Showing papers in "Geophysical Research Letters in 1992"
TL;DR: In this paper, a new simulation model of upper atmospheric dynamics is presented that includes self-consistent electrodynamic interactions between the thermosphere and ionosphere and uses the resultant electric fields and currents in calculating the neutral and plasma dynamics.
Abstract: A new simulation model of upper atmospheric dynamics is presented that includes self-consistent electrodynamic interactions between the thermosphere and ionosphere. This model calculates the dynamo effects of thermospheric winds, and uses the resultant electric fields and currents in calculating the neutral and plasma dynamics. A realistic geomagnetic field geometry is used. Sample simulations for solar maximum equinox conditions illustrate two previously predicted effects of the feedback. Near the magnetic equator, the afternoon uplift of the ionosphere by an eastward electric field reduces ion drag on the neutral wind, so that relatively strong eastward winds can occur in the evening. In addition, a vertical electric field is generated by the low-latitude wind, which produces east-west plasma drifts in the same direction as the wind, further reducing the ion drag and resulting in stronger zonal winds.
949 citations
TL;DR: The explosive June 1991 eruptions of Pinatubo produced the largest sulfur dioxide cloud detected by the Total Ozone Mapping Spectrometer (TOMS) during its 13 years of operation: approximately 20 million tons of SO2, predominantly from the cataclysmic June 15th eruption.
Abstract: The explosive June 1991 eruptions of Mount Pinatubo produced the largest sulfur dioxide cloud detected by the Total Ozone Mapping Spectrometer (TOMS) during its 13 years of operation: approximately 20 million tons of SO2, predominantly from the cataclysmic June 15th eruption. The SO2 cloud observed by the TOMS encircled the earth in about 22 days (about 21 m/s); however, during the first three days the leading edge of the SO2 cloud moved with a speed that averaged about 35 m/s. Compared to the 1982 El Chichon eruptions, Pinatubo outgassed nearly three times the amount of SO2 during its explosive phases. The main cloud straddled the equator within the first two weeks of eruption, whereas the El Chichon cloud remained primarily in the Northern Hemisphere. The measurements indicate that Mount Pinatubo has produced a much larger and perhaps longer-lasting SO2 cloud; thus, climatic responses to the Pinatubo eruption can exceed those of El Chichon.
514 citations
TL;DR: In this article, 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.
Abstract: 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.
421 citations
TL;DR: In this article, a complementary approach is tried: regions are identified based on the plasma characteristics as observed by low-altitude satellites using an automated identification scheme applied to approximately 60,000 individual satellite passes through the dayside oval, probability maps are computed for observing various types of plasma precipitating into the ionosphere.
Abstract: A long standing and vexing problem in magnetospheric physics is mapping the ionosphere to the magnetosphere. To date almost all information on the topic has come from magnetic field mappings. Herein a complementary approach is tried: regions are identified based on the plasma characteristics as observed by low-altitude satellites. Using an automated identification scheme applied to approximately 60,000 individual satellite passes through the dayside oval, probability maps are computed for observing various types of plasma precipitating into the ionosphere. The synthesis of the various individual maps allows the construction of an ionospheric map into the magnetosphere based on plasma characteristics. The results suggest a cusp which is about 2.5 hours in total local time extent; an LLBL which typically extends to about 0900 MLT; and a mantle which reaches to about 0800/1600. Consistent with earlier results, the extension of the nightside region of discrete aurora into the dayside is far more substantial at dusk than at dawn.
401 citations
TL;DR: SAGE II satellite measurements of the Mt. Pinatubo eruption cloud in the stratosphere during June, July, and early August 1991 show that aerosols in the tropics reached as high as 29 km altitude with most of the cloud between 20 and 25 km as mentioned in this paper.
Abstract: SAGE II satellite measurements of the Mt. Pinatubo eruption cloud in the stratosphere during June, July, and early August 1991 show that aerosols in the tropics reached as high as 29 km altitude with most of the cloud between 20 and 25 km. The most optically thick portions of the cloud covered latitudes from 10 deg S to 30 deg N during the early part of this period. By late July, high stratospheric optical depths were observed to at least 70 deg N, with the high values north of about 30 deg N from layers below 20 km. High pressure systems in both hemispheres were observed to be correlated with the movement of volcanic material at 21 km into the westerly jet stream at high southern latitudes and similarly to high northern latitudes at 16 km. By August, the entire Southern Hemisphere had experienced a 10-fold increase in optical depth relative to early July due to layers above 20 km. Initial mass calculations using SAGE II data place the aerosol produced from this eruption at 20 to 30 megatons, well above the 12 megatons produced by El Chichon.
