bluuebunny/crossref_metadata_2025 huggingface dataset:
Radial diffusion in the Uranian radiation belts: Inferences From satellite absorption loss models
Authors: Hood L. L.
Published: February 2008

Phase space density profiles for protons and electrons with first invariants µ≤100 MeV/G (integral invariants K = 0.3 and 0.6 G1/2 RU) previously derived from measurements by the low energy charged particle (LECP) detector on Voyager 2 during the 1986 Uranus encounter are analyzed using solutions of the time‐averaged radial diffusion equation in a dipolar magnetic field. These profiles are selected for their consistency with an absence of local sources of particles. A loss model consisting of absorption by the major inner satellites Miranda, Ariel, and Umbriel is assumed and the corresponding form of the time‐averaged radial diffusion coefficient D(L) (taken to be of the form D(L) = DoLn, where n is an integer) is determined by a minimum‐variance fit to the phase space density profiles. Satellite macrosignatures present in the experimentally derived profiles are approximately reproduced in several cases lending credence to the loss model and indicating that magnetospheric distributed losses are not as rapid as satellite absorption near the minimum satellite L shells for these particles. The latter inference implies an upper limit of approximately 10 cm−3 for the neutral hydrogen number density near the orbit of Ariel, based on a comparison of charge exchange lifetimes with the calculated satellite absorption lifetimes. The preferred forms for D(L) are characterized by a low‐order L dependence (∼L³–L4) and an amplitude (Do ≃ 10−11–10−10 RU² s−1). The inferred L dependence is least consistent with “terrestrial‐type” diffusion mechanisms including magnetic impulses and electrostatic field fluctuations of purely magnetospheric origin. “Jovian‐type” mechanisms including the ionospheric dynamo electric field mechanism and the centrifugal interchange instability in regions of strong negative radial plasma density gradients predict L dependences that are more consistent with the inferred form. Of these, the ionospheric dynamo mechanism is most viable at Uranus.
Upward ion flow in ionospheric holes on Venus
Authors: Hartle R. E. , Grebowsky J. M.
Published: February 2008

Ion composition measurements made by the orbiter ion mass spectrometer on the Pioneer Venus Orbiter are used to determine the ion flow characteristics within two ionospheric holes on the nightside of Venus. A comparison of the altitude profiles of the observed ion densities with those expected under diffusive equilibrium conditions indicates that the major ions O+, NO+, and O2+ and the minor ions H+ and He+ flow upward, away from Venus, along the axis of the holes. The result is substantiated by a quantitative evaluation of the ion flow speeds appearing in expressions derived from the equations for conservation of mass and momentum of the ions and electrons. The resulting equations for the ion flow speeds are functions of the observed ion and neutral species densities and the ion and electron temperatures. The analysis reveals that all ion species flow upward in the holes because the upward force produced by the plasma pressure gradient exceeds all downward forces. Furthermore, the ion flow speeds are found to increase with altitude as a result of the continual acceleration caused by the net upward force. In the acceleration region, where the observing altitudes were limited to the range extending from the [O+] peak (∼170 km) to 250 km, none of the ion components attain their acoustic speeds, although the light ions come within about a factor of 2. The light ions H+ and He+ attain the greatest speeds because as minor ions they experience the largest polarization electric fields produced in the ion‐electron gas. Although upward ion flow is inferred from the ion composition measurements, the nature of the ion source required to maintain such flow remains a puzzle. For example, the classical diffusion rate of O+ across the magnetic field, into the base of the holes, is about an order of magnitude less than what is needed to feed the upward O+ flux.
Current‐driven ion cyclotron turbulence: Evolution of the electron distribution and wave spectrum
Authors: Muschietti L. , Dum C. T.
Published: February 2008

The evolution of the drifting electron distribution responsible for exciting electrostatic ion cyclotron waves in a magnetized collisionless plasma is studied by means of a computer code. Quasi‐linear equations including cyclotron damping on the ions are used for investigating two situations: (1) initial value problem in an infinite homogeneous plasma and (2) open configuration with fresh electrons streaming along the field lines into a region of instability. In both situations a partial plateau is progressively dug through the top of the distribution by velocity diffusion of the electrons along the magnetic field. Cyclotron damping limits the extension of the plateau toward small velocities whereas the high velocity end terminates with a strong positive slope. The asymptotic relaxed flat‐topped distribution can be directly calculated from an equation based on marginal stability. Due to wave pileup the level of ion cyclotron waves is greatly enhanced in the inhomogeneous situation. For an isothermal plasma the critical drift is 19 υi (with υi the ion thermal velocity); for drifts 25, 30 υi, the waves are found to reach eф/T = 0.02, 0.06, respectively. In the first case the resonant heating of the ion tail is negligible. Consequences for ion cyclotron waves and hydrogen energization in the downward and upward current regions of the topside ionosphere are discussed. It is argued that localized parallel electric fields may play an important role in the generation of the observed large waves.
Drifting black aurorae?
Authors: Schoute‐Vanneck H. , Scourfield M. W. J. , Nielsen E.
Published: February 2008

