bluuebunny/crossref_metadata_2025 huggingface dataset:
Pitch angle distributions of geomagnetically trapped MeV helium ions during quiet times
Authors: Fritz Theodore A. , Spjeldvik Walther N.
Published: February 2008

During geomagnetically quiet conditions energetic radiation belt helium ion fluxes at MeV energies have been found to exhibit characteristic radial profiles and large pitch angle anisotropies. Compiling data from many experiments we deduce a systematic dependence of this anisotropy with helium ion energy. When the observed pitch angle distributions are approximated by j(α0) = j⊥ sinn α0, α0 being the equatorial pitch angle, an empirical relation n = 7 + 9.1 log10(E), with the helium ion energy E, in MeV, is deduced over the energy range 0.59–9 MeV total ion energy (148–2250 keV per nucleon). These values are obtained for L shells in the range L ∼ 2 to 5, and possible radial variations of n are averaged over. The results are compared with theoretical expectations, and we suggest a qualitative explanation for the observed trend.
Measurement of the rugged invariants of magnetohydrodynamic turbulence in the solar wind
Authors: Matthaeus William H. , Goldstein Melvyn L.
Published: February 2008

Measurements of the total energy, cross helicity, and magnetic helicity of the solar wind at 1, 2.8, and 5 AU are presented. These quantities are the three rugged invariants of three‐dimensional ideal incompressible MHD turbulence theory. The theoretical technique for measuring the magnetic helicity from the matrix of two‐point correlations is shown. The length scales characterizing the magnetic helicity are found to be equal to or greater than those which characterize the magnetic energy. The magnetic helicity typically lies at scales larger than the magnetic correlation length, consistent with the expectations of the inverse cascade and selective decay hypotheses of three‐dimensional MHD turbulence. At smaller scales, the magnetic helicity oscillates in sign. Our measurements of the cross helicity are not fully consistent with the usual interpretation in terms of outward propagating Alfvénic functuations. Especially during the interval at 5 AU the cross helicity is found to oscillate in sign indicating fluctuations propagating both outward and inward.
Observations of structuring in the downstream region of a large spherical model in a laboratory simulated solar wind plasma
Authors: Intriligator D. S. , Steele G. R.
Published: February 2008

Laboratory experiments have been performed that show the effects of inserting a spherical conducting model, large in comparison with the Debye length, into a free streaming high‐energy (1 kV) unmagnetized hydrogen plasma. These experiments are the first laboratory measurements at energies and compositions directly relevant to solar wind and astrophysical plasma phenomena. The incident plasma parameters were held constant. Transverse profiles of the net Langmuir probe current, plotted at various locations downstream in the model wake, are divided into three regions—the ‘shadow,’ the ‘transition,’ and the ‘boundary.’ The following new results are obtained: (1) enhancements in the ‘shadow’ exist at downstream locations where the Mach ratio is less than one; (2) turbulence exists in the ‘transition’ region on the shadow edges and outside in the ‘boundary’ region. These results appear to be attributable to the use of a high‐energy plasma for these studies. A small current enhancement is also present in the boundary and is attributable to the plasma/model interaction, in close agreement with low‐energy plasma/model interaction experiments. We speculate that many similar features observed by in situ spacecraft downstream from planetary bodies are relatively permanent and are due to the intrinsic nature of the interaction between the solar wind plasma and the obstacle rather than to transitory effects caused by the presence of discontinuities or other transitory characteristics associated with the flow.
Radio interferometric detection of a traveling ionospheric disturbance excited by the explosion of Mount St. Helens
Authors: Roberts D. H. , Rogers A. E. E. , Allen B. R. , Bennett C. L. , Burke B. F. , Greenfield P. E. , Lawrence C. R. , Clark T. A.
Published: February 2008

A large‐amplitude traveling ionospheric disturbance (TID) was detected over Owens Valley, California, on May 18, 1980, by a highly sensitive very long baseline interferometry (VLBI) radio astronomy experiment. This TID is interpreted as the response of the ionosphere to a gravity wave excited in the neutral atmosphere by the explosion of Mount St. Helens that took place at 1532 UT on that day. A model, invoking the point‐excitation of internal gravity waves in an isothermal atmosphere, which fits observations of the TID at several other stations, leads to identification of the features observed in the VLBI data. Small‐amplitude higher‐frequency changes in the ionosphere were detected for several hours after the passage of the large‐amplitude Mount St. Helens TID, but it is not clear whether these were excited by the passage of the gravity wave or were background fluctuations.
An electric noise component with density f−1 identified on ISEE 3
Authors: Hoang S. , Steinberg J. L. , Couturier P. , Feldman W. C.
Published: February 2008

