bluuebunny/crossref_metadata_2025 huggingface dataset:
A reexamination of important N2 cross sections by electron impact with application to the dayglow: The Lyman‐Birge‐Hopfield Band System and N I (119.99 nm)
Authors: Ajello J. M. , Shemansky D. E.
Published: February 2008

The far ultraviolet emission spectrum (120 to 210 nm) of electron‐excited N2 has been obtained in a crossed‐beam laboratory experiment. The cross section of the Lyman‐Birge‐Hopfield (LBH) band system (a¹Πg → X¹Σg+) has been remeasured using experimental techniques we have previously developed for this metastable transition. The improved laboratory data set for the a¹Πg state allows a determination of the excitation, emission, and predissociation cross section from threshold to 200 eV for use in planetary atmosphere models of the dayglow and aurora. An analytic fit to the experimental cross section allows accurate estimates to arbitrarily high excitation energy. The close agreement in both energy dependence and absolute cross section values between the emission measurements, presented here, and published electron scattering results shows cascade is small (<5%). The total excitation cross section for the N2 a¹Πg state is estimated to be 6.22 ± 1.37×10−18 cm² at 100 eV. The absence of emission bands for υ′ ≥ 7 suggests the predissociation yield is unity. The excitation function of each vibrational level is found to have the same shape to within 5%. In the low‐energy region, ε < 20 eV, differences in excitation threshold lead to a significant departure of the relative vibrational cross sections from the Franck‐Condon distribution. Thus the relative LBH vibrational population distribution in a planetary dayglow or aurora is affected by the energy distribution of the electron flux; and we show that atmospheric models need to include this threshold effect. The N I (119.99 nm) cross section has also been remeasured and found to be 3.48 ± 0.77×10−18 cm² at 200 eV on the basis of a comparison with Lyman α emission from dissociative excitation of H2.
Specularly reflected ions, shock foot thicknesses, and shock velocity determinations in space
Authors: Gosling J. T. , Thomsen M. F.
Published: February 2008

The magnetic foot of a quasi‐perpendicular, supercritical collisionless shock is spatially coincident with and caused by gyrating ions nearly specularly reflected from the shock. The reflected ions are turned around by the upstream magnetic field and returned to the shock after completing a partial gyration. An expression is derived for the turnaround distance of specularly reflected ions for arbitrary orientations of the incident velocity vector and the upstream magnetic field. This expression is then used to derive a formula for calculating the shock speed in the spacecraft frame from a single point measurement of the time required for the magnetic foot to transit a spacecraft. The derived formulas for turnaround distance and shock speed differ from previously published equations for these parameters and in some geometries give quantitatively very different results.
High‐level spacecraft charging in the low‐altitude polar auroral environment
Authors: Gussenhoven M. S. , Hardy D. A. , Rich F. , Burke W. J. , Yeh H.‐C.
Published: February 2008

Regions of intense keV electron precipitation, such as inverted‐V structures, at times colocate with ionospheric plasma depletion regions in the high‐latitude polar ionosphere. When Defense Meteorological Satellite Program (DMSP) F6 and F7 satellites, at 840 km, enter these regions in darkness, ion signatures of high spacecraft‐to‐ambient plasma potential differences (several hundred volts negative) are observed with the new SSJ/4 ion detectors. A systematic survey of charging events and the environment in which they occur was made using the DMSP F6 and F7 precipitating ion and electron detectors, the SSIE thermal plasma probes, and the SSM (F7 only) vector magnetometer. The charging events of November 26, 1983, are analyzed in detail since they occurred on both satellites. Critical levels of number flux and average energy for the precipitating electrons, and the threshold density of the thermal ionospheric ions are defined for different levels of spacecraft charging.
Collisional time scales for temperature and velocity exchange between drifting Maxwellians
Authors: Hernandez R. , Marsch E.
Published: February 2008

The Coulomb collision transfer rates for energy and momentum exchange between drifting Maxwellians are evaluated. We discuss the associated time scales which depend on the masses, densities, temperatures, and velocities of the two colliding species. Inclusion of finite mass density effects of the minor species generally leads to shorter time scales, as compared to the test particle approach, for any differential speed and temperature equalization. We find that two particle species may, by collisions, attain a temperature ratio which can range between 1 (thermodynamic equilibrium) and the inverse ratio of their mass densities. This result suggests the possibility of preferentially heating the species with the lower mass density by Coulomb collisions, assuming that their differential speed can be externally maintained against frictional deceleration.
High‐latitude convection on open and closed field lines for large IMF By
Authors: Moses J. J. , Crooker N. U. , Gorney D. J. , Siscoe G. L.
Published: February 2008

S3‐3 electric field observations for August 23, 1976, show a single convection cell engulfing the northern polar cap. The flow direction is that for a positive IMF By component. The particle data indicate that nearly half the duskside sunward flow occurs on closed field lines whereas the dawnside flow is entirely on open field lines. We interpret this in terms of an IMF By‐induced deformation in the polar cap boundary, where the deformation moves with the convective flow. Thus convection streamlines cross the deformed polar cap boundary, but no flow crosses the boundary because it is carried by the flow. Since southern hemisphere convection is expected to occur with the opposite sense of rotation, we predict that closed field lines will be forced to tilt azimuthally. On the nightside the tilt produces a y component of the magnetic field in the same direction as the IMF for either sign of IMF By. Our interpretation is consistent with observations of a greater y component in the plasma sheet than the tail lobes, which are difficult to understand in terms of the common explanation of IMF penetration. Alternatives to this interpretation are also discussed.
The hidden ion population: Revisited
Authors: Olsen R. C. , Chappell C. R. , Gallagher D. L. , Green J. L. , Gurnett D. A.
Published: February 2008

