POLE-TO-POLE SOLAR WIND DENSITY FROM ULYSSES RADIO MEASUREMENTS K. ISSAUTIER, N. MEYER-VERNET, M. MONCUQUET and S. HOANG Observatoire de Paris, DESPA, URA 264 CNRS, 92195 Meudon, France (Received 16 July 1996; accepted 3 February 1997) Abstract. We present new in situ measurements of solar wind electron density as a function of heliolatitude. The data were obtained on Ulysses during its fast transit from south solar pole to north solar pole, at heliocentric distance about 1.5 AU, near the 1996 solar activity minimum. The density is measured accurately using the method of quasi-thermal noise spectroscopy with the Ulysses radio experiment, at a higher time resolution than the particle analysers on board. At low heliolatitudes (22 S to 21 N) the histogram of our data shows three main classes of flows with densities centered at 3.5, 7, and 12 cm 3 , close to the values previously found by near-ecliptic space probes, in the region where fast coronal hole wind alternates with slower streamer belt wind. Poleward of 22 latitude where Ulysses encountered fast wind coming from coronal holes, the histogram of our data shows a single class of flow centered at 2.9 cm 3 with a roughly normal distribution. We find a density nearly independent of latitude, with the mean density from the south coronal hole 10% larger than that from the north, which may stem from a genuine north/south asymmetry and/or from the small decrease in solar activity during the time of the observations. We finally compare the data with some analytical models. 1. Introduction For many years, most in situ interplanetary observations have been made near the ecliptic plane, restricting the detailed three-dimensional heliospheric exploration to a narrow latitude band of 7 . A 3-D picture could only be obtained by indirect methods, such as radio sounding or scintillation, or precarious extrapolations of low latitude data. The Ulysses spacecraft explored for the first time a wide range of heliolatitudes. While approaching perihelion Ulysses achieved a pole-to-pole transit in less than one year near the solar activity minimum. During this quiet period, the heliospheric current sheet (HCS) was flattened and near equatorial, making the solar wind structure relatively simple. Moreover, the heliocentric distance remained nearly constant at about 1.5 AU. These circumstances provided a unique opportunity to study the latitudinal variation of solar wind parameters at solar minimum. At heliolatitudes poleward of 22 S and 21 N, Ulysses encountered continuous fast wind in a speed range of 700 to 800 km s 1 (Phillips et al., 1995), which originates from low density and temperature regions in the solar atmosphere known as coronal holes. At lower latitudes, Ulysses observed alternating streams of high and low speed flows, corresponding respectively to low and high densities, due to its crossings of the warped and tilted HCS, which is the extension of the coronal equatorial streamer belt in the interplanetary space. Solar Physics 172: 335–343, 1997. c 1997 Kluwer Academic Publishers. Printed in Belgium.