bluuebunny/crossref_metadata_2025 huggingface dataset:
Ionospheric projections of magnetospheric regions under low and high solar wind pressure conditions
Authors: Newell Patrick T. , Meng Ching‐I.
Published: February 2004

The projection of magnetospheric regions into the dayside ionosphere as determined by particle precipitation characteristics was studied for dependence on solar wind parameters. It was found that the solar wind kinetic pressure p dramatically affected the map of magnetospheric projections. Under the constraint that p ≥ 4 nPa (yielding
 = 5.9 nPa), the area of the cusp (magnetic latitude times magnetic local time extent) was 4.83 degree‐hours; whereas under the constraint that p ≤ 2 nPa (yielding
 = 1.5 nPa), the cusp area was only 1.01 degree‐hours. The ionospheric footprint of the low‐latitude boundary layer was similarly affected. Various possible correlations of p with other solar wind variables, including n, υ, and |Bz|, proved unable to account for the pressure effect. Because one criterion for identifying the cusp is high fluxes, the effect of nυ was investigated with particular care, both in examples and statistically. Again, p itself had by far the most striking effect. Thus we concluded that some physical mechanism is needed to account for the pressure effect. One possibility is that increased direct solar wind plasma penetration of the magnetopause occurs under high‐p conditions in the manner suggested by various proponents of impulsive penetration models. An alternative, which we find promising, is that, regardless of the original interplanetary magnetic field (IMF) strength, a high‐p solar wind leads to a large compression factor for the magnetosheath field, which is the field actually in contact with the magnetosphere. From this latter viewpoint, the chief effect of high particle pressure is simply to enhance the effectiveness of the interaction of the IMF with the magnetosphere.
Particle acceleration in the dynamic magnetotail: Orbits in self‐consistent three‐dimensional MHD fields
Authors: Birn Joachim , Hesse Michael
Published: February 2004

The acceleration of protons in a dynamically evolving magnetotail is investigated by tracing particles in the fields obtained from a three‐dimensional resistive MHD simulation. The MHD simulation, representing plasmoid formation and ejection through a near‐Earth reconnection process, leads to cross‐tail electric fields of up to approximately 4 mV/m with integrated voltages across the tail of up to approximately 200 kV. Energization of particles takes place over a wide range along the tail, due to the large spatial extent of the increased electric field together with the finite cross‐tail extent of the electric field region. Such accelerated particles appear earthward of the neutral line over a significant portion of the closed field line region inside of the separatrix, not just in the vicinity of the separatrix. Two different acceleration processes are identified: a “quasi‐potential” acceleration, due to particle motion in the direction of the cross‐tail electric field, and a “quasi‐betatron” effect, which consists of multiple energy gains from repeated crossings of the acceleration region, mostly on Speiser‐type orbits, in the spatially varying induced electric field. The major source region for accelerated particles in the hundreds of keV range is the central plasma sheet at the dawn flank outside the reconnection site. Since this source plasma is already hot and dense, its moderate energization by a factor of approximately 2 may be sufficient to explain the observed increases in the energetic particle fluxes. Particles from the tail are the source of beams at the plasma sheet/lobe boundary. The temporal increase in the energetic particle fluxes, estimated from the increase in energy gain, occurs on a fast timescale of a few minutes, coincident with a strong increase in Bz, despite the fact that the inner boundary (“injection boundary”) of the distribution of energized particles is fairly smooth.
Electron cyclotron wave generation by relativistic electrons
Authors: Wong H. K. , Goldstein M. L.
Published: February 2004

We show that an energetic electron distribution which has a temperature anisotropy (T⊥b > T∥b), or which is gyrating about a DC magnetic field, can generate electron cyclotron waves with frequencies below the electron cyclotron frequency. Relativistic effects are included in solving the dispersion equation and are shown to be quantitatively important. The basic idea of the mechanism is the coupling of the beam mode to slow waves. The unstable electron cyclotron waves are predominantly electromagnetic and right‐hand polarized. For a low‐density plasma in which the electron plasma frequency is less than the electron cyclotron frequency, the excited waves can have frequencies above or below the electron plasma frequency, depending upon the parameters of the energetic electron distribution. This instability may account for observed Z mode waves in the polar magnetosphere of the Earth and other planets.
Use of FUV auroral emissions as diagnostic indicators
Authors: Germany G. A. , Torr M. R. , Torr D. G. , Richards P. G.
Published: February 2004

