bluuebunny/crossref_metadata_2025 huggingface dataset:
Three‐dimensional position and shape of the bow shock and their variation with Alfvénic, sonic and magnetosonic Mach numbers and interplanetary magnetic field orientation
Authors: Peredo M. , Slavin J. A. , Mazur E. , Curtis S. A.
Published: February 2004

A large set of bow shock crossings (i.e., 1392) observed by 17 spacecraft has been used to explore the three‐dimensional shape and location of the Earth's bow shock and its dependence on solar wind and interplanetary magnetic field (IMF) conditions. This study investigates deviations from gas dynamic flow models associated with the magnetic terms in the magnetohydrodynamic (MHD) equations. Empirical models predicting the statistical position and shape of the bow shock for arbitrary values of the solar wind pressure, IMF, and Alfvénic Mach number (MA) have been derived. Individual crossings have been rotated into aberrated GSE coordinates to remove asymmetries associated with the earth's orbital motion. Variations due to changes in solar wind dynamic pressure have been taken into consideration by normalizing the observed crossings to the average value 〈p〉 = 3.1 nPa. The resulting data set has been used to fit three‐dimensional bow shock surfaces and to explore the variations in these surfaces with sonic (MS), Alfvénic (MA) and magnetosonic (MMS) Mach numbers. Analysis reveals that among the three Mach numbers, MA provides the best ordering of the least square bow shock curves. The subsolar shock is observed to move Earthward while the flanks flare outward in response to decreasing MA; the net change represents a 6‐10% effect. Variations due to changes in the IMF orientation were investigated by rotating the crossings into geocentric interplanetary medium coordinates. Past studies have suggested that the north‐south extent of the bow shock surface exceeds the east‐west dimension due to asymmetries in the fast mode Mach cone. This study confirms such a north‐south versus east‐west asymmetry and quantifies its variation with MS, MA, MMS, and IMF orientation. A 2‐7% effect is measured, with the asymmetry being more pronounced at low Mach numbers. Combining the bow shock models with the magnetopause model of Roelof and Sibeck (1993), variations in the magnetosheath thickness at different local times are explored. The ratio of the bow shock size to the magnetopause size at the subsolar point is found to be 1.46; at dawn and dusk, the ratios are found to be 1.89 and 1.93, respectively. The subsolar magnetosheath thickness is used to derive the polytropic index γ according to the empirical relation of Spreiter et al. (1966). The resulting γ = 2.3 suggests the empirical formula is inadequate to describe the MHD interaction between the solar wind and the magnetosphere.
Variations of the mean phase velocity of 1‐m ionospheric plasma waves with the plasma electron temperature
Authors: Chen Pei‐Ren , Yi Luo , Nielson Erling
Published: February 2004

The phase velocities of 1‐m electrostatic plasma waves excited by the two‐stream instability in the ionospheric E region were measured with a coherent radar. Simultaneously, plasma temperatures were measured with an incoherent radar inside the sampling volume of the coherent radar. The phase velocities, when measured in units of the ion acoustic velocity, were shown to decrease in magnitude from 1.0 toward 0.7 as the electron temperature of the plasma increases from 500° to 3000°K. Recent theoretical predictions are in accord with this experimental result.
Balloon measurements above the south pole: Study of ionospheric transmission of ULF waves
Authors: Bering E. A. , Benbrook J. R. , Byrne G. J. , Liao B. , Theall J. R. , Lanzerotti L. J. , Maclennan C. G.
Published: February 2004

We report here an experimental study of the Hughes and Southwood model of the transmission of electromagnetic signals from the ionosphere/magnetosphere to the lower atmosphere. The electric field data were obtained from one of eight high‐latitude balloon payloads launched above the south geographic pole during the South Pole Balloon Campaign in the 1985–1986 austral summer. The magnetic field data are from the South Pole magnetometer. The balloon payloads were instrumented with double‐probe electric field detectors and bremsstrahlung X ray detectors. One of the events from flight 7 (January 8, 1986, at 1500–1800 UT (1130–1430 MLT)) had an amplitude in the range of 10–20 mV/m and a period of several minutes. In the magnetosphere, the wave was probably an Alfvén mode. The wave event was superimposed on background fluctuations that can be attributed to turbulence. The results agree with the predictions of the model in that the best coherence is observed between parallel components of E versus B. The results disagree with the predictions of the model in two respects. First, we find a frequency structure not predicted by the model, and second, we find intervals where the electric and magnetic field polarizations are of opposite handedness. The first discrepancy was modeled by taking account of the measured electrostatic turbulence. The second problem remains a puzzle.
Nonequilibrium, large‐amplitude MHD fluctuations in the solar wind
Authors: Roberts D. Aaron , Wiltberger Michael J.
Published: February 2004

