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At least 451 records · Page 25
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NASA NTRS1989

The influence of grain growth in circumstellar dust envelopes on observed colors and polarization of some eruptive stars

R CrB stars are classical examples of stars where dust envelope formation takes place. Dust envelope formation was detected around the Kuwano-Honda object (PU Vul) in 1980 to 1981 when the star's brightness fell to 8(sup m). Such envelopes are also formed at nova outbursts. The process of dust envelope formation leads to appreciable variations in optical characteristics, which are seen in specific color and polarization variations in the course of light fading and the appearance of IR radiation. It is shown that the model of a circumstellar dust envelope with aligned particles of changing size can be successfully applied to explain most phenomena observed at the time of light minima for a number of eruptive stars. The polarization may arise in a nonspherical dust envelope or be produced by alignment of nonspherical particles.

Efimov, Yu. S.↗
NASA NTRS1991

Climatic impact of volcanic eruptions

Studies have attempted to 'isolate' the volcanic signal in noisy temperature data. This assumes that it is possible to isolate a distinct volcanic signal in a record that may have a combination of forcings (ENSO, solar variability, random fluctuations, volcanism) that all interact. The key to discovering the greatest effects of volcanoes on short-term climate may be to concentrate on temperatures in regions where the effects of aerosol clouds may be amplified by perturbed atmospheric circulation patterns. This is especially true in subpolar and midlatitude areas affected by changes in the position of the polar front. Such climatic perturbation can be detected in proxy evidence such as decrease in tree-ring widths and frost rings, changes in the treeline, weather anomalies, severity of sea-ice in polar and subpolar regions, and poor grain yields and crop failures. In low latitudes, sudden temperature drops were correlated with the passage overhead of the volcanic dust cloud (Stothers, 1984). For some eruptions, such as Tambora, 1815, these kinds of proxy and anectdotal information were summarized in great detail in a number of papers and books (e.g., Post, 1978; Stothers, 1984; Stommel and Stommel, 1986; C. R. Harrington, in press). These studies lead to the general conclusion that regional effects on climate, sometimes quite severe, may be the major impact of large historical volcanic aerosol clouds.

Rampino, Michael R.↗
NASA NTRS1991

Ashfall dispersal for the 16 September 1986, eruption of Lascar, Chile, calculated by a turbulent diffusion model

A two dimensional model for the dispersal of volcanic ash, modified from that of Suzuki (1983), is applied to the September 16, 1986 eruption of Lascar, in northern Chile. The result is an isopach map predicting ground concentrations of ash. The map shows two local maxima along distinct dispersal axes and predicts that the deposit was detached from the source. Predicted concentrations are in agreement with a single reported thickness measurement.

Glaze, Lori S.↗
NASA NTRS1992

Lidar Observations of Stratospheric Clouds After Volcanic Eruption of Pinatubo

A very large increase of backscattered light from the stratospheric aerosol layer was observed by using a ruby laser in Beijing (39 degrees 54 minutes N, 116 degrees 27 minutes E) from the end of July 1991 to March 1992. It was concluded that this increase was almost certainly due to the volcanic eruption of Mt. Pinatubo in the Philippines in June 1991. The measuring instruments used are described. Information is given in graphical form for vertical profiles, fluctuation of the maximum backscattering ratio above 20 km during the nine month period, and the time variation of the integrated backscattering coefficient at a height of 15 to 30 km.

Sun, Jinhui↗
NASA NTRS1992

Potential climate impact of Mount Pinatubo eruption

The GISS global-climate model is used to make a preliminary estimate of Mount Pinatubo's climate impact. Assuming the aerosol optical depth is nearly twice as great as for the 1982 El Chichon eruption, the model forecasts a dramatic but temporary break in recent global warming trends. The simulations indicate that Pinatubo occurred too late in the year to prevent 1991 from becoming one of the warmest years in instrumental records, but intense aerosol cooling is predicted to begin late in 1991 and to maximize late in 1992. The predicted cooling is sufficiently large that by mid 1992 it should even overwhelm global warming associated with an El Nino that appears to be developing, but the El Nino could shift the time of minimum global temperature into 1993. The model predicts a return to record warm levels in the later 1990s. The effect is estimated of the predicted global cooling on such practical matters as the severity of the coming Soviet winter and the dates of cherry blossoming next spring.

Hansen, James↗
NASA NTRS1990

The role of Magnetic Reconnection in flares and prominence eruptions

Magnetic Reconnection is often invoked as the primary mechanism for driving a flare or a prominence eruption. This paper argues that a catastrophic loss of mechanical equilibrium, rather than reconnection, is probably the primary mechanism for driving these phenomena. However, reconnection is still essential in order for any significant amount of energy to be released. To illustrate this idea, some recent results are presented from an MHD simulation based on a catastrophe mechanism first proposed by Van Tend and Kuperus. In order for this mechanism to be effective, a substantial amount of reconnection must occur within a few Alfven-scale times. Such rapid reconnection is plausible since the loss of mechanical equilibrium can generate flows which drive the reconnection at a rapid rate.

