bluuebunny/crossref_metadata_2025 huggingface dataset:
Substorm trigger conditions
Authors: Horton W. , Wong H. V. , Van Dam J. W.
Published: September 2002

Critical conditions for the onset of fast interchange dynamics in the stressed geotail during the growth phase of the substorm are derived. We compare the ideal MHD interchange stability condition [Hurricane, 1997] with that for kinetically modified interchange‐ballooning motions. It is shown that fast interchange growth is possible only in the near‐Earth boundary of the plasma sheet where the local plasma pressure‐to‐magnetic pressure ratio (beta) is near unity since compressibility stabilizes the high‐β geotail. The growth rate is proportional to the local current density and exceeds the ion bounce frequency in the local region of β ≲ 1. Only after sufficient thinning of the current sheet will the kinetic theory growth rate exceed the bounce frequency of the ions, which is the effective condition for the substorm MHD‐space‐timescale unloading of the plasma energy stored in the geotail.
Properties of interstellar dust
Authors: Mathis John S.
Published: September 2002

The reference abundance of the interstellar medium (the abundance of elements in gas + dust), until recently assumed to be solar, is discussed because of its great importance for the composition of dust. For some important elements (O, N, and probably more), three lines of reasoning (observed gas‐phase abundances, H II regions, and stellar abundances) suggest that the reference abundance is only ∼70% of solar. Carbon is especially crucial. This change in the reference abundance, coupled with recently observed gas‐phase abundances of C, present grain theories with a major challenge. Diagnostics of the nature of dust are considered, with emphasis on continuous interstellar extinction, the carriers of the 217.5 nm feature, some infrared extinction features, and the “Unidentified Infrared Bands” in the range 3.28 μm ≤ λ ≤ 11.3 μm. Laboratory materials have provided important data. The low abundance of carbon in dust probably requires that the carriers of the 217.5‐nm feature have the same distribution of shapes in all directions. There are comments about grain theories and the implications of observed gas‐phase abundances in cool and warm interstellar clouds.
Coherent HF radar backscatter characteristics associated with auroral forms identified by incoherent radar techniques: A comparison of CUTLASS and EISCAT observations
Authors: Milan S. E. , Davies J. A. , Lester M.
Published: September 2002

Backscatter from decameter‐wavelength field‐aligned F region irregularities, as measured by the Cooperative UK Twin Located Auroral Sounding System (CUTLASS) Finland HF coherent radar, is compared with common volume plasma parameters and the electric field deduced by the European Incoherent Scatter (EISCAT) UHF incoherent radar system, for a 12 hour period from June 18 to June 19, 1996. During this interval we find an excellent agreement between irregularity Doppler velocity and bulk ion drift resolved along the CUTLASS beam. Backscatter is found to exist only in regions of nonzero electric field, as the E×B instability growth rate is dependent on E. Following a substorm expansion phase onset, backscatter largely disappears for a period of several hours, thought to be a consequence of nondeviative absorption of the HF radio wave in the D region or a quenching of the F region instability mechanism by enhanced E region Pedersen conductivity. Finally, the presence of auroral arcs within the scatter volume increases the intensity of backscatter returns and introduces a subsidiary peak, displaced from the preexisting peak, in the backscatter spectra; this subsidiary peak results in an increase in the apparent spectral width of the backscatter. We show how this allows the location of precipitation features within the field of view to be determined.
Magnetic signatures of the distant polar cusps: Observations by Polar and quantitative modeling
Authors: Tsyganenko N. A. , Russell C. T.
Published: September 2002

Polar cusps are an essential element of the magnetospheric topology. In the outermost regions of the cusp the magnetosheath plasma becomes entrained on magnetospheric field lines and penetrates to low altitudes along field lines on the dayside, causing a significant local depression of B, clearly visible in magnetometer data taken at high latitudes. This study addresses the spatial distribution of the magnetic depression associated with the polar cusp, as derived from a large set of Polar magnetometer data taken in 1996–1998. The depression ΔB was derived as the difference between the total magnitude of the observed field and the one calculated from the International Geomagnetic Reference Field model of the internal field, combined with an external field model. In the noon sector the cusp depression is clearly visible in plots of ΔB versus solar magnetic latitude as a relatively narrow region of large negative ΔB (down to ∼ −80 nT), extending from R ∼ 8 − 9 RE (Polar apogee) to R ∼ 5 – 6 RE. At closer geocentric distances the cusp depression fades out because of a rapid increase of the geomagnetic field toward Earth. The cusp depression also shallows as one moves away from the noon meridian and completely disappears at solar magnetic longitudes ±40 – 60°. The second part of the paper deals with a mathematical representation of the observed polar cusp depression in quantitative models of the geomagnetic field. A method is suggested on the basis of the field deformation technique, making it possible to incorporate a realistic structure of the polar cusps in global models of the magnetosphere.
Dispersive width of the Alfvénic field line resonance
Authors: Streltsov A. V.
Published: September 2002

