FREQUENCY DRIFT OF 3-KHZ INTERPLANETARY RADIO EMISSIONS - EVIDENCE OF FERMI ACCELERATED TRAPPED RADIATION IN A SMALL HELIOSPHERE

NEITHER the termination shock wave formed where the solar wind ceases to be supersonic, nor the slightly more distant heliopause, where the wind runs into the interstellar medium, have been directly observed, but estimates based on observed cosmic-ray modulations and on pressure balance between the two media1 suggest that they are 50-200AU from the Sun. We argue here that the well-known interplanetary radio emission of 2-3 kHz in frequency 2-4 is trapped in the electromagnetic cavity formed by the heliopause5,6, and furthermore that the fluctuating solar wind will cause the frequency of this trapped radiation to increase at a rate dependent on the geometry of the cavity. Applying this interpretation to the previously unexplained frequency drift, amounting to ∼1 kHzyr-1, of the 3-kHz burst4, we estimate an average heliopause distance of 60-100 AU.This agrees with recent data from Pioneer 10 and Voyager 2, suggesting that the termination shock is located at a distance of ∼50 AU, and implies that Voyager 1 may reach the shock in about 1993 and the heliopause as early as 1996. © 1990 Nature Publishing Group.