The line-of-sight magnetic field in a limb prominence was first measured successfully by Zirin and Severny(1961) using Zeeman splitting. Subsequent measurements using both the Zeeman and Hanle effects (see Leroy,1989) show that the magnetic field in prominences is about 3 to 30 G (strongest in active prominences), is approximately horizontal, and makes a small angle with the long axis (. 25◦). In other words, the magnetic field in a filament lies parallel to the photospheric PIL beneath. This may also be seen from Hα images of the chromospheric fine structure. Foukal (1971) inferred the direction of the magnetic field from the orientation of Hα
fibrils, which emanate from plagettes highlighting concentrations of opposite polarity magnetic flux on either side of the PIL. He found that the fibrils point in opposite directions on either side of the PIL, and indicate a magnetic field along the filament axis. Later,Martin, Bilimoria, and Tracadas(1994) found that filaments always form within these so-called “filament channels” where the horizontal magnetic field—as outlined by the fibrils—is aligned with the PIL axis, and there is no Hαstructure crossing the PIL.
We define the chirality of a filament as either “dextral” or “sinistral” (Martin, Bilimoria, and Traca- das,1994), according to whether the axial magnetic field in the filament points to the right or left respectively, when viewed from the side with positive polarity in the photosphere. This definition is illustrated in the left column of Figure5.1. The middle column shows the corresponding dir- ection of Hαfibrils that would be expected for each chirality type, and the right column shows
5.1 Observational Background 83
Figure 5.1: Filaments with dextral and sinistral chirality (after Figure 10 ofMartin,1998). Left column shows definition of chirality by the direction of the axial magnetic fieldBrelative to the underlying photospheric polarities (red). Thick field lines show the corresponding skew of low- to mid-latitude coronal arcades. Middle column shows corresponding Hαfibril patterns originating from plagettes on either side of the PIL, and right column shows examples of each filament type in BBSO Hαimages.
examples of each type. The examples are taken from the data set of BBSO Hα images used in this chapter (Section5.3). While the resolution of these images makes the fibril directions difficult to trace, the chirality may be more easily determined from the structure of the filament “barbs”, appendages which break off to the side of the main filament body and reach down to the photo- sphere.Martin, Bilimoria, and Tracadas(1994) showed that for any given filament most of these barbs are in the same direction, either left-bearing or right-bearing. Further, they found a tight one-to-one correspondence between this structural type and the magnetic chirality: dextral fila- ments have dominantly right-bearing barbs and sinistral filaments have dominantly left-bearing barbs. This is shown in Figure5.1. Note that some filaments are observed to have some barbs of the minority type, either due to short-lived perturbations in the filament structure, or because of the observer’s viewing angle as the filament rotates with the Sun (Gaizauskas, Mackay, and Harvey,2001). However,Martin, Bilimoria, and Tracadas(1994) demonstrate that each filament is of either dextral or sinistral magnetic type, and filaments of different types may not share the same filament channel. The majority direction of barbs may therefore be used to determine the chirality of a filament from the Hα images. This technique is used in Section 5.3. Notice that the chirality of a filament may be determined in this way even without knowing the photospheric polarities on either side of the PIL, because of the asymmetry of the barb structure. If the filament is thought of as a highway with travel in the direction ofB(Martin,1998), then a dextral filament
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Figure 5.2: Schematic view of filaments observed on the Sun on 1980 July 15, showing the mag- netic field vector measured in the high latitude filaments and the underlying photospheric polar- ities (Figure 5 fromLeroy, Bommier, and Sahal-Brechot,1983, reproduced with kind permission of Springer Science and Business Media).
has exits to the right and a sinistral filament has exits to the left.
In a comparison between filaments and their overlying X-ray coronal arcades observed in Yohkoh SXT,Martin and McAllister(1996) found a one-to-one correspondence between the skew direc- tion of the arcades and the chirality of the filaments beneath. Their results show that for a low- to mid-latitude filament the coronal field has the same sense of chirality as the filament itself, as shown in the left column of Figure5.1. Note however thatMcAllister et al.(1998) found the ma- jority of coronal arcades on the polar crown to have opposite skew to their underlying filaments. These arcades were oriented as would be expected from driving by differential rotation (Martens and Zwaan,2001); this behaviour is also found in our simulations and will be considered later in this chapter.