bluuebunny/crossref_metadata_2025 huggingface dataset:
Current‐driven instabilities in a laminar perpendicular shock
Authors: Lemons Don S. , Gary S. Peter
Published: February 2008

The linear theory of fully electromagnetic plasma instabilities driven by currents flowing across a magnetic field is investigated, with applications to the laminar perpendicular bow shock. The theory utilizes the local approximation, assumes unmagnetized ions and magnetized electrons, and includes the effects of gradients in the magnetic field magnitude and density. Oblique propagation is considered, and wave numbers may be of the order of or larger than the reciprocal electron Larmor radius. An increase in the electron beta does not significantly affect the ion acoustic instability but does reduce the growth rates of the Buneman and the modified two‐stream instabilities. If Te » Ti within the shock, results indicate that the ion acoustic instability is the most important growing mode, for any reasonable beta. If Te < Tι throughout the shock, further work is necessary to determine which instability is most important.
An algorithm for generating fluctuations having any arbitrary power spectrum
Authors: Owens A. J.
Published: February 2008

A calculational scheme is given for generating fluctuations which have any specified power spectrum. The fast computer‐based algorithm makes use of a random number generator and fast Fourier transform (FFT) routine.
Low‐energy plasma observations at synchronous orbit
Authors: Lennartsson Walter , Reasoner David L.
Published: February 2008

The University of California at San Diego Auroral Particles Experiment on the ATS 6 satellite in synchronous orbit has detected a low‐energy plasma population which is separate and distinct from both the ring current and the plasma sheet populations. The density and temperature of this low‐energy population are highly variable, with temperatures in the range kT = 1–30 eV and densities ranging from less than 1 cm−3 to more than 10 cm−3. The occurrence of a dense low‐energy plasma is most likely in the afternoon and dusk local time sectors, whereas n > 1 cm−3 is seen in the local night sector only during magnetically quiet periods. These observations suggest that this plasma is the outer zone of the plasmasphere. During magnetically active periods this low‐energy plasma is often observed flowing sunward. In the dusk sector, strong sunward plasma flow is often observed for 1–2 hours prior to the onset of a substorm‐associated particle injection.
Long‐term variations of selected solar wind properties: Imp 6, 7, and 8 results
Authors: Feldman W. C. , Asbridge J. R. , Bame S. J. , Gosling J. T.
Published: February 2008

Variations in solar wind ion characteristics observed between March 18, 1971, and January 6, 1977, are presented in order to study long‐term trends in large‐scale interplanetary structures. Salient features observed during this period include the following. (1) Large amplitude, broad, corotating high‐speed streams appeared in January 1973 and disappeared in mid‐1976 and resulted in enhanced average flow speeds during this epoch of solar activity cycle 20. (2) Both proton number and total energy fluxes were generally larger after September 1972 than before August 1972. This increase is thought to result from an areal expansion of both polar coronal holes in response to the increasing polar field magnitudes and to lead to a compression of the low‐speed equatorial disclike region postulated to surround the sun in interplanetary space. (3) The occurrence frequency of density enhancements not obviously produced by interplanetary compressions did not decrease with decreasing solar activity. It therefore seems unlikely that all such enhancements are caused by coronal transient events. (4) The average solar wind He abundance varied from about 3 to 5% and back to 3% during activity cycle 20. This variation lagged behind that of the smoothed sunspot number by about 1–2 years. He abundance variations, like all other known systematic solar wind ion variations, occurred preferentially during low‐speed and/or high‐density flow conditions. This result may be caused by a systematic time variation in the mixture of coronal flow types leading to low‐speed conditions at 1 AU. Near solar maximum, energetic transient disturbances may dominate, while near solar minimum, spatially structured high‐density expansions may dominate.
Transport properties of monatomic carbon
Authors: Biolsi Louis
Published: February 2008

Because of ablation injection, monatomic carbon is an important flow field constituent of ablative flow during planetary entry, particularly near the surface of the entry vehicle. Therefore knowledge of the transport properties for carbon species is required for calculating the flow field properties and surface heating. In this study, the transport collision integrals for each of the 18 molecular C2 states that dissociate into ground state carbon atoms are obtained. This is accomplished by fitting two‐body semiempirical interaction potentials, for which the collision integrals are tabulated, to experimental and/or accurate theoretical calculations for each of these states. The collision integrals are used to calculate the diffusion coefficient, viscosity, and translational contribution to the thermal conductivity of monatomic carbon at 1‐atm pressure from 1000°K to 25,000°K.
Characteristics of energetic electrons in the vicinity of the magnetospheric boundary layer at Vela orbit
Authors: Palmer I. D. , Hones E. W.
Published: February 2008

