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Sequence of Hα images of two active regions (NOAA 8731 and NOAA 8732) acquired by Big Bear Solar Observatory on 15(a), 16(b), 18(c), and 19(d) October 1999. The arrow in (a) indicates the location where a filament started to form. In (c) it is possible to see a barb of the filament. The field of view is ~280 × 280 Mm2. North is on the top and West at the right.
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