bluuebunny/crossref_metadata_2025 huggingface dataset:
Wave dynamics in mantle plume heads and hotspot swells
Authors: Bercovici David
Published: February 2008

Laboratory experiments with thermal plumes in fluid with temperature‐dependent viscosity suggest that wave‐like instabilities can form in the horizontally flowing, disk‐shaped head of the plume. The waves propagate radially outward from the axis of the plume and appear to be most intense in a finite band near the perimeter of the plume head. A simple theoretical model shows that interfacial waves in a highly viscous fluid may occur if the plume‐head is comprised of temperature‐dependent‐viscosity fluid that cools as it flows between two boundaries. The model suggests that the waves arise as an oscillatory instability and that wave formation is most robust in the colder regions of the plume‐head, as indicated by the experiments. The theory also predicts that the instability will only occur above some critical plume‐head flow velocity, and that mantle plume conditions are generally supercritical.
The pulsating magnetosphere and flux transfer events
Authors: Potemra T. A. , Zanetti L. J. , Elphinstone R. , Murphree J. S. , Klumpar D. M.
Published: February 2008

A unique positioning of the GOES 5, GOES 6, AMPTE/CCE, and Viking satellites on the dayside of the magnetosphere has provided the opportunity to study the relationship of periodic variations in magnetic fields, energetic particle fluxes, and images of UV auroral forms. On March 25, 1986, at about 1725 UT, two cycles of 10‐min‐period magnetic field oscillations were observed by all four satellites and by the Huancayo magnetic ground station. The UV images acquired by Viking showed intense emissions in a wide area near noon at 1730 UT, but near dawn 11 min later. We interpret these observations as being associated with anti‐sunward‐moving periodic compressions of the magnetopause, which precipitated low‐energy electrons that produced the enhanced UV emissions. In the midst of the longer‐period variations, the magnetic field intensity measured by CCE near the magnetopause decreased sharply for a 105‐sec period. This may be interpreted as being due to a flux transfer event that occurred during the longer (10‐min) periodic compressions of the magnetosphere. These observations support the view that the magnetosphere often varies in a periodic way because of its own resonant processes and processes driven by the solar wind. A wide range of phenomena is associated with these variations including dayside auroral emissions, magnetic field variations throughout the magnetosphere and on the Earth's surface, and flux transfer events.
Magnetic field maxima in the low latitude boundary layer
Authors: Sonnerup B. , Paschmann G. , Phan T. ‐D. , Lühr H.
Published: February 2008

The magnetic field often exhibits a maximum in the Earth's low‐latitude boundary layer. We show examples of this behavior, using data from the AMPTE/IRM spacecraft, and argue that two fundamentally distinct causes exist for the excess field: (i) a depression, within the layer, of the population of medium‐energy ions of magnetospheric origin; (ii) field curvature effects associated with undulations of the magnetopause itself.
The status of the world's public‐domain digital topography of the land and ice
Authors: Wolf Michael , Wingham Duncan
Published: February 2008

There has been no reasonably comprehensive survey of what digital, elevation models of the Earth's land surface exist in the public‐domain. We have performed a survey of these data, and we report on the coverage and cost of the data we have established. To qualify for inclusion in our coverage, we required the data to be available to the general public, obtainable from a specific, identified institution, and to have a determined cost. We have established that at 100 m resolution coverage exists of most of the the developed world. In the United States and Australia, the data are available at cost, at around 3000 km2 £−1. In the remaining countries, the data are available from mapping agencies with varying commercial pricing strategies. The total cost of the data we have identified is £ 1,688,312. For much of the world we are unable to confirm the existence of such data, and our experience is that in these regions it will prove difficult to obtain digital, elevation data, if, indeed, it exists at all.
Polar stratospheric clouds over McMurdo, Antarctica, during the 1991 spring: Lidar and particle counter measurements
Authors: Adriani A. , Deshler T. , Gobbi G. P. , Johnson B. J. , Di Donfrancesco G.
Published: February 2008

Lidar and balloonborne particle counter measurements were performed simultaneously on two days when polar stratospheric clouds were observed in late August 1991 at McMurdo, Antarctica. Both nitric acid trihydrate and ice clouds were observed in the lower stratosphere between 10 and 23 km in different formation stages and with different cooling rate; however in all cases the size distributions were bimodal. Comparison of scattering ratios measured by lidar and calculated from particle size distributions are in good agreement; however, discrepancies were observed when the lower stratosphere was highly perturbed by wave activity. Lee waves generated by air flowing over the Trans Antarctic Mountains induced ice cloud formation at altitudes as high as 20 km. No PSCs were observed after the end of August in 1991.
Seismic transmission survey: Mapping in two‐layered media
Authors: Katsumata Kei
Published: February 2008

I propose a new concept of a seismic survey which takes advantage of mode‐converted transmission waves. This method furnishes us with valuable information independent of any other survey which have been used to explore the Earth's structure. In the seismic reflection survey, both not mode‐converted and mode‐converted reflection waves are used in order to obtain the section of the reflectivity. In the seismic transmission survey, refraction waves with mode changes from P to SV or from SV to Pin propagating through the interfaces are used in order to obtain the section of the transmissivity.

