bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of neutral atoms from the solar wind
Authors: Collier Michael R. , Moore Thomas E. , Ogilvie Keith W. , Chornay Dennis , Keller J. W. , Boardsen S. , Burch James , El Marji B. , Fok M.‐C. , Fuselier S. A. , Ghielmetti A. G. , Giles B. L. , Hamilton D. C. , Peko B. L. , Quinn J. M. , Roelof E. C. , Stephen T. M. , Wilson G. R. , Wurz P.
Published: January 2004

We report observations of neutral atoms from the solar wind in the Earth's vicinity with the low‐energy neutral atom (LENA) imager on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft. This instrument was designed to be capable of looking at and in the direction of the Sun. Enhancements in the hydrogen count rate in the solar direction are not correlated with either solar ultraviolet emission or suprathermal ions and are deduced to be due to neutral particles from the solar wind. LENA observes these particles from the direction closest to that of the Sun even when the Sun is not directly in LENA'S 90° field of view. Simulations show that these neutrals are the result of solar wind ions charge exchanging with exospheric neutral hydrogen atoms in the postshock flow of the solar wind in the magnetosheath. Their energy is inferred to exceed 300 eV, consistent with solar wind energies, based on simulation results and on the observation of oxygen ions, sputtered from the conversion surface in the time‐of‐flight spectra. In addition, the sputtered oxygen abundance tracks the solar wind speed, even when IMAGE is deep inside the magnetosphere. These results show that low‐energy neutral atom imaging provides the capability to directly monitor the solar wind flow in the magnetosheath from inside the magnetosphere because there is a continuous and significant flux of neutral atoms originating from the solar wind that permeates the magnetosphere.
Storm‐substorm relationship: Contribution of the tail current to Dst
Authors: Ohtani S. , Nosé M. , Rostoker G. , Singer H. , Lui A. T. Y. , Nakamura M.
Published: January 2004

The Dst index has been conventionally used as a measure of the storm intensity, which ideally assumes that the associated ground magnetic disturbance is caused by the ring current. The present study examines the contribution of the tail current to Dst, focusing on the occurrence of geosynchronous dipolarization close to the Dst minimum, in other words, the start of the storm recovery phase. The Sym‐H (referred to as Dst(Sym‐H) hereafter) index rather than the conventional Dst index is used because of its higher time resolution (1 min). For the June 1998 storm event, dipolarization started at two GOES satellites and the Geotail satellite in the near‐Earth tail when Dst(Sym‐H) reached its minimum. This result indicates that the source current was located outside of geosynchronous orbit, and therefore the recovery of Dst(Sym‐H) can be attributed to the reduction of the tail current rather than the decay of the ring current. A statistical study based on 59 storm events (79 GOES events) confirms the tendency for geosynchronous magnetic field to dipolarize at the Dst(Sym‐H) minimum. It is therefore highly likely that the Dst(Sym‐H) minimum is misidentified as the start of the ring current (storm) decay at a time when the ring current may actually be intensifying owing to substorm‐associated injection. From the magnitude of the Dst(Sym‐H) recovery during the interval of geosynchronous dipolarization, the contribution of the tail current to Dst(Sym‐H) at the Dst(Sym‐H) minimum is estimated to be 20–25%. However, the contribution of the tail current may be even larger because the tail current may not return to preintensification levels and may continue to contribute to Dst(Sym‐H) after dipolarization. The trigger of dipolarization (substorm) and the subsequent recovery of Dst(Sym‐H) tend to take place in the course of the reduction of the southward interplanetary magnetic field (IMF) BZ. It is therefore suggested that the ring current (storm) recovers after the substorm since the magnetospheric convection weakens because of weaker southward IMF BZ.
Double cusp: Model prediction and observational verification
Authors: Wing Simon , Newell Patrick T. , Ruohoniemi J. Michael
Published: January 2004

Recent modeling of the entry of solar wind plasma into the magnetosphere and ionosphere has adequately simulated the large‐scale particle precipitation features in the observed cusp, mantle, polar rain, and open‐field line low‐latitude boundary layer regions. The assumption of a simple dawn‐dusk electric field limited the models to the near‐noon region and southward interplanetary magnetic field (IMF) case. Here, we present an improved model that incorporates the electric field obtained from statistical convection patterns. When the IMF is strongly duskward/dawnward and weakly southward, the model predicts the occurrence of a double cusp near noon: one cusp at lower latitude and one at higher latitude. The lower‐latitude cusp ions originate from low‐latitude magnetosheath, whereas the higher‐latitude ions originate from the high‐latitude magnetosheath. The lower‐latitude cusp is located in the region of weak azimuthal E × B drift, resulting in a dispersionless cusp, as would be observed from a typical meridional trajectory of a polar‐orbiting satellite. The higher‐latitude cusp is located in the region of strong azimuthal and poleward E × B drift. Because of a significant poleward drift, the higher‐latitude cusp dispersion has some resemblance to that of the typical southward IMF cusp. This prediction was subsequently confirmed in a large case study with Defense Meteorological Satellite Program (DMSP) data. Occasionally, the two parts of the double cusp have such narrow latitudinal separation that they give the appearance of just one cusp with extended latitudinal width. From the 40 DMSP passes selected during periods of large (positive or negative) IMF By and small negative IMF Bz ,30 (75%) of the passes exhibit double cusps or cusps with extended latitudinal width. The double‐cusp result is consistent with the following new statistical results: (1) the cusp latitudinal width increases with |IMF By| and (2) the cusp equatorward boundary moves to lower latitude with increasing |IMF By|.
Instability at the electron inertial scale
Authors: Seyler C. E. , Wu K.
Published: January 2004

