bluuebunny/crossref_metadata_2025 huggingface dataset:
The spin and inertia of Venus
Authors: Spada Giorgio , Sabadini Roberto , Boschi Enzo
Published: February 2004

One of the most remarkable rotational features of Venus is the presence of a relatively large offset between spin and maximum inertia axis. According to the most recent gravity field models, this offset may amount to ∼0.5°, considerably larger than the one which characterizes the Earth's Chandler wobble (∼0.3″). At present, it is not clear whether the offset of Venus is the result of the limited resolution of the gravity models or, alternatively, it may be regarded as an evidence in support to the existence of a wobble. Purpose of this note is to demonstrate on quantitative grounds that an offset between spin axis and maximum inertia axis is to be expected on the time‐scales of mantle convection, due to the peculiar rotational characteristics of Venus. We also show that the inertia perturbation associated to the rising of hot thermal plumes from the core‐mantle boundary to the surface is large enough to maintain offsets ranging between 0.1 and 1°. Of course, this does not preclude that other processes, such as Venusquakes or periodic atmospheric variabilities, may contribute significantly to the observed offset on shorter time‐scales.
Precipitating electrons: Evidence for effects on mesospheric odd nitrogen
Authors: Callis Linwood B. , Boughner Robert E. , Baker Daniel N. , Mewaldt Richard A. , Bernard Blake J. , Selesnick Richard S. , Cummings Jay R. , Natarajan Murali , Mason Glenn M. , Mazur Joseph E.
Published: February 2004

Observations of electron fluxes made by the PET and LICA instruments aboard SAMPEX have been used with NO measurements made by HALOE aboard UARS to provide evidence of mesospheric and lower thermospheric NO formation due to precipitating electrons. Results indicate significant NO increases from 70 to 120 km which are associated with the occurrence of enhanced electron populations in the outer trapping regions of the magnetosphere, 2.5 ≤ L ≤ 7, which precipitate into the atmosphere.
Incoherent scatter radar, ionosonde, and satellite measurements of equatorial F region vertical plasma drifts in the evening sector
Authors: Fejer Bela G. , de Paula Eurico R. , Scherliess Ludger , Batista Inez S.
Published: February 2004

Studies of equatorial F region evening vertical plasma drifts using different measurement techniques have produced conflicting results. We examine the relationship of incoherent scatter radar and ionosonde drift observations over the Peruvian equatorial region, and AE‐E satellite drifts for different geophysical conditions. Our data show that there is large day‐to‐day variability on the ratios of radar and ionosonde drifts, but on the average the measurements from these two techniques are in fair agreement during low and moderate solar flux conditions. For high solar activity, however, the Jicamarca evening drifts during equinox and December solstice are significantly larger than the ionosonde drifts. These results can be explained by the different height ranges of the radar and ionosonde measurements, and the increase of the upward drift velocity with height below the F region peak. This altitudinal variation is related to the longitudinal gradient of the zonal plasma drifts as a result of the curl‐free electric field condition. Our results also indicate that during equinox the increase of the vertical prereversal velocity enhancement with solar activity is largely longitude independent.
Shocks in the polar wind
Authors: Demars H. G. , Barakat A. R. , Schunk R. W. , Thiemann H.
Published: February 2004

The occurrence of shock waves in the terrestrial polar wind was predicted many years ago by a time‐dependent three‐dimensional model based on hydrodynamic equations. These shocks were seen to occur for counterstreaming ion populations and for cases when a convecting flux tube entered a region of sharply increasing electron temperature, such as the dayside cusp. Other studies conducted at about the same time showed that the shocks induced by counterstreaming ion populations may simply be artifacts of the adopted hydrodynamic model. The validity of shocks induced by electron temperature enhancements has remained an open question. Using a macroscopic particle‐in‐cell (PIC) code, we have verified the hydrodynamic prediction that sudden electron temperature enhancements can launch shock waves in a convecting flux tube of plasma. Our simulation follows a flux tube as it convects antisunward across the dayside auroral oval, the polar cap, and the nightside auroral oval. The electron temperature at 2000 km altitude is assumed to be relatively low (3000 K) in the subauroral ionosphere and in the polar cap but much higher (7000 K) in the dayside and nightside auroral oval. As the flux tube enters the auroral oval, either on the dayside or the nightside, forward and reverse shock pairs in the H+ component of the plasma are created at the bottom of the flux tube and propagate upward until they exit the simulation region at the top. The forward and reverse shock fronts propagate at speeds greater than and less than the drift speed of the H+ gas, respectively.
SCIFER‐Transverse ion acceleration and plasma waves
Authors: Kintner Paul M. , Bonnell John , Arnoldy Roger , Lynch Kristina , Pollock Craig , Moore Tom
Published: February 2004