A study is made of intensity and anisotropy characteristics of energetic electron populations (≳30 keV) near the magnetotail surface at the Vela orbit (r ≈ 18 RE). Inferring from a unidirectional (ξu) or bidirectional (ξb) anisotropy of the electron flux that the electrons are on open or closed magnetic field lines, we find that the field lines in the magnetospheric boundary layer adjacent to the plasma sheet were closed in about one third of the boundary layer crossings studied and were open in about one eighth of those crossings. No closed field lines can be inferred for the boundary layer crossings contiguous to the high‐latitude tail (lobe), but open field lines are inferred for a few such crossings. These observations support the idea that closed boundary layer flux tubes are swept from the subsolar surface of the magnetosphere along its flanks and into the tail and that the plasma sheet is generated at least in part by the incorporation of these closed flux tubes into the tail. Electron intensities in the portion of the boundary layer contiguous to the plasma sheet were less by a factor of ∼2.5 than those within the plasma sheet and greater than those in the neighboring magnetosheath by about the same factor. Thus the plasma sheet is probably the source of the electrons in these regions. But electron intensities in the boundary layer adjoining the tail lobes were less by a factor of ∼2 than those in the neighboring magnetosheath, suggesting that the magnetosheath is the source in these regions. In the magnetosheath, highly collimated beams (ξu ≳ 1.0) of energetic electrons are seen near the magnetopause, most commonly on the duskside, and occur less than 25% of the time in which significant fluxes are measured. The beams are sporadic, lasting typically 5–30 min. The beams are aligned with the magnetopause, consistent with streaming of electrons along field lines draped around the surface of the magnetosphere, and are generally directed antisunward. It is suggested that energetic electrons observed in the magnetosheath originate from magnetospheric populations which escape from closed field lines in a corridor between x ≈ −10 RE in the plasma sheet and the nose of the magnetosphere. Their escape and/or subsequent propagation into a weakening magnetosheath field gives rise to a collimated beam through conservation of the first adiabatic invariant. A collimated beam sometimes coexists with an approximately isotropic component, suggesting that the isotropic component may result in part from relaxation of the collimated beam due to pitch angle scattering.
Currents and electric fields in the ionosphere due to field‐aligned auroral currents
Authors: Nisbet John S. , Miller Mark J. , Carpenter Lynn A.
Published: February 2008

Static electric field and current patterns due to the field‐aligned Birkeland currents in the auroral zone have been examined. The model uses currents approximating those reported by Iijima and Potemra (1976a,b) as input to a global model of the ionospheric conductivities in which interhemispheric coupling along field lines is included. The model reproduces the main features of the high‐latitude current and voltage system and the penetration of these currents within the plasmasphere. The calculated values of the dawn to dusk electric field across the polar cap varied from 17 mV/m for the winter solstice to 4.6 mV/m for the summer solstice. Electric fields in the auroral zone are compared with satellite measurements under similar conditions. The electric fields due to the current system decrease from maximum values of 4.5 mV/m at 45° latitude to 0.45 mV/m at 20° and 0.32 mV/m at the equator. The maximum layer current density in the equatorial electrojet due to the current system was calculated to be of the order of 15 mA/m at around 1600 MLT. The energy inputs from the given input currents were calculated to vary from 8 × 1010 W in the winter hemisphere to 4 × 1010 W in the summer hemisphere.
Plasma near Venus from the Venera 9 and 10 wide‐angle analyzer data
Authors: Verigin M. I. , Gringauz K. I. , Gombosi T. , Breus T. K. , Bezrukikh V. V. , Remizov A. P. , Volkov G. I.
Published: February 2008

Preliminary results of ion and electron plasma measurements near Venus are presented and discussed. The data were obtained with wide‐angle plasma analyzers carried on the Venera 9 and 10 spacecraft. On the basis of 33 bow shock crossings the position of the shock is quite stable and agrees well with theoretical predictions of Spreiter et al. with H/r0 = 0.01 and a stagnation point altitude of ∼500 km. This observation lends strong support to the assumption that the solar wind interacts with the upper ionosphere of Venus and not with a planetary magnetic field. These spacecraft are the first to explore the optical umbra of Venus. Close to the planet a stable population of electrons and an ill‐defined population of positive ions were found; this region is called the corpuscular umbra. The corpuscular umbra and the transition region are separated by a zone which contains both positive ions and electrons and is characterized by a flow velocity reduced in comparison with that of the transition region. This zone is called the corpuscular penumbra. The distribution of plasma density behind the bow shock (including the optical umbra of the planet) is given, and the existence of a Venusian plasma magnetic tail is revealed.
Interplanetary scintillation at large elongation angles: Response to solar wind density structure
Authors: Erskine F. T. , Cronyn W. M. , Shawhan S. D. , Roelof E. C. , Gotwols B. L.
Published: February 2008

