bluuebunny/crossref_metadata_2025 huggingface dataset:
A model for the source location and beam geometry of Neptune's principal smooth radio component
Authors: Wang Liyun , Carr Thomas D.
Published: February 2004

Two empirical models were developed for the location and beam structure of Neptune's right‐hand (RH) polarized corotating smooth component radio source. The O8 magnetic field model was assumed. In both models the usual assumptions were also made that (1) the source is distributed within a region such that X mode emission toward the spacecraft would be RH elliptically polarized, (2) the resultant beam is the sum of elementary hollow cone beam components for which the cone axes are tangent to the magnetic field at the individual source elements, and (3) all the source elements lie just above the surface on which the electron cyclotron frequency equals the frequency of the emitted radiation. (The resultant beam was found also to be confined to a hollow cone, indicating a relatively small source area.) In the second model, which is the more realistic of the two, an additional requirement was imposed that all ray paths to the spacecraft must lie entirely outside this surface, i.e., within the radio horizon. At each of several frequencies between about 40 and 400 kHz, the adjustable model parameters were varied to yield calculated intensity versus time curves at the spacecraft location that most closely matched the observed intensity versus time curves. The model indicated that the source is located within or near the the northern auroral zone at L = 20. At 116 kHz it was found that the half angle of the hollow cone emission beam was approximately 80° (at its midpoint) with a beam wall thickness of about 15°. At higher frequencies the beam half angle and wall thickness were somewhat less than these values. A new component of the Neptunian radio emission is also described. This component is left‐hand polarized and is of extremely narrow bandwidth. It is believed to be confined within an unusually thin‐walled hollow cone emission beam. It is referred to as the LNT component. Evidence is presented suggesting that it is O mode emission from the same source that produces the X mode smooth component.
Direct allowance for the effect of photoelectrons in ionospheric modeling
Authors: Titheridge J. E.
Published: February 2004

Model calculations for the ionospheric E and F1 regions yield electron densities which are much too small, if no allowance is made for the production of secondary ionization by primary photoelectrons. Full calculations of this secondary production are quite difficult, since the upward and downward photoelectron fluxes must be determined as a function of energy at each height. Early studies showed that secondary ionization increases the total production rate by about 30% in the F2 region, while more recent studies show increases of around 100% in the E and F1 regions. Use of a fixed correction factor (for a given height and zenith angle) is not satisfactory, however, since the amount of secondary production varies greatly for different radiation bands. This paper describes a new approach in which a secondary production factor is determined for each ion and each radiation band. These factors (ns) are defined by the initial photon energy, and the mean energy of the final secondary electrons. For each radiation band the effective production efficiencies, for each ion, are increased by the factor 1 + ns. Modeling of the ionosphere then proceeds normally, with no other changes and no increase in computer time. All results automatically include a full allowance for secondary production, for any assumed values of zenith angle, atmospheric model, or EUV fluxes. Comparison with recent, full photoelectron calculations shows that this procedure gives reliable results, with errors which are less than those due to current uncertainties in the solar fluxes, the photoionization cross sections and the electron collision cross sections.
Structure and dynamics of inner cometary plasmas
Authors: Sharma A. S. , Milikh G. M.
Published: February 2004

The inner cometary coma is a weakly ionized plasma, and its structure and dynamics are governed mainly by ionization due to solar radiation and solar wind electrons and losses due to the radiative processes, recombination, and transport. At comet Halley a narrow ion density depletion region was observed by spacecraft as well as ground‐based instruments and has been linked to the dynamics of the plasma and radiation. A model of the cometary plasma consisting of water group ions, bulk electrons, and energetic electrons produced mainly by photoionization is presented. The dominant losses in the inner coma are the radiation from the excitation of rotational and vibrational levels of water molecules and the recombination of the plasma. The electron energy losses due to these processes peak near 4000 K, and at temperatures higher than this value a localized cooling leads to further cooling arising from increased radiation loss and consequently to a thermal instability. The resulting increase in recombination leads to an ion density depletion, and the estimates for this depletion at comet Halley agree with the observations. This instability is sensitive to the plasma conditions and the transport processes, that is, diffusion and thermal conductivity. There is no direct measurement of the electron temperature in this range, and the electron temperature profile from MHD simulations has been used to develop a model of the inner cometary plasma, which yields the localization of the thermal instability and, hence, the observed ion density depletion region. The resulting electron temperature profile is also consistent with that obtained from the temperature dependence of the electron‐ion recombination rate.
Thermospheric winds over Japan: Comparison of ionosonde and radar measurements
Authors: Igi Seiji , Ogawa Tadahiko , Oliver W. L. , Fukao Shoichiro
Published: February 2004

