bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear competition between the whistler and Alfvén fire hoses
Authors: Hellinger P. , Matsumoto H.
Published: February 2004

We examine a competition between the whistler and Alfvén fire hoses driven by bi‐Maxwellian protons with Tp‖ > Tp⊥, where Tp‖ and Tp⊥ are proton temperatures, parallel and perpendicular to the background magnetic field, respectively. We extend the work of Hellinger and Matsumoto [2000] using a two‐dimensional hybrid simulation that includes both the instabilities. In the simulation the whistler fire hose initially dominates and saturates in a quasi‐linear manner. The Alfvén fire hose is not strongly affected by the presence of the whistler fire hose and grows even when the former is saturated. However, as the Alfvén fire hose grows and saturates via a conversion to Alfvén waves that heat the protons, the waves driven by the whistler fire hose get strongly damped and disappear. The Alfvén waves damp as well, so that at the end of the simulation the wave activity is low. The strong decay of the generated waves translates to an important decrease of the proton temperature anisotropy.
A global MHD and empirical magnetic field model investigation of the magnetospheric cusp
Authors: Fenrich F. R. , Luhmann J. G. , Fedder J. A. , Slinker S. P. , Russell C. T.
Published: February 2004

Magnetic field configurations, cusp locations, merging, and interconnection regions for the Lyon‐Fedder global magnetohydrodynamic (MHD) simulation model and the Tsyganenko 96 (T96_01) empirical model are presented and compared for three different interplanetary magnetic field (IMF) directions (north, south, and dusk directed). Field line configurations indicate a significantly larger magnetosphere in the T96_01 model compared to the MHD model for all IMF directions. In each of the models the polar cusp is identified by depressions in the magnetic field residual and by the boundary between dayside closed and open field lines. It is found that the MHD model exhibits a much stronger residual magnetic field depression in the cusp region than does the T96_01 model. The MHD field depression is associated with the presence of diamagnetic magnetosheath plasma that enters the cusp via the MHD merging process. In the northward IMF case, MHD merging above both poles results in an MHD open‐closed field line boundary located at 89° invariant latitude with MHD cusp plasma and field depressions centered on closed field lines near 81°. The T96_01 model is very different from this with its north IMF open‐closed boundary located at 79° invariant latitude. For the southward IMF case the T96_01 open‐closed boundary at 74° invariant latitude lies slightly poleward of the MHD open‐closed boundary at 73°. In this case the MHD cusp plasma is centered near 75° invariant latitude. In the dusk‐directed IMF case the MHD cusp plasma shifts ∼2 hours in magnetic local time (MLT) toward dusk and remains poleward of the MHD open‐closed boundary. In this case the MHD dayside open‐closed boundary extends from dawn to dusk, while the T96_01 dayside open‐closed boundary extends only from dawn to ∼13 MLT and near local noon is coincident with the MHD boundary at ∼74° invariant latitude. Differences between the two models are due in large part to differences in the form of their interconnectivity with the interplanetary magnetic field lines. For each IMF direction the MHD merging locations and T96_01 strongest interconnection regions are identified and are found to be very different. On the basis of comparisons with numerous case and statistical studies of the cusp it is concluded that the MHD model provides a fairly realistic representation of the cusp and dayside magnetic field configuration, while the T96_01 model is limited in its ability to model the cusp region.
Correlation studies of compressional Pc5 pulsations in space and Ps6 pulsations on the ground
Authors: Vaivads A. , Baumjohann W. , Georgescu E. , Haerendel G. , Nakamura R. , Lessard M. R. , Eglitis P. , Kistler L. M. , Ergun R. E.
Published: February 2004

Compressional Pc5 pulsations in space and Ps6 pulsations on the ground are common features observed in the morning sector. Here we use a conjunction study of Equator‐S, Geotail, and ground stations in Canada to show that Ps6 pulsations can be the ground counterpart of compressional Pc5 pulsations observed by satellites. Because strong Ps6 pulsations are associated with optical omega‐band signatures, we also suggest that the omega‐band counterparts in space might be compressional pulsations on the Pc5 scale. We also discuss the magnetic field configuration that makes all these observations consistent.
Excitation of low‐frequency fluctuations at the magnetopause by intermittent broadband magnetosheath waves
Authors: De Keyser J. , Čadež V.
Published: February 2004

We study the excitation of magnetic field fluctuations at the magnetopause by incident low‐frequency waves (frequencies below the ion gyrofrequency) that are highly intermittent and that have a broad frequency spectrum, like those observed in the magnetosheath. We present simulations of the transient response of the subsolar magnetopause to such magnetosheath waves in the context of linear resistive magnetohydrodynamics (MHD) in low‐beta plasmas. The incident waves excite local Alfvén waves in resonant sheets within the magnetopause transition region wherever the condition for resonant MHD wave mode conversion is satisfied. Resonant sheet locations depend on the dominant frequencies in the incident wave spectrum. Because of the intermittency of the incident waves, the resonant sheets are rarely driven to saturation. The number of resonances, their position, their thickness, and the resonant wave amplitude all vary with time in response to the temporal changes in the incident wave spectra. The net result is that the low‐frequency fluctuation level at the magnetopause is enhanced throughout the transition region rather than in a single or in a few resonant sheets, while the resonant amplitudes never reach excessively high values. This also implies that a fraction of the incident wave energy is resonantly absorbed throughout the magnetopause layer, instead of the very local absorption associated with an incident monochromatic wave. Resonances never develop into very narrow length scale structures, so that the MHD approach remains justified.
Estimating superparamagnetism in marine sediments with the time dependency of coercivity of remanence
Authors: Smirnov Alexei V. , Tarduno John A.
Published: February 2004

