Physical parameters of a flare on 22 December 1999 derived from the H alpha line with a modified cloud method

The flare of 22 December 1999 was observed in the H alpha line using the im aging spectrograph in the solar tower of Nanjing University. We present a n ew technique different from the classical cloud model to fit the H alpha li ne profile which avoids using the background profile. We obtain the four pa rameters of the flare chromosphere: the source function, the optical thickn ess at line center, the line-of-sight velocity and the Doppler width. The o bserved asymmetry profiles have been reproduced well by the theoretical one s based on our model. A discussion is made about the reliability of the res ults we have obtained using the present method.