bluuebunny/crossref_metadata_2025 huggingface dataset:
A simple mechanical model for resonance absorption: The Alfvén resonance
Authors: Hollweg Joseph V.
Published: February 2004

We consider resonance absorption of magnetohydrodynamic waves, and the Alfvén resonance layer in particular. We show that the dissipative layer can be modeled as a simple mechanical system consisting of a few harmonic oscillators which are coupled by friction. The mechanical model reproduces known results for the externally driven system in steady state, such as the structure of the dissipative layer, the “waves” of heating which propagate across the layer, and the fact that the total heating is independent of time. The total work done on the oscillators by the driver is always positive; the external driver sees the total system as a single damped oscillator driven exactly at resonance. Nonetheless, some of the oscillators return energy back to the driver. The total kinetic energy of all the oscillators and the total potential energy are nearly independent of time, because the integrals, across the dissipative layer, of the square of the velocity and the square of the displacement, are truly constants in time. Waves of kinetic and potential energy propagate across the system in the same sense as the waves of heating. We also investigate an initial value problem in which the driver is turned on at t = 0. There is no single number representing the time required for the dissipative layer to reach a steady state. The waves of heating which are found in the steady state are also present in the buildup phase. However, if the driver is turned off after the system has reached a steady state, then the waves of heating are less obvious. We consider the effects of a nonlinear frictional coupling between the oscillators, designed to mimic the effects of Kelvin‐Helmholtz instabilities. The nonlinear coupling has surprisingly little effect on the system. The total steady state heating rate is the same as in the linear system; even with nonlinear dissipation, the dissipative layer adjusts itself to absorb a predetermined amount of energy being pumped in by the external driver. The waves of heating which are found in the linear system are still present. We find no evidence of chaotic behavior.
Cause of postterminator flux transfer events
Authors: Kawano H. , Russell C. T.
Published: February 2004

The local time at which flux transfer events (FTEs) are observed affects the interplanetary magnetic field (IMF) dependence of their occurrence [Kawano and Russell, 1997]. Subsolar FTEs occur mainly during southward IMF, consistent with the day side reconnection model. Postterminator FTEs occur during both southward and northward IMF but are not simply generated by pressure pulses convected to the magnetopause from the foreshock. This paper evaluates other possible generation mechanisms of these postterminator FTEs. We attribute the generation of the postterminator FTEs when IMF Bz < 0 to an “equatorial reconnection line,” which is tilted according to the IMF By, just as we do for subsolar FTEs. When IMF Bz > 0, we find it unlikely that the postterminator FTEs are generated by local reconnection. Two possible sources are cusp reconnection and the “equatorial reconnection,” with re‐reconnection in the subsolar region [Nishida, 1989]. When the IMF is northerly directed but mostly equatorial, the “equatorial reconnection line” model fits better with observations. When the IMF is more northward, both models can explain observations we have at hand.
Isotopic composition of solar wind neon measured by CELIAS/MTOF on board SOHO
Authors: Kallenbach R. , Ipavich F. M. , Bochsler P. , Hefti S. , Hovestadt D. , Grünwaldt H. , Hilchenbach M. , Axford W. I. , Balsiger H. , Bürgi A. , Coplan M. A. , Galvin A. B. , Geiss J. , Gliem F. , Gloeckler G. , Hsieh K. C. , Klecker B. , Lee M. A. , Livi S. , Managadze G. G. , Marsch E. , Möbius E. , Neugebauer M. , Reiche K.‐U. , Scholer M. , Verigin M. I. , Wilken B. , Wurz P.
Published: February 2004

We present first results taken from the high‐resolution mass time‐of‐flight spectrometer (MTOF) of the charge, element, and isotope analysis system (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO) spacecraft launched in December 1995, concerning the abundance ratios of neon isotopes in the solar wind. We obtain the isotopic ratios 20Ne/22Ne = (13.8 ± 0.7) and 20Ne/21Ne = (440 ± 110), which agree with the values obtained from the Apollo foil solar wind experiments and which have been derived from measurements of solar particles implanted in lunar and meteoritic samples.
Galactic cosmic ray intensity variations at a high latitude sea level site 1937–1994
Authors: Ahluwalia H. S.
Published: February 2004

We have created an extended cosmic ray data string (1937–1994) by combining data obtained with ion chambers at Cheltenham/Fredericksburg (1937–1972) and Yakutsk (1953–1994). Both represent high‐latitude, sea level sites, with an atmospheric cutoff of about 4 GV. Their common median rigidity of response to galactic cosmic ray spectrum is 67 GV. This data string represents the longest continuous cosmic ray intensity variations record yet. Therefore it is useful for the cosmic ray modulation studies over a longer time period. We show that there is no striking quantitative correspondence between the amplitudes of solar activity cycles and cosmic ray modulation over six cycles. However, an inverse correlation is apparent between the two. Moreover, the results on the durations of the two are ambiguous. Flat‐topped recovery pattern predicted for even cycles (A > 0), at lower rigidities, is not observed in the ion chamber data. We speculate that three solar activity cycle quasi‐periodicity may be present in the extended data string. Its origin is not identified yet. The quasi‐periodicity is shown to exist also in the planetary index Ap data. As such, it may represent a characteristic feature of the solar wind source region on the Sun as yet unexplored. The odd cycles, with a negative polarity in the solar northern hemisphere, yield a better linear correlation between galactic cosmic ray intensity decrease and Ap than even cycles and A > 0 epochs.
Electric fields with a large parallel component observed by the Freja spacecraft: Artifacts or real signals?
Authors: Chust Thomas , Louarn Philippe , Volwerk Martin , de Feraudy Hervé , Roux Alain , Wahlund Jan‐Erik , Holback Bengt
Published: February 2004

