bluuebunny/crossref_metadata_2025 huggingface dataset:
Cause of the localized maximum of X‐ray emission in the morning sector: A comparison with electron measurements
Authors: Østgaard N. , Stadsnes J. , Bjordal J. , Vondrak R. R. , Cummer S. A. , Chenette D. L. , Schulz M. , Pronko J. G.
Published: February 2004

The Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on board the Polar satellite has provided the first global scale views of the patterns of electron precipitation through imaging of the atmospheric X‐ray bremsstrahlung. While other remote sensing techniques like ultraviolet (UV) and visible imaging sense emissions that are dominantly produced by low‐energy electrons (<10 keV), the PIXIE X‐ray images used in this study respond to electrons of energy above ∼3 keV. From a statistical study of global X‐ray emission a localized maximum in the morning sector, delayed with respect to substorm onset, is found to be a common feature during substorms. The time delay of this morning precipitation relative to substorm onset strongly indicates that this localized maximum is caused by electrons injected in the midnight sector drifting into a region in the dawnside magnetosphere where some mechanism effectively scatters the electrons into the loss cone. In this study we have examined two isolated substorms that occurred on July 31, 1997, and September 4, 1997, to investigate these features in more detail. PIXIE images are used to examine the global structures of the two events. Particle measurements from several low‐altitude satellites, NOAA and DMSP, have provided fine structure information of the spatial and spectral development of the morning precipitation. We find good spatial correlation between X‐ray emission and electron measurements for both these events. Comparison of measured electron spectra and electron spectra derived from X‐ray measurements by PIXIE are also presented and correlates fairly well. The technique for deriving electron characteristics from the X‐ray measurements are described and discussed. We find that both the electron spectra and the X‐ray spectra can be represented by double or single exponentials. The electron spectra measured in the early stage of the localized morning maximum of X‐ray emission strongly indicate that the scattering of >2–10 keV electrons by waveparticle interaction into the loss cone is the main mechanism for this precipitation.
Global storm time auroral X‐ray morphology and timing and comparison with UV measurements
Authors: Anderson P. C. , McKenzie D. L. , Brittnacher M. J. , Chen M. W. , Hairston M. , Thomsen M. F.
Published: February 2004

The Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on NASA's Polar spacecraft provides the first global images of the auroral oval in X‐rays and allows very accurate measurements of the timing of geomagnetic disturbances to a degree of temporal resolution not available from previous imagers due to its photon counting characteristics. On October 19, 1998, a magnetic cloud associated with a CME encountered the Earth's magnetopause near 0500 UT, generating a magnetic storm that reached a minimum value in Dst of −139 nT. The z component of the interplanetary magnetic field (IMF) (Bz) remained remarkably steady for the first 10 hours of the storm as did the solar wind particle pressure. The PIXIE and UVI instruments on the Polar spacecraft were both imaging the auroral oval from 0800 to 1800 UT; six distinct impulsive auroral enhancements were observed by the imagers during this time period. Global imaging combined with geosynchronous particle observations allowed classification of the geomagnetic disturbances associated with the events. Only two of the events were classified as substorms; one was classified as a poleward boundary intensification, one was a convection bay, and one was a pseudobreakup. A sixth event occurred after a dramatic northward turning of the IMF at the end of the 10‐hour Bz south period but was very weak and transient. The effects of the northward turning were counteracted by a simultaneous increase in the By component of the IMF. The first sign of significant substorm activity occurred over 8 hours after the cloud encountered the Earth and was not associated with any change in the solar wind magnetic field or particle pressure. The cross polar cap potential remained large (> 100 kV), and most of the X‐ray emissions observed were associated with enhanced earthward convection caused by large cross‐tail electric fields; 50% were collected from the 0000 – 0600 magnetic local time (MLT) sector.
Composition of quasi‐stationary solar wind flows from Ulysses/Solar Wind Ion Composition Spectrometer
Authors: von Steiger R. , Schwadron N. A. , Fisk L. A. , Geiss J. , Gloeckler G. , Hefti S. , Wilken B. , Wimmer‐Schweingruber R. R. , Zurbuchen T. H.
Published: February 2004

Using improved, self‐consistent analysis techniques, we determine the average solar wind charge state and elemental composition of nearly 40 ion species of He, C, N, O, Ne, Mg, Si, S, and Fe observed with the Solar Wind Ion Composition Spectrometer on Ulysses. We compare results obtained during selected time periods, including both slow solar wind and fast streams, concentrating on the quasi‐stationary flows away from recurrent or intermittent disturbances such as corotating interaction regions or coronal mass ejections. In the fast streams the charge state distributions are consistent with a single freezing‐in temperature for each element, whereas in the slow wind these distributions appear to be composed of contributions from a range of temperatures. The elemental composition shows the well‐known first ionization potential (FIP) bias of the solar wind composition with respect to the photosphere. However, it appears that our average enrichment factor of low‐FIP elements in the slow wind, not quite a factor of 3, is smaller than that in previous compilations. In fast streams the FIP bias is found to be yet smaller but still significantly above 1, clearly indicating that the FIP fractionation effect is also active beneath coronal holes from where the fast wind originates. This imposes basic requirements upon FIP fractionation models, which should reproduce the stronger and more variable low‐FIP bias in the slow wind and a weaker (and perhaps conceptually different) low‐FIP bias in fast streams. Taken together, these results firmly establish the fundamental difference between the two quasi‐stationary solar wind types.
Self‐consistent hybrid simulations of the interaction of the heliosphere with the local interstellar medium
Authors: Müller Hans‐Reinhard , Zank Gary P. , Lipatov Alexander S.
Published: February 2004