Characteristics of eastward drifting forms, previously described in the literature as black aurorae, have been identified in low‐light level TV camera data. The TV field of view was within the field of view of STARE and that of an all‐sky camera. On the basis of these observations we propose that these euroral forms are a manifestation of folds or waves on the borders of auroral bands propagating along the dark regions between neighboring auroral bands. Conditions under which the folds or waves occur are compatible with their formation by the Kelvin‐Helmholtz electrostatic instability.
The onset of the new solar modulation cycle in 1987–1988 as a function of heliocentric radius and latitude
Authors: Webber W. R. , Lockwood J. A.
Published: February 2008

Observations of the onset of the new modulation cycle in 1987 using the counting rates of E > 60 MeV particles at the IMP, Voyager and Pioneer spacecraft indicate that the decrease at Earth was rapid after the intensity maximum in early 1987 and was closely correlated with the increase in the average tilt of the heliospheric current sheet. Several small Forbush decreases were seen at Earth in 1987–1988 superimposed on a relatively smooth overall decrease. Similar episodic decreases were seen at Voyager 2 (V2) and Pioneer 10 (P10) following these decreases at Earth by an appropriate time delay corresponding to a propagation speed of about 400 km/s. The relative effects of these transient decreases as compared with the overall decrease was more pronounced at larger radii. By the end of 1988 the intensity had decreased by about 40% at Earth, 30% at V2 and 18% at P10. This overall decrease was accompanied by an increase in the average integral radial gradient as well as a decrease in the radial dependence of the gradient out to about 40 AU so that by the end of 1988 the gradient was ∼2.0%/AU, essentially independent of radius. V1, about 30°N of the heliographic equatorial plane, observed none of the decrease seen in the equatorial plane. However, a prominent 27‐day variation of peak‐to‐peak amplitude of ∼5% was observed. As a result, the intensity at V2 in 1988 decreased below that of V1. Consequently, the latitudinal gradient present since 1985 almost completely disappeared by the end of 1988. The onset of the new solar modulation cycle seems to be related to the complete altering of the solar magnetic structure as observed on the surface of the sun. This is manifested in the heliosphere by an overall change of the average tilt of the current sheet from 9° to ∼50° between 1987 and late 1988 occurring in several distinct steps which are in association with distinct changes in the magnetic structure at the Sun and with Forbush decreases observed at Earth. The relationship between the changes in the average current sheet tilt and the changes in the latitudinal and radial gradients is investigated.
Propagation of a nonrelativistic electron beam in three dimensions
Authors: Okuda Hideo , Ashour‐Abdalla Maha
Published: February 2008

The propagation of a nonrelativistic electron beam injected into space plasma from a conductor was studied by means of an electrostatic three‐dimensional simulation model. Only the injection of an electron beam along a magnetic field was considered. Contrary to one‐ and two‐dimensional simulations, an overdense electron beam whose density is much larger than the ambient plasma density can escape a spacecraft without causing significant charging. It was found that the radial electric field arising from the space charge of the beam electrons can give rise to a rotation of both the beams and ambient electrons with respect to ambient ions. The rotation speed can easily exceed the electron thermal speed even when the beam density is smaller than the ambient density. Instabilities arising from such a rotating electron beam are discussed.
The configuration of the auroral distribution for interplanetary magnetic field Bz northward: 2. Ionospheric convection consistent with Viking observations
Authors: Jankowska K. , Elphinstone R. D. , Murphree J. S. , Cogger L. L. , Hearn D. , Marklund G.
Published: February 2008

Views of the northern hemisphere auroral distribution obtained by the Viking satellite present a qualitative means of inferring the convective patterns which occur during interplanetary magnetic field (IMF) Bz northward. The approach is taken whereby upward field‐aligned currents are assumed to be coincident with large‐scale discrete auroral features and on this basis possible convective patterns are deduced. While the patterns are not unique solutions, they are found to be consistent with merging theory predictions. That is, for Bz northward the auroral observations support the possibility of three and/or four cell patterns. When the IMF azimuthal angle is 90° (270°), a clockwise (anticlockwise) cell is found to be located in the polar region between the two standard viscous cells. When IMF Bx dominates and is in a toward orientation, convection stagnates, whereas if Bx is negative, a four‐cell pattern may form with sunward flow at very high latitudes. The concept of using global auroral images as an additional tool when developing convection models could prove to be necessary in order to extend beyond the few isolated measurements taken in situ by satellites.
Polar region Birkeland current, convection, and aurora for northward interplanetary magnetic field
Authors: Zanetti L. J. , Potemra T. A. , Erlandson R. E. , Bythrow P. F. , Anderson B. J. , Murphree J. S. , Marklund G. T.
Published: February 2008

