Solar wind protons: Three‐dimensional velocity distributions and derived plasma parameters measured between 0.3 and 1 AU

Abstract

A survey of solar wind three‐dimensional proton velocity distributions as measured by the Helios solar probes between 0.3 and 1 AU is presented. A variety of nonthermal features like temperature anisotropies, heat fluxes, or proton double streams has been observed. The relative speed of the second proton component increases on the average with increasing wind speed and decreasing heliocentric radial distance and shows a correlation with the local Alfvén speed. A marked anisotropy in the core of proton distributions with a temperature larger perpendicular than parallel to the magnetic field (T∥c < T∥c) is a persistent feature of high speed streams and becomes most pronounced in the perihelion (≈0.3 AU). Fairly isotropic distributions have only been measured very close to and directly at magnetic sector boundaries. Low and intermediate speed distributions usually show a total temperature anisotropy T∥p/T⊥p > 1 frequently caused by ‘high‐energy shoulders’ or a resolved second proton component. No clear radial gradient of the temperature anisotropy could be established in these cases. The average dependence of the proton temperature on heliocentric radial distance is given by a power law R−α, where α ≈ 1 for T⊥p and 0.7 < α < 1 for T⊥p are compatible neither with isothermal nor adiabatic expansion. Flattest radial temperature profiles are obtained in high‐speed streams. These observations indicate that local heating or considerable proton heat conduction occurs in the solar wind. Some consequences of nonthermal features of proton distributions for plasma instabilities are discussed as well as kinetic processes that may shape the observed distributions.

Phase‐steepened Alfvén waves, proton perpendicular energization and the creation of magnetic holes and magnetic decreases: The ponderomotive force

“…Presumably it is the wave phase steepening (creation of a high-frequency component) that leads to proton perpendicular energization and the T ? /T k > 1 anisotropies in high-speed streams previously noted by Bame et al [1975] and Marsch et al [1982]. The Ponderomotive Force accelerates ions in the perpendicular direction, extracting energy from the ever-present Alfvén waves in the solar wind.…”

Magnetic fields and plasmas in the inner heliosphere: Helios results

“…Thus, the RMS variability of the magnetic ÿeld is 3-6 times greater at Mercury than it is as Earth. (see also Musmann et al, 1977;Marsch et al, 1982). The means and standard deviations of the distributions of magnetic ÿeld strength B, density N , and bulk speed V are 30:5 ± 11:2 nT, 59:8 ± 44:4 cm −3 , and 423 ± 112 km=s, respectively for a period of approximately half a solar cycle.…”

Mirror‐mode storms inside stream interaction regions and in the ambient solar wind: A kinetic study

“…All the parameters used as input for the dispersion relation solver are averaged values observed by STEREO from mirror-mode storm events observed in 2007 and 2008, except for the proton temperature anisotropy. Because STEREO has not yet provided temperature anisotropies (T ⊥ /T || ), we assume proton temperature anisotropies between 1 and 3 based on previous solar wind measurements at 1 AU [Winterhalter et al, 1994;Liu et al, 2006;Marsch et al, 1982;Marsch, 1991;Gary et al, 2002].…”