bluuebunny/crossref_metadata_2025 huggingface dataset:
Neutral winds in the lower thermosphere from Dynamics Explorer 2
Authors: Killeen T. L. , Nardi B. , Purcell P. N. , Roble R. G. , Fuller‐Rowel T. J. , Rees D.
Published: February 2008

Doppler line profile measurements of the OI λ557.7 nm “green line” emission, made by the Fabry‐Perot interferometer on Dynamics Explorer 2, have provided altitude profiles of the meridional component of the lower‐thermospheric neutral wind. The wind inversion technique of Nardi [1991] has been used to extract the neutral wind profiles from the line‐of‐sight measurements. Individual λ557.7 nm Doppler line profiles and inverted volume‐emission‐rate and neutral‐wind profiles are presented. Neutral wind measurements from the ∼120 km altitude level, obtained on multiple orbital passes over the summer hemisphere polar region, have been merged to produce a synthesized averaged “vector” wind field in geomagnetic coordinates. The wind pattern exhibits a region of anticyclonic vorticity in the daytime sector of the magnetic polar cap. Averaged winds of ∼300 m/sec in the equatorial direction are observed in the early morning sector. The satellite winds are in reasonable agreement with the predictions of the NCAR and UCL thermosphere general circulation models, with significant regional discrepancies evident in both magnitude and direction.
Low frequency magnetic signals associated with Langmuir waves
Authors: Kellogg Paul J. , Goetz K. , Lin N. , Monson S. J. , Balogh A. , Forsyth R. J. , Stone R. G.
Published: February 2008

With the URAP experiment on Ulysses, we have observed low frequency signals with a magnetic component in close time correlation with electrostatic Langmuir waves at the plasma frequency. In most, if not all, of these cases, the Langmuir waves are part of a Type III solar burst. We investigate this effect and show that the low frequency waves are in the whistler mode and are most likely due to nonlinear effects involving Langmuir waves.
An estimate of the relative magnitude of small‐scale tracer fluxes
Authors: Bacmeister Julio T. , Schoeberl Mark R. , Loewenstein Max , Podolske Jim R. , Strahan Susan E. , Chan K. Roland
Published: February 2008

The wind and constituent measurements from the polar aircraft data are used to compute the flux spectra. Although there is variation from flight to flight, the flux spectra generally fit a −2 to −1.5 power law as expected theoretically. This result suggests that tracer fluxes from small scale features do not substantially contribute to the overall tracer budget relative to the fluxes from the larger scales.
b‐Value, aseismic deformation and brittle failure within an isolated geological object: Evidences from a dome structure loaded by fluid extraction
Authors: Volant Philippe , Grasso Jean‐Robert , Chatelain Jean‐Luc , Frogneux Michel
Published: February 2008

Studies of b‐values are usually performed either at rock sample scale (laboratory experiments) or at crustal scale (earthquakes). But interpretations at crustal scale are extrapolated from small scale experimental laws with well defined boundary conditions, to a larger object with no clear boundary conditions. We examine variations of the b‐value in time and space at spatial (10×10×10 km3) and temporal (20 years) scales intermediate between laboratory analyses and tectonic processes, in a dome structure that is an isolated geological object with defined boundaries and known geomechanical properties. Seismic activity (about 1000 events with magnitude ≤ 4.5) and aseismic displacements (6–7 cm of cumulative subsidence) have been induced by gas extraction in an area where no displacement had previously been reported. We find no agreement between temporal variations in b‐values and results from laboratory experiments: there is no correlation between b‐values and stress histories, nor between b‐values and the spatial migration of seismicity. Aseismic slips introduce anomalies in b‐value behaviour when seismic instabilities are a second order process compared to the whole deformation. These observations imply that when changes in b values have been used for earthquake forecasting, false alarms can be explained by the occurrence of aseismic displacement.
Diffusive shock acceleration and the March 1991 solar events
Authors: Naidu N. , Quenby J. J. , Keppler E. , Korth A. , Witte M. , Krupp N. , Fränz M. , Blake J. B. , Mazuk S. , Moussas X.
Published: February 2008

Spectral and anisotropy measurements are reported for protons between 0.5 and 1.4 MeV during the particle increases associated with the passage of 3 interplanetary shocks past Ulysses between March 19 and March 31 1991. The data are interpreted in terms of diifusive shock acceleration in the travelling solar induced disturbances to the solar plasma.
Rotational discontinuities in anisotropic plasmas
Authors: Omidi N.
Published: February 2008

The kinetic structure of rotational discontinuities (RDs) in anisotropic plasmas with T⟂/T‖ > 1 is investigated by using a one‐dimensional electromagnetic hybrid code. To form the RD, a new approach is used where the plasma is injected from one boundary and reflected from the other, resulting in the generation of a traveling fast shock and an RD. Unlike the previously used methods, no a priori assumptions are made regarding the initial structure (i.e. width or sense of rotation) of the rotational discontinuity. The results show that across the RD both the magnetic field strength and direction, as well as the plasma density change. Given that such a change can also be associated with an intermediate shock, the Rankine‐Hugoniot relations are used to confirm that the observed structures are indeed RDs. is found that the thickness of RDs is a few ion inertial lengths and is independent of the rotation angle. Also, the preferred sense of rotation is in the electron sense; however, RDs with a rotation angle larger than 180° are found to be unstable, changing their rotation to a stable ion sense.
Sporadic electromagnetic emissions in the Akr frequency range associated with electrostatic plasma turbulence
Authors: Malingre M. , Pottelette R. , Dubouloz N. , Lindqvist P. A. , Holmgren G. , Aparicio B.
Published: February 2008

