bluuebunny/crossref_metadata_2025 huggingface dataset:
Geomagnetic main field analysis at the core‐mantle boundary: Spherical harmonics compared with harmonic splines
Authors: Benton Edward R. , Kohl Benjamin C.
Published: February 2008

An optimum truncation level, N, in a spherical harmonic analysis of the geomagnetic main field at the core mantle boundary is determined by harmonic spline analysis. Specifically, we find that value of N at which the two analyses are closest in a well defined sense and, for that value of N, we examine how "close" the two models are. Depending slightly on the definition of closeness, optimum N is found to be either 10 or 11. For those values the two analyses give remarkably similar results, showing that the conveniences of spherical harmonics can be retained with little penalty.
P‐wave travel‐times from the Tonga Subduction Zone to stations on the oceanic side of the Tonga‐Kermadec Trench
Authors: Prasad G. , Bock G.
Published: February 2008

P‐waves propagating from shallow earthquakes in the Tonga‐Kermadec seismic zone to regional stations in Raratonga and Niue are associated with large negative travel‐time residuals (observed minus Jeffreys‐Bullen time) of the order of −10 s. For focal depths below 100 km the magnitude of residuals decreases sharply, to about −2.5 s for depths near 200 km. The pattern of residuals is explained by a model of oceanic lithosphere possessing an upper mantle P‐wave velocity of 8.3 km/s and having its base at a depth near 100 km.
Array analysis of seismic signals
Authors: Goldstein Peter , Archuleta Ralph J.
Published: February 2008

In this paper we present a method for analyzing seismic signals recorded at an array of seismometers. The method is based on the multiple signal characterization (MUSIC) method. Four important features of this approach are: (1) it has the ability to resolve multiple closely spaced sources, (2) it works with both stationary, and nonstationary signals, (3) it is most sensitive to the strongest sources, and (4) it provides a geometric interpretation of the solution to the direction finding problem. We have developed a numerical algorithm that extends the MUSIC method to nonstationary and correlated signals. We present results using this algorithm to measure directions of arrival and amplitudes of multiple plane waves in a homogeneous medium.
Internal wave stress‐shear correlations: A choice of reference frames
Authors: Jacobs David C. , Cox Charles S.
Published: February 2008

A numerical simulation of internal waves using an eikonal approach is analyzed for a relation between the short wavelength, relatively high frequency waves, and the velocity and shear resulting from a background of low vertical wavenumber waves through which the short waves propagate. The background consists predominantly of quasi‐inertial motions. The results indicate a strong correlation between the horizontal group velocity of the short waves and the shear of the background when the short waves have been refracted to 10 meters or less in vertical wavelength. It appears that rapid refraction of the short waves to dissipative scales (< 5 m) then occurs as they become oriented with the shear. The directional anisotropy of the short high frequency waves at dissipation scales suggests an average u‐w correlation. Thus the high Reynolds stresses predicted by Müller [1976] are primarily associated with the shear of long wave inertial motions. In nature, the u‐w correlation would only be seen if the averaging is done in a reference frame that is allowed to rotate with the shear. A fixed frame averaging would give a near zero result, thus accounting for the absence of stress‐shear correlations in the fixed frame measurements of Ruddick and Joyce [1978]. Vertical profiler data of horizontal velocity are also examined for indications of a correlation in the short wave velocity field relative to the long wave shear and velocity fields. These observations are dominated by low frequency, quasi inertial oscillations. There is no consistent evidence of a directional anisotropy in the short wave field which is dependent on the long wave shear, but weak evidence of velocity polarization such that the long and short waves are polarized in the same direction.
Geomagnetic activity and local modulations of cosmic rays circa 1 GV
Authors: Ely J. T. A. , Huang T. C.
Published: February 2008

Solar‐sector synchronous modulations of the particulate cosmic radiation reaching the earth's atmosphere have been studied using low altitude satellite and surface data. The flux in the broad maximum of the galactic cosmic ray differential spectrum (near 1 Gv rigidity) exhibits an intermittent north‐south asymmetry in mid and high geomagnetic latitudes. During the 1964 and 1965 years of sunspot minimum, this modulation had a negative rigidity dependence and strong correlations with geomagnetic disturbance index, ap, and interplanetary magnetic field direction. Taken together with other features, this ap dependence is consistent with the hypothesis that reconnection of the interplanetary and geomagnetic fields should produce a local north‐south asymmetry (NSA) independent of much larger scale NSA's associated with cosmic ray gradients in the heliosphere. This finding is also consistent with suggestions that solar activity influence on atmospheric processes may be mediated by the resulting modulations of upper tropospheric ionization.
The vertical ozone distribution above the GDR‐Research Base, Antarctica in 1985
Authors: Gernandt H.
Published: February 2008

Results of balloon borne soundings with electrochemical ozone sondes carried out at 70,77° S; 11,85° E are discussed. During polar spring in 1985 the high time resolution of soundings (about 12 launches per months) gives a detailed picture on the vertical distribution and height variations of different maxima of ozone partial pressure near the margin of the ozone hole crossing the continental coast above Queen Maud Land in recent time.
Non‐Maxwellian ion velocity distributions observed using EISCAT
Authors: Lockwood M. , Bromage B. J. I. , Horne R. B. , St‐Maurice J.‐P. , Willis D. M. , Cowley S. W. H.
Published: February 2008

