bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear standing shear Alfvén waves in the Earth's magnetosphere
Authors: Rankin R. , Frycz P. , Tikhonchuk V. T. , Samson J. C.
Published: February 2004

We present theory and numerical simulations of strong nonlinear effects in standing shear Alfvén waves (SAWs) in the Earth's magnetosphere, which is modeled as a finite size box with straight magnetic lines and (partially) reflecting boundaries. In a low β plasma it is shown that the ponderomotive force can lead to large‐amplitude SAW spatial harmonic generation due to nonlinear coupling between the SAW and a slow magnetosonic wave. The nonlinear coupling leads to secularly growing frequency shifts, and in the case of driven systems, nonlinear dephasing can lead to saturation of the driven wave fields. The results are discussed in the context of their possible relevance to the theory of standing ionospheric cavity wave modes and field line resonances in the high‐latitude magnetosphere.
O+‐O collision frequency in high‐speed flows
Authors: Pesnell William Dean , Omidvar Kazem , Hoegy Walter R. , Wharton Larry E.
Published: February 2004

Throughout much of the terrestrial thermosphere and ionosphere, the motions of the neutral and ionized constituents are closely coupled and relative velocities are small, of the order of 100 m s−1 or less. This is particularly true at midlatitudes to low latitudes where typical velocities in the neutral gas due to tidal forcing are only 20‐50 m s−1. However, the solar wind‐magnetosphere interaction drives a large‐scale convection pattern in the polar ionosphere. When the rapid adjustment of the plasma to changes in the solar wind is combined with the slower response of the more massive neutral gas, large relative velocities on the order of 1 km s−1 can exist for substantial lengths of time. This will be more common during periods of high geomagnetic activity, as a result of the greater number of magnetic substorms and other particle precipitation events. When a significant relative velocity is present, the calculation of interaction parameters of the two gases passing through each other, such as collision frequency, must include that velocity. These effects are usually neglected when interpreting wind and ion drift observations. We show how the collision frequency is affected by a directed relative velocity between any two gases interacting with a power law or exponential potential energy curves. The directed velocity increases the collision frequency at all temperatures for most ion‐neutral interactions. For certain power law potentials, such as the charge quadrupole, the collision frequency is decreased. We present an analytic solution for the high‐speed collision integral using the resonance charge exchange cross section.
Method for confining the magnetic field of the cross‐tail current inside the magnetopause
Authors: Sotirelis T. , Tsyganenko N. A. , Stern D. P.
Published: February 2004

A method is presented for analytically representing the magnetic field due to the cross‐tail current and its closure on the magnetopause. It is an extension of a method used by Tsyganenko (1989b) to confine the dipole field inside an ellipsoidal magnetopause using a scalar potential. Given a model of the cross‐tail current, the implied net magnetic field is obtained by adding to the cross‐tail current field a potential field B = −▽γ, which makes all field lines divide into two disjoint groups, separated by the magnetopause (i.e., the combined field is made to have zero normal component with the magnetopause). The magnetopause is assumed to be an ellipsoid of revolution (a prolate spheroid) as an approximation to observations (Sibeck et al., 1991). This assumption permits the potential γ to be expressed in spheroidal coordinates, expanded in spheroidal harmonics and its terms evaluated by performing inversion integrals. Finally, the field outside the magnetopause is replaced by zero, resulting in a consistent current closure along the magnetopause. This procedure can also be used to confine the modeled field of any other interior magnetic source, though the model current must always flow in closed circuits. The method is demonstrated on the T87 cross‐tail current, examples illustrate the effect of changing the size and shape of the prescribed magnetopause and a comparison is made to an independent numerical scheme based on the Biot‐Savart equation.
Effect of magnetic field geometry on the wave signature of the pickup of interstellar neutrals
Authors: Williams L. L. , Zank G. P.
Published: February 2004

By using the technique of Johnstone et al. (1991), the asymptotic wave spectrum induced by the pickup of interstellar ions in the heliosphere is derived. This approach is independent of quasi‐linear theory. The asymptotic wave spectrum is calculated from the standard resonance condition by assuming that the asymptotic ion distribution is bispherical and by using conservation of energy. The calculation places no restriction on either the ratio of Alfvén speed to solar wind flow speed or the particle pitch angle. Spectra for various geometries (relative orientations of the magnetic field and solar wind velocity) are calculated. Absolute upper limits to the expected wave enhancements are calculated. These upper limits are compared to the background fluctuation spectrum extrapolated from 0.87 AU, assuming a spectral index of −5/3 and radial dependences of −2 and −3. The heliocentric radial distances at which the peaks in the various geometry‐dependent wave spectra are expected to be observable above the background are identified. Peaks due to pickup hydrogen at the proton gyrofrequency are found to be always stronger relative to the background in parallel than in perpendicular geometries. Depending on how the background varies with heliocentric radius, this peak should be observable at 5‐10 AU in parallel geometries; 10‐50 AU in perpendicular geometries. The parallel‐geometry excitations consist only of sunward propagating waves. In perpendicular geometries a peak also occurs, this time near 0.1 of the proton gyrofrequency (corresponding to the peak calculated by Lee and Ip (1987)). However, the steep spectral dependence of the ambient wave field makes it unlikely that this peak will be observed inside 20 AU. A similar result holds for the case of pickup‐helium induced excitation at the helium gyrofrequency. Inside 10 AU, peaks at these lower frequencies tend to be obscured by the steep frequency dependence (∝ −5/3) of the ambient spectrum and then are only observable in parallel geometries.
Modulation of high‐energy cosmic rays in the heliosphere
Authors: Hall D. L. , Humble J. E. , Duldig M. L.
Published: February 2004

