Quasiperpendicular High Mach Number Shocks

Abstract

Shock waves exist throughout the Universe and are fundamental to understanding the nature of collisionless plasmas. Reformation is a process, driven by microphysics, which typically occurs at high Mach number supercritical shocks. While ongoing studies have investigated this process extensively both theoretically and via simulations, their observations remain few and far between. In this Letter we present a study of very high Mach number shocks in a parameter space that has been poorly explored and we identify reformation using in situ magnetic field observations from the Cassini spacecraft at 10 AU. This has given us an insight into quasiperpendicular shocks across 2 orders of magnitude in Alfvén Mach number (M_{A}) which could potentially bridge the gap between modest terrestrial shocks and more exotic astrophysical shocks. For the first time, we show evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted time scale of ~0.3τ_{c}, where τ_{c} is the ion gyroperiod. In addition, we experimentally reveal the relationship between reformation and M_{A} and focus on the magnetic structure of such shocks to further show that for the same M_{A}, a reforming shock exhibits stronger magnetic field amplification than a shock that is not reforming.

Suprathermal Electrons at Saturn's Bow Shock

“…This is possible because the evolution of solar wind parameters with heliospheric distance makes Saturnʼs bow shock one of the strongest in the Solar System (e.g., Russell 1985), and such occasions occur under rare solar wind conditions where the near-Saturn Interplanetary (solar) Magnetic Field (IMF) strength drops to ∼0.1 nT and the shock Alfvén Mach number increases to order 100. On such occasions Cassini has witnessed electron acceleration at a quasi-parallel shock crossing , and provided evidence for shock reformation controlled by specular ion reflection (Sulaiman et al 2015). The electron acceleration result indicates that there may not be an electron injection problem at high Mach number quasiparallel collisionless shocks, in agreement with some theories (e.g., Amano & Hoshino 2010).…”

“…However, the Cassini data have provided 31 examples of shock-acceleration of electrons that span a Mach number range that enters the high-Mach number regime of young SNR shocks, which does not occur at other heliospheric shocks frequently encountered by spacecraft (Sulaiman et al 2015). These data represent an opportunity to determine the conditions under which electron acceleration occurs, over a range of Mach numbers.…”

“…These two upstream parameters are not continuously measured by Cassini due to instrument pointing constraints. However, Cassini studies to date have shown how (and to what extent) the influence of variations in key parameters can be assessed, even when dealing with hundreds of events Sulaiman et al 2015). Step-like" features in ELS spectra are due to onboard spacecraft averaging in response to telemetry constraints.…”

“…Magnetic field data taken during the mission to date reveal 871 unambiguous bow shock crossings (Sulaiman et al 2015). Electron data taken by LEMMS is available for 856 of these 871 crossings.…”

“…Recently reported observations made by the Cassini spacecraft during its orbital tour of Saturn have shown that the shock wave that stands in the solar wind sunward of the planet (Saturnʼs bow shock) is occasionally able to bridge the gap to the high-Mach number regime of young SNR shocks Sulaiman et al 2015). This is possible because the evolution of solar wind parameters with heliospheric distance makes Saturnʼs bow shock one of the strongest in the Solar System (e.g., Russell 1985), and such occasions occur under rare solar wind conditions where the near-Saturn Interplanetary (solar) Magnetic Field (IMF) strength drops to ∼0.1 nT and the shock Alfvén Mach number increases to order 100.…”