bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical survey of auroral latitude Pc 5 spectral and polarization characteristics
Authors: Ziesolleck C. W. S. , McDiarmid D. R.
Published: February 2004

Recent magnetometer and HF radar observations have indicated the existence of multiple, discrete field line resonances with remarkably stable frequencies of 1.3, 1.9, 2.6, and 3.4 mHz, apparently driven by MHD waveguide/cavity modes in the magnetosphere. Given the dynamic nature of the magnetosphere, the apparent stability of these frequencies is difficult to understand, and it is therefore important to examine this question on the grounds of a larger database. In this paper we present a statistical survey of the occurrence of Pc 5 field line resonance frequencies based on 1 year of data from several stations of the CANOPUS magnetometer array. Pure state filtering techniques were used to investigate various statistical aspects of auroral latitude Pc 5 characteristics, with emphasis on the occurrence of preferential, discrete spectral peaks. A number of case studies exploiting the full spatial extent of the CANOPUS array were carried out to complement the statistics. Although the results provide further support for the existence of discrete (spatially monochromatic) field line resonances, we find that the apparently stable frequencies do not seem to be particularly distinguished from other Pc 5 frequencies, except perhaps the 1.9‐mHz peak. Discrete field line resonance characteristics were also observed for other sets of frequencies, and we therefore conclude that the apparently stable waveguide/cavity mode frequencies do not necessarily represent a unique set. The implications of these results with respect to various possible Pc 5 wave generation mechanisms are discussed.
Interaction between two shock waves in a collisionless plasma: Description of the behavior of the heliospheric termination shock
Authors: Ziemkiewicz J.
Published: February 2004

A shock in a collisionless plasma should be treated as a place where the superthermal particles are continuously produced. The Rankine‐Hugoniot conditions modified so as to describe the loss of the energy flux during the production process are presented. The model of the interaction between the two colliding shocks in the collisionless magnetized plasma is used to describe the behavior of the heliospheric termination shock. Both forward‐reverse and reverse‐reverse shock pairs are considered. In consequence of these interactions the termination shock is in a constant in and out motion with velocities equal to 100–200 km/s. The dependence of the interaction parameters on the production efficiency of the superthermal population is discussed.
Cosmic electron spectra 1987–1994
Authors: Evenson Paul , Huber David , Patterson Evelyn Tuska , Esposito Joseph , Clements David , Clem John
Published: February 2004

We report cosmic electron spectra obtained on balloon flights in August of the years 1987, 1990, 1992, and 1994 in an extension of the series of observations with the same instrument begun in 1968. Electron fluxes at approximately 6 GeV have returned to levels seen in the decade of the 1980s in spite of the reversal of solar magnetic polarity in 1990, which in some models should have produced a return to the much lower fluxes observed in the 1970s. We know of no satisfactory theoretical interpretation of these results.
Role of magnetic reconnection in Venus ionopause activity
Authors: Dreher J. , Birk G. T. , Schindler K. , Otto A.
Published: February 2004

The role of magnetic reconnection in the dynamics of the Venus dayside ionopause is investigated. The dispersion relation of the resistive tearing mode for the asymmetric ionopause equilibrium is derived and solved. It is shown that typical signatures of this mode correspond to helical magnetic flux tubes (flux ropes) as observed by Pioneer Venus Orbiter. Nonlinear numerical MHD‐simulations were carried out in addition to the analytical investigations. The results of these simulations support our view that magnetic reconnection plays an essential role in the dynamic interaction of the solar wind with the Venus ionosphere.
Nonsteady state ionosphere‐plasmasphere coupling of superthermal electrons
Authors: Khazanov G. V. , Liemohn M. W.
Published: February 2004

Numerical solutions to the nonsteady state kinetic equation which describes the transport of superthermal electrons in the ionosphere and plasmasphere between the magnetically conjugate regions of the ionosphere are presented. The distribution function in time, distance along arbitrary geomagnetic field lines, energy, and pitch angle are among the parameters calculated by the model. This model represents a unified approach by self‐consistently coupling the interaction of the two hemispheres and the trapping of superthermal electrons in the plasmasphere. Our calculations take into account the various ionization and excitation processes and the effect of an inhomogeneous magnetic field (i.e., magnetic mirroring of precipitating electrons and focusing of escaping electrons along magnetic field lines). Omnidirectional flux spectra and pitch angle distributions are shown for various L shells and situations, and the features are described in detail. Nonsteady state calculations predict that a depleted flux tube can take several hours to reach steady state levels again. Plasmaspheric transparencies are calculated for different conditions of illumination, scattering processes in the conjugate ionospheres, and field‐aligned gradients of the thermal plasma density. Plasmaspheric transparency is found to be a complicated function of not only the plasmaspheric thermal plasma but also the ionospheric sources and scattering processes. A phenomenological model is used to describe the energy transmission, reflection, and deposition in the plasmasphere. By studying the ionosphere and plasmasphere as one system rather than two separate ones, substantial corrections are introduced in the values of key parameters describing photoelectron fluxes.
Magnetohydrodynamic and hybrid simulations of broadband fluctuations near interplanetary shocks
Authors: Agim Y. Z. , Viñas A. F. , Goldstein M. L.
Published: February 2004

