Measurement of Airgun Waveforms

Summary. Airgun waveforms in the deep sea were measured from 160 in³ and 300 in³ guns with known firing pressure and depth, known geometry of source and receiver, and a recording system with known impulse response. The waveforms were compared with waveforms predicted from bubble oscillation theory and were found to be similar.     

A New look at the ionosphere of Jupiter in light of the UVS occultation results

Simple photochemical models cannot reconcile Jupiter's ionospheric electron density profiles with the observed neutral atmosphere. The location of the peak electron density predicted when the neutral atmosphere determined by theVoyager Ultraviolet Spectrometer is combined with simple models falls about 1000km lower than the peak determined by radio occultation. The locations and magnitudes of the peaks in electron density can be accounted for by including the effects of vertical transport of ions in the ionospheric models. This vertical transport may be induced by meridional winds in the neutral atmosphere or external electric fields. It is probable that precipitating particles and an altitude-variable H₂ vibrational temperature play important roles in determining the character of the iono?phere. In view of the complex relationship between the ionosphere and neutral atmosphere, an attempt to infer one from the other cannot succeed. However, combining independent information on the two leads to new insights into the coupling of the neutral atmosphere, the ionosphere and the magnetosphere.     

Numerical studies of vibratory pile driving

The processes underlying pile installation by vibratory loading rather than impact loading are complex. However, numerical studies using simple models for soil behaviour do throw some light on the basic dynamic processes involved. As better soil models become widely used, quantitative analyses using the techniques described, or simplifications of them, should be feasible.     

Mariner 10 observations of hydrogen Lyman alpha emission from the venus exosphere: Evidence of complex structure

The Ultraviolet Spectrometer Experiment on the MARINER 10 spacecraft measured the hydrogen Lyman α emmission resonantly scattered in the Venus exosphere at several viewing aspects during the encounter period. Venus encounter occurred at 17:01 GMT on 5 February 1974. Exospheric emissions above the planet's limb were measured and were analyzed with a spherically symmetric, single scattering, two-temperature model. On the sunlit hemisphere the emission profile was represented by an exospheric hydrogen atmosphere with T_c = 275±50 K and n_c = 1.5 × 10⁵ cm^?3 and a non-thermal contribution represented by T_H = 1250±100 K with n_H = 500±100 cm^?3. The observations of the dark limb showed that the spherically symmetric model used for the sunlit hemisphere was inappropriate for the analysis of the antisolar hemisphere. The density of the non-thermal component had increased at low altitudes, < 12,000 km, and decreased at high altitudes, > 20,000 km, by comparison. We conclude that the non-thermal source is on the sunward side of the planet. Analysis of the dark limb crossing suggests that the exospheric temperature on the dark side is <125 K if the exospheric density remains constant over the planet; upper limits are discussed. An additional source of Lyman α emission, 70 ± 15 R, was detected on the dark side of the planet and is believed to be a planetary albedo in contrast to multiple scattering from the sunlit side. Our analysis of the MARINER 10 data is consistent when applied to the MARINER 5 data.     

The February 1986 solar activity: A comparison of GIOTTO,VEGA-1, and IMP-8 solar wind measurements with MHD simulations

Large disturbances in the interplanetary medium were observed by several spacecraft during a period of enhanced solar activity in early February 1986. The locations of six solar flares and the spacecraft considered here encompassed more than 100° of heliolongitude. These flares during the minimum of cycle 21 set the stage for an extensive multi-spacecraft comparison performed with a two-dimensional, magnetohydrodynamic (MHD) numerical experiment. The plasma instruments on the European Space Agency (ESA)'s GIOTTO spacecraft, on its way to encounter Comet Halley in March 1986, made measurements of the solar wind for up to 8 hours per day during February. We compare solar wind measurements from the Johnstone Plasma Analyzer (JPA) experiment on GIOTTO with the MHD simulation of the interplanetary medium throughout these events. Using plasma data obtained by the IMP-8 satellite in addition, it appears that an extended period of high solar wind speed is required as well as the simulated flares to represent the interplanetary medium in this case. We also compare the plasma and magnetometer data from VEGA-1 with the MHD simulation. This comparison tends to support an interpretation that the major solar wind changes at both GIOTTO and VEGA-1 on 8 February, 1986 were due to a shock from a W05° solar flare on 6 February, 1986 (06:25 UT). The numerical experiment is considered, qualitatively, to resemble the observations at the former spacecraft, but it has less success at the latter one.     

Ice in the Southern Coalsack

To better understand the conditions under which ice mantles form on grains in molecular clouds, three globules in the Southern Coalsack have been searched for the presence of H₂O ice. Given the total lack of star formation in the Coalsack, it is an ideal site for studying unprocessed icy molecular mantles. In our sample of eight field stars lying behind the Coalsack we detect strong H₂O ice absorption in the lines of sight to two stars and possible weak absorption in four others. We estimate H₂O ice column densities or upper limits for these lines of sight. Compared to dark clouds such as Taurus, the Coalsack H₂O ice column densities are lower than expected given the quiescent nature of the Coalsack region. It is possible that the chemical evolution of the Coalsack may simply be at too early a stage for significant ice mantles to appear on the grains, except perhaps in the densest parts of some of the globules. Alternatively, the presence or absence of ice absorption may be related to the distribution of dust along each line of sight, specifically, the relative contributions of dense globules and a more extended diffuse component. For example, our observations are consistent with an ice threshold extinction similar to that observed in the Taurus dark cloud if extinction amounting to   A _V∼5  towards Globules 2 and 3 arises in the extended component. Globule 1 appears to have no extended component.     

Mineral magnetic analyses as a means of lithostratigraphic correlation and provenance indication of glacial diamicts: Intra- and interunit variation

Mineral magnetic analyses are applied to a series of diamict samples taken from the Quaternary sequence in the Isle of Man in order to assess the relative magnitude of the intra-and inter-unit variation of the magnetic properties. The results are analysed both qualitatively and quantitatively and show that the level of intra-unit variation is significantly less than the level of inter-unit variation. The intra-unit variation detected appears to show some relationship to the basic stratigraphy. Previous correlations based upon field evidence of diamict units exposed upon the east and west coasts of the Isle of Man are supported by their mineral magnetic properties. The statistically significant differences in the magnetic mineralogy of the stratigraphically lower Wyllin and Shellag Tills and the upper Orrisdale, Ballavarkish and Ballaquark Tills suggests distinct provenances for these two suites of sediments, allowing potential source areas to be suggested. These source areas are consistent with published data on ice flow directions in the Irish Sea Basin during the Devensian.