bluuebunny/crossref_metadata_2025 huggingface dataset:
On the generation of 3‐m irregularities during equatorial spread F by low‐frequency drift waves
Authors: Huba J.D. , Ossakow S.L.
Published: February 2008

We consider the linear theory of low‐frequency drift waves for parameters typical of the ionosphere during equatorial spread F. These waves are considered to be generated by density gradients associated with long‐wavelength primary plasma fluid instabilities (e.g., collisional Rayleigh‐Taylor). We include electron‐ion, electron‐neutral, and ion‐ion collisions in our analysis. We find that a collisional drift wave can occur in the 3‐meter regime but that it is heavily damped by ion viscosity. For density gradient scale lengths of ≳75 m it is required that n <2 × 10³ cm−3 for wave excitation to occur.
CO2and CO electron vibrational cooling rates
Authors: Porter H.S. , Mayr H.G.
Published: February 2008

We calculate new electron cooling rates due to electron vibrational excitation of CO2 and CO and rotational excitation of CO. Our results are expressed as closed form analytic expressions that are both convenient for numerical calculations and valid over a wide range of electron and neutral temperatures. We find that our cooling rates and their temperature dependences differ significantly from other calculations. For CO2, these differences are related mainly to the assumptions about the approach to equilibrium and not to the actual cross‐section choices. For electron temperatures below 1000°K and gas temperatures near 300°K (representative of conditions in the Venus atmosphere, where collisional cooling of the electrons dominates) our CO2 cooling rate is as much as an order of magnitude larger than previously considered in ionospheric models.
Geomagnetopause surface fluctuations observed by Voyager 1
Authors: Lepping R. P. , Burlaga L. F.
Published: February 2008

As Voyager 1 moved out of the dawnside of the earth's magnetosphere on September 5, 1977 (at position (−2.6, −16.5, 1.1) earth radii in GSE), it crossed the magnetopause apparently seven times, despite the high spacecraft speed of 11 km/s. Normals to the magnetopause and their associated error cones were well determined for each of the crossings through the use of a minimum variance analysis of the internal magnetic field. The oscillating nature of the ecliptic plane component of these normals indicates that most of the multiple crossings were due to a wavelike surface disturbance moving tailward along the magnetopause. We modeled the wave, which was aperiodic, as a sequence of sine waves with amplitude Ai, wavelength λi, and speed Vi. These quantities were determined for two pairs of intervals from the measured slopes, occurrence times, and relative positions of six magnetopause crossings. The average amplitude was A =2100−500+3800 km, and the wavelengths were of the order of 47,000−12,000+30,000km. The wave speed was approximately 340−95+210 km/s, and typical periods were in the neighborhood of 170±60 s. The magnetopause thickness was estimated to lie in the range 300–700 km with higher values possible. The estimated amplitude of these waves was obviously small compared to their wavelengths; this conclusion is independent of any bulk normal motion of the magnetopause that might have been present.
Rotation of hydromagnetic waves between the magnetosphere and the ground
Authors: Andrews M. K. , Lanzerotti L. J. , Maclennan C. G.
Published: February 2008

We show, via an analysis of ground‐based magnetic field data and fixed‐frequency VLF signals propagating in the whistler mode in the magnetosphere, that magnetospheric hydromagnetic waves (having periods of ∼60–120 s) observed on the ground have their wave ellipse orientations rotated by π/2 from their orientations in the magnetosphere. This analysis provides a detailed nonstatistical confirmation of recent theoretical predictions of the rotation effect of the ionosphere/atmosphere/ground on hydromagnetic waves.
Electric fields, electron precipitation, and VLF radiation during a simultaneous magnetospheric substorm and atmospheric thunderstorm
Authors: Bering E.A. , Rosenberg T.J. , Benbrook J.R. , Detrick D. , Matthews D.L. , Rycroft M.J. , Saunders M.A. , Sheldon W.R.
Published: February 2008

A balloon payload instrumented with a double‐probe electric field detector and an X ray scintillation counter was launched from Roberval, Quebec, Canada (L =4.1) at 0828 UT (0328 LT) on July 9, 1975. A magnetospheric substorm was observed locally between 0815 and 1100 UT, which produced a maximum ΔB of ∼500 nT at ∼0930 UT. A single‐cell atmospheric thunderstorm developed northeast of Roberval beginning around 0925 UT which was most intense from ∼1000 to 1035 UT. Detailed study of the electrical properties of the thunderstorm, the X ray precipitation data, and VLF spheric data leads to three conclusions. First, the electrical coupling from the thunderstorm to the magnetosphere increases with frequency from dc to the VLF; for the observed storm the amplitude at the ionosphere of thunderstorm produced electric fields was not significant at frequencies below 0.1 Hz. Second, the atmospheric conductivity above the thunderstorm was observed to be about one‐half the fair weather value prior to 1000 UT; decreased to about one‐quarter the fair weather value at about 1000 UT; and remained depressed after the end of the thunderstorm. This result was contrary to that expected on the basis of previous work and is one which merits considerably more investigation. Third, the data show a high probability that half‐hop whistlers initiated by sferics from the thunderstorm triggered energetic electron precipitation from the magnetosphere.
Dependence of hydromagnetic energy spectra on solar wind velocity and interplanetary magnetic field direction
Authors: Wolfe A. , Lanzerotti L.J. , Maclennan C.G.
Published: February 2008