The results from a numerical investigation of the dependence of the eigenfrequency and structure of Alfvénic field line resonance (FLR) layers on the dispersion of the resonant Alfvén waves are presented. The investigation is based on the linear, reduced, two‐fluid MHD‐kinetic model considered in the dipole magnetic field geometry with a realistic distribution of the background plasma parameters along the entire auroral flux tube. It is shown that (1) dispersion of the small‐scale Alfvén wave due to the finite electron inertia (inertial dispersions) decreases the eigenfrequency of the Alfvénic FLR and broadens the FLR layer in the direction along the transverse gradient in the background Alfvén speed; (2) dispersion of the wave due to the finite plasma temperature (kinetic dispersion) increases the FLR eigenfrequency and broadens the resonance layer in the opposite direction; and (3) on the field line where the effect of the kinetic dispersion balances the effect of the inertial dispersion, the eigenfrequency becomes independent of the transverse wavelength of the resonant wave. As a result, a FLR occurring on such a particular field line develops fields and currents with smaller transverse scale sizes and larger amplitudes than FLRs on field lines where inertial and kinetic dispersions are not in a balance.
Substorm dipolarization and recovery
Authors: Baumjohann W. , Hesse M. , Kokubun S. , Mukai T. , Nagai T. , Petrukovich A. A.
Published: September 2002

On the basis of ∼2 years of Geotail data, we use a superposed epoch approach to study the average behavior of plasma and magnetic fields at different radial distances, between 11 and 31 RE, during 66 substorms in the premidnight sector. Magnetic field dipolarization is first seen in the innermost region (11–16 RE) around substorm onset and subsequently moves tailward at a rate of 35 km/s. Fast earthward and tailward ion bulk flows in the central plasma sheet indicate that during substorm expansion the near‐Earth neutral line is located between 21 and 26 RE, with a tendency to be closer to 21 RE near substorm onset. About 45 min after onset, the tailward moving dipolarization front reaches the distance range where the near‐Earth neutral line is located. Thereafter the near‐Earth neutral line disappears beyond 31 RE. This is the classical signature of the start of the recovery phase. We conclude that substorm recovery sets in when the tailward moving dipolarization front reaches the near‐Earth neutral line, because the near‐Earth neutral line cannot operate in a dipolar field geometry.
The effect of electron Coulomb collisions on the incoherent scatter spectrum in the F region at Jicamarca
Authors: Sulzer Michael P. , González Sixto
Published: September 2002

The fact that the incoherent backscatter spectrum narrows when the radar beam is nearly perpendicular to the magnetic field is well known and has been used at Jicamarca for more than 30 years to measure very accurate line‐of‐sight velocities. Recently it has become clear that these spectra are narrower than expected. We have explained this effect and also the small change to the spectral shape required at somewhat larger angles to correct the ratio of electron to ion temperature seen in some studies. Coulomb collisions affecting the motion of the electrons are responsible for the additional spectral narrowing. We have carried out very accurate simulations of electron motion resulting in incoherent scatter spectra which are qualitatively similar to spectra resulting from other types of collisions, and to those predicted in an analytic solution for the Coulomb case [Woodman, 1967]. However, we found that the spectrum of the velocity time series in the radar line of sight departs significantly from the nearly Lorentzian form expected with simple collisional models. This causes the collisional effects to extend to somewhat shorter scale lengths, or further from perpendicular to the magnetic field than expected. In order to investigate the collisional process more closely, we performed another simulation combining the effects of electron‐ion collisions and a simple friction model (Langevin equation) in an adjustable combination. This one showed that the effect of electron‐ion collisions alone would result in collisional effects extending several degrees farther from perpendicular to the field than when both kinds of collisions are included. Collisions affecting the speed of the electrons tend to limit the size of the effect at larger angles from perpendicular. Thus the effect of these collisions on the incoherent scatter spectrum cannot be accurately predicted from simple models but depends on the detailed physics of the collisions.
Auroral ion acceleration from lower hybrid solitary structures: A summary of sounding rocket observations
Authors: Lynch K. A. , Arnoldy R. L. , Kintner P. M. , Schuck P. , Bonnell J. W. , Coffey V.
Published: September 2002

