Ulysses plasma parameters : latitudinal , radial , and temporal variations

Observations by the Ulysses SWOOPS plasma experiment are used to investigate spatial and temporal gradients during the mission, with emphasis on more recent high latitude observations including the recent South Pole to North Pole passage during solar minimum. Compared to lower latitudes, the high latitude solar wind had higher average speed, proton temperature, and momentum flux, and lower number flux density. As the average momentum flux observed in the high speed wind was 21% greater than at the equator, during solar minimum the distance to the heliopause will be comparatively less in the solar equatorial plane than over the poles. The long term temporal gradients of momentum flux over the life of the mission are considerably larger than the latitudinal gradient observed by Ulysses during solar minimum. A modest North-South high latitude asymmetry is observed in the plasma parameters; the velocity is on the average 13 km/s to 24 km/s greater at Northern latitudes than at Southern, and temperature is also higher. The North-South temperature asymmetry is greater than can be explained by the North-South velocity difference and the dependence of solar wind temperature upon speed. The power law dependence of temperature on heliocentric distance, r, at high latitudes is in range r −0.81to−1.03 , where r −0.81 is the Southern latitude result and r −1.03 the Northern. The parameter T/n 1/2 , where T is temperature and n is proton number density, can be better predicted from speed than can temperature alone. Comparison with calculations based on source models and magneto-graph data indicate that the expansion of open coronal field lines close to the Sun was greater in the Southern hemisphere than in the Northern; this anticorrelation with the expansion factor is consistent with previous observational and theoretical work.