bluuebunny/crossref_metadata_2025 huggingface dataset:
Induced magnetosphere of comet Halley: 2. Magnetic field and electric currents
Authors: Israelevich P. L. , Ershkovich A. I.
Published: February 2004

An induced magnetosphere of a comet rotates around the Sun‐comet line along with the interplanetary magnetic field (IMF) vector. During the Giotto flyby near comet Halley the direction of the IMF changed several times. For this reason, the trajectory of the Giotto spacecraft, being represented in the IMF‐related coordinate system, covered rather well the transverse cross section of the comet Halley magnetosphere. As a result, two‐dimensional distributions of both the magnetic field and electric current density have been calculated in the transverse cross section of the induced magnetosphere of comet Halley. The distributions reveal the following facts. The magnetic barrier is axially symmetric (within the accuracy of the grid ∼ 10³ km). Such a behavior is associated with the fact that the magnetic field strength in the barrier is governed by the dynamic pressure of the solar wind and does not depend on the ion‐neutral friction (which affects only the location of the magnetic field maximum). Along with draping about the dayside of a comet, magnetic field lines also drape about its flanks (where the magnetic field turns out to be enhanced). As a result, the Lorentz electric field inside the cometary ionosphere decreases. Lines of the electric current drape about the cometary ionosphere in a manner resembling the magnetic field lines. The region of the magnetic barrier in front of the contact surface is shown to be an electric load for the MHD generator arising as a result of the solar wind interaction with a comet.
A unifying comparison of nearly scatter free transport models
Authors: Schwadron N. A. , Gombosi T. I.
Published: February 2004

Gombosi et al. (1993) recently derived a modified telegrapher's equation for charged particle transport under the influence of isotropic scattering. This equation obeys causality and disallows upstream diffusion for particles with random velocities smaller than the bulk flow velocity. The acausal diffusion equation was obtained to lowest order in the expansion of smallness parameters. The paper by Gombosi et al. (1993) prompted responses from Pauls et al. (1993) and Earl (1993). This paper is written to explain the differences between the methods, assumptions, and results of Gombosi et al. (1993), Pauls et al. (1993), and Earl (1993) and presents a new method of obtaining approximate solutions. It is shown that the assumptions used by Gombosi et al. (1993) and Pauls et al. (1993) are physically equivalent. In these papers, a second‐order expansion is made by introducing smallness parameters, not by truncating an eigenfunction series. Earl (1993) estimates the overall behavior of a dispersion relation for the two lowest‐frequency modes by truncating an eigenfunction series and by using empirical approximations motivated by a Monte Carlo simulation. Earl's (1993) approximations are mathematically not equivalent to the smallness parameter expansion introduced by Gombosi et al. (1993). We have developed a new solution method which is both functionally consistent with Earl's (1993) solutions and with the smallness parameter expansion. Because the new solution method involves both causal telegrapher's equation propagation and diffusion, it becomes clear that Earl's (1993) coherent pulse velocity is smaller than the modified telegrapher's coherent velocity because diffusion limits the efficiency of coherent propagation. In our solution method, the solution of the modified telegrapher's equation is obtained as the causal limit of solutions accurate to second order in the smallness parameter expansion. In order to investigate the coherent velocity, we have also developed “wavenumber eigenfunctions” which account for all the pitch angle dependence in our Boltzmann equation. Using truncation, Earl (1993) obtains approximations for the wavenumber dependence of the lowest two frequency modes, which correspond to two of the wavenumber eigenmodes. We find that a consequence of including only two wavenumber eigenmodes is that one obtains solutions which disobey causality at sufficiently short times. Furthermore, the coherent velocity of the two eigenmodes is strongly dependent on wavenumber and approaches the particle velocity in the limit of large wavenumber for both isotropic and anisotropic scattering processes. We conclude that Earl's (1993) solutions and solutions obtained using the new solution method implicitly assume weak acausality and reasonable behavior in the temporal regime, t < 4τ. The solutions are not strictly consistent with the behavior of the lowest two frequency modes but have similar behavior in the regime of low wavenumber.
On the sources of energization of molecular ions at ionospheric altitudes
Authors: Peterson W. K. , Abe T. , Fukunishi H. , Greffen M. J. , Hayakawa H. , Kasahara Y. , Kimura I. , Matsuoka A. , Mukai T. , Nagatsuma T. , Tsuruda K. , Whalen B. A. , Yau A. W.
Published: February 2004

