bluuebunny/crossref_metadata_2025 huggingface dataset:
Global magnetohydrodynamic simulation of a comet crossing the heliospheric current sheet
Authors: Yi Yu , Walker Raymond J. , Ogino Tatsuki , Brandt John C.
Published: February 2004

A disconnection event (DE) of the plasma tail is one of the most spectacular phenomena observed in comets. Yet it has remained an important unsolved problem in planetary astronomy and space physics. The solar wind is thought to play a major role in the creation of comet plasma tail DEs. Comparison of the solar wind conditions and 16 DEs in Halley's comet shows that DEs are associated primarily with crossings of the heliospheric current sheet (HCS) and apparently not with any other properties of the solar wind, such as high‐speed streams [Yi et al., 1994]. In order to present a mechanism that explains the DE in terms of the local conditions at the comet, a three‐dimensional resistive compressible magnetohydrodynamic (MHD) simulation was carried out to test the effect of the HCS crossing on the comet plasma tail dynamics. We have focused on the fact that the thickness of the HCS is about 10,000 km [Winterhalter et al., 1994], which was neglected in the previous simulations. Such a narrow discontinuity cannot be described by a global simulation using large grid cells, due to the intrinsic numerical diffusion in the system. Therefore, we have used an approximation to calculate the induced current across the HCS when it is compressed inside the cometosheath. This produces results different from previous simulations. However, they satisfy all the observational constraints. The results show that frontside magnetic reconnection between the reversed interplanetary magnetic fields [Niedner and Brandt, 1978] can reproduce the evolution morphology of a DE, including ray formation, when a comet crosses the HCS. This supports the association of DEs with HCS crossings.
Spectral characteristics of the Farley‐Buneman instability: Simulations versus observations
Authors: Oppenheim Meers , Otani Niels
Published: February 2004

The Farley‐Buneman instability is a collisional two‐stream instability observed in the E region ionosphere at altitudes in the range of 95–110 km. While linear theory predicts the dominant wavelengths, it cannot fully describe the behavior of this nonlinearly saturated instability as observed by radar and rocket measurements. We simulate the behavior of this instability in the plane perpendicular to the Earth's magnetic field, using a two‐dimensional hybrid code which models electron dynamics as a fluid and ion dynamics with a particle‐in‐cell approach. The results show the growth, saturation, and nonlinear behavior of the instability for a much longer period of time than was possible with the pure particle codes used in previous studies. This paper describes the spectra from these simulations and compares them to the observed spectra. Both the simulations and observations show that (1) type I spectra result from saturated two‐stream waves for a broad range of elevation angles, (2) the phase velocity of these waves is below that predicted by linear theory, (3) mode coupling leads to type II‐like spectra without the presence of a plasma density gradient as often thought necessary, (4) longer wavelengths due to mode coupling develop, and (5) spectral power decreases at a rate of 0.3 dB/degree of elevation angle.
Energetic neutral atoms propagating toward the Earth: Analysis of the reduction rate due to ionospheric and atmospheric interactions
Authors: De Michelis P. , Orsini S.
Published: February 2004

Recently, several theoretical studies have shown that it is possible to obtain global images of the Earth's inner magnetosphere through detection of Energetic Neutral Atoms (ENA). These ENA, when precipitating toward the Earth, interact with the ionospheric and atmospheric particles through both elastic and inelastic collisions. These various interactions, which cause energy loss and deflection of the original particles, put critical constraints on the possibility of obtaining reliable magnetospheric images by ENA detection from low‐altitude orbits. It follows that the estimate of a lower‐altitude limit for the ENA imaging technique is crucial for this kind of studies. In the present study the ENA interaction processes are analyzed, and the rate of ENA destruction is estimated as a function of altitude for different energies and solar activities, along a radial line of sight on the magnetic equator, at MLT = 0000.
Trapped anomalous cosmic rays near the geomagnetic cutoff
Authors: Looper M. D. , Blake J. B. , Klecker B. , Hovestadt D.
Published: February 2004

Observations made with the mass spectrometer telescope (MAST) instrument aboard the SAMPEX spacecraft have previously been used to map the belt of geomagnetically trapped anomalous cosmic rays at L ≈ 2. Here we use the heavy ion large telescope (HILT) instrument to extend these observations to lower energies, closer to the peak of the interplanetary spectrum at 1 AU, for oxygen and neon. At lower energies than observed by MAST we see a distinct peak in the oxygen spectrum at the energy corresponding to the geomagnetic cutoff for singly charged incident ions arriving from the west, at each L where that is in the HILT energy range. This is due to the suppression of the particle source at energies below the cutoff, and the subsequent filling‐in of lower energies via slowing down of ions that had energies above the cutoff when originally trapped. In addition, our observations of oxygen confirm the MAST discovery that the pitch angle distribution at SAMPEX altitudes is nearly isotropic outside the loss cone, and we see similar variations with time, trends with L, and general level of trapped flux at the energies where the two instruments' responses overlap. The normalization of the trapped oxygen and neon fluxes as functions of L, when compared with previously published models, suggests more efficient trapping toward low L and/or additional losses at high L.
Magnetic storm modeling in the Earth's electron belt by the Salammbô code
Authors: Bourdarie Sebastien , Boscher Daniel , Beutier Thierry , Sauvaud Jean‐Andre , Blanc Michel
Published: February 2004

