bluuebunny/crossref_metadata_2025 huggingface dataset:
Early‐time plasma expansion characteristics of ionized clouds in the ionosphere
Authors: Schunk R. W. , Szuszczewicz E. P.
Published: February 2008

The collisionless expansion of high‐density Ba+, Li+, and Ba+‐Li+ plasma clouds into low‐density O+ background plasmas was studied using a one‐dimensional Vlasov‐Poisson model in order to elucidate the very early‐time plasma expansion characteristics. Simulations were conducted for a range of Ba+‐Li+ mixtures, cloud/background density ratios, and electron/ion temperature ratios. Simulations were also conducted for cloud expansions against both stationary and flowing O+ background plasmas to simulate gas releases from moving spacecraft. The following results were obtained from the numerical simulations: (1) The front of an expanding high‐density Ba+ cloud acts as an “electrostatic snowplow,” and both O+ density and temperature peaks are pushed ahead of the expanding Ba+ cloud. (2) The strength of the electrostatic snowplow is increased for elevated cloud electron temperatures. (3) The effect of a flowing O+ background plasma is too slow the Ba+ expansion and change the O+ response. For small O+ drift velocities the Ba+ snowplow still occurs, for moderate O+ drift velocities ion density peaks propagate into and away from the cloud, and for large O+ drift velocities the O+ plasma quickly penetrates the Ba+ cloud and there are small density perturbations. (4) The Li+ cloud expansion is faster than the Ba+ expansion by approximately the square root of the heavy‐to‐light ion mass ratio, and the Li+ electrostatic snowplow is weaker. (5) As with a Ba+ cloud, an expanding Li+ cloud pushes an O+ density enhancement ahead of it, but some of the light Li+ ions can penetrate this O+ enhancement with the result that an Li+ plateau forms and propagates ahead of the propagating O+ enhancement. (6) For Li+ plasma expansions against a rapidly drifting O+ plasma, the two plasmas quickly penetrate each other with minor density perturbations. (7) For Ba+‐Li+ cloud expansions, with Li+ minor, into an O+ plasma, the Ba+‐O+ interactions are not affected by the presence of the light minor ion. This expansion scenario is led by suprathermal forerunner Li+ ions, then a propagating Li+ density plateau, then a propagating O+ density peak, and finally the main front of the expanding Ba+ cloud. A linear instability analysis including a constant magnetic field indicates that some of the expansion scenarios with elevated electron temperatures are unstable. When the plasma is unstable, the ion‐ion acoustic wave is the most unstable mode. Depending on the conditions, waves can be excited in the expanding plasma cloud by penetrating O+ ions and in the background O+ plasma by penetrating cloud ions. The numerical simulations are limited in that the simulation domain is relatively small, the initial plasma density gradients are fairly steep, the expansions are one‐dimensional, collisions are ignored, and only very early times are simulated.
Generation of superthermal electrons by electron beams
Authors: Rowland H. L.
Published: February 2008

A series of computer simulations is reported studying the coupling of a continuously injected cold electron beam into a warm background plasma. The simulations are performed in a finite length nonperiodic system. It is shown that strong turbulence determines the coupling of the beam to the ambient plasma and leads to the formation of superthermal electrons with velocities comparable to that of the injected beam. These electrons are accelerated both in the direction of the beam and back toward the gun. The superthermal electrons can carry return currents greater than the beam current. For active beam injection experiments in space, these particles provide a direct indication of the presence of the beam plasma interaction.
stability of a spherical shell distribution of pickup ions
Authors: Freund H. P. , Wu C. S.
Published: February 2008

The stability of a spherical shell (complete or incomplete) is investigated. The study is motivated by the research of cometary pickup ions. The present discussion is restricted to low‐frequency electromagnetic waves propagating along the ambient magnetic field. It is found that a complete shell is stable, whereas an incomplete shell can be unstable. The numerical results are reported and discussed.
Ion beam driven electrostatic shocks
Authors: Pottelette R. , Treumann R. A. , Bauer O. H. , Lebreton J. P.
Published: February 2008

Electrostatic shock waves have been detected during the injection of an artificial xenon ion beam into the collisionless ionospheric plasma. The beam was directed quasi‐perpendicular to the ambient magnetic field. The ion source was situated on a small ejectable payload. The properties of the plasma/ion beam interaction were recorded on the main payload as a function of the separation distance to the ion source. The shock structure develops at distances where the apparent azimuthal speed of the beam fan is comparable to the phase velocity of ion acoustic waves sustained by background O+ ions. The shock is characterized by a decrease in the local density up to about 10% of the background density, strong shear motion visible in the convection electric field, field‐aligned potential drops of the order of several tens of millivolts, increased electron temperatures, and fluxes of reflected ions. Probably suprathermal electron fluxes along the magnetic field were also present, but these have not been measured. The Mach number is determined to M = 1.09, and thus the shock is weak and turbulent as a result of ion acoustic turbulence which has been observed throughout the shock region. It is suggested that this turbulence is driven by an ion‐ion instability excited by reflected background ions. Estimates of the number density of reflected ions show that roughly 50% of the background ion population may be reflected from the shock potential. On a larger spatial scale these reflected ions may be responsible for the excitation of the observed lower hybrid and ion cyclotron harmonic waves in front of the leading edge of the beam. No shock has been found on the trailing edge of the beam, suggesting that different physical conditions apply there.
Generation and propagation of kilometric radiation in the auroral plasma cavity
Authors: Pritchett P. L. , Winglee R. M.
Published: February 2008

