bluuebunny/crossref_metadata_2025 huggingface dataset:
Ground‐based observations of F region aurora associated with the magnetospheric cusp
Authors: Deehr C.S. , Sivjee G.G. , Egeland A. , Henriksen K. , Sandholt P.E. , Smith R. , Sweeney P. , Duncan C. , Gilmer J.
Published: February 2008

A multinational program of optical observations of the dayside aurora was carried out at Longyearbyen and Ny Ålesund, Svalbard, during the winter of 1978–1979. Observations were made of the temporal and spatial variations of the particle precipitation region characterized by a predominance of [O I] 6300‐Å emission previously mapped by satellites and associated with the magnetospheric cusp. This region expands and moves equatorward at a rate proportional to the negative excursion of the H component of the disturbance geomagnetic field measured at a station in the nightside at the latitude of maximum disturbance (approximately 10° farther south than the latitude of the cusp). The intensity ratios of 5577‐Å [O I], 6300‐Å [O I], and 4278‐Å N2+, normally used to determine the energy of the incoming particles, are often not quantitatively useful for this purpose, owing to the predominating enhancement of N2+ emissions by resonantly scattered sunlight; however, comparisons of relative values show that there is a considerable variation in the energy of the incoming electrons during magnetic activity. The derivation of realistic values of energy of auroral primaries must rely on a combination of spectrometer and meridian‐scanning photometer observations. A few examples of such a derivation yield characteristic energies of auroral primaries which vary between 2 keV and <100 eV in the cusp aurora. The higher values are associated with occasional rayed bands, and the lower with the large, structureless emitting region associated with the cusp. Because of the high altitude of this region, resonance scattering is the most important source of N2+ emission, and the process O+(²D) + N2 → N2+ (A²Σ) + O is an important source of N2+ ions. Thus the observed N2+ 4278‐Å emission is greater than would be expected for impact excitation alone, and the ‘midday gap’ in 5577‐Å emission is simply a result of the low characteristic energy of the incoming particle spectrum. The rayed bands occurring occasionally throughout the cusp region are spectroscopically very similar to polar cap arcs and appear simultaneously with increasing nightside activity, an indication that they could not have drifted from the nightside. Spectrometry of auroral and atmospheric emissions reveals enhanced atomic emissions as well as redistribution of vibrational and rotational emissions which should be expected for sunlit aurora produced in the F region by relatively low energy (100 eV) particles precipitating in the cusp region. F region neutral winds display a strong, steady cross‐polar flow in the direction of the magnetic noon‐midnight meridian with vortices near 6 and 18 hours MLT. The wind strength is relatively greater on the dawnside and on the nightside, increasing with magnetic activity.
A magnetohydrodynamic model for corotating interplanetary structures
Authors: Whang Y.C.
Published: February 2008

This model treats interplanetary structures as a two‐time scale problem, a small‐scale incompressible Alfvénic fluctuation superposed on a large‐scale background flow, and uses the complete magnetohydrodynamic equation and the full energy equation to describe the interplanetary structures. The small‐scale fluctuation is governed by a system of linear equations, and the large‐scale background flow by a system of nonlinear partial differential equations in a heliocentric coordinate system corotating with the sun. For steady state corotating structures the model assumes the background flow to be field aligned and proposes to treat the nonlinear system as composed of two subsets: one dealing with the expansion of the solar wind in stream tubes, the other with the interaction between neighboring stream tubes. The interaction process governs the variations in the cross‐sectional area and the direction of each stream tube. This process is required to satisfy the divergence‐free condition of the magnetic field and a dynamical equilibrium of momentum transverse to the streamline direction. The interaction subset has a singularity at the Alfvénic point. The expansion process is required to satisfy the equations of mass and energy conservation, and the equation of motion tangential to the streamline direction. The expansion subset has its singularity at Parker’s critical radius. The two subsets should be solved simultaneously for numerical solutions. We obtain the characteristic equation of the interaction subset, and discuss the method of solutions for three‐dimensional corotating interplanetary structures. The viscous attenuation of the small‐time‐scale incompressible Alfvénic fluctuation is also studied by using the WKB approximation.
Interplanetary pitch angle scattering and coronal transport of solar energetic particles: New information from Helios
Authors: Bieber J.W. , Earl J.A. , Green G. , Kunow H. , Müller‐Mellin R. , Wibberenz G.
Published: February 2008

Solar particle data from the Kiel experiment on Helios 2 at 0.5 AU were compared with the predictions of a model which includes the effect of a postulated temporal profile of injection and which describes the coherent interplanetary transport of energetic particles. During an event on March 28, 1976, good agreement between observations and predictions was obtained for 5 MeV protons and for ∼0.5 MeV electrons. For both species, this correspondence led to a mean free path of 0.7±0.1 AU, independent of rigidity, and a focusing length of 0.86±0.1 AU. The latter value is substantially larger than the value of ∼0.25 AU, which is expected if the large‐scale interplanetary magnetic field has its nominal spiral pattern. The large‐scale dependence of scattering was investigated with the aid of a least squares criterion for agreement between the data and the model. The strongly anisotropic pitch angle distributions that were recorded throughout this nearly coherent event were most accurately described when the rate of scattering was assumed to go through a minimum centered at 90° pitch angle. Coherent interplanetary propagation and close proximity of Helios to the sun made it possible to derive reliable temporal profiles of particle injection at the coronal footpoint of the interplanetary field line through the spacecraft. Within the uncertainties, the injection of electrons had the same temporal dependence as the flux of 7‐GHz radio noise had during the rising phase of the ‘great burst’ that accompanied the flare. After the radio maximum, the radio intensity decayed more rapidly than the electron injection. The injection of protons took place later and on a slower time scale than that of electrons.
A simple analysis of spacecraft charging involving blocked photoelectron currents
Authors: Besse Arthur L. , Rubin Allen G.
Published: February 2008

