Nonresonant electromagnetic instabilities in space plasmas: interplay of Weibel and firehose instabilities

In coronal outflows and solar winds, the presence of the interplanetary magnetic field and heat fluxes combined with jets and shock waves give rise to important thermal anisotropies and energetic counterstreaming motions of plasma shells. Such anisotropic structures of plasma quickly lead to the onset of the kinetic electromagnetic instabilities which are dependent solely on bulk properties of the plasma and not on resonant interaction with charged particles. Here the interplay between the Weibel and firehose instabilities, both driven by an excess of kinetic energy in the direction of the ambient magnetic field, is considered. Their growth rates and thresholds are evaluated and compared for the electron temperature anisotropies in the solar wind. It is shown that the instability of the Weibel-type, which is improperly known as the “oblique firehose” instability, is the most efficient mechanism of isotropisation limiting the increase of particle velocity anisotropy and thus confirming the observations. These instabilities can explain the origin of interplanetary magnetic field fluctuations, which are expected to enhance along the temperature anisotropy thresholds.status: publishe