Compressible MHD simulations in one dimension with three‐dimensional vectors are used to investigate a number of processes relevant to problems in interplanetary physics. The simulations indicate that a large‐amplitude nonequilibrium (e.g., linearly polarized) Alfvénic wave, which always starts with small relative fluctuations in the magnitude B of the magnetic field, typically evolves to flatten the magnetic profile in most regions. Under a wide variety of conditions B and the density ρ become anticorrelated on average. If the mean magnetic field is allowed to decrease in time, the point where the transverse magnetic fluctuation amplitude δBT is greater than the mean field B0 is not special, and large values of δBT/B0 do not cause the compressive thermal energy to increase remarkably or the wave energy to dissipate at an unusually high rate. Nor does the “backscatter” of the waves that occurs when the sound speed is less than the Alfvén speed result, in itself, in substantial energy dissipation, but rather primarily in a phase change between the magnetic and velocity fields. For isolated wave packets the backscatter does not occur for any of the parameters examined; an initial radiation of acoustic waves away from the packet establishes a stable traveling structure. Thus these simulations, although greatly idealized compared to reality, suggest a picture in which the interplanetary fluctuations should have small δB and increasingly quasi‐pressure balanced compressive fluctuations, as observed, and in which the dissipation and “saturation” at δBT/B0 ≈ 1 required by some theories of wave acceleration of the solar wind do not occur. The simulations also provide simple ways to understand the processes of nonlinear steepening and backscattering of Alfvén waves and demonstrate the existence of previously unreported types of quasi‐steady MHD states.
What is the source of observed annual variations in plasmaspheric density?
Authors: Guiter S. M. , Rasmussen C. E. , Gombosi T. I. , Sojka J. J. , Schunk R. W.
Published: February 2004

Plasmaspheric densities have been observed previously to be higher in December than in June, with the ratio varying between 1.5 and 3.0 and with larger variations at lower L shells. In order to search for the cause of the observed annual variations, we have modeled plasmaspheric density, using a time‐dependent hydrodynamic model. On an L = 2 field line with geomagnetic longitude equal to 300°, the modeled plasmaspheric densities were a factor of 1.5 times higher in December than in June. The modeled December to June density ratio was found to increase slightly with L shell, in contrast to observations; this discrepancy may be due to the fact that outer plasmaspheric flux tubes are never completely full. In addition, for an L = 2 field line with geomagnetic longitude equal to 120°, the modeled plasmaspheric density was higher in June than in December by a factor of about 1.2. Various numerical tests were also performed in order to examine the sensitivity of plasmaspheric density to various parameters. In particular, a large vertical neutral wind was applied in order to raise the O+ profile, which had the effect of raising plasmaspheric density by a factor of 6. This in conjunction with a theoretical analysis suggests that plasmaspheric density levels are very sensitive to O+ levels in the upper ionosphere. We conclude that annual variations in plasmaspheric density are due to similar variations in ionospheric O+.
Interaction between biased sunlit electron collectors in an infinite‐Debye‐length magnetoplasma: Electron temperature threshold effect
Authors: Hilgers A.
Published: February 2004

The interaction between two electrically coupled collectors in an infinite‐Debye‐length magnetoplasma is studied by the use of a simple semianalytical model of particle collection by spherical probes. Because the ambient magnetic field confines the orbits of the particles, the perturbations due to photoelectron emission or screening of the ambient electron flux are “guided” along the magnetic field direction. When one collector is used as a density probe, depending on the size of the other collector with respect to the mean Larmor radius of the photoelectrons, the model predicts either an electron temperature threshold or a satellite potential threshold in order that the photoelectron current between the collectors overcomes the ambient electron current to the probe. This explains the variation of the modulation patterns of biased Langmuir probe measurements observed in the vicinity of auroral arcs.
Localized and strongly unstable plasma regions in the auroral E region ionosphere and implications for radar experiments
Authors: Haldoupis C. , Koehler J. A. , Sofko G. J. , Danskin D. W. , André D. , Mu J.
Published: February 2004

The results in this paper were obtained with SAPPHIRE, a new auroral Doppler radar experiment designed to study meter‐scale E region irregularities. SAPPHIRE is a dual 50‐MHz continuous wave, phased array, multibeam, bistatic system which is capable of performing cross‐beam measurements from two widely different directions. There are two transmitters, each of which probes the auroral electrojet plasma over a large spatial target grid area of multiple intersections that determine 16 scattering regions or cells. Initial observations using untapered antenna arrays showed a class of scatter characterized by a narrow power spectrum peaking at the same Doppler shift in all, or several, observing cells simultaneously. These are strong echoes ranging in lifetime from a few tens of seconds to a few minutes and occurring preferentially in the midnight and morning magnetic time sectors. The analysis showed that this scatter is strongly anisotropic in azimuth and comes from localized regions of spatially coherent large‐amplitude plasma waves that produce mostly type III, but also type I and the rare type IV, radar auroras. By using many events and analyzing a large number of Doppler spectra, we found that type III echoes are the strongest observed, having on the average relative intensities at least 15 dB higher than the type I echoes. The observations relate to the “short discrete radar auroras” which are known to originate in spatially confined, dynamic plasma regions. The possibility exists that the large free energy for instability in these active regions is provided from intense electric fields and/or very sharp electron density gradients, both expected to occur at times near the edges of discrete auroral arcs. Finally, the present results confirm that, because of the large dynamic range of radio auroral echoes, strong scattering regions lead to the complete domination, at times, by backscatter through antenna sidelobes. For the localized regions of strong type III and type I echoes, this means that the conventional 3‐dB antenna beam width scale size of the scattering region is unrealistic. Obviously, this has important implications for the radar auroral experiments and the interpretation of observations.
On the propagation of transversally small‐scale standing Alfven waves in a three‐dimensionally inhomogeneous magnetosphere
Authors: Klimushkin Dmitrii Yu. , Leonovich Anatolii S. , Mazur Vitalii A.
Published: February 2004

