Obtain Stokes profiles maps and Ca H II filtergrams of filaments in emerging flux regions and of off-limb protuberances to provide context and binding information to spectropolarimetric observations of the chromosphere in the He 10830 multiplet. Such information is crucial to draw the magnetic configuration of solar prominences/filaments.
This proposal requires continuous, uninterrupted observations for about 5 hours a day during 10 days. We also demand very high signal- to-noise ratios to detect the smallest magnetic flux elements along the polarity inversion line below filaments. The ground-based campaign will run from August 29 to September 7, 2012 using the Tenerife Infrared Polarimeter installed at the Vacuum Tower Telescope at the Observatotio de Izaña, in Tenerife (Spain). During the campaign we will record H alpha slit-jaws images to align the He 10830 data with
the Hinode data, so it would be derivable to obtain NFI halpha context images, and if possible, Dopplergrams using the same line.
Motivation
----------
One of the manifestations of the buoyant rise of magnetic flux from the convection zone outwards are solar filaments (solar prominences when observed above the solar limb). Various models for the magnetic structure of prominences have been proposed (for a review see e.g. Mackay et al. 2010). Among them, we shall highlight the so-called dip models and flux-rope models. In the first one the cool material of the prominence is supported by many magnetic dips via a magnetic tension force, by MHD-wave pressure (e.g., Pecseli & Engvold 2000), or by the presence of tangled magnetic fields in very small scales (van Ballegooijen and Cranmer 2010). The second one suggest that the prominence is
supported by a helical magnetic field structure that emerged from below the photosphere or was generated by magnetic reconnection of pre-existing magnetic fields lines. In this configuration, the prominence material is suspended via the magnetic tension force. However, it is not jet clear what is the most probable magnetic configuration of the filament and what physical mechanisms are supporting the filament plasma against gravity. This is partly due to the lack of observations showing how the photospheric and chromospheric magnetic fields evolve during the filament formation and emergence. To alleviate this problem, we need to simultaneously observe flux emergence events in the photosphere and the chromosphere and to record filament formation and evolution from its earliest stages.
Our group at the IAC has been investigating magnetic flux emergence processes since few years ago. Some interesting preliminary results can be found in Asensio Ramos & Trujillo Bueno (2010). In this case we analyzed an emerging flux region and found out that magnetic fields connect both polarities crossing the polarity inversion line and that the fields are smoother in the chromosphere than in the photosphere. We detect strong downflows at the loop-like structures foot-points. Interestingly, we find out that the filamentary structure seen in the chromosphere may be produced by a density enhancement in a relatively uniform magnetic field rather than by a (filamentary) magnetic field structure. Recent observations of filaments also show the presence of supersonic downflows along the filaments in the chromosphere (e.g., Merenda et al. 2007; Katsukawa et al. 2012). However, due to the discontinuity in the observations we cannot associate such supersonic
flows as a prevalent property of solar filaments. We have also observed that the chromospheric magnetic plasma is systematically redshifted at both sides of the polarity inversion line. We still have no definitive clues about its origin.
To shed some light to these open issues we have allocated time in the Vacuum Tower Telescope (VTT) in Tenerife in order to obtain to get high-quality spectropolarimetric data of emerging magnetic flux events in the 10830 A spectral region using Tenerife Infrared Polarimeter (TIP). This spectral region contains two spectral lines, the photospheric Si I line and of the chromospheric lines of the He I 1083.0 nm triplet. These lines will allow us to obtain empirical information of the 3D geometry of the magnetic field. The He I 1083.0 nm multiplet is particularly very well suited because its polarization signals are sensitive to atomic level polarization and to the joint action of the Hanle and Zeeman effects. Reliable inversions can be performed with
the HAZEL code (Asensio Ramos, Trujillo Bueno and Landi DeglfInnocenti, 2008).
Our primary aim is to determine the magnetic field configuration of solar filaments. This will help us discriminate between the different models proposed in the literature. To this end, we need to know the photospheric magnetic fields configuration and evolution. We also need to know the response in the chromosphere using high cadence Ca II H filtergrams and Halpha Dopplergrams for both, off-limb and on-disk prominences. Hinode is the only facility able to provide such information. The data will certainly help to understand the physics
of solar filaments. |
|