bluuebunny/crossref_metadata_2025 huggingface dataset:
Weak auroral emissions and particle precipitations in the dusk auroral oval
Authors: Ono Takayuki , Hirasawa Takeo , Meng Ching‐I.
Published: February 2008

Faint auroral displays in the low‐latitude region of the duskside auroral oval were examined by using 5577 Å, 6300 Å, and 4861 Å auroral images from three monochromatic all‐sky television cameras at Syowa Station, Antarctica, and simultaneous precipitating auroral particle data obtained by the U.S. Air Force/Defense Meteorological Satellite Program (USAF/DMSP) F6 satellite. In the low‐latitude region of the duskside auroral oval, we found three types of auroral displays with weak optical intensity: (1) proton auroras, (2) pulsating auroras, and (3) faint discrete auroral arcs distinct only in the 6300 Å emission. In usual cases, the energy input into this region is mostly carried by proton precipitations to produce proton auroras mainly at wavelengths of 4861 Å and 5577 Å. Pulsating features are sometimes observed in the diffuse auroral region in the dusk sector. Comparing auroral images with the nearly simultaneous data of precipitating auroral particles, we confirmed that the pulsating auroras are associated with the intensification of precipitating electron flux from the central plasma sheet. Furthermore, electrons are the main contributors to the energy input into the duskside auroral oval in this case. We also found that discrete auroras sometimes appeared in the 6300 Å images, but not in images at other wavelengths. They appear in the equatorial part of the dusk auroral oval. These 6300 Å discrete auroras correspond to weak precipitation spikes of low‐energy electrons simultaneously measured by DMSP satellites. The flux and average energy of these electron spikes are about 108/(cm² sr s) and 100 eV, respectively. They are intense enough to excite 6300 Å emissions but not 5577 Å emissions, as detected from the ground observations.
Vertical plasma drifts in the F region at the magnetic equator
Authors: Namboothiri S. P. , Balan N. , Rao P. B.
Published: February 2008

HF Doppler observations of vertical plasma drifts in the equatorial F region at Trivandrum (dip: 0.9°S), conducted during 1984–1986, are presented. The observations reveal a fairly consistent diurnal variation pattern with the drift velocities directing upward during day and downward during night. The diurnal pattern is dominated by the characteristic prereversal enhancement that exhibits the most striking seasonal, solar, and magnetic activity effects. Under high solar activity, the average peak velocity of the enhancement is found to drop from ∽40 m s−1 in equinox to ∽10 m s−1 in summer. Likewise, the average equinoctial peak velocity drops almost by a factor of 3 as the solar activity index (S10.7) falls from 125 to 70 units. The peak velocity has a unique magnetic activity dependence; it drops by more than a factor of 2 as the activity changes from quiet to moderate but assumes well above the quiet day value for high activity. The corresponding variations for the other times of the day are nowhere as pronounced as for the period of the prereversal enhancement. The observations presented here are found to be in broad agreement with those reported for Jicamarca and are also in accordance with the model predictions based on the F region dynamo.
"Electron acceleration at nearly perpendicular collisionless shocks, 1, One‐dimensional simulations without electron scale fluctuations""
Authors: Krauss‐Varban D. , Burgess D. , Wu C. S.
Published: February 2008

Under certain conditions electrons can be reflected and effectively energized at quasi‐perpendicular shocks. This process is most prominent close to the point where the upstream magnetic field is tangent to the curved shock. The energization process has been observed at the Earth's bow shock, but is also considered to occur elsewhere in space. A theoretical explanation of the underlying physical mechanism has been proposed which assumes conservation of magnetic moment and a static, simplified shock profile. We perform test particle calculations of the electron reflection process in order to examine the results of the theoretical analysis without imposing these restrictive conditions. A one‐dimensional hybrid simulation code generates the characteristic field variations across the shock. Special emphasis is placed on the spatial and temporal length scales involved in the mirroring process. The simulation results agree generally well with the predictions from adiabatic theory. The effects of the cross‐shock potential and unsteadiness are quantified, and the influence of field fluctuations on the reflection process is discussed.
ISEE 3 observations during the CDAW 8 intervals: Case studies of the distant geomagnetic tail covering a wide range of geomagnetic activity
Authors: Richardson I. G. , Owen C. J. , Cowley S. W. H. , Galvin A. B. , Sanderson T. R. , Scholer M. , Slavin J. A. , Zwickl R. D.
Published: February 2008

