Tilt angles of solar filaments over the century: 1919-2014

We study the distribution of solar filaments using the data from Meudon Observatory 1919-2003 together with Kislovodsk Mountain Astronomical Station 1979-2014. We scanned and digitized the observed H-alpha synoptic maps line, on which we isolated and digitized filament locations. We also carried out the digitization of filaments form daily observations of solar activity in 1965-2014. Data on each filament contain information of the location, length, area, position and other geometric characteristics. We obtained the distributions of the number and the length of filaments with time as well as their latitudinal drift with solar cycle. We also analyzed of the longitudinal distribution, the asymmetry over the northern and southern hemispheres and other characteristics.
We analyzed the tilt angles to the equator. On average, the Eastern ends of the filaments are closer to the poles than Western ones by some 10 degrees in latitude, that we prescribed positive tilts. To the contrary, in the sub-polar regions with latitudes greater than 50 degrees the filament tilt angles are mainly negative. The portion of filaments in the middle-latitude is not large (less than 30%). The tilt angles vary with the phase of solar cycle attaining theory maximum at maximum sunspot activity phase. The highest values for low-latitudinal filaments (below 40 degrees from the equator) have been attained in the middle of 20th century in solar cycles 18-19 at the time of maximum activity. We therefore propose using statistical properties of solar filaments as an additional coherent measure for manifestation of the solar cycle which covers all latitudes and has a systematic calibrated almost a century long available data series.