bluuebunny/crossref_metadata_2025 huggingface dataset:
Two‐dimensional computer simulations of electrostatic solitary waves observed by Geotail spacecraft
Authors: Miyake T. , Omura Y. , Matsumoto H. , Kojima H.
Published: February 2004

We performed computer simulations of electrostatic solitary waves (ESW), which correspond to broadband electrostatic noise (BEN), observed by Geotail spacecraft in the magnetotail. According to Geotail/WFC data analysis, ESW are composed of sequences of impulsive solitary waves, and their potentials are revealed to have uniform structures in the direction perpendicular to the ambient magnetic field. We performed a series of two‐dimensional electrostatic particle simulations with a cold electron beam drifting against background hot electrons along the ambient magnetic field. In these simulations, spatial potentials are excited by the initial electron beam instability. They coalesce with each other and form potential troughs uniform in the direction perpendicular to the ambient magnetic field, as predicted by the Geotail observations. We varied the magnitude of the ambient magnetic field and found that this parameter critically affects the ESW formation process. These simulation results indicate that ESW are excited by the electron beam instability in the presence of the static magnetic field with a certain magnitude. ESW are also subject to the enhanced thermal fluctuations due to the limited number of superparticles in the simulations.
Tidal influence on midlatitude airglow: Comparison of satellite and ground‐based observations with TIME‐GCM predictions
Authors: Shepherd G. G. , Roble R. G. , Zhang S.‐P. , McLandress C. , Wiens R. H.
Published: February 2004

WINDII, the Wind imaging interferometer on the Upper Atmosphere Research Satellite measures winds and emission rates from selected excited metastable species. Measurements of the 558‐nm emission from atomic oxygen provide both the wind from the Doppler shift, and the atomic oxygen concentration from the emission rate. Thus the tides and their influence on atomic oxygen are measured with the same instrument. Ground‐based airglow measurements provide vertical integrals of the same quantities and coordinated observations were obtained between WINDII and ground‐based instruments at Bear Lake (42.5°N, 212°E) for O(1S) 558 nm winds and O2(b1Σ) (0,1) band emission rate and temperature. The TIME‐GCM model has recently incorporated airglow photochemistry, so that direct comparisons may be made with airglow observations, without inverting those observations to atomic oxygen distributions. In this study, the influence of tides on airglow emission at midlatitude is studied through the comparison of the above data sets with the TIME‐GCM model, extending earlier studies conducted for the equatorial region. At the vernal equinox the upward propagating diurnal tide is found to be the dominant influence on airglow diurnal variation. At solstice the diurnal tide does not penetrate to as high an altitude, so that the dominant influence is then the in situ semidiurnal tide. This conclusion is consistent with both WINDII observations and TIME‐GCM predictions, whose data sets agree extremely well with one another. The ground‐based results agree well in the local time variation pattern, but the amplitudes observed are larger than for WINDII or the TIME‐GCM by roughly a factor of 2. This difference illustrates very clearly the differences between a tidal pattern observed at a single site for a few nights, and a global pattern that is first zonally averaged, and then combined in local time over about 1 month, as is done with the WINDII data. The agreement of these averaged data with the TIME‐GCM model strongly suggests that they accurately represent the behavior of the zonally averaged atmosphere.
Modeling and observations of dayside auroral hydrogen emission Doppler profiles
Authors: Lorentzen D. A. , Sigernes F. , Deehr C. S.
Published: February 2004

We present a three‐dimensional, collision by collision model of the transport and energy degradation of energetic protons incident on the atmosphere. A test of the model was carried out using NOAA 12 satellite measurements of incoming energetic protons as input and simultaneous spectrometric measurements of the resulting dayside auroral hydrogen emission. There is a good match between the calculated and the observed emission profiles. The emission peak of the narrow and symmetric emission profile is positioned 4 Å to the blue of the unshifted line center, which is characteristic of the monoenergetic proton precipitation associated with the magnetospheric cusp/cleft, and the “velocity filter” resulting from dayside magnetic field line merging and convection. The intensities of the emission was reproduced using a proton number flux of approximately one‐half of that which was obtained by NOAA. The portion of the emission profile to the red of the unshifted line was well within the instrumental broadening. From this we conclude that the contribution of upward moving hydrogen atoms to the observed dayside H emission must be of the order of 10 percent.
Statistical study of emissions near ƒp and 2ƒp in the dayside and nightside auroral region and polar cap
Authors: Menietti J. D. , Cairns Iver H. , Piker C. W. , Averkamp T. F.
Published: February 2004