An electric noise component with an f−1 spectrum is observed with the SBH radioastronomy receivers on ISEE 3 at frequencies lower than the plasma frequency fp. On the Z antenna (electrical length for long waves is 7 m) this component is 5–10 times more intense than the predicted thermal noise level. Its spectral density is proportional to fpf−1(Tc)1/2, where Tc is the core electron temperature and f is the observing frequency. On the S antenna (90 m tip to tip) the new component is much weaker and most probably represents the high‐frequency part of a noise spectrum found by Kellogg (1981) with an antenna of the same length. This author interpreted it as mostly due to electron acoustic waves and Doppler shifted ion acoustic waves, but this interpretation has not been confirmed by more accurate calculations (Couturier et al., 1982). Kellogg's spectrum also shows an f−1 frequency dependence and, if extrapolated assuming the same law as for the Z antenna, approximately fits our S antenna observations. The S antenna f−1 noise is deeply spin modulated with the minimum electric field in the direction of the solar wind flow as seen in the spacecraft frame of reference. The modulation factor decreases with increasing frequency, becomes negligible when the new component intensity becomes negligible as compared with thermal noise, and increases with the solar wind velocity. The f−1 component shows some of the properties which are expected from shot noise (direction of minimum intensity) but its spectral index is −1 while shot noise is supposed to show a spectral index −2.
Active‐region evolution and solar rotation variations in solar UV irradiance, total solar irradiance, and soft X rays
Authors: Donnelly R. F. , Heath D. F. , Lean J. L.
Published: February 2008

Variations in the solar UV spectral irradiance, the total solar irradiance, and solar soft X ray emission over days and weeks are analyzed by using concurrent measurements from the NIMBUS 7 and GOES satellites. The UV variations at wavelengths (160–400 nm) of interest to ozone photochemistry and the physics of the middle atmosphere are emphasized. The nonflare variations studied are caused by two types of processes: (1) the birth, growth, peak, and decay of solar active regions and (2) solar rotation, for example, when portions of active regions are occulted near the solar limb. The observations are interpreted by using simple empirical models that relate ground‐based observations of the size and location of sunspots and plages to the full‐disk temporal variations. The major dips observed in the total solar irradiance on August 20–23 and November 6–10, 1979, and April 8, May 25, and September 1, 1980, are accompanied by only average UV and X ray enhancements. The models show that the active regions associated with the sunspots responsible for the major dips in the total solar irradiance caused strong but not outstanding emission enhancements at UV and X ray wavelengths. The solar rotation variations are studied as a function of the central meridian distance of solar active regions where the central meridian distance width at half the maximum is narrowest for decreases in the total solar irradiance (∼83°), slightly wider and also a function of wavelength for UV enhancements (∼95° at 198 nm), wider still for the 10.7 cm radio flux (∼150°), and extremely wide for soft X rays (≳205°).
The hydromagnetic oscillation of individual shells of the geomagnetic field
Authors: Poulter E. M. , Nielsen E.
Published: February 2008

Ground‐based magnetometer observations of Pc 5 micropulsation periods suggest these events are resonant oscillations of geomagnetic field lines. Although most individual events are essentially monochromatic, the average periods increase with latitude and hence field line length. Walker et al. (1979) have use STARE (Scandinavian twin auroral radar experiment) radar observations of ionospheric electric fields in support of the monochromatic resonance theory. Transient Pc 5 events whose period increases with latitude have also been observed by STARE. These events have the characteristics of toroidal eigenmode oscillations of individual shells of the geomagnetic field. The period variation with latitude is also consistent with that derived from averaged periods and time of flight calculations.
The semiannual oscillation in the thermosphere as a conduction mode
Authors: Walterscheid R. L.
Published: February 2008

The substantially semiannual character of the density variation and its rather weak latitudinal dependence are puzzling features of the seasonal variation of density in the thermosphere. These features are puzzling because the annual harmonic of the seasonal variation of thermospheric heating is considerably greater than the semiannual and because the heating for both harmonics is predominately latitude dependent. A model of the globally uniform component of the semiannual oscillation in thermospheric density is developed. This component is a conduction mode of oscillation modified by thermal damping. It is shown that this oscillation is essentially driven by Joule heating at high latitude. Model calculations give results which agree well with observations. It is suggested that the observed prominance of the semiannual oscillation is a consequence, primarily, of (1) the stronger forcing for the semiannual conduction mode over the annual and (2) the suppression of the latitudinally dependent components by seasonal variations in composition.
Magnetic flux ropes in the Venus ionosphere: Observations and models
Authors: Elphic R. C. , Russell C. T.
Published: February 2008

In this paper we examine the detailed magnetic structure of ropes and quantitatively compare it with a general flux rope model. First, the seemingly chaotic ionospheric magnetic signature is shown to be ordered in a local coordinate system unique to each rope. This signature is also consistent with that expected of a flux rope. We have selected a special subset of the flux rope data that provides detailed information on rope magnetic structure. This subset, referred to as the small impact parameter subset, consists of flux rope traversals on which the spacecraft passed through the center of the structure. We have formulated a model of flux rope structure and have fit it successfully to cases from the small impact parameter subset. From these modeled cases we have inferred that flux ropes are comprised of primarily field‐aligned currents with maximum current densities of a few tens of microamps/m² and that parallel electric fields of less than a microvolt/m can drive these currents given the background ionospheric conductivity. We have also found that Joule heating rates within flux ropes are much smaller than ambient photoelectron heating. These modeled cases also suggest that flux ropes are stable to pinch‐related instabilities like the sausage and planar kink modes, but are unstable to the long‐wavelength helical kink mode.
Observations of upstream ions and low‐frequency waves on ISEE 3
Authors: Sanderson T. R. , Reinhard R. , Wenzel K.‐P. , Roelof E. C. , Smith E. J.
Published: February 2008

