bluuebunny/crossref_metadata_2025 huggingface dataset:
A new approach to laser heating in high pressure mineral physics
Authors: Boehler R. , Chopelas A.
Published: February 2008

Single crystals of olivine, forsterite, and enstatite were heated to over 2000 K between 100 and 250 kbar with a 120 W CO2‐laser in a diamond anvil cell using a hydrostatic, inert gas pressure medium. The conversion to some of their high pressure polymorphs, β‐phase, γ‐phase, perovskite, majorite, ilmenite, and clinoenstatite yielded single crystals large enough to obtain high quality Raman spectra, thus providing a quick, economic, and precise method to measure phase diagrams without pressure quenching.

The phase boundaries between olivine or forsterite and β‐phase and between γ‐phase and perovskite were accurately bracketed. Our preliminary results on the latter transition are within the P‐T uncertainties of previous measurements using multi‐anvil devices [Ito and Takahashi, 1989], constraining the temperature at 670 km depth to 1900±100 K which is about 300 K higher than using their best estimate of the phase boundary.

The unquenchable phase of MgSiO3‐clinopyroxene was synthesized at ∼170 kbar; Raman spectra show that this phase is of C2/c symmetry. Its stability field is between 50 and ∼165 kbar at ambient temperature.

MgSiO3 perovskite was synthesized above 230 kbar and ∼2000 K. Raman spectra were measured from 75 to 470 kbar. Above 370 kbar, two modes disappear and two others change slope with pressure, suggesting a change in symmetry from Pbnm at room temperature.
Spectra of gravity wave density and wind perturbations observed during ALOHA‐90 on the 25 March flight between Maui and Christmas Island
Authors: Hostetler Chris A. , Gardner Chester S. , Vincent Robert A. , Lesicar Drazen
Published: February 2008

The airborne Na lidar and Christmas Island MF radar were used to measure the spectra of gravity wave density and wind perturbations during ALOHA‐90 on the 25 March mission. Wave activity was especially strong near Christmas Island with measured wind variances between 1000 and almost 1400 (m/s)². The vertical and meridional wave number spectra of horizontal wind perturbations exhibited power law shapes with slopes of −3.17 and −2.22, respectively. The temporal frequency spectrum appeared to be influenced by strong Doppler effects and had a shallow slope near −1. The magnitudes of the spectra were 4.9 × 105 (m/s)²/(cyc/m) at the vertical wave number 2π/(4 km), 8.4 × 106 (m/s)²/(cyc/m) at the meridional wave number 2π/(200 km) and 7 × 105 (m/s)²/(cyc/m) at the temporal frequency 2π/(1 h).
Dissociative recombination of the ground state of N+2
Authors: Guberman Steven L.
Published: February 2008

Large scale theoretical quantum chemical calculations are reported for the dissociative recombination with an electron of the lowest vibrational level of the ground state of the nitrogen molecular ion. The calculated dissociative recombination rate is 1.6 × 10−7 × (Te/300)−.37cm3/sec for 100 < Te< 1000K and is in excellent agreement with prior microwave afterglow experiments but disagrees with recent merged beam results. The dissociation limit yields mostly 4S + 2D atoms in disagreement with plasma flow tube results. The dominant mechanism for dissociative recombination imparts sufficient energy to the product atoms to allow for escape from the Martian atmosphere.
Depth of the ash flow deposit in the Valley of Ten Thousand Smokes, Katmai National Park, Alaska
Authors: Kienle Jürgen
Published: February 2008

In anticipation of a drill hole, seismic refraction and gravity profiles acquired more than 20 years ago have been reinterpreted in light of new density data to determine the thickness of the ash flow deposit in the Valley of the Thousand Smokes (VTTS). The data show the ash flow is at least 170m thick and most likely welded in its lower portion. Its average density is ∼1800 kg m−3. Seismic velocities range from 0.6 to 1 km/sec for the deposit's unwelded 30‐to‐50m‐thick top section and 1.8 to 2.5 km/sec for the lower welded section. The data confirm the first realistic estimate of the volume of the ash flow of about 11 km³ by Curtis (1968), based on a geomorphic analysis of stream profiles.
Lyapunov exponent of magnetospheric activity from AL time series
Authors: Vassiliadis D. , Sharma A. S. , Papadopoulos K.
Published: February 2008

A correlation dimension analysis of the AE index indicates that the magnetosphere behaves as a low‐dimensional chaotic system with a dimension close to 4. Similar techniques are used to determine if the system's behavior is due to an intrinsic sensitivity to initial conditions and thus is truly chaotic. The quantity used to measure the sensitivity to initial conditions is the Lyapunov exponent. Its calculation for AL shows that it is nonzero (0.11±0.05 min−1). This gives the exponential rate at which initially similar configurations of the magnetosphere evolve into completely different states. Also predictions of deterministic nonlinear models are expected to deviate from the observed behavior at the same rate.
Paleohydrological processes in Siling‐CO (lake) in the Qing‐Zang (Tibetan) Plateau based on the physical properties of its bottom sediments
Authors: Kashiwaya Kenji , Yaskawa Katsumi , Yuan Baoin , Liu Jiaqi , Gu Zhaoyan , Cong Shaoguang , Masuzawa Toshiyuki
Published: February 2008

