bluuebunny/crossref_metadata_2025 huggingface dataset:
Brightness distribution of midlatitude E region echoes detected at the Chung‐Li VHF radar
Authors: Huang Chien‐Ming , Kudeki Erhan , Franke Steven J. , Liu C. H. , Röttger Jürgen
Published: February 2004

A three‐sensor interferometry method is employed to parameterize the brightness distribution of VHF radar returns from midlatitude E region plasma irregularities monitored by the Chung‐Li radar in Taiwan. The angular half‐width of the brightness distribution in the direction parallel to the geomagnetic field is estimated as ∼0.2°. In the orthogonal direction the angular half‐width is measured as ∼0.5°‐1°. The results suggest that meter scale field‐aligned plasma irregularities responsible for the observed echoes were organized in localized patches with 2 to 4 km cross‐field scales.
Impulsive ion injections in the morning auroral region
Authors: Clemmons J. H. , Carlson C. W. , Boehm M. H.
Published: February 2004

Low altitude (less than 1000 km) measurements of ions precipitating into the morning auroral region are presented and analyzed. The ion fluxes exhibited time‐energy signatures consistent with impulsive injection onto high‐altitude field lines, followed by time‐of‐flight dispersion. The origin of these ions is investigated through the detailed examination of these signatures in conjunction with simultaneous measurements of precipitating electrons and a magnetic field model. A model is developed which indicates that the source for these particles was located in or near the magnetopause boundary layer, with the position deduced to be in the midlatitude flank region about 20–30 RE tailward of the Earth. The model explains the existence of multiple injections on a given field line as due to a quasi‐periodic source, with the periodicity being about 100–200 s at the source. Several mechanisms are examined in an attempt to explain the injections, with a mechanism related to the propagation of waves on the surface of the boundary layer found to be the most plausible. The observations and results are compared to those of similar experiments and some unifying ideas are discussed.
Magnetic flux rope versus the Spheromak as models for interplanetary magnetic clouds
Authors: Farrugia C. J. , Osherovich V. A. , Burlaga L. F.
Published: February 2004

Magnetic clouds form a subset of interplanetary ejecta with well‐defined magnetic and thermodynamic properties. Observationally, it is well established that magnetic clouds expand as they propagate antisunward. The aim of this paper is to compare and contrast two models which have been proposed for the global magnetic field line topology of magnetic clouds: a magnetic flux tube geometry, on the one hand, and a spheromak geometry (including possible higher multiples), on the other. Traditionally, the magnetic structure of magnetic clouds has been modeled by force‐free configurations. In a first step, we therefore analyze the ability of static force‐free models to account for the asymmetries observed in the magnetic field profiles of magnetic clouds. For a cylindrical flux tube the magnetic field remains symmetric about closest approach to the magnetic axis on all spacecraft orbits intersecting it, whereas in a spheromak geometry one can have asymmetries in the magnetic field signatures along some spacecraft trajectories. The duration of typical magnetic cloud encounters at 1 AU (1 to 2 days) is comparable to their travel time from the Sun to 1 AU and thus magnetic clouds should be treated as strongly nonstationary objects. In a second step, therefore, we abandon the static approach and model magnetic clouds as self‐similarly evolving MHD configurations. In our theory, the interaction of the expanding magnetic cloud with the ambient plasma is taken into account by a drag force proportional to the density and the velocity of expansion. Solving rigorously the full set of MHD equations, we demonstrate that the asymmetry in the magnetic signature may arise solely as a result of expansion. Using asymptotic solutions of the MHD equations, we least squares fit both theoretical models to interplanetary data. We find that while the central part of the magnetic cloud is adequately described by both models, the “edges” of the cloud data are modeled better by the magnetic flux tube. Further comparisons of the two models necessarily involve thermodynamic properties, since real magnetic configurations are never exactly force‐free and gas pressure plays an essential role. We consider a poly tropic gas. Our theoretical analysis shows that the self‐similar expansion of a magnetic flux tube requires the poly tropic index γ to be less than unity. For the spheromak, however, self‐similar, radially expanding solutions are known only for γ equal to 4/3. This difference, therefore, yields a good way of distinguishing between the two geometries. It has been shown recently (Osherovich et al., 1993a) that the polytropic relationship is applicable to magnetic clouds and that the corresponding polytropic index is ∼0.5. This observational result is consistent with the self‐similar model of the magnetic flux rope but is in conflict with the self‐similar spheromak model.
The double oval UV auroral distribution: 1. Implications for the mapping of auroral arcs
Authors: Elphinstone R. D. , Murphree J. S. , Hearn D. J. , Cogger L. L. , Sandahl I. , Newell P. T. , Klumpar D. M. , Ohtani S. , Sauvaud J. A. , Potemra T. A. , Mursula K. , Wright A. , Shapshak M.
Published: February 2004

