Observational Evidence for Solar Wind Proton Heating by Ion‐Scale Turbulence

Abstract

Based on in situ measurements by Wind spacecraft from 2005 to 2015, this letter reports for the first time a clearly scale-dependent connection between proton temperatures and the turbulence in the solar wind. A statistical analysis of proton-scale turbulence shows that increasing helicity magnitudes correspond to steeper magnetic energy spectra. In particular, there exists a positive power law correlation (with a slope ∼0.4) between the proton perpendicular temperature and the turbulent magnetic energy at scales 0.3 ≲ k p ≲ 1, with k being the wave number and p being the proton gyroradius. These findings present evidence of solar wind heating by the proton-scale turbulence. They also provide insight and observational constraint on the physics of turbulent dissipation in the solar wind. Plain Language Summary The solar wind is a tenuous magnetized plasma that serves as a natural laboratory of nearly collisionless turbulence. It is streaming outward from the Sun and has temperatures much higher than those from a spherically expanding ideal gas, implying a heating process must occur. The heating has been extensively discussed in past decades, but still not well understood. Based on in situ measurements, we reveal a scale-dependent connection between proton temperatures and the turbulence with enhanced magnetic helicity signature. The connection is particularly strong for the proton temperature in the direction perpendicular to the background magnetic field. These observations provide implications for the physics of turbulent dissipation and heating in a collisionless plasma.

Dependence of Solar Wind Proton Temperature on the Polarization Properties of Alfvénic Fluctuations at Ion-kinetic Scales

“…In summary, our observations suggest that the properties of Alfvénic fluctuations at ion-kinetic scales determine the level of proton heating from turbulent dissipation. This interpretation is consistent with recent studies showing that larger magnetic helicity signatures at ion-kinetic scales are associated with larger proton temperatures and steeper spectral exponents (Pine et al 2020;Zhao et al 2020bZhao et al , 2021. Our findings, therefore, provide new evidence for the importance of local kinetic processes in determining proton temperature in the solar wind.…”

On the violation of the zeroth law of turbulence in space plasmas

“…2018; Zhao et al. 2020), the latter of which is known to be more imbalanced than the slow wind. Each of these observations is well explained by the helicity-barrier hypothesis, at least qualitatively: we reproduce double-kinked spectra with the first break at a non-universal scale above , while steeper spectra and larger-scale breaks result from the energy growing in time (figure 5).…”

“…The position, shape and cause of the ion-kinetic transition has been a decades-long puzzle with numerous proposed explanations (Schekochihin et al 2009;Sahraoui et al 2010;Meyrand & Galtier 2012;Lion, Alexandrova & Zaslavsky 2016;Voitenko & De Keyser 2016;Woodham et al 2018); observations show widely varying break positions and slopes (Leamon et al 1998) often followed by a spectral flattening at yet smaller scales (Sahraoui et al 2009;Bowen et al 2020a;Duan et al 2021). In addition, larger-scale transitions to steeper spectra correlate with higher-amplitude fluctuations, lower β, higher proton-scale magnetic helicity and fast-wind regions (Smith et al 2006;Bruno, Trenchi & Telloni 2014;Vech et al 2018;Zhao et al 2020), the latter of which is known to be more imbalanced than the slow wind. Each of these observations is well explained by the helicity-barrier hypothesis, at least qualitatively: we reproduce double-kinked spectra with the first break at a non-universal scale above k ⊥ ρ i = 1, while steeper spectra and larger-scale breaks result from the energy growing in time (figure 5).…”

Cross-scale Correlations in Imbalanced Solar Wind Turbulence: Parker Solar Probe Observations

“…Statistical studies by Wind and Advanced Composition Explorer measurements later revealed that higher proton temperature is related to larger inertial-scale magnetic energy density (Smith et al 2006;Vech et al 2018). Recent statistical studies by Wind and Parker Solar Probe (PSP) observations further revealed that higher proton temperature is linked to larger proton-scale magnetic energy density (Zhao et al 2020(Zhao et al , 2022. On the other hand, cross and magnetic helicities at inertial and kinetic scales, respectively, are often employed to describe the imbalance and handedness of solar wind turbulence.…”