bluuebunny/crossref_metadata_2025 huggingface dataset:
The potential effects of volcanic aerosols on cirrus cloud microphysics
Authors: Jensen Eric J. , Toon Owen B.
Published: February 2008

The potential impact of volcanic aerosols on nucleation of ice crystals in upper tropospheric cirrus clouds is examined from a microphysical perspective. The sulfuric acid aerosols which form in the stratosphere are presumably transported into the troposphere by sedimentation and tropopause folding. The tropospheric volcanic aerosol size distribution is estimated from 10 μm lidar backscatter measurements [Post, 1986] and in situ measurements [Pueschel et al., 1992b], Microphysical simulations suggest that at temperatures below about −50°C the concentration of ice crystals which nucleate may be as much as a factor of 5 larger when volcanic aerosols are present. Our simulations suggest that the presence of volcanic aerosols may increases the net radiative forcing (surface warming) of certain types of cirrus near the tropopause by as much as 8 W/m2. Further observations are required to determine whether these effects actually occur, and their global impact.
Penetration of Mt. Pinatubo aerosols into the north polar vortex
Authors: Rosen James M. , Kjome Norman T. , Fast Hans , Khattatov Vyacheslav U. , Rudakov Vladimir V.
Published: February 2008

Observations of the Mount Pinatubo cloud as it moved northward and into the winter polar vortex were made with a balloon borne two‐wavelength backscattersonde. Some volcanic debris had arrived at far northerly latitudes below 20 km by October and was apparently incorporated into the initial vortex. Subsequent measurements did not show a significant increase in the central vortex aerosol until after a mid January disturbance, at which time the backscatter profiles began to increase. Above 20 km and near the center of the vortex there was no significant increase in aerosol through mid March. The column stratospheric aerosol mass loading as calculated from individual soundings at the wall of the vortex during March 1992 ranged from 18 to 24 megatonnes per unit area where the unit of area is that of the earth. This indicates that a significant amount of material was transported northward by the end of the winter.
Analytic description of the electron temperature behavior in the upper ionosphere and plasmasphere
Authors: Khazanov G. V. , Nagy A. F. , Gombosi T. I. , Koen M. A. , Cariglia S. J.
Published: February 2008

Approximate analytic solutions to the well‐known and commonly used time‐dependent electron energy balance equation for the upper ionosphere and plasmasphere have been obtained and are discussed. The various potential heating sources for the terrestrial plasmasphere are summarized and the corresponding electron temperatures and related characteristic heating and cooling times are calculated. A comparison between the analytic expressions for the temperature variations and relevant measurements shows excellent agreement.
Variation of sea floor depth with age: A test of models based on drilling results
Authors: Johnson H. Paul , Carlson Richard L.
Published: February 2008

Previous analyses of the variation of depth with age in oceanic basins have demonstrated a systematic square root of age dependence in crust younger than 80 Ma. These studies used depths determined from marine seismic data, which have uncertainties due to the assumed sediment velocity. As an alternative, we compiled the basement depths from all DSDP and ODP drill sites that sampled ‘normal’ ocean crust in the Atlantic, Pacific and Indian Oceans, up to recent drilling (Leg 136). We applied further criteria to the data, rejecting any site not formed at a mid ocean ridge, or that did not penetrate extrusive basalts of MORB composition. The remaining high quality data set is small (77 sites), but adequate to test models of the thermal structure of the upper mantle. Our new results also show that the square root of age (t½) variation incorrectly estimates the observed depth for crustal ages older than 90 Ma. However, our data suggest a plate thickness of 104 ± 9 km and a basal temperature of 1400 ± 140 °C, and agree, at all ages, with the recent inversion by Stein and Stein, 1992.
Oceanic crust in the Japan Basin of the Japan Sea by the 1990 Japan‐USSR Expedition
Authors: Hirata Naoshi , Karp Boris Ya. , Yamaguchi Takashi , Kanazawa Toshihiko , Suyehiro Kiyoshi , Kasahara Junzo , Shiobara Hajime , Shinohara Masanao , Kinoshita Hajimu
Published: February 2008

In September of 1990, a seismic refraction and reflection survey was conducted in the Japan Basin, in the northeastern part of the Japan Sea, as a part of the Japan‐USSR joint expedition. Twenty‐six ocean bottom seismometers (OBSs) were deployed on two 200‐km long lines. Explosives and an airgun were fired as controlled seismic sources on the two mutually perpendicular lines. Airguns were also used as the source for the multi‐channel reflection profiles.