Two‐dimensional particle simulations and three‐dimensional ray‐tracing calculations are used to investigate the generation and propagation of electromagnetic radiation produced by the electron cyclotron maser instability under conditions that prevail in the auroral plasma cavity. The simulations incorporate a continual flow of primary energetic electrons along the magnetic field. In such a driven system the maser instability leads to a quasi‐steady state in which as much as 7% of the incident flux of electron energy is converted into radiation. The maximum radiation intensity occurs within 20‐30 km from the injection point of the primary electrons, and the radiation consists of discrete wave packets with a spatial extent of 4‐6 km parallel to the field. The magnetic field gradient is shown to be unimportant for these quasi‐local properties of the maser instability. The ray path calculations are performed in a dipolar field geometry and retain the relativistic corrections to the wave dispersion. For the case where the energetic electrons are the dominant species, amplifications of the order of e10 are achieved within path lengths of less than 100 km. This indicates that the maser instability can produce the observed amplification of auroral kilometric radiation above cosmic background levels within a distance smaller than the dimensions of the auroral cavity.
A statistical study of ELF‐VLF plasma waves at the magnetopause
Authors: Tsurutani Bruce T. , Brinca Armando L. , Smith Edward J. , Okida Roy T. , Anderson Roger R. , Eastman Timothy E.
Published: February 2008

A statistical study of the broadband ELF‐VLF plasma waves at the magnetopause has been performed using ISEE 1 plasma wave data. It is found that enhanced wave intensities are detected at 85% of all magnetopause crossings. Although wave amplitudes are highly variable from event to event and even within an event (particularly during high‐intensity events), the wave spectra averaged over many passes are remarkably similar at dawn, noon, and dusk local hours. Thus to first order, the average wave intensity and spectral shape are independent of local time. At select frequencies, the intensities are 10−8, 10−13, and 3 × 10−17 V2/m2 Hz at 101, 103, and 105 Hz and 3 × 10−4 and 5 × 10−8 nT2/Hz at 101 and 102 Hz, respectively. The average wave intensity is independent of latitude, magnetosheath field strength, and magnetopause position. The only parameter found to be correlated with wave intensity is the magnitude of the Z component of the magnetosheath magnetic field. The broadband wave intensities increase with increasing negative Bz. These observational results put strong constraints on any proposed generation mechanism for the broadband magnetopause boundary layer waves. Reasonable mechanisms should be able to explain the lack of local time, latitude, and interplanetary parameter dependences. The dependence of IMF Bz and possible correlation with magnetic reconnection should be a principal feature in the model. These observations support the conjecture that cross‐field diffusion of magnetosheath plasma by wave particle interaction with these waves (Tsurutani and Thorne, 1982) is the steady state source of the low‐latitude boundary layer, and that pitch angle scattering and the consequential particle precipitation into the ionosphere is the mechanism for the dayside aurora. For cyclotron resonant interaction with E>100 eV electrons during southward IMF Bz events, a total precipitation rate of 1.0 to 1.2 ergs/cm2 s is determined.
Steady state magnetic field configurations for the Earth's magnetotail
Authors: Hau L.‐N. , Wolf R. A. , Voigt G.‐H. , Wu C. C.
Published: February 2008

We present a two‐dimensional, force‐balanced magnetic field model in which flux tubes have constant pVγ throughout an extended region of the nightside plasma sheet, between approximately 36 RE geocentric distance and the region of the inner edge of the plasma sheet. We have thus demonstrated the theoretical existence of a steady state magnetic field configuration that is force‐balanced and also consistent with slow, lossless, adiabatic, earthward convection within the limit of the ideal MHD (isotropic pressure, perfect conductivity). The numerical solution was constructed for a two‐dimensional magnetosphere with a rectangular magnetopause and nonflaring tail. The primary characteristics of our steady state convection solution are (1) a pressure maximum just tailward of the inner edge of the plasma sheet and (2) a deep, broad minimum in equatorial magnetic field strength Bze, also just tailward of the inner edge. Our results are consistent with Erickson's (1985) convection time sequences, which exhibited analogous pressure peaks and Bze minima. Observations do not indicate the existence of a Bze minimum, on the average. We suggest that the configurations with such deep minima in Bze may be tearing‐mode unstable, thus leading to substorm onset in the inner plasma sheet.
Properties of 6300‐Å auroral emissions at South Pole
Authors: Rairden R. L. , Mende S. B.
Published: February 2008

