bluuebunny/crossref_metadata_2025 huggingface dataset:
Centrifugal acceleration of the polar wind
Authors: Horwitz J. L. , Ho C. W. , Scarbro H. D. , Wilson G. R. , Moore T. E.
Published: February 2004

The effect of parallel ion acceleration associated with convection was first applied to energization of test particle polar ions by Cladis (1986). However, this effect is typically neglected in “self‐consistent” models of polar plasma outflow, apart from the fluid simulation by Swift [1990]. Here we include approximations for this acceleration, which we broadly characterize as centrifugal in nature, in our time‐dependent, semikinetic model of polar plasma outflow and describe the effects on the bulk parameter profiles and distribution functions of H+ and O+. For meridional convection across the pole the approximate parallel force along a polar magnetic field line may be written as Fcent,pole = 1.5m(Ei/Bi)²(r²/ri³) where m is ion mass, r is geocentric distance; and Ei, Bi and ri refer to the electric and magnetic field magnitudes and geocentric distance at the ionosphere, respectively. For purely longitidinal convection along a constant L shell the parallel force is Fcent,long = Fcent,pole [1‐(r/(riL)]3/2/[1‐3r/(4riL)]5/2. For high latitudes the difference between these two cases is relatively unimportant below ∼5 RE. We find that the steady state O+ bulk velocities and parallel temperatures strongly increase and decrease, respectively, with convection strength. In particular, the bulk velocities increase from near 0 km s−1 at 4000 km altitude to ∼10 km s−1 at 5 RE geocentric distance for a 50‐mV/m ionospheric convection electric field. However, the centrifugal effect on the steady O+ density profiles depends on the exobase ion and electron temperatures: for low‐base temperatures (Ti = Te = 3000 K) the O+ density at high altitudes increases greatly with convection, while for higher base temperatures (Ti = 5000 K, Te = 9000 K), the high‐altitude O+ density decreases somewhat as convection is enhanced. The centrifugal force further has a pronounced effect on the escaping O+ flux, especially for cool exobase conditions; as referenced to the 4000‐km altitude, the steady state O+ flux increases from 105 ions cm−2 s−1 when the ionospheric convection field Ei = 0 mV/m to ∼107 ions cm−2 s−1 when Ei = 100 mV/m. The centrifugal effect also decreases the time scale for approach to steady‐state. For example, in the plasma expansion for Ti = Te = 3000 K, the O+ density at 7 RE reaches only 10−7 of its final value ∼1.5 hours after expansion onset for Ei = 0. For meridional convection driven by Ei = 50 mV/m, the density at the same time after initial injection is 30‐50% of its asymptotic level. The centrifugal acceleration described here is a possible explanation for the large (up to ∼10 km s−1 or more) O+ outflow velocities observed in the midaltitude polar magnetosphere with the Dynamics Explorer 1 and Akebono spacecraft.
ROSAT observations of the Jupiter aurora
Authors: Waite J. H. , Bagenal F. , Seward F. , Na C. , Gladstone G. R. , Cravens T. E. , Hurley K. C. , Clarke J. T. , Elsner R. , Stern S. A.
Published: February 2004

Röntgen satellite (ROSAT) high‐resolution imager (HRI) and position sensitive proportional counter (PSPC) observations of Jupiter obtained in April 1991 and May 1992 reveal soft X ray emissions apparently associated with Jupiter's aurora and similar to X ray emissions observed earlier by the Einstein Observatory. The HRI images show emission mainly from Jupiter's northern hemisphere at all Jovian longitudes observed, and there is some indication of a longitudinal modulation of the emission in phase with the well‐known ultraviolet modulation of the northern aurora. The PSPC data reveal a very soft spectrum. Comparison of the observed spectrum with models for both electron bremsstrahlung radiation and line emission from S and O ions indicates that the line spectrum gives a much better statistical fit to the observed spectrum. The X ray observations presented here therefore support the hypothesis that ion precipitation is the most likely cause of the Jovian X ray emissions, a result first suggested by the Einstein results [Metzger et al., 1983].
Energy transfer between energetic ring current H+ and O+ by electromagnetic ion cyclotron waves
Authors: Thorne Richard M. , Horne Richard B.
Published: February 2004

Electromagnetic ion cyclotron (EMIC) waves in the frequency range below the helium gyrofrequency can be excited in the equatorial region of the outer magnetosphere by cyclotron resonant instability with anisotropic ring current H+ ions. As the unducted waves propagate to higher latitudes, the wave normal should become highly inclined to the ambient magnetic field. Under such conditions, wave energy can be absorbed by cyclotron resonant interactions with ambient O+, leading to ion heating perpendicular to the ambient magnetic field. Resonant wave absorption peaks in the vicinity of the bi‐ion frequency and the second harmonic of the O+ gyrofrequency. This absorption should mainly occur at latitudes between 10° and 30° along auroral field lines (L ≥ 7) in the postnoon sector. The concomitant ion heating perpendicular to the ambient magnetic field can contribute to the isotropization and geomagnetic trapping of collapsed O+ ion conics (or beams) that originate from a low‐altitude ionospheric source region. During geomagnetic storms when the O+ content of the magnetosphere is significantly enhanced, the absorption of EMIC waves should become more efficient, and it may contribute to the observed acceleration of O+ ions of ionospheric origin up to ring current energies.
Two different types of enhanced ion acoustic fluctuations observed in the upper ionosphere
Authors: Forme F. R. E. , Fontaine D. , Wahlund J. E.
Published: February 2004

