bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimated solar wind‐implanted helium‐3 distribution on the Moon
Authors: Johnson Jeffrey R. , Swindle Timothy D. , Lucey Paul G.
Published: September 2002

Among the solar wind‐implanted volatiles present in the lunar regolith, ³He is possibly the most valuable resource because of its potential as a fusion fuel. The abundance of ³He in the lunar regolith at a given location depends on surface maturity, the amount of solar wind fluence, and titanium content, because ilmenite (FeTiO3) retains helium much better than other major lunar minerals. Surface maturity and TiO2 maps from Clementine multispectral data sets are combined here with a solar wind fluence model to produce a ³He abundance map of the Moon. Comparison of the predicted ³He values to landing site observations shows good correlation. The highest ³He abundances occur in the farside maria (due to greater solar wind fluence received) and in higher TiO2 nearside mare regions.
Multi‐tube model for interplanetary magnetic clouds
Authors: Osherovich Vladimir A. , Fainberg J. , Stone R. G.
Published: September 2002

Measurements of the polytropic index γ inside a magnetic cloud showed that there are two non‐equal tubes inside the cloud [Fainberg et al., 1996; Osherovich et al., 1997]. For both tubes, γ < 1, but each tube has its own polytrope. We test equilibrium solutions which are a superposition of solutions with cylindrical and helical symmetry [Krat and Osherovich, 1978] as a new paradigm for a multi‐tube model. Comparison of magnetic and gas pressure profiles for these bounded MHD states with observations suggests that complex magnetic clouds can be viewed as multiple helices embedded in a cylindrically symmetric flux rope.
TDLAS Trace Gas Measurements within Mountain Waves Over Northern Scandinavia during the POLSTAR Campaign in Early 1997
Authors: Schilling T. , Lübken F. ‐J. , Wienhold F. G. , Hoor P. , Fischer H.
Published: September 2002

Trace gas measurements of CO and N2O were performed over Northern Scandinavia during a mountain wave event in the night 1./2. February 1997. The data were taken during the POLSTAR I campaign on board the DLR research aircraft Falcon by the newly developed tunable diode laser absorption spectrometer TRISTAR. Strong tropospheric winds almost perpendicular to the Scandinavian mountain ridge led to the formation of mountain waves. Due to the steep gradient in the CO mixing ratio around the tropopause these mountain waves resulted in extensive modulations in the CO signal. Peak‐to‐peak amplitudes of up to 70 ppbv and horizontal wavelengths in the order of 10 to 20 km were recorded. Vertical fluxes of CO in the tropopause region are calculated using the CO data and the vertical wind data taken from the standard meteorological equipment of the DLR aircraft. The vertical eddy exchange coefficient shows values comparable to or even greater than those calculated for tropopause folds. We conclude that mountain wave events seem to play an important role as a mechanism for troposphere/ stratosphere exchange of chemical constituents.
Recent changes in the diurnal cycle of precipitation over the United States
Authors: Dai Aiguo
Published: September 2002

Hourly precipitation data from 1963 to 1993 of the contiguous United States were analyzed for secular trends in the diurnal cycle of precipitation. During the period the winter daytime and nighttime precipitation frequencies and the diurnal amplitude of precipitation frequency increased by ∼30–47% over the Southwest, while the winter precipitation frequencies and the diurnal amplitude of precipitation intensity decreased by similar amounts over the Northwest. During this period, summer afternoon precipitation frequency increased by ∼30–60% in the Southwest and decreased by ∼15–30% in the Southeast.
Eurasian snow cover variability and northern hemisphere climate predictability
Authors: Cohen Judah , Entekhabi Dara
Published: September 2002

We present observational evidence demonstrating dynamic interactions and feedbacks between multi‐seasonal snow cover and winter‐time circulation anomalies over mid‐high latitudes. The cooling effect of snow cover is associated with a strengthened and more expansive Siberian high with more frequent, topographically constrained intrusions west and north. Early‐season snow cover variability leads to altered general circulation patterns consistent with the dominant mode of winter variability observed in the Northern Hemisphere troposphere. The implications of the surface‐atmosphere coupling for seasonal to interannual predictability are also discussed.
An improved method to deduce the unloading component for magnetospheric substorms
Authors: Sun W. , Xu W.‐Y. , Akasofu S.‐I.
Published: September 2002

Because our earlier analysis [Sun et al., 1998] confirmed that the directly driven component prevails during the growth phase, an attempt has been made in this paper to deduce the unloading component as accurately as possible. First, the directly driven pattern is determined by taking the average of patterns of the equivalent current function during the growth phase of substorms, then the correlation method is applied to calculate time variations of the magnitude of the directly driven component. Next, the method of natural orthogonal components is used to analyze the residual part, which equals the difference between the total current and the directly driven component. In the present method, the pattern and the magnitude of the unloading component are much improved. The correction in the magnitude of two components is ∼ 50%. Time variations of the magnitude of the unloading component thus obtained will be crucial in understanding the unloading process. The effect of the variable directly driven pattern, which is ∼ 11% of the magnitude of the directly driven component, also has been estimated quantitatively. This improved method has the advantage of allowing study of individual substorms over the earlier method.
A search of UARS data for ozone depletions caused by the highly relativistic electron precipitation events of May 1992
Authors: Pesnell W. Dean , Goldberg Richard A. , Jackman Charles H. , Chenette D. L. , Gaines E. E.
Published: September 2002

