# Solar cycle indices from the photosphere to the corona: measurements and underlying physics

## TL;DR

This paper reviews various solar cycle indices derived from observations of the solar atmosphere, discussing their measurements, physical basis, and how they relate to solar magnetism and activity over different timescales.

## Contribution

It provides a comprehensive overview of solar cycle indices from the photosphere to the corona, linking observational data with underlying physical processes.

## Key findings

- Most indices are highly correlated with the solar cycle.
- Differences in indices reveal fine details and trends in solar activity.
- Physical observables are explained through underlying solar physics.

## Abstract

A variety of indices have been proposed in order to represent the many different observables modulated by the solar cycle. Most of these indices are highly correlated with each other owing to their intrinsic link with the solar magnetism and the dominant eleven year cycle, but their variations may differ in fine details, as well as on short- and long-term trends. In this paper we present an overview of the indices that are often employed to describe the many features of the solar cycle, moving from the ones referring to direct observations of the inner solar atmosphere, the photosphere and chromosphere, to those deriving from measurements of the transition region and solar corona. For each index, we summarize existing measurements {\bf and typical use}, and for those that quantify physical observables, we describe the underlying physics.