bluuebunny/crossref_metadata_2025 huggingface dataset:
Supply of thermal ionospheric ions to the central plasma sheet
Authors: Cully C. M. , Donovan E. F. , Yau A. W. , Opgenoorth H. J.
Published: May 2003

New observations from the Akebono satellite [Cully et al., 2002] evaluate the magnitude and solar wind dependence of the thermal component of terrestrial ion outflow. In this paper, we address the eventual supply of <20 eV ions to the central plasma sheet (CPS) by tracing their trajectories using empirical models of the electric and magnetic fields. We show the flux of ionospheric origin ions across the surface bounding the CPS to be substantial and largely controlled by the convection electric field. We find that under times of enhanced convection, ionospheric ions preferentially supply the region where substorm activity is thought to originate, and that a reduction in the convection (as caused, for example, by a northward turning of the interplanetary magnetic field (IMF)) immediately and dramatically reduces this supply. This effect arises due to the modification of the trajectories of “in‐flight” ions. It is postulated that this sudden reduction in CPS ion supply in response to a northward IMF turning may have consequences for substorm triggering.
Global observations of nitric oxide in the thermosphere
Authors: Barth C. A. , Mankoff K. D. , Bailey S. M. , Solomon S. C.
Published: May 2003

Nitric oxide density in the lower thermosphere (97–150 km) has been measured from the polar‐orbiting Student Nitric Oxide Explorer (SNOE) satellite as a function of latitude, longitude, and altitude for the 2 1/2 year period from 11 March 1998 until 30 September 2000. The observations show that the maximum density occurs near 106–110 km and that the density is highly variable. The nitric oxide density at low latitudes correlates well with the solar soft X‐ray irradiance (2–7 nm), indicating that it is the solar X‐rays that produce thermospheric nitric oxide at low and midlatitudes. Nitric oxide is produced at auroral latitudes (60°–70° geomagnetic) by the precipitation of electrons (1–10 keV) into the thermosphere. During high geomagnetic activity, increased nitric oxide may be present at midlatitudes as the result of meridional winds that carry the nitric oxide equatorward.
Latitude dependence of zonal plasma drifts obtained from dual‐site airglow observations
Authors: Martinis C. , Eccles J. V. , Baumgardner J. , Manzano J. , Mendillo M.
Published: May 2003

All‐sky imagers located at Tucumán, Argentina (26.9°S, 65°W, 14.2°S dip latitude), and Arequipa, Perú (16.5°S, 71.5°W, 2.7°S dip latitude), are used to track 630 nm airglow depletion motions in the first use of multisite airglow imagers for studies of low‐latitude plasma dynamics. A new image analysis technique yields a consistent determination of nighttime zonal plasma drifts from all‐sky images of the depletion motions. The observed eastward plasma drifts are smaller at Arequipa than at Tucumán in the postsunset period. During the postmidnight hours, the opposite pattern occurs. These observations are interpreted using the simple plasma drift model and coupled ionosphere‐electric field model of Eccles [1998a, 1998b]. The observed zonal plasma drifts result from low‐latitude electrodynamics with a mix of influences from E and F region conductivities and neutral wind shears in altitude and latitude. Analysis of the observations suggests that postsunset zonal drifts near the magnetic equator (Arequipa) are strongly influenced by the E region dynamo, while the F region dynamo is the main cause of zonal drifts observed closer to the Equatorial Ionization Anomaly (Tucumán). The observed altitude‐latitude behavior of the plasma drifts gives the first two‐dimensional evidence for the so‐called F region plasma vortex's influence at equatorial and low latitudes obtained using optical imaging techniques. With this framework a synthesis is offered for seemingly inconsistent zonal drift observations in the published literature.
MHD field line resonances and global modes in three‐dimensional magnetic fields
Authors: Cheng C. Z.
Published: May 2003

By assuming a general isotropic pressure distribution P = P(ψ, α), where ψ and α are three‐dimensional scalar functions labeling the field lines with B = ∇ψ × ∇α, we have derived a set of MHD eigenmode equations for both global MHD modes and field line resonances (FLR). Past MHD theories are restricted to isotropic pressures with P = P(ψ) only. The present formulation also allows the plasma mass density to vary along the field line. The linearized ideal MHD equations are cast into a set of global differential equations from which the field line resonance equations of the shear Alfvén waves and slow magnetosonic modes are naturally obtained for general three‐dimensional magnetic field geometries with flux surfaces. Several new terms associated with ∂P/∂α are obtained. In the FLR equations a new term is found in the shear Alfvén FLR equation due to the geodesic curvature and the pressure gradient in the poloidal flux surface. The coupling between the shear Alfvén waves and the magnetosonic waves is through the combined effects of geodesic magnetic field curvature (κs = 2κ · B × ∇ψ/B2) and plasma pressure as previously derived. The properties of the FLR eigenfunctions at the resonance field lines are investigated, and the behavior of the FLR wave solutions near the FLR surface are derived. Numerical solutions of the FLR equations for three‐dimensional magnetospheric fields in equilibrium with high plasma pressure will be presented in a future publication.
Forecasting solar wind structures and shock arrival times using an ensemble of models
Authors: Fry C. D. , Dryer M. , Smith Z. , Sun W. , Deehr C. S. , Akasofu S.‐I.
Published: May 2003

