By Simon Opie (Mullard Space Science Laboratory, University College London)
Where and under what conditions the transfer of energy between electromagnetic fields and particles takes place in the solar wind remains an open question. In this paper we resolve to find a quantitative and causative link between turbulence in the solar wind and the occurrence of temperature anisotropy in the proton distribution as measured by Solar Orbiter’s Proton Alpha Sensor (PAS) which is part of the Solar Wind Analyser’s (SWA) suite of instruments. We define and derive the radial rate of strain ΓR as a dynamical measure of the driving of temperature anisotropy by bulk plasma motions. Intervals in the data unstable to the oblique firehose and mirror-mode instabilities are on average characterised by high absolute values of ΓR. We attribute this observation to the proposition that temperature anisotropies associated with these kinetic instabilities are the result of strong, intermittent velocity shears in the turbulent solar wind that cause shearing of the frozen-in magnetic field, with a local double-adiabatic impact on the particle distributions.
We show the distribution of ΓR as bin averages in T⊥/T∥–β∥ parameter space, where T is the temperature, β is the ratio of plasma pressure to magnetic pressure, and the subscripts represent measurement of the quantity either perpendicular (⊥) or parallel (||) to the magnetic field. We overplot the instability thresholds for the Oblique Firehose (OF), Alfvén/Ion cyclotron (A/IC), and Mirror-mode (M) instabilities. We see that the areas of parameter space beyond the thresholds for the oblique firehose and mirror-mode instabilities are well defined by extreme values in the distribution of ΓR.
See publication for details:
Opie, S., Verscharen, D., Chen, C.H.K., Owen, C.J., Isenberg, P.A., Sorriso-Valvo, L., Franci, L., Matteini, L., 2024. Temperature anisotropy instabilities driven by intermittent velocity shears in the solar wind.
https://doi.org/10.1017/s0022377824001375