AN INTERCOMPARISON OF PLASMA TURBULENCE AT 3 COMETS - GRIGG-SKJELLERUP, GIACOBINI-ZINNER, AND HALLEY

We examine and intercompare the LF plasma wave turbulence at three com ets: Grigg-Skjellerup (GS), Giacobini-Zinner (GZ), and Halley (H). All three have power spectral peaks at the local ion cyclotron frequency (the pump wave) at similar to 10(-2) Hz, and a power-law fall-off at h igher frequencies that suggest the development of turbulent cascades [ Acuna, 1986]. The power laws for the three comets are approximately f( -1.9), f(-1.9) and f(-2.1), respectively. However, other than the simi larities in the power spectra, we find the magnetic field turbulence i s considerably different at the three comets. Phase steepening is demo nstrated to occur at the trailing edges of the GS waves. This is proba bly due to nonlinear steepening plus dispersion of the left-hand mode components. A coherency analysis of GZ turbulence indicates that it is primarily composed of right-handed mode components, i.e., the turbule nce is ''whistler-mode''. This too can be explained by nonlinear steep ening plus dispersion of the magnetosonic waves. At the level of GS an d GZ turbulence development when the spacecraft measurements were made , classical three-wave processes, such as the decay or modulation inst abilities do not appear to play important roles. It is most likely tha t the nonlinear steepening and dispersive time scales are more rapid t han three-wave processes, and the latter had not had time to develop f or the relatively ''new'' turbulence. The wave turbulence at Halley is linearly polarized. The exact nature of this turbulence is still not well understood at this time. Several possibilities are suggested, bas ed on our preliminary analyses.