摘要:

The following papers were presented as contributed talks at the Chapman Conference on Plasma Waves and Instabilities in Magnetospheres and at Comets held in Sendai/Mt. Zao, Japan, October 12–16, 1987. The tutorials and invited talks given at the conference will appear in an AGU monograph at about the same time as the issue of theJournal of Geophysical Research.The purposes of the conference were (1) to discuss recent discoveries of plasma waves and instabilities at comets and nonlinear processes and turbulence in plasmas, particularly computer simulation results, and (2) to compare and contrast instabilities in the magnetospheres of the Earth, Jupiter, Saturn, and Uranu

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00001

年代:1989

数据来源: WILEY

摘要:

The free energy in oxygen or hydrogen ions freshly created in the solar wind stimulates low‐frequency electromagnetic waves whose growth does not always maximize at parallel propagation. Exploration of the wave vector plane discloses the frequent occurrence of islets of oblique growth unconnected to the unstable parallel modes. Contour plots of the growth rate, real frequency, polarization, and magnetic compression characterize the oblique wave behavior for large values (≥45°) of the initial pitch angle of the cometary particles. Although wave‐particle (Landau and cyclotron) resonances feed most of the surveyed oblique instabilities, some are seemingly fluidlike. The results, obtained from the numerical solution of the kinetic dispersion and wave equations, imply that newborn ions can easily excite significant oblique hydromagnetic wave activity. Cometary environments provide the adopted plasma model, but the study is helpful in the interpretation of other low‐frequency wave observations

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00003

年代:1989

数据来源: WILEY

摘要:

The ion pickup process is examined in a multifluid hydrodynamic approximation which allows for relative motion, along the magnetic field, of the newly created ions and the original fluid. If this relative motion is appreciable, the cometary ion flow will not be exactly antisunward, and its reaction on the solar wind will deflect the solar wind in the opposite direction. Fluid equations are given, but they would necessitate calculating the flow of several fluid species.

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00015

年代:1989

数据来源: WILEY

摘要:

The properties of 50‐ to 100‐s low‐frequency plasma waves and the parent 65‐ to 95‐keV H2O group ion pitch angle distributions are presented for spatial regions distant (3.5‐7.0 × 105km) from comet Giacobini‐Zinner where the cometary ion densities are low and the solar wind interaction is weak. The presence of interplanetary discontinuities in this interval causes the interplanetary magnetic field angle α to vary over a range from ∼35° to 90°, allowing the possible generation of several new wave modes which are theoretically expected, but as yet undetected. It is determined that whenx= 45° ± 20° (Parker spiral orientations), the waves are typically elliptically polarized and propagate at 20°‐50° relative toB0. The wave properties are, however, not limited to the above range. They can exhibit even more extreme polarizations and propagation directions. The waves have periods near the H2O group ion cyclotron period and are left‐hand polarized in the spacecraft frame. In the relatively rare instances when wave steepening is detected, the steepened edge is near the trailing portion of the wave. The wave period, sense of polarization, and steepening are consistent with the properties of waves detected closer to the comet and identify them as being generated by the right‐hand resonant instability. The high degree of ellipticity is in sharp contrast to waves detected closer to the comet, however. The energetic ions are not isotropic in pitch angle (in the plasma frame), in general agreement with recent numerical modeling. When α increases to 70°‐90°, the right‐hand resonant mode is observed to cut off, as theoretical studies indicate. However, the left‐hand cyclotron resonant mode is not detected. Although the detected energetic 65‐ to 95‐keV ion fluxes are enhanced during these intervals (because of the instantaneous pickup by the solar wind), particle pitch angle scattering away from the initial ring distribution is very weak, consistent with the o

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00018

年代:1989

数据来源: WILEY

摘要:

Magnetic pulses with durations near the local proton cyclotron period (in the spacecraft frame) have been detected during the ICE encounter with comet Giacobini‐Zinner. The pulses typically last only a single cycle (solitary wave), are transverse, noncompressional oscillations, and have peak‐to‐peak transverse amplitudes of 2‐3 nT. Occasional large pulses with amplitudes of 5‐7 nT have been detected. The waves typically propagate at angles 2°‐15° relative toB0and are highly elliptically (linearly) polarized. The pulses have been detected when α, the angle between the ambient magnetic field and the solar wind velocity, is 90° ± 30° and when ICE was 350,000‐700,000 km from the comet. At the present time it is uncertain whether such pulses are generated exclusively under large α conditions or whether the presence of large‐amplitude heavy ion cyclotron waves during more moderate α conditions masks their presence. It is also unclear whether these pulses are superposed on top of cometary turbulence or simply a part of it. The above observations will be compared to recent theoretical predictions of cometary waves generated du

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00029

年代:1989

数据来源: WILEY

摘要:

Spectral characteristics of low‐frequency plasma turbulence in a cometary environment have been determined using cross‐spectral analyses of magnetic field and plasma measurements made on board the Giotto spacecraft upstream of comet P/Halley. Two upstream regions have been identified, where the mean solar wind magnetic field was approximately parallel and perpendicular to the solar wind flow velocity direction, respectively. The two regions also differ with respect to magnetic field lines having been disconnected (connected) to the cometary bow shock in the quasi‐parallel (quasi‐perpendicular) regions. In the quasi‐perpendicular region the mean magnetic field and solar wind flow direction as well as the Giotto velocity relative to the comet are approximately mutually perpendicular to each other. Thus there is no Doppler shift between the observations made in the spacecraft and plasma frame of reference for parallel propagating waves in the quasi‐perpendicular region. In both regions the turbulence spectrum can be described by a power law with a spectral index steeper than that of a Kolmogoroff‐Obukov spectrum. In the quasi‐parallel region the observed turbulence clearly exhibits an Alfvénic character with dynamic alignment between magnetic field and flow found throughout the frequency range (1‐100 mHz) analyzed. Wave propagation is sunward for all spectral components, at least in the quasi‐parallel region. The turbulence cascade observed is probably driven by right‐hand resonant pickup water group ion instabilities in the quasi‐parallel region, while in the quasi‐perpendicular region, left‐hand ion cyclotron ring or Alfvén/ion anisotropy instabilities might be a possible driving mechanism. The polarization of the pump wave in the quasi‐parallel region is almost linear, while at higher frequencies almost linear as well as right‐ and left‐handed polarization is found. In the quasi‐parallel, region evidence for water group ion cyclotron harmonics has been found with the first harmonic exhibiting a slow mode character and all harmonics almost propagating along the mean magnetic field. In the quasi‐perpendicular connected region, no clear evidence has been found for the existence of backstreaming protons, reflected off the cometary bow shock

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00037

年代:1989

数据来源: WILEY

摘要:

Relationships between simultaneous plasma wave, magnetic field, and energetic heavy ion data obtained by the International Cometary Explorer (ICE) spacecraft in the large‐scale solar wind particle pickup region surrounding comet Giacobini‐Zinner are examined. In particular, we investigate the conditions under which electrostatic emissions at frequencies of a few kilohertz and electromagnetic waves at a few tens of hertz are observed. It is shown that the data are consistent with the view that the kilohertz electrostatic emissions result from a beam‐type instability excited by the pickup photoelectron population when the angle α between the magnetic field and the plasma velocity vectors is not too large (α<60°). The data also suggest that the few tens of hertz electromagnetic waves may be excited by a ring‐type instability associated with the pickup ion population, which occurs when the magnetic field is near to orthogonality with the fl

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00049

年代:1989

数据来源: WILEY

摘要:

The ICE spacecraft observed in the vicinity of comet Giacobini‐Zinner electrostatic noise with burstlike features seemingly modulated by the relative orientation of the interplanetary magnetic field and the solar wind velocity (Richardson et al., this issue). We discuss the adequacy of ion acoustic‐like instabilities stimulated by photoelectron or newborn ion beams to the interpretation of the electrostatic bursts, and we conclude that the mode fed by the photoelectrons exhibits characteristics compatible with the observati

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00060

年代:1989

数据来源: WILEY

摘要:

Stationary solutions of the derivative nonlinear Schroedinger equation are discussed and classified by using a pseudopotential formulation. The solutions consist of a rich family of nonlinear Alfvén waves and solitons with parallel and oblique propagation directions. Expressions for the envelope and the phase of nonlinear waves with periodic envelope modulation, and “hyperbolic” and “algebraic” solitons are given. We evaluate the propagation angle for the slightly modulated elliptic, periodic waves and for oblique solitons. Also, we present periodic stationary waves which may arise in numerical simulations using periodic boundary conditions. The parallel and oblique stationary solutions discussed here can serve as a starting point for studies of modulational and decay instabilities and for a stability analysis of the

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00065

年代:1989

数据来源: WILEY

摘要:

Electrons backstreaming into Earth's foreshock generate waves near the plasma frequencyfpby the beam instability. Two versions of the beam instability exist: the “reactive” version, in which narrow‐band waves grow by bunching the electrons in space, and the “kinetic” version, in which broadband growth occurs by a maser mechanism. Recently, it has been suggested that (1) the backstreaming electrons have steep‐sided “cutoff” distributions which are initially unstable to the reactive instability, (2) the back reaction to the wave growth causes the instability to pass into its kinetic phase, and (3) the kinetic instability saturates by quasi‐linear relaxation. In this paper we present two‐dimensional simulations of the reactive instability for Maxwellian beams and cutoff distributions. The results of the simulations are consistent with suggestions 1 and 2 above; suggestion 3 is not addressed here. We demonstrate that the reactive instability is a bunching instability and that the reactive instability saturates and passes over into the kinetic phase by particle trapping. A reactive/kinetic transition is shown to most likely occur within 1 km and 50 km of the bow shock. We suggest that the frequency of the intense narrow‐band waves decrease from abovefpto perhaps 0.9fp(dependent on the beam density) with increasing penetration into the high beam speed region (υb/Ve>10) of the foreshock, before the wave frequency rises again as the waves become broadband deeper in the foreshock. Both the simulation results and numerical solutions of the dispersion equation indicate that for the observed beam parameters the center frequency of the waves near the foreshock boundary should be between 0.9fpand 0.98fp, rather than abovefpas previously believed. The simulation results indicate that the effects of spatial inhomogeneity are vital for a quantitative understanding

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00079

年代:1989

数据来源: WILEY