bluuebunny/crossref_metadata_2025 huggingface dataset:
Segmentation and crustal structure of the western Mid‐Atlantic Ridge flank, 25°25′–27°10′N and 0–29 m.y.
Authors: Tucholke Brian E. , Lin Jian , Kleinrock Martin C. , Tivey Maurice A. , Reed Thomas B. , Goff John , Jaroslow Gary E.
Published: February 2004

We conducted a detailed geological‐geophysical survey of the west flank of the Mid‐Atlantic Ridge between 25°25′N and 27°10′N and from the ridge axis out to 29 Ma crust, acquiring Hydrosweep multibeam bathymetry, HAWAII MR1 sidescan‐sonar imagery, gravity, magnetics, and single‐channel seismic reflection profiles. The survey covered all or part of nine spreading segments bounded by mostly nontransform, right‐stepping discontinuities which are subparallel to flow lines but which migrated independently of one another. Some discontinuities alternated between small right‐ and left‐stepping offsets or exhibited zero offset for up to 3–4 m.y. Despite these changes, the spreading segments have been long‐lived and extend 20 m.y. or more across isochrons. A large shift (∼9°) in relative plate motion about 24–22 Ma caused significant changes in segmentation pattern. The nature of this plate‐boundary response, together with the persistence of segments through periods of zero offset at their bounding discontinuities, suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt. Within segments, there are distinct differences in seafloor depth, morphology, residual mantle Bouguer gravity anomaly, and apparent crustal thickness between inside‐corner and outside‐corner crust. This demands fundamentally asymmetric crustal accretion and extension across the ridge axis, which we attribute to low‐angle, detachment faulting near segment ends. Cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal‐thickness changes of at least 1–2 km every 2–3 m.y. These are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension, controlled by retention and quasiperiodic release of melt from the upwelling mantle. Detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes, creating crustal domes analogous to “turtleback” metamorphic core complexes that are formed by low‐angle, detachment faulting in subaerial extensional environments.
Evolution of the stress field in southern California and triggering of moderate‐size earthquakes: A 200‐year perspective
Authors: Deng Jishu , Sykes Lynn R.
Published: February 2004

Changes in stress in southern California are modeled from 1812 to 2025 using as input (1) stress drops associated with six large (7.0≤M<7.5) to great (M≥7.5) earthquakes through 1995 and (2) stress buildup associated with major faults with slip rates ≥3 mm/yr as constrained by geodetic, paleoseismic, and seismic measurements. Evolution of stress and the triggering of moderate to large earthquakes are treated in a tensorial rather than a scalar manner. We present snapshots of the cumulative Coulomb failure function (ΔCFF) as a function of time for faults of various strike, dip, and rake throughout southern California. We take ΔCFF to be zero everywhere just prior to the great shock of 1812. We find that about 95% of those well‐located M≥6 earthquakes whose mechanisms involve either strike‐slip or reverse faulting are consistent with the Coulomb stress evolutionary model; that is, they occurred in areas of positive ΔCFF. The interaction between slow‐moving faults and stresses generated by faster‐moving faults significantly advanced the occurrence of the 1933 Long Beach and 1992 Landers events in their earthquake cycles. Coulomb stresses near major thrust faults of the western and central Transverse Ranges have been accumulating for a long time. Future great earthquakes along the San Andreas fault, especially if the San Bernardino and Coachella Valley segments rupture together, can trigger moderate to large earthquakes in the Transverse Ranges, as appears to have happened in the Santa Barbara earthquake that occurred 13 days after the great San Andreas shock of 1812. Maps of current ΔCFF provide additional guides to long‐term earthquake prediction.
Shear and compressional wave structure of the East Pacific Rise, 9°–10°N
Authors: Christeson G. L. , Shaw P. R. , Garmany J. D.
Published: February 2004

We use shear and compressional arrivals recorded by 45 ocean bottom seismograph receivers to model regional structure of the 9°–10°N region of the East Pacific Rise. Modeling indicates that shear conversion occurs at the base of layer 2A, producing Ps (energy that travels through oceanic crust as P and converts to S at the base of layer 2A on upgoing path) and pS (energy that travels as P through layer 2A and converts to S at the base of layer 2A on downgoing path) arrivals. The travel times of these arrivals require a shear wave velocity within layer 2A of 0.4–0.8 km/s (Poisson's ratio of 0.46–0.49). Waveform inversion was used to model the Poisson's ratio structure of layers 2B and 3 for nine instruments with good shear arrivals. Poisson's ratio within layer 2B was highly variable, with values as low as 0.24 and a mean value of 0.263. Some of this variability might be due to lateral variability in layer 2A structure. The mean Poisson's ratio of layer 3 was 0.271. According to the cracking model of Shaw [1994], the low Poisson's ratios within the upper portion of layer 2B indicate that thick cracks (aspect ratio α = 0.1) extend to depths of ∼ 1.5–1.7 km in this region. Two‐dimensional travel time modeling indicates that the southwestern portion of our study area is associated with anomalously low seismic velocities, with layer 2B and layer 3 velocities reduced by up to 11% from the regional values. Cracking theory suggests that these low velocities could be caused by porosities of 0.3–5.5%, depending on the crack aspect ratio; a maximum porosity of 1.5% is predicted from our one Poisson's ratio measurement in this area. The anomalous velocities are located near the western discordant zone left by the 9°03′N overlapping spreading center. We suggest that shearing associated with on‐axis rotation of the overlap basins is responsible for the low seismic velocities. The pattern of anomalies indicates that faulting extends to distances of 10–15 km from the basins.
Färoe‐Iceland Ridge Experiment 1. Crustal structure of northeastern Iceland
Authors: Staples Robert K. , White Robert S. , Brandsdóttir Bryndis , Menke William , Maguire Peter K. H. , McBride John H.
Published: February 2004