In this paper we present a review of sounding rocket observations of the ion acceleration seen in nightside auroral zone lower hybrid solitary structures. Observations from Topaz3, Amicist, and Phaze2 are presented on various spatial scales, including the two‐point measurements of the Amicist mission. From this collection of observations we will demonstrate the following characteristics of transverse acceleration of ions (TAI) in lower hybrid solitary structures (LHSS). The ion acceleration process is narrowly confined to 90° pitch angle, in spatially confined regions of up to a few hundred meters across B. The acceleration process does not affect the thermal core of the ambient distribution and does not directly create a measurable effect on the ambient ion population outside the LHSS themselves. This precludes observation with these data of any nonlinear feedback between the ion acceleration and the existence or evolution of the density irregularities on which these LHSS events grow. Within the LHSS region the acceleration process creates a high‐energy tail beginning at a few times the thermal ion speed. The ion acceleration events are closely associated with localized wave events. Accelerated ions bursts are also seen without a concurrent observation of a localized wave event, for two possible reasons. In some cases, the pitch angles of the accelerated tail ions are elevated above perpendicular; that is, the acceleration occurred below the observer and the mirror force has begun to act upon the distribution, moving it upward from the source. In other cases, the accelerated ion structure is spatially larger than the wave event structure, and the observation catches only the ion event. The occurrence rate of these ion acceleration events is related to the ambient environment in two ways: its altitude dependence can be modeled with the parameter B2/ne, and it is highest in regions of intense VLF activity. The cumulative ion outflow from these LHSS TAI is consistent with Freja statistics for VLF‐type premidnight auroral upflow.
Sources of the solar wind electron strahl in 1995
Authors: Ogilvie K. W. , Burlaga L. F. , Chornay D. J. , Fitzenreiter R. J.
Published: September 2002

The electron “strahl” is a narrow beam of electrons formed along the interplanetary magnetic field in the solar wind, associated with coronal holes. Observations of this feature, covering the energy range 10–1000 eV, are being made continuously by a specially designed part of the Solar Wind Experiment (SWE) instrument on the Wind spacecraft. During 1995, when corotating streams were regularly observed, the strahl flux was always observed to increase to a maximum following the stream interface marking the separation of the interacting flows. It was not seen before a stream interface, meaning that the strahl does not cross the stream interface. The strahl is associated with corotating streams and their sources rather than with the magnetic sectors. The source of the strahl observed by Wind was the southern coronal hole during the first half of the year and the northern coronal hole during the last half, as indicated by the polarity of the magnetic field.
A note on current closure
Authors: Vasyliūnas Vytenis M.
Published: September 2002

Contrary to what is sometimes assumed, it is mathematically impossible to write down a magnetic field of a current system that is not closed. When the Biot‐Savart law is integrated over a current that ostensibly is not closed, the resulting magnetic field is that of a system containing a closure current which is uniquely determined but may neither be physically realistic nor correspond to the desired approximation.
Analysis of electron auroras based on the Monte Carlo method: Application to active electron arc auroras observed by the sounding rocket at Syowa Station
Authors: Onda Kunizo , Ejiri Masaki , Itikawa Yukikazu
Published: September 2002

A downward electron differential number flux, the absolute photoemission rate for the (0, 1) band of the first negative band system of the molecular nitrogen ion, and the number density of thermal electrons were simultaneously measured by the sounding rocket S‐310JA‐8 launched toward active auroral arcs at a substorm expansion phase on April 4, 1984, from Syowa Station in Antarctica. We apply the Monte Carlo method to analyze these observed results. The MSIS‐86 model is employed to represent the atmospheric number density and temperature in the aurora observed by this experiment. Only N2, O, and O2 are taken into account as constituent elements of the atmosphere. Electrons are injected downward into the upper atmosphere at the altitude of 200 km, at which the downward electron differential number flux was measured. An initial electron energy is considered in the range of 0.1‐18 keV. It is assumed that an initial pitch angle is uniformly distributed in the range of [0, π/2]. Excitation and ionization rates of N2, O, and O2 are calculated as a function of altitude, the initial pitch angle, and the initial electron energy. Production and emission rates of the N2+ 1N (0, 1) band are deduced by using these calculated rates. Time variation of the observed absolute intensity of this band is reasonably well reproduced by the Monte Carlo method combined with the measured electron number flux.
Storm time variation in the magnetospheric magnetic field
Authors: Ostapenko A. A. , Maltsev Y. P.
Published: September 2002

We used the database of Fairfield et al. [1994], which includes several tens of thousands three‐component magnetic measurements, to study the external magnetic field at distances of 10 RE > x > −30 RE under three levels of storm activity: Dst > 0, −50 < Dst < 0, and Dst < −50 nT. The field pattern is obtained for a number of magnetospheric planes. In the major part of the equatorial plane a depression of the magnetic field takes place, expanding and growing with the increase of storm activity. For the midnight sector the point of the deepest depression is located at the distance of a few Earth radii. In the magnetotail lobes the Bx field component also grows with storm activity, the most pronounced growth being displayed in the near‐Earth lobe regions. The external field in the noon‐midnight meridian plane resembles the field produced by the cross‐tail current, intensifying with the enhancement of the storm time depression.
Velocity distributions of C atoms in CO+ dissociative recombination: Implications for photochemical escape of C from Mars
Authors: Fox J. L. , Hać Aleksander
Published: September 2002