The model consists of only a horizontally layered elastic medium. To correct the normal move‐out of the transmission waves, I apply the method which have been developed to process the mode‐converted reflection waves. The approximation error is estimated by comparing with the exact solution in the case of a horizontally two‐layered medium. Numerical simulations prove that this method is available for the two‐layered medium with the discontinuity which has the inclination of 30 degrees.
Production of polar cap electron density patches by transient magnetopause reconnection
Authors: Lockwood M. , Carlson H. C.
Published: February 2008

Some implications are considered of recent theoretical work concerning the excitation of dayside ionospheric convection by magnetic reconnection at the dayside magnetopause. In particular, transient bursts of such reconnection (‘flux transfer events’) are considered as a cause of polar cap ‘patches’ of enhanced plasma density. Examples of such patches, as observed at European longitudes by the EISCAT radar, are presented and used to discuss the implications of the proposed mechanism.
The effect of two—dimesional terrain with point source on resistivity surveys
Authors: Shi‐zhe Xu
Published: February 2008

This paper uses the boundary element method to solve the problem of the effect of two‐dimensional terrain with point current source on resistivity surveys. First, the 3‐D boundary value problem of the electric field of a point source on 2‐d terrain is converted into a 2‐D boundary value problem using a Fourier transform. Then, the 2‐D boundary value problem is transformed into a boundary integral equation by means of Green's formula. The boundary element method is used to solve the integral equation. Finally, using an inverse Fourier inverse transform, we get the electric potential in 3‐D space.

Examples calculated for two terrain models show that the results from this method compare well with those from analytic and finite element method. If the terrain has a clear strike, this method can be used for terrain corrections for resistivity surveys. An example of a terrain correction for electric sounding is given. This method can be run on a personal computer and takes a few minutes to correct a profile of data on an IBM AT—grade computer.
Impact of recent North Atlantic Anomalies on surrounding land areas based on dendroclimatic evidence
Authors: D'Arrigo Rosanne D. , Jacoby Gordon C. , Cook Edward R.
Published: February 2008

Tree rings are useful indicators of interannual to decadal‐scale climate variability. In this paper we show that tree‐ring data from the northwestern Atlantic sector may reflect the great negative salinity (GSA) and related positive sea ice anomalies of the 1960's–70's. Along with cold surface air temperatures at coastal land stations, there were declines in ring widths in the 1970's in temperature‐sensitive birch trees in southern Greenland, white spruce in Labrador, and white spruce and white pine from Newfoundland. Tree‐ring and other data from both sides of the North Atlantic also indicate a decadal‐scale warm event, with subsequent cooling, around the 1820's–1830's.
A determination of the rate constant for the reaction of HOCL with O‐atoms at room temperature
Authors: Vogt Rainer , Schindler Ralph N.
Published: February 2008

A discharge fast flow system with mass spectrometric detection was used to determine the rate constant of the title reaction to be k(1) = (1.3±0.2)×10−13 cm3 molecule−1 s−1. This experimental value is roughly a factor of 20 higher than previously recommended. It is suggested that the title reaction should be considered as a removal process in modeling stratospheric HOCl.
Observations of plasma sheet expansion at substorm onset, R = 15 to 22 Re
Authors: Lyons L. R. , Huang C. Y.
Published: February 2008

We have used a large number of auroral magnetograms to identify four isolated substorms and estimate their onset times. At the onsets, ISEE‐1 was in the vicinity of magnetic midnight at radial distances of 15.6 to 21.8 Re and very near the outer boundary of the plasma sheet. We find that, for each event, the plasma sheet expanded, and the magnetic field dipolarized at the inferred onset time. Our most definitive event occurred while ISEE was at a geocentric radial distance of 21.8 Re. This result conflicts with previous understanding, though further verification of the result is required. Our observations show very similar characteristics to those observed at synchronous orbit, and they are consistent with an extension of a portion of the substorm current wedge to the radial distance of the satellite. If this explanation is correct, ISEE must have been within the longitude range of the substorm current wedge at the onsets.
The MHD structure of the plasmasheet boundary: (1) Tangential momentum balance and consistency with slow mode shocks
Authors: Cattell C. A. , Carlson C. W. , Baumjohann W. , Lühr H.
Published: February 2008

Differences between observations in the near and distant tail and MHD simulations have motivated a statistical study of the MHD structure of the plasmasheet boundary in the near tail, utilizing data from the AMPTE/IRM spacecraft. The relationship between the change in the tangential velocity predicted by the Rankine‐Hugoniot relations for a discontinuity with a normal magnetic field and the measured change in the tangential velocity for ∼80 crossings is presented. The measured change is almost always much less than the predicted value. This suggests that either: (1) There is not usually a normal component of the magnetic field across the boundary; combined with previous work on pressure balance, this implies that the boundary is a tangential discontinuity; or (2) The boundary is not usually well‐modeled as a planar, time‐stationary MHD discontinuity. This is very different from the case of the deep tail where the plasmasheet boundary is usually a slow mode shock tailward of the neutral line. It agrees with MHD simulations, including the effects of the ionosphere, which do not have slow mode shocks earthward of the neutral line. These results suggest that the magnetic connection to the earth inhibits the formation of slow mode shocks. There is a small subset of the crossings which are consistent with the identification of part of the plasmasheet boundary as a slow mode shock. These may be cases where the substorm neutral line formed in close proximity to the satellite.
Observations of a new SAGE II aerosol extinction mode following the eruption of Mt. Pinatubo
Authors: Thomason Larry W.
Published: February 2008

