bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma depletions in the Jovian magnetosphere: Evidence of transport and solar wind interaction
Authors: McNutt Ralph L. , Coppi Paolo S. , Selesnick Richard S. , Coppi Bruno
Published: February 2008

A series of plasma voids (“dropouts”) was observed by the Plasma Science (PLS) experiment in Jupiter's magnetosphere during the Voyager 2 encounter with that planet. A reexamination of Voyager 2 data has led us to conclude that the dropout phenomenon cannot be a manifestation of a plasma wake produced by Ganymede. Rather, the appearance of the dropouts is attributed to changes in the upstream solar wind conditions and the global state of the magnetosphere; the proximity of Voyager 2 to Ganymede at the time is considered to be coincidental. We suggest that these dropouts are evidence of a state of “bubbling” of the magnetosphere that alternates with “laminar” states in which, as in the case of the Voyager 1 encounter with Jupiter, voids are not present and that these states correspond to different processes by which plasma is transported out of the system. The nature of these states is related to changes in the magnitude of the upstream solar wind ram pressure. In the bubbling state, this pressure is higher than in the laminar state and drives an intermittent instability. The analysis presented is one of the first attempts to introduce, in space physics, recently acquired theoretical notions of the physics of finite‐β plasmas of which the Jovian magnetospheric plasma is an important example.
Simultaneous measurements of the thermospheric wind profile at three separate positions in the dusk auroral oval
Authors: Mikkelsen I. S. , Larsen M. F. , Kelley M. C. , Vickrey J. , Friis‐Christensen E. , Meriwether J. , Shih P.
Published: February 2008

On March 20, 1985, two rockets were launched from Søndre Strømfjord, Greenland, into the dusk auroral oval. Three trimethyl aluminium trails were released to measure the neutral wind profiles between 95 and 190 km of altitude at two points separated by 190 km normal to the invariant latitude circles and at a third point separated from the first two by 300 km along the invariant latitude circles. Two barium/strontium clouds were released at 250 km of altitude, extending two of the neutral wind profiles to this altitude. In the E region the tip of the wind vector traced an ellipse as a function of increasing altitude with maximum wind speeds of 100–150 m/s in the southeastward and northwestward directions. The F region winds were southward with speeds of 100–200 m/s. The zonal wind component between 115 and 140 km of altitude had a horizontal gradient in the southeastward direction, whereas the meridional wind component at the same heights was constant over the spatial extent covered by the measurements. We interpret the observed E region wind field as being part of a gravity wave with a period of 3 hours as estimated from the ellipticity of the wind hodograms. The wind vectors rotated 540° clockwise with increasing height, indicating that the wave energy is propagating upward. The Fabry‐Perot interferometer at Søndre Strømfjord was first able to detect the F region winds 45 min after the releases and measured winds of 100–400 m/s mainly in the southeastward or antisunward direction. The geomagnetic conditions were quiet, with Kp not exceeding 2 for the 24 hours preceding the experiment. The incoherent scatter radar at Søndre Strømfjord observed a contracted plasma convection pattern associated with positive By and Bz components of the interplanetary magnetic field. Intermittently, a By < 0 pattern was observed. We suggest that the observed gravity wave was excited by the temporal variations of the large‐scale plasma convection, although a wave source in the mesosphere or below cannot be excluded.
A simple spectral model of the dynamics of the Venus ionosphere
Authors: Singhal R. P. , Whitten R. C.
Published: February 2008

A two‐dimensional model of the ionosphere of Venus has been constructed by expanding pertinent quantities in Legendre polynomials. The model is simplified by including only a single ion species (O+). Horizontal plasma flow velocity and plasma density have been calculated as a coupled system. The calculated plasma flow velocity is found to be in good agreement with observations and the results of earlier studies. Solar zenith angle dependence of plasma density, particularly on the nightside, shows some features which differ from results of earlier studies and observed values. Effects of raising or lowering the ionopause height and changing the nightside neutral atmosphere have been discussed.
Field‐aligned structure of the storm time Pc 5 wave of November 14–15, 1979
Authors: Takahashi K. , Fennell J. F. , Amata E. , Higbie P. R.
Published: February 2008

Magnetic field data from the four satellites SCATHA (P78‐2), GOES 2, GOES 3, and GEOS 2 have been analyzed to examine the magnetic field‐aligned structure of a storm time Pc 5 wave that occurred on November 14–15, 1979. The wave had both transverse and compressional components. At a given instance the compressional and the radial components oscillated in phase or 180° out of phase, and the compressional and the azimuthal components oscillated +90° or −90° out of phase. In addition, each component changed its amplitude with magnetic latitude: the compressional component had a minimum at the magnetic equator, whereas the transverse components had a maximum at the equator and minima several degrees off the equator. A 180° relative phase switching among the components occurred across the latitudes of amplitude minima. From these observations the field line displacement of the wave is confirmed to have an antisymmetric standing structure about the magnetic equator with a parallel wave length of a few earth radii. We also observed other intriguing properties of the wave such as different parallel wave lengths of different field components and small‐amplitude second harmonics near the nodes. A dielectric tensor appropriate for the ring current plasma is found to give an explanation for the relation between the polarization and the propagation of the wave. However, plasma data available from SCATHA does not support either the drift mirror instability of A. Hasegawa (1969) or the coupling between a drift mirror wave and a shear Alfvén wave as discussed by A. D. M. Walker et al. (1982).
Observation of nonlinear wave decay processes in the solar wind by the AMPTE IRM Plasma Wave Experiment
Authors: Koons H. C. , Roeder J. L. , Bauer O. H. , Haerendel G. , Treumann R. , Anderson R. R. , Gurnett D. A. , Holzworth R. H.
Published: February 2008

