Cross-phase Spectra Research Articles

Characteristics of fluctuations in the equatorial and polar directions of a compact dipole plasma driven at steady state

The fluctuations in the plasma potential (Vs̃) and electron density (Nẽ) are measured in the steady-state dipole plasma confined in a spherical vacuum chamber using a single permanent dipole magnet. Measurements are made from a radial distance r = 0.5–17 cm, from the surface of the magnet in the equatorial (θ=90°) and polar (θ=180°) directions, using a 4-pin probe setup, and have been characterized by power spectra, cross-phase, cross-coherence, induced transport, and energy spectra. The fluctuations are stronger near the surface of the dipole magnet for θ=90°, thereafter their magnitude decreases gradually with the radial distance. However, a peaked profile of potential fluctuations with the peak around r = 9 cm is observed for θ=180°. The locations of strong fluctuations are around the locations of smaller values of the electron neutral collision frequency. Frequency-resolved power spectra show that the fluctuations in the dipole plasma, predominantly, belong to the very low-frequency range. The values of cross-phase (≤45°) and |Vs̃/Te|/|Nẽ/Ne|≤1 indicate the presence of drift wave instability in both planes, where Te and Ne are equilibrium values of the electron temperature and density. The radial variation of fluctuation-induced electron flux (Γr) verifies the “inward diffusion” of electrons in the equatorial plane.

Observing the cold plasma in the Earth's magnetosphere with the EMMA network

We illustrate a semi-automated procedure to detect the field line resonance (FLR) frequencies and the derived equatorial plasma mass densities in the inner magnetosphere from ULF measurements recorded at the European quasi-Meridional Magnetometer Array (EMMA). FLR frequencies are detected using the standard technique based on cross-phase and amplitude ratio spectra from pairs of stations latitudinally separated. Equatorial plasma mass densities are then inferred by solving the toroidal MHD wave equation using the TS05 Tsyganenko magnetic field model and assuming a 1/r dependence of the mass density along the field line. We also present a statistical analysis of the results obtained from 165 non-consecutive days of observations at 8 station pairs covering the range of magnetic L -shells 2.4-5.5 and encompassing a wide range of geomagnetic conditions. The rate of FLR detection maximizes around local noon at each pair of stations, reaching the highest values (~95%) around L = 3. A clear diurnal modulation of the FLR frequency is observed at all L values. At the lowest latitudes, the variation is characterized by a rapid decrease in the early morning hours, a stagnation in the middle of the day, and an increase in the evening hours. At higher latitudes, a continuous and more pronounced decrease of the FLR frequency is observed during all daytime hours reflecting a permanent state of recovery of flux tubes depleted by events of enhanced magnetospheric convection. Consistently, the radial profiles of the inferred equatorial mass density show a density increase from morning to afternoon which gets more pronounced with increasing distance and with the level of the preceding geomagnetic activity. The results also confirm the formation of the plasmapause at geocentric distances that decrease as the disturbance level increases. Mean mass density distributions in the equatorial plane are also shown in 2-D maps for different geomagnetic conditions, as well as for a representative stormy day.

A novel approach to estimating the Doppler shift frequency from quadrature mixer output.

Doppler backscattering systems (DBSs) have been widely used in magnetic confinement fusion devices to measure the density fluctuations and propagation velocity of turbulence. However, the received signals of a DBS usually include both zero-order reflection and backscattering components, which results in interference in calculating the Doppler shift frequency from the backscattering components. A novel method is introduced here for estimating the Doppler shift frequency by separating the zero-order reflection and backscattering components using the cross-phase spectrum between the I-signal and Q-signal from a quadrature mixer, based on the difference in symmetrical characteristics between the zero-order reflection and backscattering signal spectra. It is proven that this method is more effective than traditional approaches, such as multiple signal classification and fast Fourier transformation, for extracting Doppler shift information.

Radial and poloidal correlation reflectometry on Experimental Advanced Superconducting Tokamak.

An X-mode polarized V band (50 GHz-75 GHz) radial and poloidal correlation reflectometry is designed and installed on Experimental Advanced Superconducting Tokamak (EAST). Two frequency synthesizers (12 GHz-19 GHz) are used as sources. Signals from the sources are up-converted to V band using active quadruplers and then coupled together for launching through one single pyramidal antenna. Two poloidally separated antennae are installed to receive the reflected waves from plasma. This reflectometry system can be used for radial and poloidal correlation measurement of the electron density fluctuation. In ohmically heated plasma, the radial correlation length is about 1.5 cm measured by the system. The poloidal correlation analysis provides a means to estimate the fluctuation velocity perpendicular to the main magnetic field. In the present paper, the distance between two poloidal probing points is calculated with ray-tracing code and the propagation time is deduced from cross-phase spectrum. Fluctuation velocity perpendicular to the main magnetic field in the core of ohmically heated plasma is about from -1 km/s to -3 km/s.

