bluuebunny/crossref_metadata_2025 huggingface dataset:
Testing the thermospheric neutral wind suppression mechanism for day‐to‐day variability of equatorial spread F
Authors: Mendillo Michael , Meriwether John , Biondi Manfred
Published: February 2004

The determination of the physical processes that cause the day‐to‐day variability of equatorial spread F (ESF) has long been one of the outstanding problems in terrestrial space physics. Within the context of the Rayleigh‐Taylor instability model for ESF, mechanisms that either enhance or inhibit the growth of a seed perturbation offer potential sources of variability that can be tested. In this study the hypothesis that enhanced thermospheric meridional winds play a critical role in suppressing ESF is examined during the Multi‐Instrumented Studies of Equatorial Thermospheric Aeronomy (MISETA) campaign of September 1998. New, high‐time‐resolution Fabry‐Perot interferometer (FPI) observations at 6300‐Å nightglow made at Arequipa (Peru) provided the neutral wind measurements during the critical postsunset hours that had been sampled only sparsely in earlier morphology studies. Evidence of local ESF activity was obtained using GPS‐based observations of phase fluctuations (Fp) and 6300‐Å all‐sky optical images from the same site. Additional GPS measurements of Fp and total electron content (TEC) from Bogota (Colombia) and Santiago (Chile) were used to determine the full flux tube development of ESF plumes and to characterize the F region morphology of the interhemispheric Appleton anomaly. Correlative studies between the nightly magnitudes of the meridional winds (Um), ESF activity (Fp), and indices describing the strength (Is) and asymmetry (Ia) of the Appleton anomaly offered no convincing evidence for the wind suppression mechanism. The best available precursor for premidnight ESF appeared to be the strength of the electrodynamically driven Appleton anomaly pattern at sunset. If one assumes that the required seed perturbation for ESF onset is essentially always available, then for all practical purposes, the magnitude of the eastward electric field that causes upward drift is both the necessary and sufficient parameter to forecast ESF with reasonable success. These results reconfirm 60 years of study pointing to the dominance of electrodynamical processes in the onset and growth of plasma instabilities at low latitudes.
Rapid enhancement of radiation belt electron fluxes due to substorm dipolarization of the geomagnetic field
Authors: Fok Mei‐Ching , Moore Thomas E. , Spjeldvik Walther N.
Published: February 2004

The classical pure radial diffusion mechanism appears not to fully explain the frequently observed rapid enhancement in the timescales of minutes to hours in the radiation belt electron fluxes in the Earth's magnetosphere. We here consider other physical mechanisms, such as energization mechanisms associated with substorm processes, to account for these sudden increases. A three‐dimensional electron kinetic model is used to simulate the dynamics of the geomagnetically trapped population of radiation belt electrons during a substorm injection event. In the past this model has been extensively used to study dynamics of energetic ions in the ring current. This work, for the first time, constitutes the development of a combined convection and diffusion model to radiation belt electrons in the 0.04–4 MeV kinetic energy range. The Tsyganenko 89 geomagnetic field model is used to simulate the time‐varying terrestrial magnetosphere during the growth phase elongation and the expansion phase contraction. We find that inductive electric field associated with the magnetic reconfiguration process is needed in order to transport substorm electrons into the trapped particle region of the magnetosphere. The maximum enhancement in energetic electron fluxes is found to be located around the geosynchronous orbit location (L = 6.6), with up to 2 orders of magnitude enhancement in the total fluxes (0.04–4 MeV). Although this enhancement in the inner magnetosphere is very sensitive to the temperature and, to a less extent, density of the source population in the plasma sheet, we suggest that the substorm‐associated energization in the magnetotail and the subsequent adiabatic acceleration in the earthward region account for the enhanced MeV electrons (killer electrons) seen at the geosynchronous orbit during storms and substorms.
Different types of low‐latitude boundary layer as observed by Interball Tail probe
Authors: Vaisberg O. L. , Smirnov V. N. , Avanov L. A. , Waite J. H. , Burch J. L. , Gallagher D. L. , Borodkova N. L.
Published: February 2004

Interball Tail probe crossed dayside magnetopause during late winter to middle autumn months. The high‐latitude magnetopause was crossed outbound orbit, while the low‐latitude magnetopause was crossed inbound. We analyze 31 low‐latitude magnetopause/low‐latitude boundary layer (LLBL) crossings recorded in 1996 in the fast mode of SCA‐1 ion spectrometer. Data from the MIF/FM‐3 magnetometers and ELECTRON spectrometer are also used. The majority of magnetopause/LLBL crossings fall into two categories: (1) highly structured LLBL (14 cases) and (2) weakly structured LLBL (9 cases). The highly structured LLBL usually consists of short‐time (0.5–5 min) transients including fast‐moving (of an order of magnetosheath velocity) plasma as well as density transients that are nearly stationary relative to the magnetospheric plasma. The observed ensemble of transients shows evolution of plasma parameters from magnetosheath‐like to magnetosphere‐like. Fast‐moving transients frequently have flux transfer event (FTE) magnetic signatures. This type of LLBL is usually accompanied by various reconnection signatures and is more frequently observed when the interplanetary magnetic field (IMF) and/or magnetosheath magnetic field has a negative Bz component. The weakly structured LLBL shows a significantly smaller variation of number density and hotter plasma temperature. Strong velocity shear is frequently observed at the magnetopause in the weakly structured case. Dispersed ion signatures are often seen within this type of LLBL. The ion and electron temperatures within the weakly structured LLBL are more elevated in respect to the magnetosheath values compared to the highly structured LLBL. The weakly structured LLBL has a tendency to occur at positive IMF and/or magnetosheath magnetic field Bz. The cause of significant differences between the two kinds of LLBL lies, apparently, in the significantly larger distance of the plasma entry site from the observation point, for weakly structured LLBL, as opposed to highly structured LLBL. While the highly structured LLBL originates at low latitudes, the weakly structured LLBL is most probably formed at high latitudes.
Survey of thermal ions in the Io plasma torus with the Galileo spacecraft
Authors: Frank L. A. , Paterson W. R.
Published: February 2004