We present results of a theoretical study of evolution of a spectrum of finite amplitude right‐hand elliptically polarized magnetohydrodynamic (MHD) waves. The analysis includes use of one‐and‐a‐half‐dimensional solutions of the equations that describe compressible MHD together with one‐and‐a‐half‐dimensional hybrid simulation of the phenomenon. The motivation of the study is to understand the origin and properties of finite amplitude waves often observed in the vicinity of collisionless shocks in the heliosphere. The solutions of the MHD equations are compared with both the results of the hybrid simulations and observations previously reported by Viñas et al. (1984) in the vicinity of a quasi‐parallel interplanetary shock. The initial conditions of the MHD solutions were constructed to model the observed spectrum of magnetic and velocity fluctuations; plasma parameters were also chosen to replicate the observed parameters. For the typical parameters of β = 0.5, δB/B0 = 0.25 and a spectrum of parallel propagating, circularly polarized dispersive waves, initially the density and magnetic energy density correlations grow due to the (nonlinear) ponderomotive effect. The spectral features below the ion cyclotron frequency are established quickly on the Alfvénic timescale but then persist and match closely the observed fluctuations. The parametric decay instabilities that subsequently appear further enhance the density fluctuations and produce a high‐frequency magnetic power spectrum consistent with the spacecraft observation. The MHD and hybrid simulations extend the previous picture of wave generation by a beam‐driven ion cyclotron instability to the fully nonlinear stage.
On removing molecular ions from Venus
Authors: Luhmann J. G. , Kasprzak W. T. , Grebowsky J. M.
Published: February 2004

Acceleration or “pickup” of exospheric atomic oxygen ions by the interplanetary convection electric field is a generally accepted mechanism for the observed removal of O+ from Venus. However, heavier escaping molecular ions (e.g., O2+, CO2+, N2+, CO+, and NO+) in high abundances were also detected in the wake by the Pioneer Venus Orbiter (PVO) neutral mass spectrometer (ONMS) operating in its ion mode. It was recently demonstrated that pickup of O+ at low velocities from the terminator upper ionosphere could explain some characteristics of the Venus ionospheric “tail rays.” Since the PVO ion mass spectrometer data indicate that a significant molecular ion component also contributes to the terminator ionosphere above the collisional region (≥250 to 300 km altitude), we apply the tail ray model to study both the associated low‐altitude O+ flows and the behavior of heavier ions of similar origin. The predicted flow vectors show dawn/dusk asymmetries similar to those in the ONMS observations. Further, the heavier ions achieve higher peak energies, thus improving their chances of detection by the ONMS which has an energy threshold of ∼36 eV in the spacecraft frame. The appeal of this explanation is that no exotic or complicated interpretations are required, and that a broad set of diverse observations fit a common scenario. The same mechanism could in principle be operating at Mars where molecular ions were also detected in the wake on Phobos 2.
Low‐energy ion spectral peaks detected by CRRES in the plasma sheet
Authors: Rubin A. G. , Burke W. J. , Hardy D. A.
Published: February 2004

We have examined energy‐versus time color spectrograms compiled by the low‐energy plasma analyzer (LEPA) during 140 orbits of the Combined Release Radiation Effects Satellite (CRRES). Over the period of interest, the apogee of CRRES' orbit precessed from near dawn to near midnight. During more than half of the orbits LEPA detected low‐energy ion spectral peaks (LISPs) that fell into two categories: isotropic and field aligned. Isotropic LISPs were detected most frequently in the 0200–0500 magnetic local time (MLT) sector and relatively high levels of Kp. They always were detected in the company of >10 keV plasma sheet electrons and appeared in one or, at most, two LEPA energy channels. Field‐aligned LISPs were evenly distributed over the sampled MLT sector and magnetic activity levels. They could be either bidirectional or monodirectional. They too were detected along with plasma sheet electrons whose spectra may or may not contain significant fluxes at energies ≥ 10 keV. Although some field‐aligned LISPs were detected in one or two energy channels, others were quite spread in energy. Isotropic LISPs are interpreted as signatures of CRRES charging. Field‐aligned LISPs cannot be due to charging. On the basis of other measurements at geostationary orbit, we interpret them as being of ionospheric origin. Their detection in the postmidnight sector suggests that they were initially accelerated perpendicular to (ion conics) rather than along (ion beams) the Earth's magnetic field.
Kinetic localization of beam‐driven Langmuir waves
Authors: Muschietti L. , Roth I. , Ergun R. E.
Published: February 2004

The bursts of Langmuir waves observed in the auroral ionosphere are interpreted as localized wave packets of tens to a hundred wavelengths along the geomagnetic field, which interact with streaming, energetic electrons. We investigate the interaction at a kinetic level and show by means of self‐consistent, particle‐in‐cell simulations with open boundary conditions that the interaction itself can lead to wave localization. Ballistic perturbations of the distribution function and nonlinear orbit corrections combine to modulate the perturbed density that drives the waves. The resulting kinetic localization takes place within shorter timescales and for smaller wave amplitudes than the focusing via ponderomotive force.
Propagation to the ground at high latitudes of auroral radio noise below the electron gyrofrequency
Authors: Horne Richard B.
Published: February 2004

