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May 2005

Volume 12, Issue 5, Articles (05xxxx)

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back to top Ionospheric, Solar-System, and Astrophysical Plasmas

Possible two-step solar energy release mechanism due to turbulent magnetic reconnection

Quan-Lin Fan, Xue-Shang Feng, and Chang-Qing Xiang

Phys. Plasmas 12, 052901 (2005); http://dx.doi.org/10.1063/1.1862249 (5 pages) | Cited 1 time

Online Publication Date: 7 April 2005

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In this paper, a possible two-step solar magnetic energy release process attributed to turbulent magnetic reconnection is investigated by magnetohydrodynamic simulation for the purpose of accounting for the closely associated observational features including canceling magnetic features and different kinds of small-scale activities such as ultraviolet explosive events in the lower solar atmosphere. Numerical results based on realistic transition region physical parameters show that magnetic reconnections in a vertical turbulent current sheet consist of two stages, i.e., a first slow Sweet–Parker-like reconnection and a later rapid Petschek-like reconnection, where the latter fast reconnection phase seems a direct consequence of the initial slow reconnection phase when a critical state is reached. The formation of coherent plasmoid of various sizes and their coalescence play a central role in this complex nonlinear evolution. The “observed” values of the rate of cancellation flux as well as the approaching velocity of magnetic fragments of inverse polarity in present simulation are well consistent with the corresponding measurements in the latest observations. The difference between our turbulent magnetic reconnection two-step energy release model and other schematic two-step models is discussed and then possible application of present outcome to solar explosives is described.
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96.60.qe Flares
52.35.Vd Magnetic reconnection
52.35.Ra Plasma turbulence

Equilibrium properties and exact solutions for two-dimensional nonlinear force-free magnetic fields with mass flow

A. H. Khater and S. M. Moawad

Phys. Plasmas 12, 052902 (2005); http://dx.doi.org/10.1063/1.1890965 (9 pages) | Cited 4 times

Online Publication Date: 15 April 2005

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The steady state equilibrium properties of force-free magnetic fields (FFMFs) with mass flow in Cartesian geometry and invariant in a given direction are investigated. For constant Mach number (defined in Sec. 2) flows, several classes of exact analytic solutions associated with solitonlike, antisolitonlike, kinklike, and antikinklike configurations are obtained. For nonconstant Mach number flows, it is shown that the equilibrium flow is impossible in some cases of FFMFs, and exact equilibria can be obtained in the other cases. The magnetic surfaces have a circular cross section for two-dimensional flows.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Sb Solitons; BGK modes
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Fi Transport properties

Conversion of ordinary and extraordinary waves into upper hybrid waves in inhomogeneous plasmas

Kyung-Sub Kim, Eun-Hwa Kim, Dong-Hun Lee, and Kihong Kim

Phys. Plasmas 12, 052903 (2005); http://dx.doi.org/10.1063/1.1896285 (5 pages) | Cited 2 times

Online Publication Date: 28 April 2005

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Linear mode conversion of ordinary and extraordinary waves into upper hybrid waves has been investigated by adopting a time-dependent numerical model. In order to solve the wave equations as an initial-valued problem, the finite difference method is used in both time and space. It is examined how wave coupling occurs in a cold magnetized plasma, where inhomogeneity lies perpendicular to the ambient magnetic field, by analyzing time histories of both electric and magnetic field components. The results show that electromagnetic energy of ordinary and extraordinary waves is transferred into electrostatic energy when the resonant condition at upper hybrid resonances is satisfied.
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52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Xz Magnetized plasmas
02.70.Bf Finite-difference methods

Wavelength and decay length of density overshoot structure in supercritical, collisionless bow shocks

R. Saxena, S. D. Bale, and T. S. Horbury

Phys. Plasmas 12, 052904 (2005); http://dx.doi.org/10.1063/1.1900093 (6 pages)

Online Publication Date: 9 May 2005

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Above some critical Mach number, quasiperpendicular collisionless shocks are known to exhibit “overshoot” and “undershoot” structure thought to be associated with the motion of ions trapped at the shock front. Using spacecraft potential data from the Cluster spacecraft, the overshoot/undershoot density structure at 56 crossings of the quasiperpendicular bow shock is studied. The envelope of the absolute value of the density, in most cases, decays exponentially and these envelopes are fitted to a decaying function from which we calculate the decay length scale. The overshoot/undershoot wavelength is also estimated using the zero crossings of the density profile and a good correlation between the average wavelength and the convected ion gyroradius is found: the wavelength is approximately two to three times the ion gyroradius. There is no evidence of a strong correlation between the wavelength and the ion inertial length. Similar results are found for the decay length, which also seems to be ordered by the convected ion gyroradius.
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52.35.Tc Shock waves and discontinuities
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.25.Fi Transport properties
95.30.Qd Magnetohydrodynamics and plasmas
94.30.cs Plasma motion; plasma convection
94.30.Va Magnetosphere interactions
96.60.Vg Particle emission, solar wind

Second-order quasilinear theory of cosmic ray transport

A. Shalchi

Phys. Plasmas 12, 052905 (2005); http://dx.doi.org/10.1063/1.1895805 (10 pages) | Cited 44 times

