bluuebunny/crossref_metadata_2025 huggingface dataset:
Relationship of topside ionospheric ion outflows to auroral forms and precipitation, plasma waves, and convection observed by Polar
Authors: Hirahara M. , Horwitz J. L. , Moore T. E. , Germany G. A. , Spann J. F. , Peterson W. K. , Shelley E. G. , Chandler M. O. , Giles B. L. , Craven P. D. , Pollock C. J. , Gurnett D. A. , Pickett J. S. , Persoon A. M. , Scudder J. D. , Maynard N. C. , Mozer F. S. , Brittnacher M. J. , Nagai T.
Published: February 2004

The POLAR satellite often observes upflowing ionospheric ions (UFIs) in and near the auroral oval on southern perigee (∼5000 km altitude) passes. We present the UFI features observed by the thermal ion dynamics experiment (TIDE) and the toroidal imaging mass angle spectrograph (TIMAS) in the dusk‐dawn sector under two different geomagnetic activity conditions in order to elicit their relationships with auroral forms, wave emissions, and convection pattern from additional POLAR instruments. During the active interval, the ultraviolet imager (UVI) observed a bright discrete aurora on the duskside after the substorm onset and then observed a small isolated aurora form and diffuse auroras on the dawnside during the recovery phase. The UFIs showed clear conic distributions when the plasma wave instrument (PWI) detected strong broadband wave emissions below ∼ 10 kHz, while no significant auroral activities were observed by UVI. At higher latitudes, the low‐energy UFI conies gradually changed to the polar wind component with decreasing intensity of the broadband emissions. V‐shaped auroral kilometric radiation (AKR) signatures observed above ∼200 kHz by PWI coincided with the region where the discrete aurora and the UFI beams were detected. The latitude of these features was lower than that of the UFI conies. During the observations of the UFI beams and conies, the lower‐frequency fluctuations observed by the electric field instrument were also enhanced, and the convection directions exhibited large fluctuations. It is evident that large electrostatic potential drops produced the precipitating electrons and discrete auroras, the UFI beams, and the AKR, which is also supported by the energetic plasma data from HYDRA. Since the intense broadband emissions were also observed with the UFIs, the ionospheric ions could be energized transversely before or during the parallel acceleration due to the potential drops.
Two‐dimensional HaIl‐MHD simulation of current sheet dynamics during substorm growth phase
Authors: Yoon Peter H. , Lui A. T. Y. , Ziebell L. F.
Published: February 2004

In this paper we study the dynamical evolution of the current sheet in the near‐Earth geomagnetic tail region during the substorm growth phase by means of HaIl‐MHD simulation. Special attention is paid to the development of cross‐tail electric current components attributed to the electrons and ions. It is found that the induced component of the electric field generated during the current sheet thinning process leads to E×B drift of both the electrons and ions toward the dawnward direction, which augments the electron diamagnetic drift but opposes the ion diamagnetic drift. This effect causes the electrons to carry a higher and higher portion of the cross‐tail current as time progresses, even if the bulk of the initial current may be carried by the ions. However, the,increase of electron current strongly depends on the boundary condition and on the location along the distant‐tail/near‐Earth axis. It is found that close to the near‐Earth simulation boundary, a sizable normal magnetic field component Bz persists even in the late phase of the simulation. This limits the north‐south electric field Ez to a relatively low value, and as a result, the cross‐tail current is largely carried by the ions there. However, toward the distant tail, Bz is much weaker, and thus the electron current is seen to increase quite rapidly.
SPEAR 3 flight analysis: Grounding by neutral gas release, and magnetic field effects on current distribution
Authors: Mandell M. J. , Jongeward G. A. , Cooke D. L. , Raitt W. J.
Published: February 2004

The Space Power Experiment Aboard Rockets (SPEAR) 3 experiment was launched on March 15, 1993, to test grounding devices for negative payloads. In this paper we review two aspects of the high‐altitude flight data and compare them with preflight predictions. The SPEAR 3 neutral gas release experiment studied a grounding mechanism observed on previous flights during attitude control system (ACS) firings. Preflight calculations using Paschen law physics generalized to three dimensions predicted that the high rate gas release (about one order of magnitude below normal ACS) would reduce the rocket potential to within 200–300 V of plasma ground. The flight data is well fit by a value of −225 V. Orientation relative to Earth's magnetic field had no effect on the floating potential or grounding operations but had a large effect on the portion of the current collected by the boom. We compare these flight measurements with preflight calculations made with the DynaPAC computer code.
Heliospheric tomography using interplanetary scintillation observations: 3. Correlation between speed and electron density fluctuations in the solar wind
Authors: Asai K. , Kojima M. , Tokumaru M. , Yokobe A. , Jackson B. V. , Hick P. L. , Manoharan P. K.
Published: February 2004