During geomagnetically active times, the suprathermal mass spectrometer on the Akebono satellite frequently observes upflowing molecular ions (NO+, N2+, O2+) in the 2‐3 Earth radii geocentric distance regions in the auroral zone. Molecular ions originating at ionospheric altitudes must acquire an energy of the order of 10 eV in order to overcome gravitation and reach altitudes greater than 2 RE. This energy must be acquired in a time short compared with the local dissociative recombination lifetime of the ions; the latter is of the order of minutes in the F region ionosphere (300‐500 km altitude). Upflowing molecular ions thus provide a test particle probe into the mechanisms responsible for heavy ion escape from the ionosphere. In this paper we analyze the extensive complement of plasma, field, and wave data obtained on the Akebono satellite in a number of upflowing molecular ion events observed at high altitudes (5000 ‐ 10,000 km). We use these data to investigate the source of energization of the molecular ions at ionospheric altitudes. We show that Joule heating and ion resonance heating do not transfer enough energy or do not transfer it fast enough to account for the observed fluxes of upflowing molecular ions. We found that the observed field‐aligned currents were too weak to support large‐scale field‐aligned current instabilities at ionospheric altitudes. The data suggest but in the absence of high‐resolution wave measurements in the 300 to 500 km altitude range cannot ascertain the possibility that a significant fraction of escape energy is transferred to molecular ions in localized regions from intense plasma waves near the lower hybrid frequency. We also compared the energization of molecular ions to that of the geophysically important O+ ions in the 300 to 500 km altitude range, where the energy transfer to O+ is believed to occur via small‐scale plasma instabilities, ion resonance, and ion‐neutral frictional heating. Direct observation of energy input to the ionosphere from all of these sources in combination with in situ measurements of the density and temperature of neutral and ionized oxygen in the 300 to 500 km range are required to determine the relative importance of these energy sources in providing O+ with sufficient energy to escape the ionosphere.
A kinetic model of the formation of the hot oxygen geocorona: 1. Quiet geomagnetic conditions
Authors: Shematovich V. I. , Bisikalo D. V. , Gérard J. C.
Published: February 2004

A model of the hot oxygen geocorona in the transition region near the exobase is described. It is based on a Monte Carlo solution of the nonlinear Boltzmann equation for hot oxygen atoms produced by chemical processes usually considered as a source of hot oxygen (photodissociation of O2 and dissociative recombination of O2+ and NO+ ions). The evolution of the system is described stochastically as a series of random Markovian processes. The energy distribution function of the thermal and non‐thermal O(³P) atoms and of the nonthermal O(¹D) atoms is calculated from the thermospheric collision‐dominated region to the exosphere where the gas flow is virtually collisionless. The model is applied to equatorial latitudes for conditions of low solar and geomagnetic activity. Numerical simulations show that the distribution function of thermal oxygen is increasingly perturbed by collisions with the hot oxygen population at high altitudes and departs significantly from a Maxwellian distribution at all altitudes. The number density and temperature of the nonthermal oxygen atoms are derived from their microscopic distribution function and found to be in qualitative agreement with previous theoretical and experimental estimates.
A model of the steady state magnetic field in the magnetosheath
Authors: Kobel E. , Flückiger E. O.
Published: February 2004

For given parameters of the interplanetary plasma near Earth the magnetic field in the magnetosheath can be described by fluctuations superimposed upon an average static field configuration. Using potential theory, we derived analytical expressions describing the steady state magnetic field in the transition region between the bow shock and the magnetopause. The model contains five parameters, namely, the stand‐off distance of the magnetopause, the stand‐off distance of the bow shock, and the three components of the interplanetary magnetic field. The shape of the magnetopause and of the bow shock is modeled by a paraboloid with rotational symmetry around the Sun‐Earth axis. Compared to other models, our procedure leads to simple analytical formulae which make it most suitable for studies of the effects of the magnetosheath on charged particle propagation.
Modeling the Arecibo nighttime F2 layer: 2. Ionospheric gradients
Authors: Meléndez‐Alvira Daniel J. , Burnside R. G. , Walker J. C. G.
Published: February 2004

The servo model is extended and used to fit horizontal gradients in the F2 layer height and density and to estimate the zonal Pedersen current and its zonal and meridional gradient. Horizontal gradients were measured from the Arecibo Observatory during the following five nights: August 16‐17 and 17‐18, 1982; and October 4‐5, 5‐6, and 9‐10, 1983. The model gradients are driven by nonzero current gradients, which are applied as needed to fit the measured gradients in the F2 peak. The gradient in the peak height is accurately reproduced; the peak density gradient is calculated self‐consistently in the model. The divergence of the Pedersen current can be deduced when the current flows zonally and is found to differ from zero. This is a consequence of zonal divergence of the model zonal current. Expressions are derived for the divergence of the Hall current and for the curl of the current in the presence of ionospheric gradients. The vertical vorticity of the F region current is determined from the radar and optical measurements and the mass spectrometer/incoherent scatter (MSIS) neutral densities. Both neutral and plasma motions generate current vorticity equally as expected from the F region dynamo. The measured velocity gradients produce more current gradients and vorticity than the measured conductance gradients. The measured height gradient in the perpendicular‐north plasma drift (∂zυ⊥N) is the dominant term in the vorticity and drives the two current shears that cause vorticity. Geometrical factors increase (∂zυ⊥N) by about 13% in the zonal gradient of the meridional current over the meridional gradient of the zonal current. This causes anticorrelation between the former current shear and the vorticity. The two measured shears generally follow each other, have opposing vorticities and large uncertainties. The nighttime current may be irrotational or have constant vorticity. Large current gradients occur in conjunction with observed descents of the F2 peak height. The gradients are interpreted as due to the midnight pressure bulge at low latitudes. Short‐period gravity waves of meteorological origin are ruled out as they were not observed and are limited in their ability to reach ionospheric heights. The harmonic analysis used to obtain horizontal wind gradients is largely unaffected by spatially uniform wind accelerations. Therefore the deduced spatial variations in the measured winds are unlikely to be due to temporal variations.
Observations of solar‐wind‐driven progression of interplanetary magnetic field BY‐related dayside ionospheric disturbances
Authors: Stauning P. , Clauer C. R. , Rosenberg T. J. , Friis‐Christensen E. , Sitar R.
Published: February 2004

