bluuebunny/crossref_metadata_2025 huggingface dataset:
Tests of a boundary layer model of field‐aligned currents using S3–3 data
Authors: Cattell C. , Somoza M. , Siscoe G. , Roeder J.
Published: February 2004

Data obtained from the low altitude, polar orbiting S3–3 satellite are utilized to test the model of field‐aligned currents and electric fields within the auroral zone developed by Siscoe and Maynard [1991]. This model, for which the cross polar cap potential drop is the input parameter, combines the boundary layer‐driven field‐aligned current model of Siscoe et al. [1991] for Region 1 with a model for Region 2 currents based on Vasyliunas [1972]. Many general features of the observations are reproduced by the model. The main discrepancies may be explained by two simplifications which were made in the model: (1) the time independence of the boundary layer, in particular, of the scale length of magnetosphere‐ionosphere coupling, l and (2) the uniform ionospheric conductivity. When the expected time dependence of l is taken into account, the disagreement between the model gradients, latitudinal extents, and maxima in the fields and those observed in the S3–3 data in the poleward (Region 1) auroral zone is readily accounted for in the context of a refined boundary layer model. The fact that the observed Region 2 currents are usually larger and narrower than the predicted currents may be due to the underestimate of the Region 1 current explained above and the fact that the ionospheric conductivity was constant. These results suggest that the two most important modifications needed to improve the model are the inclusion of local‐time dependence in the magnetosphere‐ionosphere coupling parameter (as was done by Phan et al. [1989]) and of latitude and local time dependence in the ionospheric conductivity. There is also some evidence that the low latitude boundary layer (LBL) parameters have cross polar cap potential dependencies which must be included in a model of this type. The S3–3 field data used the convection reversal as a marker for the polar cap boundary, whereas the model used the poleward edge of the low‐latitude boundary layer. Comparison of the energetic particle signatures with the electric field data indicated that this was a small effect on the determination of the cross polar cap potential drop. For the events shown herein, the potential drop across the low‐latitude boundary layer was usually less than a few kV due to the existence of sunward convection in the equatorward portion of the layer.
Ionospheric signature of the cusp as seen by incoherent scatter radar
Authors: Nilsson H. , Yamauchi M. , Eliasson L. , Norberg O. , Clemmons J.
Published: February 2004

Measurements with the Søndre Strømfjord incoherent scatter radar, co‐ordinated with the observations by the Freja satellite, have been performed during three campaigns, April 1993, February 1994, and May–June 1994. Radar signatures of various types of magnetosheath particle injections in the cusp‐cleft region are investigated. The measurement days represent very different geomagnetic conditions, from very quiet to a Kp index of 7+. On three occasions both Freja and the radar detected the cusp. A unique cusp signature is found for a relatively stable cusp, distinguishing it from the many other soft precipitation features seen around noon. The signature includes extremely high electron temperatures in a latitudinally well‐defined region with a sharp equatorward border, some F region electron density enhancement, ion outflow, and mainly poleward plasma flow. Enhanced ion temperatures are also seen in the vicinity of, but not exactly coincident with, the electron temperature enhancements. Other day side precipitation features observed with an intense soft component are narrow arcs, which usually have an accompanying accelerated electron component of several hundred eV to some keV energy. These are typically seen in, or bordering, convection regions where the plasma flow vorticity implies upward field‐aligned currents.
Harmonics of the ion acoustic frequency in the heater induced ion spectrum
Authors: Kohl H. , Rietveld M. T.
Published: February 2004

Odd harmonics of the ion acoustic frequency in the 224MHz radar ion line spectrum have been found during HF heating experiments at Tromsø. This new feature appears in a narrow height range near the matching height where the ion line is characterized by enhanced ion acoustic shoulders and cascade or line spectra are observed in the plasma line channel. An explanation and a first estimate are given in terms of the parametric decay theory. A numerical calculation using an extended Zakharov model also demonstrates the existence of such ion line harmonics.
Energetic (>0.2 MeV) electron bursts in the deep geomagnetic tail observed by the Goddard Space Flight Center experiment on ISEE 3: Association with geomagnetic substorms
Authors: Richardson I. G. , Owen C. J. , Slavin J. A.
Published: February 2004

