bluuebunny/crossref_metadata_2025 huggingface dataset:
Broadband ELF plasma emission during auroral energization: 1. Slow ion acoustic waves
Authors: Wahlund J.‐E. , Eriksson A. I. , Holback B. , Boehm M. H. , Bonnell J. , Kintner P. M. , Seyler C. E. , Clemmons J. H. , Eliasson L. , Knudsen D. J. , Norqvist P. , Zanetti L. J.
Published: February 2004

High‐resolution measurements by the Freja spacecraft of broadband extremely low frequency (BB‐ELF) emission from dc up to the lower hybrid frequency (a few kHz) are reported from regions of transverse ion acceleration (TAI) and broad‐energy suprathermal electron bursts (STEB) occuring in the topside ionospheric auroral regions. A gradual transition of the broadband emission occurs near the local O+ cyclotron frequency (ƒO+ ≈ 25 Hz) from predominantly electromagnetic below this frequency to mostly electrostatic above this frequency. The emission below 200 Hz often reach amplitudes up to several hundred mV/m and density perturbations (δn/n) of tens of %. An improved analysis technique is presented, based on the quantity |δE/(δn/n)| versus frequency and applied to the Freja plasma wave measurements. The method can be used to infer the dispersion relation for the measured emission as well as give estimates of the thermal plasma temperatures. The BB‐ELF emission is found to consist partly of plasma waves with an ion Boltzmann response, which is interpreted as originating from the so‐called slow ion acoustic wave mode (SIA). This emission is associated with large bulk ion (O+) temperatures of up to 30 eV and low electron temperatures (1–2 eV) and therefore occurs during conditions when Te/Ti ≪ 1. The BB‐ELF emissions also contain other wave mode components, which are not equally easy to identify, even though it is reasonably certain that ion acoustic/cyclotron waves are measured. The ion Boltzmann component is characterized by a dominantly perpendicular polarization with respect to the Earth's magnetic field direction and a small magnetic component with amplitudes around 0.1–1 nT. The ion Boltzmann component dominates the lower‐frequency part (30–400 Hz) of the BB‐ELF emissions. The BB‐ELF emission have often an enhanced spectral power when certain waveform signatures, interpreted as solitary kinetic Alfvén waves (SKAW), or when large‐amplitude electric fields, possibly related to black aurora, are encountered in regions often associated with large‐scale auroral density depletions. A scenario where the SKAW provides the original free energy and via the BB‐ELF emission causes intense transverse ion heating (TAI) is suggested.
Bernstein‐Greene‐Kruskal analysis of electrostatic solitary waves observed with Geotail
Authors: Krasovsky V. L. , Matsumoto H. , Omura Y.
Published: February 2004

Electrostatic solitary waves (ESW) relevant to the high‐frequency component of broadband electrostatic noise spectra observed by Geotail spacecraft in the magnetotail are analyzed. A general description of the ESW, as stationary solutions of Vlasov‐Poisson equations, is developed on the basis of the Bernstein‐Greene‐Kruskal (BGK) technique. It is shown that the structure and relationships among basic parameters of a localized perturbation are primarily determined by resonant and nonresonant plasma screenings of bunched electrons trapped by a potential pulse moving in a plasma. The dependence of the typical width of the pulse on its drift velocity and pulse amplitude is found, which can be treated as a nonlinear dispersion relation for the localized perturbation. The theoretical results are compared with the Geotail data and a particle simulation.
Latitudinal variation of quasi‐biennial oscillation modulation of the semidiurnal tide in the lower thermosphere
Authors: Jarvis M. J.
Published: February 2004

Perturbations in the geomagnetic field at the Earth's surface can result from variations in the neutral wind field in the lower thermosphere through dynamo action in the interfused ionospheric E region. Thus spectral analysis of the Sq variations in ground‐based geomagnetic data extending continuously over many years, and from several sites worldwide, has been carried out to investigate the variation with latitude of quasi‐biennial oscillation (QBO) effects in the semidiurnal tide in the lower thermosphere. This QBO modulation of the tide appears to be greatest around ∼60° latitude. The phase of the oscillation is largely latitude independent and implies QBO modulation of the upward propagating semidiurnal tide close to its source region. The semidiurnal tide appears to propagate upward more easily during the easterly phase of the tropical stratospheric QBO at 30 mbar than during the westerly phase.
Global modeling of the plasmasphere following storm sudden commencements
Authors: Lambour R. L. , Weiss L. A. , Elphic R. C. , Thomsen M. F.
Published: February 2004