We have used sidereal diurnal variations observed by neutron monitors and surface and underground muon telescopes to examine the north‐south anisotropy and radial density gradient of high‐energy galactic cosmic rays between 1957 and 1985. We have derived the average amplitude and rigidity spectrum of the anisotropy for this period and estimate the radial gradient to be best represented as a function of rigidity P by 1.02 × (P/10)−0.5% AU−1 for rigidities less than ≈450 GV. The temporal variation of both the north‐south anisotropy and the radial density gradient show a solar cycle variation. Possible dependence of the anisotropy on the solar magnetic polarity state has been examined under the assumptions of a static and a nonstatic rigidity spectrum, with conflicting results. We show that the analysis is affected by a further sidereal variation and that the responsible anisotropy is related to the asymmetric heliospheric modulation of a galactic anisotropy.
Proton flow in the Martian magnetosheath
Authors: Kallio E. , Koskinen H. , Barabash S. , Lundin R. , Norberg O. , Luhmann J. G.
Published: February 2004

The Automatic Space Plasma Experiment with a Rotating Analyzer (ASPERA) measurements on board the Phobos 2 spacecraft gave, for the first time, a three‐dimensional (3‐D) picture of the proton flow around Mars. The measurements from the circular orbits of Phobos 2 are well suited to study the bow shock at the terminator region, the nightside magnetosheath, and the tail region. Moreover, measurements from the elliptical orbits offer dayside magnetosheath data. In this work, all circular orbits (11) where there was enough information for 3‐D velocity calculations are analysed. The solar wind deflection at the bow shock and the disappearance of the flow near the optical shadow of Mars are found to be typical features on all circular orbits. A dawn‐dusk asymmetry is detected in many cases as well. When the results are compared to a gasdynamic model, the locations of the observed boundaries and the general behavior of the flow are found to be quite consistent with the model. The region where proton particle flux decreases significantly, referred to as a magnetopause near the optical shadow of Mars, was typically found near the magnetic field maximum. The magnetopause was thus inside the so‐called magnetic tail boundary, which is defined to be at the broad magnetic minimum between the bow shock and the central current sheet. The magnetic tail boundary may be related to the O+ pick‐up ions because the mass loading boundary also lies between the shock and the magnetopause. Because the proton flow may behave differently in the dayside than in the nightside magnetosheath, the 3‐D velocities are calculated on two elliptical orbits as well. However, in these cases the nature of the flow is not possible to determine as reliably as near the terminator.
Wave rectification in plasma sheaths surrounding electric field antennas
Authors: Boehm M. H. , Carlson C. W. , McFadden J. P. , Clemmons J. H. , Ergun R. E. , Mozer F. S.
Published: February 2004

Combined measurements of Langmuir or broadband whistler wave intensity and lower‐frequency electric field waveforms, all at 10‐µs time resolution, were made on several recent sounding rockets in the auroral ionosphere. It is found that Langmuir and whistler waves are partially rectified in the plasma sheaths surrounding the payload and the spheres used as antennas. This sheath rectification occurs whenever the HF potential across the sheath becomes of the same order as the electron temperature or higher, for wave frequencies near or above the ion plasma frequency. This rectification can introduce false low‐frequency waves into measurements of electric field spectra when strong high‐frequency waves are present. Second harmonic signals are also generated, although at much lower levels. The effect occurs in many different plasma conditions, primarily producing false waves at frequencies that are low enough for the antenna coupling to the plasma to be resistive.
Low‐frequency instabilities and the resulting velocity distributions of pickup ions at comet Halley
Authors: Karimabadi H. , Krauss‐Varban D. , Omidi N. , Fuselier S. A. , Neugebauer M.
Published: February 2004

