bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of the stratospheric hydrogen peroxide concentration profile using far infrared thermal emission spectroscopy
Authors: Chance K. V. , Johnson D. G. , Traub W. A. , Jucks K. W.
Published: February 2008

We have made the first unequivocal measurement of hydrogen peroxide in the stratosphere, a concentration profile obtained from a balloon platform using Fourier transform thermal emission spectroscopy in the far infrared. Measurements were made using the 112 cm−1RQ5 branch of the rotational‐torsional spectrum, with some confirmation from the 94 cm−1RQ4 branch. The atmospheric spectra were obtained from a balloon flight on September 26–27, 1989, launched from Fort Sumner, NM.The volume mixing ratio of H2O2 is 1.6 × l0−10 at 38.4 km, decreasing to 0.6 × l0−10 at 23.8km, with uncertainties of about 16% (1σ). Our measurements are compared to a recent stratospheric model calculation.
Introduction to ALOHA‐90: The airborne lidar and observations of the Hawaiian Airglow Campaign
Authors: Gardner Chester S.
Published: February 2008

The middle atmosphere is a region of complex photochemical and dynamic interactions. Stretching from roughly 30 to 100 km altitude, this is the transition region between the stratosphere and thermosphère and is perhaps the least understood region of the earth's atmosphere. A variety of interesting and important structures are found in this region including the ozone layer, noctilucent clouds, the airglow layers, the meteoric metal layers and the recently discovered sporadic Na and Fe layer phenomena.
Total Ozone trends deduced from Nimbus 7 Toms data
Authors: Stolarski Richard S. , Bloomfield Peter , McPeters Richard D. , Herman Jay R.
Published: February 2008

The Total Ozone Mapping Spectrometer (TOMS) on the Nimbus 7 satellite has been measuring the total column amount of ozone over the globe for more than 11 years. Recent improvements in the data analysis have led to a technique for determining and removing drift in the calibration such that the data at the end of the record are precise to ±1.3% (2σ) relative to the data at the beginning of the record. A statistical model, including terms for seasonal variation, linear trend, quasi‐biennial oscillation, solar cycle and second‐order autoregressive noise has been fit to the TOMS time series of total ozone data. The linear trend obtained when this statistical model is fit to the TOMS data averaged between 65N and 65S latitudes is −0.26±0.14%/year or −3% over the 11.6 year time period from November, 1978 through May, 1990. The trend is near zero (0.0002±0.2 %/year) at the equator and increases towards both poles. At 50N the annually averaged trend is −0.5±.21%/year. The 50N trend over the 11.6 year time period shows a strong seasonal variation from more than −0.8%/year in winter and early spring (February and March) to about −0.2%/year in summer (July and August).
Degree‐2 pattern of attenuation structure in the upper mantle from apparent complex frequency measurements of fundamental spheroidal modes
Authors: Suda Naoki , Shibata Naoki , Fukao Yoshio
Published: February 2008

We present preliminary results of an analysis of global lateral variations in elastic and anelastic structure of the upper mantle. Using the Sompi method, we have analyzed IDA records to measure apparent complex frequencies of fundamental spheroidal modes 0S21–0S34, which are primarily sensitive to the structure of the transition zone. The apparent complex frequencies have then been inverted into local frequency and attenuation maps using our new inversion method. We have obtained consistent degree‐2 patterns in the local attenuation maps as well as in the local frequency maps. The high‐attenuation anomalies correlate with regions of slow P‐wave velocity in the lower mantle and with regions of high hotspot density. This may represent a continuation of lower‐mantle upwelling into the transition zone.
Seismic velocity gradients in the upper mantle
Authors: Kennett B. L. N.
Published: February 2008

The radial velocity gradients for P‐ and S‐waves in the upper mantle are of considerable interest for petrological studies of the upper mantle but are difficult to constrain directly. Comparison of three different styles of velocity model, constructed within a common parametrisation to give a good fit to the travel times of P‐ and S‐waves to 30°, provide a useful framework for constructing bounds on the radial gradients. For depths from 250–1000km a somewhat subjective set of bounds are suggested for the velocity gradients and velocity contrasts at the major discontinuities in an ‘average’ mantle.

One of the most significant factors affecting the estimation of radial gradients is the presence of small‐scale heterogeneity in the mantle. Modest heterogeneity, around 1 percent perturbations on scale lengths of about 300km, can give rise to horizontal velocity gradients which can overwhelm the radial gradients in the shallower parts of the mantle and still be a quarter of the size of the radial gradients at greater depths. However such scales of heterogeneity are unlikely to simulate the presence of additional upper mantle discontinuities for ray paths travelling near vertical.
Post‐glacial rebound and asthenosphere viscosity in Iceland
Authors: Sigmundsson Freysteinn
Published: February 2008