Nonlinear wave decay processes have been detected in the solar wind by the plasma wave experiment aboard the Active Magnetospheric Particle Tracer Explorers (AMPTE) IRM spacecraft. The main process is the generation of ultralow‐frequency ion acoustic waves from the decay of Langmuir waves near the electron plasma frequency. Frequently, this is accompanied by an enhancement of emissions near twice the plasma frequency. This enhancement is most likely due to the generation of electromagnetic waves from the coalescence of two Langmuir waves. These processes occur within the electron foreshock in front of the earth's bow shock.
Riometer absorption and spectral index for precipitating electrons with exponential spectra
Authors: Stoker P. H.
Published: February 2008

The altitude profiles computed by Berger et al. (1974) for energy deposition in the atmosphere by electrons and electron‐produced X rays are transformed to electron density profiles in order to calculate the absorption of cosmic radio noise at 20, 30 and 51.4 MHz. The electrons are precipitated isotropic over the downward hemisphere with exponential spectra having e‐folding energies of 10, 50 and 200 keV. It is shown that the absorption of a wide angle (± 34°) riometer antenna is about 1.4 times larger than zenithal absorption, that a relationship A(ƒ) = k(ƒ)N0.45 exists between precipitation electron flux N and riometer absorption, with k(ƒ) a frequency dependent constant, and that the spectral index n, defined by A(ƒ) ∝ ƒ−n, is ≃2.0 for all three the e‐folding energies if the precipitation is uniform over the field of view of the antenna, but attains values well below 2.0 for the higher e‐folding energies only if the effective electron collision frequency is enhanced by hyperthermal electrons. The spectral index for wide beam riometer absorptions decreases also to values much less than 2.0 if a narrow strip of the upper atmosphere across the field of view is strongly ionized. It appears then that the frequency dependence of the absorption may not be related uniquely by A(ƒ) ∝ ƒ−n. The absorption event recorded at Sanae, Antarctica, on June 27, 1982, at 0122 not only illustrates that the spectral index n is not an unique parameter when electron precipitation is not uniform in the field of view of a riometer antenna but suggests also highly wavelength selective scattering of cosmic radio noise from the more luminous parts of the radio sky by the Bragg condition as suggested by D'Angelo (1976).
Simulation of amplitude‐modulated circularly polarized Alfvén waves for beta less than one
Authors: Machida S. , Spangler S. R. , Goertz C. K.
Published: February 2008

We study the nonlinear properties of the amplitude‐modulated circulary polarized Alfvén wave for β < 1. The temporal behavior of the wave packet of the electromagnetic hybrid simulation is compared with a numerical solution of the derivative nonlinear Schrödinger (DNLS) equation. It is shown that the left‐hand‐polarized mode evolves into a shocklike structure due to the modulational instability. However, both cyclotron damping and a snowplow effect near the steepened wave packet suppress its further steepening, contrary to the predictions of the DNLS equation. For the right‐hand mode, formation of the shock does not take place, and the initial time development is well described by the DNLS equation. The daughter Alfvén wave and ion acoustic waves are excited due to the decay instability at a later time. Heating or acceleration of the particles takes place for both left‐ and right‐hand waves. Energy transfer from the wave to the particles occurs effectively when substantial modulation in the wave amplitude is present. The significance of this result is briefly discussed.
High‐energy cosmic ray intensity waves
Authors: Jacklyn R. M. , Duldig M. L. , Pomerantz M. A.
Published: February 2008

A new mode of galactic cosmic ray modulation has been observed. It manifests itself as waves of isotropic intensity variations of high‐energy (∼150 GeV) galactic cosmic rays. The wave periods are in synchronism with the alternating toward and away polarity states of the sectored interplanetary magnetic field. Since data with the requisite precision for observing the phenomenon first became available in 1982, isotropic intensity waves have occurred during the latter half of three successive years when the long‐term modulation was maximum (1982) and recovering rapidly (1983 and 1984), but were not readily discernible in 1985, after the steady state intensity had recovered almost to the solar minimum level. The variational spectrum Δj(p)/j(p) = kpγ (where p is magnetic rigidity) is hard, with γ ≈ 0. Although the largest Forbush decrease ever recorded occurred while the 1982 episode was in progress, isotropic intensity waves are not causally related to that effect. However, the isotropic intensity waves observed thus far are in phase in the southern hemisphere with much smaller concurrent variations of the north‐south anisotropy. The new phenomenon bespeaks a difference, not yet identified, in the plasma regimes on opposite sides of the neutral sheet.
Field‐aligned electron flux oscillations that produce flickering aurora
Authors: McFadden J. P. , Carlson C. W. , Boehm M. H. , Hallinan T. J.
Published: February 2008

