High-resolution spectra of solar magnetic features. I, Analysis of penumbral fine structure

The Swedish Vacuum Telescope on La Palma was used to obtain spectra of the magnetic-sensitive Fe I 630.25 nm line under conditions of exceptional angular resolution (0.32 arcsec) and high spectral resolution (FWHM 2.5 pm). Simultaneous 0.02 s CCD exposures of both the spectrum and the slit-jaw image effectively 'freeze' the atmospheric seeing motions and permit unambiguous identification of the spectra of the various penumbral structures. These spectra reveal the magnetic field strength in penumbral filaments through an intensity fit of the Zeeman splitting of this line. The observations show that: (1) the field strength varies from about 2100 G near the umbra-penumbra boundary to about 900 G at the outer edge of the penumbra, (2) the observed fluctuation of penumbral magnetic field is much less dramatic than the fluctuation in intensity, (3) there is a suggestion of a rapid change in field inclination between some light and dark filaments near the edge of the penumbra, and (4) there is no obvious correlation between Doppler shift (in part due to the Evershed flow) and filament intensity. 43 refs.