About this Research Topic
A great early success of dynamo theory was the demonstration that convection and rotation are enough to produce the large-scale magnetic fields seen on the Sun. The question became: how do the global-scale flows interact with the magnetic field inside the Sun to produce the magnetic fields we actually see on the surface? This is not a new question and progress occurs by comparing observations (what do the flows and magnetic fields on the Sun actually look like) and theory (how can we understand what we see and place limits on what is hidden from us beneath the solar surface).
Dynamo theory has concentrated on a few observations where: the Sun rotates differentially; the activity varies over a roughly 11 year period; the sunspot “butterfly wings” propagate equatorwards; sunspot groups emerge with a strongly preferred E-W magnetic polarity in each hemisphere during one cycle; and tend to emerge with one magnetic polarity slightly more north than the other polarity in each cycle . It is worth noting that these observational “laws” are close to a century old, or more.
These observational trends, or laws, became the basis for Parker’s dynamo-wave (DW) model, the Babcock-Leighton (B-L) dynamo model, and the flux transport dynamo (FTD) that was motivated by observations of the Sun’s meridional circulation - a relatively recent four decades years ago. Therefore the models of the Sun’s dynamo which can validly be compared to observations are thus 25 years old (FTD), almost 50 years old (B-L) or over 60 years (DW). This partly explains why the observations they are based on are even older.
There is a wealth of extremely high quality observations which have been systematically obtained in the last 40 years. Observations of the quality we now have for 3 1/2 solar cycles could only be dreamed of by the pioneers in dynamo theory. These observations include synoptic maps of the magnetic and doppler velocity fields. There have been discoveries, for example observations of the photospheric magnetic field has revealed structures at scales smaller than the global Sun but larger than supergranules with associated coronal counterparts. The existence of data covering a number of cycles has also revealed an ‘extended' solar cycle. The “extended solar cycle” can be seen in many observables, from coronal activity to sub-photospheric zonal flows -- the essential property which they have in common is that they represent a spatial pattern in the activity which extends considerably beyond one cycle and extend to much higher latitudes than permitted by analysis of sunspot groups.
In parallel to the observational extensions, there has also been a boom in how observations can be analysed - for instance local correlation tracking applied to good quality intensity images allows the mapping of horizontal flows with good time and spatial resolution, which has allowed the imaging of giant cells. Helioseismology has revealed the subsurface flows and in particular torsional oscillations as part of the extended cycle. Perhaps most useful of all are the synoptic magnetic maps which now exist for cycles 21-23 and half way through cycle 24. The simplest possible analysis of these synoptic maps has allowed strong constraints to be placed on the strength of the subsurface toroidal field.
The main point of this Frontiers Topic is to point out the opportunity which currently exists for the excellent quality data obtained in the last 40 years, together with amazing analysis techniques such as LCT, to be brought to bear on our understanding of the dynamo. This proposal hopes to bring observational and numerical analysis together in a new push to understand the critical processes at play in the generation of the Sun’s magnetic field and its variability. The expected outcome of this topic will be a demonstration of the wealth of high quality observations which are now available constrain dynamo theory. This includes whether they can either be understood in terms of our one of the current theories or whether they require the dynamo theories to be revised. Interest in this topic extends far beyond the solar domain.
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