bluuebunny/crossref_metadata_2025 huggingface dataset:
A stationary planetary‐scale feature in the tropical mesopause temperature field
Authors: Venne D. E. , Dartt D. G. , Rodgers C. D.
Published: February 2008

Three years of daily global temperature grids at 5, 7, 9, and 12 scale heights have been retrieved from Nimbus 6 Pressure Modulator Radiometer measurements. A stationary pattern is observed in the tropical 12 scale height (about 82 km altitude) seasonal and monthly mean temperatures that has an approximately wavenumber four zonal structure. This pattern has warm centers located near the equator and cold centers positioned some 10 to 20 degrees of latitude away towards the winter pole. The amplitude of this pattern varies in a semiannual fashion, with maxima occurring during late winter and late summer. The structure and behavior of the feature compares well with the seasonal changes in tropical deep convective storms, leading to speculation that the feature may be driven by tropospheric gravity waves.
Hydromagnetic waves and the ionosphere
Authors: Kivelson Margaret G. , Southwood David J.
Published: February 2008

The effect of the ionosphere on the detectability, polarization, structure and decay of low frequency magnetospheric hydromagnetic waves has been a topic of interest for thirty years. There are two classes of signal behavior in the atmosphere and ionosphere, one where the ionosphere has little effect and one where the ionospheric conductivity controls the reflection, the amplitude detected on the ground and rate of signal decay. It has been assumed that the latter type of behavior is pertinent in most practical applications. Recent interest in global modes of the magnetospheric cavity requires a reassessment of the assumption. We investigate one simple case, the hydromagnetic surface wave, and discuss what happens in more complicated modes. We conclude that signals in the vicinity of field line resonance will be strongly coupled to the local ionosphere but far away will not.
An experimental method for directly determining the interconnectivity of melt in a partially molten system
Authors: Daines Martha J. , Richter Frank M.
Published: February 2008

An experimental method for directly determining the degree of interconnectivity of melt in a partially molten system is discussed using an olivine‐basalt system as an example. Samarium 151 is allowed time to diffuse through mixtures of olivine and basalt powder which have texturally equilibrated at 1350°C and 13 to 15 kbars. The final distribution of samarium is determined through examination of developed radiographs of the samples. A uniform distribution of samarium throughout an entire sample implies that melt is present as an interconnected network because diffusion of samarium along melt‐free grain boundaries is shown not to occur in the time frame of these experiments. Quantitative determination of the degree of interconnectivity is done by establishing a one to one correspondence between a sample and its radiograph. Results suggest an interconnected melt network is established at melt fractions at least as low as 1 wt % and all melt is completely interconnected at melt fractions at least as low as 2 wt % for the system examined.
Tidal current effects on temperature in diffuse hydrothermal flow: Guaymas Basin
Authors: Little Sarah A. , Stolzenbach Keith D. , Grassle Frederick J.
Published: February 2008

A twelve day record of temperature collected from the diffuse flow area of a Guaymas Basin hydrothermal site exhibits variations, from a minimum of 3.05°C to a maximum of 4.87°C, whose periodicity is correlated with tides measured at the nearby town of Guaymas. A simple model, based on the hypothesis that temperature variations result as changes in tidal bottom currents induce changes in the height of the thermal boundary layer, is in good quantitative agreement with observed temperatures for most of the record. The success of this model illustrates that the effects of tidal currents can be strong enough to dominate the time variability of a temperature signal at a fixed point in hydrothermal flow. Therefore, tidal currents must be taken into account when using temperature measurements to estimate time varying heat fluxes from hydrothermal diffuse flow regions.
Stochastic modeling of seafloor morphology: A parameterized Gaussian model
Authors: Goff John A. , Jordan Thomas H.
Published: February 2008

Stochastic methods of analysis are useful for quantifying ensemble properties of small‐scale bathymetric features such as abyssal hills. In this paper we model the seafloor as a stationary, zero‐mean, Gaussian random field completely specified by its autocovariance function. We formulate an anisotropic autocovariance function that has five free parameters describing the amplitude, anisotropic orientation and aspect ratio, characteristic length, and Hausdorff (fractal) dimension of seafloor topography. Parameters estimated from various seafloor regions by an inversion of Sea Beam data indicate that the seafloor exhibits a wide range of stochastic characteristics within the constraints of the model. Synthetic topography can be generated at arbitrary scale and resolution from the Gaussian model using a Fourier method. Color images of these synthetics are useful for illustrating the stochastic behavior of the model.
Synchrotron radiation from Neptune: Neptune's magnetic field and electron population
Authors: de Pater Imke , Goertz Christoph K.
Published: February 2008

On the basis of radio measurements of Neptune at 20 cm, one can infer the presence of ∼ 0.4 mJy synchrotron radiation. By assuming a dipole‐like magnetic field, with the energetic electrons confined to the magnetic equator, we estimate Neptune's magnetic dipole moment to be ∼ 1 G R³N. Its electron population should be between that in Uranus' and Jupiter's magnetospheres. If the electrons are injected by the solar wind, local acceleration processes in the magnetosphere are required to energize the particles, because adiabatic diffusion alone is not sufficient.
Shear wave measurements for improved characterization of shock‐Induced phase transformations in Carrara marble
Authors: Aidun John B. , Gupta Y. M.
Published: February 2008