We examine the dynamics of the outer plasmasphere during 10 post‐ssc events by comparing observations of cold, dense ions from Los Alamos magnetospheric plasma analyzers on board three widely spaced geosynchronous satellites with output from the Magnetospheric Specification and Forecast Model (MSFM). The MSFM is a data‐driven, operational space weather specification and forecast code originally designed to facilitate U.S. Air Force spacecraft operations. For this study we modified the MSFM to include a cold plasmaspheric ion population that was subject to the effects of ionospheric refilling. We utilized the electron density model of Carpenter and Anderson [1992] and the assumption of charge neutrality to initialize the plasmaspheric proton density within a specified plasmapause. This configuration was then allowed to evolve under the effects of E × B drift and refilling. The modified MSFM clearly shows the development and westward transport of duskside plasmaspheric plumes/tails during periods of enhanced convection and the eastward transport of these structures during decreasing activity. We present a detailed comparison between the data and the model output for one case and a “statistical” analysis of the comparison for all 10 cases. We also compare the model results with previously published observations of plasmapsheric ions and models of plasmaspheric dynamics. The MSFM was able to systematically reproduce the geosynchronous observations with good accuracy in both local time placement and density level. We find the modified MSFM to be an improvement upon previous plasmaspheric models and a useful tool in the interpretation of spatially and temporally separated geosynchronous observations.
Thermospheric neutral winds at southern mid‐latitudes: A comparison of optical and ionosonde hmF2 methods
Authors: Dyson P. L. , Davies T. P. , Parkinson M. L. , Reeves A. J. , Richards P. G. , Fairchild C. E.
Published: February 2004

During the first 6 days of March 1995, measurements of the ionospheric electron density were made with a digisonde, and thermospheric winds were measured with a Fabry‐Perot interferometer. This was a period of low solar activity and moderate to high magnetic activity. The ionograms have been scaled and the traces inverted to obtain the electron density profile and the peak height of the F2 layer (hmF2). Modeling has been employed to derive equivalent thermospheric neutral winds at hmF2. The derived neutral winds are in very good agreement with the measured optical winds most of the time. The winds follow a strong diurnal pattern with poleward winds during the day, weak winds near dawn and dusk, and strong equatorward winds peaking near local midnight. On most nights the peak equatorward wind speed was around 200 m s−1, but on March 1 it did not exceed 110 m s−1. For these magnetic and solar activity conditions the wind at the F2 peak altitude (∼350 km) from the HWM93 empirical wind model[Hedin et al., 1996] did not exceed 90 m s−1 at any time but was in generally good agreement with the hmF2 wind during the day and with both measured winds on the nights of March 1 and 2. The good agreement between the optical and hmF2 winds was obtained by using the recommended Burnside factor of 1.7 to multiply the O+‐O collision frequency, but better agreement was obtained either by using a Burnside factor of 2.0 or by increasing the atomic oxygen density by 20%. Recent suggestions of much lower Burnside factors could be tolerated only if there were large systematic errors in the measurements or large electric fields.
Heliosphere in the magnetized local interstellar medium: Results of a three‐dimensional MHD simulation
Authors: Linde Timur J. , Gombosi Tamas I. , Roe Philip L. , Powell Kenneth G. , DeZeeuw Darren L.
Published: February 2004

The results of a three‐dimensional adaptive magnetohydrodynamic (MHD) model of the interaction of a magnetized solar wind with a magnetized very local interstellar medium in the presence of neutral interstellar hydrogen are presented. The interplanetary magnetic field is approximated by the Parker spiral, and the direction of the interstellar magnetic field is taken to be arbitrary. It is demonstrated that magnetic field interaction has a very pronounced effect on the structure of the global heliosphere. In particular, it is shown that the interaction of the interstellar wind with the shocked solar wind significantly depends on the direction of the interstellar magnetic field. This effect is mainly manifested in the distances to the heliospheric boundaries and the shape of the heliosphere. Depending on the orientation of the interstellar magnetic field the upstream location of the termination shock is expected to be at 80 ± 10 AU. The termination shock is predicted to be weak in agreement with the available body of observations. It is found that under quiet solar conditions the spiral structure of the interplanetery magnetic field remains imprinted in the solar wind far beyond the termination shock. Numerical simulations indicate that magnetic fields have a stabilizing effect on the heliopause.
Structure and dynamics of magnetic reconnection for substorm onsets with Geotail observations
Authors: Nagai T. , Fujimoto M. , Saito Y. , Machida S. , Terasawa T. , Nakamura R. , Yamamoto T. , Mukai T. , Nishida A. , Kokubun S.
Published: February 2004

Fast tailward ion flows with strongly southward magnetic fields are frequently observed near the neutral sheet in the premidnight sector of the magnetotail at 20–30 RE for substorm onsets in Geotail observations. These fast tailward flows are occasionally accompanied by a few keV electrons. With these events, we study the structure and dynamics of magnetic reconnection. The plasma sheet near the magnetic reconnection site can be divided into three regions: the neutral sheet region (near the neutral sheet with the absolute magnitude of Bx of < 5 nT), the boundary region (near the plasma sheet/tail lobe boundary with the absolute magnitude of Bx is near or > 10 nT), and the off‐equatorial plasma sheet (the rest). In the neutral sheet region, plasmas are transported with strong convection, and accelerated electrons show nearly isotropic distributions. In the off‐equatorial plasma sheet, two ion components coexist: ions being accelerated and heated during convection toward the neutral sheet and ions flowing at a high speed almost along the magnetic field. In this region, highly accelerated electrons are observed. Although electron distributions are basically isotropic, high‐energy (higher than 10 keV) electrons show streaming away from the reconnection site along the magnetic field line. In the boundary region, ions also show two components: ions with convection toward the neutral sheet and field‐aligned ions flowing out of the reconnection region, although acceleration and heating during convection are weak. In the boundary region, high‐energy (10 keV) electrons stream away, while medium‐energy (3 keV) electrons stream into the reconnection site. Magnetic reconnection usually starts in the premidnight sector of the magnetotail between XGSM = −20 RE and XGSM = −30 RE prior to an onset signature identified with Pi 2 pulsation on the ground. Magnetic reconnection proceeds on a timescale of 10 min. After magnetic reconnection ends, adjacent plasmas are transported into the postreconnection site, and plasmas can become stationary even in the expansion phase.
Ground observations and possible source regions of two types of Pc 1‐2 micropulsations at very high latitudes
Authors: Dyrud L. P. , Engebretson M. J. , Posch J. L. , Hughes W. J. , Fukunishi H. , Arnoldy R. L. , Newell P. T. , Horne R. B.
Published: February 2004