A new method for investigating the interaction of the solar wind with the partially ionized local interstellar medium (LISM) is presented. The solar wind and the interstellar plasma are modeled using a two‐dimensional (2‐D) hydrodynamic numerical code. The plasma is coupled to the neutral hydrogen (of both interstellar and solar wind origin) via resonant charge exchange. To model the neutral H distribution, we use a nonstationary 2.5‐D particle mesh, method to solve the Boltzmann equation, which is coupled self‐consistently to the interstellar and solar wind plasma. Numerical self‐consistency is achieved by iterating the plasma and neutral H distributions between the two numerical schemes until a steady state is achieved. Results from three test applications are presented and discussed, including the first one‐shock kinetic simulation. The simulations are able to reproduce the main features of the heliosphere such as shock structure, hydrogen wall, and heating, deceleration and filtration of neutral hydrogen. In addition, they enable the study and interpretation of the non‐Maxwellian hydrogen distribution function. Traces of fast neutrals originating inside the termination shock and the heliosheath/heliotail region can be found far upstream of the outer heliosphere. The influence of different interstellar plasma boundary values on the heliosphere is highlighted in the comparison of two supersonic simulations and one subsonic simulation. In particular, by comparing the simulated energetic neutral atom (ENA) fluxes at 1 AU of the supersonic and subsonic models, it is found that the subsonic flux is significantly underabundant in the energy range 10 – 60 eV compared to the supersonic case. This may offer an important diagnostic for determining whether the heliosphere possesses a bow shock or not.
Ionospheric storm of early November 1993 in the Indian equatorial region
Authors: Sastri J. Hanumath , Jyoti N. , Somayajulu V. V. , Chandra H. , Devasia C. V.
Published: February 2004

An investigation is made of the response of equatorial ionosphere in the Indian (75°E) sector to the major magnetic storm of November 3, 1993, using data from the ionosonde and magnetometer networks spanning the region 0.3–34.5°N dip. Some outstanding and new aspects of the storm time ionospheric behaviour are revealed. An anomalous and striking positive gradient in ƒ0F2 from the magnetic equator to 34.5° dip developed under counter electrojet (CEJ) condition in the morning on November 4, corresponding to the early stage of the storm main phase. This storm effect is attributed to plasma transport by a poleward surge in transequatorial winds due to large scale atmospheric gravity waves (AGWs) launched by auroral heating. Remarkable wave‐like variations in hpF2 and ƒ0F2 immediately followed at locations away from dip equator till local sunset, with concomitant disruptions in the development of the equatorial ionization anomaly (EIA). Rapid variations in meridional neutral winds due to large‐scale AGWs are assessed as the cause of the oscillations in hpF2 and the associated cyclic sequence of development and inhibition of EIA as the outcome of the combined effects of plasma transport due to meridional winds and the plasma “fountain” driven by E×B drift. Just after the onset of the storm recovery phase at 1800 LT, a sudden and anomalous drop (54–57%) in ƒ0F2 prevailed throughout the anomaly region over the interval 1915–2330 LT, with an apparent time delay in occurrence toward the magnetic equator. This premidnight collapse of equatorial F region is interpreted in terms of horizontal transport of plasma across the equator toward the opposite hemisphere by an equatorward surge in meridional winds. The case study showed that besides disturbances in the zonal electric field due to prompt penetration and ionospheric disturbance dynamo effects, perturbations in neutral meridional winds played a prominent role in the ionospheric storm of November 4, 1993, in the Indian equatorial region.
Enhancement of equatorial energetic electron fluxes near L = 4.2 as a result of high speed solar wind streams
Authors: Hilmer R. V. , Ginet G. P. , Cayton T. E.
Published: February 2004