Using plasma wave data sampled by the Freja spacecraft from the topside ionosphere during auroral conditions, the possible existence of electric fields with an intense parallel component (a few tens of millivolts per meter) with respect to the Earth's magnetic field is discussed. When Freja crosses large‐amplitude solitary electromagnetic structures (ΔE ≈ 100 mV/m and ΔB ≈ 10 nT, identified as being solitary kinetic Alfvén waves), strong electric spikes are sometimes detected along a direction almost parallel to the static magnetic field. The possible sources of errors due to the plasma inhomogeneities and/or to the magnetic connection between the electric probe and the spacecraft body are reviewed and discussed. In particular, using an indirect technique based on the reconstruction of the electric field hodograms, it is shown that these sources of errors have no influence on our conclusions. Unless unknown mechanisms strongly affect the validity of double‐probe measurements in some circumstances, it is then concluded that an electric field with a parallel component 2–3 orders of magnitude larger than expected from the theory of kinetic Alfvén waves can develop in the topside ionosphere.
On the development of abnormally large postsunset upward drift of equatorial F region under quiet geomagnetic conditions
Authors: Sastri J. Hanumath
Published: February 2004

The regular postsunset enhancement of upward drift of the equatorial F region is observed to be abnormally large on certain quiet days (Ap ≤ 5) as manifested by an anomalous dusktime increase of F region height. We explored the origin of this extreme form of the quiet‐time variability of postsunset vertical drift/height through case studies using data from the ionosonde and magnetometer networks in the Indian equatorial region. It is found that on the days with an unsually large postsunset increase of F region height near the magnetic equator, the diurnal profile of the equatorial electrojet (EEJ) strength is severly distorted (with a shift, in some cases, of Sq(H) phase from the usual time interval, charactersitic of the abnormal quiet days) with enhanced EEJ conditions in the postnoon period (1300–1600 LT). This is accompanied, near the magnetic equator, by higher values of F layer peak height (hpF2) and lower values of peak electron density (ƒ0F2) in the early evening period (1600–1800 LT), compared with the monthly median/quiet day mean values. These changes in EEJ and hpF2/ƒ0F2 are consistently seen in all eight cases studied. It is suggested that the perturbations in plasma density distribution of equatorial F region increase the therinospheric zonal wind and its local time gradient and the ratio of flux‐tube‐integrated Pedersen conductivity of the F to E region. These modifications just prior to sunset of the properties of the equatorial thermosphere‐ionosphere prompt an efficient F region dynamo action, resulting in the observed abnormally large dusktime increase of F region height.
Ion cyclotron waves during a great magnetic storm observed by Freja double‐probe electric field instrument
Authors: Bräysy T. , Mursula K. , Marklund G.
Published: February 2004

Evolution of the great magnetic storm in April 1993 is studied using observations of electromagnetic ion cyclotron (EMIC) waves by the F1 double‐probe electric field instrument onboard the Freja satellite. The almost continuous operation of the F1 instrument in the overview mode allowed us to follow the global EMIC wave activity at low altitudes above the ionosphere during several subsequent days covering the initial (compression), main, and recovery phases of the storm. During the initial phase of the storm the spatial occurrence of EMIC waves has a postnoon high‐latitude maximum, in agreement with earlier statistical results. A sudden and dramatic change of this pattern was observed with the start of the storm main phase. During the main phase, wave amplitudes were greatly enhanced and the active wave region moved to considerably lower latitudes to the late evening MLT sector. Also, the existence of heavy ions in the later main phase changed the distribution of wave frequencies dramatically. Most interestingly, a number of oxygen band EMIC waves were observed during a limited period of about 7 hours in the later main phase. The observed asymmetric MLT distribution of these oxygen band waves implies that the oxygen loss rate is faster than the drift rate. The results suggest that the EMIC waves play a crucial role in the main and early recovery phase of a great storm.
Analysis of the bimodal nature of solar wind—magnetosphere coupling
Authors: Smith James P. , Horton Wendell
Published: February 2004

It has been shown that the optimal linear prediction filter relating the solar wind electric field and the geomagnetic activity, as measured by the AL index, is both bimodal and dependent on the level of activity in the magnetosphere. Further studies truncated the prediction filter to a five‐parameter model containing two low‐pass‐filtered delta functions of arbitrary amplitude and delay time. The present study elaborates on the nature of the bimodal response by using the five‐parameter model to quantify the effects of the level of geomagnetic activity on each of the modes of the filter individually. We find that at all levels of activity, the second mode, occurring at ∼ 1 hour, is relatively unchanged. The first mode, however, has a necessary one‐parameter dependence on the level of activity in the magnetosphere. The amplitude of the first mode is shown to increase with respect to activity, and this dependence is sufficient in characterizing the changing properties of the magnetosphere with respect to activity.
Are whistler ducts created by thunderstorm electrostatic fields?
Authors: Rodger Craig J. , Thomson Neil R. , Dowden Richard L.
Published: February 2004

