bluuebunny/crossref_metadata_2025 huggingface dataset:
Onset conditions of equatorial (range) spread F at Fortaleza, Brazil, during the June solstice
Authors: Sastri J. H. , Abdu M. A. , Batista I. S. , Sobral J. H. A.
Published: February 2004

Case studies are made of the ionospheric, thermospheric, and geomagnetic conditions associated with the occasional onset of range spread F at Fortaleza (4°S, 38°W, dip latitude −1.8°, and magnetic declination 21°W), Brazil, in the June solstice, a season in which frequency spread F is typically seen in the postsunset hours (after 2000 LT). It is found that the F layer experiences quite consistently a large vertical drift early in the evening hours (1815–1915 LT) on days of range spread F in comparison to days of only frequency spread F. The anomalously large dusk time vertical plasma drift occurs under both geomagnetically disturbed and quiet conditions. There is no significant change in the pattern of meridional neutral winds (at and prior to the time of onset of range spread F) estimated from the data of F layer peak height (hmax) at the low‐latitude station, Cachoeira Paulista (23°S, 45°W, and dip 26.5°S), using a modified form of the servo model. The results strongly suggest that though the meridional (poleward) wind is very effective in inhibiting the growth of Rayleigh‐Taylor (R‐T) instability and hence range spread F at Fortaleza during the June solstice as shown by Maruyama [1996], its variability does not play an important role in creating favorable conditions for R‐T instability on a day‐to‐day basis. The prerequisite for the occasional occurrence of range spread F at Fortaleza in the June solstice seems to be the presence of an impulsive and large F layer vertical plasma drift, a condition favorable for destabilizing the bottomside F region through R‐T instability mechanism because of the high altitude of the layer.
Foreshock Langmuir waves for unusually constant solar wind conditions: Data and implications for foreshock structure
Authors: Cairns Iver H. , Robinson P. A. , Anderson Roger R. , Strangeway R. J.
Published: February 2004

Plasma wave data are compared with ISEE 1's position in the electron foreshock for an interval with unusually constant (but otherwise typical) solar wind magnetic field and plasma characteristics. For this period, temporal variations in the wave characteristics can be confidently separated from sweeping of the spatially varying foreshock back and forth across the spacecraft. The spacecraft's location, particularly the coordinate Dƒ downstream from the foreshock boundary (often termed DIFF), is calculated by using three shock models and the observed solar wind magnetometer and plasma data. Scatterplots of the wave field versus Dƒ are used to constrain viable shock models, to investigate the observed scatter in the wave fields at constant Dƒ, and to test the theoretical predictions of linear instability theory. The scatterplots confirm the abrupt onset of the foreshock waves near the upstream boundary, the narrow width in Dƒ of the region with high fields, and the relatively slow falloff of the fields at large Dƒ, as seen in earlier studies, but with much smaller statistical scatter. The plots also show an offset of the high‐field region from the foreshock boundary. It is shown that an adaptive, time‐varying shock model with no free parameters, determined by the observed solar wind data and published shock crossings, is viable but that two alternative models are not. Foreshock wave studies can therefore remotely constrain the bow shock's location. The observed scatter in wave field at constant Dƒ is shown to be real and to correspond to real temporal variations, not to unresolved changes in Dƒ. By comparing the wave data with a linear instability theory based on a published model for the electron beam it is found that the theory can account qualitatively and semiquantitatively for the abrupt onset of the waves near Dƒ = 0, for the narrow width and offset of the high‐field region, and for the decrease in wave intensity with increasing Dƒ. Quantitative differences between observations and theory remain, including large overprediction of the wave fields and the slower than predicted falloff at large Dƒ of the wave fields. These differences, as well as the unresolved issue of the electron beam speed in the high‐field region of the foreshock, are discussed. The intrinsic temporal variability of the wave fields, as well as their overprediction based on homogeneous plasma theory, are indicative of stochastic growth physics, which causes wave growth to be random and varying in sign, rather than secular.
A global magnetohydrodynamic simulation of the Jovian magnetosphere
Authors: Ogino Tatsuki , Walker Raymond J. , Kivelson Margaret G.
Published: February 2004

We have developed a three‐dimensional global magnetohydrodynamic simulation of the interaction between the solar wind and a rapidly rotating magnetosphere and applied it to Jupiter. For fixed solar wind dynamic pressure the rotating model Jovian magnetosphere extends farther toward the Sun and has greater extent in the east–west direction than a model without rotation but is little different in the north south direction. There is a pronounced dawn‐dusk asymmetry with the dawn magnetopause displaced farther from Jupiter. The middle Jovian magnetosphere contains a thin plasma sheet dominated by rotating plasmas. On the day side this plasma sheet thickens near the magnetopause. Near the dawnside magnetopause where rotating Jovian convection is opposed to the solar wind induced flow, a pressure ridge forms where the magnetospheric flow slows and forms a stagnation region. In the magnetotail the rotating flow is diverted tailward. For x < −100RJ x‐type and o‐type neutral lines form. When the solar wind pressure was decreased, the boundaries moved away from Jupiter and the dayside field lines became stretched into a more tail‐like configuration. A flow vortex formed in the evening middle magnetosphere.
Pitch angle diffusion at Jupiter's moon Ganymede
Authors: Williams D. J. , Mauk B.
Published: February 2004

