摘要:

In the first study to employ dual‐spacecraft cross‐spectral techniques in the magnetosheath, we have analyzed ISEE 1 and ISEE 2 magnetic field and plasma data from an afternoon magnetosheath crossing to determine dominant MHD wave modes and directions. Our principal discovery is of guided MHD waves traveling along the flow‐modified wave characteristics (the curves along which wave energy is propagated in a flowing medium ‐ see, for example,Akhiezer et al.[1975]). Wave polarizations were determined by transforming the magnetic field to a field‐ and boundary‐aligned coordinate system. We have inferred wave propagation directions using the time delays between the passage of coherent structures across the spacecraft. Weaker broadband coherent oscillations in the direction radial from the planet could be associated with large scale motions of the shock and/or magnetopause, but the dominant signals appear not to be from such a source. Separate broadband Alfven and slow magnetoacoustic waves have been identified in the inner sheath, propagating across the field and flow toward the magnetopause. In the outer magnetosheath the most prominent disturbances were narrow‐band Alfven waves propagating downstream and along the field. The phase propagation directions detected for the largest amplitude signals were closely aligned with the flow‐modified wave characteristics, the group propagation directions. From this result, we infer that the sources of the dominant wave activity are localized MHD disturbances on the bow shock and not spatially coherent distributed sources such as bulk boundary motion. We suggest that the flow‐modified characteristics of the two field‐guided MHD modes may play an important role within the magnetosheath, determining the directions along which energy is transmitted between the solar wind an

ISSN:0148-0227    

DOI:10.1029/90JA01397

年代:1991

数据来源: WILEY

摘要:

Electrons can be efficiently energized at interplanetary shocks and planetary bow shocks. The acceleration and reflection process is extremely sensitive to the angle θBnbetween the upstream magnetic field and the shock normal, and is most prominent at θBn∼ 90°. The mechanism has been investigated by theoretical and simulation means, and can be interpreted as a fast Fermi process or as gradient drift acceleration. Previous work has been carried out for plane shocks only, and is expanded here to take into account the global curvature of a shock. Simple estimates suggest that this curvature may have a strong limiting effect on the acceleration to high energies, i.e., above several keV in case of the Earth's bow shock. We perform two‐dimensional test particle calculations to address this question, and evaluate the reflected electron flux as a function of θBnat the shock surface. The shock profile is derived from hybrid code simulations, and modified to include the first order effects of a global curvature in the vicinity of θBn= 90°. At low energies, the calculated fluxes exhibit a cut‐off and a maximum, which can give rise to observed bump‐on‐tail distributions in the electron foreshock. Results at high energy show that while individual electrons gain less energy in a curved shock, concerning the flux this fact is largely offset by two‐dimensional focusing effects. Electrons that drift into the shock over a wide area converge and stream out within a narrow spatial area, thus greatly enhancing the flux of reflected electrons. A κ distribution of suprathermal solar wind electrons (of index κ = 6) is capable of producing the observed large fluxes of reflected electrons at the Earth's bow shock up to energies of 10 to 15 keV, even when the global shock curvature is accounted for. Beyond this energy range observed spectra are harder, as has been found previously for a plane shock. As a likely reason, the solar wind seed population may be denser than modele

ISSN:0148-0227    

DOI:10.1029/90JA01728

年代:1991

数据来源: WILEY

摘要:

A recent attempt to measure the presence of interstellar pickup protons in the distant solar wind through their effects on the variations within pressure‐balanced structures found no evidence of these particles (Burlaga et al., 1990). That study concluded that the pickup protons, expected to be correlated with the solar wind density through the charge exchange ionization process, had become smeared in the radial direction. The resulting constant pressure rendered these particles undetectable by the analysis used. This paper presents an improved analysis which takes into account the cooling of these particles in the expanding solar wind. This improvement does not change the conclusions of the previous work. The possibility that quasi‐linear perpendicular diffusion is responsible for the inferred smearing across field lines is then investigated. It is shown that this mechanism can easily produce the required transport over the radial dimension of such a struct

ISSN:0148-0227    

DOI:10.1029/90JA01994

年代:1991

数据来源: WILEY

摘要:

Coupled spatially homogeneous quasilinear kinetic equations are derived which describe the evolution of the energetic ion omnidirectional distribution function and the intensities of magnetohydrodynamic waves propagating parallel and antiparallel to the ambient magnetic field. The energetic ions are assumed to be nearly isotropic and possess speeds much greater than the Alfvén speed. For application to pickup ions the equations may also include an energetic ion injection rate and wave excitation or damping caused by isotropization of the newborn ions. The wave kinetic equations may be integrated to yield explicit expressions for the wave intensities, which may be substituted into the ion kinetic equations to yield a single self‐consistent energy diffusion equation for the energetic ions. The theory represents the first treatment of stochastic (second‐order Fermi) acceleration in which the back reaction of the ions on the turbulence is included self‐consistently. Numerical solutions of the kinetic equations are presented for four cases of pickup ions in the solar wind which illustrate the essential features of the evolution: (1) interstellar pickup helium near a heliocentric radial distance of 1 AU; (2) interstellar pickup hydrogen near 10 AU; (3) water group pickup ions downstream of the bow wave of Comet Giacobini‐Zinner for parameters observed during the International Cometary Explorer flyby; (4) water group pickup ions downstream of the bow wave of Comet Halley for parameters observed during the Giotto flyby. The helium calculation reveals some modification of the solar wind wave spectrum and energy diffusion of the ions; although adiabatic deceleration is not included, acceleration rates are qualitatively consistent with the observed spectrum at 1 AU (Möbius et al., 1985). The hydrogen calculation shows extreme damping of the solar wind wave spectrum in the cyclotron‐resonant frequency range and a reduction in the acceleration rate of most of the ions. It is suggested that this behavior is responsible for an underabundance of hydrogen relative to the minor ions in the anomalous cosmic ray component, which is thought to originate from pickup ions accelerated at the solar wind termination shock. Wave damping is small at comet G‐Z, and the calculated energy spectra do not appear to be in quantitative agreement with the observed spectra (Richardson et al., 1987). At Comet Halley, on the other hand, wave damping is substantial and the calculated spectra appear to be in general agreement with the observations (McKenna‐Lawlo

ISSN:0148-0227    

DOI:10.1029/90JA02096

年代:1991

数据来源: WILEY

摘要:

In this paper we examine the stability of a steady solar wind dissipatively heated by Alfvén waves whose relative amplitude is saturated at a given level by nonlinear processes. It is shown that long‐wavelength compressional modes can be driven unstable by dissipative heating arising from short‐wavelength saturated Alfvén waves. Analytic expressions are derived for the marginal stability condition and the growth rates in the unstable region for the case of a moderate to low β plasma. These are supplemented by a numerical solution of the full MHD dispersion equation, including dissipative Alfvénic effects, which confirms the approximate analysis. It is shown that the growth time of the instability can be of the order of 7 times the characteristic period of an Alfvén wave for a wide range of parameters appropriate to the solar wind. The implication is that the compressional instability driven by dissipative Alfvén waves could play a significant role in the large‐scale heating and dynamics of the solar wind, particularly in the supers

ISSN:0148-0227    

DOI:10.1029/90JA01834

年代:1991

数据来源: WILEY

摘要:

Equatorial thermospheric tidal temperatures and densities inferred from Atmosphere Explorer E (AE‐E) mass spectrometer data are compared with theoretical predictions from the National Center for Atmospheric Research Thermosphere/Ionosphere General Circulation Model (TIGCM) for solar minimum, solstice conditions. The thermospheric diurnal and semidiurnal tides are excited in situ by solar heating and by ion‐neutral momentum coupling. Semidiurnal tides are also generated by upward propagating waves excited by heating in the lower atmosphere. The model calculations include all of these sources. The TIGCM reproduces the gross tidal features observed by the satellite, including the midnight temperature anomaly, and the diurnal phases are in good agreement for the densities of atomic oxygen and molecular nitrogen. However, for the neutral temperature, the predicted phases are 1–2 hours earlier than observed. In addition, the diurnal temperature and density amplitudes predicted by the model are considerably weaker than indicated by the AE‐E measurements. The semidiurnal variations found in the observations agree well with the model for December solstice but not for June. The present results indicate that upward propagating tides from the lower atmosphere are responsible for at least half of the amplitude of the semidiurnal tide in the upper therm

ISSN:0148-0227    

DOI:10.1029/90JA02149

年代:1991

数据来源: WILEY

摘要:

The first satellite observations of the total field‐aligned component of the quasi‐dc Poynting flux are presented for two passes over the polar region, one in the noon sector and one in the afternoon. The energy input due to electron precipitation is also presented. In the noon pass the downward Poynting flux in the auroral oval was comparable to the kinetic energy input rate. The peak electromagnetic energy input rate of 6 ergs/(cm² s) equaled the peak particle input while the integrated electromagnetic value along the trajectory was 60% that of the particles. In the afternoon pass the peak electromagnetic energy input was also about 6 ergs/(cm² s), but the peak particle energy was 6 times this value. The average electromagnetic input was 10% of the particle input for the pass. In this study we can measure the Poynting flux only over a limited range of scale sizes; thus the contribution to the total energy budget in the polar cap cannot be determined. Both passes show small regions characterized by upward Poynting flux suggesting a neutral wind dynamo. There is also evidence during part of the noontime pass that the external generator acted in opposition to an existing wind field since the Poynting flux was greater than the estimate of Joule heating from the electric field measurement alone (i.e., from ΣpE²). In the course of deriving Poynting's theorem for the geophysical case we also present a proof that ground magnetometer systems respond primarily to the Hall current which does not depend upon geometric cancellation between the field generated by Pedersen and field‐aligned

ISSN:0148-0227    

DOI:10.1029/90JA01837

年代:1991

数据来源: WILEY

摘要:

In general, the eigenmodes of a cold inhomogeneous magnetoplasma have magnetic field perturbations with components both parallel and transverse to the background magnetic field direction. Recently, constraints have been derived which the medium must satisfy for the field perturbation to be either entirely compressional, or entirely transverse. Here these criteria are investigated to yield all planar and axisymmetric magnetic field configurations allowing pure decoupled transverse and compressional oscillations. For example, in an axisymmetric dipole field the azimuthal field perturbations decouple exactly, but solely poloidal transverse ones do not. The magnetic field geometry and background plasma density for the most useful decoupled solutions are listed in a table at the end of the paper.

ISSN:0148-0227    

DOI:10.1029/90JA01787

年代:1991

数据来源: WILEY

摘要:

We consider the classical linear theory of electromagnetic wave growth in a warm plasma for waves propagating parallel to a uniform ambient magnetic field. Wave growth rates γ are calculated for ion‐driven right‐hand mode waves for Kappa (bi‐Lorentzian) and Maxwellian particle distribution functions. Systematic calculations of γ are carried out for various values of the spectral index κ, the temperature anisotropy A+= 1 ‐ (T⊥+/ T∥+), and the ratio β+of plasma pressure to magnetic pressure, appropriate to the solar wind. When the anisotropy is low (A+≪ 1), the wave growth is limited to frequencies below the proton gyrofrequency, and the growth rate increases dramatically as the spectral index κ is reduced (i.e., as the high‐energy tail becomes more pronounced). The growth rate for any Kappa distribution greatly exceeds (often by several orders of magnitude) that for a Maxwellian with the same bulk properties. For large thermal anisotropy (T∥+, ≫ T⊥+) the growth rate from either distribution is greatly enhanced. In comparison to a Maxwellian distribution the growth rates from a Kappa distribution are generally larger, and significant wave growth occurs over a broader range of frequencies. Evidently, the theory is important in the study of microinstabilities in space plasmas in which a high‐energy tail distr

ISSN:0148-0227    

DOI:10.1029/90JA01629

年代:1991

数据来源: WILEY

摘要:

Recent measurements of auroral electrons and intense Langmuir waves on a high‐altitude sounding rocket show that these waves were produced by dispersive bursts of low‐energy electrons. The sounding rocket was launched northward into the evening auroral zone during a substorm expansion and crossed several discrete auroral arcs having electron energy peaks greater than 25 keV. The large‐amplitude Langmuir waves, which appeared in ∼ 100‐ms bursts and had amplitudes greater than 200 mV/m, were seen only during periods of enhanced low‐energy (0.3–3.0 keV), field‐aligned electron precipitation. The enhanced electron flux displayed a dispersive signature in which the higher‐energy electrons arrived before the lower‐energy electrons. A wave‐particle correlator, a new instrument that performed a direct correlation of the arrival times of electrons with the phase of the high‐frequency wave field, detected electron bunching at the wave frequency during the bursts of intense Langmuir waves. The electron bunching events, which had amplitudes of a few percent, provided a direct identification of the energy (velocity) of the resonant electrons and therefore established the parallel wavelength (∼15 m). The electron bunching events were detected at or near the energy at which a positive slope in the electron distribution function was seen. During a dispersive burst, the phase velocity (or wavelength) of the Langmuir waves changed in response to the changing velocity at which the positive slope occurred. We conclude that the velocity dispersion in the low‐energy, field‐aligned electrons created the unstable electron distribution that was responsible for Langmuir wave growth. The proposed growth mechanism is similar to that suggested for Langmuir wave growth durin

ISSN:0148-0227    

DOI:10.1029/90JA01596

年代:1991

数据来源: WILEY