In this study we examine a large database of relatively intense plasma waves observed at frequencies near the plasma frequency ƒp and near 2ƒp (the PF and H components, respectively) by the DE 1 spacecraft. These data are from a period of approximately 2.5 years, corresponding to over 2600 orbits, and specifically include observations in the auroral regions and over the polar caps. The data show PF and H emissions during approximately 100 passes, or approximately 4% of passes. The PF and H emissions are observed exclusively at latitudes corresponding to the polar cap and auroral regions, with a clear preponderance for the polar cap. The emissions are observed at all magnetic local times but with a concentration near local midnight. No association is observed with the Ap and Dst indices. Polarization data available from a subset of these passes show that PF emissions have a polarization that is consistent with whistler mode on 10 passes and with the z mode on 25 passes. It was not possible to discern the polarization for the remaining ∼80 cases. This preponderance of z‐mode PF emission is unexpected on the basis of previous observations in the auroral regions. The polarization of the H emissions near 2ƒp was z mode on all 11 passes in which definitive measurements were possible. The PF emissions are observed to have a magnetic component (typically with E/cB ∼ 20), but no magnetic components were detected for the weak H emissions near 2ƒp. We believe that the enhanced PF emissions are sometimes primarily z mode, sometimes primary whistler mode, and sometimes a relatively equal mixture of whistler and z‐mode emission. Cold plasma theory shows that under certain conditions (nearly field‐aligned propagation for small ratios of ƒp/ƒg) the ratio of E/cB can be quite large for the z mode (E/cB ∼ 10) near ƒp, as it can be for the whistler mode near ƒp. Thus both the z mode and whistler mode can explain the E/cB observations in some cases. Our current thinking is that both the PF and H emissions are produced in the z mode by direct linear instabilities involving streaming nonthermal electrons, probably with significant temperature anisotropies and a beam distribution. Preliminary examination of plasma wave data from the Polar satellite reveal similar observations of H component waves on a number of polar cap passes at different altitudes from DE 1. These data have yet to be analyzed in detail but show both similarities to and differences from the DE 1 observations, perhaps due to the different spacecraft locations.
Inference of Europa's conductance from the Galileo Energetic Particles Detector
Authors: Paranicas C. , Cheng A. F. , Williams D. J.
Published: February 2004

On Galileo's first approach to Europa on December 19, 1996, the Energetic Particles Detector (EPD) observed decreases in the count rates of low‐energy ions in Europa's plasma wake. We understand these decreases to be “microsignatures” of Europa, that is, the signatures of recent satellite absorptions of energetic ions. Using a previously developed simulation of count rates measured by a detector, we compare simulated data with measurements from Europa's immediate and distant wakes. We find that if Europa is considered to be an inert, solid body absorber of charged particles, we cannot account for the depth of observed microsignatures using our model. Instead, if Europa, its ionosphere, or its magnetosphere is sufficiently conducting that Jovian field lines become “hung up” on the moon with contact times of the order of the bounce time of energetic ions, then we can explain the decreases in the data using our model. For a conducting Europa, our simulation gives the best order‐of‐magnitude agreement with a global radial diffusion of the order of DLL ≈ 10−10L4. Simulation results put a lower limit of ΣE ≈ 8 mho on the global conductance of Europa and/or its environs, ∼50% the value for Io. Finally, higher‐energy microsignatures in the EPD data can be understood when we include the effects of a bend in magnetic field lines near Europa that causes a local curvature drift perpendicular to the direction of plasma motion.
Inversion of plasmaspheric EUV remote sensing data from the STP 72‐1 satellite
Authors: Meier R. R. , Nicholas A. C. , Picone J. M. , Melendez‐Alvira D. J. , Ganguli G. I. , Reynolds M. A. , Roelof E. C.
Published: February 2004

Observations of the extreme ultraviolet emission of helium ions at 30.4 nm can be used to study the global shape of the plasmasphere and its dynamical response to geomagnetic forcing. In order to retrieve number densities of plasmaspheric He+ from such observations, we have developed a new inversion technique based on discrete inverse theory, which uses the optical data to optimize a parameterized model of the He+ distribution. We apply this inversion technique to several orbits of data obtained from the Naval Research Laboratory extreme ultraviolet photometric experiment launched on the STP 72‐1 satellite in October 1972. The inversion is limited to nighttime conditions where contamination from the topside ionosphere is minimal and where a simple parameterization of the He+ number density is applicable. We obtain excellent fits to the data; however, some of the retrieved model parameters have large uncertainties due to inadequate sampling of the plasmasphere. Our study shows that improved sampling using observations from different locations and view directions would significantly enhance the accuracy of the retrieved model parameters. Using a newly developed three‐dimensional imaging tool to visualize the plasmaspheric regions being sampled remotely, we demonstrate that emission features observed from two of the STP 72‐1 orbits originate beyond the plasmasphere. Estimated number densities of this feature are roughly consistent with observations of cold plasma seen at geosynchronous orbit by in situ experiments.
Traveling compression regions in the midtail: Fifteen years of IMP 8 observations
Authors: Taguchi S. , Slavin J. A. , Lepping R. P.
Published: February 2004