SCIFER encountered the cleft ion fountain within the cleft ionosphere at 1000 MLT and 1400 km altitude where it was possible to investigate the fine structure of transverse ion acceleration (TIA). Latitudinally narrow (30 km) regions of TIA were found to be closely correlated with broadband low frequency electric fields and reduced ionospheric density. The low frequency electric fields extended up to a few kHz with the largest amplitudes of about 10–20 mV/mp‐p occurring below 400 Hz. No spectral features ordered by the ion cyclotron frequencies were observed. Outside regions of TIA the ionospheric density was typically 2 × 10³ cm−3 while inside regions of TIA the density dropped to 5 × 10² cm−3. The correlation between TIA, reduced ionospheric density and broadband low frequency electric fields is so exact, sometimes within a few hundred meters, we interpret the broadband low frequency electric fields as current‐driven electrostatic waves, perhaps a mixture of ion cyclotron and ion acoustic waves.
Observational constraints on the formation of type ia polar stratospheric clouds
Authors: Tabazadeh A. , Toon O. B. , Gary B. L. , Bacmeister J. T. , Schoeberl M. R.
Published: February 2004

On January 11, 1989 during AASE I, Type Ia polar stratospheric clouds (PSCs) were observed over a vast area in the Arctic. Synoptic scale temperature histories for this flight are obtained using the Goddard trajectory model. Mesoscale temperature fluctuations (MTF) are divided into two major categories of background and lee waves. The statistics of background MTFs are derived from measurements by the microwave temperature profiler aboard the ER‐2 aircraft. A forecast model is used to determine the occurrence of lee waves. The MTFs are then superimposed upon the Goddard synoptic scale temperatures. Statistical correlations between temperature histories and Type Ia PSC observations indicate that MTFs can not be solely responsible for the formation of Type Ia PSCs in the stratosphere. Instead, on a synoptic scale, the exposure time of air masses to temperatures below the nitric acid trihydrate (NAT) frost point for ∼1 day or more should be the main criteria for predicting the occurrence of Type Ia clouds in the stratosphere.
VLF wave generation using VLF heating and the cubic nonlinearity of the ionosphere
Authors: Barr Richard
Published: February 2004

On the west coast of Norway, two VLF transmitters, Omega Norway and JXN, are located within 6 km of each other. JXN has an output power of 150 kW at 16.4 kHz and may be expected to modify the ionospheric plasma in the region where the signals from Omega Norway also propagate. Nonlinear wave interactions have been observed between the signals from the two transmitters with clear signals being observed at Lycksele, Sweden, on 20.75 kHz (twice the frequency of the upper sideband of JXN‐‘unique’ frequency of Omega Norway 12.1 kHz). Calibration results are presented which show the nonlinearity is a feature of the ionospheric plasma and not the receiving equipment. This nonlinear technique may be an effective means of generating ELF signals from suitably sited VLF transmitters of the appropriate frequency.
Temperature and the Seismic/Aseismic Transition: Observations from the 1992 Landers Earthquake
Authors: Williams Colin F.
Published: February 2004

An important constraint on the size and destructive potential of earthquakes is the depth extent of rupture. Laboratory studies of the transition from unstable to stable sliding, along with observed relationships between surface heat flow and the thickness of the seismogenic crust, provide strong evidence for the significance of temperature in determining the maximum nucleation depth of large earthquakes. The June 28, 1992, Mw 7.3 Landers earthquake ruptured fault segments within 20 km of 11 pre‐existing heat flow measurements, and shallowing of the base of aftershock seismicity along strike correlates with an increase in heat flow. Crustal geotherms estimated from these measurements place the base of seismicity along the 250°C isotherm. This temperature is consistent with predictions from laboratory studies of the frictional stability of Westerly granite, but estimated temperatures for the seismic‐aseismic transition along other faults within the San Andreas fault system are in the range of 350 to 400°C. Variations in country rock and fault gouge composition, together with higher slip rates, may account for this difference, although part of the Landers seismogenic crust might remain unruptured.
Simplified analysis of horizontal stresses in a buttressed forearc sliver at an oblique subduction zone
Authors: Wang Kelin
Published: February 2004

Forearc slivers are commonly present at obliquely convergent margins. Such slivers are often buttressed at their leading edges, but no quantitative model of the buttress effect exists. I consider the horizontal stresses in a buttressed sliver using a simple plan‐view two‐dimensional elastic model. The driving force created by the subducting plate is approximated by a body force in the direction of plate convergence. An Airy stress function solution relates the stress field to the boundary and body forces that allows the relative importance of different forces driving and resisting the sliver to be evaluated. Oblique subduction can easily cause arc‐parallel compression in the buttressed sliver, especially near the leading edge, because the arc‐parallel component of the driving force by the subducting plate is integrated over a much larger distance than its arc‐normal component. For the Southwest Kuril arc, the stress field predicted by the simple model agrees with that inferred from geological structures. It is possible that the model can also explain the stress field of the Cascadia forearc as constrained by earthquake focal mechanisms and in situ stress measurements.
Molecular excitation in sprites
Authors: Green B. D. , Fraser M. E. , Rawlins W. T. , Jeong L. , Blumberg W. A. M. , Mende S. B. , Swenson G. R. , Hampton D. L. , Wescott E. M. , Sentman D. D.
Published: February 2004

We have determined the molecular internal energy distribution in the N2 B³IIg state from the fluorescence measured during the observations of sprites during 1995. Spectrally resolved data from two different instruments and three different sprites are compared with theoretical spectra to obtain excited state vibrational distributions. Energy dependent electron excitation cross‐sections and laboratory data were used to estimate the energies of electrons producing the red sprite radiance. Implications for chemical production in the mesosphere and critical future measurements are discussed.
Experimental evidence for unstable waves in the lower E/Upper D‐region excited near the bisector between the electric field and the drift velocity
Authors: Blix T. A. , Thrane E. V. , Kirkwood S. , Dimant Y. S. , Sudan R. N.
Published: February 2004

