(a) Longitude and latitude variations of DMSP F12 N_i measured at nightside 21:30 LT around the June Solstice of 1996. The thick black lines denote that geomagnetic declination is zero and the thin gray lines are geomagnetic inclination isolines. (b) The distribution of N_i in geomagnetic coordinates. Two black dotted lines denote the solar terminators at 300 km height at 21:30 LT (the upper line) and 20:30 LT, respectively. (c) N_i longitude fluctuations with longitudinal averages removed for each geomagnetic latitude.