bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear evolution of equatorial spread F: 1. On the role of plasma instabilities and spatial resonance associated with gravity wave seeding
Authors: Huang Chao‐Song , Kelley Michael C.
Published: February 2004

We have used a computer simulation to study properties of large‐scale equatorial F region irregularities produced by gravity waves by separating such different processes as the spatial resonance effect and the Rayleigh‐Taylor instability. Our purpose is to show their relative importance in the production of strong ionization perturbations. When a gravity wave propagates perpendicular to the magnetic field, it generates a polarization electric field. If there is no amplification by the Rayleigh‐Taylor instability, the amplitude of the electric field remains almost constant for a long time. If spatial resonance occurs, after one wave period strong ionization perturbations with a relative density amplitude of 58% are produced. However, the associated electric field is limited and the gravity wave‐induced perturbation does not grow into topside plasma bubbles when the Rayleigh‐Taylor instability is absent. A zonally propagating gravity wave can, however, initiate the Rayleigh‐Taylor instability in the bottomside F region. The initiation does not depend on the spatial resonance mechanism. After initiation, the Rayleigh‐Taylor instability amplifies nonlinearly the perturbations induced by the seed gravity wave, and results in topside plasma bubbles. Spatial resonance can speed up the formation of bubbles. It is thus concluded that the Rayleigh‐Taylor instability mechanism is the most important for production and rise of plasma bubbles and that seeding by gravity waves can occur even without the spatial resonance effect.
The whistler‐mode bow wave of an asteroid
Authors: Gurnett Donald A.
Published: February 2004

The Galileo spacecraft has flown by two asteroids, Gaspra in 1990 and Ida in 1993. In both cases the magnetometer detected magnetic field perturbations that are believed to be produced by an interaction of the asteroid with the solar wind. Kivelson et al. have proposed that these perturbations are caused by whistler‐mode waves excited by the solar wind flow around the asteroid. This paper presents an analysis of whistler‐mode waves generated by the interaction of a small object with the solar wind. Three cases are considered, with the solar wind magnetic field (1) parallel, (2) perpendicular, and (3) at an arbitrary angle to the velocity vector. Using the Cerenkov condition and the quasi‐longitudinal approximation for the whistler mode, the angular limits of the wave pattern are determined. For the parallel case the waves are confined to the upstream region, and for the perpendicular case the waves are confined to two wedge‐shaped regions extending downstream from the magnetic field line through the object. For intermediate magnetic field orientations the accessibility region maintains the wedge‐shaped configuration, with the leading edge of the wedge roughly following the direction of the magnetic field. The outer boundary of the accessibility region is a caustic surface, along which large field amplitudes are expected, similar to the bow wave of a ship. In all cases the interaction involves a characteristic wavelength, λc = ƒcc²/(ƒp²V), that is comparable to the size of an asteroid.
Thermospheric response to morningside diffuse aurora: High‐resolution three‐dimensional simulations
Authors: Sun Zi‐Ping , Turco R. P. , Walterscheid R. L. , Venkateswaran S. V. , Jones P. W.
Published: February 2004

We have carried out numerical simulations of the thermospheric response to the morningside intense diffuse aurora, using a three‐dimensional (3‐D), nonhydrostatic, composition‐dependent, regional‐scale, high‐resolution numerical model. A diffuse aurora is represented in the model by its equivalent 3‐D features of ion drag and joule heating. The diffuse aurora propagates westward within the model grid, which is fixed in the terrestrial frame. Our numerical results show that the neutral atmospheric response to the diffuse aurora is much weaker in a 3‐D simulation than in a two‐dimensional (2‐D) simulation. For example, the predicted zonal flow is weaker by a factor of roughly 2. The fidelity of the 3‐D results is supported by a recent study (Brinkman et al., 1995) in which zonal winds predicted from a 2‐D model were shown to be larger than observations by a factor of 2. Our numerical results also show that the inertial gravity waves that are generated in the 3‐D flow control the energy dissipation and reduce the dynamical response in the source region. These waves propagate in both the north‐south and east‐west directions. However, the longitudinal propagation rate is relatively small compared to the latitudinal propagation rate. The simulated westward drift of the diffuse aurora creates a distinct displacement of the propagating disturbances, which further reduces the dynamical response in the source region. The study described here underscores the importance of 3‐D high‐resolution treatments of upper atmospheric physics to analyze and interpret dynamical, compositional, and electrodynamic observations in this region.
Equatorial plasma convection from global simulations of the Earth's magnetosphere
Authors: Mobarry C. M. , Fedder J. A. , Lyon J. G.
Published: February 2004