Synoptic interplanetary scintillation (IPS) index measurements were taken at 34.3 MHz during May–December 1974 using the University of Iowa Cocoa Cross radiotelescope on a ‘grid’ of 150 selected radio sources covering solar elongation angles up to 180°. Over 80 of these sources displayed definite IPS. The solar elongation dependence of the 34.3‐MHz IPS index is consistent with the elongation angle dependence measured at higher frequencies. Large enhancements (factors of ≳2) of the IPS index are found to coincide with the solar wind (proton) density increases greater than 10 cm−3 as measured by Imp 7 and 8 for nearly all observed IPS sources throughout the sky. These ‘all‐sky’ IPS enhancements appear to be caused by increased contributions to the scintillation power by turbulent plasma in regions close to the earth (≲0.3 AU) in all directions. Correlation analysis confirms the IPS response to solar wind density and indicates that the events are due primarily to the corotating solar wind turbulent plasma structures which dominated the interplanetary medium during 1974. The expected IPS space‐time signature for a simple model of an approaching corotating turbulent structure is not apparent in our observations. In some cases, the enhancement variations can be attributed to structural differences in the solar wind density turbulence in and out of the ecliptic.
Imp 6 measurements in the distant polar cusp during substorms
Authors: Fairfield D. H. , Hones E. W.
Published: February 2008

The Imp 6 spacecraft was able to make measurements in the distant dayside cusp during substorms when the cusp was located slightly equatorward of its normal location and the geomagnetic dipole was tilted in the appropriate direction. High time resolution data from the magnetic field, plasma, energetic particle, and VLF wave experiments yield support for both reconnection and diffusion as methods of particle entry to the magnetosphere. The support for reconnection is indirect evidence from particles which flow down cusp field lines near the noon meridian. This evidence (1) indicates an acceleration process to explain enhancements of 400 to 600‐km/s protons above their magnetosheath intensities and (2) suggests convection of field lines over the polar cap as a means of explaining the lack of low‐energy protons near the low‐latitude boundary of the cusp. Support for diffusion comes from indications of turbulent flows and fluctuating magnetic fields near the outer boundary of the entry layer. Magnetic field fluctuations below the proton gyrofrequency increase at larger radial distances and higher latitudes within the cusp. Perturbation vector is perpendicular to in the lower β plasma at lower altitudes but is more randomly oriented in the higher β plasmas of the distant cusp. Ion cylotron waves are frequently seen in the entry layer and outer cusp. An abrupt change in the intensity of both whistler waves and electrostatic waves near half‐integral multiples of the electron gyrofrequency seems to be characteristic of the outer boundary of the cusp. This boundary does not seem to be a standing shock.
Probable wavelengths and relative intensities of MgO infrared bands
Authors: Murty P. S.
Published: February 2008

The need for experimental data on some of the spectroscopic properties of MgO which may be useful during a further search of MgO in the lower E region of the ionosphere is emphasized. Predicted relative intensities of the bands of the A¹Π‐X¹Σ+ transition of MgO from 1.0 to 10 µm are calculated to assist in the search for the presence of this system in the atmosphere.
The Mg II equatorial airglow altitude distribution
Authors: Gérard J. C. , Monfils A.
Published: February 2008

Four occasional measurements of the altitude distribution of the 2796–2803‐Å doublet equatorial airglow have been made with the astronomical TD‐1A satellite during winter 1973–1974. The observations indicate the presence of layers of Mg+ ions in the evening twilight F region in the African sector. Horizontal column densities reaching 8 × 109 Mg+ ions/cm² are observed with peak altitudes varying from 230 to 480 km. Steep topside gradients with scale heights of about 5 km are observed on three occasions.
Diffusion model for the upper atmosphere of Venus
Authors: Mayr H. G. , Harris I. , Hartle R. E. , Hoegy W. R.
Published: February 2008

On the basis of a linear theory it is shown that wind‐induced diffusion associated with large‐scale circulation in the Venus atmosphere leads to nighttime bulges in O (consistent with the Dickinson and Ridley model), He, and H. The effect on He is very large with a 243‐day rotation period of the atmosphere, yielding an upper limit for the maximum/minimum diurnal density ratio of ∼1000. This ratio is much smaller for H (max/min of ∼4) due to the greater importance of exospheric flow, which acts (like for He) as a damping mechanism. For the faster rotation period of 4 days consistent with the motions of the cloud tops, the diurnal variation of O is significantly reduced, contributing to an increase by a factor of two in the horizontal wind velocity. The same effect significantly reduces the He variations to an increase by a factor of 7 from day to night. The results suggest that composition and wind measurements provide valuable information on the rotation rate of the cytherean thermosphere.
Compression of Jupiter's magnetosphere by the solar wind
Authors: Smith Edward J. , Fillius R. Walker , Wolfe John H.
Published: February 2008

