WG1 – Sub-surface magnetic connections Karel Schrijver: Instrumentation is definitely improving --- there will soon be much more high quality data available for the community’s use. Unfortunately, the increase in available data is not accompanied by a corresponding increase in the number of scientists analyzing this data. The increased data also requires increased attentiveness to how much, and in what form this data should be provided to the community to be of most use --- the instrument teams would very much like input from the community as they are developing their strategy for making the data accessible and useful. Unresolved physics of active region magnetic fields (ARs): During the emergence phase of ARs, a theoretical picture of AR fields as structures anchored well below the visible surface describes observations very well. On the other hand, as AR’s decay, they behave as though they have absolutely no connection to the interior, and can be described remarkably well with e.g., flux transport models. What is the physics of this disconnection ---- when do individual ARs lose all knowledge of their preivious sub-surface moorings? At what depth below the surface does this “dynamic disconnection” occur?

Sketches courtesy of L. Lundquist

Other questions addressed: What is the source of AR magnetic fields Other questions addressed: What is the source of AR magnetic fields? What is the role of a surface dynamo? Are Quiet Sun fields simply decayed ARs, or is there some component of the field generated at the surface? What is the physical basis of Joy’s law? If the “braiding” of magnetic fieldlines is the primary source of magnetic heating in the corona, why don’t we see evidence for this braiding in TRACE images? Fernando Moreno-Insertis: Described current efforts in 3D MHD modeling of AR-scale flux emergence from below the visible surface out into the solar corona --- he explained in detail how difficult this particular calculation was, and focused on two parallel modeling efforts: The first method is to simplify the energetics of the problem, and run well-posed, albeit simplified numerical experiments to understand the basic physics of the emergence process. Using this approach, he described how the magnetic connectivity (from below the surface into the corona) evolves when a twisted magnetic flux rope emergence into a field-filled corona. The axis of the flux rope need not fully emerge to have a significant amount of the tube’s flux enter the corona, and to see significant changes in the corona’s magnetic topology.

The second approach is to model the energetics as realistically as possible --- This requires a numerical solution of the optically thick radiative transfer equation throughout the surface layers, and is very computationally expensive. These calculations have been performed at AR spatial scales, but the trade-off is that one can simulate only a portion of the atmosphere (domain is severely restricted). He then presented an impressive set of simulations by Cheung et al. which followed the evolution of an emerging rope in the presence of surface convection, and described the criterion for which flux can emerge cohesively through the surface layers without being severely distorted and pumped back down into the interior by the convective flow field. Abbett --- Presented an intermediate approach where large domain sizes can be maintained. The energetics, in this case, is treated semi-realistically, with major simplifications made to the treatment of radiation at the surface, along with empirical treatments of coronal heating consistent with Pevtsov’s Law. Parchevsky --- Presented a helioseismological analysis of sub-surface flows in and around sunspots. He showed with a well defined numerical experiment how the suppression of acoustic sources inside sunspots can almost entirely explain the reduced oscillation amplitude observed in sunspots. Further, the amplitude ratio of acoustic waves exhibits a radial dependence: the bigger radius, the bigger suppression.