In the aftermath of the eruption of Mt. Pinatubo, multiwavelength stratospheric aerosol extinction measurements by the satellite‐borne Stratospheric Aerosol and Gas Experiment (SAGE II) revealed the presence of a previously unobserved mode of aerosol that exhibited high extinction but a small inferred particle size. This mode may represent a transitional phase between the very small aerosol created by gas‐to‐particle conversion and a quasi‐steady state, post‐volcanic aerosol that exhibits both large extinction and large particle size. The presence of a transitional small aerosol mode may have a significant impact on chemical and radiative processes in the stratosphere.
Oceanwide gravity anomalies from GEOS‐3, Seasat and Geosat altimeter data
Authors: Rapp Richard H. , Bašić Tomislav
Published: February 2008

Three kinds of satellite altimeter data have been combined, along with 5'×5' bathymetric data, to calculate a 0°.125 ocean wide gridded set of 2.3×106 free‐air gravity anomalies. The procedure used was least squares collocation that yields the predicted anomaly and standard deviation. The value of including the bathymetric data was shown in a test around the Dowd Seamount where the root mean square (rms) difference between ship gravity measurements decreased from ±40 mgal to ±20 mgal when the bathymetry was included. Comparisons between the predicted anomalies and ship gravity data is described in three cases. In the Banda Sea the rms differences were ±20 mgal for two lines. In the South Atlantic rms differences over lines of 2000 km in length were ±7 mgal. For cruise data in the Antarctica region the discrepancies were ±12 mgal. Comparisons of anomalies derived from the Geosat geodetic mission data by Marks and McAdoo [1992] with ship data gave differences of ±6 mgal showing the value of the much denser Geosat geodetic mission altimeter data.
Cratering mechanics on Venus: Pressure enhancement by the atmospheric “ocean”
Authors: Brackett Robert A. , McKinnon William B.
Published: February 2008

The mass per unit area of the Venusian atmosphere is ∼106 kg/m2, equivalent to a 1‐km‐deep water ocean. An important effect of this “ocean” on cratering is pressure enhancement due to multiple reverberations during the compression stage. We consider impactors with diameters, D, less than a scale height (∼15 km). Such impactors drive before them bow shocks with standoff distances of Δ ∼ 0.05 D into the lowermost atmosphere. Reflection of this shock at normal incidence from the basalt surface raises the pressure in the atmosphere and surface, but initially not as much as a direct impact would. Several shock reverberations should occur, however, as the shock crossing time in the trapped, compressed atmospheric layer is much less than either the shock crossing time in the impactor or the time for the impactor to penetrate the atmospheric layer. With each reverberation we estimate new Hugoniots for shocked atmosphere, impactor, and target. The first is treated as an ideal gas of γ= 1.2–1.3, and the latter are modeled with Murnaghan equations of state and Grüneisen Γs ∝ ρ−1. The asymptotic pressure enhancement for a 20 km/s carbonaceous chondrite impactor (modeled as serpentine) striking basalt at 20 km/s, relative to the atmosphereless 1‐D impedance match pressure (∼550 GPa), is a factor of ∼1.4. This maximum enhancement factor increases with velocity, reaching 1.6 for serpentine moving at typical Venus‐impacting cometary velocities (∼65 km/s), and is greater for more compressible impactors (comets). Each reverberation sends a shock pulse into the surface or impactor that overtakes the previous ones, hence at the end of the compression stage both impactor and target are shocked to a higher pressure than would occur in the absence of the atmosphere. The impactor is effectively denser or staffer than in the atmosphereless case, but as the energy and momentum delivered are essentially the same, entering efficiency should be unaffected to first order. However, fusion and vaporization efficiency should be enhanced for impactors whose impact velocities are below (whether initially or due to atmospheric drag) the threshold for complete melting or vaporization, respectively, in the absence of an atmosphere.
High resolution magnetic susceptibility imaging of geological thin sections: Pilot study of a pyroclastic sample from the Bishop Tuff, California, U.S.A.
Authors: Thomas I. M. , Moyer T. C. , Wikswo J. P.
Published: February 2008

High resolution magnetic susceptibility imaging is a new technique for studying the magnetic properties of geological thin sections. The two‐dimensional distribution of both remanent and induced magnetization can be determined with a spatial resolution (< 1.0 mm) that is similar to the size of phenocrysts in the sample. Amongst many problems in rock magnetism to which it could be applied, the technique holds great potential for understanding the origin of the anisotropy of magnetic susceptibility in pyroclastic flows. Preliminary tests on a single sample of ignimbrite indicate that secondary iron‐titanium oxide particles deposited within vesicle walls prior to their collapse are responsible for the bulk susceptibility.
Coulomb plasticity within the fault zone
Authors: Byerlee J. D. , Savage J. C.
Published: February 2008

