bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct plasma measurements in the Io torus and inner magnetosphere of Jupiter
Authors: Bagenal Fran , Sullivan James D.
Published: February 2008

A model of the Io plasma torus has been constructed using the in situ plasma measurements of Voyager 1. A sharp gradient in plasma temperature of ∼7 × 105 K RJ−1 at 5.7 RJ divides the torus into two parts, a cold inner region, where the ions are closely confined to the centrifugal equator, and a warm outer region, which includes the orbit of Io and has a thickness scale height of 1 RJ. The outer edge of the warm torus is defined by a drop in plasma density near 7.5 RJ. The bulk motion of the plasma, i.e., the average velocity vector, is within 1% of the value expected on the basis of strict corotation in the inner part of the torus but probably deviates by 5 to 10% from corotation outside the torus. This breakdown from corotation may occur at the outer boundary of the warm torus. The energy per charge spectra show well‐resolved peaks in the inner part of the torus but strongly overlapping peaks in the outer part. In the inner torus there is a significant variation in the abundances of different ionic species over spatial scales <104 km. However, in the plasma sheet of the middle magnetosphere the ionic composition appears to be uniform from 12 to 42 RJ and is strongly dominated by ions with a ratio of atomic mass to charge of 16. These ions are most probably some combination of O+ and S2+ ions. One consequence of the observation is that the Alfven speed is uniformly low in the outer part of the torus, with values less than 250 km s−1.
The Jovian ring
Authors: Jewitt David C. , Danielson G. Edward
Published: February 2008

The results of further measurements of the Jovian ring system are presented. The system has three major components: the bright ring, the faint sheet, and the out‐of‐plane halo. The bright ring has an outer radius of 1.81±0.01 RJ, an inner radius 1.72±0.01 RJ, an eccentricity not greater than 0.003 and a normal optical depth 3 × 10−5. The faint sheet extends from the inner edge of the bright ring to the surface of Jupiter. Its optical depth is approximately 7 × 10−6. Three arguments are presented to show that a halo of material envelops the above two rings and extends 104 km above the ring plane. A simple model is invoked to account for the halo by means of interactions between the Jovian magnetic field and charged ring particles less than 0.5 µm in diameter. The source of small particles is probably within the bright ring itself and may be due to micrometeorite impact into larger ring bodies. Small particles evolve in towards Jupiter under Poynting Robertson and other drag forces. The outer edge of the ring system is defined by the satellite 1979J1.
Magnetic field‐aligned electron distributions in the dayside cusp
Authors: Zanetti L. J. , Potemra T. A. , Doering J. P. , Lee J. S. , Hoffman R. A.
Published: February 2008

Observations of low‐energy electron fluxes made over a 6‐year period with the photoelectron spectrometer on board the AE‐C and AE‐D spacecraft have been used to compile the first detailed survey of electron pitch angles in the low‐altitude cusp. One 16‐point energy spectrum from 2 to 500 eV was obtained every ¼ s with the photoelectron spectrometer instrument (with energy resolution ΔE/E=2.5%) providing a pitch angle resolution of about 6° at the 4‐rpm spacecraft spin rate. The location of the cusp was verified by the presence of protons detected with the low‐energy electron instrument. Observations were generally acquired below a 300‐km altitude with a few cases near 1000‐km altitude. Isotropic fluxes of precipitating electrons with Maxwellian energy spectra and characteristic energies of a few tens of electron volts were observed in the low‐altitude cusp as expected. However, the outstanding result that has emerged from this study is the presence of low‐energy electrons with pitch angles less than 15° streaming down into the cusp ionosphere. The energies of these streaming electrons sometimes appear as a peak superimposed upon the ‘normal’ cusp Maxwellian background of isotropic electrons. The streaming electron fluxes appear in about half the cusp observations, so we believe them to be stable features (i.e., they are not short bursts of particles). Using the velocity distribution contours computed from these photoelectron spectrometer measurements, we conclude that the streaming electrons cannot be explained in terms of a cusp plasma falling through an electric potential above the Atmosphere Explorer spacecraft.
Mechanism for the auroral red lower border
Authors: Benesch William
Published: February 2008

A model is proposed for the generation of the red lower border of type B auroras that results from the effect of increased collision frequency at the lower altitudes on the distribution of excitation within the nitrogen molecule. There are two characteristic spectral distributions for the red sequence (Δυ=3) of the molecular nitrogen first‐positive system, one for normal auroras and one for laboratory (higher pressure) discharges. The laboratory spectrum is shifted in wavelength by the intersystem collisional transfer of excitation with respect to the auroral spectrum, and such a shift manifests itself to the human eye as a two‐fold increase in luminous flux in the red. Recent investigations have suggested that it is the λ5577‐Å cutoff that is responsible for the red border and have placed the lower limit of the green line emission at about 100 km. The present enhancement mechanism, however, is consistent with the onset of added bright red emission at 85 km.
Mesospheric winds at low‐ and mid‐latitudes in the southern hemisphere
Authors: Vincent R. A. , Ball S. M.
Published: February 2008

