bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagation directions of hydromagnetic waves in interplanetary space: Pioneer 10 and 11
Authors: Parker G.D.
Published: February 2008

The real and imaginary parts of spectral matrices of magnetic field fluctuations near 4 and 5 AU are used separately to generate estimates of hydromagnetic wave normal directions over the frequency range 10−5 to 10−2 Hz in the spacecraft frame. Both procedures suggest the predominance of the Alfvén mode over the fast and slow MHD modes among transverse magnetic field perturbations. Both procedures also show that the Alfvén mode is more common among transverse perturbations at 4 AU than at 5 AU. We develop statistical techniques for estimating uncertainties in computed wave normal directions and conclude that the real parts of the spectral matrices are more useful than the imaginary parts in specifying wave normals at times of magnetic quiet in the solar wind near 4 and 5 AU.
Photometry and polarimetry of Saturn's rings from Pioneer Saturn
Authors: Esposito Larry W. , Dilley James P. , Fountain John W.
Published: February 2008

We present a profile of the average normal optical depth for Saturn's rings between 1.22 and 2.35 Rs. In the A and B rings, horizontal inhomogeneities make these values deceptive. A thinner component of the B ring with optical depth less than 0.08 covers ½% to 4% of its surface area. In the A ring the more transparent component has optical depth τs >0.10 and covers more than 7% of its area. These thinner parts of the rings would rarely be apparent from earth‐based observations. The particles in the C ring are larger than 15µ and different from those in the B and A rings. The C ring is either homogeneous with high albedo (in the red)
and forward scattering phase functions or shows a gradient in albedo with distance from Saturn. In the latter case the red albedo ranges from at the inner edge of ring C to at the outer edge, and the phase function is both moderately forward and backward scattering. Polarimetry of Saturn's rings provides only an upper limit for the B ring polarization, p <15%, which is consistent with predictions from ground‐based data. In the outer A ring, the polarization is negative p(blue) = −7±2%; p(red) = −1±2%. Azimuthal variations are seen on the darkside of the A ring with a peak‐to‐trough ratio of 1.25. The position angle of the maximum is close to that seen from earth. No variations are seen in the B and C rings with an upper limit of 6%. The measured color of the A ring supports the many particle thick model of Saturn's rings.
An empirical model of the Venusian outer environment 2. The shape and location of the bow shock
Authors: Smirnov V. N. , Vaisberg O. L. , Intriligator D. S.
Published: February 2008

Venera 9 and Venera 10 bow shock crossings are analyzed for solar zenith angles in the range from ∼25° to ∼153°. The best fit to 62 observed crossings gives the location of the subsolar point of the bow shock as ∼7600 km from the center of Venus or 1.27 Rv. Comparison of Venera 9 and Venera 10 bow shock crossings with those observed by the Pioneer Venus orbiter indicate that both the secular variation and the latitudinal asymmetry may be responsible for the closer shock crossings of the Venera 9 and Venera 10 spacecraft.
The character of the thermal emission from Venus
Authors: Apt Jerome , Brown Robert A. , Goody Richard M.
Published: February 2008

Images of the thermal infrared emission from 10.6 to 12.6 µm from the earth‐facing hemisphere of Venus were made from earth on each available opportunity over a 26‐day period in 1977 and a 106‐day period in 1978–1979, using a 1.5 m telescope. Compact, variable thermal features exist at latitudes higher than about 50°, some of which repeat at 5‐day intervals. No such repeatability is observed for features near the equator. Day‐to‐day variations in limb darkening and the contrast of high latitude features appear to differ in the conjunction and post‐conjunction images. A strong, solar‐fixed component of the emission exists and exhibits wave number 1, 2, and 4 components. The thermal tides provide constraints upon the atmosphere structure. The solar‐fixed structure shows significant asymmetries with respect to the equator.
Distribution and source of the UV absorption in Venus' atmosphere
Authors: Pollack James B. , Toon Owen B. , Whitten Robert C. , Boese Robert , Ragent Boris , Tomasko Martin , Esposito Larry , Travis Larry , Wiedman Donald
Published: February 2008

Information about the nature of the UV absorbing agents in Venus' atmosphere has been obtained from a comparison of model predictions with a variety of photometric data obtained from the Pioneer Venus probes and orbiter as well as from the earth. The theoretical simulations were performed with numerically accurate radiative transfer codes that incorporated spacecraft constraints on the properties of aerosols and gases in Venus' atmosphere. We find that at least two UV absorbers are needed: gaseous SO2, which is a major source of opacity at wavelengths shortward of 0.32 µm, and a second absorber, which dominates above 0.32 µm. The concentration of the second absorber increases sharply with increasing optical depth within the first few optical depths and then has a relatively flat profile throughout most of the remainder of the upper main cloud layer; but it is largely absent from the middle and lower cloud regions. Several mechanisms appear to be responsible for producing UV contrast features. These include changes in the optical depth of the upper haze layer, which leads to brightness differences between equatorial and polar regions, and changes in the depth over which the second UV absorber is depleted in the uppermost part of the main clouds, which creates UV features at equatorial latitudes. Atmospheric dynamics acts as a driving force for the above variations in the radiative properties of the clouds through dynamically induced temperature changes and changes in vertical mixing. Many UV‐absorbing materials, including sulfur, various iron compounds, and nitrogen dioxide gas, have spectral absorption properties that are in serious disagreement with those deduced for the second UV absorber. One possibly tenable candidate for the second absorber is gaseous Cl2, whose spectral absorption characteristics closely match the empirically derived ones and whose expected altitude profile is crudely consistent with the sharp decreases in the concentration of the second absorber near the cloud tops and bottom of the upper clouds. Approximately 1 ppm of Cl2 is required. Whether this amount of Cl2 can be generated from the photodissociation of HCl is not clear, with the peak Cl2 mixing ratio being strongly dependent on the exact amount of HCl and other gases that are present in the upper clouds.
Earth‐based radar imagery of Venus
Authors: Campbell D. B. , Burns B. A.
Published: February 2008