Results from the Färoe‐Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s−1). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.
Determination of earthquake source mechanisms using teleseismic 30–140 s waves: The January 17, 1994, Northridge earthquake
Authors: Zhang Jiajun , Kuge Keiko , Lay Thorne , Tsuboi Seiji
Published: February 2004

Complete seismic waveforms with periods between 30 and 140 s recorded at teleseismic distances (≥10°) are used to determine the source mechanism of the January 17, 1994, Northridge earthquake (Ms=6.8). An algorithm called the Earth simplifying transformation (EST) is applied to the mainshock (primary event) data to calibrate and remove propagation effects of Earth's lateral and radial heterogeneities that are not predicted by a reference Earth model. The procedure involves frequency domain division of each pair of observed and synthetic waveforms for an auxiliary event with a well‐constrained source mechanism in proximity to the primary event, with the synthetics being computed for a reference Earth model. If the auxiliary event source mechanism is correct, the differences between the data and synthetics represent the residual effects of the Earth's structure that are not predicted by the Earth model. The frequency dependent deconvolution filters are convolved with the corresponding observed signals for the primary event. The resulting waveforms are then used to determine the source mechanism of the primary event by forward modeling and by centroid‐moment‐tensor inversion, with excellent fits to the complete waveforms being achieved. The resulting solution for the Northridge event has a centroid depth of 12±1 km and a mechanism with strike, ϕ=115°, dip, δ=40°, and rake, λ=95°, with 5° uncertainty for each parameter. The EST procedure holds promise for reliable source inversion for small earthquakes in a region calibrated by large events with well‐determined source parameters.
Multispectral thermal infrared mapping of sulfur dioxide plumes: A case study from the East Rift Zone of Kilauea Volcano, Hawaii
Authors: Realmuto V. J. , Sutton A. J. , Elias T.
Published: February 2004

The synoptic perspective and rapid mode of data acquisition provided by remote sensing are well suited for the study of volcanic SO2 plumes. In this paper we describe a plume‐mapping procedure that is based on image data acquired with NASA's airborne thermal infrared multispectral scanner (TIMS) and apply the procedure to TIMS data collected over the East Rift Zone of Kilauea Volcano, Hawaii, on September 30, 1988. These image data covered the Pu‘u ‘O ‘o and Kupaianaha vents and a skylight in the lava tube that was draining the Kupaianaha lava pond. Our estimate of the SO2 emission rate from Pu‘u ‘O ‘o (17–20 kg s−1) is roughly twice the average of estimates derived from correlation spectrometer (COSPEC) measurements collected 10 days prior to the TIMS overflight (10 kg s−1). The agreement between the TIMS and COSPEC results improves when we compare SO2 burden estimates, which are relatively independent of wind speed. We demonstrate the feasibility of mapping Pu‘u ‘O ‘o ‐ scale SO2 plumes from space in anticipation of the 1998 launch of the advanced spaceborne thermal emission and reflectance radiometer (ASTER).
The German Continental Deep Drilling Program KTB: Overview and major results
Authors: Emmermann Rolf , Lauterjung Jörn
Published: February 2004

The German Continental Deep Drilling Program (KTB) was designed to study the properties and processes of the deeper continental crust by means of superdeep borehole. Major research themes were (1) the nature of geophysical structures and phenomena, (2) the crustal stress field and the brittle‐ductile transition, (3) the thermal structure of the crust, (4) crustal fluids and transport processes, and (5) structure and evolution of the central European Variscan basement. The project was conducted in distinct phases: a preparatory phase (1982–1984), a phase of site selection (1985–1986), and a pilot phase (1987–1990), which included sinking of a pilot borehole to 4000 m and a 1‐year experimentation program. The main phase (1990–1994) comprised drilling of a superdeep borehole which reached a final depth of 9101 m and a temperature of ∼265 °C, and three subsequent large‐scale experiments in the uncased‐bottom hole section. Among the outstanding results are the following (1) A continuous profile of the complete stress tensor was obtained. (2) Several lines of evidence indicate that KTB reached the present‐day brittle‐ductile transition. (3) The drilled crustal segment is distinguished by large amounts of free fluids down to midcrustal levels. (4) The role of postorogenic brittle deformation had been grossly underestimated. (5) Steep‐angle seismic reflection surveys depict the deformation pattern of the upper crust. (6) High‐resolution seismic images of the crust can be obtained with a newly developed technique of true‐amplitude, prestack depth migration. (7) The electrical behavior of the crust is determined by secondary graphite (±sulfides) in shear zones.
Shock propagation in the outer heliosphere: 1. Pickup ions and gasdynamics
Authors: Zank G. P. , Pauls H. L.
Published: February 2004

