bluuebunny/crossref_metadata_2025 huggingface dataset:
Generation of auroral kilometric radiation in upper hybrid wave‐lower hybrid soliton interaction
Authors: Pottelette R. , Treumann R. A. , Dubouloz N.
Published: February 2008

Sporadic bursts of auroral kilometric radiation (AKR) associated with strong bursty electrostatic turbulence in the vicinity of the lower hybrid frequency have been frequently recorded in the AKR source region by the Viking satellite. The variation time scale of these emissions is typically 1 s, long enough for lower hybrid waves to grow to amplitudes of several hundred millivolts per meter and to evolve nonlinearly into solitons. On the basis on these observations it is suggested that formation of lower hybrid solitons may play a role in the generation of AKR. A theoretical model is proposed which is based on the direct acceleration of electrons in the combined lower hybrid soliton and upper hybrid wave fields. The solitons act as sporadic, localized antennas allowing for efficient conversion of the electrostatic energy stored in upper hybrid waves into electromagnetic radiation at a frequency above the X mode cutoff. Excitation of lower hybrid waves is due to the presence of energetic electron beams in the auroral zone found to be associated with steep plasma density gradients. Upper hybrid waves can be excited by a population of energetic electrons with loss cone distributions. The power of the electromagnetic radiation obtained is only noticeable in regions where the plasma frequency is less than the electron gyrofrequency. The theory predicts spectral power densities of the order of 10−11 to 10−9 W m−2 Hz−1 in the source region, in good agreement with the Viking observations. The sporadic nature of the radiation derives from lower hybrid soliton collapses which occur on ∼1‐s time scales.
A model of the ionosphere of Titan
Authors: Keller C. N. , Cravens T. E. , Gan L.
Published: February 2008

We have developed a one‐dimensional photochemical model to study both the composition and density structure of Titan's ionosphere. The model neutral atmosphere developed by Yung et al. (1984) and Yung (1987) was used as a basis to model the chemistry of Titan's ionosphere. Ionization rates due both to photoionization by solar EUV flux and to electron impact ionization by photoelectrons and Saturnian magnetospheric electrons were included. The major neutral species (nitrogen and methane) are ionized to produce N2+, N+, CH4+, CH3+, CH2+, and CH+ ions. Ion‐neutral chemistry converts these ions to the major ion species H2CN+ (in agreement with Atreya (1986), Strobel (1985), and Ip (1990a)), as well as C2H5+, CH5+, HCN+, and more complex hydrocarbon ions (CnHm+). Inclusion of both forms of ionization gave a peak electron density of ≈ 3030 cm−3 at an altitude of ≈ 1175 km along the terminator. The radio occultation experiment on board Voyager 1 (Lindal et al., 1983) put limits on the maximum ionospheric electron density of 5 × 10³ cm−3 and 3 × 10³ cm−3 along the morning and evening terminators, respectively. The total external pressure (i.e., dynamic, thermal, and magnetic) upstream of Titan at the time of the Voyager encounter is of the order of the maximum ionospheric thermal pressure. As a consequence one can expect that the solar wind interaction with Venus during periods of high solar wind dynamic pressure might provide a good analogy for the interaction of Titan with the Saturnian magnetospheric plasma.
Simulations of coronal disconnection events
Authors: Linker Jon A. , Van Hoven Gerard , McComas David J.
Published: February 2008

The lack of evidence for magnetic disconnection of coronal mass ejections (CMEs) from the Sun has long been a puzzle, as it implies a buildup of the interplanetary magnetic field (IMF) magnitude over time. Such a buildup is ruled out by observations. Magnetic reconnection above helmet streamer configurations could provide a mechanism for maintaining the observed relative constancy of the IMF [McComas et al., 1989]; McComas et al. [1991] showed observational evidence of reconnection above a streamer. We investigate this interpretation using time‐dependent MHD simulations. We model the opening of new magnetic flux on the Sun (as might occur in a CME or other transient event) as an increase in magnetic flux at the poles of a simulated corona. We find that this perturbation can in fact cause reconnection above an equatorial helmet streamer, and the resultant density signature is similar to the observations of McComas et al. [1991].
Relationship between wave energy and free energy from pickup ions in the comet Halley environment
Authors: Huddleston D. E. , Johnstone A. D.
Published: February 2008

The free energy available from the implanted heavy ion population at comet Halley is calculated by assuming that the initial unstable velocity space ring distribution of the ions evolves toward a bispherical shell. Ultimately this free energy adds to the turbulence in the solar wind. We obtain upstream and downstream free energies separately for the conditions observed along the Giotto spacecraft trajectory. The results indicate that the waves are mostly upstream propagating in the solar wind frame. The total free energy density always exceeds the measured wave energy density because, as expected in the nonlinear process of ion scattering, the available energy is not all immediately released. An estimate of the amount which has been released can be obtained from the measured oxygen ion distributions and again it exceeds that observed. The difference may be expressed in terms of “damping” of the waves and enables the damping rate to be calculated. We extend the theoretical analysis to calculate the k spectrum of the cometary‐ion‐generated turbulence. It provides an almost complete quantitative explanation of the observed spectrum. The differences at long wavelengths could be the result of the firehose instability or induced scattering on solar wind ions.
MHD modeling of magnetotail instability for anisotropic pressure
Authors: Hesse Michael , Birn Joachim
Published: February 2008

