Electrons in the Young Solar Wind: First Results from the Parker Solar Probe

Abstract

The Solar Wind Electrons Alphas and Protons experiment on the Parker Solar Probe (PSP) mission measures the three-dimensional electron velocity distribution function. Halekas et al.We derive the parameters of the core, halo, and strahl populations utilizing a combination of fitting to model distributions and numerical integration for ∼ 100, 000 electron distributions measured near the Sun on the first two PSP orbits, which reached heliocentric distances as small as ∼ 0.17 AU. As expected, the electron core density and temperature increase with decreasing heliocentric distance, while the ratio of electron thermal pressure to magnetic pressure (β e ) decreases. These quantities have radial scaling consistent with previous observations farther from the Sun, with superposed variations associated with different solar wind streams. The density in the strahl also increases; however, the density of the halo plateaus and even decreases at perihelion, leading to a large strahl/halo ratio near the Sun. As at greater heliocentric distances, the core has a sunward drift relative to the proton frame, which balances the current carried by the strahl, satisfying the zero-current condition necessary to maintain quasineutrality. Many characteristics of the electron distributions near perihelion have trends with solar wind flow speed, β e , and/or collisional age. Near the Sun, some trends not clearly seen at 1 AU become apparent, including anti-correlations between wind speed and both electron temperature and heat flux. These trends help us understand the mechanisms that shape the solar wind electron distributions at an early stage of their evolution.

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.…”

“…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).…”

“…However, we do not know much about the evolution of halo electrons and their properties with heliocentric distance, or the way these properties correlate with the solar wind speed. Recent reports of PSP data have provided the first radial profiles of electron properties in the corona, within the interval 0.17 -0.4 AU, showing a clear anti-correlation of the halo temperature with the solar wind speed (Halekas et al, 2020).…”

Plasma Double Layers at the Boundary Between Venus and the Solar Wind

“…Blue boxes indicate bow shock crossing times (vertical lines in Figures 1a-1d). In Figure 1i (following the method of Halekas et al, 2020). Figures 2f and 2l show electron core temperature determined by the same fits.…”

Constraining Ion-Scale Heating and Spectral Energy Transfer in Observations of Plasma Turbulence

“…We consider frequencies up to 100 Hz (k ⊥ ρ i ∼ 10), to avoid the SCM noise floor. Average plasma properties are computed for each interval using solar wind electrons alphas and protons (SWEAP) investigation data [62]; n i and T i from the SWEAP-solar probe cup (SPC) [72] and n e , T e from SWEAP-solar probe analyzers PHYSICAL REVIEW LETTERS 125, 025102 (2020) 025102-2 (SPAN) electron fits [73,74]. On average, β i (β e ) is 0.26 (0.74) with a standard deviation of 0.13 (0.25), with…”