bluuebunny/crossref_metadata_2025 huggingface dataset:
The variation with Te and Ti of the velocity of unstable ionospheric two‐stream waves
Authors: Farley Donald , Providakes Jason
Published: February 2008

It is generally accepted that unstable ionospheric plasma waves moving at the ion‐acoustic velocity (“two‐stream” waves) are responsible for the so‐called type 1 VHF radar echoes commonly observed at equatorial and auroral latitudes. These same waves apparently are also the source of type 4 echoes, which have sharply peaked spectra with unusually large Doppler shifts and are seen at auroral latitudes during sufficiently disturbed conditions. But how, exactly, is the observed Doppler shift, or equivalently the ion‐acoustic velocity Cs, related to the electron and ion temperatures? The expression usually quoted, with occasional caveats, is the isothermal result Cs² = K(Te + Ti)/mi. The validity of the isothermal assumption has not been of much concern until recently, when the first simultaneous independent measurements of the temperatures and Cs were made in Scandinavia. We argue here that, in fact, the electrons should usually be treated as adiabatic, with three degrees of freedom, while the ions may or may not be adiabatic (with only one degree of freedom), depending upon the temperatures, the altitude, and the radar frequency. In other words, the ion effects generally should be calculated kinetically. The differences between the two models in the computed wave velocity are substantial (∼20–40%). A comparison between European Incoherent Scatter (EISCAT) temperatures and wave velocities measured with the Cornell University Portable Radar Interferometer (CUPRI) shows good agreement with the model given here.
Upstream waves at Uranus
Authors: Russell C. T. , Lepping R. P. , Smith C. W.
Published: February 2008

Since the Mach number of the solar wind increases with increasing heliocentric distance, the ratio of thermal to magnetic pressure, or beta, of the Uranian magnetosheath is expected to be much higher than in the terrestrial magnetosheath. Consistent with this expectation the magnetosheath is observed to be extremely turbulent, and we may expect many particles to leak back upstream into the solar wind and/or be scattered from the bow shock. In accord with the expected presence of backstreaming particles, waves of the type associated with terrestrial backstreaming particles are seen outbound along the trajectory of Voyager in the pre‐shock solar wind with frequencies close to 10−3 Hz. The wave frequency is close to that expected for upstream waves based on measurements closer to the Sun. Upstream from the bow shock the magnetic field was found to be much weaker than expected from observations in the inner solar system. The cause of this depression is unlikely to be the upstream particles; rather the cause is probably intrinsic to the solar wind such as reconnection across the heliospheric current sheet.
Model of the high‐latitude ionospheric convection pattern during southward interplanetary magnetic field using DE 2 data
Authors: Hairston M. R. , Heelis R. A.
Published: February 2008

Data from the polar‐orbiting satellite DE 2 are used to calculate one‐dimensional electrostatic potential distributions across the polar cap region. Using passes that lie within ±3 hours MLT of the dawn‐dusk line, various parameters of the polar potential distribution (location and magnitude of the maxima and minima, location of the zero potential point, etc.) are analyzed in relation to each other and to the interplanetary magnetic field (IMF). The resulting dependences are used to derive a two‐dimensional model of the distribution of the electrostatic potential in the high‐latitude ionosphere during times of southward IMF. This model can be generated using as inputs either the ionospheric potential parameters or, based on the relationships analyzed here, the IMF conditions. The capabilities of the resulting mathematical model are illustrated, and the importance of retaining a flexibility in the model to accommodate individual observations is emphasized.
Magnetic field line draping in the plasma depletion layer
Authors: Sibeck D. G. , Lepping R. P. , Lazarus A. J.
Published: February 2008

Magnetosheath magnetic field lines drape about the magnetosphere. Even when the interplanetary magnetic field lies parallel to the ecliptic plane, the magnetosheath magnetic field just outside the dawn and dusk flanks of the magnetosphere is deflected far out of the ecliptic plane. We present simultaneous IMP 8 solar wind and ISEE 1/2 observations for a northern dawn ISEE 1/2 magnetopause crossing on November 6, 1977. During this crossing, ISEE 1/2 observed quasi‐periodic pulses of magnetosheathlike plasma on northward magnetic field lines. The ISEE 1/2 observations were originally interpreted as evidence for strong diffusion of magnetosheath plasma across the magnetopause and the Kelvin‐Helmholtz instability at the inner edge of the low‐latitude boundary layer. An alternate explanation, in terms of magnetic field merging and flux transfer events, has also been advocated. We provide a third interpretation in terms of quasi‐periodic magnetopause motion which causes the satellites to repeatedly exit the magnetosphere and observe draped northward magnetosheath magnetic field lines in the plasma depletion layer.
Average spatial distributions of energetic particles in the midaltitude cusp/cleft region observed by Viking
Authors: Kremser G. , Lundin R.
Published: February 2008