In terms of a three‐dimensionally inhomogeneous model of the magnetosphere, we have investigated theoretically the process of propagation across the geomagnetic field of monochromatic standing Alfven waves that are transversally small‐scale ones. For this purpose we introduced an orthogonal coordinate system tied in a natural manner to the geomagnetic field. It is shown that a three‐dimensionally inhomogeneous magnetosphere is composed of sectors that differ from each other by the propagation character of the waves with a different sign of azimuthal wave number. The study revealed the existence of transversally small‐scale waves produced near the toroidal surface in one sector and absorbed near this same surface in another sector of the magnetosphere. These phenomena have no analog in the theory of the axisymmetric magnetosphere where oscillations come into being near the poloidal surface and propagate toward the toroidal surface.
Demagnetization of electrons in the electromagnetic field structure, typical for quasi‐perpendicular collisionless shock front
Authors: Gedalin M. , Gedalin K. , Balikhin M. , Krasnosselskikh V.
Published: February 2004

We consider the motion of electrons in the electromagnetic field structure which is typical for the ramp of the quasi‐perpendicular collisionless shock. The total energy that an electron can gain due to the crossing of the stationary shock front is determined by the cross‐shock potential in the de Hoffman‐Teller frame. When the shock is wide the electrons are accelerated by the electric field along the magnetic field, while the perpendicular energy gain is adiabatic υ⊥² ∝ B. If, however, the ramp is narrow enough, the electrons are effectively demagnetized, and a considerable fraction of the cross‐shock potential energy can be transferred directly into the electron perpendicular energy, causing substantial overadiabatic energization. This effect may be closely related to the strong heating of electrons in the supercritical shocks.
Two‐stream modeling of plasmaspheric refilling
Authors: Guiter S. M. , Gombosi T. I. , Rasmussen C. E.
Published: February 2004

Plasmaspheric refilling on an L = 4 flux tube was studied by using a time‐dependent, hydrodynamic plasmaspheric flow model in which the ion streams from the two hemispheres are treated as distinct fluids. In the model the continuity, momentum, and energy equations of a two‐ion (O+ and H+), quasi‐neutral, currentless plasma are solved along a closed geomagnetic field line; diffusive equilibrium is not assumed. Collisions between all stream pairs and with neutral species are included. The model includes a corotating, tilted dipole magnetic field and neutral winds. Ionospheric sources and sinks are accounted for in a self‐consistent manner. Electrons are assumed to be heated by photoelectrons. The model flux tube extends from 200‐km altitude in one hemisphere to 200‐km altitude in the other hemisphere. Initially, the upwelling streams pass through each other practically unimpeded. When the streams approach the boundary in the conjugate ionosphere, a shock develops there, which moves upward and dissipates slowly; at about the same time a reverse shock develops in the hemisphere of origin, which moves upward. After about 1 hour, large shocks develop in each stream near the equator; these shocks move toward the equator and downward after crossing the equator. However, these shocks are probably artificial, because counterstreaming flows occur in each H+ fluid, which the model can only handle by creating shocks.
Ground‐based observations of ion/neutral coupling at Thule and Qânâq, Greenland
Authors: Thayer J. P. , Crowley G. , Niciejewski R. J. , Killeen T. L. , Buchau J. , Reinisch B. W.
Published: February 2004

During December 1988, 24 hours of darkness and clear sky conditions permitted continuous observations of the O I (6300 Å) airglow by a Fabry‐Perot interferometer located at Thule Air Base, Greenland. Thus a continuous record of the F region neutral winds was obtained for that month. During this same time period, a digital ionosonde located at Qânâq, Greenland (110 km north of Thule Air Base), was in operation measuring electron density profiles and F region ion drifts. This combination of ground‐based observations allowed the investigation of ion/neutral coupling at a temporal resolution of about 15 min. Interplanetary magnetic field (IMF) data from the IMP 8 satellite were also available from December 16 to 24 and indicated intervals of Bz northward IMF conditions during this period. Here we investigate the observed response of the neutral wind to convection changes in the ion drift inside the polar cap for southward and northward IMF Bz conditions. In particular, we establish a control day illustrating the typical antisunward neutral wind and ion drift patterns observed for southward Bz over Thule and Qânâq, and we compare it with observations made when the IMF Bz is directed northward. The observations during periods of northward Bz display sunward directed ion drifts over the polar cap accompanied by decreasing antisunward directed neutral winds. We investigate these times of northward Bz further and demonstrate that the ion drag term alone cannot describe the observed response in the neutral wind during northward IMF.
Radial alignment simulation of solar wind streams observed by Pioneers 10 and 11 in 1974
Authors: Pizzo V. J. , Intriligator D. S. , Siscoe G. L.
Published: February 2004

