scholarly journals Magnetohydrodynamic Turbulence in the Earth’s Magnetotail From Observations and Global MHD Simulations

2021 ◽  
Vol 8 ◽  
Author(s):  
Mostafa El-Alaoui ◽  
Raymond J. Walker ◽  
James M. Weygand ◽  
Giovanni Lapenta ◽  
Melvyn L. Goldstein
Keyword(s):  
Turbulent Flows ◽  
Plasma Sheet ◽  
Power Spectra ◽  
Wave Number ◽  
Magnetospheric Substorms ◽  
Fluid Turbulence ◽  
Mhd Simulations ◽  
Sheet Plasma ◽  
Power Spectral ◽  
Theoretical Evidence

Magnetohydrodynamic (MHD) turbulent flows are found in the solar wind, the magnetosheath and the magnetotail plasma sheet. In this paper, we review both observational and theoretical evidence for turbulent flow in the magnetotail. MHD simulations of the global magnetosphere for southward interplanetary magnetic field (IMF) exhibit nested vortices in the earthward outflow from magnetic reconnection that are consistent with turbulence. Similar simulations for northward IMF also exhibit enhanced vorticity consistent with turbulence. These result from Kelvin-Helmholtz (KH) instabilities. However, the turbulent flows association with reconnection fill much of the magnetotail while the turbulent flows associated with the KH instability are limited to a smaller region near the magnetopause. Analyzing turbulent flows in the magnetotail is difficult because of the limited extent of the tail and because the flows there are usually sub-magnetosonic. Observational analysis of turbulent flows in the magnetotail usually assume that the Taylor frozen-in-flow hypothesis is valid and compare power spectral density vs. frequency with spectral indices derived for fluid turbulence by Kolmogorov in 1941. Global simulations carried out for actual magnetospheric substorms in the tail enable the results of the simulations to be compared directly with observed power spectra. The agreement between the two techniques provides confidence that the plasma sheet plasma is actually turbulent. The MHD results also allow us to calculate the power vs. wave number; results that also support the idea that the tail is turbulent.

Power Spectra Density Replication Control Strategy for 2-Axis Hydraulic Shaker Based on HV Estimator

2014 ◽  
Vol 596 ◽  
pp. 610-615
Author(s):  
Yu Chen ◽  
Qiang Li Luan ◽  
Zhang Wei Chen ◽  
Hui Nong He
Keyword(s):  
Response Function ◽  
Control Algorithm ◽  
Power Spectra ◽  
Time History ◽  
Good Effect ◽  
System Response ◽  
Power Spectral ◽  
Iterative Correction ◽  
Replication Control ◽  
The Given

Hydraulic shaker, equipment of simulating laboratory vibration environment, can accurately replicate the given power spectral density (PSD) and time history with an appropriate control algorithm. By studying method Hv estimator of frequency response function (FRF) estimation, a FRF identification strategy based on the Hv estimator is designed to increase the convergence rapidity and improve the system response function specialty. The system amplitude-frequency characteristics in some frequency points or frequency bands have large fluctuation. To solve this issue, a step-varying and frequency-sectioning iterative correction control algorithm is proposed for the control of 2-axial exciter PSD replication tests and the results show that the algorithm has a good effect on the control of hydraulic shaker, and can achieve reliable and high-precision PSD replication.

An Optical Instrument for Measuring Fluctuating Velocities in Highly Turbulent Flows

1978 ◽  
Vol 29 (2) ◽  
pp. 98-113 ◽  
Author(s):  
M. Gaster ◽  
J.F.M. Maybrey
Keyword(s):  
Turbulent Flows ◽  
Light Sources ◽  
Optical Instrument ◽  
Surface Reflection ◽  
Power Spectral ◽  
Optical Layout ◽  
Wide Range ◽  
Laser Anemometer ◽  
Sampling Zone ◽  
Highly Turbulent Flows

SummaryFlow measurement by optical devices of various types, particularly those involving laser light sources, have received considerable attention over the last few years. Different schemes employing a wide range of optical layouts have evolved and the resulting signals have been processed in a number of ingenious ways. We report new experimental work on an optical instrument that can be considered as the forerunner of the laser anemometer in the belief that in certain circumstances this particular optical layout offers some real advantages over the majority of laser anemometers. One important advantage of this system is the ease with which both the shape and size of the sampling zone can be independently controlled. Another is the ability to position the sampling region very close to a boundary without having to contend with the surface reflection difficulties that often prevent such measurements being made with laser optics. The instrument measures the velocities of small particles suspended in the fluid in much the same way as the laser anemometer. In unsteady flows this results in a series of velocity estimates generated at random time instants. These intermittent samples of the velocity are used to form power spectral density estimates by methods recently developed for the analysis of randomly sampled records (Gaster and Roberts, 1975 & 1976). This method of analysis could well be applied to the processing of the signals generated by laser anemometers operating in the burst counter mode.

scholarly journals A physical mechanism producing suprathermal populations and initiating substorms in the Earth's magnetotail

