bluuebunny/crossref_metadata_2025 huggingface dataset:
Use of aerosol microphysical measurements to model IR backscatter in support of GLOBE
Authors: Patterson Edward M. , Bowdle David A.
Published: February 2008

We have used the GAMETAG Pacific mid‐tropospheric aerosol data set to calculate aerosol optical extinction coefficients (σ) at two wavelengths (0.55 μm and 1 μm) and volume backscatter coefficients (β) at 4 wavelengths (1 μm, 9.11 μm, 9.25 μm, and 10.6 μm). At an altitude of 5 km over the Pacific, northern hemispheric mean values of β for 10.6 μm are near 10−10 m−1sr−1 at an altitude of 5 km, with southern hemispheric values approximately an order of magnitude lower. The 9.11 μm values are roughly a factor of 3 higher than the 10.6 μm values; 9.25 μm values are approximately the same as 9.11 μm values. For our data, averaging times of 5–10 min are necessary for the calculated β values to be representative of β values as seen by a satellite lidar system. Under the assumptions of this study the molecular form of the sulfate aerosol is not a major determining factor in the calculated β values at 10.6 μm but could be significant at 9.11 μm. A study of relationships among the optical parameters indicates that visible and near‐visible values of β and σ may be useful in predicting 9.11‐ and 10.6 μm backscatter, so that short wavelength aerosol data bases from satellites and Nd‐YAG lidars may be useful in extending the data base of direct backscatter measurements at CO2 wavelengths.
On the satellite retrieval of Saharan dust optical thickness over land: Two different approaches
Authors: Tanré D. , Legrand M.
Published: February 2008

Two approaches are presented to retrieve the desert aerosol optical thickness over land from satellite. One approach uses the infrared imagery, and the other uses contrast reduction in visible imagery. The retrieved aerosol optical thicknesses from Meteosat data are shown in good agreement with simultaneous ground‐based measurements. Advantages as well as deficiencies of both methods are compared.
Observed high‐altitude pressure waves from an underground and a surface explosion
Authors: Banister John R. , Hereford William V.
Published: February 2008

Measurements of high‐altitude pressure waves from underground and surface explosions are of interest, as these waves can affect ionospheric electron density and be a source of infrasonic signals. Canisters, equipped with parachutes, were dropped from aircraft to determine time histories of pressure waves radiated by the ground surface above an underground nuclear explosion and a surface chemical explosion. These canisters contained transducers to document pressure and acceleration histories as well as a transmitter to relay information to ground receiver stations. We found observed pressure histories from the underground explosion to be consistent with histories calculated from surface ground motion records. The peak blast wave overpressures from the chemical explosion agreed with predicted scaled values. Observed time histories, however, had shorter positive phase duration than predicted with scaling.
Charge transfer during individual collisions in ice growing from vapor deposition
Authors: Caranti G. M. , Avila E. E. , Ré M. A.
Published: February 2008

Measurements of charge transfer during individual collisions between ice spheres and a target growing from vapor deposition have been performed in order to establish the importance of fragmentation in the charging process. It is found that when the target is a few degrees colder than the environment, the charge pulses have mixed signs but the number of positive ones becomes dominant as the target temperature decreases. This behavior is reproduced in collisions with dielectric particles and a target in the same growth conditions. Reversal in the temperature gradient reversed the sign of the charge transfer. Direct evidence of fragments produced during collisions suggests that they are responsible for the measured charge transfer.
Reconsidering caledonian deformation in southwest Spitsbergen
Authors: Bjørnerud M. , Decker P. L. , Craddock C.
Published: February 2008

Geologic thinking about the Svalbard Caledonides has long been shaped by the hypothesis that large‐scale strike‐slip plate motions assembled the archipelago in mid‐Paleozoic time. However, the first detailed studies of upper Proterozoic meta‐sedimentary rocks along one of the postulated strike‐slip zones, in northern Wedel Jarlsberg Land, southwest Spitsbergen, have produced no evidence for major Caledonian transcurrent shear in the region. An angular unconformity divides the greenschist‐facies rocks into two sequences and chronicles a period of intense deformation in late (?) Proterozoic time. Rocks above the unconformity (conglomerates, dolomites, greenschists, and Vendian diamictites) provide a detailed record of early to mid‐Paleozoic deformation. The widely distributed conglomerates allow definition of regional strain patterns and construction of a possible deformation path for the study area. Deformed conglomerate clasts range from oblate and parallel with bedding, to prolate and aligned parallel to the axes of northwest trending folds. Microstructural evidence indicates that net orogen‐parallel elongation of clasts resulted from modification of an early subhorizontal flattening fabric by later progressive coaxial flattening in a vertical plane. This is consistent with macroscopic evidence for two distinct, though perhaps regionally diachronous, phases of deformation: a period of ductile thrusting and recumbent folding followed by upright to northeast vergent folding, in a progressive trend toward more brittle behavior. The structural fabric of the rocks may record their incorporation into an accretionary prism, in a process closely linked with blueschist emplacement in central west Spitsbergen. The rocks provide no evidence for vorticity about a vertical axis, as might be expected near a major strike‐slip zone. Stratigraphic correlation of Proterozoic rocks on both sides of the postulated fault zone also argues against large‐scale strike‐slip displacement during Caledonian deformation in southwest Spitsbergen. The results of this study and others call for critical reexamination of the prevailing tectonic model for Caledonian Svalbard.
Simulation of Foreland Basin Stratigraphy using a diffusion model of mountain belt uplift and erosion: An example from the central Alps, Switzerland
Authors: Sinclair H. D. , Coakley B. J. , Allen P. A. , Watts A. B.
Published: February 2008

