bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence of the ionospheric response to the QBO
Authors: Chen Pei‐Ren
Published: February 2008

The yearly intensity variation of the day‐to‐day variability of the equatorial ionization anomaly (EIA) has been investigated. It is revealed that this yearly intensity variation is weakly anticorrelated with the solar activity and geomagnetic aa indices but obviously modulated by the QBO (Quasi‐Biennial Oscillation): in the east phase of the QBO the day‐to‐day variability of the EIA increases; in the west phase of the QBO the day‐to‐day variability of the EIA decreases. These effects seem to imply that it is not the solar bursts and/or geomagnetic storms but other disturbances to which the day‐to‐day variability of the EIA is to be ascribed. A possible explanation for this phenomenon is proposed.
Nonlinear inversion of phase and amplitude coherence functions at NORSAR for a model of nonuniform heterogeneities
Authors: Flatté Stanley M. , Wu Ru‐Shan , Shen Zheng‐Kang
Published: February 2008

A Marquardt, nonlinear, iterative inversion is applied to obtain the best‐fit parameters for 7 spectral models of heterogeneities under NORSAR using the observed TCF (Transverse Coherence Functions) and ACF (Angular Coherence Functions) at NORSAR. The simplest model is a single‐layer model with one power‐law spectrum; the most complex model is a two‐overlapped‐layer model with two different power‐law spectra specified by 6 unknown parameters. F‐tests are performed for different pairs of models; the two‐layer model with two different spectra is a significant improvement over the single layer, one spectrum model. The final results of nonlinear inversion agree with the Flatté‐Wu spectral model for heterogeneities under NORSAR.
Chaos in the plasma sheet
Authors: Goertz Christoph K. , Smith Robert A. , Shan Lin‐Hua
Published: February 2008

We discuss a simple dynamical model of the magnetotail, in which the electric field in the current sheet evolves in response to a solar‐wind‐induced electric field at the magnetopause, and depends on the specific entropy of the plasma in the plasma sheet. The entropy varies due to nonadiabatic heating in a region where ULF waves are absorbed by resonant mode conversion. The feedback between temperature and entropy change leads to an evolution which exhibits chaos when the solar wind electric field is neither very small nor very large. The onset of chaos may be quite sudden or may proceed through a sequence of period doublings.
On the equipartition of kinetic energy in plate tectonics
Authors: Olson Peter , Bercovici David
Published: February 2008

Kinetic energy in present‐day plate motions is partitioned approximately equally between poloidal and toroidal parts. The poloidal part is excited by buoyancy forces and is associated with cellular flow in the mantle. Most of the toroidal part is derived from strike‐slip motion at transform faults and from oblique subduction at convergent margins. Plate spin accounts for only a few percent of the toroidal energy of the lithosphere. Using a simple kinematic model, we demonstrate that a square‐shaped plate, drifting without spin, has equal poloidal and toroidal energies. We then show that the most probable state for an array of many rectangular plates is equipartition of poloidal and toroidal kinetic energy, provided the drift directions of individual plates in the array are random. Our model predicts that convergent margins occupy 42% of plate boundaries, while divergent and transform margins occupy 25% and 33%, respectively. This distribution is in good agreement with the actual distribution along the boundaries of the Pacific, India, Cocos and Nazca plates, which contain more than 90% of the lithosphere kinetic energy. According to these calculations, equipartitioning of plate kinetic energy implies a large degree of decoupling between plates and a disorganized pattern of convection in the mantle.
Chaos and stochasticity in space plasmas
Authors: Ashour‐Abdalla Maha , Baker Daniel N.
Published: February 2008

This special issue of Geophysical Research Letters presents a collection of papers dealing with stochasticity and nonlinear dynamics in space physics. Its 20 papers represent contributions from a large number of research groups and cover a wide range of topics in space plasma research. Despite this diversity and the use of many different methodologies, the papers have as a common theme the recognition that apparent randomness in space plasma systemssometimes has an underlying order which is describable by ordinary nonlinear coupled differential equations.
Seismic evidence for magma in the vicinity of Mt. Katmai, Alaska
Authors: Ward Peter L. , Pitt Andrew M. , Endo Eliot
Published: February 2008

P‐wave traveltime delays of as much as 0.9 sec are consistently observed at one seismic station from local and regional earthquakes 70 to 150 km deep. This station is on the southwest flank of Mt. Trident, the most recently active volcano within Katmai National Park, Alaska. Delays from local shallow earthquakes are typically less than 0.3 sec, suggesting that most of the major delay results from anomalous material at depths of more than a few kilometers. This station is located near the center of a bowlshaped low in the Bouguer gravity field that is approximately 15 km in diameter and more than 25 mgals deep. These anomalies suggest, but do not prove, the presence of considerable amounts of magma in the shallow part of the crust that could have been the source for all magma erupted in the vicinity of Mt. Katmai and Mt. Trident this century.
The loss of CF2O on ice, NAT, and sulfuric acid solutions
Authors: Hanson D. R. , Ravishankara A. R.
Published: February 2008

