Recovering the unsigned photospheric magnetic field from Ca II K observations

Abstract

Context.A number of studies have aimed at defining the exact form of the relation between magnetic field strength and Ca II H and K core brightness. All previous studies have, however, been restricted to isolated regions on the solar disc or to a limited set of observations. Aims. We reassess the relationship between the photospheric magnetic field strength and the Ca II K intensity for a variety of surface features as a function of the position on the disc and the solar activity level. This relationship can be used to recover the unsigned photospheric magnetic field from images recorded in the core of Ca II K line. Methods. We have analysed 131 pairs of high-quality, full-disc, near-co-temporal observations from the Helioseismic and Magnetic Imager (SDO/HMI) and Precision Solar Photometric Telescope (Rome/PSPT) spanning half a solar cycle. To analytically describe the observationally-determined relation, we considered three different functions: a power law with an offset, a logarithmic function, and a power law function of the logarithm of the magnetic flux density. We used the obtained relations to reconstruct maps of the line-of-sight component of the unsigned magnetic field (unsigned magnetograms) from Ca II K observations, which were then compared to the original magnetograms. Results. We find that both power-law functions represent the data well, while the logarithmic function is good only for quiet periods. We see no significant variation over the solar cycle or over the disc in the derived fit parameters, independently of the function used. We find that errors in the independent variable, usually not accounted for, introduce attenuation bias. To address this, we binned the data with respect to the magnetic field strength and Ca II K contrast separately and derived the relation for the bisector of the two binned curves. The reconstructed unsigned magnetograms show good agreement with the original ones. RMS differences are less than 90 G. The results were unaffected by the stray-light correction of the SDO/HMI and Rome/PSPT data. Conclusions. Our results imply that Ca II K observations, accurately processed and calibrated, can be used to reconstruct unsigned magnetograms by using the relations derived in our study.

How faculae and network relate to sunspots, and the implications for solar and stellar brightness variations

“…The power-law exponent has been reported for the Ca II K (Schrijver et al 1989;Harvey & White 1999;Loukitcheva et al 2009;Chatzistergos et al 2019b) and Mg II k lines (Barczynski et al 2018). However, since the passband of the Mg II k filtergrams used in the Barczynski et al (2018) study differs from the spectral sampling of the Mg II h&k doublet in the derivation of the F MgII , the reported exponents are of limited relevance to the F MgII .…”

“…Various studies have noted that at chromospheric passbands, the relationship between the intensity excess of chromospheric features and the underlying photospheric magnetic flux density can be described by a power law with an exponent that is below unity (e.g. Schrijver et al 1989;Harvey & White 1999;Loukitcheva et al 2009;Kahil et al 2017;Barczynski et al 2018;Chatzistergos et al 2019b). In fact, part of the purpose of the h 2 term in the YSK model (Eq.…”

Analysis of full-disc Ca II K spectroheliograms

“…Ermolli et al 2011;Chapman et al 2012;Fontenla & Landi 2018) and studies of the relation between the Ca II K brightness and the photospheric magnetic field (e.g. Loukitcheva et al 2009;Chatzistergos et al 2019d, and references therein).…”

Magnetic Activity of F-, G-, and K-type Stars in the LAMOST–Kepler Field

“…The chromospheric Ca II H and K emissions are well known to correlate well with the stellar actual magnetic flux and thus are the most commonly used indicator of chromospheric activity for cool stars (Saar & Schrijver 1987;Chatzistergos et al 2019). The Mount Wilson program measured the chromospheric Ca II H and K emissions of more than 1000 stars for over four decades (Wilson 1963;Duncan et al 1991;Baliunas et al 1995).…”