Amplitude and orientation of prominence magnetic fields from constant-α magnetohydrostatic models

We analyze outputs from three-dimensional models for three observed filaments, which belong to the quiescent, intermediate and plage class respectively. Each model was calculated from a constant-alpha magnetohydrostatic extrapolation, assuming that the prominence material is located in magnetic dips, so that the field is nearly horizontal throughout the prominence body and feet. We calculate the spatial distribution of the magnetic field amplitude B and orientation theta with respect to the filament axis, neither of which were imposed a priori in the models. In accordance with past magnetic field measurements within prominence bodies, we also obtain nearly homogeneous magnetic fields, respectively of about B similar to 3; 14 and 40 G for the quiescent, intermediate and plage prominence, with a systematic weak vertical field gradient of partial derivativeB/partial derivativez similar to 0.1-1.5 x 10(-4) G km(-1). We also find that the inverse polarity configuration is dominant with theta similar to -20degrees to 0degrees, which is slightly smaller than in some observations. We also report some other properties, which have either rarely or never been observed. We find at prominence tops some localized normal polarity regions with theta < + 10°. At prominence bottoms below 20 Mm in altitude, we find stronger field gradients &PARTIAL;B/&PARTIAL;z ∼ 1-10 x 10(-4) G km-1 and a wider range of field directions θ ∼ -90° to 0°. These properties can be interpreted by the perturbation of the prominence flux tube by strong photospheric polarities located in the neighborhood of the prominence. We also report some full portions of prominences that have the normal polarity. The latter are simply due to the local curvature of the filaments with respect to their average axis, which was used to define θ. These results could either be used as predictions for further testing of this class of models with new observations, or as quantitative tools for the interpretation of observations which show complex patterns.