A particularly favorable radial lineup between spacecraft in Earth orbit and Pioneers 10 and 11 (near the ecliptic plane at ∼5.5 AU and ∼4.5 AU, respectively) occurred in mid‐1974, when the solar corona was in a stable and well‐defined warped‐dipole configuration. The radial alignment study reported here differs from previous applications of the technique in two respects: (1) It is the first time a two‐dimensional (2‐D) MHD model has been tested over such a lengthy propagation interval; the 2‐D capability is crucial for treatment of the nonradial shearing motions occurring across the stream interface. (2) The three‐dimensional (3‐D) structure observed in the white light corona at that time is related to the systematic patterns of nonradial flow deflections appearing at the Pioneer corotating interaction region (CIR) fronts. Comparison of predicted and observed flows for pairs of streams in two successive rotations reveals that when the parent coronal hole projects far across the solar equator in a predominantly north‐south orientation (i.e., nearest the 2‐D idealization), the mapping is accurate down to details of the flow structures. But where the spacecraft tracks along a latitudinal boundary of a hole or the associated stream front is inclined at a shallow angle to the equator, the numerical projections deviate systematically from the observations. Among the sources of error are 3‐D dynamical interactions neglected in the model, differential rotation effects, and slow temporal evolution of the coronal structures. A characteristic pattern of north‐south and east‐west deflections indicative of the 3‐D geometry appears across the CIRs, but these patterns evidently reflect primarily the local, as opposed to global, orientation of the 3‐D stream fronts. Such patterns appear common in CIRs observed by Pioneer during this period. These findings thus hold special relevance for the analysis of Ulysses observations, since the present coronal configuration is similar to that of 1974.
A theoretical study concerning the solar cycle dependence of the nightside ionosphere of Venus
Authors: Dobe Zoltan , Nagy Andrew F. , Fox J. L.
Published: February 2004

We modeled the chemical and physical processes taking place in the nightside ionosphere of Venus by solving the one dimensional coupled continuity and momentum equations for 12 ion species [CO2+, O2+, O+, H+, NO+, CO+, N2+, N+, He+, C+, O+ (²D) and O+ (²P)]. We investigated the relative importance of the two major processes responsible for maintaining the nightside plasma densities: atomic ion transport from the dayside and impact ionization due to energetic electron precipitation. We compared our model calculations with electron density and ion composition observations obtained by instruments aboard the Pioneer Venus Orbiter during both high and moderate solar cycle conditions. These studies lead us to conclude that day‐to‐night atomic ion transport is dominant during high solar activity, while during moderate solar activity conditions the combined effects of the electron precipitation and reduced day‐to‐night ion transport are responsible for maintaining the nightside ionosphere of Venus.
Bounded anisotropy fluid model for ion temperature evolution applied to AMPTE/IRM magnetosheath data
Authors: Denton Richard E. , Li Xinlin , Phan Tai‐Duc
Published: February 2004

The proton temperature ratio T⊥/T∥ measured by the AMPTE/CCE spacecraft under conditions of high solar wind dynamic pressure follows closely the relation T⊥/T∥ = (T⊥/T∥)β ≡ 1 + 0.85 β∥−0.48. On the basis of this relation, a bounded anisotropy fluid model was developed which describes the temperature evolution of a collisionless anisotropic (T⊥ ≠ T∥) plasma including the effect of energy exchange due to pitch angle scattering by cyclotron waves. This model has been shown to well predict the ion temperature evolution within the magnetosheath close to the magnetopause as observed by CCE. The model was also successful in explaining the temperature evolution of data from the AMPTE/IRM spacecraft averaged over more general solar wind conditions, despite the fact that much of the IRM data lay well below the CCE relation. Here, we make a more thorough comparison of the bounded anisotropy model with the IRM data. Three example IRM crossings are shown: (1) a case with the temperature ratio T⊥/T∥ closely following the CCE relation, (2) a case with T⊥/T∥ closely following the functional form of the CCE relation, but at a lower value, and (3) a case where T⊥/T∥ often falls well below and evolves in a manner unrelated to the CCE relation. These three cases roughly exhibit the range of typical behavior in the entire IRM data set. In all three cases, it is clear that the CCE relation represents an approximate upper bound on the anisotropy and that the ion cyclotron waves control the temperature evolution when the anisotropy is driven up to this bound. The bounded anisotropy model is successful in explaining the temperature evolution in the first two cases and the gross features of the third. In the third case, large fluctuations in temperature were probably due to temporal changes in magnetosheath conditions.
One‐ and two‐dimensional simulations of whistler mode waves in an anisotropic plasma
Authors: Devine P. E. , Chapman S. C. , Eastwood J. W.
Published: February 2004