The crustal structure deduced is that of a typical oceanic basin: the crustal thickness is about 8.5 km including 2 km of sediment. We obtained a more detailed crustal structure than that obtained previously. From the dense airgun shooting data, the crustal structure is well resolved to show layer 1A, layer 1B, layer 2A, 2B, 2C, layer 3, and the mantle. The crust basically consists of laterally homogeneous layers but the Moho deepens slightly westward.
New observations of Meddy Movement south of the Tejo Plateau
Authors: Zenk Walter , Schultz Tokos Kathy , Boebel Olaf
Published: February 2008

Mediterranean salt lenses (meddies) are a dominant factor in the salt budget of the Atlantic at middepth. In spite of their important role, their juvenile migration has not yet been directly observed. For the first time, two RAFOS float trajectories show strong evidence of a meddy along the Iberian continental slope off Lisbon. Over six weeks we obtained drift observations from two levels (629, 847 dbar). Both instruments recorded a series of loops with an azimuthal speed O (30 cm s−1) at a radius of about 25 km. Relatively high propagation speeds of several centimeters per second indicate the meddy was probably carried along with the undercurrent of Mediterranean Water. The Tejo Plateau, a prominent feature of the continental slope and a natural obstacle for the spreading Mediterranean Water tongue, appears to act as a deflector for advected meddies possibly formed by interaction of the undercurrent with the canyon‐rich topography farther south.
Statistical assessment of the preferred longitudinal bands for recent geomagnetic reversal records
Authors: Laj C. , Mazaud A. , Weeks R. , Fuller M. , Herrero‐Bervera E.
Published: February 2008

Records of recent polarity reversals recorded in sediments show a preponderance of transitional VGP's situated in the longitudinal sector over the Americas and, to a lesser extent, over its antipode. A series of tests based on the methods of circular statistics show that this distribution is not consistent with the statistical fluctuations of a random probability distribution. The longitudinal bias in the distribution, therefore, requires a physical explanation.
The effects of multiple phase transitions on Venusian mantle convection
Authors: Steinbach Volker , Yuen David A.
Published: February 2008

The effects of multiple phase‐transitions on the dynamics of the Venusian mantle have been investigated with a two‐dimensional finite‐element method. A depth‐dependent thermal expansivity has been employed in a purely basal heating configuration with an aspect‐ratio of four. The addition of the olivine → spinel transition promotes layered convection more so than models with a single spinel → perovskite phase‐change. The shifts in the phase‐transition depths and the presence of a conductive lid increase the tendency of the Venusian mantle circulation to flow through the transition zone. The potentially lower Rayleigh number in Venus from lack of volatiles will also enhance whole mantle convection. The style of convection in Venus may have changed from layered convection in the past to whole‐mantle convection today. This transition might have been responsible for the major resurfacing event on Venus.
Sensitivity of the lateral correlation function in deep seismic reflection data
Authors: Holliger Klaus , Carbonell Ramon , Levander Alan R.
Published: February 2008

In the absence of multiple scattering the lateral correlation function of the reflected wavefield can be related to the lateral correlation function of the medium. We have analyzed the lateral correlation function of a series of synthetic seismograms from stochastic models of reflective lower crust with the structural and petrophysical statistics of the Ivrea Zone. Our results show that in the presence of Ivrea‐type lower crustal heterogeneity the lateral correlation function of the backscattered wavefield is insensitive to the lateral correlation function of the scattering medium, which suggests that multiple scattering is important in the reflective lower crust. This interpretation is consistent with lower crustal reflections commonly being shorter than a Fresnel zone, the absence of clear diffractions and the poor performance of standard migration algorithms on deep seismic reflection data.
Shear‐wave splitting in the Rio Grande Rift
Authors: Sandvol Eric , Ni James , Ozalaybey Serdar , Schlue John
Published: February 2008

Fast directions and lag times associated with SKS shear‐wave splitting are determined for six digital stations in the Rio Grande Rift (RGR). The mean fast direction for three stations in the central RGR is about 45° oblique to the axis of the RGR. In contrast, fast directions approximately parallel to the axis of the RGR are found for two stations in the southern RGR (∼10°), and one station in the northern RGR (∼26°). The fast directions for southern stations are not aligned with the NE‐SW extension direction of the Oligocene to early Miocene stage of extension. The significant N‐S lateral variation in the SKS shear‐wave splitting parameters is difficult to explain by fossil anisotropy because the mantle structure beneath the RGR is underlain by hot material with low seismic‐wave velocities. Hence the fast direction of azimuthal anisotropy is probably related to the preferred orientation of olivine created by the present deformation in the upper mantle. Because none of the fast directions are parallel to the present E‐W extension direction of the RGR the dominant upper mantle flow direction is not perpendicular to the axis of the RGR. The pattern of the fast directions along the RGR provides evidence for a rising asthenosphere beneath the central to southern RGR and suggests that thermally driven small‐scale convection may be faster for flow parallel to the rift than perpendicular to it.
Unroofing of the Himalayas: A view from apatite fission‐track analysis of Bengal fan sediments
Authors: Corrigan Jeff D. , Crowley Kevin D.
Published: February 2008

Apatite fission‐track analysis of Early Miocene to Quaternary distal Bengal Fan sediments from ODP Leg 116 provides a unique perspective on denudation processes associated with the Tertiary Himalayan orogeny. These sediments yield pooled apatite fission‐track ages that are 0‐ to 10‐m.y. older than depositional ages. Mean track lengths range from 13.2 to 14.8 μm. These young ages and long track lengths indicate that rocks in the upper crust of Bengal Fan source areas cooled rapidly (≥15 °C/m.y.), implying denudation rates of several hundreds of meters per million years. These observations support the hypothesis that nearly continuous unroofing of a large source area, similar to the dimensions of the present Himalayan and southern Tibetan Plateau deformation front, and an efficient transport system, similar to the present Himalaya‐Ganges‐Brahmaputra system, supplied sediment to the Bengal Fan since the Early Miocene.
Elastic wave attenuation and velocity of Berea Sandstone measured in the frequency domain
Authors: Shankland T. J. , Johnson P. A. , Hopson T. M.
Published: February 2008

