bluuebunny/crossref_metadata_2025 huggingface dataset:
Global, quasi‐steady dynamics of the distant solar wind: 2. Deformation of the Heliospheric Current Sheet
Authors: Pizzo V. J.
Published: February 2004

A three‐dimensional (3‐D) magnetohydrodynamic (MHD) numerical model is used to trace global deformations of the heliospheric current sheet (HCS) caused by large‐scale dynamical interactions associated with corotating solar wind flows. Configurations incorporating the tilted‐dipole geometry are investigated out past 30 AU for a variety of dipole tilt angles, α. Inclusion of the full, 3‐D interplanetary dynamics allows north‐south displacements and the east‐west warping of the HCS by advective corotational effects to be accurately assessed for the first time. It is found that large‐scale spatial correlations between velocity and density imposed at the coronal source (i.e., the geometric arrangement whereby regions of high‐speed low‐density material lying adjacent to areas of slow, dense flow interact obliquely under the influence of solar rotation) result in a distinctive pattern of deformation of the HCS. For an α = 30° tilted‐dipole example, it is shown that typical zonal variations in radial velocity lead to significant folding of the HCS within about 5 AU of the Sun. By 10 AU, additional sharp bends appear near the latitudinal extremes of the HCS surface, where it is overtaken by shock fronts driven by 3‐D corotating interaction regions (CIRs). Moreover, the model suggests that inside about 20 AU, major plasma structures are systematically organized about the HCS, such that the greatest concentrations of material and magnetic field (the centroids of the 3‐D CIR structures) are coincident with the folded crests of the HCS (near heliographic latitudes λ = ±α). Thus, in these circumstances many of the more interesting dynamical features inconveniently lie well away from the heliographic equator. At larger heliocentric distances, where neighboring CIRs begin to interact strongly, the warping of the HCS abates dramatically and the association between folds in the HCS and major field and density concentrations is weakened and ultimately breaks down. On the basis of the model calculations, quasi‐steady deformations and related phenomena should figure prominently in the interplanetary medium, except during those periods when the magnetic dipole and solar spin axes are nearly aligned ( α ≲ 10°). Even then, they may be of significance if there are any substantial local warps or kinks in the streamer belt.
Simultaneous retrieval of the solar EUV flux and neutral thermospheric O, O2, N2, and temperature from twilight airglow
Authors: Fennelly J. A. , Torr D. G. , Richards P. G. , Torr M. R.
Published: February 2004

We present a method to retrieve neutral thermospheric composition and the solar EUV flux from ground‐based twilight optical measurements of the O+(²P) 7320 Å and O(¹D) 6300 Å airglow emissions. The parameters retrieved are the neutral temperature, the O, O2, N2 density profiles, and a scaling factor for the solar EUV flux spectrum. The temperature, solar EUV flux scaling factor, and atomic oxygen density are first retrieved from the 7320‐Å emission, which are then used with the 6300‐Å emission to retrieve the O2 and N2 densities. The retrieval techniques have been verified by computer simulations. We have shown that the retrieval technique is able to statistically retrieve values, between 200 and 400 km, within an average error of 3.1 ± 0.6% for thermospheric temperature, 3.3 ± 2.0% for atomic oxygen, 2.3 ± 1.3% for molecular oxygen, and 2.4 ± 1.3% for molecular nitrogen. The solar EUV flux scaling factor was found to have a retrieval error of 5.1 ± 2.3%. All the above errors have a confidence level of 95%. The purpose of this paper is to prove the viability and usefulness of the retrieval technique by demonstrating the ability to retrieve known quantities under a realistic simulation of the measurement process, excluding systematic effects.
Core magnetic field enhancement in single X line, multiple X line and patchy reconnection
Authors: Ma Z. W. , Otto A. , Lee L. C.
Published: February 2004

Magnetic flux transfer events often show a significant increase of the magnetic field strength at the center of the events. Similar magnetic field observations have been reported for structures in or near the plasma sheet of the magnetotail at about 20 RE. We have carried out two‐dimensional (2D) and three‐dimensional (3D) simulations of single X reconnection (SXR), multipleX line reconnection (MXR), and patchy reconnection to determine and compare the amplification of the magnetic field in the center of the developing flux tubes. The various processes are achieved by appropriate choices of 2D or 3D resistivity models. The simulations show that the increase in magnetic field strength depends on both the property of the initial configuration and the particular reconnection geometry. For the chosen initial conditions the MXR process leads to a larger increase of the core magnetic field than the patchy reconnection and SXR caused by larger magnetic tensions in the MXR process. The 3D processes always lead to a larger amplification than the corresponding 2D processes. In the 3D cases, force imbalance in the y direction will accelerate plasma out of the flux tube. This process reduces the thermal pressure and leads to a further compression of the flux rope, which yields an additional increase in the interior magnetic field strength.
The heliospheric plasma sheet
Authors: Winterhalter D. , Smith E. J. , Burton M. E. , Murphy N. , McComas D. J.
Published: February 2004