The convection of plasma in the closed field region of the Earth's magnetosphere is an important consequence and a diagnostic of the coupling of solar wind momentum and energy into the magnetosphere. In the outer regions the nature of the flow can help to distinguish between different coupling processes; for example, between (1) a global reconnection process between closed geomagnetic field and the interplanetary magnetic field (IMF), or (2) a local diffusive‐viscous interaction. In the inner magnetosphere, the convection is responsible for both energization and redistribution of energetic and thermal plasmas. This paper presents the first results for convection of plasma in the closed field magnetosphere derived from a self‐consistent, three‐dimensional, magnetohydrodynamic simulation model of the Earth's magnetosphere. The results are shown as electric potential patterns in the equatorial plane. The equipotential contours, which are interpreted as plasma streamlines, demonstrate that line reconnection between closed geomagnetic field and the IMF is the dominant coupling process for southward IMF in the parameter range studied. Results are presented from three simulations, that is, southward, northward, and duskward IMF. For the southward IMF case, the quasi‐steady cross polar cap electric potential is essentially equal to the bow reconnection voltage. The results show that reconnection on the open/closed magnetic field boundary is important for northward IMF also. In this case, the reconnection process takes place at the two polar cusps, and is an inherently unsteady process even though it leads to a quasi‐steady convection potential in the ionosphere. The implications of these results, for the nature of the solar wind magnetosphere coupling, are discussed. Moreover, the convection results are closely related to the polar Birkeland current systems, which are the self‐consistent consequences of the coupling process and in turn create the ionospheric convection flow.
Evidence for an Io plasma torus influence on high‐latitude Jovian radio emission
Authors: Kaiser M. L. , Desch M. D. , Brown M. E.
Published: February 2004

We report the discovery with the Ulysses unified radio and plasma wave (URAP) instrument of features in the Jovian hectometer (HOM) wavelength radio emission spectrum which recur with a period about 2–4% longer than the Jovian System III rotation period. We conclude that the auroral HOM emissions are periodically blocked from “view” by regions in the torus of higher than average density and that these regions rotate more slowly than System III and persist for considerable intervals of time. We have reexamined the Voyager planetary radio astronomy (PRA) data taken during the flybys in 1979 and have found similar features in the HOM spectrum. Contemporaneous observations by Brown (1994) show an [SII] emission line enhancement in the Io plasma torus that rotates more slowly than System III by the same amount as the HOM feature.
Generation of the magnetic field at the magnetopauses of the rapidly rotating planets
Authors: Belenkaya E. S.
Published: February 2004

This paper is a study of a possible mechanism of magnetic field amplification in the thin transition layer just outside the magnetopause of a quickly spinning planet. Here we present analysis of a new magnetospheric magnetic field source arising due to the rapid rotation. When the ionosphere and magnetopause conductivities of the rapidly roating planet are sufficiently high (but less than the field‐aligned conductivity) the circulation exists at the magnetopause in the open field line bundles. The toroidal magnetic field is generated there from the poloidal ambient magnetosheath magnetic field by differential rotation. The requirement of the α effect existence is satisfied due to the deviation from axial symmetry about the planet's rotation axis of the initial magnetosheath velocity and magnetic fields. The increasing poloidal magnetic field can be regenerated from the toroidal one by the α effect. This mechanism may act as a hydromagnetic dynamo. The generated magnetic field penetrates into the magnetosphere and adds to the magnetospheric magnetic field.
Simulation of magnetic cloud propagation in the inner heliosphere in two dimensions: 2. A loop parallel to the ecliptic plane and the role of helicity
Authors: Vandas M. , Fischer S. , Dryer M. , Smith Z. , Detman T.
Published: February 2004

