Temperature variations in the dayside magnetosheath and their dependence on ion‐scale magnetic structures: THEMIS statistics and measurements by MMS

Abstract

The magnetosheath contains an array of waves, instabilities, and nonlinear magnetic structures which modify global plasma properties by means of various wave‐particle interactions. The present work demonstrates that ion‐scale magnetic field structures (∼0.2–0.5 Hz) observed in the dayside magnetosheath are statistically correlated to ion temperature changes on orders 10–20% of the background value. In addition, our statistical analysis implies that larger temperature changes are in equipartition to larger amplitude magnetic structures. This effect was more pronounced behind the quasi‐parallel bow shock and during faster solar wind speeds. The study of two separate intervals suggests that this effect can result from both local and external drivers. This manuscript presents two separate case studies, one from using THEMIS (Time History of Events and Macroscale Interactions during Substorms) data and another from Magnetospheric Multiscale; these measurements are then supported by extensive THEMIS statistical observations. These results could partly explain the 10–20% dawn‐favored asymmetry of the magnetosheath ion temperature seed population and contribute to the dawn‐favored asymmetry of cold component ions in the cold dense plasma sheet.

Statistical Study of Solar Wind, Magnetosheath, and Magnetotail Plasma and Field Properties: 12+ Years of THEMIS Observations and MHD Simulations

“…The ions can be heated more efficiently in the downstream of the quasi‐parallel bow shock due to a variety of wave modes, wave‐particle interactions (Blanco‐Cano et al., 2006; Eastwood et al., 2002, 2003, 2005), instabilities, and reconnection in thin current sheets (Karimabadi et al., 2014; Retinò et al., 2007; Sundkvist et al., 2007), which eventually can contribute to the dawn‐dusk asymmetric proton temperature in the dayside magnetosheath (Dimmock et al., 2015; Dimmock, Osmane, et al., 2017). However, the observed level of dawn‐favored temperature asymmetry in the magnetosheath is only 10–20% and thus smaller than the 30–40% asymmetry of the cold component ions observed in the plasma sheet (Wing et al., 2005), indicating that other mechanisms, either those at the magnetopause or magnetosphere, further contribute to the dawn‐favored plasma sheet asymmetry.…”

Comparison Between Fluid Simulation With Test Particles and Hybrid Simulation for the Kelvin‐Helmholtz Instability

“…The test particle simulation agrees well with the hybrid simulation at the early nonlinear stage; however, there is large deviation in the vortex region at the later nonlinear stage. This anisotropic temperature is likely to driven small-scale kinetic waves (e.g., mirror modes and ion cyclotron waves; Dimmock et al, 2015;Dimmock et al, 2017;Nykyri et al, 2003;Nykyri et al, 2011) and secondary instabilities (e.g., firehose instability). Note in Figure 4, these two simulations have similar 3 ∕ 1 value in the vortex region, meaning this deviation may be attributed to the different magnetic field directions in fluid and hybrid simulations.…”

Dependence of Thermodynamic Processes on Upstream Interplanetary Magnetic Field Conditions for Magnetosheath Ions

“…Dimmock et al () statistically examined the relationship between the ion temperature and magnetic field fluctuations in the dayside magnetosheath and their bow shock geometry dependence. Their results led to the conclusion that the amplitude in the ion temperature variations observed in the quasi‐parallel magnetosheath is larger than that in the quasi‐perpendicular magnetosheath due to the small‐scale magnetic field fluctuations generated by the turbulence or foreshock effects.…”