High‐resolution magnetic field and plasma data gathered by ISEE 3/ICE during several sector boundary crossings are used to investigate the narrow heliospheric current sheet (≈ 3 × 10³ km to 104 km thick), together with the heliospheric plasma sheet in which it is embedded. The heliospheric plasma sheet region is identified by a significantly enhanced plasma beta caused by density enhancements and diminished magnetic field strength and is about 20 to 30 times the thickness of the current sheet. The thickness of the heliospheric plasma sheet is found to increase exponentially with its average proton density. The heliospheric current sheet is often displaced to one edge or the other of the heliospheric plasma sheet. Further, the point of maximum plasma beta in the plasma sheet, where the magnetic field strength is at a broad local minimum, is not colocated with the heliospheric current sheet. Within the plasma sheet, changes in the magnetic pressure are balanced by corresponding changes in the plasma thermal pressure as expected for a convected solar wind feature. In addition, observations show small pressure differences between the regions upstream and downstream of the plasma sheet, which are interpreted as causing the plasma sheet to move across the spacecraft.
Transversely accelerated ions in the topside ionosphere
Authors: Retterer John M. , Chang Tom , Jasperse J. R.
Published: February 2004

Data from the rocket campaigns MARIE and TOPAZ III, within regions of low‐altitude transversely accelerated ions, are interpreted to explain the acceleration of the ions. Using the Monte Carlo kinetic technique to evaluate the ion heating produced by the simultaneously observed lower hybrid waves, we find that their observed electric field amplitudes are sufficient to explain the observed ion energies in the MARIE event. Much of the uncertainty in evaluating the efficiency of a plasma wave induced particle heating process which is dependent on a velocity resonance comes from the lack of information on the phase velocities of the waves. In the case of the MARIE observations, our modeling efforts show that features in the ion velocity distribution are consistent with the wave phase velocities inferred from interferometer measurements of wavelengths. The lower hybrid waves with which low‐altitude transversely accelerated ions are associated are frequently observed to be concentrated in small‐scale wave packets called “spikelets”. We demonstrate through the scaling of the sizes of these wave packets that they are consistent with the theory of lower hybrid collapse. Using the Monte Carlo technique, we find that if the lower hybrid field energy is concentrated in these wave packets, it is still adequate to accelerate the ionospheric ions to the observed energies.
Progressing polar convection disturbances: Signature of an open magnetosphere
Authors: Stauning P. , Friis‐Christensen E. , Rasmussen O. , Vennerstrøm S.
Published: February 2004

On basis of comparison of data collected from geomagnetic observatories in the polar regions with solar wind data collected by the IMP 8 satellite, the characteristic patterns for the dynamical interaction of the solar wind with the Earth' magnetosphere have been found for various interplanetary conditions. The interplanetary magnetic field (IMF) is a key parameter for the interaction. The variations in the dayside ionospheric currents associated with changes in the IMF BY component often produces recognizable signatures in the ground magnetic variations. These signatures help to identify related space and ground data sets. An important element for the interpretation of the geomagnetic signatures is their dynamical appearance as seen from latitudinally extended chains of stations in the polar cap. The frequent observations of poleward progressing polar convection disturbances during IMF BZ<0 conditions strongly support the concept of an "open" magnetosphere during southward directed IMF.
Vital phase of space science
Authors: Parker E. N.
Published: February 2004

Space science began with the indirect phase where the activity in space was inferred from such terrestrial phenomena as geomagnetic storms, ionospheric variations, and fluctuations in the cosmic ray intensity. The direct phase was initiated with spaceflight placing instruments directly in space and permitting the direct observation of UV and X rays, as well as precision observations of solar luminosity variations. The evidence from these many direct studies, together with the historical record of terrestrial conditions, shows that the variations of the luminosity of the Sun affect the terrestrial atmosphere at all levels, with devastating changes in climate tracking the major changes in the activity level and luminosity of the Sun. The quantification and understanding of this vital connection should be the first priority of space science and geophysics, from oceans and atmosphere through the ionosphere, magnetosphere, and all the way to the convective zone of the Sun. It becomes the vital phase of space science, focused on the basic science of the changing habitability of Earth.
The good old days
Authors: Axford W. I.
Published: February 2004

Some important events leading to the emergence of our present understanding of some space physics phenomena are recounted from the author's personal perspective.
Numerical simulation of torus‐driven plasma transport in the Jovian magnetosphere
Authors: Yang Y. S. , Wolf R. A. , Spiro R. W. , Hill T. W. , Dessler A. J.
Published: February 2004

