bluuebunny/crossref_metadata_2025 huggingface dataset:
Excitation of sideband emissions by a modulated electron beam during the CHARGE 2B Mission
Authors: Sotnikov V. I. , Schriver D. , Ashour‐Abdalla M. , Ernstmeyer J.
Published: February 2004

Sideband wave emissions are secondary peaks in wave power observed at frequencies above and below the primary peak. Observed sideband amplitudes are usually reduced compared to the primary wave amplitude. During the recent CHARGE 2B ionospheric rocket mission, sideband waves were observed during an electron beam injection experiment. The beam current was modulated causing a peak in wave power at the modulation frequency and at certain times sidebands were observed along with the primary wave emission. A nonlinear coupling mechanism between the primary wave and a lower‐frequency wave is explored here to explain the presence of the sideband emissions. Comparison of the theoretical results with the wave data confirm this mechanism as a viable explanation for the observed sidebands.
Analysis of the effect of artificial ion masses on current‐driven electrostatic instabilities
Authors: Mälkki Anssi
Published: February 2004

The electrostatic ion cyclotron (EIC) and ion acoustic (IA) waves can be driven unstable by an electron current. The critical drift velocity of the electrons, at which the wave modes become unstable, depends on the parallel and perpendicular temperatures of the electrons and ions, and on the ion mass. We show that in plasma simulations the critical drift velocity for the electrostatic ion cyclotron wave has an additional dependence on the plasma magnetization, Ωe/ωpe. This is negligible for naturally occurring ions, but becomes increasingly important when ion mass mi is reduced below the proton mass (Mp = 1836me). We have solved numerically the dispersion equation for the critically unstable EIC and IA waves for ion to electron mass ratios 40 ≤ mi/me ≤ 1836, and show how the reduction of the mass ratio affects the critical drifts. The temporal and spatial growth rates of the excited waves and changes in the wave behavior when the ion mass is reduced are discussed. It is shown that a careful selection of the ion to electron mass ratio has important consequences with regard to the interpretation of results of two‐dimensional numerical simulations of electrostatic wave instabilities, since in certain parameter regimes the EIC wave behavior is strongly altered below a magnetization‐dependent ion to electron mass ratio.
Interpretation of satellite airglow observations during the March 22, 1979, magnetic storm, using the coupled ionosphere‐thermosphere model developed at University College London
Authors: Parish H. F. , Gladstone G. R. , Chakrabarti S.
Published: February 2004

The University of California, Berkeley, extreme ultraviolet spectrometer aboard the U.S. Air Force STP 78‐1 satellite measured emission features in the Earth's dayglow due to neutral and ionized species in the atmosphere, in the 35‐140‐nm range. The spectrometer was operating between March 1979 and March 1980, including the period of the magnetic storm on March 22, 1979. Some of these measurements are interpreted using the predictions of the three‐dimensional time‐dependent coupled ionosphere‐thermosphere model developed at University College London. The observations show a reduction in the atomic oxygen 130.4‐nm airglow emission at high northern latitudes following the storm. Model simulations show that this reduction in 130.4‐nm emission is associated with an increase in the O2/O ratio. Analysis of model results using electron transport and radiative transport codes shows that the brightness of 130.4‐nm emission at high latitudes due to resonantly scattered sunlight is approximately twice that due to photoelectron impact excitation. However, the observed decrease in the brightness at high northern latitudes is mainly due to a change in the photoelectron impact source, which contributes approximately 75% of the total, as well as its multiple scattering component; for the photoelectron impact source at 70° latitude and 200 km altitude, the reduction in multiple scattering is 1.5 times greater than the reduction in the initial excitation. The reduction in the airglow emission is visible only in the northern hemisphere because the south pole was not sunlit over the storm period. The comparison of model results with observations suggests that 130.4‐nm emission may be a useful as a tracer for global changes in the concentration of atomic oxygen.
Diffuse minor ions upstream of simulated quasi‐parallel shocks
Authors: Trattner K. J. , Scholer M.
Published: February 2004

We have performed a number of one‐dimensional hybrid (particle ions, massless fluid electrons) simulations of quasi‐parallel collisionless shocks in order to elucidate the origin of diffuse upstream minor ions. Minor ions are treated self‐consistently by the hybrid code; however, a number of runs have also been performed wherein the minor ions are treated as test particles. We have investigated the dependence of the density ratio bias factor fnof; (the alpha to proton ratio of the diffuse ions normalized to the solar wind ratio) on shock Mach number, solar wind beta (thermal to magnetic pressure) and on angle ΘBno between shock normal and magnetic field and found strong dependencies on all parameters. An addition of 5% alpha particles changes the shock structure, resulting in a significant modification of the density ratio bias factor as compared to the test particle cases. In the case of a low beta solar wind (β ≃ 0.1) the solar wind alpha particles penetrate the shock ramp rather unaffected and gyrate in the downstream magnetic field as a whole. The alpha particle beam returns occasionally to the shock ramp and spatially localized clouds of alpha particles can be found in the upstream region. With increasing Mach number the gyroradius of the alpha particles increases and it becomes easier for the alpha particle beam to return from downstream to the shock; a high density of clouds is found upstream. In the case of a higher beta solar wind (β≳0.5) and moderate Mach numbers, no localized alpha particle clouds leave from downstream into the upstream region. This can be attributed to the smoother shock transition in the case of higher beta shocks. The effect of beta and Mach number is counteracting: high Mach number shocks can lead even at higher beta to minor ion clouds in the upstream region. Higher beta shock simulations result in an increase of fnof; with beta and ΘBno. The increase with beta is due to the fact that diffuse ions originate mainly from the high‐energy tail of the distribution in the upstream rest frame. It is suggested that large ΘBno implies a more tangential downstream field and the alpha particles reach the shock during their downstream gyration easier.
Single‐particle dynamics of the parabolic field model
Authors: Wang Zuo‐Ding
Published: February 2004

