bluuebunny/crossref_metadata_2025 huggingface dataset:
Temporal study of wake formation behind a conducting body
Authors: Meassick S. , Chan C. , Qian Y. , Sroda T. , Azar T. , Allen R. C.
Published: February 2008

The temporal evolution of the wake of a conducting body is studied experimentally in a pulsed plasma device. Three‐dimensional measurements of the plasma potential, density, particle energy distribution, and ion currents are measured throughout the near‐ and mid‐wake regions during the wake formation. It is found that the potential behind the conducting body is initially negative. This negative potential is caused by the higher mobility of the electrons, allowing them to flow into the ion free wake region. The negative potential in the wake region induces an electric field that pulls ions into the region behind the conducting body. However, the dominant factor in determining the length of the near wake is the thermal energy spread of the ions. At later times, as the sheath forms around the conducting body, ions are deflected by the potential gradient in the sheath region. This deflection, in addition to the thermal energy spread of the ions, determines the length of the near wake.
The evolution of field‐aligned currents as a function of substorm phase
Authors: Chun Francis K. , Russell Christopher T.
Published: February 2008

ISEE 1 and 2 measurements of magnetic changes across nightside field‐aligned currents at mid altitudes in the inner magnetosphere are examined to determine the average ionospheric extrapolated characteristics and properties of region 1 and 2 currents as a function of substorm phase. The properties of these currents studied include current intensity, density, layer width, and velocity and are consistent with earlier studies at lower altitudes. In general, region 1 and 2 currents behave differently during substorm growth and expansion, but are comparable in the recovery phase. During a substorm, region 1 current intensity ranges from 0.4 to 0.6 A/m and peaks during the expansion phase. Region 2 intensity varies from 0.15 to 0.35 A/m and reaches a maximum during recovery. The density of region 2 currents remains essentially steady at 1.4 μA/m², while region 1 decreases from 5 to 1.3 μA/m². The width of these current sheets throughout a substorm remains in the range from 100 to 500 km. The velocity of these sheets is for the most part inwards with speeds from 50 to 200 m/s. Even though these current sheet crossings were not originally selected as substorm associated events, they all occurred during one of the phases of a substorm. This indicates that substorms play a major role in the generation of nightside region 1 and 2 currents detected by the ISEE spacecraft. These results are consistent with a model in which these currents flow to couple the outer magnetosphere and the ionosphere in response to the stresses applied by both dayside and nightside reconnection. Region 1 currents are responsive to variations in the stress caused by both reconnection regions, whereas region 2 currents are responsive to only nightside reconnection.
Solar influence on ELF‐VLF emission observed at conjugate stations
Authors: Sato Natsuo , Sasahara Toshiya , Suzuki Hiroyuki , Maezawa Kiyoshi , Saemundsson Thorsteinn
Published: February 2008

Characteristics of the long‐term variation of daytime (06–16 hours MLT) ELF‐VLF emissions at 750 Hz and 2 kHz are examined, using 7 and 4 years of digital data obtained at a conjugate pair of stations, Syowa in Antarctica and Husafell in Iceland. The emission intensity shows obvious quasi‐periodic fluctuations, with periods of 1 day (maximum around noon), ∼27 days, 1 year (minimum around local winter), and ∼11 years (not yet confirmed but reasonably apparent in the 7 years of data). The diurnal and seasonal variations are strongly dependent on sunlight in the topside ionosphere. The emission intensity is also affected by geomagnetic activity with quasi‐periodicities of ∼27 days and ∼11 years. It is suggested that sunlight in the ionosphere affects the propagation of waves whereas the solar activity (geomagnetic activity) affects their generation.
Ionospheric disturbances at the equatorial anomaly crest region during the March 1989 magnetic storms
Authors: Huang Yinn‐Nien , Cheng Kang
Published: February 2008

On March 6,1989, the largest sunspot group since 1982 came into view as it moved out of the eastern limb of the Sun. It was highly active during March 8–18, and a great many transient ionospheric and geomagnetic variations were triggered by this sunspot group. The intensive ionospheric observations at Lunping Observatory and Chungli Ionosphere Station during this period recorded 30 solar flares manifested as shortwave fade‐outs, sudden frequency deviations, and solar flare effects and three storm sudden commencement (SSC)‐type geomagnetic storms, among which the March 13 SSC‐type geomagnetic storm triggered an unusually severe ionospheric disturbance. The ionospheric total electron content, the critical frequency of the F2 layer, f0F2, and the virtual heights at given frequencies all show wavelike up‐and‐down oscillations of the ionosphere. This oscillatory ionospheric motion is explained as due to the compression and expansion of the plasmasphere.
Waves in Venus's middle and upper atmosphere: Implications of Pioneer Venus Probe data above the clouds
Authors: Seiff Alvin , Kirk Donn B.
Published: February 2008