An analytical model of the satellite photosheath is obtained in the presence of spacecraft charging. The combined potential is described by a combination of monopole and dipole contributions. This simple monopole‐dipole model predicts the saddle point behavior found in computer simulations. Applications to daylight charging and sheath effects on particle measurements are described.
Observations of field‐aligned currents in association with strong convection electric fields at subauroral latitudes
Authors: Rich Frederick J. , Burke William J. , Kelley Michael C. , Smiddy Michael
Published: February 2008

An examination of regions of strong convection electric fields (E >50 mV/m) equatorward of the auroral zone has been made on the basis of data from the S3‐2 satellite in 1976. Only seven examples were found, which indicates a low occurrence rate even after accounting for the gaps in data coverage. While this phenomenon did not correlate with the magnitude of Kp or AE, it is always substorm related. The electric field and magnetic field data indicate that the electric field is generated by plasma sheet ions being driven closer to the earth than plasma sheet electrons by magnetospheric convection in the evening sector. The existence of a net field‐aligned current into the ionosphere through the region indicates the presence of latitudinal conductivity gradient. The depletions of ionospheric particles near regions of strong field‐aligned currents (≥1 µA/m²) into the ionosphere indicate that upward moving ionospheric electrons are carrying the current. The evacuation of the E region approximately 1 min after the onset of the electric field as suggested by Banks and Yasuhara (1978) is not supported by the data, but it may occur on a time scale of ∼30 min. If both the F region and E region are depleted in the strong convection region, the result may be a naturally occurring ‘hole’ in the ionosphere.
F region airglow: Are ground‐based observations consistent with recent satellite results?
Authors: Cogger L.L. , Walker J.C.G. , Meriwether J.W. , Burnside R.G.
Published: February 2008

Because previous interpretations of ground‐based airglow measurements have used photochemical rate coefficients quite different from those deduced from the Atmosphere Explorer series of measurements, the question arises of whether the AE rates are as successful at explaining ground‐based data as they are at explaining satellite data. In this paper we use the AE photochemistry to interpret simultaneous observations made at the Arecibo Observatory of the O I (6300 Å) and N I (5200 Å) airglow surface brightness and electron density and temperature profiles measured by incoherent scatter radar. We find that theory and experiment are in agreement for the 5200‐Å emission. However, we are unable to obtain complete agreement for the 6300‐Å nightglow and suggest that the source of the discrepancy lies with one of the parameters used to calculate the production rate of O(¹D). The data show clear evidence of an asymmetrical behavior of the ionosphere between times when the F layer is descending and times when it is ascending. This asymmetry probably reflects the effects of transport on molecular ion densities in the bottom side of the F region. Our results support a low rate for the quenching reaction between N(²D) and O. Calculations indicate that measurements of airglow surface brightnesses are relatively insensitive to the abundance of atomic nitrogen in the atmosphere.
Solar cycle‐dependent north‐south field configurations observed in solar wind interaction regions
Authors: Rosenberg Ronald L. , Coleman Paul J.
Published: February 2008

This study investigates the structure of the normal or ‘north‐south’ field component BN in the ‘interaction regions’ of the interplanetary magnetic field (IMF). An interaction region corresponds to the region of interaction between a solar wind stream of lower velocity and the higher‐velocity stream overtaking it. A computer program was developed to locate interaction regions by testing for the enhancement in field magnitude known to be characteristic of these regions. The results of our statistical analysis of Pioneer 10 and 11 IMF data taken in 1972–1973 indicated a systematic configuration of enhanced southward BN which reversed to enhanced northward BN in the rear portion of the interaction regions. Our superposed epoch analyses of BN in the interaction regions in data taken with other spacecraft from 1964 through 1973 indicated a similar systematic configuration of BN. In pre‐1970 data the leading portion of the interaction regions had enhanced northward BN, on the average. The BN configuration reversed phase near the beginning of 1970 as did the dipolar‐related dominant polarity effect found by Rosenberg and Coleman in 1969. The evidence indicates that the BN configuration in corotating interaction regions is related to the interplanetary magnetic field being parallel to the global heliospheric current sheet near it and as far as a quarter (1.5 days at 1 AU) of a typical sector length away from it. Our results indicate that two thirds of interaction regions occur in the first quarter of a sector. The parallelism of the field to the current sheet implies that the field is southward following a positive to negative sector crossing, or vice versa, both above and below the solar equatorial plane for the phase of the solar‐dipolar cycle beginning in 1970. When the azimuthal component BT reverses or tends to reverse direction, due to azimuthal stress, preceding the plasma interface of the interaction region, BN reverses from a southward to a northward direction, so that the field remains parallel to the global current sheet. The components BT and BN are also enhanced by compression in the interaction region. Preliminary interpretations in terms of the warped current sheet are discussed for the dependence of BN on heliographic latitude found by Coleman and Rosenberg in 1971 and the effect on atmospheric vorticity of a sector crossing passing the earth found by Wilcox and Svalgaard and their colleagues in 1974. The characteristics of the interaction region found in this study define in detail the structure of the classic event in the solar wind that is closely related to interplanetary‐magnetospheric interactions. In turn, knowledge of this structure may allow investigators to finally choose between the many competing interaction models.
Direction‐of‐arrival measurements of auroral kilometric radiation and associated ELF data from Isis 1
Authors: James H.G.
Published: February 2008

