The link between shocks, turbulence, and magnetic reconnection in collisionless plasmas

Abstract

Global hybrid (electron fluid, kinetic ions) and fully kinetic simulations of the magnetosphere have been used to show surprising interconnection between shocks, turbulence, and magnetic reconnection. In particular, collisionless shocks with their reflected ions that can get upstream before retransmission can generate previously unforeseen phenomena in the post shocked flows: (i) formation of reconnecting current sheets and magnetic islands with sizes up to tens of ion inertial length. (ii) Generation of large scale low frequency electromagnetic waves that are compressed and amplified as they cross the shock. These "wavefronts" maintain their integrity for tens of ion cyclotron times but eventually disrupt and dissipate their energy. (iii) Rippling of the shock front, which can in turn lead to formation of fast collimated jets extending to hundreds of ion inertial lengths downstream of the shock. The jets, which have high dynamical pressure, "stir" the downstream region, creating large scale disturbances such as vortices, sunward flows, and can trigger flux ropes along the magnetopause. This phenomenology closes the loop between shocks, turbulence, and magnetic reconnection in ways previously unrealized. These interconnections appear generic for the collisionless plasmas typical of space and are expected even at planar shocks, although they will also occur at curved shocks as occur at planets or around ejecta. V C 2014 AIP Publishing LLC. [http://dx.

Plasma flow patterns in and around magnetosheath jets

“…These flows are superposed to the usual background flow of ambient plasma in the magnetosheath (mostly in the direction of propagation), thereby slowing plasma down. That deceleration is, however, small on average, so that no sunward flows become apparent in the statistical (median) results, contrary to simulations by Karimabadi et al (2014) and case study observations by Plaschke et al (2017).…”

“…4a. Note, however, that this minimum in median v is very shallow and far from exhibiting the sunward plasma motion (negative v ) in the ambient magnetosheath, seen in simulations by Karimabadi et al (2014) as a reaction to jet penetration. Not even the lower quartiles of second spacecraft v measurements become negative.…”

“…Also within the magnetosheath, jets are expected to alter ambient plasma due to their excess velocity. Simulation results by Karimabadi et al (2014) show that fast jets push slower ambient magnetosheath plasma out of their way. As a result, that ambient plasma performs an evasive motion around jets, such that it is further slowed down or even pushed in a sunward direction in the vicinity of jets.…”

The dawn–dusk asymmetry of ion density in the dayside magnetosheath and its annual variability measured by THEMIS

“…Thus, although the ion density dawn-dusk asymmetry does vary with the 11-year solar cycle, the behaviour of the plasma properties during each solar cycle have to be taken into account. With the increased sophistication and resolution of numerical models and simulations (Alfthan et al, 2014;Karimabadi et al, 2014;Palmroth et al, 2015), in conjunction with the long-term monitoring of solar wind conditions, we should be well equipped to resolve the drivers of this asymmetry.…”

Dawn‐dusk asymmetry in bursty hot electron enhancements in the midtail magnetosheath

“…Therefore, it is plausible that during Alfvénic solar wind, the IMF directions change Figure 6. In addition to HFAs and FBs, there are other types of density or dynamic pressure perturbations created in association with the quasi-parallel bow shock, such as spontaneous HFAs [Zhang et al, 2013], magnetosheath filamentary structures [Omidi et al, 2014], transient flux enhancements or jets [e.g., Němeček et al, 1998;Hietala et al, 2012], or large-scale low-frequency electromagnetic wavefronts [Karimabadi et al, 2014]. But these perturbations can occur without IMF discontinuities.…”