The interaction between the solar wind and newborn cometary ions is studied using a new analytical theory as well as one‐ and two‐dimensional hybrid simulations. Using the observed parameters upstream of the comet Halley, a detailed study of wave excitation and the resulting particle distributions is presented. Linear theory as well as simulations show that a variety of modes such as the fast magnetosonic mode, high frequency whistlers and obliquely propagating Alfvén ion cyclotron waves can be excited. However, parallel propagating waves are found to be dominant in the wave spectrum and to control the scattering of the pickup ions. Several features of the observed distributions of pickup protons are explained. In particular, it is shown that the observed asymmetric pitch angle distribution for the pickup protons is due to the small saturation amplitude of the waves for the given parameters. Water group associated waves can lead to energy diffusion and further pitch angle scattering of protons. This effect is most likely to be important in the vicinity of the bow shock of comet Halley where the density of water group ions becomes comparable to that of protons. It is shown that the observed increase in the radius of the proton velocity shell just outside the bow shock can be due to water group waves. The nearly isotropic proton pitch angle distribution observed by Neugebauer et al. [1989] just outside the bow shock may, however, be related to the presence of a rotational discontinuity which has been identified in the magnetic field data. Just outside the bow shock, simulations show that parallel propagating water group waves can steepen with attached whistler wave packets. The steepening process at parallel propagation is a transient effect, in an important contrast to the case of steepening at oblique angles. The smaller beam densities at comet Halley appears to be the main reason not only why waves at comet Halley have smaller amplitudes but also why oblique, steepening magnetosonic waves have not been detected at comet Halley, whereas they have been seen at comet Giacobini‐Zinner.
Time dependence of ponderomotive plasma energization in the magnetosphere
Authors: Allan W.
Published: February 2004

The time‐dependent second‐order force generated by standing Alfvén waves in a dipolar magnetospheric geometry is used to derive the time‐dependent specific parallel energy (parallel energy per proton mass) of magnetospheric plasma particles in the wave fields. Time‐dependent energy signatures are evaluated at the dipole equatorial plane (EP), in order to provide a means of identifying ponderomotive energization of plasma in spacecraft data sets. Starting magnetic latitudes for particles arriving at the EP are also derived. In general, specific energies increase rapidly at the EP soon after the driving wave is switched on, reaching a plateau as particles arrive from increasingly higher latitudes. Oscillations in specific energy occur at twice the driving wave frequency. At geosynchronous orbit these oscillations are unlikely to be detectable, but at high L values (where L is the McIlwain parameter) the oscillation amplitudes can be significant. For the fundamental mode with reasonable amplitude, ions from the ionosphere can reach the EP in a few wave cycles at high L values. Higher harmonic standing waves include negative acceleration regions (or potential barriers) on a magnetic field line which create “stop bands” of magnetic latitudes from which particles cannot reach the EP. (In some cases ionospheric ions are “shielded” from reaching the EP.) The stop bands in turn create characteristic gaps in the time series of specific energy at the EP. At high L values, O+ ions from the ionosphere can just overcome the third harmonic potential barrier and can be accelerated to the EP with energies ∼2 keV for reasonable wave amplitudes of the order of 5 mV m−1.
Application of the centrifugal impulse model to particle motion in the near‐Earth magnetotail
Authors: Delcourt D. C. , Martin R. F.
Published: February 2004

Particles traveling in the geomagnetic tail do not conserve their magnetic moment (first adiabatic invariant) due to significant field variations on the length scale of their Larmor radius. We examine the possibility of describing these magnetic moment changes by the action of an impulsive centrifugal force on the timescale of the particle cyclotron turn. Trajectory calculations demonstrate that such a centrifugal impulse model adequately describes the nonadiabatic particle behavior for situations where the κ parameter (defined as the square root of the minimum curvature radius‐to‐maximum Larmor radius ratio) is of the order of 1 to 2. In particular, it is shown that this behavior can be organized by another parameter (referred to as κα), which is proportional to κ but depends upon the particle pitch angle, namely, one obtains: systematic magnetic moment enhancements for κα ≪ 1, large gyrophase effects with possibly prominent damping of the magnetic moment for κα ∼ 1, and nearly constant magnetic moment for κα ≫ 1. More generally, we show that the centrifugal impulse approximation applies to an intermediate orbit regime at the transition between the fully adiabatic (magnetic moment conserving) regime and that regime where particles experience meandering motions about the field minimum. It applies to ion transport in the near‐Earth magnetotail where the magnetic field lines evolve from dipolar to taillike configurations and where the κ parameter nears unity. In this region of space the model predictions are in agreement with numerical results, revealing both enhanced trapping (due to magnetic moment enhancement) and possible precipitation (due to magnetic moment damping) of plasma sheet ions depending upon their pitch and phase angles.
The Ionospheric Focused Heating Experiment
Authors: Bernhardt P. A. , Siefring C. L. , Rodriguez P. , Haas D. G. , Baumback M. M. , Romero H. A. , Solin D. A. , Djuth F. T. , Duncan L. M. , Hunton D. E. , Pollock C. J. , Sulzer M. P. , Tepley C. A. , Wagner L. S. , Goldstein J. A.
Published: February 2004

