bluuebunny/crossref_metadata_2025 huggingface dataset:
Evolution of subkilometer scale ionospheric irregularities generated by high‐power HF waves
Authors: Basu S. , Costa E. , Livingston R. C. , Groves K. M. , Carlson H. C. , Chaturvedi P. K. , Stubbe P.
Published: February 2004

Electron density irregularities in the F region excited by the European incoherent scatter (EISCAT) high‐power high‐frequency (HF) facility in Ramfjordmoen, Norway, have been studied in detail by observing scintillations of 250‐MHz satellite signals traversing the HF beam. Spaced antenna measurements were performed to determine the temporal structure of scintillation and the drift of the irregularities. Studies of scintillation spectra indicate that even though the onset occurs nearly simultaneously for all irregularity scale sizes, the electron density irregularities with wavelengths in the range of 50–15 m attain their saturation amplitudes within tens of seconds, whereas longer scale‐size irregularities with wavelengths exceeding 100 m saturate in the timescale of minutes. The threshold power density requirement for the generation of irregularities of different scales has also been studied by the variation of the heater power. It is found that for large‐scale (>100 m) irregularities the threshold power density varies inversely as the irregularity wavelength with a power law index of 4 as predicted by the theories of self‐focusing instability.
Meridional wind in the auroral thermosphere: Results from EISCAT and WINDII‐O(1D) coordinated measurements
Authors: Lathuillère C. , Lilensten J. , Gault W. , Thuillier G.
Published: February 2004

Neutral thermospheric winds calculated from European incoherent scatter (EISCAT) radar data have been compared with winds measured by wind imaging interferometer (WINDII) in O(1D) emission during 11 passes of the WINDII fields of view near the radar facility. For the eight occasions when geomagnetic activity was low the average difference in the meridional winds measured by the two methods is less than 10 m/s. The EISCAT calculations were done with and without a “Burnside factor” of 1.7, and agreement with WINDII is somewhat better when the Burnside factor is not included. The three passes corresponding to disturbed conditions show poor agreement. In addition, agreement between EISCAT and WINDII is better when unfiltered EISCAT winds are used, rather than the 2‐hour running mean used in earlier work. This finding suggests that the short‐term oscillations seen by EISCAT are real oscillations of the neutral atmosphere.
Transients associated with recurrent storms
Authors: Crooker N. U. , McAllister A. H.
Published: February 2004

From August 1993, to April 1994, in the declining phase of solar cycle 22, a broken series of eight storms recurred with successive passages of the same sector boundary in the course of 10 solar rotations. Three of the strongest of these storms, on November 3–4, 1993, February 21, 1994, and April 17, 1994, are the subjects of recent community studies. All three have been associated with X ray events in Yohkoh data signaling the launch of coronal mass ejections (CMEs). Because of the clear morphology of these events, the associations are more obvious than past associations with flares. We find additional X ray events that can be associated with the remaining five storms in the sequence. The associations range from nearly certain for three of the four strongest storms to doubtful for the weakest storm. No X ray events occurred on the two rotations with no storms. Of the eight storms in total, four were associated with large arcades near the south polar crown, three with smaller‐scale events in active regions bordering the northern coronal hole extension, and one with a medium arcade equatorward of the north polar crown. These sites apparently map out to the same sector boundary at 1 AU. Although recurrent storms are usually associated with high‐speed streams from coronal holes, we infer that the southward interplanetary magnetic field responsible for peak activity is often brought by CMEs from the base of the sector boundary between coronal holes. Coupled with corotating interaction regions (CIRs) created by any oncoming high‐speed flow from the following coronal hole, the transient CMEs fall into the recurrence pattern and can account for the unpredictably wide range of recurrent storm strengths.
Revised Landau damping rates of magnetohydrodynamic waves in hot magnetized equilibrium plasmas and its consequences for cosmic ray transport in the interplanetary medium
Authors: Schlickeiser R. , Fichtner H. , Kneller M.
Published: February 2004

A relativistic theory of transverse oscillations in hot, magnetized, isotropic equilibrium plasmas is presented. The special theory of relativity enters the description with the requirement that the particle masses depend on the particle velocities which themselves are limited by the speed of light. Using the relativistic Maxwell‐Boltzmann‐Jüttner (MBJ) distribution correctly describing the equilibrium state of a plasma, we formulate the dispersion relations for subluminal as well as superluminal waves, using the Trubnikov representation. For Landau damping of transverse wave modes, the so‐called cyclotron damping, an analytical expression generalizing earlier results to a relativistically correct form is derived. The theory is applied to an electron‐proton plasma, the main constituents of the interplanetary medium. Special consideration is given to the regime of weak damping of low‐and high‐frequency waves in the interplanetary plasma. The resulting dispersion relations and damping rates are used to determine cosmic ray transport parameters in the interplanetary medium, in particular the mean free path of solar energetic particles and the related modulation parameter of galactic cosmic rays in a simple diffusion‐convection modulation model. It is shown that at particle rigidities below about 109 V/c the mean free path and the modulation parameter are significantly different for cosmic ray protons, negations, and positrons because of the different handedness of the particles in the ordered interplanetary magnetic field and the different Landau damping rates of right‐handed and left‐handed circularly polarized plasma waves.
Cosmic radio noise absorption related to structures in auroral luminosity
Authors: Stoker P. H. , Mathews M. J. , Scourfield M. W. J.
Published: February 2004

