bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic theory of geomagnetic pulsations: 2. Ion flux modulations by transverse waves
Authors: Chen Liu , Hasegawa Akira
Published: February 2004

Ion flux modulations by ultralow‐frequency radially polarized geomagnetic pulsations are examined theoretically based on the gyrokinetic analysis of Chen and Hasegawa (1991). The theoretical results thus contain important effects such as plasma anisotropy and inhomogeneities, finite Larmor radii, realistic magnetic field, magnetic trapping, and wave mode structures. The predicted properties are consistent with the satellite observations (Takahashi et al., 1990) and further support the drift‐Alfvén ballooning mode as a primary instability candidate. Our analysis, furthermore, demonstrates that, in the case of highly energetic ions, it is crucial to include the finite‐Larmor‐radius effects self‐consistently in order to properly analyze and compare with the satellite observations.
Evidence of electron impact ionization on the nightside of Venus from Pioneer Venus Orbiter Ion Mass Spectrometer measurements near solar minimum
Authors: Kar J. , Hartle R. E. , Grebowsky J. M. , Kasprzak W. T. , Donahue T. M. , Cloutier P. A.
Published: February 2004

Nightside ion composition measurements from the Pioneer Venus Orbiter ion mass spectrometer are compared for solar maximum conditions encountered during the early part of the mission and near solar minimum conditions that prevailed just prior to the end of the mission. A drastic decrease in the O+ ion abundance takes place as the solar activity level approaches solar minimum conditions. This indicates a greatly reduced flux of O+ ion transport across the terminator. At the same time, distinct O2+ layers were observed with peak densities only slightly reduced from solar maximum conditions. The relationship between the solar minimum O+ and O2+ profiles provides the first clear in situ evidence for a nightside ionosphere maintained primarily by electron impact, presumably penetrating solar wind electrons. Observation of excess production of mass 28 ions (over chemical production) provides further evidence of the presence of electron impact ionization. A modest change in He+ densities in the region of the predawn bulge does not have a counterpart in neutral helium. Absent a corresponding change in ionizing electron fluxes, this result indicates that during solar maximum the ionization of nightside helium is primarily due to transport of He+ while electron impact can account for He+ densities as solar minimum is approached. Further, the behavior of hydrogen ions over a solar cycle suggests that impact ionization was significant in producing H+ in 1992, but less than that for He+ ions in that year.
Empirical model of the Io plasma torus: Voyager measurements
Authors: Bagenal Fran
Published: February 2004

We present a description of plasma conditions in the Io plasma torus, between 5 and 10 RJ, based on Voyager 1 observations obtained in March 1979. The model includes updated analyses of Plasma Science (PLS) data obtained along the spacecraft trajectory as well as Ultraviolet Spectrometer (UVS) observations of composition made remotely from Jupiter. The plasma characteristics observed along the spacecraft trajectory have been extrapolated along magnetic field lines by numerically solving the equations of diffusive equilibrium to produce radial profiles of plasma properties at the centrifugal equator as well as maps of the densities of the major ionic species in a meridian plane. The diffusive equilibrium distribution of plasma along magnetic field lines depends mainly on T∥. Unfortunately, we only have measurements of T⊥ and must make assumptions about the thermal anisotropy of the plasma. We assume the thermal populations and the suprathermal electrons to be isotropic. The suprathermal ions have probably been recently picked‐up and are expected to be highly anisotropic. Varying the thermal anisotropy of the hot ions between A=T⊥/T∥=1 to 5 has a minor effect on the plasma maps but makes a significant difference to the fraction of hot ions in the plasma when integrated over a complete shell of magnetic flux. We have found that the vertical extrapolation of plasma density is insensitive to the geometry of different magnetic field models except inside 5 RJ (where the plasma scale height is comparable to uncertainties in the location of the centrifugal equator) and outside 8 RJ (where the magnetospheric current sheet significantly perturbs the magnetic field). The radial profile of flux tube content (N L²) exhibits the same “precipice”, “ledge,” and “ramp” features as previous studies as well as confirming small ‐ scale features which indicate local sources of plasma in the cold torus and near the orbit of Europa. The observations of O++ and molecular (SO2+ or S2+) ions inside 5.4 RJ, far from Io, in a region of cold dense plasma, remain difficult to explain, indicating either strong temporal variability in the Io plasma source or a strong source of plasma, possibly from the dissociation of dust, inside Io's orbit. Further evidence of a Europa source are the decrease in the ratio of sulfur to oxygen ions and the increase in plasma temperature outside 8 RJ.
Studies of the E region neutral wind in the quiet auroral ionosphere
Authors: Brekke Asgeir , Nozawa Satonori , Sparr Trygve
Published: February 2004

