Role of Magnetosonic Solitons in Perpendicular Collisionless Shock Reformation

Abstract

The nature of the magnetic structure arising from ion specular reflection in shock compression studies is examined by means of 1d particle in cell simulations. Propagation speed, field profiles and supporting currents for this magnetic structure are shown to be consistent with a magnetosonic soliton. Coincidentally, this structure and its evolution are typical of foot structures observed in perpendicular shock reformation. To reconcile these two observations, we propose, for the first time, that shock reformation can be explained as the result of the formation, growth and subsequent transition to a super-critical shock of a magnetosonic soliton. This argument is further supported by the remarkable agreement found between the period of the soliton evolution cycle and classical reformation results. This new result suggests that the unique properties of solitons can be used to shed new light on the long-standing issue of shock non-stationarity and its role on particle acceleration.Introduction.-Collisionless shocks have been intensively studied since the late 1950's by virtue of of the role they are believed to play in plasma heating and charged particle acceleration (see, e. g., Refs. [1][2][3][4][5][6][7] and references therein). One of the most important features in high Mach number shocks is the specular reflection of upstream ions, which serves as an energy dissipation mechanism [8] to satisfy to shock conservation equations: ion specular reflection is paramount to both ion acceleration and shock structure.Further to its role in ion acceleration, ion specular reflection is responsible for the non-stationarity of quasiperpendicular shocks. This temporal variability has been demonstrated through numerical simulations (see, e. g., Refs [9-13]), observations [14][15][16][17] and experiments [18]. Although four different non-stationarity mechanisms have been suggested in full generality [5], the most likely candidate for explaining shock reformation in a 1-d exactly perpendicular shock (θ = 90• , with θ the angle between the upstream magnetic field and the shock normal) is the so-called self-reformation mechanism (see, e. g., Ref. [7]). This self reformation cycle can be summarized as follows. First, ions reflected by the shock form a foot ahead of the ramp. Due to the gyro-motion, these reflected ions pile up upstream of the foot at a distance slightly smaller than an ion gyro-radius ahead of the shock ramp, and create local magnetic field and density maxima there. Through a feedback mechanism, this foot then grows until it becomes as large as the initial shock ramp, effectively becoming the new ramp. Finally, this new ramp reflects incoming ions and the process repeats, with the shock advancing in a step-wise fashion. Numerical simulations suggest that the onset of this non-stationary reformation process is conditioned

Emergence of MHD structures in a collisionless PIC simulation plasma

“…The expansion speed of our blast shell remains below that considered in Ref. [7] and no shock reformation takes place. We use the same setup as in Ref.…”

“…Collisionless plasmas support energetic structures that are not captured by a single-fluid MHD theory and that can play a vital role in the thermalization of plasma. Examples are magnetosonic solitons [6,7] and the beams of shock-reflected particles ahead of the bow shock [8], which enforce a non-stationarity of the shock [9][10][11][12]. Single-fluid MHD simulations are nevertheless used to solve problems in collisionless plasma based on the argument that they can describe the plasma dynamics on a large enough scale.…”

Expansion of a radially symmetric blast shell into a uniformly magnetized plasma

“…Shocks with such a low Mach number reflect only a small fraction of the inflowing upstream ions. The beam they formed was not energetic enough to enforce the cyclic shock reformation, which is observed for collisionless magnetized shocks with a higher Mach number [10][11][12][13][14][15][16][17][18][19][20].…”

Quasi-perpendicular fast magnetosonic shock with wave precursor in collisionless plasma

“…Previous one-dimensional PIC simulations studied the propagation of MHD shocks across a perpendicular magnetic field. Shocks reached a steady state [1][2][3] if they moved slow enough to avoid a self-reformation [4]. Selfreformation is a process that is not captured by an MHD model.…”

“…The precursor softened the transition of the ambient plasma into the shocked ambient one and we could not observe a strong acceleration of ions by the shock passage. An absent shock-reflected ion beam implied that the quasi-perpendicular shock did not reform by driving solitons upstream [4].…”