Mesospheric wind measurements were made by using the partial reflection technique at a tropical location (Townsville, 19°S), and at a mid‐latitude location (Adelaide, 35°S) in June and November 1978. It is shown that the prevailing winds were typical of expected winter and summer type circulations in June and November, respectively, but there was a large difference in the amplitudes and structure of the diurnal tide between the two sets of observations. In November the tide was stable with a large amplitude and it appeared to ‘break’ at a height of 86 km. The propagating S1,1, mode dominated. In June, however, the tide was weak with large day‐to‐day variability and the evanescent S1,−2 mode was dominant. The semi‐diurnal tide amplitudes were relatively small but internal gravity wave amplitudes were significant. Spectral analyses showed that in the 2 to 8 hour period band the wave energy decreased as f−k with k in the range 1–1.5 and it is also shown that the waves were losing energy as a function of height. The inferred energy loss rates were ∼0.05 W kg−1 and a turbulent viscosity of about 500 m² s−1 was estimated near the mesopause at both sites and in both seasons.
ISEE 1 observations of thermal plasma in the vicinity of the plasmasphere during periods of quieting magnetic activity
Authors: Horwitz J. L. , Baugher C. R. , Chappell C. R. , Shelley E. G. , Young D. T. , Anderson R. R.
Published: February 2008

Thermal (≲100 electron volts) ion observations made with the plasma composition experiment on ISEE 1 are combined with plasma density profiles obtained from plasma frequency measurements made with the plasma wave experiment to conduct an investigation of thermal plasma behavior in the vicinity of the plasmasphere during periods of quieting magnetic activity. Normally, the principal thermal ion population in the plasmasphere consists of cold (kT ≲ 1 eV), isotropic distributions with ion species in the order of dominance H+ :He+ :O+, while outside the plasmapause, the observed E ≲ 100 eV ion distributions usually are field‐aligned in structure, have characteristic energies E ≲ 10 eV and H+ :O+ :He+ order of dominance in fluxes. During periods in which the magnetic activity quiets, the above two regions are separated by a new region in which, at times, low‐energy (∼1–2 eV) H+ and He+ are found flowing along the magnetic field lines. On other occasions following quieting magnetic activity, pancake distributions (peak fluxes at 90° pitch angle) are observed in this region. Other complex distributions have been seen, and these complexities and the limitations of the data coverage preclude a satisfactory simple interpretation. It seems plausible to identify this region as the site of plasmasphere refilling. However, the data presumably also contain evidence of the quiet time rotation of the plasmasphere bulge region into the morning sector.
A search for interplanetary He II, 304‐A emission
Authors: Paresce Francesco , Fahr Hans , Lay Günter
Published: February 2008

A region of the sky around the antisolar direction in the earth's shadow was systematically surveyed for the first time at high sensitivity and angular resolution by the extreme ultraviolet telescope on the Apollo‐Soyuz mission. The signal from the 170‐620 A channel of the instrument, which is sensitive primarily to the He II, 304‐A and He I, 584‐A resonance lines, shows a sharp decrease in intensity as the line of sight sweeps into the shadow region. Once the line of sight is completely contained within the shadow, a small residual signal above instrumental background and independent of view direction and spacecraft position is observed. Its intensity is consistent with the expected signal from the interplanetary He I, 584‐A emission measured by the 500‐780 A channel of the extreme ultraviolet telescope. An upper limit at the 2σ confidence level to a possible He II, 304‐A component that can escape detection by our instrument corresponds to 0.02 R (rayleigh) (1.6 × 10³ photons cm−2 s−1 sr−1) or to a column density of 109 ions cm−2 scattering at line center. This flux conceivably could be generated by a number of emission sources whose characteristics we investigate in detail. We find that the two mechanisms most likely to approach the observed limit are multiple scattering of solar radiation by He+ in the plasmasphere and resonance scattering of sunlight by interplanetary He+ produced by photoionization of interstellar neutral He. For the latter source, our upper limit can be used to set an upper limit of 109 s to the e‐folding time for the decrease of the suprathermal He+ velocity toward the bulk solar wind speed, if line radiation dominates the solar continuum beyond a full line width. If the opposite is true, the upper limit becomes 107 s. These high values conflict with commonly believed theories of the underlying relaxation process, and thus a drastic revision would be required if a 304‐A intensity of 0.02 R is confirmed as due to this source.
Equatorially trapped plasma populations
Authors: Olsen Richard Christopher
Published: February 2008

Plasma observations made at 5.5RE. by the USCD detectors on the SCATHA satellite reveal the existence of a thermal plasma population trapped within a few degrees latitude of the magnetic equator, defined here as the minimum B surface. These measurements were restricted to the 1000 to 2000 LT sector by the spacecraft orbit. The ions show a higher degree of anisotropy than the electrons, with a FWHM of 10° to 25°, narrowing with increasing energy. The electron distribution shows a width of 20° to 60°, again narrowing with increasing energy. The 20‐ to 100‐eV ion fluxes typically show temperatures in the 20‐ to 50‐eV range, and densities of 1–10 cm−3. The electron population typically extends from 50 to 500 eV, with temperatures of 100‐200 eV and densities also in the 1‐ to 10‐cm−3 range. Lower energy field‐aligned populations are occasionally found in both ions and electrons at the same location. These could be sources for the warmer pancake plasma if heated primarily in the perpendicular direction. Equatorial noise in the 20‐ to 200‐Hz range is seen in the electric field data about 90% of the time in association with the trapped plasma, with an intensity corresponding to the strenth of the trapped fluxes.
On the charge exchange loss of energetic charged particles in the Jovian magnetosphere
Authors: Ip W.‐H.
Published: February 2008

