bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of seasonal and interannual variations of internal gravity wave intensity in the thermosphere over Saint Santin
Authors: Gavrilov Nikolai M. , Richmond Arthur D. , Bertin François , Lafeuille M.
Published: February 2004

Seasonal and interannual variations in the intensity of internal gravity waves (IGW) at altitudes of 100‐300 km are investigated using measurements of electron density, ion drift velocity parallel to the geomagnetic field, and ion temperature with the Saint Santin incoherent scatter radar between 1967 and 1987. IGW with periods of less than 6 hours produce ion temperature amplitudes of 5‐80 K and ion velocity amplitudes along the geomagnetic field of 3‐17 m/sec. Vertical profiles of IGW intensity have maxima in the altitude regions 140‐180 and 250‐280 km and vary on annual and interannual timescales. Usually, the IGW intensity is at its maximum in winter and minimum in summer. Strong interannual variations are also observed, whose long‐term trends sometimes appear to follow the solar cycle, either positively or negatively, but are often irregular. Model calculations show that maxima of IGW intensity during Solar minimum can be explained by the diminution of atmospheric density and ion drag, which leads to increased amplitudes of IGW propagating to the thermosphere from lower layers of the atmosphere.
Propagation of solar neutrons through the atmosphere of the Earth
Authors: Shibata S.
Published: February 2004

The propagation of solar neutrons through the atmosphere of the Earth has been calculated by a Monte Carlo method using a nuclear interaction model which can reproduce the existing data obtained by accelerator experiments. With this calculation, it has been found that the elastic scattering process plays an important role in the propagation of solar neutrons in the atmosphere at an incident energy below ∼ 200 MeV. By applying this calculation to the analysis of a solar neutron event observed at Mount Norikura Cosmic Ray Observatory on June 4, 1991, it is possible to interpret the event as γ rays and neutrons having been produced impulsively at the time of the solar flare. The event observed at Jungfraujoch on June 3, 1982 can be explained similarly.
The decay of suprathermal ion fluxes during the substorm recovery phase
Authors: Kistler L. M. , Larson D. J. , Möbius E. , Baumjohann W.
Published: February 2004

During a magnetospheric substorm, the energetic particle flux in the plasma sheet increases by as much as 2 orders of magnitude. During the recovery phase the ion flux decreases. The decay rate is energy dependent, with the decay time constant decreasing for increasing energies. By comparing the decay rates of different species, we are able to test whether the decay rate is organized by total energy, energy per charge, velocity, or rigidity. Using the suprathermal energetic ion charge analyzer (SULEICA) instrument on Active Magnetospheric Particle Tracer Explorers Ion Release Module (AMPTE IRM), we have determined the decay rates for the species H+, He+, O+, and He++ for individual cases and by using a superposed epoch analysis. We find that the decay rate is organized by energy per charge. We have calculated the decay rates predicted by two different models: adiabatic cooling in a radially expanding plasma sheet, and particle drift loss, or “leakage,” from the plasma sheet. Both models predict that the decay rates will be organized by energy per charge, and in both cases the predicted decay rate is faster than that actually observed. This indicates that the production of energetic particles continues at a reduced rate in the the tail during the recovery phase, as the neutral line retreats down the tail.
Coupling of microprocesses and macroprocesses due to velocity shear: An application to the low‐altitude ionosphere
Authors: Ganguli G. , Keskinen M. J. , Romero H. , Heelis R. , Moore T. , Pollock C.
Published: February 2004

Recent observations indicate that low‐altitude (below 1500 km) ion energization and thermal ion upwelling are colocated in the convective flow reversal region. In this region the convective velocity V⊥ is generally small but spatial gradients in V⊥ can be large. As a result, Joule heating is small. The observed high level of ion heating (few electron volts or more) cannot be explained by classical Joule heating alone but requires additional heating sources such as plasma waves. At these lower altitudes, sources of free energy are not obvious and hence the nature of ion energization remains ill understood. The high degree of correlation of ion heating with shear in the convective velocity (Tsunoda et al., 1989) is suggestive of an important role of velocity shear in this phenomenon. We provide more recent evidence for this correlation and show that even a small amount of velocity shear in the transverse flow is sufficient to excite a large‐scale Kelvin‐Helmholtz mode, which can nonlinearly steepen and give rise to highly stressed regions of strongly sheared flows. Furthermore, these stressed regions of strongly sheared flows may seed plasma waves in the range of ion cyclotron to lower hybrid frequencies, which are potential sources for ion heating. This novel two‐step mechanism for ion energization is applied to typical observations of low‐altitude thermal ion upwelling events.
Langmuir turbulence in the auroral ionosphere: 1. Linear theory
Authors: Newman D. L. , Goldman M. V. , Ergun R. E. , Boehm M. H.
Published: February 2004

