Iver H. Cairns , P. A. Robinson , and G. P. Zank, PASA, 17 (1), 22.
Next Section: OUTER HELIOSPHERIC RADIO EMISSIONS
Title/Abstract Page: Progress on Coronal, Interplanetary,
Previous Section: EARTH'S FORESHOCK
TYPE II SOLAR RADIO BURSTS
Coronal type II solar radio bursts often appear as two bands with a frequency ratio 
in the range 
MHz that drift slowly downwards in frequency at a rate consistent with an MHD shock moving through the solar corona and driving radiation near fp and 2fp (Wild 1950, Nelson & Melrose 1985). Similar, slow-drift emissions have been observed in the solar wind at frequencies from 
kHz - 20 MHz and directly associated with shock waves measured in situ by spacecraft (Cane et al. 1982, 1987, Lengyel-Frey 1992, Reiner et al., 1997, 1998). It has long been hypothesised that the type II radiation is produced at fp and 2fp by nonlinear processes involving Langmuir waves which are driven by electron beams accelerated at the shock (Wild 1950, Nelson & Melrose 1985). Indeed, Cairns (1986a) pointed out that in the rest frame of the propagating shock the situation is qualitatively identical to Earth's bow shock and that the reflection/acceleration of electrons into the foreshock region upstream of the type II shock should result naturally in electron beams by the same physics described above for Earth's foreshock. This suggests that a foreshock model for type II bursts is the natural way to develop the theory. However, as described next, multiple unresolved observational questions have hindered the development of a theory for type II bursts and it is only in the last three years that data from the Wind spacecraft have started to clarify the situation significantly.
Direct experimental proof that type II bursts are generated in foreshock sources upstream from shock waves is provided by two recent lines of evidence. First, Reiner et al. (1997, 1998) showed that the frequency of type II radiation varies inversely with time in the solar wind, corresponding to an emitter radiating at fp and 2fp while moving outward with constant speed in the solar wind (whose density varies inversely with heliocentric distance squared in steady-state). They found that the radiation frequency reached the local plasma frequency at approximately the same time as an interplanetary, CME-driven shock was observed in situ. These observations are consistent with generation of fp and 2fp radiation in a foreshock region directly upstream from the shock. However, Reiner et al. (1998) presented no evidence that the spacecraft had passed through an active source region. Second, Bale et al. (1999) provided the first direct observations of energetic electrons and bursty Langmuir waves in an active type II source region upstream of a CME-driven shock. These observations specifically show the production of radiation in a source region moving toward the spacecraft, the arrival of energetic electrons first anti-parallel to 
and then parallel to which drove high levels of Langmuir waves, and the disappearance of the streaming electrons and Langmuir waves when the shock wave passed over the spacecraft. Figure 9 illustrates these results schematically, demonstrating obvious strong similarities to Earth's foreshock.
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Unresolved issues include the following. (1) Do multiple classes of interplanetary type II bursts exist, associated with different spectral properties (broadband, smooth events detected by ISEE-3 (Cane et al. 1982) versus Reiner et al.'s (1997, 1998) narrowband, intermittent events measured by Wind) or shock waves interacting with different structures in the solar wind such as co-rotating interaction regions (CIRs) or dense filaments (Reiner et al. 1997)? (2) Why are the source regions of interplanetary type II events so rarely encountered and why are most interplanetary and coronal type II events bursty and intermittent? (3) Why are most coronal and interplanetary type II bursts apparently associated with distinct shocks, as argued on statistical grounds (Gopalswamy et al. 1998) but more compellingly by frequency-time analyses which often show distinct starting times and shock speeds for the coronal and interplanetary exciters (Reiner & Kaiser 1999)? These authors suggest that coronal type II bursts are associated with blast wave shocks while the interplanetary events are generated by CME-driven shocks. A plausible but simple explanation is that perhaps blast waves typically use up their energy (by heating, compressing, and accelerating the plasma particles) and cease to exist at relatively low altitudes, while CMEs provide a large inertia and energy reservoir to drive shocks far into the interplanetary medium (4) How are the multiple fine structures (Nelson & Melrose 1985) of coronal and interplanetary type II bursts produced? (5) Can the combination of SGT and nonlinear processes explain the growth of Langmuir waves and radio emission in type II sources, as expected by analogy with Earth's foreshock and type III bursts? Further research is required to resolve these issues. It is anticipated that the next few years will lead to a greatly improved theoretical understanding of type II bursts, both in the corona and the solar wind.
Next Section: OUTER HELIOSPHERIC RADIO EMISSIONS
Title/Abstract Page: Progress on Coronal, Interplanetary,
Previous Section: EARTH'S FORESHOCK
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