Pickup ions are expected to play a fundamental role in determining the global structure of the heliosphere since they dominate the internal energy of the outer solar wind beyond the ionization cavity. The dynamical effect of pickup ions on the propagation characteristics of interplanetary shocks is addressed here. It is found that (1) forward and reverse shocks in a heliosphere mediated by pickup ions have higher speeds than expected of shocks in an adiabatic solar wind without pickup ions; (2) identical initial conditions evolve differently in a pickup ion mediated heliosphere compared to an adiabatic heliosphere, producing more complicated velocity, density and temperature structure, and often with multiple forward‐reverse shock pairs; (3) pickup ions act to weaken and dissipate shocks in the outer heliosphere although the velocity jumps can remain large; (4) density fluctuations in the outer pickup ion mediated heliosphere do not correlate with velocity fluctuations as well as they do in the inner heliosphere, and (5) the amplitudes of density fluctuations in a pickup ion mediated heliosphere are considerably smaller than those found in an adiabatic heliosphere without pickup ions.
Some seasonal hemispheric similarities in ƒoF2 quasi‐2‐day oscillations
Authors: Altadill D. , Apostolov E. M. , Alberca L.
Published: February 2004

The annual variations of the amplitude, period, and probability of existence of ƒoF2 quasi‐2‐day oscillations at middle latitudes in both northern and southern hemispheres are investigated. The ƒoF2 hourly data for the period 1977–1982 for stations Kiev (50.5° N, 30.5° E) and Wakkanai (45.39° N, 141.69° E) for the northern hemisphere and Kerguelen (49.35° S, 70.24° E) and Campbell Island (52.6° S, 169.1° E) for the southern hemisphere are used for analysis. It is found that there is a strong tendency for hemispheric similarities in the seasonal variations of oscillation amplitude, period, and probability of existence. The annual variations of the oscillation amplitude are modulated by the 12‐month hemispheric and semiannual geomagnetic waves; the dominant period of oscillation is maximum in summer (50–51 hours) and minimum in winter (47–49 hours), and the probability of existence has a maximum in the summer half year and minimum in the winter half year. The seasonal similarities in the annual variations of the oscillation period and the probability of existence show the possible influence of the planetary 2‐day wave in the middle neutral atmosphere on the electron density variations in the F region.
Attachment rate effects during electron attachment chemical releases
Authors: Scales W. A. , Myers T. J. , Bernhardt P. A. , Ganguli G.
Published: February 2004

The effects of attachment rate on the evolution of electron depletions produced during ionospheric electron attachment chemical releases are studied by using a numerical simulation model. Varying the attachment rate is observed to have important effects on small‐scale processes in the depletion boundary layer as well as the large‐scale evolution of the depletion. Increasing the attachment rate is observed to increase the amplitude of electrostatic waves in the depletion boundary. The increase in amplitude of these waves is accompanied by an increase in heating of negative ions produced by the electron attachment. Also, the wavelength of waves in the boundary layer is seen to decrease as the attachment rate increases, since steeper density gradients and more highly sheared electron flows are produced. This result indicates that the attachment rate and the size of the release are important factors in setting the scale size of irregularities during the experiments.
Helios 1 and 2 observations of particle decreases, ejecta, and magnetic clouds
Authors: Cane H. V. , Richardson I. G. , Wibberenz G.
Published: February 2004

We have used >60 MeV/amu particle data from Helios 1 and 2 to demonstrate the close association between ejecta (as defined, for example, by regions of depressed solar wind proton temperature) and short‐term (<3 days duration) particle decreases. Of 84 short‐term decreases of ≥4%, we find that 88% were associated with an ejecta, and 70% of these were also associated with a shock. It is clear that the presence of a particle decrease is a robust signature for identifying ejecta in the ecliptic, at least within 1 AU. Conversely, ejecta are evidently important in the production of short‐term cosmic ray depressions. The absence of a sample of well‐defined ejecta without an associated particle decrease suggests that extended regions of open field geometry are rare inside ejecta, at least on the ∼0.005 AU scale sizes probed by these particles. Sixty‐three percent of the ejecta decreases were associated with smooth magnetic field rotations characteristic of magnetic clouds. Our results suggest that there is no fundamental difference in the particle response to ejecta with or without magnetic cloud signatures. We find that some ejecta observed at multiple spacecraft have a magnetic cloud signature at one spacecraft but not at another. The most likely explanation is that magnetic clouds are a substructure of ejecta and the field structure observed depends on where the ejecta is intercepted. We also find that ejecta probably typically extend much less in longitude than the 100° inferred from single‐spacecraft studies.
A study of the CDAW 9C substorm of May 3, 1986, using magnetograrn inversion technique 2, and a substorm scenario with two active phases
Authors: Mishin V. M. , Block L. P. , Bazarzhapov A. D. , Saifudinova T. I. , Lunyushkin S. B. , Shirapov D. Sh. , Woch J. , Eliasson L. , Marklund G. T. , Blomberg L. G. , Opgenoorth H.
Published: February 2004

