Collisional Age Research Articles

Anisotropic Heating and Cooling within Interplanetary Coronal Mass Ejection Sheath Plasma

This study presents the first comprehensive investigation of the relationship between heating and cooling, temperature anisotropy, turbulence level, and collisional age within interplanetary coronal mass ejection (ICME) sheaths, which are highly compressed, heated, and turbulent. Using Wind spacecraft data, we analyze 333 ICME sheaths observed at 1 au from 1995 to 2015. The proton temperature within the ICME sheaths has a log-normal probability distribution. Irrespective of instability growth rates, plasma unstable to proton-cyclotron (PC) and firehose instabilities appear to be statistically hotter, at least by a factor of 5 to 10, compared to stable plasma. We also observe relatively enhanced magnetic fluctuations and low collisional age, especially in regimes unstable to PC and firehose instabilities at low proton betas β p ≤ 2. In the case of high beta β p ≥ 2, we observe high magnetic fluctuations close to the instabilities and less collisional age to the plasma unstable to firehose instability rather than near the mirror mode and PC threshold. Our findings suggest that heating processes dominate over cooling processes in producing proton temperature anisotropy in the ICME sheath region. Moreover, collisional age and magnetic fluctuations are critical in maintaining anisotropic and isotropic conditions.

Expanding-box Quasilinear Model of the Solar Wind

The expanding-box model of the solar wind has been adopted in the literature within the context of magnetohydrodynamics, hybrid, and full particle-in-cell simulations to investigate the dynamic evolution of the solar wind. The present paper extends such a method to the framework of self-consistent quasilinear kinetic theory. It is shown that the expanding-box quasilinear methodology is largely equivalent to the inhomogeneous steady-state quasilinear model discussed earlier in the literature, but a distinction regarding the description of wave dynamics between the two approaches is also found. The expanding-box quasilinear formalism is further extended to include the effects of a spiraling solar-wind magnetic field as well as collisional age effects. The present finding shows that the expanding-box quasilinear approach and the steady-state global-kinetic models may be employed interchangeably in order to address other more complex problems associated with the solar-wind dynamics.

Anterograde Collisional Analysis of Solar Wind Ions

Owing to its low density and high temperature, the solar wind frequently exhibits strong departures from local thermodynamic equilibrium, which include distinct temperatures for its constituent ions. Prior studies have found that the ratio of the temperatures of the two most abundant ions—protons (ionized hydrogen) and α-particles (ionized helium)—is strongly correlated with the Coulomb collisional age. These previous studies, though, have been largely limited to using observations from single missions. In contrast, this present study utilizes contemporaneous, in situ observations from two different spacecraft at two different distances from the Sun: the Parker Solar Probe (PSP; r = 0.1–0.3 au) and Wind (r = 1.0 au). Collisional analysis, which incorporates the equations of collisional relaxation and large-scale expansion, was applied to each PSP datum to predict the state of the plasma farther from the Sun at r = 1.0 au. The distribution of these predicted α–proton relative temperatures agrees well with that of values observed by Wind. These results strongly suggest that, outside of the corona, relative ion temperatures are principally affected by Coulomb collisions and that the preferential heating of α-particles is largely limited to the corona.

Paleomagnetic and geochronological results of the Risong Formation in the western Lhasa Terrane: Insights into the Lhasa-Qiangtang collision and stratal age

