bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetohydrodynamic simulations of the motion of magnetic flux tubes through a magnetized plasma
Authors: Cargill P. J. , Chen J. , Spicer D. S. , Zalesak S. T.
Published: February 2004

Magnetohydrodynamic simulations of the evolution of a flux tube accelerated through a stationary magnetized plasma are presented. As the flux tube moves through the external plasma, its shape becomes distorted and reconnection can take place between the flux tube and external fields. The coupling between the moving flux tube and the external plasma is generally efficient, with simulated flux tube velocities many times smaller than those expected from frictionless motion. The reconnection between the flux tube and external field takes place when there is a unidirectional external field component in the direction of flux tube propagation. The reconnection is intrinsically nonsymmetric around the flux tube boundary. The principal reconnection site is at the rear of the flux tube, where strong vortices convect the external field toward the flux tube. Drag coefficients (CD) that parameterize this interaction have been determined. When the flux tube is continually accelerated, CD > 1 is appropriate, consistent with previously used ad hoc values. Examples of when the flux tube is accelerated for a short time but allowed to continue interacting with the external plasma are presented. It is shown that in the absence of reconnection, the coupling time is several Alfvén wave transit times across the flux tube. However, when reconnection takes place, this coupling can cease to occur, and the flux tube may move frictionlessly (CD ≈ O). The results are discussed in terms of interplanetary magnetic clouds, and it is suggested that the observations of comoving coronal mass ejection and solar wind plasma can be accounted for by drag between the two.
Pc 1–2 and Pc 4–5 pulsations observed at a network of high‐latitude stations
Authors: Plyasova‐Bakounina T. A. , Kangas J. , Mursula K. , Molchanov O. A. , Green A. W.
Published: February 2004

Analysis of magnetospheric Pc 1 and Pc 2 (in brief Pc 1–2) pulsations at a network of six northern hemisphere stations at latitudes 65°–76° CGM shows that their average frequency decreases with increasing latitude of the station. Their maximum occurrence is at L=6–9 and 1200–1600 LT. The ionospheric ducting is weak for f < 0.3 Hz. Occasionally the Pc 1 intensity at a station is modulated by a simultaneously arriving quasi‐periodic Pc 4–5 signal. A mechanism for generating micropulsations which is common to both high‐frequency and low‐frequency waves is presented and discussed, the proposed mechansim being that a broadband signal from the solar wind is amplified in the magnetosphere in the frequency bands of the field line resonance (Pc 4–5) and in the He+ cyclotron frequency (Pc 1–2) range.
Alfvén solitons in the Earth's ionosphere and magnetosphere
Authors: Pokhotelov O. A. , Pokhotelov D. O. , Gokhberg M. B. , Feygin F. Z. , Stenflo L. , Shukla P. K.
Published: February 2004

Nonlinear equations of the Kadomtsev‐Petviashvili type are derived for Alfvén and magnetosonic waves propagating at a small angle to the Earth's magnetic field. This represents a generalization of the derivative nonlinear Schrödinger equation to the two‐dimensional case. The theory is applied to explain the observations of stable Alfvén‐like solitons by means of the Aureol 3 satellite during the Russian seismic impact experiments. It may also be useful for theoretical interpretations of the chevron structures of the “pearls” that are occasionally observed in the Earth's magnetosphere.
Spatial evolution of the magnetic field spectral exponent in the solar wind: Helios and Ulysses comparison
Authors: Marsch E. , Tu C.‐Y.
Published: February 2004

The spatial evolution of the spectral exponent of magnetic field fluctuations in the solar wind is investigated by a comparison of spectra and length functions obtained from Helios for in‐ecliptic and from Ulysses for high‐latitude observations. A similar radial evolution trend is found in both data sets for the Bz component of the magnetic field, which is least affected by compressive interactions. Yet the fluctuations seem to evolve more slowly in the polar flows as compared with the Alfvénic fluctuations found by the Helios experiments in coronal‐hole‐related flows near the Sun in the ecliptic plane.
Magnetic field structure in the comet Grigg–Skjellerup pileup region
Authors: Israelevich P. L. , Ershkovich A. I. , Neubauer F. M. , Coates A. J.
Published: February 2004

The position of the Giotto spacecraft relative to comet Grigg‐Skjellerup is found from the draping geometry based on the vector magnetic field data. It is shown that Giotto passed the cometary nucleus on the nightside. The behavior of the magnetic field vectors exhibits two different types in the pileup region. In the outer pileup region, the magnetic field vectors converge toward the nightside in accordance with the flank draping observed earlier near comet Halley. The existence of the flank draping is a consequence of a three‐dimensional plasma flow around the comet: a slippage of magnetic field lines on the flanks of the obstacle results in this effect. Because of this slippage, only a fraction of the incoming magnetic flux takes part in the formation of the magnetic barrier and the magnetic tail, whereas most of the magnetic flux bypasses a comet from both sides. In the inner pileup region, the magnetic field vectors diverge toward the nightside. This effect is attributed to the plasma flow into the cavity behind the obstacle.
Hydrogen peroxide and methylhydroperoxide distributions related to ozone and odd hydrogen over the North Pacific in the fall of 1991
Authors: Heikes Brian G. , Lee Meehye , Bradshaw J. , Sandholm S. , Davis D. D. , Crawford J. , Rodriguez Jose , Liu S. , McKeen S. , Thornton D. , Bandy A. , Gregory G. , Talbot R. , Blake D.
Published: February 2004