We present heretofore unreported Viking observations of sporadic AKR bursts occurring well above the electron gyrofrequency together with spiky electrostatic emissions recorded just above the lower hybrid frequency, which exhibit a high degree of correlation in time. Both types of waves show simultaneous large amplitude variations of the order of 30‐40 dB on time scales of a few hundreds of ms. Possible interpretations which may account for the observed correlation are investigated. Arguments are given which suggest that such events could be the signature of small scale non‐linear structures in the plasma.
temporal change in Coda Q−1 in the Hida Region, central Japan
Authors: Hiramatsu Y. , Ando M. , Takeuchi F.
Published: February 2008

Coda Q−1 was determined from the decay rate of coda waves in the bands of 1.5 to 24 Hz in the Hida region, central Japan. The data consist of routinely made observations for events from 1985 to 1991 (367 events), with magnitudes less than 4.0 and depths shallower than 30 km. We found a systematic temporal variation in coda Q−1. Coda Q−1 increased with time in the volcanic area. The interpretation of the systematic temporal change in coda Q−1 in the Hida region is that aseismic fractures due to magma accumulation can modify the scattering environment and consequently the coda Q−1 estimates.
Charge‐exchange born He+ ions in the solar wind
Authors: Gruntman Michael A.
Published: February 2008

The effect of charge transfer between solar wind alpha‐particles and hydrogen atoms of interstellar origin is revisited. Singly‐charged helium ions born in the charge transfer carry important information on processes in the solar wind and the heliosphere. The velocity distribution of such He+ ions is substantially different from that of He+ pick‐up ions due to ionization of the interstellar helium atoms. Estimates of the expected abundances of the charge‐exchange born He+ in the solar wind are presented, and the possibility of measuring this plasma component on deep space missions is discussed.
Geomagnetic poles over the past 10,000 years
Authors: Ohno Masao , Hamano Yozo
Published: February 2008

Locations of the geomagnetic pole over the past 10,000 years have been calculated by averaging the VGP positions obtained from paleomagnetic data. The distribution of the geomagnetic pole was elongated to the direction parallel to the meridian of 45° and 225° longitude, and westward movement of the pole was predominant throughout this period. The time sequence of the polar motion can be divided into three intervals, the intervals between ca. 10,000 B.P. and ca. 7000 B.P., between ca. 7000 B.P. and ca. 3700 B.P., and between ca. 3700 B.P. and the present. During the period between ca. 7000 and ca. 3700 B.P., the range of the movement of the geomagnetic pole was limited within 5 degrees around the geographical pole. Before and after this period, the movement was very active, fluctuating over 10 degrees. The results of the last 2000 years show good consistency with the geomagnetic pole calculated from archaeomagnetic data by Merrill and McElhinny [1983].
Detection of land degradation with polarimetric SAR
Authors: Ray Terrill W. , Farr Tom G. , van Zyl Jakob J.
Published: February 2008

Land degradation is a crucial problem facing the human race. With an ever‐increasing population placing increasing stress on agricultural lands, land impoverishment has the potential for adversely impacting the food supply in many regions of the world. The Manix Basin Area of the Mojave desert has been cropped using center pivot irrigation, but since 1973 many fields have been abandoned for economic reasons. Data were collected using the JPL Airborne Synthetic Aperture Radar (AIRSAR), a multi‐spectral radar polarimeter. Analysis of these data revealed unusual polarization responses which we attribute to the formation of wind ripples on the surfaces of fields which had been abandoned for more than 5 years. This conjecture was confirmed through field observations, and the observed polarization responses were effectively modelled using a second‐order small perturbation model. These results demonstrate the usefulness of remote sensing techniques supported by limited field work for study of land degradation at synoptic scales.
Interaction of reflected ions with the firehose marginally stable current sheet: Implications for plasma sheet convection
Authors: Pritchett P. L. , Coroniti F. V.
Published: February 2008

The firehose marginally stable current sheet, which may model the flow away from the distant reconnection neutral line, assumes that the accelerated particles escape and never return to re‐encounter the current region. This assumption fails on the earthward side where the accelerated ions mirror in the geomagnetic dipole field and return to the current sheet at distances up to about 30RE down the tail. Two‐dimensional particle simulations are used to demonstrate that the reflected ions drive a “shock‐like” structure in which the incoming flow is decelerated and the Bz field is highly compressed. These effects are similar to those produced by adiabatic choking of steady convection. Possible implications of this interaction for the dynamics of the tail are considered.
An inversion for radial viscosity structure using seismic tomography
Authors: King Scott D. , Masters Guy
Published: February 2008

