Detection of the Horizontal Divergent Flow prior to the Solar Flux Emergence

It is widely accepted that solar active regions including sunspots are formed by the emerging magnetic flux from the deep convection zone. In previous numerical simulations, we found that the horizontal divergent flow (HDF) occurs before the flux emergence at the photospheric height. This Paper reports the HDF detection prior to the flux emergence of NOAA AR 11081, which is located away from the disk center. We use SDO/HMI data to study the temporal changes of the Doppler and magnetic patterns from those of the reference quiet Sun. As a result, the HDF appearance is found to come before the flux emergence by about 100 minutes. Also, the horizontal speed of the HDF during this time gap is estimated to be 0.6 to 1.5 km s^-1, up to 2.3 km s^-1. The HDF is caused by the plasma escaping horizontally from the rising magnetic flux. And the interval between the HDF and the flux emergence may reflect the latency during which the magnetic flux beneath the solar surface is waiting for the instability onset to the further emergence. Moreover, SMART Halpha images show that the chromospheric plages appear about 14 min later, located co-spatial with the photospheric pores. This indicates that the plages are caused by plasma flowing down along the magnetic fields that connect the pores at their footpoints. One importance of observing the HDF may be the possibility to predict the sunspot appearances that occur in several hours.Comment: 32 pages, 8 figures, 3 tables, accepted for publication in Ap

Two-step Emergence of the Magnetic Flux Sheet from the Solar Convection Zone

We perform two-dimensional MHD simulations on the solar flux emergence. We set the initial magnetic flux sheet at z=-20,000 km in the convection zone. The flux sheet rises through the convective layer due to the Parker instability, however, decelerates beneath the photosphere because the plasma on the flux sheet piles up owing to the convectively stable photosphere above. Meanwhile, the flux sheet becomes locally unstable to the Parker instability within the photosphere, and the further evolution to the corona occurs (two-step emergence model). We carry out a parameter survey to investigate the condition for this two-step model. We find that magnetic fluxes which form active regions are likely to have undergone the two-step emergence. The condition for the two-step emergence is 10^21 - 10^22 Mx with 10^4 G at z=-20,000 km in the convection zone.Comment: 41 pages, 15 figures, 1 table, Accepted for publication in Ap

Numerical Experiments on the Two-step Emergence of Twisted Magnetic Flux Tubes in the Sun

We present the new results of the two-dimensional numerical experiments on the cross-sectional evolution of a twisted magnetic flux tube rising from the deeper solar convection zone (-20,000 km) to the corona through the surface. The initial depth is ten times deeper than most of previous calculations focusing on the flux emergence from the uppermost convection zone. We find that the evolution is illustrated by the two-step process described below: the initial tube rises due to its buoyancy, subject to aerodynamic drag due to the external flow. Because of the azimuthal component of the magnetic field, the tube maintains its coherency and does not deform to become a vortex roll pair. When the flux tube approaches the photosphere and expands sufficiently, the plasma on the rising tube accumulates to suppress the tube's emergence. Therefore, the flux decelerates and extends horizontally beneath the surface. This new finding owes to our large scale simulation calculating simultaneously the dynamics within the interior as well as above the surface. As the magnetic pressure gradient increases around the surface, magnetic buoyancy instability is triggered locally and, as a result, the flux rises further into the solar corona. We also find that the deceleration occurs at a higher altitude than in our previous experiment using magnetic flux sheets (Toriumi and Yokoyama). By conducting parametric studies, we investigate the conditions for the two-step emergence of the rising flux tube: field strength > 1.5x10^4 G and the twist > 5.0x10^-4 km^-1 at -20,000 km depth.Comment: 42 pages, 13 figures, 2 tables, accepted for publication in ApJ. High-resolution figures will appear in the published versio

Evolution and Flare Activity of Delta-Sunspots in Cycle 23

The emergence and magnetic evolution of solar active regions (ARs) of beta-gamma-delta type, which are known to be highly flare-productive, were studied with the SOHO/MDI data in Cycle 23. We selected 31 ARs that can be observed from their birth phase, as unbiased samples for our study. From the analysis of the magnetic topology (twist and writhe), we obtained the following results. i) Emerging beta-gamma-delta ARs can be classified into three topological types as "quasi-beta", "writhed" and "top-to-top". ii) Among them, the "writhed" and "top-to-top" types tend to show high flare activity. iii) As the signs of twist and writhe agree with each other in most cases of the "writhed" type (12 cases out of 13), we propose a magnetic model in which the emerging flux regions in a beta-gamma-delta AR are not separated but united as a single structure below the solar surface. iv) Almost all the "writhed"-type ARs have downward knotted structures in the mid portion of the magnetic flux tube. This, we believe, is the essential property of beta-gamma-delta ARs. v) The flare activity of beta-gamma-delta ARs is highly correlated not only with the sunspot area but also with the magnetic complexity. vi) We suggest that there is a possible scaling-law between the flare index and the maximum umbral area