Measurements of energetic electrons that produce flickering aurora were made by a pair of sounding rockets launched during a slowly evolving auroral breakup. Both payloads passed through a broad inverted‐V structure. A component of the electron distribution function was closely aligned with the magnetic field over a broad energy range that extended from low energies up to the inverted‐V differential energy flux peak. High time resolution measurements of the field‐aligned component showed the presence of order of magnitude coherent flux oscillations. Source altitudes between 4000 and 8000 km were derived from velocity dispersion of the flux oscillations. A ground‐based TV camera recorded visual flickering in the vicinity of the payloads' auroral footprints during periods when flux oscillations were present. Measurements are compared with previous observations of electron flux oscillations, and possible sources for the field‐aligned component are discussed.
Interplanetary magnetic field By effects on the magnetic field at synchronous orbit
Authors: Nagai T.
Published: February 2008

GOES 3 magnetic field data are analyzed to reveal interplanetary magnetic field (IMF) By effects on the magnetic field configuration near the magnetic equatorial plane at synchronous orbit. The magnetic equatorial plane on the nightside moves upward (northward) in the evening sector and downward (southward) in the morning sector for the positive IMF By. The magnetic equatorial plane moves in the opposite sense for the negative IMF By. A perturbation appears in the azimuthal component of the magnetic field in the midnight sector (2100–0300 LT), and its magnitude is a fraction 0.3 of the IMF By near midnight. These changes are attributable to the IMF By‐induced asymmetry of the reconnection. Observations of substorm‐associated field‐aligned current effects depend strongly on the geometry of synchronous spacecraft relative to these currents so that the IMF By‐induced distortion of the magnetic equatorial plane produces apparent IMF By effects on D observations during substorms.
Parallel electric fields from ionospheric winds
Authors: Nakada M. P.
Published: February 2008

The possible production of electric fields parallel to the magnetic field by dynamo winds in the E region is examined, using a jet stream wind model. Current return paths through the F region above the stream are examined as well as return paths through the conjugate ionosphere. The Wulf geometry with horizontal winds moving in opposite directions one above the other is also examined. Parallel electric fields are found to depend strongly on the width of current sheets at the edges of the jet stream. If these are narrow enough, appreciable parallel electric fields are produced. These appear to be sufficient to heat the electrons which reduces the conductivity and produces further increases in parallel electric fields and temperatures. Calculations indicate that high enough temperatures for optical emission can be produced in less than 0.3 s. Some properties of auroras that might be produced by dynamo winds are examined; one property is a time delay in brightening at higher and lower altitudes.
Electrostatic charge on a dust size distribution in a plasma
Authors: Houpis Harry L. F. , Whipple Elden C.
Published: February 2008

The capacitance of a grain immersed in a steady state plasma containing a size distribution of dust particles is calculated. Assuming the equilibrium potential has been obtained by a simple balance of electron and ion collection currents, the grain charge is also obtained. It is shown that the validity of the analytical treatment given here for the linearized Poisson's equation is confined to a certain region of λD ‐ R (Debye length; interparticle spacing radius) space. Outside this region, a numerical solution of the full nonlinear Poisson's equation will be needed. Within the valid linear region and starting at very small λD (or R), the capacitance at first exhibits a monotonic increase with increasing λD (or R). Eventually, the capacitance reaches a maximum, and this is followed by a monotonic decrease. The charge density of the dust in the plasma, however, is found to be only a function of the Debye length; there is no significant dependence on the interparticle spacing. Finally, the results of similar calculations by Goertz and Ip (1984) and Whipple et al. (1985) for a dusty plasma with a single grain size are shown to be qualitatively different from the results given here. The reason for this is the limited λD ‐ R space investigated by these authors and the inconsistent use of a linearized calculation to interpret dust behavior in a nonlinear regime.
The effect of photoelectrons on boom‐satellite potential differences during electron beam ejection
Authors: Lai Shu T. , Cohen Herbert A. , Aggson Thomas L. , McNeil William J.
Published: February 2008

Data taken on the SCATHA satellite at geosynchronous altitudes during periods of electron beam ejection in sunlight showed that the potential difference between an electrically isolated boom and the satellite main body was a function of beam current, energy, and boom‐sun angle. The potential difference decreased as the boom area illuminated by the sun increased; the maximum and minimum potential differences were measured when minimum and maximum boom areas, respectively, were exposed to the sun. It is shown that photoelectrons, created on the boom, could be engulfed in the electrostatic field of the highly charged satellite main body. Theoretical calculations made using a simple current balance model showed that these electrons could provide a substantial discharging current to the main body and cause the observed variations in the potential difference between the main body and the booms.
Characteristics of a reflecting layer below the classical D region
Authors: Bain W. C. , Kossey P. A.
Published: February 2008

Rasmussen et al. (1980) described low‐altitude reflections observed with a VLF/LF ionospheric sounder, apparently from a C layer, which could be interpreted as having come from ionospheric layers of constant conductivity. In the example given in their paper, they found a layer that extended from 63 to 69 km in height. This brief paper presents an improved method for calculating such layers which shows that, in fact, the best fitting layer for their example extends from 60 to 66 km.
The evolution of a coronal streamer prior to mass ejection
Authors: Wolfson Richard , Conover Charles , Illing Rainer M. E.
Published: February 2008

