bluuebunny/crossref_metadata_2025 huggingface dataset:
Variation in apparent attenuation of the Earth's normal modes due to lateral heterogeneity
Authors: Davis J. Peter
Published: February 2008

Study of 3242 full first order synthetic seismograms of isolated normal mode multiplets shows that constructive and destructive interference of the singlets comprising a multiplet can account for the noise‐corrected variance of free oscillation attenuation measurements in the frequency band 3.0‐4.0 mHz. This interference gives rise to a distribution of synthetic Q measurements which is sensitive to the short wavelength velocity features of a model. While individual attenuation measurements depend also on the moment tensor, the overall variance of Q fluctuations is insensitive to the characteristics of the source employed and depends little on the length of the time series processed. The Woodhouse‐Dziewonski model M84A of upper mantle velocity structure causes a scatter in the synthetics large enough to interfere significantly with any signal from lateral Q structure. Correlation of individual synthetics with data is poor, however, which suggests that either there are remaining uncertainties in the velocity model, that mode‐mode coupling occurs in the data in a way that is poorly understood, or that lateral attenuative structure contributes to the observed scatter. Resolution of any such Q structure will be difficult given the present data set.
A note on the nature of the distant geomagnetic tail magnetopause and boundary layer
Authors: Gosling J. T. , Thomsen M. F. , Swift D. W. , Lee L. C.
Published: February 2008

Recently reported plasma and magnetic field measurements of the distant geomagnetic tail magnetopause and boundary layer (i.e. mantle) are compared with numerical simulation results for an "open" boundary. Most aspects of the observations are consistent with the simulation results provided that the normal magnetic field component at the magnetopause is generally small.
Differential excitation cross section measurement of O(¹D) at 20 eV electron impact
Authors: Shyn T. W. , Sharp W. E.
Published: February 2008

The differential excitation cross section of O(¹D) from the ground state has been measured at 20 eV electron impact by a crossed‐beam method. The angular range covered was from 30 to 150 degrees. Atomic oxygen was generated by a thermal dissociation method (iridium oven). The measured total excitation cross section is (1.5±0.7)×10−17 cm² which agrees, within the accuracy of the measurement, with the theoretical value (1.7 × 10−17 cm²) calculated by Henry et al. (1969).
Profiling the Gulf Stream with a current meter mooring
Authors: Hall Mindy M. , Bryden Harry L.
Published: February 2008

A current meter mooring was deployed for one year at the mean position of the Gulf Stream at 68°W. With instruments from 400 m to 4000 m, it provides the first unambiguous evidence that the Gulf Stream extends from top to bottom of the water column. Using the strong cross‐stream temperature structure to derive a horizontal coordinate related to temperature, we estimate the width and transport of the Gulf Stream for four individual events and we determine an average profile of the vertical and cross‐stream structure of the Gulf Stream. A conservative estimate of transport for this average profile indicates that the Gulf Stream has a transport of 103 × 106 m³ s−1 at 68°W.
Simultaneous electric field changes in the equatorial electrojet in phase with polar cusp latitude changes during a magnetic storm
Authors: Somayajulu V. V. , Reddy C. A. , Viswanathan K. S.
Published: February 2008

The Doppler frequency of the VHF radar signals backscattered from the drifting ionization irregularities in the equatorial electrojet is proportional to the east‐west electric field Ey in the equatorial electrojet. Using the VHF backscatter radar observations at Thumba during the geomagnetic storm of February 16, 1980, it is shown that the decreases and increases of the storm‐time electric fields in the equatorial electrojet are completely in phase with the decreases and increases in the mean latitude of the polar cusp region. This observation has important implications for understanding the magnetosphere‐ionosphere interactions, particularly the effect of the magnetospheric electric fields on the low latitude ionosphere.
The effect of El Chichón on wind variability in the troposphere, stratosphere, and mesosphere over Alaska
Authors: Balsley B. B. , Garello R.
Published: February 2008

Data from the Poker Flat, Alaska MST radar combined with standard NWS balloon observations at Fairbanks, Alaska show that low‐frequency (3‐11 day) wind fluctuations within the troposphere and lower stratosphere for the period January‐March 1983 were noticeably weaker than those for other similar periods between 1979‐1984. Similar effects can be detected in the higher‐frequency (1‐6 hr and 8‐48 hr) wind fluctuations obtained from the radar data alone.

In contrast to these decreases in the troposphere and lower stratosphere wind variations, the magnitude of the tidal period (8‐48 hr) atmospheric wind fluctuations observed by radar in the mesosphere during June‐July were significantly increased in 1983 relative to surrounding years. All of these variations appear to be reasonably well associated with the presence of enhanced aerosol concentrations in the lower stratosphere following the explosion of El Chichón volcano in April 1982.
A new mechanism for excitation of electrostatic ion cyclotron waves and associated perpendicular ion heating
Authors: Ganguli G. , Palmadesso P. , Lee Y. C.
Published: February 2008

A new mechanism for exciting the kinetic ion cyclotron waves in the presence of a nonuniform electric field perpendicular to the external magnetic field is given. Application of this instability to various space plasmas is discussed. The new instability mechanism may provide a more efficient agent for perpendicular ion heating than other EIC generation processes, since the linear growth rate is insensitive to the temperature ratio.
The Atmosphere Explorer optical glow near perigee altitudes
Authors: Yee J‐H. , Abreu V. J. , Dalgarno A.
Published: February 2008

