bluuebunny/crossref_metadata_2025 huggingface dataset:
Dayside auroral intensifications during an auroral substorm
Authors: Lui A. T. Y. , Venkatesan D. , Rostoker G. , Murphree J. S. , Anger C. D. , Cogger L. L. , Potemra T. A.
Published: February 2008

Variations in the dayside and nightside auroral luminosity during the substorm activity on October 15, 1986 are examined using images obtained by the ultraviolet (UV) imager on board the Viking spacecraft. It is observed that the intensity and dynamic behavior of the localized auroral emissions around the 14 MLT sector can be comparable to that found for nightside discrete arc structure seen during auroral‐breakup conditions. For the event studied in this paper, spatial periodicities in luminosity are observed in both the dayside and nightside auroral oval during substorm activity with the azimuthal separation between bright spots on the dayside being approximately 1.5 hours of MLT in the postnoon quadrant and approximately 0.5 hour of MLT in the midnight sector. However, we have no direct evidence that these groups of spots are necessarily linked causally. We suggest that such features could be a manifestation of a Kelvin‐Helmholtz instability located at the sites of a velocity shear zone.
Limits on modes of lithospheric heat transport on Venus from impact crater density
Authors: Grimm Robert E. , Solomon Sean C.
Published: February 2008

We present a formalism by which the size‐frequency distribution of impact craters on Venus may be used to estimate upper bounds on the mean global rates of volcanic resurfacing and lithospheric recycling on that planet over the past several hundred million years. The principal assumptions are that an upper bound on the rate of crater production on Venus may be estimated and that craters are volcanically obliterated only when flows completely cover their rims. The impact crater density reported from Venera 15‐16 observations, if valid for the entire Venus surface, then indicates a mean volcanic flux no greater than 2 km³/y, corresponding to a maximum average rate of resurfacing of about 4 km/b.y. The fraction of global heat loss due to such a rate of volcanic resurfacing is negligible. For the lowest estimated crater retention age of the surface imaged by Venera 15‐16, if globally representative, the rate of lithospheric recycling on Venus does not exceed 1.5 km²/y, corresponding to 25% of the global heat loss. More likely estimates of the mean surface age limit the contribution to global heat loss of any lithospheric recycling to be less than 10%. These results support the hypothesis that simple conduction dominates heat transport at lithospheric levels in the Venus interior.
Dimethyl sulfide over the western Atlantic Ocean
Authors: Van Valin C. C. , Berresheim H. , Andreae M. O. , Luria M.
Published: February 2008

Dimethyl sulfide and anthropogenic pollutants were measured over the western Atlantic Ocean during January and February 1986, 50 to 150 km east of Boston, MA and in the vicinity of Bermuda in June 1986. The highest DMS concentration measured in the vicinity of Boston was 18 pptv; all other values within the mixed layer (ML) were between 0.5 and 12 pptv. The free troposphere (FT) values were 0.5–5.0 pptv. Sea to air fluxes were estimated at 36 to 66 ng m−2 min−1. FT values measured near Bermuda were 1–17 pptv, and most values were <10 pptv. The Bermuda ML values (35–195 pptv) were as much as 10 times greater than those found near Boston. A steep vertical gradient was observed within the ML in the mid‐ocean measurements, indicating that DMS removal rates were faster than the ML mixing times. Several variables may be of consequence in the differences observed between the Boston and Bermuda measurements; the most important of these appear to be the air mass travel time over the ocean and the sea surface temperature.
The size distribution of Pacific Seamounts
Authors: Smith Deborah K. , Jordan Thomas H.
Published: February 2008

An analysis of wide‐beam, Sea Beam and map‐count data in the eastern and southern Pacific confirms the hypothesis that the average number of "ordinary" seamounts with summit heights h ≥ H can be approximated by the exponential frequency‐size distribution: v(H) = vo e−βH. The exponential model, characterized by the single scale parameter β−1, is found to be superior to a power‐law (self‐similar) model. The exponential model provides a good first‐order description of the summit‐height distribution over a very broad spectrum of seamount sizes, from small cones (h < 300 m) to tall composite volcanoes (h > 3500 m). The distribution parameters obtained from 157,000 km of wide‐beam profiles in the eastern and southern Pacific Ocean are vo = (5.4 ± 0.65) × 10−9m−2 and β = (3.5 ± 0.21) × 10−3 m−1, yielding an average of 5400 ± 650 seamounts per million square kilometers, of which 170 ± 17 are greater than one kilometer in height. The exponential distribution provides a reference for investigating the populations of not‐so‐ordinary seamounts, such as those on hotspot swells and near fracture zones, and seamounts in other ocean basins. If we assume that volcano height is determined by a hydraulic head proportional to the source depth of the magma column, then our observations imply an approximately exponential distribution of source depths. For reasonable values of magma and crustal densities, a volcano with the characteristic height β−1 = 285 m has an apparent source depth on the order of the crustal thickness.
High resolution images of Venus from ground‐based radar
Authors: Jurgens R. F. , Slade M. A. , Robinett L. , Brokl S. , Downs G. S. , Franck C. , Morris G. A. , Farazian K. H. , Chan F. P.
Published: February 2008

