Solar wind observations over Ulysses' first full polar orbit

TLDR

This study examines solar wind plasma and magnetic field observations from Ulysses' first full polar orbit to characterize the high‑latitude solar wind during decreasing and low solar activity. The authors compare observations across nearly all heliolatitudes and two intervals at the same radial distances, and analyze six years of data from the declining phase to the recent minimum, to separate radial and latitudinal variations and assess hemispheric asymmetries. After removing radial gradients, high‑latitude solar wind parameters vary little with latitude, indicating uniformity from polar coronal holes, while hemispheric asymmetries driven by source differences during the solar cycle reveal more energy in the polar wind during the declining phase, with larger mass flux but similar speeds, implying energy injection below the acceleration critical point, and the study supplies statistics for over 20 parameters to facilitate comparison with future observations.

Abstract

This study examines solar wind plasma and magnetic field observations from Ulysses' first full polar orbit in order to characterize the high‐latitude solar wind under conditions of decreasing and low solar activity. By comparing observations taken over nearly all heliolatitudes and two different intervals covering the same radial distances, we are able to separate the radial and latitudinal variations in the solar wind. We find that once the radial gradients are removed, none of the high‐latitude solar wind parameters show much latitudinal variation, indicating that the solar wind emanating from the polar coronal holes is extremely uniform. In addition, by examining nearly 6 years of data starting in the declining phase of the last solar cycle and extending through the most recent solar minimum, we are able to address hemispheric asymmetries in the observations. We find that these asymmetries are most likely driven by differences in the solar wind source over the solar cycle and indicate that more energy goes into the polar solar wind during the declining phase of the solar cycle than around minimum. Because the mass flux is larger in the declining phase while the speeds are very similar, we conclude that this energy is introduced at an altitude below the solar wind acceleration critical point. Finally, we provide details of the statistics of over 20 solar wind parameters so that upcoming observations from Ulysses' second polar orbit, during much more active times on the Sun, can be readily compared to the quieter first orbit results.