Browsing by Author "Solanki, Parth K."

We present a new method of Stokes inversion of spectropolarimetric data and evaluate it by taking the example of a SUNRISE/IMaX observation. An archive of synthetic Stokes profiles is obtained by the spectral synthesis of stateof- the-art magnetohydrodynamics (MHD) simulations and a realistic degradation to the level of the observed data. The definition of a merit function allows the archive to be searched for the synthetic Stokes profiles that best match the observed profiles. In contrast to traditional Stokes inversion codes, which solve the Unno-Rachkovsky equations for the polarized radiative transfer numerically and fit the Stokes profiles iteratively, the new technique provides the full set of atmospheric parameters. This gives us the ability to start an MHD simulation that takes the inversion result as an initial condition. After a relaxation process of half an hour solar time we obtain physically consistent MHD data sets with a target similar to the observation. The new MHD simulation is used to repeat the method in a second iteration, which further improves the match between observation and simulation, resulting in a factor of 2.2 lower mean chi(2) value. One advantage of the new technique is that it provides the physical parameters on a geometrical height scale. It constitutes a first step toward inversions that give results consistent with the MHD equations.

In this review, we look back upon the literature, which had the GREGOR solar telescope project as its subject including science cases, telescope subsystems, and post-focus instruments. The articles date back to the year 2000, when the initial concepts for a new solar telescope on Tenerife were first presented at scientific meetings. This comprehensive bibliography contains literature until the year 2012, i.e., the final stages of commissioning and science verification. Taking stock of the various publications in peer-reviewed journals and conference proceedings also provides the historical context for the reference articles in this special issue of Astronomische Nachrichten/Astronomical Notes. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In 2013 June, the two scientific instruments on. board the second SUNRISE mission witnessed, in detail, a small-scale magnetic flux emergence event as part of the birth of an active region. The Imaging Magnetograph Experiment (IMaX) recorded two small (similar to 5 '') emerging flux patches in the polarized filtergrams of a photospheric Fe I spectral line. Meanwhile, the SUNRISE Filter Imager (SuFI) captured the highly dynamic chromospheric response to the magnetic fields pushing their way through the lower solar atmosphere. The serendipitous capture of this event offers a closer look at the inner workings of active region emergence sites. In particular, it reveals in meticulous detail how the rising magnetic fields interact with the granulation as they push through the Sun's surface, dragging photospheric plasma in their upward travel. The plasma that is burdening the rising field slides along the field lines, creating fast downflowing channels at the footpoints. The weight of this material anchors this field to the surface at semi-regular spatial intervals, shaping it in an undulatory fashion. Finally, magnetic reconnection enables the field to release itself from its photospheric anchors, allowing it to continue its voyage up to higher layers. This process releases energy that lights up the arch-filament systems and heats the surrounding chromosphere.

We present a comparison of high-resolution images of the solar surface taken in the MgII k and Ca II H channels of the Filter Imager on the balloon-borne solar observatory SUNRISE. The Mg and Ca lines are sampled with 0.48 nm and 0.11 nm wide filters, respectively. The two channels show remarkable qualitative and quantitative similarities in the quiet Sun, in an active region plage and during a small flare. However, the Mg filtergrams display 1.4-1.7 times higher intensity contrast and appear more smeared and smoothed in the quiet Sun. In addition, the fibrils in a plage are wider. Although the exposure time is 100 times longer for Mg images, the evidence suggests that these differences cannot be explained only with instrumental effects or the evolution of the solar scene. The differences at least partially arise because of different line-formation heights, the stronger response of Mg k emission peaks to the higher temperatures, and the larger height range sampled by the broad Mg filter used here. This is evidently manifested during the flare when a surge in Mg evolves differently than in Ca.