Hydrogen peroxide and methylhydroperoxide were measured in the troposphere over the western North Pacific as part of the airborne portion of NASA's Global Tropospheric Experiment/Pacific Exploratory Mission‐West A field mission. The flights circled the North Pacific, focusing on the western Pacific, and extended from 300 to 13,000 m altitude. The hydroperoxides were uniquely separated and quantified using a high‐pressure liquid chromatography system in conjunction with a continuous enzyme fluorometric instrument. Results show a latitudinal gradient in both peroxides at all altitudes; for example, between 3 and 5 km, H2O2 median values decrease from 1700 to 500 parts per trillion by volume (pptv) in going from 0°–15°N to 45°–60°N, and the corresponding decrease in CH3OOH was 1100 to 200 pptv. Concentration maxima are observed in both species at altitudes of 2 to 3 km with H2O2 concentrations below 1 km lower by 30%, 10% for CH3OOH, and even lower, by a factor of 10, for both above 9 km. The H2O2 to CH3OOH ratio increased with altitude and latitude with ratios <1 in the tropical surface layer and >2 at midlatitude high altitude. Highest peroxide concentrations were encountered over the Celebes Sea in air which was impacted by aged biomass fire and urban pollutants. CH3OOH was below the level of detection in stratospheric air. H2O2 exceeded SO2 95% of the time, with the exceptions generally above 9 km. Above 3 km, O3 increases with decreasing H2O2 and CH3OOH. Below 3 km the O3‐CH3OOH trend is the same but O3 increases with increasing H2O2. The measurements are compared with predictions based upon a photochemical steady state zero‐dimensional model and a three‐dimensional mesoscale time‐dependent model. These models capture observed trends in H2O2 and CH3OOH, with the possible exception of H2O2 below 2 km where surface removal is important. A surface removal lifetime of 3.5 days brings the observed and zero‐dimensional model‐estimated H2O2 into agreement. The steady state model suggests a strong correlation between the ratios of NO/CO or HO2/HO and the ratio of H2O2/CH3OOH. The observed hydroperoxide ratios bracket the modeled relationship with occasionally much lower H2O2 than expected.
Land‐atmosphere interaction
Authors: Dickinson R. E.
Published: February 2004

The basic elements of land‐atmosphere interaction (LA‐I) are the exchanges of moisture and energy between these two systems. Historically, many of the important aspects of this interaction have been treated in the areas related to micrometeorology, agriculture and forest meteorology, planetary boundary layer, and hydrology. More recently, LA‐I has also become recognized as important for studies of biogeochemical cycling, climate, mesoscale meteorology, and numerical weather prediction. Initial recognition of the importance of LA‐I in these latter areas extends back at least two decades, but major advances have occurred over the period covered by this review. Land has been recognized as, in principle, a major element of the climate system since the initiation of the World Climate Research Programme (WCRP), and land was included in simple form in even the earliest General Circulation Model (GCM) climate studies.
Density depletion in an anisotropic magnetosheath
Authors: Denton Richard E. , Lyon John G.
Published: February 2004

We use a two dimensional model of the magnetosheath, not including the bow shock, to examine the effects of pressure anisotropy on magnetosheath structure. The simulation plane includes the Earth‐Sun direction and that of the incoming interplanetary magnetic field. Thus the crucial physical effects of compression and draping are both included. Flow of magnetic flux and plasma around the magnetopause in the third direction is modeled by a finite electric field at the magnetopause which allows plasma and flux to cross the magnetopause. The magnetpause is a flux surface, which is a reasonable approximation for low shear conditions. We use three models for the plasma pressure: isotropic adiabatic, double adiabatic, and the bounded anisotropy model [Denton et al., 1994; 1995]. In an isotropic plasma, density depletion results from draping which produces parallel (to B) flow around the magnetopause. This flow is greatly enhanced by the parallel pressure gradient. In an anisotropic plasma, enhancement of density depletion comes about not by the ordinary parallel pressure gradient, but by action of the mirror force. Because this mirror force is large for an anisotropic plasma, use of double adiabatic or bounded anisotropy pressure equations results in greater density depletion than that resulting from isotropic adiabatic theory.
Concerning possible effects of ionospheric conductivity upon the occurrence patterns of impulsive events in high‐latitude ground magnetograms
Authors: Sibeck D. G. , Greenwald R. A. , Bristow W. A. , Korotova G. I.
Published: February 2004

Previous studies have interpreted the occurrence patterns of impulsive events in the high‐latitude dayside ionosphere in terms of transient processes occurring near the dayside magnetopause. In particular, the fact that most impulsive ground magnetometer events occur prior to local noon has been interpreted as evidence for a dawn/dusk asymmetry in transient processes at the magnetopause. In this paper, we seek to raise the awareness of the community that the high‐latitude distribution of the Hall conductivity also influences the signatures observed by ground magnetometers. In particular, we will show that impulsive event occurrence patterns bear a striking resemblance to reported statistical patterns for the Hall conductivity. We suggest that this similarity be taken into account when interpreting occurrence patterns for impulsive ground events.
Some new perspectives on quasi‐static convection formalism
Authors: Liu W. William
Published: February 2004