We have carried out Monte Carlo calculations to determine the velocity distributions of C atoms produced by dissociative recombination of CO+ using recent data for the branching ratios of various allowed channels and ion and electron temperatures appropriate to the Martian thermosphere. We find that the fractions of 12C atoms with velocities greater than the escape velocity are ∼ 0.66 and ∼ 0.62, and those for 13C are ∼ 0.47 and ∼ 0.48 at the plasma temperatures characteristic of the exobases at low and high solar activities, respectively. The ratio of the escape fractions of 13C and 12C, which is a measure of the isotope effect inherent in the CO+ dissociative recombination mechanism, is thus in the range 0.72–0.77. Using model thermospheres and ionospheres for low solar activity from Viking, and for high solar activity from the Mars Thermospheric General Circulation Model, we have computed the global escape flux of C due to dissociative recombination of CO+ as (3 − 5) × 105 cm−2 s−1. This value is of the same order as current estimates for the escape flux of C in all forms due to sputtering by O+ pickup ions at the current epoch but may be much less than that due to sputtering at earlier times in the history of the planet.
The induced magnetosphere of comet Halley: 4. Comparison of in situ observations and numerical simulations
Authors: Israelevich P. L. , Gombosi T. I. , Ershkovich A. I. , DeZeeuw D. L. , Neubauer F. M. , Powell K. G.
Published: September 2002

A comprehensive comparison between the results of a multiscale three‐dimensional adaptive MHD model of the comet Halley magnetosphere and in situ observations by the Giotto mission is presented. It is shown how a steady state model can describe the effects of the varying IMF direction on the cometary magnetosphere. The simulation results reproduce the observed profiles of the magnetic field and plasma velocity very well. There are only two discrepancies. First, the magnitudes of both the magnetic field strength and plasma velocity are slightly but systematically lower in numerical simulation as compared with the experiment. This can be attributed to some inaccuracy in our knowledge of plasma interaction parameters (ion‐neutral momentum exchange rate, photoinization and recombination rates). Second, the observed and simulated magnetic field profiles show some differences in the anticipated ion pile‐up region along the outbound trajectory. This means that the ion pile‐up region on the outbound leg (if it existed) differed significantly from that on the inbound leg.
Study of the evening plasma drift vortex in the low‐latitude ionosphere using San Marco electric field measurements
Authors: Eccles J. Vincent , Maynard Nelson , Wilson Gordon
Published: September 2002

San Marco D electric field measurements have been averaged in terms of the equivalent
plasma drifts for days near the 1988 autumn equinox. The observed plasma drifts provide a satellite‐based composite in altitude and local time of the evening enhancements in the zonal and vertical drifts and the shear in the zonal drift. The satellite data reveal details of the postsunset plasma drift vortex, which have been studied in radar and chemical release experiments. The averaged drift data show a proportional relationship between the prereversal enhancement of the vertical drift and the zonal derivative of the zonal drift. This relationship suggests that the vertical drift enhancement is caused by a curl‐free response to the rapid increase of the zonal drift, which in turn is tied to the F region zonal winds and the E region sunset. The data also show a curl‐free drop‐off in altitude of the vertical plasma drift above and below the peak drift at 450 km altitude during the prereversal enhancement. Comparisons of the averaged data with results of coupled ionosphere‐electrodynamics models are used to examine the underlying causes of the evening plasma drifts. The evening plasma drift vortex is shown to be a single drift structure with a single source. The source of the evening drift vortex resides on the bottomside of the integrated F region, just as originally supposed by Rishbeth [1971]. This dominant single source, which is produced by the divergence of the wind‐driven current of the F region, causes the evening enhancement of the eastward plasma drift, the shear in the eastward drift, and the prereversal enhancement of the vertical drift.
MHD waves in a three‐dimensional dipolar magnetic field: A search for Pi2 pulsations
Authors: Lee Dong‐Hun , Lysak Robert L.
Published: September 2002

ULF pulsations have been numerically studied in a new three‐dimensional dipole model, which allows a realistic Alfven speed profile for the plasmasphere and outer magnetosphere in the tailward region. This model includes more realistic boundary conditions at the outer boundary, allowing for partial reflection at the magnetopause and escape of wave energy down the tail. We investigate how Pi2 modes develop in time when an impulse associated with the substorm onset is assumed. It is shown that discrete compressional modes, which are initially excited by the plasmasphere, persistently arise in the nightside region. Our numerical results suggest that Pi2 pulsations are strongly associated with these virtual resonances in the plasmasphere. Wave spectra are examined both at the equator and the ionosphere. We discuss and compare them with current theoretical and observational characteristics.
Variations, with peak emission altitude, in auroral O2 atmospheric (1, 1)/(0, 1) ratio and its relation to other auroral emissions
Authors: Sivjee G. G. , Shen D. , Yee J.‐H. , Romick G. J.
Published: September 2002