Online Publication Date: 10 May 2005

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The problem of pitch-angle diffusion close to 90° is well known in cosmic ray astrophysics. If the pitch-angle Fokker–Planck coefficient for pure slab geometry is calculated, the quasilinear approximation results in vanishing pitch-angle scattering. For a realistic wave spectrum with a steep dissipation range this vanishing coefficient generates an infinitely large parallel mean free path. It is well known from numerical simulations that the 90° problem is a problem of quasilinear theory and not a problem of reality. In the current paper quasilinear theory is used to calculate corrections of the unperturbed orbit. These corrections can be resubstituted into transport theory to calculate a second-order pitch-angle Fokker–Planck coefficient. The second-order quasilinear theory is an applicable theory which agrees with simulations for pitch-angle diffusion.
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52.25.Fi Transport properties
52.20.Dq Particle orbits
95.30.Qd Magnetohydrodynamics and plasmas
96.50.S- Cosmic rays
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Learning about coronal heating from solar wind observations

S. Peter Gary, Ruth M. Skoug, and Charles W. Smith

Phys. Plasmas 12, 056501 (2005); http://dx.doi.org/10.1063/1.1863192 (5 pages) | Cited 9 times

Online Publication Date: 7 April 2005

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Vlasov theory describing the interaction of Alfvén-cyclotron fluctuations and ions in the collisionless solar wind predicts that alpha particles should be strongly scattered perpendicular to the background magnetic field when the alpha/proton relative velocity vαp is negative or has a sufficiently small positive value relative to the Alfvén speed vA. This theory also predicts that, if vαp/vA is positive and sufficiently large, it is the protons which are scattered in the perpendicular direction, although less strongly. Here proton and alpha particle anisotropies measured in the solar wind near 1 AU (AU, astronomical unit) by the plasma and magnetic field instruments on the Advanced Composition Explorer spacecraft are reported which are consistent with these predicted signatures. This indicates that Alfvén-cyclotron heating of ions is active in the solar wind; by using this medium to study this fundamental process, a greater understanding may be obtained of how Alfvén-cyclotron scattering contributes to heating of ions in the solar corona.
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52.50.Qt Plasma heating by radio-frequency fields; ICR, ICP, helicons
52.70.Nc Particle measurements
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.25.Gj Fluctuation and chaos phenomena
52.25.Fi Transport properties
96.60.Vg Particle emission, solar wind
96.60.P- Corona

Nonlinear evolution of the firehose instability in a magnetic dipole geotail geometry

H. Vernon Wong, B.-Y. Xu, W. Horton, J. Pratt, and J. W. Van Dam

Phys. Plasmas 12, 056502 (2005); http://dx.doi.org/10.1063/1.1888786 (11 pages) | Cited 1 time

Online Publication Date: 5 May 2005

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Bursty bulk flows increase the parallel pressure faster than the perpendicular pressure in the high-β central plasma sheet driving the firehose instability [ S. Ji and R. A. Wolf, J. Geophys. Res. 108, 1191 (2003); S. Ji and R. A. Wolf, J. Geophys. Res.30, 2242 (2003) ]. A nonlinear partial differential equation is derived and an initial-value code developed to investigate the firehose anisotropy-driven turbulence in the Earth’s geotail. It is essential to include dispersive ion kinetic effects in order to limit the range of linearly unstable parallel wave numbers and to achieve a stationary nonlinear turbulent state. The nonlinear dynamics of the firehose instability provides a possible explanation for ultralow-frequency Pi-2 fluctuations associated with bursty bulk flows and substorms.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Ra Plasma turbulence
52.30.-q Plasma dynamics and flow
52.25.Fi Transport properties
52.25.Gj Fluctuation and chaos phenomena
52.72.+v Laboratory studies of space- and astrophysical-plasma processes
94.30.cl Magnetotail
94.30.cq MHD waves, plasma waves, and instabilities
94.30.Lr Magnetic storms, substorms
02.30.Jr Partial differential equations

Phenomenology treatment of magnetohydrodynamic turbulence with nonequipartition and anisotropy

Ye Zhou and W. H. Matthaeus

Phys. Plasmas 12, 056503 (2005); http://dx.doi.org/10.1063/1.1887187 (8 pages) | Cited 11 times

Online Publication Date: 9 May 2005

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Magnetohydrodynamics (MHD) turbulence theory, often employed satisfactorily in astrophysical applications, has often focused on parameter ranges that imply nearly equal values of kinetic and magnetic energies and length scales. However, MHD flow may have disparity magnetic Prandtl number, dissimilar kinetic and magnetic Reynolds number, different kinetic and magnetic outer length scales, and strong anisotropy. Here a phenomenology for such “nonequipartitioned” MHD flow is discussed. Two conditions are proposed for a MHD flow to transition to strong turbulent flow, which are extensions of (i) Taylor’s constant flux in an inertial range and (ii) Kolmogorov’s scale separation between the large and small scale boundaries of an inertial range. For this analysis, the detailed information on turbulence structure is not needed. These two conditions for MHD transition are expected to provide consistent predictions and should be applicable to anisotropic MHD flows, after the length scales are replaced by their corresponding perpendicular components. Second, it is stressed that the dynamics and anisotropy of MHD fluctuations are controlled by the relative strength between the straining effects between eddies of similar size and the sweeping action by the large eddies, or propagation effect of the large-scale magnetic fields, on the small scales, and analysis of this balance, in principle, also requires consideration of nonequipartition effects.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.27.Jv High-Reynolds-number turbulence
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