Current sheets having a transverse scale near the electron inertial length c/ωp are shown to be unstable. Two general classes of modes are found to occur, the single and double tearing modes and a current shear driven current‐convective interchange mode. The tearing modes are global modes that relax the equilibrium to a lower magnetic energy state. The current‐convective interchange mode is localized near points at which k · B0 = 0. The interchange mode has features in common with the collisionless tearing modes but is a local rather than global instability. An extensive analysis of the linear stability is presented, and the results are confirmed from linear initial value and eigenvalue calculations. Nonlinear three‐dimensional initial value simulations are performed to determine the effects of the mode on the equilibrium. The δE/δB spectrum of the field fluctuations is computed and compared to satellite observations. The results support the hypothesis that short‐scale broadband extremely low‐frequency (BBELF) waves associated with Alfvénic structures are possibly the result of current decay through the electromagnetic current‐convective instability.
Solar wind preconditioning in the flank foreshock: IMP 8 observations
Authors: Sibeck D. G. , Decker R. B. , Mitchell D. G. , Lazarus A. J. , Lepping R. P. , Szabo A.
Published: January 2004

We use IMP 8 plasma, magnetic field, and energetic ion observations within the Earth's foreshock from January through August 1995 to determine the effects of energetic ion bursts on the ambient solar wind for comparison with model predictions. Owing to the spiral interplanetary magnetic field orientation, the events are far more common upstream from the prenoon than postnoon bow shock. Pressures associated with the energetic ions depress foreshock magnetic field strengths and plasma densities. The magnitude of the depression is proportional to the intensity of energetic ions. The excavated plasma and magnetic field sometimes pile up in narrow regions of enhanced plasma densities and magnetic field strengths, but depressed flow velocities, just outside the foreshock cavities. Typical amplitudes of the depressions and enhancements at IMP 8 far upstream from the bow shock are far less (20% and 10%, respectively) than those seen in past case studies of events observed just outside the bow shock. The cavities occur preferentially during high‐speed solar wind streams but show no clear dependence upon other solar wind parameters. The distribution of burst durations resembles those for interarrival times for interplanetary magnetic field discontinuities, magnetopause motion, and flux transfer events, suggesting causal relationships between these phenomena.
Time domain analysis of plasma turbulence observed upstream of a quasi‐parallel shock
Authors: Coca D. , Balikhin M. A. , Billings S. A. , Alleyne H. S. C. K. , Dunlop M.
Published: January 2004

This paper presents, for the first time, an analysis of space plasma turbulence based on the NARMAX system identification approach. Fundamental nonlinear processes in the low‐frequency turbulence observed in the terrestrial foreshock by Active Magnetospheric Particle Tracer Explorers United Kingdom Satellite (AMPTE UKS) and Ion Release Module (AMPTE IRM) are studied using time domain identification methods developed for nonlinear dynamical systems. It is shown, directly from the experimental data, that the cubic nonlinearity has a significant influence on the steepening of the nonlinear low‐frequency waves and on the dependence of the phase velocity upon the wave amplitude. In comparison with a previous frequency domain approach, the present method requires only short data sets.
Special features of the September 24–27, 1998 storm during high solar wind dynamic pressure and northward interplanetary magnetic field
Authors: Clauer C. Robert , Alexeev Igor I. , Belenkaya Elena S. , Baker Joseph B.
Published: January 2004

The geomagnetic storm on September 24 – 27, 1998, was initiated by a sudden compression of the magnetosphere in response to the solar wind dynamic pressure pulse. Simultaneous with the pressure increase, the interplanetary magnetic field (IMF) became strongly northward. Several unexpected magnetospheric responses to this sudden impulse were observed. First, for ∼ 30 min following the sudden impulse, the entire auroral oval became active and thick, while the polar cap area decreased to less than 1/2 of its original size. The second unusual observation associated with the sudden impulse is the global magnetic perturbation measured by low‐latitude magnetic stations. The field shows an asymmetric increase in the axial component (parallel to the dipole axis) with the strongest enhancement measured on the night side and at local magnetic noon the perturbation is small or slightly negative. This is very unusual since sudden compressions are generally measured by low‐latitude stations to have the largest enhancement of the field on the dayside. The main phase of the geomagnetic storm begins in the second hour on September 25, 1998, following the southward turning of the IMF. The auroral oval becomes thin and moves equatorward increasing the polar cap area by more than a factor of 3. The theoretical analysis presented in this paper suggests that the response to the sudden impulse is an electrodynamic effect produced by a “transition” current system in response to the northward turning of the IMF.
Reconnection at the high‐latitude magnetopause during northward interplanetary magnetic field conditions
Authors: Onsager T. G. , Scudder J. D. , Lockwood M. , Russell C. T.
Published: January 2004