The magnetic meridional component of the thermospheric neutral wind is derived from the F2 layer height measured from ionograms collected at Kokubunji, Japan and compared with the winds measured by the middle and upper atmosphere radar at Shigaraki, Japan and the winds predicted by the empirical Horizontal Wind Model 1990 (HWM90). Good agreement is found between the ionosonde and the radar winds. This agreement supports the previous finding of the radar study that the winds over Japan are smaller in amplitude than the winds reported elsewhere by a factor of 1.5–2. On the other hand, disagreement is found at some local times between the ionosonde and HWM90 winds. The ionosonde winds in the period from 1986 to 1988 are, on average, poleward in the daytime and equatorward in the nighttime. The daytime‐nighttime difference remains at about 100 m/s summer or winter, but this diurnal pattern is superposed upon a daily average drift of about 10 m/s southward in summer and 10 m/s northward in winter, suggesting a consistent summer‐to‐winter circulation pattern. This limited study validates the ionosonde wind method as applied at Japanese longitudes and opens the way for comprehensive studies of the thermospheric wind over Japan from the long and continuous ionogram libraries from the Japanese ionosonde chain.
Modeling the linear transient response of the magnetotail to variations in the plasma mantle
Authors: Richard Robert L. , Zelenyi Lev M. , Ashour‐Abdalla Maha
Published: February 2004

A simple generalization of large‐scale kinetics is used to model the linear transient response (LTR) of the magnetotail plasma parameters to changes in a mantle particle source. A change in the source intensity results not only in changes in magnetotail plasma density but also in temporary changes of the order of a few hundred kilometers per second in the local bulk flow velocities which last for 10–20 min. The effect that dominates the initial stage of the magnetotail response, particularly in the distant tail, should be the ballistic effect of fast ion beams. While LTR does not include the self‐consistent responses of the magnetic and electric fields to the changes in plasma parameters, its effects may be significant for timescales of minutes.
Sudden impulses at subauroral latitudes: Response for northward interplanetary magnetic field
Authors: Russell C. T. , Ginskey M.
Published: February 2004

The response of subauroral H component magnetograms at the time of passage of interplanetary shocks under northward interplanetary magnetic field (IMF) conditions is used to examine the behavior of the magnetosphere when it is suddenly compressed. At subauroral latitudes, near 55° or L = 3, the response has some similarities and some important differences from the low‐latitude response. The first effect is a preliminary impulse which appears to be due to the Hall current driven in the ionosphere by the arrival of the first Alfven waves along the magnetic field lines from the distant equatorial magnetosphere that has been set in motion by the penetration of the shock wave into the magnetosphere. Next there is a general increase of the magnetic field everywhere as the magnetosphere is compressed to its new size and the information of this compression propagates to the ground. This increase takes about 5 min followed by a 10‐min relaxation to the final asymptotic value. This general increase is accompanied by two more localized current systems: one transient and one steady. The transient current appears to be a dual‐vortex current system launched from noon toward the nightside. The other steady system is a fixed double‐cell convection system. We identify the former with the propagation of the magnetopause indentation associated with the enhanced pressure in the solar wind. We attribute the latter, steady system to high‐latitude reconnection with northward IMF.
Fast ionospheric response to enhanced activity in geospace: Ion feeding of the inner magnetotail
Authors: Daglis Ioannis A. , Axford W. Ian
Published: February 2004

The role of the ionospheric particle source in the dynamic processes of the geospace was neglected, underestimated, or questioned for a long period. Recent studies have indicated a fast and effective feeding of the magnetosphere with energetic ionospheric‐origin ions during periods of enhanced auroral electrojets, i.e., during the expansion and the late growth phase of substorms. Such a feeding with ionospheric ions can alter the plasma composition in the geospace system and thus influence its dynamics and evolution during substorms and magnetic storms. This paper addresses the issue of short‐timescale (rather than long‐term) feeding of the magnetosphere with ionospheric ions. We consider this feeding to be a result of the ionospheric response to increased auroral current dissipation due to increased solar wind ‐ magnetosphere coupling or increased magnetospheric activity. We review and combine old and recent observations of both the ionospheric outflow and the ionospheric ions in the magnetosphere, along with transport and energization mechanisms for the relatively cold ionospheric outflowing ions and the potential implications of their enhanced presence in the magnetosphere for the dynamics of geospace processes (substorms/storms). We suggest that a fast feeding of the near magnetotail with ionospheric ions can lead to a transient localized dominance of heavy ionospheric ions (namely O+) and consequently to an ionospheric regulation of the evolution of dynamic geospace processes. Accordingly, we suggest that the role of the ionosphere within the dynamic geospace coupling be given more emphasis: the ionosphere should be regarded not only as a material source for the magnetosphere but also as an active element in dynamic geospace processes.
Observations of thermospheric horizontal neutral winds at Watson Lake, Yukon Territory (Λ = 65°N)
Authors: Niciejewski R. J. , Killeen T. L. , Solomon Stanley C.
Published: February 2004

