bluuebunny/crossref_metadata_2025 huggingface dataset:
Vertical ionization drift velocities and range type spread F in the evening equatorial ionosphere
Authors: Abdu M. A. , de Medeiros R. T. , Bittencourt J. A. , Batista I. S.
Published: February 2008

Range type spread F indices in the post sunset hours, determined from ionograms, are compared with F layer vertical velocities, also deduced from ionograms, for a period of one year, over the magnetic equatorial station Fortaleza (geographic coordinates: 4°S, 38°W; dip latitude: 1.8°S), Brazil. Besides good correlation between the two parameters, the results suggest the presence of a certain threshold value for the vertical ionization velocity and a corresponding threshold height for the base of the evening F layer, as one of the necessary conditions for the occurrence of spread F irregularities valid for southern solstice (summer) and equinoctial months. The nature of the possible variabilities in the amplitude of the initial perturbation in the ionization (seeding mechanism), which is another prerequisite for the generation of irregularities, is discussed briefly.
Numerical modeling of the energy spectrum of the cosmic ray Forbush Decrease
Authors: Nishida Atsuhiro
Published: February 2008

We have studied the change in the cosmic‐ray density caused by propagating solar‐wind disturbances (characterized by enhancement in velocity and decrease in diffusion coefficient) by solving the modulation equation formulated by Parker, Gleeson, and Axford. The independent variables are radial distance and particle energy. The passage of the region of the enhanced solar wind speed leaves behind it net heating of cosmic ray particles. The cosmicray density is reduced principally by the sweeping effect of the propagating solar wind disturbance, and the rigidity spectrum of the percentage reduction in density or intensity is roughly proportional to the rigidity dependence of the reciprocal of the diffusion coefficient D. If D is proportional to βP, the calculated spectrum is close to P−0.8, which is compatible with the observationally derived spectrum of the Forbush decrease. The density reduction is expected to be preceded by a weak precursory increase at higher energies (T > 1 GeV) but by a precursory decrease at lower energies. The latter is the consequence of the particle heating by the solar wind disturbance, but it often seems to be masked by enhancement of particles of the solar origin.
Acceleration of low‐energy protons and alpha particles at interplanetary shock waves
Authors: Scholer M. , Ipavich F. M. , Gloeckler G. , Hovestadt D.
Published: February 2008

We have investigated low‐energy protons and alpha particles in the energy range 30 keV/charge to 150 keV/charge associated with three different interplanetary shock waves in the immediate preshock and postshock region. The data were obtained with the Max‐Planck‐Institut/University of Maryland sensor system on ISEE 3. In particular, we present spatial distributions in the preshock and postshock medium as measured in the spacecraft frame and after transformation into the solar wind and the shock frame of reference, respectively, the dependence of the phase space density at different energies on distance from the shock, and the form of the distribution function of both species immediately at the shock. In the preshock region particles are flowing in the solar wind frame of reference away from the shock and in the postshock medium the distribution is more or less isotropic in this frame of reference. This is similar to findings at the earth's bow shock during times when diffuse particles are present. The distribution function in the postshock region can be represented by a power law in energy with the same spectral exponent for both protons and alpha particles. In the preshock medium the phase space densities falls off exponentially (after subtraction of a background value) with distance from the shock and the e‐folding distance scale of the proton phase space densities increases approximately proportional to the square root of energy. In addition, there is a much slower intensity increase further upstream over a time period of several hours. Although the spectra of diffuse bow shock associated particles are different from the spectra of the interplanetary shock‐associated particles in the immediate vicinity of the shock, it is concluded that the first‐order Fermi acceleration process can consistently explain the data. It is shown that independent of the acceleration mechanisms invoked, the mean free path of the low energy ions in the preshock medium is considerably smaller than the mean free path determined by the turbulence of the background interplanetary medium. This suggests that these particles generate a wave field of their own making, which in turn scatters the particles.
splitting and divergence of STARE auroral radar velocities
Authors: Whitehead J. D. , Jerkic H. M. , Nielsen E.
Published: February 2008

A study of the power spectra of auroral radar echoes shows that they are sometimes asymmetrical. This is probably due to two waves (at different heights) having different line‐of‐sight velocities. The velocity splitting occurs over a limited region of space (∼300 km in geographic extent) that moves toward the equator. The velocity splitting seems to be associated with divergence of the STARE drift velocities. This has been calculated and found to have significant time and space variations—much too large to be explained by magnetic field changes. The divergence is greatest at times when the power spectrum is asymmetrical and more than one velocity may be present. Several possible corrections (including a few unlikely ones) have been tried, but none reduce the magnitude of the divergences appreciably. We must conclude that STARE does not measure the electric field exactly and that a correction velocity of the order of 25 m s−1 and divergence of the order of 10−4 s−1 is required. We have not been able to find what it depends on, except its association with velocity splitting.
Lower hybrid drift instability with temperature gradient in a perpendicular shock wave
Authors: Zhou Y. M. , Wong H. K. , Wu C. S. , Winske D.
Published: February 2008