A study of the major changes in the solar wind during the Pioneer 10 and 11 encounters and their influence on the size of the Jovian magnetosphere is reported. Simultaneous sets of encounter data acquired by the Jet Propulsion Laboratory vector helium magnetometer, the Ames Research Center plasma analyzer, and the University of California at San Diego trapped radiation detector have been compared with data acquired simultaneously in interplanetary space by the sister spacecraft. Of particular interest were four intervals during which it appeared that the spacecraft had reentered the magnetosheath near 50 RJ after having first entered the magnetosphere near 100 RJ. The principal outcome of the study is that in three of these cases the reentries into the magnetosheath occurred when high‐speed solar wind streams and their associated interaction regions were expected to arrive at Jupiter. Thus the study supports the hypothesis advanced previously that the Jovian magnetosphere had undergone a large‐scale compression. The results are contrary to an alternative hypothesis that the Pioneers had traversed a spatial region located inside the magnetosphere possibly associated with plasma outflow. The fourth case, which was observed by Pioneer 11 outbound, appears to have occurred during quiet interplanetary conditions. However, a detailed reinvestigation of magnetic field and plasma data during this interval shows that the spacecraft had reentered the magnetosheath and not a region interior to the magnetosphere. The reentry into the magnetosheath and the subsequent return to the magnetosphere were separated by an interval of 10 hours and would have been expected to occur when the spacecraft was at its highest magnetic latitude. It is concluded, tentatively, that this reentry was the result of a large‐scale north‐south motion intrinsic to the Jovian magnetosphere. The question of whether or not the magnetic field just inside the magnetopause is sufficiently strong to withstand the pressure of the incident solar wind has been reexamined within the context of this present study. The field appears able to hold off the solar wind both at 100 RJ and near 50 RJ. The compressibility of the Jovian magnetosphere is enhanced because the field inside the magnetopause is not the planetary field but is principally caused by currents inside the magnetosphere, presumably the equatorial current sheet. The possible acceleration of energetic trapped radiation when the magnetosphere was compressed has been investigated. Comparison of the increased particle fluxes and the magnetic field shows that gyrobetatron acceleration can be discounted. Based on the measured time difference between the particle enhancement and the arrival of the magnetopause at the spacecraft, an estimate is derived for the average plasma density inside the magnetosphere of 1–10 cm−3. Finally, the characteristic time constants appropriate to an electric circuit model of Jupiter's magnetosphere have been estimated as being in the range between 15 and 50 hours.
Effect of anomalous transport coefficients on the thermal structure of the storm time auroral ionosphere
Authors: Fontheim Ernest G. , Ong R. S. B. , Roble R. G. , Mayr H. G. , Baron M. J. , Hoegy W. H. , Wickwar V. B. , Vondrak R. R. , Ionson J. A.
Published: February 2008

By analyzing an observed storm time auroral electron temperature profile it is shown that anomalous transport effects strongly influence the thermal structure of the disturbed auroral ionosphere. Such anomalous transport effects are a consequence of plasma turbulence, the existence of which has been established by a large number of observations in the auroral ionosphere. The electron and composite ion energy equations are solved with anomalous electron thermal conductivity and parallel electrical resistivity coefficients. The solutions are parameterized with respect to a phenomenological altitude‐dependent anomaly coefficient A and are compared with an observed storm time electron temperature profile above Chatanika. The calculated temperature profile for the classical case (A = 1) disagrees considerably with the measured profile over most of the altitude range up to 450 km. It is shown that an anomaly coefficient with a sharp peak of the order of 104 centered around the F2 peak is consistent with observations.
Electrostatic emissions between electron gyroharmonics in the outer magnetosphere
Authors: Hubbard Richard F. , Birmingham Thomas J.
Published: February 2008

Electrostatic emissions are often observed by spacecraft in the outer magnetosphere at frequencies between low electron gyroharmonics. The emissions include the well‐known odd half‐harmonic emissions as well as weaker emissions which occur near the electron plasma frequency. We have constructed a scheme for classifying the emissions and have shown that a theoretical model which has been previously used to explain ‘3/2’ emissions can be extended to the other classes of emissions. All of the emissions therefore appear to be generated by the same basic mechanism: an unstable electron plasma distribution consisting of cold electrons (<100 eV) and hot loss cone electrons (∼1 keV). Each emission class is associated with a particular range of model parameters; the wide‐band electric field data can thus be used to infer the density and temperature of the cold plasma component. The model predicts that gyroharmonic emissions near the plasma frequency require large cold plasma densities.
Diurnal variation of cosmic ray intensity, 2. Calculation of the diffusion coefficients
Authors: Sari J. W.
Published: February 2008

Diffusion coefficients for cosmic ray propagation in the interplanetary magnetic field are obtained for particles of energies typically observed by ground‐based neutron monitors. The estimates are made in the context of present quasi‐linear and nonlinear theories. Recent observations of the interplanetary magnetic field fluctuations are employed. It is found that the diffusion coefficients derived from the theoretical models generally differ by less than 50%. These differences are less than experimental uncertainties arising from variations in the relevant solar wind observables, where the observables are obtained over scale sizes of the order of a few cosmic ray mean free paths.
Moon‐magnetosphere interaction and estimates of possible lunar core size
Authors: Goldstein Bruce E.
Published: February 2008