One of the CDAW 9C substorms is investigated in this paper using the database reported by Hones et al. and supplemented with magnetogram inversion technique (MIT) 2 data. These latter have provided information about the dynamics of the open tail magnetic flux, current systems in the ionosphere, and the size and dynamics of the current wedge. We have identified the growth, expansion, and recovery phases of this substorm, with characteristics expected from a generally accepted scenario. However, specific signatures were observed in the interval (0919‐0935) UT, i.e., between the growth and expansion phases, indicating the concurrent development of the substorm onset and corresponding instabilities in the innermost current sheet, and small‐scale cross‐tail current disruptions without the open tail reconnection. In addition to signatures of small‐scale dipolarization, an increase of the open tail magnetic flux, and a current system of the type close to DP 2 were observed at (0919‐0935) UT, which is more likely to suggest predominance of the tail‐stretching process than magnetic collapse. This fact was interpreted in terms of a relevant simple model as a signature of the growth of the energy input from the solar wind which ensures the observable disturbance power. Hence the disturbance at (0919‐0935) UT was more likely a driven one than an unloading one. The aforementioned signatures make it possible to identify the interval (0919‐0935) UT as the “phase of multiple onsets” or (equivalently) the “first active phase,” which was previously defined by Mishin [1991, and references therein] as one of the four standard phases of a typical substorm (in addition to the expansion phase). Thus the case study supports the substorm scenario with two active phases and, accordingly, with two different kinds of physics. This case study illustrates also the informativity of MIT 2 data and their ability to effectively complement the database traditionally used in substorm studies.
Relationships between plasma depletion and subsolar reconnection
Authors: Anderson B. J. , Phan T.‐D. , Fuselier S. A.
Published: February 2004

Observations in the subsolar magnetosheath show that the plasma depletion layer (PDL) is less pronounced for southward than for northward IMF. Since subsolar plasma depletion indicates pile‐up of magnetic flux, the degree of plasma depletion should depend on the relative rates of flux transport via subsolar reconnection and flux advection by the solar wind flow. To identify the factors affecting plasma depletion, we consider the ratio D = Er/Esw where Er is the reconnection electric field and Esw is the imposed solar wind electric field. For a quasi‐perpendicular subsolar bow shock D can be expressed in terms of magnetosheath parameters. Since PDL formation is suppressed when the subsolar bow shock is quasi‐parallel, we restrict attention to quasi‐perpendicular conditions. We show that D increases with increasing reconnection efficiency, magnetic shear at the magnetopause, and the magnetosheath magnetic field but decreases with increasing total perpendicular pressure (particle plus magnetic field) in the magnetosheath. By combining observations of the subsolar quasi‐perpendicular magnetosheath from AMPTE/IRM and AMPTE/CCE we verify that the degree of plasma depletion is inversely correlated with D. Furthermore, we show that the greater prevalence of plasma depletion in the CCE data implies that the reconnection efficiency is lower for the CCE events and specifically that the reconnection efficiency depends roughly inversely on magnetosheath β, for β > 1. Finally, the results show that all of the changes brought about by plasma depletion act to increase Er so that some subsolar reconnection is likely to occur even for low magnetic shear. Thus the percentage of the magnetosheath magnetic field that participates in reconnection near the subsolar region does not act as a rectifier but remains positive for all shear angles, decreasing monotonically as the magnetic shear at the subsolar magnetopause changes from high to low shear. This implies that the equatorward polar cap convection cells observed during northward IMF and conventionally thought to be driven by a viscous interaction may be due at least in part if not wholly to subsolar low shear reconnection.
Numerical simulations of gravity waves imaged over Arecibo during the 10‐day January 1993 campaign
Authors: Hickey Michael P. , Walterscheid Richard L. , Taylor Michael. J. , Ward William , Schubert Gerald , Zhou Qihou , Garcia Francisco , Kelly Michael C. , Shepherd G. G.
Published: February 2004

