SH51C-0278 0800h
Alpha Particle Heating in the Solar Wind: ACE Observations
Alfv\'en-cyclotron fluctuations of successively shorter wavelengths in collisionless plasmas transfer energy via cyclotron resonances to ions of successively larger charge-to-mass ratios. Theories and simulations predict that this interaction leads to $T_{\perp}/T_{\parallel} >$ 1 for each resonant ion species, where the subscripts refer to directions relative to the background magnetic field. Here observations of the solar wind near 1 AU by the plasma and magnetic field instruments on the ACE spacecraft are reported which show the predicted signatures of such wave-ion interactions in the proton and alpha particle anisotropies. These results support the theory that Alfv\'en-cyclotron fluctuations are an important source of heavy ion heating in the corona.
http://nis-www.lanl.gov/~pgary/
SH51C-0279 0800h
Proton Landau Damping of Oblique Alfven Waves in the Solar Wind
The role of Landau damping on protons in dissipation of Alfven waves in the solar wind is studied. It is found that in a finite-beta plasma, Landau resonant interaction of oblique Alfven waves with protons can lead to their effective damping. The problem of power-law spectra of MHD waves in the solar wind plasma is discussed.
SH51C-0280 0800h
An Extension of the Quasi-linear Theory on Shock Acceleration to Low-energy Ions
The quasi-linear theory on self-consistent wave generation and ion acceleration upstream of an interplanetary traveling shock by M.A. Lee (1983) applies for highly super-Alfv\'{e}nic ions amplifying pure Alfv\'{e}n waves in the solar wind. Data from the CELIAS/STOF sensor on board SOHO and from the magnetometer on board ACE indicate that slightly super-Alfv\'{e}nic ions interact with proton cyclotron waves in the upstream vicinity of the shock. We extend the theory by Lee (1983) to apply for slightly super-Alfv\'{e}nic ions and for proton cyclotron waves. In addition to our study of the upstream wave-particle interactions, the wave generation and dissipation processes in the shock transition region and in the downstream plasma are explored.
SH51C-0281 0800h
Heating and Acceleration of Minor Ions in the Expanding Solar Wind
We present hybrid simulations of the interaction of Alfven waves with protons, alpha particles and a small abundance of oxygen 5+ using the expanding box model. The simulations test the sweeping mechanism of the heating and acceleration of the solar wind by cyclotron resonance with Alfven waves. The numerical simulations indicate that oxygen (and other minor ions) are efficiently heated in perpendicular direction and accelerated but are able to absorb only a limited amount of available energy in the Alfven waves. The presence of oxygen ions has a minimal influence on alpha particles and protons. However, for the parameters used in the simulations the heating and acceleration of alpha particles and protons are not very efficient. We also explore the role of the radial stretching which occurs in the acceleration region of the wind. This amounts to an expansion of the box also in the radial direction and leads to important parallel cooling. The combination of the perpendicular heating by Alfven wave and the parallel cooling induced by the radial expansion leads to the strong temperature anisotropies of oxygen ions. The simulations are discussed within the context of observations and theoretical models of the evolution of MHD turbulence and ion thermodynamics in the outer corona and accelerating solar wind.
SH51C-0282 0800h
Turbulence and the Third Moment of Fluctuations: Kolmogorov's 4/5 law and its MHD Analogues in the Solar Wind
The assertion that the third moment of fluctuations at lag L, is proportional to L in the inertial range is fundamental to Kolmogorov's early and late theories of inertial-range turbulence, and all models of intermittent hydrodynamic turbulence obey it. Kolmogorov's "4/5 law" more specifically states that the third moment of longitudinal fluctuations is equal to minus 4/5 times the length scale times the energy dissipation rate per unit mass, E. Longitudinal fluctuations are the component of velocity parallel to the lag direction, e.g. fluctuations in radial velocity parallel to the solar wind. For solar wind measured by a single spacecraft using the Taylor hypothesis, the longitudinal fluctuation is [-Vr(t+L)+ Vr(t)]. The 4/5 law then states that $<$[Vr(t+L/V)- Vr(t)]3$>$ = + 4/5 EL.. The pdf of the fluctuations cannot be symmetric in an energy-conserving cascade, i.e., in an inertial range. We are not aware of a theory of the shape of the pdf that would characterize the sign and scaling of the third moment. MHD equivalents of the 4/5 law using the Elsasser variables, have been proposed by Politano and Pouquet and others and discussed in Biskamp's book on MHD turbulence. Since the third moment has such a crucial role in turbulence theory, we thought we ought to find out how it actually behaves in the solar wind, and if it is related to the energy dissipation rate as theories imply. We used ACE solar wind data to calculate signed third moments at lags from 64 seconds to days. They are indeed proportional to lag and positive in sign. Dissipation rates inferred from these moments compare well with the energy dissipation rates computed with other measures of the heating of the solar wind as inferred from Helios and Voyager and from the observed power spectrum of IMF fluctuations.