The Salammbô code, which solves the three‐dimensional phase‐space diffusion equation for the electron radiation belt, was used to explain the dynamic conditions present during a geomagnetic storm. With a simple injection model, we have characterized the dynamic behavior for relativistic electrons in the outer belt. The particles in the range 100 keV – 500 keV are diffused throughout the belt, with a shape essentially dependent on the radial diffusion coefficients. Particles with higher energies are “created” by acceleration of slower particles near the plasmapause location. The calculated shape of the fluxes in an L versus time grid for the 1 MeV electrons looks globally like those measured aboard the CRRES satellite.
Ionosphere‐protonosphere thermal plasma fluxes estimated from Magion 2 and ionosonde data in August–September 1990
Authors: Skoblin M. G. , Förster M.
Published: February 2004

The daytime thermal plasma fluxes at the topside of the midlatitude ionosphere were assessed during a quiet period and a recovery phase of a magnetic storm in summer 1990. Using in situ measurements of electron concentration and temperature and a crude estimate of mean ion mass performed by the Magion 2 subsatellite in an altitudinal range from 1200 to 2500 km, in addition to data supplied by ground‐based ionosondes, electron concentration profiles were drawn by means of a theoretical one‐dimensional steady state ionospheric model. Latitudinal variations of daytime ion fluxes at the 1000‐km‐height level during quiet periods revealed a peak at L = 2.5 ÷ 3.0 with maximum values up to 7 × 108 cm−2 s−1. The difference in flux versus latitude behavior between quiet and poststorm periods was clearly observable for L > 3. The uncertainty in the resultant ion fluxes depends upon the accuracy of the electron temperature and ƒoF2 measurements, as well as on the neutral wind velocity approximation, and was estimated to be about 50 percent. A comparison of the ion flows obtained with either direct measurements or indirect estimations made earlier show reasonable agreement.
Numerical simulations of gravity wave modulation of midlatitude sporadic E layers
Authors: Huang Chao‐Song , Kelley Michael C.
Published: February 2004

We have used a computer simulation to study gravity wave modulation of midlatitude sporadic E (Es) layers. It is shown that a horizontally stratified Es layer may be deformed by gravity waves and become a large‐scale wavelike structure. Spatial resonance is not required for significant modulations to form. For a southward propagating gravity wave the south side sections of the wavelike deformed Es layer may overturn and appear as field‐aligned features stretching about 10 km in altitude. The deformed Es layer drifts at a velocity larger than neutral wind velocity but smaller than the phase velocity of the gravity wave. The scale length of the wavelike Es layers is determined by the horizontal wavelength of the gravity wave. If there are two or more Es layers modulated simultaneously by a gravity wave, the height dependence of the amplitude of the wave must be taken into account. The Es layer at lower altitude is weakly disturbed if the gravity wave has only a small amplitude there. In contrast, the Es layer at higher altitude may be deeply modulated and appear as field aligned, since the amplitude of the gravity wave has increased greatly in comparison with the amplitude at lower altitude. The numerical results may be used to explain the midlatitude E region field‐aligned irregularities observed by the Middle and Upper atmospheric (MU) radar.
Nonlinear evolution of equatorial spread F: 4. Gravity waves, velocity shear, and day‐to‐day variability
Authors: Huang Chao‐Song , Kelley Michael C.
Published: February 2004

Production and evolution of plasma bubbles seeded by gravity waves in the equatorial F region are studied under different conditions using a computer simulation. The problem of day‐to‐day variability in the occurrence of equatorial spread F (ESF) is discussed. It is shown that gravity waves over a wide range of amplitude and wavelength are a very effective seed mechanism for production of ESF. However, there is the day‐to‐day variability in the occurrence of ESF even if gravity waves are omnipresent. The maximum height of the F peak arid the bottomside background electron density gradient can significantly influence production and rise of plasma bubbles. It is found also that the timing of the seed gravity wave is critical to ESF generation. If à gravity wave exists in the F region during the upward drifting of the F layer, the gravity wave can initiate the Rayleigh‐Taylor instability and result in topside bubbles. In contrast, gravity‐wave‐induced perturbations in a descending ionosphere evolve into large‐scale, wavelike structures but not bubbles. Gravity waves can propagate in any direction, but only the gravity waves propagating with an azimumal angle range of about 10° relative to the zonal direction are expected to be capable of generating ESF bubbles. These factors are all variable from day to day. Their variation must cause variation of resulting ESF. In addition, we show first that in agreement with linear analysis, a velocity shear in the plasma drift can generate plasma structures in the bottomside F layer. For reasonable ionospheric parameters, the shear‐determined wavelength of the Rayleigh‐Taylor instability is a few tens of kilometers. The plasma structures organized by velocity shear appear as bottomside spread F but cannot evolve into topside bubbles. Velocity shear provides a possible explanation of generation of bottomside spread F and secondary plumelike structures on the west wall of large‐scale plasma upwelling.
Behavior of heavy ions in a collisionless parallel shock generated by the solar wind and planetary plasma interactions
Authors: Shimazu Hironori , Tanaka Motohiko , Machida Shinobu
Published: February 2004

