bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma waves observed at low altitudes in the tenuous Venus nightside ionosphere
Authors: Strangeway R. J. , Russell C. T. , Ho C. M. , Brace L. H.
Published: February 2008

The Pioneer Venus Orbiter Electric Field Detector (OEFD) measured many plasma wave bursts throughout the low altitude ionosphere during the final entry phase of the spacecraft. Apart from 100 Hz bursts observed at very low altitudes (∼130 km), the bursts fall into two classes. The first of these is a wideband signal that is observed in regions of low magnetic field, but average densities, in comparison to the prevailing ionospheric condition. This wideband signal is not observed in the 30 kHz channel of the OEFD, but is restricted to the 5.4 kHz channel and lower. Since these bursts are observed with roughly constant burst rate above 160 km altitude, we attribute them to ion acoustic mode waves generated by precipitating solar wind electrons. The second type of signal is restricted to 100 Hz only, and is observed in regions of low electron beta, consistent with whistler‐mode waves. These waves could be generated by lightning in the Venus atmosphere if the vertical component of the magnetic field >3.6 nT. Unfortunately, the spacecraft spin axis is mainly horizontal, and only that component of magnetic field can be measured. Alternatively, the 100 Hz bursts could be generated locally through gradient drift instabilities, provided the ambient magnetic field is horizontal. Because the ionosphere is very different during the entry phase, compared to the ionosphere as observed early in the Pioneer Venus mission, any conclusions regarding the source of the plasma waves detected during entry phase cannot be applied directly to the earlier observations.
Surface wave dispersion and upper mantle structure beneath southern Germany from joint inversion of network recorded teleseismic events
Authors: Stange Stefan , Friederich Wolfgang
Published: February 2008

We have determined the dispersion of fundamental mode Rayleigh waves in Southern Germany in the frequency range 4–50 mHz by simultaneously inverting data from 66 teleseismic events recorded at a network of up to 9 broadband stations. Three different inversion experiments were performed: determination of (1) isotropic phase velocity, of (2) azimuthally anisotropic phase velocity assuming a plane wave traveling across the network, and (3) isotropic phase velocity but now allowing for non‐plane properties of the wavefield. In experiments (1) and (2), for each event amplitude, phase and deviation from the great circle azimuth were estimated as well. We have obtained remarkably stable results for phase velocity over the whole frequency range. Between 7 and 25 mHz azimuthal anisotropy is definitely below 0.5 percent. Below 7 and above 25 mHz we found up to 3 percent anisotropy but with negligible variance reduction compared to the isotropic inversion experiment (1). Admitting non‐plane wavefields in the inversion still yields nearly identical phase velocities. On inversion of the phase velocity curve in terms of shear‐wave velocity we obtained a rather shallow asthenosphere with minimum υSof 4.28 km/s at about 130 km depth covered by a thin lid with υSof 4.63 km/s.
Density and morphology of impact craters on Tessera Terrain, Venus
Authors: Ivanov M. A. , Basilevsky A. T.
Published: February 2008

Densities of impact craters on tessera, which is complex ridged terrain of tectonic origin, and on the remainder of the planet, which is mostly volcanic plains, were studied using Magellan images for about 96% of the surface of Venus. The density of large (D>16 km) impact craters on tessera is higher by a factor of about 1.4 than on the remainder of the planet. This means that the tessera crater retention age is larger than the age of the plains. This is in agreement with the well known fact that tessera is embayed by the surrounding volcanic plains. The density of small (D<16 km) impact craters on tessera is lower than on the remainder of the planet, evidently an observational bias caused by a difficulty in recognizing the small craters on rough tessera terrain. The absence of recognizable tectonic deformation in most of the large on‐tessera craters means that during the period of crater emplacement most of the studied tesserae were tectonically stable and did not undergo noticeable degree of deformation.
Observation of intense wave bursts at very low altitudes within the Venus nightside ionosphere
Authors: Strangeway R. J. , Russell C. T. , Ho C. M.
Published: February 2008

Intense ELF (100 Hz) bursts were detected by the Pioneer Venus Orbiter plasma wave instrument during the final operations of the spacecraft prior to atmospheric entry. These bursts were detected at ∼130 km altitude around 0400 local time. The wave activity lasted for several tens of seconds. Furthermore the bursts were not symmetric about periapsis, unlike instrument noise caused by neutral impacts on the spacecraft. The bursts had a vertical attenuation scale height of the order 1 km, consistent with that expected for whistler‐mode waves propagating through a collisional ionosphere. Since the decay of the signals appears to be due to attenuation, the source must persist for several tens of seconds. The wave bursts could therefore be the signature of electromagnetic radiation entering the bottomside ionosphere from several distant sources, as would be expected if lightning were a relatively persistent phenomenon within the Venus atmosphere.
Sensing atmospheric water vapor with the global positioning system
Authors: Rocken Christian , Ware Randolph , Van Hove Teresa , Solheim Fredrick , Alber Chris , Johnson James , Bevis Mike , Businger Steven
Published: February 2008