Radar images of Venus with best resolution near 1.3 km have been formed using the Goldstone Deep Space Stations as a ground‐based synthetic aperture radar system. Twelve days of observations were successful during the 1986 inferior conjunction, and we have selected three observations for initial processing to demonstrate the new capability. The three images show several remarkable surface features including regions of high Fresnel reflectivity in the plains region.
Lithologic column of the “Arkoma” Drillhole and its relation to the Cajon Pass Deep Drillhole
Authors: Silver Leon T. , James Eric W.
Published: February 2008

The 1795 m "Arkoma" Federal 1‐26 well, 48.5 m from the Cajon Pass Deep Drillhole (CPDDH), provides additional lithologic and structural data pertinent to that project. Basement surface was encountered 158 m deeper than in the CPDDH. Rock units correlate well between the holes; the offset persists to 1128 m. Below this depth both lithologic unit thicknesses and fault zones correlate between holes on subhorizontal projections. A combination of previously unrecognized high‐angle and low‐angle faults of several ages are required to explain the structures. Blind low angle faults may be regionally important.
Constraints on the geometries of structures within the sedimentary rocks at the Cajon Pass Scientific Drillsite, California
Authors: Pezard Philippe A. , Weldon Ray J. , Anderson Roger N. , Wilkinson Craig R. , Ollier Gilles R.
Published: February 2008

An analysis of downhole geophysical logs, and their relation to surface geological data constrain the structure in the first 700 m of the Cajon Pass scientific drillhole. Several angular folds, mapped in a small canyon 150 m to the southeast of the drillsite, are associated with a steep northeast dipping reverse fault at depth. This fault is inferred to cut the scientific drillhole in basement at 700 m, and to intercept a nearby oil‐exploration well, Arkoma Federal 1‐26, within the sediments at 350 m. The folds mapped at the surface are thought to have developed in Late Miocene to Early Pliocene time, during the Squaw Peak thrusting event. The inferred reverse fault offsets the sediment‐basement contact 180 m between the two holes.
In situ permeability and fluid pressure measurements at ∼2 km depth in the Cajon Pass research well
Authors: Coyle Brian J. , Zoback Mark D.
Published: February 2008

Two in situ bulk permeability tests were conducted in the metamorphic basement rocks in the Cajon Pass research well. In one interval, from 1829 m to 1905 m, we measured an effective permeability of 0.5 × 10−18 m² (0.5 × 10−6 darcy). Over an interval from 1829 m to 2115 m, we measured an average permeability of 1.67 × 10−18 m² (1.67 × 10−6 darcy). In addition, fluid pressures 5% over hydrostatic were determined through analysis of pressure build‐up in the larger test interval. The in situ permeability measured is one to three orders of magnitude greater than the permeability measured in core samples; a difference we attribute to the presence of fluid conducting fractures. A large fracture at 2076 m with an aperture of 26.6 cm appears to be a zone of localized high permeability. The permeability of this fracture is as high as 1.8 × 10−16 m² (0.18 × 10−3 darcy). In light of the low permeability and fluid pressure measured, the transport of significant amounts of frictionally generated heat by hydraulic conduction is extremely unlikely.
Crack‐like rupture processes observed in near‐fault strong motion data
Authors: Yomogida Kiyoshi
Published: February 2008

Strong motion data observed directly above a rupture zone of the Michoacan, Mexico, earthquake in 1985 provide, for the first time, clear evidence for crack‐like rupture process in a major earthquake. The observed simple ramp‐function type displacements cannot be explained by dislocation models such as a step‐function dislocation, but require a crack‐like slip function which grossly satisfies stress conditions on the fault (i.e., stress drops to some finite level after a passage of rupture front). This conclusion is required because the records contain sufficient near‐field terms to distinguish between the two models.
Large‐scale structure of the electric field and field‐aligned currents in the auroral oval From the Intercosmos‐Bulgaria‐1300 satellite data
Authors: Podgorny I. M. , Dubinin E. M. , Izrailevich P. L. , Nikolaeva N. S.
Published: February 2008

Electric fields and currents were studied at the night side of the auroral oval. The electric field distribution between the field‐aligned current sheets is determined by the Pedersen conductivity of the ionosphere. The ionosphere is a load in an electric circuit whose generator is located in the magnetosphere. The site of electric field generation is assumed to be the current sheet of the magnetospheric tail. The plasma in the tail is assumed to be polarized under the influence of a Lorentz force. The polarization field is projected along geomagnetic field lines to the auroral zone and causes the Hall current that constitutes a westward electrojet.
A theoretical interpretation of upstreaming electrons and elevated conics on auroral field lines
Authors: Ashour‐Abdalla Maha , Schriver David
Published: February 2008

Recent VIKING satellite observations in the auroral zone have shown the association of elevated ion conics (conics with a low energy cutoff above zero) with upward streaming electrons in the presence of low frequency electric field fluctuations. A self‐consistent particle simulation was developed which assumed the presence of a steady state electric field on auroral zone field lines capable of accelerating ions up the magnetic field lines. Results from this study show that a low frequency ion‐ion two stream instability can be excited. This low frequency instability creates a fluctuating electric field which heats the ions oblique to the magnetic field forming distributions similar to the elevated ion conies. The ion‐ion waves also interact resonantly with electrons and accelerates them in the direction of the ion beam.
Teleseismic investigation of the January 22, 1988 Tennant Creek, Australia, earthquakes
Authors: McCaffrey Robert
Published: February 2008