In an earlier study we modeled selected FUV auroral emissions (O I (1356 Å), N2 Lyman‐Birge‐Hopfield (LBH) (1464 Å), and LBH (1838 Å)) to examine the sensitivity of these emissions and their ratios to likely changes in the neutral atmosphere. In this paper we extend that study to examine the dependence of these same emissions and their ratios on the shape of the energy distribution of the auroral electrons. In particular, we wish to determine whether changes in energy spectra might interfere with our determination of the characteristic energy. Modeled column‐integrated emissions show relatively small (<30%) dependences on the shape and width of the incident energy spectrum, provided the average energy and total energy flux of the energy distribution are held constant. Long‐wavelength FUV emissions, which are relatively unaffected by O2 absorption losses, exhibit virtually no dependence on the shape of the incident energy distribution. Changes in ratios of FUV short‐ to long‐wavelength emissions as a function of characteristic energy are much larger than those due to changes in energy distribution. As a result, the determination of characteristic energy using these emission ratios is relatively unambiguous. We also examine the relative intensities of the aurora and the dayglow for various conditions. The intensities of modeled FUV auroral emissions relative to the dayglow emissions are presented as a function of solar zenith angle and incident energy flux. Under certain conditions (energy flux ≤ 1 erg cm−2 s−1 and solar zenith angle ≤50°) the dayglow will be the limiting factor in the detection of weak auroras.
Auroral ionospheric signatures of the plasma sheet boundary layer in the evening sector
Authors: Burke W. J. , Machuzak J. S. , Maynard N. C. , Basinska E. M. , Erickson G. M. , Hoffman R. A. , Slavin J. A. , Hanson W. B.
Published: February 2004

We report on particles and fields observed during Defense Meteorological Satellite Program (DMSP) F9 and DE 2 crossings of the polar cap/auroral oval boundary in the evening MLT sector. Season‐dependent, latitudinally narrow regions of rapid, eastward plasma flows were encountered by DMSP near the poleward boundary of auroral electron precipitation. Ten DE 2 orbits exhibiting electric field spikes that drive these plasma flows were chosen for detailed analysis. The boundary region is characterized by pairs of oppositely‐directed, field‐aligned current sheets. The more poleward of the two current sheets is directed into the ionosphere. Within this downward current sheet, precipitating electrons either had average energies of a few hundred eV or were below polar rain flux levels. Near the transition to upward currents, DE 2 generally detected intense fluxes of accelerated electrons and weak fluxes of ions, both with average energies between 5 and 12 keV. In two instances, precipitating ions with energies >5 keV spanned both current sheets. Comparisons with satellite measurements at higher altitudes suggest that the particles and fields originated in the magnetotail inside the distant reconnection region and propagated to Earth through the plasma sheet boundary layer. Auroral electrons are accelerated by parallel electric fields produced by the different pitch angle distributions of protons and electrons in this layer interacting with the near‐Earth magnetic mirror. Electric field spikes driving rapid plasma flows along the poleward boundaries of intense, keV electron precipitation represent ionospheric responses to the field‐aligned currents and conductivity gradients. The generation of field‐aligned currents in the boundary layer may be understood qualitatively as resulting from the different rates of earthward drift for electrons and protons in the magnetotail's current sheet.
The magnetosheath region adjacent to the dayside magnetopause: AMPTE/IRM observations
Authors: Phan T. ‐D. , Paschmann G. , Baumjohann W. , Sckopke N. , Lühr H.
Published: February 2004

We have studied 38 low‐latitude, dayside (0800‐1600 LT) magnetopause crossings by the AMPTE/IRM satellite to investigate the variations of key plasma parameters and the magnetic field in the magnetosheath region adjacent to the dayside magnetopause. We find that the structures of the key plasma parameters and the magnetic field and the dynamics of plasma flows in this region depend strongly on the magnetic shear across the magnetopause, that is, on the angle between the magnetosheath magnetic field and the geomagnetic field. When the magnetic shear is low (<30°), a magnetosheath transition layer, also called the “plasma depletion layer,” of 10‐min average width exists where the magnetosheath magnetic field piles up against the magnetopause. In this region the plasma density and plasma β as well as the proton and electron temperatures are lower than in the magnetosheath proper. The condition for the onset of the mirror instability is generally not met in the magnetosheath transition layer, where the plasma β often falls below 1, while it is marginally satisfied in the magnetosheath proper, where usually β>1. When the magnetic shear across the magnetopause is high (>60°), the near‐magnetopause magnetosheath is more disturbed. The magnetic field in this case does not pile up in the immediate vicinity of the magnetopause, and no systematic variations in the plasma parameters are observed in this region until the encounter of the magnetopause current layer; that is, there is no magnetosheath transition layer. Also in contrast to the low‐shear case, the mirror instability threshold is marginally satisfied throughout the magnetosheath. The plasma flow pattern in the magnetosheath region adjacent to the dayside magnetopause is also found to depend strongly on the magnetic shear across the magnetopause: the magnetosheath flow component tangential to the magnetopause is enhanced and rotates to become more perpendicular to the local magnetic field as the low‐shear magnetopause is approached. This flow behavior may be consistent with the formation of a stagnation line instead of a stagnation point at the subsolar magnetopause. Enhancement and rotation of the magnetosheath flow on approach to the magnetopause are rarely observed when the magnetic shear across the magnetopause is high. In essence, our observations provide evidence for high (low) rate of transfer of magnetic flux and mass across the magnetopause when the magnetic shear is high (low). The relationships between the electron and proton temperature anisotropies and β in the near‐magnetopause magnetosheath region are also examined. It is found that Te⊥/Te∥ remains close to 1 for the entire range of βe, whereas Tp⊥/Tp∥ is generally anticorrelated with βp∥. However, no universal relationship seems to exist between Tp⊥/Tp∥ and βp∥.
A model of the large‐scale distribution of the neutral sheet current and its substorm‐associated changes
Authors: Nakai H. , Kamide Y.
Published: February 2004

