Nanoflare heating model for collisionless solar corona

The problem of coronal heating remains one of the greatest unresolved problems in space science. Magnetic reconnection plays a significant role in heating the solar corona. When two oppositely directed magnetic fields come closer to form a current sheet, the current density of the plasma increases due to which magnetic reconnection and conversion of magnetic energy into thermal energy takes place. The present paper deals with a model for reconnection occurring in the solar corona under steady state in collisionless regime. The model predicts that reconnection time in the solar corona varies inversely with the cube of magnetic field and varies directly with the Lindquist number. Our analysis shows that reconnections are occurring within a time interval of 600 s in the solar corona, producing nanoflares in the energy range 10 ²¹–10 ²³ erg /s which matches with Yohkoh X-ray observations.     

Skin friction measurements on structured surfaces using Clauser-chart method and Oil film interferometry

Measurements of skin friction have been performed on flat and hexagonal concave surfaces using the conventional Clauser-chart method and the Oil film interferometry. The values of shear stress coefficients measured by the conventional Clauser-chart method on a flat plate were found to be up to 13 % higher from the ones deduced by the Oil film interferometry. The velocity profiles required for the Clauser-chart were obtained by using hot wire anemometry. The analysis of the results suggested that the conventional Clauser-chart method cannot be used to predict shear stresses acting on the hexagonal concave surfaces due to the existence of strong pressure gradients. Oil film interferometry not only provides accurate and direct values of shear stress coefficients but also helps to visualize the flow above the surface.     

Electrochemical solid electrolyte gas sensors — hydrocarbon and NOx analysis in exhaust gases

For the in situ measurement of the free oxygen concentration and the equilibrium oxygen partial pressure oxygen sensors based on zirconia solid electrolytes are widely used in order to monitor and control technical high temperature processes. Similarly combustibles (HC, CO) and NO_x can be determined in non equilibrated oxygen containing gas mixtures of exhausts by mixed potential sensors and amperometric solid electrolyte sensors. It is expected that their long-term stability is similar to that of oxygen sensors. In both cases the electrode material with the desired electrochemical and catalytic properties is the key component. Different electrode materials made of perovskites (La_1-xSr_xCr_1-yGa_yO_3-δ) and composites (Au/Metal oxide) were investigated in different combustibles including CO, C₃H_6/8, C₇H₈ and CH₄. The response behaviour of mixed potential sensors is determined by the catalytic activity of the measuring electrode, which is closely connected with the defect structure and depends on the measuring conditions. Furthermore the electrode response can be understood by electrokinetic data. Gas symmetrical mixed potential sensors with electrodes made of Au/Nb₂O₅ composites show maximum sensitivity. By using Ptreference electrodes without equilibrium behaviour the sensors are applicable in lean and rich mixtures as well. In the amperometric sensor mode the consecutive determination of oxygen and NO_x or combustibles at two working electrodes is possible. The catalytic activity of the oxygen pumping electrode should be low in order to avoid the decomposition of NO and HC respectively. Alternatively, the electrochemical reduction of NO can be performed at a single working electrode, made of materials with improved NO selectivity, without the previous reduction of oxygen. Paper presented at th 8th EuroConference on Ionics, Ixia, Rhodos, Greece, Sept. 15–21, 2002.     

Noncollinear magnetic order in quasicrystals

Based on Monte Carlo simulations, the stable magnetization configurations of an antiferromagnet on a quasiperiodic tiling are derived theoretically. The exchange coupling is assumed to decrease exponentially with the distance between magnetic moments. It is demonstrated that the superposition of geometric frustration with the quasiperiodic ordering leads to a three-dimensional noncollinear antiferromagnetic spin structure. The structure can be divided into several ordered interpenetrating magnetic supertilings of different energy and characteristic wave vector. The number and the symmetry of subtilings depend on the quasiperiodic ordering of atoms.     

Self-consistent description of the inner crust of a neutron star with allowance for superfluidity effects

Within the Wigner-Seitz approximation, a self-consistent fully quantum-mechanical calculation of the structure of the inner crust of a neutron star is performed over a wide range of densities with allowance for superfluidity effects. Within the approach used, the Wigner-Seitz cell consists of a nuclear-like cluster surrounded by a nearly uniform neutron gas. An effective energy functional is constructed by matching, at the cluster surface, the realistic phenomenological nuclear functional for the cluster due to S. A. Fayans and his coauthors and the energy functional calculated microscopically for neutron matter. The microscopic component of the functional is calculated within the Brueckner method by using the v18 Argonne interaction.     

Non-parametric identification of non-linear oscillating systems

The problem of system identification from a time series of measurements is solved by using non-parametric additive models. Having only few structural information about the system, a non-parametric approach may be more appropriate than a parametric one for which detailed prior knowledge is needed. Based on non-parametric regression, the functions in the additive models are estimated by a penalized least-squares approach using backfitting. The optimal smoothing parameters are determined via generalized cross-validation, making this approach completely adaptive to the data. The procedure is applied to identify the non-linear restoring force of vibrationally excited helical wire rope isolators.     

Maximal operators of Fejér means of double Walsh-Fourier series

The main aim of this paper is to prove that the maximal operator of the Fejér mean of the double Walsh-Fourier series is not bounded from the Hardy space H _1/2 to the space weak-L _1/2. This paper was written during the visit of the author at the College of Nyíregyháza in Hungary.     

Experimental characterization of bi-directional terahertz emission from gold-coated nanogratings

The THz radiation emission of Au-coated nanogratings (fused silica substrate, 30?nm Au layer thickness, 500?nm grating constant) upon fs laser irradiation (785?nm, 150?fs, 1?kHz,???1?mJ/pulse) is observed in both directions along the laser beam axis (forward and backward) and for both, illumination of the Au/air or the Au/silica interface. THz radiation along the laser beam propagation is emitted in a narrow solid angle of about 15°?fwhm independent on the laser pulse fluence, the angle of incidence and the nanograting profile. The bar width and groove depth of the nanograting as well as the angle of laser beam incidence strongly affect the THz radiation yield. The energy of single THz light pulses is measured absolutely (2?fJ in the 0.3?C0.38?THz range) using a highly sensitive and fast superconducting transition edge sensor. The bi-directional emission of THz radiation is in agreement with the model assumption of surface plasmon polaritons propagating simultaneously on both Au layer interfaces (Au/air and Au/silica).