Global Positioning System (GPS) receivers, water vapor radiometers (WVRs), and surface meteorological equipment were operated at both ends of a 50‐km baseline in Colorado to measure the precipitable water vapor (PWV) and wet delay in the line‐of‐sight to GPS satellites. Using high precision orbits, WVR‐measured and GPS‐inferred PWV differences between the two sites usually agreed to better than 1 mm. Using less precise on‐line broadcast orbits increased the discrepancy by 30%. Data simulations show that GPS measurements can provide mm‐level separate PWV estimates for the two sites, as opposed to just their difference, if baselines exceed 500 km and the highest accuracy GPS orbits are used.
Pointed water vapor radiometer corrections for accurate global positioning system surveying
Authors: Ware Randolph , Rocken Christian , Solheim Fredrick , Van Hove Teresa , Alber Chris , Johnson James
Published: February 2008

Delay of the Global Positioning System (GPS) signal due to atmospheric water vapor is a major source of error in GPS surveying. Improved vertical accuracy is important for sea level and polar isostasy measurements, geodesy, normal fault motion, subsidence, earthquake studies, air and ground‐based gravimetry, ice dynamics, and volcanology. We conducted a GPS survey using water vapor radiometers (WVRs) pointed toward GPS satellites to correct for azimuthal variations in water vapor. We report 2.6 mm vertical precision on a 50‐km baseline for 19 solution days. Kalman filter or least‐square corrections to the same data do not account for azimuthal distribution of water vapor and are degraded by 70%.
Laboratory flow reactor measurements of the reaction SO3 + H2O + M → H2SO4 + M: Implications for gaseous H2SO4 and aerosol formation in the plumes of jet aircraft
Authors: Reiner Th. , Arnold F.
Published: February 2008

The reaction SO3 + H2O + M has been investigated with a tubular flow reactor at a pressure of 85 Torr. Sulfuric acid has been identified as the most probable product of the reaction. Both, reactant SO3 as well as the product H2SO4 have been measured quantitatively using chemical ionization mass spectrometry. An upper limit for the reaction rate coefficient of 2.4 × 10−15 cm³s−1 has been inferred from the measurements. Implications for sulfuric acid and aerosol formation in the plumes of jet aircraft are discussed.
Is the AE index the result of nonlinear dynamics?
Authors: Prichard D. , Price C. P.
Published: February 2008

Recently there as been much interest in determining if the global geomagnetic system, as characterized by AE and AL indices, can be described by a low dimensional, possibly chaotic, non‐linear dynamical system. By using a statistical method for detecting nonlinearity, we show that for one interval which has been previous analyzed (the first 8192 minutes of January 1983) there is no evidence for low dimensional behavior.
Airborne Arctic Stratospheric Expedition II: An overview
Authors: Anderson James G. , Toon Owen B.
Published: February 2008

The sudden onset of ozone depletion in the antarctic vortex set a precedent for both the time scale and the severity of global change. The Airborne Antarctic Ozone Experiment (AAOE), staged from Punta Arenas, Chile, in 1987, established that CFCs, halons, and methyl bromide, the dominant sources of chlorine and bromine radicals in the stratosphere, control the rate of ozone destruction over the Antarctic; that the vortex is depleted in reactive nitrogen and water vapor; and that diabatic cooling during the Antarctic winter leads to subsidence within the vortex core, importing air from higher altitudes and lower latitudes. This last conclusion is based on observed dramatic distortion in the tracer fields, most notably N2O.

In 1989, the first Airborne Arctic Stratospheric Expedition (AASE‐I), staged from Stavanger, Norway, and using the same aircraft employed for AAOE (the NASA ER‐2 and the NASA DC‐8), discovered that while NOx and to some degree NOy were perturbed within the arctic vortex, there was little evidence for desiccation. Under these (in contrast to the antarctic) marginally perturbed conditions, however, ClO was found to be dramatically enhanced such that a large fraction of the available (inorganic) chlorine resided in the form of ClO and its dimer ClOOCl.

This leaves two abiding issues for the northern hemisphere and the mission of the second Airborne Arctic Stratospheric Expedition (AASE‐II): (1) Will significant ozone erosion occur within the arctic vortex in the next ten years as chlorine loading in the stratosphere exceeds four parts per billion by volume? (2) Which mechanisms are responsible for the observed ozone erosion poleward of 30°N in the winter/spring northern hemisphere reported in satellite observations?
The oceanic contribution to the Earth's seasonal angular momentum budget
Authors: Dickey J. O. , Marcus S. L. , Johns C. M. , Hide R. , Thompson S. R.
Published: February 2008

