According to \emph{Wind} observations between June 2004 and May 2019, this Letter investigates the proton and alpha particle temperatures in the space of ($θ_d$, $V_d/V_A$) for the first time, where $θ_d$ and $V_d$ are the radial angle and magnitude of alpha$-$proton differential flow vector respectively, $V_A$ is the local Alfvén speed. Results show that the temperatures significantly depend on $θ_d$ as well as $V_d/V_A$. In case of low proton parallel beta ($β_{p{\parallel}} < 1$), it is found that the proton perpendicular temperature is clearly enhanced when $θ_d$ is small ($\lesssim 45^\circ$) and $V_d/V_A \gtrsim 0.5$. On the contrary, the perpendicular temperature of alpha particles is considerably enhanced when $θ_d$ is large ($\gtrsim 90^\circ$) or $V_d/V_A$ is sufficiently small. The maximum of proton parallel temperature takes place at $θ_d \sim 90^\circ$ accompanied by higher $β_{p{\parallel}}$ and by larger turbulence amplitude of magnetic fluctuations in inertial range. This study should present strong evidence for cyclotron resonance heating of protons and alpha particles in the solar wind. Other mechanisms including Landau resonance and stochastic heating are also proposed, which tend to have different ($θ_d$, $V_d/V_A$) spaces than cyclotron resonance heating.
We extend previous theories of stochastic ion heating to account for the motion of ions along the magnetic field. We derive an analytic expression for the ion-to-proton perpendicular temperature ratio in the solar wind for any ion species, assuming that stochastic heating is the dominant ion heating mechanism. This expression describes how this temperature ratio depends upon the average velocity of the ions along the magnetic field direction and the ratio of the parallel proton pressure to the magnetic pressure. We compare our model with previously published measurements of alpha particles and…
We consider the radial evolution of perpendicular ion heating due to the violation of magnetic moment conservation caused by Alfven and kinetic Alfven wave turbulence. This process, referred to as stochastic heating, can be quantified by the ratio between the average velocity fluctuations at the ion gyroradius and the perpendicular ion thermal speed $ε\equiv δv / v_{t\perp}$. Using 17 years of Helios observations, we constrain how much energy could be dissipated by this mechanism between 0.29 and 0.98 au. We find that stochastic heating likely operates throughout the entire inner heliosphere, …
Parker Solar Probe Observations of Preferential Heating of Protons over Alpha Particles near Turbulent Coherent Structures
Yunnan University（云南大学）；University College London（伦敦大学学院）；Mullard Space Science Laboratory, University College London（穆拉德空间科学实验室，伦敦大学学院）；University of Delaware（特拉华大学）；CNR, Institute for Plasma Science and Technology (ISTP)（意大利国家研究委员会等离子科学与技术研究所（ISTP））；KTH - Royal Institute of Technology（瑞典皇家理工学院）；China University of Petroleum (East China)（中国石油大学（华东））；State Key Laboratory of Deep Oil and Gas, China University of Petroleum (East China)（中国石油大学（华东）深油气资源与地质国家重点实验室）；Beihang University（北京航空航天大学）；National Institute of Natural Hazards, Ministry of Emergency Management of China（中国应急管理部自然灾害防治研究院）；National University of Defense Technology（国防科技大学）
Spacecraft measurements show that protons undergo substantial perpendicular heating during their transit from the Sun to the outer heliosphere. In this paper, we use {\em Helios~2} measurements to investigate whether stochastic heating by low-frequency turbulence is capable of explaining this perpendicular heating. We analyze {\em Helios~2} magnetic-field measurements in low-$β$ fast-solar-wind streams between heliocentric distances $r=0.29$ AU and $r=0.64$ AU to determine the rms amplitude of the fluctuating magnetic field, $δB_{\rm p}$, near the proton gyroradius scale $ρ_{\rm p}$. We then e…
The velocity of alpha particles relative to protons can vary depending on the solar wind type and distance from the Sun (Marsch 2012). Measurements from the previous spacecraft provided the alpha-proton's differential velocities down to 0.3 au. Parker Solar Probe (PSP) now enables insights into differential flows of newly accelerated solar wind closer to the Sun for the first time. Here, we study the difference between proton and alpha bulk velocities near PSP perihelia of Encounters 3-7 when the core solar wind is in the field of view of the Solar Probe Analyzer for Ions (SPAN-I) instrument. …