We examine the relationship of energetic equatorial electron flux enhancements occurring near L = 4.2 and 6.6 associated with 26 well‐defined high‐speed solar wind streams (HSSWS) detected by Wind between December 1994 and September 1996. Events were selected for having high‐energy (>2 MeV) geosynchronous electron daily average fluxes surpassing 103 cm−2 s−1 sr−1 for at least a day as measured by GOES 7 or GOES 9. Los Alamos differential‐energy electron data from SOPA (0.2 – 2.0 MeV) at L = 6.6 and the GPS BDD‐II dosimeters (0.2 – 3.2 MeV) at L = 4.2 illustrate that flux dropouts are typically observed in all energy channels at both equatorial altitudes within the first day of each event. While SOPA consistently records postdropout flux enhancements, GPS dosimeters detect equatorial postdropout enhancements in 1.6–3.2 MeV electron fluxes in only 15 of 26 events and all are either concurrent (1 event) with or follow (14 events) the geosynchronous increases of electrons with similar values of the first adiabatic invariant, μ ∼ 2.1 × 103 MeV G−1. In addition, 10 of 15 GPS growth periods produced electron enhancements above predropout levels. For all 26 events the phase space density for electrons of similar μ is consistently greater at geosynchronous altitude than at GPS equatorial altitude. The critical factor leading to GPS L = 4.2 electron flux enhancements is elevated geomagnetic activity levels of Kp ∼ 3.0 – 3.5 and above for extended periods. A combination of enhanced solar wind ram pressure, electric field (with B‐south), and velocity also appears to be necessary. If outward phase space density gradients are combined with the large electric fields generally accompanying elevated Kp, then sufficient conditions may exist to promote the inward radial diffusive transport of equatorial electrons that ultimately lead to electron flux enhancements at GPS altitudes. Comparison of observed and theoretically estimated electron growth rates is consistent with this picture of inward radial transport for these equatorially mirroring particles with μ ∼ 2.1 × 103 MeV G−1 at L = 4.2.
Similarity transformations for fitting of geophysical properties: Application to altitude profiles of upper atmospheric species
Authors: Picone J. M. , Meier R. R.
Published: February 2004

The similarity transform method provides a new, highly robust, and stable parametric representation of geophysical functions for use in retrieving such functions from remote sensing observations. The present discussion focuses on the approximation of altitude profiles of upper atmospheric species concentration and on the development of parametric forward models for use with discrete inverse theory (DIT). Of equal importance, the similarity transform approach provides a framework for extracting generic profile shape information, in the form of a nondimensional shape function, from observations or detailed numerical simulations. In this way the method facilitates analysis of general characteristics of species concentration variations with altitude and with other geophysical parameters. For DIT retrievals of concentration profiles from observations a similarity transformation‐based forward model embeds the generic (“basis”) shape information directly into a parametric representation of each species profile. The presentation covers the extraction of nondimensional shape functions from discrete data or simulations, the basic forward model representation, and generalizations of the basic approach. We include simple examples of similarity transform fitting calculations in which the species concentration profiles to be approximated are generated by the Mass Spectrometer Incoherent Scatter Empirical 1990 (MSISE‐90) atmospheric model, as are the basis profiles that define the shape information.
Multipoint observations of global magnetospheric compressions
Authors: Korotova G. I. , Kivelson M. G. , Sibeck D. G. , Potemra T. A. , Stauning P.
Published: February 2004

The magnetic field experiment on the Viking satellite detected long‐period compressional oscillations with amplitudes ranging from 2 to 5 nT and periods ranging from 7 to 23 min on nine Viking orbits covering a wide range of polar and subauroral latitudes during the period from March to August 1986. We present a case study of an event on June 18, 1996, in which similar long‐period pulsations were also observed by a global network of more than 50 ground magnetometers and by the GOES‐5 and GOES‐6 spacecraft. The magnetic field variations throughout the auroral, midlatitude, and equatorial regions, at Viking, GOES‐5, and GOES‐6 were approximately in phase, whereas the variations at some morningside stations were a half‐wave period out of phase. We interpret these long‐period oscillations as evidence for periodic compressions and inflations of the magnetosphere driven by solar wind pressure pulses. The relative time delays of the peak variations observed by GOES‐5, GOES‐6, Viking, and the ground‐based stations are consistent with an event originating on the prenoon magnetopause whose signatures spread both poleward and antisunward on both sides of local noon.
Statistical study of auroral kilometric radiation fine structure striations observed by Polar
Authors: Menietti J. D. , Persoon A. M. , Pickett J. S. , Gurnett D. A.
Published: February 2004

We have conducted a statistical survey of a semirandom sample of the auroral kilometric radiation (AKR) data observed by the plasma wave instrument wideband receiver on board the Polar spacecraft. We have determined that AKR fine structure patterns with very narrowband, negative drifting striations occur in approximately 6% of the high‐resolution wideband spectrograms when AKR is present. Positive sloping striations are also observed, but at a much lower rate. More than 8200 AKR stripes have been scaled. The stripes are predominantly found in the 40‐ to 215‐kHz frequency range and have a frequency extent of about 4 kHz and a duration of usually less than 2 s. The majority of the stripes have drift rates between −8 and −2 kHz/s, with a peak in the distribution between −6 and −4 kHz/s. There is also a much smaller group of striations with positive drift rates of up to about 5 or 6 kHz/s. We have further investigated the change of drift rate with frequency. Almost all striations are observed in the lowest two frequency bands of the wideband receiver (ƒ< 215 kHz). There is an increase in the statistical drift rate with increasing frequency. The statistical slope of the striations increases with frequency from about −4.4 kHz/s at 75 kHz to about −5.7 kHz/s at 170 kHz. This frequency dependence of the drift rate is consistent, under certain conditions, with a production mechanism stimulated by an upward propagating electromagnetic ion cyclotron wave, as had been suggested earlier. However, such a changing drift rate is also compatible with a stimulated source region that propagates upward along the magnetic field line at the velocity of an ion beam accelerated by a local, upward directed electric field, as is typically observed in the auroral region. An explanation for this association is not apparent at this time.
Relationship between the Northern Hemisphere Joule heating and geomagnetic activity in the southern polar cap
Authors: Ballatore P. , Lanzerotti L. J. , Lu G. , Knipp D. J.
Published: February 2004