We present European incoherent scatter observations of several events that show enhanced ion acoustic fluctuations in the altitude region 400–1500 km. The incoherently scattered spectra have one or both the ion acoustic lines enhanced, and sometimes a peak close to the zero frequency is observed. It is shown that these observations can be separated into two types depending on the surrounding physical conditions. The first type is related to strongly enhanced electron temperatures, ion outflows with large vertical velocities, an altitudinal extend of 300–700 km, and auroral arcs and precipitating particles of 100 eV to 10 keV. The spectra are in that case strongly asymmetric. The second type corresponds to slightly enhanced electron temperatures, no ion outflows, spatial extends of 100–200 km, and an apparent lack of precipitating particles of less than 1 keV. The corresponding spectra show mostly both the ion lines enhanced. These observations are compared with the existing interpretations.
Kinetic Alfvén waves as a source of plasma transport at the dayside magnetopause
Authors: Lee L. C. , Johnson Jay R. , Ma Z. W.
Published: February 2004

As the shocked solar wind with variable plasma density and magnetic field impinges on the dayside magnetopause, it is likely to generate large‐scale Alfvén waves at the solar wind magnetosphere interface. However, large gradients in the density and magnetic field at the magnetopause boundary effectively couple large‐scale Alfvén waves with kinetic Alfvén waves. In this paper we propose that the wave power converted into kinetic Alfvén waves may play an important role in plasma transport at the dayside magnetopause and in electron acceleration along field lines. The transport can occur because, unlike the magnetohydrodynamic (MHD) shear Alfvén wave, the kinetic Alfvén wave has an associated parallel electric field which breaks down the “frozen‐in” condition and decouples the plasma from field lines. We calculate the average deviation of the plasma from the field line from which we estimate the diffusion coefficient associated with these “bundles” of decoupled plasma to be approximately 109m²/s. The parallel electric field also may lead to acceleration of electrons along field lines in the magnetopause boundary and may possibly provide an explanation for observed counterstreaming electron beams characterized by energies of 50‐200 eV.
Distributions of the interplanetary magnetic field revisited
Authors: Feynman Joan , Ruzmaikin Alexander
Published: February 2004

The adequacy of the power spectrum to characterize the variations of a parameter depends on whether or not the parameter has a Gaussian distribution. We here perform very simple tests of Gaussianity on the distributions of the magnitudes of the interplanetary magnetic field, and on the distributions of the components; that is, we find the first four cumulants of the distributions (mean, variance, skewness, and kurtosis) and their solar cycle variations. We find, consistent with other recent analyses, that the traditional distributions of the 1‐hour averaged magnitude are not distributed normally or lognormally as has often been assumed and the 1‐hour averaged z component is found to have a nonzero kurtosis. Thus the power spectrum is insufficient to completely characterize these variations and polyspectra are needed. We have isolated variations in the 1/f frequency region of the spectrum and show that the distributions of the magnitudes have nonzero skewness and kurtosis, the magnitudes are not distributed lognormally, and the distributions of the components have nonzero kurtosis. Thus higher‐order spectra are again needed for a full characterization.
Observations of ionospheric electric fields above atmospheric weather systems
Authors: Farrell W. M. , Aggson T. L. , Rodgers E. B. , Hanson W. B.
Published: February 2004

We report on the observations of a number of quasi‐dc electric field events associated with large‐scale atmospheric weather formations. The observations were made by the electric field experiment onboard the San Marco D satellite, operational in an equatorial orbit from May to December 1988. Several theoretical studies suggest that electric fields generated by thunderstorms are present at high altitudes in the ionosphere. In spite of such favorable predictions, weather‐related events are not often observed since they are relatively weak. We shall report here on a set of likely E field candidates for atmosphere‐ionosphere causality, these being observed over the Indonesian Basin, northern South America, and the west coast of Africa; all known sites of atmospheric activity. As we shall demonstrate, individual events can often be traced to specific active weather features. For example, a number of events were associated with spacecraft passages near Hurricane Joan in mid‐October 1988. As a statistical set, the events appear to coincide with the most active regions of atmospheric weather.
Direct experimental measurement of electron impact ionization‐excitation branching ratios: 2. Angular distribution of secondary electrons from N2 at 100 eV
Authors: Goembel L. , Yang J. , Doering J. P.
Published: February 2004

The coincidence ionization experiment used previously to measure ionization‐excitation branching ratios for the X ²Σg+, A ²Πu, and B ²Σu+ states of N2+ at a fixed secondary electron emission angle of 74° has been modified to allow the secondary emission angle to be scanned. Angular distributions of electrons which have excited the three available electronic states of the N2+ ion have been measured at 100 eV. For the small, primary electron scattering angles where most ionization events occur the distribution for the X ²Σg+ state is found to have a peak at 57° with respect to the incident beam direction, while that for the A ²Πu state has a broad peak near 75°. The distribution for the much less intense B ²Σu+ state is intermediate between the two. The partial branching ratios deduced for the 74° fixed angle case have been corrected to reflect the more accurate partial branching ratios obtained by integrating the secondary electron angular results. The corrected values for the X ²Σg+ A ²Πu, and B ²Σu+ states are 0.56±0.05, 0.35±0.03, and 0.09±0.02, respectively. These results show that the A ²Πu state branching ratio is much smaller than deduced from a number of previous studies
Shock normal determination for multiple‐ion shocks
Authors: Kessel R. L. , Coates A. J. , Motschmann U. , Neubauer F. M.
Published: February 2004

