Anna Tenerani

THE TEARING MODE INSTABILITY OF THIN CURRENT SHEETS: THE TRANSITION TO FAST RECONNECTION IN THE PRESENCE OF VISCOSITY

This paper studies the growth rate of reconnection instabilities in thin current sheets in the presence of both resistivity and viscosity. In a previous paper, Pucci and Velli (2014), it was argued that at sufficiently high Lundquist number S it is impossible to form current sheets with aspect ratios L/a which scale as

Models of coronal heating, turbulence and fast reconnection

L/a\sim S^\alpha

The Parametric Instability of Alfvén Waves: Effects of Temperature Anisotropy

with

Coupling between whistler waves and slow-mode solitary waves

\alpha > 1/3

Evolving Waves and Turbulence in the Outer Corona and Inner Heliosphere: The Accelerating Expanding Box

because the growth rate of the tearing mode would then diverge in the ideal limit

Activation of MHD reconnection on ideal timescales

S\rightarrow\infty

Marginal Stability of Sweet–Parker Type Current Sheets at Low Lundquist Numbers

. Here we extend their analysis to include the effects of viscosity, (always present in numerical simulations along with resistivity) and which may play a role in the solar corona and other astrophysical environments. A finite Prandtl number allows current sheets to reach larger aspect ratios before becoming rapidly unstable in pile-up type regimes. Scalings with Lundquist and Prandtl numbers are discussed as well as the transition to kinetic reconnection

‘Ideally’ unstable current sheets and the triggering of fast magnetic reconnection

Coronal heating is at the origin of the EUV and X-ray emission and mass loss from the sun and many other stars. While different scenarios have been proposed to explain the heating of magnetically confined and open regions of the corona, they must all rely on the transfer, storage and dissipation of the abundant energy present in photospheric motions, which, coupled to magnetic fields, give rise to the complex phenomenology seen at the chromosphere and transition region (i.e. spicules, jets, ‘tornadoes’). Here we discuss models and numerical simulations which rely on magnetic fields and electric currents both for energy transfer and for storage in the corona. We will revisit the sources and frequency spectrum of kinetic and electromagnetic energies, the role of boundary conditions, and the routes to small scales required for effective dissipation. Because reconnection in current sheets has been, and still is, one of the most important processes for coronal heating, we will also discuss recent aspects concerning the triggering of reconnection instabilities and the transition to fast reconnection.

“Ideal” tearing and the transition to fast reconnection in the weakly collisional MHD and EMHD regimes

We study the stability of large-amplitude, circularly polarized Alfven waves in an anisotropic plasma described by the double-adiabatic/CGL closure, and in particular the effect of a background thermal pressure anisotropy on the well-known properties of Alfven wave parametric decay in magnetohydrodynamics (MHD). Anisotropy allows instability over a much wider range of values of parallel plasma beta (β ∥) when ξ = p 0⊥/p 0∥ > 1. When the pressure anisotropy exceeds a critical value, ξ ≥ ξ* with ξ* 2.7, there is a new regime in which the parametric instability is no longer quenched at high β ∥, and in the limit β ∥ 1, the growth rate becomes independent of β ∥. In the opposite case of ξ < ξ*, the instability is strongly suppressed with increasing parallel plasma beta, similarly to the MHD case. We analyze marginal stability conditions for parametric decay in the (ξ, β ∥) parameter space and discuss possible implications for Alfvenic turbulence in the solar wind.

Fast Magnetic Reconnection: “Ideal” Tearing and the Hall Effect

The interplay between electron- and ion-scale phenomena is of general interest for both laboratory and space plasma physics. In this paper, we investigate the linear coupling between whistler waves and slow magnetosonic solitons through two-fluid numerical simulations. Whistler waves can be trapped in the presence of inhomogeneous external fields such as a density hump or hole where they can propagate for times much longer than their characteristic time scale, as shown by laboratory experiments and space measurements. Space measurements have detected whistler waves also in correspondence to magnetic holes, i.e., to density humps with magnetic field minima extending on ion-scales. This raises the interesting question of how ion-scale structures can couple to whistler waves. Slow magnetosonic solitons share some of the main features of a magnetic hole. Using the ducting properties of an inhomogeneous plasma as a guide, we present a numerical study of whistler waves that are trapped and transported inside propagating slow magnetosonic solitons.