摘要:

Following the recent optical discovery of intense sodium D‐line emission from Mercury, we explore the scenario of an extended exosphere of sodium and other metallic atoms. It is shown that the strong effect of solar radiation pressure acceleration would permit the escape of Na atoms from Mercury's surface even if they are ejected at velocity less than the surface escape velocity. Fast photoionization of the Na atoms is effective in limiting the tailward extension of the sodium exosphere, however. The subsequent loss of the photoions to the magnetosphere could be a significant source of the magnetospheric plasma. The recirculation of the magnetospheric charged particles to the planetary surface could also play an important role in maintaining an extended sodium exosphere as well as a magnetosphere of sputtered metallic ion

ISSN:0094-8276    

DOI:10.1029/GL013i005p00423

年代:1986

数据来源: WILEY

摘要:

Solar ultraviolet flux changes associated with the 27‐day solar rotational period cause corresponding variations in mesospheric ozone near the maximum of the 11‐year sunspot cycle. This statement is based on a correlation and spectral analysis of ozone mixing ratios, deduced from Solar Mesospheric Explorer satellite‐based measurements of 1.27‐µm O2airglow emission and solar flux observations made from the same spacecraft in 1982. With the Lyman α flux taken as an indicator of solar ultraviolet variability, spectral analysis shows a primary period of 27.1 days with a secondary period of 13.5 days. The 27.1‐ day period is observed in the ozone mixing ratio data together with other periods, including 13.5 days. Both a classical statistical analysis and a time series treatment show that for 244 days there is a correlation between ozone and solar flux near 50 km and between 65 and 70 km. Calculations predict a positive correlation over the entire mesosphere if there is no change in temperature accompanying the solar flux. Lack of correlation is temperat

ISSN:0094-8276    

DOI:10.1029/GL013i005p00427

年代:1986

数据来源: WILEY

摘要:

It is shown that from the appearance of certain photographs taken from STS‐5, it is possible to make several useful statements about the production mechanism of the visible glow around the shuttle, and presumably other space vehicles. These are: 1) the electronic excitation of particles in the gas phase by either electrons or fast neutrals is probably not important, 2) the interaction of thermal atoms and molecules with the surfaces does not generate the glow, and 3) translational energy accommodation of fast particles is efficient enough that a single surface collision reduces the kinetic energy below the threshold for glow production. There is no glow without the direct interaction of fast particles with the surfaces, and the sum of the contributions of the three above processes does not exceed 14% of the observed surface intensit

ISSN:0094-8276    

DOI:10.1029/GL013i005p00431

年代:1986

数据来源: WILEY

摘要:

Gas ionization induced by a fast plasma injection has been observed with the Space Experiments with Particle Accelerators (SEPAC) Experiment on Spacelab‐1. When an impulsive high‐density plasma was injected from the orbiter, waves near the lower‐hybrid frequency were enhanced, and the surrounding gas drifting with the orbiter was ionized for several tens of milliseconds after the plasma injection. The long‐duration gas ionization was observed only when the plasma flux incoming to the orbiter cargo bay and the orbital velocity perpendicular to the magnetic field were relatively large. This effect has been explained by the concept of critical velocity ionization (CVI) for the gas drifting with the orbiter, although the gas velocity perpendicular to the magnetic field was less than the Alfvén critical

ISSN:0094-8276    

DOI:10.1029/GL013i005p00434

年代:1986

数据来源: WILEY

摘要:

It is well known that intense HF‐radiowaves of the ordinary mode transmitted from the ground enhance plasma waves near the reflection height in the ionosphere. These enhancements are observed as an increase in the returned power of radars having a wavevector equal to one half the wavevector of the enhanced plasma waves. It is expected that strong Landau‐damping will suppress such enhancements if kλD≃ 0.25 ( k and λDcorrespond to the wavevector of the Langmuir wave and the Debye‐length) which was the case for Langmuir‐waves detected by the EISCAT/UHF radar during a recent HF‐modification experiment at Tromsø. Enhanced plasma waves were observed unexpectedly at a low heater frequency of fHF= 2.76 MHz. Enhancements at this frequency were not observed during previous attempts and were believed to be damped too strongly to be observable. The observed power threshold can marginally be matched with theory under the assumption of low but not unreal electron and ion temperatures. The observed spectra indicate that the interaction is more intricate than expected under the gi