Recent observations from the EISCAT incoherent scatter radar have revealed bursts of poleward ion flow in the dayside auroral ionosphere which are consistent with the ionospheric signature of flux transfer events at the magnetopause. These bursts frequently contain ion drifts which exceed the neutral thermal speed and, because the neutral thermospheric wind is incapable of responding sufficiently rapidly, toroidal, non‐Maxwellian ion velocity distributions are expected. The EISCAT observations are made with high time resolution (15 seconds) and at a large angle to the geomagnetic field (73.5°), allowing the non‐Maxwellian nature of the distribution to be observed remotely for the first time. The observed features are also strongly suggestive of a toroidal distribution: characteristic spectral shape, increased scattered power (both consistent with reduced Landau damping and enhanced electric field fluctuations) and excessively high line‐of‐sight ion temperatures deduced if a Maxwellian distribution is assumed. These remote sensing observations allow the evolution of the distributions to be observed. They are found to be non‐Maxwellian whenever the ion drift exceeds the neutral thermal speed, indicating that such distributions can exist over the time scale of the flow burst events (several minutes).
Transitional geomagnetic field behavior: Volcanic records from French Polynesia
Authors: Roperch Pierrick , Chauvin Annick
Published: February 2008

Here we report the behavior of the earth's magnetic field for transitions recorded on volcanic islands from Polynesia in the South Central Pacific Ocean. Detailed transitional fields are observed on Huahine Island; the data suggest the record of an excursion N‐N following the Kaena event. The Jaramillo termination and the Brunhes onset are recorded in a volcanic sequence from Tahiti Island. The transitional field is mainly characterized by low intensities. While the beginning of the transition appears to be consistent with the zonal model of reversal, large deviations from a north‐south planar distribution of the intermediate directions show that non dipolar fields without preferential axisymmetry control the middle of the transition.
Non‐existence of gravitationally controlled partial corotation in cosmogonic plasmas
Authors: Vasyliunas Vytenis M.
Published: February 2008

If a corotating magnetospheric plasma is in equilibrium under the action of gravitational, inertial, and electromagnetic forces alone (neglecting pressure), the corotation speed must be proportional to the local Keplerian orbital speed, a result used by Alfvén as the basis for the "cosmogonic shadow effect" in the formation of the solar system (leading to Alfvén's well‐known 2/3 law for asteroid‐belt or planetary‐ring structures). The electric field implied by this distribution of corotation speeds has components parallel to the magnetic field which require non‐zero electron pressures and thus invalidate the initial neglect of pressure forces. Inclusion in the stress balance of the force on ions and electrons exerted by the parallel electric field leads to a state of isorotation along lines of the magnetic field modified by the vorticity of the plasma, with a corotation frequency presumably governed by interaction with the ionosphere and having no particular relation to the gravitational orbital period.
AMPTE‐IRM observations of magnetic cavities near the magnetopause
Authors: Lühr Hermann , Klöcker Norbert
Published: February 2008

On 4 September 1984 the AMPTE‐IRM spacecraft encountered shortly after the inbound crossing of the magnetopause a magnetic cavity of natural origin. The field strength decreased abruptly from 100 nT to values below 5 nT within the cavity. A striking feature of the data is the sudden disappearance of wave emissions that had characterized the magnetopause crossing. Simultaneously with the decrease in field strength the plasma density increased so that pressure balance is nearly maintained. A magnetic cavity of similar appearance was detected on 23 September 1984.
On the conjugacy of the aurora: High and low latitudes
Authors: Mizera P. F. , Gorney D. J. , Evans D. S.
Published: February 2008

On December 5, 1983 the polar orbiting satellites NOAA‐7 and DMSP‐F6, while in opposite hemispheres, each crossed a narrow structure of auroral electron precipitation at nearly the same Universal Time (UT) and at the same high magnetic latitude local time location. The Operational Line Scanner (OLS) image taken by DMSP‐F6 of the northern polar region showed that this enhancement was associated with a sun‐aligned arc (SAA) which extended poleward from the midnight sector auroral zone and passed across the polar cap very near to the magnetic pole. The DMSP‐F6 plasma measurements showed particle precipitation at background levels in the regions adjacent to the SAA suggesting that those magnetic field lines connected to the interplanetary medium via the northern tail lobe. NOAA‐7 observations show that the analogous precipitation structure in the southern hemisphere was embedded in a region of low intensity ion and electron precipitation which had the characteristics of the plasma sheet population. Comparisons between the boundaries of the lower latitude auroral zone precipitation and the electron distributions at those boundaries were also made. The results of these comparisons together with those made for the high latitude SAA argue for a high degree of inter‐hemisphere conjugacy in the electron precipitation over a wide range of invariant magnetic latitudes.
Uptake of carbonyl sulfide by silver nitrate impregnated filters: Implications for the measurement of low level atmospheric H2S
Authors: Cooper David J. , Saltzman Eric S.
Published: February 2008