Optical (6300 Å) observations of the polar cap region were recorded during the April to August austral winters of 1983 through 1986 with an image‐intensified all‐sky camera located at South Pole Station. Auroral structures seen in these all‐sky images are correlated with the signatures registered by a meridian slit (keogram) imager. The keograms provide a convenient survey of auroral activity, but the all‐sky images are required in order to determine the precise auroral morphology associated with the commonly observed keogram traces. Typical images of auroral activity around the diurnal period are presented. During morning and afternoon (magnetic local time) the auroral oval is usually seen crossing zenith, and frequent meridional movements appear on the keograms. During the midday interval, discrete poleward moving arcs occasionally appear, likely associated with flux transfer events or similar changes in the plasma flow pattern. Nightside keograms show the characteristic signatures of Sun‐aligned arcs, which are well defined in the associated all‐sky images. Two thirds of the Sun‐aligned arcs are observed on the dawnside of local midnight. The arcs drift overhead at velocities of typically less than 250 m/s with reference to a Sun‐oriented noncorotating frame. No significant preference is shown for easterly (dawnward) or westerly (duskward) drift directions. These Sun‐aligned arcs occur with greater frequency than previously thought: we observe a total of 114 such events during 219 diurnal periods of image collection. The available interplanetary magnetic field measurements, though limited, indicate Bz ≥ 0 for two thirds of the Sun‐aligned arcs. While no correlation is found between By and their local time distribution, there is a suggestion that dawnward motions appear preferentially with By > 0 and duskward motions with By < 0.
Ghost images of auroral X ray sources observed by directional balloon‐borne detectors
Authors: Ogura Koichi , Kodama Masahiro
Published: February 2008

Diffusion characteristics of auroral X rays propagating through the atmosphere have been investigated by means of a Monte Carlo simulation. The results are used to estimate the defocusing effect of multiple photon scatterings on auroral X ray images detected at balloon altitudes. It is predicted that the auroral X ray image at an atmospheric depth of 6–14 g/cm2 is occasionally accompanied by a spurious ghost image in the zenith when an auroral X ray source near the zenith is viewed by a collimated detector inclined with respect to the zenith. The ghost image is enhanced with increasing zenith angle of the collimator axis. The maximum spurious contribution occurs for a collimator with ±20° opening angle and 40° zenith angle. The magnitude of the spurious contribution also depends on the atmospheric depth and the threshold energy of the detector.
Transition in the dispersive properties of beam‐plasma and two‐stream instabilities
Authors: Dum C. T.
Published: February 2008

The linear dispersion relation and the quasi‐linear evolution of the electron beam‐plasma instability are studied in intermediate regimes between the limits of the gentle bump‐on‐tail instability of Langmuir waves and purely reactive instability. The ranges of beam and plasma parameters which correspond to narrow‐banded plasma waves, broadband waves and waves with frequencies substantially below the electron plasma frequency are determined in a systematic and practical manner. The transition between these regimes and the changes in the topology of the dispersion relation are described by analytic theory and by detailed numerical solutions. All branches of the dispersion relation, including weakly damped modes which may be important in nonlinear wave coupling, are considered. Dissipative and reactive instability mechanisms are differentiated analytically and by comparing solutions of the exact dispersion relation with hybrid models using fluid components. Consequences for observations, especially in the electron foreshock, and for the design and analysis of simulation experiments are pointed out. The theory is also applicable to instabilities with ion beams or current driven instabilities. The study of the effects of increasing bulk temperature (or decreasing drift) applies in particular to the evolution from reactive Buneman instability to kinetic ion sound instability.
Plasmoid‐associated energetic ion bursts in the deep magnetotail: Numerical modeling of the boundary layer
Authors: Scholer M. , Jamitzky F.
Published: February 2008

Single particle orbits are numerically followed in a time dependent two‐dimensional resistive MHD model of tail reconnection. In this model, near‐Earth reconnection disconnects the plasma sheet and the tailward departing closed field line region is surrounded by a thickening layer of disconnected lobe field lines. Orbits starting in this layer are traced backward in time, and their interaction with the current sheet close to the X line is studied. The model predicts the spatial distribution of energetic protons in the boundary layer surrounding plasmoids. It is argued that lower energy protons are injected after their acceleration onto lobe field lines closer to the neutral line than higher energy ions. Thus the low‐energy proton boundary layer will be found closer to the separatrix than the higher energy proton layer. This is confirmed by the numerical calculations. Further downtail from the neutral line the energy dispersion due to the dawn‐dusk electric field becomes larger and leads to the usual inward dispersion. Applying Liouville's theorem, we calculate the distribution function of accelerated ions in the boundary layer. Comparison with measurements obtained in the deep tail indicates that proton intensities observed in the layer around plasmoids at energies above ∼30 keV can be obtained by the neutral line acceleration of a suitable lobe ion distribution.
The coalescence of magnetic flux ropes and reconnection in the magnetotail
Authors: Richard Robert L. , Walker Raymond J. , Sydora Richard D. , Ashour‐Abdalla Maha
Published: February 2008