We present three examples of correlated observations of 35‐ to 1600‐keV upstream ions and low‐frequency waves measured on ISEE 3, located between ∼76 and ∼200 RE from the earth's bow shock. The observations show that when no upstream waves are present, the particle anisotropies are beamlike, aligned along the magnetic field after transformation into the
drift velocity reference frame. When waves are present, the pitch angle distributions are broadened, the amount of broadening being correlated with the amount of wave activity. These waves are transverse, circularly polarized, and have a period of ∼30 s. From our observations we conclude that during long‐lasting events an equilibrium is reached where the region between the spacecraft and the bow shock contains a significant amount of low‐frequency wave activity, together with significant pitch angle scattering. The amount of scattering decreases with upstream distance, ranging from strong scattering at the bow shock to weak scattering at ISEE 3.
Auroral hiss, Z mode radiation, and auroral kilometric radiation in the polar magnetosphere: DE 1 observations
Authors: Gurnett D. A. , Shawhan S. D. , Shaw R. R.
Published: February 2008

The polar‐orbiting DE 1 spacecraft is now providing the first measurements of high‐latitude auroral phenomena in the radial distance range between about 2 and 5 RE, where several important types of auroral plasma wave emissions are believed to be generated. This paper describes the initial observations from the DE 1 plasma wave instrument in this interesting region. Three principal types of plasma wave emissions are discussed: auroral hiss, Z mode radiation, and auroral kilometric radiation. Whistler mode auroral hiss emissions are observed on essentially every pass over the auroral zone. The auroral hiss usually has a characteristic ‘funnel‐shaped’ frequency‐time spectrum which can be explained by a simple whistler mode propagation effect if the radiation is emitted from a spatially localized source below the spacecraft. Ray path studies show that the auroral hiss is propagating upward from a source, the lower boundary of which is an altitude of about 0.7–0.9 RE. The DE 1 observations have identified broadband Z mode emissions in the low‐density region over the auroral zone and polar cap. This radiation usually has a sharply defined upper cutoff near the electron gyrofrequency and extends downward in frequency to a cutoff called the fL = 0 cutoff. The Z mode radiation is sometimes difficult to distinguish from the auroral hiss, which occurs in the same general frequency range with about the same intensity. The Z mode radiation probably corresponds to the noise previously identified as ‘continuum radiation’ in the Hawkeye polar region data. The auroral hiss can usually be distinguished from the Z mode radiation by the sharp upper cutoff of the whistler mode at the local electron plasma frequency. Broadband emissions identified as auroral kilometric radiation are frequently observed over the evening auroral regions at frequencies between about 100 kHz and 400 kHz. These emissions are very intense, usually 30–50 dB above the intensity of the auroral hiss and Z mode radiation, and are highly variable, sometimes disappearing completely. The auroral kilometric radiation usually occurs at frequencies above the electron gyrofrequency, consistent with earlier measurements which indicate that this radiation is propagating in the free‐space R‐X mode.
The nonlocal theory of periodic density drift instabilities
Authors: Bernhardt P. A. , Thomsen M. F. , Gary S. Peter
Published: February 2008

The nonlocal theory of electrostatic density drift instabilities is developed for an arbitrary, periodic variation in plasma density. Linear Vlasov theory is applied to Fourier series representations of the plasma density, and the resulting equations are solved numerically. The growth rates of the universal drift instability are compared by using local and nonlocal theories. It is demonstrated that the local theory is inadequate if the nonlocal eigenfunctions of the electrostatic modes have significant amplitude over spatial regions wider than the regions of large plasma density gradients. Nonlocal effects can enhance wave‐particle diffusion of a triangular‐shaped variation and reduce wave‐particle dissipation of a square‐shaped irregularity.
Thermospheric odd nitrogen: 1. NO, N(4S), and O(³P) Densities from rocket measurements of the NO δ and γ bands and the O2 Herzberg I bands
Authors: McCoy Robert P.
Published: February 2008

Airglow emissions from the upper atmosphere were measured with instruments on a sounding rocket launched on November 7, 1979. The nitric oxide delta and gamma bands and the molecular oxygen Herzberg I bands were observed at evening twilight with a pair of ultraviolet spectrometers and a photometer. Nightglow emission of NO and O2 were measured on the upleg. Twilight NO gamma band fluorescence and NO delta band chemiluminescence were measured on the downleg. The altitude profiles of the NO and O2 nightglow emissions were inverted to yield the density of O(³P) between 90 and 120 km and the density of N(4S) between 100 and 200 km. The twilight NO gamma band emission profile was inverted to yield the density of nitric oxide between 112 and 200 km. By viewing in the solar and antisolar directions on the two legs of the flight the measurements provide an indication of the temporal variation of the N(4S) density. These observations provide the first simultaneous measurements of the thermospheric densities of NO, N(4S), and O(³P).
The Great Solar Anomaly ca. 1780–1800: An error in compiling the record?
Authors: Sonett C. P.
Published: February 2008

