R. Schlichenmaier

Kiepenheuer-Institut für Sonnenphysik, Freiburg, Baden-Württemberg, Germany

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Publications (80)201.85 Total impact

  • R. Schlichenmaier, M. Franz
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    ABSTRACT: Context. In solar physics, spectropolarimeters based on Fabry-Pérot interferometers are commonly used for high spatial resolution observations. In the data pipeline, corrections for scattered light may be performed on each narrow band image. Aims: We elaborate on the effects of stray-light corrections on Doppler maps. Methods: First, we demonstrate the basic correction effect in a simplified situation of two profiles that suffer from stray light. Then, we study the correction effects on velocity maps by transforming a Hinode SP map into a two-dimensional spectroscopic data set with i(x,y) at each wavelength point, which mimicks narrow band images. Velocity maps are inferred from line profiles of original and stray-light corrected data. Results: The correction of scattered light in narrow band images affects the inferred Doppler velocity maps: relative red shifts always become more red, and relative blue shifts become more blue. This trend is independent of whether downflows have dark or bright intensities. As a result, the effects of overcorrection produce both downflows and upflows. Conclusions: In 2D spectropolarimetry, corrections for scattered light can improve the image intensity and velocity contrast but inherently produce downflow signatures in the penumbra. Hence, such corrections are justified only if the properties of the stray light (seeing, telescope, and instrument) are well known.
    Astronomy and Astrophysics 07/2013; · 5.08 Impact Factor
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    Johannes Löhner-Böttcher, Rolf Schlichenmaier
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    ABSTRACT: Context. The presence of the moat flow around sunspots is intimately linked to the mere existence of sunspots. Aims. We characterize the moat flow (MF) and Evershed flow (EF) in sunspots to enhance our knowledge of sunspot structures and photospheric flow properties. Methods. We calibrated HMI synoptic Doppler maps and used them to analyze 3h time averages of 31 circular, stable, and fully developed sunspots at heliocentric angles of some 50$^{\circ}$. Assuming axially symmetrical flow fields, we infer the azimuthally averaged horizontal velocity component of the MF and EF from 51 velocity maps. We studied the MF properties (velocity and extension) and elaborate on how these components depend on sunspot parameters (sunspot size and EF velocity). To explore the weekly and monthly evolution of MFs, we compare spots rotating from the eastern to western limbs and spots that reappear on the eastern limb. Results. Our calibration procedure of HMI Doppler maps yields reliable and consistent results. In 3h averages, we find the MF decreases on average from some 1000 $\pm$ 200m/s just outside the spot boundary to 500m/s after an additional 4 Mm. The average MF extension lies at 9.2 $\pm$ 5 Mm, where the velocity drops below some 180m/s. Neither the MF velocity nor its extension depend significantly on the sunspot size or EF velocity. But, the EF velocity does show a tendency to be enhanced with sunspot size. On a time scale of a week and a month, we find decreasing MF extensions and a tendency for the MF velocity to increase for strongly decaying sunspots, whereas the changing EF velocity has no impact on the MF. Conclusions. On 3h averages, the EF velocity scales with the size of sunspots, while the MF properties show no significant correlation with the EF or with the sunspot size. This we interpret as a hint that the physical origins of EF and MF are distinct.
    Astronomy and Astrophysics 01/2013; · 5.08 Impact Factor
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    Morten Franz, Rolf Schlichenmaier
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    ABSTRACT: Aims: We search for penumbral magnetic fields of opposite polarity and for their correspondence with downflows. Methods: We used spectropolarimetric HINODE data of a spot very close to disk center to suppress the horizontal velocity components as much as possible. We focus our study on 3-lobe Stokes V profiles. Results: From forward modeling and inversions, we show that 3-lobe profiles testify to the presence of opposite magnetic fields. They occur predominately in the mid and outer penumbra and are associated with downflows in the deep layers of the photosphere. Conclusions. Standard magnetograms show that only 4% of the penumbral area harbors magnetic fields of opposite polarity. If 3-lobe profiles are included in the analysis, this number increases to 17%.