Probe data showing the presence of waves in Venus's middle and upper atmosphere [Seiff et al., 1980; Seiff and Kirk, 1982] are critically reevaluated and extended to 138 km, near the level of in‐situ data taken by the Pioneer Venus orbiter. Uncertainties in temperature are determined. They are typically about 0.1 times amplitude, thus supporting the reality of large amplitude oscillations approaching 40 K at 120 km. Growth rates above 100 km follow approximately the inverse square root of density until “saturation” occurs (in the sense that lapse rates become adiabatic in the expanding segment of the wave). The waves then break at the 120 km level, providing a source for the “friction” required in models to match the observed day‐night temperature contrast in Venus's lower thermosphere [Seiff, 1982; Bougher, 1984]. The data correlate to an unexpected degree with temperatures from the Pioneer Venus orbiter atmospheric drag (OAD) experiment taken at altitudes of 140 to 165 km, which, especially for the night probe, extend not only the mean temperature structure, but also the oscillation structure of the probe data at the same local Venus time. OAD temperatures depend on local Venus time and altitude, but, in the limited number of observations, appear independent of observing date over periods of up to 11 days, and correlate as described with probe data taken 65 to 137 days earlier. These observations lead to the suggestion that the thermospheric waves are solar‐fixed, induced either by the major subsidence across the terminators or as continuations upward of waves in the middle atmosphere. The wave structure in the large probe sounding below 100 km is similar to, but does not quantitatively support the solar‐tidal model of Pechmann and Ingersoll, which gives much larger amplitudes and different wave phases.
Effects of dawn‐dusk pressure asymmetry on convection in the central plasma sheet
Authors: Lui W. W. , Rostoker G.
Published: February 2008

The central plasma sheet (CPS) of the Earth's magnetosphere contains a thermally differentiated plasma, whose ion temperature exceeds severalfold its electron temperature. The gradient/curvature drift of ions and electrons of the CPS plasma tend to separate the two species in the dawn‐dusk direction. Because of the temperature difference, the dusk CPS, which gains a surplus of hotter ions, is expected to have a larger plasma pressure than its dawnside counterpart. Preliminary analysis of ISEE 1 plasma data confirmed the presence of such a pressure asymmetry in the CPS during quiet and the growth‐of‐driven conditions. The closure of the electric current created by this pressure asymmetry usually requires the flow of a field‐aligned current, whose effects on the E×B drift of the CPS plasma are the central focus of this paper. The dawn‐dusk asymmetry in the magnetosphere generally leads to an azimuthally asymmetric ionospheric convection, with close resemblance to the streamline structure first identified by Heppner (1977). The field‐aligned current associated with the asymmetry has a strong effect in shielding the inner magnetosphere, in which the imposed electric field decreases exponentially for polytropic indices greater than 1. The asymmetry also affects the location of the inner edge of the plasma sheet. We present a simplified model in this paper, using an analytic paradigm to portray the general properties of the asymmetry‐induced effects. A limited attempt is made to examine how a departure from the idealized condition would change the solution of the problem.
Prebreakup arcs: A comparison between theory and experiment
Authors: Rothwell P. L. , Silevitch M. B. , Block L. P. , Fälthammar C.‐G.
Published: February 2008

We have developed a model describing the structure of a prebreakup arc based on an ionospheric Cowling channel and its extension into the magnetosphere. A coupled two‐circuit representation of the substorm current wedge is used which is locally superimposed on both westward and eastward electrojets. We find that brighter, more unstable prebreakup arcs are formed in the premidnight (southwest of the Harang Discontinuity) than in the postmidnight (northeast of the Harang Discontinuity) sector. This contributes to the observed prevalence of auroral activity in the premidnight sector. Also, our model predicts that the north‐south dimensions of the current wedge in the ionosphere should vary from a few kilometers at an invariant latitude (Λ) of 62° to hundreds of kilometers above Λ = 68°. Comparison of the model results with the extensive observations of Marklund et al. (1983) for a specific arc observed just after onset shows good agreement, particularly for the magnitude of the polarization electric field and the arc size. We conclude that this agreement is further evidence that the substorm breakup arises from magnetosphere‐ionosphere coupling in the near magnetosphere and that the steady state model developed here is descriptive of the breakup arc before inductive effects become dominant.
VLF versus MF heating of the lower ionosphere
Authors: Dowden R. L. , Adams C. D. D.
Published: February 2008

The perturbation in the signal strength of a VLF transmitter by another transmitter heating the ionosphere on the great circle path to the receiver is calculated for two cases: The NAA‐Palmer path heated by the VLF transmitter, NAU, and the Omega Norway‐Skibotn path heated by the MF transmitter, “Heating,” near Tromsø. Comparison implies either that VLF heating is over 100 times more effective than MF heating in terms of the perturbation produced, per RF heating watt, in VLF propagation in the Earth‐ionosphere waveguide or that the observed effect at Palmer is fortuitous or spurious.
Evolution of magnetic configurations in the plasma sheet during a substorm on March 19, 1978
Authors: Sun W. , Kan J. R. , Akasofu S.‐I.
Published: February 2008