The data obtained by the ISEE 3 spacecraft during the eight Coordinated Data Analysis Workshop 8 (CDAW 8) intervals provide an excellent opportunity to study the structure and dynamics of the distant geomagnetic tail under a wide range of geomagnetic activity ranging from intervals of magnetic quiet punctuated by isolated substorms to extended intervals of strong disturbance. By examining the properties of the plasma sheet, evidence has been found for the persistence of reconnection in the tail during long intervals of magnetic quiet, with the neutral line lying ≈100 to 200 RE or more downtail. The suggestion that the distant tail plasma sheet is populated exclusively by tailward moving closed flux tubes under quiet geomagnetic conditions is therefore not supported. However, a “slow plasma sheet” regime is also found during such conditions, in which closed flux tubes move slowly tailward in a thick region adjacent to the magnetopause, presumably due to some form of “viscous” momentum transfer from the magnetosheath. This process does not appear to simultaneously transfer mass into the tail, and there is some indication that the Kelvin‐Helmholtz instability is involved. The observations strongly suggest that the closed flux tubes originate from the closed field line plasma sheet region earthward of the neutral line rather than, for example, from the near‐Earth low‐latitude boundary layer. Plasmoids are observed in the distant tail following disturbance enhancements, the time of their appearance being generally consistent with disconnection from the near‐Earth region at the time of the enhancement. Their structure is entirely consistent with the “neutral line” model. However, not all enhancements in geomagnetic activity result in the observation of plasmoids. In particular, the CDAW 8 data suggest that during extended intervals of strong activity, the neutral line may reside in the near‐Earth tail (<60 RE from Earth) on an essentially continuous basis and that some disturbance enhancements may then relate to an increase in the reconnection rate at a pre‐existing neutral line, rather than to new neutral line, and plasmoid, formation. The plasma sheet downtail of the neutral line then thickens and may engulf the spacecraft, but no plasmoid signatures are present.
Ballooning instability and structure of diamagnetic hydromagnetic waves in a model magnetosphere
Authors: Miura A. , Ohtani S. , Tamao T.
Published: February 2008

A linear eigenmode analysis of ballooning instability of an Alfvén wave and a drift‐Alfvén wave is performed for a curved magnetic field line configuration in a finite‐β plasma to examine stability of the tail plasma sheet and to find a generation mechanism of diamagnetic storm time Pc 5 pulsations, which are characterized by a large azimuthal mode number (m>50). A curved magnetic field line is obtained from a self‐consistent model of magnetic field and plasma pressure distributions, which is constructed on the basis of a two‐dimensional equilibrium model of the tail plasma sheet. An eigenmode equation is solved along the field line in order to obtain the eigenfrequency and eigenmode structure of the ballooning instability of the Alfvén and the drift‐Alfvén waves. Only fundamental mode is unstable to the ballooning instability, which is driven by the pressure gradient combined with the unfavorable magnetic field line curvature, while higher harmonic modes are stable. The eigenfunction of the unstable wave (fundamental mode) is evanescent or decaying exponential toward the ionosphere along the field line and strongly confined near the equator with its plasma pressure and magnetic pressure being out of phase. The stable higher harmonic modes, on the other hand, have standing Alfvén mode structures along the field line and have frequencies determined by oscillation periods of the standing Alfvén modes. In the absence of the coupling to the drift wave, the unstable fundamental wave is aperiodic with zero real frequency. When the unstable fundamental wave is coupled to the drift wave, however, the unstable wave (drift‐Alfvén wave) has a real frequency determined by an ion diamagnetic drift speed. The obtained oscillation period of a few hundred seconds for the unstable drift‐Alfvén wave with an azimuthal model number m = 50, its westward propagation, diamagnetic relationship between the perturbed magnetic and plasma pressures, and strong spatial confinement of the unstable wave near the equator suggest to us that the unstable drift‐Alfvén wave destabilized by the ballooning instability is a strong candidate mechanism for explaining the observed storm time Pc 5 pulsations. By changing parameters of the equilibrium plasma sheet configuration it is found that the growth rate of the ballooning instability increases as the pressure gradient in the plasma sheet increases.
Electrostatic lower hybrid waves excited by electromagnetic whistler mode waves scattering from planar magnetic‐field‐aligned plasma density irregularities
Authors: Bell T. F. , Ngo H. D.
Published: February 2008

Recent satellite observations demonstrate that high amplitude, short wavelength (5 m ≤ λ ≤ 100 m) electrostatic waves are commonly excited by electromagnetic whistler mode waves propagating in regions of the magnetosphere and topside ionosphere where small‐scale magnetic‐field‐aligned plasma density irregularities are thought to exist. A new theoretical model of this phenomenon is presented, based upon passive linear scattering in a cold magnetoplasma. In this model the electrostatic waves are excited by linear mode coupling as the incident electromagnetic whistler mode waves scatter from the magnetic‐field‐aligned plasma density irregularities. The excited short wavelength waves are quasi‐electrostatic whistler mode waves, a type of lower hybrid wave, whose wave normal lies near the whistler mode resonance cone where the wave refractive index becomes very large. For simplicity the case of planar irregularities is considered in which the electron density varies in only a single direction, roughly perpendicular to B0, the Earth's magnetic field. The amplitude of the excited electrostatic lower hybrid waves is calculated for a wide range of values of input electromagnetic wave frequency, wave normal direction, electron plasma frequency, gyrofrequency, ion composition, and irregularity scale and density enhancement. Results indicate that high amplitude lower hybrid waves can be excited over a wide range of parameters for irregularity density enhancements as low as 5% whenever the scale of the irregularity is of the same order as the lower hybrid wavelength. It is shown that lower hybrid waves can be excited only when the planar irregularities are aligned with B0 within a small angle 2χc, where χc is equal to the complement of the resonance cone half‐angle. For the frequencies and L shells considered, χc ≤ 8°. Predictions of the theory are shown to be consistent with satellite data. A VLF “radar” method is demonstrated whereby the lower hybrid wave excitation phenomenon can be used as a diagnostic tool to determine the small scale irregularity structure of the medium. The effective damping of the input electromagnetic wave due to the excitation of the lower hybrid waves is also considered. It is found that this form of damping may be a dominant factor for whistler mode waves throughout large regions of the magnetosphere.
On the electromagnetic stability of isotropic populations
Authors: Brinca Armando L.
Published: February 2008

