The Minimum of Solar Cycle 23: As Deep as It Could Be?

TL;DR

This study introduces a new binning method for sunspot data to analyze the impact of solar cycle strength on sunspot properties, revealing that the 2008 solar minimum was not as deep as it could have been.

Contribution

It presents a novel approach to analyze sunspot data based on activity level, distinguishing two populations of sunspots and examining their contributions to solar magnetism during cycle minima.

Findings

Sunspot size distribution has two populations: active regions and pores/ephemeral regions.

Only active regions' properties vary with activity level.

The 2008 minimum was not as deep as it could have been from a heliospheric perspective.

Abstract

In this work we introduce a new way of binning sunspot group data with the purpose of better understanding the impact of the solar cycle on sunspot properties and how this defined the characteristics of the extended minimum of cycle 23. Our approach assumes that the statistical properties of sunspots are completely determined by the strength of the underlying large-scale field and have no additional time dependencies. We use the amplitude of the cycle at any given moment (something we refer to as activity level) as a proxy for the strength of this deep-seated magnetic field. We find that the sunspot size distribution is composed of two populations: one population of groups and active regions and a second population of pores and ephemeral regions. When fits are performed at periods of different activity level, only the statistical properties of the former population, the active regions, is found to vary. Finally, we study the relative contribution of each component (small-scale versus large-scale) to solar magnetism. We find that when hemispheres are treated separately, almost every one of the past 12 solar minima reaches a point where the main contribution to magnetism comes from the small-scale component. However, due to asymmetries in cycle phase, this state is very rarely reached by both hemispheres at the same time. From this we infer that even though each hemisphere did reach the magnetic baseline, from a heliospheric point of view the minimum of cycle 23 was not as deep as it could have been.