We have developed a model describing the quasistatic evolution of a coronal helmet streamer as it is inflated with excess mass. The model produces a sequence of magnetostatic equilibria for a magnetic field configuration that includes a current sheet in the equatorial plane as well as distributed volume currents. The equilibrium solutions are given as Legendre series, and the height at which the current sheet starts is adjusted iteratively to maintain constant flux in the closed‐field region. We have fit our model to SMM coronagraph/polarimeter observations showing the slow growth of a coronal streamer prior to its disruption in association with an eruptive prominence and mass ejection on August 18, 1980. The results suggest that the early phase of this event is consistent with the quasi‐static evolution of the corona in response to the slow addition of mass to the closed‐field region of the streamer.
Solid hydrocarbon aerosols produced in simulated Uranian and Neptunian stratospheres
Authors: Khare B. N. , Sagan C. , Thompson W. R. , Arakawa E. T. , Votaw P.
Published: February 2008

Optical constants n and k are measured for thin hydrocarbon films produced from charged particle (RF plasma) irradiation of (1) 100% CH4; (2) 7% CH4, 93% H2; (3) 0.5% CH4, 99.5% H2; (4) 0.0002% CH4, 99.3% H2 (with impurities); and (5) 3 to 25% CH4, 25% He, remainder H2 — all at submillibar pressures. In all experiments, yellow to deep brown‐red solid products are synthesized which are hypothesized to be, at least in part, the unidentified visible and near‐UV chromophores in the stratospheres of Uranus and Neptune. Results for experiments 2, 3, and 4 are in good mutual accord, but are significantly different from experiments 1 and 5. He in the precursor gases affects the product composition. Typical solid products for experiments 5 show, at 0.55 µm wavelength, n = 1.60 ± 0.05, 3 × 10−2 ≥ k ≥ 3 × 10−3, and [C/H] ≃ 0.7. These results are, for n and k respectively, consistent with and in excellent agreement with those derived from high phase angle Voyager 2 photometry of Uranus (Pollack et al., this issue). Aerosols produced directly from the atmosphere by precipitating magnetospheric charged particles may be competitive with those produced by UV and charged particle irradiation of simple hydrocarbon condensates. The optical and chemical properties of aerosols in the Uranian and Neptunian atmospheres may evolve toward higher values of n and k and higher carbon content as the particles sediment through changing radiation and thermal environments.
Electrostatic waves in the magnetosphere of Uranus
Authors: Kurth W. S. , Barbosa D. D. , Gurnett D. A. , Scarf F. L.
Published: February 2008

During its encounter with Uranus the plasma wave receiver on Voyager 2 observed electrostatic waves similar in many respects to those observed in other planetary magnetospheres. The most prominent type observed was the Bernstein mode emissions between harmonics of the electron cyclotron frequency. As is the case at other planets, the most intense Bernstein waves were observed near the magnetic equator of the planet, even though the tilt of the Uranian magnetic moment with respect to the rotational axis is very large. A small offset in the location of these electrostatic waves from the equator predicted by the offset, tilted dipole magnetic field model suggests some warping of the magnetic equator due to ring currents or external currents flowing on the magnetopause. Other examples of electrostatic Bernstein waves were observed closer to the planet and at higher magnetic latitudes. The energy of resonant electrons is calculated to be a few hundred electron volts, and measurements of electrons with this energy indicate the critical flux required to drive the Bernstein mode is available. The existence of the Bernstein modes near the upper hybrid resonance frequency leads to estimates of the electron density at several locations within the Uranian magnetosphere, and these compare well with densities measured by the plasma science investigation. In addition to the Bernstein modes, a number of highly sporadic emissions were observed in the vicinity of the Miranda L shell. While the absolute determination of the mode of these waves is uncertain, it is likely that some are electrostatic modes. Since this region of the Uranian magnetosphere is very perturbed and interesting, we shall attempt to identify possible modes associated with the waves.
Thermospheric dynamics during November 21–22, 1981: Dynamics Explorer measurements and thermospheric general circulation model predictions
Authors: Roble R. G. , Killeen T. L. , Spencer N. W. , Heelis R. A. , Reiff P. H. , Winningham J. D.
Published: February 2008

Time‐dependent aurora and magnetospheric convection parameterizations have been derived from solar wind and aurora particle data for November 21–22, 1981. These parameterizations are used to drive the auroral and magnetospheric convection models that are embedded in the National Center for Atmospheric Research thermospheric general circulation model (TGCM). The TGCM has been used to calculate the time‐dependent global circulation, temperature and compositional structures for the geomagnetic moderately disturbed period. The results of the TGCM calculation are compared with measurements made by the Dynamics Explorer 2 satellite along its orbital path during several passes over the northern and southern hemisphere polar caps. TGCM input parameterizations of auroral particle precipitation and ion drift generally agree with satellite measurements of these inputs for the period. There is also good agreement between TGCM‐predicted neutral winds and DE 2 neutral wind observations showing the dominant influence of magnetospheric convection on the high‐latitude circulation. On the eveningside of the auroral oval there is a transition from a magnetospheric convection‐driven wind pattern at high magnetic latitudes to one that is driven principally by solar EUV and UV heating at middle and low latitudes. This transition is consistent with the measured latitudinal falloff of electric field strength. The comparison of experimental measurements and time‐dependent model calculations demonstrates that the great natural variability that is observed in the large‐scale structure and dynamics in the thermosphere can be interpreted successfully in terms of variations in the thermospheric forcing mechanisms that are incorporated in the TGCM.
Thermal filamentation instability driven by the auroral electrojet current
Authors: Kuo S. P. , Lee M. C.
Published: February 2008