Superparamagnetic grains, ultrafine (<50 nm) particles that do not retain a magnetic remanence at room temperature, can form through several biogeochemical pathways in marine sediments. Knowing the distribution of such particles therefore provides a means of better understanding the past diagenetic history of sediments and evaluating their potential record of environmental change. Herein we employ magnetic hysteresis measurements in a new way to determine the abundance of superparamagnetic (SP) grains. We start with a phenomenological model that describes a dependency of coercivity of remanence (Hcr) with measurement time as a function of superparamagnetic grain content. Calculated time dependencies of Hcr agree with experimental data, suggesting that the model can be used to derive quantitative SP estimates. When applied to pelagic sediments of the western equatorial Pacific Ocean (Ocean Drilling Program Site 805C), the time dependencies of Hcr reveal a 25–30% SP increase starting just above the modern iron redox boundary (IRB). Next, we compare our approach with a previous method that detects SP grains through the thermal demagnetization of saturation remanence acquired at very low temperature (20 K). We find this method fails to record the SP changes detected using the time dependency of Hcr and suggest that the cause is a sensitivity of the low‐temperature data to small differences in magnetic mineralogy that accompany diagenesis. Specifically, differential maghemitization causes varying suppression of the remanence changes associated with the Verwey transition at ∼120 K. The removal of maghemite shells on magnetic grains below the IRB results in greater remanence changes at the Verwey transition and a corresponding SP underestimate using the low‐temperature method. In addition, some iron oxyhydroxides paramagnetic at room temperature can become ferromagnetic at very low temperatures, resulting in an SP overestimate in oxic sediments above the IRB. The increase of SP content in the zone of magnetic mineral reduction observed with the time dependency of Hcr may be related to the breakdown of larger magnetic grains and the activity of dissimilatory iron‐reducing bacteria. Superparamagnetism measured by the time dependency of Hcr may be useful for tracking the history of these biogeochemical processes in marine sediments.
Mantle viscosity from Monte Carlo inversion of very long baseline interferometry data
Authors: Spada G.
Published: February 2004

Using a Monte Carlo approach, we determine the mantle viscosity profiles which best fit the currently observed very long baseline interferometry (VLBI) baseline lengths in North America. Our inversion, which is based on the deglaciation model ICE‐1 and on the most recent VLBI global solution (we employ the Goddard Space Flight Center VLBI solution 1102, which contains data through July 1998 (Ma and Ryan, 1998)), demonstrates that the observations are best fit for a shallow upper mantle with viscosity below the traditional reference value of 1021 Pa s. The lower mantle viscosity cannot be unequivocally determined by means of VLBI data alone. In fact, our inversion indicates that two distinct solutions are possible. The first is characterized by a lower mantle with viscosity of a few times 1021 Pa s, a value in agreement with that derived by the global set of relative sea level curves. The second admissible solution corresponds to a lower mantle with viscosity close to 1023 Pa s. We have found that the viscosity of the transition zone may vary between 5×1021 and 5×1022 Pa s, i.e., larger than the lowest admissible values found for the lower mantle viscosity. The results of the Monte Carlo inversion are essentially stable with respect to time, since similar viscosity profiles are suggested if the VLBI global solution 1014 is employed, which contained data through July 1995. However, the increased precision of VLBI data since that time has somewhat narrowed the set of admissible viscosity profiles above the 670‐km depth discontinuity. Choosing various subsets of solution 1102, we have also tested the robustness of the results presented, with reference to possible data contaminations due to tectonic deformations.
Modeling surface deformation observed with synthetic aperture radar interferometry at Campi Flegrei caldera
Authors: Lundgren P. , Usai S. , Sansosti E. , Lanari R. , Tesauro M. , Fornaro G. , Berardino P.
Published: February 2004

Satellite radar interferometry of Campi Flegrei caldera, Italy, reveals a pattern of subsidence during the period 1993–1998. Interferograms spanning the first half of the observation period (1993–1995) have a lower amplitude and average rate of subsidence than those spanning either the second half (1995–1998) or the entire period (1993–1998), consistent with observations of a slowing down or reversal of subsidence during the first half of the observation period. We calculate a time series of deformation images relative to a reference image on the basis of a least squares inversion. During the observation period the maximum subsidence progresses at a rate of roughly 38±2 mm/yr, with periods of no apparent subsidence in late 1996 to early 1997. To understand the characteristics of the source, we jointly invert pairs of ascending and descending differential interferograms spanning similar time intervals (first half, second half, or entire interval) of the period 1993–1998. In each case the joint inversion fits the two unwrapped interferograms with a similar subhorizontal rectangular contracting tensile dislocation striking roughly N98°E with dimensions ∼4×2 km and located beneath the city of Pozzuoli at a depth of 2.5–3 km. Inversion for a spheroidal or Mogi point source also produced reasonable fits but with progressively poorer overall fits to the data, respectively. Our inversion assuming a simple source in an elastic half‐space does not include the possible effects of local structure on the surface deformation, a factor that may also reduce the need for an asymmetric source. The solution we find is consistent with other studies that suggest subsidence due to hydrothermal diffusion as the primary deformation mechanism during this phase of caldera deflation.
Origin of lateral variation of seismic wave velocities and density in the deep mantle
Authors: Karato Shun‐ichiro , Karki Bijaya B.
Published: February 2004

