bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma bulk flow in Jupiter's dayside middle magnetosphere
Authors: Sands Mark R. , McNutt Ralph L.
Published: February 2008

Using the plasma data obtained during the Voyager 1 encounter and the full response function of the Plasma Science (PLS) experiment, convective plasma velocities have been determined in the dayside middle magnetosphere of Jupiter (10 RJ < r < 25 RJ). We find that temperature anisotropies have very little effect on plasma velocity determination and that the plasma data are well approximated by converted, isotropic Maxwellian ion distribution functions. The insensitivity of the analysis to any thermal anisotropies which may exist allows a good determination of the bulk plasma flow velocity. In addition to the subcorotational azimuthal flow there exists a substantial nonazimuthal component of plasma flow. This nonazimuthal flow is mostly aligned (antialigned) with the local magnetic field but also exhibits a cross‐field component. The velocity pattern is inconsistent with enhanced plasma outflow in the active sector, as suggested by the corotating convection model of plasma transport. The contribution of field‐aligned flow along the curved magnetic field lines to the stress on the magnetic field is evaluated. In the region studied such flow contributes up to one half the stress produced by the azimuthal plasma flow.
Theory and simulation of collisionless parallel shocks
Authors: Quest K. B.
Published: February 2008

A one‐dimensional theory of shocks propagating parallel to the ambient magnetic field in a collisionless plasma is presented. We show that shock formation and plasma heating can result from parallel propagating electromagnetic ion beam‐driven instabilities for a wide range of Mach numbers and upstream plasma conditions. The plasma upstream from a parallel shock overtakes the slower, downstream plasma in the absence of any electrostatic or electromagnetic wave fields. An interface region is created that is unstable to electromagnetic ion beam‐driven growth. The waves grow until they can scatter and couple the upstream and downstream plasma populations and can provide the necessary entropy increase for the shock. The marginal firehose state is reached downstream from the shock if the Alfvén Mach number MA is high enough (MA ≡ Uu/CA, where Uu is the upstream shock velocity and CA is the upstream Alfvén speed). A one‐dimensional compression of the ions along the magnetic field line direction results for weaker shocks. We solve a set of jump conditions and show that the marginal firehose state is first reached at a small magnetosonic Mach number (less than 2). We derive various conditions for creating and maintaining the shock via the ion beam‐driven instability and show that these conditions are roughly equal. The results from many one‐dimensional hybrid simulation runs are compared with the predictions of the theory and are shown to be in good agreement. At low Mach numbers, resonant group‐standing waves are the most strongly amplified and scatter the ions over several wavelengths. The transition in the ion pressure tensor is smooth, going from an isotropic upstream state to an anisotropic firehose state downstream in several wavelengths. The ions are compressed in one dimension at small Mach numbers, and there is little change in the perpendicular ion temperature across the shock. At high Mach numbers a small fraction of the upstream ions are backscattered by large‐amplitude magnetic waves at the shock front. The backscattered ions resonantly generate waves in the foreshock region, which are then convected downstream and are strongly compressed and amplified at the shock front. The bulk of the ion population passes through the shock and is nonresonantly scattered and heated by the amplified downstream waves. The change in the pressure tensor across the shock is abrupt; most of the entropy increase occurs within one or two wavelengths beyond where the waves are compressed and amplified. We compare the theoretical predictions with recently published observations of quasi‐parallel interplanetary shocks and the Earth's quasi‐parallel bow shock and find good agreement. Some theoretical limits on the fraction of ions that can be scattered back upstream are derived, and the implications for cosmic ray injection, particle acceleration, and astrophysical shocks are discussed.
A model for multiple throat structures in the polar cap flow entry region
Authors: Moses J. J. , Siscoe G. L. , Heelis R. A. , Winningham J. D.
Published: February 2008

A two‐dimensional ionospheric convection model has been developed to produce convection patterns for southward interplanetary magnetic field (IMF) and a positive or negative IMF y component. The model consists of a movable, shear convection reversal boundary with a gap in it where flux enters the polar cap. The sign of IMF By determines the dayside gap geometry. We use this simple model to simulate measured ionospheric flows from the DE 2 satellite. Roughly 35% of DE 2 passes that cross the dayside between 0800 and 1400 hours MLT cannot be modeled with a single narrow flow entry region. By comparing model calculations and the measured ion flows, we show that the dayside flow entry region to the polar cap typically spans several hours in local time. The electric field can concentrate along portions of the polar cap entrance and weaken between the concentrated regions, thus forming multiple “throats.”
The location of the high‐latitude boundary for magnetospheric convection is not arbitrary
Authors: Atkinson G.
Published: February 2008