We investigate the relationship of local daytime geomagnetic energy (measured in the period range ∼30–300 s at a mid‐latitude ground‐based station) to interplanetary parameters during two time intervals when the solar wind/interplanetary field conditions were quite different. We find that the geomagnetic energy levels in the entire period band are consistent with production by a Kelvin‐Helmholtz instability source at the earth's magnetopause. We further find that at the shorter periods (∼30–60 s) the energy may also have a source contribution from the mechanism proposed by Greenstadt, which involves wave energy transfer from the earth's bow shock to the magnetopause. This result represents the first evidence of frequency dependencies in the interplanetary source mechanisms for hydromagnetic energy production in the magnetosphere.
The dynamics of equatorial irregularity patch formation, motion, and decay
Authors: Aarons J. , Mullen J.P. , Whitney H.E. , MacKenzie E.M.
Published: February 2008

Using scintillation observations from a series of equatorial propagation paths as well as backscatter and airglow data, the development, motion, and decay of equatorial irregularity patches have been studied. Assembling the results of earlier studies in the field with our observations, we find the following: the patch has limited east‐west dimensions with a minimum of 100 km. Several patches may be melded together to reach an extent of 1500 km. Its magnetic north‐south dimensions are often greater than 2000 km; the most intense irregularities (as evidenced by the Jicamarca radar at the dip equator) are from 225 to 450 km in altitude, although irregularities are found as high as 1000 km. The patch initially has a westward expansion following the solar terminator, then, maintaining its integrity, moves eastward. Evidence over a limited series of experiments suggests that premidnight patches are formed within 1½ hours after ionospheric sunset in the absence of special magnetic conditions. From Ascension Island (∼16°S dip latitude) the individual patches can be clearly distinguished. The decay of patches in the midnight time period was studied, pointing to a rapid decrease in scintillation intensity in this time period.
Solar flux variation of the thermospheric molecular oxygen density
Authors: Kayser D.C.
Published: February 2008

Measurements of the thermospheric molecular oxygen density obtained from the Atmosphere Explorer (AE) satellites and composition data obtained previously from rocket‐based experiments provide complimentary information on the distribution of O2 between 120 and 200 km as a function of solar flux. The AE measurements provide a description of the constituent's short‐term behavior near solar minimum. With this knowledge, the rocket experiment data, which cover a wider range of solar activity and altitudes, can be analyzed for long‐term variations. At mid‐latitudes and low latitudes the 200‐km O2/N2 density ratio decreases ∼35% as the F10.7 index increases from 70 to 150. The O2 density at 200 km shows little variation but at 120 km decreases ∼30% for the same increase in F10.7. Additionally, the analysis gives evidence for photochemically induced departures from diffusive equilibrium in the molecular oxygen density.
High‐latitude convection: Comparison of a simple model with incoherent scatter observations
Authors: Sojka J.J. , Foster J.C. , Raitt W.J. , Schunk R.W. , Doupnik J.R.
Published: February 2008

We have compared a simple model of plasma convection at high latitudes with data obtained from simultaneous measurements made by the incoherent scatter facilities at Chatanika, Alaska and Millstone Hill, Massachusetts in June 1978 during moderately disturbed conditions. The measured horizontal plasma drift velocities were averaged for four days to emphasize gross features of the convection pattern and reduce the effects of substorms. The convection model includes the offset of 11.5° between the geographic and geomagnetic poles, the tendency of plasma to corotate about the geographic pole, and a constant dawn/dusk magnetospheric electric field mapped to a circle about a center offset by 5° in the anti‐sunward direction from the magnetic pole. The radius of the circle corresponds to 17° of latitude and the electric potentials are aligned parallel to the noon/midnight meridian within the circle. Equatorward of the circle the potential diminishes radially and varies inversely as the fourth power of sine magnetic co‐latitude. A consequence of these two offsets and the sunward alignment of the magnetospheric electric field is that our model predicts different diurnal convection patterns when viewed at different longitudes in the geographic frame. The concurrently observed diurnal distributions of horizontal plasma convection velocities are different for Chatanika and Millstone Hill even though the measurements cover approximately the same range of magnetic latitudes. We find there is good agreement between our simple model and the gross features of these two diurnal patterns.
Solar wind ions accelerated to 40 keV by shock wave disturbances
Authors: Gosling J.T. , Asbridge J.R. , Bame S.J. , Feldman W.C. , Paschmann G. , Sckopke N.
Published: February 2008

Observations in the solar wind with the LASL/MPI fast plasma experiment on ISEE 1 and 2 reveal the common presence of ions with energies extending from ∼100 eV up to at least 40 keV in a broad region, typically ∼107 km wide, following interplanetary shocks. Peak differential fluxes up to ∼5 × 10³ (cm² s sr keV)−1 at 28 keV are observed either at the shock or within the first ∼1.5 hours following shock passage. In the solar wind frame the distribution function f(ν) of these ions is roughly isotropic, peaks near zero velocity, and above ∼5 keV can adequately be characterized as power law in energy with a spectral index of ∼2.7. The effective ‘temperature’ of these ions generally exceeds 108 °K. These suprathermal interplanetary ions are almost certainly solar wind ions which have been accelerated by some mechanism associated with the shock wave disturbance. Present evidence leads the authors to favor stochastic particle acceleration involving electrostatic and/or electromagnetic turbulence in the postshock flow.
Airglow observations of the OI 7774 Å multiplet at Arecibo during a magnetic storm
Authors: Burnside Roger G. , Meriwether John W. , Walker James C.G.
Published: February 2008