Park and Helliwell [1971] suggested that whistler ducts might be caused by plasma interchange of geomagnetic flux tubes driven by thundercloud electrostatic fields. At present, this is the most favored whistler duct creation mechanism. Theoretical formulations have been developed to calculate high‐altitude electric fields due to electrified clouds, taking into account the presence of the ionosphere. Previous studies have calculated the values of these fields, making use of approximated conductivity profiles. Calculations are undertaken using more accurate representations of more modern conductivity profiles than those used previouly. Preference is given to experimental rather than theoretically derived conductivity profiles. These calculations indicate that under “typical” conductivity conditions the high‐altitude electric fields from even giant thunderclouds are too small to create a realistic whistler duct in a realistic period. This is the case for both day and night conditions.
Time—dependent three‐dimensional Petschek—type reconnection: A case study for magnetopause conditions
Authors: Biernat H. K. , Semenov V. S. , Rijnbeek R. P.
Published: February 2004

We illustrate the implications of a generalized Petschek‐type reconnection model for conditions prevailing at the magnetopause. Petschek's model is generalized through the introduction of a space‐ and time‐varying reconnection rate. Furthermore, the model can incorporate skewed magnetic fields with different magnitudes, a velocity shear, and different densities on either side of the current sheet. Here we study a situation typical for the dayside magnetopause, in which all these features are present. On a qualitative level, it is shown that the physical manifestations of reconnection include phenomena which have been observed at the magnetopause, such as accelerated plasma flows, flux transfer events, and surface waves. To aid a quantitative and more detailed comparison with experimental data, we present the results in a format which is similar to that used to view the plasma bulk parameters and magnetic field data obtained from spacecraft measurements. In particular, at several fixed positions in space we analyze the time behavior of the magnetic field and plasma parameters resulting from a localized pulse of reconnection. This exercise reveals that although there is broad agreement between the observations and expectations based on the model, several discrepancies and unexplained features still remain, such as the increase in the total pressure associated with some observations of flux transfer events. These are presumably the result of various simplifying assumptions made in the model. We also reach the new conclusion that application of the stress balance relation does not necessarily guarantee the correct identification of a rotational discontinuity traversal, and we suggest an additional identifying criterion.
Formation of imbedded rotational discontinuities with nearly field aligned normals
Authors: Vasquez Bernard J. , Hollweg Joseph V.
Published: February 2004

We present hybrid numerical simulations of a small number of low‐frequency (much less than the proton gyrofrequency) Alfvén waves and, for the first time, show how to produce imbedded rotational discontinuities (RDs) with a small angle (θBn) between their asymptotic and normal fields which are stable against dispersion. When the initial waves are linearly polarized and give a waveform whose Fourier components of B2 have wave vectors within ≈10° of the background magnetic field B0, the waveform tends to steepen even for small amplitudes (|δB|/B ≪ 1). This produces RDs with normals nearly along B0 and nearly constant B between the RDs. However, the waveform is not steady because these RDs widen continually due to the presence of dispersive intermediate or ion cyclotron modes which are permanently attached to their edges. This commonly occurs in simulations of isolated RDs which have small θBn. When the initial waveform is modified by including a transverse field component with a wavenumber equaling that from the Fourier component of B2 along B0 so that the field moves on an “elliptical” (B not constant) arc, the dispersive intermediate modes no longer develop, and the waveform and imbedded RDs evolve toward a steady state with nearly constant B and a “circular” arc polarization. Spherically polarized waveforms can be made in a similar manner. Only this type of waveform produces RDs without intermediate modes on their edges for all parameters. Oblique fast modes do form, but these have phase speeds which are faster than the Alfvén phase speed and so detach from the RD's edge and do not cause the RD to widen continually. The appearance of RDs with small θBn varies greatly with the time of development, and this can explain the diversity of hodogram shapes of such RDs in the solar wind. We finally conclude that Alfvénic fluctuations with imbedded RDs must evolve through a succession of arc‐polarized or spherically polarized waveforms as they travel outward from the Sun: Otherwise, the RDs with small θBn in the solar wind would have widen to the point where they are beyond recognition.
Mapping between the ionospheric and the tail electric fields in a time‐dependent Earth's magnetosphere
Authors: Toivanen P. K. , Koskinen H. E. J. , Pulkkinen T. I.
Published: February 2004

