Shear Motion Of The Footpoints In Two-Ribbon Flares

Yingna Su

CHarvard-Smithsonian Center for Astrophysics

We present two statistical investigations relevant to the shear motion of the footpoints in two-ribbon flares observed by TRACE (in 1998-2005). We have selected 50 well-observed X- and M- class two-ribbon flares as our data sample. The results of the first study are: 1) 86% (43 out of 50) of these flares show both strong-to-weak shear change of footpoints and ribbon separation. Shear motion of footpoints is thus a common feature in two-ribbon flares; 2) the initial and final shear angles of the footpoints in this type of flare are mainly in the range from 50° to 80° and 15° to 55°, respectively; 3) in 10 out of the 14 events with both measured shear angle and corresponding hard X-ray observations, the cessation of shear change is 0-2 minutes earlier than the end of the impulsive phase. This may suggest that the change from impulsive to gradual phase is related to magnetic shear change. We then selected 20 flares with measured shear angles and corresponding CMEs, to do our second study adressing the question of what determines the intensity of solar flare/CME events. We found that the magnetic flux and change of shear angle show comparably strong correlations with the peak flare flux and CME speed, while the intial shear angle does not. This result indicates that the intensity of solar flare/CME events may depend mainly on the released magnetic free energy rather than the total magnetic free energy stored prior to the eruption.