bluuebunny/crossref_metadata_2025 huggingface dataset:
A simulation study on reconnection and small‐scale plasmoid formation
Authors: Scholer Manfred , Roth David
Published: February 2008

Magnetic reconnection in a plane current sheet is investigated by means of a two‐dimensional time dependent compressible resistive MHD code. The system length is large compared to the current sheet half thickness. The anomalous resistivity is allowed to grow with either the electrical current density or the drift velocity. Reconnection is not forced, but is initiated by a small localized resistivity which is subsequently switched off. Fast reconnection develops self‐consistently with slow shocks extending from the X point and a fast mode expansion upstream. A slow mode expansion fan leads upstream of the slow mode shocks to a plasma flow toward the X point. In the case of current dependent resistivity the reconnection rate decreases again, which leads to a flattening of the separatrices and a long current layer. Secondary tearing produces a new pair of X points which are active sites of reconnection. For low plasma β the two developing plasmoids immediately merge into a single one. For high plasma β, reconnection proceeds at all three X points leading to a figure‐8 magnetic island structure. In the case of drift velocity dependent resistivity and low plasma β values the secondary tearing occurs at locations of low density along the x axis (large resistivity). The plasmoids merge, and new X points develop further out. Continuous development of plasmoids and their subsequent coalescence leads to a large‐scale plasmoid filling the whole computational box. For high plasma β values (small compressibility) the resistivity (and the reconnection rate) at the initial X point stays constant, the current sheet does not elongate, and no plasmoid develops, although the system length is large. The results are discussed in relation to recent observations by ISEE 3 in the distant tail.
Low‐frequency modulation of plasmas and soft electron precipitation near the dayside cusp
Authors: Basinska Ewa M. , Burke William J. , Basu Sunanda , Rich Frederick J. , Fougere Paul F.
Published: February 2008

On December 2, 1983, the HILAT satellite trajectory was approximately aligned with the noon‐midnight meridian while crossing the northern polar cusp/cleft region. During that time we observed (1) an increase in scintillations of radio signals, (2) strongly variable fluxes of precipitating low‐energy electrons, (3) gradients in the thermal ion density, (4) fluctuations in the east‐west component of the ion drift velocities superimposed on the noon sector convection pattern, and (5) field‐aligned currents. In the region corresponding to the highest level of radio scintillations we also detected fluctuations in electron fluxes on a time scale of half a second superimposed on a low‐energy inverted‐V event. The analysis of these measurements, which provide information on both large‐ and small‐scale dynamics of the dayside cusp, leads to the following conclusions: (1) the large‐scale convection pattern is characteristic for the northern hemisphere high‐latitude ionosphere when the interplanetary magnetic field has negative Y and Z components; (2) the most intense scintillations and the cusp field‐aligned currents occur on recently opened magnetic field lines; (3) plasma created by increased fluxes of precipitating low‐energy electrons convects antisunward with velocity of ∼0.5 km/s; (4) enhanced chemical reactions due to the Joule heating associated with large electric fields maximize at the equatorward edge of the cusp, leading to the net depletion in total electron content and plasma density measurements; (5) plasma irregularities throughout the F region are formed due to a combined effect of structured soft electron precipitation and current‐convective instabilities; and (6) if the subsecond fluctuations of the ion drift measurements seen during the inverted‐V event are due to static variations in the ambient electric field, there are potential drops of a few tens of volts between the altitude of the satellite (∼830 km) and the lower ionosphere.
Kinematic properties of the F region ion velocity field inferred from incoherent scatter radar measurements at Arecibo
Authors: Burnside R. G. , Walker J. C. G. , Sulzer M. P.
Published: February 2008

A new multiple‐frequency incoherent scatter observing technique has improved the precision of F region ion velocity measurements at Arecibo. This technique makes it possible to analyze velocity data without making the assumption that the horizontal gradients in each component of the velocity vector are zero. By assuming that spatial gradients in ion velocity are constant and that the magnetic field line through the F region is equipotential it is possible to determine completely the variation of the ion velocity vector in the plane of the magnetic meridian. The horizontal divergence of the velocity vector is generally observed to be small. At night the most significant spatial variation in the ion velocity field is usually converging flow parallel to the magnetic field; this reverses to diverging flow in the daytime. Neglect of these horizontal gradients in the vertical ion velocity can result in systematic errors in the evaluation of the meridional ion velocity as large as 30 m s−1.
Absolute differential and integral electron excitation cross sections of the atomic oxygen ³P and 5P states at 30 eV
Authors: Gulcicek Erol E. , Doering John P.
Published: February 2008