Forecasting the time of arrival at Earth of interplanetary shocks following solar metric type II activity is an important first step in the establishment of an operational space weather prediction system. The quality of the forecasts is of utmost importance. The performances of the shock time of arrival (STOA) and interplanetary shock propagation models (ISPM) were previously evaluated by Smith et al. [2000] for 36 solar events. Here we use 173 solar events between February 1997 and October 2000 to set thresholds for the Hakamada–Akasofu–Fry version 2 (HAFv.2) model and then present the results of a comparison of the performance of this model to the STOA and ISPM solar wind models. Each model predicts shock arrival time (SAT) at the Earth using real‐time metric type II radio frequency drifts and coincident X‐ray and optical data for input and L1 satellite observations for verification. Our evaluation of input parameters to the models showed that the accuracy of the solar metric type II radio burst observations as a measure of the initial shock velocity was compromised for those events at greater than 20° solar longitude from central meridian. The HAF model also calculates the interplanetary shock propagation imbedded in a realistic solar wind structure through which the shocks travel and interact. Standard meteorological forecast metrics are used. A variety of statistical comparisons among the three models show them to be practically equivalent in forecasting SAT. Although the HAF kinematic model performance compares favorably with ISPM and STOA, it appears to be no better at predicting SAT than ISPM or STOA. HAFv.2 takes the inhomogeneous, ambient solar wind structure into account and thereby provides a means of sorting event‐driven shock arrivals from corotating interaction region (CIR) passage.
Energetic ions, large diamagnetic cavities, and Chapman‐Ferraro cusp
Authors: Fritz Theodore A. , Chen Jiasheng , Siscoe George L.
Published: May 2003

Extremely large diamagnetic cavities with a size of as large as 6 RE have been observed in the dayside high‐altitude cusp regions. These diamagnetic cavities were associated with strong magnetic field turbulence. Associated with these cavities are >40 keV ions that are more typical of the trapped ring current and radiation belt populations than the solar wind. The charge state distribution of these cusp cavity ions was indicative of their seed populations being a mixture of ionospheric and solar wind particles. In April 1999, the cusp diamagnetic cavities were observed by the Polar spacecraft almost in every orbit, indicating that such cavities are always there day by day. Some of the diamagnetic cavities were independent of the interplanetary magnetic field directions, suggesting that the cusp diamagnetic cavities are different from the magnetospheric sash predicted by MHD simulations. During a high solar wind pressure period on 21 April 1999, the Polar spacecraft observed lower energetic (>20 keV/e) ion fluxes in the dayside high‐latitude magnetosheath than that in the neighboring cusp cavities. By their geometry cusp magnetic field lines are connected to all of the magnetopause boundary layers. These energetic particles in the cusp diamagnetic cavity together with the cusp's connectivity probably have significant global impacts on the geospace environment.
Toward a self‐consistent treatment of the cyclotron wave heating and acceleration of the solar wind plasma
Authors: Laitinen T. , Fichtner H. , Vainio R.
Published: May 2003

We have developed a steady state two‐fluid description of the radial solar wind expansion close to the coronal base. The model is formulated self‐consistently with respect to the radial evolution of turbulence by supplementing the moment equations of the plasma with an equation describing the radial evolution of the Alfvén wave power as originally suggested by Tu et al. [1984]. We assume that there is sufficient wave power in the ion‐cyclotron frequency range and determine the resulting acceleration and heating rates of the thermal plasma in the corona and interplanetary space. In improvement of earlier models of the nonlocal dynamics of the expanding corona, the dissipation frequency is determined self‐consistently from the ion‐cyclotron damping rate. After demonstrating that the time‐independent modeling can correctly reproduce the solutions obtained with (in the above sense) non‐self‐consistent models, the significance of the self‐consistent treatment is pointed out. The new solutions are studied in detail, and their implications for the transport of solar energetic particles are discussed.
Dynamic response of Earth's magnetosphere to By reversals
Authors: Kabin K. , Rankin R. , Marchand R. , Gombosi T. I. , Clauer C. R. , Ridley A. J. , Papitashvili V. O. , DeZeeuw D. L.
Published: May 2003

In this paper we use a three‐dimensional magnetohydrodynamic (MHD) model to study the response of the magnetosphere‐ionosphere system to a sudden change in the sign of the By component of the interplanetary magnetic field. We find that after such a change the magnetosphere responds as a linear system and the ionospheric potential can be very accurately represented by a linear superposition of the initial and final distributions of the ionospheric potential. Although heuristic in nature, this representation of ionospheric potentials significantly simplifies the analysis of the time‐dependent output of the model. The accuracy of this decomposition of the ionospheric potential is discussed. This decomposition also helps to define, in a very intuitively clear way, the bow shock to ionosphere communication time and the ionospheric reconfiguration time. We find that in our model simulation the reconfiguration time depends on the strength of the interplanetary magnetic field (IMF) and the solar wind speed but not on the ionospheric conductivities. The communication time in our model has a typical value of 300–400 s and the reconfiguration time is approximately 300–450 s. These characteristic times are generally consistent with observations and earlier modeling results.
Detection of artificially generated ULF waves by the FAST spacecraft and its application to the “tagging” of narrow flux tubes
Authors: Wright D. M. , Davies J. A. , Yeoman T. K. , Robinson T. R , Cash S. R. , Kolesnikova E. , Lester M. , Chapman P. J. , Strangeway R. J. , Horne R. B. , Rietveld M. T. , Carlson C. W.
Published: May 2003

