Solar wind radial and latitudinal structure: Electron density and core temperature from Ulysses thermal noise spectroscopy

Abstract

We present new in situ solar wind plasma measurements obtained during Ulysses fast transit from the south solar pole to the north one, which took place 1 year before the 1996 sunspot minimum. The data were obtained with the radio receiver of the Unified Radio and Plasma Wave Experiment, using the method of quasi‐thermal noise spectroscopy, which is relatively immune to spacecraft potential perturbations and whose density measurements are independent on gain calibrations. We analyze the electron density and the core electron temperature. We deduce their radial profiles in the steady state fast solar wind; southward of 40° latitude, between 1.52 and 2.31 AU, the total electron density varies as n e ∝ r (−2.003±0.015) , while the core temperature varies as T e ∝ r (−0.64±0.03) . This allows to estimate the interplanetary electrostatic field using a simplified fluid equation. We also study, poleward of 40° (where the variance of both parameters are very low), the histograms of the electron density and core temperature scaled to 1 AU, assuming the above determined radial variation. Each histogram shows a single class of flow with a roughly normal distribution. We find a mean electron density of 2.65 cm −3 in the southern hemisphere which is about 8% larger than in the northern one. The core temperature histogram is centered at a mean of 7.5 × 10 4 K in the south, and of 7 × 10 4 K in the north. This small asymmetry may be due to a genuine solar asymmetry between the two hemispheres and/or to a temporal variation since solar activity slightly decreased during the Ulysses exploration.