Showing papers by "Krishnan Balasubramaniam published in 2005"

Sequential Chromospheric Brightenings beneath a Transequatorial Halo Coronal Mass Ejection

Abstract: Analyses of multiwavelength data sets for a solar eruption at ~21:30 UT on 2002 December 19 show evidence for the disappearance of a large-scale, transequatorial coronal loop (TL). In addition, coronal manifestations of the eruption (based on SOHO EIT and LASCO images) include large-scale coronal dimming, flares in each associated active region in the northern and southern hemispheres, and a halo CME. We present detailed observations of the chromospheric aspects of this event based on Hα images obtained with the ISOON telescope. The ISOON images reveal distant flare precursor brightenings, sympathetic flares, and, of most interest herein, four nearly cospatial propagating chromospheric brightenings. The speeds of the propagating disturbances causing these brightenings are 600-800 km s-1. The inferred propagating disturbances have some of the characteristics of Hα and EIT flare waves (e.g., speed, apparent emanation from the flare site, subsequent filament activation). However, they differ from typical Hα chromospheric flare waves (also known as Moreton waves) because of their absence in off-band Hα images, small angular arc of propagation (<30°), and their multiplicity. Three of the four propagating disturbances consist of a series of sequential chromospheric brightenings of network points that suddenly brighten in the area beneath the TL that disappeared earlier. SOHO MDI magnetograms show that the successively brightened points that define the inferred propagating disturbances were exclusively of one polarity, corresponding to the dominant polarity of the affected region. We speculate that the sequential chromospheric brightenings represent footpoints of field lines that extend into the corona, where they are energized in sequence by magnetic reconnection as coronal fields tear away from the chromosphere during the eruption of the transequatorial CME. We report briefly on three other events with similar narrow propagating disturbances that were confined to a single hemisphere.

Full Stokes Spectropolarimetry of Hα in Prominences

Abstract: We report on spectropolarimetric observations of Hα in prominences made with the Telescope Heliographique pour l'Etude du Magnetisme et des Instabilites Solaires and the High Altitude Observatory/Advanced Stokes Polarimeter. Stokes Q and U show the expected profile shape from resonance scattering polarization and the Hanle effect. In contrast, most of the time, Stokes V does not show the antisymmetric profile shape typical of the Zeeman effect but a profile that indicates the presence of strong atomic orientation in the hydrogen levels, to an extent that cannot be explained by invoking the alignment-to-orientation transfer mechanism induced by the prominence magnetic field. We found that the largest signal amplitudes of Stokes V (comparable to that of Stokes Q and U) could be produced by a process of selective absorption of circularly polarized radiation from the photosphere, which requires that the prominence be in the vicinity of an active region. Although recent observations of active region filaments indicate such a selective absorption mechanism as a plausible explanation of the anomalous signals observed, the particular set of conditions that must be met suggest that a different explanation may be required to explain the almost ubiquitous symmetric V signal observed in Hα prominences. Therefore, we speculate that an alternative mechanism inducing strong atomic orientation at the observed level could be due to the presence of electric fields inducing an electric Hanle effect on Hα. Although we are still working toward a careful modeling of this effect, including both electric and magnetic fields, we present some preliminary considerations that seem to support this possibility.

A multi-frequency eddy current inversion method for characterizing conductivity gradients on water jet peened components

Abstract: This paper describes a multi-frequency eddy current inversion procedure for characterizing specimens that are water jet peened. Multifrequency inductance data was obtained by using well-characterized eddy current probes. The inversion uses a multi-layer axisymmetric finite element model as the forward model and the conductivity of each layer is found through interpolation of the inductance‐conductivity data generated by the forward model. Skin depth approximation was used to isolate the integral effects of the conductivity variation on the inductance signal. Inverted conductivity profiles of the water jet peened specimens was found to resemble the predicted profiles. Information regarding the shape of residual stress gradients and relative intensities of peening were inferred from the conductivity profiles. q 2005 Elsevier Ltd. All rights reserved.

Ray based model for the ultrasonic time-of-flight diffraction simulation of thin walled structure inspection

Abstract: It is necessary to size the cracklike defects accurately in order to extend the life of thin-walled (<10 mm) components (such as pressure vessels) particularly for aerospace applications. This paper discusses the successful application of ray techniques to simulate the ultrasonic time-of-flight diffraction experiments for platelike structures. For the simulation, the diffraction coefficients are computed using the geometric diffraction theory. The A and B scans are simulated in near real time and the different experimental parameters can be interactively controlled due to the computational efficiency of the ray technique. The simulated results are applied to (1) defect signal identification for vertical defects, (2) inspection of inclined defects, and (3) study the effect of pulse width or probe frequency on experimental results. The simulated results are compared with laboratory scale experimental results.

