3-D magnetic configurations for filaments and flares: The role of "magnetic dips" and "bald patches"

The 3-D magnetic configuration of a filament and of a low energy flare is reconstructed, using linear magnetohydrostatic (lmhs) extrapolations. In both cases, we find observational signatures of energy release at the locations of computed "bald patches" separatrices, characterised by field lines which are tangent to the photosphere. The filament was observed on Sept. 25, 1996, in Hα with the MSDP on the German VTT, Tenerife, as well as in Si IV with SOHO/SUMER. It is modeled as a twisted flux-tube deformed by the magnetic polarities observed with SOHO/MDI. The shape and location of the computed dipped field lines are in good agreement with the shape of the filament and its feet observed in Hα. Some "bald patches" (BPs) are present where the distribution of dips reaches the photosphere. We show that some of the large scale field lines rooted in BPs can be related to bright fine structures in Si IV. We propose that the plasma there is heated by ohmic dissipation from the currents expected to be present along the BP separatrices. The flare was observed on May 18, 1994, in soft X-rays with Yohkoh/SXT, and in Ha at Mitaka (Japan). The magnetic field is directly extrapolated from a photospheric magnetogram from Kitt Peak Observatory. The intersections with the photosphere of the computed separatrices match well the bright Ha ribbons. The later are associated to three BPs, with overlaying dipped field lines. We show that enhanced densities are present in these dips, which can be correlated with dark Hα fibrils. Both cases show the importance of dipped field lines and BPs in the solar atmosphere. Energy release via ohmic dissipation as well as reconnection along BP separatrices is proposed to provide heating observed as UV brightenings in filament channels and even as small flares.