We measured the first‐order loss rate coefficients for CF2O on solid surfaces, ice and HNO3‐treated ice, and on sulfuric acid solutions that coated the inside wall of a flow tube. We determined an upper limit for the uptake coefficient, γ, of ∼3×10−6 for CF2O on ice and HNO3‐treated ice. A value for γ on a 60% (by weight) sulfuric acid solution of 3(+2/−3)× 10−6 was measured and we observed a higher value over 40% acid, γ = 6×10−5 (± 30%). The loss of CF2O on stratospheric particles is a very slow process and will be insignificant for events that occur on timescales of a year, such as the annual polar chlorine activation phenomena.
Shallow faults mapped with seismic reflections: Lost River Fault, Idaho
Authors: Ali Mubarik , Miller Richard D. , Steeples Don W.
Published: February 2008

A high‐resolution seismic‐reflection survey, conducted at the intersection of Arentson Gulch road and the western splay of the Lost River fault scarp in central Idaho, defines a bedrock surface about 80 m deep which is segmented by several faults forming graben structures. Six meters of total fault displacement can be interpreted on the bedrock reflector while only 1 to 2 m of displacement can be observed on a shallower refracting interface and the surface fault scarp. This relatively small displacement suggests the western splay has either been active only recently or extremely infrequently since deposition of the bedrock, or that strike‐slip motion may be present. A westward deflection of the major activity along the Lost River fault was probably responsible for the gap in 1983 surface faulting between the Warm Spring and Thousand Springs segments. The inconsistency in total bed displacement based on reflection, refraction, and fault‐scarp evidence suggests tectonic activity on the western splay spans more than just a single episode.
Early solar mass loss: A potential solution to the weak sun paradox
Authors: Graedel T. E. , Sackmann I.‐J. , Boothroyd A. I.
Published: February 2008

The weak sun paradox, in which early planetary temperatures are known to have remained above the freezing point of water despite presumably lower early solar luminosity, has customarily been resolved by imposing high early CO2 atmospheric abundances. Speculative new solar models incorporating early solar mass loss provide a possible solution to the case of ‘missing’ solar lithium; at the same time, they predict higher early (4.5–3.8 Gyr BP) solar luminosities. These higher luminosities have the potential to produce planetary temperatures within the liquid water range indicated by observations without requiring extremely high CO2 concentrations.
Diapirism and the origin of high TiO2 mare glasses
Authors: Hess Paul C.
Published: February 2008

High TiO2 mare picritic glasses are derived from cumulate source regions that are only modestly endowed with ilmenite‐enriched crystallization products. These sources are mobilized by the heat derived from the primitive interior and evolve into diapirs which rise adiabatically from depths in excess of 700 km. As these diapirs undergo pressure‐release melting, they also stir in significant portions of the surrounding mantle.
Mineral compositions in pristine lunar highland rocks and the diversity of highland magmatism
Authors: Bersch Michael G. , Taylor G. Jeffrey , Keil Klaus , Norman Marc D.
Published: February 2008

High precision electron microprobe analyses of olivine and pyroxene in pristine lunar highland rocks confirm the dichotomy between ferroan anorthosites and the Mg‐suite. Ferroan‐anorthosites plot as coherent trends, consistent with formation in a complex global magma system. Lack of coherent compositional trends in the Mg‐suite rocks indicates derivation from numerous magmas.
Element partitioning between majorite and liquid: A secondary ion mass spectrometric study
Authors: Yurimoto Hisayoshi , Ohtani Eiji
Published: February 2008

The secondary ion mass spectrometry (SIMS) was applied to measure the trace element abundances in majorite and the ultramafic liquid equilibrating at high pressure and temperature for the first time, and was proved to be useful for the high pressure experimental petrology. The analysis by SIMS was made to determine the abundance of 15 lithophile elements (Li, Be, B, Sc, Ti, V, Cr, Mn, Y, Zr, La, Sm, Gd, Yb, and Hf), one siderophile (Co), and two chalcophile elements (Cu, Zn). The electron microprobe (EPMA) analysis was also made to obtain the major element abundance. Some lithophile elements (Al, Be, and V) are enriched in majorite, whereas the lithophile elements (Li, B, Na, K, Ca, Ti, Zr, Y, La, Sm, Gd, and Hf), and siderophile and chalcophile elements (Co, Cu, Zn) are favored in the liquid. These results give important constraint for the genesis of komatiites and chemical heterogeneity in the Archean mantle.
In situ measurements of fluid flow in DSDP Holes 395A and 534A: Results from the Dianaut Program
Authors: Morin Roger H. , Hess Alfred E. , Becker Keir
Published: February 2008