The characteristics of traveling compression regions (TCRs) in the midtail lobes near expansion‐phase onsets have been analyzed utilizing the data set returned by the IMP 8 magnetic field investigation between 1973 and 1990. Examination of the 15 years for which we have the AL index led to the identification of 565 isolated substorm events with expansion phases for which IMP 8 was in the tail lobes and returned magnetic field measurements. For 31 of the 565 events a typical TCR variation was observed at IMP 8. These TCR perturbations have been previously studied in the ISEE 3 distant tail measurements and consist of a field compression during which BZ increases and then decreases after peaking, although BZ may not reach a positive value in many cases. For 59 of the substorms a field compression was seen at IMP 8 during which the BZ peak before the field maximum is lacking, i.e., the BZ just decreases rapidly, and the variation is unipolar. These 90 (31 plus 59) substorms occurred in the region for X=−26 to −38 RE, |Y|≤15 RE, and ZN ≤ 20 RE all in the solar wind aberrated GSM coordinate system. The “unipolar BZ compression region” has a phase of flat BZ preceding the sharp southward tilting interval. The typical timescale of this phase is 60–100 s. We interpret this flat BZ phase as a field compression created by the rapid growth of the plasmoid bulge in the Z dimension underneath IMP 8 just prior to the plasmoid being ejected tailward. The fractional occurrence frequency of the TCRs observed in our substorm database for each year was calculated by dividing the number of the substorms producing TCRs by all of the IMP 8 lobe substorm events for each year. This ratio tends to be high (maximum 41.7%) in the years of low sunspot number, and low (minimum 7.7%) in the years of high sunspot number. For years when the occurrence ratio is low, the TCRs are observed primarily within |Y| = 10 RE. When the occurrence ratio is high, the TCRs are found across a wider region in Y with a peak in the premidnight. These results suggest that the plasmoid formation is controlled by solar cycle related parameters in such a manner that the near‐Earth neutral line occurs at a wider region in the dawn‐dusk direction for the years of low sunspot number.
A magnetosphere‐thermosphere‐ionosphere electrodynamics general circulation model
Authors: Peymirat C. , Richmond A. D. , Emery B. A. , Roble R. G.
Published: February 2004

A new simulation model of the thermosphere, the ionosphere, and the magnetosphere is presented. This model, which we call the magnetosphere‐thermosphere‐ionosphere electrodynamics general circulation model (MTIE‐GCM), calculates the three‐dimensional structure of the thermosphere and the ionosphere, the two‐dimensional magnetospheric plasma convection in the equatorial plane of the magnetosphere, and the couplings among the thermosphere, the ionosphere, and the magnetosphere. The electrodynamic couplings induced by the auroral precipitation, the region 2 field‐aligned currents, and the neutral winds are self‐consistently computed. The major drawback of the model is the use of a dipole magnetic field in the magnetosphere such that we do not expect to reproduce observations exactly but rather to predict only correct orders of magnitude. The MTIE‐GCM is run for solar maximum conditions and moderate magnetic activity in a sample case, where the electric potential is imposed in the polar cap but calculated everywhere else, to study the electrodynamic couplings occurring at high latitudes. The neutral winds increase the north–south component and decrease the east–west component of the ionospheric electric field, corresponding to an enhancement of the shielding effect by 10%. The region 2 field‐aligned currents and the distribution of the magnetospheric pressure present smaller modifications, suggesting that the magnetosphere acts partly as a current generator. However, the results could possibly be different if the polar cap electric potential were allowed to change due to the neutral winds.
Characteristics of the solar wind controlled auroral emissions
Authors: Liou K. , Newell P. T. , Meng C.‐I. , Brittnacher M. , Parks G.
Published: February 2004

We performed a high‐time resolution (5 min) correlative study of the energy deposition rate in the northern auroral zone with the concurrent solar wind plasma and interplanetary magnetic field (IMF) observations for a 4 month period from March 30 to July 29, 1996. Auroral power, inferred by auroral emissions, was derived from images acquired by the ultraviolet imager (UVI) on board the Polar satellite, and the interplanetary parameters were based on Wind observations. It is found that dayside aurorae in the afternoon sector (65°–80° magnetic latitude (MLAT) and 1300–1800 magnetic local time (MLT)) are more active for large IMF cone angles and large solar wind electric fields. This result can be attributed to the manifestation of the antiparallel magnetic field merging in different locations and the partial “penetration” of the IMF on the dayside magnetopause. The integrated nightside (60°–75° MLAT and 2000–0100 MLT) auroral brightness is moderately correlated with the north–south component of the IMF and the solar wind speed with correlation coefficients of 0.49 and 0.35, respectively. The mean nightside auroral power is found to be approximately linearly proportional to the IMF Bz with a constant slope of 2 GW/nT. The solar wind speed, however, affects the nightside auroral power for both polarities of IMF Bz. Interestingly, the solar wind dynamic pressure shows no effect on the nightside auroral brightness. All these findings indicate that both reconnection and viscous‐like interaction mechanisms play an important role in producing auroral emissions in the night sector. It is also found that the nightside auroral brightness responds to the southward turning of the IMF with a peak delay time of ∼60 min. This result favors the model of loading‐unloading magnetosphere. We also found that a negative IMF By condition favors the nightside auroral activity, and we attributed this effect to the partial penetration of the IMF By. Finally, the response function for nightside aurora is given as ∼VB4Tsin4θc2) with a median correlation coefficient of 0.63, indicating that there may be other factors other than the solar wind and IMF responsible for lightening up the northern–southern hemispheric sky.
Heliospheric magnetic field strength out to 66 AU: Voyager 1, 1978–1996
Authors: Burlaga L. F. , Ness N. F. , Wang Y.‐M. , Sheeley N. R.
Published: February 2004