Seasonal variations in the speed of the Earth's rotation manifest themselves as fluctuations in the length of the day (LOD) with an amplitude of about 1000 microseconds (µs). We know from previous work that at least 95% of these variations can be accounted for in terms of angular momentum exchanged between the atmosphere and the solid Earth. Here we examine the respective contributions of the Antarctic Circumpolar Current (ACC) and the global oceans to the Earth's seasonal angular momentum budget, using in situ data from the Drake Passage and results from both the oceanic regional model (Fine Resolution Antarctic Model—FRAM) of Webb et al. [1991] and the global oceanic model of Maier‐Reimer et al. [1993] as analyzed by Brosche et al. [1990]. The estimated annual contribution of the ACC (2–4 µs) is much smaller than the total variation in the oceanic models or the existing LOD‐AAM residual (both ∼15–20 µs). The estimated semi‐annual ACC contribution (3–8 µs) is offset by counter‐currents further north in both oceanic models, which exhibit larger semi‐annual variations in planetary angular momentum. Further refinements in the Earth's seasonal angular momentum budget, therefore, will require the full (planetary plus relative) contribution of the global oceans in addition to that of the ACC.
The North Atlantic Oscillation signature in deuterium and deuterium excess signals in the Greenland Ice Sheet Project 2 Ice Core, 1840–1970
Authors: Barlow L. K. , White J. W. C. , Barry R. G. , Rogers J. C. , Grootes P. M.
Published: February 2008

The Greenland Ice Sheet Project 2 (GISP2) core can enhance our understanding of the relationship between parameters measured in the ice in central Greenland and variability in the ocean, atmosphere, and cryosphere of the North Atlantic Ocean and adjacent land masses. Seasonal (summer, winter) to annual responses of δD and deuterium excess isotopic signals in the GISP2 core to the seesaw in winter temperatures between West Greenland and northern Europe from A.D. 1840 to 1970 are investigated. This seesaw represents extreme modes of the North Atlantic Oscillation, which also influences sea surface temperatures (SSTs), atmospheric pressures, geostrophic wind strength, and sea ice extents beyond the winter season. Temperature excursions inferred from the δD record during seesaw/extreme NAO mode years move in the same direction as the West Greenland side of the seesaw. Symmetry with the West Greenland side of the seesaw suggests a possible mechanism for damping in the ice core record of the lowest decadal temperatures experienced in Europe from A.D. 1500 to 1700. Seasonal and annual deuterium excess excursions during seesaw years show negative correlation with δD. This suggests an isotopic response to a SST/ land temperature seesaw. The isotopic record from GISP2 may therefore give information on both ice sheet and sea surface temperature variability. Cross‐plots of δD and d show a tendency for data to be grouped according to the prevailing mode of the seesaw, but do not provide unambiguous identification of individual seesaw years. A combination of ice core and tree ring data sets may allow more confident identification of GA and GB (extreme NAO mode) years prior to 1840.
Nonlinear aspects of core‐mantle interaction
Authors: Zhang Keke , Gubbins David
Published: February 2008

Core‐mantle interaction is now recognised as having a major influence over core dynamics and geomagnetic behaviour. We argue that nonlinear effects will dominate longer‐term phenomena and be relatively independent of the type of mechanism: whether it be thermal, electromagnetic, or topographic. A detailed study of thermal interaction for non‐magnetic rotating convection exhibits locking of the convection to the boundary and resonance between the convective modes and the boundary condition. We speculate that core convection and the geomagnetic field will lock to boundary anomalies giving configurations that are stable for long periods of time, possibly interspersed by intervals of rapid change as the natural time variation of rotating convection asserts itself. The instabilities that arise in the thermal case are rather general and are illustrated here mathematically by a simple analogue equation and physically by a pendulum with variable gravitational force. This generality suggests the same nonlinear behaviour may arise in topographic and electromagnetic coupling.
HF Doppler and ionosonde observations on the onset conditions of equatorial spread F
Authors: Jayachandran B. , Balan N. , Rao P. B. , Sastri J. H. , Bailey G. J.
Published: February 2008

The relative importance of height, vertical drift velocity, and electron density gradient of the postsunset bottomside (5.5 MHz) equatorial F region for the onset of spread F is studied using simultaneous HF Doppler radar and ionosonde observations. The study conducted for the periods January‐March of 1984 and 1985 shows that the height of the F layer, determined by the time history of the prereversal enhancement of the drift velocity, is the deciding factor for the onset of equatorial spread F (ESF) with little contribution from the electron density gradient. Maximum growth rate of linear collisional Rayleigh‐Taylor instability occurs at the time of peak height rather than at the time of peak velocity confirming that, for the onset of ESF, the layer should attain a threshold height. The threshold (group) height of the 5.5 MHz layer falls from ∼450 km in 1984 (mean F10.7 equals 120) to ∼350 km in 1985 (mean F10.7 equals 70); the corresponding evening peak upward drift velocities decrease from about 30 m s−1 in 1984 to about 20 m s−1 in 1985. The significant fall of the thresholds with the declining solar activity is due to the decrease in the ion‐neutral collision frequency with declining solar activity; the fall of the thresholds is reflected in large decreases in the intensity and duration of the spread F.
Prediction of the location and form of the auroral zone: Wandering of the auroral zone out of high latitudes
Authors: Oguti T.
Published: February 2008