The cusp/cleft region provides an entrance for magnetosheath particles into the magnetosphere and a sink for magnetospheric particles. In addition, strong acceleration and/or scattering of particles takes place. The Swedish satellite Viking crossed this region at midaltitudes. Measurements from this spacecraft were used to determine the average spatial distributions of H+ and He++ ions in the energy/charge range 2 keV/e ≤ E/Q ≤ 60 keV/e and of electrons with 7 keV ≤ E ≤ 97 keV. The data supply information on the structure of the midaltitude cusp/cleft region, the particle sources, and dynamical processes. Four different parts can be distinguished: (1) The cusp extends from about 76° to 82° invariant latitude (INL) and from 0800 to 1400 MLT. It is characterized by the presence of magnetosheath origin particles and important electron acceleration signatures. The magnetosheath particles presumably enter via the entry layer. (2) A smaller region inside the cusp (77°–82° INL, 1000–1330 MLT) contains magnetosheath origin ions without electron acceleration. This is regarded as the cusp proper connected to the exterior cusp. (3) Poleward of the cusp magnetosheath origin ions are still present, but no magnetosheath electrons. This region is related to the plasma mantle. (4) Another region without magnetosheath origin ions but with strong electron acceleration extends equatorward of the cusp and probably constitutes part of the cleft, likely to be connected to the low‐latitude boundary layer. The observations provide evidence for the interaction of energetic particles with the magnetopause current layer in which ions are accelerated and the particles are scattered on field lines threading through the cusp/cleft region.
On the plasma environment of solitary waves and weak double layers
Authors: Koskinen Hannu E. J. , Lundin Rickard , Holback Bengt
Published: February 2008

Observations of charged particles during events of solitary waves and weak double layer‐like structures by the Swedish magnetospheric research satellite Viking are discussed. The observations took place in relatively weak acceleration regions at altitudes between 1 and 2 RE. The total accelerating potential was typically somewhat less than 1 kV. The characteristics of ion and electron spectra indicate that the main part of the potential was below the satellite but some field‐aligned acceleration of electrons had also taken place above it. The composition of the ion beams was dominated by protons with a variable concentration (0–30%) of oxygen ions. While it is difficult to determine the exact parameters of the background plasma, there is clear evidence of the presence of cool background electrons and ions. It is proposed that the weak double layer‐like structures are structures in the cool ambient ions and convect slowly upward with the ion population. On the other hand, the structures accelerate auroral electrons toward Earth and beam ions from the ionosphere upward. It is suggested that the structures are likely to contribute to the perpendicular heating of the ion beams.
Observations of upflowing ionospheric ions in the mid‐altitude cusp/cleft region with the Viking satellite
Authors: Thelin B. , Aparicio B. , Lundin R.
Published: February 2008

Measurements of positive ions (0.1–10 keV) from the polar‐orbiting Viking satellite have been obtained in the cusp/cleft region and have been tabulated in MLT versus invariant latitude plots to get a synoptic picture of the occurrence of upgoing auroral positive ions. A distinction was made between ion distributions with peak fluxes along B (ion beam) and those exhibiting flux maxima that are not field‐aligned (conics). Both beams and conies are shown to be common auroral phenomena, whose frequencies of occurrence in MLT, invariant latitude, and altitude were studied. During the period of study (March‐June 1986) the ion beams were more frequent (about a factor of 2) in the dusk sector than in the dawn sector. The ion conies, however, were more frequent (about 2–3 times) in the dawn sector than in the dusk sector. This effect seemed to increase with magnetic activity but was mostly unchanged with the sign of the interplanetary magnetic field By component. An investigation was also made of the invariant latitude dependence for beams and conics, where the dawn sector beams have a tendency to be located toward higher invariant latitudes for both positive and negative By components. The ion beams were observed primarily above 5000 km with a frequency of occurrence increasing with altitude up to the satellite apogee at about 13,500 km. The ion conics were observed from above 7000 km to be steadily increasing in altitude.
A three‐dimensional time‐dependent model of the plasmasphere
Authors: Rasmussen C. E. , Schunk R. W.
Published: February 2008

A three‐dimensional, time‐dependent, nonlinear, hydrodynamic model of the plasmasphere has been developed which includes the self‐consistent coupling of conjugate hemispheres within the plasmasphere as well as the effects of cross‐L drifts due to convection electric fields. The inner region of the plasmasphere was modeled from an L of 1.5 to an L of 3.5 to 4.5, depending on local time. In this first study of the global plasmasphere, the model was run for solar minimum conditions until diurnally reproducible results were obtained, indicating that the plasmasphere was fully filled. The results of the model were then compared with applicable measurements of the plasmasphere. It was found that the model tends to overestimate densities somewhat, although it is within a factor of 2 of an average of June and December whistler observations. The model predicts densities within a factor of 2 of most of the satellite observations as well. As anticipated, the diurnal variation of the plasmasphere was found to depend on magnetic latitude. At low latitudes the diurnal variation in density was relatively small, with the largest densities occurring in the afternoon time sector. However, near the plasmapause, the effects of changes in volume of drifting tubes of plasma due to cross‐L drifts led to a factor of 3 variation in equatorial density, with the highest densities occurring near local midnight where the volume was lowest. Various limitations of the current model and suggested improvements are also discussed.
Electron beam injection during active experiments: 2. Collisional effects
Authors: Winglee R. M.
Published: February 2008