Using a simple model of the neutral sheet current, the relationship between the large‐scale distribution of the neutral sheet current and the lobe magnetic field at X=−18 RE is examined. The current is assumed to flow duskward along parabolic curves and return dawnward through the semicircular paths along the high‐latitude magnetopause, and the intensity of the neutral sheet current is expressed by using a power law function of the distance from the Earth. A series of tests are performed to find the relative importance of physical parameters in the model. It is found that the lobe magnetic field depends significantly on the radial gradient and the flow‐line curvature of the neutral sheet current as well as on the current intensity. We predict that the radial gradient of the current intensity becomes steeper after the expansion onset of substorm.
Change in the radiative output of the Sun in 1992 and its effect in the thermosphere
Authors: White O. R. , Rottman G. J. , Woods T. N. , Knapp B. G. , Keil S. L. , Livingston W. C. , Tapping K. F. , Donnelly R. F. , Puga L. C.
Published: February 2004

Ground and space measurements of the solar spectral irradiance at radio, visible, UV, and X ray wavelengths show a large decline in the first 6 months of 1992. This sustained drop in the solar output is important in understanding the connection between the emergent magnetic flux on the Sun and the radiative output as well as in understanding the effects of such change in the upper atmosphere of the earth. We present preliminary estimates of the observed changes as the means to spur inquiry into this solar event in the declining phase of solar cycle 22. Typical decreases are 15% in Lyman α and 40% in 10.7‐cm radio flux. Mass spectrometer and incoherent scatter model calculations at 600 km in the thermosphere indicate a 30% decrease in the temperature and a 3X decrease in the density of the thermosphere near the altitude where both the Upper Atmosphere Research Satellite (UARS) and Hubble Space Telescope are flying. Decrease of the orbital period of the UARS shows the expected effect of decreasing density at flight altitude. Work in progress indicates that the output change results from the decline in solar magnetic flux to a lower level of activity in the southern hemisphere of the Sun.
Observations of power line harmonic radiation by the low‐altitude AUREOL 3 satellite
Authors: Parrot Michel
Published: February 2004

During the past decade, most scientific satellites have been dedicated to the study of the auroral zones, but low‐altitude satellites also allow the study of the anthropogenic effects in the mid‐latitude regions. AUREOL 3 was such a low‐altitude polar‐orbiting satellite, with an apogee and perigee of 2000 and 400 km, respectively, which observed power line harmonic radiation (PLHR). The wave experiment named ARCAD 3 provided the waveforms of five components of the electromagnetic field (three magnetic and two electric) in a frequency range between O and 1.5 kHz. The telemetry was recorded at different stations. During the lifetime of the mission, 940 records, each lasting approximately 12 min, were processed in order to obtain spectrograms of the waveforms. On these spectrograms, five events with magnetospheric lines have been detected, which in all cases are separated by 50 Hz (with an accuracy of 1 %); lines are not at exact harmonics of the 50‐Hz power system, but drift in frequency with time. The drift, which is between 1 and 8 Hz/s, is not constant during an event. A common property of these five events is that they were recorded during periods of very low magnetic activity and were observed mainly in the night sector. Comparison of the characteristics of these PLHR with existing theories concerning the general problem of VLF triggered emissions and sideband stability of whistler mode waves, shows that the drift in frequency is not due to a Doppler effect, but most probably to a non linear interaction between electrons and coherent waves, occurring in the topside ionosphere.
Dynamics of three‐dimensional plasma clouds with coupling to the background ionosphere
Authors: Ma T.‐Z. , Schunk R. W.
Published: February 2004

A three‐dimensional, time‐dependent model with a two‐grid system was developed to study the expansion of a plasma cloud in the F region and topside ionosphere. The model maintains an adequate resolution for the released cloud motion and its interaction with the immediate environment, and it includes the effect due to the coupling with the distant part of the ionosphere (i.e., E region). Simulations were performed using realistic background ionospheric density profiles in both the E and F regions. The results show that the cloud coupling to the underlying E region affects the perpendicular cloud motion the most. The distant coupling acts to reduce the perturbation potential and perpendicular velocity and delays or eliminates the striations. These simulation results are consistent with simple analytical approximations. The simulation results also show that the distant coupling has a very small effect on “localized” phenomena, such as the cloud expansion along the
field and the electrostatic snowplow. The cloud‐induced electric potential is attenuated in the lower E region. The electrons flow along the
field, carrying the current to the E region and back to the cloud. The current closure is demonstrated in three dimensions for the first time for such a problem. The perpendicular current flowing through the plasma cloud is closed by the field‐aligned electron current and the background perpendicular (mainly Pedersen) current in both the E and F regions. The “image cloud” formation in the E region is also clearly demonstrated. The variation of the density change in the “image cloud” along the
field and the features of the image cloud are shown.
Empirical modeling of the quiet time nightside magnetosphere
Authors: Lui A. T. Y. , Spence H. E. , Stern D. P.
Published: February 2004

