Ih. Cairns et Pa. Robinson, ION-ACOUSTIC-WAVE FREQUENCIES AND ONSET TIMES DURING TYPE-III SOLAR RADIO-BURSTS, The Astrophysical journal, 453(2), 1995, pp. 959-972
Conflicting interpretations exist for the low-frequency ion acoustic (
S) waves often observed by ISEE 3 in association with intense Langmuir
(L) waves in the source regions of type III solar radio bursts near 1
AU. Two indirect lines of observational evidence, as well as plasma t
heory, suggest they are produced by the electrostatic (ES) decay L -->
L' + S. However, contrary to theoretical predictions, an existing ana
lysis of the wave frequencies instead favors the electromagnetic (EM)
decays L --> T + S, where T denotes an EM wave near the plasma frequen
cy. This conflict is addressed here by comparing the observed wave fre
quencies and onset times with theoretical predictions for the ES and E
M decays, calculated using the time-variable electron beam and magneti
c field orientation data, rather than the nominal values used previous
ly. Field orientation effects and beam speed variations are shown anal
ytically to produce factor-of-three effects, greater than the differen
ce in wave frequencies predicted for the ES and EM decays; effects of
similar magnitude occur in the events analyzed here. The S-wave signal
s are extracted by hand from a sawtooth noise background, greatly impr
oving the association between S waves and intense L waves. Very good a
greement exists between the time-varying predictions for the ES decay
and the frequencies of most (but not all) wave bursts. The waves occur
only after the ES decay becomes kinematically allowed, which is consi
stent with the ES decay proceeding and producing most of the observed
signals. Good agreement exists between the EM decay's predictions and
a significant fraction of the S-wave observations while the EM decay i
s kinematically allowed. The wave data are not consistent, however, wi
th the EM decay being the dominant nonlinear process. Often the observ
ed waves are sufficiently broadband to overlap simultaneously the freq
uency ranges predicted for the ES and EM decays. Coupling the dominanc
e of the ES decay with this frequency overlap provides support for a p
revious suggestion that fundamental emission occurs when the EM decay
is stimulated by the ES decay product waves. The periods in which the
ES and EM decays produce observable S waves are consistent with the ob
served and (independently) predicted times of fundamental and harmonic
radio emission. This supports interpretation of fundamental emission
as stimulated EM decay and harmonic emission as the coalescence L + L'
--> T of beam-generated L waves and L' waves produced by the ES decay
, where T denotes an electromagnetic wave at twice the plasma frequenc
y. Analysis of the electron beam data reveals that the time-varying be
am speed is consistent with ballistic beam propagation with minimal en
ergy loss, implying that the beam propagates in a state close to time-
and volume-averaged marginal stability. This confirms a central tenet
of the stochastic growth theory for type III bursts. The observationa
l data are thus shown to be uniformly consistent with the ES decay bei
ng the dominant nonlinear process for type III bursts near 1 AU, as ex
pected theoretically. Further work is required to determine whether th
e ES decay appreciably limits the beam's energy loss over and above th
e effects of linear stochastic growth theory.