bluuebunny/crossref_metadata_2025 huggingface dataset:
Satellite estimation of spectral UVB irradiance using TOMS derived total ozone and UV reflectivity
Authors: Eck T. F. , Bhartia P. K. , Kerr J. B.
Published: February 2004

A method for satellite remote sensing of spectral UVB radiation incident at the earth's surface for snow and/or ice free areas has been developed. Measurements of total ozone and UV reflectivity from the Nimbus‐7 Total Ozone Mapping Spectrometer (TOMS) instrument have been applied to this technique. Comparison of satellite estimates with ground based measurements of spectral UVB irradiance show differences which are comparable to differences between near simultaneous measurements made with two or more ground based co‐located instruments.
Correlated brightness variations in solar radiative output from the photosphere to the corona
Authors: Lean J. L. , Mariska J. T. , Strong K. T. , Hudson H. S. , Acton L. W. , Rottman G. J. , Woods T. N. , Willson R. C.
Published: February 2004

Correlated brightness variations are shown to occur in time series of coronal soft X‐rays exclusive of prominent active regions, chromospheric ultraviolet radiation, and the photospheric total solar irradiance corrected for sunspot effects. These temporal correlations suggest that upwardly extending magnetic fields may have a large scale impact on the solar atmosphere in addition to their demonstrable role of generating localized active regions. The correlations have implications for improving and extending solar spectrum variability models.
Evidence for muon‐induced production of 10Be in near‐surface rocks from the Congo
Authors: Brown Erik Thorson , Bourlès Didier L. , Colin Fabrice , Raisbeck Grant M. , Yiou Françoise , Desgarceaux Sophie
Published: February 2004

Distributions of 10Be in vein quartz from a lateritic weathering profile from the Congo provide evidence for significant in situ production of 10Be by reactions other than neutron‐induced spallation. After consideration of geomorphological and nuclear processes which might contribute to the observed depth variability, we conclude that cosmic ray muon‐induced reactions are the most likely mechanism, and that such reactions lead to 1% to 3% of 10Be production at the surface. This small muon‐induced component has only minor implications for the use of 10Be for quantification of exposure histories of surficial rocks. Nevertheless, the muogenic component has the potential to expand the utility of cosmogenic nuclides in examination of surficial processes, overcoming some of the limitations associated with the use of the neutron‐produced component.
GPS detection of ionospheric perturbations following the January 17, 1994, Northridge Earthquake
Authors: Calais Eric , Minster J. Bernard
Published: February 2004

Sources such as atmospheric or buried explosions and shallow earthquakes producing strong vertical ground displacements are known to produce pressure waves that propagate at infrasonic speeds in the atmosphere. At ionospheric altitudes low frequency acoustic waves are coupled to ionospheric gravity waves and induce variations in the ionospheric electron density. Using Global Positioning System (GPS) data recorded by the permanent GPS network operating in southern California, we computed ionospheric electron content time series for several days preceding and following the January 17, 1994, Mw=6.7 Northridge earthquake. We observe an anomalous signal beginning several minutes after the earthquake with time delays that increase with distance from the epicenter. The signal frequency and phase velocity are consistent with results from numerical models of atmospheric‐ionospheric acoustic‐gravity waves excited by seismic sources as well as previous electromagnetic sounding results. We believe that these perturbations are caused by the ionospheric response to the strong ground displacement associated with the Northridge earthquake.
Monitoring a temporal change of seismic velocity in a volcano: Application to the 1992 eruption of Mt. Merapi (Indonesia)
Authors: Ratdomopurbo Antonius , Poupinet Georges
Published: February 2004

Multiplets, i.e. events with similar waveforms, are selected from shallow earthquakes recorded on Merapi volcano (Indonesia) before the eruption of February 2nd, 1992. Two multiplet families are found with their sources close to the summit. Their seismograms are analyzed using the Moving Window Cross Spectrum technique which measures the precise time delay between seismic phases in the entire seismogram. For both families of multiplets, a gradual decrease in the arrival times of coda waves is observed as a function of the date prior to the eruption: coda waves are becoming progressively faster (up to 1.2 per cent) as the time interval to the eruption shortens. This observation is interpreted as the consequence of an increase in the seismic velocity inside the volcano. The increase in velocity started in May 1991 and was observed until September 1991, 4 months before the eruption. This velocity increase may be related to an increase in pressure in the magma chamber or in the conduits and to the resulting closure of the surrounding cracks.
Thermal infrared lightcurves of the impact of comet Shoemaker‐Levy 9 fragment R
Authors: Friedson A. J. , Hoffmann W. F. , Goguen J. D. , Deutsch L. K. , Orton G. S. , Hora J. L. , Dayal A. , Spitale J. N. , Wells W. K. , Fazio G. G.
Published: February 2004

The impact of fragment R was observed at thermal infrared wavelengths of 7.85, 10.3 and 12.2 µm from the NASA/Infrared Telescope Facility on July 21 UT, using the MIRAC2 mid‐infrared array camera. Thermal emission at the three wavelengths was sampled sequentially using a 1.8% circular variable filter, with an average time interval of 17 seconds between observations at different wavelengths. Continuous imaging of Jupiter in this mode began at 5:08 UT and extended to 5:55 UT. We present calibrated lightcurves for the three wavelengths. Clear evidence for enhanced emission from the impact region first appears at 5:41 UT, with the peak in emission at all three wavelengths occurring ∼3.5 minutes later. The information content of the data is presented in terms of plots of the product of emissivity times angular size versus source temperature for each wavelength. Assuming that the peak in the lightcurves is due to rotation of the hot impact site into view from Earth, we estimate a diameter of ∼1900 km for the source emitting area at 5:44:30 UT and estimate a lower limit on the source temperature at this time of ∼1350 K. This lower limit drops to 800 K if the diameter of the emitting region was actually a factor of two larger.
Vortex streets driven by sheared flow and applications to black aurora
Authors: Shukla P. K. , Birk G. T. , Bingham R.
Published: February 2004