The Ionospheric Focused Heating rocket was launched on May 30, 1992. The sounding rocket carried an instrument and chemical payload along a trajectory that crossed the intersection of the beams from the 430‐MHz incoherent scatter radar and the 5.1‐MHz high‐power radio wave facility near Arecibo. The release of 30 kg of CF3Br into the F region at 285 km altitude produced an ionospheric hole that acted like a convergent lens to focus the HF transmissions. The power density inside the radio beam was raised by 12 dB immediately after the release. A wide range of new processes were recorded by in situ and ground‐based instruments. Measurements by instruments flying through the modified ionosphere show small‐scale microcavities (<1 m) and downshifted electron plasma (Langmuir) waves inside the artificial cavity, electron density spikes at the edge of the cavity, and Langmuir waves coincident with ion gyroradius (4 m) cavities near the radio wave reflection altitude. The Arecibo incoherent scatter radar showed 20 dB or greater enhancements in ion acoustic and Langmuir wave turbulence after the 5.1‐MHz radio beam was focused by the artificial lens. Enhancements in airglow from chemical reactions and, possibly, electron acceleration were recorded with optical instruments. The Ionospheric Focused Heating experiment verified some of the preflight predictions and demonstrated the value of active experiments that combine high‐power radio waves with chemical releases.
The source of Jovian auroral hiss observed by Voyager 1
Authors: Morgan D. D. , Gurnett D. A. , Kurth W. S. , Bagenal F.
Published: February 2004

Observations of auroral hiss obtained from the Voyager 1 encounter with Jupiter have been reanalyzed. The Jovian auroral hiss was observed near the inner boundary of the warm Io torus and has a low‐frequency cutoff caused by propagation near the resonance cone. A simple ray tracing procedure using an offset tilted dipole of the Jovian magnetic field is used to determine possible source locations. The results obtained are consistent with two sources located symmetrically with respect to the centrifugal equator along an L shell (L ≃ 5.59) that is coincident with the boundary between the hot and cold regions of the Io torus and is located just inward of the ribbon feature observed from Earth. The distance of the sources from the centrifugal equator is approximately 0.58 ± 0.01 RJ. Based on the similarity to terrestrial auroral hiss, the Jovian auroral hiss is believed to be generated by beams of low energy (∼tens to thousands of eV) electrons. The low‐frequency cutoff of the auroral hiss suggests that the electrons are accelerated near the inferred source region, possibly by parallel electric fields similar to those existing in the terrestrial auroral regions. A field‐aligned current is inferred to exist at L shells just inward of the plasma ribbon. A possible mechanism for driving this current is discussed.
Generation of plasma waves by a Tethered Satellite System with satellites elongated in the direction of flight for arbitrary oblique geometry
Authors: Lüttgen A. , Neubauer F. M.
Published: February 2004

One of the effects of the coupling of a tethered satellite system to the ambient plasma is the excitation of plasma waves in the Earth's ionosphere. The generation of wave wings has been described by various authors using linear theory to investigate the electrodynamical behavior of a conducting body in a moving magnetized plasma. In this paper an analytical description of these waves including oblique geometry is given by extending the perpendicular geometry model of Estes (1988) with satellites elongated in the direction of flight. We include the case of fully arbitrary geometry, i.e., the magnetic field, direction of flight, and direction of the tether are not perpendicular to each other. We derive far‐field equations for the magnetic field aligned current density and the associated wave impedance where a two‐dimensional Fourier transform has to be done numerically. We find that the limits of the frequency bands for propagating waves (Barnett and Olbert, 1986) change due to changes in the geometry of the magnetic field. Our calculations are limited to the lowest band, i.e., frequencies below the ion cyclotron frequency. The solution for the far field can be described as wave wings which are tilted by a small angle dependent on the system's velocity and the Alfvén velocity. For the associated wave impedance we find strong dependence on the geometry of the magnetic field.
Whistler mode waves in the Jovian magnetosheath
Authors: Lin Naiguo , Kellogg P. J. , Thiessen J. P. , Lengyel‐Frey D. , Tsurutani B. T. , Phillips J. L.
Published: February 2004

During the Ulysses flyby of Jupiter in February 1992, the spacecraft traversed the Jovian magnetosheath for a few hours during the inbound pass and for a few days during the outbound pass. Burstlike electromagnetic waves at frequencies of ∼0.1‐0.4 of the local electron cyclotron frequency have been observed by the Unified Radio and Plasma Wave (URAP) experiment. The waves were more often observed in the regions which were probably the outer or middle magnetosheath, especially near the bow shock, and rarely seen in the magnetosphere/magnetosheath boundary layer. The propagation angles of the waves are estimated by comparing the measurements of the wave electric and magnetic fields in the spacecraft spin plane with the corresponding values calculated using the cold plasma dispersion relation under local field and plasma conditions. It is found that the waves propagate obliquely with wave angles between ∼30° and 50°. These waves are likely to be the whistler mode waves which are excited by suprathermal electrons with a few hundred eV and a slight anisotropy (T⊥/T∥ ∼1.1‐1.5). They are probably similar in nature to the lion roars observed in the Earth's magnetosheath. Signature of coupling between the mirror mode and the whistler mode have also been observed. The plasma conditions which favor the excitation of the whistler mode instability during the wave events exist as observed by the plasma experiment of Ulysses.
Pickup protons and pressure‐balanced structures: Voyager 2 observations in merged interaction regions near 35 AU
Authors: Burlaga L. F. , Ness N. F. , Belcher J. W. , Szabo A. , Isenberg P. A. , Lee M. A.
Published: February 2004