In this paper we use the concept of the Faraday loop to connect the ionosphere and the tail along the geomagnetic field lines to study the coupling between ionospheric and magnetospheric electric fields. The formulation using the Faraday loop shows that the coupling consists of three contributions: One is the familiar mapping of the ionospheric field to the tail along equipotential magnetic field lines. The other two are parallel potential differences between the tail and the ionosphere distributed across the magnetic field lines and magnetic flux transport across the Faraday loop due to time evolution of the magnetic field, which gives rise to an inductive electric field perpendicular to the magnetic field. Application of this method requires a model for the ionospheric electric field, a model distribution of the parallel potential differences, and a time‐evolving magnetic field model. In the present study the ionospheric electric field pattern is described using a statistical model. The parallel potential differences are modeled by Gaussian distributions in latitude as narrow longitudinally elongated ridges. The magnetic field model describes the time evolution of the geomagnetic field during the substorm growth phase. We show that the effects of parallel potential differences and the inductive fields are of the same order as the mapped ionospheric field, and hence they must be taken into account when the large‐scale coupling is studied.
Longitudinal and interhemispheric variations of auroral ionospheric electrodynamics in a realistic geomagnetic field
Authors: Gasda S. , Richmond A. D.
Published: February 2004

We investigate the influences of nondipolar features of the geomagnetic field (field strength, orientation, and magnetic coordinate distortion) on auroral ionospheric electrodynamics. We present a conceptual model in which both the electric potential and the distribution functions of precipitating auroral particles are invariant in the magnetic latitude/magnetic local time reference frame, and we explore the predictions of this model concerning longitudinal (or universal time) and interhemispheric (north/south) variations of auroral electrodynamic parameters at ±68° magnetic latitude for a fixed magnetic local time and solar zenith angle. The conductances, electric fields and currents, ion drift velocity components, and Joule heating tend to have two minima and maxima with respect to longitude in the northern hemisphere but have a single minimum and maximum in the southern hemisphere. Particularly large variations are found for the field‐aligned current density (maximum/minimum = 1.76) and the regional Joule heating (maximum/minimum = 1.88), with maxima near northwest Iceland and northwest Alaska, and minima over north‐central Siberia and Hudson Bay. The variations of field‐aligned current intensity may imply a tendency for more frequent field‐aligned electron acceleration and thus brighter aurora near northwest Iceland and northwest Alaska than elsewhere. The longitudinal variations of Joule heating may contribute significantly to universal time variations of total hemispheric Joule heating. Whether or not the conceptual model is correct, at least some of the various electrodynamic parameters must have longitudinal and hemispheric variations of the general magnitudes the model predicts.
The role of precipitation losses in producing the rapid early recovery phase of the Great Magnetic Storm of February 1986
Authors: Kozyra J. U. , Fok M.‐C. , Sanchez E. R. , Evans D. S. , Hamilton D. C. , Nagy A. F.
Published: February 2004

The possible role of precipitation losses in eroding stormtime ring current is subject to debate. To explore this controversy, the recovery phase of the February 6–10, 1986, great magnetic storm is examined, when intense ion precipitation was observed at midlatitudes by NOAA‐6 and DMSP satellites. This storm period is particularly interesting because the ring current exhibits distinctive two‐phase decay as seen in the Dst index, the early rapid timescale decay corresponding to the intense ion precipitation period described above. Hamilton et al. [1988] concluded, from close agreement between the observed timescale for ring current decay and the theoretical timescale for O+ charge exchange loss, that rapid early recovery phase of this storm resulted from the charge exchange loss of high energy O+; the second and longer decay phase was equated with H+ charge exchange loss. A model of the ring current evolution during this great magnetic storm [Fok et al., 1995] failed to reproduce the observed ring current decay rates, a puzzling result because charge exchange losses were well represented in the ring current model and initial and boundary conditions were taken from the same data set used in the Hamilton et al. [1988] study. A simple energy balance calculation for the global ring current is carried out using (1) either an energy input predicted from upstream solar wind parameters or one calculated from the drift‐loss model output, (2) collisional loss timescales extracted from the drift‐loss model, and (3) precipitation losses estimated from NOAA‐6 and DMSP observations. The energy balance model replicates the evolution of the ring current energy content derived from Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) observations when ion precipitation losses are included and model energy input function is reduced to agree with predictions based upon upstream solar wind parameters. The O+ charge exchange losses and observed global precipitation losses were of equal magnitude in early recovery of the ring current during this great magnetic storm. Later longer decay timescales in the model resulted from a combination of O+ and H+ charge exchange losses; O+ charge exchange losses remained important throughout the model time interval. The present model produces agreement with the AMPTE/CCE estimates of ring current kinetic energy content versus time. Disagreement between the Dst* inferred from the AMPTE/CCE particle measurements and the observed Dst* is an interesting issue needing further explanation.
The “thermospheric spoon”: A mechanism for the semiannual density variation
Authors: Fuller‐Rowell T. J.
Published: February 2004

A mechanism is proposed to explain the cause of the global, semiannual thermospheric density variation. It is suggested that the global‐scale, interhemispheric circulation at solstice acts like a huge turbulent eddy in mixing the major thermospheric species. The effect causes less diffusive separation of the species at solstice, which tends to raise molecular nitrogen and oxygen densities and reduce atomic oxygen density, compared with equinox. The increased mean mass, at solstice, reduces the density scale height at a given altitude. This “compression” of the atmosphere at solstice can explain the mean amplitude of the semiannual density anomaly. Since ionospheric loss rates are affected by neutral composition, the proposed mechanism also leads to a similar ionospheric density variation.
Dependence of the large‐scale, inner magnetospheric electric field on geomagnetic activity
Authors: Rowland D. E. , Wygant J. R.
Published: February 2004