On the basis of a forward extrapolation of secular variations in the geomagnetic field, in particular a rapid decrease in the dipole moment, wandering of the dipole axis direction and westward drifts of the nondipole components, the change in the location and form of the auroral zone for the next 1000 years is estimated. The auroral zone will tend to expand to lower latitudes because of a decrease in the dipole moment, rotate westward and become considerably deformed. In 1000 years it will no longer be a circular zone in each high latitude. A large part of Europe may be within the auroral zone in 500 years from now, and the United States and even Japan may also be in the auroral zone in 1000 years.
Structure and properties of the subsolar magnetopause for northward interplanetary magnetic field: Multiple‐instrument particle observations
Authors: Song P. , Russell C. T. , Fitzenreiter R. J. , Gosling J. T. , Thomsen M. F. , Mitchell D. G. , Fuselier S. A. , Parks G. K. , Anderson R. R. , Hubert D.
Published: February 2008

The structure and properties of the subsolar magnetopause for northward interplanetary magnetic feild (IMF) are studied with measurements from 10 different instruments for three ISEE crossings. Data show that the overall structure and properties are similar for the three crossings, indicating the magnetopause is relatively well determined in the subsolar region for strongly northward IMF. The measurements from different instruments are consistent with each other and complementary based on the current knowledge of space plasma physics. The combined data set suggests that the magnetopause region is best organized by defining a sheath transition layer and steplike boundary layers. The sheath transition layer contains mostly magnetosheath particles. The magnetosheath, magnetospheric, and ionospheric populations are mixed in the interior boundary layers. This result, which is consistent with previous studies, is now supported by observations of a much broader spectrum of measurements including three‐dimensional electron, energetic particle, heavy ion and plasma wave. Some new features are also found: even for quiet subsolar magnetopause crossings, transient or small‐scale structures still occur sporadically; slight heating may occur in the boundary layers. Some outstanding issues are clarified by this study: the electron flux enhancements in the lowest energies in the boundary layers and magnetosphere are ionospheric electrons and not photoelectrons from the spacecraft; for northward IMF, they are photoelectrons, but for southward IMF they may be secondary electrons; and the density measurements from differential and integral techniques are similar, leaving no room for a significant “invisible” population.
Stability of the Halley cometosheath with resistivity and plasma motion
Authors: Srivastava Krishna M. , Tsurutani Bruce T. , Goldstein Bruce E. , Sharma V.
Published: February 2008

The MHD stability of the cometary inner sheath determined by the balance between the inward Lorentz body force and the outward ion‐neutral drag force is investigated by numerically solving the wave equations which include resistivity, plasma motion and plasma pressure with the help of two‐point boundary value method. The eigenvalues and the eigenfunctions are obtained numerically by treating the cometary inner sheath as a layer of finite thickness, bounded by the contact surface, that is, the diamagnetic cavity boundary. To gain an insight into the problem, certain limiting cases of the wave equations are also discussed. The diamagnetic cavity boundary and the adjacent layer of about 100‐km thickness of comet Halley is found to be unstable. The effects of finite plasma pressure, dissociative recombination, mass loading due to photoionization, resistivity, and plasma motion are found to be stabilizing but are unable to quench the instability completely. An estimate of τc/τi shows that it lies in the range of 10 to 20 or even higher which appears to be sufficient for the effective penetration of the magnetic field perturbations into the cavity surface. Motion of the Halley ionopause has been confirmed by observations: according to Neubauer (1987), Halley ionopause seemed to have strong ripples with a wavelength of several hundred kilometers.
Electron ‐ whistler interaction at the Earth's bow shock: 1. Whistler instability
Authors: Veltri P. , Zimbardo G.
Published: February 2008

The interaction of whistler waves with the electrons in the Earth's quasi‐perpendicular bow shock of November 7, 1977, is considered. A Monte Carlo simulation which includes the effect of electrostatic noise is used to obtain the electron distribution function fnof;e across the shock layer. The simulated fnof;e is rather anisotropic and in the shock foot has a marked loss cone structure which is evident in the data, too; therefore we study the instability of fnof;fe with respect to the emission of whistler modes. Strong whistler emission from the electrons is found, with frequency and spatial dependencies of the growth rate in good agreement with the observed magnetic noise. Most of the whistlers are generated in the shock foot, where the loss cone in the electron distribution function is found. This indicates that in quasi‐perpendicular collisionless shock, besides the reflected ion beam and the field‐aligned electron beam, an important source of whistler mode waves is the electron loss cone due to reflection from the magnetic ramp.
Lobe cell convection as a summer phenomenon
Authors: Crooker N. U. , Rich F. J.
Published: February 2008

Patterns of average potential over the high‐latitude ionosphere in winter show that the dusk convection cell dominates the dawn cell, consistent with the presence of a day‐night conductivity gradient, as predicted by a number of models. However, in the summer hemisphere, when IMF By is strongly positive, the dusk cell so dominates the dawn cell that the latter nearly disappears; and when IMF By is strongly negative, the cells are most nearly equal. The difference between winter and summer can be explained by the addition in summer of a single lobe cell, that is, a cell confined to open field lines, circulating within the dusk cell of the two‐cell pattern when By is positive and within the dawn cell when By is negative. The result is consistent with predictions of the overdraped lobe model, that lobe cells occur in only one hemisphere at a time, and that their occurrence is controlled by dipole tilt.
Low‐Mach number quasi‐parallel shocks: Upstream waves
Authors: Scholer Manfred , Fujimoto Masaki
Published: February 2008

