The Enhancement of Proton Stochastic Heating in the Near-Sun Solar Wind

Abstract

Stochastic heating is a non-linear heating mechanism driven by the violation of magnetic moment invariance due to large-amplitude turbulent fluctuations producing diffusion of ions towards higher kinetic energies in the direction perpendicular to the magnetic field. It is frequently invoked as a mechanism responsible for the heating of ions in the solar wind. Here, we quantify for the first time the proton stochastic heating rate Q ⊥ at radial distances from the Sun as close as 0.16 au, using measurements from the first two Parker Solar Probe encounters. Our results for both the amplitude and radial trend of the heating rate, Q ⊥ ∝ r −2.5 , agree with previous results based on the Helios data set at heliocentric distances from 0.3 to 0.9 au. Also in agreement with previous results, Q ⊥ is significantly larger in the fast solar wind than in the slow solar wind. We identify the tendency in fast solar wind for cuts of the core proton velocity distribution transverse to the magnetic field to exhibit a flat-top shape. The observed distribution agrees with previous theoretical predictions for fast solar wind where stochastic heating is the dominant heating mechanism.

Constraining Ion-Scale Heating and Spectral Energy Transfer in Observations of Plasma Turbulence

“…Several competing mechanisms have been suggested for ion-scale dissipation: e.g., stochastic heating [42], Landau damping [13,17,76,77], and cyclotron resonance [10,16,18]. Correlations between signatures of the transition range and parameters associated with stochastic heating were not found [42,63]. Measurements of 3D ion distributions will enable comparison of damping rates associated with, e.g., Landau and cyclotron resonance, with our observed dissipation rates [1,8,[10][11][12][13][14]17,18,76,82,83].…”

“…Measurements of 3D ion distributions will enable comparison of damping rates associated with, e.g., Landau and cyclotron resonance, with our observed dissipation rates [1,8,[10][11][12][13][14]17,18,76,82,83]. Additionally, heating processes may leave clear signatures in the distribution function: e.g., diffusive shells in the case of cyclotron resonance [84] or flattening associated with resonant damping or perpendicular stochastic heating [63,85]. Use of electric fields will enable analysis of alignment over the transition range, enabling further constraint of energy transfer, heating rates, and mechanisms [86][87][88].…”

“…Surveying transition-range steepening in E k illustrates the dramatic effect that it takes on the turbulent cascade. Data and fitting.-The PSP mission [60] provides a set of in situ measurements [61,62] enabling detailed studies of turbulence in the inner heliosphere [23,[63][64][65]. Preliminary observations show a steep f −3 -f −4 spectrum of the magnetic field fluctuations at ion scales [66][67][68], but instrumental sensitivity has precluded further detailed study.…”

Proton Temperature Anisotropy Variations in Inner Heliosphere Estimated with the First Parker Solar Probe Observations

“…In addition, we think the momentderived temperature values could be more physical because they do not impose the assumption of a Maxwellian shape on the velocity distribution functions. Other studies of the proton velocity distribution functions seen by PSP are reporting a significant non-Maxwellian kurtosis near E1 perihelion (Martinović et al 2020) , and other significant deviations from the two-Maxwellian model (Case et al 2020). This implies that different model functions other than bi-Maxwellian (for example, Wilson III et al (2019a,b) found bi-Kappa works better to fit electron halo and strahl VDFs) might be considered in the future work.…”

Effects of the Background Turbulence on the Relaxation of Ion Temperature Anisotropy in Space Plasmas

“…However, steep spectra (larger α values) have been typically observed in association with small values of the plasma beta and large levels of turbulent fluctuations, both recently in near-Sun environment by Parker Solar Probe, and also previously in the near-Earth environment by WIND [82]. Moreover, Parker Solar Probe measurements have also shown an enhanced perpendicular proton heating possibly due to stochastic heating related to the strong turbulent fluctuations particularly in the fast solar wind (see e.g., Martinović et al [83]) that could compete with all the mentioned mechanisms, including the heating mechanism suggested in this paper.…”