bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact of an improved longwave radiation model, RRTM, on the energy budget and thermodynamic properties of the NCAR community climate model, CCM3
Authors: Iacono Michael J. , Mlawer Eli J. , Clough Shepard A. , Morcrette Jean‐Jacques
Published: September 2002

The effect of introducing a new longwave radiation parameterization, RRTM, on the energy budget and thermodynamic properties of the National Center for Atmospheric Research (NCAR) community climate model (CCM3) is described. RRTM is a rapid and accurate, correlated k, radiative transfer model that has been developed for the Atmospheric Radiation Measurement (ARM) program to address the ARM objective of improving radiation models in GCMs. Among the important features of RRTM are its connection to radiation measurements through comparison to the extensively validated line‐by‐line radiative transfer model (LBLRTM) and its use of an improved and validated water vapor continuum model. Comparisons between RRTM and the CCM3 longwave (LW) parameterization have been performed for single atmospheric profiles using the CCM3 column radiation model and for two 5‐year simulations using the full CCM3 climate model. RRTM produces a significant enhancement of LW absorption largely due to its more physical and spectrally extensive water vapor continuum model relative to the current CCM3 water continuum treatment. This reduces the clear sky, outgoing longwave radiation over the tropics by 6–9 W m−2. Downward LW surface fluxes are increased by 8–15 W m−2 at high latitudes and other dry regions. These changes considerably improve known flux biases in CCM3 and other GCMs. At low and midlatitudes, RRTM enhances LW radiative cooling in the upper troposphere by 0.2–0.4 K d−1 and reduces cooling in the lower troposphere by 0.2–0.5 K d−1. The enhancement of downward surface flux contributes to increasing lower tropospheric and surface temperatures by 1–4 K, especially at high latitudes, which partly compensates documented, CCM3 cold temperature biases in these regions. Experiments were performed with the weather prediction model of the European Center for Medium Range Weather Forecasts (ECMWF), which show that RRTM also impacts temperature on timescales relevant to forecasting applications. RRTM is competitive with the CCM3 LW model in computational expense at 30 layers and with the ECMWF LW model at 60 layers, and it would be relatively faster at higher vertical resolution.
A nonlinear regional prediction experiment on a short‐range climatic process of the atmospheric ozone
Authors: Peicai Yang , Xiuji Zhou , Jianchun Bian
Published: September 2002

A regional prediction experiment on the distribution of the atmospheric ozone column over China is performed by means of a spatiotemporal artificial neural network system (STANNS) built by using the monthly mean values of Total Ozone Mapping Spectrometer (TOMS) data from the Nimbus 7 satellite. The experiment region is situated from 72.5°E to 128.75°E and from 22°N to 49°N and divided into a rectangular grid net of 16×10. The data used cover 174 months from November 1978 to April 1993; those from the first 150 months were used to construct STANNS, and those from the last 24 months were used to test the prediction ability. The experiment results are encouraging. All the values of the correlation coefficient between the prediction and the actual fields for 24 or more samples exceed 90% at the lead time of not more than 6 months. This shows that there exists a good agreement between the prediction and the actual fields. In addition, some important features in the ozone distribution, such as the unusual depletion area over the Tibetan Plateau in summer, are also predicted very well. The results also show that, in a rough sense, the prediction accuracy decreases with the increasing of lead time.
Spatial distributions of upper tropospheric water vapor measurements from the UARS Microwave Limb Sounder
Authors: Stone Elizabeth M. , Pan Liwen , Sandor Brad J. , Read William G. , Waters Joe W.
Published: September 2002

We characterize measurements of upper tropospheric (∼300–150 hPa) water vapor obtained from the Microwave Limb Sounder (MLS) instrument on board the Upper Atmosphere Research Satellite (UARS) in terms of their spatial and temporal variability. We present the climatology of water vapor mixing ratio for October 1991‐June 1997, which includes the seasonal means, the root‐mean‐square deviations, and the seasonal differences. The climatology of the divergent wind field is compiled to examine the relationship of upper tropospheric moisture fields and the tropical circulations. The tropical wet and dry patterns are latitudinally distributed in accordance with the Hadley circulation, and longitudinal distributions correspond well to the Walker circulation. We quantify the frequency of dry tropical observations in seasonal maps. The most prominent dry regions are located in the subtropics. On the 316 hPa surface there is some occurrence of low values of humidity throughout the majority of the tropics, while at 215 hPa it is primarily the subsidence regions that show dry values. Subtropical dry observations are more frequent in the Southern Hemisphere winter than in the Northern Hemisphere winter. The seasonal cycle of MLS water vapor is compared with Stratospheric Aerosol and Gas Experiment II (SAGE II) measurements. Good agreement is found between these two data sets for measurements near the extratropical tropopause. Potential vorticity analysis is used to indicate whether extratropical measurements were influenced primarily by stratospheric or tropospheric air. Frequency distributions display distinct characteristics and seasonal dependence for these two regions.
On the nature of large‐scale mixing in the stratosphere and mesosphere
Authors: Shepherd Theodore G. , Koshyk John N. , Ngan Keith
Published: September 2002

