Showing papers in "Geophysical Research Letters in 1993"
TL;DR: In this article, the velocity field of present-day deformation in Central Asia is modelled using a set of four rotating blocks (Siberia, Tarim, Tibet, India) on a spherical earth.
Abstract: The velocity field of present‐day deformation in Central Asia is modelled using a set of four rotating blocks (Siberia, Tarim, Tibet, India) on a spherical earth. A best‐fit is inverted on the basis of estimated shortening‐rates across the main thrust zones (Himalayas, Tien Shan) and measured slip‐rates along the principal strike‐slip faults (Altyn Tagh and Karakorum) separating those blocks. The fit to the data implies that nearly all the present convergence between India and Asia can be accounted for by slip‐partitioning on these four zones, with as much as 50% absorbed by northeastwards extrusion of Tibet. This suggests that localised deformation governs the present mechanical behaviour of the Central Asian lithosphere.
881 citations
TL;DR: In this article, the contribution from anelasticity is shown to be important in the Earth's mantle particularly for shear wave velocities, and it is shown that the temperature anomalies associated with low velocity anomalies in the mantle will be significantly smaller than previously considered on the basis of anharmonic effect alone.
Abstract: Temperature dependence of seismic wave velocities comes both from anharmonicity and anelasticity. The contribution from anelasticity is shown to be important in the Earth's mantle particularly for shear waves. In the low Q (Qµ∼100) regions in the upper mantle, the correction due to anelasticity will roughly double the temperature derivatives due to anharmonicity alone. The correction for anelasticity will also be important in the deep mantle where Q is larger, if temperature derivatives due to anharmonicity will decrease significantly with pressure. These results imply that the temperature anomalies associated with low velocity anomalies in the mantle will be significantly smaller than previously considered on the basis of anharmonic effect alone and that the amplitude of velocity anomalies will be significantly larger for shear waves than for compressional waves.
844 citations
TL;DR: In this article, the authors used submicron magnetite cubes with mean sizes of 0.037, 0.076 and 0.22 µm which were initially non-stoichiometric (oxidation parameter z from 0.2 to 0.3).
Abstract: At the Verwey transition (Tv≈110–120 K), magnetite transforms from monoclinic to cubic spinel structure. It has long been believed that magnetic remanence and susceptibility would change markedly at Tv in the case of coarse grains but only slightly or inappreciably in the case of fine (<1 µm) grains. We find on the contrary that remanence changes at Tv by 50–80% in both large and small crystals, if they are stoichiometric. However, minor surface oxidation suppresses the transition, and the fact that fine grains oxidize more readily leads to an apparent size dependence. Our experiments used submicron magnetite cubes with mean sizes of 0.037, 0.076, 0.10 and 0.22 µm which were initially non-stoichiometric (oxidation parameter z from 0.2–0.7). A saturation isothermal remanent magnetization (SIRM) given in a 2.5 T field at 5 K decreased steadily during zero-field warming to 300 K with little or no indication of the Verwey transition. After the oxidized surface of each crystal was reduced to stoichiometric magnetite, the SIRM decreased sharply during warming by 50–80% around 110 K. The change in SIRM for the 0.22 µm grains was almost identical to that measured for a 1.5 mm natural magnetite crystal. Thus a 1012 change in particle volume does not materially affect the remanence transition at Tv but oxidation to z=0.3 essentially suppresses the transition. The effect of the degree of oxidation on Tv provides a sensitive test for maghemitization in soils, sediments and rocks.
479 citations
TL;DR: In this paper, the authors modeled the formation of a new electron radiation belt at about or = 25 that resulted from the Storm Sudden Commencement (SSC) of March 24, 1991 as observed by the Combined Release and Radiation Effects Satellite (CRRES) satellite.
