A statistical study into the spatial distribution and dawn‐dusk asymmetry of dayside magnetosheath ion temperatures as a function of upstream solar wind conditions

Abstract

The magnetosheath contains the shocked solar wind and behaves as a natural filter to the solar wind plasma before it reaches the magnetosphere. The redistribution of kinetic energy at the bow shock results in significant thermalization of the solar wind plasma, resulting in a magnetosheath temperature profile which is highly nonhomogeneous and nonisotropic and differs between the dawn and dusk flanks. The present study attempts to study the spatial distribution of magnetosheath ion temperature as a function of upstream solar wind conditions. We pay particular attention to the dawn/dusk asymmetry in which we attempt to quantify using experimental data collected over a 7 year period. We also compare these data to simulated data from both the Block‐Adaptive‐Tree‐Solarwind‐Roe‐Upwind‐Scheme (BATS‐R‐US) MHD code and a kinetic hybrid model. We present evidence that the dawn flank is consistently hotter than the dusk flank for a variety of upstream conditions. Our statistical data also suggest a dependency on solar wind speed such that the level of asymmetry increases with faster speeds. We conclude that the dawn‐favored asymmetry of the magnetosheath seed population is insufficient to explain the dawn asymmetry (30–40%) of cold component ions in the cold, dense plasma sheet, and therefore, other mechanisms are likely required.

The dawn–dusk asymmetry of ion density in the dayside magnetosheath and its annual variability measured by THEMIS

“…To compile our data sets, we extracted magnetosheath observations from the complete THEMIS catalogue using our magnetospheric data analysis tool (see Dimmock and Nykyri (2013); Dimmock et al (2014Dimmock et al ( , 2015a). We quantitatively showed the asymmetry during typical solar wind conditions (Fig.…”

“…To summarise these asymmetries: magnetic field strength, plasma velocity, temperature anisotropy and mirror mode occurrence favour the dusk-flank and magnetic field turbulence, ion total temperature and ion number density are stronger on the dawn-side (Paularena et al, 2001;Němeček et al, 2003;Longmore et al, 2005;Walsh et al, 2012;Dimmock and Nykyri, 2013;Dimmock et al, 2014Dimmock et al, , 2015a. Remaining issues are the following: (1) what are their underlying physical mechanisms, and (2) what role do they play in driving magnetospheric plasma properties such as the formation of the cold dense plasma sheet.…”

“…We utilised our existing data analysis tool developed over the past 3 years (see Dimmock and Nykyri, 2013;Dimmock et al, 2014Dimmock et al, , 2015a to compile annual data sets of magnetosheath ion density measurements between 2008 through 2014. We also investigate the solar wind parameter dependence for each year separately to shed further light on the physical driver.…”

“…The physical processes leading to magnetosheath dawn-dusk asymmetries (Paularena et al, 2001;Longmore et al, 2005;Walsh et al, 2012;Dimmock and Nykyri, 2013;Dimmock et al, 2014Dimmock et al, , 2015a, are fundamental in solar wind-magnetosphere coupling as they can directly impact plasma transport processes at the magnetopause. Our results imply that the role they play is time dependent and this needs to be taken into consideration in the analysis of long duration data sets of both magnetosheath and magnetospheric measurements.…”

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.…”