During active beam experiments, the presence of high neutral densities at low altitudes and/or during thruster firings has been observed to modify the spacecraft charging and the properties of the beam. Two‐dimensional (three velocity) electromagnetic particle simulations with ionizing collisions incorporated are used to investigate the modification of the beam‐plasma interaction as the neutral density is increased. It is shown that when the spacecraft is uniformly immersed in a neutral cloud, most of the ionization is produced by direct ionization by the beam and its secondaries, rather than via vehicle‐induced or wave‐induced ionization for the neutral densities considered. This ionization enhances the return current into the spacecraft with the spacecraft potential being reduced well below the beam energy when the collision period is shorter than the spacecraft charging time. Neutral densities of about 1011‐1012 cm−3 are required to produce this reduction in the spacecraft potential for typical ionospheric and beam parameters. At these densities, the beam is able to propagate away from the spacecraft with little distortion except in the case of thruster firings where the beam can be subject to large space‐charge oscillations near the boundaries of the neutral cloud. The ionization of neutrals in the beam region also modifies the wave emissions and spatial profile of return currents into the spacecraft, both of which tend to become localized to the beam region for high neutral densities. In the case where neutrals are not present in the beam region, for example, during a thruster firing away from the beam, most of the ionization is vehicle induced and higher neutral densities are required to produce the same reduction in spacecraft potential.
Postsunset rise of F layer height in the equatorial region and its relation to the F layer dynamo polarization fields
Authors: Goel M. K. , Singh S. S. , Rao B. C. N.
Published: February 2008

The ionosonde data are studied for equatorial station, Thumba, to delineate various features of the evening height rise of F layer. Sharp increase of h′F and hpF2 is observed in the postsunset period for high solar activity. Seasonal variation is observed in this increase of h′F and it is maximum for equinox months. For summer months, there is a delay of about an hour in the time of occurrence of h′F (peak) as compared to winter and equinox months. This delay is shown to be associated with the delay in sunset times in the conjugate E regions. As for magnetic activity dependence, it is found that this height increase is less pronounced for disturbed days for winter and equinox whereas for summer it is marginally higher over the quiet day values. Further, it is observed that the value of h′F (peak) during disturbed periods is almost at the same value of 350 km for all the three seasons. Thus the seasonal variation of magnetic activity effects appears to be mainly governed by the average seasonal variation for quiet times. The increase in F layer height is due to zonal eastward electric fields developed after sunset which is believed to be due to F region dynamo fields. While the main driving force for these fields is the zonal neutral winds, the development of these fields depend on the ratio of the F region to E region conductivity and the longitudinal gradient in the E region conductivity. Experimental observations of both the neutral winds and ionospheric conductivities are examined for their variation with solar activity, season and magnetic activity as both these factors will contribute for the various observed features of the height rise. Through model calculations it is shown that the E region density (conductivity) gradient in the postsunset period is higher by a factor of 2 for high solar activity compared to low solar activity and hence it is partly responsible for the observed solar activity variation of the postsunset height rise of the F layer. The importance of the studies using available ionosonde data for understanding the F region dynamo electric fields is emphasized.
Reconstructing the emission at 10.7 cm from individual active regions in terms of optical data
Authors: Oster Ludwig
Published: February 2008

A detailed statistical analysis of F10.7 emission and its relation to manifestations of optical activity as evidenced by plages and sunspots has been carried out. The data were selected from days with complete radio and optical coverage during the years 1976 to 1982. Starting with individual peaks as shown by east–west scans at 10.7 cm, we find an excellent correlation (0.93) with the reported spot area for the individual active region (AR) from which the radio flux emanated. The correlation with the reported plage area is 0.84; the combination of plage and spot areas (0.96) only slightly improves the correlation with spot area alone. An equally close correlation of F10.7 exists with the 3/4 power of spot areas. These results suggest that the F10.7 emission is tightly bound to the magnetic flux of ARs. Center‐to‐limb variations indicate that the emitting region at 10.7 cm can be described as an optically thin volume with, in first order, spherical symmetry. Next, calculated on the basis of the sum of individual areas, one finds the expected total emission from the Sun for a carefully selected sample to be of comparable quality of correlation with spot and plage areas (0.88 and 0.80, respectively). Several tests were carried out to determine time variations of the reconstruction parameters, in particular, concerning a possible dependence of the optical data on the choice of observing station. We were unable to find in the data any evidence for the existence of a significant third component in the F10.7 emission, that is, a component that is not localized yet still follows roughly the solar cycle variation (network contribution). Finally, we compared for our sample days these findings with published daily indices for solar activity, that is, plage index and relative sunspot number, and conclude that the latter, with a correlation of 0.94, is overall the most trustworthy stand‐in for F10.7 data.
The dynamics of the ionosphere of Mars at large solar zenith angles
Authors: McCormick P. T. , Whitten R. C.
Published: February 2008

A model of the ion chemistry and dynamics of the ionosphere of Mars at large solar zenith angles has been constructed using techniques previously developed for Venus. It is found that a substantial nighttime ionosphere should occur even when the ionopause height is as low as 250 km. At low altitudes the interaction of the plasma flow with the neutral atmosphere appears to result in considerable “noise” in the flow patterns. Transterminator plasma velocities are smaller than for Venus, although they may exceed the local sonic speed. The effect of a nocturnal source of suprathermal electrons is also discussed.
Wave excitation downstream of the low‐β, quasi‐perpendicular bow shock
Authors: Brinca A. L. , Sckopke N. , Paschmann G.
Published: February 2008