We have examined the relationship between solar wind speed and electron density fluctuations on scale sizes around 100 km in the heliocentric distance range of 0.3 to 0.8 AU using interplanetary scintillation (IPS) data obtained at the Solar‐Terrestrial Environment Laboratory. The solar wind properties derived from the IPS data are biased by line of sight integration through a three‐dimensional structured solar wind. Therefore we have applied a computer‐assisted tomography (CAT) method to deconvolve the line of sight integration and reconstruct the solar wind structure. The analysis was made for the solar wind speed V and electron density fluctuations δNe in the solar activity minimum phase when high‐speed regions are separated from an equatorial low‐speed region by a sharp velocity gradient. From results of the CAT analysis we derived the best fit power law relation of δNe ∝ V−γ with γ = 0.5 ± 0.15, indicating that fractional density fluctuations δNe/Ne in the high‐speed wind are larger than those in the low‐speed wind. Combining this relation with results of previous workers [Coles et al., 1995; Manoharan, 1993; Celnikier et al., 1987; Jackson et al., this issue], we suggest that the fractional density fluctuation level of the high‐speed wind evolves with heliocentric distance.
Testing the formulation of Park and Dejnakarintra to calculate thunderstorm dc electric fields
Authors: Rodger Craig J. , Thomson Neil R. , Dowden Richard L.
Published: February 2004

Park and Dejnakarintra [1973] presented a formulation to calculate high‐altitude electric fields due to electrified clouds, taking into account the presence of the ionosphere. Calculation of these fields is necessary to test the hypothesis that whistler ducts might be caused by plasma interchange of geomagnetic flux tubes driven by thundercloud electrostatic fields. To test the accuracy of the formulation, the calculated dc electric fields are compared with those measured during a thunderstorm campaign reported by Holzworth et al. [1985]. Using a conductivity profile measured during their campaign, it is shown that the formulation of Park and Dejnakarintra produces electric field predictions which are consistent with the measurements of Holzworth et al. This indicates the formulation developed by Park and Dejnakarintra is valid. The comparisons agree with measurements which show that large negative screening charges commonly exist around the top of thunderclouds, reducing the high altitude fields. These calculations indicate that the effective thunderstorm charge used in such calculations needs to be significantly reduced to account for screening charges. Such reductions lead to serious increases (up to a factor of 10) in the time required for high‐altitude thunderstorm electric fields to produce whistler ducts.
Numerical investigation of hydrodynamic instabilities of the heliopause
Authors: Wang Chi , Belcher John W.
Published: February 2004

The heliopause is the interface between the solar wind plasma and the very local interstellar medium (VLISM) and manifests itself as a tangential discontinuity across which the flow velocity and the plasma density jump (except at the nose). Hydrodynamic instabilities of either the Rayleigh‐Taylor type or the Kelvin‐Helmholtz type will likely develop at the heliopause. To our knowledge, previous analytical studies of these instabilities were confined to linear perturbation analyses, and most existing numerical simulations did not obtain the Kelvin‐Helmholtz type instability of the heliopause, probably due to large numerical dissipation. In this paper we use the piecewise parabolic method (PPM) in our hydrodynamic simulation to study the stability of the heliopause. The PPM can capture shocks and discontinuities within 1–2 grid points with negligible numerical dissipation. For simplicity, magnetic fields, interstellar neutrals, cosmic rays, etc., are neglected in our model. We thus focus our attention on the general pattern of the Kelvin‐Helmholtz instability at the heliopause. In both the “one‐shock” and “two‐shock” models, the Kelvin‐Helmholtz instability occurs at the heliopause and leads to nonlinear oscillations of the heliopause and the termination shock with a timescale of the order of 102 years. The excursion of the heliopause at the nose as a result of these oscillations is of the order of tens of astronomical units, with much smaller excursions for the termination shock. Growth rates from the simulations are in reasonable agreement with theoretical estimates. The possible stabilizing influence of the magnetic field, neglected in the present model, is discussed.
Ionospheric convection at Casey, a southern polar cap station
Authors: Smith P. R. , Dyson P. L. , Monselesan D. P. , Morris R. J.
Published: February 2004

A digital ionosonde (Digisonde Portable Sounder 4) located at Casey, Antarctica (66.3°S, 110.5°E, −80.8° corrected geomagnetic latitude) has been operational since early 1993 and has accumulated 3 years of plasma drift measurements, providing an excellent data set for studying the characteristics of ionospheric convection flow at a southern polar cap station. The purpose of this study is to investigate the influence of the IMF on the F region ionospheric convection over Casey and to compare it to the Heppner‐Maynard satellite‐derived electric field models. We find clear dependencies in the drift on the sign and strength of the IMF By and Bz components and with Kp. Antisunward flow dominates during Bz south conditions, turning to have a sunward component around noon when Bz is northward. The By component causes the entire convection system to rotate and distorts the dayside flow in the proximity of the throat, with a dawnward (duskward) component for By negative (positive). Comparison with the Bz south Heppner‐Maynard BC, DE, and A patterns is favorable at most times, although we predict a rounder, more dominant dusk (dawn) cell and a smaller crescent‐shaped dawn (dusk) cell for By < 0 (By > 0). There is a dependence on Kp when Bz is south in both the model and the drifts, flow directions becoming more antisunward and velocities becoming higher on the dayside as Kp increases. This implies the polar cap is expanding under conditions of enhanced reconnection. When Bz is north, the F region drift agreement with the BCP(P) and DEP(P) models is excellent on the dawn (dusk) side for By < 0 (By > 0) but diverges on the opposite side as the pattern flow lines twist sunward. Separation of the drifts into Bz weakly (<3 nT) and strongly (>3 nT) northward cases did not reveal any appreciable difference in the observed drift velocities.
Energetic solar particle dropouts detected by Ulysses at 1.63 AU: A possible encounter with the Earth's distant magnetotail
Authors: Ashford Stephen M. , Anderson Kinsey A. , Lin Robert P. , Sommers Jeneen R. , Phillips John L.
Published: February 2004

