bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐frequency magnetic fluctuation spectra in the magnetosheath and plasma depletion layer
Authors: Denton Richard E. , Gary S. Peter , Anderson Brian J. , Fuselier Stephen A. , Hudson Mary K.
Published: February 2004

Recent observations have delineated several different kinds of enhanced magnetic fluctuation spectra below the proton cyclotron frequency in the terrestrial magnetosheath. A model is presented that represents the variation of plasma parameters across the plasma depletion layer and into the magnetosheath proper. Using this model, we find that many of the properties of the observed spectra follow directly from the predictions of linear Vlasov instability theory. The observed progression of spectral features is a natural progression from mirror mode to merged (in frequency range) proton and He2+ cyclotron modes to bifurcated (in frequency range) cyclotron modes as plasma convects earthward in the magnetosheath. The necessary change in dispersion surface topology from separated proton and He2+ cyclotron surfaces at low beta to merged surfaces at high beta is described.
Balloon observations of nightside Pc 5 quasi‐electrostatic waves above the South Pole
Authors: Liao B. , Benbrook J. R. , Bering E. A. , Byrne G. J. , Theall J. R. , Lanzerotti Louis J. , Maclennan Carol G.
Published: February 2004

We report here a unique Pc 5 band quasi‐electrostatic wave event (∼300‐s period) observed near local geomagnetic midnight at an invariant latitude of 75°. The electric field signal was obtained from one of the eight high‐latitude balloon payloads launched above the south geographic pole during the South Pole Balloon Campaign in the 1985‐86 austral summer. The balloon payloads were instrumented with double‐probe electric field detectors and bremsstrahlung X ray detectors. The electric field data from one flight of particular interest have been compared with ground‐based magnetometer and micropulsation data in an attempt to understand the nature of the wave event. The Pc 5 waves were linearly polarized in the electric field, the electric field components had amplitudes of 20 to 30 mV/m, and the event persisted for an interval of more than 3 hours from 0000 to 0330 UT (2030 to 2400 MLT) on December 22, 1985. The magnetic activity was quiet during this time period. Detailed power spectra are presented in the paper. No evidence was found suggesting that the event was produced by an artifact. The event was not associated with atmospheric neutral waves, weather processes, or upward propagating gravity waves. The event was produced in the ionosphere by a process other than the convection of irregularities. We suggest that ULF magnetosonic waves originating at the magnetopause produced the signals that were observed.
Simulations of phase space distributions of storm time proton ring current
Authors: Chen Margaret W. , Lyons Larry R. , Schulz Michael
Published: February 2004

We use results of guiding‐center simulations of ion transport to map phase space densities of the stormtime proton ring current. We model a storm as a sequence of substorm‐associated enhancements in the convection electric field. Our pre‐storm phase space distribution is an analytical solution to a steady‐state transport model in which quiet‐time radial diffusion balances charge exchange. This pre‐storm phase space spectra at L∼2‐4 reproduce many of the features found in observed quiet‐time spectra. Using results from simulations of ion transport during model storms having main phases of 3, 6, and 12 hr, we map phase space distributions from the pre‐storm distribution in accordance with Liouville's theorem. We find stormtime enhancements in the phase space densities at energies E∼30‐160 keV for L∼2.5‐4. These enhancements agree well with the observed stormtime ring current. For storms with shorter main phases (∼3 hr), the enhancements are caused mainly by the trapping of ions injected from open night side trajectories, and diffusive transport of higher‐energy (≳ 160 keV) ions contributes little to the stormtime ring current. However, the stormtime ring current is augmented also by the diffusive transport of higher‐energy ions (E≳ 160 keV) during storms having longer main phases (≳ 6 hr). In order to account for the increase in Dst associated with the formation of the stormtime ring current, we estimate the enhancement in particle‐energy content that results from stormtime ion transport in the equatorial magnetosphere. We find that transport alone cannot account for the entire increase in |Dst| typical of a major storm. However, we can account for the entire increase in |Dst| by realistically increasing the stormtime outer boundary value of the phase space density relative to the quiet‐time value. We compute the magnetic field produced by the ring current itself and find that radial profiles of the magnetic field depression resemble those obtained from observational data.
Retrieval of absolute thermospheric concentrations from the far UV dayglow: An application of discrete inverse theory
Authors: Meier R. R. , Picone J. M.
Published: February 2004

The photoelectron‐excited far ultraviolet dayglow provides a means for remote sensing of N2, O, O2, and temperature in the terrestrial thermosphere. This paper describes a model based on the maximum likelihood method of nonlinear discrete inverse theory, which extracts information on the state of the thermosphere from limb scans of the dayglow. We show that concentrations between about 150 and (at least) 350 km can be retrieved to a high degree of accuracy and precision, independent of instrument absolute calibration. Also, the retrieved concentrations are not strongly sensitive to errors in the photoelectron excitation cross sections. The model will allow the routine development of climatological databases on the thermosphere from satellite remote sensing missions.
Ion‐recombination nucleation and growth of ice particles in noctilucent clouds
Authors: Sugiyama Takuya
Published: February 2004