Observations from August 2 and 3, 1991, of poleward progressing, dayside convection disturbances accompanied by geomagnetic perturbations and ionospheric radio wave absorption have been analyzed and compared to variations in the solar wind parameters as observed from the IMP 8 satellite. The convection disturbances appear to start at dayside cusp latitudes from where they progress antisunward to high latitudes. The reported observations have enabled calculations of the progression directions and velocities and precise estimates of the delays between solar wind variations as measured by the IMP 8 satellite and ionospheric convection changes as observed from an array of polar magnetic observatories. The progressing ionospheric disturbance events occur during intervals of southward interplanetary magnetic fields (negative interplanetary magnetic field (IMF) BZ component); they are found to be closely related to variations of the east‐west component BY of the IMF. The close coupling between the solar wind and the polar ionosphere(s) is explained in an open magnetospheric model in which the geomagnetic field extending from a localized region of the dayside polar cap merges with the southward interplanetary field. Variations in the IMF BY component are reproduced in corresponding modulations of the east‐west component of the plasma flow at the ionospheric foot points of the connecting “open” field lines. The perturbations of the plasma flow persist while the open field lines are convected with the ionospheric plasma across part of the dayside polar cap. The observed geomagnetic perturbations result from the combined effects of field‐aligned currents and horizontal ionospheric currents, notably the convection‐related Hall currents. The associated radio wave absorption events are explained as the result of E region electron heating by the horizontal electric fields associated with the convection enhancements.
Relationship between energy flux Q and mean energy 〈E〉 of auroral electron spectra based on radar data from the 1987 CEDAR Campaign at Sondre Stromfjord, Greenland
Authors: Strickland D. J. , Hecht J. H. , Christensen A. B. , Kelly J.
Published: February 2004

The incoherent scatter radar at Sondre Stromfjord, Greenland, measured electron density profiles from 90 to 500 km during four auroral events over a 3‐hour period on February 28, 1987. The profiles were obtained with the radar pointed along the magnetic field near zenith at 15‐s intervals. Under the assumption that proton/H atom precipitation was unimportant during these events a representation of the incident electron flux was obtained by fitting calculated profiles with measured profiles in the vicinity of their peaks (lower E region). Maxwellian and Gaussian electron distributions with high‐ and low‐energy tails were used to generate the calculated profiles. The distributions were specified in terms of average energy 〈E〉 and energy flux Q. We find that we can clearly distinguish between profiles that result from a Maxwellian incident electron spectrum and those that result from a Gaussian spectrum. Interpreting Gaussian and Maxwellian spectra as representative of discrete and diffuse aurora, respectively, the measurements indicated good correlation between 〈E〉 and Q for discrete aurora, while essentially no correlation was observed for diffuse aurora. This is consistent with current understanding that discrete auroras are produced by electrons accelerated by magnetic field‐aligned potential drops whereas diffuse auroras are produced by pitch angle diffusion of plasma sheet electrons into the loss cone.
Unusual locations of Earth's bow shock on September 24–25, 1987: Mach number effects
Authors: Cairns Iver H. , Fairfield Donald H. , Anderson Roger R. , Carlton Victoria E. H. , Paularena Karolen I. , Lazarus Alan J.
Published: February 2004