SH51C-0283 0800h
Spectral Maps of Collisionless-MHD Turbulence in $k{_\parallel}$-$k_{\perp}$ Space
For turbulence in fluids, the energy cascade is famously depicted by a one-dimensional (omnidirectional) wavenumber spectrum that is divided into three regions: an energy subrange at small k, an inertial subrange at intermediate k, and a dissipation subrange at large $k$. Eddy-eddy interactions produce a net flow of energy from small-$k$ to large-$k$. For turbulence in collisionless plasmas, the magnetic field ${\vec B}$ of the plasma defines a coordiante system with behavior parallel to${\vec B}$ differing from behavior perpendicular to ${\vec B}$. A map of MHD turbulence in $k_\parallel$-$k_\perp$ space is considered. Owing to the magnetic field, every turbulent fluctuation in a plasma has an eddy nature and an Alfven-wave nature. The wave nature of the fluctuations has two major effects. First, the wave nature splits the inertial subrange into two regions in $k_\parallel$-$k_\perp$ space: a region where eddy-eddy interactions happen more quickly than Alfven-wave effects (the Kolmogorov-turbulence region at $k_\perp$ $\gg$ $k_\parallel$) and a region where eddy-eddy interactions happen more slowly than Alfven-wave effects (the Kraichnan-turbulence region at $k_\perp$ $\ll$ $k_\parallel$). Second, the wave nature of the fluctuations allows wave-particle interactions to dissipate the turbulence. The boundary ("inner scale") between the inertial subrange and the dissipation subrange is a curve in $k_\parallel$-$k_\perp$ space that depends on where electron Landau damping, ion Landau damping, or ion cyclotron damping dominates over eddy-eddy energy transfer, which differs for left-hand-polarized (Alfven-cyclotron branch) and right-hand-polarized (magnetosonic-whister branch) turbulent fluctuations. For solar-wind turbulence, the two $k_\parallel$-$k_\perp$ maps are drawn: one for the left-hand fluctuations in the turbulence and one for the right-hand fluctuations in the turbulence.
SH51C-0284 0800h
Anisotropic MHD/EMHD Turbulence in the Solar Wind and the Interstellar Medium
The solar wind and the interstellar medium is permeated by large-scale magnetic fields that render magnetohydrodynamic (MHD) turbulence anisotropic. In the weak-turbulence limit in which three-wave interactions dominate, analytical and high-resolution numerical results based on random scattering of shear-Alfv\'en waves propagating parallel to a large-scale magnetic field demonstrate rigorously an anisotropic energy spectrum that scales as $k_{\perp}^{-2}$, instead of the famous Iroshnikov-Kraichnan spectrum of $k^{-3/2}$ for the isotropic case. Even in the absence of a background magnetic field, when the energy spectrum is globally isotropic, anisotropy is found to develop with respect to the local magnetic field. Collisionless turbulence is studied in electron magnetohydrodynamics (EMHD), where whistler waves mediate the anisotropic energy cascade. Comparisons are made with MHD turbulence, especially with respect to global and local anisotropy in both inertial and dissipation ranges. The anisotropy of the energy cascade necessarily implies anisotropy of turbulent heating. Scalings of collisional and collisionless energy spectrum and dissipation rate will be discussed.