The view that the sunspot index time sequence is of lessened validity prior to about 1850 is shown to be consistent with an error in the 1780–1800 time period when phase, offset of the Hale carrier sinusoid from zero mean, and the amplitude modulation envelope all show a lack of conformity with respect to the remainder of the record subsequent to the Maunder minimum. This lack of conformity is consistent with an error that delays one or more of the 11‐year cycle minima but recovers phase subsequently. This type of error creates a strongly coherent set of companion changes in offset of the carrier from zero mean and in the amplitude modulation envelope.
Off‐perpendicular propagation of irregularities in middle latitude ES layers
Authors: Tanaka Takashi , Venkateswaran S. V.
Published: February 2008

This paper develops the linear gradient drift instability theory of middle latitude Es‐layer irregularities, taking into account the appropriate field geometry and allowing for the fact that, in addition to a predominant perpendicular component, the propagation vector of an irregularity can have a small but significant component parallel to the magnetic field as well. The theory is used to calculate the growth rates and phase and group velocities of the unstable waves as a function of the off‐perpendicular propagation angle. Group propagation effects are also considered.
Finite parallel wavelengths and ionospheric structuring
Authors: Sperling J. L.
Published: February 2008

Much large‐scale fluid structuring in the ionosphere has been attributed to the flutelike Rayleigh‐Taylor and E×B gradient drift instabilities. The finite extent of the ionosphere and the spatial variation of plasma parameters within the ionosphere suggest that these instabilities can be expected to vary along magnetic field lines. The variations are taken into account by assuming a nonzero component of wave number parallel to the ambient magnetic field. The accompanying electric fields are not purely electrostatic but imply mode magnetic fields that may permit plasma transport across density gradients that are larger than classical cross‐field diffusion. This enhanced diffusion, which is most effective for sufficiently large and tenuous plasma clouds, can limit the minimum size of striations to a larger value than classical considerations alone permit. Finite parallel wave number has the additional effect of allowing ion free energy to be transferred to parallel electron motion and so the Rayleigh‐Taylor and E×B gradient drift instabilities can contribute to structuring at conjugate points along magnetic field lines where electron energy is deposited. Also, the transfer of free energy indicates that long‐term structural persistence requires a continuous source of ion free energy. Some finite parallel wavelength effects, particularly those relating to transport, can be included in present two‐dimensional striation simulations.
Auroral electron interaction with the atmosphere in the presence of conjugate field‐aligned electrostatic potentials
Authors: Prasad Sheo S. , Strickland Douglas J. , Chiu Yam T.
Published: February 2008

Interaction of auroral electrons with the atmosphere in the presence of field aligned electrostatic potentials in conjugate atmospheres has been studied. Both pitch angle and energy distributions of auroral electrons have been obtained for the first time by numerically solving multiangle equation of transfer at all energies in a realistic three component atmosphere. In addition to the generation of the auroral primary electrons, the electric field reflects backscattered electrons at energies below the total potential drop. Higher energy electrons scattered in the upward directions are able to penetrate the electric potential in the ionosphere of their origin and appear in the conjugate hemisphere where their energy is restored by the local electric field. Solutions of the transport equation, taking into account these effects of the electric field in an iterative manner, indicate that the auroral population above the atmosphere has three components: (1) an isotropic low energy component whose flux decreases sharply with energy, (2) a flux with predominantly 90° pitch angle centered near the beam energy, and (3) a predominantly downward flux containing the primary electrons and the electrons from the conjugate hemisphere. The isotropic component was solely due to the reflecting action of the electric field. These results from the theory are in good agreement with available observations.
Observations of LHR noise with banded structure by the Sounding Rocket S29 Barium‐GEOS
Authors: Koskinen Hannu E. J. , Holmgren Gunnar , Kintner Paul M.
Published: February 2008

We report here the measurement of electrostatic noise near the lower hybrid frequency made by the sounding rocket S29 barium‐GEOS. The noise is related to the spin of the rocket and reaches well below the local lower hybrid resonance frequency. Above the altitude of 300 km the noise shows banded structure roughly organized by the hydrogen cyclotron frequency. Simultaneously with the banded structure a signal near the hydrogen cyclotron frequency is detected. This signal is also spin modulated. The character of the noise strongly suggests that it is locally generated by the rocket payload disturbing the plasma. If this interpretation is correct, we expect plasma wave experiments on other spacecrafts to observe similar phenomena.
Neutron high‐energy spectra at seven different depths in the atmosphere from 0 to 40 mbar near the geomagnetic equator
Authors: Bhatt V. L.
Published: February 2008