During a severe geomagnetic storm the OI 7774 Å triplet was observed to have an intensity of the order of 15R, reduced to the zenith. Theoretical calculations using electron density profiles obtained with the Arecibo incoherent scatter radar show that the measured intensities cannot be explained by either radiative or ion‐ion recombination reactions. A possible correlation between the airglow enhancement and Dst has been found. Excitation by heavy neutral atoms or ions, for example, oxygen or O+, precipitating into the thermosphere is put forward as a possible explanation for enhanced 7774 Å emission during this magnetic storm.
Electrodynamic structure of the late evening sector of the auroral zone
Authors: Burke W.J. , Hardy D.A. , Rich F.J. , Kelley M.C. , Smiddy M. , Shuman B. , Sagalyn R.C. , Vancour R.P. , Widman P.J.L. , Lai S.T.
Published: February 2008

Detailed observations from a late evening sector pass of S3‐2 over the northern auroral zone are presented and compared with nearly simultaneous DMSP imagery. High time resolution measurements are presented from electric field, magnetic field, energetic electron (80 eV ≤E ≤17 keV), and thermal plasma detectors. The region of diffuse aurorae was characterized by nearly isotropic electron fluxes and slowly varying (≤20 mV/m) electric fields. The Birkeland currents in this region were predominantly into the ionosphere with embedded, narrow sheets of upward currents. Simultaneous magnetometer and satellite potential fluctuations show that the upward current sheets are real and that these upward currents are probably carried by precipitating, low‐energy plasma sheet electrons. Poleward of the diffuse aurorae, the covective electric field varied rapidly in magnitude, anticorrelating with the intensity of precipitating electrons. An inverted‐V structure was observed above a visible arc. An intense downward current sheet (>10 µA/m²) was found near the equatorward edge of the arc. At this place the ionospheric thermal plasma was near marginal stability for the onset of O+ ion cyclotron turbulence. An electric field component parallel to B of ∼10 mV/m was measured in this region of strong downward current. The plasma and electric field distributions in the vicinity of the arc suggest that the auroral electrojet was distributed over the region of diffuse auroral but was most intense near the equatorward boundary of the auroral arc.
Time analysis of volcanic activity on Io by means of plasma observations
Authors: Mekler Yuri , Eviatar Aharon
Published: February 2008

A model of Io volcanism in which the probability of activity obeys a binomial distribution is presented. Observed values of the electron density obtained over a 3‐year period by ground‐based spectroscopy are fitted to such a distribution. The best fit is found for a total number of 15 volcanoes with a probability of individual activity at any time of 0.143. The Pioneer 10 ultraviolet observations are reinterpreted as emissions of sulfur and oxygen ions and are found to be consistent with a plasma much less dense than that observed by the Voyager spacecraft. Late 1978 and the first half of 1979 are shown to be periods of anomalous volanicity. Rapid variations in electron density are related to enhanced radial diffusion.
The oxygen polar corona
Authors: Yee J.H. , Hays P.B.
Published: February 2008

At high‐latitude regions, energetic oxygen atoms are produced by a nonthermal mechanism associated with the precipitated energetic atomic oxygen ions during disturbed periods. The upward moving nonthermal atoms can redistribute themselves at higher altitudes. For the storm of December 17, 1971, assuming the energetic O+ ions are uniformly precipitated between 50° and 70° magnetic latitudes, the nonthermal oxygen atoms become dominant over the ambient at about 700 km on the dayside and at about 600 km on the nightside at 60° magnetic latitude. Globally, a high number density atomic oxygen corona appears between 50° and 70° magnetic latitudes above the exobase, and the maximum is located at a few degrees poleward from the center of the band (i.e., 63° magnetic latitude).
Joint two‐dimensional observations of ground magnetic and ionospheric electric fields associated with auroral zone currents 1. Three‐dimensional current flows associated with a substorm‐intensified eastward electrojet
Authors: Baumjohann W. , Untiedt J. , Greenwald R.A.
Published: February 2008

Two‐dimensional distributions of ground magnetic and ionospheric electric fields in the evening sector auroral oval have been simultaneously observed by the Scandinavian Magnetometer Array and the Scandinavian Twin Auroral Radar Experiment (Stare) radars, respectively, on February 15, 1977. They were associated with varying, substorm‐intensified, eastward electrojet current flows. Using both data sets, we have derived three‐dimensional models for the current systems of the western, middle, and eastern segment of the eastward electrojet. We conclude that the substorm‐intensified eastward electrojet was a nearly pure Hall current driven by northward electric fields. The observed eastward increase of the current in the western segment of the electrojet was due to a gradual enhancement of the Hall conductivity. Here, the electrojet was fed by a broad sheet of net downward field‐aligned current. During one period, the eastern‐terminating part of the eastward electrojet diverged up the field lines in a rather local area because of a strong longitudinal decrease in the northward‐directed electric field. On another occasion, it diverged northward within the ionosphere and joined the westward‐flowing current because of a rotation of the northward electric field with increasing latitude through west‐ to southward. These two observed mechanisms of current divergence in the region where eastward and westward electrojets coexist may shed some new light on the controversy over the existence of upward field‐aligned current flow in the Harang discontinuity.
Dynamics and spatial scale of auroral absorption spikes associated with the substorm expansion phase
Authors: Nielsen E.
Published: February 2008