Quiet time auroral E region neutral wind data obtained by European Incoherent Scatter Radar (EISCAT) have been analyzed in order to establish the background mean neutral velocities as well as tidal influence on the wind. There is a rather strong and persistent mean eastward wind present in all seasons in the E region below 120 km. It is strongest in summer (∼60 m s−1), weak in fall and spring, and weaker in winter (∼20 m s−1). The zonal wind is westward above 120 km, in general stronger in winter (∼70 m s−1) and weaker in the other seasons (∼30 m s−1). The northward mean wind is rather small (∼10 m s−1). It is shifted gradually from being mainly southward in winter to becoming northward in summer. The mean vertical velocity is of the order of 5 m s−1 in the auroral E region. The amplitude of the diurnal tides of the vertical component have broad minima in the height region between 90 and 120 km. The horizontal diurnal tide is dominant in the upper E region, while the semidiurnal tide has a maximum at 110 km, in particular in the eastward component. The 8‐ and 6‐hour tides are rather constant in amplitude by height. The results obtained are in good agreement with other results published on the basis of incoherent scatter radar measurements. The comparison with theoretical models underlines the need for more modeling of the auroral E region neutral tides including the ionized plasma as well as electric fields and mean background winds. Theoretical studies of the vertical motion with respect to the influence of tides are especially lacking in this important region.
Thin current sheets in the magnetotail during substorms: CDAW 6 revisited
Authors: Pulkkinen T. I. , Baker D. N. , Mitchell D. G. , McPherron R. L. , Huang C. Y. , Frank L. A.
Published: February 2004

The global magnetic field configuration during the growth phase of the CDAW 6 substorm (March 22, 1979, 1054 UT) is modeled using data from two suitably located spacecraft and temporally evolving variations of the Tsyganenko magnetic field model. These results are compared with a local calculation of the current sheet location and thickness carried out by McPherron et al. (1987) and Sanny et al. (this issue). Both models suggest that during the growth phase the current sheet rotated away from its nominal location, and simultaneously thinned strongly. The locations and thicknesses obtained from the two models are in good agreement. The global model suggests that the peak current density is ∼120 nA/m2, and that the cross‐tail current almost doubled its intensity during this very strong growth phase. The global model predicts a field configuration that is sufficiently stretched to scatter thermal electrons, which may be conducive to the onset of ion tearing in the tail. The electron plasma data further support this scenario, as the anisotropy present in the low‐energy electrons disappears close to the substorm onset. The electron contribution to the intensifying current in this case is of the order of 10% before the isotropization of the distribution.
Turbulence and transport in the magnetopause current layer
Authors: Drake J. F. , Gerber J. , Kleva R. G.
Published: February 2004

The current convective instability is identified as the dominant mechanism for producing turbulence and transport in the magnetopause current layer. A linear stability analysis demonstrates that the instability should be robustly unstable for the parameters of the magnetopause current layer. Three‐dimensional nonlinear simulations indicate that the current layer evolves to a strongly turbulent state. The current convective instability is driven unstable by the cross‐field gradient of the parallel current in collisionless plasma. A simple physical picture of the instability is presented. For narrow current layers with scale length L less than an ion Larmor radius ρi, the growth rates exceed the ion gyrofrequency even when Ti ≫ Te. A set of nonlinear fluid equations are derived in the unmagnetized ion limit ρi > L. In three‐dimensional simulations based on these equations the current layer evolves to a turbulent state with layers of extended but irregular clifflike structures with transverse scale length of the order of ρes, the electron Larmor radius based on Ti. The anomalous cross‐field transport rate D⊥ of current resulting from this instability is given by D⊥ ∼ ρes υy with υy the electron parallel streaming velocity. The present linear theory and simulations are limited to electrostatic disturbances. The implications of these results for understanding the structure of the magnetopause current layer are discussed.
Source regions of long‐period pulsation events in electron precipitation and magnetic fields at South Pole Station
Authors: Paquette J. A. , Matthews D. L. , Rosenberg T. J. , Lanzerotti L. J. , Inan U. S.
Published: February 2004

Pulsation events with long (100‐1000 s) periods with a consistent frequency in both particle precipitation and surface geomagnetic field variations have been reported in the past from measurements made at various geomagnetic latitudes. An examination of broad beam riometer and magnetometer data from South Pole Station for the interval from 1982 to 1989 revealed nearly 200 such events. The onset times of these events were determined, and the results compared with predictions based on the work of Coroniti and Kennel (1970). This mechanism ascribes the occurrence of correlated magnetic and precipitation pulsations to ULF modulation of equatorial VLF wave‐particle interactions. For this reason, VLF data from South Pole Station were also examined. Taking into consideration the ULF wave and particle transit times from an interaction region near the magnetic equator to the ground leads to an expectation that the onset of pulsations in the magnetometer data will lag the onset of pulsations in the riometer data by several minutes. This disparity in onset times, together with modulation of VLF emissions in the 0.5‐1 kHz band, serves as an important indicator of whether or not an event can be explained by the above‐cited theory. While about a third of the events fit the prediction of Coroniti and Kennel, another third do not. In these events, the onset of magnetic and precipitation pulsations is nearly simultaneous, and possible alternative generation mechanisms are explored. In the remaining third of the events, magnetic pulsations begin substantially earlier than precipitation pulsations. Events of this type appear at first to be inexplicable in terms of any transit time argument. However, data from the imaging riometer at South Pole Station indicate that this third class of events is probably not physically distinct from the first two but is the result of the differing areas to which the riometer and magnetometer are sensitive and can be accounted for by considering the effects of transverse motion of a persistent precipitation region.
Interhemispheric asymmetry of the high‐latitude ionospheric convection pattern
Authors: Lu G. , Richmond A. D. , Emery B. A. , Reiff P. H. , de la Beaujardière O. , Rich F. J. , Denig W. F. , Kroehl H. W. , Lyons L. R. , Ruohoniemi J. M. , Friis‐Christensen E. , Opgenoorth H. , Persson M. A. L. , Lepping R. P. , Rodger A. S. , Hughes T. , McEwin A. , Dennis S. , Morris R. , Burns G. , Tomlinson L.
Published: February 2004