Highly relativistic electron precipitation (HRE) events containing significant fluxes of electrons with E > 1 MeV have been predicted by models to deplete mesospheric ozone. For the electron fluxes measured during the great HRE of May 1992, depletions were predicted to occur between altitudes of 55 and 80 km, where HOx reactions cause a local minimum in the ozone number density and mixing ratio. Measurements of the precipitating electron fluxes by the particle environment monitor (PEM) tend to underestimate their intensity; thus the predictions of ozone depletion should be considered an estimate of a lower limit. Since the horizontal distribution of the electron precipitation follows the terrestrial magnetic field, it would show a distinct boundary equatorward of the L = 3 magnetic shell and be readily distinguished from material that was not affected by the HRE precipitation. To search for possible ozone depletion effects, we have analyzed data from the cryogenic limb array etalon spectrometer and microwave limb sounder instruments on UARS for the above HRE. A simplified diurnal model is proposed to understand the ozone data from UARS, also illustrating the limitations of the UARS instruments for seeing the ozone depletions caused by the HRE events. This diurnal analysis limits the relative ozone depletion at around 60 km altitude to values of < 10% during the very intense May 1992 event, consistent with our prediction using an improved Goddard Space Flight Center two‐dimensional model.
Coordinated ISTP satellite and ground observations of morningside Pc5 waves
Authors: Ohtani S. , Rostoker G. , Takahashi K. , Angelopoulos V. , Nakamura M. , Waters C. , Singer H. , Kokubun S. , Tsuruda K. , Hughes W. J. , Potemra T. A. , Zanetti L. J. , Gary J. B. , Lui A. T. Y. , Williams D. J.
Published: September 2002

This paper reports the result of a coordinated data analysis of a morningside Pc5 event observed at different altitudes in the magnetosphere and also on the ground. The event took place during 1400–1500 UT of April 29, 1993. The Geotail satellite was located in the boundary region and observed a 5‐min quasi‐periodic magnetic oscillation. The oscillation was mostly transverse to the background magnetic field. A 90° phase lag between the magnetic field and electric field variations was not clear, suggesting that the oscillation was not a standing wave and that Geotail was located in or close to the excitation region. The plasma flow vector rotated clockwise on the equatorial plane viewed from the north as expected for a magnetospheric surface wave on the morningside. At geosynchronous altitude, the GOES satellites also observed a 5‐min magnetic oscillation but with a significantly smaller amplitude than at the Geotail position. Five‐minute magnetic oscillations were also detected at Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) and Magnetometer Array for Cusp and Cleft Studies (MACCS) ground stations in the same local time sector as the satellites, even equatorward of a region 2 field‐aligned current observed by the Freja magnetometer data. From the phase analysis of ground signatures, the wave is inferred to propagate westward (antisunward) at a velocity of 18° in longitude per minute. The propagation speed mapped to the equator, 400 km/s, is in the range of the expected flow speed of the magnetosheath. It is inferred that in the present event, the Kelvin‐Helmholtz instability at the magnetopause, rather than at the inner edge of the boundary layer, excited an oscillation at the single frequency in a large area from the boundary region to deep inside the magnetosphere.
Signatures of nonlinear charged particle dynamics in Geotail comprehensive plasma instrument observations
Authors: Holland D. L. , Paterson W. R. , Frank L. A. , Kokubun S. , Yamamoto Y.
Published: September 2002

In this paper we present simulation results and observational data from the Geotail comprehensive plasma instrument of an ion distribution function signature which arises due to nonlinear particle dynamics in the quiet time magnetotail. The signature manifests itself as peaks and valleys in the ion distribution function whose separation scales as the fourth root of the particle energy. The Geotail observations represent the first independent corroboration of this signature since it was seen in ISEE 1 data by Chen et al. [1990]. The simulations demonstrate that the signature is present in the pitch‐angle‐resolved distribution even in the case of perfectly symmetric particle sources in the northern and southern hemispheres. When combined with magnetometer data, we show how the peaks and valleys may be used to determine the current sheet thickness using a single satellite. The current sheet thickness determined in this fashion is less than but consistent with other measurements of the current sheet.
Case studies on the wave propagation and polarization of ELF emissions observed by Freja around the local proton gyrofrequency
Authors: Santolík Ondřej , Parrot Michel
Published: September 2002

Using a multicomponent measurement of electric and magnetic fields in the ELF range, we investigate the polarization and propagation characteristics of several types of electromagnetic emissions around the local proton gyrofrequency. The data of the low‐orbiting Freja satellite are examined in the auroral and subauroral regions as well as inside the plasmasphere. We confirm previous results on the properties of the high‐latitude hiss with a sharp lower cutoff just below the local proton gyrofrequency. The waves propagate downward from higher altitudes outside the plasmasphere and reflect at the local two‐ion cutoff frequency. With an enhanced data representation we are able to characterize the reflected upgoing waves on both poleward and equatorward edges of the hiss emission. The high‐latitude emissions in a large band below the local proton gyrofrequency contain right‐hand circularly polarized waves propagating nearly parallel to the terrestrial magnetic field. We believe these emissions can originate by tunnelling of the right‐hand‐polarized hiss below the two‐ion crossover frequency. We report high‐latitude “frequency drift phenomena” below the local proton gyrofrequency. The frequency of these faint emissions increases by several tens of hertz per latitude degree. The electromagnetic noise at low frequencies is always found with a high degree of a nearly linear polarization. Upgoing ion whistlers are observed to follow each other with time delays as low as about 100 ms.
A model for the reflection of Alfvén waves at the source region of the Birkeland current system: The tau generator
Authors: Vogt J. , Haerendel G. , Glassmeier K. H.
Published: September 2002

