bluuebunny/crossref_metadata_2025 huggingface dataset:
The influence of critical Moho Reflections on strong ground motions recorded in San Francisco and Oakland during the 1989 Loma Prieta Earthquake
Authors: Somerville Paul , Yoshimura Joanne
Published: February 2008

The amplitudes of strong ground motions from the Loma Prieta earthquake recorded in the San Francisco and Oakland areas exceeded the levels predicted by standard empirical attenuation relations. Preliminary analysis of accelerograms having known trigger times strongly suggests that the elevation of ground motion amplitudes in the distance range of approximately 40 to 100 km was due to critical reflections from the base of the crust. These reflections, which are identified on the basis of their arrival times and phase velocity, and by comparison with simulated accelerograms, were large and occurred at relatively close range because of the deep focal depth of the earthquake and the strong velocity gradient at the base of the crust. These motions were further amplified, presumably by impedance contrast effects, at soft soil sites in San Francisco and Oakland. The effect of the critical reflections in amplifying peak accelerations of the Loma Prieta earthquake in the San Francisco and Oakland regions was as large as the effect of soft soil site conditions. Focal depth has an important influence on strong motion attenuation at distances beyond about 40 km, and empirical attenuation relations derived from shallow crustal earthquakes may underpredict the ground motions of deeper crustal events in this distance range. Further analyses using an expanded data base that includes recordings of aftershocks are required to rigorously test the proposed explanation of the ground motions recorded in San Francisco and Oakland, and the conclusions drawn from that explanation.
Complex fault interactions in a restraining bend on the San Andreas Fault, southern Santa Cruz Mountains, California
Authors: Schwartz S. Y. , Orange D. L. , Anderson R. S.
Published: February 2008

The unusual oblique thrust mechanism of the October 18, 1989 Loma Prieta earthquake focused attention on the complex tectonic setting of this segment of the San Andreas Fault. Near the mainshock epicenter, the San Andreas Fault curves to the left defining a restraining bend. The large thrust component of the mainshock focal mechanism is consistent with the horizontal compression expected across restraining bends. However, repeated “Loma Prieta type” earthquakes cannot exclusively produce the observed topography of the southern Santa Cruz Mountains, the highest point of which experienced subsidence during the 1989 earthquake. In this paper, we integrate seismic, geomorphic and tectonic data to investigate the possibility that motions on faults adjacent to the San Andreas Fault play an important role in producing the observed topography. The three‐dimensional geometry of active faults in this region is imaged using the Loma Prieta preshock and aftershock sequences. The most conspicuous features of the fault geometries at depth are: 1) the presence of two distinct zones of seismicity corresponding to the San Andreas and the Sargent‐Berrocal Fault Zones 2) the concave upward shape of the Loma Prieta rupture surface, 3) the reduction in dip of the deepest portions of the rupture plane as the mainshock hypocenter is approached, 4) the apparent transfer of shallow slip in some areas from faults in the San Andreas Fault Zone to those in the Sargent‐Berrocal Fault Zone, and 5) the presence of a deep northeasterly dipping plane associated with the Sargent‐Berrocal Fault Zone. We find that a model of fault interactions which involves displacement on faults in both the San Andreas and the Sargent‐Berrocal Fault Zones is consistent with Loma Prieta coseismic displacements, preshock and aftershock seismicity and observed topography.
Seismomagnetic effect generated by the October, 1989, ML, 7.1 Loma Prieta, California, Earthquake
Authors: Mueller R. J. , Johnston M. J. S.
Published: February 2008

A differentially connected array of proton magnetometers operated within the epicentral region of the October 18, 1989, ML 7.1 Loma Prieta earthquake for 12 years from 1974 to 1986. The closest magnetometer station was located 7.3 km from the epicenter of the earthquake and within 3 km of the site where anomalous ULF magnetic noise measurements were observed. Following the earthquake, the magnetometers were reinstalled with sensors replaced in the original undisturbed sensor holders. Comparison of pre‐1986 total intensity magnetic field data with data obtained during the months following the earthquake indicate local offsets of about 1 nT may have been generated at stations nearest the epicenter. Tests on other continuous differenced data from 1983 to present indicate that offsets determined could be biased by as much as 0.7 nT. The offsets can be approximately fit with a simple seismomagnetic model of the earthquake for which 1.9 m of right lateral and 1.3 m of dip slip (southwest side up) occurred on a fault patch between 6 km and 18 km deep and 45 km long. The total rock magnetization is assumed to be 1.5 A/m. Since the offset has persisted following the earthquake, an alternate explanation in terms of electokinetic effects is unlikely even though transient ground water flow occurred following the earthquake. Comparison of pre‐1986 and similar post‐seismic total magnetic field noise does not indicate any change caused by aliasing of ULF (0.01 Hz‐10 Hz) magnetic noise in the vicinity of the Loma Prieta earthquake.
Implication from the aftershocks of the 1989 Loma Prieta Earthquake
Authors: Tajima Fumiko , Sen Mrinal K.
Published: February 2008