Context. Some models for the topology of the magnetic field in sunspot penumbrae predict regions free of magnetic fields or with only dynamically weak fields in the deep photosphere. Aims. We aim to confirm or refute the existence of weak-field regions in the deepest photospheric layers of the penumbra. Methods. We investigated the magnetic field at log tau(5) = 0 is by inverting spectropolarimetric data of two different sunspots located very close to disk center with a spatial resolution of approximately 0.4-0.45 ''. The data have been recorded using the GRIS instrument attached to the 1.5-m solar telescope GREGOR at the El Teide observatory. The data include three Fe I lines around 1565 nm, whose sensitivity to the magnetic field peaks half a pressure scale height deeper than the sensitivity of the widely used Fe I spectral line pair at 630 nm. Before the inversion, the data were corrected for the effects of scattered light using a deconvolution method with several point spread functions. Results. At log tau(5) = 0 we find no evidence of regions with dynamically weak (B < 500 Gauss) magnetic fields in sunspot penumbrae. This result is much more reliable than previous investigations made on Fe I lines at 630 nm. Moreover, the result is independent of the number of nodes employed in the inversion, is independent of the point spread function used to deconvolve the data, and does not depend on the amount of stray light (i.e., wide-angle scattered light) considered.

Observations with the balloon-borne Sunrise/Imaging Magnetograph eXperiment (IMaX) provide high spatial resolution (roughly 100 km at disk center) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona, we extrapolate these photospheric measurements into the upper solar atmosphere and analyze a 22-minute long time series with a cadence of 33 seconds. Using the extrapolated magnetic-field lines as tracer, we investigate temporal evolution of the magnetic connectivity in the quiet Sun's atmosphere. The majority of magnetic loops are asymmetric in the sense that the photospheric field strength at the loop foot points is very different. We find that the magnetic connectivity of the loops changes rapidly with a typical connection recycling time of about 3 +/- 1 minutes in the upper solar atmosphere and 12 +/- 4 minutes in the photosphere. This is considerably shorter than previously found. Nonetheless, our estimate of the energy released by the associated magnetic-reconnection processes is not likely to be the sole source for heating the chromosphere and corona in the quiet Sun.

We present the first high-resolution solar images in the Mg ii k 2796 angstrom line. The images, taken through a 4.8 angstrom broad interference filter, were obtained during the second science flight of Sunrise in 2013 June by the Sunrise Filter Imager (SuFI) instrument. The Mgii k images display structures that look qualitatively very similar to images taken in the core of Ca ii H. The Mg ii images exhibit reversed granulation (or shock waves) in the internetwork regions of the quiet Sun, at intensity contrasts that are similar to those found in Ca ii H. Very prominent in Mgii are bright points, both in the quiet Sun and in plage regions, particularly near the disk center. These are much brighter than at other wavelengths sampled at similar resolution. Furthermore, Mg ii k images also show fibril structures associated with plage regions. Again, the fibrils are similar to those seen in Ca ii H images, but tend to be more pronounced, particularly in weak plage.

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 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 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-m 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(-1) for the fast component in the vicinity of footpoints of filamentary structures. The slow component presents velocities close to rest. (C) 2016 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim

Improved measurements of the photospheric and chromospheric three-dimensional magnetic and flow fields are crucial for a precise determination of the origin and evolution of active regions. We present an illustrative sample of multi-instrument data acquired during a two-week coordinated observing campaign in August 2015 involving, among others, the GREGOR solar telescope (imaging and near-infrared spectroscopy) and the space missions Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS). The observations focused on the trailing part of active region NOAA 12396 with complex polarity inversion lines and strong intrusions of opposite polarity flux. The GREGOR Infrared Spectrograph (GRIS) provided Stokes IQUV spectral profiles in the photospheric Si i.1082.7 nm line, the chromospheric He I lambda 1083.0 nm triplet, and the photospheric Ca I lambda 1083.9 nm line. Carefully calibrated GRIS scans of the active region provided maps of Doppler velocity and magnetic field at different atmospheric heights. We compare quick-look maps with those obtained with the " Stokes Inversions based on Response functions" (SIR) code, which furnishes deeper insight into the magnetic properties of the region. We find supporting evidence that newly emerging flux and intruding opposite polarity flux are hampering the formation of penumbrae, i.e., a penumbra fully surrounding a sunspot is only expected after cessation of flux emergence in proximity to the sunspots. (C) 2016 WILEY-VCH Verlag GmbH& Co.KGaA, Weinheim