A new variable angle and incident energy electron energy loss spectrometer coupled to a supersonic jet expansion microwave discharge source has been constructed to study the electron scattering cross sections of stable and unstable atmospheric species. This apparatus was specifically designed to operate at low incident electron energies and at high scattering angles to observe forbidden transitions. In this paper we report the first measurements of the absolute differential and integral cross sections of the atomic oxygen (2p4 ³P‐3p ³P), 10.99 eV, and (2p4 ³P‐3p 5P), 10.76 eV, transitions at a single electron impact energy of 30 eV. The direct excitation cross section ratio for the ³P and 5P states was found to be 6:1 which confirms previous suggestions that electron impact on O I is responsible for the observed 8446 Å and 7774 Å, O I emissions in the dayside cusp region. Additional sources of O I 3p 5P are necessary to explain the nightside auroral 8446 Å/7774 Å ratio. The absolute cascade‐free direct excitation integral cross sections for O I(³P‐³P) and O I(³P‐5P) at 30 eV are 4.0 × 10−18 cm² and 0.67 × 10−18 cm², respectively.
A possible energy source to power stable auroral red arcs: Precipitating electrons
Authors: Slater D. W. , Gurgiolo C. , Kozyra J. U. , Kleckner E. W. , Winningham J. D.
Published: February 2008

Results of coincident measurements by ground‐based photometers and the low‐altitude plasma instrument on board the Dynamics Explorer 2 satellite are presented that demonstrate the association of precipitating low‐energy electrons with stable auroral red (SAR) arcs. A search of available data has yielded 23 instances of DE 2 overflights during the presence of SAR arcs being monitored by the photometers. For each case, downward fluxes of electrons (E < 10 eV) are found to be enhanced along field lines penetrating the arcs in relation to regions both north and south of the features. Modeling of the atmospheric response to these influxes indicates that these electrons can represent a major source of the energy required to establish temperature profiles within the ionospheric electron gas that are sufficient to produce the recorded 6300‐Å (O(¹D)) emission rates. The sensitivity of these results to uncertainties of the assumed spacecraft potential and thermospheric composition has been investigated and found to be important but does not alter the conclusion that precipitating electrons are a fundamental link in the production of SAR arcs.
Relaxation of magnetotail plasmas
Authors: Bhattacharjee A.
Published: February 2008

A quasi‐thermodynamic model is presented for the relaxation of magnetotail plasmas during substorms, followed by quiet times. It is proposed that the plasma relaxes to a state of low‐potential energy subject to a small number of global constraints. The constraints are exactly preserved by all ideal motions and, approximately, by a wide class of nonideal motions of the plasma undergoing magnetic reconnection. A variational principle which minimizes the free energy predicts the relaxed state. Exact, two‐dimensional solutions of the relaxed state are obtained. A universal feature of the exact solutions is a chain of magnetic islands along the tail axis. Sufficient conditions for the stability of relaxed states are obtained from the second variation of the free‐energy functional.
A possible explanation of the electron temperature enhancement in the wake of a satellite
Authors: Singh Nagendra , Samir U. , Wright K. H. , Stone N. H.
Published: February 2008

A numerical simulation was performed in an attempt to explain the enhancement of electron temperature in the wake of an ionospheric satellite in terms of counterstreaming plasma expansions. The simulation was conducted using full kinetic treatments (Vlasov‐Poisson code) for both the plasma electrons and ions. The calculations indicate that at a distance of a few Debye lengths from the surface of the body the ratio [Te(wake)/Te(ambient)] reaches a value of ∼2. This result is in good agreement with some in situ and laboratory experimental results.
Generation of solar wind proton tails and double beams by Coulomb collisions
Authors: Livi S. , Marsch E.
Published: February 2008

A kinetic model is presented for the generation of proton heat flux tails and double beams by Coulomb collisions in the solar wind. The combined action of the large‐scale interplanetary magnetic field (mirror effect) and of the collisional scattering (runaway) is shown to be able to produce variously skewed velocity distributions. All intermediate cases between the exospheric and isotropic adiabatic expansion can be simulated as a function of the collisionality of the plasma. Good qualitative agreement is achieved between in situ measurements and model results in the collisional domains of the solar wind.
Electron impact ionization in the vicinity of comets
Authors: Cravens T. E. , Kozyra J. U. , Nagy A. F. , Gombosi T. I. , Kurtz M.
Published: February 2008

The solar wind interacts very strongly with the extensive cometary coma, and the various interaction processes are initiated by the ionization of cometary neutrals. The main ionization mechanism far outside the cometary bow shock is photoionization by solar extreme ultraviolet radiation. Electron distributions measured in the vicinity of comets Halley and Giacobini‐Zinner by instruments on the VEGA and ICE spacecraft, respectively, are used to calculate electron impact ionization frequencies. Ionization by electrons is of comparable importance to photoionization in the magnetosheaths of Comets Halley and Giacobini‐Zinner. The ionization frequency in the inner part (radial distance ≈ 104 km) of the cometary plasma region of comet Halley is several times greater than the photoionization value. Tables of ionization frequencies as functions of electron temperature are presented for H2O, CO2, CO, O, N2, and H.
The electrostatic effect for the collisionless tearing mode
Authors: Hoshino Masahiro
Published: February 2008