Brief (< 1‐hour duration) 0.2‐ to 2‐MeV electron bursts were detected in the geomagnetic tail out to 238 RE from Earth by the Goddard Space Flight Center medium energy cosmic ray experiment on ISEE 3 in 1982–1983. We examine their association with geomagnetic substorms and illustrate several examples in conjunction with simultaneous ISEE 3 plasma, magnetic field, and low‐energy (35–56 keV) ion data. The majority (>80%) of electron bursts in the plasma sheet and plasma sheet boundary layer (PSBL) were associated with AL intensifications and/or particle injections observed at geosynchronous orbit, indicative of a relationship with substorm onsets. Bursts were detected following less than 50% of apparent substorms, presumably because a burst was detected only if the spacecraft was located within the region of field lines reconnected at the substorm neutral line. In the deep tail (∼ 200 RE from Earth), peak energetic electron intensities were typically observed near the outer edge of the PSBL closely following substorm onset. This finding suggests that the electrons were accelerated on newly reconnected field lines and lost rapidly from the tail. Unlike low‐energy (tens of keV) ions, energetic electrons (at least with intensities above the detector background) were not a general feature of the PSBL or the plasma sheet outside times of enhanced substorm activity. Most plasmoids (∼80%) in the deep tail were not associated with enhanced energetic electron fluxes, a suggestion that electrons accelerated at the substorm neutral line during disconnection of the plasmoid may leak out from the plasmoid. This possibility is consistent with flux‐rope‐like magnetic structures in plasmoids which connect to open field lines at the flanks of the tail. Electron anisotropies can be examined during the more intense bursts. These show either weak flows consistent with trapping on closed field lines connected to the Earth or stronger, tailward flows which suggest flow away from an acceleration region earthward of the spacecraft. Weak flows were observed in the plasma sheet within ∼90 RE from Earth, consistent with previous ISEE 3 studies suggesting that this is the typical location of the distant neutral line. We confirm earlier observations that electron bursts in the magnetosheath are found predominantly close to the magnetopause, are associated with low‐energy ion enhancements, and show strong tailward flows. Since most of these bursts were detected within ∼90 RE of Earth, the region where intense plasma sheet electron bursts were also observed, this finding suggests that the plasma sheet may be source of the magnetosheath bursts.
Ionospheric signature of the tail neutral line during the growth phase of a substorm
Authors: Moses J. J. , Slavin J. A. , Heelis R. A.
Published: February 2004

An isolated substorm occurred on October 21, 1981 at the end of a large geomagnetic storm. Observations of particles and fields were made during the presumed growth phase of this substorm by the DE 2 satellite. The ionospheric convection at the time indicates a pattern commonly observed during the maximum of substorms with convection velocities averaging ∼500 m/s in an eastward channel of flow in the premidnight sector. Despite the convection signature, geomagnetic conditions were quiet with the AL index >−50 nT and very weak field‐aligned current. We will present an in‐depth study of the nightside gap region within the channel of eastward flow. We will discuss the possible effects of a neutral wind flywheel on the convection. Also, we will present a detailed analysis of the electrodynamic structures within the nightside gap which indicates a magnetospheric source. Possible ionospheric signatures of the tail neutral line are discussed.
VLF remote sensing of the auroral electrojet
Authors: Cummer S. A. , Bell T. F. , Inan U. S. , Zanetti L. J.
Published: February 2004

We investigate a new potential technique to determine the position of the auroral electrojet from ground‐based VLF amplitude and phase measurements. The chief advantage of this technique over conventional ground magnetometer measurements is that it can provide data on a continental scale with a small number of receiving stations and with a minimum of data processing. At the edge of the auroral zone, where the electrojet current system flows, high‐energy (E > 300 keV) precipitating electrons cause local electron density enhancements in the ionosphere which cause phase and amplitude perturbations in VLF waves propagating in the Earth‐ionosphere waveguide. Continuous measurements of the amplitude and phase of signals from the Omega North Dakota VLF transmitter were made in Nome, Alaska. Using a two‐dimensional model of VLF propagation which accounts for ionospheric disturbances caused by the electron precipitation associated with the electrojet, the amplitude and phase signatures of electrojet incursion across each propagation path were predicted. Seventeen nights of simultaneous VLF amplitude and phase data and ground magnetometer data were examined and catalogued based on the degree of temporal correlation between the two data sets and the degree to which the VLF events matched the propagation simulations. Of the nights exhibiting activity, more than 60% exhibited excellent correlation between the magnetometer and VLF events, and the majority of these showed good agreement with the model results. An additional estimate of the electrojet position was provided for one of the studied nights by field‐aligned current measurements from the Freja satellite. A comparison of these independent means of determining the electrojet position shows that they are in good agreement for the night examined.
A transport model for the diffusive shock acceleration and modulation of anomalous cosmic rays in the heliosphere
Authors: le Roux J. A. , Potgieter M. S. , Ptuskin V. S.
Published: February 2004

Standard transport theory is used to investigate the acceleration and modulation of anomalous cosmic rays during solar minimum periods. With the developed anomalous cosmic ray transport model, pickup ions are transformed into anomalous cosmic rays by diffusive shock acceleration at the heliospheric termination shock. The emphasis is on explaining the 1987 cosmic ray spectra at large radial distances in the equatorial plane. Energy density calculations of cosmic ray spectra at the termination shock suggest that the termination shock might be strongly mediated by anomalous cosmic rays. Therefore, in the spirit of nonlinear shock acceleration theory, the termination shock is modeled as a hyperbolic tangent. Calculations show that realistic anomalous cosmic ray intensity radial gradients can only be achieved, within the framework of our assumptions, for a termination shock structure with a compression ratio of 3.2 <s <4.0 and a scale length of ∼1.2 AU. In addition, simulations of anomalous and galactic cosmic ray spectra revealed as follows: (1) For radial distances of less than at least 24 AU from the Sun it should be generally difficult, although not impossible, to identify anomalous protons, for example, in the observed 1987 proton spectra. (2) Anomalous protons should be more easily detectable, however, in the proton spectra seen at 42 AU by Pioneer 10 during 1987 and should be clearly visible at larger radial distances. (3) The best way to identify anomalous protons in observed proton spectra is to look for a clear flattening in the spectral slopes in the energy interval of ∼20–100 MeV. Recent 1994 observations by Pioneer 10 at ∼60 AU and by Voyager 1 and Voyager 2 support the model prediction of very flat proton spectra in the above mentioned energy interval at large radial distances. Within the framework of our assumptions this might be an indication that the average termination shock position during 1994 was in the vicinity of 80 AU as assumed in the model.
VLF and LF signatures of mesospheric/lower ionospheric response to lightning discharges
Authors: Inan U. S. , Slingeland A. , Pasko V. P. , Rodriguez J. V.
Published: February 2004

