Abstract
Abstract Instabilities described by linear theory characterize an important form of wave–particle interaction in the solar wind. We diagnose unstable behavior of solar wind plasma between 0.3 and 1 au via the Nyquist criterion, applying it to fits of ∼1.5M proton and α particle Velocity Distribution Functions (VDFs) observed by Helios I and II. The variation of the fraction of unstable intervals with radial distance from the Sun is linear, signaling a gradual decline in the activity of unstable modes. When calculated as functions of the solar wind velocity and Coulomb number, we obtain more extreme, exponential trends in the regions where collisions appear to have a notable influence on the VDF. Instability growth rates demonstrate similar behavior, and significantly decrease with Coulomb number. We find that for a nonnegligible fraction of observations, the proton beam or secondary component might not be detected, due to instrument resolution limitations, and demonstrate that the impact of this issue does not affect the main conclusions of this work.
Highlights
Instabilities, driven by departures from local thermodynamic equilibrium (LTE), are frequently credited with affecting the behavior of rapidly evolving plasma systems, e.g. the expanding solar wind (Matthaeus et al 2012)
We find that the traditional way of organizing Helios observations over radial distance (Matteini et al 2007; Hellinger et al 2011, 2013) does not provide a complete description of the evolution of linear instabilities, neither in terms of their occurrence rate nor the growth rate
The database from 15 years of Helios observations of ion Velocity Distribution Functions (VDFs) processed by the Plasma in a Linear Uniform Magnetized Environment (PLUME) dispersion solver provides sufficiently robust statistics of the inferred behavior of linear instabilities between 0.3 and 1 au
Summary
Instabilities, driven by departures from local thermodynamic equilibrium (LTE), are frequently credited with affecting the behavior of rapidly evolving plasma systems, e.g. the expanding solar wind (Matthaeus et al 2012) To quantify these departures, the underlying charged particle Velocity Distribution Functions (VDFs) are typically modelled as bi-Maxwellians, having anisotropic temperatures T⊥,j and T ,j with respect to the local magnetic field B, relative field-aligned drifts between each pair of constituent VDF components i and j being ∆vi,j = (Vi −Vj)·B/|B|, and temperature disequilibrium between species Ti = Tj (Marsch et al 1982). A stable VDF consisted of an almost isotropic core and mildly shifted isotropic beam could potentially be fitted as a single population with T ,c T⊥,c, highly susceptible to firehose (FH) instability To ensure this issue does not have a misleading effect on our analysis, we detail the procedure of diagnosing insufficiently well resolved observations, and remove them from consideration of the results.
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