The magnetic field of Jupiter's moon Ganymede, provides a unique, large B value mirror point near the equator of Jovian field lines that intersect the moon. Ganymede creates a large loss cone in the distributions of all charged particles populating those field lines, emptying them of particles that mirror at radial distances < ∼ 8 RJ. Such a loss cone is seen when the Galileo energetic particles detector (EPD) views along the field line toward Ganymede. When the EPD views in the antimoon direction, the loss cone is nearly filled in for low energy electrons and remains empty at high energies. Because these electrons travel between their Ganymede mirror points and their near‐Jupiter mirror points much faster than they drift across the Ganymede magnetosphere, they are effectively trapped between the moon and Jupiter for several bounce periods. Thus the amount of filling observed in the antimoon viewing loss cone gives a direct measure of the amount of pitch angle scattering occurring during the electrons half‐bounce period. Using general wave‐particle scattering theory, we have fit the pitch angle distributions within the antimoon viewing loss cone and have extracted values of the diffusion coefficient and its variation with energy. We find weak diffusion to dominate, yielding lifetimes of ∼103 s at low energies to ∼104 s at high energies. The diffusion coefficient, assumed independent of pitch angle, varies as ∼E−1 or equivalently ∼γ−3.5 (E is the electron kinetic energy and γ is the relativistic correction factor) at energies above ∼50 keV. Finally, we note that the existence of loss cones in the antimoon direction is evidence that Jovian field lines intersecting Ganymede effectively move with the moon.
Resonance‐generated particle distributions in magnetotail x line configurations
Authors: Harold J. B. , Chen J. , Joyce G.
Published: February 2004

We present the results of a series of test particle simulations that model particle acceleration both in self‐consistent magnetospheric fields obtained by magnetohydrodynamic (MHD) simulations and in simple x line fields. In particular, we investigate the physical mechanisms responsible for the formation of beam‐like features (“beamlets”) in the accelerated outgoing particle distributions reported by previous authors. We find that in our simulations these effects arise from the x line, where the field gradients are strong; despite this finding they can be readily explained by using a simple analysis based on the phase space partitioning and resonances in one‐dimensional current sheets, consistent with the conclusions of previous authors. We confirm the expected scaling using both two‐and one‐dimensional models of the x line with imposed uniform electric field. However, we find that the formation of these features is critically dependent on both the field geometry and the electric field imposed at the x line. In particular, use of an MHD‐generated nonuniform electric field model eliminates the energized particle population, while use of current sheets with a self‐consistent width blurs the features until they are no longer easily discernible. This finding calls into question the degree to which the beamlet structures (those that depend on magnetic field gradients along the tail with a uniform electric field) can be observed in physical systems.
SAMPEX observations of energetic electron precipitation in the dayside low‐latitude boundary layer
Authors: Blanchard G. T. , Lyons L. R. , Blake J. B. , Rich F. J.
Published: February 2004

We address the problem of the magnetic topology of the dayside low‐latitude boundary layer by using conjugate observations from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) and the Defense Meteorological Satellite Program Flight 10 (DMSP F10) satellites while they are in coplanar polar orbits in the 0900‐2100 magnetic local time (MLT) meridian. In the region identified in the DMSP observations as the low‐latitude boundary layer (LLBL), we observe energetic electron precipitation at SAMPEX in detectors measuring electrons in the energy ranges E > 150 keV and E > 1.05 MeV. This precipitation is in the region that would otherwise be identified as the polar cap. We find that the equatorward edge of the energetic electron precipitation coincides precisely with the equatorward edge of the LLBL as these boundaries move over a wide range of invariant latitude, We investigate the hypothesis that the energetic electron precipitation is due to pitch angle scattering in the magnetopause current layer of magnetospheric electrons on open drift paths and find that this hypothesis is reasonable. Additional evidence supporting this hypothesis is the dependence of the precipitating flux on the solar wind clock angle. A consequence of this hypothesis is that the low‐latitude boundary layer in the 0900‐1000 MLT sector lies completely on open magnetic field lines except, perhaps, when the interplanetary magnetic field is predominantly northward.
Polar observations of convection with northward interplanetary magnetic field at dayside high latitudes
Authors: Maynard N. C. , Burke W. J. , Weimer D. R. , Mozer F. S. , Scudder J. D. , Russell C. T. , Peterson W. K. , Lepping R. P.
Published: February 2004