We have used 1‐year's data from the recently installed Magnetometer Array for Cusp and Cleft Studies (MACCS) in Arctic Canada and from two stations of the developing “conjugate” array of Automated Geophysical Observatories (AGOs) in Antarctica to study ULF waves in the Pc 1–2 (100–600 mHz) frequency band at cusp and polar cap latitudes (Λ ∼ 74° – 80°). In this paper we focus on the spectral properties and latitudinal and local time distributions of Pc 1–2 events observed during 1994 and use these along with several case studies to infer the source locations of the two major wave types we have observed. We found little variation in center‐band frequency of the Pc 1–2 waves we observed, but the average event bandwidth was distinctly wider at stations near 80° MLAT than at stations near 75° MLAT. Broadband waves, with diffuse spectral character, dominated at the higher latitudes, but their occurrence was confined at most stations to within 4 hours of local magnetic noon. Waves with narrower bandwidth were much more common in our data set, and were the statistically dominant wave type at the lower‐latitude MACCS stations. Their occurrence was also limited to the dayside but extended both later and more widely in local time than the more broadband waves. These multistation observations, combined with data from the DMSP, IMP 8, and Geotail satellites, suggest the possibility that these two wave types originate in quite different regions near the magnetospheric boundary; the more narrowband waves in the subsolar and postnoon equatorial region, and the more broadband waves in the high‐latitude plasma mantle (and possibly at the poleward edge of the cusp). The cusp itself appears to not be a significant source of Pc 1–2 wave activity that can be detected by ground observatories.
Identification of magnetospheric particles that travel between spacecraft and their use to help obtain magnetospheric potential distributions
Authors: Whipple E. C. , Halekas J. S. , Scudder J. D. , Paterson W. R. , Frank L. A. , Sheldon R. B. , Maynard N. C. , Weimer D. R. , Russell C. T. , Tsuruda K. , Hayakawa H. , Yamamoto T.
Published: February 2004

We identify magnetospheric ions from Hydra on Polar and comprehensive plasma instrument on Geotail that have traveled between these spacecraft at three times at half‐hour intervals from 1800 to 1900 UT on August 19, 1996. During this time the activity level was low, the magnetosphere was quiet, and inductive electric fields should have been minimal. The identification is made by first using the magnetic fields measured by Magnetic Field Experiment (Polar) and magnetic field instrument (Geotail), together with the Tsyganenko 1996 magnetic field model to calculate the mapping of pitch angles between spacecraft. We then make a one‐parameter search in energy, and equivalently in the space defined by the first two adiabatic invariants, to find matchings in the ion velocity distribution functions as predicted by Liouville's theorem. We find matchings at these three times, with each matching occurring over the appropriate ranges of pitch angles and at a unique difference in kinetic energy. We interpret the difference in kinetic energy as a measure of the potential difference between the regions of the two spacecraft. These differences are consistent with the potential changes along each spacecraft path during this time from the electric fields measured by electric field instrument (Polar) and electric field detector (Geotail) and from a map of estimated magnetospheric potential distribution for the existing activity levels. Spacecraft positions when plotted in the (U, B, K) coordinate system confirm that ion travel between the spacecraft was possible during this time period. This technique for identifying particles traveling between spacecraft should be a valuable tool for correlating spectral features seen in particle data by different spacecraft in the magnetosphere.
Galileo‐measured depletion of near‐Io hot ring current plasmas since the Voyager epoch
Authors: Mauk B. H. , McEntire R. W. , Williams D. J. , Lagg A. , Roelof E. C. , Krimigis S. M. , Armstrong T. P. , Fritz T. A. , Lanzerotti L. J. , Roederer J. G. , Wilken B.
Published: February 2004

The first mass‐discriminated, hot ion distribution moments (pressure, energy intensity) are determined for hot >50‐keV ions in Jupiter's inner magnetosphere at the outer edge of Io's plasma torus by using the Galileo energetic particle detector (EPD) data. These hot plasmas were significantly depleted during the Galileo encounter in 1995 as compared with the Voyager epoch of 1979. The depletion of the hot ions is apparently caused by enhanced charge exchange losses of hot ions, perhaps associated with enhanced emissions of neutral gases from the volcanoes of Io. Such neutral gas enhancements could simultaneously explain increases, reported elsewhere, in the densities of the cooler Io torus plasmas. The hot plasma changes may explain why radial transport interchange turbulence has been observed by Galileo in the Io torus regions, whereas such turbulence was not apparent during the Voyager encounters in 1979. The hot ion depletion could also play a role in explaining the apparent differences between the Jovian auroral configuration observed in recent years by the Hubble space telescope and ground observers and the configuration observed by Voyager. This possibility is much less certain, however.
Trapped particles in the polar wind
Authors: Demars H. G. , Barakat A. R. , Schunk R. W.
Published: February 2004

