bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiple morphodynamic equilibria in tidal embayments
Authors: Schuttelaars H. M. , de Swart H. E.
Published: February 2004

The possible morphodynamic equilibria of tidal embayments are investigated within the framework of a one‐dimensional model The equilibria are defined by a steady profile of the erodible bottom. The extension with respect to earlier studies is that the embayments have arbitrary lengths L with respect to the tidal wavelength. This implies a much richer dynamics due to the possibility of tidal resonance and new sediment transport contributions that are caused by internally generated overtides and residual currents. If the system is only forced by an externally prescribed M2 tide at the seaward boundary, a unique morphodynamic equilibrium is obtained for all embayment lengths smaller than the frictional length scale of the tide. Bottom friction causes tidal resonance to occur for a shorter length than a quarter of the frictionless tidal wavelength. This shift is smaller than would occur in the case of a fixed bed profile since the equilibrium condition induces larger water depths. If an externally prescribed overtide is added to the forcing, more than one type of morphodynamic equilibria can be found. For L values smaller than the M4 resonance length scale the bottom profiles are strongly concave, with locally large water depths, and the water motion resembles a standing tidal wave. For longer embayments another type of equilibria, characterized by a weakly concave bottom profile and a traveling tidal wave, appears. For sufficiently strong amplitudes of the externally prescribed M4 tide, multiple morphodynamic equilibria are found. The maximum L, beyond which morphodynamic equilibria cease to exist, decreases with increasing influence of external overtide and bottom friction. These model results show an overall good agreement with field observations.
Study of the dynamics of wind‐driven transports into the Yellow Sea during winter
Authors: Riedlinger S. K. , Jacobs G. A.
Published: February 2004

Synoptic wind stresses and surface heat flux force a three‐dimensional numerical ocean model of the Yellow Sea/East China Sea dynamics. The basin response to winter winds is examined through the model output fields. Northerly wind bursts are a frequent winter occurrence in this region. The wind bursts develop a southward flowing current along the Chinese coast as the wind stress forces fluid out of the Bohai Sea. A north‐south pressure gradient develops as surface water travels from the Bohai Sea southward, decreasing the elevation in the north relative to the elevation in the south. Also, a westward pressure gradient across the Yellow Sea basin develops in response to the wind forcing. The southward pressure gradient forces a bottom flow in the direction opposite to the wind stress. This return flow is concentrated in a deep trough located on the eastern side of the basin. After the northerly wind ceases the northward transport increases pressure in the north with the result of slowing or stopping the flow northward. At times, pressure increases in the north sufficiently to force a southward flow in the trough. Following strong northerly wind events, northward transport into the Yellow Sea occupies the entire trough and may extend into the northern Yellow Sea/Bohai Sea. During periods of calm winds, northward flow usually does not penetrate beyond 34°N in the Yellow Sea. This flow is northward on the western side of the island of Cheju. The flow turns eastward north of Cheju and then southward, passing through the Cheju‐Korea Strait. The wind stress along the Korean coast often deviates from the northerly wind stress over the rest of the Yellow Sea. Thus the flow along the Korean coast is not as closely related to the northerly wind bursts as the flow along the Chinese coast. The winter circulation in the Yellow Sea is predominately a barotropic response to winds with fluctuations due to Ekman flow.
Tidal asymmetry in an estuarine pycnocline: Depth and thickness
Authors: Cudaback Cynthia N. , Jay David A.
Published: February 2004

Tidal variations in estuarine stratification are revealed by the depth and thickness of the density interface. The depth of the interface may be predicted using an inviscid two‐layer model that combines baroclinic estuarine circulation with barotropic tidal currents [Helfrich, 1995]. Here we present results from a two‐layer model modified to include the effects of bottom friction and interfacial mixing. Modeled layer thickness and speed compare favorably with prior analytic studies [Farmer and Armi, 1986; Pratt, 1986]. We use a bulk Richardson number criterion to estimate the thickness of the pycnocline from two‐layer model results; the predicted pycnocline depth and thickness compare remarkably well with observations. We also investigate the effects of changing bottom friction and barotropic currents on the pycnocline thickness.
The generation of internal waves on the continental shelf by Hurricane Andrew
Authors: Keen Timothy R. , Allen Susan E.
Published: February 2004

Observed currents, temperature, and salinity from moored instruments on the Louisiana continental slope and shelf reveal multiple baroclinic oscillations during Hurricane Andrew in August 1992. These measurements are supplemented by numerical models in order to identify possible internal wave generation mechanisms. The Princeton Ocean Model is run with realistic topography, stratification, and wind forcing to extend the observations to Mississippi Canyon and other areas on the shelf. A two‐layer isopycnal model is used with idealized topography and spatially uniform winds to isolate internal waves generated in and around the canyon. The combination of the observations and the results from the numerical models indicates several possible mechanisms for generating long internal waves: (1) near‐inertial internal waves were generated across the slope and shelf by dislocation of the thermocline by the wind stress; (2) interaction of inertial flow with topography generated internal waves along the shelf break, which bifurcated into landward and seaward propagating phases; (3) downwelling along the coast depressed the thermocline; after downwelling relaxes, an internal wave front propagates as a Kelvin wave; and (4) Poincaré waves generated within Mississippi Canyon propagate seaward while being advected westward over the continental slope. These processes interact to produce a three‐dimensional internal wave field, which was only partly captured by the observations.
Coherence of internal tide modulations along the Hawaiian Ridge
Authors: Mitchum Gary T. , Chiswell Stephen M.
Published: February 2004

