Precise determination of cooling times of post-flare loops from the detailed comparison between Hα and soft X-ray images

Abstract

A detailed study of the evolution and cooling process of post-flare loops is presented for a large X9.2 solar flare of 2 November 1992 by using Hα images obtained with Domeless Solar Telescope at Hida Observatory and soft X-ray images of Yohkoh Soft X-ray Telescope (SXT). The detailed analysis with a new method allows us to determine more precise values of the cooling times from 107 K to 104 K plasma in the post-flare loops than in previous works. The subtraction of sequential images shows that soft X-ray dimming regions are well correlated to the Hα brightening loop structure. The cooling times between 107 K and 104 K are defined as the time difference between the start of soft X-ray intensity decrease and the end of Hα intensity increase at a selected point, where the causal relation between Hα brightening and soft X-ray dimming loops is confirmed. The obtained cooling times change with time; about 10 min at the initial stage and about 40 min at the later stage. The combined conductive and radiative cooling times are also calculated by using the temperature and density obtained from SXT data. Calculated cooling times are close to observed cooling times at the beginning of the flare and longer in the later stage.