This study of dayside electrodynamics provides a first view of quasi‐dc electric fields measured by Polar at middle altitudes. Three high‐latitude passes chosen for analysis, occurred while Polar was in the midday sector and the interplanetary magnetic field (IMF) maintained a steady northward orientation at clock angles near 40° in the YGSM – ZGSM plane. At different times during the passes, Polar encountered electron and ion fluxes from the central plasma sheet (CPS), the boundary plasma sheet, the polar rain, and various magnetospheric boundary regions. Although electric fields measured within the boundary layers were highly variable, we derived average components of plasma convection. The results compare favorably with convection characteristics of four‐cell patterns observed at ionospheric altitudes with northward IMF. We show that Polar encountered lobe cells of both positive and negative polarity in the early postnoon sector. The negative potential extends well into the prenoon sector. Inverse, energy‐dispersed fluxes of He++ were detected in regions of sunward convection. Sunward convection extended into the region of polar rain north of the merging line's projection. The magnetic merging signatures indicate that the process is both time varying and patchy. Particle and field measurements are consistent with the afternoon, auroral convection cell closing in the low‐latitude boundary layer, which extend many tens of RE tailward of the dusk terminator. Taken in a larger context, the Polar measurements provide new insight concerning the evolution of high‐latitude convection from distorted two‐cell patterns at large IMF clock angles to four cells at low clock angles.
Two spacecraft observations of a reconnection pulse during an auroral breakup
Authors: Petrukovich A. A. , Sergeev V. A. , Zelenyi L. M. , Mukai T. , Yamamoto T. , Kokubun S. , Shiokawa K. , Deehr C. S. , Budnick E. Y. , Büchner J. , Fedorov A. O. , Grigorieva V. P. , Hughes T. J. , Pissarenko N. F. , Romanov S. A. , Sandahl I.
Published: February 2004

At 1130 UT on November 28, 1995, two spacecraft, Interball‐Tail and Geotail, were in a favorable position to study the plasma sheet activity and an auroral breakup observed on the ground near the spacecraft ionospheric footpoints. Both spacecraft were near the neutral sheet, and they were nearly aligned along the magnetic meridian. During the auroral breakup observed at the equatorward half of the auroral oval (also registered as an AKR burst at Interball) both spacecraft simultaneously detected signatures of a reconnection pulse: The earthward plasma streaming and magnetic field dipolarization were observed at 12 RE at Interball, while the tailward energetic ion beam, then the tailward flow and the passage of a plasmoid were observed at 28 RE at Geotail. This pulse seem to proceed inside of the plasma sheet closed field lines, in the region of small (∼1 nT) background magnetic field at the neutral sheet. At Interball position the onset of fast earthward ion flow, likely initiated by the reconnection pulse, was followed by other manifestations (dipolarization, enhancements of the magnetic turbulence and the energetic particle flux, the intensification of field‐aligned currents). Auroral observations showed initial brightening delayed an approximately l min after the commencement of the reconnection pulse. The auroral intensification was not accompanied by a significant magnetic disturbance on the ground, and therefore the event can be classified as the pseudobreakup. We estimate magnetic flux transport characteristics and possible location of the onset region in the plasma sheet. We conclude that observations during this event are consistent with the initiation of an auroral breakup by some disturbance (e.g., Alfven wave) generated by the reconnection pulse that commenced in the neutral sheet at ∼15 RE distance.
Dayside ionospheric plasma convection, electric fields, and field‐aligned currents derived from the SuperDARN radar observations and predicted by the IZMEM model
Authors: Kustov A. V. , Papitashvili V. O. , Sofko G. J. , Schiffler A. , Feldstein Y. I. , Gromova L. I. , Levitin A. E. , Belov B. A. , Greenwald R. A. , Ruohoniemi M. J.
Published: February 2004

A recent deployment of the Super Dual Auroral Radar Network (SuperDARN) HF radar network provides excellent opportunities to construct two‐dimensional maps of the ionospheric convection over large areas in both the northern and southern polar regions. The Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation electrodynamic model (IZMEM) is a potentially useful tool for predicting, also on a global scale, the ionospheric plasma convection patterns, electric fields, magnetic disturbances, ionospheric and field‐aligned currents. Comparisons of the IZMEM predictions with satellite and incoherent scatter radar data show that the model's performance is reasonably good, but the model needs more extensive and accurate verifications. In this paper, several events under relatively stable IMF conditions (Bz < 0 and Bz > 0) are studied and discussed. The SuperDARN/IZMEM ion drift velocities are found to be in reasonable agreement in both magnitude and direction; the average difference (over the individual SuperDARN convection map) between the predicted magnitude of the ion drift velocity and the measured magnitude is about 50%, while the difference in the direction is typically less than 25°. The IZMEM predictions of the location, direction, and magnitude of field‐aligned currents also agree well with the SuperDARN observations. It is concluded that simultaneous use of SuperDARN data and IZMEM model can improve the specification of the polar ionospheric convection for space weather applications.
Ionization mechanisms in CRRES chemical releases: 1. In situ measurements and model results
Authors: Hunton D. E. , Wolf P. J. , Shadid T. M.
Published: February 2004