Intense bursts of Langmuir waves with electric fields of 50‐500 mV/m have been frequently observed at altitudes >500 km in the auroral ionosphere. These bursts are driven by 20 eV ‐ 4 keV field‐aligned electrons, which are embedded in an approximately isotropic nonthermal tail of scattered electrons. The Langmuir bursts are often observed at altitudes where the ionosphere is moderately magnetized (Ω ≈ ωpe). Both the moderate magnetization and the scattered electrons have a major influence on the linear dispersion and damping of Langmuir waves. In particular, the linear dispersion is topologically different depending on whether the magnetic field is subcritical (Ω < ωpe) or supercritical (Ωe > ωpe). The correct dispersion and damping can account for the observed polarization of the Langmuir waves, which is very nearly parallel to the geomagnetic field. Inferred properties of the linear instability driven by the field‐aligned electrons are discussed. The linear dispersion and damping derived here provide the basis for a nonlinear turbulence study described in a companion paper (Newman et al., this issue).
A comparison of midlatitude Pi 2 pulsations and geostationary orbit particle injections as substorm indicators
Authors: Yeoman T. K. , Freeman M. P. , Reeves G. D. , Lester M. , Orr D.
Published: February 2004

Both the injection of energetic particles at geostationary orbit and ground magnetic observations of Pi 2 wave activity are characteristic indicators of the onset of the substorm expansion phase. Occurrence statistics for the appearance of electron and proton particle injection at three geostationary spacecraft and for the detection of midlatitude magnetic Pi 2 pulsations in a 3‐hour local time sector have been compiled from 240 hours of data. Throughout this interval a signature was detected on one or more of the instruments on average every 65 min. It is demonstrated that the detection of geostationary orbit particle injections and the detection of ground‐based Pi 2 pulsations are correlated at a very high significance level, and that both appear to be effective substorm indicators. However, a small percentage of events (∼10% in each case) may be identified as a Pi 2 event but not as an injection event or viceversa, without any obvious explanation, such as the local time of the observing instrumentation. A number of possible explanations for the discrepancies between the two data sets are discussed.
Proton and helium cyclotron anisotropy instability thresholds in the magnetosheath
Authors: Gary S. Peter , Convery Patrick D. , Denton Richard E. , Fuselier Stephen A. , Anderson Brian J.
Published: February 2004

Both the protons and the helium ions of the terrestrial magnetosheath typically display T⊥ > T∥, where ⊥ and ∥ denote directions perpendicular and parallel to the background magnetic field. Observations of the highly compressed magnetosheath show an inverse correlation between these ion temperature anisotropies and the parallel proton β. Computer simulations have demonstrated that these correlations are due to wave‐particle scattering by electromagnetic ion cyclotron anisotropy instabilities. These correlations correspond to linear theory thresholds of the proton cyclotron and the helium cyclotron instabilities. This paper uses linear Vlasov theory and the assumption of a constant maximum growth rate to obtain closed‐form expressions for these thresholds as a function of the relative helium density and the parallel proton β in a parameter model of the magnetosheath.
Signatures of flux erosion from the dayside magnetosphere
Authors: Sibeck D. G.
Published: February 2004

The rate of merging and the strength of the region 1 Birkeland currents increase during periods of southward interplanetary magnetic field (IMF). Fringe fields of the Birkeland currents depress dayside magnetospheric magnetic field strengths and remove magnetic flux from the dayside magnetosphere, thereby allowing the dayside magnetopause to move inward and the cusp equatorward. We use previously derived fits to the magnetopause location as a function of IMF Bz, the condition of pressure balance at the magnetopause, and an idealized model of region 1 Birkeland currents to estimate that strong southward IMF turnings will produce ∼13‐ to 26‐nT depressions in the geosynchronous magnetic field strength over periods of 30‐60 min. We then present three case studies of geosynchronous magnetic field strength variations during periods of nearly constant solar wind dynamic pressure and southward IMF. The dayside magnetospheric magnetic field strength was depressed ∼10 nT during a period of strongly southward IMF (Bz = −6 nT), but only ∼5 nT during two more typical periods of slightly southward IMF (Bz = −2 to −3 nT). The depressions correspond to periods of enhanced AL index, which we interpret as evidence for directly driven solar wind‐magnetosphere interaction rather than the unloading of energy stored within the magnetotail. Dayside geosynchronous magnetic field strengths are weakly correlated with IMF Bz.
Dynamics of the generation of magnetic‐field‐aligned electric fields by drift currents
Authors: Fridman Sergey V.
Published: February 2004

A study is made of the process of generation and evolution of parallel electric fields on closed geomagnetic field lines. Charge separation that is due to the different character of the drift of energetic electrons and protons of inhomogeneous magnetospheric plasma in the geomagnetic field is considered to be the factor responsible for the generation of the electric field. In a magnetic flux tube that drifts together with the cold plasma, this separation of charges is manifested as the injection of energetic electrons or protons. A numerical self‐consistent calculation of the evolution of the distribution functions of particles and of the parallel electric field is carried out for the injection of energetic electrons into the geomagnetic flux tube. An analytical investigation of the macroscopic and microscopic processes accompanying such injection is carried out. The calculations take into account cold ionospheric particles and energetic magnetospheric protons and electrons. It was found that an intense parallel electric field is produced in the form of two collisionless rarefaction shock waves that separate from the equatorial plane in the northward and southward directions. Below the front of each shock wave, there is mainly the ionospheric plasma, while behind the front there are the upward accelerated ionospheric ions and the hot magnetospheric plasma. Microscopic structure behind the shock front is formed by electrostatic oscillations of a large amplitude. Generation conditions for intense longitudinal fields are formulated: The difference in densities of injected energetic electrons and protons near the equatorial plane must exceed the initial density of cold particles, and the current‐associated velocity of cold electrons must be larger than the ion sound velocity. It is shown that these conditions can be satisfied, owing to a drift mechanism of charge separation. A rarefaction shock wave is excited with substantially smaller field‐aligned currents as compared with anomalous resistance or double layers. The observed manifestations of the rarefaction shock waves are auroral plasma cavities.
Comparison of theories for gravity wave induced fluctuations in airglow emissions
Authors: Walterscheid R. L. , Schubert G. , Hickey M. P.
Published: February 2004

