The dissipation range for interplanetary magnetic field fluctuations is formed by those fluctuations with spatial scales comparable to the gyroradius or ion inertial length of a thermal ion. It is reasonable to assume that the dissipation range represents the final fate of magnetic energy that is transferred from the largest spatial scales via nonlinear processes until kinetic coupling with the background plasma removes the energy from the spectrum and heats the background distribution. Typically, the dissipation range at 1 AU sets in at spacecraft frame frequencies of a few tenths of a hertz. It is characterized by a steepening of the power spectrum and often demonstrates a bias of the polarization or magnetic helicity spectrum. We examine Wind observations of inertial and dissipation range spectra in an attempt to better understand the processes that form the dissipation range and how these processes depend on the ambient solar wind parameters (interplanetary magnetic field intensity, ambient proton density and temperature, etc.). We focus on stationary intervals with well-defined inertial and dissipation range spectra. Our analysis shows that parallel-propagating waves, such as Alfven waves, are inconsistent with the data. MHD turbulence consisting of a partly slab and partly two-dimensional (2-D) composite geometry is consistent with the observations, while thermal paxticle interactions with the 2-D component may be responsible for the formation of the dissipation range. Kinetic Alfven waves propagating at large angles to the background magnetic field are also consistent with the observations and may form some portion of the 2-D turbulence component.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JA03394

Observational constraints on the dynamics of the interplanetary magnetic field dissipation range

Kinetic plasma physics of the solar corona and solar wind are reviewed with emphasis on the theoretical understanding of the in situ measurements of solar wind particles and waves, as well as on the remote-sensing observations of the solar corona made by means of ultraviolet spectroscopy and imaging. In order to explain coronal and interplanetary heating, the microphysics of the dissipation of various forms of mechanical, electric and magnetic energy at small scales (e.g., contained in plasma waves, turbulences or non-uniform flows) must be addressed. We therefore scrutinise the basic assumptions underlying the classical transport theory and the related collisional heating rates, and also describe alternatives associated with wave-particle interactions. We elucidate the kinetic aspects of heating the solar corona and interplanetary plasma through Landau- and cyclotron-resonant damping of plasma waves, and analyse in detail wave absorption and micro instabilities. Important aspects (virtues and limitations) of fluid models, either single- and multi-species or magnetohydrodynamic and multi-moment models, for coronal heating and solar wind acceleration are critically discussed. Also, kinetic model results which were recently obtained by numerically solving the Vlasov‐Boltzmann equation in a coronal funnel and hole are presented. Promising areas and perspectives for future research are outlined finally.

/pdf/kinetic-physics-of-the-solar-corona-and-solar-wind-bduwadfu6p.pdf

Kinetic Physics of the Solar Corona and Solar Wind

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Gravitation And Cosmology

Measurements of stellar mass-loss rates are used to assess how wind strength varies with coronal activity and age for solar-like stars. Mass loss generally increases with activity, but we find evidence that winds suddenly weaken at a certain activity threshold. Very active stars are often observed to have polar starspots, and we speculate that the magnetic field geometry associated with these spots may be inhibiting the winds. Our inferred mass-loss/age relation represents an empirical estimate of the history of the solar wind. This result is important for planetary studies as well as solar/stellar astronomy, since solar wind erosion may have played an important role in the evolution of planetary atmospheres.

http://iopscience.iop.org/article/10.1086/432716/pdf

New Mass-Loss Measurements from Astrospheric Lyα Absorption

The interaction of the solar wind with the local interstellar medium (LISM) is attracting renewed interest, thanks to the possibility that the Voyager spacecraft may, in the not too distant future, cross the heliospheric termination shock. This has spurred the development of increasingly sophisticated models which attempt to describe various aspects of the physics underlying the interaction of the solar wind and the LISM. A comprehensive review of the subject is presented here.

/pdf/interaction-of-the-solar-wind-with-the-local-interstellar-wg8st723sd.pdf

Interaction of the solar wind with the local interstellar medium: a theoretical perspective

First results from a two-dimensional (axisymmetric) time-dependent gas dynamic model for the interaction of the solar wind with the local interstellar medium are presented The model includes the mutual influence of the interstellar and interplanetary plasma (protons and electrons) and the neutral interstellar hydrogen atoms Neutrals created by charge exchange with the solar wind are ignored; this allows us to approximate the dynamical evolution of the system with the isotropic fluid equations A supersonic interstellar wind is assumed The global structure of the heliosphere and interaction region is described for both the plasma and neutral gas The self-consistent model is compared to a noninteracting gas dynamic model, and dramatic differences in the size of the heliosphere are found The distribution of neutral hydrogen in the heliosphere is also discussed

Interaction of the solar wind with the local interstellar medium

A distinct fluid description for neutral hydrogen originating from either the interstellar medium, the heliosheath, or the solar wind has been developed to describe the detailed interaction of the solar wind with the local interstellar medium (LISM) [Williams et al., 1995]. Such a multifluid description for the hydrogen serves to capture the highly anisotropic nature of the neutral distribution. The neutral multifluid is coupled to a hydrodynamic plasma through charge exchange and the time-dependent model is solved in two spatial dimensions. This approach is used (1) to elucidate the nature of the solar wind - LISM interaction more carefully and precisely than has been done previously; (2) to identify precisely the role played by the three identifiably distinct neutral populations in determining the global structure and dynamics of the heliosphere (it is found, for example, that the heliopause is weakly time-dependent for the two-shock model); (3) to contrast these results with those obtained from the simpler, computationally more efficient model developed by Pauls et al. [1995]; and (4) to investigate the differences in global heliospheric structure and neutral properties between a two-shock (which results from a supersonic interstellar wind impinging on the heliosphere) and a one-shock (i.e., for a subsonic interstellar wind) model. In particular, observational criteria are presented that may allow one to distinguish between a one-shock and a two-shock heliosphere.