The location of the high‐latitude boundary for magnetospheric convection to the dayside on almost‐dipolar field lines cannot be arbitrarily selected. The correct choice is near the high‐latitude boundary of the composite Alfvén layer (the thick layer for a distribution of particle energies). It is argued that other choices lead to extensive regions of β > 1 plasma and hence are unphysical. Differences, supported by observation, in the resulting convection model include no gaps between Birkeland current sheets, no plateau in electric field measurements across the auroral zones, the existence of a strong Harang discontinuity on the nightside, and the movement of the convection region to lower latitudes with increasing cross‐magnetosphere potential. The above theoretical boundary is related to observed boundaries.
Characterization of Alfvenic fluctuations in the magnetopause boundary layer
Authors: Rezeau L. , Morane A. , Perraut S. , Roux A. , Schmidt R.
Published: February 2008

The European Space Agency GEOS 2 spacecraft happened to cross the magnetopause several times, at various local times. Intense electric and magnetic fluctuations, in the ultralow‐frequency (ULF) range (0‐10 Hz) have been detected during each such crossing, with a peak at the magnetopause and still large amplitudes in the adjacent magnetosheath and magnetopause boundary layer. By applying spectral analysis and correlations to the electric and magnetic fluctuations, and a minimum variance analysis to the magnetic fluctuations, we investigate the nature of these fluctuations which appear as short‐lasting bursts in the spacecraft frame. Having reviewed possible interpretations, we show that the observed electric and magnetic signatures are consistent with small‐scale (L ≈ ion Larmor radius) Alfvenic field‐aligned structures passing by the spacecraft at high speed. It is suggested that these structures correspond to nonlinear Alfvenic structures.
A statistical model of auroral ion precipitation
Authors: Hardy David A. , Gussenhoven M. S. , Brautigam D.
Published: February 2008

The average global characteristics of precipitating auroral ions were determined using the data from the SSJ/4 detectors on the F6 and F7 satellites of the Defense Meteorological Satellite Program (DMSP). For this study the high‐latitude region was divided into spatial elements in magnetic local time (MLT) and corrected geomagnetic latitude (CGL). One such spatial matrix was created for each of seven levels of magnetic activity as defined by Kp. Approximately 26.5 million, individual, 1‐s spectra were used to determine the average ion spectrum over the energy range from 30 eV to 30 keV in each spatial zone and at each level of activity. Where appropriate, the spectra were extrapolated to 100 keV to provide a more complete estimate of the total integral energy flux, number flux, and average energy. The global patterns of the integral energy flux, integral number flux, and average energy derived from the average spectra vary smoothly with latitude, MLT, and activity. For a given Kp value the higher levels of integral energy flux occur in C‐shaped regions symmetric about a meridian running prenoon to premidnight. The maximum integral energy flux is found premidnight; the minimum peak integral energy flux in latitude is found prenoon. Except in the prenoon region, the overall level of integral energy flux increases with Kp with the premidnight maximum increasing by a factor of 6.1 from Kp = 0 to Kp ≥ 6−. The maximum integral number flux is centered at noon and is taken to be the center of the cusp. The maximum value does not vary significantly with activity, although its location moves to lower latitude with increasing Kp. A minimum in the average energy also occurs in the noon meridian, several degrees poleward of the integral number flux maximum. The separation in latitude between the integral flux maximum and the average energy minimum of the cusp increases with Kp. The maximum average energy occurs on the eveningside of the oval near the equatorward boundary of the ion precipitation and shifts toward noon with increasing activity. The average integral number flux for the precipitating auroral ions is 1‐2 orders of magnitude less than that for the precipitating auroral electrons at all latitudes, MLTs, and activities. The integral energy flux of the ions, on the eveningside of the oval, can equal or exceed that for the electrons near the equatorward edge of the auroral region. Integrated globally, the ratio of the ion to electron number flux varies from 0.024 to 0.015 over the range of activity from Kp = 0 to Kp ≥ 6−. The ratio for the integral energy flux ranges from 0.11 to 0.17 with no trend in activity.
Structure of the bipolar plasma sheath generated by SPEAR I
Authors: Katz I. , Jongeward G. A. , Davis V. A. , Mandell M. J. , Kuharski R. A. , Lilley J. R. , Raitt W. J. , Cooke D. L. , Torbert R. B. , Larson G. , Rau D.
Published: February 2008

The Space Power Experiment Aboard Rockets I (SPEAR I) biased two 10‐cm radius spheres as high as 46,000 V positive with respect to an aluminum rocket body. The experiment measured the steady state current to the spheres and the floating potential of the rocket body. Three‐dimensional calculations performed using NASCAP/LEO and POLAR 2.0 show that both ion‐collecting and electron‐collecting sheaths were formed. The rocket body potential with respect to the ionospheric plasma adjusted to achieve a balance between the electron current collected by the spheres and the secondary electron‐enhanced ion current to the rocket body. This current balance was obtained with a large ion‐collecting sheath that enveloped most of the electron‐collecting sheath and reduced the area for collection of ionospheric electrons. The calculated current is in agreement with the flight measurement of a steady state current of less than 1/10 A. The calculations show that the rocket body was driven thousands of volts negative with respect to the ionospheric plasma. The calculated rocket potential is within the uncertainty of that inferred from ion spectrometer data. The current flowed through the space plasma. There was almost no direct charge transport between the spheres and the rocket body.
Thermodynamic properties of the heliospheric plasma
Authors: Whang Y. C. , Behannon K. W. , Burlaga L. F. , Zhang Shuli
Published: February 2008