This paper continues studies of the cylindrical magnetic clouds' propagation in the interplanetary medium. In our first paper devoted to this topic (Vandas et al., 1995) we dealt with the cloud with the axis perpendicular to the ecliptic plane and derived time dependencies of its velocity, field magnitude, and temperature as well as its shape for different initial conditions. Here, analogously, we present simulations for the cloud with the axis parallel to the ecliptic plane and show that the propagation of these clouds practically does not depend on the inclination of their axes to the ecliptic plane. We made a new conclusion concerning the helicity of the magnetic field inside the cloud. Because of the magnetic interaction with the background field, the cloud is shifted to the side where it meets with the external interplanetary magnetic field (IMF) polarity that is opposite to that within the cloud. The net effect of the time dependent Lorentz, inertial, and pressure gradient forces probably causes the complementary deformation of the whole cloud.
Onset of collisionless magnetic reconnection in sheared field reversals
Authors: Kuznetsova M. M. , Büchner J. , Zelenyi L. M.
Published: February 2004

An extended analysis of the kinetic stability theory of a current sheet in a curved and sheared magnetic field reversal with respect to a tearing mode perturbation is presented. The coupling between quasi‐parallel and quasi‐perpendicular mode components is newly taken into account. We obtain that an unstable branch of tearing mode instability with a growth rate less than the electron bounce frequency can be excited. The dependencies of the growth rate of this instability on the shear (Bs) and normal (Bn) field components as far as on the wave length of the mode are illustrated. The resulting tearing mode destabilization takes place favorably for an inclination of the magnetic field near the quasi‐neutral plane at about 45° with respect to the neutral plane. Our theory reveals that for proper geometry of magnetic field lines in field reversal region a collisionless ion tearing mode can be triggered and magnetic flux ropes are formed. The growth of the unstable tearing mode can be accompanied by the generation of magnetic field aligned electric fields and ion acceleration.
Formation of thin current sheets during plasma sheet convection
Authors: Pritchett P. L. , Coroniti F. V.
Published: February 2004

Fully kinetic simulations in two spatial dimensions are used to demonstrate that the attempt to establish convection in the near‐Earth magnetotail can drive the cusp or hinge region to form a thin current sheet which is embedded within a thicker plasma sheet. The simulation model incorporates a realistic near‐Earth magnetic field configuration including dipolelike and taillike regions and allows plasma to precipitate at the earthward boundary. The imposition of an electrostatic convection field leads to a slow earthward convection of plasma and a tailward transfer of magnetic flux, with the inductive electric field opposing and almost canceling the external field. This flux transfer leads to the formation of a deep minimum in the equatorial Bz field in the near‐Earth plasma sheet. As a consequence of the difference in dynamics between electrons and ions, the central plasma sheet in the cusp region becomes negatively charged, which leads to the formation of a strong polarization electric field (Ez). The growth of the polarization field causes the electrons to carry the dominant current in the thin current sheet, whose characteristic half thickness is of the order of the local ion gyroradius and whose current density is an order of magnitude larger than for the usual tail current sheet. The negative space charge electric field associated with the thin current sheet and the related field‐aligned currents are consistent with the polarization Harang equatorward electric field.
In situ generation of intense parallel electric fields in the lower ionosphere
Authors: St.‐Maurice J.‐P. , Kofman W. , James D.
Published: February 2004