Results of three‐dimensional MHD simulations of magnetotail dynamics with anisotropic pressure are presented. The pressure tensor is assumed to be gyrotropic, satisfying a modified double adiabatic approximation including Ohmic heating. It is found that these constraints tend to stabilize the tail with respect to the resistive tearing instability. Including an increasing level of additional isotropization results in a gradual transition to the fast instability found in an isotropic model, which is discussed for comparison. Possible isotropization mechanisms yielding magnetotail instability are discussed also. The stabilizing anisotropies are found primarily in the plasma sheet boundary region. This fact indicates a possibly important role of isotropization mechanisms operating in this region in destabilizing the magnetotail and initiating a tearing instability without the necessity of changes in fluctuation levels in the neutral sheet.
Nonadiabatic transport features in the outer cusp region
Authors: Delcourt D. C. , Moore T. E. , Sauvaud J. A. , Chappell C. R.
Published: February 2008

The dayside to nightside circulation of plasma along the magnetopause inside the magnetosphere is examined by means of three‐dimensional single‐particle codes. It is demonstrated that particles incident upon the outer cusp region experience transient non‐adiabatic motions, owing to a localized minimum in the field magnitude. Here, possibly large magnetic moment changes yield injection into the loss cone of fractions of the incoming population or, alternatively, enhanced bouncing motions at high altitudes. It is shown that particles gaining access to the magnetotail over the polar cap are progressively extracted from the weak field region by the large‐scale convection electric field. In this latter case, the trajectory simulations suggest an implicit “entry boundary” into the nightside magnetosphere, which corresponds to the sunward edge of field lines featuring monotonic decrease of the field magnitude along their length.
The N2+ first negative system in the dayglow from Spacelab 1
Authors: Torr Marsha R. , Torr D. G. , Richards P. G.
Published: February 2008

During the Spacelab 1 shuttle mission, the first spectral images of the N2+ first negative system in the dayglow were made with the Imaging Spectrometric Observatory, an array of imaging spectrometers that covered a wavelength range extending from the extreme ultraviolet to the near infrared. These data have been the subject of a comprehensive study involving additional ground‐based observations, modeling of the causative mechanisms, and assessments of the predicted N2+ first negative system airglow on a semiglobal basis. Two surprising findings from these spectral images are that the bands of the N2+ first negative system exhibit very unusual vibrational distributions and that the intensities are significantly higher than anticipated. The spectra show unexpected populations up to ν″ = 4, a result that was not anticipated on the basis of prior theories. It is found that an apparent rotational temperature of ∼3000 K is needed to explain the data, while the closest apparent vibrational temperature is ∼6000 to 8000 K. Conventional thermospheric theory is not able to account for these vibrational distributions which appear to be present throughout the 10‐day shuttle mission and in quite different vehicle orientations. In this paper we show examples of these spectra and then concentrate on comparing the measured absolute intensities of the 0–0 band at 3914 Å with the results from our interhemispheric thermospheric airglow model. After appropriate corrections are included for radiation backscattered from the atmosphere and the ground, the intensities are found to be larger than current theory predicts for midday conditions by factors greater than 3. At large solar zenith angles, these intensity data are consistent with the Atmosphere Explorer results. The question arises as to whether the enhanced midday intensities are due to unexplained processes in the natural dayglow or whether sources of N2+ exist in the shuttle environment which can lead to large intensity and vibrational population enhancements. Like the enhanced vibrational populations, the enhanced intensities are present over the course of the 10‐day mission and do not appear to be associated with any particular vehicle attitude or viewing geometry. The intensity variation with solar zenith angle and viewing elevation appears to support the existence of a significant induced N2+ component near the shuttle, which fluoresces when the shuttle is sunlit.
Plasma irregularities caused by cycloid bunching of the CRRES G‐2 barium release
Authors: Bernhardt P. A. , Huba J. D. , Pongratz M. B. , Simons D. J. , Wolcott J. H.
Published: February 2008

The Combined Release and Radiation Effects Satellite (CRRES) spacecraft carried a number of barium thermite canisters for release into the upper atmosphere. The barium release labeled G‐2 showed evidence of curved irregularities not aligned with the ambient magnetic field B. The newly discovered curved structures can be explained by a process called cycloid bunching. Cycloid bunching occurs when plasma is created by photoionization of a neutral cloud injected at high velocity perpendicular to B. If the injection velocity is much larger than the expansion speed of the cloud, the ion trail will form a cycloid that has irregularities spaced by the product of the perpendicular injection speed and the ion gyroperiod. Images of the solar‐illuminated barium ions are compared with the results of a three‐dimensional kinetic simulation. Cycloid bunching is shown to be responsible for the rapid generation of both curved and field‐aligned irregularities in the CRRES G‐2 experiment.
Heating of oxygen ions by resonant absorption of Alfvén waves in a multicomponent plasma
Authors: Le Quéau D. , Roux A.
Published: February 2008

