bluuebunny/crossref_metadata_2025 huggingface dataset:
Gravity waves in the upper atmosphere of Venus
Authors: Mayr H. G. , Harris I. , Kasprzak W. T. , Dube M. , Varosi F.
Published: February 2008

We present a spectral model in terms of spherical and Fourier harmonics to describe gravity waves in the thermosphere of Venus. This is an extension of a transfer function model for a dissipative multiconstituent atmosphere on Earth which describes quasi three‐dimensional perturbations on the globe, including acoustic‐gravity waves. Owing to the differences in the vertical temperature distributions on Earth and Venus, there are significant differences in the propagation characteristics of gravity waves. For example, the ducted wave mode that is excited in the Earth's thermosphere through reflections from the surface and the temperature minimum at the mesopause is absent on Venus. On Venus, as is the case on Earth, wave reflection from the surface can occur. For both planets, the most important gravity wave mode is propagating quasi horizontally, near the local speed of sound. The model is used to interpret density perturbations in CO2, O and He observed with the orbital neutral mass spectrometer on Pioneer Venus [Niemann, 1977; Kasprzak et al., this issue] which leads to the following conclusions: (1) For localized sources (2 km thick) in the lower atmosphere (at 80 km or below) and thermosphere (at 130 km), of the same magnitude day and night, the computed wave amplitudes at satellite altitudes are about a factor of three to five larger during the night than during the day; in substantial agreement with the observations. The difference is due to the large night to day increase in the thermospheric temperature. (2) Waves excited in the lower atmosphere propagate more obliquely, and geometric attenuation as well as dissipation are less important. Thus extended wave trains are produced that can simulate the data. (3) Waves excited in the thermosphere (at 130 km) are rapidly attenuated. Although such signatures are also observed in the data, they could also be produced by an impulsive perturbation propagating up from the lower atmosphere. (4) For the wavelengths observed, the wave periods are between 30 and 60 min. (5) Due to wind induced diffusion, the computed wave amplitudes for the three species are of comparable magnitude, that of CO2 being largest; and He varies out of phase with the heavier gases; reproducing the observed behavior.
Colliding plasma structures: Current sheet and perpendicular shock
Authors: Burgess D. , Schwartz S. J.
Published: February 2008

We present results of a one‐dimensional simulation of the interaction of a current sheet and a supercritical perpendicular shock in a collisionless plasma. The low‐field region of the current sheet passes through the shock, allowing, for a short time, the complete reflection of the incident flow at the transition of the magnetic field to its downstream value. The counterstreaming ion beams relaunch the shock, while the low‐field region remains downstream. The plasma in the low field region is unshocked but develops a complex multistream configuration, which in time develops to a more uniform distribution. We discuss the implications of the simulation results for observations within the magnetosheath and of active current sheets upstream of the bow shock.
Evolution of diamagnetic cavities in the solar wind
Authors: Thomas V. A. , Brecht Stephen H.
Published: February 2008

Two‐dimensional hybrid (particle ions and fluid electrons) simulations of a finite plasma beam moving through a collisionless plasma parallel to an ambient magnetic field are considered. When the relative drift speed exceeds twice the local Alfven velocity, an electromagnetic ion beam instability is excited which produces an energetic plasma in the interaction region with β larger than unity. This energetic plasma pushes out the magnetic field, creating a diamagnetic cavity and launching outgoing Alfven waves. Application of the results to recent observations of hot diamagnetic cavities in the solar wind is discussed.
Observations of the plasma environment during an active ionospheric ion beam
Authors: Pollock C. J. , Arnoldy R. L. , Erlandson R. E. , Cahill L. J.
Published: February 2008

Data obtained during an active rocket‐borne ion beam injection experiment (ARCS 3), which was carried out in the polar cap ionosphere over Sondre Stromfjord, Greenland, are presented and discussed. The experiment consisted of a main instrumented sounding rocket payload and a separated subpayload from which two Ar+ ion beam generators were operated. These generators produced neutralized ion beams which were aligned generally perpendicular and antiparallel to the ambient geomagnetic field lines, as the two spinning payloads moved apart along (0–1 km) and across (0–0.5 km) field lines. Ion flux data obtained aboard the main payload show certain characteristics which are directly attributable to known beam‐injection geometry parameters, and others which are not so easily understood. Specifically, when the beam is injected transverse to the geomagnetic field, strong ion fluxes near 90° magnetic pitch angle are observed, as expected. The particle energies in these cases were different from those expected, however, with separate ion flux components appearing near 100 and 15 eV/q. The expected beam energy was near 200 eV/q, so that subpayload charging to levels on the order of 100 V is inferred to have taken place during transverse beam injections. During antiparallel injections a component of ∼200 eV/q ions is observed to be moving up the field line, in addition to two other components: one transverse component at ∼200 eV/q and a second transverse component near 15 eV/q. In this case, no spacecraft charging is inferred. The mechanism giving rise to the 15‐eV/q transverse ion components, observed during both transverse and antiparallel injections, is an open question. The existence of ion fluxes at magnetic pitch angles other than those anticipated during both transverse and antiparallel beam injections indicate ion emissions from the subpayload which are much less beamlike than expected based on preflight laboratory measurements and that beam‐plasma interactions tend to isotropize the beam very near the emitting payload. In addition, photometric, electric field, and ion flux data are presented which support the picture of a spatially asymmetric and highly localized region of strong beam‐plasma interactions surrounding the beam‐emitting subpayload.
Multistream hydrodynamic modeling of interhemispheric plasma flow
Authors: Rasmussen C. E. , Schunk R. W.
Published: February 2008