The assimilative mapping of ionospheric electrodynamics technique has been used to derive the large‐scale high‐latitude ionospheric convection patterns simultaneously in both northern and southern hemispheres during the period of January 27‐29, 1992. When the interplanetary magnetic field (IMF) Bz component is negative, the convection patterns in the southern hemisphere are basically the mirror images of those in the northern hemisphere. The total cross‐polar‐cap potential drops in the two hemispheres are similar. When Bz is positive and |By| > Bz, the convection configurations are mainly determined by By and they may appear as normal “two‐cell” patterns in both hemispheres much as one would expect under southward IMF conditions. However, there is a significant difference in the cross‐polar‐cap potential drop between the two hemispheres, with the potential drop in the southern (summer) hemisphere over 50% larger than that in the northern (winter) hemisphere. As the ratio of |By|/Bz decreases (less than one), the convection configuration in the two hemispheres may be significantly different, with reverse convection in the southern hemisphere and weak but disturbed convection in the northern hemisphere. By comparing the convection patterns with the corresponding spectrograms of precipitating particles, we interpret the convection patterns in terms of the concept of merging cells, lobe cells, and viscous cells. Estimates of the “merging cell” potential drops, that is, the potential ascribed to the opening of the dayside field lines, are usually comparable between the two hemispheres, as they should be. The “lobe cell” provides a potential between 8.5 and 26 k V and can differ greatly between hemispheres, as predicted. Lobe cells can be significant even for southward IMF, if |By| > |Bz|. To estimate the potential drop of the “viscous cells,” we assume that the low‐latitude boundary layer is on closed field lines. We find that this potential drop varies from case to case, with a typical value of 10 kV. If the source of these cells is truly a viscous interaction at the flank of the magnetopause, the process is likely spatially and temporally varying rather than steady state.
A comparison of energetic ions in the plasma depletion layer and the quasi‐parallel magnetosheath
Authors: Fuselier Stephen A.
Published: February 2004

Energetic ion spectra measured by the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) downstream from the Earth's quasi‐parallel bow shock (in the quasi‐parallel magnetosheath) and in the plasma depletion layer are compared. In the latter region, energetic ions are from a single source, leakage of magnetospheric ions across the magnetopause and into the plasma depletion layer. In the former region, both the magnetospheric source and shock acceleration of the thermal solar wind population at the quasi‐parallel shock can contribute to the energetic ion spectra. The relative strengths of these two energetic ion sources are determined through the comparison of spectra from the two regions. It is found that magnetospheric leakage can provide an upper limit of 35% of the total energetic H+ population in the quasi‐parallel magnetosheath near the magnetopause in the energy range from ∼10 to ∼80 keV/e and substantially less than this limit for the energetic He2+ population. The rest of the energetic H+ population and nearly all of the energetic He2+ population are accelerated out of the thermal solar wind population through shock acceleration processes. By comparing the energetic and thermal He2+ and H+ populations in the quasi‐parallel magnetosheath, it is found that the quasi‐parallel bow shock is 2‐3 times more efficient at accelerating He2+ than H+. This result is consistent with previous estimates from shock acceleration theory and simulations.
Theoretical calculations of ion acceleration in the vicinity of comet Giacobini‐Zinner
Authors: McKenzie M. L. , Cravens T. E. , Ye G.
Published: February 2004

Ionization of cometary neutral molecules produces ions which are picked up by the solar wind. The cometary ion pickup process for comet Giacobini‐Zinner is studied in two ways: (1) with a test particle method in which trajectories are numerically calculated for several thousand ions whose initial locations were chosen randomly with probability proportional to the neutral density and (2) with a quasi‐linear diffusion model. The cometary ion distribution function was calculated with the test particle model at several locations upstream of the bow shock and for several types of magnetic fluctuations (or waves). These waves were allowed to propagate in both directions along the magnetic field at the Alfven speed. Both pitch angle scattering and energy diffusion are evident in the derived ion distributions. The monochromatic waves result in less ion acceleration than turbulent fluctuations with about the same amplitude. The calculated ion distribution functions are in reasonable agreement with the distributions measured in the vicinity of comet Giacobini‐Zinner in 1985 by particle detectors on the ICE spacecraft when the ratio of power in sunward propagating Alfven waves to the power in antisunward propagating waves is assumed to lie between about 20% and 50%. However, the quasi‐linear diffusion model results agree best with the measured distribution functions when the power ratio is only about 3%.
Influence of plasma sheet thickening toward the tail flanks on magnetotail stability and dynamics
Authors: Birn Joachim , Hesse Michael
Published: February 2004