The altitude variation of the Atmosphere Explorer optical glow intensity suggests that two different processes are responsible for the glow. One, dominant for altitudes above 180 km, has an emission brightness proportional to the ambient atomic oxygen density whereas the other, dominant at altitudes below 160 km, produces an emission whose intensity is proportional to the product of the densities of any of N2, O2 or NO. The first mechanism apparently has two components, one from the surface recombination of O and H and the other from a process similar to that producing the Shuttle glow. Unless the efficiency of the second mechanism is much enhanced on the Shuttle its contribution to the Shuttle glow is negligible.
Time‐space variation of seismic S‐wave coda attenuation (Qc−1) and magnitude distribution (b‐values) for the Petatlan Earthquake
Authors: Novelo‐Casanova D. A. , Berg E. , Hsu V. , Helsley C. E.
Published: February 2008

Average S‐wave coda‐Q (Qc) at 6 Hz and to about 10 sec after the S‐wave arrival shows: (1) temporal variation positively correlated with the magnitude distribution parameter b for foreshocks and aftershocks of the 1979 Petatlan, Mexico earthquake; (2) spatial variation through the aftershock area, positively correlated with b, high Qc ‐b values at the two activity centers, intermediate values in between and landward, and low values toward the Middle America Trench, which remain low for two weeks after the main shock; and (3) five‐foreshock average Qc = 135±1.6% and eleven‐aftershock average Qc= 175±1.3% for the eastern asperity. The b values—and possibly Qc values—seem to correlate with the level of aftershock activity.
Competing atomic processes in Ba and Sr injection critical velocity experiments
Authors: Newell Patrick T. , Torbert R. B.
Published: February 2008

The critical ionization velocity effect requires a superthermal electron population to ionize through collisional impact. Such superthermal electrons can however lose energy to competing atomic processes, as well as to ionization, thus limiting the efficiency of the effect. Considering Ba and Sr magnetospheric injection experiments designed to test the CIV theory, we find that in both cases roughly 60% of the superthermal electron energy is lost on exciting line radiation. Moreover, energy loss to background neutral oxygen places a strict limit on the injected cloud densities for which critical velocity effects are possible; a finding which explains the consistently negative results in radial injection experiments.
Deep earthquakes and the thermal assimilation of subducting lithosphere
Authors: Wortel Rinus
Published: February 2008

Two problematic aspects of deep seismic zones are investigated: the maximum depth (about 700 km) of seismic activity and the relative maximum in seismic energy release in the 500‐700 km depth range. It is shown that both features can be accounted for by a model based on thermal assimilation. This implies that no barrier is required at a depth of 650‐700 km to explain the observations.
Adiabaticity and viscosity in deep mantle convection
Authors: Quareni Francesca , Yuen David A. , Saari Marc R.
Published: February 2008

We have studied steady convection with adiabatic and viscous heating for variable viscosity in the Boussinesq limit using the mean‐field theory. A strong nonlinear coupling is found between the thermodynamic constants governing adiabatic heating and the rheological parameters. We have established the range of rheological values for which adiabaticity would occur throughout the mantle. Too large an activation volume, greater than 6 cm³/mol for the cases examined here, would produce unreasonably high temperature at the bottom of the mantle (>6000 K) and superadiabatic gradients, especially in the lower mantle. Radiogenic heating plays a profound role in controlling dynamically mantle temperatures. Present values for the averaged mantle heat production would yield objectionably high temperatures in the lower mantle.
Dimethylsulfoxide and dimethylsulfone in the marine atmosphere
Authors: Harvey George R. , Lang Russell F.
Published: February 2008

New isolation and detection methods were developed to measure dimethylsulfoxide (DMSO) and dimethylsulfone (DMSO2) in marine rain and marine air masses. Central equatorial Pacific rain contained 1 to 10 µg/ℓ of each of these compounds. Uncontaminated air sampled off Miami contained 2 to 6 ng/m³ of each component. These concentrations suggest that DMSO and DMSO2 may be as significant as dimethylsulfide (DMS) in marine sulfur transport. In fact, DMSO was observed to undergo disproportionation in illuminated seawater or distilled water to DMS and DMSO2. This latter observation implies a partially reversible loop in the sulfur transport cycle and complicates the calculation of the flux of sulfur into the marine boundary layer.
Destiny of earthward streaming plasma in the plasmasheet boundary layer
Authors: Green James Lauer , Horwitz James L.
Published: February 2008

Propagation characteristics of earthward streaming protons in the plasmasheet boundary layer are investigated in a model magnetosphere from the analysis of single particle trajectories. The phase space of initial proton distributions within the plasmasheet boundary layer out to 40 Earth radii (Re) can be separated into four major components. In a region outside the loss cone, protons with pitch angles less than about 2.6° can make it to the auroral zone at S3‐3 altitudes and below. Protons at all other pitch angles, with speeds greater than about 1100 km/s, reflect (or mirror) at high latitudes near the Earth and return tailward, often convecting toward the inner plasmasheet. Protons with velocities as high as 800 km/s, regardless of pitch angle, are found to mirror such that they are "trapped" in the ring current region of the magnetosphere. These results are dependent on the magnitude and direction of the convection electric field, such that during geomagnetic storms with higher convection electric fields, higher energies of the plasmasheet boundary layer protons would be expected to reach the low‐latitude portions of the ring current.
The gas‐phase reaction of chlorine nitrate with water vapor
Authors: Atkinson Roger , Tuazon Ernesto C. , Mac Leod Hélène , Aschmann Sara M. , Winer Arthur M.
Published: February 2008

