Wind-Ulysses in-situ thermal noise measurements of solar wind electron density and core temperature at solar maximum and minimum Original Research Article

The radio receivers on the Wind and Ulysses spacecraft in the solar wind continuously record spectra of the quasi-thermal plasma noise near the electron plasma frequency, from which the electron density and core temperature can be determined using the method of quasi-thermal noise spectroscopy. Such in-situ thermal noise measurements were obtained by Wind in the in-ecliptic solar wind upstream of the Earth, and by Ulysses during its two pole-to-pole fast latitude scans, in 2000–2001 at solar maximum and in 1994–1995 at solar minimum. We present histograms of these measurements performed over common time spans of several months by the two spacecraft. We compare and discuss these histograms, together with those provided by Ulysses in 1990–1991 at solar maximum, thus extending our study to over a full solar activity cycle. From the resulting distributions, we classify the solar wind flow into distinct populations both in density and temperature. Their variations in number and importance with the solar activity and heliolatitude are also investigated.