Five pressure‐balanced structures, each with a scale of the order of a few hundredths of an astronomical unit (AU), were identified in two merged interaction regions (MIRs) near 35 AU in the Voyager 2 plasma and magnetic field data. They include a tangential discontinuity, simple and complex magnetic holes, slow correlated variations among the plasma and magnetic field parameters, and complex uncorrelated variations among the parameters. The changes in the magnetic pressure in these events are balanced by changes in the pressure of interstellar pickup protons. Thus the pickup protons probably play a major role in the dynamics of the MIRs. The solar wind proton and electron pressures are relatively unimportant in the MIRs at 35 AU and beyond. The region near 35 AU is a transition region: the Sun is the source of the magnetic field, but the interstellar medium is the source of pickup protons. Relative to the solar wind proton gyroradius, the thicknesses of the discontinuities and simple magnetic holes observed near 35 AU are at least an order of magnitude greater than those observed at 1 AU. However, the thicknesses of the tangential discontinuity and simple magnetic holes observed near 35 AU (in units of the pickup proton Larmor radius) are comparable to those observed at 1 AU (in units of the solar wind proton gyroradius). Thus the gyroradius of interstellar pickup protons controls the thickness of current sheets near 35 AU. We determine the interstellar pickup proton pressure in the PBSs. Using a model for the pickup proton temperature, we estimate that the average interstellar pickup proton pressure, temperature, and density in the MIRs at 35 AU are (0.53 ± 0.14) × 10−12 erg/cm³, (5.8 ± 0.4) × 106 °K and (7 ± 2) × 10−4 cm−3, respectively.
Interaction of global merged interaction region shock with the heliopause and its relation to the 2‐ and 3‐kHz radio emissions
Authors: Whang Y. C. , Burlaga L. F.
Published: February 2004

We use Voyager 2 plasma and magnetic field data together with a one‐fluid magnetohydrodynamics model to study the interactions of the 1991 global merged interaction region (GMIR) shock with the termination shock and the heliopause. The 1991 GMIR is an extraordinarily large global solar wind structure in radial, longitudinal, and latitudinal extents. It has a strong shock at the leading edge. After its penetration through the termination shock, the GMIR shock first propagates through the subsonic solar wind, then interacts with the heliopause. The interaction produces a transmitted shock propagating outward in the interstellar medium, and a reflected shock propagating backward in the subsonic solar wind. We identify the reflected shock and the transmitted shock as the possible source of the radio noise detected at Voyagers (Gurnett, et al., 1993). The plasma frequency behind the reflected and the transmitted shock can be responsible for the 2‐ and 3‐kHz radio emissions, respectively. The two bands of radio noise are emitted from sources on both sides of the heliopause starting at about the same time. If the emissions picked up by Voyager are due to fp radiation, then the heliopause is located at R∼130 AU. If the emissions are due to 2fp radiation, then R∼150 AU. Because the relative speed of the interstellar plasma with respect to the sun appears to be sub‐Alfvenic, it is very unlikely there is a fast‐mode bow shock of the heliosphere.
Theory and observation of auroral substorms: A magnetohydrodynamic approach
Authors: Liu W. W. , Xu B.‐L. , Samson J. C. , Rostoker G.
Published: February 2004

A theory of auroral substorm dynamics is constructed on the basis of MHD wave processes in the ionosphere‐magnetosphere system. The basic view is that the substorm commences in the nightside near‐Earth magnetosphere through a collapse of plasma equilibrium. The collapse releases a significant amount of free energy embedded initially in a collection of compressional waves. It is suggested that substorm dynamics after the collapse are determined by the evolution of these waves. We first investigate the quantitative ramifications of the waves in a two‐dimensional box in the GSM yz cross section of the magnetotail. The model is constructed to allow the study of radiation of substorm wave energy into the solar wind and also encompasses the essential elements of resonant interaction in the plasma sheet boundary layer. The natural boundary condition leading to radiative loss is introduced. It is found that wave radiation into the solar wind can relax the magnetospheric system in less than a hour. The resonant Alfvén modes driven by the normal compressional modes in the box are studied through the construction of proper dispersion equation. By studying the field‐aligned current generated by resonances, we establish the auroral pattern expected to result from the coupling. Following the theoretical study, we examine an auroral substorm observed by the CANOPUS photometer array on February 20, 1990. It is found that, among the testable theoretical predictions, there exists a general agreement with the observations. We did find, however, that electron‐ and proton‐induced aurora oscillate essentially in phase, thus implying a more complicated precipitation process.
Determination of ionospheric conductivities from FUV auroral emissions
Authors: Germany G. A. , Torr D. G. , Richards P. G. , Torr M. R. , John S.
Published: February 2004

The purpose of this paper is to examine the viability of using LBH emission ratios to infer auroral conductances and to quantify the strengths and weaknesses of this technique. We show that column‐integrated Hall and Pedersen conductances may be determined from a single remote measurement of a pair of auroral LBH emissions, one in the region of strong O2 absorption (1464 Å) and one lying outside of this region (1838 Å). The dependence of the determined conductivities on incident average energy, total energy flux, and changes in solar and magnetic activity levels is examined. We show that, for energies above 5 keV, auroral conductances may be scaled by the square root of the incident energy flux with errors less than about 20%. For lower energies, however, the scaling may deviate significantly from a power of 0.5. We provide appropriate scaling factors as a function of average energy. We also note that the choice of either a Gaussian or Maxwellian distribution can significantly affect low‐energy (< 1 keV) conductance calculations. Finally, we quantify the conditions under which the mean energy of a Gaussian energy distribution may differ significantly from the characteristic energy.
Solar flare effects at Ebre: Unidimensional physical, integrated model
Authors: Curto J. J. , Amory‐Mazaudier C. , Torta J. M. , Menvielle M.
Published: February 2004