Recently, Robinson et al. [2000] presented a brief report on the artificial generation of ULF waves by high power ionospheric modification and their subsequent detection by the Fast Auroral Snapshot (FAST) spacecraft. The radio frequency “heating” facility at Tromsø was employed to impose a 3‐Hz modulation on the current system that constitutes the auroral electrojet, resulting in the injection of field‐guided ULF waves into the magnetosphere. The electric and magnetic field signatures of the waves were detected directly by the FAST spacecraft. Furthermore, a signature in the downgoing field‐aligned electron flux, also observed by the satellite, was postulated to have resulted from the interaction of the ULF waves with the upper boundary of the ionospheric Alfvén resonator. This paper evaluates these results in the context of the prevailing ionospheric conditions during the experiment and discusses the significance of the substorm activity, which occurred during this interval. The technique of artificial ULF wave injection, which can be used to “tag” narrow flux tubes, could play an important part in overcoming mapping issues between ground‐based and space‐based observations of phenomena that couple the ionosphere and magnetosphere.
FAST observations of ion solitary waves
Authors: McFadden J. P. , Carlson C. W. , Ergun R. E. , Mozer F. S. , Muschietti L. , Roth I. , Moebius E.
Published: May 2003

Measurements from the FAST spacecraft are used to show that ion solitary waves observed at the lower edge of the acceleration region travel at velocities faster than the associated auroral proton beams. The parallel phase velocity is consistent with the acoustic speed in the reference frame of the proton beam, strongly suggesting these waves are an ion acoustic mode. Their high phase velocity places them outside the ion beam population and rules out the ion two‐stream instability as their source. These low‐altitude structures may arise out of turbulence generated at the lower edge of the acceleration region. Their preferential observation at FAST altitudes may result from their high velocity combined with weak Landau damping that is restricted to the tenuous hot plasma sheet ions near the loss cone. Three different methods for estimating the velocity of these structures are examined. For the FAST antennae configuration it is found that signal delays between Langmuir probes operated in either current mode or voltage mode cannot provide valid estimates of the velocities. Instead, velocities are estimated by measuring the energy shift in the electron distribution within the negative potential well of the solitary wave. Using the measured wave potential and electric field, the scale size and velocity of the structures are calculated. Asymmetric solitary waves, sometime described as weak double layers, are also examined and shown to have no significant net potential. These new velocity estimates contrast sharply with reports based upon Viking observations and differ by about a factor of 2 from recent estimates deduced from Polar observations. These results are discussed in the context of previous estimates along with possible sources of error.
A kinetic exospheric model of the solar wind with a nonmonotonic potential energy for the protons
Authors: Lamy H. , Pierrard V. , Maksimovic M. , Lemaire J. F.
Published: May 2003

In solar wind kinetic exospheric models the exobase level is defined as the altitude where the mean free paths of the coronal protons and electrons become larger than the density scale height. For the region above this exobase, kinetic exospheric models have been developed assuming that the charged particles of the solar wind move collisionless in the gravitational, electric, and interplanetary magnetic fields, along trajectories determined by their energy and pitch angle. In these models the exobase was usually chosen at a radial distance of ∼5–10 Rs, above which the total potential energy of the protons is a monotonic decreasing function of the radial distance. Although these models were able to explain many characteristics of the solar wind, they failed to reproduce the bulk velocities observed in the fast solar wind, originating from the coronal holes, without postulating proton and electron temperatures at the exobase in clear disagreement with recent measurements obtained with the SOHO satellite. Moreover, since the number density is lower in the coronal holes than in the other regions of the solar atmosphere, the altitude of the exobase is located deeper in the corona at a radial distance ∼1.1–5 Rs. At these smaller radial distances, the gravitational force is larger than the electric force acting on the protons up to a radial distance rm. Therefore the total potential energy of the protons is first attractive (increasing with altitude) and then repulsive (decreasing with altitude). We describe a new exospheric model with a nonmonotonic total potential energy for the protons and show that lowering the altitude of the exobase below the maximum of the potential energy accelerates the solar wind protons to large velocities. Since the density is lower in coronal holes and the exobase is at lower altitude, the solar wind bulk velocities predicted by our new exospheric model are enhanced to values comparable to those observed in the fast solar wind.
Winds in the high‐latitude lower thermosphere: Dependence on the interplanetary magnetic field
Authors: Richmond A. D. , Lathuillère C. , Vennerstroem S.
Published: May 2003

