Feller, Alex

[["dc.bibliographiccitation.artnumber","A2"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","596"],["dc.contributor.author","Borrero, J. M."],["dc.contributor.author","Asensio Ramos, A."],["dc.contributor.author","Collados, M."],["dc.contributor.author","Schlichenmaier, R."],["dc.contributor.author","Balthasar, H."],["dc.contributor.author","Franz, M."],["dc.contributor.author","Rezaei, R."],["dc.contributor.author","Kiess, C."],["dc.contributor.author","Orozco Suarez, D."],["dc.contributor.author","Pastor, A."],["dc.contributor.author","Berkefeld, T."],["dc.contributor.author","von der Luehe, O."],["dc.contributor.author","Schmidt, D."],["dc.contributor.author","Schmidt, W."],["dc.contributor.author","Sigwarth, M."],["dc.contributor.author","Soltau, D."],["dc.contributor.author","Volkmer, R."],["dc.contributor.author","Waldmann, T."],["dc.contributor.author","Denker, C."],["dc.contributor.author","Hofmann, Albrecht W."],["dc.contributor.author","Staude, J."],["dc.contributor.author","Strassmeier, K. G."],["dc.contributor.author","Feller, A."],["dc.contributor.author","Lagg, A."],["dc.contributor.author","Solanki, Parth K."],["dc.contributor.author","Sobotka, M."],["dc.contributor.author","Nicklas, H."],["dc.date.accessioned","2018-11-07T10:04:58Z"],["dc.date.available","2018-11-07T10:04:58Z"],["dc.date.issued","2016"],["dc.description.abstract","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."],["dc.identifier.doi","10.1051/0004-6361/201628313"],["dc.identifier.isi","000390797900033"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/14275"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/38805"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Deep probing of the photospheric sunspot penumbra: no evidence of field-free gaps"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A3"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","596"],["dc.contributor.author","Verma, M."],["dc.contributor.author","Denker, C."],["dc.contributor.author","Balthasar, H."],["dc.contributor.author","Kuckein, C."],["dc.contributor.author","Manrique, S. J. Gonzalez"],["dc.contributor.author","Sobotka, M."],["dc.contributor.author","Gonzalez, N. Bello"],["dc.contributor.author","Hoch, S."],["dc.contributor.author","Diercke, A."],["dc.contributor.author","Kummerow, P."],["dc.contributor.author","Berkefeld, T."],["dc.contributor.author","Collados, M."],["dc.contributor.author","Feller, A."],["dc.contributor.author","Hofmann, Albrecht W."],["dc.contributor.author","Kneer, F."],["dc.contributor.author","Lagg, A."],["dc.contributor.author","Loehner-Boettcher, J."],["dc.contributor.author","Nicklas, H."],["dc.contributor.author","Pastor Yabar, A."],["dc.contributor.author","Schlichenmaier, R."],["dc.contributor.author","Schmidt, D."],["dc.contributor.author","Schmidt, W."],["dc.contributor.author","Schubert, M."],["dc.contributor.author","Sigwarth, M."],["dc.contributor.author","Solanki, Parth K."],["dc.contributor.author","Soltau, D."],["dc.contributor.author","Staude, J."],["dc.contributor.author","Strassmeier, K. G."],["dc.contributor.author","Volkmer, R."],["dc.contributor.author","von der Luehe, O."],["dc.contributor.author","Waldmann, T."],["dc.date.accessioned","2018-11-07T10:04:59Z"],["dc.date.available","2018-11-07T10:04:59Z"],["dc.date.issued","2016"],["dc.description.abstract","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."],["dc.identifier.doi","10.1051/0004-6361/201628380"],["dc.identifier.isi","000390797900035"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/14276"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/38807"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/312495/EU/High-Resolution Solar Physics Network/SOLARNET"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Horizontal flow fields in and around a small active region The transition period between flux emergence and decay"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A8"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","596"],["dc.contributor.author","Joshi, J."],["dc.contributor.author","Lagg, A."],["dc.contributor.author","Solanki, S. K."],["dc.contributor.author","Feller, A."],["dc.contributor.author","Collados, M."],["dc.contributor.author","Orozco