The lower hybrid instability is studied in the perpendicular bow shock geometry including finite beta effects and an electron temperature gradient. The flute (k∥ = 0) mode, which for constant electron temperature is stable at the shock, is destabilized for sufficiently large temperature gradient (εTρi ≳ 1). Numerical solutions are presented for cases in which the ion distribution is either a drifting Maxwellian or consists of two Maxwellians to represent the effect of reflected ions at the shock. Implications of the results concerning ion and electron heating and electron acceleration at the bow shock are also discussed.
Numerically simulated two‐dimensional auroral double layers
Authors: Borovsky Joseph E. , Joyce Glenn
Published: February 2008

Electrostatic plasma double layers are numerically simulated by using a magnetized 2½ dimensional particle‐in‐cell system, periodic in one direction and bounded by reservoirs of Maxwellian plasma in the other. The simulations of both oblique and two‐dimensional double layers show periodic instability, indicate Debye length rather than gyroradii scaling, and reveal low‐frequency electrostatic turbulence and electron beam excited electrostatic electron‐cyclotron waves. Simulated electric field and electron beam data resemble referenced satellite measurements of oppositely directed electric fields within auroral regions and of precipitating electrons in inverted ν events. Estimates are given for the thicknesses of auroral double layers and for the separations within multiple auroral arcs. The temporal modulation of accelerated beams is investigated; the possibilities of the double‐layer production of ion conies and of ion precipitation are hypothesized; and the consequences of electrostatic plasma wave excitement are discussed. Simulations including the atmospheric backscattering of electrons predict the presence of an ionospheric sheath that acts to accelerate ionospheric ions upward.
The dissociative recombination of O2+: The quantum yield of O(¹S) and O(¹D)
Authors: Abreu Vincent J. , Solomon S. C. , Sharp W. E. , Hays P. B.
Published: February 2008

Data from the visible airglow experiment on the Atmosphere Explorer‐E satellite have been used to determine the quantum yield of O(¹S) and O(¹D) from the dissociative recombination of O2+. A range of values between 0.09 and 0.23 has been obtained for the quantum yield of O(¹S). It is shown that the quantum yield of O(¹S) depends on the ratio of electron density to atomic oxygen density. This suggests that the quantum yield of O(¹S) may depend on the degree of vibrational excitation of the recombining O2+. The quantum yield of O(¹D) has been measured to be 1.23±0.42, with no dependence on the electron‐oxygen ratio.
Conjugate observations of Pc 5 electric fields with a geostationary satellite and a ground radar facility
Authors: Grard Réjean , Villain Jean‐Paul
Published: February 2008

Pulsating electric fields in the Pc 5 frequency range have been recorded at magnetically conjugate locations in the magnetosphere and in the ionosphere. This coordinated investigation has been conducted with the geostationary satellite GEOS 2 in the vicinity of the geomagnetic equator and the STARE backscatter radar facility located in northern Scandinavia. Such simultaneous electric field measurements can be understood in the light of the field line resonance theory. The profile of the resonant region is described, and the polarization of the pulsating fields is defined. It is shown that the phase difference between the magnetospheric and ionospheric signals and the determination of the spacecraft invariant magnetic latitude are consistent with theoretical predictions made for the fundamental mode.
Inelastic scattering effects on photoelectron spectra and ionospheric electron temperature
Authors: Stamnes K. , Rees M. H.
Published: February 2008

Three sets of inelastic scattering cross sections have been used by various investigators in energy degradation computations of photoelectrons. The effects of the three sets upon photoelectron spectra in the ionosphere, electron heating rates and temperature, and dayglow emissions are examined for solar minimum and solar maximum conditions. Comparison of individual cross sections shows large differences among the three sets but the overall inelastic scattering, summed over states and species is sufficiently similar such that the effect on the electron heating rate and temperature is only minor. Photoelectron spectra differ in details at energies below about 20 eV. The major difference is found in the excitation and emission rates of some features in the dayglow spectrum.
Structure of the quasi‐parallel bow shock: Results of numerical simulations
Authors: Kan J. R. , Swift D. W.
Published: February 2008

Quasi‐parallel bowshock structures are simulated by using a one‐dimensional nonperiodic hybrid code in which the ion dynamics are treated exactly while the electron dynamics are omitted by neglecting the electron inertia (me = 0) and pressure. The simulations are initialized with the upstream condition specified by θ1 = 10°, β1 = 0.5, and MA1 = 2 to 4 with 0.5 increment while the downstream condition is obtained from the Rankine‐Hugoniot relations, where θ1 is the angle between the shock normal and the magnetic field, β1 is the ratio of kinetic to magnetic pressures, and MA1 is the Alfvén Mach number. The main results of our simulations follow: (1) For MA1, < 3, the magnetic field profile of the shock is laminar or quasi‐laminar. The upstream waves are right‐hand polarized whistlers with ω ≃ 3.2Ωni in the plasma frame, where Ωni is the ion gyrofrequency based on the normal field component Bn. This corresponds to a wave frequency ƒ ≃ 0.25 Hz and wavelength λ ≃ 310 km under solar wind conditions. The downstream waves are absent or low in amplitudes. (2) For MA1 > 3, the magnetic field profile of the shock is turbulent. The upstream waves are again right‐hand polarized whistlers with wavelengths varying from λ ∼ 310 km upstream to λ ≳ 1250 km downstream. The downstream waves are predominantly the right‐hand polarized fast magnetosonic waves (ω ≲ Ωi). (3) The transition from the laminar (subcritical) to the turbulent (supercritical) shock structures is shown to result from the firehose instability which occurs when MA1 ≳ 3. (4) The dissipation mechanism for the quasi‐parallel shock in our simulation is identified with the nonadiabatic compression of ions streaming through the low frequency whistler waves (ω ≳ Ωi), resulting in conversion of ion streaming energy to ion thermal energy. (5) Significant ion heat flux flowing upstream occurs when MA1 ≳ 3 due primarily to the backscattering of highly nonadiabatically heated ions.
Low‐energy (<100 eV) ion pitch angle distributions in the magnetosphere by ISEE 1
Authors: Nagai T. , Johnson J. F. E. , Chappell C. R.
Published: February 2008