Using a uniform velocity/density scaling, we have inverted the seismically inferred 3‐dimensional structure of the whole mantle for the radial viscosity structure which best fits the geoid. We are able to obtain 60–72% variance reductions for three different S‐wave tomographic models. The resulting viscosity structures are remarkably similar, showing a high viscosity lid, a low viscosity zone in the transition region and a high viscosity lower mantle. A resolution analysis indicates that the viscosity structure in the upper mantle is well resolved by the data, however the resolution in the lower mantle is poorer. Our models are in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400–670 km as opposed to 100–400 km.
Buffering of stratospheric circulation by changing amounts of tropical ozone a Pinatubo Case Study
Authors: Kinne S. , Toon O. B. , Prather M. J.
Published: February 2008

Stratospheric aerosol from Mount Pinatubo heated the tropical lower stratosphere by about 0.3 K/day mainly due to absorption of terrestrial infrared radiation. This heating was dissipated by (1) an observed increase in stratospheric temperatures, which enhanced the radiative cooling, (2) additional mean upward motion, observed for the aerosol cloud, which led to adiabatic cooling and (3) reductions in ozone concentrations resulting from enhanced upward motions. Each of these processes operated on a different time scale: maximum temperatures were observed after about 90 days; maximum ozone losses of about −1.5 ppm occurred after 140 days when the enhanced vertical velocities effectively lifted the ozone profile by about 2 km. We believe this shows that ozone plays an important role in buffering vertical motion in the tropical lower stratosphere, and hence the residual Brewer Dobson circulation of the whole stratosphere.
Multiple brightenings of transient dayside auroral forms during oval expansions
Authors: Fasel G. J. , Minow J. I. , Smith R. W. , Deehr C. S. , Lee L. C.
Published: February 2008

Poleward moving transients have been proposed to be ionospheric signatures of plasma transfer events taking place at the dayside magnetopause. They are usually observed to brighten at the equatorward edge of the dayside auroral oval and fade as they move into the polar cap. This paper reports the observation of a new type of poleward moving dayside auroral transient which has several cycles of intensity variations. Observations of these transients show a series of intensifications in brightness along the arc or rayed band during poleward motion accompanied by a brightening in the auroral oval. As they reach their extreme poleward position they brighten and than fade from view. This brightening sequence may be explained by multiple reconnection of the magnetic flux tube associated with the transient.
Viscous flow model of a subduction zone with a faulted lithosphere: Long and short wavelength topography, gravity and geoid
Authors: Zhong Shijie , Gurnis Michael
Published: February 2008

The essential features of observed topography, free air gravity, and the geoid over subduction zones have been reproduced with a dynamically self consistent viscous flow model of a subduction zone with a faulted lithosphere. A fault between the overriding and subducting plates is crucial to understanding lithospheric deformation and gravity in a subduction zone. The topography for models with a fault is characterized by a deep and narrow trench and a broad and shallow back‐arc depression, quite different from viscous models without faults which only have a broad and large depression over subducting slabs. For a model in which a slab is 600 km long and 80 Myr old, the resulting trench is approximately 3 km deep and 100 km wide and the back‐arc basin is about 1.5 km deep and 600 km wide; the model is characterized by a long wavelength geoid high of about 30 m over the slab, a local geoid low of 10 m over the trench and a 150 mgal gravity low over the trench. The fact that trench depth and width are reproduced in models with a small resisting shear stress, 50 bars, suggests that convergent plate boundaries are probably weak.
Simulation of heavy ion ring and shell distributions downstream of the bow shock
Authors: Motschmann Uwe , Glassmeier Karl‐Heinz
Published: February 2008

The behavior of heavy solar wind ions at the bow shock is studied using one‐dimensional electromagnetic hybrid computer simulations. Beginning from a synchronous flow of different ion species protons and heavy ions jump to different downstream velocities. Three significant downstream regions are found: (i) Immediately downstream of the shock the differential velocity between the protons and the heavy ions generates coherent pickup of the heavy ion minor component and this region is characterized by a gyrating heavy ion beam. (ii) In the intermediate downstream region pitch‐angle scattering transforms the heavy beam within a few heavy ion gyrations to a ring‐like distribution in case of a quasi‐perpendicular magnetic field or to a shell‐like distribution in case of quasi‐parallel magnetic field. (iii) In the far downstream region energy diffusion thermalizes these distributions to become Maxwellians. Though in (ii) the heavy ion distributions are unstable their contribution to the downstream magnetic wave spectrum is unimportant.
Air‐sea CO2 fluxes in the equatorial Pacific in January‐March 1991
Authors: Lefevre Nathalie , Dandonneau Yves
Published: February 2008

The partial pressure of CO2 in surface seawater and in air were continuously measured during the cruise ALIZE II (January 1991–March 1991) in the equatorial Pacific from Panama to Nouméa via Tahiti. It provides a large set of data for estimating the air‐sea CO2 flux in the equatorial Pacific. An average flux is calculated between 2.5N and 2.5S. This estimation ranges from 5 to 8.5 mmol m−2d−1 according to the CO2 exchange coefficient used.
The effect of turbulence on atmospheric gradient parameters estimated from ground‐based radiometric and space geodetic measurements
Authors: Davis James L.
Published: February 2008

