JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. A4, PAGES 3513-3525, APRIL 1, 1989 ComputerSimulations of Cometary-Ion/Ion Instabilities and Wave Growth S. PETER GARY AND KAZUHIRO AKIMOTO Earth and Space Seienee• Di•ision, Los illamos National Laboratory, New Mexico DAN •rINSKE Applied Theoretical Physics Division, Los AlamosNational Laborato•, New Mexico The growth of electromagnetic fluctuations driven by newborn cometary ions is studied by means of hybrid computer simulations of homogeneous plasmas. The simulations axe one- dimensional with the direction of spatial vaxiation in the direction of the axnbientmagnetic field Bo. Newborn ions are injected into the simulations at a constant rate, and axe given a velocity relative to the solax wind which makesa nonzeroangle a with respectto Bo. For relatively strong free energy injection rates corresponding to oxygen ion injection at relatively small values of a, the fluctuating field energy density exhibits exponeatial temporal growth of the ion/ion right-hand resonant instability. At the end of this growth regime there is a peak value of the fluctuating magnetic fieldenergy density; at 0 ø _( a • 45 ø this quantity is approximately proportional to cos sa. For relatively weak free energy injection rates corresponding to oxygenion injection at relatively large a and proton injection at all a values, there are two distinct regimes. At early times the fluctuating field energy density exhibits lineax temporal growth, whereas at later times the fluctuating field energyapproaches a constaal, value in ax• asymptoticregime. The exponential growth regime corresponds to fluctuating magnetic field energy density spectra with clear peaks at the oxygen ion cyclotron resonance wave number in agreementwith observations.In contrast, spectra of the lineax temporal growth regime exhibit less distinct peaks at shorter wavelengths whichthen show a transferof wave energy to longer wavelengths (an apparent "inverse cascade") as the asymptotic regime is approad•ed. This absence of a clear spectral peak at the proton cy- clotron resonance frequency is sirnilax to cometaxy observations. The simulations and observations share two additional properties: a linear correlationbetweencometaxy proton energy density and proton resonant magnetic fluctuations, as well as a shift of magnetic fluctuation energy of the oxygen-ion/proton modes to shorter wavelengths and weaker amplitudes as a increases toward 90 ø . 1. INTRODUCTION At cometocentric distances beyond the bow wave or shock, cometsty ion densities are small compared to the solar wind density, and collisionless processes dominate the plasma physics. The appearanceof newborn cometsty ions represents the sudden introduction of a source of free en- ergy in the solar wind. Under such conditions, linear the- ory predicts that there may exist a variety of cometsty ion instabilities at wr < f•p, the proton cyclotron frequency [l/Vinske and Gary, 1986; Gary and l/Vinske, 1986; Gary and Schriver, 1987; Goldstein and l/Vong, 1987; Lakhina, 1987; Thorne and Tsurutani, 1987; Brinca and Tsurutani, 1987a, b, 1988a, b; Price et al., 1988; Gary and Madland, 1988]. Computer simulations of the distant cometsty environment show that these instabilities lead to significantly enhanced [e.g., Bame et al., 1986; Tsurutani and Smith, 1986a; Yu- moto et al., 1986; Johnstone et al., 1987; Glassmeier et al., 1989]and downstream [Glassmeier et al., 1987] of the bow wave or shock. Furthermore, recent research has provided a qualitative comparisonof theoretical predictions and exper- imental observations of the fluctuationamplitudes [Galeev et al., 1986; Gary et al., 1988; Le et al., 1989]. Computer simulations which model cometsty ion creation as a function of time may be subdivided into two cate- gories: the inhomogeneous plasma computations of Omidi and Winske [1986, 1987, 1988] which are primarily con- cerned with macroscopic issuessuch as mass loading and wave steepening to shock formation, and the homogeneous plasma studies of Gary et al. [1986a,1988]whichaddress the microscopic problems associated with ion distribution functions and fluctuation amplitudes. This paper describes plasmaand field fluctuations [Omidi et al., 1986; Omidi and our continuing research in the second category' a study of Winske, 1986, 1987; Wu et al., 1986; Gary et al., 1986a].Re- cent spacecraftvisits to cometsGiacobini-Zinner and Halley have observed such enhanced fluctuations both upstream Copyright 1989 by the American Geophysical Union. Paper number 88JA04307. 0148-0227/89 / 88JA-04307505.00 magnetic fluctuation amplitudes and spectra in the distant cometsty environment as a function of a, the angle between the solarwind flow velocity and the interplanetary magnetic field Bo = •.Bo, whichwe hereafter call the injectionangle. Linear theory of cometsty ion instabilities, reviewed in Section 2, shows that two kinds of modes are particularly important. If a = 0ø so that the cometsty ions initially form a beam distribution, and if cometsty ion densitiesare sufficiently tenuous, the ion/ion right-hand resonant insta- 3513