1973 SEPTEMBER 7 2-RIBBON FLARE .2. PHYSICAL-PROPERTIES OF THE LOOPS IN THE ARCADE

Spectral data of selected lines between 170 and 340 A for a large two-ribbon flare of 1973 September 7 are presented. The flare arcade consisted of four prominent loops. Loop intensities and densities decreased by approximately a factor of 2 during one hour of observations, beginning 15 minutes after Solrad flux maximum, suggesting that the mass and energy supply are strongly maintained during the early hours of the gradual phase. Observed emission measures in Ca XVII loops give Ne = 7 × 1010 cm-3 compared with 2 × 1010 cm-3 in Fe XIV loops. The Fe XIV density sensitive 219/211 line ratio gives similar densities implying a filling factor of ≈50% near flare maximum to unity 1 hour later. These results imply that the upper and outer parts of the rising loop system are hotter and denser than the lower and inner parts of the arcade. Electron densities at the loop apex were ∼50% greater than those at the loop footpoints. The loop aspect ratio (i.e., the ratio of the loop cross section diameter to the loop foot-point separation) varies from 0.1 to 0.3, being largest for the higher temperature lines. The derived loop parameters are compared with scaling laws. Loop cooling rates are calculated and compared with observed cooling times. Finally, the observed energy density is compared with that calculated from a reconnection model.