The Rice convection model has been modified for application to the transport of Io‐generated plasma through the Jovian magnetosphere. The new code, called the RCM‐J, has been used for several ideal‐MHD numerical simulations to study how interchange instability causes an initially assumed torus configuration to break up. In simulations that start from a realistic torus configuration but include no energetic particles, the torus disintegrates too quickly (∼50 hours). By adding an impounding distribution of energetic particles to suppress the interchange instability, reasonable lifetimes were obtained. For cases in which impoundment is insufficient to produce ideal‐MHD stability, the torus breaks up predominantly into long fingers, unless the initial condition strongly favors some other geometrical form. If the initial torus has more mass on one side of the planet than the other, fingers form predominantly on the heavy side (which we associate with the active sector). Coriolis force bends the fingers to lag corotation. The simulation results are consistent with the idea that the fingers are formed with a longitudinal thickness that is roughly equal to the latitudinal distance over which the invariant density declines at the outer edges of the initial torus. Our calculations give an average longitudinal distance between plasma fingers of about 15°, which corresponds to 20 to 30 minutes of rotation of the torus. We point to some Voyager and Ulysses data that are consistent with this scale of torus longitudinal irregularity.
Fluorescence excitation function of Ni 1200‐Å and cross sections for fluorescence photon‐photoion coincidence produced through photoexcitation of N2
Authors: Wu C. Y. Robert
Published: February 2004

The absolute fluorescence excitation function of the Ni 1200‐Å emission through photoexcitation of N2 has been measured in the 250‐620 Å region using synchrotron radiation as the continuous tunable light source. The dissociative photoionization excitation of N2 involving the production of Ni 1200‐Å emission and N+ ion has also been studied at several selected photon wavelengths shortward of 360 Å using a coincidence technique to detect the Ni 1200‐Å fluorescence photon and the N+ ion. The maximum quantum yields for producing these processes are of the order of 6 × 10−2. It was found that dissociative excitation processes can be competitive with the dissociative ionization excitation processes for the multiple‐electron transition states. The present results suggest that the electronic states of N2 in the 320‐340 Å region consist of both ion states of N2+ (Langhoff et al., 1981) and neutral states of N2, which could be the Rydberg states converging to N2++ (Sambe and Ramaker, 1986).
Electric currents in the subsolar region of the Venus lower ionosphere
Authors: Cole K. D. , Hoegy W. R.
Published: February 2004

The ion and electron momentum equations, along with Ampere's law, are solved for the ion and electron drift velocities and the electric field in the subsolar Venus ionosphere, assuming a partially ionized gas and a single ion species having the ion mean mass. All collision terms among the ions, electrons and neutral particles are retained in the equations. A general expression for the evolution of the magnetic field is derived and compared with earlier expressions (Luhmann et al., 1984; Cloutier, 1984; Cravens et al., 1984). Subsolar region data in the altitude range 150‐300 km from the Pioneer Venus Orbiter are used to calculate altitude profiles of the components of the current due to the electric field, gradients of pressure, and gravity. Altitude profiles of the ion and electron velocities as well as the electric field, electrodynamic heating, and the energy density are determined. Only orbits having a complete set of measured plasma temperatures and densities, neutral densities, and magnetic field were considered for analysis; the results are shown only for orbit 202. The vertical velocity at altitudes above 220 km is upgoing for orbit 202. This result is consistent with observations of molecular ions at high altitudes and of plasma flow to the nightside, both of which require upward velocity of ions from the dayside ionosphere. Above about 230 km the momentum equations are extremely sensitive to the altitude profiles of density, temperature, and magnetic field.
The ion cyclotron anisotropy instability and the inverse correlation between proton anisotropy and proton beta
Authors: Gary S. Peter , Lee Martin A.
Published: February 2004

Magnetosheath observations have shown an inverse correlation between the proton temperature anisotropy and the parallel proton β. Computer simulations have produced similar correlations and have demonstrated that they are due to wave‐particle scattering by ion cyclotron anisotropy instabilities. Three derivations of this correlation in an electron/proton plasma yield physical insight into this relationship.
Meteor observations with the European Incoherent Scatter UHF Radar
Authors: Pellinen‐Wannberg Asta , Wannberg Gudmund
Published: February 2004

The European Incoherent Scatter (EISCAT) UHF radar, which operates at a nominal frequency of 930 MHz, is introduced as a powerful meteor radar. Its high sensitivity is utilized to detect transient enhanced ionization trails caused by meteors of all orientations, in contrast to conventional HF and VHF backscatter radars, which observe only the meteor trails oriented approximately normal to the radar beam. A comparison shows that EISCAT observes almost as high hourly rates of meteors as meteor radars do, in spite of its beamwidth being only 0.5° compared to their 100°. Two different kinds of echoes are seen in the data. About three fourths of them are strongly Doppler‐shifted transient echoes, which we interpret as head echoes, since they move with meteor velocities. The remaining echoes are long‐lived. They come from the ionized trails left behind the meteors, which drift slowly or not at all. There are fundamental differences between the scattering process producing both types of echoes and the scattering from trails observed with the meteor radars. For example, we have never seen the trail echoes appear after a head echo. All our shower echoes are also strongly nonspecular. We postulate that some time is required for the trails to expand and for the ionization within them to approach the thermal equilibrium state in order to give detectable incoherent echoes. Due to the very small beamwidth our radar cannot follow the motion of an individual meteor, as the conventional radars can, but we can obtain a statistical profile of what happens at different heights as the meteoroids penetrate the atmosphere. The altitude distribution of ionized trails shows that they are observable over the whole measurement range from about 65 to 165 km, while the head echoes can be detected only within a narrow altitude range. The latter effect is probably related to the radio meteor ceiling effect.
Lunar tidal winds at Adelaide and Saskatoon at 80 to 100 km heights: 1985‐1990
Authors: Stening R. J. , Manson A. H. , Meek C. E. , Vincent R. A.
Published: February 2004