Shear waves have been measured in shock‐induced, high‐pressure phases of Carrara marble (polycrystalline calcite). The shear modulus decrease associated with the calcite‐CaCO3(II) transition is comparable to that observed under hydrostatic loading. Beyond this transition, the shear modulus in the shocked marble increases with compression, even with the occurrence of the CaCO3(II)‐CaCO3(III) transition. Longitudinal wave speeds over the entire compression range examined are in reasonable agreement with previous shock wave and hydrostatic measurements in polycrystalline CaCO3. A preliminary determination of the bulk modulus from the shear and longitudinal wave speeds in the shocked marble is in reasonable agreement with hydrostatic measurements on polycrystalline and single crystal calcite, suggesting that the transformation to CaCO3(III) may be complete during shock compression. The usefulness of shear wave measurements in studying shock‐induced phase transformations in solids is indicated.
Global marine temperature variation and the solar magnetic cycle
Authors: Newell Nicholas E. , Newell Reginald E. , Hsiung Jane , Zhongxiang Wu
Published: February 2008

Global and hemispheric marine temperatures for 1856‐1986 have been subjected to harmonic analysis. We investigated a prominent 22‐year peak in the Fourier Transform by subtracting from the raw data the inverse transform of the periods longer than 26.2 years (the fifth harmonic). The shorter periods that remain are dominated by the 22‐year period the approximate amplitude and phase of which we determined with a 10‐year running mean filter. We suggest that this period may be related to the solar magnetic cycle. The low frequency data show a major dip centered around the period 1905‐1910. These results should be considered in any analysis of global marine temperature for trends.
Cyclic behavior at quasi‐parallel collisionless shocks
Authors: Burgess D.
Published: February 2008

Large scale one‐dimensional hybrid simulations with resistive electrons have been carried out of a quasi‐parallel (θBn = 30°) high Mach number collisionless shock. The shock initially appears stable, but then exhibits cyclic behavior. For the magnetic field the cycle consists of a period when the transition from upstream to downstream is steep and well defined, followed by a period when the shock transition is extended and perturbed. This cyclic shock solution results from upstream perturbations, caused by backstreaming gyrating ions, convecting into the shock. The cyclic re‐formation of a sharp shock transition can allow ions, at one time upstream because of reflection or leakage, to contribute to the shock thermalization. The simulations are simplistic in a number of ways for the quasi‐parallel shock, but their results suggest a model which may explain qualitatively several features of observations.
Ion heating and acceleration by magnetosonic waves via cyclotron subharmonic resonance
Authors: Terasawa Toshio , Nambu Mitsuhiro
Published: February 2008

We investigate a subharmonic‐resonant interaction between ions and magnetohydrodynamic waves propagating perpendicular to the background magnetic field, Bo. This interaction occurs at frequencies which are subharmonics of the cyclotron frequency of ions (Ωi/n, where n is an integer, 2,3,4,...). We have found that this interaction becomes quite effective if the wave magnetic field δB exceeds ∼10% of Bo. Further, when δB exceeds ∼50% of Bo, ions have stochastic orbits in the wave and are accelerated with quite high efficiency. This subharmonic resonant interaction can play quite significant roles in the ion heating/acceleration processes in space plasmas, where large amplitude waves (δB/Bo ≳ 0.1) are not unusual. Possible observational features of the subharmonic resonance which we suggest are: (1) “absorption lines” in the broad background spectrum of compressible geomagnetic pulsations, and (2) enhancement of energetic ion fluxes relating to the large fluctuation of the magnetic field magnitude.
Field and temperature behavior of remanence in synthetic goethite: Paleomagnetic implications
Authors: Rochette P. , Fillion G.
Published: February 2008

Synthetic powders of pure goethite exhibit an antiferromagnetic behavior and negligible isothermal remanent magnetization up to 4 Tesla. However thermoremanence up to 0.3 Am²kg−1 can be imparted to the samples. High field measurements show median acquisition field of 9–13 T and suggest saturation fields of the order of 10² T. Large increase in remanence at low temperature is an intrinsic feature of goethite. The weak ferromagnetism observed is linked to a bulk origin for spin imbalance rather than to an effect of very fine grain‐size or surface defects. The results are compared with properties of rocks bearing goethite and hematite.
Oceanic heat flux in the Fram Strait measured by a drifting buoy
Authors: Perovich Donald K. , Tucker Walter B. , Krishfield Richard A.
Published: February 2008

As one component of the Arctic Environmental Drifting Buoy, two thermistor strings were installed through the ice to measure ice temperatures and determine oceanic heat fluxes as the buoy drifted from the Arctic basin into the Greenland Sea. Ice temperature data between 14 Dec 1987 and 2 Jan 1988 were retrieved. During this period the AEDB progressed from approximately 81°N 4°E to 77°N 5°W. This constituted the most rapid displacement of the entire drift, coinciding with the entry of the floe into the marginal ice zone of Fram Strait. Once in the MIZ, water temperatures increased, most notably at a depth of 16 m, where values changed from −1.8°C to more than 2°C. Bottom ablation rates of 34 mm/day were observed between 21 and 28 Dec. During this excursion into warmer water, the oceanic heat flux increased by a factor of 18, from 7 W/m² to 128 W/m².
Resolution experiments for NW Pacific subduction zone tomography
Authors: Spakman Wim , Stein Seth , van der Hilst Rob , Wortel Rinus
Published: February 2008

