Statistical Study of Foreshock Bubbles, Hot Flow Anomalies, and Spontaneous Hot Flow Anomalies and Their Substructures Observed by MMS

Abstract

Ubiquitous throughout the universe, shocks form when a supersonic flow encounters an obstacle and are important particle accelerators through different acceleration mechanisms (e.g., Helder et al., 2012;Lee et al., 2012). On Earth, a bow shock forms when the supermagnetosonic solar wind encounters the magnetosphere. At the quasi-parallel bow shock (where the angle between the shock normal and the interplanetary magnetic field (IMF) is less than 45°), some solar wind particles are reflected by the bow shock and populate the region upstream known as the foreshock (e.g., Eastwood et al., 2005). Interactions between these foreshock particles and the incoming solar wind particles can generate ULF waves that permeate the foreshock (e.g., Eastwood et al., 2005;Wilson et al., 2016). In addition, transient kinetic phenomena are also observed in the foreshock (see review by Zhang et al., 2022), such as hot flow anomalies (HFAs) (

Statistical Study of Foreshock Density Holes

“…We establish an event list of 411 DHs with Magnetospheric Multiscale 1 (MMS1) in the solar wind from October 2017 to April 2018 and October 2018 to May 2019 (∼15 months). We use the solar wind time intervals from Vu et al (2022) through machine learning and note that the presence of the foreshock region is not a requirement to investigate whether DHs could be solar wind structures.…”

“…We use the solar wind time intervals from Vu et al. (2022) through machine learning and note that the presence of the foreshock region is not a requirement to investigate whether DHs could be solar wind structures.…”

Magnetospheric Field‐Aligned Current Generation by Foreshock Transients: Contribution by Flow Vortices and Pressure Gradients

“…However, the spatial scale of foreshock transients like HFAs is typically a few R E (e.g., Facskó et al., 2009; Lin et al., 2022; Vu et al., 2022), much smaller than the solar wind impulses or shocks. They also have sub‐scale structures with complicated pressure variation inside their core region (e.g., Vu et al., 2022; S. Wang et al., 2012). Therefore, considering their significant pressure variation across a rather small spatial scale, they may result in strong pressure gradients near the magnetopause.…”

“…For major magnetopause disturbances, such as those caused by solar wind impulses or shocks, global MHD simulations (e.g., Fujita, Tanaka, Kikuchi, Fujimoto, Hosokawa, & Itonaga, 2003; Fujita, Tanaka, Kikuchi, Fujimoto, & Itonaga, 2003; Keller et al., 2002; Slinker et al., 1999) and observations (e.g., Glassmeier et al., 1989; Moretto et al., 2002; Tian et al., 2016) have shown that the flow vortices are a dominant driver of the associated FACs. However, the spatial scale of foreshock transients like HFAs is typically a few R E (e.g., Facskó et al., 2009; Lin et al., 2022; Vu et al., 2022), much smaller than the solar wind impulses or shocks. They also have sub‐scale structures with complicated pressure variation inside their core region (e.g., Vu et al., 2022; S. Wang et al., 2012).…”

“…However, the spatial scale of foreshock transients like HFAs is typically a few R E (e.g., Facskó et al, 2009;Lin et al, 2022;Vu et al, 2022), much smaller than the solar wind impulses or shocks. They also have sub-scale structures with complicated pressure variation inside their core region (e.g., Vu et al, 2022;S. Wang et al, 2012).…”