Fabry‐Pérot interferometer observations of the thermospheric O I (6300 Å) emission have been conducted from an airglow observatory at a dark field site in the southeastern Yukon Territory, Canada, for the period November 1991 to April 1993. The experiment, operated in an unattended, remote fashion, has resulted in a substantial data set from which mean neutral winds have been determined. Dependent upon geomagnetic activity, the nocturnal location of the site is either equatorward of the auroral oval or within the oval boundaries. The data set is rich enough to permit hourly binning of neutral winds based upon the Kp geomagnetic disturbance index as well as the season. For cases of low geomagnetic activity, the averaged, vector horizontal neutral wind exhibits the characteristics of a midlatitude site displaying antisunward pressure‐gradient‐driven winds. As the geomagnetic activity rises the late afternoon and evening winds slowly rotate sunward in an anticlockwise direction, initially remaining near 100 m/s in speed but eventually increasing to 300 m/s for Kp > 5. For the higher levels of activity the observed neutral wind flow pattern resembles a higher‐latitude polar cap pattern characterized by ion drag forcing of thermospheric neutral gases. In addition, rotational Coriolis forcing on the dusk side enhances the ion drag forcing, resulting in dusk winds which trace out the clockwise dusk cell plasma flow. On the dawn side the neutral winds also rotate in an anticlockwise direction as the strength of geomagnetic disturbances increase. Since the site is located at a transition latitude between the midlatitude and the polar cap, the data set provides a sensitive test for general circulation models which attempt to parameterize the contribution of magnetospheric processes. A comparison with the vector spherical harmonic (VSH) model indicates several regions of poor correspondence for December solstice conditions but reasonable agreement for the vernal equinox.
A case study of the ionospheric storm dusk effect
Authors: Buonsanto Michael J.
Published: February 2004

The initial ionospheric storm positive phase often appears as a large enhancement in F region electron density at middle latitudes in the evening hours following a storm sudden commencement. The cause of this dusk effect is still not well understood, though several mechanisms have been proposed. This phenomenon is studied using incoherent scatter radar data collected at Millstone Hill and total electron content data from a north‐south chain of stations during the May 26‐27, 1990, storm. Nine‐position Millstone Hill radar data allows estimation of gradients in electron density and in velocity components above the station. From these the motion term in the continuity equation is calculated and used along with wind, electric field data, and modeling to assess the relative importance of the proposed mechanisms during this storm. It is shown that a combination of mechanisms appear to have been involved, including a traveling atmospheric disturbance, advection of high‐density plasma from lower latitudes, and neutral composition changes.
N2+ (X), N2+ (A), and N2+ (B) Production in e− + N2 collisions
Authors: Van Zyl B. , Pendleton W.
Published: February 2004

A variety of cross‐section data for electron‐impact ionization of N2 have been analyzed, including those for total N2+ production and for the Meinel and first‐negative emissions from excited N2+ ions. These results are used to infer the relative production rates of the N2+ (X), N2+ (A), and N2+ (B) product ions during the reactions, yielding branching‐ratio fractions of 0.320±0.147, 0.535±0.112, and 0.145±0.019, respectively, for 100‐eV electrons. The data are compared with the relative formation rates of these ions recently obtained using electron‐energy spectroscopy coincidence measurements. The significant discrepancy between these two procedures is discussed, and the results obtained from our data analysis are recommended for auroral modeling at the present time.
Energy distribution of fast nitrogen atoms in the nighttime terrestrial thermosphere
Authors: Sharma R. D. , Kharchenko V. A. , Sun Y. , Dalgarno A.
Published: February 2004

Energetic nitrogen atoms play a significant role in the chemistry and thermal balance of the thermosphere. The nocturnal energy distribution is calculated and the importance of including the center of mass motion is demonstrated. The center of mass motion gives rise to an extended component of fast atoms. The calculation of the energy distribution includes the loss of energetic nitrogen atoms in reactions with molecular oxygen. The density of energetic atoms and their energy distribution are strong functions of local time, altitude, and latitude.
The Triton‐Neptune plasma interaction
Authors: Hoogeveen Gary W. , Cloutier Paul A.
Published: February 2004

The Voyager 2 encounter with Neptune and Triton in August 1989 showed a large ionosphere at Triton. Subsequent studies have tried to explain the production of such high levels of ionization but have ignored the possible plasma dynamics originating from the interaction between Neptune's magnetosphere and Triton. This study applies knowledge gained from studying the solar wind‐Venus interaction to this case. We find that observations made by Voyager 2 can be explained by downward convection of magnetospheric plasma into Triton's atmosphere, with the flow momentum transferred to the neutral atmosphere near an altitude of 650 km. We show that momentum transfer is accomplished as both the convective velocity and the magnetic field go to zero. The mechanism by which the ionosphere is produced was identified previously as impact ionization from hot electrons originating in Neptune's magnetosphere. These precipitating hot electrons are shown in this study to be unaffected by a magnetic field below roughly 650 km. This is a result not previously anticipated, and one which implies that the plasma interaction between Neptune's magnetosphere and Triton cannot be ignored.
Sources of the infrared radiation generated by the interaction of fast O(3P) with H2O in space
Authors: Bernstein Lawrence S. , Elgin James B. , Pike Charles P. , Knecht David J. , Murad Edmond , Zehnpfennig T. F. , Galica G. E. , Stair A. T.
Published: February 2004

