Coronal Electron Temperature Inferred from the Strahl Electrons in the Inner Heliosphere: Parker Solar Probe and Helios Observations

Abstract

The shape of the electron velocity distribution function plays an important role in the dynamics of the solar wind acceleration. Electrons are normally modelled with three components, the core, the halo, and the strahl. We investigate how well the fast strahl electrons in the inner heliosphere preserve the information about the coronal electron temperature at their origin. We analysed the data obtained by two missions, Helios spanning the distances between 65 and 215 R S , and Parker Solar Probe (PSP) reaching down to 35 R S during its first two orbits around the Sun. The electron strahl was characterised with two parameters, pitch-angle width (PAW), and the strahl parallel temperature (T s ). PSP observations confirm the already reported dependence of strahl PAW on core parallel plasma beta (β ec )(Berčič et al. 2019). Most of the strahl measured by PSP appear narrow with PAW reaching down to 30 o . The portion of the strahl velocity distribution function aligned with the magnetic field is for the measured energy range well described by a Maxwellian distribution function. T s was found to be anti-correlated with the solar wind velocity, and independent of radial distance. These observations imply that T s carries the information about the coronal electron temperature. The obtained values are in agreement with coronal temperatures measured using spectroscopy (David et al. 1998), and the inferred solar wind source regions during the first orbit of PSP agree with the predictions using a PFSS model Badman et al. 2019).

Parker Solar Probe observations of suprathermal electron flux enhancements originating from Coronal Hole boundaries

“…We therefore conclude that the most likely explanation of these peaks is due to their association with the CH boundaries themselves. While other enhancements in flux are observed during the PSP encounter (Figure 1), this is expected based on previous studies relating suprathermal electron properties (such as temperature) to different sources (Hefti et al 1998;Macneil et al 2017;Berčič et al 2020;Maksimovic et al 2020). Thus CH boundaries are not the sole cause of enhancements in suprathermal electron flux close to the Sun, and this is largely unsurprising.…”

“…This fitting technique was motivated by the exospheric models' prediction that , in absence of collisions and wave-particle interactions, remains unchanged from the solar corona, and preserves information about the electron VDF at its origin. For more details about the fitting procedure see Section 3.1 in Berčič et al (2020).…”

“…The absence of such peaks at 1 AU may be due to the fact that trailing CH boundary solar wind typically forms rarefactions, and again these are less developed near the Sun. Finally, the studies discussed above (Maksimovic et al 2020;Berčič et al 2020;Halekas et al 2020) indicate that suprathermal electrons near the Sun more faithfully reflect source region properties than those at 1 AU. Thus if the conditions of the CH boundary do intrinsically produce peaks in suprathermal flux, then examining the near-Sun solar wind provides the greatest chance of observing these peaks.…”

“…Suprathermal electrons have a far longer collisional mean free path than thermal particles in the solar wind, which has lead to research on their capacity to transmit source region signatures (particularly electron temperature) to 1 AU (e.g., Hefti et al 1999;Macneil et al 2017). Evidence has emerged from PSP observations to suggest that the properties of suprathermal, and even thermal, electrons at distances 0.3 AU are reflective of their source (Halekas et al 2020;Maksimovic et al 2020;Berčič et al 2020). In particular, core, halo, and strahl temperatures in the inner heliosphere appear to anti-correlate with solar wind speed, suggesting that at these distances, these electrons retain the electron temperature signatures of their source.…”

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.…”