Formation of ice particles in the polar summer mesosphere is simulated with the proton hydrate, H+(H2O)n, as the origin. In an ion‐recombination nucleation scheme, electrons in the mesosphere play a role in removing heavy proton hydrates from the ionic system to the background neutral system, making an ionic system a steady source of neutral embryonic nuclei for condensation. Referring to the laboratory data of n ≤ 6, a model of reaction coefficients up to the hydration order of n = 1000 is constructed with the help of both the nucleation theory and the unimolecular reaction theory. For a model nucleation layer 600 m in thickness with a minimum temperature of 125 K, produced fluxes of embryonic nuclei are calculated as a strong function of [H2O], 10³ cm−2 s−1 at [H2O] = 2.16 parts per million by volume, for example. In the time dependent simulations, stable limit cycle oscillations of the cloud formation are found with periods of 3 to 4 days depending of the magnitude of diffusion. A cloud becomes bright after an active nucleation with a delay of ∼ 1.3 days which is a sedimentation time of embryonic nuclei. Deposition of water vapors in ice particles switches off the nucleation, and evaporation of particles at the bottom of a cloud recovers the nucleation activity with a lag of diffusion‐advection time. Out of phase oscillations are predicted between the cloud brightness and the polar mesospheric summertime echo from the region of heavy proton hydrates, which will be a crucial evidence for the in situ origin of the clouds.
Growth rates for modulational instabilities of radio waves in highly collisional ionospheric plasmas
Authors: Shukla P. K. , Vladimirov S. V.
Published: February 2004

Accounting for ponderomotive force and differential Joule heating nonlinearities as well as relativistic electron mass variation, we consider the parametric coupling between a large‐ amplitude electromagnetic wave and nonresonant density and temperature perturbations. A nonlinear dispersion relation that is appropriate for the modulational instability is presented. Some interesting regimes for the thermal modulational instability are identified, and expressions for their increments are obtained. The results can be useful in understanding the phenomena of enhanced density and temperature fluctuations that have been observed during the artificial modification of the Earth's ionosphere by high‐power radio waves.
Simultaneous measurements of waves and precipitating electrons near the equator in the outer radiation belt
Authors: Imhof W. L. , Robinson R. M. , Collin H. L. , Wygant J. R. , Anderson R. R.
Published: February 2004

An investigation of wave‐particle interactions is made using several simultaneous electron and wave measurements performed at near‐equatorial positions from the CRRES satellite. Bursts of electron precipitation were observed, most frequently at local times near dawn. Examples of bursts are presented in which the fluxes of the precipitating electrons and the wave intensities are correlated with coefficients as high as 0.7. During bursts the frequencies of the enhanced waves spanned a wide range from 311 Hz to 3.11 kHz, and the energies of the enhanced electrons were in the range 1.7 keV to 288 keV. The changes of the precipitating fluxes were generally less pronounced at the lowest energies. On the basis of electron‐cyclotron resonant calculations using the cold plasma densities and ambient magnetic fields taken from the CRRES measurements it was found that the wave frequencies and precipitating electron energies were generally consistent with those expected from electron resonance with parallel propagating whistler waves. The electron data of principal concern here were acquired in and about the loss cone with narrow angular resolution spectrometers covering the energy range 340 eV to 5 MeV. The wave data included electric field measurements spanning frequencies from 5 Hz to 400 kHz and magnetic field measurements from 5 Hz to 10 kHz.
Statistical study of ULF wave occurrence in the dayside magnetosphere
Authors: Cao M. , McPherron R. L. , Russell C. T.
Published: February 2004

Ultralow‐frequency (ULF) waves are observed almost everywhere in the dayside magnetosphere. The mechanism by which these waves are generated and transformed in the dayside magnetosphere is still not understood. Here we report a statistical study of these waves based on magnetic field data from the ISEE 1 spacecraft. Data from the first traversal of the spacecraft through the entire dayside magnetosphere have been examined to determine the spatial distribution of wave occurrence. Successive 20‐min segments of data were transformed to a field‐aligned coordinate system. The parallel component was detrended and all three components of the field spectrally analyzed. Wave occurrence was defined by the presence of significant peaks in the power spectra. Wave events were categorized by three wave frequency bands: Pc 3 with T∼10‐45 s; Pc 4 with T∼45‐150 s; the short‐period part of the Pc 5 wave band with T∼150‐324 s. Properties of the spectral peaks were then entered into a data base. The data base was next sorted to determine the spatial occurrence pattern for the waves. Our results show that Pc 3 waves most frequently occur just outside synchronous orbit and are approximately centered on local noon. Pc 4 waves have a similar distribution with its peak further out. Pc 5 waves have high occurrence rate at the two flanks of the magnetosphere. Peaks in spectra obtained near the magnetopause are less clearly defined than those deeper in the magnetosphere.
A revised analysis of micron‐sized particles detected near Saturn by the Voyager 2 plasma wave instrument
Authors: Tsintikidis D. , Gurnett D. , Granroth L. J. , Allendorf S. C. , Kurth W. S.
Published: February 2004