Long time series of sea level from tide gauges along the north side of the Hawaiian Ridge and shorter series of dynamic heights inferred from inverted echo sounders moored just north of the main Hawaiian Islands are examined for evidence of internal tides at the M2 frequency. We find that the amplitudes and phases of the M2 tidal components have low‐frequency variability, which is consistent with a superposition of an internal tide with the larger barotropic tide. Further, the low‐frequency variability is correlated with low‐frequency changes in the depth of the pycnocline, which suggests a simple physical mechanism to account for the low‐frequency modulations in the internal tidal amplitude. These modulations are coherent for long distances along the Hawaiian Ridge, indicating a coherent generation of the internal tide that is consistent with acoustic observations in the North Pacific and with recent analyses of sea surface heights from satellite altimetry.
Role of morphologic feedback in surf zone sandbar response
Authors: Plant N. G. , Freilich M. H. , Holman R. A.
Published: February 2004

Several aspects of feedback mechanisms associated with surf zone sandbar response have been characterized using bathymetric surveys, sampled approximately monthly over a 16‐year period at the Army Corps of Engineers' Field Research Facility (North Carolina). The measured bathymetry was alongshore averaged and modeled by the superposition of two Gaussian‐shaped sandbars on an underlying planar slope. A third, half‐Gaussian‐shaped bar represented steepening at the shoreline. The rms error between the measured bathymetry and the profile model was 0.10 m (estimated over 322 different surveys). The model explained 99% of the profile variance that remained after first removing the linear, cross‐shore trend from each observed profile. Bar response, which was extracted from the modeled profiles, was compared to a local hydrodynamic forcing variable Γ (Γ was defined as the ratio of the wave height to water depth, evaluated at bar crest locations). At low values of Γ (i.e., nonbreaking conditions), bars migrated onshore, and their amplitude tended to decay. At high values of Γ (i.e., breaking conditions), bars migrated offshore, with relatively little change in amplitude. The transition between onshore and offshore migration occurred at a value of Γ that was consistent with the onset of wave breaking. Bar migration was associated with a stabilizing feedback mechanism, which drove bar crests toward an equilibrium position at the wave breakpoint. However, we observed that the rate of bar response showed no reduction for any nonzero choice of Γ, indicating that bars never reached equilibrium. Systematic bar amplitude decay was observed under nonbreaking conditions. Bar amplitude decay could drive Γ farther away from breaking conditions, allowing further bar amplitude decay. This is a destabilizing feedback mechanism, potentially leading to bar destruction.
Cross‐shelf eddy heat transport in a wind‐free coastal ocean undergoing winter time cooling
Authors: Pringle James M.
Published: February 2004

A steady state cross‐shelf density gradient of a wind‐free coastal ocean undergoing winter time cooling is found for cooling and geometries which do not vary in the along‐shelf direction. The steady state cross‐shelf density gradient exists even when the average density of the water continues to increase. The steady state density gradient can be attained in less than a winter for parameters appropriate to the mid‐Atlantic Bight. The cross‐shelf eddy‐driven buoyancy fluxes which cause this steady state gradient are found to depend critically on bottom friction and bottom slope, and the coastal polyna solutions of Chapman and Gawarkiewicz [1997] are significantly modified by this dependence in the limit of polynas with a large alongshore extent. Bottom friction retards the cross‐shelf propagation of eddies, so that the buoyancy transport is no longer carried by self‐advecting eddy pairs but mixed across the shelf by interacting eddies. The eddy interaction changes the length scale of the eddies until it is the lesser of the Rhines arrest scale or an analogous frictional arrest scale. The estimates of the steady state cross‐shelf density gradient are found to compare well with numerical model results.
Nonlinear energy fluxes and the finite depth equilibrium range in wave spectra
Authors: Resio Donald T. , Pihl Jorgen H. , Tracy Barbara A. , Vincent C. Linwood
Published: February 2004

Results using a finite depth representation of Webb's [1978] transformation of Herterich and Hasselmann's [1980] equation for the rate of change of energy density in a random‐phase, spatially homogeneous, finite depth wave spectrum show that the equilibrium range in finite depth preserves a k−2.5 form consistent with Resio and Perrie's [1991] deepwater results and that the relaxation time toward an equilibrium range in shallow water is considerably faster than in deep water. Results from this finite depth nonlinear energy transfer representation compared to previously calculated results of analytical spectral situations show agreement, and the finite depth Zakharov [1968] and Herterich and Hasselmann [1980] forms are shown to be numerically equivalent. Spectral analyses of matching wave spectra sets at sites in 8 and 18 m depths at Duck, North Carolina, show a k−2.5 shape in the equilibrium range and show energy gains above the spectral peak and at high frequencies with energy loss in the midrange of frequencies near the spectral peak, consistent with four‐wave interactions. Spectral energy losses between these two sites correlate with spectral energy fluxes to high frequencies, again consistent with four‐wave interactions. The equilibrium range coefficient shows strong dependence on friction velocity at both gages.
Observations of intratidal variability of flows over a sill/contraction combination in a Chilean fjord
Authors: Valle‐Levinson Arnoldo , Jara Fernando , Molinet Carlos , Soto Doris
Published: February 2004