The spin‐modulated noise from the dipole connected to the sounder receiver on the Isis 1 spacecraft has been used along with the computation of ray paths to deduce the direction of propagation of auroral kilometric radiation (AKR). On the basis of three passes with fixed‐frequency measurements at 0.48 MHz it is concluded that in the generation region at about 3500‐km altitude the wave vectors make an angle between about 60° and 90° with respect to the local magnetic field. That is, they are predominantly downcoming. Also, in using the sounder records the plasma frequency fN is found to be very much less than the gyrofrequency fH in the generation region (fN ≲50 kHz, fH ≃450 kHz). When these Isis observations are combined with previous evidence on AKR, from which it was concluded that it propagates predominantly in the right‐hand polarized X mode, serious disagreement with a number of theories, particularly those that predict the left‐hand‐polarized O mode, results. However, two theories of a quite different nature, direct cyclotron radiation and a three‐wave interaction involving an ELF/VLF wave, give some prospect of accommodating all the observations.
Detection of a corona of fast oxygen atoms during solar maximum
Authors: Yee J.H. , Meriwether J.W. , Hays P. B.
Published: February 2008

A series of twilight interferometric observations of the near infrared O+(²P) doublets at 7320 and 7330 Å between April 1979 and October 1979 detected excessive amounts of emission at shadow heights above 550 km. The scale height deduced from the vertical brightness profile determined from data taken on September 26, 1979, when the F10.7was 231, showed a marked increase above 550 km. The equivalent temperature was estimated to be 4000°K or higher. Observations of the emission line shape with the high‐resolution capability of the interferometer substantiated these results by a very apparent progressive broadening of the emission profile between times of low and high shadow height. We conclude from these results that there exists an atomic oxygen corona overlying the thermosphere during the solar maximum period.
Solar corona electron density distribution
Authors: Esposito Pasquale , Edenhofer Peter , Lueneburg Ernst
Published: February 2008

Three and one‐half months of single‐frequency (f≐2.2 × 109 Hz) time delay data (earth‐to‐spacecraft and return signal travel time) were acquired from the Helios 2 spacecraft around the time of its solar occultation (May 16, 1976). Following the determination of the spacecraft trajectory the excess time delay due to the integrated effect of free electrons along the signal's ray path could be separated and modeled. An average solar corona, equatorial, electron density profile, during solar minimum, was deduced from time delay measurements acquired within 5–60 solar radii (Rs) of the sun. As a point of reference, at 10 Rs from the sun we find an average electron density of 4500 el cm−3. However, there appears to be an asymmetry in the electron density as the ray path moved from the west (preoccultation) to east (post‐occultation) solar limb. This may be related to the fact that during entry into occultation the heliographic latitude of the ray path (at closest approach to the sun) was about 6°, whereas during exit it became −7°. The Helios electron density model is compared with similar models deduced from a variety of different experimental techniques. Within 5–20 Rs of the sun the models separate according to solar minimum or maximum conditions; however, anomalies are evident.
Magnetic field orientation and suprathermal ion streams in the Earth's foreshock
Authors: Greenstadt E.W. , Russell C.T. , Hoppe M.
Published: February 2008

The different populations of backstreaming ions found earlier with ISEE instruments outside the earth's bow shock were organized by the orientation of the IMF to the shock in the plane containing BSW and the solar ecliptic X axis (the B‐X plane). The associations of ion classes with field geometry and shock structure are shown by means of computer‐drawn three‐dimensional sketches. Particles of several keV, or about 10 times the solar wind streaming energy, left the shock as ‘reflected’ beams in a guiding center direction determined by BSW and the interplanetary drift velocity. Their area of departure was the part of the shock where the transition from quasi‐perpendicular to quasi‐parallel structure began. Slightly farther toward the quasi‐parallel portion of the shock, particles of lower energy escaped upstream, widening the ion spectra, exciting small magnetic waves, and forming a class of ‘intermediate’ ions. Finally, in direct connection with the quasi‐parallel structure, ions leaving the shock formed the ‘diffuse’ group, containing a wide distribution of energies including high values well beyond any recorded in the other two classes. Sources of the various ion groups are postulated.
High‐voltage differential charging of geostationary spacecraft
Authors: Prokopenko S.M.L. , Laframboise J.G.
Published: February 2008