Digitized all‐sky white light images of auroral optical emissions, recorded by a low‐light level TV system at Sanae (70.3°S, 2.4°W, L = 4.0), have been mapped onto the angular sensitivity functions of the inner four beams of a 16‐element imaging riometer. Cosmic radio waves of 5–10 m wavelengths appeared to be absorbed more strongly in directions through regions adjacent to discrete auroral arcs than through the arc regions themselves. These stronger absorptions may be due either to enhanced electron temperatures, caused by the Farley‐Buneman two‐stream instability in the presence of large electric fields around auroral arcs in the E region, or to D region absorption caused by energetic electrons precipitating along magnetic field lines passing through regions adjacent to auroral arcs. The two auroral events before local magnetic midnight reported in this paper started with high luminosity and small ionospheric absorption. The absorption increased as the event developed. Auroral luminosities and structures were changing in all four viewing directions during the event on May 10, 1992, with variations in absorptions following variations in luminosity after a delay of 30 s. The event on June 8, 1992, involved a pulsating arc structure. Changes in absorption appeared to be delayed relative to changes in luminosity that varied from 0 to 60 s in a particular viewing direction. The auroral event after local magnetic midnight on April 14, 1993, differed from the former two events in the appearance of pulsating auroral patches and in slower temporal variations, but, occasionally, changes in absorption of cosmic radio noise in the four beam directions were still delayed relative to changes in luminosity. The observed spatial and temporal differences in regions of optical emissions and ionospheric absorption should be of magnetospheric origin rather than ionospheric origin on account of the long delay times.
The average shape of the Magnetopause: A comparison of three‐dimensional global MHD and empirical models
Authors: Elsen R. K. , Winglee R. M.
Published: February 2004

We present a predictive model of the magnetopause, constructed using three‐dimensional global MHD simulations of the magnetosphere over a range in solar wind dynamic pressure and the north‐south component of the interplanetary magnetic field (IMF BZ). The magnetopause is identified in the simulations using electric current intensity, thermal pressure, and magnetic field line mapping. The magnetopause is fitted by least squares to an ellipse in each plane, and a fully three‐dimensional magnetopause surface is constructed by assuming an elliptical cross section at intermediate clock angles. There is considerable flaring of the magnetopause dependent on BZ in the noon‐midnight meridian plane due to reconnection but virtually no flaring in the equatorial plane. Despite this asymmetry, when averaged over clock angle, the model shows a general agreement with recent (axisymmetric) empirical magnetopause models and predicts observed magnetopause crossings with about the same accuracy as the empirical models. Unlike these empirical models, our model can directly investigate asymmetries between the meridian and equatorial planes. The subsolar distance, directly measured from the simulations, varies linearly with BZ for both northward and southward interplanetary magnetic field, as do distances to the magnetopause on the flanks. Magnetospheric plasma pressure causes these distances to scale with dynamic pressure by a power law whose exponent is a bit less than −1/6 on the dayside, but little deviation from this ideal Chapman‐Ferraro value is seen in the magnetotail. These uniform changes with Bz lead to approximate conservation of (1) magnetopause shape in the dayside equatorial plane, (2) projected area in the dayside meridian plane, and (3) magnetopause cross‐sectional area in the dawn‐dusk plane. The dawn‐dusk magnetopause cross section is nearly circular on average, but cross sections in the tail are elongated in the Z direction on average.
Tidal oscillations of the Arctic upper mesosphere and lower thermosphere in winter
Authors: Oznovich I. , McEwen D. J. , Sivjee G. G. , Walterscheid R. L.
Published: February 2004

The upper mesosphere and lower thermosphere of the northern polar cap was investigated using Fabry‐Perot interferometer measurements of the horizontal wind velocity in the lower thermosphere, meridian scanning photometer observations of atomic oxygen green line brightness in the lower thermosphere, and Michelson interferometer records of the infrared Meinel OH bands brightness and rotational temperature in the upper mesosphere. Tidal airglow oscillations in the absence of local solar heating were studied using a superposed epoch analysis of observations obtained around three new‐Moon intervals (November 9–22, 1993, December 7–20, 1993, and January 4–17, 1994) in Eureka (80°N). Tidal harmonics were uncovered with an iterated least chi‐squared fit, and their existence in the data tested using a goodness‐of‐fit probability against a null hypothesis of no oscillations. No single dominant tide was found at all times and altitudes of the winter Arctic upper mesosphere and lower thermosphere. The horizontal wind velocity of the lower thermosphere exhibited all the first three harmonics of the tide, with the largest‐amplitude oscillations shown by the diurnal component of the meridional wave speed. The atmospheric layer near the mesopause that contributes to the OH airglow emission experienced terdiurnal oscillations throughout the winter season. Theoretical polarization relations for evanescent tides, considerations of energy density propagation with altitude, and a day‐by‐day analysis of the zonal and meridional wind speed and temperature all indicate that the terdiurnal tide observed on November was an evanescent zonally symmetric tide. Other data indicate propagating tides, migrating tides, or a mixture of both.
A comparison between Pc 3–4 pulsations observed by GOES 7 and the CANOPUS magnetometer array
Authors: Ziesolleck C. W. S. , McDiarmid D. R. , Feng Q.
Published: February 2004