We interpret the low‐frequency wave observations of the AMPTE/IRM emission in the downstream region of a marginally supercritical, quasi‐perpendicular bow shock. A companion paper on ion heating comprehensively describes the corresponding plasma and wave magnetic field measurements. A simple model of the shocked plasma consisting of a background hydrogen magnetoplasma with anisotropic core protons permeated by a hotter dilute beam of anisotropic protons reproduces the basic features of the wave polarization and frequency spectra: wave activity below the proton cyclotron frequency, sometimes displaying a double‐humped structure, with unusual (in space plasmas outside the magnetosphere) dominance of (plasma frame) left‐hand‐polarized modes. The core and beam anisotropies stimulate electromagnetic (left‐hand) ion cyclotron waves in distinct spectral regions, whereas the large downstream Alfvén speed (with respect to the beam drift speed) precludes an efficient cyclotron resonant generation of the right‐hand magnetosonic modes.
Source location of the Jovian hectometric radiation via ray‐tracing technique
Authors: Ladreiter Hans Peter , Leblanc Yolande
Published: February 2008

Ray tracing using realistic magnetic field and plasma models has been performed to locate the Jovian hectometric radiation (HOM) sources. To consider only a few possible source locations, we have assumed that the radiation mechanism is the cyclotron maser instability (CMI), and taken into account the characteristics of the HOM including the recent results by Ladreiter and Leblanc (1989) (hereafter paper 1). The emission is assumed to escape from the planet in the extraordinary wave mode within a thin beam of hollow cones whose apexes are distributed uniformly in longitude at the northern and southern hemispheres at altitudes where the gyrofrequency fc is less but nearly equal to the observed wave frequency. The rays are traced from possible sources located at f/fc = 1.05 and labeled by the dipole shell parameter L. For each source, we have determined the emergence wave normal angle θcrit above which the rays no longer travel within the HOM emission beam nearby the magnetic equator (paper 1). We show that the HOM sources cannot be located on field lines within the Io flux tube. However, wave refraction due to the Io torus plays a crucial role for the propagation and beaming of the waves toward the magnetic equator. It is found that HOM sources located at field lines 15 ≤ L ≤ 30 agree best with the observations when assuming that the cone half angle of emission is about 70°–90° with respect to the local magnetic field. Thus, the source is associated with the tail field aurora where the magnetic field lines connect the polar region to the Jovian magnetic tail and is extended in altitude from 2 to 7 Jovian radii (RJ). Our results explain in detail the radio phenomenology of the hectometric radiation, in particular the strong latitudinal beaming, the lowest observed frequencies (≈ 40 kHz), the solar wind control, the absence of HOM emission when Voyager 1 was within the Io torus, and the absence of Io control on the HOM. The results are discussed in the context of the other Jovian radio sources, namely the sources of the non‐Io decametric and kilometric emissions.
On the topology of flux transfer events
Authors: Hesse Michael , Birn Joachim , Schindler Karl
Published: February 2008

We present a topological analysis of a simple model magnetic field of a perturbation at the magnetopause that shares magnetic properties with flux transfer events. The aim is to clarify a number of topological aspects that arise in the case of fully three‐dimensional magnetic fields. Since the properties discussed can be assumed to be typical for a large class of perturbations, it seems appropriate to illustrate them in terms of a field model that is assumed rather than computed from a special dynamic theory. We show that a localized perturbation at the magnetopause can in principle open a closed magnetosphere by establishing magnetic connections across the magnetopause by the formation of a ropelike magnetic field structure. For this purpose we consider a global topological model of a closed magnetosphere as the unperturbed state. When the perturbation is introduced, it will be evident that, although the model field exhibits neutral points, these are not involved in the localized changes of magnetic connection brought about by the flux rope. The topological substructure of the model flux rope is discussed in detail. The flux rope consists of a tangled subset of magnetic filaments of different magnetic connections. We shall find that the topological substructure of the flux rope becomes increasingly complicated for decreasing magnetic shear field at the magnetopause, i.e., when the inside and outside fields are getting close to antiparallel, leading to a topological mixture reminiscent of chaos. This chaotic mixture of field topologies may provide a mechanism for enhanced dissipation and thus suggests an explanation why magnetic reconnection could be more effective when the magnetosheath and magnetospheric fields are nearly antiparallel. The results bear also on the choice of an appropriate description of magnetic reconnection in flux transfer events.
Northward IMF and patterns of high‐latitude precipitation and field‐aligned currents: The February 1986 Storm
Authors: Rich F. J. , Hardy D. A. , Redus R. H. , Gussenhoven M. S.
Published: February 2008