SH51C-0285 0800h
Effect of Latitudinal Dependence of Boundary Conditions on Transport of Turbulence in the Heliosphere
A four equation MHD turbulence model describes the radial evolution of fluctuation energy, correlation scale, temperature, and cross-helicity in a specified spherically expanding solar wind flow. This model is solved numerically along every radial direction in our simulation domain, spanning the region from 0.3 AU to 100 AU, varying inner boundary conditions and parameters to account for latitudinal structure. The model involves, as parameters, the plasma shear, wind speed, and strength of pick-up ion driving, Karman-Taylor constants, a constant that depends upon turbulence geometry, and another that specifies the ratio of kinetic to magnetic energy in the fluctuations. Magnetic variance, correlation length, cross helicity and plasma temperature are given latitudinal dependence along the inner boundary at 0.3 AU. The solar wind speed, proton number density, and temperature profiles are chosen to be consistent with observations over Ulysses' first full polar orbit [McComas et al., J. Geophys. Res., 105, 10419, 2000]. A simple model of pick-up ions is employed at present, which we plan to improve, following Isenberg et al [ApJ, 592, 564 2003]. The early indication shows that the simulation results thus obtained can be brought into good agreement with Voyager and Ulysses observations using parameters and boundary conditions that are consistent with observations. An interesting feature is that a relatively high magnetic variance is required at high latitude at inner boundary to make reasonable comparisons with observations. Heating is suppressed in the inner heliosphere and at high latitudes by the cross helicity effect, and the Alfvenicity of the turbulence almost completely vanishes by 10 AU.
SH51C-0286 0800h
The Turbulent Correlation Length in the Distant Solar Wind
A model of turbulent evolution in the distant solar wind has recently been presented which incorporates the detailed quasilinear generation of fluctuations by interstellar pickup protons to drive the turbulence [{\it Isenberg et al., ApJ}, 592, 564, 2003; {\it Isenberg, ApJ}, submitted, 2004]. The dissipation of the turbulent fluctuations at the standard Kolmogorov rate has been shown to provide a reasonable agreement with observed heating of the core solar wind protons out to almost 75 AU [{\it Smith et al., ApJ}, submitted, 2004]. In this model, the correlation length of the turbulence $\lambda (r)$ is a fundamental quantity, since it controls the turbulent dissipation rate. Although analysis of observations out to 30 AU indicates that $\lambda$ steadily increases with {\it r}, the model calls for this quantity to decrease beyond about 10 AU due to the turbulent driving by the pickup protons. Here, we investigate modifications to the present model which will bring the correlation length more in line with the observations. These modifications will, in turn, affect the predicted turbulent intensities and solar wind temperatures. We will present and discuss the modified model and compare its results with solar wind observations.
SH51C-0287 0800h
Rapid Density Fluctuations in the Solar Wind
Using the EFW experiment on the Cluster spacecraft, rapid electron density fluctuations ( up to 2.5 Hz) in the solar wind have been studied, with the aim of understanding their nature and cause. The density fluctuation spectra obtained from the EFW probe potential variations are in rough agreement with earlier, OGO 5, measurements of ion density fluctuations. The electric fields corresponding to the electron density fluctuations are extremely small compared with what would be obtained if the electron fluctuations were not cancelled out by nearly equal ion density fluctuations, so that the fluctuations have the nature of ion acoustic waves. The electric field spectrum is more concentrated toward larger wave number,than the density spectrum, as would also be expected for ion acoustic waves. Correlation with magnetic fluctuations is weak, as is expected as magnetic fluctuations are known to be nearly incompressible. Nevertheless there are indications of pressure balance structures. Pressure balance structures are the nearly perpendicular propagation limit of ion acoustic waves. As ion acoustic waves are strongly damped in plasmas like the solar wind, it has always been a puzzle as to why they are found there. We speculate that these high frequency waves are created by mode conversion from magnetic fluctuations, and may represent part of the dissipation process for these.
