bluuebunny/crossref_metadata_2025 huggingface dataset:
Absolute experimental differential and integral electron excitation cross sections for atomic oxygen: 3. The (³P →³D°) transition (989 Å) from 20 to 200 eV with improved values for the (³P →³S°) transition (1304 Å)
Authors: Vaughan S. O. , Doering J. P.
Published: February 2008

The absolute differential and integral electron excitation cross sections for the atomic oxygen ³P → ³D° transition (989 Å) have been measured at incident energies from 20 to 200 eV. A variable angle and incident energy electron spectrometer coupled to a microwave discharge free supersonic jet expansion source was used for the experiments. The source produced an O/O2 density ratio typically greater than 2.5. The differential cross sections were determined by comparison to computer‐fitted recalculations of the differential cross section for the atomic oxygen ³P →³S° transition (1304 Å). The method used for the computer fit and the results of the fitting calculations are discussed for the ³P →³S° atomic oxygen transition (1304 Å). The O I ³P →³S° (989 Å) differential cross sections vary smoothly with the incident energy and are flatter with scattering angle at the lower incident energies, as expected. The absolute integral cross section is found to agree with the theoretical shape at higher energies. Previous estimates of this cross section from fluorescence data were larger than the present values by approximately a factor of 2. The present lower value appears to provide much better agreement with aurora and dayglow 989 Å emission intensities.
Characteristics of cometary picked‐up ions in a global model of Giacobini‐Zinner
Authors: Kimmel C. D. , Luhmann J. G. , Phillips J. L. , Fedder J. A.
Published: February 2008

Energetic ions observed during the International Cometary Explorer (ICE) spacecraft flyby of comet Giacobini‐Zinner provide information about both the constitution of comets and the plasma physical processes associated with their interaction with the solar wind. In this investigation the details of ion “pickup,” in the limit where small‐scale fluctuations in the plasma and magnetic field are neglected, are modeled by following the motion of a large number of initially cold, heavy (mass 18) ions in a global magnetohydrodynamic model of the local plasma and magnetic field. The results indicate how the background or macroscopic velocity and magnetic field structure of the comet can affect the average spatial and spectral characteristics of the observed cometary ions. These effects, which occur by virtue of forces associated with the compression and the curvature of the magnetic field in the presence of the stagnating plasma flow, can explain the double maxima in the time series of the energetic ion flux observed along the ICE trajectory.
Atomic hydrogen and solar Lyman α flux deduced from STP 78‐1 UV observations
Authors: Anderson D. E. , Paxton L. J. , McCoy R. P. , Meier R. R. , Chakrabarti Supriya
Published: February 2008

Simultaneous observations of the Lyman α and 1026 Å airglow by a spectrometer on the STP 78‐1 satellite have been analyzed. A spherical model of the multiple scattering of solar Lyman α and Lyman β radiation was utilized which includes the effect of pure absorption by O2 and a nonisothermal temperature profile. Analysis of data near 0° latitude yields an exobase temperature Tc = 1300 ± 200 K, exobase density Nc = 2.6 ± 0.2 × 104 cm−3, and escape flux Fc = 2 ± 0.5 × 108 cm−2 s−1. The solar Lyman α line center flux determined from analysis of the Lyman α airglow was 6.0 ± 1.2 × 1011 photons cm−2 s−1 Å‐1. Analysis of the 1026 Å airglow showed that this emission is dominated by multiple scattering of solar Lyman β photons by atomic oxygen and that the Lyman β airglow contributes less than 20% to the observed emission.
Velocity of iron ions in the solar wind
Authors: Schmid J. , Bochsler P. , Geiss J.
Published: February 2008

From a set of approximately 7000 spectra provided by the ISEE 3 ion composition instrument at the time near the maximum of solar cycle 21 we have been able to derive iron velocities and to compare them with velocities of helium ions obtained with the same instrument. We find a strong correlation among these velocities (rcor = 0.975 ± 0.001). Whereas no significant velocity differences between helium and iron are found at low solar wind speeds, it appears that iron tends to flow at a somewhat lower speed (δυ ∼ 10 km/s) in high‐speed streams.
Simultaneous observation of Pc 3–4 pulsations in the solar wind and in the Earth's magnetosphere
Authors: Engebretson M. J. , Zanetti L. J. , Potemra T. A. , Baumjohann W. , Lühr H. , Acuna M. H.
Published: February 2008

