bluuebunny/crossref_metadata_2025 huggingface dataset:
First observations of precipitation with a spatial interferometer
Authors: Chilson P. B. , Palmer R. D. , Larsen M. F. , Ulbrich C. W. , Fukao S. , Yamamoto M. , Tsuda T. , Kato S.
Published: February 2008

This paper presents the initial results of a study of precipitation using spatial interferometry (SI) at the MU radar in Japan. On April 30, 1992, data were collected using the VHF Doppler radar at the facility during the passage of a frontal system. A linear variation in the phase was identified in the cross‐spectra of the returned signal in the Doppler velocity ranges corresponding to contributions from both precipitation and refractive index irregularities of the turbulent air. The portions of the spectra which are attributed to precipitation are broadened by the range of particle sizes and the presence of turbulence. This leads to a larger underestimation in the slope of the phase than would be attributed to turbulent fading alone. The results present the first radar interferometry study of precipitation.
Magnetostriction in aluminium‐substituted titanomagnetites1
Authors: Özdemir Özden , Moskowitz Bruce M.
Published: February 2008

Using the strain gauge technique, the magnetostriction constants λS have been measured on sintered polycrystalline specimens of titanomagnetites with Fe2.4‐δ Ti0.6 A��δ O4 compositions (0.05 ≤ δ ≤ 0.20) in the temperature range between 25°C and Tc. The cell‐edge, saturation magnetization, and Curie temperature decrease with increasing aluminium concentration. The room‐temperature λS values are 95.7, 70.6, 52.9 and 25.2×10−6 for δ=0.05, 0.10, 0.15 and 0.20 respectively. A linear relationship is found between the magnetostriction constant and the aluminium content at room temperature. The constant λS(T) is proportional to approximately the third power of the saturation magnetization MS(T). The values of the index n in λS (T) ∼ MSn(T) are 2.72, 3.16, 3.35 and 3.60 for δ=0.05, 0.10, 0.15 and 0.2 respectively. This if the first reported measurement of λS(T) at elevated temperatures for A��‐substituted titanomagnetites. Such data are urgently needed in theoretical and experimental magnetic domain structure studies in rock magnetism.
Long‐term and recent anomalous temperature changes in Australia
Authors: Balling Robert C. , Idso Sherwood B. , Hughes Warwick S.
Published: February 2008

Temperature records from rural station networks are being assembled and analyzed for many Northern Hemispheric locations. In this study, we examine the annual mean, maximum, and minimum temperatures from a network of rural stations in Australia over the period 1911–1990. Our findings show general cooling from 1911 to 1978 with a steady decrease in the diurnal temperature range. The cooling in the mean temperature results from a decrease in maximum temperature that is larger than the increase in minimum temperature. However, in the late 1970s, maximum, minimum, and mean temperatures all rose sharply, although the daily temperature range continued its steady decline. This discontinuity in the Australian temperature record is similar in magnitude and timing to discontinuities found in the Northern Hemispheric climate system. Whether it is related to the buildup of greenhouse gases or some other cause, is unknown.
Evidence for the existence of a mid‐crustal reflector in the Beppu‐Shimabara Graben, Kyushu, Japan
Authors: Iidaka Takashi , Miura Katsumi , Ikami Akira
Published: February 2008

The Beppu‐Shimabara graben, in Kyushu, Japan, has active seismic regions and active volcanoes. Studies using triangulation, gravity surveys and mechanism solutions of large earthquakes show that the graben is in a state of extensive stress associated with rifting. A temporary seismic study was carried out from February 20 to March 8 in 1992 in the region, during which time many seismic signals were observed. The microearthquake seismograms recorded inside the graben have a later phase dominant on the horizontal component following the direct S‐wave. Two‐dimensional ray‐tracing was applied to investigate the cause of the later phase. The phase was identified as an S‐reflected S phase (SxS‐phase) from a discontinuity with a depth of 20 km. The reflection points are located inside the Beppu‐Shimabara graben that is in a state of extensive stress associated with rifting.
Crustal age between the Clipperton and Clarion Fracture Zones
Authors: Eittreim Stephen L. , Ragozin Nikita , Gnibidenko Helios S. , Helsley Charles E.
Published: February 2008

Magnetic anomalies of 52–80 Ma age have been mapped in the equatorial Pacific in a region where they were previously unrecognized. These anomalies, now at 7° N are best modeled with crustal rocks of negative inclination, apparently because the crust was formed at a spreading ridge that was south of the magnetic equator in Late Cretaceous‐Early Tertiary time. Magnetic anomalies recorded along a 5900‐km trackline that follows a flowline of crustal generation show one long period, from 12 to 31 Ma, of apparent constant half‐spreading rate over the 0 to 80 Ma represented.
Shattering and modification of sea‐salt particles in the marine atmosphere
Authors: Mouri Hideaki , Okada Kikuo
Published: February 2008