A great increase of the ionizing radiation during solar flares results in an immediate increase of the ionization production rate, electron densities and electric currents in the ionosphere, followed simultaneously by disturbances of the magnetic elements at ground level (solar flare effects (sfe)). In this paper an attempt is made to model sfe phenomena combining several semiempirical models derived from satellite and radar data obtained during the last two decades. The model allows us to quantify model values of the phase difference between the sfe and Sq vectors, for comparison to the measurable quantity. It explains the cause of the change in magnetic perturbation during a flare at Ebre Observatory (40.8° latitude N, 0.5° longitude E). Large phase shift of the magnetic vector observed before noon, result from a descent of the “center of gravity” of the conducting mass that, combined with a very different regime of neutral winds in the lower and in the middle parts of the dynamo region, produce a change in the direction of the integrated currents.
One‐dimensional hybrid satellite track model for the Dynamics Explorer 2 (DE 2) satellite
Authors: Deng Wei , Killeen T. L. , Burns A. G. , Johnson R. M. , Emery B. A. , Roble R. G. , Winningham J. D. , Gary J. B.
Published: February 2004

A one‐dimensional hybrid satellite track model has been developed to calculate the high‐latitude thermospheric/ionospheric structure below the satellite altitude using Dynamics Explorer 2 (DE 2) satellite measurements and theory. This model is based on Emery et al. (1985) satellite track code but also includes elements of Roble et al. (1987b) global mean thermosphere/ionosphere model. A number of parameterizations and data handling techniques are used to input satellite data from several DE 2 instruments into this model. Profiles of neutral atmospheric densities are determined from the MSIS‐90 model and measured neutral temperatures. Measured electron precipitation spectra are used in an auroral model to calculate particle impact ionization rates below the satellite. These rates are combined with a solar ionization rate profile and used to solve the O+ diffusion equation, with the measured electron density as an upper boundary condition. The calculated O+ density distribution, as well as the ionization profiles, are then used in a photochemical equilibrium model to calculate the electron and molecular ion densities. The electron temperature is also calculated by solving the electron energy equation with an upper boundary condition determined by the DE 2 measurement. The model enables calculations of altitude profiles of conductivity and Joule heating rate along and below the satellite track. In a first application of the new model, a study is made of thermospheric and ionospheric structure below the DE 2 satellite for a single orbit which occurred on October 25,1981. The field‐aligned Poynting flux, which is independently obtained for this orbit, is compared with the model predictions of the height‐integrated energy conversion rate. Good quantitative agreement between these two estimates has been reached. In addition, measurements taken at the incoherent scatter radar site at Chatanika (65.1° N, 147.4° W) during a DE 2 overflight are compared with the model calculations. A good agreement was found in lower thermospheric conductivities and Joule heating rate.
Plasma flow and magnetic field characteristics near the midtail neutral sheet
Authors: Nakamura R. , Baker D. N. , Fairfield D. H. , Mitchell D. G. , McPherron R. L. , Hones E. W.
Published: February 2004

Using IMP 6, 7, and 8 magnetic field and plasma data, we have determined statistical occurrence properties of bulk flow and magnetic field orientation near the midtail neutral sheet. Characteristics of bulk plasma flow and magnetic field significantly change according to the radial distance down the tail. High‐speed flow events (V > 300 km/s) are essentially restricted to the region tailward of X = −25 RE and are predominantly sunward or tailward. The low‐speed flows were nearly equally likely to be in any direction, with the occurrence rate of duskward and sunward flow being larger than that of tailward and dawnward flow. Duskward flow occurrence is highest in the region Earthward of X = −25 RE, while sunward flow occurrence is highest in the region tailward of X = −25 RE. The significance of the dawn‐to‐dusk flow in the near‐Earth region obtained in our study supports the idea that there exists a very effective mechanism to accelerate ions in the dawn‐to‐dusk direction and hence the relief of pressure buildup in the near‐Earth region. During high‐speed flow events the relationship between BZ polarity and plasma flow direction is largely consistent with that expected from the magnetic reconnection processes associated with substorms. There are also significant numbers of negative BZ events that are not associated with tailward flow. Mechanism other than substorm neutral line should therefore also taken into account to explain general BZ polarity in the midtail region.
On quasi‐thermal fluctuations near the plasma frequency in the outer plasmasphere: A case study
Authors: Lund E. J. , LaBelle J. , Treumann R. A.
Published: February 2004