During the later stages of the auroral substorm the luminosity distribution frequently resembles a double oval, one oval lying poleward of the normal or main UV auroral oval. We interpret the double oval morphology as being due to the plasma sheet boundary layer becoming active in the later stages of the substorm process. If the disturbance engulfs the nightside low‐latitude boundary layers, then the double oval configuration extends into the dayside ionospheric region. The main UV oval is associated with the inner portion of the central plasma sheet and can rapidly change its auroral character from being diffuse to discrete. This transition is associated with the substorm process and is fundamental to understanding the near‐Earth character of substorm onset. On the other hand, the poleward arc system in the nightside ionosphere occurs adjacent to or near the open‐closed field line boundary. This system activates at the end of the optical expansion phase and is a part of the recovery phase configuration in substorms where it occurs. These two source regions for nightside discrete auroral arcs are important in resolving the controversy concerning the mapping of arcs to the magnetosphere. The dayside extension of this double oval configuration is also investigated and shows particle signatures which differ considerably from those on the nightside giving clues to the magnetospheric source regions of the aurora in the two local time sectors. Near‐Earth substorm onsets are shown to be coupled to processes occurring much further tailward and indicate the importance of understanding the temporal development of features within the double oval. Using “variance images,” a new technique for the investigation of these dynamics is outlined.
Neutron measurements in near‐Earth orbit with COMPTEL
Authors: Morris D. J. , Aarts H. , Bennett K. , Lockwood J. A. , McConnell M. L. , Ryan J. M. , Schönfelder V. , Steinle H. , Peng X.
Published: February 2004

The fast neutron flux in near‐Earth orbit has been measured with the COMPTEL instrument on the Compton Gamma Ray Observatory (CGRO). For this measurement one of COMPTEL's seven liquid scintillator modules was used as an uncollimated neutron detector with threshold of 12.8 MeV. The measurements cover a range of 4.8 to 15.5 GV in vertical cutoff rigidity and 3° to 177° in spacecraft geocenter zenith angle. One of the measurements occurred near the minimum of the deepest Forbush decrease ever observed by ground‐level neutron monitors. After correction for solar modulation, the total flux is well fitted by separable functions in rigidity and zenith angle. With the spacecraft pointed near the nadir the flux is consistent with balloon measurements of the atmospheric neutron albedo. The flux varies by about a factor of 4 between the extremes of rigidity and a factor of 2 between the extremes of zenith angle. The effect of the spacecraft mass in shielding the detector from the atmospheric neutron albedo is much more important than its role as a source of additional secondary neutrons. The neutron spectral hardness varies little with rigidity or zenith angle and lies in the range spanned by earlier atmospheric neutron albedo measurements.
An explanation for auroral structure and the triggering of auroral kilometric radiation
Authors: Calvert W.
Published: February 2004

Auroral electron precipitation can be attributed energetic electrons which have been scattered into the loss cone by the cyclotron maser instability, whereupon the structure of the diffuse aurora can then be attributed to the enhanced scattering which should then occur in density depletions as a result of the increased wave gain of the cyclotron maser instability at lower densities. The resulting precipitation, moreover, then presumably enhances the density depletions in which it occurs, thus causing arc‐shaped density depletions as a result of particle drifts which then account for the formation of diffuse auroral arcs. Discrete arcs, on the other hand, have previously been attributed to radio lasing caused by wave feedback within such depletions, and the onset of discrete arcs at substorm onset can then be attributed to the onset of radio lasing within the arc‐shaped density depletions caused by the diffuse aurora prior to substorm expansion. Substorm onset should therefore be delayed depending upon the amplitude of the incoming waves being amplified by the cyclotron maser instability to produce the diffuse aurora, thus accounting for the observed triggering of auroral kilometric radiation (AKR) by external waves and also suggesting that the similar triggering of adjacent discrete arcs by the resulting AKR then accounts for the subsequent latitudinal expansion of the aurora during a substorm.
Interplanetary magnetic field control of magnetotail field: IMP 8 data and MHD model compared
Authors: Kaymaz Zerefsan , Siscoe George , Luhmann Janet G. , Fedder Joel A. , Lyon John G.
Published: February 2004

Magnetic field patterns in the magnetotail's cross‐sectional plane at ∼30 RE derived from IMP 8 data are compared with corresponding patterns derived from the Fedder‐Lyon MHD model of magnetosphere‐solar wind interaction. The comparisons emphasize features attributable to the influence of the interplanetary magnetic field (IMF). They reveal considerable correspondences in field asymmetries, nonuniform field perturbations, and current sheet twisting. Both data‐based and MHD‐based patterns are qualitatively similar to patterns obtained by superposing a uniform field on a dipole field, but they show that the field perturbations are stronger at the equatorial flanks of the tail than in the high‐latitude lobes. These quantitative details go beyond the superposition model. From the point of view of interpretation, the fact that the MHD‐based patterns reproduce the distinctive nonsuperposition features seen in the data‐based patterns indicates that these features result from MHD motions within the magnetosphere distributing the field that is merged at the magnetopause (i.e., the “penetrating” field). From the point of view of modeling, the agreement indicates that MHD modeling captures essential aspects of the solar wind‐magnetosphere coupling.
Particle dynamics in a spatially varying electric field
Authors: Rothwell Paul L. , Silevitch Michael B. , Block Lars P. , Fälthammar Carl‐Gunne
Published: February 2004

