Impacts of spontaneous hot flow anomalies on the magnetosheath and magnetopause

Abstract

Spacecraft observations and global hybrid (kinetic ions and fluid electrons) simulations have demonstrated that ion dissipation processes at the quasi‐parallel bow shock are associated with the formation of structures called spontaneous hot flow anomalies (SHFAs). Previous simulations and recent spacecraft observations have also established that SHFAs result in the formation of magnetosheath filamentary structures (MFS). In this paper we demonstrate that in addition to MFS, SHFAs also result in the formation of magnetosheath cavities that are associated with decreases in density, velocity, and magnetic field and enhancements in temperature. We use the results of a global MHD run to determine the change in the magnetosheath properties associated with cavities due to ion kinetic effects. The results also show the formation of regions of high flow speed called magnetosheath jets whose properties as a function of solar wind Mach number are described in this study. Comparing the properties of the simulated magnetosheath cavities and jets to past spacecraft observations provides good agreement in both cases. We also demonstrate that pressure variations associated with cavities and SHFAs in the sheath result in a continuous sunward and antisunward magnetopause motion. This result is consistent with previous suggestions that SHFAs may be responsible for the generation of ion cyclotron waves and precipitation of ring current protons in the outer magnetosphere.

Cavitons and spontaneous hot flow anomalies in a hybrid-Vlasov global magnetospheric simulation

“…Dynamics of SHFA-triggered erosion of other planetary bow shocks may be different due to different magnetic field strengths and spatial scales. The fact that multiple SHFAs are needed for the formation of large magnetosheath cavities is in agreement with the results of Omidi et al (2016).…”

“…The large magnetosheath cavity that develops in this run has similar features to the magnetosheath cavities reported by Omidi et al (2016), with decreased values of magnetic field magnitude, ion density, and high temperature. Observational work presented in Katırcıoǧlu et al (2009) shows the existence of such structures in the Earth's magnetosheath.…”

“…Time T 2 = 1105 s shows the phase when, as the result of SHFAs surviving crossing the bow shock, a large magnetosheath cavity has formed (yellow area around x = −4 R E , z = −33 R E in panel f). Magnetosheath cavities such as this have been found in observations (Katırcıoǧlu et al, 2009) and hybrid simulations (Omidi et al, 2016). "Chains" of SHFAs and cavitons are seen also upstream of the bow shock that add to the large magnetosheath cavity.…”

“…In a recent study, Omidi et al (2016) demonstrated that SHFAs can result in the formation of magnetosheath cavities (Katırcıoǧlu et al, 2009), which are associated with decreases in plasma density, bulk velocity and magnetic field magnitude, and enhancements in temperature. However, we still know very little about how the arrival of cavitons and SHFAs can modify the bow shock structure and the magnetosheath.…”

Cavitons and spontaneous hot flow anomalies in a hybrid-Vlasov global magnetospheric simulation

“…When crossing the bow shock, the large SHFAs create a region of enhanced temperature, decreased plasma density and weakened magnetic field, with an 15 associated increase in plasma β. This downstream region, which grows as it is convected tailward along the shock, was termed a magnetosheath cavity, following the terminology introduced in Katırcıoǧlu et al (2009) and Omidi et al (2016) for a similar phenomenon. This structure is strengthened subsequently by additional SHFAs which cross the shock at the same position.…”