During the METAL campaign conducted at Andøya Rocket Range in September–October 1991, detailed in situ measurements were performed by means of plasma probes in the TURBO rocket payload. During one of the salvoes (30 September 1991), strong plasma density fluctuations were detected above 78 km, especially on downleg. The results are shown to be in good agreement with the new instability predicted from the kinetic theory of Dimant and Sudan (1995). The new instability leads to the excitation of long wavelength plasma waves along the bisector between the drift velocity Vd and the electric field E.
Sub‐bottom temperature anomalies detected by long‐term temperature monitoring at the TAG Hydrothermal Mound
Authors: Kinoshita M. , Matsubayashi O. , Von Herzen R. P.
Published: February 2004

During Aug. 1994–Feb. 1995, geothermal measurements were carried out on the TAG hydrothermal mound (26°0′N, 44°49′W) of Mid‐Atlantic Ridge. Two geothermal probes made reliable measurements. Six transient anomalies in subbottom temperatures were detected 20 m south of the central black‐smoker complex (CBC), having durations 40 to 400 hours and magnitude 0.2°C to > 30°C. These anomalies are probably evidence for short‐period fluctuations in the fluid circulation beneath the mound. The time variability in subbottom temperatures near CBC suggests that instantaneous heat flow measurements may not represent the time average of heat flow there. Although it is difficult to specify the origin of thermal anomalies during the drilling period by the Ocean Drilling Program, at least one of them seems to correlate with the drilling events because it was also accompanied by transient temperature anomalies at CBC and 50 m east of CBC observed by other instruments.
Seasonal dependence of mesospheric gravity waves ( <100 Km) at Peach Mountain Observatory, Michigan
Authors: Wu Q. , Killeen T. L.
Published: February 2004

We present results from a 14‐month study of all‐sky camera observations of the Hydroxyl (OH) nightglow made at the Peach Mountain Observatory, Michigan (42.3°N; 83.7°W) Spatial variations in the observed OH airglow images have been used to assess gravity‐wave (GW) occurrence frequency at ∼85 km altitude as a function of season. A Strong seasonal dependence of mesospheric GW activity is observed, with peak activity in the summer months and much reduced activity during the winter months. Gravity waves (as defined by observed coherent variations in relative OH brightnesses of >∼7.5) were found to be present on about 70% of the clear‐sky nights during the summer months. During the spring, fall, and winter months, however, the observed GW occurrence frequency was very low (<10%). Most of the GWs were observed to propagate towards the eastward hemisphere. We suggest that the tropospherically‐generated GWs are anisotropic (eastward) thus passing through to the mesosphere only in the summer and being filtered out by the intervening neutral winds during other seasons. It is also possible that the GWs are able to reach higher altitudes without breaking because of their smaller amplitudes at lower altitudes during the summer season relative to the winter season.
Intercomparison of tropospheric OH radical measurements by multiple folded long‐path laser absorption and laser induced fluorescence
Authors: Brauers T. , Aschmutat U. , Brandenburger U. , Dorn H.‐P. , Hausmann M. , Heßling M. , Hofzumahaus A. , Holland F. , Plass‐Dülmer C. , Ehhalt D. H.
Published: February 2004

An intercomparison of in‐situ OH measurements by differential optical absorption spectroscopy (DOAS) and laser‐induced fluorescence spectroscopy (LIF) was carried out in August 1994 in a clean rural environment in North‐East Germany. A large data set of temporally overlapping OH measurements with well defined measurement errors was obtained and compared. Both instruments encountered the same air masses, except when the wind came from NNW and caused a perturbation of the DOAS measurements. Excluding that wind sector, the weighted regression analysis of 137 data pairs (70% of all available data pairs) yields a linear relationship between the DOAS and LIF measurements with a correlation coefficientr = 0.90. The unity slope (1.01±0.04) and the non‐significant intercept (0.28±0.15) × 106 cm−3 demonstrate that both OH instruments agreed excellently in their calibrations and accurately measured OH.
Response of the atmospheric angular momentum and the length of the day to the surface temperature increase for an Aqua Planet Model
Authors: Satoh Masaki , Yoshida Shigeo
Published: February 2004

We examine the effect of the surface temperature increase on the atmospheric angular momentum to study relations between the decadal variations in the atmospheric angular momentum and those of the rotation of the solid earth. We use an atmospheric general circulation model forced by simple surface temperature distributions (an aqua planet model). We vary the temperature distribution in two ways and investigate the responses. In one set of calculations, the surface temperature is increased uniformly globally. In the other set of calculations, the surface temperature is increased locally near the equator. We have found that a globally uniform warming results in an increase of the atmospheric angular momentum. This is because the tropopause becomes higher and the depth of tropospheric westerly shear increases as the surface temperature increases. The altitude of the jet (maximum of the westerlies) increases, while its latitude does not change. These dependencies can be explained by the dynamics of the Hadley circulation. We have also found that a warming localized near the equator results in an westerly jet built up around the equatorial tropopause and the increase in the angular momentum. The results of the two cases investigated here show that the effect of warming on the atmospheric angular momentum is too small to explain the decadal variations of the length of the day. Furthermore, they show that the sense of the change is opposite to that proposed by Lambeck and Cazenave [1976], which is based on observational data of the lower atmosphere.
Strong scattering of short‐period seismic waves by the core‐mantle boundary and the P‐diffracted wave
Authors: Bataille Klaus , Lund Fernando
Published: February 2004