A thermal instability leading to the filamentation of auroral electro‐jet currents and giving rise to purely growing magnetic field‐aligned density irregularities in the E region of the high‐latitude ionosphere is investigated. The physical process of the instability is through the modification of the electron‐neutral collision frequency due to the electron temperature perturbation in the electrojet. A dispersion relation of the instability is derived, from which the threshold electrojet current and the growth rate of the instability are determined. It is found that they become independent of the scale sizes of the irregularities for scale sizes larger than about 13 m. The proposed instability can, thus, be considered to be one of the mechanisms responsible for those observed relatively large‐scale E region irregularities (Pfaff et al., 1984).
Low‐frequency waves and electric fields in the magnetospheric current sheets
Authors: Hruška A.
Published: February 2008

Low‐frequency disturbances in a plasma immersed in a force‐free magnetic field B0 characterized by a zero‐order field‐aligned current J are analyzed in the shortwave approximation. Parallel electric field E∥ (always small in comparison with E⊥) often substantially modifies the properties of waves in the typical current sheets in the near‐Earth magnetosphere. The mode of oscillations corresponding in the limit J = 0 to the Alfvén mode is partly or totally absorbed on the auroral oval field lines. Conditions under which optimum absorption occurs are pointed out. The second mode of oscillations, which correspond to the compressional wave in the limit J = 0, is not substantially affected by the magnetospheric current sheets.
Analysis of Pioneer Venus Orbiter ultraviolet spectrometer Lyman α data from near the subsolar region
Authors: Paxton L. J. , Anderson D. E. , Stewart A. I. F.
Published: February 2008

Pioneer Venus Orbiter ultraviolet spectrometer data from 20 orbits which span 3 years have been analyzed to determine the atomic hydrogen number density and vertical flux at the exobase as a function of solar zenith angle, F10.7 index, and spacecraft latitude. From 1979 through 1981 the exobase number density nc and flux ϕc are remarkably constant at nc = 6.0 ± 1.5 × 104 cm−3 and ϕc = 7.5 ± 1.5 × 107 cm−2 s−1 in the subsolar region. The integrated vertical column density above 110 km is 3.6 ± 1 × 1013 cm−2. An empirical relationship is determined between the line center solar flux at H Lyman α, πF0, and the F10.7 index. At 1 AU, πF0 = (2.0 + 1.1 × 10−2F10.7) × 1011 photons cm−2 s−1 Å−1.
Outer zone electron precipitation produced by a VLF transmitter
Authors: Vampola A. L. , Adams C. D.
Published: February 2008

By means of high‐resolution pitch angle measurements made by a magnetic‐focusing electron spectrometer on the S3‐3 satellite while in the drift loss cone region of the magnetosphere, characteristics of fluxes of 108‐ to 654‐keV electrons precipitated in the inner zone, in the slot region, and in the outer zone of the magnetosphere are all shown to be consistent with the precipitation's having been produced by the same ground‐based VLF transmitter, UMS. Pitch angle measurements are used to locate the longitude of precipitation. The temporal pattern of transmitter operation obtained from synoptic data from a ground‐based VLF receiver is used along with drift rate calculations to predict the electron energies as a function of L shell which should be observable by the S3‐3 instrument. The predicted energy response is then compared with the in situ observations, getting complete agreement. Finally, wave‐particle resonance calculations are made for each of the three regions. The study indicates that ground‐based VLF transmitters, which have previously been shown to produce precipitation in the inner zone and slot regions, are almost certainly instrumental in precipitating electrons in the outer zone also.
Growth of correlation in compressible two‐dimensional magnetofluid turbulence
Authors: Dahlburg R. B. , Picone J. M. , Karpen J. T.
Published: February 2008

Spectral transfer has been proposed as the primary mechanism for generating outward propagating Alfvén waves in the solar wind. This process has been investigated extensively for incompressible magnetofluids, but the issue of whether it occurs in compressible magnetofluids such as the solar wind remains unresolved. We report the results of direct numerical simulations of nonisentropic, compressible, two‐dimensional magnetohydrodynamic turbulence which indicate that for systems with finite initial cross‐helicity the correlation between the fluctuating velocity field and the fluctuating magnetic field grows as a function of time. We support the interpretation of this growth of correlation as a turbulent process by examination of modal wave number spectra.
Current‐driven lower hybrid waves at the inner edge of the ring current
Authors: LaBelle J. , Treumann R. A.
Published: February 2008