Strong constraints can be placed on the origin of heterogeneity of seismic wave velocities and density if the observed ratios of various parameters are compared with mineral physics predictions. They include the shear to compressional wave velocity heterogeneity ratio,Rs/p≡ δ logVs/δ logVp, the bulk sound to shear wave velocity heterogeneity ratio,Rϕ/s≡ δ logVϕ/δ logVs, and the density to velocity heterogeneity ratio,Rρ/s,p≡ δ log ρ/δ logVs,p. Using mineral physics considerations, we calculate these ratios in the lower mantle corresponding to the thermal and chemical origin of velocity and density heterogeneity. Both anharmonic and anelastic effects are considered for thermal origin. Anharmonic effects are estimated from the theoretical calculations as well as from laboratory measurements which show a marked increase inRs/pwith pressure from ∼1.5 to ∼2.1 in the lower mantle. Such a trend is marginally consistent with seismological observations showing an increase inRs/pwith depth (from ∼1.7 to ∼3.2 in the lower mantle). However, anharmonic effect alone cannot explain inferred lowRρ/s(<0.2–0.4), and the values ofRs/ppredicted from anharmonic effects are systematically smaller than seismological observations. Anelastic effects must be included to account for these geophysical observations. A new formulation of anelastic effect is developed in which the role of nonlinear temperature dependence of anelasticity is included. When anharmonic and anelastic effects are combined, thermal effects on velocity and density can account for most of the geophysical observations in Earth's lower mantle. Effects other than temperature variations (such as chemical heterogeneity) are required only in the very deep portions of the mantle (deeper than ∼2000–2300 km). A puzzling observation is very large values ofRs/p(>2.7) and corresponding negative values ofRϕ/s(andRρ/s) in the deep lower mantle which cannot be accounted for by thermal or simple chemical heterogeneity such as the heterogeneity in the Fe/(Fe + Mg) and/or Mg/(Mg + Si) ratios. Possible causes of anomalies in this region are discussed, including the role of anisotropy and a combined effect of heterogeneity in Fe and Ca content.
Marginal stability of atmospheric eruption columns and pyroclastic flow generation
Authors: Kaminski Édouard , Jaupart Claude
Published: February 2004

Explosive volcanic eruptions frequently generate fall and flow deposits simultaneously, which can be attributed to a marginally stable atmospheric column in transitional conditions between the buoyant and collapse regimes. This behavior is reproduced by laboratory experiments and numerical simulations. Ten well‐documented eruptions are used to test theoretical models of explosive eruptions. Three types of deposits, fall, flow, and composite deposits made of intercalated flow and fall units, are observed in these eruptions. Estimates of mass discharge rate and initial volatile concentration in the magma are available for each eruptive phase. Using the simple assumptions that (1) the mass fraction of gas in the mixture is equal to the initial volatile content of magma and (2) jet expansion outside the vent is unconstrained by crater dimensions, theoretical predictions are not consistent with the data. Agreement between data and theory may be achieved by appealing to imperfect degassing of pyroclasts, which lowers the gas content of the erupted mixture. The effective amount of continuous gas phase carrying pyroclasts in suspension depends on the size distribution of pyroclasts. In coarse pyroclast populations a large amount of magmatic gas remains trapped in bubbles within the pyroclasts and is not involved in the bulk volcanic flow. A new regime diagram based on estimates of the effective gas content in the erupted mixture allows good agreement with the observations. For given mass flux and initial dissolved volatile content, changes of the size distribution of pyroclasts may have a strong effect on atmospheric column behavior.
Crustal thickness of V‐shaped ridges south of the Azores: Interaction of the Mid‐Atlantic Ridge (36°–39°N) and the Azores hot spot
Authors: Escartín Javier , Cannat Mathilde , Pouliquen Gaud , Rabain Aline , Lin Jian
Published: February 2004

