Statistical Study of Foreshock Density Holes

Abstract:

Density holes (DHs) are one type of the transient structures observed in Earth’s foreshock region. Inside the density hole, the density and magnetic field decrease below the ambient solar wind values. There are still many outstanding questions about density holes. E.g., What is their occurrence rate and its dependence on solar wind/IMF conditions? What are the similarities and differences between foreshock density holes and other foreshock transients such as HFAs? What is the generation mechanism of density holes? What role do waves play in the formation of DHs? By studying this structure, we could have a better understanding of Earth’s foreshock region. This statistical study is aiming at determination of the characteristics of foreshock density holes, their occurrence rate and its dependence on solar wind/IMF conditions. We use quantitative criteria to automatically identify ~250 foreshock density holes based on data from the Magnetospheric Multiscale (MMS) spacecraft from October 2017 to May 2019. The results show that the mean duration of the events is ~20 s and the mean electron density depletion is ~0.35. Most events have small magnetic shear angles. Density holes’ occurrence rate is higher under higher solar wind speed and larger magnetic shear angle. The electron temperature inside DHs could be 3 times higher than that in the ambient solar wind. Most foreshock density holes are observed in the quasiparallel shock region. Within 72% of the density holes, Te_para/Te_perp increases compared to the ambient solar wind.