ISEE 1 and IMP 8 data are used to identify 19 crossings of Earth's bow shock during a 30‐hour period following 0000 UT on September 24, 1987. Apparent standoff distances for the shock are calculated for each crossing using two methods and the spacecraft location; one method assumes the average shock shape, while the other assumes a ram pressure‐dependent shock shape. The shock's apparent standoff distance, normally ∼ 14 RE, is shown to increase from near 10 RE initially to near 19 RE during an 8‐hour period, followed by an excursion to near 35 RE (where two IMP 8 shock crossings occur) and an eventual return to values smaller than 19 RE. The Alfvén MA and fast magnetosonic Mms Mach numbers remain above 2 and the number density above 4 cm−3 for almost the entire period. Ram pressure effects produce the initial near‐Earth shock location, whereas expansions and contractions of the bow shock due to low Mach number effects account, qualitatively and semiquantitatively, for the timing and existence of almost all the remaining ISEE crossings and both IMP 8 crossings. Significant quantitative differences exist between the apparent standoff distances for the shock crossings and those predicted using the observed plasma parameters and the standard model based on Spreiter et al.'s (1966) gasdynamic equation. These differences can be explained in terms of either a different dependence of the standoff distance on Mach number at low MA and Mms, or variations in shock shape with MA and Mms (becoming increasingly “puffed up” with decreasing MA and Mms, as expected theoretically), or by a combination of both effects. Global MHD simulations, to be presented elsewhere, confirm that both effects occur and are significant. Ram pressure‐induced changes in the shock's shape are discussed but found to be quantitatively unimportant for the shock crossings analyzed. Approximate estimates of both the deviation of the shock's standoff distance from the standard model and of the shock's shape are determined independently (but not consistently) for Mms ∼ 2.4. The estimates imply substantial changes in standoff distance and/or shock shape at low MA and Mms. Mach number effects can therefore be quantitativwely important in determining and predicting the shape and location of the bow shock, even when MA and Mms remain above 2. This study confirms and generalizes previous studies of Mach number effects on Earth's bow shock. Statistical studies and simulations of the bow shock's shape and location should be performed as a function of Mach number, magnetic field orientation, and ram pressure.
Interplanetary magnetic field control of mantle precipitation and associated field‐aligned currents
Authors: Xu Dingan , Kivelson Margaret G. , Walker Ray J. , Newell Patrick T. , Meng C. ‐I.
Published: February 2004

Dayside reconnection, which is particularly effective for a southward interplanetary magnetic field (IMF), allows magnetosheath particles to enter the magnetosphere where they form the plasma mantle. The motions of the reconnected flux tubes produce convective flows in the ionosphere. It is known that the convection patterns in the polar cap are skewed to the dawnside for a positive IMF By (or duskside for a negative IMF By) in the northern polar cap. Correspondingly, one would expect to find asymmetric distributions of mantle particle precipitation, but previous results have been unclear. In this paper the correlation between By and the distribution of mantle particle precipitation is studied for steady IMF conditions with southward IMF. Ion and electron data from the DMSP F6 and F7 satellites are used to identify the mantle region and IMP 8 is used as a solar wind monitor to characterize the IMF. We study the local time extension of mantle precipitation in the prenoon and postnoon regions. We find that, in accordance with theoretical expectations for a positive (negative) IMF By, mantle particle precipitation mainly appears in the prenoon region of the northern (southern) hemisphere. The mantle particle precipitation can extend to as early as 0600 magnetic local time (MLT) in the prenoon region but extends over a smaller local time region in the postnoon sector (we did not find mantle plasma beyond 1600 MLT in our data set although coverage is scant in this area). Magnetometer data from F7 are used to determine whether part of the region 1 current flows on open field lines. We find that at times part of the region 1 sense current extends into the region of mantle particle precipitation, and is therefore on open field lines. In other cases, region 1 currents are absent on open field lines. Most of the observed features can be readily interpreted in terms of the open magnetosphere model.
The whistler heat flux instability: Threshold conditions in the solar wind
Authors: Gary S. Peter , Scime Earl E. , Phillips John L. , Feldman William C.
Published: February 2004

Solar wind electrons are observed often to consist of two components: a core and a halo. The anisotropies and relative average speeds of these two components correspond to a heat flux that has the potential to excite several different electromagnetic instabilities; wave‐particle scattering by the resulting enhanced fluctuations can limit this heat flux. This manuscript describes theoretical studies using the linear Vlasov dispersion equation for drifting bi‐Maxwellian component distributions in a homogeneous plasma to examine the threshold of the whistler heat flux instability. Expressions for this threshold are obtained from two different parametric baselines: a local model that yields scalings as functions of local dimensionless plasma parameters, and a global model based on average electron properties observed during the in‐ecliptic phase of the Ulysses mission. The latter model yields an expression for the heat flux at threshold of the whistler instability as a function of heliospheric radius that scales in the same way as the average heat flux observed from Ulysses and that provides an approximate upper bound for that same quantity. This theoretical scaling is combined with the observational results to yield a semi‐empirical closure relation for the average electron heat flux in the solar wind between 1 and 5 AU.
Hot proton anisotropies and cool proton temperatures in the outer magnetosphere
Authors: Gary S. Peter , Moldwin Mark B. , Thomsen Michelle F. , Winske Dan , McComas David J.
Published: February 2004

