arXiv subjects

M. Verma

Publications and source records attributed to M. Verma.

43 records · Page 3Linked to original sources

arXiv2016

Fitting peculiar spectral profiles in He I 10830 Å absorption features

The new generation of solar instruments provides better spectral, spatial, and temporal resolution for a better understanding of the physical processes that take place on the Sun. Multiple-component profiles are more commonly observed with these instruments. Particularly, the He I 10830 Å triplet presents such peculiar spectral profiles, which give information on the velocity and magnetic fine structure of the upper chromosphere. The purpose of this investigation is to describe a technique to efficiently fit the two blended components of the He I 10830 Å triplet, which are commonly observed when two atmospheric components are located within the same resolution element. The observations used in this study were taken on 2015 April 17 with the very fast spectroscopic mode of the GREGOR Infrared Spectrograph (GRIS) attached to the 1.5-meter GREGOR solar telescope, located at the Observatorio del Teide, Tenerife, Spain. We apply a double-Lorentzian fitting technique using Levenberg-Marquardt least-squares minimization. This technique is very simple and much faster than inversion codes. Line-of-sight Doppler velocities can be inferred for a whole map of pixels within just a few minutes. Our results show sub- and supersonic downflow velocities of up to 32 km/s for the fast component in the vicinity of footpoints of filamentary structures. The slow component presents velocities close to rest.

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arXiv2014

Near-infrared spectropolarimetry of a delta-spot

Sunspots harboring umbrae of both magnetic polarities within a common penumbra (delta-spots) are often but not always related to flares. We present first near-infrared (NIR) observations (Fe I 1078.3 nm and Si I 1078.6 nm spectra) obtained with the Tenerife Infrared Polarimeter (TIP) at the Vacuum Tower Telescope (VTT) in Tenerife on 2012 June 17, which afford accurate and sensitive diagnostics to scrutinize the complex fields along the magnetic neutral line of a delta-spot within active region NOAA 11504. We examine the vector magnetic field, line-of-sight (LOS) velocities, and horizontal proper motions of this rather inactive delta-spot. We find a smooth transition of the magnetic vector field from the main umbra to that of opposite polarity (delta-umbra), but a discontinuity of the horizontal magnetic field at some distance from the delta-umbra on the polarity inversion line. The magnetic field decreases faster with height by a factor of two above the delta-umbra. The latter is surrounded by its own Evershed flow. The Evershed flow coming from the main umbra ends at a line dividing the spot into two parts. This line is marked by the occurrence of central emission in the Ca II 854.2 nm line. Along this line, high chromospheric LOS-velocities of both signs appear. We detect a shear flow within the horizontal flux transport velocities parallel to the dividing line.

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arXiv2013

High-resolution spectroscopy of a giant solar filament

High-resolution spectra of a giant solar quiescent filament were taken with the Echelle spectrograph at the Vacuum Tower Telescope (VTT; Tenerife, Spain). A mosaic of various spectroheliograms (Hα, Hα +/- 0.5Å and Na D2) were chosen to examine the filament at different heights in the solar atmosphere. In addition, full-disk images (He I 10830Å and Ca II K) of the Chromspheric Telescope and full-disk magnetograms of the Helioseismic and Magnetic Imager were used to complement the spectra. Preliminary results are shown of this filament, which had extremely large linear dimensions (~ 740'') and was observed in November 2011 while it traversed the northern solar hemisphere.

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arXiv2013

Evaluating local correlation tracking using CO5BOLD simulations of solar granulation

Flows on the solar surface are linked to solar activity, and LCT is one of the standard techniques for capturing the dynamics of these processes by cross-correlating solar images. However, the link between contrast variations in successive images to the underlying plasma motions has to be quantitatively confirmed. Radiation hydrodynamics simulations of solar granulation (e.g.,CO5BOLD) provide access to both the wavelength-integrated, emergent continuum intensity and the 3D velocity field at various heights in the solar atmosphere. Thus, applying LCT to continuum images yields horizontal proper motions, which are then compared to the velocity field of the simulated (non-magnetic) granulation. In this study, we evaluate the performance of an LCT algorithm previously developed for bulk-processing Hinode G-band images, establish it as a quantitative tool for measuring horizontal proper motions, and clearly work out the limitations of LCT or similar techniques designed to track optical flows. Horizontal flow maps and frequency distributions of the flow speed were computed for a variety of LCT input parameters including the spatial resolution, the width of the sampling window, the time cadence of successive images, and the averaging time used to determine persistent flow properties. Smoothed velocity fields from the hydrodynamics simulation at three atmospheric layers (log tau=-1,0,and +1) served as a point of reference for the LCT results. LCT recovers many of the granulation properties, e.g.,the shape of the flow speed distributions, the relationship between mean flow speed and averaging time, and also--with significant smoothing of the simulated velocity field--morphological features of the flow and divergence maps. However, the horizontal proper motions are grossly underestimated by as much as a factor of three. The LCT flows match best the flows deeper in the atmosphere at log tau=+1.