Observations with the 12.6‐cm radar system at the Arecibo Observatory in 1975 and 1977 have yielded images covering approximately 25% of the surface. The resolutions of these images range from 5 to 20 km. Most of the apparent large features correlate to some degree with features in the Pioneer Venus altimetry data. The correlation is especially good for the continental‐sized elevated region, Ishtar Terra, in the north and Beta Regio at 30°N latitude, for which a volcanic origin has been suggested. Over 30 circular features have been observed and tentatively identified as craters. Whether they are of volcanic or impact origin cannot be determined at the relatively poor resolution attained here. A comparison of their characteristics and statistics with those of known impact craters on the moon, Mercury, and Mars suggests that at least some of them may be of impact origin. Assuming that all of them are impact craters and that we have detected most of the larger ones with diameters above 100 km, the small number found indicates that Venus may have a relatively young surface.
A low‐order global gravity field of Venus and dynamical implications
Authors: Ananda M. P. , Sjogren W. L. , Phillips R. J. , Wimberly R. N. , Bills B. G.
Published: February 2008

An estimate of sixth‐degree and sixth‐order harmonic coefficients of the global gravity field of Venus has been obtained by processing the long periodic variations of the mean orbital elements of the Pioneer Venus orbiter. Approximately 220 days of data have been included in this reduction, which provides almost complete longitudinal coverage. Our estimate of oblateness (C20) is −5.97±3.2 × 10−6. The amplitudes of other coefficients are similar to the predicted coefficients using Kaula's rule under equal stress assumption. Atmospheric density values as a function of altitude have been obtained to help model drag perturbation. A radial acceleration map at 100 km above the Venus surface has been generated, and correlation between gravity anomalies and major topographic features is observed. Spectral analysis of the harmonic model suggests that the interior density anomalies are similar to those on earth. The orientation angles of the principal axes of the moments of inertia have been computed, and deviation of the maximum moment of inertia axis from the spin axis is observed to be small (<5°). The minimum moment of inertia axis passes through the Aphrodite and Beta regions.
Limits on the possible intrinsic magnetic field of Venus
Authors: Russell C. T. , Elphic R. C. , Slavin J. A.
Published: February 2008

Magnetic field measurements obtained by the Pioneer Venus orbiter at low altitudes in the solar wind wake region are examined to search for possible surface‐correlated features and any possible intrinsic magnetic moment. The observed field variations in the wake do not resemble our expectations for a solar wind interaction with even a weak intrinsic magnetic field. There is little orbit‐to‐orbit persistence of features in the magnetic records. The magnetic field measurements in the wake have been averaged in 10° × 10° bins to minimize the effects of external field sources. The average fields in these 37 bins appear to be randomly oriented and consistent with zero mean in the region mapped. Using these 37 averaged vector fields, we obtain a maximum intrinsic magnetic dipole moment of 4.3±2.0 × 1021 G cm³, approximately an order of magnitude less than previous estimates. A more conservative estimate of the probable error of the mean is 5.5 × 1021 G cm³. Thus the Pioneer Venus measurements are consistent with zero planetary moment. The present measurements are also far below estimates based on angular momentum, the so‐called ‘magnetic Bode's law’ and far below the dynamo scaling law of Busse.
Field‐aligned currents in the Earth's magnetotail
Authors: Frank L. A. , McPherron R. L. , DeCoster R. J. , Burek B. G. , Ackerson K. L. , Russell C. T.
Published: February 2008