Studies of tracer transport in the stratosphere have shown that adiabatic quasi‐horizontal tracer evolution is controlled primarily by the large‐scale low‐frequency component of the flow. This behavior is consistent with the concept of chaotic advection, wherein the Eulerian velocity field is spatially coherent and temporally quasi‐regular on timescales over which the Lagrangian evolution is chaotic. In this study, winds from a middle atmosphere general circulation model (the Canadian Middle Atmosphere Model) are used to compare and contrast the nature of tracer evolution in the stratosphere and mesosphere. It is found that the concept of chaotic advection is relevant in the stratosphere but not in the mesosphere. The explanation for this behavior is the increased strength of gravity wave activity in the mesosphere as compared with the stratosphere, which leads to shallower kinetic energy spectra on synoptic scales and a much shorter Eulerian correlation time. The shallower kinetic energy spectra imply that tracer evolution in the mesosphere is spectrally local, in contrast with the spectrally nonlocal regime that prevails in the stratosphere. This means that tracer advection calculations in the mesosphere are controlled primarily by the gravity wave spectrum and are intrinsically resolution dependent.
Retrieving cloud top structure from infrared satellite data
Authors: van Hees Richard M. , Lelieveld Jos
Published: September 2002

A new retrieval method to detect steep temperature gradients between the convective overshoots of cumulonimbus clouds and the surrounding cirrus has been applied to determine gradients and their orientation in the image plane of infrared data of the advanced very high resolution radiometer (AVHRR). These orientations are used to derive cloud elevations which are brightened by the Sun or are in shadow, which strongly affects the visible signal. The impact of temperature gradients on the visible radiances is illustrated by two examples that indicate deep convective overshoots. Both examples show that the illuminated side of the overshoots can exceed the cloud top reflectivity by 50%, while the shadows account for <50% of the cloud top reflectance. The shadows usually extend several pixels beyond the base of the overshoots. Here we show that statistical analyses of cloud optical depth are affected by the cloud top structure, based on 1 month of high‐resolution AVHRR satellite data. The contribution of shadow side pixels with steep temperature gradients (≥16 K) can exceed 30% for small optical depths (<3). The contribution of illuminated sides of cloud top structures and cloud sides with steep temperature gradients can exceed 70% for large optical depths (>32).
Impact of the convection triggering function on single‐column model simulations
Authors: Xie Shaocheng , Zhang Minghua
Published: September 2002

We analyze the causes of the large thermal biases in the simulation of the National Center for Atmospheric Research's Community Climate Model Version 3 (CCM3) single‐column model (SCM) when it is forced with the Atmospheric Radiation Measurement Program's (ARM) summer 1995 Intensive Observing Period (IOP) data. We have found that deficiencies of the convection triggering function used in the model can explain a significant part of the biases. In the model, convection is triggered whenever there is positive convective available potential energy (CAPE), which happens to occur during daytime due to solar heating of the land surface. In observations, however, convection takes place only when the large‐scale dynamic condition is favorable to the release of CAPE. On the basis of observations we propose a new triggering function for the CCM3 deep convection scheme. We assume that model convection occurs only when the large‐scale dynamic forcing makes positive contributions to the existing positive CAPE. Improved simulation results are obtained when the new triggering function is implemented in the model. We further test the new triggering function using the ARM Southern Great Plains summer 1997 IOP data and the Global Atmospheric Research Program's Atlantic Tropical Experiment phase III observations. These tests confirm the results obtained from the ARM 1995 experiments.
A correlated‐k distribution scheme for overlapping gases suitable for retrieval of atmospheric constituents from moderate resolution radiance measurements in the visible/near‐infrared spectral region
Authors: Buchwitz Michael , Rozanov Vladimir V. , Burrows John P.
Published: September 2002

An accurate and fast radiative transfer scheme for the simulation of the spectral radiance measurements of a new generation of multichannel moderate resolution satellite spectrometers, such as GOME/ERS‐2 (240–790 nm) and SCIAMACHY/ENVISAT‐1 (240–2385 nm), has been developed. Representative absorption cross sections of the individual molecular line‐absorbers CH4, CO, CO2; H2O, N2O, and O2 for relatively narrow spectral intervals (≤0.2 nm) have been determined using a combined correlated‐k (c‐k) distribution and exponential sum fitting of transmittances (ESFT) scheme. The speed of the c‐k calculations mainly depends on the channel‐dependent spectral resolution of the instruments. For the measurements to be made by SCIAMACHY, this may be up to 800 times faster than line‐by‐line calculations. The agreement between c‐ k radiances and line‐by‐line reference radiances is in the range 1–2%. The dominant error (∼1%) is attributed to the correlation assumption made when applying the k distribution method to inhomogeneous atmospheres. As the commonly used c‐k methods for overlapping gases are shown to result in large radiance errors at SCIAMACHY spectral resolution or do not permit individual representative absorption cross sections to be defined for each absorber separately, a new and novel method for the accurate simulation of overlapping absorption has been developed, verified, and validated. This technique enables overlapping line absorption to be considered for arbitrary spectral correlation of the monochromatic absorption cross sections of the individual gases. The new c‐k scheme is based on a linear mixing of two radiance terms determined assuming correlation and anticorrelation of the monochromatic absorption cross sections of the overlapping gases.
Relationships between cloud condensation nuclei spectra and aerosol particles on a south‐north transect of the Indian Ocean
Authors: Cantrell Will , Shaw Glenn , Leck Caroline , Granat Lenart , Cachier Hélène
Published: September 2002