Sampling of sea‐salt particles was carried out over the western Pacific Ocean. The collected particles were studied individually by X‐ray spectrometry. Sea‐salt particles with radii r ≥ 0.5 μm show a considerable range in their weight ratios of S/Na and Ca/Na, while Cl/Na ratios remain constant at the seawater value. Moreover, there is a tight correlation between S/Na and Ca/Na. This property is attributable to fractionary recrystallization within evaporating drops. Particles with r < 0.5 μm exhibit a similar trend. However, these particles tend to show smaller Cl/Na ratios and larger S/Na ratios than those with r ≥ 0.5 μm. Chemical reaction between sea salt and acidic material, where Cl− is replaced by (SO4)2− and (NO3)−, is the most plausible explanation.
Turbulent relaxation of magnetic fields: 1. Coarse‐grained dissipation and reconnection
Authors: Tetreault David
Published: February 2008

A nonlinear, stochastic MHD model is suggested as an explanation for magnetic reconnection in the nearly collisionless environment of space plasma. The magnetic field line stochasticity (diffusion) results from the existence of overlapping k · B0(x)=0 resonances and provides the anomalous resistivity driving the reconnection. The field lines diffuse in bundles which derive directly, via a nonlinear MHD instability, from the turbulent mixing of the large‐scale fields. The model is a purely self‐consistent MHD model in which the dynamical invariants of one fluid MHD are satisfied. No ad hoc or kinetic instability‐driven anomalous resistivity is required. Anomalous dissipation and reconnection are generated by a coarse‐graining of the stochastic fields. The model is an example of “topological dissipation” of magnetic fields (Parker, 1972). Reconnection rates at a fraction of the Alfven speed are predicted, and are in accord with previous conjecture (Parker, 1973).
Turbulent relaxation of magnetic fields: 2. Self‐ organization and intermittency
Authors: Tetreault David
Published: February 2008

The intermittent nature of the stochastic MHD reconnection model of magnetic fields presented by Tetreault (this issue) is developed. It is shown that the large‐scale magnetic field topology relaxes in spatially localized regions or “domains”. This occurs where the magnetic fluctuations, initially generated nonlinearly in the form of field line bundles, self‐organize into coherent structures. The self‐organization is driven by the tendency for parallel currents flowing along field lines within a bundle to attract each other. The self‐organized structures take the form of flux tubes or ropes which in cross section are localized, topologically closed, magnetic island structures. The emergence of the structures out of the underlying MHD turbulence is an intermittent, macroscopic signature of the reconnection process and of a phase transition in the magnetic field topology. The structures are derived from a coarse‐grained variational principle satisfying the dynamical invariants of MHD, and as such, their emergence can be expected to occur in a wide variety of magnetic field topologies. The model may have geophysical and astrophysical application, including reconnection and associated flux transfer events (FTEs) in the Earth's dayside magnetopause, as well as flux emergence and the occurrence of solar flares on the Sun. Quantitative comparison (spatial scales, speeds, time scales, thresholds, intermittency, and spatial distribution) with satellite data of magnetopause FTEs is favorable.
Ponderomotive mass transport in the magnetosphere
Authors: Allan W.
Published: February 2008

It has recently been suggested that the nonlinear ponderomotive force of hydromagnetic waves in the magnetosphere may cause significant mass transport along the ambient magnetic field. To investigate this further, a two‐dimensional nonlinear hydromagnetic computer code is applied to box magnetospheres with two different radial variations of β, the ratio of plasma pressure to magnetic pressure. It is found that the rate at which density structure develops is dependent on the local sound speed. For fundamental standing waveforms along the magnetic field both the size of maximum equatorial density enhancements and the time for enhancements to develop decrease with increasing sound speed. Density enhancements are limited at each radial position by the propagation of density fronts from the equator toward the ionospheres. The propagation is confined to a given field line, supporting the idea that the fronts propagate in the slow magnetosonic mode. Simple predictors Dsm (for the maximum relative density change) and tsm (for the time to reach maximum density) derived from second‐order theory are tested against the numerical simulations and are found to give quite good results. A dipolelike magnetosphere model is then set up with plausible plasma and magnetic field parameters. Radial variations of Dsm and tsm are obtained for observationally reasonable models of cavity modes and Alfvén resonances. It is found that maximum density enhancements in the inner plasmasphere and in the plasma sheet are relatively small. However, large density enhancements are obtained in the outer plasmasphere and plasma trough. The size of these enhancements depends on the mean ion temperature and the effective ion mass in that region and is also likely to depend on how strongly slow magnetosonic waves are Landau damped.
Spatial and temporal behavior of ULF pulsations observed by the Goose Bay HF Radar
Authors: Walker A. D. M. , Ruohoniemi J. M. , Baker K. B. , Greenwald R. A. , Samson J. C.
Published: February 2008