Waves near the lower hybrid frequency have been observed in association with the diamagnetic current at the inner edge of the ring current (LaBelle et al., this issue). Here we interpret these waves in terms of the modified two‐stream instability, generalized to include two ion components: the cold ions of the background plasma and the hot drifting ring current ions. Solution of the dispersion relation yields a weak instability. Since the waves are Landau damped by the background ions, the instability depends sensitively on the background ion temperature as well as the drift velocity. The general characteristics of the observed waves are determined fairly well by the linear theory. Estimates of the saturation level suggest that the waves saturate due to flattening of the current ion distribution function.
Linear instabilities in multicomponent plasmas and their consequences for the auroral zone
Authors: Schriver David , Ashour ‐Abdalla Maha
Published: February 2008

Ion conics are commonly observed along auroral zone field lines and involve all major terrestrial ion species, including hydrogen, helium, and oxygen. It is believed that low‐frequency plasma waves, driven unstable by field‐aligned currents, transversely heat the ion distributions via wave‐particle interactions, creating the ion conics. Considered here are low‐frequency, oblique, electrostatic instabilities found in a mixed plasma that includes an electron beam streaming through a background of electrons, hydrogen, and oxygen. The addition of oxygen not only modifies the lower hybrid frequency, but allows the existence of an ion‐ion (Buchsbaum) hybrid mode with a frequency between the hydrogen and oxygen gyrofrequencies. Because of its low frequency, the ion‐ion hybrid instability can be effective in transversely heating heavy ions. When the electron beam drift speed is greater than 3 times the background electron thermal velocity(Vo > 3vte) and the electron gyrofrequency to plasma frequency ratio is less than 10 (Ωe/ωpe < 10), the lower hybrid instability dominates. However, for Ωe/ωpe > 20, which is a condition commonly found in the auroral zone night side region, the ion‐ion instability has the largest growth rates; in these regions, heavy ion transverse heating can occur. When 10 < Ωe/ωpe < 20 the hydrogen to oxygen density ratio determines which instability dominates.
An analysis of satellite observations of the O I EUV dayglow
Authors: Link R. , Chakrabarti S. , Gladstone G. R. , McConnell J. C.
Published: February 2008

An analysis of satellite STP78‐1 measurements at solar maximum of the latitudinal variation of O I EUV dayglow emissions at 1356, 1304 and 989 Å is presented. The day glow intensity measurements corroborated by Atmosphere Explorer‐E data indicate that the exospheric temperature was about 220–240 K higher than predicted by the MSIS‐83 thermospheric model. In addition, the temperature variation with latitude is not as large as predicted by MSIS‐83 for day 80, 1979. The 1304‐ and 1356‐Å intensity measurements are consistent with electron impact excitation cross‐section measurements of Zipf and Erdman (1985). The 989‐Å excitation cross section of Zipf and Erdman (1985) requires a branching ratio of 7×10−4 for transitions other than 989‐Å in order to produce agreement with our data. The Vaughan and Doering (1987) direct 3s' ³D° excitation cross section, about a factor of 3 smaller than the optical excitation cross section of Zipf and Erdman (1985), is also compatible with our 989‐Å data for a branching ratio near 1×10−4. The solar 1304‐Å spectral irradiance inferred from the observations is about a factor of 2 lower than the rocket measurements of Mount et al. (1980), but is consistent with Solar Mesosphere Explorer results. Our measurements also suggest that enhancement of the 1304‐Å electron impact excitation cross section due to radiative entrapment of cascade‐feeding photons is negligible.
Effects of dissipation on rotational discontinuities
Authors: Wu C. C.
Published: February 2008

The effects of dissipation on rotational discontinuities have been studied as an initial‐value problem. Starting with a rotational discontinuity, we apply resistivity and viscosity and follow the resulting evolution of the system by numerically integrating in time the viscous, resistive MHD equations. The results show that by including dissipation, a rotational discontinuity is unstable and will evolve to an MHD intermediate shock of the kind whose shock frame fluid velocities are subfast, super‐Alfvénic, and superslow ahead, and subfast, sub‐Alfvénic, and superslow behind the shock. (However, with small dissipation, the intermediate shock and the rotational discontinuity would be very close.) The results also show that there are a larger class of shocklike structures, which do not satisfy Rankine‐Hugoniot relations, in the time‐dependent dissipative MHD equations. The results also indicate that the corresponding Riemann problem is not well‐posed. In addition, the admissibility of MHD equations is discussed. The conclusion is that all entropy satisfying shocks along the shock curve are physical. This differs from the conventional view that intermediate shocks are not allowed. This also differs from the admissibility criterion of Oleinik [1957] and Liu [1981]. However, their criterion is valid for a strictly hyperbolic system, whereas the set of MHD equations is nonstrictly hyperbolic. Regarding the magnetospheric physics, our study suggests that instead of rotational discontinuities, intermediate shocks should exist at the magnetopause. Thus one has a different signature from that of a rotational discontinuity to compare with space observations. The results also provide an explanation for the observation that the sense of the magnetic field rotation across the magnetopause depends on the relative orientation of the magnetosheath and the magnetospheric field, and the rotation is less than 180°.
Linear stability of the H+ ‐ O+ two‐stream interaction in a magnetized plasma
Authors: Bergmann R. , Roth I. , Hudson M. K.
Published: February 2008