Wind observations in the summertime lower thermosphere at high southern latitudes, measured by the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite, are statistically analyzed in magnetic coordinates and correlated with the interplanetary magnetic field (IMF) to determine influences of IMF‐dependent ionospheric convection on the winds. Effects are clearly detectable down to 105 km altitude. Above 125 km the wind patterns show considerable similarity with ionospheric convection patterns, and the speed of the averaged neutral wind in the polar cap often exceeds 300 m/s. The correlation between the IMF Bz component and the diurnal harmonic of the winds is generally best when the IMF is averaged over the preceding 1–4.5 hours. The magnetic‐zonal‐mean zonal wind below 120 km correlates best with the IMF By component when the latter is averaged over approximately the preceding 20 hours. The wind has a significantly stronger rotational than divergent component. Around and above 120 km a dusk‐side anticyclonic wind vortex is prominent, consistent with earlier findings. Around 140 km and higher the dusk‐side vortex intensifies for negative Bz, but around 120 km it is the dawn‐side cyclonic vortex that responds more strongly to Bz variations. The dependence of the wind on the IMF is nonlinear, especially with respect to IMF Bz. For positive Bz the difference winds are largely confined to the polar cap, while for negative Bz the difference winds extend to subauroral latitudes. A significant correlation between the diurnal Bz‐dependent neutral and convection velocity components exists above 108 km, when the convection velocity is suitably rotated in magnetic local time (MLT) with respect to the wind. The rotation that maximizes the correlation ranges from −1.5 hours at 130 km (wind preceding convection) to nearly +6 hours at 108 km (wind lagging convection). The rotated diurnal Bz‐dependent wind pattern projects onto the diurnal Bz‐dependent ionospheric convection pattern with about 60% the amplitude of the latter above 125 km, decreasing to about 17% at 108 km. On timescales of ∼20 hours, a By‐dependent magnetic‐zonal‐mean zonal wind generally exists, with maximum wind speeds at 80° magnetic latitude, typically 10 m/s at 105 km, increasing to about 60 m/s at 123 km and 80 m/s at 200 km. In the southern hemisphere the wind is cyclonic when the time‐averaged By is positive and anticyclonic when By is negative; the wind direction is expected to be opposite in the northern hemisphere.
Changes of Schumann resonance parameters during the solar proton event of 14 July 2000
Authors: Roldugin V. C. , Maltsev Y. P. , Vasiljev A. N. , Shvets A. V. , Nikolaenko A. P.
Published: May 2003

The ELF measurements in Russian observatories Lovozero (the Kola Peninsula) and Lekhta (Karelija) during the solar proton event of 14 July 2000 show the decrease of frequencies of the first and second Schumann resonance modes of ∼0.4 Hz and the increase of the first‐mode bandwidth from 0.8 to 1.5 Hz. The solar X‐ray burst, preceding the proton precipitation, is accompanied by the increase of the first‐mode frequency. Approximate formulas for frequencies and qualities of the two first modes are found for a two‐layer model of the Schumann resonator (SR). The changes of the SR parameters are a response of the Earth‐ionosphere resonator to large‐scale disturbances of the polar ionosphere.
On nonlinear response of minor species with a layered structure to gravity waves
Authors: Huang Tai‐Yin , Hickey Michael , Tuan Tai‐Fu
Published: May 2003

We investigate the temporal and spatial variations of the local and integrated response of minor species and OH emission to a small‐scale gravity wave. A Chapman‐like function is used to model the unperturbed profiles of minor species like ozone, hydrogen, and OH emission. Because the gravity waves that we simulate do not violate the nonacceleration conditions, the waves will not cause a secular variation in the minor species concentrations. We therefore use the Krylov‐Bogoliubov‐Mitropolsky averaging method to remove the higher‐order secular terms in our perturbation expansion. A vertical drift velocity, second order in nature, is required to remove the secular terms. The equivalence of this vertical drift velocity to the Eulerian drift is demonstrated. Using the perturbation method to treat the response of minor species to a small‐scale gravity wave, we compute the first‐ and second‐order perturbation terms and find that the second‐order terms will also be important for narrow minor species profiles having large gradients (or small‐scale heights).
Radial dependence of intermittency in the fast polar solar wind magnetic field using Ulysses
Authors: Pagel C. , Balogh A.
Published: May 2003

The Ulysses spacecraft, in a polar orbit of the Sun, spent almost 3 years sampling the pure polar coronal fast wind at solar minimum between 1994 and 1996. Turbulence in coronal fast wind, unaffected by stream shears with slow solar wind, evolves more slowly than fast wind turbulence in the ecliptic and is a more homogenous system. However, it is still a highly intermittent and turbulent medium. We use the Castaing probability distribution to study the intermittent properties of the fast wind magnetic field components at different time lags within the inertial range and for the first time present results on how intermittency in the magnetic field components depends on radial distance from the Sun. We find that for all distances, the transverse magnetic field components are significantly more non‐Gaussian than the radial component. We also find that non‐Gaussian behavior at a given time lag increases with distance but that this can be explained by the corresponding extension of the inertial range to lower frequencies so that the non‐Gaussianity is constant at the integral scale, agreeing with theoretical predictions. We also find that the rate at which non‐Gaussian deviations propagate through the cascade decreases slightly with distance.
Interchange instability in the presence of the field‐aligned current: Application to the auroral arc formation
Authors: Golovchanskaya I. V. , Maltsev Y. P.
Published: May 2003