Satellite potentials in the outer plasmasphere range from near zero to +5 to +10 V. Under such conditions ion measurements may not include the low energy core of the plasma population. In eclipse, the photoelectron current drops to zero, and the spacecraft potential can drop to near zero volts. In regions where the ambient plasma density is below 100 cm−3, previously unobserved portions of the ambient plasma distribution function can become visible in eclipse. A survey of the data obtained from the retarding ion mass spectrometer (RIMS) on Dynamics Explorer 1 shows that the RIMS detector generally measured the isotropic background in both sunlight and eclipse in the plasmasphere. Absolute density measurements for the “hidden” ion population are obtained for the first time using the plasma wave instrument observations of the upper hybrid resonance. Agreement in total density is found in sunlight and eclipse measurements at densities above 80 cm−3. In eclipse, agreement is found at densities as low as 20 cm−3. The isotropic plasma composition is primarily H+, with ∼10% He+, and 0.1 to 1.0% O+. A low energy field‐aligned ion population appears in eclipse measurements outside the plasmasphere, which is obscured in sunlight. These field‐aligned ions can be interpreted as field‐aligned flows with densities of a few particles per cubic centimeter, flowing at 5–20 km/s. The problem in measuring these field‐aligned flows in sunlight is the masking of the high energy tail of the field‐aligned distribution by the isotropic background. Effective measurement of the core of the magnetospheric plasma distributions awaits satellites with active means of controlling the satellite potential.
Low‐latitude Pc 3 magnetic pulsations observed at conjugate stations (L ∼ 1.5)
Authors: Yumoto Kiyohumi , Saito Takao , Tanaka Yoshihito
Published: February 2008

In order to clarify characteristics of low‐latitude Pc 3 magnetic pulsations with 10–45 s periods, we carried out a simultaneous observation with standardized rulfmeters (ring‐core‐type ULF flux gate magnetometer) at conjugate stations, Moshiri in Japan, and Birdsville in Australia (L ∼ 1.5). Amplitude of low‐latitude Pc 3's with 20 min to 1 hour duration changed concurrently at the conjugate stations. Diurnal amplitude variations of the Pc 3's which appear primarily in the morning hours are found to be related with the shaded and sunlit ionospheres. There are three distinct intervals in the diurnal variation of the Pc 3 polarizations at the low‐latitude stations. (1) Before sunrise, the major axes of the Pc 3 polarization ellipses associated with coherent H‐ and D‐wave packets are almost always in the NW‐SE quadrant at both the northern and southern stations. (2) In the sunlit ionosphere during 0630–1100 local time (LT), the Pc 3 pulsations show predominantly “mirror” polarizations of standing field‐line oscillations, i.e., predominantly left‐handed (right‐handed) with the major axes in the NW‐SE (NE‐SW) quadrant in the northern (southern) hemisphere. (3) In the afternoon, the D component of Pc 3 is often smaller than and not coincident with the H component. Therefore mixed polarizations are observed, but the major axes are almost oppositely directed at the conjugate northern and southern stations. Sometimes the polarization reversal across local noon appears simultaneously at the low‐latitude conjugate stations. We suggest that Pc 3 source waves, originating outside the magnetosphere, are propagating westward in the morning and eastward in the evening and can couple with various HM oscillations in the plasmasphere.
Two‐dimensional numerical modeling of the cosmic ray storm
Authors: Kadokura Akira , Nishida Atsuhiro
Published: February 2008

A numerical model of the cosmic ray storm in the two‐dimensional heliosphere is constructed incorporating the drift effect. We estimate the effect of a flare‐associated interplanetary shock and the disturbed region behind it (characterized by enhancement in velocity and magnetic field, and decrease in mean free path) on the density and anisotropy of cosmic rays in the heliosphere. As the disturbance propagates outward, a density enhancement appears on the front side, and a density depression region is produced on the rear side. The effect of drift on the cosmic ray storm appears most clearly in the higher‐latitude region. For the parallel (antiparallel) state of the solar magnetic field which corresponds to the pre‐(post‐) 1980 period, the density in the higher‐latitude region decreases (increases) before the shock arrival. The maximum density depression near the earth for the parallel state is greater than for the antiparallel state, and the energy spectrum of the density depression in percentage is softer for the parallel state than for the antiparallel state. Prior to the arrival of the shock, the phase of solar diurnal anisotropy begins to shift to the earlier hours, and its amplitude becomes greater for both polarity states. North‐south anisotropy also becomes greater because of the enhanced drift for both polarity states.
Numerical simulation of nonoscillatory mirror waves at the Earth's magnetosheath
Authors: Price Channon P. , Swift Daniel W. , Lee Lou‐Chuang
Published: February 2008

