Prominence Cavities

Organisateurs : Laurel A. Rachmeler (HAO, NCAR), Guillaume Aulanier (LESIA)

The magnetic structure of prominences is greatly debated in the solar community. Generally they are believed to be either twisted flux ropes or sheared arcades with little or no twist (e.g. Gibson and Fan, 2006 ; Aulanier and Demoulin, 1998, Van Ballegooijen, 2004). Because to date coronal magnetic field observations have been unavailable, the answer to this question has relied on extrapolations from photospheric fields and on high energy observations where bright structures are thought to follow lines of magnetic force.
The High Altitude Observatory (HAO) in the National Center for Atmospheric Research (NCAR) in the United States has recently deployed a new instrument, the Coronal Multi-Channel Polarimeter (CoMP), which measures linear and circular polarization in coronal infra-red (IR) lines off the limb of the sun (Tomczyk et al, 2008). These measurements are signatures of the magnetic field, and may be used to differentiate between the flux rope and the sheared arcade prominence models. With the use of a new FORWARD suite of IDL codes, developed at HAO with international collaborators (ISSI workshop), we can now create simulated CoMP, EUV, and other observables that can be directly compared to observational data.
The goal of this workshop is to bring together numerical modelers and observers from several world-leading groups, including HAO and LESIA, to test various theoretical, analytic, and numerical models of the magnetic field surrounding prominences and to analyze the CoMP coronal polarization signatures from each model.
The polarization signatures in the corona are not trivial to invert into magnetic field, thus it is extremely useful to forward model theoretical magnetic field configurations which can be directly compared to observable quantities. These can then also be compared to other observables calculated in the FORWARD code, such as EUV images. Material at prominence temperatures is not within the range of the Fe XII 1074.7 nm coronal line (the line calculated in the CoMP FORWARD model), and so we will mainly focus on the magnetic morphology of the plasma surrounding the prominence itself.
It has been shown that when a prominence on the limb is along the line of sight, a coronal cavity is often observed in EUV pass-bands (Hudson et al, 1999). Coronal cavities in some models, including those models used at LESIA, are fundamental aspects of prominence magnetism. Although relatively low in intensity, they are only depleted in density by about a factor of two compared to surrounding bright structures, and are brighter than coronal holes. Because of this low density, cavities are extremely sensitive to orientation (Gibson et al, 2010), and the same is true of the polarization signature. In order to address the question of sheared vs. twisted fields, we need to observe prominences where the axis is along the line of sight. Thus, the appearance of a cavity in
EUV is a good indication that the prominence is in the correct orientation and would be an ideal candidate for comparison to models.
There are multiple questions that can be addressed during the workshop, and they can be tailored to the specific participants. Among these questions are :
•Do different models of prominences predict different CoMP linear and circular polarization signals ?
•How do CoMP polarization signals correspond to EUV images — e.g. discrete O or C-shaped bright loops inside cavities ? This question is motivated by recent Solar Dynamics Observatory (SDO) observations of such bright and dynamic features.
•Do prominence tops correspond to the center of these concentric circles and/or CoMP linear polarization depletions, and hence the axis of the magnetic field ? This prediction comes from idealized highly twisted and/or nearly axisymmetric flux rope models. Although we cannot directly forward model the prominences in this particular wavelength of polarized light, we can „paint‟ the dips in the field which are expected to correspond with the bright prominence material.
•Can we distinguish not only between twisted and untwisted ropes, but also put limits on the amount of twist in these ropes ?
•What is the relationship between prominences and cavities, are they separate or unified magnetic structures ?
As stated earlier, the data from CoMP are new, but will to greatly enhance the field of prominence and cavity magnetometry. We want to bring this new data directly to the modelers to help solve the outstanding questions about prominence magnetic topology.