摘要:

Model calculations, as well as optical observations, have shown that hot oxygen is the dominant neutral constituent up to about 3000 km in the upper atmosphere of Venus. Consequently O+is expected to be the dominant ion up to about 3000 km in the upper ionosphere of Venus. Under normal circumstances, it is difficult to verify this result, since the solar wind interaction terminates the Venus ionosphere at altitudes below about 500 km for subsolar conditions. However, the upper boundary of the Venus ionosphere can move up to very high altitudes during some rare events when the solar wind dynamic pressure is extremely low. An analysis of Pioneer Venus measurements during such events indeed shows O+to be the dominant ion at altitudes even as high as 2800 km. This result provides ionospheric evidence of hot oxygen in the upper atmosphere of Venus.

ISSN:0094-8276    

DOI:10.1029/92GL01804

年代:1992

数据来源: WILEY

摘要:

The firehose marginally stable current sheet, which may model the flow away from the distant reconnection neutral line, assumes that the accelerated particles escape and never return to re‐encounter the current region. This assumption fails on the earthward side where the accelerated ions mirror in the geomagnetic dipole field and return to the current sheet at distances up to about 30REdown the tail. Two‐dimensional particle simulations are used to demonstrate that the reflected ions drive a “shock‐like” structure in which the incoming flow is decelerated and theBzfield is highly compressed. These effects are similar to those produced by adiabatic choking of steady convection. Possible implications of this interaction for the dynamics of the tail are c

ISSN:0094-8276    

DOI:10.1029/92GL01617

年代:1992

数据来源: WILEY

摘要:

It has recently been found that a one‐dimensional current sheet equilibrium with a non‐zero confection electric field,Ey, and a non‐zero normal magnetic field component,Bn, can reach a point of catastrophe through either the reduction of the drift velocity, υD=cEy/Bnor the increase ofBn. This point of catastrophe coincides with a value of κA≃ 0.7, where κAis the self‐consistent value of κ = (Rmin/ρA max)½corresponding to ions of average energy. (HereRminis the minimum field‐line radius of curvature and ρA maxis the maximum gyroradius for ions of average energy.) The point of catastrophe was found to be preceded by a twisting of the current sheet field‐lines into the dawnward direction, i.e. by the development of ay‐component ofB, with odd symmetry inz, and a sign oppositeBx, where positivexis earthward, positiveyis in the dawn‐to‐dusk direction and positivezis northward. Since the loss of the current sheet would cause the local configuration to become more dipolar, it was suggested that the catastrophic loss of the local current sheet equilibrium could correspond to local current disruption and dipolarization. In this paper, observations of some of the signatures predict

ISSN:0094-8276    

DOI:10.1029/92GL01615

年代:1992

数据来源: WILEY

摘要:

Electron precipitation induced by oblique whistler waves is investigated using a new test particle simulation code based on gyro‐averaged equations of motion. Results indicate that highly oblique waves can efficiently pitch angle scatter suprathermal electrons (10–100 eV). AtL= 3, for example, in a single encounter with a wave with power densityS∼ 8 pW/m2and propagating at ψ = 60° with respect to the Earth magnetic fieldBo, average pitch angle scattering of 100 eV electrons is ∼ 0.1°. In comparison, the average scattering of energetic (100 keV) electrons by a parallel propagating wave with the sameSis ∼ 0.01°. Estimates indicate that the precipitated electron energy fluxes resulting from the interaction of ∼ 100 eV electrons with oblique waves can be up to 30 times larger than that due to the precipitation of 100 keV electrons by parallel pr

ISSN:0094-8276    

DOI:10.1029/92GL01811

年代:1992

数据来源: WILEY

摘要:

Falling sphere measurements were part of a study to investigate the relationship between Noctilucent Clouds and Polar Mesospheric Summer Echoes within the context of interactions between the ionized and the neutral atmosphere. During this investigation there was one measurement of a very low mesopause temperature. This temperature is believed to be the lowest value ever observed in the terrestrial environment. The reasons why this cold mesopause measurement is considered to be valid are presented.

ISSN:0094-8276    

DOI:10.1029/92GL01506

年代:1992

数据来源: WILEY

摘要:

In this study it is documented for the first time that the atmospheric ozone column density is decreased during solar proton events, GLEs (Ground Level Events), events with protons having energy exceeding 50 MeV and penetrating down to the ground. The ozone decrease was significant at the polar cap stations, Longyearbyen and Barentsburg at 78° N, whereas it was not detectable at the auroral zone stations Murmansk (69° N) and Tromsø (70° N). The duration of the depletion was limited to the days of the proton precipitation, and the depletion may be explained by the ability of energetic protons to produce nitric and hydrogen oxides in the stratosphere and their catalytic reactions with ozone. Similar events are identified in the Antarc

ISSN:0094-8276    

DOI:10.1029/92GL01876

年代:1992

数据来源: WILEY

摘要:

We measure an upper limit for stratospheric HBr from three balloon flights. The observations were made with the FIRS‐2 far‐infrared Fourier transform spectrometer. The 1σ upper limits from the 1988, 1989 and 1990 balloon flights are 13 pptv at 35 km, 7 pptv at 32 km, and 3 pptv at 31 km, respectively. Combining all 3 flights, the weighted average 1σ upper limit for HBr is 4 pptv at 32 km. This value is significantly smaller than the only other previously published spectroscopic value of 20 ± 7 pptv (2σ), but is consistent with (albeit well in excess of) a theoretical estimate which predicts roughly 0.4 pptv at this a

ISSN:0094-8276    

DOI:10.1029/92GL01800

年代:1992

数据来源: WILEY

摘要:

SAGE II multiwavelength aerosol extinction measurements are used to estimate mass‐ and extinction‐to‐backscatter conversion parameters. The basis of the analysis is the principal component analysis of the SAGE II extinction kernels to estimate both total aerosol mass and aerosol backscatter at a variety of wavelengths. Comparisons of coincident SAGE II extinction profiles with 0.694‐μm aerosol backscatter profiles demonstrate the validity of th

ISSN:0094-8276    

DOI:10.1029/92GL01619

年代:1992

数据来源: WILEY

摘要:

As determined by impactor samplers flown on ER‐2 and DC‐8 aircraft, black carbon aerosol (BCA) mass loadings in the stratosphere average 0.6 nanograms per standard cubic meter, or 0.01% of the total aerosol. Upper tropospheric BCA increases to 0.3%. Low stratospheric concentration is commensurate with present commercial air traffic fuel consumption, given the following assumptions: the BCA emissions are 0.1 grams per kilogram of fuel consumed, 10% of route mileage is above the tropopause, and average BCA stratospheric residence time is about one year. Taking BCA into account, the stratospheric single scatter albedo is ≈0.99. Using parameters for planned supersonic commercial aircraft, whose emissions will be predominantly in the stratosphere, we show that such traffic will double stratospheric BCA concentration. This would reduce the aerosol single scattering albedo by one percent, and double the BCA surface area that is available for heterogeneous chem

ISSN:0094-8276    

DOI:10.1029/92GL01801

年代:1992

数据来源: WILEY

摘要:

Observations of the stratospheric plume from the May 18, 1980, Mount St. Helens eruption suggest that it spread in the crosswind direction as an intrusive gravity current, as it was transported downwind. Grain size analyses of the plinian tephra are consistent with this model, suggesting that to distances of many hundreds of kilometers, turbulent atmospheric diffusion played a secondary role in plume spreading and tephra dispersal.

ISSN:0094-8276    

DOI:10.1029/92GL01639

年代:1992

数据来源: WILEY