Bottom sediments in Siling‐co in central Tibet reveal that late in the last glacial period the area around the lake was arid, water inflow was small and the level of the lake was low. In the early postglacial period it became markedly humid and the water level rose rapidly because of large inflows from surrounding basins. The climate was humid and the level of the lake high during the hypsithermal. At about 2,000–4,000 yr B.P. it again became arid within a short time and the water level dropped sharply. After this short arid interval and low water level, it again became humid and the water inflow increased sharply. Large inflows continued for a time after which it again began to get arid.
Chaos and ion heating in a slow shock
Authors: Lin Y. , Lee L. C.
Published: February 2008

Hybrid‐code simulations of slow shocks indicate that there exists a critical number, Mc, such that slow shocks with an intermediate Mach number MI ≥ Mc have a rotational wave train and slow shocks with MI < Mc do not have such a wave train. We propose an ion heating mechanism of slow shocks, which is associated with the chaotic motion of particles in the downstream wave field. For a coherent electromagnetic wave propagating along the downstream magnetic field, corresponding to switch‐off shocks, the particle motions are not chaotic. For an oblique wave, the interaction between the particles and the wave field may lead to chaotic particle motions. Such particles may be greatly thermalized within one wavelength after they are incident into the downstream wave field. The results can be used to explain the existence of the critical intermediate Mach number observed in the hybrid simulations.
Detection of Philippine Sea Plate Motion by very long baseline interferometry
Authors: Matsuzaka Shigeru , Tobita Mikio , Nakahori Yoshiro , Amagai Jun , Sugimoto Yuji
Published: February 2008

The Geographical Survey Institute (GSI) deployed a mobile Very Long Baseline Interferometry (VLBI) system at an established site in Chichijima (the biggest island in the Bonin islands), twice in an interval of two years. Two sessions were conducted in the experiments each year by connecting the reference base station at Kashima Space Research Center of Communications Research Laboratory (CRL). The results from the data of four experiments indicate that the Chichijima site moved 7.4±0.8cm in the direction of N67°W in the two years. This is the first successful direct measurement of the motion of Philippine Sea plate (PHS) by any method. Our result is consistent with the predicted rigid motion of PHS from the newest plate velocity model (NUVEL‐1).
The determination of source parameters for small earthquakes from a single, very broadband seismic station
Authors: Fan Guangwei , Wallace Terry
Published: February 2008

The installation of very broadband seismic stations makes it possible to recover the source parameters of small earthquakes (2.5 < ML < 5.0) which occur at local and regional distances. If the gross crustal structure along the travel path is known, it is possible to use the P, SV and SH displacement waveforms from a single station to determine the seismic moment tensor. Although the details of the crustal structure strongly affect the body waveforms at regional distances, the signature of the seismic source orientation on the waveform is robust at frequencies less than 1–3 Hz. We explore the trade‐offs between crustal model, hypocentral depth and filtering for a linear moment tensor inversion procedure. The procedure is tested on two small earthquakes which occurred in the Rio Grande Rift and were recorded at the IRIS/USGS station ANMO. The agreement between the single station moment tensor inversion fault plane parameters and those determined from local first motions is excellent.
Propagation of perturbation energy fluxes in the subsolar magnetosheath: AMPTE IRM observations
Authors: Lin N. , Engebretson M. J. , Baumjohann W. , Luehr H.
Published: February 2008

We have studied the propagation properties of perturbation energy fluxes of subsolar magnetosheath fluctuations. The Poynting flux, kinetic energy flux, and enthalpy flux are calculated using magnetic field and plasma measurements from the AMPTE IRM satellite during five intervals in 1984. The results are then compared with a disturbance indicator R of the magnetic field in the same magnetosheath region. It is shown that during disturbed periods with large transverse variations (low R level), the perturbation Poynting flux and the kinetic energy flux increase, and a significant portion of the fluxes consistently propagates toward the magnetopause. The Poynting flux of those fluctuations which consist of mainly compressional perturbations does not appear to propagate in any certain direction. The enthalpy flux of the perturbations does not propagate in any certain direction in any of the cases. The kinetic energy flux appears to be more important in exciting harmonic ULF waves in the dayside magnetosphere. What portion of this energy flux is transferred into the magnetosphere needs further investigation.
Modification of lower crust by continental rift magmatism
Authors: Wendlandt Richard F. , Baldridge W. Scott , Neumann E.‐R.
Published: February 2008

We document a systematic inverse relationship between the volume of erupted volcanic rocks associated with continental rifts and the seismic compressional wave velocity contrast across the Moho. This relationship is interpreted as evidence that magmatism significantly modifies the composition(s) of the lower crust (and upper mantle) during rifting processes. The relationship is not influenced by the current thermal state of the lithosphere, being observed for both modern rifts (Rio Grande, Kenya, Rhine) and paleorifts (Oslo), and may also characterize continental flood basalt provinces (Columbia River and Snake River plain). The trend is reconciled by igneous and metamorphic processes that collectively modify the lower crust to more mafic bulk compositions, resulting in higher compressional velocities, and that decrease the density of the residual mantle, resulting in lower compressional velocities. These results support the view that most mantle‐derived magma in continental rift settings is trapped in the crust.
Emission of elemental gold particles from Mount Erebus, Ross Island, Antarctica
Authors: Meeker Kimberly A. , Chuan Ray L. , Kyle Philip R. , Palais Julie M.
Published: February 2008