The dynamics and spatial scale of auroral absorption events, named ‘spikes,’ have been investigated with a new multi‐narrow‐beam riometer experiment. Measurements of the components of the ionospheric electric field perpendicular to the geomagnetic field at the time of a spike were made with the auroral radar system Stare. An absorption spike is caused by a ribbon‐shaped enhancement in the ionospheric electron density causing maximum absorption at an altitude near 90 km. The velocity of a ribbon is generally poleward directed and of magnitude between 300 and 3000 m/s. The half width at half maximum and the length of a ribbon are characterized by dimensions of 10 to 50 km and >400 km, respectively. The electric field within a ribbon is so weak that its poleward motion cannot be explaned as E × B drift in the ionosphere. Equatorward of the ribbon the meridional component of the electric field points equatorward. The height of maximum absorption suggests an effective mean energy of the precipitating electrons of about 30 keV. Finally, we find that these events typically occurred during dusk to midnight in magnetic local time. Events which occurred at early local times were associated with high values of Kp.
A macroscopic profile of the typical quasi‐perpendicular bow shock: Isee 1 and 2
Authors: Greensadt E. W. , Russell C.T. , Gosling J.T. , Bame S.J. , Paschmann G. , Parks G.K. , Anderson K.A. , Scarf F.L. , Anderson R.R. , Gurnet D. A. , Lin R.P. , Lin C.S. , Rème H.
Published: February 2008

Field and particle properties of the bow shock recorded on November 5, 1977, identified earlier as stable features by comparison of Isee 1 and 2 data, are examined macroscopically. The extent to which the shock was in a ‘typical’ state is described, and multidiagnostic observations are combined to define the shock profile, which, in this known case, is reminiscent of composite portraits assembled from earlier studies. A foot, a principal wave front gradient, and a postfront overshoot characterized the magnetic field. Various particle measurements revealed corresponding structural elements, including a bimodal ion energy distribution whose two peaks were at roughly the solar wind streaming energy and 2 to 3 times the streaming energy. The electrons were thermalized before the principal sharp magnetic gradient; the ions were partially randomized at the gradient, but the ion thermalization process was not completed until long after the gradient was crossed. The second peak of the ion distribution represented a separate group of reflected particles upstream of the main magnetic gradient that retained its identity for an appreciable distance behind the front before being merged by thermalization into the general distribution. This separate group included a northward proton flux in front of, but not immediately behind, the front. The group ultimately became thermalized through a series of very regular oscillations. The decrease in solar wind bulk velocity at the front exceeded the sum of that calculated from an idealized first‐order model of the rise in potential opposing the flow, plus the proton heating, leaving proton reflection and wave radiation and dispersion as candidates to account for the discrepancy. Plasma wave activity, at ion acoustic frequencies, was most intense in the foot and at the front and appeared to be enhanced at the outer boundary of the reflected particles.
The ultraviolet dayglow 1. Far UV emissions of N and N2
Authors: Meier R.R. , Strickland D.J. , Feldman P.D. , Gentieu E.P.
Published: February 2008

Far ultraviolet rocket spectra of N I and N2 dayglow emissions have been analyzed by using AE‐E photoelectron spectra, laboratory‐measured excitation cross sections, and photochemical models of atomic nitrogen. A self‐consistent picture of both optically thick and thin emission features is found by using a model in which the principal production mechanism for N I 1200‐Å and 1493‐Å photons is photodissociative excitation of N2. The aeronomic data require that ∼50–70% of the excited 4P atoms produced dissociatively have velocities within the Doppler core of the ambient nitrogen atoms, contrary to the expectation that those atoms are produced with large excess kinetic energy. The derived atomic nitrogen density has a maximum density of 2.7 × 107 cm−3 at 170 km, a value that is within 40% of that from recent models of odd nitrogen photochemistry.
IMF sector behavior deduced from geomagnetic data
Authors: Matsushita S. , Trotter D.E.
Published: February 2008

Interplanetary magnetic field (IMF) sector structures, such as ‘toward’ the sun and ‘away’ from the sun on each day, have been objectively estimated from daily and monthly mean values of the horizontal component of the geomagnetic variation field at Godhavn during the period 1926–1970. The agreement between this estimation and actual satellite observations of the sector structures of the interval 1964‐1970 is 88, 79, and 58% in summer, equinox, and winter, respectively. A remarkable agreement (more than 95%) is obtained for the summers of 1964, 1969, and 1970. Various types of IMF sector behavior are examined by taking this seasonal factor into consideration. Approximately 27‐day recurrences of the same structure are often found, and 5‐ to 14‐day consecutive occurrences of the same sector are frequently noted. Furthermore, the total number of occurrences for each estimated sector in each year shows an apparently good correlation with smoothed sunspot numbers and geomagnetic aa index. After a brief introduction of the production mechanism of sector effects on polar geomagnetic fields the limitations and merits of IMF sector inference from geomagnetic data are emphasized.
Rocket observations at the northern edge of the eastward electrojet
Authors: Cahill Laurence J. , Arnoldy R.L. , Taylor William W.L.
Published: February 2008