The equatorially orbiting AMPTE (Active Magnetospheric Particle Tracer Explorers) CCE and IRM satellites have made numerous observations of Pc 3–4 magnetic field pulsations (10‐ to 100‐s period) simultaneously at locations upstream of the earth's bow shock and inside the magnetosphere. These observations show solar wind/interplanetary magnetic field (IMF) control of two categories of dayside magnetospheric pulsations: (a) Harmonically structured, azimuthally polarized pulsations are commonly observed from L = 4 to 9 in association with upstream waves. Their periods (including harmonics) appear to be governed by local resonant conditions. Varying levels of broadband compressional wave power are associated with these pulsations, most prominently near local noon, (b) More monochromatic compressional pulsations are clearly evident on occasion, with periods identical to those observed simultaneously in the solar wind. Harmonically structured, azimuthally polarized pulsations also occur in conjunction with these compressional pulsations, but again display frequencies characteristic of local resonant conditions. As in earlier studies there is clear control of the occurrence of pulsations both inside and outside the magnetosphere by the cone angle of the IMF, and harmonic pulsation amplitude increases when the solar wind velocity increases. The observations reported here are consistent with a high‐latitude (cusp) entry mechanism for wave energy related to harmonically structured pulsations.
Magnetotails at unmagnetized bodies: Comparison of comet Giacobini‐Zinner and Venus
Authors: McComas D. J. , Gosling J. T. , Russell C. T. , Slavin J. A.
Published: February 2008

Both Comet Giacobini‐Zinner (G‐Z) and Venus have magnetotails consisting of draped interplanetary magnetic field lines. This field line draping is caused by a velocity shear between regions of greater flow speeds away from the bodies and lesser flow speeds near to the bodies. Data obtained within the Venus magnetotail by the Pioneer Venus Orbiter and within the G‐Z tail by the International Cometary Explorer traversal of G‐Z have previously been combined with stress balance considerations to infer many of the physical characteristics of these two magnetotails. In the present paper we compare and contrast these physical characteristics and thereby examine those aspects of the interactions with the solar wind and draped magnetotail forming processes which are common at the two bodies, and those which are different. We find that the near ionopause environs play a crucial role in the tail formation process at both Venus and G‐Z and that draping at the two very different sized bodies occurs on ionopause scale sizes. On the other hand, ion densities, downtail mass fluxes, tailward J × B forces, and lobe betas are factors of ∼104, 50, 100, and 20 times greater in the G‐Z tail than in Venus', while bulk flow speeds and ion temperatures are factors of ∼15 and 240 times lower. These large quantitative differences in the properties within the two magnetotails are attributable to the significantly greater upstream mass loading of the solar wind by the extended neutral atmosphere at G‐Z (comets in general) compared to the gravitationally bound atmosphere of Venus.
The evolution of spectral densities in weakly inhomogeneous plasmas
Authors: Rönnmark Kjell
Published: February 2008

Theoretical studies of waves and instabilities are often performed for an infinite, homogeneous plasma, and the results are given in terms of eigenmodes (plane waves) extending over all space. Real plasmas are, however, always finite and inhomogeneous, and observations of waves in space are normally presented as frequency power spectra which depend on the time and place of observation. The purpose of this study is to reconcile these apparently disparate descriptions of plasma waves. A model describing inhomogeneous spectral densities in a homogeneous plasma is first discussed in some detail. When this model is generalized to inhomogeneous plasmas, it is found that the incompressibility of the spectral densities in k space results in a remarkably simple transfer equation. A method which allows the observable frequency power spectrum to be calculated from a space and time dependent dispersion relation and appropriate boundary conditions is also outlined.
Energetic magnetospheric ions at the dayside magnetopause: Leakage or merging?
Authors: Sibeck D. G. , McEntire R. W. , Lui A. T. Y. , Lopez R. E. , Krimigis S. M. , Decker R. B. , Zanetti L. J. , Potemra T. A.
Published: February 2008

We elaborate upon the leakage model for the escape of energetic magnetospheric particles into the magnetosheath. Unlike the merging model, no interconnection (or merging) of magnetospheric and magnetosheath magnetic field lines is required. Because outer magnetospheric energetic particle drift paths intersect the magnetopause, the leakage model requires the continual escape of ions at postnoon local times and electrons at prenoon local times, regardless of solar wind conditions. It also predicts a division between dawnward and duskward streaming ions at the point where most magnetosheath magnetic field lines make their closest approach to the magnetopause, typically near 1500 LT. Like the merging model, the leakage model predicts equatorward streaming just inside the magnetopause. We study the motion of an escaping energetic ion at a planar magnetopause to show that, without scattering, ions must move dawnward and northward in a duskward magnetosheath magnetic field and dawnward and southward in a dawnward magnetosheath magnetic field. Scattering permits some ions to move duskward. We present new observations of streaming ions outside the dayside magnetopause made by the Charge Composition Explorer satellite, a part of the Active Magnetospheric Particle Tracer Explorers program. To place these observations in context, we have performed a statistical study of previous particle observations both inside and outside the dayside magnetopause. The ensemble of observations indicates that energetic magnetospheric ions of all species continually escape from the dayside magnetosphere and stream along magnetosheath magnetic field lines, even when no merging is expected. The magnetosheath magnetic field controls the direction in which the ions stream: they move away from the magnetosphere. The results of this work indicate that energetic particle observations at the dayside magnetopause need not be taken as evidence for merging of magnetosheath and magnetospheric magnetic field lines.
Ion and electron velocity distributions within flux transfer events
Authors: Thomsen M. F. , Stansberry J. A. , Bame S. J. , Fuselier S. A. , Gosling J. T.
Published: February 2008

