An official website of the United States government
Abstract Recent observations of the solar wind ions by the SPAN-I instruments on board the Parker Solar Probe (PSP) spacecraft at solar perihelia (Encounters) 4 and closer find ample evidence of complex anisotropic non-Maxwellian velocity distributions that consist of core, beam, and “hammerhead” (i.e., anisotropic beam) populations. The proton core populations are anisotropic, withT⊥/T∥ > 1, and the beams have super-Alfvénic speed relative to the core (we provide an example from Encounter 17). Theα-particle population shows similar features to the protons. These unstable velocity distribution functions (VDFs) are associated with enhanced, right-hand (RH) and left-hand (LH) polarized ion-scale kinetic wave activity, detected by the FIELDS instrument. Motivated by PSP observations, we employ nonlinear hybrid models to investigate the evolution of the anisotropic hot-beam VDFs and model the growth and the nonlinear stage of ion kinetic instabilities in several linearly unstable cases. The models are initialized with ion VDFs motivated by the observational parameters. We find rapidly growing (in terms of proton gyroperiods) combined ion-cyclotron and magnetosonic instabilities, which produce LH and RH ion-scale wave spectra, respectively. The modeled ion VDFs in the nonlinear stage of the evolution are qualitatively in agreement with PSP observations of the anisotropic core and “hammerhead” velocity distributions, quantifying the effect of the ion kinetic instabilities on wind plasma heating close to the Sun. We conclude that the wave–particle interactions play an important role in the energy transfer between the magnetic energy (waves) and random particle motion, leading to anisotropic solar wind plasma heating.
more »
« less
This content will become publicly available on September 24, 2026
Preferential Energization of Solar Wind Ions Below the Alfvénic Surface
Abstract Understanding how solar wind ions are heated and accelerated remains a central question in heliophysics. Observations consistently show that alpha particles are hotter and faster than protons, particularly close to the Sun. This suggests that kinetic processes play a dominant role in shaping ion distributions. Theoretical models have proposed that much of this preferential energization occurs near the Alfvén critical surface, a spatially varying boundary where the solar wind transitions from magnetically dominated to thermally dominated flow. With in situ measurements from Parker Solar Probe (PSP), it is possible to explore this question near the Sun directly. In this study, we analyze PSP observations from near-perihelion encounters to statistically compare the thermodynamic properties of protons and alpha particles across the Alfvén surface. We find that ion temperature anisotropies, differential flows, and heating signatures are coherently organized by the local Alfvén Mach number (MA). In the sub-Alfvénic regime, alpha particles exhibit strong perpendicular temperature heating and elevated alpha-to-proton temperature ratio, which decline across the Alfvénic transition. Meanwhile, proton heating peaks nearMA ∼ 1, consistent with anisotropic wave–particle energization. These findings highlight the Alfvén surface as a key boundary for ion energization and momentum exchange, and establishMAas a fundamental organizing parameter in the kinetic evolution of solar wind ions in the inner heliosphere.
more »
« less
- Award ID(s):
- 2300961
- PAR ID:
- 10677328
- Publisher / Repository:
- IOP Science
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 991
- Issue:
- 2
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L35
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Recent in situ observations from Parker Solar Probe (PSP) near perihelia reveal ion beams, temperature anisotropies, and kinetic wave activity. These features are likely linked to solar wind heating and acceleration. During PSP Encounter 17 (at 11.4Rs) on 2023 September 26, the PSP/FIELDS instrument detected enhanced ion-scale wave activity associated with deviations from local thermodynamic equilibrium in ion velocity distribution functions (VDFs) observed by the PSP/Solar Probe Analyzers-Ion. Dense beams (secondary populations) were present in the proton VDFs during this wave activity. Using bi-Maxwellian fits to the proton VDFs, we found that the density of the proton beam population increased during the wave activity and, unexpectedly, surpassed the core population at certain intervals. Interestingly, the wave power was reduced during the intervals when the beam population density exceeded the core density. The drift velocity of the beams decreases from 0.9 to 0.7 of the Alfvén speed, and the proton core shows a higher temperature anisotropy (T⊥/T∥ > 2.5) during these intervals. We conclude that the observations during these intervals are consistent with a reconnection event during a heliospheric current sheet crossing. During this event,α-particle parameters (density, velocity, and temperature anisotropy) remained nearly constant. Using linear analysis, we examined how the proton beam drives instability or wave dissipation. Furthermore, we investigated the nonlinear evolution of ion kinetic instabilities using hybrid kinetic simulations. This study provides direct clues about energy transfer between particles and waves in the young solar wind.more » « less