Electron dynamics has not been self‐consistently considered in collisionless tearing mode theories to date because of the mathematical complexity of the Vlasov‐Maxwell equations. We have found using computer simulations that electrostatic fields play an important role in the tearing mode. Vlasov theory, including the electrostatic field, is investigated for topologies with both antiparallel and nonantiparallel magnetic field lines. The electrostatic field influences the resonant current in the neutral sheet which is a non‐MHD effect, and modifies the linear growth rate. At the magnetopause, where the field lines are not antiparallel, the electrostatic effect acts to raise the linear growth rate of the tearing mode. On the other hand, in the magnetotail, where magnetic field lines are antiparallel, the electrostatic effect reduces the tearing mode growth rate.
The magnetic field of the equatorial magnetotail: AMPTE/CCE observations at R < 8.8 RE
Authors: Fairfield D. H. , Acuna M. H. , Zanetti L. J. , Potemra T. A.
Published: February 2008

The Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetic field experiment has provided extensive measurements of the vector field near the equatorial plane in the inner magnetotail. Taillike distortions from the dipole field direction may reach 80° near the AMPTE/CCE apogee of 8.8 RE. The Bz (north‐south) field component in dipole coordinates was always positive within 0.5 RE of the equatorial current sheet, indicating that neutral lines were never seen inside of 8.8 RE. South of −0.5 RE, Bz values are sometimes southward due to extremely taillike fields and the southward tilt of the tail in dipole coordinates during northern hemisphere winter. Fields are most taillike near midnight and during times of high Kp. At 8.5 RE the equatorial field magnitude depressions are roughly half the dipole field strength of 51 nT. These depressions are larger at lesser distances reaching −40 nT at 3.4 RE for Kp ≤ 2‐ and −80 nT for Kp ≥ 3+. The equatorial crossing points of midnight field lines are computed using the observed field depressions. Mapping auroral boundaries along these distorted field lines indicates that most auroral arcs map to magnetospheric locations beyond 12 RE, but that low latitude arcs may well occur on closed field lines earthward of this distance. Evidence is found confirming that a cross‐tail field component in the inner magnetotail is related to the interplanetary sector structure but, contrary to earlier results, the effect is found to be larger during geomagnetically disturbed times.
Evolution of the ring current during two geomagnetic storms
Authors: Lui A. T. Y. , McEntire R. W. , Krimigis S. M.
Published: February 2008

The progressive developments in the radial profiles of the particle pressure, plasma beta, and electric currents of the storm time ring current are investigated with data from the medium energy particle analyzer on the AMPTE Charged Particle Explorer spacecraft. Measurements of ions from 25 keV to 1 MeV, which carry 70–85% of the energy density of the entire ring current population, are used in this work. Two geomagnetic storms in September of 1984 are selected and four traversals of the equatorial ring current region during the course of each storm are studied. It is shown that enhancements in the particle pressure occur initially in the outer region and reach the inner region in the late phase of the storm. Structures suggestive of multiple particle injections are seen in the pressure profile. The leading and trailing edges of the particle injection structures are associated, respectively, with the depressions and enhancements of the westward current densities of the ring current. Plasma beta occasionally increases to values of the order of 1 in some regions of the ring current from prestorm values of the order of 0.1 or less. It is also found that the location of the maximum ring current particle pressure can be several earth radii from where the most intense westward ring current flows. This is a consequence of the dominance of pressure gradient current over the current associated with the magnetic field line curvature and particle anisotropy.
Height‐integrated Pedersen and Hall conductivity patterns inferred from the TIROS‐NOAA satellite data
Authors: Fuller‐Rowell T. J. , Evans D. S.
Published: February 2008

The series of polar‐orbiting National Oceanic and Atmospheric Administration spacecraft TIROS, NOAA 6, and NOAA 7 have been monitoring the particle influx into the atmosphere since late 1978. This data base has been used to construct statistical global patterns of height‐integrated Pedersen and Hall conductivities for a discrete set of auroral activity ranges. The observations of energy influx and “characteristic electron energy” have been binned in a 1° latitude and 2° magnetic local time grid and ordered by an auroral activity index. This index is an estimate of the energy deposited into a single hemisphere by incident particles, a parameter generated directly from the particle observations and, therefore, internally consistent with the statistical patterns that are constructed. An average electron spectrum is associated with each characteristic energy, which enables a height profile of ionization rate in the upper atmosphere to be determined. The use of a pressure coordinate system insures that the normalized ionization rate profiles are independent of atmospheric model parameters. To create the statistical pattern of height‐integrated conductivities, however, vertical profiles of atmospheric temperature and composition are assumed, and the ion density enhancements are evaluated from a chemical balance between ion production and recombination based on an “effective” recombination coefficient. The data base can also provide the statistical pattern of particle heating rates and ionization rates over a three‐dimensional grid suitable as input to more sophisticated ionospheric and neutral thermospheric codes.
Ionosphere and ground‐based response to field‐aligned currents near the magnetospheric cusp regions
Authors: Lanzerotti L. J. , Hunsucker R. D. , Rice D. , Lee L. C. , Wolfe A. , Maclennan C. G. , Medford L. V.
Published: February 2008