A number of independent and diverse ionospheric electric field, current, particle, and plasma wave observations can be interpreted in terms of intense parallel current density bursts that would require parallel electric fields of the order of up to a few millivolts per meter to exist at ionospheric altitudes as low as 140 km at times. While such observations are rare, they indicate that the upper limits on the values of parallel current densities and electric fields could be extremely high. In this paper, we theoretically explore the conditions under which the ionosphere could possibly sustain very large parallel fields and current densities. Since this is a first attempt at this question, we look for the simplest possible requirements; that is, we study the possibility for the presence of a quasi‐static electric field on timescales greater than the electron collision time and the Alfven transit time. Our initial focus is on two‐dimensional linear situations, which should apply if the parallel fields are less than about 0.1 mV/m. Nonlinear effects are invoked in the event that stronger parallel fields would be implied by the observations or when very strong horizontal gradients in conductivities are required. We link the problem to a magnetospheric source through the introduction of intense fluxes of medium to strong energy (hundreds of electron volts) electrons with and without an abrupt latitudinal cutoff. When considering the presence of an abrupt latitudinal change in the precipitating energy spectrum, we also require the presence of a “background” perpendicular electric field which is assumed to be uniform on latitudinal scales 1 km or greater. One important conclusion is that for the generation of parallel fields intense enough to generate ion‐acoustic waves along the geomagnetic field as well as for the generation of shears of the order of meters per second per meter, 100‐m horizontal gradient scales are required. Another result is that for larger horizontal gradient scales, a substantial fraction of the returning currents carried by thermal electrons will flow along the same magnetic field lines that precipitating electrons are coming from; this leads to a substantial reduction in the net parallel current densities when compared to the currents borne by each type of carriers.
Long‐term enhancements in solar wind speed
Authors: Gazis P. R.
Published: February 2004

Long‐term enhancements in solar wind speed over timescales of the order of a year appear to be a common feature throughout the heliosphere over heliocentric distances that range from less than 0.72 AU to greater than 60 AU. The origin of these events remains to be determined, but they are almost certainly associated with long‐term variations at the solar wind source, in contrast with smaller‐scale structures such as corotating interaction regions (CIRs), merged interaction regions (MIRs), and global merged interaction regions (GMIRs), which are dynamical in origin. We present a survey of the long‐term speed enhancements observed at the Pioneer Venus Orbiter (PVO), IMP 8, Voyager 2, and Pioneer 10 between 1974 and 1994 and compare this with published reports of smaller‐scale events such as MIRs. We examine several of these long‐term speed enhancements in detail to identify and characterize aspects of their structure, and then describe how that structure evolves with heliocentric distance. Finally, we discuss some of the implications of these events.
Regions of abnormally low proton temperature in the solar wind (1965–1991) and their association with ejecta
Authors: Richardson I. G. , Cane H. V.
Published: February 2004

Occurrences of solar wind plasma with abnormally low proton temperatures (Tp) have long been associated with the interplanetary manifestations (which we term “ejecta”) of coronal mass ejections (CMEs). We survey the National Space Science Data Center Omni solar wind database for 1965–1991, and data from the Helios 1 and 2 spacecraft for more limited periods, to identify plasma in which Tp is less than the temperature expected (Tex) from the well‐established correlation between the solar wind speed and Tp for normal solar wind expansion. The occurrence rate of low‐temperature plasma (specifically with Tp/Tex ≤ 0.5) is shown to be correlated with solar activity levels. Individual low‐temperature regions have durations from 1 to ∼80 hours with a mean of ∼10 hours. Around one third are encounters with the heliospheric plasma sheet (HPS). These events are observed most frequently during the increasing phase of solar activity when the HPS lies closer to the ecliptic. The abnormally low temperatures may be intrinsic to the HPS or may provide support for the proposal that the coronal streamer belt underlying the HPS is a frequent source of ejecta. The remaining events have an occurrence rate which shows a particularly clear correlation with solar activity levels and with the CME rate at the Sun (when CME observations are available), consistent with an association with ejecta. These events also show greater than chance associations with other ejecta signatures. We suggest that the Omni plasma data can provide information on the rate of ejecta passing the Earth, and hence give an indication of the CME rate, for a period commencing before spacecraft coronagraph CME observations became available in the early 1970s. Our findings suggest that Tp depressions may provide a more comprehensive indication of the presence of ejecta than other ejecta signatures, such as bidirectional solar wind electron heat fluxes and energetic ion flows, which alone do not identify all ejecta.
Finite lifetimes of ideal poloidal Alfvén waves
Authors: Mann Ian R. , Wright Andrew N.
Published: February 2004