To constrain the Lhasa-Qiangtang collisional age, a combined paleomagnetic and geochronologic investigation has been performed on the Risong Formation redbeds and volcanic rocks in the Wuma area of the western Lhasa terrane (LT). A U-Pb dating result from the volcanic interlayer at the bottom of the Risong Formation redbeds yields the youngest weighted mean 206 Pb/ 238 U age of 110.6 ± 1.3 Ma, and the two detrital zircon U-Pb chronological results from the Risong Formation redbeds provide the youngest weighted mean 206 Pb/ 238 U age of 106.9 ± 3.3 Ma (the top strata of section A) and 120.5 ± 3.0 Ma (the top strata of section D), respectively. Our new geochronological results suggest that the studied Risong Formation redbeds formed during the Early Cretaceous, rather than during the Late Jurassic as assigned by 1:250,000 scale Wuma regional geological survey report. Characteristic remanent magnetization (ChRM) directions from two different limbs of folds concentrate in two different groups, and both site-mean directions pass positive fold tests. One group including 32 sites yields a tilt-corrected site-mean direction that is D = 356.1°, I = 33.1° with α 95 = 2.3°, corresponding to a paleopole at 75.2°N, 278.1°E with dp/dm = 1.5°/2.6° and a paleolatitude of 18.1° ± 1.5°N for the study area (32.41°N, 83.39°E). The other group including 24 sites provides a tilt-corrected site-mean direction that is D = 60.5°, I = 48.5° with α 9 5 = 2.0°, corresponding to a paleopole at 38.7°N, 159.6°E with dp/dm = 1.7°/2.6° and a paleolatitude of 29.5° ± 1.7°N for the study area (32.41°N, 83.39°E). After syntectonic-sedimentation-correction, the Fisherian site-mean inclination of 41.7° ± 1.6° and corresponding paleolatitude of 24.0° ± 1.2°N obtained from 56 sites of two different limbs are consistent with the inclination-only mean of 39.6 ± 3.1° and corresponding paleolatitude of 22.5° ± 2.2°N deduced from the ChRM directions of all the 56 paleomagnetic sites, suggesting that the observed inclination discrepancy of the Risong Formation redbeds may result from the syntectonic sedimentation. Comparing the Early Cretaceous paleolatitudes observed from the western LT with those from the western Qiangtang terrane shows insignificant paleolatitude difference, supporting that the Lhasa-Qiangtang collision in the western part occurred at or before the Early Cretaceous. • New dating shows the sampled Risong Formation redbeds formed during the Early Cretaceous rather than the Late Jurassic. • The Wuma sampling area was located at ~22.5° ± 2.2°N during ~120.5–106.9 Ma. • The Lhasa-Qiangtang collision occurred before ~120 Ma in the western part.

The Sunward Electron Deficit: A Telltale Sign of the Sun’s Electric Potential

Abstract As the Parker Solar Probe explores new regions of the inner heliosphere, it travels ever deeper into the electric potential of the Sun. In the near-Sun environment, a new feature of the electron distribution emerges, in the form of a deficit in the sunward suprathermal population. The lower boundary of this deficit forms a cutoff in phase space, at an energy determined by the electric potential drop between the observation point and the outer heliosphere. We explore the characteristics of the sunward deficit and the associated cutoff, as well as the properties of the plasma in which we observe them. The deficit occurs in ∼60%–80% of electron observations within ∼0.2 au, and even more frequently in plasma with low β, low collisional age, and a more anisotropic electron core population. At greater distances, the deficit rapidly disappears, as the suprathermal halo grows, with these two trends likely related. The cutoff energy varies linearly with the local electron core temperature, confirming a direct relationship to the ambipolar electric potential. Meanwhile, the cutoff width varies with β and collisional age, suggesting that energy diffusion plays a role in erasing the deficit. The nearly ubiquitous occurrence of the sunward deficit in the inner heliosphere suggests that we may need to reconsider the functional forms commonly used to represent electron distributions in this environment.

Solution to the debris disc mass problem: planetesimals are born small?