A comparison is undertaken of theories for the gravity wave induced fluctuations in the intensity of airglow emissions and the associated temperature of the source region. The comparison is made in terms of Krassovsky's ratio ηE for a vertically extended emission region (ηE is the ratio of the vertically integrated normalized intensity perturbation to the vertically integrated normalized intensity‐weighted temperature perturbation). It is shown that the formulas for ηE in the works by Tarasick and Hines (1990) and Schubert et al. (1991) are in agreement for the case of an inviscid atmosphere. The calculation of ηE using the theory of Tarasick and Hines (1990) requires determination of their function χ; we show that χ is simply related to the “single‐level” Krassovsky's ratio η of Schubert et al. (1991). The general relationship between χ and η is applied to a simple chemical‐dynamical model of the O2 atmospheric airglow and the altitude dependence of these quantities is evaluated for nonsteady state chemistry. Though the Tarasick and Hines (1990) formula for ηE does not explicitly depend on the scale heights of the minor constituents involved in airglow chemistry, ηE implicitly depends upon these scale heights through its dependences on chemical production and loss contained in χ. We demonstrate this dependence of ηE for the OH nightglow on atomic oxygen scale height by direct numerical evaluation of ηE; in this case the dependence originates in the chemical production of perturbed ozone.
Electron dynamics in two‐ and one‐dimensional oblique supercritical collisionless magnetosonic shocks
Authors: Savoini Philippe , Lembege Bertrand
Published: February 2004

Two‐ and one‐dimensional fully electromagnetic, bounded, particle (for both electrons and ions) codes are used in order to study electron dynamics in collisionless magnetosonic shocks propagating in supercritical regime and quasi‐perpendicular direction (90° > θ0 > 45°); θ0 is the angle between the shock normal and the upstream magnetic field. The purpose of the study consists in comparing electrons behavior in one‐dimensional (“pseudo‐oblique”) nonresistive shocks and in two‐dimensional resistive oblique shocks. Resistive effects related to plasma microinstabilities can be self‐consistently included in two‐dimensional particle codes in contrast with one‐dimensional particle codes. Present two‐dimensional results reproduce local electron distribution functions (in particular, downstream “flat tops”) in a self‐consistent way and in good agreement with observational results. On the other hand, one‐dimensional results exhibit either local enlarged Maxwellian distributions with a partial tail, or a flat top distribution according to the particle density n. These results emphasize that (1) the differences observed between one‐ and two‐dimensional codes may be explained in terms of a critical particle density nc used in the one‐dimensional code; (2) the evidence of flat tops in both two‐ and one‐dimensional results (provided that n > nc) proves that the macroscopic potential jump at the shock front is mainly responsible for their formation; (3) microscopic effects (herein related to the self‐consistent cross‐field/field‐aligned currents instabilities) may represent a complementary mechanism for filling the flat top distribution; (4) some relaxation of the unstable electron flat top distribution (T∥/T⊥ ≫ 1) is observed when penetrating further into the downstream region, which means that the main filling mechanisms are localized in the ramp of the shock. Moreover, a detailed study of two‐dimensional results shows that both resistive and nonresistive configurations can be easily distinguished for θ0 ≈ 90°, but not any more for large deviations of θ0 from 90° for which the self‐consistent magnetic field rotates noticeably out of the coplanarity plane at the shock front.
Anisotropic Alfvén‐ballooning modes in Earth's magnetosphere
Authors: Chan Anthony A. , Xia Mengfen , Chen Liu
Published: February 2004