Context. The solar magnetic field is responsible for all aspects of solar activity. Thus, emergence of magnetic flux at the surface is the first manifestation of the ensuing solar activity. Aims. Combining high-resolution and synoptic observations aims to provide a comprehensive description of flux emergence at photospheric level and of the growth process that eventually leads to a mature active region. Methods. The small active region NOAA 12118 emerged on 2014 July 17 and was observed one day later with the 1.5-m GREGOR solar telescope on 2014 July 18. High-resolution time-series of blue continuum and G-band images acquired in the blue imaging channel (BIC) of the GREGOR Fabry-Perot Interferometer (GFPI) were complemented by synoptic line-of-sight magnetograms and continuum images obtained with the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Horizontal proper motions and horizontal plasma velocities were computed with local correlation tracking (LCT) and the differential affine velocity estimator (DAVE), respectively. Morphological image processing was employed to measure the photometric and magnetic area, magnetic flux, and the separation profile of the emerging flux region during its evolution. Results. The computed growth rates for photometric area, magnetic area, and magnetic flux are about twice as high as the respective decay rates. The space-time diagram using HMI magnetograms of five days provides a comprehensive view of growth and decay. It traces a leaf-like structure, which is determined by the initial separation of the two polarities, a rapid expansion phase, a time when the spread stalls, and a period when the region slowly shrinks again. The separation rate of 0.26 km s(-1) is highest in the initial stage, and it decreases when the separation comes to a halt. Horizontal plasma velocities computed at four evolutionary stages indicate a changing pattern of inflows. In LCT maps we find persistent flow patterns such as outward motions in the outer part of the two major pores, a diverging feature near the trailing pore marking the site of upwelling plasma and flux emergence, and low velocities in the interior of dark pores. We detected many elongated rapidly expanding granules between the two major polarities, with dimensions twice as large as the normal granules.

The accumulation of amyloidogenic proteins is a pathological hallmark of neurodegenerative disorders. The aberrant accumulation of the microtubule associating protein tau (MAPT, tau) into toxic oligomers and amyloid deposits is a primary pathology in tauopathies, the most common of which is Alzheimer's disease (AD). Intrinsically disordered proteins, like tau, are enriched with proline residues that regulate both secondary structure and aggregation propensity. The orientation of proline residues is regulated by cis/trans peptidyl-prolyl isomerases (PPIases). Here we show that cyclophilin 40 (CyP40), a PPIase, dissolves tau amyloids in vitro. Additionally, CyP40 ameliorated silver-positive and oligomeric tau species in a mouse model of tau accumulation, preserving neuronal health and cognition. Nuclear magnetic resonance (NMR) revealed that CyP40 interacts with tau at sites rich in proline residues. CyP40 was also able to interact with and disaggregate other aggregating proteins that contain prolines. Moreover, CyP40 lacking PPIase activity prevented its capacity for disaggregation in vitro. Finally, we describe a unique structural property of CyP40 that may permit disaggregation to occur in an energy-independent manner. This study identifies a novel human protein disaggregase and, for the first time, demonstrates its capacity to dissolve intracellular amyloids.

Convective flows are known as the prime means of transporting magnetic fields on the solar surface. Thus, small magnetic structures are good tracers of turbulent flows. We study the migration and dispersal of magnetic bright features (MBFs) in intergranular areas observed at high spatial resolution with SUNRISE/IMaX. We describe the flux dispersal of individual MBFs as a diffusion process whose parameters are computed for various areas in the quiet-Sun and the vicinity of active regions from seeing-free data. We find that magnetic concentrations are best described as random walkers close to network areas (diffusion index, gamma = 1.0), travelers with constant speeds over a supergranule (gamma = 1.9-2.0), and decelerating movers in the vicinity of flux emergence and/or within active regions (gamma = 1.4-1.5). The three types of regions host MBFs with mean diffusion coefficients of 130 km(2) s(-1), 80-90 km(2) s(-1), and 25-70 km(2) s(-1), respectively. The MBFs in these three types of regions are found to display a distinct kinematic behavior at a confidence level in excess of 95%.

