bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal coupling of conjugate ionospheres and the tilt of the Earth's magnetic field
Authors: Richards P. G. , Torr D. G.
Published: February 2008

It is known, from predawn observations conducted over 20 years ago, that there is strong thermal coupling between conjugate ionospheres at mid‐latitudes. Nevertheless, most previous theoretical studies of interhemispheric coupling have concentrated on mass flux coupling, which has been universally found to be weak. Little attention has been paid to interhemispheric thermal coupling and the effects of the displacement of the magnetic and geographic axes. While these previous studies provided useful insights into the process of plasma exchange between conjugate ionospheres, several interesting phenomena have been overlooked. In this paper we examine the thermal coupling between conjugate ionospheres and confirm that it is very strong. Solar illumination in one hemisphere is accompanied immediately by heating in the conjugate hemisphere, and because the electron density is usually small in the dark hemisphere, the heating from the conjugate hemisphere can produce electron temperatures which are 1000°K higher in the dark than in the sunlit hemisphere. The high temperatures have little effect on NmF2 but cause a sharp rise in the O+ density in the topside ionosphere and the consequent rapid expulsion of H+ into the plasmasphere. The morning, upward H+ fluxes in the dark ionosphere that are produced as a result of conjugate heating are as large as the daytime fluxes. The tilt of the earth's magnetic field means that the conjugate illumination effects can actually be observed in the dark hemisphere 2 hours in local time before they appear in the sunlit hemisphere, and 5 hours before local illumination. In addition to the above effects that are caused by the longitudinal displacement of the conjugate points, there is a strong seasonal asymmetry in the overall behavior of the H+ fluxes at the longitude studied due in part to the differing day lengths at the conjugate points, which differ by 15° in latitude.
Star of Condor: A Strontium Critical Velocity Experiment, Peru, 1983
Authors: Wescott E. M. , Stenbaek‐Nielsen H. C. , Tallinan T. , Föppl H. , Valenzuela A.
Published: February 2008

Star of Condor was a critical velocity experiment using strontium vapor produced in a radial shaped charge. The radial shaped charge was carried to 571.11 km altitude on a Taurus‐Tomahawk rocket launched from Punto Lobos, Peru, and detonated in the plane of the magnetic field lines so that all ranges of pitch angles from parallel to B to perpendicular to B were covered. Strontium has a critical velocity of 3.3 km/s, and from observation, 42.5% of the neutral Sr gas had a velocity component perpendicular to B exceeding that value. No Sr ion emissions were detected shortly after the burst with usual TV integration times. However, about 10 min after the detonation a faint field‐aligned streak was discovered with long TV integration times. We estimate the brightness as 5 R, which, combined with the streak geometry, implies an ion production of 2.4×1019 ions. This is only 0.0036% ionization of the Sr vapor. All the ions could easily have been produced by thermal ionization from the original detonation thermal distribution. The breakup of the Sr gas into small bloblike structures may have allowed the high‐energy electrons to escape before an ionization cascade could be produced. For whatever reason, the Alfvén mechanism proposed for space plasmas in the absence of laboratory walls did not produce an ionization cascade in the experiment.
An MHD simulation of the effects of the interplanetary magnetic field By component on the interaction of the solar wind with the Earth's magnetosphere during southward interplanetary magnetic field
Authors: Ogino T. , Walker R. J. , Ashour‐Abdalla M. , Dawson J. M.
Published: February 2008

The interaction between the solar wind and the earth's magnetosphere has been studied by using a time‐dependent three‐dimensional magnetohydrodynamic model in which the interplanetary magnetic field (IMF) pointed in several directions between dawnward and southward. When the IMF is dawnward, the dayside cusp and the tail lobes shift toward the morningside in the northern magnetosphere. The plasma sheet rotates toward the north on the dawnside of the tail and toward the south on the duskside. For an increasing southward IMF component, the plasma sheet becomes thinner and subsequently wavy because of patchy or localized tail reconnection. At the same time the tail field‐aligned currents have a filamentary layered structure. When projected onto the northern polar cap, the filamentary field‐aligned currents are located in the same area as the region 1 currents with a pattern similar to that associated with auroral surges. Magnetic reconnection also occurs on the dayside magnetopause for southward IMF. The steady dayside reconnection mainly occurs in the antiparallel merging region nearest the subsolar point and drives strong convection near the magnetopause as reconnected field lines flow tailward. This causes an enhancement of the polar cap and region 1 field‐aligned currents. The polar cusp field‐aligned currents evolve from the polar cap (NBz) field‐aligned currents as the IMF is rotated from northward to southward.
Signatures in the dayside aurora of plasma transfer from the magnetosheath
Authors: Sandholt P. E. , Deehr C. S. , Egeland A. , Lybekk B. , Viereck R. , Romick G. J.
Published: February 2008

