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Physics and Astronomy75%

Selected Abstracts

New low profile cavity-backed Hilbert slot antenna

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 12 2006
Guido Biffi Gentili
Abstract In this paper, the use of a cavity-backed Hilbert surface as an efficient slot-like narrowband radiating aperture is suggested, to exploit its own unique electromagnetic characteristics in the context of radio frequency identification (RFID) applications. The proposed meta-slot antenna (MSA) consists of a Hilbert surface window (HSW) made of a cascade of space-filling slot inclusions of order three and backed by a very shallow cavity that contributes to its surface impedance and radiation characteristics. A straight longitudinal microstrip line printed on a thin, low permittivity substrate, whose ground plane includes the HSW, feeds the aperture through distributed electromagnetic coupling. As illustrated by simulation results and measurements, the antenna behaves as a magnetic current sheet over a ground plane, with a highly frequency-selective impedance response and unidirectional radiation pattern. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2577,2581, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21995 [source]

General relativistic force-free electrodynamics: a new code and applications to black hole magnetospheres

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Jonathan C. McKinney
ABSTRACT The force-free limit of magnetohydrodynamics (MHD) is often a reasonable approximation to model black hole and neutron star magnetospheres. We describe a general relativistic force-free (GRFFE) formulation that allows general relativistic magnetohydrodynamic (GRMHD) codes to directly evolve the GRFFE equations of motion. Established, accurate and well-tested conservative GRMHD codes can simply add a new inversion piece of code to their existing code, while continuing to use all the already-developed facilities present in their GRMHD code. We show how to enforce the E·B= 0 constraint and energy conservation, and we introduce a simplified general model of the dissipation of the electric field to enforce the B2,E2 > 0 constraint. We also introduce a simplified yet general method to resolve current sheets, without much reconnection, over many dynamical times. This formulation is incorporated into an existing GRMHD code (harm), which is demonstrated to give accurate and robust GRFFE results for Minkowski and black hole space,times. [source]

Explosive reconnection in magnetars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2003
Maxim Lyutikov
ABSTRACT The X-ray activity of anomalous X-ray pulsars and soft ,-ray repeaters may result from the heating of their magnetic corona by direct currents dissipated by magnetic reconnection. We investigate the possibility that X-ray flares and bursts observed from anomalous X-ray pulsars and soft ,-ray repeaters result from magnetospheric reconnection events initiated by development of the tearing mode in magnetically dominated relativistic plasma. We formulate equations of resistive force-free electrodynamics, discuss the relation of the latter to ideal electrodynamics, and give examples of both ideal and resistive equilibria. Resistive force-free current layers are unstable towards the development of small-scale current sheets where resistive effects become important. Thin current sheets are found to be unstable due to the development of the resistive force-free tearing mode. The growth rate of the tearing mode is intermediate between the short Alfvén time-scale ,A and a long resistive time-scale ,R: ,, 1/(,R,A)1/2, similar to the case of non-relativistic non-force-free plasma. We propose that growth of the tearing mode is related to the typical rise time of flares, ,10 ms. Finally, we discuss how reconnection may explain other magnetar phenomena and ways to test the model. [source]

Cancelling magnetic feature and filament activation

ASTRONOMISCHE NACHRICHTEN, Issue 7 2006
L. Contarino
Abstract We report in this paper the analysis of the evolution of a magnetic fragment observed in NOAA 9445 on 5 May, 2001. This magnetic fragment emerged laterally to a filament which later split into two parts. The bifurcation site coincided with the magnetic fragment location and the part of the filament which split was later destabilized and a flare occurred. The magnetic flux variations in the magnetic fragment and in the surrounding area were analyzed and, considering their trends and other observational signatures (H, brightenings and associated plasma motions), we could infer that it was a cancelling magnetic feature (CMF). We determined some geometrical and physical parameters of the CMF (area, magnetic flux variation, cancellation speed and flux cancellation rate) using high resolution magnetograms taken by BBSO. We compared the observed parameters of the CMF with the parameters of low-lying reconnection current sheets given in the model proposed by Litvinenko (1999) and found good agreement between observed and theoretical values. Therefore, we conclude that a low-lying magnetic reconnection process might be the cause of the filament activation. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]