The Polar spacecraft had a prolonged encounter with the high‐latitude dayside magnetopause on May 29, 1996. This encounter with the magnetopause occurred when the interplanetary magnetic field was directed northward. From the three‐dimensional electron and ion distribution functions measured by the Hydra instrument, it has been possible to identify nearly all of the distinct boundary layer regions associated with high‐latitude reconnection. The regions that have been identified are (1) the cusp; (2) the magnetopause current layer; (3) magnetosheath field lines that have interconnected in only the Northern Hemisphere; (4) magnetosheath field lines that have interconnected in only the Southern Hemisphere; (5) magnetosheath field lines that have interconnected in both the Northern and Southern Hemispheres; (6) magnetosheath that is disconnected from the terrestrial magnetic field; and (7) high‐latitude plasma sheet field lines that are participating in magnetosheath reconnection. Reconnection over this time period was occurring at high latitudes over a broad local‐time extent, interconnecting the magnetosheath and lobe and/or plasma sheet field lines in both the Northern and Southern Hemispheres. Newly closed boundary layer field lines were observed as reconnection occurred first at high latitudes in one hemisphere and then later in the other. These observations establish the location of magnetopause reconnection during these northward interplanetary magnetic field conditions as being at high latitudes, poleward of the cusp, and further reinforce the general interpretation of electron and ion phase space density signatures as indicators of magnetic reconnection and boundary layer formation.
Substorm injection of relativistic electrons to geosynchronous orbit during the great magnetic storm of March 24, 1991
Authors: Ingraham J. C. , Cayton T. E. , Belian R. D. , Christensen R. A. , Friedel R. H. W. , Meier M. M. , Reeves G. D. , Tuszewski M.
Published: January 2004

The great March 1991 magnetic storm and the immediately preceding solar energetic particle event (SEP) were among the largest observed during the past solar cycle, and have been the object of intense study. We investigate here, using data from eight satellites, the very large delayed buildup of relativistic electron flux in the outer zone during a 1.5‐day period beginning 2 days after onset of the main phase of this storm. A notable feature of the March storm is the intense substorm activity throughout the period of the relativistic flux buildup, and the good correlation between some temporal features of the lower‐energy substorm‐injected electron flux and the relativistic electron flux at geosynchronous orbit. Velocity dispersion analysis of these fluxes between geosynchronous satellites near local midnight and local noon shows evidence that both classes of electrons arrive at geosynchronous nearly simultaneously within a few hours of local midnight. From this we conclude that for this storm period the substorm inductive electric field transports not only the usual (50–300 keV) substorm electrons but also the relativistic (0.3 to several MeV) electrons to geosynchronous orbit. A simplified calculation of the electron ε × B and gradient/curvature drifts indicates that sufficiently strong substorm dipolarization inductive electric fields (≳ 10 mV/m) could achieve this, provided sufficient relativistic electrons are present in the source region. Consistent with this interpretation, we find that the injected relativistic electrons have a pitch angle distribution that is markedly peaked perpendicular to the magnetic field. Furthermore, the equatorial phase space density at geosynchronous orbit (L = 6.7) is greater than it is at GPS orbit at the equator (L = 4.2) throughout this buildup period, indicating that a source for the relativistic electrons lies outside geosynchronous orbit during this time. Earthward transport of the relativistic electrons by large substorm dipolarization fields, since it is unidirectional, would constitute a strong addition to the transport by radial diffusion and, when it occurs, could result in unusually strong relativistic fluxes, as is reported here for this magnetic storm.
Proton impact excitation of SO2
Authors: Kiehling Jennifer E. , Ammirati Thomas F. , Moore Matthew D. , Monce Michael N. , Hansel Elizabeth F.
Published: January 2004

Because of the interest in atomic emissions observed from the Io plasma torus, we have investigated the spectrum produced by proton impact on SO2 in the wavelength region of 380–800 nm. The proton energy was varied from 50 to 225 keV. The spectrum shows numerous S, O, S+, and O+ lines. A weak series of SO2 bands has also been tentatively identified. In addition, Balmer series lines are seen, which indicates charge‐transfer ionization of the SO2 target. Preliminary absolute emission cross sections on the order of 10−18 cm2 have been measured for the most dominant line in the spectrum, an oxygen triplet at 777 nm.
High‐resolution spectroscopy of the Io plasma torus during the Galileo mission
Authors: Thomas Nicolas , Lichtenberg G. , Scotto M.
Published: January 2004

We present Doppler‐resolved spectra of the Io plasma torus (IPT) acquired in September 1997 and October 1999. The spectra are used to demonstrate the decrease in T⊥(S+) with increasing radial distance beyond 6.0 RJ. In October 99, emissions of S III, O II, and S II were observed within a relatively short period allowing estimates of the mixing ratios of the three major species in the IPT immediately after the Galileo 124 encounter with Io. The results suggest that near the position of the ribbon feature, the three major species were nearly equally abundant. Comparison of the S II line ratio (which is electron density dependent) with the S II intensity suggests that the peak electron density in the ribbon may be just outside the position of the peak intensity of S II (the S II “ribbon”). Ion temperatures for S III were also derived. The data indicate that S III is not in thermal equilibrium with S II in the IPT (thereby supporting previous Galileo measurements). It has also been possible to study deviations from corotation in the IPT at high spatial resolution.
Electron densities near Io from Galileo plasma wave observations
Authors: Gurnett D. A. , Persoon A. M. , Kurth W. S. , Roux A. , Bolton S. J.
Published: January 2004