We have revisited the ideal MHD interchange instability for the case of the nonaligned gradients of the plasma pressure p and unit magnetic flux tube volume V, i.e., the background field‐aligned current (FAC) included. Several possibilities of a directional difference between ∇p and ∇V in the Earth's plasma sheet are considered. The growth of an interchange perturbation leading to the pressure‐gradient‐driven FAC striation is illustrated through numerical simulation in a simple model of the nightside region 1 FAC by Karty et al. [1984] and Yang et al. [1994]. It is shown that the FAC is being striated by the interchange process on a timescale from minutes to tens of minutes. The application to the quiet time discrete arc formation is discussed.
Hubble Space Telescope observations of sulfur ions in the Io plasma torus: New constraints on the plasma distribution
Authors: Herbert Floyd , Schneider Nicholas M. , Hendrix Amanda R. , Bagenal Fran
Published: May 2003

During the Io plasma torus crossings by the Galileo spacecraft in late 1999 and early 2000 we observed the dawn ansa of the torus using the Hubble Space Telescope (HST). Here we analyze 74 FUV HST/Space Telescope Imaging Spectrograph observations (1150 ≤ λ ≤ 1720 Å) of the dawn ansa of the Io plasma torus made during Galileo's C23, I24, and I27 torus passages. These data include spectral scans at various System III longitudes plus two undispersed images of S++ 1720 Å emission. Because of simultaneous observations by other instruments, the full value of these data will be developed by joint analysis, but here we present our results for their independent value. The S++ images show that the “ribbon” feature seen at visible wavelengths in ground‐based observations is also clearly present in the FUV. The spectral scans were fitted using a simple plasma emission model integrated along the line of sight in order to invert the line‐of‐sight density superposition under the assumption of local azimuthal symmetry. Fits to the S+ 1256 Å, S++ 1199 Å, and S+++ 1410 Å emissions reveal that the S++ and S+++ densities peak ∼0.2 RJ farther from Jupiter than does the S+ density. Moreover, the S++ vertical distribution is 25% more extended than that of S+, indicating that the field‐aligned S++ kinetic temperature T∥ is ∼50% higher. It also appears that the S+ abundance dropped by at least a factor of 2 between the September–October 1999 and February 2000 epochs, while the other two sulfur ion abundances remained unchanged.
Climatology of neutral winds in the lower thermosphere over Millstone Hill (42.6°N) observed from ground and from space
Authors: Zhang Shengpan P. , Goncharenko Larisa P. , Salah Joseph E. , Roble Raymond G. , Shepherd Gordon G.
Published: May 2003

Neutral winds in the lower thermosphere at altitudes of 94–130 km were measured in 1987–2000 by the ground‐based incoherent scatter radar (ISR) at Millstone Hill (42.6°N, 288.5°E) and in 1992–1997 by the space‐based Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS). The data from the ISR and WINDII have been separately grouped into four seasons. The daytime wind measurements from both instruments are found to be in good agreement, with similar structures and similar magnitudes. Winds from both instruments show an annual variation with a winter minimum. Winds in spring and summer are generally about a factor of two larger than those in fall and winter. The semidiurnal winds are dominant, but diurnal tides, mainly the in situ thermospheric diurnal tide, are comparable. The tidal characteristics and the background winds (the diurnal mean) derived from WINDII data show that the amplitude of the semidiurnal tide is generally 10 ms−1 larger than that of the in situ thermospheric diurnal tide, and the zonal and meridional background winds are less than 40 ms−1 and 15 ms−1 in all seasons, respectively. The TIME‐GCM simulations of wind fields at 42.5°N for the March equinox and the December solstice are in reasonable agreement with the observations, but the comparisons for the June solstice are less satisfactory. More research needs to be done to understand the cause of disagreement at the June solstice and the September equinox and to properly model the tidal effects in the lower thermosphere during those periods.
Quasi‐perpendicular shocks: Length scale of the cross‐shock potential, shock reformation, and implication for shock surfing
Authors: Scholer Manfred , Shinohara Iku , Matsukiyo Shuichi
Published: May 2003

One‐dimensional (1‐D) full particle simulations of almost perpendicular supercritical collisionless shocks are presented. The ratio of electron plasma frequency ωpe to gyrofrequency Ωce, the ion to electron mass ratio, and the ion and electron β (β = plasma to magnetic field pressure) have been varied. Due to the accumulation of specularly reflected ions upstream of the shock, ramp shocks can reform on timescales of the gyroperiod in the ramp magnetic field. Self‐reformation is not a low ωpe/Ωce process but occurs also in (ωpe/Ωce)2 ≫ 1, low β simulations. Self‐reformation also occurs in low ion β runs with an ion to electron mass ratio mi/me = 1840. However, in the realistic mass ratio runs, an electromagnetic instability is excited in the foot of the shock, and the shock profile is considerably changed compared to lower mass ratio runs. Linear analysis based on three‐fluid theory (incident ions, reflected ions, and electrons) indicates that the instability is a modified two‐stream instability between the decelerated solar wind electrons and the solar wind ions on the whistler mode branch. In the reforming low ion β shocks, part of the potential drop occurs at times across the foot, and part of the potential (∼40%) occurs over a few (∼4) electron inertial lengths in the steepened up ramp. Self‐reformation is a low ion β process and disappears for a Mach 4.5 shock at/or above βi ≈ 0.4. It is argued that the ion thermal velocity has to be an order of magnitude smaller than the shock velocity in order for reformation to occur. Since according to these simulations only part of the potential drop occurs for relatively short times over a few electron inertial lengths, it is concluded that coherent shock surfing is not an efficient acceleration mechanism for pickup ions at the low β heliospheric termination shock.
Ionospheric data assimilation: Comparison of extracted parameters using full density profiles and key parameters
Authors: Zhang Shun‐Rong , Oliver William L. , Holt John M. , Fukao Shoichiro
Published: May 2003