For an MHD description of a plasma a distinct separation between the macroscopic and microscopic spatial and temporal scales is assumed. In this paper we solve the particle dynamics with finite first and second spatial derivatives in the electric field. We find that (1) MHD (ideal and nonideal) becomes invalid for a sufficiently strong constant electric field gradient perpendicular to the magnetic field; (2) a sufficiently large second derivative in the electric field can cause heavy ions to become chaotically untrapped; (3) for an electric field with a constant gradient the ion drift velocity is equal to (E×B)/|B|2 as long as the orbit‐averaged value of E is used. There are no finite currents associated with the ion drift for such an electric field; (4) perturbation technique gives a poor approximation to the ion drift velocity even for values of the second derivative that may well occur in the magnetosphere. Results 1 and 2 provide necessary criteria for the applicability of magnetospheric MHD models of spatially varying electric fields. They also predict an asymmetry in the heavy ion fluxes, a feature that could be useful in inferring magnetospheric electric field structure. We illustrate these results by application to the Harang discontinuity. It is found that if the interplanetary magnetic field swings northward under substorm growth conditions the orbits of the equatorial O+ may dramatically change due to result 2. This effect may contribute to the substorm onset process.
Microwave proxies for sunspot blocking and total irradiance
Authors: Schmahl E. J. , Kundu M. R.
Published: February 2004

The microwave flux of the Sun is responsive to the same conditions that produce magnetically structured radiation at visible and X ray wavelengths, and so the solar flux at high radio frequencies such as 2800 MHz (10.7 cm) has been used as a proxy for solar optical variations. We have previously found that the the microwave flux time series show spectral variations that provide useful proxy information for total irradiance, and we have extended our analysis of the daily solar fluxes from Toyokawa Observatory at 1000, 2000, 3750, and 9400 MHz, in addition to the Ottawa 2800‐MHz flux, for the years 1980‐1989. An essential ingredient in our analysis is the extraction of the rotationally‐modulated microwave component, which differs from the “S component” as recently defined in the literature. The rotationally‐modulated fraction of the emission contains a significant, often dominant, contribution from gyroresonance emission, whereas the S component, as defined by the excess above the cycle minimum level, usually does not. This allows us to distinguish plage‐associated emission from spot‐associated emission in the time series of microwave flux. We show that in combination, the microwave fluxes for 1000–9400 MHz, which span the spectral peak of spot‐associated emission, provide a very good proxy for both the active cavity radiometer irradiance monitor (ACRIM) total irradiance and the sunspot‐blocked component of the irradiance, even without optical sunspot observations. Over the 1984–1989 period, this proxy has a weighted correlation with ACRIM of 95%, and an RMS deviation from the total irradiance of 0.27 w/m², slightly better than the deviation (0.35 w/m²) found using optical data.
Solar wind eddies and the heliospheric current sheet
Authors: Suess S. T. , McComas D. J. , Bame S. J. , Goldstein B. E.
Published: February 2004

Ulysses has collected data between 1 and 5 AU during, and just following solar maximum, when the heliospheric current sheet (HCS) can be thought of as reaching its maximum tilt and being subject to the maximum amount of turbulence in the solar wind. The Ulysses solar wind plasma instrument measures the vector velocity and can be used to estimate the flow speed and direction in turbulent “eddies” in the solar wind that are a fraction of an astronomical unit in size and last (have either a turnover or dynamical interaction time of) several hours to more than a day. Here, in a simple exercise, these solar wind eddies at the HCS are characterized using Ulysses data. This character is then used to define a model flow field with eddies that is imposed on an ideal HCS to estimate how the HCS will be deformed by the flow. This model inherently results in the complexity of the HCS increasing with heliocentric distance, but the result is a measure of the degree to which the observed change in complexity is a measure of the importance of solar wind flows in deforming the HCS. By comparison with randomly selected intervals not located on the HCS, it appears that eddies on the HCS are similar to those elsewhere at this time during the solar cycle, as is the resultant deformation of the interplanetary magnetic field (IMF). The IMF deformation is analogous to what is often termed the “random walk” of interplanetary magnetic field lines.
Strongly enhanced incoherent‐scatter plasma lines in aurora
Authors: Kirkwood Sheila , Nilsson Hans , Lilensten Jean , Galand Marina
Published: February 2004

