Quasi-monochromatic Oscillations Research Articles

Modeling of heat transfer from an electrically heated rotating helix in a circular cylindrical channel filled with a biomass moving at a constant velocity

The increasingly popular use of screw conveyors for thermo(chemical) processing of food or waste requires modeling the temperature distribution inside such a device. The purpose of this work is to demonstrate the temperature distribution around an electrically heated rotating helix inside a circular cylindrical channel of infinite length filled with a homogeneous biomass moving at a constant velocity in the axial direction. The non-stationary problem of temperature distribution is solved by replacing the real heat source with continuously distributed point sources, expansion of the given and required functions into a Fourier series over angular coordinate and applying integral Fourier and Laplace transforms over axial coordinate and time, respectively. The exact solution of the problem is represented as the product of the energy source determined by the Joule heat and the influence function in the form of the Fourier-Bessel series, which depends on the thermodynamic characteristics of substance and its velocity, radius, pitch and angular velocity of the helix. The temperature field consists of two components, one proportional to the time and the other forming the micro-structure of the temperature profile. On the basis of numerical calculations the analysis of space–time microstructure of the temperature field in the channel is provided. It is shown that in time the temperature undergoes low-amplitude quasi-monochromatic oscillations, the period of which is determined by the angular velocity of the spiral. The spatial temperature distribution has a plateau-like character formed by the geometry of the helix. In particular, the conditions under which the resonant amplification of the amplitude of temperature fluctuations occurs are established.

Five-Minute Solar Oscillations and Ion-Cyclotron Waves in the Solar Wind

In the present paper we study impact of the photospheric 5-minute oscillations on the ion cyclotron waves in the solar wind. We proceed from the assumption that the ion cyclotron waves in solar wind are experiencing modulation with a characteristic period of 5 minutes under the influence of Alfven waves driven by photospheric motions. The theory presented in our paper predicts a deep frequency modulation of the ion cyclotron waves. The frequency modulation is expected mainly from variations in orientation of the IMF lines. In turn, the variations in orientation are caused by the Alfven waves, propagating from the Sun. To test the theoretical predictions we have analyzed records of the ultra-low-frequency (ULF) geoelectromagnetic waves in order to find the permanent quasi-monochromatic oscillations of natural origin in the Pc1-2 frequency band (0.1-5 Hz), the carrier frequency of which varies with time in a wide range. As a result we found the so-called "serpentine emission" (SE), which was observed in Antarctic at the Vostok station near the South Geomagnetic Pole. The permanency, range of frequencies, and the deep frequency modulation of SE correspond to the qualitative properties of ion cyclotron waves in the solar wind. Clearly expressed 5-minute modulation of the carrier frequency is particularly important feature of the SE in the context of this work. We believe that we have found non-trivial manifestation of the solar 5-min oscillations on the Earth.

Foreshock-like density cavity in the outflow region of magnetotail reconnection

Abstract. During Cluster spacecraft crossing of the magnetotail, a novel density depleted cavity in association with magnetic compressions in the outflow region of reconnection was observed. It contains intense reflected field-aligned particles, which are produced by a generation mechanism similar to that of the terrestrial foreshock, and hence manifests a foreshock-like morphology. In this cavity, reflected field-aligned proton beams were observed and simultaneously the feature of magnetic-mirror loss-cone proton distributions were found. Magnetic field fluctuations, especially quasi-monochromatic oscillations, were recorded. Both the leading egde and the ULF wave boundary of the ion foreshock are identified from the time sequence of proton and magnetic field observations. Just upstream of the leading egde of the ion foreshock, reflected field-aligned electrons were detected, whose distribution has a narrow bump-on-tail pattern. However, close to the shock front, reflected electrons with a broad bump-on-tail pattern was measured. These two different manifestations of reflected electrons reveal the differences in their microscopic physics of the reflecting process. Moreover, a part of incident ions was further accelerated in the cavity due to trans-time magnetic pumping which provides another possible mechanism in the multi-step acceleration processes in reconnection.

ON ANALYSIS AND SYNTHESIS OF ELEMENTARY AND COMPLEX INFORMATIONAL BISPECTRAL STRUCTURES

The problem regarding shapes of bispectral peak of the triplet representing the triharmonic signal is discussed. This triplet can be used as both standard diagnostic signal for a bispectral analyzers and a simplest element of complex informational patterns of bispectrally organized signals. The problem of analyzing and measuring the parameters of bispectral peak is equally fundamental as the well-known problem concerning the shape and width of spectral line of quasi-monochromatic oscillations. The sectional area of bispectral peak restricts the limiting informational volume of complex bispectral patterns. Two models of an actual triplet with frequency fluctuations are analyzed. Universal bispectral-peak shapes are found for a triplet with proportional frequencies for the limiting cases of extremely slow and extremely fast frequency fluctuations. The phenomenon of bispectral-peak superlocalization is discovered for quasi-static fluctuations and analyzed for the most realistic model of 1/f frequency fluctuations. The experiment investigating the bispectral peak of the real high-frequency synthesizer is presented.

