bluuebunny/crossref_metadata_2025 huggingface dataset:
IMF orientation, solar wind velocity, and Pc 3‐4 signals: A joint distribution
Authors: Greenstadt Eugene W. , Singer Howard J. , Russell Christopher T. , Olson John V.
Published: February 2008

Separate studies using the same micropulsation data base in the period range 10–150 s have shown earlier that signal levels recorded during September, October, and November 1969 at Calgary correlated positively with both solar wind alignment of the IMF and solar wind speed, but each correlation contained enough scatter to allow for influence of the other factor. In this report, joint correlations of velocity and field direction with parameters representing hourly distributions rather than minima of IMF orientation angle display the relative effect of the two agents on magnetic pulsation signal levels. The joint correlations reduce the overall scatter and show that solar wind speeds above 200–300 km/s and angles between the IMF and the sun‐earth line of less than 50°–60° are associated with enlarged magnetic pulsation amplitudes. These threshold effects tend to support both the bow shock origin and the Kelvin‐Helmholtz amplification of daytime signal transients in the Pc 3, 4 period ranges.
Comparison of two‐dimensional and three‐dimensional mapping of ionospheric electric field
Authors: Park C. G.
Published: February 2008

Numerical techniques are used to compare two‐dimensional and three‐dimensional analyses of the spreading of ionospheric electric fields. Specific cases are considered in which a source electric field is applied at 150‐km altitude and within a limited horizontal distance L. It is found that horizontal spreading of fields at ionospheric heights can be accurately calculated by using a two‐dimensional model with height‐integrated conductivities, provided that L is 10 km or greater. For L < 10 km a full three‐dimensional model must be used. Three‐dimensional calculations must also be used to relate electric fields measured at balloon altitudes (˜30 km) to overhead ionospheric fields if L < 100 km. Calculated ionospheric fields depend strongly on horizontal conductivity gradients, fields spreading more easily in the direction of decreasing conductivity.
Ultraviolet spectrum of the aurora (2000–2800 Å)
Authors: Beiting Edward J. , Feldman Paul D.
Published: February 2008

Ultraviolet spectra between 2000 and 2800 Å of an IBC II+ aurora were obtained by a rocket‐borne Ebert‐Fastie monochromator at 15‐Å resolution over Fort Churchill, Manitoba. The υ′=0–8 progressions of the N2 Vegard‐Kaplan system, the Δυ=8 sequence of the Lyman‐Birge‐Hopfield system, and a progression attributed to the Herman‐Kaplan system were identified by a synthetic spectrum analysis. Relative populations of the first eight vibrational levels of the A³Σu+ state at high altitude were obtained from the theoretically generated spectra, and quenching rate coefficients were derived for these eight levels by using an atomic oxygen concentration derived from data from another experiment on board. For υ=0 the quenching rate coefficient was found to be 2.0 × 10−10 cm³ s−1 and was not found to increase for higher vibrational levels. The intensity of the (0, 2) band of the Herman‐Kaplan (E³Σg+ → A³Σu+) system was found to be ∼5 times the theoretically predicted value, but even so, the cascade from E³Σg+ provides less than 2% of the A³Σu+ state population.
Longitudinal variations in thermospheric composition under geomagnetically quiet conditions
Authors: Laux U. , von Zahn U.
Published: February 2008

Measurements of N2, Ar, O, and He densities obtained by the gas analyzer on board the satellite Esro 4 indicate in both hemispheres a persistent longitudinal variation of thermospheric composition in middle and high latitudes. Increases of N2 and Ar densities are strongly correlated with a reduction of He and O densities. The amplitudes of the variations are higher in the southern hemisphere than in the northern hemisphere. Universal time variations are only weakly observed in polar latitudes. We present an empirical model, which describes the latitudinal dependence of longitudinal composition variations under quiet geomagnetic conditions.
Plasma injection and diamagnetism
Authors: Gurgiolo C. , Lin C.S. , Mauk B. , Parks G. K. , McIlwain C.
Published: February 2008

A detailed study of the diamagnetic properties of magnetospheric plasma injected at synchronous altitudes is presented. Defining the magnetic induction field B = H + 4πM, we have computed the magnetization M from the plasma distribution functions and show that the diamagnetic contribution of the particles having energies of ∼100 eV to 81 keV can completely account for observed changes in the magnetic induction energy density. Computation of the resulting magnetic field H indicates that the current system in the magnetosphere is complex. Significant changes in H have been observed in a few minutes time scale. Studies of the particle density at various magnetic moments show that both protons and electrons together or individually act to cause the diamagnetic variations in B.
Electrostatic and electromagnetic gyroharmonic emissions due to energetic electrons in magnetospheric plasmas
Authors: Curtis S.A. , Wu C.S.
Published: February 2008