On the basis of resistive MHD simulations we investigate the stability and dynamic evolution of the magnetotail for configurations with various degrees of plasma sheet thickening from midnight toward the tail flanks, associated with a similar increase of the Bz component. This increase is varied by factors between 2 and 3.5, while the magnitude of Bz at midnight is left unchanged. The increase of Bz has a strong effect on the spatial extent of resistive tearing and reconnection across the tail and on the unstable evolution. For stronger thickening the cross‐tail extent of the near‐Earth neutral line, formed by reconnection, and correspondingly the amount of reconnected magnetic flux get reduced and the dipolarization effects earthward of the neutral line are more concentrated near midnight. An increase of Bz from midnight toward the flanks by more than a factor of about 3 can possibly even suppress the resistive tearing instability. This indicates the possibility of a stability transition due to a reduction of Bz in the flank regions in response to changes in the solar wind, even when the magnitude of Bz near midnight and the fluctuation level in that region are unchanged (provided that a sufficient fluctuation level exists).
Speeds of coronal mass ejections: SMM observations from 1980 and 1984‐1989
Authors: Hundhausen A. J. , Burkepile J. T. , St. Cyr O. C.
Published: February 2004

The speeds of 936 features in 673 coronal mass ejections have been determined from trajectories observed with the Solar Maximum Mission (SMM) coronagraph in 1980 and 1984‐1989. The distribution of observed speeds has a range (from 5th to 95th percentile) of 35 to 911 km s−1; the average and median speeds are 349 and 285 km s−1. The speed distributions of some selected classes of mass ejections are significantly different. For example, the speeds of 331 “outer loops” range from 80 to 1042 km s−1; the average and median speeds for this class of ejections are 445 and 372 km s−1. The speed distributions from each year of SMM observations show significant changes, with the annual average speeds varying from 157 (1984) to 458 km s−1 (1985). These variations are not simply related to the solar activity cycle; the annual averages from years near the sunspot maxima and minimum are not significantly different. The widths, latitudes, and speeds of mass ejections determined from the SMM observations are only weakly correlated. In particular, mass ejection speeds vary only slightly with the heliographic latitudes of the ejection. High‐latitude ejections, which occur well poleward of the active latitudes, have speeds similar to active latitude ejections.
Middle and upper atmosphere radar observations of ionospheric density gradients produced by gravity wave packets
Authors: Oliver W. L. , Fukao S. , Yamamoto Y. , Takami T. , Yamanaka M. D. , Yamamoto M. , Nakamura T. , Tsuda T.
Published: February 2004

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere (MU) radar, we have been able to track the passage of gravity waves and measure the horizontal electron density gradients that they produce. Our previous work on this topic, on one day of observations, verified that the gradient structure and fluctuations were consistent with a gravity wave source. The present work (1) looks at several days of observation to confirm the routine existence of these gradient structures; (2) identifies individual wave packets; (3) finds evidence that long trains of gravity wave fluctuations are composed of several juxtaposed wave packets of one or two cycles each rather than a single wave of many cycles; (4) finds that during quiet periods strong waves are present only during the daytime, but during disturbed periods they are present day and night; and (5) notes several other characterizations.
Simultaneous observation of a traveling vortex structure in the morning sector and a field line resonance in the postnoon sector
Authors: McDiarmid D. R. , Yeoman T. K. , Grant I. F. , Allan W.
Published: February 2004

Radar observations suggesting ULF field line resonances impulsively excited via a cavity or waveguide mode were obtained simultaneously in the dawn (Bistatic Auroral Radar System (BARS)) and postnoon (Sweden and Britain Radar‐Auroral Experiment (SABRE)) sectors. The primary data used to study this event are from both the European Incoherent Scatter magnetometer cross and Subauroral Magnetometer Network magnetometer arrays in the postnoon sector and from the Canadian Auroral Network for the Origin of Plasmas in the Earth's Neighborhood Program Unified Study (CANOPUS), MAgnetometer, RIometer and tellurics Array (MARIA), the GOES 7 magnetometer, and the neural network classification of Defense Meteorological Satellite Program (DMSP) particle data in the dawn sector. In the postnoon sector (Scandinavia) a decaying field line resonance was observed following a more complicated initial interval. The resonance period was 326 s (3.07 mHz). However, in the dawn sector the response was dominated by the initial interval in which the characteristics of a traveling vortex system were observed. The subsequent disturbance in this sector was of lesser amplitude and without discerned pattern. The field‐aligned current(s) of the traveling vortex structure was (were) at about 65° Eccentric Dipole Field Line (EDFL) latitude (approximately 68° International Geomagnetic Reference Field (IGRF)). The DMSP neural network data, however, indicate this position to have been well within the magnetosphere. GOES 7 was on a field line which was connected to the ionosphere near the MARIA meridian line at about 62.5° EDFL latitude (approximately 65.5° IGRF), just equatorward of the traveling vortex structure. It observed a magnetic disturbance which was initially driven and followed directly by a damped sinusoid signature of the azimuthal component. The frequency during this decay was 4.45 mHz (225‐s period). Values of ∑p, the height‐integrated Pedersen conductivity for both the postnoon and GOES 7 damped segments were deduced. These were low, particularly if the disturbances were in the second harmonic along the field line as suggested by GOES 7 data, and perhaps imply an additional loss mechanism. Although substantially understood, this pulsation event had distinctly different characteristics in the two time sectors and displayed many puzzling features. In particular, it was significantly more complicated than our initial anticipation of global excitation of field line resonances via a cavity or waveguide mode.
Excitation of high‐frequency electromagnetic waves by energetic electrons with a loss cone distribution in a field‐aligned potential drop
Authors: Fung Shing F. , Viñas Adolfo F.
Published: February 2004

