Statistical Study of Foreshock Transients and their Substructures Observed by MMS

Foreshock transients such as hot flow anomalies (HFAs), foreshock bubbles (FBs), and spontaneous hot flow anomalies (SHFAs) display heated, tenuous cores and large flow deflections bounded by compressional boundaries. THEMIS and Cluster observations show that within some cores were density enhancements that can be studied to better understand the formation and evolution processes of foreshock transients. However, closer examinations of these substructures were not feasible until the availability of the high-resolution instruments onboard the Magnetospheric Multiscale mission (MMS). We identify 116 foreshock transients (including HFAs, SHFAs, and FBs) from two MMS dayside phases to investigate their formation conditions, sizes, and substructure properties. The occurrence rate of foreshock transients is higher for higher solar wind speeds and Mach numbers, lower magnetic field strengths, more radial IMF, larger magnetic shear angles, quasi-parallel bow shock geometries, and inward-directed convectional electric fields. Foreshock transients usually span up to 3 R_E along the bow shock and extend up to 6 R_E from the bow shock. Foreshock transients with and without substructures exhibit similar formation conditions and sizes. Furthermore, substructure densities display a positive correlation with the plasma flows, no correlation with the magnetic field strengths, and a negative correlation with the electron temperature. Substructure sizes are less than half an ion inertial length most of the time; however, some are several ion inertial lengths indicating multiple formation mechanisms for substructures. Substructures within foreshock transients could be due to an intrusion of the solar wind plasma or shocked plasma from the magnetosheath, an intermediate stage of heating between ion populations, or a boundary of two connected foreshock transients.