Empirical modeling of plasma pressure and magnetic field for the quiet time nightside magnetosphere is investigated. Two models are constructed for this study. One model, referred to here as T89R, is basically the magnetic field model of Tsyganenko (1989) but is modified by the addition of an inner eastward ring current at a radial distance of ∼3 RE as suggested by observation. The other is a combination of the T89R model and the long version of the magnetic field model of Tsyganenko (1987) such that the former dominates the magnetic field in the inner magnetosphere, whereas the latter prevails in the distant tail. The distribution of plasma pressure, which is required to balance the magnetic force for each of these two field models, is computed along the tail axis in the midnight meridian. The occurrence of pressure anisotropy in the inner magnetospheric region is also taken into account by determining an empirical fit to the observed plasma pressure anisotropy. This effort is the first attempt to obtain the plasma pressure distribution in force equilibrium with magnetic stresses from an empirical field model with the inclusion of pressure anisotropy. The inclusion of pressure anisotropy alters the plasma pressure by as much as a factor of ∼3 in the inner magnetosphere. The deduced plasma pressure profile along the tail axis is found to be in good agreement with the observed quiet time plasma pressure for geocentric distances between ∼2 and ∼35 RE.
Aspect angle dependence of the radar aurora Doppler velocity
Authors: Kustov A. V. , Uspensky M. V. , Sofko G. J. , Koehler J. A. , Mu J.
Published: February 2004

The aspect angle dependence of the Doppler velocity of radar auroral echoes was studied by means of a comparison between the signals received simultaneously on four 50‐MHz CW links. In most of the events, the observed Doppler velocity decreased with increasing aspect angle but the rate of decrease was lower than that predicted by linear theory. The velocity decrease was about a factor of 0.4 between geometrical aspect angles of 1.5° and 4.0° and 0.6 to 0.7 between 3.2° and 5°. A model for the Doppler velocity behavior of the radar auroral signals is proposed, combining both propagation and scattering effects. Allowance is made for “ray spreading” arising from processes like refraction and diffraction during propagation. In addition, height integration over the scattering volume is taken into account.
Cyclotron harmonic effect on the thermal oscillating two‐stream instability in the high latitude ionosphere
Authors: Huang J. , Kuo S. P.
Published: February 2004

A thermal oscillating two‐stream instability as the generation mechanism of electron Bernstein/upper hybrid waves by the o‐mode HF heater wave in the high‐latitude ionosphere is investigated. The purely growing decay mode is described by the fluid equations, while the kinetic equations are employed to derive the coupled mode equations of the electron Bernstein/upper hybrid sidebands. It is found that the instability can be excited in the altitude regions both above and below the upper hybrid resonance layer. The parametric excitation of electron Bernstein waves is in general not effective. On the other hand, the upper hybrid sidebands of the instability excited in the “below” region have been considered by Leyser (1991) to be the pumps for the parametric excitation of downshifted maximum (DM) lines of the stimulated electromagnetic emissions (SEEs) observed in the ionospheric heating experiments. The detail analysis of the proposed work shows that the instability zone of upper hybrid waves below the upper hybrid resonance layer becomes very small when the heater frequency fnof;0 is operated near three times of the local electron cyclotron frequency fnof;c. Thus the source wave of the DM lines is effectively suppressed. This result explains the phenomenon of quenching of the DM feature in the SEE spectrum as fnof;0 is adjusted near 3fnof;c in the ionospheric modification experiments.
Convection as a free boundary problem: The substorm cycle
Authors: Atkinson G.
Published: February 2004

Magnetohydrodynamic solutions are presented for the problem of time‐dependent magnetospheric convection. They include regions of both taillike and dipolelike flux tubes and the interaction and flux transfer between them. The only external input is the rate of dayside merging. The problem is solved as a free boundary problem for both the high‐latitude boundary between taillike and dipolelike tubes and the low‐latitude boundary defined by plasma penetration. A constraint of no equatorward component to the convection is used to resolve the β problem in the region where the plasma sheet is on dipolar field lines. This constraint imposes a three‐dimensional nightside topology and determines the boundaries and the flux transfer rate into the nightside dipolar region. It also leads to a major asymmetry in the rate at which the amount of taillike flux can increase or decrease and hence accounts for the existence of the substorm phases. The narrow‐channel approximation, developed a number of years ago by the author, is used to obtain analytic solutions. Extensive comparison of the results of the theory with the observed aspects of convection and substorms is carried out. Values predicted by the model are in agreement with observations, and most major properties of the substorm are explained, including the existence of activity at two separated latitudes in the recovery phase.
Thermospheric O I 844.6‐nm emission in twilight
Authors: Bahsoun‐Hamade F. , Wiens R. H. , Shepherd G. G. , Richards P. G.
Published: February 2004

