The solar corona butterfly diagram

¹ INAF – Osservatorio Astrofisico di Torino, Pino Torinese, Torino, Italy
² University of Turin, Physics Department, Torino, Italy

^⋆ Corresponding authors: alessandro.bemporad@inaf.it, sara.bertone@inaf.it

Received: 4 April 2025
Accepted: 15 July 2025

Abstract

Context. Since the discovery of solar magnetism, the study of magnetic fields on the Sun has long been a central focus. The effects of the solar cycle on the photospheric magnetic fields have been extensively inspected, whereas those on the corona remain relatively obscure.

Aims. In this work, we applied the innovative approach demonstrated in a previous study to measure coronal magnetic fields from coronal densities, in order to study, for the first time, the evolution of these parameters over a full solar cycle.

Methods. We analysed 2595 polarised brightness (pB) images acquired by the Mauna Loa Solar Observatory from October 28, 1998 to December 31, 2008, covering almost the entire 23rd solar cycle. From these images, we derived the daily coronal electron densities and magnetic field maps. The combination of these maps allowed us to reconstruct Carrington rotation maps at 2.5 R_⊙ of coronal densities and magnetic fields, which we combined into butterfly diagrams to study their evolution over one full solar cycle.

Results. Our results show good agreement around solar minimum between the inferred location of the magnetic neutral line in the corona and potential field extrapolations. The butterfly diagrams show the formation of an asymmetric solar corona approaching the minimum in 2008, which is not matched by a similar asymmetry in the evolution of photospheric magnetic fields. The evolution of the coronal magnetic energy shows similarities, but also notable differences, compared to the photospheric magnetic energy, suggesting a connection between coronal structures and the innermost regions of the solar convective zone. Additionally, the coronal magnetic fields over the polar regions appear to be markedly different and nearly anti-correlated around solar maximum, but become comparable during the descending phases of the cycle.

Conclusions. These results offer a new framework for understanding the modulation of the solar corona by the activity cycle, as well as new perspectives in view of future observations of the solar poles, which will be acquired for the first time by the ESA Solar Orbiter mission in the coming years.