Black aurora often exhibits the signature of vortex structures. In this paper, it is proposed that pseudo‐three‐dimensional drift‐like electrostatic modes can play a very important role in those phenomena. Employing a two‐fluid model, we derive a set of nonlinear equations governing the dynamics of long wavelength inertial convective cells and coupled drift‐acoustic waves in the presence of geomagnetic field‐aligned plasma flows. It is shown that free energy stored in the latter can cause purely growing and oscillatory instabilities in the auroral ionosphere. The threshold conditions as well as expressions for the linear growth rates are presented. Furthermore, it is found that possible stationary solutions of the nonlinear equations can be represented in the form of vortex structures. It is, therefore, likely that the nonlinear low‐frequency electrostatic modes, as described here, may possibly account for the coherent vortex structures within black aurora, as they are often observed during the late recovery phase of a substorm.
Comparison of empirical magnetic field models and global MHD simulations: The near‐tail currents
Authors: Pulkkinen T. I. , Baker D. N. , Walker R. J. , Raeder J. , Ashour‐Abdalla M.
Published: February 2004

The tail currents predicted by empirical magnetic field models and global MHD simulations are compared. It is shown that the near‐Earth currents obtained from the MHD simulations are much weaker than the currents predicted by the Tsyganenko models, primarily because the ring current is not properly represented in the simulations. On the other hand, in the mid‐tail and distant tail the lobe field strength predicted by the simulations is comparable to what is observed at about 50RE distance, significantly larger than the very low lobe field values predicted by the Tsyganenko models at that distance. Ways to improve these complementary approaches to model the actual magnetospheric configuration are discussed.
Summary of field‐aligned Poynting flux observations from DE 2
Authors: Gary J. B. , Heelis R. A. , Thayer J. P.
Published: February 2004

Using DE 2 data of ion drift velocities and magnetic fields, we have calculated the field‐aligned Poynting flux (S∥) for 576 orbits over the satellite lifetime. This is the first application over an extended data set of Poynting flux observations from in situ measurements. The data has been sorted by interplanetary magnetic field conditions (northward or southward IMF) and geomagnetic activity (Kp≤3 and Kp>3) and binned by invariant latitude and magnetic local time. Our general results may be summarized as 1) the averaged S∥ is everywhere directed into the ionosphere, indicating that electric fields of magnetospheric origin generally dominate, and 2) the distribution of S∥ for southward IMF can be well explained in terms of an average two cell convection pattern, while for northward IMF a multiple cell convection pattern may be inferred. We have addressed the interesting question of the distribution of upward Poynting flux by binning only upward observations and found that average upward Poynting flux of less than 3 mW/m² may occur anywhere across the high latitude ionosphere. We have also observed a region at high latitudes in the predawn sector where the average upward Poynting flux is of significant size and occurrence frequency during southward IMF and high Kp conditions.
Thermal expansivity of MgSiO3 perovskite under high pressures up to 20 GPa
Authors: Utsumi Wataru , Funamori Nobumasa , Yagi Takehiko , Ito Eiji , Kikegawa Takumi , Shimomura Osamu
Published: February 2004

Volume measurement of MgSiO3 perovskite was made in the temperature range from 25 to 500°C as a function of pressure up to 20 GPa by in situ x‐ray diffraction, using a DIA apparatus combined with synchrotron radiation. The measured thermal expansivity ranges from 1.8 to 2.5*10−5 /K and decreases only slightly with pressure. Our present results are in good agreement with the previous lower pressure (up to 11 GPa) data of Wang et al. [1994] and the higher pressure (36 GPa) data of Funamori and Yagi [1993], but contrast with the results using diamond anvil cell by Mao et al. [1991], particularly in a low pressure range.
Deliquescence and freezing of stratospheric aerosol observed by balloonborne backscattersondes
Authors: Larsen Niels , Rosen James M. , Kjome Norman T. , Knudsen Bjørn
Published: February 2004

Stratospheric sulfate aerosols, originating from the Mt. Pinatubo volcanic eruption, have been observed during three winters in the Arctic by balloonborne backscattersondes. A measured color index, defined as the quotient between the aerosol backscatter ratios at wavelengths 940 and 480 nm, provides information of the size of the observed particles. The effects of liquid particle growth, by water vapor uptake, clearly show up as changes in the color index, whereas measurements on other days indicate the particles to be frozen. Air parcel trajectories have been calculated, providing the temperature history of the observed particles. Evidences appear of a temperature hysteresis in the freezing and melting cycle of the aerosol, indicating melting temperatures around 215–220 K in good agreement with laboratory measurements, and freezing of the particles within less than 5 K above the ice frost point. The changes in color index of the liquid particles are in good agreement with predictions from theoretical model calculations of growth by water vapor uptake.
Absence of magnetic field constraints on the source region of Jovian decametric radiation
Authors: Menietti J. D.
Published: February 2004