This paper explores the relative accuracy of deriving solar EUV flux, exospheric temperature Tex, and meridional winds using complete altitude profiles of ionosphere electron density as compared to just using key parameters such as Nmax, hmax, and ionospheric electron content. Incoherent scatter radar observations at Millstone Hill and Shigaraki and an ionospheric model are used in this study. We can make the following points: (1) With either the profile or key parameter data, the EUV flux can be inferred with little ambiguity, given that background variables such as the meridional wind are known. (2) The southward wind and Tex can be simultaneously derived from either type of data. The results, in particular the [O]/[N2] results, are similar for the two types of data. The accuracy of the wind result relies much on the accuracy of hmax as well as the geomagnetic dip angle. (3) A few key ionospheric parameters are not always sufficient to give a unique definition of ionospheric conditions. Additional parameters or even a full profile data are sometimes needed. In general, however, the key parameter pair hmax − Nmax seems to be a suitable assimilation data source, at least for retrieving the local features of the Tex and meridional wind variation.
Effect of solar wind flow geometry on solar wind parameters at 1 AU for a low‐frequency cascade driven solar wind
Authors: Mountford C. J. , Habbal S. R. , Li X.
Published: May 2003

A two‐fluid, one‐dimensional solar wind model is used to explore how the change in the expansion of a given flow tube with heliocentric distance influences the properties of the solar wind. Using a single driving mechanism, namely ion cyclotron resonance through a nonlinear Kolmogorov cascade process, for proton heating, the Alfvén wave amplitude, electron density, and temperature are kept fixed at the coronal base, while the parameters defining the form and extent of the expansion are varied. The results of this parameter study show that it is the variation of the expansion with heliocentric distance rather than the overall faster than radial expansion of the flow tube that plays a critical role in determining the asymptotic properties of the solar wind. These results can be understood in light of the conclusions derived originally by Leer and Holzer [1980] with a simple ad hoc heating function, namely that the mass flux and flow speed at 1 AU depend on whether most of the energy is deposited in the subsonic or supersonic region of the flow. With a more complex heating function, such as the one considered in this study, it is shown how the location of the critical point changes as a result of the interplay between the location of resonance interaction and the expansion of the flow tube, which subsequently affects the energy distribution within that flow tube. In particular, it is shown that for sharp expansion both enhanced and inhibited flows can occur depending on the heliocentric distance of the expansion region.
Lunar tidal winds in the upper atmosphere over Jakarta
Authors: Stening R. J. , Tsuda T. , Nakamura T.
Published: May 2003

Winds have been measured at heights from 70 to 120 km by means of a Meteor Wind Radar near Jakarta (6.4S, 106.7E) from 1993 to 1997. These data have been analysed for the lunar semidiurnal tide using a least squares fitting method. Generally lunar tidal amplitudes in the winds are about 2 m s−1 though the northward wind amplitude reaches 6 m s−1 in southern summer. Variations with height and season are shown and compared with the Global Scale Wave Model (GSWM). Often the agreement with the model is good but there is a difference in phase of the eastward wind in southern winter. The GSWM predicts a more rapid increase in northward wind magnitude with height than is observed. Year‐to‐year changes are examined for different seasons and these reveal sometimes consistent results and sometimes a variation in phase from year to year. Often the phase relations between eastward and northward winds at Jakarta are more characteristic of a Northern Hemisphere tide, as predicted by classical tidal theory, particularly in southern spring and summer.
Ulysses observations of solar energetic particles from the 14 July 2000 event at high heliographic latitudes
Authors: Zhang Ming , McKibben R. B. , Lopate C. , Jokipii J. R. , Giacalone J. , Kallenrode M.‐B. , Rassoul H. K.
Published: May 2003