Growth rates are calculated for perpendicularly propagating electrostatic and electromagnetic waves with frequencies between harmonics of the electron gyrofrequency. The source of free energy for these emissions is diffuse, power‐law distributed energetic electrons (E >10 keV) which possess a distribution function with a loss cone feature and a shoulder. This shoulder is defined as a hardening of the distribution function, that is, a less rapid decrease with increasing energy, in a given energy range as compared to energies outside of this range. Such shoulders have been observed experimentally to occur in conjunction with electrostatic gyroharmonic emissions. This available free energy is then coupled to waves supported by the background thermal plasma (T ≈ 1 keV) via the velocity dependence of the electron gyrofrequency. For electrostatic waves, both spatially homogeneous and inhomogeneous energetic electron distributions are considered. For the homogeneous case the calculated growth rates are consistent with observations of so‐called ‘n + 1/2’ electrostatic gyroharmonic emissions located between electron‐cyclotron frequency harmonics in the earth's magnetosphere. The additional free energy of the energetic electron gradient allows excitation without a shoulder in the inhomogeneous case. The electromagnetic emissions are modeled by the ordinary mode. For this case it is shown that either (1) the allowed coupling energies are less than 10 keV, and hence the source of energetic electron free energy is inaccessible or (2) when coupling is possible, the resulting growth rates are small compared to those for electrostatic emissions. This is consistent with the observations that the observed gyroharmonic emissions are electrostatic. From this study, it is concluded that electrostatic gyroharmonic excitation is possible without the cold component of plasma which is often postulated in the existing literature.
The source of electrostatic fluctuations in the solar wind
Authors: Lemons Don S. , Asbridge John R. , Bame Samuel J. , Feldman William C. , Gary S. Peter , Gosling John T.
Published: February 2008

Solar wind electron and ion distribution functions measured simultaneously with or close to times of intense electrostatic fluctuations are subjected to a linear Vlasov stability analysis. Although all distributions tested were found to be stable, the analysis suggests that the ion beam instability is the most likely source of the fluctuations.
Energetic protons in the Jovian magnetosphere
Authors: McDonald F. B. , Schardt A. W. , Trainor J. H.
Published: February 2008

The time histories, angular distributions and energy spectra of energetic protons have been measured over an energy range extending from 0.2 to 20 MeV for the four passes of Pioneers 10 and 11 through the Jovian magnetosphere. The energetic particle data from these four passes are remarkably different. Azimuthal asymmetries appear to dominate with time variations also contributing to the very complex topology. On the inbound P‐10 pass the expected corotation anisotropy was not observed in the outer magnetosphere. The simplest explanation is that the particle reference frame (i.e., the magnetospheric plasma in this region) is moving nearly radially, suggesting the existence of a planetary wind at that time. Near the dawn meridian, particle streaming away from the planet begins at ∼15 RJ. On both the P‐10 inbound and P‐11 outbound passes, there are regions where only partial corotation is achieved. In the mid‐magnetosphere, field‐aligned streaming away from the near‐equatorial current sheet region is the most prominent and puzzling feature. At mid‐latitudes in the subsolar regime, the streaming pattern is more chaotic and its magnitude is smaller. Qualitative discussions are presented for a number of possible mechanisms which could produce this streaming. In the context of our present understanding of the Jovian magnetosphere, each of three mechanisms show promise: perpendicular electric fields, a strong azimuthal intensity gradient at the equator, and strong diffusion processes. In the Jovian wind regions, proton energy spectra are generally of the form E−γ, where E is the kinetic energy of the particle with γ generally ranging between values of 3.0 and 4.2. For the remaining times, the spectra are most frequently of the form exp −P/P0, where P is the proton momentum with P0 varying mainly between 8.0 and 12 MV.
Gravity wave propagation in a diffusively separated atmosphere with height‐dependent collision frequencies
Authors: Del Genio Anthony D. , Schubert Gerald , Straus Joe M.
Published: February 2008

Numerical calculations of gravity wave propagation through a two‐component thermosphere with vertically‐varying inter‐species collision frequencies are performed using a direct integration method. Reflection of upgoing gravity wave energy into downgoing gravity waves appears to be small though nonnegligible at typical thermospheric periods and wavelengths. Coupling into diffusion waves is completely insignificant in most cases and may be determined in part by the relative abundance of the minor species. Temperature perturbation differences between species can be >10% above 300 km altitude even though coupling into diffusion waves is small. Diffusive dissipation of gravity wave energy can be significant in the lower thermosphere and may be comparable to dissipation by viscosity and heat conduction below ∼225 km altitude. Heating by diffusive dissipation may also be comparable to direct heating by solar radiation in the lower thermosphere. As a result of dissipative filtering of long‐period and short‐wavelength waves and the sensitivity of He‐N2 density perturbation phase differences to diffusion, the AE‐C satellite wave observations can only be fit by internal gravity waves with periods ≲25 min and horizontal wavelengths ≳200 km.
Geomagnetic shielding of cosmic rays for different satellite orbits
Authors: Heinrich W. , Spill A.
Published: February 2008

The geomagnetic shielding of cosmic ray particles for satellites can be described by the cutoff probability. This function has been calculated for different orbits, using the world grid of vertical cutoff rigidities by Shea and Smart (1975). The influence of the orbital parameters on the cutoff probability is discussed. Finally, the cutoff probability is used to determine averaged energy spectra of cosmic ray nuclei for some typical orbits.
Absorption into the ³Πu state of molecular oxygen
Authors: Guberman S. L. , Dalgarno A.
Published: February 2008

