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,
Contents Page: Volume 17, Number 1


Type III bursts typically start in the corona at frequencies of order 100 MHz, often as fundamental and harmonic bands differing in frequency by a factor $\sim 2$, and then drift downwards in frequency as the driving electrons move out into the increasingly dilute plasma of the solar wind (Figures 1 and 2). Interplanetary type III bursts almost never show fundamental/harmonic structure. Both coronal and interplanetary bursts usually have brightness temperatures in excess of 1010 K, with a maximum of order 1015 K, thereby requiring a coherent emission mechanism. More complete details of the radio emissions and early interpretations are provided in other reviews (Suzuki & Dulk 1985, Melrose 1980, Goldman 1983). It is now well accepted that type III bursts are associated with electron beams (Lin et al. 1981, 1986), which develop as faster electrons outrun slower electrons to form a localised hump in the electron distribution at velocities parallel to the magnetic field which are much larger than the electron thermal speed in the solar wind. Also known as bump-on-tail distributions, these beams drive electrostatic Langmuir waves which are observed to be extremely bursty (Figure 5a) (Gurnett & Anderson 1976, Lin et al. 1981, 1986, Robinson et al. 1993, Cairns & Robinson 1995a), with electric fields that often vary by more than two orders of magnitude from one sample to the next (separated by $\sim 0.5$ s).

Figure: (a) The time-varying electric fields observed by the ISEE-3 spacecraft for bursty Langmuir waves (31.1 kHz data) and associated ion acoustic-like waves at low frequencies (100 - 311 Hz data) during a type III burst on 17 February 1979 (Cairns & Robinson 1995a). Electromagnetic radiation from the type III burst provides the smooth, time-varying ``background'' in the 31.1 kHz data. (b) The observed probability distribution $P(\log E)$ of wave fields E for the Langmuir data in part (a) are shown as the solid line, while the upper and lower dashed curves show fits to the predictions of SGT without (Eq. (1)) and with, repectively, a nonlinear process active at high fields.
\begin{figure} \begin{center} \psfig{,angle=270,height=10cm}\psfig{file=fig5b.eps,height=7.0cm}\end{center}\end{figure}

A fraction of the Langmuir wave energy is transformed into radiation near fp and 2fp.

Figure 5b demonstrates that SGT describes very well the bursty Langmuir waves observed in the source region of one interplanetary type III burst (Robinson et al. 1993). The solid curve shows the $P(\log E)$ distribution calculated from the observed wave fields (Figure 5a). The upper dashed curve shows Eq. (1)'s prediction for simple SGT, while the lower dashed curve shows the SGT prediction including a nonlinear process at high electric fields which removes energy from the Langmuir waves. Excellent agreement between SGT and the data is evident. Figure 5 and similar figures for two other well-observed type III bursts argue strongly that (1) SGT applies and describes the bursty Langmuir waves very well, and (2) a nonlinear process occurs at high fields

$E \stackrel{>}{\scriptstyle\sim}2$ mV m-1.

In contrast to these successes for SGT, the data are strongly inconsistent with the standard model for plasma wave growth: first, the observed $P(\log E)$ distribution is clearly not flat and, second, most of the wave fields are small compared with the calculated thresholds $\ge 1$ mV m-1 for nonlinear processes (Robinson et al. 1993). Figure 5 is also inconsistent with the strong Langmuir turbulence process of wave collapse saturating the wave growth, since this process should produce a power-law tail at high E in the $P(\log E)$ distribution (Robinson and Newman 1990). Despite early enthusiasm (Papadopoulos, Goldstein & Smith 1974, Smith, Goldstein & Papadopoulos 1979, Thejappa et al. 1993), detailed tests of collapse theory against the observed Langmuir waves provide multiple strong arguments against collapse occurring frequently or playing a significant role in type III bursts (Robinson et al. 1993, Cairns & Robinson 1995b, 1998, Robinson 1997).

In conjunction with SGT, a particular nonlinear process, the Langmuir wave decay

$L \rightarrow L' + S$, plays a strong role in explaining the plasma waves and radio emissions of type III bursts; this process involves the decay of a Langmuir wave L into a backward-propagating Langmuir wave L' and an ion acoustic wave S. The threshold field $\sim 2$ mV m-1 inferred from the observed $P(\log E)$ distribution in Figure 5b is consistent with analytic theory for the Langmuir wave decay (Robinson et al. 1993). Moreover, the timing and detailed frequencies of a class of low frequency (

$\sim 100 - 500$ Hz) waves observed in association with intense bursts of Langmuir waves (Figure 5a) can be explained in terms of S waves produced in the Langmuir wave decay (Robinson et al. 1993, Cairns & Robinson 1995a,b). The Langmuir wave decay also produces the backscattered waves L' necessary for the standard 2fp emission process

$L + L' \rightarrow T(2f_{p})$, where T represents a radio wave, and stimulates the emission of fp radiation in the process

$L \rightarrow T(f_{p}) + S'$ through the high levels of S waves in the source plasma.

A detailed theory for the dynamic spectra of type III bursts in the corona and solar wind has recently been developed and compared with observations (Robinson & Cairns 1998a,b,c). This theory includes: (1) analytic models for radial and temporal variations in the parameters of the electron beam and the associated energy available for the Langmuir waves; (2) analytic estimates for the efficiencies with which the processes

$L \rightarrow L' + S$,

$L + L' \rightarrow T(2f_{p})$, and

$L \rightarrow T(f_{p}) + S'$ convert Langmuir wave energy into fp and 2fp radiation, as functions of the beam parameters and wave levels; and (3) analytic descriptions of the large-angle scattering of radiation by density turbulence inside the source and during propagation to the observer. Figure 6 (Robinson & Cairns 1998b) shows the dynamic spectrum predicted for an observer at 1 AU, with the only inputs being the characteristics of the electron beam and density turbulence obtained from independent data.

Figure 6: Dynamic spectrum predicted by the detailed SGT model of Robinson & Cairns (1998a,b,c), as described further in the text.
\begin{figure} \begin{center} \psfig{,height=10cm}\end{center}\end{figure}

These predictions closely resemble the observations over the entire frequency range, have volume emissivities and brightness temperatures consistent with observations, can explain the time scales for the exponential rise and decay of radiation at a given frequency from $\sim 100$ MHz to $\sim 30$ kHz, and can explain the existence and frequency ratios of fundamental/harmonic bands in the corona and their typical absence in the solar wind.

The SGT theory for type III bursts is thus well developed and has passed successfully all the observational tests yet attempted. So far, however, these detailed tests involve three well-defined type III bursts detected by the ISEE-3 spacecraft, higher time-resolution data from the Ulysses and Galileo spacecraft, and comparisons with the radial and temporal variations and the ranges of observed brightness temperature, volume emissivity, and flux of observed type III bursts (Robinson et al. 1993, Cairns & Robinson 1995a,b, Robinson & Cairns 1998a,b,c, and references therein). One question raised recently (see next section) is whether an alternative theory for the radio emission processes, involving linear mode conversion and reflection processes in density gradients, is sometimes relevant. The time is now ripe for testing the SGT theory with (1) a large sample of type III bursts, (2) high time-resolution data on individual Langmuir wave packets from the Wind spacecraft (Bale et al. 1997, Kellogg et al. 1999), and (3) polarization data.

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

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