At the time of the solar flare on the Bastille Day of 2000, the Ulysses spacecraft was at 3.17 AU from the Sun, 62° south in heliographic latitude, and 116° in longitude east of the Earth. Solar wind and magnetic field measurements by Ulysses indicate that the coronal mass ejection (CME) of this event had a limited size in both latitude and longitude, although it was a halo CME as seen in the Solar and Heliospheric Observatory coronagraph images. The event produced large fluxes of energetic particles up to energies >100 MeV at both Ulysses and the Earth. Enhancements of energetic particles were immediately observed at the Earth, with their onset times consistent with the velocity dispersion due to the streaming of particles along magnetic field lines from the location of particle acceleration in the corona to the Earth. To the contrary, at Ulysses, the energetic particles from the solar event were not detected until 4–11 hours later, and the increases of particle intensity were much more gradual. The onset times of particles in different energy channels were not organized by particle speed; rather they depended on both particle rigidity and speed, indicating that the transport of particles to Ulysses at high latitudes had a diffusive nature. The first‐order anisotropy in the 40–90 MeV proton flux was significantly larger than what is expected from the Compton‐Getting effect for many hours after the onset. The direction of the first‐order anisotropy was not along the projection of local magnetic fields onto the scan plane of the detector and it was not affected by the polarity of the field either, indicating that the particles did not arrive at Ulysses through propagation along magnetic field lines and rather much of the anisotropy was produced by cross‐field diffusion in the presence of a cross‐field density gradient pointing toward the low latitude direction. All these observations are consistent with easy particles transport across the mean heliospheric magnetic fields. The apparent difficulty for the theory is that the observations require a cross‐field diffusion that is too fast to be explained by random walk of field lines due only to supergranulation.
Response of electron temperature to field‐aligned current carried by thermal electrons: A model
Authors: Zhang Bei‐Chen , Kamide Y. , Liu Rui‐Yuan
Published: May 2003

A one‐dimensional, high‐latitude ionospheric model is constructed for solving the continuity, momentum, and energy equations numerically. With the assumption that field‐aligned currents (FACs) be carried by thermal electrons in the polar ionosphere, the response of the ionospheric temperature to upward and downward FACs is studied. The results show that at altitudes higher than the F‐region maximum, the electron temperature, Te, increases in upward FACs and decreases in downward FACs, consistent with what satellite observations indicate. These changes are caused by expansion/contraction effects due to drift of electrons along magnetic lines of force. It is also shown that Te increases in upward FACs more efficiently than it decreases in downward FACs. In downward FACs the ion temperature, Ti, increases, although the response is not as sensitive as that of electrons. For downward FAC cases, Te is also found to increase for a larger magnitude of FACs. In the present calculations, Te decreases only in the magnitude range between 0 and 130 μAm−2 for downward FACs. The temperature increase in upward FAC regions with respect to the background temperature without FACs is not simply linearly related with the intensity of FACs.
Thunderstorms' atmospheric electric field effects in the intensity of cosmic ray muons and in neutron monitor data
Authors: Dorman L. I. , Dorman I. V. , Iucci N. , Parisi M. , Ne'eman Y. , Pustil'nik L. A. , Signoretti F. , Sternlieb A. , Villoresi G. , Zukerman I. G.
Published: May 2003

Theoretical and experimental results on the influence of thunderstorms' atmospheric electric field on cosmic ray secondary components are presented. On the basis of the approach proposed by Dorman [1987], theoretical models for a correct numerical evaluation of these effects on hard muon, soft muon, and neutron monitor component are developed. For hard and soft muons the validity of the models are checked by their comparison with experimental results of the Baksan muon detector. For the first time, the effect of thunderstorms' atmospheric electric field on cosmic rays is investigated by simultaneous measurements of one‐minute neutron monitor intensity and of atmospheric electric field at the Emilio Segre' Observatory on Mt. Hermon (Israel). A series of large thunderstorms during February 2000 is investigated; for each thunderstorm the maximum atmospheric electric field intensity was ranging from 10 to about 100 kV/m. Clear correlation between field intensity and neutron monitor intensity variations is presented for total intensity and for different detected multiplicity channels. This correlation is quantitatively in agreement with the developed model which takes into account the formation of short‐living meso‐atoms by the capture of slow negative muons in the lead of the monitor. The effect is relevant only for neutron events with detected multiplicity m = 1 and evident for multiplicity m = 2; the other multiplicity channels are not influenced by neutrons from meso‐atoms.
Spread‐F/sporadic E coupling at Chung‐Li, especially for postsunset periods of sunspot maximum years
Authors: Bowman G. G. , Mortimer I. K.
Published: May 2003

Using ionogram‐recorded data, the coupling between spread‐F occurrence and enhanced Es blanketing frequencies (fbEs) (indicative of foEs changes), has been investigated for an equatorial‐ionospheric‐anomaly crest station, Chung‐Li, for the postsunset periods of sunspot‐maximum years. Apart from the evidence for coupling from inspections of the f‐plots alone, for the years analyzed (1981, 1982, 1988, and 1989), statistical results of more significance were found from superposed‐epoch analyses. Isolated spread‐F events of duration of 2 hours or less which started at or before 2200 LT were used as controls on time series. These series were made up of comparisons between 15‐min f‐plot fbEs values and interpolated monthly median fbEs values. Evidence for coupling was found for the analyses which involved 97 controls. The evidence in the f‐plots for three events involving Es patches, which investigators have examined in some detail using the VHF radar at Chung‐Li, suggests that these events are also coupled to isolated spread‐F occurrence.
Delivery of cold, dense plasma sheet material into the near‐Earth region
Authors: Thomsen M. F. , Borovsky J. E. , Skoug R. M. , Smith C. W.
Published: May 2003