Protons and alpha particles in the fast solar wind are only weakly collisional and exhibit a number of non-equilibrium features, including relative drifts between particle species. Two non-collisional mechanisms have been proposed for limiting differential flow between alpha particles and protons: plasma instabilities and the rotational force. Both mechanisms decelerate the alpha particles. In this paper, we derive an analytic expression for the rate $Q_{\mathrm{flow}}$ at which energy is released by alpha-particle deceleration, accounting for azimuthal flow and conservation of total momentum.…
Evidence for inhomogeneous heating in the interplanetary plasma near current sheets dynamically generated by magnetohydrodynamic (MHD) turbulence is obtained using measurements from the ACE spacecraft. These coherent structures only constitute 19% of the data, but contribute 50% of the total plasma internal energy. Intermittent heating manifests as elevations in proton temperature near current sheets, resulting in regional heating and temperature enhancements extending over several hours. The number density of non-Gaussian structures is found to be proportional to the mean proton temperature a…
The solar wind undergoes significant heating as it propagates away from the Sun; the exact mechanisms responsible for this heating are not yet fully understood. We present for the first time a statistical test for one of the proposed mechanisms, stochastic ion heating. We use the amplitude of magnetic field fluctuations near the proton gyroscale as a proxy for the ratio of gyroscale velocity fluctuations to perpendicular (with respect to the magnetic field) proton thermal speed, defined as $ε_p$. Enhanced proton temperatures are observed when $ε_p$ is larger than a critical value ($\sim 0.019 …
We develop a 1D solar-wind model that includes separate energy equations for the electrons and protons, proton temperature anisotropy, collisional and collisionless heat flux, and an analytical treatment of low-frequency, reflection-driven, Alfven-wave turbulence. To partition the turbulent heating between electron heating, parallel proton heating, and perpendicular proton heating, we employ results from the theories of linear wave damping and nonlinear stochastic heating. We account for mirror and oblique firehose instabilities by increasing the proton pitch-angle scattering rate when the pro…
Calculations are made of the energy supplied to the solar wind by the rapid decay of density fluctuations, identified as ion acoustic waves. It is shown that this process supplies an appreciable fraction, perhaps nearly all, of the observed heating of the solar wind. This process may be an important step in the conversion of magnetic turbulence to particle energy
Remote sensing observations of coronal holes show that heavy ions are hotter than protons and their temperature is anisotropic. In-situ observations of fast solar wind streams provide direct evidence for turbulent Alfvén wave spectrum, left-hand polarized ion-cyclotron waves, and He$^{++}$ -- proton drift in the solar wind plasma, which can produce temperature anisotropies by resonant absorption and perpendicular heating of the ions. Furthermore, the solar wind is expected to be inhomogeneous on decreasing scales approaching the Sun. We study the heating of solar wind ions in inhomogeneous pla…
The paper presents a new method for deriving turbulent heating of the solar wind using plasma moments and magnetic field data. We develop the method and then apply it to compute the turbulent heating of the solar wind proton plasma at 1AU. The method employs two physical properties of the expanding solar wind plasma, the wave-particle thermodynamic equilibrium, and the transport of entropic rate. We analyze plasma moments and field datasets taken from Wind S/C, in order to compute (i) the fluctuating magnetic energy, (ii) the corresponding correlation length, and (iii) the turbulent heating ra…
Magnetohydronamic turbulence is believed to play a crucial role in heating the laboratorial, space, and astrophysical plasmas. However, the precise connection between the turbulent fluctuations and the particle kinetics has not yet been established. Here we present clear evidence of plasma turbulence heating based on diagnosed wave features and proton velocity distributions from solar wind measurements by the Wind spacecraft. For the first time, we can report the simultaneous observation of counter-propagating magnetohydrodynamic waves in the solar wind turbulence. Different from the tradition…
