A recent study has been shown that long-period hydromagnetic (hm) waves (T~30 min) exist at middle- and low-latitude stations prior to and/or simultaneous with the polar substorm. The latitude--independent characteristic of waves has led to the suggestion that the plasmasphere oscillates as a resonant cavity (Wang et al., 1977) and this makes it necessary to examine their longitudinal extension to see if their activities are spatially localized. Magnetograms recorded at 19 stations in an east--west chain situated along a geomagnetic latitudinal circle of about 49¿ in the northern hemisphere were collected for the study. The following results have been found. (1) For waves occurring prior to the substorm of a period T~30 min the longitudinal range is about 240¿, starting clockwise from the 0400 meridian to the noon meridian. It is likely that this prestorm activity covers a much larger spatial extent than that of a polar substorm. (2) For waves with a longer period, of the order of 2 hours, which occurred simultaneously with the substorm a much smaller longitudinal range (~90¿--100¿) is observed in the nightside. This localized character of long-period hm waves suggest that it is a result of magnetotail oscillations. (3) The disturbed magnetic field sometimes exhibits sector structure.