The detailed nature of the thermal and suprathermal ion and electron distributions within magnetic flux transfer events (FTEs) is examined. Examples of both magnetosheath FTEs and magnetospheric FTEs are discussed. The detailed distributions confirm that FTEs contain a mixture of magnetosheath and magnetospheric plasmas. To lowest order, the distributions are consistent with a simple superposition of the two interpenetrating populations, with no strong interactions between them. To first order, some interesting differences appear, especially in the electron distributions, suggesting that considerable pitch angle scattering and some electron energy diffusion are also occurring. These observations should provide a useful test of analytical and numerical studies of interpenetrating plasmas.
A theoretical study of the lifetime and transport of large ionospheric density structures
Authors: Schunk R. W. , Sojka J. J.
Published: February 2008

Large‐scale density structures are a common feature in the high‐latitude ionosphere. They have been observed in the dayside cusp, polar cap, and nocturnal auroral region. Relative to background densities, the perturbations associated with large‐scale structures vary from about 10% to a factor of 100. The lifetime and transport characteristics of “large” ionospheric structures (factor of 10 to 100) were studied with the aid of a three‐dimensional time‐dependent ionospheric model. Both density depletions and enhancements were considered. A density structure was created at a specific location in the high latitude F region and the subsequent evolution was followed for different seasonal and solar cycle conditions as well as for different orientations of the interplanetary magnetic field (IMF), i.e., different convection patterns. Depending on the IMF, horizontal plasma convection can cause an initial structure to break up into multiple structures of various sizes, remain as a single distorted structure, or become stretched into elongated segments. The lifetime of an F region density structure depends on several factors, including the initial location where it was formed, the magnitude of the structure, season, solar cycle, and convection pattern (IMF). For example, in summer the effects of a large density structure can disappear in a few hours or last as long as 9 hours, while in winter the effects can persist for 24 hours. The passage of perturbed plasma flux tubes through sunlit and auroral regions can significantly increase the lifetime of plasma enhancements and can significantly reduce the lifetime of plasma depletions.
Geocoronal structure: 2. Inclusion of a magnetic dipolar plasmasphere
Authors: Bishop James , Chamberlain Joseph W.
Published: February 2008

Calculations of exospheric quantities (hydrogen atom density, satellite atom fractional density, kinetic temperature, and escape flux) at locations along the Earth‐Sun axis in the noon and midnight directions have been extended to incorporate a plasmasphere characterized by a dipolar shape and an empirical temperature profile. This interaction, evaluated with parameter values corresponding to low‐to‐moderate solar conditions, results in an increased density at outer geocoronal positions; the effect is not dramatic, though, and the resulting exosphere mimics the evaporative case closely, in spite of the control of trajectory parcel content by charge exchange collisions. A careful discussion of the handling of plasmaspheric charge exchange collisions and solar ionization is included, and the effect on the exospheric kinetic distribution is analyzed in terms of pertinent examples. In addition, the geotail is demonstrated to stem primarily from the imposition of an exopause by radiation pressure dynamics.
Electric field and plasma density measurements in the strongly driven daytime equatorial electrojet: 2. Two‐stream waves
Authors: Pfaff R. F. , Kelley M. C. , Kudeki E. , Fejer B. G. , Baker K. D.
Published: February 2008

Both primary and secondary two‐stream (Farley‐Buneman) waves have been detected by in situ electric field and plasma density probes in the strongly driven daytime equatorial electrojet over Peru. Simultaneous Jicamarca radar observations showed strong vertical and oblique 3‐m type 1 echoes, also indicative of the two‐stream mechanism. The rocket data show the two‐stream region on the topside of the unstable layer to be situated between 103 and 111 km where the electron current was the strongest. This region was characterized by broadband plasma oscillations extending past 1 kHz in the rocket frame. Furthermore, above 106.5 km, where the electron density gradient was stable, a layer of laminarlike, horizontally propagating two‐stream waves was detected. These waves were strongest near 108 km, the altitude where previous measurements have shown the electrojet current over Peru to be strongest. The peak rocket frame frequency of these waves was 25 Hz, and the observed broadband electric field and plasma density amplitudes were 2 mV/m and 1–2% rms, respectively. The electric field amplitudes were likely attenuated by a spatial filtering effect and may have been several times higher. The data suggest that these waves had phase velocities comparable to the electron drift velocity (≃500 m/s) and peak wavelengths (2–3 m) that agree with kinetic theory predictions. Distinct vertically oriented waves which may have been generated by a mode coupling process were also observed by the rocket in this region, in agreement with the simultaneous radar observations. Below the laminar two‐stream layer, in the region of the large‐scale, gradient drift driven horizontal electric fields, secondary two‐stream waves were observed that were driven by localized δE × B plasma drifts. The amplitudes of the horizontally propagating kilometer scale waves were clearly strong enough (10–15 mV/m) to drive vertically oriented secondary two‐stream (type 1) waves, as proposed by Sudan et al. (1973) and observed by the Jicamarca radar between roughly 103 and 106 km. Below 103 km, secondary two‐stream waves could not be generated because ψ (the ratio of the ion and electron collision frequencies to their gyro frequencies) becomes large, despite the fact that the observed large‐scale wave amplitudes remained strong at the lower altitudes. Observations of “flat‐topped” waveforms of the large horizontal electric field structures suggest that secondary two‐stream phase velocities may be saturated because of a limitation of the driving electric fields rather than a process intrinsic to the two‐stream instability itself.
Azimuthal brightness variation and albedo measurements of the Uranian rings
Authors: Svitek Tomas , Danielson G. Edward
Published: February 2008

