bluuebunny/crossref_metadata_2025 huggingface dataset:
An experiment on the threshold effect in the coherent wave instability
Authors: Mielke T. A. , Helliwell R. A.
Published: February 2008

A combination of simulated VLF noise, 200 Hz wide, superimposed on a variable amplitude constant frequency test signal (≈ 3 kHz) is transmitted from Siple Station, Antarctica, and received at Lake Mistissini, Quebec. As the test signal is slowly ramped up in power a “threshold” level is reached at which growth and triggering of emissions begin (coherent wave instability). Results show that sufficiently strong simulated noise surpresses the coherent wave instability, which corresponds to increasing the threshold level. This experiment is repeated at progressively lower levels of the simulated noise, until the threshold level for growth and triggering on the test signal no longer changes. At this point the simulated noise power is estimated to equal typical background noise levels due to magnetospheric hiss in the interaction region (near the equatorial plane in a duct at L ≈ 4). These results suggest that unducted magnetospheric hiss is responsible for the threshold effect.
Structural relations for time‐dependent intermediate shocks
Authors: Wu C. C. , Kennel C. F.
Published: February 2008

A quantitative description of time‐dependent magnetohydrodynamics (MHD) intermediate shocks is formulated. In non‐coplanar Riemann problems, time‐dependent 2 → 3 intermediate shocks evolve in time as a localized self‐similar structure whose strength decreases as
and whose width expands as
. We derive structural relations, similar to Rankine‐Hugoniot relations, between the plasma properties and the magnitude of the transverse magnetic field, which we hope can help identify time‐dependent 2 → 3 intermediate shocks at the magnetopause, in the solar wind, and elsewhere in space. The analytic forms of the structural relations agree with our numerical results.
D region disturbances caused by electromagnetic pulses from lightning
Authors: Rodriguez Juan V. , Inan Umran S. , Bell Timothy F.
Published: February 2008

Electromagnetic pulses from weak lightning discharges (E100 = 1 V/m, where E100 is the field strength in the radiation pattern maximum at 100 km) may substantially heat D region electrons, while only pulses with E100 ≥ 20 V/m may create electron density enhancements ≥10 % of ambient. A E100 = 20 V/m pulse from a horizontal radiator at 5 km altitude (e.g., the cloud discharge at the stepped‐leader onset) increases the electron temperature by a factor of ∼400 maximum and the electron density (in one ionization cycle) by ∼230 cm−3 maximum; the widths at half‐maximum of the heated and ionized regions are 200 km and 90 km. A E100 = 40 V/m pulse from a vertical radiator at 0 km altitude (e.g., the vertical return stroke channel) increases the electron temperature by a factor of ∼350 maximum and the electron density (in one ionization cycle) by ∼80 cm−3 maximum; the widths at half‐maximum of the heated and ionized regions are 440 km and 260 km. Several ionization cycles should occur during a typical lightning pulse.
Resolving marine gravity with ERS‐1 satellite altimetry
Authors: McAdoo David C. , Marks Karen M.
Published: February 2008

To estimate how well ERS‐1 data from the 35‐day repeat mission can resolve fine structure in the marine geoid or gravity field, we compare along‐track gravity and gridded gravity fields derived from ERS‐1 fast delivery altimeter data to those computed from Geosat Geodetic Mission altimeter observations in the Southern Ocean. We find that single passes of ERS‐1 data can resolve along‐track gravity anomalies with wavelengths as short as 28–30 km (i.e., comparable to Geosat). However, the ERS‐1 gridded gravity field can only resolve anomalies as short as 66 km (versus ∼18 km for Geosat). This two‐dimensional resolution is limited by the larger ground track spacing (∼35 km at 60° S) of ERS‐1. To resolve fine structure in the marine gravity field it is essential to have altimeter data along more closely‐spaced ground tracks, such as those planned for the ERS‐1 176‐day repeat mission in 1994.
An empirical relationship between thermal conductivity and elastic wave velocities in sandstone
Authors: Zamora Maria , Vo‐Thanh Dung , Bienfait Gérard , Poirier Jean Paul
Published: February 2008

Measurements in three samples of very clean quartz sandstone in the porosity range 4–16 %, under dry and 100 % water‐saturated conditions, show that P‐ and S‐wave velocities are linearly correlated with thermal conductivity. The experimental results agree with the theoretical relation between seismic velocities (predicted by the Kuster and Toksöz model (1974)) and thermal conductivity (predicted by weighted geometric mean).
Global decoupling of crust and mantle: Implications for topography, geoid and mantle viscosity on Venus
Authors: Buck W. Roger
Published: February 2008

The surface of Venus is so hot that the lower crust may be weak enough to allow decoupling of mantle and crust. An analytic model of such decoupling assumes that the shallow mantle forms the top boundary layers of large scale mantle convection cells. Crustal flow is driven by the motion of the mantle and by topographically induced pressure gradients. The model predicts that the lowest lowlands are sites of mantle upwelling and thinner than average crust. Highlands are places where mantle downwells and the crust is thick. Surface heat flow is inversely correlated with elevation, consistent with recent estimates of brittle layer thickness variations on Venus. If the average crustal thickness is about 20 km then the average lower crustal viscosity must be close to 1018 Pa s to allow decoupling. The observed amplitude of geoid highs over highlands requires an Earth‐like increase in mantle viscosity with depth.
An anomaly in the amplitude ratio of SKKS/SKS in the range 100–108° from portable teleseismic data
Authors: Silver Paul G. , Bina Craig
Published: February 2008