The structure of a low Mach number (Alfvén Mach number MA = 2.2) quasi‐parallel shocks is investigated using a two‐dimensional hybrid (particle ions, massless fluid electrons) code. We have run cases with average magnetic field ‐ shock normal angles of ΘBno = 20°, 30°, and 45°. Initially, phase standing dispersive whistler waves with an upstream directed group velocity are radiated by the shock ramp. As expected, the wave vector of these whistler waves is parallel to the average shock normal direction. The steepness of the shock ramp varies along the shock surface; this leads to a modulation of the whistler wave amplitude at a constant phase, i.e., perpendicular to the wave vector. Subsequently, ions are backstreaming from the shock into the upstream medium. At ΘBno = 30° these ions excite, as in the one‐dimensional hybrid simulations by Omidi et al. (1990), longer wavelength whistler waves by an electromagnetic ion/ion beam instability. The wave normal of these longer wavelength whistler waves is in between the average magnetic field direction and the shock normal. The skewing of the wave normal into the shock normal direction is attributed to a large density gradient of backstreaming ions upstream of the shock ramp in the shock normal direction. At ΘBno = 20° the shock consists of whistlers in the upstream, whose wavelength and shock directed phase velocity increases gradually toward the shock. It is suggested that in this more parallel case remnants of the dispersive whistlers continue to exist further upstream and interact with backstreaming ions. This leads to a change of the wave normal direction as the waves are convected toward the shock. In the 45° case the upstream density of backstreaming ions is greatly reduced. Only close to the shock ramp (within ∼ 5c/ωpi) does the backstreaming ion density exceed 10% of the solar wind density. Wave vectors of small wavelength whistlers in this region are almost aligned with the shock normal.
Ion cyclotron wave emission at the quasi‐perpendicular bow shock
Authors: Dendy R. O. , McClements K. G.
Published: February 2008

The power spectra of magnetic fluctuations occurring close to the ramp of the quasi‐perpendicular, low‐β bow shock indicate the presence of obliquely propagating electromagnetic waves with frequencies above the ion cyclotron frequency, Ωi. These waves appear to be associated with ion distributions consisting of a bi‐Maxwellian core and an energetic, approximately gyrotropic ring. We investigate the generation of ion cyclotron waves by distributions of this type, using particle and wave data from the AMPTE/IRM spacecraft. In the case of a monoenergetic ring, instability is possible over a broad range of frequencies ω > Ωi, with the highest growth rates occurring at propagation angles of typically 50° ‐ 80°. As the velocity spread of the ring υr increases, the growth rate of perpendicular‐propagating waves falls, complete stabilization occurring when υr is greater than about 20% of the mean ring speed u. The parallel‐propagating Alfvén ion cyclotron mode can be excited if the core is anisotropic, with T⊥ ≃ 3T∥. The maximum growth rate is obtained when υr is comparable to the core ion parallel thermal speed. However, if υr « u, the growth rate is much smaller than Ωi. Using these results, we show that certain qualitative features of the AMPTE/IRM wave data can be understood in terms of a nearly monoenergetic ion ring beam at the shock ramp, evolving into an extended ring beam, and then merging with a quasi‐bi‐Maxwellian ion core as it moves downstream.
Space and time distribution of HF excited Langmuir turbulence in the ionosphere: Comparison of theory and experiment
Authors: Dubois D. F. , Hanssen Alfred , Rose Harvey A. , Russell David
Published: February 2008

The predictions of strong Langmuir turbulence (SLT) theory are compared with radar observations of HF induced turbulence at Arecibo and Tromsø. The altitude distribution of turbulence observed in the cold start experiments of Fejer et al. (1991) imply that the ionospheric electron density profile is modified by the induced turbulence. The preconditioned observations at Arecibo and the Tromsø observations also appear to require a “disturbed” profile with several percent density fluctuations. With such density modifications postulated we conclude that SLT theory is in, at least, qualitative agreement with a large body of observations. Specifically SLT theory predicts, as part of a unified theory, and in distinction to the weak turbulence approximation, at least four unique physical signatures which can be compared to observations: (1.) A caviton continuum plus free mode peak in the plasma line power spectrum near reflection altitude for Arecibo conditions. (2.) A truncated decay‐cascade spectrum at lower altitudes (or densities). (3.) A continuous spectrum underlying the decay‐cascade spectrum. (4.) A zero frequency feature in the ion line power spectrum directly related to caviton dynamics. We find that there is sufficient ponderomotive pressure due to the Airy‐layered, induced Langmuir turbulence, to modify the electron density profile in a manner consistent with the time behavior of unpreconditioned Arecibo observations.
Does the ballooning instability trigger substorms in the near‐Earth magnetotail?
Authors: Ohtani Shin‐Ichi , Tamao Tsutomu
Published: February 2008