Measurements of the magnetic field induced in the moon while the moon is in the earth's geomagnetic tail lobes have been used to estimate lunar interior magnetic permeability and electrical conductivity and, in particular, to determine constraints on the size of a possible lunar core. We show here that the core size estimates are negligibly affected by currents due to a directed plasma flux toward the moon. Specifically, a theory for the compression of an induced lunar dipole by an incoming sub‐Alfvénic flux of cold plasma is presented for the time independent case where the external magnetic field, induced dipole moment, and plasma velocity vector are all aligned. When the incoming flow travels at 83% of the Alfvén speed, the change in the global dipole moment is no more than 5%, while higher order moments increase from zero to 10% of the amplitude of the dipole. Consequently, at very long time periods, effects due to the inertia of an incoming flow of cold plasma can be ignored if care is taken to exclude periods of unusually high plasma density from the data base used for obtaining lunar magnetic permeability and core size estimates. Additionally, we show how a recently derived theory for time dependent fluctuations in a stationary plasma can be modified to include the effects of incoming plasma flux, and we discuss how this theory is related to the approximation that we have used.
Cleft currents determined from magnetic and electric fields
Authors: Walker John K. , Daly P. W. , Pongratz M. B. , Stenbaek‐Nielsen H. C. , Whitteker J. H.
Published: February 2008

Cleft region electric fields and ionospheric observations are used to calculate cleft currents and their magnetic fields, which are compared with ground‐based magnetic observations. Rockets were launched through the postnoon cleft region from Cape Parry, Northwest Territories, Canada, on November 25 and 28, 1975. They carried energetic particle detectors, electrostatic analyzers, an electron spectrometer, and shaped charges with barium liners to determine the electric fields, electron energy flux, and energetic particles (>70 keV). Bottomside and Isis 1 and 2 topside ionograms, all‐sky photographs, and visual observations of aurora are used to determine empirical models of the cleft ionosphere. The conductivities of these ionospheric models, are calculated and are then combined to form representative cross sections of the cleft region during the rocket flights. Currents are calculated from the electric fields and ionospheric models, and then their magnetic fields are determined and compared with the observations. Because most of the east‐west electrojets are in oppositely flowing pairs, their magnetic fields cancel at the earth's surface. Currents flowing in the meridional plane are completed by field‐aligned currents flowing outward above the arcs and inward at discontinuities in the ionosphere or electric field. Some of the field‐aligned currents above the arcs could exceed the critical current of the topside ionosphere and possibly cause instabilities in these regions, which could in turn energize the auroral particles.
An observed relation between magnetic field aligned electric fields and downward electron energy fluxes in the vicinity of auroral forms
Authors: Lyons L. R. , Evans D. S. , Lundin R.
Published: February 2008

Using electron observations in the vicinity of, and over, auroral forms obtained from three rocket flights, we have found that the net downward electron energy flux generally varied as V², where V is the magnetic‐field‐aligned electric potential difference inferred from the peak in the observed energy spectra of precipitating electrons. We show that this relation implies that V is proportional to the net downward number flux of electrons from the magnetosphere to the auroral ionosphere and thus that increases in V and increases in the net downward number flux of electrons are equally responsible for the enhanced electron energy deposition responsible for auroral forms. This also indicates a direct physical connection may exist between V and the net downward electron number flux.
Three‐dimensional, time‐dependent modeling of neutral gas diffusion in a nonuniform, chemically reactive atmosphere
Authors: Bernhardt Paul A.
Published: February 2008

A time‐varying model of neutral gas expansion in a nonuniform environment is developed. The model includes diffusion in a multicomponent atmosphere, chemical reactions between the diffusing gases and the atmosphere, thermal diffusion effects, and transport due to altitude‐dependent winds. The three‐dimensional diffusion equation governing the neutral gas flow is solved numerically using Fourier transform and finite difference techniques. Examples of H2, OH, and CO2 diffusion illustrate the effects of chemical reactions and wind shears on the neutral expansion. The model may be applied to chemical releases which produce ionospheric depletions or luminescent trails.
Dayside merging and cusp geometry
Authors: Crooker N. U.
Published: February 2008

Geometrical considerations are presented to show that dayside magnetic merging when constrained to act only where the fields are antiparallel results in lines of merging that converge at the polar cusps. An important consequence of this geometry is that no accelerated flows are predicted across the dayside magnetopause. Acceleration owing to merging acts in opposition to the magnetosheath flow at the merging point and produces the variably directed, slower‐than‐magnetosheath flows observed in the entry layer. Another consequence of the merging geometry is that much of the time closed field lines constitute the subsolar region of the magnetopause. The manner in which the polar cap convection patterns predicted by the proposed geometry change as the interplanetary field is rotated through 360° provides a unifying description of how the observed single circular vortex and the crescent‐shaped double vortex patterns mutually evolve under the influence of a single operating principle.
The seasonal variation of ozone density in the Martian atmosphere
Authors: Shimazaki Tatsuo , Shimizu Mikio
Published: February 2008

A one‐dimensional, time‐dependent model has been developed to investigate seasonal variations in ozone density in the Martian atmosphere. The model utilizes data for atmospheric temperature, pressure, and water vapor abundance observed by Viking. The results indicate that a high ozone density is not produced near the winter solstice but is in rather late winter, when the amount of water vapor is still small and the solar radiation can penetrate more deeply. A similar condition met in early winter does not necessarily produce a high ozone density because of the predicted higher HO2 density. Seasonal variations in ozone abundance are discussed in terms of the abundances of H2O and H2 and the seasonal variations in their reaction products H2O2 and HOx. The roles of transport, the effects of ozone loss to the surface, and the differences between northern and southern hemisphere ozone distributions are also discussed.
Characteristics of the inverted‐V event
Authors: Lin C. S. , Hoffman R. A.
Published: February 2008