During the Weichselian glaciation Iceland was covered with an ice cap which caused downward flexure of the Earth's surface. The post‐glacial rebound in Iceland was very rapid, being completed in about 1000 years. The length of this time interval constrains the maximum value of asthenosphere viscosity in Iceland to be 1 × 1019 Pa s or less. Further clarification of the ice retreat and uplift history may reveal lower viscosity. Current changes in the mass balance of Icelandic glaciers must lead to measureable elevation changes considering this low viscosity. Expected current elevation changes around the Vatnajökull ice cap are of the order of 1 cm per year, due to mass balance change in this century.
Formation characteristics of sporadic Na layers observed simultaneously by lidar and airglow instruments during ALOHA‐90
Authors: Gardner C. S. , Kane T. J. , Hecht J. H. , Walterscheid R. L. , Yee J. H. , Niciejewski R. J. , Lowe R. P. , Turnbull D. N.
Published: February 2008

Sporadic Na (Nas) layers were observed by the airborne lidar during ALOHA‐90 on the 22, 25 and 27 March flight missions. Perturbations in the O2 and OH nighttime airglow emission intensities and temperatures were also observed by instruments on the aircraft and at Haleakala Crater (20.8°N, 156.2°W) during these events. The most striking correlation between the airglow and lidar measurements occurred during the northbound flight leg of the 25 March mission. When the Nas layer formed at 90.7 km, while the Electra aircraft was between 750 and 500 km south of Haleakala, the O2 temperatures near 95 km above the Electra and Haleakala increased by approximately 45 K. The data for this night suggest a connection between Nas and a large‐scale wave, and suggest that the wave is tidal in nature. The data also suggest that some Nas layers can form very quickly over large geographic areas. Fast chemical processes are required to generate the large amounts of atomic Na involved in some of these events.
Stochastic mixing of protons from chaotic orbits in the nightside geomagnetosphere
Authors: Horton W. , Liu C. , Hernandez J. , Tajima T.
Published: February 2008

The stochastic mixing of protons in the energy range from 1 to 30 keV in the night‐side magnetosphere is studied by calculating the local divergence rate of neighboring orbits and the two‐time velocity correlation function. The rate of divergence of neighboring bundles of trajectories is shown to have large bursts with average separation times of order 1 minute per e‐folding during the crossing of the central plasma sheet in the region beyond −50Re. For the Tsyganenko magnetosphere the net amount of orbit divergence is 15 to 20 e‐foldings in one hour. The velocity correlations are shown to decay as power laws τ−m with a distribution of m values. These results indicate that for short time (<1 hour) there is reversibility and memory for the protons but for longer times there is no memory for protons in the nightside magnetosphere.
Time variation in coarse materials from lake bottom sediments and secular paleoclimatic change
Authors: Kashiwaya Kenji , Fukuyama Kaoru , Yamamoto Atsuyuki
Published: February 2008

Variations in the percentage of materials coarser than 4.5 ϕ in sediment from Lake Biwa are closely related to global climatic change and are indicative of large fluctuations in sediment discharge into the lake. Coarse materials increased markedly during the interglacials (postglacial and last interglacial periods). The variations also are linked with solar insolation and have distinct periods of about 20,000 and 40,000 years which correspond to the periods of the obliquitical and precessional parameters. Periods of 60,000 and 120,000 years are major in the variations.
Shallow conductive‐component of heat flow near Novarupta Dome, Katmai, Alaska
Authors: Ballard Sanford , Carrigan Charles R. , McConnell Vicki S.
Published: February 2008

An investigation of the shallow conductive heat flow from one of the half dozen thermal areas in the vicinity of Novarupta Dome suggests that convective heat transport by hydrothermal fluids is the dominant mode of heat transfer in the shallow subsurface within the thermal areas. The thermal observations are used to qualitatively characterize the state of the hydrothermal system at depth. We conclude that there is no shallow (100–200 m), laterally extensive heat source beneath the vent region but rather that the source of the heat emanating from the thermal areas is the cooling conduit that supplied magma to the surface during the 1912 eruption. This heat is transported to the surface by vapor flow along high permeability pathways associated with the margins of the 1912 vent. The thermal observations are also used to place a lower bound on the characteristic diameter of the conduit of 50 meters.
Degassing of the 1912 Katmai magmas
Authors: Westrich H. R. , Eichelberger J. C. , Hervig R. L.
Published: February 2008

Pre‐ and post‐eruptive H2O, F, Cl, and S contents of the three 1912 Katmai magmas were inferred from analyses of melt inclusions and matrix glasses in tephra samples. With increasing silica content (andesite⇒rhyolite), pre‐emptive melt H2O increases from ≥1.0 to 3.8 wt.%, Cl increases slightly from 1700 to 1900 ppm, S decreases from 170 to ≤65 ppm, and F remains constant at 550 ppm. These variations are not consistent with a simple crystal fractionation relationship. For plausible chamber depths, the magmas were vapor undersaturated during storage and fragmented during the last few hundred meters of ascent, consistent with geologic evidence for excavation of the vent funnel within the upper 1 km. Vitrophyres of welded intravent fallback tephra ejected late in the eruption show that extensive degassing and complete welding could take place in less than the 60‐hour eruptive period. Release of HCl was twice that of the 1980 eruption of Mount St. Helens while H2SO4 output was comparable to that of the 3.5 ka Santorini eruption. Significant retention of Cl and F, which would be released along with residual H2O during high‐temperature devitrification, may explain the important vapor transport that occurred in the Valley of Ten Thousand Smokes fumaroles following emplacement of the ignimbrite.
The crustal transfer function in seismic 3‐component slowness estimation
Authors: Lokshtanov D. E. , Ruud B. O. , Husebye E. S.
Published: February 2008