Continuous ground‐based observations of the dayside aurora provide important information, complementary to the in situ measurements from satellites, on plasma transport and electromagnetic coupling between the magnetosheath and the magnetosphere. In this study, observations of the polar cusp/dayside oval aurora from Svalbard and simultaneous observations of the nightside aurora from Poker Flat, Alaska, and the interplanetary magnetic field from satellites are used to identify the ionospheric signatures of plasma transfer from the solar wind to the magnetosphere. The characteristics of motion, spatial scale, time of duration, and repetition frequency of certain day side auroral forms which occur at the time of large‐scale oval expansions (interplanetary magnetic field Bz < 0) are observed to be consistent with the expected optical signatures of plasma transfer through the dayside magnetopause boundary layer, associated with flux transfer events. Similarly, more large‐scale (time and space) events are tentatively explained by the quasi steady state reconnection process.
AE‐D measurements of the NO geomagnetic latitudinal distribution and contamination by N+(5S) emission
Authors: Gérard J. C. , Noël C. E.
Published: February 2008

Measurements of the NO γ (1, 0) band resonance scattering distribution obtained with the Atmosphere Explorer D satellite have been inverted to generate maps of the meridional distribution of nitric oxide between 90 and 160 km. It is shown that the results are better organized in terms of geomagnetic rather than geographic latitudes, especially at high latitudes. In particular, the presence of an E region maximum associated with the summer auroral zone is clearly demonstrated. The existence below the NO peak of larger concentrations in the winter than in the summer hemisphere is also observed. Comparison between calculations and rocket observations of the 2143‐Å doublet emission indicates that the quantum yield of N+(5S) is considerably less than unity. The possible contamination of the observations by this emission is shown to be negligible in these observations.
Anomalous diffusion across the magnetic field‐plasma boundary: The Porcupine Artificial Plasma Jet
Authors: Mishin E. V. , Treumann R. A. , Kapitanov V. Ya.
Published: February 2008

Observation of nearly undisturbed penetration of the Porcupine dense (nb, ≫ nion) and fast (υb = 1.7 · 104ms−1) heavy (xenon, mi = 131 mp) ion beam into the magnetized ionospheric plasma after termination of the short (τm < 1 ms) adiabatic phase requires very fast magnetic field diffusion into the beam. Collisional diffusion and instability of a sharp boundary between the magnetic field and plasma yield diffusion times too long to account for the observations. The diffusion is therefore attributed to a transverse electron drift current‐driven electrostatic instability excited by the diamagnetic current flowing in the boundary layer between the injected beam and the ambient field. In the low‐drift regime applicable to the final stage of diffusion in the outer part of the layer, electron acoustic waves are excited. In the initial strong current regime of the inner part of the layer the modified two‐stream instability becomes unstable. The anomalous collision frequencies produced in both cases by entirely different saturation mechanisms turn out to be of the order of the local lower‐hybrid (LH) frequency in the dense xenon plasma and are high enough to yield the required fast magnetic field diffusion rate. Energetically, only a very small fraction of beam energy is dissipated in the diffusion process so that no remarkable deceleration of the ion beam is observed, thus giving a satisfactory explanation of the negligible beam energy losses.
Field‐aligned plasma flow in MHD simulations of magnetotail reconnection and the formation of boundary layers
Authors: Birn J. , Hones E. W. , Schindler K.
Published: February 2008

We have used our two‐dimensional resistive MHD code to study details of the plasma flow in two classes of magnetotail reconnection models: (1) near‐earth reconnection as initiated by the sudden occurrence of anomalous dissipation, which leads to plasmoid formation and ejection consistent with phenomenological magnetospheric substorm models, and (2) distant reconnection which is forced to occur near a given location by a nonuniform inflow of plasma, energy, and magnetic flux through the high‐latitude boundaries modeling the possible effects of plasma entry through the plasma mantle, which expands until it reaches the plasma sheet. Near‐earth reconnection produces predominantly tailward flow, which occupies a major part of the plasma sheet cross section in z. Except for narrow regions close to the neutral sheet, this flow is largely field aligned. Due to the fact that the onset of this fast flow closely coincides with plasma sheet thinning, the observation at a fixed satellite location off the central neutral sheet looks like the crossing of a flow boundary layer when the satellite exits from the plasma sheet into the lobe. The distant reconnection process showed several features consistent with observations in the near and distant tail: the formation of a distant neutral sheet, rather than a single neutral line with the possibility of the occurrence of multiple neutral lines and quasi‐stagnant magnetic islands, and the splitting of the earthward flow into two boundary layers, or “separatrix layers,” inside the magnetic separatrix that limits the region of closed magnetic flux tubes.
Dual‐spacecraft measurements of plasmasphere‐ionosphere coupling
Authors: Horwitz J. L. , Brace L. H. , Comfort R. H. , Chappell C. R.
Published: February 2008

