RECENTLY PUBLISHED PAPER
Porazik, P., J. R. Johnson, I. Kaganovich, and E. Sanchez
Geophys. Res. Lett., 41, (2014)
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. 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
Eun-Hwa Kim, Iver. H. Cairns and Jay R. Johnson
Linear mode conversion of Langmuir/ to electromagnetic 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 for planetary magnetospheres. Here, we study mode conversion in warm, using a numerical electron fluid simulation code when the density gradient has a wide range of angle, , to the ambient , for a range of incident Langmuir/ wavevectors. Our results include: (1) Left-handed polarized ordinary () and right-handed polarized extraordinary () mode are produced in various ranges of for Ω = ()1 3() < 1.5, whereis the (angular) electron cyclotron frequency, is the angular frequency, is the length scale of the (linear) density gradient, and is the speed of light; (2) the mode is produced most strongly in the range, 40° < < 60°, for intermediately with Ω = 1.0 and 1.5, while it is produced over a wider range, 0° ≤ ≤ 90°, for weakly with Ω = 0.1 and 0.7; (3) the maximum total conversion efficiencies for power from the Langmuir/ mode to are of order 50%99% and the corresponding conversion efficiencies are 5%14% (depending on the adiabatic index and = / 2, where is the electron temperature and is the electron) for various Ω; (4) the mode conversion window becomes wider as Ω and increase. Hence, the results in this paper confirm that linear mode conversion under these conditions can explain the weak total circular of interplanetary type II and III solar radio bursts because a strong mode can be generated via linear mode conversion near ∼ 45°.
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.
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.
Alexander Tchekhovskoy, Anatoly Spitkovsky and Jason G. Li
MNRAS (August 01, 2013) 435 (1): L1-L5. doi: 10.1093/mnrasl/slt076
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.