The DIANAUT program provided the first opportunity to directly measure vertical fluid flow in ocean boreholes by means of a high resolution thermal flowmeter. Measurements of volumetric flow rate were obtained in DSDP (Deep Sea Drilling Project) Holes 395A and 534A. These results identified a total flow of 2300 L/hr of seawater entering Hole 395A from the seafloor that diminished to about 550 L/hr at a depth of 251 meters below seafloor (mbsf), indicating that approximately 3/4 of the original downward flow had exited the borehole and entered the open formation across the upper 140 m of basement. This information allows the upper oceanic crust at this site to be delimitated into three hydrologic units, with basalt permeabilities of 3.0 × 10−14 m2 near the sediment/basement interface decreasing sharply as a function of depth to values much less than 10−16 m2 below 440 mbsf. It is estimated that approximately 108 L of seawater have entered this well since it was drilled in 1975. Quantitative flow measurements in Hole 534A were inconclusive because of technical problems with the flowmeter packer. Nevertheless, results showed that borehole fluid was moving upward and out into the open ocean at a rate on the order of a few hundred liters per hour, roughly one order of magnitude less than that determined for Hole 395A and moving in the opposite direction. There is good correlation between these field measurements and the attendant temperature logs from each well, and the results provide strong evidence of important mass‐transport processes associated with the diverse submarine hydrologic systems in the upper oceanic crust.
Hypothesized mechanism for stratospheric QBO influence on ENSO variability
Authors: Gray William M. , Sheaffer John D. , Knaff John A.
Published: February 2008

A hypothetical mechanism is described whereby the QBO of zonal winds in the lower tropical stratosphere alters the distribution of intense deep convective activity throughout the tropical West Pacific. In conjunction with the annual cycle and the buildup of heat in the West Pacific warm pool, these QBO‐linked variations of deep convection cause variations in the Central Pacific trade winds and Walker circulation which in turn govern the occurrence of ENSO events in the tropical Pacific. Although some details of the hypothetical mechanism are tentative, empirical results present a compelling argument for the stratospheric QBO to at least partly regulate ENSO variability.
simulation of ion acceleration in a charged dust cloud
Authors: Motschmann Uwe , Sauer Konrad , Roatsch Thomas
Published: February 2008

The interaction of a plasma flow with a cloud of charged dust particles is studied using a one‐dimensional hybrid simulation model. Depending on the dust charge density and the cloud diameter, this cloud is either a weak or a strong obstacle in the plasma flow. For a weak dust obstacle a super‐magnetosonic flow is decelerated monotonically inside the cloud whereas a sub‐magnetosonic flow is accelerated. For a strong dust obstacle the flow configuration in the cloud changes dramatically: Deceleration and acceleration may change across the cloud and strong proton heating and shock generation may occur with the stationary regime being discarded. An interpretation of the proton dynamics in weak clouds is given on the basis of a simple analytic multifluid model.
Seismic image of the Ivanhoe Lake Fault Zone in the Kapuskasing Uplift of the Canadian Shield
Authors: Wu Jianjun , Mereu Robert F. , Percival John A.
Published: February 2008

The Kapuskasing uplift, located in the central Canadian shield, represents an oblique exposure of the Archean middle to lower crust. The Ivanhoe Lake fault zone, believed to be the basal thrust carrying the high‐grade rocks of the Kapuskasing zone over the low‐grade Abitibi greenstone belt, holds the key to understanding the nature and evolution of the Kapuskasing uplift. Despite numerous geological and geophysical studies, including LITHOPROBE deep seismic reflection profiles, and because of very limited bedrock exposure in the area, the shallow structure of the Ivanhoe Lake fault zone remains obscure. Here we present results obtained by reprocessing data from a LITHOPROBE seismic reflection profile across the fault zone. For the first time, the Ivanhoe Lake fault zone is clearly imaged on the seismic section as a series of west‐dipping reflectors with an average dip of 20°, which can be traced to the surface. The results support the conclusion that fault zones form good reflectors.
The plasma environment, charge state, and currents of Saturn's C and D rings
Authors: Wilson G. R.
Published: February 2008

The plasma environment of the Saturnian C and D rings is investigated by modeling the flow of ionospheric plasma from the mid‐ to low‐latitude ionosphere to the vicinity of the rings. The model used is time‐dependent and kinetic and incorporates the gravitational, centripetal, magnetic mirror and ambipolar electric forces. It was found that the plasma density near the C and D rings, at a given radial location, will experience a one to two order of magnitude diurnal variation. With a knowledge of the plasma density and temperature near these rings their charge state is investigated by use of a dust cloud charging model (Havnes et al., 1984, 1987, 1990). The associated azimuthal currents are also found. Results show that the surface charge density of the C and D rings can show significant radial and azimuthal variations, due mainly to variations in the plasma density. In addition to its plasma density and temperature dependence the surface charge density will also depend on structural features of the rings such as the ring thickness and the nature of the particle size distribution. Its magnitude may vary over seven decades. The associated azimuthal currents carried by these rings will also show large diurnal variations resulting in field‐aligned currents which close in the ionosphere as shown by Ip and Mendis [1983]. However, the resulting ionospheric electric fields will probably not produce a significant amount of plasma convection in the topside ionosphere and inner plasmasphere as proposed by these authors, due in part to the level of the currents as well as the height‐integrated Pedersen conductivities at the local times where the currents close.
Origin of low cross‐helicity regions in the inner solar wind
Authors: Bruno R. , Bavassano B.
Published: February 2008

