bluuebunny/crossref_metadata_2025 huggingface dataset:
Average spatial distributions of energetic O+, O2+, O6+, and C6+ ions in the magnetosphere observed by AMPTE CCE
Authors: Kremser G. , Stüdemann W. , Wilken B. , Gloeckler G. , Hamilton D. C. , Ipavich F. M.
Published: February 2008

Investigations of the average spatial distributions of the ionospheric ions O+ and O2+ and of the solar wind ions O6+ and C6+ provide a possibility to estimate the relative contributions of the ionosphere and the solar wind to the energetic ion population in the magnetosphere. We used measurements with the charge energy mass spectrometer (CHEM) on board the AMPTE CCE spacecraft to determine the relative fluxes of these ions near the equatorial plane as a function of the drift shell parameter L, the magnetic activity index Kp, and the local time LT. The O+ and O2+ ions have radial profiles with maxima at about L = 5 and diurnal variations with a broad maximum on the dayside. The O6+ and C6+ ion fluxes increase with L between L = 5 and L = 7 and level off farther out. The diurnal variations of the relative O6+ and C6+ fluxes exhibit a pronounced minimum on the dayside. The observations can be interpreted in terms of ion convection from the tail onto quasi‐trapped drift orbits and further radial diffusion onto closed drift shells. The maximum of the O+ and O2+ fluxes as well as the minimum of the O6+ and C6+ fluxes on the dayside can be explained by drift shell splitting. The inward transport is associated with ion losses, mainly by charge exchange. O6+ and C6+ ions are removed already at larger distances than O+ and O2+. As a result, the ionosphere contributes most of the oxygen ions in the magnetosphere. Important contributions from the solar wind are encountered in the outer magnetosphere (L > 7).
Latitudinal motions of the aurora during substorms
Authors: Craven J. D. , Frank L. A.
Published: February 2008

Sequences of auroral images obtained with Dynamics Explorer 1 are used to investigate latitudinal motions of the aurora in substorms. Average speeds of poleward motion are ∼230 m/s near local midnight for two isolated, small substorms and ∼1000 m/s during an intensification within a previously active auroral oval. The speed of poleward expansion measured at ∼6‐min temporal resolution can differ greatly from the average speed because of the episodic development of substorms. This poleward motion may be influenced by the sign of the z component of the interplanetary magnetic field. Recovery of the high‐latitude boundary of the aurora to presubstorm latitudes is first observed in the postmidnight sector. In the premidnight sector the discrete aurora can become stationary for a period of time or even continue further poleward before a retreat to lower latitudes begins. For these substorms the expansion and recovery phases are not separated by a ∼5° “poleward leap” of the aurora. During the recovery phase a prominent decrease in luminosities is first observed at intermediate latitudes within the auroral distribution. This region is bounded at higher latitudes by the discrete aurora and at lower latitudes by bright diffuse aurora. Given that magnetic field lines threading these auroral distributions map to the plasma sheet boundary layer and to the central plasma sheet, respectively, magnetic field lines at the intermediate auroral latitudes then map to the plasma sheet at distances of ≳ 22 RE.
Field‐aligned currents deduced from EISCAT radar observations and implications concerning the mechanism that produces region 2 currents
Authors: Caudal G.
Published: February 2008

Large‐scale field‐aligned currents have been determined from electric field and conductivity measurements performed with the European Incoherent Scatter (EISCAT) radar. The result, given for a quiet and stationary day, is globally consistent with the usual picture of the large‐scale morphology of field‐aligned currents involving region 1 and region 2 current systems. A comparison between the electric potential pattern (deduced from electric field measurements) and the field‐aligned current pattern in region 2 has been performed. This involved, in particular, comparing the phases (in terms of local time) of the diurnal harmonics of the field‐aligned current and potential distributions. The results imply that the field‐aligned current tends to be distributed in phase opposition, rather than phase quadrature, with the potential. This is found to be irreconcilable with the mechanism usually put forward as the generator of field‐aligned currents in the medium and inner magnetosphere (namely, charge separation due to gradient drifts of energetic particles at the ring current inner edge). We show that the loss of convected energetic electrons by precipitation due to pitch angle scattering produces azimuthal pressure gradients sufficiently pronounced that it may constitute the dominant mechanism for driving field‐aligned currents in the medium and inner magnetosphere, thus accounting for the observed phase characteristics of region 2 currents.
A new model of cometary ionospheres
Authors: Körösmezey A. , Cravens T. E. , Gombosi T. I. , Nagy A. F. , Mendis D. A. , Szegő K. , Gribov B. E. , Sagdeev R. Z. , Shapiro V. D. , Shevchenko V. I.
Published: February 2008

The coupled continuity, momentum, and energy equations were solved for ionospheric conditions appropriate for comet Halley at 1 AU. The numerical scheme used is such that any shock transition appears naturally in the solution and no a priori assumptions are necessary. Solutions were obtained for a number of different assumptions concerning electron heating rates, but all showed that the electron temperatures increase rapidly and significantly at a distance from the nucleus where collisional electron‐neutral cooling becomes unimportant. This temperature increase is accompanied by a sharp increase in both the plasma pressure and its associated polarization electric field, causing the supersonic plasma flow to go subsonic. It is not clear at this time whether or not this sonic transition is accompanied by a shock.
Substorm currents: Growth phase and onset
Authors: Kaufmann Richard L.
Published: February 2008

