub-ion magnetic holes in the solar wind. The generation of these sub-ion magnetic holes can be explained by the electron magnetohydrodynamics soliton and the electron vortex magnetic hole.