The role of heavy ions in the collisionless parallel shock of Venus and Mars is investigated where the solar wind interacts directly with the planetary ionosphere. We consider O+ ions in the planetary plasma and He++ ions (α particles) in the solar wind as the heavy‐ion species. By solving a linear dispersion relation of the ion cyclotron beam instability, which provides a dissipation in the parallel shock, we find that the dominant frequency and wavenumber are not much modified when the heavy ions are included in a realistic amount. Particle simulations are performed in order to examine the effects and dynamical behavior of the heavy ions in the parallel shock. The re‐formation of the parallel shock is observed, which confirms that the electromagnetic waves that make up the parallel shock are not affected by the heavy ions. Notable pickup of O+ ions by electromagnetic waves is not detected. Instead, strong heating of the solar wind He++ ions is observed in the parallel shock region.
Modeling of Jovian hectometric radiation source locations: Ulysses observations
Authors: Menietti J. D. , Reiner M. J.
Published: February 2004

The Unified Radio and Plasma Wave (URAP) experiment on Ulysses has provided unique high latitude measurements of Jovian hectometric radiation (HOM) during its encounter with Jupiter in February 1992. URAP was the first radio instrument in the Jovian environment with radio direction‐finding capability, which was previously used to determine the HOM source locations in the Jovian magnetosphere. These initial source location determinations were based on several assumptions, including the neglect of refractive effects, which may be tested. We have, for the first time, combined the measured incident ray‐direction at the spacecraft with a model magnetosphere to directly trace the rays back to the HOM source. We concentrate on the observations of HOM from high northern latitudes when Ulysses was at distances <15 RJ. The three‐dimensional ray‐tracing calculations presented here indicate that the HOM sources probably lie on L shells in the range 3 ≲ L < 7 (tilted dipole magnetic field model) consistent with previous determinations that ignored the effects of refraction. The ray‐tracing results, however, indicate that wave refraction due to the Io torus and the magnetic field can significantly influence the precise source location. We show that constraints on the locations imposed by the gyroemission mechanism suggest that the Io torus density may have experienced temporal and/or spatial fluctuations during the Ulysses observations of HOM. Finally, in the cold plasma approximation we demonstrate that even if the emission were nearly linearly polarized near the source region, almost circular polarization will be observed at Ulysses, in agreement with observations.
Auroral electrojet plasma irregularity theory and experiment: A critical review of present understanding and future directions
Authors: Sahr John D. , Fejer Bela G.
Published: February 2004

We review the experimental observations of meter scale plasma irregularities in the auroral E region and the status of their theoretical understanding. Most of the experimental data is derived from VHF radar scatter experiments, but sounding rockets also provide crucial information not obtainable from radars. Linear theories correctly predict the altitude of occurence, strong magnetic aspect sensitivity, marginal instability, and typical phase velocities. Subsequent nonlinear theories have been developed to account for other observed features but with less satisfying application. Further understanding of auroral electrojet irregularities is impeded by precision limitations of existing instruments, by radar data which may seem incompatible, by the usually poor knowledge of the ambient conditions during these experiments, and by some confusion in the nomenclature (e.g. “type 2”) used to describe the irregularities. We hope to clarify some of these experimental and theoretical issues. We will discuss observational “facts” that need to be refined and point out weaknesses of existing theories or their common interpretations. Finally, we will outline some avenues for future experimental and theoretical pursuit.
The 13.5‐day periodicity in the Sun, solar wind, and geomagnetic activity: The last three solar cycles
Authors: Mursula Kalevi , Zieger Bertalan
Published: February 2004

We make a detailed analysis of the 13.5‐day periodicity of the solar chromosphere, the near‐Earth solar wind, interplanetary magnetic field and geomagnetic activity during the last three solar cycles. The 13.5‐day periodicity is a real quasi‐periodicity whose amplitude varies sizably with time, attaining occasionally values larger than, for example, the amplitude of the 27‐day periodicity. In case of heliospheric and geomagnetic variables, intervals of large 13.5‐day periodicity are due to the occurrence at 1 AU of two high‐speed streams per solar rotation. According to the tilted solar dipole model, such two‐stream structures appear if the heliospheric current sheet is sufficiently narrow and tilted. We show that, during the main two‐stream structures, the interplanetary magnetic field (IMF) indeed had a persistent two‐sector structure, and the heliosheet was sizably tilted. Multiple IMF sector structure is thus excluded as the main cause for 13.5‐day periodicity in solar wind and geomagnetic activity. We determine the exact time and phase (solar longitude) of all intervals of significant 13.5‐day periodicity during the last three solar cycles. We find that even the longest intervals of two‐stream structure (up to 2 years) consist of separate activations. Each of the main activations of the 13.5‐day (as well as 27‐day) periodicity has a nearly equal length of a few (about 4) solar rotations only. This gives new, interesting information about the solar dynamics related to the development of the dipole tilt. Using the phase of the main 13.5‐day activations, we could determine the longitudinal position of the solar dipole tilt for all major activations. We note that this position can abruptly change by even 90 deg between two successive 13.5‐day activations. For each of the three solar cycles studied, the largest two‐stream structures were found in the late declining phase of the cycle. On the other hand, the main activations of the 13.5‐day periodicity of solar variables, which are due to two active solar longitudes approximately 180° apart, tend to occur around solar maxima.
A model equation for monochromatic standing Alfven waves in the axially symmetric magnetosphere
Authors: Leonovich A. S. , Mazur V. A.
Published: February 2004

