The origin of slow Alfvénic solar wind at solar minimum

Abstract

Although the origins of slow solar wind are unclear, there is increasing evidence that at least some of it is released in a steady state on over-expanded coronal hole magnetic field lines. This type of slow wind has similar properties to the fast solar wind, including a high degree of Alfvénicity. In this study a combination of proton, alpha particle, and electron measurements are used to investigate the kinetic properties of a single interval of slow Alfvénic wind at 0.35 AU. It is shown that this slow Alfvénic interval is characterised by high alpha particle abundances, pronounced alpha-proton differential streaming, strong proton beams, and large alpha to proton temperature ratios. These are all features observed consistently in the fast solar wind, adding evidence that at least some Alfvénic slow solar wind also originates in coronal holes. Observed differences between speed, mass flux, and electron temperature between slow Alfvénic and fast winds are explained by differing magnetic field geometry in the lower corona.

Highly Alfvénic slow solar wind at 0.3 au during a solar minimum: Helios insights for Parker Solar Probe and Solar Orbiter

“…These observations in the inner heliosphere and at a minimum of the solar cycle are almost in agreement with the measurements at 1 au and at a maximum of the solar activity (D'Amicis et al 2019a). This suggests that the characteristics of Alfvénic slow wind are set by the solar source of this wind, probably from overexpanded coronal holes (D'Amicis & Bruno 2015;Stansby et al 2020), and do not change, with respect to the fast and non-Alfvénic winds, during the radial evolution of the plasma, at least above the Alfvén point. In this context, Parker Solar Probe will play a crucial role in giving information about the Alfvénic slow wind in the unexplored region much closer to the Sun.…”

“…The present paper complements the work done by Stansby et al (2019bStansby et al ( , 2020, extending the analysis to areas that were not covered. In particular, by focusing on the first perihelion of the Helios mission during a minimum phase of the solar activity, in this paper we investigate and compare the characteristics of solar wind at about 0.3 au in terms of Alfvénic content, amplitude level of fluctuations, and spectral properties in different wind regimes.…”

“…The Alfvénic slow wind shares common characteristics with the fast wind, which suggests that they could have similar origin: coronal holes (D'Amicis & Bruno 2015). More recently, a thorough analysis of the Alfvénic slow wind was performed during a minimum of the solar activity in the inner heliosphere, supporting the theory that Alfvénic slow wind originates in open field lines rooted in coronal holes, where the differences with fast wind, for example the speed, could be explained by a different magnetic field geometry in the lower corona (Stansby et al 2019b(Stansby et al , 2020. Moreover, with respect to the measurements within an ascending phase of the solar cycle described in Marsch et al (1981), the Alfvénic slow wind observed in a solar minimum by Stansby et al (2019bStansby et al ( , 2020 shows almost isotropic proton distribution functions, as in the non-Alfvénic slow wind.…”

“…We find that Alfvénic slow wind at 0.3 au has the same low speed observed for the non-Alfvénic slow wind, and the proton temperature is also comparable (with the Alfvénic slow interval a bit hotter), which is much lower than that of the fast stream (see Table 1). Moreover, both slow winds are characterised by an isotropic proton distribution function, while in fast solar wind the protons are strongly anisotropic in the perpendicular direction with respect to the magnetic field (Stansby et al 2019b(Stansby et al , 2020. For the density and magnetic field magnitude, the Alfvénic slow wind is similar to the fast wind, both characterised by lower and almost constant values with respect to the non-Alfvénic slow wind, which instead shows a higher magnetic and plasma compression.…”

“…More recently, Alfvénic slow wind has been observed at 1 au during the maximum of the solar activity, and thoroughly studied also in comparison with fast and typical slow solar wind (D'Amicis & Bruno 2015;D'Amicis et al 2019a). Only recently has Alfvénic slow wind been identified during a minimum of the solar activity and in the inner heliosphere (Stansby et al 2019b(Stansby et al , 2020, but no analysis of the spectral characteristics has been presented. Therefore, for the first time, here we address the spectral properties of Alfvénic slow wind during a minimum of the solar cycle and close to the Sun.…”