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

Abstract

Introduction"Asking for the average solar wind might appear as silly as asking for the taste of an average drink. What is the average between wine and beer? Obviously mere mixing-and averaging means mixing-does not lead to a meaningful result. Better taste and judge separately and then compare, if you wish." (Reiner Schwenn, Solar Wind 5, 1982) Following this basic idea, the quasi-stationary solar wind observed in the heliosphere has been traditionally categorized into two types based on particle bulk speed (Borovsky, 2012a;Stakhiv et al., 2015;Zurbuchen, 2007), namely fast (V sw > 600 km/s) and slow (V sw < 500 km/s) wind, also characterized by different features ranging from large to small scales. Indeed, slow wind has lower proton temperature and higher density, and in general much more variable properties, with respect to fast wind (Bruno et al., 1986;Lopez & Freeman, 1986;Schwenn, 2006). Moreover, the variability in the observed relative abundance of different charge-states of solar wind ions implies an anti-correlation of the freezing-in temperature with wind speed (Geiss et al., 1995;Kasper et al., 2012), and the thermodynamics of electrons, protons and heavy ions has also been observed to vary with speed (

Domains of Magnetic Pressure Balance in Parker Solar Probe Observations of the Solar Wind

“…It is not surprising that plasma β has a strong effect on domains of approximate magnetic pressure balance, with a qualitative difference between solar wind of β < 1, for which magnetic pressure dominates over plasma pressure, and β ≈ 1, with approximate equipartition between magnetic and plasma energy density and pressure. Previous work has classified the slow solar wind as having two states with different ranges of β (e.g., Griton et al 2021;D'Amicis et al 2021). However, we do find it surprising that our results indicate a quantitative, continuous relationship between the mean domain duration T D and β (see Figure 8), rather than a step or switching function between two extreme states.…”

“…The Parker Solar Probe (PSP) spacecraft, launched in 2018 August, approached within 0.17 au of the Sun during its first three solar encounters (hereafter referred to as E1-E3) and within 0.13 au during its fourth and fifth solar encounters (E4 and E5). PSP data have revealed even stronger Alfvénicity at lower radius (D'Amicis et al 2021;Telloni et al 2021), with increasing dominance of outward-propagating compared with inward-propagating Alfvénic fluctuations (Chen et al 2020). Note that Alfvénicity can be defined in terms of various characteristics; in addition to near uniformity of |B|, it can be measured in terms of the cross-helicity, Alfvén ratio, residual energy, and alignment angle between b and v (Parashar et al 2020).…”

First Solar Orbiter observation of the Alfvénic slow wind and identification of its solar source

“…The gradual motion of the coronal magnetic footpoints of the solar wind field lines measured by Solar Orbiter, from the coronal streamer to pseudostreamer magnetic topology determines changes of the solar wind properties observed by Solar Orbiter in situ. A region of anomalous expansion rate identified as the source of the Alfvénic slow wind (Panasenco et al 2019(Panasenco et al , 2020D'Amicis et al 2021a, and references therein) may form easily when largescale pseudostreamers are present in the corona. The topology of pseudostreamers allows the formation and development of twin filament channels, related to a strong non-monotonic expansion of the open magnetic field lines from the PS coronal hole bound-aries as was shown in ; Panasenco et al (2019).…”

“…This suggests a similar origin for Alfvénic streams: open field regions on the solar surface, namely coronal holes (Stansby et al 2020;Perrone et al 2020;. Although the source regions of the Alfvénic slow wind are still an open question, regions of anomalous, larger than average, expansion rate of magnetic flux tubes near the Sun appear to be a leading candidate (D'Amicis et al 2021a). As shown in Article number, page 2 of 16 ; Panasenco et al (2019Panasenco et al ( , 2020) such regions may form easily when large scale pseudostreamers are present in the corona.…”

Flattening of the Density Spectrum in Compressible Hall-MHD Simulations

“…Still, these observations do not necessarily invalidate any of the previously mentioned theoretical models. The interior of coronal holes contains highly Alfvénic solar wind (see D'Amicis et al [31] and references therein), that is, with Alfvén wave packets propagating mostly in one direction (sunward or antisunward) and thus affecting the development of turbulence at large scales (Dobrowolny et al [32], Lithwick et al [33]). The possible effect of high cross helicity plasma on the density transition region was not taken into account by any of the mentioned theoretical models and could explain the divergence with coronal holes measurements.…”