Signatures of kinetic instabilities in the solar wind

Abstract

[1] An analysis of ion non-thermal properties in the fast solar wind based on Ulysses data is reported. The radial evolution of the main proton moments (density, temperature, and drift velocities) and their empirical correlations with other plasma parameters are investigated in detail and compared with theoretical expectations. The stability of the plasma is studied against different ion kinetic instabilities driven by ion temperature anisotropies and differential velocities, focusing on the identification of possible signatures of relevant instabilities in the observed core-beam structure of proton distributions. The temperature anisotropy of the total proton distribution appears to be constrained by fire hose instabilities, in agreement with previous studies, while if considered separately, beam and core populations exhibit opposite anisotropies, with core protons characterized by perpendicular temperatures larger than the parallel ones, possibly (marginally) unstable for ion-cyclotron instability. The evolution with distance of the drift velocity between the secondary population and the main core is found to be nonadiabatic, leading to the identification of a marginal stability path of a magnetosonic ion-beam instability. As a conclusion, we find that a large fraction of the proton distributions observed by Ulysses display signatures of either a beam or a fire hose instability, suggesting that such kinetic processes play an important role in regulating the solar wind thermal energetics during the plasma expansion.

Shaping the solar wind temperature anisotropy by the interplay of electron and proton instabilities

“…However, Shaaban et al (2015Shaaban et al ( , 2016 have recently shown that inhibiting effect induced by the anisotropic electrons is stimulated by increasing β e, . Thus, for an average value β e, = 1 indicated by the observations (Štverák et al 2008), also commonly invoked in similar investigations (Hellinger et al 2006;Matteini et al 2013), the EMIC threshold can increase considerably constraining more observational data, see dashed (green) line. On the other hand, the fit with the observations is considerably improved in the presence of suprathermal electrons, i.e., the instability thresholds enhance with decreasing the power-index κ e confirming the results in Figure 3.…”

Ion acceleration by parallel propagating nonlinear Alfvén wave packets in a radially expanding plasma

“…On the other hand, as discussed below, some important kinetic effects are not included in the present model. Of course, even if the effect of temperature anisotropy on the phase velocity of Alfvén waves is negligible, the various instabilities can be caused by temperature anisotropy (Matteini et al, 2013). In the present study, Alfvén wave packets excite the longitudinal fluctuations (deformation of the VDFs).…”

Nonequilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind (Invited Review)

“…Non-thermal properties are expected to be more enhanced closer to the Sun. Reproduced with permission from Marsch et al (1982b) and Matteini et al (2013), c John Wiley and Sons.…”