Part of the present understanding of the phenomena governing the radial evolution of the solar wind MHD turbulence can be summarized in the following scenario: outward propagating Alfvén waves are generated near the Sun and, in addition, more waves of the same type, with mixed sense of propagation, are freshly generated at interplanetary stream shears, through plasma instabilities. The nonlinear interaction between outward propagating (majority modes) and inward propagating (minority modes) Alfvén waves is then crucial for the dynamical evolution of the MHD turbulence, which is reflected in the radial dependence of the normalized cross‐helicity (σc). The evolution of this parameter toward lower values, with increasing heliocentric distance, is interpreted as a continuous production of minority modes. However, since the same effect on σc can be obtained simply by decreasing the presence of the outward modes alone, we have separated the “inward” from the “outward” contribution by using Elsässer variables (z±) and investigated the possibility that this phenomenon could be statistically related to the presence of nonpropagating field and/or density fluctuations able to reduce the v,b correlation of the majority modes. Our results support this point of view and show that while fluctuations of field intensity produce a depletion of the outward modes, fluctuations in density reinforce the inward modes. The conclusions that we draw from this analysis refer to time scales longer than 1 hour: (1) local generation of Alfvén modes is not so common in the solar wind, (2) the radial evolution of σc is mainly due to the continuous destruction of the outward modes rather than to the presence of inward modes, and (3) inward modes are likely to be representative of plasma structures convected by the solar wind rather than propagating Alfvénic fluctuations. These observations pose new theoretical problems for any model which attributes to local generation and as a consequence to e− modes, a role that they do not seem to have in the evolution of the large‐scale solar wind turbulence.
Parallel electric fields detected via conjugate electron echoes during the Echo 7 sounding rocket flight
Authors: Nemzek R. J. , Winckler J. R.
Published: February 2008

Electron detectors on the Echo 7 active sounding rocket experiment measured “conjugate echoes” resulting from artificial electron beam injections. Analysis of the electrons' drift motion after a complete bounce led to measurements of the magnetospheric convection electric field mapped to ionospheric altitudes. The magnetospheric field was highly variable, changing by tens of millivolts per meter on time scales of as little as hundreds of milliseconds. Sets of double probes on the payload measured the ionospheric electric field directly. While the smallest‐scale magnetospheric field irregularities were damped out by ionospheric conductivity, larger‐scale features were enhanced by up to 50 mV/m in the ionosphere. The mismatch between magnetospheric and ionospheric convection fields indicates a violation of the equipotential field line condition, i.e., the existence of parallel electric fields above the payload. The parallel fields occurred in regions ≈10 km across and probably supported a total potential drop of 10‐100 V. The flight took place in the premidnight diffuse auroral region and therefore was likely in a region of downward field‐aligned current. There is some evidence that the parallel fields also pointed downward and so could be responsible for creating the field‐aligned current by drawing electrons out of the ionosphere.
Plasma wave system measurements of the Martian bow shock from the Phobos 2 spacecraft
Authors: Trotignon Jean Gabriel , Grard Réjean , Savin Serguéï
Published: February 2008

The high‐resolution data of the electric field observations performed by the plasma wave system (PWS) during some of the Martian bow shock intersections by Phobos 2 were analyzed. Plasma and wave detectors are very useful instruments for locating the shock transition region and studying structures in the upstream region, such as the foot or the electron foreshock. The electron plasma oscillations that develop in the latter give access to the plasma density of the solar wind. Shock surface models derived from the PWS data are compared to those obtained by other authors, and attention is paid to similarities and differences between the electric field measurements obtained for Mars, Venus, and Earth.
ULF waves in the low‐latitude boundary layer and their relationship to magnetospheric pulsations: A multisatellite observation
Authors: Takahashi Kazue , Sibeck David G. , Newell Patrick T. , Spence Harlan E.
Published: February 2008

On April 30 (day 120), 1985, the magnetosphere was compressed at 0923 UT and the subsolar magnetopause remained near 7 RE geocentric for ∼2 hours, during which the four spacecraft Spacecraft Charging At High Altitude (SCATHA), GOES 5, GOES 6, and Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE were all in the magnetosphere on the morning side. SCATHA was in the low‐latitude boundary layer (LLBL) in the second half of this period. The interplanetary magnetic field was inferred to be northward from the characteristics of precipitating particle fluxes as observed by the low‐altitude satellite Defense Meteorological Satellite Program (DMSP) F7 and also from absence of substorms. We used magnetic field and particle data from this unique interval to study ULF waves in the LLBL and their relationship to magnetic pulsations in the magnetosphere. The LLBL was identified from the properties of particles, including bidirectional field‐aligned electron beams at ∼200 eV. In the boundary layer the magnetic field exhibited both a 5–10 min irregular compressional oscillation and a broadband (Δƒ/ƒ ∼ 1) primarily transverse oscillations with a mean period of ∼50 s and a left‐hand sense of polarization about the mean field. The former can be observed by other satellites and is likely due to pressure variations in the solar wind, while the latter is likely due to a Kelvin‐Helmholtz (K.‐H.) instability occurring in the LLBL or on the magnetopause. Also, a strongly transverse ∼3‐s oscillation was observed in the LLBL. The magnetospheric pulsations, which exhibited position dependent frequencies, may be explained in terms of field line resonance with a broadband source wave, that is, either the pressure‐induced compressional wave or the K.‐H. wave generated in or near the boundary layer.
Observations of a detached, discrete arc in association with field line resonances
Authors: Samson J. C. , Hughes T. J. , Creutzberg F. , Wallis D. D. , Greenwald R. A. , Ruohoniemi J. M.
Published: February 2008

