Solar wind plasma parameters on Ulysses: Detailed comparison between the URAP and SWOOPS experiments

The presence of several instruments measuring plasma parameters aboard Ulysses made possible an extensive comparison between them. In this paper, we focus on solar wind electron parameters measured by the Unified RAdio and Plasma (URAP) wave receiver, using the thermal noise spectroscopy method, and by the Solar Wind Observations Over the Poles of the Sun (SWOOPS) electron and ion analyzers. We compare the data sets acquired by both experiments during the ecliptic path in 1990/1991 and the first pole-to-pole fast transit of Ulysses in 1994/1995, near the maximum and minimum of solar activity, respectively. From the comparison, we find very good agreement between the total density derived by both experiments; the average offset between the two data sets differs by less than 5%. We find a systematic offset of 5% to 10% in the core electron temperature values depending on the solar wind flow speed. We present detailed observations obtained during the crossing of the stream interface on March 28, 1995, by Ulysses, when it returned to the high-speed wind region in the northern hemisphere. We find that the core electron temperature increases around 40% at the interface, which is in agreement with the statistical result obtained using IMP plasma measurements at 1 AU. We point out that SWOOPS provides reliable ion diagnostics in addition to suprathermal (halo) electron parameters and yields fundamental information on particle velocity distributions, whereas the improved quasi-thermal noise method provides total electron densities and core electron temperatures accurately.