Spectral distributions of auroral optical emissions, peaked at distinctly different heights in the thermosphere, show significant variations, with altitude, in the O2 atmospheric (1, 1)/(0, 1) band ratio. The latter increases with height in auroras peaked between 110 and 150 km and then gradually decreases at higher altitudes. To minimize ambiguities associated with auroral height determination needed for investigating this effect, four independent height‐assessment methods are employed. The first one is based on the incoherent scatter radar (ISR) soundings of the auroral ionization profile from which the height, where precipitating particles dissipate most of their energy, can be determined. Concurrent spectroscopic observations of the thermalized rotational distributions of auroral band emissions yield the ambient air temperature, and hence an independent assessment of the height, of the thermospheric region where these emissions peak. Changes in O/O2 and O/N2 ratios with height lead to changes in the ratios of auroral emissions, from these species, peaked at different heights. Finally, changes in collision frequency with height lead to changes in the brightness of the auroral emissions, resulting from radiatively allowed transitions relative to those produced from radiatively forbidden transitions. The four methods yield comparable values for the height of the thermospheric region where emissions, from each auroral event, peak. The observed variations in O2 atmospheric (1, 1)/(0, 1) with auroral height is compared with that expected from O (1D) + O2 excitation source and quenching by O2 and O. The effects of electron impact excitation of O2(b1∑g+, v′) and high rotational levels of the P branch of O2 atmospheric (0, 0) band on O2 atmospheric (1, 1)/(0, 1) ratio are discussed. Quantitative ratios of various auroral emissions, from O, N2, and N+2, peaked at different heights, that can provide an assessment of auroral heights where these emissions peak, are listed.
Dynamical investigation of three‐dimensional reconnection in quasi‐separatrix layers in a boundary‐driven magnetic field
Authors: Galsgaard Klaus
Published: September 2002

Quasi‐separatrix layers are regions in space where the mapping of field line connectivity changes especially rapidly. These layers have been suggested as special locations in three‐dimensional magnetic fields that may host magnetic reconnection. Previous investigations have been analytical and have taken different simplifying assumptions to investigate the problem. This paper takes a numerical approach to investigate the dynamical properties of quasi‐separatrix layers. The magnetic topology is stressed using drivers suggested by the analytical investigations but modified to fit the adopted boundary conditions. The experiments show that current does accumulate at specific locations in the numerical domain. The current magnitude and location depend strongly on the profile of the imposed driver, and they are found to be generated by the changes in field line parts imposed by the driving. They are therefore the manifestation of free magnetic energy in the perturbed magnetic field. After the stressing of the field has stopped, it is found that the plasma pressure is able to balance the Lorentz force of the stressed magnetic field and prevent a continued growth of the current amplitude in the current layers. Field‐line changes are produced in the experiments that include magnetic resistivity. The reconnection takes place at locations where the electric field component along the magnetic field is large. The changes in field‐line connectivity initiate flow velocities across the magnetic field lines at only a small fraction of the local Alfvén velocity.
Numerical analysis of collisionless particle motion in an observed supercritical shock front
Authors: Gedalin M. , Newbury J. A. , Russell C. T.
Published: September 2002

The well‐accepted shock model assumes that the shock front is stationary and one dimensional, in the sense that ion and electron dynamics are determined primarily by their interaction with the quasi‐stationary electric and magnetic fields in the shock front. We study the applicability of this model to an observed high Mach number supercritical shock. For this purpose we numerically analyze ion and electron dynamics in the measured magnetic field and modeled electric field (on the basis of the measurements of electron heating) and check the consistency of the numerically determined particle features with the observed shock profile. We find that the shock must be narrow enough (for given Mach number MA, kinetic‐to‐magnetic pressure ratio β, angle θ between the shock normal and upstream magnetic field, and magnetic compression Bd/Bu) to ensure that ion reflection is not too strong. We also find that the small‐scale structure is an important part of the shock: the shock front would be grossly nonstationary if it were too wide or if small‐scale features were absent. The ion energization due to the reflection and gyration is qualitatively consistent with the ion dynamics in quasi‐stationary electric and magnetic fields. However, smoothing of the downstream ion distribution is insufficient, and deviations from one dimensionality and/or nonstationarity are necessary for the shock to be stable. Electron dynamics are weakly nonadiabatic, and evolution of the collisionless electrons follows the potential and magnetic field. Gap filling cannot be studied within this collisionless approach.
Relativistic electron beam propagation in the Earth's magnetosphere
Authors: Khazanov G. V. , Liemohn M. W. , Krivorutsky E. N. , Albert J. M. , Kozyra J. U. , Gilchrist B. E.
Published: September 2002

