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

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

Title/Abstract Page: Progress on Coronal, Interplanetary,
Previous Section: Progress on Coronal, Interplanetary,
Contents Page: Volume 17, Number 1


Intense radio emissions are generated in numerous regions of our solar system, including the solar corona and solar wind, regions near shock waves, the magnetospheres and auroral regions of Earth, Jupiter, and the gaseous outer planets, and the regions of the outer heliosphere where the solar wind interacts with the local interstellar medium (e.g., Melrose 1980, 1986, McLean & Labrum 1985, Benz 1993, Kurth et al. 1984, Gurnett et al. 1993, Zarka 1998). Two basic generation mechanisms are currently believed to be responsible for these emissions, both of them involving collective plasma effects and not the single-particle process of synchrotron emission currently favoured in most theories for astrophysical radio sources. The first mechanism, cyclotron maser emission (Wu & Lee 1979, Melrose 1986, Benz 1993, Zarka 1998), involves the direct generation of x- and o-mode radiation near the electron gyrofrequency fce or its harmonics by a plasma instability driven by semi-relativistic electrons. This mechanism is believed responsible for radio emissions from the auroral regions of Earth (AKR), Jupiter and the gaseous outer planets (e.g., Melrose 1976, 1986, Wu & Lee 1979, Zarka 1998), as well as for solar ``spike'' bursts in the microwave and decimetric bands (e.g., Melrose 1986, Benz 1993). In each of these source regions, and in general for the mechanism to be effective for direct emission into escaping radiation, the electron gyrofrequency exceeds the electron plasma frequency fp (i.e.,

$f_{ce} \ge f_{p}$) and the electrons have excess energy in their motions perpendicular to the magnetic field. The second mechanism is often called called ``plasma emission'' or ``radiation at multiples of the plasma frequency'' (Ginzburg & Zheleznyakov 1959, Melrose 1980, 1986, Benz 1993) since it involves the generation of free-space radiation near fp and near 2fp; this mechanism involves a sequence of steps in which excess electron energy is first converted by a plasma instability into electrostatic Langmuir waves with frequencies near fp, after which Langmuir wave energy is converted into fp and 2fp radiation by various linear or nonlinear plasma processes.

The primary aim of this paper is to summarise the progress made in the last five years in understanding the generation and properties of type II and III solar radio bursts in the corona and solar wind, radiation from the foreshock region upstream of Earth's bow shock, and low-frequency radiation observed by the Voyager spacecraft in the outer heliosphere. These emissions are all either observed or believed to be radiation produced at multiples of fp. The properties and source environments of these emissions are briefly introduced next; they are described in more detail in Sections 3 - 6 below.

For close to 50 years Type III bursts have been associated (Figure 1) with electron beams released during solar flares that stream from the corona into the solar wind and drive Langmuir waves and radiation at fp and 2fp (Wild 1950, Ginzburg & Zheleznyakov 1959) as discussed by Melrose (1980), Goldman (1983), and Suzuki & Dulk (1985).

Figure 1: Schematic illustration of the source regions and phenomena involved in type II and III solar radio bursts in the corona and solar wind and those observed in Earth's foreshock.
\begin{figure} \begin{center} \psfig{file=Fig1.eps,height=9cm}\end{center}\end{figure}

The electron beams, Langmuir waves and radio emissions have now all been observed in situ (e.g., Gurnett & Anderson 1976, Lin et al., 1981, 1986). Similarly, for over 20 years, the foreshock region upstream of Earth's bow shock but downstream from the tangent solar wind field line has been observed to contain energetic electrons which stream away from the bow shock, driving Langmuir waves and fp and 2fp radio emissions. Type II radio bursts have long been interpreted in terms of electron beams accelerated by shock waves (Wild 1950), which then drive Langmuir waves and fp and 2fp radiation, but observational evidence has been elusive (Nelson & Melrose 1985). Very recently, however, Bale et al. (1999) and Reiner et al. (1997, 1998) have reported convincing evidence that the radiation is produced in an upstream foreshock region, strongly analogous to Earth's foreshock, as suggested previously on theoretical grounds (Cairns 1986a). Figure 2 shows schematic dynamic spectra for these three classes of emissions.