Viking magnetic field, electric field, and image data have been used to assess polar region phenomena for steady state northward interplanetary magnetic field (IMF) conditions. Regions of polar Birkeland current (NBZ currents) and convection and their extent from the vicinity of the magnetic pole are determined. Also discussed are mechanisms that could produce polar aurora in general; two suggestions are (1) converging electric fields from convection patterns alone and (2) the bifurcation of the magnetotail with its associated plasma transport and convection patterns. Macroscopic (> 1° latitude) systems of Birkeland currents and convection in the polar regions have been established for a case on April 9,1986, from Viking spacecraft data. The current systems were confined to the highest latitudes of the polar regions and occurred during strongly northward IMF with a significantly negative Bx. An arc extends across the polar region within the dawn cell of Birkeland current. The arc is located at a sunward to antisunward convection reversal that corresponds to a converging electric field. A converging electric field (▽ · E < 0) alone is suggested as the cause of this polar arc. The signature of both transverse disturbance vectors indicates that the polar region dawn NBZ Birkeland current does not connect to the dayside auroral region. It is inferred that the dawn polar region convection cell associated with this Birkeland current is also limited in the sunward direction and does not connect to the dayside auroral region convection.
Plasma fluctuations in the magnetosheath downstream from Uranus
Authors: Richardson John D. , Zhang Ming , Belcher John W. , Siscoe George L.
Published: February 2008

The Voyager plasma experiment observed large‐amplitude plasma fluctuations in the Uranian magnetosheath downstream from the planet. This is a region that has not been well sampled in the Earth's magnetosphere. These waves have periods of tens of minutes, are characterized by an anticorrelation between the plasma density and temperature, and are associated with deflections in the flow angle of the plasma. These fluctuations are observed only in regions where the magnetic field is rapidly varying. These waves have time and distance scales placing them in the MHD regime, but their characteristics are not compatible with any known solution of the MHD equations. It is suggested that these fluctuations are produced by the solar wind interaction with the magnetosphere at the bow shock, but the physics governing the production and propagation of these fluctuations is not understood.
Structure of an energetic narrow discrete arc
Authors: McFadden J. P. , Carlson C. W. , Boehm M. H.
Published: February 2008

Particle distributions, waves, dc electric fields and magnetic fields were measured by two sounding rockets at altitudes of 950 and 430 km through an energetic (>5 keV) narrow (∼10 km) stable discrete arc. Although the payloads' magnetic footprints were separated by only 50 km, differences in the arc's structure were observed including the spatial width, peak energy, and characteristic spectra. The energetic electron precipitation included both slowly varying isotropic fluxes that formed an inverted‐V energy‐time signature and rapidly varying field‐aligned fluxes at or below the isotropic spectral peak. The isotropic precipitation had a flux discontinuity inside the arc indicating the arc was present on a boundary between two different magnetospheric plasmas. Dispersive and nondispersive bursts of field‐aligned electrons were measured throughout the arc, appearing over broad energy ranges or as monoenergetic beams. Dispersive bursts gave variable source distances <8000 km. Plateauing of some of the most intense bursts suggests that waves stabilized these electrons. During the lower altitude arc crossing, the field‐aligned component formed a separate inverted‐V energy‐time signature whose peak energy was half the isotropic peak energy. This structure suggests that two separate potential drops may have formed along the same flux tube with cold plasma present between them. High time resolution measurements of electron distributions, together with measurements of the current system, plasma flows, and plasma waves, provide a detailed picture of the narrow arc. This complete set of measurements illustrates the complex structure present in narrow discrete arcs and demonstrates the advantages of high resolution measurements by multiple spacecraft.
Energetic particle observations at the Helios 1 spacecraft of shocks associated with coronal mass ejections
Authors: Cane H. V. , von Rosenvinge T. T. , McGuire R. E.
Published: February 2008

Using energetic particle data from the Helios 1 spacecraft, we have studied the particles associated with 26 coronal mass ejection (CME)/shock pairs. We divided the particle events into four classes on the basis of the relative intensities of their prompt and shock‐associated components. Events in group 1 have strong prompt and shock components and are associated with energetic CMEs originating, most commonly, with central meridian flares. Group 2 events characteristically have strong prompt components and weaker shock components and are associated with western hemisphere (i.e., favorably located) flares. Group 3 consists of events with absent prompt components and strong shock components; the CMEs probably originate in disappearing filament events near central meridian. Group 4 events show no particle enhancements and are associated with slow CMEs and slow shocks. This study supports our previous work in which solar energetic particle events were discussed in terms of a solar component and an interplanetary shock component and where the shock effects depend on the connection longitude of the observer.
The probability of observing ducted magnetospheric echoes from the ground
Authors: Ellis G. R. A. , Goldstone G. T.
Published: February 2008

Ground‐based observations of ducted magnetospheric echoes between 1985 and 1988 are used together with topside sounder observations to estimate their likely rate of occurrence in future years. The variation in the probability of observing the echoes from the ground with location, wave frequency, and time, for the years 1992–1996, is given.
Plasma wave observations during Ion Gun Experiments
Authors: Olsen R. C. , Weddle L. E. , Roeder J. L.
Published: February 2008