On February 7, 1986, during a major geomagnetic storm the Bz component of the interplanetary magnetic field (IMF) turned strongly northward for several hours. Data from the Defense Meteorological Satellite Program F6 and F7 satellites and the HILAT satellite were used to study the evolution of the pattern of high‐latitude precipitation and field‐aligned currents in response to this change. Prior to the northward IMF period, the auroral zone was observed down to mid‐latitudes and was very wide in latitude, and strong, large‐scale “region 1/region2” currents were clearly present (ΔB ∼ 1700 nanoteslas). Following the northward turning, the equatorward boundary of the auroral zone on the nightside contracted sharply poleward and polar cap arcs were observed. The strength of the region 1/region 2 currents decreased markedly and became immeasurably small at the time of the maximum contraction of the auroral oval. An NBZ current system was observed to grow and expand in the southern (summer) high latitude region over a period of more than 2 hours. Simultaneously, an irregular pattern of field‐aligned currents was observed in the northern (winter) hemisphere. During the contraction, the latitudinal width of the auroral region mapping to the central plasma sheet (CPS) decreased dramatically while the width of the area mapping to the boundary plasma regions (BPR) in the magnetosphere increased greatly. At the time of the maximum contraction the BPR extended up to a latitude of at least 87.1°. The NBZ currents expanded with and were entirely located within the BPR precipitation. Polar cap arcs were observed in both regions of BPR precipitation and polar cap precipitation and were not correlated with the location of the large‐scale field‐aligned currents. There was no indication of the CPS intruding to high latitudes, and thus no evidence for bifurcation of the magnetotail. The boundary between the CPS and the BPR showed little change. If this implies that the boundary between open and closed field lines contracted slowly or not at all, then a significant portion of the observed BPR precipitation was observed along open field lines. We interpret the pattern of particle precipitation and field‐aligned current and the implied plasma convection in terms of a distorted, two‐cell convection pattern driven by merging of closed field lines with the IMF as proposed by Crooker [1988]. When the IMF turned southward again, the pattern quickly reversed. The BPR contracted; the CPS precipitation regions expanded; the equatorward boundary of the auroral oval moved to lower latitudes; the NBZ currents disappeared in less than 30 min; and the region 1/region2 currents reappeared. Again, the BPR/CPS boundary did not move as rapidly and thus may indicate that the changes are due more to a reconfiguration within the magnetosphere than a change in the portion of the magnetosphere that is open or closed.
The excitation of plasma convection in the high‐latitude ionosphere
Authors: Lockwood M. , Cowley S. W. H. , Freeman M. P.
Published: February 2008

Recent observations of ionospheric flows by ground‐based radars, in particular by the European Incoherent Scatter (EISCAT) facility using the “Polar” experiment, together with previous analyses of the response of geomagnetic disturbance to variations of the interplanetary magnetic field (IMF), suggest that convection in the high‐latitude ionosphere should be considered to be the sum of two intrinsically time‐dependent patterns, one driven by solar wind‐magnetosphere coupling at the dayside magnetopause, the other by the release of energy in the geomagnetic tail (mainly by dayside and nightside reconnection, respectively). The flows driven by dayside coupling are largest on the dayside, where they usually dominate, are associated with an expanding polar cap area, and are excited and decay on ∼10‐min time scales following southward and northward turnings of the IMF, respectively. The latter finding indicates that the production of new open flux at the dayside magnetopause excites magnetospheric and ionospheric flow only for a short interval, ∼10 min, such that the flow driven by this source subsequently decays on this time scale unless maintained by the production of more open flux tubes. Correspondingly, the flows excited by the release of energy in the tail, mainly during substorms, are largest on the nightside, are associated with a contracting polar cap boundary, and are excited on ∼1‐hour time scales following a southward turn of the IMF. In general, the total ionospheric flow will be the sum of the flows produced by these two sources, such that due to their different response times to changes in the IMF, considerable variations in the flow pattern can occur for a given direction and strength of the IMF. Consequently, the ionospheric electric field cannot generally be regarded as arising from a simple mapping of the solar wind electric field along open flux tubes.
Low‐latitude thermospheric neutral winds determined from AE‐E measurements of the 6300‐Å nightglow at solar maximum
Authors: Burrage M. D. , Fesen C. G. , Abreu V. J.
Published: February 2008

Atmosphere Explorer E (AE‐E) measurements of the O(¹D) 6300‐Å emission in the nighttime equatorial thermosphere are used to infer the height of the F2 layer peak as a function of latitude and local time. The investigation is conducted both for northern hemisphere winter solstice and for spring equinox, under solar maximum conditions. The layer heights are used to derive magnetic meridional components of the transequatorial neutral wind, in conjunction with the MSIS‐86 model and previous Jicamarca incoherent scatter measurements of the zonal electric field. The AE‐E wind estimates indicate a predominant summer to winter flow for the winter solstice case. Comparisons are made with the empirical horizontal wind model HWM87 and with winds generated by the thermospheric general circulation model. The model predictions and experimental results are generally in good agreement, confirming the applicability of visible airglow data to studies of the global neutral wind pattern. Perhaps the most significant discrepancy occurs near midnight, where some of the data indicate a sharp decrease in the magnitude of the neutral wind. This feature, which has been observed in previous experimental studies, is not reproduced by either of the two models.
The role of high‐speed plasma flows in plasmaspheric refilling
Authors: Guiter S. M. , Gombosi T. I.
Published: February 2008