The thermospheric O I 844.6‐nm column emission rate was measured over Toronto, a midlatitude station, in the autumn of 1991 using an imaging Fabry‐Perot spectrometer. Twilight decay curves were measured on four clear evenings when the solar zenith angle was between 95° and 104°, giving corresponding column emission rates between 874 R and 130 R at 20° elevation angle in the azimuth of the Sun. The expected decay curves were calculated from the field line interhemisperic plasma model assuming only photoelectron impact excitation as the production mechanism with a cross section appropriate to an optically thin atmosphere. The agreement was good when the solar and geomagnetic activity levels were low to moderate, but the emission rate was overestimated during high activity periods. The comparison indicates that the photoelectron impact mechanism with a thin‐atmosphere cross section is sufficient to explain the twilight decay of the thermospheric O I 844.6‐nm emission.
Magnetospheric field and current distributions during the substorm recovery phase
Authors: Pulkkinen T. I. , Baker D. N. , Toivanen P. K. , Pellinen R. J. , Friedel R. H. W. , Korth A.
Published: February 2004

We have studied 11 substorm recovery phase events in which magnetic field and energetic particle data were available near the midnight sector from the GEOS 2 satellite. Comparison with the Tsyganenko magnetic field model shows that, after the expansion phase, BZ is large and decreases gradually toward the model value during the recovery phase, whereas deviations of BX and BY relative to the model values are small after the effects of the substorm current wedge have disappeared. We have modeled this sequence by using temporally evolving current systems implemented as additions to the Tsyganenko model. The tail current sheet thickness and the cross‐tail current intensity at different radial distances were varied using six free parameters in the model. The parameters were evaluated using a least squares fit for each of the 11 events separately. The results suggest that at the beginning of the recovery phase the current sheet was relatively thick close to the inner edge of the plasma sheet. Model fittings produced two different field configurations. In seven events the cross‐tail current was weak, and the field configuration was highly dipolar. In four events the near‐Earth current was weak, but stronger currents remained in the midtail region. In these latter events the field configuration at the beginning of the recovery phase included a region where BZ was negative. This negative BZ and the associated near‐Earth neutral line disappeared later as the current system developed toward the quiet time configuration. The magnetic field configuration, current distributions, and particle drift paths during the substorm recovery phase are examined and compared with those prevailing during the substorm growth phase.
Signatures of the substorm recovery phase at high‐altitude spacecraft
Authors: Baker D. N. , Pulkkinen T. I. , Hones E. W. , Belian R. D. , McPherron R. L. , Angelopoulos V.
Published: February 2004

The substorm recovery phase typically commences ∼30 min after the substorm expansion phase onset and covers a period of roughly 1 hour. Several signatures have previously been associated with the recovery phase such as plasma sheet expansion in the midtail and magnetic field return toward the quiet time configuration. However, the detailed temporal sequence during the recovery phase is still not very well established. A total of 66 events in February‐April 1979 have been investigated where ISEE 2 observed a plasma sheet expansion associated with fast earthward flows. Of these, 50 events were clearly associated with the substorm recovery phase as identified in ground magnetic records. For this data set, energetic electron (>30 keV) and proton (>145 keV) observations from two geostationary spacecraft were available in 41 cases. Of the 41 cases, 32 of the midtail plasma sheet recoveries were associated with distinctive ion or electron flux increases at geostationary orbit. These flux increases, often in both protons and electrons, were generally observed from the predusk to the postdawn sector. However, very few enhancements were found near local noon. The lack of large energy dispersion in the flux increases and the simultaneous occurrence of both electron and ion enhancements suggests that the particles do not drift from a more distant location (as in substorm expansion phase onsets) but are accelerated or redistributed locally. These events are suggested to be associated with a large‐scale reconfiguration of the near‐Earth tail as the neutral line retreats to large distances.
An improved Langmuir probe formula for modeling satellite interactions with near‐geostationary environment
Authors: Lai Shu T.
Published: February 2004

The Langmuir plasma probe model is an important tool in spacecraft current collection and charging calculations. In ideal geometries, such as a sphere or an infinitely long cylinder, the model is well understood. However, the realistic geometries of current collectors, or spacecraft, are nonideal. An empirical formula for a Langmuir probe with a given nonideal geometry would be useful. We derive such a formula for the SCATHA satellite by using the SC10 potential data obtained during electron beam emissions. The satellite rotated perpendicular to sunlight with the SC10 booms in the equatorial plane. We choose one special mode of operation during a quiet space environment. In this mode the beam current increased continuously, while the energy remained constant. We analyzed the variations of the vehicle potential responding to the unique driving factor, the beam current. To provide physical explanation to the behavior of the SC10 potential data, we model the interactions between the beam, photoelectron, and ambient currents. We present an algorithm which successfully yields an empirical Langmuir probe formula for SCATHA, from which we obtain improved estimate of ambient electron temperatures and densities. The results predicted by the improved Langmuir probe model compare favorably with the very few published measurements from the region.
Early time evolution of negative ion clouds and electron density depletions produced during electron attachment chemical release experiments
Authors: Scales W. A. , Bernhardt P. A. , Ganguli G.
Published: February 2004

