Evidence for Strong and Weak Turbulence Processes in the Source Region of a Local Type III Radio Burst

Abstract

We present the Ulysses UniÐed Radio and Plasma Wave Experiment (URAP) observations of a local type III radio burst and the associated plasma wave activity generated in the vicinity of the spacecraft. These observations represent the Ðrst clear evidence of (1) the occurrence of ion-acoustic waves as well as whistlers in close association with Langmuir waves, (2) the coexistence of strong turbulence processes (modulational instability and soliton formation) and weak turbulence processes (electrostatic decay instability), (3) the frequency ratio of the fundamental (F) and harmonic (H) components as 1 : 1.67, and (4) a very narrowband F component with relative bandwidth of *f/f D 6%, in comparison to that of the harmonic (H) component with *f/f D 38%. Based on energy considerations, it is shown that whistlers are not generated by any nonlinear e †ects involving Langmuir waves ; instead, along with Langmuir waves, they are excited directly by the type III electron beam. On the other hand, ion-acoustic waves are generated by the Langmuir waves through the electrostatic decay process, since (1) the observed frequencies agree with the predicted values, (2) the ion-acoustic occupation numbers are well below those of Langmuir waves, and (3) the normalized peak energy density of Langmuir waves is well W L /n e T e D 10~5 above the electrostatic decay threshold D7 ] 10~7. The occurrence of Langmuir waves as intense spikes with spatial scales of 300È3000 Debye lengths with peak energy densities well above the modulationalinstability threshold of D2 ] 10~7 suggest that the Langmuir waves are also saturated by strong turbulence processes. The implications of these observations for the emission mechanisms at and at f pe 2f pe and for beam stabilization processes are discussed.

Second Harmonic Emission during Solar Flares

“…Smallangle decay processes could in principle explain observations of relatively smaller frequencies, also present during type III bursts. An example of this are the low-frequency bursts detected by Ulysses in the 25È60 Hz range (see Thejappa & MacDowall 1998). A more general model, including all possible angles between incoming and outgoing Langmuir waves, is certainly necessary to obtain more deÐ-nite results.…”

Interaction of suprathermal solar wind electron fluxes with sheared whistler waves: fan instability

“…In conclusion, one possible source of the whistlers observed in Ergun et al (1998) and Thejappa and MacDowall (1998) is the free energy of the suprathermal electron tail which can excite waves through the fan instability.…”

“…Let us now estimate if sheared whistler waves or lower hybrid waves can be excited in the solar wind by the mechanism of fan instability, considering, for example, typical solar wind parameters recorded in situ by space experiments during the observations of solar type III radio bursts Thejappa and MacDowall, 1998;Ergun et al, 1998) or of dispersive bursts of field-aligned electron fluxes in the Earth's auroral region (Ergun et al, 1993). For the first case, one chooses measurements from Ergun et al (1998) …”

“…Correspondence to: C. Krafft (Catherine.Krafft@lpgp.u-psud.fr) high-energy electron fluxes and low-frequency electrostatic and electromagnetic waves (Lin et al, 1981(Lin et al, , 1986Kellogg et al, 1992a, b;Stone et al, 1995;Reiner et al, 1992;Thejappa et al, 1995;Ergun et al, 1998;Thejappa and MacDowall, 1998;Moullard et al, 1998Moullard et al, , 2001; moreover, the simultaneous identification of energetic electron fluxes, Langmuir and whistler waves was performed by some of the cited experiments. It is commonly believed that the impulsive solar electrons ejected from the corona develop a streaming anisotropy as the faster electrons catch up to the slower ones, which results in the appearance of a bump (or beam) in the tail of the electron velocity distribution.…”

Resonant three-wave interaction in the presence of suprathermal electron fluxes

“…Moreover, several in-situ measurements have evidenced their presence simultaneously with suprathermal fluxes or beams of electrons; for example, they are believed to play a role in solar type III radio bursts (Lin et al, 1981(Lin et al, , 1986Kellogg et al, 1992aKellogg et al, , 1992bReiner et al, 1992;Stone et al, 1995;Thejappa et al, 1995; Correspondence to: C. Krafft (catherine.krafft@pgp.u-psud.fr) 1998; Thejappa and MacDowall, 1998;Moullard et al, 1998Moullard et al, , 2001) and in wave-particle processes occurring in the auroral ionosphere (Ergun et al, 1993;Muschietti et al, 1997) or in the terrestrial electron foreshock, for example (Hoppe et al, 1982;Zhang et al, 1999). Many questions deserve up to now to be raised concerning the generation mechanisms of the observed waves, the processes that govern the high-energy fluxes' evolution and the role of nonlinear waveparticle and wave-wave interactions.…”