The impulsive noise that the plasma wave and radio astronomy instruments detected during the Voyager 2 swing by Saturn was attributed to dust grains striking the spacecraft. This report presents a reanalysis of the dust impacts recorded by the plasma wave instrument using an improved model for the response of the electric antenna to dust impacts. The fundamental assumption used in this analysis is that the voltage induced on the antenna is proportional to the mass of the impacting grain. Using the above assumption and the antenna response constants used at Uranus and Neptune, the following conclusions can be reached. The primary dust distribution consists of a “disk” of particles that coincides with the equator plane and has a north‐south thickness of 2Δz = 962 km. A less dense “halo” with a north‐south thickness of 2Δz = 3376 km surrounds the primary distribution. The dust particle sizes are of the order of 10 µm, assuming a mass density of 1 g/cm³. The corresponding particle masses are of the order of 10−9 g, and maximum number densities are of the order of 10−2 m−3. Most likely, the G ring is the dominate source since the particles were observed very close to that ring, namely at 2.86 RS. Other sources, like nearby moons, are not ruled out especially when perturbations due to electromagnetic forces are included. The calculated optical depth differs by about a factor of 2 from photometric studies. The current particle masses, radii, and the effective north‐south thickness of the particle distribution are larger than what Gurnett et al. (1983) reported by about 2, 1, and 1 orders of magnitude, respectively. This is attributed to the fact that the collection coefficient used in this study is smaller than what was used in Gurnett et al.'s earlier publication.
Nonlinear decay of magnetic helicity in magnetohydrodynamic turbulence with a mean magnetic field
Authors: Stribling Troy , Matthaeus William H. , Ghosh Sanjoy
Published: February 2004

We show that the magnetic helicity associated with fluctuations in homogeneous incompressible magnetohydrodynamic (MHD) turbulence with a mean magnetic field decays in time because of nonlinear processes. Evidence is obtained numerically, by use of both dissipative and nondissipative spectral method simulations. The described effect stands in contrast to expectations based on studies of MHD turbulence without an applied mean field, in which magnetic helicity is transferred nonlinearly to long wavelengths and is preserved in time because of selective decay when dissipation is present. The process of nonlinear decay is described in terms of a generalized ideal helicity invariant and is characterized by the transient production of a mean induced electric field aligned with the applied magnetic field, an effect reminiscent of the alpha effect in dynamo theory. A simple phenomenological model for the decay process is proposed.
Computer simulation of Alfvén waves and double layers along auroral magnetic field lines
Authors: Silberstein M. , Otani N. F.
Published: February 2004

A plasma simulation has been developed to model interactions between inertial Alfvén waves and double layers and to investigate their relative contributions to auroral particle acceleration. We use a novel one‐dimensional particle‐in‐cell code, with periodic boundary conditions, to model the nonlinear excitation of current‐driven weak double layers via the free energy supplied by an inertial Alfvén wave. Analysis of the simulation output shows that double layers are not the agent primarily responsible for electron acceleration. Rather, the inertial Alfvén wave accelerates groups of electrons into a steepening beam as it encounters them. As the beam electrons reenter the main distribution, decelerated by anomalous resistive effects, they are replaced by electrons farther downstream. Hence, the particles do not free‐stream over the length of the channel. Furthermore, this wave action persists even when the system is linearly stable to ion‐acoustic modes, precluding the possibility that this behavior is brought about by the formation of ion‐acoustic double layers.
Two‐dimensional simulations of ion anisotropy instabilities in the magnetosheath
Authors: McKean M. E. , Winske D. , Gary S. P.
Published: February 2004

Ion populations with large perpendicular temperature anisotropies in the magnetosheath can excite both the mirror and proton cyclotron anisotropy instabilities. We compare kinetic aspects of these two instabilities using two‐dimensional hybrid simulations, expanding upon an earlier work using one‐dimensional simulations. Three simulation runs are examined: one in which the proton cyclotron instability has a higher linear growth rate, the second in which the mirror instability grows more rapidly, and the third in which the linear growth rates are identical. The last two runs include a large density of isotropic He++ ions to suppress the cyclotron instability. We find that initial growth occurs in all three runs at approximately the frequencies, wavenumbers, and obliquities expected by linear theory. As the system evolves, the power in both instabilities shifts to longer wavelength modes, and the characteristic frequency of the proton cyclotron instability decreases. In the first two runs, the instability with the larger linear growth rate dominates the wave energy at saturation, and in the third, the two instabilities contribute about equally. The proton cyclotron instability is relatively more important to wave‐particle energy exchange than the mirror instability; its importance to proton isotropization is generally greater than that suggested by its contribution to the total wave energy, and it is almost solely responsible for heating the helium ions.
Ionospheric current system associated with the equatorial counterelectrojet
Authors: Rastogi R. G.
Published: February 2004

