Abstract
Recent reports of the first data from Parker Solar Probe (PSP) have pointed to a series of links, correlations or anti-correlations between the solar wind bulk speed (V_{mathrm{SW}}) and physical properties of plasma particles from less than 0.25 AU in the corona. In the present paper, we describe corresponding and additional links of solar wind properties, at 0.4 AU and 1.0 AU, in an attempt to complement the PSP data and understand their evolution. A detailed analysis is carried out for the main electron populations, comparing the low-energy (thermal) core and the collisionless suprathermal halo. We show that the anti-correlation observed at 0.4 AU between V_{mathrm{SW}} and the number density (average value) is maintained also at 1 AU for both the core and halo electrons. On the contrary, only the core electrons manifest a clear anti-correlation of the temperature with V_{mathrm{SW}}, while the halo temperature does not vary much. We also describe the ions, protons and helium, which have a more reduced mobility and their properties exhibit different variations with the solar wind speed. The results are used to shed more light on the mechanisms leading to a differential acceleration of these species and the origin of slow and fast wind modulation.
Highlights
Properties of the solar wind (SW) plasma particles have been measured in-situ by a multitude of spacecraft missions, e.g., Ulysses, Helios 1 and 2, Wind, Cluster, etc. (Domingo, 2002; Meyer-Vernet, 2007; Lazar, 2012), enabling indirect interpretations of plasma physics at low heliocentric distances in the corona where direct in-situ measurements were not possible (Scudder, 1992; Pierrard, Maksimovic, and Lemaire, 2001a,b)
Two solar wind types were identified by ULYSSES when flying at different latitudes: the high-speed solar wind associated to low density wind at high latitudes during minimum solar activity, and the low-speed solar wind much more variable and associated to high n, appearing at low latitudes during minimum solar activity (McComas and et al, 1998)
The velocity is strongly correlated to the temperature of the protons while it is anti-correlated to the temperature of the core and halo electrons
Summary
Properties of the solar wind (SW) plasma particles have been measured in-situ by a multitude of spacecraft missions, e.g., Ulysses, Helios 1 and 2, Wind, Cluster, etc. (Domingo, 2002; Meyer-Vernet, 2007; Lazar, 2012), enabling indirect interpretations of plasma physics at low heliocentric distances in the corona where direct in-situ measurements were not possible (Scudder, 1992; Pierrard, Maksimovic, and Lemaire, 2001a,b). The plasma beta and temperature anisotropy of these quasi-thermal electrons do not change much from 0.35 to 2 AU, and remain well localized around the equipartition conditions, i.e., βc ≈ 1 (Štverák et al, 2008) Both the number density and the temperature show a clear anti-correlation with the flow speed in the corona (Halekas et al, 2020) and in the solar wind at low distance, becoming barely noticeable at 1 AU (Maksimovic et al, 2020). Each species or population of plasma particles, including thermal or suprathermals, e.g., core and halo electrons, may show specific variations with the bulk speed of the solar wind, which can help to understand their origin in the solar corona (e.g., slow winds) or more inner corona (fast winds from coronal holes) This information is encoded in the properties of plasma particles, velocity distributions and moments of the distributions defining macroscopic parameters, which vary continuously with the solar wind expansion in the heliosphere. Understanding these variations may help to identify key physical processes which overheat the corona and accelerate the solar wind
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