The flow of plasma along open field lines at high latitudes is highly variable and depends both on conditions in the underlying ionosphere and thermosphere and on the transport of particles and energy from the magnetosphere. Past attempts to model this time variability have, for the most part, examined the response of the plasma on a stationary field line to certain prespecified boundary conditions and heat sources. While such prespecified conditions may bear some resemblance to what occurs naturally, they are artificial and cannot be expected to yield a truly quantitative understanding of the various physical processes that interact to produce the dynamic polar wind. The present study is one in a series of studies that attempts to eliminate this artificiality by coupling the mathematical description of the polar wind to a three‐dimensional time‐dependent model of the high‐latitude ionosphere. In this study, an individual flux tube of plasma is followed as it moves under the influence of combined corotation and convection electric fields. Boundary conditions at the lower end of the flux tube are obtained from the ionosphere model, which takes into account all significant particle species, chemical reactions, and heat sources that contribute to the state of the ionosphere. A multi‐ion macroscopic particle‐in‐cell code is used to model the plasma in the flux tube. A description of the behavior of H+ and O+ for the altitude range from 2000 km to about 8 Earth radii is obtained as the flux tube moves along the trajectory, which traverses regions of the subauroral ionosphere, dayside and nightside ovals, and polar cap. The goal of the study is to determine the extent to which ion trapping can occur in the polar wind and the effects that collisions, wave‐particle interactions, centrifugal acceleration, and varying ionospheric conditions have on the trapped ions. The main conclusion of the study is that O+ trapping is important and it acts to increase the O+ density at high altitudes.
Data‐derived analogues of the magnetospheric dynamics
Authors: Klimas A. J. , Vassiliadis D. , Baker D. N.
Published: February 2004

A method is presented for transforming a local‐linear prediction model into dynamical analogues of the coupling between the input and output data that enters and forms the prediction model. Two types of analogues are produced: (1) local‐linear analogues composed of readily recognized physical components and (2) nonlinear analogues suitable for prediction purposes; both are data derived. Second‐order analogues of the magnetospheric dynamics are examined in detail using the composite ISEE 3 solar wind parameterVBsfor input and simultaneousDstindex data for output. An optimized local‐linear analogue is constructed which minimizes the rms difference between analogue output and measuredDst. It is shown that the optimized local‐linear analogue reduces, in behavior, to that of the first‐orderBurton et al. [1975]Dstmodel during periods ofDstrecoveries, both large and small. During those periods, however, the analogue effectively decouples from the solar windVBsdriver. At other times the coupling strength is found in general agreement with the value deduced by Burton et al., but, at those times, the analogue behavior is beyond the scope of their first‐order equation. Nonlinear analogues are also constructed and studied. These are driven, damped, harmonic oscillators in which the dissipation rate, restoring force, and coupling strength to theVBsdriver are expressed as functions of theDstandVBsvariables; these expressions are data derived. It is found that these analogues couple to the solar wind through the expression (VBs/Dst)VBs, rather than through the usual linear dependence onVBs. The suitability of these nonlinear analogues for prediction purposes is discussed.
A low‐dimensional dynamical model for the solar wind driven geotail‐ionosphere system
Authors: Horton W. , Doxas I.
Published: February 2004

A six‐dimensional nonlinear dynamics model is derived for the basic energy components of the night‐side magnetotail coupled to the ionosphere by the region 1 currents. In the absence of solar wind driving and ionospheric dissipation the system is a 3 degree of freedom Hamiltonian system. The large ion gyroradius conductance of the quasi‐neutral sheet produces the energization of the central plasma sheet (CPS)5 while the unloading is triggered when the net geotail current or current density exceeds a critical value. For a steady southward interplanetary magnetic field (IMF) the model predicts an irregular sequence of substorms with a mean recurrence period of about 1 hour as in the Klimas et al. [1992] Faraday loop model. Here we use the new model as a nonlinear prediction filter on the Bargatze et al. [1985] database. Starting with physics calculations of the 13 physical parameters of the model, we show that the average relative variance (ARV) is comparable to that obtained with data‐based prediction filters. To obtain agreement between the predicted AL and the database AL it is essential to include the nonlinear increase of the ionospheric conductance with power deposited in the ionosphere.
Shape of the open–closed boundary of the polar cap as determined from observations of precipitating particles by up to four DMSP satellites
Authors: Sotirelis Thomas , Newell Patrick T. , Meng Ching‐I.
Published: February 2004