Volcanoes are an important source of gases and aerosols in the atmosphere. Significant quantities of trace elements are emitted as vapor species [Nriagu, 1989; Symonds et al., 1987] and are strongly enriched in the gas relative to the magma [Tazieff and Sabroux, 1983; Crowe et al., 1987]. After eruption the trace elements condense on ash and other particles or they form sublimates and agglomerates. Here we report on the emission of gold (Au) from Mount Erebus, Antarctica. Although the flux of emitted Au is low compared to other volcanoes, crystalline particulate Au has been found in the plume near the crater, in ambient air up to 1000 km from the volcano and in near surface samples. Vapor phase transport of Au may occur as a chloride species and could be an important transport mechanism in crystallizing magmatic intrusions.
Direct solar neutrons detected by neutron monitors on 24 May 1990
Authors: Shea M. A. , Smart D. F. , Pyle K. R.
Published: February 2008

The ground‐level energetic solar particle event on 24 May 1990 has an extremely impulsive onset in the particle intensity that we attribute to a significant flux of direct solar neutrons impacting the top of the atmosphere. This is the largest solar neutron event reported to date with a 25.3% increase recorded by the Climax neutron monitor in the time interval 2050–2055 UT. At least 7 neutron monitors in North America recorded this increase simultaneously, the magnitude of which is ordered by the air mass along the line of sight to the sun at the time of the event. The initial solar neutrons arrive within the same minute as the maximum in both the soft X‐ray and the H‐alpha emission observed at the earth, whereas the onset of the world‐wide increase of solar protons occurred approximately 15 minutes later.
Modeling the growth phase of a substorm using the Tsyganenko Model and multi‐spacecraft observations: CDAW‐9
Authors: Pulkkinen T. I. , Baker D. N. , Fairfield D. H. , Pellinen R. J. , Murphree J. S. , Elphinstone R. D. , McPherron R. L. , Fennell J. F. , Lopez R. E. , Nagai T.
Published: February 2008

The CDAW‐9 Event C focused upon the early part of 3 May 1986 when a large substorm onset occurred at 0111 UT. By modifying the Tsyganenko 1989 magnetic field model, we construct a model in which the near‐Earth current systems are enhanced with time to describe the observed development of the tail magnetic field during the growth phase. The cross‐tail current intensity and the thickness of the current sheet are determined by comparison with three spacecraft in the near‐Earth tail. The location of the auroral bulge as recorded by the Viking imager is mapped to the equatorial current sheet. The degree of chaotization of the thermal electrons is estimated, and the consequences to the tail stability towards ion tearing are discussed. We conclude that the mapping of the brightening region in the auroral oval corresponds to the regions in the tail where the current sheet may be unstable towards ion tearing.
Decay of ion beam driven acoustic waves into ion holes
Authors: Gray Perry C. , Hudson Mary K. , Lotko William , Bergmann Rachelle
Published: February 2008

Particle simulation is used to demonstrate the formation of ion holes and parallel potential structures in a background plasma driven by an ion beam. Recent satellite observations suggest that such potential structures which are observed along auroral field lines may form in a cool background ion population. We show that if such a background is present ion holes form in the background ion distribution and travel at the acoustic speed in the direction of the ion beam velocity. Linear stability analysis shows this system to be unstable to a beam‐acoustic mode involving the beam ions and the background electrons. These beam unstable modes nonlinearly pump the linearly stable ion acoustic modes in the background plasma seeding the formation of ion holes. The dynamics of these structures are essentially indistinguishable from the evolution of weak double layers seen in previous simulations.
The short‐term temperature response to smoke from oil fires
Authors: Westphal Douglas L. , Toon Owen B.
Published: February 2008

Simulations with a model of atmospheric dynamics, aerosols, and radiative transfer show that both ground cooling and warming are possible after one diurnal cycle beneath smoke plumes of moderate visible absorption optical depths representative of oil smoke down‐wind of Kuwait. For an elevated smoke plume, the calculated ground temperature changes range from +1 to −6 K, while within the smoke plume temperature increases range from 0 to 8 K. The response of the ground temperature and boundary layer dynamics depends strongly upon the surface characteristics with deserts exhibiting the strongest cooling of all land use types. The variability of meteorology generally precludes the possibility of a persistent smoke plume, or its effects, over any location other than the immediate surroundings of a persistent fire. For the Middle East these results suggest persistent effects only near Kuwait, with short‐term coolings of about 5 K over desert regions a few thousand kilometers downwind of Kuwait.
Mass‐loading and parallel magnetized shocks
Authors: Zank G. P. , Oughton S. , Neubauer F. M. , Webb G. M.
Published: February 2008

Recent observations at comets Giacobini‐Zinner and Halley suggest that simple non‐reacting gas dynamics or MHD is an inappropriate description for the bow shock. The thickness of the observed (sub)shock implies that mass‐loading is an important dynamical process within the shock itself, thereby requiring that the Rankine‐Hugoniot conditions possess source terms. This leads to shocks with properties similar to those of combustion shocks. We consider parallel magnetized shocks subjected to mass‐loading, describe some properties which distinguish them from classical MHD parallel shocks, and establish the existence of a new kind of MHD compound shock. These results will be of importance both to observations and numerical simulations of the comet‐solar wind interaction.
The role of corotating interaction regions in cosmic‐ray modulation
Authors: Kóta J. , Jokipii J. R.
Published: February 2008