We examined the P wave displacement spectra of nine aftershocks (M=2.5∼3.6) of the 1989 Loma Prieta earthquake recorded on PASSCAL/IRIS instruments to search for changes in the spectra that are indicative of heterogeneity in the source region. Temporal variations of spectral characteristics have been observed from the records of two events (#7 with M=2.7 and #8 with M=2.5) that occurred at an almost identical location with a time interval of 16 min and recorded at common stations. The different spectral characteristics can be attributed to the differences in the source time functions of these events. Also, the spectrum of event #2 (M=2.5) that occurred near San Francisco, is substantially richer in high frequency than those from events of a comparable size in the immediate aftershock area. This could be an indication of progressive stress concentration beyond the present aftershock area.
Experimental petrology of the lower mantle: A natural peredottte taken to 54 GPA
Authors: O'Neill Bridget , Jeanloz Raymond
Published: February 2008

The high‐pressure mineral assemblage produced from a natural peridotite sample (xMg = 0.897) has been synthesized in the laser‐heated diamond cell and identified by x‐ray diffraction at 54 GPa. Orthorhombic perovskite, (Mg,Fe)SiO3, magnesiowüstite, (Mg,Fe)O, and cubic perovskite, CaSiO3, are observed in estimated molar ratios of 1.00 : 0.46 : 0.08 (1.00 : 0.21 : 0.09 by volume) respectively, with no evidence of other phases being present. The lattice parameters measured at 54 GPa yield a density of 4.97(±0.03) Mg/m3 for the high pressure assemblage at 300 K, in good agreement with prior calculations of density for candidate lower mantle assemblages.
Probing water turbulence by high frequency Doppler sonar
Authors: Lhermitte R. , Lemmin U.
Published: February 2008

Observations of sound wave backscattering by turbulent water (free of particulates) in a water flume using a high resolution, 1.5 mm wavelength, Doppler sonar, are presented and discussed. The results show that the backscattering produced by water turbulence yields well defined Doppler spectra containing velocity variance information relevant to the study of water turbulence characteristics. Near the flume bed, the observed Doppler velocity spectrum width is greater than that expected from the turbulent velocity spectrum and other known sources of spectral broadening. The excess Doppler spectrum width may be explained assuming that the temperature structures acting as sonar targets tracing the velocity eddies, have a transient nature.
Smooth radio emission and a new emission at Neptune
Authors: Sawyer C. , Warwick J. W. , Romig J. H.
Published: February 2008

The Planetary Radio Astronomy (PRA) experiment [Warwick et al., 1977] on the spacecraft Voyager 2 observed three types of smooth emission: (1) Numerous recurrent episodes are modeled by filled emission cones from midlatitude conjugate sources. (2) An “equatorial” feature seen soon after closest approach includes electron cyclotron harmonic emission above the upper hybrid resonance, as well as smooth recurrent emission; its strange appearance is a result of rapid change in Voyager's magnetic latitude. (3) Unique “high‐latitude” emission is seen near closest approach during Voyager's single excursion to high north (+ ) magnetic latitude when fc, the electron cyclotron frequency at the spacecraft, lies in the observable range. The stronger component covers a broad band of frequencies above 2fc; its sensitivity to magnetic field identifies it as extraordinary (X) mode. The weaker component extends smoothly through f = fc and is identified as ordinary (O) mode. At each frequency f the observed sense of polarization reverses when f = 2fc.
Satellite sweeping of electrons at Neptune and Uranus
Authors: Cooper John F.
Published: February 2008

Knowledge of satellite sweeping parameters at Neptune and Uranus, and of their functional dependences on particle energy and pitch angle, can be critical in the proper identification of parent absorbers for observed absorption signatures in regions where OTD (Offset, Tilted Dipole) models are valid representations of the measured magnetic fields. In this paper, we calculate critical electron energies for longitudinal drift resonance, snowplow (i.e., strong) absorption, leapfrog, and corkscrew effects, using a reduced version of OTD that neglects nonaxial dipole offsets. Earlier analytic work on sweeping rates is extended to give the radial dependence of these rates the minimum‐L region and to set limits on diffusion of electrons with the simplifying approximation that leapfrog effects are ignored.
Triton: Topside ionosphere and nitrogen escape
Authors: Yung Y. L. , Lyons J. R.
Published: February 2008

The principal ion in the ionosphere of Triton is N+. Energetic electrons of magnetospheric origin are the primary source of ionization, with a smaller contribution due to photoionization. To explain the topside plasma scale height, we postulate that N+ ions escape from Triton. The loss rate is 3.4 × 107 cm−2 s−1 or 7.9 × 1024 ions s−1. Dissociative recombination of N+2 produces neutral exothermic fragments that can escape from Triton. The rate is estimated to be 8.6 × 106 N cm−2 s−1 or 2.0 × 1024 atoms s−1. Implications for the magnetosphere of Neptune and Triton's evolution are discussed.
Multiple electron species dynamics parallel to a high‐latitude field line
Authors: Mitchell H. G. , Ganguli Supriya B. , Palmadesso P. J.
Published: February 2008