We investigate the resolving power of ISC/NEIC P travel time data in tomographic inversions for the geometry of the subduction zones in the NW Pacific. From thermal models for the Kurile, Japan, Izu‐Bonin, Mariana and Ryukyu slabs, we generate three dimensional synthetic velocity anomalies for subducting slabs which are then projected onto a cell model for the uppermost 1400 km of the mantle. Using these synthetic models we compute synthetic delay times for ray paths corresponding to the source and receiver locations used for the actual data, add Gaussian noise, invert the synthetic data, and compare the resulting velocity structure to the initial synthetic models. This comparison is illustrated for sections through the Kuriles and the Mariana arcs. A variety of resolution artifacts are observed, which in many cases resemble features visible in the tomographic results obtained from inverting the actual ISC/NEIC data.
Internal heating and thermal constraints on the mantle
Authors: Leitch A. M. , Yuen D. A.
Published: February 2008

We have carried out numerical simulations to study the average temperature and thermal structure of an internally heated mantle. It is found that in an incompressible mantle the hottest parts are localized in regions of slow flow in the middle and upper mantle, whereas in a compressible mantle they occur over laterally extensive regions near the core‐mantle boundary (CMB). For chondritic concentrations of heat‐producing elements (hpe's), the average thermal conductivity in the mantle must be high to avoid heating the core, and the temperature of the CMB (TCMB) is low to avoid large‐scale melting. The mantle may have been extensively molten in the Archaean.
One year variations in the near Earth solar wind ion density and bulk flow velocity
Authors: Bolton Scott J.
Published: February 2008

One year periodic variations in the near Earth solar wind ion density and bulk flow velocity are reported. The variations show an inverse relationship between the ion velocity and density. The peak strength of the observed density variation ranges from 50–100% over the background. These variations imply either large scale mass loading inside the Earth's orbit or intrinsic solar modulations. Analyses of both near Earth and Pioneer Venus Orbiter spacecraft data provide a comparison at two different heliocentric distances. Several explanations for these variations are discussed.
Surface‐based remote sensing of the observed and the Adiabatic liquid water content of stratocumulus clouds
Authors: Albrecht Bruce A. , Fairall Christopher W. , Thomson Dennis W. , White Allen B. , Snider Jack B. , Schubert Wayne H.
Published: February 2008

A laser ceilometer, an acoustic sounder, and a microwave radiometer were used to estimate cloud thickness and the adiabatic and integrated liquid water content of shallow stratocumulus clouds continuously for three days using two‐minute averages. Although the observed liquid water path was close to the theoretical adiabatic value for most of the three days, there was one four‐hour period when the liquid water content dropped to about 50% of the adiabatic value. Hourly‐averaged values for a 19‐day period of intensive observations show that the cloud water content was generally close to the adiabatic value. Occasionally there were clouds greater than 300 m in depth in which the water content was clearly less than adiabatic.
Regional seismic recordings of the Soviet nuclear explosion of the Joint Verification Experiment
Authors: Priestley Keith F. , Walter William R. , Martynov Valadic , Rozhkov Mikhail V.
Published: February 2008

During August and September, 1988, the United States and the Soviet Union conducted a Joint Verification Experiment in which each country was permitted to observe a nuclear explosion at the other country's test site using hydrodynamic means. We have seismically recorded the Soviet explosion at four stations at regional distances (Δ ≤ 750km), and here report those observations. The peak‐to‐peak particle motion in the P‐wave varied from 7.54×10−1 cm/sec at Δ ≈ 160 km to 1.02 × 10−2 cm/sec at Δ ≈ 740 km. The average mb(Lg) was 5.97 corresponding to a yield of 118 KT, based on the NTS mb(Lg) ‐ yield relation of Patton [1988], The average Pn corner frequency of the Soviet explosion was 4 Hz with evidence of a second corner at about 0.7 Hz. Both of these corners could be affected by spall. Near‐regional seismograms of the Soviet JVE explosion show a large arrival within the Lg phase on the transverse component at all four sites. This may correspond to tectonic release accompanying the explosion, or to the effects of anisotropy on seismic wave propagation.
Planforms in Thermal Convection With Internal Heat Sources at Large Rayleigh and Prandtl Numbers
Authors: Weinstein Stuart A. , Olson Peter
Published: February 2008

Laboratory experiments are used to examine the effects of large amounts of internal heat generation on Rayleigh‐Bénard convection planforms in a constant viscosity, high Prandtl number fluid. Internal heat generation is simulated by lowering the boundary temperatures at a constant rate. The Rayleigh number (RaT) based on the imposed temperature difference is l.5×105 and the Rayleigh number based on the amount of internal heat generation (RaH) is varied from 0 to 3.0×106. In the absence of internal heat generation the convection planforms are the symmetric spoke pattern in which connected spokes of ascending and descending flow have, on average, equal strength. With internal heat production, the planform is asymmetric with buoyancy concentrated in isolated descending arcuate trenches and circular plumes. The upwellings are nonbuoyant, although their planform is more connected than downwellings. The amount of time dependence is observed to increase with internal heat generation, and occurs by branching and propagation of the negatively buoyant trenches.
Stability of the Martian atmosphere: Possible role of heterogeneous chemistry
Authors: Atreya S. K. , Blamont J. E.
Published: February 2008