One of the most important effects from the coupling of the solar wind to the magnetosphere‐ionosphere system is the Joule heating (JH) of the atmosphere that is produced by the energy dissipation of ionospheric currents and geomagnetic field‐aligned precipitating particles. At present, the most commonly used technique to estimate the global JH rate is the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure. Here we describe a study of the relationship of the Northern Hemisphere JH and the Southern Hemisphere polar geomagnetic index AES‐80 during a magnetic storm on October 18–23, 1995 (when both quantities are available). The purpose is to study the effects of the Northern‐Southern Hemispherical asymmetry on the correlation between JH and geomagnetic indices. Our results confirm a higher contribution to JH from regions associated with eastward currents. Moreover, we find that the best correspondence between the northern JH and the AES‐80 occurs during negative interplanetary magnetic field (IMF) BZ and By conditions. We discuss how this result is in agreement with the magnetospheric‐ionospheric model that considers, during negative IMF Bz and By, an increase of conductance in the regions associated with eastward currents in the Northern Hemisphere. Our observations related to the best estimation of Southern Hemisphere JH are in agreement with the same model too. We also find a “saturation” effect for large values of northern JH: the JH—AES‐80 correlation breaks down for intervals with JH > 190 gigawatt (GW), during the highest geomagnetic perturbations, and a negative IMF Bz that exceeds −20 nT. This “saturation” is in part attributed to the onset of hemispherical asymmetry due to the solar wind pressure with respect to the Earth‐dipole orientation under severe storm conditions.
Theoretical interpretation of Jupiter's multibanded whistler mode emission
Authors: Leubner M. P.
Published: February 2004

Three distinct bands of whistler mode emissions were detected by the Voyager 1 and Voyager 2 plasma wave instruments during the encounter with Jupiter. The broadband whistler mode hiss below 1 kHz and two narrowbanded, high‐frequency emissions near 10 kHz were identified as whistler mode chorus and half electron cyclotron emission. In the present analysis the entire electron velocity space distribution is modeled by a superposition of three anisotropic components, two kappa distributions of different spectral index, and a drifting Maxwellian distribution. Applying a linear instability analysis on this specific distribution and providing the basic plasma parameters from spacecraft observations yields a highly accurate picture of the structure of the observed discrete Jovian whistler emission spectrum. It is concluded that the generation of the hiss structure can be interpreted to be a consequence of resonating suprathermal non‐Maxwellian electrons, whereas the chorus is generated by a nearly bi‐Maxwellian population. The half electron cyclotron emission can be associated with the interaction of a drifting, anisotropic component of keV electrons. The analysis of the main emission bands of a specific full spectrum can serve as representative pattern for the interpretation of various whistler mode events in planetary magnetospheres.
The propagation of high‐m Alfvén waves in the Earth's magnetosphere and their interaction with high‐energy particles
Authors: Klimushkin Dmitri Y.
Published: February 2004

This paper is devoted to the study of Alfvén waves generated by the bounce‐drift resonance. The global structure of such waves is investigated with due regard to the curvature of field lines, the plasma nonuniformity along field lines and across magnetic shells, finite plasma pressure, and the interaction of the waves with the ionosphere. It is shown that the wave is enhanced as it propagates across magnetic shells with a growth rate dependent on the radial coordinate: The growth rate is maximal on the poloidal surface (i.e., on the magnetic shell, on which the oscillations have a poloidal character), and it decreases to zero on the toroidal surface (where the oscillations have a toroidal character). However, the amplitude maximum does not coincide with the poloidal surface; it is shifted toward the toroidal surface. Moreover, when the value of the growth rate due to the driving particles increases on the poloidal surface, the amplitude maximum of the wave is shifted toward the toroidal surface.
Determining the mass density along magnetic field lines from toroidal eigenfrequencies
Authors: Denton Richard E. , Gallagher D. L.
Published: February 2004

Toroidal eigenfrequencies can be used to remotely sense the equatorial mass density ρeq and the density dependence along a magnetic field line. Here we present improvements to the method of Schulz [1996], which allows ρeq and the power law index α (for mass density along a field line ∝ R−α, where R is the radial distance from the center of the Earth) to be determined from the y intercept and slope of a plot of toroidal frequency versus toroidal harmonic number n. Our modifications include a model form for eigenfrequencies with a fractional precision of 0.0005 for −6 ≤ α ≤ 6 and 2 ≤ L ≤ 8 (accuracy is doubtful beyond L = 5) and an iterative procedure for getting more accurate results than those found using Schulz's method. In addition, we do an analysis of the effect of random measurement errors. Observed frequencies need to be accurate to ∼6% (3%) of the fundamental frequency in order to determine ρeq (α) to a precision of 30% (unity). We then apply our method to data generated using the Global Core Plasma Model for plasmaspheric mass density; our analysis demonstrates clearly how the α index represents the mass density dependence on the outer part of the field line (R/(LRE) ≳ 2/3).
Particle transport and energization associated with substorms
Authors: Zaharia Sorin , Cheng C. Z. , Johnson Jay R.
Published: February 2004