A numerical prediction is presented for upper bounds on negative floating potentials of shaded spacecraft surfaces, using a local‐current‐balance formulation. Results include the following: (1) By influencing the velocity‐space cutoff boundaries for incident ion fluxes, the spacecraft geometry and sheath potential profile (particularly shaded‐sunlit asymmetries in the latter) have large influences on shaded‐surface potentials, which may exceed ‐ 20 kV in certain circumstances. (2) For electrically isolated surfaces in shaded cavities, negative floating potentials may exceed those on convex surfaces. (3) In some conditions, two distinct floating potentials are predicted. This implies the possibility of ‘bifurcation phenomena’ in which adjacent isolated surfaces made of the same material may follow different charging histories. It also implies that large and relatively sudden changes in surface potentials can be caused by gradual changes in either the external environment or beam emission currents.
Observation of the plasma boundary layer at lunar distances: Direct injection of plasma into the plasma sheet
Authors: Sanders G.D. , Maher L.J. , Freeman J.W.
Published: February 2008

Simultaneous data returned by the Apollo Suprathermal Ion Detector Experiments are combined to yield information on the properties of plasmas in the magnetosheath at lunar orbit. An interface between the magnetosheath and the plasma sheet has been identified. This layer is characterized by a systematic change in the magnetosheath flow properties in the vicinity of the magnetopause. The flow is observed to turn inward toward the plasma sheet and become thermalized, indicating that the magnetosheath particles can gain access to the plasma sheet at lunar distances. This layer, which we term the plasma sheet boundary layer (PSBL), is believed to be the extension of the magnetospheric boundary layer (also called plasma boundary layer) observed on the dayside. Rough order of magnitude calculation shows that the flux crossing the PSBL is capable of supplying the roughly 5 × 1025 particles/s necessary for maintenance of the plasma sheet. Measurements of the thickness of the plasma sheet boundary layer, the magnetic field, and the plasma flow velocity allow an estimate of the potential drop across this layer of 4.2 kV. Since this is much less than the potential drop of opposite polarity across the balance of the tail, we conclude that the magnetosphere is open.
An empirical model of quiet‐day ionospheric electric fields at middle and low latitudes
Authors: Richmond A. D. , Blanc M. , Emery B. A. , Wand R. H. , Fejer B. G. , Woodman R. F. , Ganguly S. , Amayenc P. , Behnke R. A. , Calderon C. , Evans J. V.
Published: February 2008

Seasonally averaged quiet‐day F region ionospheric E × B drift observations from the Millstone Hill, St. Santin, Arecibo, and Jicamarca incoherent scatter radars are used to produce a model of the middle and low‐latitude electric field for solar minimum conditions. A function similar to an electrostatic potential is fitted to the data to provide model values continuous in latitude, longitude, time of day, and day of the year. This model is intended to serve as a reference standard for applications requiring global knowledge of the mean electric field or requiring information at some location removed from the observing radars. This article contains supplementary material.
Solar flux variability in the Schumann‐Runge continuum as a function of solar cycle 21
Authors: Torr M. R. , Torr D. G. , Hinteregger H. E.
Published: February 2008

Measurements of the solar flux in the Schumann‐Runge continuum (1350–1750 Å) by the Atmosphere Explorer satellites reveal a strong dependence on solar activity. Solar intensities over the rising phase of cycle 21, increase by more than a factor of two at the shorter wavelengths (1350 Å), with a smaller change (∽10%) at 1750 Å. A significant 27 day variability is found to exist superimposed on the solar cycle variation. Because radiation in this portion of the spectrum is important to the lower thermosphere in the photodissociation of O2 and the production of O(¹D), we use the unattenuated Schumann‐Runge continuum dissociation frequency as a parameter to illustrate the magnitude and temporal characteristics of this variation. The values of this parameter, J∞(O2)SR, range from 1.5 × 10−6 s−1 for April 23, 1974, to 2.8 × 10−6 s−1 for February 19, 1979. In studies of oxygen in the lower thermosphere, it is therefore necessary to use solar spectral intensities representative of the actual conditions for which the calculations are made. Both the J∞(O2)SR parameter and the solar flux at various wavelengths over the 1350 to 1750 Å range can be expressed in terms of the F10.7 index to a reasonable approximation.
A seasonal change in the effect of field‐aligned currents at synchronous orbit
Authors: McPherron Robert L. , Barfield J. N.
Published: February 2008

The expected signature of substorm field‐aligned currents is described for synchronous satellites near 10° magnetic latitudes. The main effect is a positive D perturbation premidnight and a negative D perturbation postmidnight. This behavior is illustrated for two substorms near the fall equinox. A second pair of substorms taken from winter solstice do not show the expected behavior. The absence of this effect in winter observations is verified statistically by superposed epoch analysis. A simple explanation of this behavior based on the geometry of the plane of magnetic symmetry is presented. During disturbed times at winter solstice a synchronous satellite nominally at 10° magnetic latitude is effectively at the magnetic equator. This distortion of the magnetic equator must be considered in future models of the disturbed magnetospheric magnetic field.
Thermal energy transport in the Venus ionosphere: Classical and saturated electron temperature profiles
Authors: Merritt David , Thompson Kevin
Published: February 2008