In this paper we analyze the dynamics of single particle motion in the parabolic field model of the geomagnetotail using both numerical and analytical methods. We focus on the topology of the Poincaré surface of section and its evolution, when the field lines are strongly curved (κ < 0.53). The stochastic region is classified into RB subcells and fingers. Diffusion of intersections in RB subcells and fingers is different because the subcells and fingers correspond to destructed islands and destructed separatrices, respectively. Phenomena like differential memory and enhanced trapping and escaping are results of these structures and the time it takes the orbits to return to the reversal region. Observations in some of the previous literature are discussed.
Equatorial heating and hemispheric decoupling effects on inner magnetospheric core plasma evolution
Authors: Lin J. , Horwitz J. L. , Wilson G. R. , Brown D. G.
Published: February 2004

We have extended our previous semikinetic study of early stage plasmasphere refilling with perpendicular ion heating (Lin et al., 1992) by removing the restriction that the northern and southern boundaries are identical and incorporating a generalized transport description for the electrons (e.g., Brown et al., 1991). This allows investigation of the effects of electron heating and a more realistic calculation of electric fields produced by ion and electron temperature anisotropies. The combination of perpendicular ion heating and parallel electron heating leads to an equatorial electrostatic potential peak, which tends to shield and decouple ion flows in the northern and southern hemispheres. Unequal ionospheric upflows in the northern and southern hemispheres lead to the development of distinctly asymmetric densities and other bulk parameters. At t= 5 hour after the initiation of refilling with different source densities (Nnorth = 100 cm−3, Nsouth = 50 cm−3), the maximum potential drops of the northern and southern hemispheres are 0.6 and 1.3 V, respectively. At this time the minimum ion densities are 11 and 7 cm−3 for the northern and southern hemispheres. DE 1 observations of asymmetric density profiles by Olsen (1992) may be consistent with these predictions. Termination of particle heating causes the reduction of equatorial potential and allows interhemispheric coupling. When the inflows from the ionospheres are reduced (as may occur after sunset), decreases in plasma density near the ionospheric regions are observed while the heated trapped ion population at the equator persists.
Tail lobe ion composition at energies of 0.1 to 16 keV/e: Evidence for mass‐dependent density gradients
Authors: Lennartsson O. W.
Published: February 2004

A large set of energetic (0.1‐ to 16‐keV/e) ion composition data from the central magnetotail, obtained by the ISEE 1 spacecraft between 10 and 23 RE from Earth, is sorted according to measured total ion beta value, in order to investigate whether bulk properties of different ions vary in different ways between plasma sheet and lobes, as suggested by a comparison of certain experimental and theoretical results in the literature. Despite inevitable difficulties with extracting statistically valid data at very low beta (10−2 or lower), the results seem to support a recent theoretical model suggesting that lighter ions have a steeper density gradient than heavier ions, especially when comparing H+ ions with O+ ions. The results also indicate that ion velocity distributions are fairly isotropic even at low beta, at least those of H+ ions, although field‐aligned flows are common. The results are evaluated in the context of plasma transport and are found to lend some support to the notion that tail lobe convection may be directed inward from the dawn and dusk flanks.
Propagation of whistler waves driven by fine structured ion beams in the magnetotail
Authors: Burinskaya T. , Schriver D. , Ashour‐Abdalla M.
Published: February 2004

In a previous paper, which examined the propagation of low‐frequency whistler waves generated by ion beams in the Earth's plasma sheet boundary layer (PSBL), it was found that whistler waves driven in the PSBL are focused toward the central plasma sheet due to the global magnetotail inhomogeneities; this finding may help explain the observations of magnetic noise bursts in the tail (Burinskaya et al., 1993). In this paper the same phenomenon is examined, but this time a much more realistic model is used for the ion beams in the PSBL. While the PSBL has been modeled as a solid, homogeneous ion beam with a width of one Earth radius, observations and theoretical considerations have shown that PSBL ion beams actually have a decreasing velocity profile toward the plasma sheet and that the density of the beams within the PSBL can vary locally. We consider again the propagation and generation of electromagnetic waves but in the presence of fine structured ion beams in the PSBL. Our results show that whistler waves, generated quasi‐parallel to the background magnetic field, can be trapped locally within small spatial regions where the ion beam density is enhanced compared to the density of the adjacent PSBL region. Wave spectra and nonlinear saturation mechanisms are discussed.
The October 22, 1989, solar cosmic ray event measured at geosynchronous orbit
Authors: Nemzek R. J. , Belian R. D. , Cayton T. E. , Reeves G. D.
Published: February 2004