Low‐energy (<100 eV) ion data from the plasma composition experiment on ISEE 1 are examined statistically to study pitch angle distributions in all local times of the magnetosphere (L = 3–10). The pitch angle distributions in the data set used here can be classified into seven types; however, there are four major types, i.e., isotropic distribution, bi‐directional field‐aligned distribution, unidirectional field‐aligned distribution, and low flux. The isotropic distribution that consists of very low energy (typically <10 eV) ions is a persistent feature in the inner region. It is frequently observed with an accompanying loss cone‐like structure. The bi‐directional field‐aligned distribution consisting of warm ions (⩾10 eV) is a persistent feature on the outer dayside and it is seen just outside the isotropic distribution region of the nightside. It is noted that the loss cone‐like structure is also a common feature of this type of distribution in the noon sector. On the outer nightside the unidirectional field‐aligned distribution consisting of warm ions is the dominant signature, but in some cases only the low flux (no appreciable flux) is observed. The ‘sources’ of ions in various regions are discussed on the basis of these results and others.
Characteristics of thermal and suprathermal ions associated with the dayside plasma trough as measured by the Dynamics Explorer Retarding Ion Mass Spectrometer
Authors: Sojka J. J. , Schunk R. W. , Johnson J. F. E. , Waite J. H. , Chappell C. R.
Published: February 2008

The retarding ion mass spectrometer (RIMS) instrument on Dynamics Explorer 1 has carried out high‐altitude measurements of the polar wind. During an outbound pass through the outer plasmasphere and plasma trough at 0900 LT the RIMS instrument measured the detailed characteristics of the thermal and suprathermal ions. Three distinct populations were observed: (1) thermal ions (H+ and He+) with densities ranging from > 500 ions/cm³ to < 1 ion/cm³, temperatures around 2 eV (20,000° K), and H+/He+ density ratios of 2 to 4, (2) warm anisotropic plasma with densities ranging from 2 to 12 ions/cm³, temperatures between 10 and 15 eV, and H+/He+ density ratios of 0.1 to 6, and (3) thermal field‐aligned H+ and He+ ion streams displaying both single stream and counterstreaming flow characteristics—the polar wind. These streaming ions display temperatures ≤ 1.5 eV and ≤ 2.0 eV for H+ and He+, respectively, and respective parallel drift velocities of < 30 km/s and < 15 km/s with an H+/He+ flux ratio of 3 to 10. Best‐fitting techniques for the H+ and He+ streaming distributions indicate a supersonic flow, although the data cannot be interpreted unambiguously. These comprehensive measurements of thermal and suprathermal plasma characteristics are interpreted in terms of plasmasphere refilling processes.
Solar modulation of galactic cosmic rays: 5. Time‐dependent modulation
Authors: Perko J. S. , Fisk L. A.
Published: February 2008

A numerical technique is introduced for the solution of the time‐dependent equation for the solar modulation of galactic cosmic rays, in which transport in heliographic latitude is ignored. We illustrate this method with a model for the solar cycle variation in cosmic ray intensity. The variation is assumed to result from changes in the number of enhanced cosmic ray scattering regions, which are produced by flare‐generated shock waves in the solar wind. The model can account for the observed solar cycle variation in intensity at different energies as well as the observed spatial gradients. This model also provides a natural explanation for the cosmic ray hysteresis effect.
The current flow between the equatorial E and F region
Authors: Duhau S. , Louro A. A.
Published: February 2008

In dynamical calculations in the equatorial F region it is usually assumed that the current flowing through the base of the F region does so exclusively along the geomagnetic field lines. This hypothesis is revised in the present paper, and it is found that the major contribution to the total flow at that height is given by the current normal to the field lines.
Electron velocity distributions near interplantary shocks
Authors: Feldman W. C. , Anderson R. C. , Bame S. J. , Gosling J. T. , Zwickl R. D. , Smith E. J.
Published: February 2008

