Tearing and Kelvin-Helmholtz instabilities in the heliospheric plasma

¹ Department of Astronomy and Space Science, Univ. of Firenze, 50125 Firenze, Italy e-mail: betta@arcetri.astro.it
² Department of Physics, Univ. of Pisa, 56127 Pisa, Italy
³ Jet Propulsion Laboratory, California Institute of Technology, 91109 Pasadena, CA, USA

Received: 30 September 2005
Accepted: 9 January 2006

Abstract

We used 2.5D simulations to analyze the magnetohydrodynamic instabilities arising from an initial equilibrium configuration consisting of a plasma jet or wake in the presence of a magnetic field with strong transverse gradients, such as those arising in the solar wind. Our analysis extends previous results by considering both a force-free equilibrium and a pressure-balance condition for a jet in a plasma sheet, along with arbitrary angles between the magnetic field and velocity field. In the force-free case, the jet/wake does not contain a neutral sheet but the field rotates through the flow to invert its polarity. The presence of a magnetic field component aligned with the jet/wake destroys the symmetric nature of the fastest growing modes, leading to asymmetrical wake acceleration (or, equivalently, jet deceleration). In the case of a jet, the instability properties depend both on the magnetic field and flow gradients, as well as on the length of the jet. The results are applied to the post-termination shock jet recently found in 3D global heliospheric simulations, where our analysis confirms and explains the stability properties found in such simulations.