Data from the Canadian Auroral Network for the OPEN Unified Study (CANOPUS) array in Canada are used to analyze the magnetic fields and auroral structures which were associated with a field line resonance which occurred near local midnight and in the early morning sector. The electric fields of this resonance, which have a frequency of about 1.95 mHz, were observed by the Johns Hopkins University, Applied Physics Laboratory HF radar at Goose Bay and had a maximum at about 70.7° invariant latitude. Effects of the field line resonance were seen in the 5577 Å, meridian scanning photometer data and the magnetometer data from the CANOPUS array. The field line resonance was accompanied by precipitating energetic electrons (energies greater than 3 keV) in a narrow auroral arc which was 3°–4° equatorward of the resonance structure seen in the radar data and approximately 1° equatorward of field lines threading the inner boundary of the proton plasma sheet. The electron precipitation in this arc was modulated at the frequency of the field line resonance. The effects of the resonance are also seen as pulsations in the magnetometer data, and the horizontal polarization of the pulsations showed a change in sense of rotation across the arc. The oscillations in the precipitating electrons might have been caused by modulation in the ELF/VLF growth rates due to the presence of the magnetohydrodynamic waves associated with the resonance, by mode conversion to kinetic Alfvén waves, or by the formation of electrostatic ion cyclotron waves due to field‐aligned currents associated with a second field line resonance collocated with the arc. The evidence presented here suggests that the modulation of ELF/VLF by magnetohydrodynamic waves on field lines near the plasmapause was the most likely cause of the auroral oscillations.
Comparison of ground‐based optical observations of N2 second positive to N2+ first negative emission ratios with electron precipitation energies inferred from the Sondre Stromfjord radar
Authors: Gattinger R. L. , Jones A. Vallance , Hecht J. H. , Strickland D. J. , Kelly J.
Published: February 2008

Optical emission ratios of the N2 Second Positive Group 0,0 band to N2+ First Negative Group 0,0 band were observed in the magnetic zenith at Sondre Stromfjord in parallel with radar observations of the electron density height profile. The dependence of the optical ratios on the inferred incident electron energies was investigated to assess whether the ratios could be used as an electron energy monitor. Analysis procedures were developed to minimize the effects of atmospheric scattering as well as spectral blending with other emission features. Two independent computational techniques for inferring the electron energies from the electron density profiles were compared and found to agree sufficiently well for the current study. The observed optical emission ratios were found to vary by less than 15% over the incident electron energy range from 2 to 12 keV. Comparisons were made with the ratios predicted by several auroral electron deposition models.
Momentum transport at the Mars magnetopause
Authors: Perez‐de‐Tejada H.
Published: February 2008

The conditions leading to the transport of momentum of the shocked solarwind to the Mars magnetosphere are examined. It is argued that planetary pickup ions born in the magnetosheath and scattered across the magnetopause by local turbulent waves carry that momentum and deliver it to the magnetospheric plasma. It is further suggested that as the pickup ions experience momentum scattering interactions with the wave field in the velocity shear adjacent to the magnetosphere they are subject to a gradual internment within that region of space. The end effect of this phenomenon is that the pickup ions deliver a larger amount of momentum to the local flow than what they can subtract from it. Calculations of the efficiency of the process lead to values of the effective mean free path of the pickup ions of the order of a few hundred kilometers.
Collisionless reconnection in two‐dimensional magnetotail equilibria
Authors: Pritchett P. L. , Coroniti F. V. , Pellat R. , Karimabadi H.
Published: February 2008

A two‐dimensional (x, z) particle simulation model based on the Darwin approximation to Maxwell's equations is developed for studying collisionless reconnection in the magnetotail. The particles and fields are initialized in accord with a general equilibrium configuration which includes a pressure gradient along the tail axis and tail flaring. The model is used to investigate a number of theoretical issues regarding the spontaneous ion tearing instability under the assumption that the electron dynamics are unimportant. It is demonstrated both numerically and analytically that in a thin current sheet with ρ i0 ∼ λ (ρ i0 is the ion Larmor radius based on the lobe field and λ is the characteristic thickness of the current sheet) the growth rates in the absence of a normal field component are much smaller than expected based upon the analytic theory for a thick sheet (ρi0 ≪ λ). For such a thin current sheet the presence of a normal field Bz on axis of even a few percent strongly inhibits the growth of the instability. This result is not altered by the addition of a constant By component smaller than the lobe field. It is demonstrated further that the transition to stability occurs when the cyclotron frequency based on Bz equals the growth rate of the Bz = 0 tearing mode. This requires typically a normal field of the order of 6% of the lobe field. If a sufficiently large external perturbation of the lobe magnetic field reduces the normal field on axis below the stabilization threshold over a significant fraction of a tearing mode wavelength, then one can recover the rapid instability of the one‐dimensional neutral sheet.
Observations of reconnection of interplanetary and lobe magnetic field lines at the high‐latitude magnetopause
Authors: Gosling J. T. , Thomsen M. F. , Bame S. J. , Elphic R. C. , Russell C. T.
Published: February 2008