Previous formulations of heating and transport associated with strong magnetohydrodynamic (MHD) turbulence are generalized to incorporate separate internal energy equations for electrons and protons. Electron heat conduction is included. Energy is supplied by turbulent heating that affects both electrons and protons, and is exchanged between them via collisions. Comparison to available Ulysses data shows that a reasonable accounting for the data is provided when (i) the energy exchange timescale is very long and (ii) the deposition of heat due to turbulence is divided, with 60% going to proton…
Plasma processes occurring in the corona and solar wind can be probed by studying the thermodynamic properties of different ion species. However, most in-situ observations of positive ions in the solar wind are taken at 1 AU, where information on their solar source properties may have been irreversibly erased. In this study we aimed to use the properties of alpha particles at heliocentric distances between 0.3 and 1 AU to study plasma processes occurring at the points of observation, and to infer processes occurring inside 0.3 AU by comparing our results to previous remote sensing observations…
In fast wind or when the local Coulomb collision frequency is low, observations show that solar wind minor ions and ion sub-populations flow with different bulk velocities. Measurements indicate that the drift speed of both alpha particles and proton beams with respect to the bulk or core protons rarely exceeds the local Alfvén speed, suggesting that a magnetic instability or other wave-particle process limits their maximum drift. We compare simultaneous alpha particle, proton beam, and proton core observations from instruments on the Wind spacecraft spanning over 20 years. In nearly collision…
The fast solar wind's high speeds and nonthermal features require that significant heating occurs well above the Sun's surface. Two leading theories seem incompatible: low-frequency "Alfvénic" turbulence, which transports energy outwards and is observed ubiquitously by spacecraft but struggles to explain the observed dominance of ion over electron heating; and high-frequency ion-cyclotron waves (ICWs), which explain the nonthermal heating of ions but lack an obvious source. Here, we argue that the recently proposed "helicity barrier" effect, which limits electron heating by inhibiting the turb…
Solar wind heating rates have often been calculated by fitting plasma and magnetic field data with a set of model functions. In this letter, we show that the rates obtained by such an approach strongly depend on the rather arbitrary choice one makes for these model functions. An alternative approach, consisting in monitoring the radial evolution of the adiabatic invariants, based on locally and consistently measured plasma and magnetic field parameters, is free from such a flaw. We apply this technique to a recently released Helios proton dataset, and confirm the existence of a clear perpendic…
Various remote sensing observations have been used so far to probe the turbulent properties of the solar wind. Using the recently reported density modulation indices that are derived using angular broadening observations of Crab Nebula during 1952 - 2013, we measured the solar wind proton heating using the kinetic $\rm Alfv\acute{e}n$ wave dispersion equation. The estimated heating rates vary from $\approx 1.58 \times 10^{-14}$ to $1.01 \times 10^{-8} ~\rm erg~ cm^{-3}~ s^{-1}$ in the heliocentric distance range 5 - 45 $\rm R_{\odot}$. Further, we found that heating rates vary with the solar c…
Previous studies have shown that the observed temperature anisotropies of protons and alpha particles in the solar wind are constrained by theoretical thresholds for pressure-anisotropy-driven instabilities such as the Alfvén/ion-cyclotron (A/IC) and fast-magnetosonic/whistler (FM/W) instabilities. In this letter, we use a long period of in-situ measurements provided by the {\em Wind} spacecraft's Faraday cups to investigate the combined constraint on the alpha-proton differential flow velocity and the alpha-particle temperature anisotropy due to A/IC and FM/W instabilities. We show that the m…
We analyze measured proton and electron temperatures in the high-speed solar wind in order to calculate the separate rates of heat deposition for protons and electrons. When comparing with other regions of the heliosphere, the fast solar wind has the lowest density and the least frequent Coulomb collisions. This makes the fast wind an optimal testing ground for studies of collisionless kinetic processes associated with the dissipation of plasma turbulence. Data from the Helios and Ulysses plasma instruments were collected to determine mean radial trends in the temperatures and the electron hea…
