Back to Volume
Paper: The Subsequent Decaying Regions of NOAA AR7978
Volume: 383, Subsurface and Atmospheric Influences on Solar Activity
Page: 397
Authors: Li, Y.; Welsch, B.T.
Abstract: NOAA AR 7978 (S09E15), which emerged during solar activity minimum as an old cycle spot, evolved for five rotations without interacting with any other region. Using MDI 96min full disk magnetograms we investigate the magnetic field evolution of this long-lived active region (AR) by following a few to several days during each of the first four rotations. In three days (during CR 1911), AR 7978 emerged to reach 90% of the peak total magnetic flux found in the second rotation (CR 1912). The total flux decayed to 50% of the peak flux in ∼50 days thereafter. AR flux was well balanced between opposite polarities in CR 1912, but became increasingly unbalanced during the decay phase, with excess negative flux of ∼12% and ∼20% of the AR total flux in CR 1913 and CR 1914, respectively. Despite a dimension increase of 30%, the distance between weighted positive and negative magnetic flux-centers remained constant in the decay phase, which is unexplained. The apparent motion of the magnetic features showed organized pattern during the emerging and stable phase, but disorganized patterns at the decay phase. We conclude that the evolution of the photospheric magnetic field of the AR was dominated by changes and interactions of the internal structure responding to surrounding plasma conditions during the emerging and stable phase, but lost most of its coherent structure during the decay phase and was controlled by forces external to the AR, e.g., local convection, diffusion, and global scale flows. The AR was the source region of 29 flares including the 9 July 1996 X flare during its short emerging phase, and 20 small flares during its stable and decay phase.
Back to Volume