The analytical properties of the relevant, nonrelativistic magnetoplasma dispersion function demonstrate the stability of arbitrary particle populations of isotropic velocity distributions with respect to the propagation of electromagnetic waves along the ambient magnetic field. The corresponding stability of complete spherical‐shell velocity distributions, as frequently encountered in cometary investigations, follows.
Ca+emission in the sunlit ionosphere
Authors: Torr Marsha R. , Torr D. G. , Bhatt P. , Swift W. , Dougani H.
Published: February 2008

In the course of a program of twilight airglow observations from the McDonald Observatory in southwest Texas, the resonance fluorescence emissions from calcium ions were measured. In particular, twilight sequences were obtained during the period of December 19–22, 1987, which coincided with the Ursids meteorite shower. During this meteorite event the intensities of the Ca+ emission lines at 3934 Å increased to the point that the surface brightness profiles could be inverted to volume emission rate profiles. These profiles show evidence for strong spatial redistribution of the Ca+ over the course of three days. Prior to the onset of the meteorite activity, emissions from the Ca+ originate from below 100 km, on the occasions when the emissions are visible. By the evening of December 19 a peak is measurable at 108 km. On the morning of December 22, a high‐altitude peak was observed above 250 km, with a larger peak down at approximately 85 km. By the evening of December 22, the emission had substantially intensified, with the peak of the layer being at 80 km or below, but with emission being produced all the way up to at least 160 km. Observations of these emissions during meteor shower periods could provide a valuable tracer for the processes responsible for the transport of ions in the D, E, and F region, allowing the full altitude and latitude extent of the distribution to be determined.
Signatures of the high‐altitude polar cusp and dayside auroral regions as seen by the Viking Electric Field Experiment
Authors: Marklund G. T. , Blomberg L. G. , Fälthammar C.‐G. , Erlandson R. E. , Potemra T. A.
Published: February 2008

Electric field and satellite potential (which is a rough measure of plasma density) observations along 42 Viking orbits in the high‐altitude (2 RE) polar cusp and dayside auroral region have been examined. A useful, straightforward way to identify the polar cusp from the satellite potential is demonstrated, the results being in excellent agreement with those based on particle data. Within the cusp the plasma density usually reaches a maximum, and it is typically very homogeneous, in contrast to the irregular and lower density in the cleft and dayside auroral regions. The maxima in the plasma density are sometimes anticorrelated with the magnetic field strength, indicating a diamagnetic effect. The entire cusp and dayside auroral regions are characterized by irregular and burstlike electric fields, comprising field reversals on various scales (up to 3 min or 500 km), the larger scales, however, being rare in the cusp. Another common feature in these regions is the high correlation between mutually orthogonal components of the electric and magnetic fields, both for large‐scale variations across spatial structures (associated with the magnetosphere‐ionosphere coupling through field‐aligned currents) and for waves and pulsations in the ULF frequency range. The electric field signatures in the cusp (in the 1100–1300 MLT sector) are, however, characteristically different from the cleft and oval field signatures in that the electric field is usually less intense and less structured and not correlated with the substorm activity level.
High‐frequency waves in the cusp/cleft regions
Authors: Pottelette R. , Malingre M. , Dubouloz N. , Aparicio B. , Lundin R. , Holmgren G. , Marklund G.
Published: February 2008

Because of the high time resolution of many of its scientific instruments, the Viking satellite has detected the presence of intense wave bursts in the cusp/cleft regions, at altitudes between 1 and 2 Earth radii. These bursts are closely related to the acceleration processes of charged particles and are characterized by a strong plasma turbulence illustrated by sporadic and intense broadband electrostatic noise detected in the 4‐ to 700‐kHz frequency range of the high‐frequency wave receiver. The duration of such bursts is small, typically a few hundreds of milliseconds, corresponding to a spatial width less than 1 km at the satellite altitude. The low‐frequency part of this noise is always the most intense, with electric field amplitudes of a few tens of millivolts per meter. This noise is correlated with 0.1‐ to 1‐keV electron beams and with small‐scale field‐aligned currents and is always associated with low‐frequency (∼1 Hz) electric field fluctuations. Perpendicular acceleration of ambient plasma ions occurs in connection with the bursts. The impulsive broadband waves are detected at the equatorward edge of the polar cusp and more generally in the entire cleft region. The central part of the cusp is characterized by an increase of the frequency cutoff of the hiss emissions, related to the increase of the electron density. In addition, electrostatic electron cyclotron waves are also recorded. Terrestrial kilometric radiation is often detected in the cusp/cleft regions usually in close correlation with particle acceleration processes. The Viking satellite tends to have been mostly above the generation regions of these electromagnetic waves in the dayside auroral oval, which makes it difficult to localize precisely their source.
Experimental evidence of widespread regions of small‐scale plasma irregularities in the magnetosphere
Authors: Holmgren G. , Kintner P. M.
Published: February 2008