A model equation is proposed to describe the radial (in the direction across magnetic shells) structure of standing Alfven waves with fixed frequency and azimuthal wave number in the axially ‐ symmetric magnetosphere. A semiphenomenological derivation of this equation is given. In those limiting cases when the solution of starting equations can be obtained rigorously, it coincides qualitatively with the solution of the model equation. Its chief merit is a relative simplicity and the possibility of investigating azimuthal harmonics with an arbitrary azimuthal wave number m (except for a few fundamental harmonics with m ∼ 1). An integral representation is obtained for the solution of the model equation, important limiting cases are studied analytically, and it is investigated numerically. The dependence of the radial structure on the azimuthal wave number and the magnitude of dissipation of standing waves in the ionosphere are analyzed. It is shown that, with increasing m, the radial structure changes from field line resonance to a traveling wave‐like structure, while with increasing dissipation, the mode changes from toroidal to poloidal one.
Cometary ion pressure anisotropies at comets Halley and Grigg‐Skjellerup
Authors: Coates A. J. , Johnstone A. D. , Neubauer F. M.
Published: February 2004

Cometary water group ions are the dominant population controlling the interaction of comets with the solar wind. The distribution function of this population has important effects on the overall structure and boundaries in the comet‐solar wind interaction and on the plasma wave activity produced. In this paper we present new results on the behavior of the water group ions measured by Giotto at comets Halley and Grigg‐Skjellerup. The pressures perpendicular and parallel to the magnetic field are presented and the implications explored. We show that during the Halley inbound pass, the firehose instability may excite the observed waves with low wavenumber. Also, we find that the energy density of energetic cometary ions is greater than the observed difference between the energy released from the ring‐like distribution and the energy in the pickup‐excited waves. This indicates that another energy source must be contributing to the acceleration, probably related to the deceleration of the solar wind in the comet frame.
Total electron content variations due to nonclassical traveling ionospheric disturbances: Theory and Global Positioning System observations
Authors: Beach T. L. , Kelley M. C. , Kintner P. M. , Miller C. A.
Published: February 2004

Measurements of total electron content (TEC) variations are easy to perform. As several authors have pointed out, however, TEC variations must be interpreted carefully since TEC is an integrated quantity. Prior studies of TEC variations have considered these variations to stem from “classical” gravity‐wave interaction with the ionosphere at midlatitudes; that is, the component of gravity‐wave neutral wind perturbations along the local magnetic field moves the charged particles up and down the field lines. Recent evidence suggests that at night, in the absence of photoproduction and when E layer conductivity becomes small, electrodynamic effects come into play and gravity‐wave perturbations can yield vertical movement of the F layer, rather than movement along the field lines. Here we consider the TEC variations resulting from horizontally propagating disturbances in ionospheric height. First, we develop an analytic model, assuming sinusoidal height variations and a geostationary satellite, to estimate the magnitude of TEC variations produced. In this case, the dependence of TEC variations on the orientation of the line of sight, relative to the direction of disturbance propagation, takes on a different form from the classical case. Total electron content variations may also be observed using nonstationary satellites, such as Global Positioning System (GPS) satellites. We present GPS measurements from Puerto Rico for the night of August 22–23, 1995, which show TEC variations of ∼ ±5 × 1015 m−2 in the presence of a southwesterly propagating ionospheric height disturbance (as independently corroborated by 630‐nm airglow images at Arecibo and Ramey digisonde measurements). Although GPS satellite motion does not permit direct application of the geostationary theory, geostationary theory still allows us to make a crude estimate of the magnitude of TEC variations that result.
Substorm current in the equatorial plane of the magnetosphere deduced from ground‐based magnetometer chain records
Authors: Sun W. , Kamide Y. , Akasofu S.‐I.
Published: February 2004

The purpose of this paper is to demonstrate the feasibility of inferring the distribution of substorm electric currents in the equatorial plane on the basis of ionospheric current vectors and field‐aligned currents during substorms; the ionospheric current vectors are obtained from the ground‐based magnetometer network by using the KRM inversion algorithm [Kamide et al,. 1996] and the longitudinal and latitudinal current loops proposed by Bostrom [1964]. A simple model of the magnetosphere is used to trace field lines for our demonstration purposes. It is found that the calculated radial currents in the equatorial plane are in good agreement with the statistical results obtained by Iijima et al. [1990] on the basis of the satellite AMPTE/CCE data. We show also that the statistical azimuthal currents are mainly composed of the ring current and the cross‐tail current. The ionospheric contribution to the azimuthal component is estimated to be about 20% of the total current. Therefore both results deduced from the ground‐based magnetometer network and from AMPTE/CCE satellite data are useful in studding the distribution of substorm currents in the equatorial plane, in spite of the fact that they are based on entirely different methods and that a simple magnetosphere model is used to trace field lines.
Excitation of upper hybrid waves from ordinary‐mode electromagnetic waves via density gradient in the ionosphere
Authors: Antani S. N. , Kaup D. J. , Rao N. N.
Published: February 2004