The quadrupole ion mass spectrometer (QIMS) measured fluxes of barium and strontium ions during the CRRES G‐1 chemical release in full Sun and the G‐11b release in darkness. The barium ion fluxes detected in darkness were only a factor of 10 lower than the photoionization fluxes, indicating that the rate of nonsolar‐UV ionization was at least a factor of 8 higher than literature values of charge transfer and collisional ionization cross sections would suggest. Model calculations of the QIMS ion fluxes using new, state‐specific, calculations of the cross sections for ion‐pair formation and charge transfer [Wolf and Hunton, this issue] demonstrate that the large nonphotoionization rates are consistent with significant metastable excitation in the neutral cloud but not with a ground state population. Upper limits to the collisional ionization rates for the conditions of the releases are 4.3 × 10−3 s−1 for ion‐pair production and 2.2 × 10−3 S−1 for charge transfer. Collisions of high‐energy electrons with the neutrals early in the cloud expansion is suggested as a mechanism for producing the metastable metal atoms. A similar analysis of the strontium data indicates that photoionization, charge transfer, and collisional ion‐pair production alone cannot explain the large fluxes of strontium ions detected in the sunlit release. Associative ionization may therefore be an important ionization mechanism for that metal in sunlight. The implications of these results for analysis of space critical ionization velocity experiments are discussed.
Contribution of obliquely incident particles to neutron monitor counting rate
Authors: Clem J. M. , Bieber J. W. , Evenson P. , Hall D. , Humble J. E. , Duldig M.
Published: February 2004

We describe a new method for calculating geomagnetic cutoffs that incorporates obliquely incident primaries, and we use it to interpret a sea level neutron monitor latitude survey. Effects due to obliquely incident primaries are significant and may be responsible for anomalies observed in this and other latitude surveys. We define an “apparent” cutoff that takes these obliquely incident particles into account. Use of our apparent cutoff in a Dorman function fit to the 1994–1995 Bartol Research Institute‐University of Tasmania latitude survey data results in a significant improvement over use of the more conventional effective vertical cutoff.
Systems III and IV modulation of the Io phase effect in the Io plasma torus
Authors: Volwerk Martin
Published: February 2004

We report two new findings regarding Io control of the EUV brightness of the plasma torus around Jupiter. These new findings relate to the well‐known periodic brightenings of the torus that are tied to the orbital phase of Io (the Io phase effect), the most prominent of the periodic torus brightness variations seen in the EUV. We first show that the amplitude of the Io phase effect depends on the system III longitude of the ansa, in addition to the Io phase angle. Because the ribbon has a system III dependent motion, the brightness of the EUV ribbon, which is embedded in the torus, is a function of its closest approach to the orbit of Io. We model the observed average Io phase effect with two sources of torus power: one, a longitudinally symmetric power source that keeps the Io plasma torus at a temperature of ∼ 8 × 104 K and the other a power source located around Io. The amplitude of the Io phase effect is dependent on the distance between the ansa and lo's orbit (not Io itself). We then show that this dependence arises from a geometrical effect and self‐absorption of the 685‐Å line by the torus. The interplay of system III and system IV and its effect on the Io phase effect is investigated for connection between cumulative density variations through the Io plasma torus leading to a modification of the heat source near Io. This modification could create the brightness difference in the aligned and antialigned phase of systems III and IV.
Quasi‐periodic poleward motions of morningside Sun‐aligned arcs: A multievent study
Authors: Shiokawa K. , Ogino T. , Hayashi K. , McEwen D. J.
Published: February 2004

We study eight events of the quasi‐periodic poleward motions of Sun‐aligned auroral arcs in the high‐latitude morning sector using all‐sky image data and meridian scanning photometer data obtained at Resolute Bay (RSB; 84° magnetic latitude). Canada, during four winters from December 1992 to February 1996. More than 10% of morningside arcs observed at RSB move poleward repeatedly. The periodic arc motions occur during intervals of interplanetary magnetic field Bz >0 or ∼0, Bx >0 or <0, and By >0 or ∼0. Most of the events occur around the end of substorm‐like magnetic activity, suggesting that the source of moving arcs is related to the plasma sheet particles. Comparison with a global MHD simulation suggests that the observed poleward motion of arcs corresponds to inward (+YGSM direction) motion of source plasmas in the morningside tail flank where the plasmas from the low‐latitude boundary layer and from the plasma sheet are mixed.
A model of the formation of stable nonpropagating magnetic structures in the solar wind based on the nonlinear mirror instability
Authors: Pantellini Filippo G. E.
Published: February 2004

