Oscillations of the Hα emission in solar prominences

The time dependence of Doppler shift and line-center intensity is simultaneously observed for the H emission of three solar prominences, each one during about two hours. Doppler oscillations with periods near one hour and amplitudes between 1 and 2 km s^–1 are conspicuously visible in the recordings of all three prominences. Fourier analysis yields periods of 50, 60, and 64 min, as well as slight indications of short periods near 3 and 5 min. No oscillations are found in the line-center brightness.     

Applying speckle masking to spectra

We have applied the technique of speckle masking to spectra. The observation 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 H 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 Evershed effect, which are tightly correlated to the continuum intensity fluctuations. Our reconstruction increases the coherence between both from 0.6 to 0.8.     

Magnetic field strength and inclination in the penumbral fine-structure

The uncertainty about a possible correlation between magnetic field strength, inclination, and the continuum intensity of sunspot penumbral fine-structure has been removed from detailed analysis of a spatially very well-resolved spectrum: the darker, long penumbral lanes host a 10% stronger and 30° flatter magnetic field as compared to the field in bright penumbral locations. This finding is not only based on the high spatial resolution but also on the use of a spectral line, here Fe 6842.7 Å, obtaining its essential contribution from those deep layers where the penumbral structure is seen, i.e. the continuum intensity level. The almost perfect correlation establishes that the penumbral structure is formed by the two magnetic components mainly differing by the field inclination. The different results from other Zeeman lines, as, e.g., Fe 6302.5 Å, indicate a different field structure above the white-light penumbral layers.     

On polarimetry in solar active regions

High resolved magnetograms ( 3) were obtained 3 hrs before and 1 hr after a 1b flare, respectively, the only bright flare reported for that active region. Careful comparison between both magnetograms shows that the line-of-sight component of the active region magnetic field remains constant. In particular there is no simplification of the rather complicated field structure in connection with the flare. Magnetic flux and field gradients also do not show any variation above the 3 scale. Essential changes, however, were observed after 19 hrs without flare activity. This indicates that evolutionary field changes predominate over flare related variations.     

Spectroscopy of solar prominences from space and ground

Two quiescent solar prominences were observed in July 2000 from SUMER aboard SOHO and from the two German solar telescopes at Tenerife. Two‐dimensional images taken at the VTT simultaneously in the spectral lines Hβ at 4862 Å and Ca II at 8542 Å show no significant spatial variation of their pressure‐sensitive emission ratio. Slit spectra of the Ca II 8542 Å and He I 10830 Å lines obtained at the Gregory‐Coudé telescope yield 8000 K < T_kin < 9000 K and 3 km/s < V_n–th < 8 km/s. Among the various spectral ranges observed with SUMER, we first investigate the Lyman emission lines, which were fitted by Gaussians yielding reliable spectral radiances and line widths for the series members 5 < k < 18. A determination of the level population gives for the lower series members a Boltzmann temperature of 60 000 K, the higher members being over‐populated. This temperature indicates an origin of the Lyman lines from hot surroundings of the cool prominence body seen in the ground‐based data; this also holds for the ‘hotter’ SUMER lines.     

On the branching in the emission relations of Ca+ in prominences

Spatially well resolved prominence spectra of the three lines Ca⁺ K, H, and Ca⁺ 8542 are analysed. It is confirmed that the branching in the emission relations of Ca⁺ versus H correlates with the magnitude of non-thermal (turbulent) broadening.     

Multiple-Scale Pattern Recognition Applied to Faint Intergranular G-band Structures

Small-scale solar magnetic flux concentrations are studied in two-dimensional G-band images at very high spatial resolution and compared with Ca ii H enhancements. Among 970 small-sized G-band intergranular structures (IgS), 45% are co-spatial with isolated locations of Ca ii H excess and thus considered as magnetic (MIgS); they may be even twice as frequent as the known G-band bright points. The IgS are recognized in the G-band image by a new algorithm operating in four steps: (1) A set of equidistant detection levels yields a pattern of primary “cells”; (2) for each cell, the intrinsic intensity profile is normalized to its brightest pixel; (3) the cell sizes are shrunk by a unitary single-intensity clip; (4) features in contact at an appropriate reference level are merged by removal of the respective common dividing lines. Optionally, adjoining structures may be excluded from this merging process (e.g., chains of segmented IgS), referring to the parameterized number and intensity of those pixels where enveloping feature contours overlap. From the thus recognized IgS pattern, MIgS are then selected by their local Ca ii H contrast and their mean G-band-to-continuum brightness ratio.     

On polarimetry in solar active regions

Phase retardation originating in a telescope is measured by means of polarizer and analyzer. The amount of this retardation depends only on the declination of the telescope. The retardation axes rotate with the Coudé image.The influence of this retardation on a Zeeman triplet, is measured photoelectrically. Consequences for solar polarimetry are discussed: Strong effects originate from the miscentering of the asymmetric Zeeman triplet introduced by the Doppler compensator. This yields a non-vanishing V-Stokes parameter in the central exit slit and thus the often observed crosstalk between the U and V-Stokes parameters.