It has been suggested that the refilling of plasmaspheric flux tubes depleted during magnetic storms occurs via a pair of electrostatic shocks which propagate downward from the equatorial region of the magnetosphere toward the conjugate ionospheres. These shocks are believed to be created by the collision of outflowing ion streams during the refilling process. However, there is some question as to whether or not shocks are created in this manner. Analytical studies, as well as numerical simulations, indicate that an initial shock formation during plasmaspheric refilling is unlikely and that a counterstreaming of H+ ions is more likely to occur. This prediction of counterstreaming ion flow during flux tube refilling has been supported by observations. Up to this point in time the modeling of flux tube refilling has been done with the use of single‐fluid hydrodynamic equations which do not take into account the multistream nature of the refilling process. In this study the hydrodynamic equations have been modified to include multiple ion streams, which allows for the possibility of counterstreaming flow. It was found that counterstreaming of H+ ions does occur and also that a pair of shocks are not initially created in the equatorial region. However, shocks do form away from the equator, and the subsequent motion of these shocks creates conditions similar to those predicted by the single‐stream hydrodynamic models. This condition occurs about 4 hours after refilling begins. Thus we support the conclusion, found in earlier studies, that the refilling of the plasmasphere occurs from the equatorial region downward. Also, it was found that the rate of refilling is not substantially altered by counterstreaming flow. However, during the first 4 hours of refilling, the plasma flow characteristics were found to be substantially different from those previously reported.
The relationship between ULF geomagnetic pulsations and ionospheric Doppler oscillations: Derivation of a model
Authors: Poole A. W. V. , Sutcliffe P. R. , Walker A. D. M.
Published: February 2008

Several papers have been written about the observed relationship between ground based magnetometer pulsation measurements and simultaneous oscillations of Doppler frequency shifts in ionospherically reflected radio frequency echoes. In this paper we derive the mechanisms which relate these observations for the vertical incidence case at low to mid latitudes. We investigate the effects of oscillating electric and magnetic fields at ionospheric heights on the phase path of the reflected radio wave, which gives rise to the Doppler shifts. We identify three main mechanisms at work in the ionosphere, which are applicable at all latitudes; however, our numerical computations do not take account of certain high‐latitude effects. By quantifying the electric and magnetic pulsation fields for a partially reflected downcoming Alfvén wave, we derive a quantitative phase and amplitude relationship between the rate of change of phase path on Doppler velocity and the pulsation magnetic field measured on the ground. A surprising result is that the dominant mechanism is not necessarily the vertical component of bulk plasma movement, as has been previously suggested. In many cases, the dominant mechanism is compression and rarefaction of plasma frozen onto the field lines as they oscillate under the action of the field‐aligned component of the pulsation magnetic field.
Large‐scale solar wind fluctuations in the inner heliosphere at low solar activity
Authors: Bavassano B. , Bruno R.
Published: February 2008

In this study we investigate the properties of solar wind fluctuations with time scales from 1 hour to 3 days in the inner heliosphere (0.3 to 1.0 AU) close to the minimum of solar cycle 21. Hourly averages of plasma and magnetic field data from Helios spacecraft have been used. Two different ranges in time scale have been considered separately and it has been found that the features of the fluctuations depend dramatically upon the scale. At scales from 1 to 12 hours, Alfvénic correlation is dominant. Alfvénic fluctuations are observed in all the regions investigated and in any solar wind condition. The highest degree of Alfvénicity is found in fast streams near the Sun. No strong evidence of pressure balance structures, as found by other studies in the outer heliosphere, has been obtained. At scales from 0.5 to 3 days the amount of Alfvénic fluctuations is greatly reduced. A positive correlation between fluctuations in thermal and magnetic pressures is progressively built up inside 0.7‐0.8 AU by the interaction between flows with different speed.
The Venus precursor: An environmental effect on the Pioneer Venus Spacecraft
Authors: Knudsen William C. , Luhmann Janet G. , Russell Christopher T. , Scarf Frederick L.
Published: February 2008