New evidence is presented of disturbances of the electrical conductivity of the nighttime mesosphere and the lower ionosphere in association with lightning discharges. In addition to extensive documentation of the characteristics of a class of events heretofore referred to as early/fast VLF events [Inan et al., 1993], our data reveal a new feature of these events, consisting of a postonset peak that typically lasts for 1–2 s. We also report the observation of short‐duration VLF or LF perturbations, in which the amplitude of the subionospheric signal exhibits a sudden change within 20 ms of the causative lightning discharge, and recovers back to its original level in < 3 s. These short‐duration events have characteristics similar to the previously observed rapid onset, rapid decay VLF signatures [Dowden et al.., 1994]. Both the typical and rapidly recovering events are observed primarily when the causative lightning discharge is within ±50 km of the VLF or LF great circle propagation path, indicating that the scattering from the localized disturbance is highly collimated in the forward direction. The latter in turn implies that for the parameters in hand, the transverse extent of the disturbance must be at least ∼ 100–150 km. The measured VLF signatures are compared with the predictions of a three‐dimensional model of subionospheric VLF propagation and scattering in the presence of localized ionospheric disturbances produced by electromagnetic impulses and quasi‐electrostatic (QE) fields produced by lightning discharges. The rapidly recovering or short‐duration events are consistent with the heating of the ambient electrons by quasi‐static electric fields, in cases when heating is not intense enough to exceed the attachment or ionization thresholds. When no significant electron density changes occur, the conductivity changes due to heating alone last only as long as the QE fields, typically less than a few seconds. When heating is intense enough so that attachment or ionization thresholds are exceeded, reductions or enhancements in electron density can respectively occur, in which case the medium would relax back to the ambient conditions with the time scales of the local D region chemistry, typically 10–100 s.
Effects on the Jovian magnetosheath arising from solar wind flow around nonaxisymmetric bodies
Authors: Erkaev N. V. , Farrugia C. J. , Biernat H. K.
Published: February 2004

We investigate the MHD structure of the Jovian magnetosheath along the Sun‐Jupiter line and, particularly, the region where the interplanetary magnetic field (IMF) exerts a large influence on the magnetosheath flow (the “magnetic barrier”). We do this by integrating numerically the dissipationless MHD equations in their “magnetic string” formulation. The lack of axisymmetry of the magnetospheric obstacle introduces corresponding asymmetries in the Jovian magnetosheath. The dominant effect on the flow is produced by the IMF component orthogonal to Jupiter's rotational equator. The thicknesses of the magnetosheath and magnetic barrier depend sensitively on the orientation of the IMF, decreasing monotonically as the inclination of the IMF to the rotational equator decreases. The magnetic barrier practically disappears when the IMF vector lies in the equator. For an arbitrary orientation of the IMF the magnetosheath magnetic field along the stagnation streamline is not only compressed as the magnetopause is approached but also rotates smoothly toward the direction of the Jovian rotation axis. This effect is absent in the case of flow around axisymmetric obstacles, such as the terrestrial magnetosphere.
Pickup ion energization by shock surfing
Authors: Lee Martin A. , Shapiro Vitali D. , Sagdeev Roald Z.
Published: February 2004

Energization at a quasi‐perpendicular shock is described for ions which approach the shock with a speed much less than that of the incoming plasma. These ions may be trapped between the shock electrostatic potential and the upstream Lorentz force and accelerated by “surfing” along the shock surface, before eventually escaping the shock into the upstream or downstream plasma. The process is described in detail, extending previous work on the mechanism, and energy gains are calculated. It is pointed out that pickup ions in the solar wind are ideally configured, so that a reasonable fraction of the ions can be accelerated by this mechanism at cometary bow shocks, the solar wind termination shock, and interplanetary traveling shocks. The mechanism may provide the required “injection” or preacceleration at quasi‐perpendicular shocks for subsequent diffusive shock acceleration.
Electron acceleration by inertial Alfvén waves
Authors: Thompson B. J. , Lysak R. L.
Published: February 2004

