Motions, fields, and flares in the 1989 March active region

The active region of 1989 March, NOAA AR 5395, was the most prolific flare-producing region in recent history (although the 1989 October region produced considerably more energetic particles). The region produced some 200 flares, of which 50 were class M flares and 10 were class X. We present here the results of joint observations of this region at the Big Bear and Huairou Solar Observatories. While some cloudy periods occurred, the region was observed every day from limb to limb for significant periods, and nine of the 10 class X flares were recorded. The resultant long hours of coverage in various modes and wavelengths give a good overview of the flows and spot evolution in the photosphere, revealing effects not hitherto observed. The salient characteristic of this region is that it was a great δ group, dominated by spots of following (f) polarity, which moved rapidly westward, producing large changes in magnetic structure which increased the shear and led to great flares. Aside from its great size, the region was unusual in that normally p spots dominate and move westward. In this case there was a 4:1 flux imbalance; 80% of the flux measured was of f polarity. Also, it is rare for large regions to occur at such a high latitude (33° N), although evidence is accumulating that exceptional regions may occur there. The new phenomena observed were the following: 1. The major f spot in the region moved with a near-constant acceleration, eventually reaching 0.25 km s^-1. While there is no obvious connection between the acceleration rate and flare occurrence, there is sufficient kinetic energy to power the flares. We have found rapid spot motion in all other superactive regions. 2. Small p and f spots move out from either side of the large f spot, and curl around it in curved trajectories. 3. The moving penumbral material coalesces into new umbrae. Although flares occurred in all the segments of the neutral line in this region, the class X flares occurred exclusively at three sites: (1) A δ spot F7-P4 where new pores were emerging and streaming out from F1. (2) A δ spot P1-F4 where the magnetic gradient built up as the p spots merged into the growing P1. (3) A δ spot (P3-F6) where the magnetic field lines are highly sheared.