V. Martínez Pillet

SUNRISE: Instrument, Mission, Data, and First Results

The SUNRISE balloon-borne solar observatory consists of a 1 m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system, and further infrastructure. The first science flight of SUNRISE yielded high-quality data that revealed the structure, dynamics, and evolution of solar convection, oscillations, and magnetic fields at a resolution of around 100 km in the quiet Sun. After a brief description of instruments and data, the first qualitative results are presented. In contrast to earlier observations, we clearly see granulation at 214 nm. Images in Ca II H display narrow, short-lived dark intergranular lanes between the bright edges of granules. The very small-scale, mixed-polarity internetwork fields are found to be highly dynamic. A significant increase in detectable magnetic flux is found after phase-diversity-related reconstruction of polarization maps, indicating that the polarities are mixed right down to the spatial resolution limit and probably beyond.

The Sunrise Mission

The first science flight of the balloon-borne Sunrise telescope took place in June 2009 from ESRANGE (near Kiruna/Sweden) to Somerset Island in northern Canada. We describe the scientific aims and mission concept of the project and give an overview and a description of the various hardware components: the 1-m main telescope with its postfocus science instruments (the UV filter imager SuFI and the imaging vector magnetograph IMaX) and support instruments (image stabilizing and light distribution system ISLiD and correlating wavefront sensor CWS), the optomechanical support structure and the instrument mounting concept, the gondola structure and the power, pointing, and telemetry systems, and the general electronics architecture. We also explain the optimization of the structural and thermal design of the complete payload. The preparations for the science flight are described, including AIV and ground calibration of the instruments. The course of events during the science flight is outlined, up to the recovery activities. Finally, the in-flight performance of the instrumentation is discussed.

Optical Tomography of a Sunspot. II. Vector Magnetic Field and Temperature Stratification

An observational determination of the three-dimensional magnetic and thermal structure of a sunspot is presented. It has been obtained through the application of the SIR inversion technique (Stokes Inversion based on Response functions) on a low-noise, full Stokes profile two-dimensional map of the sunspot as observed with the Advanced Stokes Polarimeter. As a result of the inversion, maps of the magnetic field strength, B, zenith angle, γ, azimuth, χ, and temperature, T, over 25 layers at given optical depths (i.e., an optical tomography) are obtained, of which those between log τ5 = 0 and log τ5 = -2.8 are considered to provide accurate information on the physical parameters. All over the penumbra γ increases with depth, while B is larger at the bottom layers of the inner penumbra (as in the umbra) but larger at the top layers of the outer penumbra (as in the canopy). The corrugation of the penumbral magnetic field already observed by other authors has been confirmed by our different inversion technique. Such a corrugation is especially evident in the zenith angle maps of the intermediate layers, featuring the presence of the so-called spines that we further characterize: spines are warmer and have a less inclined magnetic field than the spaces between them and tend to have a smaller gradient of γ with optical depth over the entire penumbra, but with a field strength which is locally stronger in the middle penumbra and locally weaker in the outer penumbra and beyond in the canopy. In the lower layers of these external parts of the sunspot, most of the field lines are seen to return to the solar surface, a result that is closely connected with the Evershed effect (e.g., Westendorp et al., the third paper in this series). The Stokes V net area asymmetry map as well as the average B, γ, and T radial distributions (and that of the line-of-sight velocities; see the third paper in this series) show a border between an inner and an outer penumbra with different three-dimensional structure. We suggest that it is in this middle zone where most of a new family of penumbral flux tubes (some of them with Evershed flow) emerge interlaced (both horizontally and vertically) among themselves and with the background magnetic field of the penumbra. The interlacing along the line of sight is witnessed by the indication of many points in the outer penumbra showing rapid transitions with height between two structures, one with very weak and inclined magnetic field at the bottom of the photosphere and the other with a stronger and less inclined magnetic field. Over the whole penumbra, and at all optical layers, a constant but weak deviation from radiality of some 5° is detected for the azimuth of the vector magnetic field, which may be in agreement with former detections but which is not significantly higher than the size of the errors for this parameter.