Do anthropogenic emissions affect cloud microphysical properties? One parameter needed to answer the preceding question is a link between the aerosol number distribution and the cloud condensation nuclei (CCN) supersaturation spectrum for polluted and unpolluted air masses. On a south‐north cruise in the Indian Ocean in March of 1998, we found that measured CCN spectra and CCN spectra calculated from the ambient aerosol number distributions disagreed by as much as a factor of 2. The aerosol soluble fraction inferred from the relationship between measured and calculated CCN spectra ranged from 0.2 to 0.05 and exhibited a broad minimum at the Intertropical Convergence Zone (ITCZ). The concentration of CCN, non‐sea‐salt SO42−, NH4+, black carbon, and organic carbon increased by an order of magnitude as we passed through the ITCZ.
A probe for measuring wetting front migration in rocks
Authors: Salve Rohit , Wang Joseph S. Y. , Tokunaga Tetsu K.
Published: September 2002

To facilitate investigations of flow dynamics in unsaturated fractured rocks, we have designed a probe to qualitatively monitor saturation changes in borehole environments. This flexible probe is able to fit the cylindrical shape of boreholes and requires no backfill. Field results suggest that the design of this electrical resistivity probe (ERP) is suitable for determining changes in relative wetness along boreholes in unsaturated fractured rock environments. The performance of these probes was compared with that of psychrometers under a variety of tests; these probes were found to be significantly cheaper in addition to providing data that had a greater spatial and temporal resolution.
Macroporous drainage of unsaturated swelling soil
Authors: Askar Azza , Jin Yee‐Chung
Published: September 2002

A general procedure for modeling one‐dimensional unsteady water flow through unsaturated swelling soil has been developed. The generalized equation is based on Richards’ [1931] equation for unsaturated flow. The developed equation and its numerical solution used in the procedure can be applied to the study of soil drainage and redistribution where the flux of water is downward toward the water table. In this case the water content at each point in the profile decreases with time. The derivation of the relationship between volume changes and corresponding water losses reflects the effect of different shrinkage phases for a clayey soil. The model partitions the soil into two domains: macropores and micropores. To verify the model, results are compared with two other models. The model is then applied, using Regina clay as a media, to demonstrate the effect of volume change on moisture profiles during drainage. The results demonstrate that errors will occur if the classical theory of water movement in a nonswelling soil is employed to describe water flow in an unsaturated soil that exhibits volume change during wetting and drying.
Energetic oxygen ion bursts in the distant magnetotail as a product of intense substorms: Three case studies
Authors: Zong Q.‐G. , Wilken B. , Woch J. , Mukai T. , Yamamoto T. , Reeves G. D. , Doke T. , Maezawa K. , Williams D. J. , Kokubun S. , Ullaland S.
Published: September 2002

On the basis of Geotail high energy particle ‐ low energy particle detector (HEP‐LD) observations, this paper reports on three energetic (144 – 4000 keV) oxygen burst sequences detected in the distant tail (Xgse = −40 to −66 RE) and their relation to substorm signatures. Those energetic oxygen ion bursts lasted only 20 to 30 min and exhibited strong beam‐like structures. Two of the events (at about 1000 and 1900 UT on February 13, 1994) occurred in tailward flowing plasma after the flow direction changed from earthward to tailward; geostationary and ground based observations detected intense substorm activity during these periods, and the local magnetic field component Bz assumed predominantly negative values. The occurrence of a magnetic field with southward polarity and oxygen bursts embedded in tailward flowing plasma is consistent with the basic signatures of reconnection (formation of a neutral line) in the magnetotail. The third energetic oxygen burst with earthward flow was observed relatively close to Earth on August 27, 1993, X= −40 RE. No reversal in the plasma flow direction was seen, and the magnetic field polarity was essentially positive throughout the event. This is consistent with a “near‐Earth” neutral line that had formed beyond X=−40 RE. We conclude that (1) a large amount of heavy ions from the ionosphere can be transferred to the distant tail and accelerated to high energies during substorm activity and that (2) these oxygen O+ ions from the polar ionosphere can be considered as “tracer ions” in the substorm dynamical process.
Pyroclastic stratigraphy of the Soufriere Hills Volcano, Montserrat ‐ Implications for the present eruption
Authors: Roobol M. J. , Smith A. L.
Published: September 2002

The stratigraphy of the Soufriere Hills volcano is dominated by dense‐andesite block and ash flow deposits. Forty‐one radiocarbon ages permit a reconstruction of the eruptive history over the past 31,000 years. A single stratigraphic member yielding radiocarbon ages of 22,000 to 20,000 years B.P. is composed of block and ash flow deposits with semi‐vesiculated clasts concentrated in the upper third of each deposit. The present eruption is also producing block and ash flow deposits with both dense and semi‐vesiculated clasts. It marks a return to a more explosive driven type of activity as occurred 22,000 to 20,000 years ago. Deposits of this member have a broad sheet‐like form and volumes in excess of any produced so far by the present eruption.
Overview of the eruption of Soufriere Hills Volcano, Montserrat, 18 July 1995 to December 1997
Authors: Young Simon R. , Sparks R. Steven J. , Aspinall Willy P. , Lynch Lloyd L. , Miller Angus D. , Robertson Richard E. A. , Shepherd John B.
Published: September 2002