The three January 22, 1988, earthquakes are the largest to occur within the Australian continent in 20 years and produced two colinear but discontinuous WNW trending scarps enclosing a shorter ENE trending scarp. Teleseismic long period P and SH and short period P waves are used to constrain the locations and source parameters for the three events. The third earthquake has a seismic moment (1019Nm) equal to that of the 1968 Meckering event and most likely ruptured the southern of the WNW trending faults. Relative relocations suggest that the two smaller earthquakes initiated north or NW of the third event. Centroid depths are less than 6 km, but most likely 3 to 4 km, and source durations are 4 to 6s so it is probable that each event broke the surface. All three earthquakes have predominantly thrust mechanisms and nodal planes that strike west to WNW, consistent with faults dipping southward at 30° to 40°. Only the first has a nodal plane that could support a north dip for the ENE trending fault, as proposed by other researchers on the basis of ground deformation and preliminary aftershock locations. However, source directivity evident in the short period P waves and relocations are interpreted better if this event, like the others, ruptured a SW dipping fault plane. The ENE trending scarp is interpreted to be a tear fault in the hanging wall that may have acted as a barrier to slip on a single continuous SW dipping fault, resulting in this unusual, at least for Australia, sequence of three large similar earthquakes.
Relativistic electrons near geostationary orbit: Evidence for internal magnetospheric acceleration
Authors: Baker D. N. , Blake J. B. , Callis L. B. , Belian R. D. , Cayton T. E.
Published: February 2008

At times, relativistic electron fluxes in Earth's outer magnetosphere are not obviously related to an external (Jovian or solar) source. This finding suggests that an internal magnetospheric acceleration mechanism may operate under some circumstances. A possible mechanism identified for Jupiter's magnetosphere could also be considered in the terrestrial case. Such a model requires the substorm‐ generation of a spectrally‐soft electron component with subsequent inward radial diffusion (violating the third adiabatic invariant). A large electron energy gain transverse to the magnetic field occurs in this process. Eventually, deep within the magnetosphere, substantial pitch angle scattering occurs violating all adiabatic invariants. Then, at low L‐values, there occurs an energy‐preserving outward transport of energetic electrons near the mirror points. This leads to a return of the accelerated population to the outer magnetosphere. Such low‐altitude processes should result in “conic”quot; or “butterfly” pitch angle distributions at very high energies as the electrons execute trans‐L diffusion at the mirror altitudes and then are magnetically focussed near the equator. Data collected concurrently at geostationary orbit at three widely‐spaced local times during a relativistic electron event show a butterfly pitch angle distribution, while lower energy electrons simultaneously show pancake‐like distributions. The butterfly pitch angle distributions appear in ∼ 25% of the examined relativistic electron events, thereby providing support for acceleration by a recirculation process.
Electron velocity space hole modes and narrowband electrostatic noise in the distant tail
Authors: Coroniti Ferdinand V. , Ashour‐Abdalla Maha
Published: February 2008

Narrowband electrostatic noise (NEN) is frequently observed upstream of the slow shocks in the distant tail. We suggest that NEN represents a non‐standard plasma wave mode which results from a hole in the low energy electron velocity distribution. A very simple model which plausibly indicates how a hole might be formed by the interaction of the electrons with the slow shock is presented. The hole mode is destabilized by a positive slope in the parallel electron distribution and/or by a weak beam of energetic ions which escape upstream from the slow shock.
dayside Pi 2 pulsations at low latitudes
Authors: Sutcliffe Peter R. , Yumoto Kiyohumi
Published: February 2008

In this paper we investigate the occurrence of dayside Pi 2 geomagnetic pulsations at low and mid latitudes. The technique of data adaptive filtering is used to identify Pi 2's concealed by the presence of typical daytime Pc type pulsations. Convincing new evidence is presented demonstrating that Pi 2 pulsations occur simultaneously in both the nightside and dayside hemispheres at low latitudes. Dayside Pi 2's are occasionally identified at mid latitudes. These results have implications with regard to the source mechanism for low latitude Pi 2 pulsations and allude to a global cavity mode.
Concentrations and radiocarbon signatures of dissolved organic matter in the Pacific Ocean
Authors: Druffel Ellen R. M. , Williams Peter M. , Suzuki Yoshimi
Published: February 2008

We present evidence suggesting that only a portion of the dissolved organic carbon (DOC) in the seawater analyzed previously by Williams and Druffel (1987) was oxidized by the UV‐radiation method. High temperature catalytic (HTC) methods (Sugimura and Suzuki, 1988) used to reoxidize the central North Pacific gyre water samples reveal that the total DOC (DOCHTC) is about twice that of the UV‐oxidizable DOC (DOCuv). Indications from the original study suggest that this additional DOC contains higher concentrations of radiocarbon than in the DOCuv(Williams and Druffel, 1987). This evidence implies that DOC is older and thus more refractory with respect to biological utilization, yet is more chemically reactive with respect to photooxidation, than the fraction resistant to UV (DOCres).