Excitation of upper hybrid (UH) waves from ordinary‐mode (O‐mode) electromagnetic waves via large‐scale electron density gradient in the upper hybrid resonance (UHR) region of the ionosphere is investigated. The model used is that of a driven harmonic oscillator in which the electron drift velocity induced by the O‐mode beats with the local electron density to produce a source current that drives the excited UH waves. It is found that strong excitation of UH waves occurs in the UHR region, implying enhanced damping of the incident O‐mode.
Trapped energetic ions in Jupiter's inner magnetosphere
Authors: Anglin J. D. , Burrows J. R. , Mu J. L. , Wilson M. D.
Published: February 2004

Ion measurements made with the high‐flux telescope at ∼1 MeV/nucleon during the passage of the Ulysses spacecraft through Jupiter's inner magnetosphere are presented. Stable pancake‐shaped pitch angle distributions were observed for protons inside ∼17 RJ. They are fitted by a time‐independent model in which an assumed directional flux at the magnetic equator is transformed to the spacecraft's position using Liouville's equation and the O6CS magnetic field model. Nongyrotropic features are fitted with a flow in the corotation direction. The derived equatorial pitch angle distribution is approximately independent of radial distance and has broad shoulders. However, it is not as isotropic as that predicted for strong diffusion. From a flux‐composition fitting analysis, we find (1) the equatorial proton omnidirectional flux decreases approximately exponentially with magnetic equatorial distance RM and is nearly longitudinally symmetric, (2) the equal energy per nucleon abundance ratios for O/H, S/H, and S/O decrease with RM, (3) the ion spectral index softens linearly with RM, (4) the phase space densities of protons and oxygen at a constant magnetic moment increase rapidly with RM, and (5) the sulfur phase space density turns up inside 10 RJ. The data are best fitted with similar spectral indices for oxygen and sulfur. Thus there is likely a source of energetic sulfur in the Io torus. Particle losses throughout the region are required to fit the radial variation of the phase space densities with a physically reasonable radial diffusion coefficient. Further work is needed to determine whether these losses are consistent with weak equatorial pitch angle diffusion.
Theory of magnetospheric standing hydromagnetic waves with large azimuthal wave number: 4. Standing waves in the ring current region
Authors: Walker A. D. M. , Pekrides H.
Published: February 2004

There is a rich variety of observations of long‐period pulsations in the ring current region. One class of such pulsations is characterized by large azimuthal wave numbers and close confinement to a field line. In this paper, computations are made of the eigenfrequencies and fields of standing oscillations which can exist in a realistic self‐consistent magnetohydrodynamic model of such a region. The model, which depends on a few simple parameters, allows for the shape of the field line to be distorted from a dipole. In this paper it is limited to isotropic pressure. The magnetic field and transverse gradient of pressure can be defined. It is assumed that growth and loss processes are small and in balance because the plasma has reached a quasi‐linear equilibrium state. The computations show that a rich variety of compressional modes can exist. They are much more complex than simple coupled Alfvén and magnetosonic modes which are often invoked in such circumstances. At some points along the field line, such modes may be closely coupled while at other points they are essentially uncoupled with different wavelengths. This leads to modes with complex nonharmonic structures. It is noted that the transverse pressure gradient allows an MHD instability, related to the flute instability, to exist in appropriate circumstances. Such calculations should allow good comparison of theory and observation.
The location of the high‐latitude polar cusp and the shape of the surrounding magnetopause
Authors: Zhou X.‐W. , Russell C. T.
Published: February 2004

Hawkeye magnetic field measurements have been used to determine the location of the magnetopause in the vicinity of the polar cusp. We can clearly distinguish field lines above the cusp from those below the cusp and thus can well identify the statistical location of the cusp. As expected, we find that the location of the polar cusp depends strongly on the dipole tilt angle. However, there appears to be no indentation of the magnetopause surrounding the cusp in contrast to models of the cusp that are frequently drawn. At the magnetopause on the noon meridian the cusp is at a latitude of nearly 82° for northward interplanetary magnetic field (IMF) and 80° for southward IMF. This is more swept back than in models such as that of N. Tsyganenko but less swept back than in the computer simulation of C. C. Wu. In our study, both geocentric solar magnetospheric (GSM) and solar magnetic (SM) coordinates are used. GSM coordinates order the high‐altitude polar cusp position slightly better than SM coordinates.
Using steady state vibrational temperatures to model effects of N2* on calculations of electron densities
Authors: Pavlov A. V. , Buonsanto M. J.
Published: February 2004