A new hypothesis is proposed for recycling Martian CO to CO2. The same hypothesis can satisfactorily explain the recently observed depletion in CO in the middle atmosphere of Mars. The mechanism involves oxidation of carbon monoxide through heterogeneous chemistry in the presence of aerosols. It is further suggested that H2O ice aerosols in the atmosphere of Mars are particularly effective in this process. The thrust for suggesting this mechanism came from the extensive presence of aerosols in the Martian atmosphere detected by the Auguste‐Spectrophotometer Interferometer experiment on the Phobos spacecraft, combined with similar results from earlier missions, the detection of relatively low CO mixing ratios in the low to middle atmosphere by the Infrared Spectrometer experiment on the Phobos spacecraft, and the fact that earlier proposed mechanisms for recycling CO2 require either unacceptably high values of the eddy diffusion coefficient or a high water vapor abundance in the middle atmosphere of Mars. The mechanism proposed in this paper might have an analog in the Antarctic ozone hole problem, and it points out a need for laboratory measurements of appropriate sticking coefficients and rate constants.
Temperature and wind measurements and model atmospheres of the 1989 Airborne Arctic Stratospheric Expedition
Authors: Chan K. R. , Bowen S. W. , Bui T. P. , Scott S. G. , Dean‐Day J.
Published: February 2008

The ER‐2 Meteorological Measurement System provides accurate in‐situ measurements of atmospheric state variables. During the Airborne Arctic Stratospheric Expedition (AASE) the ER‐2 flew over the polar region on 14 occasions in January and February, 1989. Vertical temperature profiles, during aircraft takeoff at ≈ 60°N and during mid‐flight descent and ascent at high latitudes, are presented. Latitudinal variations of the horizontal wind measurement are illustrated and discussed. Based on observational data, model atmospheres at 60° and 75°N, representative of the environment of the AASE campaign, are developed.
Airborne lidar observations in the wintertime Arctic stratosphere: Polar stratospheric clouds
Authors: Browell E. V. , Butler C. F. , Ismail S. , Robinette P. A. , Carter A. F. , Higdon N. S. , Toon O. B. , Schoeberl M. R. , Tuck A. F.
Published: February 2008

Polar stratospheric cloud (PSC) distributions in the wintertime Arctic stratosphere and their optical characteristics were measured with a multi‐wavelength airborne lidar system as part of the 1989 Airborne Arctic Stratospheric Expedition. PSCs were observed on 10 flights between January 6 and February 2, 1989, into the polar vortex. The PSCs were found in the 14–27 km altitude range in regions where the temperatures were ≤195 K. Two types of aerosols with different optical characteristics (Types Ia and Ib) were observed in PSCs thought to be composed of nitric acid trihydrate. Type Ia PSCs typically exhibited low scattering ratios (1.2–1.5) and high aerosol depolarizations (30–50%) at 603 run, while Type Ib PSCs had higher scattering ratios (3–8) and lower aerosol depolarizations (0.5–2.5%). Water ice PSCs (Type 2) were observed to have high scattering ratios (>10) and high aerosol depolarizations (>10%) at temperatures ≤190 K.
An analysis of lidar observations of polar stratospheric clouds
Authors: Toon Owen B. , Browell E. V. , Kinne S. , Jordan J.
Published: February 2008

We interpret lidar observations by Browell et al. (1990) using single scattering calculations for nonspherical particles and aerosol microphysical calculations. Many of the lidar observations are consistent with particles containing 10 ppbv of condensed nitric acid vapor and an equivalent mass of water. The lidar observations of these Type 1 clouds identify two subtypes, whose properties we deduce. Type Ib particles arc spherical, or nearly spherical, and typically have radii near 0.5 (μm Type Ia particles are not spherical, and have a spherical volume equivalent radius exceeding 1.0 μm. Several factors may cause variations in the size of the particles. The UK) St significant factors are the cooling rate and the degree to which the air parcels cool below the condensation point. Specific examples in which cooling rate and cooling point nrny have led to variations in particle size are found in the Browell et al. (1990) data set. Condensation of 1 ppmm of water or less is quantitatively sufficient to account for the magnitude of the lidar backscatter observed from water ice clouds. The ice particles are not spherical in shape. The sizes of particles in water ice clouds cannot be determined because they are much larger than the wavelength of the lidar.
In situ measurements of BrO in the Arctic stratosphere
Authors: Toohey D. W. , Anderson J. G. , Brune W. H. , Chan K. R.
Published: February 2008