The parallel dynamics of the plasma along high‐latitude field lines is a critical feature of the ionosphere‐magnetosphere coupling problem. We have created a time‐dependent, large scale simulation of these dynamics in a region from the ionosphere well into the magnetosphere. Including multiple electron species has allowed us to dynamically simulate the diode‐like response of the field‐line plasma to the parallel currents coupling the ionosphere and magnetosphere. Downward (return) currents flow with small resistance, while upward currents produce kilovolt‐sized potential drops along the field due to the effect of the converging magnetic field on the high energy magnetospheric electrons.
The dipole tilt angle effect on the magnetosphere of Neptune – A laboratory simulation
Authors: Minami Shigeyuki , Hashimoto Kumiko , Takeya Yoshio
Published: February 2008

Recent observations of Neptune by Voyager 2 reveal that the dipole tilt angle is almost 50 degrees – close to that of Uranus. When comparing the magnetospheres of planets, it is therefore important to understand configurations of magnetic field lines with large dipole tilt angles. This paper deals with the laboratory characteristics of Neptune's magnetosphere whose dipole tilt angle dominates the configuration of its magnetic field lines and particle intrusions from interplanetary space. The dipole tilt angle has a considerable effect on the formation of the asymmetrical structure of the field lines and the cylindrical tail current sheet in the magnetosphere. The experiment also shows the effect of the x‐component of the interplanetary Magnetic field to form an asymmetrical magnetospheric structure.
On the quality of eddy heat flux calculations in the vicinity of the Antarctic lower stratosphere
Authors: Rosen Richard D. , Salstein David A. , Miller Alvin J.
Published: February 2008

The ability of analyses from the National Meteorological Center and the European Centre for Medium Range Weather Forecasts to depict the dynamics of the high latitude Southern Hemisphere lower stratosphere is examined by considering time series of daily values of a meridional eddy heat flux term at 100 mb during 1986. Both analyses yield series of this quantity that although highly variable are nevertheless well correlated with each other. Coupled with the credible spatial structure of the heat flux field in the Analyses, this agreement offers support for the realism of the eddy heat flux calculations. Despite the possible presence of low‐frequency problems in the analyses, we conclude that they are adequate for studying important atmospheric phenomena that occur in this region of the Southern Hemisphere at sub‐monthly time scales.
Increased cloudiness in the United States during the first half of the Twentieth Century: Fact or fiction?
Authors: Karl Thomas R. , Steurer Peter M.
Published: February 2008

A new source of cloud and sunshine data, in conjunction with an inspection of the observing practices, instruments, and daily temperature range over the United States, are used to confirm or contradict the large increases of cloudiness previously reported. This analysis indicates that changes in observing practices are primarily responsible for the reported increase of cloudiness during the first half of the this Century. The response of the sunshine instruments and the surface daily temperature range over the United States are not consistent with the reported increases during the first half of this Century. Since the late 1940s however, the increase of cloud cover is confirmed by an inspection of the sunshine data, daily temperature range, changes in instruments, and the instructions to the observers. The types of changes in the cloud observing practices during the late 1930s and 1940s would tend to lead to increases in reported cloud amounts. Although we have not yet produced a combined sunshine/cloudiness index of cloud cover over the Twentieth Century, some cogent inferences can be made at this time:

Monotonic trends of cloudiness on the Century time‐scale do not exist in the United States.

Fewer clouds were present during the 1930s and early 1950s.

There has been a tendency toward increased since 1948.
Depositional history of the Sulu Sea from ODP Sites 768, 769 AND 771
Authors: Nichols Gary , Betzler Christian , Brass Garrett , Huang Zehui , Linsley Braddock , Merrill Dean , Müller Carla , Nederbragt Alexandra , Pubellier Manuel , Sajona Fernando , Scherer Reed , Shibuya Hidetoshi , Shyu Jih‐Ping , Smith Randall , Solidum Renato , Spadea Piera
Published: February 2008

Holes were drilled at three Sites in the Sulu Sea on Ocean Drilling Program Leg 124. Site 768 lies in the deeper part of the SE sub‐basin and Sites 769 and 771 lie on the flanks of the Cagayan ridge. The results indicate that the Sulu Basin originated in the late early Miocene (c.18.8 Ma) in a backarc setting. The Cagayan Ridge was a site of early to early middle Miocene arc volcanism with the deposition of a thick sequence of andesitic to basaltic volcaniclastic deposits. In the basin center an early Miocene pelagic sequence is interrupted by a thick unit of rhyolitic to dacitic pyroclastic flows. Middle to late Miocene sedimentation is more continental in character with thick quartz‐rich turbidites in the basin center. Only the hemipelagic claystone related to these terrigenous turbidites were deposited on the Cagayan Ridge. A decrease in the supply of clastic detritus from arc and continental sources and a change in the level of the carbonate compensation depth in the upper Pliocene resulted in pelagic carbonate deposition throughout the late Pliocene and Pleistocene.
Finite difference seismic modeling of axial magma chambers
Authors: Swift Stephen A. , Dougherty Martin E. , Stephen Ralph A.
Published: February 2008