A special campaign was conducted at the Sondre Stromfjord incoherent scatter radar facility in October 1985 to study hydromagnetic and atmospheric gravity wave phenomena near the cusp regions of the magnetosphere. During a day when the convection reversal boundary was measured just to the north of the radar, a field‐aligned current filament of inferred amplitude ∼2 × 105 A was measured at the boundary. Both ion and electron heating appeared to accompany the current filament. Magnetic field data acquired in the southern conjugate hemisphere are used to conclude that the current filament was probably on closed field lines. The current filament may be evidence of the magnetosphere boundary layer on closed field lines, perhaps produced by the injection of magnetosheath plasmoids onto the field lines by a flux transfer‐type reconnection process.
Characteristics of the Marslike limit of the Venus‐solar wind interaction
Authors: Luhmann J. G. , Russell C. T. , Scarf F. L. , Brace L. H. , Knudsen W. C.
Published: February 2008

Many authors have already noted that Mars' interaction with the solar wind may be like Venus', e.g., that of a supermagnetosonic plasma flowing past an effectively unmagnetized body having an ionosphere. However, at Mars, the incident solar wind dynamic pressure usually exceeds the peak ionospheric plasma pressure, while at Venus this condition prevails only when the incident dynamic pressure is extraordinarily high. With the aim of predicting what might be expected at Mars, this study examines the subset of Pioneer Venus Orbiter observations obtained during intervals of extremely high solar wind dynamic pressure. The characteristic features of this limit of the Venus‐solar wind interaction include a bow shock position that is not notably different from the norm, altitude profiles of the dayside upper ionosphere density without a sharp increase in gradient at the ionopause, and dayside electron temperatures that rapidly increase with altitude to consistently exceed the temperatures above 200 km which are present for lower solar wind pressures. Depleted nightside ionosphere densities, and a large‐scale horizontal magnetic field in both the dayside and nightside ionospheres, are among the previously identified responses to high dynamic pressure. Here, emphasis is placed on the dayside ionosphere because some data are available for the dayside Martian ionosphere from the Viking mission. The density and ion temperature trends are found to be similar to those seen in the Viking data. Overall, the Venus observations at high dynamic pressure provide a framework for reassessing the available Mars observations. In particular, this study shows that the observed absence of a distinct ionopause “cutoff” in the dayside plasma density gradient cannot be construed as evidence for an intrinsic magnetic field which stops the solar wind at higher altitudes. Similarly, the rules of thumb that the electron temperature is given by twice the ion temperature and that the ionospheric magnetic field pressure can only double the total ionospheric pressure, which have been applied to assess Mars' ability to standoff the solar wind, are not justified. Observations at Venus thus illustrate the expected modification of the common pressure balance picture of the solar wind interaction at an unmagnetized planet with a weak ionosphere. The available Mars observations appear to be consistent with this modified picture.
Plasma transport in the auroral return current region
Authors: Ganguli Supriya B. , Palmadesso Peter J.
Published: February 2008

The classical and anomalous transport properties of a multifluid plasma consisting of H+, O+, and electron populations in the presence of auroral field‐aligned return currents are investigated, using a multimoment fluid model with anomalous transport coefficients. This approach offers the possibility of simulating large‐scale dynamic phenomena without neglecting the important macroscopic consequences of microscopic processes such as anomalous resistivity and turbulent heating. The macroscopic effects of the electrostatic ion cyclotron (EIC) instability (perpendicular ion heating) and of an EIC‐related anomalous resistivity mechanism which heats the electrons are included in the present version of the model. The responses of the outflowing polar wind plasma to the application of current, with and without instabilities, are exhibited. The simulations show that the electron drift velocity corresponding to a return current of 0.65 µA/m² is above the threshold for EIC waves. Downward electron heat flow competes with upward convection and adiabatic effects to determine the direction of the electron temperature anisotropy. Resistive electron heating lowers the critical drift velocity for marginal EIC stability and leads to enhanced ion heating. The model predictions are compared with the experimental observations where possible.
An analysis of solar wind fluctuations between 1 and 10 AU
Authors: Vellante M. , Lazarus A. J.
Published: February 2008

A cross‐correlation analysis between different parameters of the solar wind plasma based on Voyager 1 and 2 data from 1 to 9.5 AU evidences, as a predominant feature of the microscale structure of the solar wind, a persistent presence of time periods characterized by high anticorrelation between the magnetic field magnitude and the proton density. These events are present at least 25% of the total time, and their frequency of occurrence progressively increases with increasing heliocentric distance. These results have been satisfactorily interpreted as being due to the presence of nonpropagating structures with internal pressure balance and characteristic time scale of ≲10 hours. Alfvénic fluctuations are confirmed to dominate the microscale at 1–2 AU, but they seem to become less important at larger heliocentric distances.
Collisional interchange instability: 1. Numerical simulations of intermediate‐scale irregularities
Authors: Zargham S. , Seyler C. E.
Published: February 2008