We report accelerator mass spectrometry (AMS) Δ14C measurements of humic, fulvic and hydrophilic acid fractions isolated from water collected at 180 m in the North Pacific (19°N, 158°W) using XAD macroreticular resins. Δ14C values of the humic material are less than those of DOCuv from a similar depth 1200 km further north (Williams and Druffel, 1987) indicating that these humic substances are part of the ‘old’ recycled DOCuv in the ocean.
Fractal dimension: A descriptor of ice keel surface roughness
Authors: Bishop Garner C. , Chellis Suzanne E.
Published: February 2008

Measured under‐ice acoustic profile data and several large‐scale geometric parameters of ice keels are used to construct and partition an ensemble of large‐scale relief features into two subsets: a subset of keel‐like features (e.g., ice keels) and a subset of nonkeel‐like features. The draft data of each feature are regarded as a realization of a nonstationary random process while the draft increment data are regarded as a realization of a stationary zero mean random process. A maximum likelihood estimator technique (MLE) and a technique based on the variance function are used to calculate fractal dimensions for keel‐like and nonkeel‐like features. It is shown that for the same feature, the MLE technique and the variance function based technique yield similar values for the fractal dimension (D) and that for keel‐like features 1.2 < D < 1.7 while for nonkeel‐like features 1.2 < D < 1.6. The use of D in feature classification is also indicated.
Anisotropies of partial anhysteretic remanence and susceptibility in compacted black shales: Grainsize‐ and composition‐dependent magnetic fabric
Authors: Jackson Mike , Sprowl Don , Ellwood Brooks
Published: February 2008

We have quantified the magnetic fabric of the highly‐compacted Heebner Shale (Pennsylvanian) of Kansas in terms of the anisotropies of partial anhysteretic remanent magnetization (pARM) and low‐field susceptibility. Hysteresis loops exhibit almost purely paramagnetic behavior, and the ratio of high‐field linear susceptibility to low‐field susceptibility is very close to one; the susceptibility anisotropy therefore reflects the fabric of the paramagnetic matrix, while the remanent anisotropies are due to the preferred orientations of ferrimagnetic grains. Both the paramagnetic and ferrimagnetic fabrics are dominated by a horizontal foliation, with a weak northeasterly lineation, but the pARM anisotropies are generally much larger than the susceptibility anisotropy. The degree of foliation measured with pARM's decreases markedly with increasing coercivities, indicating stronger foliation in the coarser grainsize fraction. In contrast, the pARM mean lineations are grainsize‐independent, suggesting a post‐compactional origin, related to tectonic compression. The susceptibility lineation appears to have a slightly different azimuth, and may therefore be due to paleocurrents.
On the stability of rotational discontinuities
Authors: Richter Peter , Scholer Manfred
Published: February 2008

The stability of symmetric rotational discontinuities in which the magnetic field rotates by 180° is investigated by means of a one‐dimensional self‐consistent hybrid code. Rotational discontinuities with an angle Θ > 45° between the discontinuity normal direction and the upstream magnetic field are found to be relatively stable. The discontinuity normal is in the x direction and the initial magnetic field has a finite y component only in the transition region. In the case of the ion (lefthanded) sense of rotation of the tangential magnetic field, the transition region does not broaden with time. In the case of the electron (right‐handed) sense of rotation, a damped wavetrain builds up in the By component downstream of the rotational discontinuity and the discontinuity broadens with time. Rotational discontinuities with smaller angles, Θ, are unstable. Examples for a rotational discontinuity with Θ = 30° and the electron sense of rotation as well as a rotational discontinuity with Θ = 15° and the ion sense of rotation show that these discontinuities disintegrate into waves. These waves travel approximately with Alfvén velocity in the upstream direction and are therefore phase standing in the simulation system. The magnetic hodograms of these disintegrated discontinuities are S‐shaped. The upstream portion of the hodogram is always right‐handed; the downstream portion is always left‐handed.
Three‐Dimensional Convection Planforms With Internal Heat Generation
Authors: Travis Bryan , Weinstein Stuart , Olson Peter
Published: February 2008

Thermal convection planforms in an infinite Prandtl number, isoviscous fluid layer with basal and internal heating are determined using three‐dimensional finite difference calculations. With basal heating only, at Rayleigh number RaT=1.5×105 the planform consists of a weakly time‐dependent spoke pattern, with connected networks of equally buoyant up‐ and down‐wellings. With volumetric heat sources the planform is strongly time variable. It consists of negatively buoyant flow concentrated in thermal trenches [discrete, arcuate sheets] and spots [cylindrical columns], separated by diffuse, nonbuoyant upwelllngs. Rapid planform changes occur primarily by trench propagation, as observed experimentally by Weinstein and Olson [this issue]. The analogy between thermal trenches in internally heated convection and convergent plate boundaries on Earth suggests that the global pattern of subduction may evolve by propagation of Wadati‐Benioff zones along the strike of the slab.
ATLAS instrument characterization: Accuracy of the AASE and AAOE nitrous oxide data sets
Authors: Loewenstein M. , Podolske J. R. , Strahan S. E.
Published: February 2008

ATLAS, the Airborne Tunable Laser Absorption Spectrometer, was used to measure nitrous oxide in the 1987 Airborne Antarctic Ozone Experiment (AAOE) and in the 1989 Airborne Arctic Stratospheric Expedition (AASE). After the AASE, a detailed study of the ATLAS characteristics was undertaken to quantify the error inherent in the in situ measurement of atmospheric N2O. Using the latest calibration of the ATLAS (June 1989) and incorporating the recognized errors arising in the flight environment of ATLAS, we have established that for both the AASE and the AAOE most of the acquired N2O data sets are accurate to ±10% (2 sigma). Data from two of the earlier AAOE flights had a larger uncertainty.
The polar stratospheric cloud event of January 24: Part 2, PHotochemistry
Authors: Jones R. L. , Solomon S. , McKenna D. S. , Poole L. R. , Brune W. H. , Toohey D. W. , Anderson J. G. , Fahey D. W.
Published: February 2008

