Session VM1: Mini-conference on Interface between Fluid and Kinetic Processes in Laboratory, Space and Astrophysics II

The Physics of the Solar Wind Termination Shock: What are the Ingredients?

After a 27 year journey, Voyager 1 crossed the solar wind termination shock, the first boundary separating the solar system from the rest of the galaxy. Voyager's \textit{in situ} observations both before and after the shock crossing presented some surprises, and only some of these have so far been explained. In this talk, we discuss some of these observations and some of the physical effects (magnetic fields, charge exchange, etc.) that must be included in MHD modeling of the solar wind termination and the global heliosphere in order to understand the observations. Previous models and observations of the termination shock and global heliosphere will also be discussed.   

The interaction of cosmic rays with the background plasma during shock acceleration

Acceleration of cosmic rays (CR) at shock fronts takes place through the interaction of CR, which must be modelled kinetically, with the background thermal plasma, which can be modelled as a magnetised fluid. The CR gain energy from the difference between the fluid velocities upstream and downstream of the shock. In order to match observations, the scattering of CR by fluctuations in the magnetic field must take place on the scale of a CR Larmor radius and the magnetic field must in many cases be greater than that far upstream of the shock. We show that CR naturally excite an instability which can non-linearly amplify the magnetic field by orders of magnitude. We explain how the instability works, why it can grow beyond $\delta $B/B$\sim $1, and show that as well as producing very large fields it also produces voids in the background plasma of very low density and very low magnetic field. We discuss the relationship to observation and how CR might react on the background plasma to produce observable macroscopic structures including filaments and beams.   

Kinetic Models of the Cyclotron Resonant Wave-Particle Interaction in Heliospheric Plasmas

The resonant cyclotron interaction between gyrating ions and MHD waves is fundamental to understanding the behavior of many plasma phenomena in the heliosphere. Close to the Sun, the heating and acceleration of the solar wind in the corona is likely caused by the resonant dissipation of ion cyclotron waves. In the outer heliosphere, the ionization of inflowing interstellar neutrals creates rapidly streaming ``pickup ions'' whose isotropization generates waves which heat the core proton plasma there. Between these extreme positions, this interaction governs the energetic particle scattering required for shock acceleration, transport of solar energetic particles, and modulation of cosmic rays, and also contributes to shaping the distributions of the various thermal and suprathermal ion populations in the solar wind. Recent quasilinear models without some of the traditional simplifications have recognized the importance of a more complete kinetic treatment of this interaction, in particular the ion diffusion along velocity-space resonant surfaces which include the effects of wave dispersion. We will present the basic concepts involved in this interaction and discuss a number of applications to plasma processes in the heliosphere.   

Kinetic dissipation of high-frequency MHD turbulence

The dissipation of MHD turbulence at small scales by kinetic processes is critical for a number of problems in astrophysical plasmas, including the heating of the solar corona and the acceleration of particles in solar flares. The nature of such kinetic dissipation in turn depends critically on the frequencies of the turbulent fluctuations at the small-scale end of the inertial range. In linear theory, collisionless dissipation of waves with frequencies much less than the ion cyclotron frequency is unable to increase the perpendicular temperature of ions due to magnetic-moment conservation. On the other hand, waves at frequencies comparable to the ion cyclotron frequency dissipate via cyclotron damping and can cause perpendicular ion heating. A key problem for the kinetic dissipation of turbulence is thus to determine the fraction of turbulent power that reaches the cyclotron frequency. This problem is particularly important for the solar wind, where there is direct observational evidence that perpendicular ion heating occurs. This presentation will describe a weak turbulence theory for low-beta MHD plasmas that predicts the fraction of the turbulent power that is dissipated via the cyclotron resonance. The possible role of cyclotron damping of MHD turbulence in solar flares will also be briefly discussed.   

Wave-Wave and Wave-Particle Nonlinear Behavior Observed in Laser-Plasma Interaction Experiments

Controlling laser-plasma interactions is of critical importance to achieving fusion ignition at the National Ignition Facility. The high laser intensities encountered in these plasmas are typically sufficient to exceed the threshold for parametric instabilities, such as stimulated Raman scattering (SRS), where an intense laser wave decays into a Langmuir wave (LW) and a scattered light wave. The saturation of SRS occurs as the LW dissipates energy into other waves (wave-wave coupling) or into the electrons (wave-particle coupling). While such phenomena are readily observed in fully kinetic simulations, the ability to detect the subtle signatures of wave-wave and wave-particle nonlinear behavior in experiments is masked by the plasma inhomogeneity found in these small scale ($\sim$ 1 mm) plasmas. To overcome this, an experimental test-bed was developed using a diffraction-limited laser beam to interact with a preformed plasma and drive SRS in well-characterized and homogeneous conditions. This novel technique has enabled the observation of Langmuir Decay Instability cascade, the nonlinear frequency shifts due to electron trapping, as well as the observation of scattering from a trapped electron-acoustic mode. These results validate our fundamental understanding of these processes, and help guide the development of predictive capabilities in this field.   

Spectral Energy Transfer and Dissipation of Magnetic Fluctuations in the Solar Wind

We will give an overview of observations of magnetic fluctuations in the solar wind, followed by an overview of various physical models that have been put forward to explain the formation and dynamics of the magnetic fluctuation spectra. We will discuss in detail the physical processes might be present in the relatively high frequency range where both the proton and electron damping of the solar wind magnetic fluctuations might play an important role. Assumptions, limitations and the subsequent results from different approaches will be presented. Results from many different numerical simulations designed for understanding the wave-particle interactions in the solar wind conditions will be described as well.