Abstract
Abstract. Excitation of Weibel magnetic fields in an initially non-magnetized though anisotropic plasma may trigger other low-frequency instabilities fed by pressure anisotropy. It is shown that under Weibel-like stable conditions the Weibel-like thermal fluctuation magnetic field allows for restricted Firehose-mode growth. In addition, low-frequency Whistlers can also propagate in the plasma under certain anisotropic conditions. When the Weibel-like mode becomes unstable, Firehose instability ceases but Mirror modes take over. This will cause bubble structures in the Weibel-like field in addition to filamentation.
Highlights
There are just three celebrated fundamental very low-frequencymagnetic instabilities in hot anisotropic plasmas, the well-known Firehose mode (Vedenov et al, 1961; Treumann and Baumjohann, 1997, for a plasma physics textbook), its complementary equivalent, the Mirror mode, and the Weibel instability (Weibel, 1959; Yoon and Davidson, 1987, and others)
The latter Weibel-like mode acts in non-magnetized plasmas when, for some not further specified reason, the plasma exhibits a thermal anisotropy with higher temperature in one than in the two other directions. In both the Firehose and Weibel-like cases the cause of a higher equivalent temperature in one direction can be a fast streaming of the plasma with kinetic energy exceeding the transverse thermal energy. This can be provided by beam or counter-streaming beam configurations and has been made responsible for the generation of magnetic fields under various nondynamo conditions occurring, for instance, in shock waves, preferentially in relativistic shocks
In the present note we briefly examine this situation at the example of the classical Weibel instability, showing that there exists such a competition which may become important in limiting the growth of the Weibel-like modes, allowing other fluctuations to propagate on the magnetic Weibel-like background field which may contribute to distribution of magnetic fields in a larger volume
Summary
There are just three celebrated fundamental very low-frequency (electro-)magnetic instabilities in hot anisotropic plasmas, the well-known Firehose mode (Vedenov et al, 1961; Treumann and Baumjohann, 1997, for a plasma physics textbook), its complementary equivalent, the Mirror mode, and the Weibel instability (Weibel, 1959; Yoon and Davidson, 1987, and others). The first is a general bulk plasma mode excited in an external magnetic field by a thermal anisotropy with larger magnetically parallel than perpendicular temperature, T⊥ > T , resulting in Alfvén waves which radiate along the magnetic field On the contrary, the latter Weibel-like mode acts in non-magnetized plasmas when, for some not further specified reason, the plasma exhibits a thermal (pressure) anisotropy with higher temperature (kinetic energy) in one than in the two other directions. While Weibel-like modes provide a non-dynamo mechanism to produce quasi-stationary and non-propagating magnetic fields in an otherwise nonmagnetized plasma, the Firehose mode grows on an existing field and propagates along the field at Alfvén velocity VA transporting energy away from the region where it is excited, filling a large volume with magnetic fluctuations and contributing to turbulence and other effects. A similar analysis examining the newly identified electromagnetic modes (Schlickeiser et al, 2011; Schlickeiser and Skoda, 2011; Felten et al, 2013; Felten and Schlickeiser, 2013a, b, c), though being highly desirable, lies outside this brief communication
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