a) Magnetic flux in NOAA 10720 as a function of time: the thin (thick) line indicates the negative (positive) flux. b) Horizontal velocity fields measured in NOAA 10720 on 17 January 2005; the fov is 400 × 200 arcsec. c) Magnetic helicity rate measured in NOAA 10720 during the period analyzed. d) Magnetic helicity accumulation in NOAA 10720. The vertical dotted lines indicate the time of occurrence of the X3.8 flare.
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