We present the first global simulations of the modulation of galactic cosmic rays by a three‐dimensional solar wind with corotating interaction regions. The cosmic‐ray transport equation is solved in a computed wind and magnetic‐field model. The results show both the previously‐neglected small‐scale response to CIR's and global, drift‐dominated, effects which are similar to previous models. Our model predicts a correlation between the local magnetic field and the rate of decrease of the cosmic‐ray intensity which is similar to that observed. This is found to be due to inhibited diffusion. We suggest that such small‐scale variations are local and that they do not change significantly the global cosmic‐ray structure except, perhaps, near sunspot maximum.
Evidence for denitrification in the 1990 Antarctic spring stratosphere: II. Lidar and aerosol measurements
Authors: Deshler T. , Adriani A. , Hofmann D. J. , Gobbi G. P.
Published: February 2008

Balloonborne aerosol soundings from McMurdo Station, Antarctica in 1990, documented the occurrence of polar stratospheric clouds (PSCs) during both fast and slow cooling events as well as the background stratospheric sulfate aerosol. PSCs formed under slow cooling predominated and in this case the size distributions were found to be bimodal with mode radii of 0.08 and 2–3 µm, similar to previous measurements in Antarctica. The aerosol soundings were also compared to lidar measurements at McMurdo in three cases. In the one PSC layer formed from fast cooling, the best agreement between measured and calculated scattering ratio was found using an index of refraction of 1.37, suggesting an amorphous nitric acid/water composition rather than crystalline nitric acid trihydrate. For the background aerosol, calculations using an index of 1.5 were generally in best agreement with the measured values.
Exospheric transport restrictions on water ice in lunar polar traps
Authors: Hodges R. Richard
Published: February 2008

There is little doubt that at least 1017 g of water has accreted on the moon as a result of the reduction of ferric iron at the regolith surface by solar wind protons, the vaporization of chondrites, and perhaps comet impacts. Lacking an efficient escape mechanism, most of this water (or its progeny) is probably on the moon now. If the water were to have migrated to permanently shaded cold traps near the lunar poles, ice deposits with densities greater than 1000 g cm−2 would cover the traps, providing accessible resources. However, exospheric transport considerations suggest that the actual amount of water ice in the cold traps is probably too small to be of practical interest. The alternative is global assimilation of most of the water into the regolith, a process that must account for about 30 micromoles of water per gram of soil.
Expected extreme ultraviolet spectrum of the lunar surface
Authors: Edwards Bradley C. , Priedhorsky William C. , Smith Barham W.
Published: February 2008

The moon was recently observed to be a source of very soft x‐ray emission. The emission was most intense at wavelengths longer than 62 Å and was attributed to Thomson scattering of solar x‐rays. This observation prompted us to study the emissions expected from the lunar surface in the wavelength range between 90 and 500 Å. Photons in this wavelength range scatter inefficiently. Instead, the solar x‐rays are absorbed in the first several microns of lunar regolith. The absorbed x‐rays can excite the surface elements and result in fluorescent emission. We find that much of the L‐ and M‐shell extreme ultraviolet fluorescence, in the wavelength range between 90 and 500 Å, have higher peak intensities than the scattered solar spectrum. The total integrated fluorescent emission is also higher than the total scattered solar radiation. The L‐shell fluorescent radiation can be an indicator of the surface abundances of Si, Al, Mg and other major lunar elements.
The atomic sodium exosphere/coma of the Moon
Authors: Ip W.‐H.
Published: February 2008

A Monte Carlo model is used to study the random walk process of sodium atoms created by meteoroid impacts on the lunar surface. The collisional surface interaction is parameterized by a thermal accomodation coefficient of 0.5. Sodium atoms launched with initial velocities well below the surface escape velocity of 2.38 km s−1 will be recycled and contribute to the formation of an exosphere with a temperature of about 500 K on the sunlit side. For large launch velocities (>2 km s−1) there will be systematic transport of the escaping sodium atoms to the antisolar direction as a result of the acceleration effect of the solar radiation pressure. An extended coma with a significant elongation in the antisolar direction can form this way. Recent spectroscopic and imaging observations are consistent with this picture. The present model calculations suggest that meteoroid impact effect (and solar wind sputtering) should be the major supplier of the lunar sodium exosphere; the possible existence of an efficient loss mechanism of the sodium atoms during surface interaction is also indicated.
The resonance structure of low latitude Pc3 geomagnetic pulsations
Authors: Waters C. L. , Menk F. W. , Fraser B. J.
Published: February 2008

The spectral difference in ULF wave amplitude between closely spaced meridional ground stations may be used to measure the eigenfrequency of magnetospheric field lines [Baransky et al., 1985]. A more reliable technique based on the crossphase spectrum has been used to identify eigenfrequencies and study the temporal evolution of local field line resonances. Pc3 (22–100 mHz) pulsations recorded with two pairs of low latitude ground stations have been specifically examined. Resonances and harmonics whose frequencies are in excellent agreement with model calculations of standing toroidal field line resonances, when ionospheric mass loading at low latitudes is taken into account, were identified virtually every day. This points to a diagnostic technique for monitoring temporal variations in ionospheric and magnetospheric plasma parameters.
Rheology of the Earth's mantle and geodynamical processes
Authors: Birger Boris I.
Published: February 2008