Here we present a new approach for estimating the slowness vector of incident P‐waves from records of a single or an array of 3‐component (3C) seismometers. The slowness vector is extracted by using a Maximum Likelihood (ML) inversion scheme where the signal model incorporates the transfer function of the layered crust beneath the receivers. The unknown time function of the incident plane wave is excluded from the inversion. In the case of array observations both polarization and moveout times are utilized. The scheme was tested on records of NORESS 3‐component stations from teleseismic and local events and the obtained slowness estimates are closer to the expected values than estimates derived by conventional methods.
Preliminary magnetostratigraphy of the red clay underlying the loess sequence at Baoji, China
Authors: Evans M. E. , Wang Y. , Rutter N. , Ding Z.
Published: February 2008

Magnetic remanence and bulk susceptibility measurements are reported from 93 samples spanning the uppermost 27m of the Red Clay underlying the loess sequence at Baoji, Shaanxi province, China. If a new, previously undetected, short (about 20ka) excursion or reversed interval near 3.3Ma be accepted, then a reasonable fit to the standard time scale emerges. A linear accumulation rate of 1.5cm/ka is implied and the correlation coefficient is 0.992. No significant hiatus occurs between the Red Clay and the overlying loess, so the 27m investigated reach back to about 4.3Ma. Fourier analysis of the bulk susceptibility data yields a strong peak near 400ka in agreement with astronomical calculations of the orbital parameters responsible for climatic forcing.
Experimental dissolution‐precipitation creep in quartz aggregates at high temperatures
Authors: Cox S. F. , Paterson M. S.
Published: February 2008

Rapid compaction creep has been achieved in fine‐grained quartz aggregates during hydrothermal isostatic pressing (HTIP) at 1200K, a confining pressure of 300MPa and a pore water pressure of 200 MPa. While raising the pressure and temperature over a period of three hours, the porosity decreased by over 50%. During subsequent HTIP for periods up to 4 hours, further porosity reduction occurred at compaction creep rates between 10−5 and 10−6 s−1. Changes in grain shapes, grainsize distribution, pore geometry and microstructures on grain surfaces, together with an absence of substantial brittle or intragranular plastic deformation, indicate that compaction creep has occurred largely by dissolution‐precipitation processes. Interpenetration of grains and grain‐shape truncation are well‐developed at grain contacts. Arrays of fine‐scale ridges, plateaus and flat‐bottomed pores on interfaces indicate that the fluid distribution on actively dissolving interfaces is a dynamic structure that is not controlled entirely by equilibrium wetting angles.
A study of the frequency coherence of stratospheric and tropospheric radar echoes made with Chung‐Li VHF radar
Authors: Chu Yen‐Hsyang , Franke Steven J.
Published: February 2008

In the past few years the technique of Frequency Domain Interferometry(FDI) has been developed on VHF radar. By using this technique, the characteristics of a very thin atmospheric layer structure, which is embedded in the radar volume and can not be resolved by the conventional VHF radar with only one operational frequency, can be determined through the calculation of the coherence and the phase from two echo signals with slightly different operational frequencies. In this article the results of a frequency domain interferometry experiment carried out on Chung‐Li radar are presented. The use of four discrete frequencies and three different frequency separations distinguishes this study from previous investigations. It is shown experimentally that the signal‐to‐noise power ratio indeed affects significantly the magnitude of the observed coherence. For the first time the dependence of coherence on frequency separation is also investigated. The results show that coherence decreases with frequency spacing with the roll‐off rate ranging from −0.2/MHz to −1.2/MHz, depending on the atmospheric structure. After best fitting the theoretical model to the observed coherence taken from a height range with no atmospheric layer existing, the effective range resolution can be found. Employing this information, the thicknesses of two prominent and lasting layers locating at height 7.2 km and 7.8 km are estimated through the least square fitting process, and found to be 114 and 186 meters, respectively.
Isolation of source and receiver statics and scale factors in waveform inversion of marine refraction data
Authors: Shaw Peter R.
Published: February 2008

In this paper I present a method that uses multiple ocean bottom instruments to separate variations in seismic amplitudes and travel times caused by near‐source topography and variable source amplitudes from similar variations related to the overall one‐dimensional seismic structure. The results indicate that the standard correction for topography, sediment thickness, and source size can be improved with this method, reducing uncertainties in the velocity solution.
Changes of mantle sources in the course of a rift evolution: The Afar Case
Authors: Vidal Ph. , Deniel C. , Vellutini P. J. , Piguet P. , Coulon C. , Vincent J. , Audin J.
Published: February 2008

