The size of penumbral fine structure

I present power spectra of penumbral and granular intensity variations from a speckle-restored G-band image sequence of sunspot NOAA 9407 taken on April 1, 2001 with the Dutch Open Telescope on La Palma. I compare spatial power spectra of the sunspot penumbra with spatial power spectra from granulation with and without filigree. Relative to the granular power distribution, the penumbral power spectrum is enhanced over a wide range in spatial frequency peaking at 0."35. For smaller scales, the penumbral power distribution closely resembles that of the granular intensity variations. In contrast, the power spectrum of granulation with filigree exhibits increased power down to the resolution limit of 0."22, indicating the presence of unresolved magnetic elements

Temperature mapping of sunspots and pores from speckle reconstructed three colour photometry

The two-dimensional temperature distribution in a highly structured sunspot and in two small umbrae is determined from a three-colour photometry in narrow spectral continua. Disturbing influences from the earths atmosphere are removed by speckle masking techniques, yielding a spatial resolution limited by the telescopes aperture. The corresponding colour temperatures are consistent over a range of more than 2000 K, although the numerical correction introduced by the reconstruction differs largely for the three colours. Part of the scatter in the temperature relation disappears when convoluting the final images with artificial PSFs that compensate for the different, colour dependent spatial resolution. The remaining spread in the scatter plots does not reflect noise, but is related to local variations of the temperature difference between the continuum emitting layers. This is most obvious for a small umbra which yields branches in the scatter plots the bluer of which corresponding to the limb-side umbral border. Here, the hot rim of a Wilson depressed umbra becomes visible. The temperature map of the large spot shows that the bright umbral dots do not reach the temperature of the non-spot surroundings. Instead, they exceed the 2000K cooler umbral temperature minimum by 900-1300 K. The filamentary structure of the surrounding penumbra has spatial temperature fluctuations of typically 700 K, a value which fits earlier observed ontrasts. However, the mean temperatures of 5650K in the dark and 6250K in the bright penumbral fine structures exceed former findings. Exceptionally bright penumbral grains are 250K than the mean solar surface and thus exceed even brightest granules

Applying speckle-masking to spectra

We have applied the technique of speckle masking to spectra. The obser- vation of elongated solar structures avoids the problem of missing information in one-dimensional spectra. Image motion perpendicular to the slit was diminished by a one-dimensional image stabilization system. The remaining influence of the earth's atmosphere was removed by a modified speckle-masking algorithm, adapted to the single spatial dimension occurring in the spectra. The reconstructed spectra achieve the diffraction limit of the telescope and the spectrograph. The first application of this technique to observations of spicules and penumbral filaments reveals more details and also yield line profiles which differ from those before reconstruction. The Ha emission in spicules shows line-of-sight velocities two times larger than in the unprocessed spectra. The non-magnetic line Fe 709.03 nm shows penumbral line widths, reflecting mostly the line asymmetry from the Ever- shed effect, which are tightly correlated to the continuum intensity fluctuations. Our reconstruction increases the coherence between both from 0.6 to 0.8

Opposite polarity field with convective downflow and its relation to magnetic spines in a sunspot penumbra

We discuss NICOLE inversions of Fe I 630.15 nm and 630.25 nm Stokes spectra from a sunspot penumbra recorded with the CRISP imaging spectropolarimeter on the Swedish 1-m Solar Telescope at a spatial resolution close to 0.15". We report on narrow radially extended lanes of opposite polarity field, located at the boundaries between areas of relatively horizontal magnetic field (the intra-spines) and much more vertical field (the spines). These lanes harbor convective downflows of about 1 km/s. The locations of these downflows close to the spines agree with predictions from the convective gap model (the "gappy penumbra") proposed six years ago, and more recent 3D MHD simulations. We also confirm the existence of strong convective flows throughout the entire penumbra, showing the expected correlation between temperature and vertical velocity, and having vertical RMS velocities of about 1.2 km/s.Comment: Accepted for publication in A&A (06-March-2013). Minor corrections made in this version

Proxy magnetometry with the Dutch Open Telescope

Superb movies from the Dutch Open Telescope (DOT) on La Palma have proven the validity of the open concept of this innovative telescope for high-resolution imaging of the solar atmosphere. A five- camera speckle-burst registration system is being installed that should permit consistent and synchronous speckle reconstruction at multiple wavelengths including the G band, Ca II K and Ha, and provide tomo- graphic high-resolution imaging of the magnetic topology of the solar atmosphere up to the transition region. Other plans include use of a birefringent Ba II 4554 filter built at Irkutsk. However, the DOT funding remains insecure

A Multi-Channel Speckle Imaging System for the DOT

The Dutch Open Telescope (DOT) had its initial observing campaign in September 1999. Although only a simple video system was used, the results demonstrated the excellent high-resolution capabilities of the combination of the open-telescope concept, the DOT optics, the La Palma site, and the speckle masking algorithm used to overcome the remaining image degradation due to the earth's atmosphere. The latter technique was therefore selected as primary DOT observing mode. The DOT data-acquisition system is now being rebuilt into multi- channel imaging optics with fast digital 10-bit cameras and large-volume speckle burst storage. It will simultaneously map the deep photosphere in the G band and the chromosphere in Ca II K and Ha. In addition, trial Doppler imaging in the Ba II 4554 line with a narrow-band (80 mÅ ) Lyot filter from Irkutsk is so promising that usage of this filter is now planned as well

Solar Magnetometry with the dutch open telescope

The Dutch Open Telescope (DOT) has become op- erational at the Roque de los Muchachos Observa- tory on La Palma. The rst image sequences taken with this innovative telescope demonstrate its capa- bility for tomographic high-resolution imaging of the magnetic topology of the solar atmosphere up to the transition region over the large eld of view permit- ted by consistent speckle restoration. We review the science needs for such imaging and describe the DOT solution to the problems posed by the earth atmo- sphere and the solar physics niche lled by the DOT

Impact of Stark Shifts on the Radiation Cooling of Cu-Dominated Plasmas

We study the impact of Stark line shifts reported recently for Cu I transitions on the radiative cooling of Cu-dominated plasmas. The observed detuning in absorption between the hot core and cold shell of the arc leads to a reduction in radiation reabsorption compared to the case where Stark line shifts are neglected. Using a modeling based on a phenomenological treatment of the Stark line shift, we show that this reduction is below 2%