The source region of the Birkeland current system, also called the generator of the auroral current circuit, is probably located in the outer equatorial magnetosphere on closed magnetic field lines. Alfvén waves have often been suggested as carriers for field‐aligned currents, momentum, and electromagnetic energy which is converted to particle kinetic energy and ohmic heat at the polar end of the auroral flux tubes. In this report we present a model for the interaction of Alfvén waves with the plasma in the generator region: the tau generator. We show that existing concepts like current and voltage generators correspond to extreme values of the intrinsic parameter τ. A frequency dependent reflection coefficient can be found which has considerable effect on the evolution of magnetospheric oscillations. Furthermore, we discuss the physical significance of the parameter τ and relate it to some measure for the effective height of the generator region.
Extremely intense whistler mode waves near the bow shock: Geotail observations
Authors: Zhang Y. , Matsumoto H. , Kojima H. , Omura Y.
Published: September 2002

Very intense whistler mode waves with amplitudes up to ∼1 nT were frequently recorded by the Waveform Capture on board the Geotail satellite in the bow shock. The intense whistler waves cover a frequency range from 0.02 to 0.56Ωe (10 ∼ 400 Hz) with durations from tens to hundreds of milliseconds. These intense whistler waves propagate in a direction quasi‐parallel to the ambient magnetic field with an average angle of 22°. Most (∼80%) of the intense whistler waves propagate in a downstream direction, while only 20% of them propagate toward solar wind. Quasi‐perpendicular shocks with shock normal angles from 66° to 86° are associated with the intense whistler waves. The quasi‐parallel propagation indicates that the electron cyclotron resonance is the dominant mechanism for the excitation of the intense whistler waves. It is found that loss cone electrons with temperature anisotropy at the foot of quasi‐perpendicular bow shocks are the major source of the intense whistler waves. The electron beams, formed by an electric field within the bow shock, are very likely the source of those whistlers propagating in an upstream direction. Trapping of electrons by these intense whistler waves may partially explain the flat‐topped electron distribution at the downstream edge of the bow shock.
Systematic modeling of soft‐electron precipitation effects on high‐latitude F region and topside ionospheric upflows
Authors: Su Y.‐J. , Caton R. G. , Horwitz J. L. , Richards P. G.
Published: September 2002

An ionospheric plasma fluid transport model is used to investigate the effects of soft (<1 keV) electron precipitation on high‐latitude F region/topside ionospheric O+ upflows. In this paper we present a systematic modeling study of ionospheric effects of varying soft‐electron precipitation, focusing on the resulting upward O+ ion velocities and fluxes, as well as the elevated ion and electron temperatures, due to the precipitation. Recent satellite observations [Seo et al., 1997] suggest an inverse relationship between upward O+ fluxes and the characteristic energy of the precipitating electrons for the same energy flux level. The modeling results presented here show this inverse relationship explicitly. Our interpretation is that a declining characteristic energy at constant energy flux increases the number of precipitating electrons available to heat the thermal electrons, and thus enhances the thermal electron temperature and hence the ambipolar electric field for propelling the upward O+ flows. The modeled increase of the thermal electron temperature with enhanced auroral electron precipitation is also generally consistent with the Seo et al. [1997] topside ionospheric plasma measurements. In addition, the modeling results presented here illustrate characteristic temporal development responses, showing dramatic increases in velocity, Mach number, and flux values during the first 10–13 min after the precipitation is turned on. By ∼1 hour after the initiation of a soft‐electron precipitation event the ion upward velocities and fluxes approach nearly stable, asymptotic values.
Multistation observations of auroras: Polar cap substorms
Authors: Mende S. B. , Frey H. U. , Geller S. P. , Doolittle J. H.
Published: September 2002

Double‐wavelength (630.0 and 427.8 nm) all‐sky imagers were operated at the Automatic Geophysical Observatory (AGO) sites P1 and P4, (magnetic latitude=80°), South Pole Station (magnetic latitude=74°), and P3 (magnetic latitude=72°) during the austral winter of 1994 and 1995. A new data format was developed to present the 24 hour auroral displays from the station arrays, which provided 24 hour optical data during the Antarctic winter night for several months. Auroras at these stations exhibit the typical magnetic local time (MLT) features of high‐latitude precipitation. On the nightside (MLT), substorm poleward expansions were observed to reach very high latitudes. A typical event (June 3, 1994, 0400–0500 UT) started equatorward from the South Pole and propagated to the high‐latitude limit of the field of view at P1 and P4. The morphological appearance of these high‐latitude auroral substorm features was similar to that of substorm auroras observed at lower latitudes. The intense 427.8 nm emission accompanied by a weak 630 nm emission showed that the particles were energetic. The interplanetary magnetic field (IMF) Bz component was slightly negative before and during this event, emphasizing that these were substorm auroras and not polar cap auroras, which tend to occur during times of prolonged positive IMF Bz. Statistical analysis of the 1995 AGO optical data shows that over 20% of substorms are seen to penetrate to regions greater than 80° of latitude.
A new approach in incoherent scatter F region E × B drift measurements at Jicamarca
Authors: Kudeki Erhan , Bhattacharyya Santanu , Woodman Ronald F.
Published: September 2002