A simple model for the formation of stable nonpropagating structures in a magnetized collisionless plasma is presented. The model describes the evolution of an electron‐proton plasma from an initially spatially uniform, but unstable, configuration toward a final nonuniform and nonpropagating stable configuration. The model is based on the following hypothesis: (1) one‐dimensionality and spatial periodicity, (2) cold electrons, (3) bi‐Maxwellian protons as initial condition, (4) conservation of magnetic moment for all protons, (5) conservation of energy for magnetically non trapped protons, (6) spatial pressure balance, (7) evolved structure has a crenellated shape, (8) slow growth of the structure. Given these assumptions all the macroscopic properties of the plasma (density, pressure, and magnetic field) in the saturated state can be computed explicitly. The model shows that a spatially uniform and homogeneous plasma that is unstable against the linear mirror mode can form stable non propagating structures. Thus one can consider the model as a model for the nonlinear mirror instability where the magnetic trapping of protons in the low magnetic field region is the important saturation mechanism. A simple expression for the magnetic field saturation amplitude is found. The pressure balance, between high and low magnetic field regions, which is needed for the evolved structure to be a stable one, is obtained solely through betatron cooling of the trapped protons. Modification of the trapped protons energy due to the Fermi effect seems to be of secondary importance. The model predicts that the evolved structures are characterized by narrow and deep magnetic wells except in the case of very low magnetic pressure (ratio of thermal to magnetic pressure β ≳ 10) where the opposite situation becomes possible. This enforces the idea according to which the proton mirror instability is the driving mechanism for the formation of magnetic holes in high β(≳ 1) plasmas.
On the internal radial structure of field line resonances
Authors: Mann Ian R.
Published: February 2004

We examine the radial scales developed inside field line resonances (FLRs) when they are driven by both broad and narrow frequency bandwidth fast mode sources. The finest FLR radial scales are always limited by ionospheric dissipation, being determined primarily by the height‐integrated Pedersen conductivity ∑p. We estimate likely FLR radial scale sizes in both dayside and nightside ionospheric conditions, and confirm the accuracy of these estimates using the wave Doppler shifts observed on inbound/outbound passes of Active Magnetospheric Particle Tracer Explorers CCE [Anderson et al, 1989]. Dayside broadband FLR events can have radial scale lengths significantly shorter than their overall widths, suggesting they may possess several radial amplitude nodes and antinodes. Further, we examine the Kelvin‐Helmholtz (KH) stability of FLRs due to their azimuthal velocity shear. We estimate a FLR toroidal velocity threshold, for particular ∑p, beyond which the KH growth rate is sufficiently large to disrupt the FLR. For typical magnetospheric conditions, FLRs are not likely to be disrupted by driving secondary KH vortices. For large‐amplitude FLRs in regions of high ∑p, however, it may be possible for FLRs to be disrupted by the KH instability and to develop into large‐scale KH vortices. We further speculate on the possible link between auroral zone FLR internal radial scales and the observed optical widths of discrete auroral arcs.
A survey of Pc 5 pulsations in the dayside high‐latitude regions observed by Viking
Authors: Potemra Thomas A. , Blomberg Lars G.
Published: February 2004

We have examined magnetic and electric field observations acquired with the polar orbiting Viking satellite near its 3 RE apogee and at local times from dawn through noon to dusk. Our objective was to determine the statistical properties of Pc 5 (150–600 s period) pulsations at different dayside local times and their relationship to large‐scale Birkeland currents. The Viking data examined in detail here were acquired from 41 orbits during the period from March 26, 1986, to July 19, 1986. Pulsations could be readily identified on prenoon orbits as evidenced in a series of 10 consecutive orbits in one case and six consecutive orbits in another. The waveforms in the postnoon hours are more complicated than in the morning, and this made it more difficult to identify pulsations in this region. The simultaneous electric and magnetic field measurements from Viking were used to distinguish the magnetic variations associated with large‐scale Birkeland currents from those associated with waves. These observations also made it possible to derive values for Pedersen and wave conductivities and Poynting fluxes for the waves. The average wave period for the 40 cases studied here is 4.8 min (frequency of 3.5 mHz), the average magnetic field amplitude is 30 nT, the average electric field amplitude (for the 11 cases when the electric field observations were available) is 7 mV/m, and the average incident Poynting flux is 85 μW/m2. The principal characteristics of the waves include the following: (1) From morning to noon, the magnetic field pulsations are polarized primarily in the east–west direction, and the associated electric field oscillations lag by 60°–80° and are polarized close to the north–south direction. Consequently, these pulsations are interpreted as the fundamental modes of resonant toroidal magnetic field oscillations. (2) In the afternoon hours, the magnetic field pulsations have a strong north–south component and include higher‐frequency components (possibly the second harmonic). (3) In both the morning and afternoon hours, the pulsations occur 2°–5° equatorward of the interface between the counterflowing region 1 and region 2 Birkeland currents. (4) The average wave periods increase from ∼3 min near 0800 magnetic local time (MLT) to ∼5 min near noon. (5) The average wave amplitudes range from ∼40 nT near 0900–1000 MLT to ∼25 nT near 1300–1600 MLT.
Isotropic and perpendicular energization of oxygen ions at energies below 1 eV
Authors: Norqvist Patrik , Oscarsson Tord , André Mats , Blomberg Lars
Published: February 2004

