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Radio emissions from the outer heliosphere

The first radio emissions observed to come from the outer heliosphere were observed at frequencies of tex2html_wrap_inline524 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 tex2html_wrap_inline526 kHz. Figure 20.5 presents the Voyager 1 plasma wave observations from 1982 until the end of 1993.

  figure135
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 tex2html_wrap_inline352 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 tex2html_wrap_inline530 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 tex2html_wrap_inline534 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 tex2html_wrap_inline536 W. This power is greater than that in Jovian radio emissions ( tex2html_wrap_inline538 W), the Earth's AKR ( tex2html_wrap_inline540 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 tex2html_wrap_inline354 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 tex2html_wrap_inline354 and tex2html_wrap_inline356 radiation as in type III solar radio bursts (Lectures 10 and 11) and Earth's tex2html_wrap_inline356 radiation. This proposed source region is now considered most unlikely due to tex2html_wrap_inline354 falling off as tex2html_wrap_inline362 in the solar wind, with tex2html_wrap_inline554 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.

  figure149
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 tex2html_wrap_inline556 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].

  figure153
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 tex2html_wrap_inline558 for the GMIRs associated with the two radiation events, tex2html_wrap_inline560 km s tex2html_wrap_inline396 , and the time delay tex2html_wrap_inline564 between the onset of the radiation and the Forbush decrease at Earth the distance to the source can be estimated from the obvious equation

equation159

Substituting these numbers into the equation yields tex2html_wrap_inline566 AU. Correcting for the change in shock speed across the shock, yields tex2html_wrap_inline568 AU [Gurnett et al., 1993].

Gurnett et al.'s [1993] model for the radio emissions therefore involves the GMIR shock starting to produce tex2html_wrap_inline354 and tex2html_wrap_inline356 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 tex2html_wrap_inline354 and tex2html_wrap_inline356 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].

  figure166
Figure 20.8: Dynamic spectrum of tex2html_wrap_inline354 and tex2html_wrap_inline356 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 tex2html_wrap_inline354 and tex2html_wrap_inline356 emission from the undisturbed solar wind (below tex2html_wrap_inline586 Hz and especially prior to day 280) and the inner heliosheath interior to the heliopause. Second, the emissions drifting rapidly from tex2html_wrap_inline496 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 tex2html_wrap_inline590 and 6 kHz are produced when the shock is in the outer heliosheath beyond the heliopause. Note that current best estimates of tex2html_wrap_inline354 in the VLISM yield tex2html_wrap_inline596 kHz.

Comparing Figures 20.5 and 20.8, it is very appealing to interpret the 2 kHz component as tex2html_wrap_inline354 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 ( tex2html_wrap_inline600 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.


next up previous
Next: Concluding remarks Up: The Outer Heliosphere Previous: The termination shock

Iver Cairns
Wed Oct 20 15:39:59 EST 1999