Since 1996 incoherent scatter F region plasma drift measurements at Jicamarca have been implemented using a new signal processing approach replacing the traditional pulse‐to‐pulse correlation method. The new method, based on Doppler spectrum estimation and nonlinear least squares fitting to model spectra obtained from incoherent scatter theory, improves the instrumental sensitivity remarkably under low signal‐to‐noise conditions. With the new method it has become possible to obtain very high quality drifts data at nearly all hours of the day throughout most F region heights. Altitudinal smoothing of the drifts data to reduce measurement noise is no longer necessary, and studies of the height variations of drifts can be performed with much greater certainty than before. Small‐amplitude gravity wave oscillations have been detected at F region heights and a vortical circulation of the F region plasma has been observed in the post sunset period.
Energy transport during O+ energization by ELF waves observed by the Freja satellite
Authors: Vaivads Andris , André Mats , Norqvist Patrik , Oscarsson Tord , Rönnmark Kjell , Blomberg Lars , Clemmons James H. , Santolík Ondrej
Published: September 2002

We present a study of strong ELF waves correlated with O+ ion heating. We investigate the kinetic energy flux of oxygen ions and electrons parallel to the ambient magnetic field and the Poynting flux along the magnetic field of waves in different frequency regions. We show that the Poynting flux of lower hybrid waves and waves with frequencies below the oxygen gyrofrequency is downward but much less than the upgoing oxygen ion energy flux. The Poynting flux of ELF waves with frequencies at about half the local proton gyrofrequency is also downward, and it is comparable to, or larger, than the upward oxygen energy flux. Our results are consistent with a scenario where ELF waves with frequencies around half the proton gyrofrequency are generated above the satellite and transfer energy to oxygen ions in a region below the satellite.
Simultaneous satellite and ground‐based observations of a discretely driven field line resonance
Authors: Lessard M. R. , Hudson M. K. , Samson J. C. , Wygant J. R.
Published: September 2002

An analysis is presented of a set of toroidal field line resonances observed on the ground by CANOPUS magnetometers and scanning auroral photometers on December 13, 1990, following a substorm onset at 0750 UT and intensification at 0850 UT. Magnetic and electric field data from the CRRES satellite provide evidence that the resonance was also observed in the magnetosphere. To our knowledge, this is the first report of discretely driven resonances observed by ground‐based magnetometers and photometers and confirmed using satellite data. A spectral peak at 2.1 mHz is present in all data sets at approximately the same invariant latitude and universal time, indicating that CANOPUS and CRRES are observing the same resonance. Peaks are also present at 1.4 and 1.7 mHz in the ground‐based magnetometer and CRRES data at a slightly higher latitude with corresponding spectral peaks apparent in the photometer data. The ground signature for each resonance indicates an antisunward phase velocity, suggesting that the excitation source is in the vicinity of the dayside magnetosphere, consistent with a waveguide model of the magnetosphere but not with a cavity model. This fact, combined with a possibly enhanced solar wind dynamic pressure, suggests that the substorm was not directly responsible for exciting the resonances. The interaction of the resonances with the substorm remains unclear except for the luminosity fluctuations associated with the resonances.
Three‐dimensional, one‐fluid, ideal MHD model of magnetosheath flow with anisotropic pressure
Authors: Erkaev N. V. , Farrugia C. J. , Biernat H. K.
Published: September 2002

We present a three‐dimensional, one‐fluid, steady state magnetohydrodynamic (MHD) model of magnetosheath flow near the subsolar line with unequal plasma pressures perpendicular (P⊥) and parallel (P‖) to the magnetic field (P⊥ > P‖). Aside from an assumption on the total pressure normal to the magnetopause, our analytical‐numerical method is completely general and is an extension of our isotropic, “magnetic string” MHD model, which we describe in detail here. The MHD equations are closed by a relation between P⊥ and P‖ as in the Bounded Anisotropy Model [Denton et al. 1994] corresponding to the threshold of the electromagnetic proton cyclotron wave instability. We take an IMF oriented perpendicular to the solar wind velocity. As boundary conditions, we have Rankine‐Hugoniot relations at the bow shock and a no‐flow condition at the magnetopause. We obtain steady state profiles of the magnetic field and plasma parameters for upstream sonic and Alfvén Mach numbers equal to 10, and compare them with the isotropic case (P‖ = P⊥). Anisotropy slightly thickens the magnetosheath. In the anisotropic model, the density, the parallel and perpendicular temperatures, plasma pressures, and betas all decrease toward the magnetopause. Isotropic profiles lie between those of quantities perpendicular and parallel to the field. Anisotropy has considerable effect on the density profile, which lies below that in the isotropic limit throughout the magnetosheath. Density depletion results from stretching of magnetic field lines, which is caused by field‐aligned plasma flow. Approaching the magnetopause, the tangential component of velocity parallel to the magnetic field decreases, while the tangential component perpendicular to the magnetic field increases. These are features characterizing a stagnation line flow at the magnetopause. The acceleration along the magnetic field is produced by the gradient of P‖ and the mirror force, which depends on anisotropy. They both make substantial contributions and are responsible for the changes we see from isotropy. The acceleration perpendicular to magnetic field is also larger than in the case of isotropy and is caused by the gradient of total pressure, the magnetic strength, and the mirror force. In addition, acceleration in both directions is affected by the decreasing density.
Electromagnetic ion cyclotron waves in the near‐Earth magnetotail
Authors: Chaston C. C. , Hu Y. D. , Fraser B. J.
Published: September 2002

