Disambiguated magnetic field measurements in a quiescent prominence
Résumé
The magnetic field vector was measured in a prominence by interpretation of the Hanle effect, following the method described in Bommier, V., Leroy, J.L., and Sahal-Bréchot, S., 1981, A&A, 100, 231, but with the difference that the measurements were spatially resolved in the prominence body, because the prominence was observed with the French THEMIS telescope (Izaña, Tenerife, Canarias). The prominence was observed on two following days, and the measurement ambiguity was resolved by selecting the closest pair in the ambiguous solutions, whose symmetry changes along the days due to solar rotation. The method successfully worked. The observed line was He I D3. The polarisation was integrated along the line profile for accuracy purposes, and the two line components were not resolved. The magnetic field was then assumed to be horizontal, as shown by Athay et al. (1983, Solar Phys. 89, 3). The magnetic field strength and the α angle between the field vector and the prominence long axis were unambiguously obtained for nearly each pixel. A vertical gradient of +0.5e-4 G/km (increasing with height) is found above h=34 Mm, in excellent agreement with Leroy et al. (1983, Sol. Phys. 83, 135). The new result is that the α angle is found to decrease with height, in accordance with the theoretical model by Aulanier & Démoulin (2003, A&A, 402, 769). The second new result is that measurements are now obtained close to the solar limb, lower than 20 Mm. In these regions, the magnetic field strength gradient is found negative -0.6e-4 G/km (decreasing with height), contrarily to the model. In this region, the α angle is also found decreasing with height, but in a quicker manner and with more scattered values. The prominence is found of Inverse Polarity at all heights.