We have carried out a theoretical analysis of the stability and parallel structure of coupled shear Alfvén and slow magnetosonic waves in Earth's inner magnetosphere (i.e., at equatorial distances between about five and ten Earth radii) including effects of finite anisotropic plasma pressure. Multiscale perturbation analysis of the anisotropic Grad‐Shafranov equation yields an approximate self‐consistent magnetohydrodynamic (MHD) equilibrium. This MHD equilibrium is used in the numerical solution of a set of eigenmode equations which describe the field line eigenfrequency, linear stability, and parallel eigenmode structure. We call these modes anisotropic Alfvén‐ballooning modes. The main results are: (1) The field line eigenfrequency can be significantly lowered by finite pressure effects. (2) The parallel mode structure of the transverse wave components is fairly insensitive to changes in the plasma pressure, but the compressional magnetic component can become highly peaked near the magnetic equator as a result of increased pressure, especially when P⊥ > P∥ (here P⊥ and P∥ are the perpendicular and parallel plasma pressure). (3) For the isotropic (P∥ = P⊥ = P) case ballooning instability can occur when the ratio of the plasma pressure to the magnetic pressure, β = P/(B²/8π), exceeds a critical value β0B ≈ 3.5 at the equator. (4) Compared to the isotropic case the critical beta value is lowered by anisotropy, either due to decreased field line bending stabilization when P∥ > P⊥ or due to increased ballooning‐mirror destabilization when P⊥ > P∥ (5) We use a β‐δ stability diagram to display the regions of instability with respect to the equatorial values of the parameters β and δ, where β¯=(1/3)(β∥+2β⊥) is an average beta value and δ = 1 − P∥/P⊥ is a measure of the plasma anisotropy. The diagram is divided into regions corresponding to the firehose, mirror and ballooning instabilities. It appears that observed values of the plasma pressure are below the critical value for the isotropic ballooning instability but it may be possible to approach a ballooning‐mirror instability when P⊥/P∥ ≳ 2.
Vibrational distribution of N2+A ²Πu from 100‐eV electron impact on N2
Authors: Doering J. P. , Goembel L. , Yang J.
Published: February 2004

Measurements of the electron excitation of the Meinel band system of N2+ produced by electron impact on N2 have been hampered by the long lifetime of the N2+ A 2Πu upper state, the spectroscopically inconvenient near‐infrared region where the bands are located, as well as overlap by the N2 first positive system. In the present work, an experiment in which the scattered projectile electron and the low‐energy secondary electron are detected in coincidence was used to allow the vibrational distribution of the A 2Πu state produced by 100‐eV electron impact on N2 to be determined directly in excitation without the need for optical measurements. The coincidence energy loss spectrum was measured for a typical secondary electron energy of 5 eV over a range of secondary electron ejection angles. The results of a band contour analysis show that the population of the ν′=0‐5 vibrational levels is consistent with that predicted by the Frank‐Condon factors for ionization of N2 to the N2+ A 2Πu state.
Double vision: Remote sensing of a flux transfer event with ISEE 1 and 2
Authors: Walthour D. W. , Sonnerup B. U. Ö. , Elphic R. C. , Russell C. T.
Published: February 2004

A flux transfer event recorded by the ISEE 1 and 2 spacecraft in the northern hemisphere near local noon is examined using analysis techniques developed recently for determination from single satellite magnetometer data of the orientation and cross‐sectional shape of a two‐dimensional disturbance moving along the magnetopause. The analysis is performed independently on the magnetic field data recorded by each satellite, enabling us to check the consistency of the results and thus to benchmark the method. The analysis is also extended to utilize the dual magnetometer measurements for determination of the event speed and size (which for a single spacecraft requires use of three‐dimensional plasma velocity data to obtain a deHoffmann‐Teller frame velocity) and for accommodating acceleration of the magnetopause normal to itself, which results in a curved spacecraft trajectory in the frame of reference moving with the disturbance. The analysis of the disturbance reveals that while its overall size is about 34,000 km in a direction parallel to the magnetopause but perpendicular to the event axis, the actual size of the bulge causing the field disturbance may have been as small as 9000 km with a dimension perpendicular to the magnetopause of the order of 1200 km. The bulge is found to be traveling at a speed of about 140 km/s toward the northern dawn quadrant of the dayside magnetopause. While plasma data from ISEE 2 provides evidence of plasma jetting, which suggests the event may be associated with reconnection, the orientation of the flux tube axis obtained from the analysis is significantly different from the expected reconnection line direction. A discussion of possible interpretations of this bulge orientation and motion is provided.
Evidence for polarization electric fields in the daytime F region above Millstone Hill
Authors: Buonsanto Michael J.
Published: February 2004

Persistent anticorrelations between the ion drift components V⊥N and V∥ are seen in the daytime F region near solar maximum above Millstone Hill. The possibility that all of these anticorrelations may be spurious has been ruled out by calculating the correlations and anticorrelations which are introduced by simply adding random errors to the line‐of‐sight velocity measurements for each experiment considered. Variations in hmF2 follow V∥ during most of these intervals, suggesting that a neutral wind causes the hmF2 variations and is responsible for the anticorrelations between V⊥N and V∥ by setting up polarization electric fields by the wind dynamo mechanism. It is widely believed that such F region polarization electric fields cannot occur or are very weak during the daytime, as they are shorted out by the E region conductivity. However, calculations of the E and F region Pedersen conductivities using Millstone Hill electron density profiles and the mass spectrometer/incoherent scatter (MSIS) 86 model show how the F region Pedersen conductivities increase from solar minimum to solar maximum, so that the height‐integrated Pedersen conductivities are larger in the F region than in the E region under winter daytime conditions at solar maximum. Thus the E region conductivity is not capable of fully shorting out the F region polarization electric fields above Millstone Hill during daytime at solar maximum. These are apparently the first calculations of ionospheric conductivities above Millstone Hill using incoherent scatter electron density profiles.
Filamentary current structures in the postnoon sector: Observations from UARS
Authors: Bythrow P. F. , Anderson B. J. , Potemra T. A. , Zanetti L. J. , Winningham J. D. , Chenette D. L.
Published: February 2004