We present results from self‐consistent, one‐ and two‐dimensional, electromagnetic simulations of the electron whistler mode instability relevant to the near‐Earth nightside plasma sheet region during geomagnetically disturbed times. Specifically, we study the evolution of energetic, anisotropic (T⊥ > T∥) electron distributions that are injected into the nightside ring current region at geomagnetically disturbed times, the resulting growth of electron whistler mode waves, and subsequent electron pitch angle diffusion via electron whistler wave‐particle interactions. Growth of whistler mode waves from an initial pitch angle anisotropy (T⊥ ≈ 4 T∥) is studied in the strong pitch angle diffusion regime (defined as having scattering times much shorter than a typical electron bounce time in the near Earth's dipolar field). The quasi‐linear and subsequent nonlinear evolution of waves and the corresponding migration of electrons in velocity space is followed over timescales such that ion motion may be neglected. Our simulations contain wave frequencies and growth rates that are a significant fraction of the electron gyrofrequency (ω ∼ 0.5ωce, γ ∼ 0.1ωce being typical) and the simultaneous evolution of waves propagating both parallel and nonparallel to the ambient magnetic field direction. Effects due to these are not usually accounted for in applications of quasi‐linear theory to the problem of electron whistler wave‐particle interactions, so that our self‐consistent simulations of the electron whistler instability provide an important insight into the applicability of quasi‐linear theory to the velocity space diffusion of electrons due to electron whistler wave‐particle interactions. We examine the dependence of whistler mode wave growth rates, nonlinear wave mode saturation, and pitch angle diffusion rates on the β value of the hot electron species which contains the resonant population, and we compare the differences between results of one‐ and two‐dimensional simulations. In general, we have found significantly larger average growth rates in a one‐dimensional than in a two‐dimensional geometry, by up to a factor of ∼2–3, with the difference between such growth rates becoming larger as β increases. As a result, we find pitch angle diffusion rates are significantly larger (by up to a factor of ∼10) in a one‐dimensional geometry, and pitch angle diffusion rates increase as β increases. During the early wave growth period of the instability, pitch angle diffusion rates Dα have been found to scale to magnetic wave energy Bw², approximately Dα ∝ Bw². We also show that in the self‐consistently evolved system (containing several wave modes), the pitch angle diffusion is still consistent with a usual quasi‐linear approach in which resonant particles are taken to diffuse along surfaces of constant wave frame energy.
Time‐varying reconnection: Implications for magnetopause observations
Authors: Semenov Vladimir S. , Lebedeva Valentina V. , Biernat Helfried K. , Heyn Martin F. , Rijnbeek Richard P. , Farrugia Charles J.
Published: February 2004

We discuss the implications of results arising from an analysis of a Petschek‐type reconnection model for the interpretation of data obtained at the terrestrial magnetopause. In this model, reconnection is initiated through the introduction of a reconnection electric field in the diffusion region. The magnitude of the electric field is considered to be small compared to the product of characteristic values of the magnetic field strength and Alfvén speed in the system; that is, we study the case of weak reconnection only. Outside the diffusion region, the behavior of the plasma is governed by the ideal MHD equations. Petschek's original analysis is generalized through the introduction of a spatially and temporally varying reconnection rate, that is, the reconnection line has a finite length and the reconnection electric field along it varies in time. Additionally, the magnetic fields on either side of the current sheet (although uniform initially) may have arbitrary strength and are skewed relative to each other. New features are that (1) the plasma velocity may have a shear across the current layer, and (2) the densities on either side of the current sheet may be different in general. The reconnection electric field initiates a localized disruption of the current sheet, and the associated disturbances are propagated into the system by MHD waves. With this model we are able to explain and interpret various features observed at the terrestrial magnetopause, such as accelerated plasma flows and flux transfer events. We describe magnetic field signatures predicted by our model. We also show that reconnection is capable of generating surface waves. A property of our model is that it predicts a displacement of the magnetopause when time‐dependent reconnection is occurring.
Analysis of steepened magnetosonic waves using wavelet transforms and neural networks
Authors: Muret P. , Omidi N.
Published: February 2004

In this study, wavelet transforms and neural networks are used to analyze the formation and evolution of steepened magnetosonic waves (shocklets) generated in hybrid (fluid electrons, particle ions) simulations. These waves model the shocklets observed upstream of planetary bow shocks and at comets. Specifically, the usefulness of wavelet transforms and neural networks for understanding the nature of the steepening process, and the further evolution of shocklets into less coherent structures is investigated. Previous studies had suggested that the nonlinear steepening process is associated with the excitation of higher frequency waves within the original wave. This hypothesis, however, could not be directly substantiated using Fourier transforms. In order to continue with the analysis of shocklets, it has become necessary to implement new techniques using wavelet transforms and neural networks. Wavelet transforms are tailored to the analysis of localized structures such as shocklets, while neural networks have the ability to model nonlinear dynamical systems. Application of wavelet transforms has verified the presence of higher frequency waves within the steepening shocklet and has identified them as the forward propagating and the backward propagating magnetosonic waves as well as the backward propagating Alfvén ion‐cyclotron mode. The wavelet transform has also located the source of the whistler wave packet attached to the shocklet to be the region of steep magnetic field gradient. In order to understand the further evolution of shocklets, neural networks have been applied in the analysis. Used in conjunction with a translation invariant transform, neural networks have been successfully trained to identify shocklets. This allows for the scanning of large data sets as well as the development of a classification system for shocklets. A multinetwork classification system using various techniques, including wavelet preprocessing, has been developed to analyze the further evolution of shocklets and their components. Identification and classification of neural networks have increased our understanding of shocklet evolution. The techniques involving wavelet transforms and neural networks that have been employed in this study show considerable potential for the study of not only shocklets, but also other wave phenomena in space plasmas.
Analysis of Lyman α and He I 584‐Å airglow measurements using a spherical radiative transfer model
Authors: Bush Brett C. , Chakrabarti Supriya
Published: February 2004