We tested the feasibility of using finite difference methods to model seismic propagation at ∼10 Hz through a two‐dimensional representation of an axial magma chamber with a thin, liquid lid. Our technique produces time series of displacement or pressure at seafloor receivers to mimic a seismic refraction experiment and snapshots of P and S energy propagation. The results indicate that our implementation is stable for models with sharp velocity contrasts and complex geometries. We observe a high‐energy, downward‐traveling shear phase, observable only with borehole receivers, that would be useful in studying the nature and shape of magma chambers. The ability of finite difference methods to model high‐order wave phenomena makes this method ideal for testing velocity models of spreading axes and for planning near‐axis drilling of the East Pacific Rise in order to optimize the benefits from shear wave imaging of sub‐axis structure.
40Ar−39Ar dating of the Beja Gabbro: TIming of the accretion of southern Portugal
Authors: Ruffet Gilles
Published: February 2008

The 40Ar−39Ar dating of the amphibole from the Beja gabbro (Southern Portugal) yields a plateau age at 336.4 ± 0.8 Ma (2σ level). The corresponding calculated isotopic closure temperature is around 800°C. The comparison of this temperature with the magnetic blocking temperature (∼570°C) allows an estimation of a probable thermoremanent acquisition age for the characteristic magnetization component of the Beja gabbro between 335Ma and 315Ma, assuming cooling rates between 10°C/Ma and 100°C/Ma. These results, combined with palaeomagnetic results from the Beja gabbro and Late Paleozoic rocks from Southern Portugal [Perroud et al., 1985], suggest that the southermost part of Spain and Portugal was separated from Northern Iberia in Early Carboniferous times and was accreted to Europe during the Late Carboniferous.
Heat transfer from Atlantic waters to sea ice in the Arctic Ocean: Evidence from dissolved argon
Authors: Moore R. M. , Spitzer W.
Published: February 2008

In an attempt to determine whether the temperature and salinity properties of Arctic Ocean waters above the Atlantic water temperature maximum are the result of heat transfer to sea‐ice, dissolved Ar has been measured as a temperature tracer. Consistent with such a hypothesis, it is found that there is a transition from supersaturation of Ar in the upper waters to undersaturation below a depth of 275m. Using the known dependence of the solubility of Ar on T and S, and assuming that the water was originally equilibrated with the atmosphere at 760mm Hg, it has been calculated that ca. 0.6° C of cooling can be attributed to transfer of heat to sea‐ice.
Frequency domain interferometry observations of tropo/stratospheric scattering layers using the MU radar: Description and first results
Authors: Palmer R. D. , Woodman R. F. , Fukao S. , Larsen M. F. , Yamamoto M. , Tsuda T. , Kato S.
Published: February 2008

This paper describes the first results of the implementation of frequency domain interferometry (FDI) using the Middle and Upper (MU) atmosphere radar. A new method of using this FDI data is introduced, which is shown to be useful in studying the dynamics of scattering layers in the troposphere and lower stratosphere. Using this method, the effective altitude resolution of the MU radar has been improved.
Electric current in a unipolar sunspot with an untwisted field
Authors: Osherovich V. A. , Garcia H. A.
Published: February 2008

We apply the Return Flux (RF) sunspot model to a round, unipolar sunspot observed by H. Kawakami (1983). Solving the magnetohydrostatic problem using the gas pressure deficit between the umbral and quiet‐Sun atmospheres as a source function, we find a distribution of electric current density in an untwisted, unipolar sunspot as a function of height and radial distance from the sunspot center. Maximum electric current density is about 32 m A m−2 at the bottom of the sunspot.
A photochemical origin of acetic acid in the troposphere
Authors: Madronich Sasha , Chatfield Robert B. , Calvert Jack G. , Moortgat Geert K. , Veyret Bernard , Lesclaux Robert
Published: February 2008

Significant concentrations of formic and acetic acids are known to exist in the troposphere. These acids contribute between 16 and 35% of the free acidity in North American precipitation and between 25 and 98% of the free acidity of precipitation in remote areas. Their origin is still relatively uncertain. For formic acid, several gas phase and aqueous phase chemical production mechanisms have been postulated, and seem to explain at least roughly the observed concentrations. However, for acetic acid, the analogous production mechanisms appear to be far too slow, and it has been suggested that this acid may be emitted directly by vegetative matter. Based on recent laboratory and theoretical studies, we have considered the possible gas phase production of acetic acid via the reaction of peroxy acetyl radicals with HO2 and organic peroxy radicals. A highly detailed gas phase chemical mechanism coupled with a simple 1‐D model was used to predict organic acid concentrations for one set of conditions (Amazon boundary layer) where extensive field measurements are available for model initialization and comparison. The results indicate that the gas phase photooxidation of non‐methane‐hydrocarbons can produce amounts of acetic acid in the range of several parts per billion, comparable to the observed values.
Thermal structure of the primitive ionosphere
Authors: Ashihara O. , Shimizu M. , Shimazaki T.
Published: February 2008

