DE-1 VLF OBSERVATIONS DURING ACTIVITY WAVE INJECTION EXPERIMENTS

We report on coordinated high-altitude satellite observations in suppo rt of one of the first space-based VLF wave injection experiments, nam ely the USSR Aktivny mission. The Activny satellite (A) was designed t o carry a VLF transmitter (nominal frequency approximately 10 kHz, tra nsmitter power approximately 10 kW) coupled to a 20-m-diameter loop an tenna in a nearly polar orbit (83-degrees inclination, apogee approxim ately 2500 km, perigee approximately 500 km). We focus our attention o n conjunction experiments between the Activny and DE 1 satellites. Bec ause of problems in the deployment of the loop antenna, the radiated p ower capability of the antenna was significantly reduced. Although thi s substantially reduced the expectation of receiving detectable signal levels on the satellite, the DE 1/Activny conjunction experiments wer e nevertheless carried out as a means of possibly placing an upper lim it on the radiated power. During the period November 1989 through Apri l 1990, a total of 10 DE 1/Activny wave injection sessions were conduc ted. During each session the Activny transmitter operated at 10.537 kH z with 1 s On - 1 s Off format, for a period of 6 min centered around the conjunction time. During three conjunction periods (December 12, 2 6, and 27, 1989) both DE 1 and Activny were in the southern hemisphere , and DE 1 was at relatively low altitudes (ranging from 6211 to 14,81 0 km), thus providing the best conjunction possibilities according to the ray tracing criteria developed above. On most days, Omega transmit ter signals as well as commonly occurring natural wave phenomena such as whistlers (0+) and hiss were clearly seen well above the background level, but there was no evidence of the Activny 1 s On/1 s Off patter n. Though no Activny signals were detected by the LWR on the DE 1 sate llite, the experimental constraints allow us to place an upper limit o n the total power radiated by the Activny transmitter in the whistler- mode. Using experimental parameters, and the minimum detectable signal level of 0.05 muV/m for LWR, we find the upper limit on the total pow er radiated by the Activny satellite in the whistler-mode to be approx imately 10 mW. Several recommendations for future space-based wave inj ection experiments are presented.