Experiments in charge control on the AF/NASA P78‐2 (SCATHA) satellite were conducted with a plasma/ion source in the inner magnetosphere. These experiments were monitored with plasma wave instruments capable of high temporal and frequency resolution in the 0–6 kHz frequency range. Ion gun experiments revealed two distinct classes of behavior. Nonneutralized ion beam operation at 1 mA, 1 kV resulted in arcing signatures (spiky in time, broad frequency range), coincident with induced satellite potentials of −600 to −900 V. This signature disappeared when the accelerating voltage was switched off or the beam was neutralized (at which time the satellite body approached potentials of a few volts). The signal is attributed to arcing between differentially charged surfaces. An additional feature was noted in the 100‐kHz channel of the wave receiver. During emission of dense, low‐energy plasma, a signal is generated which may be at the upper hybrid, or plasma frequency for the local plasma.
The distortion of the magnetotail equilibrium structure by a net cross‐tail magnetic field
Authors: Birn J.
Published: February 2008

Observations have shown that in addition to a y component associated with the flaring of the tail, a net y component of the magnetic field can persist in the geomagnetic tail, roughly proportional to the interplanetary By. We use the tail equilibrium theory outlined by Birn (1987) to construct self‐consistent three‐dimensional models of the tail that include this effect. We find that the net ByN field varies only weakly along field lines and across the tail. The presence of the net ByN in the neutral sheet implies that a part of the cross‐tail current becomes field‐aligned, flowing from the southern to the northern hemisphere for ByN > 0 and from north to south for Byn < 0. This field‐aligned current is proportional to ByN for small values. For a typical value of 2% of the lobe field strength we find a total current of 4.4×105 A flowing toward one hemisphere at x = −20RE. This current, however, is not expected to reach the Earth, because it decreases strongly along each field line away from the neutral sheet, due to a conversion into perpendicular current. The deformations of magnetic flux surfaces, implied by the models, are not consistent with an entirely closed magnetotail boundary. They suggest that in addition to traditional lobe field lines, interconnected with the solar wind field at large distances or high latitudes, there may be a class of field lines extending from the Earth into the magnetosheath region through the low latitude flanks of the tail. These field lines, which might even be associated with positive Bz at a neutral sheet crossing, effectively reduce the region of closed flux. For typical values of ByN of 2% of the lobe field this effect is small, but for values above about 10%, not extremely unusual, it can be appreciable. For ByN > 0 this opening occurs preferably on the north/dawn and south/dusk sides, producing an asymmetry and an apparent tilt of the closed field line plasma sheet region around the tail axis relative to the neutral sheet. We find also a topological connection of the low latitude region near the magnetopause with the plasma sheet boundary layer, separating closed plasma sheet and open lobe field lines. For ByN > 0 this interconnection takes place on the north/dusk and south/dawn sides of the low latitude boundary region.
Kinetic hydromagnetic instabilities due to a spherical shell distribution of pickup ions
Authors: Wu C. S. , Yoon Peter H.
Published: February 2008

The stability of a spherical shell ion distribution function against low‐frequency hydromagnetic waves is discussed by relaxing the assumption of parallel propagation which was imposed in the preceding discussions published in the literature. It is found that Alfvén waves as well as magnetosonic waves with oblique angles of propagation can be excited by a shell distribution. In the present analysis, the emphasis is placed on the case in which the thickness of the spherical shell is small.
Nonlocal analysis of low‐frequency waves in the plasma sheet
Authors: Seboldt W.
Published: February 2008

This paper investigates the spectrum of MHD waves associated with plasma sheet configurations and applies the results to the plasma sheet in the Earth's magnetotail. We represent the plasma sheet by an inhomogeneous continuous one‐dimensional static equilibrium model (Harris sheet) and discuss its response to small perturbations in the framework of ideal MHD theory including compressibility. The wave frequency is calculated from a generalized singular eigenvalue problem which in general contains a continuous spectrum with eigensolutions becoming singular at frequency‐dependent positions from the neutral sheet. The singular modes lead to time‐asymptotically damped solutions for the physical perturbations by the process of phase mixing. In the special case of two‐dimensional perturbations, the spectrum displays discrete eigenfrequencies with smooth eigensolutions in addition to a continuous part. The application of the theory to the Earth's magnetotail on the basis of the hypothesis that observed waves in the plasma sheet belong to the undamped part of the spectrum leads to the result that the lowest two‐dimensional modes are natural candidates for the observed oscillations with periods of the order of minutes. The lowest eigenmode of the discrete spectrum has an antisymmetric wavy structure and is consistent with the mentioned periods if the wavelength along the tail is of the order of the plasma sheet thickness.
Observations on the DE 1 spacecraft of ELF/VLF waves generated by an ionospheric heater
Authors: James H. G. , Inan U. S. , Rietveld M. T.
Published: February 2008