384 citations
TL;DR: On 24 March 1991 instrumentation aboard CRRES observed the almost instantaneous injection of electrons and protons with energies above 15 MeV into the L-region 2 < L < 3.
Abstract: On 24 March 1991 instrumentation aboard CRRES observed the almost instantaneous injection of electrons and protons with energies above 15 MeV into the L-region 2 < L < 3. The energy spectrum of the injected electrons, a power law (E−6) peaked at 15 MeV and continued to at least 50 MeV
344 citations
TL;DR: The Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay often sees F-region drifts or electric fields which are associated with field line resonances in the Earth's magnetosphere.
Abstract: The Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay often sees F-region drifts or electric fields which are associated with field line resonances in the Earth's magnetosphere. These resonances are seen in the interval from local midnight to morning, and have discrete, latitude-dependent frequencies at approximately 1.3, 1.9, 2.6–2.7, and 3.2–3.4 mHz. We show that these frequencies are compatible with MHD waveguide modes, with antisunward propagation and reflection at the magnetopause and at turning points on dipolar field lines.
330 citations
TL;DR: In this article, the presence of a filamentary structure has been detected in daily global tropospheric water vapor flux values, where the filamentary structures have lengths many times their widths and persist for many days while being translated through the atmosphere.
Abstract: Computations of daily global tropospheric water vapor flux values show the presence of a filamentary structure The filaments, here called rivers, have lengths many times their widths and persist for many days while being translated through the atmosphere They are present in data analyzed for both 1981 and 1991 The water vapor flux maxima coincide quite closely to reflectivity features (averaged from wavelengths of 380 and 360 nm) as revealed by the Total Ozone Mapping Spectrometer (TOMS) It is suggested that the filamentary structure may also be present in other trace constituents
321 citations
TL;DR: In this article, an analysis of the Northern Hemisphere winter surface temperature patterns after the 12 largest volcanic eruptions from 1883-1992 shows warming over Eurasia and North America and cooling over the Middle East which are significant at the 95 percent level.
Abstract: An examination of the Northern Hemisphere winter surface temperature patterns after the 12 largest volcanic eruptions from 1883-1992 shows warming over Eurasia and North America and cooling over the Middle East which are significant at the 95 percent level. This pattern is found in the first winter after tropical eruptions, in the first or second winter after midlatitude eruptions, and in the second winter after high latitude eruptions. The effects are independent of the hemisphere of the volcanoes. An enhanced zonal wind driven by heating of the tropical stratosphere by the volcanic aerosols is responsible for the regions of warming, while the cooling is caused by blocking of incoming sunlight.
303 citations
TL;DR: In this paper, the five largest magnetic storms that occurred between 1971 to 1986 were studied to determine their solar and interplanetary causes, and all of the events were associated with high speed solar wind streams led by collisionless shocks.
Abstract: The five largest magnetic storms that occurred between 1971 to 1986 are studied to determine their solar and interplanetary causes. All of the events are found to be associated with high speed solar wind streams led by collisionless shocks. The high speed streams are clearly related to identifiable solar flares. It is found that: (1) it is the extreme values of the southward interplanetary magnetic fields rather than solar wind speeds that are the primary causes of great magnetic storms, (2) shocked and draped sheath fields preceding the driver gas (magnetic cloud) are at least as effective in causing the onset of great magnetic storms (3 of 5 events) as the strong fields within the driver gas itself, and (3) precursor southward fields ahead of the high speed streams allow the shock compression mechanism (item 2) to be particularly geoeffective.
301 citations
TL;DR: In this article, the changes in biologically active ultraviolet (UV) radiation reaching the Earth's surface from 1979 to 1989 were analyzed using satellite measurements of total atmospheric ozone, and the calculated increases were on average substantially larger than earlier estimates, particularly at mid and high latitudes of both hemispheres.
