Investigating the Effect of Solar Ambient and Data Characteristics on Ca ii K Observations and Line Profile Measurements

Abstract

We analyzed state-of-the-art observations of the solar atmosphere to investigate the dependence of the Ca ii K brightness of several solar features on spectral bandwidth and spatial resolution of the data. In particular, we study data obtained at the Swedish Solar Telescope with the Crisp Imaging Spectropolarimeter and Chromospheric Imaging Spectrometer instruments. The analyzed data, which are characterized by a spectral bandwidth of 0.12 Å and a spatial resolution of 0.″078, were acquired close to the disk center by targeting a quiet-Sun area and an active region. We convolved the original observations with Gaussian kernels to degrade their spectral bandwidth and spatial resolution to the instrumental characteristics of the most prominent series of Ca ii K observations available to date. We then studied the effect of data degradation on the observed regions and on parameters derived from Ca ii K line measurements that are largely employed as diagnostics of the solar and stellar chromospheres. We find that the effect of degrading the spectral resolution of Ca ii K observations and line profiles depends on both the employed bandwidth and observed solar region. Besides, we found that the spatial degradation impacts the data characterized by a broad bandwidth to a larger extent compared to those acquired with a narrow band. However, the appearance of the observed solar regions is only slightly affected by the spatial resolution of data with bandwidths up to 1 Å and in the range [3,10] Å. Finally, we derived relationships that can be used to intercalibrate results from observations taken with different instruments in diverse regions of the solar atmosphere.

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

“…Bertello et al (2020) employed the image cross-correlation technique, instead of FFT, on Ca II K data acquired at Mount Wilson Observatory (MWO;1915-1985 and found that the chromospheric plages give 0.63% slower rotation than the photospheric sunspots. Furthermore, studies comparing active regions in the photosphere and their counterparts in the chromosphere highlight their great spatial correspondence (e.g., Babcock & Babcock 1955;Loukitcheva et al 2009;Chatzistergos et al 2019c;Murabito et al 2023), which would suggest that there is no significant change in the rotational profile between photosphere and chromosphere.…”