The electron cyclotron maser instability (CMI) driven by momentum space anisotropy, ∂f/∂p⊥ > 0, has been invoked to explain many aspects, such as the modes of propagation, harmonic emissions, and the source characteristics of the auroral kilometric radiation (AKR). Recent satellite observations of AKR sources indicate that the source regions are often imbedded within the auroral acceleration region characterized by the presence of a field‐aligned potential drop. In this paper we investigate the excitation of the fundamental extraordinary mode radiation due to the accelerated electrons. The momentum space distribution of these energetic electrons is modeled by a realistic upward loss cone as modified by the presence of a parallel potential drop below the observation point. On the basis of linear growth rate calculations we present the emission characteristics, such as the frequency spectrum and the emission angular distribution as functions of the plasma parameters. We will discuss the implication of our results on the generation of the AKR from the edges of the auroral density cavities.
Observations of ionosphere heating in the TSS‐1 subsatellite presheath
Authors: Katz Ira , Melchioni Enrico , Mandell Myron , Oberhardt Marilyn , Thompson Don , Neubert Torsten , Gilchrist Brian , Bonifazi Carlo
Published: February 2004

The first flight of the Tethered Satellite System (TSS‐1) was to investigate the mechanical and electrical dynamics of a conducting satellite deployed from the orbiter via a tether whose core was a conducting wire [Dobrowolny and Melchioni, 1993; Dobrowolny, 1987; Dobrowolny and Stone, 1994). In the TSS‐1 system the satellite deployed from the orbiter radially away from the Earth. The relative motion between the tether and Earth's magnetic field generated an electromotive force (EMF) that is the product of orbiter velocity, Earth's magnetic field, and the length of the deployed tether. This EMF drove a current through the tether. Electrons were collected on the satellite's electrically conductive skin and traveled through the tether to the orbiter, where they either went to orbiter structural ground or were emitted into the ionosphere via active electron emission. During TSS‐1 this electron emission was accomplished mainly by the 100 mA, 1‐keV fast pulsed electron gun (FPEG) of the shuttle electrodynamic tether system (SETS) (Williamson et al., 1988; Banks et al., 1994; V. M. Aguero et al., manuscript in preparation, 1994]. The FPEG electron emission was much higher than either ambient ion collection at the orbiter end or electron collection at the satellite. Potentials of the orbiter with respect to the ambient plasma were obtained from measurements from the Shuttle Potential and Return Electron Experiment (SPREE) (Oberhardt et al., 1993a, b, 1994), the SETS tether current voltage monitor (Thompson et al., 1993), and the Agenzia Spaziale Italiana deployer and satellite core equipment (Bonifazi et al., 1988, 1994). Despite the limited tether deployment length of 268 m the TSS‐1 system proved capable, during certain events, of generating satellite potentials sufficient to illuminate a previously unexplored aspect of plasma physics: that of an ion repelling, electron attracting, moving probe in a magnetoplasma. During such events the satellite boom‐mounted Langmuir probe flown as part of the Research on Electrodynamic Tethers experiment (Dobrowolny et al., 1994) measured an increase in the electron plasma temperature in the quasi‐neutral ionospheric region beyond the satellite sheath. This observed heating of the presheath electrons was distinctly different from the acceleration of the electrons in the sheath, which was also observed when the sheath expanded such that the probe was completely in the sheath. We show that the observed elevated electron temperatures are consistent with the formation of a Bohm stable electron collecting sheath.
Theory of ballooning‐mirror instabilities for anisotropic pressure plasmas in the magnetosphere
Authors: Cheng C. Z. , Qian Q.
Published: February 2004