Alfvén waves reflected by the ionosphere and by inhomogeneities in the Alfvén speed can develop an oscillating parallel electric field when electron inertial effects are included. These waves, which have wavelengths of the order of an Earth radius, can develop a coherent structure spanning distances of several Earth radii along geomagnetic field lines. This system has characteristic frequencies in the range of 1 Hz and can exhibit electric fields capable of accelerating electrons to several keV. These electric fields have the potential to accelerate electrons in several senses: via Landau resonance, bounce or transit time resonance as discussed by André and Eliasson [1992] or through the effective potential drop which appears when the transit time of the electrons is much smaller than the wave period, so that the electric fields appear effectively static. A time‐dependent model of wave propagation is developed which represents inertial Alfvén wave propagation along auroral field lines. The disturbance is modeled as it travels earthward, experiences partial reflections in regions of rapid variation, and finally reflects off a conducting ionosphere to continue propagating antiearthward. The wave experiences partial trapping by the ionospheric Alfvén resonator, which is the effective cavity formed between the ionosphere and the Alfvén speed peak discussed earlier by Polyakov and Rapoport [1981] and Trakhtengerts and Feldstein [1981, 1984, 1991] and later by Lysak [1991, 1993]. Results of the wave simulation and an accompanying test particle simulation are presented, which indicate that inertial Alfvén waves are a possible mechanism for generating electron conic distributions and field‐aligned particle precipitation. The model incorporates conservation of energy by allowing electrons to affect the wave via Landau damping, which appears to enhance the effect of the interactions which heat electron populations.
Identification of the poleward boundary of the auroral oval using characteristics of ion precipitation
Authors: Troshichev O. A. , Shishkina E. M. , Meng C.‐I. , Newell P. T.
Published: February 2004

Electron and ion precipitation data from DMSP F6 and F7 spacecraft are compared in the polar region for BZ < 0 and BZ > 0. We used the electron and ion number fluxes averaged over 0.5° latitude, normalized by the respective flux maxima in each crossing. Analysis of these averaged and normalized fluxes shows that two regions of large‐scale precipitation are present. The first is the “regular” auroral oval, where electron and ion precipitation is highly correlated irrespective of interplanetary magnetic field (IMF) polarity. The other is the polar cap, wherein for BZ > 0 the diffuse electron and ion precipitation is one or more orders of magnitude weaker than in the oval, although isolated spikes of accelerated electrons occur, and for BZ < 0, only an unstructured flux of low‐energy polar rain electrons with no easily resolvable ion precipitation is observed. For northward IMF the poleward boundary of the regular oval can be identified as a boundary where the normalized ion flux falls below 0.2. At the same boundary, diffuse electrons of Ee < 1 keV disappear when BZ > 0. Using this criterion, the poleward boundary of the polar cap can be defined computationally. It is suggested that for northward IMF, the poleward boundary of the oval may represent the mapping of a separatrix which, according to recent research results, can exist inside a closed magnetosphere at large distances (∼ X = −100 RE), namely, where a divergence exists between plasma flowing sunward and antisunward.
The interaction of the atmosphere with the space shuttle thruster plume: The NH(A‐X) 336‐nm emission
Authors: Viereck Rodney A. , Murad Edmond , Knecht David J. , Pike Charles P. , Bernstein Lawrence S. , Elgin James B. , Broadfoot A. Lyle
Published: February 2004

Observations of the optical emissions from the space shuttle's thrusters have been examined. Particular attention has been paid to the interaction of the thruster plume with the atmosphere. Emissions from CN, CH, C2, HNO, and NO2 have been observed near the nozzle of the thruster in the vacuum core region of the plume, but these emissions are the direct result of the combustion process. Other emissions including OI and NH have been observed in the downstream region of the plume, where the plume effluents interact with the atmosphere. The NH emission is one of the most dominant UV/visible wavelength emissions observed in the plumes. This emission was observed to extend several thousand meters from the shuttle, and detailed analysis shows that the total intensity of the emission depends on the ram angle (angle in the shuttle reference frame between the plume effluents and the ramming atmosphere) and altitude, indicating an interaction process with the atmosphere. Data from two observational experiments are presented. The Air Force Maui Optical Site (AMOS) experiment includes ground‐based spectral and spatial measurements of the shuttle plumes as the thrusters were fired over the AMOS site on top of Haliakala Volcano on the island of Maui in the mid‐Pacific. The GLO experiment was flown in the payload bay of the space shuttle and also includes spectral and spatial measurements of the shuttle plumes. During both of these experiments, the primary reaction control system (PRCS) engines (870 lb (394 kgf) thrust) and Vernier reaction control system (VRCS) engines (25 lb (11 kgf) thrust) were fired at various angles relative to the ram, thus providing a range of collision velocities (4.5–11 km/s) between the thruster plume and the atmosphere. In this report the dependence of the NH emission on ram angle, thruster size, and distance from the shuttle is presented and analyzed using a three‐dimensional Monte Carlo simulation of the plume‐atmosphere interactions called spacecraft/orbiter contamination representation accounting for transiently emitted species (SOCRATES). The chemical reactions deemed most likely involve collisions of the plume products HNC, HNCO, and CH2NH with atmospheric O, and all of these processes are examined. The ram‐angle dependence is used to determine a threshold energy required for the reaction that leads to the NH emission and to conclude that the most likely reaction involves CH2NH collisions with O.
Front structure of the Alfven wave radiated into the magnetosphere due to excitation of the ionospheric E layer
Authors: Surkov V. V.
Published: February 2004