We discuss Voyager 1 (V1) observations of the heliospheric magnetic field strength from 1978 through 1996. During this period the distance of V1 from the Sun increased from ≈3 AU to 66 AU and its heliographic latitude increased from ≈5°S to 33°N. The magnetic field strength profile observed by V1 is consistent with Parker's spiral field model when one considers (1) the solar cycle variation of the observed magnetic field strength at 1 AU, B1(t) (which is a measure of the source field strength) and (2) the latitudinal and solar cycle variations of the solar wind speed,V(t, θ). Both B1(t) and V(t, θ) make significant contributions to the variation of the magnetic field strength variations observed by V1. There is no evidence for a “magnetic flux deficit” increasing with distance from the Sun. There is a solar cycle variation of the magnetic field strength in the outer heliosphere, which will affect the modulation of cosmic rays.
Estimating the size and timing of maximum amplitude for cycle 23 from its early cycle behavior
Authors: Wilson Robert M. , Hathaway David H. , Reichmann Edwin J.
Published: February 2004

On the basis of the lowest observed smoothed monthly mean sunspot number, cycle 23 appears to have conventionally begun in May 1996, in conjunction with the first appearance of a new cycle, high‐latitude spot group. Such behavior, however, is considered rather unusual, since, previously (based on the data‐available cycles 12–22), the first appearance of a new cycle, high‐latitude spot group has always preceded conventional onset by at least 3 months. Furthermore, accepting May 1996 as the official start for cycle 23 poses a dilemma regarding its projected size and timing of maximum amplitude. Specifically, from the maximum‐minimum and amplitude‐period relationships we infer that cycle 23 should be above average in size and a fast riser, with maximum amplitude occurring before May 2000 (being in agreement with projections for cycle 23 based on precursor information), yet from its initial languid rate of rise (during the first 6 months of the cycle) we infer that it should be below average in size and a slow riser, with maximum amplitude occurring after May 2000. The dilemma vanishes, however, when we use a slightly later‐occurring onset. For example, using August 1996, a date associated with a local secondary minimum prior to the rapid rise that began shortly thereafter (in early 1997), we infer that the cycle 23 rate of rise is above that for the mean of cycles 1–22, the mean of cycles 10–22 (the modern era cycles), the mean of the modern era “fast risers,” and the largest of the modern era “slow risers” (i.e., cycle 20), thereby suggesting that cycle 23 will be both fast rising and above average in size, peaking before August 2000. Additionally, presuming cycle 23 to be a well‐behaved fast‐rising cycle (regardless of whichever onset date is used), we also infer that its maximum amplitude likely will measure about 144.0 ± 28.8 (from the general behavior found for the bulk of modern era fast risers; i.e., 5 of 7 have had their maximum amplitude to lie within 20% of the mean curve for modern era fast risers). It is apparent, then, that sunspot number growth during 1998 will prove crucial for correctly establishing the size and shape of cycle 23.
Solar wind in the distant heliosphere
Authors: Whang Y. C.
Published: February 2004

This paper presents a quantitative MHD model to study the effects of pickup protons on the solar wind. The model includes the kinetic theory solution of interstellar neutral hydrogen, the ionization process for the production of pickup protons, the spiral interplanetary magnetic field, and the full MHD equations. In order to study the effects of pickup protons, we first obtain the kinetic theory solutions of interstellar neutral hydrogen. This solution is then used to calculate the flow of interstellar hydrogen and the production of pickup protons. We study a one‐dimensional, steady state model of the pickup proton solar wind in the upwind direction. The solar wind proton parameters predicted by this solution agree reasonably well with observational data from Voyager 2. Throughout the heliosphere the temperature of pickup protons is higher than the temperature of the solar wind protons by two orders of magnitude. Outside the ionization radius, the solar wind temperature and the fast speed begin to increase and the bulk speed and the Mach number of the wind begin to decrease. The component of the equation of motion along the streamline direction can be written in the form of a Bernoulli equation; it can clearly explain the deceleration of the solar wind. In the distant heliosphere the sum of the kinetic energy and the thermal energy decreases with the heliocentric distance due to the charge exchange process; at the same time, the thermal energy of the solar wind plasma increases due to heating by the pickup process. The combined action causes a significant radial decrease in the kinetic energy of the wind.
Electron dissipation in collisionless magnetic reconnection
Authors: Hesse Michael , Winske Dan
Published: February 2004

A study of the electron dynamics in the dissipation region of collisionless magnetic reconnection is presented. The investigation is based on a new 2.5‐dimensional electromagnetic particle‐in‐cell simulation code. This code is applied to the problem of reconnection in two differently sized current sheets: one with a thickness of the ion inertial length and the other with electron inertial length thickness. The complete set of contributions to the reconnection electric field is calculated directly from the particle information. The two cases lead to quite different results. In the ion scale, sheet reconnection is significantly slower, and the dissipation is provided virtually exclusively by electron quasi‐viscous effects. The electron scale sheet reconnects much faster, involving a bifurcation of the reconnection region and the formation of a magnetic island. In this latter case, dissipation appears to be primarily provided by electron inertial effects and here foremost by bulk electron acceleration. Finally, an attempt to represent the effects of electron pressure‐based dissipation in a transport model is presented also.
Propagation of a spheromak: 2. Three‐dimensional structure of a spheromak
Authors: Vandas M. , Fischer S. , Dryer M. , Smith Z. , Detman T.
Published: February 2004