We have adapted the single‐ion Viñas and Scudder (1986) solution to the Rankine‐Hugoniot problem to a multiple‐on solution. Using this technique, we can calculate a shock normal direction, shock speed, best estimate of the upstream and downstream magnetic field and plasma asymptotic states, and θBn, the angle between the shock normal and the upstream magnetic field. We test the multi‐ion solution with a theoretical case but are restricted to a perpendicular shock in order to close the multi‐ion Rankine‐Hugoniot equations. For this test case both single‐ion and multi‐ion solutions are equally valid. We examine parameter regimes to look for differences between single‐ion and multi‐ion solutions of the R‐H equations, and we find that the largest differences occur for quasi‐parallel shocks, small values of solar wind speed, large values of heavy ion density, and very strong and very weak shocks. For both the inbound and outbound crossing of comet Halley we have a slow solar wind speed, small values of water group ions and fairly weak shocks. We examine both the quasi‐perpendicular inbound crossing and the quasi‐parallel outbound crossing at comet Halley. For the inbound crossing we have good resolution data and make an accurate determination. However, it is likely that the shock normal determination was influenced by the interplanetary disturbances flowing from the Sun that permeate the inbound crossing region and that force the normal to an antisunward direction. For the outbound case the FIS experiment is not providing data because it was damaged by cometary dust soon after closest approach. However, the results are more definitive: all of the normals are within 31° of each other, which makes the analysis essentially independent of interval. In both cases there are differences between the single‐ and multi‐ion solutions with the multi‐ion solution being the more reliable.
The art of mapping the magnetosphere
Authors: Stern David P.
Published: February 2004

A comprehensive review is presented of the mathematical methods used to represent magnetic fields in the Earth's magnetosphere, of the way existing data‐based models use these methods and of the associated problems and concepts. The magnetic field has five main components: the internal field, the magnetopause, the ring current, the tail and Birkeland currents. Methods of representing separately each of these are discussed, as is the deformation of magnetic fields; Appendix B traces the connection between deformations and the Cauchy integral. A summary section lists the uses of data‐based models and their likely future evolution, and Appendix A supplements the text with a set of problems.
A theoretical model for the broad upshifted maximum in the stimulated electromagnetic emission spectrum
Authors: Huang J. , Kuo S. P.
Published: February 2004

A second‐order four‐wave interaction process including two pump photons, an upper hybrid plasmon, and an electron Bernstein plasmon is studied. The pump is the second harmonic of the HF heater in the plasma. It is found that, when the heater wave frequency f0 is above a harmonic of the electron cyclotron frequency fc, frequency‐upshifted upper hybrid waves and frequency‐downshifted electron Bernstein waves can be excited above the upper hybrid resonance layer via the considered process. The process occurs in a local region where the heater wave frequency is about the mean of the upper hybrid wave frequency and the electron Bernstein wave frequency. Moreover, in this interaction process, a low‐frequency electrostatic oscillation in the frequency range of the lower hybrid wave is generated through nonlinear coupling of the HF heater wave with the excited high‐frequency electrostatic waves. However, this wave does not satisfy the linear dispersion relation of the lower hybrid wave and is thus a driven wave. The excited frequency‐upshifted upper hybrid waves can then scatter off field‐aligned density irregularities to generate O ‐ mode emissions with frequencies around 2f0 − nfc This is consistent with observations of the broad upshifted maximum (BUM) feature in the stimulated electromagnetic emission (SEE) spectrum. The concomitantly excited frequency‐downshifted electron Bernstein waves are found to have much smaller amplitudes; hence their scattering products are also relatively weak. This explains why only BUM lines are detected. Furthermore, the driven low‐frequency fluctuations can also be the scatterers to convert the upper hybrid waves into emissions with frequencies aroundy fBUM + fLH and fBUM − fLH, where fLH is the lower hybrid wave frequency. This is suggested to be the generation mechanism of the second BUM feature which appears when the shoulder of the BUM feature is not very high.
Rapidly convergent Lambda Operator Method for solving resonance line scattering in planetary atmospheres: 1. The one‐dimensional slab
Authors: Griffioen E. , McConnell J. C. , Shepherd G. G.
Published: February 2004

The study of planetary atmospheric atomic and molecular (resonance) line emissions often require sophisticated radiative transfer models to properly account for scattering and absorption effects. In this work a relatively new and simple, yet powerful, numerical technique for solving radiative transfer problems has been developed and coded for a two‐level atom in an one‐dimensional Cartesian coordinate system. Angular scattering is assumed to be isotropic, and the method has been tested for monochromatic, complete frequency redistribution, and angle‐averaged frequency redistribution. Formally, the solution is based on the commonly known lambda operator method. Rapid and robust convergence is attained using an intermediate operator that is readily evaluated. In addition, bilinear extrapolation based on previous iterates is found to further increase the rate of convergence. The one‐dimensional model intensity versus frequency profiles are found to compare favorably with those of other proven models over a large range of optical depths.
Middle‐ and low‐latitude emissions from energetic neutral atom precipitation seen from ATLAS 1 under quiet magnetic conditions
Authors: Tinsley B. A. , Rohrbaugh R. P. , Ishimoto M. , Torr M. R. , Torr D. G.
Published: February 2004