The kinetics of the reaction of chlorine nitrate (ClONO2) with water vapor have been investigated in large‐volume (2500‐L and 5800‐L) Teflon or Teflon‐coated environmental chambers to assess the importance of this reaction as a loss process for odd nitrogen in the stratosphere. These data show that the upper limit rate constant for the homogeneous gas phase reaction between ClONO2 and H2O is <2 × 10−21 cm³ molecule−1 s−1 at 298 ± 1 K. With this rate constant, the homogeneous reaction of ClONO2 with water vapor will be of negligible importance under stratospheric conditions.
Observations of long period earthquakes accompanying hydraulic fracturing
Authors: Bame Dorthe , Fehler Michael
Published: February 2008

Waveforms of most seismic events accompanying hydraulic fracturing have been reported to contain clear P and S waves and have fault plane solutions consistent with shear displacement across a fault. This observation is surprising since classical hydraulic fracturing theory predicts the creation of a tensile opening of a cavity in response to fluid pressure. Very small long period events, similar to long period earthquakes observed at volcanoes, were found to occur during four hydraulic fracturing experiments carried out at Fenton Hill, New Mexico. Since the long period earthquakes occur in the same region as the shear type events, it is concluded that the unusual character of the long period earthquake waveforms is due to a source effect and not a path effect. The occurrence of long period earthquakes during hydraulic fracturing could indicate tensile fracturing. Many waveforms of these events are identical, which implies that these events represent repeated activation of a given source. A proposed source for these long period events is the sudden opening of a channel that connects two cracks filled with fluid at different pressures. The sizes of the two cracks differ, which causes two or more peaks to appear in the spectra, each peak being associated with one physical dimension of each crack. From the frequencies at which spectral peaks occur, crack lengths are estimated to be between 3 and 20m.
Double‐diffusive convection in magma chambers: Single or multiple layers?
Authors: Spera Frank J. , Yuen David A. , Clark Stephen , Hong H.‐J.
Published: February 2008

An approximate set of equations applicable to double‐diffusive convection in magma chambers has been solved in the steady state for a range of geologically relevant parameters. No evidence for the steady state existence of layered convection for conditions appropriate to magma chambers has been found. At large ratios of the thermal to chemical diffusivity (Lewis number, Le) the heat transport is independent of the Lewis number and equal to that found for thermal convection in a single component fluid. The vertical upward transport of light component varies directly with both the Rayleigh and Lewis numbers (Nuc ∼ Ra0.23 Le0.37).
A statistical correlation between Ridge Crest offsets and spreading rate
Authors: Abbott D.
Published: February 2008

Ridge crest offsets follow the statistical distribution, N = A exp [−B ln (L/Lo)], where N is the cumulative number of ridge crest offsets with lengths greater than L, Lo is the cutoff length, and A and B are constants. We measured A and B for the slow spreading mid‐Atlantic ridge, the intermediate spreading Juan de Fuca Ridge, and the fast spreading east Pacific Rise. The predicted mean distances between offsets greater than 4 km long are 30, 32, and 74 km on the slow, intermediate and fast spreading ridge crests. These distances are in agreement with observations of other workers and calculations of modal ridge lengths.
Ion distribution functions in the vicinity of comet Giacobini‐Zinner
Authors: Cravens T. E.
Published: February 2008

Photoionization of neutral molecules in the coma surrounding a comet produces heavy ions which contaminate and mass‐load the solar wind. Cometary ion distribution functions in the vicinity of comet Giacobini‐Zinner (G/Z) are calculated using a Monte Carlo method. The distribution function calculated behind the shock has both cold (≈ 2 keV) and hot (≈ 40 keV) components.
Giacobini‐Zinner magnetotail: ICE magnetic field observations
Authors: Slavin J. A. , Smith E. J. , Tsurutani B. T. , Siscoe G. L. , Jones D. E. , Mendis D. A.
Published: February 2008

The ICE magnetic field observations at Comet Giacobini‐Zinner have revealed the presence of a well developed magnetotail with a diameter of ∼ 10,000 km at a distance of 7,800 km downstream of the nucleus. The tail is composed of two lobes of opposite magnetic polarity separated by a ∼ 1,500 km thick plasma sheet. A ∼ 250 km diameter region of twisted, high intensity magnetic fields was observed in the plasma sheet and may correspond to a cometary “flux rope.” The magnetotail is bounded by a thin, ∼ 200 km wide magnetopause‐like current layer. The field magnitude increases by a factor of 2 between the outer portions of the lobe and the central regions where peak fields of ∼ 60 nT were measured. The magnetic field in the lobes of the near‐tail region is highly flared with the field making a mean angle of about 20° with the symmetry axis. Flaring is observed to be least in the high field regions of the central tail and greatest in the outer portions of the lobes where minimum variance analyses on the magnetopause crossings provided flare angles of 20‐40°. The cross‐tail current layer is quite broad and occupies the full width of the plasma sheet. Minimum variance analysis indicates that it was inclined to the plane of the ecliptic by 43°. This current sheet orientation is consistent with the observed direction of the IMF in the Y‐Z plane during the ICE fly‐by. It is concluded on the basis of these results that the ICE magnetic field observations downstream of Giacobini‐Zinner have confirmed the Alfven field line draping model of type I cometary tails.
3‐dimensional structure of the Indian Ocean inferred from long period surface waves
Authors: Montagner Jean‐Paul
Published: February 2008