A non‐linear integral rheological model consistent with general creep theory and laboratory studies of rock creep is proposed to describe the rheology of the Earth's mantle. This model is universal, i.e., relevant to all strains, stresses and timescales. For constant stresses the model behaves like a power law non‐Newtonian fluid. However, the model differs significantly if stresses change with time, because it has a memory, in contrast with the power law fluid model. A linear anisotropic integral rheological law that relates stress perturbation to strain perturbation is obtained for small‐strain flows superimposed on a basic steady large‐strain convective flow in the mantle. The rheological parameter in this law depends on the basic flow. The small‐strain deformations associated with seismic waves and postglacial rebound are considered. In a pure power law fluid the effective viscosity associated with the convective flow is equal to the effective viscosity associated with postglacial rebound, whereas in the proposed model, the effective convective viscosity is about three orders of magnitude larger than the effective postglacial viscosity, at least for the lower mantle.
A genetic link between remagnetization and potassic metasomatism in the Devonian Onondaga Formation, Northern Appalachian Basin
Authors: Lu Gang , McCabe Chad , Hanor Jeffrey S. , Ferrell Ray E.
Published: February 2008

Whole‐rock chemical analyses indicate that the Onondaga Formation along a Rochester‐Syracuse (New York) transect has a marked eastward increasing trend in K/Al ratio, which correlates with an increase in authigenic magnetite content and degree of diagenetic illitization. An eastward increase in the content of some authigenic minerals including K‐feldspar, quartz, and Fe‐rich chlorite in the rocks is indicated by petrographic and X‐ray diffraction studies. These observations suggest a link between remagnetization and potassic alteration of Fe‐bearing silicates in the Onondaga. Two hypothetical reactions are proposed in which magnetite authigenesis is related to the breakdown of Fe‐bearing smectite in the presence of potassium. Products of these reactions include authigenic K‐feldspar, illite, quartz and chlorite, as well as magnetite. Our results suggest that remagnetization was triggered by a basin‐wide migration of K‐rich fluids during the Alleghenian Orogeny.
A fault model for the 1989 Kilauea South Flank Earthquake from leveling and seismic data
Authors: Arnadottir Thora , Segall Paul , Delaney Paul
Published: February 2008

The geometry of the fault that ruptured during the M6.1 south flank earthquake on Kilauea volcano in 1989 is determined from leveling data. The elastic dislocation, in a homogeneous elastic half‐space, that best fits the data is found using a nonlinear inversion procedure. The best fitting model is a gently dipping thrust fault that lies at 4 km depth. This is significantly shallower than the 9 km hypocentral depth determined from the local seismic network. Two‐dimensional finite‐element calculations indicate that at least part of this discrepancy can be attributed to the focusing of the surface deformation by the upper few kilometers of compliant, low‐density lavas. We conclude that it is important to include realistic elastic structure to estimate source geometry from geodetic data.
Physical variation of water vapor, and the relation with carbon dioxide
Authors: Egan W. G. , Hogan A. W. , Zhu H.
Published: February 2008

Analysis of the long term NOAA carbon dioxide flask sample records to examine the exchange among the continental Antarctic air mass and other air masses shows a meteorological variation of carbon dioxide concentration. There is an inverse relation between the seasonal variation of carbon dioxide concentration and water vapor at all stations examined. Well established diffusion coefficients indicate an interaction of water and carbon dioxide vapor on the molecular scale. Laboratory experiments using a Fourier transform spectrometer show carbon dioxide to be removed from an airstream in proportion to water vapor precipitated. We propose that interaction of carbon dioxide and water vapor in the atmosphere provides temporary sinks that can influence the balance of the carbon dioxide budget.
A simulation study of multiple ion wave generation downstream of lOW Mach number quasiperpendicular shocks
Authors: Motschmann Uwe , Raeder Joachim
Published: February 2008

The behavior of minor ions just downstream of a low Mach number quasiperpendicular shock is investigated both theoretically and by computer simulations. Because all ions see the same cross shock electric field their deceleration depends on their charge to mass ratio, yielding different downstream velocities. It is shown that these differences in velocity can lead to coherent wave structures in the downstream region of quasiperpendicular shocks with a narrow transition layer. These waves are shown to be multi ion hybrid waves in contrast to mirror waves and ion cyclotron waves. Under favorable conditions these waves should be observable both at interplanetary shocks and at planetary bowshocks.
A new inversion method of Coda waveforms to determine spatial distribution of Coda scatterers in the crust and uppermost mantle
Authors: Nishigami Kin'ya
Published: February 2008

We have developed a new inversion method of coda waveforms from local earthquakes to estimate the inhomogeneous spatial distribution of scattering coefficient in the crust and the uppermost mantle. A preliminary analysis by this method was applied to the data observed at the Hokuriku district, central Japan. Inhomogeneous structures have been revealed in both horizontal and vertical directions as follows: (1) Strong scatterers are distributed in some localized parts of the surface layer and the upper crust; (2) Some of the strong scatterers seem to be related to major active faults; (3) Horizontal variation of scattering coefficient is smaller in the uppermost mantle than in the crust. We have found that the method presented here is effective enough to investigate deterministically the inhomogeneity in the shallow part of the crust.
Geophysical implications from relocations of Tibetan Earthquakes; Hot lithosphere
Authors: Zhao Lian‐She , Helmberger Donald V.
Published: February 2008