Magnetic clouds are commonly modeled as flux ropes and, less frequently, as spheromaks or toroids. In a previous paper, we performed a numerical study of the propagation of a spheromak in the inner heliosphere using a three‐dimensional, self‐consistent, MHD description. In this second paper on propagating spheromaks we study the interaction of their initial internal field (depending on the orientation of their axes) with the ambient interplanetary magnetic field and their three‐dimensional structure near 1 AU. It is shown by our simulation that a spheromak with a poloidal field expands into a toroid during its propagation. It develops into an oblate shape by virtue of its momentum interaction with the solar wind. Draping of magnetic field lines around it causes its polar axis (evolving into the toroid's hole) to be aligned with the radial direction irrespective of its initial orientation. This means that a hypothetical spacecraft can cross the resulting toroid only once, thereby observing signatures similar to a flux rope crossing. Double crossings are excluded even near the Sun (0.3 AU). Therefore the observations of magnetic clouds which resemble a double flux rope crossing must be explained in a way other than within the spheromak or toroidal model.
Magnetospheric plasma regimes identified using Geotail measurements: 2. Statistics, spatial distribution, and geomagnetic dependence
Authors: Christon S. P. , Eastman T. E. , Doke T. , Frank L. A. , Gloeckler G. , Kojima H. , Kokubun S. , Lui A. T. Y. , Matsumoto H. , McEntire R. W. , Mukai T. , Nylund S. R. , Paterson W. R. , Roelof E. C. , Saito Y. , Sotirelis T. , Tsuruda K. , Williams D. J. , Yamamoto T.
Published: February 2004

We investigate the spatial distribution, solar‐terrestrial correlation, and basic statistics of a long sequence of plasma regime identifications through the probability of observation P at downtail distances of ∼30 < xagsm < 210 RE, in an aberrated GSM coordinate system. The data are from Geotail spacecraft measurements obtained from October 1992 through October 1994. Earth's local plasma environment has been classified into five principal plasma regimes: PS, the plasma sheet, LB, the lobe (vacuum lobe, with only trace plasma content), and BL, the magnetospheric boundary layer, which together comprise the magnetosphere proper, and MS, the magnetosheath, and SW, the nearby solar wind. Eastman et al. [this issue] describe the process, rationale, and instrumentation used in the regime identifications. In the middle magnetotail (30 < xagsm < 100 RE): At geomagnetic activity index values Kp < 3o, (a) The average magnetospheric morphology appears nominal, with no large‐scale dawn‐dusk asymmetries; (b) PS is the regime most likely to be observed, and the spatial interrelationships of the PS, LB, BL, and MS appear similar to those commonly observed within −20 RE of Earth; that is, the magnetosphere is inside and the magnetosheath is outside; and (c) P(BL) increases gradually with increasing xagsm. At Kp ≥ 3o, (d) The centroid of P(PS) lies duskward of the XAGSM axis, producing a distinct dawn‐dusk asymmetry; (e) P(BL) increases sharply at xagsm > 60 RE and remains high tailward; and (f) At xagsm < 75 RE, MS is rarely observed for |YAGSM| < 20 RE. In the distant magnetotail (xagsm > 100 RE): (a) Conditions appear more chaotic. PS, BL, or MS can be observed anywhere inside the nominal magnetotail domain; (b) Compared to the middle magnetotail, PS and BL are observed more dawnward of the XAGSM axis and P(PS) is significantly lower (most distinctly at Kp > 3o); (c) P(PS) is centered dawnward of the XAGSM axis for both low and high Kp; and (d) At xagsm ≥ 170 RE, P(BL) is greater than P(PS) or P(MS), with P(MS) ≈ P(PS). P(LB) > 0, but low, out to xagsm ≥ 200 RE. In general, the downtail distributions of the magnetospheric plasma regimes depend more strongly on xagsm than on Kp, however the YAGSM distribution of P(PS) depends strongly with both xagsm and Kp. Magnetotail width varies weakly with xagsm and Kp.
The effect of field‐aligned density variations on time‐dependent MHD wave coupling in the magnetotail
Authors: Wright Andrew N. , Allan W.
Published: February 2004

We investigate linear MHD wave mode coupling in the cold plasma limit when the equilibrium density (ρ) varies along the equilibrium magnetic field (B). Our numerical simulations indicate that efficient fast/Alfvén mode coupling persists even when (B·∇)ρ≠0. The time‐dependent picture we develop asymptotes to a normal mode for large t and also recovers the (B·∇)ρ = 0 singularity qualitatively. When applied to waves in the magnetotail, we find that Alfvénic disturbances will have an extended structure across the magnetic field and that Alfvén wave packets should be excited on plasma sheet boundary layer (PSBL) or mantle field lines. Alfvén waves driven on PSBL field lines are probably the most observable signature at Earth of wave coupling in the distant tail.
Atomic and molecular emissions in the middle ultraviolet dayglow
Authors: Bucsela Eric J. , Cleary David D. , Dymond Kenneth F. , McCoy Robert P.
Published: February 2004