Numerical simulations of the generalized Rayleigh‐Taylor instability are presented. The model and simulations are applicable to bottomside and topside spread F, unstable barium cloud dynamics, and collisional interchange instability in general. The principal result is that the evolution of the instability tends to an anisotropic state consisting of nearly sinusoidal (quasi‐periodic) variation along the effective electric field, and shocklike structures propagating perpendicular to Eeff along the extrema of the quasi‐periodic structures. The spectral properties of the nonlinear state are analyzed using one‐dimensional power spectra calculated along spatial trajectories for selected angles to Eeff. In this way a direct comparison to in situ probe data can be made. The inherent anisotropy of the nonlinear state is reflected in major qualitative differences between the spectra taken parallel to and perpendicular to Eeff. The fundamental finding of the present work is that anisotropy in interchange dynamics is much greater than had been previously reported. This strong anisotropy can explain much of the spectral and spatial structural characteristics of both bottomside and topside spread F. In a companion paper a comparison of the simulation results to various in situ data sets is given.
Magnetotail equilibrium theory: The general three‐dimensional solution
Authors: Birn J.
Published: February 2008

We have reduced the general magnetostatic equilibrium problem for the geomagnetic tail to the solution of ordinary differential equations and ordinary integrals. The theory allows the integration of the self‐consistent magnetotail equilibrium field from the knowledge of four functions of two space variables, x and y: the neutral sheet location zo (x,y), the total pressure
, the magnetic field strength Bo (x,y), and the z component of the magnetic field Bzo (x,y) at the neutral sheet. The self‐consistency of the equilibrium problem thus reduces the effort to construct a tail model from empirical knowledge of four functions (Bx, By, Bz, and p) of three space coordinates to four functions of two coordinates only. All functions can be (and are in fact to a large extent) obtained from observations. The basic assumption of the quasi‐static theory with isotropic pressure is that spatial variations in the x and y directions can be treated as small in comparison with variations in the z direction perpendicular to the plasma sheet. This assumption restricts the validity of the theory to average, quiet configurations of the interior tail excluding, in particular, regions of fast flow and strong y gradients such as the plasma mantle and the low‐latitude boundary layer. The theory allows one to include the effects of field‐aligned currents and distortions due to an average cross‐tail By field (in addition to an antisymmetric By of a flaring tail) not present in earlier models.
Origin and evolution of fluctuations in the solar wind: Helios observations and Helios‐Voyager comparisons
Authors: Roberts D. A. , Goldstein M. L. , Klein L. W. , Matthaeus W. H.
Published: February 2008

Using hour‐averaged data from the Helios and Voyager spacecraft, we have investigated the origin and evolution of low‐frequency interplanetary fluctuations from 0.3 to 20 AU. Alfvénic fluctuations in the inner solar system are found to be generally outward traveling from the Sun and at times quite pure, in general agreement with previous work. The correlation between velocity and magnetic field fluctuations can be high even on scales longer than the transit time from the Sun to the spacecraft, indicating a solar origin for the initial outward traveling waves. However, the fluctuations become substantially less Alfvénic by 1 AU, with the larger scales evolving more rapidly, and this evolution continues in the outer heliosphere. Near the Sun it is regions with small velocity gradients, rather than specifically the trailing edges of high‐speed streams, that exhibit the purest Alfvénic fluctuations. Density and magnetic field magnitude fluctuations inside 1 AU show the anticorrelation characteristic of pressure balance structures previously found in the outer heliosphere. The lower frequency positive correlation between density and field observed farther out in association with the growth of compression regions is not generally present inside 0.4 AU. Fluctuations have a somewhat higher magnetic than kinetic energy at scales of less than a day, but at lower frequencies, kinetic energy is already dominant by 0.3 AU. These results support the view that outward propagating Alfvénic fluctuations are generated near the Sun and that substantial dynamical evolution, probably involving shear‐generated nonlinear couplings, is important at all heliocentric distances examined.
Concerning the ionospheric modification of magnetospheric hydromagnetic waves: Case studies
Authors: Glassmeier Karl‐Heinz , Junginger Hans
Published: February 2008

Correlated ground magnetic and geostationary satellite electric field measurements of four ULF pulsation events occurring at different local times within the Earth's magnetosphere are analyzed with special emphasis on the influence of the ionosphere on ground‐based observations. Using a simplified rule for the projection of the electric field observations down to the ionosphere, we find two occasions where the ionospheric rotation angle is about 90° and two occasions where it significantly deviates from this value. A possible explanation of these results is given in terms of a recently developed theory concerning the influence of an ionosphere with arbitrary nonuniform horizontal conductivity distribution and in terms of propagation of hydromagnetic waves in noncoplanar magnetic field geometries. The need for further study of this latter point is strongly emphasized.
Electrodynamic effects of thermospheric winds from the NCAR Thermospheric General Circulation Model
Authors: Richmond A. D. , Roble R. G.
Published: February 2008

