Comparison of the wave energy transport at comets p/Halley and p/Giacobini-Zinner

Using magnetic field, plasma density and flow observations from spacecraft flybys of two comets, Elsässer variables are determined in order to study wave propagation directions. The inbound path of the Giotto spacecraft flyby of comet p/Halley is investigated outside the bow shock (between 10:36 and 19:11 UT on March 13, 1986, corresponding to distances from 3.3 × 106 to 1.2 × 106 km from the nucleus) and the inbound and outbound path of the ICE spacecraft flyby of comet p/Giacobini-Zinner is investigated outside of the bow wave (between 00:00 and 9:24 UT and between 12:25 and 23:30 UT on September 11, 1985, corresponding to distances from 8.5 × 105 to 1.2 × 105 km from the nucleus and from 1.0 × 105 to 9.3 × 105 km from the nucleus, respectively). The interaction of cometary pick-up ions and the solar wind is expected to generate waves which are assumed to be dominated by field-aligned propagating Alfvén waves. This predicted wave propagation combined with observed wave modes generated by the ion cyclotron resonance instability are compared with observed wave properties. Three mhd-modes are possible: right-hand polarized waves propagating toward the Sun (RH−) and left-hand polarized waves propagating toward (LH−) and away from the Sun (LH+). Plasma conditions dictate that the RH− mode is the most unstable mode and satisfactory agreement is found between observed and predicted energy flow directions in regions outside the bow shock at Halley and the bow wave at Giacobini-Zinner, where local cometary ion pick-up conditions dominate.