Mixing ratios of BrO have been measured in the arctic lower stratosphere with an instrument mounted on the NASA ER‐2 aircraft. Observations from fourteen flights above the Arctic Circle in January and February of 1989 defined mixing ratios within the vortex of 4±2 parts per trillion by volume (pptv) at a potential temperature of 400 K, rising to 8±2 pptv at 470 K. These values are twice as large as values found at equivalent potential temperatures at lower latitudes, and are comparable to the mixing ratios found inside the antarctic polar vortex. Within the statistical uncertainty of the measurements, no BrO was observed in darkness at any time either inside or outside of the vortex, indicating that active bromine was sequestered in long‐lived reservoirs, probably BrONO2 and BrCl. These measurements, in conjunction with measurements of ClO, demonstrate that the interaction of bromine and chlorine could represent a major sink for ozone in the presence of sunlight.
Comparison of 10.7 cm radio flux with SME solar Lyman alpha flux
Authors: Barth Charles A. , Tobiska W. Kent , Rottman Gary J. , White Oran R.
Published: February 2008

Measurements of the solar Lyman alpha flux that were made over a seven‐and‐one‐half‐year period between October 11, 1981 and April 13, 1989 have been compared with ground‐based measurements of the solar 10.7 cm radio flux made over the same time period. There is a long‐term correlation between these two measures of solar flux during the declining part of the solar cycle. During the solar minimum period, there is only a poor correlation between the two solar fluxes because the 10.7 cm radio flux reaches a minimum of 65 × 10−22 W m−2 Hz−1 and does not vary below this value while the Lyman alpha flux continues to decline and show long‐term and short‐term variations. During the early ascending phase of the new solar cycle, there is again a correlation between the two fluxes, although the constant of proportionality between the two is different from the constant during the declining phase of the previous solar cycle. Somewhat later, during the period November 25, 1988 — April 13, 1989 (last period when observations of Lyman alpha were made) a medium‐term correlation exists and the proportionality of the two indices is once again similar to what it was during the declining phase of the previous solar cycle. A study of the correlation of the 10.7 cm flux with the Lyman alpha for a 999‐day period during the declining phase showed that for the short‐term (27‐day) variation there is a correlation between the two fluxes but the proportionality between them varies from one solar rotation to the next. The conclusion is that the solar 10.7 cm radio flux is not a useful index for the prediction of solar Lyman alpha flux for the short‐term, 27‐day variations.
Remote sensing of wind speed at sea surface level using HF skywave echoes from decametric waves
Authors: Gaffard C. , Parent J.
Published: February 2008

Using an HF skywave radar to study the decametric sea waves, it appears that their amplitudes do not saturate as the wind speed increases. A measurement method, based on the Kahma sea wave spectrum, is proposed and radar estimations of the wind speed are compared with in situ measurements performed on board a ship. The rms difference between the two measurements is 2.5m/s and is probably mainly due to the spatial variability in the wind speed.
Kelvin‐Helmholtz instability for supersonic shear flow at the magnetospheric boundary
Authors: Miura Akira
Published: February 2008

It is demonstrated by means of a MHD simulation that a finite thick velocity shear layer with super‐Alfvénic velocity jump is unstable to the Kelvin‐Helmholtz (KH) instability no matter how large the sonic Mach number, a result suggesting that the tail flank boundary is unstable to the KH instability. For supersonic shear flow the unstable mode becomes damped‐oscillatory in the magnetosheath. For both subsonic and supersonic shear flows, the energy flux density into the magnetosphere by the KH instability is large enough to replenish the plasma in the low latitude boundary layer with the tailward flow kinetic energy of observed intensity. A significant fraction of the energy flux density can reach deeper into the magnetosphere and its intensity is comparable to an energy flux density required for excitation of a ULF wave in the magnetosphere.
Reconnaissance techniques for determining soil‐gas radon concentrations: An example from Prince Georges County, Maryland
Authors: Reimer G. M.
Published: February 2008

Preliminary field measurements of soil‐gas radon (Rn) concentrations can provide an initial assessment of the Rn potential in any location and aid in designing a larger or more detailed sampling program. Radon reconnaissance requires some special considerations because a large area must be covered in a short period of time and analyses must be made soon after collection because of Rn decay. A simple approach to collection and field analysis consists of a small‐diameter probe pounded into the ground to a depth of at least 0.75 m. Analysis is by an alpha‐scintillometer. Soil‐gas samples collected along a traverse in Prince Georges County, Maryland, demonstrate the utility of the technique. Although this region in the Coastal Plain province might be considered to possess the type of geologic setting that would not contribute to significant Rn potential, the reconnaissance sampling revealed Rn soil‐gas concentrations of up to 2500 pCi/L (picocuries per liter) indicating that the potential exists for indoor accumulations in excess of 4 pCi/L.
Broadband source spectrum, seismic energy, and stress drop of the 1989 MacQuarie Ridge Earthquake
Authors: Houston Heidi
Published: February 2008