Measurements made with the University of California, Berkeley/Air Force Geophysics Laboratory electric field instrument on the CRRES spacecraft are examined to determine the average structure of the inner magnetospheric electric field for different geomagnetic activity levels. Data were gathered between L=2.5 and L=8.5 over the period from January to October 1991 in the local time sector between 1200 and 0400 magnetic local time. The average dawn‐dusk component of the electric field was nearly always duskward, scaling in magnitude from 0.05 mV/m to 1.5 mV/m as Kp increased from 0 to 9‐. The data indicate that the electric field is shielded out of the inner magnetosphere, with the shielding distance starting at about L=5.0 for Kp=1 and moving 0.5 RE earthward for every unit increase in Kp. There is evidence that the electric field penetrates below L=2.5 for Kp ≥ 5. The most interesting aspect of this statistical study is the development for moderate to high Kp of a region of enhanced electric field between L=3.5 and L=6. For moderately active conditions the electric field does not decrease monotonically as one moves earthward. Instead, it increases to a broad local maximum near the position where the ring current is typically observed to be the strongest, falling off earthward of that position. The electric field magnitude can be a factor of 2 or more larger at this location than at higher L values. These results are discussed in the context of large‐scale flows and the effects of hot plasma in the inner magnetosphere.
A study of density fluctuations in the solar wind acceleration region
Authors: Yamauchi Y. , Tokumaru M. , Kojima M. , Manoharan P. K. , Esser R.
Published: February 2004

Interplanetary scintillation (IPS) measurements of the density fluctuations in the fast and slow solar wind were made at 2 GHz and 8 GHz in 1994 at the Kashima Space Research Center, Communications Research Laboratory. Using the observations which cover the distance range from 5 to 76 solar radii (Rs), we investigate the radial evolution of the dissipation scale length of the density fluctuations, the so‐called “inner scale.” Our IPS observations reveal that the inner scale shows different radial profiles between the inside and outside of the acceleration region. The size of the inner scale outside 25 Rs increases linearly with radial distance, showing good agreement with previous observations made at r ≥ 50 Rs. The inner scale inside 25 Rs, on the other hand, deviates from the linear relation. We simulate the radial variation of the inner scale using a solar wind acceleration model and then compare the results to that of the observed inner scale. We find that, in the low‐speed solar wind, the radial profile of the computed inner scale is in good agreement with that of the observed inner scale and that the solar wind acceleration causes the deviation of the inner scale from a linear relation. However, in the high‐speed wind, we cannot reproduce the radial profile of the observed inner scale with the acceleration model, even though the line‐of‐sight integration effect on IPS measurements is taken into account. We propose that this disagreement is due to the density and magnetic field fluctuations not being correlated in the high‐speed solar wind, as shown at greater heliocentric distances by Helios measurements.
A numerical investigation of choked flow dynamics and its application to the triggering mechanism of long‐period events at Redoubt Volcano, Alaska
Authors: Morrissey Meghan M. , Chouet Bernard A.
Published: February 2004

We use numerical simulations of transonic flow through a crack to study the dynamics of the formation of shock waves downstream from a nozzle‐like constriction inside the crack. The model solves the full set of Navier‐Stokes equations in two dimensions via an explicit multifield finite difference representation. The crack walls are assumed to be perfectly rigid, and elastic coupling to the solid is not considered. The simulations demonstrate how the behavior of unsteady shock waves near the walls can produce recurring step‐like pressure transients in the flow, which in turn induce resonance of the fluid‐filled crack. The motion of the shock waves is governed primarily by smooth, low‐amplitude pressure fluctuations at the outlet of the crack. The force induced on the walls scales with the amplitude of the shock, which is a function of the magnitude of the inlet pressure, aperture of the constriction, and thickness of the boundary layer. The applied force also scales in proportion to the spatial extent of the shock excursion, which depends on the fluctuation rate of outlet pressure. Using the source parameters of long‐period (LP) events at Redoubt Volcano, Alaska, as a guide for our simulations, we infer that coupling of the shock to the walls occurs for crack inlet to outlet pressure ratios pi/po>2.31 and that the position of the shock front becomes most sensitive to outlet pressure fluctuations for flow regimes with pi/po>2.48. For such regimes, fluctuations of outlet pressure of up to ±0.5 MPa at rates up to 3 MPa/s are sufficient to induce pressure transients with magnitudes up to 12.5 MPa over 0.1–2.5 m of the walls within ∼0.5 s. These flow parameters may be adequate for triggering the LP events in the precursory swarm to the December 14, 1989, eruption of Redoubt. According to the flow model the recurrence rate and amplitudes of L.P events are inferred to be a manifestation of the response of a shallow hydrothermal reservoir to the sustained injection of superheated steam from a magma column roofing below this reservoir.
Volcanic gas emissions from Mount Erebus and their impact on the Antarctic environment
Authors: Zreda‐Gostynska Grazyna , Kyle Philip R. , Finnegan D. , Prestbo Kimberly Meeker
Published: February 2004