Standing second harmonic poloidal Alfvén waves can be excited by drift‐bounce resonance with energetic particle populations in the Earth's magnetosphere. Using a cold, ideal, MHD model, we study the temporal evolution of the resulting poloidal Alfvén waves. Imposing an azimuthal dependence of exp (iλy) in a “box” model of the magnetosphere, we describe poloidal waves, using a large azimuthal wavenumber λ. In homogeneous media, poloidally polarized waves simply oscillate in time. However, if these waves are excited in a nonuniform medium, we find that their polarization rotates from poloidal to toroidal in time. This polarization change is driven by magnetic field gradients which develop as the poloidal wave fields phase mix in time. Asymptotically, all the initial poloidal wave energy is ultimately transferred to a toroidal polarization. On the basis of this phase mixing we define a poloidal lifetime as the time taken for the poloidal and toroidal amplitudes to become equal. We find that the lifetime is given by τ = λ/ (dωA/dx). The irreversible change from a poloidal to a toroidal polarization is in agreement with early studies [Radoski, 1974] but contrary to a recent report [Ding et al., 1995]. Our results support the findings of Radoski. Consequently, poloidal Alfvén waves in the Earth's magnetosphere may have a finite poloidally polarized lifetime, after which they become dominantly toroidal, determined by their azimuthal wavenumber and the local natural Alfvén frequency gradient.
Magnetic storm response of lower thermosphere density
Authors: Forbes J. M. , Gonzalez R. , Marcos F. A. , Revelle D. , Parish H.
Published: February 2004

Measurements of atmospheric density near 200 km from the Satellite Electrostatic Triaxial Accelerometer (SETA) experiment are used to delineate the temporal, seasonal‐latitudinal, and day/night dependences of the response to magnetic storm‐related perturbations in high‐latitude energetic inputs. Five periods of geomagnetic activity are analyzed and yield consistent results which can be interpreted within the framework of recent thermosphere‐ionosphere simulations by Fuller‐Rowell et al. [1995]: In response to a change in magnetic activity level from quiet (Kp ≈ 1–2) to active (Kp ≈ 4–7) conditions, an increase in daytime (1030 LT) density of order 50–70% occurs between 60 and 80° geographic latitude in the summer hemisphere, with about half the maximum response in the winter hemisphere. This difference is mainly due to the difference in ionization/conductivity levels (and hence joule heating rates) between the hemispheres. On the dayside, penetration of the disturbance at about the 50% intensity level is realized at the equator, whereas in the winter hemisphere equatorward penetration is much weaker. These effects are connected with the prevailing solar‐driven circulation; the net summer‐to‐winter meridional flow facilitates equatorward advection of the disturbance bulge in the summer hemisphere but hinders advection in the winter hemisphere. In both hemispheres the daytime component of the solar‐driven diurnal circulation tends to oppose equatorward penetration to the same degree. However, on the nightside (2230 LT) penetration at nearly the 100% level of both summer and winter disturbance bulges are realized to within 20° of the geographic equator. This behavior is associated with the equatorward advection in both hemispheres consistent with the nighttime component of the solar‐driven circulation. Comparisons with the MSISE90 model [Hedin, 1991] show it to capture the salient features of the daytime behavior but exhibits little day/night asymmetry, in contrast to the experimental results.
Ulysses observations of microstreams in the solar wind from coronal holes
Authors: Neugebauer M. , Goldstein B. E. , McComas D. J. , Suess S. T. , Balogh A.
Published: February 2004

During its south polar passage in 1994, the Ulysses spacecraft continuously sampled the properties of the solar wind emanating from the south polar coronal hole. At latitudes poleward of ∼−60°, the solar wind speed had an average value of 764 km/s and a range of 700–833 km/s. The principal variations in the vector velocity were associated with either outward propagating Alfvén waves with periods up to about half a day or with longer‐period high‐ or low‐speed “microstreams.” The microstreams had an amplitude of ∼40 km/s and a mean half width of 0.4 days, and they recurred on timescales of 2–3 days (power spectral peaks at 1.9 and 3.3 days). The density and temperature profiles showed the expected evidence of pileup and compression on the leading edges of high‐speed microstreams, although no forward or reverse shocks were observed. The particle fluxes were nearly the same for both the fast and slow microstreams. The higher‐speed microstreams had higher proton temperatures and higher alpha‐particle abundances than did the slower microstreams. The absence of latitude variations in the thickness or the recurrence rate suggests that the microstreams are caused by temporal rather than long lived (> a few days) spatial variations in the source region at the Sun. Some speculations are made about the possible cause of the microstreams.
Model‐observation comparison study of multiple polar cap arcs
Authors: Zhu L. , Valladares C. E. , Sojka J. J. , Schunk R. W. , Crain D. J.
Published: February 2004

