The target filament is framed on the ISOON image. In the DOT and THÉMIS images at the upper left corner, it has been rotated to horizontal. In the other images, it has his primary orientation. These images are of 2004 October 6, except the ones from the Meudon Solar Tower that are of 2004 October 7, though pointed on the same filament. The filament is like a matter "wall" standing in the Solar Corona, anchored in the surface by "feet". When seen "on the disk" as here, it absorbs the disk radiation at certain wavelengths, in particular in the Ha hydrogen line where it then appears in dark (DOT-THÉMIS-Meudon-DST images). On the contrary, it does not absorb the continuum (DOT image), where only the granulation appears. It does not absorb also the H line of ionized calcium (DOT image), but this image shows clearly the neighbor activity plages that are poorly visible only in the Ha image. In the UV images (TRACE and SOHO-EIT), the filament location appears as a dark channel, broader than it. The THÉMIS image provides the magnetic field below and around the filament, and the MSDP images display the Ha line center and wings intensity, and the radial velocity field.
THEMIS map of the magnetic field below and around the same filament, on October 8. One has : the vertical component (or "longitudinal"), with the outgoing field in hot colors (yellow, red), and the incoming field in cold colors (blue, green), and the horizontal component (or "transverse") represented by dashes proportional to its intensity and giving its direction. In principle, a filament separates two opposite "polarities" (the "green" from the "red"), but some "parasitic polarities" are visible ("green" in "red" and vice versa) at the filament feet (appearing like thorns on the disk) as predicted by theory.
The filament that absorbs the solar radiation when observed "on the disk", appears on the contrary in emission (radiating) when it "passes at the limb" under the effect of the solar rotation. As it becomes visible above the limb, it is then called "prominence", but prominence and filament are nothing else than the same object seen under two different angles. This object is composed of cold and dense matter hanging againts gravity by the magnetic field and embedded in the Solar Corona that is 100 times hotter and thinner. A recent magnetohydrodynamical model (Aulanier G. & Démoulin P., 1998, Astronomy & Astrophysics 329, 1125) shows that the parasitic polarities are essential for explaining the formation of the magnetic structure that supports the filament. They play a fundamental role in the formation and stability of a filament that may evolve in "Coronal Mass Ejection" where charged particles (electrons, ions) are ejected in the interplanetary space. The prevision of such events is a science still very much in limbo, the "space weather forecast".
THEMIS map below and around another filament (October 16). The upper layer displays the longitudinal component using the same color scale as on the previous map. The lower layer is a 3D plot of the magnetic field vectors superimposed to the Ha map that shows the filament in dark.
These vector magnetic field maps are the exact superimposition of Ha maps showing the absorbing filament and of vector magnetic field maps obtained at the photospheric level (below the filament) by using the photospheric line of neutral iron at 6302 Å. Such an exact superimposition, that enables to see the parasitic polarities at the filament feet, has been possible only because the two lines Ha and 6302 Å are simultneously observed, a THÉMIS facility unique in the world still for a long time This campaign has been initiated by Thierry Roudier (CNRS-UMR5572/LAT), who has organized and managed it, with the collaboration of Sylvain Rondi (CNRS-UMR5572/LAT) at DOT, Jean-Marie Malherbe and Christian Coutard (CNRS-UMR8109/LESIA) at Meudon, Véronique Bommier (CNRS-UMR8112/LERMA) at THÉMIS (October 5-11), Brigitte Schmieder (CNRS-UMR8109/LESIA) at THÉMIS (October 12-19), Arturo López Ariste (CNRS-UPS-THÉMIS) at THÉMIS, Guillaume Molodij (CNRS-UMR8109/LESIA) at Sac Peak, of all the THÉMIS S.L. staff, in particular Alberto Sainz Dalda, and of other international collaborations : Pit Sütterlin & R. Rutten (DOT, La Palma), Steve Keil (DST, Sac Peak), Carl Henney & Ruth Kneale (SOLIS, Kitt Peak), Tin Henry (ISOON, Sac Peak), Emily Stein (SOHO/SOC), S.E. Gregory (SOHO-MDI), Stein Vidar Hagfors Haugan & Joe Gurman (SOHO-EIT), Andrzej Fludra (SOHO-CDS), Karel Schrijver (TRACE), Sara Martin. The targets have been determined by using the full sun Ha observations of Meudon Observatory (France), Big Bear Observatory (USA), Kanzelhöhe Solar Observatory (Austria), Catania Astrophysical Observatory (Italy).