We report on the scattering and excitation mechanisms of the terrestrial exospheric H I 1216‐Å and He I 584‐Å airglow emissions by comparing simulations from a radiative transfer model with spectroscopic measurements from an Earth‐orbiting satellite. The purpose of these comparisons are twofold: to assess the sensitivity of the input parameters to the model results and to test the applicability of the model to airglow analysis. The model incorporates a spherically oriented atmosphere to account for the extended scale heights of the exospheric scatterers as well as to properly mimic scattering across the terminator region from the dayside to the nightside hemispheres. Spectroscopic Lyman α and He I 584‐Å data were obtained by the STP78‐1 satellite that circumnavigated the Earth in a noon/midnight orbit at an altitude of 600 km. The “best fit” analysis of the Lyman α data acquired on March 25, 1979, requires scaling the hydrogen density distribution obtained from the MSIS‐90 (Hedin, 1991) atmospheric model by 45–50%, the exospheric temperature by 90–100%, and the Lyman α solar flux predicted by EUV91 model (Tobiska, 1991) by 1.9–2.0. Similar analysis of the He I 584‐Å data acquired on March 5, 1979, requires scaling the helium density distribution obtained from the MSIS‐90 (Hedin, 1991) atmospheric model by 60–80% and the exospheric temperature by 105–115% while using a line center 584‐Å solar flux of 1.44×1010 photons cm−2 s−1 Å−1.
Observations of ionospheric escape on Venus' nightside
Authors: Mihalov J. D. , Russell C. T. , Kasprzak W. T. , Knudsen W. C.
Published: February 2004

A population of low‐energy (0–250 V E/q ions with tailward directed velocity vectors and energies above that for escape from Venus is evident in nightside data from the Ames plasma analyzer on the Pioneer Venus Orbiter spacecraft. Good correlations with solar wind parameters were not obtained for the magnitudes of these ion fluxes, but tendencies for occurrence at times of tailward oriented magnetic fields and for alignment of the ion flows with the magnetic field were found. These tendencies seemed to be enhanced for higher‐energy ions. In a few cases where comparisons were made, the ion fluxes were consistent with simultaneous O+ measurements by the neutral mass spectrometer experiment on the spacecraft. The mean flux observed of the escaping nightside ions, averaged over an approximately 10‐week‐long spacecraft nightside season, was < 2×106 cm−2 s−1.
Fine structure of auroral roar emissions
Authors: LaBelle J. , Trimpi M. L. , Brittain R. , Weatherwax A. T.
Published: February 2004

In September 1994, a narrowband downconverting receiver was installed at Circle Hot Springs, Alaska, to complement ongoing stepped frequency receiver observations of auroral roar, a relatively narrowband emission at frequencies roughly 2 and 3 times the ionospheric electron gyrofrequency (ƒce). By good fortune, auroral roar events were captured at 0957 UT (0057 LT) on September 6, 1994, and at 1015 UT (0115 LT) on October 23, 1994. The narrowband recording with a center frequency of 2.853 MHz and bandwidth of 10 kHz revealed that these auroral roar events were composed of multiple narrowband features which drift in frequency in a complicated pattern. Both rising tones and falling tones were observed, and the slopes of the discrete features varied from nearly flat to 100 kHz/s. When the downconverted signals are played through a loudspeaker, they resembled structured VLF emissions such as chorus. These data show that auroral roar, like auroral kilometric radiation, is at least sometimes characterized by fine frequency structures previously not resolved by stepped frequency analyzers. The existence of such fine structures has significance for theories of the generation mechanism of auroral roar, which remains unexplained.
Results from active spacecraft potential control on the Geotail spacecraft
Authors: Schmidt R. , Arends H. , Pedersen A. , Rüdenauer F. , Fehringer M. , Narheim B. T. , Svenes R. , Kvernsveen K. , Tsuruda K. , Mukai T. , Hayakawa H. , Nakamura M.
Published: February 2004

A low and actively controlled electrostatic potential on the outer surfaces of a scientific spacecraft is very important for accurate measurements of cold plasma electrons and ions and the DC to low‐frequency electric field. The Japanese/NASA Geotail spacecraft carries as part of its scientific payload a novel ion emitter for active control of the electrostatic potential on the surface of the spacecraft. The aim of the ion emitter is to reduce the positive surface potential which is normally encountered in the outer magnetosphere when the spacecraft is sunlit. Ion emission clamps the surface potential to near the ambient plasma potential. Without emission control, Geotail has encountered plasma conditions in the lobes of the magnetotail which resulted in surface potentials of up to about +70 V. The ion emitter proves to be able to discharge the outer surfaces of the spacecraft and is capable of keeping the surface potential stable at about +2 V. This potential is measured with respect to one of the electric field probes which are current biased and thus kept at a potential slightly above the ambient plasma potential. The instrument uses the liquid metal field ion emission principle to emit indium ions. The ion beam energy is about 6 keV and the typical total emission current amounts to about 15 µA. Neither variations in the ambient plasma conditions nor operation of two electron emitters on Geotail produce significant variations of the controlled surface potential as long as the resulting electron emission currents remain much smaller than the ion emission current. Typical results of the active potential control are shown, demonstrating the surface potential reduction and its stability over time.
Implications of variable mass loading in the Io torus: The Jovian flywheel
Authors: Pontius D. H.
Published: February 2004