We present observations of low‐energy O+ ion distributions obtained by the Freja satellite at altitudes between 1100 and 1600 km in the dayside high‐latitude magnetosphere. The particle observations are obtained by the Freja ion mass spectrometer designed to observe ion energies up to several keV. We show that such instruments can also be used to accurately measure temperatures less than 1 eV, by operating the instrument at a fixed energy level of a few eV. When the mass spectrometer is operated at an energy approximately equal to the energy of O+ ions moving with the satellite velocity (several km/s), the satellite motion and the detector viewing directions become important. The idea is that the satellite ram effect together with different instrument viewing directions related to the satellite spin can allow sampling of different regions of ion velocity space. We find that isotropic O+ energization dominates at mean energies below about 0.4 eV, while energization perpendicular to the geomagnetic field dominates at higher energies. The isotropic ion distributions may exhibit a bulk motion up or down along the geomagnetic field, while the perpendicularly heated distributions are moving upward. The perpendicular energization occurs in the dayside auroral region including the cusp/cleft, the source region of the so‐called cleft ion fountain. The perpendicular heating to a few eV can be associated with weak broadband low‐frequency electric wave fields, similar to the stronger emissions causing O+ energization to hundreds of eV. However, at low energies other mechanisms may also be important.
On origin of outshifted plasma lines during HF modification experiments
Authors: Mishin E. , Hagfors T. , Kofman W.
Published: February 2004

Outshifted plasma lines, observed during injections of intense HF radiowaves into the ionosphere, are due to scattering on Langmuir waves with an unusually large spectral width shifted above the injected frequency. Incoherent scatter radar observations reveal that these waves sometimes are located at altitudes exceeding by several kilometers the altitude of the reflection layer of the ordinary HF wave, thus making an explanation in terms of the so‐called “free mode” problematic. We show that these observations may be consistently described by the creation, due to the ponderomotive force, of density depletions in the Airy pattern of the Z mode. The generation of Langmuir plasmons by accelerated suprathermal electrons inside the depletions causes the wide spectral width of these plasma line features and their mean frequency shift.
A study of the dispersion of the electron distribution in the presence of E and B gradients: Application to electron heating at quasi‐perpendicular shocks
Authors: Balikhin M. , Krasnosel'skikh V. V. , Woolliscroft L. J. C. , Gedalin M.
Published: February 2004

The dynamics of electrons in the presence of E and B field gradients is considered. The results are applied to electron heating in quasi‐perpendicular shocks. The main focus is on the transitions between adiabatic and overadiabatic heating and from perpendicular to oblique geometry. Contrary to previous results, it is shown that the divergence of initially close electron trajectories always takes place in the quasi‐perpendicular collisionless shock front. In the case of negligible electric field gradients at the shock front, the divergence rate depends only upon the magnetic field gradient and results in adiabatic heating. If the electric field gradient is not negligible, divergence rate depends on increasing electric field gradients. In this case, the divergence rate in the first part of the ramp will exceed the “adiabatic” divergence rate. This enhancement is significant for scales of the electric field of about 4–6 c/ωpe. Under the conditions of the Earth's bow shock the magnetic field and particularly its gradients affect the electron trajectory divergence rate and thus the process of thermalization of electrons.
Lion roars in the magnetosheath: The Geotail observations
Authors: Zhang Y. , Matsumoto H. , Kojima H.
Published: February 2004

Lion roars in the magnetosheath are studied based on the waveform data recorded by the Geotail plasma wave instrument (PWI). It is found that about 30% (type A) of the lion roars are associated with the mirror waves or the decrease of the ambient magnetic field (B0). About 70% (type B) of the lion roars are not associated with the decrease of the B0. Both types of the lion roars cover a frequency range from 0.02Ωe to 0.75Ωe. However, the dominant frequencies are around 0.12Ωe, where Ωe is the electron cyclotron frequency. The amplitudes of the lion roars range from a few picoteslas to a few hundreds of picoteslas. Their average amplitude is about 110 pT. Most of these lion roars propagate in a slightly oblique direction (θκB ∼ ±10°) with respect to the B0. One important feature of these lion roars is that they propagate mostly in a single direction, either in the direction of or in the opposite direction to the B0. This suggests that the sources of the lion roars are very likely away from the location where they are observed. Five percent of the lion roars (only the type B lion roars) propagate in two opposite directions simultaneously. This indicates a possibility that the lion roars are generated locally in the vicinity of the spacecraft for these cases. Those highly oblique lion roars (the type B, θκB up to 87°) are usually observed near the bow shock and they are very likely the downstream propagating whistlers excited in the bow shock region. The nearly monotonic decrease of the wave normal distributions of the lion roars suggests that the Landau damping may play an important role. The relatively narrow wave normal distribution of the types A lion roars than that of the type B lion roars suggests that the type A lion roars may be closer to their sources than the type B lion roars.
Preliminary analysis of two‐hemisphere observations of sidereal anisotropies of galactic cosmic rays
Authors: Hall D. L. , Munakata K. , Yasue S. , Mori S. , Kato C. , Koyama M. , Akahane S. , Fujii Z. , Fujimoto K. , Humble J. E. , Fenton A. G. , Fenton K. B. , Duldig M. L.
Published: February 2004