This paper is concerned with the problem of the generation of low‐frequency Alfven and fast magnetosonic (FMS) waves due to excitation of the conductive gyrotropic E layer of the ionosphere. The wave source is a system of currents excited in the ionosphere (e.g., by an acoustic wave due to an earthquake or any other reason). A theoretical model is considered using equations with the constant Hall and Pedersen conductivities for the E region; the upper regions are described by equations for a cold collisionless plasma. The case of the vertical magnetic field is investigated. The main purpose of this work is to research the influence of the dispersion properties of the ionosphere on the front structure of the Alfven and FMS waves radiated into the Earth's magnetosphere. It has been shown that the main features of the signal are brought about by processes such as the oscillatory current diffusion across the E layer. The function describing the wave front is an oscillatory one; the oscillation period increasing in time. The universal character of this dependence allows us to deduce that in this frequency range there is a strong dispersion of signals passing the ionosphere. This unique effect can be explained by the affect of the Hall conductivity on the E layer.
The solar flare radiation responsible for sudden frequency deviation and geomagnetic fluctuation
Authors: Liu J. Y. , Chiu C. S. , Lin C. H.
Published: February 2004

Data recorded by the Geostationary Operational Environmental Satellites, frequency deviations derived from a CW‐HF Doppler sounding system, and geomagnetic field strength variations obtained from ground‐based magnetometers were analyzed to study the ionospheric and geomagnetic solar flare effects. A model of flare radiation was constructed from synthetic flux intensities and satellite X ray observations, and corresponding ionospheric frequency deviation and electron density at various altitudes were calculated. The evolution of solar EUV and X ray radiation and associated maximum values responsible for frequency deviation and geomagnetic field strength for various flux intensities of the excess solar radiation were studied in detail. It was found that during a solar flare, not only the magnitude of solar radiation, but also the rate of its change dramatically affects the maximum ionospheric frequency deviation. However, the results confirmed that only the intensity of solar radiation determines the magnitude of geomagnetic field strength.
Magnetic flux redistribution in the storm time magnetosphere
Authors: Maltsev Y. P. , Arykov A. A. , Belova E. G. , Gvozdevsky B. B. , Safargaleev V. V.
Published: February 2004

The commonly used formula H = DCF + DR, where DCF and DR are the effects of the magnetopause and ring current, respectively, neglects contribution of the cross‐tail current to the Dst variation. The formula allows us to explain satisfactorily the observed relation of the Dst variation to the ring current intensity but faces difficulties in explaining other experimental facts. First, the equatorward shift of the auroral oval cannot be caused by the sole enhancement of the ring current. Second, the observed relation of the Dst growth rate to the southward IMF component [Burton et al., 1975] does not have any quantitative explanation up to now. We suggest using a different formula, H = (2μ0psw)1/2 + DR‐Fout/2S. The formula is obtained from the conditions of magnetic flux conservation and pressure balance. The flux Fout is directed mainly to the nightside of the magnetosphere. Hence the term Fout/2S describes the effect of the crosstail current and a part of the magnetopause currents. During quiet periods, each term in the right‐hand side of our formula is of the order of tens of nanoteslas. During storm time, each term can rise to hundreds of nanoteslas. The flux Fout grows after the interplanetary magnetic field (IMF) becomes southward owing to the flux transport from the dayside to the magnetotail. The growth rate is described by the formula dFout/dt = U − Fout/τF + ηF, where U is the electric potential difference between the dawnside and duskside of the magnetosphere and τF and ηF are constant. The voltage U depends linearly on the IMF southward component. Combining the latter formula with the expression for H yields a relationship between the Dst growth rate and the IMF southward component close to the observed one. Since the auroral oval is mapped predominantly to the plasma sheet of the magnetotail, the growth of Fout during a storm allows us to explain the equatorward shift of the auroral oval. Another prediction from our theory is the erosion of the stable trapping region in which the equatorial cross section S is related to the flux Fout by the equation S1/2[S(2μ0psw)1/2 + Fout] = 3π3/2(ME + MRC), where ME and MRC are the magnetic moments of the Earth and ring current, respectively. Growth of Fout leads to the decrease of S and to the earthward movement of the dayside magnetopause. During storms this effect can be stronger than that of the region 1 Birkeland current, also moving the magnetopause earthward.
Eigenfrequencies of geomagnetic field lines and implications for plasma‐density modeling
Authors: Schulz Michael
Published: February 2004