The hot protons of the outer magnetosphere typically exhibit a temperature anisotropy such that T⊥/T∥ > 1, where perpendicular and parallel symbols denote directions relative to the background magnetic field. If this anisotropy is sufficiently large, the electromagnetic proton cyclotron anisotropy instability may be excited. This instability is studied using linear Vlasov theory and one‐dimensional hybrid simulations for a homogeneous plasma model representative of conditions in the outer magnetosphere with a hot anisotropic proton component (denoted by subscript h) and a cool, initially isotropic proton component (subscript c). Linear theory yields an instability threshold condition on the hot proton temperature anisotropy whereas the simulations imply an upper bound on T⊥h/T∥h; both the threshold and the upper bound have similar scalings with the maximum growth rate γm, the parallel β of the hot component, β∥h, and the relative density of the hot component nh/ne. An analysis of plasma observations from the Los Alamos magnetospheric plasma analyzer (MPA) in geosynchronous orbit finds that the maximum value of the hot proton temperature anisotropy approximately satisfies the predicted scalings with β∥h and nh/ne and yields the proportionality factor that quantifies this upper bound. The simulations are also used to examine the heating of the cool protons by the proton cyclotron instability. The simulations yield a scaling for the dimensionless late‐time cool proton average temperature Tc/T∥h as (nh/ne)/β∥h0.5. Analysis of MPA data shows that the observed values of Tc/T∥h have similar scalings and again yield the proportionality factor which quantifies this relationship.
Determination of auroral heat fluxes and thermal ion outflows using a numerical ionospheric model and incoherent‐scatter radar data
Authors: Min Q.‐L. , Watkins B. J.
Published: February 2004

A comprehensive one‐dimensional model of the polar ionosphere has been used in conjunction with incoherent‐scatter radar data from Sondrestrom, Greenland, to determine downward heat fluxes and thermal ion outflows at very high latitudes. For periods of very quiet geomagnetic activity the model closely simulates the observed time‐dependent behavior of the electron density, ion and electron temperatures. To obtain this similarity between model and data, the upper boundary conditions of the model, namely downward heat flux, and magnetic field‐aligned ion flows, are continually adjusted with time to provide a best fit with data. The heat fluxes and ion flows are determined indirectly from this fitting procedure. The technique has been applied to a 10‐hour daytime data set for February 12, 1990, to search for enhanced downward heat fluxes and outward thermal ion fluxes associated with dayside auroral oval. Variations of heat flux ranged from about 2×109 to 2×1010 eV cm−2 s−1, and vertical outward fluxes of ionization ranged from about zero to 8×108 cm−2 s−1. For both quantities the peak values occurred when the radar site was located under the dayside auroral oval. It is suggested that these marked upward thermal ion flows in the dayside auroral ionosphere may be associated with energetic O+ ion outflows that have been observed at high altitudes with spacecraft.
Modulational instability of lower hybrid waves at the magnetopause
Authors: Shapiro V. D. , Shevchenko V. I. , Cargill P. J. , Papadopoulos K.
Published: February 2004

The role of lower hybrid waves at the magnetopause is reexamined. It is found that for the maximum observed wave power, the lower hybrid waves are unstable to a modulational instability on the magnetosheath side of the magnetopause. The modulational instability leads to localized field structures oriented predominantly along the magnetic field. Such patchy lower hybrid turbulence has been observed by some spacecraft. As a result of the large Ti/Te ratio, the waves saturate by ion heating; as a result, unlike other settings (e.g. comets, critical ionization phenomena), energetic electrons are not expected. The stochastic electron transport in the presence of such turbulence is analyzed and results in strongly anisotropic electron diffusion, with the dominant direction across the magnetic field. The diffusion rate exceeds significantly that expected from quasi‐linear considerations and, for magnetopause parameters, also exceeds the rate discussed by Sonnerup (1980).
Postmodern view of M‐regions
Authors: Crooker N. U. , Cliver E. W.
Published: February 2004

The discovery of coronal holes in the early 1970s was heralded as the discovery of the mysterious M‐regions responsible for recurrent geomagnetic activity. High‐speed flow from the holes, sweeping past Earth on successive solar rotations, was directly correlated with sustained increases in activity indices. We argue that this view of M‐regions as coronal holes is incomplete because it ignores work dating from the 1960s demonstrating that peak recurrent activity coincides with passage of corotating interaction regions between high‐ and low‐speed flows. Thus, in an important way, M‐regions include the boundary between coronal holes and coronal streamers, since streamers supply the slowest flows. In addition, we suggest that coronal mass ejections propagating through the streamer belt can contribute to the peak intensities of recurrent storms. Finally, we demonstrate that both peak recurrent activity and the following sustained activity that correlates with the high‐speed flows are controlled by the Russell‐McPherron effect: They are prominent only when the azimuthal component of the Parker spiral field projects a southward component in Earth's tilted dipole frame.
Monte Carlo simulation of the terrestrial hydrogen exosphere
Authors: Hodges R. Richard
Published: February 2004

Methods for Monte Carlo simulation of planetary exospheres have evolved from early work on the lunar atmosphere, where the regolith surface provides a well defined exobase. A major limitation of the successor simulations of the exospheres of Earth and Venus is the use of an exobase surface as an artifice to separate the collisional processes of the thermosphere from a collisionless exosphere. In this paper a new generalized approach to exosphere simulation is described, wherein the exobase is replaced by a barometric depletion of the major constituents of the thermosphere. Exospheric atoms in the thermosphere‐exosphere transition region, and in the outer exosphere as well, travel in ballistic trajectories that are interrupted by collisions with the background gas, and by charge exchange interactions with ionospheric particles. The modified simulator has been applied to the terrestrial hydrogen exosphere problem, using velocity dependent differential cross sections to provide statistically correct collisional scattering in H‐O and H‐H+ interactions. Global models are presented for both solstice and equinox over the effective solar cycle range of the F10.7 index (80 to 230). Simulation results show significant differences with previous terrestrial exosphere models, as well as with the H distributions of the MSIS‐86 thermosphere model.
Beat, modulational, and decay instabilities of a circularly polarized Alfvén wave
Authors: Hollweg Joseph V.
Published: February 2004