Exospheric neutral and electron temperatures have been estimated for the primitive upper atmosphere and ionosphere with various oxygen content in the scheme of our previous model [Shimizu and Shimazaki, 1976]. The exospheric neutral temperature has been shown to be rather insensitive to the change of oxygen content, justifying our previous assupmtion for the temperature variation, while the exospheric electron temperature has been found to be quite sensitive to the compositional change, mainly owing to the strong dependence of electron density on the oxygen concentration.
The effect of HF‐induced plasma instabilities on ionospheric electron temperatures
Authors: Showen R. L. , Behnke R. A.
Published: February 2008

A connection is shown between the rise in electron temperature and the presence of plasma instabilities during an ionospheric heating experiment. The heating of the electron gas is due to the combined effects of both ohmic and instability energy deposition. The instabilities are generated by the intense electric fields near the altitude of reflection of the O mode HF wave. The dominant instability is the decay of the HF wave into a Langmuir wave and an ion acoustic wave. These daughter waves then rapidly lose energy to the local electrons. When the HF transmitter is cycled on and off to measure the rate of change of electron temperature, the plasma instability intensity has an overshoot behavior. This change in instability level in time permits a distinction between the ohmic and the instability energy sources.
Magnetic reconnection: Acceleration, heating, and shock formation
Authors: Hayashi Takaya , Sato Tetsuya
Published: February 2008

Numerical analysis of two‐dimensional compressible MHD magnetic reconnection has shown that (1) the reconnection process is strongly dependent on boundary conditions but weakly dependent on internal conditions, (2) a slow mode shock layer is formed on the downstream side of the magnetic separatrix, and (3) plasma is heated up to a considerable degree near the X type neutral line and is accelerated up to the Alfvén speed by the slow mode shock.
Dynamic MHD modeling of the solar wind disturbances during the August 1972 events
Authors: Dryer M. , Candelaria C. , Smith Z. K. , Steinolfson R. S. , Smith E. J. , Wolfe J. H. , Mihalov J. D. , Rosenau P.
Published: February 2008

A time‐dependent one‐dimensional MHD theoretical model (Steinolfson et al., 1975a, b) is tested by using plasma and magnetic field observations of Pioneer 9 and Pioneer 10 during the August 1972 events on the sun and in the interplanetary medium. These spacecraft were nearly aligned along a common heliocentric radius during these events, considered now to be the most spectacular and best‐documented events during solar cycle 20. The observations of Pioneer 9 at 0.78 AU were used as ‘input’ for the theoretical model. The plasma and magnetic field forcing functions were superimposed upon a preexisting ambient solar wind at this inner boundary, and the response was simulated as far as 8 AU. The simulated ‘output’ at 2.2 AU is compared directly with the Pioneer 10 observations at 2.2 AU. Qualitative comparison is good, although several limitations of the one‐dimensional theory are noted.
The interplanetary modulation and transport of Jovian electrons
Authors: Conlon T. F.
Published: February 2008

Simultaneous measurements by Pioneer 11 of the 3‐ to 6‐MeV Jovian electron flux, interplanetary magnetic field magnitude, and solar wind speed during the pre‐Jupiter encounter period reveal that electron transport across the average field direction was greatly inhibited in corotating interaction regions (CIR's) and enhanced in rarefaction regions. Since CIR's are regions of compressed solar wind plasma, these results suggest that cross‐field transport is affected by the degree of compression of the solar wind. The ‘stochastic field’ model of cross‐field diffusion provides a plausible explanation of the effect of compression on the cross‐field diffusion coefficient K⊥. The solution of a convection‐diffusion equation for a model that includes the effects of CIR's and assumes that Jupiter is a time‐independent point source of electrons describes the spatial dependence of the peak electron fluxes and the time‐intensity profiles of individual electron flux increases. It has been noted that Jovian electrons are detected at the earth even when the interplanetary magnetic field does not appear to connect the earth directly to Jupiter. By extending the above model to 1 AU from the sun we find that cross‐field diffusion of the electrons provides a plausible explanation for this observation.
Energetic protons associated with interplanetary active regions 1–5 AU from the Sun
Authors: Pesses M. E. , Van Allen J. A. , Goertz C. K.
Published: February 2008

About 100 energetic proton (0.61 ≤ Ep ≤ 3.41 MeV) events have been observed at heliocentric distances between 1 and 5 AU with the University of Iowa instrument on Pioneer 11. Absolute intensities, anisotropies, and crude energy spectra are measured. The particle intensity in the events has no significant dependence on heliocentric distances. Beyond ∼2.4 AU, power spectra of the counting rates show strong peaks at about 26 and also usually 13 days corresponding to corotation of the solar wind structure. The number of events observed per unit time interval is 10 times greater in the close vicinity of interplanetary active regions (IAR) (Smith and Wolfe, 1976) than elsewhere in interplanetary space. The frequency of occurrence of events has a marked tendency to peak within ±5 hours of the time that IAR edges are crossed. Events observed in the vicinity of IAR have, on the average, greater particle intensities, softer energy spectra, and smaller time widths at half maximum intensity than events observed elsewhere. Many of the IAR‐associated events have particle anisotropies corresponding to the net flow of particles toward the sun along the interplanetary magnetic field. The phenomenological evidence strongly favors local acceleration in the interplanetary medium by an as yet unidentified mechanism in magnetohydrpdynamic shocks. The sources of the shock‐accelerated particles are apparently corotating streams of solar emitted protons, most of which have energies Ep < 0.61 MeV.
Comparison of magnetic field perturbations at high latitudes with charged particle and IMF measurements
Authors: McDiarmid I. B. , Burrows J. R. , Wilson Margaret D.
Published: February 2008