Aims. We aim to investigate the long-term temporal evolution of the magnetic field of the solar-type star xi Bootis A, both from direct magnetic field measurements and from the simultaneous estimate of indirect activity indicators. Methods. We obtained seven epochs of high-resolution, circularly-polarized spectra from the NARVAL spectropolarimeter between 2007 and 2011, for a total of 76 spectra. Using approximately 6100 photospheric spectral lines covering the visible domain, we employed a cross-correlation procedure to compute a mean polarized line profile from each spectrum. The large-scale photospheric magnetic field of the star was then modelled by means of Zeeman-Doppler Imaging, allowing us to follow the year-to-year evolution of the reconstructed magnetic topology. Simultaneously, we monitored the width of several magnetically sensitive spectral lines, the radial velocity, the line asymmetry of intensity line profiles, and the chromospheric emission in the cores of the Ca II H and H alpha lines. Results. During the highest observed activity states, in 2007 and 2011, the large-scale field of xi Bootis A is almost completely axisymmetric and is dominated by its toroidal component. The toroidal component persists with a constant polarity, containing a significant fraction of the magnetic energy of the large-scale surface field through all observing epochs. The magnetic topologies reconstructed for these activity maxima are very similar, suggesting a form of short cyclicity in the large-scale field distribution. The mean unsigned large-scale magnetic flux derived from the magnetic maps varies by a factor of about 2 between the lowest and highest observed magnetic states. The chromospheric flux is less affected and varies by a factor of 1.2. Correlated temporal evolution, due to both rotational modulation and seasonal variability, is observed between the Ca II emission, the H alpha emission and the width of magnetically sensitive lines. The rotational dependence of polarimetric magnetic measurements displays a weak correlation with other activity proxies, presumably due to the different spatial scales and centre-to-limb darkening associated with polarimetric signatures, as compared to non-polarized activity indicators. Better agreement is observed on the longer term. When measurable, the differential rotation reveals a strong latitudinal shear in excess of 0.2 rad d(-1).

Magneto-static models may overcome some of the issues facing force-free magnetic field extrapolations. So far they have seen limited use and have faced problems when applied to quiet-Sun data. Here we present a first application to an active region. We use solar vector magnetic field measurements gathered by the IMaX polarimeter during the flight of the SUNRISE balloon-borne solar observatory in 2013 June as boundary conditions for a magneto-static model of the higher solar atmosphere above an active region. The IMaX data are embedded in active region vector magnetograms observed with SDO/HMI. This work continues our magneto-static extrapolation approach, which was applied earlier to a quiet-Sun region observed with SUNRISE I. In an active region the signal-to-noise-ratio in the measured Stokes parameters is considerably higher than in the quiet-Sun and consequently the IMaX measurements of the horizontal photospheric magnetic field allow us to specify the free parameters of the model in a special class of linear magneto-static equilibria. The high spatial resolution of IMaX (110-130 km, pixel size 40 km) enables us to model the non-force-free layer between the photosphere and the mid-chromosphere vertically by about 50 grid points. In our approach we can incorporate some aspects of the mixed beta layer of photosphere and chromosphere, e.g., taking a finite Lorentz force into account, which was not possible with lower-resolution photospheric measurements in the past. The linear model does not, however, permit us to model intrinsic nonlinear structures like strongly localized electric currents.

We use seeing-free high spatial resolution Ca II H data obtained by the SUNRISE observatory to determine properties of slender fibrils in the lower solar chromosphere. In this work we use intensity images taken with the SUFI instrument in the Ca II H line during the second scientific flight of the SUNRISE observatory to identify and track elongated bright structures. After identification, we analyze theses structures to extract their morphological properties. We identify 598 slender Ca II H fibrils (SCFs) with an average width of around 180 km, length between 500 and 4000 km, average lifetime of approximate to 400 s, and average curvature of 0.002 arcsec(-1). The maximum lifetime of the SCFs within our time series of 57 minutes is approximate to 2000 s. We discuss similarities and differences of the SCFs with other small-scale, chromospheric structures such as spicules of type I and II, or Ca II K fibrils.