ABSTRACTDebris belts on the periphery of planetary systems, encompassing the region occupied by planetary orbits, are massive analogues of the Solar system’s Kuiper belt. They are detected by thermal emission of dust released in collisions amongst directly unobservable larger bodies that carry most of the debris disc mass. We estimate the total mass of the discs by extrapolating up the mass of emitting dust with the help of collisional cascade models. The resulting mass of bright debris discs appears to be unrealistically large, exceeding the mass of solids available in the systems at the preceding protoplanetary stage. We discuss this ‘mass problem’ in detail and investigate possible solutions to it. These include uncertainties in the dust opacity and planetesimal strength, variation of the bulk density with size, steepening of the size distribution by damping processes, the role of the unknown ‘collisional age’ of the discs, and dust production in recent giant impacts. While we cannot rule out the possibility that a combination of these might help, we argue that the easiest solution would be to assume that planetesimals in systems with bright debris discs were ‘born small’, with sizes in the kilometre range, especially at large distances from the stars. This conclusion would necessitate revisions to the existing planetesimal formation models, and may have a range of implications for planet formation. We also discuss potential tests to constrain the largest planetesimal sizes and debris disc masses.

Discovery of Vestige Sedimentary Archives of the India‐Asia Collision in the Eastern Yarlung Zangbo Suture Zone

AbstractThe lack of syncollisional sediments in the eastern Yarlung Zangbo suture zone (YZSZ) has impeded our understanding of the India‐Asia collision, particularly as to whether the collision was synchronous or diachronous. In this paper, we present evidence for the vestige sedimentary archives of the initial India‐Asia collision retained in the previously proposed “Cretaceous Langxian Mélange” in the eastern YZSZ, southern Tibet. Field investigations of marbles and intercalated meta‐siliciclastic rocks, together with analyses of petrography, cathodoluminescence image, geochronology, trace elements, and Nd isotopes show that (1) marbles occur as thin‐bedded to massive individual layers consistently dipping to the south at 50–80°, (2) marble and intercalated meta‐siliciclastic rocks exhibit the same deformational (foliations S0 and S1) and metamorphic (green‐schist facies) history with nearly same detrital‐zircon age spectra and εNd(0) values, (3) protoliths of the meta‐sedimentary rocks were sourced from both India and Asia and deposited during the early Eocene, and (4) protoliths of the marbles were probably precipitated in a marine environment. These lines of evidence suggest that the previously proposed mélange is indeed not the Cretaceous mélange, but the vestige of syncollisional sedimentary strata that was originally deposited in situ in a syncollisional foreland basin. The estimated collisional age of prior to ~56 Ma in the eastern YZSZ is almost consistent with the 60–56 Ma age estimated from the western‐central YZSZ and Himalaya, indicating a (quasi‐) synchronous initial India‐Asia collision at ~60–56 Ma.

Proton-proton collisional age to order solar wind types

Context. The properties of a solar wind stream are determined by its source region and by transport effects. Independently of the solar wind type, the solar wind measured in situ is always affected by both. This means that reliably determining the solar wind type from in situ observations is useful for the analysis of its solar origin and its evolution during the travel time to the spacecraft that observes the solar wind. In addition, the solar wind type also influences the interaction of the solar wind with other plasma such as Earth’s magnetosphere. Aims. We consider the proton-proton collisional age as an ordering parameter for the solar wind at 1 AU and explore its relation to the solar wind classification scheme developed by Xu & Borovsky (2015, J. Geophys. Res.: Space Phys., 120, 70). We use this to show that explicit magnetic field information is not required for this solar wind classification. Furthermore, we illustrate that solar wind classification schemes that rely on threshold values of solar wind parameters should depend on the phase in the solar activity cycle since the respective parameters change with the solar activity cycle. Methods. The categorization of the solar wind following Xu & Borovsky (2015, J. Geophys. Res.: Space Phys., 120, 70) was taken as our reference for determining the solar wind type. Based on the observation that the three basic solar wind types from this categorization cover different regimes in terms of proton-proton collisional age acol, p-p, we propose a simplified solar wind classification scheme that is only based on the proton-proton collisional age. We call the resulting method the PAC solar wind classifier. For this purpose, we derive time-dependent threshold values in the proton-proton collisional age for two variants of the proposed PAC scheme: (1) similarity-PAC is based on the similarity to the full Xu & Borovsky (2015, J. Geophys. Res.: Space Phys., 120, 70) scheme, and (2) distribution-PAC is based directly on the distribution of the proton-proton collisional age. Results. The proposed simplified solar wind categorization scheme based on the proton-proton collisional age represents an equivalent alternative to the full Xu & Borovsky (2015, J. Geophys. Res.: Space Phys., 120, 70) solar wind classification scheme and leads to a classification that is very similar to the full Xu & Borovsky (2015, J. Geophys. Res.: Space Phys., 120, 70) scheme. The proposed PAC solar wind categorization separates coronal hole wind from helmet-streamer plasma as well as helmet-streamer plasma (slow solar wind without a current sheet crossing) from sector-reversal plasma (slow solar wind with a current sheet crossing). Unlike the full Xu & Borovsky (2015, J. Geophys. Res.: Space Phys., 120, 70) scheme, PAC does not require information on the magnetic field as input. Conclusions. The solar wind is well ordered by the proton-proton collisional age. This implies underlying intrinsic relationships between the plasma properties, in particular, proton temperature and magnetic field strength in each plasma regime. We argue that sector-reversal plasma is a combination of particularly slow and dense solar wind and most stream interaction boundaries. Most solar wind parameters (e.g., the magnetic field strength, B, and the oxygen charge state ratio no7+/no6+) change with the solar activity cycle. Thus, all solar wind categorization schemes based on threshold values need to be adapted to the solar activity cycle as well. Because it does not require magnetic field information but only proton plasma measurements, the proposed PAC solar wind classifier can be applied directly to solar wind data from the Solar and Heliospheric Observatoty (SOHO), which is not equipped with a magnetometer.

