Investigating the nature of the link between magnetic field orientation and proton temperature in the solar wind

Abstract

Solar wind fluctuations are a mixture of propagating disturbances and advected structures that transfer into the interplanetary space the complicated magnetic topology present at the basis of the corona. The large-scale interplanetary magnetic field introduces a preferential direction in the solar wind, which is particularly relevant for both the propagation of the fluctuations and their anisotropy and for the topology of the structures advected by the wind. This paper focusses on a particular link observed between angular displacements of the local magnetic field orientation from the radial direction and values of the proton temperature. In particular, we find that observations by Helios and Wind show a positive correlation between proton temperature and magnetic field orientation. This is especially true within Alfvénic wind characterized by large-amplitude fluctuations of the background field orientation. Moreover, in the case of Wind, we found a robust dependence of the perpendicular component of the proton temperature on the magnetic field angular displacement. We interpret this signature as possibly due to a physical mechanism related to the proton cyclotron resonance. Finally, by simulating the sampling procedure of the proton velocity distribution function (VDF) of an electrostatic analyzer, we show that the observed temperature anisotropy is not due to instrumental e ects.