I compute the broadband source spectrum at periods from 1 to 50 seconds using teleseismic P body waves of the May 23, 1989 Macquarie Ridge earthquake (Mw = 8.1) recorded by the GDSN, GEOSCOPE, and IDA networks. The average source spectrum is obtained by windowing, tapering, and Fourier‐transforming P waves, removing from the spectra the effects of attenuation, geometrical spreading, and radiation pattern, and averaging logarithmically over the stations. The source spectrum for the strike‐slip Macquarie Ridge earthquake is higher than an average source spectrum of seven recent large earthquakes (scaled to be comparable to a Mw = 8.1 earthquake) by a factor of 2 to 3 at periods of 1 to 20 seconds. These other earthquakes were underthrusting events in subduction zones. Using Haskell's formulation assuming a point source with no directivity, I estimate the seismically radiated energy from the source spectrum by integrating the square of the source spectrum in velocity and scaling the result. The seismic energy thus estimated for the Macquarie Ridge earthquake is 3 to 8 ×1023 ergs. An Orowan stress drop can be obtained from the seismic energy and moment. The Orowan stress drop for the Macquarie Ridge earthquake is about 20 to 50 bars, much higher than similarly determined stress drops of other recent large earthquakes. There is a correlation between the Orowan stress drops and time since the last earthquake of comparable or larger magnitude for seven recent large earthquakes. This correlation suggests that a healing process operates that may control the mechanical strength of the fault and is important on time scales of tens to hundreds of years.
Amplification of electromagnetic ion cyclotron waves along a wave path in the Earth's multicomponent magnetosphere
Authors: Hu Y. D. , Fraser B. J. , Olson J. V.
Published: February 2008

In this report, we consider the amplification of electromagnetic ion cyclotron waves along a geomagnetic field line in the multicomponent magnetosphere, assuming that the waves propagate parallel to the background magnetic field. We find it is possible for the ring‐current protons (energy ∼10–100 keV), which supply the free energy to stimulate the waves, to resonate with the waves not only in the equatorial region but also off the equator. An instability, caused by a thermal anisotropy, may occur in separated regions on and/or off the equator. The positions of the source regions along the wave path depend on the concentration of cold heavy ion species. The significant off‐equator source regions may be located at geomagnetic latitudes where the waves, with frequencies greater than the He+ gyrofrequency on the equator, are in a local He+ pass band.
Moment tensor rate functions for the 1989 Loma Prieta Earthquake
Authors: Ruff Larry J. , Tichelaar Bart W.
Published: February 2008

The fault plane and overall coseismic slip of the 1989 Loma Prieta, California, earthquake (Ms=7.1) are well determined [Plafker and Galloway, 1989]. Teleseismic waves can be used to determine the time history of moment release. We invert a data set of ten broadband P and SH waves for the most general point source description: the five moment tensor rate functions. The linear inversion also provides formal estimates of model uncertainty. While the moment tensor rate functions suggest a different focal mechanism for the first few seconds of the rupture process, it is not statistically significant at the 95% level. We can thus proceed to invert for one single time function (the source time function), and five scalars (the moment tensor). The major double couple that we find (strike 138°±6° clockwise from North, dip 76°±5°, slip angle 120°±10°) agrees with the results of Plafker and Galloway [1989]. The minor double couple is small (1%). The best point source depth is about 10 km. Several broadband P waves show that a small precursor occurred a few seconds before the main pulse of moment release. The duration of the source time function is 9 s, with a moment of 2±0.5 ×1019 Nm. A bilateral rupture with velocity of 2.8 km/s and 9 s duration encompasses the aftershock region.
Venus: Crater distributions at low northern latitudes and in the Southern Hemisphere from new Arecibo observations
Authors: Campbell D. B. , Stacy N. J. S. , Hine A. A.
Published: February 2008

Arecibo high resolution radar data covering approximately 23% of the surface of Venus was analyzed for circular features with crater‐like appearances. Overlapping Venera 15/16 radar data, with its more normal to the surface viewing geometry, was used to verity crater diameters measured from the Arecibo data. A comparison of the diameter‐frequency distribution of the possible impact features in the Arecibo data for northern latitudes south of 40°N and for the southern hemisphere with the distribution from the Venera 15/16 data for the northernmost quarter of the planet indicated similar distributions for large crater‐like features with diameters > approximately 30 km, but a deficiency of smaller craters in the Arecibo data. The average crater density measured for all craters in the northernmost quarter of the planet from the Venera 15/16 data was 1.27 per 106 km2 while the average for the 106 ×l06km2 covered by the Arecibo data was 0.95 per 106 km2. For craters with diameters less than 22km, the densities were 0.59 per million square kilometers for the Venera data compared with 0.33 per 106 km2 for the average of the two Arecibo data regions. All circular features in the Arecibo data with a crater‐like appearance that could not be excluded as being impact features were used in this study so the result represents an upper limit on the crater densities in the size interval considered. One possible process responsible for the density differences of small craters is a difference in the rate of volcanic resurfacing. An increase in the resurfacing rate of approximately 50% would explain the lower density of small craters in the area covered by the Arecibo data compared with the Venera data, but would not account for the differences being confined to the < 30 km crater diameter range.
Constraining sediment thickness in the San Francisco Bay Area using observed resonances and P‐to‐S conversions
Authors: Hough S. E.
Published: February 2008

Between October 20, 1989, and October 27, 1989, aftershocks of the Loma Prieta earthquake were recorded at five locations on a variety of site conditions in the West Oakland, California region. We show evidence of P‐to‐S conversions that have comparable characteristics on both mud and alluvium sites, suggesting that these conversions are generated at the base of the alluvium layer. The inferred transfer function for the sedimentary layer and the travel‐time observations for direct and converted phases can be used to constrain the depth of the sedimentary layers. Both the amplitude and many characteristics of the observed transfer functions on mud and alluvium are well predicted with one‐dimensional modelling. The results suggest that, in some cases, separate resonance peaks result from multiple sediment layers.
Tectonic release from the Soviet Joint Verification Experiment
Authors: Walter William R. , Patton Howard J.
Published: February 2008

