bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo models for the terrestrial exosphere over a solar cycle
Authors: Tinsley B. A. , Hodges R. R. , Rohrbaugh R. P.
Published: February 2008

Improved Monte Carlo models for the concentration and velocity distribution for hydrogen in the terrestrial exosphere have been formulated for minimum, medium, and maximum solar cycle conditions. Both the plasmaspheric source of hot hydrogen and the classical exobase source have been included, along with solar radiation pressure and photoionization. At solar minimum the hydrogen from charge exchange of hot ions in the plasmasphere exceeds that from the exobase source not only for escape but for the population at geocentric distances greater than two earth radii (RE). At about 2 RE the equivalent temperature is about 50% greater than that of the exobase, a situation similar to the “two‐temperature” exosphere observed and calculated for Venus. At solar maximum the exobase source dominates. The concentration at about 2 RE geocentric and above varies little from solar maximum to solar minimum, although the exobase concentration increases by about a factor of 10. The diurnal variation at the exobase is a factor of 2.1 at solar maximum and 2.7 at solar minimum at low latitudes. This variation is mostly smoothed out at 2 RE, but above 10 RE a nighttime enhancement by about a factor of 2 develops, constituting a slight “geotail” of the type found earlier by observation and by calculations of the effects of radiation pressure. The non‐Maxwellian velocity distributions show “flattened” tops compared to Maxwellian distributions for components in the transverse directions between 2 RE and 8 RE which appear to be due to the importance there of satellite particles with high apogees and the depletion of satellite particles with low perigees.
Correlation of geomagnetic storm sudden commencement with triggered magnetospheric particle bursts
Authors: Tholen S. M. , Armstrong T. P. , Lanzerotti L. J.
Published: February 2008

A previous investigation (Tholen and Armstrong, 1986) demonstrated that the occurrence of a storm sudden commencement (SSC) significantly increases the probability of observing energetic particle bursts in the magnetosphere, particularly in the magnetotail. Reported here is an evaluation of all such observed particle bursts following SSCs between 1973 and 1983. Data from five magnetometer stations near L = 4 were used to investigate the details of the correlations between SSC amplitudes, occurrence times, particle burst characteristics, and triggering probability. The results indicate that SSC amplitude apparently controls the probability of a subsequent particle burst if the observation location is in the magnetosheath or magnetotail; the observed intensity of the burst, however, appears to be unrelated to the size of the SSC.
DE 1 observations of harmonic auroral kilometric radiation
Authors: Mellott M. M. , Huff R. L. , Gurnett D. A.
Published: February 2008

The plasma wave instrument on board the DE 1 spacecraft has observed several intervals of auroral kilometric radiation during which harmonic structure is clearly present. We present evidence, some of which is based on unique capabilities of the DE instrument, which argues strongly that the harmonic structures are natural rather than instrumental in origin. The harmonic emissions occur infrequently, but when present may persist for intervals of up to an hour. The emissions are relatively narrow band and consist of a relatively weak fundamental (10−14–10−11 V²/m² Hz) accompanied by an even weaker second harmonic. The ratio of power in the fundamental band to the power in the harmonic ranges from 10 to 100. In all cases, polarization data indicate that the fundamental is a left‐hand ordinary (L‐O) mode emission while the harmonic is right‐hand extraordinary (R‐X) mode. These observations are consistent with predictions based on the cyclotron maser mechanism.
The importance of accurate secondary electron yields in modeling spacecraft charging
Authors: Katz Ira , Mandell Myron , Jongeward Gary , Gussenhoven M. S.
Published: February 2008

Spacecraft charging has commonly been attributed to electrons with several kilovolts of energy impinging upon spacecraft surfaces. Recent experimental evidence from the SCATHA satellite has shown that charging correlates well with electrons of energies greater than 30 keV. In this paper it is shown that the SCATHA observations are consistent with the model of charging in which a satellite is immersed in a Maxwellian plasma, particle collection is orbit limited, and dominant surface effects are the emission of secondary and backscattered electrons. The energy dependence of the secondary yield for multikilovolt incident electrons determines the charging threshold. In the past, inadequate representations of the secondary yield have led experimenters to question the validity of the charging model. The accuracy of the secondary electron yield formulation based on electron stopping power, such as the one in NASA Charging Analyzer Program (NASCAP), gives good agreement with the SCATHA results. A Maxwellian representation of the magnetospheric plasma is justified by choosing effective temperatures and densities that minimize the error in calculating charging current densities.
An analysis of the shocklike electrostatic noise observed during AMPTE solar wind ion releases
Authors: Ma T. Z. , Gurnett D. A. , Omidi N.
Published: February 2008