We represent a well‐developed fault by a layer of granular material (fault gouge) confined between two competent fault blocks. Slip on such a fault involves plastic shearing of the fault gouge. That is, the fault gouge behaves as a Coulomb material, and the plastic flow is accomplished by slip on the two sets of Coulomb shears appropriate to the stress state and the frictional properties of the gouge. Neither set of Coulomb shears is coplanar with the fault zone. Generally, the initial plastic flow is self arresting, and increased shear stress is required to drive further shear. But as the shear stress is increased a stress state is reached within the gouge in which the plane of maximum shear stress is parallel to the plane of the fault zone. Then the fault gouge shears at whatever rate is required to keep the shear stress across the fault from increasing further as was demonstrated experimentally by Mandl et al. [1977]. In a mature fault previous slip cycles have presumably left the fault gouge in a stress state such that the plane of maximum shear is parallel to the fault plane. Then, with additional shear loading, unlimited shear deformation is possible at the shear stress corresponding to the onset of Coulomb failure, and the fault appears to slip as if friction on the plane of the fault were less than on planes of other orientations. Slip on the fault does not imply that the fault plane coincides with a Coulomb shear as is sometimes assumed.
ionospheric modification with a VLF transmitter
Authors: Inan U. S. , Rodriguez J. V. , Lev‐Tov S. , Oh J.
Published: February 2008

Detectable heating of the nighttime D region by a 28.5 kHz signal is observed in 16 out of 144 cases with events occurring under conditions of moderate to low D region electron densities as represented by the unperturbed VLF signal levels. Three dimensional modeling of the effects of NAU heating on a probe VLF signal predicts values in general agreement with observations and suggests that maximum effects should occur under tenuous D region conditions.
Magnetotelluric evidence for a brittle‐ductile transition, peninsular ranges batholith, southern California?
Authors: Park Stephen K. , Jiracek George R. , Johnson Kevin M.
Published: February 2008

A magnetotelluric traverse of the Peninsular Ranges in southern California has revealed a pervasive zone of lower resistivity beginning at a uniform depth of 10 km and extending to depths of 60–90 km. Resistivities above 10 km depth are similar to those found in batholiths; very high values correspond to outcrops of crystalline basement. Because seismicity below 11–12 km is sparse, others have concluded that the brittle‐ductile transition is shallow beneath the range. The zone of low resistivity in the lower crust corresponds well to the ductile region, and we conclude that the lower values are caused by fluids trapped below the transition. Because the range has experienced vertical uplifts during the Pliocene era and the top of the low resistivity zone is flat, the present brittle‐ductile transition must have been formed in the last 5 M.y. A possible source for the fluids is the rift to the east in the Salton Trough.
Optical remote sensing of the ion convection pattern in the high‐latitude ionosphere from a polar orbiting satellite
Authors: Minow Joseph I. , Smith Roger W.
Published: February 2008

Presented is a technique where on a routine basis a single satellite borne instrument can be used to determine quasi‐instantaneous ion convection patterns over the entire high‐latitude ionosphere. The remote sensing technique utilizes the Doppler shift information in the optical emissions originating from thermospheric ions drifting in response to the Earth's magnetic and convection electric fields. The feasibility of the technique is demonstrated using a computer simulation. The simulation shows that a satellite can obtain information with sufficient spatial resolution to obtain quasi‐instantaneous estimates of ion convection patterns. The results are accurate enough to distinguish between features present in currently accepted averaged models of ion convection.
Resistivity due to weak double layers: A model for auroral arc thickness
Authors: Prakash Manju , Lysak Robert L.
Published: February 2008

We have calculated the resistivity due to a sequence of fluctuating weak double layers aligned parallel to the ambient magnetic field line. The average response of an electron drifting through a one‐dimensional randomly oriented array of WDLs is studied using a test particle approach. The average is taken over the randomly fluctuating values of the electric field associated with the double layers. Based on our calculations, we estimate that at 350 eV electron energy the thickness of the visual auroral arc is ≃ 2.5 km and that of the auroral fine structure as ≃ 250 m when mapped down to the ionosphere. The significance of our calculations is discussed in the context of magnetosphere‐ionosphere coupling.
Effect of altered boundary conditions on GCM studies of the climate of the Last Glacial Maximum
Authors: Hyde William T. , Peltier W. Richard
Published: February 2008

Since publication of the paper by Bard et al. (1990) it has been known that GCM studies of the climate of the last glacial maximum (LGM), employed lower boundary conditions appropriate to this time but astronomical parameters of an era 3000 years later. The LGM boundary conditions from CLIMAP were for 18 kyr BP on the 14C timescale. The GCM simulations employed the insolation regime appropriate to 18 kyr BP on the sidereal timescale whereas the appropriate LGM insolation regime is that of 21 kyr BP. These studies also used the CLIMAP ice sheet reconstruction. However, on the basis of most recent analyses the reconstruction by Tushingham and Peltier (1991) is to be preferred. Hyde et al. (1989) showed that a simple EBM compared favourably with the NCAR CCM, when both were used to simulate the temperature distribution of the LGM. Here we shall employ the same EBM to study the effect on LGM climate of the timing mismatch, and of the different horizontal extents of the different ice sheet reconstructions. In each case the climatic effect is found to be significant. Thus we cannot claim an accurate LGM simulation unless the orbital and terrestrial inputs match to within 1,000 years and unless we employ the best possible ice sheet reconstruction in the analysis.
Simultaneous equatorial measurements of waves and precipitating electrons in the outer radiation belt
Authors: Imhof W. L. , Robinson R. M. , Collin H. L. , Wygant J. R. , Anderson R. R.
Published: February 2008