ISSN:0094-8276    

DOI:10.1029/GL013i005p00438

年代:1986

数据来源: WILEY

摘要:

In light of the Voyager 2 mission, the ringlets of Uranus seem unlikely to be accompanied by the swarm of 10 to 18 "shepherding" satellites generally supposed. Consequently, coherent motion (i.e., bulk circulation) in the ringlets again deserves serious consideration. Moreover the radio occultation data show the rings to be composed of larger boulders but few of the small particles one would expect if these boulders bumped and ground against one another as they would have to if the motions were incoherent. Recent evidence that Neptune's ringlets may be incomplete arcs further points in this direction. Dense (‘liquid’) rings have been proposed by Borderies, Goldreich and Tremaine. We point out here that such rings would be essentially viscosity free if executing coherent circulation and hence endure for geologically long time sca

ISSN:0094-8276    

DOI:10.1029/GL013i005p00442

年代:1986

数据来源: WILEY

摘要:

Mariner 9 and Viking Orbiter observations of the topography and mass of layered deposits in the north polar region of Mars suggest a density of about 1 gm/cm³. Consideration of the self‐compaction of these deposits further suggests that this low density cannot be explained by a high porosity sediment. Rather, a significant component of ice is probably required to account for the low value. The density further implies that the silicate fraction, regardless of its density (i.e., its composition), represents a volumetrically almost negligible component of the deposit. The most likely values, 5% silicate dust and 95% ice, agree well with a theoretical model for the formation of the deposits through cyclic incorporation of the perennial polar ice cap. Additional tests of this model may be posed in advance of future spacecraft missions to Ma

ISSN:0094-8276    

DOI:10.1029/GL013i005p00444

年代:1986

数据来源: WILEY

摘要:

The effects of solid‐solid phase changes on subsolidus convection in the large icy moons of the outer solar system are considered. Phase transitions affect convection via processes that distort the phase change boundary and/or influence buoyancy through thermal expansion. Linear stability analyses are carried out for ice layers with a phase change at the midplane. Two exothermic phase transitions (ice I ‐ ice II, ice VI ‐ ice VIII) and two endothermic transitions (ice I ‐ ice III, ice II ‐ ice V) are considered. For the exothermic cases, the phase change can either impede or enhance whole‐layer convection. For the endothermic cases, the phase change always inhibits whole‐layer convective overturn and tends to enforce two‐layer convection. These results place some constraints on possible models of icy satellite evolutio

ISSN:0094-8276    

DOI:10.1029/GL013i005p00448

年代:1986

数据来源: WILEY

摘要:

The presence of a mean flow vorticity field is found to affect significantly mixed‐layer deepenin

ISSN:0094-8276    

DOI:10.1029/GL013i005p00452

年代:1986

数据来源: WILEY

摘要:

The motions of the subducting and the overriding plates influence the spatial and temporal distribution of back‐arc spreading. Cenozoic plate motions in hot spot‐fixed and no‐net‐rotation reference frames were studied with attention to correlations between changes in motion and episodes of back‐arc spreading in the Western Pacific. The results suggest that major back‐arc opening occurs when both the overriding plate retreats from the trench in an absolute sense and the subducting plate undergoes a significant speed‐up. Neither phenomenon alone is sufficient to initiate spreading. Three major plate velocity increases can be identified in the Cenozoic : 1) the Pacific plate 5‐9 Ma; 2) the Indian plate at 27 Ma; and 3) the Pacific plate at 43 Ma, due to its shift from northerly to more westerly motion. At the present time, the Indian and Philippine are the only overriding plates that are retreating from their Pacific trenches and back‐arc spreading occurs only on these two retreating plates. Although the Indian plate has been retreating for at least 25 Ma, back‐arc spreading began only following the Pacific plate speed‐up 5‐9 Ma. Earlier, during the Indian plate speed‐up, no overriding plates were retreating strongly and no back‐arc spreading episodes are preserved from this time. For the earliest Pacific plate shift at 43 Ma, the Eurasian plate was not advancing, thus creating the only favorable plate kinematic conditions in the Cenozoic for back‐arc basin formation in this region. It is unclear whether extension in the Japan Sea is

ISSN:0094-8276    

DOI:10.1029/GL013i005p00456

年代:1986

数据来源: WILEY