Magnetic field models are used to study electric currents that flow during growth phases and onsets of magnetospheric substorms. Large cross‐tail currents between altitudes of about 7 and 10 RE are required near midnight during growth phase in order to produce the observed magnetic field perturbations at synchronous altitude. Considerations of the particle fluxes needed to carry growth phase currents showed that the required current can be carried by the drift of a particle population whose energy density is about 20 keV/cm³. This energy density frequently is present at synchronous altitude after injection events. However, the model calculations require establishment of the large currents beyond synchronous altitude during growth phase. No observations of substantial flux increases during growth phase within the region of interest could be found. As a partial explanation of this problem, we found that a modest increase in particle energy density can produce a substantial increase in cross‐tail current. The increased currents carried by unaccelerated preexisting particles in the changing growth phase magnetic field can play a significant role in altering the magnetic field at synchronous altitude. This process involves a positive feedback effect, with preexisting particles carrying more cross‐tail current as soon as any perturbation begins to stretch tail field lines. It is concluded that more extensive observations of the changes in particle fluxes and pitch angle distributions during growth phase are needed in the equatorial region near 8 RE. Such observations also will help determine whether the energy of the plasma that is injected near synchronous altitude during substorms primarily is introduced slowly as fluxes build up during growth phase or primarily is introduced suddenly by the local conversion of magnetic field energy to particle energy at onset. In the first case, an injection event primarily would represent the inward motion of a population which already exists at substorm onset. In the latter case, strong impulsive acceleration would be an intrinsic part of the injection process. For our calculations, substorm onset is modeled by diverting current to the ionosphere in a wedge near midnight. It is found that field lines within the wedge collapse dramatically even if only a portion of the cross‐tail current is diverted. At the same time, a satellite outside the current wedge sees field lines become more taillike. It is suggested that diversion of only the electron cross‐tail current to the ionosphere is enough to initiate a substorm. Ion drift is reduced substantially within the wedge as field lines become more dipolar even if the ion energy density remains large. Finally, it is noted that very strong drift shell splitting effects should be seen if cross‐tail current is diverted only in a wedge near midnight.
Energy dependence of the equatorward cutoffs in auroral electron and ion precipitation
Authors: Newell Patrick T. , Meng C. ‐I.
Published: February 2008

The spectral dependence in the equatorward cutoff of diffuse auroral electron and ion precipitation is investigated using the Defense Meteorological Satellite Program F6 and F7 low‐altitude polar‐orbiting satellites. These equatorward boundaries are determined at 100, 1000, and 3000 eV during very quiet conditions and during normal conditions (low‐level magnetic activity) in the dawn, dusk, premidnight, and prenoon sectors. We find that under normal conditions the low‐energy ions cut‐off equatorward of the higher energy ions at all local times, with the largest effect at dawn and the smallest at dusk. Electrons exhibit greater variability; however, at premidnight, low‐energy electrons cut‐off equatorward of higher energy electrons, while at dawn the electron cutoff‐ordering becomes λ100 ≫ λ3000 > λ1000, where the subscripts are energies in electron volts. During times of prolonged and profoundly quiet conditions, the dispersion, particularly for ions, disappears; that is, the cutoffs at all three energies coincide. It is argued that these observations provide evidence against the predictions of the often considered model of steady‐state Alfvén layers arising from a constant source in the magnetotail convected earthward by a Stern‐Volland cross‐tail electric field toward a magnetic dipole. For example, the predicted ion dispersion is never observed. Investigating the response to magnetic disturbances following times of profound quiet, we find prompt, near simultaneous reaction over a wide local time range, including dawn and dusk. Following a return to quiet magnetic conditions, the precipitation relaxes to predisturbance patterns in the premidnight region within 1 to 2 hours but takes several hours longer at dawn. The electron precipitation cutoffs are equatorward of the ion precipitation cutoffs in all magnetic local time sectors except dusk, where the ion precipitation extends further equatorward.
Spectral broadening of VLF transmitter signals and sideband structure observed on Aureol 3 satellite at middle latitudes
Authors: Tanaka Y. , Lagoutte D. , Hayakawa M. , Lefeuvre F. , Tajima S.
Published: February 2008

Electric and magnetic field wave data acquired on Aureol 3 satellite demonstrate the existence of a spectral broadening effect in which VLF transmitter signals from Alpha station (geographic coordinates, 50.5°N, 137°E) in USSR undergo a significant spectral broadening on electric fields as they propagate through the ionosphere up to the spacecraft in the altitude range of 500–2000 km at middle latitudes (L ∼ 2). The spectral broadening phenomena may be divided into two types: (1) spectrally broadened components occurring without any association with ELF/VLF emissions under disturbed ionospheric conditions and (2) spectrally broadened components with predominant sideband structure in association with ELF emissions. Bicoherence computation results suggest a nonlinear mode coupling between the transmitter signal and ELF emission which produces sidebands that are quasi‐electrostatic in nature. However, faint spectral broadened components in both types 1 and 2 may be connected with Doppler shift of quasi‐electrostatic whistler mode waves with a broad spectrum of k near the resonance cone, due to scattering of the transmitter signals from ionospheric irregularities in the F region.
Simulation of the electron acoustic instability for a finite‐size electron beam system
Authors: Lin C. S. , Winske D.
Published: February 2008