Underway velocity measurements were carried out for the first time in a Chilean fjord using an acoustic Doppler velocimeter with the purpose of elucidating the intratidal variability of flows through a pass, Paso Galvarino. The pass included a sill, where the bottom sloped by roughly 30%, and a coastline contraction of ∼90%. The relatively small dimensions of the pass allowed for rapid sampling of the flow evolution throughout the tidal cycle. The backscattered sound signal from the velocimeter and from an echo sounder were used to describe the vertical excursions of the pycnocline throughout the domain and to identify regions of enhanced vertical mixing within the pass. The spatial variability of the flow in the pass was consistent with uniform two‐layer flow. At the narrowest section of the contraction the pycnocline dropped sharply around both maximum flood and maximum ebb, while the flow accelerated downstream relative to the tidal flow. The slope of the pycnocline changed sign from flood to ebb, which was atypical of other fjord observations but could be explained by the transitions from subcritical to supercritical flow. These transitions switched location at either side of the narrowest section of the contraction. Leeward of this section, increased sound backscatter suggested intensified turbulence that extended over a greater area during ebb than during flood because the distance between the point of pycnocline drop and the end of the pass was longer during ebb. Enhanced vertical mixing within the pass was reflected in the tidally averaged fields by a three‐layer flow that consisted of near‐surface and near‐bottom flow converging toward the pass and flow around the pycnocline diverging away from the pass.
Recent Arctic change simulated with a coupled ice‐ocean model
Authors: Zhang Yuxia , Hunke Elizabeth C.
Published: February 2004

A high‐resolution coupled ice‐ocean model, forced with 1983–1997 European Center for Medium‐Range Weather Forecasts data, is used to explore recent Arctic change. In response to changes in atmospheric circulation, stronger cyclonic circulation is present in Arctic sea ice and upper ocean in the late 1980s and early 1990s as compared to the early 1980s, manifested as the weakening of the Beaufort Gyre and the shifting of the Transpolar Drift Stream. Corroborating previous studies, ice divergence in the central Arctic Ocean is highly correlated with surface atmospheric vorticity in summer, suggesting that summer atmospheric circulation is more important than winter for inducing interannual variability of the central Arctic ice divergence and growth rate. The weakening of the summer atmospheric cyclonic circulation from the earlier period to the later period over the Canadian Basin leads to decreased ice divergence there, which then has significant impact on the ice growth rate by reducing ice formation in fall and winter. For the 15 year period, variability in the spatial distribution of ice concentration and thickness is largely determined by the ice dynamics, which is dominated by the atmospheric circulation, except over the Greenland and Labrador Seas, where the ice thermodynamics plays a more important role. The model simulation supports the recent observations of increased presence of Atlantic Water in the Arctic Ocean. The spatial pattern of warming and salinization of the Arctic Atlantic layer follows the pathways of the strengthened boundary currents along the continental slopes and over the ridges, thereby slowly spreading more Atlantic Water downstream from the eastern Arctic into the western Arctic. The integrations with and without surface temperature restoring indicate that the restoring leads to a warmer ocean surface temperature. However, the restoring has little impact on its interannual variability for the 15 year period.
Shear instability in the St. Lawrence Estuary, Canada: A comparison of fine‐scale observations and estuarine circulation model results
Authors: Bourgault Daniel , Saucier Francois J. , Lin Charles A.
Published: February 2004

A three‐dimensional numerical model was used to predict the timing and the location of shear instabilities in the St. Lawrence Estuary. This model suggests that significant mixing occur during flood tides in the upper estuary. This mixing is associated with a strong bottom density current made of the cold Gulf of St. Lawrence intermediate waters flowing under the St. Lawrence mixed surface waters. Guided by these results, a field experiment was undertaken in summer 1997 to verify this and to document the conditions that favor the development of instabilities. The instabilities were found as predicted and documented from acoustic imaging, current profiler, and density measurements. The instabilities first develop in the form of wavelike disturbances before they break, like Kelvin‐Helmholtz instabilities. The unstable waves have wavelength of ≈140–150 m and extend vertically between 10 and 25 m. The fine‐scale observations of the semidiurnal evolution of the vertical structure of currents and density at the experimental site are compared with the numerical results. The model reproduces accurately the tidal variability of the currents but underestimates by a factor of 2 the amplitude of the density fluctuations. The general patterns of the shear squared S2 and the buoyancy frequency squared N2 are reasonably well reproduced by the model, but their intensities are ≈2 times smaller than the observations. This difference is attributed to the limited vertical resolution of the model at the pycnocline. However, the modeled Richardson numbers, Ri ≡ N2S−2, are reasonably well reproduced and appeared to be useful for the prediction of instabilities in such a complex environment.
Analysis of FORTE data to extract ionospheric parameters
Authors: Roussel‐Dupré Robert A. , Jacobson Abram R. , Triplett Laurie A.
Published: February 2004