Evolution of the magnetic field configuration in the plasma sheet is modeled for an intense substorm event on March 19, 1978. The model is based on the idea that the substorm enhanced field‐aligned currents are initiated in the ionosphere in response to an enhanced magnetospheric convection (Kan et al., 1988). The field‐aligned currents in the model are determined from the ground‐based magnetometer data with a time resolution of 5 min. The substorm field‐aligned currents are assumed to close in the plasma sheet to complete the substorm current circuit. It is shown that the magnetic field produced by the substorm current system in the model can reproduce several important substorm signatures observed in the plasma sheet. These signatures include the taillike reconfiguration in the plasma sheet during the growth phase, the dipolarization of the plasma sheet associated with the substorm expansion onset, and the formation of a new X line. A shortcoming of the model is that the plasma dynamics in the plasma sheet have been ignored. In spite of this shortcoming, however, the model demonstrates that the ionosphere, in response to an enhanced magnetospheric convection, can cause the plasma sheet to change its magnetic configuration to result in the substorm signatures observed in the plasma sheet. The present study shows that it is possible for the ionosphere to play an active role in causing the observed reconfigurations of the plasma sheet during substorms.
An observation of a heliospheric magnetic cycle dependence for the integral radial gradient of E > 60 MeV cosmic rays
Authors: Webber W. R. , Lockwood J. A.
Published: February 2008

We find that the magnitudes of the integral radial gradients of >60 MeV cosmic ray particles as measured between Pioneer 10 (P10) and IMP and between Voyager 2 (V2) and IMP (also P10 and V2) during a period of negative solar magnetic polarity are a strong function of the cosmic ray intensity. These gradients increase from ∼1.5%/AU at the time of peak intensity in 1987 to ∼3%/AU at the times of minimum intensity in 1982 and 1990. During this time a strong radial dependence of the differential radial gradient was also observed, correlated with the appearance of a latitudinal gradient as measured between V2 and V1. In an earlier period of positive polarity from 1977 to 1982 we found practically no dependence of the magnitude of the integral radial gradient on the cosmic ray intensity. This gradient remained at ∼3%/AU during the entire period and no significant radial dependence of the differential gradient was observed. These specific changes in the behavior of the gradient in opposite polarity cycles do not seem to be predicted by either the drift‐wavy current sheet or simple diffusion‐convection models of the solar modulation, although it is interesting to note that the dependence of the magnitude of the radial gradient on intensity observed during the negative polarity period from 1982 to 1990 is consistent with the predictions of simple diffusion‐convection models. These results are discussed in the context of our understanding of the causes for the asymmetry in the 11‐year modulation cycles, particularly with regard to how particles enter the heliosphere and how, once inside the heliosphere, they move from the polar to the equatorial regions.
An unusual nonlinear system in the magnetosphere: A possible driver for auroral pulsations
Authors: Davidson G. T. , Chiu Y. T.
Published: February 2008

Many aspects of the relation between spatial and temporal structures in morningside aurorae can be explained with the aid of a nonlinear model that describes the interactions between energetic electrons and VLF waves. The simplest form of the model consists of three differential equations, with time derivatives generated in reference frames moving with either the energetic electrons, the cold plasma, or the VLF waves that interact with the energetic electrons. Although unstable solutions that could explain the origin of spatial structures are not found, the existence of spatial structures having sufficiently fine scales can provide a repetitive perturbation that maintains the temporal variations. An analysis of the behavior of solutions of the nonlinear system reveals that a strange attractor does not occur in the autonomous system; but when the repetitive perturbations are regarded as external periodic forcing terms, unusual forms of nonlinear behavior are revealed. If the frequencies of the forcing terms are not commensurate with the natural periods of the system, the solutions present some of the characteristics of deterministic chaos. The nearest analogy is a driven nonlinear oscillator with asymmetric damping. However, because of the stiffness of the system, even those solutions that are not truly chaotic (in the commonly accepted usage) will be indistinguishable from chaos over tens to hundreds of pulsation cycles. In certain cases, when damping terms are reduced to zero, the system displays a form of behavior with many of the important characteristics of chaos, but the phase trajectories do not appear to be bounded. Variation of the forcing amplitude produces regions of unbounded phase trajectories, which do not approach stable limit cycles, separated by narrow regions of classical period doubling.
Observations of upper atmospheric weather during solar minimum winter
Authors: Hagan M. E. , Barth C. A. , Tobiska W. K. , Manson A. H. , Vincent R. A. , Buonsanto M. J. , Burnside R. G. , Wickwar V. B.
Published: February 2008

We report on a wide variety of thermospheric and ionospheric observations from three consecutive January World Day campaign periods. Despite remarkably similar geophysical conditions characterizing the in situ forcing of the upper atmosphere during these solar minimum campaigns, we find significant variability in the observations of the ionosphere and thermosphere particularly at low latitudes in the American sector. In addition, we present further observational evidence of the unexpected exospheric temperature suppression at low latitudes initially reported by Hagan and Salah (1988). We discuss the lower and upper atmospheric coupling mechanisms of plausible importance to the interpretation of the observed thermospheric weather patterns. We report evidence that lower thermospheric [NO] (nitric oxide number density) and upward propagating atmospheric tides affected the thermospheric energy and momentum budgets during the campaign periods.
A purely growing electromagnetic mode operative in the geomagnetic tail
Authors: Wu C. S. , Yoon Peter H. , Ziebell L. F. , Chang C. L. , Wong H. K.
Published: February 2008