Fast solar particles are used to trace the topology of the interplanetary magnetic field during a solar event detected by the heliosphere instrument for spectra composition and anisotropy at low energy (HISCALE) on the Ulysses spacecraft on January 2, 1991. Two sharp‐edged dropouts, lasting for 10 and 25 min, were detected in the fluxes of solar ions from ∼130 keV to >1.8 MeV and halo (heat flux) electrons from 71–461 eV, while simultaneously the flux of high‐energy, 38–315 keV solar electrons and 56–78 keV ions remained constant. The halo electrons and 130 keV to >1.8 MeV solar ions are traveling with similar speeds, ∼4 × 106 to 3 × 107 m/s, much slower than the energetic solar electrons (>108 m/s), and faster than the 56–78 keV ions, suggesting that the dropout field lines were first disconnected from the Sun and then reconnected, with the distance to the reconnection point and time of the reconnection such that >38 keV electrons had already repopulated the dropout field lines. At the time Ulysses was 0.63 AU from the Earth on its way to Jupiter, ∼2° above the ecliptic with a Sun‐Earth‐spacecraft angle of ∼172.8°, approximately where the Earth's magnetotail would be expected to be if it extended to 15,000 Earth radii. We consider the possibility that Ulysses encountered interplanetary field lines connected to the magnetotail.
Incoherent scatter from space shuttle and rocket engine plumes in the ionosphere
Authors: Bernhardt P. A. , Huba J. D. , Swartz W. E. , Kelley M. C.
Published: February 2004

Enhanced echoes from the 430 MHz radar at Arecibo were observed during burns of the space shuttle orbital maneuver subsystem (OMS) engines near 317 km altitude. Similar radar signatures of enhanced backscatter were also obtained by the Millstone Hill radar observing the plume of a Centaur engine burning in the ionosphere. A theoretical model of incoherent scatter is presented to explain the radar backscatter observations. The theory considers molecular ion beams generated in the exhaust plume as a result of charge exchange between the ambient O+ ions and the high‐speed exhaust molecules (primarily H2O). The field‐aligned gyromotion of the pickup ions affects the radio wave scattering from the random thermal fluctuations of electron density. Numerical calculations are carried out for plasmas modified by the space shuttle or Centaur engines, and reasonable agreement with observations is found for the total scattered power. Incoherent backscatter spectra respond to characteristics of the exhaust plume such as vector flow velocity, temperature, and composition. The nonequilibrium velocity distributions for the ions in the pickup ion plume are similar to the distributions found in strongly convecting auroral region ionospheres. The incoherent scatter from the plume ions can be used to validate techniques used to study naturally disturbed plasmas. The predictions of our radar scatter calculations will be tested in future experiments using the space shuttle OMS engines over incoherent scatter radars located at equatorial latitudes and midlatitudes.
The effect of Poisson statistics on studies of turbulence in the solar wind
Authors: Johnstone A. D. , Krauklis I. C.
Published: February 2004

Studies of Alfvén turbulence in the solar wind require the use of magnetic field and particle data. However, particle data is subject to a source of noise not present in magnetic field data, i.e. statistical noise because the bulk parameters of the plasma are calculated from a finite number of counts. The derivation of bulk parameters is unavoidably affected by Poisson statistics which add variance to the real value. The magnitude of the effects of Poisson noise on the frequency spectrum of Alfvén waves in the solar wind has been calculated by numerical simulation for the case of cometary pickup ion turbulence observed at comet Halley. The results show that Poisson noise is “white” and flattens the frequency spectrum at high frequencies, where its power spectral density exceeds the solar wind turbulence. If a power law is fitted to the spectrum, the power law index can be reduced significantly even if the flattening is not obvious in the spectrum, and the effect can be significant even when the particle count rates seem high. In Elsässer variable studies the influence of Poisson noise is seen as a tendency to equality of the two spectra at high frequencies since each contains uncorrelated noise. The simulations show that Poisson noise effects must be taken into account in the interpretation of Alfvén wave observations. A concrete result is that the difference between the spectral indices obtained separately from magnetic and particle data at comet Halley can be explained quantitatively by Poisson noise.
Multispacecraft observations of sudden impulses in the magnetotail caused by solar wind pressure discontinuities: Wind and IMP 8
Authors: Collier M. R. , Slavin J. A. , Lepping R. P. , Ogilvie K. , Szabo A. , Laakso H. , Taguchi S.
Published: February 2004