During the 1988/89 Airborne Arctic Stratospheric Expedition (AASE), observations of the chemical composition, aerosol characteristics and atmospheric state were obtained from two aircraft, a NASA ER‐2 and a DC‐8. In this paper we present a diagnosis of observations obtained using the ER‐2 on January 24, 1989, using a Lagrangian coupled microphysical‐photochemical model. We interpret the high chlorine monoxide mixing ratios observed from the ER‐2 on the afternoon of January 24, 1989 as a result of in situ heterogeneous release of reactive chlorine from the reservoirs HCl and ClONO2 on type 1 polar stratospheric cloud particles observed to be present at that time. This essential element in theories of polar ozone depletion has never before been observed directly in the stratosphere.
On the influence of polar stratospheric cloud formation on chemical composition during the 1988/89 Arctic winter
Authors: Jones R. L. , McKenna D. S. , Poole L. R. , Solomon S.
Published: February 2008

The northern winter polar vortex is more disturbed dynamically and warmer than the Antarctic equivalent, and correspondingly fewer polar stratospheric clouds (PSCs) are observed to form. However, the rapid flow of stratospheric air through slow moving synoptically forced PSC regions can result in exposure of both vortical and extra vortical air to PSCs intermittently throughout the winter months. This periodic exposure to PSCs may be sufficient to perturb the chemical composition of large volumes of northern hemisphere air. The synoptic forcing also leads to marked meridional flow which has a profound effect on chemical composition, having major impacts on both short term ozone depletion and the longer term recovery to lower ClOx abundances. Accurate simulation of the air flow is thus essential for the reliable calculation of ozone loss in polar regions.
Rapid whistler‐mode wave growth resulting from frequency‐time curvature
Authors: Helliwell R. A. , Mielke T. , Inan U. S.
Published: February 2008

The rapid (<50 ms) temporal growth of ducted whistlers is simulated using controlled injection of VLF pulses from the Siple Station, Antarctica transmitter. The results show that, when the frequency‐time function of the injected pulse has a positive slope and negative curvature, producing a kind of “chirp” such that it approximates the f(t) shape of a lightning‐generated whistler at frequencies above the ‘nose’ frequency, growth up to a saturation level (20–30 dB) commonly occurs within <50 ms as opposed to 200–300 ms that is required for monochromatic input signals. The phenomenon is explained in terms of second‐order‐resonance theory [Helliwell, 1967; Carlson et al., 1985; Chang et al., 1983] where the frequency variation of the pulse matches the changing cold plasma parameters, facilitating enhanced resonance interactions over extended portions of the field line.

Magnetospheric whistler‐mode signals that originate in ground sources (e.g. lightning, VLF transmitters) often stimulate or trigger nonlinear responses in the form of amplified signals, narrowband variable frequency emissions and complex sidebands [Helliwell, 1988]. One such effect commonly associated with nose whistlers is the growth of the whistler in the upper part (above the ‘nose’) of its frequency range and the associated emission triggering that tends to occur at the whistler's upper cutoff frequency (usually at 0.5 fH, where fH is the equatorial electron gyrofrequency). An example of the dynamic spectrum of such an event is shown in Figure 1a where the growth of the whistler is represented by the darkening and broadening of the upper segment of the trace. The whistler triggered emission is, in essence, a narrowband oscillation of slowly‐varying center frequency. What makes this phenomenon remarkable is the relatively short time (∼50 ms) of growth (∼20dB) of the whistler compared with the time (200–400 ms) required for a monochromatic signal under comparable conditions to exhibit the same growth [Helliwell, 1988]. Since the triggered emissions associated with whistlers and constant frequency signals (such as those from VLF transmitters) are comparable in their intensities (as observed on the ground or on satellites) and spectral characteristics, one might expect the mechanisms of their generation to be the same. On the other hand, since the typical peak intensities of whistlers excited by lightning impulses are likely to exceed the signal level injected from ground‐based transmitters that have been used in such experiments, one might simply attribute the observed difference in behavior to unknown nonlinear effects (none is suggested here) related to the high intensity of the input signals. In this paper, we describe a new experiment where the upper part (i.e. frequencies above the nose frequency [Helliwell, 1965]) of a one‐hop nose whistler is simulated using the Siple Station experimental VLF transmitter [Helliwell, 1988] to reproduce two‐hop nose whistlers that may exhibit the rapid growth described above (e.g. Figure 1a).
The November 20,1960 Peru Tsunami Earthquake: Source mechanism of a slow event
Authors: Pelayo Aristeo M. , Wiens Douglas A.
Published: February 2008