Quantal calculations are carried out of the wave function of the excited ³Πu state of molecular oxygen, and the X³∑g−–³Πu transition dipole moment is determined. The resulting absorption cross sections are evaluated, and it is demonstrated that absorption into the ³Πu state is not a significant source of atmospheric opacity. It is shown also that radiative association of oxygen atoms through emission from the ³Πu state is not an important source of infrared airglow.
The core electron temperature profile between 0.5 and 1.0 AU in the steady‐state high speed solar wind
Authors: Feldman W.C. , Asbridge J.R. , Bame S.J. , Gosling J.T. , Lemons D.S.
Published: February 2008

The radial dependence of the core electron temperature in the steady‐state high speed solar wind between 0.5 and 1.0 AU is measured using Mariner 10 and Imp 6, 7, and 8 instrumentation. The observed temperature profile is best characterized by two separate power‐law functions which apply in the radial ranges between 0.47 and 0.62 AU and between 0.62 and 1.0 AU, respectively. The separate values of the logarithmic temperature derivatives are αi = −1.14±0.24 and α0 = +0.28 ±0.13, respectively. Whereas the inner value of α, αi, agrees with a previous estimate (αc = −0.9±0.1) using data measured within high speed compression zones of simple high speed streams, the outer value of α, α0, is significantly more positive than any previous theoretical or experimental estimate. A likely explanation is a modest thermal coupling between the protons and electrons by way of kinetic instabilities driven by the generally observed, double streaming proton velocity configurations. The core electron temperature of about 1 × 105 K measured using Mariner 10 instrumentation at 0.47 AU is also significantly less than any value predicted by current models of the high speed solar wind at that distance.
On the validity of the Navier‐Stokes equations for thermosphere dynamics calculations
Authors: Conrad James R. , Schunk Robert W.
Published: February 2008

We studied the validity of using the Navier‐Stokes approximation for the viscous stress tensor and heat flow vector in global thermospheric flow situations. Our approach was based on the complete 13‐moment system of transport equations, which is applicable to both collision‐dominated and collisionless flows. We found that the classical Navier‐Stokes expression for the heat flow is valid at all altitudes, i.e., the heat flow is proportional to the neutral temperature gradient. However, the Navier‐Stokes expression for the stress tensor is not valid above about 400 km for the off‐diagonal elements and above 150 km for the diagonal elements due to the direct effect of heat flow on stress. Nevertheless, the corrections to the Navier‐Stokes stress tensor expression have less than a 10% effect on the wind speed for a wide range of thermospheric conditions.
Relevance of southward magnetic fields in the neutral sheet to anisotropic distribution of energetic electrons and substorm activity
Authors: Lui A.T.Y. , Meng C.‐I.
Published: February 2008

The implications of southward magnetic fields at the magnetotail neutral sheet to the development of streaming anisotropy of energetic electrons and magnetospheric substorm activity are examined. Magnetic field and energetic particle measurements from the Imp 6 spacecraft, the AE index, and global auroral images from DMSP spacecraft are utilized in this study. Criteria are developed to identify events of southward magnetic fields at the neutral sheet which imply the presence of X‐type magnetic neutral lines. Several features of the observations suggest that the southward magnetic fields and the implied X‐type neutral lines are associated with magnetic bubbles in the neutral sheet region. It is found that the signatures of magnetic bubbles are sometimes detected in association with tailward streaming and flux enhancement of energetic electrons (47 keV < E < 350 keV). A cigar‐shaped anisotropy in the energetic electron distribution is frequently but not always observed before the onset of tailward streaming of energetic electrons. The tailward streaming is magnetic field‐aligned and occurs in the form of bursts, suggesting that the generating process is activated somewhat quasi‐periodically and is not in a steady state. Signatures of magnetic bubbles are also detected without any substantial enhancement or detectable tailward streaming of energetic electrons. By comparing Imp 6 observations with the AE index and global auroral images from DMSP spacecraft, it is found that signatures of magnetic bubbles in the neutral sheet are observed during substorms as well as during quiet geomagnetic conditions, indicating that magnetic bubbles are intrinsic features of the neutral sheet in the magnetotail regardless of substorm activity.
HF‐induced parametric decay in presence of field‐aligned irregularities
Authors: Rypdal K. , Cragin B.L.
Published: February 2008

A nonlinear theory for the saturation of the HF‐induced parametric decay instability is formulated in terms of the normal modes of a field‐aligned plasma wave duct. Coupling coefficients between modes are derived, and a saturation spectrum is computed numerically for a typical (δn/n0 ∼ 0.04, Λ⊥ ∼ 4.5 m) irregularity. The computed spectrum shows that unstable waves may propagate at larger angles to the B field than would be the case in a regular medium. The relevance of this work to the experiment of Muldrew and Showen (1977), concerning the height of the HF‐induced plasma line at Arecibo, is discussed. A possible explanation is also found for the sharpness of the ‘decay line’ feature in comparison with the ‘broad bump’ in the observed backscatter spectrum.
Measurements of plasma flow at the dawn magnetopause by Voyager 1
Authors: Lanzerotti L.J. , Krimigis S.M. , Bostrom C.O. , Axford W.I. , Lepping R.P. , Ness N.F.
Published: February 2008