The Io plasma torus exhibits a persistent lag from corotation with Jupiter, a phenomenon associated with mass loading due to local ionization of neutrals ejected from Io. The observed lag is the sum of two effects: slippage of the high‐altitude neutral atmosphere relative to the planet and slippage of the torus relative to the neutrals. However, the relative weights of the two effects have not been well constrained. Recent observations show that the lag is very steady and insensitive to the position of Io. Models of torus chemistry suggest that mass loading should be concentrated in the vicinity of Io. If the lag is indeed a consequence of mass loading, then this inhomogeneity should presumably lead to more pronounced spatial and temporal variations than are observed. I show that because the neutral atmosphere has a much larger moment of inertia than the torus, it acts as a massive flywheel that requires several hundred hours to adjust to a changing source. By further implication, the effects of local time variations in the mass loading are also smoothed out, and the observed lag reflects the longitude‐averaged source rate. This also confirms earlier predictions that most of the observed lag is accountable to slippage of the neutral atmosphere. Implications for radial transport are also discussed.
Upgoing electrons produced in an electron‐proton‐hydrogen atom aurora
Authors: Decker Dwight T. , Basu B. , Jasperse J. R. , Strickland D. J. , Sharber J. R. , Winningham J. D.
Published: February 2004

The first test of a self‐consistent theory for the combined electron‐proton‐hydrogen (H) atom aurora is presented. Specifically, the upgoing differential electron flux that results from precipitating protons and electrons is modeled and compared to observations. The observations are from the Low Altitude Plasma Instrument (LAPI) on board the Dynamics Explorer 2 satellite. Excellent agreement between data and theory is produced when a kappa distribution is used to extrapolate the observed proton spectra to energies beyond the instrument's energy cutoff (27 keV). More definitive tests of the model will require more comprehensive observations as well as improvements in available proton‐H atom cross sections. However, it is concluded that data clearly supports the theoretical prediction that the secondary electron spectra due to precipitating protons and H atoms is much softer than that produced by precipitating electrons. Also, the implication that the incoming proton spectra have a high‐energy power law tail raises the possibility that previous determinations of the total ion energy flux are too low.
A statistical study of the motions of auroral arcs in the high‐latitude morning sector
Authors: Shiokawa K. , Yumoto K. , Hayashi K. , Oguti T. , McEwen D. J.
Published: February 2004

Motions of auroral arcs in the high‐latitude morning sector (0300–0900 magnetic local time) have been studied statistically for magnetically quiet periods, using the data from all‐sky images for two winters from two ground‐based stations at magnetic latitudes (MLAT) of 84° and 78°. Most of the observed arcs are in a sun‐aligned direction. Most of the arcs observed at 84° MLAT move duskward with a typical velocity of ∼500 m/s. A similar duskward motion of the arcs with a slightly higher velocity is dominant at 78° MLAT, though some arcs move antisunward at this latitude. It is suggested that the observed duskward auroral motion is related to poleward shrinkage of the morningside oval on closed field lines during magnetically quiet conditions.
Diagnosis of general magnetic reconnection
Authors: Van Hoven G. , Hendrix D. L. , Schnack D. D.
Published: February 2004

During the course of an investigation of solar coronal heating, we were motivated to consider magnetic reconnection of the atmospheric magnetic field as a possible resistive heating source in the topological‐dissipation model proposed by Parker (1972). It was apparent that a quantitative diagnostic was needed which could establish the existence of three‐dimensional magnetic reconnection and provide quantitative information on the extent of its dissipative effects. A new diagnostic of general magnetic reconnection (GMR), as described by Schindler et al. (1988), is derived and applied to numerical simulations of the tearing mode and Alfvén waves. MHD simulations of Parker's heating model are then described, and the existence of reconnection is shown through the use of the GMR diagnostic and other indicators of magnetic reconnection.
Measurement of geocoronal Balmer alpha in the auroral zone
Authors: Kerr R. B. , Hecht J. H.
Published: February 2004

A Fabry‐Perot spectrometer is used at Sondre Stromfjord, Greenland, during periods of quiet auroral conditions to isolate the resonant fluorescent component of the geocoronal atomic hydrogen Balmer alpha emission. The data demonstrate that we can detect a weak, 8‐rayleigh emission with signal‐to‐noise ratio of 6 using 25‐min of integration. Moreover, these observations have sufficient spectral resolution to determine effective exospheric temperatures with statistical uncertainties of about 35%. Energetic neutral hydrogen, due presumably to neutralized proton precipitation, appears in the line profile background, and, as such, is distinct from the quiescent, less energetic, geocoronal component excited by resonant fluorescence of solar Lyman beta photons.
Evidence of upward H+ flow in the Venus dayside ionosphere
Authors: Ghosh Suchita , Mahajan K. K.
Published: February 2004

