# Carrington cycle 24: The solar chromospheric emission in a historical and stellar perspective

## TL;DR

This study presents a detailed record of solar chromospheric Ca II H&K emission during cycle 24, revealing unusually deep minimum and low maximum activity, with implications for understanding solar UV flux variations and their impact on Earth's atmosphere.

## Contribution

It provides the first high-resolution, calibrated S-index record of cycle 24 using moonlight, enabling direct comparison with historical data and other stars, highlighting unique solar activity features.

## Key findings

- Cycle 24 had an unusually deep and long minimum in 2008/09.
- The cycle 24 maximum was notably low compared to previous cycles.
- Chromospheric emission variations differ from coronal and sunspot activity.

## Abstract

We present the solar S-index record of cycle 24, obtained by the TIGRE robotic telescope facility and its high-resolution spectrograph HEROS (R$\approx$20,000), which measures the solar chromospheric Ca II H&K line emission by using moonlight. Our calibration process uses the same set of standard stars as introduced by the Mt. Wilson team, thus giving us a direct comparison with their huge body of observations taken between 1966 and 1992, as well as with other cool stars. Carrington cycle 24 activity started from the unusually deep and long minimum 2008/09, with an S-index average of only 0.154, by 0.015 deeper than the one of 1986 (<S>=0.169). In this respect,the chromospheric radiative losses differ remarkably from the variation of the coronal radio flux F10.7cm and the sunspot numbers. In addition, the cycle 24 S-amplitude remained small, 0.022 (cycles 21 and 22 averaged: 0.024), and so resulted a very low 2014 maximum of <S>=0.176 (cycles 21 and 22 averaged: 0.193). We argue that this find is significant, since the Ca II H&K line emission is a good proxy for the solar far-UV flux, which plays an important role in the heating of the Earth's stratosphere, and we further argue that the solar far-UV flux changes change s with solar activity much more strongly than the total solar output.