We have re‐examined the Voyager 1 and 2 planetary radio astronomy radio emission data in light of new magnetic field models. A previous study based on earlier models [Genova and Aubier, 1985] indicated that an apparent offset (∼70° measured at the orbit of Io) exists between the instantaneous Io flux tube and the source region of Jovian decametric radiation (DAM). Our new survey indicates that no such offset is necessary based on more recent magnetic field models. In addition, examples of DAM that are probably due to 2nd harmonic gyroemission were observed, as well as emission that may be due to mechanisms other than the cyclotron maser instability.
Comparing simultaneous stratospheric aerosol and ozone lidar measurements with SAGE II data after the Mount Pinatubo Eruption
Authors: Yue G. K. , Poole L. R. , McCormick M. P. , Veiga R. E. , Wang P.‐H. , Rizi V. , Masci F. , D'Altorio A. , Visconti G.
Published: February 2004

Stratospheric aerosol and ozone profiles obtained simultaneously from the lidar station at the University of L'Aquila (42.35°N, 13.33°E, 683m above sea level) during the first 6 months following the eruption of Mount Pinatubo are compared with corresponding nearby Stratospheric Aerosol and Gas Experiment (SAGE) II profiles. The agreement between the two data sets is found to be reasonably good. The temporal change of aerosol profiles obtained by both techniques showed the intrusion and growth of Pinatubo aerosols. In addition, ozone concentration profiles derived from an empirical time‐series model based on SAGE II ozone data obtained before the Pinatubo eruption are compared with measured profiles. Good agreement is shown in the 1991 profiles, but ozone concentrations measured in January 1992 were reduced relative to time‐series model estimates. Possible reasons for the differences between measured and model‐based ozone profiles are discussed.
Numerical modeling of tracer transport within and out of the lower tropospheric Arctic region
Authors: Kao C.‐Y. J. , Barr S. , Quintanar A. , Langley D. , Glatzmaier G. A. , Malone R. C.
Published: February 2004

The dispersion of passive tracer within and out of the Arctic region is studied using a three‐dimensional general circulation model with a Lagrangian particle module. Emphasis is placed on the lower troposphere by releasing numerical tracer particles in a layer near the surface within the Arctic circle to mimic the potential release of aerosol particles from the Arctic Ocean and the observed variability of Arctic haze. This study is also intended to investigate the characteristics of the quasi‐horizontal mixing in transporting the air from high latitudes to mid‐ and low latitudes; a process which has extensively been studied mostly for the stratospheric regime. The model calculation shows that the Lagrangian passive tracer contours can correlate well with the isentropic potential vorticity field. This indicates that the identified large‐scale mixing processes due to Rossby wave breaking and chaotic advection in the stratosphere may also occur in the lower troposphere.
Nonsphericity of dust‐like tropospheric aerosols: Implications for aerosol remote sensing and climate modeling
Authors: Mishchenko M. I. , Lacis A. A. , Carlson B. E. , Travis L. D.
Published: February 2004

The nonsphericity of dust‐like tropospheric aerosols causes us to question the applicability of using conventional Mie theory to compute their radiative properties. In this paper we compare T‐matrix computations of light scattering by polydispersions of randomly oriented nonspherical aerosols and Mie computations for equivalent spheres. We demonstrate that even moderate nonsphericity results in substantial errors in the retrieved aerosol optical thickness if satellite reflectance measurements are analyzed using Mie theory. On the other hand, the use of Mie theory for nonspherical aerosols produces negligible errors in the computation of albedo and flux related quantities, provided that the aerosol size distribution and optical thickness are known beforehand. The first result can be explained by large nonspherical‐spherical differences in scattering phase function, while the second result follows from small nonspherical‐spherical differences in single‐scattering albedo and asymmetry parameter. Our results demonstrate that no cancellation of errors occurs if one consistently uses Mie theory in the retrieval algorithm and then in computing the albedo for the retrieved aerosol optical thickness.
Evidence for warming of Atlantic water in the Southern Canadian Basin of the Arctic Ocean: Results from the Larsen‐93 Expedition
Authors: Carmack Eddy C. , Macdonald Robie W. , Perkin Ronald G. , McLaughlin Fiona A. , Pearson Richard J.
Published: February 2004

Potential temperature (θ) and salinity (S) data obtained along the perimeter of the southern Canadian Basin north of the East Siberian Sea in 1993 aboard the CCGS Henry Larsen show higher temperatures in waters of Atlantic origin than in available climatological data for the Canadian Basin. In particular, a front is observed near the Mendeleyev Ridge which separates the cooler Atlantic waters of the Canada Basin from the warmer Atlantic waters observed in the Makarov Basin. The front is further characterized by a change in the θ/S slope of Arctic thermocline water, and by thermohaline intrusions (θ and S reversals) within the Atlantic layer. The idea that this warm variety of Atlantic water has come recently from the Eurasian Basin is supported by its higher level of the tracer CFC‐11.
Atmospheric gradients from very long baseline interferometry observations
Authors: MacMillan D. S.
Published: February 2004