Winds data from the two sites were analyzed for lunar tides in 2‐year groups. The variation with height of the phase and amplitude of the tide showed considerable variability from year to year, yet in general, seasonal variations at particular heights were similar to those determined previously. Phases of the tide at the two sites were compared in order to estimate whether the tide was symmetric or antisymmetric. For this purpose it was found necessary to use only 1‐year data sets. The predominance of symmetric tides during solstice months and antisymmetric tides at the equinox came as a surprise, yet the result is in broad agreement with phase relations of the lunar geomagnetic tide.
High‐voltage electron collection by a spherical satellite
Authors: Sheldon J. W.
Published: February 2004

The numerical solution to the cold electron flow equations is presented for the region surrounding an ionospheric satellite at high positive potential. The momentum, continuity, and Poisson equations are solved simultaneously for satellite potentials of 1000, 10,000, 30,000, and 100,000 kT (kT = 0.17 eV). The electron velocity vectors in the sheath region are presented. The electron current density to the satellite surface is also reported. Electrons impact the satellite surface with average velocity vectors having angles of incidence (relative to the surface normal) that increase from the satellite's magnetic pole to equator. A toroidal flow develops in the vicinity of the magnetic equator and expands away from the satellite surface as the surface potential is increased. The current density to the satellite surface increases with increasing satellite potential and decreases with increasing polar angle.
Operator splitting for multidimensional magnetohydrodynamics
Authors: Murawski K. , Goossens M.
Published: February 2004

Operator splitting and direct time integration of the ideal MHD equations are compared by using a finite difference flux‐corrected transport method. The comparison is made for impulsively generated nonlinear waves in a coronal plasma. It suggests that operator splitting and direct time integration both lead to physically acceptable results and provide a good agreement with results obtained with the fast Fourier transform method.
MHD modeling of magnetotail instability for localized resistivity
Authors: Hesse Michael , Birn Joachim
Published: February 2004

We present results of a three‐dimensional MHD simulation of magnetotail evolution initiated by a sudden occurrence or increase of spatially localized resistivity as the major expected consequence of some localized microinstability. Because of the absense of a quantitative model, possible variations of resistivity levels with current density, or the reduction thereof, are not incorporated in the present investigation. The emphasis of the study is on an investigation of the changes to the overall evolution brought about by this localization, in particular, on the disruption and diversion of the cross‐tail current and the nonlinear evolution of the magnetotail instability. The immediate consequences of the occurrence of the localized resistance and the resulting electric field are a reduction and diversion of the electric current around the region of high resistivity, associated with an increase of Bz (“dipolarization”) at the earthward edge and a decrease of Bz at the tailward edge of this region. These effects, however, are localized and do not involve a reduction of the total cross‐tail current and hence do not lead to the global development of a “substorm current wedge,” which includes not only the reduction of the cross‐tail current but also the buildup of a global field‐aligned current system of “region 1” type (toward the Earth on the dawnside and away on the duskside of the tail). Such signatures develop at a later time, as consequences of a three‐dimensional tearing instability, which is triggered by the occurrence of the resistivity. These features are found in combination with plasmoid formation and ejection, quite similar to results of earlier simulations with uniform resistivity. Differences are found in the timescale of the evolution, which tends to be shorter for localized resistivity, and in the propagation of the dipolarization effects in the equatorial plane. Whereas for uniform resistivity the temporal increase in Bz tends to propagate tailward, apparently due to a pileup effect in the near‐Earth region, an earthward propagation is found for the localized resistivity. This propagation results from an earthward convection of the gradient in Bz, which is set up by the reconnection process further tailward.
VHF radar and rocket observations of equatorial spread F on Kwajalein
Authors: Hysell D. L. , Kelley M. C. , Swartz W. E. , Farley D. T.
Published: February 2004

VHF radar data from the Summer 1990 Equatorial Spread F campaign on Kwajalein are presented. The Cornell 50 MHz portable radar interferometer (CUPRI) operated concurrently with the Altair UHF incoherent scatter radar throughout July and August and supported two sounding rocket flights on July 30 and August 2. This experiment provided the first opportunity to simultaneously diagnose equatorial spread F using the three prime experimental techniques: VHF/UHF coherent scatter, incoherent scatter, and in situ probe measurements of electric field and density fluctuations. The intensity of the coherent echoes observed was consistent with typical Jicamarca spread F observations, but chains of periodic, large‐scale plasma upwellings were observed more often and for much longer durations on Kwajalein than have been seen over Peru. CUPRI also measured Doppler frequencies in one upwelling corresponding to 1200‐m/s plasma drift velocities. This measurement agrees with recent observations of supersonic drift rates at the magnetic equator by spacecraft. Near the most active localized plasma upwellings, interferometer data reveal that the zonal drift rate of plasma irregularities can vary sharply in space, as one would expect for two‐dimensional incompressible flow. We introduce a semiempirical model of the three‐dimensional spectrum of F region irregularities that is consistent with the one‐dimensional spectra of density fluctuations observed by sounding rockets and with the axial ratio of irregularities determined recently. Normalized to data from one of the rocket flights on Kwajalein, the model predicts the 3‐m scattering cross‐section measured by CUPRI to within a few decibels.
Oxygen in the heliosphere
Authors: Luhmann J. G.
Published: February 2004

