ISSI International Team

Current status, unsolved problems and primary focus

The existence of such filamentary structures in the quiet Sun regions has been a subject of debate for many years, but eventually was confirmed by high-resolution observations (e.g. Keller et al. 1990) and numerical simulations (Nordlund 1983). Recent observational data from the Hinode space mission revealed formation of magnetic flux tubes of kilogauss field strength by convective collapse (Nagata et al. 2008). The observed events are consistent with the process of field intensification by flux advection, radiative cooling, and strong downflow found in MHD simulations. Hinode observations also indicated that the unresolved structures of horizontal magnetic field can be as small as 50 km (Ishikawa et al. 2008). Numerical simulations also showed that the ubiquitous horizontal field may be a manifestation of magnetic field generation by local dynamo (Danilovic et al. 2010). However, the simulations of Steiner et al. (2008) suggest that the horizontal internetwork fields are a natural consequence of flux expulsion in the vertical direction, without any need to invoke surface dynamo action. Thus, the role of local dynamo is a subject of debate.

An interesting question is if such filamentary flux tubes represent ultimate building blocks of the solar magnetism or there are much smaller "hidden magnetism" structuring (de Wijn et al. 2009). The magnetic Reynolds number of the solar plasma is very high, making possible the existence of magnetic structures on the scale of only few meters (Sanchez Almeida 1998). How this "sub-grid scale" structuring may affect the observational results and numerical simulations is uncertain. It has been suggested that the traditional two-component "standard model", in which isolated magnetic flux tubes are embedded into non-magnetic plasma, is too simplistic (Stenflo 2009), although there is no general consensus as to whether or not this is the case. The analysis of asymmetries of observed line polarization profiles provides an important diagnostic tool to infer the properties of unresolved magnetic structures, and has predicted their richness (Sanchez Almeida et al. 1996; Sanchez Almeida and Lites 2000). This is one of the venues we will explore to constrain properties of the filamentary structure at the smallest scales. Certainly, the formation and dynamics of the filamentary magnetic structures in quiet Sun regions is one of the central problems of solar magnetism. Comparing high-resolution and spectro-polarimetric observations with realistic MHD simulations is a promising approach to solving this problem. This includes analysis of synthetic Stokes calculated for the MHD simulations (Khomenko et al. 2005), high-resolution imaging spectroscopy and spectropolarimetric (Hanle and Zeeman) observations.

In sunspots the filamentary structure of magnetic field is apparent. Recent numerical simulations showed that the filamentary magnetic structure and dynamics of the sunspot umbra and penumbra may be a natural consequence of magneto-convection in strong field regions (Heinemann et al. 2007; Scharmer et al. 2008; Kitiashvili et al. 2009a; Rempel et al. 2009b). The numerical simulations reproduce the basic dynamics of the sunspot umbra and the penumbra and have revealed the nature of umbral dots and the Evershed outflow (Schuessler and Voegler 2006; Kitiashvili et al. 2009b; Nordlund and Scharmer 2009; Rempel et al. 2009a). The simulation describe details of the filamentary magnetic structures and their dynamics, such as the appearance of high-speed "Evershed cloud" and moving "sea-serpent" field lines (Sainz Dalda and Bellot Rubio 2008; Kitiashvili et al. 2010).

The recent results demonstrate the power of the realistic radiative MHD simulations of the complicated turbulent processes in the upper convective layer and photosphere. They not only explain the observed phenomena, but also make interesting predictions. For instance, the simulation of the turbulent magneto-convection in strong inclined magnetic fields, modeling the Evershed effect, indicated that the high-speed flows along the magnetic filaments may have a coherent organization, which appears quasi-periodically on a large scale, and that this organization becomes more prominent when the mean magnetic field in penumbra is stronger (Kitiashvili et al. 2009b). Observations have provided hints of such large-scale organization of the Evershed flows (Rimmele and Marino 2006), but this effect has not been investigated, and its implications for the sunspot structure and dynamics are unclear.

Perhaps, one of the most important problems of the solar magnetism is the understanding of the mechanism of formation of sunspots and active regions. Helioseismology results from MDI and Hinode (Kosovichev et al. 2000; Zhao et al. 2001; Zhao et al. 2010) seem to confirm Parker's idea that sunspots represent bundles of filamentary magnetic structures held together by converging downdrafts (Parker 1979). Such downdrafts have been observed around small-scale elements on the Sun and reproduced in the simulations. But, in sunspots they may be hidden from direct observations by the Evershed flows in the penumbra. The initial attempts to simulate the whole sunspot structure in realistic turbulent conditions show that such structure is quickly destroyed by convection unless it is held by the boundary condition that fixes the magnetic flux concentrated in a small area at the bottom (Rempel et al. 2009b). The simulations of emerging magnetic flux in the turbulent convective environment also have not been able to reproduce the formation of sunspot-like structures; however, they appear to explain quite well the formation of small-scale filamentary flux tubes and transient kG horizontal field (Cheung et al. 2008; Stein et al. 2010). These simulations made a prediction of formation of a horizontal magnetic flux sheet just below the visible photospheric layer, which can be tested by local helioseismology.

Thus, the recent results of high-resolution observations and numerical simulations have provided a significant new insight into the physics of the filamentary magnetic structures on the Sun. The understanding of the small-scales will then be the key to understand the global structure and stability of sunspots and large-scale magnetic structures (Schlichenmaier 2009), and also coupling to the chromosphere and corona (Wedemeyer-Bohm et al. 2009). It is remarkable that this progress is achieved by a synergy of observations and modeling, and we expect that this synergy will stimulate further progress in the field.

The proposed investigation will focus on the origin and physical properties of filamentary magnetic structures and their role in large-scale magnetic phenomena and dynamics on the Sun. This investigation is particularly important and timely because of the simultaneously operating space missions Hinode and SDO, and large solar telescopes, which provide unprecedented coverage of the solar magnetism from smallest scales to the global-Sun scale.

References

Site hosted by ISSI