Azimuthal asymmetries in the atmospheric refractive index can lead to errors in estimated vertical and horizontal station coordinates. Generally these gradients are not modeled in standard very long baseline interferometry (VLBI) geodetic analysis. Daily average gradient effects can be as large as 50 mm of delay at a 7° elevation. To model gradients, the constrained estimation of gradient parameters was added to the standard VLBI solution procedure. Here the analysis of two sets of data is summarized: the set of all geodetic VLBI experiments from 1990–1993 and a series of 12 state‐of‐the‐art R&D experiments run on consecutive days in January 1994. In both cases, when the gradient parameters are estimated, the overall fit of the geodetic solution is improved at greater than the 99% confidence level. Repeatabilities of baseline lengths ranging up to 11,000 km are improved by 1 to 8 mm in a root‐sum‐square sense. This varies from about 20% to 40% of the total baseline length scatter without gradient modeling for the 1990–1993 series and 40% to 50% for the January series. Gradients estimated independently for each day as a piecewise linear function are mostly continuous from day to day within their formal uncertainties.
Comet Shoemaker‐Levy 9: No effect on the Io plasma torus
Authors: Brown Michael E. , Moyer Elisabeth J. , Bouchez Antonin H. , Spinrad Hyron
Published: February 2004

Observations of the Io plasma torus made before, during, and after the impact of Comet Shoemaker‐Levy 9 with Jupiter reveal no comet‐induced changes. Three weeks of high spectral‐resolution ground‐based visible spectroscopy show no changes larger than typical day‐to‐day variations in the torus densities, ion temperatures, or rotation velocities. Comparison with six months of identically obtained data from 1991 and 1992 also shows no differences.
Near‐IR imaging observations of the cometary impact into Jupiter
Authors: Takeuchi Satoru , Hasegawa Hitoshi , Watanabe Jun‐ichi , Yamashita Takuya , Abe Masanao , Hirota Yuka , Nishihara Eiji , Okumura Shi‐ichiro , Mori Atsushi
Published: February 2004

We carried out observations of Jupiter during the impact of comet Shoemaker‐Levy 9 with Jupiter at the Okayama Astrophysical Observatory with a near‐infrared camera, OASIS. In this paper we report on result of absolute photometry of radiation from the impacts of fragment C,D and K at 2.35µm and discues on the impact phenomena. Lightcurve of the K impact event had four stages. We interpret the first as flash from the comet entry and the second as radiation from shock wave and/or high temperature plume itself, seen directly by Earth. The third, in which rapid increment of the infrared radiation was observed, may be caused by the increase of optical depth due to dust grain formation in the expanding and cooling plume. The fourth shows slowly decrease of the flux, which may be re‐entry phenomena of the plume.
Long term periodicity in solar wind velocity during the last three solar cycles
Authors: Gazis P. R. , Richardson J. D. , Paularena K. I.
Published: February 2004

Solar wind measurements from the Pioneer 10, Pioneer 11, Voyager 2, IMP 8, and Pioneer Venus Orbiter (PVO) spacecraft were examined to search for long‐term periodicities during the last three solar cycles. For the time of the last solar maximum, these measurements confirm the existence of the periodic 1.3‐year enhancements in solar wind velocity reported by Richardson et al. [1994]. For most of the preceding two solar cycles, long‐term velocity enhancements occurred that were similar in structure but lacked the 1.3‐year periodicity. It appears that long‐term enhancements in solar wind velocity, with durations on the order of a few months to a year, are a common feature throughout the heliosphere.
Methane and carbon monoxide infrared emissions observed at the Canada‐France‐Hawaii Telescope during the collision of comet SL‐9 with Jupiter
Authors: Maillard J.‐P. , Drossart P. , Bézard B. , de Bergh C. , Lellouch E. , Marten A. , Caldwell J. , Hilico J.‐C. , Atreya S. K.
Published: February 2004

Observations with the Fourier Transform Spectrometer were conducted in spectral ranges from 1.6 to 4.7 µm from July 17 to 21 (UT) on the hot plumes appearing on the limb as well as hours or days after the impacts. We present here an analysis of the methane emission observed at 3.3 µm some 10 min after the C impact, indicating the presence of a very small (less than 100 km wide) hot region with temperatures in the 750–1500 K range within the 0.1‐ to 0.01‐mbar region. We also report the detection of CO emission at 4.7 µm 4.5 hrs after the L impact, indicative of a temperature of 274±10 K at the ∼1016 CO molec cm−2 level. The observations suggest that the stratospheric temperature decreases with depth by at least 30 K over two CO pressure decades.
Preliminary results of Galileo direct imaging of S‐L 9 impacts
Authors: Chapman Clark R. , Merline William J. , Klaasen Kenneth , Johnson Torrence V. , Heffernan Catherine , Belton Michael J. S. , Ingersoll Andrew P.
Published: February 2004

Direct Galileo imaging data were obtained of the Jupiter impact sites for Comet Shoemaker‐Levy 9 fragments K, N, and W during their early, high‐energy phases. Initial ∼5s‐long flashes for all 3 impacts result from radiant bolides; analogous, abrupt onsets of luminosity observed by the Galileo photopolarimeter for other impacts must also be the bolide phase. The 3 bolides were dim at 0.56 or 0.89µm (few percent of total Jupiter) and had similar amplitudes, despite huge late‐stage differences observed from Earth. Subsequent, continuous luminosity lasting ∼40s for K and ∼10s for N is optical radiation as the initial bolide train erupts into a “fireball”. The K light curve may show (a) two impacts 10s apart or (b) delayed evolution of the fireball.
The limitation and applicability of Musher‐Sturman Equation to two‐dimensional lower hybrid wave collapse
Authors: Tam Sunny W. Y. , Chang Tom
Published: February 2004