Simultaneous wave and precipitating electron measurements near the equator in the outer radiation belt have been made from the CRRES satellite. The electron data of principal concern here were acquired in and about the loss cone with narrow angular resolution spectrometers covering the energy range 340 eV to 5 MeV. The wave data included electric field measurements spanning frequencies from 5 Hz to 400 kHz and magnetic field measurements from 5 Hz to 10 kHz. This paper presents examples in which the variations in electron fluxes in the loss cone and the wave intensities were correlated. These variations in electron flux were confined to pitch angles less than about 30°. The association between the flux enhancements and the waves is consistent with wave‐induced pitch angle diffusion processes.
Microwave magnetisation
Authors: Walton D , Share J , Rolph T C , Shaw J
Published: February 2008

We have successfully magnetised and demagnetised magnetic grains with microwaves. We show that the rapidity of the heating and cooling process significantly reduces the problem of mineral alteration that often occurs during conventional heating.
New error calibration tests for gravity models using subset solutions and independent data: Applied to GEM‐T3
Authors: Lerch F. J. , Nerem R. S. , Chinn D. S. , Chan J. C. , Patel G. B. , Klosko S. M.
Published: February 2008

Orbit error projections based on the error covariance estimates of Goddard Earth Model (GEM)‐T3 have been shown to be reliable through their projection on observation residuals within independent data sets. Special geopotential solutions were developed based upon the same data set and weighting used in the GEM‐T3 gravity model, but with a significant satellite data set eliminated from the solution. These subset gravity models are then used to compute the observation residuals within orbital solutions for the omitted satellite and the results are compared to their predicted values based on the error covariance of these models. To ensure meaningful results, the tests were designed so that the observation residuals are dominated by geopotential modeling errors. This yields a reliable test of the error estimates of the subset solutions and hence tests the data weighting used in the construction of these models (GEM‐T3 and subset solutions alike). The error estimates for GEM‐T3 are based upon an optimal data weighting method and have been obtained in a separate calibration process. The test results shown here indicate that the GEM‐T3 error estimates for the gravity parameters are calibrated and that the predicted orbit errors correspond well with actual orbit accuracies. Test results of the complete GEM‐T3 model with totally independent high precision DORIS Doppler tracking data acquired on the French SPOT‐2 satellite confirms these conclusions.
Remote sensing studies of the terrain northwest of Humorum Basin
Authors: Hawke B. Ray , Peterson Chris A. , Lucey Paul G. , Taylor G. Jeffrey , Blewett David T. , Campbell Bruce A. , Coombs Cassandra R. , Spudis Paul D.
Published: February 2008

We have used near‐infrared reflectance spectra and Earth‐based radar data to investigate the composition and origin of the various geologic units northwest of Humorum basin as well as the stratigraphy of the Humorum pre‐impact target site. The results of our spectral analysis indicate that at least a portion of the inner, mare‐bounding ring is composed of pure anorthosite. Other highlands units in the region are dominated by noritic anorthosite. The anorthosites on the inner ring may have been derived from a layer of anorthosite that exists at depth beneath a more pyroxene‐rich unit. Both Gassendi G and F craters expose mare material from beneath a highlands‐rich surface unit that was emplaced as a result of the Letronne, Gassendi, and other impact events. This ancient basalt unit was emplaced after the formation of Humorum basin but prior to the Orientale impact.
Melting in the Mg2SiO4‐H2O system at 3 to 12 GPa
Authors: Luth Robert W.
Published: February 2008

The melting of forsterite (Mg2SiO4) in the presence of H2O was studied from 3 to 12 GPa in a multiple‐anvil apparatus to constrain the maximum temperature of the hydrous solidus in peridotitic systems relevant to the Earth's mantle. The solidus has a negative slope to 12 GPa, despite the stabilization of Phase E (a hydrous magnesium silicate) coexisting with forsterite and vapour at the solidus at > 9 GPa. Abundant quench vapour deposits in all the run products indicate extensive solubility of silicate in the vapour; however, there is no direct evidence for incongruent solution in the vapour to 9 GPa. At > 9 GPa, the coexistence of Fo + PhE + V at the solidus requires Mg/Si≠2 in the vapour. Silicate melts and hydrous vapours remain immiscible phases in this system to >12 GPa.