Following the suggestion that the magnetospheric charged particles may suffer charge exchange loss in the vicinity of the Io torus owing to the presence of neutral atomic and molecular clouds emitted from Io, some quantitative estimates are given to the resulting variation in the pitch angle distribution as well as the lossy radial diffusion between 5 and 8 RJ. Comparison of the theoretical calculations with the Voyager observations indicates that the low‐energy particle measurements are compatible with the existence of neutral clouds of molecules and atoms distributed between 5.6 RJ and 6 RJ (and beyond) with a total number density of ≲180 cm−3. This result reinforces the deduction from other observations that the neutral atoms and molecules ejected from Io into the Jovian magnetosphere could be quite extensive and effective in regulating the overall magnetospheric processes.
Solar wind flow about the terrestrial planets 1. Modeling bow shock position and shape
Authors: Slavin James A. , Holzer Robert E.
Published: February 2008

General technique for modeling the position and shape of planetary bow waves are reviewed. A three‐parameter method was selected to model the near portion (i.e.,x′ > −1 Rob) of the Venus, earth, and Mars bow shocks and the results compared with existing models using 1 to 6 free variables. By limiting consideration to the forward part of the bow wave, only the region of the shock surface that is most sensitive to obstacle shape and size was examined. In contrast, most other studies include portions of the more distant downstream shock, thus tending to reduce the planetary magnetosphere in question to a point source and constrain the resultant model surfaces to be paraboloid or hyperboloid in shape to avoid downstream closure. It was found by this investigation that the relative effective shapes of the near Martian, Cytherean, and terrestrial bow shocks are ellipsoidal, paraboloidal, and hyperboloidal, respectively, in response to the increasing bluntness of the obstacles that Mars, Venus, and earth present to the solar wind. The position of the terrestrial shock over the years 1965 to 1972 showed only a weak dependence on the phase of the solar cycle after the effects of solar wind dynamic pressure on magnetopause location were taken into account. However, the bow wave of Venus was considerably more distant around solar maximum in 1979 than at minimum in 1975–6 suggesting a solar cycle variation in its interaction with the solar wind. Finally, no significant deviations from axial symmetry were found when the near bow waves of the earth and Venus were mapped into the aberrated terminator plane. This finding is in agreement with the predictions of gas dynamic theory which neglects the effects of the IMF on the grounds of their smallness. Farther downstream where the bow wave position is being limited by the MHD fast mode Mach cone, an elliptical cross section is expected and noted in the results of other investigations.
Solar wind helium ions: Observations of the Helios solar probes between 0.3 and 1 AU
Authors: Marsch E. , Mühlhäuser K.‐H. , Rosenbauer H. , Schwenn R. , Neubauer F. M.
Published: February 2008

A survey of solar wind helium ion velocity distributions and derived parameters as measured by the Helios solar probes between 0.3 and 1 AU is presented. Nonthermal features like heat fluxes or He2+ double streams and temperature anisotropies have been frequently observed. Fairly isotropic distributions have only been measured close to sector boundaries of the interplanetary magnetic field. At times in slow solar wind, persistent double‐humped helium ion distributions constituting a temperature anisotropy T∥α/T⊥α > 1 have been reliably identified. Distributions in high‐speed wind generally have small total anisotropies (T∥α/T⊥α ≳ 1) with a slight indication that in the core part the temperatures are larger parallel than perpendicular to the magnetic field, in contrast to simultaneous proton observations. The anisotropy tends to increase with increasing heliocentric radial distance. The average dependence of helium ion temperatures on radial distance from the sun is described by a power law ∼ R−β with 0.7 ≲ β ≲ 1.2 for T∥α and 0.87 ≲ β ≲ 1.4 for T⊥α. In fast solar wind the T⊥α profile is compatible with nearly adiabatic cooling. Pronounced differential ion speeds Δvαp have been observed with values of more than 150 km/s near perihelion (0.3 AU). In fast streams Δvαp tends to approach the local Alfvén velocity vA, whereas in slow plasma values around zero are obtained. Generally, the differential speed increases with increasing proton bulk speed and (with the exception of slow plasma) with increasing heliocentric radial distance. The role of Coulomb collisions in limiting Δvαp and the ion temperature ratio Tα/Tp is investigated. Collisions are shown to play a negligible role in fast solar wind, possibly a minor role in intermediate speed solar wind and a distinct role in low‐speed wind in limiting the differential ion velocity and temperature.
Whistler‐mode interactions with plasma sheet electrons
Authors: Edgar B. C. , Koons H. C.
Published: February 2008

We present a unique set of wave and particle measurements taken by the S3‐3 satellite on August 2, 1976, from which we infer that signals from a ground‐based VLF transmitter were ducted up to the equatorial plasma sheet region by a detached region at L ∼ 4.5. The waves resonated with the ∼ 1 keV plasma sheet electrons, producing narrowband emissions that were received in the conjugate hemisphere by the S3‐3 satellite at L ∼ 3.
Adiabatic charged particle motion in rapidly rotating magnetospheres
Authors: Northrop T. G. , Birmingham T. J.
Published: February 2008