Radio waves at frequencies between 1525 and 5925 Hz were observed on the DE 1 satellite during a pass over the Heating facility of the Max‐Planck‐Institut für Aeronomie (MPAe Heating) near Tromsø, Norway. The waves were detected during a 2‐min period on December 12, 1981, and measurements indicated pulse stretching by a few hundred milliseconds and spectral broadening of about 10 Hz. Measured signal delays are compared with those calculated using three‐dimensional ray tracing. The observed signal delays and pulse distortion are not consistent with that expected on the basis of propagation in a smooth magnetosphere between the assumed “polar electrojet antenna” (PEJ) in the ionospheric D/E region and the satellite at about 11,000 km altitude. Scattering by density irregularities may be responsible for the observed long delays and spectral broadening, as has been speculated in previous interpretations of spacecraft observations of signals from ground‐based ELF/VLF transmitters. Although the MPAe Heating ELF transmissions were 100% square‐wave modulated, odd harmonies of the ELF modulation frequencies were not observed at the satellite; this remains unexplained. Similar spectral broadening and signal delays were observed on 10.2‐kHz signals from the Omega Norway transmitter during the same DE 1 pass. Using the Omega transmitter as a reference, the PEJ radiated power is estimated to be about 30 W.
A helically distorted MHD flux rope model
Authors: Theobald Michael L. , Montgomery David
Published: February 2008

A flux rope model is proposed which has a variable degree of helical distortion from axisymmetry. The basis for this suggestion is a series of numerical and analytical investigations of magnetohydrodynamic states which result when an axial electric current is directed down a dc magnetic field. The helically distorted states involve a flow velocity and seem to be favored because of their lower rate of energy dissipation. Emphasis is on the magnetometer and particle energy analyzer traces that might be characteristic of such flux ropes. It is shown that even a fractionally small helical distortion may considerably alter the traces in minimum‐variance coordinates. In short, what may be fairly common MHD processes can render a flux rope almost unrecognizable under standard diagnostics, even if the departures from axisymmetry are not great.
Resonant Alfvén wave heating of the plasma sheet boundary layer
Authors: Harrold B. G. , Goertz C. K. , Smith R. A. , Hansen P. J.
Published: February 2008

The exchange of energy between the plasma mantle and the plasma sheet boundary layer (PSBL) is examined with a one‐dimensional magnetotail model. The energy exchange occurs via Poynting flux generated by the localized mode conversion of a surface wave to an Alfvén wave. This Poynting flux propagates through the lobe and into the PSBL where it is absorbed by two processes. The first arises from a gradient in the plasma β causing a smooth absorption of Poynting flux. The second process results from the localized mode conversion of the decaying surface wave to an Alfvén wave, causing a localized absorption of energy. A numerical solution of the linearized ideal MHD equations is obatined by assuming an adiabatic equation of state. The energy flux absorbed within the PSBL is of the order of 1 µW/m² (assuming a fluid displacement of 1 RE at the plasma mantle) and varies with central plasma sheet temperature in a manner capable of triggering a thermal catastrophe in the boundary layer.
Source of O mode radio emissions from the dayside of Uranus
Authors: Menietti J. D. , Curran D. B.
Published: February 2008

During the inbound trajectory toward Uranus the Planetary Radio Astronomy instrument on board the Voyager 2 spacecraft observed narrowband smooth (n‐smooth) emission at frequencies centered near 60 kHz and O mode emission (the “dayside source”) in a frequency range narrowly confined around 160 kHz. By assuming empirical models of the plasma density for the dayside magnetosphere of Uranus, and by using cold plasma theory together with observational constraints, we have performed ray‐tracing calculations to determine the source location of the O mode emission. The “dayside source” appears to originate along magnetic field lines with a footprint near the north magnetic pole. Sources of nightside, high‐frequency, broadband smooth (b‐smooth) emission observed by Voyager after encounter are believed to exist near the conjugate footprint of these same field lines. This would indicate that the particle population supplying the free energy source has energies at least as high as a few keV and the density in the source region satisfies the condition 0.3 < fp/fce < 1.0 where fp and fce are the electron plasma frequency and gyrofrequency, respectively.
Interchange instability of the Earth's plasmapause
Authors: Huang T. S. , Wolf R. A. , Hill T. W.
Published: February 2008

We reexamine the stability of low‐frequency electrostatic waves with k·B = 0 at the plasmapause, using an extension of the procedure of Richmond (1973), an approach that is based on computing individual particle motions and includes the line‐tying effect of the ionosphere. We derive expressions for the complex frequency as a function of wave number. The effect of the polarization (inertial) current accompanying the wave is included in an approximate way. The instability is caused by the sharp change in plasma pressure at the plasmapause, but its growth rate is limited by ionospheric conductivity and, for very short wavelengths, by the inertia of the magnetospheric particles. The growth rate generally increases with decreasing ripple wavelength, until that wavelength becomes small compared to the thickness of the plasmapause. The ring current (hot and warm plasma) particles suppress wave growth significantly only for long wavelengths. According to our linear analysis, an interchange ripple with wavelength ≲2000 km that forms on a sharp plasmapause (≲0.1 RE) should grow by ≳10 e folds as it traverses the nightside of the Earth. This result is consistent with Richmond's (1973) conclusion that the interchange instability limits the thickness of the plasmapause on the nightside of the Earth.
Comparison of picked‐up protons and water group ions upstream of comet Halley's bow shock
Authors: Neugebauer M. , Coates A. J. , Neubauer F. M.
Published: February 2008