Small‐scale (≤ 1 km) plasma irregularities have previously been observed in situ from the E region to an altitude of 8000 km. In this paper we report results from the Viking plasma wave experiments which extends the measurements of high‐latitude irregularities in two ways: (1) we have acquired electron density fluctuation measurements up to an altitude of 13,500 km and (2) for the first time a measurement technique was used that made a phase velocity deduction possible from in situ measurements. The spacecraft was equipped with two spatially separated Langmuir probes, each with an ability to measure relative probe current fluctuations with frequencies from dc to about 400 Hz. Under certain assumptions the current fluctuations could be interpreted as relative plasma density fluctuations, δne/ne. Data from this “interferometric” instrument has been used to infer the distribution and nature of plasma irregularities along Viking orbits. It is demonstrated that the interferometric technique offers great advantages compared to single point measurements in this kind of study. It is shown that the observed small‐scale plasma irregularities are “nondispersive” and convecting with the background plasma. They exhibit a power law frequency spectrum as observed in the satellite reference frame. The spectral index varies with location. An attempt to map source regions by identifying regions of high power and shallow spectrums is made. It is concluded that plasma irregularities, in accordance with our selection criteria, can be present in all regions of space encountered by Viking.
Large‐scale variations of thermal electron parameters in the solar wind between 0.3 and 1 AU
Authors: Pilipp W. G. , Miggenrieder H. , Mühläuser K.‐H. , Rosenbauer H. , Schwenn R.
Published: February 2008

Variations of thermal electron parameters in the solar wind are investigated using data obtained from the Helios probes in the years from 1974 to 1976, shortly before solar minimum, at heliocentric distances ranging from 0.3 to 1 AU. The main part of the present analysis is based on Helios 2 data obtained in 1976. Variations across the sector structure of the interplanetary magnetic field (IMF) and across the plasma stream structures are studied. These studies support the hypothesis that the thermal electron properties, i.e., the electron temperatures, the core temperatures, the heat flux, and the normalized heat flux, are strongly correlated with the distance from the neutral sheet in the IMF (implying also a correlation with the plasma stream structures). The results do not indicate significant electron heating in compression regions. Also, power laws for the variations of the electron parameters with distance from the Sun as well as polytrope laws are derived for different solar wind structures and distance ranges. The core temperatures, determined by bi‐Maxwellian fits to the electron distributions below 20 or 30 eV, mostly decrease faster with distance from the Sun than the electron temperatures Te∥ and Te⊥, parallel and perpendicular to the magnetic field, respectively, as determined from the entire electron distributions. Most of the Helios 2 observations indicate that the radial profiles of electron temperatures and of electron heat flux tend to be flatter in high‐speed streams embedded within the interior of magnetic sectors than in the slow‐ and intermediate‐speed solar wind and in compression regions mostly at or near sector boundaries. For high‐speed streams, the Helios data indicate a flattening of the radial profiles for Te⊥ with increasing distance from the Sun. For Te∥ this effect is not as evident in the case of Helios 2 data but is indicated by Helios 1 data. Analogous trends are observed for most of the derived polytrope indices for the core and electron temperatures. The data provide evidence that in high‐speed streams the electrons are heated not only by the electron heat flux but also by external energy sources. Finally, estimated decreases with heliocentric distance from 0.3 to 1 AU of the interplanetary electrostatic potential could provide an increase of the solar wind bulk velocity of about 5 to 20 km s−1.
VLF wave emissions by pulsed and DC electron beams in space: 2. Analysis of Spacelab 2 results
Authors: Reeves G. D. , Banks P. M. , Neubert T. , Harker K. J. , Gurnett D. A.
Published: February 2008

Experiments investigating the generation of radio frequency waves in space plasmas by the injection of artificial electron beams into ionospheric plasmas were conducted in July and August of 1985 on the Spacelab 2 space shuttle mission. Among the results were the production of broadband electromagnetic emissions from continuous and square‐wave‐modulated, low‐power (1‐keV, 50/100‐mA) electron beams and the observation of narrow‐band radiation from pulsed beam operations. Observations from the Spacelab 2 experiments were presented by Reeves et al. (1988b). This paper is a sequel to that earlier work and presents analysis of the observations. Observations of narrow‐band radiation are compared with the predictions of a theory of wave generation by pulsed electron beams (Harker and Banks, 1987). Good agreement between observations and predictions is found for the s = 0, root 2 solutions which represent Cherenkov radiation with wave normal angles less than the Gendrin angle. For the broadband emissions, predictions using a single‐ion, cold plasma theory are compared with the general features observed in the data. Broadband emissions from dc and pulsed electron beams also have features which can be understood in terms of the characteristics of whistler mode waves which are produced by the Cherenkov resonance. All observations were made within the near field of waves in this mode. Waves observed outside the predicted region of propagation are identified as near‐field components. Study of wave amplitudes in a coordinate system defined by the orbiter velocity vector indicates the presence of a wake structure behind the beam. Large‐amplitude waves observed in the beam and beam‐wake regions may include additional, electrostatic contributions to the wave fields. The relative contributions of electromagnetic and electrostatic fields are discussed, and an order of magnitude estimate of the Poynting flux is presented.
Electrodynamic properties of auroral surges
Authors: Robinson R. M. , Vondrak R. R.
Published: February 2008