We present suprathermal electron data recorded by the Pioneer Venus (PV) orbiter retarding potential analyzer for two orbits with periapsis in the subsolar region of Venus. In the first of the two orbits, the PV spacecraft passed entirely through the Venus bow shock transition region, both inbound and outbound from periapsis, and in the second it only grazed the upstream border of the transition region. In both orbits, two electron populations are observed, a low‐temperature population interpreted as an electron sheath population trapped in the positive spacecraft potential well and a high‐temperature population interpreted as the ambient solar wind electron population. The density of the sheath (low temperature) electron population is observed to increase and to decrease almost identically in both orbits by a factor of about 5 as the spacecraft approaches and recedes from a location near periapsis. The density evidently depends only on the coordinates of the spacecraft and not on the location of the bow shock. We interpret the increase and decrease in sheath electron density as responsible for the increase and decrease in negative current recorded by the PV Langmuir probe experiment as the PV spacecraft passes through the subsolar region. Brace et al. (1985) have called the region of increased Langmuir probe current the Venus precursor region and have suggested that the cause is mass loading of the solar wind upstream of the bow shock. The observed gradual increase and decrease in density of the solar wind (high temperature) electron population, previously interpreted as evidence for upstream mass loading of the solar wind (Knudsen, 1985), is now interpreted as having resulted from the near‐grazing passage of the spacecraft into or through the bow shock transition region. The increase in density of the cold sheath electrons as the spacecraft approaches periapsis in the subsolar region is interpreted as having been caused by an increase in production rate of secondary electrons from the spacecraft surfaces. The increased production rate is caused by an as yet unestablished process, or processes, operating in the vicinity of Venus. We see in the behavior of the two suprathermal electron populations no evidence for an increase in upstream solar wind plasma density produced by mass loading. An increase in density as small as a few percent would not be detectable.
A theory of self‐consistent two‐dimensional tail equilibria for quasi‐neutral e− H+ O+ mixtures
Authors: Kiessling Michael , Ziegler Harald J.
Published: February 2008

The two‐dimensional equilibrium theory of isothermal current‐carrying three‐species plasmas is evaluated. That theory allows a first step to investigate self‐consistently the influence of realistic oxygen ion admixtures on the equilibrium structure of two‐dimensional models of the Earth's quiet magnetotail. The equations are solved explicitly in the tail approximation. The results are discussed both in view of possible observational implications and in view of the consequences for theoretical studies on the structure and stability of the Earth's magnetotail.
Magnetospheric interchange motions
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2008

We describe flux tube interchange motion in a corotating magnetosphere, adopting a Hamiltonian formulation that yields a very general criterion for instability. We derive expressions for field‐aligned currents that reveal the effect of ionospheric conductivity on interchange motion and we calculate instability growth rates. The absence of net current into the ionosphere demonstrates the bipolar nature of interchange flow patterns. We point out that the convection shielding phenomenon in the terrestrial magnetosphere is a direct consequence of the interchange stability of the system. We conclude the paper with an extended analysis of the nature of interchange motion in the Jovian system. We argue that centrifugally driven interchange drives convection and does not give rise to diffusion of Io torus plasma. Neither a large‐scale convection nor other forms of unstable interchange overturning appear adequate to explain the plasma distributions detected at Jupiter.
Effects of ion two‐stream instability on auroral ion heating
Authors: Roth I. , Hudson M. K. , Bergmann R.
Published: February 2008

The ion two‐stream instability develops between upflowing H+ and O+ in the auroral acceleration region, if the two species are accelerated to different velocities in a semistationary electrostatic potential. Parallel propagating modes are unstable only for small relative drifts, while oblique modes remain unstable at larger drifts and, consequently, higher altitudes. The effects of oblique modes on ion heating are investigated, and it is found that O+ heats primarily by trapping in the direction of wave propagation during the early, coherent phase of wave growth in the temporal evolution problem. After undergoing trapping along the magnetic field direction, H+ heats primarily by quasi‐linear diffusion when the spectrum broadens nonlinearly. The parallel ion distributions evolve by formation of a high‐energy (low‐energy) tail on oxygen (hydrogen) along with bulk slowing of hydrogen, which results in more energetic oxygen than hydrogen. Total heating is greatest in the direction of wave propagation, and this effect is greater for hydrogen than for oxygen.
Motions of the ring current inner edge inferred from plasma measurements
Authors: LaBelle J. , Paschmann G.
Published: February 2008

At the inner edge of the ring current (3‐7 RE) the three dimensional plasma instrument from the AMPTE/IRM satellite sometimes obtains measurements of the pressure gradient and the resulting diamagnetic drift in the 20 eV to 40 keV ions [LaBelle et al., 1988]. To the extent that the boundary may be considered a stationary structure moving past the satellite, a comparison of these measurements, along with simultaneous density and magnetic field measurements, allows the radial (parallel to the pressure gradient) ring current velocity to be inferred. In this work, 11 ring current traversals have been selected from the 1985‐1986 AMPTE/IRM data set corresponding to 1700‐2200 LT. Only relatively quiet times have been considered since large errors may be incurred during active times if the ion composition is not monitored. These cases show a correlation between the radial motion of the ring current inner edge and its radial distance. It is speculated that during quiet times the motion may be purely convective, which implies westward electric fields of the order of 0.2–0.3 mV/m, consistent with previous measurements in this region.
The density of cometary protons upstream of comet Halley's bow shock
Authors: Neugebauer M. , Goldstein B. E. , Balsiger H. , Neubauer F. M. , Schwenn R. , Shelley E. G.
Published: February 2008

