NOCTURNAL VARIATIONS OF GEOMAGNETIC HORIZONTAL FIELD AT EQUATORIAL STATIONS

The nocturnal variations of the horizontal component (H) of the geomag netic field at equatorial stations Koror, Trivandrum, Addis-Ababa and Ibadan during any of the seasons of the period 1958-59 showed a steady decrease after sunset throughout the nighttime hours. However, at Hua ncayo a steady decrease after sunset to sunrise is seen only during th e D-months. During the J and E months decrease is seen after sunset ti ll about 22-23 hr and a slow and steady increase from midnight hours t o sunrise. The nocturnal variations of H field at Huancayo show a rema rkable similarity with tile corresponding variations of the horizontal electric field in the ionosphere as determined by tile Doppler shifts of backscatter echoes from tile E and F layers over Jicamarca. Thus, genuine changes in the equatorial electric field and current in the io nosphere over the American sector during the nighttime hours are clear ly indicated.