The stability of the near‐Earth magnetotail against ballooning (or configurational) instability is examined in the framework of the MHD approximation. It is emphasized that a change in plasma pressure induced by a meridional electric field drift δun is an important factor that determines the stability. We have to consider two ways in which plasma pressure changes, that is, a convective change −δun · ▽P0, where P0 is background plasma pressure, and plasma expansion/compression ‐P0▽ · δun. Since δun is perpendicular to the magnetic field and its magnitude is inversely proportional to the magnetic field strength, δun diverges/converges in usual tail magnetic field configurations. For the instability, the convective change must overwhelm the effects of the plasma expansion/compression. However, near the equator in the near‐Earth tail, the latter may overcompensate for the former. We describe the ballooning instability in terms of a coupling between the Alfvén and slow magnetosonic waves in an inhomogeneous plasma and derive instability conditions. The result shows that the excessive curvature stabilizes, rather than destabilizes, perturbations. It is also found that the field‐aligned flow stabilizes perturbations, as well as the field‐aligned current. We infer that under quiet conditions, the plasma pressure gradient in the near‐Earth tail is not sharp enough to trigger the instability. The plasma sheet is expected to become more stable during the substorm growth phase because of an increase in the field line curvature associated with the plasma sheet thinning. In the region closer to the Earth, including the ring current, the plasma pressure gradient may be localized in a limited range of the radial distance during the growth phase. However, recently reported plasma and magnetic field parameters before substorm onsets do not provide very convincing evidence that the ballooning instability is the triggering mechanism of substorms.
Solar wind control of the distant magnetotail: ISEE 3
Authors: Fairfield D. H.
Published: February 2008

During a 40‐day period in 1983, ISEE 3 was located about 225RE behind the Earth and remained within 12 RE of the nominal tail axis. During this time the spacecraft spent at least 70% of its time in the magnetotail with occasional excursions into the magnetosheath. However, during five geomagnetically disturbed intervals of 1‐3 days duration during this period, ISEE 3 remained within the magnetosheath for extended intervals, even when it was very near the center of an average tail. Simultaneous observations of the solar wind direction and thermal pressure suggest that nonradial solar wind flow associated with interacting solar wind streams moves a compressed tail away from the nominal position at these times and explains most of these observations. However, during several few‐hour intervals of strongly northward interplanetary magnetic field (IMF) within these periods, the solar wind is more radial and cannot explain the residence of the spacecraft in the magnetosheath. At these times ISEE 3 seems to be moving back and forth between two regions, one a higher‐density, lower‐temperature magnetosheathlike region but with density somewhat lower than the normal magnetosheath, the other a lower‐density, higher‐temperature taillike region but with density higher than the normal tail. Both regions have large Bz components and Bx components that tend to vary as if the spacecraft were moving from one hemisphere of the tail to the other. It is suggested that the magnetotail at these times of northward IMF consists mostly of field lines that close Earthward of the spacecraft with a narrow remaining tail at 225 RE waving back and forth across the spacecraft. This suggestion is supported by recent MHD simulations of J. A. Fedder and J. G. Lyon (submitted manuscript) for northward IMF conditions that show a completely closed tail that extends only to 155 RE. If relatively rare intervals of long‐duration, very northward IMF can eliminate the extended tail, it seems likely that more common, less northward IMF might well have very important, though less drastic, effects on the tail configuration.
Particle propagation in the inner heliosphere
Authors: Kallenrode May ‐Britt
Published: February 2008

For 27 energetic particle events observed on one of the Helios spacecraft we present fits on the intensity and anisotropy time profiles performed with the model of focused transport. The events have local mean free paths λ∥ between 0.02 and ≥1 AU, for most events λ∥ lies within or above the consensus range between 0.08 and 0.3 AU. The propagation in general can be described as focused or diffusive transport, therefore in the majority of events interplanetary scattering plays an important role. Comparing with results from earlier studies we find that scatter‐free events can be observed inside 0.5 AU as well as close to 1 AU, therefore extreme scattering properties seem not to be related to radial changes in scattering conditions. In addition, any radial variation in the scattering mean free path seems to be small compared to the event‐to‐event variations. A comparison of fits on ∼18‐MeV protons and ∼1‐MeV electrons shows that the scattering mean free path λp of the protons is related to the electron mean free path λe, whereby the average ratio λp/λe is 1.6±0.9. This latter observation has important consequences for our understanding of the underlying scattering mechanism, in particular as it is in contradiction with conventional results from quasi‐linear theory.
Interpretation of high‐speed flows in the plasma sheet
Authors: Chen C. X. , Wolf R. A.
Published: February 2008

Pursuing an idea suggested by Pontius and Wolf (1990), we propose that the “bursty bulk flows” observed by Baumjohann et al. (1990) and Angelopoulos et al. (1992) are “bubbles” in the Earth's plasma sheet. Specifically, they are flux tubes that have lower values of pV5/3 than their neighbors, where p is the thermal pressure of the particles and V is the volume of a tube containing one unit of magnetic flux. Whether they are created by reconnection or some other mechanism, the bubbles are propelled earthward by a magnetic buoyancy force, which is related to the interchange instability. Most of the major observed characteristics of the bursty bulk flows can be interpreted naturally in terms of the bubble picture. We propose a new “stratified fluid” picture of the plasma sheet, based on the idea that bubbles constitute the crucial transport mechanism. Results from simple mathematical models of plasma sheet transport support the idea that bubbles can resolve the pressure balance inconsistency, particularly in cases where plasma sheet ions are lost by gradient/curvature drift out the sides of the tail or bubbles are generated by reconnection in the middle plasma sheet.
Effects of an atmospheric gravity wave on the midlatitude ionospheric F layer
Authors: Millward G. H. , Moffett R. J. , Quegan S. , Fuller‐Rowell T. J.
Published: February 2008