The propagation of auroral radio noise at frequencies below the ionospheric electron gyrofrequency (ƒce ≈ 1.4 MHz) at high latitudes is investigated using the HOTRAY ray tracing code. Two mechanisms are described whereby energy in this frequency range may access the ground. It is assumed that the radiation is generated by an upgoing loss cone in regions of depleted electron density above the ionosphere according to the theory of Wu et al. (1989). It is demonstrated that since the waves are generated with ƒpe < ƒ < ƒce, where ƒpe is the electron plasma frequency, they correspond to Z mode waves. Ray tracing shows that for a density model where ƒpe < ƒce at high altitudes and ƒpe > ƒce in the ionosphere and for a dipole magnetic field, Z mode waves are reflected in the topside ionosphere and do not have access to the ground. However, Z mode waves can access the second radio window where energy can be mode converted into whistler mode waves which do propagate to the ground. Typically, the range of initial wave normal angles is a few degrees and the resulting latitudinal spread of emissions on the ground is a few degrees. This suggests that any emissions observed on the ground should be closely confined in latitude to the generation region. For a density model where ƒpe < ƒce throughout the ionosphere it is shown that Z mode waves can propagate through the ionosphere and hence to the ground for ƒLcut < ƒ < ƒce where ƒLcut is the left‐hand cutoff frequency. Due to refraction these waves are spread over a few tens of degrees in latitude but the spread should decrease with decreasing frequency. This suggests there is low‐frequency cutoff to this mechanism below which waves can only reach the ground by mode conversion.
Studies of the E region neutral wind in the disturbed auroral ionosphere
Authors: Nozawa Satonori , Brekke Asgeir
Published: February 2004

Mean neutral wind velocity vectors observed at auroral E region altitudes by European incoherent scatter radar (EISCAT) for disturbed days are found to be rather similar to the mean wind velocity vectors observed on quiet days. It is demonstrated that the main difference in the E field between quiet and disturbed days is an enhancement in the diurnal component by a factor of 3. The semidiurnal, 6‐, and 8‐hour components display only minor differences. The amplitudes of the tidal components of the neutral wind increase in general above 109 km during ionospheric disturbances in particular for the diurnal tide of the zonal wind. The disturbance pattern of the neutral wind has a stronger equatorward component above ∼109 km than the corresponding quiet time pattern. In general, on disturbed days there is an additional eastward motion from the late evening until prenoon. During the daytime the northward wind is enhanced, while in the afternoon stronger westward velocities are seen above 109 km. In the afternoon sector until late evening during disturbed conditions there is also a southward component above ∼109 km. Below 109 km the semidiurnal tide appears to be enhanced during disturbed conditions. It is found that only part of the disturbance in the E region neutral wind can be explained by ion drag while Joule heating and particle precipitation may play an additional role. Satisfying agreement is found between the variations of the diurnal components of the E region neutral wind observed in the present work and the recent model predictions by Fesen et al. [1991].
Correlator measurements of megahertz wave‐particle interactions during electron beam operations on STS
Authors: Gough M. P. , Hardy D. A. , Oberhardt M. R. , Burke W. J. , Gentile L. C. , McNeil B. , Bounar K. , Thompson D. C. , Raitt W. J.
Published: February 2004

We report on the analysis of megahertz modulation of electrons as measured by the Shuttle Potential and Return Electron Experiment (SPREE) during dc firing of the shuttle electrodynamic tether system (SETS) fast pulsed electron generator (FPEG). The SPREE and FPEG were flown aboard the space shuttle Atlantis flight STS 46 as part of the Tethered Satellite System (TSS 1) mission. The principal data reported here are from the SPREE multiangular electrostatic analyzers (ESAs) and Space Particle Correlator Experiment (SPACE). The ESAs, mounted on rotary tables, measured electrons and ions in the energy range from 10 eV to 10 keV over a solid angle of 2π sr. The SPACE is a signal processing system that analyzes the pulse stream from the SPREE ESAs to identify bunching of the electrons and ions produced by coherent wave‐particle interactions (WPIs). The SPACE detects modulations in the electron fluxes in frequency range 0‐ to 10‐MHz. This paper concerns 2‐ to 4‐MHz modulations of the electron flux detected by the SPACE when the FPEG was firing in a dc mode at pitch angles close to 90°. During such operations, FPEG emitted a current of 100 mA at an energy of 1 keV. For these times, electrons with energies from 10 to 1850 eV were measured by the SPREE. For energies between ∼10 and 100 eV the electron flux is basically isotropic. At higher energies the flux increases for pitch angles near 90°. The electron distribution functions generally decrease monotonically with increasing energy up to 100 eV. At energies > 100 eV the distributions either monotonically decrease or exhibit a peak or plateau at energies near the beam emission energy. Megahertz modulations were observed for electrons with energies from 10 to 1180 eV, on both positive and negative slopes in the distribution function and throughout the 2π sr sampled by the ESAs. The occurrence and strength of the modulations exhibit no clear dependence on the pitch angle at which the electrons are measured. However, they appear to be limited to low parallel velocities (< 3×106 m s−1 ) where beam‐generated waves are in resonance with suprathermal electrons.
Effects of ring current disturbances on global MHD oscillations
Authors: Lee D. H. , Kim K. H. , Min K. W. , Lysak R. L.
Published: February 2004