We have analyzed 1 month of GOES 7 and Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer data and compared Pc 3–4 wave activity seen at geosynchronous orbit with simultaneous ground Pc 3–4 pulsations at auroral latitudes. We found that at geosynchronous orbit the majority of coherent Pc 3–4 events were azimuthally or radially polarized. A number of transverse events revealed clear field line resonance (FLR) signatures in the latitudinal power, polarization, and phase variations. However, the FLR characteristics often appeared to be masked by ionospheric screening and/or wave propagation effects. The azimuthal phase propagation was found to be antisunward for most events, although sunward phase propagation has also been observed. The ground Pc 3–4 occurrence, polarization, and longitudinal phase characteristics associated with transverse waves were found to be similar to those of auroral latitude Pc 5 pulsations. These similarities between transverse Pc 3–4 and auroral latitude Pc 5 characteristics may point to a common source mechanism for these pulsations. One event appeared consistent with the excitation of multiple waveguide modes. Most compressional Pc 3–4 activity appeared to be relatively broadband and virtually unpolarized. However, the center frequency of the dominant band was found to be well correlated with the interplanetary magnetic field magnitude. Power spectra from the ground stations usually contained significant signal power within the frequency band of the compressional perturbation seen by GOES 7. It is shown that at some frequencies, the broadband compressional waves may excite local field line resonances, whereas at other frequencies the compressional signal is transmitted to the ground without obvious resonance structures. The ground Pc 3–4 occurrence, spectral, polarization, and longitudinal phase characteristics associated with compressional waves were found to differ in several respects from the signal characteristics associated with transverse waves. These differences between transverse and compressional Pc 3–4 waves argue against upstream waves being the sole source of auroral latitude Pc 3–4 pulsations.
A durable reduction of cosmic ray intensity in the outer heliosphere
Authors: Van Allen James A. , Randall Bruce A.
Published: February 2004

This paper reports Pioneer 10 (P10) and Pioneer 11 (P11) observations of the intensity J(Ep > 80 MeV) of galactic cosmic rays in the heliosphere near the heliographic equator during the 24‐year period 1972–1996 and out to a heliocentric radial distance of 65 AU. It updates previous P10/P11 determinations of the time dependence of the radial gradient of intensity and emphasizes the recent 10‐year period, especially the consequences of the great Forbush decrease in 1991. A fresh analysis compares P10 and P11 data with comparable data from IMP 8 at 1.0 AU. For this purpose, we have made a critical study of the data from three different instruments on IMP 8 and have developed a new time‐dependent reference level of intensity at 1.0 AU for the period 1974–1996. Using this reference, we find that as of late 1996, recovery of intensity following the 1991 Forbush decrease has been markedly less complete in the outer heliosphere than at 1.0 AU. As a consequence, the mean radial gradient between 4 and 65 AU is now only about +0.3% AU−1. Our findings favor the latitudinal wedge model of the heliosphere [Van Allen and Mihalov, 1990] and suggest that the modulation boundary of the heliosphere is far beyond 65 AU. Generally concordant, but less decisive, evidence of a similar nature has been reported previously by Van Allen [1993], Van Allen [1996], and Webber and Lockwood [1995b].
Large‐scale transport of metallic ions and the occurrence of thin ion layers in the polar ionosphere
Authors: Bedey D. F. , Watkins B. J.
Published: February 2004

A necessary condition for the formation of thin metallic ion layers in the high‐latitude ionosphere, when strong convective electric fields are present, is that these fields have a magnetically westward component. However, observations have shown that proper field direction does not guarantee the occurrence of a layer. A sufficient abundance of metallic ions is also required. We assert that the abundance of metallic ions (and hence occurrence of thin layers) is strongly influenced by large‐scale ion transport, which at high latitudes is determined by the spatial and temporal structure of the large‐scale convective electric field. A simple model is presented which indicates that on the dayside, ions should be lifted from the nominal background metallic layer below 100 km into the lower F region, where they flow horizontally in a narrow vertical stream (∼50 km) toward the nightside. Upon entering the nightside this stream of metallic ions precipitates within a limited band of geomagnetic latitudes, leading to spatially selective enhancements of ion abundance within the E region. Given an appropriately directed convective electric field, the rate of occurrence of detectable thin ion layers should be greater at those locations where precipitation has occurred and, conversely, should be less where precipitation has not occurred. We suggest that this process controls the abundance of metallic ions and therefore explains the fact that layers are sometimes absent in observations, despite the presence of westwardly directed electric fields.
Electrodynamics of midlatitude spread F: 1. Observations of unstable, gravity wave‐induced ionospheric electric fields at tropical latitudes
Authors: Miller C. A. , Swartz W. E. , Kelley M. C. , Mendillo M. , Nottingham D. , Scali J. , Reinisch B.
Published: February 2004