During an intense geomagnetic storm (Kp 7+) that began at ∼1830 UT on October 1, 1991, the UARS satellite encountered the dayside postnoon auroral oval. On two consecutive crossings of the northern hemisphere between 2040 and 2240 UT, the vector magnetometer detected region 1 and 2 Birkeland and ionospheric currents in the postnoon sector. Low‐energy electron events were observed near 1400 MLT within a narrow portion of the region 1 current system. Simultaneous magnetic field measurements revealed the presence of intense (∼20 μA/m²) bipolar filament current structures embedded in the auroral oval. The upward‐directed currents were associated with the more concentrated region of precipitating electrons. Ions associated with the more intense flux of low‐energy electrons exhibited a dispersion signature typical of an ion velocity filter. The dispersion, aligned along the orbit, exhibited higher‐energy ions at lower latitudes and earlier local times. The colocation of filament currents and ion dispersion signatures at such late postnoon local times is not consistent with typical E × B “cusp” dispersions. These features more likely result from dayside boundary wave phenomena.
Magnetosphere‐ionosphere response to an enhanced energetic coupling with the solar wind
Authors: Jacquey Christian , Sauvaud Jean‐André
Published: February 2004

A remote sensing study of the dynamics of the cross‐tail current has been performed using ISEE magnetic field data during a 12‐hour period when these spacecraft were in the tail lobe. IMP 8 data taken simultaneously in the interplanetary medium showed that the solar wind was continuously supplying power to the magnetosphere. On a global scale the magnetosphere responded directly, as evidenced by the good general correlation between the auroral electrojets and the interplanetary magnetic field (IMF). A detailed study of the ISEE and ground magnetic fields reveals the occurrence of four main successive cross‐tail current disruptions associated with substorm onsets. As a general trend, each disruption was followed by a cross‐tail current increase while in the meantime the intensity of the auroral electrojet decreased. A new substorm started when the cross‐tail current again reached a high value and while the polar ionosphere was relaxing to a quieter state. These observations also apply to the last substorm of the series, which coincided with a northward turning of the IMF. These results strongly support a cross‐tail current threshold instability as the main cause of auroral substorms.
Energetic ion distributions on both sides of the Earth's magnetopause
Authors: Paschalidis N. P. , Sarris E. T. , Krimigis S. M. , McEntire R. W. , Levine M. D. , Daglis I. A. , Anagnostopoulos G. C.
Published: February 2004

The AMPTE/CCE spacecraft, with an apogee of ∼8.8RE and an inclination of ∼4.3°, sampled the outer dayside equatorial magnetosphere for extended time periods and often crossed into the magnetosheath whenever the solar wind pressure was sufficiently high to compress the magnetopause to <8.8RE. We have analyzed ion distributions on both sides of the magnetopause in order to investigate any local time and energy dependence, giving information about physical processes at the magnetopause and the bow shock. Particle measurements are from the CHEM (1.5 to 300 keV/e) and MEPA (10 keV to 2 MeV) instruments. The wide total energy range (1.5 keV to 2 MeV) covered describes the magnetospheric distributions quite well, and for the purpose of this study, adequately describes the high‐energy part of the shocked solar wind. Thus both solar wind and magnetospheric components can be recognized in a mixed particle distribution. Case studies of representative magnetopause crossings at dawn, noon, and dusk, as well as a survey of several other crossings, indicated: (1) a local time and energy dependence of magnetosheath spectra at energies ≥50 keV; spectra were harder at the duskside than at the dawnside and also correlated with magnetospheric activity, (2) constantly much higher intensities in the magnetosphere than in the magnetosheath at energies >10 keV and an earthward gradient in the subsolar magnetosheath. In addition to the steady state magnetosheath population there exists a burst‐type component indicative of a magnetospheric source, and most of the time this is recognized as a flux transfer event. Overall, the results about the origin of the ≥50 keV magnetosheath ions are consistent with the continuous leakage of magnetospheric particles across a tangential discontinuity magnetopause, locally distributed according to magnetospheric drift paths. Magnetic reconnection, although present, should not be a dominant source on average, because it is not continuous in time. Fermi acceleration should not be dominant because it predicts the opposite local time asymmetry, and shock drift acceleration should be a minor contributor at E≥50 keV because of upper‐energy cutoff limitations. Our observations also indicate a significant magnetospheric contribution to energies as low as ∼10 keV, where the magnetosphere‐magnetosheath intensity gradient reverses. However, in order to examine the relative strength and local time distribution of all possible sources at these energies, a detailed analysis is required.
The relationship between magnetospheric convection and ionospheric heating
Authors: Zhu Xiaoming
Published: February 2004