    Astronomy and Astrophysics 12/2012; · 5.08 Impact Factor
  • 12/2012;
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    ABSTRACT: This paper describes the main characteristics of GRIS (GREGOR Infrared Spectrograph), the grating spectrograph installed in the recently inaugurated (May 2012) 1.5-meter GREGOR telescope located at the Observatorio del Teide in Tenerife. The spectrograph has a standard Czerny-Turner configuration with parabolic collimator and camera mirrors that belong to the same conic surface. Although nothing prevents its use at visible wavelengths, the spectrograph will be initially used in combination with the infrared detector of the Tenerife Infrared Polarimeter (TIP-II) in standard spectroscopic mode as well as for spectropolarimetric measurements.
    Astronomische Nachrichten 11/2012; 333:872. · 1.40 Impact Factor
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    ABSTRACT: In this contribution we describe some recent observations of high-speed magnetized flows in the quiet Sun granulation. These observations were carried out with the Imaging Magnetograph eXperiment (IMaX) onboard the stratospheric balloon SUNRISE, and possess an unprecedented spatial resolution and temporal cadence. These flows were identified as highly shifted circular polarization (Stokes V) signals. We estimate the LOS velocity responsible for these shifts to be larger than 6 km s-1, and therefore we refer to them as supersonic magnetic flows. The average lifetime of the detected events is 81.3 s and they occupy an average area of about 23 000 km2. Most of the events occur within granular cells and correspond therefore to upflows. However some others occur in intergranular lanes or bear no clear relation to the convective velocity pattern. We analyze a number of representative examples and discuss them in terms of magnetic loops, reconnection events, and convective collapse.
    05/2012;
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    ABSTRACT: In July 2009, the leading spot of the active region NOAA11024 was observed simultaneously and independently with the 'Goettingen' FPI at VTT and CRISP at SST, i.e., at two different sites, telescopes, instruments and using different spectral lines. The data processing and data analysis have been carried out independently with different techniques. Maps of physical parameters retrieved from 2D spectro-polarimetric data observed with 'Goettingen' FPI and CRISP show an impressive agreement. In addition, the 'Goettingen' FPI maps also exhibit a notable resemblance with simultaneous TIP (spectrographic) observations. The consistency in the results demonstrates the excellent capabilities of these observing facilities. Besides, it confirms the solar origin of the detected signals, and the reliability of FPI-based spectro-polarimeters.
    04/2012;
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    ABSTRACT: In this contribution we describe some recent observations of high-speed magnetized flows in the quiet Sun granulation. These observations were carried out with the Imaging Magnetograph eXperiment (IMaX) onboard the stratospheric balloon {\sc Sunrise}, and possess an unprecedented spatial resolution and temporal cadence. These flows were identified as highly shifted circular polarization (Stokes $V$) signals. We estimate the LOS velocity responsible for these shifts to be larger than 6 km s$^{-1}$, and therefore we refer to them as {\it supersonic magnetic flows}. The average lifetime of the detected events is 81.3 s and they occupy an average area of about 23\,000 km$^2$. Most of the events occur within granular cells and correspond therefore to upflows. However some others occur in intergranular lanes or bear no clear relation to the convective velocity pattern. We analyze a number of representative examples and discuss them in terms of magnetic loops, reconnection events, and convective collapse.
    02/2012;
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    N. Bello González, F. Kneer, R. Schlichenmaier
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    ABSTRACT: Aims: We measure proper motions of fine structures in a forming sunspot to infer information about the dynamics of flux emergence at the sub-photospheric level. Methods: The active region NOAA 11024 was observed with the Vacuum Tower Telescope at Observatorio del Teide/Tenerife over several days in July 2009. Here, we concentrate on a two-hour sequence taken on July 4, when the leading spot was at an early stage of its evolution. Speckle reconstructions from Ca ii K images and polarimetric data in Fe i λ6173 allow us to study proper motions of umbral fine structures. Results: We detect three prominent features: (1) A light bridge, divided by a dark lane along its axis, shows proper motions in opposing directions on its sides, with velocities of ~100-500 m s-1. The flows are seen in both the Ca ii K and the broadband time sequences. (2) Umbral dots in one umbral region outline a vortex with speeds of up to 550 m s-1. The direction of the motion of the umbral dots is different from that in the light bridge. (3) At one rim of the umbra, the fine structure of the magnetic field moves horizontally with typical velocities of 250-300 m s-1, prior to the formation of the penumbra. Conclusions: We report on shear and vortex motions in a forming sunspot and interpret them as tracers of twist relaxation in magnetic flux ropes. We suggest that the forming sunspot contains detached magnetic flux ropes that emerge at the surface with different amounts of twist. As they merge to form a sunspot, they untwist giving rise to the observed shear and vortex motions.