By using the two‐hemisphere network of underground muon telescopes we have examined the average sidereal daily variations in the count rates recorded by 48‐component muon telescopes. The telescopes respond to primary cosmic rays with rigidities between ∼140 and 1700 GV and view almost the entire celestial sphere. We have modeled the data by using Gaussian functions, and we have related the Gaussian parameters to the recent tail‐in and loss cone anisotropy model proposed by Nagashima et al. [1995a, b] to explain the sidereal daily variations. We have used the model parameters to derive the rigidity and latitude spectra of the galactic anisotropies and find them to be qualitatively in agreement with Nagashima et al.'s predictions. The results indicate, however, that the tail‐in anisotropy is asymmetric about its reference axis, whereas the loss cone anisotropy is more symmetric. We show that these characteristics of the galactic anisotropies may explain the north–south asymmetry observed in the amplitude of the sidereal diurnal variation derived from Fourier analysis techniques.
Particle acceleration at corotating interaction regions in the three‐dimensional heliosphere
Authors: Desai M. I. , Marsden R. G. , Sanderson T. R. , Balogh A. , Forsyth R. J. , Gosling J. T.
Published: February 2004

We have investigated the relationship between the energetic (∼1 MeV) proton intensity (J) and the magnetic compression ratio (C) measured at the trailing edges of corotating interaction regions observed at Ulysses. In general, our results show that the proton intensity was well correlated with the compression ratio, provided that the seed intensity remained constant, consistent with predictions of the Fermi model. Specifically, our results indicate that particles were accelerated to above ∼1 MeV in energy at or near the trailing edges of the compression regions observed in the midlatitude southern heliosphere, irrespective of whether the bounding reverse shocks were present or not. On the basis of this, we conclude that shock acceleration is probably not the only mechanism by which particles are accelerated to above ∼1 MeV in energy at compression or interaction regions (CIRs). On the basis of magnetic field measurements obtained near the trailing edges of several midlatitude CIRs, we propose that particles could have been accelerated via the Fermi mechanism by being scattered back and forth across the trailing edges of the compression regions by large‐amplitude Alfvén waves. Our results also show that the proton intensity was well correlated with the compression ratio during low solar activity periods but was essentially independent of C during periods of high solar activity. We suggest that the correlation between J and C was not observed during solar active periods because of significant variations in the seed intensity that result from sporadic contributions from transient solar events. In contrast, the correlation was observable during quiescent periods probably because contributions from transients had decreased dramatically, which allowed the CIRs to accelerate particles out of a seed population whose intensity remained relatively unperturbed.
Dispersive, nonradiative field line resonances in a dipolar magnetic field geometry
Authors: Streltsov A. V. , Lotko W.
Published: February 2004

Results from a numerical study of field line resonances (FLRs), formed by dispersive Alfvén waves standing between southern and northern ionospheres along auroral magnetic field lines, are presented. Dispersion of the Alfvén wave is due to the finite electron inertia at low altitudes and the finite electron temperature at high altitudes. Previous numerical studies of the phenomenon based on linear, magnetically incompressible, two‐fluid MHD in a slab geometry (with constant ambient magnetic field and temperature, the so‐called “box” model) are extended here to a dipolar magnetic field geometry. The new computations quantify earlier qualitative conclusions from the box model that the electric field at low altitudes is significantly amplified in nonradiative, dispersive FLRs, with an associated field‐aligned potential drop sufficient to accelerate electrons up to several hundreds of eV. The dipolar results also provide further insights into the role of the transverse plasma inhomogeneity in the formation of the dispersive, nonradiative FLRs: even a moderate transverse inhomogeneity of the background plasma, localized to the equatorial section of the flux tube, significantly increases the growth rate of resonances. Similarities and differences between results obtained from the new dipolar model and the previous box model are discussed, as well as the relation between the new results and observations.
Nonlinear shear Alfvén resonances in a dipolar magnetic field
Authors: Voronkov I. , Rankin R. , Tikhonchuk V. T. , Samson J. C.
Published: February 2004

The effect of the ponderomotive force (PF) on the temporal evolution of shear Alfvén field line resonances (FLRs) is considered for a magnetic dipole geometry appropriate to the Earth's magnetosphere. We derive a set of equations which describes the coupling of shear Alfvén and slow mode waves and show that in a dipole field, the PF initiates a spectrum of standing slow mode waves, rather than just the fundamental mode that arises in a Cartesian box model magnetosphere. Magnetic field aligned slow mode density perturbations lead to a nonlinear temporal phase shift between the compressional driver and shear Alfvén wave. This results in nonlinear saturation of the wave fields of FLRs and may occur well in advance of linear saturation as a result of ionospheric dissipation. We derive expressions for the nonlinear frequency shifts caused by the slow mode spectrum and determine the timescale for nonlinear saturation of the shear Alfvén wave fields. Finally, we compare our results with previous estimates made in a box model magnetosphere and show that our main conclusions remain valid.
Numerical comparison of two schemes for the generation of ELF and VLF waves in the HF heater‐modulated polar electrojet
Authors: Kuo S. P. , Faith James , Lee M. C. , Kossey Paul
Published: February 2004