The regional broadband seismic recordings of the Soviet Joint Verification Experiment underground nuclear explosion show evidence of tectonic release in the form of Love waves at long periods and high‐frequency SmS phases on the transverse components. We present the results of some simple forward modeling of this data in order to quantify both the moment and mechanism of the tectonic release. Using 10–20 second Love wave amplitude and phase data, we estimate the tectonic release moments to be 3.5 × 1015 N‐m for a north striking right‐lateral strike‐slip fault, and 7.0×1015 N‐m for a northwest striking reverse fault. Using the results of these end member tectonic models, the Rayleigh waves can be satisfied with explosion moments of 1.7×1016 N‐m (F=0.31) and 2.4×l016 N‐m (F=0.44), respectively. To resolve the ambiguity in tectonic mechanism we used the reflectivity method to generate synthetics and compare the relative amplitudes of the SH and SV parts of the SmS phase. While the results favor the strike‐slip mechanism of tectonic release, application of source inversion techniques to this data and other regional data will be needed to evaluate more realistic tectonic release models involving, for example, oblique fault slip and dip.
Near‐field high resolution strain measurements prior to the October 18, 1989, Loma Prieta Ms 7.1 Earthquake
Authors: Johnston M. J. S. , Linde A. T. , Gladwin M. T.
Published: February 2008

High resolution strain recordings were made in deep boreholes throughout California prior to, during, and following, the October 18, 1989, Ms 7.1 Loma Prieta earthquake. The nearest dilational strainmeters (sensitivity 1010) and 3‐component tensor strainmeters (sensitivity 10−9) were 37 km to 42 km, respectively, from the main shock. High quality data, including details of strain offsets, were recorded on both instruments through the earthquake. These data have been searched for indications of short‐, intermediate‐, and long‐term strain redistribution and/or fault slip that might have indicated imminent rupture. Short‐ and intermediate‐term changes in both tensor strain and dilational strain (≤, several nanostrain, if any) during the minutes to months before the earthquake are at least 1000 times smaller than that generated by the earthquake itself. If short‐term preseismic slip did occur at the nucleation point of the earthquake during the previous week, and if the type of slip is similar to that during the earthquake, its moment could not be more than 1024 dyne‐cm. Stated another way, slip equivalent to that expected for a M 5.3 earthquake could have occurred in the hypocentral region without the strainmeters detecting it at these distances and azimuthal positions. Long‐term changes appear to have occurred in mid 1988 and mid 1989. These changes were both followed by ML 5 earthquakes in the hypocentral region on June 27, 1988, and August 8, 1989, respectively, and, since they correspond approximately to changes in geodetic strain rate over the epicenter, may indicate precursory strain redistribution in the epicentral region. Minor postseismic strain recovery (≈ 14%) occurred in the month following the main shock.
Variations in the mode of great earthquake rupture along the Central Peru Subduction Zone
Authors: Beck Susan L. , Nishenko Stuart P.
Published: February 2008

The historic record for the central Peru subduction zone suggests significant variations in the earthquake size during the last 400 years. During this century there have been four great underthrusting earthquakes along the central Peru seismic zone. From north to south these are the 17 October 1966 (Mw=8.1), 24 May 1940 (M∼8), 3 October 1974 (Mw=8.1), and 24 August 1942 (M∼8.2) earthquakes. Modified Mercalli intensity data and tsunami observations for the earthquakes in this century are compared with the 29 October 1746 and 20 October 1687 earthquakes. The 1746 earthquake had maximum intensity values between 9° and 13°S while the 1687 event had maximum values between 12° and 14°S suggesting that the two events failed different segments of the subduction zone. We find that the 1746 event occurred along the segiment that includes both 1940 and 1966 earthquakes. The size of the 1746 event is estimated to be Mw ∼8.8 based on the ratio of near‐field tsunami heights for the 1746 and 1966 earthquakes. The 1687 earthquake probably raptured the 1974 segment as well as the adjacent segment to the south where there is at present a gap between the 1942 and 1974 rupture zones. The size of the 1687 event is estimated to be Mw∼8.7 based on both far‐field and near‐field tsunami height ratios of the 1687 and 1974 events. Both 1746 and 1687 earthquakes appear to be much larger than the events of this century. In contrast to the simple, single asperity nature of the 20th century earthquakes, these older and larger events may represent multiple‐asperity ruptures along the Peru subduction zone. Hence, variations in the mode of earthquake rupture from cycle to cycle along the central Peru seismic zone may explain the significant difference in earthquake size during die last 400 years.
The stably‐stratified outermost core revisited
Authors: Lay Thome , Young Christopher J.
Published: February 2008

The P‐wave velocity structure of the outermost 200 km of the outer core (the E' region) is analyzed using a suite of differential travel‐times for SKS, SKKS and S‐waves bottoming below, within, and above this depth range. Core‐penetrating phases with paths beneath Alaska exhibit similar SKS‐SKKS differential travel‐time patterns to phases sampling the core beneath the central Pacific, but very different SKS‐S differential travel‐times. These patterns can be explained by a core velocity model in which the outermost 50 to 100 km has 1–2% slower P‐wave velocities than standard Earth models, perhaps due to a stably‐stratified chemical boundary layer, combined with 3% lateral variations in S‐wave velocity in the lowermost mantle.
Numerical estimation of interactive effects in single‐domain magnetite
Authors: Sprowl Donald R.
Published: February 2008

