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Modification of the loss cone for energetic particles

posted Dec 3, 2014, 1:01 PM by Peter Porazik   [ updated Dec 3, 2014, 1:06 PM ]

Porazik, P., J. R. Johnson, I. Kaganovich, and E. Sanchez

Geophys. Res. Lett., 41, (2014)

doi:10.1002/2014GL061869.

The optimal pitch angle which maximizes the penetration distance, along the magnetic field, of relativistic charged particles injected from the midplane of an axisymmetric field is investigated analytically and numerically. Higher-order terms of the magnetic moment invariant are necessary to correctly determine the mirror point of trapped energetic particles, and therefore the loss cone. The modified loss cone resulting from the inclusion of higher-order terms is no longer entirely defined by the pitch angle but also by the phase angle of the particle at the point of injection. The optimal orientation of the injection has a nonzero component perpendicular to the magnetic field line, and is in the plane tangential to the flux surface. Numerical integration of particle orbits were carried out for a relativistic electron in a dipole field, showing agreement with analytic expressions. The results are relevant to experiments, which are concerned with injection of relativistic beams into the atmosphere from aboard a spacecraft in the magnetosphere.

A model for falling-tone chorus

posted Apr 21, 2014, 11:34 AM by Eun-Hwa Kim   [ updated Apr 23, 2014, 7:18 AM by Rualdo Soto ]

A. R. Soto-Chavez, G. Wang, A. Bhattacharjee, G. Y. Fu and H. M. Smith

Geophys. Res. Lett. 41 (2014)

Motivated by the fact that geomagnetic field inhomogeneity is weak close to the chorus generation region and the observational evidence that falling-tone chorus tend to have large oblique angles of propagation, we propose that falling-tone chorus start as a marginally unstable mode. The marginally unstable mode requires the presence of a relatively large damping, which has its origins in the Landau damping of oblique waves in this collisionless environment. A marginally unstable mode produces phase-space structures that release energy and produce wave chirping. We show that the present model produces results in reasonable agreement with observations.

Linear mode conversion of Langmuir/z-mode waves to radiation in plasmas with various magnetic field strength

posted Dec 16, 2013, 11:15 AM by Eun-Hwa Kim

Eun-Hwa Kim, Iver. H. Cairns and Jay R. Johnson

Phys. Plasmas 20, 122103 (2013)http://dx.doi.org/10.1063/1.4837515

Linear mode conversion of Langmuir/z waves to electromagnetic radiation near the plasma and upper hybrid frequency in the presence of density gradients is potentially relevant to type II and III solar radio bursts, ionospheric radar experiments, pulsars, and continuum radiation for planetary magnetospheres. Here, we study mode conversion in warm, magnetized plasmasusing a numerical electron fluid simulation code when the density gradient has a wide range of angle, δ, to the ambient magnetic field, B 0, for a range of incident Langmuir/z wavevectors. Our results include: (1) Left-handed polarized ordinary (oL) and right-handed polarized extraordinary (xR) mode waves are produced in various ranges of δ for Ω0 = (ωL/c)1 / 3(ωc e) < 1.5, whereωc e is the (angular) electron cyclotron frequency, ω is the angular wave frequency, L is the length scale of the (linear) density gradient, and c is the speed of light; (2) the xR mode is produced most strongly in the range, 40° < δ < 60°, for intermediately magnetized plasmas with Ω0 = 1.0 and 1.5, while it is produced over a wider range, 0° ≤ δ ≤ 90°, for weakly magnetized plasmas with Ω0 = 0.1 and 0.7; (3) the maximum total conversion efficiencies for wave power from the Langmuir/z mode to radiation are of order 50%99% and the corresponding energyconversion efficiencies are 5%14% (depending on the adiabatic index γ and β = T e/m e c 2, where T e is the electron temperature and m e is the electron) for various Ω0; (4) the mode conversion window becomes wider as Ω0 and δ increase. Hence, the results in this paper confirm that linear mode conversion under these conditions can explain the weak total circularpolarization of interplanetary type II and III solar radio bursts because a strong xR mode can be generated via linear mode conversion near δ ∼ 45°.

Linear dispersion relation for the mirror instability in context of the gyrokinetic theory

posted Oct 23, 2013, 2:27 PM by Eun-Hwa Kim   [ updated Oct 23, 2013, 2:28 PM ]

Peter Porazik and Jay R. Johnson 

Phys. Plasmas 20, 104501 (2013); http://dx.doi.org/10.1063/1.4822339

The linear dispersion relation for the mirror instability is discussed in context of the gyrokinetic theory. The objective is to provide a coherent view of different kinetic approaches used to derive the dispersion relation. The method based on gyrocenter phase space transformations is adopted in order to display the origin and ordering of various terms.

Magnetic reconnection process in transient coaxial helicity injection

posted Sep 17, 2013, 8:47 AM by Eun-Hwa Kim   [ updated Oct 23, 2013, 2:28 PM ]

F. Ebrahimi, E. B. Hooper, C. R. Sovinec, and R. Raman

Phys. Plasmas 20, 090702 (2013); http://dx.doi.org/10.1063/1.4821974 (4 pages)

The physics of magnetic reconnection and fast flux closure in transient coaxial helicity injection experiments in NSTX is examined using resistive MHD simulations. These simulations have been performed using the NIMROD code with fixed boundary flux (including NSTX poloidal coil currents) in the NSTX experimental geometry. Simulations show that an X point is formed in the injector region, followed by formation of closed flux surfaces within 0.5 ms after the driven injector voltage and injector current begin to rapidly decrease. As the injector voltage is turned off, the field lines tend to untwist in the toroidal direction and magnetic field compression exerts a radial J × B force and generates a bi-directional radial Etoroidal×Bpoloidal pinch flow to bring oppositely directed field lines closer together to reconnect. At sufficiently low magnetic diffusivity (high Lundquist number), and with a sufficiently narrow injector flux footprint width, the oppositely directed field lines have sufficient time to reconnect (before dissipating), leading to the formation of closed flux surfaces. The reconnection process is shown to have transient Sweet-Parker characteristics.

Time-dependent 3D magnetohydrodynamic pulsar magnetospheres: oblique rotators

posted Sep 16, 2013, 1:36 PM by Eun-Hwa Kim   [ updated Sep 16, 2013, 1:37 PM ]

Alexander Tchekhovskoy, Anatoly Spitkovsky and Jason G. Li

MNRAS (August 01, 2013) 435 (1): L1-L5. doi: 10.1093/mnrasl/slt076

Abstract

The current state of the art in pulsar magnetosphere modelling assumes the force-free limit of magnetospheric plasma. This limit retains only partial information about plasma velocity and neglects plasma inertia and temperature. We carried out time-dependent 3D relativistic magnetohydrodynamic (MHD) simulations of oblique pulsar magnetospheres that improve upon force free by retaining the full plasma velocity information and capturing plasma heating in strong current layers. We find rather low levels of magnetospheric dissipation, with < 10 per cent of pulsar spin-down energy dissipated within a few light cylinder radii, and the MHD spin-down that is consistent with that in force free. While oblique magnetospheres are qualitatively similar to the rotating split-monopole force-free solution at large radii, we find substantial quantitative differences with the split-monopole, e.g., the luminosity of the pulsar wind is more equatorially concentrated than the split-monopole at high obliquities, and the flow velocity is modified by the emergence of reconnection flow directed into the current sheet.

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