    Astronomy and Astrophysics 02/2012; · 5.08 Impact Factor
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    Reza Rezaei, Nazaret Bello Gonzalez, Rolf Schlichenmaier
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    ABSTRACT: We study the formation of a sunspot penumbra in the active region NOAA11024. We simultaneously observed the Stokes parameters of the photospheric iron lines at 1089.6 nm with the TIP and 617.3 nm with the GFPI spectropolarimeters along with broad-band images using G-band and CaIIK filters at the German VTT. The formation of the penumbra is intimately related to the inclined magnetic field. Within 4.5 h observing time, the magnetic flux of the penumbra increases from 9.7E+20 to 18.2E+20 Mx, while the magnetic flux of the umbra remains constant at about 3.8E+20 Mx. Magnetic flux in the immediate surroundings is incorporated into the spot, and new flux is supplied via small flux patches (SFPs), which on average have a flux of 2-3E+18 Mx. The spot's flux increase rate of 4.2E+16 Mx/s corresponds to the merging of one SFP per minute. We also find that during the formation of the spot penumbra: a) the maximum magnetic field strength of the umbra does not change, b) the magnetic neutral line keeps the same position relative to the umbra, c) the new flux arrives on the emergence side of the spot while the penumbra forms on the opposite side, d) the average LRF inclination of the light bridges decreases from 50 to 37 deg, and e) as the penumbra develops, the mean magnetic field strength at the spot border decreases from 1.0 to 0.8 kG. The SFPs associated with elongated granules are the building blocks of structure formation in active regions. During the sunspot formation, their contribution is comparable to the coalescence of pores. A quiet environment in the surroundings is important for penumbral formation. As remnants of trapped granulation between merging pores, the light bridges are found to play a crucial role in the formation process. They seem to channel the magnetic flux through the spot during its formation. Light bridges are also the locations where the first penumbral filaments form.
    Astronomy and Astrophysics 11/2011; · 5.08 Impact Factor
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    Matthias Rempel, Rolf Schlichenmaier
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    ABSTRACT: We review our current understanding of sunspots from the scales of their fine structure to their large scale (global) structure including the processes of their formation and decay. Recently, sunspot models have undergone a dramatic change. In the past, several aspects of sunspot structure have been addressed by static MHD models with parametrized energy transport. Models of sunspot fine structure have been relying heavily on strong assumptions about flow and field geometry (e.g., flux-tubes, "gaps", convective rolls), which were motivated in part by the observed filamentary structure of penumbrae or the necessity of explaining the substantial energy transport required to maintain the penumbral brightness. However, none of these models could self-consistently explain all aspects of penumbral structure (energy transport, filamentation, Evershed flow). In recent years, 3D radiative MHD simulations have been advanced dramatically to the point at which models of complete sunspots with sufficient resolution to capture sunspot fine structure are feasible. Here overturning convection is the central element responsible for energy transport, filamentation leading to fine-structure and the driving of strong outflows. On the larger scale these models are also in the progress of addressing the subsurface structure of sunspots as well as sunspot formation. With this shift in modeling capabilities and the recent advances in high resolution observations, the future research will be guided by comparing observation and theory.
    Living Reviews in Solar Physics 09/2011; · 13.83 Impact Factor
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    Rolf Schlichenmaier, Reza Rezaei, Nazaret Bello Gonzalez
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    ABSTRACT: Solar magnetic fields are generated in the solar interior and pop up at the solar surface to form active regions. As the magnetic field appears on the surface, it forms various structures like small magnetic elements, pores, and sunspots. The nature of these formation processes is largely unknown. In this contribution we elaborate on the formation of sunspots and particularly on the formation of penumbrae. We report on observations that we obtained at the German VTT on July 4, 2009 on the formation of the spot in AR 11024. This data set is accomplished with data from SOHO/MDI which offers an entire time coverage. Moreover, the evolution of AR 11024 is compared with a particular event of penumbra formation in AR 11124 around November 13, 2010, using intensity images from SDO/HMI. We conclude that two processes contribute to the increase of the magnetic flux of a sunspot: (1) merging pores, and (2) emerging bipoles of which the spot polarity migrates towards and merges with the spot. As the penumbra forms the area, magnetic flux, and maximum field strength in the umbra stay constant or increase slightly, i.e. the formation of the penumbra is associated with flux emergence and an flux increase of the proto-spot. If two pores merge or if a pore merges with a proto-spot a light bridge is created. This initial light bridge dissolves in the further evolution.