Measurements of atmospheric hydrogen sulfide are often made by first collecting H2S on a paper filter impregnated with silver nitrate. Using this technique on a recent shipboard study, we observed anomalous breakthrough characteristics of sulfide, which caused us to suspect the presence of an artifact. We have investigated the interaction of various sulfur species with these filters as precursors for the artifact sulfide. Laboratory experiments indicate that OCS generates sulfide on the filters with an efficiency of 1‐2% under conditions comparable to those used for field sampling in the marine boundary layer. The magnitude of the artifact increases with increasing temperature and is insensitive to humidity and insolation. It appears likely that OCS is responsible for our shipboard results. Since much of the data on H2S concentrations in marine air have been obtained with this technique, they are most likely in error, producing overestimates of background H2S concentrations.
Analysis of Organic matter from cloud particles
Authors: Bank Shelton , Castillo Raymond
Published: February 2008

Organic matter collected from filtration of two separate cloud events was analysed by Fourier Transform Infrared Spectroscopy. Particles collected from different size filters were separated by color and each type of particle gave rise to a characteristic spectrum. The major constituents were identified as complex proteins and cellulose. Additionally, some degraded material (likely protein) and an unidentified orange‐brown material were present. Finally some trace components were identified as wax, oil, silicon oil, polyvinyl chloride, calcium carbonate, clay, sand and polyethylene.
Comment on “Paleomagnetism of Early Jurassic rocks, Watchung Mountains, Newark Basin: Evidence for complex rotations along the border fault”
Authors: Kodama Kenneth P.
Published: February 2008

Van Fossen et al. [1986] interpret discordant paleomagnetic directions from the eastern and western limbs of the Watchung Syncline to suggest complex strain near the Border Fault. If a case can be made that there is a 10 my age difference between the eastern and western limb magnetizations, and the western limb magnetization has not been thermally reset by the Preakness Basalt, then apparent polar wander during the Triassic/Jurassic can be used to explain the discordance. The western limb magnetization may record the sharp cusp predicted by the PEP model of Gordon et al. [1984] and date it to be mid‐Hettangian in age.
Introduction: Background and implications of the linear heat flow‐heat production relationship
Authors: Morgan Paul , Sawka Wayne N. , Furlong Kevin P.
Published: February 2008

The discovery in the late 1960s of the linear heat flow‐heat production relationship was a major advance in the understanding of the thermal structure of continents. However, although heat flow provinces have now been defined in all continents except Antarctica, the full implications of the relationship to deep thermal structure and the limitations of the relationship to applications in all terranes are not yet clear. In addition, despite major advances in the geochemical factors controlling the distribution of heat production in the crust, a geochemical rationale for the relationship is lacking. The relationship was originally interpreted in terms of one‐dimensional models, but new two‐ and three‐dimensional model studies suggest more complex interpretations in terms of lower crustal heat flow. Contributions to these problems in this special issue represent new observational, geochemical and modeling studies of the relationship and a renewed attempt to understand the thermal and geochemical implications of the relationship.
Factors controlling the distribution of heat production in selected UK granites
Authors: Webb P. C. , Tindle A. G. , Barritt S. D.
Published: February 2008

The variation of heat production with respect to magma evolution is examined in four UK granites in which fractional crystallization accounts for the range of rock compositions present. Accessory minerals are the dominant hosts of U and Th in these granites and their role in controlling variations of these heat producing elements is assessed. Melt compositional parameters determine which accessory minerals precipitate (solubility control) and influence the partitioning of radioelements between early‐crystallizing (fractionating) minerals and residual liquid. As a result, parental magma compositions and the effect of critical properties, such as alkalinity, aluminosity and volatile contents (particularly F‐complexing) on magma evolution are considered important in controlling the primary distribution of radioelements in granites.
Sources of electromagnetic radiation from fracture of rock samples in the laboratory
Authors: Cress Glynn O. , Brady B. T. , Rowell Glen A.
Published: February 2008

Rock, when fractured in the laboratory, emits electrical signals with a broad maximum in the power density spectrum in the band from 900 Hz to 5 kHz. Quartz‐free basalt rocks radiate both light and low‐frequency electrical signals as intensely as quartz‐bearing rocks, suggesting that the piezoelectric effect of quartz is at most a minor contributor to the total power radiated. Newly created rock surfaces acquire a local net charge distribution when exoelectrons are expelled and removed from the vicinity of the new surfaces by collisional and other processes. The rotational, vibrational, and linear motion of rock fragments with charged surfaces is the major contributor to the observed power density spectrum of the low‐frequency electrical signals. Neither optical spectral lines nor high‐frequency electromagnetic radiation, characteristic of electric discharges, occur at fracture. Exoelectron bombardment of the ambient fluid surrounding the sample, not electric discharge, is the excitation source for the light emitted at fracture.
Plasma parameters in the near wake of the space shuttle
Authors: Raitt W. J. , Eccles J. V. , Thompson D. C. , Banks P. M. , Williamson P. R. , Bush R. I.
Published: February 2008

Results are presented from a Spherical Retarding Potential Analyzer (SRPA) and a spherical Langmuir Probe (LP) which were flown on the Spacelab‐2 (SL‐2) mission of the Space Shuttle Orbiter in August, 1985. The probes were mounted in such a position that for certain Orbiter orientations, the wake edge formed by the starboard payload bay sill swept across the probes at a roll rate of once per orbit about an inertially stabilized pointing direction. Profiles of O+ number density and electron number density across the wake edge showed that, after correction for the spatial separation of the sensors, the charge particle number densities declined into the wake at a faster rate than that predicted by theoretical models. At angles of more than 10° into the wake from its geometric edge we find two electron populations having different mean random energies and electrical potentials. One of these populations is suggested to be the result of selective filtering of the high energy tails of the ambient ionospheric population, the other a result of photo‐emission from the payload bay surfaces.
Transverse ion heating in multicomponent plasmas
Authors: Ashour‐Abdalla M. , Okuda H. , Kim S. Y.
Published: February 2008