The classical equation for electron thermal conductivity, which gives a heat flux proportional to the temperature gradient, breaks down in regions of the Venus ionosphere where the temperature scale height becomes comparable to or less than the electron mean free path. Assuming that the actual flux is approximately given by the harmonic mean of the classical and the free molecular flow heat flux, we calculate model temperature profiles based on the Pioneer Venus data. We find that the high dayside and nightside temperatures can be maintained, in the absence of a magnetic field, with somewhat lower heat inputs than those suggested by previous investigators who also neglected the magnetic field. The required heat influx on the nightside is only about one fourth that on the dayside, in sharp contrast to the results of earlier calculations.
A new determination of the ultraviolet heating efficiency of the thermosphere
Authors: Torr Marsha R. , Richards P. G. , Torr D. G.
Published: February 2008

Experimental measurements of relevant parameters over the last 5 years have brought us to the point where an accurate reassessment of the thermospheric heating efficiency for solar ultraviolet photons is possible. We report here the outcome of a steady state calculation applicable to mid‐latitudes. The results are found to be significantly different from an earlier determination, both in magnitude and in variation with altitude. The heating efficiency is approximately 50% near the peak of the extreme ultraviolet energy deposition and falls to approximately 10% near 400 km. The implications for our understanding of the thermospheric energy budget are significant. We quantify the various energy channels by which solar energy is transferred to the neutral atmosphere and compare summer/winter and solar maximum/solar minimum conditions.
Pioneer Venus plasma wave observations: The solar wind‐Venus interaction
Authors: Scarf F. L. , Taylor W. W. L. , Russell C. T. , Elphic R. C.
Published: February 2008

We briefly describe the Pioneer Venus plasma wave instrument and continue with a discussion of wave observations throughout the typical. near‐noon and near‐midnight orbits. This is followed by a detailed comparison of the bow shock turbulence characteristics at earth and at Venus. We then turn to analyze the wave‐particle interactions detected near the dayside ionopause, and we show that the whistler mode Landau damping develops when the B field direction changes so that the Whistler becomes oblique.
Measurements of thermal structure and thermal contrasts in the atmosphere of Venus and related dynamical observations: Results From the four Pioneer Venus Probes
Authors: Seiff A. , Kirk Donn B. , Young Richard E. , Blanchard Robert C. , Findlay John T. , Kelly G. M. , Sommer S. C.
Published: February 2008

Thermal structure of the atmosphere of Venus, and differences in structure with latitude (up to 60°) and clock hour (from midnight to 8 A.M.) have been measured in situ from an altitude of 126 km to the surface by instruments on the four Pioneer Venus entry probes. Several indications from the preliminary analyses are confirmed by the current analysis: Thermal contrasts below 45 km are a few K, with the mid‐latitudes warmer than both equatorial and the high latitudes. Sizeable temperature and pressure differences with latitude develop in the clouds (25 K and 20 mbar at the 200 mbar level). At 30° latitude, diurnal differences were small throughout the lower atmosphere from midnight to 7 A.M. A major stable layer 25 km deep exists just below the clouds. Waves of global extent were observed within this layer. A locally stable layer is indicated in the deep atmosphere, between 10 and 20 km, at latitudes up to 30°. In the middle cloud and immediately below the deep stable layer, the atmosphere is approximately neutrally stable, and there is evidence for convective overturning below the stable layer. Just above the clouds, the lapse rate becomes stable, and a ‘stratosphere’ begins which extends upwards to 110 km, becoming isothermal above 85 km. The stratospheric temperature profiles were essentially the same in three widely separated soundings. Upward of 110 km, there is evidence of large amplitude temperature oscillations with altitude, believed to signify the presence of large amplitude waves, perhaps thermal tides. By comparing data of several experiments, it is found that the large diurnal variations in the upper atmosphere begin at an altitude ∼115 km. Agreement of structure data from other Pioneer Venus experiments with the present results is generally excellent. Our measurements of the winds derived from Doppler data agree well with DLBI results and indicate a retrograde zonal velocity of 113 m/s at 63 km altitude and 30° latitude. The zonal winds predicted at cloud levels from pressure differences between 60° latitude and the mid‐latitude probes by assumption of cyclostrophic balance are in first order agreement with the observed winds. At latitudes below ∼30°, however, cyclostrophic balance of the zonal winds is not the dominant process. At altitudes from 60 to 105 km, the measured pressure differences and the assumption of cyclostrophic balance indicate zonal wind velocities peaking at 155 m/s at 68 km, remaining above 120 m/s up to 95 km, then decreasing rapidly.
Venus upper atmosphere structure
Authors: Keating G. M. , Nicholson J. Y. , Lake L. R.
Published: February 2008