V‐shaped ridges propagating along the Mid‐Atlantic Ridge axis south of the Azores and Iceland hot spots indicate that ridge‐hot spot interactions produce temporal and spatial variations in melt supply to the ridge axis. Estimates of relative crustal thickness variations associated with the ridges south of the Azores hot spots, based on gravity and bathymetry data collected during the SudAçores cruise (1998), provide constraints on the rate of propagation of these melt anomalies and on the variations in melt production along the axis and in time. The maximum apparent crustal thickness along the Azores V ridge is ∼14 km near the Azores, decreasing to normal crustal thickness of ∼6 km toward the south. This crustal thickness variation may be explained by enhanced melt production associated with the propagation of a mantle temperature anomaly that initiated ∼10 Myr ago at the Azores hot spot. The temperature anomaly decreased as it propagated southward, reaching ambient mantle temperatures at the present time at its predicted location under the axis. The excess melt was emplaced on axis forming discrete, shallow (<1000 m) oceanic plateaus (∼100 km in diameter at ∼37.5°N) that are isostatically compensated. The numerous seamounts, lack of normal faults, and smooth basement at the summit of these plateaus suggest high effusion rates that persisted for ∼5 Myr or less, with little or no tectonic strain. As the melt anomaly propagated along axis, the magmatic activity at the plateaus ceased, resulting in rifting of the plateau and onset of normal seafloor spreading. The variations in crustal thickness inferred for the V ridges south of the Azores are at least twice that inferred for the Iceland structures. In both cases the V ridges record temporal variations in temperature and/or mantle flux that affect melt production under the ridge axis, but the fluctuations are larger for the Azores than for the Iceland hot spot.
Superelevation of flowing avalanches around curved channel bends
Authors: McClung D. M.
Published: February 2004

Flowing avalanches are those with a dense core of flowing granular material at the base, which dominates the dynamics causing friction. Sometimes when avalanches descend, they encounter curved channel bends, which cause the flowing material to superelevate or climb up on the wall of the channel bend so that the level of the flowing snow is higher on the outside of the bend than at the center of the channel. This can provide important information about avalanche speeds. The conventional approach for estimating speeds is mathematically equivalent to one developed from equations based on fluid mechanics with a balance between fluid pressure force and centrifugal force to yield a simple expression which relates channel radius, avalanche speed, and superelevation geometry. In this paper, the conventional theory is replaced by one which relates avalanche speed to basal friction (as appropriate for flowing snow), passive snow pressure (accounting for friction between snow particles), channel radius, and superelevation geometry. It is shown that the conventional theory is physically unrealistic for flowing snow modeled as a frictional material. Also, the conventional theory will result in lower speed estimates than the new formulation for the same amount of superelevation.
Fluctuations of the paleomagnetic field during chron C5 as recorded in near‐bottom marine magnetic anomaly data
Authors: Bowers Nancy E. , Cande Steven C. , Gee Jeffrey S. , Hildebrand John A. , Parker Robert L.
Published: February 2004

Near‐bottom magnetic data contain information on paleomagnetic field fluctuations during chron C5 as observed in both the North and South Pacific. The North Pacific data include 12 survey lines collected with a spatial separation of up to 120 km, and the South Pacific data consist of a single long line collected on the west flank of the East Pacific Rise (EPR) at 19°S. The North Pacific magnetic profiles reveal a pattern of linear, short‐wavelength (2 to 5 km) anomalies (tiny wiggles) that are highly correlated over the shortest (3.8 km) to longest (120 km) separations in the survey. Magnetic inversions incorporating basement topography show that these anomalies are not caused by the small topographic relief. The character of the near‐bottom magnetic profile from anomaly 5 on the west flank of the EPR, formed at a spreading rate more than twice that of the North Pacific, displays a remarkable similarity to the individual and stacked lines from the North Pacific survey area. Over distances corresponding to 1 m.y., 19 lows in the magnetic anomaly profile can be correlated between the North and South Pacific lines. Modeling the lows as due to short polarity events suggests that they may be caused by rapid swings of the magnetic field between normal and reversed polarities with little or no time in the reversed state. Owing to the implausibly high number of reversals required to account for these anomalies and the lack of any time in the reversed state, we conclude that the near‐bottom signal is primarily a record of paleointensity fluctuations during chron C5. Spectral analysis of the North Pacific near bottom lines shows that the signal is equivalent to a paleointensity curve with a temporal resolution of 40 to 60 kyr, while measurements of the smallest separations of correctable dips in the field suggest a temporal resolution of 36 kyr.
Seismic wave observations with the Global Positioning System
Authors: Nikolaidis Rosanne M. , Bock Yehuda , de Jonge Paul J. , Shearer Peter , Agnew Duncan Carr , Van Domselaar Matthijs
Published: February 2004

We describe the direct measurement of ground displacement caused by the Hector Mine earthquake in southern California (Mw 7.1, October 16, 1999). We use a new method of instantaneous positioning, which estimates site coordinates from only a single epoch of Global Positioning System (GPS) data, to measure dynamic as well as static displacements at 24 stations of the Southern California Integrated GPS Network (SCIGN), with epicentral distances from 50 to 200 km. For sites outside the Los Angeles basin the observed displacements are well predicted by an elastic half‐space model with a point shear dislocation; within the sedimentary basin we observe large displacements with amplitudes up to several centimeters that last as long as 3–4 min. Since we resolve the GPS phase ambiguities and determine site coordinates independently at each epoch, the GPS solution rate is the same as the receiver sampling rate. For the SCIGN data this is 0.033 Hz (once per 30 s), though sample rates up to 2 Hz are possible with the SCIGN receivers. Since the GPS phase data are largely uncorrelated at 1 s, a higher sampling rate would offer improved temporal resolution of ground displacement, so that in combination with inertial seismic data, instantaneous GPS positioning would in many cases significantly increase the observable frequency band for strong ground motions.
Unmixing magnetic assemblages and the magnetic behavior of bimodal mixtures
Authors: Carter‐Stiglitz Brian , Moskowitz Bruce , Jackson Mike
Published: February 2004

