Progress on Coronal, Interplanetary, Foreshock, and Outer Heliospheric Radio Emissions

Iver H. Cairns , P. A. Robinson , and G. P. Zank, PASA, 17 (1), 22.

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Title/Abstract Page: Progress on Coronal, Interplanetary,
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OUTER HELIOSPHERIC RADIO EMISSIONS

Figure 4 illustrates schematically the radio emissions observed by the Voyager 1 and 2 spacecraft at heliocentric distances greater than 12 AU (Kurth et al. 1984, 1987, Cairns et al. 1992, Gurnett et al. 1993, Gurnett & Kurth 1995). The emissions have occurred in two large outbursts approximately 9 - 11 years apart, together with some weaker events. The emissions come in two classes, the ``transient'' emissions which drift to higher frequencies $\sim 3.5$ kHz from initial frequencies $\sim 2.4$ kHz over a time period $\sim 180$ days, and the ``2 kHz component'' which remains in the frequency range

$\sim 1.8 - 2.4$ kHz and lasts longer than the transient emissions ($\sim 2$ years). These emissions involve emitted powers

$\stackrel{>}{\scriptstyle\sim}10^{13}$ W and are the most powerful emissions generated in our solar system (Gurnett et al. 1993). In contrast, the radio emissions from the auroral regions and magnetospheres of Jupiter and Earth involve emitted powers

$\stackrel{<}{\scriptstyle\sim}10^{11}$ W and

$\stackrel{<}{\scriptstyle\sim}10^{9}$ W, respectively, and the most intense type II and III solar radio bursts involve powers

$\stackrel{<}{\scriptstyle\sim}10^{11}$ W (D.A. Gurnett and M.L. Kaiser, personal communications, 1997). Noting that the power in the solar wind's kinetic energy flux is

$\sim 5 \times 10^{16}$ W, it appears that a global interaction involving the solar wind is a natural way to account for the observed power.

McNutt (1988) suggested that the emissions are triggered by the arrival of solar wind disturbances in the vicinity of the termination shock or the heliopause. First interpreted in terms of unusually fast solar wind streams, the trigger is now widely accepted to be a fast-moving global region of compressed magnetic field and density which drives a shock in the outer heliosphere (Gurnett et al. 1993). These so-called global merged interaction regions (Burlaga et al. 1991), or GMIRs, result from the merging of multiple CMEs and associated shocks and magnetic field enhancement, produced by solar activity, in the distant solar wind beyond about 20 AU. The GMIRs produce major decreases in the flux of cosmic rays and other energetic particles (Forbush decreases), due to scattering and mirroring effects by the GMIR's enhanced and turbulent magnetic fields, that are observed as the GMIR passes the Earth and spacecraft. Gurnett et al. (1993) and Gurnett & Kurth (1995) demonstrated that the two major outbursts of radio emission started approximately 415 days after the two largest Forbush decreases measured thus far at Earth and that a GMIR with associated Forbush decreases and a shock wave was observed by multiple widely-spaced spacecraft between 1 and 53 AU for each radio event. Moreover, taking into account the measured shock speeds

$\sim 800 - 850$ km s-1, the observed time delays $\sim 415$ days, and plausible estimates for slowing of the shock beyond the termination shock, Gurnett et al. (1993) estimated that the source region lies

$\sim 110 - 180$ AU from the Sun. These distances are plausible for the heliopause (e.g., Zank 1999a).

Gurnett et al.'s (1993) model for the radio emissions thus involves the GMIR shock starting to produce fp and 2fp emission after it traverses the heliopause: the transient emissions come from a putative density enhancement near the nose of the heliopause, while the 2 kHz component comes from other regions of the outer heliosheath. Figures 3 and 10 illustrate this model, which can be tested directly using the output from global plasma simulations of the outer heliosphere (Zank 1999a, and references therein) and a model for a shock's propagation through the outer heliosphere.