During the ATLAS 1 mission spectral observations were made at middle and low latitudes of features expected from the precipitation of energetic neutral atoms. The Imaging Spectrometric Observatory was used at night in the UV and visible with maximum gain. The tangent ray heights of the look directions ranged from near 100 km to near 200 km, and the geomagnetic conditions were quiet during the observations, which were made March 28 to April 3, 1992. The N2+1N 391.4‐nm and O I 130.4 and 135.6‐nm emissions were observed at all latitudes, with lower emission rates at lower magnetic dip latitudes, except that enhancements in the O I lines were seen within 30° of the dip equator due to radiative recombination of ionospheric plasma. The latitude profile observed for the N2+1N emission did not show an equatorial or midlatitude peak. This implies that the source of energetic neutrals is more consistent with prompt charge exchange loss of freshly injected trapped ions with relatively low mirror heights (i.e., ions on higher L shells with equatorial pitch angle distributions nearly isotropic to the loss cone) than loss of highly eroded populations of particles with high mirror heights (i.e., ions on lower L shells with pancake equatorial pitch angle distributions). The N2+1N emission rates have been compared with models of atmospheric emission due to fluxes of O/O+ and H/H+ in the thermosphere, as produced by energetic neutral oxygen or hydrogen atom precipitation. Energy deposition rates are inferred that range from about 12 (±3) × 10−4 erg cm−2 s−1 near 45° geomagnetic latitude to about 3 (±1) × 10−4 erg cm−2 s−1 near the geomagnetic equator. These can be compared with energy deposition rates a factor of 10² higher during magnetic storms.
Evidence for ion transport and molecular ion dominance in the Venus ionotail
Authors: Intriligator D. S. , Brace L. H. , Cloutier P. A. , Grebowsky J. M. , Hartle R. E. , Kasprzak W. T. , Knudsen W. C. , Strangeway R. J.
Published: February 2004

We present analyses from the five Pioneer Venus Orbiter plasma experiments and the plasma wave experiment when a patch of plasma with enhanced densities was encountered in the near‐Venus ionotail during atmospheric entry at an altitude of ∼ 1100 km in the nightside ionosphere. Our analyses of the thermal and superthermal ion measurements in this plasma feature provides the first evidence that at times molecular ions in the 28‐32 amu mass range are dominant over atomic mass species thus yielding evidence for a transport mechanism that reaches into the lower ionosphere. Analysis of plasma analyzer (OPA) observations at this time indicates the presence of ions measured in the rest frame of the spacecraft at ∼27 and 37 volt energy per unit charge steps. In the rest frame of the planet these superthermal ions are flowing from the dawn direction at speeds (assuming they are O2+) of ∼8 km/s and with a flow component downward (perpendicular to the ecliptic plane) at speeds of ∼2 km/s. OPA analyses also determine the ion number flux, energy, flow angles, and angular distributions. Plasma wave bursts appear to indicate that plasma density decreases within and on the equatorward edge of the patch of enhanced plasma densities are associated with ion acoustic waves and relative ion streaming.
Formation and eruption of Sun‐aligned arcs at the polar cap‐auroral oval boundary
Authors: Berg G. A. , Kelley M. C. , Mendillo M. , Doe R. , Vickrey J. , Kletzing C. , Primdahl F. , Baker K. D.
Published: February 2004

This paper reports on results from a study of the poleward edge of the auroral oval in the morning sector using a comprehensive blend of in situ and ground‐based measurements. Three rockets, equipped to measure electric and magnetic fields, energetic particles, and plasma density flew into an auroral display whose dynamical features were recorded with a digital image intensified all‐sky camera as well as with an incoherent scatter radar. In addition, a number of DMSP satellite measurements bracketed the launch time. Evidence is presented here that in a condition of declining magnetic activity Sun‐aligned arcs are injected into the polar cap at velocities ∼7 km/s from locations of periodic brightening along the morningside of the auroral oval. The multipoint in situ measurements allow some separation of temporal and spatial effects and strongly suggest a poleward contraction of the convection pattern of about 0.25° INVL in 70 s. The most equatorward of the two brightest arcs studied erupted into a region which already was characterized by strong sunward convection. The most poleward, however, pushed into a region that had been convecting in an antisunward direction at velocities exceeding 1 km/s less than 2 min earlier, and it is likely that sunward convection subsequently pertained poleward of that arc as well. We believe that these events mark the reconfiguration of the magnetosphere into a system characterized by a smaller polar cap.
Downshifted maximum features in stimulated electromagnetic emission spectra
Authors: Leyser T. B. , Thidé B. , Waldenvik M. , Veszelei E. , Frolov V. L. , Grach S. M. , Komrakov G. P.
Published: February 2004