Abstract: We model the rapid (about 1 min) formation of a new electron radiation belt at L about or = 25 that resulted from the Storm Sudden Commencement (SSC) of March 24, 1991 as observed by the Combined Release and Radiation Effects Satellite (CRRES) satellite Guided by the observed electric and magnetic fields, we represent the time-dependent magnetospheric electric field during the SSC by an asymmetric bipolar pulse that is associated with the compression and relaxation of the Earth's magnetic field We follow the electrons using a relativistic guiding center code The test-particle simulations show that electrons with energies of a few MeV at L greater than 6 were energized up to 40 MeV and transported to L about or = 25 during a fraction of their drift period The energization process conserves the first adiabatic invariant and is enhanced due to resonance of the electron drift motion with the time-varying electric field Our simulation results, with an initial W(exp -8) energy flux spectra, reproduce the observed electron drift echoes and show that the interplanetary shock impacted the magnetosphere between 1500 and 1800 MLT
407 citations
TL;DR: In this article, an empirical vapor pressure equation was derived and allowed prediction of vapor pressures between 170 k and the triple point of water with an accuracy of approximately 2 percent, within experimental uncertainty, with the most reliable equation derived from thermodynamic principles.
Abstract: New measurements of ice vapor pressures at temperatures between 170 and 250 K are presented and published vapor pressure data are summarized. An empirical vapor pressure equation was derived and allows prediction of vapor pressures between 170 k and the triple point of water with an accuracy of approximately 2 percent. Predictions obtained agree, within experimental uncertainty, with the most reliable equation derived from thermodynamic principles.
405 citations
TL;DR: The first measurements of inorganic chlorine gases in the marine atmosphere using a new tandem mist chamber method were reported in this article, where surface air was sampled during four days including one diel cycle in January, 1992, at Virginia Key, Florida.
Abstract: We report the first measurements of inorganic chlorine gases in the marine atmosphere using a new tandem mist chamber method. Surface air was sampled during four days including one diel cycle in January, 1992, at Virginia Key, Florida. Concentrations of HCl* (including HCl, ClNO3, ClNO2, and NOCl) were in the range 40 to 268 pptv and concentrations of Cl2* (including Cl2 and any HOCl not trapped in the acidic mist chamber) were in the range <26 to 254 pptv Cl. Concentrations of Cl2* increased during the night, and decreased after sunrise as HCl* concentrations increased by similar amounts. The measurements suggest an unknown source of either HOCl or Cl2 to the marine atmosphere. Photochemical model calculations indicate that photolysis of the observed Cl2* would yield a chlorine atom (Cl•) concentration of order 104–105 cm−3. Oxidation by Cl• would then represent a significant sink for alkanes and dimethylsulfide (DMS) in the marine boundary layer. The cycling of Cl• could provide either a source or a sink for O3, depending on NOX levels.
334 citations
TL;DR: In this article, the authors use a monte carlo simulation technique to estimate the amount of tectonic tilting at the time of magnetization along with a 95% confidence interval.
Abstract: Most proposed fold test formulations use significance tests to try pre-tilting and post-tilting remanence hypotheses. We suggest that it is better to consider the fold test as a parameter estimation problem. Making the usual assumption that the distribution of remanence vectors was originally roughly parallel, we propose, using a monte carlo simulation technique, to estimate the amount of tectonic tilting at the time of magnetization along with a 95% confidence interval. If, for example, this confidence interval included 100% then one could not rule out pre-tilting remanence. In the older terminology, the fold test is positive. The k-ratio test of McElhinny [1964] is often said to be conservative in that if a study passes the k-ratio test then it certainly passes a more rigorous test. We show with a typical counter-example that this assertion is incorrect. Observational uncertainty of bedding directions is easily included in this formulation.
318 citations
TL;DR: In this article, a model for the response of sedimentary layers to ambient seismic noise is presented, where the noise sources are modeled as a random distribution (in time and space) of point forces located on the Earth's free surface.
Abstract: For over thirty years, attempts have been made to gain information about sediment amplification during earthquakes from observations of ambient seismic noise. While the results of several feasibility studies have been encouraging, theoretical support for the technique is scant. We present a model for the response of sedimentary layers to ambient seismic noise. The noise sources are modeled as a random distribution (in time and space) of point forces located on the Earth's free surface. This model is applied to a site where observed noise spectral ratios, relative to a rock site, have previously been shown to reveal the fundamental resonant frequency of a soft clay layer. Approximating the sediment site as a single layer over a half-space, the horizontal noise spectrum predicted by our model reveals the fundamental resonance and first harmonic of the layer. We also examine an estimate of site response proposed by Nakamura (1989), which is formed by dividing the horizontal-component noise spectrum by that of the vertical component. Nakamura's estimate applied to both observed and predicted noise-spectra was also successful in identifying the fundamental resonance, with a slight (<10%) shift toward lower frequencies. Future work is needed to determine the generality of our results, and to elucidate the influence of the simplifying assumptions.