The densities, temperatures, and bulk flow velocities are reported for a series of six passages of the Galileo spacecraft through Io's torus in Jupiter's magnetosphere. These observations were gained with the plasma analyzers on December 7, 1995, and July 1–2, August 12, September 14, October 11, and November 25, all of the latter in 1999. These plasma analyzers provided high‐resolution measurements of the energy/charge (E/Q) spectra of the three‐dimensional velocity distributions of the thermal ions that included determinations of the mass/charge (M/Q) of the primary ions with a mass spectrometer. The four dominant ions in the hot torus were two populations of ions with M/Q = 16 and two smaller populations with M/Q = 8 and 32, respectively. The identification of these four ions was based upon a best fit to the E/Q spectra of the measured three‐dimensional ion velocity distributions. The two distributions with M/Q =16 were characterized by two different temperatures, in the ranges of 20 to 30 eV and 60 to 80 eV, respectively. On the basis of expectations of higher temperatures with higher masses for the pickup ions the cooler population is identified as O+, and the hotter as S++. The two smaller populations with M/Q = 8 and 32 are identified as O++ and S+, respectively. The temperatures were ∼20 eV for the O++ and 60 to 80 eV for the S+. The densities and temperatures of the ions in the hot torus remained constant during the period July through November 1999. However, the ion densities during the initial passage on December 7, 1995, were greater by factors of ∼3, which support the presence of long‐term density variations of the torus plasmas but with relatively small fluctuations in the temperatures. A complete survey of System III longitudes was acquired with the set of six passages. The presence of an “active sector” at longitudes in the approximate range of 180° to 230° as originally found with remote observations of the torus brightnesses is confirmed with these Galileo measurements. In addition, further evidence for the importance of interchange motions for radial plasma transport was also evident in several of the Galileo passages, a dynamical process which was first identified in fields and particles measurements during the first passage through the torus on December 7, 1995. A persistent lag with an average in the range of ∼2 to 4 km/s in the azimuthal flows relative to that for rigid corotation was detected and supports the previously proposed System IV coordinate system that has a slightly slower rotation rate relative to that for System III.
Effects of thunderstorm‐driven runaway electrons in the conjugate hemisphere: Purple sprites, ionization enhancements, and gamma rays
Authors: Lehtinen N. G. , Inan U. S. , Bell T. F.
Published: February 2004

The presence of energetic runaway electron beams above thunderstorms is suggested by observations of terrestrial gamma ray flashes [Fishman et al., 1994], as well as by theoretical work [Roussel‐Dupré and Gurevich, 1996; Lehtinen et al., 1999], although such beams have not been directly measured. In this paper we consider possible measurable effects of such beams in the conjugate hemisphere as a means to confirm their existence and quantify their properties. High‐density relativistic runaway electron beams, driven upward by intense lightning‐generated mesospheric quasi‐static electric fields, have been predicted [Lehtinen et al., 2000] to be isotropized and thermalized during their interhemispherical traverse along the Earth's magnetic field lines so that only ∼10% of the electrons which are below the loss cone should arrive at the geomagnetically conjugate ionosphere. As they encounter the Earth's atmosphere, the energetic electrons would be scattered and produce light and ionization, much like a beam of precipitating auroral electrons. A Monte Carlo approach is used to model the interaction of the downgoing electrons with the conjugate atmosphere, including the backscattering of electrons, as well as production of optical and gamma ray emissions and enhanced secondary ionization. Results indicate that these conjugate ionospheric effects of the runaway electron beam are detectable and thus may be used to quantify the runaway electron mechanism.
Kinetic structure of the post plasmoid plasma sheet during magnetotail reconnection
Authors: Arzner K. , Scholer M.
Published: February 2004