To improve the lateral resolution of the first global 3 ‐ dimensional models of seismic wave velocities, regional studies have to be undertaken. The dispersion of Rayleigh waves along 86 paths across the Indian Ocean and surrounding regions is investigated in the period range 40 ‐ 300 s. The regionalization of group velocity according to the age of the sea floor shows an increase of velocity with age up to 150 s only, similar to the results in the Pacific Ocean. But here, this relationship vanishes more quickly at long period. Therefore the correlation of the deep structure with surface tectonics seems to be shallower in the Indian Ocean than in the Pacific Ocean. A tomographic method is applied to compute the geographical distributions of group velocity and azimuthal anisotropy and then the 3‐D structure of S‐wave velocity. Horizontal wavelengths of 2000 km for velocity and 3000 km for azimuthal anisotropy distribution can be resolved. Except for the central part of the South East Indian ridge which displays high velocities at all depths, the inversion corroborates a good correlation between lithospheric structure down to 120 km and surface tectonics: low velocities along the central and southeast Indian ridges, velocity increasing with the age of the sea floor, high velocities under African, Indian and Australian shields. At greater depths, the low velocity zones under the Gulf of Aden and the western part of the Southeast Indian ridges hold but the low velocity anomaly of the Central Indian ridge is offset eastward. The low velocity anomalies suggest uprising material and complex plate boundary.
The distribution of cosmogenic 7BE in salt marsh sediments
Authors: Casey William H. , Larsen Ingvar L. , Olsen Curtis R.
Published: February 2008

Two related factors affect the vertical distribution and inventory of 7Be (53‐day half life) in salt marsh sediments: (i) the movement of pore water, and (ii) the presence of vegetation. Infiltration of rainwater into unsaturated sediment distributes 7Be over several centimeters depth without disturbing the sediment fabric. The sediment inventory of 7Be expected from its atmospheric flux could only be accounted for in vegetated areas where foliage traps 7Be from rainfall and surface flow. These factors lead to heterogeneous vertical and lateral distributions of 7Be throughout the marsh, and may also affect the sediment distribution of other trace substances, such as 210Pb, 137Cs, and heavy metals.
Mantle viscosity beneath the Galapagos 95.50°W propagating rift
Authors: Schubert Gerald , Hey R. N.
Published: February 2008

Detailed geophysical surveys in the vicinity of the Galapagos 95.5°W propagating rift tip establish the opening history of the rift and its velocity of propagation. These data together with a theory for mantle upwelling into slowly widening lithospheric cracks constrain the viscosity of the asthenosphere beneath the propagating rift to be less than about 1017 to 1018 Pa s.
The sodium exosphere and magnetosphere of Mercury
Authors: Ip W.‐H.
Published: February 2008

Following the recent optical discovery of intense sodium D‐line emission from Mercury, we explore the scenario of an extended exosphere of sodium and other metallic atoms. It is shown that the strong effect of solar radiation pressure acceleration would permit the escape of Na atoms from Mercury's surface even if they are ejected at velocity less than the surface escape velocity. Fast photoionization of the Na atoms is effective in limiting the tailward extension of the sodium exosphere, however. The subsequent loss of the photoions to the magnetosphere could be a significant source of the magnetospheric plasma. The recirculation of the magnetospheric charged particles to the planetary surface could also play an important role in maintaining an extended sodium exosphere as well as a magnetosphere of sputtered metallic ions.
Influence of non‐dipole field on determination of Plio‐Pleistocene true polar wander
Authors: Schneider David A. , Kent Dennis V.
Published: February 2008

Some studies of global paleomagnetic data have found an offset of the magnetic pole during the Plio‐Pleistocene which has been interpreted as indicating a period of rapid True Polar Wander, with a rate of movement comparable to the present‐day rate of polar motion deduced from astronomical observations. We show that much of the polar offset determined from the paleomagnetic data may be due to deviations in pole position caused by persistent non‐dipole zonal components of the geomagnetic field. A correction of paleomagnetic poles for the long‐term non‐dipole field reduces the polar offset and thus suggests a slower or shorter episode of True Polar Wander over the past 5 million years.
Polarity transition records and the acquisition of remanence: A cautionary note
Authors: Hoffman Kenneth A. , Slade Steven B.
Published: February 2008

Simulations of natural smoothing of paleomagnetic reversal records suggest that illustrative features of actual field behavior may be made undiscernible even when relatively short integration times are associated with the acquisition of magnetic remanence. The results of this exercise further caution against the sole use of logs of component behavior (i.e. declination, inclination and intensity) for analysis, since time‐averaging of the complete vector field not only smooths, but also can significantly alter the component paleomagnetic waveforms.
On the current‐voltage relationship of the magnetospheric generator at intermediate spatial scales
Authors: Vickrey J. F. , Livingston R. C. , Walker N. B. , Potemra T. A. , Heelis R. A. , Kelley M. C. , Rich F. J.
Published: February 2008