We present a derivation of the quasi‐thermal electrostatic fluctuation power spectrum in a multi‐Maxwellian plasma and show sample calculated spectra. We then apply this theory, which has been successfully applied in other regions of space, to spectra from two AMPTE IRM passes through the duskside plasmasphere. We show that the plasma line that is often seen in this region is usually quasi‐thermal in origin. We obtain a refined estimate of the plasma frequency and infer a cold electron temperature which is consistent within a factor of 2 with both models and previous measurements by other techniques, but closer investigation reveals that details of the plasma line cannot be explained with the usual two isotropic Maxwellian model.
Reflection of Alfvén waves in the solar wind
Authors: Krogulec M. , Musielak Z. E. , Suess S. T. , Nerney S. F. , Moore R. L.
Published: February 2004

We have revisited the problem of propagation of toroidal and linear Alfvén waves formulated by Heinemann and Olbert (1980) to compare WKB and non‐WKB waves and their effects on the solar wind. They considered two solar wind models and showed that reflection is important for Alfvén waves with periods of the order of one day and longer and that non‐WKB Alfvén waves are no more effective in accelerating the solar wind than WKB waves. There are several recently published papers that seem to indicate that Alfvén waves with periods of the order of several minutes should be treated as non‐WKB waves and that these non‐WKB waves exert a stronger acceleration force than WKB waves. The purpose of this paper is to study the origin of these discrepancies by performing parametric studies of the behavior of the waves under a variety of different conditions. In addition, we want to investigate two problems that have not been addressed by Heinemann and Olbert, namely, calculate the efficiency of Alfvén wave reflection by using the reflection coefficient and identify the region of strongest wave reflection in different wind models. To achieve these goals, we investigated the influence of temperature, electron density distribution, wind velocity, and magnetic field strength on the waves. The obtained results clearly demonstrate that Alfvén wave reflection is strongly model dependent and that the strongest reflection can be expected in models with the base temperatures higher than 106 K and with the base densities lower than 7 × 107 cm−3. In these models as well as in the models with lower temperatures and higher densities, Alfvén waves with periods as short as several minutes have negligible reflection so that they can be treated as WKB waves; however, for Alfvén waves with periods of the order of one hour or longer reflection is significant, requiring a non‐WKB treatment. We also show that non‐WKB, linear Alfvén waves are always less effective in accelerating the plasma than WKB Alfvén waves. Finally, it is evident from our results that the region of strongest wave reflection is usually located at the base of the models and hence that interpretation of wave reflection based solely on the reflection coefficient can be misleading.
Causes of plasma flow bursts and dayside auroral transients: An evaluation of two models Invoking reconnection pulses and changes in the Y component of the magnetosheath field
Authors: Lockwood M. , Cowley S. W. H. , Sandholt P. E. , Løvhaug U. P.
Published: February 2004

Longitudinal flow bursts observed by the European Incoherent Scatter (EISCAT) radar, in association with dayside auroral transients observed from Svalbard, have been interpreted as resulting from pulses of enhanced reconnection at the dayside magnetopause. However, an alternative model has recently been proposed for a steady rate of magnetopause reconnection, in which the bursts of longitudinal flow are due to increases in the field line curvature force, associated with the By component of the magnetosheath field. We here evaluate these two models, using observations on January 20, 1990, by EISCAT and a 630‐nm all‐sky camera at Ny Ålesund. For both models, we predict the behavior of both the dayside flows and the 630‐nm emissions on newly opened field lines. It is shown that the signatures of steady reconnection and magnetosheath By changes could possibly resemble the observed 630‐nm auroral events, but only for certain locations of the observing site, relative to the ionospheric projection of the reconnection X line: however, in such cases, the flow bursts would be seen between the 630‐nm transients and not within them. On the other hand, the model of reconnection rate pulses predicts that the flows will be enhanced within each 630‐nm transient auroral event. The observations on January 20, 1990, are shown to be consistent with the model of enhanced reconnection rate pulses over a background level and inconsistent with the effects of periodic enhancements of the magnitude of the magnetosheath By component. We estimate that the reconnection rate within the pulses would have to be at least an order of magnitude larger than the background level between the pulses.
Prediction of the interplanetary magnetic field strength
Authors: Zhao Xuepu , Hoeksema J. Todd
Published: February 2004

A new model of the coronal and interplanetary magnetic field can predict both the interplanetary magnetic field strength and its polarity from measurements of the photospheric magnetic field. The model includes the effects of the large‐scale horizontal electric currents flowing in the inner corona, of the warped heliospheric current sheet in the upper corona, and of volume currents flowing in the region where the solar wind plasma totally controls the magnetic field. The model matches the MHD solution for a simple dipole test case better than earlier source surface and current sheet models. The strength and polarity of the radial interplanetary magnetic field component predicted for quiet time samples in each year from 1977 to 1986 agree with observations made near the Earth's orbit better than the hybrid MHD‐source surface model (Wang and Sheeley, 1988). The results raise the question of whether coronal holes are the only solar source of the interplanetary magnetic field in the solar wind. If some interplanetary flux originates outside coronal holes, the model can match the observed field using the accepted 1.8 saturation correction factor for λ5250 Å magnetograph observations. Requiring open flux to come exclusively from coronal holes requires an additional factor of two.
Electron number density fluctuations near the plasmapause observed by the CRRES Spacecraft
Authors: LeDocq M. J. , Gurnett D. A. , Anderson R. R.
Published: February 2004