A kinetic‐MHD eigenmode analysis of ballooning‐mirror instabilities is performed for anisotropic pressure plasmas in the magnetosphere. The energetic particle kinetic effects and the coupling between the ballooning and mirror modes are taken into account. Without energetic trapped particle kinetic effects the ballooning‐mirror modes with symmetric field‐aligned structure of parallel perturbed magnetic field δB∥ and electrostatic potential Φ have lower β instability threshold than the antisymmetric modes. Pressure anisotropy with (P⊥/P∥ > 1) reduces the β threshold for ballooning‐mirror instabilities. In the limit that the wave frequency is smaller than the energetic trapped particle magnetic drift frequency, the symmetric ballooning‐mirror mode is completely stabilized by the energetic trapped particle kinetic effects. However, the antisymmetric ballooning‐mirror mode is only weakly influenced by the energetic trapped particle kinetic effects and has the lowest β threshold. For symmetric modes the energetic trapped particles experience a bounce‐averaged wave structure due to their rapid bounce motion, and their nonadiabatic kinetic pressure response cancels with their fluid pressure response so that they do not contribute to the mode stability. Physically, the energetic trapped particles precess very rapidly across the
field, and their motion becomes very rigid with respect to low‐frequency symmetric MHD perturbations. For antisymmetric modes the energetic trapped particle kinetic pressure response from the northern hemisphere cancels with that from the southern hemisphere in a bounce period, and thus the instability β thresholds is mainly determined by the energetic particle fluid free energy. The field‐aligned perturbed magnetic field structure of the antisymmetric mode changes from a ballooning mode with dominant transverse magnetic field components at P⊥/P∥ = 1 to a mixed ballooning‐mirror mode with comparable transverse and compressional components near the equator as P⊥/P∥ increase. With large equatorial plasma β (β∥ ≥ O(1)) and pressure anisotropy (P⊥/P∥ > 1) the field‐aligned wave structure of antisymmetric ballooning‐mirror mode resembles the multisatellite observation of a long lasting compressional Pc 5 wave event during November 14‐15, 1979 [Takahashi et al., 1987].
Hybrid simulations of collisionless reconnection in current sheets
Authors: Hesse Michael , Winske Dan
Published: February 2004

Thin current sheets, with thicknesses of the order of a few ion gyroradii, are believed to exist quite commonly at the Earth's magnetopause as well as in the magnetotail current sheet just prior to substorm onset. Analytical theory predicts that sheets of this kind can become the locus of an ion‐tearing instability. We use a modified hybrid simulation code that treats the ions fully kinetically and the electrons as a fluid with a model for the full pressure tensor to study the ion tearing mode development in one‐dimensional current sheets of various thicknesses, with varying initial electron to ion temperature ratios. Further, we investigate the electron anisotropy dependence of the ion tearing mode growth rate and compare the growth rates found in the numerical simulations to linear growth rates for the pure ion‐tearing instability derived from analytic theory. Estimates of the numerical accuracy of the simulations and of possible electron distribution functions resulting in the modeled anisotropies are also presented.
Properties of the fluctuating magnetic helicity in the inertial and dissipation ranges of solar wind turbulence
Authors: Goldstein M. L. , Roberts D. A. , Fitch C. A.
Published: February 2004

We investigated the inertial and dissipation ranges of the reduced magnetic helicity spectrum of solar wind fluctuations and have found that this spectrum appears insensitive to solar cycle variations and changes in solar wind flow parameters. In the inertial range of the spectrum, the reduced helicity is large but random and independent of heliocentric distance between 0.3 and 10 AU. At small scales, in the dissipation range of the spectrum, a correlation appears to exist between the average value of the normalized reduced magnetic helicity and the polarity of magnetic sectors, suggesting that these fluctuations, if outward propagating, are predominantly right‐hand polarized. In the inertial range the statistical properties of the normalized magnetic helicity are well approximated by a simple model of the magnetic field in which the total magnetic field vector randomly walks with only small variations in magnitude. The behavior of the inertial range spectrum is very similar to that seen in three‐ and two‐and‐a‐half‐dimensional simulations of the incompressible and compressible equations describing magnetohydrodynamic turbulence, consistent with the paradigm that the solar wind is a turbulent magnetofluid.
Ultraviolet plume instrument imaging from the LACE satellite: Analysis of broadband UV auroral limb data
Authors: Evans J. S. , Strickland D. J. , Anderson D. E. , Conway R. R. , Smathers H. W. , Bakeris D. F. , Swanson R. A. , Chubb T. A. , Cardon J. G.
Published: February 2004

Southern auroral limb images have been recorded by the wide‐angle UV camera of the Ultraviolet plume instrument on board the Low‐power Atmospheric Compensation Experiment satellite. Tangent altitudes within each image have been determined using recorded stars along with the known position of the satellite. Range to the observed aurora has been determined by stereoscopic techniques using images of the same auroral forms collected over several seconds. An auroral model has been used in conjunction with simulated annealing techniques to fit two limb profiles taken from a single image. The auroral model provides altitude profiles of volume emission for the relevant features based on electron transport calculations starting with a user‐specified precipitating electron spectrum. Volume emission is then distributed within the observing region, followed by line‐of‐sight integration producing limb profiles. The fitting procedure provides distributions of the energy characteristics of the aurora over the relevant observing region. The results of the fitting process suggest that a quantitative description of the aurora has been achieved in terms of the most probable precipitating electron spectral characteristics and the spatial distribution of this precipitation.
Ionospheric ion velocity distributions and associated transport properties in the presence of auroral electric field gradients
Authors: St. ‐Maurice J. ‐P. , Winkler E. , Hamza A. M.
Published: February 2004

