摘要:

A parameter study is designed and carried out to illustrate the physical effects that can be studied through analysis and interpretation of coronal minor ion spectral line observations. It is shown that minor ion line width, together with the coronal Ly α line width and coronal white light observations, can yield important information concerning the transport and dissipation of energy carried outward from the coronal base by hydromaghetic waves. Although it is difficult to infer minor ion velocities through the Doppler dimming technique, the application of this technique using both radiatively and collisionally excited lines can provide constraints on the acceleration of coronal minor ions. It is concluded that the observation of coronal minor ion spectral lines represents an important component of a concerted observational approach to the solar wind acceleration problem. It must be emphasized, however, that the measurement of line widths is the most important coronal minor ion observation to obtain

ISSN:0148-0227    

DOI:10.1029/JA092iA12p13377

年代:1987

数据来源: WILEY

摘要:

We argue, using new and previous experimental evidence that the height‐integrated flux of O+ions transported from the dayside Venus ionosphere into the nightside ionosphere is strongly reduced (shut off) at solar cycle minimum (SCmin) and that the electron impact source becomes the predominant nightside ionization source during this phase of the solar cycle. The height‐integrated transterminator O+flux, which is probably the dominant nightside ionization source at solar cycle maximum (SCmax), is shut off at SCmin by compression of the dayside ionopause to low altitude. We suggest that the ionopause is depressed because the dayside ionospheric kinetic pressure field is reduced by a factor of approximately 3 from its SCmax value and that the typical SCmin solar wind dynamic pressure, which is not less than the typical SCmax value, cannot be balanced by the ionospheric pressure field at altitudes above approximately 250

ISSN:0148-0227    

DOI:10.1029/JA092iA12p13391

年代:1987

数据来源: WILEY

摘要:

As Voyager 2 moved inward toward Jupiter in the vicinity of Ganymede's orbit, the low‐energy ion and electron fluxes recorded by the Plasma Science Experiment (PLS) intermittently fell to very low levels. The low fluxes, which lasted for times of the order of 10 min and recurred over an 8‐hour interval, have been called “voids.” Early interpretations associated the voids with Ganymede wake effects. Further analysis has demonstrated that the location and extent of the voids are inconsistent with such an interpretation. In this study, we have reexamined the PLS data in conjunction with data from the magnetic field experiment and the low‐energy charged particle experiment (LECP) in the relevant interval. The LECP data showed that the PLS voids were accompanied by large enhancements of the flux of energetic electrons and ions. No systematic signatures were found in the magnetic data. We suggest that increased energetic electron fluxes in the void regions intermittently charged the spacecraft negatively to values between a few kV and tens of kV. For a realistic phase space distribution of cold and hot particles in the Jovian magnetosphere, spacecraft charging can produce dropouts in the measured cold ion and electron fluxes and enhancements in the measured fluxes of hot particles consistent with the observations. Spacecraft charging would not produce a signature detectable by the flux gate magnetometer. We propose that the plasma density in the nominal voids is not exceptionally low and that the near‐Ganymede anomalies arose from an unusual combination of enhanced fluxes of ∼10 to ∼100 keV electrons in a low‐density plasma environment. Further work is needed to explain why this phenomenon occurred intermittently and only over a limited portion of the V

ISSN:0148-0227    

DOI:10.1029/JA092iA12p13399

年代:1987

数据来源: WILEY

摘要:

A detailed numerical study is conducted to understand the kinetic processes associated with solar wind mass loading due to pickup of cometary ions and the formation of cometary bow shocks. Because in this study we are most interested in phenomena which take place over time and spatial scales associated with ion dynamics, electrons are treated as a massless fluid. On the other hand, solar wind protons and heavy cometary ions are treated kinetically. It is found that solar wind deceleration and pickup of cometary ions take place through both the macroscopic electromagnetic fields embedded in the solar wind, as well as the microscopic fields associated with low‐frequency electromagnetic waves that are generated by the unstable velocity distribution function of the cometary ions. These electromagnetic waves can grow to very large amplitudes and their nonlinear evolution is controlled by their wave normal angle. At parallel propagation where the waves are noncompressional, parametric instabilities seem to be operative, while the oblique, compressional waves are found to steepen and form shocklets. As for the nature of cometary bow shocks, three different regimes are found, based on the shock normal angle. For shock normal angles at or near 90° (quasi‐perpendicular), a typically thin transition region (shock) is formed through steepening of fast magnetosonic pulses. This shock is a pure proton shock in that no sharp change in density or velocity of the cometary ions is observed across the shock. At intermediate shock normal angles (oblique), a much wider transition region is seen where the solar wind is gradually heated and decelerated. This region, whose width is much larger than the gyroradius of cometary ions, is associated with a high level of turbulence due to ion pickup instabilities and is unlike a typical fast magnetosonic shock. Finally, at small shock normal angles (quasi‐parallel), a narrow transition region is found which is similar to a parallel terrestrial shock. The formation of this shock is not due to wave steepening but rather is the result of scattering and heating of the solar wind protons by the large‐amplitude electromagnetic waves that are generated through the relative drift between the protons and the cometary ions. These results are compared and shown to be in qualitative agreement with the recent observations a

ISSN:0148-0227    

DOI:10.1029/JA092iA12p13409

年代:1987

数据来源: WILEY

摘要:

We use stationary point analysis to compute generalized critical Mach numbers for finite amplitude fast and slow shocks in classical MHD fluids. We pay particular attention to the case where the resistive and thermal conduction dissipation scale lengths are comparable and much larger than the viscous scale lengths. With both resistivity and thermal conduction, the critical Mach number at which viscosity must be invoked is determined by the condition that the downstream flow speed equal the isothermal sound speed. We also show that resistivity and thermal conduction can provide convergent stationary point solutions for nearly all slow shocks, except perhaps switch‐off shock

ISSN:0148-0227    

DOI:10.1029/JA092iA12p13427

年代:1987

数据来源: WILEY

摘要:

The present paper is a continuation of the preceding article (Lee et al., 1986) in which it is suggested that the nonadiabatic motion of the directly transmitted ions in a quasi‐perpendicular shock wave can result in an increase of the ion kinetic temperature transverse to the ambient magnetic field in the downstream. A series of computer simulations based on a hybrid code have been carried out to examine the dynamics of the transmitted ions in both the subcritical and supercritical shock waves. It is found that in both cases the directly transmitted ions can contribute to the heating process. In the case of a resistiveless supercritical shock the reflected and transmitted ions can be equally important; whereas for a subcritical shock the transmitted ions are primarily responsible for the ion heatin

ISSN:0148-0227    

DOI:10.1029/JA092iA12p13438

年代:1987

数据来源: WILEY

摘要:

This paper establishes that the deHoffmann‐Teller frame electron bulk velocityVeHT(x) is nearly parallel to the magnetic fieldB(x) everywhere through those one‐dimensional, time stationary layers which possess a normal mass flux, even though electric fields perpendicular to the magnetic field exist throughout such layes. Examples of layers of this class are the fast and slow shocks as well as rotational discontinuities. The explicitly calculated corrections to are proportional to (1) the electron pressure anisotropy, (2) the electron inertia, and (3) the bulk resistivity. The near alignment of electron bulk velocity inside layers with a normal mass flux represents, for electrons only, a nearly complete extension of the 1950 deHoffmann‐Teller theorem concerning the exact alignment of electron and ion flows with the magnetic field in both asymptotic regions outside such layers. This alignment would be perfect within such layers if the electrons were approximated as a massless, dissipationless, isotropic gas. Without such approximations this alignment is better in more oblique shocks of either type; low electron β improves the approximation in the fast‐mode shock, and higher ion sound Mach numbers enhance the approximation for slow shock layers. This approximate symmetry provides a simple description of the causes of currents within the layers and an overview of the two‐fluid “mixing” which is caused by passage through them. It also permits a clarification of the coplanarity theorems often used in shock work. A clear comparison is also provided of the relative efficiencies of the electron pressure anisotropy, inertia, and resistivity in the slippage between the electron flow lines and the magnetic tubes of force known as “frozen flux violations.” The natural scale lengths for amplification and twists of the magnetic field in the fast and slow shocks are recovered, starting with the field‐aligned approximation. This calculation also correctly recovers the polarization of the fast‐ and slow‐mode shock layers, in addition to allowing a direct calculation of the initial scale length for such twists within the shock layer. Finally, in the regime of validity of the field‐aligned electron flow approximation it is demonstrated that testing with electric and magnetic field profiles for the existence of a deHoffmann‐Teller frame at magnetopause crossings is generally superior to ion bulk velocity‐magnetic field tests of tangential stress balance. Such tests can be performed whenever the scale lengths of the layer are much larger than the electron gyroradius, regardless of the size of the layer