Measurements are presented showing strong tailward flow of ions along the dawn magnetopause as the Voyager 1 spacecraft crossed the earth's magnetosphere boundary following launch on September 5, 1977. With one exception all of the observed flows occur outside the magnetopause. The particle flux measurements at E ≳30 keV, together with the observed magnetic field signatures of the boundary crossing, are consistent with a minimum tailward ion energy flow of ∼(2–7) × 1017 ergs/s at the time of observation. High‐time resolution particle data indicate that the ion flow can vary on a time scale of ∼400 ms. These results, together with recent results from several other spacecraft, show conclusively that a source of energetic particles exists sunward of the dawn‐dusk meridian.
Joule heating in the high‐latitude mesosphere
Authors: Banks Peter M.
Published: February 2008

The contribution made by Joule dissipation to heating of the daytime high‐latitude upper mesosphere is discussed. During solar proton precipitation events in regions of large electric fields, Joule dissipation can be substantially larger than the local solar heating rate. Altitude profiles of Joule dissipation are presented for the polar cleft region for the August 4, 1972, solar proton event.
Parametric decay instability in high‐speed solar wind streams
Authors: Ling K.M. , Abraham‐Shrauner B.
Published: February 2008

The parametric decay of an ion cyclotron pump wave in the high‐speed solar wind streams is studied. The pump wave is an exact stationary solution of the Vlasov equation and Maxwell's equations, and it decays parametrically into an ion acoustic and a product ion cyclotron wave. The case where all the waves propagate parallel to the uniform interplanetary magnetic field is considered. Two typical measurements of the high‐speed solar wind protons provide the input data for the calculation of the parametric decay instability. Although this instability has been considered important for the Alfvén wave, the calculation shows that it is probably not important for the ion cyclotron wave, since the range of wave number values where the growth rate is above threshold is very small.
Emission triggering in the magnetosphere by controlled interruption of coherent VLF signals
Authors: Chang D. C. D. , Helliwell R. A.
Published: February 2008

Man‐made VLF whistler mode (WM) waves are employed in controlled experiments on wave growth and triggering within the magnetosphere. In one such experiment, VLF pulses of 1‐s duration are interrupted by a 10‐ms gap. Two types of gap are employed. In the first, the phase of the wave following the gap is the same as that of a constant‐frequency wave with no gap. In the second type, this phase is reversed. The data indicate that the growth rate and saturation level of the postgap section of a pulse are independent of the phase shift introduced in the gap. Moreover, it is often observed that rising frequency emissions are triggered during the 10‐ms gap. They closely resemble those emissions that frequently appear near the ends of constant‐frequency signals. The gap‐induced emissions may then interact with the postgap signals. Suppressions and entrainments between those signals are often observed. An explanation of the phenomenon of gap‐induced emissions is based on the hypothesis that the radiation from the wave‐organized electrons can switch from a ‘forced’ mode to a ‘natural’ mode after termination of the triggering wave. An experiment on the relation between gap length and the development of falling emissions confirms an earlier observation that falling emissions always start with a small rise in frequency. It also shows that a postgap signal can ‘capture’ a gap‐induced falling emission when its frequency is close to that of the postgap signal at its beginning.
The latitudinal structure of Pc 5 waves in space: Magnetic and electric field observations
Authors: Singer Howard J. , Kivelson Margaret G.
Published: February 2008

The occurrence frequency and spatial structure of Pc 5 magnetic pulsations in the dawnside of the plasma trough have been studied using data from the Ogo 5 satellite. The wave magnetic fields were obtained from the University of California, Los Angeles, flux‐gate magnetometer measurements, and one component of the wave electric field was inferred from oscillations of the ion flux measured by the Lockheed light ion mass spectrometer. During portions of seven of the 19 passes comprising the survey, Pc 5 oscillations were observed in the ion flux but not in the magnetic field, and in each case the satellite was within 10° of the geomagnetic equator. Above 10° latitude, transverse magnetic and electric oscillations were both observed. The results are consistent with the model of a standing Alfvén wave along a resonant field line with the geomagnetic equator as a node of the magnetic perturbation, that is, an odd mode. The wave periods are generally consistent with the fundamental resonant period. In this study, Pc 5 oscillations were identified 3 or 4 times more frequently (per orbit) than in previous spacecraft studies which relied only on magnetic data.
Magnetosphere‐ionosphere coupling
Authors: Goertz C. K. , Boswell R. W
Published: February 2008

A simple two‐dimensional model of the magnetosphere‐ionosphere system is discussed in which a localized electromotive force applied across a magnetic field line at t=0 is shown to propagate along the magnetic field with the Alfvén velocity. The perpendicular electric field is assumed to reverse direction across the field line. Since the perpendicular electric field is limited in space, the propagation involves parallel electric fields whose magnitude depends on the characteristic scale length of the applied emf and the local plasma parameters. The electric field pulse associated with the ‘shock’ front is reflected at the ionosphere and propagates back to the source region. The finite Pedersen conductivity in the ionosphere damps the wave, and a steady state current system is established in the order of several hours. The parallel electric field can accelerate ions and electrons.
Long‐term solar wind electron variations between 1971 and 1978
Authors: Feldman W. C. , Asbridge J. R. , Bame S. J. , Gosling J. T.
Published: February 2008