Using data from the drift meter and magnetometer on board the HILAT satellite, we have examined fluctuations in high latitude electric and magnetic fields at scale sizes between 80 km and 3 km. A comparison of data from summer and winter allows us to assess the impact of changing ionospheric conductivity on the magnetospheric generator. We find that, at these scale sizes, the magnetosphere tends to behave as a constant current source that is independent of ionospheric conductivity. This characteristic was noted on both open and closed field lines. The electric field pattern, on the other hand, is much more highly structured in the winter than in the summer. This behavior implies scale size dependent potential drops on closed field lines.
Possible impact of sodium species on ClO and O3 in the upper stratosphere
Authors: Rodriguez José M. , Ko Malcolm K. W. , Sze Nien Dak
Published: February 2008

Recent measurements of the rates for reactions of NaO2 and NaOH with HCl and of the photoabsorption cross section of NaCl allow for realistic estimates of the impact of sodium chemistry on chlorine partitioning in the upper stratosphere. When sodium chemistry is incorporated into our one‐dimensional model of the upper atmosphere, the results indicate that the calculated ClO concentrations near 50 km could increase by up to a factor of two with a corresponding decrease in calculated O3 abundances of as much as 15 percent. The impact on ClO and O3 depends crucially on the assumed meteoritic source of sodium, on transport from the mesosphere, and on transformation and removal of gaseous sodium in the stratosphere. Since the fluctuations in the abundance of gaseous sodium could induce O3 variations comparable to those caused by changes in temperature and solar UV, consideration of the chemistry of sodium and other metals may be an important element in efforts toward early detection of trends in upper stratospheric O3. Future increases in stratospheric chlorine will magnify the impact of sodium species on ozone.
Seismogenesis of the 1985 Great (Ms=8.1) Michoacan, Mexico Earthquake
Authors: McNally Karen C. , González‐Ruiz Jaime R. , Stolte Christian
Published: February 2008

The size and source complexity of the great (Ms = 8.1) Michoacan, Mexico earthquake can be attributed to both a restriction of the downdip width of the seismic interface between the overriding and downgoing plates, and a uniform distribution of asperities. The seismogenic evidence of strain accumulation during the 20 years prior to the 19 September, 1985 mainshock closely resembles that observed prior to the smaller MW = 7.6 earthquakes in the adjacent Colima (1973) and Petatlán (1979) regions: the subsequent rupture zones are seismically quiescent (mb ≥4.0) for ∼ 2.5 ‐ 4 years prior to the mainshocks; the locked thrust interface is loaded by aseismic slip and normal faulting within the downgoing plate below 25‐30 km. There is evidence that the 1981 Playa Azul earthquake (MW = 7.3) is an integral stage of the evolutionary process of strain accumulation and release in the Michoacan area and could thus be considered a foreshock to the great 1985 earthquake.
A very broad band analysis of the Michoacan, Mexico, Earthquake of September 19, 1985
Authors: Ekström Göran , Dziewonski Adam M.
Published: February 2008

Long‐period seismograms from the GDSN, RSTN, and IDA network are used in a centroid‐moment tensor analysis of the September 19, 1985 Michoacan, Mexico, earthquake (Ms=8.1). The long‐period data are also used together with broad band displacement P‐waveforms in a joint very broad band inversion for the moment tensor and the moment rate distribution in time and space. Results are that the earthquake had a total scalar moment of 1.1×1028 dyne‐cm and that the moment release occurred mainly in two subevents, the second of which was located approximately 70 km southeast of the first. Observations of Earth's normal modes at ultra long periods (600 to 2000 sec) are shown to be consistent with the obtained scalar moment and source mechanism.
Ionospheric convection associated with discrete levels of particle precipitation
Authors: Foster J. C. , Holt J. M. , Musgrove R. G. , Evans D. S.
Published: February 2008

A precipitation index is described which quantifies the intensity and spatial extent of high‐latitude particle precipitation based on observations made along individual satellite passes. By sorting plasma convection data according to this index, average patterns of the ionospheric convection electric field have been derived from a data set consisting of five years' observations by the Millstone Hill radar. Reference to the instantaneous precipitation index, and the average patterns keyed to it, provides a means of characterizing the global precipitation and convection patterns throughout an event.
Multiple‐event relocation of earthquakes on and near the Gorda Ridge
Authors: Sverdrup Keith A.
Published: February 2008

Multiple‐event relocation of earthquakes associated with the Gorda ridge and Gorda basin indicates that activity tends to cluster spatially with some suggestion of linear trends in the epicenters of events near the central segment of the ridge, sub‐parallel to the ridge axis. Events in the interior of the basin may be related to relic faults formed at the ridge crest that remain zones of weakness in the plate and are subject to periodic reactivation as well as the proposed Gorda fault which crosses the plate in a NW‐SE direction.
Rupture process of the March 3, 1985 Chilean earthquake
Authors: Christensen Douglas H. , Ruff Larry J.
Published: February 2008