ISC locations for earthquakes beneath Tibet indicate a random distribution of events down to a depth of about 50 km. This distribution would be expected from a relatively cool crust which would allow the seismo‐genic zone to extend to such depths. A detailed investigation of the Tibetan earthquakes, with magnitude greater than 5.5 from 1964 to 1986, yields a distinctly different picture. Waveform modeling of depth phases indicates that only a few events from this population is actually deeper than 25 km. These few events occurred near the edges of the Plateau where active subduction is occurring as suggested by the thrust‐like nature of their mechanisms. The events, averaging the entire population, occurred earlier than indicated by the ISC by about 3 seconds which leads to about a 1.5% and 0.5% over estimation of Pn and Sn velocities respectively applying ISC tables and standard flat‐layered models. A more serious error occurs if the Pn and Sn velocities are determined by correcting for source depth but assuming the ISC origin times.
ODP Leg 137, borehole fluid chemistry in Hole 504B
Authors: Magenheim Andrew J. , Bayhurst Greg , Alt Jeff C. , Gieskes Joris M.
Published: February 2008

Borehole fluid samples collected from Site 504B, during ODP Leg 137, provide the best opportunity to date to study water‐rock interaction within the ocean crust. Chemical composition of the borehole fluids reveal the results of seawater‐basalt interaction at elevated temperatures. The borehole fluids can be described as a two component mixture, consisting of a reacted fluid component and seawater which has convectively mixed into the borehole from the seafloor. Progressive trends in fluid chemistry with time suggest that the fluid‐rock reactions have changed as the borehole has been drilled deeper into the hotter formation Based on data from this study and comparisons to fluid studies from past visits to Site 504B, indications are that these reactions are occurring within the borehole environment.
Linear theory of the Kelvin‐Helmholtz instability in the low‐latitude boundary layer
Authors: Rajaram R. , Sibeck D. G. , McEntire R. W.
Published: February 2008

We examine the stability of the magnetospheric low‐latitude boundary layer to the Kelvin‐Helmholtz instability taking into account the finite thickness of the velocity shear layer, the relative thickness and position of the “current” layer in which the magnetic field rotates from magnetosheath to magnetospheric configurations, and the collisionless and anisotropic nature of the plasmas therein. We show that three factors enhance the instability: (1) a decrease in the thickness of the shear layer, (2) a decrease in the thickness of the current layer (relative to the shear layer), and (3) the proximity of the “current layer” to the outer edge of the shear layer. Although the maximum growth rate occurs for wave numbers roughly equal to the inverse of the semithickness of the shear layer, the velocity threshold for the onset of the instability is insensitive to the thickness. However, when the current layer is displaced from the center toward the outer edge of the shear layer, the threshold is dramatically reduced and the boundary layer can be rendered unstable by relatively small shear flows, suggesting that even the region close to the subsolar point could become unstable when the sheath flow is perpendicular to the magnetospheric magnetic field. This mode can be identified by characteristic profiles across the boundary layer: the plasma density, pressure, and magnetic field strength perturbations all peak near the center of the boundary layer. The transverse magnetic field fluctuations are largest around the current layer, while the radial motions dominate near the inner edge of the boundary layer.
Intensification and fading of auroral arcs in the dusk‐midnight sector of the polar cap
Authors: Wu Q. , Rosenberg T. J. , Berkey F. T. , Eather R. H.
Published: February 2008

Observations of the aurora from South Pole station (magnetic latitude = −74.2°) have been used to study the intensification and fading of polar arcs observed near the dusk meridian. Most of the cases examined have the following features in common: (1) a preexisting auroral form intensifies for about 10 min; (2) this activation is followed by a pronounced decrease of luminosity; (3) the auroral fade terminates after 30–60 min with the onset of intense aurora which sweeps rapidly overhead. The brief intensifications involve O (630.0 nm) enhancements of 0.1–0.3 kR and N2+ (427.8 nm) enhancements of 0.3–1.5 kR giving an emission ratio which in each case is consistent with the precipitation of electrons with an average energy of 3–12 keV, assuming a Maxwellian distribution. The auroral brightening is not accompanied by riometer absorption. Ground magnetic perturbations of the order 20–40 nT occur, and in about half the instances, weak broadband impulsive hiss is present. The availability of all‐sky camera, auroral electrojet (AE) index and interplanetary magnetic field (IMF) data for some of the cases enables the following additional characterizations of these events. The preexisting form is a Sun‐ or oval‐aligned arc (or part of a multiple arc system) which disappears following the activation; equatorward drift of the arc (or system) accompanies the luminosity change. There is some evidence to suggest that the arc is poleward of the auroral oval. The brief intensification and/or the onset of fading occurs during the growth phase or near the start of the expansive phase of a substorm; termination of the fade is near the maximum in AE and is probably indicative of the beginning of the recovery phase of the substorm. For all three cases for which IMF data were available the onset of fading occurred 20–30 min after Bz turned southward. Two of these cases were preceded by auroral arc enhancement when Bx was sunward; in the third case there was no prior enhancement when Bx was zero or slightly antisunward. By was negative (dawnward) in each case. Sun‐aligned arcs are a common feature of the polar cap during northward Bz but disappear during the increasingly disturbed conditions that accompany southward Bz. The present results suggest that brief intensifications of southern hemisphere polar cap arcs near dusk may be linked in part to the sunward orientation of the IMF which favors enhanced electron fluxes in the southern lobe of the magnetotail.
Magnetosphere‐ionosphere coupling during plasmoid evolution: First results
Authors: Hesse Michael , Birn Joachim
Published: February 2008