We have examined the amplitude ratio SKKS/SKS from two deep focus events recorded by the APT89 portable seismic experiment. For one of the events (beneath the Solomon Islands), we obtain a profile of 18 stations in the distance range 99–108°. We observe large systematic variations in this ratio as a function of distance. From 99° to 106° the ratio is fairly constant and in the range 0.1 to 0.5, but it then rapidly increases to the range 0.5–3.5 between 106° and 108°. Such variations are in fact predicted by standard earth models, such as PREM, although this feature of the wavefield has not previously been observed. It is due to the momentary drop in SKS amplitude at the ray parameter where the reflected phase ScP is critically reflected. Because ray theory is not valid in this distance range, we use Full Wave theory to model the amplitudes. We have been able to match the distance pattern reasonably well if we use a model with the P‐velocity at the base of the mantle reduced by 1.5% compared to PREM. Although fewer stations are available, the second event shows the same basic distance pattern, except that the abrupt increase in the ratio occurs at a closer distance. This suggests an even lower P‐velocity for this region (about 3.5% slower than PREM) and is consistent with lateral heterogeneity at the base of the mantle of a few percent.
Evolutionary impact of sputtering of the Martian atmosphere by O+ pickup ions
Authors: Luhmann J. G. , Johnson R. E. , Zhang M. H. G.
Published: February 2008

Nonthermal processes such as dissociative recombination of ionospheric molecules are known to lead to loss of atmospheric constituents (N, O, C) at Mars where the gravitational potential is easily overcome by the energy imparted to the product atoms. Moreover, observations of escaping planetary ions on the PHOBOS‐2 spacecraft showed that the solar wind is presently scavenging significant amounts of both oxygen and molecular species as it flows past the planet. Because both the sun and the atmosphere of Mars have changed over time, the evolutionary importance of these processes cannot be estimated by simply multiplying the contemporary loss rates by the solar system age. Models of these loss mechanisms must include consideration of the evolution of the solar EUV intensity and solar wind and their effects. Here we describe calculations of solar wind‐induced loss rates for evolving solar and atmospheric conditions like those described by Zhang et al. [1992a], but including sputtering of the Martian atmosphere by reentering O+ pickup ions. The inclusion of the sputter loss increases by about 30% the cumulative, estimated loss of oxygen to that in ∼ 50 m of water (global surface depth) over the last ∼3.5 billion years, when contemporary loss mechanisms are thought to have become dominant. More significant is the result that these ions also sputter CO2 and its fragments in substantial amounts. That integrated loss is equivalent to ∼ 0.14 bar atmospheric CO2 pressure, of the order of some estimates of Mars' early atmospheric inventory.
Tropical ozone loss following the eruption of Mt. Pinatubo
Authors: Schoeberl Mark R. , Bhartia P. K. , Hilsenrath Ernest , Torres Omar
Published: February 2008

Total Ozone Mapping Spectrometer (TOMS) measurements of equatorial total ozone following the eruption of Mt. Pinatubo show a decrease of up to 6% over climatology. Ozone losses begin approximately a month following the eruption, consistent with the time required for the SO2 to convert to sulfuric acid aerosol. The thick aerosol layer interferes with the TOMS retrieval, but this interference is small and easily accounted for in the retrieval. Ozone values remain below climatology until December, 1991. Ozonesonde data from Natal, Brazil taken before and two months after the eruption support TOMS observations of ozone loss. These sondes show that the ozone loss region is confined to a 2–3 km thick layer between 24 and 28 km.
Direct measurement of over pressure of a volcanic blast on the June 1991 eruption at Unzen Volcano, Japan
Authors: Taniguchi Hiromitsu , Suzuki‐Kamata Keiko
Published: February 2008

Over pressure of a volcanic blast has been measured for the first time by a lead‐plate blastmeter at Unzen Volcano, Kyushu, Japan. The over pressure of the volcanic blast on June 8 was 0.28 bar at 2700 m distance from the crater, and that was less than 0.06 bar at 4400 m distance. The estimated explosive yield of the explosion accompanied by the blast was 1.1 × 107 kg TNT (5 × 1020 erg) calculated based on the over pressure and distribution of the blast effect. The explosion energy is as much as 1/600 of that of the blast at Mount St. Helens on 18 May 1980. The lead‐plate of the blastmeter at 2700 m has also recorded the collision with some accretionary lapillis. The maximum particle velocity of the blast was estimated to be about 75 m/sec based on the trace of the collision. The significance of over pressure for the prediction of volcanic hazards is also discussed briefly.
Changes in the Earth's rotation by tectonic movements
Authors: Vermeersen L. L. A. , Vlaar N. J.
Published: February 2008

Present‐day true polar wander and the secular non‐tidal acceleration of the earth are usually attributed to post‐glacial rebound. In the models which relate this rebound to changes in rotation, the mantle is assumed to relax passively to the melted ice‐loads. The lithosphere is usually modeled as a highly viscous upper layer in these models, having viscosities which exceed mantle viscosities by several orders of magnitude.

