Evolution of Active Regions

Abstract

The evolution of active regions (AR) from their emergence through their long decay process is of fundamental importance in solar physics. Since large-scale flux is generated by the deepseated dynamo, the observed characteristics of flux emergence and that of the subsequent decay provide vital clues as well as boundary conditions for dynamo models. Throughout their evolution, ARs are centres of magnetic activity, with the level and type of activity phenomena being dependent on the evolutionary stage of the AR. As new flux emerges into a pre-existing magnetic environment, its evolution leads to re-configuration of small-and large-scale magnetic connectivities. The decay process of ARs spreads the once-concentrated magnetic flux over an ever-increasing area. Though most of the flux disappears through small-scale cancellation processes, it is the remnant of large-scale AR fields that is able to reverse the polarity of the poles and build up new polar fields. In this Living Review the emphasis is put on what we have learned from observations, which is put in the context of modelling and simulation efforts when interpreting them. For another, modelling-focused Living Review on the sub-surface evolution and emergence of magnetic flux see Fan (2009). In this first version we focus on the evolution of dominantly bipolar ARs. Article RevisionsLiving Reviews supports two ways of keeping its articles up-to-date:Fast-track revision. A fast-track revision provides the author with the opportunity to add short notices of current research results, trends and developments, or important publications to the article. A fast-track revision is refereed by the responsible subject editor. If an article has undergone a fast-track revision, a summary of changes will be listed here.Major update. A major update will include substantial changes and additions and is subject to full external refereeing. It is published with a new publication number.For detailed documentation of an article's evolution, please refer to the history document of the article's online version at http://dx

High-resolution imaging spectroscopy of two micro-pores and an arch filament system in a small emerging-flux region

“…Measuring the magnetic flux and thus the growth and decay rates is hampered by the low cadence (96 min), the varying noise level as a function of the heliocentric angle, the geometric foreshortening of the pixel area, the angle of the magnetic field lines (assumed to be perpendicular to the surface) with the line-of-sight (LOS), and the small number of pixels having a flux density of more than 20 G. Therefore, we conservatively report only the total unsigned magnetic flux of 2.2 × 10 20 Mx (the negative magnetic flux is 1.2 × 10 20 Mx) at 08:00 UT on 2008 August 7, which attains a maximum right at the time of the high-resolution GFPI observations. The flux contained in the EFR is slightly larger than the upper limit for ephemeral regions (see van Driel-Gesztelyi & Green 2015), that is, much too small to form larger pores or even sunspots. Even though the time resolution is too coarse to provide a growth rate for the magnetic field, our measurements agree with a rapid rise time (already significant changes between the first two magnetograms) and a much slower decay rate (total unsigned flux of 1.3 × 10 20 Mx at 22:24 UT on 2008 August 7).…”

The Life Cycle of Active Region Magnetic Fields

“…above the C-class) are produced as long as high magnetic flux density/strong magnetic field concentrations are present, and the overall flare production rate decreases sharply with decreasing flux density, while the rate of CME pro-duction shows a much slower decrease with time, ı.e. CMEs keep erupting from active regions well into their decay phase (van Driel-Gesztelyi & Green 2015). This is not surprising, as during the decay phase persistent flux cancellations along inversion lines occur (Green et al 2011).…”

“…By all available observational accounts, sunspots form as large-scale flux bundles emerge from the solar interior onto the surface (see reviews by Zwaan 1978Zwaan , 1985Fan 2004Fan , 2009Lites 2009;Cheung & Isobe 2014;van Driel-Gesztelyi & Green 2015). At the scale of photospheric granulation (L ⇠ 1 Mm), the emerging flux reaches the surface as predominantly horizontal fields within the centers of granules (e.g.…”

“…For an extensive review of active region evolution (during the phase when they are visible at and above the photosphere), we encourage the reader to consult van Driel-Gesztelyi & Green (2015). For discussions of the detailed processes related to magnetic flux emergence, we refer the reader to reviews by Schmieder et al (2014) and Cheung & Isobe (2014).…”

“…The definition of what constitutes a solar active region has evolved as our un-derstanding of solar magnetism has progressed in the last century. As suggested by van Driel-Gesztelyi & Green (2015), a useful modern definition due to Kiepenheuer (1968) is '[the] totality of all observable phenomena preceding, accompanying and following the birth of sunspots'. Figure 1 shows a subset of such observations from NASA's Solar Dynamics Observatory.…”