The onset of phreatic volcanic activity at the Soufriere Hills volcano, Montserrat on 18 July 1995 followed a three‐year period of heightened volcano‐seismic activity beneath the island. Phreatic explosions gave way to continuous eruption of juvenile andesitic magma in the form of a lava dome on or around 15 November 1995. Magma production rate has varied, leading to changes in eruptive style. An explosive eruption on 17 September 1996 followed a period of enhanced dome growth and large‐scale gravitational collapses from its eastern flank. Increasing dome volume led to stressing and overtopping of the confining crater walls to the southwest, north and west during early 1997. Sustained high magma production rate since June 1997 has led to three periods of major gravitational dome collapses followed by vulcanian explosive eruptions. Dome growth re‐started immediately after the cessation of the latest of these explosive phases in October 1997 and continues as of December 1997.
Seismicity associated with dome growth and collapse at the Soufriere Hills Volcano, Montserrat
Authors: Miller A. D. , Stewart R. C. , White R. A. , Luckett R. , Baptie B. J. , Aspinall W. P. , Latchman J. L. , Lynch L. L. , Voight B.
Published: September 2002

Varied seismicity has accompanied growth and collapse of the lava dome of the Soufriere Hills Volcano, Montserrat. Earthquakes have been classified as either volcano‐tectonic, long‐period or hybrid, and daily variations in the numbers of events have mapped changes in the style of eruption. Repetitive hybrid earthquakes were common during the first months of dome growth. In July 1996 the style of seismicity changed and regular, short‐lived hybrid earthquake swarms became common. This change was probably caused by an increase in the magma flux. Earthquake swarms have preceded almost all major dome collapses, and have accompanied cyclical deformation, thought to be due to a build‐up of pressure in the upper conduit which is later released by magma moving into the dome.
Sudden enhancement and partial disruption of thin current sheets in the magnetotail due to Hall MHD effects
Authors: Ma Z. W. , Bhattacharjee A.
Published: September 2002

Multi‐satellite observations indicate the development of thin current sheets and a rapid intensification of the cross‐tail current density at near‐Earth distances during a short interval (<1 min) just before onset, after a period of sluggish growth (∼ 0.5–1.5 hr). These observational features are described by a Hall MHD simulation in which we include the effects of the electron pressure gradient and the Hall current. These Hall MHD effects decouple the spatial scales of the parallel electric field and current density. The thin current sheet exhibits an impulsive pre‐onset enhancement at near‐Earth distances on a near‐Alfvénic time scale, weakly dependent on the value of the Lundquist number. The amplitude of the pre‐onset current sheet is larger, and its subsequent disruption faster than in a resistive MHD simulation. Agreement and discrepancies between theory and observation are discussed.
Atmospheric greenhouse effect and ionospheric trends
Authors: Upadhyay Hari Om , Mahajan K. K.
Published: September 2002

Theoretical calculations predict that CO2 doubling would produce a 50 K decrease in the thermospheric temperature which can result in about 20 km decrease in the F2 peak height (hmF2) and a minor decrease in the F2 layer critical frequency (foF2) [Rishbeth and Roble 1992]. In this paper we analyze ionosonde data for some 31 stations to study the long term trends in hmF2 and foF2. Regression coefficients for hmF2 and foF2 as a function of solar activity, are obtained for each station and departures (anomalies) from expected values derived for both these parameters. An analysis of hmF2 and foF2 anomalies indicates negative trends for some stations and positive trends for others. These varied between +29 to −20 km for hmF2 and +0.7 to −0.6 MHz for foF2 in the 34 year period since 1957, the International Geophysical Year (IGY). We conclude that the present data do not provide a definitive evidence of any global long term trend in the ionosphere.
Transition between stable sliding and stick‐slip due to variation in slip rate under variable normal stress condition
Authors: He Changrong , Ma Shengli , Huang Jianguo
Published: September 2002

This paper discusses a new mechanism that may control the transition between stable sliding and cyclic stick‐slip under variable normal stress condition. With a rate‐ and state‐dependent friction law that embodies the effect of normal stress, a single degree of freedom spring‐slider system inclined to the driving direction has a velocity‐dependent critical stiffness which separates stable sliding and cyclic stick‐slip. With steady state log‐linear rate‐dependent friction, the critical stiffness decreases with increasing velocity. Accordingly, higher velocity stabilizes a system for a predetermined stiffness lower than the critical value and may change the motion from cyclic stick‐slip to stable sliding, and vice versa. Our numerical simulations in full dynamic motion confirmed such transition, and agreed well with the analytical calculation. Such transitions can also be found in experimental works.
Precipitation sensitivity to global warming: Comparison of observations with HadCM2 simulations
Authors: Hulme Mike , Osborn Timothy J. , Johns Timothy C.
Published: September 2002