From analysis of the orbiter atmospheric drag (OAD) data obtained from the orbital decay of the Pioneer Venus orbiter from December 9, 1978, to August 7, 1979, atmospheric densities have been determined and tabulated near 16°N latitude between 140 and 190 km for all times of day. Maximum daytime densities on Venus are approximately 8 × 10−13 g cm−3 at an altitude of 150 km (dropping by a factor of 24 at night) and 7 × 10−14 g cm−3 at 170 km (dropping by a factor of 82 at night). Comparative atmospheric densities on earth at 150 km are a factor of 3.5 higher during the day with only a 1% diurnal variation. An atmospheric composition, temperature, and density model based on OAD vertical structure measurements is presented. The inferred exospheric temperatures and atomic oxygen concentrations are surprisingly insensitive to model assumptions. This model indicates that atomic oxygen is the major species in the Venus atmosphere above 145 km at night and above 160 km during the day with mixing ratios near 140 km in excess of 0.1. Atomic oxygen concentrations determined from drag measurements near 170 km range from 1 × 109 cm−3 during the day to 3 × 107 cm−3 at night. Exospheric temperatures inferred from OAD measurements are found to vary from 100°K at night to approximately 300°K during the day. The very low exospheric temperatures discovered at night, lower than temperatures near 100 km, are inconsistent with the concept of a planetary thermosphere, in which temperature increases with altitude. We have adopted the term ‘cryosphere’ to define this region of cooling where temperature decreases with altitude. The phase of the diurnal temperature variation is consistent with theoretical models of an atmosphere with mean motion to the west more rapid than planetary rotation superimposed on solar‐antisolar motion. The very low nighttime temperatures and our observation of an absence of the predicted nighttime atomic oxygen bulge suggest substantial downward eddy transport of heat as well as of atomic oxygen. Finally, the OAD data indicate that the neutral upper atmosphere of Venus is unexpectedly insensitive to both solar extreme ultraviolet variations and variations in the solar wind. Thus, the Venus upper atmosphere may not be controlled by the major heating mechanisms generally assumed for planetary thermospheres.
The microphysics of the clouds of Venus: Results of the Pioneer Venus Particle Size Spectrometer Experiment
Authors: Knollenberg R. G. , Hunten D. M.
Published: February 2008

The results of the particle size spectrometer experiment on the Pioneer Venus sounder probe are presented. The vertical cloud structure is found to consist of three primary cloud regions of approximately 20 km total thickness suspended within an ubiquitous aerosol haze which extends more than 10 km above and below it. The three cloud regions are separated by sharp transition regions where both particle chemistry and microphysics exhibit change. The size distributions are multimodal in all cloud regions. Three size modes are observed in the middle and lower cloud region which are composed of aerosol, H2SO4droplets, and crystals. The crystals likely could be either sulphates or chlorides. We provide interpretations of the sources, growth characteristics, and fate of the particle species through a partitioning analysis of the LCPS size distribution data.
Analyses of atomic oxygen, the Green Line, and Herzberg Bands in the lower thermosphere
Authors: Thomas R. J.
Published: February 2008

Measured altitude profiles of atomic oxygen, 557.7‐nm emissions, and Herzberg I emission between 80 and 120 km were used to test the proposed mechanisms leading to these emissions. As generally assumed, the O2 (A³Σu+) that emits the Herzberg I bands is excited by three‐body recombination of atomic oxygen. The quenching of this state is by N2 or O2; for N2 the lifetime times the rate is 2.75 × 10−13cm³; other quenchers are insignificant. We find the atomic oxygen O(¹S) emitting the green line is excited by the Barth mechanism.
The effect of temperature on thermospheric molecular oxygen absorption in the Schumann‐Runge Continuum
Authors: Lean J. L. , Blake A. J.
Published: February 2008

Previous determinations of molecular oxygen number densities in the lower thermosphere by ultraviolet extinction in the Schumann‐Runge continuum have used absorption cross sections measured at room temperature. Recent laboratory measurements of the temperature dependence of the molecular oxygen absorption cross section in this wavelength region allow a reassessment of the previous atmospheric data. The new cross sections have been used to reanalyze data from a spectrometer flown on an Aerobee rocket from Woomera, Australia. The densities obtained are smaller by as much as 20% than those obtained from the previous analysis, in better agreement with the results of broad band ion chambers flown at the same time that were relatively insensitive to temperature.
Electric fields and plasma waves resulting from a Barium Injection Experiment
Authors: Koons H. C. , Pongratz Morris B.
Published: February 2008

A shaped‐charge, barium plasma injection experiment was performed in June 1976 from the DOE Kauai test facility. The rocket payload contained seven shaped charges, a sevenfold larger package than the typical payloads employed in previous barium plasma injections conducted in this program. An attitude control system oriented the injection perpendicular to the local geomagnetic field. The injection altitude was 450 km. The second‐stage motor section contained a payload to measure dc and ac electric fields using a 5‐m‐long, tip‐to‐tip dual spherical probe antenna. The ac electric field frequency response was from 100 Hz to 12 kHz. The dc channel responds up to 160 Hz. Shortly after the detonation, the dc electric field was parallel to the geomagnetic field. A few seconds later the measured electric field was the v × B field induced by the motion of the antenna through the plasma. A variety of plasma waves were detected for approximately 20 s following the injection. The initial impulse in the dc channel was measured to be 115 mV/m. The injection generated barium cyclotron waves at 3.4, 7.2, and 10.6 Hz. The barium gyrofrequency at the altitude of the injection was 3.3 Hz. The initial amplitude at 3.4 Hz was 25 mV/m peak‐to‐peak. These waves were evident in the data for approximately 6 s. Between 3 and 8 s after the explosion, band‐limited noise was intermittently present between 1.6 and 3.2 kHz. These emissions were lower‐hybrid‐resonance emissions in a barium plasma. The instability responsible for these emissions is tentatively identified as the modified two‐stream instability. Wide‐band noise with a decreasing intensity as a function of frequency in the band from 100 Hz to 5 kHz was present from 4 to 30 s following the explosion. The intensity maximized when the antenna was most nearly parallel to the geomagnetic field. This suggests that the waves were ion acoustic waves. Highly structured emissions occurred between 500 Hz and 2.5 kHz for several seconds beginning 17 s after the explosion. The wave mode responsible for these structure emissions has not yet been identified.
Meteoric ion production near Jupiter
Authors: Grebowsky J. M.
Published: February 2008

