P. Sütterlin

Motions of Isolated G-Band Bright Points in the Solar Photosphere

Magnetic elements on the quiet Sun are buffeted by convective flows that cause lateral motions on timescales of minutes. The magnetic elements can be observed as bright points (BPs) in the G band at 4305 A ˚ .W e present observations of BPs based on a long sequence of G-band images recorded with the Dutch Open Telescope and postprocessed using speckle-masking techniques. From these images we measured the proper motions of isolated BPs and derived the autocorrelation function of their velocity relative to the solar granulation pattern. The accuracy of BP position measurements is estimated to be less than 23 km on the Sun. The rms velocity of BPs (corrected for measurement errors) is about 0.89 km s � 1 , and the correlation time of BP motions is about 60 s. This rms velocity is about 3 times the velocity measured using cork tracking, almost certainly due to the fact that isolated BPs move more rapidly than clusters of BPs. We also searched for evidence of vorticity in the motions of G-band BPs. Subject headings: Sun: atmospheric motions — Sun: faculae, plages — Sun: magnetic fields — techniques: high angular resolution

DOT tomography of the solar atmosphere. IV. Magnetic patches in internetwork areas

We use G-band and Call H image sequences from the Dutch Open Telescope (DOT) to study magnetic elements that appear as bright points in internetwork parts of the quiet solar photosphere and chromosphere. We find that many of these bright points appear recurrently with varying intensity and horizontal motion within longer-lived magnetic patches. We develop an algorithm for detection of the patches and find that all patches identified last much longer than the granulation. The patches outline cell patterns on mesogranular scales, indicating that magnetic flux tubes are advected by granular flows to mesogranular boundaries. Statistical analysis of the emergence and disappearance of the patches points to an average patch lifetime as long as 530 ± 50 min (about nine hours), which suggests that the magnetic elements constituting strong internetwork fields are not generated by a local turbulent dynamo.

Magnetic properties of G-band bright points in a sunspot moat

We present simultaneous spectropolarimetric observations of four visible (630 nm) and three infrared (1565 nm) spectral lines from the German Vacuum Tower Telescope, together with speckle-reconstructed filtergrams in the G-band and the Ca ii H line core from the Dutch Open Telescope. After alignment of the data sets, we used the G-band intensity to locate bright points (BPs) in the moat of a regular sunspot. With the cospatial and cotemporal information provided by the polarimetric data, we characterize the magnetic, kinematic, and thermal properties of the BPs. We find that (a) 94% of the BPs are associated with magnetic fields; (b) their field strengths range between 500 and 1400 G, with a rather flat distribution; (c) the contrast of BPs in the G-band depends on the angle between the vector magnetic field and the line of sight; (d) the BPs harbor downflows of magnetized plasma and exhibit Stokes V profiles with large area and amplitude asymmetries; (e) the magnetic interior of BPs is hotter than the immediate field-free surroundings by about 1000 K at equal optical depth; and (f) the mean effective diameter of BPs in our data set is 150 km, with very few BPs larger than 300 km. Most of these properties can be explained by the classical magnetic flux tube model. However, the wide range of BP parameters found in this study indicates that not all G-band BPs are identical to stable long-lived flux tubes or sheets of kG strength.

DOT tomography of the solar atmosphere - II. Reversed granulation in Ca II H

High-quality simultaneous image sequences from the Dutch Open Telescope (DOT) in the G band and the Ca  H line are used to quantify the occurrence of reversed granulation as a constituent of the subsonic brightness pattern observed as a background to acoustic oscillations in the quiet-Sun internetwork atmosphere. In the middle photosphere re- versed granulation constitutes a much larger part of this background than at the larger heights sampled by ultraviolet radiation. The anticorrelation with the underlying granulation reaches about 50% at a temporal delay of 2-3 min, and increases with spa- tial image smoothing to mesogranular resolution. We discuss the nature of reversed granulation in terms of convection reversal, gravity waves, acoustic waves, and intergranular magnetism, suggest that the internetwork background pattern is primarily a mixture of the first two ingredients, and speculate that it is also an inverse canopy mapper.

Ellerman bombs: fallacies, fads, usage

Ellerman bombs are short-lived brightenings of the outer wings of Halpha that occur in active regions with much flux emergence. We point out fads and fallacies in the extensive Ellerman bomb literature, discuss their appearance in various spectral diagnostics, and advocate their use as indicators of field reconfiguration in active-region topography using AIA 1700 A images.