Nearly simultaneous and conjugate measurements of the thermal plasma density and temperature structure in the early evening ionosphere and plasmasphere are obtained with the DE 1 and 2 satellites. The results indicate a number of significant relationships in plasmasphere‐ionosphere thermal plasma interchange and coupling. The principal results are as follows: (1) A transition in the ionospheric electron temperature (Te) from a relatively smooth profile of low Te (<2000°K) at the base of the inner plasmasphere to enhanced and highly structured Te at higher invariant latitudes occurs near or along a plasmaspheric density gradient. (2) Plasmaspheric enhancements of the heavy ions O+ and O++ are often closely aligned with distinct ionospheric Te enhancements. (3) When the plasmaspheric light ion density profile contains a significant trough, there is a pronounced ionospheric Te peak aligned with the poleward plasmaspheric density enhancement or the poleward edge of the plasmaspheric trough. (4) The latitudinal structures of ionospheric Te and plasmaspheric TH+ are sometimes strikingly well‐correlated. (5) The plasmaspheric density ratio He+/H+ is generally of the order 0.2 and appears to be insensitive to such parameters as the invariant latitude or the plasmaspheric and ionospheric densities and temperatures. (6) Equatorward of the plasmapause region/outer plasmasphere there is no consistent relation between ionospheric and plasmaspheric density structure, with the ionospheric electron densities frequently displaying smooth profile regions across field lines where highly complex plasmaspheric density structure is seen. (7) The evolution of the plasmasphere and F region/topside ionosphere during the recovery phase of a magnetospheric storm illustrates how the time scales for replenishment of the ionosphere are significantly shorter than those of the plasmasphere. Certain of these observations are interpreted qualitatively in terms of extensions of current ideas on plasmasphere‐ionosphere coupling, in particular, ring current/plasmasphere/stable auroral red arc interactions and plasmasphere refilling. Other features are puzzles that await satisfactory theoretical explanation.
An analysis of the high‐latitude thermospheric wind pattern calculated by a thermospheric general circulation model: 2. Neutral parcel transport
Authors: Killeen T. L. , Roble R. G.
Published: February 2008

A diagnostic package developed to analyze runs made by the National Center for Atmospheric Research thermospheric general circulation model (TGCM) is used to calculate neutral parcel trajectories in the high‐latitude E and F regions. The calculations are made considering steady, time‐invariant magnetospheric convection and auroral forcings for moderate levels of geomagnetic activity (Kp ∼3 or 4 and cross‐polar cap potential of 60 kV). Individual parcels of neutral gas are tracked by interpolation through the TGCM grid in time and space. Trajectories are calculated to illustrate the predicted loci of thermospheric neutral gas parcels that encounter high‐latitude ion drag forces. The forcing histories for such parcels are determined by decomposing terms in the hydrodynamic and thermodynamic equations used in the model calculations. Trajectories, analyzed for “steady state,” diurnally reproducible runs of the TGCM, have shown that (1) the magnetospheric convection throat region plays an important regulatory role for the overall flow pattern at high latitudes, (2) the trajectories of typical F region neutral gas parcels pass through the throat region at least once and sometimes twice in a 24‐hour period as a result of the convergent geometry of the ion convection pattern, (3) parcels transiting the dayside cusp region are heated rapidly in the region of soft particle precipitation and this energy is then advected over the polar cap and deposited several hours “downstream” along the parcel trajectory heating the polar cap, (4) the “postmidnight” surge in the meridional neutral wind is a result of parcel acceleration in the convection throat region and subsequent deflection towards the corotation direction, and (5) E region parcels move less rapidly than their F region counterparts and can dwell for appreciable periods of time in the auroral oval. Few geometrically selected parcels escape to lower latitudes.
Phase scintillations due to equatorial F region irregularities with two‐component power law spectrum
Authors: Bhattacharyya A. , Rastogi R. G.
Published: February 2008

Power spectra of weak phase scintillations on a 140‐MHz signal, transmitted from the geostationary satellite ATS 6 and observed during premidnight and postmidnight periods at an equatorial station Ootacamund (magnetic dip 6°N), show that the nighttime equatorial F region irregularities in the wavelength range of about hundred meters to a few kilometers exhibit a two‐component power law spectrum. The long‐ and short‐wavelength spectral indices and the break scale at which the transition from a shallow to a steep slope occurs are determined self‐consistently using both the phase and amplitude scintillation data. As the power spectra of phase scintillations do not exhibit the effect of Fresnel filtering, they provide fairly accurate estimates of the spectral indices and the break scale. These estimated parameters are utilized in a model calculation of the dependence of the S4 index on signal frequency based on weak scattering theory. The accuracy of the estimates is borne out by the agreement of the result of this calculation with the signal frequency dependence of S4 derived from the observed amplitude scintillations on simultaneously transmitted 140‐ and 360‐MHz signals for all the events studied. The spectral indices and break scales determined here fall within the ranges obtained from in situ observations.
A simulation study of the critical ionization velocity process
Authors: Machida S. , Goertz C. K.
Published: February 2008

We studied the critical ionization velocity (CIV) process by first investigating a coupled system of equations describing the production of several ion species and electrons by impact ionization, their collisions with neutrals, and the heating of electrons. Analytic relations derived from this were tested with the help of a particle simulation including collisonal processes between neutrals and plasma particles. It was found that resistive heating of electrons plays an important role when the density of the neutrals is high and that electron heating due to lower hybrid waves is significant when the neutral density is low. In both cases we verified the control of the plasma production rate by the ratio of the beam velocity to the critical velocity.
High‐frequency waves generated by auroral electrons
Authors: McFadden J. P. , Carlson C. W. , Boehm M. H.
Published: February 2008

