Changes of the size of the open field line region during substo rms

The dimensions of both the near noon-midnight and near dawn-dusk meridians of the polar cap are examined during a number of substorms on the basis of particle data taken from the DMSP satellites. There is an annular belt of precipitation of soft electrons (∼300 eV) which extends poleward from the auroral oval. In general, the energy flux of these precipitating soft electrons is rather low and does not always cause sufficient luminosity to be detected by the present satellite-borne aurora imagers. This makes it very difficult to determine, conclusively, the open field line region on the basis of the present auroral imagers. In the present study, the open field line region is determined accurately in terms of the area of void of the soft electron precipitation. It is in this area where the so-called "polar rain" is observed. Our results show clearly that, at least for both medium and weak substorms, the dimension of the open field line region varies roughly in harmony with the AE index. The open region reaches the largest dimension at about the maximum epoch of substorms and becomes smallest during a quiet period, after slowly contracting during the recovery phase. The expansion and contraction of the open region result primarily from a decrease and increase, respectively, of the width of the soft electron precipitation belt, although the hard electron precipitation belt also expands and contracts.