Conical distributions of oxygen ions are commonly observed at high altitudes above the auroral zone and in the cusp and cleft regions. The occurrence of these oxygen conics is well correlated with the intensity of waves in the ULF frequency range (of the order of a few hertz). It is shown that the inhomogeneity along the geomagnetic field lines allows a mode coupling between downgoing Alfvén waves (either proton cyclotron or magnetosonic) and upgoing oxygen cyclotron waves. Thus the electromagnetic energy flux carried downward with a large phase velocity can be transferred into a low phase velocity left‐handed mode that resonates with oxygen ions and heats them. We estimate the corresponding rate of absorption of the incident Alfvén waves as a function of the incidence angle and of the parameter ηO+, which is essentially the product of the O+ relative concentration and the characteristic length of inhomogeneity of the magnetic field, normalized to the typical wavelength of the Alfvén waves, at the O+ gyrofrequency. For an incident magnetosonic wave (type II branch) the rate of absorption can be quite large (up to 20%), for ηO+ < 10−2. This peak in the absorption coefficient, however, is obtained for a relatively narrow angular range, when the incidence angle is close to 90°. An incident proton cyclotron wave (type III branch) is more likely to lead to efficient O+ heating because (1) two regimes, either at small or at large incidence angle, lead to an absorption rate up to 20%, (2) these maxima are only slightly dependent on the incidence angle, and (3) large absorption rates are expected over a relatively broad range of values of ηO+ (5 × 10−3 < ηO+ < 10−1), compatible with those values that are expected throughout the magnetosphere.
Global characteristics of particle precipitation and field‐aligned electron acceleration during isolated substorms
Authors: Shiokawa K. , Yumoto K.
Published: February 2008

Global features of particle precipitation and field‐aligned electron acceleration during isolated substorms were investigated using energetic particle data from the DMSP F6 and F7 satellites. The onset time and the recovery phase of each isolated substorm were determined using both the AU and AL indices and low‐latitude Pi 2 magnetic pulsations. It is found that during the recovery phase the poleward edge of the precipitation region of central plasma sheet (CPS) type electrons expands to latitudes higher than 72° around midnight, while the equatorward boundary of this region expands to latitudes lower than those before the substorm onset in the postmidnight sector of 00‐06 MLT. In the recovery phase, many accelerated electron precipitation events were embedded in the CPS‐type electron precipitation region for the nightside local times from 18 MLT to 09 MLT. Several different kinds of latitudinal dispersion of electron and ion energy were observed near the lowest latitude of the particle precipitation region. The features vary significantly depending on the magnetic local time. These energy dispersion features can be explained by injected particle motions in the magnetosphere. These energy dispersion features also show a possibility of some particle injections in the magnetotail even before the substorm onset. The field‐aligned potential differences, estimated from the peak energy of accelerated electron energy spectra, do not change on the morningside before and after the substorm onset, while they increase up to several kilovolts on the eveningside during the recovery phase. On the other hand, the field‐aligned potential differences were estimated for the accelerated electron precipitation events embedded in the CPS‐type electron precipitation region and were found to be slightly larger on the morningside than on the eveningside during the recovery phase. These features of field‐aligned potential differences can be explained by assuming that the field‐aligned electric field is formed to maintain the large‐scale region I and II currents. The electron densities above the field‐aligned potential difference were also estimated from an accelerated Maxwellian fitting procedure. They do not change clearly before the onset and in the recovery phase. This fact suggests that the supply and loss of electrons into the loss cone at magnetospheric altitudes are balanced during the substorms.
Does fast magnetic reconnection exist?
Authors: Priest E. R. , Forbes T. G.
Published: February 2008

The classical Petschek model of fast, steady state reconnection has been generalized in two families of reconnection regimes. The first family, which we refer to as “almost uniform,” models the reconnection of nearly uniform, antiparallel magnetic fields, and it includes Petschek's model as a special case. The second family, which we refer to as nonuniform, models the reconnection of strongly curved magnetic fields, and it includes separatrix jets and reversed current spikes at the ends of the diffusion region. In general, both families contain regimes having fast reconnection rates, but we show here that these fast reconnection regimes do not occur when the boundary conditions often used in numerical experiments are adopted. In 1986, D. Biskamp carried out a series of numerical experiments to check Petschek's prediction that the maximum reconnection rate should scale with the magnetic Reynolds number, Rme, as [ln (Rme)]−1. Biskamp found that the maximum reconnection rate in his experiments did not scale in this way but instead as Rme−1/2. Because this corresponds to the scaling predicted by the slow reconnection theory of Sweet (1958) and Parker (1957), Biskamp has argued that his numerical experiments show that fast reconnection does not exist at high magnetic Reynolds numbers. However, by applying boundary conditions similar to Biskamp's to the “nonuniform” family of reconnection regimes, we are able to explain Biskamp's scaling results and to explain why he did not achieve fast reconnection in his numerical experiments. Therefore, we conclude that numerical experiments with suitably designed boundary conditions are highly likely to exhibit fast reconnection and that such reconnection is a common process in astrophysical and space plasmas.
Viking investigations of auroral electrodynamical processes
Authors: Marklund Göran
Published: February 2008