We have used magnetohydrodynamic simulation methods to investigate the characteristics and relevance of the magnetic island coalescence instability for magnetotail reconnection. The study was carried out under a wide range of parameters with different initial conditions. Periodic and nonperiodic island chains in or near equilibrium were considered and their linear and nonlinear evolution compared and contrasted. We found that for the periodic two‐island configuration the reconnection rate showed little dependence on resistivity and increased with lower plasma β. For the nonperiodic case, which resembled a two‐dimensional magnetotail configuration, a chain of magnetic flux ropes whose size decreased down the magnetotail was initialized. The coalescence process led to the formation of a large magnetic island whose motion down the tail drove reconnection at an x type neutral line. In the nonperiodic case variation of parameters had different effects than in the periodic case: a lower plasma β lowered the growth rate while the rate of energy conversion was higher for lower resistivity. Our results indicate that coalescence is an efficient mode of reconnection in long island chains, and in a finite system it can help drive reconnection on a scale larger than the original islands.
Particle precipitation due to nonlinear wave‐particle interactions in the magnetosphere
Authors: Prakash Manju
Published: February 2008

Chaos and nonlinear dynamics of a bouncing proton in the presence of a kinetic Alfvén wave in the magnetosphere are studied. Particle motion is traced numerically in phase space using the Poincaré surface of section technique. With suitable initial conditions particle trajectories are calculated under paraxial approximation for several values of the wave amplitude. The trajectories show chaotic behavior at some finite amplitude of the wave. The threshold electrostatic field computed for the onset of chaos is less than the expected electric field associated with a kinetic Alfvén wave in the magnetosphere. This suggests that a kinetic Alfvén wave can lead to proton precipitation near the equatorial region.
The neutral E region zonal winds during intense postmidnight diffuse aurora: Response to observed particle fluxes
Authors: Walterscheid R. L. , Lyons L. R.
Published: February 2008

The neutral response to the postmidnight diffuse aurora was calculated using observed fluxes of electron precipitation and a high‐resolution, time‐dependent numerical model. In an earlier work, based on an idealized representation of auroral precipitation, we simulated a dramatic supersonic E region jet stream. We suggested that hydrodynamical shear instabilities of this jet were responsible for eastward propagating features on the poleward edge of the diffuse aurora called omega bands. The results of the present study are qualitatively similar to the results of the previous study: A very strong dynamically unstable E region jet is simulated for an electric field of 50 mV m−1, and the results suggest a connection with omega bands. The winds are not quite as strong as previously simulated; zonal wind speeds of ∼600 m s−1 are achieved in ∼2 hours in the present study, whereas comparable wind speeds are achieved in ∼1 hour in the earlier study. After 1 hour, the peak winds reached are ∼400 m s−1 in the present study. The smaller acceleration for the present simulations is primarily a consequence of a smaller total energy flux in the observed fluxes than in the idealized ones. The zonal winds which we simulate are 2–3 times stronger than those previously simulated for the diffuse aurora elsewhere by a model of similar complexity. We have evaluated the effects of large‐scale cross‐arc winds, which were lacking in our earlier study, and found that strong cross‐arc winds are required to prevent the formation of a strong E region zonal jet. Our present results indicate that supersonic jets are possible, but are not apt to occur very often.
Radial diffusion and losses of energetic protons in the 5 to 12 RS region of Saturn's magnetosphere
Authors: Hood L. L.
Published: February 2008

Phase space density profiles for protons with first invariants µ = 600 and 3000 MeV/G (integral invariants K = 0.30 and 0.003 G½ RS) previously derived from measurements by the University of Iowa detector G on Pioneer 11 during the 1979 Saturn encounter are analyzed using solutions of the time‐averaged radial diffusion equation in a dipolar planetary magnetic field. A series of loss models ranging from satellite absorption only to satellite plus ring E absorption and added distributed losses is assumed and the corresponding form of the time‐averaged radial diffusion coefficient D(L) (taken to be of the form D(L) = DoLn, where n is an integer) is determined in each case by a minimum‐variance fit to the data‐derived phase space density profiles. The inferred forms for D(L) are characterized by a low‐order L‐dependence (∼L³–L6) and a relatively high amplitude (Do ≃ 10−9–10−8 RS² s−1) in approximate agreement with earlier studies. Loss models that include only satellite absorption result in pronounced Dione macrosignatures in the model phase space density profiles that are not present in the data profiles, indicating a need for additional distributed losses. Absorption by ring E would be sufficient to explain the absence of a Dione macrosignature only if both the presently accepted maximum normal optical depth and mean ring particle size are increased by at least 1 order of magnitude. Charge exchange losses, estimated from observational upper limits on the concentrations of relevant neutral species, also appear to be only marginally important. Pitch angle scattering precipitation losses occurring within the orbit of Rhea at a rate of at least 0.01 times that of the strong pitch angle diffusion limit could in principle provide the necessary distributed losses.
Generation of macroscopic magnetic‐field‐aligned electric fields by the convection surge ion acceleration mechanism
Authors: Mauk B. H.
Published: February 2008