Satellite observations at midaltitudes (≈ 20,000 km) near the earth's dayside polar cusp boundary layer indicate that the upward electron beams have a narrow latitudinal width up to 0.1°. In the cusp boundary layer where the electron population consists of a finite‐size electron beam in a background of uniform cold and hot electrons, the electron acoustic mode is unstable inside the electron beam but damped outside the electron beam. Simulations of the electron acoustic instability for a finite‐size beam system are carried out with a particle‐in‐cell code to investigate the heating phenomena associated with the instability and the width of the heating region. The simulations show that the finite‐size electron beam radiates electrostatic electron acoustic waves. The decay length of the electron acoustic waves outside the beam in the simulation agrees with the spatial decay length derived from the linear dispersion equation. The ambient cold electrons in a diffusion region surrounding the beam are heated to a higher temperature by absorbing the radiated electron acoustic waves, with the heating occurring mainly in the parallel direction. In the heat diffusion region, the temperature of the cold electrons decreases with the distance from the beam with a temperature gradient length smaller than the decay length of the wave energy. This feature of electron heating is modeled by using a heat diffusion equation derived from the second‐order theory. In solving the heat diffusion equation, the wave energy inside the beam is assumed to be saturated by trapping the beam and the cold electrons. The dominant wave number is determined by assuming that the wave frequency outside the beam is equal to the frequency corresponding to the maximum growth rate inside the beam. These results are discussed with respect to the DE 1 plasma and wave observations in the polar cusp region. Using the typical parameters in the cusp boundary layer, the spatial decay length of the electron acoustic waves is estimated to be in the range of 1–10 km and the heat diffusion region is about 5 km. The results suggest that the electron acoustic waves generated by cusp upward electron beams are probably confined in a small region surrounding the beams.
Structure of reconnection boundary layers in incompressible MHD
Authors: Sonnerup B. U. Ö , Wang D. ‐J.
Published: February 2008

The incompressible MHD equations with nonvanishing viscosity and resistivity are simplified by use of the boundary layer approximation to describe the flow and magnetic field in the exit flow regions of magnetic field reconnection configurations when the reconnection rate is small. The conditions are derived under which self‐similar solutions exist of the resulting boundary layer equations. For the case of zero viscosity and resistivity, the equations describing such self‐similar layers are then solved in terms of quadratures, and the resulting flow and field configurations are described. Symmetric solutions, relevant, for example, to reconnection in the geomagnetic tail, as well as asymmetric solutions, relevant to reconnection at the earth's magnetopause, are found to exist. The nature of the external solutions to which the boundary layer solutions should be matched is discussed briefly, but the actual matching, which is to occur at Alfvén‐wave characteristic curves in the boundary layer solutions, is not carried out. Finally, it is argued that the solutions obtained may also be used to describe the structure of the intense vortex layers observed to occur at magnetic separatrices in computer simulations and in certain analytical models of the reconnection process.
Theoretical study of the high‐latitude ionosphere's response to multicell convection patterns
Authors: Sojka J. J. , Schunk R. W.
Published: February 2008

It is well known that convection electric fields have an important effect on the ionosphere at high latitudes and that a quantitative understanding of their effect requires a knowledge of the plasma convection pattern. When the interplanetary magnetic field (IMF) is southward, plasma convection at F region altitudes displays a two‐cell pattern with antisunward flow over the polar cap and return flow at lower latitudes. However, when the IMF is northward, multiple convection cells can exist, with both sunward flow and auroral precipitation (theta aurora) in the polar cap. The characteristic ionospheric signatures associated with multicell convection patterns were studied with the aid of a three‐dimensional time‐dependent ionospheric model. Two‐, three‐, and four‐cell patterns were considered and the ionosphere's response was calculated for the same cross‐tail potential and for solar maximum and winter conditions in the northern hemisphere. As expected, there are major distinguishing ionospheric features associated with the different convection patterns, particularly in the polar cap. For two‐cell convection the antisunward flow of plasma from the dayside into the polar cap acts to maintain the densities in this region in winter. For four‐cell convection, on the other hand, the two additional convection cells in the polar cap are in darkness most of the time, and the resulting O+ decay acts to produce twin polar holes that are separated by a sun‐aligned ridge of enhanced ionization due to theta aurora precipitation. For three‐cell convection, only one polar hole forms in the total electron density, but in contrast to the four‐cell case, an additional O+ depletion region develops near noon owing to large electric fields causing an increased O+ + N2 loss rate. These general distinguishing features do not display a marked universal time variation in winter.
Mass spectrometric measurements of SF6 chemical releases from sounding rockets
Authors: Hunton D. E. , Viggiano A. A. , Swider W. , Paulson John F. , Sherman C.
Published: February 2008

