Space and Atmospheric Physics, Blackett Laboratory, Imperial College London, London SW7 2AZ, United Kingdom.
Astronomy Unit, Queen Mary University of London, London E1 4NS, United Kingdom.
Phys Rev Lett. 2015 Sep 18;115(12):125001. doi: 10.1103/PhysRevLett.115.125001. Epub 2015 Sep 16.
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.
冲击波存在于整个宇宙中，是理解非碰撞等离子体性质的基础。重联是一个由微观物理驱动的过程，通常发生在高马赫数超临界激波中。虽然目前的研究已经从理论和模拟两个方面广泛地研究了这个过程，但对其观测仍然很少。在这封信中，我们研究了在一个研究很少的参数空间中的极高马赫数激波，我们使用卡西尼号航天器在 10 AU 处的原位磁场观测来识别重联。这使我们深入了解了跨越阿尔文马赫数（M_A）两个数量级的准垂直激波，这可能有助于弥合中等地球激波和更奇特的天体物理激波之间的差距。我们首次展示了由预测的约 0.3τ_c 时间尺度上的镜面离子反射控制的周期性重联的证据，其中 τ_c 是离子回旋周期。此外，我们还通过实验揭示了重联与 M_A 的关系，并关注此类激波的磁场结构，以进一步表明对于相同的 M_A，重联激波比不重联的激波具有更强的磁场放大。