Given current estimates of the temperature distribution in the mantle, a hydrous vapour can exist in the asthenospheric mantle only below ∼5 GPa (∼160 km depth). At greater depths, water would be present in a hydrous silicate melt rather than in a free vapour. This depth is a maximum, because solidus temperatures in hydrous peridotite systems would be lower than those in the endmember Mg2SiO4−H2O system. In cooler subducting slabs, a hydrous vapour could persist down to the transition zone.
Viking 2 electron observations at Mars
Authors: Johnson Francis S. , Hanson William B.
Published: February 2008

The Viking retarding potential analyzer flown to Mars in 1976 had negative retarding potential sweeps in addition to the positive ion sweeps. This paper presents an analysis of the electron mode sweeps made in Viking 2 above the ionosphere. An observation of the electron energy spectrum over the range 0 to 78 eV was recorded in 1 s and observations were made at intervals of 4 or 8 s. The portion of the orbit where measurements were made was entirely behind the shock front and in the dawn sector. Observations were made from an altitude of 15,644 km at 0304 LT down to 791 km at 0715 LT, at which point sunlight fell on the entrance grids and further analysis of the electron observations was not possible. The concentrations and temperatures were highly variable in the altitude range 14,000 to 9000 km (solar zenith angles 133° to 123°). Evidence for two Maxwellian components was present in most of the records. The general trend of concentration and temperature for the predominant component was from 2 cm−3 and 100,000 K at 15,600 km to 5 cm−3 and 220,000 K at 900 km, in good agreement with the Mars 3 observations of Gringauz et al. [1974]. The higher‐temperature component was generally characterized by a temperature near 400,000 K and concentrations near 0.1 cm−3. The electron plasma pressures near 800 km were about a factor of 20 lower than those obtained from Viking 1, the difference being much greater than expected from the normal distribution around the stagnation point in the shocked solar wind.
Bispectral analysis of equatorial spread F density irregularities
Authors: LaBelle J. , Lund E. J.
Published: February 2008

Bispectral analysis has been applied to density irregularities at frequencies 5–30 Hz observed with a sounding rocket launched from Peru in March 1983. Unlike the power spectrum, the bispectrum contains statistical information about the phase relations between the Fourier components which make up the waveform. In the case of spread F data from 475 km the 5–30 Hz portion of the spectrum displays overall enhanced bicoherence relative to that of the background instrumental noise and to that expected due to statistical considerations, implying that the observed ƒ−2.5 power law spectrum has a significant non‐Gaussian component. This is consistent with previous qualitative analyses. The bicoherence has also been calculated for simulated equatorial spread F density irregularities in approximately the same wavelength regime, and the resulting bispectrum has some features in common with that of the rocket data. The implications of this analysis for equatorial spread F are discussed, and some future investigations are suggested.
Rotating magnetic anomalies as a possible accelerator of charged particles
Authors: Liu W. William
Published: February 2008

We show, through the formulation of a simple theoretical model, that there exists a possibility that a significant number of energetic ions in the Jovian magnetosphere, and presumably in other astrophysical environments beyond our direct access, owe their origin to a large‐scale longitudinal magnetic anomaly corotating with the central body anchoring the magnetosphere. The proposed acceleration mechanism can explain the observed properties of MeV ions in the Jovian magnetosphere, if the invoked magnetic anomaly has a typical boundary thickness comparable to an ion gyroradius, which for a preheated 10‐keV heavy ion in the outer Jovian magnetosphere may be of the order of a few Jovian radii. The energy source for the acceleration is a torque from the central planet arising from a differential rotation between the ionosphere and the magnetosphere. The premise and outstanding issues pertaining to the present proposal are further discussed as motivating points for future studies.
Shock drift acceleration of energetic protons at a planetary bow shock
Authors: Giacalone Joe
Published: February 2008

We present the results of numerical orbit integrations of the interaction of suprathermal charged particles (protons) with a planetary bow shock. The primary goal of this study is to analyze the effect of the changing geometry of the shock, due to its curvature, on the kinematics of the particle/shock interaction. The model bow shock is a conic section which has as free parameters the eccentricity and the distance from the planet to the subsolar point of the shock surface. We have considered both two‐ and three‐dimensional geometries for the bow shock. Protons of various suprathermal energies are considered (we make no assumptions as to the origin of these seed particles). The protons are released in such a fashion to best sample the quasi‐perpendicular region of the Earth's bow shock (i.e., 45° ≤ θBn ≤ 90°, where θBn is the angle between the local shock normal and the incident magnetic field). The results of the numerical orbit integrations are compared to an analytical model in which the charged particles conserve their first adiabatic invariant upon crossing each plane shock and it is assumed that they enter and exit at the same point on the shock. An important parameter is the ratio of the particle's initial gyroradius to the local radius of curvature of the shock. When this parameter is very small (∼ 10−4) the curved bow shock may be modeled in this fashion. For purposes of comparison with other bow shocks, we also discuss the application of this model to the bow shocks of Jupiter and Mercury. It is shown that the effect of introducing shock curvature to the shock in the numerical simulations is to increase the reflection probability of incident suprathermal protons. It is also shown that the curvature of the planetary bow shock allows protons to enter at a given value of θBn and exit at a different value. Since θBn is a very important variable concerning charged particle motion in collisionless shock waves, this result has consequences in the characteristics of the energetic charged particle environment in the planetary foreshock as well as the injection of near thermal energy protons into seed energies for further acceleration by either shock drift or Fermi acceleration.
Distribution of ULF energy (ƒ < 80 mHz) in the inner magnetosphere: A statistical analysis of AMPTE CCE magnetic field data
Authors: Takahashi Kazue , Anderson Brian J.
Published: February 2008