This paper presents an overview of electron densities obtained near Io from the Galileo plasma wave instrument during the first four flybys of Io. These flybys were 10, which was a downstream wake pass that occurred on December 7, 1995; I24, which was an upstream pass that occurred on October 11, 1999; I25, which was a south polar pass that occurred on November 26, 1999; and I27, which was an upstream pass that occurred on February 22, 2000. Two methods were used to measure the electron density. The first was based on the frequency of upper hybrid resonance emissions, and the second was based on the low‐frequency cutoff of electromagnetic radiation at the electron plasma frequency. For three of the flybys, 10, I25, and I27, large density enhancements were observed near the closest approach to Io. The peak electron densities ranged from 2.1 to 6.8 × 104 cm−3. These densities are consistent with previous radio occultation measurements of Io's ionosphere. No density enhancement was observed during the I24 flyby, most likely because the spacecraft trajectory passed too far upstream to penetrate Io's ionosphere. During two of the flybys, I25 and I27, abrupt step‐like changes were observed at the outer boundaries of the region of enhanced electron density. Comparisons with magnetic field models and energetic particle measurements show that the abrupt density steps occur as the spacecraft penetrated the boundary of the Io flux tube, with the region of high plasma density on the inside of the flux tube. Most likely the enhanced electron density within the Io flux tube is associated with magnetic field lines that are frozen to Io by the high conductivity of Io's atmosphere, thereby enhancing the escape of plasma along the magnetic field lines that pass through Io's ionosphere.
Magnetized or unmagnetized: Ambiguity persists following Galileo's encounters with Io in 1999 and 2000
Authors: Kivelson Margaret G. , Khurana Krishan K. , Russell Christopher T. , Joy Steven P. , Volwerk Martin , Walker Raymond J. , Zimmer Christophe , Linker Jon A.
Published: January 2004

Magnetometer data from Galileo's close encounters with Io on October 11, 1999, and February 22, 2000, do not establish clearly either the existence or absence of an internal magnetic moment because they were acquired in regions where plasma currents contribute large magnetic perturbations. Data from an additional encounter on November 26, 1999, with closest approach beneath Io's south polar regions, were lost. The recent passes add to our understanding of the interaction of the torus with Io and its flux tube and tighten the limits on possible internal sources of magnetic fields. Simple field‐draping arguments account for some aspects of the observed rotations. Interpretations in terms of both a magnetized and an unmagnetized Io are considered. Data from the February 2000 pass (closest approach altitude 201 km, latitude 18°) rule out a strongly magnetized Io (surface equatorial field larger than the background field) but do not rule out a weakly magnetized Io (surface equatorial field of the order of Ganymede's but smaller than the background field at Io). Models suggest that if Io is magnetized, its magnetic moment is not strictly antialigned with the rotation axis. The inferred tilt is consistent with contributions from an inductive field analogous to that observed at Europa and Callisto. If an induced field is present, the currents would flow in the outer mantle or aesthenosphere. Wave perturbations differ on flux tubes that do or do not link directly to Io and its ionosphere suggesting that the latter flux tubes are virtually stagnant in Io's frame and that a unipolar inductor appropriately models the currents linking Io to Jupiter's ionosphere.
Morphology and time variability of Io's visible aurora
Authors: Geissler P. E. , Smyth W. H. , McEwen A. S. , Ip W. , Belton M. J. S. , Johnson T. V. , Ingersoll A. P. , Rages K. , Hubbard W. , Dessler A. J.
Published: January 2004

Clear‐filter imaging of Io during the Galileo nominal and extended missions recorded diffuse auroral emissions in 16 distinct observations taken during 14 separate eclipses over a two year period. These images show that the morphology and time variability of the visible aurora have several similarities to Io's far ultraviolet emissions. The orbital leading hemisphere of Io is consistently brighter than the trailing hemisphere, probably due to a greater concentration of torus electrons in the wake region of the satellite. The locations of the polar limb glow and the bright equatorial glows appear to correlate with Io's System III longitude. Unlike the far ultraviolet emissions, the visible aurorae are enhanced near actively venting volcanic plumes, probably because of molecular emission by SO2.
A critical height of quiescent prominences before eruption
Authors: Filippov B. P. , Den O. G.
Published: January 2004

We discuss the problem of the limiting height of quiescent prominences and the relationship of this value to the photospheric magnetic field characteristics. Solar prominences are considered preeruptive states of coronal mass ejections (CMEs). We note that in a model of inverse‐polarity filament, the equilibrium is stable only in the region where the photospheric magnetic field falls off with a height not faster than h−l. Current‐free magnetic field calculations are compared with the observed prominence heights. It is shown that the maximum stable height of a prominence depends on the vertical gradient value and never exceeds the level where the power is equal to unity in the power‐law decrease of the magnetic field. This is a new way to predict prominence eruptions and CMEs.
About the nature of long‐term microflare energy release in solar active regions
Authors: Bogod Vladimir M. , Mercier Claude , Yasnov Leonid V.
Published: January 2004