The ionospheric electric fields and currents and the associated ground magnetic variations, generated by the dynamo action of winds simulated with the National Center for Atmospheric Research, Boulder, Colo., (NCAR) Thermospheric General Circulation Model (TGCM), are modelled and compared with observations for equinox solar minimum conditions. The dynamo model uses a tilted dipole geomagnetic field and allows for field‐aligned current flow between conjugate points, but no magnetospheric dynamo effects are included. Two TGCM wind simulations are used, one of which is driven only by in situ solar ultraviolet heating and the other of which includes lower boundary forcing that mimics the effects of upward propagating semidiurnal tides, as described by Fesen et al. (1986). Without tidal forcing, the TGCM winds produce ground magnetic variations that have the general pattern of observed Sq variations but are only about half as strong. The addition of tidal forcing, as specified by Fesen et al., improves the agreement between calculated and observed magnetic variations and also between calculated and observed electric fields, although the nighttime electric fields are not well reproduced. The dynamo effects of the simulated semidiurnal tides appear to be somewhat overestimated, as has been similarly noted by previous modellers. The nightside disagreement between modelled and observed ionospheric electrodynamic drifts appears to be related to inaccurate simulation of F‐region dynamo effects, which are sensitive to the nightside electron density distribution. The field‐aligned currents between southern and northern magnetic hemispheres are substantial even at equinox, with about 290 kA total current flowing at 1700 UT. The ground magnetic effects associated with these field‐aligned currents are roughly 25% the size of the magnetic effects associated with the horizontally nondivergent component of ionospheric current. The field‐aligned current distribution is considerably more sensitive to the wind and conductivity patterns than is the horizontal ionospheric current distribution.
Distribution of whistler mode bursts at Venus
Authors: Scarf F. L. , Jordan K. F. , Russell C. T.
Published: February 2008

Several thousand impulsive whistler mode noise bursts were detected by the Pioneer Venus wave instrument during the first 10 seasons with nightside traversals at low altitudes. The altitude distribution for these events shows that essentially all of the bursts were detected when the orbiter was less than 2000 km above the planet, suggesting that the varying plasma conditions could not maintain coherent whistler mode field‐aligned guidance over greater distances. Within the 2000‐km range, the distribution of the number of events (N) versus altitude (h) shows that there are two distinct subregions: for h < 800 km we find N varying as (h − h0)−2, 0 ≤ h0 ≤ 50 km, and for 800 < h < 2000 km we find N approximately constant, with significant fluctuations that are frequently correlated with passage over surface features. These results are interpreted in terms of two types of whistler mode propagation from sources below the ionosphere; we consider direct detection of near‐field radiation for h < 800 km, and detection of ducted whistlers for the 800 < h < 2000 km range.
Heavy ion density enhancements in the outer plasmasphere
Authors: Roberts W. T. , Horwitz J. L. , Comfort R. H. , Chappell C. R. , Waite J. H. , Green J. L.
Published: February 2008

A region of density enhancements of thermal heavy ions (O+, O++, and N+) has been observed on numerous occasions by the retarding ion mass spectrometer on the Dynamics Explorer 1 satellite. Outer plasmasphere densities of heavy ions are often observed to be up to 2 orders of magnitude higher than equatorward densities within an orbital pass. This phenomenon is almost always observed in the region of the plasmasphere just inside the plasmapause and has been seen at all local times. A statistical study of these heavy ion density enhancements, covering almost 600 passes of DE 1 through the plasmasphere, shows that O+ and O++ enhancements occur over about 64% of the observed passes, with the highest frequency of occurrence being found in the late evening and morning regions. O+ enhancements tend to be seen more frequently in the morning, while O++ enhancements are more likely to be observed in the evening. O+ appears to have a higher outer plasmasphere density enhancement than O++ in all local time regions. Evidence showing that the L shell of the heavy ion density enhancement peaks is dependent upon Dst is presented. Finally, mechanisms for the creation of the heavy ion density enhancements are briefly discussed, including the “geomagnetic mass spectrometer” and plasmapause‐associated electron heating of the topside ionosphere.
Perpendicular ion heating effects on the refilling of the outer plasmasphere
Authors: Singh Nagendra , Hwang K. S.
Published: February 2008

The trapping and the thermalization of the outflowing ionospheric plasma in the outer plasmaspheric flux tubes during refilling after geomagnetic storms involves microscopic processes, which in the initial stage of the refilling are essentially collisionless. Although there are several observations on upflowing ions with anisotropic pitch angle distributions at superthermal energies in the region of the refilling, the role of such heated ions in the refilling has not been examined. It is shown here that an extended perpendicular ion heating by a low‐level background plasma noise near the ion cyclotron frequency with spectral power density ψ ∼ 10−11 V² m−2 Hz−1 can be effective in trapping the bulk of the ions in the flux tube. Such electric field noise levels can be generated by fluctuations associated with the equilibrium near marginal stability of a low‐density plasma mainly consisting of very hot ions originating from the ring current. The heated bulk ions have energies (∼ 1 eV) characteristic of the filled plasmasphere. When the turbulence level exceeds the above level, the heated ions show the features of the ion conies in the superthermal energy range observed along the field lines of refilling. In contrast to the weak extended heating, when the heating is intense and localized in the equatorial region, the trapped ions are shown to set up parallel electric fields pointing away from the equator. The potential drop associated with such fields are found to be large enough to stop interhemispheric flow by reflecting the ionospheric plasma streams.
Counterstreaming ion events in the magnetosphere
Authors: Horita R. E. , Ungstrup E. , Shelley E. G. , Anderson R. R. , Fitzenreiter R. J.
Published: February 2008