The ionospheric transfer function is derived for a spherically symmetric ionosphere with an arbitrary radial electron density profile in the limit where the radio frequencies of interest ω are much larger than the plasma frequency ωpe. An expansion of the transfer function to second order in the parameter X (= ω2pe/ω2) is carried out. In this limit the dispersive properties of the ionosphere are manifested as a frequency‐dependent time of arrival that includes quadratic, cubic, and quartic terms in 1/ω. The coefficients of these terms are related to the total electron content (TEC) along the slant path from transmitter to receiver, the product of TEC and the longitudinal magnetic field strength along the slant path, and refractive bending and higher‐order electron density profile effects, respectively. By fitting the time of arrival versus frequency of a transionospheric signal to a polynomial in 1/ω it is possible to extract the TEC, the longitudinal magnetic field strength, the peak electron density, and an effective thickness for the ionosphere. This exercise was carried out for a number of transionospheric pulses measured in the VHF by the FORTE satellite receiver and generated by the Los Alamos Portable Pulser. The results are compared with predictions derived from the International Reference Ionosphere and the United States Geological Survey geomagnetic field model.
CRRES observation of Pi2 pulsations: Wave mode inside and outside the plasmasphere
Authors: Takahashi Kazue , Ohtani Shin‐ichi , Hughes W. Jeffrey , Anderson Roger R.
Published: February 2004

The relationship between the plasma density and the mode of Pi2‐band (40–150 s) waves in the magnetosphere is studied using measurements made from the Combined Release and Radiation Effects Satellite (CRRES). We focus on wave events that occurred from 1100 to 1500 UT on February 17, 1991, on a single CRRES orbit. During this interval the low‐latitude ground station Kakioka (L = 1.2) observed a series of Pi2 pulsations within 2 hours of the local time of CRRES. When CRRES was in the plasmasphere, the poloidal components (Ey, Bx, Bz) at the spacecraft had high coherence with the Pi2 pulsations seen at Kakioka, where Ey, Bx, and Bz are, respectively, the azimuthal component of the electric field and the radial and compressional components of the magnetic field. This relationship lasted until the spacecraft completely moved into the plasmatrough. Waves were present in the plasmatrough, but they did not maintain high coherence with the ground Pi2. For a plasmaspheric event, Ey and Bz oscillated nearly in quadrature, in support of a radially standing wave mode. These results confirm previous AMPTE/CCE magnetic field studies that suggested that fast‐mode waves in the inner magnetosphere are the source of low‐latitude Pi2 pulsations. In addition, the CRRES results strongly suggest that the outer limit of the “inner magnetosphere” is the plasmapause. The observed wave properties are discussed in light of possible Pi2 mechanisms, including plasmaspheric cavity modes and braking of periodic bursty bulk flows.
Day the solar wind almost disappeared: Magnetic field fluctuations, wave refraction and dissipation
Authors: Smith Charles W. , Mullan Dermott J. , Ness Norman F. , Skoug Ruth M. , Steinberg John
Published: February 2004

On May 11, 1999, the Advanced Composition Explorer (ACE) spacecraft observed a rarefied parcel of solar wind. This has come to be known as “The Day the Solar Wind Disappeared.” Little if any change is seen in the large‐scale interplanetary magnetic field during this time, but the magnetic field fluctuations are depressed and significantly more transverse to the mean field. The high Alfvén speed resulting from the constant field intensity and low ion density enhances wave refraction, and we examine this as a possible explanation for the fluctuation properties. The solar wind possesses a very low proton β, thereby separating the cyclotron and ion inertial length scales and permitting a test of possible dissipation dynamics. We find that the test favors the ion inertial scale theories.
Similarity transformation‐based analysis of atmospheric models, data, and inverse remote sensing algorithms
Authors: Meier R. R. , Picone J. M. , Drob D. P. , Roble R. G.
Published: February 2004

The similarity transformation (ST) defines a new class of robust and stable parametric functions with embedded physical shape information to optimize flexibility in fitting or inverting data. The similarity transformation also permits the extraction of information on the shape of a particular class of physical functions, thereby providing the basis for comparing alternative models and for analyzing the information content of data. We employ these properties of similarity transformations to study differences between state‐of‐the‐art physics‐based atmospheric models (the thermosphere ionosphere electrodynamic general circulation model, or TIEGCM) and empirical atmospheric models (Mass Spectrometer Incoherent Scatter, or MSIS) and to investigate the universality of these models; we examine the role of noise in determining acceptable resolution for faithful retrieval of physical properties; and we measure the performance of MSIS‐based forward models for inversion of ultraviolet remote sensing of the neutral upper atmosphere. The similarity transform method proves to be a valuable new tool for identifying common and discrepant properties of the models. Further, the ST method shows that TIEGCM and MSISE‐90 profiles embody similar shape information and that a suitable ST parameterization can be constructed that approximates profiles from either model to within a few percent accuracy.
Multisatellite observations of rapid subauroral ion drifts (SAID)
Authors: Anderson P. C. , Carpenter D. L. , Tsuruda K. , Mukai T. , Rich F. J.
Published: February 2004