The generation of nonoscillatory mirror waves is studied using a one‐dimensional periodic hybrid electromagnetic simulation. The ion dynamics are treated exactly; the electrons are approximated as a finite pressure, massless fluid. Compression of the flux tubes in the magnetosheath causes a large pressure anisotropy, and it has been proposed that this anisotropy drives a mirror instability. The mirror waves have been identified by large amplitude fluctuations of the magnetic field, anticorrelated with pressure fluctuations. The simulations are initiated in a homogeneous high beta (beta = 2.5) plasma with the ambient magnetic field at various angles to the simulation axis. It is found that ion cyclotron waves are also driven by the pressure anisotropy, in competition with the nonoscillatory mirror waves. Simulations indicate that in a pure ¹H+ plasma the much faster growing ion cyclotron waves absorb the free energy in the anisotropy to the extent that mirror waves should not be observed. Analysis of the dispersion relations of mirror waves and ion cyclotron waves in the multi‐component plasma indicates that 4He2+ and 16O6+ ions in the solar wind should stabilize the ion cyclotron waves sufficiently that the mirror waves become the dominant instability.
Relations between morning sector Pi 1 pulsation activity and particle and field characteristics observed by the De 2 satellite
Authors: Engebretson M. J. , Cahill L. J. , Winningham J. D. , Rosenberg T. J. , Arnoldy R. L. , Maynard N. C. , Sugiura M. , Doolittle J. H.
Published: February 2008

Impulsive short‐period magnetic field fluctuations (Pi 1) observed by ground instrumentation in the morning sector auroral zone have in recent years been convincingly tied to the presence overhead of pulsating aurora. We here present simultaneous records from ground‐based magnetometer, photometer, and riometer instrumentation at Siple, Antarctica, and its conjugate point, Roberval, Quebec, and low‐altitude particle and magnetic and electric field data from the DE‐2 spacecraft. Satellite magnetometer observations support earlier findings of a link between morning sector Pi 1 and the presence of a region 2 Birkeland current sheet. The addition of energetic electron and electric field observations provides a more complete characterization of the environment of Pi 1 pulsation events. Intense fluxes of energetic electrons (from 10 keV to over 35 keV) are observed simultaneously with Pi 1 pulsations and region 2 Birkeland currents. When intense (>10−5 ergs/cm² s sr eV) fluxes of lower energy electrons (E < 1 keV) are also present, we observe strong region 2 Birkeland currents and strong Pi 1 pulsations; when lower energy electrons are less abundant, region 2 Birkeland currents and Pi 1 pulsations are considerably weaker. No Pi 1 pulsations have been found in regions without intense fluxes of energetic electrons. Simultaneous observations of magnetic and electric fields in space indicate complex localized current and field structures, which are likely to be the source of the magnetic pulses observed on the ground.
Observations of the May 30, 1984, annular solar eclipse at Millstone Hill
Authors: Salah J. E. , Oliver W. L. , Foster J. C. , Holt J. M. , Emery B. A. , Roble R. G.
Published: February 2008

The Millstone Hill incoherent scatter radar was operated on May 30, 1984, to gather data on the effects of the annular solar eclipse on the structure and dynamics of the earth's ionosphere and thermosphere. Maximum obscuration of 86% was experienced at Millstone Hill at 1700 UT, and there was only low magnetic activity coincident with the event. The observations, which were made using both the zenith‐fixed and the steerable antenna at Millstone Hill, indicated a drop of 60% in electron density shortly after peak obscuration time in the altitude region 130–250 km and a smaller decrease (15–30%) at higher altitudes. The electron temperature was found to decrease by 500–700 K in the F region during the eclipse, and relatively little variation was observed in the ion or neutral temperature. The measured three‐dimensional ion drifts in the F region were synthesized in terms of a meridional wind velocity of about 25 m/s northward at eclipse time, which represents a drop of 40–50 m/s from conditions prior to the eclipse. Electric field strengths were less than 1 mV/m and showed the same general pattern as quiet time fields at Millstone Hill. Comparisons with predictions of the neutral atmospheric response made for this eclipse by the NCAR thermospheric general circulation model (TGCM) and of the ionospheric response made by the NCAR ionospheric model indicate good agreement below 250 km for the electron density and temperature; at higher altitudes the predictions and observations can be reconciled when the effects of heat and ionization flux are taken into account for the boundary condition in the ionospheric model. The predicted neutral winds are within a factor of 2 of the measurements, but it is difficult to ascertain reliably from the data the magnitude of the perturbation fields converging onto the eclipse path.
Cosmic ray variations and turbulent flow systems: 0.3–1.0 AU; 1977–1980
Authors: Burlaga L. F. , Goldstein M. L. , McDonald F. B. , Lazarus A. J. , Mariani F. , Neubauer F. M. , Schwenn R.
Published: February 2008