Measurements of marginally unstable electron distribution functions and high‐frequency plasma waves were made on a sounding rocket flight through a quiet auroral arc. The waves appeared near the electron plasma frequency and had a large parallel electric field component such that k∥ > k⊥. The appearance of these waves was correlated with the presence of marginally unstable parallel electron distributions. Analysis has shown that the waves were produced by ∂fe/∂ν∥ > 0 rather than the small ∂fe/∂ν⊥ > 0 features. Wave levels and growth rates inside the arc were small, and nonlinear wave‐wave and wave‐particle interactions appear to have been minimal.
Multiple X line reconnection: 2. The dynamics
Authors: Fu Z. F. , Lee L. C.
Published: February 2008

The plasma dynamics associated with both the single and multiple X line reconnection processes is studied based on an MHD simulation. It is found that the plasma dynamics in the single X line reconnection is generally consistent with the reconnection models by Petschek and Sonnerup. However, the outflow plasma speed is considerably smaller than the upstream Alfvén speed predicted by the theoretical models. In the case of multiple X line reconnection, repeated formation and convection of magnetic islands are observed with a recurrence time τ ≃ 10tA/Ro, where tA is the Alfvén transit time, and Ro = V1/VA is the imposed driving rate. The magnetic reconnection process is found to be impulsive and intermittent. An exponential growth of the reconnection rate is observed during the impulsive phase. The simulation results are generally consistent with satellite observations of flux transfer events (FTE's). The BN signature associated with the FTE's is observed in our simulation. Magnetic flux tubes with a cross section of ∼1 RE² are found to occur repeatedly every 5–15 min in our simulation.
Properties of a large‐scale interplanetary loop structure as deduced from low‐energy proton anisotropy and magnetic field measurements
Authors: Tranquille C. , Sanderson T. R. , Marsden R. G. , Wenzel K. ‐P. , Smith E. J.
Published: February 2008

We present correlated particle and magnetic field measurements by the ISEE 3 spacecraft for the loop structure behind the interplanetary traveling shock event of November 12, 1978. Following the passage of the turbulent shock region, we observe strong bidirectional streaming of low‐energy protons for approximately 6 hours, corresponding to a loop thickness of about 0.07 AU. This region is also characterized by a low relative variance of the magnetic field, a depressed proton intensity, and a reduction in the magnetic power spectral density. Using quasi‐linear theory applied to a slab model, we derive a value of 3 AU for the mean free path during the passage of the closed loop. We infer from this observation that the proton régime associated with the loop structure is experiencing scatter‐free transport and that either the length of the loop is approximately 3 AU between the sun and the earth or else the protons are being reflected at both ends of a smaller loop.
The ionotail of Venus: Its configuration and evidence for ion escape
Authors: Brace L. H. , Kasprzak W. T. , Taylor H. A. , Theis R. F. , Russell C. T. , Barnes A. , Mihalov J. D. , Hunten D. M.
Published: February 2008

The Pioneer Venus Orbiter measurements of the plasma and magnetic environment of the near tail of Venus show that the ionosphere becomes increasingly filamentary with increasing altitude, apparently forming cometlike tail rays that extend several thousand kilometers behind the planet. We call this region the ionotail of Venus. The tail rays are envisioned as plumes of high‐beta plasma of ionospheric origin that are surrounded by regions of low‐density, low‐beta plasma. The ionotail appears to be in quasi‐equilibrium, with the plasma pressure in the rays approximately balanced by the magnetic pressure of the region surrounding the rays. The magnetic field in this region is approximately sunaligned as we assume are the tail rays. Magnetic field reversals observed in the tail ray boundaries suggest the presence of strong current sheets there. Unlike the lower ionosphere whose major ion is thermal O+, a detailed study of tail ray plasma between 2000 and 2500 km altitude shows that the major ions are superthermal O+, with energies in the range of 9‐16 eV. The electrons are much cooler, with energies of about 1 eV. A minor, more energetic ion component, having energies exceeding 40 eV is also observed within the tail rays and occasionally between the rays as well. These Pioneer Venus Orbiter measurements reveal an ionotail that is highly dynamic, a region in which solar wind induced magnetic fields configure the ionospheric structures and accelerate the ions beyond the planetary escape velocity. We estimate a total planetary O+ escape rate of 5 × 1025 ions/s, and we infer an H+ escape rate of about half that value, about a factor of 2 below the hydrogen escape rate due to H+ charge exchange with the hydrogen exosphere of Venus.
Enhancements of energetic ions associated with travelling compression regions in the deep geomagnetic tail
Authors: Murphy N. , Slavin J. A. , Baker D. N. , Hughes W. J.
Published: February 2008

This paper presents a representative example of an enhancement in energetic ion flux associated with the International Sun‐Earth Explorer 3 (ISEE 3) spacecraft's encounter with a traveling compression region (TCR). Data from the energetic particle anisotropy spectrometer (EPAS) instrument on ISEE 3 are studied, along with magnetic field data from the vector helium magnetometer. It is concluded that the ion enhancements seen are spatial in nature, thus supporting the idea that TCRs are the lobe signatures of plasmoids moving along the magnetotail, away from earth.
ULF geomagnetic power at cusp latitudes in response to upstream solar wind conditions
Authors: Wolfe A. , Kamen E. , Lanzerotti L. J. , Maclennan C. G. , Bamber J. F. , Venkatesan D.
Published: February 2008