For the purpose of explaining the shocklike noise detected by the Ion Release Module (IRM) instruments during the Active Magnetospheric Particle Tracer Explorers (AMPTE) solar wind ion releases, we present an analysis of electrostatic waves produced by the interaction of the ion cloud with the solar wind. This analysis considers an arbitrary direction of propagation. It is found that in the absence of cold photoelectrons the maximum growth rate occurs almost perpendicular to the solar wind velocity. The wave growth is much higher than for propagation parallel to the solar wind direction, and is caused by an ion‐ion interaction between the injected ions (Ba+ or Li+) and the solar wind protons. However, in the presence of cold photoelectrons, the growth rate for the ion‐ion interaction is comparable or less than for parallel propagation, which is primarily an ion‐electron interaction. Therefore both types of interactions are probably involved in generating the shock‐like electrostatic noise. The ion‐ion instability is more likely to occur in the later stage of the cloud expansion after the cold photoelectrons have escaped and the ion‐electron instability is more likely, or at least comparable to the ion‐ion instability, in the early stages when the photoelectrons are dominant.
Tail modeling in a stretched magnetosphere: 1. Methods and transformations
Authors: Stern David P.
Published: February 2008

A new method is developed for representing the magnetospheric field B as a distorted dipole field. Because ▽ · B = 0 must be maintained, such a distortion may be viewed as a transformation of the vector potential A. The simplest form is a one‐dimensional “stretch transformation” along the x axis, a generalization of a method introduced by Voigt. The transformation is concisely represented by the “stretch function” ƒ(x), which is also a convenient tool for representing features of the substorm cycle. One‐dimensional stretch transformations are extended to spherical, cylindrical, and parabolic coordinates and then to arbitrary coordinates. It is next shown that distortion transformations can be viewed as mappings of field lines from one pattern to another: Euler potentials are used in the derivation, but the final result only requires knowledge of the field and not of the potentials. General transformations in Cartesian and arbitrary coordinates are then derived, and applications to field modeling, field line motion, MHD modeling, and incompressible fluid dynamics are considered.
Electron cyclotron harmonic waves observed by the AMPTE‐IRM Plasma Wave Experiment following a lithium release in the solar wind
Authors: Roeder J. L. , Koons H. C. , Holzworth R. H. , Anderson R. R. , Bauer O. H. , Gurnett D. A. , Haerendel G. , Häusler B. , Treumann R.
Published: February 2008

An unexpected occurrence following the second lithium release by the AMPTE‐IRM spacecraft in the solar wind on September 20, 1984, was the appearance of electron cyclotron harmonic emissions. These emissions began about 50 s after the release and continued for several minutes. Narrow‐band emissions polarized perpendicular to the magnetic field with amplitudes of approximately 10−5 V m−1 were observed in each of the first five harmonic bands. Unpolarized diffuse emissions were also present at the same time. The diffuse emissions extended from below the lowest measured frequency channel to above the highest narrow‐band emission with a maximum below the electron cyclotron frequency. It will be shown that these observations are inconsistent with their generation by several ion beam instabilities.
Ion precipitation from the magnetopause current sheet
Authors: Lyons L. R. , Vampola A. L. , Speiser T. W.
Published: February 2008

Measurements from the polar‐orbiting S3‐3 satellite show that energetic ions frequently precipitate with isotropic pitch angle distributions at auroral latitudes on the dayside. We propose that this precipitation results from non‐guiding center motion of radiation belt ions that drift into the magnetopause current sheet. We also propose that the ion precipitation and flows of energetic ions observed in the magnetosheath originate together within the magnetopause current sheet. Ions ejected from the magnetopause toward the earth flow along open field lines that are adjacent to the separatrix between open and closed field lines. Those ejected into the magnetosheath flow along field lines that are connected to the geomagnetic field and adjacent to the separatrix between the connected field lines and purely magnetosheath fields lines. These proposals are tested by comparing the S3‐3 observations of precipitating ions with previously analyzed ISEE 1 observations of energetic ions obtained near the magnetopause and in the magnetosheath. The S3‐3 observations imply that the region of ion precipitation is often continuous as a function of local time. This suggests that at least on the dayside there is often a continuous shell of magnetospheric ions within the magnetosheath flowing from the magnetosphere. These ions are likely an important source for energetic ions in the interplanetary medium.
Hydromagnetic wave activity detected by the GEOS 2 Double‐Probe Experiment during 1979
Authors: Matthews John
Published: February 2008

I make a comparative survey of wave period and amplitude measurements for hydromagnetic disturbances detected by the GEOS 2 double‐probe experiment in 3 months during 1979. Interpretation of these data is aided through knowledge of the average mass to charge ratio of the ion component of the background magnetospheric plasma. These ratios are obtained from comparison of the observed period to a WKB period estimate based on a realistic magnetic field model, and measured plasma parameters. For data recorded in January 1979, event amplitudes and occurrence were greatest in the early afternoon, corresponding to background electron densities in the range 5–10 cm−3, and mass to charge ratios between about 1.6 and 6. Values of mass to charge ratio decrease systematically with local time, with high early morning values indicating a preponderance of heavy ions (possibly O+) at the times of the wave activity. By dusk, mass to charge ratios are close to unity, pointing to the dominance of H+ ions. For the magnetically quiet month of July 1979, amplitudes and occurrence were greatest in the morning hours, with the effects of plasma trough filling and depletion on wave periods clearly visible. Qualitative estimates show that, for this month, no clear trend exists in the local time variations of mass to charge ratio. Wave activity registered in the disturbed equinoctial period of March 6 to April 5, 1979, indicates strong excitation, particularly in the morning hours, with a broad spread in period, and no systematic change in mass to charge ratio through local time.
Venus “lightning” signals reinterpreted as in situ plasma noise
Authors: Taylor Harry A. , Cloutier Paul A. , Zheng Zirao
Published: February 2008