Measurements of NmF2 and hmF2 at Millstone Hill are compared with results from the Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation (IZMIRAN) time‐dependent mathematical model of the Earth's ionosphere and plasmasphere during the periods March 16–23 and April 6–12, 1990. The effects of vibrationally excited N2 are studied by using two approaches: (1) obtaining a full solution of the vibrational quanta continuity equation and (2) calculating an analytical solution of the steady state vibrational quanta continuity equation using a simple FORTRAN subroutine. It is shown that during both undisturbed periods and magnetic storms, the full solution and the analytical approach give very similar results for NmF2 and hmF2 derived from the IZMIRAN model, so the analytical approach can be recommended for use in ionospheric models for calculation of electron densities during both quiet and disturbed conditions.
Theory and simulation of Kelvin‐Helmholtz instability in the geomagnetic tail
Authors: Yoon Peter H. , Drake James F. , Lui Anthony T. Y.
Published: February 2004

In recent, nonlocal stability analyses of the cross‐field current instability that may operate in the geomagnetic tail, it was found that the instability is suppressed when the cross‐field ion flow is uniform but that the instability is restored only after a substantial velocity shear is introduced. However, the velocity shear also excites the Kelvin‐Helmholtz instability. Therefore, it is important to understand how the cross‐field current instability and the shear‐driven Kelvin‐Helmholtz instability mutually interact and compete with the available free energy. As a first step, with the use of Hall‐MHD theory and two‐dimensional simulation, this paper investigates the development of the Kelvin‐Helmholtz instability for a thin current sheet with sheared velocity flow. It is found that a finite north‐south field at the center of the neutral sheet inhibits the growth of the Kelvin‐Helmholtz instability to some extent. However, the timescale of its growth is still sufficiently short, so that this instability may play a part in broadening the thin current sheet at the time of substorm expansion onset.
Characteristics of electrons at the trapping boundary of the radiation belt
Authors: Imhof W. L. , Chenette D. L. , Gaines E. E. , Winningham J. D.
Published: February 2004

New measurements are presented of the L‐dependent energy threshold for electron flux isotropy observed near midnight from low‐altitude satellites. These data provide the basis for an important remote sensing of the geomagnetic field configurations near the outer edge of the radiation belt. The threshold energy for isotropy observed at low altitudes is interpreted as providing a measure of the effective scale length of inhomogeneity of the geomagnetic field near the equator at high altitude. The data presented were obtained with electron spectrometers on the UARS satellite providing flux and energy spectrum measurements for energies between 30 keV and 5 MeV simultaneously at a variety of pitch angles. On certain orbits the UARS satellite achieved fine L shell resolution and nearly simultaneous measurements at different local times. With this previously unavailable L shell resolution the threshold energy for isotropy versus L shell curves near midnight have shown slopes steeper than 20 MeV per unit L on 20% of the cases, using the International Geomagnetic Reference Field (IGRF) 85 magnetic field model. On 66% of the satellite passes the slopes at the two boundary crossings within 01 to 06 hours in magnetic local time and about 10 min in elapsed time were within a factor of 3 of each other. The L shell at which isotropy occurs above a given threshold energy is found to be generally lower and the slope steeper at times of high geomagnetic disturbance. For the analysis presented here, if isotropy occurs when the radius of field line curvature is less than 10 times the gyroradius of the particles, the scale lengths of inhomogeneities were always found to be less than 2900 km, and in 99% of the cases the scale lengths were greater than 73 km.
New results on the site of initiation of coronal mass ejections and an interpretation of observation of their interaction with streamers
Authors: Eselevich V. G. , Tong Y.
Published: February 2004

The place of origin of coronal mass ejections (CMEs) are streamer belts with a neutral line (NL) and streamer belts without a NL. In some months during 1985–1987, up to 80–90% of the total number of CMEs were produced near streamer belts with a NL. As one approaches maximum solar activity, along with an increase in the number of CMEs, the proportion of CMEs places of origin lie near streamers without a NL is as high as 80%. Because of the inaccuracy in determining the longitude, some CMEs can be referred to both categories. Then the CMEs whose places of origin can be associated solely with streamers without NL will make up 20–25%. Transformation processes of streamer belts with NL and without NL and also processes of disappearance and formation of streamer belts without NL are likely to be an important factor contributing to an enhancement of CME generation. By considering six selected events, it is shown that whether the streamer decays or does not decay with the CME passsage depends on the position of the streamer relative to the CME span on the limb.
Dynamical polar cap: A unifying approach
Authors: Newell Patrick T. , Xu Dingan , Meng Ching‐I. , Kivelson Margaret G.
Published: February 2004

Various polar cap configurations have been proposed as typical for northward interplanetary magnetic field conditions (IMF BZ > 0), including that (1) a central region empty of plasma sheet precipitation and arcs remains even after several days of profound magnetic quiet; (2) the polar cap is routinely closed for northward IMF; (3) the polar cap is open, with arcs and polar rain interspersed; and (4) the polar cap is closed, but that arcs separated by no precipitation are spread throughout the polar cap. We have studied eight cases of BZ > 0 intervals using DMSP precipitating particle data and high time resolution (15.36 s) IMF data under a variety of conditions. It turns out that all of the first three proposed configurations can occur, but each under a specific set of circumstances. When BZ > 0 and BZ ∼ |By| continuously, the polar cap maintains a central finite open region void of precipitation. When BZ > |By| for about 4 hours, the polar cap completely closes, with plasma sheet precipitation across' the entire polar cap, and no significant gaps in precipitation. Because the rate at which dayside merging can open flux (∼105 Wb/s) greatly exceeds the quiet time rate of flux closure (which we estimate to be ∼7 × 103 Wb/s), the polar cap proves to be highly dynamic, with southward turnings of about 6–7 min sufficient to balance extended intervals without merging. For this reason, it is comparatively rare that the polar cap completely closes, because even entire days with profound magnetic quiet generally include brief southward fluctuations in BZ. Finally, the configuration of polar cap arcs separated from the oval by several degrees of void or polar rain is created only for BZ < 0 after a prolonged or intense interval of BZ > 0.
Probing properties of the magnetospheric hot plasma distribution by whistler mode wave injection at multiple frequencies: Evidence of spatial as well as temporal wave growth
Authors: Carpenter D. L. , Sonwalkar Vikas S. , Helliwell R. A. , Walt M. , Inan U. S. , Ikeda M. , Caudle D. L.
Published: February 2004