This study considers the characteristics and generation mechanisms of bursts of polarized transverse electromagnetic waves observed over the frequency range from 50 up to 500 mHz in the near‐Earth magnetotail from ISEE 1 and ISEE 2. Since this range contains the cyclotron frequencies of the ion components found in this region of space, we loosely term these bursts “electromagnetic ion cyclotron waves” (EMICs). Nearly all wave events fall in the frequency and wavelength ranges of 50 to 150 mHz and 1000 to 6700 km, respectively, with amplitudes of the order of 1 nT and may exhibit both left‐and right‐hand polarizations. These waves propagate at wavenormal angles, θk<45°, and exhibit wave amplitudes where the ratio of the amplitude parallel (b‖) to the background magnetic field (B0) to that perpendicular (b⊥) to B0 varies proportionally with θk, and this ratio is typically less than 0.5. ISEEl LEPEDEA distribution function data indicate that the polarized bursts are often accompanied by anisotropic ion distributions and or significant field‐aligned currents. It is demonstrated that a positive correlation between the wave properties predicted from linear Vlasov theory and the observed field fluctuations exists for most wave events considered when the plasma is modeled using both bi‐Maxwellian and bi‐Lorentzian functional forms. The occurrence of these wave bursts is commonly synchronous with the decreasing phase of large‐scale surface waves permeating the magnetotail, thereby suggesting that these waves provide a means of energy dissipation for global processes.
Collisionless dissipation wavenumber: Linear theory
Authors: Gary S. Peter
Published: September 2002

The collisionless dissipation wavenumber kd of a plasma fluctuation may be defined as the minimum value of wavenumber at which damping becomes significant. Linear Vlasov theory is used to determine kd for both magnetosonic and Alfvén fluctuations in a spatially homogeneous, collisionless plasma with Maxwellian velocity distributions. Alfvén mode properties at maximum dissipation are independent of Te/Tp, and the corresponding magnetosonic mode properties are weakly dependent on this temperature ratio, so theory predicts that the proton beta βp is the primary parameter determining kd. Theory yields kdc/ωp = Sk/βpαk, where ωp is the proton plasma frequency and Sk and αk are fitting parameters; for the Alfvén mode αk ≃ 0.3, whereas for the magnetosonic mode αk ≃ 0.8. This implies that Alfvén fluctuations are the more likely source of collisionless dissipation for long wavelength magnetic turbulence cascading to shorter wavelengths at βp ≲ 1, and magnetosonic fluctuations are more likely to dissipate such turbulence at βp ≳ 1.
Electron pitch angle distributions following the dipolarization phase of a substorm: Interball‐Tail observations and modeling
Authors: Smets R. , Delcourt D. , Sauvaud J. A. , Koperski P.
Published: September 2002

We investigate Interball‐Tail observations of electron pitch angle distributions after the dipolarization phase of a substorm. For 10 keV electrons we observe beamlike, coniclike, and perpendicularly peaked distributions at L ∼ 11, L ∼ 9, and L ∼ 7, respectively. We examine the efficiency of betatron heating and Fermi acceleration associated with adiabatic transport of the electrons during the substorm dipolarization phase. This dipolarization phase was modeled using transition between different Kp levels within a realistic magnetic field model. The calculations reproduce well the evolution of the high‐energy electron flux in the parallel and perpendicular directions. They also reproduce well the pitch angle distribution observed by Interball‐Tail at 10 keV, after the dipolarization phase. It is shown that Fermi acceleration is the leading process, compared to betatron heating. The production of the coniclike distributions is narrowly linked to the existence of a transition region between dipolelike and taillike magnetic fields, at about L ∼ 9.
Discharging spacecraft through neutral gas release: Experiment and theory
Authors: Walker D. N. , Amatucci W. E. , Bowles J. H. , Fernsler R. F. , Siefring C. L. , Antoniades J. A. , Keskinen M. J.
Published: September 2002