Pca-interpolation methods for inversion of solar stokes profiles. i. inversion of photospheric profiles

Abstract: We present a method for the analysis of solar Stokes profiles based on the technique of principal component analysis (PCA) We consider the development of a PCA/interpolation technique for obtaining estimates of the solar magnetic field vector when the underlying physical parameter distribution is not well determined Unlike previous applications of PCA, we consider separate distributions of the PCA components as applied to the source of the underlying plasma, namely, the umbra, penumbra, and quiet photosphere We also isolate the PCA for the Fe I λ63015 line In some previous work, PCA was applied to the entire 6300-6305 A spectral region that includes the telluric lines The method is tested on photospheric (Fe I λ63015) observations of NOAA Active Region 9240 and compared with the results of the traditional High Altitude Observatory Milne-Eddington inversion code We also consider a PCA-perturbation analysis of the data and how it can assist the interpretation of the results, discuss current challenges faced by the inversion codes, and suggest important areas for future development

A point source correlation technique for automatic discontinuity identification and sizing using time of flight diffraction

Abstract: This paper proposes a technique for automatic discontinuity location and sizing using the ultrasonic time of flight diffraction technique. Here, the crack tips are modeled as point sources of diffracted waves in a homogenous, isotropic medium. The diffraction arcs are modeled using a ray based approach and the modeled arcs are correlated with the experimental B-scan data. The points of high correlation provide information about the location of the crack tips. A statistical echo separation procedure to isolate the diffraction arcs in the B-scan image is discussed. This paper also addresses the issue of application of this time of flight diffraction technique to a thin section (less than 12 mm [0.47 in.]), wherein the echoes from the various sources (lateral wave, back surface reflection, diffraction from crack tips and so on) interfere with each other, making it difficult to identify diffracted signals from the discontinuity tips.

Patch Element Model for the Evaluation of Displacement Fields Within an Elastic Solid from a Non-Contact Immersion Transducer: Application to the 2004 Ultrasonic Benchmark Problem

Abstract: The ultrasonic benchmark problem requires models to predict, given a reference pulse waveform, the pulse echo response of cylindrical voids of various radii located in an elastic solid for various incidence angles of a transducer immersed in water. We present a conceptually simple yet reliable numerical technique to determine these internal fields in any region of interest within the elastic solid for the specified angles made by the transducer in water. The technique, equivalent to evaluating the Rayleigh‐Sommerfeld integral but through a computationally less demanding procedure, regards the transducer as made of elemental rectangular/square patches and uses the well‐known expression for the radiation pattern of an elemental patch to obtain the total transducer radiation field. A ray‐based method is adopted to propagate the elementary radiation field across a fluid‐solid interface. The FBH is treated in terms of explicit patch element reflectors, its response is evaluated and validated with measurements. The P‐wave scattering charactieristics of spherical voids are evaluated using the exact separation of variables method and the patch model for the transducer is used to determine their response. The advantages of the patch model for the transducer in the context of the benchmark problems in particular and non‐destructive evaluation in general are indicated.

Plate waves structural health monitoring of composite structures

Abstract: Layered composite plate‐like structures are finding an increasing range of applications in the aerospace industry. Structural Health Monitoring (SHM) of such structures is seen as a paradigm that will embrace efficient non‐destructive testing/evaluation techniques. The present study demonstrates two techniques that have the potential for the SHM of multi‐layered composite structures. The first technique is based on multi‐transmitter‐multi‐receiver (MTMR) technique with tomographic methods used for data reconstruction. In the MTMR, the possibility of SHM using algebraic reconstruction techniques (ART) for tomographic imaging with Lamb wave data measured in realistic materials is examined. Commercially available narrow bandwidth PZT crystals were used as sensors on multi‐layered quasi‐isotropic and cross‐ply composite plates with and without defects. Defects (through holes and low velocity impact delaminations) were synthetic and have been chosen to simulate impact damage in composite plates. To achieve reasonable image quality, conventional cross‐hole configuration is replaced by a new modified cross‐hole configuration that also optimizes the number of sensors. The second technique is a single‐transmitter‐multi‐receiver (STMR) technique that is more compact and uses reconstruction techniques that are analogus to synthetic aperture techniques. Here, the phase shifting is performed on the individual signals based on the guided wave dispersion relationships. The reconstruction algorithm uses summation of the phase shifted signals to image the location of defects, portions of the plate edges, and any reflectors from inherent structural features of the component.

Application of TOFD Technique to Thin Sections Using ESIT and PSCT

Abstract: It is difficult to accurately size the defects that are oriented at an angle (that is not normal to the wave) using conventional amplitude based ultrasonic techniques. Since Time of Flight Diffraction (TOFD) is based on the diffraction of ultrasound at defect edges, defect sizing using this technique is amplitude independent. However, most of the TOFD based assessment relies on manual sizing, whose accuracy depends on quality of image and the operator’s experience. Also, the utilization of TOFD for sections less than 15 mm has reportedly several difficulties. In this paper, we report our attempts to size the vertical and inclined defects using an in‐house TOFD system built to inspect thin sections (6–10 mm). To improve sizing, automated defect sizing techniques termed Embedded Signal Identification Technique (ESIT) and Point Source Correlation Technique (PSCT) were developed. A ray tracing based model was also developed for a) optimizing the experimental parameters for thin sections, b) interpreting the r...

Directional Filter Bank for NDE Image Segmentation

Abstract: This paper introduces a Directional Filter Bank (DFB) for segmentation of C‐Scan images from fiber‐reinforced composites The DFB is a two‐channel decomposition employing the Quincunx sampling matrix and the Diamond Half Band filter pair The incident ultrasonic field on a fiber‐reinforced composite is backscattered by the fibers leading to the appearance of a characteristic pattern (oriented along the fiber direction) in the C‐Scan image The fiber pattern can cloud the defects, such as delaminations etc, thus making their detection difficult The DFB is a sub‐band coding technique that has been applied to segment the fiber pattern from the defect features in the C‐Scan image of the composite