Abstract: Recent satellite measurements of total atmospheric ozone were analyzed to deduce the changes in biologically active ultraviolet (UV) radiation reaching the Earth's surface from 1979 to 1989. The calculated increases are on average substantially larger than earlier estimates, particularly at mid and high latitudes of both hemispheres.
TL;DR: In this article, the pressure dependence of the thermal expansion coefficient, α, previously reported as 5.5 ± 0.5 by Chopelas and Boehler [1989] is refined, using systematics in the volume dependence of (∂T/∂P)s measured for a large number of materials at high pressures and high temperatures.
Abstract: The pressure dependence of the thermal expansion coefficient, α, previously reported as (∂lnα/∂lnV)T = 5.5 ± 0.5 by Chopelas and Boehler [1989] is refined, using systematics in the volume dependence of (∂T/∂P)s measured for a large number of materials at high pressures and high temperatures. Since (∂ln (∂T/∂P)s/∂(V/V0))T is found to be constant and material independent over a very large compression range, (∂lnα/∂lnV)T is proportional to the compression, V/V0. We find α decreases by a factor of 5 for MgO throughout the mantle, reaching a value of 1.0 · 10−5 K−1 at its bottom. Densities of perovskite (PV) and magnesiowustite (MW) are calculated for lower mantle conditions using our new α(P, T), a room temperature finite strain equation, and recent data on the Mg-Fe partitioning in the PV-MW system. Both minerals have nearly identical densities to those of PREM throughout the entire lower mantle, which allows variable PV:MW ratios. A lower mantle made entirely of PV with a molar ratio of Mg:Fe of 88:12 would be about 0.11 g/cm3 or 2.5% denser than this mixture, but this density would just be within the uncertainty in PREM. A change in chemistry at 660 km depth to a PV mantle requires a thermal boundary which would improve the match in the densities between PV and PREM. These density agreements therefore preclude evaluation of a mineralogical model for the lower mantle using density comparisons. Recent measurements on melting of Fe, FeO, and FeS, however, suggest temperatures at the core-mantle boundary below 3500 K, which tends to favor a geotherm without a large thermal boundary at 660 km depth.
TL;DR: In this paper, the authors analyzed over 1300 km of high resolution along-axis gravity profiles at ridges with half-spreading rates ranging from 1.2 to 5.5 cm/yr.
Abstract: We analyze over 1300 km of high resolution along-axis gravity profiles at ridges with half-spreading rates ranging from 1.2 to 5.5 cm/yr. The results show consistently higher along-axis gradients of mantle Bouguer anomaly at the slow-spreading Mid-Atlantic Ridge (MAR) (0.3–1.2 mgal/km) than at the intermediate- to fast-spreading Cocos-Nazca Ridge and East Pacific Rise (EPR) (0.1–0.2 mgal/km). The regional peak-to-trough amplitude of mantle Bouguer anomaly is also greater along the MAR (30–60 mgal) than the Cocos-Nazca Ridge and the EPR (10–20 mgal). With increasing spreading rate, the regional peak-to-trough amplitude of axial seafloor depth decreases from 1000–1700 m to 200–700 m. 3-D numerical experiments suggest that mantle contributions to the gravity can be significant only near large-offset transforms. At the more commonly observed non-transform offsets, gravity anomalies will reflect crustal thickness variations.
The along-axis gravity data thus indicate that the amplitude of along-axis crustal thickness variation decreases with increasing spreading rate. We propose that this spreading rate dependent crustal accretion style may originate in the mantle: finite-amplitude mantle upwelling is intrinsically plume-like (3-D) beneath a slow-spreading ridge but more sheet-like (2-D) beneath a fast-spreading ridge. Such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate. Small amplitude 3-D upwellings may occur at a fast-spreading ridge, but their effects on crustal thickness variations will be significantly reduced by along-axis melt flows along a persistent low-viscosity crustal magma chamber. In contrast, the large crustal thickness variations due to 3-D mantle upwellings will be maintained at a slow-spreading ridge because less along-axis melt flows can occur in the colder and more rigid crust there.
TL;DR: As a result of the eruption of Mt. Pinatubo (June 1991), direct solar radiation was observed to decrease by as much as 25-30% at four remote locations widely distributed in latitude as discussed by the authors.