Cometary protons picked up by the solar wind were detected by the high energy range spectrometer of the Giotto ion mass spectrometer starting at a cometocentric distance of ∼12×106 km. On the average, the density of cometary protons varied approximately as the inverse square of the cometocentric distance, reaching a value of 0.11 cm−3 just outside the bow shock. The data can be successfully fit to models that include substantial amounts of both slow (∼1 km/s) and fast (≥8 km/s) H atoms beyond the bow shock. Large local variations in the density of picked‐up protons can be explained on the basis of variations in the direction of the interplanetary magnetic field in upstream regions where pitch angle scattering was weak.
Equatorial ionospheric irregularities produced by the Brazilian Ionospheric Modification Experiment (BIME)
Authors: Klobuchar J. A. , Abdu M. A.
Published: February 2008

On two separate evenings in September 1982, rockets were launched into the bottomside equatorial F2 region off the coast of Natal, Brazil, to inject chemicals, consisting of mainly H2O and CO2, to create a hole in ionization. The chemicals were injected near the height where the density gradient was steepest, and at a time when the F2 region was rising rapidly, to see whether plasma bubble irregularities could be generated from instabilities triggered by the ionization hole. On both occasions, hole‐induced depletions in total electron content (TEC) of more than 1016 el/m² were observed over horizontal distances of at least 60 km from the chemical injection point. The eastward drifts of these artificial depletions were observed by the time difference in the TEC features observed at various TEC monitoring stations, and from the changing range of oblique ionosonde echoes observed by an ionosonde located 300 km magnetically east of the chemical release point. Their subsequent evolution into plasma bubble irregularities was demonstrated from the observations of spread F echoes, strong‐amplitude scintillation, and TEC depletion at distances of from 300 to 500 km eastward of the release points. The fact that similar behavior of the ionosphere was observed during the evenings of both rocket chemical releases, and on no other nights of the campaign, is strong evidence of successful artificial generation of bubble irregularities by chemical injection into the bottomside F2 region.
Plasma waves in the magnetotail of Uranus
Authors: Kurth W. S. , Gurnett D. A. , Scarf F. L. , Mauk B. H.
Published: February 2008

As the Voyager 2 spacecraft left Uranus during its encounter with the planet on January 24, 1986, it passed through the magnetotail over the distance range of about 11 RU to 79 RU. During this interval the plasma wave receiver on board measured only very weak plasma wave intensities. The most remarkable of these waves are found predominantly inside of about 23 RU with an additional event occurring between about 48 and 56 RU. The frequency of these magnetotail plasma waves is generally below about 100 Hz, extending down to the 10‐Hz lower frequency limit of the receiver. In each case the upper frequency limit of the emissions lies below the local electron cyclotron frequency. The plasma waves are found prior to the first neutral sheet crossing and between the second and third neutral sheet crossings, as defined by the Voyager magnetometer investigation. This timing suggests the waves are found only in the negative solar magnetospheric lobe of the tail, corresponding to field lines which intersect the planet near the southern magnetic pole—that which is continuously in the nightside hemisphere during the encounter epoch. The observations are not conclusive as to the mode of propagation of the waves. However, the spectrum is similar in some respects to broadband electrostatic waves observed in similar regions of Earth's magnetotail.
On the source of diffuse, suprathermal ions observed in the vicinity of the Earth's bow shock
Authors: Gosling J. T. , Thomsen M. F. , Bame S. J. , Russell C. T.
Published: February 2008

Using the fast plasma experiment (FPE) data from ISEE 2, we have searched for “diffuse,” suprathermal ion events that occurred when the interplanetary magnetic field was nearly radial and when the spacecraft was upstream from the Earth's bow shock at local times greater than 1530. For such conditions the field lines sampled should not approach closer than ∼4 RE to the Earth's magnetopause according to present models of the flow of the solar wind around the magnetosphere. A number of diffuse ion events satisfying these selection criteria have been found, two of which are described in detail in this paper. At energies below 40 keV (the highest energy sampled by the FPE) these events are representative of the vast majority of diffuse ion events detected by the FPE independent of field orientation and local time. In particular, the events are observed in association with large‐amplitude, low‐frequency magnetic field fluctuations; the suprathermal particle velocity distributions are roughly isotropic in the solar wind frame; and the spectra appear to extend smoothly to energies beyond 40 keV. Owing to the special field geometry selected for these events, it is highly unlikely that the events studied could be the result of leakage of suprathermal ions from the magnetosphere. Rather, the suprathermal ions are almost certainly accelerated out of the solar wind by processes operating in the vicinity of the bow shock. The fact that these specially selected events have characteristics that are typical of diffuse ion events in general as observed by the FPE suggests that shock processes are the prime cause of diffuse ion events observed in the vicinity of the bow shock at energies below 40 keV.
Suprathermal ions observed upstream of the Venus bow shock
Authors: Moore K. R. , McComas D. J. , Russell C. T. , Mihalov J. D.
Published: February 2008