Recently, measurements were made of mesospheric gravity waves in the OI (5577 Å) nightglow observed from Arecibo, Puerto Rico, during January 1993 as part of a special 10‐day campaign. Clear, monochromatic gravity waves were observed on several nights. By using a full‐wave model that realistically includes the major physical processes in this region, we have simulated the propagation of two waves through the mesopause region and calculated the O(1S) nightglow response to the waves. Mean winds derived from both UARS wind imaging interferometer (WINDII) and Arecibo incoherent scatter radar observations were employed in the computations as were the climatological zonal winds defined by COSPAR International Reference Atmosphere 1990 (CIRA). For both sets of measured winds the observed waves encounter critical levels within the O(1S) emission layer, and wave amplitudes, derived from the requirement that the simulated and observed amplitudes of the O(1S) fluctuations be equal, are too large for the waves to be gravitationally stable below the emission layer. Some of the model coefficients were adjusted in order to improve the agreement with the measurements, including the eddy diffusion coefficients and the height of the atomic oxygen layer. The effect of changing the chemical kinetic parameters was investigated but was found to be unimportant. Eddy diffusion coefficients that are 10 to 100 times larger than presently accepted values are required to explain most of the observations in the cases that include the measured background winds, whereas the observations can be modeled using the nominal eddy diffusion coefficients and the CIRA climatological winds. Lowering the height of the atomic oxygen layer improved the simulations slightly for one of the simulated waves but caused a less favorable simulation for the other wave. For one of the waves propagating through the WINDII winds the simulated amplitude was too large below 82 km for the wave to be gravitationally stable, in spite of the adjustments made to the model parameters. This study demonstrates that an accurate description of the mean winds is an essential requirement for a complete interpretation of observed wave‐driven airglow fluctuations.
Auroral optical emissions during the solar magnetic cloud event of October 1995
Authors: Sivjee G. G. , Shen D.
Published: February 2004

The solar magnetic cloud of October 18, 1995, swept by the Earth's magnetosphere, starting at about 2200 UT. This event triggered prolonged auroral displays in the terrestrial atmosphere. The optical emissions from these auroras were recorded with a CCD spectrograph, operating at the radar site in Sondrestromfjord, Greenland. Continuous spectroscopic measurements were made for about 10 hours, from 2200 UT on October 18, 1995, until local dawn set in at 0800 UT on October 19, 1995. These measurements show markedly different spectral characteristics from those of normal auroras observed at other times in various sectors of the auroral oval and in the polar cap region. The differences are most pronounced in the relative brightnesses of the ion and neutral line emissions from the thermospheric oxygen atoms as well as in the O2 At band emissions. Other differences between the normal auroras and the magnetic cloud induced auroras relate to the temperature of the thermospheric height regions where the N2 1PG and the N2+ M band emissions peak in these auroras. Least squares synthetic profiles fit to the rotational distributions of the N2 1PG and the N2+ M bands observed during the solar magnetic cloud event, coupled with auroral model calculations constrained to reproduce the measured emission brightnesses, indicate a higher than normal height for the emissions from the auroras associated with this event. Further evidence for the higher emission altitude is derived from the detailed comparison of the spectral distribution observed during this period with various other measurements of the relatively high‐altitude auroral emissions. All these observations point to the precipitation in the polar terrestrial atmosphere of electrons with an average energy of about 500eV ± 100eV during the interaction of the solar magnetic cloud with the near‐Earth space environment. Such an auroral electron precipitation event is unusual and occurs infrequently; it is similar to the rare type A global red aurora.
The Wind magnetic cloud and events of October 18–20, 1995: Interplanetary properties and as triggers for geomagnetic activity
Authors: Lepping R. P. , Burlaga L. F. , Szabo A. , Ogilvie K. W. , Mish W. H. , Vassiliadis D. , Lazarus A. J. , Steinberg J. T. , Farrugia C. J. , Janoo L. , Mariani F.
Published: February 2004

Late on October 18, 1995, a magnetic cloud arrived at the Wind spacecraft ≈ 175 RE upstream of the Earth. The cloud had an intense interplanetary magnetic field that varied slowly in direction, from being strongly southward to strongly northward during its ≈ 30 hours duration, and a low proton temperature throughout. From a linear force free field model the cloud was shown to have a flux rope magnetic field line geometry, an estimated diameter of about 0.27 AU, and an axis that was aligned with the Y axis(GSE) within about 25°. A corotating stream, in which large amplitude Alfven waves of about 0.5 hour period were observed, was overtaking the cloud and intensifying the fields in the rear of the cloud. The prolonged southward magnetic field observed in the early part of the cloud produced a geomagnetic storm of Kp = 7 and considerable auroral activity late on October 18. About 8 hours in front of the cloud an interplanetary shock occurred. About three‐fourths the way into the cloud another apparent interplanetary shock was observed. It had an unusual propagation direction, differing by only 21° from alignment with the cloud axis. It may have been the result of the interaction with the postcloud stream, compressing the cloud, or was possibly due to an independent solar event. It is shown that the front and rear boundaries of the cloud and the upstream driven shock had surface normals in good agreement with the cloud axis in the ecliptic plane. The integrated Poynting flux into the magnetosphere, which correlated well with geomagnetic indices, jumped abruptly to a high value upon entry into the magnetic cloud, slowly decreased to zero near its middle, and again reached substantial but sporadic values in the cloud‐stream interface region. This report aims to support a variety of ISTP studies ranging from the solar origins of these events to resulting magnetospheric responses.
HF‐radar observations of the dayside magnetic merging rate: A Geospace Environment Modeling boundary layer campaign study
Authors: Baker K. B. , Rodger A. S. , Lu G.
Published: February 2004