We have studied the response of the ionospheric F region to intense horizontal gradients in the convection electric field, using a kinetic theory of the ion gas. In the boundary regions where the electric field changes rapidly in space, we have found that the local ion velocity distribution can become markedly asymmetric in a direction that is perpendicular to that of the magnetic field. The degree of asymmetry depends on the local electric field and on the magnitude of the shears. There is also a marked contrast between situations for which the electric field increases in its own direction as opposed to when it decreases along that direction. An integral part of our kinetic solution is the presence of a time‐dependent ion density, associated with a change in the net charge and therefore with the divergence of the electric field. This is a result of current continuity requirements for the problem at hand. A more unexpected result is that the mean ion drift differs markedly from the local value of the E × B drift as well. This behavior could lead to strong Hall currents in the regions of convection shears and cause a Farley‐Buneman type of instability at F region heights. Other findings include the fact that the ion temperature varies markedly from one situation to another and is strongly three‐dimensionally anisotropic. Finally, strong ion heat flows can be induced in the E × B direction even though the plasma is strongly magnetized.
Observation of anomalous slow‐mode shock and reconnection layer in the dayside magnetopause
Authors: Walthour D. W. , Gosling J. T. , Sonnerup B. U. Ö. , Russell C. T.
Published: February 2004

Plasma and magnetic field data from the ISEE 2 spacecraft recorded during an outbound crossing of the dayside, northern hemisphere magnetopause on October 29, 1979, provide evidence for a slow shock (SS) in the observed reconnection layer. This layer is found to be bounded on the magnetosheath side by the SS; near the magnetospheric side of the layer, a second current sheet is found that may have been a rotational discontinuity (RD). The direction of the accelerated plasma flow, the earthward sense of the normal magnetic field across the SS and RD, and the relative orientation of the SS and the RD all indicate that the reconnection site was located south of the spacecraft. Quantitative tests show that, allowing for experimental uncertainties, data taken upstream and downstream of the SS are consistent with coplanarity and other Rankine‐Hugoniot (RH) conditions. Examination of the flow parameters indicates two anomalous properties of the SS: the upstream flow, viewed in the deHoffmann‐Teller frame, is superalfvénic and the downstream plasma is firehose unstable. In comparison to the long‐wavelength slow‐mode phase speed, however, the flow in the upstream region is super slow, while in the downstream region it is subslow, as required for a slow‐mode shock. Further properties of the shock include a large decrease in total enthalpy across it, indicating the escape of a sizable heat flux from the shock structure, and the occurrence of a polarization reversal of the tangential magnetic field within the shock layer, a feature that is predicted by linear double‐polytropic Hall‐MHD and results from a large increase in pressure anisotropy from the upstream region, where p∥ ≅ p⊥, to the downstream region, where p∥ > p⊥. Quantitative tests of the RD‐like discontinuity show that it satisfies the necessary RH conditions within experimental uncertainties and indicate that the flow across it is earthward, that is, it is the same as for the SS. Thus the RD is propagating behind the SS in the direction away from the Earth. This reversal in the order of the discontinuities is attributed to the reversal of the slow and intermediate phase speeds produced by the pressure anisotropy in the region between the RD and the SS. The linear dispersion relation and polarization properties of dispersive MHD waves in an anisotropic plasma are examined in the appendix.
Effect of By on neutral line ridges and dynamical source ordering
Authors: Martin R. F. , Speiser T. W. , Klamczynski K.
Published: February 2004

In this paper we study the effect of a uniform magnetic field By in the cross‐tail y direction on the dynamics and distribution function of energetic ions in a current sheet model including a neutral line. Martin and Speiser (1988) have previously shown that a “ridge” in the velocity space distribution function is a remote neutral line signature in a two‐dimensional field without By. Our results show that as By is increased, using nominal tail parameters, there is very little change in the ridge signature for small By (up to about 4 times Bz). For intermediate values (up to about 10 times Bz) the ridge becomes observable further from the neutral line, while close to the X line the ridge is strongly modified. For large By (of the order of Bx) the ridge evolves into new neutral line signatures which depend strongly on whether the observation point is above or below the current sheet. These results are used to estimate By < 2 nT in the AMPTE/IRM event modeled by Speiser and Martin (1994), which is consistent with onboard magnetometer measurements. We further show that the cause of the new structures appearing at large By is the same source separation effect that produced the original ridge and that all these signatures are relatively insensitive to a Small parallel electric field. Finally, we point out a dynamical ordering which occurs at large By: Using high‐resolution simulations, we find a common boundary separating regions of positive and negative initial x, y, and z position, as well as initial pitch angle. This boundary is purely dynamical in nature and is independent of modeled source distributions.
Elliptically polarized Jovian decametric radiation: An investigation of the electron cyclotron maser mechanism
Authors: Willes A. J. , Melrose D. B. , Robinson P. A.
Published: February 2004

