Vortex configurations and critical parameters in superconducting thin films containing antidot arrays: Nonlinear Ginzburg-Landau theory

Using the non-linear Ginzburg-Landau (GL) theory, we obtain the possible vortex configurations in superconducting thin films containing a square lattice of antidots. The equilibrium structural phase diagram is constructed which gives the different ground-state vortex configurations as function of the size and periodicity of the antidots for a given effective GL parameter $\kappa^{*}$. Giant-vortex states, combination of giant- and multi-vortex states, as well as symmetry imposed vortex-antivortex states are found to be the ground state for particular geometrical parameters of the sample. The antidot occupation number $n_o$ is calculated as a function of related parameters and comparison with existing expressions for the saturation number $n_s$ and with experimental results is given. For a small radius of antidots a triangular vortex lattice is obtained, where some of the vortices are pinned by the antidots and some of them are located between them. Transition between the square pinned and triangular vortex lattices is given for different values of the applied field. The enhanced critical current at integer and rational matching fields is found, where the level of enhancement at given magnetic field directly depends on the vortex-occupation number of the antidots. For certain parameters of the antidot lattice and/or temperature the critical current is found to be larger for higher magnetic fields. Superconducting/normal $H-T$ phase boundary exhibits different regimes as antidots are made larger, and we transit from a plain superconducting film to a thin-wire superconducting network. Presented results are in good agreement with available experiments and suggest possible new experiments.Comment: 15 pages and 20 figure

A Magnetic Flux Tube Oscillation Model for QPOs in SGR Giant Flares

Giant flares from soft gamma-ray repeaters (SGRs) are one of the most violent phenomena in neutron stars. Quasi-periodic oscillations (QPOs) with frequencies ranging from 18 to 1840 Hz have been discovered in the tails of giant flares from two SGRs, and were ascribed to be seismic vibrations or torsional oscillations of magnetars. Here we propose an alternative explanation for the QPOs in terms of standing sausage mode oscillations of flux tubes in the magnetar coronae. We show that most of the QPOs observed in SGR giant flares could be well accounted for except for those with very high frequencies (625 and 1840 Hz).Comment: 15 pages,1 figures,1 table, accepted for publication in The Astrophysical Journa

Nonlinear effects in resonant layers in solar and space plasmas

The present paper reviews recent advances in the theory of nonlinear driven magnetohydrodynamic (MHD) waves in slow and Alfven resonant layers. Simple estimations show that in the vicinity of resonant positions the amplitude of variables can grow over the threshold where linear descriptions are valid. Using the method of matched asymptotic expansions, governing equations of dynamics inside the dissipative layer and jump conditions across the dissipative layers are derived. These relations are essential when studying the efficiency of resonant absorption. Nonlinearity in dissipative layers can generate new effects, such as mean flows, which can have serious implications on the stability and efficiency of the resonance

Evidence for storm-time ionospheric ion precipitation in the cusp with magnetosheath energy

We present evidence for a sporadic precipitation into the north polar cusp of ionospheric O+ and He+ ions accelerated up to the magnetosheath flow speed during a magnetic storm. This is deduced from data obtained on board the Interball-Auroral satellite showing that the energy/charge ratios of the H+, He++, He+ and O+ populations are similar to those of ion masses. These measurements pertain to a very disturbed magnetic period. A storm was in progress with a Dst reaching -149nT during the cusp measurements, while the AE index reached values higher than 1000nT. This result is discussed in terms of ion circulation from the magnetosphere to the magnetosheath and back to the magnetosphere. We suggest that the acceleration of O+ and He+ ions up to a magnetosheath-like velocity is directly linked to the large By component of the IMF.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetosheath; storms and substorms

Eigenfrequencies and optimal driving frequencies of 1D non-uniform magnetic flux tubes

The eigenfrequencies and the optimal driving frequencies for flux tubes embedded in uniform but wave-carrying surroundings are calculated, based on matching conditions formulated in terms of the normal acoustic impedances at the Bur tube boundary. The requirement of the equality of the normal acoustic impedance of the transmitted wave field with the normal acoustic impedance of the outgoing wave field selects the eigenmodes, while the equality of the ingoing and the transmitted normal acoustic impedance selects the optimal driving frequencies (Keppens 1996). Even if the flux tube is uniform, the eigenfrequencies can be complex due to leakage of wave energy into the surroundings. The case of uniform flux tubes has been considered previously (e.g. Cally 1986), and serves as a testcase of our formalism. We extend Cally\u27s results by taking a radial stratification of the flux tube into account. The non-uniformity of the flux tube can introduce another cause for energy loss, namely resonant absorption internal to the flux tube. When resonant absorption occurs. we must incorporate the appropriate jump conditions over the dissipative layer(s). This can be done using a simple numerical scheme as introduced by Stenuit et al. (1995)