Global compressional MHD wave properties are numerically studied in a three‐dimensional box model. In this model, the effect of the ring current is introduced as a disturbance near the plasmapause. It is assumed to be constant in the azimuthal direction (axisymmetric) and have pressure gradients in the meridional plane. We investigate how global and transverse modes are affected by any small changes in the ring current in the ideal MHD regime. It is found that the spectrum of global cavity modes, produced by an impulsive source, may be significantly distorted due to the perturbations of the ring current. However, field line resonances (FLR) are found to remain the same as long as the distribution of the ring current does not vary rapidly in the azimuthal direction and becomes almost axisymmetric. It is suggested that the interaction of the ring current with an axisymmetric global mode may help to explain the observed feature that transverse waves have relatively strong spectral harmonic peaks, while discrete global cavity modes have rarely been confirmed even when the source is presumed impulsive. In addition, we discuss the general characteristics of the magnetospheric global modes in the system with complicated boundaries and highly nonuniform plasmas.
Magnetometer array for cusp and cleft studies observations of the spatial extent of broadband ULF magnetic pulsations at cusp/cleft latitudes
Authors: Engebretson M. J. , Hughes W. J. , Alford J. L. , Zesta E. , Cahill L. J. , Arnoldy R. L. , Reeves G. D.
Published: February 2004

We have used magnetometer data from 10 locations in Arctic Canada and Greenland, covering over 5 hours in magnetic local time at magnetic latitudes from 75° to 79°, to characterize the dayside patterns of enhanced long‐period ULF (10‐ to 600‐s period) wave power at cusp/cleft latitudes. We conclude the following: (1) In agreement with earlier single‐station studies, we find that the most common wave type is broadband noise (Pi 1‐2). Distinct Pc 3‐4 activity and more sustained monochromatic Pc 5 activity are most apparent when this broadband noise is weak. (2) Multistation observations also make clear that strong, broadband Pi 1‐2 signals are both temporally and spatially structured: Although their amplitude is somewhat larger near local noon and near nominal cusp latitudes, they often occur simultaneously (to within a few minutes) at all stations. They are thus not local signals, and cannot be interpreted as evidence of passage of an auroral region or boundary over an individual magnetic observatory. In particular, we have found no evidence for a distinctive “cusp” signature in broadband ULF waves in this frequency range. (3) The occurrence of strong broadband Pi 1‐2 signals at these latitudes appears to be controlled largely by solar wind velocity. We found good correlations between the occurrence of strong Pi 1‐2 signals and high solar wind velocity, and we also noted some dependence on the cone angle of the interplanetary magnetic field for moderate to low solar wind velocities. We speculate that there may be an additional dependence on enhanced levels of trapped plasma in regions topologically connected to the very high latitude dayside ionosphere, such as the entry layer, high‐latitude dayside field minimum regions, or plasma mantle. Available satellite data on the level of trapped energetic electron fluxes at geosynchronous orbit showed that broadband power levels appeared to correlate with enhanced flux levels on the time scale of days, but not on shorter time scales, suggesting that any such dependence is not directly related to substorm injections.
Coherent radar estimates of average high‐latitude ionospheric Joule heating
Authors: Kosch M. J. , Nielsen E.
Published: February 2004

The Scandinavian Twin Auroral Radar Experiment (STARE) and Sweden and Britain Radar Experiment (SABRE) bistatic coherent radar systems have been employed to estimate the spatial and temporal variation of the ionospheric Joule heating in the combined geographic latitude range 63.8°–72.6° (corrected geomagnetic latitude 61.5°–69.3°) over Scandinavia. The 173 days of good observations with all four radars have been analyzed during the period 1982 to 1986 to estimate the average ionospheric electric field versus time and latitude. The AE dependent empirical model of ionospheric Pedersen conductivity by Spiro et al. (1982) has been used to calculate the Joule heating. The latitudinal and diurnal variation of Joule heating as well as the estimated mean hemispherical heating of 1.7×1011 W are in good agreement with earlier results. Average Joule heating was found to vary linearly with the AE, AU, and AL indices and as a second‐order power law with Kp. The average Joule heating was also examined as a function of the direction and magnitude of the interplanetary magnetic field. It has been shown for the first time that the ionospheric electric field magnitude as well as the Joule heating increase with increasingly negative (southward) Bz.
Magnetosphere cyclotron maser: Backward wave oscillator generation regime
Authors: Trakhtengerts V. Y.
Published: February 2004