In this paper we compare the long‐term impulsive microflares (microbursts (MB) in the range 1000 MHz) using RATAN‐600 observations and impulsive noise storm (NS) emission on frequencies 233 and 164 MHz using Nancay radioheliograph data. It is shown that these processes result from destruction of current sheets at the top of a coronal loop as a result of the development of plasma instability sequences. The coincidence in time between the generation of microbursts and coronal mass ejections is of special interest. The impulses of NS radio emission at meter waves arise after the appearance of the MB at decimeter waves. The temporal parameters of the processes are determined. It is shown that the MB duration is determined by the spreading time of the magnetic flux in the reconnection region along the current sheet. The flux of energy in the form of fast electrons is equal to 9.2 × 104 ergs cm−2 s−1.
Statistical analysis of coronal shock dynamics implied by radio and white‐light observations
Authors: Reiner M. J. , Kaiser M. L. , Gopalswamy N. , Aurass H. , Mann G. , Vourlidas A. , Maksimovic M.
Published: January 2004

For 19 solar eruptive events we present a statistical comparison of the shock dynamics derived from the measured frequency drift rates of metric and decametric‐hectometric (D‐H) type II radio bursts with the dynamics of the associated coronal mass ejection (CME). We find that the shock speed parameters derived from the D‐H type II radio emissions generated in the high corona (∼2–4 R⊙; R⊙ = 696,000 km) are well correlated with the corresponding CME plane‐of‐the‐sky speeds (correlation coefficient = 0.71). On the other hand, we find no obvious correlation between the shock speed parameters derived from the metric type II radio bursts, generated in the middle corona (1.4–2 R⊙), and the corresponding CME speeds (correlation coefficient = −0.07). In general, we also find no clear correlation between the shock speed parameters derived from the metric type II bursts and the D‐H radio emissions (correlation coefficient = 0.3). However, the metric type II radio bursts sometimes include a second component that is possibly related to the D‐H radio emissions. These statistical comparisons of the shock dynamics, implied by the observed metric and D‐H type II frequency drift rates, provide further evidence for two distinct coronal shocks. Our statistical analyses are preceded by two specific examples that illustrate the methodology used in this study.
Near‐Sun and near‐Earth manifestations of solar eruptions
Authors: Gopalswamy N. , Lara A. , Kaiser M. L. , Bougeret J.‐L.
Published: January 2004

We compare the near‐Sun and near‐Earth manifestations of solar eruptions that occurred during November 1994 to June 1998. We compared white‐light coronal mass ejections, metric type II radio bursts, and extreme ultraviolet wave transients (near the Sun) with interplanetary (IP) signatures such as decameter‐hectometric type II bursts, kilometric type II bursts, IP ejecta, and IP shocks. We did a two‐way correlation study to (1) look for counterparts of metric type II bursts that occurred close to the central meridian and (2) look for solar counterparts of IP shocks and IP ejecta. We used data from Wind and Solar and Heliospheric Observatory missions along with metric radio burst data from ground‐based solar observatories. Analysis shows that (1) most (93%) of the metric type II bursts did not have IP signatures, (2) most (80%) of the IP events (IP ejecta and shocks) did not have metric counterparts, and (3) a significant fraction (26%) of IP shocks were detected in situ without drivers. In all these cases the drivers (the coronal mass ejections) were ejected transverse to the Sun‐Earth line, suggesting that the shocks have a much larger extent than the drivers. Shocks originating from both limbs of the Sun arrived at Earth, contradicting earlier claims that shocks from the west limb do not reach Earth. These shocks also had good type II radio burst association. We provide an explanation for the observed relation between metric, decameter‐hectometric, and kilometric type II bursts based on the fast mode magnetosonic speed profile in the solar atmosphere.
Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Extraction of the equatorial ion distribution
Authors: Brandt Pontus C:son , Barabash Stas , Roelof Edmond C. , Chase Christopher J.
Published: January 2004

Energetic neutral atom (ENA) images obtained by the imager on board the Astrid satellite in the polar cap at 1000 km during a moderate magnetic storm (Dst ≥ −80 nT) on February 8, 1995, are simulated using a parameterized model of the equatorial ion distribution and a six‐component Chamberlain exosphere with parameters from the Mass Spectrometer Incoherent Scatter Extended (MSISE‐90) model. By changing the ion parameters until a matching ENA image is obtained, one can extract the equatorial ion distribution. Four consecutive images from different view points several minutes apart are simulated assuming H+ and O+, respectively, as parent ions. The optimal set of parameters is extracted by minimizing the χ2 difference between simulated and observed ENA images using Powell's minimization algorithm. The optimal equatorial model ion distribution consists of O+ peaked around dusk. The lower intensity of fluxes obtained from vantage points closer to the pole is an effect of the loss cone of the parent ion distribution being empty.
Dual‐spacecraft observations of standing waves in the magnetosheath
Authors: Balikhin M. A. , Schwartz S. , Walker S. N. , Alleyne H. St. C. K. , Dunlop M. , Lühr H.
Published: January 2004

