Total electron temperature derived from quasi-thermal noise spectroscopy in the pristine solar wind from Parker Solar Probe observations

Abstract

Aims. We applied the quasi-thermal noise (QTN) method to Parker Solar Probe (PSP) observations to derive the total electron temperature (Te). We combined a set of encounters to make up a 12-day period of observations around each perihelion from encounter one (E01) to ten (E10), with E08 not included. Here, the heliocentric distance varies from about 13 to 60 solar radii (R⊙). Methods. The QTN technique is a reliable tool to yield accurate measurements of the electron parameters in the solar wind. We obtained Te from the linear fit of the high-frequency part of the QTN spectra acquired by the RFS/FIELDS instrument. Then, we provided the mean radial electron temperature profile, and examined the electron temperature gradients for different solar wind populations (i.e. classified by the proton bulk speed, Vp, and the solar wind mass flux). Results. We find that the total electron temperature decreases with the distance as ∼R−0.66, which is much slower than adiabatic. The extrapolated Te based on PSP observations is consistent with the exospheric solar wind model prediction at ∼10 R⊙, Helios observations at ∼0.3 AU, and Wind observations at 1 AU. Also, Te, extrapolated back to 10 R⊙, is almost the same as the Strahl electron temperature, Ts (measured by SPAN-E), which is considered to be closely related to or even almost equal to the coronal electron temperature. Furthermore, the radial Te profiles in the slower solar wind (or flux tube with larger mass flux) are steeper than those in the faster solar wind (or flux tube with smaller mass flux). The more pronounced anticorrelation of Vp–Te is observed when the solar wind is slower and located closer to the Sun.

Quantifying the Energy Budget in the Solar Wind from 13.3 to 100 Solar Radii

“…, and the electric potential with respect to infinity is then ef e = 5/2 k B T e , which exactly replicates the electron enthalpy term in the electron energy flux. However, for the observed electron temperature dependence, T e = T 0 r − α , with α ; 0.5-0.66 (Halekas et al 2022;Liu et al 2023), we would instead expect an electric potential of ef e = (2 + α)/α k B T e , consistent with that in fact observed by PSP (Halekas et al 2021(Halekas et al , 2022. One can regard this as a polytropic behavior of the electrons, with γ = 1 + α/ 2 ; 1.25-1.33, consistent with the electron polytropic index derived from other considerations (Dakeyo et al 2022).…”

Fundamental–Harmonic Pairs of Interplanetary Type III Radio Bursts

“…The presence of a standard deviation in this context is attributed to the variability of the pre-event background, which is dependent on the electron density/temperature during each specific period of the bursts. In combination with the fact that the measurements were conducted during active CEs, this may provide an explanation for the observed shallower scaling law in comparison to the one obtained by Liu et al (2023), i.e., f −3 . An important point to clarify is that we did not differentiate between the quasi-thermal noise (QTN) and galactic noise.…”

“…The F component is observed continuing into low frequencies (<1 MHz), while H is observed less at low frequencies. This may be partly due to the domination of QTN close to the local plasma frequency f p (the spectral tail extends beyond f p depending on the electron density/temperature; Zaslavsky et al 2011;Liu et al 2023), which ranged between ∼400 kHz and ∼1 MHz, on average, between the first and 10th CEs. For reference, the f p at a spacecraft located at 1 au is ∼20 kHz.…”

Solar Wind Density and Core Temperature Derived from the PSP Quasi-thermal Noise Measurements

“…Note that the ARF involved in the calculation of the electron QTN has been accurately determined using the AWAS software for PSP in M22. In addition, the antenna gain factor Γ = 0.33 has been adopted as in Moncuquet et al (2020) and Liu et al (2023).…”