Anticorrelation between the Bulk Speed and the Electron Temperature in the Pristine Solar Wind: First Results from the Parker Solar Probe and Comparison with Helios

Abstract

We discuss the solar wind electron temperatures T e as measured in the nascent solar wind by Parker Solar Probe during its first perihelion pass. The measurements have been obtained by fitting the high frequency part of Quasi-Thermal Noise spectra recorded on-board by the Radio Frequency Spectrometer (Pulupa et al. 2017). In addition we compare these measurements with those obtained by the electrostatic analyzer Halekas & al. (2019). These first electron observations show an anti-correlation between T e and the wind bulk speed V : this anti-correlation is most likely the remnant of the well-known mapping observed at 1 AU and beyond between the fast wind and its coronal hole sources, where electrons are observed to be cooler than in the quiet corona. We also revisit HELIOS electron temperature measurements and show, for the first time, that an in-situ (T e , V) anti-correlation is well observed at 0.3 AU but disappears as the wind expands, evolves and mixes with different electron temperature gradients for different wind speeds.

Solar Wind Plasma Particles Organized by the Flow Speed

“…(Domingo, 2002;Meyer-Vernet, 2007;Lazar, 2012), enabling also 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 new observations from Parker Solar Probe (PSP) and Solar Orbiter (SO) are expected to confirm these predictions, and explain the main mechanisms which trigger plasma outflows and modulate the main properties of the SW plasma particles, i.e., electrons, protons and heavier ions (Berčič et al, 2020;Halekas et al, 2020;Maksimovic et al, 2020). Measured in-situ, the velocity distributions of different species (subscript s) are used to determine their macroscopic properties (moments), such as number densities n s , bulk velocities u s , temperatures T s , and, implicitly, their plasma betas β s = 8πn s k B T s /B 2 0 , or temperature anisotropy T s,⊥ /T s, , with ⊥, indicating gyrotropic directions with respect to the magnetic field.…”

“…It is less clear at larger distances probably due to mix of solar wind particles of different speeds as the solar wind expands. With the new observations of Parker Solar Probe at even lower radial distances (0.25 AU), this anti-correlation is even more obvious (Maksimovic et al, 2020). This can again be a remnant of coronal hole sources, where electrons of fast wind are observed to be cooler than in the quiet corona.…”

“…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., β c ≈ 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). These anti-correlations are often invoked to argue that slow or fast winds have different origins in the corona (Gloeckler, Zurbuchen, and Geiss, 2003), despite the very high mobility of electrons, and the fact that proton temperature shows a direct link with the solar wind speed (Elliott et al, 2012).…”

The Solar Orbiter mission

“…the model calculations of Smith et al 2012), these models (e.g. Zouganelis et al 2004Zouganelis et al , 2005 can successfully predict some aspects of the in-situ wind properties at 1 AU and explain some of the electron properties as observed in the inner heliosphere by Helios (Berčič et al 2019) and Parker Solar Probe (Moncuquet et al 2020;Martinović et al 2020;Maksimovic et al 2020b;Halekas et al 2020). The velocity filtration mechanism, which is a physical phenomenon of the solar wind (for a thorough discussion on controversies and uncontroversial mechanisms, see Cranmer et al 2017), like any other coronal heating theory requires converting some other form of energy (i.e.…”

Sensitivity of solar wind mass flux to coronal temperature

“…Habbal et al 1993;Esser et al 1995) and in situ measurements of solar wind temperatures at 1 AU have been significantly distorted from their coronal values (e.g. Marsch et al 1983;Stansby et al 2019b;Maksimovic et al 2020).…”