We interpret the long‐tail‐in‐time (up to 3 minutes) decay of short‐period Pdiff as being due to multiple scattering within D″, which, for this purpose, is assumed to be an heterogeneous region with a low velocity zone just next to the core‐mantle boundary. A simple multiple scattering theory, generalized for a two‐dimensional spherical geometry, provides good agreement with observations for values of the scattering and attenuation coefficients (η) of about 10−3 km−1.
Inverse modeling of biogenic isoprene emissions
Authors: Chang Michael E. , Hartley Dana E. , Cardelino Carlos , Chang Wen‐Ling
Published: February 2004

Using current biogenic emission estimates, Urban Airshed Model simulations for Atlanta, Georgia substantially underpredict isoprene concentrations relative to observations. In this work, an inverse method is used to determine the biogenic isoprene emissions for Atlanta that minimize the difference between the model‐simulated and the observed isoprene concentrations. The resulting isoprene emissions are 2 to 10 times higher than any of the accepted emission estimates. Overall, the concurrent ozone simulation improved when the biogenic isoprene emissions were increased. These higher isoprene emissions significantly increase the concentration of model‐simulated ozone within the plumes of nitrogen oxides emitted from large point sources. These results should be considered when developing control strategies for urban ozone.
Volcanic aerosols and interannual variation of high clouds
Authors: Song Naihui , Starr David O'C. , Wuebbles Donald J. , Williams Allen , Larson Susan M.
Published: February 2004

Interannual variability of high‐level cloudiness (HC) is examined using global outgoing longwave radiation (OLR). Variations of HC are analyzed versus a measure of global stratospheric aerosol amount and an El Nino index. Volcanic aerosols are apparently associated with widespread increases of up to 10% in an OLR‐based HC index. The most significant effects occurred in middle latitudes and persisted for several years after major eruptions. El Nino is found to be associated with decreased cloud activity in the subtropics. This study suggests that volcanic aerosols can significantly modify global cloudiness, and that stratospheric aerosol loading can be an important variable controlling the interannual variations of high level clouds and climate.
Experimental investigation of spectral global irradiance measurement errors due to a non ideal cosine response
Authors: Gröbner J. , Blumthaler M. , Ambach W.
Published: February 2004

Due to limitations in the diffusers used for global irradiance measurements, the method usually applied to correct global irradiance measurements is investigated. Based on sky radiance measurements, the assumption of a homogeneous sky radiance distribution generally used to calculate the diffuse part of the global cosine error is accurate to within ±1.5% in the UVB for varying atmospheric and geographic conditions. At longer wavelengths, the assumption of a homogeneous sky radiance distribution should not be used for global cosine corrections since it underestimates the true diffuse cosine error by up to 10% at 500 nm. The accuracy of diffuse irradiance measurements using shading disks is investigated and its dependence on different aerosol amounts is shown. The cosine correction of global irradiance measurements is shown to be insensitive to errors in the determination of the fraction of direct to global irradiance. The uncertainty in the cosine correction of global irradiance measurements is therefore less than ±2% in the UVB.
Redistribution of magnetic helicity at the Sun
Authors: Ruzmaikin A. A.
Published: February 2004

Evolution of magnetic loops associated with filaments and coronal mass ejections involves a redistribution of solar magnetic helicity. Two mechanisms of the helicity redistribution are discussed. The first one involves magnetic reconnections among magnetic loops and can introduce helicity into an erupting magnetic field accompanied with an encapture of helicity by the Sun. The second one involves the MHD helicity redistribution in the Sun and indicates that the magnetic helicity of each hemisphere of the Sun oscillates about a mean with the half‐period of the solar cycle (11 years), but does not change sign from one 11 year period to the next.
Observed changes in the diurnal temperature and dewpoint cycles across the United States
Authors: Knappenberger Paul C. , Michaels Patrick J. , Schwartzman Peter D.
Published: February 2004

We analyzed hourly coterminous U.S. airport data, beginning in 1948, for changes in daily temperature and dewpoint regimes. We found an ubiquitous phase shift in the daytime cycle of warming, towards later in the day. Overall day and night temperature changes were very consistent with the results of Karl et al. [1993] even though this is an entirely different source of data, and one that is not specifically deurbanized, although we attempted to use stations that were not from downtown airports. We divided our results into eastern and western subsets as a first approximation for high and low sulfate aerosol conditions, and found evidence that was consistent with a sulfate effect on overall temperature, but inconsistent with modeled estimates of the effect of sulfates on the intradiurnal regime. It is difficult to establish the causes of the observed intradiurnal phase shift of warming time although urbanization and anthropogenic emissions are likely to be involved. The nature of our finding is subtle, and is consistent with other results that show very modest (and nonobvious) responses to greenhouse changes. Further research will ultimately clarify its causes and effects.
The impacts of SO2 oxidation in cloud drops and in haze particles on aerosol light scattering and CCN activity
Authors: Hegg D. A. , Majeed R. , Yuen P. F. , Baker M. B. , Larson T. V.
Published: February 2004