The heating of heavy ions such as ionospheric oxygen ions in the presence of auroral electrons was studied theoretically and by using numerical simulations. It was found that the ion‐ion hybrid mode becomes unstable when the electron drift speed is too small to excite the lower hybrid instability. The linear dispersion relation shows that for electron drift, Ωo < k∥vo < ΩH, the ion‐ion hybrid mode becomes unstable and for k∥vo > ΩH, the lower hybrid mode is excited. Here ΩH and Ωo are the hydrogen and oxygen gyrofrequencies and vo is the electron drift speed along the magnetic field lines. The results of the numerical simulations confirm the presence of these two instabilities and show that both light and heavy ions are heated perpendicularly with respect to the magnetic field. In particular, the formation of a high energy tail was observed for heavy ions, and the bulk light ions were heated.
Scientific results from the Viking ultraviolet imager: An introduction
Authors: Anger C. D. , Murphree J. S. , Jones A. Vallance , King R. A. , Broadfoot A. L. , Cogger L. L. , Creutzberg F. , Gattinger R. L. , Gustafsson G. , Harris F. R. , Haslett J. W. , Llewellyn E. J. , McConnell D. J. , McEwen D. J. , Richardson E. H. , Rostoker G. , Sandel B. R. , Shepherd G. G. , Venkatesan D. , Wallis D. D. , Witt G.
Published: February 2008

The Ultraviolet Imager on the Viking Spacecraft provides global images of aurora in both night and day hemispheres at rates of up to three per minute. First results summarized in this and a set of eight companion papers reveal new aspects of the patterns and dynamics of the aurora, including eastward as well as westward expanding substorm intensifications, but very little evidence of bulk westward motion of auroral surges; rapid poleward and equatorward expansions during some substorms, resulting in the formation of an intense eye‐shaped bulge containing diffuse aurora and north‐south aligned arc segments; substorm‐like intensifications at the point where transpolar arcs join the nightside oval; rapid small scale auroral brightenings, or “hot spots” over a wide range of local times; and very rapid changes in auroral forms in the noon sector.
An ultraviolet auroral imager for the Viking spacecraft
Authors: Anger C. D. , Babey S. K. , Broadfoot A. Lyle , Brown R. G. , Cogger L. L. , Gattinger R. , Haslett James W. , King R. A. , McEwen D. J. , Murphree J. S. , Richardson E. H. , Sandel Bill R. , Smith K. , Jones A. Vallance
Published: February 2008

The V5 ultraviolet auroral imager is one of a complement of experiments launched into polar orbit aboard the Swedish spacecraft Viking in February of 1986. The imager consists of two intensified wide‐angle f/1 cameras, identical except in passband, with electronic despinning providing exposure times of one second. Critical to electronic despinning is the use of tapered fiber optic bundles to remove distortions generated when the spherical focal surface of the Burch configuration camera is projected onto the plane of the charge coupled detector. Data acquired by V5 are telemetered to a ground station in Kiruna, Sweden for archiving and real time display using a VAX11/750 system.
Observations of large‐scale Birkeland currents with Viking
Authors: Potemra T. A. , Zanetti L. J. , Erlandson R. E. , Bythrow P. F. , Gustafsson G. , Acuña M. H. , Lundin R.
Published: February 2008

The Viking spacecraft carries a high‐resolution Magnetic Field Experiment that acquires approximately 53 vector samples per second. It has four automatically switchable ranges from ±1024 to ±65,536 nT (full scale) and resolutions commensurate with a 13 bit A/D converter in each range (±0.125 to ±8 nT). Data acquired on March 25, 1986 (just a month after Viking's launch) at ∼ 13,000 km altitude over the morning sector near 08:30 MLT indicate that the large‐scale (>0.5° invariant latitude wide) region 1 and region 2 Birkeland current systems can be readily identified. A narrow (∼0.2° wide) upward flowing Birkeland current exists poleward of the region 1 (downward) flowing current. The density of the large‐scale currents is about 0.1 µA/m², which is the value expected from a mapping of the densities of 2 to 4 µA/m² usually observed at ∼ 800 km altitudes. Small‐scale currents embedded in the region 1 system include an intense 8 µA/m² earthward‐flowing current that is only 8 km wide. That current can be associated with an upward flux of electrons with energies ≤ 1 keV that can account for nearly all of the current. Comparison of the magnetic field observations with particle measurements from the Hot Plasma Experiment confirms that the earthward‐flowing large‐scale region 1 system is carried primarily by upward‐flowing low‐energy (< 10 eV) electrons. Embedded in this low‐energy flux are smaller regions of more energetic electrons that produce the small‐scale currents. The correlation of magnetic and particle observations supports previous suggestions that the region 1 currents in the morning sector have their source in the boundary layer and that the region 2 currents are associated with the plasma sheet.
The May 2‐3, 1986 magnetic storm: First energetic ion composition observations with the Mics instrument on Viking
Authors: Stüdemann W. , Wilken B. , Kremser G. , Korth A. , Fennell J. F. , Blake B. , Koga R. , Hall D. , Bryant D. , Søraas F. , Brønstad K. , Fritz T. A. , Lundin R. , Gloeckler G.
Published: February 2008