Toroidal eigenfrequencies ωn/2π (with harmonic numbers n ≥ 1) of dipolar magnetospheric B field lines are well approximated (within a few percent at most for L ≥ 2) by the linear expression
, if the plasma density ρ is proportional to (La/r)m along the magnetic field line of interest (r being the geocentric distance, a being the planetary radius, and subscript 0 signifying evaluation at the magnetic equator). The spacings Δω/2π between consecutive eigenfrequencies are nearly equal and well approximated by the reciprocal of ∮ (ds/cA), where cA is the local Alfvén speed and s is the coordinate that measures arc length along the field line. Poloidal eigenfrequencies ωn/2π with harmonic numbers n ≥ 2 are equally well approximated by the same expression for ωn. This means that observed pulsation periodicities can be identified with their respective harmonic numbers by plotting the corresponding frequencies on a rectangular grid against possible harmonic numbers to see which reasonable identifications produce the best straight line. (Identification could alternatively be achieved, without plotting, through a modified form of linear regression.) Extrapolated to n = 3/4, the linear fit to an observed eigenfrequency spectrum (toroidal with n ≥ 1 and/or poloidal with n ≥ 2) would then yield a good estimate for the equatorial plasma density ρ0. The slope of this same linear fit would lead (when divided by ω0.75/2π) to a good estimate for the corresponding value of m, which is the exponent of La/r in the modeled plasma density distribution.
The fine structure of dispersive, nonradiative field line resonance layers
Authors: Streltsov A. , Lotko W.
Published: February 2004

The fine structure of dispersive Alfvén wave resonance layers extending along magnetic field lines from northern to southern auroral ionospheres is investigated using a magnetically incompressible, linear, two‐fluid model. The model includes effects of finite electron inertia (at low altitude) and finite electron pressure (at high altitude). Plasma parameters are chosen so that refraction by the parallel inhomogeneity causes the dispersive Alfvén wave to become trapped in the resonance layer. The parallel and perpendicular structure of these nonradiative, dispersive resonance layers is computed for the first four odd harmonics. A significant enhancement of the perpendicular and parallel components of the electric field near the ionosphere is found. The instantaneous potential drop along the magnetic field is sufficient to accelerate electrons up to several keV. The thickness of field line resonance layers in the ionosphere is estimated to be less than 5 km. The results suggest that dispersive resonance layers produce subkilometer‐scale, multiple, discrete auroral arcs.
X ray microbursts in the South Atlantic magnetic anomaly
Authors: Pinto O. , Pinto I. R. C. A. , Mendes O.
Published: February 2004

Balloon‐borne X ray measurements with high time resolution were carried out in the South Atlantic magnetic anomaly to investigate the fine structure of the precipitation in this region. Measurements were obtained on two balloon flights launched from Cachoeira Paulista, São Paulo, Brazil, on January 26 and February 1, 1994, and were supported by onboard electric field and geomagnetic field ground data. In one flight, we have found that rapid changes do occur in times of about 0.05 s during periods of a few seconds. They should be considered as the first evidence of the presence of microbursts in the low L value region of the South Atlantic anomaly. These microbursts were not correlated with sferic signatures in the same hemisphere and may be correlated with magnetic fluctuations in the geomagnetic field. Energy‐time features of the microbursts are examined in order to identify possible responsible mechanisms.
Nonlinear reflection of quasi‐monochromatic whistler waves from lower hybrid resonance level
Authors: Trakhtengerts V. Y.
Published: February 2004

Nonlinear reflection of quasimonochromatic whistler waves, which is due to their parametric interaction with an electrostatic resonant cone wave and an ionic (acoustic or cyclotron) wave near the level in the magnetosphere, where the whistler wave frequency coincides with the lower hybrid resonance (LHR) frequency, is considered. The coherent couple of these electrostatic LHR and ionic waves, which arises during the parametric decay process of the initial whistler wave, generates the reflected whistler wave. It has the wave vector directed in the opposite direction to the initial whistler wave and frequency downshifted by two ionic wave frequencies. The quantitative estimations show a high efficiency of this process in magnetosphere conditions.
A new model of interaction between the solar wind and the local interstellar wind
Authors: Khabibrakhmanov Ildar K. , Summers Danny
Published: February 2004

The interaction of subsonic plasma flow with a neutral particle cloud via charge exchange is considered. We derive a general expression for the velocity potential of the plasma flow induced by charge exchange interaction in the axisymmetric case. The particular case of the interaction of uniform plasma flow with a spherically symmetric distribution of neutral particles is considered in detail. It is shown that charge exchange interaction is able to stagnate the plasma flow completely at a finite distance along the stagnation line, and to redirect the plasma flow around the neutral cloud. We argue that this simple situation could represent the main features of the interaction of the interstellar wind with high‐speed solar hydrogen flowing from the inner heliosphere. Thus the heliopause could in part, if not as a whole, actually result from this particular form of charge exchange interaction. The degree of ionization of the local interstellar medium is shown to determine the extent of the role of the charge exchange interaction. For a partially ionized interstellar wind, charge exchange plays the main role in slowing down and stagnating the interstellar wind, while for a highly ionized interstellar wind, its role is secondary to the direct gasdynamic interaction of the two counter streaming plasmas.
In situ observations of bifurcation of equatorial ionospheric plasma depletions
Authors: Aggson T. L. , Laakso H. , Maynard N. C. , Pfaff R. F.
Published: February 2004