Electric field noise bursts detected from the Pioneer Venus Orbiter across the nightside of Venus have been interpreted in a number of papers as providing evidence of extensive lightning at Venus. Moreover, the asserted clustering of these 100‐Hz signals over mountainous topography has encouraged widespread speculation that present‐day volcanism is prevalent. Earlier studies have noted, however, that the 100‐Hz signals were often associated with ionospheric perturbations, prompting the suggestion that the electric field signals were being produced in situ and were thus unrelated to the lower atmosphere or surface. In this more detailed analysis we find a statistically persistent correlation between the 100‐Hz noise and ion troughs marking the solar wind interaction with the nightside ionosphere. Clusters of closely spaced 100‐Hz noise bursts attributed to lightning often “turn on” in the troughs, simultaneously with sharp density gradients in which the plasma concentration drops by an order of magnitude or more and the direction of the magnetic field changes significantly, within a few seconds. Analysis of the 100‐Hz noise also shows two other important statistical characteristics, namely (1) the noise is most frequently observed near 180 km and above but much less frequently at lower altitudes and (2) the noise is most often detected when the angle between the spacecraft velocity vector and the direction of the B field approaches 90°. We interpret the foregoing characteristics as evidence that the 100‐Hz noise is produced from solar wind/interplanetary magnetic field related plasma perturbations occurring near the spacecraft and that the noise is unrelated to the lower atmosphere and surface of Venus. Unfortunately, we do not have access to sufficient high‐resolution data to specifically identify the type of ionospheric noise involved.
The interaction of Io's Alfven waves with the Jovian magnetosphere
Authors: Wright Andrew N.
Published: February 2008

A numerical solution for the propagation of the Alfvén waves produced by Io is presented. The waves are shown to interact strongly with the torus and magnetic field inhomogeneities. We find substantial reflection occurs from the magnetospheric medium and only about a quarter of the wave power will reach the ionosphere on its first pass. It is concluded that both WKB and ray tracing arguments are inappropriate, contrary to previous studies. A more realistic picture may be that of a whole field line or L shell resonating in an eigenmode. We discuss the Alfvén structure behind Io and comment upon some possible features that it may exhibit. In particular it may be possible to produce decametric arcs that are more closely spaced than ray tracing permits by exciting higher harmonic eigenmodes of Io's L shell.
Chaotization of the electron motion as the cause of an internal magnetotail instability and substorm onset
Authors: Büchner J. , Zelenyi L. M.
Published: February 2008

A new mechanism is proposed that is able to explain the sudden disruption of the magnetotail energy storage and the explosive onset of isolated magnetospheric substorms. During the growth phase the magnetotail accumulates additional magnetic flux and evolves in such a way that the special parameter κe = (Bn/Bo)(L/ρeo)1/2 describing the ratio of characteristic frequencies of the electron motion along and across the field reversal region quickly decreases. Here Bn and Bo are the components of magnetic field across and along the central plasma sheet, L is the thickness of the field reversal region, and ρeo is the Larmor radius of thermal electrons in the field Bo. When κe reaches a critical value κe|crit ≅ 1.6, a nonlinear interaction between the degrees of freedom of electron motion becomes strong enough to cause a chaotization of the electron orbits. This effect results in a fast growth of a collisionless tearing mode instability even within the framework of the WKB approximation, i.e., in a wide interval of possible wavelengths 1 > kL > Bn/Bo. The proposed theory‐ gives the mechanism of the formation of new reconnection region in the near‐Earth magnetotail, based exclusively on intrinsic properties of the magnetotail evolution. This reveals the natural relation between the onsets of isolated substorms and an internal large‐scale magnetotail instability of the tearing type considered in this paper.
Absolute experimental differential and integral electron excitation cross sections for atomic oxygen: 4. The (³P → 3s″³P°), (³P → 2s 2p5 ³P°), (³P → 4d′ ³P°) Autoionizing Transitions (878, 792, and 770 Å) and five members of the (³P → nd ³D°) Rydberg series (1027 Å)
Authors: Vaughan S. O. , Doering J. P.
Published: February 2008

Absolute differential and integral direct electron excitation cross sections have been measured for the three most intense autoionizing ³P → ³P° transitions in atomic oxygen (878, 792, and 770 Å) and the first five members of the ³D° Rydberg series whose origin is the 1027 Å transition. Incident electron energies from 30 to 200 eV were used. Absolute values for the differential cross sections were calculated using the previously measured atomic oxygen ³P → ³S° (1304 Å) cross section of Vaughan and Doering (1987) as a secondary standard. The cross sections were much more strongly forward peaked at the higher laboratory incident energies than at low energies. The integral cross sections for the 878‐ and 792 Å transitions were identical within experimental error, both in shape and absolute magnitude. The peak values were 1.1‐1.4 × 10−17 cm² at the maximum near 50 eV, about a factor of 3 lower than the total cross section deduced by Zipf and Kao (1986) from emission experiments. The 770 Å transition was approximately a factor of 4 weaker with the same shape. The ³D° Rydberg series cross sections decrease uniformly with increasing principal quantum number as expected. The peak of the 1027 Å origin transition is approximately 3.5 × 10−18 cm² at 50 eV, in satisfactory agreement with the emission cross section of Zipf and Erdman (1985) The present cross sections for the higher energy states suggest that the autoionizing transitions are a significant source of O+(4S) ions in the thermosphere, but not as strong a source as has been inferred on the basis of previous emission measurements.
Linear theory of driven reconnection
Authors: Horton W. , Tajima T.
Published: February 2008