Foreland basin stratigraphy can be considered as the result of three interacting processes: thrust deformation, which builds the tectonic load, sedimentary and erosional processes which redistribute that load, and the flexural response of the lithosphere. The resultant stratigraphy of foreland basins is commonly composed of a small number of shallowing and coarsening upward cycles bounded by regional unconformities. To understand the development of these unconformities, we present a simple model of these three processes, coupling an evolving thrust wedge on a linear elastic plate with erosion and sedimentation defined by the diffusion equation applied to topography. Our model demonstrates the development of regional unconformities without recourse to either eustasy or complex viscoelastic models for the continental lithosphere. The model describes the thrust wedge‐foreland basin system in terms of four parameters: (1) the effective elastic thickness of the foreland plate (Te), (2) the sediment transport coefficient (K), (3) the thrust wedge advance rate, (4) the surface slope of the thrust wedge. The model is applied to the Oligocene‐Miocene North Alpine Foreland Basin (NAFB) of eastern Switzerland. The stratigraphy of the NAFB can be simplified into two large‐scale shallowing upward cycles separated by an unconformity at the base of the Burdigalian (22 Ma). Geological information is taken from the NAFB to estimate suitable values for the parameters listed above. Assuming a linear elastic lithospheric rheology, the Te value is estimated at 10±5 km from decompacted sediment columns. Data to constrain the sequential development of the thrust wedge come from structural geology. The early stages (40–24 Ma) of compression involved a relatively low‐angle thrust wedge with an advance rate of approximately 2–4 mm/yr. At about 24 Ma the wedge advance slowed down and thickened by underplating crystalline basement of the foreland plate. The value for the transport coefficient has been estimated from previous studies. Prior to attempting to simulate the broad‐scale geometry of the NAFB the role of each parameter was assessed individually. The values for Te and K are held constant throughout the simulation of the NAFB at 7.5 km and 500 m²/yr, respectively. The geometry of the base Burdigalian unconformity is reproduced by variations in the parameters describing the thrust wedge. The surface slope angle of the wedge is increased from 2.5° to 4° over 0.2 m.y., and the rate of thrust advance is decreased from 2 mm/yr to 0.2 mm/yr, during the thickening event between 24 and 23.8 Ma. The rejuvenation of the internal parts of the thrust load causes backtilting of the foreland basin sediments and, after a time lag, erosion of the distal stratigraphy over the forebulge, so simulating the base Burdigalian unconformity.
Constraints on unroofing rates in the High Himalaya, eastern Nepal
Authors: Hubbard Mary , Royden Leigh , Hodges Kip
Published: February 2008

Thermobarometric data for samples across the Main Central thrust zone in eastern Nepal show an inversion in temperature but not in pressure. These data have been interpreted to represent a portion of the paleogeotherm at the time of Main Central thrust deformation. A 40Ar/39Ar age on hornblende (closure temperature (Tc)=500±50°C) constrains the timing of this deformation to be ∼21±0.2 Ma. The 40Ar/39Ar ages of other minerals (muscovite, Tc=350°C, age (t)=12.0±0.2 Ma; K‐feldspar, Tc=220°C, t=8.0±0.2 Ma) from the same location further constrain the cooling history of this region. Together the geochronologic and thermobarometric data yield an average unroofing rate of 1.2±0.6 mm/yr for the High Himalaya of eastern Nepal.

Simple thermal models show that these geochronologic and thermobarometric data are consistent with a wide range of different initial geotherms, applied boundary conditions and magnitude of radiogenic heat production. The variation through time of the unroofing rates can only be poorly constrained, however. The unroofing histories were found to be largely insensitive to the details of the assumed initial geotherm, fairly sensitive to the magnitude of radiogenic heat production, and extremely sensitive to the nature of the boundary conditions applied below the fault zone. This study underscores the difficulty in constraining uplift histories on the basis of cooling rates even when thermobarometric data are available to supplement geochronologic constraints on the cooling history of the region.
Profile observations of long‐lived trace gases in the Arctic vortex
Authors: Schmidt U. , Bauer R. , Khedim A. , Klein E. , Kulessa G. , Schiller C.
Published: February 2008

Five sets of vertical profiles of long‐lived trace gases were measured in the stratosphere at high northern latitudes (68° N) during three field campaigns of the CHEOPS ‐ Project. Large whole air samples were collected by means of balloon‐borne cryogenic samplers analysed for their content of N2O, CH4, CFCl3, CF2Cl2, C2F3Cl3, CCl4, CH3Cl and CH3CCl3. The measured polar profiles will be compared with mean vertical distributions derived from a series of observations at midlatitudes (44° N). The difference indicates a pronounced effect of subsidence in the Arctic winter stratosphere with a net downward shift of the Arctic vertical profile by about 7 km over the time period from November until February. Owing to this efficient downward transport, the abundance of chlorine in the form of reservoir and reactive species, is increased in the lower Arctic stratosphere to about twice that observed at midlatitudes.
Observations of PSCs in polarized light
Authors: Herman M. , Santer R. , Gonzalez L. , Lecomte P. , Verwaerde C.
Published: February 2008