Two upstream solar wind pressure discontinuities that were associated with storm sudden commencements have been examined to determine their effect on the geomagnetic tail lobe field. During the two events, occurring on March 9, 1995, and August 17, 1995, the Wind spacecraft was located in the upstream region monitoring the solar wind, and the IMP 8 spacecraft was in the geomagnetic tail lobe observing the tail response. The two events occurred during periods with northward or weak southward interplanetary magnetic field. In each case, the data suggest that the magnetic field in the tail lobe increased in magnitude directly in response to the external solar wind pressure increase. It is shown that a simple model in which a uniform magnetic field is compressed by a step function constriction accurately predicts characteristic timescales, which are of the order of a couple minutes, and the magnetic field profiles. The inferred flaring angles are consistent with model predictions, and the changes in the flaring angle across the discontinuities correspond to expectations based on changes in the subsolar magnetopause position and tail width. Overall, the results of this study indicate that the magnetotail maintains an approximate MHD equilibrium even as it responds rapidly to interplanetary pressure discontinuities.
Arguments against modulational instabilities of Langmuir waves in Earth's foreshock
Authors: Cairns Iver H. , Robinson P. A. , Smith N. I.
Published: February 2004

The nonlinear dispersion equation for monochromatic pump Langmuir waves is solved analytically and numerically and the results are applied to Langmuir‐like waves growing in Earth's foreshock. It is shown that modulational instabilities and parametric decay instabilities occur in distinct regions of W − κλD space, confirming and extending earlier work. Here W is the ratio of electric field to thermal energy density and κ is the pump wavenumber. In particular, for W ≲ 10−3 and Te ≳ 3Ti modulational instability occurs only for waves with κλD ≲ (me/9Mi)1/2, where me and Mi are electron and ion masses; decay is relevant at higher wavenumbers. Beam‐driven waves in the foreshock are shown to lie well outside the region of parameter space for which modulational instability can proceed. In contrast, the beam‐driven waves have wavenumbers appropriate for the decay. The beam‐driven waves are also shown theoretically and observationally to have bandwidths much larger than the nonlinear growth rate for modulational instability and the parametric decay. The absence of a random‐phase version of modulational instability and the decrease in growth rate caused by finite bandwidth effects provide two more arguments against modulational instability occurring frequently in the foreshock. Modulational instability may, however, be possible for very rare, intense (W ≳ 10−2) wave packets with small wavenumbers (κλD ≲ (me/9Mi)1/2) and small bandwidths (Δω/ωp ≲ 10−3). Furthermore, the large Langmuir bandwidths predicted and observed require that the parametric decay cannot proceed, except perhaps for very intense wave packets with W > 10−2. Instead the random‐phase, weak turbulence decay is predicted to occur, consistent with some previous observations and theoretical suggestions. Nonlinear plasma theory and our current understanding of foreshock electron beams and waves thus argue strongly against modulational instability and/or parametric decay proceeding or being important for the great majority of beam‐driven Langmuir wave packets in Earth's foreshock.
Mode conversion at the Jovian plasma sheet boundary
Authors: Perraut Sylvaine , Roux Alain , Louarn Philippe , Gurnett Donald A. , Kurth William S. , Khurana K. K.
Published: February 2004

The plasma wave data obtained by Galileo in Jupiter's magnetosphere often exhibit three distinct frequency bands in the frequency range between a few hertz and a few kilohertz. It is shown that these emissions are generally electromagnetic. They are identified by relating their characteristic frequencies to the solutions of the cold plasma dispersion relation. Four modes are possible: X, Z, O, and whistler. Knowing the electron gyrofrequency ƒce measured by the fluxgate magnetometer, we have considered two different hypotheses for the observed lower‐frequency cutoff of the intermediate frequency emissions which occur below ƒce. Under these assumptions, characteristic frequencies have been computed from the cold plasma theory and compared with the set of cutoff frequencies derived from the observations. Consistency checks lead to the identification of the intermediate frequency band as being on O mode with a low‐frequency cutoff at the electron plasma frequency ƒp. Below the O mode, Galileo detects whistler mode emissions (below ƒp). Above ƒce the observed emission is consistent with being X mode. An attempt is made to identify the source of the O mode radiation. Quasi‐electrostatic waves are sometimes identified below the upper hybrid frequency when the plasma sheet boundary is crossed. We suggest that these electrostatic waves, which are presumably generated by field‐aligned electron beams flowing along plasma sheet boundary, are successively mode converted into Z and later O mode. Thus the O mode observed mostly outside the plasma sheet is generated by mode conversion of primary electrostatic waves.
Geotail survey of ion flow in the plasma sheet: Observations between 10 and 50 RE
Authors: Paterson W. R. , Frank L. A. , Kokubun S. , Yamamoto T.
Published: February 2004

One‐minute samples of the velocity distributions of ions are used to compute plasma moments for the 18‐month period between December 1, 1994 and June 1, 1996. Statistical analyses of these measurements are given for the plasma sheet and its boundary layer at radial distances from Earth between 10 and 50 RE. Analysis concentrates on examination of parallel and perpendicular components of bulk flow and is directed toward understanding processes that affect transport. On average, the perpendicular components of the bulk flow are small, less than 15 km/s. On the basis of this study, two distinct patterns emerge. First, at radial distances from Earth less than 30 RE the mean perpendicular flow indicates a well‐defined pattern of convection that is a combination of drift toward Earth and drift toward the flanks. The average electric field across the tail is inferred to be 0.1 mV/m. Second, there is a notable increase in the variability of the perpendicular flow speeds beyond 25 RE. Although relatively large bulk flows with speeds in the range 200 to 800 km/s are sometimes found in the tail at lesser radial distance, these are primarily field‐aligned and thus do not contribute substantially to convection or to the transport of magnetic flux.
Statistical imaging of the Venus foreshock using VLF wave emissions
Authors: Crawford G. K. , Strangeway R. J. , Russell C. T.
Published: February 2004

