Alexandra Tritschler

IMAGING SPECTROPOLARIMETRY WITH IBIS: EVOLUTION OF BRIGHT POINTS IN THE QUIET SUN

We present the results from first spectropolarimetric observations of the solar photosphere acquired at the Dunn Solar Telescope with the Interferometric Bidimensional Spectrometer. Full Stokes profiles were measured in the Fe I 630.15 nm and Fe I 630.25 nm lines with high spatial and spectral resolutions for 53 minutes, with a Stokes V noise of 3 × 10–3 the continuum intensity level. The data set allows us to study the evolution of several magnetic features associated with G-band bright points (BPs) in the quiet Sun. Here we focus on the analysis of three distinct processes, namely the coalescence, fragmentation, and cancellation of G-band BPs. Our analysis is based on an SIR inversion of the Stokes I and V profiles of both Fe I lines. The high spatial resolution of the G-band images combined with the inversion results helps to interpret the undergoing physical processes. The appearance (dissolution) of high-contrast G-band BPs is found to be related to the local increase (decrease) of the magnetic filling factor, without appreciable changes in the field strength. The cancellation of opposite-polarity BPs can be the signature of either magnetic reconnection or the emergence/submergence of magnetic loops.

Two-dimensional spectroscopy of a sunspot - II. Penumbral line asymmetries

We present, analyse, and interpret line asymmetries from Fe I 557.6 nm of a sunspot penumbra at a heliocentric angle of 23 ◦ with high spatial (0.5 arcsec) and spectral (λ/� λ = 250 000) resolution. The data set is described and presented in the first paper of this series (Tritschler et al. 2004). Line bisectors are used to quantify the line asymmetries. Our findings are: (1) For averaged limb and center side bisectors the shift increases linearly with the bisector intensity level, but the limb side bisector is more inclined than the center side bisector. (2) Individual bisectors exhibit kinks, such that the bisector at high intensity levels is shifted towards the red for both, limb and center side bisectors. Some of the kinks produce bisector reversals in the outer center side penumbra. The bisector properties and their intriguing differences between center and limb side can be explained if one assumes downflows in deep atmospheric layers (log τ> −1). This is demonstrated by synthetic bisectors. The differences between the two penumbral sides are due to projection effects of non-horizontal flow channels. Our findings also imply that bisectors reversals are not due to elevated channels, but due to the presence of downflows. Along a specific center side flow filament the bisector shift is found to be largest in the line wing, except for the outer end of the filament, where a kink at high bisector intensities toward the red is found. This is consistent with an upflow at the inner footpoint, a deep lying horizontal flow, and, after a spatial distance of 4 arcsec, with a downflow at the end of the flow filament.

LINE SHAPE EFFECTS ON INTENSITY MEASUREMENTS OF SOLAR FEATURES: BRIGHTNESS CORRECTION TO SOHO MDI CONTINUUM IMAGES

Continuum intensity observations obtained with the Michelson Doppler Imager (MDI) on board the SOHO mission provide long time series of filtergrams that are ideal for studying the evolution of large-scale phenomena in the solar atmosphere and their dependence on solar activity. These filtergrams, however, are not taken in a pure continuum spectral band, but are constructed from a proxy, namely a combination of filtergrams sampling the Ni I 6768 A line. We studied the sensitivity of this continuum proxy to the shape of the nickel line and to the degradation in the instrumental transmission profiles. We compared continuum intensity measurements near the nickel line with MDI proxy values in three sets of high-resolution spectro-polarimetric data obtained with the Interferometric Bidimensional Spectrometer, and in synthetic data, obtained from multi-dimensional simulations of magneto-convection and one-dimensional atmosphere models. We found that MDI continuum measurements require brightness corrections which depend on magnetic field strength, temperature and, to a smaller extent, plasma velocity. The correction ranges from 2% to 25% in sunspots, and is, on average, less than 2% for other features. The brightness correction also varies with position on the disk, with larger variations obtained for sunspots, and smaller variations obtained for quiet Sun, faculae, and micropores. Correction factors derived from observations agree with those deduced from the numerical simulations when observational effects are taken into account. Finally, we found that the investigated potential uncertainties in the transmission characteristics of MDI filters only slightly affect the brightness correction to proxy measurements.