314 citations
TL;DR: In this article, the precipitable water vapor (PWV) and wet delay in the line-of-sight to GPS satellites were measured at both ends of a 50-kin baseline in Colorado.
Abstract: Global Positioning System (GPS) receivers, water vapor radiometers (WVRs), and surface meteorological equip- ment were operated at both ends of a 50-kin baseline in Colorado to measure the precipitable water vapor (PWV) and wet delay in the line-of-sight to GPS satellites Using high pre- cision orbits, WVR-measured and GPS-inferred PWV differences between the two sites usually agreed to better than 1 min Using less precise on-line broadcast orbits increased the discrepancy by 30% Data simulations show that GPS mea- surements can provide ram-level separate PWV estimates for the two sites, as opposed to just their difference, if baselines exceed 500 km and the highest accuracy GPS orbits are used
301 citations
TL;DR: In this article, a laboratory study of peat columns revealed that CH 4 emission rates initially increased and then decreased as the water table was lowered from the peat surface to a depth of 50 cm, with the release of CH 4 trapped in pores.
Abstract: Water table position, through the creation of aerobic and anaerobic conditions in the soil profile, plays an important role in controlling CH 4nflux from wetlands. A laboratory study of peat columns revealed that CH 4emission rates initially increased and then decreased as the water table was lowered from the peat surface to a depth of 50 cm, with the release of CH 4ntrapped in pores. There was a strong hysteresis between CH 4nflux on the falling and rising water table limbs (falling g rising). When expressed as seasonal average values, there was a strong relationship (rs 0.08 n 0.74) between log CH 4nflux and water table position for sites within 5 wetland regions in boreal‐subarctic Canada. The regression coefficients were similar among regions (0.022 n 0.037), but there were differences in the regression constants (0.47 n 1.89). CH 4nflux from drained, forested peatland soils decreased as the water table depth increased, and several sites were transformed from sources to sinks of CH 4. Global CH 4nemissions to the atmosphere may have been reduced by a 1 Tg yr m1nby peatland drainage during the last 100 yr.
286 citations
TL;DR: In this paper, microstructural data on experimentally deformed natural olivine rocks (Anita Bay and Aheim dunite), together with the other reliable experimental data, indicate that the experimental stress-recrystallized grain size relationship in olivined-rocks is largely independent of water content and temperature, and is only slightly dependent on the flow properties of the material.
Abstract: New microstructural data on experimentally deformed “wet” and “dry” natural olivine rocks (Anita Bay and Aheim dunite), together with the other reliable experimental data, indicate that the experimental stress-recrystallized grain size relationship in olivine-rocks is largely independent of water content and temperature, and is only slightly dependent on the flow properties of the material. The experimental data cover a stress range of 30–300 MPa, water contents from <30 ppm to 300 ppm, and temperatures in the range 1100–1650°C. Local melt contents of up to 10 volume% cannot be demonstrated to have a significant effect on the stress—grain size relationship.
TL;DR: In this article, the vertical profiles of aerosol have been measured approximately biweekly since June 1991 at Laramie, Wyoming (41°N) using balloon-borne instruments, and the total number concentration and concentration of particles > 0.15 to 10.0 µm were measured using balloonborne instruments.
Abstract: Vertical profiles of aerosol have been measured approximately biweekly since June 1991 at Laramie, Wyoming (41°N). Both the total number concentration and concentration of particles >0.15 to 10.0 µm were measured using balloonborne instruments. The aerosol size distributions were best represented when bimodal lognormal distributions were fit to the data. After an early short-lived intense aerosol layer, the stratospheric maximum surface area and mass (40 µm² cm−3, 160 ppbm) was observed to occur approximately 180 days after the eruption. The aerosol was then observed to remain relatively homogeneous both in altitude and time during 1992, with the maximum surface area and mass remaining relatively constant between 20 to 30 µm² cm−3 and 30 to 60 ppbm.