The “convection surge” computer model presented previously (concerning the dramatic, nonadiabatic, magnetic‐field‐aligned energization of ions near the Earth's geosynchronous orbit in the presence of strong, transient, magnetic‐field‐perpendicular inductive electric fields) has been extended to include the self‐consistent generation of magnetic‐field‐aligned electric fields. The field‐aligned electric potential is obtained by imposing the quasi‐neutrality condition using approximated electron distribution forms. The ions are forced to respond self‐consistently to this potential. It is found that field‐aligned potential drops up to 1 to 10 kV can be generated depending on electron temperatures and on the mass species of the ions. During transient periods of the process, these large potential drops can be confined to a few degrees of magnetic latitude at positions close to the magnetic equator. Anomalous, sometimes dramatic, additional magnetic‐field‐aligned ion acceleration also occurs in part as a result of a quasi‐resonance between the parallel velocities of some ions and the propagating electric potential fronts. It is speculated that the convection surge mechanism could be a key player in the transient, field‐aligned electromagnetic processes observed to operate within the middle (e.g., geosynchronous) magnetosphere.
Simulations and observations of heating of auroral ion beams
Authors: Winglee R. M. , Dusenbery P. B. , Collin H. L. , Lin C. S. , Persoon A. M.
Published: February 2008

In the auroral zone, quasi‐static parallel electric fields produce beams of ionospheric ions (e.g., H+, He+ and O+), which flow outward into the magnetosphere, providing a significant source of ions for the ring current and plasma sheet. Because the velocities to which these beams are accelerated is dependent on the mass of the ions, differential flows between the various ion species can develop which are unstable to an ion‐ion streaming instability. Particle simulations and observations from DE 1 are used to investigate the heating of the ion beams produced by this instability. It is shown that there is net transfer of energy from the light ions to the heavy ions, with the heavy ions reaching maximum velocities near the beam velocity of the light ions. Bulk heating of the heavy ions occurs when their relative density is low while high‐energy tails are produced when their relative density is high. The heating is primarily parallel to the magnetic field if the difference in the heavy and light ion beam velocities is subsonic while both perpendicular and parallel heating can occur if it is supersonic. In the latter case, very strong heating of an intermediate ions species such as He+ can also occur. Comparison with observations shows features consistent with heating via the ion‐ion instability including perpendicular heating in the supersonic regime and parallel heating in the subsonic regime and a change in the heating between these regimes as the ratio of the H+ beam speed to the local sound speed is observed to decrease. This heating is, however, not always observed in association with enhanced wave emissions. This lack of waves is attributed to reabsorption of the waves as the ions become heated.
Electron content over Tucuman
Authors: Ezquer Rodolfo G. , de Adler N. Ortiz
Published: February 2008

The first morphological study of electron content (NF) over Tucumán (26.9° S, 65.4° W) (dipole geomagnetic coordinates: Latitude, −15.62°, Longitude, 4.64°, L = 1.08) using ATS 3 and GOES 3 geosynchronous satellite signals has been done, for the period April 1982 through March 1983 near the maximum of the second highest solar cycle. The NF over Tucumán is characterized by a day to day temporal variability, by semiannual variation, by equatorial anomaly influence, by different behaviors during magnetic storms and by occasional postsunset second maximum. Data of Tucumán and other stations were used for a comparative study. The maximum value of NF occurs later at Tucumán and has a similar order of magnitude in comparison to that observed at other stations. The winter anomaly is only present at northern hemisphere stations.
A numerical model of the electrodynamics of plasma within the contaminant gas cloud of the Space Shuttle Orbiter at low Earth orbit
Authors: Eccles J. Vincent , Raitt W. John , Banks Peter M.
Published: February 2008

This paper presents results from a two‐dimensional, finite‐difference model used to solve for the time evolution of low beta plasma within the neutral contaminant cloud in the vicinity of space platforms in low earth orbit. The model of the ambient and contaminant plasma dynamics takes into account the effects of the geomagnetic field, electric fields, background ionosphere, ion‐neutral collisions, chemistry, and both Pederson and Hall currents. Net ionization and charge exchange source terms are included in the fluid equations to study electrodynamic effects of chemistry within a moving neutral cloud in the low earth orbit ionosphere. The model is then used with complete water cloud chemistry to simulate the known outgassing situation of the space shuttle Orbiter. A comparison is made of the model results with plasma observations made during daytime on OSS‐1/STS‐3 mission. The reported density enhancements of the OSS‐1 mission are unattainable with normal photoionization and charge exchange rates of simple water cloud chemistry used in the two‐dimensional model. The enhanced densities are only attained by a generic chemistry model if a net ionization rate 1000 times higher than the photoionization rate of water is used. It is also shown that significant plasma buildup at the front of the contaminant neutral cloud can occur due to momentum transfer from the neutral outgas cloud to the plasma through elastic collisions and charge exchange. The currents caused by elastic and reactive collisions result in the generation of a small polarization electric field within the outgas cloud. The polarization electric field, in combination with the geomagnetic field, causes the plasma to drift with the contaminant neutral cloud but at a slower speed in the stationary reference frame. The slow drifting is consistent with OSS‐1 plasma probe interpretations where a high relative speed between the Orbiter and ions is inferred. Also, if there is a fast net ionization process within the outgas cloud, the drifting plasma allows for nonlinear buildup of plasma density with respect to the net ionization rate.
A study of electron impact excitation of NO: The vacuum ultraviolet from 40 to 170 nm
Authors: Ajello J. M. , Pang K. D. , Franklin B. O. , Howell S. K. , Bowring N. J.
Published: February 2008

