Secular variations in the solar corona shape according to observations during a solar activity minimum epoch

TL;DR

This study analyzes the solar corona's shape during solar minimum periods from 1867 to 2006, introducing a new flattening index that reveals long-term variations linked to solar magnetic field configurations.

Contribution

A new flattening index for large coronal streamers is proposed, showing smooth changes over 140 years and correlating with solar magnetic field configurations.

Findings

Index was minimal during cycles 17-19, indicating a dipole magnetic field dominance.

Coronal structure shifted to quadrupole configuration at the start of 20th and 21st centuries.

The variations in corona shape are related to changes in solar magnetic field and geomagnetic activity.

Abstract

Analysis of the solar corona structure during the periods of minimum solar activity from 1867 till 2006 has been carried out. A new flattening index for the large coronal streamers has been proposed. It has been shown that the index has been smoothly changing during the last 140 years. The minimal value of the index occurred during activity cycles 17--19; this was the period when the solar corona most of all corresponded to the dipole configuration of the global magnetic field of the Sun. At the beginning of the 20th and the 21st centuries, the corona structure corresponded to the quadrupole configuration. The reasons for the variations in the solar corona structure and its relation to geomagnetic activity are discussed.