Suprathermal ions with arrival directions quite distinct from those of the solar wind have been detected upstream of the Venus bow shock. We examine the possibility that these events could be caused by instrumental or spacecraft effects or that they could be either solar wind disturbances, planetary pickup ions, or Suprathermal ions upstream of the Venus bow shock and conclude that they are consistent with upstream suprathermal ions associated with the bow shock that are observed down‐stream from the points of intersection of their extrapolated trajectories with the shock.
Undriven magnetic reconnection in an isolated current sheet
Authors: Scholer Manfred
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. A spatially limited resistivity is applied in the center of the current sheet. The resistivity falls off along the current sheet with some e‐folding distance xl. For sufficiently small xl, quasi‐steady reconnection with a small diffusion region develops. The reconnection exhibits all Petscheklike features, such as two pairs of slow mode shocks attached to the diffusion region, a strongly converging flow toward the field line reversal region, and a weak fast mode expansion of the inflowing plasma. At larger xl values, long current sheets develop; i.e., the length of the diffusion region is always such as to occupy the largest permitted space. The flow in the long current sheet cases is also of the fast mode expansion type. However, the flow turns in these cases upstream of the separatrices into a wedge‐shaped reconnection layer, whereas in the case with a small diffusion region the separatrices are well outside the reconnection layer. The reconnection rate measured in terms of the inflow Alfvén Mach number well upstream of the diffusion region decreases with increasing current sheet length and reaches the Sweet‐Parker rate for a current sheet corresponding to the system size. Starting the simulation with a small xl and increasing xl as a function of time to the total system size inevitably also leads to long extended current sheets with a correspondingly small reconnection rate. It is thus impossible to retain a diffusion region with a comparable width and length in a large‐sized system with spatially extended finite resistivity. By artificially limiting the length of the diffusion region, the maximum reconnection rate has been determined for several simulation runs with different magnetic Reynolds numbers Rm. Over a limited Rm range the numerically obtained rate is consistent with the classical Petschek rate.
A Fluid Finite Ion Larmor Radius Model of the magnetopause layer
Authors: Stasiewicz K.
Published: February 2008

A model of the magnetopause current layer is constructed on the basis of fluid equations for collision‐free plasmas with finite ion Larmor radius (FLR). The model provides self‐consistent solutions for the plasma flow vector, magnetic field, and electric currents inside the magnetopause layer. This is the first fluid model that offers explanations for some observations at the terrestrial magnetopause that are inexplicable by earlier models. In particular, it is shown that the erosion of the dayside magnetosphere can be explained by the normal component of the gyroviscous stress tensor that is related to the intensity of field‐aligned currents inside the magnetopause layer. It is found that the sense of rotation of the magnetic field across the magnetopause is determined by the ratio of the normal component of the Alfvén velocity to the normal flow velocity ξ = Bn/(µoρ)½Vn. For |ξ|>1 the sense of rotation corresponds to electron polarization, and |ξ|<1 yields proton polarization. It is argued that the case |ξ| = 1 corresponds to the formation of transient flux transfer events. The observed departures from MHD jump conditions across the magnetopause are explained by additional, finite Larmor radius terms in the moment equations. An expression is also derived for the characteristic thickness of the magnetopause layer.
A surface chemistry model for the altitude dependence of the N2 Lyman‐Birge‐Hopfield glow on spacecraft
Authors: Cuthbertson J. W. , Langer W. D.
Published: February 2008

Low‐orbiting spacecraft have been observed to cause UV emission in the N2 Lyman‐Birge‐Hopfield bands. We examine the chemical processes expected to underlie this emission. Recombination between incident and adsorbed N atoms has been proposed as the source of radiating N2*; however, the cubic dependence of the intensity, as [N2]³ or [N2]²[O], has not been explained. We suggest that this can be explained by a model where adsorption of nitrogen on the spacecraft surface is balanced mainly by the removal of N from the surface by atomic oxygen. On the basis of these assumptions we construct a mathematical model for the production of N2*. We show that for large enough reaction efficiencies our model predicts a magnitude and altitude dependence for the emission which can explain the measurements reported by other investigators.
Characteristics of short‐duration electron precipitation bursts and their relationship with VLF wave activity
Authors: Imhof W. L. , Voss H. D. , Mobilia J. , Walt M. , Inan U. S. , Carpenter D. L.
Published: February 2008