The existence of localized regions of intense lower hybrid waves in the auroral ionosphere recently observed by rocket and satellite experiments can be understood by the study of a non‐linear two‐timescale coupling process. Such type of wave‐wave interactions was first described by Musher and Sturman [1975]. In this Letter, we demonstrate that the leading non‐linear term in the standard Musher‐Sturman equation vanishes identically in strict two‐dimensions (normal to the magnetic field). Instead, the new two‐dimensional equation is characterized by a much weaker non‐linear term which arises from the ponderomotive force perpendicular to the magnetic field, particularly that due to the ions. The old and new equations are compared by means of time‐evolution calculations of wave fields. The results exhibit a remarkable difference in the evolution of the waves as governed by the two equations. Such dissimilar outcomes motivate our investigation of the limitation of Musher‐Sturman equation in quasi‐two‐dimensions. We show that the equation may be applicable only if κ∥²/κ⟂²≫ω²/Ωe², where ω² ≲ ωA² or Ωi² In addition, assumptions of quasi‐steady slow‐mode responses by ions and electrons require ω²/κ²νi² and ω²/κ∥²νe² ≪ 1 respectively. Only within all these limits can Musher‐Sturman equation adequately describe the collapse of lower hybrid waves.
Observations of IMF and seasonal effects in high‐latitude convection
Authors: Ruohoniemi J. M. , Greenwald R. A.
Published: February 2004

We describe strong interplanetary magnetic field (IMF) and seasonal effects in the convection of nightside ionospheric plasma. The findings are based on a statistical analysis of observations made with the JHU/APL HF radar located at Goose Bay, Labrador. For positive sign of the IMF dawn‐dusk component, i.e., By>0, the dawn cell is more crescent‐shaped and the dusk cell more round while for By<0 these pairings of size and shape are reversed. The more extreme crescent/round cell dichotomy is obtained for By>0. The return flows associated with the crescent‐shaped cell dominate at midnight MLT (Magnetic Local Time); the reversal in the zonal velocity in the 67°–69° Λ (magnetic latitude) interval occurs 2½ hr earlier for By>0. The seasonal dependence of nightside convection resembles in important respects the By dependence. Greater latitudinal velocity shears occur in the morning/afternoon sector for summer/winter and the return flow of this sector dominates at midnight. The zonal flow reversal occurs 2½ hr earlier in summer than in winter. The maximum effects are obtained on the nightside for the pairings [By>0,summer] and [By<0,winter]; the first produces the more structured cell in the morning, the second in the evening, and this cell dominates the return flow at midnight. The difference in the zonal flow reversals for these pairings exceeds 4 hr in MLT.
Palomar observations of the R impact of comet Shoemaker‐Levy 9: II. Spectra
Authors: Nicholson P. D. , Gierasch P. J. , Hayward T. L. , McGhee C. A. , Moersch J. E. , Squyres S. W. , Van Cleve J. , Matthews K. , Neugebauer G. , Shupe D. , Weinberger A. , Miles J. W. , Conrath B. J.
Published: February 2004

We present mid‐infrared spectroscopic observations from Palomar observatory of the impact of fragment R of comet P/Shoemaker‐Levy 9 with Jupiter on 21 July 1994. Low‐resolution 8–13 µm spectra taken near the peak of the lightcurve show a broad emission feature that resembles the silicate feature commonly seen in comets and the interstellar medium. We use this feature to estimate the dust content of the impact plume. The overall infrared spectral energy distribution at the time of peak brightness is consistent with emission from an optically‐thin layer of small particles at ∼600 K. Integrating over the spectrum and the lightcurve, we obtain a total radiated energy from the R impact of ≥ 2 × 1025 ergs and a plume mass of ≥ 3 × 1013 g.
Multiple‐Branch Model of the open magnetopause
Authors: Vasyliūnas Vytenis M.
Published: February 2004

When the magnetosphere is magnetically open, the magnetopause must cut across open field lines and is usually identified with a rotational discontinuity or finite‐amplitude Alfvén wave. The merging of magnetospheric and interplanetary magnetic fields in the nose region of the magnetosphere generates two pairs of such Alfvén waves; the two interior ones propagate down to the ionosphere, and the two exterior ones propagate into the incoming magnetosheath plasma and thus can be identified with the magnetopause in the dayside. However, continuing this magnetopause into the nightside magnetotail region would require reversing the direction of magnetic‐field rotation at some point on the propagating wave, which is not possible. To resolve this problem, I propose that the magnetopause on the nightside is not a continuation of that on the dayside but is a distinct surface formed by a separate Alfvén wave generated near the downstream edge of the dayside polar cusp. This multiple‐branch model also explains the boundary of the exterior cusp (identified with the continuation of the dayside magnetopause) and accounts for the non‐stagnant flow within the exterior cusp (the flow simply proceeds across the nightside magnetopause into the plasma mantle region). Where the two branches are adjacent to the magnetotail, their spatial separation can be estimated as typically no more than about 2 RE, not likely to have been noticed unless specifically looked for. A possible mechanism for generating the nightside Alfvén wave is the expansion of plasma within the polar cusp as the flux tubes are advected into the rapidly increasing volume of the magnetotail.
Study of alternative tracer tests in characterizing transport in fractured rocks
Authors: Tsang Y. W.
Published: February 2004