Data are presented on the properties of picked‐up cometary protons and water group (WG) ions observed upstream of the bow shock of comet Halley by the ion mass spectrometer and Johnstone plasma analyzer experiments on the Giotto spacecraft. The number of WG ions exceeded the number of cometary protons at cometocentric distances r < 1.3×106 km, while the WG mass density exceeded the proton mass density when r < 6×106 km. The small scale variations of proton and WG densities were well correlated, which argues against the "expanding halo" model which has been used to explain the quasi‐periodicity of energetic particles observed by VEGA and Giotto. Two parameters are used to describe the pitch angle distributions of the picked‐up ions: a 10% width, which is the full angular width at 10% of the maximum of the pitch angle distribution, and a mean width. The cometocentric distance profiles of both parameters exhibited a great deal of scatter, and both parameters showed a steeper average radial gradient for WG ions than for protons. The 10% widths of WG ions were consistent with less than 10∶1 anisotropies for r < 2.5×106 km, but the proton anisotropy did not drop below 10∶1 until the spacecraft entered the “foreshock” region at r = 1.4×106 km. After subtraction of the variation in field direction, the mean width of the proton pitch angle distribution was nearly independent of distance everywhere outside the shock. The WG mean width, on the other hand, increased with increasing WG density (as expected) and with increasing angle α between the interplanetary field and the solar wind velocity vector (an unexpected result). No increase in shell radii, due to either adiabatic compression or first‐order Fermi acceleration, could be discerned for either ion species until the spacecraft was very close to the bow shock. The thickness of the picked‐up proton shell slowly increased as r decreased inside 2.5×106 km, whereas the water group shell thickness remained fairly constant until just outside the shock. While some of the differences between the two ion species can be attributed to the greater ionization scale length of cometary H atoms compared to WG atoms and molecules, the very different dependences of their pitch angle and energy diffusion rates on ion densities and on α are difficult to understand in terms of presently available theories and models.
The structure of resistive‐dispersive intermediate shocks
Authors: Hau L.‐N. , Sonnerup B. U. Ö
Published: February 2008

The structure of intermediate shocks is studied on the basis of the resistive, nonviscous two‐fluid equations. Electron inertia effects are neglected so that the generalized Ohm's law contains only the Hall current and the electron pressure terms in addition to the usual resistive term and the electric field. As for the case of purely resistive MHD, reported recently by Hau and Sonnerup (Journal of Geophysical Research, 94, 6539, 1989), fixed‐point analysis is performed to examine the nature of the magnetic structure near the upstream and downstream states of the intermediate shock. The one‐dimensional, steady state, resistive Hall MHD equations are then integrated numerically to generate complete shock structures which are presented in the form of magnetic hodograms. These hodograms describe fast and slow shocks in addition to intermediate shocks. As expected, the calculations show that the main effect of Hall currents is to remove the symmetry between left‐hand and right‐hand polarized shock structures found in the purely resistive case and sometimes to convert the smooth shock transitions obtained from the resistive MHD model into transitions that incorporate oscillatory standing wave train structures at their upstream and/or downstream edge. The magnetic structure in the plane of the shock near the possible upstream and downstream states of the intermediate shock, which in the case of purely resistive MHD is either a node or a saddle, can be either a node, a saddle or a spiral point, the latter corresponding to a standing wave train, when the Hall term is included. As a result, the number of possible types of magnetic hodogram topology increases from 3 in the resistive case, to a total of 20. However, it appears that the constraints provided by the shock jump conditions make certain of these topologies unattainable: only 13 of the 20 cases have been found and are reported in the paper. The relationship between small‐amplitude dispersive waves in the flow upstream or downstream of a shock and the nature of the corresponding fixed point is also discussed.
On the search for evidence of fast mode compressions in the near‐Earth tail: ISEE observations
Authors: Moortgat K. T. , Cattell C. A. , Mozer F. S. , Elphic R.
Published: February 2008

A search for earthward propagating fast mode compressions in the near‐Earth tail has been conducted. ISEE electric and magnetic field data were studied for substorm and quiet times when the spacecraft was close to the neutral sheet, near midnight, and at radial distances of 5.5 to 13 RE. Due to ISEE orbit characteristics, the set of events covered 12½ hours, during which time the satellite remained between 8.5 and 13 RE. Contrary to earlier ideas, no evidence for fast mode compression waves was found during the substorm events studied here, since plasma data and spacecraft potential data showed no density increases at the times of rapid magnetic field increases. For the one compression event for which a temporal offset between the ISEE 1 and ISEE 2 magnetic field structures could be clearly determined, the propagation velocity was ∼140 km/s, well below the fast mode speed of ∼400 km/s for the measured composition. During substorm events, the largest (EB)/|B|² convection flows were earthward with magnitudes of 100–350 km/s.
A survey of upwelling ion event characteristics
Authors: Pollock C. J. , Chandler M. O. , Moore T. E. , Waite J. H. , Chappell C. R. , Gurnett D. A.
Published: February 2008