Oxygen within the heliosphere, whether neutral or ionized, comes from both the external source in the local interstellar medium (LISM) and internal sources. If transient cometary sources are neglected, Jupiter is the strongest of the internal sources by virtue of its corotation and charge exchange driven neutral wind. O+ pickup ions are born where the penetrating neutrals lose an electron as a result of photoionization, charge exchange with solar wind protons, or solar wind electron impact ionization. The region of the heliosphere from which these pickup ions arise depends on the phase of the solar cycle as well as on the velocities of the neutrals. The present model of neutral O populations and their ionization suggests that Jovian pickup ions can dominate the inner heliospheric O+ population if the LISM neutrals are strongly filtered at the heliopause, or are excluded by strong ionizing fluxes from the Sun. Other heliospheric species such as sulfur and nitrogen may have similar origins. These planetary sources need to be taken into account in the interpretation of interplanetary gas and ion composition observations.
Cross‐tail current, field‐aligned current, and By
Authors: Kaufmann Richard L. , Lu Chen , Larson Douglas J.
Published: February 2004

Orbits of individual charged particles were traced in a one‐dimensional magnetic field model that included a uniform cross‐tail component Byo. The effects of Byo on the cross‐tail current distribution jy (z), the average cross‐tail drift velocity 〈νy(z)〉, and the average pitch angle change 〈Δα〉 experienced during current sheet encounters were calculated. The addition of a Byo that exceeded several tenths of one nanotesla completely eliminated all resonance effects for odd‐N orbits. An odd‐N resonance involves ions that enter and exit the current sheet on the same side. Pitch angles of nearly all such ions changed substantially during a typical current sheet interaction, and there was no region of large cross‐tail drift velocity in the presence of a modest Byo. The addition of a very large Byo guide field in the direction that enhances the natural drift produces a large jy and small 〈Δα〉 for ions with all energies. The addition of a modest Byo had less effect near even‐N resonances. In this case, ions in a small energy range were found to undergo so little change in pitch angle that particles which originated in the ionosphere would pass through the current sheet and return to the conjugate ionosphere. Finally, the cross‐tail drift of ions from regions dominated by stochastic orbits to regions dominated by either resonant or guiding center orbits was considered. The ion drift speed changed substantially during such transitions. The accompanying electrons obey the guiding center equations, so electron drift is more uniform. Any difference between gradients in the fluxes associated with electron and ion drifts requires the presence of a Birkeland current in order to maintain charge neutrality. This plasma sheet region therefore serves as a current generator. The analysis predicts that the resulting Birkeland current connects to the lowest altitude equatorial regions in which ions drift to or from a point at which stochastic orbits predominate. The proposed mechanism appears only in analyses that include non‐guiding‐center effects.
Stochastic acceleration of energetic ions in Jupiter's magnetosphere
Authors: Barbosa D. D.
Published: February 2004

An equation governing the combined radial diffusion and stochastic acceleration of super‐Alfvénic ions by magnetohydrodynamic waves in Jupiter's outer magnetosphere is derived. The formulation is based upon a total energy invariant of the adiabatic transport which applies to an isotropic distribution undergoing rapid pitch angle scattering by waves. An analytic solution to the double diffusion equation is obtained and numerical results are presented for two models of ion injection. The first model assumes S+ and O+ are injected throughout a broad region of space through photoionization of Jupiter's magnetospheric neutral wind and obtain an initial energy corresponding to the local corotation energy after pickup by the planetary magnetic field. The second model assumes a monoenergetic distribution of energetic protons is implanted in the middle magnetosphere by the action of field‐aligned potential drops in Jupiter's auroral ionosphere. For both light and heavy ions the injection process creates a seed population of particles which are further accelerated nonadiabatically by the MHD waves and adiabatically through radial diffusion. A comparison of the theoretical results with a recent data analysis of Voyager low‐energy charged particle measurements is made with very good agreement, thus providing a rigorous quantitative account of and definitive explanation for the high‐energy ion component of Jupiter's magnetosphere.
Nonlinear input/output analysis of the auroral electrojet index
Authors: Price C. P. , Prichard D. , Bischoff J. E.
Published: February 2004

We analyze the auroral electrojet index for the days 30‐31 October 1978 as the output of a nonlinear system driven by various aspects of the interplanetary magnetic field (IMF) and solar wind (speed and density). The nonlinear input/output analysis takes the form of a prediction scheme; we compare the results to an autoregressive (linear) predictor and to a nonlinear predictor based only on the output signal. Different combinations of the solar wind variables are used as input functions. We find that only a few of the input combinations show any evidence whatsoever for nonlinear coupling between the input and the output for this interval.
Statistical analysis of diffuse ion events upstream of the Earth's bow shock
Authors: Trattner K. J. , Möbius E. , Scholer M. , Klecker B. , Hilchenbach M. , Lühr H.
Published: February 2004