Energetic (>6 keV) electron data from the SEEP payload on the low altitude (∼200 km) polar orbiting S81‐1 satellite indicate a high rate of occurrence of short duration (<0.6 s) electron precipitation bursts. Characteristics of events observed at night (2230 MLT) versus daytime (1030 MLT) and at midlatitudes (2<L<3) versus higher latitudes (L>3) were distinctly different in several ways. For 2<L<3 the daytime bursts occurred approximately uniformly in longitude and were equally distributed between the northern and southern hemispheres. The nighttime bursts in the same L shell range occurred approximately twice as often on a worldwide basis and were observed predominantly in the northern hemisphere and at longitudes of 260°E to 320°E. In a significant number of the nighttime events at 2<L<3 the median electron energy increased with time during the burst, but most of the other spectra showed no well‐defined trend. During some of the nighttime bursts broad peaks were observed in the energy spectra, but these peaks were not so evident in the daytime bursts. On higher L shells, L>3, narrow electron precipitation bursts (<0.3 s duration) were frequently observed poleward of the plasmapause, more often near noon than near midnight and much more frequently than at 2<L<3. Comparison of the electron data with simultaneous VLF wave data from Palmer (L≃2.4) and Siple (L≃4.3) stations in Antarctica indicated a varying degree of association of electron bursts with whistlers and chorus emissions. Several of the electron bursts observed at nighttime and at 2<L<3 were correlated with lightning‐generated whistlers observed at Palmer Station. When daytime bursts at higher latitudes (L>3) were observed on satellite passes within ±50° of the Siple meridian, chorus was invariably detected at Siple, but correlation of electron bursts with individual chorus spectral elements was not evident. The lack of such correlation may be due to the limited spatial extent of flux tubes excited by individual chorus elements which are possibly generated without mutual coherence in multiple high altitude magnetospheric locations. Within one hour of all four of the satellite passes within 400 km of Siple with the highest rate (≥10 individual bursts per pass) of burst occurrence, overhead electron precipitation was clearly detected by ground based sensors at Siple Station. Quasi‐periodic bursts of several seconds duration were observed on one or more of the photometer, riometer, and magnetic pulsation sensors and many of these bursts were correlated with clustered VLF chorus bursts. Hence, there is at least association of chorus with electron precipitation.
Basic properties of solar wind MHD turbulence near 0.3 AU analyzed by means of Elsässer variables
Authors: Tu C.‐Y. , Marsch E. , Thieme K. M.
Published: February 2008

Magnetic field and plasma data, obtained by the Helios 1 and 2 spacecraft in the solar wind near 0.3 AU during the years 1975 to 1976, have been analyzed by calculating 12 kinds of spectra related to the Elsässer variables, δZ+ = δV + δVA, and δZ− = δV − δVA, where δV and δVA are the bulk velocity and Alfvén velocity fluctuations, respectively. For small amplitude Alfvén waves the fluctuation variable δZ+ simply relates to outward propagation and δZ− to an inward sense of propagation, if the ambient magnetic field B0 is directed inward. The frequency range analysed in this paper is 6×10−6 Hz to 6×10−3 Hz. It is found that (1) the autocorrelation length for δZ− is much larger than for δZ+ in both the high‐speed and low‐speed wind. (2) The power spectra of δZ−, especially in high‐wind speed, are steeper in the low‐frequency range and flatten in the high‐frequency range. (3) In the low‐frequency range, the power spectra for the components of δZ+ tend to be isotropic with respect to the three polarization directions, while the spectra of δZ− are dominated by the radial component. In the high‐frequency domain, the spectra of both δZ+ and δZ− are dominated by the transverse component in high‐speed wind and are more isotropic in low‐speed wind. (4) The spectra related to the residual energy or the cross‐correlation in low‐speed flows have a power law with the slope near to −5/3. However, in high‐speed flows the corresponding data are widely distributed in a cloud of points with an upper envelope near to the spectrum of δZ−. The origin of all these spectra and their importance for the solar wind physics have also been discussed. Several generation mechanisms are suggested as candidates. In the flat part of e− spectrum, the fluctuations may be generated by non‐local (in wave number space) interactions with the low‐frequency part of the e+ spectrum, or just by parametric decay of the high‐frequency part of the e+ spectrum. The steep part of e− (f) may be related to small‐scale stream tubes, or be influenced by pressure waves, nonlinear cascading, and the interaction with the outgoing Alfvén waves.
The 206‐year cycle in tree ring radiocarbon data and in the thermoluminescence profile of a recent sea sediment
Authors: Castagnoli G. Cini , Bonino G. , Provenzale A.
Published: February 2008

We study the secular variations observed (1) in the thermoluminescence (TL) profile of a recent sea sedimentary core, (2) in the Δ14C time series from tree rings, and (3) in the mean annual sunspot number Rz. We show that a regular oscillation with a period of approximately 206 years is present in the radiocarbon signal. The comparison between the Δ14C secular oscillation and the 206‐year TL modulation produced by the beats of two TL waves with close‐by decennial periods is considered. The role of the 206‐year radiocarbon wave and its possible relationships with the solar activity (as represented by the annual sunspot number Rz) are finally discussed.
ULF waves at comets Halley and Giacobini‐Zinner: Comparison with simulations
Authors: Le G. , Russell C. T. , Gary S. P. , Smith E. J. , Riedler W. , Schwingenschuh K.
Published: February 2008