We propose that lithospheric processes unrelated to post‐glacial rebound and taking place under non‐isostatic conditions are also able to induce non‐negligible influences on the earth's rotation. Examples of such processes are mountain building and erosion, foundering flexure of oceanic basins and lithospheric snapbacking resulting from detachment of subducting slabs. Lithospheric and crustal rheologies and intraplate‐stresses are the dominant factors in these mechanisms, contrary to the mantle rheologies which are assumed to dominate the process of post‐glacial rebound.
Field‐aligned current signatures during the March 13–14, 1989, Great Magnetic Storm
Authors: Fujii R. , Fukunishi H. , Kokubun S. , Sugiura M. , Tohyama F. , Hayakawa H. , Tsuruda K. , Okada T.
Published: February 2008

Characteristics of field‐aligned currents (FACs) in the evening and morning regions during the March 13–14, 1989, great magnetic storm have been determined using magnetic and electric field data obtained from the EXOS D spacecraft. This storm began with an SSC at 0128 UT on March 13, and the second SSC occurred at 0747 UT on the same day. The storm continued until March 14. The equatorward boundary of the FAC region began to move equatorward right after the first SSC in both the evening and morning sectors, but the poleward boundary did not immediately respond to the SSC. The equatorward boundary of the FAC system reached as low as below 48° invariant latitude, which corresponds to L = 2.2, and the latitudinal width of the FAC region increased greatly, particularly in the morning sector (∼33° in invariant latitude). In the evening sector the conventional current system characterized by a pair of upward region 1 and downward region 2 FACs changed into complicated patterns consisting of many pairs of upward and downward FACs with the development of the storm, particularly around 22 UT on March 13 when an intense eastward electrojet was observed as low as 50° invariant latitude on the ground. In the morning sector an additional large‐scale upward FAC was observed poleward of the conventional downward region 1 and upward region 2 FAC system throughout the storm. In addition, a pair of FACs with a narrow latitudinal width (∼1.5°) was observed at the poleward boundary of the extra upward FAC.
The role of upstream waves in supercritical quasi‐parallel shock re‐formation
Authors: Scholer M. , Burgess D.
Published: February 2008

It has been demonstrated by hybrid simulations of collisionless shocks that the shock itself is not stationary but exhibits a cyclic behavior. We have performed a number of one‐dimensional hybrid simulations of collisionless shocks and of the interaction of two plasma streams in order to assess the role of upstream waves in the re‐formation process. We found that when upstream waves and reflected ions are present, the waves inevitably steepen up and lead to shock re‐formation. To investigate the interaction of the reflected ions with upstream waves, the hybrid code is then used in two stages. In the first stage a shock together with upstream diffuse particles and waves is generated. In the second stage, upstream wave trains are isolated, and their subsequent interaction with a finite length ion beam as well as with ions emitted from a moving point source is investigated. The results show that the original wave amplitude grows and the wave steepens. This is due to a deceleration and deflection of the beam ions once they encounter a wave crest with a large local value of the angle ΘBn between the magnetic field and the direction of beam propagation: the deflection leads to a local density increase and due to the compressibility to a corresponding magnetic field increase. This sets up a positive feedback loop during which the background ions are also decelerated and can even get reflected. It is concluded that shock re‐formation can be caused by the interaction of reflected ions with low‐frequency upstream waves.
Kelvin‐Helmholtz instability at the magnetospheric boundary: Dependence on the magnetosheath sonic Mach number
Authors: Miura Akira
Published: February 2008

It has recently been demonstrated, by means of a two‐dimensional MHD simulation, that a finite thick velocity shear layer with super‐Alfvénic velocity jump at the magnetospheric boundary is unstable to the Kelvin‐Helmholtz (K‐H) instability no matter how large the magnetosheath sonic Mach number (MS); a result suggesting that the tail flank boundary of the magnetosphere is unstable to the K‐H instability. In order to investigate this consequence further, the dependence of the development of the K‐H instability on MS is studied in detail. For all magnetosheath sonic Mach numbers a velocity boundary layer is formed by the instability inside of the magnetopause, and it becomes wider for a smaller magnetosheath sonic Mach number. A flow vortex is excited at the inner edge of the velocity boundary layer for all sonic Mach numbers, and the magnetopause boundary is more highly nonlinearly corrugated by the instability for a smaller sonic Mach number. The net energy and momentum flux densities into the magnetosphere are calculated just prior to the saturation stage; for 1.0 < MS < 3.0 the energy flux density into the magnetosphere is approximated by 0.054 MS ρ0 CS³/2 = 0.045V0p0 (where ρ0 is the unperturbed magnetosheath plasma density, p0 is the unperturbed magnetosheath pressure, V0 is the unperturbed magnetosheath flow velocity, and CS is the magnetosheath sound speed), and the momentum flux density into the magnetosphere or the tangential (shearing) stress at the boundary is approximated by 0.083p0. The anomalous viscosity by the instability decreases in the absolute magnitude with increasing MS; this result suggests that the dayside (except the subsolar region) and the dawn‐dusk magnetopauses, where the magnetosheath flow remains subsonic, are the most viscous parts of the boundary, although the tail flanks are also found to be viscous enough for the viscous interaction. The structure of the weak shock in the magnetosheath developed from the K‐H instability and the asymptotic eigenmode structure of the instability are elucidated. The relevance of the simulation results to the viscous interaction and a ULF wave generation is finally discussed.
An assessment of lightning or in situ instabilities as a source for whistler mode waves in the night ionosphere of Venus
Authors: Strangeway R. J.
Published: February 2008

