Quantitative filtergram imagery of the solar acoustic oscillations

Abstract

The solar birefringent filter FPSS is used at the focus of a 28cm diameter solar refractor, at Meudon Observatory, to produce images in a large field of view, with a spectral resolution of 4.6 × 10⁴. The instrument is calibrated for absolute Doppler velocity measurements in the wings of the strong line NaD1, and is presently used for quantitative Imagery of the radial velocity motions in the photosphere with a resolving power of near 0.8 arc sec. The coherent five minutes acoustic oscillations of large spherical harmonics degree l (of a few hundreds) are directly imaged. The power spectra of the velocity structures are derived over the fields: Fourier transform spectra are computed. Several maxima are observed which characterize at least 3 superimposed coherent oscillations systems of typical elements diameters 14, 4.3 and 2. 1 Mm and probably a system with 22 Mm. Vertical velocities are derived. Spatial filtering by optical and by computerized techniques produces separate images for each of these coherent oscillating modes.