Magnetospheric convection occurs in response to a combination of factors. It is driven by the solar wind‐magnetosphere interaction, causes Joule heating in the ionosphere, and energizes particles as they convect earthward. While the equation governing magnetospheric convection under the quasi‐static condition is well known, there is a lack of a formal proof that the energy processes named above are properly balanced in the convection formalism. In particular, the question has not been answered concerning the conservation of energy in a steady state convection. In this paper, I prove that energy conservation holds for steady state convection of an isotropic plasma. The consideration of energetics also sheds light on the nature of and relationship between region 1 and region 2 currents. It is further proven that the quasistatic convection formalism is conformal invariant, a property which can be exploited to develop more efficient algorithms for modeling magnetospheric convection. Several examples are presented to illustrate the conformal transformation method.
The SAPPHIRE‐North radar experiment: Observations of discrete and diffuse echoes
Authors: Kustov A. V. , Koehler J. A. , Sofko G. J. , Danskin D. W.
Published: February 2004

Since February 1995, regular observations have been carried out by the University of Saskatchewan radar group using a new 50‐MHz Doppler radar system called SAPPHIRE‐North. The SAPPHIRE‐North radar monitors the ionospheric processes in the transitional region between the auroral oval and the polar cap over Cambridge Bay, Northwest Territories. In this paper, results of the first 2 months of observations are presented. Echoes were registered at about 10% of observational time. Diurnal echo occurrence exhibited two distinct maxima, one close to noon and another prior to midnight. The noon echoes were usually of a diffuse type, while midnight echoes were of a discrete type. Midnight discrete echoes were found to be associated with an arrival of poleward expanding substorm bulges into the SAPPHIRE‐North field of view. Noon diffuse echoes were most likely due to electric field enhancements in the vicinity of the cusp/cleft region. Salient spectral features of both the diffuse and discrete echoes are presented and compared with auroral zone results. The role of ionospheric refraction for observations of these high‐latitude echoes is discussed.
Twenty‐five milliamperes: A tale of two spacecraft
Authors: Van Allen James A.
Published: February 2004

This paper is a participant's account of the spacecraft Pioneer 10 and Pioneer 11, their historic missions to Jupiter, Saturn, and the outer heliosphere; and the observations by the University of Iowa radiation instruments on both. Each of these instruments has a mass of 1.64 kg and draws 25 mA from the electrical power supply of the spacecraft. It is shown that a mere 25 mA, if properly applied, can yield some remarkable results. The flow of data is continuing at the time of writing.
Curiosity and chance
Authors: Haerendel Gerhard
Published: February 2004

The author recounts some stations of his professional life, which was to a large extent dedicated to active experimentation with plasma clouds in space and to the development of theoretical concepts for some intriguing plasmaphsical processes. The human and political environment of this work receives some coverage as well as his opinion of state and future of his discipline.
An unplanned career in space physics
Authors: Hunten Donald M.
Published: February 2004

This paper recounts a career that started out in nuclear physics and migrated through spectroscopy of aurora and airglow, study of the Earth's upper atmosphere, and spectroscopy of planetary atmospheres to work with sounding rockets and experiments on planetary spacecraft.
The electrical and precipitation characteristics of morning sector Sun‐aligned auroral arcs
Authors: Kletzing C. A. , Berg G. , Kelley M. C. , Primdahl F. , Torbert R. B.
Published: February 2004

Two sounding rockets launched from Greenland on March 31, 1987, present one of the most detailed pictures of polar cap auroral arc dynamics ever obtained. The electrodynamics show a clear picture of energy flux carried by energetic, precipitating electrons accompanied by a comparable magnitude Poynting flux immediately adjacent to the electron precipitation. The result is a W‐shaped region of energy flux at the locations of auroral arcs. In addition, the current carried by the energetic, precipitating electrons is somewhat greater than that indicated by the magnetometer measurements, suggesting that the background ionosphere in the polar cap may have closed the upward electron current with a downward current along the same field lines. The measurements are compared to a model of quasi‐static auroral acceleration based on that by Lyons [1981]. It is found that this model reproduces well most of the electrodynamic features; energy flux, electric fields, and parallel currents determined by the model are in generally good agreement with the in situ measurements.
Electromagnetic proton cyclotron instability: Proton velocity distributions
Authors: Gary S. Peter , Vazquez Victor M. , Winske Dan
Published: February 2004