Burst rate and polarization studies have shown that at least half of the impulsive 100‐Hz signals detected at low altitudes in the night ionosphere of Venus by the Pioneer Venus Orbiter electric field detector are whistler mode waves. At the Earth, whistler mode waves are generated both by lightning and by in situ instabilities, and we investigate the likelihood of either being a source for the VLF bursts at Venus. Because the magnetic field is weak, the nightside ionosphere at Venus can be a high βe plasma, where βe is the ratio of thermal to magnetic energy density for electrons. For βe ≈ 1, electron thermal speeds are comparable to whistler mode phase speeds and Landau resonant instabilities will not apply to the Venus ionosphere. However, gyroresonant instabilities may operate at Venus. Linear stability analysis shows that the damping by ambient electrons is weak for βe ≪ 1 and whistler mode waves could be generated by either lightning or instabilities driven by superthermal electrons if βe is low. On the other hand, the waves would be associated with high βe if they are generated by temperature anisotropy in the ambient electron population. The dependence of 100‐Hz wave intensity on the plasma parameters shows that the waves are most intense for low βe, and hence the waves cannot be generated through temperature anisotropy of the ambient electrons. The 100‐Hz waves are also most intense when the whistler mode damping is weakest, but not when the growth rate due to superthermals is greatest. Moreover, the 100‐Hz wave intensity does not depend on electron density, as would be expected if superthermal electrons were responsible for the waves, since the growth rates are largest when the beam to ambient density ratio is greatest. Consequently, whistler mode instabilities do not appear to explain the 100‐Hz waves at Venus, and atmospheric lightning is the most likely source for these waves.
Magnetic field models from energetic particle data at Neptune
Authors: Selesnick R. S.
Published: February 2008

The locations of features in the Voyager 2 energetic particle data from Neptune are combined with uncertainties in the multipole expansion of the planetary magnetic field to derive new magnetic field models that are consistent both with various interpretations of the particle features and with the magnetic field data. While assumptions as to the origin of the features must be made, they do not provide sufficient constraints to obtain significant new information on any of the unknown multipole coefficients. However, the magnetic L shell positions of the particle features, which are interpreted primarily as absorption signatures of Neptune's satellites, can, in general, be brought into agreement with expected values.
The spatial extent of radial magnetic pulsation events observed in the dayside near synchronous orbit
Authors: Engebretson M. J. , Murr D. L. , Erickson K. N. , Strangeway R. J. , Klumpar D. M. , Fuselier S. A. , Zanetti L. J. , Potemra T. A.
Published: February 2008

We have used simultaneous observations from the AMPTE CCE satellite, in an elliptical orbit with apogee at 8.8 RE, and GOES 5 and GOES 6, in a geostationary orbit at 6.6 RE, to investigate the radial and longitudinal extent of magnetic pulsation events with predominantly radial polarization. Twenty‐one events were selected by visual inspection of color‐coded Fourier spectrograms produced from data from all three satellites during a several month interval in fall 1984 when the apogee of AMPTE CCE was on the dayside; sixteen events were observed at all three satellites. Local time of the observed events ranged from 0900 to 1900 MLT, but the apparent longitudinal extent of the oscillation region varied considerably from event to event, ranging from the minimum resolution of 1.5 hours MLT (the local time separation of GOES 5 and GOES 6) to 8 hours MLT. Plasma wave data from AMPTE CCE indicated the waves occurred in regions of density characteristic of the outer plasmasphere (∼10 cm−3) and quite far outside the L shell region where densities reached 400 cm−3. These events occurred during magnetically quiet times usually after magnetic storms; interplanetary magnetic field data, when available, indicated an either roughly radial or northward orientation during the events. Wave onset often (but not always) occurred within one hour after sharp drops in the AE index to values of 100 or below. There was no apparent correlation of wave onset or amplitude with plasma beta, which ranged from 0.23 to 1.09 during the nine events presented here. Our frequent observation of the simultaneous onset of waves at different local times with considerably different frequencies reinforces the belief that the onset of these pulsations is determined by an instability that covers some longitudinal extent but that the frequencies are determined by local Alfven resonance conditions, not by the bandwidth of an external source. The data suggest that local plasma density increases associated with plasmaspheric refilling are the immediate cause of local instabilities leading to wave onset; the increase in density may alter the field line resonance conditions to allow the free energy of ∼ 100‐keV trapped ions to drive waves via the drift‐Alfven‐ballooning‐mirror mode instability.
Incoherent scatter plasma lines at angles with the magnetic field
Authors: Fredriksen Åshild , Bjørnå Noralv , Lilensten Jean
Published: February 2008

