摘要:

Numerical solutions of a magnetohydrodynamic model are carried out to describe the nonlinear interaction of corotating high‐speed streams near the solar equatorial plane. Two problems are studied. The first problem is to simulate the evolution of an idealized high‐speed stream. Numerical solutions are obtained to represent the variations of flow velocity, magnetic field, plasma density, temperature, and conduction heat flux in the interaction region. They demonstrate that the dynamical interaction and heat conduction process are responsible for the thermal structure of a high‐speed stream. The second problem deals with the formation of corotating shock waves near the leading edge of a broad stream resulting from the merging of characteristic curves. Corotating shocks do not necessarily occur in pairs; a reverse shock can be formed without a forward shock n

ISSN:0148-0227    

DOI:10.1029/JA086iA05p03263

年代:1981

数据来源: WILEY

摘要:

This investigation has studied the structure of the interplanetary sector boundaries observed by Helios 1 within sector transition regions during the time interval from December 1974 to April 1975. A sector transition region is the region of variable magnetic field magnitude and direction observed in or near the ecliptic plane between magnetic sectors in the interplanetary magnetic field (IMF). Such regions are found generally to be complex in character, consisting of intermediate (nonspiral) average field orientations in some cases, as well as a number of large‐angle (120° ≤ω ≤180°) directional discontinuities (DD's) on the fine scale (time scales<1 h). Such DD's are found to be more similar to tangential than rotational discontinuities, to be oriented on average more nearly perpendicular than parallel to the ecliptic plane (the mean tilt is ∼58°), to be accompanied usually by a large dip (≳80%) inB(≡|B|), and, with a most probable thickness of 3 × 104km, significantly thicker structures on average (by a factor of 10 or more) than ordinary DD's previously studied in the IMF. It is hypothesized that the observed structures represent multiple traversals of the global heliospheric current sheet caused by local fluctuations in the position of the sheet. There is evidence that such fluctuations are sometimes produced by wavelike motions or surface corrugations of scale length 0.05–0.1 AU, superimposed on the large‐scale structure. The observed steep inclinations could be produced by a combination of current sheet warping at the sun, a global wave structure, and the observed small‐scale fluctuations. No radial distance dependence is found for the current sh

ISSN:0148-0227    

DOI:10.1029/JA086iA05p03273

年代:1981

数据来源: WILEY

摘要:

Initial observations of the dayside magnetopause thickness using UCLA magnetometers on board Isee 1 (International Sun‐Earth Explorer) and Isee 2 indicate that this boundary may only be a few ion gyroradii thick during periods of observed southward interplanetary magnetic field (IMF). Such a thin current layer can destabilize the collisionless tearing mode, a probable first step in the reconnection sequence. We have calculated the linear growth rate assuming typical magnetopause parameters and find that it is a sensitive function of IMF orientation, magnetopause thickness, and electron number density. We also show that the finite size of the dayside magnetopause necessitates the existence of a two‐dimensional tearing mode wave vector spectrum, a consequence of the inability of a thermal electron to maintain Landau resonance with the wave for a growth period. Implications regarding reconnection are discus

ISSN:0148-0227    

DOI:10.1029/JA086iA05p03289

年代:1981

数据来源: WILEY

摘要:

We calculate the linear dispersion relation for a collisionless plasma in a sheared one‐dimensional current sheet and extend the existing calculations to include β ∼1 plasmas, where β is the ratio of the thermal pressure to the magnetic pressure. DefiningBzas the component of the magnetic field which reverses sign as a function ofxandByas that component which is nonzero but does not reverse sign (the guide‐field), we find that the tearing mode eigenstructure and temporal growth rate are a sensitive function of the ratiosls/lG, ls/ln, andls/lT, wherelsis the shearing length of the magnetic field,lGis the gradient scale length of the guide fieldBy,lnis the number density gradient scale length, andlTis the temperature gradient scale length. In particular, if β ≳1, andls

ISSN:0148-0227    

DOI:10.1029/JA086iA05p03299

年代:1981

数据来源: WILEY

摘要:

It is shown that it is useful to define double layers and shocks so that the ion phase spaces of double layers are shown to be just the mirror image (about zero ion velocity) of the ion phase spaces for laminar electrostatic shocks. The distinguishing feature is the direction of the free ion velocity. It is also shown that double layers can exist without the presence of trapped ions. The Bohm condition for double layers, that the ion drift velocity on the high potential side must be greater than the ion sound velocity, is shown to be related to a requirement of a lower limit on the Mach number of laminar electrostatic shocks.

ISSN:0148-0227    

DOI:10.1029/JA086iA05p03307

年代:1981

数据来源: WILEY

摘要:

Observations of pancake (peak flux near 90° pitch angle) distributions of low‐energy (≲100 eV) ions are reported. Pancake distributions occur often in H+and He+simultaneously while O+fluxes are either undetectable or field‐aligned. These H+and He+pancake distributions typically display characteristic energies of the order of 10 eV and are frequently mixed with higher density, colder (κT≲3 eV), isotropic, quasi‐Maxwellian components. They appear often within the outer regions of the plasmasphere, and seem to occur most frequently on the dayside and near the magne

ISSN:0148-0227    

DOI:10.1029/JA086iA05p03311

年代:1981

数据来源: WILEY

摘要:

The magnetic structure of Jupiter's magnetopause, as observed by the space probes Pioneer 10 and 11, is compared with terrestrial magnetopause structures from the OGO 5 mission. The Jovian magnetopause thickness, deduced from a rapid triple crossing, is found to lie in the range 3500–5200 km, while the adjoining plasma boundary layer was 5600–8400 km thick. Comparison with the terrestrial situation suggests that the dayside magnetopause thickness in both cases is a few times the ion gyroradius and that the dayside boundary layer is also of about the same width. The magnetopause normal vector and normal magnetic field component are determined for each crossing by use of minimum variance analysis. The results indicate a blunt and floppy mangetopause surface, for the most part with an insignificant normal magnetic field component. Only for three of the 14 crossings did this component exceed 1.5 times its error estimate. In two of these cases, the magnetic field tangential to the magnetopause displayed the characteristic features of a rotational discontinuity. Such structures are occasionally also seen at the earth's magnetopause and are presumably associated with an open (reconnecting) magnetosphere. Some of the Jovian crossings also display other features that have been observed at the earth: initial field rotation in the ‘wrong’ sense at the outer edge of the magnetopause; diamagnetic effects and field‐aligned currents at the inner edge of the plasma bound

ISSN:0148-0227    

DOI:10.1029/JA086iA05p03321

年代:1981

数据来源: WILEY

摘要:

We study, in a steady state situation, the relationships that can exist between charge exchange (CE) and the emission of electromagnetic ion cyclotron waves resonating with energetic ring current protons. The CE process between an energetic proton and a cold hydrogen atom is considered in both its aspects: as a loss of particles and as a source of anisotropy in velocity space. This anisotropy, which is required in order that the waves be amplified, can be generated by the CE process itself because of its dependance on the proton pitch angle. By an iterative procedure we establish a formula for the wave field which includes the possible CE effect. This last effect depends in particular on the intensity of the proton source (dn/dt). One deduces from it that there exists a critical value of the source intensity (dn/dt)cwhich requires discriminating between three different equilibrium situations. When (dn/dt)(dn/dt)c, a strong coupling can exist between wave‐particle interaction and CE, in a weak diffusion regime. In this last case one establishes a formula for the integrated wave field that only depends on three easily accessible parameters: the McIlwain parameterL, the cold plasma densityn0, and the wave peak frequencyfM. A preliminary study of some GEOS 1 data seems to indicate this effective coupling. Also we discuss, from this point of view, some aspects of the emission of the stable auroral red arcs. Finally, when (dn/dt)≫(dn/dt)c, protons undergo strong diffusion, and the CE effect has negligible effect on the wave emiss

ISSN:0148-0227    

DOI:10.1029/JA086iA05p03335

年代:1981

数据来源: WILEY

摘要:

Simultaneous quasi‐periodic variations of electrons (Ee>22 keV), ions (Ei>27 keV), and the geomagnetic field in the Pc 5 period range were observed on board the geostationary satellite GEOS 2. Fifty‐four events occurred between August 1978 and July 1979. Two types of events could be distinguished. During the first type, electrons and ions reached their flux maxima and minima simultaneously (‘in‐phase events’). During events of the second type the electron flux had minima at the ion flux maxima and vice versa (‘out‐of‐phase events’). During all events the total magnetic induction had minima at the time of the ion flux maxima and vice versa. The in‐phase events occurred preferentially around noon. They had longer average periods and durations than the out‐of‐phase events that appeared around dusk. The phase relationship between electrons and ions was found to depend on the slope of the electron pitch angle distributions. For pitch angles α ≷90° the variations of the electron intensityJwere in phase with the ion intensity variations fordJ/dα≶0 and out of phase fordJ/dα≷0. The ion intensity variations were associated with azimuthal asymmetries that can be interpreted as the east to west movement of regions of intense ion fluxes. The observations are in reasonable agreement with the predictions of the drift mirror instability theory. In terms of this theory the two different types of quasi‐periodic events can be regarded as the result of different responses of the electrons to the disturbance conditions around noon and dusk. The drift mirror instability is associated with drifting energetic proton bunches that generate Alfvén waves. An estimate of the wave amplitudes yielded value

ISSN:0148-0227    

DOI:10.1029/JA086iA05p03345

年代:1981

数据来源: WILEY

摘要:

A comprehensive numerical study of ion acoustic double layers has been performed for both periodic and nonperiodic systems by means of one‐dimensional particle simulations. For a nonperiodic system, an external battery and a resistance are used to model the magnetospheric convection potential and the ionospheric Pedersen resistance. It is found that the number of double layers and the associated potential buildup across the system increase with the system length. The potential jump ΔϕDacross a single double layer is approximatelyeΔϕD/Te≈ 1 and the distance between two consecutive double layers is typically 1000 Debye length, whereeandTeare the electronic charge and temperature. There is little interaction among double layers, and each double layer behaves almost independently of the others. The maximum increase of plasma energy through the enhanced resistance associated with double layer formation is found to be about 15% of the initial plasma energy for the parameters used in the simulations. The resistivity takes its maximum value at the instance when the double layers are just formed and then drops to almost zero afterwards, indicating the presence of a superconducting phase due to the formation of a quasistable Bernstein‐Green‐Kruskal type equilibrium. The equilibrium is, however, unstable with respect to the emission of ion acoustic solitons, and the double layers eventually decay as the solitons carry the energy away from the double layers propagating through a plasma toward the downstrea

ISSN:0148-0227    

DOI:10.1029/JA086iA05p03357

年代:1981

数据来源: WILEY