Solar wind electron velocity distributions measured across interplanetary shocks using the Los Alamos plasma analyzer on ISEE 3 have been studied to understand electron heating mechanisms for weak and intermediate strength collisionless shocks. This study thus complements earlier studies of electron heating at the earth's bow shock, an example of a highly supercritical collisionless shock. At the weakest interplanetary shocks (downstream to upstream density ratio N(d/u) ≲ 2 and velocity difference V(d‐u) ≲ 70 km/s), heating perpendicular to the magnetic field B predominates over parallel heating. Such heating may be a consequence of conservation of magnetic moment across the shock. At the stronger interplanetary shocks (N(d/u) ≳ 2 and V(d‐u) ≳ 70 km/s), heating parallel to B is dominant and the downstream velocity distributions are flat‐topped, similar to what is observed downstream of the earth's bow shock. This similarity suggests that electron heating in all collisionless shocks with N(d/u) ≳ 2 and V(d‐u) ≳ 70 km/s results in part from an acceleration parallel to B produced by the macroscopic shock electric field, followed by beam driven plasma instabilities.
Pedersen density drift instabilities
Authors: Gary S. Peter , Cole Thomas E.
Published: February 2008

This paper describes the linear kinetic theory of electrostatic drift instabilities driven by Pedersen and density drift velocities. The model uses a uniform magnetic field
; a weak, uniform density gradient in the x direction; and a weak, uniform electric field in the y direction. Weak charged‐neutral collisions are represented by the addition of BGK model terms to the Vlasov equation. The resulting local dispersion equation is used to study the properties of the associated instabilities at kai ≳ 1 (where k is the wave number and ai is the ion gyroradius). Results show that the E×B gradient drift instability at kai ≃ 1 may grow in the auroral ionosphere primarily in the vicinity of 200 km and only if the electron density is sufficiently small.
On the control of magnetospheric convection by the spatial distribution of ionospheric conductivities
Authors: Senior Catherine , Blanc Michel
Published: February 2008

A self‐consistent semianalytical model of magnetospheric convection including the effect of the latitude and local time variations of ionospheric conductivities is presented. The motions of the inner edge of the magnetospheric ring current, and the associated field‐aligned currents, produced by the externally imposed dawn‐to‐dusk potential drop across the magnetospheric cavity are computed by using a linear approximation. The coupling between the different diurnal harmonics in the local time variations of fields and currents produced by the local time dependence of ionospheric conductivities is described by an appropriate matrix formalism. The calculations show that the enhancement of auroral conductivities by electron precipitation in the auroral zone significantly enhances both the typical duration and the absolute amplitude of the penetration of convection electric fields to midlatitudes. Furthermore, the local time variations of the convection electric field generated at midlatitudes by a sudden increase of the dawn‐to‐dusk potential drop are in good agreement, both at the initial time and after the steady state is reached, with the available statistical models of the disturbance midlatitude electric field. The amplitude of the steady state field seems sufficient to explain these observations, thus confirming that the concept of the shielding of midlatitudes from the convection electric fields is basically correct but was overestimated in earlier analytical calculations. The large subauroral electric fields observed by several satellites are also reproduced in the model either by a decrease of the subauroral conductivities below the midlatitude values or by the consideration of a very narrow latitudinal extent of the auroral zone. The overall consistency between the results of the model and the electric field observations thus supports the idea that a large class of phenomena related to magnetospheric convection in the dipole regions of the magnetosphere can be described in a reasonably realistic manner by a linear theory.
Maximum entropy spectral analysis of artificial sinusoidal signals
Authors: Vellante M. , Villante U.
Published: February 2008

We applied the Burg's algorithm to the spectral analysis of a signal that is the sum of 11 sinusoids as intermixed with different noise levels. We paid particular attention to those features that for single sinusoids represent the crucial point of this technique and found that, for a multicomponent signal, the frequency shiftings are smaller than for single sinusoids and the splitting phenomena tend to occur at much higher orders of the prediction error filter. For a noise power smaller than 17% of the power of the lowest amplitude component, all the expected peaks were easily identified and most of the power was concentrated in very narrow ranges of frequency. The behavior of the final prediction error might provide useful suggestions only for higher noise levels and the choice of the limits for the application of the Akaike's criterion is ambiguous and strongly dependent on the noise level. An analysis of individual signal components does not confirm the results of previous investigations in that the splitting hierarchy appears independent on the wave parameters. We also examined the effects of the progressive addition of new components and found that it progressively leads to smaller frequency shifts and higher orders of appearance of splitting phenomena.
Ionospheric signatures of magnetospheric convection
Authors: Foster John C.
Published: February 2008

Convection plays a major role in determining the characteristics of the high‐latitude ionosphere and the processes that drive magnetospheric convection are reflected in the ionospheric configuration. Average patterns of ionospheric convection observed with the Chatanika incoherent scatter radar, when mapped along magnetic field lines to the equatorial plane, delineate regions of earthward convection within the magnetotail that either proceed sunward to the dayside magnetopause where reconnection with the IMF may occur or that join the antisunward circulation along the magnetospheric flanks at dusk and dawn. Summer and winter patterns of ionospheric convection and F region density are examined in terms of the corresponding patterns of magnetospheric circulation. It is found that ionospheric ionization troughs at dusk and dawn are associated with sunward convection of low‐density plasma from the night sector and that plasma in the vicinity of the dawn and dusk electric field reversals circulates at high latitudes away from the solar ionization source at noon. Convection into the cleft from low latitudes populates the polar cap with high density solar‐produced plasma. Winter densities in this region were three times those seen in summer. Plasma exiting the polar cap is convected away from midnight in the equatorward portion of the regions of sunward convection at auroral latitudes. Characteristic features of the polar cap plasma serve to identify plasma following such convection trajectories.
Correlation of auroral hiss and upward electron beams near the polar cusp
Authors: Lin C. S. , Burch J. L. , Shawhan S. D. , Gurnett D. A.
Published: February 2008

