SIMULTANEOUS OBSERVATIONS OF VLF GROUND TRANSMITTER SIGNALS ON THE DE-1 AND COSMOS-1809 SATELLITES - DETECTION OF A MAGNETOSPHERIC CAUSTIC AND A DUCT

Khabarovsk transmitter signals (15.0 kHz, 48 degrees N, 135 degrees E) were observed on the high-altitude (similar to 15000 km) Dynamic Expl orer 1 (DE 1) and the low-altitude (similar to 960) km COSMOS 1809 sat ellites during a 9-day period in August 1989. On 7 out of 9 days the l inear wave receiver (LWR) on the DE 1 satellite detected signals from the Khabarovsk transmitter. In addition, the DE 1 satellite also detec ted signals from the Alpha transmitter (11.9-15.6 kHz) in Russia and a n Omega transmitter (10.2-13.6 kHz) in Australia, as well as natural V LF emissions such as hiss, chorus, whistlers, and wideband impulsive s ignals. On two days, August 23 and 27, 1989, observations of the Khaba rovsk transmitter signals were simultaneously carried out at high alti tude on the DE 1 satellite and at low altitude on the COSMOS 1809 sate llite. Analysis of data from these 2 days has led to several new resul ts on the propagation of whistler mode signals in the Earth's magnetos phere. New evidence was found of previously reported propagation pheno mena, such as (1) confinement of transmitter signals in the conjugate hemisphere at ionospheric heights (similar to 1000 km), (2) observatio n of direct multipath propagation on both DE 1 and COSMOS 1809, (3) de tection of ionospheric irregularities of less than or equal to 100 km scale size with a few percent enhancement in electron density, believe d to be responsible for the observed multipath propagation. We report the first detection of an exterior caustic surface near L similar to 3 .5 for VLF ground transmitter signals injected into the magnetosphere; the location of the caustic surface depended on the signal frequency, and the electric and magnetic fields decreased by several hundred dec ibels per L shell in the dark (shadow) side of the caustic. We also re port the first direct detection of a magnetospheric duct at L = 2.94 w hich was believed to be responsible for the ducted propagation of Khab arovsk signals observed on the COSMOS 1809 satellite; the measured duc t parameters were: Delta L similar to 0.06 and Delta N-e similar to 10 - 13%. The duct width at the equator was similar to 367 km. Our study also indicates that duct end points can extend down to at least simil ar to 1000 km. The peak electric and magnetic fields of ducted Khabaro vsk transmitter signals at similar to 1000 km were 520 mu V/m and 36 p T respectively. Estimated field strengths of these signals inside the duct at the geomagnetic equator were 57 mu V/m and 12 pT for electric and magnetic field respectively. The results of two-dimensional ray tr acing simulations were consistent with the observations of the nonduct ed whistler-mode propagation of Khabarovsk (15 kHz) and Alpha (11.9 kH z) signals from the transmitter location to the DE 1 and COSMOS 1809 s atellites. Our results have direct implications for the question of ac cessibility of waves injected from the ground to various regions of th e ionosphere and the magnetosphere. In situ measurements of electric a nd magnetic fields of Khabarovsk transmitter signals inside a duct may well prove to be the critical measurements needed to differentiate be tween the small signal and large signal theories of wave particle inte ractions.