RADIO EMISSIONS FROM THE OUTER HELIOSPHERE

For nearly fifteen years the Voyager 1 and 2 spacecraft have been dete cting an unusual radio emission in the outer heliosphere in the freque ncy range from about 2 to 3 kHz. Two major events have been observed, the first in 1983-84 and the second in 1992-93. In both cases the onse t of the radio emission occurred about 400 days after a period of inte nse solar activity, the first in mid-July 1982, and the second in May- June 1991. These two periods of solar activity produced the two deepes t cosmic ray Forbush decreases ever observed. Forbush decreases are in dicative of a system of strong shocks and associated disturbances prop agating outward through the heliosphere. The radio emission is believe d to have been produced when this system of shocks and disturbances in teracted with one of the outer boundaries of the heliosphere, most lik ely in the vicinity of the the heliopause. The emission is believed to be generated by the shock-driven Langmuir-wave mode conversion mechan ism, which produces radiation at the plasma frequency (f(p)) and at tw ice the plasma frequency (2f(p)). From the 400-day travel time and the known speed of the shocks, the distance to the interaction region can be computed, and is estimated to be in the range from about 110 to 16 0 AU.