Calculations with an explicit microphysical model show significant sulfate production in both clouds and unactivated haze but significant impact on particle light scattering and CCN production only for the cloud scenarios.
Parameterization of the droplet effective radius of warm layer clouds
Authors: Pontikis C. A.
Published: February 2004

Two pertinent parameterizations representing respectively the cloud droplet effective radius of maritime and continental warm layer clouds are presented. They are obtained by assuming that warm layer clouds are adiabatic. In order to derive these parameterizations, a theoretical expression of the mean adiabatic cloud droplet effective radius is first developed. This expression depends upon the cloud droplet concentration, the liquid water path and a parameter A that relates linearly the liquid water content to the distance above cloud base. Secondly, an analytic expression of the adiabatic liquid water content is derived and reveals that A depends exclusively upon the cloud base characteristics. By using climatological cloud base values to determine the A value as well as two “standard” droplet concentrations representing respectively droplet concentrations in maritime and continental clouds, one obtains parameterized expressions of the droplet effective radius that depend exclusively upon the liquid water path. The maritime parameterization is validated by using observational data.
γ‐Ray emission produced by a relativistic beam of runaway electrons accelerated by quasi‐electrostatic thundercloud fields
Authors: Lehtinen N. G. , Walt M. , Inan U. S. , Bell T. F. , Pasko V. P.
Published: February 2004

In an experiment described by Fishman et al. [1994], high energy photons of atmospheric origin were detected by the Burst and Transient Source Experiment (BATSE) detectors, located on the Compton Gamma Ray Observatory (CGRO). In this paper we assess the possibility that the bursts may be bremsstrahlung produced by relativistic (>1 MeV) runaway electron beams accelerated in an avalanche process by quasi‐electrostatic thundercloud fields. We consider the height‐dependent density profile of the relativistic electrons specified as a function of time in the context of a previously reported runaway model [Bell et al., 1995]. The electron beam is modeled as a vertical cylinder with radius 10 km, and numerical estimates are provided of γ‐ray fluxes which would be observed at the satellite. The predicted fluxes at the satellite altitude and at horizontal distances of up to 500 km from the source are found to be comparable to the experimental data.
Baroclinic wave variations observed in MLS upper tropospheric water vapor
Authors: Stone Elizabeth M. , Randel William J. , Stanford John L. , Read William G. , Waters Joe W.
Published: February 2004

Upper tropospheric water vapor measurements from the UARS Microwave Limb Sounder are used to investigate the structure and evolution of eastward traveling medium‐scale wave features in Southern Hemisphere summertime. The extratropical Southern Hemisphere summer circulation pattern is frequently dominated by medium scale waves which exhibit life cycles of baroclinic growth and barotropic decay. The water vapor field during such life cycles is examined here and found to be well correlated with meteorological fields derived from European Centre for Medium Range Weather Forecasts global analyses. From mid January to mid February 1992 several episodes of growth and decay in the amplitude of eastward traveling waves are found in the water vapor and meteorological data at levels of the upper troposphere, with zonal waves four, five and six being predominant modes. The water vapor data are compared with derived potential vorticity (PV) fields, with strong anticorrelation observed in middle and high latitudes. The results are consistent with model paradigms for the structure and evolution of baroclinic disturbances, coupled with the known characteristics of high PV and low water vapor mixing ratios in lower stratospheric air parcels and the reverse for upper tropospheric air.
Observations of large reductions in the NO/NOy ratio near the mid‐latitude tropopause and the role of heterogeneous chemistry
Authors: Keim E. R. , Fahey D. W. , Del Negro L. A. , Woodbridge E. L. , Gao R. S. , Wennberg P. O. , Cohen R. C. , Stimpfle R. M. , Kelly K. K. , Hintsa E. J. , Wilson J. C. , Jonsson H. H. , Dye J. E. , Baumgardner D. , Kawa S. R. , Salawitch R. J. , Proffitt M. H. , Loewenstein M. , Podolske J. R. , Chan K. R.
Published: February 2004

During the 1993 NASA Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE), anomalously low nitric oxide (NO) was found in a distinct sunlit layer located above the mid‐latitude tropopause. The presence of a significant amount of reactive nitrogen (NOy) in the layer implies the systematic removal of NO, which is without precedent in stratospheric in situ observations. Large increases in measured chlorine monoxide (ClO) and the hydroperoxyl radical (HO2) also were observed in the layer. Heterogeneous reaction rate constants of chlorine nitrate (ClONO2) with hydrogen chloride (HCl) and H2O to form nitric acid (HNO3) on sulfate aerosol are enhanced in the NO removal layer by local increases in H2O and aerosol surface area. The associated conversion of NOx (= NO + NO2) to HNO3 is the most likely cause of the observed low NO and NOx/NOy values and high ClO values.
Influence of fluids on the elastic properties of sandstone at high pressure and temperature
Authors: Lebedev E. B. , Ryzhenko B. N. , Dorfman A. M. , Zebrin S. R. , Sokolova N. T. , Burkhardt H. , Mörig R. , Wulff A.
Published: February 2004