    02/2011;
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    ABSTRACT: Using the IMaX instrument on board the SUNRISE stratospheric balloon telescope, we have detected extremely shifted polarization signals around the Fe I 5250.217 Å spectral line within granules in the solar photosphere. We interpret the velocities associated with these events as corresponding to supersonic and magnetic upflows. In addition, they are also related to the appearance of opposite polarities and highly inclined magnetic fields. This suggests that they are produced by the reconnection of emerging magnetic loops through granular upflows. The events occupy an average area of 0.046 arcsec2 and last for about 80 s, with larger events having longer lifetimes. These supersonic events occur at a rate of 1.3 × 10–5 occurrences per second per arcsec2.
    The Astrophysical Journal Letters 10/2010; 723(2):L144. · 6.35 Impact Factor
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    Rolf Schlichenmaier, Nazaret Bello González, Reza Rezaei
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    ABSTRACT: The generation of magnetic flux in the solar interior and its transport to the outer solar atmosphere will be in the focus of solar physics research for the next decades. One key-ingredient is the process of magnetic flux emergence into the solar photosphere, and the reorganization to form the magnetic phenomena of active regions like sunspots and pores. On July 4, 2009, we observed a region of emerging magnetic flux, in which a proto-spot without penumbra forms a penumbra within some 4.5 hours. This process is documented by multi-wavelength observations at the German VTT: (a) imaging, (b) data with high resolution and temporal cadence acquired in Fe I 617.3 nm with the 2D imaging spectropolarimter GFPI, and (c) scans with the slit based spectropolarimeter TIP in Fe I 1089.6 nm. MDI contiuum maps and magnetograms are used to follow the formation of the proto-spot, and the subsequent evolution of the entire active region. During the formation of the penumbra, the area and the magnetic flux of the spot increases. The additional magnetic flux is supplied by the adjacent region of emerging magnetic flux: As emerging bipole separate, the poles of the spot polarity migrate towards the spot, and finally merge with it. As more and more flux is accumulated, a penumbra forms. From inversions we infer maps for the magnetic field and the Doppler velocity (being constant along the line-of-sight). We calculate the magnetic flux of the forming spot and of the bipole footpoints that merge with the proto-spot. We witness the onset of the Evershed flow and the associated enhance of the field inclination as individual penumbral filaments form. Prior to the formation of individual penumbral sectors we detect the existence of 'counter' Evershed flows. These in-flows turn into the classical radial Evershed outflows as stable penumbra segments form. Comment: 6 pages, 4 figures, submitted to IAU S 273
    Proceedings of the International Astronomical Union 09/2010;
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    Morten Franz, Rolf Schlichenmaier
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    ABSTRACT: We investigated the horizontal and the vertical component of the Evershed flow (EF). To this end, we computed average Stokes V profiles for various velocity classes in penumbrae at different heliocentric angles. Our results show that for blueshifted profiles an additional lobe with the same polarity as the spot is present in the blue side of the average Stokes V profile. The amplitude of the additional lobe grows with increasing blueshift and with increasing heliocentric angle. For small redshifts, the profiles show an additional lobe with the opposite polarity as the spot on the red side of the average Stokes V profile. Even at disk center, the original polarity of the average Stokes V profile is reversed for strong redshifts. The transition between the different types of Stokes V profiles is continuous and indicates that not only the vertical, but also the horizontal EF is a magnetized stream of plasma in a magnetic background field.