The March 3, 1985 central Chile earthquake (Ms=7.8) ruptured a well studied seismic gap along the Chilean subduction zone. The epicenter of this event is located near the center of an approximately 300 km long region which ruptured in a great event in 1906 (Mw=8.2). The northern portion of the 1906 zone has since ruptured in 1971 (Ms=7.5) and 1973 (Ms=6.7). We have determined the rupture history of the 1985 central Chile earthquake from the deconvolved source functions of body waves (both P and PP). The source functions show one major pulse of moment release, the onset of which occurs about 16 seconds after an initial small pulse. The azimuthal directivity in the P waves indicates that the rupture front propagated from the epicenter southward and that the moment release of the major pulse is concentrated in a region between the epicenter and approximately 75 km south of the epicenter. We conclude that the region of high moment release is the dominant asperity and was loaded by slip in the relatively weaker northern region.
Small‐scale impacts into rock: An evaluation of the effects of target temperature on experimental results
Authors: Smrekar Suzanne , Cintala Mark J. , Hörz Friedrich
Published: February 2008

A series of cratering and catastrophic fragmentation experiments has been performed, involving the impact of aluminum and stainless ‐ steel spheres into warm (∼298K) and cold (∼100K) granodiorite targets. Although some vague hints of a thermal effect might be found in some of the results, in no case was there a substantial difference between the warm and cold series. Since these experiments were well within the strength‐dominated regime of impact phenomena, variations due to low target temperatures in more energetic events will probably be negligible. Thus, there appear to be no significant temperature‐dependent mechanical effects during impact into solid rock over a wide range of temperatures prevalent in the solar system.
Inversion of long period regional body waves for crustal structure
Authors: Wallace Terry C.
Published: February 2008

Long‐period regional distance (2° < Δ < 12°) body waves are strongly influenced by the waveguide nature of the crust. If the seismic source is known, it is possible to parameterize the waveforms in terms of gross structural properties of crustal thickness and Pn velocity. Since the regional distance waveforms are a product of interference of multiple reflected, and mode converted phases, changes in crustal thickness or Pn velocity changes the relative timing of the various phases. Approximate Green's function for a suite of crustal structures can be constructed by stretching or squeezing the Green's function of an average crustal model, which is the basis of an interative, least squares inversion procedure for crustal structure. The crustal structure of the western U.S. is inverted as a test of the technique.
Multiple auroral arcs and Birkeland currents: Evidence for plasma sheet boundary waves
Authors: Bythrow P. F. , Doyle M. A. , Potemra T. A. , Zanetti L. J. , Huffman R. E. , Meng C. I. , Hardy D. A. , Rich F. J. , Heelis R. A.
Published: February 2008

Quasi‐periodic structures of small‐scale Birkeland currents, energetic electrons, ion drifts, and auroral forms in the dawn sector have been observed simultaneously with the HILAT spacecraft. These structures map to the Low Latitude Boundary Layer using a model geomagnetic field. This mapping is supported by the observed characteristics of the energetic electrons and the relationship between precipitating electrons, Region 1 Birkeland currents, and plasma drift. These observations are interpreted as the result of quasi‐periodic variations of the Low Latitude Boundary Layer. Values of potential, estimated from assumed characteristics of a large‐scale wave propagating in this layer, agree with those determined from the HILAT particle and drift measurements. These results support the view that wave propagation at the Low Latitude Boundary Layer/Plasma Sheet interface can be a source of multiple auroral forms at low altitudes. We suggest that the Kelvin‐Helmholtz instability may be the ultimate source of these waves.
Critical ionization velocity effects in the inner coma of comet Halley: Measurements by Vega‐2
Authors: Galeev A. A. , Gringauz K. I. , Klimov S. I. , Remizov A. P. , Sagdeev R. Z. , Savin S. P. , Sokolov A. Yu. , Verigin M. I. , Szego K.
Published: February 2008

Plasma density and plasma wave measurements aboard the "Vega‐2" spacecraft discovered two plasma density enhancements in the inner coma of comet Halley that are accompanied by bursts of plasma waves in the lower‐hybrid frequency range. These events are explained by the critical ionization velocity (CIV) effects of Alfvén. The persistent bursty plasma wave activity and the intermittent plasma density growth toward the cometary nucleus indicate that the CIV effect contributes to the ionization of the cometary gas.
An MHD simulation of the interaction of the solar wind with the outflowing plasma from a comet
Authors: Ogino Tatsuki , Walker Raymond J. , Ashour‐Abdalla Maha
Published: February 2008

The interaction between the solar wind and the outflowing plasmas from a comet has been studied by using a two‐dimensional time‐dependent magnetohydrodynamic (MHD) simulation. The model reproduced several features of the comet‐solar wind interaction predicted by earlier theories and observed on the recent cometary probes. These include the formation of the contact surface and the cometary magnetotail. For a constant interplanetary magnetic field (IMF) the cometary plasma captures field lines which drape over the comet to form an antiparallel magnetic field configuration in the tail and a thin plasma sheet. Eventually, tail magnetic reconnection begins to occur at several points. When the IMF orientation is reversed dayside magnetic reconnection occurs at the subsolar point and a large disturbance propagates down the tail.
Importance of on scale observations of first arriving Rayleigh wave trains for source studies: Example of the Chilean Event of March 3, 1985, Observed on the Geoscope and Ida Networks
Authors: Monfret Tony , Romanowicz Barbara
Published: February 2008

A comparative moment tensor inversion study of the Chilean event of March 3, 1985 is performed using very long period Rayleigh wave data from the IDA and GEOSCOPE networks. It is shown that the perturbing effects of lateral heterogeneity on the propagation of the Rayleigh waves, especially in the amplitudes, can be minimized if first arriving trains R1 and R2 only can be used. More stable estimates of the main source parameters can then be obtained as well as an estimate of source directivity compatible with the aftershock distribution and body wave results.
Harmonic structure of compressional Pc5 pulsations at synchronous orbit
Authors: Higuchi T. , Kokubun S. , Ohtani S.
Published: February 2008