The low energy electron (LEE) experiment, consisting of an array of 19 electrostatic analyzers, was conducted on board the polar orbiting satellite Atmosphere Explorer D (AE‐D). Electron energy spectra and angular distributions obtained simultaneously with high temporal resolutions (∼62 ms) have been used to study structures of inverted‐V events. The principal results found are as follows: (1) Inverted‐V events occur in the auroral as well as the polar cap latitudes. The low‐latitude edge of the occurrence region varies from 62° near midnight to 80° invariant latitude at local noon. (2) The latitudinal width regularly observed is about 0.5° invariant latitude, and the longitudinal width can extend to at least 15° (3) The monoenergetic peak of the energy spectrum is found for pitch angles from 0° to 180° minus the local loss cone. (4) The temperature of precipitating electrons is linearly proportional to the peak energy. (5) The flux is field aligned at the peak energy and trapped above the peak energy. Immediately below the peak energy, the pitch angle distribution is V‐shaped, peaking toward both small and large pitch angles. At energies further below the peak energy the distribution function becomes isotropic. (6) Fast irregular flux fluctuations are frequently observed at energies below the monoenergetic peak. These fluxes produce a secondary monoenergetic peak in the spectrum. The occurrence map indicates that the inverted‐V occurrence region is not limited to the particle trapping boundary. The origin of the inverted‐V precipitating electrons appears to be in the plasma sheet and the neutral sheet. Most of the energy and pitch angle structures can be interpreted as electrons accelerated by an electrostatic field, while the V‐shaped pitch angle distributions suggest that particles were trapped between an electric potential and their mirror points. However, the existence of the monoenergetic peak at pitch angles larger than about 25° cannot be explained simply by an electrostatic field. The structural details are compared with predictions of three existing theories of creating the electrostatic field, and the results are qualitatively consistent with the theory of anomalous resistivity.
Pulsating auroras: Evidence for noncollisional thermalization of precipitating electrons
Authors: Stenbaek‐Nielsen H.C. , Hallinan T.J.
Published: February 2008

Observations with a stereo television system show that many pulsating auroras have vertical extents of less than 2 km. This is significantly less than has been calculated based on collisions between precipitating electrons and the atmosphere. Assuming that the pulsating auroras are caused by precipitating electrons, the data suggest that noncollisional interactions, localized in the lower E region (90–107 km), where most of the pulsations are observed, may play a dominant role in thermalizing the particles. Proximate pulsating forms may be at different altitudes, but invariably, the pulsating auroras appear at lower altitudes than interspersed nonpulsating forms. The altitude is essentially constant during an individual pulsation as well as between succeeding similar pulses in a series, but the average altitude of all pulsations appears to increase toward dawn.
Auroral pulsations from atmospheric waves
Authors: Luhmann J.G.
Published: February 2008

A mechanism for producing quasi‐periodic fluctuations in the intensity of auroral light is proposed in which pressure waves in the neutral atmosphere drive the pulsations. A model is constructed to describe how a horizontal movement of the atmosphere can cause the energy deposition by a steady particle distribution to vary with time if the isodensity surfaces of the local atmosphere become nonplanar. When the distortions are wavelike corrugations and the horizontal motion is steady and also has a component normal to the phase front, periodic variations in auroral brightness can result. The pulse shape and the magnitude of the pulsations depend on both the energy spectrum and the pitch angle distribution of the particles and the amplitude and altitude distribution of the atmospheric waves. It is suggested that the required atmospheric oscillations can be provided by acoustic‐gravity waves which are generated by active arcs near the poleward edge of the auroral oval. This mechanism can also produce aperiodic pulsations by convection of irregular atmospheric density fluctuations, some of which may result from interfering acoustic‐gravity wave trains.
Simulation of auroral double layers
Authors: Hubbard Richard F. , Joyce Glenn
Published: February 2008

The plasma double layer is a possible mechanism for the acceleration of high‐energy auroral electrons. Some of the basic properties of double layers are deduced from particle‐in‐cell computer simulations and are related to parallel electric field structures above the auroral regions. We conclude that a large potential difference applied to a finite length plasma will be concentrated in a shocklike localized region instead of occurring over the entire length of the system. The initial stage of the formation of the double layer is dominated by a large‐potential pulse. Once the double layer is formed, the entire potential φ0 is dropped over a distance LDL ≃6(eφ0/kTe)1/2λD. For the parameters above the auroral region this distance is about 1 km.
Particle ionization rates from total solar eclipse rocket ion composition results in the South Atlantic Geomagnetic Anomaly
Authors: Abdu M.A. , Batista Inez S. , Sobral José H.A.
Published: February 2008