Emission rates of SO2, HCl, and HF from the active volcano Mount Erebus, Antarctica, increased between 1986 and 1991; SO2 from 7.7 to 25.9 Gg yr−1, HCl from 6.9 to 13.3 Gg yr−1 and HF from 4.0 to 6.0 Gg yr−1. The emission rates of halogens from Mount Erebus are high relative to SO2 emissions and are accompanied by relatively high emissions of trace gases and aerosols (Na, K, As, Zn, In, As, Se, and Au). Many elements (S, Cl, and metals) found in the Erebus plume are common impurities in Antarctic snow. Using a model which assumes a homogeneous distribution of the volcanic gas plume over Antarctica, we suggest that Erebus could be a source of the impurities. We calculate that Erebus could potentially contribute between 4 and 14 ng g−1 snow of Cl at the south pole, and between 11 and 36 ng g−1 snow of Cl at Dome C. Excess Cl (Cl in excess of that derived from marine NaCl aerosols) recorded in snow and firn cores from south pole and Dome C could be mainly derived from Erebus. Similarly, our predicted concentrations of Erebus‐derived Cu, Zn, Cd, V, As, and Au in Antarctic snow are close to those reported. Trace element and Pb isotope compositions of Erebus aerosols are similar to those collected in remote regions of Antarctica. The volcanic gas plume emitted from Erebus appears to make a significant contribution to the Antarctic atmosphere and can be detected in the snow deposited over a wide area of the continent.
Kinematic constraints on thrust‐belt curvature from syndeformational magnetizations in the Lagos del Valle Syncline in the Cantabrian Arc, Spain
Authors: Van der Voo Rob , Stamatakos John A. , Parés Josep M.
Published: February 2004

A detailed paleomagnetic study in a single, first‐order, refolded syncline in the western hinge zone of the Variscan foreland fold and thrust belt of northern Spain was carried out to constrain the tectonic deformation of the Cantabrian Arc. Results show a complex pattern of structural deformation and acquisition of at least two secondary syntectonic magnetizations. On the basis of local and regional fold tests we document an Early Permian (B) component that postdates earlier (F1) Westphalian and Stephanian thrusting and folding and a Late Carboniferous to earliest Permian (C) component that is synfolding, acquired during Fl. The B component only resides in the carbonates and is carried by magnetite. The C component is carried by magnetite in the gray carbonates and by hematite in the red beds. Both B and C components reveal significant vertical‐axis rotations that indicate that an originally cylindrical Lagos del Valle Syncline was refolded (F2) about a vertical axis after acquisition of the B component magnetization. The pattern of rotations suggests that the entire Cantabrian Arc may have originated as a much more linear fold and thrust belt, which was later modified by significant oroclinal bending with relative rotations totaling at least 120°. The temporal distribution of remagnetization and deformation also suggests a protracted Variscan orogeny in the area, lasting from at least the Late Carboniferous into the Permian.
Magnetostratigraphic dating of river terraces: Rapid and intermittent incision by the Yellow River of the northeastern margin of the Tibetan Plateau during the Quaternary
Authors: Li Ji‐Jun , Fang Xiao‐Min , Van der Voo Rob , Zhu Jun‐Jie , Niocaill Conall Mac , Ono Yugo , Pan Bao‐Tian , Zhong Wei , Wang Jian‐Li , Sasaki Taka , Zhang Yu‐Tian , Cao Ji‐Xiu , Kang Shi‐Chuang , Wang Jian‐Ming
Published: February 2004

Up to 23 paleosols have been identified in loess sequences that overlie fluviatile sediments on seven terraces of Huang He (the Yellow River) and Daxia He in the Linxia Basin at the northeastern margin of the Tibetan Plateau. The magnetostratigraphic record in the oldest sequence appears to be fairly complete and spans the entire Quaternary. It includes evidence for the Jaramillo and Olduvai normal polarity subchrons within the Matuyama chron, as well as the Cobb Mountain and “stage 54” (formerly Gilsa) events. Age constraints within the Brunhes are provided by 14C and thermoluminescence/optical stimulated luminescence dating. The magnetostratigraphic chronology of the Quaternary sediments indicates that terraces were formed at about 1.66, 1.4, 1.1, 0.6, 0.12, 0.055, and 0.01 Ma. The terraces may represent climatically induced pauses in long‐term tectonically induced incision by the rivers. On the basis of terrace heights and ages, incision rates were highest from 1.66 to 1.4 Ma (∼1 m/kyr), decreased from 1.4 to 0.12 Ma (∼ 0.1 m/kyr), and increased from 0.12 Ma to present (∼0.75 m/kyr). If river incision is attributed solely to uplift of the Tibetan Plateau, then this uplift appears to have been rapid as well as intermittent following the first evidence of the existence of Huang He in this area at about 1.75 Ma. However, the effects of uplift cannot be fully separated from incision caused by changes in climate.
Fractal and multifractal measures of natural and synthetic fracture networks
Authors: Berkowitz Brian , Hadad Avichay
Published: February 2004