Measurements with the Magnetospheric Ion Composition Spectrometer (MICS) onboard the polar orbiting Swedish satellite VIKING clearly show a strong asymmetry between the dusk‐ and dawnside ion distributions obtained after the onset of a magnetic storm on May 2‐3, 1986. The eveningside of the inner ring current region is quickly filled by protons with energies of some tens of keV. The morningside continues for several hours to show the quiet‐time inner ring current proton spectra in which protons below ∼100 keV have been removed due to charge‐exchange processes. The data obtained at high latitudes will be compared with measurements of the CHEM instrument on the AMPTE/CCE spacecraft near the equatorial plane. The observations are discussed in context of the convection of injected ions into the inner magnetosphere during magnetically active periods.
Solar wind mass‐Loading at comet Halley: A lesson from Venus?
Authors: Breus T. K. , Krymskii A. M. , Luhmann J. G.
Published: February 2008

Recent observations at comet Halley show that the region within which cometary ions become the dominant component lies outside of the magnetic field‐free cavity. This behavior resembles that found at Venus under conditions where the incident solar wind dynamic pressure exceeds the ionospheric pressure. On these occasions the magnetosheath magnetic field is found well inside of the region where planetary ions are observed. Although scaling and the details of formation of the inner boundary of the magnetic field are different for these two objects, the processes by which the interplanetary magnetic field penetrates into the ionospheres at Venus and at Comet Halley are in many ways analogous.
On the response of ionospheric magnetisation to solar wind dynamic pressure from Pioneer Venus measurements
Authors: Kar J. , Mahajan K. K.
Published: February 2008

The variation of the average low altitude dayside ionospheric magnetisation with solar wind dynamic pressure is studied using the magnetometer measurements from the Pioneer Venus Orbiter. The data analysed, includes the first three dayside periapsis passes by the orbiter Passes with even low magnetisation values have been included in the analysis. The results indicate a statistically linear relationship between the ionospheric magnetisation and the solar wind dynamic pressure even below the critical pressure for magnetisation adopted earlier by Phillips et al. (1984). Also similar features in the low altitude magnetic field structure are seen to persist for the whole range of solar wind pressures. These observations point to a continuous rather than time evolving mechanism of generation of ionospheric magnetic fields on Venus.
Comment on “Wave propagation effects and the Earth's structure in the lower mantle”
Authors: Young Christopher J. , Lay Thorne
Published: February 2008

In a recent contribution, Haddon and Buchbinder introduced a new shear velocity model for the lower mantle which, combined with effects of velocity inhomogeneity at the base of the mantle, they concluded to be consistent with observed SH arrivals preceding the core reflected phase ScSH at distances beyond 70°. Using Kirchhoff wave theory, they computed synthetic waveforms for the radially symmetric version of their model (which has a 1.4% discontinuous velocity decrease at 2300 km depth) that seem to match some SH data, even without allowing for lateral heterogeneity. However, synthetic waveforms for this model computed using generalized ray theory and reflectivity do not match the Kirchhoff synthetics or the data, casting doubt upon the conclusions drawn in their study. Earlier models by Lay and Helmberger, involving a 2.75% discontinuous velocity increase at a depth of about 2600 km, require only slight modification in order to match all of the SH wave observations.
Geoid anomalies across Ascension Fracture Zone and the cooling of the lithosphere
Authors: Gibert Dominique , Camerlynck Christian , Courtillot Vincent
Published: February 2008

Twenty‐three SEASAT altimeter profiles crossing the Ascension fracture zone are used together with seafloor ages inferred from a kinematic model to estimate a geoid slope‐age relation between ages of 5 and 50 Ma. The data do not definitely contradict a half space cooling model (slope = −9.5 ± 3 cm/Ma) if biases are accepted, resulting in large scatter. However the data would seem to support a puzzling explanation in terms of a plate cooling model involving two distinct thermal thicknesses, one of ≈70 km for ages < 35 Ma, the other of at least 100 km for ages > 35 Ma. Although preliminary, this result is similar to that found in the Pacific with a much larger data set.
Vertical profile of krypton‐85 in the stratosphere
Authors: Sanak Joseph , Jegou Alain , Lambert Gérard
Published: February 2008

A vertical profile of Krypton‐85 from 12 to 40 km height was obtained from a balloon‐borne experiment. The results display a clear concentration gradient between the troposphere and 22 km. Above this altitude, the concentration varies very little, in agreement with theoretical predictions.
On the 3‐dimensional structure of plasmoids
Authors: Hughes W. J. , Sibeck D. G.
Published: February 2008