An unambiguous determination of the wave modes observed in plasma turbulence requires the determination of the dispersion characteristics of the wave. The method adopted for the determination of the wave dispersion relation is based upon the study of the phase difference of the waves at two closely separated spacecraft [Balikhin and Gedalin, 1993]. It is applied to observations of waves in the magnetosheath magnetic field made by AMPTE UKS and AMPTE IRM. We show that the observed waves are propagating in the sunward direction but are convected toward the magnetopause by the plasma flow. As a result, the observed waves are quasi‐standing in the flow. These waves have been previously identified as either slow or mirror modes depending the procedure adopted for their analysis. They may also be an experimental observation of the mirror and slow (MIAOW) waves identified near the magnetopause boundary in the hybrid simulations of Omidi and Winske [1995]. The observed waves possess short wavelengths (λ) such that RBi/λ ≈ 1 (where RBi is the ion Larmor radius). Thus they may only be studied analytically within the framework of the kinetic approximation. Both the nonlinear amplitudes and possibly the strong underlying inhomogeneity are responsible for the significant differences between the properties of the observed modes and those resulting from linear homogeneous kinetic theory.
Acceleration of ionospheric O+ ions on open field lines in the low‐latitude boundary layer and the cusp region.
Authors: Krauklis I. , Johnstone A. D. , Peterson W. K.
Published: January 2004

A common observation made by the toroidal imaging mass‐angle spectrograph on the polar orbiting Polar Spacecraft is the acceleration of ionospheric O+ and H+ to similar velocities on open field lines in the low‐latitude boundary layer (LLBL) and cusp regions. A detailed study of nine events made in the midaltitude cusp region is presented here and the acceleration can be split into two regions. On open field lines in the LLBL both H+ and O+ undergo rapid upward acceleration within approximately 0.75° invariant latitude. The acceleration aH+/ao+ ≈ 2 is inconsistent with the proposed electrostatic acceleration. Intense electrostatic waves often associated with the acceleration is consistent with energy transfer from the faster H+ to the slower O+ population via two‐stream instabilities. In the cusp region injected magnetosheath plasma is a possible free energy source for the acceleration of ionospheric ion populations: The component temperature ratio T‖/T⊥. of the upgoing, mirrored injected magnetosheath ions is generally lower than the downgoing ions consistent with a two‐stream instability driven energy transfer from the magnetosheath to the ionospheric ion populations.
Empirical model of proton fluxes in the equatorial inner magnetosphere: Development
Authors: Milillo A. , Orsini S. , Daglis I. A.
Published: January 2004

We present an empirical model of the inner magnetosphere, based on proton measurements performed by the charge‐energy‐mass instrument in the energy range 1–300 keV and in the L range 3–9 during 3 years of the Active Magnetospheric Particle Tracer Explorers/CCE mission (1985 to 1987, i.e., near solar minimum). We have fitted the average proton fluxes at low geomagnetic activity (AE < 100 nT) and obtained a multi parametric function, which is the sum of five different parts and depends on energy, L shell, magnetic local time, and a few independent factors. The model is able to accurately reproduce the average perpendicular proton fluxes in the equatorial inner magnetosphere. The various segments of the model function reproduce the effects of physical processes, such as diffusion, on magnetospheric plasma. We describe the methodology followed for the development of the model. Empirical models like this one are critical in our effort to exploit global neutral atom imaging measurements of ongoing and future missions. Furthermore, they serve as a benchmark of theoretical models and numerical simulations of the inner magnetosphere.
Naturally enhanced ion acoustic fluctuations seen at different wavelengths
Authors: Forme F. , Ogawa Y. , Buchert S. C.
Published: January 2004

We present European Incoherent Scatter (EISCAT) Svalbard Radar observations of enhanced incoherent scatter ion‐acoustic spectra with transmitted frequencies slightly shifted relative to the radar central frequency. This study confirms the initial findings that (1) the naturally enhanced ion acoustic lines sometimes show asymmetric power profiles and (2) the amplitude enhancement of upgoing and downgoing ion‐acoustic waves occurs at somewhat different altitudes and different power levels. These observational facts are modeled using the parametric decay of beam driven Langmuir wave in an inhomogeneous medium. It is shown that the density gradient with altitude as well as the collisions contribute to an altitude variation of ion‐acoustic spectra and may account for the observed signatures. It is also responsible for the transition from upshifted to downshifted enhanced ion lines. Our model shows that the power radiated by enhanced spectra varies with the wavelength and that the power difference between two wavelengths varies with altitude.
Imaging observations of midlatitude ionospheric disturbances during the geomagnetic storm of February 12, 2000
Authors: Sahai Y. , Shiokawa K. , Otsuka Y. , Ihara C. , Ogawa T. , Igarashi K. , Miyazaki S. , Saito A.
Published: January 2004

Regular all‐sky imaging observations of the F region OI 630 nm nightglow emission are carried out at Rikubetsu (43.5°N) and Shigaraki (34.8°N), Japan since 1998. In this paper, we present observations of the OI 630 nm emission images from these two sites with a cluster of other observations during the geomagnetic storm of February 12, 2000. The airglow observations were obtained between 1500 UT and 2000 UT (0000–0500 LT) on this night. Images from both Rikubetsu and Shigaraki show the presence of mesoscale‐enhanced airglow bands moving slowly (∼20–30 m s−1) to the southwest direction. The mesoscale band structures were also detected by an extensive network of GPS signal observations over Japan. The airglow observations at Shigaraki also show intense OI 630 nm emission in the south with several small‐scale intensity depletion structures. We suggest that these small‐scale intensity depletion structures (scale size 30–50 km) are caused by possible nonlinear interaction between the mesoscale TIDs and enhanced regions of the equatorial ionospheric anomaly. The ionosonde observations from Wakkanai (45.4°N) and Kokubunji (35.7°N) show spread F signatures between 1130 and 2100 UT and 1215 and 1715 UT, respectively, whereas no spread F was observed at Yamagawa (31.2°N) and Okinawa (26.3°N). The phase fluctuations of GPS signal were observed at latitudes higher than 30°N for 1400–1900 UT. These observations suggest enhanced storm time ionospheric disturbances were confined in the midlatitudes.
Ion composition variations in the inner magnetosphere: Individual and collective storm effects in 1991
Authors: Fu S. Y. , Wilken B. , Zong Q. G. , Pu Z. Y.
Published: January 2004