The phenomenon of the equatorial counterelectrojet current (CEJ) is studied utilizing the D, H, and Z components of the magnetic field at a chain of geomagnetic observatories in the Indo‐Russian zone extending from 0 to 75° N magnetic dip. Very significant changes in both H and D components are shown to occur on CEJ days at all stations even north of Sq focus. The current system associated with CEJ consists of two vortices, one counterclockwise in the morning and another clockwise in the evening hours. The counterelectrojet events during December solstices seem to be associated with the disappearance of the Sq current loop in the Indo‐Russian sector and on a number of consecutive days. The cause of the counterelectrojet is suggested to be the changes in various modes of the tidal movements in the upper atmosphere.
Thruster firing effects in the shuttle environment: 2. Positive ion composition
Authors: Hunton D. E. , Machuzak J. S.
Published: February 2004

Measurements of positive ion composition in the payload bay of the Space Shuttle confirm that the vernier reaction control system rocket engines cause major changes in the shuttle plasma environment. Previously reported measurements of increases and decreases in ambient and contaminant ion fluxes are corroborated and are shown to depend on the attitude of the shuttle, the direction of exhaust plume flow, and the pointing direction of the mass spectrometer sampling orifice. The column density of exhaust neutrals and the length of the effective column are both roughly estimated from the magnitude and time response of the reduction in ambient O+ flux. The measurements are consistent with control of the contaminant ion motions by the Earth's magnetic field. Contaminant ions are detected only if the mass spectrometer is approximately aligned with the plane of ion motion perpendicular to the magnetic field vector, and only if the plume gases expand into a region of space from which the pickup trajectories are directed back into the instrument sampling orifice. The degree to which contaminant ions are picked up by the magnetic field depends on the chemical dynamics of the ion‐molecule reaction that formed them. The appearance of large fluxes of ions at mass 28 cannot be explained by the chemistry of O+ reacting with the principal exhaust neutral species. Electron impact ionization of neutral N2 to give N2+ is proposed as a possible source for this mass 28 ion.
The impact of background noise on the determination of the fractal and statistical properties of cosmic ray time series
Authors: Bergamasco L. , Serio M. , Osborne A. R.
Published: February 2004

This paper investigates the influence of background white noise on the determination of the colored random noise properties of cosmic ray experimental signals. We consider fractal methods (Grassberger and Procaccia method, scaling exponent method and fractal length method) and statistical methods (comparison with the Gaussian and Rice curves, and the multivariate scaling analysis technique). The investigation is carried out using two experimental time series with different color (spectral indices α = 1.6 and α = 1.2) and, as a reference, several computer simulations of pure noise‐free colored random noise signals with 0 ≤ α ≤ 3 with the same number of points. The results of the investigation is that the presence of noise in a experimental signal “deceives” the algorithms employed in the various analyses causing them to sample noiselike properties (e.g., an infinite or very large fractal dimension and Gaussian or quasi‐Gaussian behavior), irrespective of the physical characteristics of the particular data set.
Euler potentials of current‐free fields expressed in spherical harmonics
Authors: Stern David P.
Published: February 2004

Given a magnetic field B = −∇γ with γ expanded in spherical harmonics, it is shown that analytic Euler potentials may be derived for B if γ is asymmetrical but contains only the contribution of a single index n. This work generalizes a result for sectorial harmonics with n = m, derived by Willis and Gardiner (1988).
Downdrafting plasma flow in equatorial bubbles
Authors: Laakso Harri , Aggson Thomas L. , Pfaff Robert F. , Hanson William B.
Published: February 2004

The electric field experiment carried aboard the San Marco D equatorial ionospheric satellite regularly measured updrafting in plasma depletion channels or “equatorial bubbles” which form on the bottomside of the nightside equatorial F region. We report here observations of downdrafting vertical plasma velocities inside such depletion regions in the nightside equatorial ionosphere. Both updrafting and downdrafting motions can be expected on the basis of a generalized gradient drift/collisional Rayleigh‐Taylor instability process in the ionospheric F region. Although the gravitation can only drive upward plasma flow in plasma depletion regions, both background westward zonal electric fields and upward vertical neutral winds can cause an occurrence of downdrafting (i.e., a downward motion of the plasma within the bubble) if those parameters are strong enough. We show that as the background zonal electric field becomes westward (often after ∼2100 LT) in the equatorial ionosphere, the plasma interior to an existing bubble at altitudes of ∼400 km and less at the magnetic equator may assume a downdrafting motion, while at higher altitudes in the same bubble channel, the plasma flow remains upward. Such a simultaneous occurrence of the updrafting and downdrafting plasma flow in a single bubble channel may lead to the pinching off of the upper part of the depletion region from the lower altitude regions, causing the decay of a bubble or the formation of a “dead” bubble.
The induced magnetosphere of comet Halley: Interplanetary magnetic field during Giotto Encounter
Authors: Israelevich P. L. , Neubauer F. M. , Ershkovich A. I.
Published: February 2004