The magnetometer aligned along the spin axis of the Isis 2 satellite has been used to study magnetic field perturbations in the dawn‐dusk sectors of the auroral oval and in the polar cap. The field perturbations have been compared with simultaneous particle measurements and with measurements of the interplanetary magnetic field (IMF). Northern hemisphere passes were selected during a period when the satellite orientation could be determined to an accuracy of about ±0.1°. In about 80% of the passes examined, there are significant and systematic deviations from a model field in the oval and in the polar cap. The magnitude of the perturbations, ΔB, in both the oval and the polar cap, is correlated with the z component of the IMF. The perturbations in the polar cap also suggest a dependence on the y component of the IMF, being larger on the morning side than on the afternoon side when By is positive and larger on the afternoon side than on the morning side when By is negative. The perturbations are discussed in terms of the tilt of the magnetic field direction, either toward or away from the sun, relative to the direction of the model field, and it is suggested that the tilt direction can be related to the direction of convection in the magnetosphere. When the poleward edge of the plasma sheet is compared with the latitude where the direction of the perturbation changes from sunward to antisunward, it is found that the former typically extends poleward of the latitude where ΔB changes sign. It is suggested that the region between the poleward edge of the plasma sheet and the latitude where ΔB changes sign is the low‐altitude signature of the magnetospheric boundary layer.
Frequency analysis of 4‐ to 6‐keV electrons associated with an auroral arc
Authors: Murphree J. S. , Anderson H. R.
Published: February 2008

Theoretical considerations indicate that auroral electrons may be modulated by an electrostatic wave generated by instabilities in the ionosphere. A high time resolution (HTR) experiment has been designed to detect such variations and covers the frequency ranges of 0–2.5 kHz and 230 kHz to 9.9 MHz. The energy range sampled is 4–6 keV, and the pitch angle coverage is designed to be from 0° to 180°. The results of the experiment set an upper limit of approximately 5% on the electron modulations in the appropriate frequency ranges and establish the necessity for high‐intensity fluxes before this upper limit can be lowered. The experiment was launched on a Nike Tomahawk rocket on the evening of March 14, 1974, and the HTR operated as expected from lift‐off. Fluxes peaked at a few times 107 el/cm² s sr keV during the brightest part of the arc. No modulation was observed in the high‐frequency range, a finding which is consistent with the stabilization of the plasma by the low flux of energetic electrons. Suspected modulation around 2 kHz was observed, but no event is of sufficiently long duration or of sufficiently high power level to be defined unequivocally as real electron modulation. The suspected modulation is not confined to any single pitch angle range and is not associated with the high flux gradients (where modulation may be expected) at the northern and southern edges of the arc.
Nonneutral and quasi‐neutral diffusion of weakly ionized multiconstituent plasma
Authors: Hill R. J.
Published: February 2008

A theory is developed for ambipolar diffusion in a multiconstituent weakly ionized plasma. The implications of the presence of negative ions and of differing positive ion and negative ion diffusion coefficients are found. In particular, the presence of numerous negative ions is found to enhance the diffusion of the electrons. Furthermore, this theory of multiconstituent ambipolar diffusion is extended to include the nonneutral case in which the scale length of the inhomogeneity is smaller than or of the order of the electron Debye length.
Rocket measurement of thermospheric photoelectron energy spectra
Authors: McMahon W. J. , Heroux L.
Published: February 2008

Relative values of thermospheric photoelectron energy distributions between 2 and 100 eV were measured over the altitude range 100–220 km by using a 127° cylindrical electrostatic deflection analyzer flown on a rocket. The measured photoelectron spectra display distinct features that are theoretically predicted (Victor et al., 1976; Jasperse, 1977) and are similar to those obtained in recent satellite experiments (Doering et al., 1975, 1976). The valleylike structure in the energy range 2–5 eV attributed to resonant‐vibrational excitation of N2 was observed in more detail and over a greater altitude range than that measured previously. The prominence of this low‐energy valley decreases with increasing altitude above about 150 km and disappears above 200 km, in general agreement with theory. However, in contrast to theoretical predictions, the low‐energy valley also appears to become less prominent as altitude decreases below about 140 km, down to the lowest observations at 100 km.
Momentum source signatures in thermospheric neutral composition
Authors: Trinks H. , Mayr H. G. , Philbrick C. R.
Published: February 2008