Relations among the relative counting rates of cosmic ray protons and the magnetic fields and flows observed by Helios 1 between 0.3 AU and 1 AU in the period January 1977 to August 1980 are discussed. Isolated corotating or transient flows are associated with short‐term depressions in cosmic ray intensity, but the effects are localized in longitude. Systems of corotating streams do not produce a long‐term modulation effect, although temporary, short‐term depressions in cosmic ray intensity are associated with individual streams and interaction regions. Spectra of fluctuations of the magnetic field and magnetic helicity for systems of corotating flows often do not have the form f−5/3 at frequencies ≥10−5 Hz, and accordingly, the hydromagnetic fluctuations in corotating streams near the sun do not appear to represent fully developed turbulence at low frequencies. Systems of transient streams containing shocks that might encircle the sun are associated with long‐lasting decreases in cosmic ray intensity. The magnetic field fluctuations in such flow systems are often turbulent, with f−5/3 spectra extending from below 10−5 Hz to the Nyquist frequency of the data. The turbulence is likely to be confined to sheets with thicknesses of the order of 0.1 AU, possibly associated with shocks. We conclude that long‐term modulation of cosmic rays inside 1 AU is associated with turbulence in transient disturbances that probably encompass a large longitudinal extent around the sun. In mixed flow systems the effects are intermediate between those of corotating and transient systems. The net modulation may be relatively small, with large short‐term variations associated with individual streams.
Observed associations of substorm signatures at South Pole, at the auroral zone, and in the magnetotail
Authors: Hones E. W. , Rosenberg T. J. , Singer H. J.
Published: February 2008

Rapid poleward excursions of auroras and associated ionospheric currents and ionospheric radio absorption are observed quite often at the South Pole Station (λm = −75°). The auroral phenomena appear at the equatorward horizon and move rapidly (up to 3 km/s) toward the station, frequently passing overhead and proceeding farther poleward. We find that such an event typically occurs some 30–60 min after the onset of a substorm expansive phase as indicated by magnetic records from the auroral zone. We also find that these auroral excursions over South Pole Station are quite closely coincident with thickenings (recoveries) of the magnetotail plasma sheet observed by the ISEE 1 and 2 satellites at distances of 15–20 RE in the magnetotail. In short, these South Pole events have the signatures of the “poleward leap” of the auroral electrojet, a late feature of substorms that was identified several years ago primarily from auroral zone magnetic records and magnetotail plasma data and that has been taken to signify the retreat of the substorm neutral line outward through the tail. We argue that they are, indeed, further examples of that phenomenon, here manifested primarily by their high‐latitude (polar cap) aspects. Identification of the poleward leap as such in data from the well‐instrumented South Pole Station provides an important new opportunity and motivation to study this late‐stage feature of substorms and its association with plasma phenomena in the magnetotail.
A self‐consistent model of Jupiter's magnetodisc including the effects of centrifugal force and pressure
Authors: Caudal Gérard
Published: February 2008

This paper describes a self‐consistent axisymmetric stationary MHD model of the Jovian magnetosphere, in which both the centrifugal force and the pressure gradients exerted on the plasma are taken into account. The outer boundary condition is chosen so as to represent the dayside (noon meridian). The plasma distribution within the magnetosphere is represented by two Maxwellian components: a cold plasma (several tens of eV) produced at the orbit of Io and diffusing outward under centrifugally driven interchange instability, and a hot (30 keV) low‐density plasma, originating from the outer regions of the magnetosphere and diffusing inward in response to it. Outside the Io torus, this process is assumed to be sufficiently rapid that the amount of particles of each population per unit flux tube is independent of distance. The self‐consistent solution to the model, with input parameters suitable for the Voyager 1 encounter, is presented and compared to observations. It is found to reproduce very satisfactorily the large‐scale features of the dayside magnetic field, plasma density, and plasma pressure, thus supporting the relevance of the plasma distribution that was assumed as input to the model. Among the main conclusions, the pressure gradients are found to constitute the major source of azimuthal currents, thus producing, at all distances, at least 68% of the total current (integrated over latitude) per unit radial distance. The centrifugal current is of secondary importance, although its distribution is more closely confined to the equator. Several runs of the model made with different outer boundary conditions have also permitted study of the compressibility of the magnetosphere, thus showing that due to the presence of high beta plasma within it, the Jovian magnetosphere is much more sensitive to variations of the solar wind pressure than is the terrestrial magnetosphere, in accordance with the observed large variability of the Jovian magnetopause location.
The near‐Earth cross‐tail current sheet: Detailed ISEE 1 and 2 case studies
Authors: McComas D. J. , Russell C. T. , Elphic R. C. , Bame S. J.
Published: February 2008

Three near geomagnetic tail current sheet crossings of the ISEE 1 and 2 satellites, on April 5, 1979, are examined in detail. All are associated with the passage of an interplanetary shock and the region of variable solar wind pressure behind it. The general geometry of field reversing current sheets is discussed, and this geometry is examined for the cases studied, by using the ISEE 1 and 2 coorbiting satellite data sets. A new technique is employed which removes the effects of a variable sheet normal velocity for the first time. This allows us to calculate firm upper bounds on the current sheet thicknesses, and by utilizing certain physically motivated assumptions, determine the most probable actual sheet thicknesses, and inclinations of the field lines within these sheets. Current density profiles derived with this technique show the main cross‐tail current sheet to be a structure that is many thermal ion‐gyroradii thick and which is sometimes imbedded in a region that is three or more times thicker and contains much smaller current densities. These profiles also exhibit a considerable amount of fine structure in the sheet which appears as narrow peaks in the current density distributions. Possible explanations for these structures, and for the overall sheet structure itself, are examined.
The wave magnetic field amplitude threshold for nonlinear trapping of energetic gyroresonant and Landau resonant electrons by nonducted VLF waves in the magnetosphere
Authors: Bell T. F.
Published: February 2008