Observations from several satellites in geosynchronous orbit, combined with solar wind observations from the ACE spacecraft, demonstrate that cold, dense plasma sheet material can be delivered into the inner magnetosphere under two scenarios: (1) a sudden southward IMF preceded by an extended interval of northward field and (2) a very strong magnetospheric compression caused by a large increase in the solar wind dynamic pressure, also under conditions of northward IMF. Examination of 30 “superdense” plasma sheet events observed during 15 months in 2001 and 2002 shows that these two mechanisms are probably responsible for the occurrence of most superdense events, if “northward IMF” is generalized to “northward or very weak IMF Bz.” Previous work has shown that a superdense plasma sheet contributes to a strong ring current and geomagnetic disturbance if it is accompanied by strong magnetospheric convection (which requires southward IMF). Hence this study suggests that the formation of a strong geomagnetic storm should be favored by an extended interval of strong southward IMF that is preceded by an earlier interval of northward field or very weak Bz, during which the plasma sheet fills with cool, dense plasma.
A source of auroral electrons and the magnetospheric substorm current systems
Authors: Akasofu Syun‐Ichi
Published: May 2003

Auroral electrons are field‐aligned current carriers and thus changes of their flux are related to the magnetospheric substorm current systems. Therefore a study of processes associated with substorms can provide insight into a source of auroral electrons. This paper attempts to identify basic processes associated with substorm onset by synthesizing/integrating the three well‐established observations at substorm onset: a sudden brightening of an auroral arc, a sudden growth of the westward electrojet, and dipolarization. It is demonstrated that when these observations are combined, they reveal basic processes associated with substorms and a source of auroral electrons. First of all, it is shown that, together with the southward‐directed electric field Es, the associated Pedersen current Ip and its equatorial closing component Ir, the pair of the upward/downward field‐aligned current sheets in the night sector forms a meridional current system (Boström's Case 2 current system); since Es · Ip > 0, driving electric field Ed must be present to drive Ir on the equatorial plane, so that Ed · Ir < 0. Thus, an enhancement of Boström's Case 2 current system can increase the upward current sheet, explaining a sudden brightening of auroral arcs. Also, it is noted that Ed drives the westward electrojet, which is mainly the Hall current. It is argued that the westward electrojet cannot be the diverted cross‐tail current, and the dipolarization must be caused by the “return current” from the westward electrojet; the return current must be located in the equatorial plane of the magnetosphere. In fact, the diversion of the cross‐tail currents explains little about the aurora. Further, the dipolarization sometimes results in an overdipolarization and even a negative dipolarization, which cannot be explained by the diversion of the cross‐tail current. Therefore there must be an internal dynamo process that enhances Boström's current system on the equatorial plane. This explains many aspects of auroral arcs during substorms and provides a source of auroral electrons. The enhanced internal dynamo process during the expansive phase must be closely related to a short‐lived unloading process. The disruption (not the diversion) of the cross‐tail current can release magnetic energy stored during the growth phase for the unloading process. A chain of processes associated with the substorm warrants further study.
Plasma instabilities in meteor trails: 2‐D simulation studies
Authors: Oppenheim Meers M. , Dyrud Lars P. , vom Endt Axel F.
Published: May 2003

Field‐aligned plasma density irregularities detected as nonspecular echoes by radars with large aperture‐power products indicate the presence of plasma turbulence within meteor trails. This paper presents two‐dimensional simulations of meteor trail instabilities and compares these results with theory and observations. In particular, this paper describes techniques for simulating trail turbulence and then discusses two sample cases using realistic plasma density gradients, masses, and atmospheric conditions appropriate for a 102‐km altitude. In the first case, the trail lies along the geomagnetic field, B. In the second, it lies perpendicular to B and is subject to a small external electric field pointing parallel to it. These cases show the spontaneous development of instabilities leading to turbulence and field‐aligned irregularities. These irregularities can create nonspecular echoes with broad spectral lines and small Doppler shifts similar to those observed by radars with large aperture‐power products. The simulations also show turbulence‐enhanced cross‐field diffusion rates. Finally, the paper describes simulations of trails containing multiple ion species and shows how turbulent mixing greatly reduces species fractionation.
Field‐aligned electron acceleration in Alfvén waves
Authors: Wright Andrew N. , Hood Alan W.
Published: May 2003

The field‐aligned current of standing Alfvén waves is mainly carried by electrons travelling parallel to the magnetic field. During the upward current phase, magnetospheric electrons travel downward to the ionosphere. In large‐amplitude Alfvén waves, where current densities reach a few μAm−2 above the ionosphere, the electrons achieve energies of the order of keV. This problem has been addressed recently in terms of two‐fluid theory. The present paper builds on these studies by employing a distribution function formulation. When the electron motion is dominated by the parallel velocity component, we find the B/n curve is central to interpreting the solution: B/n has a peak (i.e., d(B/n)/dℓ = 0, where ℓ is path length along the field line) below which ionospheric electrons are trapped. Above the peak we find the parallel electric field is balanced by the convective plasma acceleration, as suggested by Rönnmark [1999] and has a value of the order of mV/m for ∼1 RE above the B/n peak. The maximum E∥ occurs where d2(B2/n2)/dℓ2 = 0 and is located a couple of density scale heights beyond the B/n peak.