Observations of incoherent‐scatter plasma‐line intensities, measured in the E‐region with the European Incoherent Scatter UHF radar, during auroral precipitation, are presented. Intensities up to 200 times the thermal level were observed in a restricted frequency interval from 5.5 to 6.5 MHz. Intensities were lower at both higher and lower frequencies. The intensities are compared with quantitative estimates based on model suprathermal electron fluxes for the prevailing conditions. The minimum in suprathermal electron fluxes between 2 and 4 eV, which is caused by the excitation of vibrational levels in N2, is found to result in a minimum in expected plasma‐line damping and a maximum in intensity between 5.5 and 6.5 MHz, for the EISCAT UHF wavelength. Good agreement is found between the calculated and observed intensities.
Magnetosphere‐ionosphere‐thermosphere coupling: Effect of neutral winds on energy transfer and field‐aligned current
Authors: Lu G. , Richmond A. D. , Emery B. A. , Roble R. G.
Published: February 2004

The assimilative mapping of ionospheric electrodynamics (AMIE) algorithm has been applied to derive the realistic time‐dependent large‐scale global distributions of the ionospheric convection and particle precipitation during a recent Geospace Environment Modeling (GEM) campaign period: March 28‐29, 1992. The AMIE outputs are then used as the inputs of the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model to estimate the electrodynamic quantities in the ionosphere and thermosphere. It is found that the magnetospheric electromagnetic energy dissipated in the high‐latitude ionosphere is mainly converted into Joule heating, with only a small fraction (6%) going to acceleration of thermospheric neutral winds. Our study also reveals that the thermospheric winds can have significant influence on the ionospheric electrodynamics. On the average for these 2 days, the neutral winds have approximately a 28% negative effect on Joule heating and approximately a 27% negative effect on field‐aligned currents. The field‐aligned currents driven by the neutral wind flow in the opposite direction to those driven by the plasma convection. On the average, the global electromagnetic energy input is about 4 times larger than the particle energy input.
Average equatorial zonal and vertical ion drifts determined from San Marco D electric field measurements
Authors: Maynard Nelson C. , Aggson Thomas L. , Herrero Federico A. , Liebrecht M. Carmen , Saba Jack L.
Published: February 2004

San Marco D electric field measurements have been averaged in terms of their equivalent ion drift to produce an average pattern of equatorial zonal and vertical ion drifts. Variations with season, solar activity, Kp, lunar phase and longitude have been analyzed. Similarities and some differences from previous Jicamarca, DE 2 and AE‐E results are seen. Confirmation is given of the dominance of the F region dynamo in the 1900‐2100 local time region. The daytime zonal ion drift is larger for high F10.7 values than that for low values. There is little variation between high and low values of Kp. Superrotation is evident in this data set and is larger at equinox compared to solstice. At the June solstice there are significant differences between the average ion drifts in the longitude sector where the geomagnetic equator is north of the geographic equator (Indian sector) and the sector where the geomagnetic equator is south of the geographic equator (Peruvian sector). The daytime upward velocity is larger in the Indian sector than in the Peruvian sector, and it reverses later in the evening in the Indian sector. Daytime westward zonal velocities are larger and the nighttime eastward velocities are smaller in the Indian sector. A presunrise enhancement is seen in the downward velocity in the Indian sector but not in the Peruvian sector. Significant variations are also seen with the phase of the moon. In light of current theory, the lunar variations suggest a complex interaction of E and F region dynamo sources with conductivity, changing in phase and character with latitude.
Quasi‐static evolution of the magnetosphere: The substorm growth phase
Authors: Kropotkin A. P. , Lui A. T. Y.
Published: February 2004

The growth phase of a substorm is marked by “energy loading” in the magnetosphere prior to energy dissipation in the substorm expansion phase. This loading takes place primarily in the geomagnetic tail and is identified by the growth of total magnetic flux in the tail lobes, the increase of magnetic field intensity in the near‐Earth portion of the tail, and the field line stretching leading to thinning of the plasma sheet in that region where a “neck” is thus formed. The last two processes are shown here to be natural consequences of the magnetic flux accumulation by considering theoretically a quasi‐static evolution of tail equilibrium. One of the model inferences in the growth phase is a strong diminishment of the dimension of the transition region in the nightside where field lines change from dipolar to tail‐like. This accounts for the growth phase development often seen in the near‐synchronous region. This study also reveals a qualitative difference between the growth phase and the “quiet” condition or the “ground state”: the tail lobe flux should exceed a certain threshold for the neck formation to occur and the size of the transition region to diminish.
Hot ions in Jupiter's magnetodisc: A model for Voyager 2 low‐energy charged particle measurements
Authors: Kane M. , Mauk B. H. , Keath E. P. , Krimigis S. M.
Published: February 2004