Perturbations in the structure of the neutral atmosphere were measured in narrow latitude bands near the dayside polar cusp by mass spectrometers aboard the S3‐1 and Esro 4 satellites. The disturbances are characterized by an in‐phase variation of both the lighter and the heavier species. They are observed to occur in latitude bands of about 5°–10° extent in the altitude region of 200 km during periods of increased geomagnetic activity. It is suggested that thermospheric winds driven by momentum sources associated with ion convection are the predominant cause for these disturbances. The narrow structure features associated with the momentum source are superimposed on the thermospheric changes caused by Joule heating. Both effects may be present simultaneously, resulting in a complex response pattern of the neutral gas composition in the thermosphere.
Use of the Jacchia (1977) Empirical Model in a dynamical model of upper‐atmospheric helium
Authors: Straus Joe M.
Published: February 2008

This brief report describes an investigation of the effects of using the Jacchia (1977) model in place of the Jacchia (1971) model for specifying the background gas pressure in the theoretical model of thermospheric helium described by Straus et al. (1977). Changes in the global transport of helium caused by a small increase in the latitudes at which the background gas pressure extrema occur lead to much better agreement of the model predictions with data taken by the mass spectrometers on board the Esro 4 and Ogo 6 satellites.
Comparison of incoherent scatter radar and photometric measurements of the energy distribution of auroral electrons
Authors: Vondrak Richard R. , Sears Robert D.
Published: February 2008

Simultaneous radar and photometric measurements of the mean energy of precipitating electrons have been made at Chatanika, Alaska, during a variety of auroral conditions. The radar measurements of the altitude profile of electron density were used to obtain the differential energy spectrum of the precipitating flux. Photometric measurements of 4278‐ and 6300‐Å, emission intensities over the same region and time span were analyzed in terms of the mean energy parameter, as was previously described by Rees and Luckey (1974). The mean energy parameter values determined by the two techniques show good agreement, which tends to confirm quantitatively the theoretical predictions of Rees and Luckey. The energy parameter range measured was ∼3–15 keV, and the auroral intensities ranged from about IBC I to IBC III (<1 to ∼ 10 kR of 4278‐Å intensity).
Geometrical MHD wave coupling
Authors: Hollweg Joseph V. , Lilliequist Carl G.
Published: February 2008

Refraction and/or magnetic field curvature can lead to ‘geometrical mode coupling’ between the MHD modes. The effect is linear, but it is a finite‐wavelength effect. It is studied here for a simple configuration which is amenable to analysis and which illustrates the basic features of the coupling. In the solar wind the geometrical coupling may be operative only in the solar corona or in the interaction regions of high‐speed streams. In the latter case the geometrical coupling may provide an explanation for the non‐Alfvénic fluctuations but only for long‐period waves (greater than several hours) in small interaction regions (<0.1 AU).
Axisymmetric ideal MHD stellar wind flow
Authors: Heinemann M. , Olbert S.
Published: February 2008

The ideal MHD equations are reduced to a single equation under the assumption of axisymmetric flow. A variational principle from which the equation is derivable is given. The characteristics of the equation are briefly discussed. The equation is used to rederive the theorem of Gussenhoven and Carovillano.
Longitudinal phase variations of Pc 4‐5 micropulsations
Authors: Olson John V. , Rostoker Gordon
Published: February 2008

A set of 61 relatively uncontaminated Pc 4‐5 micropulsations from the interval July 1 through October 10, 1974, were examined for longitudinal phase changes. The data were obtained from the east‐west line of magnetometers operated by the University of Alberta over the summer and early fall of 1974. From our analysis a direct correlation between longitudinal phase shift and frequency is found which is of the form m = (1.4 ± 0.4) f + 0.26, where f is the frequency in millihertz and m is the azimuthal wave number derived from the measured phase shift. This relationship indicates a phase velocity which is independent of frequency and whose magnitude is Vϕ ≃ 14 km/s. The sign of the phase shift, and thus the direction of apparent phase velocity, changes abruptly 1–2 hours before local noon. The signals appear to propagate away from this prenoon region toward the terminators; that is, they move westward in the morning hours and eastward during the afternoon hours. Both the magnitude and the sign of this phase velocity are consistent with proposed generation mechanisms which invoke the Kelvin‐Helmholtz instability at the magnetopause boundary.
Models for quasi‐periodic electric fields and associated electron precipitation in the auroral zone
Authors: Petelski E. F. , Fahleson U. , Shawhan S. D.
Published: February 2008

On four rocket flights, quasi‐periodic electric fields were observed in the nighttime auroral oval and in the polar cleft. These fields had periods of between 0.5 and 3 s and amplitudes from 2 to 30 mV/m and exhibited left‐hand and right‐hand elliptical polarization. Electric field intensifications often coincided with bursts of energetic electron precipitation, on occasion modulated with nearly the same period as the fields. The events in the auroral oval were associated with substorms and visual auroral activity. The spectral and polarization properties of the observed fields suggest that they represented the electric components of Pc 1 or Pi 1 micropulsations. To model these waves and the concurrent electron flux variations, different mechanisms for wave excitation, electron acceleration, and wave‐particle interaction are considered. One likely interpretation attributes the micropulsations to Birkeland current chopping by an unstable double layer located at ≃1‐RE altitude. The double layer is also assumed to accelerate the observed electrons, the electron flux variations being due either to the inherent variations of the double layer or to its interaction with the micropulsations. Because of the sparcity of parameters measured so far the models are provisional. Follow‐up experiments are proposed to investigate the problem further.
High‐resolution altitude profiles of the auroral zone energy dissipation due to ionospheric currents
Authors: Brekke A. , Rino C. L.
Published: February 2008

