PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT NOTE = "ABSTRACT.TXT file is a digitized version of the abstracts from the Journal of Geophysical Research Vol 98, No. A12, December 1, 1993 (copyright AGU). 2011-05-04 A.Raugh Removed illegal tab characters" PUBLICATION_DATE = 1993-12-01 END_OBJECT END *****File ABSTRACT.TXT JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 20,911-20,920, DECEMBER 1, 1993 The Change of Giotto's Dynamical State During the P/Grigg-Skjellerup Flyby Caused by Dust Particle Impacts MARTIN PATZOLD Institut fur Geophysik und Meteorologie, Universitat zu Koln, Cologne, Germany MICHAEL K. BIRD Radioastronomisches Institut, Universitat Bonn, Bonn, Germany PETER EDENHOFER Institut fur Hochfrequenztechnik, Ruhr-Universitat Bochum, Bochum, Germany The Giotto spacecraft was decelerated by impacting cometary dust during its flight through the coma of comet P/Grigg-Skjellerup on July 10, 1992. The change in radial velocity determined by two-way Doppler measurements made on July 8 (pre encounter) and July 11 (post encounter) was (-0.4 +/- 0.1) mm/s. An abrupt frequency drop of 138 Hz was observed in the one-way real-time data during the actual encounter phase at 1530:42.8 UT, 6 s after the expected time of closest approach. This frequency discontinuity is attributed to a disturbance in the spacecraft's radio system and is unrelated to the spacecraft deceleration inferred from the more reliable two-way Doppler measurements. The spacecraft spin period changed by (-0.8 +/- 0.1) ms and a nutation of amplitude (0.10 degrees +/- 0.01 degrees) was generated. This relatively large nutation angle, coupled with the very modest deceleration, is most easily generated by a single dust particle. Assuming that the total impacted mass was concentrated in one single large particle, it is possible to determine a range for the total impacted real mass (20-39 mg), the total change in velocity delta v (approximately = 1 mm/s), the momentum enhancement factor epsilon (0 to 2.0), and the impact coordinates on the spacecraft. It is deduced that the particle most likely impacted on the upper end of the spacecraft toward the antenna tripod. No definite solution could be found for impacts on the bumper shield. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 20,921-20,935, DECEMBER 1, 1993 Low-Frequency Electromagnetic Plasma Waves Comet P/Grigg-Skjellerup: Overview and Spectral Characteristics KARL-HEINZ GLASSMEIER Institut fur Geophysik und Meteorologie, Technische Universitat Braunschweig, Braunschweig, Germany FRITZ M. NEUBAUER Institut fur Geophysik und Meteorologie, Universitat zu Koln, Cologne, Germany Large-amplitude electromagnetic plasma waves are one of the dominant features of the solar wind-comet interaction. Wave characteristics strongly depend on parameters such as the solar wind flow and Alfven velocities and the angle between flow and interplanetary magnetic field as well as the production rate. With respect to the latter the flyby of the spacecraft Giotto at comet P/Grigg-Skjellerup provides a unique possibility to study such waves in further detail. Pickup ion-related wave signatures have been observed up to a distance of 600,000 km from the nucleus. Peak spectral power in the spacecraft frame of reference occurs at frequencies mainly somewhat below the water group ion gyrofrequency. From this the waves are determined to be mainly left-hand polarized waves, causing one-sided pitch angle diffusion outbound. The wave activity strongly increases close to the comet; upstream it exhibits a quadratic dependence on the water group pickup ion free energy. Furthermore, a phenomenological study of the wave characteristics provides a unique description of the fine-structure of the interaction region. Indications of steepened magnetosonic waves have been found in the outbound magnetosheath region. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 20,937-20,953, DECEMBER 1, 1993 Low-Frequency Electromagnetic Plasma Waves at Comet P/Grigg-Skjellerup: Analysis and Interpretation F.M. NEUBAUER Institut fur Geophysik und Meteorologie, Universitat zu Koln, Cologne, Germany K.