Goose Bay HF‐radar data have been used to determine the dayside reconnection electric field which transports energy from the solar wind into the Earth's magnetosphere and ionosphere. The speed of the ionospheric plasma flow perpendicular to the open/closed boundary is determined in the rest frame of the boundary along each of the 16 beam directions of the HF radar. The observations were made during one of the Geospace Environment Modeling program's boundary layer campaigns. The period from 1200 to 1600 UT on March 29, 1992, was one of generally southward interplanetary magnetic field (IMF). The y component of the IMF was negative for most of the time. Despite the generally steady IMF conditions, the merging rate observed by the radar shows a great deal of temporal structure. The radar observations have been compared with the results from the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. Initially, the merging inferred from the radar observations accounts for a significant portion of the total polar cap potential drop, suggesting that a majority of the potential drop was generated within the radar field of view and must therefore be due to magnetic merging at the magnetopause. At the end of the period, however, the potential drop derived from the radar measurements is distinctly less than that derived from the AMIE procedure. At that time, however, satellite and ground magnetometer data show that a substorm was in progress, and there is substantial evidence for a strong nightside contribution to the polar cap potential drop. An additional feature that appears in this data set is that the orientation of the open/closed magnetic field separatrix with respect to magnetic latitude is well correlated to the y component of the IMF.
Radial mass transport and rotational dynamics
Authors: Pontius Duane H.
Published: February 2004

The main source of plasma in the Jovian magnetosphere is Io and the dominant sink is outward radial transport. Conservation of angular momentum leads to a decrease in angular velocity with distance, but ionospheric coupling resists any departure from corotation. Competition between these effects predicts a radial dependence for the lag that agrees with observations out to some moderate distance in the middle magnetosphere. However, the observed angular velocity farther out appears to saturate at roughly half the planetary angular frequency, in contrast to the theoretical prediction that it continue to decrease asymptotically to zero. This paper reviews the theory of this phenomenon and explores three of its underlying assumptions as candidates for explaining this discrepancy: (1) Field line stretching from the assumed dipole configuration; (2) a variation in the ratio of the average angular velocity to the average weighted by mass outflux; and (3) a nonlinear response in the atmospheric component of the system.
Direct measurements of the helium photoionization rate from sounding rockets
Authors: Ogawa H. S. , Phillips E. , Judge D. L.
Published: February 2004

A method of directly measuring the absolute helium photoionization rate coefficient independent of detailed solar EUV spectra and photoionization cross sections has been demonstrated. Three such measurements were obtained using windowless rare‐gas ionization cells operated in an optically thin mode. The cells were launched on Nike/Terrier boosted Black Brant sounding rockets from the White Sands Missile Range in 1983, 1989, and 1990. The rate coefficients were determined for altitudes from 150 to 330 km and were extrapolated to give values of 11.1 × 10−8 s−1 ± 11%, 21.3 × 10−8 s−1 16%, and 13.2 × 10−8 s−1 ± 11% at the “top” of the Earth's atmosphere. The solar activity levels were characterized by absolute F10.7 values of 119, 172, and 142 (10−22 W m−2 Hz−1), respectively. The top of the atmosphere solar EUV flux values inferred from the measured ionization rates were found to be consistent with absolute flux measurements obtained simultaneously with a neon ionization cell operated in the optically thick mode.
Synchrotron evidence for Amalthea's influence on Jupiter's electron radiation belt
Authors: de Pater Imke , Schulz Michael , Brecht Stephen H.
Published: February 2004

High‐resolution radio images of Jupiter at wavelength λ = 20 cm obtained with the Very Large Array (VLA) in June 1994 (a few weeks before comet Shoemaker‐Levy 9 collided with the planet) are compared with detailed model calculations. All major features of the radio emission can be explained or simulated through model calculations. In particular, we infer how the pitch angle distribution of high‐energy electrons varies with L value. The electron pitch angle distribution seems to undergo a dramatic change at Amalthea's orbit: A fraction of the electron population is redistributed in pitch angle (isotropized) there, so that fewer electrons mirror near the magnetic equator and more electrons mirror off the equator at L ≲ 2.65 than beyond. The isotropic component leads to the high‐latitude emission regions, while the decreased number of equatorially mirroring electrons results in a “shoulder” or flattening in the radio intensity pattern at L ∼ 2.5, as is observed. Perhaps Amalthea's motion through Jupiter's magnetic field induces Alfvén or whistler wings or electrostatic high‐frequency waves which lead to the “observed” pitch angle scattering. Jupiter's ring absorbs 80%–100% of electrons with small pitch angles that diffuse through the region it occupies. The observed effect of this absorption is that the high‐latitude emission peaks remain distinct from the equatorial maximum. Ring absorption causes nearly all electrons at L ≲ 2 to be narrowly confined to the magnetic equator, a distribution which accounts for the east‐west asymmetry, which is very prominent at certain central meridian longitudes. The azimuthal variation (east‐west asymmetry) over a Jovian rotation is completely determined by the magnetic field configuration, as was suspected by many researchers in the past but never modeled succesfully before. We infer, however, that Connerney's O6 magnetic field model from 1992–1993 is slightly oversimplified, since the radiation characteristics cannot be completely matched at all Jovian longitudes: Deviations appear in particular at longitudes λcml ∼ 140°–180° and λcml ∼ 300°–340° (corresponding to λIII ∼ 30°–90° and λIII ∼ 210°–270° in Jovicentric coordinates).
Surface waves on the low‐latitude boundary layer inner edge and travelling convection vortices
Authors: Lyatsky W. B. , Sibeck D. G.
Published: February 2004