The recent Voyager encounter with Uranus has produced new insights and observational constraints on the structure, composition, and evolution of the Uranian rings. We have made careful measurements of the brightness variation as a function of longitude for the α, β, γ, δ, and ε rings. These measurements showed a substantial variation in azimuthal brightness because of the varying width of the rings. The single scattering albedo of the ε ring particles was found to be 3.9% ± 0.6% (for the Lambert surface) or 2.3% ± 0.4% assuming the “lunar‐type” phase function. The optical depth at apoapse of the ε ring was found to be 0.40 ± 0.05. Note that this determination of the optical depth is independent of the occultation measurement from Earth or from the spacecraft. This value is the optical depth for reflected light, whereas occultation data provide the forward scattering optical depth. It is consistent with varying amounts of mutual shadowing of ring particles due to changing width of the ring along its orbit. The single scattering albedo of the α, β, γ, and δ rings is consistent with the ε ring, however, with bigger error bars (between 1.5 and 2.0% ±0.5% assuming the “lunar‐type” phase function).
Temperature and aerosol structure of the nightside Uranian stratosphere from Voyager 2 photopolarimeter stellar occultation measurements
Authors: West R. A. , Lane A. L. , Hord C. W. , Esposito L. W. , Simmons K. E. , Nelson R. M. , Wallis B. D.
Published: February 2008

The Voyager 2 photopolarimeter experiment measured the ultraviolet flux from the star γ Pegasi as the star emerged from behind the Uranus limb at planetocentric latitude 68.9°N on the nightside. We have compared these measurements to model atmospheres with, and without, an aerosol component. If the atmosphere is free of aerosols, the temperature is 85 ± 2.3 K at a pressure of 2.7 mbar, 90 ± 6 K at 1 mbar, and 96 ± 13 K at 0.37 mbar, and the radius of the 1‐mbar level is 25,219 ± 6.3 km. The radius value is close to the inferred radius (25,225 km) of the 1‐mbar level at latitude 69° in the southern hemisphere (G. Lindal, private communication, 1987). This agreement argues for hemispheric symmetry in the shape of the 1‐mbar geopotential and, therefore, in the latitude profile of the zonal wind velocity which was not measured in the northern hemisphere. Our measurement, when coupled with a 1‐mbar equatorial radius of 25,734 km (G. Lindal, private communication, 1986), leads to an effective planetary oblateness of ϵ= 0.0230 ± 0.0004, slightly smaller than the value (0.024) determined from earlier work. We place upper limits to the amount of aerosol that can form a discrete layer or haze‐top boundary; the aerosol extinction coefficient, kext ≤ ∼10−4 km−1, if the layer is at an altitude equal to or higher than the 1‐mbar altitude. Our ability to sense haze boundaries diminishes rapidly below 3 mbar. A well‐mixed haze may be present. Candidates for this haze include dust from meteor and ring particle infall, photochemically formed diacetylene condensing at temperatures below 103 K, and other photochemical species which condense at temperatures higher than about 80 K. We show that dust from the rings and meteors contributes no more than a few tenths of a percent to the extinction profile. Current predictions for the diacetylene abundance reveal that diacetylene aerosols may contribute as much as a few tens of percent to the extinction profile and can be accommodated by our observations. More detailed photochemical‐transport‐aerosol models are needed to explore these questions further.
Polarization and localization of the Uranian radio sources
Authors: Lecacheux Alain , Ortega‐Molina Arturo
Published: February 2008