We examine the magnetic topology that results from reconnection at a new near‐earth neutral line when the magnetic field in the plasma sheet has a finite dawn‐dusk component. A dawn‐dusk magnetic field component has been observed in the plasma sheet which correlates with the IMF direction. If we include such a component in the standard model, the resulting magnetic structures are very different from the conventional plasmoid model which (with the exception of Hones et al. [1982]) has invariably been drawn as a two‐dimensional structure in the noon‐midnight meridional plane. In the three‐dimensional model, reconnected plasma sheet field lines form a magnetic flux‐rope like structure which stretches across the tail but whose ends are connected to the auroral ionosphere, one end in each hemisphere. When open lobe field lines reconnect at the near‐earth neutral line, they form loops of flux that are topologically linked around the flux rope and which will tend to pull the flux rope tailwards. Nevertheless the flux ropes remain attached to the earth until further reconnection near the flanks of the tail releases them. The particle signatures expected in such structures are similar to those observed in plasmoids. Furthermore, the model explains other magnetic signatures seen in the tail, suggesting that plasmoids are one of a complete spectrum of magnetic structures that can arise from near‐earth reconnection.
Ionospheric currents associated with a Sun‐aligned arc connected to the auroral oval
Authors: Robinson R. M. , Vondrak R. R. , Friis‐Christensen E.
Published: February 2008

On the evening of 14 November, 1985, the Sondrestrom incoherent scatter radar made measurements of ionization and drifts associated with a sun‐aligned polar cap arc near local midnight. All‐sky photographs at 630.0 nm showed that the arc, which appeared to move westward across the Sondrestrom meridian, was connected to the auroral oval. The radar measured height‐integrated Hall and Pedersen conductances in the arc of 2 to 3 mhos. The plasma drift within the arc was anti‐sunward with velocities greater than 1 km/sec, indicating a westward electric field across the arc of about 60 mV/m. The radar data imply a model current system that includes a northward Hall current within the sun‐aligned arc and a westward electrojet in the auroral oval to the south. The model currents produce ground magnetic perturbations that are consistent with those measured by stations in the Greenland magnetometer chain; in particular the sun‐aligned arc is detectable by a negative deflection in the D component of the magnetometers. Divergence of the currents at the edges of the auroral forms yield field‐aligned currents that are upward on the west edge of the arc and downward on the east edge. An upward current is required at the point where the polar cap arc joins the auroral oval because of the gradient in the conductivity and electric field at that location. This upward current implies the presence of enhanced electron precipitation which can account for the observed brightening of polar cap arcs at the point where they intersect the auroral oval.
Proton auroras observed at the equatorward edge of the duskside auroral oval
Authors: Ono Takayuki , Hirasawa Takeo , Meng Ching I.
Published: February 2008

The contribution of precipitating protons to the formation of the duskside auroral oval is examined. Auroral images at wavelengths of 5577 Å (OI) and 4861 Å (Hβ) are compared with auroral particle data obtained by the DMSP F6 and F7 satellites. On the basis of the particle data, we found a proton‐dominant region in which the input energy carried by protons exceeded that of electrons. The proton‐dominant region is located at the equatorward edge of the duskside auroral oval. A direct comparison of data from auroral photoemissions and auroral particle precipitations reveals that the region of diffuse 5577 Å and 4861 Å auroras equatorward of the quiet discrete arcs coincides with the proton‐dominant region. Furthermore, we found a spot‐like structure of the proton aurora drifting westward at a speed of 0.4 km/sec.
On MHD intermediate shocks
Authors: Wu C. C.
Published: February 2008

Contrary to the usual belief that MHD intermediate shocks are extraneous, we show in this paper by numerical solutions of resistive MHD equations that at least some of the intermediate shocks are admissible and can be formed through nonlinear steepening from a continuous wave. The result is of importance not only to the MHD shock theory but also to studies such as magnetic field reconnection models.
Variation in ejecta size with ejection velocity
Authors: Vickery Ann M.
Published: February 2008

A knowledge of the size‐velocity distribution of ejecta from impact craters is important for a number of processes that occur or may have occurred in the solar system, such as planetary accretion, collisional evolution of asteroids, the ejection of meteorites from parent bodies larger than asteroids (such as the lunar meteorites and the SNC meteorites, which may be from Mars), and massive biological extinctions on the Earth. In order to estimate the size‐velocity distribution of impact crater ejecta, the sizes and ranges from the primary crater were measured for the secondary craters of twelve large craters on Mercury, the Moon and Mars. The ballistic equation for spherical bodies was used to convert the ranges to velocities, and the velocities and crater sizes were used in the appropriate Schmidt‐Holsapple scaling relation to estimate ejecta sizes. Assuming that d ⧜ v− β, lines were fitted to the log (maximum size) vs. log (ejection velocity) using the least squares method to determine the velocity exponent. Certain problems with data collection, such as partial burial of some craters and interference from other craters and topographic features, made it impossible to determine a simple, unique relation between size and velocity. The velocity exponents are generally between −1 and −3, with an average value of approximately −1.9. The complicating effect of secondaries produced by clusters of impactors implies that the calculated lines are probably steeper than the slopes of the “true” fragment size‐velocity distributions.
Effects of near source bathymetry on teleseismic P waveforms
Authors: Wiens Douglas A.
Published: February 2008

Teleseismic P waveforms from submarine earthquakes show water reverberations that are often poorly modeled by synthetic seismograms. This study suggests that considerable misfit results from the non‐horizontal nature of the seafloor in the epicentral region and examines this effect for a planar dipping water‐crust interface. Geometric ray theory synthetics calculated for a double‐couple source show that P waveforms are extremely sensitive to seafloor orientation. Because of the large impedance contrast across the boundary, seafloor dips as small as 0.5° significantly modify the ray paths, affecting the take‐off angle at the source, reflection and transmission coefficients at the interface, geometric spreading, and travel time.