Ion dynamics during magnetotail reconnection is studied by means of a two‐dimensional, large‐scale hybrid simulation (macroparticle ions, inertialess electron fluid). The initial setup is a realistic two‐dimensional equilibrium with a normal magnetic field component through the current sheet and a flaring lobe field. Reconnection is initiated by a localized resistivity in the near‐Earth region. As in MHD, a plasmoid develops and is ejected downtail. The ion kinetic structure in the post plasmoid plasma sheet is studied in detail. In a region of about 10 RE from the neutral line the ions are demagnetized and are picked up by the electron fluid ejected from the X line. Although the magnetic field in this region is reminiscent of slow mode shocks, no shocks occur, and the structure can be described by a standing large‐amplitude whistler. The Hall current leads to a cross‐tail magnetic field up to 40% of the lobe field. This may have important consequences for mapping of low‐altitude features into the tail. Further away from the neutral line over a distance up to 30 RE a thin current sheet develops in which the lobe ions perform quasi‐adiabatic orbits. This current sheet becomes instable and disrupts; the instability is driven by the free energy contained in the non‐Maxwellian velocity distributions in the current sheet. In the region of current sheet breakup the post plasmoid plasma sheet is hot and moves with a bulk speed close to the local Alfvén speed. In order to delineate the acceleration and heating process, individual ions are followed during the simulation. There is no indication for slow mode shocks in the simulation system within the simulation time. This suggests that slow mode shocks are not to be expected in the geomagnetic tail within several tens of RE from the X line.
Characterization of Joule heating in structured electric field environments
Authors: Walker David N. , Amatucci William E. , Ganguli Gurudas I.
Published: February 2004

We have recently performed a detailed characterization of ion Joule heating perpendicular to an axial magnetic field in the laboratory in a simulated ionospheric plasma environment which contains localized electric field structuring. Since Joule heating is often regarded as an important mechanism contributing to energization of outflowing heavy ions observed by higher‐altitude auroral satellites, this work has particular relevance to space physics issues, and, to our knowledge, has not been investigated systematically in a controlled environment. Since transverse (to B) ionospheric electric fields are often spatially and temporally structured, with scale lengths often as small as an ion gyroradius, the ability to systematically vary the spatial extent and magnitude of an electric field region and to observe the effect on ion energy is important. The experiment makes use of a concentric set of separately biasable ring anodes which generate a radial electric field with controllable scale length perpendicular to an ambient axial magnetic field. Joule heating results from ion‐neutral collisions occurring within this transverse, dc electric field. Until there is sufficient neutral pressure to raise the ion‐neutral collision frequency (vin) to an observable Joule heating threshold, ion cyclotron wave heating, which is induced by shear in E × B rotation, can be the primary channel for ion energization. We have discussed in earlier papers the conditions under which this occurs, and we have treated the transition between the two forms of ion heating. We concentrate primarily in this work on constructing the fields themselves and on the relationship between the subsequent collisional heating and the Pedersen conductivity as an initial indication of the validity of the measurement results. We are able to demonstrate that measurable heating is produced by even relatively small scale structures of the order of the ion gyroradius. In addition, we show that measured heating is consistent with predictions of Joule heating as a function of ion‐neutral collisions. Finally, this work can have major implications for ionospheric studies where large‐scale electric fields are often assumed in the calculation of Joule heating.
Simultaneous satellite and ground observations of transient events near the morningside magnetopause
Authors: Kim K.‐H. , Lin N. , Cattell C. A. , Song Y. , Lee D.‐H. , Engebretson M. J. , Kawano H. , Kokubun S. , Mukai T. , Tsuruda K.
Published: February 2004

We present observations of transient events on April 13, 1996, using simultaneous measurements in the solar wind, near the magnetopause, at geosynchronous orbit, and on the ground at high latitudes. On this day, Geotail passed outbound through the morningside magnetopause. When Geotail was in the magnetosphere, a clear bipolar perturbation in the magnetic field component normal to the nominal magnetopause and transient outbound (from the magnetosphere to the magnetosheath)/inbound (from the magnetosheath to the magnetosphere) magnetopause crossings were observed. When Geotail was in the magnetosheath, inbound/outbound crossings were observed. During the bipolar magnetic field signature, Geotail observed asymmetric bipolar (less inward/more outward) flow normal to the nominal magnetopause accompanied by sunward flow. Inward flow was observed when the total magnetic field of the event was enhanced, whereas outward flow was observed when the total magnetic field was depressed. We observed that some of the transient outbound/inbound crossings show significantly different signatures in the plasma flow and magnetic field, that is, weak inward flow and enhanced magnetic field before the outbound crossing and strong outward flow and decreased magnetic field after the inbound crossing. The field and flow variations during the transient outbound/inbound crossings are qualitatively similar to those during the bipolar magnetic field signature. These observations suggest that the bipolar field signature and the transient magnetopause crossings would be similar phenomena on the magnetopause. We discuss whether the observed field and flow variations during the transient events are caused by inward/outward magnetopause motion due to compression/expansion of the magnetosphere by comparing the data sets obtained from different regions of the solar wind‐magnetosphere system.
Ion temperature enhancement in the wakes of ionospheric spacecraft
Authors: Samir U. , Israelevich P. , Wright K. H. , Stone N. H.
Published: February 2004