In part 1 of our series exploring the role of electrical forces in midlatitude spread F, we present observations of an electrodynamically driven traveling ionospheric disturbance which passed over Arecibo Observatory between 22 and 24 AST on January 26, 1993. The total electric potential differences driving the wave were of the order of 1 kV. Our analysis indicates that this disturbance is the result of a midlatitude F region plasma instability seeded by a thermospheric gravity wave. Two novel measurements, in addition to typical incoherent scatter observations, were crucial to this determination: the use of 6300 Å airglow images from the coupling, energetics, and dynamics of atmospheric regions (CEDAR) allsky imager to track the two‐dimensional, mesoseale dynamics of the disturbance and the use of a portable ionosonde to simultaneously measure the fieldline integrated ionospheric conductivity in the conjugate hemisphere. We have also determined that this disturbance, like several previously observed midlatitude disturbances, is consistent with our theoretical knowledge of the basic instability of the midlatitude ionosphere described originally by Perkins [1973].
Horizontal plasma flow at midlatitudes: More mechanisms and the interpretation of observations
Authors: Miller Clark A. , Kelley Michael C.
Published: February 2004

The midlatitude ionosphere has been known for decades to exhibit correlated (or anticorrelated, depending on the coordinate system) perpendicular and parallel meridional ion velocities. This correlation is indicative of horizontal plasma flow, despite the highly magnetized character of the midlatitude F region. This coupling of the perpendicular and parallel dynamics is a consequence of extremely weak atmosphere‐ionosphere coupling and the strong tendency for the neutral atmosphere to flow horizontally. Three physical mechanisms have previously been identified as effecting this coupling: ion drag, the F region dynamo, and enhanced gravity‐driven diffusion. We point out some difficulties with one of these processes and present two additional mechanisms, wind‐driven ionospheric equilibrium and gravity wave seeding of plasma instabilities, which can also result in horizontal plasma flow. Although these additional mechanisms further complicate the observational distinction between the possible mechanisms, we argue that an appropriately clustered array of instruments can still distinguish which mechanism is responsible for any particular geophysical occurrence.
On the theory of Alfvén waves in the solar wind
Authors: Khabibrakhmanov Ildar K. , Summers Danny
Published: February 2004

Parallel propagating finite‐amplitude Alfvén waves in a nonhomogeneous medium are considered. The analysis is based on an ideal magnetohydrodynamic (MHD) description with an arbitrary geometry for the background magnetic field. Examination of the characteristic equations of the system reveals, as expected, six waves: forward and backward propagating Alfvén waves and fast/slow magnetosonic waves. However, although these waves are well classified in a homogeneous medium (in particular, the pure Alfvén wave is strictly incompressible and circularly polarized), such a simple classification with regard to compressibility and polarization is found to be impossible in a nonhomogeneous medium. We derive exact forms for the wave action invariant equation and the amplitude conservation law for Alfvén waves and find these equations to be inconsistent, reaffirming that the pure incompressible Alfvén wave does not exist in a nonhomogeneous medium. Construction of a consistent Alfvén wave solution is outlined that incorporates predominantly large‐amplitude incompressible Alfvén waves with small‐amplitude compressible fluctuations. Such a solution corresponds to strong fluctuations in the magnetic field vector and weak fluctuations in the magnetic field strength, as typically observed in the solar wind. In order to counteract the tendency of compressible fluctuations to grow without bound in the ideal MHD model, we include a Landau damping term obtained from kinetic theory.
Distant tail behavior during high speed solar wind streams and magnetic storms
Authors: Ho C. M. , Tsurutani B. T.
Published: February 2004

We have examined the ISEE 3 distant tail data during three intense (Dst < −100 nT) magnetic storms and have identified the tail response to high‐speed solar wind streams, interplanetary magnetic clouds, and near‐Earth storms. The three storms have a peak Dst ranging from −150 to −220 nT and occur on January 9, February 4, and August 8, 1983. During the storm onsets, the fast solar wind and magnetic field dynamic pressure (B2/8π+ΣnikTi) fluctuations moved the tail across the spacecraft multiple times. The magnetotail is strongly compressed by the outside sheath pressure. The lobe field strength can usually be predicted by the pressure balance. The strongest lobe field magnitude detected is 37 nT during storm main phase on January 10, which is higher than the sheath field by 5 – 10 nT. The sheath plasma pressure accounts for the higher lobe field strengths. However, for the February 4 storm, we find that three tail lobe encounters are not in static balance with sheath pressure. During the storm times, the field magnitudes of the lobe and plasma sheet increase by a factor of 3–5 relative to the quiet time. The temperature and density in both regions also increase by factors of 2–3, but with little plasma β changes, as one would expect. Under the assumption of tail flux conservation, increased sheath pressure implies a reduced tail size. Besides the tail size changes, the location of the nominal tail axis is controlled by solar wind flow orientation. This study shows that more than 70% of tail in‐and‐out events are predicted by either of these external mechanisms (changes of tail size due to the external pressure and the solar wind directional changes). Nine tail plasma sheet jettings and 12 slow‐mode shocks have been detected during the three storms. One remarkable feature of the jettings is very strong earthward flow (up to 1200 km/s) and tailward flow (up to 1500 km/s). The solar wind speed for these events was only ∼900 km/s. Both tail flow events have the highest speeds found to date. The preponderance of such a strong earthward flow indicates that during magnetic storms, magnetic reconnection occurs at locations well beyond the distance of ISEE 3. Through the interface of slow‐mode shocks between the tail lobe and the plasma sheet/boundary layer, magnetic energy is being converted into plasma thermal and kinetic energy by the magnetic merging process. The predicted downstream plasma jetting speed (978 km/s) is consistent with the observations (1000 km/s) in the boundary layer. One surprising feature is that this reconnection process seems to be quite prominent during the storm recovery phase. One possible suggestion is that the dynamics of the distant tail are not at all related to magnetic storms and substorms but is an aftereffect, releasing extra magnetic tail energy by field sloughing via these reconnection events, accompanying the plasma sheet expansion.
On estimating the amplitude of Jovian whistlers observed by Voyager 1 and implications concerning lightning
Authors: Hobara Y. , Kanemaru S. , Hayakawa M. , Gurnett D. A.
Published: February 2004