Sulfur hexafluoride was released from a small reservoir on the side of a sounding rocket equipped with a negative ion mass spectrometer. The effects of the release were observed only at altitudes between 100 km and the rocket's apogee at 128 km. Below these altitudes the SF6 was swept away by the flow of atmospheric gases past the rocket. The maximum ion signals were observed at 115–118 km, where the mean free path equaled the distance from the reservoir to the mass spectrometer. The negative ions that resulted from the SF6 release were SF6−, SF5−, F−, O−. At 155 km the relative intensities of these ions were 1.0, 0.3, 0.07, and 0.06, respectively. A large fraction of the SF6− sampled by the mass spectrometer was probably in a metastable state. The other observed ions were formed by dissociative electron attachment to SF6, collision‐induced dissociation of SF6−*, and ion‐molecule reactions. The mass spectrometer sampled a nonequilibrium distribution of ions in the first milliseconds after they were formed. In addition, the large draw‐in field of the mass spectrometer probably altered the identities and relative intensities of the ions. Laboratory measurements with a selected ion flow tube apparatus show that the reaction SF5− + O → F− + SF4O proceeds with rate constant 1.1 × 10−11 cm³ s−1 at 300 K.
Energetic interplanetary shocks, radio emission, and coronal mass ejections
Authors: Cane H. V. , Sheeley N. R. , Howard R. A.
Published: February 2008

The interplanetary shocks which generate detectable low‐frequency (< 1 MHz) radio emission, represent as a group, the most energetic shocks produced by the sun. For all interplanetary (IP) shocks which generated so‐called IP type II events, we find, when observations were available, that the associated solar events involved fast (>500 km/s) coronal mass ejections (CMEs). In comparison with the set of all CMEs detected by the Solwind coronagraph, the CMEs associated with IP type II events are the most massive and energetic. The majority (>50%) belong to the structural classes described by the Solwind researchers as “curved front” or “halo.” Evidence presented suggests that these are the same class viewed from a different perspective. Our results are consistent with there being a close relationship between interplanetary shocks and fast CMEs.
Low altitude observations of dispersionless substorm plasma injections
Authors: Newell Patrick T. , Meng Ching‐I.
Published: February 2008

The substorm injection boundary model proposed by McIlwain (1974) and since adopted by many researchers describes the phenomenology of plasma introduction into the middle magnetosphere during a magnetic substorm: at the time of the substorm onset, both electrons and ions of all energies share a common Earthward boundary. Such an injection should leave a clear signature in the auroral precipitation data. We used the Defense Meteorological Satellite Program (DMSP) F6 and F7 satellites to search for such signatures in the ≤ 30‐keV auroral electron and ion data. Clear examples of an equatorward leap of the previously energy dependent equatorward cutoff in auroral precipitation to a cutoff boundary common to ions of all energies measured (i.e., dispersionless up to 30 keV) and electrons up to several keV (usually less than 10 keV) were observed in the near‐midnight sector at a time corresponding to substorm onset. A collection of 10 such events occurring over a 1‐month interval was studied, providing unusually direct confirmation that the phenomenological model of an initially dispersionless boundary accurately describes at least some injections. The DMSP data show the injection boundary to be dispersionless over a wide longitudinal range, but often only a dispersive injection is seen at dawn.
One‐dimensional hybrid simulations of boundary layer processes in the AMPTE solar wind lithium releases
Authors: Chapman S. C. , Schwartz S. J.
Published: February 2008

As part of the Active Magnetospheric Particle Tracer Explorers (AMPTE) mission, releases of ∼1025 atoms of lithium, and later barium, were made in the solar wind and magnetosheath. Initial photoionization produced a release plasma of sufficient densities that a diamagnetic cavity was seen to form at the release center, disrupting the ambient field and flow over a localized region (approximately tens of kilometers for Li, approximately hundreds of kilometers for Ba). Since the gyroradii of both the oncoming solar wind protons and the release ions were either greater than or of the order of the scale size of this perturbation while the electron gyroradii remained small, a hybrid description, where the ions are treated as particles, and the electrons as a charge‐neutralizing fluid, is appropriate. Here we present the results of one‐dimensional simulations for lithium using a hybrid description in order to investigate the process of momentum transfer between the two ion species that occurs locally over the boundary layer which forms between them. We find that a well‐defined field and plasma structure evolves in which the bulk of the lithium ions are moved en masse by a "snowplough" type process, those remaining either being accelerated to higher speeds to produce “taillike” structures or remaining upstream to form part of the snowplough momentum balance. Field and ion density structures found in the simulations are in good gross agreement with observations. We also obtain an analytical estimate of the snowplough speed which is in good agreement with that obtained computationally. When set in the context of the three‐dimensional release geometry, features resolved by our one‐dimensional simulation are found to have clear parallels in the AMPTE data.
Steepened magnetosonic waves at comet Giacobini‐Zinner
Authors: Tsurutani Bruce T. , Thorne Richard M. , Smith Edward J. , Gosling J. T. , Matsumoto Hiroshi
Published: February 2008

