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

Abstract

Alfvénic fluctuations in solar wind are an intrinsic property of fast streams, while slow intervals typically have a very low degree of Alfvénicity, with much more variable parameters. However, sometimes a slow wind can be highly Alfvénic. Here we compare three different regimes of solar wind, in terms of Alfvénic content and spectral properties, during a minimum phase of the solar activity and at 0.3 au. We show that fast and Alfvénic slow intervals share some common characteristics. This would suggest a similar solar origin, with the latter coming from over-expanded magnetic field lines, in agreement with observations at 1 au and at the maximum of the solar cycle. Due to the Alfvénic nature of the fluctuations in both fast and Alfvénic slow winds, we observe a well-defined correlation between the flow speed and the angle between magnetic field vector and radial direction. The high level of Alfvénicity is also responsible of intermittent enhancements (i.e. spikes), in plasma speed. Moreover, only for the Alfvénic intervals do we observe a break between the inertial range and large scales, on about the timescale typical of the Alfvénic fluctuations and where the magnetic fluctuations saturate, limited by the magnitude of the local magnetic field. In agreement with this, we recover a characteristic low-frequency 1/f scaling, as expected for fluctuations that are scale-independent. This work is directly relevant for the next solar missions, Parker Solar Probe and Solar Orbiter. One of the goals of these two missions is to study the origin and evolution of slow solar wind. In particular, Parker Solar Probe will give information about the Alfvénic slow wind in the unexplored region much closer to the Sun and Solar Orbiter will allow us to connect the observed physics to the source of the plasma.

On Alfvénic Slow Wind: A Journey From the Earth Back to the Sun

“…The low magnetic compressibility found by Perrone, Bruno, et al. (2020) are in agreement with both results in the inner heliosphere (Bavassano, Dobrowolny, Fanfoni, et al., 1982) and at 1 AU (Perrone et al., 2017) in fast solar wind. The presence of small‐scale coherent structures during PSP’s first encounter has also been highlighted by employing the partial variance of increments (PVI) technique (Greco et al., 2008).…”

“…Moreover, Perrone, Bruno, et al. (2020) have studied magnetic properties at ion scales in three selected periods characterized by different switchbacks activity. They found that this range of scales appears to be characterized by a high level of intermittency, with the presence of non‐compressive coherent events, such as current sheets, vortex‐like structures, and wave packets identified as ion cyclotron modes.…”

“…Please refer to D’Amicis, Matteini, and Bruno (2019) for Wind data; Perrone, D’Amicis, et al. (2020) for Helios data; and Bale et al. (2019) for PSP data.…”

The Effect of Solar-Wind Turbulence on Magnetospheric Activity

“…[106]. These results were further supported by another study by the same authors [107] who presented the first statistical evidence of the presence of a slow Alfvénic solar wind during maximum of solar activity and found to be very similar to the fast wind on many respects and not only for the Alfvénic content of the fluctuations [44,[108][109][110][111][112]. [107] demonstrated that the nature of these kind of fluctuations plays a major role in the geomagnetic activity rather than the type of wind selected on the basis of the flow speed.…”

Energy Supply for Heating the Slow Solar Wind Observed by Parker Solar Probe between 0.17 and 0.7 au

“…Matteini et al (2018) found that the presence of the −1 frequency range could be a result of the saturation of the amplitude of the magnetic fluctuations in the fast wind. D' Amicis et al (2019) reported that the slow solar wind with high Alfvénicity share common characteristics with the fast wind, and Perrone et al (2020) further observed that Alfvénic slow wind shows a break between the inertial range and large scales where the magnetic fluctuations saturate. It seems plausible that this −1 frequency range is related to the observations of Alfvénic switchbacks (de Wit et al 2020;Horbury et al 2020;Mozer et al 2020;McManus et al 2020), close the Sun, that provide most of the energy-containing range in the slow wind.…”