3-D MAGNETIC CONFIGURATIONS SUPPORTING PROMINENCES - I - THE NATURAL PRESENCE OF LATERAL FEET

It is now commonly accepted that prominence plasma is supported in mag netic dips, in particular in twisted flux-tubes. But present two-dimen sional models are unable to explain the observed presence and structur e of prominences feet. This requires three-dimensional models. We mode led the field using linear force-free field equations. Combining a sma ll number of harmonics, and using observational constraints, we have f ound the area in the parameter space where prominences are likely to b e present. Then, adding 3-D harmonics, we show that feet appear period ically underneath the prominence body. For great helicity, the paramet er space is mostly fulfilled by configurations which have feet alterna ting between both sides of the prominence axis, as observed. The theor etical photospheric field has a quasi-bipolar pattern and the prominen ce stands above a magnetic corridor containing only small parasitic po larities. The lateral feet are formed by dips in the vicinity of these small polarities. These configurations show in a natural fashion a nu mber of well-established as well as more recent observationnal aspects of prominences, in particular the vector magnetic field measurements in prominences and the chirality patterns (the dextral/sinistral, righ t/left bearing, skew of the overlying coronal arcade and fibril organi zation in prominence channels).