Recent century‐long experiments performed with global climate models have simulated observed trends in global‐mean temperature quite successfully when both greenhouse gas and aerosol forcing has been included. The performance of these same experiments in simulating observed global‐scale changes in precipitation has not previously been examined. Here we use a gridded terrestrial precipitation dataset for the period 1900 to 1996 to examine the extent to which observed global and zonal‐mean precipitation sensitivities to global warming have been captured by a series of model simulations recently completed by the UK Hadley Centre. There are signs that the model has been able to reproduce at least some of the observed zonal‐mean variations in the precipitation sensitivity to warming. Questions remain both about the quality of the observed precipitation data and about the spatial scale at which anthropogenically‐forced global climate models can be expected to reproduce observed variations in precipitation.
Europa's surface composition and sputter‐produced ionosphere
Authors: Johnson R. E. , Killen R. M. , Waite J. H. , Lewis W. S.
Published: September 2002

The efficient sputtering and decomposition of Europa's regolith by energetic charged particles produces an atmosphere representative of its surface composition. In addition to O2 and H2 from the decomposition of ice, we show that molecules representative of organics and salts will be present in ionic form at levels detectable using an ion mass spectrometer on an orbiting spacecraft. Such an instrument can also measure isotope ratios to determine surface age.
Evidence of an extratropical atmospheric influence during the onset of the 1997–98 El Niño
Authors: Yu Lisan , Rienecker Michele M.
Published: September 2002

The major 1997–98 El Niño episode was initiated by a series of very energetic westerly wind bursts in late 1996 and early 1997. Downwelling oceanic Kelvin waves were subsequently generated and propagated rapidly eastward across the equatorial Pacific Ocean and induced significant warming in the eastern basin. By analyzing newly available wind products derived from SSM/I satellite observations, we found indications that these westerly wind bursts were embedded within the active phase of the tropical intraseasonal oscillation (the Madden‐Julian Oscillation) but their amplitudes were greatly enhanced in the western Pacific sector. The local enhancement involved the development of equatorial twin cyclones which themselves were induced by northerly cold surges from East Asia/Western North Pacific into the tropical Pacific, demonstrating an extratropical atmospheric influence on tropical processes.
Ion heating, electron acceleration, and the self‐consistent parallel E‐field in downward auroral current regions
Authors: Jasperse John R.
Published: September 2002

Downward auroral current regions are characterized by: upflowing field‐aligned electrons; transverse ion heating; depleted plasma densities; downward‐pointing parallel E‐fields and current densities; intense ELF and VLF plasma wave activity; and the absence of hot magnetospheric particles. In this paper, we present a kinetic model that explains all of these features. We imagine a geomagnetic flux tube of finite length imbedded in a downward current region and impose appropriate kinetic boundary conditions. We assume that broadband ELF waves exist on the flux tube and heat the ions by cyclotron resonance near the ion gyrofrequency. We carry out an iterative, steady‐state solution for the one‐particle distribution functions, fα(s, ν⟂, ν∥), and the self‐consistent parallel E‐field, E∥(s). When this theory is applied to the Gorney et al. [1985] data set, we find good agreement between theory and experiment for the ion conic, the electron beam, and the downward‐pointing E∥(s).
IMP 8 observations of a poleward plasma circulation: Evidence of entry into the plasma mantle?
Authors: Kolpak M. A. , Paularena K. I. , Richardson J. D. , Siscoe G. L.
Published: September 2002

Plasma data from IMP 8 are used to map the spatial structure of plasma flow in Earth's magnetosheath. Plasma and magnetic field data from instruments on ISEE‐3 and WIND are used to normalize the IMP 8 magnetosheath data to upstream solar wind conditions. For the majority of the data, the magnitude and direction of plasma flow inside the magnetosheath agree well with gas‐dynamic models. About 8% of the ISEE‐3‐normalized velocity vectors and 3% of the WIND‐normalized velocity vectors exhibit a poleward circulation in the YZ plane. The observed circulation is consistent with predictions for flow within the plasma mantle. The occurrence frequency of these poleward flows increases further down the tail, consistent with predictions that the mantle width increases tailward. Densities are lower where this flow is observed, consistent with expectations for the plasma mantle, but temperatures are higher and the magnetic field magnitudes lower, the opposite of theoretical expectations.
Ice Age storm trajectories inferred from radar stratigraphy at Taylor Dome, Antarctica
Authors: Morse D. L. , Waddington E. D. , Steig E. J.
Published: September 2002

The Taylor Dome ice core, East Antarctica, provides a climate record that extends through the previous interglacial. We use ice dynamics modeling and geochemical records to infer the local accumulation rate history. A pronounced accumulation reduction occurred during the Last Glacial Maximum (LGM). The spatial gradient of accumulation across the surface of Taylor Dome indicates the modern local prevailing storm direction. Radar internal layering shows that this gradient was reversed during the LGM, suggesting a reorganization of synoptic weather systems occurred, possibly linked to the advance and retreat of the West Antarctic Ice Sheet.
On the kinetic structure of the magnetotail reconnection layer
Authors: Scholer M. , Lottermoser R.‐F.
Published: September 2002

