Bulletin of the American Physical Society
2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009; Denver, Colorado
Session D6: Ion Heating in Turbulent Plasmas |
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Sponsoring Units: GPAP DPP Chair: Michael Brown, Swarthmore College Room: Governor's Square 16 |
Saturday, May 2, 2009 3:30PM - 4:06PM |
D6.00001: Scenarios for Ion Heating in Low Frequency MHD Turbulence Invited Speaker: Dissipation of a magnetohydrodynamic cascade is of considerable interest in understanding the solar corona and solar wind, as well as in diverse circumstances in astrophysical and laboratory plasma physics. The problem is considerably complicated when the collisionality of the medium is low, due to the potential role of a variety of kinetic processes. Familiar cases such as cyclotron heating or linear Vlasov decay are typically described for homogeneous plasmas, and substantial generalizations may be required for the structured conditions that occur naturally in intermittent turbulence. A first step toward understanding dissipation is a description of the cascade, which when a large scale magnetic field is present is expected to be highly anisotropic in a way that favors perpendicular spectral transfer. Parallel cascade may not be entirely negligible however, especially through compressive channels. Perpendicular cascade gives rise to structures such as current sheets. Substantial evidence, using test particles, but recently in observations and in kinetic simulations, suggests that heating can occur due to ion interaction with these structures. In particular, perpendicular heating of ions is expected, and parallel heating of electrons. This may give rise to a two stage dissipation mechanism that first removes some energy near ion scales, and then much of the remaining energy at electron scales. Between these scales, wave energy may sometimes be enhanced due to reversible or linear kinetic processes. This perspective, while not fully explored in terms of self consistent inhomogeneous plasma physics, maybe provide a perspective on how low frequency waves can generate an MHD cascade that links ultimately to kinetic scales, where fluctuation energy vanishes in favor of increased entropy and internal energy. A number of observations are consistent with this picture. [Preview Abstract] |
Saturday, May 2, 2009 4:06PM - 4:42PM |
D6.00002: Ion Heating from Magnetic Reconnection in Laboratory Plasmas Invited Speaker: In many laboratory plasmas ions are much hotter than expected from classical heating mechanisms such as electron-ion collisions. Frequently, ion heating is observed during magnetic reconnection as result of conversion of magnetic energy into thermal energy. In many cases, magnetic reconnection and the associated heating occur impulsively, such as during sawtooth crashes in the tokamak and the Reversed Field Pinch (RFP). Even though this phenomenon has been observed for a long time specific mechanisms of the energy conversion are poorly understood. In this presentation I will cover examples of ion heating from many laboratory plasmas, including RFP, spherical tokamak, spheromak, reconnection experiments, and linear machines. Several mechanisms of ion heating including viscous damping of turbulent plasma flows and ion-cyclotron heating will be discussed. [Preview Abstract] |
Saturday, May 2, 2009 4:42PM - 5:18PM |
D6.00003: Ion Heating in the Solar Corona and Solar Wind Invited Speaker: The solar corona is the hot, ionized outer atmosphere of the Sun that expands into interplanetary space as a supersonic solar wind. This tenuous medium is a unique laboratory for the study of magnetohydrodynamics (MHD) and plasma physics with ranges of parameters that are inaccessible on Earth. The last decade has seen significant progress toward identifying and characterizing the processes that heat the corona and accelerate the solar wind, but the basic physics is still unclear. Some key clues about the mechanisms responsible for energizing the plasma have come from UV spectroscopy of the extended corona (i.e., using a combination of an occulting coronagraph and a spectrometer). There is evidence for preferential acceleration of heavy ions in the fast solar wind, ion temperatures exceeding 100 million K, and marked departures from Maxwellian velocity distributions. These collisionless departures from thermal equilibrium point to specific types of kinetic processes. This presentation reviews the measurements (both telescopic and from `in situ' probes) that constrain theoretical explanations and provides a current survey of the landscape of proposed ideas for ion energization. Many of the suggested processes are related to the dissipation of MHD waves (e.g., ion cyclotron waves), and many involve multiple steps of energy conversion between waves, turbulence, current sheets, and other nonlinear plasma features. A discussion of future measurements that could help to test, refine, and possibly winnow down the list of competing models will also be presented. [Preview Abstract] |
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