The distribution of ULF energy (frequency < 80 mHz) in the inner magnetosphere (L=2.5–6.5) has been studied using magnetic field measurements made with the Active Magnetospheric Particle Tracers Explorers Charge Composition Explorer (AMPTE CCE) spacecraft during its whole mission of over 4 years. The magnetic field time series was split into consecutive 10‐min segments and subjected to fast Fourier transform (FFT), and the spectral data were archived along with the Kp index and the satellite location. This data base was then used to examine the spatial distribution of ULF energy. Overall, the oscillation energy is localized to the dayside except for compressional oscillations associated with substorms. From the spatial distributions of wave power and spectral structures we identify several pulsation types, including multiharmonic toroidal oscillations; equatorial compressional Pc 3 oscillations; second harmonic poloidal oscillations; and nightside compressional oscillations. The toroidal oscillations are observable even in the plasmasphere, and using their frequencies we can determine the statistical radial profile of the plasma mass density and Alfvén velocity. We find a clear signature of the plasmapause in the profiles of these average parameters. Of particular interest among the other oscillation types are the equatorial compressional Pc 3 oscillations that have an average spectral peak around 30 mHz and are present in the prenoon sector and from L ∼ 3 to ∼ 5. The oscillations are similar to the compressional Pc 3 previously observed at and near geostationary orbit and might represent magnetohydrodynamic waves propagating from the parallel portion of the bow shock.
Excitation of Pc 5 pulsations in the morning sector by a local injection of particles in the magnetosphere
Authors: Saka O. , Iijima T. , Yamagishi H. , Sato N. , Baker D. N.
Published: February 2008

We have determined a characteristic of Pc 5 pulsation in the morning sector by use of ground magnetometer and riometer (cosmic noise absorption) data, in conjunction with data acquired with satellites which include the magnetic fields above the ionosphere and electron fluxes at geosynchronous orbit. We have found that the onset of a flux increase in energetic electrons of 30 keV to 200 keV at geosynchronous orbit almost coincides with the onset of Pc 5 pulsation activity and riometer absorption on the ground. We have ascertained that when the Pc 5 pulsation occurs on the ground, the large‐scale Birkeland current system observed at ionospheric altitude splits into a number of small‐scale Birkeland current pairs. We could infer that the electron flux enhancement, presumably supplied from the tail plasma sheet associated with the substorm onset, provides stress to cause the background large‐scale plasma vortex to split into the small‐scale vortices. We suggest that the field‐aligned currents in the small‐scale vortices propagate along the field lines and sustain the standing Alfvenic oscillations at several different, but neighboring shells of the field lines
Radiative association of N and O atoms at low temperatures
Authors: Sun Y. , Dalgarno A.
Published: February 2008

The rate coefficients for the radiative association of N(4S) and O(³P) atoms are calculated for temperatures between 1 K and 2000 K. Below 200 K the effects of the interaction between the B²Π and C²Π states have to be included. Experimental spectroscopic data are analyzed to determine the mixing of the states and to infer the radiative and predissociation lifetimes for the quasi‐bound rotational levels through which the radiative association occurs at low temperatures. At 20 K the rate coefficient is 6.5×10−17 cm³ s−1, and at 200 K it is 2.3×10−17 cm³ s−1.
Solar wind from a corona with a large helium abundance
Authors: Leer Egil , Holzer Thomas E. , Shoub Edward C.
Published: February 2008

Observations of quasi‐steady high‐speed solar wind streams show that the proton mass flux density at 1 AU is remarkably constant, varying by less than 10% over long time periods. These observations are problematic, for simple theoretical models predict that the proton mass flux density is a sensitive function of the coronal base temperature, which is not expected to be unvarying to the degree required by the observations. In this paper we investigate the possibility that the presence of alpha particles in the coronal base region can reduce the sensitivity of the proton mass flux to the base temperature. The equations of mass and momentum conservation are solved for electrons, protons, and alpha particles using a variety of assumed temperature profiles for each species. A wide range of base conditions are considered. We find that for an alpha particle to proton density ratio at the base as small as 10%, alpha particles can reduce the sensitivity of the proton mass flux density to variations in the base temperature. We also study the effects of enhanced collisional coupling and of Alfvén waves on the flux of protons and alpha particles. As an aid to future observational determination of the alpha particle density in the corona, we present calculations of the intensities of the resonantly scattered lines He II λ304 and H I λ1216 for selected models.
Comparison of measured and modeled solar EUV flux and its effect on the E‐F1 region ionosphere
Authors: Buonsanto M. J. , Solomon S. C. , Tobiska W. K.
Published: February 2008