Enhancements of the temperature of electrons in spacecraft plasma wakes have been reported for numerous cases [Samir and Wrenn, 1972; Troy et al., 1975; Oran et al., 1975], and this phenomenon has been discussed both empirically [Samir and Stone, 1986; Stone and Samir, 1986] and theoretically [Singh et al., 1987]. However, very few measurements seem to have been made of the ion temperature within plasma wakes, possibly because the great majority of ion measurements were focused on obtaining geophysical parameters and hence were confined to the region ahead of the spacecraft. Recently, however, an enhancement of the temperature of ions was discovered in data obtained in the wake of the space shuttle during the Spacelab 2 mission [Sorensen et al., 1997]. At the time of that publication this was the only known observation of this type. Herein we report an additional case of ion temperature enhancement in a plasma wake. The data were taken during the tethered satellite system reflight (TSS‐IR) mission in the wake of the tethered satellite during passive (no current flow) operations. The measurements were obtained with the Differential Ion Flux Probe [Stone, 1977, Stone et al., 1985].
Validating the solar EUV Proxy, E10.7
Authors: Tobiska W. Kent
Published: February 2004

A demonstration of the improvement in thermospheric densities using the daily E10.7 proxy compared to F10.7 is shown. The daily altitude decay for the Solar Mesosphere Explorer (SME) satellite from April 1, 1982 through August 9, 1983, using both proxies and the actual altitude data, are compared. The F10.7 case finished 2 km lower than each of the E10.7 and actual altitude cases which were nearly identical. During active solar conditions, daily F10.7 can overestimate the EUV energy input into the atmosphere by up to 60% and also underestimate it by as much as 50%. Progress is shown towards validating E10.7 as a more accurate proxy compared to F10.7 for use in atmospheric density calculations that are applicable to satellite drag problems. In support of the validation of the E10.7 proxy, an operational prototype hardware/software platform for visualizing of the near‐Earth space environment was created. This platform uses a data‐driven, data visualization environment. Platform development continues so as to accommodate not only historical data but also nowcast and forecast data streams. Upgrades to the SOLAR2000 Research Grade model are continuing in order to improve the correlation coefficients from multiple linear regressions in several wavelength regions. E10.7 is used in applications that incorporate F10.7, including empirical thermospheric models, ionospheric models, and general representations of solar activity ranging from climate research to engineering applications.
Observations of geomagnetic cutoff variations during solar energetic particle events and implications for the radiation environment at the Space Station
Authors: Leske R. A. , Mewaldt R. A. , Stone E. C. , von Rosenvinge T. T.
Published: February 2004

Data from the polar‐orbiting Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite have been used to measure the location of the geomagnetic cutoff for low‐energy protons and alpha particles during several large solar energetic particle events from mid‐1992 to late 1998. When fluxes are sufficiently high, the cutoff latitude can be measured up to four times per orbit, allowing the variability of the cutoff to be observed on relatively short timescales. We find significant changes in the cutoff location, often by more than 5° in less than 1 day, and these changes are well correlated with geomagnetic activity as measured by either Dst or Kp. Spacecraft in intermediate‐inclination orbits such as the International Space Station (ISS) graze the geomagnetic polar cap at certain longitudes each day. Calculations show that a 5° suppression in the average geomagnetic cutoff increases by more than a factor of 2.5 the time that the ISS spends in the polar cap exposed to energetic particles. Since the Station is only vulnerable at certain longitudes, however, real‐time monitoring of the cutoff location from a polar‐orbiting spacecraft could be used to provide advance notice of the polar cap location and conditions, sometimes hours before the Space Station itself reaches high magnetic latitudes.
A new nonlinear approach to the theory of E region irregularities
Authors: St.‐Maurice J.‐P. , Hamza A. M.
Published: February 2004

In the study of E region irregularities the standard procedure is to Fourier analyze the irregularities in both time and space, that is, to describe them as a superposition of plane waves. This introduces difficulties when the amplitude of the plane waves becomes large, thereby adding nonlinear terms to the original equations and forcing all the plane waves to become coupled to one another. In the present work we stay away from Fourier analysis and use the standard fluid description of the instabilities in the limit of perturbed electric fields that are strictly perpendicular to the geomagnetic field. We obtain a nonlinear generalization of the standard results whereby the diffusion‐like operator found in linear theory is now a function of the density itself. Therefore, as the structures grow, the net electric field seen by the ambient plasma inside the structures changes in time: it rotates and its amplitude decreases. Consequently, one possible saturation mechanism for the instabilities is a reduction in the net electric field inside the structures, which brings them to threshold velocity conditions. This being stated, other nonlinear saturation mechanisms remain possible if they require smaller saturation amplitudes than the present work. Either way, our work is consistent with intermittency and implies that the largest amplitude structures in the medium should be rotated away from zero flow angle conditions by a measurable amount. Finally, we show that when compared to an irregularity‐free situation, there should be a measurable reduction in the average Hall current carried by the plasma, while the average Pederseri current should not be affected.
Broadband transverse waves below 1 Hz in the afternoon sector of the magnetosphere
Authors: Matsui H. , Hayashi K. , Mukai T. , Ohtani S. , Kokubun S. , Yamamoto T. , Matsumoto H. , Tsuruda K.
Published: February 2004