SH51C-0288 0800h
Solar Wind Heating: Critical Tests of a Turbulence Theory
We review observations of solar wind heating in the outer heliosphere as measured by the Voyager 2 and Pioneer 11 spacecraft and described previously by a theory of pickup ion wave excitation and turbulent transport. That theory was most recently applied to observations by Smith et al. [2001] and with significant revision of the pickup proton component by Isenberg et al. [2003]. We extend the application of the theory to include time variation of solar wind parameters as recorded by the Omnitape dataset of 1 AU measurements. We also extend the range of heliocentric distances made available by the more recent Voyager data. By averaging Omnitape observations over several solar rotations and using the resulting values as input to the theory, we are able to reproduce the variability of the thermal proton temperatures observed in the outer heliosphere. This is seen to be a direct result of the dependence of energy injection by pickup protons upon bulk solar wind parameters such as Alfven speed and wind speed and the fact that these parameters persist in a predictable manner from 1 AU to the outer heliosphere. There is also evidence of latitudinal effects during solar minimum that may be explainable by using high-latitude observations by Ulysses as input for the theory. Smith et al., JGR, A106, 8253-8272 [2001] Isenberg et al., ApJ, 592, 564-573 [2003]
SH51C-0289 0800h
Dissipation Range Observations in Interplanetary Magnetic Clouds
In two earlier papers Leamon et al. [1998a,b] examined the properties of the dissipation range for interplanetary magnetic fluctuations at 1 AU. In the first paper they focused on 33 1-hour samples of open field line measurements chosen without any regard for context other than being sufficiently well-behaved for sufficient time to yield good spectra. All 33 intervals were chosen from WIND/MFI measurements in the solar wind near 1 AU. They found that the dissipation range typically set in at frequencies slightly greater than the proton cyclotron frequency, had consistently steeper forms than the associated inertial range spectra with power law indexes generally between -3 and -5, were consistently more compressive than the inertial range, and possessed wave vectors more nearly field aligned than in the inertial range. In the second paper they chose to examine intervals from within a single magnetic cloud. They found that the cloud spectra showed generally less steepening in the dissipation range than did the open field line examples. Inertial range fluctuations were significantly less compressive in the cloud examples. Very little energy was seen to reside with wave vectors parallel to the mean magnetic field in either the inertial or dissipation ranges. We have examined 30 additional magnetic clouds observed by ACE in order to develop a more statistically significant characterization of magnetic cloud dissipation range spectra near 1 AU. We find that the Leamon results characterize frequently observed aspects of cloud spectra, but that they constitute a common example within a range of possible results. In an effort to better understand the in situ heating of magnetic clouds, we present the statistics we have gathered and compare these results with typical open field line observations. Leamon et al., JGR, A103, 4775--4787, 1998a Leamon et al., GRL, 25, 2505--2509, 1998b
SH51C-0290 0800h
CLUSTER Observations of Langmuir Wave Decay in the Terrestrial Foreshock
The parametric decay of Langmuir waves is often proposed as one of the processes saturating the growth of beam-excited Langmuir waves in the Earth's foreshock. We investigated this process by analysis of high frequency electric field data obtained in the foreshock by the WBD instrument of CLUSTER. A typical observed electric field waveform contains a superposition of several waves with frequencies close to $\omega_p$ often accompanied by bursts of weaker ion-acoustic waves at lower frequencies. Such observations are consistent with the three-wave decay if the frequencies of waves satisfy a resonance condition $f_3 = f_2 + f_1$. In this study, we proved by statistical analysis that this condition is satisfied for a large number of observed triple-peaked spectra and that the relative number of such triplets increases with wave amplitude. This finding is consistent with the presumption that the decay instability can only proceed if the wave amplitude exceeds certain threshold and our analysis provides a rough estimate of this threshold value. Since the solar wind plasma is not strictly Maxwellian, but contains a significant supra-thermal component, two types of parametric decay are possible here: the classical one, where a Langmuir wave decomposes into Langmuir and ion-sound waves, or an alternative process, where one of the product waves is an electron-acoustic wave. In this presentation, we bring an experimental argument in favor of the latter alternative.
SH51C-0291 0800h
Scale-Invariance and Intermittency in the Solar Wind Alfv\'enic Turbulence: Wind Observations
In the "Alfv\'enic" regime, i.e. for frequencies below the local proton cyclotron frequency, solar wind MHD turbulence exhibits what appears like an inertial domain, with power-law spectra and scale-invariance, suggesting as in fluid turbulence, a nonlinear energy cascade from the large "energy containing" scales towards the small scales where dissipation by kinetic effects is presumed to act. However, the intermittent character of solar wind fluctuations is much more important than in ordinary fluids. Indeed, the fluctuations consist of a mixture of random fluctuations and small-scale "singular" or coherent structures. This intermittency modifies significantly the scaling exponents of actual power-law spectra, which are directly related to the physical nature of the energy cascade taking place in the solar wind. The identification of the most intermittent structures and their relation to dissipation represents then a crucial problem in the framework of turbulence. We present here a new approach to study the scaling laws and intermittency based on the use of Wavelet transforms on simultaneous WIND 3s resolution particle and magnetic field data from the 3DP and the MFi experiments respectively. Using the Haar wavelet transform, spectra and structure functions are calculated. We show that this powerful technique allows: (1) for a systematic elimination of intermittency effects on spectra and structure functions and thus for a clear determination of the actual scaling properties in the inertial range, and (2) for a direct and systematic identification of the most active, singular structures responsible for the intermittency in the solar wind. We finally discuss the various effects which may be important for the formation of these structures in the absence of collisions.