Statistical study of foreshock Density Holes

Density holes (DHs), characterized by the correlating depletion of plasma density and magnetic field strength, are one of the transient structures frequently observed upstream of Earth's bow shock. The low dynamic pressure of DHs may disturb the bow shock and magnetopause, and have potential geoeffects. In this paper, we perform a statistical study of 411 density hole events, identified by the Magnetospheric Multiscale Mission. The average occurrence rate of DHs is similar to 5.4 events/day. All DHs are associated with foreshock ions. DHs tend to occur on the dawn side than on the dusk side. The occurrence rate is higher for faster solar wind, larger magnetic shear angle across DHs, and lower magnetic field strength. The spatial scale of DHs is several R-E. Within DHs, electron heating and density depletion are correlated. Most DHs do not have significant flow deflections. Most DHs with discontinuities have the convection electric field pointing toward the discontinuity on at least one side. About two thirds of the 411 events show different characteristics compared to other types of foreshock transients that also have correlated density and field strength depletion.