Results available on ion composition and electron densities from the rocket measurements carried out during the total solar eclipse of November 12, 1966, at Cassino, Brazil, are subjected to a detailed numerical analysis using the presently known ion chemistry models for the D and E regions. Solutions of the continuity equations applicable to the individual ionic species are carried out under equilibrium conditions to determine the ion production rates that can best represent the rocket measurements of the ion composition and electron density carried out to represent the full sun conditions. The ion production rates so derived are then inserted in a set of time dependent solutions of the continuity equations to predict the ion density height distribution during the eclipse corresponding to the available rocket measurements. A comparison between the calculated results and the observations during the totality is then made to determine the residual ion production rates in the lower ionosphere, from which the part that cannot be accounted for by solar radiation is attributed to the precipitation of high‐energy particles in the South Atlantic geomagnetic anomaly. The results are discussed in the light of the previous estimates of the ion production rates based on the nighttime occurrence of blanketing type Es in the anomaly region.
The auroral ionosphere: Comparison of a time‐dependent model with composition measurements
Authors: Gerard J.‐C. , Rusch D.W.
Published: February 2008

A time‐dependent model of the auroral ionosphere including the odd nitrogen species, NO, N(²D), and N(4S), is used for comparison with data from a coordinated rocket‐satellite measurement of an auroral event. The chemical scheme and the adopted rate coefficients have been shown to be compatible with daytime mid‐latitude ionospheric chemistry. The electron flux and neutral atmospheric parameters measured on the satellite are used to compute the appropriate ionization and dissociation rates. The calculated NO+, O2+, O+, Ne, and NO densities agree well with the rocket measurements. The calculated N2+ densities are larger than the measured densities by a factor of 3 at most altitudes. The calculations show that the nitric oxide content of the aurora (∼1.2 × 109 NO molecules/cm³ at 105 km) is below the saturation value.
A new inversion method for remote sounding of planetary atmospheres
Authors: Abbas Mian M.
Published: February 2008

A new inversion method for remote sounding of planetary atmospheres is presented which appears to have several significant advantages over the conventional methods. This method is applicable to high‐resolution observations where the spectral lines are fully resolved and is based on matching the calculated slopes of the spectral line profiles with slopes of the observed line shapes. The method is applied to inversion of ozone absorption lines in the earth's atmosphere, and the results are compared with those obtained by a conventional method. The proposed method is seen to provide a significant improvement in the overall accuracy of the retrieved profiles, with higher vertical resolution and higher levels which may be probed.
Theory and analysis of the ‘super whistler’
Authors: Bernhardt P.A.
Published: February 2008

Whistlers observed on the ground with frequency cutoffs greater than one‐half the equatorial electron gyrofrequency along their field‐aligned propagation paths have been termed super whistlers. The rarity of the super whistler is a consequence of the relatively small number of ducts which allow their propagation. The upper cutoff frequency of the super whistler is directly related to the physical structure of the duct. In addition to the path location and equatorial electron density that can be deduced from ordinary whistlers, super whistlers also provide information on density gradients along the magnetic field lines near the equator. Analysis of one set of super whistlers shows that immediately after a magnetic disturbance the equatorial plasma near L=4.5 is in an intermediate state between the collisionless and diffusive equilibrium distributions. The plasma distribution approaches that of diffusive equilibrium after a period of low geomagnetic activity.
Self‐consistent magnetotail theory: General solution for the quiet tail with vanishing field‐aligned currents
Authors: Birn J.
Published: February 2008

The three‐dimensional theory of the quiet magnetotail by Birn et al. (1977a) is revised and generalized. The general static solution is presented for the case of vanishing field‐aligned currents holding for quiet times with small bulk velocities and negligible pressure anisotropy. Since the knowledge of particle entry, transport, and loss and of the interaction of the magnetosphere with the solar wind is too small to determine the solution by appropriate initial and boundary conditions, the free functions of the general solution have been adjusted to the observational material inside the tail. It is shown that the quiet tail data can be fitted by a self‐consistent model without flux loss through the boundary (closed magnetosphere). Restrictions following from self‐consistency provide further (indirect) evidence for plasma sheet thickening towards the flanks. The effect of dipole tilt on the neutral sheet position and tail aberration can also be included in the model. It turns out that the curvature of the field lines in the x, y plane is connected with the occurence of field‐aligned currents.
Effects of geomagnetic disturbances on neutral winds and temperatures in the thermosphere observed over Millstone Hill
Authors: Babcock Richard R. , Evans John V.
Published: February 2008

Ionospheric measurements of electron density, electron and ion temperature, and vertical drift made at the Millstone Hill incoherent scatter radar have been used to derive the neutral exospheric temperature and the thermospheric neutral wind speed along the magnetic meridian. These two parameters were used as input to a dynamic model for the neutral atmosphere over Millstone from which values of the zonal and meridional winds at 300 km were derived. Of 85 days of data available over the 6 year period 1970‐1975, 21 days were geomagnetically active (daily average AE ≥ 300 γ). In analyzing the disturbed days, heating of the ions by the neutral winds, differences in the neutral density from MSIS model predictions, and electric field effects were taken into consideration. All disturbed days show three distinct effects, viz., enhanced nighttime temperatures, strong equatorward nighttime winds, and transition of the zonal winds from eastward to westward much earlier in the nighttime sector than usual. The strong equatorward winds transport high latitude heat away from the auroral zone. They usually are found to maximize after midnight and probably represent an extension to mid‐latitudes of the ‘midnight surge’ seen at auroral latitudes (Bates and Roberts, 1977). In the daytime sector, some disturbed days exhibit poleward winds, which are not as strong as normal, but this does not seem to be a consistent feature.
The transport of resonance radiation in a nonisothermal medium—the effect of a varying Doppler width
Authors: Strickland D.J.
Published: February 2008

