JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. A2, PAGES967-975, FEBRUARY 1, 1990 Pc 5 Pulsationsin the Outer Dawn Magnetosphere Seen by ISEE 1 and 2 D. G. MITCHELL, • M. J. ENGEBRETSON, 2 D. J. WILLIAMS, • C. A. CATTELL, 3 AND R. LUNDIN 4 A long-lasting Pc 5 pulsation at the dawn flank of the magnetosphere is studied usingparticleand field instrumentation from the ISEE 1 and 2 satellites. Electricfield and particle modulationsignatures were clearer than magnetic field variations, consistent with the satellites' position in latitudenear the equatorial node of a fundamental resonance. Pulsationflow velocities along the ISEE 1 trajectorywere calculated from particle characteristics usingdata from several instruments and from electric and magnetic field data. Theseflow ve- locities were all consistent with each other,but the velocities derived from plasma and energetic particle obser- vations were a factor of 2.5 largerthan velocities derivedfrom the fieldsdata. We have not beenable to find the source of this discrepancy; one possibility is that the field near the spacecraft differs from the large-scale field. In contrast to observations of pulsations during magnetic storms, which ofteninvolve resonant or gyrat- ing particle behavior, particles at all energies sampled (10 eV to 200 keV) appeared to respond passively to the pulsation throughout mostof the periodof interest. Comparison of data from the two spacecraft, which traveled from --- 15RE to ---7RE with a time separation of --- 1 hour,suggests the propagation of relatively broadband pulsation energy from the magnetopause/low latitude boundary layerand subsequent resonance of independent L shells in the fundamental toroidal mode after the cessation of powerinput. INTRODUCTION A large body of literature has developed in recentyears to characterize long-period pulsations in the Earth's outer mag- netosphere. These ULF pulsations, firstobserved in ground records as sinusoidal oscillations in geomagnetic records, have now been observed by magnetometers on a large variety of satellites, by ground magnetometers andradars, andincreasingly by other satel- lite instrumentation as well. Early studies by Brownet al. [1968] and Sonnerup et al. [1969] of a waveeventduringa magnetic stormnoteda correlation be- tween variations in energetic particle densities and magnetic field magnitude. Further studies revealed that suchstorm-associated waves typically occur in theafternoon sector during thestorm main phase[BarfieM and Coleman, 1970;Barfield and McPherron, 1972, 1978; Barfield et al., 1972; Engebretson and Cahill, 1981], and are intimately related to the arrival of substorm-injected hot plasma. They are oftenpolarized predominantly in the radialdirec- tion and have at times large compressional components. Pc 5 waves observed during relatively quietmagnetic conditions havebeen found to be qualitatively differentfrom those observed duringmagnetic storms [Kokubun et al., 1977].Wavesobserved during quiet timeshave beenobserved in both morning and af- ternoon sectors [Lanzerotti et al., 1974, 1975; Kokubunet al., 1976; Singer et al., 1979, 1982; Takahashi and McPherron, 1982, 1984]. These arepredominantly azimuthally polarized and oftenhave little compressional component; in most cases, magnetospheric parti- clepopulations do not appear to be the free-energy source for these pulsations. The interaction of magnetospheric plasma with these pulsations hasbeen studied theoretically by Southwood and Hughes[1983] and Tamao [1978, 1984] and in a series of papers by Southwood and Kivelson [1981, 1982] and Kivelsonand Southwood [1983, IThe Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland. 2Department of Physics, Augsburg College, Minneapolis, Minnesota. 3Space Science Laboratory, University of California, Berkeley. 4Swedish Institute of Space Physics, Kiruna, Sweden. Copyright 1990 by the AmericanGeophysical Union. Paper number 89JA00915. 0148-0227/90/89 JA-00915505.00 1985].Experimental studies of pulsations using measurements of plasma populations,energetic particles,and/or electricfields to supplement magneticfield measurements include several of the above aswell asHughes et al. [1979],Suet al. [1979, 1980], Kaye and Shelley [1981], Kremser et al. [1981], Walker et al. [1982], Waite et al. [1986], Engebretson et al. [1986a], and Takahashi et al. [1985, 1987]. In this paper we examine the plasma and energetic particle response to a long-lived Pc 5 pulsation occurring on a nearly radial pass of the ISEE 1 and 2 satellites along the dawn meridianand close to the geomagnetic equator. As is typical for observations of quiettime pulsations, charged particles from 10 eV to 200 keV appear to exhibit a passive response to the pulsationdisturbance for most of the period studied. We utilize data from several in- struments on ISEE 1 to independently characterize the pulsations, and find the results generally to be in excellent agreement. These instrumentsinclude the Medium Energy Particle Instrument (MEPI) for energetic ions [Williamset al., 1978];the Low Ener- gy Proton and Electron Differential Energy Analyzer(LEPEDEA) for plasma ionsand electrons [Frank et al., 1978]; the quasi-static electric field experiment[Mozer et al., 1978]; and the flux gate magnetometer [Russell, 1978]. There is a quantitative discrepan- cy between the magnitude of the electric field and the convective velocityderivedfrom the ion distribution. We have beenunable to discover an instrumental or analysis error source for this dis- crepancy; one possibility is that the near and distant electric fields differ from one another. We also exploit the dual satellite capability of the ISEE mis- sion to study the time history, and by inference the energy source, of this pulsationevent. For this measurement we rely upon the Fast PlasmaExperiment[Bameet al., 1978]. Theoretical studies of field line resonant pulsations by Chen and Hasegawa [1974] and Southwood [1974]assumed a monochromatic drivingsource, such as the Kelvin-Helmhøltz instability at or near the mag- netopause. The characteristics predictedby thesestudies agreed with nearlyall of the data available at that time, based on ground studies and data from synchronous orbit, which found many es- sentially monochromatic oscillations. Studies using elliptically or- biting spacecraft indicated, however, that many ULF pulsations had L shell dependent periods [Singer et al., 1979, 1982; Linet al., 1986;Cahill et al., 1986;Engebretson et al., 1986b].The ob- servations reportedhere are consistent with theselatter studies, namely, that several shells of _<0.5 Re thickness canbe set into simultaneous resonance. In addition, comparison of data from 967