Spatial and temporal fluctuations in the refractive index of air affect estimates of atmospheric parameters determined from data obtained from radio signals which have propagated through the atmosphere. These effects will become increasingly important as we continue to improve the accuracy of microwave space geodetic techniques. Using a straightforward expression for the covariance of a general vector of atmospheric parameters, I calculate the effects on estimates of spatial gradients for a frozen Kolmogorov turbulence field moving over a site at constant velocity. Numerical calculations are performed to investigate the implications for three techniques: ground‐based microwave radiometry, geodesy with the Global Positioning System, and Very Long Baseline Interferometry. The results indicate that a prevailing wind with no true static gradients may give rise to apparent azimuthal asymmetries when gradient parameters are estimated. Moreover, care must be taken in comparing gradient parameters determined using data from these different techniques, because these techniques sample the atmosphere on different spatial and temporal scales.
Sampling bias in VGP longitudes
Authors: Egbert Gary D.
Published: February 2008

I derive probability densities for virtual geomagnetic pole (VGP) longitudes for a general statistical model of local magnetic field variations. I show that even for very simple statistically homogeneous models of secular variation, the distribution of VGP longitudes is peaked 90° away from the sampling longitude. Thus, when sites are distributed unevenly, a non‐uniform overall distribution of VGP longitudes is to be expected. Analysis of the recent geomagnetic field indicates that this bias can be significant, particularly when random errors in paleomagnetic data are allowed for. It is possible that some of the deviations from uniformity seen in recent compilations of paleomagnetic reversal and secular variation data are a statistical artifact resulting from the distortion of the VGP transformation, and the non‐uniformity of paleomagnetic sampling sites.
The discovery and significance of a fossilized radiogenic argon wave (Argonami) in the Earth's crust
Authors: Hyodo Hironobu , York Derek
Published: February 2008

We have discovered a curious “fossilized” asymmetric radiogenic argon wave trapped in single grains of biotite in the contact zones between a diabase dike (≈570 Ma) and the Grenville country rock (tonalitic gneiss ≈1000 Ma) near Mattawa, Ontario, Canada. The phenomenon is presumably the result of the interplay between the partial pressure of the ambient radiogenic argon and the opening and closing (blocking) of biotite to argon movement during the period of thermal (and fluid?) perturbation following the dike intrusion. The wavelength is roughly equal to one dike width (≈30 m). The only previously reported example of such a wave (which we call an “argonami”) was one discovered in 1970 by Wanless and his colleagues in a study of biotites from a traverse across the Grenville Front. The two argonamis are remarkably similar, even in their asymmetry, but the wavelength scales differ by roughly two order of magnitude, presumably reflecting the relative sizes of the Mattawa dike and the continental‐scale Grenville Front. The occurrence of the argonami phenomenon suggests that in some instances, the analysis of excess argon may become a useful tool for the unraveling of complex tectono‐thermal and fluid‐migratory events taking place at plate boundaries and igneous contacts.
A high resolution gravity model for Venus: GVM‐1
Authors: Nerem R. S. , Bills B. G. , McNamee J. B.
Published: February 2008

A spherical harmonic model of the gravitational field of Venus complete to degree and order 50 has been developed using the S‐band Doppler tracking data of the Pioneer Venus Orbiter (PVO) collected between 1979 and 1982. The short wavelengths of this model could only be resolved near the PVO periapse location (∼14°N latitude), therefore a priori constraints were applied to the model to bias poorly observed coefficients towards zero. The resulting model has a half‐wavelength resolution of 400 km near the PVO periapse location, but the resolution degrades to greater than 1000 km near the poles. This gravity model correlates well with a degree 50 spherical harmonic expansion of the Venus topography derived from a combination of Magellan and PVO data. New tracking data from Magellan's gravity mission should provide some improvement to this model, although a complete model of the Venusian gravity field will depend on tracking of Magellan after the circularization of it's orbit using aerobraking.
Measurements of jet aircraft emissions at cruise altitude I: The odd‐nitrogen gases NO, NO2, HNO2 and HNO3
Authors: Arnold F. , Scheid J. , Stilp Th. , Schlager H. , Reinhardt M. E.
Published: February 2008

Using a novel aircraft‐borne automatic mass spectrometer, the odd‐nitrogen gases NO, NO2, HNO2 and HNO3 were for the first time measured in a young exhaust‐trail of a jetliner at cruise‐altitude. The measurements, which took place at an altitude of 9.5 km and a distance of about 2 km from a DC‐9 jetliner, revealed NO and NO2 to be present with abundances as large as 780 and 150 ppbv, respectively. The acids HNO2 and HNO3 reached abundances of 0.52 and 0.46 ppbV, which implies that only about 0.05% of the emitted reactive nitrogen experienced rapid conversion to the stable odd nitrogen reservoir nitric acid. Hence, most of the emitted odd‐nitrogen remained in the reactive form NO and NO2, which affect ozone. Nitrous acid (HNO2) turns out to be an excellent tracer for young jet‐aircraft plumes since it is short‐lived and reaches only a very low atmospheric background abundance. The low HNO3‐emission implies that HNO3‐H2O nucleation and condensation in jet‐aircraft plumes is hardly favoured by the additional HNO3. However, it may still occur due to H2O emissions.
Periodic magma movement in the conduit with a barrier: A model for the volcanic tremor
Authors: Honda Satoru , Yomogida Kiyoshi
Published: February 2008

