The first radio emissions observed to come from the outer heliosphere were observed at frequencies
of 
kHz by the Voyager spacecraft in 1983 [Kurth et al., 1984]. At this time, the
spacecraft were just outside the orbit of Saturn, where the average solar wind plasma frequency is
kHz. Figure 20.5 presents the Voyager 1 plasma wave observations from 1982 until the
end of 1993.
Figure 20.5: Dynamic spectrum of Voyager 1 data, showing the 1983-1984 and 1992-1993 outbursts of
radiation from the outer heliosphere [Gurnett and Kurth, 1995]. The dark, very uniform band from
kHz is interference from the spacecraft power supply.
The radio emissions occurred in two major, sporadic outbursts, one in the period 1983-1984 and one in
the period 1992-1993, albeit with with several, minor events that are close to the noise level.
Two classes of radio emissions are identified: (1) ``transient emissions'' which drift steadily
upward in frequency from 
kHz to a maximum near 3.5 kHz, over a period of about
180 days; (2) the ``2 kHz component'' which remains in the frequency range 
kHz, shows no frequency drift, and is more uniform, longer-lasting, and slowly varying than
the transient emissions.
These are the most powerful radio emissions in our solar system, having a total power 
W.
This power is greater than that in Jovian radio emissions ( 
W), the Earth's
AKR ( 
W), and type III solar radio bursts. The most likely source of this power is the
solar wind's ram energy.
The emissions were quickly interpreted in terms of a source in the outer heliosphere beyond the
planets, based on Voyager observations in the Jovian and Saturnian magnetospheres, the frequencies and
intensity of the emissions, and the appearance of the emissions only after the average value of
in the solar wind decreased below about 2 kHz [e.g., Kurth et al., 1984; Macek et al., 1991].
Indeed the foreshock region sunward of the termination shock was considered in detail as the source
of the radiation [e.g., Macek et al., 1991; Cairns and Gurnett, 1992; Cairns et al., 1992], with the
theory involving foreshock electron beams, Langmuir waves and the production of and 
radiation
as in type III solar radio bursts (Lectures 10 and 11) and Earth's radiation. This
proposed source region is now considered most unlikely due to falling off as 
in the
solar wind, with 
Hz at 100 AU on average, making it very difficult to produce radiation at
frequencies of 2 - 4 kHz with this emission mechanism.
Figure 20.6 [Gurnett and Kurth, 1995] provides strong evidence for McNutt's [1988] idea that the radio events are triggered by solar wind disturbances when they reach the vicinity of the termination shock and heliopause.
Figure 20.6: Cosmic ray counts from the Deep River Neutron Monitor are compared with the flux densities of
the outer heliospheric radiation measured by Voyager 1 [Gurnett and Kurth, 1995].
The figure shows that each major radio event follows 
days after one of the two largest decreases
in the cosmic ray flux observed at Earth (Forbush decreases), both of which were associated with periods of
unusually high solar activity and multiple CMEs. Figure 20.7 demonstrates that a global disturbance developed
in the distant solar wind and then propagated further out [Gurnett et al., 1993].
Figure 20.7: Cosmic ray counts from the Deep River Neutron Monitor and the Pioneer and Voyager spacecraft are
compared as functions of heliocentric distance and time for the disturbance that apparently triggered
the 1992-1993 radio emissions [Gurnett et al., 1993].
This global disturbance results
from the merging of multiple CMEs and associated shocks and magnetic field enhancements into a single entity,
called a ``global merged interaction region'' (GMIR). This GMIR, at least, is preceded by a shock wave (Figure 20.7).
Taking the shock speeds 
for the GMIRs associated with the two radiation events, 
km s 
,
and the time delay 
between the onset of the radiation and the Forbush decrease at Earth the distance to
the source can be estimated from the obvious equation
Substituting these numbers into the equation yields 
AU. Correcting for the change in
shock speed across the shock, yields 
AU [Gurnett et al., 1993].
Gurnett et al.'s [1993] model for the radio emissions therefore involves the GMIR shock starting to produce
and radiation after it traverses the heliopause, with the transient emissions coming from a putative
density enhancement near the nose of the heliopause, while the 2 kHz component comes from other regions of the
outer heliosheath. This model can be tested directly using the plasma density structures obtained from modern global
simulations of the outer heliosphere. Figure 20.8 shows the dynamic spectrum predicted for a GMIR shock that
produces and radiation in an upstream foreshock as it moves through the global 3-D density
structures obtained from Zank et al.'s [1996] simulation code [Cairns and Zank, 1999].
Figure 20.8: Dynamic spectrum of and radiation generated upstream of a shock
moving with constant isotropic speed through the 3-D plasma density structures shown in Figures 20.3 and
20.4 [Cairns and Zank, 1999].
A number of emissions
can be identified in Figure 20.8. First, the emissions below 1 kHz are and emission from the
undisturbed solar wind (below 
Hz and especially prior to day 280) and the inner heliosheath
interior to the heliopause. Second, the emissions drifting rapidly from 
kHz are associated with
the shock moving up the density ramp at the heliopause. Third, the intense, uniform and slow varying
emissions with constant frequencies 
and 6 kHz are produced when the shock is in the outer heliosheath
beyond the heliopause. Note that current best estimates of in the VLISM yield 
kHz.
Comparing Figures 20.5 and 20.8, it is very appealing to interpret the 2 kHz component as
radiation from the outer heliosheath, consistent with Gurnett et al.'s [1993] model. Unfortunately, however,
the only drifting emissions in Figure 20.8 occur when the shock drifts up the heliopause density ramp and occur
far too rapidly to be consistent with the observed time scale for the transient emissions ( 
days).
At the present time, then it appears that a reasonable theoretical explanation exists for one of the two observed classes of radiation but not for the second class [Cairns and Zank, 1999]. For phenomena that lie on the true border between astrophysics and space physics this is not unexpected. A number of teams are working on this radiation and the plasma environment of the outer heliosphere. Further progress is therefore expected.