The November 20,1960 Peru earthquake excited a tsunami that was anomalously large for an earthquake of moderate magnitude (M 6.75 PAS). Body wave first motions combined with observed Love to Rayleigh wave spectral ratios indicate slip along a shallowly dipping thrust fault rather than a single force mechanism suggestive of a landslide. Deconvolution of SH waveforms shows a time function consisting of 4 subevents with a total duration of 110 s. Using this time function, body waveform inversion yields a very shallowly dipping thrust faulting mechanism (strike 0°, dip: 6°, slip: 100°) and a seismic moment of 1.9 × 1027 dyn cm. However, the time function obtained from the band limited body waveforms cannot fit the surface wave data which show a large increase in amplitude with period and a moment of 3.4 × 1027 dyn cm. Both types of data can be modeled by combining the body wave time function and a 130 s cosine time function representing a longer period component. This event represents the largest earthquake to occur along this relatively aseismic portion of the Peruvian coast. The tsunami excitation is not anomalous relative to the measured seismic moment and moment magnitude (MW 7.6). The disparity between tsunami height and surface wave magnitude results from underestimation of the earthquake size by conventional magnitude scales due to the long source duration.
A current disruption mechanism in the neutral sheet: A possible trigger for substorm expansions
Authors: Lui A. T. Y. , Mankofsky A. , Chang C.‐L. , Papadopoulos K. , Wu C. S.
Published: February 2008

A linear analysis is performed to investigate the kinetic cross‐field streaming instability in the Earth's magnetotail neutral sheet region. Numerical solution of the dispersion equation shows that the instability can occur under conditions expected for the neutral sheet just prior to the onset of substorm expansion. The excited waves are obliquely propagating whistlers with a mixed polarization in the lower hybrid frequency range. The ensuing turbulence of this instability can lead to a local reduction of the cross‐tail current causing it to continue through the ionosphere to form a substorm current wedge. A substorm expansion onset scenario is proposed based on this instability in which the relative drift between ions and electrons is primarily due to unmagnetized ions undergoing current sheet acceleration in the presence of a cross‐tail electric field. The required electric field strength is within the range of electric field values detected in the neutral sheet region during substorm intervals. The skew in local time of substorm onset location and the three conditions under which substorm onset is observed can be understood on the basis of the proposed scenario.
Distinguishing Alfvén waves from quasi‐static field structures associated with the discrete aurora: Sounding rocket and HILAT satellite measurements
Authors: Knudsen D. J. , Kelley M. C. , Earle G. D. , Vickrey J. F. , Boehm M.
Published: February 2008

We present and analyze sounding rocket and HILAT satellite measurements of the low frequency (< 1 Hz) electric and magnetic fields δE and δB perpendicular to the Earth's magnetic field B0 in the auroral oval. By examining the time‐domain field data it is often difficult to distinguish temporal fluctuations from static structures which are Doppler shifted to a non‐zero frequency in the spacecraft frame. However, we show that such a distinction can be made by constructing the impedance function Z(f) = μ0|δE(f)/δB(f)|. Using Z(f) we find agreement with the static field interpretation below about 0.1 Hz in the spacecraft frame, i.e. Z(f) = Σp−1 where Σp is the height‐integrated Pedersen conductivity of the ionosphere. Above 0.1 Hz we find Z(f) > Σp−1, which we argue to be due to the presence of Alfvén waves incident from the magnetosphere and reflecting from the lower ionosphere, forming a standing wave pattern. These waves may represent an electromagnetic coupling mechanism between the auroral acceleration region and the ionosphere.
On the interannual variability of trace gases in the middle atmosphere
Authors: Gray L. J. , Chipperfield M. P.
Published: February 2008

A parameterization of the mechanisms believed to be responsible for the quasi biennial oscillation (QBO) in the zonal winds of the equatorial lower stratosphere has been added to a fully interactive dynamical radiative photochemical two dimensional model of the atmosphere. In addition to a well defined QBO in the zonal winds, temperature and column ozone, the model exhibits a strong QBO signal in many of the other modelled trace gases. Large year to year variations, in some cases up to 70 %, are predicted. The importance of taking this prediction into account when measuring and interpreting the distributions of these gases is emphasized.
The global geodynamic effect of the MacQuarie Ridge Earthquake
Authors: Gross Richard S. , Chao B. Fong
Published: February 2008

Besides generating seismic waves, which eventually dissipate, an earthquake also generates a static displacement field everywhere within the Earth. This global displacement field rearranges the Earth's mass distribution, causing the Earth's rotational properties and gravitational field to change. The size of these changes depends, in general, upon the size of the earthquake. The Macquarie Ridge earthquake of May 23, 1989 is considered to be the largest earthquake to have occurred since the 1977 Sumba and Tonga events. As such, the coseismic effect of this earthquake upon the Earth's length‐of‐day, polar motion, and low‐degree harmonic coefficients of the gravitational field are computed. It is found that this earthquake should have caused the length‐of‐day to decrease by 0.06 μsec, the position of the mean rotation pole to shift 0.11 milli‐arcsec towards 323 °E longitude, and selected degree l = 2–5 gravitational field coefficients to change by about 1 part in 1013. These changes are all smaller than can be detected by current observational techniques. However, changes of this size could perhaps be detected in the future with the implementation of proposed improvements to the techniques of monitoring the Earth's rotation, and (especially for the low‐degree gravitational field coefficients) with the placement of GPS receivers onboard orbiting spacecraft.
Pn attenuation for a spherically symmetric Earth model
Authors: Sereno Thomas J. , Given Jeffrey W.
Published: February 2008