A Nike‐Tomahawk rocket was launched north over two quiet, late evening auroral arcs in March 1975. A northward magnetic disturbance was observed on the ground under the rocket trajectory. South of the arcs the northward electric field was 60 mV/m, indicating strong westward plasma flow. An eastward electrojet current layer was penetrated in the upward flight. Precipitating electrons were observed over each arc. The electric field decreased to below 20 mV/m over each arc and recovered to 50 mV/m between the arcs. Using the observed electron flux and a model of the ionosphere, the Hall and Pedersen conductivities were calculated. These conductivities were used, with the observed electric field, to calculate the eastward and northward components of the horizontal ionospheric currents. The eastward current calculated south of the first arc agreed well with the observed eastward electrojet current. The power dissipated by the Pedersen current Σp|E|² was also calculated and compared with the power carried by the precipitating electrons. The Joule power decreased abruptly in the auroral arcs, as the precipitating electron power increased. The total dissipated power was approximately the same inside the arcs, between them, and south of the luminosity. North of the aurora the electric field and dissipated power remained low. Field‐aligned currents carried by the observed electrons were about a factor of 3 lower than those inferred from the magnetic field measurements. Likewise, current continuity arguments to keep the auroral current system divergence free required a larger field‐aligned current than that obtained from the particle detectors.
Normals of non‐WKB Alfvén waves in the solar wind
Authors: Heinemann M.
Published: February 2008

A theoretical treatment of wave normals of monochromatic non‐WKB Alfvén waves in the solar wind is developed. The method is based on the one‐fluid ideal MHD equations governing small‐amplitude, undamped, poloidal Alfvén waves propagating in the solar equatorial plane of a Weber and Davis spiral field. Numerical results for waves propagating from the sun to 1 AU are presented. The purpose of the calculations is to determine whether the inclusion of non‐WKB effects can significantly alter the geometrical optics result of Völk and Alpers that the wave normals tend to align themselves with the radial direction as the waves propagate through the solar wind. The conclusion is that there are differences, depending on the wave period, but that they are relatively small: the wave normals at 1 AU of waves of any period which have evolved from a reasonable distribution of normals in the corona are confined to within about 20° from the radial direction. The implication is that any lack of radial alignment is unlikely to be explained by appeal to finite wavelength effects.
On the morphology of auroral zone radio wave scintillation
Authors: Rino C.L. , Matthews S.J.
Published: February 2008

This paper describes the morphology of midnight sector and morning sector auroral zone scintillation observations made over a two‐year period using the Wideband satellite, which is in a sun‐synchronous, low‐altitude orbit. No definitive seasonal variation was found. The nighttime data showed the highest scintillation occurrence levels, but significant amounts of morning scintillation were observed. For the most part the scintillation activity followed the general pattern of local magnetic activity. The most prominent feature in the nighttime data is a localized amplitude and phase scintillation enhancement at the point where the propagation vector lies within an L shell. A geometrical effect due to a dynamic slab of sheetlike structures in the F region is hypothesized as the source of this enhancement. The data have been sorted by magnetic activity, proximity to local midnight, and season. The general features of the data are in agreement with the accepted morphology of auroral zone scintillation.
The occurrence rate, polarization character, and intensity of broadband Jovian kilometric radiation
Authors: Desch M.D. , Kaiser M.L.
Published: February 2008

We describe the major observational features of one new component of Jupiter's radio emission spectrum, the broadband kilometer wavelength radiation, or bKOM. This study, using the Voyager Planetary Radio Astronomy (PRA) experiments, reveals that the overall occurrence morphology, dynamic spectra, and polarization character of bKOM are strong functions of the latitude and/or local time geometry of the observations. The postencounter data show a decline in the mean occurrence rates and power level of bKOM and, in particular, a depletion in the occurrence rate at those same longitudes where the detection rate is a maximum before encounter. Additionally, the polarization sense undergoes a permanent reversal in sign after encounter, whereas the time‐averaged wave axial ratio and degree of polarization remain relatively unchanged. Finally, no evidence of any control by Io is found. The strong dependence of the morphology on local time suggests a source whose beam is nearly fixed relative to the Jupiter‐sun line.
Airborne studies of equatorial F layer ionospheric irregularities
Authors: Weber E.J. , Buchau J. , Moore J.G.
Published: February 2008