Surface waves on the inner boundary of the low‐latitude boundary layer (LLBL) are considered as a possible cause for dayside high‐latitude travelling convection vortices (TCVs). A dispersion equation for the surface waves with regard to their attenuation in the ionosphere is deduced. The current system of the surface wave is consistent with a pair of upward and downward field‐aligned currents. The wave velocity depends upon the wavelength; taking the latter to be equal to 1000 km in the ionosphere, which is typical for the TCVs, we obtain the velocity of these waves to be to about 50–100 km/s in the magnetosphere, which corresponds to several kilometers per second in the ionosphere. The surface waves on the LLBL inner boundary may be generated by a sharp deflection of this boundary from the equilibrium position. A cause of this may be sudden impulses of the solar wind dynamic pressure or bursty reconnection at the dayside magnetopause. Surface waves may play an important role in the relaxation of the magnetosphere. The velocity of the surface waves on the LLBL inner edge is close to or slightly less than the magnetosheath plasma velocity near the magnetopause; the movement of the magnetosheath plasma along the magnetopause with a speed more than that of the surface waves may favor the formation of impulsive TCVs. It follows from the considered mechanism that the disturbance maximum is to be observed on the LLBL inner edge, and the TCV velocity has to increase with the wavelength that might be verified from experimental data.
Global perspective of ionospheric traveling convection vortices: Case studies of two Geospace Environmental Modeling events
Authors: Moretto T. , Friis‐Christensen E. , Lühr H. , Zesta E.
Published: February 2004

Two very different events of high‐latitude ionospheric traveling convection vortices (TCV) are studied on the basis of a very broad and dense collection of groundbased magnetometer data. A new display technique is introduced and used for the analyses. Many of the previously reported features of these events are confirmed, but some new features are revealed. Both events exhibit pronounced postnoon/prenoon asymmetry. Much substructure and complex dynamics of the vortex structures are also evident. One of the events is observed to be closely related to a very sudden and strong change in the large‐scale polar ionospheric convection system. The difference between the two events, not only in this point, indicates the existence of at least two classes of TCV events, the distinction of which requires global analysis.
Ions and electrons of the lower‐latitude D region
Authors: Kull A. , Kopp E. , Granier C. , Brasseur G.
Published: February 2004

A positive and negative ion chemical steady state model which combines about 400 ion‐chemical and photochemical reactions with a two‐dimensional neutral model for minor atmospheric constituents is presented in the height range 50–84 km, for latitudes −50° to +50° in January and September. The ion model yields concentrations of the main positive and negative ions and of electrons. Two important parameters for the description of the lower ionosphere were deduced, namely, the ratio λ of electrons to negative ions and the ƒ+ parameter, describing the ratio of the positive cluster ions to positive molecular ions. In addition, the parameterization of ƒ+ indicates a strong dependency on the water vapor concentration. At the transition height (ƒ+ = 1) at midlatitudes the water concentration can be expressed as a function of the electron density and the temperature: [H2O] = 1.09 × 10−6[ρ]0.6T−3[e], with the units of the water vapor density [H2O], the air density [ρ], and the electron concentration [e] in cm−3 and T in degree kelvin.
Coherent structures in the magnetotail triggered by counterstreaming electron beams
Authors: Mottez F. , Perraut S. , Roux A. , Louarn P.
Published: February 2004

We describe wave observations carried out on board Galileo during the crossing of a flux rope in the Earth magnetotail. Waveform observations give evidence for transient electrostatic structures. Numerical simulations of the evolution of counterstreaming electron beams with a core of hotter ions have been performed with parameters consistent with Galileo observations during the flux rope crossing. A strongly unstable mode, with a zero real part of the frequency is found to develop and to lead to the generation of coherent electrostatic structures with velocities between ±υt, the electron thermal velocity. The characteristics of the simulated structures are shown to be similar to those observed in the magnetotail.
An analysis of tidal and planetary waves in the neutral winds and temperature observed at low‐latitude E region heights
Authors: Zhou Qihou H. , Sulzer Michael P. , Tepley Craig A.
Published: February 2004