Two‐dimensional electrostatic particle‐in‐cell simulations are used to study the early time evolution of electron depletions and negative ion clouds produced during electron attachment chemical releases in the ionosphere. The simulation model considers the evolution in the plane perpendicular to the magnetic field and a three‐species plasma that contains electrons, positive ions, and also heavy negative ions that result as a by‐product of the electron attachment reaction. The early time evolution (less than the negative ion cyclotron period) of the system shows that a negative charge surplus initially develops outside of the depletion boundary as the heavy negative ions move across the boundary. The electrons are initially restricted from moving into the depletion due to the magnetic field. An inhomogenous electric field develops across the boundary layer due to this charge separation. A highly sheared electron flow velocity develops in the depletion boundary due to E × B and ▽N × B drifts that result from electron density gradients and this inhomogenous electric field. Structure eventually develops in the depletion boundary layer due to low‐frequency electrostatic waves that have growth times shorter than the negative ion cyclotron period. It is proposed that these waves are most likely produced by the electron‐ion hybrid instability that results from sufficiently large shears in the electron flow velocity.
Analytical models of the magnetic field of disk‐shaped current sheets
Authors: Tsyganenko N. A. , Peredo M.
Published: February 2004

Disk‐shaped current distributions are useful tools in modeling the magnetospheres of Earth and other planets and have also been adapted for modeling the geotail. Such models usually start with an axisymmetric vector potential but may be modified to account for observed asymmetries, variable thickness, and warping in response to an inclined orientation of the planetary dipole axis. Models of this type until now either have lacked the ability to simulate a sharp inner edge of the current and to control accurately its falloff with distance or did not allow a simple analytical representation. Here existing methods will be reviewed, after which a new class of models which overcomes the above deficiencies and also allows the modeling of current disks of finite thickness flanked by current‐free regions will be presented.
Contribution of low‐energy ionospheric protons to the plasma sheet
Authors: Delcourt D. C. , Moore T. E. , Chappell C. R.
Published: February 2004

The magnetospheric transport of low‐energy ionospheric ions is examined by means of three‐dimensional particle codes. Emphasis is placed on the behavior of polar wind and cleft originating protons. It is demonstrated that, via nonadiabatic motion inside the neutral sheet, these ions can significantly contribute to the populations of the plasma sheet. The importance of this contribution is found to depend critically upon the dynamics of particles originating from the highest latitudes, as these possibly have access to the distant tail. Hence it is shown that polar wind H+ expelled into the magnetosphere at very low energies (in the electron volt range) preferentially feed the plasma sheet during quiet times, experiencing accelerations up to several kiloelectron volts upon return into the inner magnetosphere. In contrast, during disturbed times, the intensifying magnetospheric convection confines this population to low L shells where it travels in a nearly adiabatic manner. As for the protons originating from the cleft fountain, the simulations reveal that they can be transported up to the vicinity of the distant neutral line in the nightside sector. Via interaction with the neutral sheet, these ionospheric ions are rapidly raised to the characteristic plasma sheet energy range. The density levels contributed by these populations are quite substantial when compared to those measured in situ. These simulations establish an active role of low‐energy ionospheric ions in the overall magnetospheric dynamics.
Energy density of ionospheric and solar wind origin ions in the near‐Earth magnetotail during substorms
Authors: Daglis Ioannis A. , Livi Stefano , Sarris Emmanuel T. , Wilken Berend
Published: February 2004

Comprehensive energy density studies provide an important measure of the participation of various sources in energization processes and have been relatively rare in the literature. We present a statistical study of the energy density of the near‐Earth magnetotail major ions (H+, O+, He++, He+) during substorm expansion phase and discuss its implications for the solar wind/magnetosphere/ionosphere coupling. Our aim is to examine the relation between auroral activity and the particle energization during substorms through the correlation between the AE indices and the energy density of the major magnetospheric ions. The data we used here were collected by the charge‐energy‐mass (CHEM) spectrometer on board the AMPTE/CCE satellite in the near‐equatorial nightside magnetosphere, at geocentric distances ∼7‐9 RE. CHEM provided the opportunity to conduct the first statistical study of energy density in the near‐Earth magnetotail with multispecies particle data extending into the higher energy range (≥ 20 keV/e). The use of 1‐min AE indices in this study should be emphasized, as the use (in previous statistical studies) of the (3‐hour) Kp index or of long‐time averages of AE indices essentially smoothed out all the information on substorms. Most distinct feature of our study is the excellent correlation of O+ energy density with the AE index, in contrast with the remarkably poor He++ energy density ‐ AE index correlation. Furthermore, we examined the relation of the ion energy density to the electrojet activity during substorm growth phase. The O+ energy density is strongly correlated with the pre‐onset AU index, that is the eastward electrojet intensity, which represents the growth phase current system. Our investigation shows that the near‐Earth magnetotail is increasingly fed with energetic ionospheric ions during periods of enhanced dissipation of auroral currents. The participation of the ionosphere in the substorm energization processes seems to be closely, although not solely, associated with the solar wind/magnetosphere coupling. That is, the ionosphere influences actively the substorm energization processes by responding to the increased solar wind/magnetosphere coupling as well as to the unloading dissipation of stored energy, with the increased feeding of new material into the magnetosphere.
Effect of magnetospheric convection on thermal plasma in the inner magnetosphere
Authors: Khazanov G. V. , Rasmussen C. E. , Konikov Yu. V. , Gombosi T. I. , Nagy A. F.
Published: February 2004