The structure of the reconnection layer in the magnetotail is studied by solving the Riemann problem of the decay of a current sheet with a normal magnetic field component using 2‐D hybrid and Hall MHD simulations. In the first run a segment of the reconnection layer obtained from a hybrid simulation of reconnection is used as initial state in a periodic system. The thin central current sheet becomes instable in the direction parallel to the current sheet: the electrical current becomes filamentary and a warped magnetic field structure develops. In a second run, where the initial state is a Harris sheet with a superposed normal magnetic field component, a similar behavior is found. In both cases the boundary of the layer is not bounded by slow mode switch‐off shocks and no large‐amplitude rotational wave train develops within the layer, in contrast to 1‐D hybrid simulations of the same Riemann problem. The corresponding 2‐D Hall MHD simulation results in a stable layer bounded by slow‐mode switch‐off shocks.
Low‐frequency electromagnetic turbulence observed near the substorm onset site
Authors: Shinohara I. , Nagai T. , Fujimoto M. , Terasawa T. , Mukai T. , Tsuruda K. , Yamamoto T.
Published: September 2002

On the basis of wave and plasma observations of the Geotail satellite, the instability mode of low‐frequency (1–10 Hz) electromagnetic turbulence observed at the neutral sheet during substorms has been examined. Quantitative estimation has also been made for the anomalous heating and resistivity resulting from the electromagnetic turbulence. Four possible candidates of substorm onset sites, characterized by the near‐Earth neutral line, are found in the data sets obtained at substorm onset times. In these events, wave spectra obtained by the search‐coil magnetometer and the spherical double‐probe instrument clearly show the existence of electromagnetic wave activity in the lower hybrid frequency range at and near the neutral sheet. The linear and quasi‐linear calculations of the lower hybrid drift instability well explain the observed electromagnetic turbulence quantitatively. The calculated characteristic electron heating time is comparable to the timescale of the expansion onset, while that of ion heating time is much longer. The estimated anomalous resistivity fails to supply enough dissipation for the resistive tearing mode instability.
Deconvolution of interplanetary transport of solar energetic particles
Authors: Ruffolo D. , Khumlumlert T. , Youngdee W.
Published: September 2002

We address the problem of deconvolving the effects of interplanetary transport on observed intensity and anisotropy profiles of solar energetic particles with the goal of determining the time profile and spectrum of particle injection near the Sun as well as the interplanetary scattering mean free path. Semi‐automated techniques have been developed to quantitatively determine the best fit injection profile, assuming (1) a general piecewise linear profile or (2) a Reid profile of the form [C/(t − t0)] exp[−A/(t − t0) − (t − t0)/B]. The two assumptions for the form of the injection profile yielded similar results when we tested the techniques using ISEE 3 proton data from the solar flare events of July 20, 1981 (gradual flare), and January 2, 1982 (impulsive flare). For the former event, the duration of injection was much shorter for protons of higher energy (75–147 MeV), which may be interpreted as indicating that the coronal mass ejection‐driven shock lost the ability to accelerate protons to ∼100 MeV after traveling beyond a certain distance from the Sun.
Low‐frequency impulsive auroral hiss observations at high geomagnetic latitudes
Authors: LaBelle J. , Weatherwax A. T. , Perring J. , Walsh E. , Trimpi M. L. , Inan U. S.
Published: September 2002

Wave receivers in Antarctic Automatic Geophysical Observatories (AGOs) measure spectral features of low‐frequency impulsive auroral hiss emissions which cannot be observed at northern hemisphere sites or near manned Antarctic stations due to man‐made interference. From observations at AGO‐P1 (∼ 80° invariant latitude) we distinguish two spectral types of LF impulsive auroral hiss: normal events, in which the power spectral density is a decreasing function of frequency throughout the LF band, presumably peaking in the VLF band below the frequency range of the LF/MF/HF receiver; and LF cutoff events, for which the power spectral density peaks in the LF band with little or no power at VLF. Out of a large sample of impulsive auroral hiss events observed at AGO‐P1, 64% are the normal type, 14% are the LF‐cutoff type, and 22% are a mixture of both. A hiss propagation model similar to that of Makita [1979] was used to evaluate approximately whether the observed occurrence statistics of LF cutoff auroral hiss are consistent with previous models of auroral hiss generation and propagation. The results indicate that LF cutoff auroral hiss can be explained by propagation effects and suggest that the observed LF cutoff occurrence statistics are consistent with LF impulsive auroral hiss originating at or ducted to altitudes of 2000–5000 km.
Interferometry investigations of VHF backscatter from plasma irregularity patches in the nighttime E region using the Chung‐Li radar
Authors: Chu Yen‐Hsyang , Wang Chien‐Ya
Published: September 2002

Backscatter spectra with extremely narrow spectral width of only 2 to 7 m/s and small mean Doppler velocity of 10 to 30 m/s associated with plasma irregularity patches in nighttime sporadic E layers have been obtained with the Chung‐Li VHF radar. The radar interferometer shows these echoes to be highly aspect sensitive with striated structures in the horizontal and azimuthal planes. By considering the spatial distributions of the Es echoes and the geometry of the effective radar beam, we propose a schematic model in which multiple thin and tilted Es layers with steep electron density gradients are drifting across the illuminated region. The results show that the layers appear to be tilted toward east at angles of 3° to 14°, and their horizontal drift velocities (obtained by tracking the patches with the interferometer phase) imply primarily eastward motions at speeds of 7.5 to 56.2 m/s.
Earthward flow bursts in the inner magnetotail and their relation to auroral brightenings, AKR intensifications, geosynchronous particle injections and magnetic activity
Authors: Fairfield D. H. , Mukai T. , Brittnacher M. , Reeves G. D. , Kokubun S. , Parks G. K. , Nagai T. , Matsumoto H. , Hashimoto K. , Gurnett D. A. , Yamamoto T.
Published: September 2002