The Low‐Energy Charged Particle (LECP) instrument on the Voyager 2 spacecraft acquired a comprehensive set of directional and energy‐dependent information on the nature of hot ions in the Jovian magnetodisc. The LECP measurements in the energy range 30 keV to 5 MeV, where the ion pressure dominates the total plasma pressure, have been successfully fit to a two‐species convected k distribution function model for hot ions in the Jovian magnetodisc in the vicinity of neutral sheet crossings. The regions where the model could be used ranged from 60 to 30 RJ on the dayside (inbound) and 75 to 125 RJ on the nightside (outbound). With this model, the full angular and spectral information from the lowest‐energy LECP detectors has been deconvolved using a nonlinear least squares technique to reveal the heavy ion pressure, density, and temperature distinct from the corresponding hot proton parameters. The pressure is dominated by heavy ions in the outer magnetosphere. The temperature of protons remains nearly constant at 20 keV (dayside) and 10 keV (nightside), whereas the heavy ion temperature shows a distinct increase with radial distance paralleling the corotation or pickup energy of heavy ions. A neutral wind of heavy atoms, originating in the near‐Io regions and ionized during their flight through the outer magnetosphere by solar radiation, may be the seed population for the heavy ions measured by the LECP. The convection velocity of the plasma is subcorotational, reduced from the rigid value by a factor of ∼2, but increases with increasing distance from 30 to 60 RJ in the dayside region and from 75 to 85 RJ in the nightside region. The trend stops beyond 85 RJ in the nightside region, but there is still a substantial corotational flow that extends from 85 RJ to at least 130 RJ. In all the regions studied, the particle anisotropies in the LECP scan plane below ∼2 MeV are believed to result primarily from the Compton‐Getting effect and not from gradient anisotropies or particles executing nonadiabatic orbits as they encounter the neutral sheet. Gradient anisotropies are not important even in the distant nightside neutral sheet region (>85 RJ) below ∼2 MeV. The large flow velocities and increasing heavy ion temperatures are consistent with a strong corotational electric field and imply that the mass loading due to lower‐energy heavy ion plasma via outward transport from Io is insufficient to disrupt corotation within ∼60 RJ during the Voyager 2 encounter.
Coupling of magnetospheric cavity modes to field line resonances: A study of resonance widths
Authors: Mann Ian R. , Wright Andrew N. , Cally Paul S.
Published: February 2004

By using a box model for the magnetosphere and by using a matrix eigenvalue method to solve the cold linearized ideal MHD equations, we examine the temporal evolution of the irreversible coupling between fast magnetospheric cavity modes and field line resonances (FLRs). By considering the fast mode frequency to be of the form ωf = ωfr − iωfi, and using a Fourier transform approach, we have determined the full time‐dependent evolution of resonance energy widths. We find that at short times the resonances are broad, and narrower widths continue to develop in time. Ultimately, an asymptotic resonance Alfvén frequency full width at half maximum (FWHM) of ΔωA = 2ωfi develops on a timescale of τfi = ωfi−1. On timescales longer than τfi, we find that the resonance perturbations can continue to develop even finer scales by phase mixing. Thus, at any time, the finest scales within the resonance are governed by the phase mixing length Lph(t) = 2π (tdωA/dx)−1. The combination of these two effects naturally explains the localisation of pulsations in L shells observed in data, and the finer perturbation scales which may exist within them. During their evolution, FLRs may have their finest perturbation scales limited by either ionospheric dissipation or by kinetic effects (including the breakdown of single fluid MHD). For a continually driven resonance, we define an ionospheric limiting timescale τI in terms of the height‐integrated Pedersen conductivity ΣP, and hence derive a limiting ionospheric perturbation scale LI = 2π (τIdωA/dx)−1, in agreement with previous steady state analyses. For sufficiently high ΣP, FLR might be able to evolve so that their radial scales reach a kinetic scale length Lk. For this to occur, we require the pulsations to live for longer than τk = 2π (LkdωA/dx)−1. For t < τk, τI, kinetic effects and ionospheric dissipation are not dominant, and the ideal MHD results presented here may be expected to model realistically the growth phase of ULF pulsations.
Dayside poleward moving auroral forms: A statistical study
Authors: Fasel Gerard J.
Published: February 2004

This paper presents the first comprehensive study of poleward moving auroral forms (PMAFs). PMAFs observed near the dayside auroral oval around geomagnetic noon have been the focus of many previous case studies. These ionospheric signatures are thought to be the product of solar wind‐magnetospheric interactions. This paper examines PMAFs using 12 years of ground‐based optical data from Longyearbyen, Svalbard. Examined are the distribution, lifetime, and time between successive PMAFs around geomagnetic noon. Changes in solar wind parameters are studied to see how they influence PMAF occurrence. The important results found from this study are (1) 15% of the PMAFs observed are PMAF1 events, 84% are PMAF2 events, and 1% are PMAF3 events, (2) PMAF1 events appear to favor slower solar wind speeds; higher solar wind speeds may lead to an increase of PMAF2 events, (3) PMAFs have a mean life time of 5 min, (4) the mean time between successive PMAF events is 6 min, (5) the PMAFs occur mostly for IMF Bz<0 conditions (117 PMAF events) but can also occur for steady IMF Bz>0 conditions (PMAF 41 events); another 31 PMAF events had an IMF Bz component, that when averaged was positive, (6) the distributions of successive PMAF events and that of inter‐FTE intervals are not different within a 95% confidence level, (7) there is a significant decrease in PMAF activity at geomagnetic noon, and (8) PMAFs are not likely to be caused by dynamic pressure variations in the solar wind.
New sources for the hot oxygen geocorona: Solar cycle, seasonal, latitudinal, and diurnal variations
Authors: Hickey M. P. , Richards P. G. , Torr D. G.
Published: February 2004