Stable single‐domain (SSD) grains were mixed separately with superparamagnetic, pseudosingle‐domain, and multidomain (MD) magnetite/maghemite particles in order to test the linearity of various magnetic parameters as a function of mixing ratio. Hysteresis loops, isothermal remanent magnetization acquisition curves, DC demagnetization curves, and low‐temperature thermal demagnetization curves were measured on the mixtures. The experiments demonstrate that magnetization parameters are linearly dependent on the mixing ratio, while more complex parameters, e.g., coercivities, do not behave linearly as a function of mixing ratio. Armed with linearity, we apply a mathematical technique which, given a database of type curves, uses singular value decomposition to solve for the various concentrations of the magnetic phases in the mixture and a Monte Carlo simulation to determine the error in the inversion. We then test the technique on numerical mixtures, on the physical mixtures, and on a small set of natural samples from Lake Pepin, Minnesota. Finally, the magnetic behavior of the mixture of MD and SSD grains is considered, and two more mixture strains of MD and SSD grains (numerically produced) are considered to facilitate this discussion.
Fault slip rates and initiation age based on diffusion equation modeling: Wasatch Fault Zone and eastern Great Basin
Authors: Mattson Ann , Bruhn Ronald L.
Published: February 2004

Models of the evolution of fault scarp morphology provide time elapsed since slip initiated on a faulted surface and may therefore provide more accurate estimates of slip rate than the rate calculated by dividing scarp offset by the age of the ruptured surface. To accomplish this task, linear and nonlinear models of sediment transport are calibrated from the morphology of Lake Bonneville shoreline scarps and fault scarps formed by multiple, surface‐rupturing earthquakes along the Wasatch Fault Zone (WFZ). Profile modeling of scarps formed by several events distributed through time is done using a constant slip rate (CSR) solution and yields a value ofA/κ (1/2 slip rate/diffusivity). Time elapsed since slip initiated on a fault is determined by establishing a value for κ and measuring total scarp offset. CSR nonlinear modeling (κ0= 2.8 ± 1.1 m2/kyr, WFZ) of faults along the west slope of the Oquirrh Mountains indicates a slip rate of ∼0.1 mm/yr since 50 to 65 ka, which is corroborated by cosmogenic dating (10Be/26Al age = 75 ka). The slip rate along the west flank of the Stansbury Mountains varies from 0.04 to 0.2 mm/yr for time frames of 10 to >100 ka, with the most recent rupture on the northern portion of the fault zone ∼10 ka. Scarp analysis of the southern end of the Nephi segment, WFZ, suggests either temporal clustering or variable slip rate as indicated by differences in the short‐term (1.3 mm/yr for 4.3 ka) versus long‐term (0.4 mm/yr for 70 ka) slip rates.
Impact of variable inflow on the dynamics of a coastal buoyant plume
Authors: Yankovsky Alexander E. , Hickey Barbara M. , Münchow Andreas K.
Published: February 2004

The impact of buoyant discharge variations on the dynamics of coastal buoyancy‐driven currents is studied using a primitive equation numerical model (SPEM5). First, variable discharge is introduced as harmonic fluctuations of the inflow velocity at the tidal (period 12 hours) and subinertial (period 10 days) frequencies. Tidal fluctuations produce only minor effects on the buoyant plume compared to the case of constant inflow, while subinertial fluctuations substantially modify the anticyclonic bulge. A partially detached anticyclonic plume forms when discharge subsides after reaching its peak value. Such a plume has maximum offshore extension some distance downstream of the mouth with the lightest water separated from the coast. A secondary bulge forms during the low runoff interval. When high discharge resumes, this secondary bulge is shifted offshore and enhanced for some time. An individual high‐discharge event is next considered, where both the net transport of the inflow and the absolute value of its density anomaly increase and then return to their initial (background) values over 5 and 10 day time intervals. This event also generates a partially detached plume (especially with the 10 day duration). In this case, the lightest water occupies the downstream part of the bulge and is separated not only from the coast but also from the mouth. The effect of variable discharge is more dramatic with a uniform downstream current of 0.1 m s−1. Under such conditions, constant buoyant discharge does not form a well‐pronounced anticyclonic bulge. In contrast, variable discharge produces an almost circular anticyclone during the high‐runoff interval. As runoff decreases, this anticyclone separates from the source and either continues to propagate downstream as an individual eddy or is modified by the next cycle of increasing discharge. Observational evidence for both the partially detached bulge near the mouth and the anticyclone propagating downstream from its source is presented in this study. One feature was observed at the mouth of the Columbia River estuary; the second feature was observed off the southern New Jersey coast ∼150 km south of its source, the Hudson estuary.
On the subsurface CFC maxima in the subtropical North Pacific thermocline and their relation to mode waters and oxygen maxima
Authors: Mecking Sabine , Warner Mark J.
Published: February 2004