Direction of the interplanetary magnetic field (IMF) which governs the orientation of the Halley's magnetosphere was restored by using Giotto magnetic field data. The Giotto trajectory was a twisted curve in the coordinate system rotating with IMF vector and covered rather uniformly the transverse cross section of the comet. Magnetic field vectors show a regular structure arising from the field line draping about the cometary ionosphere. Violations of the field line draping occur only at the moments of rapid changes of IMF orientation.
Cusp latitude magnetic impulse events: 2. Interplanetary magnetic field and solar wind conditions
Authors: Konik R. M. , Lanzerotti L. J. , Wolfe A. , Maclennan C. G. , Venkatesan D.
Published: February 2004

The interplanetary magnetic field (IMF) conditions and solar wind plasma parameters prevailing during the magnetic impulse events identified by Lanzerotti et al. (1991) at the near cusp latitude stations Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica, are examined. The impulse events are found to occur during periods of high IMF variability. The prevailing IMF orientation, averaged over 11‐min periods during the events, shows no distinct anisotropies. However, by examining the IMF orientation when the IMF variability is small a south‐east anisotropy is found. Examining the IMF orientation on timescales of less than 1 min reveals that the impulse events are characterized by a more southward oriented IMF than is to be expected statistically. The duration of the events is found to be controlled by both solar wind speed and by the IMF By. The IMF By control of the duration of nonconjugate events is in accordance with the effect expected from the presence of magnetic tension in a reconnection setting. The amplitude of the events is found to be strongly related to solar wind speed. These findings suggest that sporadic reconnection is among the mechanisms producing the events. On the basis of the amplitudes of the impulse events, it is argued that the fractional changes in dynamic pressure must be of the order of 1 to be able to associate the impulse events with changes in the dynamic pressure. Such variability in the dynamic pressure at the time of the magnetic impulse events is not observed. It is concluded that a change in the solar wind dynamic pressure, while not among the primary mechanisms generating the magnetic impulse events, may account for between 15% and 30% of them. On the other hand, reconnection processes are able to produce impulse events of the required magnitude. It is argued then that magnetic reconnection is likely responsible for generating a minimum of 50%‐70% to a maximum of 90% of the events. However, this conclusion leaves parts of the picture unclear. Lanzerotti et al. (1991) found that magnetic impulse events with a unipolar deflection in the vertical component of the magnetic field possessed a local time dependence; positive deflections occurred in local morning, while negative deflections occurred in local afternoon. This dependence can be explained by postulating that the magnetic impulse events are associated with vortices generated by the Kelvin‐Helmholtz instability in the low‐latitude boundary layer. As such, it is suggested that the Kelvin‐Helmholtz instability may mediate the process by which sporadic reconnection produces a ground signature.
Shocks in the outer heliosphere: Voyager 2 observations from 18.9 AU to 30.2 AU (1986‐1989)
Authors: Burlaga L. F.
Published: February 2004

A search for shocks observed by Voyager 2 from 1986.0 to day 233, 1989, in the region between 18.9 AU and 30.2 AU revealed only five candidate fast forward shocks and no reverse shocks. No shock was observed during 1987 (near solar minimum). Only one candidate shock was observed during 1986, when the heliospheric current sheet (HCS) was still close to the equator. This, the weakest of the shocks, was associated with a weak corotating merged interaction region (CMIR). Only two shocks were observed during 1988, when solar activity and the inclination of the HCS were increasing. One of these shocks was associated with a CMIR, the other with a local merged interaction region (MIR) (a transient MIR localized in space). The shocks observed between 18.9 AU and 28.2 AU from 1986 through 1988 were relatively weak, the estimated magnetoacoustic Mach number being ≤ 1.5 for each of the three shocks. Since the shocks observed between 20 AU and 29 AU during the 3‐year period centered around solar minimum were weak and few in number, shocks were not important in determining the structure, dynamics, and thermodynamics of the equatorial solar wind. The strongest shock in this study was observed during 1989 at the front of a global merged interaction region (GMIR) (a MIR that extends around the Sun and is presumably formed by the coalescence of several transient ejecta and other interaction regions). A weak shock was also observed inside this GMIR.
Interplanetary magnetic field dependency of stable sun‐aligned polar cap arcs
Authors: Valladares C. E. , Carlson H. C. , Fukui K.
Published: February 2004