Treating the magnetic stress introduced into the magnetosphere by the solar wind at the magnetopause, we show the direct relation between the magnetospheric convection pattern and the field perturbations. The Birkeland currents are caused by the field perturbations and follow directly from Ampere's law, of course. The ionospheric heating is given by the rate at which the stress in the convecting field does work on the ionosphere. The parallel electric fields may then be deduced from the plasma microphysics and the established currents if desired.
Some topics and historical episodes in geomagnetism and aeronomy
Authors: Fukushima N.
Published: February 2004

Both the American Geophysical Union (AGU) and the International Union of Geodesy and Geophysics (IUGG) were established in 1919, to promote the study of physical problems in and above the Earth, for national and international managements. The initial internal structure within IUGG (especially for geomagnetism and space science dealt with in IAGA, the International Association of Geomagnetism and Aeronomy) owes very much to AGU's appropriate proposals. After the IGY (International Geophysical Year 1957‐58), it was necessary for both AGU and IAGA to modify their internal structures to cope with a rapid progress in space physics with technological development for in‐situ observations by spacecraft. A serious discussion was held in 1920's ‐ 30's about the possible existence of a non‐potential part in the geomagnetic field and its time‐variations in connection with the air‐Earth current. The curl‐freeness of the geomagnetic field cannot be experimentally proved insofar as the differential operation is simply substituted by spatial differences of the observed magnetic field values. It was shown that the archeo‐secular variation of the geomagnetic dipole axis in historic time could be known also from a comparison of the records of ancient auroras preserved in the Oriental and Occidental places. In the history of magnetic storm studies, there was a severe debate between two groups of workers, the Scandinavian school (the importance of field‐aligned currents at high latitudes was emphasized, based on an early study of Birkeland, and later advocated by Alfvén and his colleagues) and the British school (headed by Chapman, attributing the ground magnetic effect rather to horizontal currents in the ionosphere). However, their severe debate was rather meaningless without in‐situ magnetic field measurements above the ionosphere. It is emphasized in this paper that we have to pay more attention to the change in the magnetic declination, in particular its seasonal dependence. The declination change at the storm sudden commencements shows specific local‐time and seasonal dependences, reflecting the solar‐wind interaction with the geomagnetic field. The seasonal dependence of the daily declination change on quiet days enables us to infer transequatorial field‐aligned currents in the magnetosphere at low and middle latitudes. A review is given also for the significant contribution of MAGSAT to the study of three‐dimensional electric currents in the near‐Earth environmental space. There are five Appendices to supplement the above subjects, i.e. on the origin of the words “geomagnetism” and “aeronomy”; the transition from IATME to IAGA; Birkeland's last days in Tokyo; and Harang discontinuity; and the VII General Assembly of IUGG in September 1939 in Washington, D. C.
From ionosonde to rocket sonde
Authors: Friedman H.
Published: February 2004

The 75th Anniversary of the AGU marks the discovery of the basic features of solar control of the ionosphere. A quarter of a century later our understanding of ionospheric physics was revolutionized by the early rocket era of upper air research that brought the discoveries of solar X ray and XUV radiation.
Consequences of magnetotail ion dynamics
Authors: Ashour‐Abdalla Maha , Zelenyi Lev M. , Peroomian Vahé , Richard Robert L.
Published: February 2004

The trajectories of a large ensemble of particles are calculated in a modified Tsyganenko magnetic field model with a uniform cross‐tail electric field. The model magnetotail can be divided into several distinct dynamical regimes of ion motion. Near Earth, where the field lines are dipolar the adiabatic formalism is adequate. In the mid‐tail and distant tail, guiding‐center theory breaks down and must be replaced by a quasi‐adiabatic formalism. There is an important transition region between the adiabatic and quasi‐adiabatic regions where ion trajectories become more complicated and no simple analytical description holds. This wall region is characterized by rapid ion acceleration and a major loss of particles to the dusk flank. The moments of the ion distribution function are constructed from the ion trajectories, including density, temperature, and pressure in the x‐z and x‐y planes. In the noon‐midnight meridian plane, parameters are relatively constant except near the Earth, while the x‐y plots show strong gradients across the magnetotail. Magnetotail plasma convects earthward, drifts toward dusk, and is squeezed out of the tail in the near‐Earth region. A thin current sheet forms in the quasi‐adiabatic region, and the pressure tensor has significant off‐diagonal terms at its edges. These terms are the result of quasi‐adiabatic ion trajectories which lead to azimuthally asymmetric distribution functions capable of maintaining approximate stress balance across the current sheet. Simplified analytical descriptions provide further physical insight into ion dynamics that are observed.
Compressible fluctuations in the equatorial solar wind with a spiral magnetic field
Authors: Lou Yu‐Qing
Published: February 2004