We describe a new direction finding method for studying low‐frequency radio emissions when observed from spacecraft. The method is based on the polarization measurement of the received radiation and works if the radio source contains a circularly polarized component. It is especially suitable in the case of the nonthermal, low‐frequency emissions of the planets, which are known to have a large degree of circular polarization. In order to apply this method to the Voyager 2 observations of the Uranian kilometric radiation (UKR), we develop a quantitative model of the polarization response of the planetary radio astronomy experiment (PRA) aboard the Voyager spacecraft. The model parameters, valid in the low‐frequency range where the antennas are electrically short, are obtained by analyzing several Voyager 1 and 2 maneuvers that occurred during the Saturn and Uranus encounters. We find that the PRA orthogonal monopoles can be accurately modeled by two crossed, quasi‐orthogonal dipoles; the plane of the equivalent dipoles is displaced by 23.3° ± 0.5° from the physical antenna plane in the opposite direction to the magnetometer boom. By analyzing an inbound maneuver, when the spacecraft was at 8 RU from the planet, the source of the low‐frequency UKR component (or smooth low‐frequency component (SLF)) is found to lie in the northern (dayside) magnetic hemisphere, in a restricted range of magnetic longitudes. By assuming that the radio source emits at the local electron gyrofrequency, an independent analysis of the intrisic (not due to the antenna geometry) polarization reversal, observed near the magnetic equator crossing, leads to the same source location. After correction for the antenna response, the SLF emission is found to be 100% circularly polarized, the sense of polarization being in accordance with an O mode emission. The direct localisation of the broadband component (or smooth high‐frequency component (SHF)), obtained from an outbound spacecraft maneuver at 12 RU from Uranus, is in agreement with a radio source in the southern (nightside) magnetic hemisphere. However, the analysis of the polarization reversal, due to the slowly change of the source‐antenna geometry when the spacecraft was passing round the planet at 6 RU, gives inconsistent results, suggesting that more than a single source were observed at that time. The observed circular polarization degree is large, and the polarization sense corresponds to an X mode emission. During one of the spacecraft maneuvers, the SLF radio source could be directly mapped with an angular resolution corresponding to a small fraction of one planetary radius. The source location is found to depend on the frequency, the higher frequencies being observed at lower altitudes from the planet, in agreement with the usually assumed morphology for the planetary auroral radio sources. Despite of the high precision of the method, more refined results could not be obtained, probably because of propagation effects close to the radio source.
Beaming of Uranian nightside kilometric radio emission and inferred source location
Authors: Zarka Philippe , Lecacheux Alain
Published: February 2008

Taking advantage of the large range of magnetic latitudes explored by Voyager 2, due to the 60° tilt of the Uranian dipole field with respect to the rotation axis of the planet, we study the beaming pattern of the broadband kilometric radio emission detected over Uranus' nightside. Using the simple physical assumptions of an emission close to the local gyrofrequency and in a time‐stationary average lobe (i.e., assuming that the 10 min‐averaged intensity received does not vary with time for a given observing geometry), we constrain the radiation source to a high south magnetic latitude region (∼80°, or L∼40) magnetically connected to the magnetospheric tail. We discuss the different source locations derived by other authors from different analyses. We determine then the beaming pattern of the main components (smooth and bursty) of the Uranian nightside radio emission and their variations with frequency. It appears to be a hollow cone whose axis is tangent to the magnetic field in the source region, with a large aperture (semi‐apical angle ∼50°) and with an angular thickness (at −3 dB) increasing with decreasing frequency from about 30° at 750 kHz to more than 55° below ∼350 kHz; the bursty emission occurs about 90° away from the magnetic field. The motion of the spacecraft relative to the radiation lobe explains most of the observed emission characteristics. The origin of the bursty emission and its possible link to the smooth component are discussed. The results are compared with the present experimental and theoretical knowledge of planetary radio emission beaming.
VLF wave stimulation by pulsed electron beams injected from the space shuttle
Authors: Reeves G. D. , Banks P. M. , Fraser‐Smith A. C. , Neubert T. , Bush R. I. , Gurnett D. A. , Raitt W. J.
Published: February 2008

Among the investigations conducted on the space shuttle flight STS 3 of March 1982 was an experiment in which a 1‐keV, 100‐mA electron gun was pulsed at 3.25 and 4.87 kHz. The resultant waves were measured with a broadband plasma wave receiver. At the time of flight the experimental setup was unique in that the electron beam was square wave modulated and that the shuttle offered relatively long times for in situ measurements of the ionospheric plasma response to the VLF pulsing sequences. In addition to electromagnetic response at the pulsing frequencies the waves exhibited various spectral harmonics as well as the unexpected occurrence of “satellite lines” around those harmonics. Both phenomena occurred with a variety of different characteristics for different pulsing sequences.
Ground‐level detection of low‐ and medium‐frequency auroral radio emissions
Authors: Benson Robert F. , Desch Michael D. , Hunsucker Robert D. , Romick Gerald J.
Published: February 2008