Using a recently developed theory, we calculate the wave magnetic field amplitude threshold Bτ necessary to allow the nonlinear trapping of energetic gyroresonant and Landau resonant electrons by VLF whistler mode waves in the magnetosphere, propagating at an arbitrary angle, ψ, with respect to the earth's magnetic field, B0. This theory predicts that the amplitude threshold is governed near the magnetic equatorial plane by the gradient of ψ with respect to distance, z, along B0. Using commonly accepted models of the magnetosphere and computer raytracing techniques, we determine the function ψ(z) for magnetic shells in the range 2 ≤ L ≤ 5 and for frequencies 5 kHz ≤ f ≤ 17.8 kHz. We then use the functions ψ(z) to calculate Bτ. It is found that the minimum values of Bτ along each L shell generally occur at points of “second‐order” resonance where both υz = υR and
, where υz is the particle velocity along the z axis and υR is the resonance velocity. In the case of gyroresonance, for a given frequency it is found that for most L shells accessible to the input rays, there is a single point of second‐order resonance near the magnetic equator, and Bτ there is larger than that associated with ducted waves of the same frequency on the same L shell. However, over a narrow range of L shell, as many as three points of second, order resonance can exist on each magnetic shell for the case of nonducted waves, suggesting that as many as three regions of VLF emission generation can exist on the same magnetic field line. The results of the calculations agree qualitatively with experimental data concerning triggered VLF emissions, which indicates that Bτ is generally larger for nonducted waves than for ducted waves and that nonducted waves tend to trigger multiple VLF emissions. Calculations also indicate that as many as three points of second order resonance can exist for energetic electrons experiencing a nonlinear Landau resonance with nonducted waves on certain L shells. Minimum threshold values of Bτ for the nonlinear Landau resonance can be lower than that for the nonlinear gyroresonance interaction, suggesting the possibility that nonducted waves may sometimes trigger VLF emissions via the Landau resonance mechanism.
Simulation of the thermospheric tides at equinox with the National Center for Atmospheric Research Thermospheric General Circulation Model
Authors: Fesen C. G. , Dickinson R. E. , Roble R. G.
Published: February 2008

Numerical calculations of the thermospheric tidal winds and temperatures at equinox are presented. The calculations were made using the National Center for Atmospheric Research thermospheric general circulation model (TGCM), which includes the effects of viscosity, conductivity, diffusion, ion drag, winds, and temperature gradients. The thermospheric diurnal and semidiurnal tides are excited in situ by solar heating and by ion‐neutral momentum coupling. The semidiurnal tidal calculations also include the effects of upward propagating waves generated by heating in the lower atmosphere. This semidiurnal propagating component is modeled by use of the classical tidal perturbations as lower boundary conditions. The model is tuned by adjusting the propagating tidal forcing term until calculated semidiurnal wind and temperature fields best approximate incoherent scatter observations. The tidal TGCM results are consistent with previous theoretical work and successfully reproduce high‐altitude temperature and meridional velocity data, but they give significantly lower magnitudes for velocities and temperatures near 160 km than are seen by observations.
Energetic neutral particles from Jupiter and Saturn
Authors: Cheng Andrew F.
Published: February 2008

The Voyager 1 spacecraft has detected energetic neutral particles escaping from the magnetospheres of Jupiter and Saturn. These energetic neutrals are created in charge exchange reactions between radiation belt ions and ambient atoms or molecules in the magnetosphere. If the Io torus is assumed to be the dominant Jovian source region for energetic neutrals, the Voyager observations can be used to infer upper limits to the average ion intensities there below about 200 keV. No readily interpretable in‐situ measurements are available in the Io torus at these energies. The middle and outer Jovian magnetospheres may also be a significant source of energetic neutrals. At Saturn, the observed neutral particle count rates are too high to be explained by charge exchange between fast protons and H atoms of the Titan torus. Most of the energetic neutrals may be produced by charge exchanges between heavy ions and a neutral cloud containing H2O in Saturn's inner magnetosphere. If so, the Voyager measurements of energetic neutral fluxes would be the first detected emissions from this region of Saturn's magnetosphere.
The reaction H + C4H2: Absolute rate constant measurement and implication for atmospheric modeling of Titan
Authors: Nava David F. , Mitchell Mark B. , Stief Louis J.
Published: February 2008