We describe experimental and theoretical research related to the mitigation of spacecraft charging by the release of neutral gas through specially designed nozzles. The experiments were conducted in the large Space Physics Simulation Chamber (SPSC) at the Naval Research Laboratory. A realistic near‐Earth space environment can be simulated in this device for which minimum scaling needs to be performed to relate the data to space plasma regimes. The environment of the SPSC is similar to that encountered by spacecraft in low Earth orbit. The experimental arrangement consists of an aluminum cylinder which can be biased to high voltage (0.4 < V < 10 kV). The cylinder incorporates a neutral gas release valve designed for millisecond release times, a pressure‐regulated neutral gas reservoir, and variable Mach number nozzles. After the cylinder is charged to high voltage the neutral gas is released, inducing a breakdown of the gas in the strong electric field about the cylinder. Collection of ions from the newly created dense plasma, along with secondary electron emission from the cylinder surface, provides the return current necessary for grounding the body. We treat the breakdown theoretically as a Townsend discharge and use the fundamental assumption of exponential electron growth as one proceeds from the cathode toward the anode during neutral gas breakdown. In addition, the nozzle release of neutral gas is modeled, and a simple linear spatial dependence of the applied potential is assumed. This basic model produces quite good results when compared to the experiment.
On the relationship between the magnetic and VLF signatures of the substorm expansion phase
Authors: Smith A. J. , Freeman M. P. , Wickett M. G. , Cox B. D.
Published: September 2002

We firmly establish a certain class of post‐midnight ELF/VLF wave events as a signature of the substorm expansion phase. These substorm chorus events (SCEs) have been observed near L = 4 at Halley, Antarctica (76°S, 27°W), by the VELOX instrument. By examining 4 years (1992–1995) of nearly continuous Halley VELOX data, we identified 973 SCEs. From this database a subset of 258 events was selected, occurring within ±1 hour of local magnetic midnight at Halley, that is, between 0200 and 0400 UT. The epochs of these events were used to perform a superposed epoch analysis on the three components of the Halley fluxgate magnetometer data. A significant negative bay in the magnetic H component was observed to start at the SCE epoch, as expected for a ground station near the westward electrojet of the substorm current wedge. Positive and negative bays, respectively, were seen in the D and Z components. We interpret the detailed features of the composite magnetograms and conclude that the SCE is an unambiguous signature of substorm expansion phase onset. Given this association of the SCE with the substorm expansion phase based on the qualitative features of the superposed epoch ground magnetic signature, this paper also provides the most complete quantitative description to date of both the VLF and magnetic substorm signatures observed at a near‐auroral zone ground station.
Magnetic proxy climate results from the Duanjiapo loess section, southernmost extremity of the Chinese loess plateau
Authors: Florindo Fabio , Zhu Rixiang , Guo Bin , Yue Leping , Pan Yongxin , Speranza Fabio
Published: September 2002

We report mineral magnetic results from a 7.5 m loess sequence (150 samples) from the southernmost extremity of the Chinese loess plateau (which includes the last two glacial cycles). In this area the loess sediments experienced particularly intense weathering processes. The magnetic assemblage is dominated by a mixture of pseudo‐single domain (PSD) and multidomain (MD) magnetite with associated superparamagnetic (SP) grains of either magnetite or maghemite in the paleosols and weathered loess horizons. All the rock magnetic parameters fluctuate in parallel with marine sediment δ18O data over the last 150 kyr, thus reflecting changing global paleoclimatic conditions. This relationship is also supported by the evidence of Milankovitch cycles in the magnetic susceptibility record. Paleorainfall estimates, when compared with other studies from the Chinese loess plateau, underline the (more) humid character of this region during the last ∼130 kyr.
The age of Lathrop Wells volcanic center: An 40Ar/39Ar dating investigation
Authors: Heizler Matthew T. , Perry Frank V. , Crowe Bruce M. , Peters Lisa , Appelt Robert
Published: September 2002

The 40Ar/39Ar age spectrum analyses of basalt whole rock and partially degassed sanidine xenocrysts for the Lathrop Wells volcanic center, Nevada, yield eruption ages of 77.3 ± 6.0 and 76.6 ± 4.9 ka (2σ) for the Ql1 and Ql2 flows, respectively. Basaltic samples were analyzed in four to six replicate runs and yield normally distributed plateau ages. Miocene ash flow tuff xenoliths were collected from the basaltic units with the majority of samples coming from the Ql2. The sanidines from the Ql2 xenoliths yielded apparent plateau ages (n = 10) ranging from 69 ± 17 to 309 ± 11 ka with a strong cluster (n = 6) at ∼77 ka. Because the diffusion dimension of sanidine is approximated by the physical crystal size, cracking of the sanidine following the development of an argon concentration profile may result in complex argon age spectra. Experimental studies exploring the natural and laboratory argon systematics of degassed sanidine reveal that crystal breakage and HF acid etching lead to anomalously old apparent ages. These new ages for the Ql1 and Ql2 flows are analytically indistinguishable and agree well with published eruption ages determined by 3He and 36Cl cosmogenic surface exposure dating methods. We conclude that the Ql1 and Ql2 flows are temporally equivalent. Additionally, the agreement between our eruption ages for these units and the published 76 ± 20 ka [Zreda et al., 1993] 36Cl exposure age for the Lathrop Wells scoria cone (Qs3) suggests that the cone may be temporally equivalent to the Ql1 and Ql2 flows. We believe the simplest and most reasonable interpretation is that the volcanic center is monogenetic.
A new conceptual model for fluid flow in discrete fractures: An experimental and numerical study
Authors: Waite Miles E. , Ge Shemin , Spetzler Hartmut
Published: September 2002