Ilford G5 emulsions were exposed at 6 altitudes from 4 to 40 mbar at λ = 7.8° N during 1961 to 1966. Atmospheric neutrons ∼8 to 200 MeV are studied by using three different parameters. Rate of production of cosmic ray and evaporation stars at 6 different altitudes are given. Neutron differential energy spectra as well as integrated spectra at 0, 4, 6, 10, and 40 mbar are presented; the 0 mbar spectrum is obtained by extrapolation. Neutron energy spectra for E > 50 MeV using another parameter are given for 7 different altitudes. All spectra show maxima near 40 MeV, and large drops near 22 MeV as well as 100 MeV. Thus all spectra show clear structures having different slopes in the low, medium, and high‐energy range. Our results at λ = 7.8° N are compared with others near λ = 42° N. For the high‐energy range latitude variation factor is in good agreement with theoretical calculations and our neutron energy distributions are in excellent agreement with the experimental results of other workers, confirming all the important features given above. Results are discussed and possible explanations are offered for the important features as well as the actual shape of the neutron energy spectra based on the production spectra of neutrons in the atmosphere.
A two‐dimensional model of the ionosphere of Venus
Authors: Cravens T. E. , Crawford S. L. , Nagy A. F. , Gombosi T. I.
Published: February 2008

A substantial nightside ionosphere has been observed on Venus during most orbits of the Pioneer Venus Orbiter. However, on some orbits during which the solar wind dynamic pressure was large, the nightside ionosphere seems to have almost disappeared, existing only as irregular patches of low‐density plasma. We interpret these observational results using a two‐dimensional theoretical model of the ionosphere of Venus in which empirical horizontal velocities are adopted. We show that the degree to which horizontal transport of ions from day to night can maintain the nightside ionosphere depends on two parameters: (1) the flow velocities and (2) the ionopause height at the terminator. We also investigate the role of electron precipitation in supporting a nightside ionosphere. Our model also provides indirect evidence for an enhanced deuterium to hydrogen ratio on Venus.
A first comparison of STARE and EISCAT electron drift velocity measurements
Authors: Nielsen E. , Schlegel K.
Published: February 2008

The Scandinavian twin auroral radar experiment (STARE) has a demonstrated capability of providing estimates of the ionospheric electron drift velocities, which are useful for a wide range of geophysical studies. The accuracy of such estimates has for the first time been tested by comparison with simultaneous velocity measurements made with the European incoherent scatter facility (EISCAT). The magnitudes of the estimated drift velocities are in agreement with the EISCAT measurements for small velocities (<700 ms−1), but we find the estimates to be increasingly too low as the velocities become larger. The directions of the estimated vectors are in agreement with the EISCAT measurements for all drift magnitudes. The measured phase velocities are even lower than those predicted by the kinetic theory of the two stream instability. Agreement can only be obtained for relatively large neutral densities. The data are not inconsistent with an assumption that the possible phase velocities in the plasma are limited upward by the ion‐acoustic velocity (which is an increasing function of the ionospheric electric field). Power spectral observations and an improved modelling with the kinetic theory suggest that a backscatter region, extended in altitude and allowing several values of k∥ to influence observations, leads to an improved theoretical fit to the measurements.
The response of thermospheric atomic nitrogen to magnetic storms
Authors: Engebretson M. J. , Mauersberger K.
Published: February 2008

Neutral composition data obtained by the open‐source neutral‐mass spectrometer on the polar orbiting Atmosphere Explorer D satellite during the periods October 31 to November 15, 1975, and January 5 to 15, 1976, are used to characterize the response of thermospheric atomic nitrogen densities to geomagnetic activity. These periods provided nearly simultaneous polar and low‐latitude data coverage. At low and middle latitudes near dawn and at all latitudes on the dayside, N densities at 400 km appear to vary like those of a species of atomic mass 14: N is observed to increase with increasing geomagnetic activity in a manner similar to that of O. At auroral latitudes near dawn, however, there is a more complex dependence on the amplitude of the disturbance. For substorm‐scale activity, N tends to increase during periods of auroral heating but exhibits a sharp temporary decrease afterward. During large storms there is a significant but short‐lived increase of N at auroral latitudes. Our results support current models showing that N is produced and transported out of the auroral zone during geomagnetic disturbances.
Generation of auroral arc elements in an inverted‐V arc due to ion cyclotron turbulence
Authors: Kan J. R. , Okuda H.
Published: February 2008

By using a two‐dimensional particle simulation it is shown that the electrostatic ion cyclotron turbulence driven by the field‐aligned current on auroral field lines can lead to striations of the current into east–west aligned current sheets of ion gyroradius scale (La ∼ ρi). Physical mechanism for the striation can be identified with nonlinear mode couplings of ion cyclotron waves resulting in condensation of the wave energy into a zero‐frequency mode. It is proposed that the auroral arc elements (La ∼ ρi), which collectively give rise to the inverted‐V arc (Lυ ≫ ρi), can be produced by the east–west aligned striations of the field‐aligned current on inverted‐V field lines.
Observations of ion streaming during substorms
Authors: Lui A. T. Y. , Eastman T. E. , Williams D. J. , Frank L. A.
Published: February 2008