We present a one‐dimensional flow model in which compressible viscous fluid runs through the dike‐like conduit with a barrier. The barrier is assumed to open or close above or under a certain threshold pressure, respectively. Constant flow rate at one end and a constant pressure at the other end are set as boundary conditions, which mimicks the fluid flow from a reservoir to an open space. The behaviour of the system depends on various parameters such as viscosity, flow rate and threshold pressure. For a set of parameters plausible for the magma flow in cracks, the system oscillates periodically even in the existence of viscous damping. Spectra of the far field displacement excited by the stress due to this oscillation show a prominent frequency peak in each case. The time interval during which the barrier is kept open (We call the ”open period”) becomes longer, as the flow rate increases. This results in the elongation of peak period expected in the spectra of far field displacement. The length of the open period is affected by both the flow rate and the interference of the pressure waves in the conduit generated by the previous opening‐closure of the barrier. This model may have an important implication for volcanic tremors whose dominant frequency and continuation time change temporarily.
Ponderomotive effects on ion acceleration in the auroral zone
Authors: Li Xinlin , Temerin M.
Published: February 2008

Low frequency (ω < Ωi), large amplitude (E > 100 mV/m) Alfvén waves occur in the auroral zone. Such waves have a ponderomotive effect, on both ions and electrons. In the region between the large wave field and the ionosphere, the ponderomotive force accelerates electrons downward and ions upward, which produces an ambipolar electric field. The combined effect is to produce a differential acceleration between O+ and H+ with the O+ accelerated more out of the ionosphere. The typical resulting energization for O+ is tens of eV, which is sufficient for the ions to escape the ionosphere. We demonstrate this by means of analysis and test‐particle calculation.
A potential new source of OH and odd‐nitrogen in the atmosphere
Authors: Toumi R.
Published: February 2008

HOx (OH+HO2) and NOx (NO+NO2) radicals play a key role in controlling global ozone. However, several atmospheric measurements of these species are larger than current model calculations suggest. The oxidation of H2 and N2O by the second electronically excited state of oxygen, O2(b1Σg+), is theoretically possible and proposed here as an atmospheric source of these radicals. If only 1% of the quenching of O2(b1Σg+) by H2 and N2O results in the proposed oxidation products (OH and NO+NO2), then model calculations predict a significant production of these radicals in exactly those regions of the atmosphere were current models seem to fail.
Fast magnetic reconnection with small shock angles
Authors: Yan M. , Lee L. C. , Priest E. R.
Published: February 2008

The steady state fast magnetic reconnection process with small separatrix angles proposed by Pestchek (1964) and generalized by Priest and Forbes (1986) is studied by a two‐dimensional incompressible magnetohydrodynamic (MHD) code. In the code a uniform tangential magnetic field and normal plasma speed are specified on the inflow boundary. On the outflow boundary the tangential flow speed and tangential magnetic field are specified to be those in the Priest‐Forbes' model in order to obtain different reconnection regimes. In our simulations both a uniform and a nonuniform resistivity are used. For a nonuniform resistivity model in which the resistivity in the outflow region is highly reduced, our simulations are in many aspects consistent with those in the analytical results. A steady state magnetic reconnection configuration with small separatrix angle is obtained. The plasma is heated and accelerated by the current sheets associated with slow shocks. We also obtain various regimes predicted by Priest and Forbes for different boundary conditions on the outflow boundary which are characterized by a parameter b0. They are a weak fast‐mode expansion (b0=0), slow‐mode compression (b0 < 0), slow‐mode expansion (b0 ≥ 0), and a hybrid regime of fast‐mode and slow‐mode expansion (0 < b0 < 1). The width and length of the current sheet for different parameters obtained in the simulations are found to be consistent with theoretical values. When the magnetic Reynolds number or the reconnection rate given on the inflow boundary increases, the diffusion region becomes smaller. However, for cases with a uniform resistivity imposed in the simulation domain it is found that the diffusion region tends to lengthen indefinitely and no steady state configuration is obtained. Magnetic islands are usually formed at the later stage of the simulation.
Energetic ions as remote probes of X type neutral lines in the geomagnetic tail
Authors: Speiser T. W. , Martin R. F.
Published: February 2008