Energetic particle flux enhancement events observed by satellites during substorms are studied by considering the interaction of particles with earthward propagating electromagnetic pulses of westward electric field and consistent magnetic field of localized radial and azimuthal extent in a background magnetic field. The energetic particle flux enhancement is mainly due to the betatron acceleration process: particles are swept by the earthward propagating electric field pulses via the E×B drift toward the Earth to higher magnetic field locations and are energized because of magnetic moment conservation. The most energized particles are those which stay in the pulse for the longest time and are swept the longest radial distance toward the Earth. Assuming a constant propagating velocity of the pulse, we obtain analytical solutions of particle orbits. We closely examine substorm energetic particle injection by computing the particle flux and comparing with geosynchronous satellite observations. Our results show that for pulse parameters leading to consistency with observed flux values, the bulk of the injected particles arrive from distances of less than 9 RE, which is closer to the Earth than the values obtained by the previous model (Li et al., 1998).
Small‐scale vortices in the high‐latitude F region
Authors: Huber Matthias , Sofko George J.
Published: February 2004

The dayside outer magnetosphere is a region of direct interaction with the shocked solar wind in the magnetosheath, resulting in plasma and hydromagnetic waves and spatial turbulence which map along the Earth's magnetic field lines down to the ionosphere where the waves and/or turbulence can be detected directly by radar scattering from the associated electron density variations. In a previous paper by Schiffler et al. [1997] it was reported that double‐peaked (D‐P) velocity spectra measured by the Super Dual Auroral Radar Network (SuperDARN) HF radars are a signature of backscatter from the ionospheric footprint of the outer low‐latitude boundary layer (LLBL). In the present statistical study, Saskatoon HF radar data are used to determine the ionospheric location at which D‐P HF spectra originate and to confirm the previous result that the majority of the D‐P spectra are associated with the ionospheric cusp/cleft region. Employing high spatial (15 km) and high temporal (3 s) resolution modes, we establish an upper limit of ∼ 26 km on the scale size and ∼4 s on the lifetime of the structures producing the echoes showing D‐P spectra. We postulate a number of mechanisms that could cause D‐P spectra and conclude that ionospheric vortices appear to be the most likely explanation of the radar observations.
Statistical nature of impulsive electric fields associated with fast ion flow in the near‐Earth plasma sheet
Authors: Tu J.‐N. , Tsuruda K. , Hayakawa H. , Matsuoka A. , Mukai T. , Nagano I. , Yagitani S.
Published: February 2004

Statistical characteristics of impulsive electric field (IEFD) associated with fast earthward ion flows in the inner central plasma sheet were studied by using electric field data obtained from the double‐probe instrument on board the Geotail satellite. It is shown that the strongest electric field is produced in the region between XAGSM = −15 RE and −9 RE where the fast earthward flow is decelerated. The IEFDs are short‐lived with a timescale of ∼2 min on the average, compatible with the timescale of other signatures of substorm fine structures. This strong cross‐tail electric field, coincident with the braking of the fast flow and correlated with the magnetic field dipolarization, has important implications for the particle acceleration in the near‐Earth plasma sheet.
Alfvén waves and wings in nonuniform plasmas
Authors: Sallago P. A. , Platzeck A. M.
Published: February 2004

The problem of a source moving in a plasma with background fields varying in one direction, in the magnetohydrodynamic approximation, is analyzed. The variation of the background fields is assumed to be perpendicular to the background velocity and induction magnetic fields. Since Alfvén wings are closely related to Alfvén waves, the existence of Alfvenic perturbations, in this case, is searched. Instead of linearizing the MHD equations and searching monochromatic waves, the conditions that the group velocity be parallel to the background magnetic induction field, in the reference system in which the plasma is locally at rest, the perturbation be incompressible, and the total pressure remain constant, are imposed. Two modes of propagation are found. Since it is possible to define an invariant direction, the methodology of stream functions is used for the analysis of Alfvén wings. Their existence, when the variation of the background fields satisfies some conditions, is proved, and the relations among the density, pressure, magnetic field, plasma velocity, and electric current density in the wing are found. Physical situations in which these results can be applied, with such a particular spatial dependence of the background fields, are restricted. However, as an approximation, it can be applied to spacecraft or space tethers moving in a circular orbit, if one takes into account that the density and the induction magnetic field change with the altitude. The method can be extended to cases in which the background fields vary in a more general way.
Association of energetic neutral atom bursts and magnetospheric substorms
Authors: Jorgensen A. M. , Kepko L. , Henderson M. G. , Spence H. E. , Reeves G. D. , Sigwarth J. B. , Frank L. A.
Published: February 2004