Vector electric field measurements from the San Marco D satellite are utilized to investigate the bifurcation of ionospheric plasma depletions (sometimes called “bubbles”) associated with nightside equatorial spread F. These depletions are identified by enhanced upward ExB convection in depleted plasma density channels in the nighttime equatorial ionosphere. The in situ determination of the bifurcation process is based on dc electric field measurements of the bipolar variation in the zonal flow, westward and eastward, as the eastbound satellite crosses isolated signatures of updrafting plasma depletion regions. We also present data in which more complicated regions of zonal velocity variations appear as the possible result of multiple bifurcations of updrafting equatorial plasma bubbles.
A study of Uranus' bow shock motions using Langmuir waves
Authors: Xue S. , Cairns I. H. , Smith C. W. , Gurnett D. A.
Published: February 2004

During the Voyager 2 flyby of Uranus, strong electron plasma oscillations (Langmuir waves) were detected by the plasma wave instrument in the 1.78‐kHz channel on January 23–24, 1986, prior to the inbound bow shock crossing. Langmuir waves are excited by energetic electrons streaming away from the bow shock. The goal of this work is to estimate the location and motion of Uranus' bow shock using Langmuir wave data, together with the spacecraft positions and the measured interplanetary magnetic field. The following three remote sensing analyses were performed: the basic remote sensing method, the lag time method, and the trace‐back method. Because the interplanetary magnetic field was highly variable, the first analysis encountered difficulties in obtaining a realistic estimation of Uranus' bow shock motion. In the lag time method developed here, time lags due to the solar wind's finite convection speed are taken into account when calculating the shock's standoff distance. In the new trace‐back method, limits on the standoff distance are obtained as a function of time by reconstructing electron paths. Most of the results produced by the latter two analyses are consistent with predictions based on the standard theoretical model and the measured solar wind plasma parameters. Differences between our calculations and the theoretical model are discussed.
Equatorial E and F region zonal electric fields in the postsunset period
Authors: Hari S. S. , Viswanathan K. S. , Subbarao K. S. V. , Krishna Murthy B. V.
Published: February 2004

The E and F region eastward electric fields in the postsunset period obtained from VHF backscatter radar and ionosonde observations respectively at the magnetic equatorial station at Trivandrum are compared. It is found that the F region eastward electric field is greater than the E region electric field indicating latitudinal gradients in the E region electric field.
On the thermal evolution of the Earth's core
Authors: Buffett Bruce A. , Huppert Herbert E. , Lister John R. , Woods Andrew W.
Published: February 2004

The Earth's magnetic field is sustained by dynamo action in the fluid outer core. The energy sources available to the geodynamo are well established, but their relative importance remains uncertain. We focus on the issue of thermal versus compositional convection, which is inextricably coupled to the evolution of the core as the Earth cools. To investigate the effect of the various physical processes on this evolution, we develop models based on conservation of energy and the assumption that the core is well mixed by vigorous convection. We depart from previous numerical studies by developing an analytical model. The simple algebraic form of the solution affords insight into both the evolution of the core and the energy budget of the geodynamo. We also present a numerical model to compare with the quantitative predictions of our analytical model and find that the differences between the two are negligible. An important conclusion of this study is that thermal convection can contribute significantly to the geodynamo. In fact, a modest heat flux in excess of that conducted down the adiabatic gradient is sufficient to power the geodynamo, even in the absence of compositional convection and latent‐heat release. The relative contributions of thermal and compositional convection to the dynamo are largely determined by the magnitude of the heat flux from the core and the inner‐core radius. For a plausible current‐day heat flux of Q = 3.0 × 1012 W and the current inner‐core radius, we find that compositional convection is responsible for approximately two thirds of the ohmic dissipation in the core and thermal convection for the remaining one third. The proportion of ohmic dissipation produced by thermal convection increases to 45% with an increase in Q to 6.0 × 1012 W. In the early Earth, when the inner core was smaller and the heat flux probably greater than the present values, thermal convection would have been the dominant energy source for the dynamo. We also calculate the history of inner‐core growth as a function of the heat flux. For example, the inner core would have grown to its present size in 2.8 × 109 years if the average heat flux was Q = 4.0 × 1012 W. The model does not require the heat flux to be constant.
Re‐Rebuttal to the Reply of Varotsos et al.
Authors: Mulargia Francesco , Marzocchi Warner , Gasperini Paolo
Published: February 2004

The Reply by Varotsos et al. [1996a] incorrectly understates the performance of our “predictive rule” and incorrectly overstates VAN's performance.
Numerical simulation of a Zakharov‐Boussinesq system of equations to study Langmuir turbulence in the ionosphere
Authors: Sharma R. P. , Stubbe P. , Verga A. D.
Published: February 2004