Simultaneous plasma and wave data obtained by the DE‐1 satellite are used to study a correlation between electrostatic auroral hiss emissions at several kilohertz and upward electron beams at altitudes between 2 and 4 RE near the dayside polar cusp. Among five randomly selected DE‐1 passes, intense electrostatic hiss emissions at frequencies below the electron plasma frequency are found to be associated with strong upward electron beams for every pass. The frequency‐time spectrum of auroral hiss near the polar cusp is sometimes characterized by a funnel shape, suggesting that the radiation is emitted from a wave source below the spacecraft. At the center of the enhanced wave region, the electron distribution function above 20 eV is characterized by two components: a hot Maxwellian component and an upward electron beam. The beams generally have a peak energy around 50 eV, a temperature around 20 eV and a density of the order of 1 cm−3. The observed distribution functions are fitted by a drifting Maxwellian function for the electron beam and an isotropic Maxwellian function for the hot component. The empirically fitted plasma parameters are then used to solve the linear dispersion equation of electrostatic waves. The instability analyses indicate that whistler waves propagating with wave normal angles near a small resonance cone can be easily excited by low energy (<100 eV) upward electron beams. The frequencies of large growth rates are found below the electron plasma frequency, in agreement with the observations. On the basis of the model that cusp auroral hiss emissions are whistler waves propagating near the resonance cone, ray path studies indicate that the low altitude boundary of the wave source of cusp hiss is located at about 1 RE.
Thermal electron heating rate: A derivation
Authors: Hoegy Walter R.
Published: February 2008

The thermal electron heating rate, Qe, is an important heat source term in the ionospheric electron energy balance equation, representing heating by photoelectrons or by precipitating higher energy electrons. A formula for the thermal electron heating rate is derived from the kinetic equation by using the electron‐electron collision operator as given by the unified theory of Kihara and Aono. This collision operator includes collective interactions to produce a finite collision operator with an exact Coulomb logarithm term. The derived heating rate Qe is the sum of three terms, Qe = Qp + S + Qint, which are, respectively, (1) primary electron production term giving the heating from newly created electrons that have not yet suffered collisions with the ambient electrons, (2) a heating term evaluated on the energy surface meυ²/2 = ET at the transition between Maxwellian and tail electrons at ET, and (3) the integral term representing heating of Maxwellian electrons by energetic tail electrons at all energies > ET. Published ionospheric electron temperature studies have used only the integral term Qint with differing lower integration limits. There can be a significant numerical difference between Qe and Qint. Use of the incomplete heating rate could lead to erroneous conclusions regarding electron heat balance, since Qe is greater than Qint by as much as a factor of 2. The sensitivity of the heating rate to the method of calculating the energetic (tail) electron distribution function by using either a linear or a quadratic collision operator is demonstrated. Choice of the transition energy, ET, between the thermal (Maxwellian) population and the energetic tail electrons significantly affects the magnitude of the individual heating rate terms. The net heating rate Qe is less sensitive to the value of ET than are the individual terms.
F layer ionization patches in the polar cap
Authors: Weber E. J. , Buchau J. , Moore J. G. , Sharber J. R. , Livingston R. C. , Winningham J. D. , Reinisch B. W.
Published: February 2008

Ground‐based optical and digital ionosonde measurements were conducted at Thule, Greenland to measure ionospheric structure and dynamics in the nighttime polar cap F layer. These observations showed the existence of large‐scale (800–1000 km) plasma patches drifting in the antisunward direction during a moderately disturbed (Kp ≥ 4) period. Simultaneous Dynamics Explorer (DE‐B) low‐altitude plasma instrument (LAPI) measurements show that these patches with peak densities of ∼106 el cm−3 are not locally produced by structured particle precipitation. The LAPI measurements show a uniform precipitation of polar rain electrons over the polar cap. The combined measurements provide a comprehensive description of patch structure and dynamics. They are produced near or equatorward of the dayside auroral zone and convect across the polar cap in the antisunward direction. Gradients within the large scale, drifting patches are subject to structuring by convective instabilities. UHF scintillation and spaced receiver measurements are used to map the resulting irregularity distribution within the patches.
Observations relating changes in thermospheric composition to depletions in topside ionization during the geomagnetic storm of September 1982
Authors: Miller N. J. , Mayr H. G. , Spencer N. W. , Brace L. H. , Carignan G. R.
Published: February 2008