A new generation regime of the magnetosphere cyclotron maser is considered, based on phase coherence effects in wave‐particle systems with step‐like deformations of electron velocity distribution functions. Such deformations appear during cyclotron interactions of noise‐like whistler wave emissions and energetic electrons at the boundaries between resonant and nonresonant particles. The new regime is similar to the backward wave oscillator in laboratory electronic devices. This regime applies to the generation of chorus emissions and may explain the connection between chorus and hiss, their fast growth rates, and the temporal succession of chorus elements.
The mosaic structure of plasma bulk flows in the Earth's magnetotail
Authors: Ashour‐Abdalla M. , Zelenyi L. M. , Peroomian V. , Richard R. L. , Bosqued J. M.
Published: February 2004

Moments of plasma distributions observed in the magnetotail vary with different time scales. In this paper we attempt to explain the observed variability on intermediate timescales of ∼10–20 min that result from the simultaneous energization and spatial structuring of solar wind plasma in the distant magnetotail. These processes stimulate the formation of a system of spatially disjointed, highly accelerated filaments (beamlets) in the tail. We use the results from large‐scale kinetic modeling of magnetotail formation from a plasma mantle source to calculate moments of ion distribution functions throughout the tail. Statistical restrictions related to the limited number of particles in our system naturally reduce the spatial resolution of our results, but we show that our model is valid on intermediate spatial scales Δx × ΔZ ∼ 1 RE × 1000 km. For these spatial scales the resulting pattern, which resembles a mosaic, appears to be quite variable. The complexity of the pattern is related to the spatial interference between beamlets accelerated at various locations within the distant tail which mirror in the strong near‐Earth magnetic field. Global motion of the magnetotail results in the displacement of spacecraft with respect to this mosaic pattern and can produce variations in all of the moments (especially the x‐component of the bulk velocity) on intermediate timescales. The results obtained enable us to view the magnetotail plasma as consisting of two different populations: a tail ward‐Earthward system of highly accelerated beamlets interfering with each other, and an energized quasithermal population which gradually builds as the Earth is approached. In the near‐Earth tail, these populations merge into a hot quasi‐isotropic ion population typical of the near‐Earth plasma sheet. The transformation of plasma sheet boundary layer (PSBL) beam energy into central plasma sheet (CPS) quasi‐thermal energy occurs in the absence of collisions or noise. This paper also clarifies the relationship between the global scale where an MHD description might be appropriate and the lower intermediate scales where MHD fails and large‐scale kinetic theory should be used.
Persistent north–south asymmetry of the daily interplanetary magnetic field spiral
Authors: Sabbah I.
Published: February 2004

We have studied the north–south asymmetry of interplanetary magnetic field (IMF) and plasma data taken near 1 AU during the 26‐year period 1965–1990. The IMF azimuthal component is larger in magnitude than the radial IMF component north of the current sheet, while the reverse of this effect is true south of the current sheet. This should produce an asymmetry in the IMF spiral with larger angle north of the current sheet than south of the current sheet. We find that the IMF spiral angle is larger north of the current sheet than south of the current sheet. We confirm this north–south asymmetry in the IMF spiral angle to be persistent, using three different criteria to define away and toward field days. The asymmetry is 3.0° ± 0.7° for “well‐defined” polarity days. The field azimuthal component is higher north of the current sheet than south of the current sheet, while the reverse is true for the solar plasma density.
Characterization of low frequency oscillations at substorm breakup
Authors: Holter Ø , Altman C. , Roux A. , Perraut S. , Pedersen A. , Pécseli H. , Lybekk B. , Trulsen J. , Korth A. , Kremser G.
Published: February 2004

Field and particle data recorded on the geostationary satellite GEOS 2 are used to investigate the electric and magnetic signatures of a substorm characterized by a dispersionless injection of energetic electrons and ions. Three types of field variations are observed: (1) Long‐period oscillations with period of ∼ 300 s, interpreted as oscillations of entire field lines. These oscillations develop as second harmonic standing waves and correspond to coupled shear Alfvén‐slow magnetosonic modes. They grow after the most active period of the breakup. (2) Short‐period transient oscillations with periods of ∼ 45–65 s, interpreted as wave modes trapped in a current layer which develops prior to the substorm breakup and is disrupted at breakup. These oscillations also correspond to a coupled shear Alfvén‐slow magnetosonic mode (coupled via magnetic field curvature effects in a high‐β plasma). The short‐period transient oscillations are only observed during the most active period of the breakup. (3) A nonoscillatory sharp increase observed on both the parallel magnetic component and the energetic ion flux, averaged over one satellite rotation, interpreted as evidence for the fast magnetosonic mode which in view of the simultaneous large impulsive increase in the azimuthal electric field, appears to propagate radially outwards, transporting the substorm breakup downtail.
Generation of radiation by upper hybrid pump waves
Authors: Stenflo L. , Shukla P. K.
Published: February 2004

It is shown that large‐amplitude high‐frequency electrostatic pump waves can parametrically excite magnetic field aligned electron‐cyclotron waves as well as magnetosonic and/or Alfvén waves propagating at an arbitrary angle to the external magnetic field lines. The results lead to a new proposal for the origin of the nonthermal electromagnetic waves in space plasmas.
Anomalous magnetosheath properties during Earth passage of an interplanetary magnetic cloud
Authors: Farrugia C. J. , Erkaev N. V. , Biernat H. K. , Burlaga L. F.
Published: February 2004