Three field‐aligned current sheets are directly observed with plasma and magnetometer instrumentation on board the ISEE spacecraft located at ∼20 RE in the postmidnight sector of the magnetotail. These current sheets are encountered at the northern plasma sheet boundary as the plasma sheet expanded past the spacecraft positions during the recovery phase of a magnetic substorm. This expansion speed, Vz in solar magnetospheric coordinates, is 14 km/s. The corresponding convection electric fields E⊥ are derived from the three‐dimensional proton velocity distributions as measured with the quadrispherical Lepedea plasma instrumentation. The average current densities within the three field‐aligned current sheets are +3.3 × 10−9, −1.3 × 10−8, and +1.1 × 10−8 A/m², in order of decreasing distance to the plasma sheet. Current densities in the first and third sheets are directed into the ionosphere, and the current carriers are ionospheric electrons drifting away from the ionosphere. The second, or central, current sheet is directed away from the ionosphere; however, the source of the current‐carrying electrons is unclear. This central current sheet is thought to be associated with ionospheric electron precipitation producing discrete auroral arcs via an acceleration mechanism at intermediate altitudes. The thicknesses of the three magnetotail current sheets are 0.36, 0.55, and 0.74 RE, again in order of decreasing distance from the plasma sheet. Current intensities, the products of current densities and sheet thicknesses, of the field‐aligned current sheets are measured simultaneously with magnetometers and are +0.010, −0.028, and +0.018 A/m, respectively. These values agree reasonably well with those derived from the direct plasma measurements. Electron drift velocities VD within the current sheets are in the range 0.05 to 0.1 Ve, where Ve is the electron thermal velocity. The ion sound velocity Cs ≃VD. These plasma parameters, including the fact that the protons are hot with Tp ≃3Te, appear to provide an unfavorable situation for any substantial steady state anomalous resistivity at this location of the magnetotail. Current‐driven ion cyclotron and/or ion cyclotron drift instabilities may be responsible for broadband electrostatic noise previously observed at the boundary of the plasma sheet.
Linear theory of tearing in a high‐β plasma
Authors: Quest Kevin B. , Coroniti Ferdinand V.
Published: February 2008

We calculate the linear dispersion relation for a collisionless plasma in a sheared one‐dimensional current sheet and extend the existing calculations to include β ∼1 plasmas, where β is the ratio of the thermal pressure to the magnetic pressure. Defining Bz as the component of the magnetic field which reverses sign as a function of x and By as that component which is nonzero but does not reverse sign (the guide‐field), we find that the tearing mode eigenstructure and temporal growth rate are a sensitive function of the ratios ls/lG, ls/ln, and ls/lT, where ls is the shearing length of the magnetic field, lG is the gradient scale length of the guide field By, ln is the number density gradient scale length, and lT is the temperature gradient scale length. In particular, if β ≳1, and ls <lG, ln, and lT, then the thickness of the layer over which particles are resonantly accelerated by the induction electric field is ∼ρ, a thermal gyroradius. In this limit the growth rate reduces to that derived by Laval et al. (1966). If the above conditions are not satisfied, plasma gradients may electrostatically stabilize the mode.
Altitude dependent model of the auroral beam and beam‐generated electrostatic noise
Authors: Maggs J. E. , Lotko W.
Published: February 2008

A height dependent model of the auroral beam and ionosphere is used to investigate the electrostatic noise generated by the auroral beam. The beam pitch angle is assumed to evolve with the first adiabatic invariant conserved. The beam generates whistler noise over most of the field line and upper hybrid noise only at low altitudes. Only a very narrow range of beam source densities generates noise without producing such large power fluxes that nonlinear effects are important. The strongest power fluxes in the auroral arc are likely to occur in the altitude range of a few thousand kilometers.
The effect of horizontal winds upon the chemical composition of the lower thermosphere at high latitude
Authors: Rodrigo R. , Nisbet J. S. , Battaner E.
Published: February 2008

A theoretical model for neutral compounds in the polar latitude lower thermosphere has been developed. The results are compared with a previous middle latitude model in order to study the special polar region effects. The main difference between both models is the inclusion of horizontal winds which are very large and cannot be ignored at high latitudes. Their effects are even more important than diffusion itself. Absolute values for the atomic oxygen concentration are considerably lower, in agreement with recent measurements, and the consequences on the green line airglow are examined. The vertical winds induced by horizontal motions are much more important than those induced by direct solar heat input, so that a very different pattern with respect to middle latitudes is obtained.
Global distribution of the Pedersen and Hall Currents and the electric potential pattern during a moderately disturbed period
Authors: Kamide Y. , Akasofu S.‐I.
Published: February 2008

The distribution of the Pedersen and Hall currents and the electric potential pattern over the entire polar region is determined for the first time by using a computer code developed by Kamide et al. [1981] on the basis of magnetic field data obtained from the IMS Alaska meridian chain of observatories.
IMF enhancements associated with streams: An analysis of magnetic field behavior
Authors: Villante U.
Published: February 2008

We examined the interplanetary magnetic field behavior in the field enhancement regions associated with the high velocity stream fronts observed during the primary mission of Helios 2 (January–April 1976). The principal conclusions of our investigation are the following: (1) The relative variation of the field magnitude ranges between 1.3 and 1.8 and is independent of heliocentric distance at least between 0.5 and 1 AU; (2) Close to the sector boundaries the field enhancements are consistent with a field perturbation occurring in a plane perpendicular to both the solar equator and the radial direction; (3) The field line has occasionally the form of a cylindrical helix rather than a simple planar geometry; (4) There is no compelling evidence to relate, in the inner solar system and in the period of interest, the field enhancements with the shape and location of the sector boundary surface.
Guided magnetospheric propagation of Alfvén waves in the Pc 4–5 frequency range
Authors: Fejer J. A.
Published: February 2008