Energy densities are computed for geomagnetic field fluctuations in the Pc 3–4 range using H component magnetometer data recorded at South Pole station over a three‐month period from June 3 to September 4, 1982. Hourly values of the energy densities in the Pc 3 and Pc 4 period bands are found to be highly correlated during geomagnetic local daytime hours. The results of multivariate analyses between geomagnetic energies and upstream solar wind quantities show that the most important quantity in controlling the magnitude of the field fluctuations is the solar wind speed, with the IMF Bz component being of next importance. We conclude that the Kelvin‐Helmholtz instability at the dayside magnetopause is, statistically, the dominant energy source that contributes to ULF hydromagnetic wave activity at cusp latitudes.
Density fluctuation spectra in magnetohydrodynamic turbulence
Authors: Montgomery D. , Brown M. R. , Matthaeus W. H.
Published: February 2008

It is shown that within the framework of nearly incompressible magnetohydrodynamics, but not within that of neutral‐fluid hydrodynamics, a k−5/3 inertial‐range wave number density fluctuation spectrum is to be expected at the same times that k−5/3 kinetic energy and magnetic energy cascade spectra are present. A previous discrepancy between theory and observation in the local interstellar medium and solar wind is thereby resolved.
The velocity field of a coronal mass ejection: The event of September 1, 1980
Authors: Low B. C. , Hundhausen A. J.
Published: February 2008

A coronal mass ejection with the appearance of two sets of overlapping loops occurred at about 0600 UT on September 1, 1980, over the northwest limb of the sun. It was one of the fastest events observed by the Solar Maximum Mission coronagraph during the 1980 epoch, with apparent radial velocity components of several features approaching 1000 km s−1. A study of the slow evolution of Hα prominence filaments and coronal structures in the northwest solar sector suggests that the mass ejection resulted from the disruption of a helmet streamer in association with, possibly, two filaments to give rise to the double‐loop structure. This event is well covered by 10 coronagraph images of good quality so that the complex velocity field, defined by the apparent motions of many different parts of the mass ejection, can be mapped out as a function of space and time. The results of such an analysis are presented and related to current concerns in the theoretical understanding of mass ejections. In particular, it is concluded that a self‐similar description of the velocity field is a gross oversimplification and that although some evidence of wave propagation can be found, the bright features in this mass ejection are plasma structures moving (presumably) with frozen‐in magnetic fields, rather than waves propagating through plasmas and magnetic fields.
On the generation of magnetosheath lion roars
Authors: Lee L. C. , Wu C. S. , Price C. P.
Published: February 2008

A theoretical model is proposed to discuss the electron dynamics associated with the mirror waves and their effects on the generation of the observed lion roars in the magnetosheath. It is pointed out that the usual double‐adiabatic theory of hydromagnetics is not applicable to the electrons in mirror waves. Although the electron magnetic moment is conserved, the energy of each electron in the mirror waves is expected to be constant (because of the high electron speed along the magnetic field). Assuming an initial electron temperature anisotropy, we can show that in the low field region the electron temperature and thermal anisotropy are higher than the initial values, whereas in the high field region the electron temperature and anisotropy are lower. This point can lead to a theoretical explanation of the important features of the observed lion roars. The present discussion complements the existing theories in the literature.
Filamentary structures in the magnetotail lobes
Authors: Huang C. Y. , Frank L. A. , Peterson W. K. , Williams D. J. , Lennartsson W. , Mitchell D. G. , Elphic R. C. , Russell C. T.
Published: February 2008

Filamentary structures consisting of energetic plasma have been observed in the magnetotail lobes. Using plasma, energetic particle, composition, and magnetic field data from the ISEE 1 and 2 spacecraft, we have established that these structures may be related to polar cap auroras. The filaments consist of energetic (several keV) plasma with a quasi‐isotropic or field‐aligned velocity distribution similar to that in the plasma sheet boundary layer. Composition analysis reveals that the origin of the plasma is the plasma sheet or plasma sheet boundary layer. The gradients in the energetic particle fluxes show that the structures are approximately aligned in the XZ plane, so that the possibility of simple plasma sheet flapping can be ruled out. Field‐aligned currents are associated with the filamentary structures. The current densities when mapped along the magnetic field lines are comparable with low‐altitude measurements over the polar cap. While the occurrence of the filaments is not correlated with interplanetary conditions, the convection pattern within the filaments appears to be related to the interplanetary magnetic field direction. When the IMF BZ is positive, the E × B convection is in the sense of a dusk‐to‐dawn electric field. Our observations of field‐aligned currents and plasma convection are consistent with those made by polar‐orbiting satellites.
A comparison of magnetic signatures and DMSP auroral images at substorm onset: Three case studies
Authors: Gelpi C. , Singer H. J. , Hughes W. J.
Published: February 2008

