摘要:

From a synthesis of data from the Plasma Science and Ultraviolet Science instruments on the Voyager 1 spacecraft we present a radial profile of O++abundance between 4.9 and 42 RJ. We observe a sharp rise in O++mixing ratio near 7.5 RJ, coincident with a sharp rise in effective electron temperature at the outer boundary of the Io plasma torus. Beyond 8.5 Rj the O++mixing ratio is found to be roughly constant which indicates freezing of the ionization prevailing at the outer edge of the hot torus.

ISSN:0094-8276    

DOI:10.1029/92GL00070

年代:1992

数据来源: WILEY

摘要:

The Jovian aurora is the most powerful planetary aurora in the solar system; to date, however, it has not been possible to establish conclusively which mechanisms are involved in the excitation of the auroral emissions that have been observed at ultraviolet, infrared, and soft X ray wavelengths. Precipitation of Iogenic heavy sulfur and oxygen ions, downward acceleration of electrons along Birkeland currents, and a combination of both of these mechanisms have all been proposed to account for the observed auroral emissions. Modeling results reported here show that precipitating auroral electrons with sufficient energy to be consistent with the Voyager UVS observations will produce bremsstrahlung X rays with sufficient energy and intensity to be detected by the Solar Flare X Ray and Cosmic Ray Burst Instrument (GRB) on board the Ulysses spacecraft The detection of such bremsstrahlung X rays at Jupiter would provide strong evidence for the electron precipitation mechanism, although it would not rule out the possibility of some heavy ion involvement, and would thus make a significant contribution toward solving the mystery of the Jovian aurora.

ISSN:0094-8276    

DOI:10.1029/92GL00052

年代:1992

数据来源: WILEY

摘要:

As a step to the study of the large‐scale flow in the magnetosheath, the MHD flow past an obstacle is investigated. A time asymptotic method is used to obtain 3D steady‐state solutions. The results indicate the formation of a depletion layer near the obstacle due to the increase of the magnetic field. Along the earth‐sun line the plasma density increases first and then decreases from the post bow shock to the magnetopause. The local density maximum in front of the magnetopause may correspond to what was recently observed. When the interplanetary magnetic field direction is tilted from the solar wind flow, the IMF influences the shape of the bow shock as well as the location of the stagnation point at the magnetopause in a way consistent with observation. In addition, the results show the existence of a magnetosheath current in the post parallel‐shock

ISSN:0094-8276    

DOI:10.1029/91GL03007

年代:1992

数据来源: WILEY

摘要:

Seismo‐Electromagnetic (SEM) waves observed by low‐altitude satellites passing over seismic regions were studied. The data of the COSMOS‐1809 satellite were analysed over the earthquake region in Armenia during the period from January 20 to February 17, 1989. Intense EM radiation at frequencies below 450 Hz was observed at the L‐shells of the earthquake, during 12 orbits out of the 13 that passed within 6° in longitude from the epicenter, and during 1 out of 6 in the range of 6°–8° longitude away from this region. The other orbits, which passed 10°–12° from the epicentre, showed no effect. To complete this study, we used the emissions observed by another low‐altitude satellite (AUREOL‐3).It is shown that during the event the seismic region is permanently radiating; the intensity and the envelope shape of the wave depend on its time relatively to the time of the earthquake. Their frequency spectra are compared to the average spectrum recorded in the same geomagnetic regions. Similar wave intensities and spectral distributions were observed on the two satellites during

ISSN:0094-8276    

DOI:10.1029/91GL02775

年代:1992

数据来源: WILEY

摘要:

We demonstrate that conventional ionosondes can provide long‐term observations of intermediate, descending, and transitional layers in the 100–200 km altitude region of the ionosphere. Using 15 consecutive days of observations at Townsville, Australia, during the SUNDIAL campaign of September 1989, we tracked the “birth” of the layers at altitudes above 150 km and their systematic downward motion to the 110 km region at rates between 4 and 5 km/hr. The observations are compared with NCAR TIGCM simulations, and the results show: (i) that the layering process is identifiable with meridional wind‐shear‐node convergence of ions; (ii) that zonal wind controls of the layers are insignificant under the prevailing conditions; and (iii) that electric fields play an important role in the effectiveness of the ion‐convergence and downward transport processes at altitudes above 125 km. The measurement and modelling comparisons are the first of a kind, providing insight into the relative roles of winds and electric fields, and opening possibilities for determining the global characteristics of the layers and their cause‐effect roles in the dynamics of the lower ionospheric‐the