Strong VLF wave emissions have been shown to be an intrinsic property of the Venus foreshock and foreshocks in general. In this work, we use these measured wave emissions to construct statistical “images” of the Venus foreshock. This analysis method allows us to develop a macroscopic picture of the wave properties, the inferred particle distributions, and their evolution as a function of position within the foreshock as well as compare our observations to Earth. The electron foreshock emissions at Venus are parallel polarized Langmuir mode waves with the same peak amplitude at the foreshock boundary as terrestrial emissions (10 mV/m). However, the wave characteristics differ markedly between the upstream and downstream foreshocks for near Parker spiral interplanetary magnetic field (IMF) orientations (∼35° for Venus). Additionally, there is a dramatic decrease in wave intensity for distances beyond ∼15 Rv from the point of tangency along the foreshock boundary. These characteristics in the wave emissions provide strong observational evidence supporting reflection and energization of solar wind electrons at the shock as the dominant source for providing upstream electrons, not leakage. In the ion foreshock the wave emissions consist of parallel polarized ion acoustic‐like waves with similar intensities and spectral characteristics to terrestrial emissions. However, these waves are situated much deeper in the foreshock than expected from terrestrial observations. Surprisingly, no emissions are observed in regions where field aligned ion distributions are expected. Rather, the emissions are confined to a region where diffuse ion distributions are expected.
Wind observations of the influence of the Sun's magnetic field on the interplanetary medium at 1 AU
Authors: Sanderson T. R. , Lin R. P. , Larson D. E. , McCarthy M. P. , Parks G. K. , Bosqued J. M. , Lormant N. , Ogilvie K. , Lepping R. P. , Szabo A. , Lazarus A. J. , Steinberg J. T. , Hoeksema J. T.
Published: February 2004

We combine data from several instruments on the Wind spacecraft with ground‐based observations of the position of the heliospheric current sheet for the first 2.5 years of the mission, which covers the period from the end of solar cycle 22, through solar minimum, and into the start of cycle 23. We use data from the three‐dimensional (3‐D) plasma instrument (3DP), magnetometer (MFI), and Solar Wind Experiment (SWE). We examine the energetic ion and electron increases associated with corotating interaction regions (CIR) and relate them to the position and shape of the current sheet. The characteristics of the energetic particle increases associated with the observed CIRs change several times during the mission. We identify these periods and find that we can relate these particle signatures of the CIRs to the plasma conditions which themselves depend upon the changing tilt and warp of the current sheet. We examine the additional increases associated with magnetic clouds and impulsive solar flares that were observed at irregular intervals during the mission. We show how the Sun's magnetic field influences the interplanetary medium at 1 AU, as variations in the shape and inclination of the source surface current sheet propagate out to 1 AU. We identify the three major sources of these variations, (1) the long‐term, slowly varying tilt of the current sheet with a timescale of the solar cycle, (2) the medium‐term small but significant annual movement of the position of the Earth relative to the current sheet due to the inclination of the plane of the ecliptic with respect to the heliographic equator, and (3) the short‐term warps and bulges which come and go due to the appearance of active regions, lasting only a few solar rotations.
SuperDARN interferometry: Meteor echoes and electron densities from groundscatter
Authors: André Dieter , Sofko George J. , Baker Kile , MacDougall John
Published: February 2004

The SuperDARN radars are now able to measure the angles of arrival of the backscattered radiation. We describe the analysis procedure and present results for meteor scatter from slant ranges below 500 km and ground scatter from either the E region or the F region at greater slant ranges which can be used to determine peak electron densities and their heights. The angle of arrival measurements can also be used to identify “unwanted” backscatter from the backward lobes of the antenna radiation pattern. Electron densities can also be measured in the back lobes.
Structure of the low‐latitude boundary layer: A case study with Geotail data
Authors: Fujimoto M. , Mukai T. , Kawano H. , Nakamura M. , Nishida A. , Saito Y. , Yamamoto T. , Kokubun S.
Published: February 2004

A case study of the structure of the low‐latitude boundary layer (LLBL) on dawnside (0600–0900 MLT) is reported. It is shown that the LLBL consists of two regions, the sheath‐like region and the mixing region. The sheath‐like region is where cold ions from the magnetosheath are flowing tailward. Detailed study has shown that this region is produced by reconnection and subsequent draping of open field lines over the dayside magnetosphere [Fujimoto et al., 1997]. The mixing region, on the other hand, is characterized by mixing of cold ions from the magnetosheath with the magnetospheric hot ions. Thermal electrons (< 500 eV) enhanced bidirectionally along field lines are detected to accompany these mixed ions. The balanced bi‐directional flux is taken to indicate the closed topology of the field lines. An interesting finding is that the flow direction in this region tends to be sunward. Flow in the mixing region and the adjacent plasma sheet is faster sunward. than in the ring current region, suggesting that these two constitute a channel of sunward returning convection in the dayside outer magnetosphere for this particular case. More new information on the mixing region from the present study is the ion distribution function showing a three‐component feature and autocorrelation between plasma density and the degree of electron bidirectional heating. These observational facts would offer clues for understanding of the mixing region formation, which still remains open.
Neutral density depletions attributed to plasma bubbles
Authors: Illés‐Almár E. , Almár I. , Bencze P.
Published: February 2004