Martin and Speiser (1988) have predicted ridges in the velocity space distribution function as a signature of the interaction of energetic ions with an X type neutral line in the geomagnetic tail. In this paper we study the properties of these ridges as the observation point is moved relative to the X line, as phase angle is varied, and the effect of the ridges on initial distributions with a loss cone. With the ridges, one can remotely sense not only the presence of an X line, but also, potentially, the distance from the X line, whether the observer is earthward or tailward of the X line, and the vertical position within the current sheet. For example, we find that for single particle dynamics in a current sheet with neutral line, the phase space ridge is predicted to be found throughout the current sheet if it is not destroyed by collective behavior. The ridge is predicted to be found for distributions plotted in ν⊥, ν∥ space and also in νx, νy, νz space. Additionally, valleys are found in νx, νy, νz space, which are also signatures of a neutral line, and which depend on the initial flowing distribution. An initially tailward flowing distribution causes asymmetries in distributions earthward versus tailward of the neutral line. These asymmetries are due to the fact that part of the distribution below (at smaller pitch angles than) the ridge comes from initially earthward (tailward) particles when the modeling point is earthward (tailward) of the neutral line. The accelerated beam is not predicted inside the current sheet at x = L (one separatrix distance earthward of the neutral line), but close to the sheet center a perpendicular bulk flow is predicted. Spatially, the ridge is found to move to larger pitch angles as the observation point approaches the neutral line in the plasma sheet boundary layer. As the observation point moves toward the current sheet center, the ridge stays at about the same pitch angle, but then tends to be diminished very near the center where a perpendicular bulk flow is found. Chaotic pitch angle scattering can fill even a relatively large (10°), initially empty loss cone. This loss cone filling is more complete when orbits are calculated in a thicker current sheet. Thus chaotic pitch angle scattering may be a dominant mechanism to produce nightside proton isotropy in the auroral zones, as suggested by Sergeev et al. (1983).
Outer heliospheric radio emissions: 2. Foreshock source models
Authors: Cairns Iver H. , Kurth William S. , Gurnett Donald A.
Published: February 2008

Low‐frequency radio emissions in the range 2–3 kHz have been observed by the Voyager spacecraft during the intervals 1983–1987 and 1989 to the present while at heliocentric distances greater than 11 AU. New analyses of the wave data are presented, and the characteristics of the radiation are reviewed and discussed. Two classes of events are distinguished: transient events with varying starting frequencies that drift upward in frequency and a relatively continuous component that remains near 2‐kHz. Evidence for multiple transient sources and for extension of the 2‐kHz component above the 2.4‐kHz interference signal is presented. The transient emissions are interpreted in terms of radiation generated at multiples of ƒp when solar wind density enhancements enter one or more regions of a foreshock sunward of the inner heliospheric shock. Theoretically, and based on extrapolated Langmuir levels in planetary foreshocks, the foreshock should contain sufficient levels of Langmuir waves to produce the radio emissions in path lengths small compared with 1 AU and the estimated radial size of the foreshock. Enhancements of the solar wind density by factors greater than 4 are required for the radiation to be generated at the observed frequencies and to propagate into the inner heliosphere, as observed, from a source outside about 50 AU. Solar wind density enhancements by factors of 4–10 are observed. The radiation can therefore be generated in sources beyond the present locations of the Voyager spacecraft. Propagation effects, the number of radiation sources, and the time variability, frequency drift, and varying starting frequencies of the transient events are discussed in terms of foreshock sources. The available data appear to be qualitatively and semiquantitatively consistent with this foreshock theory for transient events. A similar foreshock theory involving transient density enhancements can explain the levels, characteristic frequency, and inward propagation of the 2‐kHz component in terms of a source outside 50 AU. However, the theory encounters significant difficulties in explaining the relatively constant frequency and presence of the radiation, as well as its different properties relative to the transient emissions. Remedies for these problems are briefly explored. However, the 2‐kHz component quite likely has a different generation mechanism and/or a different source region than the transient emissions.
On the standing wave mode of giant pulsations
Authors: Takahashi K. , Sato N. , Warnecke J. , Lühr H. , Spence H. E. , Tonegawa Y.
Published: February 2008

Both odd‐mode and even‐mode standing wave structures have been proposed for giant pulsations. Unless a conclusion is drawn on the field‐aligned mode structure, little progress can be made in understanding the excitation mechanism of giant pulsations. In order to determine the standing wave mode, we have made a systematic survey of magnetic field data from the AMPTE CCE spacecraft and from ground stations located near the geomagnetic foot point of CCE. We selected time intervals when CCE was close to the magnetic equator and also magnetically close to Syowa and stations in Iceland, and when either transverse or compressional Pc 4 waves were observed at CCE. Magnetograms from the ground stations were then examined to determine if there was a giant pulsation in a given time interval. One giant pulsation was associated with a compressional wave, while no giant pulsation was observed in association with transverse wave events. The CCE magnetic field record for the giant pulsation exhibited a remarkable similarity to a giant pulsation observed from the ATS 6 geostationary satellite near the magnetic equator (Hillebrand et al., 1982). In agreement with Hillebrand et al., we conclude that the compressional nature of the giant pulsation is due to an odd‐mode standing wave structure. This conclusion places a strong constraint on the generation mechanism of giant pulsations. In particular, if giant pulsations are excited through the drift bounce resonance of ions with standing Alfvén waves, ω ‐ mωd = ±Nωb, where ω is the wave frequency, m is the azimuthal wave number, ωd is the ion drift frequency, N is an integer, and ωb is the ion bounce frequency, then the resonance must occur at an even N.
Neutral atomic oxygen density from nighttime radar and optical wind measurements at Millstone Hill
Authors: Buonsanto M. J. , Tung Y. ‐K. , Sipler D. P.
Published: February 2008