Figure 10: Profiles of fp as a function of heliocentric distance R predicted by global simulations (Zank et al. 1996) along the nose direction, defined by the VLISM's velocity vector relative to our solar system, and at rightangles to that direction. These profiles have been modified near $R \sim 50$ AU by superposing the GMIR-driven shock wave hypothesized to produce the outer heliospheric radiation as fp and/or 2fp radiation near and beyond the heliopause.
\begin{figure} \begin{center} \psfig{file=Fig10.eps,height=10cm}\end{center}\end{figure}

The variations in fp with heliocentric distance predicted by Zank et al.'s (1996) global simulation code are shown in Figure 10; this simulation uses input parameters for the plasma and neutral characteristics of the solar wind and VLISM obtained from published observations. Current estimates for fp in the VLISM are 1.6 - 3.5 kHz (Zank 1999a). Figure 11 shows the dynamic spectrum predicted for a GMIR shock that produces fp and 2fp radiation in an upstream foreshock as it moves through the global 3-D plasma structures obtained from Zank et al.'s (1996) simulation code (Cairns & Zank 1999).

Figure 11: Theoretical predictions (Cairns & Zank 1999) for the dynamic spectra of radio emissions generated at fp and 2fp upstream of a shock moving isotropically at 600 km s-1 through the plasma environment given by the global simulations of Zank et al. (1996).
\begin{figure} \begin{center} \psfig{file=fig11.ps,angle=270,height=12cm}\end{center}\end{figure}

The shock is assumed to leave the Sun at the time origin and to move at a constant, isotropic speed of 600 km s-1.

A number of emissions can be identified in Figure 11 (Cairns & Zank 1999). First, the emissions below 500 Hz prior to day 280 are fp and 2fp emission from the undisturbed solar wind (the shock reaches the nose of the termination shock on day 280) while the emissions below 1 kHz after day 280 are 2fp emissions from the inner heliosheath (the band from 500 Hz to 1 kHz) and the superposition of fp emission from the inner heliosheath with emission from regions of the shock still in the undisturbed solar wind. Second, the relatively weak emissions drifting rapidly to higher frequencies (ranging from $\sim 1$ to 6 kHz) are produced when the shock moves up the density ramp at the heliopause (see Figure 10b). Different stripes correspond to emission from different angles $\theta$ relative to the axis direction in Figure 3. Third, the intense, relatively broadband, and slowly varying emissions in two bands with frequencies $\sim 3$ and 6 kHz are produced when the shock is in the outer heliosheath beyond the heliopause.

Comparing Figures 4 and 11, it is very appealing to interpret the 2 kHz component as fp radiation produced in the outer heliosheath, consistent with Gurnett et al.'s [1993] model (Cairns & Zank 1999). Note that current estimates of fp in the VLISM are

$\sim 1.6 - 3.5$ kHz (Zank 1999a), not inconsistent with Figure 11 and fp in Zank et al.'s (1996) simulations. In contrast, the only upwards-drifting emissions in Figure 11 occur when the shock moves up the density ramp at the heliopause; these emissions drift far too rapidly to be consistent with the observed time scale of transient emissions ($\sim 180$ days). A detailed explanation for the transient emissions therefore does not exist at the present time (Cairns & Zank 1999).

The simplest qualitative interpretation remains that transient emissions are generated when a GMIR shock moves up a density ramp beyond the heliopause (e.g., Gurnett et al. 1993), but the location and nature of the density ramp need to be identified. With the the heliopause density ramp itself apparently ruled out by Figure 11 (Cairns & Zank 1999) and the absence of Gurnett et al.'s (1993) putative density ramp in current steady-state simulations (Zank et al. 1996, Linde et al. 1998), new ideas are required. One possibility is that solar cycle effects lead to the generation of large scale density waves in the outer heliosheath (Zank 1999b) and that traversal of the density waves by the GMIR shock leads to the observed transient emissions. Further work on the theory and simulation of these emissions and the outer heliosphere is required, including the possible role of SGT, as well as on in situ observations of the plasmas, global plasma structures, and radio source regions in the outer heliosphere by the Voyager spacecraft and their successors.


Next Section: CONCLUSIONS
Title/Abstract Page: Progress on Coronal, Interplanetary,
Previous Section: TYPE II SOLAR RADIO
Contents Page: Volume 17, Number 1

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