Isothermal magnetosheath electrons due to nonlocal electron cross talk

Abstract

Heating of the various plasma species at the Earth's collisionless bow shock is not fully understood. Although the total amount of heating is constrained by the one-fluid Rankine-Hugoniot relations in terms of local plasma conditions, the partition of energy between, e.g., electrons and ions, is influenced by particle kinetics which are not considered in the Rankine-Hugoniot approach. Additionally, in this paper we demonstrate the impact of nonlocal effects. Here we model the effects of shock-heated electrons which traverse the magnetosheath to arrive at another point on the bow shock surface. We construct the distribution of electrons immediately downstream of each point on the bow shock from two populations: local solar wind electrons entering the magnetosheath for the first time and previously shocked electrons which have traversed the magnetosheath. We determine the self-consistent cross-shock potential at each point such that the resulting electron distribution gives a downstream plasma which is neutral and has zero parallel current. This leads to very little variation in the downstream electron temperature everywhere around the curved bow shock, in spite of large differences in the total (Rankine-Hugoniot) downstream temperature. Consequently, the partition of heating between electrons and ions at the shock, i.e., the equation of state, contains nonlocal contributions. We show that heating properties at the point where the shock is exactly perpendicular controls electron heating throughout the rest of the shock surface.

Comment on “Electron demagnetization and heating in quasi‐perpendicular shocks” by Mozer and Sundkvist

“…upstream have traversed the local shock from the downstream side. Thus, this portion of the distribution is not the corresponding portion of the ambient pristine solar wind, but a mapping of electrons in the magnetosheath which have encountered some other portion of the bow shock [Mitchell and Schwartz, 2013a]. Accordingly, much of the v ∥ < 0 region in Figure 1 (bottom) should actually be filled by a preexisting and nonlocally heated electron population.…”

“…In particular, the dHT potential sets the overall scale of the spread in velocity. In the model of Mitchell and Schwartz [2013a] the potential actually adjusts to respond to the velocity scale of electrons arriving from deeper in the magnetosheath. Those electrons fill a large portion of velocity space.…”

MMS Observations of Electrostatic Waves in an Oblique Shock Crossing

“…The role of electrostatic waves in energy conversion within the shock has been the subject of debate for decades. Some theorize that macroscopic, quasi‐static electric fields have the strongest influence on particles within collisionless shock environments (Eastwood et al, ; Hull et al, ; Mitchell & Schwartz, ; Scudder et al, ). In the direction parallel to the magnetic field in particular, quasi‐static electric fields play a critical role in decelerating ions while inflating the electron phase‐space distribution, thereby increasing the electron temperature (Goodrich & Scudder, ).…”

Quantified energy dissipation rates in the terrestrial bow shock: 1. Analysis techniques and methodology

“…Thus, the ramp currents [e.g., Sagdeev, 1966;Gary, 1981], voids in electron velocity distributions [e.g., Hull et al, 1998Hull et al, , 2000Hull et al, , 2001Mitchell and Schwartz, 2014], and reflected ion currents [e.g., Scholer et al, 2003;Tsubouchi and Lembège, 2004;Matsukiyo and Scholer, 2006;Muschietti and Lembège, 2013] are all capable of producing instabilities that radiate electromagnetic waves.…”

“…These studies did not focus on or measure the high-frequency fields responsible for wave-particle interactions. However, they often invoke these microscopic processes as a mechanism to fill in the unobserved inaccessible voids in phase space predicted by Liouville mapping [e.g., Hull et al, 1998Hull et al, , 2000Hull et al, , 2001Schwartz et al, 2011;Mitchell and Schwartz, 2014]. Note that the quasi-static fields studies were often able to explain the average characteristics in the downstream electron velocity distributions.…”