The electron impact induced fluorescence spectrum of NO at 200 eV has been measured in a crossed‐beam experiment. The wavelength range studied spanned the range of the vacuum ultraviolet (VUV) from 40 to 170 nm. The basis of the absolute cross sections was the newly established Ly α cross section standard from electron impact dissociation of H2. By comparison with this standard the cross section of the N I (120.0 nm) transition produced by dissociative excitation of 200 eV was established to be 1.73 ± 0.38 × 10−18 cm². The cross section of all other features in the VUV were measured. The extreme ultraviolet spectrum from 40 to 120 nm consists entirely of emissions from excited N I, N II, O I, and O II atomic dissociation fragments. In addition, in the far ultraviolet (FUV) from 120 to 170 nm we report a measurement of the cross section of the NO+(A¹Π → X¹Σ+) Baer‐Miescher (BM) bands. We develop a band model of the FUV spectrum which consists of the BM bands, including the known variation of the electronic transition moment and the β′ bands (B′²Δi → X²Πr). Identification of the (4, 0) β′ band has allowed an estimate of the electronic emission cross section of the B′²Δi → X²Πr transition.
Ion beam generation at the plasma sheet boundary layer by kinetic Alfvén waves
Authors: Moghaddam‐Taaheri E. , Goertz C. K. , Smith R. A.
Published: February 2008

The kinetic Alfvén wave, an Alfvén wave with a perpendicular wavelength comparable to the ion gyroradius, can diffuse ions both in velocity and coordinate spaces with comparable transport rates. This may lead to the generation of ion beams in the plasma sheet boundary layer (PSBL). To investigate the ion beam generation process numerically, a two‐dimensional quasi‐linear code was constructed. Assuming that the plasma β (the ratio of plasma pressure to the magnetic pressure) varies from β = 1 to β ≪ 1 across the magnetic field, the dynamics of the ion beam generation in the PSBL was studied. It was found that if we start with an ion distribution function which monotonically decreases with velocity along the magnetic field and a density gradient across the magnetic field, ions diffuse in velocity‐coordinate space until nearly a plateau is established along the diffusion path. Depending on the topology of the magnetic field at the lobe side of our simulation system, i.e., open or closed field lines, the ion distribution function may or may not reach a steady state. If the field lines are open there, i.e., if the diffusion extends into the lobe, the double diffusion process may provide a mechanism for continuously transferring the ions from the central plasma sheet to the lobe. We comment on the effect of the particle loss on the establishment of the pressure balance in the plasma sheet.
Stability properties of a cometary plasma tail
Authors: Malara Francesco , Einaudi Giorgio , Mangeney Andrè
Published: February 2008

The linear stability of the plasma tail of a comet is numerically investigated. The zeroth‐order model of the tail plasma configuration is based on the result of a companion paper (Malara et al., this issue), discussing the possible modeling of the data obtained during the ICE encounter with comet Giacobini‐Zinner. Effects both of finite resistivity and of velocity shear are taken into account. The tail appears to be unstable against Kelvin‐Helmholtz‐like modes in which a certain amount of reconnection occurs. Because of the velocity flow shear the perturbation extends far beyond the singular layer and affects in depth the magnetic lobes. The relative importance of the Kelvin‐Helmholtz turbulence in the lobes and the level of reconnection, the typical wavelengths, and the growth time of the mode depend on the particular cometary tail considered.
The relative line strength and intensity of the N II 2143 doublet
Authors: Bucsela Eric J. , Sharp William E.
Published: February 2008

The doublet emission from N II at 2139.7 Å and 2143.6 Å was observed by a 1/4‐m scanning spectrometer with 3.1 Å resolution in the daytime, high‐altitude thermosphere during moderate levels of solar activity. The spectrometer viewed the Earth's limb 5° below the local horizontal to give a nominal tangent height of 152 km. Both sub band heads of the nitric oxide gamma band system were resolved in the data at the resolution used. The emission features from N II are clearly evident on the short wavelength shoulder of the (1, 0) band. Synthetic profiles of the (1, 0) gamma band and the (0, 3) delta band of nitric oxide were fitted to the data using a chi‐square analysis. These contributions were removed from the data leaving a residual emission, considered to be the N II doublet. A chi‐square minimization of the data relative to a synthetic intensity profile was done. The minimum was for a line strength ratio between the 2139 Å and 2143 Å lines of 0.58±0.08. The mean solar EUV flux deduced from the intensity of the N II emission in this experiment is lower than other reported observations, consistent with a lower solar activity level.
A representation of the magnetic neutral line on the solar source surface in terms of the Sun's axial dipole at the center and two equatorial dipoles in the photosphere
Authors: Saito T. , Ōki T. , Olmsted C. , Akasofu S.‐I.
Published: February 2008