Neutral density measurements in the equatorial thermosphere by the accelerometer of the San Marco V satellite in 1988 indicate from time to time sudden density depletions of 10–20 s. As the occurrence of such events maximizes between 1700 and 0300 hours local solar time, we interpret them as manifestations of the crossings of plasma bubbles by the satellite.
Particle propagation channel detected at 4.7 AU inside a corotating interaction region
Authors: Maia D. , Malandraki O. , Pick M. , Sarris E. T. , Kasotakis G. , Lanzerotti L. J. , MacLennan C. G. , Trochoutsos P. C.
Published: February 2004

This study presents an interplanetary particle (electrons and ions) event which was detected at 4.7 AU by the instruments on the Ulysses spacecraft inside a corotating interaction region between a fast and slow solar wind stream. A particle propagation channel is identified where the particles can propagate from the Sun nearly scatter free within a region bounded by tangential discontinuities. This channel is found to be rooted in the vicinity of a solar active region where the flare associated with the interplanetary event was produced. The ions (energy over 600 keV) detected inside this channel are shown to be of solar origin: dispersion occurs in the proton arrival time versus proton energy, there is a strong anisotropy in the proton pitch angle distribution, and a hard energy‐spectrum for the protons. The electrons were already present when the spacecraft crossed the structure. The chemical composition is also indicative of a solar origin, with a decrease in the 4He/H (4He in the energy range 389–1278 keV and H in the range 480–1204 keV) abundance ratio by a factor of 2 inside the channel. Mapped back to the solar surface, the size of the propagation channel is estimated to be of the order of 30,000 km. The channel is found to be magnetically quiet, without a preferred direction for the minimum of variance.
Formation of spherically polarized Alfvén waves and imbedded rotational discontinuities from a small number of entirely oblique waves
Authors: Vasquez Bernard J. , Hollweg Joseph V.
Published: February 2004

We present two‐and‐one‐half‐dimensional (2½‐D) hybrid numerical simulations of a small group of oblique Alfvén waves with linear polarization. These are the first simulations of nonplanar Alfvén waves which evolve to a nearly constant‐B state (spherical polarization) with imbedded rotational discontinuities (RDs). Initially, B varies with position in the wave group, and we consider only cases where the Fourier components of B2 are entirely oblique to the background magnetic field B0. When propagating in different directions but with group speeds in the same direction along B0, Alfvén waves generally evolve nonplanar waveforms with nearly constant magnetic intensity B. In this waveform, the magnetic field vectors move on a sphere of radius B and have a spherical rather than arc polarization. Most Alfvénic fluctuations in the solar wind are spherically polarized. We also find analytical second‐order solutions from the magnetohydrodynamic equations for linearly polarized Alfvén waves in 2½‐D and 3‐D. For moderate‐wave amplitudes (|δB|/B0 ≲ 0.5), these show that the second‐order driven wave solution can only remove variations of B when the Fourier components of B2 are oblique to B0. Large‐amplitude (|δB|/B0 ∼ 1) waves also evolve to constant magnetic intensity, but higher‐order terms produce imbedded RDs with properties similar to those seen in the solar wind. The RDs are steady, and their normals are oblique to B0. The transverse extent of the RDs is approximately of order of the average wavelength of the Alfvén waves, and the RDs are either locally planar or fully nonplanar. RDs can appear at less than twice per cycle in waveforms composed of two or more waves. If the magnetic fields are sampled along a single line through the simulation box, as a single spacecraft would observe Alfvénic fluctuations in the solar wind, the rate depends on the direction that the line takes through the box. We also suggest that all Alfvénic fluctuations are spherically polarized, and instances of arc‐polarized fluctuations occur when spacecraft sample locally planar structures.
Central plasma sheet ion properties as inferred from ionospheric observations
Authors: Wing Simon , Newell Patrick T.
Published: February 2004