We have analyzed the fractal and multifractal nature of a series of 17 natural fracture trace maps, representing a wide variety of scales, geological settings, and lithologies, as well as a number of typical synthetic fracture networks in which fracture locations, orientations, and lengths are drawn from various probability distribution functions. Recent studies have shown that multifractal methods can be used to investigate fracture networks at greater depth, since the fractal dimension represents only part of the scaling spectrum characterizing each network. We find that the real and synthetic fracture maps display fractal and multifractal properties. Moreover, the properties of the synthetic networks are very similar to those of the real networks, with nontrivial fractal dimensions and multifractal spectra. We suggest that different fracturing mechanisms can lead to two or more distinct subranges over which a fractal dimension can be defined, while the heterogeneity of the rock, and the nature of the fracturing mechanisms, lead to multifractal properties. We also find that the fractal dimension of a synthetic fracture network is relatively insensitive to parameters such as fracture length and orientation but can be controlled by appropriate choice of the relative fracture density.
Multiphase groundwater flow near cooling plutons
Authors: Hayba Daniel O. , Ingebritsen Steven E.
Published: February 2004

We investigate groundwater flow near cooling plutons with a computer program that can model multiphase flow, temperatures up to 1200°C, thermal pressurization, and temperature‐dependent rock properties. A series of experiments examines the effects of host‐rock permeability, size and depth of pluton emplacement, single versus multiple intrusions, the influence of a caprock, and the impact of topographically driven groundwater flow. We also reproduce and evaluate some of the pioneering numerical experiments on flow around plutons. Host‐rock permeability is the principal factor influencing fluid circulation and heat transfer in hydrothermal systems. The hottest and most steam‐rich systems develop where permeability is of the order of 10−15 m2. Temperatures and life spans of systems decrease with increasing permeability. Conduction‐dominated systems, in which permeabilities are ≤10−16 m2, persist longer but exhibit relatively modest increases in near‐surface temperatures relative to ambient conditions. Pluton size, emplacement depth, and initial thermal conditions have less influence on hydrothermal circulation patterns but affect the extent of boiling and duration of hydrothermal systems. Topographically driven groundwater flow can significantly alter hydrothermal circulation; however, a low‐permeability caprock effectively decouples the topographically and density‐driven systems and stabilizes the mixing interface between them thereby defining a likely ore‐forming environment.
Multiscale spectral analysis of bathymetry on the flank of the Mid‐Atlantic Ridge: Modification of the seafloor by mass wasting and sedimentation
Authors: Goff John A. , Tucholke Brian E.
Published: February 2004

The results of a multiscale spectral analysis of bathymetric data on the flank of the Mid‐Atlantic Ridge are described. Data were collected during two cruises using Hydrosweep multibeam (tens of kilometers to ∼0.2 km scale range) and Mesotech scanning pencil‐beam sonar attached to remotely operated vehicle Jason (∼1 km to ∼0.5 m scale range). These data are augmented by visual data which enabled us to identify bathymetric profiles which are over unsedimented or thinly sedimented crust. Our analysis, therefore, is focused primarily on statistical characterization of basement morphology. Work is concentrated at two sites: site B on ∼24 Ma crust in an outside‐corner setting, and site D on ∼3 Ma crust in an inside‐corner setting. At site B we find that an anisotropic, band‐limited fractal model (i.e., the “von Kármán” model proposed for abyssal hill morphology by Goff and Jordan [1988]) is not sufficient to describe the full range of scales observed in this study. Our observations differ from this model in two ways: (1) strike and cross‐strike (dip) spectral properties converge for wavelengths smaller than ∼300 m, and (2) in both strike and dip directions the fractal dimension changes at ∼10 m wavelength, from ∼1.27 at larger scales to ∼1.0 at smaller scales. The convergence of strike and dip spectral properties appears to be associated with destruction of ridge‐parallel fault scarps by mass wasting, which develops canyon‐like incisions that cross scarps at high angles. The change in fractal dimension at ∼10 m scale appears to be related to a minimum spacing of significant slope breaks associated with scarps which are created by faulting and mass wasting. At site D, although there is no significant abyssal hill anisotropy, the spectral properties at all scales are consistent with the von Kármán model. The fractal dimension at this site (∼1.15) is less than at site B. This difference may be reflect different morphology related to crustal formation at inside‐corner versus outside‐corner position or, more likely, differences in the degree of mass wasting. The smoothing of seafloor morphology by sediments is evident in Hydrosweep periodograms where, relative to basement roughness, spectral power decreases progressively with decreasing wavelength.
A test of the double‐shearing model of flow for granular materials
Authors: Savage J. C. , Lockner D. A.
Published: February 2004

The double‐shearing model of flow attributes plastic deformation in granular materials to cooperative slip on conjugate Coulomb shears (surfaces upon which the Coulomb yield condition is satisfied). The strict formulation of the double‐shearing model then requires that the slip lines in the material coincide with the Coulomb shears. Three different experiments that approximate simple shear deformation in granular media appear to be inconsistent with this strict formulation. For example, the orientation of the principal stress axes in a layer of sand driven in steady, simple shear was measured subject to the assumption that the Coulomb failure criterion was satisfied on some surfaces (orientation unspecified) within the sand layer. The orientation of the inferred principal compressive axis was then compared with the orientations predicted by the double‐shearing model. The strict formulation of the model [Spencer, 1982] predicts that the principal stress axes should rotate in a sense opposite to that inferred from the experiments. A less restrictive formulation of the double‐shearing model by de Josselin de Jong [1971] does not completely specify the solution but does prescribe limits on the possible orientations of the principal stress axes. The orientations of the principal compression axis inferred from the experiments are probably within those limits. An elastoplastic formulation of the double‐shearing model [de Josselin de Jong, 1988] is reasonably consistent with the experiments, although quantitative agreement was not attained. Thus we conclude that the double‐shearing model may be a viable law to describe deformation of granular materials, but the macroscopic slip surfaces will not in general coincide with the Coulomb shears.
Element transport from slab to volcanic front at the Mariana arc
Authors: Elliott Tim , Plank Terry , Zindler Alan , White William , Bourdon Bernard
Published: February 2004