In this paper we extensively reexamine the amplitude of many whistlers detected by the Voyager 1 and try to deduce information about the causative lightning discharges with the use of our ray‐tracing computations taking into account the amplitude. As a result, we have derived the frequency spectra and mean radiation power of the causative lightning discharges and have also applied statistical method to the analysis. We can summarize our findings in the following. The average power flux spectral density of the whistlers falls in a range from 10−12.7V2m−2Hz−1 to 10−11.0V2m−2Hz−1. We calculated the total decrease of the whistler amplitude from the bottom of the ionosphere toward the spacecraft, as a range from about 30 to 40 dB. One of the strongest estimated lightning events exhibits a frequency dependence comparable to the terrestrial one, but its peak frequency seems to be similar to the upward current strokes on the Earth. Moreover, the rather smooth profile obtained implies a small possibility of the presence of stratified layers in the Jovian ionosphere. Other events possibly have features similar to those of the terrestrial return strokes. We calculated the mean radiation power per flash of the lightning in the Jovian atmosphere for a 1‐kHz bandwidth over 60 ms, as a range from the order of 102 to 105 W. The probability distribution of the radiation power in Jupiter is found to follow a lognormal distribution, just as in the terrestrial case.
Direct experimental measurement of electron impact ionization‐excitation branching ratios 4. Branching ratios and cross sections for O2+ at 100 eV
Authors: Doering J. P. , Yang J.
Published: February 2004

The 100‐eV ionization of molecular oxygen has been studied by the (e,2e) electron‐electron coincidence method. Ionization to the ground state and five excited states of O2+ has been detected. Of these six electronic states, ionization to three, the X2Πg, a4Πu, and b4Σg− states, was found to account for approximately 95% of the O2+ production. Relative triple differential cross sections for the production of O2+ in these three states were measured as a function of ejected electron energy, ejected electron angle, and incident electron scattering angle. Ionization to the three higher energy states, B2Σg−, 2Πu, and c4Σu−, was found to be much weaker, and it was only possible to estimate the branching ratios to these states. Ionization to the A2Πu state, the upper state of the second negative bands of O2+, was not observed, as this state is only weakly excited and is overlaid by the stronger a4Πu state. The branching ratios were found to be 0.540, 0.297, and 0.122 for the X2Πg, a4Πg, and b4Σg− states, respectively. The total branching ratio for the B2Σg−, 2Πu, and c4Σu− states was 0.042 (all values ±20%). The branching ratio to the A2Πu state was estimated to be <1%. The cross sections for the X2Πg, a4Πu, and b4Σg− states were calculated using the previously measured O2+ production cross section for 100‐eV electrons on O2. The results were 92.2 (±20%) × 10−18 cm2, 50.8 (±20%) × 10−18 cm2, and 22.1 (±20%) × 10−18 cm2, respectively. The b4Σg− state cross section was in satisfactory agreement with the recalculated optical fluorescence data on the O2+ first negative band emission cross section of 37.1 (±25%) × 10−18 cm2 [McConkey and Woolsey, 1969].
Modulation of electromagnetic ion cyclotron instability due to interaction with ring current O+ during magnetic storms
Authors: Thorne Richard M. , Horne Richard B.
Published: February 2004

We demonstrate that the observed enhancement in the fractional composition of ring current O+ ions during magnetic storms can have a strong controlling effect on the excitation of electromagnetic ion cyclotron (EMIC) waves. For modest storms, when o+ 30%, strong EMIC excitation can occur in the frequency band above the oxygen gryofrequency, , due to cyclotron resonance with anisotropic ring current H+ ions. The path‐integrated gain obtained from ray tracing is sufficient to drive wave amplitudes into the nonlinear regime in a region near the equatorial duskside plasmapause. The excited wave energy is found to be absorbed efficiently at high latitudes via cyclotron resonant interactions with energetic O+ leading to perpendicular heating of the O+ population. Intense waves generated near the equator should therefore not be detectable at low altitudes once the density of O+ has been enhanced during the main phase of a storm. Cyclotron absorption will also enhance the anisotropy of energetic resonant O+ ions. We show that such enhanced anisotropy can excite cyclotron instabilities at frequencies below Ωo+ which are able to propagate to low altitudes and be detectable either on the ground or on low altitude satellites during the storm main phase. For the most intense storms, when the concentration of O+ can attain values o+ 60%, cyclotron absorption by resonant O+ can become so severe as to totally suppress wave excitation in the band above . The most rapid loss process for the ring current (i.e., that due to wave particle scattering) could therefore be suppressed during the main phase of such storms. This raises the interesting question of whether the main phase Dst depression might be modulated by the relative concentration of energetic O+ through the process of resonant interaction with EMIC waves.
Turbulent transport and deposition of the Ito pyroclastic flow: Determinations using anisotropy of magnetic susceptibility
Authors: Baer E. M. , Fisher R. V. , Fuller M. , Valentine G.
Published: February 2004