This is the second of two papers on the use of whistler mode wave injection to investigate properties of the magnetospheric hot plasma. Paper 1, [Sonwalkar et al., this issue] emphasized the use of signals at a single frequency to identify longitudinal structures ranging from 100 to 25,000 km in extent in ∼1–10 keV electrons drifting azimuthally through whistler ducts. This short paper discusses and illustrates the use of wave injection at multiple discrete frequencies to study temporal changes in magnetospheric hot electrons with parallel (gyroresonant) velocities in various nonoverlapping ranges. As in paper 1, the data studied were acquired during a special 9‐hour period of 1.9 – 2.9 kHz VLF transmissions from Siple Station, Antarctica, to Lake Mistissini, Canada, on January 23–24, 1988. The amplitudes of the leading edges of constant frequency pulses at 1900, 2150, and 2400 Hz were found to vary independently with time. This is interpreted as evidence of a spatial amplification process that accompanied the well known and more readily identifiable phenomena of exponential temporal growth to a saturation level. Evidence of wave‐hot plasma interactions showed a dependence on df/dt of the input signal frequency versus time format; in general, the slow frequency ramps showed the highest amplitudes and the fast ramps and parabolas the lowest, in agreement with past work.
High‐resolution television observations of black aurora
Authors: Trondsen T. S. , Cogger L. L.
Published: February 2004

In view of a recent revival of interest in the black aurora and related phenomena, and a striking lack of information on the phenomenon in the literature, we present high spatial and temporal resolution optical observations of the black aurora made by the University of Calgary portable auroral imager. A variety of black auroral phenomena, such as black vortices, arcs, and eastward drifting black auroral patches and arc segments, were observed in the evening and midnight sector diffuse auroral oval during a field trip to Rabbit Lake, Saskatchewan, from February 25 to March 7, 1995. Observed spatial and temporal characteristics are reviewed, with statistical data presented in the form of histograms.
A study of the effect and consequence of solar cycle variation on low‐energy galactic cosmic rays using Pioneer 10 H Lyman α data as a solar cycle proxy
Authors: Scherer Klaus , Gangopadhyay P. , Judge D. L. , Gruntman Mike
Published: February 2004

The correlation of the 1981–1990 (∼ 23 to ∼ 50 AU) data of three instruments, the ultraviolet photometer (UV), the charged particle instrument (CPI), and the trapped radiation detector (TRD), on board Pioneer 10 is investigated. The reduction of the data necessary for applying a fast Fourier transform (FFT) algorithm is described. A cross correlation function between the hydrogen Lyman a backscattered radiation, used as a solar cycle proxy, and the cosmic ray fluxes is obtained from the FFT algorithm. The time lag in the anticorrelation function between the galactic cosmic rays and the Lyman α line is used to determine the distance to the solar wind termination shock (75 AU), under the assumption that the cosmic ray modulation boundary for low‐energy particles and the termination shock are coincident. The ratio of the upwind termination shock distance to the downwind shock distance is determined to be 2/3 using the Voyager 1 position in 1996.
Tearing instability, Kelvin‐Helmholtz instability, and magnetic reconnection
Authors: Chen Q. , Otto A. , Lee L. C.
Published: February 2004

Two closely related macroscopic instabilities at the magnetospheric boundary layer are the tearing instability and the Kelvin‐Helmholtz (KH) instability. It has been suggested that a coupling of these modes may lead to larger reconnection rates and thus may explain magnetic flux transfer events at the dayside magnetopause. In the framework of two‐dimensional compressible MHD simulations we study the linear and nonlinear evolution of these instabilities. When the velocity of shear flow is below the Alfvén speed, it is verified that the tearing mode is stabilized by the flow shear and the reconnection rate is reduced. For super‐Alfvénic shear velocities the tearing mode ceases its growth, and the KH instability develops consistently with analytical results. A reasonably low resistivity has no significant influence on the growth of the KH mode. Only for nonlinear amplitudes of the KH mode is reconnection driven by the KH vortices. Thus our results do not confirm a linear interaction between the KH and the tearing mode in terms of vortex‐induced reconnection in two dimensions. Particularly, a reasonably small resistivity does not lower the threshold velocity for the KH instability to a level at which it could be excited for typical magnetopause conditions. Therefore the evolution of the KH mode (and vortex‐induced reconnection) with k vectors aligned with a southward interplanetary magnetic field component appears rather unrealistic for the subsolar magnetopause where the magnetosheath flow is slow.
Exo‐plasmaspheric refilling due to ponderomotive forces induced by geomagnetic pulsations
Authors: Feygin F. Z. , Pokhotelov O. A. , Pokhotelov D. O. , Bräysy T. , Kangas J. , Mursula K.
Published: February 2004