Flow and transport calculations are carried out by numerical simulation for different tracer designs: single‐well radially diverging /converging (huff‐puff), single well radially converging, and two‐well injection‐withdrawal (doublet) in a 2D fracture zone. The fractured rocks are conceptualized as a dual‐continuum: the well‐connected fractures forming a heterogeneous continuum for advective transport, and the less permeable matrix forming a second continuum for tracer diffusion. Results show that the huff‐puff design is a good diagnostic test for matrix diffusion. The two‐well doublet design averages over a large volume and corrects for the extreme sensitivity to spatial heterogeneities of the single well converging test, but requires prior knowledge of presence or absence of matrix diffusion to give reliable estimate of transport parameters. Results of this study demonstrate that using a suite of different tracer designs is important to reduce the uncertainty in association with solving the inverse problem of tracer test interpretation to characterize transport in fracture rocks.
Sensitivity analysis of solute transport in fractured porous media
Authors: Toran Laura , Sjoreen Andrea , Morris Max
Published: February 2004

We conducted a sensitivity analysis using a numerical model of fractured porous media to find out how porous media and fracture parameters affect solute transport. The 2‐dimensional, saturated fracture flow and transport code FRACTRAN was used to conduct the simulations. Seven parameters were considered: matrix hydraulic conductivity, matrix porosity, retardation of the matrix, gradient of the flow field, fracture retardation, fracture aperture, and fracture probability (which incorporates fracture spacing and fracture length). A Latin‐hypercube design was used to select a matrix of parameter values that minimized correlations among the design parameters. The results were summarized by examining least square fit of the relationship between the seven parameters and 50% breakthrough time and spread of the breakthrough curves, with nearly identical results. The matrix parameters, in particular hydraulic conductivity and porosity, had the greatest effect. As expected, fracture probability was nearly equivalent in importance. These results indicate that field characterization in fractured porous media should not only emphasize fracture location, which strongly influences directions of contaminant transport, but also matrix properties, which have a major influence on contaminant residence times and breakthrough concentrations.
Dual porosity fracture flow and transport
Authors: Dershowitz W. , Miller Ian
Published: February 2004

In rock masses where fracture flow greatly exceeds flow through the rock matrix, the matrix provides a storage porosity which needs to be considered for transient flow, and for solute transport. This paper presents a dual‐porosity, single permeability approach for evaluation of flow and transport in fractured rock. The fracture network flow approach is based on a generic matrix block similar to the method of Warren and Root (1963), applied to realistic fracture geometries. The solute transport approach is based on a probabilistic concept of matrix diffusion as a process of Brownian motion, implemented using a probabilistic particle tracking technique.
Estimating the abundance of ClO from simultaneous remote sensing measurements of HO2, OH, and HOCl
Authors: Johnson D. G. , Traub W. A. , Chance K. V. , Jucks K. W. , Stachnik R. A.
Published: February 2004

Using a simple photochemical model we derive the mixing ratio profile of ClO in the altitude range 20–38 km from simultaneous measurements of HO2, OH, HOCl, temperature, pressure, and ozone. The measurements were made with the FIRS‐2 far‐infrared spectrometer during a balloon flight on September 29, 1992. We compare the derived ClO with the ClO profile obtained by the SLS instrument while flying on the same gondola. The good agreement between the two profiles validates our simple model and confirms the relevant rate constants and photolysis cross sections.
Direct determination of large‐scale magnetospheric field‐aligned currents with SuperDARN
Authors: Sofko G. J. , Greenwald R. , Bristow W.
Published: February 2004

The SuperDARN (SuperDual Auroral Radar Network) radars measure large‐scale 2D images of the plasma convection velocity v in the high‐latitude F‐region. From the 2D convection maps, plasma flow vorticity can be directly calculated. We show that the vorticity is a direct measure of Jm/ΣP, the “magnetospheric” component Jm of the field‐aligned current (FAC) per unit height‐integrated Pedersen conductivity ΣP. The maps of Jm/ΣP show clearly the Region 0, 1, and 2 currents, uncontaminated by the “ionospheric” FAC component Ji caused by ionospheric Pedersen and Hall conductivity gradients (which are important in the nightside substorm sector). The generation of the large‐scale magnetospheric FAC images of Jm/ ΣP from the SuperDARN convection vorticity is explained and examples of dayside FAC patterns for northward and southward IMF conditions are presented for the SuperDARN radar pair in Canada at Saskatoon and Kapuskasing. By summer, 1995, FAC images will be available from three northern hemisphere SuperDARN radar pairs, with a field‐of‐view extending over some 12 hours MLT (from western Canada to Scandinavia), and from one Antarctic radar pair.
The importance of dynamical feedbacks on doubled CO2‐induced changes in the thermal structure of the mesosphere
Authors: Portmann R. W. , Thomas G. E. , Solomon S. , Garcia R. R.
Published: February 2004