This paper presents the numerical study of a one‐dimensional driven and damped Zakharov‐Boussinesq system of equations (ZBSE) to study Langmuir turbulence in the ionosphere. In ZBSE, the high‐frequency wave dynamics is identical to the Zakharov system of equations (ZSE), but the ion acoustic wave equation is modified by taking into account ion wave spatial dispersion and ion nonlinearity. The two models, namely, ZBSE and ZSE, have been compared in order to study the effect of ion wave spatial dispersion and ion nonlinearity on the saturated state of Langmuir and ion acoustic turbulence. The turbulent state in the wave number as well as frequency domain has been studied at different heights for ionospheric F layer parameters corresponding to the European Incoherent Scatter (EISCAT) VHF radar case. We find that in some cases, at low driver strength, these terms affect the wave number spectra. The frequency spectra are affected not only at low, but also at higher driver strengths.
Observations of iron, silicon, and other heavy ions in the geostationary altitude region during late March 1991
Authors: Grande M. , Perry C. H. , Blake J. B. , Chen M. W. , Fennell J. F. , Wilken B.
Published: February 2004

Following the great sudden storm commencement of March 24, 1991, observations were made aboard CRRES of Fe, Mg, Si, and other heavy ions in the energy range of several tens to a few hundred keV/n. The charge states of these heavy ions were observed to change abruptly. For example, the dominant Fe charge state changed at one point from q = 9+ to q = 16+. We suggest that ions from the solar corona, retaining the charge state frozen in near the Sim, were convected inward to the location of CRRES from the plasma sheet within minutes. The ions were energized in the convection process, and the ionic charge state was unaltered by charge exchange. The very large magnetospheric electric fields associated with the highly disturbed geomagnetic conditions were required to enable the ions to reach CRRES. Guiding center simulations were carried out which support this hypothesis. At least two ion populations were observed corresponding to temperatures at two different coronal locations, and the observed changes in heavy‐ion charge state were caused by a sequence of solar wind parcels sweeping over the Earth's magnetosphere. The signature of a change from low to high temperature seems to fit well the suggestions of Tsurutani and Gonzales [1994], of a fast solar wind stream tamped by a slower one, as a generator of great storms.
Current disruption in the Earth's magnetosphere: Observations and models
Authors: Lui A. T. Y.
Published: February 2004

Observations and models of current disruption in the Earth's magnetosphere are briefly reviewed. At the approach of current disruption onset, the cross‐tail current sheet shows a rapid growth in the current density, a large upsurge in the duskward ion bulk speed to nearly the ion thermal speed, an increase in the plasma pressure and its isotropy, a rise in the plasma beta, and a decrease in the current sheet thickness to a length scale comparable to the thermal ion gyroradius. During current disruption, there are (1) large changes in the local magnetic and electric fields, (2) significant magnetic and electric fluctuations over a broad frequency range, (3) magnetic field‐aligned counterstreaming electron beams, (4) ion energization perpendicular to the magnetic field, and (5) reduction in the cross‐tail current by an amount similar to that built up during the growth phase. Observations further indicate that regions of local reversal of the north‐south magnetic field component are not necessarily sites of intense particle energization. Remote sensing of disruption activities shows that at least some current disruptions are not caused by a disturbance propagating earthward from the tail beyond 10 RE downstream. The timescale involved is comparable to or shorter than the ion gyroperiod. Current disruption thus has spatial and temporal scales outside the MHD regime. Several models for current disruption are briefly discussed. Two roles are considered for the cross‐field current instability proposed for current disruption. It can provide anomalous resistivity for magnetic reconnection as advocated by the traditional viewpoint or act singly to instigate global changes of the magnetosphere during the initial substorrn expansion phase. The latter role is elaborated by showing that the instability may modify significantly the local current density and any such process will alter the force equilibrium in the current sheet and give rise to an efficient plasma and energy transport on a global scale. Furthermore, such a process can generate field‐aligned current with intensity comparable to those associated with an auroral breakup arc at substorrn expansion onset. This scenario leads to a new emphasis that in addition to magnetic reconnection, rapid conversion of magnetic energy into particle energy in magnetotail systems may take place without a magnetic X line or separatrix playing the key role in energy conversion.
Phenomenology and physics of magnetospheric substorms
Authors: Rostoker Gordon
Published: February 2004

This paper provides a phenomenological and theoretical framework for understanding magnetospheric substorms based on the boundary layer dynamics model developed by Rostoker and Eastman [1987] updated to be consistent with modern observations. The model is designed to account for both the directly driven and storage‐release aspects of substorm activity. The essence of the model is that enhanced frontside reconnection leads to both growth of the directly driven electrojets and storage of energy in the magnetic field and particle drift in the tail. The growth of particularly intense cross‐tail current near the inner edge of the plasma sheet is terminated by an abrupt collapse marking the onset of the expansive phase of the substorm. This collapse normally takes place in the midnight sector inside ∼ −12 RE. The collapse of this cross‐tail current is consistent with the breakdown of shielding associated with region 2 field‐aligned currents. The expansive phase proceeds through antiearthward propagation in the tail of a wave triggered by the collapse. When the wave reaches the neutral line in the tail, it enhances the reconnection rate leading to stronger velocity shear along the interface between the low latitude boundary layer and central plasma sheet. This velocity shear is responsible for activity at the high latitude edge of the expanded auroral oval. The physics of the processes characterizing the model requires a reexamination of the Harang discontinuity as it would appear that there are actually two regions of electric field reversal in the nightside magnetosphere both of which play important roles in the substorm process.