Plasma data from Helios A and B, IMP, ISEE 3, Pioneer 10 and 11, and Voyager 1 and 2, together with an MHD simulation model, have been used to study on a global scale the temporal and radial evolution of the solar wind's thermodynamic properties (temperature T, number density n, and entropy s). The study has given the following results: (1) Near and inside 1 AU, T and n are negatively correlated almost all the time. Outside ≈5 AU, the T,n relation evolves to become positively correlated. (2) At 1 AU, T, p, n, and βi are dependent on the sunspot number. During the minimum of the sunspot cycle, the negative correlations between T and n are very strong. (3) The radial evolution of temperature and entropy shows that a heating process takes place for the heliospheric plasma. Between 1 and 10 AU, the entropy increases by ≈4×10−16 erg/K/proton. In addition, from the combined Voyager 1 and 2 observations between 1 AU and 10 AU, the solar rotation average proton temperature was found to decrease as r−0.46±0.09, a rate much slower than the r−4/3 rate predicted by the adiabatic expansion of the solar wind. We interpret this heating to be largely due to the cumulative effect of the shock process with increasing distance from the Sun. (4) The simulation model is also used to extrapolate the thermodynamic properties of the solar wind from Voyager 2 to the region of the outer heliosphere bounded by the termination shock. As the heliocentric distance increases by a factor of 10, the temperature decreases by a factor of 3, and the shock process raises the entropy by 4×10−16 erg/K/proton.
Processes responsible for the compositional structure of the thermosphere
Authors: Burns A. G. , Killeen T. L. , Roble R. G.
Published: February 2008

A diagnostic postprocessor analysis package used with runs of the National Center for Atmospheric Research thermospheric general circulation model (NCAR‐TGCM) has been extended to include the terms of the neutral composition equation that is solved by this model. The purpose of the development of this capability is to quantify the relative importance of the various physical and chemical mechanisms that force changes in neutral thermospheric composition for given geophysical situations. Such mechanisms include photodissociation of molecular oxygen and three‐body recombination of atomic oxygen, molecular and eddy diffusion and vertical and horizontal wind advection. Compositional term analysis calculations are presented using a diurnally‐reproducible TGCM run for a day of moderate geomagnetic activity (Kp = 3) near December solstice. Principal results for F region altitudes are in general agreement with previous work and include the following: (1) upper thermospheric composition is controlled by three major processes of vertical advection, horizontal advection and molecular diffusion; (2) the time scale for changes of 1/e in the mass mixing ratio for N2 and O resulting from these processes is typically ∼25–30 hours; (3) large gradients exist in the mass mixing ratios for N2 and O in the summer hemisphere high‐latitude region that, when combined with horizontal winds blowing across the magnetic pole, led to significant composition changes due to horizontal mass advection; (4) vertical advection is the major cause of the changes in mass mixing ratio of N2 and O at low and middle latitudes, with cooling and subsidence leading to decreases (increases) in the mixing ratios of N2 (O) at night, and heating and expansion leading to increase (decreases) during the daytime; (5) upward winds caused by Joule heating in the morning and evening sectors of the auroral oval cause large increases in the mixing ratio of N2 in these regions due to vertical mass advection. The principal results at ∼120 km altitude include the following: (1) photodissociation is an important term for O and O2 in this altitude region, becoming roughly commensurate with molecular diffusion, vertical and horizontal advection; (2) significant increases in the mass mixing ratio of O in the summer hemisphere occur due to photodissociation, especially in the daytime at middle latitudes; (3) downward molecular diffusion of O is important in controlling the mass mixing ratio for O, particularly in the high‐latitude winter hemisphere; (4) a complicated morphological pattern of horizontal and vertical advection maintains the mass mixing ratios at these altitudes; (5) vertical advection resulting from Joule heating in the morning and evening sectors of the auroral zone is of importance for the composition of the high‐latitude lower thermosphere; (6) typical time scales for maximum changes of 1/e in the mass mixing ratio of N2 are of the order of 60–90 hours.
Effect of RF on type II electrojet instability
Authors: Sharma A. K.
Published: February 2008

Gradient drift instability in the equatorial electrojet is susceptible to the presence of a high‐power radio wave. When the latter has a turning point near the region of instability, interaction becomes an efficient four‐wave process involving four electrostatic waves. The high‐frequency waves exert a low‐frequency pressure gradient force on the electrons, destabilizing further the unstable mode. The effect is important when υ0²/υd² ≳ νiω0Δ/νeωpi², where υ0 and υd are electron oscillatory and dc drifts, νi and νe are ion and electron collision frequency, and Δ is a frequency mismatch.
Linear study of the beam‐plasma interaction as a source mechanism for the broadband electrostatic emissions observed in the electron foreshock
Authors: Canu Patrick
Published: February 2008