Ionospheric ion upwelling in the vicinity of the dayside cleft has been studied, based primarily on data from the Dynamics Explorer 1 spacecraft. Using retarding ion mass spectrometer low‐energy ion data and plasma wave instrument dc electric field data, bulk ion plasma parameters, including ion species density and field‐aligned bulk velocity and flux, have been derived at points within a number of observed upwelling ion events for the ion species H+, He+, O+, and O++. The ion species bulk parameters near the source latitude are examined and compared. We find that the upwelling plasma is rich in O+, which typically comprises ∼90% of the particle density, followed by H+ at somewhat less than 10%, and then He+ and O++, each comprising ∼1% of the upwelling ion particle density. The upwelling O+ ion flux is also commonly dominant over that associated with the other species, with normalized values near the source region which are typically near 109 cm−2 s−1. The fractional upward H+ flux is not as small as the fractional H+ density due to the much larger H+ upward flow velocities. Integration of the product of the normalized upward ion species flux and the upwelling ion occurrence probability (Lockwood et al., 1985) over the source area yields an estimate of the source strength of this low‐altitude cleft region magnetospheric plasma source of 2.6×1025 ions s−1.
Auroral radar measurements at 16‐cm wavelength with high range and time resolution
Authors: Schlegel Kristian , Turunen Tauno , Moorcroft D. R.
Published: February 2008

Auroral radar measurements performed with the EISCAT facility are presented. Backscatter cross sections of the irregularities produced by the two‐stream (Farley‐Buneman) or gradient drift plasma instabilities have been recorded with a range separation of 1.5 km, corresponding to a spacing of successive values in height of about 0.4 km. The apparent height profiles of the backscatter have a width of about 5–6 km and occur between 95 and 112 km altitude, with a mean at 104 km. Very often, fast motions of the backscatter layers are observed which can be explained as fast moving ionospheric structures controlled by magnetospheric convection. The maximal time resolution of the measurements is 12.5 ms. The statistics of the backscatter amplitudes at this time resolution is close to a Rice distribution with a Rice parameter a ≈ 3.7. The observed backscatter spectra do not change significantly in shape when the integration time is reduced from 5 s to 100 ms.
A large‐amplitude traveling ionospheric disturbance excited by the space shuttle during launch
Authors: Noble S. T.
Published: February 2008

The ionosphere was monitored during the fourth space shuttle (STS 4) launch in June 1982 by the Arecibo incoherent scatter radar. A long‐lived, large‐amplitude, traveling ionospheric disturbance with dominant wave modes of ∼15 and 75 min was observed shortly after the launch. The disturbance wave train is likely the product of a variety of wave modes. The disturbance front traveled with an average group speed of >628 m/s. Such speeds are typical of fast moving shock waves and ducted gravity waves. Either one or both could be responsible for the signatures observed near the leading edge of the STS 4 wave train. Later arriving waves, with their inherently lower propagation speeds, are attributed to additional gravity wave modes. These waves, however, were not explicity identified in this study. Although atmospheric waves are excited along the entire flight path, the most intense region of excitation is located along a relatively short flight segment (∼70 km) near the launch site where all primary thrusters are firing and over 70% of the propellants are expended. Not since the nuclear bomb tests of the late 1950s and early 1960s has an artificial source of atmospheric gravity waves been more available for upper atmospheric studies. The routine launching of high thrust vehicles provides an excellent opportunity to observe the propogation characteristics of atmospheric waves under controlled conditions and to acquire information on the nature of the upper atmosphere.
Scaling laws for the spectrum of interchange instabilities in the high‐latitude ionosphere
Authors: Keskinen M. J.
Published: February 2008

The Naval Research Laboratory (NRL) high‐latitude ionosphere‐magnetosphere mesoscale coupling model has been used to develop constraints on and scaling laws associated with the spectrum of interchange‐like instabilities and processes believed to be operating in the high‐latitude ionosphere. In this model, current closure in the ionosphere is given by Pedersen currents, while in the magnetosphere, closure is achieved with polarization currents. For Pedersen current‐driven interchange‐like instabilities with growth rate γ, we treat the limit υ < γ, where υ is the ion collision frequency averaged over a flux tube. The spectrum scaling laws for interchange‐like instabilities are developed using conservation laws implied by the fundamental nonlinear equations contained in the NRL coupling model. For the density spectrum implied by the interchange instability in the high‐latitude ionosphere we solve for the spectral index and develop scaling relationships for the outer scale with ambient ionospheric quantities. For a density spectrum of the form (1 + (k/k0)²)−n/2 we show that 1 < n < 3 in the steady state regime, where k is wave number perpendicular to the geomagnetic field and k0 is the outer scale wave number. The bounds on the spectral indices as predicted by the approach are shown to be consistent with experimental observations. Applications to mesoscale dynamics, structure, and stability of convecting inhomogeneities and large‐scale plasma density structures in the high‐latitude ionosphere have been made.
The behavior of a convergent plate boundary: Crustal deformation in the South Kanto District, Japan
Authors: Scholz Christopher H. , Kato Teruyuki
Published: February 2008