Electrons in the Young Solar Wind: First Results from the Parker Solar Probe

Abstract The Solar Wind Electrons Alphas and Protons experiment on the Parker Solar Probe (PSP) mission measures the three-dimensional electron velocity distribution function. We derive the parameters of the core, halo, and strahl populations utilizing a combination of fitting to model distributions and numerical integration for ∼100,000 electron distributions measured near the Sun on the first two PSP orbits, which reached heliocentric distances as small as ∼0.17 au. As expected, the electron core density and temperature increase with decreasing heliocentric distance, while the ratio of electron thermal pressure to magnetic pressure (β e ) decreases. These quantities have radial scaling consistent with previous observations farther from the Sun, with superposed variations associated with different solar wind streams. The density in the strahl also increases; however, the density of the halo plateaus and even decreases at perihelion, leading to a large strahl/halo ratio near the Sun. As at greater heliocentric distances, the core has a sunward drift relative to the proton frame, which balances the current carried by the strahl, satisfying the zero-current condition necessary to maintain quasi-neutrality. Many characteristics of the electron distributions near perihelion have trends with solar wind flow speed, β e , and/or collisional age. Near the Sun, some trends not clearly seen at 1 au become apparent, including anticorrelations between wind speed and both electron temperature and heat flux. These trends help us understand the mechanisms that shape the solar wind electron distributions at an early stage of their evolution.

Effect of a weak ion collisionality on the dynamics of kinetic electrostatic shocks

In strictly collisionless electrostatic shocks, the ion distribution function can develop discontinuities along phase-space separatrices, due to partial reflection of the ion population. In this paper, we depart from the strictly collisionless regime and present a semi-analytical model for weakly collisional kinetic shocks. The model is used to study the effect of small but finite collisionalities on electrostatic shocks, and they are found to smooth out these discontinuities into growing boundary layers. More importantly, ions diffuse into and accumulate in the previously empty regions of phase space, and, by upsetting the charge balance, lead to growing downstream oscillations of the electrostatic potential. We find that the collisional age of the shock is the more relevant measure of the collisional effects than the collisionality, where the former can become significant during the lifetime of the shock, even for weak collisionalities.