We present the first conjugate observations of subauroral ion drifts (SAID) in the magnetosphere (∼9000 km altitude) and ionosphere and coincident measurements by four ionospheric satellites. The parameters measured include ion drifts, energetic precipitating electrons and ions, and the magnetic field perturbations associated with field‐aligned currents. Observations indicate that SAID are very coherent features that occur simultaneously over a large magnetic local time (MLT) range, from at least 1600 to 2400 MLT. The equatorward extent of SAID, the ion precipitation, and the region 2 field‐aligned currents (FAC) flowing into the ionosphere are all shown to be coincident at all MLT locations where SAID are observed. They also appear to be closely related to the conductivity distribution in the subauroral ionosphere and the midlatitude trough. This is interpreted as an indication that their latitudinal distribution is a consequence of the subauroral conductivity structure and the movement of the plasma sheet ion and electron boundaries. Conjugate measurements at diverse altitudes when mapped along field lines are nearly identical, indicating the absence of significant field‐aligned potential drops. Temporally separated SAID measurements in similar MLT regions show a reduction with time in the field‐aligned current densities with little reduction in the potential drop across the SAID. We interpret the results as an indication that the magnetosphere acts as a current generator in which large FAC are initially required to support the electric field gradient in a SAID event. Subsequent evolution in the E and F regions produces large conductivity gradients that are in the right sense to remove the intense FAC requirement but maintain the large subauroral electric fields. The reported potential drops in the subauroral region can be a significant fraction of the total, up to 60 kV or more, and must be taken into account when deriving any magnetospheric convection pattern.
Simulations of diffuse aurora with plasma sheet electrons in pitch angle diffusion less than everywhere strong
Authors: Chen Margaret W. , Schulz Michael
Published: February 2004

To investigate the spatial and spectral structure of the diffuse aurora during a model geomagnetic storm characterized by random impulses in the cross‐magnetospheric convection electric field, we simulate the bounce‐averaged drift motion and precipitation of plasma sheet electrons. Bounce‐averaged drift trajectories are computed from a Hamiltonian formulation in which we have treated the plasma sheet electrons as though they were undergoing strong pitch angle diffusion in Dungey's model magnetosphere (dipole field plus uniform southward Bz). Using the simulation results, we map phase space densities from near the nightside neutral line according to Liouville's theorem, modified to account for particle losses consistent with the postulated pitch angle scattering. We consider three different idealized scattering rate models for the plasma sheet electrons: (1) strong diffusion everywhere, (2) an MLT‐independent model for diffusion that is less than everywhere strong, and (3) an MLT‐dependent scattering rate model with the same azimuthal average as the MLT‐independent model. We evaluate the precipitating energy flux and mean energy at ionospheric altitude h = 127.4 km, as well as the ionospheric Hall and Pedersen conductances, for the three different scattering rate models considered, and we compare the results with each other and with available observations. The limit of everywhere strong pitch angle diffusion yields a simulated diffuse aurora that seems too intense at its maximum (storm time energy flux of ∼5–10 erg cm−2 s−1) and seems too concentrated near midnight in magnetic local time (MLT). The distribution of energy flux above half maximum extends from ∼2000–0700 MLT throughout the model storm. Corresponding maxima in simulated Hall (∼18–34 mhos) and Pedersen (∼9–11 mhos) conductances also seem too large. Our model for pitch angle diffusion that is less than everywhere strong yields a more realistic maximum energy flux (∼2 erg cm−2 s−1) and yields a realistically broader distribution in MLT, essentially because the plasma sheet electrons of interest live longer before precipitating. An even more realistic MLT distribution of precipitating energy flux is obtained by modulating the latter scattering rate model with an MLT dependence based on the observed statistical distribution of waves with respect to MLT. With such a model we can account for intensifications of diffuse auroral electron precipitation found near dawn and late in the morning quadrant in both statistical and storm event studies.
Electron densities determined by inversion of ultraviolet limb profiles
Authors: Dymond K. F. , Budzien S. A. , Nicholas A. C. , Thonnard S. E. , McCoy R. P. , Thomas R. J.
Published: February 2004

We present electron density profiles derived by inversion of ultraviolet limb radiances observed on November 24, 1999, by the Low‐Resolution Airglow and Aurora Spectrograph instrument on the Advanced Research and Global Observing Satellite. The solar 10.7‐cm radio flux was 181 solar flux units, and the daily ap was 21, indicating moderate geomagnetic activity. The O+ density profile, which is approximately equal to the electron density profile in the F region ionosphere, was determined by inverting the limb radiance profile of O I 911‐Å emission of atomic oxygen. The 911‐Å emission is produced by radiative recombination of O+ ions and electrons. Sounding rocket and satellite measurements of the Earth's extreme ultraviolet dayglow indicated significant contamination in the 900‐ to 920‐Å passband by emissions from atomic, molecular, and ionized nitrogen [Gentieu et al., 1979, 1981; Chakrabarti et al., 1983]. As a result of these observations, the radiative recombination emission was thought to be of little use for ionospheric sensing during the daytime. Feldman et al. [2001] have recently measured spectra in the 905‐ to 1184‐Å passband and show that the contamination at F region altitudes is less than that present in the earlier observations [Gentieu et al., 1979, 1981; Chakrabarti et al., 1983]. We found the contamination of the O I 911‐Å emission to be negligible at F region altitudes and have been able to use the 911‐Å emission to accurately characterize the ionospheric state. We have compared the peak electron density and peak height determined by inversion of the 911‐Å altitude profiles with nearly coincident ionosonde measurements, and we find the measurements from the two techniques to be in good agreement, demonstrating the accuracy of this technique for sensing the ionospheric state.
Did the solar eclipse of August 11, 1999, show a geomagnetic effect?
Authors: Korte M. , Lühr H. , Förster M. , Haak V. , Bencze P.
Published: February 2004