Afar is a favorable area in which to investigate the interactions between lithosphere and asthenosphere during the opening of a continental rift. The present work focuses on the territory of Djibuti Republic where volcanism has occurred over the past 25 Ma. With the exception of the oldest rhyolites which contain a large crustal‐derived component, the Sr, Nd, Pb isotopic compositions of basalts indicate mantle sources without significant crustal contamination. As the rifting process goes on, the sources are evolving from an ancient and isotopically heterogeneous lithospheric subcontinental component (87Sr/86Sr = 0.707 and 206Pb/204Pb = 17.5) to a component having HIMU characteristics (87Sr/86Sr = 0.7035 and 206Pb/204Pb > 19). The influence of the asthenospheric depleted mantle is not established. The present data confirm that a mantle plume is associated with the rifting.
A case study of the cusp electrodynamics by the Aureol‐3 satellite: Evidence for FTE signatures?
Authors: Bosqued Jean M. , Berthelier Annick , Berthelier Jean J. , Escoubet Christophe P.
Published: February 2008

Particle and field data from a pass of the AUREOL‐3 satellite through the polar cusp, several minutes after the southward turning of the IMF, are analyzed in detail. Superposed on the classical cusp, characterized by the typical ion and electron precipitations, several very narrow arcs are detected where large fluxes of electrons and ions, accelerated to 2–4 keV, precipitate simultaneously. These localized arcs correspond to the upward current sheets of a succession in latitude of narrow, alternatively upward and downward field‐aligned current sheets. The data suggest that the satellite has crossed the ionospheric footprints of 2 adjacent flux transfer events separated by 100–150 km in latitude. Electric spikes and electromagnetic turbulence are typically associated with the region of downward currents.
A reevaluation of spectral ratios for Lunar Mare TiO2 mapping
Authors: Johnson Jeffrey R. , Larson Stephen M. , Singer Robert B.
Published: February 2008

The empirical relation established by Charette et al. [1974] between the 400/560 nm spectral ratio of mature mare soils and weight percent TiO2 has been used extensively to map titanium content in the lunar maria [e.g., Johnson et al., 1977; Johnson et al., 1991b]. Relative reflectance spectra of mare regions show that a reference wavelength further into the near‐IR, e.g., > 700 nm, could be used in place of the 560 nm band to provide greater contrast (a greater range of ratio values) and hence a more sensitive indicator of titanium content. An analysis of 400/730 nm ratio values derived from both laboratory and telescopic relative reflectance spectra suggests that this ratio provides greater sensitivity to TiO2 content than the 400/560 nm ratio. The increased range of ratio values is manifested in higher contrast 400/730 nm ratio images compared to 400/560 nm ratio images. This potential improvement in sensitivity encourages a reevaluation of the original Charette et al. [1974] relation using the 400/730 nm ratio.
Extreme magma homogeneity in the 1783–84 Lakagigar Eruption: Origin of a large volume of evolved basalt in Iceland
Authors: Sigmarsson Olgeir , Condomines Michel , Grönvold Karl , Thordarson Thorvaldur
Published: February 2008

Precise measurements of U and Th concentrations and Sr, Th and O isotopes in a suite of samples from the 1783–84 Lakagigar eruption, reveal an extreme homogeneity in the 15 km3 lava flow. However, geochemical constraints suggest that the quartz‐tholeiite magma results from the assimilation of approximately 20% of the lower crust by a mantle derived olivine‐tholeiite magma. The constant magma composition thus implies vigorous convection and efficient mixing before eruption, probably in a reservoir at the crustmantle boundary. The same deep reservoir probably also fed a shallow magma chamber below Grimsvötn, inducing simultaneous activity in both Lakagigar and Grimsvötn central volcano.
Kinetic simulation of the Kelvin‐Helmholtz instability at the Venus ionopause
Authors: Thomas V. A. , Winske D.
Published: February 2008

Two dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the kinetic evolution of the Kelvin‐Helmholtz instability at a boundary similar to the ionopause at the planet Venus. The magnetic field has been chosen to be essentially transverse to the plasma flow velocity, which is the most unstable case for the Kelvin‐Helmholtz instability. Effects analogous to those found in MHD simulations are recovered, including a mode cascade to longer wavelengths. In addition, isolated structures on the order of the ion gyroradius are formed which can cross the boundary in either direction. These structures may be related to flux ropes commonly observed near the ionopause of Venus. The time evolution of these structures may represent transport across the boundary layers and the smearing of the shear layer may give rise to internal structure in the boundary. Similar processes occur in magnetized layers akin to the Earth's magnetopause.
Remote sensing studies of the Orientale Region of the Moon: A pre‐Galileo view
Authors: Hawke B. R. , Lucey P. G. , Taylor G. J. , Bell J. F. , Peterson C. A. , Blewett D. T. , Horton K. , Smith G. A. , Spudis P. D.
Published: February 2008