Natural radio emissions in the frequency range of several hundred kilohertz were detected at ground level in the auroral region. The measurements were made during a ground‐level observing program conducted near Fairbanks, Alaska, in the spring of 1986 in an attempt to detect auroral cyclotron‐maser generated whistler mode waves. The expectation for detecting such emissions was based on earlier theoretical work and satellite observations concerning auroral radio emissions. Four commercial communication receivers were used to obtain narrowband (3 kHz) samples of the radio noise spectrum at 150, 291, 500, and 700 kHz. Signals attributed to auroral processes were obtained on 7 of 15 nights of operation. Such signals were distinguished from man‐made interference by their bursty time structure and wide bandwidth (hundreds of kilohertz) and/or comparable signal intensities on orthogonal antennas. A one‐to‐one correlation with geophysical phenomena was not always readily evident. Emissions were observed even when magnetic activity was weak‐to‐moderate, and they were often absent during the strongest magnetic activity. Most of the strongest emissions, however, were observed in a small time interval around midnight magnetic local time (≈ ± 30 min). Such a tight ordering of the phenomena near magnetic midnight suggests a close tie‐in with auroral processes, even though the necessary conditions for the wave generation process are not readily apparent in conventional ground‐based geophysical records. The present results support earlier occasional reports in the literature, over the last 3 decades, of radio noise from the aurora at frequencies ∼ 100 kHz and indicate the importance of extending the upper frequency of ground‐based high‐latitude VLF stations to the vicinity of 1 MHz. Such observations may provide the means to continuously monitor cyclotron maser‐generated whistler emissions from the ground and, in turn, to determine under what conditions this specific instability process is initiated.
Low‐altitude observations of the conjugate polar cusps
Authors: Candidi M. , Meng C.‐I.
Published: February 2008

A set of 118 pairs of nearly simultaneous polar cusp observations over opposite hemispheres, taken by two identically instrumented satellites on orbits that are very close to each other, is used to determine the morphology of the conjugate polar cusps. It is found that the general mirror image picture does not extend to the precision of a fraction of a degree in magnetic latitude (MLAT). Only 54% of the equatorial boundary observations and 32% of the poleward boundary observations are within ±1° of the conjugate location. The influences of the Earth's magnetic dipole tilt, of geomagnetic activity, and of the IMF parameters on the cusp asymmetry are investigated; no unique parameter is found to better organize the conjugacy of the two cusps. Rather, all of these parameters appear to have a slight effect on the relative locations of the opposite polar cusps. The difference in the MLATs between boundaries of conjugate polar cusp regions depends on the latitude of the cusp location. Furthermore, the projection of the boundaries of the polar cusp regions onto the ionosphere does not follow the offset circle that describes the configuration of the average auroral oval.
On the dispersion of ion cyclotron waves in H+−He+ + solar wind‐like magnetized plasmas
Authors: Cuperman S. , Ofman L. , Dryer M.
Published: February 2008

We investigate by computer simulation experiments the nonlinear behavior of mixed H+−He+ + plasma systems under the particular physical conditions found by the linear theory to allow the parallel propagation of only weakly damped electromagnetic ion cyclotron waves at the alpha particle cyclotron frequency. Here the waves are generated by anisotropic proton populations. The essentially nonlinear results of our computer simulations indicate a strong damping of the parallel propagating electromagnetic ion cyclotron waves and a significant dip in the energy spectrum, both centered at about ω = Ωα.
Corotation lag of Saturn's magnetosphere: Global ionospheric conductivities revisited
Authors: Cheng Andrew F. , Waite J. H.
Published: February 2008

Eviatar and Richardson [1986] calculated the Pedersen conductance of Saturn's ionosphere and obtained values far smaller than previous estimates. They concluded that the low Pedersen conductance explained the corotation lag in the magnetosphere. We recalculated the Pedersen conductance and confirmed the original estimates. The magnetosphere should corotate with the ionosphere. We suggest that the eddy diffusion coefficient in Saturn's upper atmosphere may be small enough to allow the ionosphere to be significantly despun relative to the interior.
Implications of the GSFC Q3 model for trapped particle motion
Authors: Acuña M. H. , Connerney J. E. P. , Ness N. F.
Published: February 2008

The adiabatic motion of energetic charged particles in the Uranian magnetosphere is studied by numerical computation of field geometric invariants associated with the GSFC Q3 magnetic field model. Comparison of the computed L shell values along the Voyager 2 trajectory with those associated with the Uranian satellites Ariel, Umbriel, and Miranda as a function of their orbital position allows the prediction of times and locations where absorption effects of charged particles should be observable. Systematic differences in the observed and predicted counting rate (flux) minima suggest that the “age” of an absorption event must be taken into account and that a better understanding of the particles’ drift history is needed to explain the observed features. As a consequence of the large tilt of the dipole term with respect to the rotation axis, the Uranian moons sweep a large range of L shells during their orbital motion and occupy several different L minima defined in the text. Comparisons with previous results obtained from the offset tilted dipole model of Ness et al. (1986) yield a complex picture regarding the predicted and observed times for absorption signatures, depending on the minimum L value used for comparison. In spite of this complexity, a general pattern is evident in which the predicted locations for the absorption signatures fall inside (i.e., at smaller radial distances) the observed features, thus suggesting that radial diffusion effects as well as the drift history of the trapped particles must be taken into account to explain the energetic charged particle observations. It is thus expected that observed absorption signatures which are “young” with respect to the location of the absorbing body and the Voyager 2 spacecraft better reflect the geometry of the magnetic field.
An unusual charging event on ISEE 1
Authors: Olsen R. C. , Whipple E. C.
Published: February 2008