A detailed analysis of HF radar data of a ULF pulsation event in the postmidnight sector on January 11, 1989, has been carried out using techniques which allow the instantaneous amplitude and phase to be determined as functions of geomagnetic latitude, longitude, and time. Field line resonances with several different frequencies occur simultaneously at different latitudes. These can be associated with cavity mode frequencies of 1.3 mHz, 1.9 mHz, 2.7 mHz, and 3.3 mHz. In addition there is a resonance at 0.8 mHz which does not fit well with a cavity picture. These frequencies are constant to better than 10% over a local time period of nearly 4 hours. They show a packet structure as would be expected if they were triggered by a succession of impulses. The phase changes arbitrarily from packet to packet, but the frequency remains constant. The position of the maximum of the resonance as a function of time changes systematically. It is shown that this arises as the length of the field line changes with time; the resonance remains on the field line having appropriate length and Alfvén speed. The field‐aligned currents driven by the resonances can be as large as 5 μA m−2 at ionospheric heights. The data support a picture of modes driven by solar wind impulses. It may be more appropriate to speak of a waveguide rather than a cavity with the phase velocity of the mode matching the velocity of the impulse along the magnetopause. A difficulty associated with this picture is that the great reproducibility of the frequencies is not consistent with the variability of the magnetopause, which forms one of the boundaries of the assumed resonator. It is, however, difficult to conceive of other resonators, for example in the magnetotail, which would provide a better explanation of the observations.
Survey of transient magnetic field events in the dayside magnetosphere
Authors: Kawano H. , Kokubun S. , Takahashi K.
Published: February 2008

Transient magnetic variations in the dayside magnetosphere are examined by using data acquired with Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE). These events have a time scale of 1 to 15 min, and their signature is similar to that of the flux transfer events (FTEs) reported in the literature. From inspection of data covering the entire dayside at L = 6.0 ∼ 9.4 during 1984 and 1986, 305 events were found, mostly near the apogee of the spacecraft (8.8 RE). The events are distributed in the 0700–1600 magnetic local time (MLT) sector with an occurrence maximum at 1000–1100 MLT. The statistical properties of these events include the following: (1) at northern latitudes the radial magnetic field perturbation is first positive and then becomes negative, while at southern latitudes the reversed sequence is seen; this radial field polarity is the same as that of FTEs; (2) the rotation of the magnetic field perturbation in the plane perpendicular to the ambient field is predominantly left handed (right handed) in the prenoon (afternoon) sector; (3) events with larger total magnetic field perturbations are observed closer to the apogee; (4) short‐duration events are localized near the magnetopause and tend to occur in association with high geomagnetic activities, while long‐duration events are observed at a wide range of distance from the magnetopause and independent of geomagnetic activity; (5) simultaneously obtained IMP 8 magnetic field data indicate that the transient events, especially the short‐duration events, occur in association with southward orientation of the interplanetary magnetic field. Regardless of the generation mechanism, the MLT dependence of the rotational polarity suggests that the source of the magnetic field perturbation, presumably located near the magnetopause, moves azimuthally tailward (dawnward or duskward) from the subsolar magnetopause. We suggest that the short‐duration events are caused by transient and patchy reconnection, while the long‐duration events are caused by changes in solar wind pressure.
The beta dependence of the collisionless tearing instability at the dayside magnetopause
Authors: Ding D. Q. , Lee L. C. , Kennel C. F.
Published: February 2008

The β dependence of the collisionless tearing instability at the dayside magnetopause current sheet is investigated analytically for various current sheet models, where β is the ratio of plasma pressure to magnetic pressure. In the present models the magnetosheath field (Bs) is separated from the magnetospheric field (Bm) by a current sheet with an antiparallel magnetic field component in the z direction and a common guiding magnetic field component (By) in the y direction. Four different current sheet models are considered. For a symmetric current sheet with equal magnetic field strength (|Bm| = |Bs|) and plasma density on its two sides and By = 0 (case A), it is found that the collisionless tearing instability has a strong β dependence. An increase in β leads to a significant decrease in the tearing growth rate. For an asymmetric current sheet with fixed |Bm| and βm on the magnetospheric side and By = 0 (case B) or By = const ≠ 0 (case C), it is found that the collisionless tearing growth rate is not very sensitive to the values of βs where βs is the ratio of plasma pressure to magnetic pressure in the magnetosheath. An increase of βs only slightly changes the tearing growth rate. For an asymmetric current sheet with fixed |Bm| and βm and a large, nonconstant By (case D), it is found that the collisionless tearing growth rate displays a strong βs dependence. The present study shows that the β dependence of the collisionless tearing instability at the dayside magnetopause is controlled by the magnetic field profile across the magnetopause current sheet.
Loss of MHD equilibrium caused by the enhancement of the magnetic By component in Earth's magnetotail
Authors: Hau L. ‐N. , Voigt G. ‐H.
Published: February 2008

The purpose of this paper is to propose that an enhancement of the magnetic By component in Earth's magnetotail might lead to the disruption of the near‐Earth cross‐tail currents and thus to the onset of a magnetospheric substorm. We show, within the context of two‐dimensional MHD equilibrium theory, that the magnetic field energy density, By²/2μo, acts as an additional pressure in the tail plasma sheet that is able to shear magnetotail field lines and, thus, to reduce the normal magnetic field component in the equatorial plane, Bze, perpendicular to the tail current sheet. We solve the Grad‐Shafranov equation for two‐dimensional (∂/∂y = 0) magnetospheric MHD equilibria with a nonzero By component. We compute sequences of quasi‐static equilibria by increasing either |By(A)| or the thermal plasma pressure p(A) in a magnetosphere including the Earth's dipolar field. As a result, the magnetosphere can change from a more “dipolar” tail structure to a tail configuration with highly sheared and/or stretched field lines. Furthermore, increasing By beyond critical values may lead to catastrophic configurations in the near‐Earth plasma sheet whereby the equatorial component, Bze, exhibits a minimum or becomes zero. Such magnetic field configurations can also be achieved by an enhancement of the plasma pressure via the process of adiabatic sunward magnetotail convection. Loss of MHD equilibrium occurs when a quasi‐static magnetic field sequence reaches a critical configuration beyond which no acceptable equilibrium solution exists. Such a senario, as a result of an enhanced (toroidal) shear magnetic field, By, has been proposed as a possible mechanism for driving solar flares or prominence eruptions.
A particle model for magnetotail neutral sheet equilibria
Authors: Burkhart G. R. , Drake J. F. , Dusenbery P. B. , Speiser T. W.
Published: February 2008