Imp solar wind electron data measured between 1971 and 1978 were studied with the aim of determining long‐term variations near the earth. Two separate sets of parameter variations were observed: (1) in 1976–1977 the solar wind density, the electron temperature, and the interplanetary electrostatic potential were all enhanced, and (2) the halo density and associated electron parameters were all depressed during a 1½‐year period centered on the last 6 months of 1976. Although interpretation of these results in terms of corresponding coronal and interplanetary variations is not unique, it may be significant that measured solar wind parameters near the minimum of solar cycle 20 agree better with the Hartle‐Sturrock model of the coronal expansion than they do during other epochs.
The lower hybrid drift instability at the magnetopause
Authors: Gary S. Peter , Eastman Timothy E.
Published: February 2008

A correlation of plasma theory, plasma data, and fluctuation observations is used to demonstrate that the lower hybrid drift instability is the most likely source of field fluctuations observed between 1 and 100 Hz at the magnetopause.
Models of electron temperature in the topside ionosphere for low and medium solar activity conditions
Authors: Mahajan K.K. , Pandey V.K.
Published: February 2008

Electron temperature models for the topside ionosphere, which make use of the protonospheric heat flux ϕC, deduced from the experimental electron temperature profiles, are presented for low as well as medium solar activity conditions. The parameter ϕC is found to show inverse correlation with electron concentration Ne in the topside ionopshere. This relationship between Ne and ϕC at 400 km is used to obtain ϕC when Ne is known. The value of ϕC can be used to build the electron temperature profile in the topside ionosphere if Te at some lower height is known. The Te value at a lower height, say at 300 km, can be obtained from earlier models based upon empirical Te‐Ne relationships. The electron temperature obtained from our present models are compared with the measured temperatures and are found to show reasonable agreement.
Magnetospheric substorms—definition and signatures
Authors: Rostoker G. , Akasofu S.‐I. , Foster J. , Greenwald R.A. , Kamide Y. , Kawasaki K. , Lui A.T.Y. , McPherron R.L. , Russell C.T.
Published: February 2008

For many years, researchers have utilized definitions of the substorm phenomenon that are not consistent among one another, and this has created great difficulties in comparing the results reported in the literature by the various researchers. In August 1978, nine magnetospheric physicists active in the field of substorm research met in Victoria, British Columbia, Canada, to attempt to reach a consensus on an acceptable definition for a magnetospheric substorm. This paper reports the agreements reached at the Victoria workshop and presents an operational definition of the magnetospheric substorm and a critique of the various signatures by which researchers can identify the time sequence and spatial extent of the substorm.
Electrostatic wave growth: Secondary peaks in a measured auroral electron distribution function
Authors: Kaufmann R.L.
Published: February 2008

Small secondary peaks are frequently measured in energetic (25 eV <E <15 keV) electron distribution functions during rocket flights over bright auroras. One sample distribution function that was measured at 250 km is analyzed in this paper. The purpose of the analysis is to see if the observed secondary peaks are large enough to drive electrostatic instabilities. Two specific features of the distribution function are analyzed in detail: a secondary parallel peak, characterized by a region of positive ∂f/∂ν∥, and a secondary perpendicular peak, characterized by positive ∂f/∂ν⊥. The parallel peak produces positive growth rates for electrostatic whistler and ion cyclotron waves, but these growth rates are too small to generate significant wave amplitudes within an arc. The perpendicular peak produces much larger growth rates for electrostatic electron cyclotron and upper hybrid waves when these two modes are interacting. We conclude that the perpendicular peak was the last feature to produce a strong instability as the electrons moved down to the rocket. The largest waves should be seen as rockets reach an altitude where ωUH = 2|ωce|. The plasma detected at 250 km appears to have evolved until the growth lengths of waves that are driven by the perpendicular peak are comparable to the dimension of an arc. Once generated, the upper hybrid and electron cyclotron waves will also resonate with electrons which have small pitch angles, and they can therefore spread the parallel peak well beyond the point at which it produces significant growth rates.
The suprathermal electron contributions to high‐latitude Birkeland Currents
Authors: Maier Eugene J. , Kayser Susan E. , Burrows J. Ronald , Klumpar David M.
Published: February 2008

Observations obtained from instruments on the Isis 2 spacecraft in polar orbit at 1400 km have been used to investigate the identity of the charge carriers in the large‐scale field‐aligned current systems present in the high‐latitude region. The vertical current is inferred from observed perturbations to the east‐west component of the local geomagnetic field obtained by the magnetometer. Simultaneous observations of the charged particle flux were obtained by the soft particle spectrometer (in the 0.005‐ to 15‐kev range) and by the retarding potential analyzer in the suprathermal energy range (≧1 eV). The flux of electrons in the suprathermal energy range has been studied in an attempt to identify and obtain consistency between the velocity distribution function of the charge carriers and the inferred currents. The observed flow of upwelling suprathermal electrons increased substantially when currents were inferred of either direction. The mean net current density due to suprathermal electrons was 0.4±0.2 µA/m² when the expected current was upward, compared to a mean expected value of 0.2±0.1 µA/m² inferred from the magnetometer. This mean net current density was derived by combining the observed much larger downward flux of electrons, primarily energetic, with an observed upward flux of suprathermal electrons. The partial cancellation of the charge transfer due to the precipitating particles by a return current of suprathermals results in reasonable agreement between the observed net particle flow and the magnetic field perturbation. When the expected field‐aligned currents were downward, the mean current density due to suprathermal electrons was −0.8±0.2 µA/m², compared to a magnetometer inferred value of −0.4±0.1 µA/m². The suprathermal electrons therefore contribute substantially to field‐aligned currents, producing most, if not all, of the inward currents.
Characteristics of ionospheric bubbles determined from aspect sensitive scatter spread F observed with Alouette 1
Authors: Muldrew D.B.
Published: February 2008