Suárez, D."],["dc.contributor.author","Schlichenmaier, R."],["dc.contributor.author","Franz, M."],["dc.contributor.author","Balthasar, H."],["dc.contributor.author","Denker, C."],["dc.contributor.author","Berkefeld, T."],["dc.contributor.author","Hofmann, A."],["dc.contributor.author","Kiess, C."],["dc.contributor.author","Nicklas, H."],["dc.contributor.author","Pastor Yabar, A."],["dc.contributor.author","Rezaei, R."],["dc.contributor.author","Schmidt, D."],["dc.contributor.author","Schmidt, W."],["dc.contributor.author","Sobotka, M."],["dc.contributor.author","Soltau, D."],["dc.contributor.author","Staude, J."],["dc.contributor.author","Strassmeier, K. G."],["dc.contributor.author","Volkmer, R."],["dc.contributor.author","von der Lühe, O."],["dc.contributor.author","Waldmann, T."],["dc.date.accessioned","2019-07-09T11:43:08Z"],["dc.date.available","2019-07-09T11:43:08Z"],["dc.date.issued","2016"],["dc.description.abstract","Aims. The fine-structure of the magnetic field in a sunspot penumbra in the upper chromosphere is to be explored and compared to that in the photosphere. Methods. Spectropolarimetric observations with high spatial resolution were recorded with the 1.5-m GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The observed spectral domain includes the upper chromospheric Hei triplet at 10 830 Å  and the photospheric Sii 10 827.1 Å  and Cai 10 833.4 Å  spectral lines. The upper chromospheric magnetic field is obtained by inverting the Hei triplet assuming a Milne-Eddington-type model atmosphere. A height-dependent inversion was applied to the Sii 10 827.1 Å  and Cai 10 833.4 Å  lines to obtain the photospheric magnetic field. Results. We find that the inclination of the magnetic field varies in the azimuthal direction in the photosphere and in the upper chromosphere. The chromospheric variations coincide remarkably well with the variations in the inclination of the photospheric field and resemble the well-known spine and interspine structure in the photospheric layers of penumbrae. The typical peak-to-peak variations in the inclination of the magnetic field in the upper chromosphere are found to be 10°–15°, which is roughly half the variation in the photosphere. In contrast, the magnetic field strength of the observed penumbra does not vary on small spatial scales in the upper chromosphere. Conclusions. Thanks to the high spatial resolution of the observations that is possible with the GREGOR telescope at 1.08 microns, we find that the prominent small-scale fluctuations in the magnetic field inclination, which are a salient part of the property of sunspot penumbral photospheres, also persist in the chromosphere, although at somewhat reduced amplitudes. Such a complex magnetic configuration may facilitate penumbral chromospheric dynamic phenomena, such as penumbral micro-jets or transient bright dots."],["dc.identifier.doi","10.1051/0004-6361/201629214"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/14289"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/58836"],["dc.language.iso","en"],["dc.notes.intern","Merged from goescholar"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/312495/EU/High-Resolution Solar Physics Network/SOLARNET"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Upper chromospheric magnetic field of a sunspot penumbra: observations of fine structure"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A59"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","596"],["dc.contributor.author","Felipe, T."],["dc.contributor.author","Collados, M."],["dc.contributor.author","Khomenko, E."],["dc.contributor.author","Kuckein, C."],["dc.contributor.author","Asensio Ramos, A."],["dc.contributor.author","Balthasar, H."],["dc.contributor.author","Berkefeld, T."],["dc.contributor.author","Denker, C."],["dc.contributor.author","Feller, A."],["dc.contributor.author","Franz, M."],["dc.contributor.author","Hofmann, A."],["dc.contributor.author","Joshi, J."],["dc.contributor.author","Kiess, C."],["dc.contributor.author","Lagg, A."],["dc.contributor.author","Nicklas, H."],["dc.contributor.author","Orozco Suárez, D."],["dc.contributor.author","Pastor Yabar, A."],["dc.contributor.author","Rezaei, R."],["dc.contributor.author","Schlichenmaier, R."],["dc.contributor.author","Schmidt, D."],["dc.contributor.author","Schmidt, W."],["dc.contributor.author","Sigwarth, M."],["dc.contributor.author","Sobotka, M."],["dc.contributor.author","Solanki, S. K."],["dc.contributor.author","Soltau, D."],["dc.contributor.author","Staude, J."],["dc.contributor.author","Strassmeier, K. G."],["dc.contributor.author","Volkmer, R."],["dc.contributor.author","von