New and detailed experimental and theoretical results concerning the prominent downshifted maximum (DM) feature in spectra of stimulated electromagnetic emissions are reported. The experimental results were obtained at the Sura ionospheric modification facility in Russia by transmitting a powerful high‐frequency ordinary mode pump wave into the ionospheric F region. We present detailed experimental results of the dependence of the DM on the pump frequency. Different frequency components of the DM have slightly different growth times after pump turn‐on and are suppressed in different pump frequency ranges at harmonics of the electron cyclotron frequency. The pump frequency range near the cyclotron harmonics in which the DM does not develop, decreases significantly with increasing harmonic, from several kilohertz at the fourth harmonic to an extremely narrow range of 0.2 kHz or less at the seventh harmonic. We discuss conditions for plasma wave propagation to explain this dependence on the cyclotron harmonics.
Contribution to the nonlinear theory of plasma wave excitation inside the electron foreshock region
Authors: Sotnikov V. I. , Fiala V. , Décréau P. M. E. , Pivovarov V. V. , Shapiro I. V.
Published: February 2004

The broadband electrostatic waves inside the electron foreshock region well away from its boundaries were explained as a beam mode for which the beam velocity is lower than the thermal velocity of the background plasma. We discuss here the nonlinear saturation of this instability. The waves propagating along the beam are stabilized by a plateau formation on the electron distribution function. The waves that propagate at an angle to the beam are stabilized in a different way. In the course of the development of the instability the beam velocity spreads and an initially negative energy mode, which is initially unstable, experiences increased damping by the beam particles and finally is stabilized due to this effect. We give an estimation of the wave energy in both cases, and we present a numerical modelling to substantiate our analytical estimates. The saturated wave energy level is found to be comparable in both cases and agrees with the levels observed in the foreshock.
HF radar signatures of the cusp and low‐latitude boundary layer
Authors: Baker K. B. , Dudeney J. R. , Greenwald R. A. , Pinnock M. , Newell P. T. , Rodger A. S. , Mattin N. , Meng C.‐I.
Published: February 2004

Continuous ground‐based observations of ionospheric and magnetospheric regions are critical to the Geospace Environment Modeling (GEM) program. It is therefore important to establish clear intercalibrations between different ground‐based instruments and satellites in order to clearly place the ground‐based observations in context with the corresponding in situ satellite measurements. HF‐radars operating at high latitudes are capable of observing very large spatial regions of the ionosphere on a nearly continuous basis. In this paper we report on an intercalibration study made using the Polar Anglo‐American Conjugate Radar Experiment radars located at Goose Bay, Labrador, and Halley Station, Antarctica, and the Defense Meteorological Satellite Program (DMSP) satellites. The DMSP satellite data are used to provide clear identifications of the ionospheric cusp and the low‐latitude boundary layer (LLBL). The radar data for eight cusp events and eight LLBL events have been examined in order to determine a radar signature of these ionospheric regions. This intercalibration indicates that the cusp is always characterized by wide, complex Doppler power spectra, whereas the LLBL is usually found to have spectra dominated by a single component. The distribution of spectral widths in the cusp is of a generally Gaussian form with a peak at about 220 m/s. The distribution of spectral widths in the LLBL is more like an exponential distribution, with the peak of the distribution occurring at about 50 m/s. There are a few cases in the LLBL where the Doppler power spectra are strikingly similar to those observed in the cusp.
Nonlinear evolution of a large‐amplitude circularly polarized Alfvén wave: High beta
Authors: Ghosh S. , Viñas A. F. , Goldstein M. L.
Published: February 2004

The nonlinear dynamics following saturation of the parametric instabilities of a monochromatic field‐aligned large‐amplitude circularly polarized Alfvén wave is investigated via direct numerical simulation in the case of high plasma β and no wave dispersion. The magnetohydrodynamic (MHD) code permits nonlinear couplings in the parallel direction to the ambient magnetic field and one perpendicular direction. Compressibility is included in the form of a polytropic equation of state. Turbulent cascades develop after saturation of two coupled oblique three‐wave parametric instabilities; one of which is an oblique filamentationlike instability reported earlier. Remnants of the parametric processes, as well as of the original Alfvén pump wave, persist during late nonlinear times. Nearly incompressible MHD features such as spectral anisotropies appear as well.
Acceleration of interstellar pickup ions in the disturbed solar wind observed on Ulysses
Authors: Gloeckler G. , Geiss J. , Roelof E. C. , Fisk L. A. , Ipavich F. M. , Ogilvie K. W. , Lanzerotti L. J. , von Steiger R. , Wilken B.
Published: February 2004

Acceleration of interstellar pickup H+ and He+ as well as of solar wind protons and alpha particles has been observed on Ulysses during the passage of a corotating interaction region (CIR) at ∼4.5 AU. Injection efficiencies for both the high thermal speed interstellar pickup ions (H+ and He+) and the low thermal speed solar wind ions (H+ and He++) are derived using velocity distribution functions of protons, pickup He+ and alpha particles from < 1 to 60 keV/e and of ions (principally protons) above ∼60 keV. The observed spatial variations of the few keV and the few hundred keV accelerated pickup protons across the forward shock of the CIR indicate a two stage acceleration mechanism. Thermal ions are first accelerated to speeds of 3 to 4 times the solar wind speed inside the CIR, presumably by some statistical mechanism, before reaching higher energies by a shock acceleration process. Our results also indicate that (1) the injection efficiencies for pickup ions are almost 100 times higher than they are for solar wind ions, (2) pickup H+ and He+ are the two most abundant suprathermal ion species and they carry a large fraction of the particle thermal pressure, (3) the injection efficiency is highest for protons, lowest for He+, and intermediate for alpha particles, (4) both H+ and He+ have identical spectral shapes above the cutoff speed for pickup ions, and (5) the solar wind frame velocity distribution function of protons has the form F(w) = F0w−4 for 1 < w < ∼5, where w is the ion speed divided by the solar wind speed. Above w ∼ 5‐10 the proton spectrum becomes steeper. These results have important implications concerning acceleration of ions by shocks and CIRs, acceleration of anomalous cosmic rays, and particle dynamics in the outer heliosphere.
Two‐dimensional hybrid simulations of impulsive plasma penetration through a tangential discontinuity
Authors: Savoini P. , Scholer M. , Fujimoto M.
Published: February 2004