Recent geophysical investigations revealed the existence of zones in the Earth's crust with anomalous low seismic velocities. The nature of these zones is still under debate, although there are some indications that they might be correlated with changes of porosity and hydraulic permeability as well as with the presence of fluids. To test this hypothesis an experimental study of the influence of fluids on the elastic properties of sandstone as a model rock system for crustal conditions was performed. The investigations were conducted at a hydrostatic pressure of 300 MPa and temperature up to 850°C. They revealed the strong influence of different fluids (neutral, acid, alkaline) on the elastic properties of sandstone as a result of mineral reactions, phase transition and of changes of the rock microstructure. To evaluate the phase changes the computer modeling was performed. Together with investigations of the role of volatiles in geochemical processes including the influence of mineralized fluids on mineral reactions these studies will contribute to the petrophysical and geochemical interpretation of geophysical measurements.
Magnetic field orientation effects on the standoff distance of Earth's bow shock
Authors: Cairns Iver H. , Lyon J. G.
Published: February 2004

Three‐dimensional, global MHD simulations of solar wind flow onto a prescribed magnetopause obstacle are used to show that a bow shock's nose location as and the relative subsolar magnetosheath thickness Δms/amp are strong functions of the IMF cone angle θ (between vsw and Bsw) and the Alfven Mach number MA For a given MA the shock is more distant for higher θ (restricted to the interval 0–90° by symmetries), while as/amp and Δms/amp increase with decreasing MA for θ ≳ 20° but decrease with decreasing MA for θ ∼ 0°. Large differences in Δms/amp are predicted between θ = 0° and 90° at low MA, with smaller differences remaining even at MA ∼ 10. The θ = 0° results confirm and extend the previous work of Spreiter and Rizzi [1974]. The simulations show that successful models for the subsolar shock location cannot subsume the dependences on MA and θ into a sole dependence on Mms. Instead, they confirm a recent prediction [Cairns and Grabbe, 1994] that as/amp and Δms/amp should depend strongly on θ and MA for MA ≲ 10 (as well as on other MHD variables). Detailed comparisons between theory and data remain to be done. However, preliminary comparisons show good agreement, with distant shock locations found for low MA and large θ ≳ 45° and closer locations found for θ ≲ 20° even at MA ∼ 8.
Global ionosphere perturbations monitored by the Worldwide GPS Network
Authors: Ho C. M. , Mannucci A. J. , Lindqwister U. J. , Pi X. , Tsurutani B. T.
Published: February 2004

For the first time, measurements from the Global Positioning System (GPS) worldwide network are employed to study the global ionospheric total electron content (TEC) changes during a magnetic storm (November 26, 1994). These measurements are obtained from more than 60 world‐wide GPS stations which continuously receive dual‐frequency signals. Based on the delays of the signals, we have generated high resolution global ionospheric maps (GIM) of TEC at 15 minute intervals. Using a differential method comparing storm time maps with quiet time maps, we find that significant TEC increases (the positive effect) are the major feature in the winter hemisphere during this storm (the maximum percent change relative to quiet times is about 150%). During this particular storm, there is almost no negative phase. A traveling ionospheric disturbance (TID) event is identified that propagates from the northern subauroral region to lower latitudes (down to about 30°N) at a speed of ∼460 m/s. This TID is coincident with significant increases in the TEC around the noon sector. We also find that another strong TEC enhancement occurs in the pre‐dawn sector in the northern hemispheric subauroral latitudes, in the beginning of the storm main phase. This enhancement then spreads into almost the entire nightside. The nighttime TEC increase in the subauroral region is also noted in the southern hemisphere, but is less significant. These preliminary results indicate that the differential mapping method, which is based on GPS network measurements, appears to be a powerful tool for studying the global pattern and evolution process of the entire ionospheric perturbation.
Mid‐latitude noctilucent cloud observations by lidar
Authors: von Cossart G. , Hoffmann P. , von Zahn U. , Keckhut P. , Hauchecorne A.
Published: February 2004

Regular zenith‐directed Rayleigh lidar soundings at Juliusruh (54.63°N, 13.38°E) in summer 1995 indicated the presence of overhead noctilucent clouds (NLC) during three nights in June. For two of these nights, NLC were also sighted visually near the northern horizon. For one of the lidar‐observed NLC events, temperature profiles were obtained immediately prior to the appearance of the NLC. These show at 81 km a strong cooling of about 30 K within two hours. The wind velocity and direction during the NLC events were inferred from daytime common‐volume MF radar wind measurements. During the occurrence of the NLCs the interpolated MF radar wind was south‐westwards directed with velocities between 9–32 m/s. Different from high latitudes, NLCs can be observed by lidar at mid‐latitudes in total darkness. Therefore, mid‐latitude observations hold the promise to allow a characterisation of NLC particle size distributions and particle densities even with comparatively simple, multi‐color lidars.
Anharmonicity of forsterite and the thermal pressure of insulators
Authors: Anderson Orson L.
Published: February 2004

