Results 1 - 10 of 13451
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[en] The authors have observed the eclipsing millisecond pulsar PSR 1957 + 20 on 32 occasions during the interval of March--September 1988 at the Arecibo Observatory. They have made radio observations at 318 MHz and 430 MHz, primarily, but we also have obtained a small amount of data at 606 MHz. The authors report on these observations here, concentrating on the frequency dependence of the eclipse duration and the posteclipse delay
[en] Low-frequency electric-mode excitation in the earth-ionosphere cavity is considered with the aid of Maxwell-equation vector mode transformation with allowance for field penetration into the ionosphere and magnetosphere. It is shown that for atmospheric current generators, the ionosphere can exhibit the properties of a conductor or an insulator, depending on frequency. The penetration of atmospheric electric fields from the earth into the ionosphere can cause corresponding unitary variations of geophysical parameters
[en] We formulate a current conserving theory for frequency dependent noise power under dc bias. In calculating the noise power we explicitly take into account of the displacement current operator. It is shown that the equilibrium frequency dependent noise satisfies the relation where is the frequency dependent conductance including both conduction current and displacement current contribution. (paper)
[en] Two possibilities whereby phase slip centers can generate electromagnetic oscillations of frequency appreciably lower than the Josephson frequency of each phase slip center are considered. One possibility is that the entire structure can move along the sample at a constant velocity, and the other is that the oscillations can result from inequality of the Josephson frequencies of the phase slip centers that are under unequal conditions; in particular, one of the centers may be near the sample boundary
[en] High-degree solar mode frequencies as measured by ring diagrams are known to change in the presence of the strong magnetic fields found in active regions. We examine these changes in frequency for a large sample of active regions analyzed with data from the Michelson Doppler Imager on board the Solar and Heliospheric Observatory spacecraft, spanning most of solar cycle 23. We confirm that the frequencies increase with increasing magnetic field strength, and that this dependence is generally linear. We find that the dependence is slightly but significantly different for active regions with different sunspot types.
[en] The control of the Casimir force between two parallel plates can be achieved through adjusting the frequency-dependent electromagnetic properties of materials of the two plates. We show that, for different plate separations, the main contribution to the Casimir force comes from different frequency regions: For smaller (larger) separation, it comes from the higher (lower) frequency region. When the separation of the plates increases, the Casimir force can vary from attractive to repulsive and/or vice versa, by selecting the two plates with suitable electromagnetic properties. We discuss how a restoring Casimir force, which varies from repulsive to attractive by increasing the separation, can be realized and that the stable equilibrium is formed at zero Casimir force.
[en] We examine the scattering of acoustic p-mode waves from a thin magnetic fibril embedded in a gravitationally stratified atmosphere. The scattering is mediated through the excitation of slow sausage waves on the magnetic tube, and only the scattering of the monopole component of the wave field is considered. Since such tube waves are not confined by the acoustic cavity and may freely propagate along the field lines removing energy from the acoustic wave field, the excitation of fibril oscillations is a source of acoustic wave absorption as well as scattering. We compute the mode mixing that is achieved and the absorption coefficients and phase shifts. We find that for thin tubes the mode mixing is weak and the absorption coefficient is small and is a smooth function of frequency over the physically relevant band of observed frequencies. The prominent absorption resonances seen in previous studies of unstratified tubes are absent. Despite the relatively small absorption, the phase shift induced can be surprisingly large, reaching values as high as 15° for f modes. Further, the phase shift can be positive or negative depending on the incident mode order and the frequency.
[en] Finite-difference time-domain methods are increasingly being used to develop, model and analyze the response of materials, including engineered metamaterials that may contain superconductors. Though simple and useful expressions for the time-domain susceptibility exist for basic metals and dielectrics, the time-domain response for a superconductor has not been developed, mainly because the frequency-dependent expressions themselves are rather complex. In this paper we present a simple approximate expression for the time-domain susceptibility of a superconductor for the ħ/2Δ time scale (where Δ is the BCS energy gap) that fulfils causality requirements, and demonstrate its ability to model the transmission and reflection of a fully-gapped superconductor in the THz region. By allowing Δ to be a function of current, we also show how this model function can be used to describe nonlinear microwave response in superconductors. (paper)
[en] This paper presents the first frequency scanning array on a flexible substrate constructed using printed electronics methods and systems. Devices were designed using a printed wide band stacked ellipsoid antenna and phase shifter network with 50 ohm matching power splitters which was developed using Ansys Electromagnetics. These devices were produced in both 1 × 4 and 1 × 8 antenna patch configurations. The resulting devices were then tested to demonstrate beam steering as a function of frequency at 4.35 GHz, 5.32 GHz, 6.27 GHz, and 7.2 GHz and compared. Additionally, bending tests were performed to demonstrate the usability of the devices in a flexible, internet of things oriented application. The demonstrated devices have a multibeam receiving and transmitting capability while being inexpensive and easy to produce in a flexible package. (paper)