ISSN:0094-8276    

DOI:10.1029/91GL02774

年代:1992

数据来源: WILEY

摘要:

This report describes a transient luminosity observed at the altitude of the airglow layer (about 95 km) in coincidence with a lightning flash in a tropical oceanic thunderstorm directly beneath it. This event provides new evidence of direct coupling between lightning and ionospheric events. This luminous event in the ionosphere was the only one of its kind observed during an examination of several thousand images of lightning recorded under suitable viewing conditions with Space Shuttle cameras. Several possible mechanisms and interpretations are discussed briefly.

ISSN:0094-8276    

DOI:10.1029/91GL03168

年代:1992

数据来源: WILEY

摘要:

In this paper we present the first rotary spectra of the horizontal wind shear measured in the stratosphere with a resolution of 25m. A low frequency gravity wave propagating upward with a vertical wavelength of ≈ 1000 m is clearly identified. This result is quite consistent with many previous observations and may explain why the horizontal velocity spectra may be biased and indicates a power variation law versus vertical wave number steeper than those of both temperature and vertical velocity. Nevertheless, around the small vertical wavelengths (100 m), the asymptotic slope of temperature and horizontal velocity spectra are both close to ‐3 and so in better agreement with the saturated model predicti

ISSN:0094-8276    

DOI:10.1029/92GL00044

年代:1992

数据来源: WILEY

摘要:

A hypothetical mechanism is described whereby the QBO of zonal winds in the lower tropical stratosphere alters the distribution of intense deep convective activity throughout the tropical West Pacific. In conjunction with the annual cycle and the buildup of heat in the West Pacific warm pool, these QBO‐linked variations of deep convection cause variations in the Central Pacific trade winds and Walker circulation which in turn govern the occurrence of ENSO events in the tropical Pacific. Although some details of the hypothetical mechanism are tentative, empirical results present a compelling argument for the stratospheric QBO to at least partly regulate ENSO variabilit

ISSN:0094-8276    

DOI:10.1029/91GL02950

年代:1992

数据来源: WILEY

摘要:

We present new seismological evidence for the depth of depression of the volcano‐crust and crust‐mantle (Moho) boundaries and the nature of the volcano‐crust interface beneath the southeast flank of Mauna Loa volcano, Hawaii, from a profile of S to P converted waves from deep local earthquakes. Three‐component seismic array observations of these converted waves indicate the presence of a low velocity layer (LVL) at a depth of about 10 km, corresponding to the base of the volcanic pile. The thickness of the LVL is estimated to be approximately 1 km. The LVL and the Moho both deepen significantly from southwest to northeast beneath Mauna Loa's southeast flank. The observed polarity reversal of the LVL conversion compared to the Moho conversion strongly supports the existence of a buried sediment layer at the volcano‐ocean crust interface, at a depth that is consistent with the depth distribution of thrust earthquakes. Our results conflict with models of lithosphere flexure at Hawaii, indicating a depression of the lithosphere greater than that predicted by flexure models. The thickness of underlying oceanic crust is observed to be relatively constant, so this discrepancy cannot be accounted for by crustal underplat

ISSN:0094-8276    

DOI:10.1029/91GL03040

年代:1992

数据来源: WILEY

摘要:

The temporal evolution of the source mechanism is retrieved for the October 13, 1963 Kurile islands earthquake by applying centroid moment tensor inversions to successively different narrow frequency windows between 2 and 12.5mHz. The lateral heterogeneity effect is corrected using model M84A of Woodhouse and Dziewonski [1984]. To the first approximation the frequency‐dependent scalar moment follows an ω−2decay law and the origin time shift is frequency independent, which can be simply explained by an isoscele source time function 140s long and a total moment release of 62 × 1027dyne.cm. The expansion of the moment rate functions in terms of spline functions enables to retrieve a smooth temporal change of the moment tensor. This second inversion yields a total moment of 71 × 1027dyne.cm with a peak activity at about 70s after the onset in a total source time of 250s. The mechanism has remained unchanged during the main stage. The occurence of back‐slip to the amount of 6.5% of the total moment is implied for the

ISSN:0094-8276    

DOI:10.1029/91GL01692

年代:1992

数据来源: WILEY