The hot (Th ∼ 10 keV) protons of the outer magnetosphere are frequently observed to have T⊥/T‖ > 1, where the subscripts denote directions relative to the background magnetic field. This anisotropy leads to the growth of the electromagnetic proton cyclotron anisotropy instability, which implies two concise scaling relations: an upper bound on T⊥h/T‖h and an expression for the apparent temperature of warm (Tω ∼ 10 eV) protons heated by this instability. One‐dimensional hybrid simulations of this instability in a homogeneous plasma have been used to examine the velocity distribution response of both hot and warm protons. Results are that the enhanced fluctuations from this instability not only preserve the initially bi‐Maxwellian character of a hot proton distribution but also drive a hot distribution which is initially nongyrotropic rapidly toward the gyrotropic condition. These results further support the validity of the hot proton temperature anisotropy upper bound. Although wave‐particle interactions yield nongyrotropic ƒω(υ⊥) distributions, the ƒω(υ‖) remain Maxwellian‐like as they are heated by the enhanced fluctuations.
Discrete, stimulated auroral kilometric radiation observed in the Galileo and DE 1 wideband data
Authors: Menietti J. D. , Wong H. K. , Kurth W. S. , Gurnett D. A. , Granroth L. J. , Groene J. B.
Published: February 2004

The Galileo spacecraft observed intense auroral kilometric radiation during the second Earth encounter in 1992. High‐resolution frequency‐versus‐time spectrograms obtained by the wideband receiver of the plasma wave instrument on board the spacecraft often show discrete, negative‐slope features each extending over a period of several seconds. These features may be due to impulsive wave generation and intrinsic velocity dispersion and/or a source stimulation by plasma waves traveling up the magnetic field line through the source region. We present several examples of these signatures, seen also on the Dynamics Explorer 1 satellite, and examine scenarios for their generation.
The polytropic index for the solar wind at Earth's bow shock
Authors: Grabbe Crockett L.
Published: February 2004

Analysis of spacecraft shock‐crossing data on the magnetosheath thickness to determine the polytropic index γ of the solar wind is shown to overestimate the value for γ when the gasdynamic (GD) equations are used for the shock jump. This is done by comparing GD estimates for γ extrapolated from observed data points to values obtained when more appropriate MHD equations are used. As a result of this bias a shock‐crossing data analysis carried out recently using the GD equations appears to have obtained a value for γ that is over 0.5 too large. This is in part associated with the breakdown of GD with respect to nonzero angle between the bow‐shock normal and the magnetic field for finite Alfven Mach numbers. The magnetosonic phenomenological model (sonic Mach number replaced with the magnetosonic Mach number in the GD equations), which was also recently used to determine γ, is found to reduce this bias by 70–90%. This analysis also strongly suggests that the phenomenological model inadequately treats variations in the angle between the bow‐shock normal and the magnetic field, which may also produce systematic errors in the experimental determination of γ using this model.
Modeling the entry of magnetosheath electrons into the dayside ionosphere
Authors: Wing S. , Newell P. T. , Onsager T. G.
Published: February 2004

It has recently been shown that it is possible to quantitatively model the entry of magnetosheath ions and their access to the dayside ionosphere with surprisingly good results. In the same model, electrons had access to the region poleward of the cusp at unrealistically high intensities. We improve the previous model by imposing the constraints of charge quasi‐neutrality and introducing more realistic electron magnetosheath populations. It turns out that no potential drop within the cusp proper is either needed or observed in order to enforce charge neutrality, since ions, as well as electrons, can enter freely, and they originally have the same density in the magnetosheath. Poleward of the magnetic cusp, ion entry is sharply curtailed because of the tailward magnetosheath flow, and the potential required and observed rises rapidly. This potential eliminates access of the “core” population of the magnetosheath electrons to the ionosphere. The typical polar rain signature observed at low altitudes fits best with the suprathermal solar wind electron population (either halo or both halo and strahl components). The model clearly shows that ions previously identified at low altitude as “mantle” do indeed cross the magnetopause tailward of the magnetic cusp, that is, the ionospheric mantle signature consists of ions originating in the high‐altitude mantle. A single ion spectrum within the low‐altitude cusp proves to consist of magnetosheath ions which have crossed the frontside magnetopause from a range of positions which commences with the merging site and extends to the magnetic cusp, but which is typically only 1–3 RE wide along the direction of the field line convection.
Measurement of the magnetotail reconnection rate
Authors: Blanchard G. T. , Lyons L. R. , de la Beaujardière O. , Doe R. A. , Mendillo M.
Published: February 2004

A technique to measure the magnetotail reconnection rate from the ground is described and applied to 71 hours of measurements from 20 nights. The reconnection rate is obtained from the ionospheric flow across the polar cap boundary in the frame of reference of the boundary, measured by the Sondrestrom incoherent scatter radar. For our measurements, the polar cap boundary is located using 6300 Å auroral emissions and E region electron density. The average experimental uncertainty of the reconnection rate measurement is 11.6 mV m−1 in the ionospheric electric field. By using a large data set, we obtain the dependence of the reconnection rate on magnetic local time, the interplanetary magnetic field, and substorm activity, with much higher accuracy. We find that two thirds of the average polar cap potential drop occurs over the 4‐hour segment of the separatrix centered on 2330 MLT, that the linear correlation between the reconnection electric field and the half‐wave rectified dawn‐dusk solar wind electric field VBs peaks between 1.0 and 1.5 hours, with a maximum linear correlation coefficient of 0.46 at 70 min; and that following substorm expansion phase onset, the reconnection electric field becomes larger than the experimental uncertainty, with an average delay of 23 min. The 70‐min delay of the reconnection rate with respect to VBs is a typical convection time for a flux tube across the polar cap. This result indicates that reconnection in the magnetotail is influenced by the solar wind electric field VBs on the field line being reconnected.
Spatial modulation of electron energy and density by nonlinear stationary inertial Alfvén waves
Authors: Knudsen David J.
Published: February 2004