A flux-matched gyrokinetic analysis of DIII-D L-mode turbulence

Previous nonlinear gyrokinetic simulations of specific DIII-D L-mode cases have been found to significantly underpredict the ion heat transport and associated density and temperature fluctuation levels by up to almost one of order of magnitude in the outer-core domain, i.e., roughly in the last third of the minor radius. Since then, this so-called shortfall issue has been subject to various speculations on possible reasons and furthermore motivation for a number of dedicated comparisons for L-mode plasmas in comparable machines. However, only a rather limited number of simulations and gyrokinetic codes has been applied to the original scenario, thus calling for further dedicated investigations in order to broaden the scientific basis. The present work contributes along these lines by employing another well-established gyrokinetic code in a numerically and physically comprehensive manner. Contrary to the previous studies, only a mild underprediction is observed at the outer radial positions which can furthermore be overcome by varying the ion temperature gradient within the error bars associated with the experimental measurement. The significance and reliability of these simulations are demonstrated by benchmarks, numerical convergence tests, and furthermore by extensive validation studies. The latter involve cross-phase and cross-power spectra analyses of various fluctuating quantities and confirm a high degree of realism. The code discrepancies come as a surprise since the involved software packages had been benchmarked repeatedly and very successfully in the past. Further collaborative effort in identifying the underlying difference is hence required.

Overview of frequency bandwidth determination techniques of useful signal in case of leaks detection by correlation method

In this paper an overview of useful signal detection methods on the background of intense noise and limits determination methods of useful signal is presented. The following features are considered: peculiarities of usage of correlation analysis, cross-amplitude spectrum, coherence function, cross-phase spectrum, time-frequency correlation function in case of frequency limits determination as well as leaks detection in pipelines. The possibility of using time-frequency correlation function for solving above named issues is described. Time- frequency correlation function provides information about the signals correlation for each of the investigated frequency bands. Data about location of peaks on the surface plot of a time- frequency correlation function allows making an assumption about the spectral composition of useful signal and its frequency boundaries.

Infrasonic component of volcano-seismic eruption tremor

Air-ground and ground-air elastic wave coupling are key processes in the rapidly developing field of seismoacoustics and are particularly relevant for volcanoes. During a sustained explosive volcanic eruption, it is typical to record a sustained broadband signal on seismometers, termed eruption tremor. Eruption tremor is usually attributed to a subsurface seismic source process, such as the upward migration of magma and gases through the shallow conduit and vent. However, it is now known that sustained explosive volcanic eruptions also generate powerful tremor signals in the atmosphere, termed infrasonic tremor. We investigate infrasonic tremor coupling down into the ground and its contribution to the observed seismic tremor. Our methodology builds on that proposed by Ichihara et al. (2012) and involves cross-correlation, coherence, and cross-phase spectra between waveforms from nearly collocated seismic and infrasonic sensors; we apply it to datasets from Mount St. Helens, Tungurahua, and Redoubt Volcanoes.

Remote sensing the plasmasphere, plasmapause, plumes and other features using ground-based magnetometers

The plasmapause is a highly dynamic boundary between different magnetospheric particle populations and convection regimes. Some of the most important space weather processes involve wave-particle interactions in this region, but wave properties may also be used to remote sense the plasmasphere and plasmapause, contributing to plasmasphere models. This paper discusses the use of existing ground magnetometer arrays for such remote sensing. Using case studies we illustrate measurement of plasmapause location, shape and movement during storms; refilling of flux tubes within and outside the plasmasphere; storm-time increase in heavy ion concentration near the plasmapause; and detection and mapping of density irregularities near the plasmapause, including drainage plumes, biteouts and bulges. We also use a 2D MHD model of wave propagation through the magnetosphere, incorporating a realistic ionosphere boundary and Alfven speed profile, to simulate ground array observations of power and cross-phase spectra, hence confirming the signatures of plumes and other density structures.

Analysis of air-ground coupling using seismo-acoustic cross-spectral analysis

Air-ground and ground-air coupling are key processes in the rapidly developing field of seismo-acoustics and are particularly relevant in volcano geophysics. Volcanic eruptions often simultaneously generate sustained seismic and infrasonic signals, e.g., by fluid flow simultaneously occurring in the subsurface and atmosphere. Building upon recent work by Ichihara etal. (2012), we show that cross correlation, coherence, and cross-phase spectra between waveforms from nearly collocated seismic and infrasonic sensors provide new information about air-ground and ground-air coupling. Combining this method with infrasound array processing can provide insight into the geophysical processes generating a range of seismo-acoustic signals. For example, we show that seismic tremor recorded during an eruption at Mount St. Helens is dominated by air-ground coupled infrasound between 5 and 15 Hz. We anticipate that cross-spectral analysis and similar techniques will have wide applicability to arbitrary seismo-acoustic sources and in exploiting the growing volume of seismo-acoustic data. Ichihara et al. (2012), “Monitoring volcanic activity using correlation patterns between infrasound and ground motion,” Geophys. Res. Lett. 39, L04304, doi:10.1029/2011GL050542.