Abstract: As a result of the eruption of Mt. Pinatubo (June 1991), direct solar radiation was observed to decrease by as much as 25–30% at four remote locations widely distributed in latitude. The average total aerosol optical depth for the first 10 months after the Pinatubo eruption at those sites is 1.7 times greater than that observed following the 1982 eruption of El Chichon. Monthly-mean clear-sky total solar irradiance at Mauna Loa, Hawaii, decreased by as much as 5% and averaged 2.4% and 2.7% in the first 10 months after the El Chichon and Pinatubo eruptions, respectively. By September 1992 the global and northern hemispheric lower tropospheric temperatures had decreased 0.5°C and 0.7°C, respectively compared to pre-Pinatubo levels. The temperature record examined consists of globally uniform observations from satellite microwave sounding units.
TL;DR: In this paper, it was shown that the global average greenhouse effect of the aerosols exceeds the albedo effect, causing a surface heating, and the climate forcing is a function of aerosols size distribution, but the size dependence can be described well by a single parameter: the area-weighted mean radius, r(eff).
Abstract: It is illustrated how climate forcing by stratospheric aerosols depends on aerosol properties. The climate forcing is a function of aerosols size distribution, but the size dependence can be described well by a single parameter: the area-weighted mean radius, r(eff). If r(eff) is greater than about 2 microns, the global average greenhouse effect of the aerosols exceeds the albedo effect, causing a surface heating. The aerosol climate forcing is less sensitive to other characteristics of the size distribution, the aerosol composition, and the altitude of the aerosols. Thus stratospheric aerosol forcing can be defined accurately from measurements of aerosol extinction over a broad wavelength range.
TL;DR: In this paper, daily and weekly composites of aerosol optical thickness (AOT) at a wavelength of 0.5 micrometers have been analyzed to monitor the spatial and temporal variability of the aerosol layer and its optical thickness since the major eruption of Mt. Pinatubo on June 15, 1991.
Abstract: The NOAA/NESDIS operational aerosol optical thickness product has provided an exceptional view of the development of the Mt. Pinatubo stratospheric aerosol layer. The product is derived from reflected solar radiation measurements of the Advanced Very High Resolution Radiometer onboard the NOAA/11 polar orbiting environmental satellite. The greater the optical thickness, the greater the amount of reflected solar radiation. Daily and weekly composites of aerosol optical thickness (AOT) at a wavelength of 0.5 micrometers have been analyzed to monitor the spatial and temporal variability of the aerosol layer and its optical thickness since the major eruption of Mt. Pinatubo on June 15, 1991. These analyses show that: the volcanic aerosol layer circled the Earth in 21 days; there are inhomogeneities in the layer that seem to remain after over two months of circling the Earth; using an AOT of 0.1 to define the layer, it covered about 42% of the Earth's surface area after two months, over twice the area covered by the El Chichon aerosol layer two months after its eruption; the layer is confined to the latitude zone 20S to 30N, with occasional patches seen at somewhat higher latitudes; the largest mean optical thickness of the layer was 0.31, occurring on August 23rd; the mass of SO2 required to produce this aerosol optical thickness is 13.6 megatons; and, the globally averaged net radiation at the top of the atmosphere may be reduced by about 2.5 Wm−2 (cooling effect of at least 0.5°C) once the aerosol is distributed globally over the next two to four years.
TL;DR: In this paper, the authors calculate the solar wind-induced loss rates for evolving solar and atmospheric conditions like those described by Zhang et al. (1992), but including sputtering of the Martian atmosphere by reentering O(+) pickup ions, are described.
Abstract: Calculations of solar wind-induced loss rates for evolving solar and atmospheric conditions like those described by Zhang et al. (1992), but including sputtering of the Martian atmosphere by reentering O(+) pickup ions, are described. The inclusion of the sputter loss increases by about 30 percent the cumulative estimated loss of oxygen to that in about 50 m of water (global surface depth) over the last 3.5 billion years. These ions also sputter CO2 and its fragments in substantial amounts. That integrated loss is equivalent to about 0.14 bar atmospheric CO2 pressure, of the order of some estimates of Mars' early atmospheric inventory.
