Tilt Angles of Solar Filaments over the Period of 1919 – 2014

The spatial and temporal distributions of solar filaments were analyzed using data from the Meudon Observatory for the period of 1919 – 2003 and the Kislovodsk Mountain Astronomical Station for the period of 1979 – 2014. We scanned H α solar synoptic charts on which the filaments were identified and digitized. The data on each filament comprise its location, length, area, and other geometrical characteristics. The temporal distributions of the number and total length of the filaments were obtained. We also found latitudinal migration of filament locations with the solar cycle and analyzed the latitudinal distribution and asymmetry of filaments in the northern and southern hemispheres as well as other properties of their distribution. The tilt angles of filaments with respect to the solar equator ( τ ) were analyzed. On average, the eastern tips of filaments are closer to the poles than the western ones ( τ ≈ 10 ∘ ). On the other hand, the filaments in the polar regions (latitude | θ | > 50 ∘ ) usually have negative tilts ( τ < 0 ∘ ). The tilt angles vary with the phase of the 11-year sunspot cycle and show their highest values in the activity maximum. In the century-long modulation of the solar activity (Gleissberg cycle), the mean tilt angles of filaments in the mid-latitude zone ( θ ≈ ± 40 ∘ ) were largest in the middle of the 20th century in Solar Cycles 18 – 19. We hereby propose that these statistical properties of solar filaments can be used as a coherent measure of the solar cycle that covers all latitudes and for which an almost century-long systematically calibrated data series is available.