A balloon borne polarimetric experiment, RADIBAL, was launched from Kiruna to observe the properties of polar stratospheric cloud particles, from limb scannings of the radiance and of the polarization of the scattered sunlight. On February 4, 1990 during the CHEOPS 3 campaign, when the stratospheric temperature was about 190 K, a polar stratospheric cloud (PSC) was sampled. At the top of the PSC, an upper thin layer no more than 2 km thick, was shown to be composed of rather large, micron sized particles, as assessed from the forward scattering feature. Difficulties in retrieving the polarization from calculations for spherical particles suggest that they might be crystalline particles. Below this layer, the PSC consisted very probably of spherical particles. In this part of the cloud, the polarization features indicate that, from the lower to the upper levels, the size distribution of the particles narrowed, suggesting a preferential growth process on the smaller class of particles. This preliminary analyzis, however, is unable to derive a simple model that makes definitively consistent the radiance and the polarization observations, so that the results have to be examined cautiously within the context of prevailing PSC models.
Evolution of fluid driving forces and composition within collisional orogens
Authors: Koons P. O. , Craw D.
Published: February 2008

Subaerial collisional mountain belts have a predictable asymmetric two‐sided wedge geometry. The major driving forces for crustal fluid flow in this framework arc thermal and topographic gradients. It is possible to predict both a time‐averaged fluid state and the evolution of the fluid regime during orogenesis, with three distinct fluid flow regimes: the outboard toe where material is incorporated into the orogen, a transitional zone of concentrated deformation, and a zone of rapid exhumation along the inboard slope adjacent to the indentor. As rocks move into the outboard wedge, relatively low temperature, compaction and head driven fluid flow dominates. Mid‐crustal material undergoes disequilibrium dehydration as rock‐fluid packets move into the transition zone, producing a dominantly aqueous ‘metamorphic fluid’. In the inboard region, rapid uplift produces geothermal gradients in excess of 80°C/km which encourage vigorous free convection. Advecting fluids evolve through immiscibility towards increasing CO2 content during uplift of the rock‐fluid packets. Mixing of thermally driven and head driven fluids occurs at depths of 5–10 km. The Southern Alps of New Zealand, the European Alps and the Himalaya represent variants of this orogenic‐hydrothermal system.
Anomalous ion layers in the high‐latitude winter E region
Authors: Kirkwood S.
Published: February 2008

The characteristics of an anomalous ionisation layer appearing during winter daytime at 69.6°N, 19.2°E are described. The layer is located at about 100 km altitude, is about 15 km thick and appears not to result from energetic particle precipitation. It has been observed on 80% of winter days when measurements have been made and was usually present for several hours on each of those days, whenever the solar zenith angle was less than 93°. The layer cannot be explained by ionisation of the major atmospheric constituents by solar EUV or X‐ray radiation. Possible explanations in terms of the minor constituents NO and metallic atoms are considered but much larger number densities of those constituents would be required than have been observed.
Singularities in mass‐loaded MHD flow: The cometary bow shock
Authors: Khabibrakhmanov I. Kh. , Coates A. J. , Galinsky V. L.
Published: February 2008

We present a one‐dimensional model of the mass‐loading of the solar wind by cometary ions which predicts a singularity in the mass‐loaded flow at M=2. Further, a subshock occurs when the flow speed reaches M≈ 1.15. The shape of the cometary bow shock in two dimensions is predicted, by requiring that the flow Mach number of the shock is 2 taking the velocity component normal to the shock surface. The Mach number results compare favourably with observations at comet Halley.
Non thermal nitrogen atoms in the Earth's thermosphere 1. Kinetics of hot N(4S)
Authors: Shematovich V. I. , Bisikalo D. V. , Gérard J. C.
Published: February 2008

A nonequilibrium kinetic model is used to calculate the energy distribution of translationally hot nitrogen atoms in the thermosphere. It is found that dissociation by solar EUV photons and photoelectrons provide an important source of non‐maxwellian high energy N(4S) atoms. We show that quenching of metastable N(2D) atoms by atomic oxygen also produces significant amounts of hot N(4S) atoms. A numerical simulation of the hot N(4S) atom kinetics show that these atoms are present in sufficient quantity in the thermosphere to play a role in the odd nitrogen chemistry and energetics of this atmospheric region. The steady state population of hot nitrogen is obtained by taking into account the relaxation by elastic collisions and high energy reaction with O2 to form nitric oxide. It is found that the fraction of hot N atoms reacting with O2 is about 15% of the production rate of atomic nitrogen by N2 dissociation by photon and photoelectron impact and quenching of N(2D) metastable atoms by atomic oxygen.
Large scale lunar horizon glow and a high altitude lunar dust exosphere
Authors: Zook Herbert A. , McCoy James E.
Published: February 2008