Dayglow spectra in the middle ultraviolet, obtained during a sounding rocket flight from White Sands Missile Range in 1992, have been analyzed to determine the altitude distributions of thermospheric atomic and molecular species and to address a number of problems related to airglow excitation mechanisms. Among the atomic and molecular profiles retrieved are the N2 second positive, N2 Vegard‐Kaplan and NO gamma band systems, and the OI 297.2 nm, OII 247.0 nm, and NII 214.3 nm emissions. A self‐consistent study of the emission profiles was conducted by comparing observed intensities with one another and to forward models. Model photoelectron and photon fluxes were generated by the field line interhemispheric plasma model (FLIP) and two solar flux models. Neutral densities were obtained from mass‐spectrometer/incoherent scatter (MSIS)‐90. The results from the data analysis suggest that the major species' densities are within 40% of MSIS values. Evidence for the accuracy of the modeled densities and fluxes is seen in the close agreement between the calculated and observed intensities of the N2 second positive emission. Analysis of the OI 297.2 nm emission shows that the reaction N2(A) + O is the dominant source of O(1S) in the daytime thermosphere. The data imply that the vibrationally averaged yield of 0(1S) from the reaction is 0.43±0.12, which is smaller than the laboratory value measured for the N2(A, v′ = O) level. The cause of a disagreement between model and data for the Vegard‐Kaplan emission is unclear, but the discrepancy can be eliminated if the N2(A)+O quenching coefficient or the A state lifetime is increased by a factor between 2 and 4. The observed intensity of OII 247.0 nm is greater than expected by a factor of 2, implying possible inadequacies in the EUVAC and/or EUV91 solar models used in the analysis.
Sounding the flanks of the Earth's bow shock to −230 RE: ISEE 3 observations of terrestrial radio sources down to 1.3 times the solar wind plasma frequency
Authors: Steinberg J.‐L. , Lacombe C. , Hoang S.
Published: February 2004

We sound the Earth's bow shock and magnetosheath to X(GSE)≃−230 RE by analyzing the propagation of low‐frequency terrestrial radio emissions through these regions to ISEE 3 in the interplanetary (IP) medium in the 0300‐0400 LT sector less than 200 RE from Earth. Two sources are seen tailward of the Earth's direction, the farther the lower the frequency and sometimes away from the shock: the spike of low‐frequency (LF) bursts from 1.3 to 2ƒp.sw (ƒp.sw is the solar wind plasma frequency) and the auroral kilometric radiation (AKR) above 2ƒp.sw. IP scattering can help explaining how radiation can reach the spacecraft while traveling sunward from X ≃−230 RE; but only if the whole of the apparent source on the shock flank is far enough tailward. The minimum frequency at which the spike can be located is in most cases within 10% of the maximum plasma frequency ƒmax.sh across the shock calculated from the Rankine‐Hugoniot equations. Models of the bow shock‐magnetosheath density distribution (ƒmax.sh (X)) are thus built so that the LF radiation becomes occulted for X < Xocc as ƒ decreases thus explaining some very remote apparent sources. However these new models do not account so well as our previous ones [Steinberg and Hoang, 1993] for the apparent direction of the AKR source at ƒ ≳ 2ƒp.sw which appears closer to Earth than the burst spike. This probably implies that the auroral and the LF burst spike radiations cross the shock at about the same time but at different places.
Reconnection modes for near‐radial interplanetary magnetic field
Authors: Belenkaya E. S.
Published: February 2004

The principal purpose of the present paper is to report about a new magnetospheric reconnection structure arising for the near‐radial interplanetary magnetic field (IMF) orientations. This structure includes simultaneous existence of two modes of the solar wind‐magnetosphere coupling (two‐ and three‐dimensional reconnections). For the southward IMF two two‐dimensional merging sites exist: one on the noon equatorial magnetopause and the other in the magnetotail. Reconnection takes place along merging line connecting them. As IMF rotates from southward toward a radial orientation, the merging site where lobe fields reconnect becomes three‐dimensional and moves from the tail to the high‐latitude dayside magnetosphere (northward for IMF Bx > 0 and southward for IMF Bx < 0), while the merging site where interplanetary and closed field lines reconnect remains two‐dimensional arid shifts along the magnetopause from subsolar point to the south for IMF Bx > 0 and to the north for IMF Bx < 0. In the vicinity of two‐dimensional magnetic null on the magnetopause, reconnection occurs at merging line simultaneously with the three‐dimensional reconnection at the second neutral point in the magnetosphere. For the strong northward IMF two three‐dimensional merging sites exist in the cusp regions of the magnetosphere. Proposed construction is supported by calculations in the paraboloid magnetospheric model.
Latitudinal structure within Jovian hectometric radiation
Authors: Higgins Charles A. , Thieman James R. , Fung Shing F. , Green James L. , Candey Robert M.
Published: February 2004

Jovian hectometric radio emission (HOM: 300–3000 kHz) has a number of persistent structural features associated with it as observed by the Voyager 1, Voyager 2, Ulysses, and Galileo spacecraft for specific jovigraphic latitudes (−4° to +7.1°) and local times (0.3 to 10.5 hours). Most notable are the presence of HOM emission between 270° and 120° central meridian longitude (CML) and the region of reduced emission intensity (a “gap”) between 120° and 270°. We displayed the Ulysses and Galileo data using time‐frequency occurrence probability spectrograms and show that the observed HOM emission features are nearly identical to those observed by the Voyager spacecraft. This implies that the HOM structure is long‐lived and fixed in its longitudinal position within the Jovian magnetosphere. HOM structure depends on small changes in the observer's jovigraphic latitude, so the different jovigraphic latitudes of the spacecraft were used to probe the HOM beam structure. Prom this analysis we found that the CML width of the main HOM gap is directly correlated to the latitude of the spacecraft. We conclude that the latitudinal thickness of the HOM beam is about 12°, extending from −5° to +7° magnetic latitude.
Properties of ELF emissions in the dayside magnetopause
Authors: Song P. , Zhu Z. , Russell C. T. , Anderson R. R. , Gurnett D. A. , Ogilvie K. W. , Strangeway R. J.
Published: February 2004