This is the first analysis, using a statistically significant data set, of the morphological dependence of the presence, orientation, and motion of stable sun‐aligned polar cap arcs upon the vector interplanetary magnetic field (IMF). For the one winter season analyzed we had 1392 all‐sky 630.0‐nm images of 2‐min resolution containing a total of 150 polar cap arcs, all with corresponding values of the IMF as measured by IMP 8 or ISEE 2. After demonstrating an unbiased data set with smooth normal distributions of events versus the dimensions of time, space, and IMF component, we examine IMF dependencies of the properties of the optical arcs. A well‐defined dependence for Bz is found for the presence/absence of stable Sun‐aligned polar cap arcs. Consistent with previous statistical studies, the probability of observing polar cap aurora steadily increases for larger positive values of Bz, and linearly decreases when Bz becomes more negative. The probability of observing Sun‐aligned arcs within the polar cap is determined to vary sharply as a function of the arc location; arcs were observed 40% of the time on the dawnside and only 10% on the duskside. This implies an overall probability of at least 40% for the whole polar cap. 20% of the arcs were observed during “southward IMF conditions”, but in fact under closer inspection were found to have been formed under northward IMF conditions; these “residual” positive Bz arcs had a delayed residence time in the polar cap of about what would be expected after a north to south transition of Bz. A firm dependence on By is also found for both the orientation and the dawn‐dusk direction of motion of the arcs. All the arcs are Sun‐aligned to a first approximation, but present deviations from this orientation, depending primarily upon the location of the arc in corrected geomagnetic (CG) coordinates. The arcs populating the 06‐12 and the 12‐18 quadrants of the CG coordinate system point toward the cusp. The By dependency of the arc alignment is consistent with a cusp displacement in local time according to the sign of By. We found that the arc direction of motion depended both on By and the arc location within the polar cap. For a given value of By, two well‐defined regions (or cells) exist. Within each cell the arcs move in the same direction toward the boundary between the cells. The arcs located in the duskside move dawnward; those in the dawnside move duskward. The relative size of these dusk and dawn regions (or cells) are controlled by the magnitude of By. This persistent dusk‐dawn motion of the polar cap arcs is interpreted in terms of newly open flux tubes entering the polar cap and exerting a displacement of the convective cells and the polar cap arcs that are embedded within them.
Comment on “Low‐energy particle layer outside the plasma sheet boundary” by G. K. Parks et al.
Authors: Feldstein Ya. I. , Galperin Yu. I.
Published: February 2004

We consider the mapping schemes where the plasma sheet boundary layer (PSBL) in the tail is projected to the auroral oval as inconsistent with the available data. We conclude that the low‐energy layer (LEL) structure discovered by Parks et al. (1992) at the outer edge of the PSBL is consistent with the mapping scheme by Feldstein and Galperin (1985) where the boundary plasma sheet (BPS) (which includes the PSBL proper and presumably can also include the newly discovered LEL) is mapped to the polar diffuse aurora (PDA). Note that this definition of BPS plasma domain in the tail (which includes only the PSBL and LEL, as distinct from the central plasma sheet (CPS)), which projects to the bulk of the region of the discrete aurora, or the auroral oval), differs substantially from the BPS as defined by Winningham et al. (1975). In Winningham's definition the BPS refers to the structured accelerated electrons precipitation region at ionospheric altitudes, and it sometimes was improperly interpreted as indicating the mapping of the nightside auroral oval to the plasma sheet outer boundary.
Model calculation of energetic neutral atoms precipitation at low altitudes
Authors: Orsini S. , Daglis I. A. , Candidi M. , Hsieh K. C. , Livi S. , Wilken B.
Published: February 2004

It has become apparent during recent years that global magnetospheric imaging through the detection of energetic neutral atoms (ENA) will be a crucial tool for the advancement of magnetospheric research via remote sensing technique. This report presents model calculations of ENA precipitation as detectable from a satellite orbiting at low altitudes (<1500 km). The ENA generation via charge exchange between energetic ions and geocoronal neutrals is simulated in the inner magnetosphere, between 1.5RE and 10RE, where both neutral densities and ion fluxes are significantly high. AMPTE/CCE energetic ion measurements and the geocoronal neutral H altitude profile are used to simulate the charge exchange process leading to ENA production. We discuss the relative contribution of ring current and radiation belts, as well as the relative contribution of H+ and O+ to the ENA generation.
Effect of electron dynamics on collisionless reconnection in two‐dimensional magnetotail equilibria
Authors: Pritchett P. L.
Published: February 2004

Analytic theory has yielded contradictory predictions regarding the stabilizing role of electron dynamics on the collisionless tearing instability in a field‐reversal region with finite normal magnetic field component Bz. Two‐dimensional (x, z) simulations in which both electrons and ions are treated as particles demonstrate that, for the case of thin current sheets (Ri/ λ ∼ 1, where Ri is the ion Larmor radius based on the lobe field and λ is the current sheet half thickness) and with weakly chaotic electron orbits, the ion tearing mode is indeed stabilized under the mild condition that kρe ≲ 1, where ρe is the electron Larmor radius in the Bz field.
Rate coefficient for the reaction N + NO
Authors: Fox J. L.
Published: February 2004

