Foreshock density holes and their connection with other foreshock transients

Abstract

Foreshock is the region upstream of the bow shock that is magnetically connected to it. In this region the particles reflected from the bow shock interact with the incoming solar wind plasma, making this place full of foreshock transients. The well-known transients, such as hot flow anomalies (HFAs), spontaneous hot flow anomalies (SHFAs), foreshock bubbles (FBs), foreshock cavities, foreshock cavitons and foreshock compressional boundaries (FCBs), are all characterized by low-density and hot cores with one or two compressional edges. Due to the low dynamic pressure in the core region, foreshock transients can disturb the bow shock, magnetosheath and magnetopause, and may have potential geoeffects. Foreshock density holes (DHs) as members in this transients' family are rarely studied since they were first observed in 2006. This dissertation presents an observational case and statistical study of characteristics and occurrence preferences of foreshock density holes via Magnetospheric Multiscale (MMS) spacecraft. Density holes are automatically selected using a computer program based on quantitative criteria. The statistical study shows that density holes are quite common in the foreshock, with an average occurrence rate of ~ 5.4 events/day. There are more DHs for higher magnetic shear angle, fast solar wind speed, lower plasma density and weaker magnetic field strength. In addition, selected density holes are categorized into other types of foreshock transients if they show the same features. Nearly two thirds of events in our list can only be identified as DHs, indicating it is a unique transient type. Furthermore, this dissertation investigates a new formation mechanism of HFAs through the interaction between low-density flux tubes and Earth's bow shock. The process is illustrated by a two-dimensional magnetohydrodynamics (MHD) model without any discontinuity prerequisites. Low density inside the flux tubes from the solar wind deform the bow shock to balance pressures, generating HFA-like structures bounded by two compressional (shock) boundaries. HFAs observed by MMS and Time History of Events and Macroscale Interactions During Substorms (THEMIS) satellites with low-density and high-density (compared to ambient solar wind density) cores are both reproduced. A double-core HFA at the extreme flank region is also analyzed. The observations and MHD simulations are in good agreement. The total ram pressure plays an essential role as an indicator of HFAs deforming the bow shock.