Photoelectron‐enhanced plasma lines at angles with the geomagnetic field are investigated numerically and experimentally with the EISCAT UHF radar. Numerical simulations of enhanced plasma line intensities are carried out for a power law photoelectron distribution and a set of different temperatures and angles with the magnetic field. The calculations indicate that plasma line measurements may be successful at large angles even in the E region, because of the low electron temperature in this region. A filter bank power profile experiment was set up to measure upshifted and downshifted plasma line intensities as a function of angle with the field, from small to intermediate angles, and also for a fixed large angle (74°) with the field. As predicted by the simulations, plasma lines were detected at large angles also in the E region at 120 km altitude. To study the angular variation of the plasma line intensities, the photoelectron flux was calculated from a model based on the Boltzmann transport equation with parameters describing the ionospheric conditions present during the experiment. A power law flux was fitted to the calculated data and used in the numerical calculations to model the plasma line intensities for comparison with the experimental results. When allowing for a pitch angle dependence in the flux, the plasma line temperatures can be predicted to within a very good accuracy at altitudes where remnants of the N2 excitation dip are no longer present in the photoelectron distribution.
Rotational polarities of sudden impulses in the magnetotail lobe
Authors: Kawano H. , Yamamoto T. , Kokubun S. , Lepping R. P.
Published: February 2008

A sudden impulse (SI) is a sudden change in the magnetic field strength which is caused by a change in the solar wind pressure and is observed throughout the magnetosphere. In this report we have examined the rotations of the magnetic field vectors at times of SIs in the magnetotail lobe, by using IMP 6, 7, and 8 magnetometer data. The following properties have been found: (1) At the time of SI the arrowhead of the magnetic vector tends to rotate in one plane. (2) The plane of rotation tends to include the unperturbed magnetic field vector. (3) The plane of rotation tends to be aligned with the radial direction from the magnetotail axis. (4) The magnetic vectors have a particular rotational polarity: when the plane of rotation is viewed so that the Sun is to the right of the viewed plane and the magnetotail axis is to the bottom, the arrowhead of the vector tends to rotate counterclockwise in this plane. These magnetic vector properties are consistent with those expected when part of an increase in solar wind lateral pressure squeezes the magnetotail axisymmetrically while moving tailward.
The variation of reconnection rate at the dayside magnetopause and cusp ion precipitation
Authors: Lockwood M. , Smith M. F.
Published: February 2008

A method is presented which allows estimation of the variation of the rate of magnetic reconnection at the day side magnetopause. This is achieved using observations of the cusp particle precipitation made by low‐altitude polar‐orbiting spacecraft. In this paper we apply the technique to a previously published example of a cusp intersection by the DMSP F7 satellite. It is shown that the cusp signature in this case was produced by three separate bursts of reconnection which were of the order of 10 min apart, each lasting roughly 1 min. This is similar to the variation of reconnection rate which is required to explain typical flux transfer event signatures at the magnetopause.
Propagation and nonlinear interaction of low‐frequency electrostatic waves in the polar cap E region
Authors: Pécseli H. L. , Trulsen J. , Bahnsen A. , Primdahl F.
Published: February 2008

Low‐frequency electrostatic fluctuations in the polar cap E region were detected by an instrumented rocket payload. The basic characteristics of the fluctuations, such as speed and direction of propagation, were obtained by a correlation analysis. The bispectrum, in particular, provided indications of nonlinear couplings. Harmonic wave interactions were observed for a wide altitude range. Evidence for cascading of wave energy toward small wavelengths was obtained. The accuracy of the statistical analysis was tested by a Monte Carlo simulation.
Approximate magnetotail equilibria with parallel flow
Authors: Young Robert , Hameiri E.
Published: February 2008

Approximate equilibrium solutions for the two‐dimensional magnetohydrodynamic equilibrium equations with flow along magnetic field lines applied to modeling the Earth's magnetic tail are constructed. An alternative version of the condition for whether a given magnetospheric configuration is open or closed is presented. Also developed are examples of open and closed magnetospheric configurations with flow occurring in a layer modeling the plasma sheet boundary layer. Examples of finite‐size compressible plasmoids traveling in the magnetotail are presented. The relation between the profile of the total pressure (thermal plus magnetic) and the shape of the plasmoid is also discussed.
Spatial characteristics of low‐latitude Pc3–4 geomagnetic pulsations
Authors: Ziesolleck C. W. S. , Fraser B. J. , Menk F. W. , McNabb P. W.
Published: February 2008

In order to identify the generation mechanisms of low‐latitude Pc3–4 geomagnetic pulsations, data were obtained from a meridional chain of induction magnetometers spanning L values from 1.4 to 2.7 (−30° to −52° geomagnetic latitude). The spatial structure of Pc3–4 signal parameters was examined by means of spectral, polarization and interstation phase analysis. This paper describes three typical individual events whose spectral, polarization and phase characteristics indicate the existence of field line resonances at low latitudes within the plasmasphere. The spatial phase structure shows a local minimum and indicates phase motion toward the resonance region. Resonance region widths of ΔL = 0.2 to ΔL = 0.8, corresponding to north–south ionospheric scale lengths of 250 and 1500 km or more, respectively, are seen. The coupling of field line resonances to global compressional modes is considered to be a likely generation mechanism of these pulsations.
Upflowing ionospheric ions in the auroral region
Authors: Lu G. , Reiff P. H. , Moore T. E. , Heelis R. A.
Published: February 2008