A recent paper [Bouhifd et al., 1996] has given the measured thermal expansivity, α, of forsterite up to 1900 K, which is 2.5 times the Debye temperature, Θ. Using the measured α, Bouhifd et al. [1996] found that the αKT curve (where KT is the isothermal bulk modulus) is parallel to the T axis, showing that (∂(αKT)/∂T)P = 0. Using these results, I show that (∂Cv/∂T)P > 0 at high T, requiring Cv to be anharmonic, is in agreement with other recent work. Further, I show that Bouhifd et al.'s [1996] work requires that all vestiges of anharmonicity be absent in the thermal pressure and the thermal expansivity of forsterite. Thus it is proven that forsterite is a solid that is both anharmonic in Cv and quasiharmonic in the thermal pressure in the high‐temperature region.
Crustal deformation at the Nankai Subduction Zone, southwest Japan, derived from GPS measurements
Authors: Tabei Takao , Ozawa Taku , Date Yuki , Hirahara Kazuro , Nakano Takehide
Published: February 2004

The interseismic strain accumulation process at the Nankai subduction zone, the plate‐boundary region between the Philippine Sea and the Eurasian plates off Southwest Japan, has been revealed by GPS measurements spanning five years. Large strains characterized by a contraction of 2.2 ∼ 3.4 × 10−7/yr have accumulated in a narrow region extending parallel to the strike of the plate boundary. The contraction axes lie in a northwest‐southeast direction nearly parallel to the direction of relative motion of the Philippine Sea plate with respect to the Eurasian plate. GPS‐derived deformation data, including both rate and direction, are consistent with the expected earthquake deformation cycle at the Nankai subduction zone. Current interplate coupling in this zone is strong; much of the relative plate motion has been taken up by deformation of the marginal parts of the Eurasian plate.
Deformation related to the 1994 Liwa Earthquake derived from geodetic measurements
Authors: Duquesnoy Th. , Bellier O. , Kasser M. , Sébrier M. , Vigny Ch. , Bahar I.
Published: February 2004

The deformation field around the southern Great Sumatran Fault (GSF) in the vicinity of the February 15 1994 Mw=6.8 Liwa earthquake has been determined by geodetic survey. Geodetic trilateration and Global Positioning System (GPS) measurements were carried out in July 1991 in the Ranau Lake‐Liwa area. A second geodetic survey reoccupied the same stations in September 1994, seven months after the Liwa earthquake. Despite the limited number of benchmarks, most of the measured displacement appears to be related to the 1994 earthquake. In addition, we show that most of this co‐seismic deformation is localized along a narrow zone corresponding to the GSF segment with a right lateral displacement of about 70 cm.
Self‐organized fluid flow through heterogeneous networks
Authors: Bernabé Yves
Published: February 2004

The purpose of this paper is to assess the coupled effect of fluid flow and dissolution/precipitation reactions in heterogeneous porous media at semi‐macroscopic scales (i.e. several tens of grains). Are the dissolution/precipitation processes roughening or smoothing at these scales? Does self‐organization arise from the coupling of these processes with fluid flow? What is the effect of heterogeneity (i.e. of the variance of the pore size distribution assuming a constant mean)? The dissolution/precipitation processes were modeled as producing a change of the local pore radius proportionally to the local flux. It was found that both precipitation and dissolution lead to more heterogeneous, anisotropic pore structures (parallel to the macroscopic flow for dissolution, perpendicular for precipitation). One new interesting observation was that precipitation generated an almost uniform flux field. To a lesser extent, dissolution also tended to smooth the pressure gradient field. The possibility of uniform flux or pressure gradient fields could have important implications in hydrology or in technical applications such as filter plugging or acid treatment of oil reservoirs.
The acceleration of slow coronal mass ejections in the high‐speed solar wind
Authors: Gosling J. T. , Riley Pete
Published: February 2004

Using a one‐dimensional hydrodynamic code we have simulated the radial evolution of heliospheric disturbances produced by slow coronal mass ejections (CMEs) embedded in much faster leading and trailing ambient solar wind. We find that pressure gradients induced by initial speed differences between slow CMEs and faster ambient wind can produce large accelerations of the CMEs, eventually nearly up to the speed of the ambient wind. The compressions, rarefactions, and shocks associated with this acceleration persist to large heliocentric distances. Comparison with observations reveals that such effects helped accelerate two of the six high‐latitude CME events observed by Ulysses.
Seismic velocity, density, thermal conductivity and heat production of cores from the KTB Pilot Hole
Authors: Huenges Ernst , Lauterjung Jörn , Bücker Christian , Lippmann Erich , Kern Hartmut
Published: February 2004

This paper presents a compilation of about 50000 petrophysical data measured on core samples from the Continental Deep Drilling Project (KTB) of the Federal Republic of Germany. The pilot hole provided a quasi‐continuous core profile, 4000 m in length, for measurement of several petrophysical parameters and analysis of quantitative mineral compositions, with sampling rates between 0.5 and 5 m. The variances of the data within the lithological units, which at KTB are mainly gneisses and metabasites, are greater than the experimental errors and are influenced by rock fabric, possible in‐situ variations, e.g., pressure and water content, or the small‐scale mineral composition. High space resolution measurements of velocity and density provide an example how relaxation effects, heterogeneity, and anisotropy can influence the petrophysical data. Density, thermal conductivity and the content of uranium, thorium and potassium are related to the contents of quartz and amphibole. Porosity and sound velocity, measured under normal conditions, do not match those expected from consideration of the mineral composition. Petrophysical properties measured in the field are normally representative of larger volumes than those measured in the laboratory. The extensive petrophysical investigations on KTB‐cores provide variances that are related to the representative rock units and could thus help address this scaling problem. The variances could, for example, be used for constructing velocity profiles or input parameters for modelling.
patterns of dust transport to the Grand Canyon
Authors: de P. Vasconcelos Luis A. , Kahl Jonathan D. W. , Liu Desong , Macias Edward S. , White Warren H.
Published: February 2004