Electrostatic cleanliness requirements on ISEE 1 were expected to prevent negative charging in sunlight. This has largely been true, but on three occasions, ISEE 1 has been observed to charge to significant negative potentials in sunlight. Data from the two electric field experiments and from the plasma composition experiment on ISEE 1 show that the spacecraft charged to close to −70 V in sunlight at about 0700 UT on March 17, 1978. Data from the electron spectrometer experiment show that there was a potential barrier of some −10 to −20 V about the spacecraft during this event. The potential barrier was effective in turning back emitted photoelectrons to the spacecraft. Potential barriers can be formed by differential charging on the spacecraft or by the presence of excess space charge in the plasma. The shape of the barrier suggests that it is due to the former, even though electrostatic cleanliness specifications imposed on ISEE were intended to eliminate differential charging. Modeling of this event showed that the barrier could not be produced by the presence of space charge but that it was most likely produced by differential charging of the solar arrays.
Ion‐ion waves in the auroral region: Wave excitation and ion heating
Authors: Dusenbery P. B. , Martin R. F. , Winglee R. M.
Published: February 2008

The ion‐ion mode is excited in plasmas whenever two cold ion populations drift relative to one another. Such conditions occur in the high‐latitude auroral region associated with discrete auroral arcs due to the existence of parallel electric fields. These fields accelerate cold ionospheric ions (primarily H+ and O+ and on occasion He+) upward while accelerating plasma sheet electrons downward. In a previous study, two distinct regimes were found for the parallel propagating ion‐ion mode depending upon the relative ion drift: (1) two‐stream and (2) transition. Generalized analytic stability conditions are derived for the ion‐ion mode when fast (light mass, e.g., H+) and slow (heavier mass, e.g., O+) ion distributions are present. In particular, marginal stability conditions are derived for the parallel propagating ion‐ion waves in both regimes and peak growth conditions for the waves in the two‐stream regime. These conditions are compared to numerical results. The effects of varying relative ion concentration, relative ion mass, and relative ion‐electron temperature on the characteristic regimes of the ion‐ion and ion acoustic modes are investigated. A study of oblique propagation is also presented. Results show that for parallel waves (1) the threshold of the instability occurs at a smaller drift for O+ than for He+ interactions when a finite ion temperature is assumed, and (2) the maximum growth rate, as well as the range of ion drift over which enhanced growth occurs, varies with ion concentration and species. For obliquely propagating waves it is found that whenever the ion‐ion mode is in the two‐stream regime for parallel propagation, peak growth occurs on axis. When in the ion‐ion transition and ion acoustic regimes, however, peak growth occurs off axis. Furthermore, the range of wave normal angles excited depends upon ion composition, relative ion drift, and ion concentration. These results are used to suggest that ion temperature relative to H+ drift energy remains approximately constant throughout the acceleration region. In addition, the nature of the wave emissions and occurrence of differential ion heating are discussed in connection with this mode.
ULF electromagnetic turbulence in the high‐latitude topside ionosphere
Authors: Berthelier J. ‐J. , Machard C. , Cerisier J. ‐C. , Berthelier A. , Bosqued J. M.
Published: February 2008

Results obtained by the Aureol 3 satellite on electromagnetic waves in the ELF range in the high‐latitude topside ionosphere show the existence of a new type of turbulence. This turbulence is observed on both the electric and magnetic wave components and appears as bursts of 0.1 s to a few seconds typical duration in the polar cusp and auroral zone, associated with field‐aligned currents and low energy electron precipitations. The two possible interpretations of these observations, either in terms of a filamentary structure of the field‐aligned currents or of Alfven waves are discussed. For either of these models the observed values of the parallel component of the electric field are much larger than those predicted by classical models of the electrodynamics of the lower ionosphere. These results are compared with recent observations and might indicate the existence of acceleration mechanisms at low altitudes.
Forward‐reverse shock pairs associated with coronal mass ejections
Authors: Whang Y. C.
Published: February 2008

For some coronal mass ejections (CMEs), their interaction with the ambient solar wind can produce a forward‐reverse shock pair. The high‐speed mass ejecta compresses the plasma near the top of the CME on both sides of the tangential discontinuity which separates the CME plasma from the ambient solar wind plasma. The front of the compressed CME plasma propagates in the reverse direction relative to the ejecta flow, it may steepen to from a reverse slow shock. The front of the compressed solar wind plasma also propagates in the forward direction relative to the ambient solar wind and it may steepen to form a forward shock. The forward‐reverse shock pair associated with CMEs moves outward in interplanetary space and evolves into a pair of fast shocks. The interplanetary manifestation of some CMEs is pictured as a magnetic cloud accompanied by a shock pair: a forward shock precedes the cloud and a reverse shock either within or behind the cloud.
A three‐dimensional diffusion/convection model of the large scale magnetic field in the Venus ionosphere
Authors: Luhmann J. G.
Published: February 2008