A modeling study of the atmospheric response to a single short burst of enhanced ion convection at high latitudes, undertaken using the Sheffield/University College London/Space Environment Laboratory coupled ionosphere/thermosphere model, has revealed a large‐scale atmospheric gravity wave (AGW) moving equatorward from a source in the dawn sector auroral zone. The wave propagates to midlatitudes, perturbing the ionosphere and creating a traveling ionospheric disturbance. Analysis of the interaction between the thermosphere and ionosphere during the passage of the AGW at midlatitudes is undertaken and reveals a complex height‐dependent response. At lower altitudes the field‐aligned velocity of the ions follows closely the field‐aligned wind. Above the F peak, diffusion processes become important and the field‐aligned ion velocity shows fluctuations which exceed those in the wind. Changes in NmF2 and hmF2, during the interaction, are due to redistribution of plasma alone with changes in production and loss insignificant. As the F layer is lifted by the positive surge in the gravity wave, NmF2 decreases, due to a divergence in the ion flux, itself caused by the combination of a divergent neutral wind and an increase in the effects of diffusion with altitude. The slab thickness also increases. Subsequently, the opposite happens as hmF2 falls below its equilibrium value.
Hybrid simulations of thin current sheets
Authors: Burkhart G. R. , Dusenbery P. B. , Speiser T. W. , Lopez R. E.
Published: February 2008

A one‐dimensional, hybrid simulation code is used to study current sheets with a nonzero normal magnetic field Bz and a dawn‐to‐dusk electric field Ey. Such configurations are dependent upon only two parameters: we use the normalized normal magnetic field
and normalized electric field
, where υT is the thermal velocity of ions prior to their interaction with the current sheet and nb is the number density outside the current sheet (at the simulation boundary). A third parameter that is relevant to the motion of particles in current sheets is κA, the value of κ = (Rmin/ρmax)1/2 for particles of average energy. We find that if either
is close to or greater than 1, or if κA is close to 1, a rotational mode develops in which the z = 0 current rotates with the ion sense about the normal magnetic field, while for small values of both
or κA, the configuration is quasi‐steady. To achieve values of κA of the order of or larger than 1, we decrease the value of
uniformly. We find that the magnetic field fluctuations and particle distribution functions are similar in many respects to what was observed in the day 240, 1986, AMPTE/CCE current disruption event (Takahashi et al., 1987), an event that appears to be located at the site of initiation of current disruption and related particle energization (Lui et al., 1992).
Formation mechanisms for low‐altitude Es and their relationship with neutral Fe Layers: Results from the METAL Campaign
Authors: Kirkwood S. , von Zahn U.
Published: February 2008

The METAL campaign was a multi‐instrument campaign conducted in September‐October 1991 that was designed to investigate the relationship between neutral and ionized metallic layers in the high‐latitude lower ionosphere. Measurements included electron density profiles and electric fields from the EISCAT UHF incoherent scatter radar, ionosonde measurements of Es layers, neutral Fe profiles from lidar and rocket observations of winds (by chaff release), and measurements of plasma density and ion composition (by mass spectrometer). Es layers were observed on eight occasions. The measurements are here compared with simulations of Es formation by tidal wind shear and electric fields. The main features of the Es layers are found to be consistent with formation by tidal wind shear alone in one case and by the combined action of wind shear and electric fields in the remaining cases. On three occasions, layers were seen to descend rapidly, in one case to below 90‐km altitude, and the rate of descent was found to be consistent with the expected action of the observed electric fields. Sporadic neutral Fe layers were observed on seven occasions, including four cases in close correlation with Es. However, the neutral layers were generally not at exactly the same altitude as the Es, nor did they follow the same altitude changes. The results indicate that both wind shears and electric fields are important for the formation of thin layers and the vertical transport of metal ions in the high‐latitude lower thermosphere. They show that neutral Fes layers, although they may be formed in close association with Es layers, can persist and migrate independently of Es layers. This latter observation suggests that partial reionization of neutral layers by intense auroral particle precipitation may explain some observations of multiple Es layers.
Broad trends in geomagnetic paleointensity on Hawaii during Holocene time
Authors: Mankinen Edward A. , Champion Duane E.
Published: February 2008

Paleointensity determinations have been obtained from 22 basaltic lava flows on the island of Hawaii using the Thelliers' method. Radiocarbon dating indicates that these flows erupted at intervals ranging from about 200 to 1000 years, and results of the experiments provide an estimate of broad trends in geomagnetic paleointensity during Holocene time in the vicinity of Hawaii. Most of the samples were obtained from quickly cooled flow margins and, as a consequence, typically contain two titanomagnetite populations. Only two of the 79 samples analyzed failed to yield an estimate of the paleofield, demonstrating that paleointensities can be obtained from such samples if carefully selected. Virtual dipole moments calculated for the flows are compared with a published curve of dipole field intensity that was constructed using worldwide archeomagnetic data. The large nondipole fields that were previously postulated for the vicinity of Hawaii are confirmed, and the present data indicate that they were present from about 5000 years B.P. to perhaps as recently as the past 200 years. The data indicate, however, that these nondipole sources must have been virtually absent between about 12,000 and 5000 years B.P. as they are at the present time.
Integrated crustal structure across the south central California Margin: Santa Lucia escarpment to the San Andreas Fault
Authors: Howie John M. , Miller Kate C. , Savage William U.
Published: February 2008

