Solar prominences: the need for dynamical modelling

Recent observations from space have shown that the ultraviolet lines of hydrogen (the socalled Lyman lines) are very asymmetric. This new and very puzzling result has now been explained by a group of scientists from the Max-Planck-Institut für Astrophysik and from the Astronomical Institute of the Academy of Sciences in Ondrejov, Czech Rep. It was found that for the modelling a large degree of fine structure together with a strong dynamical behaviour of the structures is required.

Fig. 1: Prominence seen on the solar limb in the light of one line of neutral helium. The white line gives the position of the spectrograph slit used for the Lyman line observations.

Fig. 2: Image of a filament observed against the solar disc in the H-alpha line with the Swedish 1-m Solar Telescope (SST). The picture shows a high degree of fine structure with many narrow fibrils which produce the filament configuration (courtesy of O. Engvold).

Fig. 3: Line profiles of Lyman-alpha (upper part) and Lyman-beta (lower part) of the prominence observed on May 25,2005 by SUMER. The spectra are taken at different positions along the slit .

Fig. 4: Theoretical profiles of the Lyman-alpha to Lyman-gamma lines at a few selected positions along the threads. The full lines represent the profiles which result from the model with 10 individual threads. The dot-dashed lines give the profiles obtained for a single-thread model at the same position.

Solar prominences are relatively cool structures having around 8000 K which extend high into the corona which is several million K hot. They can be observed best in the strong H-alpha line of neutral hydrogen. They can also be seen in other spectral lines and an example is shown in our Fig. 1. When seen against the solar disc in H-alpha they appear as dark structures called filaments which usually have very detailed fine structure.A typical example is given in Fig. 2.

With the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) spectrograph onboard of SOHO (Solar and Heliospheric Observatory) they have been observed recently in the Lyman lines of hydrogen. The position of the spectrograph slit is shown by the solid white line in Fig. 1. Some examples of the profiles taken at several positions along the slit of the lowest two of these lines are given in Fig. 3. One surprising result of these new observations was the fact that in many cases these line profiles are highly non-symmetric.

Theoretical models for prominence fine structures have been developed by Ulrich Anzer in collaboration with Petr Heinzel and Stanislav Gunar from the Astronomical Institute in Ondrejov. With these models we were able to reproduce the observed asymmetries. For this investigation we used a configuration which consisted of several individual thread structures which had randomly selected line-of-sight (LOS) velocities and the threads were also randomly displaced in the direction perpendicular the LOS. We applied detailed radiative transfer calculations to these models. From our calculations we found that rather small LOS velocities in the range between -10 km/s and +10 km/s were sufficient to produce large asymmetries. Some examples can be seen in Fig. 4. The physical mechanism which produces these results lies in the fact that the different threads have different relative velocities. Therefore the emission from one such thread will be absorbed by the threads in front of it and this absorption is Doppler shifted with respect to the emitter. This then can lead to a large asymmetry of the profiles. The interesting point is that already small velocities can give such asymmetries. The reason for this is that on the one hand the line profiles are strongly reversed with two narrow peaks on the two sides, and that on the other hand the absorption profiles are very steep.

These new results give strong support for the necessity to include fine structure dynamics in realistic prominence models.

Ulrich Anzer


Stanislav Gunar, Petr Heinzel, Ulrich Anzer and Brigitte Schmieder, "On Lyman-line asymmetries in quiescent prominences", 2008, Astronomy & Astrophysics 490, 307 - 313