This paper presents the results of a theoretical study of the sensitivity of Pn attenuation to the elastic and anelastic structure of the uppermost mantle for frequencies between 1 and 15 Hz and distances between 200 and 1000 km. Synthetic seismograms are computed using wavenumber integration for an elastic model consisting of a crastal layer over an upper mantle with a slight linear velocity gradient (approximately equal to the earth‐flattening transformation of a spherically symmetric homogeneous upper mantle). These are compared to synthetic seismograms for a crustal layer over a mantle halfspace (pure elastic head wave). We find that earth sphericity alone causes a significant departure in Pn attenuation from that of a canonical head wave (almost two orders of magnitude at 1000 km and 15 Hz), and that high frequencies attenuate less rapidly than low frequencies if the spherical earth velocity gradient is greater than or equal to zero. This implies that methods that assume frequency‐independent Pn geometric spreading will produce biased estimates of the anelasticity of the upper mantle. This is demonstrated using synthetic seismograms computed for an anelastic spherical earth model. These results show that the velocity structure must be carefully considered when relating observed Pn spectral amplitudes to the anelastic structure of the upper mantle.
High latitude pulsating aurorae
Authors: Craven M. , Burns G. B.
Published: February 2008

We report here the significant occurrence of pulsating aurorae in the high latitude post‐noon auroral oval. Observations of aurorae at Davis, Antarctica (Inv. 74.5°S, Geog. 78.0°E, 68.6°S) during the 1988 Austral winter indicate pulsating auroral activity peaks at around 14.5% in the period 1330–1400 magnetic local time (MLT). Post‐noon high latitude pulsating aurora is typically a less than 250 R enhancement on a 1 kR background in the OI emission at 558 nm. Pulsation periods tend to be longer at 20–30 s compared to average auroral zone fluctuations of around 2–10 s. Similar to auroral zone stations, the optical fluctuations are always accompanied by Pi(c) magnetic pulsations. Weak Kp and IMF trends are present in the data.
Note on apparent systematic and periodic errors in Geosat orbits
Authors: Sirkes Ziv , Wunsch Carl
Published: February 2008

Apparent errors in GEOSAT orbits are estimated directly from the measurements. There are technical difficulties in such estimates from quasi‐periodically gapped data. The dominant orbit errors display a line spectrum, in which the once/orbit error peak is split in a complex way into a series of narrow lines, with other errors being present as well. The spatial pattern of the errors is not random, displaying differences between mean ascending and descending orbits which are coherent over thousands of kilometers. Orbit errors do not decorrelate within a few orbit periods.
Analysis of teleseismic body waves radiated from the Loma Prieta Earthquake
Authors: Langston C. A. , Furlong K. P. , Vogfjord K. S. , Clouser R. H. , Ammon C. J.
Published: February 2008

Broad‐band data from IRIS and ORFEUS data centers, the ARCESS array and SCP station are analyzed to infer fault plane geometry and the character of rupture during the October 17, 1989, Loma Prieta earthquake. Interference of P and sP seen in the P wave forms is consistent with the 18 km source depth inferred from local observations. A minimum source depth of 14 km is inferred from interpretation of the S wave arrival time, relative to instrument trigger, using horizontal strong motion accelerations observed at Corralitos, California. P, sP, S and sS polarities and relative amplitudes are used in a comprehensive grid search to infer that oblique right‐lateral faulting occurred on a steeply dipping (56°–60°) section of the San Andreas fault with an equal amount of thrust component consistent with aftershock locations. A suite of point moment tensor inversions for source depths spanning the observed depth of aftershock hypocenters yields similar mechanisms. The best moment tensor in terms of wave form fit and minimum non‐double couple component occurs for a source depth of 8 km, substantially shallower than the locally inferred hypocenter. Seismic moment for this model is 2.3 × 1026 dyn‐cm. The slow growth of the P displacement pulse with time and impulsive sP suggests a working model for fault rapture where rupture initiates at 18 km and propagates upwards and outwards along the fault. The P wave forms show the effects of variations in rupture along the plane through observation of a cascade of successively larger subevents.
Long‐period and broad‐band teleseismic body‐wave modeling of the October 18, 1989 Loma Prieta Earthquake
Authors: Barker Jeffrey S. , Salzberg David H.
Published: February 2008

Inversion of long‐period, teleseismic body waves for the Loma Prieta earthquake yields a nearly double‐couple moment tensor corresponding to an oblique mechanism with strike 130°, dip 73° and rake 146°. The long‐period centroid depth of the source is 18 km, and the seismic moment is 2.8×1019 Nm. The source time history consists of two pulses with a total duration of 8–9 sec. Preliminary forward modeling of broad‐band, teleseismic body waves indicates the occurrence of at least two sub‐sources. Constraining the fault orientation to the long‐period inversion result, the first source was located at 18 km depth, and the second source was located updip at about 14 km depth. The second source has a seismic moment 2.7 times that of the first, and occurred 2.0 sec after the first. The precise location of the second source will require further modeling.
The October 17, 1989, Loma Prieta, California, Earthquake and its aftershocks: Geometry of the sequence from high‐resolution locations
Authors: Dietz Lynn D. , Ellsworth William L.
Published: February 2008

Hypocenters of the Loma Prieta sequence form a dipping zone that rises from the mainshock hypocenter and is parallel to the mainshock nodal plane. Most aftershocks cluster around the perimeter of the zone, surrounding a relatively aseismic center which approximates the region of mainshock rupture. At its southeastern end, the dipping aftershock zone warps into a vertical surface that corresponds to the San Andreas fault. In the central and northwestern parts of the zone at depths above ∼10 km, the aftershocks define numerous disjoint fault structures. The large component of reverse‐slip observed in this event agrees with a simple model for slip on a dipping plane within a compressional fault bend. We do not believe that the Loma Prieta earthquake occurred on the Sargent fault. However, we are unable to conclude whether it ruptured the principal plate boundary fault or a less frequently active fault.
Tomographic P‐wave velocity images of the Loma Prieta Earthquake asperity
Authors: Lees Jonathan M.
Published: February 2008