This paper demonstrates the variability of thermospheric sources of hot oxygen atoms. Numerical calculations were performed for day and night, high and low solar activity, summer and winter, and low‐ and middle‐latitude conditions. Under most conditions, reactions involving metastable species are more important hot O sources than previously considered dissociative recombination of O2+ and NO+. All the hot O sources are an order of magnitude lower at midnight than at noon. At night, dissociative recombination of O2+ and NO+ are the most important sources. Quenching of vibrationally excited N2 (N2*) by O is the most important metastable source at night. Above 300 km, hot O sources increase by an order of magnitude between solar minimum and solar maximum. For a given level of solar activity, the high‐altitude total production rate of hot O kinetic energy is greater during winter than during summer, indicating a dominance of cooler hot O sources during summer. The N2* source dominates at low altitudes. At high altitudes it is almost negligible at solar minimum, but increases to become the dominant source at solar maximum. Atomic oxygen quenching of N(²D) is a large source at solar minimum and is still important at solar maximum. Overall, seasonal variations are small compared to solar cycle, diurnal and latitudinal variations. While quenching of metastable species is more important at midlatitudes than at low latitudes, there is little latitudinal variation in hot O production due to dissociative recombination of NO+ and O2+.
Excitation of electron acoustic waves by a gyrating electron beam
Authors: Sotnikov V. I. , Schriver D. , Ashour‐Abdalla M. , Ernstmeyer J. , Myers N.
Published: February 2004

The excitation of electron acoustic waves by a gyrating electron beam in a plasma with cold and hot electron components has been examined. Different instabilities are possible due to Cerenkov, normal, and anomalous Doppler resonances. The Cerenkov and anomalous Doppler resonances lead to the excitation of intermediate‐ and short‐wavelength parts of the electron cyclotron sound dispersion branch with frequencies well above the electron cyclotron frequency but below the electron plasma frequency, i.e., Ωe ≪ ω < ωpe. The instability due to the normal Doppler resonance leads to the excitation of intermediate‐ and short‐wavelength parts of this branch as well. In addition, due to the normal Doppler resonance, waves with much larger wavelengths and frequencies ω ≳ Ωe can also be excited. These results are applied to active beam injection in the Earth's low‐altitude ionosphere during the CHARGE 2B rocket experiment, but also have applications to other regions of the magnetosphere where waves are observed in the frequency range between the electron gyrofrequency and electron plasma frequency.
Magnetic fluctuations associated with tail current disruption: Fractal analysis
Authors: Ohtani S. , Higuchi T. , Lui A. T. Y. , Takahashi K.
Published: February 2004

The objective of the present study is to assess the mechanism of substorm‐associated tail current disruption on the basis of magnetic field observations in the near‐Earth tail. We examined 15 events observed by the Charge Composition Explorer (CCE) of the Active Magnetospheric Particle Tracer Explorers (AMPTE), with an emphasis on the August 28, 1986, event. In these events the satellite observed magnetic fluctuations to start almost simultaneously with ground substorm onsets, strongly suggesting that these fluctuations are related to the trigger of substorms. In this study we applied the new method, fractal analysis, to these fluctuations. This method enables us to examine fluctuations quantitatively and to pick up characteristic timescale(s) of fluctuations, even if fluctuations are far from sinusoidal. The results are summarized as follows: (1) Whereas before the onset of tail current disruption, magnetic fluctuations are suppressed in each of the magnetic components, after the onset, the magnitude of the H (north‐south) component fluctuations is about 30% larger than the magnitudes of the fluctuations of the other components. (2) The magnetic fluctuations have a characteristic timescale, which is several times the proton gyroperiod. The first result suggests that observed magnetic fluctuations are actually related to changes in the tail current intensity, that is, tail current disruption. This result also indicates that the microprocess of tail current disruption should be described in terms of turbulent perturbation electric currents, although away from the onset region the effects of tail current disruption may be approximated by those of an orderly decrease in the tail current intensity. The second result strongly suggests that tail current disruption is driven by a certain instability, which grows most rapidly around that characteristic time scale, and in which ions should play an important role.
Velocity‐space distributions of wave‐accelerated auroral electrons
Authors: Bryant D. A. , Perry C. H.
Published: February 2004

The possibility that auroral electrons might be accelerated by electrostatic wave turbulence is further explored. Electron velocity distributions resulting from lower hybrid wave turbulence are modeled as functions of velocities parallel and perpendicular to the magnetic vector. The model, which is highly stylized with focus on the underlying physics, is found to account readily for the wide range of experimentally observed features such as “beams,” “conics,” and “widened loss cones.” Consideration of the energetics of the process leads to the suggestion that the acceleration may be essentially a plasma boundary phenomenon.
The magnetotail: A rough and leaky plasma waveguide for terrestrial radio waves
Authors: Steinberg J.‐L. , Hoang S.
Published: February 2004