The shape of the open–closed boundary is studied using auroral oval crossings from up to four DMSP satellites, providing a maximum eight‐point determination per hemisphere. The spectra of both precipitating ions and electrons are examined, and boundary crossings are determined by visual inspection. A subset of crossings with between six‐ and eight‐point determinations observed during intervals spanning 15 to 58 min are used to form a cubic spline approximation to the open–closed boundary for each interval. The variability in the size, shape, and location of the boundary is characterized. Approximately half the time the points can be well fit by a circle, but for the remaining intervals the shape is more complex. The speed at which the boundary moves is estimated, and the accuracy of determining the amount of open flux from n observed boundary crossings is tested. When only one point is used to determine the open flux (by assuming an offset circle), the standard deviation of the relative error is 33%, dropping to 16% for a four‐point measurement. The variation of the open flux with measures of solar wind coupling is tested and found to be roughly proportional. These findings have implications for space weather applications which require the construction of a data set of open flux.
A large southward magnetic field of −23.5 nT in the January 10, 1995, plasmoid
Authors: Nagai T. , Fujimoto M. , Nakamura M. S. , Nakamura R. , Saito Y. , Mukai T. , Yamamoto T. , Nishida A. , Kokubun S.
Published: February 2004

This paper reports a large southward magnetic field of −23.5 nT in the Earth's magnetotail at a radial distance of 46 RE, observed with Geotail on January 10, 1995. The large southward field was part of a bipolar Bz signature of a plasmoid in association with a well‐isolated substorm under steady solar wind conditions. Magnetic field and plasma observations are analyzed and compared with results from a hybrid simulation for magnetic reconnection. The large southward field is produced by the piling up of field lines due to colliding fast tailward flowing plasmas. This study implies that the piling‐up mechanism plays an important role in the evolution of plasmoids traveling in the magnetotail.
Frequency doubling in ultralow frequency wave signals
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2004

We report a new theory to explain the long‐observed frequency‐doubling phenomenon detected in the compressional component of “high‐m” ultralow‐frequency signals seen in the Earth's magnetosphere. We show that the effect is a nonlinear response directly associated with resonant particles. The phase relationship observed between the linear signal and the nonlinear frequency‐doubled signal is predicted by the theory. Our explanation and the fit with observation mean that the occurrence of frequency doubling provides critical evidence of the fact that ring current particles drive high‐m waves.
Height‐resolved Joule heating rates in the high‐latitude E region and the influence of neutral winds
Authors: Thayer J. P.
Published: February 2004

Height‐resolved and height‐integrated estimates of the Joule heating rate in the high‐latitude E region that include the effects of the neutral wind have been derived from measurements made by the Sondrestrom incoherent‐scatter radar. Analyzed in detail are two Sondrestrom radar data sets that represent solar minimum, daytime conditions with periods of moderate to strong geomagnetic activity. These measurements show much more structure in the height‐resolved Joule heating rate when neutral winds are included in the analysis. The neutral wind impact was present during all periods of elevated Joule heating and displayed an altitude‐dependent influence that led to both positive and negative contributions within the E region. Most often, the wind impact was to create a much narrower region of Joule heating. The influence of the neutral wind on the height‐integrated Joule heating rate from 90 to 140 km was significant, at times, with observed reductions of 40% and observed enhancements of as much as 400%. However, the height‐integrated Joule heating rate often did not reflect the degree of neutral wind influence on the local Joule heating rate because the altitude‐dependent behavior of the wind would tend to cancel out during the height‐integration process. Evidence of electric field behavior controlling the neutral wind influence on the Joule heating rate was also observed. During directional changes in the electric field, the neutral winds tended to enhance the Joule heating rate, while directionally steady electric fields resulted in an overall reduction of the Joule heating rate by the neutral wind.
Origin of coronal and interplanetary shocks: A new look with Wind spacecraft data
Authors: Gopalswamy N. , Kaiser M. L. , Lepping R. P. , Kahler S. W. , Ogilvie K. , Berdichevsky D. , Kondo T. , Isobe T. , Akioka M.
Published: February 2004

We have investigated type II radio bursts in the solar corona using data from ground‐based radio telescopes (>18 MHz) and from the Radio and Plasma Wave experiment (WAVES) on board the WIND spacecraft (<14 MHz). The wavelength range of the WAVES experiment includes the 2‐ to 14‐MHz band, previously unobserved from space. We found that all 34 coronal type II bursts observed over a period of 18 months (November 1, 1994, to April 30, 1996), decayed within a few solar radii and did not propagate into the interplanetary medium. On the other hand, most of the accompanying type III radio bursts observed by the ground‐based instruments were observed to continue into the interplanetary medium as the electron beams propagated freely along open magnetic field lines. Over the same period of time, other instruments on board the WIND spacecraft detected about 18 interplanetary shock candidates, which seem to be unrelated to the coronal type II bursts. This result confirms the idea that the coronal and interplanetary shocks are two different populations and are of independent origin. We reexamine the data and conclusions of Gosling et al. [1976], Munro et al. [1979], and Sheeley et al. [1984] and find that their data are consistent with our result that the coronal type II bursts are due to flares. We also briefly discuss the implications of our result to the modeling studies of interplanetary shocks based on input from coronal type II radio bursts.
Substorm electron injections: Geosynchronous observations and test particle simulations
Authors: Birn J. , Thomsen M. F. , Borovsky J. E. , Reeves G. D. , McComas D. J. , Belian R. D. , Hesse M.
Published: February 2004