Using measurements in the frequency domain we have measured quality factor Q and travel times of direct and side‐reflected elastic waves in a 1.8 m long sample of Berea sandstone. The frequency domain travel time (FDTT) method produces the continuous‐wave (cw) response of a propagating wave by stepwise sweeping frequency of a driving source and detecting amplitude and phase of the received signal in reference to the source. Each separate travel path yields a characteristic repetition cycle in frequency space as its wave vector‐distance product is stepped; an inverse fast Fourier transform (IFFT) reveals the corresponding travel time at the group velocity. Because arrival times of direct and reflected elastic waves appear as spikes along the time axis, travel times can be obtained precisely, and different arrivals can be clearly separated. Q can be determined from the amplitude vs. frequency response of each peak as obtained from a moving window IFFT of the frequency‐domain signal. In this sample at ambient conditions compressional velocity VP is 2380 m/s and QP is 55.
Phase locking of nordeste precipitation with sea surface temperatures
Authors: Sperber Kenneth R. , Hameed Sultan
Published: February 2008

Interannual variations of precipitation in several regions around the globe are known to be influenced by sea surface temperatures. We describe a procedure that elicits the same frequencies of variations in the sea surface temperatures as in precipitation. The modes of variation in the two fields are phase locked, allowing us to identify regions in the oceans that vary in unison with changes in precipitation on different time scales. The method is illustrated by application to the northeast section of Brazil, the Nordeste, where flood and drought occur frequently.
Estimation of crustal Poisson's ratio from broad band teleseismic data
Authors: Clarke Timothy J. , Silver Paul G.
Published: February 2008

Knowledge of Poisson's ratio constitutes an important constraint on the physical properties of the continental crust. To date there are only a few locations in the world where direct measurements of this parameter have been made. In this paper we describe a single station technique for measuring crustal Poisson's ratio from the boundary interaction phases in broad band teleseismic waveforms. In principle we can derive an estimate from one seismogram, although in practice using several seismograms helps overcome the low signal to noise ratio of the boundary interaction phases. We apply our method to 6 North American stations using 9 deep focus earthquakes in various locations. By performing a nonlinear search for the optimum Poisson's ratio and Moho depth, we are able to make a stable estimate of Poisson's ratio in each case. The estimates range from 0.22 to 0.315, representing the extremes that are usually found on the North American continent. The low value at station COL (Fairbanks, AL), is corroborated by a refraction survey in the same region. Because of the small number of seismograms needed, this technique is especially well suited for portable teleseismic experiments. In addition, we will be able to obtain estimates at each of the 100+ permanent broad band stations that are to be shortly deployed, ensuring that the number of measurements of this important parameter can quickly increase by an order of magnitude.
Fractal magnetization of continental crust
Authors: Pilkington M. , Todoeschuck J. P.
Published: February 2008

Many geophysical variables exhibit fractal or at least scaling behaviour, notably in having power spectra proportional to some power of frequency. Power spectra of magnetic susceptibility logs from boreholes are proportional to spatial frequency f−2 for wavelengths from 0.1 to 100 m. If the power spectrum of the susceptibility or magnetization of the crust is known then the power spectrum of the magnetic field at the surface can be calculated. We show that if magnetization is scaling with a single scaling exponent then the observed f−3 power spectra of aeromagnetic data implies a three‐dimensional f−4 susceptibility distribution for length scales of 1 to 1000 km. One way to reconcile the two results is to introduce anisotropy in the form of a f−4 variation for the horizontal direction and f−2 for the vertical.
Shapes of Venusian “pancake” domes imply episodic emplacement and silicic composition
Authors: Fink Jonathan H. , Bridges Nathan T. , Grimm Robert E.
Published: February 2008

The main evidence available for constraining the composition of the large circular “pancake” domes on Venus is their gross morphology. Laboratory simulations using polyethylene glycol show that the height to diameter (aspect) ratios of domes of a given total volume depend critically on whether their extrusion was continuous or episodic, with more episodes leading to greater cooling and taller domes. Thus without observations of their emplacement, the compositions of venusian domes cannot be uniquely constrained by their morphology. However, by considering a population of 51 venusian domes to represent a sampling of many stages during the growth of domes with comparable histories, and by plotting aspect ratio versus total volume, we find that the shapes of the domes are most consistent with episodic emplacement. On Earth this mode of dome growth is found almost exclusively in lavas of dacite to rhyolite composition, strengthening earlier inferences about the presence of evolved magmas on Venus.
The polar cap environment of outflowing O+
Authors: Horwitz J. L. , Pollock C. J. , Moore T. E. , Peterson W. K. , Burch J. L. , Winningham J. D. , Craven J. D. , Frank L. A. , Persoon A.
Published: February 2008