We explore the relationship between mid‐latitude and synchronous orbit magnetic signatures and the location of the auroral surge at the onset of three isolated magnetospheric substorms. Mid‐latitude data come from the Air Force Geophysics Laboratory Magnetometer Network, synchronous orbit data from the satellites GOES 2 and 3, and auroral data from Defense Meteorological Satellite Program auroral images and auroral magnetograms. We find that the surge forms with its western edge ∼1 hour west of the longitude where the major axis of the mid‐latitude Pi 2 polarization ellipse is along magnetic north and where the D component perturbation of the magnetic bay is near zero. These observations are in qualitative agreement with a current wedge consisting of a localized upward current in the west and a longitudinally distributed current in the east. At synchronous orbit the longitude of the surge head appears to separate a region to the west where the magnetic field becomes more taillike from one to the east where the magnetic field becomes more dipolar.
Longitudinal effects on the eigenfrequencies of low‐latitude Pc 3 pulsations
Authors: Sutcliffe P. R. , Hattingh S. K. F. , Boshoff H. F. V.
Published: February 2008

Pc 3 pulsations were recorded at three stations along the L ≅ 1.78 shell and spanning 35° in longitude during July 1983. The pulsations recorded at Gough Island (10°W) had higher frequencies than those recorded at two stations in South Africa (20°E and 25°E). Local time effects or small differences in the magnetic field configuration do not account for the differences in resonant frequency. We conclude that the higher pulsation frequency at Gough Island is due to a lower plasma mass density on that field line relative to the others and that oxygen plays a significant role. Factors which may contribute to the lower plasma mass density are the lower rate of production of O+ by solar ultraviolet radiation, F region heating by energetic particle precipitation, and the greater loss of O+ caused by meridional winds.
On the role of the quasi‐parallel bow shock in ion pickup: A lesson from Venus?
Authors: Luhmann J. G. , Russell C. T. , Phillips J. L. , Barnes A.
Published: February 2008

Previous observations at Venus show convincing evidence of planetary (O+) ion pickup by the large‐scale motional (−V × B) electric field in the magnetosheath when the interplanetary magnetic field is perpendicular to the solar wind flow. However, the presence of magnetic field fluctuations in the magnetosheath downstream from the quasi‐parallel bow shock should allow pickup to occur even when the upstream magnetic field B and plasma velocity V are practically coaligned. Single‐particle calculations are used to demonstrate that convecting magnetic field fluctuations similar to those observed in the Venus magnetosheath when the subsolar bow shock is quasi‐parallel can efficiently accelerate cold planetary ions by means of the electric field associated with their transverse components. This ion pickup process, which is characterized by a spatial dependence determined by the bow shock shape and the orientation of the upstream magnetic field, is likely also to occur at Mars and may be effective at comets.
Convective growth of broadband turbulence in the plasma sheet boundary layer
Authors: Dusenbery P. B.
Published: February 2008

Convective growth of slow and fast beam acoustic waves in the plasma sheet boundary layer (PSBL) is investigated. It has been shown previously that a cold ion population must be present in order to excite beam acoustic waves in the PSBL. However, growth rates are significantly enhanced when warm plasma sheet boundary layer ions are present. Net wave growth along a ray path is determined by convective growth. This quantity is calculated for particle distribution models consistent with the PSBL where the intensity of broadband turbulence is observed to peak. Total number density dependence on beam acoustic convective growth is evaluated, and it is found that even for low density conditions of ∼0.01 cm−3, a measurable level of broadband turbulence is expected. Relative drift effects between cold and warm ion populations are also considered. In particular, it is found that slow mode convective growth can be enhanced when slowly streaming cold ions are present, compared to fast ion streams.
Generation of broadband turbulence by accelerated auroral ions: 1. Parallel propagation
Authors: Dusenbery P. B. , Martin R. F.
Published: February 2008

The generation of ion beam modes by accelerated particle distributions in the high‐latitude auroral region, associated with parallel electric fields, is considered. Various particle distribution models are used, including downward accelerated electrons, a combination of trapped arid backscattered electrons, and upgoing accelerated ionospheric ions (H+ and O+). A numerical study of the excited modes implies that they can be characterized as either a nonresonant ion‐ion two‐stream mode or a resonant acoustic mode. For reasonable models of the high‐latitude auroral acceleration region, the nonresonant mode has a growth rate an order of magnitude greater than the acoustic mode. Approximate analytical expressions for the maximum growth rate and the corresponding frequency of the ion‐ion two‐stream mode are derived. Conditions are also derived that determine when the two‐stream mode evolves into an acoustic mode. The frequency range of the excited waves and the frequency at peak growth compare reasonably well with observations of broadband noise in the auroral region. The results of this study suggest that ion interactions with these waves may be a viable mechanism for the observed enhancements in the parallel temperature of accelerated auroral ions.
Lightning‐induced electron precipitation events observed at L ∼ 2.4 as phase and amplitude perturbations on subionospheric VLF signals
Authors: Inan U. S. , Carpenter D. L.
Published: February 2008