This paper compares observations and numerical simulations of magnetic fluctuations near the proton and water group ion cyclotron frequencies as a function of distance from the comets Giacobini‐Zinner and Halley. Because the Doppler shift of the ion cyclotron resonance is equal and opposite to that associated with the flow of the solar wind plasma past the observer, one can monitor the amplitude of waves due to different cyclotron resonant instabilities by examining the amplitude of waves near the gyrofrequency of the respective ions as measured in the spacecraft frame. This expectation has been checked by comparison of the data to results of a one‐dimensional electromagnetic hybrid simulation of two‐ion pickup. Distant from both comets the wave amplitudes are very weak. But as the comet is approached, the wave amplitude grows. These observations are consistent with computer simulations which predict amplitudes that depend on the properties of the injected beams and the local injection rate.
Rotation driven plasma transport: The coupling of macroscopic motion and microdiffusion
Authors: Pontius D. H. , Hill T. W.
Published: February 2008

The model of Pontius et al. (1986) for plasma transport in a corotation‐dominated magnetosphere is modified to satisfy observational constraints introduced by Richardson and McNutt (1987). As in the earlier model, small discrete flux tubes whose plasma content differs significantly from the longitudinally averaged value (transient flux tubes) move steadily under the influence of the centrifugal force. We consider the electric fields surrounding a transient flux tube of elliptical cross section and derive expressions for the translational velocity and cross‐sectional distortion of such flux tubes. To account for the observed radial decrease of flux shell content at Jupiter, we assume that plasma can pass between transient flux tubes and the background by means of single‐particle microdiffusion. The background plasma distribution is affected by the passage of a transient in two ways: direct mass loading through microdiffusion, and radial displacement to conserve magnetic flux. Upon adopting a simple form to describe the microdiffusion process, we find that there exists a particular slope of flux shell plasma content for which these effects cancel and the background maintains a steady state distribution.
Large velocity shears and associated electrostatic waves and turbulence in the auroral F region
Authors: Earle G. D. , Kelley M. C. , Ganguli G.
Published: February 2008

Broadband electrostatic waves in a frequency range from about 10 Hz to 1 kHz have been observed in conjunction with very large shears in the plasma flow velocity transverse to the ambient magnetic field in the auroral F region. The shears were detected through their perpendicular electric field signatures, which changed by as much as 200 mV/m over distances of only a few hundred meters. Power spectra of the large‐scale electric field and density fluctuations associated with the shears are in excellent agreement with previously reported DE 2 satellite observations. At smaller spatial scales, the observed low‐frequency waves were very well correlated with the shears, and had amplitudes of about 1–5 mV/m for the electric field variations and a few percent for the correlated density fluctuations. Transverse shears can be uniquely related to field‐aligned currents through the current continuity equation, and the resulting field‐aligned drift exceeds the threshold for excitation of current‐driven electrostatic ion cyclotron waves. A numerical simulation of this instability has been used to generate electric field spectra in the rocket frame of reference, and these spectra are similar to the spectra generated from the actual rocket data. Both shear and field‐aligned current are discussed as sources of the waves.
Characteristics of the sporadic sodium layers observed at 23°S
Authors: Batista P. P. , Clemesha B. R. , Batista I. S. , Simonich D. M.
Published: February 2008

The mesospheric sodium data, obtained between 1975 and 1987 at São José dos Campos (23°S, 46°W) with a laser radar, have been analyzed in order to identify the appearance of thin sporadic sodium layers. In this search, a total of 65 events were identified. The average height of the peaks is 95.0 km. The ratio of the maximum peak density to the average layer density is normally 2.5 to 3.0, but values as high as 7 have been observed in the most outstanding cases. The events last from a few minutes to several hours, although durations of 1–2 hours are more typical. The events occur more often during periods of large meteor showers, especially in August. The diurnal variation shows an increasing number of observed peaks from 1500 LT to midnight and remains almost constant from midnight to 0600 LT. In 52 out of 54 days for which sodium and ionosonde data are available there was a sporadic E layer nearly coincident with the sodium cloud. The coincidence is good for short‐lived sporadic layers, but a substantial increase in sporadic E critical and blanketing frequencies normally precedes the long‐lasting and broader ones. These results are compatible with the suggestion that the enhanced layers are produced by the wind shear distortion of sodium clouds originating in meteor deposition, but we cannot rule out the possibility of an ion conversion mechanism.
Observations of large‐amplitude, whistler mode wave ducts in the outer plasmasphere
Authors: Koons H. C.
Published: February 2008