Aspect sensitive scatter from bubbles is observed on a large percentage of equatorial Alouette 1 ionograms. The occurrence frequency of bubbles increases sharply (from zero) after sunset and reaches a maximum at about 23–24 hours local time. Near midnight, bubbles are observed in the topside ionosphere on about 65% of American and African satellite passes and on about 50% of Asian passes. The frequency of occurrence decreases gradually throughout the postmidnight and morning periods and becomes almost zero near noon. Averaged over the whole day, the occurrence of bubbles at Asian longitudes is about one half that at American and African longitudes. The occurrence of bubbles peaks at the December solstice for American longitudes and at the June solstice for Asian longitudes. The above observations imply that bubbles are initiated by field‐aligned currents at the steep density gradient at the bottom of the postsunset F layer by the mechanism proposed by K. D. Cole. After initiation the Rayleigh‐Taylor instability takes over to raise the bubbles through the F layer. Alouette 1 observations indicate that the bubbles rise to a maximum height of about 600–1000 km near midnight. The height then decreases gradually until some time after sunrise. Ray tracing, assuming aspect sensitive scatter and a circular cross section for the field‐aligned bubble, indicates that a particular, typical bubble was about 28 km in radius and intersected the vertical plane through the satellite in the magnetic east‐west direction, about 550 km below and about 750 km to the east or west of the satellite. In the topside ionosphere below Alouette 1, bubbles never seem to occupy more than about 2% of the total space.
Plasma and field signatures of poleward propagating auroral precipitation observed at the foot of the Geos 2 field line
Authors: Shepherd G.G. , Boström R. , Derblom H. , Fälthammar C.‐G. , Gendrin R. , Kaila K. , Korth A. , Pedersen A. , Pellinen R. , Wrenn G.
Published: February 2008

The 6300‐Å auroral emission intensity at Kilpisjärvi, Finland, was monitored during the 1978–1979 winter, and comparisons made with data from the geostationary Geos 2 spacecraft located on the same magnetic field line. Optical 6300‐Å events with a rise time of a few minutes and a decay time of about 1 hour were regularly observed, and four events from the night of December 18, 1978, were analyzed in detail. All‐sky camera data showed that the events arose from poleward propagating auroras associated with auroral breakup. The Geos 2 plasma and field signatures for these events were remarkably well defined and consistent. About 1 hour prior to an event the tailward component of the magnetic field at Geos 2 begins to grow; it relaxes suddenly to a dipolar configuration at the event onset. At the same time there is a short‐lived electric field enhancement up to 15 mV/m and a burst of ULF waves (a short irregular pulsation event). There is a dropout in E <500 eV electron intensity, a sudden rise in E >20‐keV intensity, and an onset of VLF activity, all of which persists for roughly 1 hour, in parallel with the decaying 6300‐Å emission. The observed electric field values are comparable to the inductive electric field expected from the observed magnetic field changes and appear to play a crucial role in the processes. The energy release associated with the 6300‐Å emission is comparable to the energy stored in the E <500‐eV electrons prior to the onset. These events appear to correspond to injection events, occurring at the onset of the locally detected expansion phase.
Correlations between neutral meridional winds and ion densities observed on AE‐E
Authors: Wharton L.E. , Spencer N.W. , Brinton H.C.
Published: February 2008

We compare the southward directed neutral meridional wind and the nighttime ion density simultaneously measured on AE‐E for altitudes from 250 to 290 km and find them to correlate when the magnetic field inclination is negative and anticorrelate when the inclination is positive. This effect is attributed to neutral winds transporting the ions up or down a field line depending upon the sign of the field‐aligned wind and the inclination. Model calculations for the ion density in the presence of a field‐aligned wind and ambipolar diffusion are presented. Six examples of this ionospheric response are analyzed and found to be in qualitative and approximate quantitative agreement with the theoretical results.
An empirical polytrope law for solar wind thermal electrons between 0.45 and 4.76 AU: Voyager 2 and Mariner 10
Authors: Sittler E.C. , Scudder J.D.
Published: February 2008