The Ito pyroclastic flow erupted about 22,000 years ago from Aira caldera in southern Kyushu, Japan. Flow directions were determined by anisotropy of magnetic susceptibility (AMS), which measured the preferential alignment of magnetite microphenocrysts, usually ≤0.25 mm in diameter. The microphenocrysts are aligned with the long axis of the clasts during strain and fracture in the vent. The grains are then deposited parallel to the flow direction with an imbrication. There is no evidence of rolling of clasts or nonflow parallel lineations, and thus AMS can be used to determine flow directions that occurred immediately before deposition. Beyond 30 km from the center of the caldera, flow directions were predominantly down paleogradient, indicating expanded flow and that the depositional system was gravity driven and largely decoupled from the transport system. Within 30 km, measured flow directions are random, indicating that sedimentation occurred from a depositional system that was closely coupled to the turbulent transport system. Individual flow directions at all sites except one varied more than the analytical error, demonstrating that a variety of flow directions existed even within a small area of the flow. This implies that the depositional system was turbulent.
Geomagnetic fluctuations during a polarity transition
Authors: Audunsson Haraldur , Levi Shaul
Published: February 2004

The extensive Roza Member of the Columbia River Basalt Group (Washington State) has intermediate paleomagnetic directions, bracketed by underlying normal and overlying reverse polarity flows. A consistent paleomagnetic direction was measured at 11 widely distributed outcrops; the average direction has a declination of 189° and an inclination of −5°, with greater variation in the inclination [Rietman, 1966]. In this study the Roza Member was sampled in two Pasco Basin drillcores, where it is a single cooling unit and its thickness exceeds 50 m. Excellent core recovery allowed uniform and dense sampling of the drillcores. During its protracted cooling, the Roza flow in the drillcores recorded part of a 15.5 Ma geomagnetic polarity transition. The inclination has symmetric, quasicyclic intraflow variation, while the declination is nearly constant, consistent with the results from the outcrops. Thermal models of the cooling flow provide the timing for remanence acquisition. The inclination is inferred to have progressed from 0° to −15° and back to −3°over a period of 15 to 60 years, at rates of 1.6° to 0.5°/yr. Because the geomagnetic intensity was probably weak during the transition, these apparently high rates of change are not significantly different from present‐day secular variation. These results agree with the hypothesis that normal secular variation persists through geomagnetic transitions. The Iow‐amplitude quasicyclical fluctuations of the field over tens of years, recorded by Roza, suggest that the geomagnetic field reverses in discrete steps, and that more than 15–60 years were required to complete this reversal.
Post‐Variscan thermal and tectonic evolution of the KTB site and its surroundings
Authors: Wagner G. A. , Coyle D. A. , Duyster J. , Henjes‐Kunst F. , Peterek A. , Schröder B. , Stöckhert B. , Wemmer K. , Zulauf G. , Ahrendt H. , Bischoff R. , Hejl E. , Jacobs J. , Menzel D. , Lal Nand , Van den haute P. , Vercoutere C. , Welzel B.
Published: February 2004

The post‐Carboniferous crustal evolution of the German Continental Deep Drilling Program (KTB) area, as summarized in this paper, could not be predicted from surface observations: deep drilling was essential for its revelation. The most conspicuous and unexpected feature discovered in the drill hole is the absence of marked gradients with respect to the pre‐Carboniferous record. There are no depth‐related differences in K‐Ar cooling ages of hornblende and white mica, in petrology or in lithology. All metamorphic rocks encountered, both at the surface as well as in the drill hole down to 9100 m depth, were below 300°C from the Carboniferous onward. The late to post‐Carboniferous deformation is essentially confined to several fault zones. A major fault zone encountered in the drill hole at 7000 m depth is linked by a prominent seismic reflector to the Franconian Lineament, the surface boundary between Variscan basement and Mesozoic cover. This fault zone probably formed in the late Paleozoic and reactivated as a reverse fault in the Mesozoic. Two important episodes of NE‐SW directed shortening by movements along reverse faults took place in the early Triassic and in the late Cretaceous, as indicated by the distribution of apatite and titanite fission‐track ages, the sericite K‐Ar ages of fault rocks, and the sedimentary record in the adjacent basins. Upper crustal slices were detached at a specific level, corresponding to the approximate position of the brittle‐ductile transition in post‐Variscan times, and form an antiformal stack that was penetrated by the KTB throughout its entire depth range.
Monte Carlo analysis of seismic reflections from Moho and the W reflector
Authors: Mosegaard Klaus , Singh Satish , Snyder David , Wagner Helle
Published: February 2004