Ion composition measurements by Dynamics Explorer 1 often show upward O+ beams at polar latitudes, with streaming energies of 1–20 eV or more. Here we utilize measurements of core (0–50 eV) and “energetic” (∼0–1 keV) ion composition, plasma waves, and auroral images from DE 1 and plasma ions and electrons from DE 2 to examine some of their properties in the context of the polar cap environment. It is found that two distinct populations of O+ beams are observed: “high‐speed” (10–30 eV or higher streaming energies) and “low‐speed” (generally <10‐eV streaming energies). The “high‐speed” polar beams show an “auroral” connection; i.e., they are observed on or near field lines threading auroral arcs seen in DE 1 images. The “low‐speed” streams are on or near field lines threading the dark polar cap and may be convected from the cleft ion fountain. The low‐speed streams are generally much more stable in energy and flux, while the high‐speed streams tend to be bursty. In general, the streams are convecting antisunward, with velocities of 5–14 km/s in the orbital plane. We sought to obtain plasma density estimates from plasma wave measurements, through analysis of features of auroral hiss as well as upper hybrid emissions. Densities in the range 1–5 el/cm³ were indicated for one segment of a DE 1 polar cap pass; however, the measurements generally indicate little auroral hiss or upper hybrid emissions in the polar cap for the other cases considered here. Estimates of electrostatic potential drops above the DE 2 satellite have been made using energy‐angle spectrograms of photoelectron data, under the assumption that the field lines of observation are “effectively” open. Potential drops often are in the 20‐ to 40‐V range. At other times the potential falls below the ∼5‐V instrument threshold, or there are insufficient photoelectron fluxes for estimation. These limited data suggest that the largest potential drops are just poleward of the cleft or near its poleward edge and there is a decline of the drop in the antisunward direction. No obvious correlation between the potential estimates and “nearby” O+ streaming energies is seen.
An experimental investigation of thermospheric structure near an auroral arc
Authors: Eastes R. W. , Killeen T. L. , Wu Q. , Winningham J. D. , Hoegy W. R. , Wharton L. E. , Carignan G. R.
Published: February 2008

Observations of thermospheric parameters, made from the Dynamics Explorer 2 (DE 2) spacecraft during three successive orbital crossings of a quiescent dusk sector auroral arc, have been compared with the predictions of three fine‐grid auroral arc models. DE 2 measured the ion and neutral winds, electron and neutral temperatures, neutral composition, and energetic auroral electron spectra (5 eV to 32 keV) at ∼320 km altitude. These observations were at high spatial and temporal resolution, suitable for comparisons with the models. The observed zonal and meridional neutral winds near the arc were greater than the model predictions, probably because of the presence of stronger electric fields and higher ion and neutral densities during the observations than were used in the models. The measured vertical winds were also larger than the corresponding model values. The DE 2 composition measurements showed a local increase of the N2/O ratio in the arc, which is interpreted to be a result of the upward motion of N2‐rich air. A reduction in the measured neutral temperature of ∼100 K in the arc, relative to temperatures on either side, was found for all three arc crossings. Of the three theoretical models examined, only one shows any tendency for the neutral thermospheric temperature to drop in the auroral arc, and the decrease calculated is significantly less than the observed temperature change. A simple calculation of the adiabatic cooling effect of the observed upward motion yields a temperature drop of ∼160 K, comparable with the observed temperature reduction (∼100 K).
Topology and signatures of a model for flux transfer events based on vortex‐induced reconnection
Authors: Liu Z. X. , Zhu Z. W. , Li F. , Pu Z. Y.
Published: February 2008

A model of the disturbed magnetic field and disturbed velocity of flux transfer events (FTEs) is deduced on the basis of the vortex‐induced reconnection theory. The topology and signatures of FTEs are calculated and discussed. We propose that the observed forms of FTE signatures depend on the motional direction of the FTE tube, the positions of the spacecraft relative to the passing FTE tube, and which part of the FTE tube (the magnetosphere part, the magnetopause part, or the magnetosheath part) the spacecraft is passing through. It is found that when a FTE tube moves from south to north along a straight line in the northern hemisphere, positive FTEs appear for most passages; however, reverse FTEs are also observed occasionally while the signatures ofBZ(BL) appear as a single peak, and the irregular FTEs always correspond to oblique line motions of the FTE tube. The velocity signatures are similar to those of the magnetic field, but in the northern hemisphere their directions are all just opposite to the magnetic field. The calculated results for the magnetic field are compared with 61 observed FTEs. The observed signatures (BNandBL) of 52 FTEs are consistent with our calculations. The results indicate that a majority of observed FTEs correspond to passages of spacecraft through the edges of FTE tubes.
Is the Earth's magnetotail balloon unstable?
Authors: Lee D. ‐Y. , Wolf R. A.
Published: February 2008

We consider the ideal MHD stability of two‐dimensional equilibrium models of the geomagnetic tail, where the magnetic field is in the xz plane and nothing depends on the coordinate y. Ballooning‐type perturbations are considered for which |ky|L≫1, where L is the scale length for variations in the xz plane. We employ the energy principle to test the stability of each flux tube of the plasma sheet for a "hard" ionospheric boundary condition (v = 0) and present a physical argument suggesting that it is the most appropriate simple boundary condition for the Earth's plasma sheet. Numerical results are presented for compressible ballooning modes that are symmetric about the center of the current sheet. For such a hard boundary condition, we found no reasonable magnetotail configuration that was unstable to compressible symmetric ballooning but stable against interchange. Therefore compressible symmetric ideal‐MHD ballooning seems incapable of substantially altering the large‐scale configuration of the magnetotail.
On the frequency of Langmuir waves in the ionosphere
Authors: Heinselman C. J. , Vickrey J. F.
Published: February 2008