Figure 2: Schematic dynamic spectra for type II and III bursts and the fp and 2fp radiation from Earth's foreshock.
\begin{figure} \begin{center} \psfig{file=Fig2.eps,height=10cm}\end{center} \end{figure}

Since the type III electrons move at speeds

$\sim (0.1 - 0.3) c$ that are much faster than the type II shock (

$\sim 500 - 1000$ km s-1), and since the solar wind plasma frequency varies inversely with heliocentric distance R (in steady-state), the type III radiation drifts much more rapidly to low frequencies than the type II radiation. Note that the type III radiation tends to be relatively continuous and broadband, type II bursts tend to be very intermittent but to show distinct fp and 2fp bands, while the foreshock 2fp radiation is present relatively continuously.

Figure 3 is a schematic of the large-scale structures predicted to result in the outer heliosphere from the interaction of the superalfvenic, supersonic solar wind and the plasma of the Very Local Interstellar Medium (VLISM): the solar wind undergoes a shock transition to a subalfvenic flow at the termination shock and is then deflected around the heliopause, a contact discontinuity which separates the (shocked) solar wind and (possibly shocked) VLISM plasmas (e.g., Zank 1999a).

Figure 3: The plasma boundaries associated with the solar wind's interaction with the VLISM: the termination shock, the heliopause, the (possible) outer bow shock, and the inner and outer heliosheath regions inside and outside, respectively, the heliopause. The dashed circle represents a global, transient shock wave, moving away from the Sun into the outer heliosphere, which may drive the outer heliospheric radio emissions.
\begin{figure} \begin{center} \psfig{file=Fig3.eps,height=9cm}\end{center}\end{figure}

An outer bow shock will exist if the VLISM plasma flows superalfvenically relative to our solar system. The inner heliosheath, between the termination shock and the heliopause, contains shocked solar wind plasma while the outer heliosheath, between the heliopause and the (possible) outer bow shock, will contain shocked VLISM plasma. The solar system moves relative to the VLISM plasma at a speed of $\sim 26$ km s-1 along the axis of symmetry in Figure 3, leading to the minimum distances between the Sun and the termination shock, heliopause, etc. lying along this axis. The outer heliospheric radio emissions occur in sporadic, transient outbursts (Kurth et al. 1984, 1987, Gurnett et al. 1993) which have been associated with global shock waves reaching the vicinity of the heliopause and then producing radio emissions (Gurnett et al. 1993); Figure 3 illustrates schematically such a moving shock. Figure 4 shows schematic dynamic spectra for the radio emissions, which are widely interpreted in terms of two components (Cairns, Kurth, & Gurnett 1992): the ``transient component'' which drifts upwards in frequency with time and the ``2 kHz component'' which remains approximately fixed in frequency and lasts longer.

Figure 4: Schematic dynamic spectra of the radio emissions observed by the Voyager spacecraft in the outer heliosphere, showing the transient emissions and the 2 kHz component.
\begin{figure} \begin{center} \psfig{file=Fig4.eps,height=9cm}\end{center}\end{figure}

Current estimates for fp in the VLISM lie in the range 1.6 - 3.5 kHz (Zank 1999a).

These four classes of emission are naturally grouped together for several reasons, despite their widely different locations and plasma environments. First, type II and III bursts and the foreshock radiation are all observed to be generated near fp and 2fp in the source region, as is also believed (but not yet demonstrated) for the outer heliospheric emissions. Second, recent observations (Reiner et al. 1998, 1999, Bale et al. 1999) show that interplanetary type II bursts are produced in foreshock regions upstream of certain travelling interplanetary shocks, directly analogous to the radiation from Earth's foreshock, as also postulated for the outer heliospheric emissions (Gurnett et al. 1993, Zank et al. 1994). Third, all four emissions are either observed or believed to be associated with electron beams and the Langmuir waves they drive. Finally, a new theory, stochastic growth theory (SGT), can explain in detail the Langmuir waves and electron beams of type III bursts and Earth's foreshock (Robinson 1992, 1995, Robinson, Cairns & Gurnett 1993, Cairns & Robinson 1997, 1998, 1999) and is a natural theory to apply to type II bursts and the outer heliospheric radio emissions for the reasons given above. The secondary aim of this paper is to summarise the major ideas of SGT and its successes, an important goal since it is envisaged that SGT may be widely applicable to plasma wave and radiation phenomena in astrophysics and space physics.

The remainder of the paper is structured as follows. Section 2 describes the ideas and theoretical predictions of SGT. Sections 3 to 6 summarise the progress made in understanding the four classes of emissions identified above, as well as the questions that currently remain unanswered. The conclusions are contained in Section 7.

Title/Abstract Page: Progress on Coronal, Interplanetary,
Previous Section: Progress on Coronal, Interplanetary,
Contents Page: Volume 17, Number 1

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