Latitude dependence of Solar Wind plasma thermal noise: Ulysses radio observations

Abstract. Thermal noise spectroscopy was performed on theURAPradioreceiverdataforthefirsttimeoutoftheeclipticdur-ingtheUlyssesspacecrafttransitfromsoutherntonorthernheli-olatitudes.Wepresenttheresultsonthesolarwindelectronden-sityandbulktemperatureobtainedfrom−43 ◦ Sto+43 ◦ N.Theyindicate a rough symmetry of the solar wind electron plasmaabout the solar equator. The latitudinal gradient of the electronbulktemperaturewasfoundtobeapproximately−830K/ ◦ lat-itude southward and −1100K/ ◦ northward. Within ±20 ◦ lati-tude, large fluctuations in the 1-AU scaled electron density andtemperature were observed and presumably linked to the coro-tating structures existing in the equatorial band. Effects of thesolar wind speed on the thermal noise spectra were highest athighlatitudesaspredicted.Theywillbefullytakenintoaccountin future work.Key words: solar wind – interplanetary medium – plasmas –Sun: radio radiation1. IntroductionSince the Ulysses launch, the radio receiver on the Unified Ra-dio and Plasma Wave (URAP) Experiment (Stone et al. 1992)hascontinuouslyrecordedthesolarwindplasmaquasi-thermalnoise (QTN), on which are sometimes superimposed variousradio and plasma waves. This noise is due to the voltage in-ducedontheantennabytherandommotionoftheambientelec-tronswhichexciteplasmawavesneartheplasmafrequency,theso-called “plasma line”. Theoretical interpretation of the QTNspectrum yields the density and temperature of both the coldand hot electron populations (Meyer-Vernet & Perche 1989).As noted by Meyer-Vernet et al. (1996), one of the main ad-vantagesofthethermalnoisespectroscopyisitsrelativeimmu-nity to the spacecraft potential and photoelectron perturbationswhich in general affect particle analyzers. This radio methodof plasma diagnostic by thermal noise spectroscopy was ap-plied on Ulysses in the ecliptic plane to study the solar wind at