A Comparison of Alpha Particle and Proton Beam Differential Flows in Collisionally Young Solar Wind

Abstract In fast wind or when the local Coulomb collision frequency is low, observations show that solar wind minor ions and ion subpopulations 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 processes 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 collisionless solar wind, we find that the normalized alpha particle drift speed is slower than the normalized proton beam speed, no correlation between fluctuations in both species’ drifts about their means, and a strong anti-correlation between collisional age and alpha–proton differential flow, but no such correlation with proton beam–core differential flow. Controlling for the collisional dependence, both species’ normalized drifts exhibit similar statistical distributions. In the asymptotic, zero Coulomb collision limit, the youngest measured differential flows most strongly correlate with an approximation of the Alfvén speed that includes proton pressure anisotropy. In this limit and with this most precise representation, alpha particles drift at 67% and proton beam drift is approximately 105% of the local Alfvén speed. We posit that one of two physical explanations is possible. Either (1) an Alfvénic process preferentially accelerates or sustains proton beams and not alphas or (2) alpha particles are more susceptible to either an instability or Coulomb drag than proton beams.

Evolution of Proton and Alpha Particle Velocities through the Solar Cycle

Abstract Relative properties of solar wind protons and α particles are often used as indicators of a source region on the solar surface, and analysis of their evolution along the solar wind path tests our understanding of physics of multicomponent magnetized plasma. The paper deals with the comprehensive analysis of the difference between proton and α particle bulk velocities at 1 au with a special emphasis on interplanetary coronal mass ejections (ICMEs). A comparison of about 20 years of Wind observations at 1 au with Helios measurements closer to the Sun (0.3–0.7 au) generally confirms the present knowledge that (1) the differential speed between both species increases with the proton speed; (2) the differential speed is lower than the local Alfvén speed; (3) α particles are faster than protons near the Sun, and this difference decreases with the increasing distance. However, we found a much larger portion of observations with protons faster than α particles in Wind than in Helios data and attributed this effect to a preferential acceleration of the protons in the solar wind. A distinct population characterized by a very small differential velocity and nearly equal proton and α particle temperatures that is frequently observed around the maximum of solar activity was attributed to ICMEs. Since this population does not exhibit any evolution with increasing collisional age, we suggest that, by contrast to the solar wind from other sources, ICMEs are born in an equilibrium state and gradually lose this equilibrium due to interactions with the ambient solar wind.

Observations of high and low Fe charge states in individual solar wind streams with coronal-hole origin

The solar wind originating from coronal holes is comparatively well-understood and is characterized by lower densities and average charge states compared to the so-called slow solar wind. Except for wave perturbations, the average properties of the coronal-hole solar wind are passably constant. In this case study, we {ocus on observations of the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) of individual streams of coronal-hole solar wind that illustrate that although the O and C charge states are low in coronal-hole wind, the Fe charge distribution is more variable. In particular, \markmeold{we illustrate that the Fe charge states in coronal-hole solar wind are frequently as high as in slow solar wind. We selected individual coronal-hole solar wind} streams based on their collisional age as well as their respective O and C charge states and analyzed their Fe charge-state distributions. Additionally, with a combination of simple ballistic back-mapping and the potential field source surface model, transitions between streams with high and low Fe charge states were mapped back to the photosphere. The relative frequency of high and low Fe charge-state streams is compared for the years 2004 and 2006. We found several otherwise typical coronal-hole streams that include Fe charge states either as high as or lower than in slow solar wind. Eight such transitions in 2006 were mapped back to equatorial coronal holes that were either isolated or connected to the northern coronal-hole. Attempts to identify coronal structures associated with the transitions were so far inconclusive.

Constraining Solar Wind Heating Processes by Kinetic Properties of Heavy Ions.

We analyze the heavy ion components (A>4 amu) in collisionally young solar wind plasma and show that there is a clear, stable dependence of temperature on mass, probably reflecting the conditions in the solar corona. We consider both linear and power law forms for the dependence and find that a simple linear fit of the form T_{i}/T_{p}=(1.35±.02)m_{i}/m_{p} describes the observations twice as well as the equivalent best fit power law of the form T_{i}/T_{p}=(m_{i}/m_{p})^{1.07±.01}. Most importantly we find that current model predictions based on turbulent transport and kinetic dissipation are in agreement with observed nonthermal heating in intermediate collisional age plasma for m/q<3.5, but are not in quantitative or qualitative agreement with the lowest collisional age results. These dependencies provide new constraints on the physics of ion heating in multispecies plasmas, along with predictions to be tested by the upcoming Solar Probe Plus and Solar Orbiter missions to the near-Sun environment.