A harmonic structure of compressional Pc5 oscillations during geomagnetically disturbed conditions is found in magnetic field data from the geostationary satellites, GOES 2 and 3. Compressional Pc5 pulsations sometimes show an interesting property that the compressional component oscillates with a frequency twice as high as that of the transverse component. In addition to these second harmonic events, waves with a transitional character between normal Pc5 waves and second harmonic events are also observed. In the normal Pc5 waves, the compressional component changes out of phase with the radial component in the northern equatorial region. In a transitional event, a small hump appears at the maximum phase of the radial component. These Pc5 oscillations with second harmonics dominant in the compressional component were observed mainly at GOES 3 and rarely at GOES 2 in the winter season. This occurrence pattern suggests that compressional Pc5 with harmonic structure is spatially confined to the equatorial region. It is also suggested that the particular modulation dominant in the compressional component reflects a finite amplitude effect in the inhomogeneity of the hot ring current plasma near the magnetic equator.
The morphology of Antarctic total ozone as seen by TOMS
Authors: Schoeberl Mark R. , Krueger Arlin J. , Newman Paul A.
Published: February 2008

A more detailed examination of the TOMS observations reveal a number of important aspects of the Antarctic ozone depletion. First we note that the presence of large scale disturbances around the edge of the "ozone hole" can lead to highly variable station observations. Second, an examination of the zonal mean total ozone for the seven Octobers shows that the large systematic decline is not simply confined to the polar region but exists at mid‐latitudes as well. Integrations of the total ozone from the South Pole northwards show that a portion of the systematic trend of decreasing Antarctic total ozone (prior to 1985) seems to be due to a redistribution of total ozone to subpolar and mid‐latitude regions. That is, decreases at high latitudes are compensated by increases at lower latitudes. The correlation between zonal mean total ozone and 70 mb zonal mean temperatures from polar to mid‐latitudes shows that the systematic decrease in total ozone is well correlated with a systematic decrease in stratospheric temperatures.
Nitrate flux on the Ross Ice Shelf, Antarctica and its relation to solar cosmic rays
Authors: Zeller Edward J. , Dreschhoff Gisela A. M. , Laird Claude M.
Published: February 2008

Nitrate flux has been determined in the snow sequence deposited at Windless Bight on the Ross Ice Shelf (Antarctica). The data were obtained from on‐site analysis of nitrate concentrations from a glaciological pit and a firn core spanning the time interval from midwinter 1971 to January 1986. The high resolution data can be combined with precipitation records collected from adjacent areas to provide a record of nitrate flux. The resulting time series contains a signal which corresponds to the two major solar events of 1972 and 1984. The concentration and flux profiles may be useful in studies of Antarctic ozone depletion.
Antarctic springtime measurements of ozone, nitrogen dioxide, and aerosol extinction by Sam II, Sage, and Sage II
Authors: McCormick M. P. , Larsen J. C.
Published: February 2008

Simultaneous vertical profiles of O3, NO2 and aerosol extinction obtained with the Stratospheric Aerosol Measurement II (SAM II), Stratospheric Aerosol and Gas Experiment (SAGE), and SAGE II satellite instruments across the southern polar vortex show that significant differences exist at all altitudes. Both gaseous species display lower concentrations within the vortex over measurement altitudes ranging from the tropopause to 60 km and 20 to 40 km for O3 and NO2 respectively. Aerosol extinction above 15 ‐ 18 km and total aerosol stratospheric column are also lower inside the vortex than outside. Total column amounts of O3 and NO2 are found to be strongly coupled to spatial location within the vortex, with minimum total values located around the vortex center. Vertical profiles selected to emphasize the observed difference across the circumpolar vortex are presented for October 13, 1981, and October 13, 1985, near 70° and 68°S latitude, respectively.
Preliminary calculation of trajectory analysis in the lower stratosphere of the Southern Hemisphere
Authors: Yamazaki Koji
Published: February 2008

A trajectory analysis of air parcels for the Southern Hemisphere winter is performed with observed data. The low‐dispersive region exists around 60°S and poleward in the lower stratosphere. This low‐dispersive feature is probably related to the low ozone density over the Antarctica in winter.
On the modelled thermal response of the Antarctic stratosphere to a depletion of ozone
Authors: Shine K. P.
Published: February 2008

Simple models are used to investigate the effect of a spring‐time depletion of ozone in the Antarctic on stratospheric temperatures. It is shown that coolings in excess of 5 K are possible in the lower stratosphere. No evidence for such changes exist. One possible mechanism that could offset this cooling involves the loss of water vapour from the lower stratosphere due to desiccation processes associated with polar stratospheric clouds.
Seven years of total ozone from the TOMS instrument‐A report on data quality
Authors: Fleig Albert J. , Bhartia Pawan K. , Wellemeyer Charles G. , Silberstein David S.
Published: February 2008

Seven years of TOMS total ozone data are currency available. Recently a new ozone retrieval algorithm based on improved ozone absorption cross section was implemented that has introduced 6‐7% discontinuity in the archived data sets at the end of the 5th year of instrument operation. Until all data are reprocessed with the new algorithm users can use a table of correction factors given in this paper to make the data set internally consistent.