The aim of this paper is to model for the first time the variation of field and flow parameters in the magnetosheath during Earth passage of an interplanetary magnetic cloud. Under typical Solar wind conditions, magnetohydrodynamic (MHD) effects on the flow of plasma in the terrestrial magnetosheath are important only in a layer adjacent to the magnetopause which is a few thousand kilometers thick (“depletion layer” or “magnetic barrier”). During the passage of an interplanetary magnetic cloud, however, conditions upstream of the bow shock depart strongly from the norm. In this case, interplanetary parameters vary slowly over a wide range of values. Values of the upstream Alfvén Mach number are much lower than those otherwise sampled (∼3 versus 8–10). Together with the magnetic shear across the magnetopause, this parameter plays a central role in determining the structure of the magnetosheath close to the magnetopause. As a consequence of sustained low values of the upstream Alfvén Mach number, the magnetic field exerts a strong influence on the flow over a very substantial fraction of the magnetosheath throughout the duration of cloud passage, i.e., for a time period of the order of 1–2 days. We apply an algorithm to integrate the ideal MHD equations, using a boundary layer technique, and compute the variations of field and flow parameters along the stagnation streamline. We choose as our example the magnetic cloud which passed Earth on January 14–15, 1988. The interaction of this cloud with the magnetosphere, as regards the resulting ionospheric flow patterns and the substorm activity, has been the subject of various investigations. Using information from these studies, we obtain results on the magnetosheath when the magnetopause is modeled, first as a tangential discontinuity and then as a rotational discontinuity. Our results are in good general agreement with recent observations on the behavior of field and flow quantities in the magnetosheath region adjacent to the magnetopause. In addition, we predict the existence of a magnetic barrier when the upstream Alfvén Mach number is low, irrespective of the magnetic shear across the magnetopause.
Diurnal modulation of the geomagnetic activity induced by the southward component of the interplanetary magnetic field
Authors: Orlando M. , Moreno G. , Parisi M. , Storini M.
Published: February 2004

The diurnal (universal time) modulation of the geomagnetic activity is investigated, using the am index along with interplanetary plasma and magnetic field data collected by a variety of spacecraft from 1965 to 1987. A direct comparison with satellite measurements of the Bs component of the interplanetary magnetic field (IMF) is made. Our results confirm the Russell‐McPherron (R‐M) model, which attributes the above phenomenon to the varying southward component (Bs) of the IMF in the geocentric solar magnetospheric system. Moreover, we compared the contributions of the equinoctial and R‐M mechanisms to the observed am modulation, showing that the latter effect is dominant.
Imaging of artificial aurora in the upper atmosphere
Authors: Mende S. B. , Fuselier S. A. , Geller S. P. , Swenson G. R. , Marshall J. A.
Published: February 2004

We use continuum (white light) and filtered (427.8 nm) images from the Atmospheric Emissions Photometric Imaging (AEPI) experiment on the Atmospheric Laboratory for Applications and Science (ATLAS) 1 shuttle mission to investigate the shape and evolution of artificial auroral patches generated by the Space Experiments with Particle Accelerators (SEPAC) electron beam experiment. Auroral patches generated by this beam experiment are complex and differ in the white light and filtered images. In the white light images, the auroral patch consists of a relatively large, diffuse, and somewhat symmetric head and a tail that is directed approximately opposite the spacecraft velocity vector. From the growth of the tail during a beam pulse, the distance from the imager to the emissions is estimated to be about 200 km, consistent with expectations from a simple model of auroral emissions in the atmosphere. In addition to the auroral patch, an intense, diffuse, and variable background glow filling essentially the entire field of view of the white light imager is seen during the beam pulses. This background glow may be caused by low‐energy electrons very near the shuttle. This glow is absent in the filtered images, in which the shape of the auroral patch differed, consisting of a relatively large, diffuse, but more asymmetric head, a tail, and a smaller and less intense spot below the head. Curvature of the magnetic field and spacecraft motion during the 1‐s filtered images allows an estimate of the relative distance from the shuttle to the head, tail, and small spot. This shape is consistent with head emissions generated relatively near the spacecraft (nearest few kilometers), tail emissions somewhat farther away, and finally the small spot emissions generated the farthest away in the lower atmosphere where the natural auroras would be created. In addition, the shape of these patches in the filtered images suggests that the risetime for the head and tail emissions in the filtered images appears to be longer than that for the small spot emission. The differences between the white light images and the filtered images are consistent with the difference between the emission parent state lifetimes and the energy requirement for the emission production. The white light images contain emissions with long lifetimes, and these emissions which are produced near the observer are swept out of the field of view because they are left behind by the shuttle. This gives a bias toward emissions generated farther from the orbiter. The filtered images contain only the very fast produced N2+ emission with a substantial component generated near the orbiter and with an inverse square law attenuated auroral spot in the lower atmosphere. The presence of the tail and the apparent risetime in the near‐field emission suggest that there is a buildup and decay time associated with the hot plasma created by the electron beam.
Three‐dimensional kinetic simulation of the Kelvin‐Helmholtz instability
Authors: Thomas V. A.
Published: February 2004

