摘要:

We present detailed measurements from the WIND 3D Plasma and Energetic Particle experiment of electron distributions in the deep foreshock, well away from the foreshock boundaries. Both electrons escaping from the magnetosheath and solar wind electrons reflected by the strong magnetic field of the shock are observed. The loss cone angle that separates the reflected electrons from the escaping magnetosheath electrons can be determined as a function of electron energy. At lower energies, the loss cone angle increases because the cross‐shock potential tends to pull electrons into the magnetosheath. The loss cone angle is typically ≲ 30° at energies ≳ 100 eV, corresponding to peak shock magnetic field to local magnetic field ratio of ≳ 5, while the loss cone variation with energy gives typical shock potentials of ∼85 V. Thus the measurements of back‐streaming foreshock electrons can be used as a tool to remotely probe the characteristics of t

ISSN:0094-8276    

DOI:10.1029/96GL02382

年代:1996

数据来源: WILEY

摘要:

We introduce a new model of solar irradiance variability at extreme ultraviolet (EUV) wavelengths. The model combines a spectral emission line database, solar emission measure distributions, and estimates from ground‐based solar images of the fraction of the Sun covered by the various types of activity to synthesize the irradiance. Using Call K‐line images, the model can be used to estimate the irradiance from EUV line emission formed in the upper chromosphere and lower transition region. Comparisons of this new model with existing empirical models reveal both similarities and disagreements in the absolute magnitude, the amplitude of the rotational modulation, and the intermediate‐term solar cycle variability of the predicted f

ISSN:0094-8276    

DOI:10.1029/96GL01481

年代:1996

数据来源: WILEY

摘要:

We present results from a 14‐month study of all‐sky camera observations of the Hydroxyl (OH) nightglow made at the Peach Mountain Observatory, Michigan (42.3°N; 83.7°W) Spatial variations in the observed OH airglow images have been used to assess gravity‐wave (GW) occurrence frequency at ∼85 km altitude as a function of season. A Strong seasonal dependence of mesospheric GW activity is observed, with peak activity in the summer months and much reduced activity during the winter months. Gravity waves (as defined by observed coherent variations in relative OH brightnesses of>∼7.5) were found to be present on about 70% of the clear‐sky nights during the summer months. During the spring, fall, and winter months, however, the observed GW occurrence frequency was very low (<10%). Most of the GWs were observed to propagate towards the eastward hemisphere. We suggest that the tropospherically‐generated GWs are anisotropic (eastward) thus passing through to the mesosphere only in the summer and being filtered out by the intervening neutral winds during other seasons. It is also possible that the GWs are able to reach higher altitudes without breaking because of their smaller amplitudes at lower altitudes during the summer season relative to th

ISSN:0094-8276    

DOI:10.1029/96GL02168

年代:1996

数据来源: WILEY

摘要:

The reaction between superthermal N(4S) atoms produced by exothermic processes and O2has been proposed to explain observations of highly rotationally excited nitric oxide in the sunlit thermosphere. We examine the importance of this mechanism using a detailed calculation of the fast N(4S) atoms energy distribution. It is shown that the hot thermal N atoms are able to produce rotationally excited NO in the upper thermosphere through the reaction of O2with N(4S). By contrast, near the NO peak at 110 km, the Maxwellian nitrogen atoms produce substantially less rovibrationally excited NO than the superthermal component. Consequently, the non Maxwellian N(4S) atoms show a clear spectral signature in the (1–0) and (2–1) bandheads at this altitude. The calculated rovibrationally excited NO concentration at 140 km is shown to be consistent with the value derived from the analysis of infrared airglow spec

ISSN:0094-8276    

DOI:10.1029/96GL02169

年代:1996

数据来源: WILEY

摘要:

The ambient water vapour and temperature conditions of stratospheric sulphate aerosol particles govern their composition and thereby influence their infrared extinction properties. This causes problems in the use of the infrared aerosol extinction as a tracer because variations in the aerosol composition modify the changes in extinction that may arise from the transport of aerosols. An improved tracer which can be derived from measurements of the infrared aerosol extinction, temperature and water vapour abundance, is the H2SO4component of aerosols. The application of this tracer to studies of stratospheric transport is demonstrated using data from instruments on the Upper Atmosphere Research Satellite and the results are compared to a contour advection calculation.