A cold, nonlinear, quasi‐neutral, two‐fluid model of field‐aligned current (FAC) sheets with a uniform tangential electric field describes stationary, spatially periodic electromagnetic perturbations (stationary inertial Alfvén (SIA) waves) which carry enhanced current, which can accelerate electrons parallel to a strong background magnetic field B0, and which cause large variations in plasma density. The waves propagate along B0 at speeds less than the Alfvén speed VA. Waves can be divided into two branches according to the sign of their field‐aligned phase velocity with respect to the direction of field‐aligned electron drift. Waves propagating parallel to electron drift can accelerate low‐energy electrons up to VA within steady state parallel sheets with widths of the order of the electron inertial length λe = c/ωpe, where ωpe is the electron plasma frequency. Waves propagating antiparallel to electron drift can accelerate electrons to velocities much greater than VA in steady state sheets which are orders of magnitude wider than λe and which are depleted in density. SIA waves also form when only an energetic subpopulation of electrons carries FAC through an ambient population initially at rest and can accelerate the ambient electrons to approach the energy of the subpopulation. These results extend the phenomenon of steady state electron acceleration by inertial Alfvén waves to include much higher energies and wider spatial scales than previously recognized. Although the relatively simple model described here omits certain effects which are known to be important in the Earth's auroral regions, several properties of SIA wave solutions suggest they may play a role in auroral arc formation, including steady state field‐aligned electron acceleration over a broad range of spatial scales and energies, density variation, and the formation of multiple parallel structures.
Equatorial plasma depletion precursor signatures and onset observed at 11° south of the magnetic equator
Authors: Weber E. J. , Basu S. , Bullett T. W. , Valladares C. , Bishop G. , Groves K. , Kuenzler H. , Ning P. , Sultan P. J. , Sheehan R. E. , Araya J.
Published: February 2004

Coordinated radio and optical measurements of the structure and dynamics of the postsunset equatorial ionosphere were conducted on October 1, 1994, from Agua Verde, Chile (11.3°S magnetic latitude (MLat)). The measurements clearly show a north‐south aligned undulation or ripple on the bottomside of the F layer at 2000 LT, appearing as an eastward propagating decrease in the 630.0‐nm airglow, resembling a traveling ionospheric disturbance in the digital portable ionosonde measurements and causing a total electron content decrease in the Global Positioning System (GPS) satellite measurements. The initial development of this feature, toward the east and away from the magnetic equator, took place in an otherwise smooth, unstructured ionosphere. Spread F began to develop in the ionograms at 2020 LT, and, at this same time, local onset of satellite signal scintillation was detected using the multiple ray paths throughout the sky available from the GPS satellite constellation transmitting at L band frequencies. UHF scintillation measurements from Ancon, Peru, along the same magnetic field line, show that intense scintillation and ionospheric irregularities had developed over the magnetic equator almost 60 min prior to their development at 11°S MLat. The observations suggest that the east‐west electric field expected to be present within the earlier developed depletion and scintillation region at the magnetic equator mapped along magnetic field lines to lower altitudes and higher latitudes, resulting in an undulation or dome‐shaped structure, before evolving into a fully developed depletion (with associated ionospheric irregularities) all along the magnetic flux tube.
Synthesis models of dayside field‐aligned currents for strong interplanetary magnetic field By
Authors: Watanabe Masakazu , Iijima Takesi , Rieh Frederick J.
Published: February 2004

Using particle and magnetic field data acquired with DMSP‐F6 and DMSP‐F7 satellites, we have investigated interplanetary magnetic field (IMF) By dependence of the global pattern of plasma regime and field‐aligned currents (FACs) on dayside high latitudes during strong IMF By (averaged |By| > 3.7 nT) and geomagnetically disturbed (mainly IMF Bz < 0) periods. From particle data we have identified five plasma regimes: inner plasma sheet, outer plasma sheet, cleft, cusp, and mantle. All the plasma domains except the inner plasma sheet show By dependence in spatial distribution. Region 1 and “traditional cusp” currents appear in cusp/mantle domains, which we call midday region 1 and region 0 currents, respectively, in this paper. These currents perfectly reverse their flow directions depending on IMF By polarity. Traditional region 1 currents occurring in cleft and outer plasma sheet almost always flow into the ionosphere in the prenoon sector and flow away from the ionosphere in the postnoon sector regardless of By polarity. Thus the midday region 1 and region 0 current system that appears at local noon is not a simple continuation of flankside region 1/region 2 current system. Midday region 1 and region 0 currents are not necessarily balanced in intensity; region 0 current intensity occasionally exceeds midday region 1 current intensity. Furthermore, intensity imbalance also appears in cleft‐associated region 1 currents; that is, region 1 current in the farside cleft from the reconnection site (“downstreamside” cleft) is larger than region 1 current in the nearside cleft (“upstreamside” cleft). On the basis of these observational facts we discuss the source mechanisms of the dayside FAC system: (1) directly coupled generation of region 0 and midday region 1 current in the cusp/mantle domains around noon and (2) generation of extra region 0 current in the tail magnetopause which is connected to the extra downstreamside cleft‐associated region 1 current.
Magnetic fields in the distant heliosphere approaching solar minimum: Voyager 1 and 2 observations during 1994
Authors: Burlaga L. F. , Ness N. F.
Published: February 2004