Diacetylene (C4H2), an intermediary product in the photodecomposition of Titan's methane, has been suggested to be the catalyst in processes converting H atoms into molecular hydrogen for subsequent escape from Titan's atmosphere. This role for C4H2 has importance concerning formation and survival of the unsaturated species in the atmosphere of Titan. The reaction kinetics of H + C4H2 are also of significance in incomplete combustion processes forming higher unsaturated hydrocarbons from acetylene and its radicals in the C2H2/O/H system. The absolute rate constant for the reaction H + C4H2 has been measured over the temperature interval 210–423 K, using the technique of flash photolysis‐resonance fluorescence. At each of the five temperatures employed in this study the results were independent of variations in [C4H2], total pressure (Argon or Nitrogen), and flash intensity (i.e., initial [H]). The rate constant results for 210 ≤ T ≤ 423 K are k = (1.39±0.25) × 10−10 exp (−1184±44/T) cm³ s−1, where the error quoted is one standard deviation. This represents the first temperature study of this reaction. The Arrhenius parameters at the high pressure limit determined here for H + C4H2 are contrasted with those for the corresponding reactions of H with C2H2 and C3H4. Implications of the kinetic results, particularly those at low temperatures, are considered for models of the atmospheric hydrocarbon chemistry of Titan. The rate of this reaction, relative to that of the analogous, but slower, reaction of H + C2H2, appears to make H + C4H2 a very feasible reaction pathway for effective conversion of H atoms to molecular hydrogen in the stratosphere of Titan.
The generation of kilometer scale irregularities in equatorial spread F
Authors: LaBelle J. , Kelley M. C.
Published: February 2008

Two rockets launched into spread F during March 1983 show that the intermediate‐scale spectrum of density irregularities sometimes exhibits a knee near 1 km scale length. This result confirms previous reports which are reviewed. Two different nonlinear theories of spread F turbulence are studied here and shown to be consistent with the experimental spectra when the knee is observed. Also, the existence or nonexistence of the knee may be effected by two linear mechanisms: the injection of kilometer scale turbulence at the steepened walls of spread F bubbles and the nonlocal role of the compressible E region in the development of spread F. This spectral feature, which seems to be associated with the most turbulent state of equatorial spread F, has not yet been identified in computer simulations.
Observations of low‐energy ions in the wake of a magnetospheric satellite
Authors: Samir U. , Comfort R. H. , Chappell C. R. , Stone N. H.
Published: February 2008

Measurements of low‐energy ions made by the retarding ion mass spectrometer (RIMS) onboard the Dynamics Explorer 1 (DE 1) satellite are used to study some aspects of “body‐plasma interactions” in the terrestrial plasmasphere. Preliminary experimental results are presented in an empirical, parametric manner, yielding the degree of ion (current) depletion in the wake of the DE 1 satellite in terms of (1) specific and average ion Mach numbers (S), (2) average ion mass, (3) body size normalized to ionic Debye length (RDi), and (4) body potential normalized to ion thermal energy (ϕN). Some results from the RIMS measurements are compared with relevant results from the Explorer 31 and the Atmosphere Explorer C ionospheric satellites. These provide semiquantitative comparisons of certain aspects of “body‐plasma” interactions in two different flow regimes, subsonic/transonic (0.4 ≦ S ≦ 2.3) and supersonic (S > 3.3). From the DE 1 measurements, it is found that the ion species act independently in expanding into the wake of the satellite, a result which is in accord with theoretical predictions. Comparisons of measurements from the magnetospheric and ionospheric satellites indicate that for a “small” body (RDi ≲ 12), the Wake depletion varies approximately linearly with RDi, while for a “large” body (RDi > 50), the variation is exponential. The DE 1/RIMS measurements were also used to examine the utility of a simple “neutral approximation” for the maximum rarefaction region (wake) of a satellite orbiting in the terrestrial plasmasphere. The results indicate that the use of such a simple approximation is much more applicable for the subsonic/transonic flow regime (e.g., the plasmasphere) than it is for a supersonic/hypersonic flow regime (e.g., the ionosphere).
Ionospheric conductances: Associations between ionospheric E region electric fields and ground magnetometer variations
Authors: Wilkinson P. J. , Nielsen E. , Luhr H.
Published: February 2008

STARE measures the two dimensional spatial electron drift distribution in the E region, with a temporal resolution of 20 s and with the added assumption of a Hall conductance, the total Hall current distribution. By using Biot‐Savart's law, the Hall currents can be integrated to give an estimate of the ground magnetometer response and when compared with the observed magnetometer response this yields the assumed Hall conductance. Data from 18 geomagnetically disturbed days were analysed and the calculated conductances show seasonal and diurnal behaviour similar to empirical conductance models. Daytime conductances agree well with solar insolation conductance estimates and night time conductances, while more variable, are in reasonable agreement with absorption measured by a narrow beam riometer, Spatial conductance gradients allow the Pedersen current to contribute to the source free currents resulting in an anticlockwise rotation of the observed horizontal ground magnetometer response relative to the response expected solely from the Hall current. Equivalently, the Hall current contributes to the ionospheric closure current. By using this information, solar insolation is identified as a significant factor for the ionospheric closure current.
On the association of magnetic clouds with disappearing filaments
Authors: Wilson Robert M. , Hildner Ernest
Published: February 2008

We present evidence that an interplanetary magnetic cloud preceding an interaction region, observed at earth January 24, 1974, is associated with the eruptive filament or disparition brusque (DB) near central meridian on January 18. The DB also was associated with a long‐decay soft X ray transient (LDE) and a long‐duration gradual‐rise‐and‐fall (GRF) radio burst. To assess whether magnetic clouds are generally associated with DBs, we present results from statistical testing of the relation of 33 magnetic clouds (and 33 control samples without magnetic clouds) to disappearing filaments near central meridian (≲45 deg (CMD). We find support for the hypothesis that magnetic clouds are the 1‐AU counterparts of either eruptive filaments or the coronal mass ejections which probably accompany them. The major result is that disappearing filaments occur more frequently on the days when magnetic clouds are launched than on control days, a result obtained with >99% confidence. There is a suggestion that clouds following shocks, probably launched at times of solar flares, are not as strongly associated with disappearing filaments as are clouds launched less violently.
Transition to unstable ion flow in parallel electric fields
Authors: Bergmann R. , Lotko W.
Published: February 2008