The influence of magnetosphere‐ionosphere coupling on the dynamic evolution of the magnetotail is investigated by a three‐dimensional resistive MHD code that includes the effects of the closure of field‐aligned currents in a simple resistive model ionosphere. Particular emphasis is on the role of this coupling during substorm evolution and the modification of the latter by the convection driven by the ionospheric electric fields. For comparison, we present results from a simulation which uses an infinitely conducting ionosphere but is otherwise identical. Comparison of the two simulations shows that the major impact of magnetosphere‐ionosphere communication is an acceleration of magnetotail evolution. Otherwise, phenomena in the two models are qualitatively similar. We conclude that ionospheric effects do not significantly affect substorm associated magnetotail dynamics.
Properties of mass‐loading shocks: 1. Hydrodynamic considerations
Authors: Zank G. P. , Oughton S.
Published: February 2008

The one‐dimensional hydrodynamics of flows subjected to mass loading are considered anew, with particular emphasis placed on determining the properties of mass‐loading shocks. This work has been motivated by recent observations of the outbound Halley bow shock (Neubauer et al., 1990), which cannot be understood in terms of simple hydrodynamical or magnetohydrodynamical descriptions. By including mass injection at the shock, we have investigated the properties of the Rankine‐Hugoniot conditions on the basis of a geometric formulation of the entropy condition. Such a condition, which is more powerful than the usual thermodynamical formulation, serves to determine those solutions to the Rankine‐Hugoniot conditions which correspond to a physically realizable downstream state. On this basis a concise theoretical description of hydrodynamic mass‐loading shocks is obtained. We show that mass‐loading shocks have more in common with combustion shocks than with ordinary nonreacting gas dynamical shocks. It is shown that for decelerated solutions to the Rankine‐Hugoniot conditions to exist, the upstream flow speed u0 must satisfy u0 > ucrit > cs, where cs is the sound speed. Besides the usual supersonic‐subsonic transition, mass‐loading fronts can also admit a decelerating supersonic‐supersonic transition, the structure of which consists of a sharp decrease in the flow velocity preceding a recovery and an increase in the final downstream flow speed. We suggest the possibility that such structures may describe the inbound Halley bow shock (Coates et al., 1987a). Both parallel and oblique shocks are considered, the primary difference being that oblique shocks are subjected to a shearing stress due to mass loading. It is conjectured that such a shearing may destabilize the shock.
The effect of the solar cycle on the maintenance of the nightside ionosphere of Venus
Authors: Kliore Arvydas J. , Luhmann Janet G. , Zhang Martina H. G.
Published: February 2008

Pioneer Venus radio occultation measurements of the nightside ionosphere of Venus collected from 1979 to 1986 have made it possible to study its behavior at times of both solar maximum and solar minimum. Although some solar maximum measurements are similar in nature to those observed at solar minimum, which have an average peak density of about 7×10³ cm−3, others show much higher peak densities, reaching values of about 4×104 cm−3. These elevated peak densities also occur at higher altitudes. The integrated electron column densities for these measurements are also much higher, indicating the presence of substantial ionization above the main peak. The magnitudes of both the peak density and the integrated content above the peak are anticorrelated with the solar wind dynamic pressure, leading to the interpretation that these enhancements during solar maximum are due to transterminator transport of O+ ions from the dayside when the solar wind dynamic pressure is low enough to permit a sufficiently high dayside ionopause. The resulting ionization peak can be many times the concentration produced by energetic electron fluxes impacting the neutral atmosphere on the nightside, which apparently form the remaining source of the nightside peak at such times during solar maximum, when transterminator flow is cut off by high solar wind pressure depressing the dayside ionopause, and during solar minimum, when the ionopause is always depressed.
Characteristics of spectra from the martian bow shock and comparison with Venus, Earth, AMPTE, Jupiter, and Saturn
Authors: Moses S. L. , Kennel C. F. , Strangeway R. J. , Grard R. , Nairn C.
Published: February 2008

We present plasma wave spectra from crossings of the Martian bow shock by the Phobos 2 spacecraft and compare them with spectra from bow shocks at other planets and the AMPTE lithium release. We find that the shock spectra of the inner planets are very similar in shape, and their wave energy densities, when normalized to the upstream electron thermal energy density, are comparable. However, the Martian shock spectrum most resembles the spectrum from AMPTE, which may indicate either the presence of pickup particles or processes involving ions with gyroradii that are large compared with the size of the interaction region. The normalized wave amplitudes at Mars are midway between the high normalized amplitudes found at Jupiter and Saturn and the lower, normalized amplitudes at Earth. This is consistent with a trend that has recently been shown to occur out to Neptune's bow shock.
Plasma and wave observations in the night sector of Mars
Authors: Nairn C. M. C. , Grard R. , Skalsky A. , Trotignon J. G.
Published: February 2008