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arXiv2013

Properties of a Decaying Sunspot

A small decaying sunspot was observed with the Vacuum Tower Telescope (VTT) on Tenerife and the Japanese Hinode satellite. We obtained full Stokes scans in several wavelengths covering different heights in the solar atmosphere. Imaging time series from Hinode and the Solar Dynamics Observatory (SDO) complete our data sets. The spot is surrounded by a moat flow, which persists also on that side of the spot where the penumbra already had disappeared. Close to the spot, we find a chromospheric location with downflows of more than 10 km/s without photospheric counterpart. The height dependence of the vertical component of the magnetic field strength is determined in two different ways that yielded different results in previous investigations. Such a difference still exists in our present data, but it is not as pronounced as in the past.

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arXiv2011

Horizontal flow fields observed in Hinode G-band images II. Flow fields in the final stages of sunspot decay

We present a subset of multi-wavelengths observations obtained with the Japanese Hinode mission, the Solar Dynamics Observatory (SDO), and the Vacuum Tower Telescope (VTT) at Observatorio del Teide, Tenerife, Spain during the time period from 2010 November 18-23. Horizontal proper motions were derived from G-band and Ca II H images, whereas line-of-sight velocities were extracted from VTT Echelle H-alpha 656.28 nm spectra and Fe I 630.25 nm spectral data of the Hinode/Spectro-Polarimeter, which also provided three-dimensional magnetic field information. The Helioseismic and Magnetic Imager on board SDO provided continuum images and line-of-sight magnetograms as context for the high-resolution observations for the entire disk passage of the active region. We have performed a quantitative study of photospheric and chromospheric flow fields in and around decaying sunspots. In one of the trailing sunspots of active region NOAA 11126, we observed moat flow and moving magnetic features (MMFs), even after its penumbra had decayed. We also noticed a superpenumbral structure around this pore. MMFs follow well-defined, radial paths from the spot all the way to the border of a supergranular cell surrounding the spot. In contrast, flux emergence near the other sunspot prevented it from establishing such well ordered flow patterns, which could even be observed around a tiny pore of just 2 Mm diameter. After the disappearance of the sunspots/pores a coherent patch of abnormal granulation remained at their location, which was characterized by more uniform horizontal proper motions, low divergence values, and diminished photospheric Doppler velocities. This region, thus, differs significantly from granulation and other areas covered by G-band bright points. We conclude that this peculiar flow pattern is a signature of sunspot decay and the dispersal of magnetic flux.

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arXiv2011

Horizontal flow fields observed in Hinode G-band images. I. Methods

Context: The interaction of plasma motions and magnetic fields is an important mechanism, which drives solar activity in all its facets. For example, photospheric flows are responsible for the advection of magnetic flux, the redistribution of flux during the decay of sunspots, and the built-up of magnetic shear in flaring active regions. Aims: Systematic studies based on G-band data from the Japanese Hinode mission provide the means to gather statistical properties of horizontal flow fields. This facilitates comparative studies of solar features, e.g., G-band bright points, magnetic knots, pores, and sunspots at various stages of evolution and in distinct magnetic environments, thus, enhancing our understanding of the dynamic Sun. Methods: We adapted Local Correlation Tracking (LCT) to measure horizontal flow fields based on G-band images obtained with the Solar Optical Telescope on board Hinode. In total about 200 time-series with a duration between 1-16 h and a cadence between 15-90 s were analyzed. Selecting both a high-cadence (dt = 15 s) and a long-duration (dT = 16 h) time-series enabled us to optimize and validate the LCT input parameters, hence, ensuring a robust, reliable, uniform, and accurate processing of a huge data volume. Results: The LCT algorithm produces best results for G-band images having a cadence of 60-90 s. If the cadence is lower, the velocity of slowly moving features will not be reliably detected. If the cadence is higher, the scene on the Sun will have evolved too much to bear any resemblance with the earlier situation. Consequently, in both instances horizontal proper motions are underestimated. The most reliable and yet detailed flow maps are produced using a Gaussian kernel with a size of 2560 km x 2560 km and a full-width-at-half-maximum (FWHM) of 1200 km (corresponding to the size of a typical granule) as sampling window.

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