The incoherent scatter radar technique provides an excellent means to study the ionization and electric fields associated with auroral precipitation events. One of the most intense and dynamic auroral events is the so‐called surge or breakup aurora that accompanies auroral substorms. For our purposes we define a surge as a transient intensification of auroral precipitation that occurs simultaneously with a pronounced negative bay in the ground magnetometer data. We present data obtained during five such events in 1980 and 1981. Measurements made by the Chatanika radar are combined with simultaneous data obtained from ground‐based magnetometers and all‐sky cameras and space‐based instruments on the Dynamics Explorer satellites. The combined data sets yield information about the auroral forms, electric fields, and currents that are associated with surges. Prior to the surge, auroral forms move equatorward, develop ray structure, and intensify. The surge is identified by an apparent poleward motion of the aurora producing aurorally associated ionization that extends over several hundred kilometers in latitude. The presurge auroral forms are embedded in a region of northward electric field. The auroral forms that comprise the surge span a region within which the meridional electric field is small and at times southward. A westward electric field is often but not always present within the surge. The behavior of the westward electric field is significantly different from the north‐south field, in that sharp spatial gradients are absent even in very disturbed conditions. Although the westward Hall currents are mostly responsible for the negative bays that accompany the surge, at times the westward Pedersen current sustained by the westward electric field can be important. Sudden variations in the H component of the ground magnetogram can be caused by motions of the aurora or by temporal variations in the fields or conductivities. We present a model that simulates the observed changes in electric field and precipitation that accompany surges. The perturbation in the electric field produced by the surge is simulated by adding negative potential in regions of intense precipitation.
Toroidal standing waves excited by a storm sudden commencement: DE 1 observations
Authors: Cahill L. J. , Lin N. G. , Waite J. H. , Engebretson M. J. , Sugiura M.
Published: February 2008

A 74‐nT sudden commencement on July 13, 1982, was observed in the magnetosphere with instruments on the Dynamics Explorer 1 satellite. Inbound, near L = 4.5, the satellite was located at 1524 magnetic local time and 20° magnetic latitude. The sudden commencement established a strong, east–west oscillation, with 100‐s period, which was observed in the magnetic field, the electric field, and the plasma flow velocity records. There was also a compressional component of this 100‐s oscillation and a rapidly damped 300‐s compressional pulsation. The compressional oscillations may be evidence of cavity resonances, excited by the sudden commencement. The cavity waves may, in turn, couple to toroidal waves in field line resonance at the satellite location. In addition the sudden commencement caused the onset of waves with frequencies from 0.1 up to at least 0.5 Hz. The observations are compared with similar reports from earlier pulsations related to sudden commencements.
Plasma flow reversals at the dayside magnetopause and the origin of asymmetric polar cap convection
Authors: Gosling J. T. , Thomsen M. F. , Bame S. J. , Elphic R. C. , Russell C. T.
Published: February 2008

A number of events have been observed in the Los Alamos/Garching fast plasma experiment data from ISEE 2 within ±3 hours of noon wherein the y component of the plasma flow within the low latitude boundary layer and magnetopause current layer is oppositely directed to that in the adjacent magnetosheath. When the y component, By, of the local magnetosheath magnetic field is positive, events of this nature are found preferentially in the northern dusk and southern dawn quadrants, and when By is negative, they are found preferentially in the opposite two quadrants. Plasma flows in these events are always also poleward, that is they are always directed away from the GSE equatorial plane. The observations are qualitatively and quantitatively consistent with previous observations of accelerated flows at the magnetopause and with models of magnetic reconnection, with the reconnection occurring at low latitudes near the GSE XY plane independent of the magnitude or the sign of the y component of the local magnetosheath magnetic field. Local magnetic shears at the magnetopause for these events range from 60 to 180 degs, which, together with the occurrence of these events at low latitudes, does not tend to support the antiparallel merging hypothesis. These observations of By‐dependent flow reversals provide a graphic demonstration of how asymmetric polar cap convection and related phenomena, such as the Svalgaard‐Mansurov effect, originate in magnetic reconnection at the dayside magnetopause.
Simulation studies of plasma waves in the electron foreshock: The transition from reactive to kinetic instability
Authors: Dum C. T.
Published: February 2008

