The MHD drift ballooning mode (DBM) instability near the inner edge of the near-Earth plasma sheet is studied by using both the one-fluid generalized progressing wave expansion method and the two-fluid approach. It is found that in the frame of reference at rest relative to the bulk plasma the DBM may become a purely growing mode in two distinct circumstances, which, for convenience, are called the DBM1 and DBM2, respectively. The &bgr; threshold for the DBM1 is identical with that derived by Ohtani and Tamao [1993> and Southwood and Kivelson [1987>, while the criterion of the DBM2 covers that of Miura et al. [1989>. Comparisons of the theory with GEOS 2 data show that the DBM2 is more easily excited in the late substorm growth phase. There is considerable evidence that the DBM is generated at expansion onsets. The characteristic features of magnetic field dipolarization can be interpreted in terms of the development of the DBM. The extremely thin current sheet cases should be studied with approaches other than those used in this work.¿ 1997 American Geophysical Union