Near‐normal‐incidence reflections have been used to image the Moho and the W reflector structure in the lithosphere, offshore northern Scotland. To determine the impedance variations at these reflectors, we use a Monte Carlo technique which allows incorporation of geologically realistic a priori information as well as an extensive exploration of the model space, after testing it on a synthetic data set. The method is based on Bayesian inversion theory. The modeled Moho consists of a series of layers with a total thickness of ∼2.4±0.3 km with an overall positive impedance contrast. Inversion of the W reflector results in a model of five‐seven layers with a total thickness of about 3.7±0.6 km and mostly nonpositive impedance contrasts. The implied fine‐scale impedance structure of the Moho is consistent with the broader velocity structure determined from previous wide‐angle reflection/refraction profiles. However, the overall nonpositive impedance contrast at the W reflector requires a structure which is overlain or underlain by a broad increase in velocity in order to match amplitudes of reflected phases observed at large offsets interpreted previously to originate at a similar depth.
The Chile ridge: A tectonic framework
Authors: Tebbens S. F. , Cande S. C. , Kovacs L. , Parra J. C. , LaBrecque J. L. , Vergara H.
Published: February 2004

A new Chile ridge tectonic framework is developed based on satellite altimetry data, shipboard geophysical data and, primarily, 38,500 km of magnetic data gathered on a joint U.S.‐Chilean aeromagnetic survey. Eighteen active transforms with fossil fracture zones (FZs), including two complex systems (the Chile FZ and Valdivia FZ systems), have been mapped between the northern end of the Antarctic‐Nazca plate boundary (Chile ridge) at 35°S and the Chile margin triple junction at 47°S. Chile ridge spreading rates from 23 Ma to Present have been determined and show slowdowns in spreading rates that correspond to times of ridge‐trench collisions. The Valdivia FZ system, previously mapped as two FZs with an uncharted seismically active region between them, is now recognized to be a multiple‐offset FZ system composed of six FZs separated by short ridge segments 22 to 27 km long. At chron 5A (∼12 Ma), the Chile ridge propagated from the Valdivia FZ system northward into the Nazca plate through crust formed 5 Myr earlier at the Pacific‐Nazca ridge. Evidence for this propagation event includes the Friday and Crusoe troughs, located at discontinuities in the magnetic anomaly sequence and interpreted as pseudofaults. This propagation event led to the formation of the Friday microplate, which resulted in the transferal of crust from the Nazca plate to the Antarctic plate, and in a 500‐km northward stepwise migration of the Pacific‐Antarctic‐Nazca triple junction. Rift propagation, microplate formation, microplate extinction, and stepwise triple junction migration are found to occur during large‐scale plate motion changes and plate boundary changes in the southeast Pacific.
Southeast Pacific tectonic evolution from Early Oligocene to Present
Authors: Tebbens S. F. , Cande S. C.
Published: February 2004

Plate tectonic reconstructions of the Nazca, Antarctic, and Pacific plates are presented from late Oligocene to Present. These reconstructions document major plate boundary reorganizations in the southeast Pacific at dirons 6C (24 Ma), 6(o) (20 Ma), and 5A (12 Ma) and a smaller reorganization at chron 3(o) (5 Ma). During the chron 6(o) reorganization it appears that a ridge propagated into crust north of the northernmost Pacific‐Antarctic Ridge, between the Chiloe fracture zone (FZ) of the Chile ridge and Agassiz FZ of the Pacific‐Nazca ridge, which resulted in a northward jump of the Pacific‐Antarctic‐Nazca (PAC‐ANT‐NAZ) mid‐ocean triple junction. During the chron 5A reorganization the Chile ridge propagated northward from the Valdivia FZ system to the Challenger FZ, through lithosphere formed roughly 5 Myr earlier at the Pacific‐Nazca ridge. During this reorganization a short‐lived microplate (the Friday microplate) existed at the PAC‐ANT‐NAZ triple junction. The PAC‐ANT‐NAZ triple junction jumped northward 500 km as a result of this reorganization, from a location along the Valdivia FZ to a location along the Challenger FZ. The chron 5A reorganization also included a change in spreading direction of the Chile and Pacific‐Antarctic ridges. The reorganization at chron 3(o) initiated the formation of the Juan Fernandez and Easter microplates along the East Pacific rise. The manner of plate boundary reorganization at chron 6(o) and chron 5A (and possibly today at the Juan Fernandez microplate) included a sequence of rift propagation, transfer of lithosphere from one plate to another, microplate formation, and microplate abandonment and resulted in northward migration of the PAC‐ANT‐NAZ triple junction. The associated microplate differs from previously studied microplates in that there is no failed ridge.
Crustal structure of northern Baffin Bay: Seismic refraction results and tectonic implications
Authors: Reid I. , Jackson H. R.
Published: February 2004