To test the applicability of such modeling, P waveforms from the March 29, 1976 Cocos Ridge intraplate earthquake were studied. The waveforms, many of which show prominent water multiples poorly modeled with flat lying bathymetry, were inverted for the best‐fitting planar ocean bottom orientation using a grid search technique. Synthetics computed for the best fitting seafloor orientation match the data much better than synthetics for a flat lying structure, and reproduce the observed azimuthal dependence of water multiple amplitudes. The derived ocean bottom dip angle and direction are in agreement with bathymetric charts, suggesting P waveforms provide constraints on the seafloor orientation in the epicentral region. Green's functions incorporating the effects of bathymetry should provide better resolution of the source parameters of submarine earthquakes.
Baseline acidity of precipitation at the South Pole during the last two millennia
Authors: Cragin J. H. , Giovinetto M. B. , Gow A. J.
Published: February 2008

Measurements of meltwater pH from annual layers of South Pole firn and ice samples ranging in age from 40 to 2000 years B.P. show that precipitation at this remote site has a higher natural acidity than that expected from atmospheric equilibrium with CO2. The average pH of deaerated (CO2‐free) samples was 5.64 ± 0.08, while air‐equilibrated samples averaged 5.37 ± 0.08, a pH that is about a factor of two more acidic than the expected background pH of 5.65. The observed “excess” acidity can be accounted for by SO42− and NO3− levels in the samples originating from non‐anthropogenic H2SO4 and HNO3. Because of the presence of these naturally occurring acids in South Pole precipitation, a pH of 5.4 is considered a more representative baseline reference pH for acid precipitation studies.
Neutral surfaces in the ocean: Implications for modelling
Authors: McDougall Trevor J.
Published: February 2008

Surfaces of neutral static stability (neutral surfaces) along which lateral ocean mixing occurs, can be quite different to potential density surfaces. For example, the lateral gradients of potential temperature may differ in the two surfaces by up to a factor of four in some extreme but important regions. Vertical mixing processes cause vertical upwelling velocities that can be modelled as vortex‐stretching terms in layered models of ocean circulation. However, since potential density surfaces are commonly inclined with respect to neutral surfaces, lateral motion along neutral surfaces also gives rise to unwanted vertical motion through isopycnals that is not related to vertical mixing processes.

When considering the conservation of potential vorticity in the real ocean, the differences between the depths of neutral surfaces and potential density surfaces becomes quite important. Neutral surface potential vorticity (NSPV), is introduced, defined to be proportional to f/h, where f is the Coriolis parameter and h is the vertical distance between adjacent neutral surfaces. NSPV is often significantly different (by factors of 2 or 3) to the two other commonly used potential vorticity variables, isopycnal potential vorticity, IPV, and fN².
Correlation of a 3,200 year old tephra in ice cores from Vostok and South Pole Stations, Antarctica
Authors: Palais J. M. , Kyle P. R. , Mosley‐Thompson E. , Thomas E.
Published: February 2008

Tephra layers in two Antarctic ice cores are correlated on the basis of their chemical compositions and estimated ages. We believe the tephra was produced about 3,200 years ago from a major explosive eruption of Candlemas Island in the South Sandwich Islands. This is the first time that tephra layers have been correlated between two widely separated ice cores and demonstrates that tephra layers may serve as stratigraphic markers for correlating, and perhaps in some cases, dating Antarctic ice cores.
Evidence for 35‐50 day low frequency oscillations in total ozone mapping spectrometer data
Authors: Sabutis Joseph L. , Stanford John L. , Bowman Kenneth P.
Published: February 2008

A large number of investigations are currently focused on low frequency oscillations (LFO) of the tropical atmosphere, with periods of one to two months. Recently, the existence of LFO in the stratosphere has been postulated by global circulation model results and also by observations based on satellite microwave and infrared data. However, the observational results are not well captured by the model calculations. The present work utilizes an independent set of satellite data (four years of data from the Total Ozone Mapping Spectrometer (TOMS), between latitudes 65N and 65S) as a check on the previous observational analyses. Evidence is found for the existence of 35‐50 day oscillations in the TOMS data over the Southeast Pacific and South Indian Oceans, corroborating the earlier observational report.
Non‐thermal plasma observations using EISCAT: Aspect angle dependence
Authors: Winser K. J. , Lockwood M. , Jones G. O. L.
Published: February 2008

Recent observations with the EISCAT incoherent scatter radar have shown large rises in dayside, auroral plasma velocities (>2 km s−1) over a wide range of latitudes and lasting about an hour. These are larger than the neutral thermal speed, and allow, for the first time, observations of a non‐thermal plasma over a range of observing angles, revealing a clear angular dependence. The observed ion temperature anisotropy, deduced by assuming a Maxwellian line‐of‐sight ion velocity distribution, is at least 1.75, which exceeds the theoretical value for a bi‐Maxwellian based on a realistic ion‐neutral collision model. The aspect angle dependence of the signal spectra also indicates non‐Maxwellian plasma.
Numerical simulations on the structure of plasmoids in the deep tail
Authors: Hautz R. , Scholer M.
Published: February 2008