During 1994, Voyager 2 (V2) was at a heliocentric distance 〈R〉 = 43.4 AU and latitude 〈δs/c〉 = 11.9°S, while Voyager 1 (Vl) was at 〈R〉 = 56.3 AU and 〈δs/c〉 = 32.5°N. The mean magnetic field strength observed by Vl during 1994 was 0.05 nT, the weakest and most distant magnetic field ever measured in situ. A bimodal distribution of azimuthal magnetic field directions was observed by V2, and a single‐peaked distribution was observed by Vl. Thus V2 was in the “sector zone” (the range of latitudes that contains the heliospheric current sheet), and Vl was in the unipolar region above it, consistent with the latitudes of the heliospheric current sheet obtained from an extrapolation of the neutral line computed from solar magnetic field observations. A lognormal distribution of the magnetic field strength (hourly averages) was observed for 95% of the Vl data and most of the V2 data. An approximately lognormal distribution of B is a general property of the heliospheric magnetic field, observed out to 58.1 AU and up to 32.6°S latitude. During 1994 an exponential tail in the distribution of the magnetic field strength was observed by both Vl and V2 for fields stronger than average. An exponential tail in the magnetic field strength distribution was observed by Vl and V2 from 1983 to 1994 at latitudes from 11°S to 33°N and at distances from 13 to 58 AU. Thus an exponential tail of the distribution of magnetic field strengths is a basic property pf the heliosphere beyond 13 AU. There was no significant latitudinal gradient in the exponential tail observed by V2 in the sector zone and observed by Vl above the sector zone during 1994. Multifractal structure in the large‐scale fluctuations of the magnetic field strength was observed by Vl for periods from 8 hours to 256 hours equaling 10.7 days for the magnetic fields stronger than average. A multiffactal spectrum is a general property of the large‐scale magnetic field strength fluctuations, from 1983 through 1994 between 13 AU and 58 AU. There remains a need for a statistical theory of the solar wind that can account for these observations and for cosmic ray models that include the effect of multifractal magnetic field strength fluctuations.
Interplanetary magnetic clouds, helicity conservation, and current‐core flux‐ropes
Authors: Kumar Ashok , Rust D. M.
Published: February 2004

A current‐core flux‐rope model for interplanetary magnetic clouds is presented which explains their average thermodynamic and magnetic properties. It is assumed that during a magnetic cloud's evolution, its total magnetic helicity, flux and mass are conserved and that the dynamics of a cloud is governed by the Lorentz self‐force acting on its curved portions. Total magnetic energy and current in a magnetic cloud decrease monotonically as it elongates. Part of this magnetic energy is lost in overcoming solar gravity, part goes into the bulk kinetic energy, and the rest can be assumed to go into heating the plasma inside the cloud. Due to this dissipation of magnetic energy as heat, the temperature of an expanding cloud goes through a maximum before the cloud leaves the corona. The temperature may reach 1.7 × 106K. As a cloud expands into interplanetary space, the total plasma beta asymptotically approaches a constant value between 0.39 and 0.52, irrespective of its initial value. Apart from explaining the heating and expansion of magnetic clouds, this model also provides expressions (scaling laws) for the magnetic field strength, temperature, radius, density, asymmetry of the magnetic field strength profile, slope of the plasma velocity profile inside clouds, and plasma beta, as functions of distance from the Sun. These theoretical results are compared with cloud data obtained between 0.3 and 4 AU from the Sun. The comparisons show a good agreement between observation and theory.
Properties of slow‐mode shocks in the distant (>200 RE) geomagnetic tail
Authors: Ho C. M. , Tsurutani B. T. , Smith E. J. , Feldman W. C.
Published: February 2004

