We measured electron density and temperature of the nighttime upper ionosphere for the several key intervals during the progress of the magnetic storm on 15 July 2000 with the Langmuir probe on Korea Multipurpose Satellite-1 (KOMPSAT-1). During the main phase of the storm KOMPSAT-1 detected near 0¿E a very deep and extensive trough of electron density centered around the geomagnetic equator. The electron density dropped sharply from ~4 ¿ 105cm-3 to less than 2 ¿ 104cm-3, with the trough region extended over 1400 km along the satellite track. Later in the recovery phase, KOMPSAT-1 observed severe distortion still persisted near 230¿E, with enhanced density in the southern hemisphere. Together with DMSP observations, we estimate the size of the observed trough to be at least 5500 km in the longitudinal direction. DMSP data indicates that the trough was caused by the enhanced eastward electric field which lifted the equatorial F-region ionosphere upward.