The electron beam‐plasma instability is analyzed in particle simulation experiments, starting with a beam of small velocity spread. The dispersion relation is solved for snapshots of the actual evolving electron distribution function, rather than for the usual models consisting of Maxwellians. As the beam broadens, the analysis shows a transition from reactive beam modes, with frequencies extending much below the plasma frequency ωe, to kinetic instability of Langmuir waves, ω ≈ ωe, which is in agreement with the frequencies and growth rates observed in the simulation. Beam evolution is also in agreement with quasi‐linear theory, except at the end of the reactive phase when trapping of beam electrons is seen. Although the spectrum temporarily narrows at this stage, there are, in contrast to previous simulations, still many modes present. The system then can proceed to a kinetic phase in which quasi‐linear theory is again applicable. This stage is identical with the evolution starting from a gentle broad beam, except that wave levels are several times higher. With higher wave levels, mode coupling effects are also more prominent, but are still unable to prevent plateau formation. In contrast to the Langmuir wave regime, the reactive broadband wave regime lasts only for a relatively short period. In the electron foreshock it could only persist if a narrow beam or a sharp cutoff feature were maintained by continued beam injection and the time‐of‐flight mechanism.
A two‐dimensional particle simulation of the magnetopause current layer
Authors: Berchem Jean , Okuda Hideo
Published: February 2008

We have developed a 2½‐dimensional (x, y, υx, υy, υz) electromagnetic code to study the formation and the stability of the magnetopause current layer. This code computes the trajectories of ion and electron particles in their self‐consistently generated electromagnetic field and an externally imposed two‐dimensional vacuum dipolar magnetic field. The results presented here are obtained for the simulation of the solar wind‐magnetosphere interaction in the subsolar region of the equatorial plane. We observe the self‐consistent establishment of a current layer resulting from both diamagnetic drift and E×B drift due to the charge separation. The simulation results show that during the establishment of the current layer, its thickness is of the order of the hybrid gyroradius ρH = (ρiρe)1/2 predicted by the Ferraro‐Rosenbluth model. However, diagnostics indicate that the current sheet is subject to an instability which broadens the width of the current layer. Ripples with wavelengths of the order of the ion gyroradius appear at the interface between the field and the particles. These perturbations are observed both on the electrostatic field and on the compressional component of the magnetic field. This instability has a frequency very close to the local ion cyclotron frequency and propagates with a phase velocity in the same direction as the electron diamagnetic drift. The nonlinear phase of the instability is characterized by the filamentation of the current layer which causes anomalous diffusion inside the central current sheet.
A Solar EUV Flux Model
Authors: Tobiska W. Kent , Barth Charles A.
Published: February 2008

A model of the solar extreme ultraviolet (EUV) irradiance variability has been developed for aeronomical use and has been named SERF2 by the Solar Electromagnetic Radiation Flux Study. The model is valid between 1981 and 1989 and is based on the Atmosphere Explorer E (AE‐E) satellite EUV data set which is correlated with independent solar emissions measured during and after the AE‐E mission. Additionally, spectral modifications are made to the model based on 18 separate rocket flights for all levels of solar activity. Two daily measured solar emissions, the H Lyman α line at 121.6 nm observed by the Solar Mesosphere Explorer satellite and the Ottawa 10.7‐cm radio flux observed at the ground, are used in the model as indices for full‐disk solar EUV chromospheric irradiance variations and transition region‐coronal irradiance variations, respectively. The model wavelength equation coefficients are presented in tabular form for 39 wavelength groups or discrete lines from 1.9 to 105.0 nm along with spectral weighting function coefficients which modify the irradiance magnitudes based upon model wavelength fits to rocket‐observed spectra. The model satisfies the general constraint of duplicating rocket‐observed EUV irradiance for a wide variety of solar activity conditions. The model development is discussed, an example calculation is given, and the comparisons with constraining rocket data sets are shown.
Distributions of He+ at middle and equatorial latitudes during solar maximum
Authors: Heelis R. A. , Hanson W. B. , Bailey G. J.
Published: February 2008

Observations of ionospheric composition made by DE 2 in the mid‐latitude ionosphere, show that during the solar maximum period of 1981, at altitudes near 900 km during the nighttime, the dominant light ion species is He+. Conditions may arise in which He+ is the dominant ion species, exceeding the O+ concentration by a factor of 2. This observation is contrasted by those at the same altitude near the dip equator. Here a deep minimum in the He+ concentration accompanies observed maxima in the O+ and H+ concentrations and O+ is always the major ion species. Calculations are used to illustrate that the distribution of neutral species at solar maximum, together with the appropriate ionization rates can easily account for the dominance of He+ at mid‐latitudes. The relative abundance of the ionospheric species is a sensitive function of local time and the associated evolution of the topside O+ concentration profile. Factors such as E×B drift may easily effect the precise nature of the observation. At the dip equator the existence of an E×B drift motion of the plasma is required to explain the He+ minimum. Here the E×B drift is such that plasma observed near 900 km during the night, has resided at altitudes below 600 km during the day. Thus the He+ concentration is only that which is produced in one day against the chemical loss process. This is contrasted with the situation at mid‐latitudes where the large flux tube volume provides a reservoir in which the He+ can accumulate each day.
Auroral beam‐plasma interactions: Particle correlator investigations
Authors: Gough M. Paul , Christiansen Peter J. , Wilhelm Klaus
Published: February 2008