When the cE × B/B² drift velocity is large enough to exceed the particle gyro velocity, adiabatic theory becomes more complicated and less useful: more complicated because there are five drifts in addition to the E × B gradient, and line curvature drifts and three more terms in the parallel equation of motion; less useful because the second and third invariants J and Φ are no longer valid in mirror geometry due to the rapid drift across field lines that destroys any semblance of periodicity in the bounce motion. (Approximate periodicity is necessary to have an adiabatic invariant.) But the special case of a rapidly rotating rigid magnetic field, with E∥ = 0, is an exception. If the field at any time is merely a rotation of that at an earlier time, the drift and parallel equations simplify, and even better, the second invariant is again valid. The drift equation now has two rather than five additional terms—a Coriolis drift and a centrifugal drift. Just as in a slowly or nonrotating mirror system, the second invariant now provides a (rapidly rotating) drift shell to which the guiding center is confined. The particle kinetic energy in the rotating frame, minus the centrifugal potential, is an exact constant of motion in a rigid rotator. This constant is well known but does not by itself put limits on radial motion toward or away from the rotation axis, hence it does not limit particle energy changes. But the drift shell does limit and make periodic any radial excursions, and this is why previous studies of particle motion, in particular examples of rigid rotators, have shown that particles experience no steady energy gain or loss.
Global characteristics of the cold plasma in the equatorial plasmapause region as deduced from the Geos 1 Mutual Impedance Probe
Authors: Décréau P. M. E. , Béghin C. , Parrot M.
Published: February 2008

Thermal plasma parameters derived by the mutual impedance experiment on GEOS are described. The experiment is well suited to the measurement of the electron density and temperature of the outer plasmasphere (when kTe/Ne < 1.6 eV/cm³). This investigation of the whole set of data supplied by GEOS 1 (4 < L < 8, 0000–2400 MLT) covers three main regions : the plasmasphere, an intermediate region of ionospheric refilling, and the plasma trough. In the plasmasphere, we observe profiles with Ne ∝ L−4, while Te stands around 10,000° K or less. The intermediate region, situated next to the plasmasphere and above it, is always present in the day sector, where the ionospheric source plays a leadingpart. In that zone, the plasma parameters, poorly known up to now, exhibit Ne values ∼ 2 to 20 cm−3, together with Te values of 20,000° K on the average, dispersed over a 5,000 to 100,000° K range during disturbances. In the night sector, the intermediate region is seen only during the recovery phase. The region of depleted density is observed at the higher L values in the night and morning MLT sectors. There, plasmas out of Maxwellian equilibrium are seen under disturbed conditions. The dynamic response of the thermal plasma parameters to temporal variations of the am index of magnetic activity follows a known scenario as concerns Ne, making apparent a night‐to‐day, MLT dependent time delay. As concerns Te, the dynamical study reveals striking features, such as the persistance of the Te modifications into the dusk sector, the interpretation of which remains to be clarified.
Solar wind energy transfer through the magnetopause of an open magnetosphere
Authors: Lee L. C. , Roederer J. G.
Published: February 2008

An expression for the total power PT transferred from the solar wind to an open magnetosphere is derived. The power is transmitted through an ‘open’ magnetopause with a nonzero normal component of the magnetic field, which is identified as a rotational discontinuity. The total power PT consists of (1) the power PEM representing the electromagnetic energy transfer and (2) the power PKE representing the rate of kinetic energy carried by particles penetrating into the magnetosphere. It is found that PEM ≃ VSWBSWψ, PKE ≃ (½MA ‐ 1) PEM and PT ≃ ½MA PEM, where VSW, BSW, and MA are the velocity, magnetic field, and the Alfvén‐Mach number in the solar wind, respectively, and Ψ is the open magnetic flux in the magnetosphere. The Alfvén‐Mach number of the flow at the magnetopause determines the nature of the local energy transfer; the power per unit area transferred from the solar wind to the magnetosphere consists mainly of kinetic energy. The electromagnetic energy rate PEM controls the near‐earth magnetospheric activity, whereas the kinetic energy rate PKE (≃ 3‐4 PEM) should dominate the dynamics of the distant magnetotail.
Simultaneous rocket probe and radar measurements of equatorial spread F—Transitional and short wavelength results
Authors: Kelley M. C. , Pfaff R. , Baker K. D. , Ulwick J. C. , Livingston R. , Rino C. , Tsunoda R.
Published: February 2008