Diverse geophysical applications exploit the existence of Langmuir waves in the natural ionosphere and magnetosphere. The frequency of these waves is often used as an absolute measure of plasma concentration in both remote sensing and in‐situ diagnostics. It is often wrongly assumed, however, that the frequency of Langmuir waves is directly interpretable in terms of the natural oscillation frequency of the thermal electron gas. This is only approximately true. Studies requiring more precision include the effects of the plasma temperature on the dispersion relation for these waves. Because this temperature dependence is also a function of the k vector of the waves, care must be taken when comparing measurements at different probing wavelengths (e.g., ionosondes and incoherent scatter radars). Herein, we point out that even further considerations are important in determining the exact frequency of Langmuir waves. The velocity distribution function of the entire electron gas must be accounted for properly. In particular, the presence of photoelectrons or auroral secondary electrons can affect the frequency of these waves. Failure to account properly for these effects may explain the unphysically large currents previously estimated by incoherent scatter measurements of the frequency asymmetry in upshifted and downshifted plasma line echoes.
Wave phenomena in the upstream region of Saturn
Authors: Orlowski D. S. , Russell C. T. , Lepping R. P.
Published: February 2008

The magnetic field data returned from the Voyager 1 and 2 transit through the upstream region of Saturn have been examined from 0.1 mHz to 0.25 Hz to search for the signatures of waves associated with both the electron and ion foreshocks. In contrast to the Earth, in the ion foreshock of Saturn we find two distinct bands of wave activity at frequencies around 0.5 and 2‐mHz in the spacecraft frame. These two waves differ considerably in their properties. The 0.5‐mHz waves are right handed, strongly elliptically polarized, while the 2‐mHz waves are elliptically or circularly polarized in the left‐handed sense. These two different waves may be associated with two different backstreaming particle distributions, possibly those reflected from the shock and those leaking from the hot magnetosheath. We also observe right‐hand elliptically polarized upstream waves propagating obliquely to the magnetic field at frequencies between 90 and 150 mHz, which are above the proton gyrofrequency in the spacecraft frame. The frequency of these waves appears to be dependent on the interplanetary magnetic field orientation. Such dependence is characteristic of the 1‐Hz waves at Earth. Discrete wave packets associated with shocklets are for the first time observed in the foreshock of an outer planet.
On the role of interplanetary sources in the evolution of low‐frequency Alfvénic turbulence in the solar wind
Authors: Bavassano B. , Bruno R.
Published: February 2008

The Alfvénic turbulence observed in the interplanetary space is mainly made up of outward traveling waves of solar origin. Inward waves, although a minor component of the turbulence, play an important role in transferring energy between different scales through nonlinear interactions with outward modes. Inward waves can only have an interplanetary origin. A good understanding of how inward waves are generated in the interplanetary space is crucial in modeling the Alfvénic turbulence evolution. Solar wind regions with high differential kinetic energy, which can be converted into turbulent energy, are the best places to search for local sources of Alfvénic turbulence. We have studied one of these regions at different heliocentric distances inside 1 AU, focusing on large‐scale fluctuations (periods approximately between 1 and 12 hours). Such a region was indicated by previous investigations as one of the best candidates for turbulence generation. In fact, fluctuations with an inward sense of correlation are observed, but a detailed analysis leads us to favor the hypothesis that they are mainly due to variations in plasma velocity, density, and magnetic field related to structures convected by the solar wind and not to real inward propagating waves. At least in the inner heliosphere, a view in which local sources of waves play a relevant role in driving the Alfvénic turbulence evolution does not seem realistic. We propose that such evolution, especially at the larger scales, is mainly governed not by interplanetary sources but rather by interactions of the outward waves of solar origin with the wind structure.
Comparison of diffusion and particle drift descriptions of radial transport in the Earth's inner magnetosphere
Authors: Riley Pete , Wolf R. A.
Published: February 2008

A comparison is made between two approaches, radial diffusion and guiding‐center drift, for describing the radial motion of charged particles in the equatorial plane of the Earth's magnetosphere. For the storm event of August 1990, a time‐dependent, observation‐based model electrostatic field is computed. This field is then used (1) to calculate the drift of a ring of monoenergetic test particles and (2) to compute the specific radial diffusion coefficient and solve the relevant diffusion equation. Because of the electric field model employed and the data used to construct it, the particles considered were restricted to those with drift periods greater than 2 hours and therefore energies less than about 130 keV at L=3. Thus our calculations bear directly on only the low‐energy part of the radiation belts. Density profiles computed from diffusion theory are compared with the results of the guiding‐center simulation for a number of initial conditions. Mediocre agreement is found when the diffusion results are compared with the guiding‐center simulation for the August 1990 event. However, for cases where (1) a number of electric fields are recovered from the power spectrum and the particle drifts are averaged or (2) a number of electric fields are applied sequentially to a single distribution, the agreement is considerably better. The main conclusion from these tests is that the diffusion formalism gives only roughly right answers for a single real storm but does much better for an average over a statistical ensemble of storms. Finally, several previously derived diffusion coefficients are compared with the present one as functions of energy.
The nonlinear evolution of field line resonances in the Earth's magnetosphere
Authors: Rankin R. , Harrold B. G. , Samson J. C. , Frycz P.
Published: February 2008