We investigate basic properties of two‐dimensional propagation and advection of compressible magnetohydrodynamic (MHD) perturbations in an axisymmetric equatorial solar wind with a spiral magnetic field. Analytical perturbation solutions at large radii are derived. In principle, numerical solutions which smoothly pass through both MHD fast and slow critical points can be constructed. In addition to wide applications of these solutions to small‐scale small‐amplitude compressible MHD fluctuations within the equatorial solar wind, we view large‐scale MHD pressure waves and pressure‐balanced structures (PBSs) as they evolve into large heliocentric distances, as a kind of nonlinear manifestations of MHD fast‐mode and slow‐mode disturbances in the solar wind, respectively. We discuss radial evolution of MHD fast waves for large‐scale MHD pressure waves in their linear phase in the distant solar wind. We argue that it is natural to interpret PBSs in terms of MHD slow‐mode disturbances at large heliocentric distances. We propose a scenario to identify PBSs with MHD slow‐mode perturbations as well as their relation to MHD pressure waves based on the dissolution of a wave packet of compressible MHD disturbances in the interplanetary space. In principle, observational verification of this suggestion can be carried out by one or several coordinated spacecrafts. Furthermore, the analysis described here provides an important basis for two‐dimensional numerical simulations of nonlinear interactions of interplanetary disturbances. Finally, we indicate that the minimum variance direction of magnetic field fluctuations in the present formulation is along the radial direction. This result is consistent with the earlier WKB Alfvén wave normal analysis but is contrary to what is observed.
Non‐MHD influences on the magnetospheric current system
Authors: Winglee R. M.
Published: February 2004

The dynamics of critical magnetospheric boundary regions is examined through combined particle and fluid simulations. The particle simulations focus on the dynamics of the magnetopause whose structure is shown to be determined by the differential penetration of magnetosheath ions and electrons. The magnetosheath electrons with their small gyroradius have only limited penetration and give rise to a sharp peak in the current density, producing a well‐defined magnetopause current sheet. The magnetosheath ions penetrate further, where they intermix with magnetospheric plasma producing the magnetopause boundary layer. An ambipolar electric field is set up by this differential penetration which draws some of the magnetospheric ions out into the current layer and beyond. In addition, the ambipolar electric field sets up surface currents along the flanks of the magnetopause. These surface currents and their analogues in the magnetotail are shown to play a major role in (1) mapping of currents into the ionosphere and (2) determining the reconnection rate at the magnetopause and magnetotail current sheets by the effective resistivity they produce on these current sheets. These surface currents which are neglected in present MHD treatments are explicitly included in a modified set of single fluid equations, called magnetoplasma dynamics (MPD). The MPD description is able to remove the need for assuming an artificial resistivity while, at the same time providing, a comprehensive description of the currents that map into the ionosphere as well as the various current sheets attached to the different magnetospheric boundary layers. Important results include (1) the mapping of region 1 and 2 currents into the noon and midnight sectors of the ionosphere, (2) their equatorward motion during the southward turning of the interplanetary magnetic field, (3) their temporary enhancement at pseudobreakup, and (4) their enhancement and poleward expansion at substorm onset.
Anomalous nighttime electron temperature events over Millstone Hill
Authors: Garner T. W. , Richards P. G. , Comfort R. H.
Published: February 2004

This paper presents a statistical analysis of anomalous, short duration, nighttime, electron temperature enhancements (NETEs) in the ionosphere over Millstone Hill (42N, 71W). These temperature enhancements are characterized by a several hundred degree increase in the topside electron temperature with a typical duration of two to four hours. The NETEs can occur anytime at night but most frequently between eighteen and twenty hours local time. Occasionally, two NETEs may be observed in the same night. The maximum temperature reached is often comparable to the daytime temperature. NETEs are most frequent in the winter, but do occur in the spring and fall. There are rare cases of the NETE occurring in the summer. There does not appear to be a clear dependency on either solar or geomagnetic activity. The NETEs association with winter suggests a relationship with conjugate photoelectrons which, if it exists, is not simple. The observed durations are too short to be attributed solely to conjugate photoelectron heating.
Motion of the heliospheric termination shock:, 2. Energy loss due to energetic particle acceleration
Authors: Barnes Aaron
Published: February 2004

We present a generalization of earlier analysis of the motion of the heliospheric termination shock in response to heliospheric disturbances (Barnes, 1993) (paper 1), to allow jump conditions that include an energy sink at the shock front. The motivation for this study is that acceleration of the anomalous cosmic ray component may in fact represent such a sink. We have idealized the situation by assuming an infinitely thin shock parameterized by a quantity λ (0 ≤ λ ≤ 1), defined as the fraction of solar wind energy that is lost due to acceleration of the energetic particle component. If the sink is strong (consuming, say, 50% or more of the incident solar wind energy), the model leads to the following principal conclusions: (1) the shocked plasma would be much denser and cooler than in the standard gasdynamic case, thereby leading to more favorable conditions for direct observation of the shocked plasma; (2) the equilibrium shock position would be slightly farther (<10%) from the Sun than in the standard model; (3) as in the gasdynamic case, the shock would normally be in motion, so that the instantaneous position of the termination shock is not determined by interstellar conditions but by the recent history of the wind that has passed through the termination shock; and (4) the response of the shock to upstream disturbances would be similar to the response in the gasdynamic case, but the speed of the new termination shock would be somewhat smaller (probably by a factor of 4 or less). We estimate that this speed is typically, approximately a few tens of kilometers per second corresponding to an inward or outward excursion of order of <1 to several AU, rather less than conventional estimates of several tens of AU.
On incoherent scatter plasma lines in aurorae
Authors: Mishin E. V. , Schlegel K.
Published: February 2004