TL;DR: In this paper, the authors compared the 20-year (1965-1984) and 26-year mean (1964-1989) monthly means of the northern hemisphere stratospheric temperatures at 30 and 50 mb beginning in June 1991 to the mean of the 26 years, with some daily zonal mean increases of almost 3 C.
Abstract: Northern-Hemisphere stratospheric temperatures at 30 and 50 mb beginning in June 1991 are compared with 20-year (1965-1984) and 26-year (1964-1989) monthly means. Significant temperature increases are shown in July, August, September, and October for latitudes from approximately 30 deg N to the equator. In September and October deviations are observed for large areas between the equator and 30 deg N, with temperature increases as high as + 3.5 C occurring at some locations. The monthly averaged zonal mean 30-mb temperatures at 20 deg N in September and October were approximately 2.5 C higher that the 26-year mean, with some daily zonal mean increases of almost 3 C. Higher values occurred equatorward of 20 deg N. These warmings are due to absorption of radiation by the aerosols produced from the June eruptions of the volcano Pinatubo (15.1 deg N, 120.4 deg E) in the Philippines. Stratospheric warmings are expected to be occurring simultaneously at southern latitudes, especially from the equator to about 20 deg S, based on satellite and lidar measurements of the locations of the new aerosol layers. These localized temperature increases should decrease in magnitude and become more global as the cloud disperses globally and spreads in altitude.
TL;DR: The characteristics of the magnetic field emissions measured at two magnetic observatories in the Republic of Georgia prior to and after the 6.9 earthquake near Spitak, Armenia, on December 7, 1988, are compared with the apparently similar emissions associated with the 7.1 earthquake that occurred near Loma Prieta, California, on October 17, 1989.
Abstract: The characteristics of the ULF magnetic field emissions measured at two magnetic observatories in the Republic of Georgia prior to and after the Ms = 6.9 earthquake that occurred near Spitak, Armenia, on December 7, 1988, are compared with the apparently similar emissions associated with the Ms = 7.1 earthquake that occurred near Loma Prieta, California, on October 17, 1989. The main features of the Spitak measurements, according to observations made at the Dusheti station (128 km to the Spitak epicenter), as compared with the Loma Prieta measurements, which were made at Corralitos, California (7 km to the Loma Prieta epicenter), are the following: (1) The intensity of ULF background activity started growing 3 to 5 days before the Spitak earthquake, whereas the corresponding increase in activity began 12 days before the Loma Prieta earthquake; (2) a substantial ULF emission burst was recorded at Dusheti starting 4 hours prior to the main shock; a similar large burst of ULF activity commenced 3 hours before the Loma Prieta event, and continued until the occurrence of the main shock; (3) ULF activity remained high for about two weeks after the Spitak earthquake, and for several months after the Loma Prieta earthquake; (4) ULF noise bursts were observed 1 to 6 hours before powerful aftershocks at Spitak during the period of enhanced activity, but there was no conclusive link between the ULF noise at Corralitos and the after-shocks. A major difference in the ULF activity preceding the two earthquakes is a difference in amplitude (0.2 nT at Spitak and 5 nT at Loma Prieta), but this is easily explained as being caused by the different distances of the observation stations from the epicenters.
TL;DR: In this paper, ground-based measurements from the CANOPUS array of meridian scanning photometers and precipitating ion and electron data from the DMSP F9 satellite show that the electron arc which brightens to initiate substorm intensifications is formed within a region of intense proton precipitation that is well equatorward (approximately four to six degrees) of the nightside open-closed field line boundary.
Abstract: Ground based measurements from the CANOPUS array of meridian scanning photometers and precipitating ion and electron data from the DMSP F9 satellite show that the electron arc which brightens to initiate substorm intensifications is formed within a region of intense proton precipitation that is well equatorward (approximately four to six degrees) of the nightside open-closed field line boundary. The precipitating protons are from a population that is energized via earthward convection from the magnetotail into the dipolar region of the magnetosphere and may play an important role in the formation of the electron arcs leading to substorm intensifications on dipole-like field lines.
TL;DR: In this article, new data obtained in the Mediterranean Sea and Pacific Ocean showed that isoprene could be produced in sea water by biological processes, leading to concentrations in the ppb range (10−9 liter of gas per liter of water).