Magnetohydrodynamic, field line resonances in the Earth's magnetosphere can have very large velocity shears and field‐aligned currents. Auroral radar measurements of high‐latitude resonances indicate that the velocities associated with the resonances in the E and F regions are often substantially greater than 1 km/s, and that the frequencies are in the interval from 1 to 4 mHz. Assuming that these resonances are oscillating at the fundamental mode frequency, and mapping these velocity fields along magnetic field lines to the equatorial plane shows that the velocity shears in the equatorial plane are of the order of 200 km/s over a radial distance of less than 2000 km (the amplitude of the velocity fluctuations is 100 km/s). Using a three‐dimensional magnetohydrodynamic computer simulation code, we show that the resonances evolve through the development of Kelvin‐Helmholtz instabilities near the equatorial plane. Within this framework, the instability is taking place on dipole magnetic field lines, and the resonances form a standing shear Alfvén wave field due to the boundary conditions which must be satisfied at the polar ionospheres. We find that the nonlinear evolution of the Kelvin‐Helmholtz instability leads to the propagation of vorticity from the equatorial plane to the polar ionosphere and that the vorticity leads ultimately to the dissipation of the resonance. This occurs within a quarter wave period of the shear Alfvén field associated with the resonances.
Spectrographic observation at wavelengths near 630 nm of the interaction between the atmosphere and the space shuttle exhaust
Authors: Broadfoot A. L. , Anderson E. , Sherard P. , Knecht D. J. , Viereck R. A. , Pike C. P. , Murad Edmond , Elgin J. E. , Bernstein L. S. , Kofsky I. L. , Rall D. L. A. , Blaha J. , Culbertson F. L.
Published: February 2008

The collision between the exhaust from the Primary Reaction Control System (PRCS) engines (870 pounds thrust) of the space shuttle and the ambient atmosphere has been observed from the Air Force Maui Optical Station (AMOS). Spectra have been obtained in the wavelength region near 630 nm. The temporal, spatial, and spectral distribution of the emission in this region has been recorded. The results reported here indicate that when the exhaust of the space shuttle interacts with the atmosphere in the ram direction, an intense, long‐lasting emission at 630 nm due to O(¹D → ³P) is generated. A substantial amount of O(¹D) is swept back onto the orbiter. Two processes are proposed for the formation of O(¹D): (1) excitation of atmospheric O(³P) by collisions with the exhaust of the space shuttle engines; and (2) charge exchange between ambient O+ and exhaust H2O. Calculations using the SOCRATES code show excellent agreement with the data.
First simultaneous measurements of neutral and ionized iron densities in the upper mesosphere
Authors: Alpers M. , Blix T. , Kirkwood S. , Krankowsky D. , Lübken F.‐J. , Lutz S. , von Zahn U.
Published: February 2008

We report on the results of a multi‐instrument study of a sporadic Fe layer and a sporadic E layer which took place in the night of September 20, 1991, at the Andøya Rocket Range (69°N, 16°E). A ground‐based lidar was used to measure mesospheric Fe densities, while simultaneously rocket‐borne probes and an ion mass spectrometer measured the profile of total ion density and ion composition, respectively. All instruments observed the sporadic layers near 97 km altitude. Radar tracking of chaff clouds established the wind field in the 90‐ to 100‐km region with high spatial resolution, while the EISCAT incoherent scatter radar measured the E fields in the F region. The major ion in the sporadic E layer turned out to be Mg+. The narrow Fe+ layer was 1 km lower than the sporadic Fe+ layer. The ratio of densities [Fe+]/[Fe] in the peaks of the sporadic layers was found to be 1.75.
Ion anisotropy instabilities in the magnetosheath
Authors: Gary S. Peter , Fuselier Stephen A. , Anderson Brian J.
Published: February 2008

Recent observations in Earth's magnetosheath have delineated several different kinds of magnetic fluctuation spectra below the proton cyclotron frequency. This paper provides a theoretical interpretation for some of these observations, describing solutions of the linear Vlasov dispersion equation for fully electromagnetic instabilities for particle distributions which model those observed in the magnetosheath. The model assumes a high‐β, homogeneous plasma with bi‐Maxwellian, anisotropic (T⊥p>T∥p) protons; a similarly anisotropic, tenuous, doubly ionized helium component; and isotropic cool electrons. The model also assumes a uniform magnetic field and no relative average drifts among the components. This model yields three growing modes: the proton cyclotron anisotropy, the helium cyclotron anisotropy, and the mirror instabilities. The regimes in βp versus T⊥p/T∥p space which correspond to maximum growth rates of the mirror and proton cyclotron anisotropy instabilities are computed, including helium densities appropriate for the magnetosheath; the results show very good agreement with the observations of mirror‐like and proton‐cyclotron‐like events. This agreement with observations implies that the transition between cyclotron and mirror fluctuation dominance is consistent with linear theory.
A dust‐driven flux tube interchange instability
Authors: Morfill Gregor E. , Goertz Christoph K. , Havnes Ove
Published: February 2008