Measurements made with the Fast Plasma Experiment on ISEE 2 in the vicinity of the high‐latitude, dusk magnetopause near the terminator plane, at a time when the local magnetosheath and tail lobe magnetic fields were nearly oppositely directed, provide direct evidence for the reconnection of the open field lines of the tail lobes with the interplanetary magnetic field (IMF). The evidence consists primarily of observations of accelerated magnetosheath plasma flowing both tailward and sunward within the high‐latitude magnetopause current layer. Observed speed changes at the magnetopause were of the order of twice the magnetosheath Alfven speed and were quantitatively consistent with the predictions of reconnection models. At times when the newly entering magnetosheath plasma observed at ISEE 2 was accelerated sunward a secondary beam of ions, presumably mirrored at low altitudes, was occasionally present. At times when the newly entering magnetosheath plasma observed by ISEE 2 was accelerated tailward a secondary beam of largely unaccelerated mantle plasma was occasionally present. Small plasma accelerations observed on reconnected field lines in the magnetosheath were associated with the presence of ions reflected at the magnetopause and moving at a speed of approximately twice the Alfven speed relative to the remainder of the magnetosheath plasma. Although previous work has anticipated that the re‐reconnection of the open field lines of the tail lobes with the IMF would be associated with northward IMF in the magnetosheath, the present reconnection event was associated with a local magnetosheath IMF which had a small southward component.
Equatorial density depletions observed at 840 km during the great magnetic storm of March 1989
Authors: Greenspan M. E. , Rasmussen C. E. , Burke W. J. , Abdu M. A.
Published: February 2008

Early on March 14, 1989, a thermal plasma probe on the Defense Meteorological Satellite Program (DMSP) F9 spacecraft detected extensive and dramatic decreases in the ion density at 840 km, near 2130 LT, during two consecutive transequatorial passes over South America. The order of magnitude decreases in the ion density extended more than 4000 km along the satellite track. The depletions were accompanied by upward and westward plasma drifts, both in excess of 100 m/s. Their onsets and terminations were marked by extremely sharp density gradients. DMSP F9 observed no similar depletions over the Atlantic during preceding orbits. A partial depletion was detected over the eastern Pacific during the following orbit. The DMSP F9 ground track passed slightly west of a Brazilian total electron content (TEC) station and two Brazilian ionosondes during the first depletion encounter. The TEC fell far below normal during the night of March 13–14. The ionosonde measurements indicate that, in the hour after sunset, before DMSP passed through the depletions, the F2 layer rose rapidly and disappeared, but at the time of the first depletion encounter, hmF2 was decreasing over one of the stations. The DMSP F8 satellite, which orbits in the dawn‐dusk meridian, made related measurements on March 13 and 14. Crossing the equator at dust on March 13, at the same longitude where DMSP F9 encountered the first depletion, DMSP F8 detected upward and westward drifts, but it measured extremely large rather then depleted ion densities. During two dawn passes over the eastern Pacific on March 14, DMSP F8 observed depletions somewhat similar to those detected by DMSP F9. Large westward drifts accompanied the depletions detected by DMSP F8. It is quite probable that the morningside depletions detected on March 14 are remnants of those detected earlier by DMSP F9 in the evening sector. We develop a phenomenological model reconciling DMSP F8, F9, and ground‐based measurements. Our calculations show that rapid upward drifts sustained for several hours can produce depletions in the equatorial ion density with sharp gradients at their high‐latitude boundaries, consistent with the data. We discuss possible contributing mechanisms for generating these upward drifts. These include direct penetration of the magnetospheric electric field to low latitudes, the electric fields generated by the disturbance dynamo, and the effects of conductivity gradients near the dusk terminator and the South Atlantic anomaly.
Absolute differential and integral electron excitation cross sections for atomic nitrogen: 1. The 4So → 3s4P (λ1200 Å) transition from 30 to 100 eV
Authors: Doering J. P. , Goembel L.
Published: February 2008

The absolute direct differential and integral electron excitation cross sections for the atomic nitrogen 4So → 3s4P (λ1200 Å) transition have been measured at incident energies of 30, 50, and 100 eV. The differential cross sections (DCS) measured versus scattering angle were integrated to give the integral cross section (ICS) as a function of incident energy. The ICS has a maximum value of 66×10−18 cm² near 50 eV. Above 50 eV, the direct excitation cross section is approximately a factor of two lower than the fluorescence cross section of Stone and Zipf (1973) when the latter is corrected for the recent change in the H2 dissociative excitation reference cross section. This difference suggests that cascade accounts for ∼50% of the fluorescence emission above 50 eV. At 30 eV, the direct cross section is only 1/5 of the fluorescence cross section, suggesting that there is a strong cascade contribution to the fluorescence cross section from forbidden transitions to the 3p 4Do and 3p 4Po states just above threshold. The consequences of these new cross section values for aeronomy are discussed.
Structure of medium Mach number quasi‐parallel shocks: Upstream and downstream waves
Authors: Krauss‐Varban D. , Omidi N.
Published: February 2008