A fundamental understanding of flow within single, or discrete, fractures is necessary so that fluid flow and chemical transport in fractured rocks can be accurately characterized. To explore discrete fracture flow, we conducted a series of experiments to study the movement of water through four artificial fractures, each with a different two‐dimensional surface topography. We then performed fluid flow simulations using a lattice gas automata (LGA) numerical model to compare with the experimental results and to obtain detailed pictures of the fluid velocity fields within these fractures. The flow experiments and LGA simulations, all performed under fully saturated and laminar flow conditions, used fracture aperture values ranging from 0.25 to 1.80 mm. Our main focus was to determine how the cubic law, derived for fluid flow through parallel plates, could be modified to accommodate a tortuous fracture geometry. Our experimental and numerical results led to the proposal of a new conceptual model. This model states that when the aperture is measured normal to the flow path and is harmonically averaged and when the tortuosity of the flow path is included in the calculation of the pressure gradient, the cubic law can adequately predict the flow rate through fractures with a nonparallel geometry. Our study has implications for interpreting laboratory fracture flow data and for improving predictive numerical models.
Life cycle of the East Carpathian orogen: Erosion history of a doubly vergent critical wedge assessed by fission track thermochronology
Authors: Sanders Carlo A. E. , Andriessen P. A. M. , Cloetingh S. A. P. L.
Published: September 2002

Apatite fission track (FT) thermochronology applied to the East Carpathian fold‐and‐thrust belt in Romania constrains the interaction between the erosion history and the tectonic evolution of the orogen. The long‐lived constant asymmetric topography of the orogen and the erosion pattern as inferred from apatite FT data are in agreement with the style of evolution predicted by doubly vergent critical Coulomb wedge models. During the Miocene, up to 4 km of erosion took place over the actively deforming orogen, and the erosion patterns forms a mirrored image of the subsurface deforming wedge. The erosion products are deposited in flanking molasse basins which subsided contemporaneously due to the growing topographic weight of the wedge. Deformation of the wedge was presumably active from the early to late Miocene, but apatite FT ages between 14 and 9 Ma and track length modeling predict an acceleration of erosion rates (0.5–0.3 mm/yr) initially in the middle to late Miocene (≈13±2 Ma). The onset of erosion coincides with the last phase of convergence and a climax in the deformation history. These features are interpreted to have been caused by underrating of the European continental margin. Convergence ceased before the Pliocene according to the activity of thrust faults. During the Pliocene and Quaternary, up to 2 km of overburden was removed by erosion, leading to isostatic rock uplift of the entire region. The use of apatite fission track “minimum ages” supports the general idea that the orogen has a diachronous evolution from north to south. The orogen in the southern Bend Zone has a more juvenile tectonic character, and the main erosion phase took place during the Pliocene and Quaternary (≈5–0 Ma). The integrated approach of large scale morphology and fission track analysis confirms the applicability of the theoretical Coulomb wedge mechanics to a natural compressive fold‐and‐thrust belt. The study complements the geometric features of the orogenic wedge with quantified erosion histories and illustrates the effects of orogen evolution on vertical movements in the entire convergent setting.
Seismic attenuation tomography of the Tonga‐Fiji region using phase pair methods
Authors: Roth Erich G. , Wiens Douglas A. , Dorman Leroy M. , Hildebrand John , Webb Spahr C.
Published: September 2002

The anelastic structure of the region surrounding the Tonga slab and Lau back arc spreading center in the southwest Pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs. Two differential attenuation methods determine δt* over the frequency band 0.1 to 3.5 Hz for earthquakes in the Tonga slab. The S‐P method measures the difference in spectral decay between P and S waves arriving at the same station. The P‐P method measures the difference in spectral decay for P waves with different paths through the upper mantle. Eight hundred sixty phase pairs are used to invert for two‐dimensional 1/Qα structure using a nonnegative least squares algorithm. A grid search method determines the Qα/Qβ ratio most compatible with both the S‐P and P‐P differential measurements. The highest attenuation (Qα = 90) is found within the upper 100 km beneath the active portions of the Lau Basin extending westward to the Lau Ridge. These regions probably delineate the source region for the back arc spreading center magmas, expected to be within the upper 100 km based on petrological considerations. The high attenuation regions also correlate well with zones of low P wave velocity determined by regional velocity tomography. Somewhat lower attenuation is found beneath the Fiji Plateau than beneath the Lau Basin. The entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km. The slab is imaged as a region of low attenuation (Qα > 900) material. A Qα/Qβ ratio of 1.75 provides the best fit between the S‐P and P‐P data sets upon inversion. Spectral stacking shows no frequency dependence within the frequency band analyzed.
Contribution of ductile flow to exhumation of low‐temperature, high‐pressure metamorphic rocks: San Juan‐Cascade nappes, NW Washington State
Authors: Feehan Jeffrey G. , Brandon Mark T.
Published: September 2002