Asymmetrical appearance of dark-cored filaments in sunspot penumbrae

Recent sunspot observations at unprecedented resolution have led to the discovery of dark cores in the bright filaments that form the penumbra (Scharmer et al. 2002). The discovery paper considered spots at disk center only, so the properties of the dark-cored filaments remain largely unknown. Here we analyze a speckle-reconstructed time series of G-band and blue continuum images of a sunspot acquired with the Dutch Open Telescope. The target was located at an heliocentric angle of 27 deg. We confirm the existence of dark-cored penumbral filaments also in spots outside the disk center, and report on distinct differences between the center and limb-side penumbra. In the inner center-side penumbra, filaments are detected as two narrow bright streaks separated by a central obscuration. These structures move together as a single entity. On the limb side, dark cores are hardly seen. The time series is used to determine the sizes (∼200-250 km), proper motions (∼280 m s -1 ), and lifetimes (≤45 min) of typical dark-cored filaments.

Ba II 4554 Å speckle imaging as solar Doppler diagnostic

We present observations testing the Dopplergram capability of a narrow-band (80 m A) Lyot lter imag- ing the solar surface in the wings of the Ba II 4554 A resonance line in combination with speckle reconstruction to obtain high angular resolution. The Ba II line is found to be an excellent tool for high-resolution Doppler map- ping thanks to opacity insensitivity to temperature variations and line-width insensitivity to thermal broadening. The resulting Dopplergrams show concentrated downflows of 1.2{2.2 km s 1 in intergranular lanes that probably mark magnetic fluxtubes. Two-wavelength prole sampling is found to suce for high-resolution Dopplergram construction. The lter will be installed as part of a multi-wavelength speckle imaging system on the new Dutch Open Telescope.

DOT tomography of the solar atmosphere. V. Analysis of a surge from AR10486

We present an analysis of high temporal and spatial resolution CaII H chromospheric limb observations obtained with the Dutch Open Telescope (DOT). We focus on a solar surge observed both by the DOT in CaII H and the Transition Region and Coronal Explorer (TRACE) satellite in the 195 A and 1600 A passbands. The surge is observed in active region AR10486 located near the solar limb, a region which two hours later produced the largest X-flare ever recorded. It consists of relatively cold gas of about 10 4 −10 5 K. In TRACE images the surge is followed for almost 2.5 h, shrinking and expanding at the same location several times. From DOT images we find outward propagating intensity disturbances, with velocities higher than 50 km s −1 , indicative of upward material motion. The latter is also suggested by the good correlation between the DOT and TRACE surge apparent height curves, their apparent time delay and a phase difference analysis. A spectral wavelet analysis of the brightness variations within and along the surge shows a predominant period of ∼6 min, the first ever reported for this kind of structures. Magnetic reconnection at the bottom of the surge as its driving mechanism is suggested by the observed inverted “Y” shape configuration and is further supported by a phase difference analysis.

Search for photospheric footpoints of quiet Sun transition region loops

Context. The footpoints of quiet Sun Transition Region (TR) loops do not seem to coincide with the photospheric magnetic structures appearing in traditional low-sensitivity magnetograms. Aims. We look for the so-far unidentified photospheric footpoints of TR loops using G-band bright points (BPs) as proxies for photospheric magnetic field concentrations. Methods. We compare TR measurements with SoHO/SUMER and photospheric magnetic field observations obtained with the Dutch Open Telescope. Results. Photospheric BPs are associated with bright TR structures, but they seem to avoid the brightest parts of the structure. BPs appear in regions that are globally redshifted, but they avoid extreme velocities. TR explosive events are not clearly associated with BPs. Conclusions. The observations are not inconsistent with the BPs being footpoints of TR loops, although we have not succeeded to uniquely identify particular BPs with specific TR loops.

The Dutch Open Telescope on La Palma

The Dutch Open Telescope (DOT) on La Palma is an innovative solar telescope combining open telescope structure and an open support tower with a multi-wavelength imaging assembly and with synchronous speckle cameras to generate high-resolution movies which sample different layers of the solar atmosphere simultaneously and co-spatially at high resolution over long durations. The DOT test and development phase is nearly concluded. The installation of an advanced speckle processor enables full science utilization including “Open-DOT” time allocation to the international community. Co-pointing with spectropolarimeters at other Canary Island telescopes and with TRACE furnishes valuable Solar-B precursor capabilities.