We present an extensive analysis of tidal and planetary waves in the altitude range of 94 to 144 km for the January 20–30, 1993, period using the temperature and winds measured by the Arecibo incoherent scatter radar (ISR). This is the first time that simultaneous observational results for the 6–8, 12, 24 hour tides and a quasi 2‐day planetary wave at E region heights have been reported at tropical latitudes. In order to derive the major oscillations from the mainly daytime data, we fill in the nighttime periods with assumed data values and large error bars when valid measurements are not obtainable and then fit the data using the measured and assumed errors as weight. Simulated results show that such a method is demonstratively better than leaving the nighttime as a gap. The salient features of the tidal analysis results include the following: (1) Despite their large day‐to‐day variabilities in amplitude, the 6–8 hour oscillations are shown to be upward propagating tides. (2) The vertical wavelength of the semidiurnal tide in the zonal wind lengthens from 25 to 110 km for the altitude range from 94 to 135 km while the vertical wavelength of the meridional wind remains constant. The semidiurnal component of the meridional wind experiences little dissipation above 106 km, which is indicative of the turbopause height. (3) The diurnal tide, which has often been assumed to be negligible in earlier observations, can be quite substantial above 110 km. (4) A 2‐day planetary wave dominates the diurnal and semidiurnal tides in the meridional wind between 97 and 108 km. Continuous ISR operation during the January 20–30, 1993, campaign also provides an opportunity to examine the tidal variability both in phase and amplitude.
Survey of flux transfer events observed with the ISEE 1 spacecraft: Dependence on the interplanetary magnetic field
Authors: Kawano H. , Russell C. T.
Published: February 2004

We have surveyed the interplanetary magnetic field (IMF) dependence of isolated bipolar perturbations of the magnetic field component normal to the magnetopause, identifiable as flux transfer events (FTEs), from nine seasons during which the ISEE 1 spacecraft completely precessed through the dayside magnetosheath and magnetosphere. The majority of subsolar FTEs are associated with southward IMF, consistent with previous studies. The ratio of the number of FTEs during spiral IMF orientation to that during ortho‐spiral IMF is roughly the same at any MLT, an unfavorable finding for the foreshock being the source of our FTEs. The curvature force controlled by IMF By tends to affect significantly the motion of FTEs near noon, causing sunward moving FTEs. This behavior is consistent with reconnection being the source of our events. The totality of these results suggest that the foreshock is not the source of FTEs on the dayside magnetopause but that reconnection at the magnetopause is the source.
Radio interferometer measurements of plasmasphere density structures during geomagnetic storms
Authors: Hoogeveen Gary W. , Jacobson Abram R.
Published: February 2004

The Los Alamos plasmaspheric drift radio interferometer is a ground‐based array that regularly measures periodic disturbances in the plasmasphere. These plasmaspheric density structures have been shown to depend on geomagnetic activity, as indicated by Kp. However, a direct storm time analysis of their behavior has not been done. This paper studies the amplitude, drift velocity, and location of these structures before, during, and after the onset of major geomagnetic storms. Distinct large‐amplitude, storm time signatures are found during the first night after onset, continuing through the third night; there were significantly more storm time signatures during nighttime than daytime. The L shells on which the disturbances existed were found to decrease after storm onset, indicating a possible shrinking of the plasmasphere.
A climatology of F region gravity wave propagation over the middle and upper atmosphere radar
Authors: Oliver W. L. , Otsuka Y. , Sato M. , Takami T. , Fukao S.
Published: February 2004

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere radar, we have been able to track the passage of gravity waves and measure their propagation characteristics. Here we develop a climatology of wave propagation based on the observation of 58 daytime experiments conducted during 1986–1994. The thermosphere seems to be continuously swept by waves detectable by an incoherent scatter radar. These waves generally come for hours on end from a consistent or slowly varying direction, which can be any direction on a given day. Statistically, the waves show a moderate preference for southward travel, with this preference being reduced or shifted to southeastward travel during disturbed times. On average, the horizontal phase trace speed remains near 240 m/s for all periods inspected (40–130 min). This speed matches the behavior expected for lossless waves with 150–200 km vertical wavelength. We find small variability in this relation for different times of day, seasons, solar and magnetic conditions, and directions of wave travel, though waves on disturbed days seem to travel moderately faster on solar minimum mornings.
Super Dual Auroral Radar Network observations of meteor echoes
Authors: Hall G. E. , MacDougall J. W. , Moorcroft D. R. , St.‐Maurice J.‐P. , Manson A. H. , Meek C. E.
Published: February 2004

Radar echoes from ranges less than 500 km are routinely observed by the Super Dual Auroral Radar Network (SuperDARN) on most days. Many of these echoes have properties which are markedly different from what one would expect from E or F region irregularities. We show that these unusual short‐range HF echoes are due to scattering off meteor trails. This explains why, among other things, the Doppler shift from the short‐range echoes taken from the SuperDARN Saskatoon antenna are consistent with the mesospheric winds observed by the Saskatoon MF radar. This means that the SuperDARN radars can be used to study neutral winds at meteor heights, a result which is especially interesting since it opens up the capability for a global coverage of mesospheric winds using the worldwide distribution of SuperDARN radars.