It is shown that in the magnetohydrodynamic approximation, neglecting pressure terms, the transverse wave fields in a uniform medium with a uniform magnetic field can be derived from linear combinations of scalar potentials of the form ϕ±(x, y, z±V At) where VA is the Alfvén speed; guidance by the magnetic field is thus perfect. It is further shown that in the ray approximation, strong guidance still exists even if the medium and the magnetic field are not quite uniform provided that the component perpendicular to the magnetic field of the propagation vector is much greater than the parallel component. If the reciprocal of the parallel component is not much greater than the spatial scale on which the medium and the magnetic field vary, then the ray approximation is only valid in the perpendicular direction. The differential equations of wave propagation along the magnetic field are derived under these conditions directly from Maxwell's equations. An alternative derivation, using an analogy to transmission lines, is also given. These equations must then be applied to each perpendicular Fourier component of a source; differently oriented Fourier components propagate differently. The resulting imperfections in guiding will be investigated elsewhere in detail.
The global distribution of nitric oxide at 200 km
Authors: Cravens T. E.
Published: February 2008

Measurements of nitric oxide at 200 km by the ultraviolet nitric oxide experiment on Atmosphere Explorer D are used to demonstrate the dependence of NO on latitude, longitude, and magnetic activity. NO is more abundant in the summer hemisphere than in the winter hemisphere and is more abundant during magnetically active times than during quiet times. A simple photochemical theory is used to show that the knowledge of local composition and temperature is sufficient to explain the variations of nitric oxide at this altitude.
Electron temperature measurements by the plasma line technique at the French Incoherent Scatter Radar Facilities
Authors: Kofman W. , Lejeune G. , Hagfors T. , Bauer P.
Published: February 2008

The results of experiments aimed at the determination of the electron temperature by a plasma line technique are presented. Using the multistatic capabilities of the French incoherent scatter radar, the plasma line frequencies were simultaneously measured at two receiving stations (Mende and Nançay) at the altitude corresponding to the maximum of the F layer. Different plasma line frequencies are measured because of different effective k vectors that appear in the thermal term of the plasma dispersion relation. We derive and apply two data analysis procedures that enable us to determine this frequency difference. Comparison of this measured frequency difference to that calculated using the ion component electron temperature demonstrates that the plasma lines could indeed be used to determine the electron temperature. A strong dependence of the power in the plasma line as a function of the angle between k vector and magnetic field is observed in agreement with the theory. The future developments of this technique with the EISCAT radar facilities are discussed.
The orientation and motion of the predawn current sheet and Jupiter's magnetotail
Authors: Goertz C. K.
Published: February 2008

From the observed times when Pioneer 10 and Voyager 1 observe particle flux maxima and minima and when the magnetic field strength is a minimum we derive a geometric description of the current sheet in Jupiter's magnetosphere. It lags the planetary rotation by an angle δ which varies as a function of radial distance as δ=22° + 0.80 (r‐16.3). The maximum latitude of the current sheet is roughly 10° inside of 60 RJ and decreases beyond. Thus there is a hinge point near 60 RJ. The position of the hinge point moves in response to the external solar wind pressure variation. We discuss the variation of δ(r) theoretically and show how it is related to the magnetic field topology and plasma flow. We conclude that the distant field lines do not corotate with Jupiter and that the Alfven speed at 50 RJ is of the order of 1500 km/s. The topology of the current sheet is affected by centrifugal forces, outward mass transport and solar wind‐magnetosphere interaction. We show how these effects can be related to different current systems.
Observations of electron gyroharmonic waves and the structure of the Io torus
Authors: Birmingham T. J. , Alexander J. K. , Desch M. D. , Hubbard R. F. , Pedersen B. M.
Published: February 2008

Narrow‐banded emissions were observed by the planetary radio astronomy experiment on the Voyager 1 spacecraft as it traversed the Io plasma torus. These waves occur between harmonics of the electron gyrofrequency and are the Jovian analogue of electrostatic emissions observed and theoretically studied for the terrestrial magnetosphere, The observed frequencies always include the component near fuhr, the upper hybrid resonant frequency, but the distribution of the other observed emissions varies in a systematic way with position in the torus. A detailed discussion of the observations is presented. A refined model of the electron density variation, based on identification of the fuhr line, is also included. Spectra of the observed waves are analyzed in terms of the linear instability of an electron distribution function consisting of isotropic cold electrons and hot loss cone electrons. The positioning of the observed auxiliary harmonics with respect to fuhr is shown to be an indicator of the cold to hot temperature ratio TC/TH. It is concluded that this ratio increases systematically by an overall factor of perhaps 4 or 5 between the inner (L ∽5 RJ) and outer (L ∽9 RJ) portions of the torus. Other relevant plasma and spectroscopic data are discussed.
Coordinates of features on the Galilean satellites
Authors: Davies Merton E. , Katayama Frank Y.
Published: February 2008