The stability of ionospheric O+‐H+ outflows accelerated by a nonambipolar parallel electric field is considered under conditions where (1) the ion motion initially develops adiabatically and (2) the ambient plasma is vertically stratified with an effective temperature that increases with altitude. Such conditions are expected near the bottom of the auroral acceleration region where ion and electron streaming instabilities first develop. It is shown for a particular equilibrium profile that the differentially accelerated ion flows become unstable within about 100 km from their entry point in the acceleration region. At O+/H+ density ratios less than about 9, the instability is dominated by a violent H+‐O+ two‐stream interaction which couples the O+ and H+ acoustic modes, and which mediates a transition to nonadiabatic acceleration. At higher altitudes and/or larger O+/H+ density ratios, a much weaker resonant instability exists, which is driven by the relative drift between electrons and O+ or H+ ions. The results suggest that the H+‐O+ two‐stream instability may be a viable mechanism for heating upflowing auroral ions.
X ray measurements at the South Atlantic Magnetic Anomaly
Authors: Pinto O. , Gonzalez W. D.
Published: February 2008

Balloon‐borne experiments using omnidirectional scintillation detectors that measured X ray with energies between 30 and 150 keV in the atmosphere were conducted during 1981 from São José dos Campos‐Brazil (geographic coordinates 23°12′S, 45°51′W and L ∼ 1.13). The measurements detected an intensification in the X ray flux in association with a strong geomagnetic storm. The enhancement in the flux in this energy range, attributed to electron precipitation, was determined to be ∼7×10−3 photons cm−2 s−1 keV−1 at 5 g cm−2. The results are compared to similar ones obtained in the region of the South Atlantic Magnetic Anomaly and their implications are discussed briefly in the context of local energetic electron precipitation.
The warped neutral sheet and plasma sheet in the near‐Earth geomagnetic tail
Authors: Gosling J. T. , McComas D. J. , Thomsen M. F. , Bame S. J. , Russell C. T.
Published: February 2008

An analysis of ISEE 2 plasma and magnetic field data indicates that the plasma sheet and neutral sheet in the near‐earth (X > −19 RE) magnetotail are warped in such a manner that in summer (winter) the neutral sheet rises above (dips below) the solar magnetospheric equatorial plane near the center of the tail but dips below (rises above) the equatorial plane along the tail flanks. In the near tail the neutral sheet crosses the equatorial plane ∼12 RE from the aberrated X axis, considerably closer to the center of the tail than has been inferred from data obtained farther (−16 > X > −40 RE) downstream. This increase in the warp with decreasing distance from the earth is consistent with theoretical predictions. In the near tail the warp is sufficiently strong when the dipole tilt angle is large that even in quiet times the upper or lower edge of the plasma sheet can be found close to the solar magnetospheric equatorial plane along the tail flanks. The seasonal dependence of the warp can produce certain dawn‐dusk asymmetries in satellite data which are more apparent than real.
Quiet time mass composition at near‐geosynchronous altitudes
Authors: Strangeway R. J. , Kaye S. M.
Published: February 2008

Mass composition data acquired from the near‐geosynchronous SCATHA spacecraft during magnetically quiet times are analyzed. The time intervals over which data were included in the study span some 4 months in the spring and summer of 1979. This allows a reasonable coverage in both L shell and magnetic local time. At the higher L shells, L > 6.5, the mass composition data are consistent with sunward convection of plasma sheet particles. Protons and alpha particle fluxes peak near 90° pitch angle. There is evidence that the alpha particle spatial distribution has a sharp inner edge near L = 6.5. At lower L values, the proton characteristics change. The density of protons above 1 keV decreases, while the lower‐energy protons show an increase in density. The oxygen ions show a similar change, in that there is a large increase in the lower‐energy oxygen ions from high L to low L, especially in the dusk and midnight local time sectors. This suggests that the ionosphere may be continuously supplying plasma to the inner magnetosphere even during magnetically quiet times.
Large amplitude wave packets observed in the ionosphere in association with transverse ion acceleration
Authors: LaBelle J. , Kintner P. M. , Yau A. W. , Whalen B. A.
Published: February 2008

Very short duration, large amplitude bursts of monochromatic waves (“spikelets”) were detected by the electric field experiment on the sounding rocket MARIE, launched in February 1985 from Churchill, Manitoba. About 35 events were detected, with an average time scale of 5 ms and an average amplitude of 100–150 mV/m. Their frequency varied between 7 and 18 kHz, and there is some evidence that the frequency is a decreasing function of altitude. The bursts are not correlated with any events on the payload, and their occurrence is not related to the rocket's spin or coning. The events were confined to the altitude range 450–650 kilometers. This coincides exactly with the altitude range for which perpendicular (90°) ion conics were detected by the particle experiment on the same payload. The “spikelets” were also correlated one‐to‐one with small (10–100 mV/m) double‐layerlike or shocklike features of similar time scale in the dc electric field data.
The average magnetic field draping and consistent plasma properties of the Venus magnetotail
Authors: McComas D. J. , Spence H. E. , Russell C. T. , Saunders M. A.
Published: February 2008

