Picture of the Month

German-Norwegian team of scientists observes heart-shaped sunspot with GREGOR, the largest European solar telescope.

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The magnetic field of a sunspot guides magnetohydrodynamic waves toward higher atmospheric layers. In the upper photosphere and lower chromosphere, wave modes with periods longer than the acoustic cut-off period become evanescent. The cut-off period essentially changes due to the atmospheric properties, e.g., increases for larger zenith inclinations of the magnetic field. These relations were now employed to develop a novel technique of reconstructing the magnetic field inclination on the basis of the dominating wave periods in the sunspot chromosphere and upper photosphere. 

On 2013 August 21st, an isolated, circular sunspot (NOAA11823) for 58min was observed in a purely spectroscopic multi-wavelength mode with the Interferometric Bidimensional Spectro-polarimeter (IBIS) at the Dunn Solar Telescope. By means of a wavelet power analysis, the dominating wave periods were retrieved to reconstruct the zenith inclinations in the chromosphere and upper photosphere.

The results shown in...

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As early as in 1611, shortly after the discovery of the telescope, sunspots were observed and scientifically analysed. Johann Fabricus (1587 - 1616) described the movement of sunspots and deduced therefrom the rotation of the sun. In 1630, Christoph Scheiner determined that the orbital period is 27 days. Today we know that the sun rotates differentially, so that spots near the equator rotate faster than those in higher latitudes.

Since 2010, the position of sunspots can be measured with unprecedented accuracy using the data obtained with the HMI on the American SDO satellite. In a study carried out at KIS, the movement of 163 stable sunspots was measured between 2010 and 2015. The chart shows the angular velocity of the spots depending on the observed latitude ϑ on the sun. The unit used was degree/day. A rotation velocity of 14.4 degrees/day is equivalent to a rotation period of 25 days. The dependency on the latitude follows the square sine function: 14.421 (+/- 0.026) - 3.116 (+/- 0.179) sin...

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A highlight of first science results with GREGOR has been achieved by detecting the small-scale geometry of the magnetic field in a sunspot penumbra. Spectro-polarimetric data with GRIS@GREGOR reveals that 35 % of the penumbra area is covered with return flux, i.e., locations where the magnetic field has opposite polarity. Since the field strength of this downward-pointing component is small, this return flux only makes up 10% of the total magnetic flux in the penumbra. A careful analysis of the depth dependence of the magnetic field reveals that the return flux is only present in the deepest layers of the photosphere, which are probed using the four iron lines in the spectral vicinity of 1564.8 nm. This is the reason why the spectropolarimeter onboard HINODE using higher-forming iron lines around 630.2 nm cannot detect the return flux, while previous observations at 1564.8 nm with the VTT did not have the necessary spatial resolution.

This finding has important implications on the...

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Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere. In an international cooperation Svetlana Berdyugina from the Kiepenheuer Institut is involved in a campaign in which simultaneous radio and optical spectroscopic observations were performed of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs...

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Our new telescope, a 14" Maksutov-Cassegrain-Telescope (14 inch correspond to an aperture of 35 cm), is now ready to be used for public relations and for educating students. The image shows Messier object number 51 or better known as the Whirlpool Galaxy (NGC 5194).

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The most prominent wave phenomena in the atmosphere above sunspots are the so-called umbral flashes and running penumbral waves. In chromospheric layers they appear as recurrent vigorous brightenings flashing in sunspot umbrae and excited waves propagating radially along penumbrae. Visually these phenomena resemble the effect of a stone falling into water.

A novel high-resolution, multi-layer study concentrating on the characteristics of running penumbral waves finally proves their photospheric origin. A very regular sunspot in NOAA 11823 was observed with the Interferometric BIdimensional Spectro-polarimeter (IBIS/DST) for one hour in various photospheric and chromospheric wavelength. The detailed analysis of wave power and propagation gives strong evidence for the scenario of running penumbral waves being upward propagating slow-mode waves guided by the field lines of the sunspot magnetic field and intimately coupled to the spectacular umbral flashes phenomena.

Running penumbral...

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The magnetic phenomena on the solar and stellar surfaces are driven by the interior dynamo of the star, which generates strong magnetic fields in the form of spots in the stellar photosphere.
The solar photosphere has been intensely studied in the past years using atomic lines, but to shed light on the darkest and coolest parts of sunspots and starspots, the use of molecular lines is favorable because atomic lines receive a strong contribution from outside the spot umbra, whereas some molecular lines can only be formed in cool spots if the effective temperature of the stellar photosphere is high enough. The choice of the adequate molecule therefore represents the key to measuring magnetic fields unambiguously in unresolved spots wherein the molecules are formed. In addition, molecules exhibit high temperature, pressure, and magnetic field sensitivities and provide a unique tool to study physical properties of cool objects and particularly their magnetic fields through phenomena such as...

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Recently, polarimetry has become a promising method in the field of exoplanets, in particular with their detection and characterization. At KIS in the Hotmol group, we investigated how transiting exoplanets might induce some polarimetric effects by breaking the symmetry of the intensity integrated over the stellar disk. More specifically, this study produced models and focused on linear polarization. As these stars are also likely to have stellar spots able to break this symmetry, we took their presence into account in our models.

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We present a method to directly detect or constrain a stochastic background of gravitational waves (SBGW) at mHz and Hz frequencies by using helioseismic observations. We demonstrate the validity of this approach by deducing a direct upper bound around 0.17 mHz on both the astrophysical and cosmological component of the SBGW.

The universe is expected to be penetrated by a stochastic background of gravitational radiation of astrophysical and cosmological origin. This background is not accessible to conventional observations based on electromagnetic waves. In recent years an increasing number of experiments have set upper limits on the amplitude of this background at various frequencies. However, the astrophysically interesting band between 10-4 - 1 Hz remains largely unexplored. As shown by Boughn & Kuhn (1984) the Sun and stars can be employed as giant hydrodynamical detectors for the gravitational-wave background. Using recent high-precision radial velocity data for the Sun and a...

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