This paper discusses a purely growing mode driven by a cross‐field current. The study was motivated by a recent article by Chang et al. (1990). The present discussion pays special attention to two aspects. One is to generalize the analysis by Chang et al. (1990) so that the unmagnetized‐ion approximation used by these authors is removed, and the other is to apply the theory to several regions in the magnetotail where the value of the plasma beta is in general very high. The present analysis is restricted to waves propagating along the ambient magnetic field. The high ion beta limit is discussed by considering two different situations. The first is to fix the strength of the ambient magnetic field but to increase the plasma temperature, and the second is to maintain the plasma temperature but to decrease the ambient magnetic field strength. It is found that in the former case the mode stabilizes when βi → ∞, but in the latter case the instability persists even if βi → ∞ (although the growth rate is significantly reduced). The present theory is applied to three regions in the magnetotail. These are: (1) the inner edge region, (2) the midtail region, and (3) the neutral sheet of a distant magnetotail. It is found that, among these three regions, for a given value of υ0 / αi, where υ0 is the net cross‐field drift speed between the electrons and the ions and αi is the ion thermal speed, the growth rate in the neutral sheet is found to be the largest.
Small‐scale plasma irregularities in the nightside Venus ionosphere
Authors: Grebowsky J. M. , Curtis S. A. , Brace L. H.
Published: February 2008

The individual volt‐ampere curves from the Pioneer Venus orbiter electron temperature probe showed evidence for small‐scale density irregularities, or short‐period plasma waves, in regions of the nightside ionosphere where the orbiter electric field detector observed waves in its 100‐Hz channel. A survey of the nightside volt‐ampere curves has revealed several hundred examples of such irregularities (approximately 2% of all the current‐voltage (I‐V) sweeps from below 2000 km and 7% of those below 200 km). The I‐V structures correspond to plasma density structures with spatial scale sizes in the range of about 100–2000 m, or alternatively they could be viewed as waves having frequencies extending toward 100 Hz. They are often seen as isolated events, with spatial extent along the orbit frequently less than 80 km. The density irregularities or waves occur in or near prominent gradients in the ambient plasma concentrations both at low altitudes where molecular ions are dominant and at higher altitudes in regions of reduced plasma density where O+ is the major ion. Electric field 100‐Hz bursts occur simultaneously, with the majority of the structured I‐V curves providing demonstrative evidence that at least some of the E field signals are produced within the ionosphere. Potential sources for the irregularities are extraionospheric particle precipitation and spatial structures and turbulence resulting from the day to night ion transport within the ionosphere.
Semikinetic and generalized transport models of the polar and solar winds
Authors: Demars H. G. , Schunk R. W.
Published: February 2008

Thermal space plasmas have generally been modeled either by direct solution of the collisionless Boltzmann equation or by solving a set of transport equations obtained by multiplying the Boltzmann equation by suitable velocity moments and integrating over velocity space. The former approach gives the spatial‐temporal variation of the distribution function itself, whereas the latter approach provides only partial information regarding the distribution function through the spatial‐temporal variation of a finite number of velocity moments. However, the transport approach can easily incorporate the effects of collisions and chemical reactions into the description, whereas a direct solution of the Boltzmann equation (the kinetic approach), in any but a collisionless regime, has proven to be difficult. A vast body of work has been done in space plasma physics in which either a transport (e.g., hydrodynamic, hydromagnetic, generalized transport) or a kinetic (e.g., fully kinetic, semikinetic) approach has been employed. In order to properly understand the significance of this work, it is critical that a better understanding be gained of the relative merits of the transport and kinetic approaches. As a first step in this direction, a comparison has been made, in as consistent a manner as possible, of a transport (bi‐Maxwellian based 16‐moment equations) and a semikinetic description of supersonic flow in the solar wind and also of both supersonic and subsonic flows in the polar wind for “steady state” conditions. The study shows: (1) remarkable agreement between the two models for supersonic collisionless flows, even for the higher‐order moments; (2) the inadequacy of the semikinetic approach for modeling subsonic flows; and (3) the superiority of the 16‐moment transport over the semikinetic approach for modeling the solar wind. Our study provides further evidence that the bi‐Maxwellian based transport equations are a useful tool for studying “thermal” space plasmas that develop non‐Maxwellian features.
Electromagnetic ion cyclotron waves observed near the oxygen cyclotron frequency by ISEE 1 and 2
Authors: Fraser B. J. , Samson J. C. , Hu Y. D. , McPherron R. L. , Russell C. T.
Published: February 2008