We present an observation of broadband transverse waves (BTW) below 1 Hz in the afternoon sector of the magnetosphere at about L ∼ 10. We find that the BTW correlate well with density enhancements and bidirectional electron flows. Characteristics of the BTW are summarized as follows. The wave amplitude varies within a few periods, which results in a broad frequency band in the Fourier spectra. Only perturbations transverse to the background magnetic field are observed. The amplitude ratio of electric to magnetic field fluctuations is close to the local Alfvén speed, so that the wave mode is suggested to be Alfvén waves. The Poynting flux is directed away from the equatorial region toward the ionosphere. We have assessed various resonance mechanisms (Landau resonance, cyclotron resonance, and resonance with bouncing electrons) but find that none gives a compelling interpretation of the observation, which thus requires future study.
Gravity wave propagation directions inferred from satellite observations including smearing effects
Authors: Brown Jason S. , Hickey Michael P.
Published: February 2004

We simulate space‐based, sublimb viewing observations of airglow brightness fluctuations caused by atmospheric gravity wave interactions with the O2 atmospheric airglow, and we demonstrate that because of the geometry associated with such observations, the brightness fluctuations observed for the optically thick 0–0 band emission will always appear stronger for waves traveling toward the observer (the satellite). The effect should be most noticeable for waves having relatively small vertical wavelengths (∼10 km) and horizontal wavelengths of 50 km or greater. For waves of short (∼100 km) horizontal wavelength, the brightness fluctuation anisotropy with respect to viewing direction may also be evident in the optically thin 0–1 band emission. We demonstrate that the waves will be observable despite the fact that an instrument requires a certain finite integration time to achieve a desired signal‐to‐noise ratio. Therefore the 180° ambiguity in wave propagation direction associated with space‐based observations may be eliminated for waves of small vertical wavelength that are dissipating in the upper mesosphere and lower thermosphere. It is these same waves that may be expected to be important to the energy and momentum budgets of the mesosphere/lower thermosphere region.
Investigation of the polytropic relationship between density and temperature within interplanetary coronal mass ejections using numerical simulations
Authors: Riley Pete , Gosling J. T. , Pizzo V. J.
Published: February 2004

Single‐point spacecraft measurements within coronal mass ejections (CMEs) often exhibit a negative correlation between electron density and temperature. At least two opposing interpretations have been suggested for this relationship. If, on one hand, these single spacecraft observations provide direct measures of the polytropic properties of the plasma, then they imply that the polytropic index for the electrons γe is often < 1. Moreover, since the electrons carry the bulk of the pressure (via their significantly higher temperature), this further implies that the dynamics of CME evolution are dominated by an effective polytropic index γeƒƒ < 1. On the other hand, γ < 1 implies that as the ejecta propagate away from the Sun and expand, they also heat up; a result clearly at odds with in situ observations. In contrast to these CME intervals, many studies have shown that the quiescent solar wind exhibits a positive correlation between electron density and temperature, suggesting that γe > 1. In this study we simulate the evolution of a variety of CME‐like disturbances in the solar wind using a one‐dimensional, single‐fluid model, to address the interpretation of the relationship between electron density and temperature within CMEs at fixed locations in space. Although we strictly impose a polytropic relationship (with γ = constant) throughout our simulations, we demonstrate that a variety of correlations can exist between density and temperature at fixed points. Furthermore, we demonstrate that the presence of only local uncorrelated random fluctuations in density and temperature can produce a negative correlation. Consequently, we conclude that these single‐point observations of negative correlations between electron density and temperature cannot be used to infer the value of γe. Instead, we suggest that entropy variations, together with the plasma's tendency to achieve pressure balance with its surroundings, are responsible for the observed profiles.
Correlating near‐Earth interplanetary magnetic fields: Foreshock effects
Authors: Kaymaz Zerefsan , Sibeck David G.
Published: February 2004

Interplanetary magnetic field (IMF) observations by ISEE 1 and IMP 8 were correlated to reveal the effects of upstream waves on IMF predictions. Past studies using spacecraft just outside the Earth's bow shock and far upstream at the L1 libration point attributed frequently poor (≤0.5) correlation coefficients to short IMF scale lengths and difficulties in estimating time delays. We find that the correlation coefficients for two near‐Earth spacecraft are actually worse than those for a spacecraft at the L1 point and one just outside the bow shock: 48% of the near‐Earth correlation coefficients for the IMF magnitude are poor (<0.5), and only 17% are good (>0.8). We attribute the poor result to two causes: (1) high‐frequency waves and diamagnetic effects in the foreshock and (2) intervals of low IMF variance. Of these two, high‐frequency waves account for 80% of the cases with poor correlation, and the intervals of nearly constant IMF account for the remaining 20% of the cases. While correlation coefficients do not increase with solar wind density while both spacecraft are in the solar wind, they do increase when one or both spacecraft lie within the foreshock. We argue that foreshock waves and intervals of low IMF variance must also have reduced correlation coefficients in previous IMF correlation studies. While the significance of the foreshock waves on the solar wind input into the magnetosphere deserves further study, there is no obstacle to predicting solar wind input into the magnetosphere during intervals with poor correlation coefficients but low IMF variance.
Plasma sheet thickness and electric currents
Authors: Kaufmann Richard L. , Ball Bryan M. , Paterson W. R. , Frank L. A.
Published: February 2004