We have acquired both an extensive data base of visible and near‐infrared spectra and multispectral images (ultraviolet and visible) of the Orientale region of the Moon. Our results show that the eastern Inner Rook Mountains are composed of anorthosite. Portions of the main ring of the Humorum basin and the inner ring of Grimaldi are also composed of anorthosite. Other deposits within the Orientale basin and the major ejecta unit outside the basin are dominated by noritic anorthosites; mature surfaces have spectra nearly identical to those taken of areas in the vicinity of the Apollo 16 site. Thus, it appears that Orientale ejecta are more mafic than materials that are thought to have originated at depth (i.e., the anorthosites in the Inner Rook Mountains). There are large areas that have mare basalt signatures mixed with highland rock types, indicating the existence of ancient, pre‐Orientale mare volcanism. Present mare surfaces in the Orientale region contain basalts of intermediate to very low TiO2 content.
Experimental discovery of scaling laws relating fractal dimensions and the length distribution exponent of fault systems
Authors: Davy Philippe , Sornette Anne , Sornette Didier
Published: February 2008

A series of experiments scaled for gravity on the formation of faults in a laboratory model of the earth's lithosphere have shown that the obtained fault patterns are self‐similar and can be characterized by various fractal dimensions. By analyzing a large set of experimental results, we discover a remarquable scaling law relating the generalized fractal dimensions Dq the fault barycenter fractal dimension ‘b’ and the exponent ‘a’ of the fault length distribution. : D0≈b is independent of ‘a’ whereas Dq≥1=b+2−a, for 2≤a≤3 as found in our experiments.
A hybrid equation of state for the quasi‐static central plasma sheet
Authors: Kan J. R. , Sun W. , Baumjohann W.
Published: February 2008

The plasma pressure calculated from the force balance equation in the Tsyganenko magnetic field model under quiet conditions is found to be in good agreement with the observed pressure distribution in the central plasma sheet. This result indicates that the central plasma sheet can exist without a minimum‐B region in a quasi‐steady state. The plasma density and temperature measured by AMPTE/ERM in the central plasma sheet under quiet conditions fall in between the predictions of the adiabatic equation of state and the isotropized magnetic‐moment equation of state. A hybrid equation of state derived from a simple combination of the adiabatic equation of state and the magnetic‐moment equation of state is shown to be in excellent agreement with AMPTE/IRM data.
Formation of non‐volcanic passive margins: A composite model applies to the conjugate Galicia and southeastern Flemish cap margins
Authors: Sibuet Jean‐Claude
Published: February 2008

One of the most controversial issues in geology today is the way the lithosphere extends. The Galicia margin is a particularly good place to adress this problem as it is fairly sediment starved, has clear upper crustal structure well imaged by a variety of seismic reflection profiles and has well control to calibrate seismic data. Observed subsidence is compared with extension of the uppermost crust of the entire margin as computed from the geometry of tilted fault blocks imaged on a series of parallel seismic lines. The pure shear (or stretching) model clearly accounts for the extension of the upper continental margin but not for the lower margin where the S reflector zone, a prominent zone of intra‐crustal reflectors, is present. I suggest that the S reflector zone is a detachment zone accomodating decreasing displacement towards the continent and that this detachment zone, active since the beginning of rifting, outcrops on the Canadian side and corresponds to the steep southeastern Flemish Cap continental slope. Pure shear is the mechanism which affects the whole lithosphere, but in the upper part of the crust, the rifting mechanism is locally simple shear where the detachment zone is active.
The role of intermediate shocks in magnetic reconnection
Authors: Lin Y. , Lee L. C. , Kennel C. F.
Published: February 2008

The structure of discontinuity layers associated with magnetic reconnection is studied by numerically solving the Riemann problem for the evolution of an initial current sheet, which separates two plasma regions with anti‐parallel magnetic field components in the z direction and a common guide magnetic field in the y direction. In the presence of a non‐zero normal component of magnetic field, Bx, the initial current sheet evolves into a system of MHD discontinuities. For the initial current sheet with a zero guide field (By = 0), steady intermediate shocks, slow shocks, slow expansion waves, or contact discontinuity are observed to develop. However, for the current sheet with a non‐zero guide field (By ≠ 0), time‐dependent intermediate shocks (TDIS), instead of steady intermediate shocks, are observed to bound the reconnection layer. The magnetic fields across the TDIS are non‐coplanar and the width of the TDIS expands self‐similarly as
The TDIS gradually evolves toward a rotational discontinuity.
Melt‐solid flow with diffusion‐controlled chemical reaction
Authors: Iwamori Hikaru
Published: February 2008

A simple model for melt‐solid two‐phase flow involving diffusion‐controlled chemical reaction is presented. The governing equations describing mass conservation were numerically integrated to clarify chemical evolution of the flow. The calculations show that the conditions for local equilibrium are violated concerning elements with small diffusion coefficient less than about 10−19 m2/s in the solid. Behavior of these elements at melting and segregation stages in the mantle should be investigated in terms of diffusivity in solid as well as partitioning between melt and solid.
Lava flows are fractals
Authors: Bruno B. C. , Taylor G. J. , Rowland S. K. , Lucey P. G. , Self S.
Published: February 2008