Three dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the kinetic evolution of the Kelvin‐Helmholtz (K‐H) instability for the simplest configuration, that of plasma with uniform density imbedded in a uniform magnetic field. The relative angle of the magnetic field and the velocity vector is varied, and for some simulations the magnetic field undergoes a rotation at the velocity shear layer. The results from the three‐dimensional simulations are generally consistent with two‐dimensional simulations, providing that the appropriate geometry is simulated in the two‐dimensional cases. The simulations suggest that even modest rotation of the magnetic field across the boundary layer strongly reduces the nonlinear development of the instability so that its possible occurrence at the dayside magnetopause is greatly inhibited.
Pitch angle scattering of diffuse auroral electrons by whistler mode waves
Authors: Villalón Elena , Burke William J.
Published: February 2004

Resonant electron‐whistler interactions in the plasma sheet are investigated as possible explanations of the nearly isotropic fluxes of low‐energy electrons observed above the diffuse aurora. Whistler mode waves, propagating near the resonance cone with frequencies near or larger than half the equatorial electron cyclotron frequency, can interact with low‐energy plasma sheet electrons. A Hamiltonian formulation is developed for test particles interacting with the coherent chorus emission spectra. We consider the second‐order resonance condition which requires that inhomogeneities in the Earth's magnetic field be compensated by a finite bandwidth of wave frequencies to maintain resonance for extended distances along field lines. These second‐order interactions are very efficient in scattering the electrons toward the atmospheric loss cone. Numerical calculations are presented for the magnetic shell L = 5.5 for wave amplitudes of ∼ 10−6 V/m, using different frequency and magnetospheric conditions.
Prediction of unspots using reconstructed chaotic system equations
Authors: Jinno Kenji , Xu Shiguo , Berndtsson Ronny , Kawamura Akira , Matsumoto Minoru
Published: February 2004

Modeling of sunspots is important since they indicate the relative activity of the Sun which in turn influence different terrestrial properties. To predict the nonlinear and chaotic behavior of sunspot time series, the problem of reconstructing underlying system equations is studied. The proposed procedure for this is (1) based on the behavior of observed time series and dimension of strange attractor, find reference system equations (in our case the modified Rössler equations) that show similar basic features as the time series (e.g., appearance of attractor, amplitude and pseudoperiod), (2) assume a general structure of the governing system equations by Taylor series expansion, (3) use the reference equation systems as initial state in an updating procedure (extended Kalman filtering) to estimate the structure of the governing system equations. Using this procedure, results show that predictions on an average up to eight months ahead can be made with good agreement for sunspot time series after identifying the governing system equations. The extended Kalman filter was shown to be an efficient tool to identify parameter values and to make updated predictions of the chaotic system.
Transition of slow shocks to fast shocks in the interplanetary space
Authors: Whang Y. C.
Published: February 2004

As a CME pushes its way through preceding slower solar wind, large disturbances in the interaction region may evolve to form transient MHD shocks. The shocks can be slow shocks in the coronal space, but they appear as fast shocks near 1 AU. We use a polytropic MHD model to study the transition of slow shocks to fast shocks in an interaction region sandwiched between a faster solar wind and a slower solar wind near the equatorial plane. The polytropic index controls the radial variation of the solar wind conditions outside the shock. The simulation results of two examples show smooth transition of the shock system from slow shocks near 0.15 AU to fast shocks near 1 AU and beyond. A parametric study of shock solutions in the dimensionless parameter space shows that the transition process is attributed chiefly to the increase of β with the heliocentric distance and to a lesser degree to the increase of the shock angle θ. Under the initial condition of small β and θ near 0.1 AU, the interaction region evolves to form a pair of slow shocks inside 0.15 AU. As the interaction region convects outward, the increases of β and θ cause a transition of the shock system between 0.15 and 0.3 AU from a slow shock pair to a double shock pair consisting of both slow and fast shocks. As the system moves outward, β and θ continue to increase, the fast shock grows stronger, and the slow shock becomes weaker. Eventually, the slow shocks fade away, and the shock system finally evolves to a pair of fast shocks.
Turbulent heating and temperature evolution in the solar wind plasma
Authors: Verma M. K. , Roberts D. A. , Goldstein M. L.
Published: February 2004

We calculate the turbulent dissipation rate of incompressive hydromagnetic fluctuations and the resulting radial evolution of temperature in the solar wind using generalizations of Kolmogorov and Kraichnan MHD turbulence phenomenologies that include the suppression of dissipation by high cross helicity. The results for the temperature evolution are compared to a variety of data sets to test the phenomenologies over a wide parameter range. Motivated by the observations, we use different power laws in radius for the amplitudes of Alfvénic and nonAlfvénic fluctuations to determine the cascade rates. To explain the observations using Kolmogorov‐like models, we found it necessary to suppress the dissipation rates for the high cross helicity streams even further than predicted by simple models; this may be due to the nonequilibrium nature of the spectrum or to other causes as yet unknown. The Kolmogorov‐like model gives rise to a significant amount of turbulent heating, implying that turbulent heating, while likely dominant only in the inner heliosphere, may be competitive with heating by shocks and the assimilation of interstellar pickup ions in the outer heliosphere. In contrast, the generalized Kraichnan phenomenology yields less turbulent heating than the Kolmogorov‐like model and seems inadequate to explain the observations. We conclude that while no existing turbulence model adequately explains the observed radial dependence of temperature in the solar wind, there appears to be sufficient energy available for turbulent heating to contribute significantly, even in the outer heliosphere.
Possible conjugate reconnection at the high‐latitude magnetopause
Authors: Feldman W. C. , Hones E. W. , Barraclough B. L. , Reeves G. D. , Belian R. D. , Cayton T. E. , Lee P. , Lepping R. P. , Trombka J. I. , Starr R. , Moersch J. , Squyres S. W. , Rich F. J.
Published: February 2004