Two years of Geotail data in the (−30 < x < −8, |y| < 15) RE region first were sorted into (x, y, β) boxes. Direct measurements of the average electron and ion current densities, symmetry assumptions, and the momentum equation were used to get three different estimates of the electric current in each box. The momentum equation method gave the most consistent results, while the other two methods provided complementary information about particle drifts. The average common drift of electrons and ions was found to be comparable to the average differential drift of ions with respect to electrons. These two components of the ion drift velocity tended to cancel on the dawnside, resulting in currents that were primarily carried by electrons moving at the common drift speed. The two ion drifts added on the duskside where ions carried most of the cross‐tail current. The particle and magnetic field measurements were used to estimate the z thickness of each β box. A concentration of the long‐term‐averaged cross‐tail current was seen near the neutral sheet. The region of nonadiabatic orbital motion had an average characteristic length scale of ∼0.4 RE. The principal plasma sheet extended to ∼2.5 RE from the neutral sheet at midnight and to ∼5 RE in the flanks. The final result is a method to create models in (x, y, z) coordinates of the long‐term‐averaged values of any of the measured fluid parameters or fields. The isotropic portion of the pressure tensor was used as an example of one parameter that can be modeled. These pressure plots showed that the z component of the long‐term‐averaged magnetic field line tension force is important everywhere, that the y component is small everywhere, and that the y component is significant in the flanks.
Requirements for detection of ionospheric bubbles in the oxygen recombination EUV airglow
Authors: Stephan Andrew W. , Chakrabarti Supriya , Cotton Daniel M.
Published: February 2004

We present simulations that show the requirements for the space‐based detection of ionospheric bubbles in the atomic oxygen recombination continuum at 911 Å. In particular, we provide limits to the instrument sensitivity and integration time required for the remote sensing of these diverse and dynamic density depletions that percolate up through the nightside equatorial ionosphere. To first order, these are also the requirements to combine a series of observations and tomographically invert the two‐dimensional structure of the depleted flux tubes. We find that with a sensitivity of 1 count per second per rayleigh, a minimum of 1‐s integration is required to obtain a 3σ detection of the depletion over a 5° zenith angle bin. We compare the results of these simulations with the characteristics of three instruments from the Space Test Program 78–1, Tomographic Experiment using Radiative Recombinative Ionospheric EUV and Radio Sources, and Advanced Research and Global Observation Satellite missions, designed to measure this emission. This comparison shows that the state of the art of ultraviolet remote sensing has advanced to the point where spectrographs can be developed to obtain multidimensional images of bubbles and other similar ionospheric density structures.
Distribution of magnetic field components in the solar wind plasma
Authors: Padhye N. S. , Smith C. W. , Matthaeus W. H.
Published: February 2004

We examine the probability distribution functions (PDFs) of fluctuations of magnetic field components using the Ulysses and Omnitape data sets to evaluate departures from Gaussian distributions. Functional fits as well as moment comparisons (kurtoses) are used in drawing conclusions concerning the degree of non‐Gaussianity. Short‐timescale fluctuations are separated, to the extent possible, from the longer‐timescale variation of the mean field, and attention is paid to data selection issues such as stationarity. At the present level of comparison it seems that departures from Gaussianity of the distributions of the fluctuations are not severe. The analysis is carried out in the mean field coordinates in which the fluctuations of components are close to being uncorrelated, and it is observed that the perpendicular components are closer to being Gaussian than the parallel one. It is shown that the kurtosis is highly exaggerated when the variation of the mean field is not taken into account. We further examine the distributions of hourly magnetic field fluctuations in fast and slow solar wind at different phases in the solar cycle in order to quantitatively describe departures from the Gaussian distribution. It is found that the kurtosis lies between 2.8 and 4.8 for all components in the mean field coordinates, in both fast and slow wind and near solar maximum and near solar minimum. It appears that the distributions of fluctuations are rather more similar with regard to their degree of non‐Gaussianity than might have been expected based upon the well‐documented differences in the characteristics of fast and slow solar wind intervals. This suggests a robust form of statistical similarity that may be associated with either in situ or source region nonlinear effects. In addition to the distributions of the fluctuations, we present the distributions of the mean field and of the variances of the components which are more likely to be influenced by solar sources than by interplanetary dynamical processes. The PDFs of variances of the magnetic field components over different subintervals approximate lognormal curves, and provide the motivation to compute the PDFs of the fluctuations using the approach of Castaing et al. [1990], which is a superposition of Gaussian distributions with variances that are distributed lognormally. The resulting PDFs of the fluctuations provide a good model to describe the small departures from Gaussian distributions seen in the observed PDFs. An additional step is taken to compute the expected PDF of the magnitude of the fluctuations from the Castaing PDFs of the components and is also seen to be in good agreement with the observed PDF.
Pulsed boundary layers observed during a Geotail magnetopause skimming orbit
Authors: Kim K.‐H. , Lin N. , Cattell C. A. , Song Y. , Wygant J. R. , Lee D.‐H. , Kawano H. , Kokubun S. , Mukai T. , Tsuruda K.
Published: February 2004

