Analysis of broad-band Hα coronagraphic observations

Context: Daily broad-band full-limb Hα images of the inner corona were obtained during solar cycle 23 (1994-2005) using the 15 cm Pic-du-Midi coronagraph. Aims: We want to automatically extract the properties and evolutions of the observed cool HI coronal structures over a wide range of sizes and light fluxes, from small jets and/or spikes to large prominences. Methods: A tool was developed to process the complete set of stored images. This paper describes the recognition techniques implemented in our software and discusses its use. It includes the removal of the parasitic diffraction ring produced by the set of different occulting disks used throughout the year. Results: We present and discuss selected results from a statistical analysis of the occurrence of parameters characterizing the observed structures applied to a large sample of observations. It illustrates the capabilities of this software when applied to our database. Strong asymmetries of the activity level over the solar poles become evident, confirming similar results from previous works. We also discuss the distribution of relative light fluxes of these structures over a wide range of sizes. Conclusions: The complete series of FITS and calibrated images, the list of the detected structures, and their geometric and luminosity evolutions are stored in the BASS2000 solar database catalogue (http://bass2000.bagn.obs-mip.fr) and are made publicly available. The Hα HI structures observed over the limb of the sun present statistical properties of great interest for understanding its eruptive activity.