The effects of E × B convection on the distribution of plasma parameters in the inner magnetosphere have been examined. Analytical solutions describing density distributions along convective trajectories in the equatorial plane have been found. These solutions suggest the following dependence of plasma concentration n on the magnetic field B along convective trajectories: n∝ Bα, where the parameter α varies between 4/3 ≤ α ≤ 2. The α = 2 case corresponds to disregarding transport parallel to B. The lower bound α = 4/3 describes the situation when parallel transport dominates over convective motions perpendicular to B. A solution has also been obtained describing the effect of convection on diffusive equilibrium profiles in the dayside plasmasphere. In addition, ion temperature variations due to adiabatic effects associated with plasma convection have been analyzed in detail. Convective drifts lead to ion temperature anisotropies with the value and sign of the anisotropy contingent on density and temperature variations, local time, and the location of the convective trajectory with respect to the location of the plasmapause. We have also found that convective motions lead to a substantial exchange of energy between the ionosphere and the plasmasphere through electron heat fluxes.
Observational signatures of the influence of the interplanetary shocks on the associated low‐energy particle events
Authors: Heras A. M. , Sanahuja B. , Sanderson T. R. , Marsden R. G. , Wenzel K. ‐P.
Published: February 2004

We have analyzed the temporal evolution of the upstream particle fluxes and anisotropies in a set of particle events associated with interplanetary shocks, detected by ISEE 3 during the period 1978‐1980 in the 147‐238 and 620‐1000 keV energy ranges. In particular, we have studied those features that can perhaps be used as signatures of the history of the influence of the shock on the particle event. We have paid special attention to the evolution of the flux anisotropy that can carry relevant information on the contribution of shock accelerated particles to the total observed flux. Our analysis shows that during most events, long‐lasting (between 5 and 36 hours) large anisotropies are observed upstream of the shock, supporting the hypothesis of continuous injection of shock accelerated particles in the interplanetary medium while the shock is propagating outward from the Sun. The evolution of the anisotropy throughout each event shows a dependence on the longitude of the parent solar source. We have interpreted the observations considering that shock accelerated particles contribute significantly to the total observed flux from the time when the shock intersects the magnetic field line that connects with the observer. The heliocentric distance of this initial intersection point has been derived from the flux anisotropy observations for each event. The values obtained show a distribution with respect to the longitude of the solar source that can be reproduced with a simple model, which assumes a spherical shock with an angular extension ≈ 100° and an archimedian spiral structure for the interplanetary magnetic field. The results of this work provide a further insight into both the importance of shock acceleration, as well as the influence of the large‐scale shock structure in low‐energy particle events.
Magnetic spectral signatures in the Earth's magnetosheath and plasma depletion layer
Authors: Anderson Brian J. , Fuselier Stephen A. , Gary S. Peter , Denton Richard E.
Published: February 2004

Correlations between plasma properties and magnetic fluctuations in the subsolar magnetosheath downstream of a quasi‐perpendicular shock have been found and indicate that mirror and ion cyclotronlike fluctuations correlate with the magnetosheath proper and plasma depletion layer, respectively (Anderson and Fuselier, 1993). We explore the entire range of magnetic spectral signatures observed from the AMPTE/CCE spacecraft in the magnetosheath downstream of a quasi‐perpendicular shock. The magnetic spectral signatures typically progress from predominantly compressional fluctuations, δB∥/δB⊥ ≈ 3, with F/Fp <0.2 ( F and Fp are the wave frequency and proton gyrofrequency, respectively) to predominantly transverse fluctuations, δB∥/δB⊥ ≈ 0.3, extending up to Fp. The compressional fluctuations are characterized by anticorrelation between the field magnitude and electron density, ne, and by a small compressibility, Ce ≡ (δne/ne)2(B/δB∥)2 ≈ 0.13, indicative of mirror waves. The spectral characteristics of the transverse fluctuations are in agreement with predictions of linear Vlasov theory for the H+ and He2+ cyclotron modes. The power spectra and local plasma parameters are found to vary in concert: mirror waves occur for β∥p (β∥p ≡ 2µ0npkT∥p/B2) ≈ 2, Ap ≡ T⊥p/T⊥ ‐ 1 ≈ 0.4, whereas cyclotron waves occur for β∥p ≈ 0.2 and Ap ≈ 2. The transition from mirror to cyclotron modes is predicted by linear theory. The spectral characteristics overlap for intermediate plasma parameters. The plasma observations are described by Ap = 0.85β∥p−0.48 with a log regression coefficient of −0.74. This inverse Ap ‐ β∥p correlation corresponds closely to the isocontours of maximum ion anisotropy instability growth, γm/Ω = 0.01, for the mirror and cyclotron modes. The agreement of observed properties and predictions of local theory suggests that the spectral signatures reflect the local plasma environment and that the anisotropy instabilities regulate Ap. We suggest that the spectral characteristics may provide a useful basis for ordering observations in the magnetosheath and that the Ap ‐ β ∥p inverse correlation may be used as a beta‐dependent upper limit on the proton anisotropy to represent kinetic effects.
Features of steady magnetospheric convection
Authors: Yahnin A. , Malkov M. V. , Sergeev V. A. , Pellinen R. J. , Aulamo O. , Vennerström S. , Friis‐Christensen E. , Lassen K. , Danielsen C. , Craven J. D. , Deehr C. , Frank L. A.
Published: February 2004