It is shown that the basic features of the neutral line configuration on the source surface (a concentric sphere of 2.5 solar radii) and their variations during a sunspot cycle can be represented fairly well in terms of an axial main dipole at the center of the Sun and two (or at most three during a very limited time period) antipodal dipoles on the photosphere. This representation is not unique, but may be physically more meaningful than the harmonic analysis results. The nature of the photospheric dipoles is also discussed. The results may have some application to the Earth's magnetic field and its variations.
A magnetohydrodynamical model of the inner coma of comet Halley
Authors: Cravens T. E.
Published: February 2008

The magnetometer onboard the Giotto spacecraft observed a diamagnetic cavity surrounding the nucleus of comet Halley. The location of the boundary of this diamagnetic cavity is determined by a balance between an inward magnetic pressure gradient force and an outward ion‐neutral drag force, associated with collisions between the outwardly flowing neutrals and the stagnated ions. A one‐dimensional, time‐dependent, magnetohydrodynamical model has been developed for the inner coma of comet Halley, and includes ion‐neutral collisions, photochemical production and loss of plasma, and finite conductivity. This model is used to investigate the plasma dynamics in the vicinity of the diamagnetic cavity boundary surface. A narrow transition layer with enhanced plasma density is shown to exist just inside the boundary, although a full understanding of this layer will require a two‐ or three‐dimensional MHD model. The flux of cometary ions into this shocklike layer is removed by electron‐ion recombination. The thickness of this layer is determined by the Mach number of the incident flow.
Associated ground‐based observations of optical aurorae and discrete whistler waves
Authors: Hansen H. J. , Scourfield W. J.
Published: February 2008

Discrete VLF wave emissions in association with auroral light intensity fluctuations have been recorded at L=4 during simultaneous operation of a broadband VLF receiver and a low light level TV system. The correlated wave‐particle interaction events are interpreted as being typical examples of the electron cyclotron resonance process responsible for the precipitation of energetic electrons in the vicinity of the plasmapause. A range of delay times between the correlated VLF wave and the optical signals, coupled with the frequency versus time characteristics of the VLF wave emissions, is consistent with the electron cyclotron source region being randomly located within 8000 km of the equatorial plane. The events are shown to be triggered by ducted whistler mode waves that originate from the opposite hemisphere to that of the station at which the observations are made. Events characterized by VLF waves of rising tone are presumed to occur in a region of enhanced plasma density commonly observed within the plasma trough. It is proposed that this region is responsible for ducting the VLF waves from the source to ground.
Ionospheric electron content over the magnetic equator during spread F conditions
Authors: Rastogi R. G. , Koparkar P. V.
Published: February 2008

Total columnal electron content in the ionosphere over the magnetic equator during the postsunset hours is found to be greatly reduced on the nights with strong equatorial spread F conditions, as compared to that on nonspread F nights. The association of spread F and reduction of electron content over the magnetic equator is suggested to be due to the additional fountain generated on the nights when electric field continues eastward for a couple of hours even after sunset.
Role of hot oxygen in venusian ionospheric ion energetics and supersonic antisunward flow
Authors: Knudsen William C.
Published: February 2008

The column heating rate of the dayside Venus ionospheric ion gas resulting from transfer of energy from the hot oxygen component of the neutral atmosphere is estimated and found equal to that which, when inserted into ionospheric models at the ionopause, raises the calculated temperature to measured values. The transfer of energy is effected through resonant charge exchange between the relatively cold ionospheric O+ ions and the hot oxygen neutrals. The hot oxygen density in the nightside hemisphere does not appear to play a significant role in the nightside ion energetics. The hot oxygen neutral gas flowing across the terminator from its dayside source to its nightside sink will exchange momentum with the antisunward flowing ionospheric gas. Although the flow rate of hot oxygen can be estimated only crudely, the estimated rate of deposition and absorption is comparable to that produced by the plasma pressure gradient and should be included in numerical studies of the terminator ionospheric wind.
Electron beam injection during active experiments: 1. Electromagnetic wave emissions
Authors: Winglee R. M. , Kellogg P. J.
Published: February 2008