We investigate electron acceleration and the flux increases associated with energetic electron injections on the basis of geosynchronous observations and test‐electron orbits in the dynamic fields of a three‐dimensional MHD simulation of neutral line formation and dipolarization in the magnetotail. This complements an earlier investigation of test protons [Birn et al., 1997b]. In the present paper we consider equatorial orbits only, using the gyrocenter drift approximation. It turns out that this approximation is valid for electrons prior to and during the flux rises observed in the near tail region of the model at all energies considered (∼ 100 eV to 1 MeV). The test particle model reproduces major observed characteristics: a fast flux rise, comparable to that of the ions, and the existence of five categories of dispersionless events, typical for observations at different local times. They consist of dispersionless injections of ions or electrons without accompanying injections of the other species, delayed electron injections and delayed ion injections, and simultaneous two‐species injections. As postulated from observations [Birn et al., 1997a], these categories can be attributed to a dawn‐dusk displacement of the ion and electron injection boundaries in combination with an earthward motion or expansion. The simulated electron injection region extends farther toward dusk at lower energies (say, below 40 keV) than at higher energies. This explains the existence of observed energetic ion injections that are accompanied by electron flux increases at the lower energies but not by an energetic electron injection at energies above 50 keV. The simulated distributions show that flux increases are limited in energy, as observed. The reason for this limitation and for the differences between the injection regions at different energies is the localization in the dawn‐dusk direction of the tail collapse and the associated cross‐tail electric field, in combination with a difference in the relative importance of E × B drift and gradient drifts at different energies. The results demonstrate that the collapsing field region earthward of the neutral line appears to be more significant than the neutral line itself for the acceleration of electrons, particularly for the initial rise of the fluxes and the injection boundary. This is similar to the result obtained for test ions [Birn et al., 1997b].
Coordinated observations demonstrating external substorm triggering
Authors: Lyons L. R. , Blanchard G. T. , Samson J. C. , Lepping R. P. , Yamamoto T. , Moretto T.
Published: February 2004

To understand the magnetospheric substorm, it is necessary to determine whether its onset is externally triggered by the interplanetary magnetic field (IMF). We analyze the relationship between the IMF and the onset of classical substorms with well‐defined onset times. A classical substorm is one that has auroral brightening and electrojet formation at onset, followed by poleward expansion of the region of bright aurora. Substorms meeting these criteria are identified using Canadian Auroral Network for the OPEN Program United Study ground photometer data. We find that a clear IMF trigger (a northward turning or a reduction in the magnitude of the y component) can be identified for 14 of the 20 substorms used in our study. All but one of the identified triggers are northward turnings. We develop a rigorous set of criteria that represents these triggers. By applying the criteria to a large set of IMF data, we find that it is essentially impossible for the observed association between triggers and substorms to happen by chance. This demonstrates that substorm triggering is a real phenomenon and not the result of the requirement that the IMF be southward before but not after a substorm. We also find that spatial structure in the plane perpendicular to the Earth‐Sun line critically affects whether or not a trigger is observed from a particular IMF monitor; the probability of seeing a trigger for the substorms in our study is 89% for monitors that are < 30 RE from the Earth‐Sun line but only 50% for monitors 30 RE to 56.7 RE from the Earth‐Sun line. Thus a well‐defined IMF trigger is associated with most of substorms considered here, and the probability of trigger identification is a strong function of IMF monitor distance from the Earth‐Sun line. Given this limitation of trigger identification due to spatial structure, our observations imply that a large majority of classical substorms are triggered by the IMF. We also obtain estimates of ∼9 min for the mean time delay between magnetopause contact of an IMF trigger and substorm onset and ∼64–72 min for the median growth‐phase period of southward IMF that precedes triggered classical substorms.
Solar wind radial and latitudinal structure: Electron density and core temperature from Ulysses thermal noise spectroscopy
Authors: Issautier Karine , Meyer‐Vernet Nicole , Moncuquet Michel , Hoang Sang
Published: February 2004

We present new in situ solar wind plasma measurements obtained during Ulysses fast transit from the south solar pole to the north one, which took place 1 year before the 1996 sunspot minimum. The data were obtained with the radio receiver of the Unified Radio and Plasma Wave Experiment, using the method of quasi‐thermal noise spectroscopy, which is relatively immune to spacecraft potential perturbations and whose density measurements are independent on gain calibrations. We analyze the electron density and the core electron temperature. We deduce their radial profiles in the steady state fast solar wind; southward of 40° latitude, between 1.52 and 2.31 AU, the total electron density varies as ne ∝ r(−2.003±0.015), while the core temperature varies as Te ∝ r(−0.64±0.03). This allows to estimate the interplanetary electrostatic field using a simplified fluid equation. We also study, poleward of 40° (where the variance of both parameters are very low), the histograms of the electron density and core temperature scaled to 1 AU, assuming the above determined radial variation. Each histogram shows a single class of flow with a roughly normal distribution. We find a mean electron density of 2.65 cm−3 in the southern hemisphere which is about 8% larger than in the northern one. The core temperature histogram is centered at a mean of 7.5 × 104 K in the south, and of 7 × 104 K in the north. This small asymmetry may be due to a genuine solar asymmetry between the two hemispheres and/or to a temporal variation since solar activity slightly decreased during the Ulysses exploration.
Global distribution of density irregularities in the equatorial ionosphere
Authors: Kil Hyosub , Heelis R. A.
Published: February 2004