Recent results from the Viking electric field experiment and their contribution to a better understanding of the aurora and of associated ionosphere‐magnetosphere processes are briefly reviewed. The high‐resolution electric field data have provided new and important results in a number of different areas, including auroral electrodynamics both on the arc scale size and on the global scale, the auroral acceleration process, the current‐voltage relationship, substorms, and the dynamics of the polar cusp. After a short introduction presenting some of the characteristic features of the high‐altitude electric field data the remainder of this paper focuses on the role of the electric field in auroral electrodynamics and in the auroral acceleration process. The relationships between the auroral emissions and the associated electric field, current, particle, and conductivity distributions are discussed for both small‐scale and large‐scale auroral distributions on the basis of results from Viking event studies and from numerical model studies. Particular attention is paid to ionospheric convection and field‐aligned current signatures associated with northward interplanetary magnetic field (IMF) auroral distributions, such as the theta aurora or those characterized by extended auroral activity poleward of the classical auroral oval. The evolution of the convection pattern as the current system changes from a typical configuration for southward IMF to a configuration that may be representative of northward IMF conditions is illustrated by numerical model results and discussed in the light of Viking observations. The role of “dc” electric fields for the auroral acceleration process has been further investigated and clarified. From a statistical study of the directly measured parallel electric field and its relation to other relevant parameters the data suggest the possible existence of both upward and downward parallel fields. Intense low‐frequency electric field fluctuations (<1 Hz) have been shown to play an important role in the auroral acceleration process. In this frequency range the electric field appears static for the electrons but not for the ions, giving rise to a selective acceleration. The fluctuations are likely to contribute to the thermal energy of the closely associated escaping ionospheric ions. Estimates of the acceleration potential based on a number of different methods generally show good agreement, providing convincing evidence of the role of “dc” electric fields in the auroral acceleration process.
A mechanism responsible for the observation of symmetric lower hybrid sidebands and a low frequency mode in the upper ionosphere
Authors: Dalkir Y. R. , Lee M. C. , Groves K. M. , Kuo S. P.
Published: February 2008

A low‐frequency (∼500 Hz) mode in conjunction with symmetric sidebands, displaced by approximately 500 Hz off the carrier frequencies of 11.90 and 12.65 kHz, has been observed in experiments conducted with the Aureol 3 satellite at altitudes of 1200 km above the Komsomolsk‐on‐Amur Alpha Station (50°N, 135°E) (Sotnikov et al., 1991 and references therein). These observations are examined and interpreted with an alternative mechanism based on the theory of Lee and Kuo [1984]. The proposed mechanism suggests that the injected VLF wave parametrically excites a purely growing mode and both the Stokes and anti‐Stokes sidebands of the lower hybrid waves. The Doppler‐ shifted frequencies of these modes resulting from the satellite motion adequately account for both the sidebands and the low‐frequency mode observed in the Aureol 3 experiments.
Three‐dimensional magnetic field configurations in the heliosphere
Authors: Lou Yu‐Qing
Published: February 2008

We obtain a class of steady three‐dimensional magnetohydrodynamic (MHD) solar wind solutions from a primary radial MHD wind which is spherically symmetric. The energetic source for modifying the given primary MHD wind is the Poynting flux into the solar corona due to interactions of rotation, random flows, and magnetic field at the solar surface. The total energy flux associated with the perturbative MHD flow is conserved beyond several solar radii for typical solar wind parameters. Our main results are as follows: (1) clarification of the issue of possible transverse magnetic field configurations in heliospheric polar regions which can significantly influence the transport of galactic cosmic rays; (2) explicit demonstration that in the presence of nonaxisymmetric MHD perturbations over the solar poles such as a typical misalignment of the rotation and magnetic axes as well as random MHD perturbations, the transverse magnetic field eventually dominates the radial component over the polar heliosphere at large radial distance; (3) an analytical MHD model for latitudinal variation of heliospheric magnetic field due to the solar differential rotation; and (4) a MHD model of a warped heliospheric current sheet and its deformation due to the solar differential rotation. Our class of MHD wind solutions provides a new theoretical basis for developing numerical MHD codes and for modeling modulation of galactic cosmic rays by the heliospheric magnetic field.
The aurora at quiet magnetospheric conditions: Repeatability and dipole tilt angle dependence
Authors: Oznovich I. , Eastes R. W. , Huffman R. E. , Tur M. , Glaser I.
Published: February 2008