The concept of driven magnetic reconnection is examined in the framework of linear theory. An imposed small‐amplitude magnetohydrodynamics (MHD) plasma flow velocity is specified at a finite distance from the magnetic neutral line in a plasma current sheet. The critical condition for driven reconnection is shown to be a function of the wavelength of the imposed flow compared with its distance from the neutral line as well as the orientation of the incoming plasma flow. Two‐dimensional, incompressible MHD is used to develop the theory, and a simple model for the collisionless reconnection problem is analyzed for the linear, driven reconnection growth rate. According to the theory, the time scale of the delay of the substorm energization is of the order of a half an hour. Other relevant evaluations of geomagnetic tail quantities are obtained.
Westward cross‐field motion of negative charge filament activated by gradient‐curvature proton drift: A cause of the westward traveling surge
Authors: Yamamoto Takashi
Published: February 2008

The cross‐field interactive dynamics of two azimuthally elongated filaments of negative and positive charge excess which are close to each other is numerically studied. The negative charge filament is assumed to exist in the premidnight sector of the plasmasheet, causing a discrete aurora at high latitude in the ionosphere. The positive charge filament responsible for a diffuse aurora is assumed to be caused by injection of the ring current energetic protons and located equatorward of the negative charge filament. In our simulation model using two‐fluid approximation, the energetic protons execute the gradient and curvature (westward) drifts in addition to the E×B drift and the electron motion is approximated by the E×B drift. The spatial and temporal development of the charge filaments is numerically followed in a perpendicular plane to the magnetic field. The simulation results show that the gradient and curvature drift of the positive charge carriers (energetic protons) can significantly increase the westward front‐velocity of the negative charge filament and also lead it poleward as seen on the ionosphere. The front‐speed of the negative charge filament is found to be on the order of a kilometer per second as geomagnetically projected on the ionosphere. It is then suggested that this physical process can be a cause of the westward traveling surge.
Spectral signature of the beam plasma discharge
Authors: Hallinan T. J. , Deehr C. S. , Hoch E. , Viereck R. , Bernstein W. , Konradi A.
Published: February 2008

Spectral measurements of the light emitted from a beam plasma discharge (BPD) in nitrogen show an abrupt increase in the ratio of N2 first positive emissions to the N2+ first negative emissions (red/blue ratio) at the onset of BPD. However, at beam currents substantially above BPD threshold, the red/blue ratio reverts to that produced by the beam itself (pre‐BPD). This is interpreted as the appearance, at BPD onset, of a large suprathermal electron population, followed by a hardening of the suprathermal energy spectrum at higher beam currents. The new results are used to interpret previous “white‐light” video images of BPD experiments. It is concluded that even larger red/blue ratios may occur at lower pressures and that a pre‐BPD glow associated with radiofrequency emissions at the upper hybrid frequency is produced by a softer electron population than that occurring in BPD. In addition to the red/blue ratio, it was found that the ratio of the intensities of the 4–1 band and the 6–3 band of the first positive emissions varies systematically with beam current. The overall behavior is similar to that of the red/blue ratio—an abrupt increase followed by a steady decrease to the pre‐BPD level. But the detailed dependence of the two ratios on current is different.
Effect of mid‐altitude ion heating on ion outflow at polar latitudes
Authors: Li Peng , Wilson G. R. , Horwitz J. L. , Moore T. E.
Published: February 2008

We use a self‐consistent, semikinetic model to describe the steady state collisionless outflow of ions along diverging magnetic field lines at high altitude. The model is similar to that used by Barakat and Schunk (1983), but modified to (1) allow for anisotropic ion temperatures at the lower boundary, and (2) eliminate lower boundary potential jumps (“double layers”). The model is used to determine what happens to a typical, predominantly light‐ion outflow when either the perpendicular or parallel (or both) ion temperatures are raised at the exobase lower boundary (4500 km altitude). Also, cases with plasma densities, bulk velocities, and temperatures characteristic of ion upwelling events (Moore et al., 1986) are shown. In addition, the escape flux of O+ ions for various combinations of electron and ion temperatures is investigated. It is shown that the O+ escape flux can be increased, to levels as high as obtained by Barakat and Schunk (1983) with electron temperatures of 104 K, by raising instead the ion temperatures, though to values considerably less than the ∼105 K observed by Moore et al. (1986).
Coupled 3‐ and 1.3‐Hz components in auroral pulsations
Authors: Hansen H. J. , Mravlag E. , Scourfield M. W. J.
Published: February 2008