A circularly polarized low‐frequency electromagnetic pump wave propagating along an ambient magnetic field is known to be unstable to the growth of several parallel‐propagating parametric instabilities. If ion‐cyclotron effects are retained in a two‐fluid description, the dispersion relation is a sixth‐order polynomial. We present a series of new analytical approximations to this dispersion relation. We emphasize new results for the beat instability that occurs as an interaction of the forward propagating upper sideband with the backward propagating lower sideband. The nature of the beat instability depends on β = (υsound/υA)² and on the sense of polarization of the pump wave. The beat and decay instabilities can occur together if the pump is left‐handed (i.e., ion resonant) and if β ≲ 1, but they cannot occur together if the pump is right‐handed. For a left‐handed pump the beat mode is the only instability if β ≳ 1. If the pump is right‐handed and β ≳ 1, then the beat instability exists only when the pump amplitude exceeds a threshold value, and the beat will be the only instability if the pump amplitude is large enough to stabilize the modulational instability. If the pump is left‐handed and β ≲ 1, then the beat mode is stabilized when the pump amplitude becomes sufficiently large. The beat instability primarily produces a forward propagating transverse wave in the upper sideband. Thus if β ≳ 1, the instabilities considered here do not produce the backward propagating waves which are thought to affect turbulence and the evolution of cross helicity in the solar wind. New analytical results are presented also for the decay and modulational instabilities when β ≈ 1.
Particle chaos and pitch angle scattering
Authors: Burkhart G. R. , Dusenbery P. B. , Speiser T. W.
Published: February 2004

One of the most important quantities that can be obtained through test particle calculations is the pitch angle scattering of particles by interaction with current sheet magnetic fields. Pitch angle scattering is a factor that helps determine the dawn‐to‐dusk current, controls particle energization, and it has also been used as a remote probe of the current sheet structure. Previous studies have interpreted their results under the expectation that randomization will be greatest when the ratio of the two timescales of motion (gyration parallel to and perpendicular to the current sheet) is closest to one. In a simple parabolic current sheet, the ratio of timescales is proportional to κ, where κ = (Rc/ρz)1/2, Rc is the field line radius of curvature at the current sheet midplane and ρz is the gyroradius at the current sheet midplane. Recently, the average exponential divergence rate (AEDR) has been calculated for particle motion in a hyperbolic current sheet (Chen, 1992). It is claimed that this AEDR measures the degree of chaos and therefore may be thought to measure the randomization. In contrast to previous expectations, the AEDR is not maximized when κ ∼ 1 but instead increases with decreasing κ. Also contrary to previous expectations, the AEDR is dependent upon the parameter bz. In response to the challenge to previous expectations that has been raised by this calculation of the AEDR, we have investigated the dependence of a measure of particle pitch angle scattering on both the parameters κ and bz. We find that, as was previously expected, particle pitch angle scattering is maximized near κ = 1 provided that κ/bz > 1. In the opposite regime, κ/bz < 1, we find that particle pitch angle scattering is still largest when the two timescales are equal, but the ratio of the timescales is proportional to bz. In this second regime, particle pitch angle scattering is not due to randomization, but is instead due to a systematic pitch angle change. This result shows that particle pitch angle scattering need not be due to randomization and indicates how a measure of pitch angle scattering can exhibit a different behavior than a measure of chaos.
Radial energy transport by magnetospheric ULF waves: Effects of magnetic curvature and plasma pressure
Authors: Kouznetsov Igor , Lotko William
Published: February 2004

The “radial” transport of energy by internal ULF waves, stimulated by dayside magnetospheric boundary oscillations, is analyzed in the framework of one‐fluid magnetohydrodynamics. (The term radial is used here to denote the direction orthogonal to geomagnetic flux surfaces.) The model for the inhomogeneous magnetospheric plasma and background magnetic field is axisymmetric and includes radial and parallel variations in the magnetic field, magnetic curvature, plasma density, and low but finite plasma pressure. The radial mode structure of the coupled fast and intermediate MHD waves is determined by numerical solution of the inhomogeneous wave equation; the parallel mode structure is characterized by a WKB approximation. Ionospheric dissipation is modeled by allowing the parallel wave number to be complex. For boundary oscillations with frequencies in the range from 10 to 48 mHz, and using a dipole model for the background magnetic field, the combined effects of magnetic curvature and finite plasma pressure are shown to (1) enhance the amplitude of field line resonances by as much as a factor of 2 relative to values obtained in a cold plasma or box‐model approximation for the dayside magnetosphere; (2) increase the energy flux delivered to a given resonance by a factor of 2–4; and (3) broaden the spectral width of the resonance by a factor of 2–3. The effects are attributed to the existence of an “Alfvén buoyancy oscillation,” which approaches the usual shear mode Alfvén wave at resonance, but unlike the shear Alfvén mode, it is dispersive at short perpendicular wavelengths. The form of dispersion is analogous to that of an internal atmospheric gravity wave, with the magnetic tension of the curved background field providing the restoring force and allowing radial propagation of the mode. For nominal dayside parameters, the propagation band of the Alfvén buoyancy wave occurs between the location of its (field line) resonance and that of the fast mode cutoff that exists at larger radial distances.
A case study of transient event motion in the magnetosphere and in the ionosphere
Authors: Korotova G. I. , Sibeck D. G.
Published: February 2004