The effects of meteoroids on the Jovian ionosphere are explored through model calculations of the Fe+ production rates associated with these particles. Dohnanyi's meteoroid mass distribution law appropriate to the near‐earth environment is extrapolated to Jupiter. For a possible meteoroid spatial density at Jupiter 10 times that at the earth, gravitational focusing could produce a cumulative meteoroid flux into the Jovian atmosphere nearly 3 orders of magnitude greater than that in the earth's atmosphere. The spatial density at Jupiter may also be down to an order of magnitude smaller. For this maximum meteoroid flux, assuming chondritic characteristics and using an ionization probability for Fe impacting H2 estimated as 0.3 times that for measured interactions with air, the ablation and impact ionization rates for Fe and Fe+ were computed. The Fe+ impact ionization rate within the Jovian atmosphere peaks above the mesopause with a magnitude ∼0.5 cm−3 s−1 and is much less than the ambient ionosphere photoionization rates near the late afternoon Pioneer 10 ionosphere occultation. Charge exchange of the ablated neutral Fe atoms with ambient ions can result in an Fe+ production rate of ∼10 cm−3 s−1 which is comparable to the ambient rates. Photoionization of metallic atoms is ignorable compared to charge exchange as a source of Fe+. Ignoring transport, steady state Fe+ density maxima of ∼104 or 106 cm−3 can be maintained when Fe+ loss is through radiative association or radiative recombination, respectively. Even if one uses an estimated lower limit to the incident meteoroid flux based on a meteoroid spatial density which does not vary with distance from the sun, the corresponding Fe+ peak densities are ∼10³ and 5 × 105 cm−3, respectively. Hence meteoric ion densities may be significant in the lower ionosphere of Jupiter.
Low energy ion pitch angle distributions observed on the dayside at geosynchronous altitudes
Authors: Comfort R. H. , Horwitz J. L.
Published: February 2008

Pitch angle distributions of low energy (20–400 eV) ions observed at geosynchronous altitudes are found to have field‐aligned components almost 90% of the time on the average from dawn to midafternoon, thereafter decreasing to just less than 50%. Pancake pitch angle component occurrence frequencies vary oppositely, increasing to a peak of 40% in late afternoon. Although the occurrence frequencies for field‐aligned components display no systematic variation with energy, pancake components are observed more often in the lower energy particles. Similarly, magnetic activity is found to have little effect on the occurrence frequencies of field‐aligned components, whereas pancake components occur more frequently under magnetically quiet conditions.
Spectral characteristics of medium‐scale equatorial F region irregularities
Authors: Livingston R. C. , Rino C. L. , McClure J. P. , Hanson W. B.
Published: February 2008

The spectral characteristics of equatorial F region irregularities with scale sizes from a few kilometers to several hundred kilometers have been measured using data from the Atmospheric Explorer‐E satellite. The spectra admit the expected power law characterization with a mean spectral index near 2. There is, however, a systematic decrease in the spectral index with increasing perturbation strength which has not previously been reported. Nearly simultaneous phase scintillation data from the Wideband satellite were also spectrally analyzed for comparison with the AE‐E data. The same systematic decrease of the spectral index with increasing perturbation strength was found in the measured phase spectral indices. A completely unambiguous comparison between the in situ and scintillation deduced turbulence levels cannot be made. However, for realistic propagation model parameters, the spectral characterization of turbulence based on the phase scintillation data is generally consistent with that observed in situ.
Hydromagnetic wave observations in the vicinity of a magnetospheric plasma density gradient
Authors: Lanzerotti L. J. , Wolfe A.
Published: February 2008

Data from the Bell Laboratories magnetometer stations in the Northern hemisphere are analyzed during a reported passage of the ISEE 1 and 2 spacecraft through the same local time sector as the ground‐based stations. The ground‐based data indicate that dominant hydromagnetic wave frequencies became higher with decreasing radial distance (decreasing latitude on the earth's surface), as was also indicated by the data recorded along the satellite track. Further, a wave resonance identified in the ground‐based data occurred at a sharp density gradient in the magnetosphere plasma as reported from ISEE particle experiments. The theoretical conditions for existence of this resonance are consistent with the satellite measurements of significant fluxes of Z >1 ions as contributors to the magnetosphere plasma population at this time.
Comparison of two modeling methods for three‐dimensional current systems
Authors: Akasofu S.‐I. , Kamide Y. , Kisabeth J.
Published: February 2008