During the PLUMEX I rocket flight from Kwajalein Island, plasma density and electric field fluctuations were measured in situ, simultaneous with ground‐based radar backscatter measurements at 0.96‐m and 0.36‐m wavelengths. The rocket penetrated an extremely turbulent topside region which had associated intense backscatter. As measured by the radar the backscatter power was decaying with time during and after the flight. The intermediate wavelength (0.1–10 km) in situ electron density measurements are described in a companion paper, while here we report the transitional and short wavelength results (λ < 100 m). These data include the first in situ equatorial spread F measurements of the electric field component of electrostatic fluctuations with wavelengths less than 1 m. At all altitudes above about 280 km, a repeatable form for the wave‐number spectrum was found for the electron density and electric field fluctuations at wavelengths less than about 100 m. The density spectrum varies approximately as k−5 and the electric field spectrum as k−3. The steepness of the density spectrum corresponds to an absence of steep edges in the density waveform on the scale of 100 m and less. These two spectral forms are shown to be consistent with an explanation involving low‐frequency waves with finite wave numbers parallel to the magnetic field (k∥). Both theory and laboratory experiments show a power law density fluctuation spectrum for gradient‐driven drift waves with negative index in the range 4.5–6.0. Since such waves do have finite k∥, and since sharp gradients exist in the spread F environment, we conclude that at sufficiently high altitudes, drift waves act on the steep gradients caused by a primary longer‐wavelength instability to create the observed spectral form. These waves may then create an anomalous diffusion as discussed by Huba and Ossakow (1981b). At lower altitudes a shallower spectral index was observed in the tens of meters range, which may be related to a collisional damping regime. This suggests an altitude threshold for the drift waves that is probably related to ion neutral collisions. The power law spectra show no marked change near k⊥ri ≈ 1 where ri is the ion gyroradius. Since low‐frequency drift waves are linearly stable for k⊥ri ≳ 1, it seems that a wave‐wave interaction (cascade) operates to deposit energy in a range where waves are linearly damped. There is a slight suggestion of a spectral change (smaller negative index) for k⊥re ≳ 0.2 which may be due to excitation of a lower‐hybrid drift wave and which may be related to the observed enhanced backscatter at wavelengths on the order of 1 m. A stability analysis shows that the plasma is near but on the stable side of the marginal stability boundary for the lower‐hybrid drift wave in the most intense region of backscatter. In regions devoid of drift waves, evidence is found for an exponential inner‐scale cutoff at a wavelength of 200 m.
Factors controlling degree of correlation between ISEE 1 and ISEE 3 interplanetary magnetic field measurements
Authors: Crooker N. U. , Siscoe G. L. , Russell C. T. , Smith E. J.
Published: February 2008

The degree of correlation between ISEE 1 and ISEE 3 IMF measurements is highly Variable. Approximately 200 two‐hour periods when the correlation was good and 200 more when the correlation was poor are used to determine the relative control of several factors over the degree of correlation. Both IMF variance and spacecraft separation distance in the plane perpendicular to the earth‐sun line exert substantial control. Good correlations are associated with high variance and distances less than 90 RE. During periods of highest variance, good correlations occur at distances beyond 90 RE up to 120 RE, the maximum range of ISEE 1‐ISEE 3 separation. Thus it appears that the scale size of magnetic features is larger when the variance is high. Abrupt changes in the correlation coefficient from poor to good or good to poor in adjacent two‐hour intervals appear to be governed by the sense of change of IMF variance: changes in correlation from poor to good correspond to increasing variance and vice versa. The IMF orientation also exerts control over the degree of correlation. During periods of low variance, good correlations are most likely to occur when the distance between ISEE 1 and ISEE 3 perpendicular to the IMF is less than 20 RE. This scale size expands to ∼50 RE during periods of high variance. Solar wind speed shows little control over the degree of correlation in the speed range 300–500 km/s.
The hidden ion population of the magnetosphere
Authors: Olsen Richard C.
Published: February 2008

Particle data from two geosynchronous satellites (Applied Technology Satellite 6 and SCATHA) show a normally hidden ion population appearing when the satellites are in the earth's shadow. Ion and electron data show the spacecraft potential dropping from +10 V in sunlight to +4 to +5 V in eclipse at local midnight, in low‐energy (Te < 1 keV) plasma sheet environments. During eclipse, a cold (T = 1 eV), dense (n = 10–100 cm−3), isotropic ion population appears which was invisible in sunlight because of the larger positive spacecraft potential. Higher‐energy populations generally cover the tails of the hidden ion populations, so they cannot be inferred from daylight data. The isotropic population appears only in a few percent of the spacecraft eclipse events, appearing only at times of low Kp (2 or less, preceded by a day with Σ Kp ≤ 20). A low‐energy (T = 1–2 eV) field‐aligned population often appears with and without the isotropic population, at slightly higher flux levels. These fluxes are visible in sunlight, but again the distribution functions obtained in eclipse differ from those that would be inferred from daylight data. Measurement of the thermal plasma population on a consistent basis, particularly in the plasma sheet, will require some method of controlling the detector potential with respect to the ambient plasma.
An analysis of the structure of Saturn's magnetic field using charged particle absorption signatures
Authors: Chenette D. L. , Davis Leverett
Published: February 2008

A new technique is derived for determining the structure of Saturn's magnetic field. This technique uses the observed positions of charged particle absorption signatures due to the satellites and rings of Saturn to determine the parameters of an axially symmetric, spherical harmonic model of the magnetic field using the method of least squares. Absorption signatures observed along the Pioneer 11, Voyager 1, and Voyager 2 spacecraft trajectories are used to derive values for the orientation of the magnetic symmetry axis relative to Saturn's axis of rotation, the axial displacement of the center of the magnetic dipole from the center of Saturn, and the magnitude of the external field component. Comparing these results with the magnetic field model parameters deduced from analyses of magnetometer data leads us to prefer models that incorporate a northward offset of the dipole center by about 0.05 Rs.
Observation and modeling of energetic particles at synchronous orbit on July 29, 1977
Authors: Baker D. N. , Fritz T. A. , Wilken B. , Higbie P. R. , Kaye S. M. , Kivelson M. G. , Moore T. E. , Stüdemann W. , Masley A. J. , Smith P. H. , Vampola A. L.
Published: February 2008