The solar eclipse of August 11, 1999, presented good conditions for the study of associated geophysical effects. Ionospheric measurements clearly show a decrease of electron density due to the reduced solar irradiation during the eclipse. However, contrary to claims elsewhere, the decreased conductivity did not cause an obvious effect in the geomagnetic recordings at the Earth's surface. Recordings of several European geomagnetic observatories and of a temporary variometer network, set up specially to observe an eclipse effect in detail, have been studied directly and in terms of equivalent currents in the ionosphere. We present the results of these studies and discuss possible current configurations that might explain the lack of an eclipse effect in geomagnetic recordings.
A study of inverted‐V auroral acceleration mechanisms using Polar/Fast Auroral Snapshot conjunctions
Authors: Janhunen P. , Olsson A. , Peterson W. K. , Laakso H. , Pickett J. S. , Pulkkinen T. I. , Russell C. T.
Published: February 2004

We present observations of electrons and ions that have been affected by an auroral plasma acceleration mechanism. In the nine events studied, nearly simultaneous and magnetically conjugate measurements were made by instruments on the Fast Auroral Snapshot (FAST) and Polar satellites (1900–4100 and 20000–37000 km altitude ranges, respectively). FAST sees inverted‐V‐type electron spectra in five out of nine events, in the remaining four events the precipitation seen at FAST is diffuse rather than inverted‐V‐type. In those inverted‐V cases where the electron distribution at Polar is quasi‐Maxwellian (two events), assuming a potential drop of appropriate magnitude between the two spacecraft can well explain the spectra. In more complicated non‐Maxwellian cases (three events), a net potential drop between FAST and Polar could explain the total FAST energy flux rather well, but it fails to explain the details of the distribution functions. Thus the “standard” explanation of inverted‐V‐type electron spectra in terms of potential drop acceleration may work in the quasi‐Maxwellian events (which represent the small‐energy end of inverted‐V spectra, with 1–2 keV energies) but not in the non‐Maxwellian events. Measurements of the upward flowing ionospheric ions at Polar also support this view. In the non‐Maxwellian events, sporadic very broadband electrostatic wave activity (4–1000 Hz frequency range) temporally correlates well with field‐aligned acceleration of the cold (but not the hot) electron population.
Ground‐based and Polar spacecraft observations of a giant (Pg) pulsation and its associated source mechanism
Authors: Wright D. M. , Yeoman T. K. , Rae I. J. , Storey J. , Stockton‐Chalk A. B. , Roeder J. L. , Trattner K. J.
Published: February 2004

Multi‐instrument observations of a Pg pulsation, which occurred on the morning of May 16, 1998, are reported. The wave signature was observed simultaneously on the ground, by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network and in the ionosphere by the Doppler Pulsation Experiment (DOPE) high resolution HF Doppler sounder. The wave occurred in the morning sector and possessed an azimuthal wave number, m, of 30±5 with a westward phase propagation. Shortly before the Pg commenced, energetic particle instruments on board the Polar spacecraft detected protons with a non‐Maxwellian energy distribution drifting westward toward the location of IMAGE and DOPE. An investigation has been undertaken to determine whether these particles were involved in the wave‐particle interaction considered responsible for generating the Pg pulsation. Proton energies of around 7 keV, which occur at the low‐energy edge of the unstable distribution (where ∂ƒ/∂W>0), satisfy the drift‐bounce resonance relation, ω − mωd, for N=1. This result indicates that this particular wave is likely to be the result of a drift‐bounce resonance mechanism and that it has an even mode standing wave structure in the magnetosphere. This result is discussed in terms of previous observations of Pgs.
Field‐aligned current effects on midlatitude geomagnetic sudden commencements
Authors: Kikuchi Takashi , Tsunomura Satoru , Hashimoto Kumiko , Nozaki Kenro
Published: February 2004