We present a comprehensive geochemical data set for the most recent volcanics from the Mariana Islands, which provides new constraints on the timing and nature of fluxes from the subducting slab. The lavas display many features typical of island arc volcanics, with all samples showing large negative niobium anomalies and enrichments in alkaline earth elements and lead (e.g., high Ba/La and Pb/Ce). Importantly, many of these key ratios correlate with a large range in 238U excesses, (238U/230Th) = 0.97–1.56. Geochemical features show island to island variations; lavas from Guguan have the largest 238U‐excesses, Pb/Ce and Ba/La ratios, while Agrigan lavas have small 238U excesses, the least radiogenic 143Nd/144Nd, and the largest negative cerium and niobium anomalies. These highly systematic variations enable two discrete slab additions to the subarc mantle to be identified. The geochemical features of the Agrigan lavas are most consistent with a dominant subducted sediment contribution. The added sedimentary component is not identical to bulk subducted sediment and notably shows a marked enrichment of Th relative to Nb. This is most readily explained by melt fractionation of the sediment with residual rutile and transfer of sedimentary material as a melt phase. For most of the highly incompatible elements, the sedimentary contribution dominates the total elemental budgets of the lavas. The characteristics best exemplified by the Guguan lavas are attributed to a slab‐derived aqueous fluid phase, and Pb and Sr isotope compositions point toward the subducted, altered oceanic crust as a source of this fluid. Variable addition of the sedimentary component, but near‐constant aqueous fluid flux along arc strike, can create the compositional trends observed in the Mariana lavas. High field strength element ratios (Ta/Nb and Zr/Nb) of the sediment poor Guguan lavas are higher than those of most mid‐oceanic ridge basalts and suggest a highly depleted subarc mantle prior to any slab additions. The 238U‐230Th systematics indicate >350 kyr between sediment and mantle melting but <30 kyr between slab dehydration and eruption of the lavas. This necessitates rapid magma migration rates and suggests that the aqueous fluid itself may trigger major mantle melting.
Isothermal magnetization of samples with stable Preisach function: A survey of hysteresis, remanence, and rock magnetic parameters
Authors: Fabian Karl , von Dobeneck Tilo
Published: February 2004

Isothermal magnetization curves, like hysteresis loops, initial curves, back field curves, acquisition curves and demagnetizing curves of isothermal remanent magnetization, are commonly used for rock magnetic purposes. In this study we investigate the relations among these curves and other useful magnetization curves (saturation initial curve and induced and remanent hysteretic magnetization curves) in order to compare coercivity and domain state parameters which can be derived from them. Most natural samples, especially sediments, are weakly magnetic and possess relatively stable Preisach functions. Their magnetization states can therefore be described by classical Preisach theory. This approach verifies well‐known rules and establishes some formerly unreported relations between isothermal magnetization curves and parameters. It is possible to point out sets of mathematically independent isothermal magnetization curves and to state theoretical interrelations between dependent curves by simply inspecting Preisach diagrams. Furthermore, we define six elemental isothermal magnetization curves from a general partition scheme of the Preisach diagram. They can be easily obtained from common measurements and generate all above mentioned curves. The experimental applicability of our results is demonstrated for three (single‐domain, pseudo‐single‐domain, multidomain) marine sediment samples. A physical rationale of the elemental curves reveals favorable properties for the investigation of interaction and domain state. As a spin‐off from the general results, a new hysteresis‐based procedure for the measurement of Hcr is presented. We also propose an apparently more robust hysteresis‐derived domain state parameter and a generalized version of the well‐known R parameter. All presented methods can be applied without actually measuring Preisach functions.
A proposed volcanic sulfur dioxide index (VSI)
Authors: Schnetzler C. C. , Bluth G. J. S. , Krueger A. J. , Walter L. S.
Published: February 2004

This paper proposes a volcanic SO2 index (VSI), on the basis of a 15‐year satellite sampling of volcanic eruptions. This index is scaled to be compatible with the commonly cited volcanic explosivity index (VEI) used to compare explosive volcanic eruptions. A range of SO2 for each VEI level is proposed so that estimates of SO2 emission can be made from the historic VEI database. Volcanic SO2 from large eruptions can be routinely measured by satellite instruments, and for the smaller eruptions, where the sensitivity of satellite remote sensing is not sufficient, ground‐based measurements are becoming more available. At the present time we do not think SO2 can be used to define an accurate climate‐perturbation index, as we have both insufficient data on volcanic SO2 and understanding of the volcano‐climate relationship.