Abstract
Two major episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager spacecraft in 1983/84 /1/ and 1992/3 /2/. This higher-frequency component is apparently triggered by solar wind transients /3,4/ with sufficiently large spatial extents and energies to continue to propagate as shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather, it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution the shock moves as time to the 2 3 power /5/ and in the other as time to the 4 5 power. Using these models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the 1992 emission is consistent with an emission region distance of ∼130 AU (assuming no additional slowing of the shock in the heliosheath). If the termination shock was at ∼70 AU when the transient shock collied with it, the true distance to the source region was probably closer to ∼115 AU.
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