The first height‐resolved altitude profiles of the Joule heating rate obtained by using the Chatanika, Alaska, incoherent scatter radar are presented. The measured Joule heating rate typically maximizes at an altitude that is well above the altitude of the current density maximum. For a reasonable model neutral atmosphere, it is also found that the maximum Joule heating rate occurs above the altitude of the corresponding Pedersen conductivity maximum. For these data the eastward auroral electrojet was centered near 130 km, while the westward electrojet was centered 10 km lower in altitude during its most intense period. It is inferred that this large difference in height between the two electrojets and the corresponding altitude change in the Joule heating will have a significant effect on the source mechanisms for such phenomena as atmospheric gravity waves, infrasonic waves, and spread F irregularities. One should expect an asymmetry in the occurrence and structure of such events with respect to the Harang discontinuity. This asymmetry is noted for aurorally excited gravity waves. A tentative calibration of the total Joule dissipation in the neutral atmosphere relative to the magnetic field disturbance vector measured by a nearby ground‐based magnetometer has been performed. The heating efficiency was found to be 10 times larger in a positive bay than in a negative bay. The heating efficiency for the negative bay event is close to the value deduced by Cole (1971) on theoretical grounds.
High‐altitude limit of the gradient drift instability
Authors: Ossakow S. L. , Chaturvedi P. K. , Workman J. B.
Published: February 2008

The gradient drift instability for F region plasma clouds is investigated retaining ion inertia effects. With the assumption that the ion gyrofrequency Ω is greater than ion inertial time changes (∂/∂t+Vi·▽) and the ion‐neutral collision frequency νin, a quadratic dispersion equation results. In the limit νin ≫ 4cE0/BL (E0 and B are the ambient electric and magnetic fields, and L is the plasma cloud density gradient scale length), the growth rate γ reduces to the usual result cE0/BL. In the opposite limit νın ≪ 4cE0/BL, γ = (cE0νin/BL)1/2. This result can explain why high‐altitude (∼450‐km) barium releases, where νin is less than unity and a factor of 10–100 smaller than it is at lower altitudes (∼200 km), striate on time scales similar to the lower altitude releases.
Signature of a parallel electric field in ion and electron distributions in velocity space
Authors: Croley D. R. , Mizera P. F. , Fennell J. F.
Published: February 2008

Charged particle data taken by the S3‐3 satellite, reported by Mizera and Fennell (1977), are presented as contours of the velocity distribution function on a velocity‐space diagram. This report focuses on the analytical technique used to interpret the particle data. Details of features exhibited by the electron and ion data in the velocity‐space representations are discussed in terms of a simple electrostatic acceleration model. The observed particle populations are separated in velocity‐space by recognizable demarcations calculated from the conservation laws in accordance with Liouville's theorem.
Energization of charged particles in Jupiter's outer magnetosphere
Authors: Goertz C. K.
Published: February 2008

It is shown that corotation of charged particles in the azimuthally asymmetric magnetic field of the Jovian magnetosphere leads to a pitch‐angle‐dependent energy loss and gain of particles that drift from noon to midnight and midnight to noon, respectively. An isotropic pitch angle distribution at noon would become dumbbell‐like at midnight. Nonadiabatic scattering relaxes the dumbbell distribution toward isotropy without changing the particle's energy. This scattering represents an increase of the first adiabatic invariant M. As the particles drift back toward noon at an increased value of M, they gain more energy than they lost during their drift from noon to midnight. The overall energy gain per rotation can be as large as a factor of 2, and in 10 rotations the total energy may be increased by several orders of magnitude. The model can account for the 10‐hour modulation of spectral index and makes a number of testable predictions.
Some characteristics of electric field momentum coupling with the neutral atmosphere
Authors: Mayr H. G. , Harris I.
Published: February 2008

It has been shown earlier that thermally driven winds due to Joule heating in the auroral zones can account for observed global scale composition effects in the thermosphere during magnetic storms. These winds are relatively small in comparison with wind fields attributed to E×B ion drifts. Using mode theory to describe the interaction of this momentum source, some response characteristics of thermospheric temperature, composition, and winds are discussed. E×B ion drifts induce horizontal winds that are predominantly divergence free and as such are, by comparison with thermally driven winds, more than a factor of 10 less effective in generating temperature and density variations. The momentum source signatures in composition (inferred from theory) reveal that all atmospheric species, including He, O, N2, and Ar, vary in phase with one another and are out of phase with Tg above 150 km, in striking contrast to the atmospheric signatures resulting from Joule heating. While the large wind velocities at higher latitudes are thus primarily produced by the ion drift momentum source, the smaller contribution from Joule heating may still be the principal cause of the observed density variations. The momentum source is relatively more effective in the lower thermosphere and, on occasion, may even produce signatures at higher altitudes as suggested from isolated satellite measurements.