A new perspective on the longitudinal variability of the semidiurnal tide

The longitudinal variability of the semidiurnal tide in the Antarctic upper mesosphere is investigated by comparison of observations from two radars at approximately opposite sides of Antarctica. Under the assumption that the tide is composed of an S = 2 (migrating) and S = 1 (westward-propagating, non-migrating) component only, the relative phases of the components are shown to vary with season such that the waves are typically in constructive interference during the winter (summer) months at longitudes around 0 degrees E (180 degrees E). We show that this has profound effects on the seasonal behaviour of the semidiurnal tide around 78 degrees S dependent on the longitude, and that no single-station observations at this latitude can be considered representative of a "zonal mean". The superposition of these two waves is used to interpret differences in previously-published ground-based climatologies of the tide. Citation: Hibbins, R. E., O. J. Marsh, A. J. McDonald, and M. J. Jarvis (2010), A new perspective on the longitudinal variability of the semidiurnal tide, Geophys. Res. Lett., 37, L14804, doi:10.1029/2010GL044015

A long-term comparison of wind and tide measurements in the upper mesosphere recorded with an imaging Doppler interferometer and SuperDARN radar at Halley, Antarctica

International audienceData from a co-located imaging Doppler interferometer and SuperDARN radar recorded since 1996 have been analysed in a consistent manner to determine daily mean winds and tides in the upper mesosphere. By comparing only days when both techniques were recording good quality data it is shown that the SuperDARN radar winds and tides correlate best with the IDI height bin 90?95 km. On timescales of one hour the winds derived from each technique correlate poorly, whereas the daily mean winds are in much better agreement suggesting that the two radars are sensitive to different parts of the gravity wave spectrum. Regression analysis reveals that the observed SuperDARN daily mean meridional wind strength is approximately 65% that recorded by the IDI while the zonal winds are of similar magnitude, in good quantitative agreement with previous studies which have shown contamination to SuperDARN-derived winds due to the significant back lobe of the radar radiation pattern. Climatologically the two techniques observe similar monthly mean winds with the SuperDARN meridional winds suppressed compared to the IDI which tends to record winds more poleward and eastward than those derived by the SuperDARN radar during the summer months, and to be slightly more equatorward during the winter. The 12-h tidal amplitude and phase in both the zonal and meridional components derived from both techniques are in excellent agreement, whereas the 24-h tides are seen much more strongly in the SuperDARN radar, especially in wintertime, with poor phase agreement. Long term comparison of the two techniques reveals a tendency for the IDI meridional winds to be more poleward during solar maximum especially during summer time; an effect which is not reproduced in the meridional winds derived from the SuperDARN radar. These results are discussed in the context of previous studies to independently determine the veracity of each technique

Superdirective Antennas of Coupled Helical Elements

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this recordUsing magnetically-coupled structures formed of subwavelength metallic helices, we demonstrate superdirective end-fire radiation in the low GHz frequency range. Numerical, experimental and analytical results are presented on superdirective dimers that are almost three times smaller compared to previously demonstrated dimers of split-ring-resonators (0.09λ compared with 0.25λ). Optimisation of such structures in terms of their size, directivity, efficiency and operational passband is demonstrated.Engineering and Physical Sciences Research Council (EPSRC

First results from the Trondheim Norway momentum-flux meteor radar

第3回極域科学シンポジウム　横断セッション「中層大気・熱圏」 11月26日（月） 国立極地研究所 ２階大会議

Electromagnetic Properties of Ultrathin Quadrifilar Spirals and Their Complementary Structures

Published

The resonant electromagnetic fields of an array of metallic slits acting as Fabry-Perot cavities

Copyright © 2006 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Applied Physics 99 (2006) and may be found at http://link.aip.org/link/?JAPIAU/99/124903/1Fabry-Perot cavities are perhaps the best known of the optical transmission resonators, with cavity field enhancement accomplished by two parallel and partially reflecting planes. Recently it has been shown that arrays of narrow slits cut into a metal substrate are similarly able to exhibit resonant transmission modes. An analysis of the field solutions and transmission properties of this resonant array is compared to the well-known etalon and dielectric slab geometries, revealing a most elegant illustration of the principles of Maxwell's electromagnetism. It is demonstrated that the matching of the propagating field to each slit-cavity mode is made possible through strong diffraction at each end. Furthermore, the interface between the slit cavities and semi-infinite space beyond acts as a high-impedance surface on resonance, reflecting the field with a positive reflection-amplitude coefficient. Metallic slit arrays have several advantages over conventional Fabry-Perot resonators with interesting application potential

Excitation of remarkably nondispersive surface plasmons on a nondiffracting, dual-pitch metal grating

Copyright © 2002 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 80 (2002) and may be found at http://link.aip.org/link/?APPLAB/80/2410/1A nondiffracting metallic lamellar grating formed from three equally spaced grooves per repeat period, with one being slightly shallower than the other two is examined at microwave frequencies. When filled with a slightly lossy dielectric, this structure supports a remarkably nondispersive surface plasmon polariton mode, which exhibits strong selective absorption of incident power. Measured reflectivities show excellent agreement with the results predicted by a rigorous coupled wave theory

Importance of diffraction in determining the dispersion of designer surface plasmons

Euan Hendry, Alastair P. Hibbins, and J. Roy Sambles, Physical Review B, Vol. 78, article 235426 (2008). Copyright © 2008 by the American Physical Society.By employing a modified modal matching approach, we obtain explicit analytical expressions relating frequency to in-plane wave vector for the surface electromagnetic mode confined at the interface between vacuum and a perfect conductor patterned with a two-dimensional square array of square holes. Our complete analytical formalism takes into account both multiple order waveguide modes and diffracted evanescent waves, hence overcoming the a priori assumptions intrinsic to previous descriptions of the dispersion of these surface waves. We validate our derived dispersion relation through comparison with that recently recorded at microwave frequencies using prism coupling. Finally, we show that diffracted evanescent waves play an important role in determining the dispersion, so that the electric field associated with “designer” surface modes is much more weakly confined to the interface than the field associated with surface plasmons on real metal surfaces