Solar wind observations over Ulysses' first full polar orbit

Abstract

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 hellolatitudes 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. IntroductionThe Ulysses spacecraft recently completed its first full, nearly polar orbit of the Sun. This orbit allowed Ulysses to measure directly the solar wind plasma and field properties of the high-latitude heliosphere for the first time. Ulysses data have already led to a host of new discoveries which defied our pre-Ulysses understanding of the heliosphere and have al- As an addition to Plate 1, we have superposed a trace of the logarithm of the proton density (green line) on the original polar plot. Owing to the large variability in the solar wind density, we smoothed the density observations for this summary plot using a one-half solar rotation-long running boxcar average. In addition, because of the large variation in radial distance, R, from the Sun over Ulysses' orbit, we have scaled the densities to their 1 AU values by multiplying by R 2, where R is in AU. Plate 1 clearly shows that the density is higher at 10,419

Latitudinal gradients of solar wind speed around the ecliptic: Systematic displacement of the streamer belt

“…Therefore the annual variation in SW speed observed at 1 AU is not due to different speeds in the two magnetic hemispheres. This conclusion is in a good agreement with the rough equality of the average SW speed originating from the northern and southern coronal holes [McComas et al, 2000].…”

Connecting the Sun and the Solar Wind: Comparison of the Latitudinal Profiles of Coronal and [ITAL]Ulysses[/ITAL] Measurements of the Fast Wind

“…The first two polar passes between 1993 and 1997 occurred during the declining phase and minimum in activity cycle of the Sun. Highlights of the scientific returns of the first completion of the pole-to-pole pass have been recently summarized by McComas et al (2000). The most noted result pertaining to the study reported here was the predominance of the fast solar wind exceeding 650 km s Ϫ1 at latitudes above ‫03(ע‬Њ-40Њ).…”

Helium energetics in the high‐latitude solar wind: Ulysses observations

“…If this occurs, then the conditions for instability are met, possibly causing the particle to slow rapidly to the observed situation of vc, p < VA. We will return to this point later in section 4, where we discuss a possible instability mechanism that may operate in this regime. We note that a similar argument is used by Goldstein et al [2000] in the context of proton-proton differential streaming to explain why the bulk of the observed proton-proton speed differences are considerably below the threshold for their proposed instability-driven damping mechanism.…”