Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session VI2: Interstellar and Solar Wind Turbulence, and Coronal and Auroral Emissions |
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Chair: Steven Spangler, University of Iowa Room: Rosen Centre Hotel Salon 3/4 |
Thursday, November 15, 2007 3:00PM - 3:30PM |
VI2.00001: Intermittency of Electron Density in Interstellar Kinetic Alfv{\'e}n Wave Turbulence Invited Speaker: Pulsar radiation pulses broaden as they propagate through interstellar space. The observed scaling of the pulse duration with distance to source indicates that the electron density fluctuations that broaden the pulse do not obey a Gaussian distribution. Instead they obey a Levy distribution, which contains an enhanced power-law tail. The physical mechanisms responsible for this tail remain to be established. We show that a Levy distribution arises from magnetic turbulence near the Larmor-radius scale where electron density actively couples to the magnetic field through kinetic Alfv{\'e}n wave (KAW) fluctuations. The analysis reveals a new type of turbulent intermittency mediated by compressive effects in electron physics rather than advection. Using analytic theory and computation we show that coherent structures form in the current, magnetic field, and density of decaying KAW turbulence. These structures avoid mixing by the turbulence because they have a strongly sheared magnetic field that refracts turbulence away from them. The required shear places these structures in the tail of the distribution function. Their probability is enhanced because decay by turbulent mixing is suppressed by the refraction. The current structures are localized but Ampere's law and KAW equipartition give coherent density and magnetic field extended spatial envelopes. These envelopes decay as $r^{-1}$ outside the current structure. This structure leads to a probability distribution function with a power-law tail. The probability of density gradient decays as $(\nabla n_e)^{-2}$, a Levy distribution. Because there are laboratory plasmas with magnetic turbulence at the Larmor-radius scale, it should be possible to look for these effects in the laboratory. [Preview Abstract] |
Thursday, November 15, 2007 3:30PM - 4:00PM |
VI2.00002: Turbulence in the Solar Wind: Theory, Simulations, and Comparisons with Observations Invited Speaker: \emph{In situ} measurements of the solar wind uniquely enable detailed comparison of turbulence in an astrophysical environment with theory and simulations. We present an analytical cascade model that follows the nonlinear flow of turbulent energy from the large driving scales in the MHD regime to the dissipative scales in the weakly collisional kinetic regime. For a large inertial range, scaling arguments suggest the turbulence remains low frequency, $\omega \ll \Omega_i$, due to the anisotropy of the MHD cascade, $k_\parallel \ll k_\perp$. Such low-frequency, anisotropic turbulence is optimally described by gyrokinetics. In this limit, the MHD Alfv\'en wave cascade transitions to a kinetic Alfv\'en wave cascade at the scale of the ion Larmor radius. Analytical cascade model results, nonlinear gyrokinetic simulations, and observational evidence support this claim, eroding the case for the importance of the ion cyclotron resonance in causing the break and steeper dissipation range of the turbulent magnetic energy spectrum in the solar wind. The analytical cascade model predicts that one expects an exponential cut-off in the energy spectrum above the spectral break, but that instrumental sensitivity limitations lend the dissipation range a power-law appearance. The observed variation of dissipation range slopes is naturally explained by the varying effectiveness of Landau damping as the plasma parameters change. Conditions under which the cyclotron resonance may play a role are identified. Nonlinear gyrokinetic simulations of solar wind turbulence support the predictions of the analytical model, producing magnetic and electric field fluctuation spectra that are consistent with satellite measurements. [Preview Abstract] |
Thursday, November 15, 2007 4:00PM - 4:30PM |
VI2.00003: The plasma properties of the solar corona, a detailed interpretation Invited Speaker: The solar corona is the plasma volume surrounding the 1.5x10$^{6}$ km diameter Sun. It consists of numerous structures of various sizes of which some last for days and reach lengths of several solar radii. Having a low electron density (usually n$_{e}\le $1x10$^{9}$ cm$^{-3})$ the corona is essentially optically thin allowing emission from all structure along a line of sight to reach an observer. When assuming that each coronal structures has its own unique temperature, a value slightly different from that of the rest, it is tempting to assume that the function describing the coronal emission measure vs. temperature ($\smallint $n$_{e}^{2}$dV where n$_{e}$ is the electron density and dV a volume element along the line of sight) is a monotonically changing function in the 7x10$^{5}$-5x10$^{6}$ K range. Essentially for the last half century this was the accepted depiction of the coronal condition. Recently, aided by spectra recorded by a high resolution stigmatic spectrometer, we studied in great details the electron temperature and emission measure properties of plasmas between 1.03R$_{/}$ and 1.5R$_{/}$ (30,000-450,000 km) and found that the commonly accepted description is lacking. In reality coronal plasmas at such heights are isothermal and could attain but one of only three temperatures, 9x10$^{5}$, 1.4x10$^{6}$ and 3x10$^{6}$ K. Furthermore, we found that in at least the two higher temperature plasma volumes to within the observational uncertainties the electron distribution is Maxwellian. The fraction of super thermal electrons, if present, in the 1.4x10$^{6}$ and the 3x10$^{6}$ K volumes are less than few percent. [Preview Abstract] |
Thursday, November 15, 2007 4:30PM - 5:00PM |
VI2.00004: Laboratory study of auroral cyclotron emission processes Invited Speaker: Electrons encounter an increasing magnetic field and increase in pitch angle as they descend towards the auroral ionosphere, according to the conservation of the magnetic moment. This process results in a horseshoe shaped distribution function in electron velocity space which has been observed by satellites [1]. Research has shown this distribution to be unstable to a cyclotron maser instability [2] and the emitted Auroral Kilometric Radiation is observed to be polarised in the extraordinary mode. Experimental results are presented based on an electron beam of energy 75keV having a cyclotron frequency of 4.45GHz, compressed using magnet coils to mimic the naturally occurring phenomenon. The emitted radiation spectrum was observed to be close to the cyclotron frequency. Electron transport measurements confirmed that the horseshoe distribution function was obtained. Measurements of the antenna pattern radiated from the output window demonstrated the radiation to be polarised and propagating perpendicular to the static magnetic field. The radiation generation efficiency was estimated to be 2{\%} in close agreement to the numerical predictions of the 2D PiC code KARAT. The efficiency was also comparable with estimates of the astrophysical phenomenon. \newline \newline [1] R. J. Strangeway et al, Geophys. Rev. Lett., 25, 1998, pp. 2065-2068 \newline [2] I Vorgul et al, Physics of Plasmas, 12, 2005, pp. 1-8 [Preview Abstract] |
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