Electron beams streaming back from the Earth's bow shock into the solar wind have been proposed as a possible source for the electron plasma waves observed by spacecraft in the electron foreshock. These waves often correspond to broadband electrostatic emissions extending both above and below the local electron plasma frequency. The linear beam‐plasma interaction is studied numerically. We focus on the generation of emissions with upper cutoff frequency of the order of or above 1.5 times the electron plasma frequency (ωpe), which were reported from ISEE observations. Beam drift velocity, thermal spread and density are varied over a range which is consistent with the observations, and the effects on the frequency bandwidth over which emissions can grow are studied. It is shown that linear instability can develop, on the so‐called beam mode, in a frequency range from ω∼0 to above twice the electron plasma frequency, when electron beam drift velocity is of the order of 1.5 to 3 times the bulk electron thermal velocity and when the ratio between the beam and the plasma temperature is very low (Tb/Tp ≤ 0.01). This large unstable frequency bandwidth decreases dramatically with increasing Tb/Tp. When Tb/Tp = 0.5, and the beam to plasma density ratio is close to Nb/Np = 0.01, positive growth rate exists on a small frequency bandwidth, Δω/ω<0.1, close to ωpe. The cold beam case does not account for some broadband emissions observed with a low‐frequency cutoff below and close to ωpe. Broadband emissions can be generated by warm electron beams, but a beam density ratio Nb/Np equal to or greater than 0.1 is required, which is 10 times greater than the upper limit ratio inferred from spacecraft observations.
Planar magnetic structures in the solar wind
Authors: Nakagawa T. , Nishida A. , Saito T.
Published: February 2008

A distinctive magnetic structure in which azimuthal and latitudinal angles of field vectors are closely related to each other has been found in the interplanetary magnetic field data obtained by Sakigake at 0.8–1.0 AU. In this structure, termed a ”planar magnetic structure” (PMS), the magnetic field vectors are nearly parallel to a fixed plane. This plane includes the spiral direction but is inclined to the ecliptic plane from 30° to 85°. The field vectors take almost all directions parallel to this plane. The PMS consists of several segments in which field directions are almost constant, and the segments are separated by tangential discontinuities where directional changes of the field vector occur abruptly without showing any preferred polarization. The ion number density, the ion temperature, and the plasma β tend to be higher in the PMS than in the surrounding plasma. The PMS events are clearly distinct from ”magnetic clouds” both in the field configuration and in the plasma conditions. During the 25‐month period from July 1985 to July 1987, eight PMS events with durations of several hours have been identified. The PMS events may be associated with newly emerging magnetic structure in the photosphere from which magnetic tongues are extended into interplanetary space.
A search for evidence of the evolution of rotational discontinuities in the solar wind from nonlinear Alfvén waves
Authors: Neugebauer M. , Buti B.
Published: February 2008

The nonlinear theory of the steepening of Alfvén waves into solitons and then into rotational discontinuities (RDs) predicts correlations between the β of the plasma, the sense of polarization of the discontinuity, and changes of magnetic field strength and plasma density across the discontinuity. The predicted correlations have been searched for in ISEE 3 field and plasma data, but no statistically significant evidence for the evolution of RDs from Alfvén solitons could be found. Approximately equal numbers of right‐ and left‐handed discontinuities were observed. Some of the rotations approached 180°, but none exceeded that value. Nearly all the RDs in the 33‐day data set were propagating outward from the Sun, which implies that sunward propagating Alfvénic fluctuations produced by stream interactions in the interplanetary medium seldom steepen into discontinuities before reaching 1 AU. The only unambiguously sunward propagating RD in the data set showed all of the expected relations for a steepened Alfvén soliton. Interplanetary processes, such as changing wave speeds and β and refraction of the discontinuities, may have changed the properties of the RDs created close to the Sun to such an extent that any original correlations are no longer detectable at 1 AU. Another possible explanation of the basically negative result is that some of the RDs produced near the Sun may have steepened from fast or slow Korteweg‐deVries solitons.
The launch of solar coronal mass ejections: Results from the coronal mass ejection onset program
Authors: Harrison R. A. , Hildner E. , Hundhausen A. J. , Sime D. G. , Simnett G. M.
Published: February 2008

We describe a set of solar coronal mass ejection (CME) events where coincident data sets from both X ray and white light instruments have been made available through deliberate planning. Using these we have been able to put tight limits on possible descriptions of the typical sequence of events, and these we relate to interpretations of models involving flares and CMEs. Our findings confirm recent suggestions that CME onsets precede any related flare activity and that the associated flaring commonly lies to one side of the CME span. The CME launch appears to be associated with minor X ray (flare precursor) activity. Although this scenario has been previously discussed (see Harrison, 1986, and references therein), the abundance of flare and CME models which are not compatible with this picture demands that confirmation be sought using programs such as this.
Simulation of electrostatic turbulence due to sheared flows parallel and transverse to the magnetic field
Authors: Nishikawa K.‐I. , Ganguli G. , Lee Y. C. , Palmadesso P. J.
Published: February 2008