Several studies in the near-Earth environment show that intermittent structures are important sites of energy dissipation and particle energization. Recent Parker Solar Probe (PSP) data, sampled in the near-Sun environment, have shown that proton heating is concentrated near coherent structures, suggesting local heating of protons by turbulent cascade in this region. However, whether electrons exhibit similar behavior in the near-Sun environment is not clear. Here, we address this question using PSP data collected near the Sun during the first seven orbits. We use the partial variance of incre…
Polytropic stellar wind models with strongly localized heating
Ruhr-Universität Bochum（波鸿鲁尔大学）；Ilia State University（伊利亚国立大学）；Evgeni Kharadze Georgian National Astrophysical Observatory（叶夫根尼·哈拉泽格鲁吉亚国家天文台）；Institut für Weltraumforschung, Österreichische Akademie der Wissenschaften（奥地利科学院空间研究所）；Institute of Radio Astronomy, National Academy of Sciences of Ukraine（乌克兰国家科学院无线电天体物理研究所）
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 $\sim 0.4$) between the proton perpendicular temperature and the turbulent magnetic energy at scales $0.3 \lesssim kρ_p \lesssim 1$, with $k$ being the wavenumber and $ρ_p$ bei…
One of the important discoveries made by Voyager-2 is the nonadiabatic radial profile of the solar wind proton temperature. This phenomenon has been studied for several decades. The dissipation of turbulence energy has been proposed as the main physical process responsible for the temperature profile. The turbulence is both convected with the solar wind and originated in the solar wind by the compressions and shears in the flows and by pick-up ions. The compression source of the solar wind heating in the outer heliosphere appears due to shock waves, which originated either in the solar corona …
Solar wind ions are observed to be heated in the directions perpendicular to the large-scale magnetic field, with preferential heating given to heavy ions. In the solar corona, this heating may be responsible for the generation of the wind itself. It is likely that this heating results from the dissipation of plasma turbulence, but the specific kinetic mechanism that produces these preferential effects is not known. Solar wind turbulence at proton scales is often characterized in terms of highly oblique kinetic Alfvén waves (KAWs), which have been thought to dissipate through the Landau resona…
Although the origins of slow solar wind are unclear, there is increasing evidence that at least some of it is released in a steady state on over-expanded coronal hole magnetic field lines. This type of slow wind has similar properties to the fast solar wind, including a high degree of Alfvénicity. In this study a combination of proton, alpha particle, and electron measurements are used to investigate the kinetic properties of a single interval of slow Alfvénic wind at 0.35 AU. It is shown that this slow Alfvénic interval is characterised by high alpha particle abundances, pronounced alpha-prot…
The SWICS instrument aboard the ACE satellite has detected frequent intervals in the slow solar wind and interplanetary coronal mass ejections (ICMEs) in which C6+ and other fully stripped ions are strongly depleted, though the ionization states of elements such as Si and Fe indicate that those ions should be present. It has been suggested that these outlier or dropout events can be explained by the resonant cyclotron heating process, because these ions all have the same cyclotron frequency as He2+. We investigate the region in the corona where these outlier events form. It must be above the i…
We generate a model description of the solar wind based on an explicit wave-turbulence-driven heating mechanism, and constrain our model with observational data. We included an explicit coronal heating source term in the general 3D magnetohydrodynamic code NIRVANA to simulate the properties of the solar wind. The adapted heating mechanism is based on the interaction and subsequent dissipation of counter-propagating Alfvén waves in the solar corona, accounting for a turbulent heating rate Q_p. The solar magnetic field is assumed to be an axisymmetric dipole with a field strength of 1 G. Our mod…
Plasma ions heating (especially minor heavy ions preferential heating) in fast solar wind and solar corona is an open question in space physics. However, Alfvén waves have been always considered as a candidate of energy source for corona heating. In this paper, by using a two-dimensional (2-D) hybrid simulation model in a low beta electron-proton-alpha plasma system, we have investigated the relationships between plasma ions heating and power spectra evolution of density and magnetic field fluctuations excited from the parametric instabilities of initial pump Alfvén waves with an incoherent sp…