During the active injection of an electron beam, a broad spectrum of waves is generated. In this paper examples of spectra from the recent Echo 7 experiment are presented. These results show that the characteristics of the emissions can change substantially with altitude. Two‐dimensional (three velocity) relativistic electromagnetic particle simulations are used to investigate the changes in the plasma conditions required to account for the observed spectral variations. It is shown that many of these variations can be accounted for by assuming that the ratio of the electron plasma frequency ωpe to cyclotron frequency Ωe is less than unity at the lower altitudes of about 200 km and near or above unity at apogee of about 300 km. In the former case, whistlers with a cutoff at ωpe, lower hybrid and plasma waves are driven by the parallel beam energy while electromagnetic fundamental z mode and second harmonic x mode and electrostatic upper hybrid waves are driven by the perpendicular beam energy through the maser instability. E×B drifts driven by perpendicular electric fields associated with the beam‐plasma interaction can also be important in generating maser emission, particularly for field‐aligned injection where there is no intrinsic perpendicular beam energy. The power in the electrostatic waves is a few percent of the beam energy and that in the electromagnetic waves a few tenths of a percent. In the latter case, where ωpe/Ωe increases above unity, emission in the fundamental z mode and second harmonic x mode become suppressed. The dominant emissions below Ωe are strong whistlers and lower hybrid waves while above Ωe the dominant waves are electrostatic Bernstein and upper hybrid waves, with frequencies near harmonics close to the spacecraft and near half harmonics downstream from the spacecraft for ωpe/Ωe ≃ 2. This difference in emission with distance is due to the local depletion of plasma near the spacecraft. Weak second harmonic plasma radiation is also identified in the simulations, but this component is probably below the noise threshold of the Echo 7 receiver.
Dispersive and viscous scale lengths in the two‐stage ion heating at quasi‐parallel collisionless shocks
Authors: Mandt M. E. , Kan J. R.
Published: February 2008

The structure of simulated quasi‐parallel collisionless shock waves can be characterized by multiple scale lengths upon which the ion heating depends. The upstream wavelength follows closely that of the phase‐standing whistler mode wave. The scale length of the principal magnetic field jump is approximately equal to or somewhat greater than the upstream wavelength. Thus, the upstream wavelength is the dispersive scale length which governs the magnetic field structure of the shock. For Alfvén Mach numbers MA ≳ 2.5, large‐amplitude waves are generated in the downstream region. Wavelengths of these downstream waves are many times longer than the upstream waves and tend to increase with increasing Alfvén Mach number. The scale length of the principal plasma density jump, which gives us the viscous scale length, is comparable to the downstream wavelength. The ion heating is seen to occur in two stages. The first‐stage heating occurs upstream of the principal density ramp and can be characterized by a power law equation of state with an index much greater than the adiabatic value of 5/3 and increases with increasing Alfvén Mach number. The second‐stage heating occurs at and downstream of the principal density jump and can be characterized by a power law index on the order of the adiabatic value. The results show that the ion heating occurs mainly around the principle density jump near the center of the shock transition region, while the increase in entropy takes place mainly in the upstream side of the shock transition region. This result is consistent with the suggestion that the ion heating is a consequence of the nonadiabatic scattering of the ions through the magnetic field structures of the shock including the upstream waves.
Investigations of a turbulence‐driven solar wind model
Authors: Isenberg Philip A.
Published: February 2008

This work presents an investigation of the properties of the one‐dimensional, two‐fluid turbulence‐driven solar wind model introduced by Hollweg and Johnson (1988). It is found that the model has serious difficulties in reproducing the observed high‐speed wind at 1 AU. In particular, the model proton temperatures are lower than those observed by a factor of 2 or more and the highest temperature models yield excessive wave intensities at 1 AU. It appears that the problem stems from the specific spatial distribution of heat deposition in the model. Thus this study does not rule out a turbulence‐driven fast solar wind, since other forms of the turbulent evolution could probably achieve better results. A three‐fluid version of the model is also presented to show that the addition of alpha particles does not significantly reduce the extreme proton temperatures displayed near the Sun by the Hollweg and Johnson work. Finally, we suggest that the additional heating needs to be located well beyond the critical point, implying that the heating mechanism for the fast solar wind is likely not the same as that heating the solar corona.
A statistical characterization of plasmasphere density structure and boundary locations
Authors: Horwitz J. L. , Comfort R. H. , Chappell C. R.
Published: February 2008

A statistical study of plasmaspheric density profiles and their boundaries is performed, using measurements of core (<50 eV) ions by the retarding ion mass spectrometer (RIMS) on Dynamics Explorer 1. The plasmasphere density profiles are classified into essentially six different categories, indicative of density structure out to the outer boundary of observable cold, isotropic plasma. The most common profiles observed are those which are relatively featureless out to this outer boundary and those which exhibit multiple plateaus and plasmapauses. The multiple plateau profiles occur predominantly in the afternoon and evening sectors, while the “featureless” profiles are most common in the midnight through morning sectors. In the multiple plateau profiles, the average MLT (magnetic local time)‐L locus of the inner “plasmapause” gradients is nearly circular at L = 3–4. Profiles with significant density troughs occur most often in the evening sector, and the troughs are widest in this sector as well. The MLT‐L local time shape of the outer boundary of RIMS‐observed cold ions (termed here low‐energy ion transition, or LEFT) is similar to previously reported average plasmapause shapes, though its bulge region occurs in the afternoon sector. This MLT‐L shape also tends to become more circular with increasing geomagnetic activity. Using simultaneous plasma density measurements from the plasma wave instrument, it is found that the LEIT typically is located between densities of 10 and 100 cm−3, with an average plasma density there of about 60 cm−3.