Ion composition variations in the inner magnetosphere during storm times are studied by using data sets obtained from the magnetospheric ion composition spectrometer (MICS) on board the Combined Release and Radiation Effects Satellite (CURES). The observations were made during the second half of the CRRES mission, which was near the maximum of solar cycle 22. Four selected storms are subjects of detailed case studies; statistical results are based on a group of moderate (50 < |Dst| < 100 nT) and large storms (|Dst| > 100 nT) storms observed in 1991. The case studies show that energetic particle enhancements occur at very low equatorial altitudes (L = 3∼4) during large storms with significant delays relative to the storm sudden commencement times (of about 20 hours). The average time duration of the particle enhancements is about 47 hours. By studying the time variation of energy spectrograms of H+, it is found that low‐energy (E < 100 keV) and high‐energy (E > 100 keV) protons show different time profiles during the development and decay of the ring current. The low‐energy part shows a dramatic intensification and a rapid decay. However, its relative contribution to the ring current defined by the density ratio N(HL+)/N during the storm maximum is almost constant. On the other hand, high‐energy protons first exhibit a flux decrease followed by a delayed increase. The density ratio
shows an anticorrelation with the storm intensity. It is confirmed that the ionospheric origin particles (e.g., O+) are important constituents of the storm time ring current. The fractional number density of O+ ions increases with the intensity of the storm. The statistical results demonstrate that the energy density of O+ is a steep function of Dst for moderate storms. However, it seems to increase very slowly with Dst, or even to be almost independent of Dst for large storms (|Dst| ≥ 120 nT). The ratios of solar wind origin He++ density to the total density show no obvious difference among large storms. The same appears for He+ ions.
Midcourse Space Experiment: Auroral enhancement of nitric oxide medium‐wave infrared emission observed by the Spatial Infrared Imaging Telescope III radiometer
Authors: Sharma R. D. , O'Neil R. , Gardiner H. , Gibson J. , Dothe H. , Duff J. W. , Wintersteiner P. P. , Kendra M.
Published: January 2004

Enhanced medium‐wave infrared nitric oxide fundamental (Δυ = −1) vibration‐rotation (VR) band emission observed by the Midcourse Space Experiment radiometer band A (6.8–10.8 μm) at nighttime in the 110‐ to 130‐km tangent altitude range on November 10, 1996, is shown to be the result of auroral dosing. The energy flux Q (ergs cm−2 s−1), the average energy 〈E〉 (keV) of the incident electrons, as well as the location of the dosed region along the line of sight (LOS) were previously derived [Strickland et al., 1997]. Combining these results with the N2+ first negative band emission at 391.4 nm gives the auroral energy deposition rate in all three spatial dimensions. The portion of the LOS NO VR band radiance transmitted by the band A filter calculated by a more elaborate version of the auroral model in the Strategic High‐Altitude Radiance Code agrees remarkably well with the enhancements in the signal observed in that band of the Spatial Infrared Imaging Telescope III radiometer.
Interplanetary sources of magnetic storms: A statistical study
Authors: Vennerstroem S.
Published: January 2004

Magnetic storms are mainly caused by the occurrence of intense southward magnetic fields in the interplanetary medium. These fields can be formed directly either by ejection of magnetic structures from the Sun or by stream interaction processes during solar wind propagation. In the present study we examine 30 years of satellite measurement of the solar wind during magnetic storms, with the aim of estimating the relative importance of these two processes. We use the solar wind proton temperature relative to the temperature expected from the empirical relation to the solar wind speed Tp/Texp, together with the speed gradient, and the interplanetary magnetic field azimuth in the ecliptic, in order to distinguish between the two processes statistically. We find that compression due to stream interaction is at least as important as the direct effect of ejection of intense fields, and probably more so. Only around 20–25% of major and large storm hours and 10–15% of medium and small storm hours are directly associated with a solar wind meeting the criteria Tp/Texp < 0.5, a criteria which previously has been found to be a good indicator of ejected material. More than 50% of the storm hours are caused by a solar wind having an enhanced proton temperature relative to what is expected on the basis of the observed speed and having clear traces of gardenhose orientation of the magnetic field in the ecliptic plane.
Role of electron physics in slow mode shocks
Authors: Daughton William , Winske Dan , Yin Lin , Gary S. Peter
Published: January 2004

