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J. Harvey, T. Bippert-Plymate, D. Branston, C. Plymate (NSO/NOAO), F. Recely (SEC/NOAA), H. Jones (GSFC/NASA)
Since mid-1996, NSO and its partners have made regular full-disk, line-of-sight component magnetograms using the 854.2 nm line of Ca II. These observations are primarily sensitive to the magnetic field about 800 km above the photosphere. Several new features of the solar magnetic field have emerged in these observations. At a sensitivity of about 3 Gauss and using one arc sec pixels, we observe diffuse, large-scale magnetic fields in the chromosphere that have no significant counterparts in the photosphere. The most obvious of these fields are found surrounding the borders of active regions and seem to correspond to the "circumfacule" phenomenon first described in 1910 by Deslandres. In other words, these fields seem to be the nearly horizontal field component associated with long chromospheric fibrils found around some active regions. The next most obvious example of large-scale chromospheric fields is a surprise. These fields are found closely associated with filament channels but, unlike the ubiquitous polarity reversal always seen in the photospheric magnetic field under filament channels, the chromospheric field has only one apparent polarity across the filament channel. However, the apparent sign of the chromospheric field reverses near the central meridian as the feature traverses the solar disk. This behavior indicates that the field is predominantly horizontal with a strong east-west component. These fields are typically seen on the poleward side of mature active regions and may last several solar rotations. They should be an important diagnostic of magnetic field patterns surrounding solar filaments. It is not clear why some filament channels show these fields while others do not. A final type of large-scale chromospheric magnetic field is at our present limit of detection and is simply a predominance of one magnetic polarity or the other over large areas of the solar disk. Such patterns can be seen in the photosphere after a large amount of spatial averaging, but require no such averaging in the chromosphere. The NSO is one of the National Optical Astronomy Observatories which are sponsored by the National Science Foundation. The NSO data used here are produced cooperatively by NSF/NOAO, NASA/GSFC, and NOAA/SEC.
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