der Lühe, O."],["dc.contributor.author","Waldmann, T."],["dc.date.accessioned","2019-07-09T11:43:09Z"],["dc.date.available","2019-07-09T11:43:09Z"],["dc.date.issued","2016"],["dc.description.abstract","Context. Active regions are the most prominent manifestations of solar magnetic fields; their generation and dissipation are fundamental problems in solar physics. Light bridges are commonly present during sunspot decay, but a comprehensive picture of their role in the removal of the photospheric magnetic field is still lacking. Aims. We study the three-dimensional configuration of a sunspot, and in particular, its light bridge, during one of the last stages of its decay. Methods. We present the magnetic and thermodynamical stratification inferred from full Stokes inversions of the photospheric Si i 10 827 Å and Ca i 10 839 Å lines obtained with the GREGOR Infrared Spectrograph of the GREGOR telescope at the Observatorio del Teide, Tenerife, Spain. The analysis is complemented by a study of continuum images covering the disk passage of the active region, which are provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. Results. The sunspot shows a light bridge with penumbral continuum intensity that separates the central umbra from a smaller umbra. We find that in this region the magnetic field lines form a canopy with lower magnetic field strength in the inner part. The photospheric light bridge is dominated by gas pressure (high-β), as opposed to the surrounding umbra, where the magnetic pressure is higher. A convective flow is observed in the light bridge. This flow is able to bend the magnetic field lines and to produce field reversals. The field lines merge above the light bridge and become as vertical and strong as in the surrounding umbra. We conclude that this occurs because two highly magnetized regions approach each other during the sunspot evolution."],["dc.identifier.doi","10.1051/0004-6361/201629586"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/14290"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/58837"],["dc.language.iso","en"],["dc.notes.intern","Merged from goescholar"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/312495/EU/High-Resolution Solar Physics Network/SOLARNET"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Three-dimensional structure of a sunspot light bridge"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A116"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","549"],["dc.contributor.author","Jafarzadeh, S."],["dc.contributor.author","Solanki, Parth K."],["dc.contributor.author","Feller, A."],["dc.contributor.author","Lagg, A."],["dc.contributor.author","Pietarila, A."],["dc.contributor.author","Danilovic, S."],["dc.contributor.author","Riethmueller, T. L."],["dc.contributor.author","Martinez Pillet, V."],["dc.date.accessioned","2018-11-07T09:30:34Z"],["dc.date.available","2018-11-07T09:30:34Z"],["dc.date.issued","2013"],["dc.description.abstract","Aims. We aim to improve our picture of the low chromosphere in the quiet-Sun internetwork by investigating the intensity, horizontal velocity, size and lifetime variations of small bright points (BPs; diameter smaller than 0.3 arcsec) observed in the Ca II H 3968 angstrom passband along with their magnetic field parameters, derived from photospheric magnetograms. Methods. Several high-quality time series of disc-centre, quiet-Sun observations from the Sunrise balloon-borne solar telescope, with spatial resolution of around 100 km on the solar surface, have been analysed to study the dynamics of BPs observed in the Ca II H passband and their dependence on the photospheric vector magnetogram signal. Results. Parameters such as horizontal velocity, diameter, intensity and lifetime histograms of the isolated internetwork and magnetic Ca II H BPs were determined. Mean values were found to be 2.2 km s(-1), 0.2 arcsec (approximate to 150 km), 1.48 < I-Ca > and 673 s, respectively. Interestingly, the brightness and the horizontal velocity of BPs are anti-correlated. Large excursions (pulses) in horizontal velocity, up to 15 