Correlated auroral and VLF wave emissions have been identified at L = 4 during simultaneous recordings of TV auroral images and broadband VLF emissions. Both these emissions are characterized by two sequential, nearly harmonic frequency components at about 1.3 and 3 Hz. A model is proposed on the basis of several assumptions. The 1.3‐ and 3.0‐Hz components are presumed to be associated with strong pitch angle scattering via the cyclotron resonance interaction, when the flux levels of two different populations of energetic electrons exceed the flux trapping limit. The 3‐Hz oscillation then arises from half‐bounce time resonance when a flux of high‐energy electrons (∼100 keV) drifts onto the flux tube of interest. In turn, the 3‐Hz oscillation gives rise to a 1.3‐Hz oscillation in the background low‐energy electrons (∼20 keV) via the half‐bounce resonance condition.
Local generation of electrostatic bursts at comet Giacobini‐Zinner: Modulation by steepened magnetosonic waves
Authors: Brinca Armando L. , Tsurutani Bruce T. , Scarf Frederick L.
Published: February 2008

The ICE spacecraft observed in the vicinity of comet Giacobini‐Zinner electrostatic noise with burstlike features seemingly modulated by the relative orientation of the interplanetary magnetic field and the solar wind velocity (Richardson et al., this issue). We discuss the adequacy of ion acoustic‐like instabilities stimulated by photoelectron or newborn ion beams to the interpretation of the electrostatic bursts, and we conclude that the mode fed by the photoelectrons exhibits characteristics compatible with the observations.
Application of bicoherence analysis in study of wave interactions in space plasma
Authors: Lagoutte D. , Lefeuvre F. , Hanasz J.
Published: February 2008

A spectral analysis at the second order (power spectrum) loses the phase information among the different Fourier components. To retain this information, the bispectrum (third order) and/or the bicoherence (normalized bispectrum) are calculated. Application to simulated data, shows the dependence of the bispectrum to amplitudes of involved waves and of the bicoherence to signal‐to‐noise ratio. Bicoherence technique is applied in the analysis of harmonics produced by an electronic receiver, as well as in the investigation of phase coherence between a ground‐transmitter signal, a natural ELF emission near the proton gyrofrequency, and the sidebands around the carrier. Strong arguments are provided that the sidebands are generated by a parametric interaction between the transmitter signal and the ELF emission.
ARCS 3 ionospheric artificial argon ion beam injections: Waves near the heavy ion gyrofrequencies
Authors: Erlandson R. E. , Cahill L. J. , Kaufmann R. L. , Pollock C. J. , Arnoldy R. L.
Published: February 2008

The low‐frequency electric field plasma wave environment associated with artificial plasma beam injections in the ionosphere has been recorded at the main payload at distances of 0 to 1000 m away from the subpayload. An argon ion beam was injected from the subpayload periodically during the ARCS 3 experiment in the direction either antiparallel or perpendicular to the magnetic field at altitudes from 250 to 405 km. During parallel argon beam injections waves below the proton gyrofrequency were not observed. During perpendicular argon beam injections the wave spectra were broadband near the subpayload while narrow‐band wave emissions near heavy ion gyrofrequencies were observed at perpendicular separation distances between 42 and 254 m. The narrow‐band waves were generally ordered by the oxygen gyrofrequency (fO = 45 Hz) although the wave spectra was different from one event to the next presumably due to the increase in perpendicular separation distance. The strongest emission lines were observed near the fourth harmonic of the oxygen gyrofrequency which was near the oxygen‐helium bi‐hybrid frequency. The narrow‐band waves appeared to be associated with both the perpendicular argon ion beam and an unexpected flux of low‐energy ions which peaked in energy near 15 eV and pitch angle near 90° with respect to the magnetic field. After discussing possible free energy sources of the waves the ARCS 3 experiment is compared to previous active plasma beam experiments.
First EISCAT Two‐Radar Plasma Line Experiment
Authors: Fredriksen Å , Bjørnå N. , Hansen T. L.
Published: February 2008

This paper reports the first EISCAT two‐radar plasma line experiment measuring photoelectron‐enhanced plasma lines with the UHF (933 MHz) and VHF (224 MHz) radars. The experiment shows that near noon at midsummer, simultaneous plasma lines can be continuously detected with the two radars in the altitude region 140–280 km. Such observations allow the electron temperature to be determined independently of observations of the ion part of the incoherent spectrum. Examples are given to illustrate this application.
Beam instability of the Z mode in the solar wind
Authors: Krauss‐Varban D.
Published: February 2008

