SH44A-01 INVITED 16:00h
The low frequency spectrum of solar wind turbulence and its relationship to wave dissipation in the solar corona
Present theories and models of coronal heating and solar wind acceleration have difficulty matching observed solar wind parameters while simultaneously satisfying important observational constraints. These constraints include the strong indication from Helios data that the source spectrum of solar wind magnetic fluctuations has a $1/f$ power spectrum. If one assumes that the spectrum at the base of the corona is $1/f$ (which might arise from a kinematical superposition of signals due to scale-invariant reconnection of magnetic structures on the solar surface), then one must construct a theory that progresses from topological rearrangement of magnetic fields to a frequency spectrum at the top of the corona that has a $1/f$ spectrum. Progress has also been made in understanding how other photospheric motions might generate the observed spectral power. One difficulty with such theories is that even when one initiates such calculations with a $1/f$ spectrum, the spectra at the out-flow boundary have evolved to be steeper than $1/f$. Damping that occurs only near the cyclotron frequency might preserve an initial $1/f$ spectrum, but that has yet to be demonstrated. One alternative (explored by {\it Dmitruk et al.} [2002] and {\it Oughton et al.} [2001]) is that the initial fluctuations give rise to a reduced magnetohydrodynamic (RMHD), quasi-two-dimensional cascade of $k_\bot$ modes that heat the corona. The fluctuating power in $k_\parallel$ then escapes into the solar wind. Simulations to date, however, cannot maintain a $1/f$ spectrum because the dissipation in the simulations is broadband. Another observation that theories of wave heating and acceleration of the corona must address is that the total energy available from waves to heat the corona and accelerate the fast solar wind appears to be inadequate. \hangindent=10pt Dmitruk, P., W.H. Matthaeus, L.J. Milano, S. Oughton, G.P. Zank, and D.J. Mullan, Coronal heating distribution due to low-frequency, wave-driven turbulence, {\it The Astrophys. J., 575} (1), 571-577, 2002. \hangindent=10pt Oughton, S., W.H. Matthaeus, P. Dmitruk, L.J. Milano, G.P. Zank, and D.J. Mullan, A reduced magnetohydrodynamic model of coronal heating in open magnetic regions driven by reflected low-frequency Alfv\'en waves, {\it The Astrophys. J., 551} (1), 565-575, 2001.
SH44A-02 16:20h
A mechanism of dissipation of the perpendicular turbulent cascade in the solar wind
We discuss a mechanism of dissipation that allows us to explain several key features of the turbulent fluctuations in the solar wind. The observational data suggest that the solar wind turbulence is dominated by fluctuations with wavevectors nearly perpendicular to the background magnetic field. This is in agreement with numerical simulations showing that the turbulent cascade tends to produce small spatial scales across the magnetic field rather than along it. The dissipation of the turbulent fluctuations is thought to be responsible for the observed perpendicular heating of the solar wind protons. The problem, however, is that the perpendicular heating is usually a signature of the cyclotron resonance, while the cross-field fluctuations cannot be immediately cyclotron-resonant with the protons. We suggest that the velocity shear associated with the cross-field fluctuations can excite a proton cyclotron instability. These unstable waves will then transfer the energy from the cross-field fluctuations to the protons thus dissipating the cascade and producing the perpendicular heating. We analyze the observed turbulence spectra and show that the threshold of the instability is consistent with the spectral break separating the inertial and dissipation ranges of the turbulence. In particular, during the periods of strong variation of the plasma beta in the solar wind, the threshold scales as the proton inertial length rather than the proton gyroradius, in agreement with the prediction of the theory. The observed turbulence spectra often have power-law dissipation ranges with an average spectral index of -3. We demonstrate that this fact is simply a consequence of a marginal state of the instability in the dissipation range.
SH44A-03 INVITED 16:33h
Review of Solar Wind Heating: Observations and Theory
Observations from Helios, Pioneer 11, and the Voyager 1/2 spacecraft provide clear evidence for heating of the solar wind protons. Several sources of energy to drive the heating are now recognized, from residual waves originating in the low solar corona that may be at least partially responsible for heating the inner heliosphere, to wind shear and shocks that provide energy to heat the solar wind inside 10 to 20 AU, to waves generated by newborn interstellar pickup ions that heat the outer heliosphere. These processes imply the need for a theory to convert energy associated with large-scale structures into the small-scale fluctuations that can interact with the protons via familiar plasma processes. We will briefly review such a theory and test it against the observations. We will examine several formalisms for computing an inferred turbulent heating rate from magnetic field measurements and compare their predictions with observations. We will also review past and recent attempts to identify the plasma processes responsible for converting magnetic energy into heat. This involves studying the so-called dissipation range and resolving its properties in comparison with the more familiar inertial range of interplanetary magnetic fluctuations. While this will not lead to a final decision on what plasma processes heat the solar wind, it will provide a list of diagnostics and observations that constrain the discussion.