Interaction of the solar wind with the local interstellar medium: A multifluid approach

By using the technique of Johnstone et al. (1991), the asymptotic wave spectrum induced by the pickup of interstellar ions in the heliosphere is derived. This approach is independent of quasi-linear theory. The asymptotic wave spectrum is calculated from the standard resonance condition by assuming that the asymptotic ion distribution is bispherical and by using conservation of energy. The calculation places no restriction on either the ratio of Alfven speed to solar wind flow speed or the particle pitch angle. Spectra for various geometries (relative orientations of the magnetic field and solar wind velocity) are calculated. Absolute upper limits to the expected wave enhancements are calculated. These upper limits are compared to the background fluctuation spectrum extrapolated from 0.87 AU, assuming a spectral index of -5/3 and radial dependences of -2 and -3. The heliocentric radial distances at which the peaks in the various geometry-dependent wave spectra are expected to be observable above the background are identified. Peaks due to pickup hydrogen at the proton gyrofrequency are found to be always stronger relative to the background in parallel than in perpendicular geometries. The parallel-geometry excitations consist only of sunward propagating waves. In perpendicular geometries a peak also occurs, this time near 0.1 of the proton gyrofrequency (corresponding to the peak calculated by Lee and Ip (1987)). However, the steep spectral dependence of the ambient wave field makes it unlikely that this peak will be observed inside 20 AU. A similar result holds for the case of pickup-helium induced excitation at the helium gyrofrequency. Inside 10 AU, peaks at these lower frequencies tend to be obscured by the steep frequency dependence (proportional to -5/3) of the ambient spectrum and then are only observable in parallel geometries.

Effect of magnetic field geometry on the wave signature of the pickup of interstellar neutrals

The deposition of energy into the solar wind by the pickup of interstellar neutrals is due to both the creation of hot, nonthermal ions and the associated generation of low frequency magnetohydrodynamic (MHD) waves. Dissipation of some fraction of the free wave energy released by ion pickup and isotropization is possible through nonlinear turbulent processes which may lead to heating of the core thermal solar wind proton distribution. Simple energy budget arguments are utilized to show that pickup ion generated wave dissipation may play a significant role in determining the solar wind radial temperature profile in the outer heliosphere. In particular, depending on the density of interstellar hydrogen in the heliosphere, there will be some radial distance beyond which the thermal solar wind core temperature increases steadily until the termination shock. Existing Pioneer and Voyager temperature profiles are consistent with this interpretation.

Dissipation of pickup‐induced waves: A solar wind temperature increase in the outer heliosphere?

We investigate the self-consistent interaction of the local interstellar cloud (LIC) and solar wind, focusing on its manifestations in the heliospheric hydrogen (H) distribution. This system is modeled hydrodynamically as a fluid proton-electron plasma and three H fluids, each arising from charge exchange production within three distinct plasma environments. Perhaps our most significant finding is that, based on the Dalgarno cross section, thermalizing H-H collisions are crucial to determining the heliospheric H distribution. Hot secondary H atoms produced from charge exchange with the solar wind will be thermalized with the bulk of the cooler LIC H distribution. This thermalization should be complete for the postheliospheric H beyond ~108 downstream from the Sun. Observed along nearby downstream interstellar sightlines, we may expect to see temperatures of order 105 K in the bulk of the postheliospheric-traversal H distribution. Recent observations along the Sirius sightline by Bertin et al. may be explainable in these terms. Based on the Dalgarno cross section for thermalizing H-proton collisions without charge transfer, this interaction should be important as well. This implies that previous charge exchange-only models have underestimated the degree of H-proton coupling and thereby the efficiency of heliospheric filtration of LIC H. The variation among published values of the charge exchange cross section is 40% at 1 eV. We find that this variation will affect predictions of the H density at 50 AU by a similar factor. We performed calculations using the larger of the charge exchange cross sections to conclude that the proton density in the LIC is not likely greater than a few ×10-2 cm-3. We show densities, temperatures, and radial velocities of the three H fluids along the sight lines of some nearby stars for one of these calculations, using an LIC proton density of 0.1 cm-3. Results from a first crude model of the effects of H-H collisions are given in an appendix.

L. L. Williams

Papers

Interaction of the solar wind with the local interstellar medium

Interaction of the solar wind with the local interstellar medium: A multifluid approach

Effect of magnetic field geometry on the wave signature of the pickup of interstellar neutrals

Dissipation of pickup‐induced waves: A solar wind temperature increase in the outer heliosphere?

The Heliospheric Hydrogen Distribution: A Multifluid Model