[Paper Review] Historical solar Ca II K observations at the Rome and Catania observatories

Theodosios Chatzistergos, I. Ermolli+6 more|arXiv (Cornell University)|Feb 20, 2019

Solar and Space Plasma Dynamics1 references4 citations

TL;DR

This study processes historically digitized Ca II K images from the Rome and Catania observatories (1964–1979 and 1970–1971, respectively) to derive plage areas, applying the same photometric and limb-darkening correction methods used for the Arcetri archive. It finds good agreement between Rome and Arcetri data, with RMS difference of 0.01 in plage area fraction, and confirms the Rome archive’s high cadence and reliability despite lower spatial resolution.

ABSTRACT

Here we present the little explored Ca II K archives from the Rome and the Catania observatories and analyse the digitised images from these archives to derive plage areas.

Motivation & Objective

• To process and analyze underutilized historical Ca II K images from the Rome and Catania observatories to derive plage areas.
• To assess the reliability and consistency of these archives with existing, better-known Ca II K datasets, particularly from Arcetri.
• To evaluate the impact of digitization artifacts and instrumental differences on plage area measurements.
• To determine whether the high-cadence Rome data can contribute to studies of transient chromospheric phenomena.
• To support gap-filling in the global Ca II K dataset for solar irradiance and climate research.

Proposed method

• Digitized Ca II K images from Rome (417×417 px, 5″/pixel) and Catania (3287×2102 to 1972×1261 px, 1.5–5″/pixel) were converted to FITS format for processing.
• Solar disc center and radius were identified using Sobel filtering on the images.
• Photometric calibration was performed using quiet Sun regions to correct for instrumental and exposure variations.
• Limb darkening was corrected by iteratively applying a running window filter and fitting 5th-order polynomials along radial, vertical, and horizontal directions after removing active regions.
• Plage regions were identified using a multiplicative factor relative to the standard deviation of quiet Sun regions.
• All processing steps were aligned with those applied to the Arcetri Ca II K archive for direct comparison.

Experimental results

Research questions

• RQ1 How do plage area measurements from the Rome and Catania Ca II K archives compare to those from the well-established Arcetri archive?
• RQ2 To what extent do digitization artifacts, such as saturation, affect the accuracy of plage area derivation in historical Ca II K data?
• RQ3 Can the high-cadence Rome data (100 images/day) provide reliable and useful information for studying transient chromospheric phenomena?
• RQ4 What is the spatial resolution and dynamic range of the processed Rome and Catania Ca II K images, and how do they compare to other archives?
• RQ5 How consistent are the plage area time series from Rome and Catania with those from Arcetri, especially during overlapping periods?

Key findings

• The Rome archive, despite lower spatial resolution (12.8±3.4″) compared to Arcetri (6.7±2.3″), yields plage area measurements that agree well with Arcetri data, with a mean difference of 0.001 in disc fraction and RMS difference of 0.01 over common days.
• Plage areas from the Catania archive are consistently lower than those from Arcetri and Rome, with a mean difference of -0.01 and RMS difference of 0.017 in disc fraction over common days.
• The Rome archive provides superior temporal sampling (1–2 observations per day, ~2983 days) compared to Catania (92 observations) and Arcetri (5133 observations over 1931–1974), making it valuable for high-cadence studies.
• Despite saturation artifacts introduced during digitization—observed in all three archives—plage area derivation remains robust and consistent across Rome and Arcetri.
• The processed images from all three archives show strong visual similarity after limb-darkening correction, though sunspots are more clearly visible in Arcetri and Catania data than in Rome data.
• The study confirms that the Rome and Catania archives are scientifically valuable and can help fill gaps in the historical Ca II K dataset for solar irradiance and climate research.