We compare proton measurements (5‐150 MeV) made on two geosynchronous satellites during the October 22, 1989 solar cosmic ray event to those from ground‐based neutron monitors. The satellite and ground instruments detected similar signatures in the solar protons for this event: an intense initial “spike” made up of two individual peaks followed by a longer, slower pulse. The height of the spike relative to the pulse was larger for higher measured energies. Most of the differences between the event's characteristics as seen on the ground and in orbit may be attributed to the different energy regimes sampled by the different detectors: < 150 MeV for the satellites, compared to > 450 MeV for the neutron monitors. The existence of the spike at low energies argues against an interpretation of the spike's origin as primary solar neutrons.
Transverse particle acceleration and diffusion in a planetary magnetic field
Authors: Barbosa D. D.
Published: February 2004

A general model of particle acceleration by plasma waves coupled with adiabatic radial diffusion in a planetary magnetic field is developed. The model assumes that a spectrum of lower hybrid waves is present to resonantly accelerate ions transverse to the magnetic field. The steady state Green's function for the combined radial diffusion and wave acceleration equation is found in terms of a series expansion. The results provide a rigorous demonstration of how a quasi‐Maxwellian distribution function is formed in the absence of particle collisions and elucidate the nature of turbulent heating of magnetospheric plasmas. The solution is applied to the magnetosphere of Neptune for which a number of examples are given illustrating how the spectrum of pickup N+ ions from Triton evolves.
Centrifugal effects near the frontside magnetopause
Authors: Delcourt D. C.
Published: February 2004

Three‐dimensional trajectory calculations are used to investigate the effects of inertial drifts due to magnetic field orientation changes in the dayside magnetosphere. It is demonstrated that the well‐known field line curvature drift and that due to curvature of the E×B path act in very distinct manners. Though negligible at low altitudes, it is shown that the latter drift may be of importance in regions adjacent to the frontside magnetopause. In these regions, it yields substantial parallel energization of low‐energy ions such as those originating from the high‐latitude ionosphere. Because this acceleration occurs prior to entry into the nightside magnetosphere, these ions are subsequently less subjected to large‐scale plasma convection and populate more distant sectors of the magnetotail. On such basis, the rarely considered drift due to curvature of the E×B path appears as an important feature of plasma transport in the magnetospheric flanks.
A new component of Jovian kilometric radio emission
Authors: Reiner M. J. , Fainberg J. , Stone R. G.
Published: February 2004

Evidence is presented for a new Jovian radio emission component in the frequency range from ∼40 to ∼200 kHz observed during the Ulysses‐Jupiter outbound pass at high Jovian southern latitudes along the dusk terminator. The new radio component (referred to as sKOM) occurs in the same frequency range as the observed broadband kilometric (bKOM) radio emission, but its characteristics are distinctly different. It has the opposite polarization, is about 100 times weaker, and has a characteristically smooth intensity profile. It is consistently observed in the longitudinal range from ∼120° to ∼230° central meridian longitude, where the intermittent bKOM is often absent, and is found to originate in the Jovian magnetosphere about 5RJ from Jupiter and at a latitude of about 35°S. Its observed right‐hand circular polarization suggests that it is generated in the O mode in the source region.
On heat flow contribution to plasma line frequency in the F region
Authors: Mishin Evgeny , Hogfors Tor
Published: February 2004

It was recently shown by Kofman et al. (1993), that an electron heat flow correction is essential in interpreting the difference between the frequency of upshifted and downshifted electron plasma lines. However, whereas the agreement with the frequency difference correction was excellent for the lowest values of the temperature gradient, there was a substantial discrepancy for the highest value of this gradient. We show that excellent agreement can be obtained in the latter case as well by taking into account the effect of runaway electrons on the heat flow. It is suggested that a proper account of the heat flow including the correction for runaways may make it possible to reliably determine the field aligned currents from incoherent scatter radars observations.
Simulations of DE 1 UV airglow images
Authors: Gladstone G. Randall
Published: February 2004