The H+ ‐ O+ two‐stream instability in a magnetized plasma is examined for application to ion beam heating in the auroral electron acceleration region. In the fluid limit (zero ion temperature) it is found that the most unstable mode coupling is between the obliquely propagating fast O+ and slow H+ cyclotron modes, even for relative ion beam velocities where parallel propagating modes are unstable. The most unstable mode has properties analogous to those of a beam‐plasma instability. Once a relative velocity is reached where obliquely propagating modes dominate, the growth rate becomes independent of the perpendicular wave vector and so there is no particular angle of maximum wave growth. Upon adding a finite ion temperature it is found that the gross features of the H+ ‐ O+ fluid interaction are retained but finite gyroradius effects cause the appearance of an angle of maximum growth. The tangent of this angle is proportional to the velocity difference between the ion beams, for large enough relative velocity, and is also a function of the H+ to O+ density ratio. The qualitative behavior of the maximum growth rate and its frequency and wave vector versus relative velocity and density ratio are examined. The implications of this behavior for H+ and O+ beam heating in the auroral electron acceleration region are discussed.
Theory of auroral zone PiB pulsation spectra
Authors: Lysak Robert L.
Published: February 2008

Changes in the auroral zone current system are often accompanied by magnetic pulsations with periods of about 1 s. These so‐called bursts of irregular pulsations (PiB) have been observed both on ground magnetograms and with in situ satellite observations. These pulsations can be understood as excitations of a resonant cavity in the topside ionosphere, where the Alfven speed has a strong gradient due to the exponential decrease of density above the ionosphere. These waves have a frequency which scales as the ratio of the Alfven speed at the ionosphere divided by the ionospheric scale height. For a pure exponential Alfven speed profile, the mode frequencies are related to zeroes of the zeroth‐order Bessel function. For other profiles of the density, and therefore Alfven speed, the frequencies are not exactly given by the simple theory, but the frequency and mode structure are similar provided the Alfven speed sharply increases above the ionosphere.
Comparison of OGO 6 measured thermospheric temperatures with the MSIS‐86 empirical model
Authors: Hedin A. E. , Thuillier G.
Published: February 2008

The thermospheric temperatures measured by the Fabry‐Perod interferometer on the OGO 6 satellite are found to be reasonably represented by the mass spectrometer/incoherent scatter 1986 (MSIS‐86) empirical model except for two anomalies, one in the South Atlantic and the other near noon local time. These anomalies are likely due to measurement problems. The OGO 6 temperature data were not used in the generation of the MSIS models, so this is an independent comparison of measured and model temperatures. The measurements were made primarily during daytime at mid to low latitudes and throughout the day at high latitudes. On average, the measured temperatures are 16 K below the MSIS‐86 model temperatures. Latitude gradients during solstices as well as for the yearly average are well represented by the model, as are high‐latitude longitudinal and magnetic activity variations.
MHD stagnation point flows in the presence of resistivity and viscosity
Authors: Gratton Fausto T. , Heyn Martin F. , Biernat Helfried K. , Rijnbeek Richard P. , Gnavi Graciela
Published: February 2008

We analyze the steady state situation in which two separate and counterstreaming plasmas (assumed to be incompressible) carrying antiparallel magnetic fields are separated by a resistive current layer. Exact solutions are presented which describe the stagnation point flow pattern and magnetic field behavior which result. They incorporate the effects of viscosity, which enables us to model flows with vorticity. The uniform plasma flow which is obtained at large distances from the current layer allows us to specify finite values of the asymptotic magnetic field. Our exact solutions complement those of a different type obtained by B. U. Ö. Sonnerup and E. R. Priest (1975) which assume potential flow, i.e., zero vorticity. The results we obtain are discussed in relation to observational features at the Earth's magnetopause.
The electromagnetic ion cyclotron instability in the Io torus
Authors: Machida S. , Goertz C. K. , Hada T.
Published: February 2008

The electromagnetic ion cyclotron waves which are expected to exist in the Jovian magnetosphere are investigated. The temperature anisotropy generated by the inward radial diffusion of hot ions gives rise to an instability of L mode waves in the off‐equatorial region of the Io torus. The resulting pitch angle scattering has been suggested as the cause for the precipitation of ions into the loss cone and auroral excitation. The linear wave dispersion is first examined, and the nonlinear wave amplitude for the saturated state is studied. Two estimates of the wave saturation level are checked by performing an electromagnetic hybrid simulation. Estimated nonlinear saturation amplitudes are compared with those resulting from linear amplification in a finite length. The result shows that the waves in the Jovian magnetosphere produced by the hot protons are mostly in a linear regime.
Alfvén surface waves excited by a sudden impulse
Authors: Uberoi C.
Published: February 2008

The time‐dependent evolution of an initial wave disturbance at the magnetospheric boundary as it is released in the presence of the sheared hydromagnetic flow in the magnetosphere is discussed for the simple case in which the shear in the Alfvén speed is uniform. The uniform‐shear assumption allows explicit solutions to be obtained which are useful in explaining the recent observations of resonant Alfvén waves excited in the magnetosphere by a sudden impulse.
Lower hybrid waves upstream of comets and their implications for the comet Halley “bow wave”
Authors: Hizanidis K. , Cargill P. J. , Papadopoulos K.
Published: February 2008

