Long-term variability of solar magnetic fields

Abstract

The solar magnetic field varies on all time scales. Recent analysis of 600 million year-old Australian varves shows terrestrial evidence for not only the familiar 22-year magnetic cycle, but variations with periods of 300–400 years as well. The Maunder minimum is but one of several long intervals showing atypical levels of solar activity. Recently a 151-day periodicity in flare activity has been found. Active regions, sunspots, ephemeral regions, and flares cover a broad range of shorter time scale variations in the solar magnetic field. Long-term variations can be interpreted in at least two ways. One outlook regards the large-scale and long-term variations of the photospheric field as more-or-less direct guides to the organization of the solar field as it evolves through a solar cycle. The slowly varying field reveals the fundamental interior structure of the Sun. An alternative view interprets the surface manifestation of the magnetic field primarily as the result of the convective motions in the solar atmosphere. The evolving distribution of photospheric flux depends upon the locations of emerging flux and the subsequent motions and interactions of the fields in the moving plasma. Information about the interior field comes largely from analysis of emerging flux. We discuss interpretation of the phenomenology of the long-term variability in the context of these contrasting views.