The growth rate of the z mode is calculated assuming a cold magnetized background plasma and a tenuous population of hot electrons. For a weak, but nonvanishing, magnetic field the growth rate is shown to coincide with that of the electrostatic Langmuir wave, i.e., the result when the influence of the ambient magnetic field is only retained for the energetic electrons. Considering the case of a beam of hot electrons, we numerically evaluate the expression for the growth rate for several cases of solar wind plasma conditions. Solution of the full 3×3 dispersion determinant allows the computation of the growth rate and real frequency shift for arbitrary beam densities and magnetic field strength. The influence of the background magnetic field is discussed, and the apparent polarization of the excited waves is calculated assuming a weak density gradient between source and observer. The effect of the beam density, direction of propagation, and magnetic field on the observable polarization is discussed.
Magnetic impulse events at high latitudes: Magnetopause and boundary layer plasma processes
Authors: Lanzerotti L. J. , Wolfe A. , Trivedi N. , Maclennan C. G. , Medford L. V.
Published: February 2008

Magnetic field data acquired at high‐latitude, near‐conjugate stations (Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica) are studied in order to examine in more detail the nature of magnetic “impulse” signatures that occur in the data and that are produced by ionosphere currents which are caused by magnetopause processes. An examination of the data, both visually and with a computer algorithm “detector,” from the 5‐month interval July–November 1985 found many such magnetic “impulse” events which could be interpreted in terms of intense field‐aligned currents above the observing stations. All the events have a half‐width in time of a few minutes, and most are reasonably conjugate. The majority of the events studied have magnetic field perturbations in the vertical component which can be interpreted in terms of field‐aligned currents directed in the same direction (either into or out of the ionosphere) in both hemispheres. The perturbations in the horizontal plane are consistent with an interpretation in terms of a single cycle of an odd mode Alfvén wave. For the events shown in detail in this paper, the impulsive magnetic signatures are found to occur for the interplanetary magnetic field BZ component positive, negative, or variable. The observations are discussed in the context of some contemporary ideas on the generation of ionospheric disturbances by magnetopause processes such as sporadic reconnection (“flux transfer events”), plasma injections into the low‐latitude boundary layer, Kelvin‐Helmholtz instability, and solar wind “pressure pulses.” Near‐equatorial data from a location in the same local time section as the high‐latitude data are used to show the gross differences in global ionospheric currents stimulated by sudden commencements and by the magnetic impulse events.
Global measures of ionospheric electrodynamic activity inferred from combined incoherent scatter radar and ground magnetometer observations
Authors: Richmond A. D. , Kamide Y. , Akasofu S.‐I. , Alcaydé D. , Blanc M. , de la Beaujardière O. , Evans D. S. , Foster J. C. , Friis‐Christensen E. , Holt J. M. , Pellinen R. J. , Senior C. , Zaitzev A. N.
Published: February 2008

An analysis of several global measures of high‐latitude ionospheric electrodynamic activity is undertaken on the basis of results obtained from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure applied to incoherent scatter radar and ground magnetometer observations for January 18–19, 1984. Different global measures of electric potentials, currents, resistances, and energy transfer from the magnetosphere show temporal variations that are generally well correlated. We present parameterizations of these quantities in terms of the AE index and the hemispheric power index of precipitating auroral particles. It is shown how error estimates of the mapped electric fields can be used to correct the estimation of Joule heating. Global measures of potential drop, field‐aligned current, and Joule heating as obtained by the AMIE procedure are compared with similar measures presented in previous studies. Agreement is found to within the uncertainties inherent in each study. The mean potential drop through which field‐aligned currents flow in closing through the ionosphere is approximately 28% of the total polar cap potential drop under all conditions during these 2 days. We note that order‐of‐magnitude differences can appear when comparing different global measures of total electric current flow and of effective resistances of the global circuit, so that care must be exercised in choosing characteristic values of these parameters for circuit‐analogy studies of ionosphere‐magnetosphere electrodynamic coupling.
Noncoplanar component of the magnetic field at low Mach number shocks
Authors: Friedman M. A. , Russell C. T. , Gosling J. T. , Thomsen M. F.
Published: February 2008

The component of the magnetic field that deviates from the plane defined by the shock normal and the upstream magnetic field is examined for low Mach number bow shocks. The integrated value of this noncoplanar component is compared to the predictions of Jones and Ellison (1987). A test of this relationship was first reported by Gosling et al. (1988) who found good agreement only at the two low Mach number shocks that were included in their study. Analysis of a more extensive collection of low Mach number shocks confirms the Jones and Ellison relationship at very low Mach numbers as well as its deterioration for higher Mach numbers. However, there also is an indication that the relationship may break down for shocks that are nearly perpendicular.
Energetic particle precipitation in the substorm growth phase measured by EISCAT and Viking
Authors: Kirkwood S. , Eliasson L.
Published: February 2008