TL;DR: In this article, the authors measured the height of a granitic fault surface as a function of position along one-dimensional profiles and showed that the profiles exhibit an "anisotropic" scaling invariance: self-affinity.
Abstract: We recorded the height of a granitic fault surface as a function of position along one-dimensional profiles. We show that the profiles exhibit an "anisotropic" scaling invariance: self-affinity. The difference between the maximum and the minimum height, and the standard deviation of the height, over a length L are propor- tional to L½, where ( 0.84. Other properties such as the Return Probability distribution or the Power Spectrum of the profile com- fort this result. This self-affne property is in good agreement with recent works on artificial fractured surfaces. Previous studies at field scale are consistent with this concept.
TL;DR: In this article, a 2.5 km deep triaxial seismometer at Cajon Pass in southern California has recorded several hundred earthquakes
Abstract: A 2.5 km deep triaxial seismometer at Cajon Pass in southern California has recorded several hundred earthquakes
TL;DR: In this paper, the photochemical age of an air mass derived from the ratio of hydrocarbons is a function of both photochemistry and atmospheric transport, and it is not possible to derive a unique value for the age from hydrocarbon ratios alone.
Abstract: An effective and commonly used technique for studying the sources, photochemistry, and even the “photochemical age” of trace species is to examine ratios of hydrocarbons by assuming the ratio is independent of transport processes. We present results from mesoscale model calculations that suggest a significant effect by atmospheric mixing on the ratio. We also show that the photochemical age of an air mass derived from the ratio of hydrocarbons is a function of both photochemistry and atmospheric transport. Without additional information, it is not possible to derive a unique value for the age of an air mass from hydrocarbon ratios alone.
Abstract: The ratio of intracloud lightning (IC) to cloud-to-ground lightning (CG) in thunderstorms is observed to vary with latitude. The reason frequently given for this behavior is that the height of the freezing level, and hence the negative charge center in thunderstorms, varies with latitude, resulting in higher ratios in the tropics compared with those in midlatitudes. This study shows that this hypothesis could well be incorrect. Analysis of cloud and lightning data indicates that the IC/CG ratio is linked to the thickness of the cold cloud region in thunderstorms (0 C to cloud top), rather than to the freezing level height.
TL;DR: The Microwave Limb Sounder (MLS) experiment on the Upper Atmosphere Research Satellite (UARS) began atmospheric measurements on September 19, 1991 and the residual SO2 from the Pinatubo eruption was detected immediately after instrument activation as mentioned in this paper.
Abstract: The Microwave Limb Sounder (MLS) experiment on the Upper Atmosphere Research Satellite (UARS) began atmospheric measurements on September 19, 1991. Residual SO2 from the Pinatubo eruption was detected immediately after instrument activation. The SO2 was 1ocated in a tropical band with peak mixing ratios of about 15 ppbv near 26 km altitude. The observed decay of SO2 has provided an estimate of the OH concentration in the plume.
TL;DR: In this paper, a short-duration upper atmospheric optical flashes were recorded on the night of 8 July, 1993 (9 July UT) from the NASA DC-8 Airborne Laboratory flying over the American Midwest.
Abstract: Short-duration upper atmospheric optical flashes were recorded on the night of 8 July, 1993 (9 July UT) from the NASA DC-8 Airborne Laboratory flying over the American Midwest. All-sky video images from an intensified silicon intensified target (ISIT) camera revealed 19 upper atmospheric flashes occurring over a period of approximately 100 min. The flashes were similar in appearance to previously reported ground and shuttle-based video observations. Detailed analysis of 12 of the events yielded these parameters: (1) duration less than or equal to 17 ms; (2) brightness 10-50 kR, roughly that of bright aurorae; (3) terminal heights 30-100 km, with a mean of approximately 60 km; (4) horizontal extent 10-50 km; (5) emission volumes greater than 1000 cu km. The relative frequency of the optical flashes was (6) 1:200-1:400 compared to negative cloud-to-ground discharges and 1:20-1:40 compared to positive cloud-to-ground discharges.