An appreciation of how large‐scale magnetic fields can be maintained in the subsolar Venus ionosphere by the solar wind interaction was previously obtained with one‐dimensional diffusion/convection numerical models. Here, the solution of the diffusion/convection or dynamo equation for the ionospheric field is generalized to three dimensions under the assumption that the field and flow at the upper boundary (in the magnetic barrier) is known from a previous gas dynamic model, and that the ionospheric plasma velocity is known. The latter is given by the combination of the antisunward convection inferred from measurements, and the downward drift calculated from the observed vertical thermal pressure gradient. The results suggest that the low‐altitude magnetosheath field draping may be distorted by the interaction with the ionosphere in such a manner that there is an apparent "focusing" of the field toward the subsolar point. Although the model resolution is too course to resolve the magnetic "belt," an ionospheric field is produced that is strongest and parallel to the overlying field in the subsolar region, as is observed.
Thermospheric winds in the geomagnetic polar cap for solar minimum conditions
Authors: Meriwether J. W. , Killeen T. L. , McCormac F. G. , Burns A. G. , Roble R. G.
Published: February 2008

A Fabry‐Perot interferometer located at Thule, Greenland (Λ = 86) has monitored the F region thermospheric neutral wind over the northern hemisphere geomagnetic polar cap during the 1985/1986 solar minimum, winter solstice period. The wind observations were obtained by determining the Doppler shift of the (O I) 15,867‐K (630.0‐nm) emission. We present a subset of the measurements made during December 1985 to January 1986. Three factors make this data set unique and particularly valuable for a study of the effects of the deposition of energy and momentum from the magnetosphere into the high‐latitude neutral thermosphere. These factors are (1) the proximity of the observing station to the geomagnetic pole, (2) the continuous nature of the coverage due to the high geographic latitude and polar night conditions, and (3) the fact that the data set was obtained near solar minimum. The measured winds are compared with the simulations of the NCAR thermospheric general circulation model (TGCM). The results show that winds in the geomagnetic polar cap have a fundamental diurnal character, in accord with model predictions, with typical speeds of ∼200 m/s, generally in an antisunward direction. A large degree of variability, however, in both the magnitude and direction of the winds is observed, including evidence for curvature in the neutral flow within the instrumental region of observation (∼400 km diameter). Acceleration of the meridional component across Thule is observed at times. This acceleration is ascribed to regions of ion‐drag forcing associated with the magnetospheric input of energy and momentum. Characteristic asymmetric wind signatures were seen that were well correlated with positive or negative changes in the By component of the interplanetary magnetic field.
Global and local Joule heating effects seen by DE 2
Authors: Heelis R. A. , Coley W. R.
Published: February 2008

In the altitude region between 350 and 550 km, variations in the ion temperature principally reflect similar variations in the local frictional heating produced by a velocity difference between the ions and the neutrals. Here we show the distribution of the ion temperature in this altitude region and discuss its attributes in relation to previous work on local Joule heating rates. In addition to the ion temperature, instrumentation on the DE 2 satellite also provides a measure of the ion velocity vector representative of the total electric field. From this information we derive the local Joule heating rate. From an estimate of the height‐integrated Pedersen conductivity it is also possible to estimate the global (height‐integrated) Joule heating rate. Here we describe the differences and relationships between these various parameters.
Magnetic fields in the venus ionosphere: General features
Authors: Krymskii A. M. , Breus T. K.
Published: February 2008

Analysis of the properties of the dayside ionosphere at Venus has shown that two situations are characteristic. One corresponds to the high dynamic pressure of the solar wind, and the other is realized at low dynamic pressure. In both regimes the ionopause manifests itself as a change of the dominant chemical component of plasma rather than the boundary of the magnetized and unmagnetized plasmas (the widely used definition). The nonstationary convection of the magnetized plasma in the Venus dayside ionosphere is analyzed qualitatively. It is argued that the observable large‐scale magnetic field in the dayside ionosphere of Venus is the solar wind magnetic field pushed down into the ionosphere during the periods of high solar wind dynamic pressure which evolves under the action of convection and diffusion. The upper boundary of the magnetic belt forms in the vicinity of the upper boundary of the photochemical region. At low dynamic pressures of the solar wind, in a region from approximately 300 km to the ionopause, an upward flux of ionospheric plasma can exist due to motion of plasma to the terminator under the day‐to‐night pressure gradient. With the plasma convection and the results of the study of the Venus ionopause stability taken into account, the destruction of the large‐scale ionospheric field is a probable source of flux ropes in the Venus lower ionosphere rather than small‐scale ionopause instabilities.