The Garcia‐Solomon two‐dimensional model was used to study the effect of doubled carbon‐dioxide on the middle atmosphere. The model has been improved to include non‐LTE CO2 cooling in the 15 micron band above 70 km and new chemical heating and heating efficiencies. The effect of doubling CO2 on the temperature is found to be large at the stratopause (about 10–12K cooling) and at the mesopause (about 6–12K cooling). In the stratosphere, dynamical feedbacks on the heating rate caused by the temperature changes are small compared to the radiative changes while in the mesosphere they can be large. In fact, calculations with the present dynamical heating rates used in the doubled CO2 energy equation indicate that the radiative forcing alone could cause a temperature increase of about 10K in the polar summer mesopause region. The dynamical feedbacks which oppose this positive radiative forcing are discussed.
An evaluation of the spatial and interannual variability of tropical precipitation as simulated by GCMs
Authors: Srinivasan G. , Hulme M. , Jones C. G.
Published: February 2004

Precipitation is one of the most difficult variables to simulate in a General Circulation Model and arguably one of the most important. The Atmospheric Model Intelcomparison Project (AMIP) provides an opportunity to examine the simulation of precipitation in a wide array of models. Monthly precipitation fields produced by a subset of 19 currently available AMIP model experiments are evaluated for the tropical region using a land‐only observed dataset for the period 1980–1988. The models show large variations in their ability to reproduce observed tropical precipitation, although spatial correlations indicate that some of the models simulate the pattern of observed precipitation fields fairly well. The correlations are strongest during boreal winter (DJF) and weakest during the boreal summer (JJA). Comparison between model and observed precipitation time series for two Central Pacific locations show that most models are unable to reliably reproduce interannual precipitation variability in this region.
A discussion of scale effects on hydraulic conductivity at a granitic site (El Berrocal, Spain)
Authors: Guimerà J. , Vives L. , Carrera J.
Published: February 2004

Scale effects on hydraulic conductivity (i.e. increase in the value of K with increasing scale of measurement) have been reported in the literature. Here, we summarize extensive tests performed at El Berrocal, a granitic site in Central Spain, to study whether such effects indeed appear. In fact, median hydraulic conductivities clearly increase with measurement scale. We argue, however, that large scale measurements may be biased towards high K values because they are performed in the most permeable zones, where pumping rates can be high. This is supported by the fact that cross‐hole transmissivities and hydraulic conductivities consistently fall below the corresponding values at the pumping interval.
Tensile stress concentration and compressive failure in cemented granular material
Authors: Wong Teng‐Fong , Wu Li‐Chun
Published: February 2004

Grain crushing and pore collapse are important micromechanical processes responsible for hydrostatic and shear‐enhanced compactions in porous rocks. These processes initiate from extensile microcracks which emanate from grain contacts. Microstructural observations indicate that such extensile cracking is inhibited in the vicinity of cemented grain contacts. The finite element technique was used to simulate the tensile stress concentration and normal stiffness in a cemented aggregate. The detrital grains were assumed to be elastically identical spheres bonded by cement layers of finite thickness. The numerical simulations show that the maximum tensile stress concentration is located near the triple junction (among grain, cement and pore space), and its magnitude is significantly less than that for an uncemented system. The development of microcracking near a cemented contact is readily inhibited unless the applied stress exceeds a critical value which is at least an order of magnitude greater than that for the onset of Hertzian fracture.
Direct measurements of soil‐gas entry into an experimental basement driven by atmospheric pressure fluctuations
Authors: Robinson Allen L. , Sextro Richard G.
Published: February 2004

To study the importance of changes in atmospheric pressure on radon entry into houses, we have simultaneously measured the soil‐gas entry into an experimental basement structure and the fluctuations in atmospheric pressure. Small amplitude (∼10 Pa), rapid (∼20 min) fluctuations in atmospheric pressure were an important driving force for soil‐gas entry because 1) the characteristic time for the propagation of a pressure disturbance in the soil gas was ∼2 min, and 2) the time‐rate‐of‐change of these small fluctuations is often larger than that of the semi‐diurnal oscillations. An analytical model has been derived for a structure with a subslab gravel layer based on a one‐dimensional solution to the transient pressure diffusion equation. This model correctly predicts the temporal response of the measured soil‐gas entry into the experimental structure, but underpredicts the amplitude.
Teleseismic tomography of the Loma Prieta Earthquake Region, California: Implications for strain partitioning
Authors: Takauchi Y. , Evans John R.
Published: February 2004

From teleseismic travel times we derive three‐dimensional velocity models of the upper 71 km in the 1989 Loma Prieta earthquake region, central California. Shallow crustal structure is consistent with local‐earthquake tomography. Horizontal velocity gradients at all depths suggest that the San Andreas fault was a deep shear locus, at least at one time. A large low‐velocity feature near the Moho beneath Loma Prieta probably is caused by a crustal root. Two low‐velocity features at about 45–70 km depth are offset right‐laterally along the San Andreas by about 45 km. Cooling of this portion of the upper mantle [Furlong et al., 1989] could have frozen in displacements in this region within a few million years after passage of the Mendocino Triple Junction. These results are consistent with Furlong et al.'s model.
The great Kurile Earthquake of October 4, 1994 tore the slab
Authors: Tanioka Yuichiro , Ruff Larry , Satake Kenji
Published: February 2004