Radio wave and optical experiments were conducted onboard a U.S. Air Force research aircraft in March 1977 and March 1978 at low magnetic latitudes to investigate the effects of F region electron density irregularities on transionospheric communications links. Imaging photometer, ionosonde, and satellite amplitude scintillation measurements were used to monitor the development and motion of F region 6300‐Å O I airglow depletions, spread F, and scintillation producing irregularities that are all associated with low‐density bubbles in the postsunset equatorial ionosphere. The 6300‐Å airglow depletions are the bottomside signature of low plasma density within the bubbles. Examples of multiple airglow depletions and their relation to variations in the F layer virtual height (h′F) and to the occurrence of amplitude scintillations on 250‐MHz satellite signals are described. Estimates of the average bottomside electron density, from simultaneous ionosonde measurements and 6300‐Å airglow intensities, show electron density decreases of ∼66% within the bubbles. These decreases are approximately the same for bubbles observed at the magnetic equator and near Ascension Island (18°S magnetic latitude). The measurements at Ascension Island show that airglow depletions extend away from the magnetic equator into the southern 6300‐Å intertropical arc. Variations in the maximum poleward extent of airglow depletions and of associated ionospheric irregularities that give rise to amplitude scintillations were observed. These latitudinal variations are interpreted, using field line mapping considerations, as variations in the maximum altitude of plasma bubbles over the magnetic equator. A north‐south flight confirms that the overall pattern of airglow depletions and associated ionospheric irregularities extends continuously across the magnetic equator to ±15° magnetic latitude.
Negative ion chemistry in the terrestrial D region and signal flow graph theory
Authors: Wisemberg J. , Kockarts G.
Published: February 2008

Aeronomic systems such as the negative ion chemistry in the D region are characterized by numerous reactions involving positive, negative, and neutral species. Classical solutions of such systems do not necessarily point out the role played by each parameter. When the chemical system is considered as a graph, it is possible to apply the signal flow graph technique, which allows a quantitative evaluation of the various paths and loops. A negative ion model is obtained for daytime conditions by using the signal flow graph technique, which is briefly described. This reference model is analyzed in terms of transmittances, path gains, and loop gains. An arbitrary variation of atomic oxygen and nitric oxide is introduced at a height of 65 km in order to show the induced effects on the negative ions and on the electron concentrations. Variations of the ratio λ between negative ions and electrons are also presented.
Conductive solar wind models in rapidly diverging flow geometries
Authors: Holzer Thomas E. , Leer Egil
Published: February 2008

A detailed parameter study of conductive models of the solar wind has been carried out, extending the previous similar studies of Durney (1972) and Durney and Hundhausen (1974) by considering collisionless inhibition of thermal conduction, rapidly diverging flow geometries, and the structure of solutions for the entire n0‐T0 plane (n0 and T0 are the coronal base density and temperature). Primary emphasis is placed on understanding the complex effects of the physical processes operative in conductive solar wind models. There are five points of particular interest that have arisen from the study: (1) neither collisionless inhibition of thermal conduction nor rapidly diverging flow geometries can significantly increase the solar wind speed at 1 AU; (2) there exists a firm upper limit on the coronal base temperature consistent with observed values of the coronal base pressure and solar wind mass flux density; (3) the principal effect of rapidly diverging flow geometries is a decrease in the solar wind mass flux density at 1 AU and an increase in the mass flux density at the coronal base; (4) collisionless inhibition of thermal conduction can lead to a solar wind flow speed that either increases or decreases with increasing coronal base density (n0) and temperature (T0), depending on the region of the n0‐T0 plane considered; and (5) there is a region of the n0‐T0 plane at high coronal base densities where low‐speed, high‐mass‐flux, transonic solar wind flows exist—a region not previously considered.
Spatial inhomogeneity of magnetosheath proton precipitation along the dayside cusp from the Arcad Experiment
Authors: Sauvaud J.A. , Galperin Yu.I. , Gladyshev V.A. , Kuzmin A.K. , Muliarchick T.M. , Crasnier J.
Published: February 2008

Soft proton (0.5–30 keV) measurements at the dayside auroral oval obtained aboard the Aureole 1 satellite are analyzed in more detail than in previous publications. Our approach in the analysis of these data is based on the distinction between soft ‘magnetosheathlike’ proton (ion) spectra (with a spectral ratio α of the differential flux at 0.5 keV to that at 5.0 keV higher than 100) and much harder ‘plasmasheetlike’ spectra (with α <100, and an average ion energy of about 5–10 keV at noon). As was shown earlier from the data of the Arcad 1 experiment aboard the Aureole 1 satellite, the plasmasheetlike proton energy spectra are regularly observed in a continuous band along the auroral oval and constitute the so‐called proton aurora, its ions presumably drifting around the earth to the dayside from the nightside plasmasheet. The dayside polar cusp crossing is identified in the data analysis as the appearance of soft proton (ion) energy spectra with α ≥100. From this approach the data are found to be consistent with the concept of the dayside cusp as a nonstationary, nonhomogeneous, AE‐dependent spot (or spots) of enhanced magnetosheathlike proton precipitation observed with the highest fluxes when its position occurs close to the noon magnetic meridian. Observational data are consistent with typical spot dimensions of the order of 200 × 500 km. This spot is considered to be an ionospheric projection of the localized and possibly time‐varying plasma injection from the magnetosheath into the magnetosphere along a newly merged magnetospheric field tube. These results confirm the earlier work based on the Arcad experiment which led to the inference of a ‘patchy’ intensity pattern of soft proton precipitation in the dayside cusp; they disagree with the concept of a free penetration of magnetosheath plasma through an ever present cleftlike magnetic field branching structure extending from the dawnside to the duskside of the magnetosphere through noon, at least during quiet magnetic conditions. Independently obtained data from other satellites are briefly summarized, and general consistency is noted with the Aureole 1 results.
Pioneer 11 Saturn encounter
Authors: Northrop T. G. , Opp A. G. , Wolfe J. H.
Published: February 2008

Pioneer 11, later named Pioneer Saturn, reached its point of closest approach to Saturn on September 1, 1979. Pioneer Saturn carried 11 operating scientific instruments. The 2.293‐GHz telemetry carrier was also used for a radio occultation experiment and a celestial mechanics experiment. The Pioneer Saturn scientific instruments, objectives, and principal investigators are listed in Table 1.