km s(-1), are present in the trajectories of most BPs. These could excite kink waves travelling into the chromosphere and possibly the corona, which we estimate to carry an energy flux of 310 W m(-2), sufficient to heat the upper layers, although only marginally. Conclusions. The stable observing conditions of Sunrise and our technique for identifying and tracking BPs have allowed us to determine reliable parameters of these features in the internetwork. Thus we find, e. g., that they are considerably longer lived than previously thought. The large velocities are also reliable, and may excite kink waves. Although these wave are (marginally) energetic enough to heat the quiet corona, we expect a large additional contribution from larger magnetic elements populating the network and partly also the internetwork."],["dc.identifier.doi","10.1051/0004-6361/201220089"],["dc.identifier.isi","000313745000116"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9989"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/31334"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Structure and dynamics of isolated internetwork Ca II H bright points observed by SUNRISE"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","A1"],["dc.bibliographiccitation.journal","Astronomy & Astrophysics"],["dc.bibliographiccitation.volume","596"],["dc.contributor.author","Sobotka, M."],["dc.contributor.author","Dudík, J."],["dc.contributor.author","Denker, C."],["dc.contributor.author","Balthasar, H."],["dc.contributor.author","Jurčák, J."],["dc.contributor.author","Liu, W."],["dc.contributor.author","Berkefeld, T."],["dc.contributor.author","Collados Vera, M."],["dc.contributor.author","Feller, A."],["dc.contributor.author","Hofmann, A."],["dc.contributor.author","Kneer, F."],["dc.contributor.author","Kuckein, C."],["dc.contributor.author","Lagg, A."],["dc.contributor.author","Louis, R. E."],["dc.contributor.author","von der Lühe, O."],["dc.contributor.author","Nicklas, H."],["dc.contributor.author","Schlichenmaier, R."],["dc.contributor.author","Schmidt, D."],["dc.contributor.author","Schmidt, W."],["dc.contributor.author","Sigwarth, M."],["dc.contributor.author","Solanki, S. K."],["dc.contributor.author","Soltau, D."],["dc.contributor.author","Staude, J."],["dc.contributor.author","Strassmeier, K. G."],["dc.contributor.author","Volkmer, R."],["dc.contributor.author","Waldmann, T."],["dc.date.accessioned","2019-07-09T11:43:07Z"],["dc.date.available","2019-07-09T11:43:07Z"],["dc.date.issued","2016"],["dc.description.abstract","A small flare ribbon above a sunspot umbra in active region 12205 was observed on November 7, 2014, at 12:00 UT in the blue imaging channel of the 1.5 m GREGOR telescope, using a 1 Å Ca ii H interference filter. Context observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), the Solar Optical Telescope (SOT) onboard Hinode, and the Interface Region Imaging Spectrograph (IRIS) show that this ribbon is part of a larger one that extends through the neighboring positive polarities and also participates in several other flares within the active region. We reconstructed a time series of 140 s of Ca ii H images by means of the multiframe blind deconvolution method, which resulted in spatial and temporal resolutions of 0.1″ and 1 s. Light curves and horizontal velocities of small-scale bright knots in the observed flare ribbon were measured. Some knots are stationary, but three move along the ribbon with speeds of 7–11 km s-1. Two of them move in the opposite direction and exhibit highly correlated intensity changes, which provides evidence of a slipping reconnection at small spatial scales."],["dc.identifier.doi","10.1051/0004-6361/201527966"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/14247"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/58832"],["dc.language.iso","en"],["dc.notes.intern","Merged from goescholar"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/312495/EU/High-Resolution Solar Physics Network/SOLARNET"],["dc.relation","info:eu-repo/grantAgreement/EC/FP7/606862/EU/Flare Chromospheres: Observations, Models and Archives/F-CHROMA"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Slipping reconnection in a solar flare observed in high resolution with the GREGOR solar telescope"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]