Lightning‐induced electron precipitation (LEP) events are studied using the Trimpi effect, in which the precipitation‐induced ionization enhancements in the lower ionosphere (D region) give rise to rapid perturbations of subionospheric VLF signals. In 1983, the phase and amplitude of signals from the NPM transmitter in Hawaii (23.4 kHz) and the Omega transmitter in Argentina (12.9 kHz) were measured at Palmer, Antarctica (L ∼ 2.4), together with the magnetospheric whistler background. The long baseline and over‐sea great circle paths from these two sources make it possible for the observed perturbations to be interpreted using a single waveguide mode theory. Analytical expressions are used to relate the magnitude of the phase perturbations to differential changes in ionospheric reflection height along a segment of the propagation path. The predicted relationship between relative perturbation sizes on the two different signals is compared with measurements. From this information, the whistler‐induced flux levels are inferred to be in the 10−4 − 10−2 erg cm−2 s−1 range and the precipitation regions are inferred to be roughly “circular” in shape, rather than elongated along L shells. Measured amplitude changes tended to be small (∼ 0.5 dB) and negative, as expected from a single‐mode theory, but the ratios of simultaneous amplitude and phase perturbations were slightly larger than the theory predicts, probably due to the effects of an additional mode(s). An assessment of the relative detectability of amplitude versus phase perturbations favors phase perturbations by ∼ 10 dB, irrespective of the detection scheme used.
Ionospheric ion cyclotron wave generation inferred from coordinated Doppler radar, optical, and magnetic observations
Authors: Prikryl P. , Koehler J. A. , Sofko G. J. , McEwen D. J. , Steele D.
Published: February 2008

Doppler spectra obtained with two cross‐beamed 50‐MHz CW radio systems often showed narrow peaks near the gyrofrequencies of the principal ions in the E region (N2+, NO+, O2+, O+). Such peaks strongly suggest the existence of electrostatic ion cyclotron waves during auroral conditions. At times when strong ion cyclotron peaks were observed by the radar, all‐sky pictures of the scattering region showed rays and coronas. Identification of the rays was made much easier because the upper portions of the rays were sunlit, so the resonance excitation of N2+ ions gave greatly enhanced 4278‐Å emissions that could be readily observed by the meridian‐scanning photometer. This intimate association of the echoes with extended ray structures suggests that the echoes may have come from the upper E region, where the conditions for exciting the ion cyclotron instability by the field‐aligned currents associated with the rays would be more favorable. There was a good correlation between the occurrence of the ion cyclotron type spectra and the E‐W magnetic perturbation, which also may have been indicative of field‐aligned currents. Finally, a close relationship was found between average plasma drifts inferred from the radar Doppler spectra and north‐south motions of optical auroral forms.
The compressional ULF foreshock boundary of Venus: Observations by the PVO magnetometer
Authors: Greenstadt E. W. , Baum L. W. , Jordan K. F. , Russell C. T.
Published: February 2008

Entries and exits of the compressional magnetic wave segments of the Pioneer Venus Orbiter data are identified as crossings of a foreshock boundary. When these crossings are plotted in an appropriate coordinate system, they form scatter diagrams representing the geometry of a composite boundary covering all solar wind conditions of the sample. When the orientation of the boundary for the average interplanetary magnetic field direction at Venus is extracted, the boundary is similar, although not identical within present uncertainties, to that found for the analogous foreshock boundary of the earth.
Ionospheric convection driven by NBZ currents
Authors: Rasmussen C. E. , Schunk R. W.
Published: February 2008

Computer simulations of Birkeland currents and electric fields in the polar ionosphere during periods of northward interplanetary magnetic field (IMF) were conducted. When the IMF z component is northward, an additional current system, called the NBZ current system, is present in the polar cap. These simulations show the effect of the addition of NBZ currents on ionospheric convection, particularly in the polar cap. When the total current in the NBZ system is roughly 25% to 50% of the net region 1 and 2 currents, convection in the central portion of the polar cap reverses direction and turns sunward. This creates a pattern of four‐cell convection with two small cells located in the polar cap, rotating in an opposite direction from the larger cells. The effects of varying ionospheric conductivity were studied by considering both a constant voltage source and a constant current source. Conductivity gradients due to seasonal and local time variations in solar production were considered, as well as gradients due to electron precipitation. When the Birkeland currents are fixed (constant current source), the electric field is reduced in regions of relatively high conductivity, which affects the pattern of ionospheric convection. Day‐night asymmetries in conductivity change convection in such a way that the two polar‐cap cells are located within the large dusk cell. When ionospheric convection is fixed (constant voltage source), Birkeland currents are increased in regions of relatively high conductivity. Ionospheric currents, which flow horizontally to close the Birkeland currents, are changed appreciably by the NBZ current system. The principal effect is an increase in ionospheric current in the polar cap.
Observations of small‐scale plasma density depletions in Arecibo HF Heating Experiments
Authors: Isham B. , Birkmayer W. , Hagfors T. , Kofman W.
Published: February 2008

