An 1 at Three Comets:

We examine and intcrcornpare the LF plasma wave turbulence at three comets: Cirjgg-Skjcllcrup, (CiS), Giacobini-Zinncr (GZ) and IIallcy (H). All three have power spectral peaks at the local ion cyclotron frequency (the pump wave) at 10-2 }Iz, and a power-law fall-off at higher frcqucncics that suggest the development of turbulent cascades (Acuna, 1986). The power law for the three comets are approximately f-].9, f-].9 and f-2 " ], respectively. However, other than the similarities in the power spectra, wc find the magnetic field turbulence is considerably different at the thre~ comets. Phase steepening is demonstrated to occur at the I.railing edges of the GS waves. This is probably 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 turbulence is " whistler-mode ". This too can be explained by nonlirwar stccpcning plus dispersion of the (rnagnctosonic waves). At the level of GS and GZ turbulence development when the spacecraft measurements were made, classical thrcc-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 arc more rapid than three-wave processes, and the latter had not had time to develop for the relatively " new " turbulence. The wave turbulence at }Iallcy is linearly polarized. The exact nature of this turbukmce is still not well understood at this time. Several possibilities arc suggested, based on our preliminary analyses. ]NT1WDUCTION Cometary waves provide us with our best opportunity in space plasma physics to study the dcvclopmcnt of plasma turbulence. In a steady flowing solar wind, instabilities associated with the pickup of freshly crcatcd ions will lead to electromagnetic wave power in a narrow frequency band. This frcc]ucncy is the local ion cyclotron frequency in the cometary rest frame (Tsurutani and Smith, 1986). Because spacecraft have had relatively low velocities relative to comets during their flybys, the spacecraft magnctomctcr rest frame is essentially the cometary frame. Thus, waves measured at frequencies higher and lower than the pump frequency (presumably duc to cascade and " inverse cascade " processes, rcspcctivcly) can bc easily studied, and the nature of the turbu]cncc established. " l'his situation dots not exist for other waves in space plasmas. Variable Doppler shifts smear out the pump frequency, and the " daughter " and " …