We undertake a comprehensive study of the extremely low frequency emissions within the magnetopause current layer using measurements obtained with ISEE 1 spacecraft. Since the emissions vary greatly, both spatially and temporally, to derive meaningful and robust statistical results is challenging. We design an averaging procedure and choose the normalized wave amplitudes for a correlation analysis. Also, we separate the parameters for the correlation with local or global properties. We found that most of the wave power in the current layer is carried by the magnetic field fluctuations, while the electric field carries relatively little energy. These emissions are best correlated with the local magnetic field shear and electron plasma beta. The power increases as the shear and beta increase. The wave is correlated with the global interplanetary magnetic field (IMF) clock angle, although it is weaker than with the local shear. The wave is weakly correlated with the IMF Bz and is not correlated with the electron temperature anisotropy. The correlation of the electric wave field with our chosen parameters is weaker than and varying from that of the magnetic wave field. These ELF emissions have the characteristics of the whistler mode dispersion with an increasing phase velocity above the lower hybrid frequency and with a cutoff at the electron cyclotron frequency. The wavelength is ∼20 to 100 km. In a case study, the electric field is polarized nearly perpendicular to the background magnetic field.
Generation of electrostatic whistler wave by extraordinary EM wave
Authors: Sharma R. P. , Kumar Atul , Tripathi Y. K. , Kumar Raj
Published: February 2004

The parametric decay instability of an extraordinary electromagnetic wave into an electron plasma wave and an electrostatic whistler wave has been studied. Explicit expressions for the homogeneous growth rate and threshold electric field have been presented for this instability. The relevance of the present parametric process has been pointed out to explain the generation of whistler mode radiations in the SL‐2 experiment as well as in auroral magnetosphere (auroral hiss).
Quantification of tailward cold O+ beams in the lobe/mantle regions with Geotail data: Constraints on polar O+ outflows
Authors: Seki Kanako , Terasawa Toshio , Hirahara Masafumi , Mukai Toshifumi
Published: February 2004

Over a wide range of the tailward distances in the lobe/mantle regions (|XGSM′|≤210 RE), the Geotail spacecraft sometimes observed cold O+ beams (COBs) streaming tailward at a velocity nearly equal to that of the major proton component. The discovery of the ionospheric ions in the distant lobe/mantle leads us to a reexamination of the conventional view which expects these O+ ions to descend into the plasma sheet because of the E×B drift. In order to clarify the quantitative property of the COBs, we have projected COB parameters onto the polar ionosphere with a simple model which takes into account the velocity filter effect alone. The statistical data set from October 1993 to March 1995 in a solar‐minimum period is used in the calculation. The calculated average COB flux in the lobe/mantle corresponds to at least ∼108 cm−2s−1 O+ flux at the 1000‐km altitude. From the comparison with polar outflows, it is suggested that the cusp/cleft originating O+ ions sometimes need significant additional energization(s) so as to supply the COBs in the distant tail. When we assume the typical flux and temperature of polar O+ outflows, the required bulk energy of upward flowing ions (UFIs) turns out to be of the order of 3 keV, which is much higher than the typical energy of cusp/cleft outflows (<1 keV). The energy difference between the tail lobe/mantle and the cusp/cleft O+ provides clues to the supply and/or energization mechanisms of the ionospheric ions. Calculations with individual COB events suggest that the rate of COB events in which the ionospheric O+ outflow flux exceeds 109 cm−2s−1 increases with the tailward distance, and it corresponds to 27% of all events in the distant tail of |XGSM′|≥150 RE.
A self‐consistent explanation for a plasma flow vortex associated with the brightening of an auroral arc
Authors: Kosch M. J. , Scourfield M. W. J. , Nielsen E.
Published: February 2004

Simultaneous observations have been made of an auroral arc by an all‐sky TV imager and of plasma flows near the arc by the Scandinavian Twin Auroral Radar Experiment. After the Harang Discontinuity, during a quiet geomagnetic interval (Kp = 1 +), the arc undergoes a sudden brightening during an eastward surge. This is accompanied by a vortex in the plasma drift velocities, poleward of the arc, of spatial extent 180×140 km. Westward drifts at higher latitudes merge with eastward drifts at lower latitudes near the arc. No radar backscatter was recorded equatorward of the arc. The plasma flow vortex is shown to correspond with an ionospheric region of diverging horizontal electric fields, which is equivalent to a downward field‐aligned current. This region may correspond to black auroras. Spatial location of the closure current region associated with an arc by ground‐based observations is a novel result.
A simple model of low‐latitude electric fields
Authors: Eccles J. Vincent
Published: February 2004