Observations of incoherent scattering of electromagnetic waves at UHF from Langmuir waves by a new scheme involving linear frequency modulation (chirping) of a UHF transmitter and the demodulation (dechirping) of the received signals have been applied during HF heating experiments. These observations show that the high power HF wave used for ionospheric modification creates small‐scale plasma depletions instantly on a time scale of 5 ms. For a plasma frequency of 5.1 MHz, plasma frequency gradient of the order of 50 kHz/km, and power density input of the HF heater wave of 8.0 × 10−5 W/m² the depletion ranged from 3 to 5%. This appears to provide direct evidence that the HF‐induced modifications involve Langmuir waves trapped in density cavities.
Polarization of low‐frequency electromagnetic radiation in the lobes of Jupiter's magnetotail
Authors: Moses S. L. , Kurth W. S. , Kennel C. F. , Coroniti F. V. , Scarf F. L.
Published: February 2008

The plasma wave instruments on the Voyager spacecraft have detected intense electromagnetic radiation within the lobes of Jupiter's magnetic tail down to the lowest frequency of the detector (10 Hz). During a yaw maneuver performed by Voyager 1 in the lobe of the Jovian magnetotail, a modulation appeared in the amplitudes of waves detected in the 10‐, 17.8‐ and 31.1‐Hz channels of the plasma wave analyzer, well below the local electron cyclotron frequency of 260 Hz. The lowest amplitudes occurred when the antenna axis was most nearly parallel to the magnetic field. Wave amplitudes in the 56.2‐Hz and higher frequency channels remained nearly constant during the maneuver. From the cold‐plasma theory of electromagnetic waves, one can conclude that the plasma frequency was between the 56.2‐ and 31.1‐Hz channels where the parallel‐polarized component of the spectrum cuts off. This implies a tail‐lobe density between 3.2 × 10−5 and 1.5 × 10−5 cm−3. The left‐hand cutoff frequency would then be below 10 Hz, consistent with either the Z mode (L, X) or whistlers (R mode) in the modulated channels. However, locally generated whistler mode waves at these frequencies would require electrons with resonant energies greater than 25 MeV, which are unlikely to exist in significant densities.
Io plasma torus electrons: Voyager 1
Authors: Sittler E. C. , Strobel Darrell F.
Published: February 2008

The analysis of in situ plasma electron observations in the Io plasma torus by the plasma science experiment during the Voyager 1 encounter with Jupiter is presented in terms of two components: a thermal (c) Maxwellian component and suprathermal (H) non‐Maxwellian component of the electron distribution function. Average electron temperatures are Te < 1 eV in the cold torus (L < 5.5), with Te ≃ 5–6 eV in the hot torus (5.5 < L < 7.6); Te rises abruptly to Te ≃ 30 eV just outside the hot torus (L > 7.6) and then continues to rise to Te > 100 eV at r > 12 RJ. In the cold torus the density ratio of the suprathermal component nH to that of the cold component nc was <10−4; but in the hot torus, nH/nC ∼ 10−3 was observed, and outside the torus, nH/nc can exceed 10−1. We present evidence that suprathermal electrons are locally produced in the hot torus. Throughout the hot torus the electron temperature Te is a factor of 10 less than the thermal ion temperature. A large difference in the hot electron pressure PH is observed between the inbound and the outbound data which is interpreted as a latitudinal gradient with PH being a maximum at the magnetic equator. If one imposes the theoretical and observational constraint that (T⊥/T∥)EQ ≤ 2 for the hot electrons, then one requires the presence of a parallel electric field E∥ > 2.5 µV/m which exceeds the ambipolar electric field E∥ < 1 µV/m produced by the centrifugally confined ions. However, if unacceptable charge imbalances in the thermal plasma are not to occur from this larger E∥, then sufficient wave turbulence in the plasma must be present to adequately scatter the thermal electrons. We infer the presence of a neutral corona around Io from the observed decrease and symmetry with respect to Io of Tc. The energy input to the torus by charge exchange and ionization in this neutral corona followed by pickup is ∼2 × 1011 W, substantially less than the EUV luminosity. In the hot torus, suprathermal electrons contribute significantly to the ionization of the more highly ionized ions (O+, O2+, S2+, and S3+).
Observations and theory of the AMPTE magnetotail barium releases
Authors: Bernhardt P. A. , Roussel‐Dupre R. A. , Pongratz M. B. , Haerendel G. , Valenzuela A. , Gurnett D. A. , Anderson R. R.
Published: February 2008

The barium releases in the magnetotail during the Active Magnetospheric Particle Tracer Explorers (AMPTE) operation were monitored by ground‐based imagers and by instruments on the Ion Release Module. After each release, the data show the formation of a structured diamagnetic cavity. The cavity grows until the dynamic pressure of the expanding ions balances the magnetic pressure on its surface. The magnetic field inside the cavity is zero. The barium ions collect on the surface of the cavity, producing a shell. Plasma irregularities form along magnetic field lines draped over the surface of the cavity. The scale size of the irregularities is nearly equal to the thickness of the shell. The evolution and structuring of the diamagnetic cavity are modeled using magnetohydrodynamics theory.
Analytical stability analysis for a two‐dimensional self‐consistent magnetotail model by use of statistical mechanics
Authors: Kiessling M. , Schindler K.
Published: February 2008

The stability analysis of two‐dimensional current‐carrying plasmas by use of statistical mechanics is described. This method is applied to a self‐consistent model configuration of the earth's magnetotail. The equations derived are solved analytically, and the results are discussed in view of spontaneous onset of magnetospheric activity.