Observations of upflowing ionospheric ions are obtained nearly simultaneously by DE 1 and DE 2 over the nightside auroral regions. At low altitudes, the mean value of the net upward ion number flux is of the order of 109 cm−2 s−1. The ionosphere is predominantly O+, and the flux of ions with energy greater than 5 eV is a very small fraction (less than 1%) of the total ion flux. At high altitudes, the upflowing ions are accelerated by a parallel electric field and heated (with characteristic energies of hundreds of electron volts). Comparing upflowing fluxes at high and low altitudes yields an estimated height of the bottom of the auroral acceleration region of 1400–1700 km for the region of peak potential drop. This low‐altitude acceleration could either be from a parallel electric field or from perpendicular acceleration. The fluxes at the edges of the arc are mostly H+ thus implying a higher‐altitude base of the acceleration region at the edges where the potential drop is lower.
Ionospheric signatures of dayside magnetopause transients: A case study using satellite and ground measurements
Authors: Denig W. F. , Burke W. J. , Maynard N. C. , Rich F. J. , Jacobsen B. , Sandholt P. E. , Egeland A. , Leontjev S. , Vorobjev V. G.
Published: February 2008

A case study is presented of coordinated ground and space measurements featuring a set of transient, auroral fragments located on the poleward side of a stable cusp/cleft arc. Optical ground data from Ny Alesund (Svalbard) and Heiss Island (Franz Josef Land) were combined with DMSP F9 satellite measurements to examine the characteristics of these auroral features. A stable red arc stretched across most of the dayside auroral zone in a region dominated by westward convection in accordance with the orientation of the IMF. Poleward of the red arc were several, westward moving auroral jets having characteristics similar to midday auroral breakup events. Such events may be ground signatures of transitory magnetic merging at the dayside magnetopause. If so, the driven convective motion of these structures should contribute to the polar cap potential. Within this limited data set we find that although the transitory structures have an inherent potential associated with the motion of the optical signatures the structures on the whole contribute a small fraction of the total polar cap potential.
Parameterization of chaotic particle dynamics in a simple time‐dependent field reversal
Authors: Chapman Sandra C. , Watkins Nicholas W.
Published: February 2008

We investigate single charged particle dynamics in the Earth's magnetotail using a simple, scale free magnetic field model which is explicitly time dependent, with a corresponding induction electric field. The time‐dependent Hamiltonian of particle motion in the simple model describes a system of two coupled oscillators which is driven. When the (time‐dependent) ratio of the oscillation frequencies is different from unity, the motion is regular, but if it approaches unity at some point on the trajectory, the motion becomes chaotic. A parameter α is found which characterizes the adiabaticity of the system, and the transition time for behavior in the system is given by at αt → 1. The condition at αt ≈ 1 is equivalent to κ ≈ 1 in the static parabolic field model discussed by previous authors (Buchner and Zelenyi, 1989). The explicit time dependence produces two possible classes of motion, ordered by α. If the reversal is thick and is folding slowly, so that α ≪ 1, the motion is a transition in behavior, from regular μ conserving to chaotic “cucumberlike” trajectories, when t ≈ 1/α. If on the other hand, the reversal is thin and folds quickly, so that α ≫ 1, the particles execute regular “ring type” trajectories once t > 1/α. Simple estimates of presubstorm magnetotail parameters indicate that electrons in a slowly thinning (15‐min time scale), thick (1 RE) sheet have α ≪ 1, whereas protons in a thin (several hundreds of kilometers) sheet which thins on a 5‐min time scale have α ≫ 1. Hence the behavior of the particles, and by implication, the field reversal which they support, will depend upon the adiabaticity of the system α as well as the “chaotization” parameter αt; this is shown only to be the case in a model which is explicitly time dependent and which includes the induction electric field.
Magnetoacoustic and Alfvén potentials for stationary waves in a moving plasma
Authors: McKenzie J. F. , Woodward T. I. , Inhester B.
Published: February 2008

The inductive interaction between a conducting body and a magnetized plasma in relative uniform motion generates a system of stationary waves in the frame of the body. This wave system is composed of both Alfvénic and magnetoacoustic perturbations associated with each of which there are corresponding electric potentials and currents. Here we develop the Green's function for each of the modes. The well‐known Alfvén “wings” are represented by delta functions which propagate the parallel components of vorticity and current along the Alfvén lines. The magnetoacoustic modes are characterized by total pressure (plasma plus magnetic) and dilatation perturbations which are propagated along the envelopes of the fast and slow mode characteristics. The concomitant electric potentials are then obtainable from a component of the momentum equation which can be written in the form of a wave equation for the potential with an Alfvénic wave operator and the magnetoacoustic pressure gradient acting as the driving term. The important consequence is that the potential associated with the compressive modes is hybrid in nature in that it is singular both on the Alfvén lines and on the magnetoacoustic characteristics so that the properties of both modes are interwoven in a complicated fashion. On the other hand, the slow mode potential and current perturbations exhibit singularities on the slow mode wings and the Alfvén lines away from both of which they decay rather gently in a two‐dimensional dipolelike fashion. By using the method of stationary phase we elucidate the detailed fine structure of the slow mode wave crests which consist of two closed, hollow wings, whose cross section reflects the topology of the slow mode group velocity surface and which emanate from the conducting body and extend out parallel and antiparallel to the background magnetic field. As an example of how Green's functions may be used to construct more general solutions and in an attempt to tackle the problem of the self‐consistent source current distribution inside the conducting body we formulate an integral equation which determines the current along a thin wire of finite length. We demonstrate that including the effect of induced fields radiated in the magnetoacoustic modes enhances the effective wave impedance of the plasma environment relative to the results of conventional treatments which only take account of the Alfvén mode.
Very long baseline interferometer measurements of plasma turbulence in the solar wind
Authors: Sakurai Takayuki , Spangler Steven R. , Armstrong John W.
Published: February 2008