SOLAR WIND COLLISIONAL AGE FROM A GLOBAL MAGNETOHYDRODYNAMICS SIMULATION

ABSTRACT Simple estimates of the number of Coulomb collisions experienced by the interplanetary plasma to the point of observation, i.e., the “collisional age”, can be usefully employed in the study of non-thermal features of the solar wind. Usually these estimates are based on local plasma properties at the point of observation. Here we improve the method of estimation of the collisional age by employing solutions obtained from global three-dimensional magnetohydrodynamics simulations. This enables evaluation of the complete analytical expression for the collisional age without using approximations. The improved estimation of the collisional timescale is compared with turbulence and expansion timescales to assess the relative importance of collisions. The collisional age computed using the approximate formula employed in previous work is compared with the improved simulation-based calculations to examine the validity of the simplified formula. We also develop an analytical expression for the evaluation of the collisional age and we find good agreement between the numerical and analytical results. Finally, we briefly discuss the implications for an improved estimation of collisionality along spacecraft trajectories, including Solar Probe Plus.

THERMALIZATION OF HEAVY IONS IN THE SOLAR WIND

Observations of velocity distribution functions from the Advanced Composition Explorer/Solar Wind Ion Composition Spectrometer heavy ion composition instrument are used to calculate ratios of kinetic temperature and Coulomb collisional interactions of an unprecedented 50 ion species in the solar wind. These ions cover a mass per charge range of 1–5.5 amu/e and were collected in the time range of 1998–2011. We report the first calculation of the Coulomb thermalization rate between each of the heavy ion (A > 4 amu) species present in the solar wind along with protons (H+) and alpha particles (He2+). From these rates, we find that protons are the dominant source of Coulomb collisional thermalization for heavy ions in the solar wind and use this fact to calculate a collisional age for those heavy ion populations. The heavy ion thermal properties are well organized by this collisional age, but we find that the temperature of all heavy ions does not simply approach that of protons as Coulomb collisions become more important. We show that He2+ and C6+ follow a monotonic decay toward equal temperatures with protons with increasing collisional age, but O6+ shows a noted deviation from this monotonic decay. Furthermore, we show that the deviation from monotonic decay for O6+ occurs in solar wind of all origins, as determined by its Fe/O ratio. The observed differences in heavy ion temperature behavior point toward a local heating mechanism that favors ions depending on their charge and mass.

The tumbling spin state of (99942) Apophis

Our photometric observations of Asteroid (99942) Apophis from December 2012 to April 2013 revealed it to be in a state of non-principal axis rotation (tumbling). We constructed its spin and shape model and found that it is in a moderately excited Short Axis Mode (SAM) state with a ratio of the rotational kinetic energy to the basic spin state energy E/E0=1.024±0.013. (All quoted uncertainties correspond to 3σ.) The greatest and intermediate principal moments of inertia are nearly the same with I2/I3=0.965-0.015+0.009, but the smallest principal moment of inertia is substantially lower with I1/I3=0.61-0.08+0.11; the asteroid’s dynamically equivalent ellipsoid is close to a prolate ellipsoid. The precession and rotation periods are Pϕ=27.38±0.07h and Pψ=263±6h, respectively; the strongest observed lightcurve amplitude for the SAM case is in the 2nd harmonic of P1=Pϕ-1-Pψ-1-1=30.56±0.01h. The rotation is retrograde with the angular momentum vector’s ecliptic longitude and latitude of 250° and -75° (the uncertainty area is approximately an ellipse with the major and minor semiaxes of 27° and 14°, respectively). An implication of the retrograde rotation is a somewhat increased probability of the Apophis’ impact in 2068, but it is still very small with the risk level on the Palermo Scale remaining well below zero. Apophis is a member of the population of slowly tumbling asteroids. Applying the theory of asteroid nutational damping by Breiter et al. (Breiter, S., Rożek, A., Vokrouhlický, D. [2012]. Mon. Not. R. Astron. Soc. 427, 755–769), we found that slowly tumbling asteroids predominate in the spin rate–size range where their estimated damping times are greater than about 0.2Gyr. The appearance that the PA/NPA rotators transition line seems to follow a line of constant damping time may be because there are two or more asteroid spin evolution mechanisms in play, or the factor of μQ (the elastic modulus times the quality factor) is not constant but it may decrease with decreasing asteroid size, which would oppose the trend due to decreasing collisional age or excitation time.