A quantitative model‐observation comparison of multiple polar cap arcs has been conducted by using a time‐dependent theoretical model of polar cap arcs. In particular, the electrodynamical features of multiple polar cap arcs with various spacings are simulated and the results are compared with the images obtained from the All‐Sky Intensified Photometer at Qaanaaq. The results show that the observed and simulated arcs are quite similar, both spatially and temporally. The results support the theory proposed by Zhu et al. [1993a, 1994b] that the structure of polar cap arcs is mainly determined by the magnetosphere‐ionosphere (M‐I) coupling processes and that the spacing of multiple polar cap arcs is closely related to the hardness of the primary magnetospheric precipitation. It is found that for the multiple polar cap arcs with both narrow and wide spacings, the associated field‐aligned currents are mainly closed by Pedersen currents. It is also found that a hard precipitation can lead to a highly structured secondary arc because of the nonlinear M‐I coupling processes.
Reinterpretation of the 6300‐Å airglow enhancements observed in ionosphere heating experiments based on analysis of Platteville, Colorado, data
Authors: Mantas George P. , Carlson Herbert C.
Published: February 2004

Airglow enhancement observations have been considered as supporting evidence of electron acceleration in ionosphere heating experiments by high‐power HF waves. Here we analyze some of the 6300‐Å airglow data from the Platteville, Colorado, heating experiments of 1970, employing new electron impact excitation rates for the O(1D) state and empirical, but in accord with experimental and theoretical constraints, plasma heating rates and show that these airglow enhancements should be attributed to excitation by thermal electrons. An important aspect of the present analysis is the excellent agreement of the observed and the calculated airglow enhancements over several complete transmitter on/off cycles of several minutes duration and an increasing airglow trend of 1 hour duration. The fact that the OI red line may be thermally excited and the scarcity of observations of simultaneous OI red and green line enhancements imply that electron acceleration, even to a few eV, may require very special experimental and ionospheric conditions that are not very often realized.
Near‐Earth magnetotail shape and size as determined from the magnetopause flaring angle
Authors: Petrinec S. M. , Russell C. T.
Published: February 2004

Knowledge of the average size and shape of the near‐Earth magnetotail is an essential element for our understanding of the magnetospheric response to the influence of the solar wind. An empirical model of the near‐Earth magnetotail has been developed, which depends upon distance downtail (xGSM), the solar wind momentum flux (ρv2sw), and the zGSM component of the interplanetary magnetic field (IMF Bz). This model has been created by using the pressure balance relation to calculate a set of flare angles for the nightside magnetopause in the region −22 RE ≤ xGSM ≤ −10 RE. Observations of the magnetic field in the lobe by ISEE 2 and simultaneous observations of the magnetic field and plasma properties of the solar wind by IMP 8 were used to determine the internal and external pressure components, respectively. Examination of calculated flare angle values reveal a dependence upon downtail distance and ρv2sw. Normalized to the median downtail distance and dynamic pressure, the angle of flare of the magnetopause is found to increase linearly with decreasing Bz when the IMF is southward, but there is little variation when the IMF is northward. The empirical function derived for the flaring angle of the magnetotail is used to determine a relation for the radius of the tail. Comparisons with previous empirical models and results are also performed. In addition, values of magnetic flux within the magnetotail are calculated for times of sudden impulse events.
A model for meteoric magnesium in the ionosphere
Authors: McNeil William J. , Lai Shu T. , Murad Edmond
Published: February 2004