TL;DR: In this paper, the authors used AMPTE/IRM and ISEE 2 data to study the properties of the inner plasma sheet (IPS), the average flow pattern in the non-BBF (quiet) IPS is constructed.
Abstract: We use AMPTE/IRM and ISEE 2 data to study the properties of the high beta (i > 0.5) plasma sheet, the inner plasma sheet (IPS). Bursty bulk flows (BBFs) are excised from the two databases, and the average flow pattern in the non-BBF (quiet) IPS is constructed. At local midnight this ensemble-average flow is predominantly duskward; closer to the flanks it is mostly earthward. The flow pattern agrees qualitatively with calculations based on the Tsyganenko ( 1987) model (T87), where the earthward flow is due to the ensemble- average cross tail electric field and the duskward flow is the diamagnetic drift due to an inward pressure gradient. The IPS is on the average in pressure equilibrium with the lobes. Because of its large variance the average flow does not represent the instantaneous flow field. Case studies also show that the non-BBF flow is highly irregular and inherently unsteady, a reason why earthward convection can avoid a pressure balance inconsistency with the lobes. The ensemble distribution of velocities is a fundamental observable of the quiet plasma sheet flow field.
TL;DR: In this article, the effects of a silicate melt and water on seismic velocities are compared at relatively small fluid fraction (less than 20 vol%) on the basis of a theoretical model of composite materials, to show that the two fluids are clearly characterized by the velocity ratio Vp/Vs.
Abstract: The effects of a silicate melt and water on seismic velocities are compared at relatively small fluid fraction (less than 20 vol.%) on the basis of a theoretical model of composite materials, to show that the two fluids are clearly characterized by the velocity ratio Vp/Vs. For a silicate melt, Vp/Vs increases with increasing fluid fraction. When a significant reflection is expected, Vp/Vs becomes much larger than 2. For water, Vp/Vs decreases as the fluid fraction increases to 10 vol.% then it increases. But it remains similar to a solid state value (about 1.8), unless the amount of water exceeds 15 vol.%. This will be a good measure to distinguish between two candidates for seismic reflectors: partially molten rocks and rocks containing free water. If a reflector can be treated as a thin low velocity layer, the velocity ratio in it can be estimated from the frequency dependence of reflection coefficient.
TL;DR: In this paper, the authors consider the foundations of 1-and 2-dimensional shock simulations in which the physical quantities are independent of a coordinate which is not parallel to the magnetic field and show analytically that the ions are effectively tied to the convected magnetic lines of force because of the presence of an ignorable spatial coordinate.
Abstract: We consider the foundations of 1- and 2-dimensional shock simulations in which the physical quantities are independent of a coordinate which is not parallel to the magnetic field. We show analytically that in such simulations the ions are effectively tied to the convected magnetic lines of force because of the presence of an ignorable spatial coordinate. This conclusion has important consequences. In particular we conclude that the acceleration of energetic charged particles at quasi-perpendicular shocks cannot be properly studied in such simulations because the role of perpendicular diffusion cannot be properly evaluated.
TL;DR: In this paper, it was shown that if magnetization is scaling with a single scaling exponent, then the observed f−3 power spectra of aeromagnetic data implies a three-dimensional f−4 susceptibility distribution for length scales of 1 to 1000 km.
Abstract: Many geophysical variables exhibit fractal or at least scaling behaviour, notably in having power spectra proportional to some power of frequency. Power spectra of magnetic susceptibility logs from boreholes are proportional to spatial frequency f−2 for wavelengths from 0.1 to 100 m. If the power spectrum of the susceptibility or magnetization of the crust is known then the power spectrum of the magnetic field at the surface can be calculated. We show that if magnetization is scaling with a single scaling exponent then the observed f−3 power spectra of aeromagnetic data implies a three-dimensional f−4 susceptibility distribution for length scales of 1 to 1000 km. One way to reconcile the two results is to introduce anisotropy in the form of a f−4 variation for the horizontal direction and f−2 for the vertical.
