  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 if 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 for 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.