The global evolution of an artificially injected relativistic electron beam is simulated and examined. The study focuses on injections originating in the upper ionosphere, magnetically mirroring above the lower atmosphere where significant energy loss occurs, and so a long‐lived population arises in the inner magnetosphere from this particle source. This investigation is conducted by solving the bounce‐averaged relativistic kinetic equation for the electron distribution function for various L shells. It is found that the beam quickly spreads in MLT due to differential drift rates, eventually morphing into a fairly uniform shell around the Earth. Wave interactions are comparable to collisional losses in reducing the beam content. It is also found that the beam total particle loss rate is a complicated function of L and, for the chosen conditions, the total beam particle counts are 73%, 77%, and 52% of the initial count at t=24 hours after injection for L=2, 3, and 4, respectively. The loss rates at this time are ∼1%/hour (of the remaining beam strength) and very slowly decreasing with time. These loss rates and other features of the beam evolution are discussed in detail. There are now four distinct stages recognized in the evolution of an injected relativistic beam: (1) the immediate loss of particles injected at pitch angles mapping to the lower thermosphere; (2) the initial loss of particles injected right next to the loss cone by collisional scattering; (3) the continuation of this collisional loss along with the spread of the beant to all local times by differential drift rates; and (4) the transformation of the beam into a fairly uniform shell covering all energies and all local times, with the loss rate mainly governed by wave scattering. Other stages may exist beyond the 1‐day limit set on these simulations. While the study dwells on beam dynamics, it is also a general examination of the leading edge population next to the loss cone. This has implications for the physics of the naturally occurring radiation belt particles, as this region of phase space regulates the actual precipitation of these particles into the atmosphere. The applicability of this model for studying the natural radiation environment around the Earth is also pondered.
Field‐aligned currents in the polar cap at small IMF Bz and By inferred from SuperDARN radar observations
Authors: Kustov A. V. , Lyatsky W. B. , Sofko G. J. , Xu L.
Published: September 2002

Routine SuperDARN observations of the ionospheric plasma convection and field‐aligned currents (FACs) in the high‐latitude ionosphere are used to study current systems established at small interplanetary magnetic field (IMF) Bz and By. By statistical averaging of available data sets we show that under this IMF condition the ionospheric convection pattern consists of two (evening and morning) convection cells that are similar in shape. The flow intensity inside the central polar cap is noticeably depressed so that plasma entering the polar cap flows around its border, predominantly along the lines of equal magnetic latitude, so that the convection cells are of a crescent‐like shape. This global pattern of plasma flow is associated with the effect of the region 0 field‐aligned currents coexisting with the region 1 and region 2 field‐aligned currents. SuperDARN observations of FACs for individual events support this conclusion. FACs were derived by analyzing the vorticity of the SuperDARN convection maps. We show that region 0 currents for small IMF Bz and By can exist in time sectors way off the magnetic noon. Thus radar observations support earlier findings from satellite magnetometer measurements of the region 0 current system at high latitudes during both the prenoon and afternoon at small IMF intensities. Because the region 0 FACs occur during small IMF intensities, it is suggested that quasi‐viscous processes play a role in their generation.
Influence of Coulomb collisions on isotopic and elemental fractionation in the solar wind acceleration process
Authors: Bodmer Roland , Bochsler Peter
Published: September 2002

In view of new observational evidence from isotope spectrometers on WIND, SOHO (Solar Heliospheric Observatory), and ACE (Advanced Composition Explorer), we explore the efficiency of isotope fractionation processes in the inner corona. We reinvestigate the role of Coulomb collisions in the acceleration of minor ions using a multifluid model. To model the main gas, we study stationary solutions for the continuity and momentum equations of electrons, protons, and alpha particles. As a closure of the system of equations, we prescribe expansion geometry and temperature profiles based on observations. The behavior of minor ions, which are treated as test particles, depends in a complicated manner on their mass and on their charge, structured by the interplay of acceleration, gravity, pressure gradient, electromagnetic fields, Coulomb drag, and thermal diffusion. We compare the fractionation effects in different solar wind regimes: In our model high‐speed solar wind emanating from polar coronal holes, Coulomb friction practically equalizes the velocities of all species, and no substantial fractionation takes place. In the case of a rapidly expanding magnetic field geometry, for example, in the vicinity of a coronal streamer, the proton flux and thus the Coulomb friction on minor ions is reduced, leading to depletion of heavy species in the solar wind. The model also predicts a substantial depletion of alpha particles relative to protons in the heliospheric current sheet, consistent with observations. In such a situation, heavy elements are depleted in the solar wind relative to protons as well, but the effect is strongest for alpha particles. Isotopic fractionation of helium of the order of 30% is possible, while the isotope effect on heavier elements amounts at most to a few percent per mass unit.
Lognormal distributions and spectra of solar wind plasma fluctuations: Wind 1995–1998
Authors: Burlaga L. F. , Lazarus A. J.
Published: September 2002

