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Conference papers

Direct Evidence of Nonstationary Collisionless Shocks in Space Plasmas

Abstract : Collisionless shocks are ubiquitous throughout the universe and are efficient particle accelerators. At Earth, the bow shock plays a fundamental role in solar wind-magnetosphere coupling by redistributing the kinetic energy of the solar wind bulk flow. However, the manner in which the shock thermalizes particles and processes the upstream plasma is dictated by the physics associated with the electromagnetic shock structure. In the low Mach number subcritical regime, the structure is well understood, but only if the Mach number is below the first critical or whistler critical Mach number. If the Mach number exceeds the second critical Mach number, a stable reflection shock can be formed. Currently thinking is that the next stage of the stable reflection shock will be a nonstationary reflection shock, when sufficient energy can no longer be dissipated to maintain a stable shock ramp. This can cause large-scale magnetic re-structuring of the shock front known as reformation. There are theoretical descriptions of this processes based on PIC simulations and a gradient catastrophe, but until now, the one applicable to conditions at the Earth's bow shock has remained unclear. The main reason is that it is necessity to experimentally resolve the smallest scale, which is on the order of the electron inertial length. Using a close separation campaign of the Cluster spacecraft, two probes crossed a shock in the correct parameter regime with a spacecraft separation of only 7km. For the first time, we present direct experimental evidence of a quasi-perpendicular nonstationary shock front. These observations demonstrate that the transition to nonstationarity is initiated by electron scale magnetic and electric field structures within the ramp consistent with the gradient catastrophe description.
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Contributor : Nathalie Pothier Connect in order to contact the contributor
Submitted on : Wednesday, February 9, 2022 - 7:26:27 AM
Last modification on : Monday, May 16, 2022 - 8:20:24 AM




A. P. Dimmock, C. T. Russell, R. Sagdeev, V. Krasnoselskikh, S. N. Walker, et al.. Direct Evidence of Nonstationary Collisionless Shocks in Space Plasmas. American Geophysical Union, 2019, San Francisco, United States. 15 pp. ⟨insu-03562454⟩



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