Two‐dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the impulsive penetration process when a magnetosheath plasma irregularity interacts with the magnetopause. The irregularity is modeled as a field‐aligned filament with an excess density impinging on the magnetopause. Simulations with strictly parallel or antiparallel magnetic fields in the magnetosheath and in the magnetosphere are performed in order to investigate ion kinetic effects during the penetration of the plasma irregularity. The results show that a MHD approach is not appropriate when describing the impulsive penetration of small‐scale irregularities. If the fields in the magnetosheath and magnetosphere are antiparallel, the magnetic field gradient near the magnetopause boundary leads to an asymmetric propagation of the magnetosheath plasma filament. In contrast, for moderate field changes across the magnetopause the parallel case exhibits an almost straight‐line propagation of the filament, while its main part is focused into a small region. In both cases, twin vortices are set up in the magnetosphere due to the filament motion. In the antiparallel case the shape of the filament is totally distorted by these vortices; these fluidlike structures stretch the magnetopause boundary and thereby increase the line to surface ratio of the boundary layer. The filament breaks in the antiparallel as well as in the parallel case into smaller parts which leads to isolated islands of magnetosheath like plasma and field in the magnetosphere of the order of the ion gyroradius.
Neutral solar wind properties: Advance warning of major geomagnetic storms
Authors: Gruntman Michael A.
Published: February 2004

Coronal mass ejections (CMEs) have been identified as a trigger for large geomagnetic storms. A clever idea to provide advance warning of a high‐speed CME approaching the Earth was recently proposed by Hsieh et al. (1992): ejected solar matter decelerates on its way from the Sun to Earth, energetic neutral atoms (ENAs) are formed in the CME plasma due to recombination, ENAs pass the decelerating CME plasma and arrive first at the Earth. We evaluate the idea to use ENAs for advance detection of the high‐speed Earth‐approaching CMEs and consider the processes involved. Charge exchange between solar wind ions and interplanetary neutral atoms contribute effectively to ENA production. Characteristics of neutral gas within Earth's orbit are updated for both neutral atoms of interstellar origin and from outgassing from interplanetary dust. Computer simulation of CME‐produced ENAs shows that the total flux of CME‐produced ENAs is slightly smaller than the intensity of the quiescent neutral solar wind (NSW) while significantly higher ENA energies make them easily distinguishable from NSW atoms. Arrival of the CME‐produced ENAs is expected 3‐4 hours before the start of a large geomagnetic storm, which provides a basis for advance storm warning and prediction of storm magnitude. The progress in development of ENA measurement technique suggests that both the quiescent NSW and CME‐produced ENAs can be reliably measured.
Lower hybrid wave phenomena associated with density depletions
Authors: Seyler C. E.
Published: February 2004

A fluid description of lower hybrid, whistler and magnetosonic waves is applied to study wave phenomena near the lower hybrid resonance associated with plasma density depletions. The goal is to understand the nature of lower hybrid cavitons and spikelets often associated with transverse ion acceleration events in the auroral ionosphere. Three‐dimensional simulations show the ponderomotive force leads to the formation of a density cavity (caviton) in which lower hybrid wave energy is concentrated (spikelet) resulting in a three‐dimensional collapse of the configuration. Plasma density depletions of the order of a few percent are shown to greatly modify the homogeneous linear properties of lower hybrid waves and account for many of the observed features of lower hybrid spikelets.
Drift shells and aurora computed using the O8 magnetic field model for Neptune
Authors: Paranicas C. , Cheng A. F.
Published: February 2004

Charged particle drift shells are calculated using the O8 magnetic field model for Neptune. Inner drift shell morphologies differ significantly from dipolar drift shells for the parts of drift shells inward of r = 2 RN. Outer drift shells (L ≳ 10), when traced down to Neptune's surface following magnetic field lines, are simple closed loops around magnetic poles. Inner drift shells (L ≲ 4), on the other hand, when traced to the surface, are also single loops but stretched in a previously unknown way: sometimes with a cusp and sometimes into two joined loops. Inner drift shell footprints on R = 1 provide the basis for identifying precipitation L shells, interpreting observed aurora, and predicting additional emissions on that part of Neptune's surface unobserved by UVS. Precipitation in a global magnetic anomaly, “ordinary” auroral precipitation near the south magnetic pole, and precipitation from field lines with no magnetic field minimum above Neptune's exobase collectively appear to account for all of the observed auroral emission regions at Neptune. To the extent that aurora can be understood in this model, it is suggested that O8 may be reasonably accurate.
Analysis of a polar auroral arc observed from Eureka (89°N) and by DMSP satellites on December 14, 1990
Authors: Huang K. , McEwen D. J. , Oznovich I.
Published: February 2004