The fluctuations in the hour averages of the speedV, densityN, and proton temperatureTat 1 AU are large, intermittent, and spiky on a scale of a year. The lognormal distribution is a good model for the distribution of measurements of these variables made at 1 AU during 1996, 1997, and 1998, near solar minimum, and during the rising phase of the solar cycle. The lognormal distribution is less satisfactory as a model for the data obtained during 1995, when corotating streams were present. For that year the distributions ofV,N, andTwere double‐peaked; nevertheless, the number of observations associated with the fast wind distribution was only 12–17%. Shocks, corotating streams, interaction regions, signatures of the sources of the streams, and the heliospheric plasma sheet are present in the data, but each of these features has its own variability. Taken together, these components and their interactions result in episodic signals and distribution functions with large tails. There remains a need for dynamical models of the solar wind that incorporate and describe both deterministic and statistical properties of the solar wind.
Trapped Energetic Electrons in the Magnetosphere of Ganymede
Authors: Eviatar Aharon , Williams Donald J. , Paranicas Chris , McEntire Richard W. , Mauk Barry H. , Kivelson Margaret G.
Published: September 2002

On May 7, 1997, the Galileo orbiter flew through the magnetosphere of Ganymede and crossed flux tubes connected at both ends to the satellite. Energetic electrons, observed during this encounter by means of the Energetic Particle Detector on board Galileo, showed double loss cones and “butterfly” type pitch angle distributions, as has been noted in past publications. In addition, as the spacecraft flew toward Ganymede, both the shape and magnitude of the spectrum changed. The intensities decreased, with the greatest depletion observed at the lowest energies, and the monotonic slope characteristic of the Jovian environment was replaced by a rollover of the spectrum at the low‐energy end. The spectra lead us to infer a strongly energy‐dependent injection efficiency into the trapping region. As on previous encounters, the pitch angle distributions confirmed the position of the magnetopause as indicated by the magnetometer measurements, but the spectra remained Jovian until the trapping region was reached. Various physical mechanisms capable of generating the observed spectra and pitch angle distributions, including downstream reconnection insertion followed by magnetic gradient drift and absorption of the lowest‐energy electrons by Ganymede and injection from Jovian flux tubes upstream are assessed.
Quasi‐linear ion cyclotron heating in the near‐Earth magnetotail
Authors: Chaston C. C. , Hu Y. D. , Fraser B. J.
Published: September 2002

Quasi‐linear studies in the near‐Earth (XGSE>‐25 RE) central plasma sheet and plasma sheet boundary layer indicate that the ion cyclotron proton anisotropy instability may lead to significant ion heating in these regions. Saturation of the anisotropy instability can yield both narrow and broadband magnetic spectra below the proton gyrofrequency with amplitudes of the order of 1 nT. These waves then heat thermal and suprathermal He+ and O+ ions via resonant and nonresonant processes to energies above 1 keV for He+ ions and up to 100 eV for O+ ions. He++ ions also experience significant heating to temperatures that may exceed that of the unstable proton component. These results are consistent with in situ observations of magnetic noise and heavy ion distributions in the near‐Earth magnetotail.
Ionospheric current signatures of transient plasma sheet flows
Authors: Kauristie K. , Sergeev V. A. , Kubyshkina M. , Pulkkinen T. I. , Angelopoulos V. , Phan T. , Lin R. P. , Slavin J. A.
Published: September 2002

The plasma flow in the central plasma sheet of the magnetospheric tail often includes short impulsive bursts. Here we investigate the ionospheric signatures of such bursts. The Wind satellite recorded several transient fast flows in the plasma sheet (at geocentric distances of ∼ 12 RE) on December 21–22, 1995. The data are compared with magnetic field observations made in the Scandinavian sector, at the ionospheric conjugate point of the satellite. Superposed epoch analysis of the satellite data suggests that most of the events are Earthward flow bursts accompanied by magnetic dipolarizations, increases in the convection electric field, and drops in the plasma pressure and density. Occasionally, also isolated tailward flow bursts within closed flux tubes can be observed. We demonstrate that in both cases the transient plasma sheet flows are systematically associated with distinct ground magnetic field variations which (after 90° rotation) have specific vortex‐like spatial distributions. The vortex patterns have similar duration to that of the flows at Wind and their longitudinal extent (≤1 hour in local time) is consistent with the azimuthal scale sizes (∼3 RE) of the transient flows reported earlier. In many cases the sense of flow rotation observed at Wind and at Wind's ionospheric footpoint agree with our expectation. Despite the care taken in accounting for the instantaneous and local currents that affect the mapping, uncertainties in the footpoint location may still be responsible of the absence of a higher degree for compliance with theory.
Parallel electric field of a mirror kinetic Alfvén wave
Authors: Nakamura Tadas K.
Published: September 2002

It has been shown that an Alfvén wave can create an electric field along the magnetic field line due to mirror effect. In a converging magnetic field, mobility of electrons is restricted by the mirror force and a parallel electric field must exist to carry a field aligned current associated with Alfvén waves. The parallel electric field created by this mechanism can be much larger than that of a kinetic or inertial Alfvén wave when the wave frequency is low enough. The essential mechanism of parallel electric field generation is reduced to the static mirror model in the limit where the wave frequency vanishes.
Self‐consistent quasi‐static parallel electric field associated with substorm growth phase
Authors: Le Contel O. , Pellat R. , Roux A.
Published: September 2002