Pioneer Saturn was launched April 5, 1973. The objectives of Pioneer Saturn and its companion spacecraft, Pioneer 10, were to penetrate the asteroid belt and to explore the environment of Jupiter. Following the successful encounter with Jupiter by Pioneer 10, Pioneer Saturn was targeted to encounter Jupiter on a trajectory that propelled the spacecraft back across the solar system to be the first spacecraft ever to encounter Saturn, nearly 5 years later.
The magnetic field of Saturn: Further studies of the Pioneer 11 observations
Authors: Acuña M. H. , Ness N. F. , Connerney J. E. P.
Published: February 2008

Analysis of magnetic field observations by the Goddard Space Flight Center high‐field flux gate magnetometer on the Pioneer 11 spacecraft during Saturn encounter yields estimates of the planetary field. The field is mainly dipolar but rather weaker than expected, with a moment equal to 0.20 G Rs³ or 4.3 × 1028 G cm³, opposite in polarity to earth's. Surprisingly, the field appears to be axially symmetric but with a small (0.04 Rs) offset to the north so that N (S) polar field intensities are 0.6 (0.4) G, respectively. The deduced polar offset appears not to be an artifact of the limited spatial extent of the observations or the presence of fields of external origin. The average stand‐off distance of the magnetopause is expected to be 20 RS, i.e., at the orbit of Titan, so that this largest of solar system satellites is immersed not only in the Saturnian magnetosphere but also at times in its magnetosheath and sometimes even in the interplanetary medium.
Plasmas in Saturn's magnetosphere
Authors: Frank L. A. , Burek B. G. , Ackerson K. L. , Wolfe J. H. , Mihalov J. D.
Published: February 2008

The solar wind plasma analyzer on board Pioneer 11 provides first observations of low‐energy positive ions in the magnetosphere of Saturn. Measurable intensities of ions within the energy per unit charge (E/Q) range 100 eV to 8 keV are present over the planetocentric radial distance range ∼4–16 Rs in the dayside magnetosphere. The plasmas are found to be rigidly corotating with the planet out to distances of at least 10 Rs. At radial distances beyond 10 Rs, the bulk flows appear to be in the corotation direction but with lesser speeds than those expected from rigid corotation. At radial distances beyond the orbit of Rhea at 8.8 Rs, the dominant ions are most likely protons, and the corresponding typical densities and temperatures are 0.5 cm−3 and 106 °K, respectively, with substantial fluctuations. Identification of the mass per unit charge (M/Q) of the dominant ion species is possible in certain regions of Saturn’s magnetosphere via the angular distributions of positive ions. A large torus of oxygen ions is located inside the orbit of Rhea, and the densities are >10 cm−3 over the radial distance range ∼4–7.5 Rs. Density maxima appear at the orbits of Dione and Tethys where oxygen ion densities are ∼50 cm−3. The dominant oxygen charge states are O2+ and O3+ in the radial distance ranges ∼4–7 Rs and 7–8 Rs, respectively. The observations are suggestive of a decrease of ion energies to values less than the instrument energy threshold of E/Q=100 eV at the apparent inward edge of the torus at 4 Rs. Ion temperatures increase rapidly from ∼2 × 105 °K at 4 Rs to 5 × 106 °K at 7.3 Rs. It is concluded that the most likely source of these plasmas is the photodissociation of water frost on the surface of the ring material with subsequent ionization of the products and radially outward diffusion. The sources associated with the satellites Dione and Tethys are probably of lesser strength. The presence of this plasma torus is expected to have a large influence on the dynamics of Saturn's magnetosphere, since the pressure ratio β of these plasmas approaches unity at radial distances as close to the planet as 6.5 Rs. On the basis of these observational evidences it is anticipated that quasi‐periodic outward flows of plasma, accompanied by a reconfiguration of the magnetosphere beyond ∼6.5 Rs, will occur in the local night sector in order to relieve the plasma pressure from accretion of plasma from the rings.
Charged particle diffusion and acceleration in Saturn's radiation belts
Authors: McKibben R. B. , Simpson J. A.
Published: February 2008