Evidence has been advanced that the rate coefficient for the reaction N + NO → N2; + O has a small positive temperature dependence at the high temperatures (900‐1500 K) that prevail in the terrestrial middle and upper thermosphere by Siskind and Rusch (1992), and at the low temperatures (100‐200 K) of the Martian lower thermosphere by Fox (1993). Assuming that the rate coefficient recommended by the Jet Propulsion Laboratory evaluation (DeMore et al., 1992) is accurate at 300 K, we derive here the low temperature value of the activation energy for this reaction and thus the rate coefficient that best fits the Viking 1 measured NO densities. We find that the fit is acceptable for a rate coefficient of about 1.3 × 10−10(T/300)0.5exp(−400/T) and better for a value of about 2.5 × 10−10(T/300)0.5exp(−600/T) cm3 s−1.
Identification of low‐frequency fluctuations in the terrestrial magnetosheath
Authors: Song P. , Russell C. T. , Gary S. P.
Published: February 2004

On the basis of MHD theory we develop a scheme for distinguishing among the four low‐frequency modes which may propagate in a high‐β anisotropic plasma such as the magnetosheath: the fast and slow magnetosonic, the Alfvén, and mirror modes. We use four parameters: the ratio of transverse to compressional powers in the magnetic field, the ratio of the wave powers in the thermal pressure and in the magnetic field, the ratio of the perturbations in the thermal and magnetic pressures, and the ratio of the wave powers in the velocity and in the magnetic field. In the test case of an Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) magnetosheath pass near the Sun‐Earth line downstream of a quasi‐perpendicular shock, the four modes can be clearly distinguished both spatially and spectrally. Near the bow shock, the waves are Alfvénic in a large frequency range, 1 to 100 mHz. In the middle and inner magnetosheath, the waves below 10 mHz are Alfvénic. The fast mode waves occur in the higher‐frequency end of the enhanced spectrum, 80 mHz for the middle magnetosheath and 55 mHz for the inner sheath. The wave enhancement in the intermediate frequencies is slow modes in the inner sheath and mirror modes in the middle sheath. This confirms the earlier report of the existence of the slow mode waves near the magnetopause. These slow waves provide evidence that the magnetopause is an active source of the waves in the sheath. We also show that the measured frequency of a wave is close to an invariant if the magnetosheath flow is in a steady state. Therefore changes in the frequencies of enhanced waves indicate emergence, or damping, or mode conversion of the waves.
DE 1 VLF observations during activity wave injection experiments
Authors: Sonwalkar V. S. , Inan U. S. , Bell T. F. , Helliwell R. A. , Molchanov O. A. , Green J. L.
Published: February 2004

We report on coordinated high‐altitude satellite observations in support of one of the first space‐based VLF wave injection experiments, namely the USSR Aktivny mission. The Activny satellite (A) was designed to carry a VLF transmitter (nominal frequency ∼10 kHz, transmitter power ∼10 kW) coupled to a 20‐m‐diameter loop antenna in a nearly polar orbit (83° inclination, apogee ∼2500 km, perigee ∼ 500 km). We focus our attention on conjunction experiments between the Activny and DE 1 satellites. Because of problems in the deployment of the loop antenna, the radiated power capability of the antenna was significantly reduced. Although this substantially reduced the expectation of receiving detectable signal levels on the satellite, the DE 1/Activny conjunction experiments were nevertheless carried out as a means of possibly placing an upper limit on the radiated power. During the period November 1989 through April 1990, a total of 10 DE 1/Activny wave injection sessions were conducted. During each session the Activny transmitter operated at 10.537 kHz with 1 s On ‐ 1 s Off format, for a period of 6 min centered around the conjunction time. During three conjunction periods (December 12, 26, and 27, 1989) both DE 1 and Activny were in the southern hemisphere, and DE 1 was at relatively low altitudes (ranging from 6211 to 14,810 km), thus providing the best conjunction possibilities according to the ray tracing criteria developed above. On most days, Omega transmitter signals as well as commonly occurring natural wave phenomena such as whistlers (0+) and hiss were clearly seen well above the background level, but there was no evidence of the Activny 1 s On/ 1 s Off pattern. Though no Activny signals were detected by the LWR on the DE 1 satellite, the experimental constraints allow us to place an upper limit on the total power radiated by the Activny transmitter in the whistler‐mode. Using experimental parameters, and the minimum detectable signal level of 0.05 µV/m for LWR, we find the upper limit on the total power radiated by the Activny satellite in the whistler‐mode to be ∼ 10 mW. Several recommendations for future space‐based wave injection experiments are presented.
Surface crossing by a group of satellites: A theoretical study
Authors: Mottez F. , Chanteur G.
Published: February 2004

In order to take advantage of multispacecraft measurements that will be provided by the Cluster mission, we introduce a new method for analyzing the local geometry of a surface crossed by a group of satellites. This analysis relies upon the determination of the normal to the surface by each spacecraft. Assuming a rigid surface with constant curvature, we give a method to estimate its radii of curvature and its velocity. Theoretically, the method could give the three components of the velocity. But because of the sensitivity of the input data to uncertainties, the tangential velocity of the discontinuity must be provided by another method.
High charge state carbon and oxygen ions in Earth's equatorial quasi‐trapping region
Authors: Christon S. P. , Hamilton D. C. , Gloeckler G. , Eastman T. E. , Ipavich F. M.
Published: February 2004