Generation of ELF and VLF waves in the HF heater modulated polar electrojet is numerically studied. Illuminated by an amplitude modulated HF heater, the electron temperature of the electrojet is modulated accordingly. This, in turn, causes the modulation of the conductivity and thus the current of the electrojet. Emissions are then produced at the modulation frequency and its harmonics. The present work extends the previous one on a thermal instability to its nonlinear saturation regime. Two heater modulation schemes are considered. One modulates the heater by a rectangular periodic pulse. The other one uses two overlapping heater waves (beat wave scheme) having a frequency difference equal to the desired modulation frequency. It is essentially equivalent to a sinusoidal amplitude modulation. The nonlinear evolutions of the generated ELF and VLF waves are determined numerically. Their spectra are also evaluated. The results show that the signal quality of the second (beat wave) scheme is better. The field intensity of the emission at the fundamental modulation frequency is found to increase with the modulation frequency, consistent with the Tromso observations.
Multipoint study of a substorm on February 9, 1995
Authors: Lui A. T. Y. , Williams D. J. , McEntire R. W. , Ohtani S. , Zanetti L. J. , Bristow W. A. , Greenwald R. A. , Newell P. T. , Christon S. P. , Mukai T. , Tsuruda K. , Yamamoto T. , Kokubun S. , Matsumoto H. , Kojima H. , Murata T. , Fairfield D. H. , Lepping R. P. , Samson J. C. , Rostoker G. , Reeves G. D. , Rodger A. L. , Singer H. J.
Published: February 2004

An extended interval of strong northward interplanetary magnetic field (IMF) was observed by the Wind spacecraft located at an upstream distance of ∼193 RE from February 8–10, 1995, with a brief break of southward IMF from 0200 to 0400 UT on February 9. This brief interval of southward IMF led to an isolated substorm of moderate intensity (∼500 nT) with expansion phase starting at ∼0431 UT. This substorm may be triggered by the northward turning of the IMF since its onset time matched well with the time expected for the arrival of the northward turning of the IMF at Earth. The substorm activities were monitored by 11 spacecraft in space (Wind, IMP 8, Geotail, six geosynchronous satellites, one DMSP satellite, and Freja) and two networks of ground stations (Canopus and SuperDARN) covering both the northern and southern hemispheres. The extensive coverage of this event provides us with results (1) showing some unusual characteristics possibly related to the isolated nature of the substorm and (2) revealing some surprising features difficult to reconcile with the traditional substorm model. In the first category is unusually long duration of the growth phase and the long time delay between substorm expansion onset and particle injection onset at the geosynchronous orbit. In the second category is new evidence for multiple particle acceleration sites during substorm expansion and for sunward flow during the late expansion phase of a substorm being unrelated to a single acceleration site (X line) moving from the near‐Earth tail to the more distant tail. We also present observations which show the possible optical signature on the ground of bursty bulk flows in the magnetotail.
Synoptic auroral distribution: A survey using Polar ultraviolet imagery
Authors: Liou K. , Newell P. T. , Meng C.‐I. , Brittnacher M. , Parks G.
Published: February 2004

The global distribution of the ultraviolet auroral emission was investigated for the period between April and July 1996 using over 17,000 imagery acquired by the ultraviolet imager (UVI) on board the Polar satellite. Average brightness of the N2 Lyman‐Birge‐Hopfield (LBH) auroral emissions at 1700 Å, which is approximately proportional to the total energy flux of precipitating electrons, was calculated with dayglow subtracted. The results of this investigation indicate that there exist two distinctive auroral emission regions, one in the premidnight sector of the auroral oval and one in the postnoon sector of the auroral oval. The maximum occurrence of nightside aurorae is found to be centered at 2230 magnetic local time (MLT) and 68° magnetic latitude (MLAT) while the dayside aurorae maximize at both 1500 MLT and 75° MLT and 1000 MLT and 75° MLAT, with the later one much weaker. This statistical auroral distribution is quite similar to previously reported distribution of discrete aurorae, suggesting that at this wavelength and at the sensitivity of the UVI detector, discrete aurorae contribute a major portion of the total emissions. The seasonal distribution of the nightside LBH auroral emissions is found to be consistent with previously reported particle result, namely nightside discrete auroral activities are more common in the dark hemisphere (winter) than in the sunlit hemisphere (summer). However, on the dayside part of auroral oval, auroral emissions are brighter in summer than in spring. The dayside auroral emissions, in particular the 1500 MLT bright spots, are also found to be correlated with the maximum region 1 upward field‐aligned currents which are most intense in summer because of a higher ionospheric conductivity produced by photoionization in the dayside region. These results point up the controlling role played by ionospheric conductivity and further illustrate how dayside and nightside aurorae behave in fundamentally different ways.