Analysis of full-disc Hαobservations: Carrington maps and filament properties in 1909–2022

Abstract

Context.Full-disc observations of the Sun in the Hαline provide information about the solar chromosphere, and in particular, about the filaments, which are dark and elongated features that lie along magnetic field polarity-inversion lines. This makes them important for studies of solar magnetism. Because full-disc Hαobservations have been performed at various sites since the second half of the 19th century, with regular photographic data having started at the beginning of the 20th century, they are an invaluable source of information on past solar magnetism.Aims.We derive accurate information about filaments from historical and modern full-disc Hαobservations.Methods.We consistently processed observations from 15 Hαarchives spanning 1909–2022. The analysed datasets include long-running ones such as those from Meudon and Kodaikanal, but also previously unexplored datasets such as those from Arcetri, Boulder, Larissa, and Upice. Our data processing includes photometric calibration of the data stored on photographic plates, the compensation for limb-darkening, and the orientation of the data to align solar north at the top of the images. We also constructed Carrington maps from the calibrated Hαimages.Results.We find that filament areas, similar to plage areas in Ca IIK data, are affected by the bandwidth of the observation. Thus, a cross calibration of the filament areas derived from different archives is needed. We produced a composite of filament areas from individual archives by scaling all of them to the Meudon series. Our composite butterfly diagram very distinctly shows the common features of filament evolution, that is, the poleward migration as well as a decrease in the mean latitude of filaments as the cycle progresses. We also find that during activity maxima, filaments cover ∼1% of the solar surface on average. The change in the amplitude of cycles in filament areas is weaker than in sunspot and plage areas.Conclusions.Analysis of Hαdata for archives with contemporaneous Ca IIK observations allowed us to identify and verify archive inconsistencies, which also have implications for reconstructions of past solar magnetism and irradiance from Ca IIK data.

Differential Rotation of the Solar Chromosphere: A Century-long Perspective from Kodaikanal Solar Observatory Ca ii K Data

“…In Figures 5((a), (b), and (c)) we show the variation of W¢ in three different bands (i) 0°-10°, (ii) 10°-20°, and (iii) 20°-30°for the northern and the southern hemispheres separately. We do note an upward trend in the rotation rate after 1980, consistent with the higher rotation rate obtained in Figure 4(c), which is again the consequence of the degraded data quality in these periods (see, e.g., Ermolli et al 2009;Chatzistergos et al 2023). Additionally, we also note this upward trend in Figure 5(d) after 1980 in the yearly averaged Ω, calculated by averaging over the disk (latitude range −30°to +30°).…”

The Bright Rim Prominences according to 2.5D Radiative Transfer

“…Finally, it is clear that the appearance of the Ca II K line in Figure 4, and especially Figure 2, somewhat contradicts what we have come to anticipate from observations of solar prominences. That is, the on-disk projection of a prominence (filament) in Ca II K is almost exclusively an absorption signature yet appears in both figures with a positive constrast against the chromosphere (see Kuckein et al 2016;Chatterjee et al 2017;Chatzistergos et al 2023). For an identical model with an internal prominence pressure of 0.1 dyn cm −2 , not explicitly shown here, we recover once more an absorption signature for the Ca II K line.…”

Developing an Automated Detection, Tracking, and Analysis Method for Solar Filaments Observed by CHASE via Machine Learning

“…Filaments are usually observed by ground-based solar Hα telescopes around the world, such as Meudon, Big Bear, Kanzelhöhe, Kodaikanal, and Huairou. These telescopes have been the workhorses of most of the current knowledge on filaments (Chatzistergos et al 2023). To study the mechanisms of solar eruptions and the plasma dynamics in the lower atmosphere, the Chinese Hα Solar Explorer (CHASE; Li et al 2019Li et al , 2022 was launched into a Sun-synchronous orbit on 2021 October 14.…”