Pc 2 electromagnetic ion cyclotron waves at 0.1 Hz, near the oxygen cyclotron frequency, have been observed by ISEE 1 and 2 between L = 7.6 and 5.8 on an inbound near‐equatorial pass in the dusk sector. The waves occurred in a thick plasmapause of width ≈1.5 RE and penetrated ≈1 RE into the plasmasphere. Wave onset was accompanied by significant increases in the thermal (0–100 eV) He+ and the warm (0.1–16 keV/e) O+ and He+ heavy ion populations. The most intense waves (8 nT) were observed in the outer plasmasphere where convection drift velocities (E × B)/B² were largest and the Alfven velocity was a minimum. Wave polarization is predominantly left‐handed with propagation almost parallel to the ambient magnetic field, and the spectral slot and polarization reversal predicted by cold plasma propagation theory are identified in the wave data. Poynting fluxes calculated during the first 15 min of the event show wave energy propagation directions both parallel and antiparallel to the field. Computations of the experimental wave spectra during the passage through the plasmapause show that the spectral slots relate to local plasma parameters, possibly suggesting an ion cyclotron wave growth source near the spacecraft. A regular wave packet structure seen over the first 30 min of the event may be attributed to the modulation of this energy source by the Pc 5 waves seen at the same time. Overall, the results are considered an example of an electromagnetic ion cyclotron wave‐particle interaction occurring during the outer plasmasphere refilling process at the time of the substorm recovery phase.
Absolute state‐selected total cross sections for the O+(4S) + CO2 reactions
Authors: Flesch G. D. , Ng C. Y.
Published: February 2008

Absolute state‐selected total cross sections for O2+, CO2+, CO+, and C+ resulting from the reaction O+(4S) + CO2 have been measured in the center‐of‐mass collision energy (Ec.m.) range of 0.2–150 eV. The cross section for O2+ drops off rapidly with increasing Ec.m. and becomes essentially zero at Ec.m. > 3 eV. The charge transfer CO2+ is overwhelmingly the major product ion at Ec.m. > 0.5 eV. The onsets measured for CO+ and C+ are consistent with the thermochemical thresholds for the formation of CO+ + 2O and C+ + 3O, respectively.
Nonadiabatic heating of the central plasma sheet at substorm onset
Authors: Huang C. Y. , Frank L. A. , Rostoker G. , Fennell J. , Mitchell D. G.
Published: February 2008

Heating events in the plasma sheet boundary layer and central plasma sheet are found to occur at the onset of expansive phase activity. The main effect is a dramatic increase in plasma temperature, coincident with a partial dipolarization of the magnetic field. Fluxes of energetic particles increase without dispersion during these events which occur at all radial distances up to 23 RE, the apogeee of the ISEE spacecraft. A major difference between these heating events and those observed at geosynchronous distances lies in the heating mechanism which is nonadiabatic beyond 10 RE but may be adiabatic closer to Earth. The energy required to account for the increase in plasma thermal energy is comparable with that required for Joule heating of the ionosphere. The plasma sheet must be considered as a major sink in the energy balance of a substorm. We estimate lobe magnetic pressures during these events. Changes in lobe pressure are generally not correlated with onsets or intensifications of expansive phase activity.
Location of the radio emitting regions of interplanetary shocks
Authors: Lengyel‐Frey D.
Published: February 2008

In this study we analyze 20 interplanetary type II radio bursts to determine the location of the type II source region relative to the interplanetary shock. We also report the first determination of a density‐distance relationship (density model) appropriate for interplanetary type II source regions. To determine source location we compare densities in type II source regions, derived from observed type II emission frequencies, to ambient solar wind densities to determine whether source regions are in the ambient, upstream solar wind or in the compressed plasma behind the shock. We compute densities in the ambient solar wind upstream of each shock by using a simple model to extrapolate solar wind plasma densities measured at 1 AU back to the shock front. Densities in the type II source regions are found to be enhanced relative to densities in the ambient solar wind by a factor which is close to the average shock density compression ratio. The simplest interpretation of this result is that the source is located in the compressed plasma within or behind the shock. Although it is possible that the emission is produced in enhanced density regions of the upstream solar wind, we feel that the weight of evidence favors the compressed postshock plasma as the source site.
Comparison of Echo 7 field line length measurements to magnetospheric model predictions
Authors: Nemzek R. J. , Malcolm P. R. , Winckler J. R.
Published: February 2008

The Echo 7 sounding rocket experiment injected electron beams on central tail field lines near L = 6.5. Numerous injections returned to the payload as “conjugate echoes” after mirroring in the southern hemisphere. We compare field line lengths calculated from measured conjugate echo bounce times and energies to predictions made by integrating electron trajectories through various magnetospheric models: the Olson‐Pfitzer Quiet and Dynamic models and the Tsyganenko‐Usmanov model. Although Kp at launch was 3−, quiet time magnetic models best fit the echo measurements. Geosynchronous satellite magnetometer measurements near the Echo 7 field lines during the flight were best modelled by the Olson‐Pfitzer Dynamic Model and the Tsyganenko‐Usmanov model for Kp = 3. The discrepancy between the models that best fit the Echo 7 data and those that fit the satellite data was most likely due to uncertainties in the small‐scale configuration of the magnetospheric models. The field line length measured by the conjugate echoes showed some temporal variation in the magnetic field, also indicated by the satellite magnetometers. This demonstrates the utility an Echo‐style experiment could have in substorm studies.
Measurement of Jovian decametric io‐related source location and beam shape
Authors: Maeda K. , Carr T. D.
Published: February 2008