All three Apollo 17 astronauts sketched a “lunar horizon glow,” seen from orbit above the Moon. It is shown that the shape of the glow is compatible with scattering of sunlight off of gas or dust at high altitudes above the Moon. Our mathematical modeling best simulates the glow with submicron dust grains whose spatial density varies with altitude above the Moon as exp(‐H/Hs), where Hs is in the range of 5 to 20 km. These dust grains are probably electrically charged and ejected above the lunar surface by local electric fields.
Sulfur isotope measurements of submicrometer sulfate aerosol particles over the Pacific Ocean
Authors: Calhoun Julie A. , Bates Timothy S. , Charlson Robert J.
Published: February 2008

The climatic importance of atmospheric submicrometer sulfate aerosol particles necessitates a thorough understanding of the various sources of sulfur to the marine atmosphere [Charlson et al., 1987; Coakley et al., 1987; Charlson et al., 1990]. Over the remote ocean, the primary source of atmospheric non‐seasalt (NSS) sulfate is thought to be marine emissions of dimethylsulfide (DMS) [Charlson et al., 1987]. Here we report the results of a study which used stable sulfur isotopes to test this hypothesis. NSS sulfate in submicrometer aerosol particles over remote parts of the South Pacific Ocean had an average δ34S value of +15.6±3.1‰, relative to the Canyon Diablo standard. The sulfur isotopic composition of these particles was 5.5‰ lighter than sulfate derived directly from seawater, but isotopically heavier than sulfate derived from most continental sources. This observed isotopic fractionation between seawater sulfate and NSS sulfate is consistent with the isotopic fractionation predicted for the transformation of seawater sulfate to atmospheric NSS sulfate via a DMS pathway (−1 to +7‰) and hence supports the hypothesis that DMS can be the source of the NSS sulfate.
Jurassic paleomagnetic constraints on the collision of the North and South China Blocks
Authors: Yang Zhenyu , Courtillot Vincent , Besse Jean , Ma Xinghua , Xing Lisheng , Xu Shujin , Zhang Jingxing
Published: February 2008

This paper reports a paleomagnetic study of Jurassic rocks from the Ordos Basin in the North China Block (NCB). A recent remagnetization and a high‐temperature and/or high‐coercivity component with dual polarities have been isolated. The Middle Jurassic pole (74°N, 233°E, A95=5°) is roughly consistent with previous results, and the Lower Jurassic pole (82°N, 286°E, A95=7°) is located between the late Triassic and Middle Jurassic poles. These results, combined with paleomagnetic results from adjacent blocks, suggest that 1) the pole for the NCB suffered an angular displacement of 47°+/− 8° between the late Triassic and Middle Jurassic, 2) the SCB underwent final accretion to the NCB in the Middle Jurassic, and 3) accretion of the NCB to Siberia was not complete until the late Jurassic and possibly even until the early Cretaceous.
The role of thermal vapor diffusion in the subsurface hydrologic evolution of Mars
Authors: Clifford Stephen M.
Published: February 2008

Various lines of geomorphic evidence suggest that large bodies of groundwater were present on Mars throughout much of its early geologic history. In this paper the hydrologic response of groundwater to the thermal evolution of the early martian crust is considered. When a temperature gradient is present in a moist porous medium, it gives rise to a vapor pressure gradient that drives the diffusion of water vapor from regions of high to low temperature. By this process, a geothermal gradient as small as 15 K km−1 could drive the vertical transport of 1 km of water to the freezing front at the base of the martian cryosphere every 106–107 years, or the equivalent of ∼10²–10³ km of water over the course of martian geologic history. Models of the thermal history of Mars suggest that this thermally‐driven vapor flux may have been as much as 3–5 times greater in the past. The magnitude of this transport suggests that the process of geothermally‐induced vapor diffusion may have played a critical role in the initial emplacement of ground ice and the subsequent geomorphic and geochemical evolution of the martian crust.
Plasma structure near the low‐latitude boundary layer: A rebuttal
Authors: Sckopke Norbert
Published: February 2008

A recent reanalysis of a well‐documented interval of plasma and magnetic field data led its authors to offer a new model for the structure of the outer magnetosphere and the magnetosheath on the northern dawnside. On November 6, 1977, ISEE 1 and 2 had observed a series of quasi‐periodic pulses of magnetosheath‐like plasma on northward oriented magnetic field lines which were originally interpreted as repeated encounters of a pulsed low‐latitude boundary layer inside a smooth magnetopause followed by a single outward crossing of the magnetopause. D. G. Sibeck and coworkers reinterpreted the ISEE observations as being due to quasi‐periodic magnetopause motion causing the satellites to repeatedly exit the magnetosphere and to observe draped northward magnetosheath magnetic field lines in the plasma depletion layer. Their model is based on qualitative arguments concerning the amount of field line draping in the magnetosheath as well as the behavior of energetic electrons near the magnetopause. It is shown in this paper that both arguments are not in accordance with the available evidence.
A theoretical study of thermospheric composition perturbations during an impulsive geomagnetic storm
Authors: Burns A. G. , Killeen T. L. , Roble R. G.
Published: February 2008