Previous studies of forced current sheet equilibria, that is, x‐independent current sheets with a constant Bz = Bn and a constant, uniform electric field Ey (Eastwood, 1972, 1974; Hill, 1975; Francfort and Pellat, 1976; Lyons and Speiser, 1985) are numerically and analytically extended to the regime υD/υT ≲ 1 and Bn/( 4πnbυT²) ≲ 1, where υD = cEy/Bn, υT the proton thermal velocity, and nb is the value of the number density outside the current sheet (at the simulation boundary). Such equilibria may be applicable to the thin current sheets observed in the current disruption region of the magnetotail during substorm growth phase and to the magnetotail neutral sheet. It is found that the current and thickness of the forced current sheet are controlled by the motions of the individual protons. For κA ≪ 1 and υD/υT ≳ 1, we numerically verify the scaling for the current sheet half thickness, a ∼ (υD/υT)−4/3 λb (Francfort and Pellat, 1976), where λb = (mic²/4πnbe²)1/2 and κA is the value of κ = (Rmin/ρmax)1/2 in the self‐consistent current sheet for protons of average energy (Rmin is the minimum field line radius of curvature and ρmax is the maximum gyroradius). The magnetic field outside the current sheet Bx0 is given by eBx0/mic = υD/λb (Hill, 1975). It is further found that if the degree of pitch angle scattering is treated as a variable parameter, there is a peak value of the current corresponding to zero isotropization of the distribution as it passes through the field reversal. The maximum reduction in the net current (or equivalently, the reduction in Bx0) due either to nonlinear particle dynamics (chaos) or to pitch angle scattering wave‐particle interactions is by a factor of
.This maximum reduction in the current can be achieved by particle nonlinear dynamics for values of κA near 1 in the regime υD/υT ≳ 1. Rather than an anomalous resistivity, the most important effect arising from the particle nonlinear dynamics is a catastrophic loss of equilibrium near κA = 1. This feature is due to the inability of proton trajectories with κ ≳ 1 to balance the gradient of the magnetic pressure and to set a length scale in the z direction. The signatures of this process, expressed in terms of the current sheet distribution function, the external distribution function, and the magnetic field structure, are charted. These signatures could be used to determine, through satellite observation, if the abrupt loss of equilibrium observed in our model can result in a disruption in the cross‐tail current.
Nonlinear generation of ion holes in auroral plasmas
Authors: Lakhina G. S. , Shukla P. K. , Verheest Frank
Published: February 2008

It is shown that ion density holes can be generated by a large‐amplitude electrostatic ion cyclotron wave on auroral field lines. The rate at which the ion holes are excited is obtained. The presence of ion holes may cause intermittent double layers to be observed in the auroral particle acceleration region. The predicted scale lengths and velocities of the double layers are in agreement with observations.
Evolution of the plasma environment induced around spacecraft by gas releases: Three‐dimensional modeling
Authors: Gatsonis Nikolaos A. , Hastings Daniel E.
Published: February 2008

A study of the neutral, plasma, and radiation environment induced around spacecraft in low Earth orbit is presented. The three‐dimensional evolution of an artificial plasma cloud created by spacecraft which release neutrals and/or plasma into the ambient ionosphere is examined. Simulations of spacecraft operations which release neutral water and create a water plasma cloud, such as thruster firings or effluent dumps, are performed. It is shown that for time scales of interest in contamination studies the flow of the released neutrals is in the free‐molecular regime. The effects of the altitude of the release, the orientation of the thrust vector with respect to the magnetic field, and the magnitude of the release velocity on the induced environment are considered. It is shown that a large water ion cloud is formed with densities of the order of the ambient oxygen ions. The role of the plasma expansion along the magnetic field lines is discussed. It is predicted that the ambient oxygen forms depletion and enhancement regions. The ultraviolet radiation intensity is estimated. The electrostatic “snowplow” effect and the mapping of the electric field to other regions of the ionosphere are demonstrated. The ambient plasma density is perturbed and forms “image” clouds. The model predictions are discussed and compared with measurements and previous results.
Determining the source region of auroral emissions in the prenoon oval using coordinated polar BEAR UV‐imaging and DMSP particle measurements
Authors: Newell Patrick T. , Meng Ching ‐I. , Huffman Robert E.
Published: February 2008