Control nets of the four Galilean satellites have been established photogrammetrically from pictures taken by the two Voyager spacecraft during their flybys of Jupiter in 1979. Coordinates of 504 points on Io, 112 points on Europa, 1547 points on Ganymede, and 439 points on Callisto are listed. Selected points are identified on U.S. Geological Survey maps of the satellites. Measurements of these points were made on 234 pictures of Io, 115 pictures of Europa, 282 pictures of Ganymede, and 200 pictures of Callisto. The systems of longitude were defined by craters on Europa, Ganymede, and Callisto. Preliminary solutions have been found for the directions of the axes of rotation of the Galilean satellites. New mean radii have been determined as 1815±5 km for Io, 1569±10 km for Europa, 2631±10 km for Ganymede, and 2400±10 km for Callisto.
Crater populations on Ganymede and Callisto
Authors: Strom Robert G. , Woronow Alex , Gurnis Michael
Published: February 2008

The discovery of heavily cratered surfaces on Ganymede and Callisto by Voyager 1 shows that like the inner Solar System, a period of heavy bombardment also occurred in the outer Solar System. Comparisons among the crater size/density curves of Ganymede, Callisto and the terrestrial planets show several striking features. The overall crater density of the most heavily cratered terrain on Ganymede is down by a factor of about 3 compared to Callisto, and when allowance is made for the difference in crater production rate due to the influence of Jupiter's gravity field it is down by a factor of nearly 6. This indicates that the oldest regions of Ganymede began recording the observed crater population at a later time than Callisto, and therefore Ganymede either experienced a large‐scale (perhaps global) diameter‐independent resurfacing event or simply developed a rigid crust capable of retaining craters later than Callisto. In either case, this process took place during the period of late heavy bombardment. Based on earlier studies of the terrestrial‐planets' cratering record, neither Ganymede nor Callisto is saturated with craters. Compared to Callisto, a diameter‐dependent loss of craters in the size range 10–40 km occurs on the grooved terrain of Ganymede and probably results from obliteration of small craters due to the formation of new ice. A similar but less severe loss also occurs on Ganymede's heavily cratered terrain and may be due to an earlier period of ice formation and/or the formation of arcuate troughs in this terrain. Seven different crater curves, in the diameter range of about 40–130 km, representing vastly different crater densities, different surface ages, different terrain types, and even different satellites all possess nearly the same distribution function. This together with other observational evidence strongly suggests that at least in this diameter range the curve basically represents its production function which is completely different from that on the terrestrial planets. This indicates that the population of bodies responsible for the period of late heavy bombardment in the inner Solar System was very different from that responsible for the late heavy bombardment in the outer Solar System. We can only speculate at this early stage that Ganymede and Callisto may principally record a population of bodies that never penetrated the inner Solar System in numbers great enough to leave a recognizable signature.
Shock‐associated low‐energy ion enhancements observed by Voyagers 1 and 2
Authors: Decker R. B. , Pesses M. E. , Krimigis S. M.
Published: February 2008

Observations of shock‐associated ≥30 keV ion enhancements are presented using data from the Low Energy Charged Particle (LECP) experiment on Voyagers 1 and 2, launched on days 248 and 232, 1977, respectively. The observations include examples of energetic storm particle (ESP) events associated with flare‐produced shocks and examples of corotating particle events (CPE) associated with forward and reverse shocks that bound corotating interaction regions in the outer heliosphere. The first well‐defined CPE are not seen at Voyagers 1 and 2 until late in 1978 when each spacecraft was at a heliocentric radial distance of ∼4 AU. Thus far, seven CPE have been identified in the Voyager 1 LECP data from launch through day 170, 1979. Most of these CPE show features similar to many of those observed at Pioneers 10 and 11, e.g., they have recurrent double‐peaked intensity enhancements showing little or no velocity dispersion at peak intensities, time durations of several days, soft (γ ≥4 for protons ∼1 MeV) energy spectra extending up to ∼5 MeV/nucleon and p/α ratios that are lowest at the reverse peaks. The new LECP measurements also show for the first time that the CPE ion spectra extend, with no sign of a low‐energy turnover, to energies as low as 30 keV/ion. If the CPE are produced by shock acceleration of an ambient particle source, the fact that the observed CPE exhibit well‐formed high intensity peaks at 30 keV/ion means that the pre‐acceleration energy of the source particles was <30 keV/ion. In several of the shock events, the lowest energy ion enhancements are confined mainly downstream of the CIR shocks, with the magnitude and duration of the upstream enhancements increasing with increasing ion energy. The similarities in the low energy ion morphologies observed near the shock during an ESP event and a CPE suggest that the same shock acceleration mechanism and propagation processes were operative in each case. These observations are consistent with the dynamical and kinematical effects expected when low energy ions are accelerated at quasi‐perpendicular shocks.
Lower‐hybrid‐drift instability and its associated anomalous resistivity in the neutral sheet of Earth's magnetotail
Authors: Pu Zu‐yin , Quest Kevin Brian , Kivelson Margaret Galland , Tu Chuan‐yi
Published: February 2008