The fault geometry, depth, and slip distribution of the Kurile earthquake of Oct. 4, 1994 are estimated using seismic waveforms, aftershock distribution, geodetic measurements, and tsunami waveforms. Previous large earthquakes in the Kurile arc had typical underthrusting focal mechanisms due to the local subduction. Seismic wave inversions of the 1994 event indicate a thrust type mechanism with a large strike‐slip component. This does not represent an underthrust event at the subduction interface. The focal depth is estimated at 50 km, just beneath the previously estimated subduction interface. We calculate crustal deformation and tsunami waveforms from the two possible fault planes of the focal mechanism solution and find that both fault planes explain the observations. We choose the north‐south striking plane, because of the occurrence of a large (Mw 7.8) intermediate‐depth earthquake on Dec. 6, 1978, northwest of the 1994 rupture area. The lineation of aftershock distributions of these two events suggests that both earthquakes were due to a tearing of the slab perpendicular to the trench. The joint inversion of geodetic and tsunami data shows that the largest slip, about 17 m, occurred at the upper end of the slab.
Climate response to indirect anthropogenic sulfate forcing
Authors: Erickson David J. , Oglesby Robert J. , Marshall Susan
Published: February 2004

A general circulation model (GCM) has been used to conduct sensitivity tests of the climatic influence imparted by a cloud albedo change hypothesized to result from anthropogenic increases in atmospheric sulfur. The global distribution of anthropogenic sulfate aerosols is computed with a simplified 3‐D transport model. The NCAR CCM1 has been run with a cloud albedo perturbation that is a function of the distribution of anthropogenic sulfur particles. We report climate statistics from the last 20 years of 30 year GCM control and experiment runs. The climate response is strongest in the northern hemisphere winter, with cooling over the North Atlantic and North Pacific oceans on the order of 2–6°C. The 500 mb geopotential height field shows a significant deepening over the Canadian provinces, enhancing the northerly flow over the North American and North Atlantic regions during boreal winter. The equilibrium climate does not, however, cool over central Europe in northern hemisphere winter, despite this region being one of the most heavily impacted areas in the world by sulfate aerosol. The anthropogenic sulfate ‘indirect’ forcing elicits a highly non‐linear climate response that can be explained through changes in the hemispheric wave train. These results may assist in explaining the long‐standing climate change issue of what causes the cooling over the North Atlantic and North Pacific over the last decades, a feature that is not explained by increases in greenhouse gases alone.
The low radio frequency limit of solar type III bursts: Ulysses observations in and out of the ecliptic
Authors: Leblanc Y. , Dulk G. A. , Hoang S.
Published: February 2004

We measure the low frequency limit flo of 1028 type III bursts and compare it with the observed plasma frequency at the spacecraft fp. We consider three periods: when Ulysses was in the ecliptic plane near the Sun, far from the Sun, and out of the ecliptic; the average plasma frequency was 20, 10, and 5 kHz respectively. The major results are: 1) During the entire period of Ulysses flight November 1990 to March 1995, 9 kHz is the lowest frequency at which type III electromagnetic radiation was ever seen, even though fp was often 3–4 kHz. 2) The distribution of cutoffs is independent of ecliptic latitude, and of fp at the spacecraft (excepting of course that flo ≥ fp) and is broadly spread between 17 and 300 kHz. 3) When fp is about 20, 10 and 5 kHz, only 15, 5 and 0% respectively of type III burst radiation approaches within a factor of two of fp.

We develop three hypotheses to explain these results: 1) that the low‐frequency cutoff of type III bursts is intrinsic to the radiation mechanism, 2) that it is due to directivity of the radiation, and 3) that it is due to propagation conditions between the burst source and the spacecraft. The evidence for intrinsic cutoffs is strong, but propagation effects play a role in some cases.
Re, Os and Ir in Antarctic unequilibrated ordinary chondrites and implications for the solar system abundance of Re
Authors: Ebihara Mitsuru , Ozaki Hiromasa
Published: February 2004

The concentrations of Re, Os and Ir in Antarctic unequilibrated ordinary chondrites (UOC) were determined by radiochemical neutron activation analysis. Large fractionations among these elements were found in Antarctic UOC. This is quite different from non‐Antarctic UOC and equilibrated ordinary chondrites (EOC). Average Re/Os and Re/Ir ratios in ordinary chondrites are 25–30% higher than those in C1 chondrites and 10–15% higher than those in C2 and C3 chondrites. In contrast, the Os/Ir ratio is not variable among these chondrite groups. It is therefore suggested that Re is fractionated among ordinary and carbonaceous chondrites. Atom abundances of Os and Ir relative to 106 Si atoms of H chondrites are nearly equal to those of C1 chondrites, but their Re value is 30% higher than the C1 value. Considering that the currently accepted value for solar system abundance of Re (0.517 atom relative to 106 Si atoms) [Anders and Grevesse, 1989] is the lowest value among (Sinormalized) Re abundances in C1, C2, C3 and H chondrites, its value should be changed to a larger value. If C2 chondrites are adopted as the solar system standard, the solar system abundance of Re is changed to 0.0589, with those of Os and Ir being 0.740 and 0.685, respectively, at the same time.
The isentropic density profile of silicate perovskite computed by the thermal pressure
Authors: Masuda Koji , Anderson O. L.
Published: February 2004

A thermodynamic method in which all physical properties (derivatives of the Helmholtz energy) are determined from seven thermoelastic parameters is used to calculate density isentropes of (Mg, Fe)SiO3perovskite for lower mantle conditions. We find that, provided the EoS constants,ρ0,KTo, andK′0, are known, the density distribution can be found from the thermal pressure and often from the thermal pressure evaluated at zero pressure. Three isentropic density distributions of perovskite are calculated and are shown to be non‐parallel with PREM's density distribution. This leads to the speculation that a lower mantle entirely composed of perovskite may not be phase homogeneous throughout its whole depth.