A model of time‐dependent one‐stream interhemispheric plasma flow is used to investigate plasmaspheric refilling. In the model the coupled time‐dependent hydrodynamic equations (continuity, momentum and energy) of a two‐ion (H+ and O+), quasi‐neutral, currentless plasma are solved for a closed geomagnetic field line. For the present set of calculations an L = 2 field line was used. A steady state solution was found and used as the initial condition in subsequent simulations of the effect of density depletions. Density depletions were modeled by reducing the densities by an arbitrary factor above 2500 km altitude, while keeping the velocities and temperatures unchanged. Shock structures develop which move up the field line, meeting and reflecting at the equator. The reflected shocks are absorbed by the dense neutral atmosphere when they reach the model flux tube boundaries. After the upwelling streams collide, refilling occurs from the equator downward for 2 to 3 hours; thereafter there is steady refilling from the ionospheres upward.
A drift‐kinetic treatment of the parametric decay of large‐amplitude Alfvén waves
Authors: Inhester B.
Published: February 2008

We derive a kinetic formulation for the parametric decay instability of a large‐amplitude, circularly polarized Alfvén wave based on a drift‐kinetic description of the plasma. The formulation is not restricted to a Maxwellian ion plasma but can be used for arbitrarily shaped ion distribution functions, e.g., those measured in the solar wind. We find that the MHD equations of the instability are recovered for the case that the ions are cold and the electrons carry all the thermal plasma energy. Even a small amount of ion thermal energy reduces the maximum growth rates of the instability below those obtained in an MHD treatment and broadens the unstable wave number range. Conversely, the presence of an ion beam or a plateau in the ion distribution function at velocities below the Alfvén velocity may give rise to enhanced growth rates. In the solar wind, the efficiency of the parametric decay instability is reduced both by kinetic effects and by the short correlation time of the driving Alfvén waves so that the instability probably cannot compete with the nonlinear energy cascade in shaping the turbulent energy spectra over short distances. However, the parametric decay instability may have an important influence on the reduction of the cross‐helicity in the solar wind observed over large scales of the order of 1 AU.
Magnetosonic waves above fc (H+) at geostationary orbit: GEOS 2 results
Authors: Laakso H. , Junginger H. , Roux A. , Schmidt R. , de Villedary C.
Published: February 2008

The fast magnetosonic waves in the frequency range 1–11 Hz (i.e., above the proton gyrofrequency Ωp) are considered. The electric wave field δE (mostly less than 0.5 mV/m) has an elliptic polarization in the plane perpendicular to B. While the magnetic wave field δB is strictly polarized along the ambient magnetic field, the ellipticity and the sense of rotation of the electric field polarization ellipses are crucial parameters for the identification of the wave mode. The waves are right‐hand polarized, and the ellipticity is usually about 0.2. The main axis of the ellipse is close to the azimuthal direction. The wave vector k is almost parallel to δE, and thus the fast magnetosonic waves propagate at small angles to the azimuthal direction. The wavelengths are usually between 150 and 300 km. The δE/δB ratio of the waves is between νA and υA(1 + ω²/Ωp²)1/2, where υA is the Alfvén velocity. In one case a burst of pure electrostatic modes near the second harmonic of the proton gyrofrequency was observed.
On the importance of ion composition to conductivities in the auroral ionosphere
Authors: Moen Jøran , Brekke Asgeir
Published: February 2008

We have used incoherent scatter radar measurements in the polar ionosphere to study the effects of ion composition variations on the E and F region conductivities and conductances. We find that these variations are of minor importance and that during solar minimum conditions the electron density above ∼170 km contributes negligibly to the Hall and less than 20% to the Pedersen conductance. This supports similar results obtained by the Chatanika and Søndre Strømfjord radars but is in contrast to theoretical predictions.
Geometry of the near‐Earth plasma sheet
Authors: Baumjohann Wolfgang , Paschmann Götz
Published: February 2008

About 275,000 plasma measurements with the AMPTE/IRM satellite in the magnetotail are used for a statistical survey on the geometry of the near‐Earth plasma sheet. Near the midnight meridian the plasma sheet has a typical thickness of 6 RE; at its flanks it is more than twice as thick. The average plasma sheet thickness does not vary much with |XGSM| during disturbed intervals, but during quiet periods the plasma sheet is considerably thinner at greater distances. The best chances to see the plasma sheet boundary layer are near |ZNS| ≈ 5 RE, except during quiet times for |XGSM| > 15 RE. In the latter case, the plasma sheet boundary layer is much closer to the neutral sheet and the central plasma sheet may often be thinner than 2 RE.
A plasma density model for Saturn based on Voyager observations
Authors: Richardson John D. , Sittler E. C.
Published: February 2008