A statistical study of diffuse energetic ion events and their related waves upstream of the Earth's bow shock was performed using data from the Active Magnetospheric Particle Tracer Explorers/Ion Release Module satellite over two 5‐month periods in 1984 and 1985. The data set was used to test the assumption in the self‐consistent model of the upstream wave and particle populations by Lee (1982) that the particle acceleration through hydromagnetic waves and the wave generation are directly coupled. The comparison between the observed wave power and the wave power predicted on the observed energetic particle energy density and solar wind parameters results in a high correlation coefficient of about 0.89. The intensity of diffuse ions falls off approximately exponentially with the distance upstream from the bow shock parallel to the magnetic field with e‐folding distances which vary from ∼ 3.3 RE to ∼ 11.7 RE over the energy range from 10 keV/e to 67.3 keV/e for both protons and alpha particles. After normalizing the upstream particle densities to zero bow shock distance by using these exponential variations, a good correlation (0.7) of the density of the diffuse ions with the solar wind density was found. This supports the suggestion that the solar wind is the source of the diffuse ions. Furthermore, the spectral slope of the diffuse ions correlates well with the solar wind velocity component in the direction of the interplanetary magnetic field (0.68 and 0.66 for protons and alpha particles) which concurs with the notion that the solar wind plays an important role in the acceleration of the upstream particles.
Motion of the heliospheric termination shock: 3. Incident interplanetary shocks
Authors: Naidu Kamcilla , Barnes Aaron
Published: February 2004

In this paper the response of the heliospheric termination shock to an incident interplanetary shock is examined. This paper is an extension of a recent study by Barnes (1993), which treated the analogous problem for an incident contact discontinuity. The termination shock is treated as a strong gasdynamic shock. The postinteraction configuration consists of a moving termination shock, a postshock contact discontinuity, and either a shock or rarefaction wave propagating the disturbance signal into the downstream medium. For a decrease in dynamic pressure a rarefaction wave propagates downstream, and the new termination shock propagates inward, while for an enhancement of dynamic pressure the termination shock moves outwards and a weak outer shock propagates into the downstream medium; speeds of motion of the termination shock are typically of the order of ∼100 km/s. The results are similar to those presented by Barnes (1993) indicating that the results of that paper are robust within the gasdynamic model, in the sense of being independent of the details of the initial disturbance.
Electron inertial effects on geomagnetic field line resonances
Authors: Wei C. Q. , Samson J. C. , Rankin R. , Frycz P.
Published: February 2004

A three‐dimensional compressible resistive magnetohydrodynamic simulation code, with inclusion of the fully generalized Ohm's law, has been developed to study the nonlinear evolution of field line resonances in Earth's magnetosphere. A simple Cartesian box model of an inhomogeneous plasma with straight geomagnetic field lines is used, and the Alfvén velocity increases monotonically from the magnetopause boundary layer toward Earth. A monochromatic fast mode compressional oblique wave is applied from the direction of the magnetopause boundary layer, pumping energy into the magnetosphere. The fast mode wave, while propagating toward Earth, is partially reflected at the turning point (located at radial distances between 8 and 10 RE in the equatorial plane) and then couples to shear Alfvén waves, leading to the formation of large‐amplitude field line resonances near Earth. The field line resonances are observed to narrow to several electron inertia lengths within several wave periods of the driver wave, and electron inertial effects become important at this stage. Final profiles near the resonance are very similar to Airy functions, indicating that electron inertial effects become important before possible nonlinear effects. The electron inertial effects lead to oscillating parallel electric field which might be potential accelerators for electrons in some types of auroral arcs.
Theory of electron acceleration by lower hybrid waves in the Io plasma torus of Jupiter
Authors: Barbosa D. D.
Published: February 2004

A theoretical model of superthermal electron acceleration in the Io plasma torus is developed. The model assumes that sulfur and oxygen ions are created out of Io's extended neutral cloud forming a ring distribution in velocity space which is unstable to the growth of lower hybrid waves traveling perpendicular to the magnetic field. The waves dissipate through Landau damping by plasma electrons accelerating them to kiloelectron volt energies. The present model computes the electron distribution function evolving under the action of the lower hybrid waves with the simultaneous occurrence of Coulomb collisions. The results show that for moderate electric field amplitudes characteristic of the lower hybrid waves observed there, a power law velocity dependence is generated along with a quasi‐isotropic pitch angle distribution due to Coulomb collisions. The power law extends up to a characteristic speed where the electron energy equals the pickup ion energy. At higher energies, an exponential distribution is prevalent with an effective temperature proportional to the wave acceleration efficiency. The model has a wide applicability to space and astrophysical plasmas where the presence of hot electrons gives rise to anomalous ionization effects.
Gradient drift eigenmodes in the equatorial electrojet
Authors: Wang X. ‐H. , Bhattacharjee A.
Published: February 2004