It has been suggested previously that at the center of the earth's plasma sheet there exists a thin nonadiabatic layer, the neutral sheet, whose thickness is of the order of the ion gyroradius. The ion distribution in such a thin sheet may have some non‐Maxwellian form. The purpose of this paper is to construct a one‐dimensional ‘non‐Maxwellian’ model for the steady neutral sheet and to examine its instability properties with respect to the lower‐hybrid‐drift (LHD) wave. A combination of a Boltzmann distribution and a modified Alpers’ distribution for ions is taken. We show that in the limit of a thin neutral sheet, this distribution is a more powerful source of free energy than the usual drifting Maxwellian. It is found that most regions within the neutral sheet are unstable. The frequency spectrum of the unstable waves is nearly the same as in the drifting Maxwellian case. As shown previously by Huba et al. (1978), the frequency spectrum is in good agreement with the observations. The growth rate, assuming a modified Alpers' distribution function, is found to be enhanced compared with that due to a drifting Maxwellian model in a portion of the sheet at some distance from the null of the field. The saturation level of the fluctuating field and the anomalous resistivity are calculated in the entire sheet by using current relaxation as the stabilizing mechanism and are found to be 3–4 times larger than those in a Maxwellian sheet.
The significance of temperature in a finite geometry electron beam‐plasma system
Authors: Strangeway R. J.
Published: February 2008

A cold plasma approximation is often employed when deriving a wave dispersion relation for a beam‐plasma system in which the beam width is considered to be a factor in the wave dispersion. This approximation may no longer be valid for a sufficiently thin beam whose width is comparable to the Larmor radius, and temperature should be included in the model. As an initial attempt to take into account warm plasma effects we treat temperature as a first‐order correction to the cold dispersion relation. Temperature dependent wave fields are found for solutions of the cold model dispersion relation, and we choose to present solutions for frequencies above the electron gyrofrequency when the electron gyrofrequency is greater than the electron plasma frequency. Since temperature enters as a correction term, some knowledge of the wave field eigenfunctions is required, and one example of both the cold model fields and the temperature corrections to these fields is given. Determining the form of the wave eigenfunctions is also a useful indicator of how electrostatic the waves are, which is of some importance in simplifying a finite geometry warm plasma dispersion relation. By comparing the correction fields to the cold model fields it is possible to determine a critical thermal velocity for which the temperature corrections become significant. A temperature of 0.25 eV for a 2.5‐keV electron beam is shown to be a reasonable limit for the validity of a cold plasma approximation. Since the temperature is not just a beam temperature but also a background plasma temperature, this result may have implications for both magnetospheric and laboratory electron beam instabilities.
Possible changes in natural Pc 1 pulsation activity caused by BART
Authors: Samadani R. , Fraser‐Smith A. C. , Villard O. G.
Published: February 2008

In a previous preliminary study of the diurnal variation of Pc 1 pulsation activity at Stanford during four months in 1975, evidence was obtained for changes in the rate of occurrence of the pulsations around the times when service was started and terminated by the San Francisco Bay Area Rapid Transit (BART) system (Fraser‐Smith et al., 1979). The present study extends this earlier work by analyzing Pc 1 pulsation data recorded at Stanford during 1976. Spectrograms of N‐S geomagnetic activity were prepared for the complete year and 15‐minute intervals containing Pc 1 pulsation activity were tabulated. The diurnal variations of the Pc 1 activity (as measured by these 15‐minute intervals) for weekdays, during which BART was in operation, and weekends, during which BART was not normally in operation, were compared. Correlations of the observed differences with the BART schedule suggest once again that the ultralow‐frequency electromagnetic noise produced by BART may be influencing the occurrence of Pc 1 pulsations along the Stanford geomagnetic meridian.
ISEE 1 observations of O++ in the magnetosphere
Authors: Horwitz J. L.
Published: February 2008

Observations of O++ by ISEE 1 have revealed occasional O++/O+ density ratios of the order of unity within the plasmasphere. However, most plasmaspheric O++/O+ density ratios are generally below 0.3. These ratios agree generally with those seen by GEOS 1. Within the plasmasphere the O++ population appears to be primarily cold and isotropic (as is typical for major ions). Outside the plasmasphere, O++/O+ field‐aligned flux ratios may be of the order of 0.1–0.2, though they are frequently much smaller, and the O++ distributions are typically field aligned. A rare observation of a unidirectional conic in O++ is also reported.
Possible evidence for coronal Alfvén waves
Authors: Hollweg Joseph V. , Bird M. K. , Volland H. , Edenhofer P. , Stelzried C. T. , Seidel B. L.
Published: February 2008

The 2.29 GHz S band carrier signals of the two Helios spacecraft are used to probe the magnetic and density structures of the solar corona inside 0.05 AU. In this paper we analyze the observed fluctuations of the electron content and Faraday rotation. A simple statistical ray analysis is employed. We conclude that (1) the observed Faraday rotation fluctuations cannot be solely due to electron density fluctuations in the corona unless the coronal magnetic field is some 5 times stronger than suggested by current estimates, and (2) the observed Faraday rotation fluctuations are consistent with the hypothesis that the sun radiates Alfvén waves with sufficient energies to heat and accelerate high‐speed solar wind streams.
An electrostatic parabolic density drift instability
Authors: Gary S. Peter , Thomsen Michelle F.
Published: February 2008

