摘要:

A very unusual change is reported in the morphology of the Jovian Ly‐α bulge, a region of enhanced Ly‐α emission near the Jovian spin equator at a fixed system III magnetic longitude (λIII) that has persisted since its discovery in the late 1970s. Observations from 1979‐March 1989 consistently showed Ly‐α emission that peaked near λIII∼100° then dropped and remained constant in the nonbulge region from λIII∼200‐360°. During this time the bulge had a full width half maximum (FWHM) of ∼100‐130°. In September 1989 and March and May 1991 this nominal morphology was substantially different: the Ly‐α brightness showed a much broader peak (FWHM ∼200°) centered near 40° meaning that the bulge encompassed nearly the entire equatorial region of the planet and the previously flat nonbulge region was virtually nonexistent. These observations show a fundamental change in the morphology of the Ly‐α bulge which was not observed in the previous 11 years. Present theories for the bulge invoke asymmetric processes or densities in the Jovian ionosphere. The change in bulge morphology may imply a significant change in the pattern of large‐scale magnetospheric convection, in turn implying a possible change in longitudinal density structure of the Io plasma torus between March and Sept 1989. The possibility that the observed change in bulge morphology is related to long‐term

ISSN:0094-8276    

DOI:10.1029/91GL02545

年代:1991

数据来源: WILEY

摘要:

IUE spectra of Jupiter's low latitude H Ly α emission line profile have been obtained on and off the “Ly α bulge” at the longitude of the central meridian (CML), near the limb of the planet, and at midlatitudes on the CML. All equatorial locations show line broadening beyond the IUE resolution of 0.14 Å. The equatorial lines are significantly broader than at midlatitudes on the CML, and near the limb the lines are broader than on the CML, both on and off the bulge. The observed line shapes are not consistent with pure resonant scattering of solar Ly α by upper atmospheric H atoms with a variable H abundance: calculated profiles near the limb have much less intense cores. The broadening of the bulge line profiles on the CML and limb compared with off‐bulge regions demonstrates that the bulge effect is a broadening of the line, not an increase in the line core flux. We propose that there is either a substantial broadened internal source and/or the upper atmosphere has considerable Doppler broadening (winds or turbulence) to increase the resonant scattering l

ISSN:0094-8276    

DOI:10.1029/91GL02091

年代:1991

数据来源: WILEY

摘要:

Previous studies of photometric time sequences from pulsating aurora have established that the O(¹S) metastable leading to the emission of the auroral green line is excited by two processes, one direct, the other indirect, with the indirect precursor having a lifetime of about 0.1s. In this paper we report on new high time‐resolution measurements which extend the observations of precursor lifetime down to about 0.01s. Simultaneous observations of pulsations in the Vegard‐Kaplan band system were also made in order to obtain direct measurements of the lifetimes of the N2(A³∑u+) metastable; these were found to be essentially equal to those of the precursor over the lifetime range from 0.01s to 0.1s, and less well‐correlated for longer lifetimes. This result identifies the O(¹S) precursor as N2(A³∑u+), at least over the altitude range

ISSN:0094-8276    

DOI:10.1029/91GL02553

年代:1991

数据来源: WILEY

摘要:

Two dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the kinetic evolution of the Kelvin‐Helmholtz instability at a boundary similar to the ionopause at the planet Venus. The magnetic field has been chosen to be essentially transverse to the plasma flow velocity, which is the most unstable case for the Kelvin‐Helmholtz instability. Effects analogous to those found in MHD simulations are recovered, including a mode cascade to longer wavelengths. In addition, isolated structures on the order of the ion gyroradius are formed which can cross the boundary in either direction. These structures may be related to flux ropes commonly observed near the ionopause of Venus. The time evolution of these structures may represent transport across the boundary layers and the smearing of the shear layer may give rise to internal structure in the boundary. Similar processes occur in magnetized layers akin to the Earth's magnetopa

ISSN:0094-8276    

DOI:10.1029/91GL02552

年代:1991

数据来源: WILEY

摘要:

We develop a 2‐D model for the standing slow‐mode expansion fan that is expected to exist downstream of the Jovian moon Io. The leading edge of the expansion fan makes an angle of 45° with the upstream magnetic field direction, and the fan width is about 114°. The plasma flow returns to its upstream direction by way of a slow‐mode shock behind Io where the MHD parameters return asymptotically to their upstream conditions. The magnetic field perturbation within the fan is much smaller than that associated with the Alfvén wing, which lies farther

ISSN:0094-8276    

DOI:10.1029/91GL02541

年代:1991

数据来源: WILEY

摘要:

The coupling of different MHD waves is important for an understanding of laboratory, space, and solar plasmas. In this paper we investigate fast and Alfvén mode coupling in the cold plasma limit. The medium is assumed to have an invariant direction (e.g., slab or axisymmetric models) and carries an arbitrary solenoidal background magnetic field. Wave fields are Fourier analysed with wave numberkβin the invariant direction. The coupled linearised wave fields are then expanded in a power series (ofkβ) to yield a decoupled hierarchy of inhomogeneous wave equations. The higher order terms in the series represent wave coupling phenomena such as the resonant excitation of Alfvén waves, and the damping of a magnetospheric cavity m

ISSN:0094-8276    

DOI:10.1029/91GL02630

年代:1991

数据来源: WILEY

摘要:

Observations from AMPTE/CCE in the Earth's magnetosheath on October 5, 1984 are presented to illustrate 0.1–4.0 Hz magnetic field pulsations in the subsolar plasma depletion layer (PDL) for northward sheath field during a magnetospheric compression. The PDL is unambiguously identified by comparing CCE data with data from IRM in the upstream solar wind. Pulsations in the PDL are dominated by transverse waves withF/FH+ ≤ 1.0 and a slot in spectral power atF/FH+ = 0.5. The upper branch is left hand polarized while the lower branch is linearly polarized. In the sheath the proton temperature anisotropy,A = T⟂/T∥− 1, is ≈ 0.6 but it is ≈ 1.7 in the PDL during wave occurrence. The properties and correlation of waves with increased anisotropy indicate that they are electromagnetic ion cy

ISSN:0094-8276    

DOI:10.1029/91GL02238

年代:1991

数据来源: WILEY

摘要:

A series of measurements of 5–30 MeV/nucleon oxygen ions made with track detector stacks on Cosmos satellites show isotropic angular distributions during solar energetic particle events. Solar‐quiet times, on the other hand, have highly anisotropic distributions suggestive of a trapped‐particle component. Detailed Monte Carlo simulations confirm this interpretation and allow us to measure the trapped and cosmic‐ray contributions to the observed fluxes. Our data are fully consistent with anomalous cosmic‐ray ions, rather than radial diffusion from the outer zone, as the source of the trapped

ISSN:0094-8276    

DOI:10.1029/91GL02551

年代:1991

数据来源: WILEY

摘要:

The CDAW‐9 Event C focused upon the early part of 3 May 1986 when a large substorm onset occurred at 0111 UT. By modifying the Tsyganenko 1989 magnetic field model, we construct a model in which the near‐Earth current systems are enhanced with time to describe the observed development of the tail magnetic field during the growth phase. The cross‐tail current intensity and the thickness of the current sheet are determined by comparison with three spacecraft in the near‐Earth tail. The location of the auroral bulge as recorded by the Viking imager is mapped to the equatorial current sheet. The degree of chaotization of the thermal electrons is estimated, and the consequences to the tail stability towards ion tearing are discussed. We conclude that the mapping of the brightening region in the auroral oval corresponds to the regions in the tail where the current sheet may be unstable towards ion

ISSN:0094-8276    

DOI:10.1029/91GL02002

年代:1991

数据来源: WILEY

摘要:

It is important to know the electric field in the tail current sheet in order to understand how particles behave and how much energy is being dissipated. The electric field is also a measurement of the reconnection rate during substorms. For the CDAW‐6 substorm period of March 22, 1979, we used the ion data from the medium energy particles experiment (MEPE) on the ISEE‐1 satellite, and studied nine measurements of the 3D distribution function centered on the center of the current sheet. The measured distribution function was then integrated to obtain the average of bulk flow velocity in the geocentric solar ecliptic (GSE) frame. This bulk flow velocity was then broken up into its components perpendicular and parallel to the magnetic field for the nine cases. It was further assumed that the perpendicular component was due, in part, to an energy dependent drift and to an energy independent electric field drift. Using the bulk flow velocities from any two energy channels we can separate out the electric and energy dependent drifts and thus obtain electric field and energy dependent components. The two lowest energy channels (34.3keVand 54.9keV) give the main results, and the 80.4keVand 118.8 keV channels are used as a cross check. We find thatExfluctuates approximately ±5mV/m, andEy± 10mV/m, in reasonable agreement with measurements by the electric field instrument [Pedersen et al., 1985], with most of the fluctuation presumably due to the motion of the current sheet. Using current sheet oscillation theory and the central current sheet data points, we can estimateEyin the frame of the current sheet and find a positive averageEywith a magnitude of ≈ 0.1mV/m, which is also consistent with that expected for reconnection in this substorm time period. TheEzcomponent has a remarkable linear correlation withBx, with a correlation coefficient of 0.91. AssumingBxvaries linearly withz, a positive ion density of 3.14×10−20/d(Re)Coulombs/m³ is implied. Such a positive space charge near the current sheet center is expected th

ISSN:0094-8276    

DOI:10.1029/91GL01999

年代:1991

数据来源: WILEY