Satellite observations of quasi‐parallel collisionless shocks document a complex behavior that has proven to be extraordinarily difficult to analyze and interpret. Recent data analyses and computer simulations, however, have revealed that these shocks can be unsteady and cyclically reforming, and have shed some light on the processes that are involved. Despite this progress, a number of issues regarding the ion thermalization at and re‐formation of quasi‐parallel shocks are still not well understood. Previous simulation studies at high Mach numbers have suggested either the convection of far upstream density perturbations into the shock or an instability at the shock interface as mechanisms contributing to the shock re‐formation. On the other hand, simulations at low Mach numbers have shown a steady shock. To understand the transition from steady low Mach number to unsteady high Mach number shocks, we have performed large‐scale one‐dimensional hybrid code simulations at increasing Mach numbers. We find that fast‐mode waves with wavelengths longer than phase‐standing whistlers are excited upstream from the shock by backstreaming ions. At increasing Mach numbers, the phase and group velocities of the dominant waves are reduced until they point back toward the shock. When there is sufficient energy flux in these waves, they lead to unsteady shock behavior and eventually to shock re‐formation. For typical solar wind conditions and an angle of θBn = 30° between the shock normal and the upstream magnetic field, we find the shock to become unsteady at MA ∼ 2.3. Various spectral diagnostics are employed to investigate the properties of the excited upstream and downstream waves. Fast‐mode wave energy that is convected through the shock is partially mode converted into downstream Alfvén and slow‐mode fluctuations. This mechanism may be responsible for some of the turbulence observed downstream of the the Earth's quasi‐parallel bow shock. An analysis of the shock dynamics indicates that there are no characteristic time scales associated with the unsteady medium Mach number shocks.
Theory and observation of triple‐root jump in spacecraft charging
Authors: Lai Shu T.
Published: February 2008

Sudden onsets of high‐voltage differential charging on spacecraft in an ambient environment may affect operations and the survival of on‐board electronics. Triple‐root jumps may be very sudden. We discuss two aspects, one theoretical and one observational. By using current balance, the theoretical parametric domain in which triple‐root jumps may occur for a surface material in a double Maxwellian electron environment has been calculated. The relation between the domain and the “critical” or “threshold” temperature for a material is revealed. We present an example of the prediction of the occurrence of a triple‐root spacecraft potential jump in a time‐varying space environment.
On the thermodynamics of the plasma sheet
Authors: Goertz C. K. , Baumjohann W.
Published: February 2008

The evidence for nonadiabatic transport in the quiet central plasma sheet is reinvestigated. In this study, we include AMPTE IRM data from the plasma sheet boundary layer and active times selected on the basis of large AE values. We find that as the plasma is transported from the plasma sheet boundary layer into the central plasma sheet, both its temperature (T) and its density (n) increase. The plasma obeys a relation p ∝ n4/3 for quiet times (AE < 100 nT) and p ∝ n5/3 for AE > 300 nT. The remarkable anticorrelation between plasma temperature (T = p/n) and density in the quiet central plasma sheet reported previously is also evident in this data set, but this may not be indicative of nonadiabatic transport. It may simply be related to the fact that the criterion used for deciding whether the spacecraft is in the central plasma sheet depends on the pressure itself or the thermal ion flux which is proportional to nT1/2. Thus low‐density and low‐temperature observations are assumed to indicate that the spacecraft is in the boundary layer. The data also indicate that the temperature in the quiet plasma sheet is usually less than 6 keV, and high‐temperature values are more likely to be observed in what is defined as the active plasma sheet. The observations also suggest that the plasma sheet contains a mixture of high‐entropy “bubbles” and low‐entropy “blobs.” It appears that these either merge or are lost from the tail before they are convected into the near‐Earth tail.
VHF and HF radar measurements of E and F region plasma drifts at the magnetic equator
Authors: Viswanathan K. S. , Namboothiri S. P. , Rao P. B.
Published: February 2008

Simultaneous observations of E region horizontal irregularity drifts by VHF backscatter radar and of F region vertical plasma drifts by HF Doppler radar conducted during daytime on a few magnetically quiet days at Trivandrum (dip 0.2°N) are presented. A comparative study of the two measurements indicates broadly (1) a resemblance in the daytime changes of the E‐W component between the electric fields and (2) evidence of quasi‐periodic electric field variations with periods ranging mostly from 1 to 2 hours. The electric fields derived from HF Doppler radar observations are somewhat lower than those deduced by VHF radar observations. The correlation coefficient for the variations of the electric fields measured by the two experimental techniques is found to be in the range of about 0.5 to 0.9. The observed difference in the E and F region electric fields at the magnetic equator is discussed in terms of the measurement uncertainties and the limitations involved in deriving E‐W electric fields. The observations are suggestive of a latitudinal variation in the E‐W component of the electric field in the equatorial ionosphere.
Theory for modeling the equatorial evening ionosphere and the origin of the shear in the horizontal plasma flow
Authors: Haerendel G. , Eccles J. V. , Çakir S.
Published: February 2008

