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

Abstract

A variety of energy sources, ranging from dynamic processes, such as magnetic reconnection and waves, to quasi-steady terms, such as plasma pressure, may contribute to the acceleration of the solar wind. We utilize a combination of charged particle and magnetic field observations from the Parker Solar Probe (PSP) to attempt to quantify the steady-state contribution of the proton pressure, the electric potential, and the wave energy to the solar wind proton acceleration observed by PSP between 13.3 and ∼100 solar radii (R ☉). The proton pressure provides a natural kinematic driver of the outflow. The ambipolar electric potential acts to couple the electron pressure to the protons, providing another definite proton acceleration term. Fluctuations and waves, while inherently dynamic, can act as an additional effective steady-state pressure term. To analyze the contributions of these terms, we utilize radial binning of single-point PSP measurements, as well as repeated crossings of the same stream at different distances on individual PSP orbits (i.e., fast radial scans). In agreement with previous work, we find that the electric potential contains sufficient energy to fully explain the acceleration of the slower wind streams. On the other hand, we find that the wave pressure plays an increasingly important role in the faster wind streams. The combination of these terms can explain the continuing acceleration of both slow and fast wind streams beyond 13.3 R ☉.

Energy Budget in the Solar Corona

“…Some contribution to wind driving may also come from the electric potential energy (the third largest energy flux), which furthermore exceeds the kinetic energy term at altitudes slightly less than 6 R e (blue curve). This is in agreement with the results found by Halekas et al (2023), which show that this form of energy can account for proton acceleration of the lower-speed streams. Enthalpy and heat fluxes (green and gray curves, respectively), on the other hand, are expected to become important only much lower, around 2-4 R e , at the temperature maximum (e.g., Antonucci et al 2005).…”

“…One viable approach is to consider that their dependence, as derived from PSP observations in the very inner heliosphere at the lowest limit of 13.3 R e also holds slightly below, down to 6.3 R e . Halekas et al (2023) showed that in the slow solar wind the perpendicular component of the proton temperature scales approximately as r −0.25 (in agreement with the results provided by Wu et al 2020), while the parallel component does not exhibit appreciable variation with heliocentric distance (in accordance with Chandran et al 2011), at least in the slowest flows. On the other hand, Zhao et al (2019) found that the near-Sun proton temperature decreases with height as r −1 .…”

“…Similar work has already been carried out in interplanetary space at different distances and latitudes (Meyer-Vernet 2007;McComas et al 2014;Liu et al 2021;Verscharen et al 2021), down to the upper limit of the solar corona (Halekas et al 2023), albeit not considering all different forms of energy. In the corona, however, this survey is still lacking.…”

Electron–Ion Heating Partition in Imbalanced Solar-wind Turbulence

“…Under the assumption that, far from the Sun, faster wind is heated more than slower wind (Hansteen & Leer 1995;Totten et al 1995;Halekas et al 2023), our results explain qualitatively various interesting properties of observed temperature profiles. Close to the Sun, proton and minor-ion temperatures correlate positively with wind speed (Burlaga & Ogilvie 1973), while the electron temperature is negatively correlated (Marsch et al 1989); this is natural if the highly ), while the dashed line shows an f i that is independent of w ⊥ (as expected from stochastic heating).…”

The Regulation of the Solar Wind Electron Heat Flux by Wave–Particle Interactions

“…The escaping population does not return to repopulate the sunward side of the electron VDF, resulting in a decrease in the phase space density on the sunward side (Jockers 1970;Landi et al 2012). This asymmetry, called a deficit or cutoff in energy, is set by the escape energy of electrons in the potential well, and it has been measured to empirically estimate the contribution of the ambipolar potential to the acceleration of the solar wind (Berčič et al 2020;Halekas et al 2020;Berčič et al 2021a;Halekas et al 2023). As an asymmetry in the local VDF, the deficit contributes to the heat flux in the near-Sun region (Halekas et al 2020(Halekas et al , 2023.…”