A method is presented for treating the transport of resonance radiation under the influence of a varying Doppler width resulting from temperature changes. The method is applied to transport of resonance photons in the earth's thermosphere for media of modest and large optical depths. The drop in temperature below the exosphere can lead to almost a factor of two reduction in the nadir viewing intensity observed, e.g., from satellites. The effect is weaker, however, for zenith viewing intensities, especially in the isothermal region.
Solar and terrestrial noble gases in magnetospheric precipitation
Authors: Lind D.L. , Geiss J. , Stettler W.
Published: February 2008

Metal foil collectors were installed on the external structure of Skylab to entrap precipitating magnetospheric particles. The foils were retrieved and the entrapped helium, neon, and argon were isotopically analyzed in a high‐resolution mass spectrometer. Solar and terrestrial helium and neon and terrestrial argon were detected. As expected, the isotopic composition of neon and argon in the high atmosphere was found to be strongly fractionated. Special techniques were used to estimate the initial particle energy of ³He. The measured ³He flux is consistent with the assumption that precipitating solar ³He is the major source of terrestrial ³He.
Precipitating electrons associated with the diffuse aurora: Evidence for electrons of atmospheric origin in the plasma sheet
Authors: Evans David S. , Moore Thomas E.
Published: February 2008

Observations were made of precipitating electrons associated with the proton, or diffuse, aurora. At energies greater than about 1 keV the electron energy distribution was Maxwellianlike, and pitch angle distribution was isotropic over the precipitation hemisphere but displayed a well‐developed atmospheric loss cone over the upgoing hemisphere. In contrast, the energy distribution of the lower‐energy electrons more nearly approached a power law, while the pitch angle distribution approached isotropy. The symmetry between the downgoing and upgoing low‐energy electron intensities shows that at these energies the downgoing electrons have their origin as backscattered and secondary electrons from an atmosphere. It is proposed that backscattered and secondary electrons, produced as a result of the general precipitation, are pitch angle scattered into stably trapped orbits while in transit along the magnetic field line by the same turbulent wave fields generally invoked to account for the precipitation. The reservoir of trapped electrons thus becomes a combination of some primordial population and the electrons produced from the atmosphere by precipitation from this reservoir. Numerical models of the equilibrium electron energy spectrum expected to exist in this situation were generated by assuming that the primordial distribution was a pure Maxwellian. Excellent agreement was obtained between modeled energy spectra and electron energy spectra observed at low altitude above the diffuse aurora and at high altitude near the L=6 geomagnetic equator.
Scale of electric field along magnetic field in an inverted V event
Authors: Cladis J.B. , Sharp R.D.
Published: February 2008

The characteristics of magnetic‐field‐aligned electrostatic‐potential differences in the structure of an inverted V event were examined by using data on the pitch angle and energy distributions of electrons and ions obtained with the polar‐orbiting S3‐3 satellite. Potential differences were found to occur simultaneously above and below the altitude of the satellite (≃7260 km). Both of these potential differences were determined from an analysis of the electron distributions. The analysis also yielded the magnetic field intensity B1 above the satellite where the potential was zero. This value of B1 indicates that the potential extends above the satellite a distance of ≃1200 km, implying an apparent electric field of ⪍1 mv/m. The total potential difference was found to have an inverted V distribution in latitude, with a peak value of ≃4.3 kV and a latitudinal width of ≃1°. Upward‐flowing H+ and 0+ ions, with pitch angles ⪍10° were observed in the structure. Their energy spectra were peaked at energies near the potential differences below the satellite but were very broad indicating perpendicular heating and an ion source within the potential region. Implications to theory are discussed.
The positron annihilation line at 511 keV in the upper atmosphere
Authors: Albernhe F. , Vedrenne G. , Martin I.M.
Published: February 2008

We present the results of a series of balloon flights made at two latitudes over Aire sur l'Adour, France (4.5 GV), and Sao José dos Campos, Brazil (12.5 GV). We used two types of omnidirectionnal detectors: a 4.5 × 5.1 cm NaI crystal and a 23 cm³ Ge‐Li high‐energy resolution detector. The two were surrounded by a 0.8‐cm thick plastic anticoincidence. These measurements allowed us to determine precisely the characteristics of the 0.511‐MeV line versus altitude and latitude. We have also determined from the analysis of the growth curves at the two latitudes an extraterrestrial component value of (4.3±1.5) × 10−2 photons/cm² s and an upper limit of 4 × 10−3 photons/cm² s for the flux from the galactic center region.