Companion papers in this series present (1) the role of equatorial E region postsunset ionosphere, (2) the origin of horizontal plasma shear flow in the postsunset equatorial ionosphere (this paper), (3) the Coloured Bubbles experiments results, and (4) computer simulations of artificial initiation of plasma density depletions (bubbles) in the equatorial ionosphere. Within this paper, equations describing the time evolution of the equatorial ionosphere are developed using flux tube integrated and flux tube weighted quantities which model the chemistry, dynamics, and electrodynamics of the equatorial ionosphere. The resulting two‐dimensional set of equations can be used to investigate equatorial electric fields neglecting small‐scale phenomena (λ < 1 km). An immediate result derived from the integrated current equations is an equation describing the physics of the shear in the horizontal flow of the equatorial plasma during the evening hours. The profile of the horizontal flow has three important contributing terms relating to the neutral wind dynamo, Hall conduction, and the equatorial electrojet current divergence. Using a one‐dimensional model of the velocity shear equation and the integrated ionospheric transport equations, a time history of the development of the shear feature during the postsunset hours is presented. The one‐dimensional model results are compared to the velocity shear measurements from the Coloured Bubbles experiments.
Pattern of electron and ion precipitation in northern and southern polar regions for northward interplanetary magnetic field conditions
Authors: Troshichev O. A. , Nishida A.
Published: February 2008

Using the data of the DMSP F6 and F7 satellites, we show that there is a systematic difference between ion precipitation in the auroral oval and in the polar cap. The oval ion precipitation is smooth, and the ratio of the total electron number flux to the total ion flux is less than 20. The polar cap ion precipitation is patchy, and the ratio is more than 20. The boundary between these two types of ion precipitation usually can be detected for both northward and southward interplanetary magnetic field by a sharp fall of the ion total number flux below the level 5×105–106 ions (cm² s sr)−1. Since this determination of the poleward boundary of the auroral oval is efficient for any conditions, we propose to define the polar cap as a region bounded by this boundary. The polar cap determined in this way has roughly the shape of an ellipse aligned along the 1100–2300 MLT meridian with daytime and nighttime boundaries at 80° and 72°, respectively. Four patterns of electron precipitation in the polar cap are identified for northward IMF conditions: (1) symmetric precipitation in both polar caps with no distinct regularity. (2) symmetric precipitation with a prevalence of electron fluxes in the morning sector of both polar caps, (3) asymmetric distribution with no correspondence between the patterns in the opposite polar regions, and (4) asymmetric distribution where the increased electron fluxes are observed in opposite sectors (morning or evening) of the opposite polar caps depending on the polarity of the BY IMF component. The energy spectra of electrons even in neighboring spikes in the same polar cap may be greatly different, and therefore discrepancy in electron spectra in opposite polar caps cannot be regarded as unique proof for the open character of the magnetic field lines.
Distribution in magnetotail of O+ ions from cusp/cleft ionosphere: A possible substorm trigger
Authors: Cladis J. B. , Francis W. E.
Published: February 2008

The transport of O+ ions from the cusp/cleft ionosphere to the magnetotail during highly disturbed times was determined by computing the guiding‐center trajectories of the ions to a distance of 6 RE from the ionosphere and the full‐motion trajectories at later times. Case histories were tallied in six planes perpendicular to the XGSM axis, three planes perpendicular to the YGSM axis, and in the center plane of the tail. At various times relative to the enhancement of the convection electric field, the following ion properties were constructed from the case histories: number density, mean energy, energy and pitch angle distributions of the flux, and ion pressure components parallel and perpendicular to the magnetic field. It was found that after about 1.7 hours the ion flux in the near‐Earth magnetotail increased dramatically and the spectrum hardened, much as observed during periods just preceding substorms. This increase is attributed to (1) the increase in the O+ outflux from the ionosphere, (2) the increased energization of the ions by the convection electric field, and (3) ion trapping, which generally occurs because the ion magnetic moments generally increase after the ions first cross the geomagnetotail center plane. Moreover, the parallel pressure of the ions exceeds the energy density of the magnetic field at XGSM < −8 RE. On the basis of the expected alterations of the magnetic and electric fields in response to this O+ pressure, a substorm trigger mechanism is suggested.
Theory of small‐scale density and electric field fluctuations in the nightside Venus ionosphere
Authors: Huba J. D.
Published: February 2008

Recently, it has been reported that small‐scale (λ ∼ 0.1‐2 km) density irregularities occur during 100‐Hz electric field bursts in the nightside ionosphere of Venus. The correlation of field and plasma fluctuations suggests that a local plasma instability may be responsible for the turbulence. In this paper we provide a detailed analysis of the lower‐hybrid‐drift instability as a mechanism to generate the observed irregularities. We develop a fully electromagnetic theory that is relevant to the finite β plasma in Venus’ ionosphere and include collisional effects (e.g., electron‐ion, electron‐neutral, and ion‐neutral collisions). The key features of the analysis that favor this instability are (1) it is a flute mode and propagates orthogonal to the ambient magnetic field, (2) it is a relatively short wavelength mode and the Doppler‐shifted frequency can be ≳ 100 Hz, (3) it can produce both electric field and density fluctuations, as well as magnetic field fluctuations in a finite β plasma, and (4) it is most unstable in low‐β plasmas (i.e., β ≲ 1) so that it is likely to occur in the low‐density, high‐magnetic‐field ionospheric holes. These features are consistent with observational results.