An intercomparison of plasma turbulence at three comds :

Wc examine and intcrcompare the LF plasma wave turtmlcnce at three comets: Grigg-Skjellertrp (GS), Giacobini Zinncr (GZ), and Ilallcy (11). All three have power spectral peaks at the local ion cyclotron frequency (the pump wave) at -10 -2 IIz, and a power-law fall-off at higher frequencies that suggest the development of turbulent cascades [Acrm4 1986]. The power laws for the three comets are approximately f ’19, f -1.9 and f -2.1, respectively. However, other than the similarities in the power spectra, wc find the magnetic field turbulence is considerably different at the three comets. Phase steepening is demonstrated to occur at the trailing edges of the GS waves. This is probably due to nonlinear steepening plus dispersion of the left-hand mode components. A cohcrmrcy analysis of GZ turbulence indicates that it is primarily composed of righthanded mode components, i.e., the turbulence is “whistlermode”. This too can be explained by nonlinear steepening plus dispersion of the magnctosonic waves. At the level of GS and GZ turbulence development when the spacecraft measurements were made, classical three-wave processes, such as the decay or modulation instabilities do not appear to play important roles. It is most likely that the nonlinear steepening and dispersive time scales are more rapid than three-wave processes, and the latter had not had time to develop for the relatively “new” turbulence. Ilc wave turbulence at I [alley is linearly polarized. Tlc exact nature of this turbulence is still not well understood at this time. Several possibilities arc suggested, based on our preliminary analyses.