The two distant ISEE 3 geomagnetic tail passes have been examined to identify all slow‐mode shocks present in the data. We find a total of 86 events from 439 plasma sheet/lobe crossings, using five criteria based on relations between the upstream lobe and the downstream plasma sheet magnetic field and plasma measurements. The statistical results of slow‐mode shock parameters such as the angle between magnetic field and shock normal, θBn, Alfven Mach number along the normal direction, MAn, and electron beta, βe, are calculated and reported. On average, the magnetic field decreases by a factor ∼2.7, the electron density increases by ∼1.7, temperature increases by ∼1.8, and the plasma flow velocity increases by ∼ 3.8 across the shocks. The average upstream θBn is ∼76°, while the downstream angle is ∼ 50°. The shocks have an average MAn ∼ 0.87 along the normal direction, and an upstream βe ∼ 0.04. In the downstream plasma sheet region, the dominant plasma flow associated with the shocks is in the tailward direction with an average speed ∼ 585 km/s. Only a few cases of Earthward downstream plasma flow have been detected. The slow shocks have thicknesses, on average, of −5380 km (about 7 ion inertial lengths) and an average tilt angle of ∼ ±22.4° between the shock normal and z axis. Using the Petschek slow shock model, the average location of the neutral lines is located in a range of ±40 RE from observation sites. About half the slow mode shock events are detected during southward IMF intervals and half during northward intervals. There is a weak substorm dependence of slow mode shocks and plasmoids, a dependence which is most obvious when Bz > +2 nT and Bz < −2 nT intervals are intercompared. We see a substorm dependence for plasma sheet/lobe crossings which suggests that the deep tail become more dynamic during substorm intervals. We have also sought the existence of large wave trains downstream of slow shocks that have been theoretically predicted by Coroniti [1971] and simulation studies. No such wave trains were observed throughout the two ISEE 3 passes of the distant tail. However, we do detect some medium amplitude transverse waves in the shock ramp regions. The waves have frequencies and polarizations similar to the plasma sheet boundary layer waves reported by Tsurutani et al. [1985]. The waves present in the shock ramp are also right‐hand ion cyclotron waves in the plasma frame. We believe that these waves are generated by the ion beams flowing away from the magnetic merging regions.
Comprehensive analysis of electron observations at Saturn: Voyager 1 and 2
Authors: Maurice S. , Sittler E. C. , Cooper J. F. , Mauk B. H. , Blanc M. , Selesnick R. S.
Published: February 2004

We present a comprehensive analysis of Voyager 1 and 2 electron observations within Saturn's magnetosphere. This analysis entails the merging of electron observations from the Plasma Science (PLS) experiment, the Low Energy Charged Particle (LECP) experiment and the Cosmic Ray System (CRS) experiment. For each encounter, the three instruments combined allow us to compute the electron energy spectra over a wide range of energies from 10 eV to ∼ 2MeV between the closest approaches and L = 18.5. The instruments use different technologies, different sensitivities, and different fields of view; however, we observe a surprisingly good matching of the data sets on a 15‐min timescale. The PLS‐LECP‐CRS spectra include the low‐energy thermal component of the magnetospheric plasma, the keV suprathermal electrons, and the high‐energy tail extending into the MeV energy range. From the combined spectra, we compute a comprehensive set of macroscopic parameters (electron density, pressure, beta factor, and electron current at the spacecraft): the analysis reveals a variety of radial gradients for these quantities and the corresponding electron populations. We also compute phase space densities over a wide range in energy and radial distances, analyzing local time symmetries, electron source distributions, and temporal variations of Saturn's magnetosphere. The ultimate goal of this study is to provide a comprehensive empirical model of the charged particle population within Saturn's magnetosphere. It will be used to support the development of the Cassini mission and to allow detailed planning of the tour design with regard to charged particle science and radiation hazards.
Foreshock structure of the slow‐mode shocks in the Earth's magnetotail
Authors: Saito Yoshifumi , Mukai Toshifumi , Terasawa Toshio , Nishida Atsuhiro , Machida Shinobu , Kokubun Susumu , Yamamoto Tatsundo
Published: February 2004

The ion distribution function in the upstream region of slow‐mode shocks observed in the Earth's magnetotail are investigated. We have found the existence of a region called the “foreshock region” upstream of slow‐mode shocks that can be clearly distinguished from the main dissipation region of the slowmode shocks. This foreshock region is characterized by counterstreaming ions: backstreaming ions that flow from the shock surface toward the upstream region along the magnetic field and the lobe cold ions that flow from the upstream region into the shock surface. Backstreaming ions may be hot plasma sheet ions escaping from the downstream region toward the upstream region. The incident cold ions are heated about 3% – 20% of the total ion heating throughout the slow‐shock transition in this foreshock region. Wave‐particle interaction with the electromagnetic ion cyclotron waves generated by the counterstreaming ions is a candidate of the cold ion heating mechanism in the foreshock region.
Optical observations of the early (t < 5 s) ion dynamics of the CRRES G1, G9, and G11A releases
Authors: Delamere P. A. , Stenbaek‐Nielsen H. C. , Hampton D. L. , Wescott E. M.
Published: February 2004

Video images of the CRRES Gl, G9, and G1lA barium releases made at 400–500 km altitudes over the Caribbean in July 1991 have been analyzed to identify morphological features formed in the very early phases of the releases with the aim of associating specific physical processes with these features. Three features are presented: the observation that the inner edge of the ion cloud is located some distance from the release point along the orbital track (“skidding”); the formation of an elongated striation at the inner edge of the ion cloud and an ion depleted region behind the striation; and the formation of a flat neutral barium disk perpendicular to the magnetic field B that is centered some tens of kilometers from the release point. We are linking these morphological features with effects associated with the polarization of the initially dense ion cloud created by solar UV illumination. A simple physical model of the release and the polarization process is presented. Model predictions are both qualitatively and quantitatively in good agreement with the observations. The skidding distance appears to be controlled by the background ionospheric density and the size of the release; the structure in the early ion cloud is due to electric fields primarily setup and maintained by the polarization of the densest part of the ion cloud; and the neutral disk is formed by charge exchange between counterstreaming ions and neutrals outside the polarized core. For the three releases analyzed, no additional internal plasma processes nor processes coupling the release to the ionosphere appear to be critical for the formation of the observed features in the initial ion cloud.
Mapping of magnetic field lines and auroral currents in the noon‐midnight meridian
Authors: Elsen R. K. , Winglee R. M.
Published: February 2004