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 …
Using one-dimensional hybrid expanding box model we investigate properties of the solar wind in the outer heliosphere. We assume a proton-electron plasma with a strictly transverse ambient magnetic field and, beside the expansion, we take into account influence of a continuous injection of cold pick-up protons through the charge-exchange process between the solar wind protons and hydrogen of interstellar origin. The injected cold pick-up protons form a ring distribution function that rapidly becomes unstable and generate Alfvén cyclotron waves. The Alfvén cyclotron waves scatter pick-up proton…
We study the propagation, reflection, and turbulent dissipation of Alfven waves in coronal holes and the solar wind. We start with the Heinemann-Olbert equations, which describe non-compressive magnetohydrodynamic fluctuations in an inhomogeneous medium with a background flow parallel to the background magnetic field. Following the approach of Dmitruk et al, we model the nonlinear terms in these equations using a simple phenomenology for the cascade and dissipation of wave energy, and assume that there is much more energy in waves propagating away from the Sun than waves propagating towards th…
In solar wind, magnetohydrodynamic (MHD) discontinuities are ubiquitous and often found to be at the origin of turbulence intermittency. They may also play a key role in the turbulence dissipation and heating of the solar wind. The tangential (TD) and rotational (RD) discontinuities are the two most important types of discontinuities. Recently, the connection between turbulence intermittency and proton thermodynamics has been being investigated observationally. Here we present numerical results from three-dimensional MHD simulation with pressure anisotropy and define new methods to identify an…
One proposed mechanism for heating the solar wind, from close to the sun to beyond 10 AU, invokes low-frequency, oblique, Alfven-wave turbulence. Because small-scale oblique Alfven waves (kinetic Alfven waves) are compressive, the measured density fluctuations in the solar wind place an upper limit on the amplitude of kinetic Alfven waves and hence an upper limit on the rate at which the solar wind can be heated by low-frequency, Alfvenic turbulence. We evaluate this upper limit for both coronal holes at 5 solar radii and in the near-Earth solar wind. At both radii, the upper limit we find is …
Plasma kinetic waves and alpha-proton differential flow are two important subjects on the topic of evolution of the solar wind. Based on the Wind data during 2005-2015, this paper reports that the occurrence of electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency significantly depends on the direction of alpha-proton differential flow Vd. As Vd rotates from anti-Sunward direction to Sunward direction, the occurrence rate of ECWs as well as the percentage of left-handed (LH) polarized ECWs decreases considerably. In particular, it is shown that the dominant polarization ch…
The steady, supersonic outflow from the Sun we call the solar wind was first posited in the 1950s and initial theories rightly linked the acceleration of the wind to the existence of the million-degree solar corona. Still today, the wind acceleration mechanisms and the coronal heating processes remain unsolved challenges in solar physics. In this work, I seek to answer a portion of the mystery by focusing on a particular acceleration process: Alfven waves launched by the motion of magnetic field footpoints in the photosphere. The entire corona is threaded with magnetic loops and flux tubes tha…
Orbits six through nine of the Parker Solar Probe have been studied to show that solar wind core electrons emerged from 15 solar radii with temperatures that were constant to within ~10% although the solar wind speed varied from 300 to 800 km/sec. After leaving 15 solar radii, the core electrons were isotropically heated as much as a factor of two below 30 solar radii by triggered and natural ion acoustic waves. To distinguish this wave heating from processes occurring inside 15 solar radii, the electron temperature is modeled as having two components, the base temperature observed at 15 solar…
