JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. A3, PAGES 3449-3454, MARCH 1, 1995 Pickup proton cyclotron turbulence at comet P/Halley G. S. Lakhina Indian Institute of Geomagnetism, Bombay Frank Verheest Sterrenkundig Observatorium, Universiteit Gent, Gent, Belgium Abstract. Low-frequency electromagnetic turbulence near the proton gyrofre- quencyobserved far upstream of comet P/Halley can be excited by a beam instability driven by relative streaming between cometary protons, solar wind protons, and water group ions. For a given solar wind velocity the growth rates peakat a certain optimum frequency shift (fromthe exactproton gyrofrequency), and the wavelengths involved can be deducedin a self-consistent way from the dispersion law. Only under ideal conditionswhen all parameters remain constant would the mode corresponding to the optimum frequencyshift grow fastest and might it be possible to observe a nearly constantfrequencymode. However, if the solar wind parameters were not constant, then a mode that was in resonance earlier would no longer remain so, and someother mode with a slightly different frequency shift might start to grow fastest, leading to a mixing of many modes. Thus only rarely would one be able to observe a single mode near the proton gyrofrequency, exactly as happensin the observations. Our self-consistent approachyields resonant instabilities with left-handed polarizations in the spacecraft frame. 1. Introduction Resonant or nonresonantlow-frequencyelectromag- netic instabilities are the natural explanation for the observed nonlinear levels of magneticfield turbulencein comefury environments [Neubauer et al., 1986;Riedler et al., 19//6; Smirk et al., 1986]. Such processes thermal- ize the streaming comefury ion distributions [$a!ideev et al., 1986,1987], leading to the pickup of comefury ions by the solar wind plasma [Carol et al., 1984,1985,1986; Winske et al., 1985; Winske and CarOl, 1986]. Higher- frequency instabilities have large initial growth rates but saturate at very low levels of turbulence. Even if we look only at the relative drifts between solar wind and comefury ions along the external mag- netic field B0 as the source of free energy, there still are several possibilities among the low-frequency modes. At the comets observedso far, P/Giacobini-Zinner, P/Halley, and P/Grigg-Skjellerup, most of the turbu- lence is characterizedby a spectral peak at the gyrofre- quency of the watergroup ions of comefury origin, and the preferredmechanism is cyclotronresonance between these pickup ions and the waves. Such resonant modes prevail far from the comefury nucleus, in the distant plasma environment, because they are the first onesto become unstable (see, for example, Lee[1989] and Tsu- Copyright 1995 by the American Geophysical Union. Paper number 94JA02907. 0148-0227/95/94JA-02907505.00 rutani [1991]). Closer to the cometary nucleus (espe- cially at distances smaller than 7 x 106km for comet P/Halley), nonresonant modes could become dominant [Lakhina, 1987;Lakhina and Verheest, 1988], although their very long wavelengths would make them very hard to observe in single-spacecraft missions. The identification of the water group ions has been rendered more easy by their notable mass difference with the protons which dominate the solar wind. How- ever, some of the observationsalso point to the possi- bility that proton cyclotron resonance might occur, as, for example, at comet P/Halley [Mazelie andNeubauer, 1993]. Single-cycle magnetic pulses or solitary waves at the proton gyrofrequency near comet P/Giacobini- Zinnerhavebeen reported by Tsurutani et al. [1989]. These observations, however, pertain essentially to bursts and not to fully developedwaves. An additional problem is that in order to fully iden- tify the modes responsible for the observedMHD tur- bulence, one would alsoneedwavelength measurements, in addition to frequencies, but this information is not available as yet. Pinpointing the modes involved was based solely on frequency measurements in the space- craft frame, only half of the criterion. In order to get someidea of the wavelengths, we re- centlyused the Giotto measurements at comet P/Grigg- Skjellerup, since these unambiguously indicated up- stream waves with periods very close to the cometary water groupion gyroperiod [Coates et al., 1993; John- stone et al., 1993; Neubauer et al., 1993]. We started from parallel modes, since these tend to be the easiest to excite, are the least damped, have the highestgrowth 3449