The compositional response of the neutral thermosphere to an impulsive geomagnetic storm has been investigated using a numerical simulation made with the National Center for Atmospheric Research thermospheric general circulation model (NCAR‐TGCM). Calculated time‐dependent changes in neutral thermospheric composition have been studied, together with detailed neutral parcel trajectories and other diagnostic information from the model, to gain a greater understanding of the physical mechanisms responsible for composition variability during geomagnetic storms and, in particular, to investigate the causes of the positive and negative ionospheric storm effects. The following principal results were obtained from this study. (1) Calculated perturbations in thermospheric composition following the onset of an impulsive geomagnetic storm were found to be in good qualitative agreement with the previous experimental statistical study of storm time thermospheric morphology by Prölss (1981). (2) During the initial (onset) phase of the simulated storm, upward vertical winds occurred in the auroral zone and downward winds occurred in the central magnetic polar cap. (3) The largest perturbations in mass mixing ratio of nitrogen (ΨN2) at F region altitudes were found to be associated with parcels of neutral gas that travelled through the cusp region and with parcels that were trapped within the auroral zone for a long time. (4) Storm time enhancements in ΨN2 were found to occur in the midnight and early morning sectors both within and equatorward of the auroral zone, and these were determined to be associated with the advective effects of the large antisunward polar cap neutral winds. The physical size of the region of enhanced ΨN2 values was determined to be related to the time taken for individual parcels of air to corotate out of the large day‐to‐night thermospheric wind jet, and thus to the magnitudes of the antisunward polar cap winds. (5) Decreases in ΨN2 were found to occur in regions of downward vertical winds and in regions into which relatively nitrogen‐poor air was advected by the horizontal wind. (6) Calculated compositional perturbations following the onset of the storm were found to propagate to mid‐latitudes due to the advective effects of equatorward winds, leading to the mid‐latitude positive ionospheric storm effects. This last result is in disagreement with the earlier study of Rishbeth et al. (1985), who found the compositional disturbance to be largely restricted to the auroral zone on a constant‐pressure surface. However, it is consistent with the more recent study undertaken by Fuller‐Rowell et al. (1991), who demonstrated that composition changes were seen well beyond the auroral oval.
Excitation of sidebands due to nonlinear coupling between a VLF transmitter signal and a natural ELF emission
Authors: Sotnikov V. I. , Fiala V. , Lefeuvre F. , Lagoutte D. , Mogilevsky M.
Published: February 2008

Symmetric sidebands are observed in the ionosphere by the AUREOL 3 satellite when it passes at a height of 1200 km above the VLF transmitter at the Komsomolsk‐on‐Amur Alpha station (50°5 N, 135° E, frequency 11.90 and 12.65 kHz). The sidebands are about 500 Hz off the carrier frequency of Alpha pulses. They are approximately 20 dB lower than the transmitter signal, and they appear only when ELF natural emission above the local proton gyrofrequency is observed. The data are presented and analyzed. The nonlinear coupling of the VLF transmitter signal to natural ELF emission is invoked to explain the symmetric sidebands. It is shown that the nonlinear current excited by the beats of VLF and ELF waves is strong enough to explain the sideband amplitude.
The OEDIPUS Experiment: Analysis of the current/voltage data
Authors: Godard R. , James H. G. , Laframboise J. G. , Macintosh B. , McNamara A. G. , Watanabe S. , Whalen B. A.
Published: February 2008

The OEDIPUS A rocket was successfully launched from the Andoya Rocket Range on January 30, 1989, toward a moderate auroral arc. In this experiment, the forward and the aft payloads constituted a tether. The forward and the aft payloads were connected through a thin isolated wire and the two subpayloads were used as a plasma diagnostic tool. Here we examine the results obtained in the current/voltage mode, when the two subpayloads act as a large double probe. In this mode, a voltage sweep from −8 V to +8 V was applied between the two payloads. The measured current/voltage characteristic has been fitted with a theoretical model of the current collected to extract the plasma parameters. The results indicate (1) a temporary contamination of the surface of the payload by the argon release system, (2) a moderate and reasonable electron temperature and (3) good correspondence with the results of the voltmeter for the measurement of the natural and induced electric fields.
Cosmic ray unidirectional latitude gradient: Evidence for north‐south asymmetric solar modulation
Authors: Chen Jiasheng , Bieber John W. , Pomerantz Martin A.
Published: February 2008

The cosmic ray diurnal anisotropy is determined separately in toward and away sectors of the interplanetary magnetic field through analysis of neutron monitor records spanning the period 1953–1988. We suggest that the dominant cause of differences (between sectors) of the anisotropy is a unidirectional latitude gradient, which is indicative of a north‐south asymmetry in the solar modulation of cosmic rays. The magnitude of the latitude gradient deduced from the observations is typically of the order of 0.7%/AU, and its direction varies in a manner not obviously related to either the 11‐year sunspot cycle or the 22‐year solar magnetic cycle. Further support for the reality of a unidirectional gradient is provided by an analysis of the cosmic ray density segregated according to sector polarity. The density is found to differ significantly in the two sector types, and the amount of difference is correlated with the concurrently measured unidirectional latitude gradient in the expected manner. We examine two hypotheses to account for a north‐south asymmetry of solar modulation: (1) that the asymmetric modulation results from a north‐south asymmetry in the winding angle of the interplanetary magnetic field spiral and (2) that the asymmetric modulation is related to a corresponding north‐south asymmetry of solar activity as indicated by sunspot number. Both hypotheses are consistent with the data, suggesting that multiple factors may give rise to north‐south asymmetric solar modulation.
Ulf waves upstream of the Venus bow shock: Properties of one‐hertz waves
Authors: Orlowski D. S. , Russell C. T.
Published: February 2008

