Energy Supply for Heating the Slow Solar Wind Observed by Parker Solar Probe between 0.17 and 0.7 au

Abstract

Energy supply sources for the heating process in the slow solar wind remain unknown. The Parker Solar Probe (PSP) mission provides a good opportunity to study this issue. Recently, PSP observations have found that the slow solar wind experiences stronger heating inside 0.24 au. Here for the first time we measure in the slow solar wind the radial gradient of the low-frequency breaks on the magnetic trace power spectra and evaluate the associated energy supply rate. We find that the energy supply rate is consistent with the observed perpendicular heating rate calculated based on the gradient of the magnetic moment. Based on this finding, one could explain why the slow solar wind is strongly heated inside 0.25 au but expands nearly adiabatically outside 0.25 au. This finding supports the concept that the energy added from the energy-containing range is transferred by an energy cascade process to the dissipation range, and then dissipates to heat the slow solar wind. The related issues for further study are discussed.

Observations of Cross Scale Energy Transfer in the Inner Heliosphere by Parker Solar Probe

“…9 between f −1 and f −5/3 regimes is seen to shift to lower frequencies with increasing heliocentric distance, consistent with earlier studies [128,130,191]. The large-scale break frequency has been shown to follow a power-law variation with heliocentric distance [8,188,192]. The power-law exponent was found to be ∼ 1.1 by [188].…”

“…Fig 14 shows the von Kármán decay rates and proton heating rates esitmated using first three encounters worth of data [211]. The red dots represent the von Kármán decay rate and the lines+shaded regions represent estimates of solar wind heating rates of protons [30,185,192]. The von Kármán decay rate is not only consistent with the energy supply rate estimated from the evolution of the large scale break frequency (see fig.…”

“…The power-law exponent was found to be ∼ 1.1 by [188]. Although [192] did not perform a fit to their data, the qualitative behaviour shown by them is similar to that shown by [8] with an exponent of ∼ 1.5. Interestingly, these variations are seen in previous studies as well [8,140,193,194].…”

“…The outer scales increases as a power-law with increasing heliocentric distance. The power-laws exponents found for outer scale vary between 0.5 − 1.5 whereas theoretical models predict the exponent to be generally less than 1 [140,188,[191][192][193][194]. Intermittency increases from inner heliosphere up to 1au.…”

Observations of cross scale energy transfer in the inner heliosphere by Parker Solar Probe

“…Although Wu et al. ( 2020 ) did not perform a fit to their data, the qualitative behaviour shown by them is similar to that shown by Bruno and Carbone ( 2013 ) with an exponent of . Interestingly, these variations are seen in previous studies as well (Horbury et al.…”