Neutral atomic oxygen densities [O] in the thermosphere are calculated from analysis of coincident Millstone Hill incoherent scatter radar and Fabry Perot interferometer measurements taken during 14 nights with widely varying geomagnetic conditions in 1990 and 1991, and the results are compared with the mass spectrometer/incoherent scatter 1986 (MSIS‐86) model. The results are generally satisfactory except during the most rapidly varying conditions, when some of our assumptions break down, such as those of a uniform ion velocity field and no vertical neutral wind. On the average, the results agree well with MSIS‐86. However, systematic differences are found on two of the geomagnetically disturbed nights, with the observed [O] much smaller than the MSIS‐86 values on one night, and much larger on another.
The ionospheric signature of Pi 2 pulsations observed by STARE
Authors: Sutcliffe Peter R. , Nielsen Erling
Published: February 2008

This study extends the work of Sutcliffe and Nielsen (1990) in which a classical Pi 2 pulsation was first isolated in Scandanavian Twin Auroral Radar Experiment (STARE) data. A high‐pass‐filtering technique is used to remove the background electric field in the STARE data and so reveal the spatial and temporal ionospheric signatures of the Pi 2 pulsation electric fields. A number of events are identified and examples presented in which pulsation electric fields up to 50 mV/m are observed. Magnetic field oscillations computed from the filtered STARE data using the Biot‐Savart law correlate well with pulsation magnetometer data. A 180° phase difference is observed between high‐ and low‐latitude X component pulsations. The ionospheric signature of a Pi 2 is located slightly poleward of the core of the auroral breakup region where the southward, westward, and northward directed background electric fields converge; the strongest pulsation fields occur in the region of equatorward directed electric fields. The ionospheric electric field patterns of the Pi 2 pulsations determined from the STARE data correlate well with those modeled for a transverse Alfvén wave incident on an east‐west aligned high‐conductivity strip in the ionosphere.
On steady field‐aligned double‐adiabatic flow
Authors: Sonnerup B. U. Ö , Hau L. ‐N. , Walthour D. W.
Published: February 2008

The four general double‐adiabatic invariants governing ideal MHD flow in a narrow flux tube are summarized and are put in a form suitable for calculation of flow in tubes of nonconstant cross‐ sectional area. Results for converging‐diverging (Laval‐type) flux tubes are developed and compared to single‐adiabatic results which are identical to those for ordinary gasdynamic flow. As in such flows, a transition from subsonic to supersonic double‐adiabatic flow requires the presence of a minimum in the flux tube area, i.e., a maximum in the field strength. With increasing subsonic values of the parallel Mach number, M∥ = ν(ρ/3p∥)1/2, as the plasma moves toward the field maximum where M∥ = 1, the density, ρ, in double‐adiabatic flow initially increases rather than decreases, behavior that is the exact opposite of that in ordinary gasdynamics. A density maximum occurs at the subsonic value M∥ = (1 ‐ p⊥/3p∥)1/2; for larger M∥ values, the density then decreases monotonically as in ordinary gasdynamics. The two pressures, p∥ and p⊥, behave in qualitatively the same way as ρ. A fifth invariant, valid for general two‐dimensional (∂/∂z ≡ 0; Vz ≠ 0; Bz ≠ 0) single‐ or double‐adiabatic field‐aligned flow, is derived. Such two‐dimensional flows over slender obstacles are then analyzed by use of linear theory. It is found that in certain parameter regimes the governing equation for the vector potential is elliptic and can be reduced to Laplace's equation by simple stretching of one coordinate. In other regimes the equation is hyperbolic, indicating the presence of standing wave patterns. The relevance of these results to flow over magnetopause undulations, including flux transfer events, is discussed briefly; actual quantitative applications will be presented in a subsequent paper (D. W. Walthour et al., submitted, 1992).
Investigation of strong Langmuir turbulence in ionospheric modification
Authors: Cheung P. Y. , DuBois D. F. , Fukuchi T. , Kawan K. , Rose Harvey A. , Russell David , Tanikawa T. , Wong A. Y.
Published: February 2008

Recent results in the search for strong Langmuir turbulence effects during ionospheric modification experiments performed at the Arecibo Observatory are presented. Indirect evidence of Langmuir wave collapse is obtained through the observation of theoretically predicted “caviton‐type” enhanced plasma waves spectra using the 430 MHz incoherent radar at Arecibo. A typical spectrum consists of a “free‐mode” peak with a frequency that is significantly higher than the heater frequency, and a broad “caviton continuum” with frequencies below the heater frequency. Free modes are freely propagating Langmuir waves radiated by collapsing cavitons during collapse. The generation and dynamics of these “free modes” will be discussed. Asymmetries between the frequency shifts and strengths of the upshifted and downshifted free‐mode lines and their dependence on the time delay following the onset of heating are explained in terms of the radiation of free Langmuir modes by cavitons and the subsequent propagation of free modes down or up the density gradient. Experimental results are compared with theoretical predictions. Results on the transition of “caviton‐type” plasma line spectra to the commonly observed “decay‐type” spectra will also be presented.
Numerical test of the weak turbulence approximation to ionospheric Langmuir turbulence
Authors: Hanssen Alfred , Mjølhus E. , DuBois D. F. , Rose H. A.
Published: February 2008

