摘要:

Measurements of interplanetary H I Lyman α over a large portion of the celestial sphere were made at the recent solar minimum by the Pioneer Venus orbiter ultraviolet spectrometer. These measurements were performed during a series of spacecraft maneuvers conducted to observe Halley's comet in early 1986. Analysis of these data using a model of the passage of interstellar wind hydrogen through the solar system shows that the rate of charge exchange with solar wind protons is 30% less over the solar poles than in the ecliptic. This result is in agreement with a similar experiment performed with Mariner 10 at the previous solar minimum

ISSN:0148-0227    

DOI:10.1029/JA095iA09p14855

年代:1990

数据来源: WILEY

摘要:

The structure of approximate transport theories for the radial behavior of interplanetary fluctuations is reconsidered. The emphasis is on theories derived under the assumption of scale separation; i.e., the correlation length of the fluctuations is much less than the scale of large inhomogeneities. In these cases the zero‐wavelength limit provides a first approximation to the spectral evolution equations for the radial dependence of interplanetary fluctuation spectra. The goal here is to investigate the structure of a recently presented (Zhou and Matthaeus, 1989) transport theory, in which coupling of “inward”‐ and “outward”‐type fluctuations appears in the leading order, an effect we call “mixing.” In linear theory, mixing‐type couplings of inward‐type and outward‐type waves are formally a nonresonant effect. However, leading order mixing terms do not vanish at zero wavelength for fluctuations that vary nearly perpendicular to the local magnetic field, or when the mean magnetic field is weak. Leading order mixing terms also survive when the dispersion relation fails and there is a nonunique relationship between frequency and wave number. The former case corresponds to nearly two‐dimensional structures; these are included, for example, in isotropic models of turbulence. The latter instance occurs when wave‐wave couplings are sufficiently strong. Thus there are a variety of situations in which leading order mixing effects

ISSN:0148-0227    

DOI:10.1029/JA095iA09p14863

年代:1990

数据来源: WILEY

摘要:

We reexamine the WKB expansion for “toroidal” Alfvén waves in the solar wind, as described by equations (9) of Heinemann and Olbert (1980). Our principal conclusions are as follows: (1) The WKB expansion used by Belcher (1971) and Hollweg (1973) is nonuniformly convergent. (2) Using the method of multiple scales (Nayfeh, 1981), we obtain an expansion which is uniform. (3) The uniform expansion takes into account the small modification to the Alfvén wave phase speed due to spatial gradients of the background. (4) Both the uniform and nonuniform expansions reveal that each “normal mode” has both Elsässer variables δz+≠ 0 and δz−≠ 0. Thus if δz−corresponds to the outgoing mode in a homogeneous background, an observation of δz+≠ 0 does not necessarily imply the presence of the inward propagating mode, as is commonly assumed. (5) Even at the Alfvén critical point (whereV= υA) we find that δz+≠ 0. Thus incompressible MHD turbulence, which requires both δz+≠ 0 and δz−≠ 0, can proceed at the Alfvén critical point (cf. Roberts, 1989). (6) With very few exceptions, the predictions of these calculations do not agree with recent observations (Marsch and Tu, 1990) of the power spectra of δz+and δz−in the solar wind. Thus the evolution of Alfvén waves in the solar wind is governed by dynamics not inc

ISSN:0148-0227    

DOI:10.1029/JA095iA09p14873

年代:1990

数据来源: WILEY

摘要:

Recently developed transport theories for small‐scale solar wind fluctuations explicitly treat convection, expansion, and other interactions with large‐scale gradients, while separating nonlinear effects associated with triple correlations of the fluctuations. To close these models, approximations for the nonlinear terms are needed, since exact treatment of the triple correlations is tantamount to a full solution of the turbulence problem, which is unavailable even for the case of homogeneous turbulence. In this paper we present a framework, based on turbulence theory, to develop approximations for the local turbulence effects that are required in transport models. Two approaches are given, based on, first, Kolmogoroff‐style dimensional analysis and, second, a wave number diffusion picture. Further extensions are given, including a unified approach to the Kolmogoroff and Kraichnan spectral theories, the inclusion of cross helicity, and a discussion of nonlinearities associated with the energy difference spe

ISSN:0148-0227    

DOI:10.1029/JA095iA09p14881

年代:1990

数据来源: WILEY

摘要:

A flux rope model is proposed which has a variable degree of helical distortion from axisymmetry. The basis for this suggestion is a series of numerical and analytical investigations of magnetohydrodynamic states which result when an axial electric current is directed down a dc magnetic field. The helically distorted states involve a flow velocity and seem to be favored because of their lower rate of energy dissipation. Emphasis is on the magnetometer and particle energy analyzer traces that might be characteristic of such flux ropes. It is shown that even a fractionally small helical distortion may considerably alter the traces in minimum‐variance coordinates. In short, what may be fairly common MHD processes can render a flux rope almost unrecognizable under standard diagnostics, even if the departures from axisymmetry are not grea

ISSN:0148-0227    

DOI:10.1029/JA095iA09p14893

年代:1990

数据来源: WILEY

摘要:

Trapping saturation of the bump‐on‐tail instability is discussed using electron plasma Vlasov simulation results. The role of electrostatic harmonic excitation is considered in detail and shown to play a decisive role in the saturation of the instability. An extensive discussion of the simulation results is given to show that the results are not significantly limited by the finite number of Fourier modes used nor by the discrete distribution of those modes in wave number. It is argued that in the leading edge of Earth's electron foreshock a narrow wave number band of unstable field modes leads to trapping saturation of the bump‐on‐tail instability while simultaneously exciting electrostatic plasma waves at harmonics of the plasma frequency in similar narrow bands of shorter wavelengths. The argument is based (1) on the observations of Lacombe et al. (1985), who found intense plasma waves at the leading edge of the foreshock with a spectral distribution sufficiently narrow to trap particles in resonance with the waves, and (2) on numerical simulations of the foreshock electron plasma which indicate that trapping saturation of the bump‐on‐tail instability leads to phase space vortex formation with consequent excitation of electrostatic harmonics. Thus it is suggested that observations of electrostatic harmonics in the leading edge of the foreshock would strongly implicate trapping as the saturation mechanism for the bump‐on‐tail instability

ISSN:0148-0227    

DOI:10.1029/JA095iA09p14905

年代:1990

数据来源: WILEY

摘要:

We consider the heating and acceleration of electrons trapped on magnetic field lines between approaching magnetic mirrors. Such a collapsing magnetic trap and consequent electron energization can occur whenever a curved (or straight) flux tube drifts into a relatively straight (or curved) perpendicular shock. Our relativistic, three‐dimensional, collisionless test particle simulations show that an initial thermal electron distribution is bulk heated while a few individual electrons are accelerated to many times their original energy before they escape the trap. Upstream field‐aligned beams and downstream pancake distributions perpendicular to the field are predicted. In the appropriate limit the simulation results agree well with a nonrelativistic analytic model of the distribution of escaping electrons which is based on the first adiabatic invariant and energy conservation between collisions with the mirrors. Space science and astrophysical applications are discus

ISSN:0148-0227    

DOI:10.1029/JA095iA09p14925

年代:1990

数据来源: WILEY

摘要:

To study the problem of electron heating in quasi‐perpendicular shocks, under the combined effects of “reversible” motion, in the shock electric potential and magnetic field, and wave‐particle interactions we have derived a diffusion equation, in the drift (adiabatic) approximation and we have solved it, using a Monte Carlo method. The results show that most of the observations can be explained within this framework. Our simulation has also definitively shown that the electron parallel temperature is determined by the dc electromagnetic field and not by any wave particle induced heating. Wave‐particle interactions are effective in smoothing out the large gradients in phase space produced by the “reversible” motion of the electrons, thus producing a “cooling” of the electrons. Some constraints on the wave‐particle interaction process may be obtained from a detailed comparison between the simulation and observations. In particular, it appears that the adiabatic approximation must be violated in order to explain the observed evolution of the perpen

ISSN:0148-0227    

DOI:10.1029/JA095iA09p14939

年代:1990

数据来源: WILEY

摘要:

From initial Pioneer Venus observations during the maximum of solar cycle 21 it was evident that the position of the Venus bow shock varies with solar activity. The bow shock radius in the terminator plane changed from 2.4 Rvto 2.1 Rvas solar activity went from maximum to minimum and, as activity has increased in cycle 22, it has increased again. Our recent studies of the subsolar region show that the altitude of the nose of the bow shock varies from 1600 km at solar minimum to 2200 km at intermediate solar activity in concert with the terminator altitude so that the shape remains constant and only the size varies during the solar cycle. Using a gas dynamic model and the observed bow shock location, we infer the variation in the size of the effective obstacle during the solar cycle. At solar maximum, the effective obstacle is larger than the ionopause as if a “magnetic barrier” exists in the inner magnetosheath. This magnetic barrier acts as the effective obstacle deflecting the magnetosheath plasma about 500 km above the surface of Venus. However, at solar minimum the effective obstacle is well below the subsolar ionopause, and some absorption of the solar wind plasma by the Venus neutral atmosphere is suggested by these observations. The dependence of the solar cycle variation of the shock position on the orientation of the interplanetary magnetic field reinforces the idea that planetary ion pickup is important in the interaction of the solar wind with Ve

ISSN:0148-0227    

DOI:10.1029/JA095iA09p14961

年代:1990

数据来源: WILEY

摘要:

The spectrum of X rays from the planet Jupiter is calculated according to an auroral electron beam model. The electrons are assumed to be accelerated by a field‐aligned potential drop and penetrate into the atmosphere as a Maxwellian beam of primaries which are scattered, degraded in energy, and merged with a population of ionization secondaries having a power law energy distribution. The soft X rays observed by the Einstein Observatory satellite are due to bremsstrahlung from the secondary electrons in the H2atmosphere. A good match to the X ray data is obtained if the power law spectral index of the secondary electrons, γe, is ≃2, yielding a power law slope for the photon flux γX= γe+ 1 ≃ 3. The X ray intensity is best reconciled with a beam of primaries having a characteristic energy 30–100 keV and penetrating the homopause with an auroral energy flux typically of 10–20 ergs cm−2s−1but no greater than

ISSN:0148-0227    

DOI:10.1029/JA095iA09p14969

年代:1990

数据来源: WILEY