Deciphering Solar Magnetic Activity. The Solar Cycle Clock

TL;DR

This paper introduces a new solar activity clock based on Hale Cycle terminator events, providing a unified framework to understand solar magnetic activity, cycle phases, and associated phenomena.

Contribution

It constructs a novel normalized solar activity clock anchored on terminator events, linking magnetic cycle progression with observable solar and geomagnetic phenomena.

Findings

Terminators mark the start of solar minimum conditions.

The solar radio flux crossing 90 sfu indicates the beginning of quiet periods.

The new clock unifies various solar magnetic phenomena across cycles.

Abstract

The Sun's variability is controlled by the progression and interaction of the magnetized systems that form the 22-year magnetic activity cycle (the "Hale Cycle'') as they march from their origin at $\sim$55 degrees latitude to the equator, over $\sim$19 years. We will discuss the end point of that progression, dubbed "terminator'' events, and our means of diagnosing them. Based on the terminations of Hale Magnetic Cycles, we construct a new solar activity 'clock' which maps all solar magnetic activity onto a single normalized epoch. The Terminators appear at phase $0 * 2\pi$ on this clock (by definition), then solar polar field reversals commence at $\sim0.2 * 2\pi$, and the geomagnetically quiet intervals centered around solar minimum, start at $\sim0.6 * 2\pi$ and end at the terminator, lasting 40% of the normalized cycle length. With this onset of quiescence, dubbed a "pre-terminator,'' the Sun shows a radical reduction in active region complexity and (like the terminator events) is associated with the time when the solar radio flux crosses F10.7=90 sfu -- effectively marking the commencement of solar minimum conditions. In this paper we use the terminator-based clock to illustrate a range of phenomena that further emphasize the strong interaction of the global-scale magnetic systems of the Hale Cycle. arXiv:2010.06048 is a companion article.