JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. A9, PAGES 7331-7337, SEPTEMBER 1, 1984 Predictions of Lithium Interactions With Earth's Bow Shock in the Presence of Wave Activity R. B. DECKER, L. VLAHOS, 1 ANDA. T. Y. Lui Applied Physics Laboratory,Johns Hopkins University, Laurel, Maryland We present the results from a test-particle simulation designedto study how lithium tracer ions injected upstream of the earth's bow shock interactwith the bow shock when waves are present in the shock's upstreamand downstream vicinity. The wave activity is assumed to consistof parallel and antiparallel propagating Alfv•n waves characterized by a frequency power spectrum P(f) -,, f- • withina frequency intervaland rangeof amplitudes defined separately in the upstream and downstream regions. At a "single encounter"(defined in this paper as the time during which an ion remains within ,-,2 gyroradii of the shock) the waves act mainlyto perturban ion'sorbit, leading to an increase (or decrease) in the number of orbital shockcrossings, and therefore increase (or decrease) the energygain (via drift along the U x B electric field) relativeto the situationwhen waves are absent. Completetransmission of the injected ion distribution is predicted with and without wave activity present when the angle q• between the shock normal and the nominal upstream magnetic field satisfies 70 ø • q• < 90 ø. A wave field of sufficient amplitude should increase ion transmission for 45 ø •< q• •< 70 ø,where significant reflec- tion (--, 50%) is possible in the absence of waves. At a fixed q•, increasing wave activity yields larger averageenergy gains, reducedpitch angle anisotropies, and increased spatial dispersion on the bow shock for boththereflected ions andthose transmitted to themagnetosheath. 1. INTRODUCTION During the Active Magnetospheric Particle Tracer Ex- plorers (AMPTE) mission, at least two releases of • 3 x 1025 lithium-6 (Li6) atoms each are planned in the solar wind near the subsolar point upstream of the earth's bow shock [Kri- mi•tis et al., 1982]. Therefore, after theseatoms are photoion- izedto Li6+ ions,one of the initial concerns regarding mag- netospheric capture of tracer ions is how this ion population is modified in passing first from the ionization point to the bow shock and then through the bow shock. Previous studiesby Decker et al. [1983a], hereinafterreferred to as paper 1, and Haerendel and Papamastorakis [1983] have explored the consequences of releasedions encounteringthe bow shock. Paper 1 presented reflection and transmission coefficients, average energy gains and pitch angle information for an en- sembleof injected ions as a function of the gardenhose angle q• between the (assumed laminar) interplanetary magnetic field (IMF) and the shocknormal. In this paper we report results from a new model that is an improvement over that in paper 1 in the following three re- spects: (1) We have employed a more realistic algorithm to inject the Li6+ ions. (2) We have included wave activity in the form of parallel and antiparallel propagating Alfv6n waves characterized by a frequency power spectrum P(f)• f-•. (3) We have calculated spatial as well as velocity space distri- butions of ions reflected and transmittedat the shock to pro- vide input for modelsthat deal with magnetosheath transport [e.g., Brinca, 1983, 1984]. The main results of our study are summarized in section 4. 2. DESCRIPTION OF MODEL 2.1. Geometry Figure la shows the geometry used in the simulationmodel. The bow shock was assumed to be locally parallel to the Y-Z • Permanently at Astronomy Program, University of Maryland, CollegePark. This paperis not subject to U.S. copyright. Published in 1984by the AmericanGeophysical Union. Paper number 4A0565. plane over an area • 12 Re (Re = 1 earth radius) in diameter centered on the origin of the X, Y, Z system (X points to- wards the sun) at the subsolarpoint. The upstream (region 1) solar wind and downstream (region 2) magnetosheathbulk flow velocities are U• =-hU•(h = ,• = shock normal) and U2, respectively. In the absence of wave activity, the upsteam (downstream) magnetic field B0•(B02 ) points toward (away from) the shockat angle ½•(½2) to fi(-h), and there exists adc electric field E = -U• x Boric = -U2 x B02/c on either sideof the shock in the shockframe.We take U• and B0• (and therefore U2 and B02 ) to lie always in the X-Y plane, which is parallel to the geocentricsolar ecliptic (GSE) plane (i.e., the xGSE-y GsE plane)only when B0• is parallelto the ecliptic plane. If B0• has a component out of theecliptic (i.e., Bz GsE • 0), then results in yOSE-zOSE planecan be obtained from our results in the Y-Z by a simple rotation through the angle arctan (BzOSE/By osE) about the X or XGsE axis. In the presence of wave activity, B•(B2) is the superposition of B0•(B02 ) and the upstream(downstream) wave field. Given the averageup- stream plasma parameters (U•, B0•, M• - Alfv6n Mach number,/• = ratio of thermal to magnetic pressure, • - ratio of specific heats), the ideal MHD equationswere solvedfor the corresponding downstream values [e.g., Walters, 1964]. All resultsshown herein were obtained by using U• = 400 km/s, B0• = 5 x 10 -5 G = 5•, M,• = 8, /• = 1 and • = 5/3 [e.g., Russell and Greenstadt, 1979], with ½• beinga variable. Once ionized, a Li6+ ion starting from rest in the shock frame has a gyroradiusP0 = U• sin ½•/fl0 in the upstream plasma frame, where fl0 =eBon/inc is the upstream gy- rofrequency, e is the unit electron charge,m the ion mass,and c the speed of light. For the chosen upstream parameters, P0 = 0.60 Re (• 3800 km) and 0.78 Re (• 5000 km) for • = 50 ø and 90ø , respectively. Since we restrict analysis to the quasi-perpendicular regime (45 ø •< • <_ 90ø), we are justified in treating the shock transition thickness 1, where 1•> (ion inertial length) • 100 km [Russell and Greenstadt,1979], as negligible in comparison with an injected Li6+ gyroradius (P0 /1 • 50 for •a = 90ø). The effects of shock curvature (which has been considered by Haerendel and Papamastorakis [1983]), and any magnetic overshoot and potential jump as- 7331