Particle precipitation into the evening auroral oval in the 1–2 hours preceding substorm onset has been studied using measurements from the EISCAT incoherent scatter radar, complemented by measurements from the Viking satellite. The ionization between 100 and 160 km altitude is found to correspond to precipitation of approximately Maxwellian populations of ions with average energies of 10–20 keV and of electrons with average energies of 1–3 keV. A high‐energy tail in the electron precipitation is found to account for the ionization below 100 km altitude in the equatorward and poleward parts of the precipitation zone. A distinct population of high‐energy electrons, with energies of several tens of keV or more, is, however, required to explain the ionization profiles in the center of the zone. The ions and separate population of high‐energy electrons are precipitated from close to the respective stable trapping boundaries (where the particle pitch angle distribution changes from isotropic to trapped). It is suggested that the latter electrons are scattered into the loss cone as the magnetic field becomes more taillike during the substorm growth phase and the radius of field line curvature in the equatorial plane becomes unusually small. The southward drifting band of high radio wave absorption, which has previously been identified by many authors as a characteristic feature of the substorm growth phase in riometer records, is found in the cases presented here to be explained mainly by the separate, high‐energy electron population. The lower‐energy electron population has the spectral characteristics expected for a plasma sheet origin but is found both equatorward and poleward of the trapping boundary for >100‐keV electrons. Southward drifting auroral arcs which also characterize the substorm growth phase are found to be located immediately poleward of the zone of higher‐energy electron precipitation, i.e., immediately poleward of the stable trapping boundary for >10‐keV electrons. This suggests that the latitude at which a substorm is initiated, which is often that of a previously southward drifting arc, may be controlled by the presence of the trapping boundary.
Flow downstream of the heliospheric terminal shock: 1. Irrotational flow
Authors: Suess Steven T. , Nervey Steven
Published: February 2008

Recent reports of remote detection of the heliospheric terminal shock place it near 50 AU. These conflict with standard models which, when combined with current data on the local interstellar medium, place the shock beyond 100 AU. Resolution of this discrepancy has led to hypotheses that invoke cosmic ray pressure, momentum exchange with interstellar neutrals, and magnetic field effects between the shock and the contact discontinuity dividing the solar wind from interstellar plasma. These hypotheses depend not only on properties of the interstellar medium, but also on the downstream three‐dimensional flow between the shock and the contact discontinuity, in the region called the “heliosheath.” We examine the downstream flow field in the absence of magnetic fields under the assumptions that the flow everywhere outside the shock can be approximated as irrotational and incompressible. We find that (1) The distance between the terminal shock and the contact discontinuity is less than the heliocentric distance to the terminal shock, effectively eliminating magnetic field effects in the heliosheath as being dynamically important, (2) the shock should be asymmetric, (3) solar wind flow in the heliosheath is approximately incompressible although it is not well approximated as being irrotational, (4) the external flow in the local interstellar medium can be approximated as incompressible only if the magnetic field contribution to the pressure is considered and if magnetic tension forces are small, and (5) the distant heliotail contains a hot, low density plasma relative to the ambient interstellar medium. Finally, we present plots of the flow topologies and their dependence on flow variables.
Magnetic field‐aligned coupling effects on ionospheric plasma structure
Authors: Heelis R. A. , Vickrey J. F.
Published: February 2008

A mathematical description of the electrical coupling and dynamics of plasma structure in the E and F regions is presented. The scale size dependence of electric field coupling along the magnetic field is examined for a realistic background ionosphere and atmosphere. It is shown that while normalized potentials map reciprocally between two altitudes, the potential disturbance caused by a fixed amplitude plasma density perturbation does not. The magnitude of electrostatic potential created by structured ionization is also shown to be strongly dependent on the altitude of the structure. The existence of plasma density structure at some altitude induces structure at other altitudes along the magnetic field in a scale size selective way. The early evolution of an F region structure that is initially confined in altitude is dominated by parallel diffusion. Low‐altitude image structure growth imposed by electrostatic fields mapped from the source is also very rapid; significant image amplitudes are reached in a matter of seconds. The altitude distribution of the evolving structure is strongly dependent on the scale size. At E layer altitudes, where the Pedersen mobility is high and parallel diffusion relatively slow, a preferential scale size for image structure is apparent for typical F region source spectra. This preferred scale size becomes smaller with increasing height. Above about 200 km altitude, however, parallel diffusion becomes increasingly important with height in determining the altitude and scale size distribution of structure resulting from a source at higher altitudes. The parallel diffusive flux can be modified by the existence of local plasma structure perpendicular to the magnetic field.
Formation, structure, and stability of MHD intermediate shocks
Authors: Wu C. C.
Published: February 2008

Contrary to the usual belief that MHD intermediate shocks are extraneous, we have recently shown by numerical solutions of dissipative MHD equations that intermediate shocks are admissible and can be formed through nonlinear wave steepening from continuous waves. In this paper, the formation, structure and stability of intermediate shocks in dissipative MHD are considered in detail. The differences between the conventional theory and ours are pointed out and clarified. We show that all four types of intermediate shocks can be formed from smooth waves. We also show that there are free parameters in the structure of the intermediate shocks, and that these parameters are related to the shock stability. In addition, we show that a rotational discontinuity can not exist with finite width, indicate how this is related to the existence of time‐dependent intermediate shocks, and show why the conventional theory is not a good approximation to dissipative MHD solutions whenever there is rotation in magnetic field.
New arcs associated with the smooth high‐frequency radiation from Uranus
Authors: Farrell W. M. , Calvert W.
Published: February 2008