We have conducted a preliminary investigation of the fractal nature of the plan‐view shapes of lava flows in Hawaii (based on field measurements and aerial photographs) as well as in Idaho and the Galapagos Islands (using aerial photographs only). Our results indicate that the shapes of lava flow margins are fractals. In other words, lava flow shape is scale‐invariant (at least within the range of scale measured, 0.5m to 2.4km). This observation has important implications for understanding the fluid dynamics of lava flows. It suggests that nonlinear forces are operating in them because nonlinear systems frequently produce fractals. Furthermore, a'a and pahoehoe flows can be distinguished by their fractal dimensions (D). The majority of the a'a flows we measured have D between 1.05 and 1.09, whereas the pahoehoe flows generally have higher D (1.14 – 1.23). We have extended our analysis to other planetary bodies by measuring flows from orbital images of Venus, Mars and the Moon. All are fractal, and have D consistent with the range of terrestrial a'a and pahoehoe values. Combining the terrestrial and extraterrestrial data, the fractal nature of lava flow outlines holds for over five orders of magnitude in scale (0.5m to 60km).
The spreading rate dependence of three‐dimensional mid‐ocean ridge gravity structure
Authors: Lin Jian , Morgan Jason Phipps
Published: February 2008

We analyze over 1300 km of high resolution along‐axis gravity profiles at ridges with half‐spreading rates ranging from 1.2 to 5.5 cm/yr. The results show consistently higher along‐axis gradients of mantle Bouguer anomaly at the slow‐spreading Mid‐Atlantic Ridge (MAR) (0.3–1.2 mgal/km) than at the intermediate‐ to fast‐spreading Cocos‐Nazca Ridge and East Pacific Rise (EPR) (0.1–0.2 mgal/km). The regional peak‐to‐trough amplitude of mantle Bouguer anomaly is also greater along the MAR (30–60 mgal) than the Cocos‐Nazca Ridge and the EPR (10–20 mgal). With increasing spreading rate, the regional peak‐to‐trough amplitude of axial seafloor depth decreases from 1000–1700 m to 200–700 m. 3‐D numerical experiments suggest that mantle contributions to the gravity can be significant only near large‐offset transforms. At the more commonly observed non‐transform offsets, gravity anomalies will reflect crustal thickness variations.

The along‐axis gravity data thus indicate that the amplitude of along‐axis crustal thickness variation decreases with increasing spreading rate. We propose that this spreading rate dependent crustal accretion style may originate in the mantle: finite‐amplitude mantle upwelling is intrinsically plume‐like (3‐D) beneath a slow‐spreading ridge but more sheet‐like (2‐D) beneath a fast‐spreading ridge. Such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate. Small amplitude 3‐D upwellings may occur at a fast‐spreading ridge, but their effects on crustal thickness variations will be significantly reduced by along‐axis melt flows along a persistent low‐viscosity crustal magma chamber. In contrast, the large crustal thickness variations due to 3‐D mantle upwellings will be maintained at a slow‐spreading ridge because less along‐axis melt flows can occur in the colder and more rigid crust there.
Lightning induced brightening in the airglow layer
Authors: Boeck W. L. , Vaughan O. H. , Blakeslee R. , Vonnegut B. , Brook M.
Published: February 2008

This report describes a transient luminosity observed at the altitude of the airglow layer (about 95 km) in coincidence with a lightning flash in a tropical oceanic thunderstorm directly beneath it. This event provides new evidence of direct coupling between lightning and ionospheric events. This luminous event in the ionosphere was the only one of its kind observed during an examination of several thousand images of lightning recorded under suitable viewing conditions with Space Shuttle cameras. Several possible mechanisms and interpretations are discussed briefly.
Frequency dependence of fracture stiffness
Authors: Pyrak‐Nolte Laura J. , Nolte David D.
Published: February 2008

The frequency dependence of fracture specific stiffness, for seismic or ultrasonic wave transmission, is shown to be a simple scaling property of fractures with spatially inhomogeneous distributions of stiffness. Different frequencies sample different subsets of the fracture geometry. Therefore the frequency dependence may be a simple consequence of fracture geometry and requires no additional dynamical mechanism. In this letter, the dynamic fracture stiffness is determined based on the displacement discontinuity model for wave transmission across a fracture. Local transmission coefficients are assumed to depend on the local static stiffness. For a fracture with a single value of static stiffness, the dynamic stiffness is frequency independent. A strongly inhomogeneous distribution of fracture stiffnesses produces a strong frequency dependence for the dynamic stiffness.
The directional dependence of magnetic fluctuations generated by cometary ion pickup
Authors: Miller Ronald H. , Gombosi Tamas I. , Gary S. Peter , Winske Dan
Published: February 2008