Tomographic inversion is applied to delay times from local earthquakes to image 3‐D velocity variations surrounding the main rupture of the 1989 Loma Prieta earthquake. The 55×45 square km region is represented by blocks of 1 km per side laterally and by 8 layers of varying thickness to 18 km depth. High quality P‐wave arrival times recorded on the USGS CALNET array from 549 crustal earthquakes with depths of O to 25 km were used as sources. Preliminary results several velocity variations (5–12%) that correlate with specific characteristics of the 1989 rupture. These include prominent high‐velocity anomalies near the mainshock hypocenter and prominent low‐velocity anomalies where the dip of the San Andreas fault appears to change significantly. The termination of prominent low velocity features existing primarily in the hanging wall to depths of 7–9 km, correlates with the top of the rupture zone. High‐velocity variations along the fault dominate where aftershock activity is high. The high velocity anomaly located at depth along the fault is interpreted as imaging the asperity on which the Loma Prieta earthquake occurred.
Seismic slip, segmentation, and the Loma Prieta Earthquake
Authors: King Geoffrey C. P. , Lindh Allan G. , Oppenheimer David H.
Published: February 2008

We have plotted the cumulative seismic slip projected onto a vertical plane for earthquakes occurring during the last 20 years along 210 km of the San Andreas fault that includes the section that moved in the Loma Prieta earthquake. These plots illustrate the differences in depth and character of the seismicity between the locked and creeping portions of the fault or fault zone and define the segment upon which the Loma Prieta earthquake occurred. Working by analogy from the relation between pre‐main shock microseismicity and presumed main shock slip regions at Parkfield and Loma Prieta, we identify a segment on the San Francisco Peninsula where we believe the M 7 1838 earthquake occurred, and which we believe may have accumulated sufficient strain that rupture should be expected in the coming decades.
Intermediate‐term prediction in advance of the Loma Prieta Earthquake
Authors: Keilis‐Borok V. I. , Knopoff L. , Kossobokov V. , Rotvain I.
Published: February 2008

The Loma Prieta earthquake of October 17, 1989 was predicted by the use of two pattern recognition algorithms, CN and M8. The prediction with algorithm CN was that an earthquake with magnitude greater than or equal to 6.4 was expected to occur in a roughly four year interval starting in midsummer 1986 in a polygonal spatial window of approximate average dimensions 600 × 450 km, encompassing Northern California and Northern Nevada. The prediction with algorithm M8 was that an earthquake with magnitude greater than or equal to 7.0 was expected to occur within 5 to 7 years after 1985, in a spatial window of approximate average dimensions 800 × 560 km. The predictions were communicated in advance of the earthquake. In previous, mainly retrospective applications of these algorithms, successful predictions occurred in about 80% of the cases.
Magnetospheric interaction with Triton's ionosphere
Authors: Strobel Darrell F. , Cheng Andrew F. , Summers Michael E. , Strickland Douglas J.
Published: February 2008

The large electron densities measured by the Voyager radio occultation experiment are attributed to the precipitation of magnetospheric electrons with energy > 10 keV. Because the ionospheric electric Pedersen conductivity of Triton is,∼ (1−2) × 104 mho and the Alfven conductance is ∼ 3.5 mho, direct connective flow of plasma into the essentially infinitely conducting ionosphere is negligible. Magnetospheric electrons are transported to Triton's ionopause by curvature drift as a result of weak magnetic field line draping in a sub‐Alfvenic plasma interaction with Triton. At the ionopause energetic electrons have a high probability of elastic and inelastic scattering and precipitate into the upper atmosphere. The average power dissipation is estimated to be ∼ (2−3) × 108 W.
Simultaneous HF‐radar and DMSP observations of the cusp
Authors: Baker K. B. , Greenwald R. A. , Ruohoniemi J. M. , Dudeney J. R. , Pinnock M. , Newell P. T. , Greenspan M. E. , Meng C.‐I.
Published: February 2008

The Geospace Environment Modeling (GEM) Program is directed toward modeling the coupled solar wind/magnetosphere/ionosphere system. The inter‐calibration of ground‐based observations of the ionosphere and satellite observations has been identified as an essential step in tying together the data to produce a global picture of geospace.

On October 10,1988 the DMSP‐F9 satellite passed through the southern hemisphere cusp while a coherent scatter HF‐radar was observing 10‐m scale irregularities present in the ionosphere. The combined data indicate that these irregularities were being generated in the cusp, and that the cusp was a region of greater than normal electric field turbulence. The radar data indicate that the cusp was co‐located with the region where the ionospheric convection rotated from sunward to anti‐sunward with increasing latitude. These observations provide an unambiguous case where simultaneous satellite and ground‐based observations of the cusp can be compared.
June cloud cover over the Arctic Ocean
Authors: Serreze Mark C. , Render Mark C.
Published: February 2008

June cloud cover over the Beaufort Sea, west‐ward to the East Siberian Sea and up to approximately 85°N, is manually charted using 12 years of satellite imagery.