Coordinated analysis of data sets from X ray, energetic electron, ionospheric convection velocity, interplanetary magnetic field (IMF), and solar wind plasma experiments yields a comprehensive picture of the stimulated precipitation of trapped relativistic electrons simultaneously onto both polar caps in response to a sharp northward‐dawnward turning of the IMF after initiation of a substorm at about 1200 UT on December 17, 1992. Although several explanations are possible, these observations are consistent with the occurrence of conjugate magnetic reconnection tailward of both geomagnetic cusps caused by the draping of northward and dawnward directed IMF over the dayside magnetopause.
Relationship between field‐aligned currents and inverted‐V parallel potential drops observed at midaltitudes
Authors: Sakanoi T. , Fukunishi H. , Mukai T.
Published: February 2004

The inverted‐V field‐aligned acceleration region existing in the altitude range of several thousand kilometers plays an essential role for the magnetosphere‐ionosphere coupling system. The adiabatic plasma theory predicts a linear relationship between field‐aligned current density (J∥) and parallel potential drop (Φ∥), that is, J∥ = KΦ∥, where K is the field‐aligned conductance. We examined this relationship using the charged particle and magnetic field data obtained from the Akebono (Exos D) satellite. The potential drop above the satellite was derived from the peak energy of downward electrons, while the potential drop below the satellite was derived from two different methods: the peak energy of upward ions and the energy‐dependent widening of electron loss cone. On the other hand, field‐aligned current densities in the inverted‐V region were estimated from the Akebono magnetometer data. Using these potential drops and field‐aligned current densities, we estimated the linear field‐aligned conductance KJΦ. Further, we obtained the corrected field‐aligned conductance KJΦC by applying the full Knight's formula to the current‐voltage relationship. We also independently estimated the field‐aligned conductance KTN from the number density and the thermal temperature of magnetospheric source electrons which were obtained by fitting accelerated Maxwellian functions for precipitating electrons. The results are summarized as follows: (1) The latitudinal dependence of parallel potential drops is characterized by a narrow V‐shaped structure with a width of 0.4°−1.0°. (2) Although the inverted‐V potential region exactly corresponds to the upward field‐aligned current region, the latitudinal dependence of upward current intensity is an inverted‐U shape rather than an inverted‐V shape. Thus it is suggested that the field‐aligned conductance KJΦC changes with a V‐shaped latitudinal dependence. In many cases, KJΦC values at the edge of the inverted‐V region are about 5–10 times larger than those at the center. (3) By comparing KJΦC with KTN, KJΦC is found to be about 2–20 times larger than KTN. These results suggest that low‐energy electrons such as trapped electrons, secondary and back‐scattered electrons, and ionospheric electrons significantly contribute to upward field‐aligned currents in the inverted‐V region. It is therefore inferred that non adiabatic pitch angle scattering processes play an important role in the inverted‐V region.
Observations of convection vortices in the afternoon sector using the SuperDARN HF radars
Authors: Bristow W. A. , Sibeck D. G. , Jacquey C. , Greenwald R. A. , Sofko G. J. , Mukai T. , Yamamoto T. , Kokubun S. , Hughes T. J. , Hughes W. J. , Engebretson M. J.
Published: February 2004

Observation of convection vortices using the new SuperDARN HF radars are presented. The velocity field derived from the overlapping fields of view of the new HF radars at Kapuskasing, Ontario, and Saskatoon, Saskatchewan, Canada, often image the portion of the convection pattern near the convection reversal boundary. Observations from near the convection reversal boundary in the afternoon sector of October 22, 1993, showed two convection vortices evolving within the field of view of both radars. The first vortex appeared at about 2120 UT and the second at about 2145 UT: 1446 MLT and 1512 MLT, respectively. The vortices were roughly 900 km in diameter and moved tailward with a velocity of about 600 m/s. At the times the vortices were observed by the radars, ground‐based magnetometers of the CANOPUS and MACCS chains show transient deflections of near 100 nT, and the GOES 6 and GOES 7 satellite magnetometers showed significant decreases in the magnetospheric magnetic field strength. Data from the Geotail satellite magnetometer lagged by an appropriate time interval indicated that there were southward turnings of the interplanetary magnetic field that coincided with the decreases of magnetospheric magnetic field strength. The observations differ in many respects from previously published vortex observations. It is theorized that the vortices were caused by the Kelvin‐Helmholtz instability at the inner edge of the low‐latitude boundary layer.