Pioneer Venus orbiter data are used to examine the properties of a class of ULF upstream waves with relatively high observed frequencies (1.1–1.3 Hz). In the spacecraft frame these waves are most often left‐hand elliptically polarized. They have amplitudes up to 3 nT (near the bow shock) and propagate obliquely to the magnetic field at angles from 10° to 50°. These waves show significant similarity in their properties to “one‐Hertz” waves identified at the Earth in the ISEE 1 and 2 observations and the whistler waves identified earlier with IMP 6 observations. The waves appear almost immediately after the spacecraft crosses the magnetic field tangent line to the bow shock surface into the region of connected field lines. The amplitude of these waves decreases with distance from the shock measured along the magnetic field line. We have used the cold plasma dispersion relation in order to study the propagation of these waves and to explain their observed polarization. Calculated group velocities indicate that those waves have sufficient upstream velocities to propagate from the shock into the solar wind. The totality of observations, including the observed wave damping and the observation of right‐handed waves, seems best explained by a source of right‐handed whistler mode waves at the bow shock.
Estimates of solar wind velocity gradients between 0.3 and 1 AU based on velocity probability distributions from Helios 1 at perihelion and aphelion
Authors: Arya Sharda , Freeman John W.
Published: February 2008

We have estimated the average velocity gradients for the solar wind protons by comparing the velocity probability distributions at 0.3 and at 1 AU Assuming a power law radial dependence, we find, for a full 6‐year data set, that the gradients for the lowest velocity ranges are about R0.1 to R0.14 and that the power index decreases steadily with increasing velocity until the slope is near zero for the high‐speed solar wind. However, upon examining the solar cycle dependence we find that this trend for the velocity gradient to decrease with increasing velocity is a characteristic primarily of the increasing solar activity and solar maximum period and is almost absent in the solar minimum data. The solar wind above 500 km/s during solar minimum shows an average acceleration similar to the slow wind, about 55 to 85 km/s/AU. On the other hand, winds above 350 km/s from the period of increasing solar activity and solar maximum show essentially no average acceleration beyond 0.3 AU.
Comparison of solar wind velocity measurements with a theoretical acceleration model
Authors: Coles W. A. , Esser Ruth , Løvhaug Unni‐Pia , Markkanen Jussi
Published: February 2008

Interplanetary radio scintillation (IPS) measurements of the solar wind velocity were made using the receiving antennas of the European Incoherent Scatter Facility (EISCAT) radar system in northern Scandinavia from June through October 1990. The observations, which cover the distance range from. 11 to 90 Rs from Sun center, were taken with sufficient density to measure the same stream at two (or more) different distances. The deduced velocities are in the range 100 ≲ u ≲ 540 km s−1. We selected from 192 observations, 16 examples of streams observed with good radial alignment, of which 12 were observed unchanged for several days. The measured velocities are compared with calculations based on a two‐fluid solar wind model with Alfvén waves. In eight cases the measurements are in good agreement with the model when a moderate amount of wave energy is added to the flow. In four cases the observed streams show low or moderate velocities below, say, 20 Rs but then accelerate fast at larger distances from the Sun. This delayed acceleration is much steeper than the acceleration in the model at these distances. In the remaining four cases the streams seem to reach their final velocities much closer to the base than in other cases, and they are not observed to accelerate much between 10 and 90 Rs. At these distances all related solar wind models we have seen give the same results; they all fit half the data, and none can fit the other half. We do not see an obvious modification to these models which will allow us to fit either the delayed acceleration or the early acceleration cases.
Compressional ULF waves in the outer magnetosphere: 1. Statistical study
Authors: Zhu Xiaoming , Kivelson Margaret G.
Published: February 2008

Using 14 months of magnetic field and plasma data observed by the ISEE 1 and 2 spacecraft, ULF waves of period 2–20 min in the outer magnetosphere were studied. Statistical properties of the ULF waves can be summarized as follows: (1) Intense compressional waves are a persistent feature near the two flanks of the magnetosphere. They are mainly polarized in a meridian plane with comparable compressional and transverse amplitudes and have larger amplitude at magnetic latitudes below 20° than at higher latitudes. The magnetic pressure perturbations for the waves are in antiphase with the plasma pressure perturbations; (2) Transverse waves polarized in the azimuthal direction (azimuthal waves) are found to be mainly a nightside phenomenon. Their appearances on the nightside magnetosphere seem to be associated with substorm activity; (3) Compressional wave power and plasma β, the ratio between the plasma pressure and the magnetic field pressure, are correlated, but ULF wave power in the above noted period range is not significantly correlated with AE, plasma pressure or VA; (4) The compressional waves are most likely to be generated internally in the regions where plasma β and field line curvature are large.
Composition of solar flare noble gases preserved in meteorite parent body regolith
Authors: Rao M. N. , Garrison D. H. , Bogard D. D. , Badhwar G. , Murali A. V.
Published: February 2008