The low‐latitude electric fields are generally modeled by numerically solving a two‐dimensional potential equation developed from assumptions of equipotential field lines and current continuity. Additional assumptions are used to derive simple formulas for the vertical electric field EL and zonal electric field Eϕ in the plane of the magnetic dip equator. Electric fields obtained from the simple model will be compared against the electric fields of the more complete numerical model to demonstrate the validity of the simple model solution and to identify the range of validity. The simple electric field model is used to directly identify the primary causes of the low‐latitude electric field structure. The observed structure has sources that can be divided into components: (1) polarization fields created by divergences in neutral‐wind dynamo currents and (2) curl‐free effects demanded by ∇ × E = 0. It is shown that the evening prereversal enhancement of Eϕ is caused by “curl‐freeness” demands of the rapidly changing EL near sunset. This supports cursory statements of Rishbeth [1971] on the cause of the postsunset enhancement. Finally, the resulting altitude variation of the zonal electric field depends on the type of source. For polarization field sources the altitude variation is approximately proportional to L2. For curl‐free sources the altitude variation is related to the rate of change of EL with respect to longitude.
The effects of meridional neutral winds on the O+‐H+ transition altitude over Arecibo
Authors: MacPherson B. , González S. A. , Bailey G. J. , Moffett R. J. , Sulzer M. P.
Published: February 2004

Improvements to the way data are taken and analyzed at the Incoherent Scatter Radar Facility at Arecibo, Puerto Rico, mean that the transition altitude, where the O+ and H+ ion densities are equal, can now be studied at all local times throughout the solar cycle. These data show that the diurnal variation of the O+‐H+ transition height is characterized by a rapid collapse following sunset, which is associated with the contraction of the plasma due to the rapidly decreasing plasma temperatures at this time. A corresponding increase in the transition altitude is seen at sunrise. The topside data also reveal that a secondary collapse, which is a nonthermal effect, is often observed at local times near midnight. Measurements taken during October 1994 and 1992 are presented and compared to results from the Sheffield University Plasmasphere Ionosphere Model (SUPIM) to study the effect of increasing solar EUV flux on the topside ionosphere. It is shown that SUPIM is able to model reasonably well the sunset and sunrise behavior of the transition altitude. In addition, the secondary collapse of the O+‐H+ transition altitude is also studied. It is shown that this secondary collapse may be directly correlated to the well‐known “midnight” collapse phenomenon of the F region over Arecibo. The magnitudes of the secondary collapse under low and moderate solar activity are compared, and it is shown that the secondary collapse is less pronounced during solar minimum conditions.
On large amplitude MHD waves in high‐beta plasma
Authors: Song P. , Russell C. T. , Chen L.
Published: February 2004

Downstream of quasi‐parallel shocks, the magnetosheath is often observed as a region of high plasma beta, of the order of 10, with large magnetic field fluctuations and relatively weak thermal pressure fluctuations. Since the wave steepening results from the nonlinearity in the plasma thermal properties, these nonlinear magnetic fluctuations that are accompanied by linear pressure fluctuations in a high‐beta plasma do not necessarily lead to wave steepening. We develop a nonlinear theory based on MHD and observational evidence to describe such a system. We first perturb the MHD equations assuming the reciprocal of beta and the perturbed pressure to be small parameters and retaining nonlinear terms for the magnetic arid velocity perturbations. We then Fourier expand the perturbations and retain terms up to the first order. A nonpropagating mode is present that has the characteristics of a series of tangential discontinuities. A purely incompressible mode is present that propagates obliquely to the background field and is nondispersive and independent of the plasma beta. The field perturbations do not need to be coplanar with the background field and the wave vector. A nonlinear compressible mode is also found. In the limit of small field perturbations, the mode is similar to the linear slow mode in high‐beta plasmas. The first harmonic field perturbation is coplanar with the background field and the wavevector. When the mode propagates along the field, the density and field strength fluctuations are small at the primary frequency of the wave and are significant at a frequency twice the primary frequency. This frequency doubling phenomenon is most severe for parallel propagation. The compressibility of the mode makes it possible to couple the energy across the magnetopause into the magnetosphere. We show observational evidence for the existence of such modes.
Fracture zone subduction and reactivation across the Puysegur ridge/trench system, southern New Zealand
Authors: Lebrun Jean‐Frédéric , Karner Garry D. , Collot Jean‐Yves
Published: February 2004

A two‐dimentional kinematic model for lithospheric thrusting that considers the flexural interaction between the underriding and overriding plates was used to assess the mechanical implications of subducting trench‐parallel fracture zones on the topography and free‐air gravity anomalies of the Puysegur ridge/trench system. Fracture zones in the underriding plate are simulated by vertical discontinuities across which bending and shearing stresses cannot be supported. The implication of reactivating fracture zones on the outer trench slope is to suppress the flexural bulge, creating topographic relief between the fracture zone and the trench, and reducing the trench gravity anomaly. In the studied region the trench subparallel L'Atalante fracture zone accounts for the lack of a flexural bulge, strong outer trench slope, and gravity gradients. The southward decrease of the trench fracture zone distance results in a decrease of the trench gravity anomaly. The effect of subducting and vertically reactivating fracture zones within the underriding plate is to segment the load of the overriding plate, creating a trough located above the underthrust fracture zone. The vertical offset and gravity modifications are functions of the fracture zone position relative to the trench. This model explains the 3500‐m‐deep troughs and the 1700‐m subsidence observed at the Puysegur ridge. We speculate that reactivating underthrust fracture zones can facilitate strike slip faults development within the upper plate in case of oblique subduction and are conducive to the formation and dispersion of terranes at convergent margins. This process can account for basal tectonic erosion.