The density, temperature, and velocity of plasma in the inner magnetosphere (L<12) of Saturn are well determined from observations by the Voyager plasma science experiment. We combine the ion and electron data sets from both Voyager flybys to produce an overview of the Saturnian plasma environment. A small spacecraft potential of −10 to −20 V is found to be a plausible mechanism for reconciling differences between the observed ion and electron densities. Electron and ion measurements from the Voyager 1 and 2 encounters are combined to produce a map of density contours inside L = 12. This contour map incorporates all the available Voyager thermal plasma data in this region under the assumption that the inner magnetosphere was stable during the nine months between encounters. In the model, heavy ions are confined to a narrow region near the equator because of their large temperature anisotropy and their low thermal velocity parallel to the magnetic field. The protons are spread much more widely in latitude. The model does not satisfactorily reproduce the densities of the suprathermal electrons observed inside L =7 , but these constitute only a few percent of the total electron density. The oxygen flux tube content decreases rapidly inside L = 5, indicating that losses occur in this region. The lifetime of neutral atoms in the inner magnetosphere is a strong function of latitude, with lifetimes varying from weeks to years.
Toward dynamic modeling of the outer electron radiation belt
Authors: Chiu Y. T. , Rinaldi M. A. , Nightingale R. W.
Published: February 2008

We show that the general solution to a simple form of the simultaneous bimodal (radial and pitch angle) diffusion equation can represent the SCATHA outer belt electron distribution to a high degree of accuracy for periods up to about 10 hours. The data representation requires diffusion parameters that are in agreement with all previous experimental and theoretical determinations of such diffusion coefficients. Such a representation satisfies the basic requirements toward the dynamic modeling of the outer electron radiation belts for L < 7 at quiet and moderate geomagnetic conditions. For L ∼ 7 the representation has difficulties with “butterfly” distributions that may signify energetic electron encounters with the magnetopause or severe shell splitting.
Dual sounding rocket observations of low‐altitude electrostatic shocks
Authors: Boehm M. H. , Carlson C. W. , McFadden J. P. , Mozer F. S.
Published: February 2008

Two sounding rockets were launched near simultaneously into the late evening auroral zone from Poker Flat and Fort Yukon, Alaska, on February 7, 1984, placing two payloads on nearly the same magnetic field lines simultaneously. The Black Brant X payload, with an apogee of 1040 km, measured dc electric fields of ∼300 mV/m, while the lower altitude Terrier‐Malemute payload (apogee 640 km) measured only typical ionospheric electric fields under ∼100 mV/m. These measurements imply a parallel potential of several kV below the higher payload, most probably occurring between the two payloads. The exact value of the parallel potential is dependent on the motion of the potential structure with respect to the payload. There is some evidence of an ∼2‐kV potential drop between the payloads in the electron data. Nonadiabatic electron trajectories are also implied. No possible resistive mechanism to support the parallel field is observed in the 100‐Hz to 10‐kHz frequency band. The existence of resistance‐producing turbulence at other frequencies or of a type not measured by the probes, or localized parallel potentials not coincident with the payload trajectories, are thus implied. The observation of apparent “secondaries” at all altitudes implies either a potential barrier of hundreds of electron volts that trapped these electrons above the region of upward electric field or that the secondaries were actually locally accelerated electrons originating out of the cold background. The plasma conditions encountered at 1000 km are unusual for sounding rockets. During the highest electric fields and lowest densities, a 5.8‐m antenna measured significantly larger dc electric fields than a 3‐m antenna. Internal consistency checks, the near‐sinusoidal nature of the spin‐dependent waveform, and the >10 λDebye separation of the booms from the payload lead to the conclusion that the longer boom measurements were correct, the payload wake having interfered with the shorter antennas.
Modeling of atmospherically induced gas phase optical contamination from orbiting spacecraft
Authors: Elgin J. B. , Cooke D. C. , Tautz M. F. , Murad Edmond
Published: February 2008

We present in this paper results of a predictive code (SOCRATES: spacecraft/orbiter contamination representation accounting for transiently emitted species) which has been developed to assess the effects of contamination on measurements aboard spacecraft in low Earth orbit. SOCRATES is a Monte Carlo code which includes in its present version scattering, collisions leading to kinetic‐to‐vibrational energy transfer, and reactive collisions. The application of this code to actual measurements aboard spacecraft in low Earth orbit makes it possible to evaluate data obtained on these platforms with a view toward extracting the data of interest from contaminated signals. Molecules considered in the present study include CO2, H2O, OH, H2, and CO.
The interaction of an artificial electron beam with the Earth's upper atmosphere: Effects on spacecraft charging and the near‐plasma environment
Authors: Neubert T. , Banks P. M. , Gilchrist B. E. , Fraser‐Smith A. C. , Williamson P. R. , Raitt W. J. , Myers N. B. , Sasaki S.
Published: February 2008

The ionization created in the Earth's upper atmosphere by an electron beam emitted from a low altitude spacecraft is described by two coupled nonlinear differential electron transport equations for the up‐going (along a magnetic field line) and down‐going differential energy flux. The equations are solved numerically, using the mass spectrometer and incoherent scatter data (MSIS) atmospheric model and the international reference ionosphere (IRI) model, yelding estimates of the differential electron energy flux density at the spacecraft location. At altitudes below 200–250 km and for beam energies around 1 keV, it is shown that secondary electrons supply a significant contribution to the return current to the spacecraft and thereby reduce the spacecraft potential. Our numerical results are in good agreement with observations from the Charge 2 sounding rocket experiment.
On WKB expansions for Alfvén waves in the solar wind
Authors: Hollweg Joseph V.
Published: February 2008