Direct numerical calculation of interactive effects among Stoner‐Wohlfarth particles is presented. Hysteresis behavior of both individual grains and the bulk sample are altered significantly by magnetic interaction between the individual grains, causing clustered single‐domain particles to behave like multi‐domain particles. Reversible interactive effects and tangential interactive fields are important and should not be neglected in future modeling.
Re‐ordered electron data in the low‐latitude boundary layer
Authors: Hapgood M. A. , Bryant D. A.
Published: February 2008

We have made a complete survey of all electron data recorded during low‐latitude boundary layer crossings of the AMPTE‐UKS spacecraft. In 27 of the 31 cases there is a clear anticorrelation between the density and the mean energy of the electrons during the transition across the boundary layer. The simplest interpretation of this anticorrelation is that, in the majority of cases, there is a smooth change of plasma state between the bounding solar and terrestrial plasmas, and that the abrupt changes frequently seen in time series data arise from boundary layer motions. Our data indicate that there are always two distinct sections to the anticorrelation. We suggest that these represent two physically distinct regions within the boundary layer and indicate how electron density and energy might vary across them. We briefly discuss an alternative interpretation in which filaments of magnetosheath plasma penetrate the boundary layer. We show that our results provide a constraint on such a model.
Magnetosheath distortion of pitch angle distributions of solar protons
Authors: Palmer I. D. , Higbie P. R.
Published: February 2008

The propagation of energetic solar protons of ≲1 MeV into the magnetosheath is investigated through three‐dimensional pitch angle distributions measured on Vela satellites. Distortions are observed in the magnetosheath as compared with isotropic or unidirectional distributions normally expected in inter‐planetary space. Two types of distortions are observed which are characterized by breaks in the distributions at μ0 < 0 or μ0 > 0, where μ is the cosine of the pitch angle. The distributions in the magnetosheath are explained by a Liouville transformation, if particle motion across the bow shock and through the magnetosheath is assumed to be adiabatic. Whether μ0 is positive or negative is determined by whether the satellite in the magnetosheath lies beyond or in front of the region of maximum magnetic field compression (or neck) in the magnetosheath, relative to the direction of the net flow of particles. The magnitude of μ0 is a measure of the field ratio between neck and satellite. Scattering effects, which must occur at the bow shock and in the magnetosheath, only perturb the adiabatic propagation of the particles. The results show that one must be cautious in inferring the true interplanetary anisotropy from that measured in the magnetosheath. While the maxima and minima of the corresponding pitch angle distributions will be the same, the distributions can be vastly different, and both anisotropy and omnidirectional intensities significantly different, too. By corollary, the pitch angle distribution and anisotropy measured in the solar wind on a field line that intercepts the bow shock sunward of the earth will in general be different from that which would be measured in interplanetary space on a field line not connected to the bow shock; to first order the effect of the bow shock can be computed by treating the motion as adiabatic.
Artificially created holes in the ionosphere
Authors: Mendillo Michael , Forbes Jeffrey M.
Published: February 2008

The artificial creation of ionospheric holes by the release of highly reactive molecules (e.g., H2 or H2O) into the F region is investigated. Through ion‐atom interchange or charge transfer reactions, H2 or H2O reacts with O+ to form OH+ or H2O+, respectively, which subsequently dissociatively recombines with electrons at a very rapid rate. The diffusion of H2 is also modified by chemical loss to the ambient atomic oxygen atmosphere. The limited spatial and temporal extent of the hole‐making process allows several approximations to be made which permit three‐dimensional analytic solutions of the continuity equations for the released particles, the O+ and e− densities, and the intermediary molecular ions. A versatile formalism is developed whereby the hole‐making capability of virtually any spatial‐temporal configuration of released particles can be determined by convolving a set of ‘destruction operators’ which can be viewed as Green's functions for the problem. Sample calculations are presented which demonstrate the competing effects of differences in the molecular weight of the released particles, the altitude of the release, the reactivity of the released molecules with the ambient atomic oxygen atmosphere, and the quantity and distribution of the released molecules. As a specific application of the techniques developed, the modification of a winter nighttime ionosphere over Millstone Hill is described by simulating the release (for example, from the space shuttle) of 1000 kg of water vapor (≃3 × 1028 molecules) near a height of 300 km.
Plasma electron observations in the wake of Venus
Authors: Yeates C. M. , Ogilvie K. W. , Siscoe G. L.
Published: February 2008

Two plasma regimes were observed in the Venus wake, one characterized by negative‐going anisotropies accompanied by high bulk speeds and low magnetic variance and the other characterized by positive‐going anisotropies accompanied by low bulk speeds and high magnetic variance. The two modes alternate in time in a stochastic bimodal fashion as interplanetary magnetic field discontinuities propagate past Venus and switch on the positive anisotropies. The positive mode probably represents enhanced planetary ion pickup which results in instabilities, the damping of which energizes the electrons in the distant wake.