On the basis of three‐dimensional measurements of energetic particles and plasma from the ISEE 1 spacecraft, the occurrence of ion beams at the plasma sheet boundary in the magnetotail previously reported by Williams (1981) is examined in relation to substorm activity. In all seven cases studied it is found that ion beams streaming earthward and/or tailward are always present at the edge of the plasma sheet adjacent to the tail lobe. Ion beams penetrating into the plasma sheet region with no detectable density gradient are also observed. The observed ion beams may occur at high energies of tens of keV, or at medium energies of a few keV, or even at energies as low as tens of electron volts. Their intensities and energies do not have any obvious relationship to the substorm onset and expansion phases or the magnitude of the substorm disturbance as indicated by the AE index. However, ion beams tend to be more intense and occur at higher energies when the plasma sheet is energized during the substorm recovery phase. The result suggests that the activation of the source for the streaming ions is not directly associated with any particular phase of a substorm.
Analysis and numerical simulation of the effect of Ion Pedersen Mobility on ionospheric barium clouds
Authors: Zalesak S. T. , Fedder J. A. , Ossakow S. L.
Published: February 2008

We analyze the effect of finite ion Pedersen mobility on the evolution of ionospheric barium clouds. We find that the Pedersen mobility provides a mechanism whereby barium can leak out of the region of elevated magnetic‐field‐line‐integrated electron density/Pedersen conductivity and hence lose the ability to structure further. The essence of the mechanism is that the finite Pedersen mobility of the barium ions allows them to separate from the cloud electrons, which have virtually no Pedersen mobility. The barium is replaced in the electron cloud by the ions constituting the ambient ionosphere, which are compressed up to the required densities by their own Pedersen mobility. (Here we take the “electron cloud” to be that defined by regions of enhanced magnetic‐field‐line‐integrated electron density, not by local electron density enhancements.) In the process of exiting the electron cloud the barium is expanded and hence attains a density lower than when it coexisted with the electron cloud. For many ionospheric parameters the barium will have in effect left the region of further structuring, which will still be proceeding in the electron cloud. As the barium leaves the electron cloud, it can form a long, thin sheet of lower‐density barium, one side of which is considerably steeper than the other, which extends from the leading (nonstructuring) edge of the cloud. An observer watching only the barium or only local electron densities would conclude that the structuring process had ceased. Further, the electron cloud itself may decay if the ions coexisting with it are subject to a fast recombination chemistry.
Bottomside sinusoidal irregularities in the equatorial F region
Authors: Valladares C. E. , Hanson W. B. , McClure J. P. , Cragin B. L.
Published: February 2008

A new category of equatorial F region plasma irregularities having properties easily distinguished from those of plasma bubbles, depletions, or broadband turbulence has been identified in the data from the retarding potential analyzer (RPA) and the ion drift meter (IDM) on the Atmosphere Explorer satellites AE‐C and AE‐E. These irregularities are observed in the bottomside at night in a narrow belt extending approximately ±12° from the dip equator and up to 7500 km or more in the east–west direction. High‐resolution data from the RPA reveal nearly sinusoidal waves in the ion number density Ni with peak to peak ΔNi/Ni amplitudes that span more than three orders of magnitude, from values less than 0.1% to more than 100%. These distinctive “bottomside sinusoidal” (BSS) irregularities typically have a central wavelength lying in the range from 300 m to 3 km, and relatively narrow spectra. The power spectral index is often large (5 or 6) at the shorter wavelengths, but the spectrum usually decays much less rapidly on the longer wavelength side. The vertical drift velocity also has a wavelike structure that appears to be anticorrelated to within a phase uncertainty of approximately ±90° with the Ni variations, such that regions of lower density plasma move upward and higher density regions move downward with respect to the mean ionospheric drift. Anticorrelated wavelike drift and density fluctuations are consistent with plasma that is Rayleigh‐Taylor unstable. Consideration of altitude localization and cross‐field ambipolar diffusion effects can introduce at least a slight maximum in the R‐T growth rate near 1 km and also sharply limit irregularity growth at smaller scale sizes, thus providing a tentative explanation of the peaked Ni spectrum observed. Ionograms from Huancayo display a typical “mid‐latitude” frequency spread signature when BSS is present.
Structure and dynamics of Saturn's outer magnetosphere and boundary regions
Authors: Behannon K. W. , Lepping R. P. , Ness N. F.
Published: February 2008