Counterstreaming ions or ions traveling simultaneously both parallel and antiparallel to the magnetic field direction have been briefly noted in the literature but have not been studied previously in depth. We have studied over 60 counterstreaming ion (CSI) events observed on the ISEE 1 satellite. Often, both oxygen and hydrogen ions at a number of energy levels are Counterstreaming, but some events show only either oxygen or hydrogen ions at one or more energy levels involved. One particularly interesting event shows only counterstreaming oxygen ions with 417 eV energy; oxygen ions of lower energy (215 eV) and higher energy (630–17,000 eV) and all the hydrogen ions between 215 and 17,000 eV energy have lower fluxes and/or nearly isotropic pitch angle distributions. This event correlates well with wave activity in the 17‐ to 100‐Hz band and is also accompanied by 200‐eV downgoing and 400‐eV upgoing electrons. Details of this and some other CSI events are presented. The CSI events were found to occur at altitudes of about 2–8 RE on L shells of about 5–12 in the evening‐to‐morning sector from about 1700 to 0900 LT; the majority of the CSI events at altitudes between 2 and 5 RE and L values from 5 to 8 involve oxygen ions, while at higher altitudes from about 5 to 8 RE and higher L values from about 8 to 12 the majority involve hydrogen ions. Some features suggest wave‐particle interaction may be involved in some CSI events.
Heat conduction limits on calorimetric effects at Mercury due to solar wind‐magnetosphere interactions
Authors: Vasyliunas Vytenis M.
Published: February 2008

Heat produced at the surface of Mercury by the highly time‐variable precipitation of charged particles need not be radiated away immediately but can also be conducted into the interior of the planet. For a given precipitated energy flux density, the rise of surface temperature can be computed taking both heat conduction and radiation into account. When the energy input varies on time scales shorter than a characteristic period, estimated to be one (terrestrial) day for conditions typical of Mercury's dark side, heat conduction dominates over radiation and the predicted surface temperature rise becomes negligibly small. Possible observable calorimetric effects are therefore confined to long time scales, and their magnitude is constrained by the average rate of energy input from the solar wind together with limits on the observable area of precipitation.
Coloration and darkening of methane clathrate and other ices by charged particle irradiation: Applications to the outer solar system
Authors: Thompson W. Reid , Murray B. G. J. P. T. , Khare B. N. , Sagan Carl
Published: February 2008

Methane clathrate is expected to be an important carbon‐containing ice in the outer solar system. We investigate the effect of electron irradiation by coronal discharge on several simple hydrocarbons enclathrated in or mixed with H2O or H2O+NH3 in simulation of the effects of the solar wind, planetary magnetospheric particles, and cosmic rays on surfaces containing these ices in the outer solar system and interstellar space. H2O+CH4 clathrate, H2O+C2H6, H2O+CH4+NH3, H2O+C2H6+NH3, and H2O+C2H2 are all initially white ices, and all produce yellowish to brownish organic products upon charged particle irradiation. Significant coloration occurs with doses of 109 erg cm−2, corresponding to short interplanetary irradiation times. Uranian magnetospheric electrons penetrate to ∼1 mm depth and deposit this dose in 8, 30, 65, 200, and 500 years into the surfaces of Miranda, Ariel, Umbriel, Titania, and Oberon, respectively. Further irradiation of the laboratory ice surface results in a progressive darkening and a more subdued color. For a conversion efficiency to solids G ≃ 1 molecule keV−1, the upper limit for the time for total destruction of CH4 and other simple hydrocarbons in the upper 1 mm is 5 × 104 years (Miranda) to 3 × 106 years (Oberon). Remote detection of CH4 is possible only when its replenishment rate exceeds the destruction rate at the depth probed by spectroscopy. Reflection spectroscopy of irradiated H2O+CH4 frost is compared with the spectra of several outer solar system objects and to other relevant organic and inorganic materials. Ultraviolet‐visible and infrared transmission spectroscopy of the postirradiation residues is presented. Persistence of color and of CH4 ice bands on Triton and Pluto suggests ongoing surface activity and/or atmospheric haze. Over 4 × 109 year time scales, ≥ 10 m of satellite and cometary surface material is processed by cosmic rays to a radiation‐hardened ice‐tholin mixture devoid of CH4. Preaccretional chemistry, exogenous materials, and endogenous organic chemistry all contribute to the spectral properties of icy satellites which accreted simple CH(O) molecules. Radiation darkening traces the deposition of mobilized or impact‐exposed carbon‐bearing volatiles on these satellites. More exhaustive experiments are necessary to work out the detailed relationships between initial composition, exposure age, and color/albedo.
Uranian ring photometry: Results from Voyager 2
Authors: Ockert Maureen E. , Cuzzi Jeffrey N. , Porco Carolyn C. , Johnson Torrence V.
Published: February 2008