Using a spatially two‐dimensional electrostatic particle simulation code, we examine the stability of a plasma equilibrium characterized by a localized transverse dc electric field and a magnetic‐field‐aligned electron drift for L ≪ Lx, where Lx is the simulation length in the x direction and L is the scale length associated with the dc electric field. It is found that the dc electric field and the field‐aligned current can together play a synergistic role to enable the excitation of electrostatic waves even when the threshold values of the field‐aligned drift and the E×B drift are individually subcritical. The simulation results indicate that a broadband turbulence is associated with such an equilibrium. This is in contrast to the current‐driven ion cyclotron instability, which is characterized by a coherent spectrum around the ion cyclotron harmonic. Further, the growing ion waves are associated with small vortices in the linear stage, which evolves to a nonlinear state dominated by larger vortices with lower frequencies.
Mapping and distortions of auroral structures in the quiet magnetosphere
Authors: Kaufmann Richard L. , Larson Douglas J. , Lu Chen
Published: February 2008

The objective of this report is to identify those features of commonly observed dayside auroras that can be explained easily by either of two processes: mapping distortions or distortions caused by nearby Birkeland currents. Mapping distortions in a quiet closed magnetosphere model yield very elongated arclike ionospheric images in the dayside auroral zone for a wide range of circular or elliptical tubelike equatorial sources. The length of a dayside auroral arc provides a good measure of the equatorial source dimension normal to the magnetopause. The orientation of the dayside arc provides a crude estimate of the solar magnetospheric×location of the equatorial source. Dayside arcs and Birkeland currents are found to map to both the dayside and the nightside magnetopause current layer and low latitude boundary layer. Nightside auroral arcs and Birkeland currents map to the magnetotail plasma sheet and plasma sheet boundary layer. Localized Birkeland currents produce characteristic distortions that are dependent on the current density gradient. Single and multiple linear and hooked auroral forms are easy to understand in terms of mapping distortions in a quiet magnetosphere. The shapes of bright twisted or folded auroral forms are more easily explained as distortions produced by localized Birkeland currents. Measurement of the size of a spiral auroral structure provides an estimate of the current magnitude. Relationships between the motion of ionospheric objects and the motion of equatorial sources are summarized by two color plates. Such motion sometimes differs markedly from what one might intuitively expect. For example, some tailward moving sources near the magnetopause can map to sunward moving ionospheric images.
Z mode radiation in Jupiter's magnetosphere: The source of Jovian continuum radiation
Authors: Barbosa D. D. , Kurth W. S. , Moses S. L. , Scarf F. L.
Published: February 2008

Voyager spacecraft observations of Z mode waves in Jupiter's magnetosphere are analyzed. The signal appears in wideband spectrograms as an intense narrowband emission separated from the broadband continuum radiation seen at higher frequencies by a deep emission gap. The assumption that the intensity minimum corresponds to the electron plasma frequency organizes the data very well and provides a consistent interpretation of all spectral features in terms of plasma resonances and cutoffs. Several examples are given which demonstrate that the continuum radiation is composed of both left‐hand and right‐hand polarized waves with distinct cutoffs observed at the plasma frequency and right‐hand cutoff frequency, respectively. A survey of a representative sample of events reveals that the Z mode peak frequency lies close to the left‐hand cutoff frequency, suggesting that the observed characteristics of the emission are the result of wave reflection at the cutoff layer. In a substantial number of events, another distinct emission occurring near the upper hybrid resonance frequency is detected simultaneously with the Z mode. The entire set of observations gives strong support to the linear mode conversion theory of upper hybrid waves to continuum radiation mediated by the Z mode via the Budden radio window mechanism.
Artificial ion beam instabilities: 1. Linear theory
Authors: Scales W. A. , Kintner P. M.
Published: February 2008

Plasma instabilities that result when a heavy ion beam is injected into the F region ionosphere are studied using linear Vlasov theory. The background plasma model used consists of a relatively cool, collisionless O+‐dominated plasma with small concentrations of H+ and He+. Due to its large gyroradius (k⊥ρb ≫ 1), the beam is assumed to be unmagnetized and homogeneous. The beam density is taken to be on the order of or less than the background density. Instabilities in the frequency range from somewhat above the background lower hybrid frequency down to the O+ gyro‐frequency are studied. The instabilities are studied for various beam drift velocities, thermal velocities, and density regimes relevant to past sounding rocket experiments. It is shown that lower hybrid and ion‐ion hybrid instabilities dominate for drift velocities significantly larger than the H+ thermal velocity. For drifts on the order of the various background ion thermal velocities, instabilities of the various background ion Bernstein modes dominate. The importance of the transition from the kinetic to the fluid version of the instabilities is also discussed. The results of the linear theory are compared to experimental observations of plasma waves from past experiments. The agreement is very favorable.
Observational test of a hot flow anomaly formation mechanism
Authors: Onsager T. G. , Thomsen M. F. , Gosling J. T. , Bame S. J.
Published: February 2008