2008 ◽  
Vol 26 (6) ◽  
pp. 1617-1639 ◽  
Author(s):  
D. V. Sarafopoulos
Keyword(s):  
Magnetic Field ◽  
Electric Fields ◽  
Plasma Sheet ◽  
Physical Mechanism ◽  
Energetic Particles ◽  
Dimensional Structure ◽  
Magnetospheric Substorms ◽  
Physical Description ◽  
Fundamental Building Block ◽  
Field Lines

Abstract. We suggest a candidate physical mechanism, combining there dimensional structure and temporal development, which is potentially able to produce suprathermal populations and cross-tail current disruptions in the Earth's plasma sheet. At the core of the proposed process is the "akis" structure; in a thin current sheet (TCS) the stretched (tail-like) magnetic field lines locally terminate into a sharp tip around the tail midplane. At this sharp tip of the TCS, ions become non-adiabatic, while a percentage of electrons are accumulated and trapped: The strong and transient electrostatic electric fields established along the magnetic field lines produce suprathermal populations. In parallel, the tip structure is associated with field aligned and mutually attracted parallel filamentary currents which progressively become more intense and inevitably the structure collapses, and so does the local TCS. The mechanism is observationally based on elementary, almost autonomous and spatiotemporal entities that correspond each to a local thinning/dipolarization pair having duration of ~1 min. Energetic proton and electron populations do not occur simultaneously, and we infer that they are separately accelerated at local thinnings and dipolarizations, respectively. In one example energetic particles are accelerated without any dB/dt variation and before the substorm expansion phase onset. A particular effort is undertaken demonstrating that the proposed acceleration mechanism may explain the plasma sheet ratio Ti/Te≈7. All our inferences are checked by the highest resolution datasets obtained by the Geotail Energetic Particles and Ion Composition (EPIC) instrument. The energetic particles are used as the best diagnostics for the accelerating source. Near Earth (X≈10 RE) selected events support our basic concept. The proposed mechanism seems to reveal a fundamental building block of the substorm phenomenon and may be the basic process/structure, which is now missing, that might help explain the persistent, outstanding deficiencies in our physical description of magnetospheric substorms. The mechanism is tested, checked, and found consistent with substorm associated observations performed ~30 and 60 RE away from Earth.

Multiple-satellite studies of magnetospheric substorms: Radial dynamics of the plasma sheet

1976 ◽  
Vol 81 (34) ◽  
pp. 5921-5933 ◽  
Author(s):  
T. Pytte ◽  
R. L. McPherron ◽  
M. G. Kivelson ◽  
H. I. West ◽  
E. W. Hones
Keyword(s):  
Plasma Sheet ◽  
Magnetospheric Substorms

Multifractal analysis of velocity and temperature fluctuations in the atmospheric surface layer

2020 ◽  
Author(s):  
Ganapati Sahoo ◽  
Soumak Bhattacharjee ◽  
Timo Vesala ◽  
Rahul Pandit
Keyword(s):  
Time Series ◽  
Turbulent Flows ◽  
Detrended Fluctuation Analysis ◽  
Multifractal Analysis ◽  
Temperature Time Series ◽  
Temperature Fields ◽  
Fluctuation Analysis ◽  
Fluid Turbulence ◽  
Detrended Fluctuation ◽  
Temperature Time

<p>The characterization of the structure of non-stationary, noisy fluctuations in a time series, e.g., the time series of the velocity components or temperature in turbulent flows, is a problem of central importance in fluid dynamics, nonequilibrium statistical mechanics, atmospheric physics and climate science. Over the past few decades, a variety of statistical techniques, like detrended fluctuation analysis (DFA), have been used to reveal intricate, multiscaling properties of such time series. We present an analysis of velocity and temperature time series, which have been obtained by measurements over the canopy of Hyytiälä Forest in Finland.<br>In our study we use DFA, its generalization, namely, multifractal detrended fluctuation analysis (MFDFA), and the recently developed multiscale multifractal analysis (MMA), which is an extension of MFDFA. These methods allow us to characterize the rich hierarchy or multi- fractality of the dynamics of the time series of the velocity components and the temperature. In particular, we can clearly distinguish these time series from white noise and the signals that display simple, monofractal, scaling with a single exponent (also called the Hurst exponent). It is useful to recall that monofractal scaling is predicted for fluid turbulence at the level of the Kolmogorov’s phenomenological approach of 1941 (K41); experiments and direct numerical simulations suggest that three-dimensional (3D) fluid turbulence must be characterised by a hierarchy of exponents for it is truly multifractal.</p><p>We present an analysis of multifractality of velocity and temperature fields that have been measured, at different heights, over the canopy of Hyytiälä Forest in Finland. In particular, we carry out a detailed study of velocity and temperature time series by using MFDFA and MMA. Results from both these methods are consistent, as they must be; but, of course, the MMA results contain more information because they account for the dependence of the multifractality on the time intervals.</p>

Seismic source decomposition

Geophysics ◽  
1983 ◽  
Vol 48 (1) ◽  
pp. 1-11 ◽  
Author(s):  
Paul L. Stoffa ◽  
Anton Ziolkowski
Keyword(s):  
Point Source ◽  
Spectral Characteristics ◽  
Patent Application ◽  
Power Spectra ◽  
Seismic Source ◽  
Three Dimensions ◽  
Far Field ◽  
Air Gun ◽  
Power Spectral ◽  
Directional Characteristics