We present a case study of the magnetospheric and ionospheric response to a series of repetitive (7–8 min) solar wind dynamic pressure and interplanetary magnetic field (IMF) strength variations generated at the bow shock. During a period of duskward and antisunward IMF orientation, the magnetospheric and ionospheric observations indicate transient events moving dawnward and duskward away from a point of origin at or after local noon. In contrast, during a subsequent period of dawnward and antisunward IMF orientation, the observations indicate events moving duskward away from a point of origin at or prior to 0900 LT. We consider explanations in terms of pressure pulse driven riplets on the magnetopause or directly driven bursts of antiparallel merging.
A correlative comparison of the ring current and auroral electrojets using geomagnetic indices
Authors: Cade W. B , Sojka J. J. , Zhu L.
Published: February 2004

From a study of the 21 largest geomagnetic storms during solar cycle 21, a strong correlation is established between the ring current index Dst and the time‐weighted accumulation of the 1‐hour auroral electrojet indices, AE and AL. The time‐weighted accumulation corresponds to convolution of the auroral electrojet indices with an exponential weighting function with an e‐folding time of 9.4 hours. The weighted indices AEw and ALw have correlation coefficients against Dst ranging between 0.8 and 0.95 for 20 of the 21 storms. Correlation over the entire solar cycle 21 database is also strong but not as strong as for an individual storm. A set of simple Dst prediction functions provide a first approximation of the inferred dependence, but the specific functional relationship of Dst (AEw) or Dst (ALw) varies from one storm to the next in a systematic way. This variation reveals a missing parametric dependence in the transfer function. However, our results indicate that auroral electrojet indices are potentially useful for predicting storm time enhancements of ring current intensity with a few hours lead time.
Kinetic simulation of the Kelvin‐Helmholtz instability in a finite sized jet
Authors: Thomas V. A.
Published: February 2004

Two‐dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the kinetic evolution of the Kelvin‐Helmholtz (K‐H) instability for a jet of material moving transverse to the magnetic field in the middle of a field reversal region. The instability shows two phases. This first phase is the development of a short scale length instability on the sides of the jet, which might be characterized as a local K‐H instability. This instability saturates at low amplitude by modest broadening of the jet profile. Then a nonlocal K‐H instability sets in where the jet exhibits a kinklike behavior which spreads and disrupts the jet in a dramatic fashion. The simulation results are discussed within the context of possible relevance to the magnetopause.
Nature and location of the source of plasma sheet boundary layer ion beams
Authors: Elphic R. C. , Onsager T. G. , Thomsen M. F. , Gosling J. T.
Published: February 2004

Onsager et al. (1991) have put forward a model of the formation of the plasma sheet boundary layer which relies on a steady source of plasma from a spatially extended plasma sheet, together with steady equatorward and earthward E×B convection of field lines due to reconnection at a downtail neutral line. This model is a synthesis of earlier proposals and it explains such features as an electron layer exterior to the ion boundary layer, ion velocity dispersion, counter streaming beams, low‐speed cutoffs in the beams. It also explains the apparent evolution of the ion beams through “kidney bean” shaped velocity‐space distributions toward quasi‐isotropic shells without invoking pitch angle scattering or energy diffusion. In this paper we explore two ramifications of the model: (1) low‐speed cutoffs observed in the ion distribution functions contain information about the distance to the neutral line downtail and (2) values of phase space density at the low‐speed cutoffs are the same as those in the distribution just earthward of the reconnection site. In principle we can map, as a function of time, the downtail neutral line distance and establish whether or not it is retreating during substorm recovery. We can also reconstruct the plasma distribution function near the neutral line to see if it is most consistent with mantle or plasma sheet plasma. We perform this analysis using ISEE Fast Plasma Experiment data for two plasma sheet recovery events, one on March 1, 1978, and the other on April 18, 1978. On March 1, 1978, we find evidence for an initial retreat from around 110 to 160 RE in the first 15 min; little further retreat occurs thereafter. On April 18, 1978, the neutral line location ranges from as little as 40 RE tailward of the satellite to as much as 200 RE, but there is no evidence for a systematic retreat. The reconstructed ion distributions for these events are most consistent with a plasma sheet origin for the March 1 case and possibly plasma mantle or low‐latitude boundary layer for the April 18 case.
Neptune inbound bow shock
Authors: Szabo Adam , Lepping Ronald P.
Published: February 2004