We examine intense hydromagnetic waves at comet Giacobini‐Zinner to investigate the mode and direction of wave propagation and thereby provide important constraints on potential mechanisms for wave origin in the vicinity of the comet. The character of the wave polarization changes from basically elliptical far from the comet to almost linear followed by either a rapid partial rotation (less than 360°) or multiple rotations (high‐frequency wave packets) in the vicinity of the bow wave. Simultaneous high‐resolution measurements of electron density fluctuations demonstrate that the long‐period (∼100 s) waves are propagating in the magnetosonic (fast MHD) mode. Principal axis analyses of the long‐period waves and accompanying partial rotations show that the sum of the wave phase rotations is 360°. This indicates that both are parts of the same wave. The change in polarization characteristics near the comet is simply a consequence of wave steepening. From the time sequence of the steepened waveforms observed by ICE, we demonstrate that the waves must propagate in the general direction along the magnetic field toward the Sun with phase velocities less than the solar wind speed. They are consequently blown back across the spacecraft and observed with a left‐hand sense of rotation. All available observations are therefore consistent with wave generation by the right‐hand resonant ion ring instability which predicts waves propagating in the ion beam (solar) direction. Based on the Wu‐Davidson linear growth rate expression, the total convective gain is shown to exhibit a weak (logarithmic) dependence on distance from the comet reaching a maximum of 30 e‐folding in the vicinity of the bow wave. Conditions necessary for the origin of multiple rotations currently are not well understood. Arguments for their being standing whistler waves consisting of a partial rotation plus multiples of 360° rotations are presented. Models of such emissions should be able to explain them as integral parts of the steepened magnetosonic waves. The large amplitudes, ΔB/|B| ∼ O(1), and small‐scale sizes (rotational discontinuities) of the cometary waves suggest that rapid pitch angle scattering and energy transfer with energetic ions should occur. Since the waves are highly compressive, Δ|B|/|B| ≃ O(0.5), one can also anticipate possible first‐order Fermi and gradient B drift acceleration.
Characteristics of wavelike fluctuations in Dynamics Explorer neutral composition data
Authors: Hedin A. E. , Mayr H. G.
Published: February 2008

Wavelike fluctuations in neutral composition data obtained with the neutral atmospheric composition system quadrupole mass spectrometer carried aboard the Dyamics Explorer (DE 2) satellite are a maximum in the vicinity of the magnetic poles. Typical rms amplitudes near the poles for N2, O, He, and Ar fluctuations in the 400‐ to 4000‐km‐wavelength band are found to be 11%, 6%, 6%, and 20%, respectively. Amplitudes near the equator are roughly a third of the polar amplitudes, and activity in the 50‐ to 400‐km‐wavelength band is roughly 20% of the longer‐wavelength activity. Wave amplitudes maximizes in the midnight and early morning magnetic local time sectors and increase with magnetic activity. The lack of a marked decrease in short‐wavelength activity in relation to long‐wavelength activity between pole and equator may indicate the presence of ducted propagation modes or local sources of activity.
Solar wind iron charge states preceding a driver plasma
Authors: Galvin A. B. , Ipavich F. M. , Gloeckler G. , Hovestadt D. , Bame S. J. , Klecker B. , Scholer M. , Tsurutani B. T.
Published: February 2008

During September 28 and 29, 1978, the ISEE 3 spacecraft observed several distinct types of high‐speed solar wind flows when a coronal hole‐associated high‐speed stream was followed by two interplanetary shocks, one of which was driven by flare ejecta contained in a “magnetic cloud” or interplanetary “plasmoid.” Using the University of Maryland/Max‐Planck‐Institut ultralow energy charge analyzer (ULECA) on ISEE 3, we present solar wind Fe and Si/S charge state and Fe density measurements for the different plasma regimes associated with the flare‐related shock and combine these measurements with the Los Alamos National Laboratory proton observations to obtain iron/hydrogen density ratios and iron/hydrogen velocity differences. We place special emphasis on the postshock shell of turbulent and compressed ambient solar wind and interplanetary magnetic fields constituting the “sheath region” preceding the driver plasma. It is generally expected that the abundance ratios and charge states of the solar wind ions in the driver plasma and the sheath remain distinct, reflecting their different origins in the solar corona. However, while the Fe/H abundance ratios observed in the sheath are consistent with the preshock solar wind values (and a factor of 2 to 6 times less than the values obtained in the driver plasma), the iron charge states observed in the sheath appear to indicate a transition between the lower charge states observed in the ambient (coronal hole associated) solar wind and the higher charge states observed in the driver plasma. These results may reflect X ray ionization of the solar wind plasma near the flare site, although other explanations are possible.
A boundary layer model for magnetospheric substorms
Authors: Rostoker Gordon , Eastman Tim
Published: February 2008

