Proton-Cyclotron and Firehose Instabilities in Inhomogeneous Solar Wind

Abstract:

The solar wind displays temperature anisotropy associated with the protons that cannot be understood by double adiabatic theory alone. In the absence of significant heat flux or collisions, the kinetic plasma instabilities driven by these proton temperature anisotropy are considered to play a crucial role in regulating the temperature anisotropy. To understand this problem, linear and quasi-linear theories as well as particle simulation methods are often employed by researchers. However, application of these research tools has hitherto been largely confined to uniform plasmas. To employ these methods to an inhomogeneous solar wind is not so straightforward. The present paper employs quasi-linear theory to investigate quasi-stationary spatial distribution, structure, and characteristics of unstable proton-cyclotron and parallel firehose modes in inhomogeneous plasma by combining the kinetic-fluid model and quasilinear kinetic theory. The present paper considers diverging magnetic field along a one-dimensional flux tube, as well as the associated density models, with various source-region boundary conditions. With such an approach, the spatial locations at which the excitation, saturation, and damping of the proton-cyclotron and firehose instabilities occur, are investigated. From the present investigation, it is seen rigorously that the solar wind is bound by the temperature anisotropy threshold condition. The present approach can be further extended to include other instabilities, and can even help interprete satellite data, which is expected to become available from the near future Solar Probe Plus and Solar Orbiter Missions.