Hot flow anomalies (HFAs) are unique plasma structures observed in the vicinity of the Earth's bow shock. They are typically characterized by high temperature plasma which is strongly deflected relative to the solar wind and flanked by shock‐like density and magnetic field enhancements. One mechanism proposed to explain these structures involves the coupling of ions reflected off the bow shock with the solar wind. This coupling would convert the relative streaming energy between the reflected and solar wind ions into thermal energy. The hot plasma would then expand to form the observed HFA. This model predicts that the initial temperature of the HFA is roughly limited by the available energy in the relative streaming between the two ion beams, and that the observed temperature and density correspond to the coupled plasma after an expansion has occurred. We present a simple test of this mechanism by comparing the measured temperature and density with the values expected immediately following the assumed coupling and after expansion. We find that the relative streaming energy is in most cases sufficient to account for the measured temperature. However, the predicted HFA temperature after adiabatic expansion to the measured density is in many cases well below the measured temperature. This analysis indicates that if the reflected ion coupling mechanism is correct, the expansion is probably not adiabatic, and in some cases an additional process may be needed to account for the observed temperature. The observed anti‐correlation between the ion temperature and density both within individual HFAs and among the different HFAs provides further evidence that processes other than adiabatic expansion are occurring.
Velocity‐dispersed ion beams in the nightside auroral zone: AUREOL 3 observations
Authors: Zelenyi L. M. , Kovrazkhin R. A. , Bosqued J. M.
Published: February 2008

Two years of data from the AUREOL 3 satellite have been used to study in detail the energetic ion beams which precipitate at low altitudes (400–2000 km) near the poleward boundary of the nighttime auroral zone. Eighty 2–20 keV events have been identified and examined; they typically display an energy‐latitude dispersion in the auroral projection of the boundary plasma sheet, with the more energetic ions precipitating at less than 0.5° invariant latitude from the polar cap boundary. Their occurrence maximum is found in the 20–04 MLT sector and is independent of the magnetic activity measured by the AE index. In all the events studied, the ion fluxes are less than 107 ions cm−2 s−1 sr−1 keV−1, and mass analysis demonstrates that they are mainly composed of energetic H+ ions. All the experimental facts are consistent with an ion acceleration by the dawn‐dusk electric field in the neutral sheet with a weak magnetic field normal component Bz in the distant magnetotail at an inferred distance of 50–100 RE. The observed velocity‐latitude ion segregation is the result of the E×B drift of the ion beams up to the ionosphere (geomagnetic filter effect) superimposed upon the intrinsic dispersive effect of the nonadiabatic energization mechanism, when a realistic dependence of Bz on downtail distance is adopted. Finally, from the striking similarity of the experimental ion distributions we believe that the ion beams observed at low altitudes close to the polar cap boundary are the signature in the auroral ion precipitation of the earthward flowing ion beams often detected along the plasma sheet boundary layer.
A theoretical study of the seasonal and solar cycle variations of stable aurora red arcs
Authors: Kozyra J. U. , Valladares C. E. , Carlson H. C. , Buonsanto M. J. , Slater D. W.
Published: February 2008

SAR arc statistics provide information on the seasonal and solar cycle variations in the subauroral region electron temperature peak and associated magnetospheric energy source. There are two sources of long‐term (solar cycle and seasonal) variability in the magnitude of the subauroral region electron temperature peak and associated SAR arc emission intensity: (1) the neutral atmosphere and ionosphere and (2) the magnetospheric energy source. The results of this study indicate that the observed seasonal variation in SAR arc intensities can be explained reasonably well by seasonal variations in the neutral atmosphere and ionosphere. True solstice effects are unlikely to result from differences in a near‐equatorial magnetospheric heat source since the same heat source supplies both the summer and the winter hemispheres at opposite ends of a common flux tube. Observed solar cycle variations in SAR arc intensity for a fixed ring current strength (as represented by the Dst index) are not consistent with variations predicted solely on the basis of a solar cycle changes in the neutral atmosphere and ionosphere. A reduction of the magnetospheric heat flux by a factor of between 5 and 20 from solar maximum to solar minimum conditions is necessary to bring model electron temperatures and 6300Å emission intensity into agreement with observational results for moderately disturbed conditions (Dst ∼ −80γ). The required reduction in the magnetospheric energy source with decreasing solar cycle is attributed to compositional changes in the magnetospheric plasma.
Electron Fermi acceleration in collapsing magnetic traps: Computational and analytical models
Authors: Gisler Galen , Lemons Don
Published: February 2008