Recent broad band‐pass infrared (2.3–3.6 μm) observations of the interaction of the exhaust plumes from space shuttle ram firing dual primary reaction control system (PRCS) engines with atmospheric atomic oxygen, O(3P), are analyzed. The results from previous laboratory and space‐based experiments that observed the IR emissions from the high‐velocity interaction of O(3P) and H2O are reexamined. Specifically, emission in the 2.7‐μm region is reinterpreted in terms of the reactive process O(3P) + H2O → OH(ν) + OH rather than the collisional excitation mechanism O(3P) + H2O → H2O (ν1 and ν3) + O(3P), which was the focus of the previous analyses. A spectral analysis is presented of a previously reported molecular beam experiment that obtained spectrally resolved emission measurements for a relative collision velocity of 8 km/s between O(3P) and H2O and that shows that OH(ν) is the dominant source of emission in the 1 to 5‐μm region. The effect of rotational radiative relaxation is discussed, and it is shown that for highly rotationally excited molecules the timescale for this process can be much faster than vibrational radiative decay, thus resulting in much narrower spectral distributions than might ordinarily be expected.
A statistical study of transient events in the outer dayside magnetosphere
Authors: Sanny J. , Sibeck D. G. , Venturini C. C. , Russell C. T.
Published: February 2004

The AMPTE CCE satellite frequently observed transient (1 ≤ τ ≤ 8 min) events marked by magnetic field strength increases and bipolar magnetic field signatures (peak‐to‐peak amplitudes ≥ 4 nT) while in the outer dayside magnetosphere. We report a survey of 59 prominant events observed from August to November 1984. The bipolar signatures and minimum variance analysis reveal that most events move poleward and antisunward, except in the immediate vicinity of local noon. Here the motion of the events appears to be better governed by the spiral/orthospiral interplanetary magnetic field (IMF) orientation than magnetic curvature forces associated with IMF By. The IMF orientation appears to have little or no influence on event occurrence or orientation. We interpret the events in terms of solar wind/foreshock pressure pulse induced ripples on the magnetopause surface. Our results can be reconciled with those obtained in previous studies which made use of ISEE 1/2, AMPTE IRM, and AMPTE UKS observations if pressure pulses produce large‐amplitude events and bursty merging (or reconnection) produces small‐amplitude events.
Cusp/cleft auroral forms and activities in relation to ionospheric convection: Responses to specific changes in solar wind and interplanetary magnetic field conditions
Authors: Sandholt P. E. , Farrugia C. J. , Stauning P. , Cowley S. W. H. , Hansen T.
Published: February 2004

This work is intended to be a first step toward a categorization of dayside auroral responses to various solar wind and interplanetary magnetic field (IMF) conditions and the corresponding states of plasma convection in the dayside magnetosphere. In this paper we relate cusp latitude auroral emissions observed during a 4‐hour interval on December 17, 1992, to solar wind data supplemented by ground magnetograms. We focus on different types of variability associated with an interplanetary shock and with an IMF directional discontinuity. We further investigate the quasi‐steady conditions related to periods of low activity versus periods of high activity, as well as periods of IMF By‐related zonal convection in the cusp region versus periods of IMF BZ‐related convection in the north‐south direction. To these various conditions the dayside aurora responds differently. These observations are discussed in relation to entry of magnetosheath plasma into the magnetosphere associated with different modes of solar wind‐magnetosphere coupling. Two main categories of persistent auroral forms in the cusp/cleft region are observed, a latitudinally wide (≥200 km) zone of weak 630.0‐nm emission with no sharp boundaries extending to magnetic latitudes of ∼78°–79° during quiet intervals and another latitudinally narrow (∼100 km) zone of strong red line emission, located equatorward of 75° MLAT during disturbed periods. These forms occur in association with sunward and antisunward convection in the dayside polar cap, respectively. It is suggested that these two signatures of electron precipitation in the noon/near‐noon sector reflect plasma entry by lobe reconnection and reconnection at low (subcusp) latitudes, respectively. The observed sudden transition from the former to the latter auroral condition may thus reflect a change of reconnection site. The associated brightening of the cusp/cleft aurora then corresponds to the larger efficiency of magnetosheath plasma entry at low magnetopause latitudes when the subcusp reconnection process is switched on, compared to the more limited plasma entry occurring at high latitude during lobe recönnection. The most intense and latitudinally narrow auroral forms at 630.0 nm are observed during intervals of strong IMF By‐related zonal convection and a strong DPY mode of ground magnetic deflection. Information on the ionospheric convection pattern in the vicinity of the magnetic field separatrix is obtained for the different IMF orientations by combining the auroral and magnetic observations in the Arctic sectors of Greenland and Svalbard.
Magnetosheath density fluctuations and magnetopause motion
Authors: Sibeck D. G. , Gosling J. T.
Published: February 2004

The interplanetary magnetic field (IMF) orientation controls foreshock densities and modulates the fraction of the solar wind dynamic pressure applied to the magnetosphere. Such pressure variations produce bow shock and magnetopause motion and cause the radial profiles for various magnetosheath parameters to sweep inward and outward past nearly stationary satellites. We report ISEE 2 observations of correlated density and speed fluctuations, and anticorrelated density and temperature fluctuations, on an outbound pass through the northern dawnside magnetosheath. Densities decreased when the magnetic field rotated southward and draped about the magnetopause. In the absence of any significant solar wind density or dynamic pressure variations, we interpret the magnetosheath fluctuations as evidence for radial magnetosheath motion induced by variations in the IMF orientation.
Coupled‐mode scenario for the magnetospheric dynamics
Authors: Sergeev V. A. , Pulkkinen T. I. , Pellinen R. J.
Published: February 2004