We have used a 2D resistive and compressible MHD code to study the formation and subsequent development of a plasmoid by near‐earth reconnection. The initial equilibrium configuration is characterized by a distant neutral line at a position, where the pressure in the plasma sheet has a minumum. The resistivity is allowed to grow with the current density. Reconnection at a near‐earth position is initiated by a small localized resistivity which is subsequently switched off. Fast reconnection develops self‐consistently and leads to closed field lines in the plasma sheet. Eventually the field lines to the distant neutral line are reconnected and the pinched‐off plasma sheet moves downtail. Continuous reconnection leads to a layer of reconnected field lines around the plasmoid which are connected to interplanetary field lines. In the case of open top and bottom boundaries secondary tearing leads to two near‐earth neutral lines and the subsequent evolution of a second plasmoid. This can result in the figure‐8 structure inferred from magnetic field data in the deep tail. A closed high latitude boundary inhibits the secondary tearing and the original plasmoid is more elongated as it is ejected down the tail.
Correlating east‐west asymmetries in the Jovian magnetosphere and the Io sodium cloud
Authors: Smyth William H. , Combi Michael R.
Published: February 2008

Three east‐west intensity asymmetries in the Jupiter system that would appear to an Earth observer in optical S+, extreme ultraviolet S++ and optical Na emissions are reviewed. The three asymmetries imply that some combination of the plasma torus number densities and electron temperature is, on the long‐term average, larger west of Jupiter than east of Jupiter. A small east‐to‐west electric field, previously proposed to explain the ion intensity asymmetries, is also shown to provide a natural explanation for the sodium intensity asymmetry. A long‐term average east‐to‐west electric field of ∼3 mV/m and a corresponding west‐to‐east offset of ∼0.16 Jupiter radii for the plasma torus at Io's orbital distance is implied. On a shorter time scale, the east‐west electric field appears to be time variable and may be readily monitored from Earth by observations of the sodium cloud.
Limits on the extent of Saturn's hydrogen cloud
Authors: Richardson John D. , Eviatar Aharon
Published: February 2008

The 30‐day reports from the Voyager Ultraviolet Spectrometer team characterize the atomic hydrogen observed near Saturn as a torus with a half width of 7 Rs extending from about 8 to 25 Rs. The source of this torus was hypothesized to be escape of hydrogen from Titan's atmosphere. Several recent papers propose that the source is actually Saturn's exosphere and that the hydrogen cloud extends inwards to the planet. We model the atomic processes occurring in the inner magnetosphere, including sputtering, ionization, charge exchange, ion‐atom interchange, recombination, and transport, and add a neutral hydrogen source to test the proposal that a hydrogen cloud exists in this region. Plasma observations in the inner magnetosphere are found to be inconsistent with the presence of a dense (20 cm−3) atomic hydrogen cloud. The presence of such a cloud in the observed plasma environment would result in a much larger proton density than observed and would remove nearly all heavy ions from the magnetosphere. Thus we conclude that the hydrogen cloud must be a torus as originally reported, and that the cloud does not extend inward to the orbits of the inner satellites of Saturn.
Rock magnetic signature of the Cretaceous‐Tertiary boundary
Authors: Worm Horst‐Ulrich , Banerjee Subir K.
Published: February 2008

Black magnetic microspherules, commonly found in Cretaceous‐Tertiary (K‐T) boundary sediments and believed to be of an impact melt origin, were found to be responsible for a strong magnetic anomaly at the K‐T boundary in Petriccio, Italy, and in almost all Deep Sea Drilling Project (DSDP) sediment cores investigated in which the K‐T boundary was present. The enriched concentrations of magnetic minerals have been detected by measurements of the magnetic susceptibility which is in a good approximation proportional to the magnetic microspherule content. A rock magnetic characterization of extracted microspherules by hysteresis, Isothermal Remanent Magnetization acquisition, demagnetization and thermomagnetic measurements describes the magnetic minerals in the microspherules as ferrites with a near magnetite composition and ‘soft’, i.e. multidomain, magnetic behavior, in agreement with earlier nonmagnetic studies. On the contrary, volcanic ash from Hawaii has distinctively ‘harder’ magnetic properties, an important finding with regard to the proposed alternative, a volcanic origin of the well‐known Ir‐anomaly at the K‐T boundary. It is suggested here that from magnetic susceptibility scanning of numerous cross‐sections of the K‐T boundary a distinct worldwide pattern might evolve that can potentially reveal the impact/volcano site because of a presumed higher microspherule concentration near the source. The advantage of mapping the microspherule distribution over the previous unsuccessful attempt using Ir‐concentrations lies in the large size (hundreds of microns) of the microspherules with relatively short airborn life times while the iridium might have adhered to small dust particles that circled the globe several times before settling.
Do slow orbital periodicities appear in the record of Earth's magnetic reversals?
Authors: Stothers Richard B.
Published: February 2008

Time‐series spectral analysis has been performed on the dates of geomagnetic reversals of the last 20 Myr BP and earlier. Possible evidence is found from the presence of high spectral peaks for two very long periodicities, 0.4 Myr and 1.3 Myr, that may be associated with slow variations of the Earth's orbital eccentricity as predicted by Berger. However, statistical significance tests and a number of other arguments do not confirm the two detections.