We analyzed measurements of ion number density made by the retarding potential analyzer aboard the Atmosphere Explorer‐E (AE‐E) satellite, which was in an approximately circular orbit at an altitude near 300 km in 1977 and later at an altitude near 400 km. Large‐scale (>60 km) density measurements in the high‐altitude regions show large depletions of bubble‐like structures which are confined to narrow local time, longitude, and magnetic latitude ranges, while those in the low‐altitude regions show relatively small depletions which are broadly distributed in space. For this reason we considered the altitude regions below 300 km and above 350 km and investigated the global distribution of irregularities using the rms deviation ΔN/N over a path length of 18 km as an indicator of overall irregularity intensity. Seasonal variations of irregularity occurrence probability are significant in the Pacific regions, while the occurrence probability is always high in the Atlantic‐African regions and is always low in the Indian regions. We find that the high occurrence probability in the Pacific regions is associated with isolated bubble structures, while that near 0° longitude is produced by large depletions with bubble structures which are superimposed on a large‐scale wave‐like background. Considerations of longitude variations due to seeding mechanisms and due to F region winds and drifts are necessary to adequately explain the observations at low and high altitudes. Seeding effects are most obvious near 0° longitude, while the most easily observed effect of the F region is the suppression of irregularity growth by interhemispheric neutral winds.
High‐latitude ionospheric perturbations and gravity waves: 2. Numerical simulations
Authors: Huang Chao‐Song , Andre Dieter A. , Sofko George J.
Published: February 2004

We have calculated ionospheric perturbations produced by gravity waves at high latitudes. It is shown that gravity waves with reasonable parameters can produce strong ionospheric perturbations in the altitude range of 200–400 km. The ionospheric perturbations become weak above 400 km due to the presence of plasma diffusion. If the horizontal perturbation velocity of the neutrals due to gravity waves increases from 20 m/s at 200‐km altitude to 30 m/s at 350‐km altitude, the amplitude of the relative ionospheric density perturbations is about 30% below 350 km. A ray tracing program has been used to trace the ionospheric ray paths of HF rays passing through the strong periodic density perturbations resulting from the gravity waves. The paths of HF rays manifest some new and important features. Rays with small elevation angles are focused and reflected by the density undulations in the bottomside F region. These rays can undergo ground scattering at the so‐called “one‐hop” range. They can also undergo ground reflection and, as such, usually are assumed to be the rays which access scattering regions at larger range by the one‐and‐a‐half hop propagation mode. In this mode the ratio of the ranges of the high‐latitude scatter and the ground scatter is about 3/2. However, there is a significant range of larger elevation angles for which the rays may be “trapped” in the highly perturbed F region. These Pedersen rays can travel large distances before being scattered and the 3/2 range ratio no longer applies. The ray tracing results presented in this paper provide a reasonable explanation of simultaneous ground scatter and auroral scatter observations made by the Saskatoon SuperDARN HF radar.
Impulsive broadband electrostatic noise in the cleft: A signature of dayside reconnection
Authors: Pottelette Raymond , Treumann Rudolf A.
Published: February 2004

Magnetosheath plasma injection events are frequently observed in the dayside oval. From the time dispersion characteristics of the injected magnetosheath ions it is possible to deduce the injection time of magnetosheath electrons. It is shown by means of specific examples taken from the Swedish Viking spacecraft data set, that this electron injection time occurs together with the generation of burst of broadband electrostatic noise (BEN). The overall duration of the BEN emissions lasts typically of the order of 1 min, probably representing the total duration of the injection events. This is within the same time domain as that inferred for flux transfer events (FTE). On a shorter timescale, BEN consists of a succession of very impulsive bursts which last a few seconds; this reveals the characteristic timescales of the elementary injection process. The BEN emissions appear to be the “messengers” of the reconnection process occurring along the magnetic field lines which are connected to the reconnection sites. The information is transmitted via solitary waves, probably of electron‐acoustic type, and consisting of isolated potential wells. From the observations, the overall extension of the injection region at the magnetopause is estimated to be of the size of a large fraction of an Earth radius. However, plasma injection appears as a noncontinuous process that takes place in narrow spatially localized regions of the size of merely more than an ion gyroradius at the magnetopause.
CRRES observations of radiation belt protons: 1. Data overview and steady state radial diffusion
Authors: Albert J. M. , Ginet G. P. , Gussenhoven M. S.
Published: February 2004