The San Juan‐Cascade (SJC) nappes were subducted to a depth of ∼18 km, metamorphosed under low‐temperature, high‐pressure conditions, and then exhumed, all within ∼16 m.y. During exhumation, penetrative deformation by solution mass transfer (SMT) resulted in a widespread spaced cleavage. Strain directions were determined for 27 sandstone samples, and absolute strain measurements for a subset of 19 samples. Z directions generally plunge moderately to the NE, and X and Y directions are scattered in the plane perpendicular to Z. SMT deformation is constrictional at the local scale, but the tensor average indicates plane‐strain uniaxial shortening at the regional scale, with Sx, Sy, and Sz equaling 1.01, 0.91, and 0.58, respectively. The average flattening plane (XY) dips 30° to the NE, and the average X direction plunges 20° to the north. The large shortening in Z was compensated by a mass‐loss volume strain of ∼47%. We present a simple one‐dimensional model that illustrates the relationship between finite strain and ductile exhumation for a steady state convergent wedge. Assuming depth‐dependent ductile flow and no reversal of principal strain rates with depth, this model indicates that ductile thinning of the SJC nappes accomplished only 13% of the total exhumation, despite a vertical shortening strain of 36%. There is no evidence that normal faulting contributed significantly to exhumation. We conclude that erosion operating at an average rate of ∼1.1 km m.y.−1 was the dominant exhumation process.
Abrupt along‐strike change in tectonic style: San Andreas Fault zone, San Francisco Peninsula
Authors: Zoback Mary Lou , Jachens Robert C. , Olson Jean A.
Published: September 2002

Seismicity and high‐resolution aeromagnetic data are used to define an abrupt change from compressional to extensional tectonism within a 10‐ to 15‐km‐wide zone along the San Andreas fault on the San Francisco Peninsula and offshore from the Golden Gate. This 100‐km‐long section of the San Andreas fault includes the hypocenter of the Mw = 7.8 1906 San Francisco earthquake as well as the highest level of persistent microseismicity along that ∼470‐km‐long rupture. We define two distinct zones of deformation along this stretch of the fault using well‐constrained relocations of all post‐1969 earthquakes based a joint one‐dimensional velocity/hypocenter inversion and a redetermination of focal mechanisms. The southern zone is characterized by thrust‐ and reverse‐faulting focal mechanisms with NE trending P axes that indicate “fault‐normal” compression in 7‐ to 10‐km‐wide zones of deformation on both sides of the San Andreas fault. A 1‐ to 2‐km‐wide vertical zone beneath the surface trace of the San Andreas is characterized by its almost complete lack of seismicity. The compressional deformation is consistent with the young, high topography of the Santa Cruz Mountains/Coast Ranges as the San Andreas fault makes a broad restraining left bend (∼10°) through the southernmost peninsula. A zone of seismic quiescence ∼15 km long separates this compressional zone to the south from a zone of combined normal‐faulting and strike‐slip‐faulting focal mechanisms (including a ML = 5.3 earthquake in 1957) on the northernmost peninsula and offshore on the Golden Gate platform. Both linear pseudogravity gradients, calculated from the aeromagnetic data, and seismic reflection data indicate that the San Andreas fault makes an abrupt ∼3‐km right step less than 5 km offshore in this northern zone. A similar right‐stepping (dilatational) geometry is also observed for the subparallel San Gregorio fault offshore. Persistent seismicity and extensional tectonism occur within the San Andreas right stepover region and at least 15 km along‐strike both to the SE and NW. The 1906 San Francisco earthquake may have nucleated within the San Andreas right stepover, which may help explain the bilateral nature of rupture of this event. Our analysis suggests two seismic hazards for the San Francisco Peninsula in addition to the hazard associated with a M = 7 to 8 strike‐slip earthquake along the San Andreas fault: the potential for a M ≈ 6 normal‐faulting earthquake just 5–8 km west of San Francisco and a M = 6+ thrust faulting event in the southern peninsula.
Controls on post‐mid‐Cretaceous landscape evolution in the southeastern highlands of Australia: Insights from numerical surface process models
Authors: van der Beek Peter , Braun Jean
Published: September 2002

The tectonic and geomorphic evolution of the southeastern Australian highlands has been a subject of controversy among geophysicists, geologists, and geomorphologists for several decades. We employ a numerical surface process model that includes long‐range fluvial transport, hillslope diffusion, and landsliding, in order to quantitatively assess the tectonic, lithological, and structural controls on the landscape evolution and denudation history of SE Australia. We constrain fluvial erosion parameters for our model by fitting river profiles. Rockfall and landsliding are the most important processes controlling the concentration of erosion in river valleys. We therefore model hillslope evolution using a very low diffusion coefficient and a threshold slope for landsliding. The “initial” (prebreakup) topography of the highlands provides a fundamental control on their subsequent evolution, together with the style and rate of fluvial incision. Secondary controls are exerted by lateral variations in lithology and local tectonic uplift along faults. Our modeling results indicate that (1) the observed highlands morphology requires that the drainage divide was established at its present location prior to opening of the Tasman Sea; whether the divide is inherited from Paleozoic orogeny or resulted from Mesozoic syn‐rift uplift cannot be established, however; (2) in contrast to traditional interpretations, escarpment retreat does not appear to be the fundamental process eroding the highlands; and (3) the observed barbed river drainage may be imprinted as a result of lithological variation; it does not necessarily evolve through capture. The driving mechanism that is most consistent with the inferred uplift pattern of the southeastern highlands appears to be magmatic underplating.