Forbes, T. G.↗
NASA NTRS1992

Observations of reduced ozone concentrations in the tropical stratosphere after the eruption of Mt. Pinatubo

Two independent sets of data, one of aerosols from an airborne lidar system, and one of ozone from ozonesonde measurements indicate that significant ozone decreases may have happened as a result of the injection of debris by the Mt. Pinatubo volcano in June 1991. The amount of this reduction maximizes at 24-25 km, near the peak of the aerosol distribution, though a deficit is seen throughout the lower stratosphere between 19 and 28 km. The greatest differences observed prior and subsequent to the eruptions at these altitudes is 18-20 percent.

Grant, W. B.↗
NASA NTRS1992

Observations of a new SAGE II aerosol extinction mode following the eruption of Mt. Pinatubo

In the aftermath of the eruption of Mt. Pinatubo, multiwavelength stratospheric aerosol extinction measurements by the satellite-borne Stratospheric Aerosol and Gas Experiment (SAGE II) revealed the presence of a previously unobserved mode of aerosol that exhibited high extinction but a small inferred particle size. This mode may represent a transitional phase between the very small aerosol created by gas-to-particle conversion and a quasi-steady state, post-volcanic aerosol that exhibits both large extinction and large particle size. The presence of a transitional small aerosol mode may have a significant impact on chemical and radiative processes in the stratosphere.

Thomason, Larry W.↗
NASA NTRS1991

An investigation of the distribution of eruptive products on the shield volcanoes of the western Galapagos Islands using remotely sensed data

Recent volcanic activity in the Galapagos Islands is concentrated on the two westernmost islands, Isla Isabela and Isla Fernandina. Difficult access has thus far prevented comprehensive geological field studies, so we examine the potential of remotely sensed data as a means of studying volcanic processes in the region. Volcan Wolf is used as an example of the analysis of SPOT HRV-1 data undertaken for each volcano. Landsat TM data are analyzed in an attempt to construct a relative age sequence for the recent eruptive activity on Isla Fernandina. No systematic variation in the surface reflectance of lava flows as a function of age could be detected with these data. Thus it was not possible to complete a study of the temporal distribution of volcanic activity.

Munro, Duncan C.↗
NASA NTRS1993

Radiative climate forcing by the Mount Pinatubo eruption

Radiative flux anomalies derived from the NASA spaceborne Earth Radiation Budget Experiment were used to determine the volcanic radiative forcing that followed the eruption of Mount Pinatubo in June 1991. They are the first unambiguous, direct measurements of large-scale volcanic forcing. The volcanic aerosols caused a strong cooling effect immediately; the amount of cooling increased through September 1991 as shortwave forcing increased relative to the longwave forcing. The primary effects of the aerosols were a direct increase in albedo over mostly clear areas and both direct and indirect increases in the albedo of cloudy areas.

Minnis, P.↗
NASA NTRS1993

Stratospheric chlorine injection by volcanic eruptions - HCl scavenging and implications for ozone

Because the output of volatile chlorine during a major volcanic event can greatly exceed the annual anthropogenic emissions of chlorine to the atmosphere, the fate of volcanic chlorine must be known. Although numerous observations have shown that volcanoes do not significantly contribute to the stratospheric chlorine burden, no quantitative explanation has been published. Hydrogen chloride (HCl) scavenging processes during the early phases of a volcanic eruption are discussed. A plume dynamics and thermodynamics model is used to show that HCl removal in condensed supercooled water can reduce HCl vapor concentrations by up to four orders of magnitude, preventing substantial stratospheric chlorine injection.

Tabazadeh, A.↗
NASA NTRS1993

Compositional heterogeneity within a dumbbell-shaped Apollo 15 green glass: Evidence for simultaneous eruption of different magmas

The Apollo 15 green glasses that occur most prominently in 15425, 15426, and 15427 consist of compositionally distinct groups. The six groups that have thus far been defined are thought to represent different batches of magma that were erupted as fire-fountains, probably at different times in lunar history; although isotopic analyses of representative glasses within some of these groups have not yet found any differences in age at the +/- 100 Ma precision. While the petrogenesis of these picritic magmas has been contentious, the discovery of a compositionally heterogeneous, dumbbell-shaped green glass raises additional questions and problems about their origin.

Delano, J. W.↗
NASA NTRS1993

In situ observations of aerosol and chlorine monoxide after the 1991 eruption of Mount Pinatubo - Effect of reactions on sulfate aerosol

Highly resolved aerosol size distributions measured from high-altitude aircraft can be used to describe the effect of the 1991 eruption of Mount Pinatubo on the stratospheric aerosol. In some air masses, aerosol mass mixing ratios increased by factors exceeding 100 and aerosol surface area concentrations increased by factors of 30 or more. Increases in aerosol surface area concentration were accompanied by increases in chlorine monoxide at mid-latitudes when confounding factors were controlled. This observation supports the assertion that reactions occurring on the aerosol can increase the fraction of stratospheric chlorine that occurs in ozone-destroying forms.