We have investigated seven images of the Uranian rings taken with the clear filter of the Voyager 2 cameras during the January 1986 encounter. The images are at phase angles of 15.5°, 21.0°, 89.5°, and 172.5°. We obtained an average reflectivity (I/F) for each of the nine main rings (6, 5, 4, α, β, η, γ, δ, and ε) at each phase angle. We compared the observed phase angle variation of I/F with model calculations for various combinations of macroscopic (backscattering) and microscopic (forward scattering) particles. The phase angle variation of I/F for all of the main rings is dominated by backscattering. The phase functions for the ring particles resemble that of the Moon; however, the observed phase functions decrease more rapidly between 21.0° and 15.5° than predicted by the lunar phase function. We obtained a single‐scattering or Bond albedo for the large particles using the best fit backscatter phase function. The average Bond albedo of the particles for all the rings was 0.014 ± 0.004. The shape of the phase variation of I/F around 172.5° phase angle was used to constrain the size distribution of the microscopic “dust‐sized” particles. The fractional area in dust‐sized particles in and around the main rings did not exceed 2 × 10−3. The observations are consistent with an average dust particle size of 1.0 ± 0.3 µm in a Hansen‐Hovenier distribution and with a power law distribution for spherical particles with spectral index (p) = 2.5 ± 0.5. The phase function of 1986U1R was not modeled, because of an anomalous azimuthal darkening in the highest phase angle image that could be due to structural variation.
The magnetic field of Uranus
Authors: Connerney J. E. P. , Acuña Mario H. , Ness Norman F.
Published: February 2008

A spherical harmonic model of the planetary magnetic field of Uranus is obtained from the Voyager 2 encounter observations using generalized inverse techniques which allow partial solutions to complex (underdetermined) problems. The Goddard Space Flight Center “Q3” model is characterized by a large dipole tilt (58.6°) relative to the rotation axis, a dipole moment of 0.228 G RU³, and an unusually large quadrupole moment. Characteristics of this complex model magnetic field are illustrated using contour maps of the field on the planet's surface and discussed in the context of possible dynamo generation in the relatively poorly conducting “ice” mantle.
Surface waves on Uranus' magnetopause
Authors: Lepping R. P. , Burlaga L. F. , Klein L. W.
Published: February 2008

Uranus' magnetosphere has a well‐developed, thick ((4.6–14) × 10³ km) magnetopause that was fully traversed twice by Voyager 2, once inbound to the planet and once outbound. This boundary appears to resemble Earth's magnetopause in approximate shape and even to the extent of supporting surface waves which were observed on the inbound pass at a distance of 18.3 RU (RU = 25,600 km). There were apparently eight partial transitions from the magnetosheath into the current sheet of the magnetopause at this time, followed by a final complete transition to the magnetosphere. Six of the estimated normal vectors to the local boundary show clear evidence of oscillations in the slope with typical angular excursions, from one partial transition to the next, of ≃90°. The vectors oscillated approximately in a plane that was severely tilted by ≃49° with respect to Uranus’ orbital plane. Analysis of these waves shows that on average the “tailward” wave speed was (76 ± 3.0) km/s, the amplitude was (4.5 ± 3.0) × 10³ km, the wavelength was (34 ± 20) × 10³ km, and the period was (6.9 ± 3.0) min. The wave properties are compared to those at Earth and Saturn, where similar surface waves have been observed.
The large‐scale structure of flare‐associated interplanetary shocks
Authors: Cane H. V.
Published: February 2008

The large‐scale structure of flare‐associated interplanetary shocks is investigated by examining the properties of 116 shocks which originated in solar flare events during a 18.7‐year period commencing mid‐May 1967. The best average representation of these shocks is an expansion which is uniform over about 100°. The highest compression ratio across the shock (∼2.7) is about 15° west of the radial from the flare site. The loose coupling of shocks and their drivers is supported by the observation that drivers are generally only detected for shocks originating near central meridian. A comparison of the numbers of shocks per year with the numbers of sudden commencement geomagnetic storms indicates that the percentage of shocks at 1 AU which originate in flare events is less than 50%. Many shock‐flare associations made in the past are probably in error.
A theory for the radiation at the third to fifth harmonics of the plasma frequency upstream from the Earth's bow shock
Authors: Cairns Iver H.
Published: February 2008

A theory is presented for the radiation at the third to fifth harmonics of the plasma frequency observed upstream from the Earth's bow shock: the radiation is produced by the process L + T′ → T in the foreshock, with the initial T′ radiation being the frequently observed second harmonic radiation (generated by another process) and the L waves being products of the decay L′ → L + S of L′ waves generated by a streaming instability. (Here L, S, and T denote Langmuir, ion acoustic, and ‘transverse electromagnetic waves, respectively.) The theory can account for the observed radiation when unusually large levels (electric fields in excess of 10 mV/m) of suitable L waves are present. Such levels of L waves are possible, in principle, but have not been reported before; the radiation is observed quite infrequently, thereby implying a requirement for unusual foreshock conditions. Predictions for the characteristics of the source regions (one to each wing of the foreshock) and the bandwidth of the radiation are given. Potential problems for the theory, relating to the large levels of L waves required to account for the radiation, are discussed.