Magnetic field changes during flares

Sasha Kosovichev called attention to changes in GONG+ observations of the X20 flare of 2 April 2001 in NOAA 9393, and Jack Harvey prepared this figure to show the temporal evolution. The left panel is the line-of-sight component of the magnetic field near the northeast limb, averaged over 30 minutes prior to the flare (2100-2129 UT). The next six panels show the difference between the absolute value of the signal averaged over subsequent 10-minute periods and the 30-minute pre-flare average. White represents increasing line-of-sight signal strength; black represents decreasing line-of-sight signal strength. The panels are averaged from left to right over 2130-39 (preflare), 2140-49 (rising phase), 2150-59 (maximum), 2200-09 and 2210-2219 (declining phase), and 2250-59 (post flare). The changes shown are a combination of seeing effects, change in perspective as the Sun rotates, line profile changes associated with the maximum phase of the flare, normal evolution of the active region field, and real field changes associated with the flare. The later panels are coincident with a strongly sheared sunspot. It is not clear if the flare field changes are in strength, direction or position, or some combination of the three. Further details are in the Science Highlights section of the December 2001 NOAO Newsletter (currently only available in PDF format).