Is there a magnetospheric ground state? Do the position and size of the auroral oval depend on the magnetic dipole tilt angle at quiet magnetospheric conditions? In order to address these questions, northern hemisphere images of the aurora at 1356 Å, obtained by Polar BEAR at solar minimum (beginning of 1987), were related to high temporal resolution IMP 8 measurements of the interplanetary magnetic field, to solar wind velocity, and to the ground‐based activity index Kp. The first problem was addressed by a two‐dimensional correlation study of the repeatability of auroral emissions in corrected geomagnetic space at conditions of minimum energy transfer from the magnetosphere. The correlation measure of auroral images was 0.6‐0.85. Error simulations indicate that given the uncertainties in pixel position and intensity, the maximum expected value of the correlation measure is 0.65‐0.9. The notion of a ground state magnetosphere is therefore supported by our data. Repeatability was found at the same level regardless of time or reconfigurations of the magnetosphere between images and independent of magnetic time sector. The second problem was addressed by relating latitudinal shifts of the aurora with dipole tilt angle without resorting to auroral boundary specification. Our data indicate that the latitude of the continuous aurora is related to the dipole tilt angle at quiet magnetospheric conditions. In the winter hemisphere a 10° increase in the dipole tilt angle causes a 1° decrease (increase) in the latitude of auroral emissions at noon (midnight). The magnetic local time distribution of the latitudinal shifts with dipole tilt angle support a simple model in which the dipole tilt angle determines the position of the center of the auroral circle along the magnetic meridian 1320‐0120 MLT (for IMF By positive) and does not affect its radius.
Periodic variation in the geomagnetic activity: A study based on the Ap index
Authors: Clúa de Gonzalez Alicia L. , Gonzalez Walter D. , Dutra Severino L. G. , Tsurutani Bruce T.
Published: February 2008

The monthly and daily samples of the Ap geomagnetic index for 51 years, 1932‐1982, were investigated by means of the power spectrum technique. In general, the results confirm previous findings about possible periodicities in the geomagnetic activity. However, in our opinion the following aspects are either new or they are being interpreted somewhat differently than other authors have done. The period around 4 years in the monthly Ap power spectrum is associated to the double peak structure observed in the geomagnetic activity variation [Gonzalez et al., 1990]. Several of the peaks shown by the daily Ap spectrum are interpreted as harmonics of the 6‐month period and other peaks as caused by the solar rotation periodicity, in such a way that the two series of Fourier sequences are consider to be juxtaposed. A strong solar cycle modulation is observed in these series, particularly in that related to the solar rotation period, which almost disappears for the solar maximum phase. The study of the seasonal variation was complemented by a superposed epoch analysis. The profiles resulting from this analysis seem to show a multiple origin of the 6‐month periodicity, so that it does not seem realistic to search for a unique cause for this well‐known seasonal variation. This conclusion is also supported by the histograms of the occurrence of storms above a given intensity level, taken over short duration intervals (i.e., 8 days). According to these histograms, for large data samples the dates with largest number of storms are spread out around those predicted by the different theoretical models, while for short intervals the semiannual periodicity may sometimes not even be present. Therefore these known mechanisms would combine to give a resulting modulation of the geomagnetic response to the randomly generated source of storms. It was also found that an additional seasonal peak seems to exist in July, with an amplitude comparable to those of the equinoctial peaks, for the range of the most intense storms (Ap ≥ 150 nT). A weak periodicity around 158 days, well correlated to that of about 155 days observed in the solar activity, has also been detected for some years during solar cycle 21.
Topology of magnetic flux ropes and formation of fossil flux transfer events and boundary layer plasmas
Authors: Lee L. C. , Ma Z. W. , Fu Z. F. , Otto A.
Published: February 2008

At the dayside magnetopause, magnetic flux ropes can form as a result of multiple X line reconnection. In this process the presence of at least two parallel X lines leads to the formation of a magnetic flux rope with each additional X line giving another flux rope. For a finite extent of these X lines the magnetic flux of the flux ropes is connected to the external magnetic field on one or the other side of the current layer. In general, such a flux rope has frayed ends, i.e., magnetic flux enters the rope from the magnetospheric as well as from the magnetosheath side and/or exits the rope to either side of the layer. However, it is shown that for an appropriate extent and location of two neighboring X lines, a simple magnetic topology can be expected, in which the major amount of magnetic flux of the rope is connected at each end to only one side of the current sheet. For a sufficient relative shift of the X lines, magnetic flux may enter a flux rope from the magnetosphere and exit into the magnetosphere. This process leads to the formation of magnetic flux ropes which contain a considerable amount of magnetosheath plasma on closed magnetospheric field lines. We discuss this process as a possible explanation for the formation of fossil flux transfer events in the magnetosphere and the formation of the low‐latitude boundary layer.
The effects of neutral inertia on ionospheric currents in the high‐latitude thermosphere following a geomagnetic storm
Authors: Deng W. , Killeen T. L. , Burns A. G. , Roble R. G. , Slavin J. A. , Wharton L. E.
Published: February 2008