We consider the heating and acceleration of electrons trapped on magnetic field lines between approaching magnetic mirrors. Such a collapsing magnetic trap and consequent electron energization can occur whenever a curved (or straight) flux tube drifts into a relatively straight (or curved) perpendicular shock. Our relativistic, three‐dimensional, collisionless test particle simulations show that an initial thermal electron distribution is bulk heated while a few individual electrons are accelerated to many times their original energy before they escape the trap. Upstream field‐aligned beams and downstream pancake distributions perpendicular to the field are predicted. In the appropriate limit the simulation results agree well with a nonrelativistic analytic model of the distribution of escaping electrons which is based on the first adiabatic invariant and energy conservation between collisions with the mirrors. Space science and astrophysical applications are discussed.
Re‐forming supercritical quasi‐parallel shocks: 1. One‐ and two‐dimensional simulations
Authors: Thomas V. A. , Winske D. , Omidi N.
Published: February 2008

Hybrid (particle ion, massless fluid electron) simulations are carried out in one and two spatial dimensions to show that the supercritical quasi‐parallel shock is not steady. Instead of a well‐defined narrow transition region (shock ramp) between upstream and downstream states that remains at a fixed position in the flow, the ramp periodically steepens, broadens, and then re‐forms upstream of its former position. During part of the re‐formation cycle, the number of ions backstreaming from the shock increases to a significant fraction (up to 40%) of the incoming ions. This dense ion beam then couples to the incident ions to form a heated region, the leading edge of which becomes the new shock ramp. Two‐dimensional simulations demonstrate that this process is not a consequence of restricting the calculations to one spatial dimension and show that the re‐formation cycle is not in phase along the shock surface, although the surface itself remains fairly laminar. Generally, the downstream wave turbulence is comparable in all three magnetic components (unlike one dimensional where Bn = const) and shows no tendency to be aligned in any particular direction. Both the two‐dimensional simulations as well as the one‐dimensional runs indicate that the downstream plasma is a complex mixture of regions of merged incident and reflected ion beams. The simulations are consistent with recent observational studies at the quasi‐parallel bow shock showing the presence of magnetic pulsations, cold ion beams, and a downstream consisting of alternating regions of low‐density hot ions and higher density cooler ions.
Seismic anisotropy in an ophiolite peridotite: Application to oceanic upper mantle
Authors: Peselnick L. , Nicolas A.
Published: February 2008

Compressional velocity Vp to 8 kbar and 275°C in a homogeneous, 0.5% porosity, little‐altered harzburgite from the Antalya ophiolite complex in Turkey was measured along the three principal kinematic axes as defined from the preferred orientations of olivine and enstatite. The observed Vp anisotropy was 0.7 km/s for pressures P from 2 to 8 kbar and temperatures T from 25° to 275°C. The seismic anisotropy at 250°C and 5 kbar (15‐km depth) calculated from the single‐crystal elastic constants and the fabric data were within 3% of the observed velocities in the core directions. Measured values of the P and T derivatives of Vp for the three cores are 14 m/s kbar and −0.6 m/s °C, which are in reasonable agreement with calculated average values from single‐crystal data. These data result in a critical thermal gradient of 7.5°C/km. A model of oceanic crust and uppermost mantle (constructed from the structural data in this ophiolite complex) permits an estimation of the azimuthal seismic anisotropy in the uppermost mantle beneath this paleocrust under assumed conditions of 250°C and 15‐km depth. The result is Δ Vp = 0.3 km/s with Vp = 8.46 and 8.16 km/s normal and parallel to the ridge, respectively. The azimuthal shear velocity anisotropy is 0.18 km/s. Measurements of Vp are also presented for three lherzolite cores from Salt Lake Crater, Hawaii. The T derivatives of Vp for these cores agree with those obtained for the harzburgite; however, the P derivatives are consistently higher. The higher values of the P derivatives for the lherzolite sample are considered to result from its higher porosity.
Island subsidence, hot spots, and lithospheric thinning
Authors: Detrick Robert S. , Crough S. Thomas
Published: February 2008

Drilling results from several western Pacific atolls indicate the long‐term subsidence of these islands is much more than would be expected from the cooling and thickening of the underlying lithosphere. This excess subsidence cannot be satisfactorily explained by isostatic adjustments to the weight of the volcano or the coral reef cap. It appears to be related to island formation atop unusually shallow areas of sea floor, like the Hawaiian swell, associated with midplate hot spots. The excess subsidence is caused by the gradual return of these shallow areas to normal depths. Several authors have suggested that the Hawaiian swell is supported by upward flow in the asthenosphere. However, this model offers no reasonable explanation for the shape of the swell or the observed rates of atoll subsidence. The regional gravity anomaly over the Hawaiian swell indicates an average depth of compensation within the lower half of the lithosphere, not within the asthenosphere, as would be expected if the swell were maintained by asthenospheric flow. While the compensating mass may extend to greater depths, most of the density changes appear to occur within the lithosphere. We propose that the Hawaiian swell is formed by lithospheric thinning over the Hawaiian hot spot. Since the asthenosphere is less dense than the lithosphere, replacement of the lower portion of the lithosphere by asthenospheric material causes isostatic uplift of the surface of the plate. After the lithosphere moves away from the hot spot, it cools and thickens, and the swell subsides. The subsidence histories of the Hawaiian swell and several Pacific atolls are in quantitative agreement with this mechanism. The main problem with this model is that it requires extremely fast rates of lithospheric heating.
Finite strain isotherm and velocities for single‐crystal and polycrystalline NaCl at high pressures and 300°K
Authors: Birch Francis
Published: February 2008