The problem of kilometer‐scale irregularities in the daytime equatorial electrojet is revisited by means of an eigenmode analysis of the gradient drift instability. Realistic physical parameters are used, including the modeled altitude variations of ion and electron collision frequencies and mobilities. The full fourth‐order system of two coupled differential equations (each of second order) for the density and electrostatic potential perturbations is solved numerically by a relaxation technique. Under some approximations, the fourth‐order system can be shown to reduce to a second‐order differential equation for the perturbed potential or density. The latter is solved using a shooting technique and provides initial guesses for numerical solutions to the full problem. It is shown that the linear growth rate peaks for kilometer‐scale waves, contrary to the findings of recent initial‐value studies. This occurs because the equilibrium velocity shear is much more effective as a damping mechanism for short‐wavelength modes than it is for the longer, kilometer‐scale modes. These results provide a natural qualitative explanation for the observed dominance of kilometer‐scale structures in the daytime electrojet spectrum.
Alfvén resonance excitation and fast wave propagation in magnetospheric waveguides
Authors: Rickard Graham J. , Wright Andrew N.
Published: February 2004

Magnetic pulsations are a robust feature of the Earth's magnetosphere. It has been suggested recently that the magnetosphere is sometimes better modeled as a waveguide rather than a cavity. This paper presents numerical simulations of linear magnetohydrodynamic (MHD) waves in an inhomogeneous, low‐β waveguide. Several features predicted by recent theoretical studies are confirmed in our simulations, notably that Alfvén resonances are driven at frequencies corresponding to the natural frequency of the fast waveguide modes with Vg ≈ 0 (ky ≈ 0).
Magnetic and electric field waves in slow shocks of the distant geomagnetic tail: ISEE 3 observations
Authors: Coroniti F. V. , Moses S. L. , Greenstadt E. W. , Tsurutani B. T. , Smith E. J.
Published: February 2004

During the first pass through the distant geomagnetic tail by ISEE 3 the slow shocks encountered on February 2 and 11, 1983, provide particularly clear examples of the magnetic field and plasma wave properties of the shock transition. The magnetic ramp contains transverse polarized magnetic field oscillations with frequencies just below the ion cyclotron frequency and amplitudes of 2‐4 nT. These waves are plausibly generated by the electromagnetic ion/ion cyclotron instability predicted by Winske and Omidi (1990). The electric field plasma waves within the shock ramp exhibit two spectral peaks. A midfrequency emission occurs near the ion plasma frequency and electron cyclotron frequency but well below the maximum Doppler shift frequency for electrostatic waves. The midfrequency waves extend into the upstream region where the spectral peak occurs at a slightly higher frequency. A new high‐frequency emission with frequencies between the maximum Doppler shift frequency and the electron plasma frequency occurs throughout the downstream region. This emission disappears at the start of the magnetic ramp and is replaced upstream by electron plasma oscillations. The high‐frequency emissions are clearly polarized parallel to the magnetic field. The polarization of the midfrequency waves is less certain; both parallel and a fairly broad angular distribution about the parallel electric fields are consistent with the measurements.
Observations of nonadiabatic acceleration of ions in Earth's magnetotail
Authors: Frank L. A. , Paterson W. R. , Kivelson M. G.
Published: February 2004

We present observations of the three‐dimensional velocity distributions of protons in the energy range 20 eV to 52 keV at locations within and near the current sheet of Earth's magnetotail at geocentric radial distances 35 to 87 RE. These measurements were acquired on December 8, 1990, with a set of electrostatic analyzers on board the Galileo spacecraft during its approach to Earth in order to obtain one of its gravitational assists to Jupiter. It is found that the velocity distributions are inadequately described as quasi‐Maxwellian distributions such as those found in the central plasma sheet at positions nearer to Earth. Instead the proton velocity distributions can be categorized into two major types. The first type is the “lima bean” shaped distribution with high‐speed bulk flows and high temperatures that are similar to those found nearer to Earth in the plasma sheet boundary layer. The second type consists of colder protons with considerably lesser bulk flow speeds. Examples of velocity distributions are given for the plasma mantle, a region near the magnetic neutral line, positions earthward and tailward of the neutral line, and the plasma sheet boundary layer. At positions near the neutral line, only complex velocity distributions consisting of the colder protons are found, whereas both of the above types of distributions are found in and near the current sheet at earthward and tailward locations. Bulk flows are directed generally earthward and tailward at positions earthward and tailward of the neutral line, respectively. Only the high‐speed, hot distribution is present in the plasma sheet boundary layer. The observations are interpreted in terms of the nonadiabatic acceleration of protons that flow into the current sheet from the plasma mantle. For this interpretation the hot, “lima bean” shaped distributions are associated with meandering, or Speiser, orbits in the current sheet. It is suggested that the colder, lower‐speed proton velocity distributions are the result of fractional or few gyromotions before ejection out of the current sheet, but this speculation must be further investigated with appropriate kinetic simulation of trajectories.
An improved solution to the “Rankine‐Hugoniot” problem
Authors: Szabo A.
Published: February 2004