The Vlasov theory for an electrostatic instability driven by a parabolic density gradient is considered. The magnetic field is taken as uniform, and the local approximation is used. The instability grows only for a sufficiently large, positive second derivative of the density. The parametric dependences of the linear growth rate are reported, as well as weakly nonlinear calculations on wave‐particle transport due to this instability.
Time‐dependent calculations of Jupiter's ionosphere
Authors: Chen Robert Hsin
Published: February 2008

Time‐dependent calculations of the vertical distribution of protons in Jupiter's ionosphere show that the accumulation of protons in the topside ionosphere produced from solar ionizing radiation overwhelms the loss to vibrationally excited molecular hydrogen at vibrational temperatures as high at 1600° K. At 2500° K the ionization is decreased over the entire planet with little diurnal variation. For Voyager 1 then, unless the H2 vibrational temperature is as high as thousands of degrees and the topside density of H2 is asymmetric and larger by orders of magnitude, dynamical processes are more likely causes of the low electron densities seen in the nightside upper ionosphere. A calculation of the H3+ density profile showed that the distribution above the turbopause is controlled by diffusion.
Turbulent electric fields in the nightside magnetosphere
Authors: Maynard N. C. , Heppner J. P. , Aggson T. L.
Published: February 2008

Electric field measurements from the long‐wire double‐probe instrument (baseline of 179 m) on ISEE 1 have shown the magnetospheric electric field on auroral L shells to be extremely turbulent during periods of magnetic activity. During intense activity these turbulent electric fields can penetrate to very low L values. The variational component of the electric field is typically larger than the DC value. Measurements are presented at frequencies up to 14 Hz. Magnitudes of over 40 mV/m (zero to peak) have been observed with spectral power levels in the 1–10 Hz range greater than 10 mV²/m² Hz. The spectral shape of the most intense events was generally flatter than that predicted by two‐dimensional hydromagnetic cascading of energy, which argues that the source of this turbulence must be driving the plasma near these frequencies. This in turn suggests that the instability is in the low‐energy plasma.
Modified disc model of Jupiter's magnetosphere
Authors: Liu Z. X.
Published: February 2008

This paper establishes a set of static magnetohydrodynamic equations in which a differential rotation of the magnetosphere and a wavy magnetodisc structure are considered. Solutions are obtained that include magnetic field, pressure, density, and temperature. By using the theoretical formulas of the magnetic field and pressure, we calculate the thickness of the plasma sheet. The main results are as follows. First, the thickness of the plasma sheet is determined by the thermal energy and the rotational energy of the plasma, and moreover, it increases with increasing thermal energy (temperature) and decreases with increasing rotational energy. Secondly, the thickness of the plasma sheet is different under adiabatic and isothermal conditions. Thirdly, in the isothermal case when β0 > 0.86 (β0 = ΩJ²L0²/2RTo), the thickness initially increases with increasing radial distance, reaches a maximum, and then decreases; for β0 < 0.86 in the isothermal case and for all β0 in the adiabatic case, the thickness decreases monotonically with increasing radial distance.
Observations of the diurnal dependence of the high‐latitude F region ion density by DMSP satellites
Authors: Sojka J. J. , Raitt W. J. , Schunk R. W. , Rich F. J. , Sagalyn R. C.
Published: February 2008

Data from the DMSP F2 and F4 satellites for the period December 5–10, 1979, have been used to study the diurnal dependence of the high‐latitude ion density at 800‐km altitude. A 24‐hour periodicity in the minimum orbital density (MOD) during a crossing of the high‐latitude region is observed in both the winter and summer hemispheres. The phase of the variation in MOD is such that it has a minimum during the 24‐hour period between 0700 and 0900 UT. Both the long term variation of the high‐latitude ion density on a time scale of days, and the orbit by orbit variations at the same geomagnetic location in the northern (winter) hemisphere for the magnetically quiet time period chosen show good qualitative agreement with the diurnal dependence predicted by a theoretical model of the ionospheric density at high latitudes under conditions of low convection speeds (Sojka et al., 1981a).
Light ion concentrations in Jupiter's inner magnetosphere
Authors: Tokar R. L. , Gurnett D. A. , Bagenal F. , Shaw R. R.
Published: February 2008

In this paper, Voyager 1 plasma wave instrument observations of lightning‐generated whistlers are combined with Voyager 1 plasma instrument heavy ion (8 ≤ A/Z ≤ 64) charge concentrations to investigate the concentration of light ions (A/Z < 8) along the whistler propagation path. Two models for light ion concentration over dipole L shells for L between 5.2 and 6.2 are obtained, one giving a constant concentration along the field line and the other corresponding to an exponential density distribution. Near the equator the light ion concentration ranges from only about 1 to 10% of the heavy ion concentration, while outside of the torus, at distances of from 1 to 1.5 RJ above the equator, the light ions are the dominant species.
Self‐consistent theory of time‐dependent convection in the Earth's magnetotail
Authors: Schindler K. , Birn J.
Published: February 2008