A method of inferring central plasma sheet (CPS) temperature, density, and pressure from ionospheric observations is developed. The advantage of this method over in situ measurements is that the CPS can be studied in its entirety, rather than only in fragments. As a result, for the first time, comprehensive two‐dimensional equatorial maps of CPS pressure, density, and temperature within the isotropic plasma sheet are produced. These particle properties are calculated from data taken by the Special Sensor for Precipitating Particles, version 4 (SSJ4) particle instruments onboard DMSP F8, F9, F10, and F11 satellites during the entire year of 1992. Ion spectra occurring in conjunction with electron acceleration events are specifically excluded. Because of the variability of magnetotail stretching, the mapping to the plasma sheet is done using a modified Tsyganenko [1989] magnetic field model (T89) adjusted to agree with the actual magnetotail stretch at observation time. The latter is inferred with a high degree of accuracy (correlation coefficient ∼0.9) from the latitude of the DMSP b2i boundary (equivalent to the ion isotropy boundary). The results show that temperature, pressure, and density all exhibit dawn‐dusk asymmetries unresolved with previous measurements. The ion temperature peaks near the midnight meridian. This peak, which has been associated with bursty bulk flow events, widens in the Y direction with increased activity. The temperature is higher at dusk than at dawn, and this asymmetry increases with decreasing distance from the Earth. In contrast, the density is higher at dawn than at dusk, and there appears to be a density enhancement in the low‐latitude boundary layer regions which increases with decreasing magnetic activity. In the near‐Earth regions, the pressure is higher at dusk than at dawn, but this asymmetry weakens with increasing distance from the Earth and may even reverse so that at distances X < ∼−10 to −12 RE depending on magnetic activity, the dawn sector has slightly higher pressure. The temperature and density asymmetries in the near‐Earth region are consistent with the ion westward gradient/curvature drift as the ions E×B convect earthward. When the solar wind dynamic pressure increases, CPS density and pressure appear to increase, but the temperature remains relatively constant. Comparison with previously published work indicates good agreement between the inferred pressure, temperature, and density and those obtained from in situ data. This new method should provide a continuous mechanism to monitor the pressure, temperature, and density in the magnetotail with unprecedented comprehensiveness.
The radial and latitudinal dependence of the cosmic ray diffusion tensor in the heliosphere
Authors: Zank G. P. , Matthaeus W. H. , Bieber J. W. , Moraal H.
Published: February 2004

The radial and latitudinal dependence of the cosmic ray diffusion tensor is investigated on the basis of a recently developed model of magnetohydrodynamic turbulence in the expanding solar wind [Zank et al., 1996a,b; Matthaeus et al., 1996]. In the ecliptic plane, decaying magnetohydrodynamic turbulence is assumed to be replenished in situ by turbulence generated through the interaction of streams (both shear and compressional effects) and by the creation of pickup ions. In the polar region, at least during solar minimum, stream interaction driven turbulence is neglected and only pickup ion driven turbulence is included. To model the perpendicular and drift elements of the cosmic ray diffusion tensor, we employ both a quasi‐linear theory (QLT) and a newly developed nonperturbative theory (NPT) to describe the field line wandering which drives perpendicular transport. A resonant quasi‐linear description is applied to the parallel component. For the QLT approach, we find that in the solar wind ecliptic plane (1) the radial diffusive length scale or mean free path (mfp) is very nearly constant until some 10 AU, after which it experiences some variation with increasing heliocentric distance; (2) the radial mfp is dominated at all radial distances by the component parallel to the mean magnetic field and the perpendicular component is completely unimportant; (3) the length scale associated with the drift component of the cosmic ray diffusion tensor is only comparable to the radial mfp beyond ∼ 10 AU; and (4) the rigidity P dependence of the radial mfp within 10 – 20 AU is weak and proportional to P1/3, but in the far outer heliosphere it is proportional to P2. For the QLT model in the polar region of the solar wind, we find that the radial cosmic ray mfp is much greater than the corresponding mfp in the ecliptic region, consistent with observed mfps for pickup ions reported by Gloeckler et al. [1995]. The polar models are, however, preliminary and assume vanishing cross‐helicity. The polar radial mfp is dominated by the parallel component, and drift length scales are never comparable to the radial mfp in the high polar latitudes. By using instead a nonperturbative model for the perpendicular and drift components of the cosmic ray diffusion tensor, it was found that the mfps for these coefficients could be significantly larger than their QLT counterparts. The increased perpendicular mfp was found to be important in the radial mfp only beyond ∼ 20 AU, which remains dominated by the parallel diffusion within this distance. Within the ecliptic, the nonperturbative model yields a radial mfp for cosmic rays that is almost constant with heliocentric distance. Similar order of magnitude differences between the radial mfps in the ecliptic and polar regions of the solar wind are found with the nonperturbative models.
Energy transfer via region 2 currents: A test of the standard magnetosphere‐ionosphere coupling theory
Authors: Siscoe George , Lu Gang
Published: February 2004

The magnetosphere‐ionosphere coupling theory associated with the names of Fejer, Vasyliunas, Swift, Wolf, and others can be used to predict that in steady state the region 2 currents transfer no net energy between the ionosphere and the magnetosphere, but when the transpolar potential drop increases, region 2 currents transfer energy away from the ionosphere into the magnetosphere. Both properties are counterintuitive and differ from the “normal” properties exhibited by region 1 currents, which in steady state or otherwise consistently deliver energy to the ionosphere to feed Joule dissipation. We extend the theory to include the effects of charge exchange so that it can be applied to magnetic storms. We conclude that charge exchange also extracts energy from the ionosphere via region 2 currents. To test these predictions of unexpected properties, we use the technique of assimilative mapping of ionospheric electrodynaics (AMIE). From AMIE‐derived values of ionospheric electrical potentials and field‐aligned currents for a well documented magnetic storm, we compute the direction and flux of energy flowing between the ionosphere and the magnetosphere. Region 2 currents indeed transfer energy out of the ionosphere during the storm main phase, when the transpolar potential drop increases to its storm time value. The rate of energy transfer is greatly suppressed late in the recovery phase when conditions approximate steady state. Also the data are consistent with charge exchange acting to extract energy from the ionosphere via region 2 currents. Thus the test confirms two of the predictions and is consistent with the third. For this storm the region 2 upward energy flux is at most about 30% of the energy that goes into the ring current to account for the growth of Dst.
Wave characteristics of geomagnetic pulsations across the dip equator
Authors: Shinohara M. , Yumoto K. , Hosen N. , Yoshikawa A. , Tachihara H. , Saka O. , Kitamura T.‐I. , Trivedi N. B. , Da Costa J. M. , Schuch N. J.
Published: February 2004