This paper presents an extension of the nonlinear least squares fitting technique of Viñas and Scudder [1986] (VS), which finds the physical and geometrical properties of nondissipational magnetohydrodynamic (MHD) shocks. The new method incorporates plasma temperature observations in the form of normal momentum flux and energy density flux conservation as well as plasma density, velocity, and magnetic field data. The new technique is capable of using known standard deviations in the individual measurement points to properly weight the fitting procedure. The new fitting code is validated through the analysis of synthetic shocks with known physical and geometrical properties. Finally, it is compared to the original VS method and the preaveraged velocity coplanarity technique.
Statistical study of the late substorm recovery phase and quiet time plasma sheet based on ISEE 1 ∼30‐keV ion observations
Authors: Traver D. P. , Mitchell D. G. , Williams D. J.
Published: February 2004

We present a statistical survey of the near‐Earth plasma sheet (r = 7‐23 RE, distance from the neutral sheet center Zns = 0‐10 RE, LT = 2100‐0200) during (late) substorm recovery phase and quiet time based on ISEE 1 medium energy particle instrument observations of first‐order ∼30‐keV ion anisotropies, comprising ∼112 hours of the 1978 tail season. The data reveal predominantly isotropic conditions, with intervals of generally weakly anisotropic (<10 highest‐to‐lowest flux ratio) field‐aligned ion flows (FAIFs) and ion flux gradients. Both earthward and tailward streaming FAIFs are observed for Zns between 0 and 10 RE and are probably unassociated with the plasma sheet boundary layer. The occurrence rate of tailward FAIF (∼4.5%) is relatively insensitive to AE conditions. The occurrence rate of earthward FAIF increases during “active” conditions to as much as 12.4% and decreases to as low as 1.3% during “quieter” conditions. An increase in the occurrence rate of tailward FAIFs is observed with increasing Z (mapping along B to the more distant current sheet). We suggest that the tailward streaming flows may arise during stretching of especially distant closed field lines during extended recovery and quiet time. Ions will lose energy as they cross the tailward moving current sheet, just as they gain energy in crossing an earthward moving field reversal region. No evidence for a quiet time distant tail source is seen in the energetic (24 keV to 2 MeV) ion data during these late recovery phase and quiet time periods. A weak local time asymmetry in the ratio of occurrence rates for tailward and earthward FAIFs (T∶E) is observed, with earthward streaming FAIFs outnumbering tailward streaming FAIFs in the dusk sector (T∶E = 0.9‐1.0) and vice versa in the dawn sector (T∶E = 1.4‐ 2.1). The occurrence rate of dawn‐to‐dusk flux anisotropies, interpreted here as gradients indicating higher ion flux toward the plasma sheet equatorial plane, increases during active conditions and decreases during quieter conditions. North‐to‐south and south‐to‐north flux anisotropies, interpreted here as dawnward and duskward flux gradients (3‐5%), are independent of AE conditions.
Phase relationships between total electron content variations, Doppler velocity oscillations and geomagnetic pulsations
Authors: Liu J. Y. , Berkey F. T.
Published: February 2004

The phase relationship between variations of ionospheric total electron content (TEC) and ground‐level ULF geomagnetic pulsations has been examined for the advection and compression mechanisms. To determine the causal mechanism, several earlier studies have examined the phase difference between oscillations of Doppler velocity in ionospherically reflected radio waves and simultaneous ULF geomagnetic pulsations. In most instances it was found that the phase relation varied from event to event. With the application of Euler’s formula this study shows that in low geomagnetic and midgeomagnetic latitudes the phase differences between variations of TEC and ULF pulsations in the northward component of the geomagnetic field due to the advection and compression mechanisms are 0o and 180o, respectively. We also found that TEC variations tend to lead ionospheric Doppler velocity oscillations by 90o. Furthermore, it is shown that the phase relationship between ionospheric Doppler velocity oscillations and ULF pulsations of the northward component of the geomagnetic field, caused by the advection and compression mechanisms, are functions of the scale length, frequency of ULF waves, and geomagnetic dip.
Implications of flow angle stabilization on coherent E region spectra
Authors: Janhunen P.
Published: February 2004

Recent perpendicular particle simulations of the Farley‐Buneman instability show good agreement with the linear dispersion relation: the phase velocity is proportional to the electric field. This is in contrast with the usually quoted nonlinear theory, which states that the phase velocity at any direction does not rise much above the ion acoustic velocity Cs. However, the simulations also produce a flow angle shift in the spatial power spectrum such that the most intense modes have phase velocities close to Cs. Using simple analytical formulae based on these simulations, we synthesize coherent spectra assuming a turbulent microstructure of the scattering volume. In this way, the observed type 1 characteristics can be reproduced, and also spectra similar to type 4 can be generated. A new interpretation of the Cs saturation observed in the equatorial electrojet is thus presented, and the difference between equatorial and auroral coherent spectra becomes understandable.