A bright and long‐lived Sun‐aligned auroral arc was observed over the north magnetic pole from 1120 to 1220 UT, December 14, 1990 at Eureka, Canada. It was monitored through its full lifetime with high resolution with an all‐sky imager and a meridian scanning spectrometer. Its measured off‐zenith 5577‐Å intensity reached 12 kR. The recorded electron flux obtained by a DMSP F9 overpass at 1124∶15 UT was 56 ergs/(cm² s sr), yielding an inferred 5577‐Å intensity in that region of the arc of at least 80 kR. The arc drifted slowly in the dusk to dawn direction during its early stages, then reversed its drift direction during later stages. The characteristics of the polar arc (its intensity, duration, and small‐scale dynamics) were very similar to those of auroral oval arcs and strongly suggest that the polar arc occurred on closed magnetic field lines. Emissions on the duskside of the arc further suggest that the region of closed magnetic field lines extended from the evening sector to the magnetic pole. The inference is that the interplanetary magnetic field was most likely Bz northward and By positive during the period of appearance of the arc.
Solar cyclic behavior of trapped energetic electrons in Earth's inner radiation belt
Authors: Abel Bob , Thorne Richard M. , Vampola Alfred L.
Published: February 2004

Magnetic electron spectrometer data from six satellites (OV3‐3, OV1‐14, OGO 5, S3‐2, S3‐3, and CRRES) have been used to study long‐term (1966‐1991) behavior of trapped energetic electrons in the inner radiation belt. Comparison of the observed energy spectra at L ≥ 1.35 for different phases of the solar cycle reveals a clear trend toward enhanced fluxes during periods of solar maximum for energies below a few hundred keV; we suggest that this is caused by an increase in the rate of inward radial diffusion from a source at higher L. In contrast, for L < 1.30, where atmospheric collisions become increasingly important, the electron flux is reduced during solar maximum; we attribute this to the expected increase in upper atmospheric densities. The electron flux above 1 MeV exhibits a systematic decay beyond 1979 to values well below the current NASA AE‐8 model. This indicates that the natural background of high‐energy electrons has previously been overestimated due to the long lasting presence of electrons produced by nuclear detonations in the upper atmosphere in the late 1950s and early 1960s.
Unusual satellite‐electron signature within the Uranian magnetosphere and its implications regarding whistler electron loss processes
Authors: Mauk B. H. , Keath E. P. , Krimigis S. M.
Published: February 2004

It has been reported that during the outbound (nightside) portion of the Voyager 2 encounter with the Uranian magnetosphere, intense whistler mode emissions were observed near the magnetic equator (λm ∼ 16°) and at L shell values between ∼ 5.5 and ∼ 9 RU. Comprehensive calculations of whistler‐driven pitch angle diffusion, in previous work, have yielded strong diffusion electron lifetimes of ∼ 1 hour for 20 to 40 keV electrons. In this paper we report on an unusual and sharply defined charged particle feature that: (1) involved electrons between 22 and 35 keV, (2) was observed during the time period of the intense whistler mode observations, (3) was aligned very accurately and sharply with the minimum L shell position (L ∼ 7.5) of the satellite Ariel, and (4) has an appearance that suggests that electrons were removed only at and beyond Ariel's minimum‐L. On the basis of our conclusion that the signature was caused by electron interactions with either Ariel or materials distributed along Ariel's orbit, the signature could not have been generated for at least 10 hours prior to its observation. Thus the calculated whistler loss times are in apparent conflict with the signature observation. A scenario of events is proposed to explain the data that involves substormlike electron acceleration on the Uranian nightside and a subsequent sculpting of the electron spatial distributions via interactions with Ariel or materials distributed along Ariel's orbit. Interactions with distributed materials, such as micron‐sized particulates, are favored because they minimize the discrepancy between the whistler‐loss calculations and the observations presented here. The possibility exists that the accurate alignment of the sharp electron feature with Ariel's minimum‐L, and the absorptionlike character of the feature, are accidental, and that the feature is caused by dynamical processes (e.g., substorms). In this case the dynamical processes must be quite dissimilar to those occurring in the Earth's magnetosphere.
Velocity dependent O atom IR excitation cross sections: Connections with flight data
Authors: Oakes D. B. , Sonnenfroh D. M. , Caledonia G. E. , Blumberg W. A. M.
Published: February 2004