Voyager 2 crossed the inbound or upstream Neptunian bow shock at 1430 spacecraft event time on August 24 in 1989 (Belcher et al., 1989). The plasma and magnetic field measurements allow us to study the solar wind interaction with the outermost gas giant. To fully utilize all of the spacecraft observations, an improved nonlinear least squares, “Rankine‐Hugoniot” magnetohydrodynamic shock‐fitting technique has been developed (Szabo, 1994). This technique is applied to the Neptunian data set. We find that the upstream bow shock normal points nearly exactly toward the Sun consistent with any reasonable large‐scale model of the bow shock for a near subsolar crossing. The shock was moving outward with a speed of 14 ± 12 km/s. The shock can be characterized as a low β, high Mach number, strong quasi‐perpendicular shock. Finally, the shock microstructure features are resolved and found to scale well with theoretical expectations.
A study of weak anisotropy in electron pressure in the tail current sheet
Authors: Lee D. ‐Y. , Voigt G. ‐H.
Published: February 2004

We adopt a magnetotail model with stretched field lines where ion motions are generally nonadiabatic and where it is assumed that the pressure anisotropy resides only in the electron pressure tensor. We show that the magnetic field lines with p⊥ > p∥ are less stretched than the corresponding field lines in the isotropic model. For p∥ > p⊥, the magnetic field lines become more and more stretched as the anisotropy approaches the marginal firehose limit, p∥ = p⊥+B²/μ0. We also show that the tail current density is highly enhanced at the firehose limit, a situation that might be subject to a microscopic instability. However, we emphasize that the enhancement in the current density is notable only near the center of the tail current sheet (z = 0). Thus it remains unclear whether any microscopic instability can significantly alter the global magnetic field configuration of the tail. By comparing the radius of the field‐line curvature at z = 0 with the particle's gyroradius, we suspect that even the conventional adiabatic description of electrons may become questionable very close to the marginal firehose limit.
Penetration characteristics of electromagnetic emissions from an underground seismic source into the atmosphere, ionosphere, and magnetosphere
Authors: Molchanov O. A. , Hayakawa M. , Rafalsky V. A.
Published: February 2004

Theoretical calculations are made on electromagnetic fields in the frequency range 10−2 to 10−2; Hz on the ground surface and above the ionosphere induced by stochastic microcurrent activity inside the future seismic sources on the assumption of cylindrical symmetry of the effective current and three types of polarization. The inhomogeneity of the ground and atmosphere conductivity and anisotropy of the ionosphere are taken into consideration. The intensity of ULF magnetic and electric precursors observed on the ground, and their spatial distribution can be explained by using the results of the present computations. It is found that only the fields from a magnetic type source can penetrate into the magnetosphere and generate propagating Alfven waves. The expected values of magnetospheric electric and magnetic field are 1‐10 μV m−1 Hz−1/2 and 1–10 pT Hz−1/2 respectively, and the horizontal scale of their distribution is about 100–200 km. Finally, these theoretical predictions are compared with the corresponding results of satellite observations.
Wave and ion evolution downstream of quasi‐perpendicular bow shocks
Authors: McKean M. E. , Omidi N. , Krauss‐Varban D.
Published: February 2004

Distribution functions of ions heated in quasi‐perpendicular bow shocks have a large perpendicular temperature anisotropy that provides free energy for the growth of Alfvén ion cyclotron (AIC) waves and mirror waves. Both types of waves have been observed in the Earth's magnetosheath downstream of quasi‐perpendicular shocks. The question of whether these waves are produced at the shock and convected downstream or whether they are produced locally in the magnetosheath has not yet been answered. If the latter were true, then under most magnetosheath conditions AIC waves should dominate the wave activity, yet frequently mirror waves either dominate or are competitive with the AIC mode. We address this question by using two‐dimensional hybrid simulations to give a self‐consistent description of the evolution of the wave spectra downstream of quasi‐perpendicular shocks. Both mirror and AIC waves are identified in the simulated magnetosheath. They are generated at or near the shock front and convected away from it by the sheath plasma. Near the shock, the waves have a broad spectrum, but downstream of the shock, shorter‐wavelength modes are heavily damped and only longer‐wavelength modes persist. The characteristics of these surviving modes can be predicted with reasonable accuracy by linear kinetic theory appropriate for downstream conditions. Throughout the downstream region, the power in compressive magnetic oscillations is of the same order as the power in transverse oscillations. We also follow the evolution of the ion distribution function. The shocked ions that provide the free energy for wave growth have a two‐component distribution function: a core population of directly transmitted ions and a smaller halo of initially reflected ions that contains the bulk of the free energy. The halo is initially gyrophase‐bunched and extremely anisotropic. Within a relatively short distance downstream of the shock (of the order of 10 ion inertial lengths), wave‐particle interactions remove these features from the halo and reduce the anisotropy of the distribution to near‐threshhold levels for the mirror and AIC instabilities. A similar evolution has been observed for ions at the Earth's bow shock.