In this paper we present evidence that short‐lived bursts of energetic neutral atoms (ENAs) observed with the Comprehensive Energetic Particle and Pitch Angle Distribution/Imaging Proton Spectrometer (CEPPAD/IPS) instrument on the Polar spacecraft are signatures of substorms. The IPS was designed primarily to measure ions in situ, with energies between 17.5 and 1500 keV. However, it has also proven to be a very capable ENA imager in the range 17.5 keV to a couple hundred keV. It was expected that some ENA signatures of the storm time ring current would be observed. Interestingly, IPS also routinely measures weaker, shorter‐lived, and more spatially confined bursts of ENAs with duration from a few tens of minutes to a few hours and appearing once or twice a day. One of these bursts was quickly associated with magnetospheric and auroral substorm activity and has been reported in the literature [Henderson et al, 1997]. In this paper we characterize ENA bursts observed from Polar and establish statistically their association with classic substorm signatures (global auroral onsets, electron and ion injections, AL drops, and Pi2 onsets). We conclude that ∼90% of the observed ENA bursts are associated with classic substorms and thus represent a new type of substorm signature.
Mirror instability thresholds in suprathermal space plasmas
Authors: Leubner M. P. , Schupfer N.
Published: February 2004

An increasing number of high‐resolution spacecraft observations identify magnetic fluctuations in various space plasma environments as mirror mode structures. In a collisionless plasma the excitation of mirror modes has to be analyzed in a kinetic approach, where details of the velocity space distributions are of considerable significance. Nonthermal, high‐energy tail distributions, commonly detected in planetary magnetospheres and the interplanetary medium, are known to be accurately modeled by the family of kappa distributions. It is demonstrated that the conventional fluid instability criterion is inadequate since suprathermal particle populations provide a strong source for the generation of mirror wave mode activity even in the low‐β limit and small temperature anisotropy. The specific structure of velocity space distributions at high energies dominates as regulating mechanism for the mirror instability threshold.
O+, O, and O2 densities derived from measurements made by the High Resolution Airglow/Aurora Spectrograph (HIRAAS) sounding rocket experiment
Authors: Dymond K. F. , McCoy R. P. , Thonnard S. E. , Budzien S. A. , Thomas R. J. , Bullett T. N. , Bucsela E. J.
Published: February 2004

We present the results of an analysis of the O II 834 Å and O I 1356 Å altitude profiles measured during a sounding rocket flight on March 19, 1992. The profiles were analyzed using a new set of models that used discrete inverse theory to seek a maximum likelihood fit to the data. Both profiles were fit simultaneously to ensure consistency of the retrieved ionosphere and thermospheric neutral density. During the analysis the thermospheric neutral density and temperature were modeled using the Mass Spectrometer Incoherent Scatter (MSIS‐86) model [Hedin, 1987]. Two parameters were used to scale the absolute MSIS O and O2 densities; the exospheric temperature was altered by varying the 10.7 cm solar flux (an MSIS‐86 input). The ionospheric O+ density was modeled by a three‐parameter Chapman layer. The retrieved MSIS scalars for the O and O2 densities were 0.47 ± 0.09 and 0.58 ± 0.14, respectively. These scalars indicate that the MSIS‐86 model predicted significantly higher O and O2 densities. The inferred exospheric temperature was 1125 K in good agreement with the MSIS‐86 prediction. The derived O density is in good agreement with the O density inferred from midultraviolet spectra observed during the same rocket flight [Bucsela et al., 1998]. The retrieved F region peak density, 1.98 ± 0.63 × 106 cm−3, peak height, 291 ± 22 km, and plasma scale height, 138 ± 24 km, all agreed with coincident digisonde measurements. Thus we have demonstrated that the ionospheric state can be accurately determined by inversion of observed O II 834 Å limb radiance profiles.
Observations of an intermediate layer during the Coqui II campaign
Authors: Bishop R. L. , Earle G. D. , Herrero F. A. , Bateman T. T.
Published: February 2004

NASA sounding rocket 21.114, launched March 7, 1998, during the Coqui II campaign, provided neutral wind and plasma density measurements of a weak intermediate layer. The layer was centered near 140 km and had an approximate peak plasma density of 2200 cm−3. The measured winds were typically less than 40 m s−1, in agreement with wind shear formation theory and coincident density observations. The data obtained during the flight allow us to explore the plasma density structure and wind field morphology of the intermediate layer. Coupled with simultaneous data from Arecibo Observatory, the upleg and downleg density profiles provide three spatially separated measurements that enable the first detailed investigation of the horizontal extent and variation of an intermediate layer.
Substorm‐associated changes in large‐scale convection during the November 24, 1996, Geospace Environment Modeling event
Authors: Lyons L. R. , Ruohoniemi J. M. , Lu G.
Published: February 2004