Empirical evidence is presented that solar wind thermal electrons obey a polytrope law of the form P=Σnγ with polytrope index γ = 1.175±0.03 (3σ). The Voyager 2 and Mariner 10 data used span the radial range from 0.45 to 4.76 AU and have a large dynamic range in density (four decades), and in temperature (over one decade), which is crucial for an unambiguous determination of γ. The polytrope index γ was found to be insensitive to systematic dependence on the bulk velocity. We find that there is a large ∼±50% variation in the entropylike stream tube constant Σ relative to its best fit value Σ0. For the Voyager 2 data set, which has 5831 hourly averages, this variation represents a nearly random statistical sample of log Σ about log Σ0, thereby reducing systematic errors. A positive correlation on average between Σ and bulk speed is found, which does introduce systematic errors (∼5%) in the determination of γ resulting in larger uncertainties than implied by a simple regression of log Te versus log n. Our results support the theoretical predictions by Scudder and Olbert (1979b) that solar wind thermal electrons in the asymptotic solar wind should obey a polytrope law with polytrope index γ = 1.16±0.06 (3σ). When restricted to high‐speed flows, these data do not support the suggestion made by Feldman et al. (1978) that γ = 1.45±0.15 (3σ). We attribute this difference to the biased sample of stream tube constants Σ encountered in that limited study and/or the smaller dynamic range of density and temperature available to the 1‐AU observer. Our results support (1) the widespread impressions in the literature that solar wind electrons behave more like an isothermal than an adiabatic gas; (2) the arguments put forward by Scudder and Olbert (1979a, b) that Coulomb collisions are the dominant stochastic process shaping observed electron distribution functions in the solar wind, and (3) the assignment of the interplanetary potential as equal to approximately 7 times the random energy of the thermal electrons.
Photometry of Saturn at large phase angles
Authors: Tomasko M. G. , McMillan R. S. , Doose L. R. , Castillo N. D. , Dilley J. P.
Published: February 2008

Limb‐darkening curves are derived from Pioneer 11's imaging photometry of a bright zone (−11° ≤latitude ≤−6°) and a dark belt (−17° ≤latitude ≤−15°) on Saturn in red and blue light at 12 phase angles between 13° and 151°. The atmosphere of Saturn scatters light only 1/3 as efficiently in the forward direction as does the atmosphere of Jupiter, and the belt on Saturn is only ∼5% fainter than the zone instead of the factor of 2 contrast seen on Jupiter in blue light. Vertically inhomogeneous models of the scattering of light in the atmosphere of Saturn were computed to fit these photometric data by using the cloud structure derived by Gehrels et al. (1980) from polarimetry in blue light at 88° phase (aerosol optical depth =1 at ∼300 mbar, aerosol scale height ≈ 1/4 gas scale height). These computations show that the absorbers of blue light in the atmosphere must be mixed with the aerosols responsible for scattering, not concentrated above them. The single‐scattering phase function of the aerosols is well represented by a combination of two Henyey‐Greenstein functions. Moderately forward scattering functions fit well in both red and blue light, although a definite difference between the two colors is found. If the particles are approximated by spheres, the small asymmetry parameter (≈ 0.32) of their phase function in red light constrains their average radius to be smaller than about 0.13 µm, less than 1/4 the size of the cloud particles on Jupiter. The aerosols on Saturn are similar in size to those in the thin, high‐altitude layer of haze reported by Stoll (1980) on Jupiter. The phase integral of the scattering models computed here is about 1.46 in red light and 1.36 in blue light. If Saturn scatters similarly at other wavelengths, its bolometric geometric albedo together with the effective temperature of 96.5 K±2.5 K reported by Orton and Ingersoll (this issue) imply that Saturn radiates between 2.3 and 4.2 times as much energy as it receives from the sun.
On the tearing mode in quasi‐neutral sheets
Authors: Coroniti F. V.
Published: February 2008

For a quasi‐neutral current sheet with a finite normal magnetic component the tearing mode is stabilized by the adiabatic motion of the magnetized electrons. If the current sheet is not absolutely quiescent but contains electromagnetic wave turbulence (or collisions) which pitch angle scatters the mirror‐trapped electrons, the tearing mode is destabilized with a growth rate proportional to the pitch angle diffusion coefficient. For observed turbulence levels in the earth's plasma sheet the tearing growth time could be 5 min.
Distribution of field‐aligned currents and expected magnetic field perturbations Resulting from auroral currents along circular orbits of satellites
Authors: Akasofu S.‐I. , Ahn Byung‐Ho , Kisabeth J.
Published: February 2008

The magnetic field produced by the auroral current system is computed along magnetic local time meridians at two altitudes, 300 and 5000 km. The computation is based on a model of a three‐dimensional current system that consists of field‐aligned currents and ionospheric currents. The input data set is constructed by using magnetic field data obtained from the International Magnetospheric Study (IMS) Alaska meridian chain of magnetic observatories. The latitudinal distribution of the field‐aligned currents and the associated Y magnetic field component are also discussed.
Suprathermal electron energy distribution within the dayside Venus ionosphere
Authors: Knudsen W. C. , Spenner K. , Michelson P. F. , Whitten R. C. , Miller K. L. , Novak V.
Published: February 2008

The suprathermal electron energy distribution for the dayside ionosphere has been derived from data returned by the Pioneer‐Venus orbiter retarding potential analyzer. The shape and magnitude of the spectrum are consistent with the assumption that solar EUV radiation is the only significant source. The magnitude of the spectrum and its variation with altitude suggest that significant vertical transport occurs, with the electrons being lost through the ionopause. In turn, significant vertical transport suggests that the effective vertical electron heat conductivity may be comparable to the field‐free value. The heat input to the thermal electron gas from the measured suprathermal electron flux is too small by a factor of at least 5 to maintain the observed electron temperature profile if the electron thermal conductivity is assumed to be close to the field‐free value. We thus infer that most of the heat is supplied by the solar wind.
The upper atmosphere of Venus during morning conditions
Authors: von Zahn U. , Fricke K. H. , Hunten D. M. , Krankowsky D. , Mauersberger K. , Nier A. O.
Published: February 2008