Ultrasonic, pressure‐volume, and shock wave measurements of NaCl are reviewed, with the purpose of representing as much as possible within a single theoretical framework, A large portion of the data is consistent, to within reasonable uncertainties, with the Eulerian formulation of finite strain, in the BE2 form. This contains three parameters, of which two, K0 and K0′, are obtainable from single‐crystal ultrasonic measurements, while the third, K0″, may be found with the aid of shock wave data. The combination of the values of Spetzler et al. (1972a) with the isotherm of Fritz et al. (1971) gives an equation of state consistent with the pressure‐volume measurements to 300 kbar, to within a kilobar or two. There remain small but possibly significant discrepancies with the high‐precision measurements of linear change at low pressures. Various two‐parameter isotherms are also examined and found to fail either at low or at high compressions: While other three‐parameter forms may be fitted to the data, they do not afford a satisfactory general treatment of wave propagation. Finite strain theory is applied to project the effective elastic coefficients of single‐crystal NaCl to 270 kbar. The anisotropy index, 2B44(B11–B12), falls from 0.7 at P = 0 to 0.12 at 270 kbar, and the usual methods of averaging to find the properties of a quasi‐isotropic aggregate diverge increasingly as pressure increases. The methods of Kröner and of Peresada give fair agreement with recent measurements of velocities in NaCl aggregates to 270 kbar.
Geometry of the subducted lithosphere beneath the Banda Sea in eastern Indonesia from seismicity and fault plane solutions
Authors: Cardwell Richard K. , Isacks Bryan L.
Published: February 2008

The spatial distribution of hypocenters in eastern Indonesia, together with 41 new and previously published fault plane solutions, can be explained by a simple model of two lithospheric plates descending into the upper mantle beneath the Banda Sea. The major one, defined by the shallow to deep hypocenters located along the Banda arc, is a laterally continuous slab that has subducted at the plate boundary defined by the Java trench‐Timor trough‐Aru trough system. The other slab descends toward the southwest to depths of about 100 km in the region of the Seram trough and may be joined to the Banda arc subduction system by the westward extension of the New Guinea Tarera‐Aidoena fault zone, which acts as an arc‐to‐arc transform. The Banda arc slab is contorted at the eastern end of the arc where the trench and the line of active volcanoes curve to the northeast. The contortion appears to be a lateral bend in the subducted slab that is continuous from the surface to depths of 600 km. Fault plane solutions in the contorted portion of the slab have P axes aligned approximately parallel to the local strike of the slab and may be related to lateral bending stresses. Similar orientations of stress axes also occur near the curved ends of other island arcs. Experiments with a lead sheet model suggest that the observed configuration of the Banda slab may be simply related to the curved end of the Banda arc and further suggest that the direction of relative motion between the Banda arc and Australia during the Neogene has been more north‐northwesterly than the direction predicted from major plate motions. The discrepancy can be resolved if Southeast Asia is not considered part of the Eurasian plate. The lack of shallow underthrusting mechanisms and the paucity of shallow earthquakes in the Timor trough‐Aru trough region may have resulted from the youthful collision of the Australian shelf with the Banda arc.
Seismological aspects of the Guatemala Earthquake of February 4, 1976
Authors: Kanamori Hiroo , Stewart Gordon S.
Published: February 2008

Detailed analyses of teleseismic surface waves and body waves from the Guatemala earthquake of February 4, 1976, show the following: (1) Left lateral displacement along a vertical fault with a strike varying from N66°E to N98°E is consistent with the teleseismic data. (2) The seismic moment was 2.6 × 1027 dyn cm. The directivity of the surface wave radiation indicates an asymmetric (1:2.3) bilateral faulting with a total length of 250 km. In modeling the displacement a rupture velocity of 3 km/s was used, and the fault curvature was included. (3) If a fault width of 15 km is assumed, the average offset is estimated to be about 2 m. This value is about twice as large as the average surface offset. (4) Although the observed directivity suggests a uniform overall displacement along the fault, the body wave analysis suggests that the earthquake consists of as many as 10 independent events, each having a seismic moment of 1.3–5.3 × 1026 dyn cm and a fault length of about 10 km. The spatial separation of these events varies from 14 to 40 km. This multiple‐shock sequence suggests that the rupture propagation is jagged and partially incoherent with an average velocity of 2 km/s. (5) The average stress drop estimated from surface waves is about 30 bars, but the local stress drop for the individual events may be significantly higher than this. (6) The complex multiple event is a manifestation of a heterogeneous distribution of the mechanical properties along the fault, which may be caused by either asperities, differences in strength, differences in pore pressure, differences in slip characteristics (stable sliding versus stick slip), or combinations of these factors. (7) This complexity has important bearing on the state of stress along transform faults and is important in assessing the effect of large earthquakes along other transform faults like the San Andreas.

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