The isotopic composition (long‐term average) of solar flare (SF) Ne has been determined by three isotope correlation techniques applied to data measured on chemically etched pyroxene separates prepared from the Kapoeta meteorite, which is known to contain implanted solar gases. The SF 20Ne/22Ne ratio obtained is 11.6±0.2 and confirms previous determinations of this SF ratio in lunar and meteoritic samples. The same SF Ne composition is also obtained by applying an ordinate intercept technique to the same data set. The ordinate intercept technique was also applied to the Ar and He data, on which the three‐isotope correlation technique cannot be applied. The isotopic composition of SF Ar and SF He so obtained are SF 36Ar/38Ar=4.9±0.1 and SF 4He/³He=3800±200, which are significantly different from the solar wind (SW) Ar and SW He values of ∼5.35 and ∼2500, respectively. Correlation between 20Ne/22Ne and 36Ar/38Ar for the same data set also gives a similar SF 36Ar/38Ar ratio of 4.8 ± 0.2. The determined SF He, Ne and Ar isotopic ratios differ from those in SW by 52%, 17% and 9%, respectively, but the elemental compositions of 4He/36Ar and 20Ne/36Ar do not show obvious differences between SF and SW. The concentration of the SF component in Kapoeta pyroxenes is ∼20% that of the SW component, which is orders of magnitude higher than expected from SW and SF proton flux measurements. Variations in elemental and isotopic composition of He, Ne and Ar in SF relative to SW are found to correlate well with a (Z/A)² dependence, indicating a rigidity‐dependent particle spectrum in solar flares.
Excitation of electrostatic wave instability by dc electric field in Earth's magnetoplasma
Authors: Mishra S. P. , Misra K. D. , Pandey R. P. , Singh K. M.
Published: February 2008

The dispersion relation for electrostatic wave propagation in an anisotropic warm collisionless magnetoplasma, in the presence of weak parallel (d c) electric field, has been derived analytically. An expression for the growth rate of the electrostatic wave and the marginal stability condition are also derived. The modifications introduced in the growth rate by the electric field and the temperature anisotropy are discussed using plasma parameters observed in the magnetospheric region (4 < L < 10). The effect of the electric field is to increase the growth rate of the electrostatic waves at different electron cyclotron harmonics, whereas the effect of the temperature anisotropy is to decrease the growth rate. The presence of parallel electric field may excite the electrostatic emissions at different electron cyclotron harmonics. The most unstable band of wave frequencies obtained with the aid of computations lies between 5 kHz and 10 kHz. These wave frequencies are well within the experimentally observed frequencies of electrostatic emissions. Therefore such a study would not only explain the observed satellite features of the electrostatic wave emissions but would also account for the diagnostics of the magnetospheric plasma parameters.
Possible wave amplitudes in shocks in the solar corona: Predictions for solar probe
Authors: Moses S. L. , Coroniti F. V. , Greenstadt E. W. , Tsurutani B. T.
Published: February 2008

Shocks and Alfven turbulence are frequently invoked as mechanisms to heat the solar corona and accelerate the solar wind. A primary objective of the Solar Probe Mission is to detect and characterize these nonthermal dissipation and energization processes. Although the solar wind in the corona is usually assumed to be sub‐Alfvenic, temporal and spatial variability in the plasma parameters could lead to conditions under which weak, fast mode shocks might form. ISEE 3 data from the flanks of the Earth's bow shock show that even low Mach number, quasi‐parallel shocks generate large‐amplitude Alfven turbulence with δB/B of the order of 1, and intense high‐frequency electrostatic plasma waves. Using a model of the corona and scaling parameters to those expected in the regions to be traversed by Solar Probe (r ≥ 4 Rs), we suggest it is possible that such shocks might produce Alfven turbulence with magnetic field amplitudes up to the order of 0.1 G and electric field amplitudes up to the order of 1 V/m; electrostatic waves near 100 kHz may have amplitudes of 0.1 V/m. Since the shock‐generated Alfven waves would be Doppler shifted to frequencies of a few kilohertz because of the high spacecraft velocity at perihelion, detection of these waves imposes severe requirements on the designs of plasma wave and magnetic field sensors on Solar Probe.
Low‐energy particle layer outside of the plasma sheet boundary
Authors: Parks G. K. , Fitzenreiter R. , Ogilvie K. W. , Huang C. , Anderson K. A. , Dandouras J. , Frank L. , Lin R. P. , McCarthy M. , Rème H. , Sauvaud J. A. , Werden S.
Published: February 2008

The ISEE spacecraft in the geomagnetic tail frequently crossed the high‐latitude boundary of the plasma sheet. On a number of these crossings on the morningside (between 15 RE and 22 RE) the ISEE instruments detected an enhanced population of low‐energy electrons and ions immediately adjacent to the plasma sheet boundary layer (PSBL). The electrons in this low‐energy layer (LEL) have energies less than a few hundred eV, and they are aligned along the magnetic field direction propagating in the tailward direction. The ions have energies less than 100 eV and are also streaming along the magnetic field direction but in the earthward direction. These particles are clearly distinguished from the bulk of the particles in the plasma sheet and the PSBL. These observations may help clarify where the various particle features in the geomagnetic tail map to in the ionosphere. It is suggested that the LEL maps to the soft (<1 keV) electron precipitation region poleward of the plasma sheet boundary.
Electrostatic potential jump across fast‐mode collisionless shocks
Authors: Mandt M. E. , Kan J. R.
Published: February 2008