ISSN:0094-8276    

DOI:10.1029/96GL02074

年代:1996

数据来源: WILEY

摘要:

The formation and development of abnormally low total ozone events (LOEs), which typically last an average of 3–4 days, are analyzed to determine conditions under which these events may form. Wavelet analysis is performed on 13 years of daily total column ozone (TOZ) obtained by the Nimbus‐7 Total Ozone Mapping Spectrometer each day of the record. With wavelet analysis, the contributions and relative phases of signal components may be ascertained at specific times of interest. Timescales relevant during LOEs are identified and the contributions and phase relations of all components present in the TOZ signal are compared for common patterns and features. For a location in Sweden, the formation of 96% of the 52 LOEs studied required that all components with timescales (τ) less than a maximum value (τmax) were present in their negative phases. This suggests that LOEs result from the simultaneous interaction of TOZ‐depleting processes. The number of interacting processes varies with season and location. In addition, two criteria are seen to prevail when LOEs form in this location. First, background conditions favorable for the development of an event exist; these occur when TOZ is in a spectral state where all components with Δτ1<τ<τmaxare in their negative phases. Second, a process takes place that can force the remaining short‐timescale components (τ<Δτ1) into their negative phases. Processes associated with “initiating” timescales (Δτ1) of ≈ 2–6 days are found to be a key factor in the initiation of LOEs in this location. Although the background conditions are not uncommon, the likelihood that both criteria are simultaneously met is low and the occurrence of LOEs is indeed rare (averaging 4.7 event

ISSN:0094-8276    

DOI:10.1029/96GL02053

年代:1996

数据来源: WILEY

摘要:

Recent investigations of the carbon kinetic isotope effect (KIE) of the reaction CH4+ Cl yielded KIECl= 1.066±0.002 at 297 K (increasing to 1.075±0.005 at 223 K) [Saueressig et al., 1995]. In order to assess the effect of the exceptionally large KIEclon δ13C of stratospheric CH4we applied a two‐dimensional, time dependent chemical transport model. The model results demonstrate the strong influence of the CH4+ Cl reaction on δ13CH4in particular in the middle and upper stratosphere, where this reaction contributes several tens of percent to the total CH4sink. The Cl sink helps to explain the relatively large overall isotope fractionation of 1.010–1.012 observed in the lower stratosphere [Brenninkmeijer et al., 1995;Brenninkmeijer et al., 1996], even though the model results predict a smaller effect than o

ISSN:0094-8276    

DOI:10.1029/96GL02139

年代:1996

数据来源: WILEY

摘要:

The occurrence of negative and positive ground flashes in Tel‐Aviv, Israel (32.05 N, 34.45 E) has been monitored through daily registrations of a CGR3‐SN5 lightning flash counter (Mackerras, 1985). Measurements were conducted from 1987 to 1995, with the most continuous data sets available for the period 1992–1995. The lightning data was augmented by radar measurements of the thunderclouds (starting from 1994) and by radiosonde data. It was found that the fractionRof positive lightning from the total ground flash count is highly variable, with a long‐term average of 0.25. It is shown that storms having a relatively large value ofRcoincide with strong vertical shear of the horizontal wind component. The dependence ofRon the intensity of the wind shearSin the cloud layer between the 0°C and −25°C isotherms, may be expressed by: logR=aS ‐ b, where a=0.1504

ISSN:0094-8276    

DOI:10.1029/96GL00709

年代:1996

数据来源: WILEY

摘要:

The first polar ice core measurements of carbon monoxide compatible with atmospheric data were obtained using an improved experimental protocol. A new CO extraction method has been developed with special care to eliminate any CO contamination. The procedure has been applied to ice core samples originating from Antarctica and Greenland in order to reconstruct past CO variations over the last 200 years. Consistent results in terms of atmospheric concentrations are obtained. We find that CO concentration started to increase over Greenland around 1850 and that, by contrast, CO levels at high southern latitudes remained fairly constant between 1860 and 1916. Based on available data on past CO sources, a scenario is proposed for the CO increase observed in Greenland. In addition, our Greenland CO results suggest that simulations of preindustrial CO distribution could have underestimated CO concentrations mainly in the northern hemisphere leading to an overestimate of the change since preindustrial times, and to an underestimate of the past interhemispheric gradient of carbon monoxide.

ISSN:0094-8276    

DOI:10.1029/96GL02137

年代:1996

数据来源: WILEY

摘要:

To examine the hypothesis that the Australian‐Antarctic Discordance overlays an anomalously cold upper mantle, we have measured Rayleigh wave group velocities along paths between the Southeast Indian Ridge and the Australian station NWAO. The group velocity peaks for events in the vicinity of 117°E, rather than for those within the discordance zone between 120 and 127°E. This variation is resolved at least for periods of 35–45 s. To model the cause, assumed velocity anomalies with simple geometry were tested using the surface wave Gaussian beam method in a forward sense. The preferred model consists of an elongated structure centered 300–500 km west of the discordance and stretching northward for at least 1000 km. The position of the anomalous structure is consistent with recent global tomographic models in this

ISSN:0094-8276    

DOI:10.1029/96GL02144

年代:1996

数据来源: WILEY