Substorm phenomena are reviewed with emphasis on the magnetospheric source region of the onset, on the morphology of the initial breakup and subsequent activations, and on the variable character of individual substorms. We provide evidence that before the substorm onset and during the following activations an intense, thin current sheet is formed at the interface between the quasi‐dipolar and taillike magnetic field regions. We infer that the initial breakup, the following multiple activations, pseudobreakups, and other short‐term activations during nonsubstorm times are all similar in morphology and have the same formation mechanism. We postulate that the elementary units of energy dissipation, impulsive dissipation events, which are localized in space and have a short lifetime of ∼1 min, are the manifestations of tail reconnection. We also emphasize the evidence that previous authors have presented in favor of this time dependence and localization. On the basis of the above, we suggest that there are two basic magnetospheric processes responsible for energy storage and dissipation during both substorm and nonsubstorm times: A global and slow quasi‐static tail reconfiguration responsible for the energy storage, and a sequence of local, sporadic, short‐term energy dissipation events. These competitive processes can be observed most the time in some part of the plasma sheet; their relative intensity determines the type of large‐scale dynamic evolution. In this scenario, the various dynamical situations are interpreted as variations in the balance between the two competing processes.
Two types of ion energy dispersions observed in the nightside auroral regions during geomagnetically disturbed periods
Authors: Hirahara M. , Mukai T. , Nagai T. , Kaya N. , Hayakawa H. , Fukunishi H.
Published: February 2004

The Akebono satellite has observed two types of energy dispersion signatures of discrete ion precipitation event in the nightside auroral regions during active geomagnetic conditions. The charged particle experiments and electric and magnetic field detectors on board Akebono provide us with essential clues to characterize the source regions and acceleration and/or injection processes associated with these two types of ion signatures. The magnetic field data obtained simultaneously by the geosynchronous GOES 6 and 7 satellites and the ground magnetograms are useful to examine their relationships with geomagnetic activity. Mass composition data and pitch angle distributions show that different sources and processes should be attributed to two types (Types I and II) of energy dispersion phenomena. Type I consists of multiple bouncing ion clusters constituted by H+. These H+ clusters tend to be detected at the expansion phase of substorms and have characteristic multiple energy‐dispersed signatures. Type II consists of O+ energy dispersion(s), which is often observed at the recovery phase. It is reasonable to consider that the H+ clusters of Type I are accelerated by dipolarization at the equator, are injected in the field‐aligned direction, and bounce on closed field lines after the substorm onset. We interpret these multiple energy dispersion events as mainly due to the time‐of‐flight (TOF) effect, although the convection may influence the energy‐dispersed traces. Based on the TOF model, we estimate the source distance to be 20–30 RE along the field lines. On the other hand, the O+ energy dispersion of Type II is a consequence of reprecipitation of terrestrial ions ejected as an upward flowing ion (UFI) beam from the upper ionosphere by a parallel electrostatic potential difference. The O+ energy dispersion is induced by the E × B drift during the field‐aligned transport from the source region to the observation point.
Tearing modes at the magnetopause
Authors: Lakhina G. S. , Schindler K.
Published: February 2004

It is shown that the shear flow present in the magnetopause current layer can destabilize the tearing modes depending upon the Mach number profiles. Using a variational principle, an heuristic stability criterion based on the form of the Mach number profile is obtained. This criterion could be used as a handy tool to search for tearing mode instability of a given magnetopause equilibrium with shear flow. The Doppler shifted magnetic structures produced by the tearing modes would be observable at ultralow frequencies in the range of ∼ 5 – 650 mHz in the satellite frame of reference.
Formation of low‐energy trapped and field‐Aligned ion distributions in substorm dipolarization events
Authors: Liu Chao , Horwitz J. L.
Published: February 2004

A dynamic semikinetic simulation is used to study the effect of substorm dipolarization events on low‐energy thermal plasmas initially distributed isotropically along closed tailward stretched magnetic field lines in the middle magnetosphere. The temporal variation of the magnetic field during dipolarization induces a perpendicular electric field that leads to E × B drift and produces centrifugal and betatron ion acceleration. We find that the dipolarization process produces field‐aligned ion streams split into components with higher and lower speeds, as generated by parallel forces identified here as the “centrifugal force” and the “Coriolis force.” The higher‐speed component leads to ion precipitation which is highly dispersed in energy during the dipolarization event. The post dipolarization evolution of the lower‐speed component produces energy‐time spectrogram features similar to the “ion bounce clusters” observed at geosynchronous orbit and noted in the “convection surge” simulations of Mauk [1986]. An entirely new feature not found in previous dipolarization simulations is the formation of an equatorially trapped population whose flux is strongly peaked at the 90° pitch angle and whose overall level sharply declines at latitudinal boundaries within 5° of the equator. These equatorially trapped ion distributions are generated through the combined influence of equatorial focusing by the parallel centrifugal force and betatron perpendicular energization. The postdipolarization latitudinal density distributions contain a local maximum, due to the trapped ion population, at the equator, where the electric potential similarly contains a small peak. Hence the substorm dipolarization process provides a new, heretofore unrecognized mechanism for producing the characteristic equatorially trapped warm ion populations observed, for example, by Horwitz and Chappell [1979] and Olsen et al. [1987].
On runaway breakdown and upward propagating discharges
Authors: Roussel‐Dupré R. , Gurevich A. V.
Published: February 2004

