Multiple Current Sheet Systems in the Outer Heliosphere: Energy Release and Turbulence

Abstract

In the outer heliosphere, beyond the solar wind termination shock, it is expected that the warped heliospheric current sheet forms a region of closelypacked, multiple, thin current sheets. Such a system may be subject to the ion-kinetic tearing instability, and hence generate magnetic islands and hot populations of ions associated with magnetic reconnection. Reconnection processes in this environment have important implications for local particle transport, and for particle acceleration at reconnection sites and in turbulence. We study this complex environment by means of three-dimensional hybrid simulations over long time scales, in order to capture the evolution from linear growth of the tearing instability to a fully developed turbulent state at late times. The final state develops from the highly ordered initial state via both forward and inverse cascades.Component and spectral anisotropy in the magnetic fluctuations is present when a guide field is included. The inclusion of a population of new-born interstellar pickup protons does not strongly affect these results. Finally, we conclude that reconnection between multiple current sheets can act as an important source of turbulence in the outer heliosphere, with implications for energetic particle acceleration and propagation.

Three-dimensional simulations of sheared current sheets: transition to turbulence?

“…The multiple current sheet system is itself anisotropic, however in the AP case fast growth of the tearing instability and associated fluctuations evolve the system towards an approximately isotropic state by tΩ i 25. However, the guide field is known to introduce anisotropy in the magnetic spectrum of an MCS system (Burgess et al 2016). In the GF case, a relative reduction in the power in the parallel direction compared to the perpendicular direction, visible also in the magnetic structure in figure 1(b), leads to an effective reduction of the spectral index for the isotropically integrated spectrum.…”

“…the development of magnetic islands of varying scales, without the clutter of following multiple field lines in three dimensions. The time evolution of the tearing instability and associated magnetic structure for these multiple current sheet systems is described in detail by Burgess et al (2016). Discussion of the effects of other instabilities on the evolution of single three-dimensional ion-scale current sheets, including the drift-kink, firehose and ion cyclotron instabilities, can be found in Gingell et al (2015).…”

“…The evolution of the simulations in time can be summarized as follows (Burgess et al 2016): (i) linear growth of the tearing instability from the symmetric, highly ordered initial state; (ii) nonlinear growth of the tearing instability, characterized by island merging and reconnection across several current sheets; (iii) saturation of the tearing instability, and relaxation towards a chaotic, 'turbulent' state. The magnetic structure at the end of the simulation for the AP, GF and SH cases is shown in figure 1.…”

“…We investigate the evolution of turbulence from a multiple current sheet system using the three-dimensional hybrid simulation code HYPSI, as used previously to study the evolution of heliospheric ion-scale current sheets (Gingell, Burgess & Matteini 2015;Burgess et al 2016). The hybrid method combines a fully kinetic, Turbulence from sheared current sheets?…”

“…Using PIC simulations Drake et al (2010) investigated particle energization from energy release in a system of closely spaced current sheets. A similar system was studied by Burgess, Gingell & Matteini (2016) using a three-dimensional hybrid simulation (particle ions with fluid electron response) including pick up ions as found in the outer heliosphere. A major result of this study was that the highly symmetric and ordered initial state of a MCS system evolves to a complex three-dimensional state, with a long-time end state which appears to become close to well-developed turbulence.…”