Wilson, J. C.↗
NASA NTRS1993

Chronology, eruption duration, and atmospheric contribution of the Martian volcano Apollinaris Patera

Results are presented of an investigation of the geology, physical properties, and topography of Apollinaris Patera which was performed on the basis of geologic mapping, thermal inertia measurements, and analysis of the color (visual wavelengths) of this Martian volcano. Initially, the main edifice was formed over a period of about 10 exp 7 yr, and then the volcano experienced a period of quiescence during which the basal scarp formed and the valleys were incised in the main flanks. Activity at the summit formed the present large outer caldera and floor materials. After, or during late stages of outer calder activity, the materials that formed the fan were erupted over 10,000 yr. After emplacement of the fan materials, the smaller, inner caldera and associated floor materials formed over an unconstrained time period. Morphologic analysis suggests that a significant portion of the main edifice is composed of poorly to nonwelded pyroclastic deposits, possibly including a phreatomagmatic component.

Robinson, Mark S.↗
NASA NTRS1993

Evolution of the symbiotic binary system AG Pegasi - The slowest classical nova eruption ever recorded

We present an analysis of new and existing photometric and spectroscopic observations of the ongoing eruption in the symbiotic star AG Pegasi, showing that this binary has evolved considerably since the turn of the century. Recent dramatic changes in both the UV continuum and the wind from the hot component allow a more detailed analysis than in previous papers. AG Peg is composed of a normal M3 giant and a hot, compact star embedded in a dense, ionized nebula. The hot component powers the activity observed in this system, including a dense wind and a photoionized region within the outer atmosphere of the red giant. The hot component contracted in radius at roughly constant luminosity from 1850 to 1985. Its bolometric luminosity declined by a factor of about 4 during the past 5 yr. Both the mass loss rate from the hot component and the emission activity decreased in step with the hot component's total luminosity, while photospheric radiation from the red giant companion remained essentially constant.

Kenyon, Scott J.↗
NASA NTRS1994

Modulations of stratospheric ozone by volcanic eruptions

We have used a time series of aerosol surface based on the measurements of Hofmann to investigate the modulation of total column ozone caused by the perturbation to gas phase chemistry by the reaction N2O5(gas) + H2O(aero) yields 2HNO3(gas) on the surface of stratospheric aerosols. We have tested a range of values for its reaction probability, gamma = 0.02, 0.13, and 0.26 which we compared to unperturbed homogeneous chemistry. Our analysis spans a period from Jan. 1974 to Oct. 1994. The results suggest that if lower values of gamma are the norm then we would expect larger ozone losses for highly enhanced aerosol content that for larger values of gamma. The ozone layer is more sensitive to the magnitude of the reaction probability under background conditions than during volcanically active periods. For most conditions, the conversion of NO2 to HNO3 is saturated for reaction probability in the range of laboratory measurements, but is only absolutely saturated following major volcanic eruptions when the heterogeneous loss dominates the losses of N2O5. The ozone loss due to this heterogeneous reaction increases with the increasing chlorine load. Total ozone losses calculated are comparable to ozone losses reported from TOMS and Dobson data.

Blanchette, Christian↗
NASA NTRS1994

Effects of the Mount Pinatubo eruption on the radiative and chemical processes in the troposphere and stratosphere

The Lawrence Livermore National Laboratory two-dimensional zonally-averaged chemical-radiative-transport model of the global atmosphere was used to study the effects of the 15 June 1991 eruption of the Mt. Pinatubo volcano on stratospheric processes. SAGE 2 time-dependent aerosol surface area density and optical extinction data were used as input into the model. By 22 December 1991, a maximum equatorial change of -1.8 percent in column ozone was derived from heterogeneous chemical processes that convert NO(x) into HNO3 on sulfuric acid aerosols. Radiative feedbacks from increased aerosol optical thickness independently changes column ozone by approximately -3.5 percent for the same period. This occurs from increasing the net heating of the lower stratosphere, which indirectly increases chemical reaction rates via their temperature dependence and from changes in actinic fluxes, which directly modify photodissociation rates. Including both heterogeneous and radiative effects changes column ozone by -5.5 percent. The model-derived change overestimates the decrease in column ozone relative to the TOMS instrument on the Nimbus 7 satellite. Maximum local ozone decreases of 12 percent were derived in the equatorial region, at 25 km. Model-derived column NO2 peaked (-14 percent) at 30 deg S in October 1991. The timing of the NO2 peak is consistent with observation, but the model underestimates the magnitude of the decrease. Local concentrations of NO(x) (NO + NO2), ClO(x) (Cl + ClO), and HO(x) (OH + HO2), in the lower stratosphere between 30 deg S and 30 deg N, were calculated to have changed by -40 percent, +100 to +160 percent, and +120 to +140 percent respectively.

Kinnison, Douglas E.↗