TL;DR: Based on a multifractal structure hypothesis for temporal rainfall processes, a general formula relating maximum possible point rainfall accumulations was derived as a function of the duration and sample size in this paper.
Abstract: Based on a multifractal structure hypothesis for temporal rainfall processes, a general formula relating maximum possible point rainfall accumulations is derived as a function of the duration and sample size. This formula appears to be in agreement with empirical observations. Such a result may reconcile some opposite points of view regarding extreme rainfall events, and suggests new ways of exploiting the scaling properties of rain processes.
TL;DR: In this paper, a model of plasma transport in the open magnetic field region of the dayside magnetosphere has been used to investigate the source regions and the distribution of solar wind plasma in the magnetospheric cusp and mantle.
Abstract: A model of plasma transport in the open magnetic field region of the dayside magnetosphere has been used to investigate the source regions and the distribution of solar wind plasma in the magnetospheric cusp and mantle. This model includes the variation in the magnetosheath properties as the plasma accelerates away from the subsolar point, the variation in the transport of magnetosheath plasma across the magnetopause as the orientation of the magnetic field line evolves following reconnection, and the transport of the magnetosheath plasma within the magnetosphere. The authors' model results are compared with low-altitude spacecraft data from a crossing of the cusp and mantle. They find qualitative and quantitative agreement between the model results and the data. This agreement suggests that the zeroth-order processes included in the model are sufficient to reproduce the commonly observed particle distributions in the low-altitude cusp and mantle regions. 16 refs., 2 figs.
TL;DR: In this paper, the authors show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern of thermal convection in a wide (8×8×1) Cartesian box.
Abstract: Numerical simulations of thermal convection in a wide (8×8×1) Cartesian box heated from below with temperature-dependent viscosity contrasts of 1000, and Rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern. With rigid upper and lower boundaries, spoke-pattern flow with small (diameter ∼ 1.5) cells is obtained, consistent with laboratory experiments and previous numerical results. However, with the arguably more realistic stress-free boundaries, the flow chooses the largest possible wavelength, forming a single square cell of aspect ratio 8, with one huge cylindrical downwelling surrounded by upwelling sheets. The addition of stress-dependence to the rheology weakens the stiff upper boundary layer, resulting in smaller cells, though still with upwelling sheets and downwelling plumes.
TL;DR: In this paper, Ulysses encountered a recurrent high-speed stream, that may also have been observed at IMP 8 at 1 AU, and observed it a total of 14 times, once in each solar rotation through June 1993 at approximately 34 deg S. The source of the high speed stream is an equatorward extension of the south polar coronal hole.
Abstract: Near-ecliptic solar wind observations by Ulysses on its way to the polar regions of the Sun, compared with those from IMP 8 at 1 AU, showed that high-speed streams decay and broaden with heliocentric distance from IMP 8 to Ulysses, as expected. In July 1992 while traveling south at approximately 13 deg S and 5.3 AU, Ulysses encountered a recurrent high-speed stream, that may also have been observed at IMP 8. The stream has been observed a total of 14 times, once in each solar rotation through June 1993 at approximately 34 deg S. The source of the high-speed stream is an equatorward extension of the south polar coronal hole. From July 1992 through June 1993, averages of solar wind peak speed increased while density decreased with heliographic latitude. Both the stream and a low-speed, high-density flow, presumably associated with the heliomagnetic (coronal) streamer belt encircling the heliomagnetic equator, crossed Ulysses with the solar rotation period until April 1993 when the spacecraft was at approximately 29 deg S heliographic latitude. After this time, as the spacecraft climbed to higher latitudes, the central portion of the streamer belt with lowest speed and highest density disappeared. Therefore, at its maximum inclination, the belt was tilted at approximately 29 deg to the heliographic equator at this point in the solar cycle.
TL;DR: In this article, a method based on thermal unblocking of low-temperature saturation remanent magnetization for a quantitative estimation of the superparamagnetic fraction (size, d < 30 nm) of magnetite produced by pedogenesis in the Chinese loess plateau was proposed.