The structure and composition of the Venus upper atmosphere between 130‐ and 650‐km altitude were measured for a solar zenith angle of approximately 60° by the neutral gas mass spectrometer on board the Pioneer Venus multiprobe bus. Below 180 km a wavelike structure is quite evident in the CO2 and He number density profiles. For altitudes above 100 km a one‐dimensional model of the Venus upper atmosphere during morningside conditions (MS model) is presented. Number densities at 150‐km altitude are as follows: CO2= 4.9 × 109, N2=1.1 × 109, CO=2.8 × 109, and He=4.8 × 106 cm−3. The exospheric temperature is 275 K. The vertical compositional structure is best described by an eddy coefficient K=1.4 × 1013na−1/2 cm²/s, where na is the total number density in units per cubic centimeter. The homopause altitudes for N2 and He are at 136 and 130 km, respectively.
Ultraviolet contrasts and the absorbers near the Venus cloud tops
Authors: Esposito Larry W.
Published: February 2008

The absorbers in the Venus atmosphere are not merely markers for the atmospheric motions; their absorption of incident sunlight determines the deposition of the energy which drives these motions. Measurements of the ultraviolet contrasts constrain the location, altitude, and identity of these absorbers near the Venus cloud tops. Spin‐scan images from the Pioneer Venus orbiter UV spectrometer (UVS) and the cloud photopolarimeter (CPP) provide a set of planetary contrast measurements in the wavelength range from 1990 Å to 3650 Å and phase angles from 33° to 130°. The planet is darkest at the point where the UVS line of sight penetrates perpendicular to the cloud tops: thus the absorbing material responsible must be deep in the atmosphere. Sulfur dioxide absorption can explain the amount of contrast seen between 2000 Å and 3200 Å. At the longer wavelengths (where the SO2 cross section has dropped by 2 orders of magnitude), the persistence of contrast requires another absorber which is deeper in the atmosphere than the SO2 (at about the 75‐mbar level). This additional absorption is strongly (but not identically) associated with the location of the SO2; we find the short and long wavelength images are remarkably similar. Part of the observed contrast is due to the high‐lying haze discovered from Pioneer Venus polarimetry, most importantly at the shortest wavelengths where CO2 is strongly absorbing. The correlation between planetary contrast and polarization does not require large‐scale clearing or major vertical motions of the cloud tops as the sole cause of the observed contrast. However, a scheme in which absorbers subject to photochemical destruction are mixed upward into the cloud top region provides a consistent explanation for the origin of these markings.
Measurements of the flux of sunlight in the atmosphere of Venus
Authors: Tomasko M. G. , Doose L. R. , Smith Peter H. , Odell A. P.
Published: February 2008

The solar flux radiometer (LSFR) aboard the Pioneer Venus (PV) sounder probe measured the intensity of sunlight in five directions to the vertical using narrow angular fields of view. The measurements in a narrow spectral channel (0.59–0.67 µm) and two broad channels (0.4–1.0 and 0.4–1.8 µm) were reduced to yield the profiles of upward and downward solar flux with a vertical resolution of 100–500 m. All the flux profiles show three distinct cloud layers with bottoms at altitudes of 57.5, 49.7, and 47.9 km. The azimuthal structure of the intensity samples at the highest altitudes implies a cloud optical depth between 3.5 and 4.0 above 64.3 km. The narrow‐band data were interpreted to give the optical depths and single scattering albedos of the clouds at a wavelength of 0.63 µm. The results for the upper cloud are generally consistent with those reported by the cloud particle size spectrometer (LCPS) experiment on PV. In the middle and lower cloud the LSFR data imply that the largest particle size mode has about one third the optical depth deduced from the LCPS data under the assumption that these particles are spherical. The total optical depth of the clouds was found to be about 25. Models of the broad‐band fluxes confirm these conclusions and indicate that about half the sunlight absorbed by Venus is absorbed above our first measurements. In addition, absorption of solar radiation occurs in the upper cloud and below 35 km; the middle and lower cloud layers absorb remarkably little sunlight. Models consistent with our measured flux profiles and the spherical albedo of Venus were used to scale our measured net fluxes to a bolometric globally averaged solar energy deposition profile, assuming that the sounder site is typical of planetwide conditions. Averaged over the planet, about 17 W/m² are absorbed at the ground (some 2.5% of the total solar energy incident on the planet). The solar net flux profile in the lower atmosphere is not very sensitive to changes in the thickness of the nearly conservative lower cloud.
Performance of entry heat shields on Pioneer Venus Probes
Authors: Pitts W. C. , Wakefield R. M.
Published: February 2008

The Pioneer Venus probes approached Venus with high relative velocity. As they entered the atmosphere, they were rapidly decelerated by aerodynamic drag, and a great deal of heat was generated. To protect the probe structure and the scientific instruments, a carbon phenolic heat shield was placed on the front of the probes. Because the design of heat shields for planetary entry is a developing technology, thermocouples were placed in the heat shields so that actual and predicted heat shield performance could be compared. The function of the heat shield is discussed, the probe environments during entry into the Venusian atmosphere are described, and some results from the heat shield experiment are presented. It was found that for the most part, the heat shields performed better than expected.