Observations of energetic (1.5 ‐ 300 keV/e) medium‐to‐high charge state (+3 ≤ Q ≤ +7) solar wind origin C and O ions made in the quasi‐trapping region (QTR) of Earth's magnetosphere are compared to ion trajectories calculated in model equatorial magnetospheric magnetic and electric fields. These comparisons indicate that solar wind ions entering the QTR on the nightside as an energetic component of the plasma sheet exit the region on the dayside, experiencing little or no charge exchange on the way. Measurements made by the CHarge Energy Mass (CHEM) ion spectrometer on board the AMPTE/CCE spacecraft at 7 < L < 9 from September 1984 to January 1989 are the source of the new results contained herein: quantitative long‐term determination of number densities, average energies, energy spectra, local time distributions, and their variation with geomagnetic disturbance level as indexed by Kp. Solar wind primaries (ions with charge states unchanged) and their secondaries (ions with generally lower charge states produced from primaries in the magnetosphere via charge exchange) are observed throughout the QTR and have distinctly different local time variations that persist over the entire 4‐year analysis interval. During Kp ≥ 3° intervals, primary ion (e.g., O+6) densities exhibit a pronounced predawn maximum with average energy minimum and a broad near‐local‐noon density minimum with average energy maximum. Secondary ion (e.g., O+5) densities do not have an identifiable predawn peak, rather they have a broad dayside maximum peaked in local morning and a nightside minimum. During Kp ≤ 2− intervals, primary ion density peaks are less intense, broader in local time extent, and centered near midnight, while secondary ion density local time variations diminish. The long‐time‐interval baseline helps to refine and extend previous observations; for example, we show that ionospheric contribution to O+3 is negligible. Through comparison with model ion trajectories, we interpret the lack of pronounced secondary ion density peaks colocated with the primary density peaks to indicate that: (1) negligible charge exchange occurs at L > 7, that is, solar wind secondaries are produced at L < 7, and (2) solar wind secondaries do not form a significant portion of the plasma sheet population injected into the QTR. We conclude that little of the energetic solar wind secondary ion population is recirculated through the magnetosphere.
Soft ion precipitation at very high latitudes during northward interplanetary magnetic field
Authors: Miyake W. , Whalen B. A. , Yau A. W. , Matsuoka A. , Hayakawa H. , Mukai T.
Published: February 2004

During extended periods of northward interplanetary magnetic field (IMF), soft ion precipitation was frequently observed at very high latitudes (>80° ILAT) on EXOS D (Akebono). The precipitating protons typically had temperatures of a few hundreds of eV, and were accompanied by precipitating electrons with temperatures of several tens of eV. The densities of the precipitating protons were of the order of 10−1 cm−3, and were usually lower than those of the electrons. During these periods, the ion convection at the highest latitudes was chaotic or weak, but often had an average antisunward component. During periods of strongly northward IMF, ion precipitation was observed at all magnetic local times (MLT). A flux minimum, sometimes below the level of detection of the Akebono suprathermal mass spectrometer instrument, was often found just poleward of the dayside auroral oval. This region is believed to coincide with the open magnetic field lines in the tail lobe. The region of ion precipitation was observed at auroral latitudes on the dawnside when the IMF By changed polarity from negative to positive; it was located at higher latitudes (>80° ILAT) on the duskside when the IMF By was positive. These observations suggest a connection between the ion precipitation and the auroral oval.
Cusp current modeling: A systematic approach
Authors: Mei Y. , Crooker N. U. , Siscoe G. L.
Published: February 2004

We describe the achievement of a significant step in a program whose ultimate goal is a self‐consistent model that matches cusp currents from both ionospheric and magnetospheric sources. Building on existing models, we calculate currents that arise from the ionosphere's ohmic response to the interplanetary electric field mapped to the polar cap. Unlike other models, the mapping is confined to the vicinity of the cusp, simulating limited direct communication with the solar wind, as mandated by recent observations. The mapped area is then inserted into the dayside gap of a purely ionospheric expanding polar cap model. The resulting two‐cell convection patterns have a pronounced kink in the antisunward flow contours at the poleward boundary of the cusp area, where it joins the expanding polar cap. A similar kink appears in the Heppner‐Maynard empirical patterns; the model implies that it marks the boundary where the ionosphere takes control from the solar wind. The model serves as a first tool for identifying Birkeland current types and causes. Three types arise: (1) Currents border the open cusp like region 1 currents bordering a steady state, mini‐polar cap. (2) Within the cusp, currents arise from curvature of the equipotentials which, when mapped to the magnetopause, correspond to flow directed away from the merging line, representing merging outflow. (3) The usual region 1 currents line the polar cap boundary along the flow reversals of the two convection cells as a result of polar cap expansion; they are the ionospheric response to the applied electric field. Thus the model predicts two types of region 1 currents, driven directly (1) and indirectly (3) by the solar wind, plus a set of pure cusp currents (2). Together they form the familiar overlapping pattern in the cusp region deduced from observations.