On January 8, 1996, the Geotail satellite skimmed along the dayside magnetopause from the dawnside to the duskside and observed a series of quasi‐periodically recurring pulsed boundary layers in the prenoon (∼0900–1000 LT) and the postnoon (∼1400–1500 LT) sectors. For both intervals the interplanetary magnetic field observed by the Wind satellite upstream of the bow shock was inferred to be southward from an estimated convection time between Wind and Geotail. During the pulsed events, Geotail observed bipolar signatures in the magnetic field component normal to the nominal magnetopause. In the prenoon sector the bipolar variation had the inward‐then‐outward polarity, while in the postnoon sector the polarity was opposite (i.e., outward‐then‐inward). The plasma flows normal to the nominal magnetopause showed bipolar signatures with the inward‐then‐outward polarity in the prenoon and postnoon sectors. They were accompanied by sunward flows for the lower‐density pulsed events and by tailward flows for the higher‐density pulsed events. We have interpreted the sunward flow as the local plasma response to a tailward moving flux tube. This may imply that the sunward plasma flow near the magnetopause does not always reflect the direction of the motion of the flux tube. On the basis of these observations, we suggest that the observed pulsed boundary layers are attributed to the passage of a train of flux transfer events propagating tailward.
Gyroresonant acceleration of electrons in the magnetosphere by superluminous electromagnetic waves
Authors: Summers Danny , Thorne Richard M. , Xiao Fuliang
Published: February 2004

Superluminous auroral kilometric radiation originates in the auroral cavity of the Earth's magnetosphere as right‐hand extraordinary (R‐X) mode emissions, with additional contributions from the left‐hand ordinary (L‐O) and left‐hand extraordinary (L‐X) modes. The three modes can propagate into the outer radiation belt and undergo gyroresonant interaction with trapped energetic electrons over a broad extent of the outer magnetosphere. We develop a general theory of quasi‐linear diffusion and construct resonant diffusion curves in velocity space for each superluminous wave mode. The potential for stochastic electron acceleration is controlled by the dispersive properties of the waves and the ratio between the electron gyrofrequency and plasma frequency. It is found that each of the R‐X, L‐O, and L‐X modes can produce significant acceleration of electrons over individual regions of parameter space. The L‐O mode is found to have the potential for accelerating electrons from ∼10 keV to ∼MeV energies, over a broad range of wave normal angles, in spatial regions extending from the auroral cavity to the high‐latitude (>30°) outer radiation belt. The R‐X mode appears to be less effective for accelerating magnetospheric electrons, since acceleration to significant energies (∼MeV) requires very small wave normal angles (<10°). The potential for significant electron acceleration in the magnetosphere by L‐X mode waves is restricted not least by the requirement of high minimum energies, e.g., 400 keV in the outer radiation belt. To assess whether the superluminous wave modes contribute significantly to the stochastic acceleration of relativistic electrons during geomagnetic storms, the present study needs to be supplemented by ray‐tracing analyses and the calculation of energy diffusion coefficients incorporating data on wave power.
Secondary instabilities in the dynamics of the Farley‐Buneman fluctuations
Authors: Smolyakov A. I. , Koustov A. V. , Makarevitch R. A.
Published: February 2004

Nonlinear dynamics of the Farley‐Buneman modes is investigated. It is shown that in the nonlinear regime, there exists a secondary nonlinear instability leading to excitation of low‐frequency, long‐wavelength modes that corresponds to the inverse energy cascade toward the longer wavelengths. The growth rate of the secondary instability is proportional to the squared amplitude of primary electron density fluctuations.
Solar minimum spectra of galactic cosmic rays and their implications for models of the near‐Earth radiation environment
Authors: Davis A. J. , Mewaldt R. A. , Cohen C. M. S. , Cummings A. C. , George J. S. , Leske R. A. , Stone E. C. , Wiedenbeck M. E. , Yanasak N. E. , Christian E. R. , von Rosenvinge T. T. , Binns W. R. , Hink P. L.
Published: February 2004

The radiation dose from galactic cosmic rays during a manned mission to Mars is expected to be comparable to the allowable limit for space shuttle astronauts. Most of this dose would be due to galactic cosmic rays with energies < 1 GeV nucleon−1, with important contributions from heavy nuclei in spite of their low abundance relative to H and He. Using instruments on NASA's Advanced Composition Explorer (ACE) spacecraft, we have made the most statistically precise measurements to date of the solar minimum energy spectra of cosmic ray nuclei with charge Z = 4–28 in the energy range ∼ 40–500 MeV nucleon−1. We compare these measurements obtained during the 1997–1998 solar minimum period with measurements from previous solar minima and with models of the near‐Earth radiation environment currently used to perform shielding and dose calculations. We find that the cosmic ray heavy‐element spectra measured by ACE are as much as 20% higher than previously published solar minimum measurements. We also find significant differences between the ACE measurements and the predictions of available models of the near‐Earth radiation environment, suggesting that these models need revision. We describe a cosmic ray interstellar propagation and solar modulation model that provides an improved fit to the ACE measurements compared to radiation environment models currently in use.
On the linear and nonlinear resistive tearing‐mode instabilities
Authors: Hau L.‐N. , Chiou S.‐W.
Published: February 2004

