The global distribution of natural and man-made ionospheric electric fields at 200 kHz and 540 kHz as observed by Ogo 6

Abstract

Our experiment on the polar-orbiting Ogo 6 spacecraft (perigee 400 km, apogee 1100 km) yielded real time analog data in several broad band channels and essentially continuous tape-recorded data from two narrow band (200 Hz) receivers operating at 200 and 540 kHz. The results show that the worldwide distribution of signals at 200 and 540 kHz falls into a number of different categories: (1) naturally generated broad band (auroral) hiss at polar latitudes with typical 200-kHz intensities of around 0.1 µV m−1 Hz−1/2, maximum intensities of up to several µV m−1 Hz−1/2, and generally lower intensities at 540 kHz; (2) nighttime mid-latitude enhancements of a few microvolts per meter, which probably result either from a superposition of signals from a number of 200- and 540-kHz stations or from interference from intense signals of much higher frequencies; (3) well-defined signal peaks associated with individual ground stations operating at 200 kHz (these upward-propagating signals appear to be guided along the earth's magnetic field and may reach an intensity of several hundred microvolts per meter); (4) striking signal enhancements in the conjugate region of a low-latitude 200-kHz station (Achkhabad), suggesting propagation in the whistler mode to the opposite hemisphere; (5) occasional signal enhancements at the magnetic equator, which are at present unexplained.