Can spicules be detected at disc centre in broad-band Ca II H filter imaging data ?

TL;DR

This study investigates whether broad-band Ca II H filter imaging data at the solar disc center can detect spicules, concluding that such data mainly reflect lower atmospheric layers and do not reveal upper chromospheric structures like spicules.

Contribution

The paper provides a detailed analysis of the formation heights and contributions of different atmospheric layers in broad-band Ca II H imaging, clarifying the limitations for spicule detection at disc center.

Findings

Broad-band Ca II H filters mainly sample lower atmospheric layers around 200 km.

Less than 10% of the signal originates from above 800 km, where spicules are located.

No visible fibrillar structures are detected in broad-band images at disc center.

Abstract

We estimate the formation height range contributing to broad-band and narrow-band filter imaging data in Ca II H to investigate whether spicules can be detected in such observations at the centre of the solar disc. We apply spectral filters of FWHMs from 0.03 nm to 1 nm to observed Ca line profiles to simulate Ca imaging data. We estimate the relative intensity contributions of off-limb and on-disc structures. We compare the synthetic Ca filter imaging data with intensity maps of Ca spectra at different wavelengths and temperature maps at different optical depths. We determine the intensity response function for the wavelengths covered by the filters of different FWHM. The intensity emitted off the solar limb is about 5% of the intensity at disc centre. For a 0.3 nm-wide Ca II H filter, up to about 1/3 of the off-limb intensity comes from emission in Hepsilon. On the disc, only about 15% of the intensity transmitted through a broad-band filter comes from the line-core region. No traces of elongated fibrillar structures are visible in imaging data at disc centre, opposite to the line-core images of the Ca spectra. The response function for a 0.3 nm-wide filter peaks at about 200 km. Relative contributions from atmospheric layers above 800 km are about 10%. The inversion results suggest that the slightly enhanced emission around the photospheric magnetic network in broad-band Ca imaging data is caused by a thermal canopy at a height of about 600 km. Broad-band Ca II H imaging data do not trace upper chromospheric structures such as spicules in observations at the solar disc because of the too small relative contribution of the line core to the total wavelength-integrated filter intensity.