The Solar Dynamo and Emerging Flux (Invited Review)

Abstract

Abstract. The largest concentrations of magnetic flux on the Sun occur in active regions. In this paper, the properties of active regions are investigated in terms of the dynamics of magnetic flux tubes which emerge from the base of the solar convection zone, where the solar cycle dynamo is believed to operate, to the photosphere. Flux tube dynamics are computed using the 'thin flux tube' approximation, and by using MHD simulation. Simulations of active region emergence and evolution, when compared with the known observed properties of active regions, have yielded the following results:(1) The magnetic field at the base of the convection zone is confined to an approximately toroidal geometry with a field strength in the range (3-10) × 10 4 G. The latitude distribution of the toroidal field at the base of the convection zone is more or less mirrored by the observed active latitudes; there is not a large poleward drift of active regions as they emerge. The time scale for emergence of an active region from the base of the convection zone to the surface is typically 2 -4 months. The equatorial gap in the distribution of active regions has two possible origins; if the toroidal field strength is close to 10 5 G, it is due to the lack of equilibrium solutions at low latitude; if it is closer to 3 × 10 4 G, it may be due to modest poleward drift during emergence. (2) The tilt of active regions is due primarily to the Coriolis force acting to twist the diverging flows of the rising flux loops. The dispersion in tilts is caused primarily by the buffeting of flux tubes by convective motions as they rise through the interior. (3) The Coriolis force also bends the active region flux tube shape toward the following (i.e., anti-rotational) direction, resulting in a steeper leg on the following side as compared to the leading side of an active region. When the active region emerges through the photosphere, this results in a more rapid separation of the leading spots away from the magnetic neutral line as compared to the following spots. This bending motion also results in the neutral line being closer to the following magnetic polarity. (4) Active regions behave kinematically after they emerge because of 'dynamic disconnection', which occurs because of the lack of a solution to the hydrostatic equilibrium equation once the flux loop has emerged. This could explain why active regions decay once they have emerged, and why the advection-diffusion description of active regions works well after emergence. Smaller flux tubes may undergo 'flux tube explosion', a similar process, and provide a source for the constant emergence of small-scale magnetic fields. positive twist in the south can be accounted for by the action of Coriolis forces on convective eddies, which ultimately writhes active region flux tubes to produce a magnetic twist of the correct sign and amplitude to explain the observations. (6) The properties of the strongly sheared, flare productive δ-spot active regions can be accounted for by the dynamics of highly twisted ...

Intense bipolar structures from stratified helical dynamos

“…Furthermore, a rich dynamic behaviour of the magnetic field is observed: bipolar spot-like structures appear, then morph into bipolar band-like structures which disappear and reappear at a different place and at a later stage evolve into spot-like structures. Such structures are similar to δ spots (see, e.g., Fisher et al 2000) and tend to show anticlockwise rotation, which is consistent with the fact that the kinetic helicity in our simulations is positive. The characteristic length and time scales of the magnetic field formed at the top surface are much smaller than the characteristic length scale (and time scale) of the dynamo-generated magnetic field.…”

A Remarkable Recent Transition in the Solar Dynamo

“…We refer to the papers by Küker et al (2001), Käpylä et al (2006) and the reviews by Fisher et al (2000); Tobias (2002);Ossendrijver (2003); Charbonneau (2010); de Jager and Duhau (2011);Schmieder et al (2014);Fletcher et al (2015). Many more papers could be mentioned but we do not consider that necessary because a treatment of dynamo theories is outside the scope and purposes of the present paper.…”

Sudden transitions and grand variations in the solar dynamo, past and future

“…It is briefly characterized by the Omega effect, which is the magnetic field strengthening in a direction parallel to the equator, leading to a field component that is mainly toroidal with a shape that somewhat resembles the Greek capital Omega (cf., e.g., Cally et al 2003), and the Alpha effect which is the formation of the poloidal field under influence of the Coriolis force leading to alpha-type field loops (Küker et al 2001). Extended descriptions and analyses are found in many review papers (we refer to the reviews by Fisher et al 2000;Tobias 2002;Ossendrijver 2003;Charbonneau 2010;De Jager & Duhau 2011).…”