We have used data from the November 24, 1996, Geospace Environment Modeling program substorm event to investigate the critical question of substorm expansion phase triggering by interplanetary magnetic field (IMF) changes that lead to a reduction in large‐scale convection. Coverage of ionospheric convection with the Super Dual Auroral Radar Network (SuperDARN) radars was excellent during this period. This has allowed us to investigate the changes in large‐scale convection associated with two expansion phase onsets that followed a 80‐min period of strongly southward and quite steady IMF. We have found that reductions in global convection occurred in association with two observed northward turnings of the IMF, and, to within observation uncertainty, these reductions occurred at the same times that the two onsets were first observed in the magnetosphere. The first (smaller) northward turning was found to give a small convection reduction and the onset of a small substorm, while the second (much larger) northward turning was found to give a large reduction in global convection and the onset of a large substorm. Taken together with other, more localized studies which also show convection reductions associated with expansion onsets, our results imply that IMF‐driven reductions in global convection may be generally associated with expansion onset. Our results also suggest that substorm expansions may represent an adjustment to conditions of reduced convection following a growth phase of enhanced convection and magnetotail energy storage. Under this suggestion, for a given amount of energy stored in the tail, a small reduction in convection would lead to a small substorm, and a large reduction in convection would lead to a large substorm.
Collisionless magnetic reconnection: Electron processes and transport modeling
Authors: Hesse Michael , Birn Joachim , Kuznetsova Masha
Published: February 2004

Particle‐in‐cell simulations are used to investigate collisionless magnetic reconnection in thin current sheets, based on the configuration chosen for the Geospace Environment Modeling (GEM) magnetic reconnection challenge [Birn et al., this issue]. The emphasis is on the overall evolution, as well as details of the particle dynamics in the diffusion region. Here electron distributions show clear signatures of nongyrotropy, whereas ion distributions are simpler in structure. The investigations are extended to current sheets of different widths. Here we derive a scaling law for the evolution dependence on current sheet width. Finally, we perform a detailed comparison between a kinetic and Hall‐magnetohydrodynamic model of the same system. The comparison shows that although electric fields appear to be quite similar, details of the evolution appear to be considerably different, indicative of the role of further anisotropies in the ion pressures.
Collisionless reconnection supported by nongyrotropic pressure effects in hybrid and particle simulations
Authors: Kuznetsova Masha M. , Hesse Michael , Winske Dan
Published: February 2004

This paper presents the detailed comparative analysis of full particle and hybrid simulations of collisionless magnetic reconnection. The comprehensive hybrid simulation code employed in this study incorporates essential electron kinetics in terms of the evolution of the full electron pressure tensor in addition to the full ion kinetics and electron bulk flow inertia effects. As was demonstrated in our previous publications, the electron nongyrotropic pressure effects play the dominant role in supporting the reconnection electric field in the immediate vicinity of the neutral X point. The simulation parameters are chosen to match those of the Geospace Environmental Modeling (GEM) “Reconnection Challenge.” It is that these comprehensive hybrid simulations perfectly reproduce the results of full particle simulations in many details. Specifically, the time evolutions of the reconnected magnetic flux and the reconnection electric field, as well as spatial distributions of current density and magnetic field at all stages of the reconnection process, are found to be nearly identical for both simulations. Comparisons of variations of characteristic quantities along the x and z axes centered around the dominating X points also revealed a remarkable agreement. Noticeable differences are found only in electron temperature profiles, i.e., in the diagonal electron pressure tensor components. The deviation in the electron heating pattern in hybrid simulations from that observed in particle simulations, however, does not affect parameters essential for the reconnection process. In particular, the profiles of the off‐diagonal components of the electron pressure tensor are found to be very similar for both runs and appear unaffected by heat flux effects. Both simulations also demonstrate that the Ey component of the electric field is nearly constant inside the diffusion region where ions are nonmagnetized. We demonstrate that the simple analytical estimate for the reconnection electric field as a convection electric field at the edge of the diffusion region very well reproduces the reconnection electric field observed in the simulations.
Charge state distributions of iron in impulsive solar flares: Importance of stripping effects
Authors: Ostryakov V. M. , Kartavykh Y. Y. , Ruffolo D. , Kovaltsov G. A. , Kocharov L.
Published: February 2004

A model of stochastic acceleration of heavy ions by Alfvén wave turbulence has been developed. It takes into account spatial diffusion, Coulomb losses, and the possibility of charge changes for ions during stochastic acceleration. The main processes influencing the ionic charge states are the stripping by thermal electrons and protons as constituents of a surrounding medium and dielectronic and radiative recombination. We have calculated energy spectra and charge distributions of nonthermal Fe ions as a sample species. The dependence of the charge distributions and energy spectra of iron on the parameters of the plasma (temperature and number density) is studied. We compare our results with measurements to date of the mean charge of iron in impulsive solar flare events and conclude that they indicate source plasma ionization temperatures between 6□×106 and 107 K.
Local interstellar medium and modeling the heliosphere
Authors: Ratkiewicz R. , Barnes A. , Spreiter J. R.
Published: February 2004

We use the best currently available estimates for interstellar parameters to update and expand the earlier study of Ratkiewicz et al. [1998]. We report (1) a more complete survey of the morphology of the heliosphere and the ways it may be modified by variation of external interstellar parameters, (2) a detailed classification of the several asymmetries and distortions associated with exterior field‐flow obliquity, including cross sections of the three‐dimensional structure in all three principal planes, (3) an analysis of the transition from supermagnetosonic to nearly transmagnetosonic flow, and (4) specific results for the heliosphere for the best current understanding of conditions in the local interstellar medium.
Periodic variations of geocoronal Balmer‐alpha brightness due to solar‐driven exospheric abundance variations
Authors: Kerr R. B. , Garcia R. , He X. , Noto J. , Lancaster R. S. , Tepley C. A. , González S. A. , Friedman J. , Doe R. A. , Lappen M. , McCormack B.
Published: February 2004

