Bulk properties of the slow and fast solar wind and interplanetary coronal mass ejections measured by Ulysses: Three polar orbits of observations

Abstract

[1] We examined plasma and magnetic field observations from all three Ulysses polar orbits of the Sun to study the properties of the slow and fast solar wind and interplanetary coronal mass ejections (ICMEs). We derived equations to characterize the radial and latitudinal variations for these three types of heliospheric plasma and identify distinguishing features in their spatial variations. Most notably, the slow-wind proton temperature falls less rapidly with distance than does the fast wind, indicating a source of enhanced heating in the low-speed wind. After removing the radial variations from the measurements, only minor latitudinal gradients were identified. The fast wind has now been shown to be only weakly dependent on solar latitude for two successive solar minima. The spatial variations in the ICME properties do not differ significantly from the slow and fast solar wind, although the variability in their parameters is much larger. We also investigated solar cycle variations in the fast polar coronal hole (PCH) flows by comparing their properties measured over Ulysses' 1st and 3rd orbits. While the latitudinal gradients were similar, slight differences were observed in the radial dependence for the proton density and magnetic field strength. Also, a slight reduction in the proton speed at 1 AU, along with more significant decreases in the proton temperature, density, dynamic pressure, and magnetic field strength, was observed for the 3rd orbit relative to that for the 1st. These results are consistent with recent observations of weaker PCH flows for the current solar minimum.

Half a Century of Kinetic Solar Wind Models

“…While these transverse temperatures are much smaller than those generally observed, however, the average temperatures <T p > and <T e > are in good agreement with those observed in the quiet or slow SW [26][27][28][29] , as indicated by the error bars in panel "d".…”

Modeling the Solar Wind at the Ulysses, Voyager, and New Horizons Spacecraft

“…A similar pattern is observed around 1997.0 and 2006.0 during which Ulysses traverses mid-latitude regions between PCHs and the equa-torial band characterized by the interaction of slow wind and PCH fast wind streams [e.g., Gosling (1996)]. Occasional coronal mass ejection (CME) and comet tail passages also contributed to the discrepancy between the model and spacecraft data [see Ebert et al (2009), Elliott et al (2012) and references therein]. The model |B| at latitudes above ±60°-70°is also systematically lower than Ulysses data by up to 30 % suggesting that we may have underestimated magnetic field values near the poles at the inner boundary, but an accurate polar magnetic field estimation is beyond the scope of our study.…”

“…At the inner boundary, the solar wind velocity changes from 800 km s −1 at the center (poles) to ∼700 km s −1 at the edges of PCHs. Empirical correlations from Ulysses data are used to estimate the PCH density and temperature as a function of solar wind radial velocity (Ebert et al 2009;Pogorelov et al 2013) using the following formulae:…”

Stellar wind interaction and pick-up ion escape of the Kepler-11 “super-Earths”

“…Given that the planets are likely to orbit close to the stellar equatorial plane (cf. Table 1), by analogy with the Sun, it is reasonable to assume that they experience mostly slow solar wind conditions, which dominate in the equatorial plane throughout most of the solar cycle for the solar wind (Ebert et al 2009). In this model, we assume a particle number density at the base of the wind of 7.0×10 8 cm −3 and a temperature of 1.625×10 6 K. The α parameter varies smoothly from a value of 1.22 near the stellar surface to 1.42 at 23 R st and is then uniform out to 1 AU.…”