In this paper we present simulations of the UV airglow observed in selected images obtained by the spinscan auroral imager experiment on the Dynamics Explorer 1 satellite. In particular, we present one example each of observed and modeled images for the dayglow and for the nightglow. The model dayglow emissions include the O I 130.4 nm triplet (excited by both photoelectron impact excitation of O atoms and resonant scattering of the solar O I 130.4 nm triplet), the O I 135.6 nm doublet (excited by photoelectron impact excitation of O atoms), and the N2 Lyman‐Birge‐Hopfield (LBH) molecular bands (excited by photoelectron impact excitation of N2 molecules). The model neglects minor contributions from H I 121.6 nm and N I 149.3 nm dayglow emissions. The model nightglow emissions are comprised of the same O I 130.4 nm and 135.6 nm features, but on the nightside they are produced through recombination of O+ ions. The model neglects a contribution to the nightglow O I 130.4 nm and 135.6 nm emissions due to recombination of O+ and O− ions. The results presented here demonstrate that the dayglow emissions are well understood and can be simulated with high accuracy. This allows us to determine relative contributions of the various dayglow emission sources over the disk of the Earth. For example, while the O I 130.4 nm emission is very bright and fairly uniform at about 15‐20 kR over much of the disk, the optically thin LBH emissions become much larger than the O I 130.4 nm emission at the limb, with a band‐integrated maximum brightness of about 35 kR. Although the image simulation code used is fairly CPU intensive, it should prove useful in studying variations in the abundances of O and N2 following geomagnetic storms or in extracting the dayglow contribution from images of the sunlit auroral oval, for instance. The nightglow emissions are not as well fit by the model, indicating that the O+‐O− mutual neutralization source is significant and should be included in future simulations. The observed tropical arcs are somewhat narrower than produced by the model simulation, probably due to the limited spatial resolution of the model. The model results indicate that the O I 130.4 nm and 135.6 nm recombination nightglow can reach several hundred rayleighs in the middle of the tropical arcs. Although swamped by dayglow emissions, the O I recombination glow on the dayside must typically exceed a kilorayleigh.
Ray tracing studies on auroral kilometric radiation in finite width auroral cavities
Authors: Gaelzer R. , Ziebell L. F. , Schneider R. S.
Published: February 2004

We investigate propagation and amplification of the auroral kilometric radiation over the geomagnetic poles using a particular model for the physical parameters in the auroral zone, which takes into account density gradients perpendicular to the geomagnetic field. The propagation of the waves is investigated with the canonical set of equations of the geometrical optics, taking into account thermal effects and considering both the energetic and the background electron populations, with the components of the dielectric tensor calculated in the locally homogeneous plasma approximation. It is shown that the spatial scale of inhomogeneity perpendicular to the magnetic field is an important factor in the amplification, although less favorable than indicated by previous studies using the method of Poeverlein to obtain the trajectory of the radiation.
The proton cyclotron instability and the anisotropy/β inverse correlation
Authors: Gary S. Peter , McKean Michael E. , Winske Dan , Anderson Brian J. , Denton Richard E. , Fuselier Stephen A.
Published: February 2004

Spacecraft observations in the strongly compressed subsolar magnetosheath show an inverse correlation between the proton temperature anisotropy (T⊥p/T∥p > Where ⊥ and ∥ denote directions perpendicular and parallel to the background magnetic field) and the parallel proton β (β∥p). This manuscript uses one‐dimensional hybrid simulations of the proton cyclotron anisotropy instability in homogeneous electron‐proton plasmas to study this correlation which may represent a limited closure relation for fluid theories of anisotropic space plasmas. The emphasis is on driven simulations which increase the temperature anisotropy by periodically reducing the magnetic‐field‐aligned velocities of the protons. The late‐time states from ensembles of both initial value and driven simulations yield very similar expressions for the proton anisotropy/β∥p inverse correlation, and provide a basis for explaining differences between sheath observations from different spacecraft. The driven simulations also yield expressions for the maximum instability growth rate and the fluctuating field energy as functions of β∥p and a parameter characterizing the anisotropy driver.
Variation of lunar sodium emission intensity with phase angle
Authors: Potter A. E. , Morgan T. H.
Published: February 2004

We report new measurements of the sodium emission intensity seen in a line of sight just above the surface of the Moon. These data show a strong dependence on lunar phase. The emission intensity decreases from a maximum around first quarter (phase angle 90°) to very small values near full Moon (phase angle 0°). This suggests that the rate of sodium vapor production from the lunar surface is largest at the subsolar point and becomes small near the terminator. However, the sodium emission near full Moon falls below that which would be expected for solar photon‐driven processes. Since the solar wind flux decreases substantially when the Moon enters the Earth's magnetotail near full Moon, while the global solar photon flux is undiminished, we suggest that solar wind sputtering is the dominant process for sodium production.
Stimulated electromagnetic emissions and anomalous HF wave absorption near electron gyroharmonics
Authors: Stubbe P. , Stocker A. J. , Honary F. , Robinson T. R. , Jones T. B.
Published: February 2004

Simultaneous observations are presented of stimulated electromagnetic emission (SEE) spectra and the anomalous absorption of a diagnostic HF wave, caused by powerful radio waves generated by the Tromsø heating facility. The pump frequency was varied in small steps around the third, fourth, and fifth harmonic of the electron gyrofrequency. Systematic variations with pump frequency were observed in the anomalous absorption data and the entirety of SEE properties. The anomalous absorption exhibits a pronounced minimum at a certain pump frequency near (or at) the gyroharmonic in question. Of the SEE properties the strength and discreteness of the downshifted maximum and the width of the downshifted continuum minimize together with the anomalous absorption. Other SEE properties (namely the broad upshifted maximum, the downshifted peak, and the broad symmetrical structure) exist only in well‐defined narrow pump frequency ranges near the absorption minimum. Significant quantitative and qualitative differences are found between the SEE properties around the third gyroharmonic on the one hand, and the fourth and fifth gyroharmonics on the other hand.
Some facets of the ionosphere and of the nightglow
Authors: Bates D. R.
Published: February 2004