Results of an experimental and theoretical investigation into the effects of the time dependent neutral wind flywheel on high‐latitude ionospheric electrodynamics are presented. The results extend our previous work [Deng et al., 1991] which used the National Center for Atmospheric Research Thermosphere/Ionosphere General Circulation Model (NCAR TIGCM) to theoretically simulate flywheel effects in the aftermath of a geomagnetic storm. The previous results indicated that the neutral circulation, set up by ion‐neutral momentum coupling in the main phase of a geomagnetic storm, is maintained for several hours after the main phase has ended and may dominate height‐integrated Hall currents and field‐aligned currents for up to 4‐5 hours. We extend the work of Deng et al. to include comparisons between the calculated time‐dependent ionospheric Hall current system in the storm‐time recovery period and that measured by instruments on board the Dynamics Explorer 2 (DE 2) satellite. Also, comparisons are made between calculated field‐aligned currents and those derived from DE 2 magnetometer measurements. These calculations also allow us to calculate the power transfer rate (sometimes called the Poynting flux) between the magnetosphere and ionosphere. The following conclusions have been drawn: (1) Neutral winds can contribute significantly to the horizontal ionospheric current system in the period immediately following the main phase of a geomagnetic storm, especially over the magnetic polar cap and in regions of ion drift shear. (2) Neutral winds drive Hall currents that flow in the opposite direction to those driven by ion drifts. (3) The overall morphology of the calculated field‐aligned current system agrees with previously published observations for the interplanetary magnetic field (IMF) BZ southward conditions, although the region 1 and region 2 currents are smeared by the TIGCM model grid resolution. (4) Neutral winds can make significant contributions to the field‐aligned current system when BZ northward conditions prevail following the main phase of a storm, but can account for only a fraction of the observed currents. (5) DE 2 measurements provide a demonstration of “local” (satellite‐altitude) flywheel effects. (6) On the assumption that the magnetosphere acts as an insulator, we calculate neutral‐wind‐induced polarization electric fields of ∼20‐30 kV in the period immediately following the geomagnetic storm.
DE 1 particle and wave observations in auroral kilometric radiation (AKR) source regions
Authors: Menietti J. D. , Burch J. L. , Winglee R. M. , Gurnett D. A.
Published: February 2008

The high‐altitude plasma instrument on board the DE 1 satellite was operating during several near crossings of the auroral kilometric radiation (AKR) source in the nightside auroral region. Observations of electron distributions indicate a region of perpendicular heating (T⊥/T∥ > 10) adjacent to and within the source region. Loss cones, trapped particles, beams, and electron conical distributions are also observed near and within the source region, which extends perpendicular to the magnetic field line for at least 20 km. Near the AKR source region wave‐particle interactions appear to have modified the observed electron distributions. We compare the observations to those predicted by recently published numerical simulations.
Three‐dimensional magnetotail equilibria by numerical relaxation techniques
Authors: Hesse Michael , Birn Joachim
Published: February 2008

A numerical method to iteratively approach three‐dimensional magnetostatic force equilibria is presented. The emphasis of the modeling is on the development of a suitable model of the Earth's magnetotail, including a portion of the inner magnetosphere, i.e., on models which violate the “tail approximation” commonly employed in analytical models. The numerical approach will be discussed and compared to methods developed for laboratory plasma physics. The method is then applied, as a first example, to the magnetotail outside of 10 RE using Tsyganenko's (1987) model for the quiet magnetosphere as an initial condition. We discuss the changes of the magnetic field necessary to yield an equilibrium configuration and the resulting distribution of the self‐consistently derived pressure. At last we will show that a self‐consistent magnetotail equilibrium based on a close approximation to Tsyganenko's model requires a region 1 type current system which is not present in the initial configuration.
Internally heated mantle convection and the thermal and degassing history of the Earth
Authors: Williams David R. , Pan Vivian
Published: February 2008