The Polar Beacon Experiment and Auroral Research (Polar BEAR) satellite included the capability for imaging the dayside auroral oval in full sunlight at several wavelengths. We compare particle observations from the DMSP F7 satellite during dayside auroral oval crossings with approximately simultaneous Polar BEAR 1356‐Å images to determine the magnetospheric source region of the dayside auroral oval. The source region is determined from the Defense Meteorological Satellite Program (DMSP) particle data, according to recent work concerning the classification and identification of precipitation source regions. The close DMSP/Polar BEAR coincidences all occur when the former satellite is located between 0945 and 1000 MLT. We found instances of auroral arcs mapping to each of several different regions, including the boundary plasma sheet, the low‐latitude boundary layer, and the plasma mantle. However, our results indicate that about half the time the most prominent auroral arcs are located at the interfaces between distinct plasma regions, at least at the local time studied here.
Loss‐cone‐driven ion cyclotron waves in the magnetosphere
Authors: Denton Richard E. , Hudson Mary K. , Roth Ilan
Published: February 2008

The theoretical properties of linear ion cyclotron waves propagating in the magnetosphere at arbitrary angles to the background magnetic field are explored. It is found that in some cases the linear wave growth of modes with oblique propagation can dominate that of the parallel propagating electromagnetic ion cyclotron (EMIC) wave. In particular, when the hot ring current protons have a loss cone and their temperature anisotropy A ≡ T⊥/T∥ ‐ 1 is reduced, the parallel propagating EMIC wave becomes stable, while the obliquely propagating loss‐cone‐driven mode persists. The growth rate of the loss‐cone‐driven mode depends strongly on the depth of the loss cone. Unlike the parallel propagating EMIC wave, it can be unstable with A = 0. Other conditions that favor the loss‐cone‐driven mode in comparison to the parallel mode are stronger background magnetic field, lower density of cold hydrogen, and a lower temperature for the hot anisotropic component of hydrogen. A simple analytical theory is presented which explains the scaling of the growth rate of the oblique mode with respect to various parameters. The loss‐cone‐driven mode is an electromagnetic mode which is preferentially nearly linearly polarized. It is nearly electrostatic in the sense that the wave electric field is aligned with the perpendicular (to B0) component of the wave vector k and k⊥ > k∥. Since the electric and magnetic wave fields are perpendicular to B0, they would be difficult to distinguish from those of a linearly polarized parallel propagating electromagnetic wave with the same k∥. However, in certain parameter ranges the loss‐cone‐driven mode can have nonzero ellipticity with a significant compressional component, in contrast to the parallel mode. The loss‐cone‐driven mode may explain recent Pc 1 observations of ion cyclotron wave events which appear to be locally generated with linear polarization (Anderson et al., 1992).
Prediction of geomagnetic activity
Authors: Goertz C. K. , Shan Lin‐Hua , Smith R. A.
Published: February 2008

A simple analytic model for the directly driven, adiabatic solar wind‐magnetosphere‐ionosphere coupling is derived on the basis of frontside reconnection and force balance in the tail. Magnetosphere‐ionosphere coupling occurs through Alfvén waves which are driven by changes of electric fields in the magnetosphere. The model provides an algorithm which calculates the directly driven part of the auroral electrojets from an observed solar wind variation. For two days in May 1979 the cross correlation between the prediction and observation of AE is greater than 0.9. This suggests that most of the geomagnetic activity during these two days was directly driven by the solar wind.
A particle simulation of “giant” undulations on the evening diffuse auroral boundary
Authors: Yamamoto Takashi , Makita K. , Meng C. ‐I.
Published: February 2008

“Giant” undulations on the equatorward boundary of the evening diffuse aurora, occasionally observed during a substorm, are numerically simulated. A “giant” undulation is defined as a waveform having the crest‐to‐trough amplitude comparable to the wavelength. The two‐dimensional particle code is used for studying the motion of magnetospheric plasma perpendicular to the geomagnetic field. Based on the observational facts, it is assumed that in the auroral magnetosphere there exist azimuthally aligned sheets of enhanced density, which are called arc sheets. (Arc sheets inside the inverted‐V structure can actually be responsible for discrete arcs.) The simulation model for giant undulations assumes the presence of an arc sheet equatorward of the proton diffuse aurora. Anomalous cross‐field diffusion of the arc sheet protons leads to polarization of the arc sheet. The polarized arc sheet is such that a negative charge sheet is sandwiched between two positive charge sheets. The polarized arc sheet is subject to a Kelvin‐Helmholtz (K‐H) instability due to a velocity shear generated in the arc sheet. The K‐H waves associated with the arc sheet can spatially modulate the equatorward boundary of the proton diffuse aurora. In the actual simulation, the temporal evolution of the diffuse auroral pattern, as displayed in photographs from the satellites, can be visualized by following the cluster of the energetic protons. The outstanding indentations characteristic of observed giant undulations are successfully reproduced. There have been a few examples of the satellite measurements of electric fields, field‐aligned currents, and fluxes of precipitating particles around the undulating diffuse auroral boundaries. The distributions of electric fields and space charges obtained from the simulation are found to be consistent with these observations. Therefore, giant undulations on the equatorward diffuse auroral boundary can be identified with spatial modulation of the energetic protons by the K‐H waves arising from the polarized arc sheet.
The spectrum of the interplanetary magnetic field near 1.3 AU
Authors: Ruzmaikin Alexander , Lyannaya Irra P. , Styashkin Valery A. , Yeroshenko Eugene
Published: February 2008