    Astronomische Nachrichten 08/2010; · 1.40 Impact Factor
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    Morten Franz, Rolf Schlichenmaier
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    ABSTRACT: To investigate the penumbral plasma flow on a small scale, spectropolari- metric data of sunspots recorded by HINODE was used. Maps of Doppler velocities were created by evaluating the bisector in the line-wing, thereby visualizing the flow pattern in the low photosphere where the Evershed effect is most pronounced. In penumbrae close to the disk center, the vertical component of the Evershed flow dominates. The latter consists of a series of elongated up-flow patterns extending radially through the entire center-side penumbra at a constant azimuth. Along this structure, strong up-flows appear in concentrated patches separated by weaker up-flows or even down-flows. The strong up-flows appear at the bright heads and the umbral side of the dark-core of the filament, while the down-flows are rather located at the penumbral side of the filament. Projection effects lead to an overall red-shift of the limb- side penumbra, but the described pattern of up- and down-flows is still ascertainable.
    08/2010;
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    Wolfgang Schmidt, Rolf Schlichenmaier, Oskar von der Lühe
    Astronomische Nachrichten 06/2010; 331(6). · 1.40 Impact Factor
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    ABSTRACT: The generation of magnetic flux in the solar interior and its transport from the convection zone into the photosphere, the chromosphere, and the corona will be in the focus of solar physics research for the next decades. With 4m class telescopes, one plans to measure essential processes of radiative magneto-hydrodynamics that are needed to understand the nature of solar magnetic fields. One key-ingredient to understand the behavior of solar magnetic field is the process of flux emergence into the solar photosphere, and how the magnetic flux reorganizes to form the magnetic phenomena of active regions like sunspots and pores. Here, we present a spectropolarimetric and imaging data set from a region of emerging magnetic flux, in which a proto-spot without penumbra forms a penumbra. During the formation of the penumbra the area and the magnetic flux of the spot increases. First results of our data analysis demonstrate that the additional magnetic flux, which contributes to the increasing area of the penumbra, is supplied by the region of emerging magnetic flux. We observe emerging bipoles that are aligned such that the spot polarity is closer to the spot. As an emerging bipole separates, the pole of the spot polarity migrates towards the spot, and finally merges with it. We speculate that this is a fundamental process, which makes the sunspot accumulate magnetic flux. As more and more flux is accumulated a penumbra forms and transforms the proto-spot into a fully-fledged sunspot. Comment: 4 pages, 2 figures, submitted to Astronomische Nachrichten
    Astronomische Nachrichten 03/2010; · 1.40 Impact Factor
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    L. R. Bellot Rubio, R. Schlichenmaier, K. Langhans
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    ABSTRACT: Recent numerical simulations of sunspots suggest that overturning convection is responsible for the existence of penumbral filaments and the Evershed flow, but there is little observational evidence of this process. Here, we carry out a spectroscopic search for small-scale convective motions in the penumbra of a sunspot located 5° away from the disk center. The position of the spot is very favorable for the detection of overturning downflows at the edges of penumbral filaments. Our analysis is based on measurements of the Fe I 709.0 nm line taken with the Littrow spectrograph of the Swedish 1 m Solar Telescope under excellent seeing conditions. We compute line bisectors at different intensity levels and derive Doppler velocities from them. The velocities are calibrated using a nearby telluric line, with systematic errors smaller than 150 m s-1. Deep in the photosphere, as sampled by the bisectors at the 80%-88% intensity levels, we always observe blueshifts or zero velocities. The maximum blueshifts reach 1.2 km s-1 and tend to be cospatial with bright penumbral filaments. In the line core, we detect blueshifts for the most part, with small velocities not exceeding 300 m s-1. Redshifts also occur, but at the level of 100-150 m s-1, and only occasionally. The fact that they are visible in high layers casts doubts on their convective origin. Overall, we do not find indications of downflows that could be associated with overturning convection at our detection limit of 150 m s-1. Either no downflows exist, or we have been unable to observe them because they occur beneath tau = 1 or the spatial resolution/height resolution of the measurements is still insufficient.
    The Astrophysical Journal 01/2010; 725:11-16. · 6.73 Impact Factor
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    ABSTRACT: While sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this article, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out a helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat.
    Solar Physics 01/2010; 267(1):1-62. · 3.26 Impact Factor

Publication Stats

435 Citations
201.85 Total Impact Points

Institutions

  • 1999–2012
    • Kiepenheuer-Institut für Sonnenphysik
      Freiburg, Baden-Württemberg, Germany
  • 1997–1998
    • Max Planck Institute for Extraterrestrial Physics
      Arching, Bavaria, Germany