Magnetospheric substorm activity is presently understood as the combined effect of energy transport and dissipation which is directly driven through the solar‐terrestrial interaction and of the unloading of energy stored in the magnetotail. Many studies of particle and field signatures of substorms in the magnetotail have led to the proposal that a neutral line forms close to the Earth at the time of expansive phase onset with the plasma sheet population on the anti‐Earthward side of the neutral line ultimately being lost to the solar wind through the ejection of the plasmoid. While the near‐Earth neutral line model has provided a useful working model for substorm phenomenology, it is not a unique description for understanding the observations. The purpose of this paper is to present an alternative framework in which substorm effects in the magnetosphere can be understood. We contend that observations of magnetic field and plasma flow variations in the magnetotail can be explained in terms of the passage of the plasma sheet boundary layer over the satellite detecting the tail signatures. We shall show, by providing a plausible mapping scheme from the high‐latitude ionosphere to the magnetotail, that field‐aligned currents and particle acceleration processes on magnetic field lines threading the ionospheric Harang discontinuity lead to the distinctive particle and field signatures observed in the magnetotail during substorms. In particular, we shall demonstrate that edge effects of field‐aligned currents associated with the westward traveling surge can lead to the negative Bz perturbations observed in the tail that are presently attributed to observations made on the anti‐Earthward side of a near‐Earth neutral line. Finally, we shall demonstrate that our model is capable of providing a physical explanation of both the driven system and the loading‐unloading system whose combined effects provide the observed substorm perturbation pattern in the magnetosphere and ionosphere. In this context, a substorm expansive phase can be interpreted as the development of a Kelvin‐Helmholtz instability on magnetic field lines which thread the velocity shear zone separating the central plasma sheet and the plasma sheet boundary layer/low‐latitude boundary layer regions.
Pc 3 pulsation eigenperiod determination at low latitudes
Authors: Hattingh S. K. F. , Sutcliffe P. R.
Published: February 2008

A realistic method of calculating the eigenperiod of Pc 3 pulsations at low latitudes is discussed. Solution of the problem requires a magnetic field model and a model for the plasma distribution along the resonating field line. The calculated eigenperiods obtained using the dipole field model and the IGRF model are found to be similar. The inclusion of the F region O+ in the plasma distribution noticeably affects the calculated eigenperiod at low latitudes. This effect decreases with increasing L value. Pulsation periods obtained from recordings made at four stations lying on a geomagnetic meridian demonstrate the importance of including O+ in the plasma model if realistic periods are to be calculated at low latitudes.
Titania's opposition effect: Analysis of Voyager observations
Authors: Thomas P. C. , Veverka J. , Helfenstein P. , Brown Robert Hamilton , Johnson T. V.
Published: February 2008

Voyager 2 obtained images of Titania over phase angles ranging from 0.8° to 150° and at sufficient resolution to investigate the photometric behavior of different surface units. The large, relatively narrow opposition surge detected from Earth was confirmed and can be successfully modeled with Hapke's (1986) photometric theory. Opposition effects do not vary greatly between bright areas (craters and ejecta) and dark areas. The primary difference in phase behavior is that the brightness of the brighter areas decreases more slowly with increasing phase angle than does the brightness of the dark areas. Thus, contrary to the situation on the Moon the fresher craters and their ejecta become less prominent in relation to the background as opposition is approached. This effect is best explained by a modest difference in single‐scattering albedo. There is evidence that the angular width of the opposition surge is slightly narrower for bright areas, a situation consistent with the interpretation that the ejecta regolith is somewhat less compacted than that in the background dark terrain.
Overview of the Uranian radio emissions: Polarization and constraints on source locations
Authors: Leblanc Y. , Aubier M. G. , Ortega‐Molina A. , Lecacheux A.
Published: February 2008

Radio emission from Uranus was discovered a few days before the closest approach of Voyager 2 with Uranus on January 24, 1986. The planetary radio astronomy experiment recorded several types of emissions in the frequency range 1–900 kHz; they differ by their spectral, temporal, and polarization characteristics. The analysis of the observations led us to distinguish two smooth radio components, one occurring in the lowest frequency range (SLF) from 20 kHz to 347 kHz, and the other one reaching the highest observed frequencies, i.e., 865 kHz (SHF). The SLF component is left‐hand polarized when observed in the northern magnetic hemisphere and right‐handed in the southern magnetic hemisphere, without polarization reversal from dayside to nightside. The SHF component is only observed in the nightside of Uranus, and it is left‐hand polarized. Several bursty emissions were observed: some of them are broadband bursts and occur in the 78‐ to 750‐kHz range (b‐bursty), others are narrow‐band bursts and occur in the 40‐ to 270‐kHz range (n‐bursty). The broadband bursts were observed at low magnetic latitudes of the spacecraft and mainly when it was in the southern magnetic hemisphere; their polarization is left‐handed. The narrow‐band bursts were observed in the northern magnetic hemisphere and are not correlated with the magnetic latitude of the spacecraft; their polarization is right‐handed. Sporadic bursts of short duration were observed mainly in the northern magnetic hemisphere with right‐hand polarization. Two periodic events were observed, ahead of the inbound bow shock and inside the magnetosphere of Uranus. Very low frequency emissions at 1.2 and 20.4 kHz are also discussed; some of them are correlated with in situ phenomena. From the polarization study we deduce the possible source locations of the main components in the frame of our hypothesis. The source regions are characterized by the latitude and longitude of the footprints of the magnetic field lines through the source expressed in the Uranographic longitude system (ULS). The SLF source would be fixed in magnetic longitude at the northern magnetic pole in the range 13°< latitudeULS<38°, 26°<west longitudeULS<70°, and 1.23<altitude (78 kHz)<1.39 RU. The emission mode would be ordinary. The SHF and b‐bursty components would be fixed in magnetic longitude at the southern magnetic pole. The SHF (at 300 kHz) would be in the range −52°< latitudeULS<−44°, 228°<west longitudeULS<256°, and 1.77<altitude<1.85 RU. The b‐bursty component would be located in a smaller region included in the SHF region in the range −48°<latitudeULS<−45° and 233°<longitudeULS<243°. For both high‐frequency components the emission mode would be extraordinary.
Plasma wave measurements in the magnetosphere of Uranus
Authors: Scarf F. L. , Gurnett D. A. , Kurth W. S. , Coroniti F. V. , Kennel C. F. , Poynter R. L.
Published: February 2008