Evidence for a downward mass flux in the penumbral region of a sunspot

Sunspots were the first extraterrestrial phenomenon found to harbour magnetic fields. But the physical nature of sunspots and their relationship to the Suns global magnetic field are still poorly understood. Perhaps the largest uncertainty is related to the outermost region of sunspots (the penumbra) and, in particular, the nature of the so-called Evershed flow-a stream of material emanating radially from sunspots at velocities of up to ∼ 6 km s -1 (ref. 5), before vanishing abruptly at the outer penumbral edges. Here we make use of a recently developed optical tomographic technique to obtain a three-dimensional model of the magnetic field and mass flow in the vicinity of a sunspot. We find that some of the magnetic field lines, together with a significant part of the Evershed mass flux, flow back towards the Sun in the deepest atmospheric layers at the outer edge of the sunspot and its surroundings. This observation should provide an important clue to our understanding of the appearance, stability and decay of sunspots, the most conspicuous tracers of the solar activity cycle.

FULLY RESOLVED QUIET-SUN MAGNETIC FLUX TUBE OBSERVED WITH THE SUNRISE/IMAX INSTRUMENT

Until today, the small size of magnetic elements in quiet-Sun areas has required the application of indirect methods, such as the line-ratio technique or multi-component inversions, to infer their physical properties. A consistent match to the observed Stokes profiles could only be obtained by introducing a magnetic filling factor that specifies the fraction of the observed pixel filled with magnetic field. Here, we investigate the properties of a small magnetic patch in the quiet Sun observed with the IMaX magnetograph on board the balloon-borne telescope SUNRISE with unprecedented spatial resolution and low instrumental stray light. We apply an inversion technique based on the numerical solution of the radiative transfer equation to retrieve the temperature stratification and the field strength in the magnetic patch. The observations can be well reproduced with a one-component, fully magnetized atmosphere with a field strength exceeding 1 kG and a significantly enhanced temperature in the mid to upper photosphere with respect to its surroundings, consistent with semi-empirical flux tube models for plage regions. We therefore conclude that, within the framework of a simple atmospheric model, the IMaX measurements resolve the observed quiet-Sun flux tube.

The Filter Imager SuFI and the Image Stabilization and Light Distribution System ISLiD of the Sunrise Balloon-Borne Observatory: Instrument Description

We describe the design of the Sunrise Filter Imager (SuFI) and the Image Stabilization and Light Distribution (ISLiD) unit onboard the Sunrise balloon borne solar observatory. This contribution provides the necessary information which is relevant to understand the instruments’ working principles, the relevant technical data, and the necessary information about calibration issues directly related to the science data.

SUNRISE/IMaX Observations of Convectively Driven Vortex Flows in the Sun

We characterize the observational properties of the convectively driven vortex flows recently discovered on the quiet Sun, using magnetograms, Dopplergrams, and images obtained with the 1 m balloon-borne SUNRISE telescope. By visual inspection of time series, we find some 3.1 ? 10?3 vortices Mm?2 minute?1, which is a factor of ~1.7 larger than previous estimates. The mean duration of the individual events turns out to be 7.9?minutes, with a standard deviation of 3.2?minutes. In addition, we find several events appearing at the same locations along the duration of the time series (31.6?minutes). Such recurrent vortices show up in the proper motion flow field map averaged over the time series. The typical vertical vorticities are 6 ? 10?3 s?1, which corresponds to a period of rotation of some 35?minutes. The vortices show a preferred counterclockwise sense of rotation, which we conjecture may have to do with the preferred vorticity impinged by the solar differential rotation.

Optical Tomography of a Sunspot. III. Velocity Stratification and the Evershed Effect