Turbulence in the solar wind plasma was studied using angular broadening measurements of 10 extragalactic compact radio sources (quasars) with a very long baseline interferometer (VLBI) at 4.99 GHz. Unlike other angular broadening studies, the measured broadening size was corrected for intrinsic source structures which were obtained from a separate VLBI observation. The solar elongations of the sources ranged from 18 RS to 243 RS, and five sources with elongations ≤ 61 RS showed varying degrees of broadening. The measured angular sizes are considerably less than predicted by the well‐known empirical relationship of Erickson [1964], as well as two other models for strength of scattering as a function of solar elongation. However, the data are in good agreement with a model for the spatial power spectrum of the turbulence proposed by Coles and Harmon [1989]. This model consists of a Kolmogorov spectrum at large scales, but with an enhancement of power near the wavenumber corresponding to the ion inertial length. Two of our sources, 1148–001 and 1253–053 (3C279), show substantial differences in the amount of scattering, even though they are at similar solar elongations (29 versus 35 RS). Data to which we have access indicate that the state of the corona along the lines of sight to these sources may have been quite different. Angular broadening measurements with VLBI interferometers currently under development (primarily the very long baseline array) will allow a global view of plasma turbulence out of the ecliptic plane and thus be complementary to the point in situ measurements with Ulysses.
A new tool for analyzing microinstabilities in space plasmas modeled by a generalized Lorentzian (Kappa) distribution
Authors: Summers Danny , Thorne Richard M.
Published: February 2008

In space plasmas, e.g., planetary magnetospheres and the solar wind, it has been observed that particle velocity distributions typically possess a non‐Maxwellian high‐energy tail that can be well modeled by a generalized Lorentzian (kappa) distribution. The generalized Lorentzian distribution is characterized by a spectral index κ, varies as {energy}−(κ+1) at high velocities, and approaches a Maxwellian distribution as κ → ∞. As a natural analogue to the widely used plasma dispersion function Z(ξ), which is based on the Maxwellian distribution, we have recently introduced a new special function Zκ*(ξ) based on the generalized Lorentzian distribution; we call Zκ*(ξ) the modified plasma dispersion function. Because Zκ*(ξ) can be expressed in simple closed form, Zκ*(ξ) is easier to use than Z(ξ) both from analytical and computational points of view. Zκ*(ξ) is, moreover, a natural tool for analyzing microinstabilities in a variety of space plasmas. In this paper we use Zκ*(ξ) to analyze three classical problems of plasma physics: Landau damping of Langmuir waves; ion acoustic instability in a current‐carrying plasma; and cyclotron resonant instability of electromagnetic R mode waves propagating parallel to an ambient magnetic field. In each case we find that results for a generalized Lorentzian plasma can differ significantly from those in a Maxwellian plasma. Previous calculations based on a Maxwellian distribution, that purport to apply to waves in space, may therefore be subject to reexamination.
Response of the ionospheric convection pattern to a rotation of the interplanetary magnetic field on January 14, 1988
Authors: Cumnock J. A. , Heelis R. A. , Hairston M. R.
Published: February 2008

Ionospheric convection signatures observed over the polar regions are provided by the DMSP F8 satellite. We consider five passes over the southern summer hemisphere during a time when the z component of the interplanetary magnetic field was stable and positive and the y component changed slowly from positive to negative. Large‐scale regions of sunward flow are observed at very high latitudes consistent with a strong z component. When By and Bz are positive, but By is greater than Bz, strong evidence exists for dayside merging in a manner similar to that expected when Bz is negative. This signature is diminished as By decreases and becomes smaller than Bz resulting in a four‐cell convection pattern displaced toward the sunward side of the dawn‐dusk meridian. In this case the sign of By affects the relative sizes of the two highest‐latitude cells. In the southern hemisphere the duskside high‐latitude cell is dominant for By positive and the dawnside high‐latitude cell is dominant for By negative. The relative importance of possible electric field sources in the low‐latitude boundary layer, the dayside cusp, and the lobe all need to be considered to adequately explain the observed evolution of the convection pattern.
Corotating MeV/amu ion enhancements at ≤1 AU from 1978 to 1986
Authors: Richardson I. G. , Barbier L. M. , Reames D. V. , von Rosenvinge T. T.
Published: February 2008