A personal perspective is given of the development of our understanding of the loss of electrons in the ionosphere, of the formation of the ionized regions, and of the collision processes pertaining to the nightglow.
A physicist in the world of geophysics and space
Authors: Simpson J. A.
Published: February 2004

This contribution to the 75th Anniversary of the AGU is an informal and anecdotal account of the author's research interests and activities as they relate to cosmic ray investigations involving geophysics and experiments in space during the period 1947 to the 1960's. It begins with the discovery of the cosmic ray nucleonic component geomagnetic latitude effect and its application to establishing the interplanetary origin of the cosmic ray intensity vs. time variations and the concept of solar modulation. The development of the cosmic ray neutron monitor contributed not only to an understanding of interplanetary electrodynamical processes, but also to the experimental evidence for a heliosphere. The author discusses the introduction of cosmic ray physics into the International Geophysical Year (1957‐58) and the post‐IGY initiation at the University of Chicago of research based on experiments carried in spacecraft.
Romancing the solar system
Authors: Sonett C. P.
Published: February 2004

This “personal history” covers the period from about 1954 to the present, beginning with the time that I witnessed the start of the U.S. space program triggered by the launch of Sputnik, and includes my experience as Project Scientist for Pioneers 1, 2, 5, and Explorer 6, my service at NASA headquarters as Chief of Lunar and Planetary Sciences, followed by my role in organization of the NASA Ames Space Sciences Division, and in 1973, I formed the Dept. of Planetary Sciences at the University of Arizona.
Inertial currents in isotropic plasma
Authors: Heinemann M. , Erickson G. M. , Pontius D. H.
Published: February 2004

The magnetospheric convection electric field contributes to Birkeland currents. The effects of the field are to polarize the plasma by displacing the bounce paths of the ions from those of electrons, to redistribute the pressure so that it is not constant along magnetic field lines, and to enhance the pressure gradient by the gradient of the bulk speed. Changes in the polarization charge during the convection of the plasma are neutralized by electrons in the form of field‐aligned currents that close through the ionosphere. The pressure drives field‐aligned currents through its gradient in the same manner as in quasi‐static plasma, but with modifications that are important if the bulk speed is of the order of the ion thermal speed; the variations in the pressure along field lines are maintained by a weak parallel potential drop. These effects are described in terms of the field‐aligned currents in steady state, isotropic, MHD plasma. Solutions are developed by taking the MHD limit of two‐fluid solutions and illustrated in the special case of Maxwellian plasma for which the temperature is constant along magnetic field lines. The expression for the Birkeland current density is a generalization of Vasyliunas' expression for the field‐aligned current density in quasi‐static plasma and provides a unifying expression when both pressure gradients and ion inertia operate simultaneously as sources of field‐aligned currents. It contains a full account of different aspects of the ion flow (parallel and perpendicular velocity and vorticity) that contribute to the currents. Contributions of ion inertia to field‐aligned currents will occur in regions of strong velocity shear, electric field reversal, or large gradients in the parallel velocity or number density, and may be important in the low‐latitude boundary layer, plasma sheet boundary layer, and the inner edge region of the plasma sheet.
Perpendicular particle simulation of the E region Farley‐Buneman instability
Authors: Janhunen P.
Published: February 2004

We study the stabilization and nonlinear quasi‐steady state of the E region Farley‐Buneman waves. We limit ourselves to the two‐dimensional, fully perpendicular case. We use a two‐dimensional explicit particle simulation code with neutral collisions. Since both perpendicular coordinates are included, we can study the dependence of temporal power spectra and backscattered power on flow angle. Unlike fluid simulation codes in general, the present method is not limited to small ambient electric field values. It is found that the phase velocity roughly obeys the linear theory: no saturation to the ion acoustic speed is seen. However, the spatial power spectrum is asymmetric as a function of the flow angle if the electric field is above the Farley‐Buneman threshold. This nonzero flow angle propagation is the most important new result. The most intense waves propagate at the k · E < 0 edge of the linearly unstable sector, with phase velocity close to the ion acoustic speed. No evidence of anomalous perpendicular wave heating is found.
Interplanetary magnetic field control of magnetotail magnetic field geometry: IMP 8 observations
Authors: Kaymaz Zerefsan , Siscoe George L. , Luhmann Janet G. , Lepping Ronald P. , Russell Christopher T.
Published: February 2004