The properties of low frequency magnetic fluctuations generated by cometary ion pickup are examined by means of one‐dimensional hybrid simulations, in which newborn ions are created at a constant rate. The helicity and direction of propagation of magnetic fluctuations are investigated for various cometary ion injection angles, α, relative to the solar wind magnetic field. The parameter η represents the relative contribution of wave energy density propagating in the direction away from the comet, parallel to the beam. For small (quasi‐parallel) injection angles, α ∼ 0° and η is of order unity, while for larger (quasi‐perpendicular) angles, α ∼ 90° and η is about 0.5. At intermediate angles, α ∼ 60°, η can vary between 0 and 1, depending on the wave number. The wave properties are consistent with the instabilities (right‐hand cyclotron resonant modes at small α, left‐hand Alfven/ion cyclotron modes at large a) expected from linear theory. Consequences of these results for particle acceleration are also discussed.
Harmonic H+ gyrofrequency structures in auroral hiss observed by high‐altitude auroral sounding rockets
Authors: Kintner P. M. , Scales W. , Vago J. , Yau A. , Whalen B. , Arnoldy R. , Moore T.
Published: February 2008

Two recent sounding rocket experiments have yielded VLF wave data with spectral structures ordered by the hydrogen gyrofrequency. The spectral structures occur near and above the lower hybrid frequency in association with auroral hiss. These structures are observed within and near regions of auroral electron precipitation and transverse ion acceleration. They are accompanied by auroral hiss but are anticorrelated with spectral peaks at the lower hybrid frequency. They are typically found above 500 km altitude, have no measurable magnetic component, and are at least occasionally short wavelength (kρi ≥ 1). Because the spectral structures appear to be electrostatic, are ordered by the hydrogen gyrofrequency, and are short wavelength, we interpret the structures as modes which connect the lower hybrid mode with the hydrogen Bemstein modes. A study of the plasma wave mode structure in the vicinity of the lower hybrid frequency is presented to substantiate this interpretation. Our results imply that these waves are a common feature of the auroral zone ionosphere above 500 km altitude and exist any time that auroral hiss exists. The absence of previous satellite observations of this phenomenon can be explained by Doppler broadening.
Height dependence of the observed spectrum of radar backscatter from HF‐induced ionospheric Langmuir turbulence
Authors: Fejer J. A. , Sulzer M. P. , Djuth F. T.
Published: February 2008

Observations of the spectrum of 430‐MHz radar backscatter from HF‐induced Langmuir turbulence with height discrimination are described. During very stable ionospheric conditions under which the height of the below‐threshold backscatter spectrum had changed by less than 300 m during a 7‐min period, a 20‐s‐long temporary increase in the HF power from 3 MW ERP to 38 MW equivalent radiated HF power resulted in subsequent strong above‐threshold spectra extending to heights up to 1200 m greater than the height of the below‐threshold spectrum for more than a minute. The generation of irregularities in the plasma density during the 20 s of enhanced HF power is suggested as a possible cause of this persistence of strong above‐threshold spectra at greater heights. The initial temporal evolution of the backscatter spectrum from Langmuir turbulence after the start of HF transmissions was observed for different heights. The observational results are compared with the predictions of existing theories of Langmuir turbulence.
Simultaneous observations of Schumann resonances in California and Australia: Evidence for intensity modulation by the local height of the D region
Authors: Sentman D. D. , Fraser B. J.
Published: February 2008

Observations are presented of the horizontal magnetic component of Schumann resonance intensities as simultaneously measured at locations in California and Western Australia during two separate intervals September 2–17, 1989, and April 14–21, 1990. For both intervals, diurnal variations of the average magnetic power over the lowest three modes of the Schumann resonances showed substantially different temporal profiles at the California and Western Australia stations, with interstation correlations of 0.51 and 0.39, respectively. A method is demonstrated for determining from these observations the average local time variation of the height of the D region. A height variation is obtained that is nearly identical for the respective analysis intervals, with a minimum height occurring at approximately 1300–1400 LT and a maximum‐to‐minimum height difference of roughly 50% of the mean. When corrected for the local D region height, the detailed diurnal intensity profiles over the analysis intervals display a greatly improved similarity, with interstation correlation coefficients increasing from 0.51 to 0.70 and from 0.39 to 0.82, respectively. Substantial agreement between the two stations after correction for D region height suggests that such observations could be used to monitor the global totality and variability of lightning, quantitatively and at time resolutions of the order of 10 min or less, in studies of global change. These results also suggest that with suitable instrumentation, in situ measurement of Schumann resonance intensities could provide a viable method for monitoring the global totality of lightning in other planetary systems capable of supporting the resonances, most notably Venus and Jupiter.
Hybrid simulation of the formation of a hot flow anomaly
Authors: Thomas V. A. , Winske D. , Thomsen M. F. , Onsager T. G.
Published: February 2008