The response of the E‐F1 region ionosphere to different solar EUV flux models is investigated theoretically using two different photochemical schemes, and the results are compared with incoherent scatter radar electron density measurements taken at Millstone Hill. The latest EUV flux model (Tobiska, 1991), which incorporates more recent measurements, has generally more flux at short wavelengths compared to the Hinteregger et al. (1981) flux model based on AE‐E satellite data. This results in better agreement with the measurements in the E‐F1 region and above. The Tobiska flux model, however, gives a smaller E region peak density, due to the influence of low Lyman ß flux in the November 10, 1988, rocket measurements of Woods and Rottman (1990). The photochemical scheme of Buonsanto (1990) has been improved and now gives results similar to the more comprehensive scheme of (Solomon et al., 1988; Solomon and Abreu, 1989; S. C. Solomon and R. G. Roble, Simulation of the global thermospheric airglow, 1, Methodology, submitted to Journal of Geophysical Research, 1992), provided that the ratios of photoelectron impact ionization to photoionization (pe/pi) given by this latter model are included. The pe/pi ratios calculated by this model and by the models of Lilensten et al. (1989) and Richards and Torr (1988) differ significantly, and work is needed to resolve these differences. In general, the photochemical model results underestimate the data, especially in winter. This result agrees with that of the earlier paper by Buonsanto and could be resolved by decreasing MSIS‐86 N2 and O2 densities in winter if additional ions were produced in the E region either by photoionization or by photoelectron impact ionization. The photoionization and photoabsorption cross sections of Conway (1988) give results in somewhat better agreement with observations than the cross sections of Torr et al. (1979). For the zenith angles considered (daytime conditions), the Chapman function method for calculating photoabsorption gives results in satisfactory agreement with a more rigorous calculation method using a formula from Rees (1989).
Oval intensification event observed by STARE and Viking
Authors: Nielsen E. , Elphinstone R. , Hearn D. , Murphree J. S. , Potemra T.
Published: February 2008

Optical emissions and electron flow patterns in the auroral oval evening sector have been investigated in the 30‐min interval prior to a major auroral intensification. The STARE radar and Viking UV image data were combined to give a dynamical picture of the auroral ionosphere just to the west of the intensification region which was bordered on its eastward side by the foot of a high‐latitude polar arc. Half an hour prior to the main intensification an auroral arc appeared equatorward of a band of eastward electrojet, which in turn was equatorward of another discrete arc system. The appearance of the new arc system was accompanied by a second eastward electrojet further equatorward. Within minutes of this appearance the older electrojet and arc system faded and the new system moved poleward to replace it. This precursor activity was followed by a major intensification, a substorm, to the east of the foot of the high‐latitude polar arc. It is speculated that the precursor is linked to changes in the current distribution in the near‐Earth nightside magnetosphere prior to the substorm onset.
Relativistic electron microbursts
Authors: Imhof W. L. , Voss H. D. , Mobilia J. , Datlowe D. W. , Gaines E. E. , McGlennon J. P. , Inan U. S.
Published: February 2008

We report the first satellite observations of relativistic (>1 MeV) electron precipitation in microbursts with measured durations of less than 1 s. Microbursts of lower‐energy electrons (10–100 keV) have been found to occur preferentially in the early daylight hours and to be closely associated with VLF chorus emissions. In contrast, the relativistic electron microbursts occurred more frequently near 2230 LT than 1030 LT, and no association was found with ELF/VLF chorus, consistent with the fact that resonant interactions with ∼ 1‐MeV electrons require significantly lower frequencies. The available data on these relativistic microbursts thus appear to indicate that many of the bursts may be due to wave‐particle interaction not with whistler mode chorus but possibly with other waveforms. The locations of many of the relativistic microbursts are concentrated at the outer edge of the trapped radiation belt, where the gyroradii of the electrons are comparable to the curvature of the magnetic field lines and stable trapping may therefore not occur. The preferred location of the microbursts, which may be primarily spatial in character, implies the possible importance of irregularities in the magnetic field lines near the trapping boundary as the responsible mechanism.
Plasma response to the emission of very weak ion beams for spacecraft potential control
Authors: Schmidt R. , Schriver D. , Ashour‐Abdalla M.
Published: February 2008

Future space missions aiming at the accurate measurement of cold plasmas and DC to very low frequency electric fields will require that the surface potential of their conductive surfaces be actively controlled to be near the ambient plasma potential. To do this, an active ion emission instrument will be flown aboard the upcoming Cluster, Geotail, and Interball satellites that involves the emission of an indium ion beam. To understand the beam‐plasma interaction that can occur for an indium ion beam injected from the satellite into magnetospheric and solar wind plasma, a linear theory parameter search has been carried out to investigate whether particular combinations of ambient plasma parameters and emission modes exist such that plasma instabilities may be excited. A restricted plasma model in the vicinity of spacecraft was used with an indium beam drifting with respect to an ion‐electron background plasma. Since the satellites will fly through different space plasma regions, the linear theory analysis was carried out using background parameters for solar wind, lobe, plasma sheet boundary layer, and central plasma sheet conditions. It was found that a beam‐driven lower hybrid type instability can be excited with maximum wave growth for beam emission and wave propagation transverse to the ambient magnetic field. The stability threshold as a function of the beam emission current, which can be varied during instrument operation, has been found for the various background plasma regions. The instability has the lowest current threshold in the central plasma sheet, and the highest in the lobe. The emission current determines the beam temperature, thus by varying the emission current above or below the threshold determined here, the instability can, in principle, be turned on and off, and the effects on other instruments on the satellite can be evaluated.