SH44A-04 16:53h
Alfvenic Turbulence Dissipation Near the Sun as Inferred From Radio Scattering Observations
Radio scattering and scintillation (IPS) observations offer a powerful technique for studying dissipation-range plasma turbulence in the near-Sun solar wind. Although the physical mechanism responsible for the low-frequency, power-law component of the density fluctuation spectrum remains uncertain, we can show that the enhanced high-frequency end of the spectrum is almost certainly the direct signature of the linear compressibility of obliquely propagating Alfven waves in the ion-cyclotron regime. The evidence for this is (1) the enhancement is just as expected from an $R^{-4}$ inward extrapolation of spacecraft magnetic spectra, and (2) IPS observations close to the Sun show large parallel ``random" velocities and an apparent wave bias that is consistent with the shear-Alfven dispersion relation. An important implication is that the observed density spectrum inner scale must be the direct signature of the dissipation of Alfven waves or Alfvenic turbulence. A passive damped-WKB model for the wave spectrum evolution is inadequate, as it predicts a stronger erosion of the spectrum by electron Landau damping than is consistent with radio and spacecraft data. Invoking a Kolmogorov cascade can counteract this erosion and push the spectral cutoff back out to near the ion inertial scale, where proton cyclotron damping becomes important. The wave energy dissipation in our damped cascade model is substantial, amounting to an equivalent base power flux of $5\times 10^5$ erg/cm$^2$-s available for extended heating between 5--20 solar radii. For a cascade at the nominal Kolmogorov rate, the electron Landau and proton cyclotron heating are about equal. The estimated proton cyclotron contribution diminishes rapidly for weaker cascades and hence can be very sensitive to the precise inner scale size.
SH44A-05 INVITED 17:06h
Turbulence and Dissipation in the Solar Wind: Wind Observations at 1 AU
The solar wind is a collisionless, strongly turbulent plasma in a supersonic and super-Alfv\'enic spherical expansion. One of its most striking features is the very large number of degrees of freedom that are excited: the electromagnetic fields and plasma properties of the solar wind show fluctuations over a very wide range of time scales, ranging from the solar rotation period down to the local electron plasma period, i.e. for frequencies $f$ from $10^{-6}$ Hz up to $f_{pe} \sim 2\cdot 10^{4}$ Hz. We present and discuss here a global spectrum (2 months average) of solar wind electromagnetic fluctuations over the above-mentioned frequency range, obtained using data from several instruments on the WIND spacecraft at 1 AU near L1. The observed power spectrum shows a power-law $f^{-5/3}$ in the Alv\'enic frequency range, between $10^{-4}$ to 0.4 Hz, but it steepens to roughly $f^{-3}$ up to the local electron cyclotron frequency ($ f_{ce} \sim 100 - 200$ Hz). It is now widely recognized that the $f^{-5/3}$ spectrum is the "inertial range" of MHD turbulence in the solar wind, resulting from a nonlinear energy cascade operating from the large "energy containing" scales to smaller scales where dissipation is presumed to act. The aim of this paper is to address both questions of turbulence and dissipation in the solar wind at 1 AU. For that, I will present new results of recent investigations using WIND data at 1 AU near L1, covering both the inertial and the $f^{-3}$ ranges. In a first part, results on the scaling properties and intermittent character of solar wind fluctuations in the inertial range will be discussed. The spectral steepening around 0.4 Hz is commonly believed to be due to collisionless ion-cyclotron damping of Alfv\'enic-like fluctuations present in the inertial range. But, is damping really the dominant effect or is the $f^{-3}$ power-law from 0.4 Hz to $f_{ce}$ sustained by some other kinetic effects that would overcome damping, as pointed out by some recent theoretical interpretations? To address this question, in a second part, i will discuss the nature of the fluctuations in this range, their properties, their possible source(s) and their role in the control of the energy transport in the solar wind.
SH44A-06 17:26h
Evidence for Cyclotron Resonant Damping of Waves by Multiple Ion Species at 1 AU
We explore the role of helium in regulating the absorption of Alfven waves near ion cyclotron frequencies. This is accomplished through a systematic study of the relationships between hydrogen and helium temperatures and temperature anisotropies as a function of plasma parameters such as differential flow speeds, plasma beta, and Coulomb collision rate. It will be demonstrated that this analysis allows us to distinguish between the roles played by adiabatic expansion, Coulmb relaxation, plasma micro-instabilities, and wave-damping in regulating solar wind ion temperatures. Our main observational result is that helium is observed to be preferentially heated relative to hydrogen in directions perpendicular to the magnetic field during intervals with low Coulomb collision rates and small diffrential flow speeds. The ratio of perpendicular temperatures, $T_{\perp \alpha}/T_{\perp p}$ grows with decreasing differential flow speed and can exceed a factor of 5. The fact that $T_{\perp \alpha}/T_{\perp p}$ can become so large and is a strong function of the differential flow speed between the hydogren and helium is interpreted in the light of recent theoretical predictions of the damping rates of cyclotron waves in multi-species plasmas.
SH44A-07 INVITED 17:39h
Test particle energization on short time scales by MHD turbulence: clues to dissipation mechanisms?
We consider the motion of test particles on short time scales in fields obtained from direct numerical simulations of three-dimensional MHD turbulence with a strong background magnetic field. We observe different properties of the velocity distribution of particles, whether electrons or protons are considered, due to the different ratio of particles' gyroradii to the MHD dissipative scales. The anisotropy of the distributions is suggestive of some of the observed properties of solar energetic particles. Although the test particle approach is incomplete, we speculate that these results could be useful on understanding possible dissipation mechanisms for models of coronal heating based on formation of current sheets and quasi-2D turbulence.