High‐velocity magnetotail flow bursts measured by the Geotail Low Energy Plasma experiment in the premidnight equatorial region between 10 and 15 RE have been compared with other magnetospheric phenomena. These bursts, typically characterized by earthward velocities approaching 1000 km/s and lasting for times of the order of l min, are associated with magnetotail dipolarizations and large magnetic field fluctuations. Using supporting measurements of the International Solar Terrestrial Physics program it is found that the flow bursts are closely associated with auroral brightenings, AKR onsets, geosynchronous particle injections, and ground magnetic activity. Flow bursts for which Polar UVI images are available showed auroral brightenings that developed near the footpoint Geotail field line. AKR intensifications usually accompanied the flow bursts in close time coincidence, whereas dispersionless geosynchronous particle injections tended to be delayed by 1–3 min. Since flow bursts often exhibit the earliest onsets of these various phenomena, it seems likely that this chain of events is initiated in the tail beyond 15 RE, presumably by magnetic reconnection. It is concluded that flow bursts are a fundamental magnetotail process of limited spacial extent that are important in energy and magnetic flux transport in the magnetosphere. Magnetotail flow bursts are intimately connected to auroral acceleration processes and AKR generation at several thousand kilometer altitude and a full explanation of substorms will have to explain this relationship.
Lunar tides in the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model
Authors: Stening R. J. , Richmond A. D. , Roble R. G.
Published: September 2002

Lunar semidiurnal tides are introduced at the lower boundary of the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). The tides are derived from the model of Vial and Forbes [1994] and interesting properties of these tides are found when they are subjected to Hough decomposition; there is considerable hemispherical antisymmetry in the September tides, and the March and September modal compositions are significantly different. A differencing method is used to isolate the lunar tidal effects in the TIEGCM, and these are compared with lunar tidal analyses of ionospheric data. The model reproduces the broad features of the lunar tide in f0F2 (maximum frequency of the F region) with phase changes around 7° magnetic dip latitude during daytime. The model and data analysis both give variations of the amplitude and phase of the lunar tide with local time. Near the equator the variation of phase with local time changes with latitude as the equatorial anomaly develops during the day. Comparison between the model predictions and analyses of data at observatories at midlatitudes produces mixed results. Here the effects of the lunar components of both electrodynamic drifts and of neutral winds need to be taken into account. Several cases of day to night changes in the phase of the lunar tide in f0F2 are noted. Large nighttime amplitudes of the lunar tide in hmF2 (height of the maximum density), more than 4 km, seem to be due to inphase action of the electrodynamic and neutral wind effects while during daytime they are out of phase. The lunar tide in the ratio of oxygen to nitrogen density [O]/[N2]is estimated and found to be of relatively minor importance. Amplitudes of the lunar tide in f0F2 may be measured at more than 0.4 MHz at some local times, but the model values are less than this. Comparison is also made with ion drift measurements made by the San Marco D satellite. The several uncertainties which underlie this work are discussed in detail.
Upper mantle seismic velocity structure beneath Tanzania, east Africa: Implications for the stability of cratonic lithosphere
Authors: Ritsema Jeroen , Nyblade Andrew A. , Owens Thomas J. , Langston Charles A. , VanDecar John C.
Published: September 2002

The assertion of cratonic stability put forward in the model for deep continental structure can be tested by examining upper mantle structure beneath the Tanzania Craton, which lies within a tectonically active region in east Africa. Tomographic inversions of about 1200 teleseismic P and S travel times indicate that high‐velocity lithosphere beneath the Tanzania Craton extends to a depth of at least 200 km and possibly to 300 or 350 km. Based on the thickness of mantle lithosphere beneath Archean cratons elsewhere, it appears that the mantle lithosphere of the Tanzania Craton has not been extensively disrupted by the Cenozoic tectonism in east Africa, thus corroborating the assertion of cratonic stability in the model for deep continental structure. The presence of thick, high‐velocity structure beneath the Tanzania Craton implies relatively low temperatures within the cratonic mantle lithosphere, consistent with relatively low surface heat flow. The thick cratonic keel is surrounded by low seismic velocity regions beneath the east African rifts that extend to depths below 400 km. Our models show a shear velocity contrast between the cratonic lithosphere and the uppermost mantle beneath the eastern branch of the rift system of about 5% to 6%, but from resolution experiments we infer that this contrast could be underestimated by as much as a factor of 1.5. We attribute about half of this velocity contrast to the depleted composition of the cratonic keel and the other half to thermal alteration of upper mantle beneath the rifts. Low‐density structures that may be required to provide buoyant support for the elevation of the Tanzania Craton must reside at depths greater than about 300–350 km.
Dike ascent in partially molten rock
Authors: Rubin Allan M.
Published: September 2002