-H. GLASSMEIER Institut fur Geophysik und Meteorologie, Technische Universitat Braunschweig, Braunschweig, Germany A.J. COATES AND A.D. JOHNSTONE Mullard Space Science Laboratory, University College London, England The propagation and polarization characteristics of low-frequency electromagnetic wave fields near comet P/Grigg-Skjellerup (P/GS) are analyzed using magnetic field and plasma observations obtained by the Giotto magnetometer experiment and the Johnstone plasma analyzer during the encounter at the comet on July 10, 1992. The results have been physically interpreted with the following new findings: (1) Broad pickup-generated wave fields exist in the magnetoplasma around comet P/GS. (2) Outside the bow wave/shock, wave fields can be pictured as plane waves with propagation angles of about 10 degrees with respect to +/- B_o, where B_o is the background magnetic field. (3) An envelope of very regular wave fields with almost exclusively left-hand polarization in the spacecraft frame exists around the bow wave/shock. More complex polarization properties are found farther away. (4) Because of the unusual interplanetary magnetoplasma characteristics the ratio M_Ar, of the local ring beam velocity component parallel to the magnetic field in the plasma frame of reference and the Alfven velocity varies between zero and a little greater than 1. As a consequence, the waves excited in the plasma frame must be predominantly left-hand Alfven waves propagating away from the Sun in contrast to the right-hand waves propagating toward the Sun in the same frame of reference at comets P/Giacobini-Zinner and P/Halley. (5) It is argued that the regular character of the waves close to but upstream of the bow wave/shock is due to the effects of nongyrotropy and/or nonlinear dispersive effects. (6) Magnetosheath waves are characterized as compressive, nonplanar waves with mixed polarization. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 20,955-20,964, DECEMBER 1, 1993 Mirror Modes and Fast Magnetoacoustic Waves Near the Magnetic Pileup Boundary of Comet P/Halley K.H. GLASSMEIER, U. MOTSCHMANN, C. MAZELLE, F.M. NEUBAUER, K. SAUER, S.A. FUSELIER, AND M.H. ACUNA Large-amplitude ultralow-frequency wave structures observed on both sides of the magnetic pileup boundary of comet P/Halley during the flyby of the Giotto spacecraft have been analyzed using suprathermal electron density and magnetic field observations. Upstream of the boundary, electron density and magnetic field magnitude variations are anticorrelated, while in the pileup region these quantities are clearly correlated. Both in front of and behind the pileup boundary the observed waves are quasi-perpendicular wave structures as a minimum variance analysis shows. A detailed comparison of our observations in the prepileup region with theoretical and numerical results shows that the mirror mode mode waves may have been generated by a mirror instability driven by the pressure anisotropy of the ring-type distributions of the heavy (water group) pickup cometary ions. A mean wavelength lambda_1 = 2071 km is found, which is about 4 times the water group ion thermal gyroradius. For the fast mode wave structures in the pileup region, analysis of wave propagation on both sides of the pileup boundary shows that mode conversion from the mirror mode to fast mode type waves at the boundary is not a possible source mechanism. However, we show that the fast mode waves may be interpreted as stationary wave structures in a multifluid plasma. The wavelength lambda_2 = 1141 km derived from the observations, when using the stationary wave hypothesis, is in good agreement with the theoretical expected wave length lambda_theo = (774 +/- 506) km computed from available ion observations. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 20,965-20,976, DECEMBER 1, 1993 Electron Plasma Environment at Comet Grigg-Skjellerup: General Observations and Comparison With the Environment at Comet Halley H. REME, C. MAZELLE, J.A. SAUVAUD, C. D'USTON, E FROMENT, R.P. LIN, K.A. ANDERSON, C.W. CARLSON, D.E. LARSON, A. KORTH, P. CHAIZY, AND D.A. MENDIS The three-dimensional electron spectrometer of the Reme plasma analyzer- complete positive ion, electron and ram negative ion measurements near comet Halley (RPA-COPERNIC) experiment aboard the Giotto spacecraft, although damaged during the comet Halley encounter in March 1986, has provided very new results during the encounter on July 10, 1992, with the weakly active comet Grigg-Skjellerup (G-S). The main characteristic features of the highly structured interaction region extending from approximately 26,500 km inbound to approximately 37,200 km outbound are: (1) a broad inbound "bow wave transition", (2) a cometosheath characterized by strong oscillations of the electron fluxes with peak-to-valley ratios up to 20:1; the levels of oscillations are much larger and generally more coherent than at comet Halley or at comet Giacobini-Zinner, despite the much lower neutral gas production rate, (3) a "mystery" region identified