As Voyager 2 traversed the magnetosphere of Uranus, the plasma wave instrument detected very significant phenomena related to local wave‐particle interactions, radio emissions, and dust impacts. Here we consider the region between the inbound and outbound bow shock traversals (covering a distance of more than 250 RU), and we demonstrate that intense plasma wave activity developed only in the inner magnetosphere (r < 12 RU); this result is similar to that found at Saturn, but it is in marked contrast with the Jupiter case, where very strong wave activity was detected out to distances of 250 RJ. The Uranus plasma wave observations in the inner magnetosphere are compared with corresponding results from the Jupiter and Saturn encounters, and it is shown that the Uranus wave measurements are unique in several significant ways. These new aspects include (1) the detection of a marked inbound‐outbound asymmetry and (2) the detection of whistler mode waves that yield the strongest wave‐particle interactions found in outer planet magnetospheres.
Proton and oxygen plasmas at Uranus
Authors: Cheng Andrew F.
Published: February 2008

Despite the presence of several large, icy moons within the Uranian magnetosphere, the Voyager 2 spacecraft did not detect any heavy ion plasma. This paper estimates the heavy ion density that would be consistent with a heavy neutral torus formed by photosputtering, charged particle sputtering, and micrometeoroid impact vaporization of icy surfaces on the moons, taking into account the large 60° angle between the satellite orbit plane and the magnetic equator. The expected heavy ion density is unobservably small. The observed proton plasma of the inner magnetosphere can be maintained by ionization of the atomic hydrogen corona and by the ionospheric proton source driven by photoelectron escape, for a plasma residence time of ∼30 days. These two proton sources are comparable near L = 5. The same 30‐day residence time for energetic protons implies an upper limit of 10−5 on the fraction of incident solar wind protons that enter the magnetosphere and penetrate to within L < 6 while conserving their first adiabatic invariants.
A study of mirror waves generated downstream of a quasi‐perpendicular shock
Authors: Lee L. C. , Price C. P. , Wu C. S. , Mandt M. E.
Published: February 2008

A large ion temperature anisotropy (Ti⊥/Ti║ ≃ 2−3), which may lead to the generation of mirror waves, is found to exist downstream of a quasi‐perpendicular shock simulated by a one‐dimensional hybrid code. In the case of the Earth's bow shock, large amplitude (δB ∼ B0) mirror waves are found to develop approximately 0.5‐1 RE (Earth radius) downstream of the shock ramp. It is found that the instability criterion for mirror waves in the downstream region is satisfied for shocks with a large Alfvén Mach number (MA ≳ 3).
On the possible role of the reaction O + HO2 → OH + O2 in OH airglow
Authors: Kaye Jack A.
Published: February 2008

Several workers have concluded that analysis of the OH infrared emission from the upper mesosphere and lower thermosphere requires population of OH(υ = 5, 6) by the reaction O + HO2 → OH(υ ≤ 6) + O2. It has been stated previously that this may be unlikely if the reaction proceeds by O atom transfer rather than H atom transfer. We show here that recent laboratory evidence argues convincingly that the mechanism of this reaction involves O atom transfer after formation of an HO3 complex and that little or no population of highly vibrational levels (υ ≥ 4) of OH are expected, although nonnegligible population of lower levels seems plausible. On this basis it appears that this reaction should not be contributing substantially to emission from OH(υ = 5, 6). The reverse reaction, OH(υ ≥ 6) + O2 → HO2 + O, which has been suggested to be a potential sink for OH(υ ≥ 6), is considered and is expected to be slow unless an HO3 complex is formed, in which case, multiple quantum deactivation of OH is also a likely process. Recent experimental evidence suggesting that OH(υ = 4, 5) may be populated by the H + O3 → OH + O2 reaction is discussed, although no firm conclusions about its importance can be reached.
Dayside long‐period magnetospheric pulsations: Solar wind dependence
Authors: Junginger Hans , Baumjohann Wolfgang
Published: February 2008