The Proton Telescope (PROTEL) instrument on the CRRES satellite made measurements of omnidirectional differential proton flux in the energy range 1–100 MeV, with full pitch angle resolution. An overview of the equatorially mirroring particle data is presented for the entire CRRES mission. We then consider the equatorially mirroring particle distribution in terms of steady state radial diffusion theory. The outer zone, 1.7 ≤ L ≤ 3, is shown to have deviated drastically from steady state profiles both before and after the magnetic storm of March 24, 1991, which rearranged the outer zone but left the inner zone, L ≤ 1.7, largely unaffected. Time‐averaged measurements are compared to theoretical steady state diffusion profiles of the inner zone, which are calculated using several different, previously introduced models of the plasmaspheric electron density and cosmic ray albedo neutron decay (CRAND). For each combination of models, a four‐parameter (electrostatic plus magnetic) radial diffusion coefficient is sought which minimizes the difference between the measurements and calculated flux values. While the best fit values found are physically implausible, nearly equally good results are obtained for values only moderately adjusted from standard, consensus values.
Exospheric dynamics of hydrogen and deuterium at solar minimum
Authors: Hodges R. Richard
Published: February 2004

The escape of hydrogen and deuterium from the Earth is regulated by eddy transport of hydrogenic gases (e.g., water vapor, methane, and molecular hydrogen) from the troposphere, through the stratosphere, and into the mesosphere and lower thermosphere where photochemistry produces atomic forms of hydrogen and deuterium. Farther up, escape creates a deficit of these atoms relative to diffusive equilibrium, and this in turn leads to upward fluxes of both species to balance escape. A Monte Carlo exosphere simulator program, which has evolved through numerous applications to the moon and terrestrial planets, has been adapted to a coordinated study of the dynamics of D and H in the terrestrial exosphere, plasmasphere, and polar wind. Results show that while the escape of hydrogen is compatible with Hunten's limiting flux at the homopause (about 2.5 × 108 cm−2 s−1), the upward flow of deuterium is suppressed by slow escape, which in turn causes an exospheric buildup. The global average escape flux of deuterium is 55% (±10%) of its limiting flux, or 1.5 × 104 cm−2 s−1. From an isotopic perspective, the escape rate of D relative to H corresponds to about 39% of the D/H ratio in SMOW.
Initial Polar magnetic field experiment observations of the low‐altitude polar magnetosphere: Monitoring the ring current with polar orbiting spacecraft
Authors: Le G. , Russell C. T.
Published: February 2004

The equatorial ring current and the magnetopause current both contribute to the magnetic field at low altitudes over the polar cap. In this region the magnetic field is dominated by the Earth's internal field and is well described by empirical models. The average change in the magnetic field strength due to external sources (the residual of the observed magnetic field strength upon subtracting the International Geomagnetic Reference Field (IGRF) 95 internal field model) is typically a few tens of nanoTesla, or a few tenths of 1% of the total magnetic field over the polar cap at the altitude of the Polar spacecraft. It is easier to measure such small differences in the total field than in the vector components because the accuracy of the residuals in the vector magnetic field depends on the accuracy of the knowledge of spacecraft pointing which is generally less well known than the position of the spacecraft. In order to isolate the ring current effects on the total field we adjust the Dst index for the contributions of the magnetopause current using the solar wind dynamic pressure, and we adjust the observed field values for the same effect using the Tsyganenko [1996] model. After these adjustments the ring current strength as measured by the adjusted Dst index is well correlated with the residual of the field strength observed in the low‐altitude polar region over a wide range of local times except when the spacecraft track is near the noon‐midnight meridian. These comparisons, using more than a full year of Polar data, demonstrate that the Tsyganenko [1996] model together with the IGRF 1995 internal field model provides a good baseline for the magnetic field at the altitude of the Polar perigee passes (∼5000 km above the Earth's surface). Further they demonstrate that total field measurements from low‐altitude polar orbiting spacecraft are potentially useful as monitors of the ring current when they cross the polar cap.
Satellite observations of lightning‐induced electron precipitation
Authors: Voss H. D. , Walt M. , Imhof W. L. , Mobilia J. , Inan U. S.
Published: February 2004

Lightning‐induced electron precipitation (LEP) from the Earth's radiation belt has been observed on numerous occasions with detectors on the low‐altitude S81‐1/SEEP satellite. A sequence of seven LEP events on September 9, 1982, and eight events on October 20, 1982, are correlated on a one‐to‐one basis with one‐hop whistlers at Palmer, Antarctica. The temporal profile within a LEP burst has a remarkable fine structure. It is shown to be associated with bunches of magnetically guided and focused 100‐to‐200 keV electrons that are repeatedly scattered by the atmosphere and bounce between the northern and southern hemispheres. The delay time between the lightning sferic and the arrival of the first electron bunch increases with increasing L as predicted by the first‐order gyroresonance theory. The global distribution of strong LEP events observed with the SEEP payload correlates with lightning activity and shows a preferred distribution at 2 < L < 3. This L shell range corresponds to the slot region in the electron radiation belt. A single LEP burst (10−3 erg s−1 cm−2) in the slot region is estimated to deplete ∼0.001% of the particles in the region covered by the burst magnetic field lines. The evidence supports the production of structured LEP by ducted rather than nonducted whistlers. It is found that ducted whistlers can be an important pitch angle diffusion mechanism for 100–250 keV electrons in the 2 < L < 3 range although a number of uncertainties in the various parameters remain to be resolved; It is suggested that observations of LEP can be a new tool to measure the presence and transverse dimensions of plasmaspheric whistler mode ducts.