Four years of IMP 8 magnetic field measurements, 1978‐1982, when ISEE 3 took upstream interplanetary magnetic field (IMF) measurements, have been analyzed to produce full cross‐section magnetic maps of the magnetotail at about 33RE downwind from Earth. This paper describes how the field geometry in the cross‐sectional plane responds to different IMF orientations: dominant By, dominant +Bz, and dominant −Bz. The dominant By case exhibits marked departures from bilateral symmetry that have the sense of superimposing a fraction of the IMF on the symmetrical tail field. However, the “superimposed” perturbation field, measured as a fraction of the IMF, is highly nonuniform: It is maximum in the flanks of the plasma sheet and minimum in the lobes. There is also a rotation of the current sheet which varies with IMF strength and with distance. The two dominant Bz cases show no systematic departures from bilateral symmetry. However, the shapes and the relative sizes of the dipolar and flaring field regions of their cross sections are markedly different. The difference field, obtained by subtracting the positive Bz case from the negative Bz case, shows that the strongest perturbations run the north‐south extent of the flanks, instead of residing in the lobes, as might be expected from the dayside reconnection model.
Electron acceleration by kinetic Alfvén waves
Authors: Kletzing C. A.
Published: February 2004

As Alfvén waves with finite extent perpendicular to the magnetic field propagate from the magnetosphere to the ionosphere, there is a region of parallel electric field in the “wave front” of the propagating wave. For short perpendicular wavelengths this parallel electric field can be large enough to accelerate electrons to auroral energies. This problem is solved for the case of uniform plasma density and background magnetic field. The parallel electric field solution is then applied to a background Maxwellian plasma to study the effects of the acceleration due to this field on the electron distribution function. Two effects are found: (1) the relatively modest acceleration of the bulk of the background electrons and (2) Fermi‐like resonant acceleration of a small component of the electrons up to velocities of the order of twice the Alfvén speed. Although both effects always occur, the response of the background electrons is a sensitive function of the magnitude, wavelength, and timescale associated with the driving perpendicular electric field. In particular, the latter effect does not produce a significant signature for all conditions. However, for reasonable values of perpendicular electric field magnitude and scale size, and plasma parameters appropriate for auroral field lines at altitudes around 7000 km near where the Alfvén speed peaks, the effect can be significant.
Plasma sheet as a shock wave: Dynamic predictions of the model
Authors: Voronov E. V. , Fridman S. V.
Published: February 2004

A study is made of some dynamic implications of our earlier model of large‐scale magnetospheric structure, in which the plasma sheet is a collisionless shock wave (Fridman and Voronov, 1993). According to our calculations, the polar boundary of the auroral oval always lies equatorward of the boundary of closed and open field lines. An abrupt enhancement of the global magnetospheric convection electric field is immediately followed by a thinning of the plasma sheet, an increase of its plasma density and by the equatorward displacement of the auroral oval polar boundary. With a decrease of the magnetospheric electric field the plasma sheet expands, and the polar boundary of the auroral oval is pulled nearer to the boundary of closed and open field lines. A typical time of change of the plasma sheet configuration in the distant tail is a few minutes, while the time of displacement of the auroral oval polar boundary amounts to several tens of minutes.
Cross‐L motion of a relativistic electron belt formed in the slot region
Authors: Korth A. , Vampola A. L.
Published: February 2004

A series of large magnetic storms during the March‐June 1991 period produced major changes in energetic particle fluxes in the magnetosphere. The most significant change was the addition of a semidurable belt of extremely energetic particles (Eel > 15 MeV and Eprot > 40 MeV) at 2.2 < L < 2.5 after the late March magnetic storm. This belt slowly moved to lower L. The width of the belt at half‐intensity remained constant at ΔL = 0.43 from 1 day after its formation through the end of the observations 6 months later. We have used data from the medium electrons A (MEA) and medium electrons B (MEB) particle spectrometers on CRRES to obtain cross‐L motion rates for the fraction of these energetic particles that occur as a background effect in the MEA and in the MEB (>6.5 MeV and >1 MeV respectively). Cross‐L motion rates for the first week after the storm sudden commencement of March 24, for the first month, and for the next 6 months have been obtained. The long‐and short‐term average rates of motion across L, ∼6 × 10−4 and ∼3 × 10−2 per day at L = 2.0 to 2.3, are similar to radial diffusion rates obtained from the distribution functions of fission electrons (Farley, 1969) and of relativistic electrons after a major magnetic storm (Tomassian et al., 1972). It is inferred from the constancy in width of the belt that radial diffusion proceeded predominantly toward lower L. The long‐term apparent decay rate for the particle flux intensity as seen by these instruments is a simple exponential in time: Nt = N0ek(1‐t/t0), where k is 0.069.
Auroral radar frequency broadening measurements at large aspect angles and comparison with theory
Authors: Haldoupis C. , Koehler J. A. , Sofko G. J. , Mu J. , André D. , Kustov A. V. , Danskin D. W.
Published: February 2004