Large quasi‐periodic fluctuations in the electron number density are often encountered near and inside the plasmapause by the CRRES satellite. A power spectral analysis reveals electron density fluctuations with frequencies ranging from 2 mHz to approximately 61 mHz. Plasma density irregularities in the outer plasmasphere sometimes have power spectral slopes near −5/3, which suggest the presence of well‐developed two‐dimensional magnetohydrodynamic turbulence. Quasi‐periodic fluctuations have been found outside the plasmasphere with spatial scales along the orbit path of approximately 750 km. A statistical analysis shows that the density fluctuation amplitudes are largest between L = 3 and L = 6, the region in which the plasmapause is located. The density fluctuation amplitudes decrease with increasing L shell for L > 6. Normalized fluctuation amplitudes greater than 10% are found in only 25% of the available data of this study. A significant magnetic local time dependence cannot be found due to incomplete data availability during the 9.00 to 14.99 MLT and the 15.00 to 20.99 MLT sectors.
Multicomponent double layers and selective acceleration of charged particles
Authors: Carlqvist Per
Published: February 2004

Multicomponent double layers, defined as layers composed of more than two kinds of charged particles, are supposed to constitute the predominant type of double layer in cosmic plasmas. A model of a steady and strong multicomponent double layer is studied in both the nonrelativistic and relativistic approximations. In particular, such properties of the layer as the structure, potential drop, and current composition are investigated. It is demonstrated that the density distribution of each kind of positive and negative particle in the nonrelativistic multicomponent layer is of the same shape as the density distribution of the ions and electrons, respectively, in Langmuir's two‐component layer. Also, the shape of the distribution of the electric field corresponds to that of the two‐component layer. In the relativistic layer the charges are distributed among two very thin layers of high positive and negative charge density close to the high‐potential and low‐potential boundaries of the layer, respectively, and a constant but low charge density in the rest of the layer. It is shown that the potential drop across the multicomponent layer is proportional to the thickness of the layer raised to 4/3 in the nonrelativistic case while it is directly proportional to the thickness in the relativistic case. Current conditions prescribing the allowed sets of current densities of the various kinds of particles in the layer are derived in the nonrelativistic and relativistic approximations. Correspondingly, abundance conditions for the particles accelerated through the layer are also obtained. The abundance of the particles accelerated by the double layer is expected generally to be different from the abundance of the ambient plasma. It is suggested that the abundance of the accelerated particles might serve as a means to detect double layers, especially in space plasmas.
Development of substorm cross‐tail current disruption as seen from the ground
Authors: Gazey N. G. J. , Lockwood M. , Smith P. N. , Coles S. , Bunting R. J. , Lester M. , Aylward A. D. , Yeoman T. K. , Lühr H.
Published: February 2004

We discuss substorm observations made near 2100 magnetic local time (MLT) on March 7, 1991, in a collaborative study involving data from the European Incoherent Scatter radar, all‐sky camera data, and magnetometer data from the Tromsø Auroral Observatory, the U.K. Sub‐Auroral Magnetometer Network (SAMNET) and the IMAGE magnetometer chain. We conclude that for the substorm studied a plasmoid was not pinched off until at least 10 min after onset at the local time of the observations (2100 MLT) and that the main substorm electrojet expanded westward over this local time 14 min after onset. In the late growth phase/early expansion phase, we observed southward drifting arcs probably moving faster than the background plasma. Similar southward moving arcs in the recovery phase moved at a speed which does not appear to be significantly different from the measured plasma flow speed. We discuss these data in terms of the “Kiruna conjecture” and classical “near‐Earth neutral line” paradigms, since the data show features of both models of substorm development. We suggest that longitudinal variation in behavior may reconcile the differences between the two models in the case of this substorm.
Transport theory and the WKB approximation for interplanetary MHD fluctuations
Authors: Matthaeus William H. , Zhou Ye , Zank G. P. , Oughton S.
Published: February 2004

An alternative approach, based on a multiple scale analysis, is presented in order to reconcile the traditional WKB approach to the modeling of interplanetary fluctuations in a mildly inhomogeneous large‐scale flow with a more recently developed transport theory. This enables us to compare directly, at a formal level, the inherent structure of the two models. In the case of noninteracting, incompressible (Alfvén) waves, the principle difference between the two models is the presence of leading‐order couplings (called “mixing effects”) in the non‐WKB turbulence model which are absent in a WKB development. Within the context of linearized MHD, two cases have been identified for which the leading order non‐WKB “mixing term” does not vanish at zero wavelength. For these cases the WKB expansion is divergent, whereas the multiple‐scale theory is well behaved. We have thus established that the WKB results are contained within the multiple‐scale theory, but leading order mixing effects, which are likely to have important observational consequences, can never be recovered in the WKB style expansion. Properties of the higher‐order terms in each expansion are also discussed, leading to the conclusion that the non‐WKB hierarchy may be applicable even when the scale separation parameter is not small.