New vertex‐early kilometric arcs have been found to be associated with the dominant smooth high‐frequency radio emission from Uranus. These arc features had vertices at about 300 kHz and bandwidths of about 200 kHz. Based on the fact that they exhibited a different temporal occurrence, spectral character, source location, and beaming pattern compared to the more dominant smooth component, it is concluded that they are a separate and independent radio component, different from those previously identified for this planet.
Bulk properties and velocity distributions of water group ions at comet Halley: Giotto measurements
Authors: Coates A. J. , Wilken B. , Johnstone A. D. , Jockers K. , Glassmeier K.‐H. , Huddleston D. E.
Published: February 2008

In the region upstream of comet Halley, pickup heavy ions of cometary origin were directly observed by the Implanted Ion Spectrometer on Giotto. Here we discuss diffusion of this population in pitch angle and in energy, during the approach to the comet and on the outbound leg. The two data sets are compared and qualitative ideas on scattering timescales are inferred. In addition the bulk parameters of these distributions have been computed and we present a comparison of the observed speed in the solar wind frame and the observed density with expectations. We find that pitch angle scattering occurs more slowly than expected with filled shells appearing at 2.5×106 km, and that significant energy diffusion does not occur until the bow shock region. Also the shell distributions downstream of the shock flow at the bispherical bulk speed (related to the Alfvén speed) along the magnetic field with respect to the solar wind in accordance with conservation of energy between the pickup ions and the wave turbulence.
A statistical study of ions and magnetic fields in the Venus magnetotail
Authors: Moore K. R. , McComas D. J. , Russell C. T. , Mihalov J. D.
Published: February 2008

Plasma and magnetic field data from 223 orbits in the first nine tail seasons of Pioneer Venus are examined to provide a statistical description of the combined ion and magnetic field properties of the Venus magnetosheath and magnetotail. This description is based solely on instrumental responses, with no a priori assumptions about existing regions or their plasma and field characteristics. Over 22,000 plasma spectra taken from 7 to 12 Rv downtail are categorized as to whether they contain shocked solar wind, pickup ions, or no discernible plasma. The magnetic field characteristics associated with each plasma spectrum category are examined, and the average two‐dimensional cross‐tail ion flow and magnetic field structures of the Venus tail are determined at a resolution of 0.25×0.25 Rv. Plasma flows everywhere tailward, slowing from 450 km/s in the sheath to less than 300 km/s at the tail axis. Weak outward deflections of 50 km/s or less are found within the tail. The magnetotail is found to be highly draped, with a field‐reversing current sheet not more than 0.25 Rv thick. At these distances the tail is filled with shocked solar wind but also contains pickup ions and a plasma component with fluxes that are not detectable by the Pioneer Venus orbiter plasma analyzer. The bulk flow and field configurations imply a number density of 1.2 cm−3 and a temperature of 9×106 °K for a current sheet composed of 90% protons and 10% O+. These conditions are at or below the limit of instrumental detection. Weak magnetic field asymmetries are associated with the plasma dropouts. The undiscernible plasma component in the tail is consistent with a planetary ion population with fluxes below the instrumental detection threshold. A high E/q plasma population previously interpreted as planetary pickup ions (O+) is found asymmetrically both within the tail and in the adjacent sheath. The undiscernible and pickup ion plasmas (plasmas of Venusian origin) are observed from 5 to 15% of the time within the tail region. This rate depends on solar EUV flux, indicating a photoionization source. Thus the Venus tail is filled with plasma that is primarily shocked solar wind at sometimes undetectable fluxes, which coexists with a photoion population that produces asymmetries in the bulk plasma and magnetic field properties.
Energy of auroral electrons and Z mode generation
Authors: Krauss‐Varban D. , Wong H. K.
Published: February 2008

When the density ratio between the background plasma and energetic auroral electrons (plasma frequencies ωec and ωeh, respectively) is of order unity or less, the dispersion relations derived from cold plasma theory are no longer valid, and new and interesting results may follow. Recently (Wong et al., 1989) we found that in low density source regions, the O mode and second harmonic X mode emissions can prevail over the X mode fundamental radiation if the energy of the suprathermal electrons is low (e.g., several hundred eV). Despite the low energy, the temporal and spatial growth rates may be substantial. The purpose of the present work is to discuss the generation of Z mode radiation in the light of these new results. Specifically, we address the questions whether the thermal effect on the Z mode dispersion can be equally important and whether the Z mode can compete for the available free‐energy source. The numerical results are based on a DGH distribution of energetic electrons (characteristic velocity α), which we have used previously to represent the loss‐cone feature of the energetic electrons. The Z mode growth rate strongly depends on the parameters that we consider, namely the density of the two electron populations, the ratio of electron plasma frequency to electron cyclotron frequency Ωe and the energy of suprathermal electrons. It will not normally prevail under conditions that are favourable for O mode and second harmonic X mode emissions. However, under suitable circumstances the growth rate of the Z mode can be substantial even for low energies of the suprathermal auroral electrons. Specifically, we find that growth generally maximizes for propagation perpendicular to the magnetic field and usually takes place at normalized wave numbers (ck/Ωe)² > 2 above a threshold (ωeh/Ωe)² ≳ 2(α/c)² if the condition ωec ≲ ωeh is satisfied. Below this threshold, growth takes place for (ck/Ωe)² < 2, and a (tenuous) cold plasma background can help sustain the instability.