The observed characteristics of radio auroral backscatter at large aspect angles have never been adequately explained by theories of plasma turbulence that otherwise seem to be satisfactory. Recently, the first nonlinear theory on the subject has appeared in the open literature. In this theory, under the action of a driving electric field, the maximum aspect angle depends on the combined contribution of frequency broadening due to mode coupling and anisotropic anomalous collisions caused by the interaction of electrons with the fluctuating electric fields in the waves. In this paper an attempt is made to test the theoretical predictions by comparing them to observations of mean frequency broadening during typical auroral electrojet conditions. The analysis was based on a large number of Doppler spectra compiled from several VHF radar backscatter experiments with magnetic aspect angles in the range from about 1 to 10 deg. The theory fails to explain echo occurrence at directions perpendicular to the E × B flow, but otherwise it can account for most of the observations. The evidence shows that the effect on large aspect angle echoes of frequency broadening alone is insufficient and that the key role in fitting the data is played by the parallel anomalous collision frequency νe∥*. The agreement between theory and experiment, however, is possible only for large enhancements of νe∥* relative to the normal electron neutral collision frequency νe, with the increase ranging between 1 and 2 orders of magnitude. This imposes difficulties because the large νe∥*/νe ratios imply a pronounced ion acoustic plasma turbulence along B inside the electrojet layer. To our knowledge, there is no observational evidence to support this theoretical implication.
Pc 3 and Pc 4 activity during a long period of low interplanetary magnetic field cone angle as detected across the Institute of Geological Sciences Array
Authors: Chi P. J. , Russell C. T. , Le G.
Published: February 2004

Strong Pc 3 and Pc 4 waves were recorded across the Institute of Geological Sciences ground magnetometer array for a period of about 12 hours on August 20, 1978, when the cone angle of the interplanetary magnetic field was less than 45°. The peak frequencies in the Pc 3 band varied little from station to station and were approximately the same as the frequency of the upstream waves simultaneously observed from ISEE 1 and 2. However, only certain pairs of stations saw signal waveforms having high coherence. In contrast, the waves in the Pc 4 band were seen on the surface of the Earth only at low latitudes and were not enhanced in the upstream waves. The peak frequency of Pc 4 waves on the ground was a strong function of latitude. With the exception of the ending one hour early of Pc 4 activity at a single station, both Pc 3 and Pc 4 signals ceased abruptly when the interplanetary magnetic field (IMF) cone angle increased to large angles. Although these observations exhibit the global control of occurrence of Pc 3 and Pc 4 waves by the IMF, the cross correlation analysis of the temporal variations of Pc 3 and Pc 4 power implies that the Pc 4 waves were from the nightside or the flank of the magnetosphere, while the Pc 3 waves were from the subsolar region.
Distance to the solar wind termination shock and the source flux of anomalous cosmic rays during 1986‐1988
Authors: Cummings A. C. , Stone E. C. , Webber W. R.
Published: February 2004

We use observations of the intensity of anomalous cosmic ray helium and oxygen in the outer heliosphere near the last solar minimum period, in conjunction with solar observations of the tilt of the neutral current sheet, to estimate that the solar wind termination shock was at 67 ± 5 AU during the time period 1986.7‐1988.5. We find the source flux of anomalous cosmic rays at the shock to be 1.7 ± 0.3 (m² s sr MeV/nucleon)−1 for 7.1‐17.1 MeV/nucleon anomalous cosmic ray oxygen and 4.3 ± 0.8 (m² s sr MeV/nucleon)−1 for 31‐57 MeV/nucleon anomalous cosmic ray helium. We also estimate a source flux of anomalous cosmic ray hydrogen of 2.3 ± 0.6 (m² s sr MeV)−1 at 134 MeV. The resulting energy density of anomalous cosmic rays at the solar wind termination shock is ∼0.03 eV/cm³, approximately 2% of the energy density of the solar wind at 67 AU.
First measurements of the two‐dimensional horizontal wave number spectrum from CCD images of the nightglow
Authors: Hecht J. H. , Walterscheid R. L. , Ross M. N.
Published: February 2004

A narrow wavelength band CCD camera, built at Aerospace, has been used to obtain images of the OH Meinel (6,2) band and the O2 atmospheric (0,1) band nightglow. The field of view of the camera is approximately 100 by 100 km at an altitude of 90 km, the nominal height of the nightglow. It is shown how Fourier techniques can be applied to these data to optimally smooth the images, to identify the presence of monochromatic waves, and to obtain both the one‐dimensional and, for the first time, the two‐dimensional horizontal wave number spectrum of gravity waves passing through the emission layers. Both measures of the spectrum depend, to a certain extent, on a technique which makes use of Krassovsky's η ratio. These techniques are applied to sample data taken from May 9, 1989, during the Arecibo Initiative in Dynamics of the Atmosphere (AIDA) campaign in Puerto Rico, and from the recent Collaborative Observations Regarding the Nightglow (CORN) campaign in Illinois. While future papers will describe these data in more detail, a brief comparison is made with recent models of the two‐dimensional horizontal wave number spectra presented by Gardner et al. (1993) and Gardner (1994).
Heat flux and thermal conduction in O+ and H+ ion flows deduced from EISCAT‐VHF radar observations in the high‐latitude topside ionosphere
Authors: Wu Jian , Taieb Charley
Published: February 2004