The tearing‐mode instabilities of Harris sheet magnetic field configuration are studied based on the numerical simulations of two‐dimensional, resistive, compressible MHD models with isotropic pressure. The linear growth rate and the eigenmode structure are calculated from the linear numerical model for various parameters: magnetic Reynolds number, Rm; the ratio of wavelength to the layer thickness; magnetic By component; and plasma beta, as well as for different equations of state. The linear solutions are then used as initial perturbations of the nonlinear numerical model to allow the full evolution of resistive tearing‐mode instability. The linear calculations show that for the range of Rm = 10–105 the fastest growth rate increases with decreasing Rm and only slightly increases with increasing plasma beta and By but that it is not sensitive to the equation of state. While the nonlinear calculations snow that only for large Rm, where the diffusion time is much larger than the linear growth time, the magnetic island may possibly grow substantially and become saturated. For small Rm the plasmoids either diminish in the late stage or do not have apparent growing; that is, the linear analysis is not meaningful for large resistivity cases. The calculations are compared to the magnetic island structure at Earth's magnetopause reconstructed from the single‐spacecraft data by Hau and Sonnerup [1999].
Plasma sheet access to geosynchronous orbit: Generalization to numerical global field models
Authors: Korth H. , Thomsen M. F.
Published: February 2004

A previous statistical analysis of the geosynchronous particle environment by Korth et al. [1999] showed that the particle fluxes are well organized by local time and the geomagnetic activity as measured by the Kp index. Regions of high and low fluxes at geosynchronous orbit are separated from each other by distinct boundaries, approximately matching the Alfvén boundary crossings of the geosynchronous satellites calculated analytically from a Volland‐Stern electric potential and a dipole magnetic field. Expanding the analysis technique from the previous work to arbitrary, numerical electric and magnetic field models, we reevaluate the Alfvén boundary crossings using several available global electric potential models and considering external magnetic field contributions. The more sophisticated numerical models do not do a better job of explaining the observed average access of plasma sheet material to the geosynchronous region than does the simple analytical model.
Stabilization of lower‐hybrid drift instability in the magnetotail by finite north‐south magnetic field component and destabilization by sheared cross‐field flow
Authors: Yoon Peter H. , Lui A. T. Y.
Published: February 2004

The problem of nonlocal lower‐hybrid drift instability in the magnetotail‐like geometry characterized by a finite north‐south (normal) magnetic field component at the center of the neutral sheet is revisited. In a preliminary paper [Lui et al., 1995] it was shown that the one‐dimensional neutral sheet of the Harris type with constant cross‐field flow speed profile, v = v0 = const, is stable with respect to perturbations in the lower‐hybrid frequency range, if the normal field component is finite, Bn > 0. They then discussed a sheared flow speed profile, v(z) = v0/[1 + (z/Δ)2], which represents a much thinner current sheet configuration, and recovered the unstable mode. In this paper, the previous work is extended in two aspects. The effects of the variation of the normal field strength, Bn, on the transition from instability to stability in the absence of the shear (Δ = ∞) is examined systematically. In addition, the eigenmodes with odd symmetry, ϕ(−z) = − ϕ(z), are included. These two extensions show that indeed, the normal field component Bn has a strong stabilizing influence on the mode. As the shear parameter Δ is reduced, the threshold value of Bn for stabilization increases in proportion to the value of Δ−1, which leads to the conclusion that the shear in the cross‐field drift speed has a destabilizing influence on the mode. Comparison of the two symmetry modes shows that the odd symmetry eigenmodes generally possess higher growth rates than the even symmetry ones.
Hybrid and Hall‐MHD simulations of collisionless reconnection: Dynamics of the electron pressure tensor
Authors: Yin L. , Winske D. , Gary S. P. , Birn J.
Published: February 2004

In this study we compare the results of two‐dimensional hybrid (particle ions, massless fluid electrons) and Hall‐MHD simulations of collisionless reconnection in a thin current sheet. Both calculations include the full electron pressure tensor (instead of a localized resistivity) in the generalized Ohm's law to initiate reconnection, and in both an initial perturbation to the Harris equilibrium is applied. As in the recent Geospace Environment Modeling (GEM) reconnection challenge studies, we find overall agreement between the two calculations in both the reconnection rate and the global configuration. Results of this study show that in addition to providing the reconnection electric field at the X point the divergence of the electron pressure tensor leads to in‐plane electric fields that exert drag forces on the ions as they enter and exit the near‐X‐point region. The in‐plane electric fields are enhanced in regions of small transverse scale along the edge of the sheet, and the resulting narrow electron current layers are demonstrated clearly. The possibility of improving magnetotail reconnection models by embedding a Hall‐MHD calculation using the electron pressure tensor model inside a large‐scale MHD simulation is suggested.