The geomagnetic sudden commencement (SC) on February 18, 1999, was preceded by a preliminary positive impulse (PPI) at noon (1146 LT) mid‐latitudes (34.9° and 26.9° geomagnetic latitude (GML)), and by a preliminary reverse impulse (PRI) near the dip equator (−0.3° and 4.9° GML) in the same local‐time sector. By assuming that the step‐like SC at a lower latitude (14.5° GML) was entirely caused by the Chapman‐Ferraro currents, we subtracted this magnetic field from the SC at midlatitudes and equatorial latitudes, to identify the magnetic fields caused by the field‐aligned currents (FACs) and ionospheric currents. We found that the PPI was composed of a positive impulse (true‐PPI) with a timescale of less than l min and a succeeding negative impulse (several minutes), with their amplitudes decreasing with decreasing latitudes. The true‐PPI occurred simultaneously with the equatorial PRI, and the succeeding negative impulse occurred with the DP 2‐type ionospheric current component of the main impulse (MI) of the SC (DP (MI)). Analysis of 46 well‐defined PPI events showed that the afternoon PPIs occurred exclusively in winter, while there was no significant seasonal dependence in the morning PPIs. None of the afternoon PPIs could be explained by the conventional SC model based on the Chapman‐Ferraro currents and the DP 2‐type ionospheric currents. We apply the Biot‐Savart law to a three‐dimensional current circuit including FACs to interpret the afternoon PPIs. Model calculations assuming a seasonal asymmetry in the ionospheric conductivity indicate that the FACs played a predominant role at midlatitudes in the winter hemisphere, while the ionospheric currents played a predominant role in the summer hemisphere. It is concluded that the true‐PPIs and succeeding negative impulses were dominated by the magnetic effects of the FACs that carry the electric fields responsible for the PRIs and DP (MIs), respectively.
Cassini Radio Detection and Ranging (RADAR): Earth and Venus observations
Authors: Lorenz R. D. , Elachi C. , West R. D. , Johnson W. T. K. , Janssen M. A. , Moghaddam M. , Hamilton G. A. , Liepack O. , Bunker A. , Roth L. E. , Wall S. D. , Dente L. , Casarano D. , Posa F.
Published: February 2004

The Cassini Radio Detection and Ranging (RADAR) was operated in scatterometric and radiometric modes during the Venus 1 and Earth swingbys to verify its functionality. At Venus, only the thermal emission from the thick absorbing atmosphere was detected. At Earth both the radar echo and the microwave emission from the surface were detected and reveal ocean surface disturbances, the rough, high, and cold Andes mountains, and surface features including a small reservoir in Brazil. Instrument performance appears to be excellent.
Dependence of magnetopause erosion on southward interplanetary magnetic field
Authors: Shue J.‐H. , Song P. , Russell C. T. , Thomsen M. F. , Petrinec S. M.
Published: February 2004

On May 4, 1998, the Wind and ACE satellites observed a period of extremely strong southward interplanetary magnetic field (IMF) followed by a period of highly variable IMF. The magnetopause moved inside the geosynchronous orbit, exposing Los Alamos National Laboratory (LANL) geosynchronous satellites and Polar to the magnetosheath. In a simple model the magnetopause erosion may be linearly proportional to the magnitude of southward IMF. A careful comparison of the predictions derived from various magnetopause location models to the in situ observations indicates a nonlinear dependence of the magnetopause erosion on IMF Bz. The observations from the Wind and ACE satellites are significantly different in the period of highly variable IMF. The predictions based on the ACE solar wind data are more consistent with the LANL observations than those based on the Wind solar wind data. The positions of the satellites show that ACE and LANL were mainly on the same side of the Sun‐Earth meridian plane. However, for the Polar observations the predictions based on the Wind data are better than those based on the ACE data. Wind and Polar were on the same side of the equatorial plane. These results suggest that we choose a solar wind monitor that is on the same side of the meridian or equatorial plane as the magnetospheric satellite for better predictions.
Propagation of the preliminary reverse impulse of sudden commencements to low latitudes
Authors: Chi P. J. , Russell C. T. , Raeder J. , Zesta E. , Yumoto K. , Kawano H. , Kitamura K. , Petrinec S. M. , Angelopoulos V. , Le G. , Moldwin M. B.
Published: February 2004

It has been thought that the preliminary reverse impulse (PRI) of the sudden commencements (SC) phenomena occurs simultaneously on the ground at different locations. A popular explanation is that the PRI propagates through the Earth‐ionosphere waveguide at the 35 ground magnetometer stations during the SC event on September 24, 1998, and found clear differences in the arrival time of PRI. We calculated the MHD wave propagation time from the location of the first compression of the magnetosphere to the low‐latitude ground stations and found good agreement with the observed PRI arrival times. Our calculation also indicates that the wavefront is seriously distorted by the inhomogeneity of the magnetosphere and the small difference in PRI arrival time between high‐latitude and low‐latitude observations cannot be an indicator of a super‐Alfvenic propagation. We also found implications that high‐latitude PRIs can be induced by the vortex of ionospheric currents at nearby latitudes, and the motion of the current vortex can affect the arrival time of high‐latitude PRIs.
Cassini Plasma Spectrometer observations of bidirectional lobe electrons during the Earth flyby, August 18, 1999
Authors: Abel G. A. , Coates A. J. , Rymer A. M. , Linder D. R. , Thomsen M. F. , Young D. T. , Dougherty M. K.
Published: February 2004