From measurements of the Jovian decametric activity that was recorded by Voyager 1 and 2 we have obtained new information on the locations of the Io‐related sources A and C (i.e., Io‐A and Io‐C) and on the shapes of their emission beams. In two instances we were able to identify the same dynamic spectral arc event in the recorded data of the two spacecraft, providing in each case an opportunity to observe the same emission beam over a wide range of frequencies from two considerably different directions. In the analysis of such a pair of correlated Voyager 1 and Voyager 2 spectral arcs, the arc centroid time was measured at each of a series of frequencies for each spacecraft, and these time values were then replotted as a function of frequency after the source‐to‐spacecraft propagation times were subtracted from them. It was found that the propagation‐corrected centroid times of each of the Voyager 1 arcs were coincident with those of the corresponding Voyager 2 arc in a particular frequency range, but not at other frequencies. From this information we have confirmed that the emission beams are in the form of thin, almost conical sheets, the cone opening angle decreasing with increasing frequency. We have also demonstrated conclusively that both the Io‐A and Io‐C sources were located near the northern foot of the magnetic flux tube that was connected to Io. By incorporating polarization information we conclude that the emission from both sources was in the X mode (both being right‐hand polarized at all frequencies). However, for other Io‐C arcs noted on the Voyager 1 record that were located near the one we investigated, the polarization sense reversed as the frequency decreased below 15 MHz. We suggest that below this frequency the emission mode had changed X to O for these arcs.
Atomic and molecular ion temperatures and ion temperature anisotropy in the auroral F region in the presence of large electric fields
Authors: Hubert D. , Kinzelin E.
Published: February 2008

The atomic O+ ion oxygen and molecular NO+ ion nitrogen temperatures and temperature anisotropy throughout a realistic auroral F region in the presence of large electric fields are studied. The O+ ion temperature is smaller than the NO+ ion temperature in the lower part of the F region, and this effect increases with the increasing of the neutral temperature as well as of the ion‐neutral drift velocity. The ion temperature anisotropy is defined as the ratio of the ion perpendicular temperature to the ion parallel temperature with respect to the magnetic field. For the O+ ions this ratio is maximum around 290 km in altitude for a constant ion‐neutral drift velocity throughout the F region. For similar conditions the NO+ ion temperature anisotropy is constant between 175 km and 250 km in altitude and decreases for higher altitudes. Application to mixed species non‐Maxwellian radar spectra analysis is discussed.
Multifractal structure of the magnetic field and plasma in recurrent streams at 1 AU
Authors: Burlaga L. F.
Published: February 2008

This paper demonstrates the existence of multifractal structure in the fluctuations of the magnetic field strength, temperature, and density in the recurrent flows at 1 AU, The multifractal scaling is observed over the range of periods from 2 hours to 32 hours for the magnetic field and density and from 2 hours to 16 hours for the temperature. The lower cutoff of 2 hours is related to the use of hour averages; it does not have physical significance. The upper cutoff of 32 hours (16 hours) represents the transition to the mesoscale regime in which the fluctuations are determined by the stream structure. Thus the multifractal structure represents fluctuations superimposed on the corotating stream profile and the heliospheric plasma sheet profile. One may think of the multifractal structure as a texture superimposed on the mesoscale profiles, the structure of the texture being related to the mesoscale structure itself. The multifractal structure of the magnetic field strength fluctuations might be produced by a multiplicative process involving the velocity fluctuations.
Atmospheric excitation of the Earth's annual wobble: 1980–1988
Authors: Chao B. Fong , Au Andrew Y.
Published: February 2008

Global meteorological analyses from the European Centre for Medium Range Weather Forecasts are employed to compute the atmospheric excitation Ψ of the polar motion for the 9‐year period of 1980–1988. Both the matter component Ψ(matter) and the motion component Ψ(motion) are computed, the former with and without the oceanic inverted barometer (IB) effect. It is found that Ψ(motion) contributes significantly to the total excitation Ψ overall and nonnegligibly to the annual signal in Ψ, or the annual wobble excitation, in particular. Our results for the annual wobble excitation, in terms of the prograde component Ψ+and the retrograde component Ψ−for January 1, are Ψ+= (16.8 milliarc seconds (mas), −93°) and Ψ−= (15.6 mas, −98°) with IB, and Ψ+= (17.3 mas, −101°) and Ψ−= (28.1 mas, −112°) without IB. These results are within the (rather large) range of previous estimates. The IB effect has a small impact on Ψ+, whereas its impact on Ψ−is considerable. The (better determined) prograde components Ψ+are then compared with that observed from the Lageos satellite laser ranging data: (17.3 mas, −61°). Although the amplitudes are nearly equal, large phase discrepancies exist between the atmospheric and this observed value. The resolution of this discrepancy awaits a better knowledge of the seasonal angular momentum budget of the Earth's surface fluid elements.
Geochemistry of igneous rocks from the Crazy Mountains, Montana, and tectonic models for the Montana Alkalic Province
Authors: Dudás Francis Ö.
Published: February 2008