Plasma wave instabilities may occur above discrete auroral arcs where the natural accelerated auroral electron component meets the cold ionospheric plasma. These wave‐particle interactions were studied on the CAESAR II rocket by measuring the modulations in energetic electron flux in the frequency ranges 0–10 kHz and 0–10 MHz. Near apogee (700 km) a strong modulation of 7.5 keV electrons at 1.4 MHz (≈ fuh) was observed where df(υ)/dυ was positive, just below the auroral beam velocity. The measurement is shown to be consistent with instabilities driven by both Landau and cyclotron resonant interactions with upper hybrid waves, and also with the earlier observation of modulations at 2.65 MHz ( ≈ fuh) seen on a low altitude rocket E2‐B (apogee 230 km).
A comprehensive investigation of compressional ULF waves observed in the ring current
Authors: Woch J. , Kremser G. , Korth A.
Published: February 2008

Eighteen months of data provided by the magnetometer and energetic particle spectrometer on board the geosynchronous satellite GEOS 2 were used to investigate compressional ULF waves with frequencies of a few megahertz associated with large anticorrelated modulations of the ion pressure. The events are classified into two groups according to the behavior of the background magnetic field/plasma configuration at the onset of the oscillations. For one group the oscillation onset is preceded by a sharp increase of the plasma energy density due to the arrival of drifting ion bunches at the satellite and a corresponding diamagnetic decrease of the magnetic field strength. The other group is observed in a steady configuration in the presence of an enhanced ring current. Besides striking differences between the two groups as regards the average event duration and wave frequency, as well as the phase relation between electron and ion flux modulations, we present evidence that both groups of events are of antisymmetric structure with respect to the geomagnetic equator (where symmetry is defined according to the field line displacement) and have a small field‐aligned wavelength. Furthermore, for both groups the frequency variation with local time and the azimuthal propagation are closely connected with the ion magnetic gradient curvature drifts, and the waves are associated with a plasma distributed anisotropically in velocity space. Therefore the waves most likely represent the excitation of a single wave mode generated via wave‐particle interactions. Using plasma parameters measured in situ we calculated the growth rate of different hydromagnetic wave modes. It was found that the plasma is able to support a wave mode similar to the drift mirror mode with a large parallel wave number, if the effects of a strongly curved magnetic field are taken into account.
Three‐dimensional driven reconnection in magnetic loops
Authors: Strauss H. R.
Published: February 2008

A numerical study of reconnection in a three‐dimensional, axially bounded magnetic flux tube is presented. An axial magnetic field is embedded in a conducting fluid, between conducting end plates. An applied flow in the end plates causes distortion and stretching of the magnetic field in the flux tube, and the creation of current sheets. After the current sheets form, the reconnection rate is fast; the merging rate M ∼ 1. The reconnection mechanism resembles flux pileup in two dimensions. It is also similar to the nonlinear behavior of a subcritically excited current‐driven MHD mode. The numerical model is relevant to a mechanism thought to be important for heating the solar corona, as well as producing solar flares. Photospheric motions distort and twist up the coronal magnetic field, causing the formation of current sheets. The present computations explicitly demonstrate how this can occur.
Particle orbits in two‐dimensional equilibrium models for the magnetotail
Authors: Karimabadi H. , Pritchett P. L. , Coroniti F. V.
Published: February 2008

Particle orbits in two‐dimensional (2‐D) kinetic equilibrium solutions for the magnetotail are considered. A comparison is made between the orbits in the equilibrium fields and those found in previous studies of the orbits where the main component of the magnetic field (Bx) was approximated as either a hyperbolic tangent or a linear function of z with the normal field (Bz) taken to be a constant (one‐dimensional models). Here, the plane z = 0 defines the neutral plane. The orbits in the 2‐D fields are found to be significantly different from those in the one‐dimensional models. The particles are generally reflected in much shorter distances and the loss region is much smaller than in the one‐dimensional fields. This holds true independent of ρ/L, where L is the current sheet thickness and ρ is the Larmor radius in the asymptotic field. For ρ/L ≳ 0.1, the fraction of integrable orbits in the equilibrium fields also becomes different from that in the one‐dimensional fields. As ρ/L becomes larger than 0.3 (with the exact value depending on the equilibrium model and particle position in the tail), the phase space becomes dominated by integrable orbits, whereas in the one‐dimensional fields the phase space remains divided into integrable and chaotic regions. Another major difference between the orbits in the 1‐D and 2‐D fields lies in the fact that the equilibrium orbits have a net drift in the y direction whereas the orbits in the 1‐D models have zero net drift. In the presence of an electric field Ey, the equilibrium orbits can be accelerated to much larger energies and behave quite differently from the orbits in the 1‐D fields. The differences between the orbits in the equilibrium and one‐dimensional fields are mainly due to the neglect of the variation of Bx with x in the one‐dimensional fields. In fact, it is found that even a weak variation of Bx with x can alter the orbits significantly. The effect of a constant‐shear field on the orbits is also examined. Finally, it is shown that the border between the integrable and chaotic orbits is in general “sticky”; i.e. if a stochastic orbit comes close to an integrable region, it can get trapped. Since the orbit is stochastic, it will escape eventually, but the detrapping time can be very long. The stickiness occurs where there are islands embedded in phase space. Thus, care must be taken in applying the diffusion formalism to such regions. The relevance of the above findings to the problem of magnetic reconnection in the tail is discussed.
Radial diffusion of low‐energy plasma ions in Saturn's magnetosphere
Authors: Barbosa D. D.
Published: February 2008