The Phobos 2 spacecraft, initially injected into an elliptical orbit around Mars on January 29, 1989, was subsequently transferred, on February 18, 1989, to a nearly circular orbit, close to that of the Phobos moon, with an areocentric radius of the order of 9600 km. The spacecraft remained in this orbit until the end of the mission, on March 27, 1989. This paper summarizes the plasma and wave observations carried out in the night sector of Mars with the plasma wave system (PWS). Embedded in the magnetic field structure of the Martian tail, cold electron enhancements (“tail rays”) with densities in the range 10–65 cm−3 are observed in association with broadband wave activity extending from a few hertz up to several kilohertz; these enhancements appear to have characteristics analogous to enhancements observed at Venus. The ion outflow through the Martian eclipse region is estimated from Langmuir probe measurements to be of the order of 1025 ions/s.
Derivation of electron densities from differential potential measurements upstream and downstream of the bow shock and in the magnetosphere of Mars
Authors: Pedersen A. , Nairn C. , Grard R. , Schwingenschuh K.
Published: February 2008

A new technique is developed where the potential difference between a conductive spacecraft at floating potential and an electric field probe, negatively biased in relation to the spacecraft, with a current of 50 nA, is used to determine electron densities in the range 1–70 cm−3. The density can be obtained with an acceptable accuracy without any knowledge of the electron energy, provided the latter does not exceed 100 eV. This technique has been applied to the Phobos 2 spacecraft data to determine characteristic densities in the solar wind near Mars, in the foot of the bow shock, in the planetosheath, and at the top of the ionosphere. In the latter region, measurements have also been made using a Langmuir probe on board the spacecraft. A magnetic barrier, with a low plasma density, is observed between the planetosheath and the top of the ionosphere. Pronounced electron density increases are observed on the flanks and in the tail, some correlated and some anticorrelated with variations in the magnetic field strength.
Comparison of observed plasma and magnetic field structures in the wakes of Mars and Venus
Authors: Dubinin E. , Lundin R. , Riedler W. , Schwingenschuh K. , Luhmann J. G. , Russell C. T. , Brace L. H.
Published: February 2008

Plasma and magnetic field observations from the Phobos 2 spacecraft at Mars and the Pioneer Venus orbiter (PVO) at Venus show that there are some notable similarities in the structure of the low‐altitude magnetotails at both of these weakly magnetized planets. In particular, it is found that when conditions in the interplanetary medium are steady and the orbit sampling geometry is appropriate, two magnetic tail lobes, with an intervening “plasma sheet” or “central tail ray” in the approximate location of the dividing current sheet, are present. This behavior is seen in both the Phobos 2 ASPERA plasma analyzer data and in the PVO Langmuir probe data. In the Phobos 2 data, the tail ray is found to be composed primarily of antisunward streaming oxygen (O+) plasma which has a bulk velocity consistent with an energy close to that of the upstream solar wind plasma. The PVO Langmuir probe experiment also detected two (or more) additional cold plasma structures flanking the central feature; Phobos 2 data, on the other hand, show a proton plasma “boundary layer” flanking the central (mostly O+) tail ray or plasma sheet, with sporadic fluxes or rays of O+ ions. If the latter considered is to be the magnetosheath (solar wind plasma) at the tail boundary, it is mainly the common central tail O+ features that suggest that there are common planetary ion acceleration and magnetotail formation processes at work in the low‐altitude wakes of Mars and Venus. On the other hand, an important contribution from picked‐up exospheric hydrogen in the wake at Mars cannot be ruled out.
Infrared emission from the electron irradiated upper atmosphere produced by the Excede: Spectral experiment
Authors: Fraser Mark E. , Green Byron David , O'Neil Robert R.
Published: February 2008

This paper presents the results and interpretation of infrared data obtained by the Excede:Spectral auroral simulation experiment. We have analyzed 2‐ to 7‐μm data from the two circular variable filter spectrometers. The dominant N2 electronic transition system identified in the data across this region is the N2(W³Δuν≤5→B³Πg,ν≤3) band system which is observed at all altitudes. Nitric oxide fundamental emission is observed in the 5‐ to 7‐μm region. The determined NO vibrational distribution can be attributed to the reaction N(²D) + O2.
A kinetic theory of driven reconnection in the Earth's magnetotail
Authors: Lakhina G. S.
Published: February 2008

The process of driven magnetic reconnection induced by perturbing the magnetopause boundary of the Earth's magnetotail is analytically studied in the framework of kinetic theory. An explicit expression for the reconnected flux is obtained. The driven reconnection can either be exponential or bursty (i.e., short lived) type. The exponential type reconnnection can occur only when the trapped electron population is less than 30%. The reconnection rates for the exponential type reconnection are either smaller than or at the most equal to the ion tearing mode instability growth rates. On the other hand, the bursty type reconnection generally occurs at rates much faster than the growth rates of the ion tearing instability. The bursty type reconnection, however, lasts typically for a period equal to the inverse of its reconnection rate. The size of the magnetic islands formed, and the magnitude and duration of the plasma flows induced along the tail axis are much larger during the exponential type than during the bursty type reconnection. Under certain conditions the bursty type reconnection is expected to be important for the onset of magnetospheric substorms and their energization.