Results are presented from two‐dimensional hybrid simulations of the interaction of a supercritical quasi‐perpendicular collisionless shock wave with current sheets embedded in the upstream flow. The current sheet normals are perpendicular to the shock normal. When the motional electric field is directed toward the discontinuity, the interaction leads to the generation of a region with high ion temperature, low magnetic field, and low density, which has many properties of a class of events variously referred to as active current sheets, hot diamagnetic cavities, or hot flow anomalies (HFAs). The simulations demonstrate that the HFA results from the interaction of ions reflected at the shock with the current sheet. The interaction does not involve an instability and is not caused by the collision of the current sheet and the shock but rather is a property of a shock with an embedded current sheet. The simulations suggest that the HFAs formed in this way are always attached to the shock and extend into both the upstream and downstream regions.
Plasma sheet instability related to the westward traveling surge
Authors: Roux A. , Perraut S. , Robert P. , Morane A. , Pedersen A. , Korth A. , Kremser G. , Aparicio B. , Rodgers D. , Pellinen R.
Published: February 2008

The detailed analysis of an isolated dispersionless substorm is performed on the basis of field and particle data collected in situ by the geostationary satellite GEOS 2 and of data from ground‐based instruments installed close to the GEOS 2 magnetic footprint. These data give evidence for (1) quasi‐periodic variations of the magnetic field configuration, which is alternatively taillike and dipolelike, (2) in‐phase oscillations of the flux of energetic electrons, which is high when the configuration is dipolelike and vice versa, (3) a gradient in the flux of energetic ions, which is, on the average, earthward but undergoes large fluctuations around this average direction, and (4) large transient fluctuations of the quasi‐dc electric field, which reverses its direction from eastward to westward. It is shown that these results are consistent with the development of an instability which leads to a westward propagating “wave”. The source of the instability is the differential drift of energetic electrons and ions in a highly stressed magnetic field configuration (in a high β plasma). Evidence is given for a system of localized field‐aligned currents flowing alternately earthward and equatorward at the leading and trailing edges of the westward propagating wave. This current system resulting from the temporal development of the instability produces the so‐called Pi 2 pulsations, at the ionospheric level. The closure of this current system in the equatorial region leads to a current antiparallel to the tail current, and therefore to its reduction or cancellation. This reduction/cancellation of the tail current restores the dipole magnetic field (dipolarization) and generates a large westward directed induced electric field (injection). Hence, dipolarization and injection are the consequences of the instability. Finally, it is suggested that the westward traveling surge observed simultaneously by all‐sky cameras, close to the magnetic field of GEOS 2, is the image of the instability in the equatorial region transmitted to the upper atmosphere by precipitating electrons.
Foreword [to Special Section on Venus and Mars]
Authors: Luhmann Janet G.
Published: February 2008

During the week of June 4–8, 1990, a Chapman Conference on Venus and Mars: Atmospheres, Ionospheres and Solar Wind Interactions was held at Balatonfüred, Hungary. The meeting was coorganized by J. G. Luhmann and M. Tatrallyay, under the auspices of the AGU and the Hungarian Academy of Sciences. Aproximately 85 participants attended some 72 presentations in oral and poster form. The topics on which the sessions focused included atmosphere evolution, present atmospheres, exospheres and ionospheres, solar wind interactions, and past and future missions. The international and multidisciplinary makeup of the audience provided the basis for a wide range of discussions and brought out a number of remaining controversies and questions to be answered by future research.
Fast acceleration of ions at quasi‐perpendicular shocks
Authors: Balogh A. , Erdős G.
Published: February 2008

Acceleration of low‐energy protons by quasi‐perpendicular shocks is investigated. The pitch angle distribution of ions in the energy range 35 keV to 1 MeV has been determined across the interplanetary shock that passed through the ISEE 3 spacecraft on November 30, 1979. Upstream of the shock a bidirectional angular distribution was observed. It is suggested that multiple crossings of the field line with the surface of the shock, forming a “magnetic bottle”, may account for such an unusual angular distribution. The shock event was modeled by integrating particle trajectories numerically. Qualitative agreement between observations and simulations supports the idea of magnetic bottle field line formation. A detailed numerical study of particle acceleration has shown that in the bottle topology the particle flux is enhanced close to the shock front, contrary to the original scatter‐free model, i.e., assuming homogeneous magnetic fields on both sides of the shock. It is suggested that multiple crossings of the field line with the shock may explain “shock spike” events.
The effects of velocity shear on the resonance absorption of MHD surface waves: Cold plasma
Authors: Yang Guang , Hollweg Joseph V.
Published: February 2008

Magnetohydrodynamic (MHD) surface waves may decay via a process called “resonance absorption,” which is a candidate for solar coronal heating. Recently, Hollweg, Yang, Cadez, and Gakovic studied the effects of velocity shear on the rate of resonance absorption of incompressible MHD surface waves. We extend the theory to a compressible but cold plasma, which is the case more applicable to the solar corona. We find that the rate of resonance absorption can be either increased or decreased significantly by velocity shear. We also find that there can exist resonances which lead to instability of the surface mode at values of the velocity shear below the Kelvin‐Helmholtz threshold. However, the resonance instability usually occurs when the density ratio across the surface is less than about 0.1. The resonant instability may therefore not be important in the solar corona, though resonance absorption remains an attractive possibility for coronal heating.