Unlike previous missions to the magnetotail (IMP 6, ISEE 1, ISEE 3, and Geotail), which effectively made observations in the lobe at a single downtail distance, the Cassini Earth swingby allowed, for the first time, near‐continuous observations covering a range of downtail distances. Bidirectional electrons are found in the northern lobe, consistent with previous studies. Enhancements in the electron fluxes are seen in a boundary layer between the lobe and sheath. These enhancements are accompanied by enhancements in ion fluxes traveling tailward. We also present what we believe to be the first observations of a returning electron population in the magnetosheath. Bidirectional electrons are observed upto 0.02 keV, while at higher energies only unidirectional electrons are observed. The low energy of the returning electrons arises as a result of the electron populations' passage through the magnetopause twice and losses due to precipitation.
Seismic waveform attributes before and after the Loma Prieta earthquake: Scattering change near the earthquake and temporal recovery
Authors: Baisch Stefan , Bokelmann Götz H. R.
Published: February 2004

During 1987–1995 several clusters of nearly identical seismic events (multiplets) occurred near the Loma Prieta source region. These multiplets allow us to investigate and demonstrate spatial and temporal changes in seismic wave character associated with the 1989 Loma Prieta main shock. For seismogram pairs we use a moving window technique to compute coherencies depending on lapse time and frequency. Post‐Loma Prieta events have reduced coherencies with pre‐Loma Prieta events in a spatially limited region close to the Loma Prieta hypocenter, while other paths remain nearly unaffected. These changes gradually recover within a time interval of 5 years after the Loma Prieta earthquake. A possible explanation for the time dependence is coseismically opened cracks which cause scattering increase for wavefields after the Loma Prieta event. Postseismic relaxation processes such as crack healing, fluid diffusion, or after deformations lead to progressive closure of these cracks with time after the main shock. Thus the scattering properties of the local crust approach the pre‐main shock state.
Strain accumulation near Yucca Mountain, Nevada, 1993–1998
Authors: Savage J. C. , Svarc J. L. , Prescott W. H.
Published: February 2004

A 50‐km aperture geodetic network centered on the proposed high‐level radioactive waste disposal site at Yucca Mountain, Nevada, was surveyed with GPS in 1993 and 1998. The average deformation rate across the area is described by the principal strain rates 22.8±8.8 nstrain yr−1 N77.6°W±13.5° and −8.8±11.9 nstrain yr−1 N12.5°E±13.5° (extension reckoned positive) and a clockwise rotation rate about a vertical axis of 9.6±7.4 nrad yr−1 relative to fixed North America. Quoted uncertainties are standard deviations. Those strain rates are consistent with the geodetic strain rates (2±12 nstrain yr−1 N87°±12°W and −22±12 nstrain yr−1 N03°±12°E) previously reported by Savage et al. [1999] for the 1983–1998 interval and with the low extension rate (5–20 nstrain yr−1) [Marrett et al., 1998] inferred from the geologic record. None of those strain rates is consistent with the 50±9 nstrain yr−1 N65°W extension rate for the area reported by Wernicke et al. [1998].
Effect of fine‐scale microstructures in titanohematite on the acquisition and stability of natural remanent magnetization in granulite facies metamorphic rocks, southwest Sweden: Implications for crustal magnetism
Authors: McEnroe Suzanne A. , Harrison Richard J. , Robinson Peter , Golla Ute , Jercinovic Michael J.
Published: February 2004

Mid‐Proterozoic granulites in SW Sweden, having opaque minerals hematiteilmenite with minor magnetite, and occurring in an area with negative aeromagnetic anomalies, have strong and stable reversed natural remanent magnetization ∼9.2 A/m, with 100% remaining after demagnetization to 100 mT. Samples were characterized by optical microscopy, electron microprobe (EMP), transmission electron microscopy (TEM), and rock‐magnetic measurements. Earliest oxide equilibrium was between grains of titanohematite and ferri‐ilmenite at 650°–600°C. Initial contacts were modified by many exsolution cycles. Hematite and ilmenite (Ilm) hosts and lamellae by EMP are Ilm 24–25, ILm 88–93, like
titanohematite, and
ilmenite above 520°C on Burton's diagram [1991]. Finer hosts and lamellae by TEM are Ilm16 ±3 and Ilm 88±4, like coexisting antiferromagnetically ordered (AF) hematite and
ilmenite below 520°C on Burton's diagram. This may be the first example of analytical identification, in one sample, of former
hematite, now finely exsolved, and AF hematite. TEM microstructures consist of gently curving semicoherent ilmenite lamellae within hematite, flanked by precipitate‐free zones and abundant ilmenite disks down to unit cell scale (1–2 nm). Strain contrast of disks suggests full coherence with the host, and probable formation at the reaction
titanohematite ‐‐‐> AF hematite +
ilmenite at 520°C. Magnetic properties are a consequence of chemical and magnetic evolution of hematite and ilmenite with bulk compositions ilmenite‐richer than Ilm 28, that apparently exsolved without becoming magnetized, down to 520°C where
hematite broke down to AF hematite plus
ilmenite, producing abundant AF hematite below its Néel temperature. Intensity of magnetization is greater than possible with hematite alone, and TEM work suggests that ultrafine
ilmenite disks in AF hematite are associated with a ferrimagnetic moment due to local imbalance of up and down spins at coherent interfaces.