We consider a new mechanism for the generation of short‐scale, λ∥ ∼ 10 cm, electron density fluctuations giving rise to plasma line enhancements measured by incoherent scatter radars in the auroral E region. It is explained as a coupling (conversion) of long‐scale Langmuir waves generated by precipitated electron beams with short‐scale ion‐acoustic waves. The latter are supposed to be driven by field‐aligned currents associated with the precipitation. This mechanism is most effective inside the thin layer of the enhanced plasma turbulence, named the plasma turbulence layer.
Chaotic proton motion driven by kinetic Alfvén waves in the magnetosphere leading to polar and equatorial proton precipitations
Authors: Ho Antony Y. , Kuo S. P. , Schmidt G.
Published: February 2004

A previous study (Prakash, 1989) to model the nonlinear interaction between the kinetic Alfven wave and a bouncing proton trapped in the magnetosphere included only the longitudinal electric field component of the kinetic Alfven wave in the analysis. Despite the simplicity of the field configuration used in that model, its results provided a possible explanation for energetic proton precipitation to the polar region by alluding to the breakdown of the second adiabatic invariant of motion as a result of the onset of chaos in the particle motion. Since the corresponding transverse component of the wave electric field is an order of magnitude larger and since the longitudinal and transverse motions of the proton are coupled, this transverse electric field component is included along with the longitudinal component in the formulation of the present work. It increases the number of coupled equations of the Hamiltonian system from two to four. By using the Lyapunov exponent analysis, the chaotic nature of this five‐dimensional system is examined. It is shown that the threshold field is lowered by at least a factor of 6 and thus is much more easily exceeded under the normal magnetospheric condition. Furthermore, the results also show that protons drift radially when chaos occurs. It may explain the observation of low‐altitude trapped energetic protons in the equatorial region (Guzik et al., 1989).
H+ and He2+ Heating at the Earth's bow shock
Authors: Fuselier S. A. , Schmidt W. K. H.
Published: February 2004

The origins of ion heating at the Earth's bow shock are explored for a wide range of Mach numbers using data from composition experiments that separate solar wind H+ and He2+ downstream from the shock. Both of these ion species are important in the production of low‐frequency waves in the downstream magnetosheath. Consistent with previous results, the majority of the H+ dissipation at the bow shock is provided by a relatively small amount of the incident solar wind distribution that specularly reflects off the shock and returns to the downstream region. Although the He2+ distribution downstream from the shock may show some evidence for a reflected‐returned component, this component has a much lower relative density compared to that of the analogous H+ component and does not have the same strong Mach number dependence. Thus in contrast to the H+ results, the He2+ dissipation at the shock is provided by the directly transmitted core of the distribution.
The magnetospheric lobe at geosynchronous orbit
Authors: Thomsen M. F. , Bame S. J. , McComas D. J. , Moldwin M. B. , Moore K. R.
Published: February 2004

On rare occasions, satellites at geosynchronous altitude enter the magnetospheric lobe, characterized by extremely low ion fluxes between 1 eV and 40 keV and electron fluxes above a few hundred eV. One year of plasma observations from two simultaneously operating spacecraft at synchronous orbit is surveyed for lobe encounters. A total of 34 full encounters and 56 apparent near encounters are identified, corresponding to ∼0.06% of the total observation time. Unlike energetic particle (E>40 keV) dropouts studied earlier, there is a strong tendency for the lobe encounters to occur postmidnight, as late as 07 local time. The two spacecraft encounter the lobe with different rates and in different seasons. These occurrence properties are not simply explicable in terms of the orbital geometry in either the solar magnetic or the geocentric solar magnetospheric coordinate system. A composite coordinate system which previously organized more energetic particle dropouts is somewhat more successful in organizing the lobe encounters, suggesting that solar wind distortion of the magnetic equatorial plane away from the dipole location and toward the antisolar direction may be largely responsible for these dropouts. Our results further suggest that this distortion persists even sunward of the dawn‐dusk terminator. However, a simple dawn‐dusk symmetric distortion does not fully account for all the seasonal and local time asymmetries in the occurrence of the lobe encounters; thus there is probably an additional dawn‐dusk asymmetry in the distorted field. The lobe encounters are strongly associated with magnetospheric activity and tend to occur in association with rare magnetosheath encounters at synchronous orbit. It thus appears that the presence of the lobe at geosynchronous orbit is the result of major, probably asymmetric modifications of the magnetospheric field geometry in times of strong disturbance.
Single coherent radar multibeam measurements of the magnetospheric convection flows in the auroral E region
Authors: Yi Luo , Chen Pei‐Ren , Nielsen Erling
Published: February 2004

This paper investigates the possibility of determining electron drift velocities in the auroral ionosphere using data from only one VHF radar station that can make simultaneous Doppler velocity measurements in several different directions relative to the electron drift velocity. A technique has been developed for determining the electron drift using Doppler velocity measurements at large flow angles; this leads to improved electron drift velocity estimates when the ionospheric flows are spatially homogeneous over the radar field of view. The results are compared with ion drift measurements made with an incoherent radar facility; agreements and differences between the two measurements as well as limitations of the analysis technique are discussed.