We exploit the differences that exist between the radiation fields of a point source and an array to design a time‐separated marine seismic source array with desired power spectral and directional characteristics, whose far‐field time signature is known precisely from measurements. The desired power spectral characteristics are created by firing a predetermined series of point source units sequentially, such that their time signatures do not overlap. The effective power spectrum of the whole series of time‐distributed signatures can be made to approximate the sum of the power spectra of the individual signatures and can, therefore, be designed to suit the desired application by the appropriate choice of source units. The desired directional characteristics of the array can be created by arranging the source unit separations such that each source unit reaches the desired spatial position at the prescribed firing instant. The key to the subsequent processing of the recorded data is to measure the pressure wave generated by each point source unit with a hydrophone placed close by, but in the linear radiation field. The position of this hydrophone relative to the source unit must be known accurately in all three dimensions. The depths of the source units and their relative spatial positions at the instants of firing must also all be known. From these measurements the far‐field signature of the sequence in any azimuth can be deduced, and the impulse response of the earth can be recovered by dividing the Fourier frequency spectrum of the recorded reflection data by that of the measured source unit sequence. This process is completely deterministic in nature and depends primarily upon our ability to monitor accurately the far‐field signature of each source unit. Field results from an initial evaluation of this method indicate that this measurement can be readily accomplished. The success of this technique is then ultimately dependent on the signal to noise ratio. [This method is the subject of a patent application.] We stress that, since the signature is known, we are not obliged to make any assumptions about the phase. In particular, we do not need to make the minimum‐phase assumption. We are therefore free to choose our parameters to optimize our desired power spectral and directional characteristics with complete disregard for the conventional requirement that the signature of an air gun source have a high primary‐to‐bubble ratio.

Human Intelligence and Power Spectral Analysis of Visual Evoked Potentials

1980 ◽  
Vol 50 (1) ◽  
pp. 192-194 ◽  
Author(s):  
Mariko Osaka ◽  
Naoyuki Osaka
Keyword(s):  
Visual Evoked Potential ◽  
Evoked Potential ◽  
Power Spectra ◽  
Power Spectral Analysis ◽  
Mentally Retarded ◽  
Power Spectral ◽  
The Difference ◽  
Mentally Retarded Children ◽  
The Relationship ◽  
Visual Evoked

The relationship between intelligence and power spectra of visual evoked potential was investigated using 8 normal and 8 mentally retarded children as subjects. The results showed the power spectrum of mentally retarded has a peak at 4 to 6 Hz, whereas that of normal has two apparent peaks at 4 and 12 Hz. It appears the peak at 12 Hz reflects the difference of intelligence.

Analysis of turbulence power spectra and velocity correlations in a pipeline with obstructions

2017 ◽  
Vol 28 (02) ◽  
pp. 1750019 ◽  
Author(s):  
A. T. da Cunha Lima ◽  
I. C. da Cunha Lima ◽  
M. P. de Almeida
Keyword(s):  
Spectral Density ◽  
Power Spectral Density ◽  
Cross Sections ◽  
Power Spectra ◽  
Low Frequency ◽  
Frequency Range ◽  
Frequency Space ◽  
Power Spectral ◽  
Power Spectral Densities ◽  
Low Frequency Power

We calculate the power spectral density and velocity correlations for a turbulent flow of a fluid inside a duct. Turbulence is induced by obstructions placed near the entrance of the flow. The power spectral density is obtained for several points at cross-sections along the duct axis, and an analysis is made on the way the spectra changes according to the distance to the obstruction. We show that the differences on the power spectral density are important in the lower frequency range, while in the higher frequency range, the spectra are very similar to each other. Our results suggest the use of the changes on the low frequency power spectral density to identify the occurrence of obstructions in pipelines. Our results show some frequency regions where the power spectral density behaves according to the Kolmogorov hypothesis. At the same time, the calculation of the power spectral densities at increasing distances from the obstructions indicates an energy cascade where the spectra evolves in frequency space by spreading the frequency amplitude.

Comment on paper by Bhartendu, "A study of atmospheric pressure variations from lightning discharges"

1968 ◽  
Vol 46 (20) ◽  
pp. 2333-2334 ◽  
Author(s):  
R. A. McCrory ◽  
C. R. Holmes
Keyword(s):  
Time Series ◽  
Power Spectra ◽  
Power Spectral Analysis ◽  
Sound Level ◽  
Hot Wire ◽  
Power Spectral ◽  
Wide Range ◽  
Spectral Components ◽  
Lightning Discharges ◽  
Level Meter

Bhartendu uses two types of microphones to provide the time series for his power spectral investigations of thunder, the hot wire microphone and a wide-range crystal microphone with amplification provided by a sound-level meter. He then takes power spectra calculated from these time series and states conclusions regarding the power spectrum of thunder. Data and discussion are presented here to demonstrate that one cannot use a single-grid hot-wire microphone for power spectral analysis without introducing spurious spectral components.

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