Strong enhancements of whistler mode hiss emissions have been observed to correlate with plasma density enhancements in the outer plasmasphere between L = 4 and L = 6. This indicates that these density enhancements are acting as whistler mode wave ducts. The wave and density observations were made simultaneously by the Aerospace swept frequency receiver aboard the AMPTE IRM spacecraft. The plasma density is determined from a narrow‐band line near the plasma frequency. The hiss emissions generally occur below 2 kHz. The ducts consist of density enhancements of more than 40%. Density gradients on the sides of the ducts range from 0.10 to 0.24 el cm−3 km−1. The half width of each duct is typically 250 km. The wave intensity maximizes about 15 km from the center of the duct toward the outside, i.e., the side with a negative density gradient with increasing distance. The wave intensity near the center of a duct is an order of magnitude higher than the wave intensity at the density minimum between ducts.
The energetic ion substorm injection boundary
Authors: Lopez R. E. , Sibeck D. G. , McEntire R. W. , Krimigis S. M.
Published: February 2008

The substorm injection boundary model has enjoyed considerable success in explaining plasma signatures in the near‐geosynchronous region. However, the injection boundary has remained primarily a phenomenological model. In this paper we examine 167 dispersionless energetic ion injections which were observed by AMPTE CCE. The radial and local time distribution of the events as a function of Kp is qualitatively similar to that envisioned in the injection boundary model of Mauk and McIlwain (1974). We will argue that particles observed during dispersionless injections are locally energized during the disruption of the cross‐tail current sheet. Therefore we identify the injection boundary, as derived from the spatial distribution of dispersionless injections, with the earthward edge of the region of the magnetotail which undergoes current sheet disruption during the substorm expansion phase. We will show that this qualitative model for the generation of the injection boundary can provide an explanation for the dispersionless nature, the double spiral shape, and the Kp dependence of the boundary.
Representations of currents and magnetic fields in isotropic magnetohydrostatic plasma
Authors: Heinemann M. , Pontius D. H.
Published: February 2008

We develop the theory describing electric currents in magnetized plasma for the case of static equilibrium with isotropic pressure. The current density is determined by pressure balance and charge conservation. We show that the current density can be expressed as the sum of the current density determined by plasma pressure gradients, Jp = c▽F × ▽P, and the force‐free current density, Jf = gB, where F = ∫ ds/B is the flux tube volume per unit flux measured from a surface at which the normal component of Jp vanishes, P is the scalar pressure, and g is a field line constant. The expression explicitly satisfies charge conservation, and its perpendicular component is the usual perpendicular current density. In the absence of force‐free currents, J is a curl; in this case it is possible to introduce a magnetic scalar potential, ψ, that satisfies the equation ▽²ψ = −4π▽ · P▽F; the magnetic field can then be determined from B = −▽ψ −4πP▽F.
The symmetry properties of planetary magnetic fields
Authors: Rädler Karl‐Heinz , Ness Norman F.
Published: February 2008

This paper provides a comparative study of the geometrical structures of the magnetic fields of Earth, Jupiter, Saturn, and Uranus, starting from the traditional multipolar representations of these fields. For Earth, Jupiter, and Saturn the centered dipole, quadrupole, and octupole contributions are included, while at Uranus, only the dipole and quadrupole contributions are considered. The magnetic fields are analyzed by decomposing them into those parts which have simple symmetry properties with respect to the rotation axis and the equatorial plane. It is found that there are a number of common features of the magnetic fields of Earth and Jupiter. Compared to Earth and Jupiter, the Saturnian field exhibits not only a high degree of symmetry about the rotation axis, by now rather well known, but also a high degree of antisymmetry about the equatorial plane. The Uranian field shows strong deviations from both such symmetries. Nevertheless, there remain features common to all four planets. The implications of these results for dynamo models are discussed. With a view to Cowling's theorem the symmetry of the fields is investigated with respect to not only the rotation axis but also to other axes intersecting the planetary center. Surprisingly, the high degree of asymmetry of the Uranian field that is observed with respect to the rotation axis reduces considerably to being comparable to that for Earth or Jupiter when the appropriate axis is employed.
Lower hybrid drift instability at the inner edge of the ring current
Authors: Roth I. , Cohen B. I. , Hudson M. K.
Published: February 2008

Excitation of electrostatic waves around and above the lower hybrid frequency at the inner edge of the ring current is investigated. The model consists of cold plasmaspheric ions; hot, dilute, inhomogeneous ring current ions; and cold charge neutralizing electrons. It is shown that in the presence of a sufficiently large density gradient the coupling of the lower hybrid with the drift cyclotron harmonic mode destabilizes the plasma. Unstable interactions also occur at higher cyclotron harmonics when the lower hybrid is replaced by an ion Bernstein mode. Additional power at frequencies above the lower hybrid frequency is attributed to the thermal enhancement by the hot ring current ions, which increases with their density gradient. Particle simulations are conducted, and comparison to AMPTE data is discussed.