The spectral power and occurrence rate of long‐period magnetospheric pulsations (predominantly fundamental mode toroidal Pc 5) observed by the electron beam experiment on board GEOS 2 are compared with IMF and solar wind parameters. No clear influences of IMF orientation and magnitude on pulsation amplitudes and occurrence rate are found. Significant correlations, however, do exist between the spectral power of pulsations and the solar wind bulk velocity, and between the spectral power and the solar wind kinetic energy flux. The sign of the latter correlation depends on the Kp index. For Kp = 0 the pulsation power decreases with increasing solar wind kinetic energy flux, whereas it increases for Kp ≥ 1. Our results are consistent with the Kelvin‐Helmholtz instability at the inner side of the low‐latitude boundary layer being the dominant mechanism for the generation of fundamental mode toroidal Pc 5 magnetospheric pulsations. Flux transfer events play only an inferior role as energy sources for these pulsations. The coupling efficiency of surface waves at the boundary layer to shear Alfvén waves near geostationary orbit seems to change significantly if the geostationary orbit is inside the plasmasphere.
Alfvén waves from an electrodynamic tethered satellite system
Authors: Estes Robert D.
Published: February 2008

The operation of an electrodynamic tethered satellite system in the ionosphere in a steady state (constant current) mode is intimately connected with the electromagnetic plasma waves the system excites, since these waves carry the electrical currents that complete the tethered system/plasma circuit. The analysis reported here determines (within cold plasma theory) the ionospheric currents and associated wave impedance based on a tethered system current distribution that takes into account the “dumbbell” geometry of the system. The analysis is not restricted to terminating satellite dimensions that are much greater than the system's velocity divided by the ion cyclotron frequency, as as been the case in almost all previous studies of the problem. When this condition is not satisfied, the “Alfvén wings” (field line sheet currents) are found to be wider than the dimensions of the terminating satellite, a result of the wave nature of the phenomenon and restrictions on perpendicular wavelengths along the line‐of‐flight direction. Previously reported large wave impedances associated with an orbiting wire, which would limit tether currents to low values, are shown to be inapplicable to a more realistic model of the tethered system. The calculated wave impedance, which depends weakly on the system dimensions, is only a few ohms or less in low Earth orbit.
Evolution of F region barium releases: Phenomenological effects of background turbulence
Authors: Hansen Paul J. , Henley John R.
Published: February 2008

The effects of a background of stationary, homogeneous, isotropic turbulence on barium releases in the F region of the ionosphere are found by inclusion of such turbulence in previously derived nonlinear model equations. An approach similar to quasi‐linear theory yields an enhanced diffusion coefficient for the release. It is shown that this turbulent diffusion can account for the large length scales observed in F region barium releases.
The latitude and radial dependence of shock acceleration in the heliosphere
Authors: Gold R. E. , Decker R. B. , Krimigis S. M. , Lanzerotti L. J. , Maclennan C. G.
Published: February 2008

Voyagers 1 and 2 have observed interplanetary shock acceleration of ≲2 MeV ions at radial distances of >25 AU and heliographic latitudes of up to ∼30°. The previously reported negative latitude gradient in the intensities of accelerated ions at these energies persists over the entire range of latitudes and radial distances measured. The energy spectra of the shock acceleration events at Voyagers 1 and 2 are similar. A dramatic decrease in the flux of ≲ 2 MeV ions in the outer heliosphere was observed by both Voyager spacecraft in 1985. The decrease was observed at ∼30° as well as in the solar equatorial plane. At the time of the decrease, Voyager 1 was at ∼22 and ∼6 AU further from the sun than Voyager 2. However, the decrease began within an ∼12 day interval at both spacecraft. Comparisons with data from the IMP 8 spacecraft near Earth show that the decrease propagated outward from the Sun at ∼570 km/s. The timing of the large decrease between the two Voyager spacecraft implies a latitude gradient in the velocity of interplanetary shocks of ∼4.2 km/s per degree of latitude above the solar equator.
E region coherent backscatter at short wavelength and large aspect angle
Authors: Moorcroft D. R. , Schlegel K.
Published: February 2008

During a special experiment carried out in June 1987, the European incoherent scatter UHF radar system was operated at an unusually low elevation angle, and detected coherent echoes from the auroral E region at times when the ion drift velocity in the F region was large. This appears to be the first time that coherent echoes have been detected at such a short radar wavelength (0.32 m; frequency 933 MHz) and at such a large angle away from perpendicular to the Earth's magnetic field (up to 6°).
Asymmetries in the location of the Venus ionopause
Authors: Phillips J. L. , Luhmann J. G. , Knudsen W. C. , Brace L. H.
Published: February 2008

Four large data sets of Venus ionopause observations, based on different definitions and observations by Pioneer Venus orbiter field and particle instruments, are compared. Regardless of the definition chosen, the ionopause is observed to be higher on average at dawn than at dusk. The results of an empirical ionosphere model and a data analysis technique which minimizes orbital bias are combined to show that most or all of this asymmetry results from the effects of orbital bias and uneven sampling of solar wind conditions. However, an additional asymmetry is found in which the ionopause is displaced in the direction opposite to the motional electric field of the solar wind. This effect is attributed to asymmetric pickup of exospheric ions by the electric and magnetic fields in the magnetosheath. This ion pickup interpretation can be understood in terms of a previously observed magnetosheath magnetic field asymmetry. Quantitative analysis indicates that the transverse momentum imparted by the solar plasma to pickup particles in their initial gyration is transmitted by the magnetosheath magnetic field to the ionosphere boundary.