Inertia-gravity waves observed by the UK MST radar

We present an analysis of eight years' data from the UK MST radar at Aberystwyth, Wales, to determine the occurrence and properties of long-period, quasi-monochromatic oscillations in the wind vector identified with inertia-gravity waves. Wave perturbations were first isolated by filtering out background wind variation in the vertical, then the continuity of the radar measurements was exploited to apply band-pass filters in time to select quasi-monochromatic oscillations with periods 4–8 hours or 12–24 hours. By searching for altitude regions where the wind vector rotates systematically with height, and fitting ellipses to such sections, the properties of inertia-gravity waves can be derived. To guard against contamination by short-intrinsic-period mountain waves, cases were discarded when the vertical velocity measured by the radar exceeded 15 cm s−1. Results show that waves in the stratosphere are dominated by upward energy propagation (clockwise rotation) and those in the troposphere by downward propagation, consistent with the dominant source for inertia-gravity waves being at tropopause level. Long-period waves (>12 hours) are observed 70% of the time in the lower stratosphere, roughly twice as often as the shorter-period waves, and 10–20% of the time in the troposphere. There is a winter maximum in occurrence in the troposphere and for the short-period stratospheric waves; however, the dominant long-period waves show little seasonal dependence. Likewise, they show only a weak correlation with jet-stream velocity or direction, unlike the other categories which are preferentially associated with strong jet streams and propagate along the jet. Inertia-gravity waves observed by the radar have typical amplitudes 1–2 m s−1 and vertical wavelength 2 km. The intrinsic wave period, estimated for non-jet-stream conditions, is similar for the two band-passed series, pointing to the importance of Doppler shifting in generating the observed oscillations. Copyright © 2007 Royal Meteorological Society

Two-feedback-loop oscillator with chaotic dynamics

This paper contains new results relating to an intensively developing field — the study of dynamic chaos. It is shown that the introduction of auxiliary wide-band feedback in an autostochastic oscillator with delay transforms chaotic oscillations into quasimonochromatic oscillations.

Trimpi events and other amplitude perturbations observed during 1991 at Kerguelen ( L = 3.7) on the subionospheric North-west Cape (NWC) signal

In the period March–November 1991, which was characterized by strong magnetic activity, two kinds of complementary VLF data were recorded at Kerguelen: (1) the amplitude of the subionospheric 22.3 kHz signal from the North-west Cape (NWC) transmitter in Australia, and (2) the integrated intensity in various VLF bands, and broadband waveforms (0.4–10 kHz). Comprehensive analysis of the data allowed one to identify and characterize four types of well-structured perturbations in the NWC signal amplitude, i.e.: classical Trimpi events associated with whistlers; hiss-induced electron precipitation events; inverted dome-shaped events identifiable with dome-shaped events recently observed at Durban by Friedel et al. (1993); and quasi-monochromatic oscillations, with a period of ∼7–15 s observed when Kerguelen is poleward of the plasmapause projection in the pre-dawn sector, similar to the zigzag effect described by Carpenter et al. (1985a). The purpose of this paper is to report on the results of investigations devoted to these various types of event; each of them is illustrated by examples. The main new results relative to classical Trimpi events concern: the dependence on severe magnetic storms (out of 2300 events identified in June and July 1991—period of maximum occurrence—48% were observed on 6 days only in the aftermath of three large magnetic storms); the correlation between amplitude perturbation sign and NWC signal amplitude connected with sunset and sunrise on the east-to-west NWC-KER path; the nocturnal variation of recovery times for plasmaspheric whistler-associated Trimpi events (the mean value increases from 30 to 42 s at night); the detection in the midnight-dawn sector of short-duration Trimpi events (recovery times in the range ∼4–15 s) probably triggered by whistler waves propagating beyond the plasmapause when the latter is equatorward of, or close to, Kerguelen, 24–48 h following the onset of intense magnetic storms. Whereas it is possible to interpret most of the characteristics and properties of classical Trimpi events in terms of electron precipitation induced by magnetospheric VLF waves—a process being favoured by the Occurrence of severe magnetic storms with probable injection of energetic particles—the dome and zigzag effects remain unexplained as yet.

Alfvén Pulses in the Ionospheric Waveguide

AbstractExact explicit solutions, describing the propagation of wideband non‐sinusoidal waveforms in the ionospheric Alfvén waveguide by means of non‐separable electromagnetic fields, are presented. The dispersive modulation of these fields is shown to result in the formation of quasimonochromatic oscillations at the wings of the Alfvén pulses. The frequencies of these oscillations are close to the cut off frequencies of the waveguide.

Laser amplification of incoherent radiation

The amplification of noise in a laser amplifier is treated theoretically. The model for the active medium and its description using density-matrix techniques, are taken from the theory of laser operation. The fundamental concern of this investigation is the spectral behavior of the radiation in the nonlinear regime; hence, the formalism is written from the onset in the frequency domain. The statistics of the light are gradually modified by the nonlinear amplification process, and expressions are derived for the rate of change of fluctuations in intensity as a measure of statistical changes. These expressions should be easily susceptible to detailed experimental observations. In addition, the range of validity of Litvak's Gaussian-statistics approximation is discussed with some detail. In the homogeneous-broadening case, the evolution of initially broadband Gaussian radiation toward quasimonochromatic oscillations with laserlike statistics is extensively explored in several numerical examples. The connections of this study with the time-domain work of Risken and Nummedal, on self-pulsing in a ring-laser configuration, are clearly established. Finally, spectral-narrowing and -rebroadening effects in Doppler-broadened media are discussed both analytically and with numerical examples. These examples show the distinct contribution of pulsations in the population ("Raman-type terms"), and saturation phenomena. The predicted narrowing-rebroadening rate is compared with expressions found in previous literature.