In the 12 hours following a worldwide storm sudden commencement at 0027 UT on July 29 there was a series of at least four magnetospheric substorms, the last and largest of which exhibited an expansion phase onset at ∼1200 UT. Data from six spacecraft in three general local time groupings (0300, 0700, and 1300 LT) were examined, and vector magnetic field data and energetic electron and ion data from ∼15 keV to >2 MeV were employed. Four primary types of studies were carried out: (1) timing and morphology of energetic particle injections; (2) variation of particle phase space densities f(µ = p²/2mB), using local magnetic field and particle flux data; (3) measurement of boundary motions, using high‐energy ion gradient anisotropies; and (4) adiabatic modeling, which included injection, large‐scale convection, corotation, and gradient drifts. For the 1200 UT substorm it is concluded that there was a substantial flux dropout in a broad sector near local midnight because of a large‐scale boundary motion, followed by a recovery to a predropout configuration. There were then several subsequent injection events with distinct onsets (extending as far eastward as 0300 LT), for which ion anisotropy information suggests an inward motion of particles from outside of geostationary orbit. Particle drift information reveals that these particles drifted azimuthally completely around the earth. It is also concluded from the phase space density studies that ‘fresh’ particles with magnetic moments of up to at least several hundred MeV/gauss were injected near geostationary orbit. The present adiabatic convection model can explain the observed injection of large magnetic moment particles from the plasma sheet into synchronous orbit, although physical elements of the normal model must be altered somewhat.
The effect on thermospheric chemistry of a resonant charge exchange reaction involving vibrationally excited N2+ ions with atomic oxygen
Authors: Abdou Wedad A. , Torr D. G. , Richards P. G. , Torr M. R.
Published: February 2008

The charge exchange of O+ (²D) with N2+ is one of the major scources of N2+ in the thermosphere. Its rate coefficient k was measured in the laboratory to be ≅ 1×10−9 cm³ s−1. Model calculations using this reaction rate yielded N2+ and NO+ concentrations in excess of those measured by the Atmosphere Explorer (AE) satellites. It was deduced that a smaller value of ∼ 1 × 10−10 cm³ s−1 was necessary in order to provide agreement between calculated and measured values of N2+ and NO+. Since the laboratory measurements were performed at high ion energies, it was suggested that the charge exchange of O+(²D) with N2 proceeded faster in the laboratory than in the thermosphere where only thermal energies are acquired by the ions. However, recent laboratory studies by two independent groups have confirmed a value of 8 ± 3×10−10 cm³ s−1 for the rate coefficient of the charge exchange of O+ (²D) with N2 at thermal energies. In this paper we draw the attention to the fact that the resonant reaction O+ (²D) + N2 (X¹Σg+)υ = 0 ⇌ N2+ (X²Σg+)υ = 5 + O(³P) has not been considered in previous studies of N2+ chemistry in the ionosphere. We show here that the inclusion of the back reaction as a new sink for N2+ resolves the discrepancy between the laboratory and aeronomically deduced results.
The dynamical evolution of the Saturnian ring spokes
Authors: Hill Jay Roderick , Mendis D. A.
Published: February 2008

It is proposed that the so‐called radial spokes observed in rotation across Saturn's B ring are composed of fine charged dust levitated off the surfaces of larger bodies by electrostatic forces when the latter are sporadically charged to high electrostatic potentials. From the observed wedge‐shaped morphology of these spokes it is deduced that the dust grains are negatively charged. The dynamical structure of a spoke feature consisting of grains of different sizes is considered. It is shown that the surface density of dust within these wedge‐shaped spokes is not uniform. Each spoke has a fine structure consisting of a number of almost straight sharp ‘ribs’ radiating out from points on the synchronous orbit. The time evolution of a spoke can be likened to the continuous unfolding of a fan whose vertex is at the synchronous orbit and one of whose edges (the corotating one) is fixed in the radial direction. The model also predicts that the measured spoke velocities correspond to phase velocities rather than particle velocities, as is generally believed. According to this model, the angular velocity of the leading edge of the fan depends on the age of the spoke. Inside the synchronous orbit it is super‐Kepler up to about 2.5 hours after formation, but sub‐Kepler between 2.5 hours and 6 hours. Those spokes surviving the ring plane crossing will once again have a super‐Kepler leading edge after 6 hours. Outside the synchronous orbit the reverse would be the case. Consequently the model anticipates the observed scatter of the measured velocities about the Kepler value as real rather than due to errors of measurement, although they lie within the expected error bars. Finally, it is shown that the resettling of the grains on the larger bodies in the ring plane following their initial levitation results in a differential transport of grains across the ring plane. A consequence of this is the establishment of different multimodal size distributions of dust within the spokes at different planetocentric distances.
The occurrence and characteristics of electron beams over the polar regions
Authors: Collin H. L. , Sharp R. D. , Shelley E. G.
Published: February 2008