The northwesternmost part of the Sagami trough, a part of the Philippine Sea‐Eurasian plate boundary, was ruptured during the great South Kanto earthquake in 1923. Very extensive and frequent geodetic measurements of crustal deformation have been made in the South Kanto district since the 1890's, and these constitute the most complete data set on crustal movements in the world. These data were reanalyzed and interpreted and according to our interpretation indicate the following sequence of events. The coseismic movements were due to oblique thrust and right lateral slip of about 8 m on a fault outcropping at the base of the Sagami trough. This was followed by postseismic deformation resulting from reversed afterslip of 20–60 cm that occurred at an exponentially decaying rate in time. The interseismic deformation is produced by steady subduction at a rate of about 1.8 cm yr−1. During subduction the top 10–15 km of the plate boundary is apparently locked, while deeper parts slip aseismically at an irregular rate. No significant precursory deformation was observed. The recurrence time for 1923 type earthquakes is 200–300 years. The Boso and Miura peninsulas are broken into a series of fault‐bound blocks that move semi‐independently of the surrounding region. The subduction zone itself, where it is exposed on land, is shown to be a wide zone encompassing several faults that are active at different times.
Periodicity in lunar seismic activity and earthquakes
Authors: Sadeh Dror , Wood Kent
Published: February 2008

Lunar seismic activity detected by Apollo seismographs clearly shows a monthly periodicity. Using 36 months of data, Lammlein et al. (1974) found that the period is 27.2 days, the lunar nodical period. A search for such periodicity in earthquakes, based on the 8 years covered in the NOAA catalog, shows an analogous effect at half the lunar sidereal period, 13.65 days. The probability that the effect occurs randomly is assessed. The highest earthquake rate occurs at the two times each month when the moon crosses declination zero. A model based upon monthly changes in tidal forces explains the main patterns in both earthquake and moonquake samples by emphasizing directional relationships rather than variations of earth‐moon separation.
Periodic earthquakes in Alaska and central America
Authors: Sadeh Dror
Published: February 2008

The Earthquake Data File Summary catalog for the years 1949–1974 was used for time analysis. Periodic changes in the number of quakes in Alaska and Central America are reported and statistically evaluated. The suspected period is 13.65 days, which is half the lunar sidereal period. Periodicities in other locations are examined and evaluated.
An explanation for sidereal periods in earthquake aftershock sequences
Authors: Hunter Roger N.
Published: February 2008

Earthquake aftershock sequences for the years 1964 through 1974 were examined for a sidereal period. High chi‐square values were found for various subsets of the data but were shown to be caused primarily by one specific sequence. Removing part of that sequence from the data eliminated the high chi‐square values when weight factors which eliminate local time fluctuations were used.
Travel time and spectrum stability in the two years preceding the Yellowstone, Wyoming, earthquake of June 30, 1975
Authors: Murdock James N.
Published: February 2008

An earthquake of magnitude ML = 6.0 occurred within the U.S. Geological Survey Yellowstone Park, Wyoming, seismic network on June 30, 1975. The two stations closest to the epicenter were Norris Junction (NJ) (8 km) and Madison Junction (MJ) (20 km). In the 2 years preceding the earthquake, 12 nuclear devices detonated at the Nevada Test Site and Novaya Zemlya were recorded by NJ, MJ, and other stations of the network. The proximity of the two stations to the epicenter, together with their recording of the nuclear shots, gives us an exceptional opportunity to test for P wave travel time and spectral changes precursive to the earthquake. The travel times at NJ and MJ were measured in relation to two stations of the Yellowstone net that were 50–60 km from the epicenter. To estimate the noise in the data reduction scheme, measurements similar to these were made at Hanford, Washington, In general, the relative arrival times at Yellowstone were as stable as, those at Hanford, a suggestion that no anomaly larger than fluctuations caused by data reduction (0.09 s) was associated with the impending earthquake. The P wave forms of the shots recorded at NJ also were stable as a function of time, and in 1975 noanomaly was found in the P wave amplitudes.
Late stage equivalence of near‐surface explosions
Authors: Schneyer G. P.
Published: February 2008

A parametric investigation of the late stage equivalence of nearsurface explosions has been performed using numerical and analytical means. The explosive sources included both nuclear and chemical energy density levels modeled as pancake‐shaped masses of wet tuff ground material containing purely internal or downward‐directed kinetic energy. All of the analyzed internalenergy sources have been shown to be members of one of the late stage equivalent families given approximately by the relationship EsVs0.233 = const, where Es is the total source energy and Vs is the total source volume. Using analytic means, a hypothesized, nondimensional expression relating the far field hydro‐dynamic shock pressure to source parameters, shock position, and shock time of arrival was derived. Recasting the calculated results into the indicatedform yields a single, late time, nondi‐mensional shock pressure‐distance curve for all of the nuclear energy density level sources (both internal and downward‐directed kinetic) analyzed.