A model for the elliptical polarization of Jovian decametric radiation is presented, based on the electron cyclotron mechanism. The aim is to determine whether the observed elliptical polarization is consistent with the radiation being generated by mildly relativistic electrons streaming along converging magnetic field lines. The growth rate for electron cyclotron maser emission is considered, assuming a drifting DGH distribution function for the streaming electrons. Constraints on the allowable parameters of this distribution function are made by the observed polarization, timescale, bandwidth, and angular range of the radiation.
Interplanetary magnetic field orientation for transient events in the outer magnetosphere
Authors: Sibeck D. G. , Newell P. T.
Published: February 2004

It is generally believed that flux transfer events (FTEs) in the outer dayside magnetosphere, usually identified by transient (∼1 min) bipolar magnetic field perturbations in the direction normal to the nominal magnetopause, occur when the magnetosheath magnetic field has a southward component. We compare the results of three methods for determining the magnetosheath magnetic field orientation at the times of previously identified UKS/IRM events: (1) the average magnetosheath magnetic field orientation in the 30‐min period adjacent to the nearest magnetopause crossing, (2) the magnetosheath magnetic field orientation observed just outside the magnetopause, and (3) the lagged interplanetary magnetic field (IMF) orientation at the time of the transient events. Whereas the results of method 2 indicate that the events tend to occur for a southward magnetosheath magnetic field, the results of methods 1 and 3 show no such tendency. The fact that the three methods yield significantly different results emphasizes the need for caution in future studies.
Quasi‐linear pitch angle diffusion coefficients: Retaining high harmonics
Authors: Albert J. M.
Published: February 2004

Diffusion coefficients for electrons pitch angle scattered by plasmaspheric hiss are reconsidered, as given by quasi‐linear theory, overcoming practical problems in keeping high cyclotron harmonics n. Upper bounds are used to identify ranges of latitude, n, and wave‐normal angle which give only negligible contributions, so that the detailed computation of these values may be avoided, resulting in large savings of computational effort. A simple way of obtaining precipitation lifetimes from the diffusion coefficients is also given, using a shooting method. The lifetimes and steady state fluxes are computed, leading to shorter lifetimes and lower fluxes than previously reported, for energies above 500 keV and McIlwain parameter L greater than 3.
Analytical model for the density distribution in the Io plasma torus
Authors: Mei Yi , Thorne Richard M. , Bagenal Fran
Published: February 2004

An analytical model is developed for the diffusive equilibrium plasma density distribution in the Io plasma torus. The model has been employed successfully to follow the ray path of plasma waves in the multi‐ion Jovian magnetosphere; it would also be valuable for other studies of the Io torus that require a smooth and continuous description of the plasma density and its gradients. Validity of the analytical treatment requires that the temperature of thermal electrons be much lower than the ion temperature and that suprathermal electrons be much less abundant than the thermal electrons; these two conditions are satisfied in the warm outer region of the Io torus from L = 6 to L = 10. The analytical solutions agree well with exact numerical calculations for the most dense portion of the Io torus within 30° of the equator.
Boundary layer polarization and voltage in the 14 MLT region
Authors: Lundin R. , Yamauchi M. , Woch J. , Marklund G.
Published: February 2004

Viking midaltitude observations of ions and electrons in the postnoon auroral region show that field‐aligned acceleration of electrons and ions with energies up to a few kiloelectron volts takes place. The characteristics of the upgoing ion beams and the local transverse electric field observed by Viking indicate that parallel ion acceleration is primarily due to a quasi‐electrostatic field‐aligned acceleration process below Viking altitudes, i.e., below 10,000–13,500 km. A good correlation is found between the maximum upgoing ion beam energy and the depth of the local potential well determined by the Viking electric field experiment within dayside “ion inverted Vs.” The total transverse potential throughout the entire region near the ion inverted Vs is generally much higher than the field‐aligned potential and may reach well above 10 kV. However, the detailed mapping of the transverse potential out to the boundary layer, a fundamental issue which remains controversial, was not attempted here. An important finding in this study is the strong correlation between the maximum upgoing ion beam energy of dayside ion inverted Vs and the solar wind velocity. This suggests a direct coupling of the solar wind plasma dynamo/voltage generator to the region of field‐aligned particle acceleration. The fact that the center of dayside ion inverted Vs coincide with convection reversals/flow stagnation and upward Birkeland currents on what appears to be closed field lines (Woch et al., 1993), suggests that field‐aligned potential structures connect to the inner part of an MHD dynamo in the low‐latitude boundary layer. Thus the Viking observations substantiate the idea of a solar wind induced boundary layer polarization where negatively charged perturbations in the postnoon sector persistently develops along the magnetic field lines, establishing accelerating potential drops along the geomagnetic field lines in the 0.5–10 kV range.