摘要:

The non‐concentric nature, anomalous depth, and terraced morphology of the nested craters that compose Cleopatra Patera are more closely analogous to volcanic craters (calderas) than multi‐ring impact structures. Associated deposits northeast and downslope of the Patera first recognized on Venera 15/16 radar images are interpreted as volcanic plains related to, and perhaps cogenetic with, Cleopatra. The plains lavas probably originated as effusions from a radial rift zone or ring fissures. Rim deposits surrounding Cleopatra are asymmetric along the structural fabric of the region, also indicating fissure‐type eruptions. Finally, a volcanic origin not only is easily reconciled with the tectonic setting of the Patera, it is almost required by the correlation between the Patera and regional structural

ISSN:0094-8276    

DOI:10.1029/GL014i001p00041

年代:1987

数据来源: WILEY

摘要:

Evidence of a tidal influence from luni‐solar gravitational fields has been high‐lighted by analysing daily precipitation data collected at many stations in the Alps and the Po Valley (Italy) over the 1970‐81 period. Tidal influence as the possible result of a modulation effect caused by luni‐solar cycles having similar lengths, does not appear to be strictly constant over time. Time variations in daily precipitation data as a function of some particular cycles show that gravitational tides affect heavy precipitation more than mean precipitation values. "Semi‐monthly" oscillations have been derived from trends in daily precipitation data grouped according to the age of distinct lunar months. The investigation of the combined effects of different luni‐solar components, relating to years of particular climatological interest, might assist in the prediction of extreme pr

ISSN:0094-8276    

DOI:10.1029/GL014i001p00045

年代:1987

数据来源: WILEY

摘要:

In this report we present dynamic spectrograms of the low‐frequency interplanetary radio emissions as observed by Voyagers 1 and 2 from 1983 through mid‐1986. The radio emissions were observed to be most intense in the latter portion of 1983 at 3 kHz but have also been detected at 2 kHz. The emission has been present almost continuously at either 2 or 3 kHz since late 1983. The spectrograms presented herein show that the phenomenon appears almost identically as observed by the two spacecraft separated by more that 10 AU, at least at the higher frequency. One feature revealed by the dynamic spectrograms which had not been noticed previously is a gradual rise in frequency of the 3‐kHz component following the onset of the late 1983 event. These new observations reinforce the conclusion that the low‐frequency emissions are freely propagating radio waves, but the two‐component spectral structure implies that the previous model of emission at twice the plasma frequency at the inner heliosphere shock is inadequate to fully account for the observations. Either an additional source region or source mechanism is

ISSN:0094-8276    

DOI:10.1029/GL014i001p00049

年代:1987

数据来源: WILEY

摘要:

A small aperture telescope has been used with the Fabry‐Perot interferometer at the Arecibo Observatory to make high spectral resolution measurements of Comet Halley emissions at 6562.72 Å (Hα) and 6300.30 Å (O¹D). Observations in March, 1986, are characterized by a highly structured spectral signature at Hα. The Hαspectra imply non‐isotropic outflow of hydrogen atoms following dissociation of the parent species. The Hαsurface brightness for a 5′.9 field of view centered on the comet head was 59 ± 13 rayleighs(R) on March 12, 1986, decreasing to 25 ± 6R on March 23 (after correction for atmospheric extinction). Calculation of atomic hydrogen production rates and outflow velocities is not straightforward due to the non‐isotropic nature of the atomic hydrogen outflow. The O(¹D) emission at 6300.3 Å was accompanied by a feature at 6300.8 Å that we attribute to NH2. The brightness of the O(¹D) emission for a 6′ field of view was 260 ± 50 R on March 15 and 17. This brightness decreased by a factor of about 2 when the comet was viewed with a 5′.9 field of view centered 6′ off the comet nucleus (sunward or tailward). The width of the O(¹D) emission line was 3.9 ± 1.5 kms−1on March 15, and 7.4 ± 2.2 km s−1on March 17. The O(¹D) spectral line profiles on March 17 were sk

ISSN:0094-8276    

DOI:10.1029/GL014i001p00053

年代:1987

数据来源: WILEY

摘要:

The temperature and density of electrons are computed for Comet Giacobini‐Zinner, and are compared with ICE observations. The computations differ from the previous model of Marconi and Mendis [1986a] in that very recently calculated cooling rates are employed and the photo‐electron collisions with neutrals are treated separately from the thermalized electrons. While Marconi and Mendis [1986a]obtained electron temperatures and densities which were high relative to ICE observations, the present calculation yields temperatures which are low relative to the ICE observations, but densities which are consistent with the observations. Plasma dissipative processes in the tail are suggested as a significant additional heat sou

ISSN:0094-8276    

DOI:10.1029/GL014i001p00057

年代:1987

数据来源: WILEY

摘要:

A new instability is proposed to explain the observed large‐scale structuring of the magnetotail AMPTE barium releases. The stability theory is based on a modified set of MHD equations which allows for large Larmor radius effects, i.e., ρi∼ Rcwhich is typical of the AMPTE releases (here ρiis the ion Larmor radius and Rcis the transverse radius of the cloud). The mode is driven by the deceleration of the cloud and the density gradient at the edge of the cloud. The predicted linear growth rate and eigenmode structure are consistent with AMPTE observa

ISSN:0094-8276    

DOI:10.1029/GL014i001p00060

年代:1987

数据来源: WILEY

摘要:

Data from the Voyager plasma science instrument is used to set limits on transport models for the inner Jovian magnetosphere. The instrument has an effective time resolution for detecting changes in density of 0.24 s, which corresponds to a spatial resolution of about 20 km (equal to the gyroradius of an oxygen ion) in the region just outside of Io. This resolution enables us to put an upper limit of about 10% on changes in density between adjacent magnetic flux tubes, which rules out transport models which invoke inward motion of near empty flux tubes to replace outward‐moving flux tubes carrying Iogenic plasm

ISSN:0094-8276    

DOI:10.1029/GL014i001p00064

年代:1987

数据来源: WILEY

摘要:

A new model is proposed for the ion beams frequently observed in the plasma sheet boundary layer. The model assumes injection of magnetosheath plasma onto closed tail field lines. The plasma then expands along the field lines and is convected towards the earth. Particle simulations of the Expansion process are used to calculate properties of these beams. The expansion produces beams of ions at energies of up to a few tens of kBTe, which are consistent with the observed energies of the beams.

ISSN:0094-8276    

DOI:10.1029/GL014i001p00068

年代:1987

数据来源: WILEY

摘要:

Solar‐sector synchronous modulations of the particulate cosmic radiation reaching the earth's atmosphere have been studied using low altitude satellite and surface data. The flux in the broad maximum of the galactic cosmic ray differential spectrum (near 1 Gv rigidity) exhibits an intermittent north‐south asymmetry in mid and high geomagnetic latitudes. During the 1964 and 1965 years of sunspot minimum, this modulation had a negative rigidity dependence and strong correlations with geomagnetic disturbance index, ap, and interplanetary magnetic field direction. Taken together with other features, this ap dependence is consistent with the hypothesis that reconnection of the interplanetary and geomagnetic fields should produce a local north‐south asymmetry (NSA) independent of much larger scale NSA's associated with cosmic ray gradients in the heliosphere. This finding is also consistent with suggestions that solar activity influence on atmospheric processes may be mediated by the resulting modulations of upper tropospheric ioniz

ISSN:0094-8276    

DOI:10.1029/GL014i001p00072

年代:1987

数据来源: WILEY

摘要:

The almost explosive growth of narrow sodium layers at altitudes near 95 km was observed by ground‐based LIDAR located at 69°N, 16°E. Starting from typical conditions at these altitudes (sodium density approximately 1 to 2× 10³ cm−3) the density is observed to increase by large factors within a few minutes in a narrow layer, the half height of which is typically 1 km only. One of the more spectacular cases involved an increase by a factor 25 within 5 min and a layer width of 0.8 km (full width at half maximum). We propose that the sodium forming these sudden layers is released from upper atmospheric dust by energetic auroral particles. Prior to the auroral bombardment the dust particles were concentrated into a narrow layer by appropriate meteorological processes. Our observations thus imply that considerable amounts of sodium are stored on the surface of upper atmospher

ISSN:0094-8276    

DOI:10.1029/GL014i001p00076

年代:1987

数据来源: WILEY