MeV/amu ion enhancements associated with corotating high‐speed solar wind streams in 1978–1986 during pre‐solar maximum to near solar minimum conditions are studied using ISEE 3/ICE, IMP 8, and Helios 1 data. Around 50% of corotating streams contain energetic ion increases. These increases extend to ∼25 MeV/amu, where they merge into the galactic cosmic ray background, and are most evident approaching solar minimum. Sunward ion streaming in the solar wind frame (first‐order anisotropy ∼20%) and positive radial intensity gradients (∼400%/AU) are consistent with acceleration in the outer heliosphere at corotating shocks followed by streaming into the inner heliosphere. The spectra and intensities show little solar cycle variation. The spectra of ions from protons to Fe at ∼2–20 MeV/amu are approximated equally well by exponentials in momentum dJ/dP ≈ exp (−P/P0), P0 = 11–16 MeV c−1 amu−1, or by distribution functions ƒ ≈ exp (−υ/υ0), υ0 = 0.18–0.25 (MeV/amu)1/2, with equivalent power law in energy slopes in the range ∼ −3 to −4. Ion abundances are correlated with the stream peak solar wind speed. In slower corotating streams (maximum solar wind speed <600 km/s), mean abundance ratios are protons/4He = 43 ± 18; 4He/O = 54 ± 23; C/O = 0.62 ± 0.06; Mg/O = 0.19 ± 0.03, and Fe/O = 0.14 ± 0.02. These show some similarity to the corresponding ratios for “solar energetic particles” (SEP) (protons/4He = 70 ± 10; 4He/O = 55 ± 3; C/O = 0.48 ± 0.02; Mg/O = 0.21 ± 0.01 and Fe/O = 0.16 ± 0.02) which are typically accelerated by shocks passing through slow solar wind. In corotating events in higher‐speed streams, these ratios become protons/4He = 19 ± 5; 4He/O = 130 ± 35; C/O = 0.89 ± 0.05; Mg/O = 0.14 ± 0.01, and Fe/O = 0.10 ± 0.01 and more closely resemble the corotating event abundance ratios measured in high‐speed streams during the mid‐1970s solar minimum (protons/4He = 17 ± 7; 4He/O ∼ 160 ± 50; C/O = 0.89 ± 0.1; Mg/O = 0.13 ± 0.03, and Fe/O = 0.096 ± 0.05). Solar wind plasma may also show similar variations in composition with solar wind speed (based on the limited solar wind composition measurements available) so that the energetic ion compositions are consistent with the acceleration of corotating event ions and SEPs from the solar wind. The ordering of corotating event and solar wind abundances by first ionization potential and their variation with solar wind speed suggest that conditions in the ion‐neutral fractionation region in the upper chromosphere determine the abundances and are associated in some way with regulation of the solar wind speed.
East‐west ion drifts at mid‐latitudes observed by Dynamics Explorer 2
Authors: Heelis R. A. , Coley W. R.
Published: February 2008

Zonal ion drifts measured from the the polar orbiting DE 2 spacecraft are examined to determine the effects of dynamo electric fields and penetration of high latitude electric fields at middle latitudes. Construction of a local time distribution from satellite data results in a mixture of local time and season as well as a range of magnetic activity encompassing Kp ≤ 2 and Kp ≥3. Thus some combination of magnetospheric effects, expected to dominate during disturbed times, are seen during both quiet and disturbed times and solar tidal influences are most easily observed during quiet times. During quiet times, at invariant latitudes near 25°, the solar diurnal tide dominates the local time distribution of the ion drift. At latitudes above 50° a diurnal component of comparable magnitude is also present, but its magnetospheric origin produces a shift in phase of almost 180° from the lower latitude diurnal tide. In the intervening region, between 20° and 50° invariant latitude, semidiurnal and terdiurnal components in the local time distribution of the drift velocity are also seen. These components are generally larger than those seen by ground based radars during quiet times and may be attributable in part to a difference in solar activity and in part to a combination of the solar tides and magnetospheric penetration fields.
Mirror and ion cyclotron anisotropy instabilities in the magnetosheath
Authors: McKean M. E. , Winske D. , Gary S. P.
Published: February 2008

Magnetosheath plasmas are typically characterized by a proton β > 1 and a proton temperature anisotropy T⊥/T∥ > 1. Such a plasma is unstable to both the ion cyclotron anisotropy and mirror instabilities, and evidence for both of these growing modes has been found in the magnetosheath. In this paper, we use one‐dimensional hybrid simulations to investigate the kinetic properties of these two instabilities. We find that, for moderate values of ion β and temperature anisotropy, the two instabilities produce similar levels of turbulence, but that the cyclotron instability apparently produces higher fluctuation levels than the mirror instability for high β or anisotropy. Although both instabilities saturate by reducing the ion temperature anisotropy, mirror waves accomplish this through the use of the wave magnetic fields, whereas cyclotron waves do so by using the wave electric fields in resonant interactions with the ions. Consequently, mirror waves preferentially affect those ions with large magnetic moment, while cyclotron waves affect a broader range of ions and thereby dominate the isotropization process even when the mirror mode is somewhat stronger. We also investigate the utility of transport ratios as mode identifiers, and find that the Alfvén ratio in particular can be useful in distinguishing between the two modes.