In order to clarify the wave characteristics of Pi2 and Pc 4–5 magnetic pulsations around the dip equator, we analyzed magnetic data from the latitudinally dense magnetometer array in Brazil. We found that the phase difference between Pi2 pulsations observed at globally separated low‐latitude stations is small, whereas Pi2 pulsations observed within the dayside dip equator region of ±2° latitude show phase lags of about 30° ∼ 50° behind those in the off‐dip equator region. Pc 4–5 magnetic pulsations at the dip equator also show the same phase character. Pi2 amplitudes are enhanced in the equatorial region, where the phase lags of pulsations must be associated with the enhancement of ionospheric conductivity. The equatorial phase lags can be explained by invoking the induction effect of the equatorial enhanced ionospheric current above the good conductor Earth.
An ionospheric conductance model based on ground magnetic disturbance data
Authors: Ahn B.‐H. , Richmond A. D. , Kamide Y. , Kroehl H. W. , Emery B. A. , de la Beaujardiére O. , Akasofu S.‐I.
Published: February 2004

An attempt is made to construct an improved ionospheric conductance model employing ground magnetic disturbance data as input. For each of the different regions in the auroral electrojets specified by different combinations of horizontal (ΔH) and vertical (ΔZ) magnetic perturbations, as well as by magnetic local time (MLT), an empirical relationship is obtained between the ionospheric conductance deduced from Chatanika radar data and magnetic disturbances from the nearby College magnetic station. The error involved in the empirical formula is generally of the order of 20–50%. However, some sectors are so poorly covered that uncertainty estimates cannot be made. The new formulas are applied to an average magnetic disturbance distribution to deduce the average conductance distribution. This is compared with a conductance model based on electron precipitation data [Hardy et al., 1987], finding good agreement in terms of the magnitude and distribution pattern. Combining our empirical relationships with the empirical formulas proposed by Robinson et al. [1987], the average energy and energy flux of precipitating electrons are also estimated. Notable similarities exist between the global distribution patterns of these and those obtained by Hardy et al. [1985]. It is proposed that the present conductance model can be used to complement more direct measurements in order to obtain the global distribution needed to study the large‐scale electrodynamics of the polar ionosphere. Several interesting characteristics about the auroral electrojet system are apparent from the empirical relationship: (1) For a given magnitude of ΔH, the electric field is relatively stronger in the eastward electrojet region than in the westward electrojet region. (2) The electric field plays a greater role in the intensification of electrojet current than the ionospheric conductance does in the poleward half of the westward electrojet, whereas the opposite trend is apparent in the equatorward half. However, no such different roles of the electric field and conductance is noticeable in the eastward electrojet region. (3) The auroral conductance enhancements tend to be largest around midnight, due to more intense particle fluxes there. (4) The mean particle energy depends on MLT but is relatively insensitive to magnetic activity.
Equinoctial asymmetries in the ionosphere and thermosphere observed by the MU radar
Authors: Balan N. , Otsuka Y. , Bailey G. J. , Fukao S.
Published: February 2004

Annual variations of the ionosphere and thermosphere studied with the middle and upper atmosphere (MU) radar during the solar maximum period 1988–1992 show that the well‐known seasonal anomaly in the electron density Ne exists only during daytime and at altitudes near the ionospheric peak and below. The observations also reveal the existence of equinoctial asymmetries in the ionosphere and thermosphere, with the asymmetry in the ionosphere changing its character with altitude during daytime. In the bottomside ionosphere the electron density Ne is slightly greater in September equinox than in March equinox. At higher altitudes the asymmetry reverses and becomes strong; the values of Ne in March equinox exceed those in September equinox by up to 100%. The electron temperature Te exhibits equinoctial asymmetries almost opposite those in Ne. The ion temperature Ti shows a weak asymmetry, in phase with the asymmetry in Ne. The field‐aligned and field‐perpendicular plasma velocities V‖ and V⊥ are also different in the two equinoxes. In the thermosphere the neutral wind and composition show consistent equinoctial asymmetries. The meridional component of the daytime poleward wind velocity (Uθ) derived from the field‐parallel plasma velocity is weaker in March equinox than in September equinox by up to 20 m s−1, and the values of the daytime [O]/[N2] ratio obtained from MSIS‐86 are larger in September equinox than in March equinox by about 20%. Model calculations carried out by incorporating the measured V⊥ and Uθ into the Sheffield University plasmasphere‐ionosphere model that uses MSIS‐86 for neutral atmosphere show that the equinoctial asymmetries in the ionosphere arise mainly from the corresponding asymmetries in the thermosphere, with major contributions from neutral winds and minor contributions from composition.