The continental margin of south central California has undergone different styles of deformation over the past 30 m.y. as the North America‐Pacific plate boundary has evolved from subduction to transform tectonics. Results presented here identify strike‐slip and compressional structures in the lower crust that probably developed during this period of tectonic transition. An integrated model of crustal structure along deep seismic line PG&E‐3 crosses the entire continental margin west of the San Andreas fault near the latitude of San Luis Obispo. A compressional wave velocity model was developed using coincident marine multi‐channel seismic and onshore/offshore wide‐angle reflection/refraction seismic data and was tested independently with gravity modeling. A high‐velocity layer (6.8–7.0 km/s, about 6 km thick) in the lower crust that extends east possibly as far as the San Andreas fault is interpreted as tectonically underplated oceanic crust. The upper and middle crust of the Patton and Sur‐Obispo Franciscan terranes extends to depths of 14 to 20 km. The velocity and density modeling indicate lower average velocities and densities for the Patton terrane, supporting earlier interpretations that it is a distinct terrane within the Franciscan complex. Abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the Santa Lucia Escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 Ma following the subduction of the Pacific‐Arguello ridge. An imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust. Convergence between the Monterey and Arguello microplates during the final stages of Farallon subduction (about 24 to 16 Ma) probably caused the imbrication.
Velocity structure of reflective lower crust beneath the Grand Banks of Newfoundland
Authors: Reid Ian
Published: February 2008

A seismic refraction experiment using air guns and ocean bottom seismometers was carried out on the southern Grand Banks, in order to study the velocity structure of highly reflective lower crust observed on a previous deep multichannel reflection profile. The data were analyzed by interactive forward modeling of travel times and amplitudes, wide‐angle normal moveout analysis and downward continuation of slant stacks. Laterally varying structure was used in the final stage of modeling. The results indicate an upper crust with a P velocity of 6.0–6.2 km/s, a zone of high velocity gradient from about 19 to 24 km depth, and a lower crustal velocity of 6.6–6.8 km/s. Moho is at 29 km. The depth of the transition zone is uncertain by 2 or 3 km, and its detailed velocity structure is not well resolved. However, the top of the zone corresponds approximately to the onset of reflectivity, while the lower crustal layer is characterized by more coherent and continuous reflections. A thin (<1 km) localized high‐velocity (around 7.2 km/s) layer is tentatively correlated with one of the reflectors in the lower crust. Bulk anisotropy of the lower crust is less than 5%. The presence of well‐defined high‐velocity layers and the lack of anisotropy tend to support mafic intrusion as the mechanism for reflectivity in the lowest part of the crust. However, there is no evidence of a thick high‐velocity underplated layer at the base of the crust.
Analysis and application of in situ pore pressure measurements in marine sediments
Authors: Fang Weijiang W. , Langseth Marcus G. , Schultheiss Peter J.
Published: February 2008

Recent measurements of in situ subseafloor pore pressure in sediment ponds on the flank of the Mid‐Atlantic Ridge using a Pop Up Pore Pressure Instrument have provided important information regarding fluid exchange processes at the seafloor as well as in situ estimates of physical properties of marine sediments. The in situ pore pressure recordings of 4–5 days duration contain three distinct components. The early part of the record is dominated by the decay of a pressure pulse associated with probe penetration. The rate of decay of this pulse is used to estimate in situ permeabilities, which range from 3.1 × 10−16 m2 to 6.5 × 10−15 m2. The recording also exhibits a low‐amplitude oscillation that is associated with tidal pressure variation on the seafloor. A study based on Biot's theory shows that amplitudes and phases of tidally induced pore pressure oscillations are determined by both the frame compressibility and the permeability of the sediments, and thus the oscillation components are used to estimate these parameters. The frame compressibilities and the permeabilities estimated from tidal oscillations range from 1.2 × 10−8 Pa−1 to 4.1 × 10−8 Pa−1 and from 1.1 × 10−15 m2 to 4.8 × 10−15 m2, respectively. Laboratory‐measured permeabilities range from 7.6 × 10−16 m1 to 1.2 × 10−15 m2. Permeabilities determined by these three methods are comparable. The third component is the ambient equilibrium pore pressure. Negative pore pressure gradients were identified on all deployments. When combined with the permeabilities of the sediments the estimated average rates of drawdown of water through the sedimentary layers in the ponds are 2–7 mm/yr. The pore pressure gradient near the seafloor can be used to construct a pore pressure profile through the sediment. Extrapolated pore pressure at the top of the basement is about 80 kPa (0.8 bar) below the hydrostatic pressure.