The large‐scale patterns of ionospheric convection and particle precipitation are described during two intervals of steady magnetospheric convection (SMC) on November 24, 1981. The unique data set used in the analysis includes recordings from the worldwide network of magnetometers and all‐sky cameras, global auroral images from the DE 1 spacecraft, and particle precipitation data from low‐altitude NOAA 6 and NOAA 7 spacecraft. The data show that intense magnetospheric convection continued during more than 10 hours under the steady southward interplanetary magnetic field without any distinct substorm signatures. All data sets available confirmed the stable character of the large‐scale magnetospheric configuration during this period. In particular, the magnetic flux threading the polar cap was stable (within 10%) during 3.5 hours of continued DE 1 observations. The dayside cusp was located at an unusually low latitude (70° CGL). The nightside auroral pattern consisted of two distinct regions. The diffuse aurora in the equatorward half of the expanded (10° wide) auroral oval was well‐separated from the bright, active auroral forms found in the vicinity of the poleward boundary of the oval. The twin‐vortex convection pattern had no signature of the Harang discontinuity; its nightside “convection throat” was spatially coincident with the poleward active auroras. This region of the auroral oval was identified as the primary site of the short‐lived transient activations during the SMC intervals. The energetic particle observations show that the auroral precipitation up to its high‐latitude limit is on closed field lines and that particle acceleration up to > 30‐keV energy starts close to this limit. The isotropic boundaries of the > 30‐keV protons and electrons were found close to each other, separating regions of discrete and diffuse precipitation. This suggests that these precipitation types originate on the very taillike and very dipolelike field lines, respectively.
Auroral latitude Pc 5 field line resonances: Quantized frequencies, spatial characteristics, and diurnal variation
Authors: Ziesolleck C. W. S. , McDiarmid D. R.
Published: February 2004

On the basis of observations with the Goose Bay Radar, several stations of the the Canadian Auroral Network for the Origin of Plasmas in the Earth's Neighborhood (OPEN) Unified Study (CANOPUS) magnetometer array, and a CANOPUS meridian scanning photometer, recent work has indicated the existence of Pc 5 ULF waves with discrete and remarkably stable frequencies (1.3, 1.9, 2.7 and 3.3 mHz with <10% variation) in the local midnight and early morning (predawn) sectors. These ULF waves were interpreted as the consequence of field line resonances driven by MHD waveguide/cavity modes, possibly excited by impulsive stimulation of the magnetopause or by Kelvin‐Helmholtz instabilities in the low‐latitude boundary layer. However, the reported stability of these frequencies over long periods of time remains a puzzle, as it seems to be inconsistent with the dynamic behavior of the magnetosphere. In order to establish if these frequencies are also observed in the dayside magnetosphere, where one of the proposed source mechanisms predicts they should be seen, we have examined the temporal and spatial characteristics of Pc 5 pulsations recorded recently with the CANOPUS magnetometer array. Pure state filtering techniques were used to determine the Pc 5 signal characteristics and their temporal and spatial variation. The results show that ULF wave signals with quantized frequencies seem to be observed throughout the dayside and dusk magnetosphere. However, frequencies both higher than and between previously reported values were also seen. The difficulty in correctly determining fundamental frequencies and their stability in the presence of polychromatic, highly amplitude‐modulated signals is noted. The signal characteristics observed in the postnoon sector were found to be significantly different compared with those seen at prenoon. While local morning and noon Pc 5 revealed typical field line resonance characteristics, toroidal mode signal characteristics prevailed in the local afternoon and evening. The reason for this asymmetry is unknown. The azimuthal wave numbers were generally small and showed a linear dependence on the Pc 5 signal frequency, implying antisunward phase velocities independent of the wave frequency.
Heliospheric current sheet inclinations predicted from source surface maps
Authors: Shodhan S. , Crooker N. U. , Hughes W. J. , Siscoe G. L.
Published: February 2004

The inclinations of the neutral line at the ecliptic plane derived from source surface model maps of coronal fields are measured for the interval from June 1976 to March 1992. The mean and median values of 53° and 57° are close to the average inclinations determined earlier from minimum variance analyses of solar wind measurements at sector boundaries, but the mode falls in the 80°‐90° bin. This result, which is based on the model assumptions implicit in deriving the source surface maps, predicts that the heliospheric current sheet typically intersects the ecliptic plane nearly at right angles, even without steepening by stream interaction regions. High inclinations dominate the solar cycle for about 7 years around solar maximum. Dips to lower inclinations occur near solar minimum, but high variance admits a wide range of inclinations throughout the cycle. Compared to the smooth solar cycle variation of the maximum latitudinal excursion of the neutral line, often treated as the tilt angle of a flat heliospheric current sheet, the noisy variation of the inclinations reflects the degree to which the neutral line deviates from a sine wave, implying warps and corrugations in the current sheet. About a third of the time the neutral line so deviates that it doubles back in longitude.