Results 1 - 10 of 85853
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[en] The equilibration of a two-level primitive equation model forced by relaxation towards a fixed axisymmetric temperature profile is studied as a function of the forcing equator-to-pole temperature difference. It was found that the mean equilibrated temperature gradient rises quickly with the forcing. It is found also that the mean eddy momentum flux convergence induces a strong barotropic jet at mid-to-high latitudes. It was suggested that, as found in previous work, the barotropic governor effect induced by the jet is responsible for the sensitivity of the equilibrated temperature gradient to the forcing
[en] Complete text of publication follows. All-sky nightglow imaging observations made over low latitude site Panhala (17degN, 74.2degE Geographic; 8.43degN Geomagnetic latitude) during February 1, 2008 revealed anomalous filamentary structure formation (referred to as 'streaks') in OI 557.7nm emission. These structures were observed on pre-midnight sector only in OI 557.7nm emission and are not seen on other wavelengths including OI 630.0nm. They were transient and extended from south to north with a tilt of ∼13deg to the geomagnetic meridian. In addition they possessed westward drift with speeds of 30-50 m/s. They are narrow with widths up to 10 km and lengths ranging from few tens of km to ∼200km. These estimations were based on an assumed height of 100 km for the OI 557.7 nm emission. A recurrent geomagnetic storm of moderate strength (minimum quick look Dst = -44) was on progress during the observation of streaks. Equatorial ionospheric soundings made over Tirunelveli (8.7degN, 77.8degE Geographic; 0.17degN Geomagnetic latitude) showed intense blanketing after midnight confirming excessive night time ionization on that day. Wide plasma depletion was observed on OI 630.0 nm emission while streaks were observed in OI 557.7nm emission. In this work we discuss the observations in detail and speculate about their possible cause. This is an extremely puzzling observation made from very low geomagnetic latitude during the main phase of a moderate recurrent geomagnetic storm in the declining phase of solar cycle. Hence understanding of such unusual and rare events is expected to substantiate our knowledge on the coupling processes at equatorial region.
[en] Trend analysis of hourly wind direction angle recorded at nine meteorological stations across Southern Canada (in the 43oN-53oN belt) identified wind direction rotation periods in the range of 7-9 days. Rotation persists during the 'summertime' season from May to mid-October during 1953-2001. Rotation with a 7.5 day period was also established in the 850 hPa geostrophic summertime wind over the Canadian province of Saskatchewan in 2000. Hodographs built from wind vectors corresponding to consecutive days of the . day period (summertime average vectors of winds binned into separate days of the period) formed nearly elliptical loops centered around the terminate point of the net westerly transport vector which ranged from 0.5 to 1.3 m/s, and was either smaller or of comparable magnitude to the zonal and meridional oscillations of wind. The observed rotations appear quasi-periodic rather than purely periodic, because nearly elliptical loops of relatively large amplitude were present at different hodographs constructed at each location for sampling periods of 6.5, 7.0, 7.5, 8.0 days. For sampling periods outside the ∼6.5-8.0 days range the rotational magnitude in hodograph appears diminished due to partial cancellation of wind vectors in corresponding hodograph bins. Quasi-periodical rotation occurs in the clockwise direction. (author)
[en] Complete text of publication follows. We have studied the spectral contents and latitudinal structure of magnetic Pc3-4 pulsations on the dayside using data acquired by the Mid-continent Magnetoseismic Chain (McMAC) at L =1.46 -3.40. We display the magnetometer data in a keogram format and find that the pulsations in the H component consist of oscillations with time-dependent amplitude and period and that these oscillations generally exhibit poleward phase propagation. By spectral analysis of Pc3 and Pc4 pulsation events identified in the keograms we find that the latitude of the amplitude peak depends on the pulsation frequency and that the phase changes by ∼180 degrees across the amplitude peak, a signature of field line resonance. Occurrence of field line resonance is further confirmed from the fact that the amplitude peak occurs at the latitude where the pulsation frequency matches the fundamental toroidal wave frequency that is determined using the cross phase technique.
[en] Transfers from geosynchronous transfer orbits (GTO) to geosynchronous orbits (GSO) using electric propulsion have been optimized in the current study. Both time-optimal and fuel-optimal trajectories are generated. Three-dimensional equations of motion are considered for the system dynamics. The indirect approach based on optimal control theory is followed and the resulting two-point boundary value problem is solved using differential evolution, a search-based global optimization technique. Optimal trajectories for various mission scenarios are obtained using differential evolution and compared to transfers presented in literature. (author)
[en] Some global atmospheric electrical parameters like atmospheric conductivity, air-earth current density, electric field, atmospheric potential, etc. have been estimated over different places of India, China and Korea of Asian continent. These calculations have been made by assuming fair weather conditions and taking into account the small scale (0.5° grid in latitude and longitude) orographic and latitudinal effects. Mean values of conductivity and air-earth current density over various places of India, China and Korea have been found to be 5.97 × 10-13, 5.46 × 10-13, 2.82 × 10-13 S m-1 and 6.58 × 10-12, 6.04 × 10-12, 3.16 × 10-12 A m-2, respectively whereas average electric field and atmospheric potential over these places are 110.9, 110.81, 112.08 V m−1 and 269.52, 272.90, 286.64 kV, respectively. It has been found from the results that latitudinal variation is very much smaller than orographic variation over these three countries of Asian region. (author)
[en] This work formulates a new computational scheme to efficiently explore the configuration space of materials and to identify a material's stable equilibrium structures. This computational tool is obtained by coupling quantum-based density functional theory (DFT) calculations (employing periodic boundary conditions) with branch-following and bifurcation (BFB) techniques. BFB is used to map equilibrium paths (stable and unstable) on the DFT energy landscape as a function of the applied load and ultimately creates 'bifurcation maps' that identify the material's stable structures and connections between them, including: soft deformation directions, transition states, transformation mechanisms, etc. This new approach has been used to study structural transitions in Si and Fe under pressure loading. The results obtained so far indicate that the new DFT–BFB methodology has the potential to provide a significant new insight into the mechanisms that drive structural phase transitions in a wide range of technologically important materials
[en] Using a non-perturbative functional method, where the quantum fluctuations are gradually set up, it is shown that the interaction of a N=1 Wess-Zumino model in 2+1 dimensions does not get renormalized. This result is valid in the framework of the gradient expansion and aims at compensating the lack of non-renormalization theorems
[en] We use two fundamental theoretical frameworks to study the finite-size (shell) properties of the unitary gas in a periodic box: (1) an ab initio quantum Monte Carlo (QMC) calculation for boxes containing 4 to 130 particles provides a precise and complete characterization of the finite-size behavior, and (2) a new density functional theory (DFT) fully encapsulates these effects. The DFT predicts vanishing shell structure for systems comprising more than 50 particles, and allows us to extrapolate the QMC results to the thermodynamic limit, providing the tightest bound to date on the ground-state energy of the unitary gas: ξS≤0.383(1). We also apply the new functional to few-particle harmonically trapped systems, comparing with previous calculations.
[en] We study the formation of bands of colloidal particles driven by periodic external fields. Using Brownian dynamics, we determine the dependence of the band width on the strength of the particle interactions and on the intensity and periodicity of the field. We also investigate the switching (field-on) dynamics and the relaxation times as a function of the system parameters. The observed scaling relations were analyzed using a simple dynamic density-functional theory of fluids