This paper also presents a comparison of TOMS results with Amundsen Scott Dobson station. Though the agreement in most years is good, in 1983‐84 the Dobson station reported unusually high values of ozone while TOMS saw the very low ozone values associated with the Antarctica ozone hole.
Vega landing sites: Venera 15/16 unit analogs from Pioneer Venus reflectivity and RMS slope data
Authors: Bindschadler Duane L. , Head James W. , Garvin James B.
Published: February 2008

Pioneer Venus radar data on surface properties have been used to compare the Vega spacecraft landing sites with the northern 1/4 of Venus mapped by the orbiters Venera 15 and 16. The regions surrounding both landing sites possess surface reflectivity and small‐scale roughness properties most similar to those of mapped volcanoes and volcanic plains regions and different surface properties than those of mapped tectonic units. Regions analogous to the Vega 1 site are relatively rare, covering 2.8% of the mapped surface. Vega 2 analogs are much more common and cover 22.6% of the surface. Neither landing site is representative of the nearby highlands of Aphrodite, but the Vega 2 landing site is similar to much of the northern plains of Venus.
Of the May 7, 1986 Andreanof Islands Earthquake source parameters
Authors: Hwang Lorraine J. , Kanamori Hiroo
Published: February 2008

Source characteristics of the May 7, 1986 Andreanof Islands earthquake (51.412°N, 174.830°W, NEIC) are investigated from WWSSN, GDSN and IDA records. First motions from over 60 stations determine one steeply dipping nodal plane. We constrained this nodal plane and inverted long‐period surface waves at a period of T=256 sec and determined the second nodal plane to be dip 18°, rake 116°, and strike 257°. This shallowly dipping thrust mechanism is consistent with plate motions in this region. Seismic moment from surface‐wave inversion is 1.3×1028 dyne‐cm corresponding to Mw=8.0. Amplitudes of body and surface waves from short‐period instruments yield magnitudes of
and Ms=7.7. The teleseismic average P‐wave moment rate spectrum from 17 short‐ and intermediate‐period instruments is slightly lower than that of an average Mw=8.0 subduction‐zone event. We constrained the fault plane as determined above to deconvolve the first 90 secs of the long‐period body wave at 11 teleseismic stations to determine the source time function and the spatial distribution of moment release. The source time function consists of 4 moment‐releasing episodes which have a total moment release of 9.4×1027 dyne‐cm. The fault ruptured bilaterally with the largest moment releasing subevent occurring between 30‐45 sec. This subevent nucleates approximately 75‐90 km west of the determined epicenter. This region corresponds to the epicentral area of the 1957 Great Aleutian earthquake which is one of the largest earthquakes in recorded history.
Seismic reflections from the upper mantle discontinuities beneath the Mid‐Atlantic Ridge observed by a seismic array in Hokkaido Region, Japan
Authors: Nakanishi Ichiro
Published: February 2008

Following a recent big deep event (mb=6.8) in the Fiji islands region, a seismic array in Hokkaido region, Japan recorded clear impulsive precursors to the P′P′ phases (P′ is another symbol for PKP and P′P′ is the surface reflection of P′) in the epicentral distance range of 70°‐72.4°. Reflection points of the phases are located 1.5°‐3.5° west of the Mid‐Atlantic ridge (13°S, 15°W). The precursors were also recorded at two WWSSN stations in Japan. Travel times and slowness measured by the array are best explained as the precursors being underside reflections from a depth of 660 km. The amplitude ratios of P′660P′/P′P′ are about 0.1, a value similar to previous observations. The observation suggests that there exists a sharp discontinuity at a depth of 660 km beneath the area close to the Mid‐Atlantic ridge.
Melt segregation driven by dynamic forcing
Authors: Ribe Neil M.
Published: February 2008

Melt segregation can be driven by suction applied to the free surface of a partially molten region (dynamic forcing). Dynamic forcing may play a role in drawing melt towards eruption centers or in maintaining melt channels within the mantle. I examine the importance of dynamic forcing by means of a model in which melt is extracted by suction from a deformable porous medium with initially uniform porosity. The suction rapidly reduces the porosity near the free surface, and the melt extraction rate drops to zero after about 10³ yr. The total volume of melt extracted during this time is only about 0.05 km³ per km² of free surface area. These results suggest that dynamic forcing probably plays a negligible role in melt segregation, and that melt channels are unlikely to exist in the mantle.
Rikitake two‐disk dynamo and paleomagnetism
Authors: Kono Masaru
Published: February 2008

The Rikitake two‐disk dynamo is known to display chaotic oscillations of the current direction which resemble occasional polarity reversals of the geomagnetic field. In order to see how well the actual behavior of the geodynamo is simulated by this simple model, statistical properties of the two‐disk dynamo system are examined in two aspects; the field intensity and the length of polarity intervals. Paleomagnetic data indicate that the occurrence frequency of the dipole intensity is well approximated by superposition of two normal distributions, and that of the polarity intervals by an exponential distribution. For various parameters tried for the Rikitake two‐disk dynamo, the distributions of the dipole intensity and of the polarity intervals are not similar to the paleomagnetically observed ones. Thus the similarity between the two‐disk dynamo and the geomagnetism appears to be limited.