Tectonic models have postulated that subduction of a flat‐lying oceanic plate was intimately linked with mid‐Eocene magmatism in the northwestern United States and have assumed that this linkage is expressed in the geochemical characteristics of igneous rocks. Geochemical data from the Crazy Mountains in south central Montana can be interpreted as indicating the presence of a subduction‐related chemical component. Because radiogenic isotopic data for these rocks restrict the major chemical events in their mantle source to the Late Cretaceous‐Early Tertiary (50–100 Ma) and the mid‐Proterozoic (1.6–1.8 Ga), subduction during one or both of these periods is the most plausible explanation for the origin of the arc‐like geochemical features. Assessment of geologic and tectonic constraints, however, indicates that Late Cretaceous‐Early Tertiary subduction cannot explain the regional distribution, the contemporaneity, and the compositional range of the Eocene igneous rocks. The Eocene magmatic event does not show interpretable age progressions, as most subduction‐related models suggest. There is considerable geographic overlap in the chemical characteristics of the Eocene igneous rocks, so that chemical zoning cannot be demonstrated south of the U.S.‐Canada border. The zone of igneous activity was diffuse and discontinuous, unlike most subduction‐related magmatic arcs. The presence of thick lithospheric mantle beneath the Archean Wyoming craton presents difficulties for models that involve subduction of a flat‐lying slab. The lithospheric mantle probably controlled the depth of subduction and probably forced the slab to depths greater than those of plausible source regions for subalkalic, mantle‐derived basalts. Flat slab subduction models cannot account for the thermal inputs required for extensive magmatism. Subduction‐related models fail to explain the CO2‐rich character of alkalic magmatism in central Montana. The arc‐like geochemical patterns in the Crazy Mountains samples could result either from subduction unrelated to Late Cretaceous‐Early Tertiary events, or from mantle geochemical processes that are unrelated to subduction. If geochemical processes unrelated to subduction are involved, trace element discrimination diagrams are misleading and are inadequate criteria for interpreting the tectonic environment. The evidence for mid‐Proterozoic chemical modification of the mantle beneath the Wyoming craton suggests that the arc‐like character could be inherited from subduction events between 1.6 and 1.9 Ga, but a specific mid‐Proterozoic subduction geometry cannot be identified. Alternative tectonic models for Eocene magmatism can be built around plausible heating or decompression mechanisms for regional magma generation. The advection of a thermal anomaly by mantle upwelling, or regional uplift related to the thickening‐flexure‐rebound cycle in the Cordillera to the west could have triggered the regional Eocene magmatic event.
Petrogenesis of Challis volcanics from central and southwestern Idaho: Trace element and Pb isotopic evidence
Authors: Norman Marc D. , Mertzman Stanley A.
Published: February 2008

Challis volcanics comprise Eocene age mafic, intermediate, and silicic lavas and ash flow tuffs erupted from fissure vents and calderas in central and southwestern Idaho. We have analyzed a suite of mafic to intermediate composition lava flows from central Idaho, and ash flow tuffs and dikes from southwestern Idaho for major elements, trace elements, and Pb isotopic compositions. These data provide constraints on the petrogenesis of the Challis volcanics and on the tectonic setting of the northwestern United States during the Eocene. The mafic to intermediate composition lavas have low Al2O3 and high MgO contents, K2O generally ≥ Na2O, and phenocryst assemblages suggesting an absarokite‐shoshonite affinity. Compatible element (e.g., Mg, Ni, and Cr) abundances in these lavas are comparable to those in oceanic basalts, consistent with a mantle source. The high concentrations of some incompatible trace elements (e.g., Rb, Th, U, and Pb) and the values of key trace element ratios (Nb/U, Ce/Pb, and Sr/Nd) in these lavas, however, are more consistent with a crustal component. Pb isotopic compositions of these lavas (206Pb/204Pb = 17.9–18.5, 207Pb/204Pb = 15.5–15.6, and 208pb/204Pb = 38.4–39.0) are independent of fractionation indicators, suggesting the magmas escaped significant assimilation during crystallization. Therefore the mixture of mantle and crustal components must have occurred prior to the crystallization history recorded by these lavas, either in the source or during the early stages of magma generation. Although Challis volcanism is obviously related to an evolving convergent margin setting, the mafic to intermediate Challis lavas do not appear to be direct products of a subduction‐related magma system. We have found no evidence for an asthenospheric mantle contribution to even the most mafic Challis basalts; rather, the trace element and Pb isotopic data suggest a more direct connection with the continental lithosphere. At this time, we favor a model for the generation of the Challis volcanics by melting entirely within the lithospheric mantle and lower crust, possibly in response to the tectonic thickening and subsequent extension experienced by the region following late Mesozoic and early Cenozoic compressional deformation.
Tectonic stress field of China inferred from a large number of small earthquakes
Authors: Xu Zhonghuai , Wang Suyun , Huang Yurui , Gao Ajia
Published: February 2008