ON THE RELATIVE SPEED AND TEMPERATURE RATIO OF SOLAR WIND ALPHA PARTICLES AND PROTONS: COLLISIONS VERSUS WAVE EFFECTS

We study the relative flow speed and the temperature ratio of alpha particles and protons and their connections to the helium ion abundance, the collisional age, and the power of transverse fluctuations within the inertial range. It is found that the alpha-to-proton temperature ratio, T{sub {alpha}}/T{sub p} , anti-correlates with the helium ion abundance. Despite a relatively high collisional age and small wave power, the ratio T{sub {alpha}}/T{sub p} can reach comparatively high values (even above 2) whenever the helium ion abundance is below about 0.02. In contrast, the differential speed of alpha particles with respect to protons is correlated with the total wave power and anti-correlated with the collisional age. Ultimately, the individual heating of each ion species is positively correlated with the total wave power. Our findings suggest that a high-friction collision could be efficient in reducing the differential speed between alpha particles and protons, but appears not to be sufficient to equalize the alpha and proton temperatures, i.e., to make T{sub {alpha}} {approx_equal} T{sub p} . This is a hint that the local wave heating process is acting on a timescale shorter than the collision time.

Hot Solar-Wind Helium: Direct Evidence for Local Heating by Alfvén-Cyclotron Dissipation

A study of solar-wind hydrogen and helium temperature observations collected by the Wind spacecraft offers compelling evidence of heating by an Alfvén-cyclotron dissipation mechanism. Observations are sorted by the rate of Coulomb interactions, or collisional age, in the plasma and the differential flow between the two species. We show that helium is preferentially heated perpendicular to the magnetic field direction by more than a factor of 6 when the flow between the species is small relative to the Alfvén wave speed and collisions are infrequent. These signatures are consistent with predictions of dissipation in the presence of multiple ion species. We also report an unexpected result: observations of efficient heating of helium parallel to the magnetic field for large differential flow relative to the sound speed.

Electron temperature anisotropy constraints in the solar wind

We have performed a statistical study of a substantial amount of electron data acquired in the solar wind to understand the constraints on electron temperature anisotropy by plasma instabilities and Coulomb collisions. We use a large data set of electron measurements from three different spacecraft (Helios I, Cluster II, and Ulysses) collected in the low ecliptic latitudes covering the radial distance from the Sun from 0.3 up to 4 AU. We estimate the electron temperature anisotropy using fits of the measured electron velocity distribution functions acquired in situ. We use a two population (core and halo) analytical model and properties of both populations are studied separately. We examine all the acquired data in terms of temperature anisotropy versus parallel electron plasma beta, and we relate the measurements to the growth rates of unstable modes. The effect of Coulomb collisions is expressed by the electron collisional age Ae defined as the number of collisions suffered by an electron during the expansion of the solar wind. We show that both instabilities and collisions are strongly related to the isotropisation process of the electron core population. In addition we examine the radial evolution of these effects during the expansion of the solar wind. We show that the bulk of the solar wind electrons are constrained by Coulomb collisions, while the large departures from isotropy are constrained by instabilities.