H_a Observation of the Great Two-Ribbon Flare of May 16 in 1981

At 07^h57^m30^s UT on May 16, 1981, a great two-ribbon proton flare occurred in Solar Active Region SESC 3106. At 08^h48^mUT, the flare reached the maximum (the area of the flare was about 1333 units). The location of this flare was Nl2, El4. The Carrington longitude was 160. It lasted about two hours. At the time when the flare occurred, Type 2 (Importance 2), Type 4 (Importance 3) radio bursts, an X1-Importance X-ray burst and a 3-Importance SWF event were also observed. At I3h 25m UT, a proton event (10Mev) was recorded in the satellitic equipment, its maximum proton flux was about 130 p/cm²/sec /ster.A Lyot-type solar monochromatic Hoc (6562.8Å) telescope mounted at the Purple Mountain Observatory was used to which, we added a set of lens in order to amplify the solar image to 10 cm. Using a shutter of Seagull Type-DF camera and Kodak SO-115 mm films, we recorded some flare phenomena on-band and off-band Ha films.From the Ha frames of the great flare of May 16, 1981, we come to the following conclusions:1. Groups 3106 and 3112 appeared as the complex structure. From May 15 to 16, we could n't find any motion of one of them with respect to the other.2. On May.16, there were many new small sunspots that emerged nearby the curvilineal neutral line in Group 3106. It may steepen the gradient of the magnetic field near the neutral line.3. About twelve minutes before the flare, the plage (near the middle segment of the filament) brightened. At the same time the plages (the two ends of the large filament) also brightened.4. About six minutes before the flare, the absorption at the eastern end of the filament was strong. Off-band Ha filtergrams show that the filaments dilated upward clearly.From the above statements, it seems that our work has presented an observational evidence in support of the newly-emerging flux model.5. The present observations confirm the earlier reports that two-ribbon flares occur on either side of the neutral line, gradually expand and deviate from the neutral line. The movement of the two-ribbon flare is often obstructed by the large sunspot, and even small sunspots. The two ribbons can easily devolop in the spotless region.6. From off-band Ha filtergrams we can see that in post-flare loop system, the material (within loop legs and heels) moved downward.