Abstract: New data obtained in the Mediterranean Sea and Pacific Ocean show that isoprene could be produced in sea water by biological processes, leading to concentrations in the ppb range (10−9 liter of gas per liter of water).
Taking into account Henry's constant for isoprene in water and the very low concentrations measured in the marine atmosphere, the superficial sea water is supersaturated in isoprene by one or two orders of magnitude. From these observations, an oceanic flux of the order of 1.2 Mt per year of isoprene can be estimated.
This is a small value, as compared with the marine fluxes of the other NMHC; it is also practically negligible in the global burden of isoprene. However, because of its short lifetime in air, isoprene in remote marine atmosphere, entirely originates from superficial seawater, it could be therefore an useful tracer of marine emissions of gaseous compounds.
TL;DR: In this paper, a compilation of oceanic crustal thickness from seismic observations collected over the past two decades is presented, showing that the average seafloor thickness is 6 km.
Abstract: A compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness, away from plateaus, is 6 km; no systematic increase of crustal thickness with spreading rate is observed. Instead, the data show large variations in crustal thickness at slow spreading ridges (3 – 8 km for half rates 30 mm/yr). The large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid-ocean ridges. Both data sets support the speculation of a transition from a 3-D structure of crustal accretion at slow ridges to a 2-D accretion pattern at fast ridges.
TL;DR: In this article, transient bursts of magnetic reconnection (flux transfer events) are considered as a cause of polar cap "patches" of enhanced plasma density, as observed at European longitudes by the EISCAT radar.
Abstract: Some implications are considered of recent theoretical work concerning the excitation of dayside ionospheric convection by magnetic reconnection at the dayside magnetopause. In particular, transient bursts of such reconnection (‘flux transfer events’) are considered as a cause of polar cap ‘patches’ of enhanced plasma density. Examples of such patches, as observed at European longitudes by the EISCAT radar, are presented and used to discuss the implications of the proposed mechanism.
TL;DR: In this paper, a simple expression for the root-mean-square variability of a process with such a spectrum over an interval of time or space is derived, which yields the powerlaw time dependence characteristic of fractal processes, but can be generalized to give the temporal variability for more general spectral behaviors.
Abstract: The power spectra of many geophysical phenomena are well approximated by a power-law dependence on frequency or wavenumber. A simple expression for the root-mean-square variability of a process with such a spectrum over an interval of time or space is derived. The resulting expression yields the powerlaw time dependence characteristic of fractal processes, but can be generalized to give the temporal variability for more general spectral behaviors. The method is applied to spectra of crustal strain (to show what size of strain events can be detected over periods of months to seconds) and of sea level (to show the difficulty of extracting long-term rates from short records).
TL;DR: In this article, the authors inverted the seismically inferred 3D structure of the whole manfie for the radial viscosity structure which best fits the geoid, and obtained 60-72% variance reductions for three different S-wave tomographic models.
Abstract: Using a uniform velocity/density scaling, we have inverted the seismically inferred 3-dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid. We are able to obtain 60-72% variance reductions for three different S-wave tomographic models. The resulting viscosity structures are remarkably similar, showing a high viscosity lid, a low viscosity zone in the transition region and a high viscosity lower manfie. A resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data, however the resolution in the lower manfie is poorer. Our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400-670 km as opposed to 100-400 kin.
TL;DR: A number of subcrustal earthquakes have been located up to 90 km depth beneath the Northern Apennines (Italy), mostly confined to the upper 20 km of the crust.
Abstract: Previously unreported subcrustal earthquakes have been located up to 90 km depth beneath the Northern Apennines (Italy). These earthquakes occur beneath a zone of abundant upper crustal seismicity, mostly confined to the upper 20 km of the crust. Although there are relatively few well located subcrustal earthquakes, there appears to be a general southwestward deepening of the hypocenters, from the Adriatic to the Tyrrhenian Sea, consistent with other geophysical data which suggest that the Adriatic lithosphere is presently subducting beneath the Northern Apennines.
TL;DR: In this article, a correlation between measured total solar irradiance (corrected for sunspot effects) and a Ca II surrogate for bright magnetic features was found, and it was estimated that the Sun's radiative output in the absence of such features to be 1365.43 w/m2, or 0.24% below its mean value for the 1980 to 1986 period.