-
Abstract We investigate the local proton energization at magnetic discontinuities/intermittent structures and the corresponding kinetic signatures in velocity phase space in Alfvénic (high cross helicity) and non-Alfvénic (low cross helicity) wind streams observed by Parker Solar Probe. By means of the partial variance of increments method, we find that the hottest proton populations are localized around compressible, coherent magnetic structures in both types of wind. Analysis of parallel and perpendicular temperature distributions suggest that the Alfvénic wind undergoes preferential enhancements ofT∥at such structures, whereas the non-Alfvénic wind experiences preferentialT⊥enhancements. Although proton beams are present in both types of wind, the proton velocity distribution function displays distinct features. Hot beams, i.e., beams with beam-to-core perpendicular temperatureT⊥,b/T⊥,cup to three times larger than the total distribution anisotropy, are found in the non-Alfvénic wind, whereas colder beams are in the Alfvénic wind. Our data analysis is complemented by 2.5D hybrid simulations in different geometrical setups, which support the idea that proton beams in Alfvénic and non-Alfvénic wind have different kinetic properties and different origins. The development of a perpendicular nonlinear cascade, favored in balanced turbulence, allows a preferential relative enhancement of the perpendicular plasma temperature and the formation of hot beams. Cold field-aligned beams are instead favored by Alfvén wave steepening. Non-Maxwellian distribution functions are found near discontinuities and intermittent structures, pointing to the fact that the nonlinear formation of small-scale structures is intrinsically related to the development of highly nonthermal features in collisionless plasmas. Our results contribute to understanding the role of different coherent structures in proton energization and their implication in collisionless energy dissipation processes in space plasmas.more » « less
-
Abstract The proton–alpha drift instability is a possible mechanism of the alpha-particle deceleration and the resulting proton heating in the solar wind. We present hybrid numerical simulations of this instability with particle-in-cell ions and a quasi-neutralizing electron fluid for typical conditions at 1 au. For the parameters used in this paper, we find that fast magnetosonic unstable modes propagate only in the direction opposite to the alpha-particle drift and do not produce the perpendicular proton heating necessary to accelerate the solar wind. Alfvén modes propagate in both directions and heat the protons perpendicularly to the mean magnetic field. Despite being driven by the alpha temperature anisotropy, the Alfvén instability also extracts the energy from the bulk motion of the alpha particles. In the solar wind, the instabilities operate in a turbulent ambient medium. We show that the turbulence suppresses the Alfvén instability but the perpendicular proton heating persists. Unlike a static nonuniform background, the turbulence does not invert the sense of the proton heating associated with the fast magnetosonic instability and it remains preferentially parallel.more » « less
-
Abstract Characterizing the plasma state in the near-Sun environment is essential to constrain the mechanisms that heat and accelerate the solar wind. In this study, we use Parker Solar Probe observations from Encounters 1 through 24 to investigate the radial evolution of solar wind plasma and magnetic field properties in this region. Using intervals with high field-of-view (>85%) coverage, we derive the radial profiles of magnetic field strength (∣B∣), proton density (N), bulk speed (V), total proton temperature (T), parallel (T∥) and perpendicular (T⊥) temperatures, temperature anisotropy (T⊥/T∥), plasma beta (β), Alfvén Mach number (MA), and magnetic field fluctuations (δB/B) for sub and super-Alfvénic regions. In super-Alfvénic regions, power laws of ∣B∣,N,V, andTas a function of the heliocentric distance are broadly consistent with previous Helios results at >0.3 au. The radial evolution of the components of the temperature tensor reveals distinct behavior:T⊥decreases monotonically with distance, whereasT∥exhibits a nonmonotonic trend—decreasing in the sub-Alfvénic region, increasing just beyond the Alfvén surface. We interpret the increase inT∥as a proxy for proton beam occurrence. We further examine the evolution of magnetic field fluctuations, finding decreasing radial/parallel fluctuations but enhanced tangential/normal/perpendicular fluctuations in the sunward direction. These fluctuations may provide free energy for beam generation and particle heating via wave–particle interactions.more » « less