A time series of the interplanetary magnetic field measured near 1.3 AU by Phobos 2 is analyzed as a fractal. The fractal dimension of the curves corresponding to the components and to the strength of the magnetic field are found to be close to 5/3. The corresponding spatial spectra are interpreted in the framework of MHD turbulence.
Derivation of atmospheric atomic oxygen and hydrogen profiles from ozone ν3 band emission
Authors: Adler‐Golden S. M. , Gruninger J. , Smith D. R.
Published: February 2008

A method is described for deriving atmospheric O and H atom density profiles in the mesopause region using ozone υ3 emissions. The 10–12 μm hot band intensity determines the O + O2 + M recombination rate; the nighttime hot band/cold band ratio determines the chemical destruction rate and hence the H atom density. Calibration of the hot band intensity versus O atom density has been accomplished using hot and cold band emissions observed under sunlit conditions. Derived density profiles are presented for the high‐latitude SPIRIT 1 and SPIRE rocket experiments and compared with mass spectrometer/incoherent scatter model predictions.
Formation and stability of the self‐consistent one‐dimensional tail current sheet
Authors: Pritchett P. L. , Coroniti F. V.
Published: February 2008

The structure and stability of thin one‐dimensional current sheets in which the ions carry most of the current and momentum are investigated. It is demonstrated that for the case of a cold current sheet the characteristic half width λ is given by λ ≈ (Bz/B0)4/3 c/ωp0, where Bz is the normal field component (assumed to be spatially uniform), B0 is the asymptotic magnitude of the reversing field, and c/ωp0 is the collisionless ion skin depth based on the asymptotic density. The trapping of the ion orbits in the reversal region leads to a strong density enhancement at the center of the current sheet given by nc/n0 ≈ c/λωp0 ≈ (B0/Bz)4/3. A two‐dimensional self‐consistent dynamical simulation model is developed which demonstrates that these idealized current sheets are unstable to kink perturbations driven by the anisotropic pressure distribution produced by the chaotic nature of the particle orbits in a field‐reversal region. The combined effects of chaotic scattering and finite thermal spread are likely to preclude the formation in the geomagnetic tail of stable current sheets with half width less than about 0.25c/ωp0 (∼200 km).
Relaxation of transient lower ionospheric disturbances caused by lightning‐whistler‐induced electron precipitation bursts
Authors: Glukhov V. S. , Pasko V. P. , Inan U. S.
Published: February 2008

A quantitative model of the relaxation of transient lower ionospheric (D region) disturbances caused by lightning‐induced electron precipitation is developed, taking advantage of known particular features of the lightning‐induced disturbances, such as the fact that they are produced in typically <1 s and decay over 10–100 s. The model represents the nighttime D region as consisting of only four kinds of charged particles (electrons, positive ions, negative ions, and positive cluster ions) and is particularly suited for description of the detailed behavior of the electron density. Application of the model to some previously modeled disturbances indicates that some of the least known chemical reaction rates in the nighttime D region altitudes may be measurable using subionospheric VLF data. In the production of secondary ionization by precipitating electron bursts, the model calculations indicate the presence of a saturation effect such that the number density of the secondary electrons is not simply equal to the ion pair production rate times the burst duration. In some cases involving precipitation of ∼1‐MeV electrons, the model predicts the formation of new layers of ionization at 50–70 km altitude that represent a different attachment‐detachment quasi‐equilibrium value from that of the unperturbed ambient. Such new layers may exist for up to ∼105 s following electron precipitation bursts.
Characterization of small‐scale structures at the magnetopause from ISEE measurements
Authors: Rezeau L. , Roux A. , Russell C. T.
Published: February 2008

High‐resolution data (covering up to 8 Hz) from the flux gate magnetometers on the two ISEE spacecraft are used to analyze ultralow‐frequency (ULF) fluctuations observed at the magnetopause and in the adjacent layers. Intersatellite correlations are computed to show that the same structure can be identified in the fluctuations observed on both spacecraft when the interspacecraft distance is small. Then the possibility of deducing the velocity of the structure from two‐point measurements is discussed; it is shown that it can be estimated only in certain cases.
On the low correlation between long‐term averages of solar wind speed and geomagnetic activity after 1976
Authors: Crooker N. U. , Gringauz K. I.
Published: February 2008

During solar cycle 20, the first full cycle with measurements of solar wind parameters, geomagnetic activity measured by Ap was found to correlate with the square of solar wind speed V, and activity measured by Dst was found to correlate with the product of V and the southward component of the interplanetary magnetic field, Bs. Both of these correlations break down during cycle 21. In the case of Ap, the much stronger variation of Bs in cycle 21 compared to cycle 20 makes clear that the Bs contribution to activity is important on yearly as well as shorter time scales. The product BsV² gives an excellent correlation with Ap over both cycles. In the case of Dst, the stronger variation of Bs in cycle 21 causes a stronger variation in BsV, which is not reflected in Dst, perhaps because Dst also depends upon solar wind dynamic pressure in a nonlinear way.
A global analytical representation of the magnetic field produced by the region 2 Birkeland currents and the partial ring current
Authors: Tsyganenko N. A.
Published: February 2008

