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Radio Emission from the Outer Heliosphere

In 1983-84, 1992-93 and 2003-04 the Voyager spacecraft observed high power radio emissions at 2-3 kHz originating in the outer heliosphere. The power of the radiation at the source has been estimated as being 10¹³ W, making it the most powerful solar system radio source. The radio emissions also act as a useful probe of the boundary between the solar wind and the interstellar medium.

Recently our group has proposed a theory for these emissions which involves combining (1) a radiation mechanism originally used for type II solar radio bursts, (2) priming of the electron velocity distribution in the emission region with a super-thermal tail via lower-hybrid drive, and (3) acceleration of electrons by a shock formed near the Sun that propagates to the heliopause and beyond during a period of high solar activity.

Our theory predicts the generation of radio emissions with similar intensities, frequencies, band widths and durations to those observed. We have also worked on predicting the timing of high power events and are able to explain these on theoretical grounds. Work is currently being undertaken on determining the precise location of the emission region based on the interstellar magnetic field and the efficiency of the priming process. We are also working on the transient component of the emission.

Publications

1. I.H. Cairns and G.P. Zank, Theories for the radio emission from the outer heliosphere, in COSPAR, The Outer Heliosphere, edited by K. Scherer, H. Fichtner, H.J. Fahr and E. Marsch, 253, Pergamon, Amsterdam, 2000.
   
2. I.H. Cairns and G.P. Zank, Turn-on of the 2-3 kHz radiation beyond the heliopause, Geophys. Res. Lett., 29(7), 47, 2002.
   
3. I.H. Cairns, J.J. Mitchell, S.A. Knock and P.A. Robinson, Towards a quantitative theory for the 2-3 kHz radio emissions from beyond the heliopause, Adv. Space Res., 34, 88, 2003.
   
4. J.J. Mitchell, I.H. Cairns and P.A. Robinson, Theory for the 2-3 kHz radiations from the outer heliosphere, J. Geophys. Res., 109, A06180, 2004.
   
5. J.J. Mitchell, I.H. Cairns, H.-R. Müller and G.P. Zank, Prediction of the timing of the 2-3 kHz radio emissions within the solar cycle, Geophys. Res. Lett., in press, 2005.

Other outer heliosphere projects

1. Priming the electron distribution with a superthermal tail
2. The propagation of radiation using ray tracing
3. Draping of the magnetic field over the heliopause

Figure. A dynamic spectrum of radio emission driven by a shock travelling out from the Sun. The predicted radiation has many properties which closely resemble the observations