In 1979–1981, the three USA spacecraft Pioneer 11 and Voyagers 1 and 2 discovered and explored the magnetosphere of Saturn to the limited extent possible on flyby trajectories. Considerable variation in the locations of the bow shock (BS) and magnetopause (MP) surfaces were observed in association with variable solar wind conditions and, during the Voyager 2 encounter, possible immersion in Jupiter's distant magnetic tail. The limited number of BS and MP crossings were concentrated near the subsolar region and the dawn terminator, and that fact, together with the temporal variability, makes it difficult to assess the three‐dimensional shape of the sunward magnetospheric boundary. The combined BS and MP crossing positions from the three spacecraft yield an average BS to MP stagnation point distance ratio of 1.29±0.10. This is near the 1.33 value for the earth's magnetosphere, implying a similar sunward shape at Saturn. Study of the structure and dynamical behavior of the outer magnetosphere, both in the sunward hemisphere and the magnetotail region using combined plasma and magnetic field data, suggest that Saturn's magnetosphere is more similar to that of earth than that of Jupiter. Also, evidence was found by Voyager 1 for tailward flowing plasma near the pre‐dawn MP, a phenomenon well known for the cases of both earth and Jupiter. That this was not observed by Voyager 2 at Saturn may have been related to the possible immersion of Saturn in Jupiter's magnetotail during a significant portion of the Voyager 2 encounter period, since the plasma flux in the Jovian tail is markedly lower than that in the solar wind on average.
Plasma distribution and magnetic field orientation in the Venus near wake: Solar wind control of the nightside ionopause
Authors: Pérez‐De‐Tejada H. , Dryer M. , Intriligator D. S. , Russell C. T. , Brace L. H.
Published: February 2008

A comparative study of the position of the Venus nightside ionopause and of the orientation of the magnetic field in the near Venus wake is presented. It is shown that when the ionopause is detected deep within the umbra the magnetic field vector above that boundary shows consistently a convergent orientation toward the sun‐Venus axis. When the ionopause is encountered outside the umbra, the magnetic field may assume directions not necessarily indicative of closure. The overall configuration of the nightside ionopause seems, in addition, to be strongly controlled by the kinetic energy density of the solar wind. At high values of this quantity the ionopause tends to occur more often at low heights.
Calculated and observed limb profiles of OI (1356 Å) dayglow
Authors: Newman A. L. , Christensen A. B. , Anderson D. E.
Published: February 2008

Limb scan observations of atomic oxygen OI (1356 Å) emissions obtained with the D sensor ultraviolet spectrometer package flown on the Defense Meteorological Satellite Program F4 satellite are analyzed with a recently developed radiative transfer model using best current estimates for the excitation cross sections, branching ratios, and photoelectron fluxes. The topside scale height of the limb profile depends mainly on the exospheric temperature T∞ used in the model computations and the peak intensity of the emission profile depends on the atomic oxygen density. To a lesser extent, the altitude of the peak and the shape of the profile vary with both T∞ and oxygen density. It is shown that more than half of the emission originates within 10 km of the tangent point altitude along a 700‐km segment of the line of sight. Comparison of calculated and observed profiles shows good agreement and suggests that limb scan observations may define the altitude distribution for some thermospheric species better than empirical models alone.
Excitation of magnetostatic fluctuations by filamentation of whistlers
Authors: Lee M. C. , Kuo S. P.
Published: February 2008

The thermal filamentation instability of whistlers that can excite magnetostatic and plasma density fluctuations simultaneously has been investigated for cases in the magnetosphere and in the ionosphere. With the available facilities at Siple, Antarctica, this instability may be excited in the magnetosphere at L = 4 within a few minutes by ducted whistler modes whose frequencies are greater than half the local electron cyclotron frequency. No significant ionospheric effects are expected to be produced by the injected Siple VLF waves. However, marked ionospheric perturbations are predicted with employment of envisioned MF transmitters at frequencies slightly less than the electron cyclotron frequency in the ionosphere.
Dependence of the spectrum of Pc 3–4 pulsations on the interplanetary magnetic field
Authors: Takahashi Kazue , McPherron Robert L. , Terasawa Toshio
Published: February 2008

Dependence of the power spectrum of Pc 3–4 magnetic pulsations observed at the ATS 6 geosynchronous satellite on the interplanetary magnetic field (IMF) has been studied. Pulsation events that were observed near noon and exhibited harmonic structure are chosen for analysis. We further select pulsation events with identical fundamental frequency to study dependence of the power of pulsations at different harmonic bands on the IMF. A weak negative correlation is observed between the IMF cone angle θXB and the power of pulsations in the frequency range 20–70 mHz. Also, a positive correlation between the intensity of the IMF 〔BIMF〕 and the power of pulsations at 50–70 mHz is found. This BIMF control is present at all ranges of the cone angle. We compare this observation with the frequency of bow shock associated upstream waves predicted from a model of wave generation by a cyclotron resonance of ions reflected at the bow shock. The predicted frequency depends on the IMF as BIMF cos² θXB. Although this relation gives a proportionality between the frequency and BIMF qualitatively consistent with the observation, it does not explain the most obvious IMF control of the spectrum of the pulsations.
Direct generation of the auroral kilometric radiation by the Maser synchrotron Instability: Physical mechanism and parametric study
Authors: Le Quéau D. , Pellat R. , Roux A.
Published: February 2008

