A new look at the nature of comet Halley's LF electromagnetic waves: Giotto observations

Abstract

All of the comet Halley high-time resolution magnetic field data have been examined to determine the nature of the “turbulence” and its difference from that of comets Giacobini-Zinner and Grigg-Skjellerup. Although much of the wave appears unpolarized, occasionally there are intervals of clear order. We find several interesting new wave polarizations: arc-, left-hand/arc- (sunglass) and left-hand circular polarized waves (in the spacecraft frame). The former two types have separations of ∼120 s between individual pulses, indicating that the waves are generated from pickup of the H2O group cometary ions. The third type of waves occurs in a wave-train and may be a detached whistler packet. The unusual polarizations could be caused by wave refraction in the highly turbulent (and high β) Halley environment or by nonlinear evolution due to strong growth rates. It is noted that some of the large amplitude waves are non-planar. These results are further details of the “linear polarization” of Halley waves determined by previous coherency analyses, and may explain some of the evolution leading to its plasma turbulence.