A simulation of radio wave propagation is used to explain deep tail observations of the circumterrestrial radio sources. The large angular diameter of these sources and their large angular distance to Earth cannot be explained by simple reflections on the magnetopause. Waves must enter the inhomogeneous magnetosheath and this requires corrugations of the magnetopause. The angular diameter observations can be interpreted quantitatively. Some variations of the source apparent direction can also be interpreted as due to the rotation of a broad beam or to a change in the relatives intensities of two primary near‐Earth sources, but some others might require distorsions of the three‐dimensional shape of the tail or motions of that tail with respect to the observer.
MHD simulations of Earth's bow shock at low Mach numbers: Standoff distances
Authors: Cairns Iver H. , Lyon J. G.
Published: February 2004

Global, three‐dimensional, ideal MHD simulations of Earth's bow shock are reported for low Alfven Mach numbers MA and quasi‐perpendicular magnetic field orientations. The simulations use a hard, infinitely conducting magnetopause obstacle, with axisymmetric three‐dimensional location given by a scaled standard model, to directly address previous gasdynamic (GD) and field‐aligned MHD (FA‐MHD) work. Tests of the simulated shocks’ density jumps X for 1.4 ≲ MA ≲ 10 and the high MA shock location, and reproduction of the GD relation between magnetosheath thickness and X for quasi‐gasdynamic MHD runs with MA ≫ MS, confirm that the MHD code is working correctly. The MHD simulations show the standoff distance as increasing monotonically with decreasing MA. Significantly larger as are found at low MA than predicted by GD and phenomenological MHD models and FA‐MHD simulations, as required qualitatively by observations. The GD and FA‐MHD predictions err qualitatively, predicting either constant or decreasing as with decreasing MA. This qualitative difference between quasi‐perpendicular MHD and FA‐MHD simulations is direct evidence for as depending on the magnetic field orientation θ. The enhancement factor over the phenomenological MHD predictions at MA ∼ 2.4 agrees quantitatively with one observational estimate. A linear relationship is found between the magnetosheath thickness and X, modified both quantitatively and intrinsically by MHD effects from the GD result. The MHD and GD results agree in the high MA limit. An MHD theory is developed for as, restricted to sufficiently perpendicular θ and high sonic Mach numbers MS. It explains the simulation results with excellent accuracy. Observational and further simulation testing of this MHD theory, and of its predicted MA, θ, and MS effects, is desirable.
Generation of whistler emissions by injection of hot electrons in the presence of a perpendicular Ac electric field
Authors: Misra K. D. , Pandey R. S.
Published: February 2004

The dispersion for the whistler mode instability in an infinite magnetoplasma in the presence of a perpendicular electric field for a loss cone type distribution function having an anisotropic temperature is obtained. The method of characteristics solutions using the perturbed and unperturbed particle trajectories have been used to determine the perturbed distribution function. Growth rates for various strengths of the loss cone index have been determined. The effect of injection of hot energetic particles of solar wind origin having a positive slope and a loss cone index j=1 has been evaluated. The ac frequency, rather than its magnitude, affects the growth rate and shifts the maxima to a higher
or frequency range, modifying the resonance conditions of charged particles. The injection of a higher number of energetic hot electrons affects the growth rate significantly, decreasing the maxima by an order of magnitude and shifting them to a lower frequency or
ranges in contrast to cold plasma injection results. This result is of importance in explaining the observed VLF emissions over a wide frequency range.
An equinoctial asymmetry in the high‐latitude thermosphere and ionosphere
Authors: Aruliah A. L. , Farmer A. D. , Fuller‐Rowell T. J. , Wild M. N. , Hapgood M. , Rees D.
Published: February 2004

A large equinoctial asymmetry has been observed in thermospheric winds and ion velocities at high latitude sites in northern Scandinavia. Throughout the solar cycle, average nighttime thermospheric meridional winds are larger in spring than autumn despite similar levels of solar insolation. The average ion velocities are also larger in spring than autumn at solar maximum, but at solar minimum this position is reversed. Numerical simulations of the thermosphere and ionosphere have not predicted such asymmetries because they generally assume forcing functions that are symmetric about the solstices. The proposed explanation lies in the annual and diurnal variation in solar wind‐magnetosphere coupling caused by changes in the orientation of the geomagnetic pole, and hence the magnetosphere, with respect to the average orientation of the IMF (the Russell‐McPherron effect). This causes a 12‐hour phase difference between the times of maximum solar wind‐magnetosphere coupling at the two equinoxes. In addition, the orientation of the geomagnetic axis with respect to the average IMF is such that  > 0 for the March equinox and By*Bz> < 0 for September. This results in a further source of asymmetry of forcing of the high‐latitude ionosphere as the result of electric fields associated with the four sign combinations of By and Bz. Several predictions arise from the explanation given: for example, a high‐latitude station measuring thermospheric neutral winds in Alaska, 180° in longitude from Kiruna, might be expected to see nighttime thermospheric winds that are larger in the autumn than in the spring.
Effect of polar cap patches on the polar thermosphere
Authors: Ma Ti‐Ze , Schunk R. W.
Published: February 2004