At Venus H+ is expected to be under diffusive equilibrium above about 400 km, but it is difficult to verify this under normal circumstances, because solar wind interaction terminates the subsolar ions around 400 km. However, under conditions of extremely low solar wind pressure, the terminal height, (called the ionopause) can move up to very high altitudes. Such events are rare, but after searching through the Pioneer Venus data, we have been able to identify some orbits with ionopause altitudes above 1000 km. Altitude distributions of O+ and H+, measured by the ion mass spectrometer experiment, have been studied for these orbits. It is found that while O+ distribution is consistent with diffusive equilibrium, H+ shows large departures providing evidence of upward flow of H+. Approximate calculations indicate that the flow is subsonic.
CIRRIS 1A global observations of 15‐µm CO2 and 5.3‐µm NO limb radiance in the lower thermosphere during moderate to active geomagnetic activity
Authors: Wise J. O. , Carovillano R. L. , Carlson H. C. , Roble R. G. , Adler‐Golden S. , Nadile R. M. , Ahmadjian M.
Published: February 2004

In this paper we present and discuss the cryogenic infrared radiance instrumentation for shuttle (CIRRIS) 15‐µm CO2 and 5.3‐µm NO data with respect to limb emission variability and within the context of latitudinal, diurnal, and geomagnetic variations during two days of observations onboard shuttle flight STS 39, April 29–30, 1991. About 50 limb emission profiles were examined for the two emissions. Enhancements were observed at high latitudes relative to midlatitudes and low latitudes at 140 km altitude for the 15‐µm CO2 emission (factor of 2–5). The high‐latitude enhancement in the 5.3‐µm NO emission was larger (factor of 11–14). The high‐latitude nighttime data were collected in the auroral zone during a class III aurora. Diurnal variations are examined at midlatitudes. A significant enhancement in the 15‐µm emission was observed between 0500 and 0700 LT at 140 and 160 km. This effect was modeled by the SHARC atmospheric generator (SAG) which uses the mass spectrometer incoherent scatter (MSIS) model. Species concentrations from the thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) and SAG models were input to the SHARC radiance code to simulate the CIRRIS limb emission data. The TIME‐GCM predicted the 15‐µm CIRRIS radiances generally well for 100 km < z < 120 km but for higher altitudes the data was consistently a factor of 2 higher. For the 5.3‐µm simulation the TIME‐GCM predicted the data well at low latitudes and midlatitudes, but some significant discrepancies were found at higher latitudes. The altitude of the peak radiance of the 5.3‐µm NO emission was found to vary between 110 to 135 km with little systematic global pattern. During high‐latitude auroral events the peak of the 5.3‐µm emission was consistently observed at higher altitudes than the peak of the 3914Å N2+ first negative emission, in agreement with previous observations.
Waves and streams in the expanding solar wind
Authors: Grappin R. , Velli M.
Published: February 2004

The expanding box model (EBM) allows the simulation of the evolution of compressible MHD turbulence within the expanding solar wind, taking into account the basic properties of expansion. Using the EBM, we follow the evolution of waves within a compressive stream shear and magnetic sector structure in the range of 0.1 to 1 AU from the Sun. We analyze the physical processes which lead in these simulations to the modulation and erosion of the wave component, combined with WKB and non‐WKB processes due to expansion. A strong erosion by stream shear corresponds indeed to one of the observed regimes in the solar wind; however, we are unable to reproduce the regime which holds during solar minimum, in which the correlation between large‐scale stream structure and turbulence remains high independently from distance to the Sun. The main point of disagreement with observations concerns the energy spectrum (it is difficult to generate and sustain small‐scale turbulence with an Alfvénic wave band present, and even more so in an expanding medium); the main point of agreement concerns the statistics of density fluctuations, which are independent of distance, and matches the observed amplitudes both within slow and fast wind. At the same time, small scales appear to be dominated in the simulations by compressible effects, which contradicts popular ideas on solar wind turbulence.
Three‐dimensional MHD simulations of the interaction between venus and the solar wind
Authors: Cable S. , Steinolfson R. S.
Published: February 2004

We have developed a three‐dimensional, single‐fluid, MHD code which has successfully simulated the major qualitative features of the Venus‐solar wind interaction. A bow shock forms at a height above the ionosphere close to that deduced from Pioneer Venus Orbiter (PVO) observations. The bow shock shows an asymmetry in the terminator plane qualitatively similar to results obtained from PVO data analyses. The magnetic field drapes over the planet and forms a two‐lobed magnetotail. The magnetic field configuration near the planet is very similar to that recorded by PVO as well. We model the Venus obstacle as a hard, conducting sphere. We simulate the case in which the IMF is tilted with respect to the incoming solar wind velocity at approximately the Parker spiral angle; this is the first simulation of this particular case which includes all spatial regions of the interaction.
Helium escape from the terrestrial atmosphere: The ion outflow mechanism
Authors: Lie‐Svendsen Ø. , Rees M. H.
Published: February 2004

We have computed global He+ escape fluxes for a range and a variety of diurnal, seasonal, universal time, and solar activity geophysical conditions. We average over the short‐term variables and compute the globally averaged escape flux for a range of cutoff latitudes, which separate regions of open and closed field lines, during one solar cycle. The global escape flux averaged over a solar cycle was computed, and we find that a cutoff latitude of about 60° or lower is sufficient to balance the outgassing from the Earth's crust.