A self‐consistent theory of time‐dependent convection in the earth's magnetotail is developed. In contrast to earlier convection models our approach takes into account that the plasma sheet particles are effectively trapped on closed geomagnetic field lines. The results demonstrate that steady state convection, although possible in principle, is unlikely to occur in the earth's magnetotail. In particular, if particle or energy losses are sufficiently small and if the outer lobe field lines have convex shape, a steady state is impossible in the framework of the present polytropic model. Quantitative models for time‐dependent convection are constructed. The time dependence introduces important consequences for energy storage, stability, and spatial dependence of the convection electric field. The results are consistent with a quasi‐periodic evolution of the tail, where periods of quasi‐static compressional convection are followed by phases of dynamic evolution.
Occurrence and recurrence in auroral activity in DMSP images
Authors: Sheehan R. E. , Carovillano R. L. , Gussenhoven M. S.
Published: February 2008

We rated the auroral activity in 749 southern hemisphere DMSP images taken during 1972 and 1973 on a qualitative scale with active (A), moderate (M), quiet (Q), and no aurora (N) categories. The overall occurrence rates of images with A, M, Q, and N ratings are 0.17, 0.55, 0.16, and 0.11, respectively. After one DMSP orbit (102 min), both A and Q conditions recur in about one half of the cases. Thus successive images showing A conditions and successive images showing Q conditions occur more often than expected from the overall rates of A and Q images. The difference between the recurrence rate and overall occurrence rate of M images is much smaller (0.63 versus 0.55), suggesting that moderate auroral activity, as seen in a series of DMSP images, occurs essentially randomly. Nearly random, short‐period (15–30 min) bursts of auroral activity have been observed by Krukonis and Whalen (1980) in montages of all‐sky photographs taken 1 min apart. Our results, which cover a much longer time span, are consistent with this behavior and confirm that auroral activity occurs on time scales that differ from the full duration of the classic substorm pattern.
Plasma shocks and energetic particles in the outer solar system: Trapping and asymmetry observations from Pioneer 10 and Pioneer 11
Authors: Intriligator D. S. , Miller W. D.
Published: February 2008

Energetic protons (0.5–20 MeV) appear to be trapped between a pair of shocks associated with the major solar flares of April 15 and April 28, 1978. Prolonged trapping (for a period of weeks) is implied by the large count rate enhancement and the large range of radial distances and longitudinal angles over which the trapping is observed. Shocks associated with both flares are detectable in the Pioneer 10 and Pioneer 11 plasma analyzer data. These shock/flare associations are different from those previously published by others studying the interplanetary events. The evidence for trapping has not been recognized heretofore most likely because of the unobservability of the April 15 flare (located ∼150°E of the sun‐earth line) and of the faintness of the signature in the plasma data of one shock in each pair. The apparent ability of a shock whose plasma signature is extremely weak to confine MeV protons in the outer solar system may have significant implications for cosmic ray studies. Contrary to earlier analyses of these data, the results of our analyses also imply significant azimuthal asymmetry in plasma and energetic particle behavior even at distances as far as 16 AU from the sun. The combination of these observations provides evidence for unexpectedly complex interactions in the outer solar system between energetic particles and solar wind plasma.
A three‐dimensional model of corotating streams in the solar wind: 3. Magnetohydrodynamic streams
Authors: Pizzo Victor J.
Published: February 2008

The focus of this paper is two‐fold: (1) to examine how the presence of the spiral magnetic field affects the evolution of interplanetary corotating solar wind streams, and (2) to ascertain the nature of secondary large‐scale phenomena likely to be associated with streams having a pronounced three‐dimensional (3‐D) structure. The dynamics are presumed to be governed by the nonlinear, polytropic, single‐fluid, 3‐D MHD equations. Solutions are obtained with an explicit, Eulerian, finite differences technique that makes use of a simple form of artificial diffusion for handling shocks. For smooth axisymmetric flows, the picture of magnetically induced meridional motions previously established by linear models requires only minor correction. In the case of broad 3‐D streams input near the sun, inclusion of the magnetic field is found to retard the kinematic steepening at the stream front substantially but to produce little deviation from planar flow. For the more realistic case of initially sharply bounded streams, however, it becomes essential to account for magnetic effects in the formulation. Whether a full 3‐D treatment is required depends upon the latitudinal geometry of the stream. Where the stream front is sharply inclined to the equator or contains significant substructure, the 3‐D effects (the modifications to planar flow occasioned by the nonradial motions driven by the interaction) can exceed the magnetic effects. Conventional radial alignment tests may be incapable of distinguishing between 3‐D and temporal effects because the former need not be accompanied by readily identifiable north‐south flow signatures, for example. The shock surfaces predicted by the model can be quite oblique and can extend to latitudes beyond the north‐south boundary of the stream. Even with magnetic and 3‐D effects accounted for, however, shock formation in the model still occurs much closer to the sun than is generally observed. It is demonstrated how the systematic nonradial flow patterns seen about a stream interface can be used to infer the orientation of the stream front. Estimates of the latitudinal transport of angular momentum flux by stream fronts suggests that the effect is likely to be so large as to preclude accurate measurement of this quantity beyond about 0.5 AU. The 3‐D transport may have a discernible influence upon average magnetic field behavior, but steady corotating flows cannot produce the large north‐south field deflections commonly observed near 1 AU.