by significant fluxes of hundreds of eV electrons, and a "mysterious" transition, about halfway between the bow wave transition/bow shock regions; they are observed as for comet Halley, confirming their large importance in the solar wind-comet interaction at 1 AU, (4) an inner region which coincides with the magnetic pileup region; at closest approach, the cold cometary electron fluxes display a prominent maximum, and (5) a sharp outbound bow shock and its possibily associated electron foreshock. These results are compared to the results obtained by the same instrument during the Giotto comet Halley fly-by. Despite the large difference in the size of the interaction regions (approximately 60,000 km for G-S, approximately 2000,000 km for Halley) due to 2 orders of magnitude difference in cometary neutral gas production rate, there are striking similarities in the solar wind interactions with the two comets. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 20,977-20,983, DECEMBER 1, 1993 Nongyrotropic Distribution of Pickup Ions at Comet P/Grigg-Skjellerup: A Possible Source of Wave Activity UWE MOTSCHMANN AND KARL-HEINZ GLASSMEIER Insitut fur Geophysik und Meteorologie, Technische Universitat, Braunschweig, Germany The pickup of newborn cometary ions in the solar wind excites strong wave activity. At comet P/Grigg-Skjellerup the interplanetary magnetic field is nearly perpendicular to the solar wind velocity; thus the pickup ions should form a ring distribution. In opposition to P/Halley, comet P/Grigg-Skjellerup is rather small, and its inhomogeneity scale is even smaller than the pickup scale. Therefore the ring will be filled incompletely, and the distribution becomes nongyrotropic. The dispersion of a nongyrotropic cometary pickup distribution in a cold proton-electron background plasma is studied. Nongyrotropy couples the gyrotropic eigenmodes and drives an instability. Left-hand polarized waves close to the cometary ion cyclotron frequency are excited. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 20,985-20,994, DECEMBER 1, 1993 Velocity Space Diffusion and Nongyrotropy of Pickup Water Group Ions at Comet Grigg-Skjellerup A.J. COATES AND A.D. JOHNSTONE Mullard Space Science Laboratory, University College London, England B. WILKEN Max-Planck-Institut fur Aeronomie, Katlenburg-Lindau, Germany F.M. NEUBAUER Institut fur Geophysik und Meteorologie, Universitat Koln, Cologne, Germany The diffusion of water group cometary ions in velocity space at comet Grigg-Skjellerup was measured during the Giotto spacecraft encounter. The evolution of the collapsed pitch angle and energy distributions during the inbound and outbound passes shows that the timescale for energy diffusion may be similar to that for pitch angle diffusion. Fully isotropic pitch angle distributions were never seen. Also the bulk parameters of the three-dimensional distributions are examined. Transformation of these parameters into a field-aligned solar wind frame allows us to test the gyrotropy of the distributions. The observations imply that there were deviations from gyrotropy throughout the encounter becoming most important near to closest approach. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 20,995-21,002, DECEMBER 1, 1993 Mass Loading and Velocity Diffusion Models for Heavy Pickup Ions at Comet Grigg-Skjellerup D.E. HUDDLESTON Jet Propulsion Laboratory, California Institute of Technology, Pasadena A.J. COATES AND A.D. JOHNSTONE Mullard Space Science Laboratory, University College London, Dorking, England F.M. NEUBAUER Institut Fur Geophysik und Meteorologie, Universitat zu Koln, Cologne, Germany We compare model predictions of cometary water group ion densities and the solar wind slow down with measurements made by the Giotto Johnstone plasma analyzer implanted ion sensor at the encounter with comet Grigg-Skjellerup (G-S) on July 10, 1992. The observed slope of the ion density profile on approach to the comet is unexpectedly steep. Possible explanations for this are discussed. We present also a preliminary investigation of the quasilinear velocity-space diffusion of the implanted heavy ion population at G-S using a transport equation including source, convection, adiabatic compression, and velocity diffusion terms. Resulting distributions are anisotropic, in agreement with observations. We consider theoretically the waves that may be generated by the diffusion process for the observed solar wind conditions. At initial ion injection, waves are generated at omega approximately Omega_i the ion gyrofrequency, and lower frequencies are predicted for diffusion toward a bispherical shell. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 21,003-21,008, DECEMBER 1, 1993 The Flow of the Contaminated Solar Wind at Comet P/Grigg-Skjellerup K.R. FLAMMER AND D.A. MENDIS Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla On July 10, 1992, the Giotto spacecraft encountered a second comet, namely, P/Grigg-Skjellerup. Despite the small production rate of this comet (approximately 2 orders of magnitude smaller than that of P/Halley and 1 order of magnitude smaller than that of P/Giacobini-Zinner) the plasma flow showed several distinct flow transitions. We have applied theoretical models, previously developed to explain the existence and location of the flow boundaries at comets P/Giacobini-Zinner and P/Halley to this new data set for comet P/Grigg-Skjellerup. The observed bow shock crossings are used to evaluate the production rate of the cometary neutrals. Using this production rate, the position of the collisiono-pause and the ionopause are estimated. The observed location of the magnetic pile-up boundary corresponds well to the theoretical calculations. We also find the reason that a magnetic field-free region was not encountered was because the ionopause, while being sharp, was inside the spacecraft trajectory which had an estimated closest approach distance greater than 120 km. Finally we have used the observed spatial profile of the solar wind magnetic field together with a simple one-dimensional, multispecies chemical model to calculate the radial density profiles of the H2O group ions outside the bow shock. Comparison of the calculated total H2O group ion density profile with observations indicates a good fit in the inner region where the H2O+ ion dominates. While the corresponding fit in the outer region, where the O+ ion dominates, becomes progressively less satisfactory with increasing cometocentric distance, it is noted that the observations themselves are believed to be less reliable at these large distances. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 21,009-21,016, DECEMBER 1, 1993 MHD Modeling Applied to Giotto Encounter With Comet P/Grigg-Skjellerup H.U. SCHMIDT AND R. WEGMANN Max-Planck-Institut fur Astrophysik, Garching, Germany F.M. NEUBAUER Institut fur Geophysik und Meteorologie der Universitat, Cologne, Germany First from a one-dimensional MHD model we derive estimates for position and shape of the bow shock applicable for arbitrary magnetosonic Mach numbers > 1 in the solar wind. This generalization is important in the case of the Giotto encounter with P/Grigg-Skjellerup with the unusual interplanetary magnetic field (IMF) of 18 nT. We compare three-dimensional model calculations with observations from six cometary missions and derive new corrected estimates for the gas production of P/Grigg-Skjellerup, which turn out to be substantially lower than those hitherto proposed assuming a large magnetosonic Mach number in the solar wind. Furthermore, we use global MHD models for this comet to determine from the measured magnetic field the position of the Giotto trajectory with respect to the comet. As a measure of the goodness of fit we use the mean square of the difference of the observed and the model field. We fit segments of the measured data of three different lengths centered near closest approach and for three different values of the unperturbed IMF. For all parameters considered, we find a unique best approximating trajectory which depends only little on the parameters. In any case, our best fitting trajectory passes through the comet at the side of the nucleus nearly straightly eastward (within +/- 15 degrees) at a distance of 400 km. It has closest approach at about 1518:59 space craft event time (SCET). JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 21,017-21,021, DECEMBER 1, 1993 Resonant Electromagnetic Ion-Ion Beam Turbulence at Comet P/Grigg-Skjellerup FRANK VERHEEST Sterrenkundig Observatorium, Universiteit Gent, Ghent, Belgium G.S. LAKHINA Indian Institute of Geomagnetism, Bombay, India The electromagnetic turbulence and associated frequencies observed far upstream of comet P/Grigg-Skjellerup during the recent Giotto spacecraft flyby can be attributed to relative streaming between the cometary ions and the solar wind, and the hypothesis that unstable parallel modes are responsible is used to deduce the wavelengths involved. Such a self-consistent approach yields resonant instabilities with wavelengths of 7000 ~ 8000 km, phase velocities which are sub-Alfvenic and polarizations which are left-handed in the spacecraft frame.