Recent observations at low altitudes made in the source region of the auroral kilometric radiation (AKR hereinafter) strongly support the maser synchrotron instability (MSI) as the relevant generation process for the AKR. This was first realized by Wu and Lee (1979). In a recent work (Le Quéau et al. (1984), hereinafter called reference 1) we have proposed an analytical treatment of the MSI. The present work is a continuation of this analytical study. First we investigate the physical process of the MSI, and simple analytical expressions, valid for any distribution functions of nonthermal electrons are given. It is shown that supraluminous X mode waves (ω/k∥c ≫ 1) can resonate with relatively low energy electrons provided that the cold plasma parameter, εc = ωpc²/ωc², is much smaller than unity. The proper frame of reference for studying the resonant coupling between the X mode and nonthermal electrons moves at a parallel velocity k∥c²/ω. Resonance curves are then circles centered around this value and electrons diffuse with almost constant parallel velocities. The difference between the MSI and the standard cyclotron theory (where resonance curves are straight lines while diffusion curves are circles centered around ω/k∥) is stressed. A parametric study of the instability conditions for such a supraluminous wave is conducted. We investigate the role played by θ, the propagation angle εb, the normalized density of energetic electrons δ, the energy width of the part of the distribution function where an inversion of population (∂f/∂υ⊥ > 0) occurs, and finally u, the parallel velocity which characterizes the maximum of ∂f/∂υ⊥ integrated along a resonance circle. In the case of the shifted loss cone distribution used here, u is simply its parallel bulk velocity. Largest growth rates are obtained for k∥c²/ω = u.
K‐coronameter observations and potential field model comparison in 1976 and 1977
Authors: Bruno R. , Burlaga L. F. , Hundhausen A. J.
Published: February 2008

Results of determining the shape and location of the heliospheric current sheet from a potential field model and from K‐coronameter observations are compared. Interplanetary magnetic field polarities as observed by IMP 8, Helios 1 and 2, and Voyager 2 spacecraft were used to test the two methods over the period May 1976 to August 1977 throughout 18 Carrington rotations. The computed heliospheric current sheets from both methods had a quasi‐stationary four‐sector structure and very similar shapes. Agreement between interplanetary magnetic field polarity and the results from the potential field model was found on 79% of the days, while agreement between the interplanetary field polarity and the polarities derived from the K‐coronameter data was found on 87% of the days.
Helios observational constraints on solar wind expansion
Authors: Marsch E. , Richter A. K.
Published: February 2008

Helios particle and magnetic field observations between 0.3 and 1 AU are used to determine plasma parameters that characterize the bulk and internal energy state of the solar wind. Quantities expected to be conserved in a time‐stationary flow with local spherical symmetry in the ecliptic plane are actually found to be invariant within measurement uncertainties. These are the total mass, energy, and angular momentum fluxes for the anisotropic solar wind plasma composited by electrons, protons, and alphas. Although individual species have nonthermal velocity distributions, the total plasma pressure is almost isotropic (p⊥/p∥ = 0.9). The total heat flux divided by the mass flux Qr/ρur is markedly smaller than thermal speeds squared υ∥,⊥² = p∥,⊥/ρ. By this reason an appropriately defined polytropic index γ is found to be almost 5/3 and rather insensitive to heliocentric distance and flow speed. This index γ does not include terms due to wave turbulence or external heat sources but is solely based on the total particle heat flux. These observational findings indicate that the heat flux beyond 0.3 AU is observationally too small to cause a strong departure from adiabaticity. The solar wind expansion may be conceived in terms of a “single particle” moving in the binding gravitational potential and in the accelerating thermal (enthalpy and heat fluxes) and magnetorotational (azimuthal kinetic energy and Poynting flux) potentials. The radial profiles of these potentials are derived from observations.
Fast moving plasma structures in the distant magnetotail
Authors: Scholer M. , Gloeckler G. , Klecker B. , Ipavich F. M. , Hovestadt D. , Smith E. J.
Published: February 2008

We report in this paper for the first time the detailed time behavior of the intensities and the angular distributions of energetic protons and electrons in the distant magnetotail of the earth at ∼220 RE and ∼110 RE. The data have been obtained by the Max‐Planck‐Institut/University of Maryland sensor system on ISEE 3 during the spacecraft’s first deep tail passage. Three energetic particle bursts are studied in detail. At the beginning of these bursts we first observe streaming electrons at an energy of 75–115 keV. These are followed after several minutes by anisotropic protons. The protons exhibit an energy dispersion with higher energy protons (∼120 keV) appearing before lower energy (∼32 keV) protons. In two of the three cases the streaming electron distributions are followed after 8–15 min by an isotropic electron distribution. At the rear side of the bursts the time dispersion effects are not seen in reverse order. Also, no gradient anisotropy effects are observed in the beginning of the proton streaming. We suggest that the satellite encounters detached plasma structures evidenced by the isotropic electrons. These structures, probably plasmoids, move with high velocities (∼800 km/s) down the tail. The energetic electrons and protons stream ahead of these fast tailward moving plasma structures, which leads to the various time dispersion effects. This allows, in principle, a determination of the source distance from the satellite.