The origins of mysterious γ ray and radio flashes recently detected by satellite‐based instruments passing over thunderstorms are examined in the context of upward propagating discharges initiated by runaway air breakdown. Preliminary calculations normalized by the recent optical measurements of so‐called sprites indicate that the runaway mechanism may well be the source of these emissions. If this is true, then upward discharges represent the first known manifestation of a fundamental, new process in plasma physics.
Alfvén waves in the vicinity of Uranus: The role of energetic proton thermal anisotropy
Authors: Ruvinsky Irena , Eviatar Aharon
Published: February 2004

Spacecraft observations in the vicinity of planetary bow shocks indicate the existence of proton beams and of magnetohydrodynamic (MHD) waves. We analyze the dispersion of such waves in the presence of energetic beams and show that the role of temperature anisotropy in the growth of low‐frequency MHD waves is significantly enhanced by weak relativistic effects. The present analysis of ion beam instability near Uranus shows that thermal anisotropy of energetic protons coupled with the relativistic dependence of their mass on energy leads to an increase of the resonant Alfvénic wave growth rate and to a dispersion curve which differs from that of the isotropic case. MHD‐like waves, excited by energetic proton beams near the planetary bow shock have (in the presence of thermal anisotropy) a dispersion relation similar to that of Alfvén waves at wavelengths of the order of 2 × 108 m and can propagate in the solar wind plasma away from Uranus.
Nonideal MHD effects in the magnetosheath
Authors: Hau L.‐N.
Published: February 2004

The double‐adiabatic or Chew‐Goldberger‐Low (CGL) laws have been found to be severely violated in the magnetosheath. In this paper we show that the nonideal MHD effects including both the heat fluxes and the nonperfect conducting effects, for example, the resistive, the ion and electron inertial terms in the generalized Ohm's law, may render the CGL laws invalid; the entropy of the gyrotropic plasmas, however, may only be altered by the heat fluxes. It is shown from a magnetosheath crossing that the observed pressure anisotropy can be largely accomplished by the nonperfect conducting effects and that the constant entropy law, which can also be reduced from the double‐adiabatic laws, is much more viable than the individual CGL relation; the slight decrease of the entropy toward the magnetopause implies a net heat loss in the system. Double polytropic closure in the magnetosheath [Hau et al., 1993] implies that these nonideal MHD effects may be approximately lumped into two empirical polytropic exponents, γ‖ and γ⊥, describing the thermodynamic behavior of the plasma; the case γ‖ = 3 and γ⊥ = 2 corresponds to the double‐adiabatic closure.
Conjugate cusp‐region ULF pulsation responses to the solar wind event of May 23, 1989
Authors: Matthews D. L. , Ruohoniemi J. M. , Dudeney J. R. , Farrugia C. F. , Lanzerotti L. J. , Friis‐Christensen E.
Published: February 2004

We have studied a hydromagnetic wave event in the early noon sector of the cusp region, using HF radar data collected in the southern hemisphere and ground‐based magnetic data from both hemispheres. There were three distinct pulsations, which appear to have been driven by changes in the solar wind, beginning with the passage of a powerful interplanetary shock. This shock triggered the first pulsation, a strong ssc, at 1346 UT. It was followed at 1402 UT by a second pulsation, which had a stable dominant frequency of 3.3 mHz and lasted about half an hour. Although this frequency was close to that of the ssc, the new pulsation was clearly differentiated from the ssc by an abrupt 180° change of phase. A further such phase discontinuity at 1434 UT (near noon MLT) marked the start of a third distinct ULF pulsation with a clearly lower frequency of 2.8 mHz, which continued for another half hour, until 1505 UT, and was correlated with the magnitude of the IMF. These latter two pulsations yielded a successful calibration of the radar observations with ground‐based magnetometer measurements; the pulsation current systems in the northern and southern hemispheres were found to be highly correlated and conjugate. The 3.3‐mHz pulsation was observed over a range of ∼10° in invariant latitude, and its amplitude showed a strong, broad maximum close to the latitude of the cusp as inferred from an independent radar‐satellite analysis of cusp signatures. The zonal E × B motion associated with the pulsation showed a linear latitudinal decrease in phase (25° per degree of latitude) while the meridional motion had nearly constant phase. The longitudinal phase variation corresponded to an m value of ∼10 with a source at later MLT. We discuss the possibility that the dominant wave activity near 3 mHz in the latter two pulsations was due to a hydromagnetic surface wave at the magnetopause stimulated by the ssc and modified by the forcing action of the solar wind.
Current sheets in the solar minimum corona
Authors: Gibson S. E. , Bagenal F. , Low B. C.
Published: February 2004