Analytic solutions to the steady state radial diffusion equation allowing for a distributed source and sink of ions are presented. The diffusion space has absorption boundaries at the ends and is divided into two regions separated by an interior boundary where a discontinuity in diffusion and loss parameters is allowed to exist. The most general solution consists of a collection of point sources with adjustable weights to approximate an arbitrary continuous distribution of ion sources. The theory is applied to the study of thermal ion diffusion in Saturn's magnetosphere. It is concluded that a relatively large transport rate is required to conform with low‐energy Voyager plasma measurements and a radial diffusion coefficient expressible as DLL = 4×10−7 s−1 (L/6)3–4 is indicated. In such a fast diffusion regime cool O+ ion densities at L = 2.8, 6, and 15 may be obtained from a single source in the form of the Dione‐Tethys neutral H2O cloud. Hot ion densities in the outer magnetosphere are obtained from ionization of Titan's neutral atomic clouds of hydrogen and nitrogen and ion pickup by the magnetospheric flow. The O+ thermal ion model is compatible with a bimodal distribution of neutral hydrogen in the magnetosphere with a Titan cloud [H] ≃ 20 cm−3 (L/20) in the outer magnetosphere and a low‐density [H] = 6 cm−3 (6/L)² hydrogen corona of either Saturnian origin or ring origin in the inner magnetosphere.
Upper hybrid mode conversion and resonance excitation of Bernstein modes in ionospheric heating experiments
Authors: Rao N. N. , Kaup D. J.
Published: February 2008

We investigate mode conversion of propagating upper hybrid waves to resonantly excited nonpropagating electrostatic electron cyclotron harmonic (Bernstein) modes and show that it can explain the absence of stimulated emission of sideband electromagnetic radiation from the ionosphere in the kilohertz frequency range less than the input pump frequency when the latter is very close to or at harmonics (third and above) of the electron cyclotron frequency. The results are in good agreement with the ionospheric heating experiments conducted at Tromsø (Norway) as well as at Arecibo (Puerto Rico).
Spatial variations in the suprathermal ion distributions during substorms in the plasma sheet
Authors: Kistler L. M. , Möbius E. , Klecker B. , Gloeckler G. , Ipavich F. M. , Hamilton D. C.
Published: February 2008

Using data from the Max‐Planck‐Institut für extraterrestrische Physik/University of Maryland Suprathermal Energetic Ion Charge Analyzer (SULEICA) on AMPTE IRM and the University of Maryland/Max‐Planck‐Institut für Aeronomie Charge Energy Mass Spectrometer (CHEM) on AMPTE CCE, we have determined the pre‐ and post‐injection Suprathermal energy spectra for the ion species H+, O+, He+, and He++ for six events in which substorm‐associated particle injections are observed in both the near‐Earth plasma sheet (7–9 RE) and farther down the tail (10–19 RE). We find similar spectral changes in both locations, with the spectra becoming harder with the injection. Post‐injection, the flux decreases exponentially with radial distance. Approximately the same gradient is observed in all species. In addition, we find that although the O+/H+ and the He++/H+ ratios increase with energy per charge, the ratios are approximately the same at the same energy per charge at the two spacecraft. The observations are difficult to explain either with a model in which the ions are accelerated at a neutral line and transported toward Earth or with a model in which the ions are accelerated in the near‐Earth region by current disruption/diversion and transported down the tail. In either case, the ions would have to be transported throughout the tail without much energization or deenergization in order to explain the energy per charge correlations. Further, earthward transport without energization would not lead to the observed radial gradient. A combination of these acceleration mechanisms, a disturbance that propagates throughout the plasma sheet, or a more global mechanism may explain the observations.
A systematic approach to modeling the geopotential with point mass anomalies
Authors: Reilly J. P. , Herbrechtsmeier E. H.
Published: February 2008

A point mass model is used to predict oceanic gravity anomalies from simulated altimeter data. Gravity anomalies from land areas are also combined with altimetry as hybrid data inputs to the mathematical model. The geocentric radii of the point mass anomaly sets are determined by use of the ‘best r‐formula’ of Hardy and Göpfert. Test control data, consisting of 1° × 1° mean free air gravity anomalies in a 30 × 30 block, are recovered with a standard deviation of about 20 mGal., using the hybrid data observations as well as altimeter observations alone. Highly significant computational advantages are evident in comparison with other methods.
The central magnetic anomaly, Manicouagan Structure, Quebec
Authors: Coles R. L. , Clark J. F.
Published: February 2008

Three‐dimensional modeling of the prominent (2000 nT) magnetic anomaly at the center of the Manicouagan structure in Quebec indicates the presence of highly magnetic rocks close to the surface over an area roughly 8 × 12 km. The models depict a tabular body of variable thickness, the shallower parts forming a crude ringlike formation about the center. The depth to the base of the body is not well determined but may be less than 3 km. The direction of magnetization is not unique but is similar to paleomagnetic directions of Triassic age obtained from other rocks within the major structure. A localized, impact‐generated magnetization produced in an underlying extensive mafic rock unit, with subsequent uplift, is suggested as the cause of the feature.