Observed and theoretical features concerning the nature of so‐called cometary “bow shocks” or “bow waves” are discussed. Collective plasma effects associated with the presence of pickup ring ions (protons and water ions) in the vicinity of the supermagnetosonic to submagnetosonic transition region in the quasi‐perpendicular limit are considered; the linear and nonlinear evolution of instabilities around the lower hybrid frequency is emphasized. It is shown that lower hybrid waves can lead to heating and produce distributions with magnitudes in reasonable agreement with Giotto data. The implications to the existence and structure of cometary bow shocks are discussed.
Numerical analysis of the coupled hydromagnetic wave equations in the magnetosphere
Authors: Krauss‐Varban Dietmar , Patel V. L.
Published: February 2008

Inside the magnetosphere two basic hydromagnetic modes, the poloidal and toroidal, are always coupled. The problem of coupled waves is of fundamental importance for nontrivial geometries and general field configurations. As a first step the box model is investigated here, using the ideal MHD equations for the cold plasma. The spectrum and eigenfunctions are calculated for several cases by applying the finite element method. Both the improper continuum eigenmodes as well as the discrete solutions are computed. Existing analytical results are confirmed, and it is found that the discrete modes situated inside the Alfvén continuum are the best candidates for explaining observed hydromagnetic resonances. Other properties of these global modes are derived and compared with experimental data, and satisfactory agreement is obtained despite the simple geometric model employed. Generally, damping of the discrete modes is found to be considerably lower than that of the continuum. The results indicate that the weakly damped global modes lose their energy directly through ionospheric Joule dissipation, and not via mode conversion to field line resonances.
IMF By dependence of region 1 Birkeland currents near noon
Authors: Erlandson R. E. , Zanetti L. J. , Potemra T. A. , Bythrow P. F. , Lundin R.
Published: February 2008

We have conducted a statistical study of large‐scale Birkeland currents, hot plasma, and the By component of the interplanetary magnetic field. Forty‐eight Viking orbits from July 19, 1986, to September 2, 1986, were used in the study during a time period when the 3‐RE geocentric apogee was located near the dayside high‐latitude region usually associated with the polar cusp. We first compared the location of magnetosheathlike electrons to the region 1 and “traditional cusp” Birkeland current systems near noon. It was found that the region 1 Birkeland current system was colocated with the region of most intense magnetosheathlike electron flux. We therefore infer that the “traditional cusp” current system, located poleward of the region 1 system, is associated with field lines that extend to the plasma mantle in the outer magnetosphere. Using this same data base a statistical study between the By component of the IMF and the flow direction of the region 1 Birkeland current system as a function of magnetic local time was performed. We determined that the flow direction of the region 1 current system near noon depends on the By component of IMF. The meridian that separates the dawnside and duskside region 1 Birkeland currents shifted to magnetic local times before noon when By was less than 0 nT, and to the afternoon when By was greater than 0 nT. The extent of the shift away from noon was dependent on the magnitude of the IMF By component.
Two classes of models for temporal variability of the Io torus
Authors: Cheng Andrew F.
Published: February 2008

The stability and temporal variability of the Io torus are investigated using spatially averaged, uniform models. Stable equilibrium torus models are found for two classes of models, one with nonlinear radial transport loss of plasma and one with linear transport. The former can be realized by centrifugally driven interchange instability, and the latter by Jovian atmospheric wind‐driven interchanges. The torus models include photoionization and recombination in addition to torus ion sputtering and other neutral injection mechanisms. Given the possible variability of the Io atmosphere, significant temporal variations may occur in the Io torus. For stable models with linear transport, significant electron cooling is predicted for ion or neutral densities more than an order of magnitude above values observed by Voyager, unless unknown electron heating mechanisms intervene. Higher densities may not be achievable. For nonlinear transport by centrifugal interchange, the torus density need not be limited by electron cooling. For either type of transport, stable torus models exist with densities smaller than Voyager values by at least a factor of 100. Observational tests are proposed that may distinguish between the two types of transport. Stability of the present Io torus does not require either nonlinear transport or centrifugally driven interchange instability.
The electrostatic charging of thin dust clouds
Authors: Wilson G. R.
Published: February 2008

The electrostatic charging of the rings of Saturn is studied by modeling the charging of a thin dense dust cloud. The model is based on a time‐dependent solution of a set of equations which include Boltzmann's and Poisson's equations. Plasma loss and photoelectron production within the cloud are included. Several aspects of the cloud's structure (thickness, average particle size, cross sectional profile, and optical depth) and the ambient plasma (density, temperature, and velocity distribution) can be varied to determine the effect of these quantities on the resulting charge and electric fields. It is found that the electric field is often more complicated in structure and stronger in magnitude within the cloud than at its edges. Thermalization of the plasma within the cloud via collisions with other plasma particles or charged dust, which might reduce the large electric fields found within the cloud, is shown to have a negligible effect because of the predominance of plasma absorption. The motion and accretion of micron and submicron dust grains within the cloud can be significantly affected because of the strong electric fields there.