A new technique has been developed to determine the average structure of the Venus magnetotail (in the range from −8 Rv to −12 Rv) from the Pioneer Venus magnetometer observations. The spacecraft position with respect to the cross‐tail current sheet is determined from an observed relationship between the field‐draping angle and the magnitude of the field referenced to its value in the nearby magnetosheath. This allows us statistically to remove the effects of tail flapping and variability of draping for the first time and thus to map the average field configuration in the Venus tail. From this average configuration we calculate the cross‐tail current density distribution and J × B forces. Continuity of the tangential electric field is utilized to determine the average variations of the X‐directed velocity which is shown to vary from −250 km/s at −8 Rv to −470 km/s at −12 Rv. From the calculated J × B forces, plasma velocity, and MHD momentum equation the approximate plasma acceleration, density, and temperature in the Venus tail are determined. The derived ion density is approximately ∼0.07 p+/cm³ (0.005 O+/cm³) in the lobes and ∼0.9 p+/cm³ (0.06 O+/cm³) in the current sheet, while the derived approximate average plasma temperature for the tail is ∼6×106 K for a hydrogen plasma or ∼9×107 K for an oxygen plasma.
A laboratory study of ion energization by eic waves and subsequent upstreaming along diverging magnetic field lines
Authors: Cartier S. L. , D'Angelo N. , Merlino R. L.
Published: February 2008

A laboratory study related to energetic upstreaming ions in the ionosphere‐magnetosphere system is described. The experiment was carried out in a cesium Q machine plasma with a region of nonuniform magnetic field. Electrostatic ion cyclotron waves were excited by drawing an electron current to a small biased exciter electrode. In the presence of the instability, ions are heated in the direction perpendicular to B. Using a gridded retarding potential ion energy analyzer, we followed the evolution of the ion velocity distribution as the ions passed through the heating region and subsequently flowed out along the diverging B field lines. As expected, the heated ions transfer their energy from perpendicular to parallel motion as they move through the region of diverging B field. Both their parallel thermal energy and the parallel drift energy increase at the expense of the perpendicular energy.
Continued observations of the H Ly α emission from Uranus
Authors: Clarke J. , Durrance S. , Atreya S. , Barnes A. , Belcher J. , Festou M. , Imhoff C. , Mihalov J. , Moos W. , Murthy J. , Pradhan A. , Skinner T.
Published: February 2008

We present four years of observations of the disk‐averaged H Ly α emission from Uranus performed with the IUE Observatory. A detailed analysis of the uncertainties of these measurements is discussed, based both on known calibration uncertainties and on a new analysis of the uncertainty in our customized extraction procedure. On the basis of roughly 30 observations we derive an average brightness of 1400 Rayleighs. The larger data base now available has allowed us to perform a more detailed analysis of the character of this emission and its functional relationship with other parameters. The observed extent and time scales of the variability of the emission are presented, and no evidence for correlation with the solar H Ly α variations is found, implying a largely self‐excited emission. Limits are derived from the minimum observed brightness and from a modeling of the atmosphere of Uranus for the possible contribution by reflected solar H Ly α emission, which we estimate to be roughly 200 Rayleighs. We therefore interpret the remaining self‐excited emission as being produced by charged particle excitation, i.e., an aurora. Studies of possible correlations between the self‐excited component of the H Ly α emission and the density and velocity of the local solar wind are presented, based on comparisons with solar wind measurements performed in the vicinity of Uranus from the Voyager 2 and Pioneer 11 spacecraft. No evidence is found for any correlation between the solar wind density and the H Ly α brightness. We estimate an upper limit to the energy of the precipitating particles based on the lack of observed H2 band emission (which sets a lower limit to the ratio H Ly α/H2) and by analogy to the auroral precipitation on Jupiter. Finally, an estimate of the total power in the precipitating particles is on the order of 1012 watts (comparable to the aurora on Saturn), and the disturbance of the upper atmosphere by the deposited energy is discussed.
Mass and energy balance of the cold io torus
Authors: Moreno M. A. , Barbosa D. D.
Published: February 2008

A new model of the cold Io torus is described. Ions and energy are injected into the system by independent processes so that the mass balance is isolated from the energy balance. The primary source of energy is local ionization and acceleration of hot pickup ions resulting from charge exchange between thermal ions and an extended cloud of Iogenic sulfur and oxygen atoms. The primary energy loss mechanism of the plasma is collisionally excited line emission at optical wavelengths. The primary ion source is radial diffusion inward from the hot torus on a time scale of 140–710 days. The primary ion loss mechanism is a novel two‐step enhanced recombination mechanism involving charge exchange between thermal ions and an extended cloud of neutral SO2 molecules, followed by rapid dissociative recombination of the resultant molecular ion. The model provides a self‐consistent solution which reconciles a number of diverse observations with known physical processes.