Polar cap patches are regions of enhanced ionization that appear when the interplanetary magnetic field is southward. They are created either in the dayside cusp or equatorward of the cusp in the sunlit hemisphere. Once formed, they convect in an antisunward direction across the dark polar cap at speeds of 100 m/s to about 2 km/s. The size of a plasma patch varies from about 100 to 1000 km, and its density relative to the background density varies from a few tens of percent to a factor of 100. Because propagating plasma patches might have a significant effect on the neutral atmosphere, a global thermospheric circulation model was used to calculate the response of the polar thermosphere to a “representative” plasma patch. The model predicts that a localized thermospheric disturbance is induced by and moves along with the propagating plasma patch. The moving disturbance is characterized by a neutral density depletion, an enhanced wind speed, an elevated neutral temperature, neutral gas up welling, and O/N2 composition changes. The thermospheric disturbance persists for a time that is much longer than the patch lifetime, and it spreads out over a region that is much larger than the patch dimensions as it dissipates.
Ulysses observation of a noncoronal mass ejection flux rope: Evidence of interplanetary magnetic reconnection
Authors: Moldwin M. B. , Phillips J. L. , Gosling J. T. , Scime E. E. , McComas D. J. , Bame S. J. , Balogh A. , Forsyth R. J.
Published: February 2004

A well‐defined, small‐scale (≈0.05 AU) magnetic flux rope was observed by Ulysses at about 5 AU in close proximity to a heat flux dropout (HFD) at the heliospheric current sheet (HCS). This magnetic flux rope is characterized by a rotation of the field in the plane approximately perpendicular to the ecliptic (and containing the Sun and the spacecraft) and with a magnetic field maximum centered near the inflection point of the bipolar signature. The edges of the flux rope are well defined by diamagnetic field minima. A bidirectional electron heat flux signature is coincident with the magnetic flux rope structure. The event occurred during a time of slightly increasing solar wind speed, suggesting that the field and plasma were locally compressed. Unlike most coronal mass ejections/magnetic clouds, this event is characterized by high proton temperatures and densities, high plasma beta, no significant alpha particle abundance increase, and a small radial size. We interpret these observations in terms of multiple magnetic reconnection of previously open field lines in interplanetary space at the HCS. Such reconnection produces a U‐shaped structure entirely disconnected from the Sun (which we associate with the HFD), a closed magnetic flux rope (which we associate with the counterstreaming electron event), and a closed magnetic loop or tongue connected back to the Sun at both ends. These observations suggest that magnetic reconnection, and its changes to magnetic field topology, can occur well beyond the solar corona in interplanetary space.
Early rocket observations of auroral Bremsstrahlung and its absorption in the mesosphere
Authors: Van Allen James A.
Published: February 2004

In the summer and autumn of 1957 the author and his colleagues at the University of Iowa conducted 10 successful balloon‐launched rocket (rockoon) flights of Geiger‐Mueller tubes during two shipboard expeditions, one to the arctic and the other to the antarctic. Summit altitudes ranged from 77 to 130 km. One flight was conducted in the equatorial zone, six in the northern auroral zone, and three in the southern auroral zone. Detailed results of each flight are presented. Auroral bremsstrahlung was detected on eight of the nine high‐latitude flights. By virtue of physical shielding, the detectors were insensitive to photons of energy E < 8 keV. Hence our derived absolute spectra refer to the high‐energy tail of the auroral electron spectrum and the resulting bremsstrahlung. For the energy range E > 8 keV, we find a typical e‐folding energy E* of about 10 keV for a differential photon number spectrum of the form dn/dE = A exp(−E/E*) with A in units of photons (cm² s keV)−1. Essentially, the same value of E* is obtained by two complementary methods: (1) The dependence of counting rate on atmospheric depth Q and (2) the inverse ratio of counting rates of an “unshielded” tube and one with an added shield. A detailed discussion of the bases for interpretation of the flight data is given in an extended appendix. For a representative set of data (flight 64) we find an omnidirectional flux of downward moving photons of 2.4×104 (cm² s)−1 at Q = 0, the integral of the above spectrum from E = 0 to E = ∞ with the explicit understanding that this result does not include the presumably much greater flux of photons having E < 8 keV and a much steeper spectrum. The corresponding electron number flux striking the top of the atmosphere is 9.3×107 electrons (cm² s)−1 with an e‐folding energy T* = 30 keV. The corresponding energy flux is 4.5 erg (cm² s)−1. Both of the two latter fluxes represent the integral from electron energy T = 0 to T = ∞ of the high‐energy tail of the electron spectrum, again with the explicit understanding that the estimates do not include the steeply rising spectrum for T ≲ 15 keV.