Three‐dimensional current systems in high latitudes are obtained by two independent methods on the basis of magnetic field data obtained from the IMS Alaska meridian chain of observatories. In spite of the totally different principles used, the two methods result in similar large‐scale distributions of the ionospheric currents and field‐aligned currents, and the estimated values are in reasonable agreement with those observed more directly by radar and satellite techniques. Thus ground‐based magnetic field records, together with an advanced computer code for separating the contribution from ionospheric and field‐aligned currents, are proven to be a powerful tool in studying the substorm current system, providing a complete set of information on the global distribution of ionospheric currents, electric fields and of field‐aligned currents as well as the convection pattern. There are, however, some differences which are mostly intrinsic to each method. These are discussed in detail in terms of advantages and disadvantages of each method.
Substorm‐related plasma sheet motions as determined from differential timing of plasma changes at the Isee satellites
Authors: Forbes T. G. , Hones E. W. , Bame S. J. , Asbridge J. R. , Paschmann G. , Sckopke N. , Russell C. T.
Published: February 2008

Comparisons of the arrivals and disappearances of plasma sheet electrons observed by the Los Alamos Scientific Laboratory/Max‐Planck‐Institut analyzers on board the Isee 1 and 2 satellites during the course of a magnetic substorm often show relative time delays between the two satellites on the order of a few seconds. Time delay measurements have been used to determine the motion of the northern plasma sheet boundary during substorms for the period February 5 to May 25, 1978. Near substorm onset the average velocity of the surface of the plasma sheet is 23±18 km/s toward the center of the sheet. And for the same times the average component of the plasma flow based upon moment integrations of the proton distribution is 20±8 km/s in the same direction. During substorm recovery the plasma sheet reappears, its surface moving outward from the center with an average velocity of 133±31 km/s as determined by timing its arrival at the two satellites. However, the corresponding proton flow speed is only 3±7 km/s. At the time of the recovery precursory appearances of the plasma sheet often occur. The time delays associated with these precursors imply that the surface of the plasma sheet moves both upwards and downwards with speeds ∼100 km/s. Both the appearance of the precursors and the large difference between the velocity of the plasma sheet surface and the velocity of the flow normal to the surface can be explained by the occurrence of surface waves at the boundary of the plasma sheet which propagate past the satellites at a velocity of 200–600 km/s in the direction of the convective flow. Estimating the tilt of the wave surfaces as ±15°, we deduce an average wavelength of 1–5 RE and a wave amplitude of ∼600–1400 km. Taking into consideration the wave structure at the time of a recovery, we compute a velocity of ∼30 km/s for the net outward motion of the plasma sheet boundaries during substorm recovery.
Inference of the ring current ion composition by means of charge exchange decay
Authors: Smith Paul H. , Bewtra N. K. , Hoffman R. A.
Published: February 2008

The analysis of data from the Explorer 45 (S³‐A) electrostatic analyzer in the energy range 5–30 keV has provided some new results on the ring current ion composition. It has been well established that the storm time ring current has a decay time of several days, during which the particle fluxes decrease nearly monotonically. In the past, ring current studies have assumed or stated that hydrogen was the dominant ion in the earth's ring current. By analyzing the measured ion fluxes during the several day storm recovery period and assuming that beside hydrogen other ions were present and that the decays were exponential in nature, we were able to establish three separate lifetimes for the ions. These fitted decay lifetimes are in excellent agreement with the expected charge exchange decay lifetimes for H+, O+, and He+ in the energy and L value range of the data. This inference technique thus establishes the presence of measurable and appreciable quantities of oxygen and helium ions as well as protons in the storm time ring current; we also find indications that He++ may also be present under these same conditions. The existence of additional ions is not ruled out by this technique.
Semiempirical modeling of thermospheric magnetic storms
Authors: Hedin A. E. , Spencer N. W. , Mayr H. G. , Porter H. S.
Published: February 2008

An improved formulation for empirical modeling of magnetic storm effects in neutral thermospheric composition and temperature is utilized in a study of two disturbed periods. The formulation, which incorporates the prior history of the heat input rather than a single phase delay, is based on a Fourier integral representation of an existing theoretical model. This results in an improved representation of the detailed time variations and a better carry‐over of model parameters from one storm to the other and provides a basis for theoretical interpretation.
Beams of protons and alpha particles ≳30 keV/charge from the Earth's bow shock
Authors: Scholer M. , Ipavich F. M. , Gloeckler G.
Published: February 2008

We have investigated two beamlike particle events (30 keV/charge to 160 keV/charge) upstream of the earth's bow shock with the Max‐Planck‐Institut/University of Maryland ultralow energy and charge analyzer on Isee 1. These beams consist of protons as well as of alpha particles, and the spectra are generally steep and are decreasing with increasing energy. During one event the spectra of both protons and alpha particles have a maximum at ∼65 keV/charge. During these events the interplanetary magnetic field through the satellite position was almost tangent to the bow shock, and application of the theory of acceleration predicts acceleration of a solar wind particle up to 60 keV/nucleon in a single reflection. The observation of reflected protons as well as alpha particles has implications for the physical reflection process usually not discussed in the acceleration theories.