A parameterized internally heated convection model for the Earth has been developed. The mantle viscosity is temperature and volatile‐content dependent. A heat flow/Rayleigh number relationship appropriate for an internally heated mantle is assumed. For each model an initial homologous temperature and bulk water content are assumed and the initial radiogenic heat production is constrained to give a present day heat flow of 0.07 W m−2. The model is run for 4.6 Gyr, and temperature, heat flow, degassing and regassing rates, stress, and viscosity are calculated. A nominal case is established which shows good agreement with accepted mantle values. The effects of changing various parameters have also been tested. All cases show rapid cooling early in the planet's history and strong self‐regulation of viscosity due to the temperature and volatile‐content dependence. The values chosen for initial temperature and volatile content have negligible effects on the results, while assumptions about present‐day heat flow, viscosity, and mantle volatile content can have important consequences. The effects of weakly stress dependent viscosity are examined within the bounds of this model and are found to be small. Mantle water is typically outgassed rapidly to reach an equilibrium concentration on a time scale of less than 200 m.y. for almost all models, the main exception being for models which start out with temperatures well below the melting temperature. For some cases the degassing efficiency is allowed to decrease with time as the mantle cools which, combined with the rapid early loss of volatiles, leads to a net regassing of the mantle to the present day and loss of water from the surface.
Global positioning system measurements of deformations associated with the 1987 Superstition Hills Earthquake: Evidence for conjugate faulting
Authors: Larsen Shawn , Reilinger Robert , Neugebauer Helen , Strange William
Published: February 2008

Large station displacements observed from Imperial Valley global positioning system (GPS) campaigns are attributed to the November 24, 1987, Superstition Hills earthquake sequence. Thirty sites from a 42‐station GPS network established in 1986 have been reoccupied during 1988 and/or 1990. Displacements at three sites within 3 km of the surface rupture approach 0.5 m. Eight additional stations within 20 km of the seismic zone are displaced at least 10 cm. This is the first occurrence of a large earthquake (MS6.6) within a preexisting GPS network. Best‐fitting uniform slip models of rectangular dislocations in an elastic half‐space indicate 130 ± 8 cm right‐lateral displacement along the northwest‐trending Superstition Hills fault and 30 ± 10 cm left‐lateral displacement along the conjugate northeast‐trending Elmore Ranch fault. The geodetic moments are 9.4 × 1025dyn cm and 2.3 × 1025dyn cm for the Superstition Hills and Elmore Ranch faults, respectively, consistent with teleseismic source parameters. The data also suggest that postseismic slip along the Superstition Hills fault is concentrated at shallow depths. Distributed slip solutions using singular value decomposition indicate near uniform displacement along the Elmore Ranch fault and concentrated slip to the northwest and southeast along the Superstition Hills fault. A significant component of nonseismic secular displacement is observed across the Imperial Valley, which is attributed to interseismic plate‐boundary deformation.
Numerical model of seismic rupture
Authors: Nielsen Stefan B. , Tarantola Albert
Published: February 2008

Simple numerical models of the cellular automaton type have been proposed recently, as an analogy for seismic faults. Those showed interesting features of spontaneous rupture evolution (Lomnitz‐Adler and Lemus‐Diaz, 1989) or even seismic recurrence (Bak and Tang, 1989). It is possible to incorporate realistic rheology and tensorial physics into this kind of model, to extend it to a portion of crust instead of a single fault‐plane and to simulate its time evolution. A numerical model based on physics of continuum media, instead of a cellular automaton, can simulate dynamic rupture in two dimensions. The medium can be submitted to increasing load which is introduced by an imposed displacement at the boundaries, space and time being discretized by a finite difference scheme. The equation of dynamics is used at each time step. A rupture criterion allows each node in the grid of the medium to fail if a threshold is reached. The brittle threshold is allowed to fluctuate from point to point in the medium according to a chosen distribution. Failure is represented by a loss of shear stiffness in the rock element concerned. Hence we distinguish two states in our medium (the unbroken, viscoelastic rheology allowing for a slight attenuation and the broken state, similar to a viscous fluid) the viscosity allows introduction of a kind of dynamic friction at the broken point. When failure takes place at a point, elastic stress is then locally released and radiates, sometimes causing rupture of neighbor points and localization of fracture along fissures or faults. This gives realistic synthetic seismograms and source slip history.
Nonlinear high‐resolution three‐dimensional seismic travel time tomography
Authors: Hole J. A.
Published: February 2008

A tomographic inversion procedure is described and applied to a synthetic three‐dimensional (3‐D) seismic refraction data set, demonstrating that tomography is capable of determining a densely sampled velocity model with large velocity contrasts. Forward and inverse modeling procedures are chosen to minimize the computational costs of the inversion. Parameterizing the linearized inversion using functions defined along the ray paths, simple backprojection with zero pixel size is shown to exactly solve the linear problem, producing the smallest model for the slowness perturbation. For small grid cells, simple backprojection closely approximates the exact solution and is a sufficient solution for an iterative nonlinear inversion. This eliminates the need to store or solve a large system of linear equations. Accurate first arrival travel times are rapidly computed using a finite difference algorithm. Forward modeling between each simple backprojection allows the procedure to correctly account for the locations of the rays. This becomes more important as the spatial resolution of the model is improved. The computational efficiency of the entire nonlinear procedure allows the model to be densely sampled, providing a spatially well‐resolved 3‐D tomographic image. The synthetic refraction survey is designed to be similar to a published 3‐D survey over the East Pacific Rise. Tests based on this example and others show that 3‐D tomography is capable of inverting a large travel time data set for detailed earth structure with large lateral velocity variations and is stable in the presence of noisy data.
Pore pressure variation within the Tuscaloosa trend: Morganza and Moore‐Sams Fields, Louisiana Gulf Coast
Authors: Weedman Suzanne D. , Guber Albert L. , Engelder Terry
Published: February 2008

