Abstract
In 1844 Schwabe discovered that the number of sunspots increased and decreased over a period of about 11 years, that variation became known as the sunspot cycle. Almost eighty years later, Hale described the nature of the Sun's magnetic field, identifying that it takes about 22 years for the Sun's magnetic polarity to cycle. It was also identified that the latitudinal distribution of sunspots resembles the wings of a butterfly showing migration of sunspots in each hemisphere that abruptly start at mid-latitudes towards the Sun's equator over the next 11 years. These sunspot patterns were shown to be asymmetric across the equator. In intervening years, it was deduced that the Sun (and sun-like stars) possess magnetic activity cycles that are assumed to be the physical manifestation of a dynamo process that results from complex circulatory transport processes in the star's interior. Understanding the Sun's magnetism, its origin and its variation, has become a fundamental scientific objective \-- the distribution of magnetism, and its interaction with convective processes, drives various plasma processes in the outer atmosphere. In the past few decades, a range of diagnostic techniques have been employed to systematically study finer scale magnetized objects, and associated phenomena. The patterns discerned became known as the ``Extended Solar Cycle'' (ESC). The patterns of the ESC appeared to extend the wings of the activity butterfly back in time, nearly a decade before the formation of the sunspot pattern, and to much higher solar latitudes. In this short review, we describe their observational patterns of the ESC and discuss possible connections to the solar dynamo as we depart on a multi-national collaboration to investigate the origins of solar magnetism through a blend of archived and contemporary data analysis with the goal of improving solar dynamo understanding and modeling.
Highlights
Specialty section: This article was submitted to Stellar and Solar Physics, a section of the journal
In 1844 Schwabe discovered that the number of sunspots increased and decreased over a period of about 11 years, that variation became known as the sunspot cycle
The patterns discerned became known as the “Extended Solar Cycle” (ESC)
Summary
Observations of the ESC began three to four decades ago when Leroy and Noens (1983) analyzed coronal data obtained at Pic du Midi from 1944 to 1974 and searched for the latitude variation of coronal activity, see Figure 2. These works collectively identified an underlying, global-scale, pattern of coronal activity that appeared to repeat over a period of about 17 years—much longer than the typical eleven-year sunspot cycle This was consistent with an earlier work of Legrand and Simon (1981), based on the analysis of 100 years of geomagnetic indices, which inferred the presence of two extended solar cycles. The orientation of the magnetic polarities in those ephemeral active regions studied by Harvey and Martin was similar to the spots that emerged for the sunspot cycle 22 instead of those belonging to sunspot cycle 21—this striking pattern was difficult to explain at the time Concerted, these observations inferred that sunspot activity was the main phase of a more extended cycle that was triggered at higher solar latitudes prior to the maximum of the given solar cycle. Similar patterns of evolution were visible in monitoring the latitudinal progression of filaments and prominences over time (e.g., Bocchino, 1933; Hansen and Hansen, 1975) that are summarized by Cliver (2014)
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