The effect of exo‐plasmaspheric refilling due to ponderomotive forces induced by geomagnetic pulsations is considered. It is shown that two maxima of high‐density cold plasma can appear on field lines near the day side magnetospheric boundary, located symmetrically with respect to the equator. When moving away from the noon meridional plane the plasma density distribution along the field line undergoes a smooth transition from two off‐equatorial maxima to one maximum at the equator. We calculate the plasma condensation due to the ponderomotive forces in this region. On the other hand, the satellite data have shown that exoplasmaspheric Pc1 pulsations have a maximum in the noon‐dusk sector. These facts bring us to a conclusion that the formation of high‐density cold plasma outside plasmasphere in the magnetospheric trough in the noon‐dusk sector may be accelerated by ponderomotive effects of Pc1 pulsations.
How wide in magnetic local time is the cusp? An event study
Authors: Maynard N. C. , Weber E. J. , Weimer D. R. , Moen J. , Onsager T. , Heelis R. A. , Egeland A.
Published: February 2004

A unique pass of the DMSP F11 satellite, longitudinally cutting through the cusp and mantle, combined with simultaneous optical measurements of the dayside cusp from Svalbard has been used to determine the width in local time of the cusp. We have shown from this event study that the cusp was at least 3.7 hours wide in magnetic local time. These measurements provide a lower limit for the cusp width. The observed cusp optical emissions are relatively constant, considering the processes which lead to the 630.0 nm emissions, and require precipitating electron flux to be added each minute during the DMSP pass throughout the local time extent observed by the imaging photometer and probably over the whole extent of the cusp defined by DMSP data. We conclude that the electron fluxes which produce the cusp aurora are from a process which must have been operable sometime during each minute but could have had both temporal and spatial variations. The measured width along with models of cusp precipitation provide the rationale to conclude that the region of flux tube opening in the dayside merging process involves the whole frontside magnetopause and can extend beyond the dawn‐dusk terminator. The merging process for this event was found to be continuous, although spatially and temporally variable.
A survey of large‐scale variations in thermospheric oxygen column density with magnetic activity as inferred from observations of the FUV dayglow
Authors: Nicholas A. C. , Craven J. D. , Frank L. A.
Published: February 2004

Brightness of the terrestrial far‐ultraviolet (FUV) dayglow is dominated by the 130.4‐nm emission of neutral atomic oxygen, O I, and variations in the brightness observed from altitudes high above the emitting region reflect variations in thermospheric oxygen density. This paper summarizes the results of an initial survey of the Dynamics Explorer 1 observations of the FUV dayglow through a presentation of 13 representative events selected to demonstrate the spatial extent and short‐term temporal stability of the brightness perturbations. The emphasis here is on the morning sector of local time and the polar cap for observations obtained in the time interval from September 23, 1981, through January 19, 1982. An analytic expression is derived for the average response of the FUV photometer to the dayglow during periods of high‐latitude magnetic quiescence. The remaining observations in this time interval are then analyzed for their deviations from the established quiet time values. Deviations of −40% to +30% are found following intervals of increased magnetic activity. The most significant decreases (−30% to −40%) are observed equatorward of the instantaneous auroral oval only after sustained periods (∼6 hours) of intense magnetic activity (average AE greater than ∼700 nT). Decreases extend equatorward from the aurora to geographic latitudes as low as ∼30° N. Decreases of lesser magnitude that do not extend as far equatorward are associated with sustained periods of more moderate activity in which the average value of AE is smaller (∼300–400 nT). Also, the spatial extent and magnitude of the decreases in the morning sector appear greater when the IMF By component is positive. In both cases, decreases are readily observed within the polar cap. Localized enhancements of +20% to +30% occur much less frequently and are detected at the middle latitudes, well equatorward of the auroral oval.
Polar rain as a diagnostic of recent rapid dayside merging
Authors: Sotirelis Thomas , Newell Patrick T. , Meng Ching‐I.
Published: February 2004

The occurrence of bright (>0.0016 ergs cm−2 s−1 sr−l) polar rain is found to be determined by interplanetary magnetic field (IMF) conditions. An automated search of 11 years of DMSP particle precipitation data (1984–1994) was used to identify polar rain. Comparison was made with 30‐min segments of appropriately lagged 15‐s IMP 8 magnetic field data. Bright polar rain away from the dayside merging line occurred almost exclusively under conditions favorable for rapid merging (Bz < 0 or |By| > 2.5|Bz|). This implies that it occurs only on recently merged field lines and that during northward IMF open field lines have a distinctly different character, threading the magnetopause much further downtail than during southward IMF. The magnitude of this result was found to depend crucially on the distance from the Earth‐Sun line of the IMF monitor, suggesting that unless the monitor is restricted to being within ∼10 RE of the Earth‐Sun line, statistical correlations with IMF are likely to suffer contamination from roughly one in five events.