A quantitative model is developed of the magnetic field produced by the electric current system of region 2 Birkeland currents, closed via the partial ring current. The distribution of j⊥ is computed from a given axially asymmetric spatial distribution of hot isotropic magnetospheric plasma over an infinitely thin L shell in an axisymmetric purely dipolar geomagnetic field, while the field‐aligned current density is found from the continuity of the net electric current. The magnetic field distribution is derived by a Biot‐Savart integral over the electric current system. An assumed cosine dependence of the plasma pressure on local time makes it possible to reduce the problem of analytical representation of the B field to two dimensions. The obtained numerical fits for the partial ring current/region 2 Birkeland current magnetic field are relatively simple, continuous, and valid throughout the whole extraterrestrial space from ionospheric heights up to tens of RE. To our knowledge, this is a first global analytical representation for the magnetic field of the partial ring current system which can be incorporated in future advanced models of the external geomagnetic field.
Effects of thruster firings on the shuttle's plasma and electric field environment
Authors: Machuzak John S. , Burke William J. , Retterer John M. , Hunton Donald E. , Jasperse John R. , Smiddy Michael
Published: February 2008

Simultaneous plasma and AC/DC electric field measurements taken during the space shuttle mission STS‐4 at times of prolonged thruster firings are analyzed and cross correlated. Depending on the orientation of the shuttle's velocity vector to the magnetic field, ion densities and electric field wave spectra were enhanced or decreased. The systematic picture of interactions within the shuttle's plasma/neutral gas environment of Cairns and Gurnett (1991b) is confirmed and extended. Waves are excited by outgassed and thruster‐ejected molecules that ionize in close proximity to the shuttle. On time scales significantly less than an ion gyroperiod, the newly created ions act as beams in the background plasma. These beams are sources of VLF waves that propagate near the shuttle and intensify during thruster firings. Plasma density depletions and/or the shuttle's geometry may hinder wave detection in the payload bay. A modified two‐stream analysis indicates that beam components propagating at large angles to the magnetic field are unstable to the growth of lower hybrid waves. The beam‐excited, lower hybrid waves heat some electrons to sufficient energies to produce impact ionization. Empirical evidence for other wave‐growth mechanisms outside the lower‐hybrid band is presented.
Heating of proton conics by resonant absorption in a multicomponent plasma: 1. Experimental evidence
Authors: Rauch Jean Louis , Lefeuvre François , Le Quéau Dominique , Roux Alain , Bosqued Jean Michael , Berthelier Jean Jacques
Published: February 2008

ELF emissions observed on the low‐altitude AUREOL 3 satellite are seen in association with H+ ions at large pitch angle. The flux in the upward direction for ∼120° pitch angle is found to be equal to or larger than the flux at ∼60° pitch angle, which provides evidence for transverse acceleration at or below the spacecraft. These emissions have a sharp lower‐frequency cutoff of the transverse components of the electric field and a narrow peak at, or, more precisely, just below the proton gyrofrequency fH+. This narrow peak is more easily seen on the parallel component and appears as a narrow line on the spectrogram of this component. A statistical study of the occurrence of this line at f ∼ fH+ is presented. It is shown that this line is observed at relatively high invariant latitude within the light ion trough where a strong depletion of thermal H+ ions occurs. Detailed analysis of ELF waves observed just below fH+ demonstrates that they propagate in the left‐hand mode. These observations are interpreted as a signature of mode conversion from a fast magnetosonic mode into a slow proton cyclotron mode. It is suggested that this slow proton cyclotron wave can accelerate protons up to a few hundreds electron volts in the transverse direction. This mode conversion process can operate over a much broader of large altitude range than covered by AUREOL 3; it is a likely candidate for explaining the formation of H+ conics. Theoretical calculations that support the above conclusions are given in a companion paper by Le Quéau et al. (this issue).
Cold ions at the Martian bow shock: Phobos observations
Authors: Dubinin E. , Lundin R. , Koskinen H. , Norberg O.
Published: February 2008

The measurements carried out by the plasma spectrometer ASPERA, on‐board the Phobos 2 spacecraft show that the Martian bow shock is characterized by a sudden increase of ionization of the neutral corona. It acts as a source of new ions that can strongly modify the process of ion heating behind the shock front. The loss of momentum of solar wind protons due to their interaction with exospheric ions may lead to an increase in the effective scale of the obstacle.
Pseudobreakup and substorm growth phase in the ionosphere and magnetosphere
Authors: Koskinen H. E. J. , Lopez R. E. , Pellinen R. J. , Pulkkinen T. I. , Baker D. N. , Bösinger T.
Published: February 2008