We reexamine the WKB expansion for “toroidal” Alfvén waves in the solar wind, as described by equations (9) of Heinemann and Olbert (1980). Our principal conclusions are as follows: (1) The WKB expansion used by Belcher (1971) and Hollweg (1973) is nonuniformly convergent. (2) Using the method of multiple scales (Nayfeh, 1981), we obtain an expansion which is uniform. (3) The uniform expansion takes into account the small modification to the Alfvén wave phase speed due to spatial gradients of the background. (4) Both the uniform and nonuniform expansions reveal that each “normal mode” has both Elsässer variables δz+ ≠ 0 and δz− ≠ 0. Thus if δz− corresponds to the outgoing mode in a homogeneous background, an observation of δz+ ≠ 0 does not necessarily imply the presence of the inward propagating mode, as is commonly assumed. (5) Even at the Alfvén critical point (where V = υA) we find that δz+ ≠ 0. Thus incompressible MHD turbulence, which requires both δz+ ≠ 0 and δz− ≠ 0, can proceed at the Alfvén critical point (cf. Roberts, 1989). (6) With very few exceptions, the predictions of these calculations do not agree with recent observations (Marsch and Tu, 1990) of the power spectra of δz+ and δz− in the solar wind. Thus the evolution of Alfvén waves in the solar wind is governed by dynamics not included in the Heinemann and Olbert equations.
A Pincer‐shaped plasma sheet at Uranus
Authors: Hammond C. Max , Walker Raymond J. , Kivelson Margaret G.
Published: February 2008

A model from Voigt et al. (1987) and an MHD simulation from Walker et al. (1989) both show that the curvature of the plasma sheet at Uranus changes as the dipole tilt varies between 38° and 22°. The models suggest that one of the two partial traversals of the uranian plasma sheet made during the outbound trajectory of Voyager 2 can be explained as an entry into the highly curved plasma sheet that develops when Uranus is near the maximum dipole tilt value of 38°; previously both partial traversals have been explained as anomalous. The spacecraft would have reversed its motion relative to the plasma sheet as the continued rotation diminished the dipole tilt and the retreating plasma sheet uncurled. As the dipole tilt approached its minimum value, spacecraft motion towards the neutral sheet resumed and the traversal of the plasma sheet was completed. Evidence from the PWS plasma wave detector suggests that the spacecraft trajectory skimmed the plasma sheet boundary layer for several hours prior to the partial immersion. The plasma sheet of the Voigt et al. model was not located near the spacecraft during this time interval. On the other hand, the MHD simulation reveals a plasma sheet that is more curved than in the Voigt et al. model; near maximum dipole tilt, the plasma sheet is pincer‐shaped. The unusual geometry implies that Voyager 2 remained near the plasma sheet boundary layer during the period suggested by the PWS data. Thus the simulation accounts easily for the first of the plasma sheet encounters previously called anomalous. The second partial immersion remains anomalous, having previously been related to substorm activity, and thus is not discussed here. The stagnation distances of the earth and Uranus at the nose of the magnetopause were used to scale the Walker et al. (1989) simulation of the terrestrial magnetosphere to represent the uranian magnetosphere. The rescaled model was found to predict the radial extent of the magnetotail of Uranus remarkably well. Previous studies which have suggested that the uranian magnetotail is blunter than that of the earth have not compared tail widths at an appropriately scaled distance down the tail.
Transition from ring to beam arc distributions of water ions near the Space Shuttle Orbiter
Authors: Cairns Iver H.
Published: February 2008

The distribution function of water ions produced near the space shuttle by charge exchange between ionospheric oxygen ions and outgassed water molecules is investigated using solutions of Liouville's equation with a source term modeling the charge exchange process. A transition from ring distributions to beamlike distributions termed “beam arc” distributions is found with decreasing distance upstream from the orbiter. This beam arc distribution corresponds to a finite section of a ring distribution and not to a conventional beam distribution. The ratio of water ion number density to oxygen ion number density is calculated; typical values within 50 m of the shuttle are in excess of 2% with a maximum value of the order of 20% for nominal parameters, suggesting that these ions must be considered when interpreting particle data from near the space shuttle. An argument for a plasma density enhancement of the order of 10% very close to the shuttle, due to kinematic effects (corresponding to pileup of plasma) and not to plasma creation, is also presented. This kinematic density enhancement is insufficient, by an order of magnitude, to explain the plasma density enhancements inferred from Spacelab 2 data.
Plasma instabilities in the high‐latitude E region induced by high‐power radio waves
Authors: Chaturvedi P. K. , Ossakow S. L.
Published: February 2008

The effect of a high‐frequency powerful pump wave on high‐latitude E region low‐frequency plasma instabilities is theoretically considered. The growth rates and threshold criteria are calculated for instabilities associated with the electrojet (Farley‐Buneman, gradient drift) and with higher‐altitude parallel currents (ion acoustic, ion cyclotron, current convective). The results are discussed in the context of present ionospheric modification (heating) experiments.