Time averaging provides a new way to study the output of global magnetospheric models and permits meaningful comparisons with ensemble‐averaged satellite data and empirical magnetospheric models. A representative static configuration is prepared by time averaging the dynamic magnetic field generated by a two‐dimensional global magnetospheric model in the noon‐midnight meridian plane developed by Winglee [1994] that uses a modified two‐fluid approximation. This two‐fluid approach incorporates additional field‐aligned currents not included in MHD but which are a vital component to the auroral current system in the noon and midnight sectors. The model produces a cyclical substorm while the IMF is southward, with a growth phase, an expansion phase, and a recovery phase. The resulting time‐averaged configurations are studied parametrically using two series of runs that vary the solar wind dynamic ram pressure and the southward component of the interplanetary magnetic field, respectively. Although the time‐dependent position of the X line can be as small as 13 RE, the average position of the X line exceeds 42 RE in all the cases studied. The auroral currents map to magnetic latitudes near 70° at midnight and move equatorward and intensify both for increased dynamic pressure and IMF BZ. These static magnetic configurations are compared with the T89M model of Tsyganenko [1989]. The peak tail current shows a variation similar to that in the T89M model but is about 30% less. Reducing the tail magnetic field of T89M to 70% of its original value gives more realistic values for BZ in the near‐Earth tail and improves equatorial mapping of closed field lines in the tail. Increased activity causes a fixed point in the tail to map to a position more equatorward in the ionosphere within the simulations but to a position more poleward in T89M.
lonization‐driven plasma dynamics in a high voltage sheath of a conducting body in the ionosphere
Authors: Singh Nagendra , Jaggernauth Ashford
Published: February 2004

A high‐voltage sheath of a conducting body in space is relevant to several space applications, such as electron beam ejections, charge neutralization on bodies, and the electrodynamic tether. The acceleration of electrons to relatively high energies inside the sheath causes ionization of the neutral gas constituents by electron‐neutral (e‐n) impacts. This in turn greatly modifies the sheath structure, including the dynamics of the charged particles inside the sheath. Artificial neutral gas releases have been used in several space experiments for the purpose of enhancing the electron collection by bodies (electrodes) in space. In this paper, the effects of ionization of a neutral gas on the plasma dynamics and the collection of electrons by a high‐voltage electrode are studied theoretically. The study is based on a one‐dimensional radial model in which time‐dependent hydrodynamic plasma equations are solved self‐consistently with the Poisson equation. The electrons originating from the ambient plasma and those produced by the ionization of the neutral gas are treated as separate fluids. The effects of the ionization on the sheath greatly depend on the neutral gas density. For sufficiently low densities, ionization has hardly any effect on the sheath. In this case, the current collection is given by collisionless sheath theories. For very high densities, collisionless description of the plasma is not valid and the current collection is limited by the e‐n collisions. For moderate densities, the ionization drives several plasma processes in the sheath, which are (1) recurrent sheath expansion, (2) recurrent evolution of potential distribution from a thin sheath to an extended distribution having a double layer and/or a “hump”, (3) formation of oscillatory density structure in the ion density profile near the low potential end of the sheath in a highly nonuniform, expanding, dense plasma, (4) enhancements and disruptions in electron current collection, and (5) plasma expansion with multiple expansion fronts. The oscillatory density structure and the recurrent behavior are interpreted in terms of an ion wave instability driven by counterstreaming electron and ion beams near the low potential end of the sheath. The instability becomes very effective when the plasma density is enhanced by the trapping of the ionization‐produced electrons inside the sheath, because its growth rate γ ∼ωpi, where ω pi is ion plasma frequency. The wave grows as it convects with the expanding ion beam. When the ion wave is amplified because of the enhanced growth rate following a substantial increase in the plasma density by the trapping, the sheath expansion is disrupted and the sheath thins. The continued ionization leads to the further expansion and the recurrent behavior.
Characteristics of localized ionospheric disturbances inferred from VLF measurements at two closely spaced receivers
Authors: Lev‐Tov Sean J. , Inan Umran S. , Smith Andy J. , Clilverd Mark A.
Published: February 2004

The very low frequency (VLF) NPM signal from Hawaii was recorded at two closely spaced (∼50 km) receivers (located at Palmer and Faraday stations) on the Antarctic Peninsula. Measurements of characteristic amplitude and phase signatures of lightning‐induced electron precipitation (LEP) events were made on three different days in March 1992. Both amplitude and, for the first time, phase measurements are quantitatively interpreted using a three‐dimensional model of VLF propagation in the Earth‐ionosphere waveguide in the presence of lower ionospheric disturbances. This is the first time such a study has been undertaken with mirrored precipitation. Differences between the amplitude and phase changes at the two sites are accounted for by the location of the LEP ionospheric disturbance transverse to the VLF signal propagation paths. The change in these differences is explained by the horizontal movement of the disturbance region and, therefore, the causative whistler duct footprint across the transmitter‐receiver paths. Trends in the amplitude and phase changes on a timescale of order 1 hour are found to be encompassed by the modeling of the passage of the day‐night terminator along the paths.