Between the base of the solar corona and the Alfven critical point, the solar-wind density decreases by approximately five orders of magnitude, but the temperature varies by a factor of only a few. In this paper, I show that such quasi-isothermal evolution out to the Alfven critical point is a generic property of outflows powered by reflection-driven Alfven-wave (AW) turbulence, in which outward-propagating AWs partially reflect, and counter-propagating AWs interact to produce a cascade of fluctuation energy to small scales, which leads to turbulent heating. Approximating the sub-Alfvenic regi…
Previous studies revealed that electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency exist widely in the solar wind, and the majority of ECWs are left-handed (LH) polarized waves. Using the magnetic field data from the STEREO mission, this Letter carries out a survey of ECWs over a long period of 7 years, and calculates the occurrence rates of ECWs with different polarization senses. Results show that the occurrence rate is nearly a constant for the ECWs with right-handed polarization, but it varies significantly for the ECWs with LH polarization. Further investigation of …
Two main models have been developed to explain the mechanisms of release, heating and acceleration of the nascent solar wind, the wave-turbulence-driven (WTD) models and reconnection-loop-opening (RLO) models, in which the plasma release processes are fundamentally different. Given that the statistical observational properties of helium ions produced in magnetically diverse solar regions could provide valuable information for the solar wind modelling, we examine the statistical properties of the helium abundance (A_He) and the speed difference between helium ions and protons (v_alpha,p) for co…
We investigate the conditions under which parallel-propagating Alfvén/ion-cyclotron (A/IC) waves and fast-magnetosonic/whistler (FM/W) waves are driven unstable by the differential flow and temperature anisotropy of alpha particles in the solar wind. We focus on the limit in which $w_{\parallel α} \gtrsim 0.25 v_{\mathrm A}$, where $w_{\parallel α} $ is the parallel alpha-particle thermal speed and $v_{\mathrm A}$ is the Alfvén speed. We derive analytic expressions for the instability thresholds of these waves, which show, e.g., how the minimum unstable alpha-particle beam speed depends upon $…
Perihelion passes on Parker Solar Probe orbits six through nine have been studied to show that solar wind core electrons emerged from 15 solar radii with a temperature of 55 plus or minus 5 eV, independent of the solar wind speed which varied from 300 to 800 km/sec. After leaving 15 solar radii and in the absence of triggered ion acoustic waves at greater distances, the core electron temperature varied with radial distance, R, in solar radii, as 1900R-4/3 electron volts because of cooling produced by the adiabatic expansion. The coefficient, 1900, reproduces the minimum core electron perpendic…
The solar wind undergoes significant heating as it propagates away from the Sun; the exact mechanisms responsible for this heating remain unclear. Using data from the first perihelion of the Parker Solar Probe mission, we examine the properties of proton and electron heating occurring within magnetic coherent structures identified by means of the Partial Variance of Increments (PVI) method. Statistically, regions of space with strong gradients in the magnetic field, $PVI \geq 1$, are associated with strongly enhanced proton but only slightly elevated electron temperatures. Our analysis indicat…
To properly describe heating in weakly collisional turbulent plasmas such as the solar wind, inter-particle collisions should be taken into account. Collisions can convert ordered energy into heat by means of irreversible relaxation towards the thermal equilibrium. Recently, Pezzi et al. (Phys. Rev. Lett., vol. 116, 2016, p. 145001) showed that the plasma collisionality is enhanced by the presence of fine structures in velocity space. Here, the analysis is extended by directly comparing the effects of the fully nonlinear Landau operator and a linearized Landau operator. By focusing on the rela…
We report analysis of sub-Alfvénic magnetohydrodynamic (MHD) perturbations in the low-\b{eta} radial-field solar wind using the Parker Solar Probe spacecraft data from 31 October to 12 November 2018. We calculate wave vectors using the singular value decomposition method and separate the MHD perturbations into three types of linear eigenmodes (Alfvén, fast, and slow modes) to explore the properties of the sub-Alfvénic perturbations and the role of compressible perturbations in solar wind heating. The MHD perturbations there show a high degree of Alfvénicity in the radial-field solar wind, with…