Data from the Lockheed experiment on the polar‐orbiting satellite S3‐3 have been used to study the morphology and some of the characteristics of narrow, field‐aligned electron beams. The beams were found to occur very commonly in a circumpolar zone corresponding to that of the statistical auroral oval. Upflowing, downflowing, and counterstreaming beams were found throughout the altitude range examined (3000–8000 km), but the variation of occurrence frequency with altitude depended on beam direction and indicated that the source region of the beams was commonly at or above 1 RE. The beams are characterized by a very narrow pitch angle distribution with a median half width at half maximum of 6° and by very soft energy spectra, and they were not usually detectable at energies above 400 eV. In contrast, their peak fluxes extended over a wide range, sometimes exceeding 1011 (cm² sr s keV)−1, although more usually they were less than 3×109 (cm² sr s keV)−1. Upflowing ions, especially ion conics, were commonly found in close association with the electron beams. It appears likely that several of the main features of the beams can be accounted for by a source model of a region of short‐lived multiple double layers.
Global distribution of ionospheric and field‐aligned currents during substorms as determined from six IMS meridian chains of magnetometers: Initial results
Authors: Kamide Y. , Ahn B.‐H. , Akasofu S. ‐I. , Baumjohann W. , Friis‐Christensen E. , Kroehl H. W. , Maurer H. , Richmond A. D. , Rostoker G. , Spiro R. W. , Walker J. K. , Zaitzev A. N.
Published: February 2008

As a part of the joint efforts of operating six meridian chains of magnetometers during the IMS, magnetic records from 70 stations are used to deduce patterns of electric fields and currents in high latitudes on March 17, 18, and 19, 1978. First of all this data set is used to examine the fidelity of the AE(12) index by comparing it with the AE(70) index and also the fidelity of the AL(70) index as a measure of the total westward electrojet intensity. The coefficients for the two correlations are found to be more than 0.8. Then the distribution of both ionospheric currents and field‐aligned currents, as well as the electric field, are for the first time determined with a time resolution of 5 min by using an appropriate conductivity distribution model. Although much improvement is still needed for better accuracy, especially in the estimation of ionospheric conductivity, it has now become possible to study the growth and decay of the three‐dimensional current system over the north polar region during individual magnetospheric substorms with sufficient time resolution. Our initial results show that the gross features of the instantaneous distributions of the ionospheric and field‐aligned currents are remarkably similar to the daily average pattern during a very weak activity and at different substorm epochs and that the large‐scale current pattern grows and decays systematically as a whole. There are, however, significant changes in local scales.
The distant interplanetary wake of Venus: Plasma observations from Pioneer Venus
Authors: Mihalov John D. , Barnes Aaron
Published: February 2008

In June 1979 the Pioneer Venus orbiter made its first series of passes through the distant solar wind wake of Venus at distances of 8–12 Rv behind the planet. During this period the plasma analyzer aboard the spacecraft detected disturbed magnetosheath plasma that intermittently disappeared and reappeared, suggesting a tattered, filamentary cavity trailing behind the planet. The magnetosheath dropouts almost always occurred inside the region of ‘magnetotail’ observed by Russell et al. Sporadic bursts of energetic ions (E/q ≳ 4 kV) are detected inside and, occasionally, outside the magnetotail; all such bursts are consistent with identification of the ion as O+ of planetary origin moving at the local magnetosheath flow speed. The morphology of the plasma dropouts and of the O+ bursts is analyzed in detail. The cavity appears to contract at times of high solar wind dynamic pressure. The intensity of the O+ component is highly variable, and appears not to be strongly correlated with solar wind dynamic pressure. The most intense bursts correspond to a flux ≲107 ions cm−2 s−1. This maximum flux, if steady and filling a cylinder 1 Rv in radius would correspond to a mass loss rate of ≲1025 ions s−1; the intermittency and variability of the flux suggest that the true mean loss rate is very much lower. The kinetic temperature of the O+ component is estimated as 105–106 K in order of magnitude.
Coordinated airborne and satellite measurements of equatorial plasma depletions
Authors: Weber E. J. , Brinton H. C. , Buchau J. , Moore J. G.
Published: February 2008

A series of experiments was conducted in December 1979 to investigate the structure of plasma depletions in the low latitude, nightime ionosphere. The measurements included all sky imaging photometer (ASIP), ionosonde and amplitude scintillation observations from the AFGL Airborne Ionospheric Observatory (AIO), and in situ ion density measurements from the Atmosphere Explorer (AE‐E) Bennett Ion Mass Spectrometer (BIMS). The AIO performed two flights along the Ascension Island (−18° MLAT) magnetic meridian: one in the southern hemisphere and one near the Ascension conjugate point in the northern hemisphere. During these flights, measurements from the AE‐E satellite at 434 km altitude are compared with simultaneous remote ionospheric measurements from the AIO. Density biteouts of approximately one order of magnitude in the dominant ion O+, were mapped to lower altitudes along magnetic field lines for comparison with 6300‐Å and 7774‐Å O I airglow depletions. Because of the different airglow production mechanisms (dissociative recombination of O2+ for 6300 Å and radiative recombination of O+ for 7774 Å) the 6300‐Å depeletions reflect plasma depletions near the bottomside of the F layer, while those at 7774 Å are located near the peak of the layer. The O+ biteouts map directly into the 7774‐Å airglow depletions in the same hemisphere and also when traced into the opposite hemisphere, which indicates magnetic flux tube alignment over north‐south distances of ∼2220 km. The 6300‐Å (bottomside) depletions are wider in longitude than the 7774‐Å (F‐peak) depletions near the equatorward edge of the Appleton anomaly. This difference in topside and bottomside structure is used to infer large‐scale structure near the anomaly and to relate this to structure, commonly observed near the magnetic equator by the ALTAIR radar.
Thin, rotating plasma disks
Authors: Cheng Andrew F.
Published: February 2008