A fast oxygen atom source has been used to study the velocity dependence of O atom infrared excitation reactions with various molecular species in a crossed beam experiment. These short wave infrared (SWIR) measurements are performed under single collision conditions, simulating the low Earth orbit environment. Such data are fundamental to the analysis and interpretation of atmospheric oxygen atom interaction with plume exhaust species and with the local environment about structures in low Earth orbit. Measurements have been performed over the oxygen atom velocity range of 6 to 12 km/s. These are the first such experimental measurements, and they may be used to validate theoretical estimates presently used in predictive models. We specifically discuss the velocity dependent cross sections for the reaction O + N2 → NO(v) + N. This reaction is endothermic for O atom velocities ≤ 8 km/s. Preliminary data are also shown for the reaction O + CO2 → CO2(ν3) + O → CO(v) + O2. Both band integral and spectrally resolved results will be presented. The spectrally resolved data provide information on the rotation/vibrational distribution of the excited states. Limited observations of infrared emissions resulting from atmospheric interactions have become available in recent years from both shuttle‐ and rocket‐borne experiments. Comparisons between our experimental database and selected data from several different flight experiments are provided. These comparisons clearly identify the kinetic mechanisms responsible for the flight observations.
Ionospheric response to the sustained high geomagnetic activity during the March '89 Great Storm
Authors: Sojka J. J. , Schunk R. W. , Denig W. F.
Published: February 2004

A simulation was conducted to model the high‐latitude ionospheric response to the sustained level of high geomagnetic activity for the great magnetic storm period of March 13‐14, 1989. The geomagnetic and solar activity indices and the DMSP F8 and F9 satellite data for particle precipitation and high‐latitude convection were used as inputs to a time‐dependent ionospheric model (TDIM). The results of the TDIM were compared to both DMSP plasma density data and ground‐based total electron content (TEC) measurements for the great storm period as well as with earlier storm observations. The comparisons showed that the overall structure of the high‐latitude ionosphere was dominated by an increased convection speed within the polar cap that led to increased ion temperatures. In turn, this enhanced the NO+ density, raised the atomic‐to‐molecular ion transition height to over 300 km, decreased NmF2, increased hmF2, and in places either increased ne at 800 km or slightly decreased it. The morphology of the ionosphere under these extreme conditions was considerably different than that modeled for less disturbed intervals. These differences included the character of the dayside tongue of ionization that no longer extended deep into the polar cap. Instead, as a result of the ion heating and consequent reduction in NmF2, a large polar hole occupied much of the polar region. This polar hole extended beyond the auroral oval and merged with the night sector midlatitude trough. The limitations associated with the applicability of the TDIM to the geomagnetic conditions present on March 13 and 14 are discussed. The primary limitations of the TDIM derive from the limited temporal resolution of the model input parameters and the lack of a suitably dynamic thermospheric specification for the great storm conditions. These limitations lead to midlatitude ionospheric storm phases that do not follow those observed.
N2 Lyman‐Birge‐Hopfield dayglow from ATLAS 1
Authors: Torr Marsha R. , Torr D. G. , Chang T. , Richards P. , Germany G.
Published: February 2004

Spectral measurements of the far ultraviolet dayglow were made from the ATLAS 1 shuttle mission using an intensified‐CCD imaging spectrometer array. The instrument imaged relatively large (177 Å) segments of the spectrum simultaneously and had the capability to image altitude in the second dimension of the two‐dimensional focal plane detector when the entrance slit of the instrument was oriented perpendicular to the limb of the Earth. The FUV channel of the instrument used a CsTe photocathode rather than the more conventionally used and more solar blind CsI photocathode material. As a result, the spectra were also measured with good sensitivity at FUV wavelengths longer than 1500 Å. In this paper, data are selected of specific interest to a study of the N2 Lyman‐Birge‐Hopfield system using two shuttle attitudes: In the first the field of view was held at a fixed tangent ray height of ∼140 km while scanning in wavelength, providing repeated FUV spectra over a range of latitudes and allowing comparison with our thermospheric airglow model. In the second observing sequence the line of sight of the instrument was scanned down through the atmosphere but covered about 75% of the full wavelength range. This sequence allows comparison of height dependence, including O2 absorption effects, with the thermospheric model. In all cases, relatively good agreement is obtained with the model (run strictly in a predictive mode) in comparisons in which the line of sight must be integrated along a path that spans a considerable range of conditions (height, local time, latitude, solar zenith angle). In the case of the data sets examined in this study the vibrational population distributions show mixed results. In one case the distribution agrees well with a theoretical model based on direct electron impact excitation without a significant cascade contribution. A second case indicates a lower population in ν′ = 3 and a higher population in ν′ = 6 than would be expected from direct impact excitation alone. This supports previous indications that conditions resulting in the cascade mechanism may not always be operative.
Magnetic nulls in the outer magnetosphere of Jupiter: Detections by Pioneer and Voyager spacecraft
Authors: Leamon R. J. , Dougherty M. K. , Southwood D. J. , Haynes P. L.
Published: February 2004

First reported during the Ulysses flyby of Jupiter in February 1992, null events are sporadic dropouts in the magnetic field strength detected in the outer Jovian magnetosphere, with the magnitude of the field strength decreasing to values that can be less than 0.2 nT. The events occur in regions where the ambient field is primarily southward, that is, aligned with the local planetary dipole field direction and are distinct from the field depressions encountered in the center of the magnetodisk in the middle magnetosphere. They also appear distinct from the field structures encountered at the magnetopause. Here we report a survey of the occurrence of such events in the magnetic field data recorded by the earlier Pioneer and Voyager spacecraft passages through the Jovian system. All of the previous spacecraft to fly through the Jovian magnetosphere recorded field null events on their inbound passes where the field magnitude dropped below 2 nT. There is little or no evidence of any field reversal in the center of events. Only Pioneer 11, which was the only spacecraft to exit on the dayside of the magnetosphere, recorded events on its outbound pass.