The stratification with optical depth of the line-of-sight (LOS) velocity of a simple, isolated, round sunspot observed with the Advanced Stokes Polarimeter (ASP; Elmore et al.) presented here completes this series of papers that investigates the stratification in optical depths of such a typical sunspot. These results have been obtained through the use of the SIR technique (Stokes Inversion based on Response functions of Ruiz Cobo & del Toro Iniesta). From these data we have confirmed that there are strong downflowing velocities at log τ5 = 0 that coincide spatially with the places where the magnetic field points downward (Westendorp Plaza et al.). Further confirmation is obtained by the application of the same method on a different sunspot, already analyzed with the Milne-Eddington inversion technique (Stanchfield, Thomas, & Lites). These downflows reconcile observations that have detected Evershed velocities outside sunspots together with suggestions of the possible return of the flow within the penumbra. The Evershed flow seems to be concentrated in elevated channels not thicker than 1 or 2 scale heights that are mostly located in the space between magnetic spines, i.e., in places where the magnetic field is more inclined, weaker in the inner-middle penumbra, and stronger in the outer penumbra and beyond the visible limits of the sunspot. This conclusion is based upon the tight correlation found between LOS velocities and the (reported in the second paper of this series) magnetic field strength and zenith angle. The upstreaming material is seen in the inner penumbra and the downstreaming in the outer penumbra. A strong increase with optical depth has been obtained for the LOS velocities that provides indications of the superposition of Evershed channels along the LOS. The differential opacity effect between the center-side and the limb-side penumbra, already reported in the second paper in this series, is also seen in the velocity maps and has suggested the comparison of the vertical mass flux through the upstreaming zones (mostly seen in the center side) and the downstreaming zones (mostly seen in the limb side), obtaining a fairly good balance between the two.

The Calibration of the Advanced Stokes Polarimeter

We describe and apply the methods that have been developed to calibrate the Advanced Stokes Polarimeter and to compensate for the polarization effects introduced by the Vacuum Tower Telescope at the National Solar Observatory/Sunspot. A seven-parameter model of the telescope is fitted to data obtained at a variety of mirror angles using observations of both the center of the solar disk and that point within a sunspot umbra at which the magnetic field is oriented as close to the line of sight as possible. The response matrix of the polarimeter itself is determined by the use of polarizing calibration optics that modify the polarization state of the beam exiting the telescope but before entering the polarimeter. A global least-squares solution is obtained simultaneously for the response matrix and the telescope parameters. A detailed gain-correction procedure is described that reduces the multiplicative gain errors in the spectral images to typically less than 1%. We have successfully recovered net-linear polarization profiles with peak amplitudes of 1 × 10-3Ic against an instrumentally produced background polarization of 1-5 × 10-2Ic. Net-polarization signals smaller than 3 × 10-4Ic are lost, even with sufficient averaging, in a background due to photometric and other calibration errors.

An active region filament studied simultaneously in the chromosphere and photosphere - I. Magnetic structure

Aims. A thorough multiwavelength, multiheight study of the vector magnetic field in a compact active region filament (NOAA 10781) on 2005 July 3 and 5 is presented. We suggest an evolutionary scenario for this filament. Methods. Two different inversion codes were used to analyze the full Stokes vectors acquired with the Tenerife Infrared Polarimeter (TIP-II) in a spectral range that comprises the chromospheric He i 10 830 A multiplet and the photospheric Si i 10 827 A line. In addition, we used SOHO/MDI magnetograms, as well as BBSO and TRACE images, to study the evolution of the filament and its active region (AR). High-resolution images of the Dutch Open Telescope were also used. Results. An active region filament (formed before our observing run) was detected in the chromospheric helium absorption images on July 3. The chromospheric vector magnetic field in this portion of the filament was strongly sheared (parallel to the filament axis), whereas the photospheric field lines underneath had an inverse polarity configuration. From July 3 to July 5, an opening and closing of the polarities on either side of the polarity inversion line (PIL) was recorded, resembling the recently discovered process of the sliding door effect seen by Hinode. This is confirmed with both TIP-II and SOHO/MDI data. During this time, a newly created region that contained pores and orphan penumbrae at the PIL was observed. On July 5, a normal polarity configuration was inferred from the chromospheric spectra, while strongly sheared field lines aligned with the PIL were found in the photosphere. In this same data set, the spine of the filament is also observed in a different portion of the field of view and is clearly mapped by the silicon line core. Conclusions. The inferred vector magnetic fields of the filament suggest a flux rope topology. Furthermore, the observations indicate that the filament is divided in two parts, one which lies in the chromosphere and another one that stays trapped in the photosphere. Therefore, only the top of the helical structure is seen by the helium lines. The pores and orphan penumbrae at the PIL appear to be the photospheric counterpart of the extremely low-lying filament. We suggest that orphan penumbrae are formed in very narrow PILs of compact ARs and are the photospheric manifestation of flux ropes in the photosphere.