We describe, formulate, and investigate quantitatively a physical process which will lead to radial plasma transport in dusty planetary magnetospheres. The process depends on plasma inhomogeneities (in particular electron temperature fluctuations) interacting with distributed fine‐grained ring material to drive currents along the magnetic field lines, which close through the ionosphere. Azimuthal electric fields generated in this way will then lead to radial plasma flows via the E×B drift. The process is investigated both for Saturn's oxygen torus and for the Io torus in Jupiter's magnetosphere.
On the coupling between the lower and the upper thermosphere during the First Lower Thermosphere Coupling Study
Authors: Fesen C. G. , Hagan M. E. , Tepley C. A. , Roble R. G.
Published: February 2008

Simultaneous observations of the neutral winds and temperatures and electron densities in the upper and lower thermosphere are presented. The measurements were made at Arecibo and Millstone Hill during the first Lower Thermosphere Coupling Study (LTCS 1) campaign in September 1987. The observations show a great deal of day‐to‐day variability possibly due to the geomagnetic activity which occurred during the period; storm effects are discernible in the data at low altitudes (near 120 km) even at low latitudes (near 18°N). Theoretical simulations of the period were attempted by the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model. A novel development was the use in the model of lower boundary conditions representative of the upward propagating semidiurnal tides based on the LTCS 1 mesosphere‐lower thermosphere observations. The upward propagating diurnal 1,1 mode was also included in the model. Simulations of the low and mid latitudes did not substantially improve with the use of lower boundary conditions derived from the measurements. The model was also unable to reproduce the observations in both the lower and upper thermosphere. This may be partially due to the effects of geomagnetic activity on the atmospheric fields, since the observational period included two intervals of geomagnetic disturbances. In the lower thermosphere, resolution of the discrepancies between the data and model predictions may require the extension of the model to lower altitudes. At present, the model predictions in the lower thermosphere are strongly dependent on the lower boundary conditions. The model predicts that diurnal waves are not negligible in analysis of lower thermosphere data; indeed, near 100 km at the model location of Arecibo, the diurnal wave amplitudes of the meridional winds were predicted to be greater than the semidiurnal. Investigation of the model predictions awaits the development of techniques to obtain observations of at least 24 hours duration in the lower thermosphere and the extension of the theoretical models to lower altitudes.
Electric field and electron density thresholds for coherent auroral echo onset
Authors: Kustov A. V. , Uspensky M. V. , Jones G. O. L. , Williams P. J. S. , Sofko G. J. , Koehler J. A.
Published: February 2008

Simultaneous observations of the Polar Geophysical Institute 83‐MHz auroral radar and the European Incoherent Scatter facility were used to study the coherent echo threshold phenomenon. It is shown that, for electric fields not far from the Farley‐Buneman instability threshold of ∼20 mV/m, the onset of echo occurrence was due to an electron density increase above some threshold value. This value was estimated to be ∼2.5 × 1011 m−3. For smaller electric fields of 5‐10 mV/m, echoes were not observed even for electron densities double the above threshold value. The importance of both factors, electric field and electron density, and some other effects are discussed in light of recent studies of coherent echo intensities and their dependence on plasma conditions at electrojet altitudes.
A wave model interpretation of the evolution of rotational discontinuities
Authors: Vasquez Bernard J. , Cargill Peter J.
Published: February 2008

The evolution of rotational discontinuities (RDs) is followed using a hybrid numerical code. An extensive parameter variation is carried out, with particular emphasis on β, Ti/Te, θB (the angle between the normal and total magnetic field), and the helicity of the RD. The RD structure is shown to have features in common with the evolution of both strongly modulated, nonlinear wave packets and linear dispersive wave propagation in oblique magnetic fields. For small θB, the RD disperses linearly giving fast and Alfvén waves upstream and downstream, respectively, and the familiar S‐shaped hodograms. At larger θB (≈ 30°), nonlinearity becomes important and strong coupling to a compressional (sonic) component can occur in the main current layer. When the ions are cold, there is a critical value of βe(=β*) when the intermediate wave train moves from the downstream side of the RD to the upstream and is replaced on the downstream side by slow modes. This is reflected in the hodograms by a change of the wave polarization on both sides, and represents an important modification to the original Goodrich and Cargill (1991) wave model of RDs. As Ti/Te increases, the spreading rate of the current layer increases for moderate θB. For large θB(≈ 60°), RDs with electron (ion) sense of rotation show increased (decreased) spreading with increasing Ti/Te. These results are applied to RDs observed in the solar wind and at the magnetopause.
On the quasi‐periodic nature of magnetopause flux transfer events
Authors: Lockwood M. , Wild M. N.
Published: February 2008