We analytically combine stress‐free current sheets with a coronal magnetostatic bulk current model. We begin by imposing a current sheet at the equator as an upper boundary condition on the modeled coronal field. We find that in order to reproduce the sharp gradients across the boundaries of helmet streamers, we also have to add current sheets along the interface between open and closed field lines. We find a description of coronal magnetic field and density in the presence of both bulk and sheet currents that matches both white light and photospheric magnetic flux observations.
Interplanetary discontinuities and Alfvén waves at high heliographic latitudes: Ulysses
Authors: Tsurutani Bruce T. , Ho C. M. , Arballo J. K. , Smith E. J. , Goldstein B. E. , Neugebauer M. , Balogh A. , Feldman W. C.
Published: February 2004

This paper presents the results of the first statistical study of interplanetary directional discontinuities at both low and high heliographic latitudes measured by the Ulysses magnetometer. There is a gradual decrease in the rate of occurrence of interplanetary discontinuities (ROIDs) with increasing radial distance. From 1 to 5 AU, an e−(r−1)/5 dependence is derived. Much of this decrease is believed to be an artifact due to the discontinuity thickening with decreasing |B|, falling outside the detection criteria. As Ulysses goes from the ecliptic plane to high (−80°) heliographic latitudes, the ROID value increases dramatically. The increase is about a factor of 5 as Ulysses moves from Jupiter at 5 AU to 2.5 AU over the south pole. There is a one‐to‐one correspondence between high ROID values and high‐speed streams. This is particularly dramatic just after the Jovian encounter when there are ∼25.4‐day period corotating streams present. Thus the increase with latitude is primarily due to Ulysses spending an increasing percentage of time within a high‐speed stream emanating from the solar coronal hole. High‐speed streams are characterized by the presence of nonlinear Alfvén waves with peak‐to‐peak transverse fluctuations as large as of 1 to 2. Over the south pole, the normalized transverse wave power can be characterized by P = 2.5 × 10−4ƒ−1.6 Hz−1 and the compressional power 1.8 × 10−4ƒ−1.2 Hz−1 for frequencies between 10−5 and 10−2 Hz. The normalized wave power spectra in different regions of the polar coronal hole streams, from midlatitudes to high heliographic latitudes, appear to be quite similar. The wave power in the ecliptic plane is somewhat lower, perhaps due to contamination from low‐speed streams. The Alfvén waves in the high‐speed stream are found to be propagating outward from the Sun, even at these large heliocentric distances (2.5–5.0 AU). The waves typically have arclike polarizations and conserve field magnitude to first order. Directional (rotational) discontinuities often form the edges of the phase‐steepened Alfvén waves, thus offering a natural explanation for the high ROID rates within high‐speed streams.
CRRES Poynting vector observations of electromagnetic ion cyclotron waves near the plasmapause
Authors: Fraser B. J. , Singer H. J. , Hughes W. J. , Wygant J. R. , Anderson R. R. , Hu Y. D.
Published: February 2004

Electromagnetic ion cyclotron waves have been observed by the Air Force Geophysics Laboratory triaxial fluxgate magnetometer on the CRRES satellite in the vicinity of the plasmapause. Specific wave events in the 1300–0100 MLT local time sector and over L = 4.8 – 6.7 at magnetic latitudes between ± 20° have been studied with respect to energy propagation. Using the wave magnetic field data and the orthogonal two‐component Berkeley electric field experiment data allows the computation of the Poynting vector. Nine events with frequencies below 0.6 Hz have been analyzed. General wave properties showed intensities in the range 4–18 μW m−2. In most events, energy propagation was predominantly away from the equator, supporting the existence of an equatorial source region. There are exceptions and one event, observed 3° from the equator, also showed some propagation back to the equator with equal energy. Four periodic wave packets were seen in another event. This event showed unidirectional propagation away from the equator over all packets with periodicity approximately one‐half of the calculated time for a typical bouncing wave packet. Consequently, the periodicity is not the signature of a classical periodic structured Pc 1–2 pulsation event of the type seen on the ground. Generally, the results for these waves with f < fHe+ may be interpreted by assuming an ion cyclotron instability source located near the equator and irregularly emitting wave trains of energy into one or both hemispheres, with minimal reflection at the ionosphere.
Magnetic storms and magnetotail currents
Authors: Alexeev I. I. , Belenkaya E. S. , Kalegaev V. V. , Feldstein Y. I. , Grafe A.
Published: February 2004

The magnetospheric magnetic field is highly time‐dependent and may have explosive changes (magnetospheric substorms and geomagnetic storms) accompanied by significant energy input into the magnetosphere. However, the existing stationary magnetospheric models can not simulate the magnetosphere for disturbed conditions associated with the most interesting magnetospheric physics events (intensive auroras, particle injection in the inner magnetosphere, and precipitations at the high latitudes, etc.). We propose a method for constructing a nonstationary model of the magnetospheric magnetic field, which enables us to describe the magnetosphere during the disturbances. The dynamic changes of the magnetosphere will be represented as a sequence of quasistationary states. The relative contributions to the Dst index by various sources of magnetospheric magnetic field are considered using a dynamic model of the Earth's magnetosphere. The calculated magnetic field is obtained by using the solar wind and geomagnetic activity empirical data of the magnetic storm of March 23–24, 1969 and the magnetic disturbance of July 24–26, 1986. The main emphasis is on the current system of the magnetospheric tail, the variations of which enable a description of the fast changes of Dst.
Ion cyclotron heating in the dayside magnetosphere
Authors: Norqvist Patrik , André Mats , Eliasson Lars , Eriksson Anders I. , Blomberg Lars , Lühr Hermann , Clemmons James H.
Published: February 2004