A new approach is proposed to calculate the self‐consistent parallel electric field associated with the response of a plasma to quasi‐static electromagnetic perturbations (ω < k‖vA, where vA is the Alfvén velocity and k‖ the parallel component of the wave vector). Calculations are carried out in the case of a mirror geometry, for ω < ωb (ωb being the particle bounce frequency). For the sake of simplification the β of the plasma is assumed to be small. Apart from this restriction, the full Vlasov‐Maxwell system of equations has been solved within the constraints described above (ω < k‖vA and ω < ωb) by [Le Contel et al., this issue] (LC0, hereafter), who describe self‐consistently the radial transport of particles during the substorm growth phase. LC00 used an expansion in the small parameter Te/Ti (Te/Ti is typically 0.1 to 0.2 in the plasma sheet) to solve the quasi‐neutrality condition (QNC). To the lowest order in Te/Ti < 1, they found that the QNC implies (1) the existence of a global electrostatic potential Χ0 which strongly modifies the perpendicular transport of the plasma and (2) the parallel electric field vanishes. In the present study, we solve the QNC to the next order in Te/Ti and show that a field‐aligned potential drop proportional to Te/Ti does develop. We compute explicitly this potential drop in the case of the substorm growth phase modeled as in LC000. This potential drop has been calculated analytically for two regimes of parameters,
and
(
being the bounce averaged magnetic drift frequency equal to
, where ky is the wave number in the y direction and
the bounce averaged magnetic drift velocity). The first regime (
) corresponds to small particle energy and/or small ky, while the second regime (
>) is adapted to large energies and/or large ky. In particular, in the limit
and
, where uy is the diamagnetic velocity proportional to the pressure gradient, we find a parallel electric field proportional to the pressure gradient and directed toward the ionosphere in the dusk sector and toward the equator in the dawn sector. This parallel electric field corresponds to a potential drop of a few hundred volts that can accelerate electrons and produce a differential drift between electrons and ions.
Cenozoic dynamics of the African plate with emphasis on the Africa‐Eurasia collision
Authors: Meijer P. Th. , Wortel M. J. R.
Published: September 2002

We investigate the forces acting on the African plate from about 56 Ma to the present‐day. We aim (1) to establish a basis for quantification of the overall dynamics of the Mediterranean/Middle East region and (2) to understand how Africa reacts to temporal variation in the forces associated with the Africa‐Eurasia collision. We use the assumption of dynamical equilibrium which implies that at any given time, the net torque of forces on Africa's northern margin must have been balanced by the other torques acting on the plate. Moreover, temporal variation in the forces on Africa's northern margin must have been balanced by contemporaneous changes in one or more of the other torques. By quantifying those “other” torques we may derive a constraint on the forces at the northern margin. We expect the Africa‐Eurasia collision to have been associated with changes in the associated forces and examine how these changes are balanced. Using alternative parameterizations, we calculate the net torque of ridge push, transform fault resistance, and basal drag. We also quantify changes in the torque of forces associated with the continent‐ocean transition. The results constitute a framework for detailed dynamic models of the Africa‐Eurasia convergence zone. Temporal changes in the ridge push torque prove to be small. The torque of transform resistance does change, but these changes do not relate to forces at the northern margin. Evidence for a relation with collision is found only for the case of basal drag. The results point to a mechanism where the plate maintains its dynamical equilibrium mainly through collision‐induced changes in absolute motion.
Excitation of high‐frequency surface waves with long duration in the Valley of Mexico
Authors: Iida Masahiro
Published: September 2002

During the 1985 Michoacan earthquake (Ms = 8.1), large‐amplitude seismograms with extremely long duration were recorded in the lake bed zone of Mexico City. We interpret high‐frequency seismic wave fields in the three geotechnical zones (the hill, the transition, and the lake bed zones) in the Valley of Mexico on the basis of a systematic analysis for borehole strong motion recordings. We make identification of wave types for real seismograms. First, amplitude ratios between surface and underground seismograms indicate that predominant periods of the surface seismograms are largely controlled by the wave field incident into surficial layers in the Valley of Mexico. We interpret recorded surface waves as fundamental‐mode Love waves excited in the Mexican Volcanic Belt by calculating theoretical amplification for different‐scale structures. Second, according to a cross‐correlation analysis, the hill and transition seismograms are mostly surface waves. In the lake bed zone, while early portions are noisy body waves, late portions are mostly surface waves. Third, using two kinds of surface arrays with different station intervals, we investigate high‐frequency surface‐wave propagation in the lake bed zone. The wave propagation is very complicated, depending upon the time section and the frequency band. Finally, on the basis of a statistical time series model with an information criterion, we separate S‐ and surface‐wave portions from lake bed seismograms. Surface waves are dominant and are recognized even in the early time section. Thus high‐frequency surface waves with long duration in the Valley of Mexico are excited by the Mexican Volcanic Belt.