The radio .astronomy explorer satdlite (RAE 1) measures noise temperatures at an altitude of about 6000 km on frequencies from 0.2 to 9.2 MHz. By taking into account the effects of ionospheric shielding on terrestrial noise propagating to RAE 1 from the earth's surface it is shown that on frequencies above the critical frequency the noise observed by RAE 1 at 6000 km over the United States during nighttime is about 6-12 dB higher when thunderstorms are reportedly active within the viewing area than when they are not. The highest level is associated with the most intense storm. When the RAE observed temperature is expressed as an equivalent average temperature of the observable source region at the surface, it appears that the noise is principally of manmade origin when thunderstorms are absent;the equivalent nonstorm temperature varies from about 65 dB (>288øK) at 3.93 MHz to 55 dB at 9.18 MHz in both autumn and winter. It is concluded that 'thundery regions' can be detected by an orbiting satellite using HF radio techniques but ionospheric effects cannot be neglected.
For a number of years the d•tection and monitoring of thunderstorms by an orbiting sa.['•ilite have been tantalizingpossibilities. Horner [ 1965] showed that significant amounts of radio energy from lightning discharges in a thunderstorm area can be expected up to altitudes of at least 1000 km, but as was pointed out by Pierce [1969], there are several major difficulties in using radio techniques for thunderstorm obser-
vations by s•it•llite.At VHF frequencies the satellite must have a highly direcrive antenna •ttii:l be in rather low orbit in order to discriminate against the generally higher cosmic noise background, whereas at HF frequencies the effects of the intervening ionosphere become important. It has been suggested that the shielding effect of the ionosphere on frequencies near the critical frequency can be used to advantage to limit the area of reception [Horner and Bent, 1969;Bent, 1969]. At the seventeenth general assembly of URSI in Warsaw in 1972, Horner [1974] suggested that a directional aerial pointed toward the earth would provide further improvement.The lack of spatial resolution is a severe iimitation in both high and low orbits. Angular resolution is sacrificed in high orbits selected to obtain broad geographical coverage, and in low orbits the satellite would move a considerable distance in the periods required tO obtain a meaningful sample of atmospherics [Pierce, 1969]. The Ariel 3 satellite, which carried an experiment designed specifically for atmospherics detection a[ high frequencies [Horner and Bent, 1969], disclosed a much greater degree of interference from terrestrial transmitters than was anticipated [Horner, 1974i. These difficulties not withstanding, HF radio noise measurements by the radio astronomy explorer (RAE i) spacecraft offer the opportunity to determine the detectibility of thunderstorms by satellite. Although the RAE 1 experi-Copyright ¸ 1975 by the American Geophysical Union. ments were not designed ...