The spatial structure of the subsurface chlorofluorocarbon (CFC) concentration maxima in the subtropical North Pacific is examined using data from the Pacific Ocean sector of the World Ocean Circulation Experiment (WOCE) Hydrographic Program collected in the period 1991–1994. Comparison with potential vorticity (PV) sections reveals that the CFC maxima coincide with the PV minimum associated with Subtropical Mode Water (STMW) in the western Pacific as suggested by earlier observations [Warner et al., 1996]. In the eastern part of the central Pacific the CFC maxima appear to be shallower than the Central Mode Water (CMW) while they are deeper than the Eastern Subtropical Mode Water in the eastern Pacific. A subsurface oxygen maximum also coincides with STMW and the CFC maxima in the west, is located between the CFC maxima and CMW in the central Pacific, and is absent in the east. The collocation in the west is explained by the fact that STMW is the densest water to outcrop in the recirculation to the south of the Kuroshio and the Kuroshio Extension while denser water is not directly ventilated in this region. Data from two post‐WOCE cruises and results from a steady state age model indicate that the CFC maxima are transient and are deepening such that the collocation of the CFC maxima and STMW does not persist in the late 1990s. Model experiments based on a smooth age profile demonstrate that the deepening of the CFC maxima is closely related to changes in the atmospheric CFC growth rates.
On the dynamics of internal waves in a nonlinear, weakly nonhydrostatic three‐layer ocean
Authors: Rubino Angelo , Brandt Peter , Weigle Rainer
Published: February 2004

Aspects of the dynamics of internal solitary waves evolving in a three‐layer ocean are investigated using a new numerical multilayer model that solves the nonlinear, weakly nonhydrostatic Boussinesq equations and uses high‐resolution in situ data. The model applications refer to two different phenomena frequently observed in the real ocean, which can be described using a three‐layer model rather than a two‐layer model. In the first application the influence of the strength of a shallow seasonal thermocline superimposed on a two‐layer permanent stratification on the structure of internal solitary waves is studied. It is found that while for small to medium wave amplitudes a decrease in the strength of the thermocline yields an increase in the simulated wavelengths, for large wave amplitudes this dependence is no longer monotonic. In particular, in the limiting case of a vanishing thermocline, first‐mode internal solitary wave solutions of the three‐layer numerical model tend to the analytical internal solitary wave solutions of the Miyata equations, a two‐layer model, in which the full nonlinearity of the shallow‐water theory up to first‐order phase dispersion is retained. In the second application that refers particularly to high‐resolution observations made north and south of the Strait of Messina in the Eurafrican Mediterranean basin the generation of internal solitary waves by the evolution of surface and subsurface water jets is investigated. The analysis of the in situ data shows in fact that from very energetic surface and subsurface jet‐like disturbances subject to strong turbulent mixing internal solitary waves emerge as robust, quasi‐nondissipative oceanic features. Idealized flow conditions aimed at approximating possible initial stages of the observed water jets are imposed to force our numerical model. In general, good agreement is found between characteristics of observed and simulated wave fields. Our investigation identifies the observed water jets as peculiar features of the complex ocean dynamics and suggests that layered numerical models can represent helpful tools in understanding fundamental processes inherent in their intricate dynamics.
Local and remote forcing of sea surface temperature in the coastal upwelling system off Chile
Authors: Hormazabal Samuel , Shaffer Gary , Letelier Jaime , Ulloa Osvaldo
Published: February 2004

An analysis is presented of records of sea surface temperature (SST), sea level and winds collected during the period 1991–1995 along the coast of northern and central Chile (sea level also from the Peru coast in 1991–1994), and of SST from remote sensing during the austral summers of 1991 and 1992 off northern Chile. Large, interannual fluctuations of SST and sea level off northern Chile are linked to vertical displacements of the equatorial thermocline during El Niño‐La Niña events. In the intraseasonal, 50‐day band, SST and sea level are intimately related all along the coast, whereby SST anomalies lag sea level perturbations by about 12 days. Such sea level fluctuations are due to free, coastal‐trapped waves, ultimately forced via equatorial Kelvin waves by 50‐day wind events in the western and central tropical Pacific. These results and those of the remote sensing analysis suggest significant remote forcing of SST fluctuations in the coastal zone off Chile by the action of the coastal‐trapped waves. Off central Chile, SST is also related to local wind stress in the intraseasonal band. Simulations with a simple, local, wind‐driven upwelling/entrainment model capture rather well observed synoptic and intraseasonal oscillations of SST there. Off northern Chile, however, this simple model is not successful in simulating observed SST. An attempt to include effects of coastal‐trapped waves into this simple, one‐dimensional model did not improve model fit to data at either location. An explanation for the apparent strong influence of these waves on SST along this coast should therefore be sought in other effects not considered by this model such as cross‐shelf and alongshore advection.
Proton precipitation during magnetic storms in August through November 1998
Authors: Walt M. , Voss H. D.
Published: February 2004