Measurements of the geocoronal Balmer‐alpha (Hα) brightness have been made at the Arecibo Observatory during 11 separate periods since 1983 using both a Fabry‐Perot interferometer and a tilting filter photometer. The tilting filter photometer is calibrated for absolute sensitivity using a constant brightness source traceable to National Institute of Standards and Techniques (NIST) standards and is used to cross‐calibrate the Fabry‐Perot interferometer. Since the observational technique has not changed since 1983, and since the data analyses technique are uniform, these data provide a measure of the solar cycle variation of Hα brightness at Arecibo. Unlike earlier studies, we discern no systematic discrepancy between the Hα brightness and estimates of the solar Lyman‐beta flux that pumps the resonant fluorescent Hα emission. Rather, we conclude that geocoronal hydrogen abundance is generally (but not always) larger than models suggest, although not systematically so. The largest Hα emission is measured during solar minimum conditions, when brightness is approximately 50% greater than during solar maximum conditions, for measurements at solar depression angles from 15° to 35°. Above about 40° solar depression (corresponding to an illuminated column with a base at approximately 2000 km), no solar cycle variation is evident, and the brightness there is persistently greater than models. Intricacies of the Lyman‐beta radiative transport problem make detection of an early morning maximum of hydrogen density near the exobase difficult using brightness data alone, although the Hα brightness is generally asymmetric with respect to local midnight, with brighter emission in the postmidnight sector. Studies of the semidiurnal (nighttime) variation demonstrate that the Hα brightness can vary by a factor of 2 over a period of days.
HF detection of slow long‐lived E region plasma structures
Authors: Jayachandran P. T. , St.‐Maurice J.‐P. , MacDougall J. W. , Moorcroft D. R.
Published: February 2004

During the equinox and winter seasons, and in the range 300–1000 km the Saskatoon Super Dual Auroral Radar Network (SuperDARN) radar often detects extended patches of coherent echoes with remarkably uniform properties and low Doppler speeds, in the range 0 to 200 m/s. Typically, these echoes last for ∼3 hours, and are observed between 1300 and 2300 MLT, at times of moderate to high Kp values. The echo Doppler shift changes systematically with azimuthal angle and a vector reconstruction of the implied drift indicates westward velocities in the range 150 to 250 m/s, well below the threshold speed associated with Farley‐Buneman waves. When ionosonde observations are available, they invariably show the presence of a thick sporadic E layer. This feature, plus the facts that the IMF By is always negative and that the echoes are equatorward of the regions of discrete precipitation (as indicated by comparison with coincident DMSP satellite observations), indicate that the echoes are associated with the diffuse aurora in regions where the electric field is of the order of 10 mV/m or less. We infer from these echo properties that the irregularities are triggered by a primary gradient‐drift mechanism which then cascades to the observed structures through weakly turbulent mode‐coupling processes. Several events were observed during special multifrequency experiments using the Saskatoon SuperDARN radar. It was found that the Doppler speed, power, and spectral width all increase systematically with increasing radar frequency. The findings for Doppler speed and power appear to arise, at least in part, from the increase in height of the radar echoes with increasing frequency. The frequency dependence of spectral width may be related to instability lifetimes; it was found to agree well with the results of numerical simulations [Keskinen et al., 1979].
The geopause in relation to the plasma sheet and the low‐latitude boundary layer: Comparison between Wind observations and multifluid simulations
Authors: Li Q. , Winglee R. M. , Wilber M. , Chen L. , Parks G.
Published: February 2004

Wind observations during the May 10, 1996 perigee are used in conjunction with multifluid global simulations to investigate the mixing of magnetosheath‐like plasma and ionospheric plasma in the dawn current sheet. Magnetosheath‐like plasma can play important roles in the formation of the low‐latitude boundary layer (LLBL) and the cold, dense plasma sheet (CDPS) identified previously in Geotail studies where the magnetosheath component is seen as a low energy component. The Wind observations show that this cold component can be seen well inside the nominal position of the magnetopause and even the CDPS, and is most pronounced near the magnetic equator. This cold plasma provides an important tracer for the geopause where the dynamics of the region inside the boundary is controlled by ionospheric plasma and the region outside is dominated by plasma of solar wind origin. The geopause position in the present case appears be about halfway between the noon‐midnight meridian and the magnetopause. In addition, it is shown that the cold magnetosheath‐like plasma has significant asymmetries between the northern and southern sides of the current sheet with enhanced cold plasma being observed in the Northern Hemisphere relative to that seen in the Southern Hemisphere. This asymmetry which is also seen in the modeling and is associated with the high dipole (> 20°) tilt during the event which allows enhanced entry of magnetosheath plasma in the Northern (summer) Hemisphere and enhanced ionospheric plasma outflows in the Southern (winter) Hemisphere. Flow asymmetries in the particle distributions that have properties similar to the magnetosheath boundary layer are also observed. However, the characteristics of the distributions indicate that the flow is between the LLBL and the CDPS as opposed to the LLBL and the magnetosheath.