Multi-instrument observations of a large scale Pc4 pulsation

Abstract. On 7 November 2005 various ground based and spaced based instruments registered five wave packets with frequencies in the Pc4 range. The most prominent of the five wave packets was observed in ground based magnetometer data spanning almost all latitudes on the dayside magnetosphere. The propagation from the dayside into the tail is deduced from Poynting flux calculations of Cluster data and an onset time analysis of the ground based magnetometer data. This suggests an upstream source. Backstreaming ions are identified to be the most probable source mechanism for this event. Due to the fortunate configuration of the Cluster satellites, the harmonic structure of the wave is analysed and compared with cross-phase spectra from ground data. We present evidence that the driving wave resonantly interacted with geomagnetic field lines. The data suggests that resonant driving occurred at stations where the driving frequency was harmonically related to the local fundamental frequency, creating FLR-like signatures.

Turbulence studies with means of reflectometry at TEXTOR

At TEXTOR, an O-mode heterodyne reflectometer system is installed and operated for the measurement of plasma density fluctuations and turbulence investigations. With two antenna arrays in the equatorial and top positions having two and three horn antennae, respectively, poloidal correlations are investigated under different plasma scenarios. From the amplitude, cross-phase and coherency spectrum, differences in the ohmic and auxiliary heated discharges are investigated. Furthermore the dynamic behaviour of the turbulence is studied in the SOC–IOC transition and in the precursor phase of a disruption. For the latter an increased integrated power spectral density was observed at the X-point of the mode compared with the O-point. Stationary m = 2 mode activity is observed for the first time at TEXTOR by reflectometry. The fluctuation level is calculated for different conditions and rises significantly increasing heating power which is consistent with the L-mode confinement degradation. Correlation measurements yield the measured phase delays which are used to calculate the poloidal phase velocity perpendicular to the magnetic field. In ohmic plasmas the turbulence rotates like a ‘rigid body’ with constant angular velocity inside the q = 2 surface. The rigid body rotation is broken up during tangential neutral beam injection. From the deduced poloidal wavenumber of the turbulence, most likely ion temperature gradient modes are the driving mechanism of the turbulence.

Time-domain triple-probe measurement of edge plasma turbulence on the Texas Experimental Tokamak Upgrade

The time-domain triple-probe method (TDTP) switches Langmuir probe states at high speeds to gather triple-probe information thereby avoiding errors which result when implementing the triple-probe method with spatially separated single state probes in an inhomogeneous plasma. Successful implementation of the TDTP method requires a switching frequency greater than the turbulence bandwidth so that aliased power does not degrade time interpolated estimates of triple-probe states. Additionally, transients between states, largely determined by the ratio of ion saturation current to probe capacitance, must be short enough to establish a probe–plasma equilibrium. These requirements have been met in the edge plasma and parts of the scrape-off layer on the Texas Experimental Tokamak Upgrade. Temperature fluctuations have been measured at two poloidally separated probes. Cross-phase and coherence spectra consistent with those of floating potential fluctuations and ion saturation current fluctuations are measured.

The resonance structure of low latitude Pc3 geomagnetic pulsations

The spectral difference in ULF wave amplitude between closely spaced meridional ground stations may be used to measure the eigenfrequency of magnetospheric field lines [Baransky et al., 1985]. A more reliable technique based on the crossphase spectrum has been used to identify eigenfrequencies and study the temporal evolution of local field line resonances. Pc3 (22–100 mHz) pulsations recorded with two pairs of low latitude ground stations have been specifically examined. Resonances and harmonics whose frequencies are in excellent agreement with model calculations of standing toroidal field line resonances, when ionospheric mass loading at low latitudes is taken into account, were identified virtually every day. This points to a diagnostic technique for monitoring temporal variations in ionospheric and magnetospheric plasma parameters.

Shot noise cross-correlation functions and cross spectra - Implications for models of QPO X-ray sources

The cross-correlation functions (CCFs) and cross spectra expected for quasi-periodic oscillation (QPO) shot noise models are calculated under various assumptions, and the results are compared to observations. Effects due to possible coherence of the QPO oscillations are included. General formulas for the cross spectrum, the cross-phase spectrum, and the time-delay spectrum for QPO shot models are calculated and discussed. It is shown that the CCFs, cross spectra, and power spectra observed for Cyg X-e2 imply that the spectrum of the shots evolves with time, with important implications for the interpretation of these functions as well as of observed average energy spectra. The possible origins for the observed hard lags are discussed, and some physical difficulties for the Comptonization model are described. Classes of physical models for QPO sources are briefly addressed, and it is concluded that models involving shot formation at the surface of neutron stars are favored by observation.

Test of surface-body wave hypothesis for the Q-frequencydependence of coda

Coda of local earthquakes from three component magnetic tape records for two regions in Europe is analysed. Cross-phase spectra and energy partition study of data suggest that surface-body wave hypothesis proposed by Aki and Chouet (1975) is inadequate to explain the Q-frequency dependence observed from the analysis of coda envelopes, using single back scattering model.