A distinct coronal downflow has been discovered in the course of a prominence eruption associated coronal mass ejection (CME) imaged by EIT ( Extreme ultraviolet Imaging Telescope) and LASCO ( Large Angle Spectrometric Coronagraph) on board SOHO ( Solar and Heliospheric Observatory) on 5-Mar.-2000. Evolution of the prominences seen by EIT was tracked into the LASCO/ C2 and C3 field-of-view where they developed as the core of a typical three-part CME. In contrast to the inflow structures reported earlier in the literatures, which were dark and were interpreted as plasma voids moving down, the downflow reported here was bright. The downflow, which was only seen in EIT FOV had an onset time that coincided with the deceleration phase of the core of the CME. The downflow showed a rapid acceleration followed by a strong deceleration. The downflow followed a curved path which may be explained by material following the apex of a contracting magnetic loop sliding down along other field lines, although other explanations are also possible. Irrespective of the detailed geometry, this observation provides support for the pinching off of the field lines drawn-out by the erupting prominences and the contraction of the arcade formed by the reconnection.

We investigate the detectability of Zeeman broadening in optical Stokes I spectra of slowly rotating sun-like stars. To this end, we apply the LTE spectral line inversion package SPINOR to very-high quality CES data and explore how fit quality depends on the average magnetic field, Bf. One-component (OC) and two-component (TC) models are adopted. In OC models, the entire surface is assumed to be magnetic. Under this assumption, we determine formal 3 sigma upper limits on the average magnetic field of 200G for the Sun, and 150 G for 61 Vir (G6V). Evidence for an average magnetic field of similar to 500 G is found for 59 Vir (G0V), and of similar to 1000G for HD68456 (F6V). A distinction between magnetic and non-magnetic regions is made in TC models, while assuming a homogeneous distribution of both components. In our TC inversions of 59 Vir, we investigate three cases: both components have equal temperatures; warm magnetic regions; cool magnetic regions. Our TC model with equal temperatures does not yield significant improvement over OC inversions for 59 Vir. The resulting Bf values are consistent for both. Fit quality is significantly improved, however, by using two components of different temperatures. The inversions for 59 Vir that assume different temperatures for the two components yield results consistent with 0-450 G at the formal 3 sigma confidence level. We thus find a model dependence of our analysis and demonstrate that the influence of an additional temperature component can dominate over the Zeeman broadening signature, at least in optical data. Previous comparable analyses that neglected effects due to multiple temperature components may be prone to the same ambiguities.

We report the detection of oscillations in slender Ca II H fibrils (SCFs) from high-resolution observations acquired with the SUNRISE balloon-borne solar observatory. The SCFs show obvious oscillations in their intensity, but also their width. The oscillatory behaviors are investigated at several positions along the axes of the SCFs. A large majority of fibrils show signs of oscillations in intensity. Their periods and phase speeds are analyzed using a wavelet analysis. The width and intensity perturbations have overlapping distributions of the wave period. The obtained distributions have median values of the period of 32 +/- 17 s and 36 +/- 25 s, respectively. We find that the fluctuations of both parameters propagate in the SCFs with speeds of 11(-11)(+49)+ km s(-1) and 15(-15)(+34) km s(-1), respectively. Furthermore, the width and intensity oscillations have a strong tendency to be either in anti-phase. or, to a smaller extent, in phase. This suggests that the oscillations of both parameters are caused by the same wave mode and that the waves are likely propagating. Taking all the evidence together, the most likely wave mode to explain all measurements and criteria is the fast sausage mode.

Ellerman Bombs are signatures of magnetic reconnection, which is an important physical process in the solar atmosphere. How and where they occur is a subject of debate. In this paper, we analyze SUNRISE/IMaX data, along with 3D MHD simulations that aim to reproduce the exact scenario proposed for the formation of these features. Although the observed event seems to be more dynamic and violent than the simulated one, simulations clearly confirm the basic scenario for the production of EBs. The simulations also reveal the full complexity of the underlying process. The simulated observations show that the Fe I 525.02 nm line gives no information on the height where reconnection takes place. It can only give clues about the heating in the aftermath of the reconnection. However, the information on the magnetic field vector and velocity at this spatial resolution is extremely valuable because it shows what numerical models miss and how they can be improved.