Abstract: We propose a method based on thermal unblocking of low-temperature saturation remanent magnetization for a quantitative estimation of the superparamagnetic [Cullity, 1972] fraction (size, d < 30 nm) of magnetite produced by pedogenesis in the Chinese loess plateau [Liu, 1988] We applied this method to the proxy climatic records of the last 130 ka from two sites 250 km apart, but separated by the mountain range Liupan-shan Xifeng to the east (357°N, 1076°E) and Baicaoyuan to the west (362°N, 1050°E) currently have humid and arid microclimates, respectively As expected, the superparamagnetic fraction increases during known warm temperature intervals at each site Furthermore, the more humid site clearly has higher overall superparamagnetic fractions during most of the last 130 ka However, during the period 5 to 10 ka ago, the relative humidity at both sites was the same within experimental errors Bulk grain size evidence confirms the magnetic data, and we suggest that the present easterly summer monsoon in China came from a more southerly direction during this time to flow parallel to Liupan-shan, resulting in very similar summer humidity at Xifeng and Baicaoyuan
TL;DR: In a recent study carried out at an isolated site in the Colorado mountains, a C5 alcohol, 2-methyl-3-buten-2-ol was found to be the most abundant volatile organic compound of biogenic origin present in the atmosphere as discussed by the authors.
Abstract: During a recent study carried out at an isolated site in the Colorado mountains, a C5 alcohol, 2-methyl-3-buten-2-ol, was found to be the most abundant volatile organic compound of biogenic origin present in the atmosphere. This finding, if generally characteristic of the natural chemical species present in the atmosphere in forested areas, has important implications. First, the presence in large quantities of a reactive chemical compound at these high levels can significantly influence the local atmospheric chemistry. Secondly, this compound, although previously identified as a pheromone for Ips typographus (spruce bark beetle), an insect predator responsible for major forest die-backs in this region, is strongly correlated with isoprene. Since isoprene is known to be emitted by the local vegetation, the observed 2-methyl-3-buten-2-ol appears also to have a vegetative rather than entomological source.
TL;DR: In this paper, the authors present evidence for approximate collocation of seismic and electrical transitions in the upper mantle by using magnetotelluric (MT) data from a lakebottom observatory in the central Canadian Shield.
Abstract: We present evidence for approximate collocation of seismic and electrical transitions in the upper mantle. More than two years of very long period magnetotelluric (MT) data were recorded at a lakebottom observatory in the central Canadian Shield. After processing to contend with non-stationary source effects, and removal of galvanic distortion, the underlying structure is 1D for periods of one hour to four days. The response was extended to periods of 100 days by appending Geomagnetic Depth Sounding data to the MT curves. Minimum structure linearised inversion, nonlinear extremal inversion, and a new genetic algorithm for nonlinear hypothesis testing, reveal discrete jumps in conductivity at depths near the major upper mantle seismic discontinuities. The jumps occur over limited depth ranges.
TL;DR: In this article, a Boltzmann formulation of the electron distribution function and Maxwell's equations for the EM fields are used to simulate the interaction of lightning radiated EM pulses with the lower ionosphere.
Abstract: A Boltzmann formulation of the electron distribution function and Maxwell's equations for the EM fields are used to simulate the interaction of lightning radiated EM pulses with the lower ionosphere. Ionization and dissociative attachment induced by the heated electrons cause significant changes in the local electron density, N(e). Due to 'slow' field changes of typical lightning EM pulses over time scales of tens of microsec, the distribution function follows the quasi-equilibrium solution of the Boltzmann equation in the altitude range of interest (70 to 100 km). The EM pulse is simulated as a planar 100 microsec long single period oscillation of a 10 kHz wave injected at 70 km. Under nighttime conditions, individual pulses of intensity 10-20 V/m (normalized to 100 km horizontal distance) produce changes in N(e) of 1-30 percent while a sequence of pulses leads to strong modification of N(e) at altitudes less than 95 km. The N(e) changes produce a 'sharpening' of the lower ionospheric boundary by causing a reduction in electron density at 75-85 km (due to attachment) and a substantial increase at 85-95 km (due to ionization) (e.g., the scale height decreases by a factor of about 2 at about 85 km for a single 20 V/m EM pulse). No substantial N(e) changes occur during daytime.