Repeat formation tester (RFT, mark of Schlumberger) pore pressure data from the lower Tuscaloosa Formation (Upper Cretaceous) are analyzed across a pressure transition zone in two gas fields of the Tuscaloosa trend, Louisiana, documenting a pressure seal within a sandstone‐dominated interval that maintains a pressure anomaly of ∼20 MPa. Pressure measurements within the formation span a depth interval of 5500–6060 m and are limited to the crests of rollover structures associated with growth faults; the onset of overpressure occurs at a depth between 5620 m and 5690 m in two fault blocks, while the strata are displaced across the fault by 100 to 120 m, suggesting that the pressure seal may be near‐horizontal, crosscutting stratigraphic boundaries. The pressure seal zone is as thin as 38 m and occurs within interbedded sandstones and shales. Overpressures in all wells of the Moore‐Sams Field and in two wells of the Morganza Field follow local hydrostatic gradients, indicating that portions of the overpressured zone are in fluid communication though isolated from shallower, normally pressured fluids. In other wells, overpressures increase with depth in a stair‐step manner to 117 MPa at depths of 5.9 km. The occurrence of the pressure seal in interbedded sandstones and shales where higher permeability is expected suggests that the sandstones of the seal zone are unusually tight. A companion petrographic study of sandstones near the pressure seal suggests that extreme compaction of the sandstones after dissolution of grain‐supporting cements may have contributed to the low permeability of the seal zone.
Textural constraints on effusive silicic volcanism: Beyond the permeable foam model
Authors: Fink Jonathan H. , Anderson Steven W. , Manley Curtis R.
Published: February 2008

Transitions between explosive and effusive phases of silicic volcanic eruptions have been related either to stratification of volatiles in the source magma body or to the loss of volatiles through the permeable host rock of the conduit. One way to distinguish between these two models is to map and analyze the vesicular and glassy textures found in silicic lava flows. In this paper we present textural observations and isotopic evidence from active and Recent silicic lava flows which show that at least some vesiculation occurs during surface advance of extrusions, after magma has reached the earth's surface. This view is in contrast to the widely promoted “permeable foam” model, which states that all volatiles escape during ascent of the magma, and that all dense glassy material in lava flows forms from the collapse of pumiceous lava, i.e., that silicic lavas emerge as highly inflated foam flows. Such interpretations which claim that silicic lavas are completely degassed upon extrusion, and that all degassing must take place on the time scale of the eruption, neglect several important pieces of evidence, including the presence of obsidian in extremely small domes, and of vesicular zones in the interiors of silicic flows; the copious loss of volatiles through eruption plumes between eruptive phases; and direct observations of surface vesiculation during growth of the Mount St. Helens lava dome. The permeable foam model also implies the unlikely requirement that explosive‐to‐effusive transitions be associated with an increase in eruption rate. We present a more comprehensive model for the emplacement of silicic extrusions that allows for early gas loss during ascent, as well as late‐stage vesiculation. We then discuss how the redistribution of volatiles during surface flow can increase explosive hazards from silicic lavas days, weeks, or months after the lava emerges from the vent.
Growth of faults by accumulation of seismic slip
Authors: Cowie Patience A. , Scholz Christopher H.
Published: February 2008

The accommodation of large strains in the upper crust is largely achieved by the accumulation of displacement on faults. Observation shows that as a fault accumulates displacement, it grows in size, i.e., its surface area and its length increase. Here we address the question: “For an increase in the amount of displacement on a fault, by how much would the length of the fault change?” It is argued by Cowie and Scholz [1992a] that the displacement on a fault is linearly related to the length of the fault. This simple result is expanded upon in this paper by assuming that a fault accumulates displacement by repeated earthquakes. Two different approaches are presented: The first approach considers the balance that must be achieved between the energy available for deformation and the work involved in creating new fault surface area as the fault grows. The available energy is provided by changes in strain energy when the fault slips. The second approach is to construct a geometrical model for fault growth using the scaling relationship between the slip during a single earthquake and the length of the rupture. The total displacement on a fault is the sum of the slips contributed by many earthquakes. The usefulness of these two approaches is that the growth of a fault over geologic time can be described by parameters that can be obtained from earthquake and fault data. The models presented here predict that (1) the maximum amount a fault can grow in a single earthquake that ruptures the entire fault is of the order of 1% of its previous length and (2) the work of faulting may account for 10% of the total energy available during an earthquake. The energy lost from the system is accounted for by work done against friction and seismic radiation. Consequences of fault growth for the segmentation and thus seismogenic potential of a fault over geologic time are discussed using predictions of the theory.