Two detailed seismic refraction lines were shot in northern Baffin Bay in order to determine the crustal structure and so provide constraints on the complex geological and tectonic history of the area. An array of air guns provided the seismic source, with a total of 16 ocean bottom seismometers as receivers. The data were analyzed by iterative forward modeling of travel times and arrival amplitudes, and the crustal velocity structure obtained was interpreted in the light of previous seismic reflection and refraction results from this area, together with other available geological and geophysical information. Plate reconstructions suggest that the region was a single entity from the Archean until the late Mesozoic, when it underwent successive stages of extension, translation, and compression. On line 2, at 76° N, the continental crust thins between Greenland and Ellesmere Island, reaching to a minimum of about 10 km beneath a narrow, linear basin, which may delineate a former transform plate boundary. On line 4, on the northern margin of Baffin Bay, there is a marked change in velocity structure across the apparent continent‐ocean boundary, with continental crust being replaced by a layer of intermediate seismic velocity that is believed to be serpentinized mantle, indicating that northern Baffin Bay was formed by amagmatic continental rifting and separation. A thick sedimentary sequence is the result of erosion of the Eurekan Orogen to the north, which was formed during the final, compressional tectonic motion. The seismic refraction results are consistent with the tectonic history inferred from plate kinematics.
Stress tensor orientation derived from fault plane solutions in the southwestern Alps
Authors: Eva E. , Solarino S. , Eva C. , Neri G.
Published: February 2004

Gephart and Forsyth's method has been applied to estimate stress orientations from earthquake fault plane solutions of the southwestern Alps, a region where the tectonic stress regime is known to be fairly complex. Fault plane solutions have been either taken from the literature or computed using data from local and regional networks. Data refer to seismic events of magnitude in the range 2.5–5.3 which occurred in the last decades at depths between 0 and 25 km. Two zones with a different stress orientation have been identified in the studied area (44.0°–45.5°N, 6.5°–8.5°E): the western zone, corresponding to the crest of the alpine belt, where a high‐dip maximum compressive stress is found, and the eastern zone (Alps chain to Po Plain transition), characterized by an almost horizontal E‐W σ1 and a nearly vertical σ3. Hypocenters of earthquakes used for stress inversion lie in the depth ranges 0–15 km and 5–25 km in the western and eastern zones, respectively. The transition between the two stress domains is very sharp, and this is also indicated by space distribution of earthquake individual misfits to the respective stress models. The findings of the present study are a good match for tectonic models which assume E‐W compression derived from the Adria‐Europe interaction and producing: (1) major thrusting processes in the eastern side of the chain and (2) secondary tensional effects at very shallow depth beneath the alpine belt crest (western zone of the area studied in this work).
Nondilatant brittle deformation of serpentinites: Implications for Mohr‐Coulomb theory and the strength of faults
Authors: Escartín J. , Hirth G. , Evans B.
Published: February 2004

We conducted deformation experiments to investigate the strength, deformation processes, and nature of the brittle‐ductile transition of lizardite and antigorite serpentinites. A transition from localized to distributed deformation occurs as confining pressure increases from ∼200 to ∼400 MPa at room temperature. Deformation in both brittle (localized) and ductile (distributed) regimes is accommodated by shear microcracks, which form preferentially parallel to the (001) cleavage. Axial microcracks (mode I) are infrequently observed. Volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant, consistent with the shear‐dominated microcracking. Three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow: (1) a lack of evidence for crystal plastic deformation, (2) a positive pressure dependence of the maximum differential stress, and (3) abundant evidence for brittle microcracking. The weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the (001) cleavage, combined with the low pressure dependence of strength. The transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite, rather than the more general Byerlee's law. If brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity, serpentinites define an end‐member style of nondilatant brittle deformation. This deformation style may result in extremely weak faults in nature, and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present.
Timing of basaltic volcanism along the Mesa Butte Fault in the San Francisco Volcanic Field, Arizona, from 40Ar/39Ar dates: Implications for longevity of cinder cone alignments
Authors: Conway F. Michael , Ferrill David A. , Hall Chris M. , Morris Alan P. , Stamatakos John A. , Connor Charles B. , Halliday Alex N. , Condit Christopher
Published: February 2004

Using high‐precision stepwise‐heating 40Ar/39Ar methods, we dated volcanic products of at least 12 vents cropping out along the northern segment of the Mesa Butte Fault in the San Francisco Volcanic Field, Arizona, where the relationship between fault and vent alignment is unambiguous. The 40Ar/39Ar isochron dates indicate that the Mesa Butte Fault hosted five episodes of volcanism over a 1 million year interval, from 1400 to 300 ka. The first episode occurred 1 km northwest of the trace of the Mesa Butte Fault, whereas the next two episodes were centered near the intersection of the Mesa Butte and Cedar Ranch faults. The final two episodes produced Red Mountain (750±100 ka) and Shadow Mountain (300±100 ka), respectively. Ages of vents representing the third (Mesa Butte) episode of volcanism (760±50 ka to 900±80 ka) indicate a systematic southwestward shift in the locus of volcanism along the Mesa Butte Fault that contrasts sharply with overall eastward younging of volcanism within the field. These data demonstrate that structures in basaltic volcanic fields are capable of hosting multiple episodes of volcanism over periods of time comparable to duration periods of composite volcanoes (106 years) but at significantly lower recurrence intervals. These results suggest that probability models of spatiotemporal recurrence of volcanism within basaltic volcanic fields should incorporate mapped structures, particularly where these structures have either hosted previous volcanic activity or are proximal to population centers or sensitive installations, such as nuclear power plants or waste repositories.