High angular resolution measurements of 155 keV protons were made from the Polar satellite during five magnetic storms in the last half of 1998. Proton precipitation was detected during all the storms studied, and in most cases protons were scattered deep into the loss cone. In one case of strong pitch angle scattering, the fluxes were isotropic for equatorial pitch angles less than 25°. During the main phases of the storms precipitation was strongest at L > 5 on the night side of the Earth, possibly due to field‐line curvature scattering of the newly injected ions. During the main and early recovery phases intense precipitation was found in the early afternoon at L > 4 where ring current models predict electromagnetic ion cyclotron waves will be generated by ring current ions. In one storm the nightside precipitation exhibited a maximum near L = 5, and this maximum moved outward during the recovery phase. In most storms the precipitation became less intense as Dst recovered.
A characterization of discrete solar wind events detected by interplanetary scintillation mapping
Authors: Moore V. , Harrison R. A.
Published: February 2004

We examine the large‐scale solar wind density and velocity characteristics which are associated with interplanetary disturbances detected by interplanetary scintillation (IPS) mapping. Our aim here is to characterize these remotely sensed events in order to provide a better foundation on which to consider both the origin of such phenomena and their influence on the near‐Earth environment. We select those events from a list published by Hewish and Bravo (1986) that appear to be Earth crossing and consider the average density and velocity in the solar wind at the time. We find density (and often velocity) enhancements in association with the majority of these IPS events, though the changes are modest. Postevent conditions commonly show a slight velocity increase and density decrease with respect to preevent conditions.
Voyager observations of O+6 and other minor ions in the solar wind
Authors: Villanueva Louis , McNutt Ralph L. , Lazarus Alan J. , Steinberg John T.
Published: February 2004

The plasma science (PLS) experiments on the Voyager 1 and 2 spacecraft began making measurements of the solar wind shortly after the two launches in the fall of 1977. In reviewing the data obtained prior to the Jupiter encounters in 1979, we have found that the large dynamic range of the PLS instrument generally allows a clean separation of signatures of minor ions (about 2.5% of the time) during a single instrument scan in energy per charge. The minor ions, most notably O+6, are well separated from the protons and alpha particles during times when the solar wind Mach number (ratio of streaming speed to thermal speed) is greater than ∼15. During the Earth to Jupiter cruise we find that that the average ratio of alpha particle number density to that of oxygen is 66 ± 7 (Voyager 1) and 71 ± 17 (Voyager 2). These values are consistent with the value 75 ± 20 inferred from the Ion Composition Instrument on ISEE 3 during the period spanning 1978 and 1982. We have inferred an average coronal temperature of (1.7 ± 0.1) × 106 K based on the ratio of O+7 to O+6 number densities. Our observations cover a period of increasing solar activity. During this time we have found that the alpha particle to proton number density ratio is increasing with the solar cycle, the oxygen to proton ratio increases, and the alpha particle to oxygen ratio remains relatively constant in time.
Ground and satellite observations of postdawn aurorae near the time of a sudden storm commencement
Authors: Egeland A. , Burke W. J. , Maynard N. C. , Basinska E. M. , Winningham J. D. , Deehr C. S.
Published: February 2004

Meridian scanning photometer measurements taken in the magnetic postdawn sector at Longyearbyen, Svalbard, between 0300 and 0630 UT on December 29, 1981, are analyzed in conjunction with particle and field data retrieved during two near passes of the Dynamics Explorer 2 (DE 2) satellite. The interval included a sudden storm commencement (SSC) at 0455 UT. Pre‐SSC optical and particle measurements showed a system of arcs that are spaced at ∼1.1° intervals in magnetic latitude, embedded within the region 1 current system and span the convection reversal. The softer particle precipitation appears to have a source near the flanks of the magnetotail while the harder, more equatorward precipitation originates closer to Earth. During the SSC period the entire sky brightened, with enhanced 630.0‐nm emissions extending from the northern horizon to south of magnetic zenith; intense but spatially separated 557.7‐nm emissions dominated the southern horizon. DE 2 detected more than an order of magnitude increase and near isotropization of ring current electron fluxes, enhanced precipitation from the plasma sheet and significantly decreased auroral zone convection. Region 1/region 2 currents remained, with wavelike structures superposed. A dual timescale response to the SSC is consistent with ground and satellite measurements. On few minute travel timescales for hydromagnetic waves to pass through the system, magnetospheric particles accelerate and precipitate to increase the ionospheric conductivity. Global, field‐aligned currents change more slowly. To maintain similar field‐aligned currents with higher ionospheric conductances requires reduced electric fields. After 0520 UT the optical emissions settled into stable, but latitudinally separated, bands of 630.0‐ and 557.7‐nm emissions characteristic of cleft and plasma sheet precipitation, respectively.
Space charge enhanced, plasma gradient induced error in satellite electric field measurements
Authors: Diebold D. A. , Hershkowitz N. , DeKock J. R. , Intrator T. P. , Lee S. ‐G. , Hsieh M. ‐K.
Published: February 2004

In magnetospheric plasmas it is possible for plasma gradients to cause error in electric field measurements made by satellite double probes. The space charge enhanced plasma gradient induced error is discussed in general terms, the results of a laboratory experiment designed to illustrate this error are presented, and a simple expression that quantifies this error in a form that is readily applicable to satellite data is derived. The simple expression indicates that for a given probe bias current there is less error for cylindrical probes than for spherical probes. The expression also suggests that for Viking data the error is negligible.
Model calculation of atmospheric emission caused by energetic O+ precipitation
Authors: Ishimoto M. , Romick G. J. , Meng C. ‐I.
Published: February 2004