We use observations of high‐energy (⪞1 MeV) protons and electrons in the core of Saturn's magnetosphere (R ⪝4 Rs) to investigate the hypothesis that the trapped radiation is accelerated and maintained by inward diffusion in violation of the third adiabatic invariant. When the intensity profiles are converted to density of particles in phase space at constant magnetic moment as a function of magnetic shell parameter L, we find that the phase space density increases inward both in narrow regions associated with satellite absorption and, more generally, over extended regions between absorption features for L ⪝4. Such behavior is inconsistent with inward diffusion as a model for population of the radiation zone and has led to the suggestion that there may be internal sources of high‐energy particles, such as cosmic ray albedo neutron decay for protons. Calculations have shown, however, that Crand is too weak to account for observed intensities of >35‐MeV protons (Cooper and Simpson, this issue), and no source has been suggested for high‐energy electrons. In our analysis we employ Monte Carlo simulations to examine the assumptions concerning particle motion implicit in a diffusion model. We find from our simulations that the regions of negative phase space density associated with satellite absorption features may be explained if diffusion proceeds in an episodic manner, via brief periods of enhanced radial motion separated by longer periods when little radial motion takes place. We further show that the general density increase inward is not inconsistent with particle propagation by a random walk process between an external source and an inner absorbing boundary if the ‘diffusion coefficient’ decreases inward from the source. Specific conditions for the particle random walk are found which determine whether the density increases or decreases inward, and it is argued that for episodic diffusion the density must increase inward. We argue that episodic diffusion, though not proven as a model, reproduces major features of the density profiles of both protons and electrons in Saturn's magnetosphere for which standard diffusion theory is unable to account.
Very energetic protons in Saturn's radiation belt
Authors: Fillius Walker , McIlwain Carl
Published: February 2008

Very energetic protons are trapped in the inner Saturnian radiation belt. The University of California at San Diego instrument on Pioneer 11 has definitely identified protons of energy >80 MeV on channel M3 and has tentatively detected protons of energy >600 MeV on channel C3. The spatial distribution of the protons is distinct from that of the trapped electrons, the main difference being that the protons are strongly absorbed by the innermost moons and that the electrons are not. We have estimated the source strength for injecting protons by the decay of cosmic ray albedo neutrons generated in the rings of Saturn. The required proton lifetime is ∼20 years.
Abundance ratios of 4He++/³He++ in the solar wind
Authors: Ogilvie K. W. , Coplan M. A. , Bochsler P. , Geiss J.
Published: February 2008

The ion composition experiment aboard the Isee 3 spacecraft has been used to measure the 4He++/³He++ ratio in the solar wind. Using 4334 observations made between August 18 and November 24, 1978, and March 24 and August 25, 1979, the average ratio 4He++/³He++ is found to be 2.1±0.2 × 10³, in agreement with previous foil measurements. Individual observations vary widely from the average and the low 4He++/³He++ ratios occur at times of low 4He++ flux.
Time sequence analysis of flickering auroras, 1. Application of Fourier analysis
Authors: Berkey F. T. , Silevitch M. B. , Parsons N. R.
Published: February 2008

Using a technique that enables one to digitize the brightness of auroral displays from individual fields of a video signal, we have analyzed the frequency content of flickering aurora. Through the application of Fourier analysis to our data, we have found that flickering aurora contains a wide range of enhanced frequencies, although the dominant frequency enhancement generally occurs in the range 6–12 Hz. Each incidence of flickering that we observed was associated with increased radio wave absorption. Furthermore, we have found that flickering occurs in bright auroral surges, the occurrence of which is not limited to the ‘breakup’ phase of auroral substorms. Our results are interpreted in terms of a recently proposed theory of fluctuating double layers that accounts for a number of the observational features.
The dynamic behavior of the Venus ionosphere in response to solar wind interactions
Authors: Brace L. H. , Theis R. F. , Hoegy W. R. , Wolfe J. H. , Mihalov J. D. , Russell C. T. , Elphic R. C. , Nagy A. F.
Published: February 2008

Measurements of electron density and temperature by the Pioneer Venus orbiter electron temperature probe (OETP) are used to describe the dynamic behavior of the Venus ionosphere and to begin to relate this complex behavior to variations in the solar wind and the ionosheath magnetic field, parameters that are also measured by orbiter instruments. The average ionopause height rises from about 330 km at the subsolar point to 700 km at the dusk terminator and 1000 km at the dawn terminator, in both cases exhibiting a stronger dependence upon solar zenith angle than that reported from Venera 9 and 10 occultation data. The ionopause on the dayside tends to expand and contract with changes in solar wind pressure, becoming asymptotic to about 290 km at pressures above 4 × 10−8 dyn/cm² and rising to over 1000 km for pressures below 5 × 10−9 dyn/cm². The solar wind pressure, after correction for solar zenith angle, agrees approximately with the magnetic field pressure applied at the ionopause, confirming earlier suggestions that the pressure is conveyed to the ionosphere primarily by the magnetic field rather than by the shocked solar wind plasma. On the nightside the ionopause is much more highly variable, sometimes falling below 200 km or rising above 3500 km. The present Pioneer Venus orbit does not permit the true configuration to be measured. Within the nightside ionosphere itself, we find extreme spatial irregularities in the form of holes, horizontally stratified layers, detached plasma clouds, and dual temperature plasma in regions of low electron density. A scenario is developed to describe the process of ion pickup on the dayside in terms of solar wind pressure discontinuities inducing wavelike structure at the ionopause, which then is penetrated by ionosheath plasma and magnetic fields that remove ionospheric plasma impulsively in the form of detached plasma clouds. The energy released in this process may be responsible for the elevated electron temperatures observed in both the dayside and nightside of the Venus ionosphere.