We present observations made in space and on the ground during the growth phase and the onset of a substorm on August 31, 1986. About 20 min after the ε parameter at the magnetopause had exceeded 1011 W, magnetic field dipolarization with an increase of energetic particle fluxes was observed by the AMPTE Charge Composition Explorer (CCE) spacecraft at the geocentric distance of 8.7 RE close to magnetic midnight. The event exhibited local signatures of a substorm onset at AMPTE CCE and a weak wedgelike current system in the midnight sector ionosphere. However, it did not lead to a full‐scale substorm expansion, as determined by several ground‐based instruments, nor did it produce large particle injections at geostationary orbit. Only after another 20 min of continued growth phase the entire magnetosphere‐ionosphere system could apparently allow the onset of a regular substorm expansion. The initial activation is interpreted in the present paper as a “pseudobreakup.” We examine the physical conditions in the near‐Earth plasma sheet using spacecraft observations and analyze the development in the ionosphere using ground‐based magnetometers and electric field observations from the STARE radar. We find that the main observable differences between pseudobreakups and ordinary breakups are the strength and consequences. Furthermore, it is shown that ionospheric activity at the time of a pseudobreakup is not necessarily as localized in longitude as generally believed.
Siple Station, Antarctica, experiments on staircase frequency ramps as approximations to continuous ramps
Authors: Mielke T. A. , Helliwell R. A.
Published: February 2008

VLF wave growth and emission triggering in the magnetosphere are known to depend on the coherence of the input signal. In these experiments the input signal may take the form of a constant frequency pulse (1 s duration) or a linearly chirped pulse (also 1 s duration) with a frequency ramp (1000 Hz/s) of either positive or negative slope. When a frequency ramp of 1000 Hz/s is approximated by a contiguous sequence of short, constant frequency steps, the growth behavior is unchanged for step durations less than 25 ms. For a step duration of 25 ms, the behavior more nearly resembles that of a pure ramp, but some evidence of individual step growth can be seen. For step durations of 50 ms and 100 ms, growth is relatively small and is confined to each individual step, with little coupling between steps. Rising ramps generally show more growth than falling ramps of similar step size and average slope, confirming earlier observations. An equivalent “sawtooth” frequency variation with an average slope of zero and a step size of 25 ms behaves much like a constant frequency pulse. An interpretation of the response to step size can be found in the theory of second‐order resonance, where the spatial rates of change of the electron gyrofrequency and the Doppler‐shifted wave frequency are equal. This theory also accounts for the previously observed “dot‐dash” anomaly, where a long constant frequency pulse shows much more growth and emission triggering than a short pulse.
On the proper motion of auroral arcs
Authors: Haerendel G. , Buchert S. , La Hoz C. , Raaf B. , Rieger E.
Published: February 2008

The ability of the tristatic UHF radar of EISCAT to measure plasma velocities in the F region has been employed for the assessment of such motions in the proximity of and relative to auroral arcs. This required real‐time tracking of the arcs with the radar upon command from the ground station monitoring the aurora. Slow relative motions of arc and plasma of the order of 30‐90 m s−1 were deduced from the observations, but the error sources are considerable. Three cases are presented, representing different situations. The arc may move into an auroral current system or out of it. The first case corresponds to an energy reduction, the second to a buildup of the energy content of the current system. Oscillatory motions, alternating between these situations, exist. The oscillations lie in the Pc 5 range and testify for an interaction of the energy release region (auroral acceleration region) and the generator via shear Alfvén waves. The events are interpreted in the light of the “fracture model” of auroral arcs developed by the first author. Although relative motions of the observed magnitude are predicted by this theory, other explanations are conceivable.
Time‐dependent magnetic annihilation at a stagnation point
Authors: Anderson C. , Priest E. R.
Published: February 2008

Magnetic reconnection is a fundamental process that can take place in astrophysical or laboratory plasmas. It occurs within regions of large magnetic gradient where the magnetic field is no longer frozen to the plasma but instead diffuses through it, releasing magnetic energy and causing a change in the connectivity of the field lines. In particular, on the Sun, magnetic reconnection is believed to play an important role in coronal heating, X ray bright points, solar flares and canceling magnetic features. Here, an exact time‐dependent solution of the MHD equations for magnetic annihilation in response to a time‐varying stagnation point flow is presented. The main assumptions in this model are that the magnetic field lines are straight (so that there is no magnetic tension acting on the plasma) and the flow that carries the field lines together is of stagnation point type. This is a reasonable model for the resistive MHD behaviour near the X point of a reconnecting field, especially when the central diffusion region is long, as in the flux pile up regime (Priest and Forbes 1986). The general solution is used to conduct a series of numerical experiments, namely, the evolution of different initial magnetic profiles in a steady flow; the effect of a sudden change in magnetic diffusivity on an initially steady state; the effects of a velocity that either increases linearly in time or ramps up from one steady value to another. The results exhibit the effects of subtle imbalances in diffusion and advection but have the following general features: (1) a diffusion layer is created, the thickness of which is determined by the nature of the plasma flow; (2) the magnetic field outside the diffusion region is determined by advection and will either exponentially increase, exponentially decay or remain in a steady state, depending on whether the initial magnetic profile B (y) ∼ y−n has n < 1, n > 1, or n = 1, respectively; and (3) the magnetic field within the diffusion layer tends to respond to the advected magnetic field at its edge.