Field observations indicate that dikes form and grow in magma source regions, but the mechanics of this process are poorly understood. I derive time‐dependent and self‐similar solutions for the growth of buoyant dikes fed by porous flow in partially molten rock. The host rock is treated as poroelastic; for basaltic (but not rhyolitic) dikes, large‐scale viscous deformation of the rock is insignificant for dikes large enough to meet the classical Griffith fracture criterion. Observed wetting angles in partially molten peridotite suggest that subcritical crack growth may be important in crack initiation and render viscous deformation insignificant for dikes that are shorter still. Melt is driven into the dike by the difference between the ambient pore pressure and the least compressive stress and driven up the dike by magma buoyancy. The volume increase of the melting reaction, driven by the local drop in pore pressure, may be more important than elastic compressibility of the pore volume and pore fluid in driving melt into the dike. Because channel flow is very efficient relative to porous flow, the dikes are thin, about 6 mm wide when they are a few kilometers tall, for an ambient melt pressure that exceeds the least compressive stress by 1 MPa. This thickness is relatively insensitive to all relevant parameters (including the dike height) because the channel flux increases as the cube of the channel thickness. One implication appears to be that dikes that traverse the lithosphere must drain large segregations of magma, rather than partially molten rock. If dikes feed compliant sills at a depth where the source region is conductively cooled, crystallization onto the dike walls could increase the observable thickness while mass balance maintains a narrow aperture for flow.
Spherical harmonic analyses of paleomagnetic data: The time‐averaged geomagnetic field for the past 5 Myr and the Brunhes‐Matuyama reversal
Authors: Shao J.‐C. , Fuller M. , Tanimoto T. , Dunn J. R. , Stone D. B.
Published: September 2002

Maxwell's multiple pole theory provides the basis for a convenient means of determining the spherical harmonics of geomagnetic fields from directional paleomagnetic data. The relationship between the Maxwell poles and axes and the corresponding spherical harmonics was given by Maxwell. We show that the distribution of virtual geomagnetic poles (VGPs) is symmetrical about the Maxwell axes and converge to their poles. Utilizing this property of VGPs and a complimentary property of the distribution of equatorial virtual poles (EVPs), which are defined as points 90° from the VGP on the site meridian, leads to a means of obtaining the Gaussian spherical harmonic coefficients from paleomagnetic data. This VGP method involves the minimization of the horizontal components of the magnetic fields at VGPs and the radial components of the magnetic field at EVPs to yield the best fitting spherical harmonic coefficients. A hybrid variant of the VGP method involving this minimization and fitting the available mean vectors measured at sites has also been developed. Tests of the VGP and hybrid methods on model fields derived from the International Geomagnetic Reference Field (IGRF) 1995 demonstrate that they are effective means of determining the Gauss coefficients for any field requiring no a priori assumptions about the field. The hybrid method has been applied in a preliminary analysis of the time‐averaged paleomagnetic data for the past 5 Myr and for the Bruhnes chron. The resulting mean fields were dominantly dipolar, but there were also persistent second order features, suggesting some long‐term smaller‐scale control over the dynamo process. The hybrid method was also applied to data from the Brunhes‐Matuyama reversal. Four models were obtained: (1) a mean field model for reversed polarity, (2) a mean field model for normal polarity, (3) a mean field model for the entire reversal, and (4) a time sequence field model. The results were consistent with the Americas being a persistent site of inward field lines in the nondipole field and suggested that the reversal is initiated by decreases in strength of dipole, quadrupole, and octupole terms. The true dipole path of the transitional field tracks across eastern Asia, but a strong radially inward flux bundle moves over Africa. The paucity of demonstrably reliable data in these reversal records, however, requires that these results be interpreted cautiously.
Numerical simulation of earthquake faults with gouge: Toward a comprehensive explanation for the heat flow paradox
Authors: Mora Peter , Place David
Published: September 2002

The particle‐based lattice solid model is used to simulate transform faults with and without fault gouge. Stick‐slip frictional behavior is observed in two‐dimensional numerical experiments of model faults both with and without gouge. When no gouge is present, the fault is strong, and the heat generation and stress drops are correspondingly high, in disaccord with observations surrounding the heat flow paradox. In contrast, when a gouge is specified, the fault is weak, and the heat generation as well as stress drops are low, in quantitative agreement with observational constraints. The heat flow is low on average and during short periods of aseismic creep. Seismic efficiencies are compatible with observationally based bounds. Counter intuitively, the fault strength decreases as the intrinsic friction between particles is increased beyond a given threshold. The mechanism for low fault strength and heat is rolling and jostling of fault gouge grains during slip. This allows macroscopic movement of the fault with only minimal slip between surfaces of the gouge grains. As this dynamical mechanism operates during seismic and aseismic slip, it provides an explanation for the lack of a heat flow anomaly in both the seismic and creeping parts of the San Andreas fault. The simulation results provide the first comprehensive and quantitative possible explanation of the heat flow paradox and suggest that fault gouge plays a fundamental role on the dynamics of earthquake faults. Whether rolling and jostling of fault gouge particles provides the explanation for the heat flow paradox in nature remains to be validated by observation evidence.