The MHD equilibrium of thin, rotating plasma disks is considered. An axisymmetric steady state is assumed. A corotation lag is included, as is a nonzero toroidal field with Bф ∝ rBr. Rigorous expressions are obtained for a characteristic Alfvén Mach number and temperature, both of which involve vertical averages. If there is strict corotation, the temperature always diverges at large radii. If there is a corotation lag, the temperature need not diverge, and the magnetodisk solutions can extend to arbitrarily large radii. With weak additional assumptions, it is shown that MA < 21/2 for a mainly poloidal field and MA < 1 for a mainly toroidal field. For the Jovian magnetodisk, MA = 0.8 from Voyager 1 inbound data, and the observation of a temperature nearly constant or slowly decreasing with r from 20 RJ to 80 RJ on the outbound pass implies a corotation lag there.
Electron velocity distributions near the Earth's bow shock
Authors: Feldman W. C. , Anderson R. C. , Bame S. J. , Gary S. P. , Gosling J. T. , McComas D. J. , Thomsen M. F. , Paschmann G. , Hoppe M. M.
Published: February 2008

A survey of two‐dimensional electron velocity distributions, ƒ(V), measured near the earth's bow shock using Los Alamos/Garching plasma instrumentation aboard ISEE 2 is presented. This survey provides clues to the mechanisms of electron thermalization within the shock and the relaxation of both the upstream and downstream velocity distributions. First, near the foreshock boundary, fluxes of electrons having a power law shape at high energies backstream from the shock. Although most often they appear as a monotonically decreasing extension of solar wind distributions in the backward hemisphere along the magnetic field direction,
, they occasionally appear as a resolved peak in energy. Within the interior of the foreshock, in addition to the hot, isotropic electrons at higher energies, field‐aligned depressions in ƒ(V) are observed at the lowest energies (E ≲ 15 eV) and twin angular peaks centered on
are observed at intermediate energies (15 eV ≲ E ≲ 45 eV). Such distributions are associated closely with 1‐Hz whistler waves. Second, within the shock, cuts through ƒ(V) along
ƒ(V∥), often show single maxima offset toward the magnetosheath by speeds comparable to, but larger than, the upstream thermal speed. When sequences of such distributions are observed in a single shock transition, offset speeds increase and peak heights of ƒ(V∥) decrease with increasing penetration toward the downstream (magnetosheath) side. Third, magnetosheath distributions generally have flat tops out to an energy, E0, with maxima substantially lower than that in the solar wind. Occasionally, cuts through ƒ(V) along
show one and sometimes two small peaks at the edge of the flat tops making them appear concave upward. The magnetosheath distributions often have strong angular anisotropies which depend on energy. For energies less than E0, ƒ(V∥) > ƒ(V⊥) at constant E, whereas for E > E0, ƒ(V∥) < ƒ(V⊥). The electron distributions characteristic of these three regions are interpreted as arising from the effects of macroscopic (scale size comparable to or larger than the shock width) electric and magnetic fields and the subsequent effects of microscopic (scale size small in comparison with the shock width) fields. In particular, our results suggest that field‐aligned instabilities are likely to be present in the earth's bow shock.
Magnetospheric convection effects at mid‐latitudes: 3. Theoretical derivation of the disturbance convection pattern in the plasmasphere
Authors: Blanc Michel
Published: February 2008

A theoretical investigation of the generating mechanisms of the disturbance E×B drifts observed at mid‐latitudes is performed, with the purpose of understanding the local time dependence of the average disturbance drifts observed above Saint‐Santin (L = 1.8, 47° geomagnetic latitude) and of extending if to other latitudes. To this end, the middle‐ and low‐latitude effects of the two possible generators of electric field disturbances, the solar wind/magnetosphere dynamo, and the ionospheric wind disturbance dynamo, computed by means of a numerical dynamo model of the ionosphere, are systematically compared with the Saint‐Santin disturbance drift pattern. The solar wind/magnetosphere dynamo is simulated as an electrostatic potential generator at 75° latitude, and its local time dependence determined by least squares fitting the drifts calculated at 45° latitude to the observed drift pattern. When the solar wind/magnetosphere dynamo only is considered, the best fit is found for a diurnal sinusoīdal shape of the 75° latitude potential corresponding to a 40 kV total potential drop across the polar cap rotated by 2 hours eastwards from the dawn‐dusk direction. But this source reproduces only one half of the 34‐m/s westward steady component observed in Saint‐Santin average disturbance drifts. The remaining gap can be filled by inclusion of Blanc and Richmond's (1980) disturbance dynamo electric field model into the fitting procedure. This fitting procedure provides a global theoretical pattern of disturbance electric fields and E×B drifts within the plasmasphere that can be compared with observations. Three latitude zones can be distinguished in the resulting picture. In the external regions of the plasmasphere, one finds an extension of the usual picture of high‐latitude plasma drifts, eastward in the morning and westward in the afternoon. The calculated drifts compare fairly well with the drifts disturbance models produced from whistler data at L = 4 and from the Millstone Hill radar. At mid‐latitudes, between 55° and 20° invariant latitude, we predict that the superposition of the two dynamos produces westward drifts at all local times; this is consistent with the Saint‐Santin and Arecibo radar drift data.