The microwave and Hα sources of the 1992 January 13 flare

We compare X-ray, microwave and Hα observations for the 1992 January 13 limb flare. The soft and hard X-ray images of this flare have been studied thoroughly by Masuda et al. (1994) with Yohkoh SXT and HXT images. We find that during the hard X-ray emission peak there is no Hα brightening on the disk nor at the limb, so the main ribbons of this flare must be beyond the limb. The microwave source maintains a fixed distance about 10″ from the optical limb in the frequency range 2.8-14.0 GHz. We interpret this limit in source position as due to the presence of a microwave limb that extends higher than the white-light limb - to a height of 7300 ± 1500 km. We believe that the high-frequency microwave emissions are occulted by this extended limb, while the soft and hard X-ray emissions are able to pass through largely unaffected. We also believe, however, that the hard X-ray footpoints are also partially occulted by the photospheric limb, despite the appearance of "footpoint sources" in HXT data shown by Masuda et al. The smooth X-ray and microwave time profiles, microwave-rich emission relative to hard X-rays, and progressive hard X-ray spectral hardening through the flare peak are all characteristics that we interpret as being a direct result of the occultation of footpoint emission.