Data taken over midlatitudes by Dynamics Explorer 2 during a geomagnetic storm are used to demonstrate that a direct correlation exists between depletions in the ratio [O]/[N2] and in the topside electron density (Ne). That correlation is most conspicuous along 45° invariant latitude where a decrease in [O]/[N2] by a factor of 5 is associated with a depletion in Ne by a factor of 8. Data comparisons were made by using densities of molecular nitrogen and atomic oxygen measured by the neutral atmosphere composition spectrometer, gas temperatures measured by the wind and temperature spectrometer, and temperatures and densities of thermal electrons measured by the Langmuir probe. The measurements were taken in the sunlit northern hemisphere during September 21–23, 1982, in the altitude range 300–500 km. During the storm, gas temperatures were elevated, while the ratio [O]/[N2] and Ne were reduced. At 60° and 45° invariant latitudes, the most significant disturbances in the ratio [O]/[N2] and Ne were depletions; but at 20° invariant latitude, depletions in the ratio [O]/[N2] and Ne were preceded by enhancements. The correlations between variations in the ratio [O]/[N2] and Ne indicate that, for this storm, ion chemistry controls the observed variations of topside Ne at the higher latitudes. However, the development at 20° invariant latitude of elevated gas temperatures and an enhancement in Ne indicates that a storm induced equatorward wind component transported plasma toward lower latitudes at a rate that initially exceeded the rate of ionization losses there.
Wave normal directions of chorus near the equatorial source region
Authors: Goldstein Bruce E. , Tsurutani Bruce T.
Published: February 2008

Wave propagation directions of post‐midnight chorus in the near‐equatorial region at L shells of 6 to 7 have been determined from OGO 5 triaxial search coil magnetometer data. Several methods were used to estimate the wave normal directions: minimum variance, imaginary part of cross‐spectral matrix, eigenvector of Hermitian cross‐spectral matrix, and fit of dispersion relations for one‐wave and two‐wave models to cross‐spectral matrix. Comparisons of the methods and results are discussed in detail. Wave propagation at all frequencies within chorus tones was found to occur most frequently along the magnetic field with median and average cone angles of 9.1° and 12.2°, respectively. All methods of analysis gave similar results. It is concluded that the wave growth is maximum for waves propagating parallel to B. Our results are consistent with emission in a narrow beam but not as a broad angular distribution. The two wave propagation model typically resulted in negligible improvements in the residual beyond that obtained by assuming that only one wave was present. In only a few cases did substantial improvement result. These cases were unusual periods when it was obvious that simultaneous rising and falling tone emissions were present and crossing. It is concluded that in most cases only one chorus wave was present. In some cases the two‐wave model gives misleading indications of waves with large energy fluxes propagating at very oblique angles near the resonance cone. This is because the wave group velocity goes to zero near the resonance cone, and if electric field data are not used, then a small amount of magnetic noise is modeled with a large wave energy flux. Some examples have been found of a lower frequency burst propagating almost exactly along the magnetic field followed by a higher frequency burst travelling at large angles to the magnetic field. These paired bursts are quasi‐periodic with a delay of about 6 s between pairs of bursts; the phenomenon is presumed to be associated with pulsating auroral patches. The physical implications are discussed.
Escape of sulfur and oxygen from Io
Authors: Cheng Andrew F.
Published: February 2008

Charged particle sputtering is reexamined as a mechanism for escape of sulfur and oxygen from Io. Corotating ions and energetic ions are considered, using the best available determinations of ion fluxes and sputtering yields. It is unclear whether molecular SO2 is the dominant escaping neutral from Io or whether atomic O and S are dominant, but the Voyager detection of SO2+ in the cold torus implies the presence of molecular SO2 there. Thin atmosphere models encounter serious difficulty in achieving adequate sulfur and oxygen escape rates, but they are not yet ruled out. Ionization and dissociation of SO2 in a sputter corona can dominate direct escape. In thick atmosphere models the neutral escape rates can be much higher, but the oxygen escape rate tends to be too high relative to the sulfur escape rate unless diffusive separation in the atmosphere is limited, perhaps by rapid mixing.
The dynamics and upstream distributions of ions reflected at the Earth's bow shock
Authors: Burgess D. , Schwartz S. J.
Published: February 2008

Using test particle trajectories in a simplified model of a supercritical, oblique, collisionless shock, we investigate the interaction of solar wind thermal ions with the earth's bow shock. We present results for shocks with shock angle (θBn) between 35° and 60° and show that their velocity space signatures are consistent with observations of many upstream ion events, particularly gyrating ions and field‐aligned ion beams. The shock is modeled as a finite, planar discontinuity in the magnetic field and in the electrostatic potential, which is also given an overshoot at the shock. We find that the shock can produce backstreaming ions by reflecting a small fraction of the incident distribution. The reflected particle trajectories are of three types: specularly reflected, multiple traversal, and multiple bounce. All backstreaming reflected ions are found to suffer specular reflection on their first encounter with the shock surface; their subsequent behavior is determined by the angle θBn and their initial conditions.
An analysis of the high‐latitude thermospheric wind pattern calculated by a thermospheric general circulation model: 1. Momentum forcing
Authors: Killeen T. L. , Roble R. G.
Published: February 2008

