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.

The supersonic solar wind, first predicted by Parker and then observed by Mariners, extends to form a heliosphere around the Sun. The energy supply from the energy containing range, the energy cascade though the inertial range, and the eventual energy dissipation are three basic processes of the energy transfer in the solar wind and have been studied for a long time. However, some basic issues remain to be discovered. Here, we review the recent progress in the mechanisms of energy transfer of the solar wind turbulence from the observational perspective. Based on the Parker solar probe observations, the energy supply mechanism by the low-frequency break sweeping is proposed to provide enough energy for the proton heating in the slow solar wind. This mechanism also works in the fast solar wind. The energy flux by the low-frequency break sweeping is consistent with that by the classical von Kármán decay mechanism. For the energy cascade in the inertial range, the scaling behavior of the third-order structure functions demonstrates the effect of the complex dynamics of the solar wind. The process of energy transfer is fundamental to understand the solar wind turbulence and help to construct the model of the space environment.

The intermittency domain is in the magnetohydrodynamic inertial range but just above the dissipation range and contains the intermittent structures that influence the spectral index. However, the nature of these structures is still under debate and how they affect the higher-order scaling behavior in the near-Sun solar wind turbulence remains to be investigated. Here we use the magnetic field data measured by Parker Solar Probe in the near-Sun solar wind. We identify the intermittency by partial variance of increments method and conduct the multiorder structure functions analyses in the intermittency domain before and after removing the intermittency. We also perform for the first time the scaling analyses on the fluctuations of magnetic field direction given by the rotational angle θ = cos − 1 ( B 1 · B 2 ) / ∣ B 1 ∣ ∣ B 2 ∣ , in which B ₁ and B ₂ are the magnetic fields measured at two instants. We find that the multifractal scaling of the magnetic field could be predicted by the log-Poisson intermittency model with 2D sheetlike structures and a simple Kolmogorov −5/3 monoscaling recovers after the removal of the intermittency. We also find a similar behavior for the scalings of θ, suggesting that magnetic discontinuities are involved in the energy transfer process of solar wind turbulence.

Switchbacks are widely acknowledged phenomena observed by the Parker Solar Probe and appear to occur in patches. Previous studies focused on the fluctuations at the magnetic reversals. However, the nature of the fluctuations inside the switchbacks remains unknown. Here we utilize the magnetic field data and plasma data measured by the Parker Solar Probe in the first four encounters. We investigate the fluctuations in the switchback intervals of 100 s with at every instant and compare them to the fluctuations in the nonswitchback intervals of 100 s with at every instant. We calculate normalized cross-helicity , normalized residual energy , correlation coefficient C _vb between and , Alfvén ratio r _A, and the amplitude of magnetic and kinetic fluctuations. We find that the switchback intervals exhibit a distribution of similar with the nonswitchback intervals. However, the r _A of switchback intervals is around 0.35, while the nonswitchback intervals have r _A around 0.65, indicating the fluctuations in the switchbacks are more magnetically dominated. We also find that the distribution pattern of pixel average amplitude of both and of switchback intervals in the C _vb– plane show a vertical stripe feature at , illustrating the possible magnetically dominant magnetic-velocity alignment structure. These results will help us to understand the nature and the formation of the switchback turbulence.

Solar wind protons undergo significant heating during the expansion of the solar wind and turbulence plays an important role in this process. It is believed that the energy is injected from the energy-containing range into the inertial range and then transferred to dissipate into heat eventually. However, the energy injection process in the heliosphere remains unclear. Here we analyze this process. We utilize Helios 2 and Ulysses measurements of the fast solar wind at different radial distances from 0.29 to 4.8 au. We obtain the perpendicular heating rate based on the gradient of the magnetic moment. We estimate for the first time the energy supply rate due to the sweeping of low-frequency break based on the identification of low-frequency break and the corresponding power spectra density profile. We find that the energy supply rate is comparable to the perpendicular heating rate of protons, which support the idea that low-frequency range becomes part of the inertial range as the solar wind turbulence ages. These results help us understand the energy supply process from the energy-containing range and the heating process of solar wind protons.