With two periods of EISCAT‐VHF radar observations, magnetically quiet and moderately disturbed, we have calculated the H+ and O+ heat fluxes in using formulae of Wu and Taieb (1993), accounting not only for temperature gradients but also for pressure gradients, velocity gradients, and diffusion‐thermal effects. They correspond to different physical processes that are calculated and compared between them. Then, the general features of the H+ and O+ ion thermal conductions are studied during the two periods of observation in the high‐latitude topside ionosphere. The analysis of the two periods of measurements revealed the following trends, to be confirmed by further observations: (1) During the two periods the H+ heat flux Q2 is always upward in the altitude range from 600 km up to about 1000 km. It is larger during the quiet period than during the disturbed period. (2) The most important dominant mechanism for H+ heat flux is the diffusion‐thermal effect due to the H+‐O+ collision, with a correcting term due to the H+ temperature gradient. (3) The O+ heat flux is always downward in the same altitude range during the two periods of observations. The absolute value during the disturbed period is slightly larger than that during the quiet period. (4) The dominant mechanism for O+ heat flux is its temperature gradient, corrected by the coupling with the H+ ion. (5) The O+ thermal conduction is less important during the quiet period than during the disturbed period, while for the H+ thermal conduction it is the contrary. (6) The H+ thermal conduction increases with altitude below about 1000 km.
Radar spectral observations of HF‐induced ionospheric Langmuir turbulence with improved range and time resolution
Authors: Sulzer M. P. , Fejer J. A.
Published: February 2004

The technique of coded long radar pulses was used to obtain radar backscatter spectra from HF‐induced Langmuir turbulence every millisecond for height intervals 150 m apart. The most interesting spectra were obtained by transmitting HF radio wave pulses of 5‐ms length every second, with a frequency of 5.1 MHz and with equivalent radiated powers (ERPs) of 40 or 60 MW ERPs. Using pulses of that length or shorter appears to have avoided the generation of density irregularities. Moreover, using these short HF pulses assured the effectiveness of the novel technique used to achieve the 1 ms temporal resolution. Under those conditions initially only a weak decay line type spectrum is observed from near the 430‐MHz radar matching height. One or two milliseconds later a broad asymmetric spectrum appears at a height greater by about 1 km, with most of the energy below the pump frequency, together with a narrow spectral “free mode” type peak above the pump frequency. Those two spectral features have been predicted by the numerical simulation spectra of DuBois et al. [1990]. In the subsequent spectra, free mode spectral peaks appear at lower heights but without accompanying broad spectra. Those free mode spectral peaks show clearly the pump standing wave structure (Airy function structure) in their distribution with height. For 50‐ms‐long HF pulses the free mode peaks are seen for only about the first 10‐20 ms of the HF pulse. At about the time when the free mode peak “disappears,” strong decay line type spectra appear at more than two heights near and above the matching height at the same frequency; both effects are tentatively attributed to the development of density irregularities in the plasma density.
An analysis of whistler waves at interplanetary shocks
Authors: Lengyel‐Frey D. , Farrell W. M. , Stone R. G. , Balogh A. , Forsyth R.
Published: February 2004

We present an analysis of whistler wave magnetic and electric field amplitude ratios from which we compute wave propagation angles and energies of electrons in resonance with the waves. To do this analysis, we compute the theoretical dependence of ratios of wave components on the whistler wave propagation angle θ for various combinations of orthogonal wave components. Ratios of wave components that would be observed by a spinning spacecraft are determined, and the effects of arbitrary inclinations of the spacecraft to the ambient magnetic field and to the whistler wave vector are studied. This analysis clearly demonstrates that B/E, the ratio of magnetic to electric field amplitudes, cannot be assumed to be the wave index of refraction, contrary to assumptions of some earlier studies. Therefore previous interpretations of whistler wave observations based on this assumption must be reinvestigated. B/E ratios derived using three orthogonal wave components can be used to unambiguously determine θ. Using spin plane observations alone, a significant uncertainty occurs in the determination of θ. Nevertheless, for whistler waves observed downstream of several interplanetary shocks by the Ulysses plasma wave experiment we find that θ is highly oblique. We suggest that the analysis of wave amplitude ratios used in conjunction with traditional stability analyses provide a promising tool for determining which particle distributions and resonances are likely to be dominant contributors to wave growth.
An explanation for the difference in cosmic ray modulation at low and neutron monitor energies during consecutive solar minimum periods
Authors: Reinecke J. P. L. , Potgieter M. S.
Published: February 2004

The phenomenon that galactic proton spectra measured during two consecutive solar minimum modulation periods intersect at a particular energy was studied with a drift model for the modulation of cosmic rays in the heliosphere. This effect causes cosmic rays at neutron monitor energies to behave differently than at lower energies during these periods. Gradient and curvature drifts of charged particles in the global heliospheric magnetic field give a natural explanation for this feature. The model shows that the crossover energy depends primarily on the particle species, the radial distance from the Sun, the minimum value of the “tilt angle” during the A < 0 minimum period, and the amount of drifting that occurs during consecutive solar minimum modulation periods.