Direct observation of solar wind proton heating from in situ plasma measurements

Aims. We determine the perpendicular and parallel proton heating rate in the solar wind, which is one of the primary goals of the Parker Solar Probe mission. Methods. To estimate the perpendicular and parallel proton heating rates from direct particle measurements by the SPAN electrostatic analyzers, the strong correlation between the proton temperature and the solar wind speed must be removed. This speed dependence is removed by normalization factors that convert the instantaneous temperature to the value it would have if the solar wind speed were 400 km s−1. One-hour and five-hour averages of the normalized perpendicular and parallel temperatures, measured on orbits 6–9, between 20 and 160 solar radii, are compared to the radial dependence they would have if there were no heating. Results. For the first time, perpendicular proton heating has been measured between 20 and 160 solar radii while there is neither heating nor cooling of the parallel protons below 70 solar radii. The extrapolated proton perpendicular temperature at one AU in a 400 km s−1 solar wind is 25 eV, which compares well with several earlier measurements. This result attests to the quality of the temperature measurements made by the particle detectors on the Parker Solar Probe. The heating rates, in ergs cm−3 s−1, that produced the observed perpendicular temperature are 6e−12 at 20 solar radii, 1e−13 at 50 solar radii, and 5e−14 at 160 solar radii.