Some anomalous auroral emissions, typically observed below 60° geomagnetic latitude during large geomagnetic storms, have distinctive spectral characteristics, that have been attributed to energetic ion/neutral particle precipitation. Mass spectrometers on satellites have observed energetic (keV) ion and neutral precipitation (up to 30 erg cm−2 s−1) below 60° geomagnetic latitude. In this study we used a model to calculate emissions with the characteristics of the incident O+ energy spectra observed from satellites and compared the emission intensities and intensity ratios of the model calculation to those from the anomalous auroral emission spectra. Using an oxygen transport model with estimated emission cross sections, we calculated the emission altitude distributions, vertically integrated column emission intensities and the spectral profile of atomic oxygen line emissions. The calculated emissions were for the N2+ and O2+ first negative(1N), N2 second positive(2P), and N2 Lyman‐Birge‐Hopfield (LBH) bands; the N I lines at 1493Å, 1744Å, and 8680Å; the N II line at 5005Å, and the O I lines at 1304Å, 1356Å, and 6300Å. Most atomic oxygen line emissions are from primary oxygen atoms traveling downward at a speed of 107 and 108 cm s−1. Because of the high speed at which the atoms travel, most ¹D is quenched before emission. Atomic oxygen line spectra at 1304Å and 1356Å display a 1Å Doppler shift and some broadening if viewed from above or below. The emission altitudes and intensities are most sensitive to the choice of elastic scattering cross sections and emission cross sections, respectively. The emission intensity ratios of both the N2+1N to N22P band and the N2LBH bands to the atomic lines, and the Doppler shift of atomic O lines strongly depend on the energy spectrum of incident O+. Seasonal and latitudinal model atmospheric differences have least impact. With an energy spectrum of incident O+, the general model results agree with the observed anomalous auroral emission spectra that were interpreted as being caused by O+ precipitation.
Identification of high‐latitude acoustic gravity wave sources using the Goose Bay HF Radar
Authors: Bristow W. A. , Greenwald R. A. , Samson J. C.
Published: February 2004

Observations of medium‐scale acoustic gravity waves using the Goose Bay HF radar are presented. Data were examined for the period of November and December 1991, and days that showed evidence of traveling ionospheric disturbances (TIDs) were selected for analysis. TIDs were identified in the HF radar data by quasi‐periodic enhancements of the power in the ground‐scattered portion of the radar signal. Various parameters were determined from the data, including the wave frequency, the wavelength, the propagation direction, and the phase velocity. It was found that multiple wave packets from multiple directions are often present simultaneously. Source times and locations were identified from data from 2 days, December 5 and 6, 1991, and these were compared with data from the Greenland magnetometer chain. The source times and locations were found to correspond to sudden magnetometer deflections, indicating the correlation of gravity wave sources with the occurrence of ionospheric electrojet current surges. Finally, the source times and locations were examined in local time and magnetic latitude, and it was found that the sources were clustered in two locations: near the magnetospheric cusp; and near 1600 MLT at about 75° magnetic latitude.
Cone structure and focusing of VLF and LF electromagnetic waves at high altitudes in the ionosphere
Authors: Alpert Ya. L. , Green J. L.
Published: February 2004

The frequency and angle dependencies of the electric field radiated by an electric dipole E = E0 cos ωt are studied through numerical calculations of |E| in the VLF and LF frequency bands 0.02fnof'b ≤ F ≤ 0.5fnof;b in a model ionosphere over an altitude region of 800‐6000 km where the wave frequency and electron gyrofrequency varies between F ∼ 4 ‐ 500 kHz and fnof;b ≃ (1.1 to 0.2) MHz respectively. It is found that the amplitudes of the electric field have large maxima in four regions: close to the direction of the Earth magnetic field line B0 (it is called the axis field E0), in the Storey ESt, reversed Storey ERevSt, and resonance ERes cones. The maximal values of E0, ERes, and ERev.St are the most pronounced close to the lower hybrid frequency, F ∼ FL. The flux of the electric field is concentrated in very narrow regions, with the apex angles of the cones Δβ ≃ (0.1 ‐ 1) deg. The enhancement and focusing of the electric field increases with altitude starting at Z > 800 km. At Z ≥ 1000 up to 6000 km, the relative value of |E|, in comparison with its value at Z = 800 km is about (10² to 104) times larger. Thus the flux of VLF and LF electromagnetic waves generated at high altitudes in the Earth's ionosphere are trapped into very narrow conical beams similar to laser beams.
Theory of magnetospheric standing hydromagnetic waves with large azimuthal wave number: 3. Particle resonance and instability
Authors: Walker A. D. M.
Published: February 2004

Theories of Pc 5 pulsations with high azimuthal wavelength generally work in artificial geometries. After discussion of a simple tutorial example in a straight field geometry a preliminary theory is developed which shows how to evaluate the effect of resonant particles on standing hydromagnetic waves which have been calculated in a realistic cylindrically symmetrical geomagnetic field. Marginal stability is assumed. A reanalysis of data from the event of October 27, 1978, is made. The theory allows estimates of a number of plasma properties to be made. These show internal consistency which is encouraging for the theory.