Observational Evidence of Shallow Origins for the Magnetic Fields of Solar Cycles

Abstract

Observational evidence for the origin of active region magnetic fields has been sought from published information on extended solar cycles, statistical distributions of active regions and ephemeral regions, helioseismology results, positional relationships to supergranules, and fine-scale magnetic structure of active regions and their sunspots during their growth. Statistical distributions of areas of ephemeral and active regions blend together to reveal a single power law. The shape of the size distribution in latitude of all active regions is independent of time during the solar cycle, yielding further evidence that active regions of all sizes belong to the same population. Elementary bipoles, identified also by other names, appear to be the building blocks of active regions; sunspots form from elementary bipoles and are therefore deduced to develop from the photosphere downward, consistent with helioseismic detection of downflows to 3-4 Mm below sunspots as well as long-observed downflows from chromospheric/coronal arch filaments into sunspots from their earliest appearance. Time-distance helioseismology has been effective in revealing flows related to sunspots to depths of 20 Mm. Ring diagram analysis shows a statistically significant preference for upflows to precede major active region emergence and downflows after flux emergence but both are often observed together or not detected. From deep-focus helioseismic techniques for seeking magnetic flux below the photosphere prior major active regions, there is evidence of acoustic travel-time perturbation signatures rising in the limited range of depths of 42-75 Mm but these have not been verified or found at more shallow depths by helioseismic holographic techniques. The development of active regions from clusters of elementary bipoles appears to be the same irrespective of how much flux an active region eventually develops. This property would be consistent with the magnetic fields of large active regions being generated in the same way and close the same depth as small active regions in a shallow zone below the photosphere. All evidence considered together, understanding the origins of the magnetic fields of solar cycles boils down to learning how and where elementary bipoles are generated beneath the photosphere.

Reviews in astronomy and space sciences

“…Martin (2018) concluded that this cited collection of observations was strong evidence for sunspots growing wider and deeper as long as their site was fed by elementary bipoles, and convective collapse could continue. The observations are consistent and there is no known contradictory observational evidence to concluding that sunspots are shallow and develop from the photosphere and downward to greater depths (Martin, 2018).…”

Does Nonaxisymmetric Dynamo Operate in the Sun?

“…Parker (1979) suggested that the magnetic buoyancy can result in formation of bipolar active regions. Currently, it becomes evident that the magnetic buoyancy is not the only mechanism forming emerging active regions of the Sun (Getling 2001;Kitiashvili et al 2010;Stein & Nordlund 2012;Leka et al 2013;Brandenburg et al 2013;Losada et al 2017;Martin 2018). In this study, we assume that the magnetic buoyancy acts on relatively small-scale parts of the axisymmetric magnetic field, perhaps, because of some kind of nonlinear instability, and contributes to generation of the non-axisymmetric magnetic field component.…”

Observations key to understanding solar cycles: a review

“…This list remains significant today. With more than 60 years of studies reviewed elsewhere (Petrovay, 2010, living review, on predicting solar cycles;Pesnell, 2012, solar cycle predictions, review;Cliver, 2014, extended solar cycle;Hathaway, 2015, review on solar cycles;and Martin, 2018, review on observational evidence of the depth at origin of active regions), these topics remain without definitive answers and important for further research: 8. The larger size and longer lifetime in the preceding (western) polarity sunspots than in the trailing (eastern) polarity sunspots in any solar cycle.…”

“…It is now becoming evident that the processes of convection and reconnection, respectively, in the photosphere and outer layers of the solar atmosphere, are continuously reorganizing to achieve the simplest version of their dynamic state. The finding of supergranulesized convection cells (Simon and Leighton, 1964;Leighton et al, 1962) in the outer one-third of the solar radius, along with increased knowledge of magnetic reconnection in different layers of the solar atmosphere, raises questions about the compatibility of these processes with the proposed long-term existence of the subsurface flux ropes (Martin, 2018).…”