ISSN:0148-0227    

DOI:10.1029/JA092iA12p13447

年代:1987

数据来源: WILEY

摘要:

A new mechanism is proposed that is able to explain the sudden disruption of the magnetotail energy storage and the explosive onset of isolated magnetospheric substorms. During the growth phase the magnetotail accumulates additional magnetic flux and evolves in such a way that the special parameter κe= (Bn/Bo)(L/ρeo)1/2describing the ratio of characteristic frequencies of the electron motion along and across the field reversal region quickly decreases. Here Bnand Boare the components of magnetic field across and along the central plasma sheet, L is the thickness of the field reversal region, and ρeois the Larmor radius of thermal electrons in the field Bo. When κereaches a critical value κe|crit≅ 1.6, a nonlinear interaction between the degrees of freedom of electron motion becomes strong enough to cause a chaotization of the electron orbits. This effect results in a fast growth of a collisionless tearing mode instability even within the framework of the WKB approximation, i.e., in a wide interval of possible wavelengths 1>kL>Bn/Bo. The proposed theory‐ gives the mechanism of the formation of new reconnection region in the near‐Earth magnetotail, based exclusively on intrinsic properties of the magnetotail evolution. This reveals the natural relation between the onsets of isolated substorms and an internal large‐scale magnetotail instability of the tearing type considered i

ISSN:0148-0227    

DOI:10.1029/JA092iA12p13456

年代:1987

数据来源: WILEY

摘要:

The magnetic field orientation just earthward of the dayside magnetopause from 95 ISEE and 11 HEOS crossings with |By|>|Bz| in the adjacent magnetosheath indicate a statistically significant ∼1 REshift of the cusps toward dawn (dusk) in the northern hemisphere and dusk (dawn) in the southern hemisphere for positive (negative) By. This small shift near the magnetopause and the contrasting large shifts of some reported ionospheric cusp signatures are explained in terms of a finite thickness magnetopause model through which the cusps shift gradually from the inner to the outer edg

ISSN:0148-0227    

DOI:10.1029/JA092iA12p13467

年代:1987

数据来源: WILEY

摘要:

Data from three instruments, the magnetometer, the charge‐energy‐mass spectrometer, and the medium‐energy particle, analyzer onboard the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (CCE) spacecraft have been used to study a compressional Pc 5 wave observed at 1925–2200 UT on day 202 (July 21) of 1986 at a radial distance of ≃8REin the postmidnight sector near the beginning of minor geomagnetic activity. The wave exhibited harmonically related transverse and compressional magnetic oscillations, modulation of the flux of medium energy protons (E≳ 10 keV), and a large azimuthal wave number (m∼ 65). These properties are similar to those of compressional Pc5 waves observed previously at geostationary orbit. The unique observations associated with the CCE event are the occurrence in the postmidnight sector, the eastward (or sunward) propagation with respect to the spacecraft, and the left‐handed polarization of the perturbed magnetic field. These are opposite to previous geostationary observations. We propose that the unique propagation and polarization are caused by a dawn‐to‐dusk dc electric field which convects the plasma sunward. The wave, probably propagating westward in the plasma rest frame, appears to propagate eastward to the observer because the electric field drift velocity is larger than the

ISSN:0148-0227    

DOI:10.1029/JA092iA12p13472

年代:1987

数据来源: WILEY