Abstract
The expansion of an initially unmagnetized planar rarefaction wave has recently been shown to trigger a thermal anisotropy-driven Weibel instability (TAWI), which can generate magnetic fields from noise levels. It is examined here whether the TAWI can also grow in a curved rarefaction wave. The expansion of an initially unmagnetized circular plasma cloud, which consists of protons and hot electrons, into a vacuum is modelled for this purpose with a two-dimensional particle-in-cell (PIC) simulation. It is shown that the momentum transfer from the electrons to the radially accelerating protons can indeed trigger a TAWI. Radial current channels form and the aperiodic growth of a magnetowave is observed, which has a magnetic field that is oriented orthogonal to the simulation plane. The induced electric field implies that the electron density gradient is no longer parallel to the electric field. Evidence is presented here that this electric field modification triggers a second magnetic instability, which results in a rotational low-frequency magnetowave. The relevance of the TAWI is discussed for the growth of small-scale magnetic fields in astrophysical environments, which are needed to explain the electromagnetic emissions by astrophysical jets. It is outlined how this instability could be examined experimentally.
Highlights
The expansion of an initially unmagnetized planar rarefaction wave has recently been shown to trigger a thermal anisotropy-driven Weibel instability (TAWI), which can generate magnetic fields from noise levels
We propose that the superposition of the electric field, which is induced by the magnetic field of the TAWI, with the radial electric field of the rarefaction wave triggers the growth of the transverse electric (TE) wave by the misalignment of the plasma density gradient and the electric field vector
The purpose of our study has been to test whether a TAWI can develop in curved rarefaction waves
Summary
The 2D3V (resolves two spatial and three momentum dimensions) PIC simulation code we employ here is based on the virtual particle-mesh numerical scheme [41]. Are solved on a grid and evolve the electromagnetic fields in time, while ∇ · E = ρ/ 0 and ∇ · B = 0 are fulfilled as constraints. The sum of the interpolated micro-currents of all CPs gives the macroscopic current J, which is used to update the electromagnetic fields through equation (2). The new fields are interpolated back to the positions of the individual CPs to update their momentum in time through equation (3). The mean speeds of the electrons and of the protons vanish initially. The electrons and protons are approximated each by ≈2500 CPs per cell. An electron with the thermal speed ve will cross the cloud diameter 2rW about eight times during t = TF, resulting in multiple bounces at the electrostatic sheath field.
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