Dust particles in the 2.5 µm to 15 µm diameter range contribute to regional haze that sometimes impairs visibility at the Grand Canyon and other National Parks in the southwestern U.S. The proportion of airborne dust that is attributable to land modification is unknown, but can be expected to increase as a consequence of the region's rapid population growth. This note examines the upwind histories of air masses bringing high coarse‐particle concentrations to the Grand Canyon over a five‐year monitoring period. Although arid and semi‐arid lands extend in all directions, and the fastest airflows generally have a northerly component, high dust concentrations are most common in air arriving from the southwest, where development has been concentrated. This empirical association suggests that the expansion of suburban and agricultural lands is raising dust levels at the Grand Canyon.
Satellite observations of atmospheric variances: A possible indication of gravity waves
Authors: Wu D. L. , Waters J. W.
Published: February 2004

The Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite has now produced the first global maps of small‐scale variances in the middle atmosphere. Initial analyses are presented here that suggest these variances are due to gravity waves, and the technique used to extract gravity wave information from saturated radiance measurements is described. Observations at 30–80km altitudes show that the variances of horizontal scales less than ∼100km are strongly correlated with upper tropospheric convection, surface topography and stratospheric jetstreams. MLS monthly averages during solstice periods suggest that the normalized variance amplitude grows exponentially with height in the stratosphere, and saturates in the mesosphere as expected from wave breaking and dissipation at these altitudes.
Gravity‐wave‐scale temperature fluctuations seen by the UARS MLS
Authors: Wu D. L. , Waters J. W.
Published: February 2004

The Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) has observed small‐ and meso‐scale temperature fluctuations during a special limb‐tracking operation. Analysis of late‐December 1994 data shows that there exist significant radiance fluctuations with horizontal scales of 10's–100's km associated with upper tropospheric convection and the stratospheric polar vortex. Phase coherence and amplitude growth with height are observed, showing the presence of vertically propagating waves. Variance maps generated for the limb‐tracking days further show that the MLS instrument can measure horizontal temperature variations at scales less than 100 km in both limb‐scanning and limb‐tracking observation modes.
Determination of the heliospheric axis orientation‐An opportunity for the Cassini Mission to Saturn
Authors: Mall U. , Fichtner H. , Hamilton D. C. , Rucinski D.
Published: February 2004

Pick‐up ion measurements that could be made during the interplanetary cruise of the Cassini spacecraft to Saturn offer a unique opportunity to determine the orientation of the upwind‐downwind axis of the heliosphere. The method and results of a feasibility study are presented. The study shows that pick‐up oxygen is the best candidate for such a measurement. Model computations are used to predict the actual pick‐up oxygen fluxes which instruments on the Cassini spacecraft would see along its trajectory for several currently debated upwind‐downwind axis orientations.
Imaging axi‐symmetric TAG‐like structures by transient electric dipole seafloor electromagnetics
Authors: Yu Liming , Edwards R. N.
Published: February 2004

We investigate the response of a conductive axi‐symmetric TAG‐like structure to a transient in‐line electric dipole‐dipole seafloor system. Coupled differential equations for the azimuthal electric and magnetic fields are solved by finite differences iteratively for an off‐axis source. We introduce an electrical model of TAG and investigate the sensitivity of first arrival data to the conductivities of the the sulfide layer and the deeper root structure. The transmitter dipole is located on one side of the mound, 200 m from its center, while receiving locations are on the mound itself, or on an offset profile 200 m from the center of the mound, on the opposite side of it from the transmitter. We show that the time taken for a first arrival to be seen at the receiver following a transition in the transmitter is a robust estimator of seafloor conductivity because it is insensitive to the effects of topography. Times of first arrivals observed on the mound are sensitive to the conductivity of both the root structure and the sulfide. Times of first arrivals at locations on the far side of the mound are sensitive to the properties of the root structure only. Travel time effects are more pronounced on transmitter‐receiver paths which pass through the TAG than on paths which graze the structure. Given sufficient data, the interior of TAG may be imaged using tomographic algorithms.
A wave‐driven nonlinear current in the E‐region ionosphere
Authors: Oppenheim Meers
Published: February 2004

Ionospheric two‐stream waves and gradient‐drift waves nonlinearly drive a large‐scale (D.C.) current in the E‐region ionosphere. This current flows parallel to, and with a comparable magnitude to, the fundamental Pedersen current. The underlying mechanism is quite simple, E‐region plasma waves generate oscillating electric fields causing electrons to E×B drift perpendicular to the wave's propagation direction. On the wave's maxima, electrons drift with the same velocity but in the opposite direction as they do on the minima. Since more electrons exist at the maxima than at the minima, a net current results. Wave‐driven currents will substantially modify the large scale dynamics of the E‐region during highly active periods. They will affect the behavior of E‐region waves, possibly causing type I radar echoes to have a Doppler velocity close to the acoustic speed. Wave‐driven currents may be the source of non‐sinusoidal electric field measurements made in gradient‐drift waves by rocket‐borne probes.