Mean principal tress axes were inferred for China using 9621 P wave first motion polarity readings from 5054 small earthquakes (1 ≤ ML < 5). The area studied was divided into 76 subregions. The mean P (compressive), B (intermediate), and T (relatively tensional) axes corresponding to composite focal mechanism solutions of multiple earthquakes for each subregion were determined by a grid test to all possible orientations of the P, B, and T axes with a step of 5° or 10°. In order to get a relatively homogeneous sampling in space we have avoided using the readings from spatially and temporally clustered earthquakes. We have rechecked most of the polarity readings by inspection of original seismograms. The focal mechanism solutions of large (M≥6) individual earthquakes are also presented for comparison. The results indicate the existence of an overall radial pattern of the present‐day maximum horizontal stress orientations throughout the continental area of China. This pattern is thought to be closely related to indentor models of continental collision between the Indian Ocean and the Eurasian plate.
Some results of heterogeneous data inversions for oceanic tides
Authors: Jourdin F. , Francis O. , Vincent P. , Mazzega P.
Published: February 2008

Many different instruments directly or indirectly observe the oceanic tidal movements. Among them, we consider here the coastal tide gauge and deep‐sea pressure recorder which provide at least the principal spectral constituents of the tidal height, the gravimeter which indirectly observes the oceanic tides through the gravitational effect generated by the deformation of the solid Earth under the ocean tidal mass load, and the satellite altimeter which samples the tides in the time domain and over the nearly whole ocean. Our present paper deals with an empirical method for the recovery of the oceanic tides by inversion of heterogeneous data sets. The aim is to take advantage of the different kinds of information coming from the measurement instruments in order to complete more accurate empirical solutions. First, tide gauge and gravity loading data are separately and jointly inverted to provide global charts of the major semidiurnal M2 wave. The results prove that the tide gauge data have a major contribution in the tidal mapping but also suggest that the gravity loading data distort the information. This is certainly due to unmodelled geophysical phenomena and instrumental errors. Second, tide gauge and satellite altimeter data are considered in a separate and a joint inversion computation to provide tidal estimations along a predefined geographical line in the North Atlantic. The results are accurate enough to make provision for the boundary conditions of an hydrodynamical tidal model. As an intrinsic advantage of the inversion method, a posteriori standard deviations and error covariances for all the solutions displayed are provided and analyzed.
Bootstrap statistics for paleomagnetic data
Authors: Tauxe Lisa , Kylstra Niels , Constable Cathy
Published: February 2008

The power and utility of paleomagnetic analyses stem largely from the ability to quantify such parameters as the degree of rotation of a rock body, or the orientation of an anisotropy axis. Until recently, estimates for uncertainty in these paleomagnetically determined parameters derived from assumptions concerning the underlying parametric distribution functions of the data. In many geologically important situations, the commonly used parametric distribution functions fail to model the data adequately and the uncertainty estimates so obtained are unreliable. Such essentials as the test for common mean require data sets consistent with a spherically symmetric underlying distribution; their application in inappropriate circumstances can result in flawed interpretations. Moreover, the almost universally used approximation for a cone of 95% confidence for the mean of a sample drawn from a Fisher distribution is quite biased even for moderate dispersions (K= 25). The availablity of inexpensive, powerful computers makes possible the empirical estimation of confidence regions by means of data resampling techniques such as the bootstrap. These resampling schemes replace analytical solutions with repeated simple calculations. We describe a bootstrap approach for the calculation of uncertainties for means or principal directions of paleomagnetic data. The method is tested on means of simulated Fisher distributions with known parameters and is found to be reliable for data sets with more than about 25 elements. Because a Fisher distribution is not assumed, the approach is applicable to a wide range of paleomagnetic data and can be used equally well on directions or associated virtual poles. We also illustrate bootstrap techniques for the discrimination of directions and for the fold test which enable the use of these powerful tests on the wider range of data sets commonly obtained in paleomagnetic investigations.
Slaty cleavage development and magnetic anisotropy fabrics
Authors: Housen Bernard A. , van der Pluijm Ben A.
Published: February 2008

The shale‐to‐slate transition preserved in the Ordovician Martinsburg Formation at the Lehigh Water Gap, Pennsylvania, provides an opportunity to study the relationship between magnetic anisotropy fabrics and the development of slaty cleavage. Our previous work has indicated that anisotropy of magnetic susceptibility (AMS) does not record changes in finite strain associated with cleavage development in these rocks but instead measures the degree of dissolution and new growth of chlorite. Additional AMS data presented in this paper lend further support to this conclusion. Conversely, anhysteretic remanent magnetization anisotropy (ARMA), which is not affected by paramagnetic chlorite, accurately reflects the strain‐induced rock fabrics associated with cleavage formation. ARMA results show that magnetite dimensional orientations vary from bedding‐parallel in shale samples to cleavage‐parallel in samples with well‐developed slaty cleavage. Samples with weak and pencil cleavage display scattered ARMA orientations which lie in between bedding and cleavage. These intermediate orientations may be due to either passive rotation of magnetite from bedding‐parallel to cleavage‐parallel or (re)crystallization of magnetite. If rotation occurred, grain rotation was highly heterogeneous in the samples with incipient cleavage. The intermediate ARMA orientations may also reflect the varying contribution of two magnetite preferred orientations, a depositional orientation parallel to bedding and a new growth orientation parallel to cleavage.