Observations of waves and particles obtained by the Freja satellite at altitudes around 1700 km in the dayside high‐latitude magnetosphere are used to study ion energization. We find that ions, including O+, during several events of intense ion energization can be heated perpendicularly to the geomagnetic field to mean energies of up to about 20 eV via the process of cyclotron resonance by broadband waves around the ion gyrofrequencies. There is a good correlation between such broadband waves and the ion energization. The waves show no spectral features at the O+ gyrofrequency. The observed wave amplitudes are used as an input to a Monte Carlo simulation to obtain the observed ion energies. The waves around the ion gyrofrequencies may be generated either by field‐aligned electrons or by nonlinear processes transferring energy from waves with lower frequencies. Not only the mean energy but also the shape of the particle distribution agrees with the cyclotron resonance heating mechanism. Other mechanisms, such as heating by lower hybrid waves or by a slowly varying electric field, are investigated but are found to be less important than cyclotron heating in this region of space.
Ion reflection at the shock front revisited
Authors: Gedalin M.
Published: February 2004

We study the process of ion reflection at the structured perpendicular shock front, in which ions cross the ramp, proceed to the overshoot, turn back to the ramp, and cross it again, being observed in the upstream region as reflected‐gyrating ions. We derive analytically the reflection condition which depends on the potential, magnetic compression ratio and increase of the vector potential across the foot and ramp. The theoretical developments are illustrated by direct numerical analysis of ion trajectories in a model structured shock front. The parameters of the model shock are chosen close to the parameters of the observed high‐Mach number supercritical shock with βi ∼ 1. The analysis shows that the foot length differs substantially from the value predicted by the specular reflection model and depends on βi as well. The foot length found for the model shock with βi ∼ 1 is in a good agreement with observations and simulations of shocks with similar parameters. It is also shown that the reflected ion velocities at the outer edge of the foot and downstream agree with measurements within the limits of the observational errors.
Low‐latitude dayside magnetopause and boundary layer for high magnetic shear: 2. Occurrence of magnetic reconnection
Authors: Phan T.‐D. , Paschmann G. , Sonnerup B. U. Ö.
Published: February 2004

Quantitative comparisons of the flow velocity change across the magnetopause (MP) with the prediction from local tangential stress balance in a one‐dimensional time‐stationary rotational discontinuity, that is, with the Walén relation, have been performed on 69 Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) low‐latitude (<30°) dayside (0800–1600 LT) high‐shear (>45°) MP crossings. It is found that in 61% of the crossings the observed flow changes agree with the prediction to better than 50%. No dependence of the occurrence of reconnection flows on local magnetic shear, local time/latitude, local tangential magnetosheath flow speed, and local magnetosheath Alfvén Mach number is found. We confirm an earlier result that the agreement with the Walén relation becomes worse with increasing magnetosheath plasma β (β is the ratio of plasma pressure to magnetic pressure) but find that the velocity change itself, predicted by the Walén relation, decreases with increasing β. Moreover, the motion and thickness of the boundary also depend on β: the higher the β value, the faster the speed and the smaller the thickness. These effects combine to make velocity changes in high β events more difficult to measure accurately, which may contribute to the poor agreement with the Walén relation in these events. The 42 events which exhibit plasma flows in reasonable agreement with the Walén relation include 21 cases where the flow direction is inconsistent with a single X line hinged at the subsolar point. The discrepancies between the former result arid dayside X line locations reported earlier may be due to a bias in selection of reconnection events in earlier studies. An average (dimensionless) reconnection rate that is substantially lower than 0.1 is inferred for the 42 events.
The influence of 10.7‐cm solar‐flux variations on midlatitude daytime ionospheric disturbance conditions
Authors: Bowman G. G.
Published: February 2004

For midlatitude regions, routinely recorded ionograms have been used to investigate whether or not the occurrence levels of (1) daytime first‐hop distortions, (2) daytime second‐hop spread, and (3) nighttime spread F are influenced by 10.7‐cm solar‐flux changes, because of associated upper atmosphere neutral‐particle density (UA‐NPD) changes. Recordings for a 5‐year period (1979–1983) were used. It was found that generally speaking, occurrence levels of these parameters were lower for high solar‐flux values and higher for low solar‐flux values as variations occurred from day to day. These are the results expected if the UA‐NPD changes have an influence on the wave amplitudes of the medium‐scale traveling ionospheric disturbances (MS‐TIDs) thought to be responsible for the recording of these parameters. This association was particularly well defined for the occurrence of the daytime second‐hop spread. It is suggested that non linear effects of the atmospheric gravity waves (AGWs) responsible for the MS‐TIDs may be responsible for generating the daytime small‐scale structures, which experimental results indicate are the ionospheric structures responsible for the second‐hop spread. These structures may possibly be field aligned.