The standard weak Langmuir turbulence approach to explain the artificial plasma line in ionospheric radio modification experiments is examined. We compare solutions of a weak turbulence approximation (WTA) derived from a version of the one‐dimensional driven and damped Zakharov system of equations (ZSE) with solutions to the same full ZSE. The electromagnetic pump field is modeled as a long‐wavelength parametric driving term. We found that from a certain distance below the O mode reflection level the wave number saturation spectra computed from the WTA agree qualitatively with those from the ZSE for weak driving strengths, in the sense that the number of cascade lines increases with increasing pump strength. However, in general, the number of cascades apparent in the WTA solutions is larger than that predicted from the full ZSE. At higher intensities of the driver the saturation spectra from the ZSE differ from the WTA cascade spectra, in that a truncation of the cascade sets in, with a subsequent filling in of the bands between the cascades. This truncation takes place far before the ZSE cascade spectra reach the so‐called “Langmuir condensate,”; contrary to earlier conjectures based mainly on dimensional analysis arguments. In the reflection region a qualitatively different process takes place: temporal cycles of large ensembles of localized events; nucleation of cavitons, collapse, and burnout constitute the basic elements of the turbulence in this region of space. No WTA exists for this region. Our findings are discussed with respect to the experiments performed at Arecibo and Tromsø, the conclusion being that the ZSE yields results closer to observations than does the WTA, in all regions of space.
Analysis of radiances from orbital gas releases
Authors: Green B. D. , Holtzclaw K. W. , Joshi P. B. , Burke H. K.
Published: February 2008

We describe here the analysis of the optical emissions resulting from the sequential release of five gases (N2, Xe, CO, NO, and methylamine) from a remote canister as observed from a separate spacecraft by ultraviolet/visible array spectrometers and a pair of infrared circular variable filter spectrometers. These spectrometers provided continuous spectral coverage of the release event between 120 nm and 22 μm. The releases occurred at 260 to 300 km altitude under conditions of no solar illumination. No emissions were detected during the N2 and Xe releases, but bright infrared emissions in both fundamental and overtone vibrational bands observed for CO and NO resulted from the highly energetic interaction of the released gases with the residual atmospheric gases. Molecular internal distributions and collisional excitation cross sections were derived from the analysis. The collisional process creates highly rotationally and vibrationally excited CO and NO. The cross sections are determined with an order of magnitude uncertainty to be 10−16 cm² for O + CO → CO(υ) + O and 10−17 cm² for O + NO → NO(υ) + O. Methylamine (released as a liquid) exhibited both IR and UV emission features attributable to OH(υ), OH(A), and complex mid‐IR emissions likely due to CH stretches and to NO and methylamine vibrational bands. This article reports our preliminary analysis and conclusions.
Eigenmode analysis of coupled magnetohydrodynamic oscillations in the magnetosphere
Authors: Fujita S. , Patel V. L.
Published: February 2008

We have performed an eigenmode analysis of the coupled magnetohydrodynamic oscillations in the magnetosphere with a dipole magnetic field. To understand the behavior of the spatial structure of the field perturbations with a great accuracy, we use the finite element method. The azimuthal and radial electric field perturbations are assumed to vanish at the ionosphere, and the azimuthal electric field is assumed to be zero on the outer boundary. The global structures of the electromagnetic field perturbations associated with the coupled magnetohydrodynamic oscillations are presented. In addition, the three‐dimensional current system associated with the coupled oscillations is numerically calculated and the following characteristics are found: (1) A strong field‐aligned current flows along a resonant field line. The current is particularly strong near the ionosphere. (2) The radial current changes its direction on the opposite sides of the resonant L shell. Unlike the field‐aligned current, the radial currents exist in the entire magnetosphere. (3) Although the azimuthal and radial currents are intense on the resonant field line, these currents do not form a loop in the plane perpendicular to the ambient magnetic field. Therefore the field‐aligned component of the perturbed magnetic field does not have a maximum at the resonant L shell.
Poynting vector measurements of electromagnetic ion cyclotron waves in the plasmasphere
Authors: LaBelle J. , Treumann R. A.
Published: February 2008

Although Pc 1–2 emissions have been measured from the ground and from satellites for a long time, relatively few studies exist in which E, B, and δN are all analyzed. In this paper, we revisit one such example: the June 6, 1985, Pc 1 event investigated by LaBelle et al. (1988), which occurred in the duskside overlap region between the plasmaspheric bulge and the ion ring current. Although this observation was made within a few degrees of the equator, equal amplitudes of R and L modes are observed, i.e., linear polarization. Poynting vector analysis of the R and L mode components show them to both be characterized by northward Poynting vector, indicating energy flux away from the equator. The value of the Poynting vector is about 3 μWm−2, consistent with comparable previous measurements when convergence of the magnetic field is taken into account. Minimum variance analysis of the magnetic field fluctuations implies that the wavevector is nearly parallel to the background B. For parallel propagation, the crossover frequencies and bi‐ion hybrid frequencies may be calculated; the waves turn out to be nearly at the crossover frequency if oxygen ions are included in the calculation. This is the simplest explanation for the linear polarization. The absence of density fluctuations associated with the waves seems to eliminate the possibility that a linear electrostatic mode is being observed in this case.