The recurrence rate of flux transfer events (FTEs) observed near the dayside magnetopause is discussed. A survey of magnetopause observations by the ISEE satellites shows that the distribution of the intervals between FTE signatures has a mode value of 3 min, but is highly skewed, having upper and lower decile values of 1.5 min and 18.5 min, respectively. The mean value is found to be 8 min, consistent with previous surveys of magnetopause data. The recurrence of quasi‐periodic events in the dayside auroral ionosphere is frequently used as evidence for an association with magnetopause FTEs, and the distribution of their repetition intervals should be matched to that presented here if such an association is to be confirmed. A survey of 1 year's 15‐s data on the interplanetary magnetic field (IMF) suggests that the derived distribution could arise from fluctuations in the IMF Bz component, rather than from a natural oscillation frequency of the magnetosphere‐ionosphere system.
Pitch angle diffusion of low‐energy electrons by whistler mode waves
Authors: Johnstone A. D. , Walton D. M. , Liu R. , Hardy D. A.
Published: February 2008

It has often been argued that electrons with energy below approximately 10 keV cannot be pitch‐angle diffused by whistler mode waves because they have less than a critical minimum parallel energy. We show that there is no theoretical justification for an energy threshold based on the grounds of either resonance or instability and present energy/pitch angle plots from the low energy plasma analyzer (LEPA) on the CRRES spacecraft which support this analysis because they have features to be expected from a whistler mode interaction. This shows that whistler mode diffusion is responsible for a substantial amount of low‐energy electron precipitation at high latitudes and is probably responsible for the diffuse aurora. Furthermore, we show that releasing lithium from the CRRES spacecraft in the outer zone was unlikely to stimulate precipitation or to trigger a substorm.
Simultaneous observation of the poleward expansion of substorm electrojet activity and the tailward expansion of current sheet disruption in the near‐Earth magnetotail
Authors: Lopez R. E. , Koskinen H. E. J. , Pulkkinen T. I. , Bösinger T. , McEntire R. W. , Potemra T. A.
Published: February 2008

In this paper we present observations of a substorm that occurred on June 7, 1985. The data consist of energetic ion and magnetic field data from AMPTE/CCE, magnetometer data from the ground stations of the EISCAT magnetometer cross, STARE radar data, and Pi 1 data from Sodankylä. CCE was in the Finnish local time sector, and the model field line threading CCE came down near the center of the EISCAT magnetometer cross. Furthermore, CCE was located near the neutral sheet and observed the disruption of the cross‐tail current in situ. At 2209 UT, Sodankylä registered a Pi 1 burst and the EISCAT stations recorded the onset of a negative bay. Simultaneously, CCE observed the onset of current sheet disruption and ion energization. The energetic ions observed during this initial burst had gyrocenters earthward of CCE. An intensification of activity occurred at 2212 UT, and it was observed essentially simultaneously on the ground and in space. It consisted of an intensification of Pi 1 activity, an intensification of the westward electrojet poleward of the initial breakup latitude (as determined from magnetometer and radar data), and magnetic turbulence and a burst of energetic ions at CCE in the magnetotail. Significantly, this second burst was composed only of ions with gyrocenters tailward of the satellite, and the magnetic turbulence was considerably weaker than that observed during the initial current disruption. This strongly suggests that the current disruption region had moved tailward of CCE. The event provides a direct observational link between the tailward expansion of active regions in the near‐Earth magnetotail and the poleward expansion of ionospheric electrojet activity during substorms.
A study of the propagation of solar energetic protons in the inner heliosphere
Authors: Wanner Wolfgang , Wibberenz Gerd
Published: February 2008

Eight solar energetic proton events observed in the inner heliosphere on the spacecraft Helios 1 or 2 are studied. Particle pitch angle diffusion coefficients and mean free paths are derived from a power spectral analysis of the turbulence of the magnetic field during the time of the events. Quasi‐linear theory of wave‐particle interaction is applied strictly throughout in combination with the slab model of fluctuations. Several points receive special attention: (1) the large difference in scattering conditions from one event to the other, which requires an individual analysis of each event; (2) the selection of the exact stretch of magnetic field data to be analyzed and the influence of this choice on the result; (3) the exclusion of data periods that contain discontinuities and other nonoscillatory structures; and (4) short‐scale variations of the overall field fluctuation level, which can be considerable. As a consequence, the mean free paths of the particles are derived from time series of the varying momentary power density spectra as an appropriate average of the scattering conditions (as an average mean free path). These results from the magnetic field spectra are then compared with results of fits of propagation models to the particle time‐intensity and time‐anisotropy profiles observed at the same time. Of eight such comparisons, half of the cases exhibit for the first time an approximate agreement between the results of the two approaches, particle analysis and magnetic field analysis. The others, however, still show the discrepancy that is cited in the literature. Among the events with good agreement are the two extreme cases in the whole Helios data set, the event of April 11, 1978, Helios 2, with a mean free path from quasi‐linear theory of 0.01 AU, and the events of June 7, 1980, Helios 1, with a mean free path from quasi‐linear theory of about 0.8 AU at a rigidity of 0.1 GV. This shows not only that quasi‐linear theory in a wave and slab model framework is indeed capable of producing correct results under certain conditions if an appropriate case‐by‐case analysis is conducted, but also that these cases can encompass a wide range of turbulence strengths. Several possibilities for the source of the discrepancy that remains for the other cases, which the theory and the assumptions applied in this work obviously do not explain, are discussed. However, at the present time no definite solution to the problem is obvious.