Absorption events associated with solar flares

During the upward period of solar cycle 23, the Imaging Riometer at Zhongshan, Antarctica (geomag. lat. 74.5 S) was used to study the solar proton events and the Xray solar flares which are associated with the absorption events. In our study, the relationship between the absorption intensity and X-ray flux is found in a power form which is consistent with the theoretical result. The imaging riometer absorption data at Ny-Ålesund, Svalbard reconfirm the above relationship. We also argue that only M-class flares can generate a significant daytime absorption.