Much of the theoretical understanding concerning the structure and essential properties of the slow mode shock has been obtained from hybrid calculations in which a full kinetic description is used for the ions while the electrons are approximated as a massless adiabatic fluid. Owing to the relatively broad spatial and relatively slow temporal scales of the slow shock, one would expect this approximation to be well justified. In this work we reexamine the importance of electron dynamics using one‐dimensional fully kinetic simulations which fully resolve all relevant spatial and temporal electron scales. The resulting shock structure and ion heating are in excellent agreement with hybrid simulations, indicating that the dissipation arising from kinetic electrons is relatively minor. However, electron heating is somewhat larger, and clear non‐Maxwellian features are observed. In the upstream region, back‐streaming electrons give rise to double‐peaked distributions, while in the downstream region, bi‐Maxwellian distributions are observed with Te‖ > Te⊥. Although the acceleration mechanism for the back‐streaming electrons is not fully understood, we present evidence that resonant wave‐particle interactions may play an important role.
Plasma conditions in auroral roar source regions inferred from radio and radar observations
Authors: Hughes J. M. , LaBelle J.
Published: January 2004

A medium frequency interferometer (MFI) has been established in Sondrestrom, Greenland, to measure the direction of arrival (DOA) of auroral roar emissions at 2.5–3.0 MHz. Combined with electron density data from the Sondrestrom incoherent scatter radar (ISR), these DOA data provide a powerful test of the most‐cited generation mechanism of auroral roar emissions. This mechanism requires that the relationship between the electron plasma frequency (fpe) and the electron gryofrequency (fce) in the source regions isfpe2 = (n2 − 1)fce2, where n = 2, 3 is the harmonic number of the emission. For each of five auroral roar events studied, to within experimental uncertainty this frequency‐matching condition is met throughout the source region of the emissions inferred from the MFI and ISR data. Furthermore, ray‐tracing analysis indicates that signals at the roar frequency are able to propagate from the source region to the location of the MFI receiver.
Ion cyclotron diffusion of velocity distributions in the extended solar corona
Authors: Cranmer Steven R.
Published: January 2004

The Ultraviolet Coronagraph Spectrometer aboard the Solar and Heliospheric Observatory has revealed strong kinetic anisotropies and extremely large perpendicular temperatures of heavy ions in the extended solar corona. These observations have revived interest in the idea that the high‐speed solar wind is heated and accelerated by the dissipation of ion cyclotron resonant Alfvén waves. This process naturally produces departures from Maxwellian and bi‐Maxwellian velocity distributions. Here it is argued that these departures must be taken into account in order to understand the resonant velocity space diffusion, the wave damping, and the formation of ultraviolet emission lines. Time‐dependent ion velocity distributions are computed for a fixed spectrum of waves in a homogeneous plasma, and the moments of the distributions are compared with simple bi‐Maxwellian models. The existence of a boundary, in parallel velocity space, between resonance and nonresonance produces an effective saturation of the velocity space diffusion that bi‐Maxwellian models could not predict. The damping of an input wave spectrum is computed for a coronal population of 1000 ion species with the above saturation effect included. For realistic levels of fluctuation power, it is concluded that waves propagating solely from the coronal base would not be able to heat and accelerate the ions that have been observed to exhibit strong energization and that local wave generation is required. Ultraviolet emission line profiles are computed for the derived non‐Maxwellian distributions, and possible unique identifiers of the ion cyclotron resonance mechanism are noted.
Hyperspectral imaging of the night airglow layer from the shuttle: A study of temporal variability
Authors: Broadfoot A. Lyle , Gardner James A.
Published: January 2004

The temporal structure of the nightglow layer at the Earth's limb was monitored by the Arizona Airglow Experiment (GLO) from the space shuttle Endeavor throughout its 12‐day STS‐69 mission in September 1995. The GLO is a wideband long slit spectrograph or hyperspectral imager that views the Earth's limb at up to 24 contiguous tangent altitudes. All 24 spectra are recorded simultaneously from a single column of gas across the limb. Hyperspectral images of four emissions, O2(0,0), O2(Hz), O I(557.7), and OH(6,2), were constructed for analysis of the temporal condition of the nightglow layer along the orbit. It was found that the emission intensity of the major emitters, O2(0,0) and OH(6,2), were modified by factors of up to 3 by upward and downward transport of the mesospheric constituents through the 90–100 km region of the mesosphere. Significant variations in intensity were found associated with vertical transport of mesospheric cells 2000–5000 km in horizontal extent. The active cells are present for less than a day. Downward transport was signified by weakening of the OH(Meinel) emission and strengthening of the O2 and O I(557.7) emissions. Upward transport regions were outlined by enhancements in the OH(Meinel) and a second source of O2(atmospheric) emission. It was found that transport of low‐level mesospheric composition into the nightglow emission altitude region controls the nightglow spectral content. The atomic oxygen concentration above 100 km was found to be constant throughout the observational sequences and consequently not responsible for nightglow variability. An atmospheric interaction with the upward transported composition was detected. This interaction is likely to be with atomic oxygen that appears to be depleted following downward transport of the lower mesospheric composition. The OH emission is a true signature of mesospheric dynamics. Therefore this analysis initiates a new approach for monitoring mesospheric dynamics. Re‐analysis of archival data is recommended to detect the OH signature identified here. Collaborative ground‐based and space‐based overflight observational opportunities will be important to understanding the driving mechanism that initiates the vertical transport.