Previous studies have shown that the wind, temperature, and composition structures calculated by the National Center for Atmospheric Research (NCAR) thermospheric general circulation model (TGCM) were in reasonable agreement with measurements made by the Dynamics Explorer 2 satellite over the southern hemisphere polar cap during October 1981. The winds at F region heights follow but generally lag behind the two‐cell ion‐drift pattern of magnetospheric convection. A diagnostic package for the TGCM has been developed to calculate the magnitude and direction of the various terms in the hydrodynamic and thermodynamic equations within the model. The package has been used to decompose the thermospheric momentum equations at each hour of universal time for a 24 hour “steady state” run of the TGCM. Displaced geomagnetic and geographic poles are considered, and a 60‐kV cross‐tail potential is assumed for the magnetospheric convection model. The individual momentum forcing terms for constant‐pressure levels z = 1 (300 km) and z = −4 (120 km) are presented to illustrate the balance of forces acting on the neutral gas at F and E region altitudes over the southern hemisphere polar cap. The results show that at F region altitudes the largest forces in the “steady state” are due to ion drag induced by sunward‐drifting ions on the dawnside and duskside of the auroral oval/polar cap boundary. There is a universal time dependence of thermospheric momentum forcing due to the diurnal oscillation of the convection pattern with respect to the solar terminator. At F region altitudes the basic balance of forces is between ion drag and pressure, whereas at E region altitudes Coriolis and advection forces also have significant contributions. The lower thermosphere cold, low‐pressure cyclonic circulation and the warm, high‐pressure anticyclonic circulation calculated by the TGCM on the dawnside and duskside of the polar cap, respectively, show a near gradient flow balance. The ion‐drag, Coriolis, and pressure forces are in approximate balance on both sides of the polar cap with the advection force enhancing and diminishing the strength of the dawn and dusk vortices, respectively.
Diurnal variation of burst precipitation effects on subionospheric VLF/LF signal propagation near L = 2
Authors: Leyser T. B. , Inan U. S. , Carpenter D. L. , Trimpi M. L.
Published: February 2008

Burst precipitation effects on subionospheric VLF/LF signal propagation are studied using data recorded at Palmer Station, Antarctica (L ∼ 2.4) between March 3 and April 7, 1983. Relatively abrupt signal amplitude changes, known as “Trimpi” effects, were observed in conjunction with magnetospheric whistlers, which are inferred to induce electron precipitation that in turn causes enhanced ionization at the 80 to 90‐km altitude level. Earlier findings that Trimpi effects occur predominantly under nighttime ionospheric conditions were confirmed by comparing the sunrise‐sunset terminator position with perturbation activity on three VLF/LF signal paths for which the arrival bearings range from magnetic north to west. Events were found to occur on 70% of the observing nights. Signal paths making larger angles with the terminator were used to deduce that most events were caused by ionospheric effects occurring within ∼1000 km of Palmer Station. Some events were observed after sunrise over the observing station, apparently due to precipitation in dark regions as far as ∼1800 km away. Observed differences between two different paths in the occurrence of this postsunrise continuation of Trimpi events suggest that the Trimpi event activity rate decreases below L ∼2. This may be due to a reduction in whistler activity and/or lower particle scattering efficiency. The average hourly occurrence rate of transmitter NSS events on the 11 most active nights studied increased from sunset to near midnight and then remained at a roughly constant level until sunrise. The influence of the whistler rate and of whistler path locations on Trimpi effects is clearly important and requires further study.
Surface solitary waves and solitons
Authors: Hollweg Joseph V. , Roberts Bernard
Published: February 2008

The solar atmosphere and solar wind are magnetically structured. The structuring can include tangential discontinuities, which can support surface waves. Such waves can be dispersive. This means that dispersion and nonlinearity can balance in such a way that solitary waves (or solitons) can result. This general point is illustrated by a two‐dimensional nonlinear analysis which explicitly demonstrates the presence of long‐wavelength solitary waves propagating on tangential discontinuities. If the waves are only weakly nonlinear, then they obey the Korteweg‐deVries equation and are true solitons.
Interannual and short‐term variations of the Venus nighttime hydrogen bulge
Authors: Taylor H. A. , Brinton H. C. , Niemann H. B. , Mayr H. G. , Hartle R. E. , Barnes A. , Larson J.
Published: February 2008

The predawn bulge in the diurnal distribution of neutral hydrogen at Venus is found to persist over the first three Venus years of the Pioneer Venus mission. Concentrations of H, derived from in situ mass spectrometer measurements of O+, H+, O, and CO2 show a diurnal bulge with peak levels near 2–5 × 107/cm³ and a night to day concentration ratio of about 200/1. The presence of short‐term fluctuations of as much as a factor of three from day to day reflects the variability in the ion and neutral concentrations often detected on the nightside. Allowing for this superimposed variability, relatively little difference in the magnitude of n(H) between 1979 and 1980 is indicated. This appears to be consistent with the fact that the solar euv flux level changed very little between successive transits of the bulge. On the other hand, there is a suggestion in some of the modulation observed in n(H) that short‐term variation in the euv flux due to solar rotation may influence the bulge characteristics. This speculation on the effectiveness of solar radiation cannot be substantiated as yet, and there is also evidence that changes in the solar wind ρυ² parameter are linked to significant changes in n(H). Although the ion and neutral variability complicates the identification of chemical equilibrium, rather similar behavior in the distributions of n(He), which is measured directly, lends confidence to the n(H) results. Overall, the attempt to understand the hydrogen and helium variability provides a stimulus for more detailed analysis of the nightside aeronomy and its relationship to both solar and solar wind inputs.
The meridional thermospheric neutral wind measured by radar and optical techniques in the auroral region
Authors: Wickwar Vincent B. , Meriwether John W. , Hays Paul B. , Nagy Andrew F.
Published: February 2008