The electrostatic potential jump across fast‐mode collisionless shocks is examined by comparing published observations, hybrid simulations, and a simple model in order to better characterize its dependence on the various shock parameters. In all three, it is assumed that the electrons can be described by an isotropic power law equation of state. The observations show that the cross‐shock potential jump correlates well with the shock strength but shows very little correlation with other shock parameters. Assuming that the electrons obey an isotropic power law equation of state, the correlation of the potential jump with the shock strength follows naturally from the increased shock compression (Δn) and an apparent dependence of the power law exponent on the Mach number which the observations indicate. We find that including a Mach number dependence for the power law exponent in the electron equation of state in our simple model, as suggested by the observations, produces a potential jump which better fits the observations. On the basis of the simulation results and theoretical estimates of the cross‐shock potential, we also discuss how the cross‐shock potential might be expected to depend on the other shock parameters, namely, βe, and θBn Given the broad range of parameters lumped together in the observational results, we feel that the qualitative agreement between them and the simulations is quite good and can be used to better elucidate the nature of the discrepancies.
On the possibility of auroral remote sensing with the Viking ultraviolet imager
Authors: Steele D. P. , McEwen D. J. , Murphree J. S.
Published: February 2008

An investigation was carried out to assess the value of ultraviolet auroral images for remote sensing of electron precipitation. We compared auroral images, obtained by both cameras of the Viking Ultraviolet Imager during April and early May 1986, with simultaneous measurements of electron precipitation from the DMSP F7 and HiLat satellites at low altitudes above the auroral zone. The camera signals in the pixels that viewed the low‐altitude satellite footprint were corrected for various instrumental and nonauroral background contributions and were converted to the equivalent signals for vertical viewing from Viking. The electron data were averaged over the image pixels and were used to normalize the imager signals to unit electron energy flux. The resulting quantity, here termed the “effective sensitivity,” showed large scatter about the mean values for both cameras that tended to mask any energy dependence except a decrease at electron energies above 6 keV. The mean effective sensitivities were 11.3 ± 1.1 digitization numbers (DN)/(erg cm−2 s−1) for the Lyman‐Birge‐Hopfield (LBH) camera and 15.9 ± 2.5 DN/(erg cm−2 s−1) for the 1304 camera. These values correspond to 39 and 55 DN per kR of λ 4278 (N2+ first negative (1 N) (0,1) band), respectively, for the two cameras and are nearly independent of electron energy below about 6 keV. A published model calculation of the LBH camera response overestimates the effective sensitivity by about 40%. The signal ratio in simultaneous images from both cameras was insensitive to electron energy in the two cases examined, consistent with the weak energy dependences of the effective sensitivities. Unusually high LBH camera effective sensitivities of 45–60 DN/(erg cm−2 s−1) were obtained during several orbits on May 2 and 3. These observations could not be accounted for by thermospheric composition changes during the geomagnetic storm on May 1 and 2 or by changes in the intrinsic gain of the camera. Some of the scatter in the measured effective sensitivities resulted from approximations required in comparing the images and particle data, which differed significantly in spatial scale. We conclude that images from either Viking camera may be used to infer the instantaneous distribution of electron energy deposition, with roughly 50% accuracy.
Outer heliospheric radio emissions: 1. Constraints on emission processes and the source region
Authors: Cairns Iver H. , Gurnett Donald A.
Published: February 2008

The Voyager 1 and 2 spacecraft observed low‐frequency radio emissions near 2 and 3 kHz during the interval 1983–1987 while at heliocentric distances from 15 to 27 AU and 11 to 20 AU, respectively. We consider the detailed theoretical and observational requirements for this radiation to be produced near multiples of the plasma frequency ƒp by nonlinear, weak turbulence, wave‐wave processes involving electrostatic Langmuir waves. Constraints on the emission processes and source characteristics are discussed. The minimum brightness temperature of the radiation, 3×1014K, requires the Langmuir waves to be generated by a plasma instability, most plausibly by an electron beam instability. This requires electron acceleration and beam production in the near vicinity of the source. Minimum Langmuir wave electric fields in the source region, based solely on the kinematics of the radiation processes, lie in the range 1–100 μV m−1 for nominal source and electron beam parameters. Path lengths for fundamental and harmonic emission processes are calculated: the observed levels of radiation can be produced in path lengths (and source dimensions) smaller than 1 AU provided the participating Langmuir waves have effective temperatures TL ∼ 1017K. These Langmuir wave levels are plausible, based on observations in planetary foreshocks and theoretical calculations. Previous source models for the radiation are discussed. Suggestions that the radiation is generated on the downstream side of the inner heliospheric shock or in the vicinity of the heliopause are shown to face severe theoretical and observational problems, based on present knowledge of the outer heliosphere. However, the upstream (sunward) side of the inner heliospheric shock is predicted theoretically to have a foreshock region containing electron beams and associated Langmuir waves. Theoretically this source region can plausibly produce radiation at multiples of ƒp with the observed brightness temperatures.