Diagnosing solar wind origins usingin situmeasurements in the inner heliosphere

Abstract

Robustly identifying the solar sources of individual packets of solar wind measured in interplanetary space remains an open problem. We set out to see if this problem is easier to tackle using solar wind measurements closer to the Sun than 1 AU, where the mixing and dynamical interaction of different solar wind streams is reduced. Using measurements from the Helios mission, we examined how the proton core temperature anisotropy and cross helicity varied with distance. At 0.3 AU there are two clearly separated anisotropic and isotropic populations of solar wind, that are not distinguishable at 1 AU. The anisotropic population is always Alfvénic and spans a wide range of speeds. In contrast the isotropic population has slow speeds, and contains a mix of Alfvénic wind with constant mass fluxes, and non-Alfvénic wind with large and highly varying mass fluxes. We split the in-situ measurements into three categories according these observations, and suggest that these categories correspond to wind that originated in the core of coronal holes, in or near active regions or the edges of coronal holes, and as small transients form streamers or pseudostreamers. Although our method by itself is simplistic, it provides a new tool that can be used in combination with other methods for identifying the sources of solar wind measured by Parker Solar Probe and Solar Orbiter.

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

“…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.…”

The origin of slow Alfvénic solar wind at solar minimum

“…and is calculated in the same manner as Stansby et al (2019b). v = vp − vp0 are the proton velocity fluctuations in the Alfvén wave frame, vp0 is chosen to maximise the value of |σc|, and b = vA (B/ |B|) is the magnetic field in velocity units.…”

“…One of the defining features of fast solar wind is a lack of variance in number density, velocity, and temperature, aside from pure Alfvénic fluctuations (Belcher & Davis 1971;Bame et al 1977). A steady background state with superimposed Alfvén waves is also observed in situ in a large amount of the slower solar wind at all stages of the solar cycle (D'Amicis & Bruno 2015; E-mail: david.stansby14@imperial.ac.uk Stansby et al 2019b), implying that close to the Sun it may also be heated and released into the heliosphere in a steady state manner on open field lines.…”

“…Some of these features are destroyed by the time the solar wind has propagated to 1 AU, necessitating the use of data from the Helios mission, which measured the solar wind from 0.3 -1 AU. Three intervals are studied, initially identified by Stansby et al (2019b) as a) typical fast solar wind, b) Alfvénic slow solar wind, and c) non-Alfvénic slow solar wind. Our comparison of both the bulk and kinetic features of fast and slow Alfvénic intervals shows that the slow Alfvénic period most likely originated from a small, overexpanded coronal hole.…”

Bimodal distribution of the solar wind at 1 AU

“…Nevertheless, slow and fast solar wind intervals are clearly identified at 1 AU. In order to identify the solar sources of individual packets of solar wind, Stansby et al (2018) used solar wind measurements closer to the Sun than 1 AU, where the mixing and dynamical interaction of different solar wind streams is reduced. After removing all the intervals identified as coronal mass ejections from the data, these latter authors found three different populations at 0.3 AU, corresponding to wind that originated: (1) in the core of coronal holes, (2) in or near active regions or edges of coronal holes, and (3) in small transients.…”