Abstract: The coincidence between the Maunder Minimum of solar magnetic activity from 1645 to 1715 and the coldest temperatures of the Little Ice Age raises the question of possible solar forcing of the Earth's climate. Using a correlation which we find between measured total solar irradiance (corrected for sunspot effects) and a Ca II surrogate for bright magnetic features, we estimate the Sun's radiative output in the absence of such features to be 1365.43 w/m2, or 0.15% below its mean value of 1367.54 W/m2 measured during the period 1980 to 1986 by the ACRIM experiment. Observations of extant solar-type stars suggest that the Ca II surrogate vas darker during the Maunder Minimum. Allowing for this, we estimate the total solar irradiance to be 1364.28 W/m2 or 0.24% below its mean value for the 1980 to 1986 period. The decrease in the global equilibrium temperature of the Earth due to a decrease of 0.24% in total solar irradiance lies in the range from 0.2° C to 0.6° C, which can be compared with the approximately 1° C cooling experienced during the Little Ice Age, relative to the present.
TL;DR: A balloonborne optical particle counters at Laramie, Wyoming during the summer of 1991 were used to study the particle size distribution and volatility of the aerosol which formed in the stratosphere following the mid-June eruptions of Mt Pinatubo as discussed by the authors.
Abstract: Measurements using balloonborne optical particle counters at Laramie, Wyoming during the summer of 1991 are used to study the particle size distribution and volatility of the aerosol which formed in the stratosphere following the mid-June eruptions of Mt. Pinatubo. Enhanced aerosol layers were observed below 20 km as early as 16 July, about 1 month after the eruption. During late July, a transient though substantial particle layer was observed in the 23 km region. High concentrations of large particles in this high altitude layer resulted in aerosol mass mixing ratios as large as 0.5 ppm, considerably larger than observed following the eruption of El Chichon. Aerosol volatility tests indicated that well over 90% of the particles were composed of an H2SO4/H2O solution in all layers observed, indicating rapid conversion of SO2 to H2SO4 and subsequent droplet growth. High concentrations of droplets suggest homogeneous or ion nucleation as the most likely aerosol production mechanism.
TL;DR: In this article, the Lehmann discontinuity in the upper mantle is explained by a change in the preferred orientation of olivine as a result of change in deformation mechanism: in the shallow upper mantle, deformation is due to dislocation creep and results in an anisotropic structure, whereas in the deeper upper mantle diffusion creep becomes dominant due to a smaller activation volume, resulting in an isotropic structure.
Abstract: The Lehmann discontinuity (∼200km) in the upper mantle is proposed to result from a change in the preferred orientation of olivine as a result of a change in deformation mechanism: in the shallow upper mantle, deformation is due to dislocation creep and results in an anisotropic structure, whereas in the deeper upper mantle, diffusion creep becomes dominant due to a smaller activation volume, resulting in an isotropic structure. For anisotropy due to dislocation-glide corresponding to a horizontal flow, the resultant anisotropy will yield VSH>VSV and VPH>vPV. Thus, for waves propagating nearly vertically (e.g., ScS waves), there will be discontinuous jumps in the S- and P-wave velocities at this discontinuity. In this model, the depth of the Lehmann discontinuity depends on the deformation conditions such as stress level, geotherm, water fugacity and grain-size, giving a natural explanation for its observed variation with tectonic setting.
TL;DR: In this paper, a simulation of the climate response to Amazon deforestation has been carried out, which shows that the initial decrease of the absorption of solar radiation by higher surface albedos is largely cancelled by a reduction in cloud cover, but consequent reduction in downward longwave has a substantial impact on surface energy balance.
Abstract: A simulation of the climate response to Amazon deforestation has been carried out. Precipitation is decreased on the average by 25 percent or 1.4 mm/day, with ET and runoff both decreasing by 0.7 mm/day. Modifications of surface energy balance through change of albedo and roughness are complicated by cloud feedbacks. The initial decrease of the absorption of solar radiation by higher surface albedos is largely cancelled by a reduction in cloud cover, but consequent reduction in downward longwave has a substantial impact on surface energy balance. Smoke aerosols might have an effect comparable to deforestation during burning season. 8 refs.