Radar observations of ion velocities in the magnetic zenith over Chatanika, Alaska, were used to determine the geomagnetic meridional component of the thermospheric neutral wind. Corrections for molecular diffusion and molecular ion contamination of the pure O+ composition assumed for the ionosphere were included in the analysis. Comparison of the averaged diurnal variation of the meridional wind showed good agreement between the two measurement techniques. Good agreement was also found for several cases of simultaneous observations. The evidence suggested that differences were caused by gravity waves. The 7 years of radar meridional wind results were examined with respect to magnetic activity, solar cycle phase, and season. During the day, the meridional component is poleward with a maximum of about 65 m/s between 1400 and 1600 local time. During the night, the wind is equatorward with a maximum of about 175 m/s between 0200 and 0500 local time. This maximum occurs after local magnetic midnight, which is about 0130 local time. When the neutral wind is averaged for 24 hours, there is a large net equatorward flow. During periods of increased magnetic activity, the nighttime wind between 2300 and 0600 local time becomes stronger toward the equator. The average increase between 0200 and 0600 local time is about 100 m/s; however, on individual days it can be as large as 400 m/s. These data pertain mostly to equinox, but the few summer and winter observations in the data set differ in the manner predicted by theory. Comparison of these results with theoretical models shows good agreement at most times, but suggests possible heating poleward of Chatanika during the morning hours. Observed exospheric temperature increases support this hypothesis.
Distant heliospheric results on interplanetary shock propagation
Authors: Mihalov John D.
Published: February 2008

Solar wind plasma data from Pioneers 10 and 11 and the Pioneer Venus Orbiter for 1978 to mid‐1981 have been examined for forward shock signatures, and associations have been made with solar flares. (IMP 8 shocks from early 1978 have similarly been included for completeness.) The numbers of flare‐associated shocks observed at the various heliocentric distances of the spacecraft (ranging from 0.7 to 24 AU) are deduced, and the shock strengths and energies are given. Significantly fewer shocks associated with flares are observed at the greater distances, presumably because more cases have decreased below the threshold for inclusion. Three cases in which the same shock was probably detected by spacecraft at different heliocentric distances indicate deceleration and weakening at the greater distances.
Near equality of ion phase space densities at Earth, Jupiter, and Saturn
Authors: Cheng A. F. , Krimigis S. M. , Armstrong T. P.
Published: February 2008

Energetic ion phase space density profiles are strikingly similar in the inner magnetospheres of Earth, Jupiter, and Saturn for ions of first adiabatic invariant near 100 MeV/G and small mirror latitudes. Losses occur inside L ≈ 7 for Jupiter and Saturn and inside L ≈ 5 at Earth. At these L values there exist steep plasma density gradients at all three planets, associated with the Io plasma torus at Jupiter, the Rhea‐Dione‐Tethys torus at Saturn, and the plasmasphere at Earth. Measurements of ion flux tube contents at Jupiter and Saturn by the low‐energy charged particle experiment show that these are similar (for oxygen ions 5 ≲ L ≲ 9) to those at earth (protons 2 ≲ L ≲ 6). Furthermore, the thermal ion flux tube contents from Voyager plasma science data at Jupiter and Saturn are also very nearly equal, and again similar to those at Earth, differing by less than a factor of 3 at the respective L values. The near equality of energetic and thermal ion flux tube contents at Earth, Jupiter, and Saturn suggests the possibility of strong physical analogies in the interaction between plasma and energetic particles at the plasma torii/plasma sheets of Jupiter and Saturn and the plasmasphere of Earth.
Magnetic field in the wake of Venus and the formation of ionospheric holes
Authors: Marubashi K. , Grebowsky J. M. , Taylor H. A. , Luhmann J. G. , Russell C. T. , Barnes A.
Published: February 2008

Magnetic field structures are analyzed for both the ionospheric hole region and the magnetosheath‐ionosphere interaction region of the nightside of Venus, in search of possible coupling between these two regimes. A magnetic coordinate system based on the directions of the solar wind and the interplanetary magnetic field (IMF) is found to order the data reasonably well, allowing consistent superposition of observational data from individual passes of the Pioneer Venus orbiter. The main findings are (1) ionospheric holes form in a zone of ±45° magnetic latitude covering possibly the entire width of the Venus wake in the longitudinal direction; (2) the magnetic field within the holes is strong and ordered, and its direction is determined by the IMF direction, being consistent with the field line draping concept; (3) the magnetic field direction in the holes, particularly at low altitudes near the terminator, shows evidence for the existence of strong plasma flow toward the antisolar meridian in the direction parallel to the magnetic equatorial plane; and (4) the average magnetic field distribution in the nightside magnetosheath also evidences strong plasma flow toward the antisolar meridian. Together these results strongly indicate that the magnetosheath plasma flow in the wake region plays an important role in forming the ionospheric holes, through deformation of the nightside ionopause. These results are combined in a model of the three‐dimensional magnetic field structure around the ionosphere of Venus.