We present a model of neutral and ionized magnesium in the equatorial ionosphere at altitudes of 50–350 km. The model was developed in an attempt to understand recent spectroscopic measurements made from the space shuttle. The model incorporates ablation of cosmic dust particles, ion and neutral chemistry, diffusion, and vertical drift due to the equatorial electric field. We find that in the absence of electric field drift, there is little or no variation in Mg and Mg+ density profiles above 120 km. With the electric field included, the high‐altitude behavior of the Mg+ ion changes dramatically. At dawn, almost all the ion density is confined to the 90‐ to 125‐km altitude range, while at dusk there is significant ion density as high as 350 km. A radiance model is developed to allow for direct comparison with measurements. The model incorporates shielding of incident solar UV by ozone and variable local time of observation and tangential height of instrument line of sight. Comparisons between model and measurement are encouraging.
Interaction of interplanetary shocks and rotational discontinuities with the Earth's bow shock
Authors: Yan M. , Lee L. C.
Published: February 2004

The magnetohydrodynamic (MHD) aspects of the interaction between the Earth's bow shock and interplanetary shocks or rotational discontinuities are systematically studied in this paper by MHD simulations. The interplanetary shocks under consideration include forward fast shocks, reverse fast shocks, forward slow shocks, and reverse slow shocks. As an incident forward shock transmits through the bow shock, a fast shock, a slow expansion wave, a slow shock, and a contact discontinuity are generated downstream of the bow shock. At the same time, the bow shock is modified and moves earthward. If the incident shock is a reverse shock, the generated fast shock becomes a fast expansion wave, and the bow shock moves away from the Earth. The generated fast shock or fast expansion wave carries most of the total pressure variation imposed by the incident shock. The contact discontinuity also carries a significant part of the pressure variation. The slow expansion wave and the slow shock are both generated with a small variation in plasma density and pressure but a large variation in magnetic field. When the solar wind Alfven Mach number is small, the density variations associated with the slow shock or slow expansion wave can be significant. Through the interaction between an incident rotational discontinuity and the bow shock, a plateau in the plasma density and thermal pressure is formed. The magnetic pressure is depressed accordingly and anticorrelated with the thermal pressure. If the incident rotational discontinuity propagates toward the Earth (Sun) in the solar wind frame, the leading (trailing) edge of the plateau consists of an intermediate shock or a time‐dependent intermediate shock and a slow shock, while the trailing (leading) edge is mainly a slow shock. The generated structure with enhanced plasma density and thermal pressure and a depressed magnetic pressure agrees very well with the observed slow‐mode structure in the magnetosheath. Strong dynamic pressure variations associated with this structure may impinge on the magnetopause as a strong pressure pulse.
The underlying Parker spiral structure in the Ulysses magnetic field observations, 1990–1994
Authors: Forsyth R. J. , Balogh A. , Smith E. J. , Erdös G. , McComas D. J.
Published: February 2004

The fundamental aim of the Ulysses space mission is to extend our understanding of the heliosphere into three dimensions. By April 1994, the spacecraft had reached a heliographic latitude of 60°S. Hourly averages of the Ulysses heliospheric magnetic field observations have been analyzed to determine to what extent the underlying field direction within 60° of the heliographic equator can be described by the Parker spiral model. At all latitudes from near the ecliptic southward to 60°S, the most probable value of the azimuthal orientation of the field lines remained in approximate agreement with the Parker model. Once Ulysses passed southward of the maximum latitude of the heliospheric current sheet at about 30°S it became possible to study the distribution of the azimuth angle in purely inward polarity southern hemisphere fields without the confusion between the northern and southern hemisphere sectors. This distribution is revealed to be highly asymmetric with a greater probability of observing field lines with an azimuth angle less tightly wound than the most probable angle. Comparison with near‐ecliptic data showed a similar asymmetry in inward polarity field sectors while in outward polarity sectors there was an asymmetry in the opposite sense. We suggest that the asymmetry in the >30°S azimuth angle distribution arises due to the presence of long‐period radially outward propagating Alfvén waves in the solar wind flows originating from the southern polar coronal hole. In addition, from studying the meridional (north‐south) orientation of the field lines we find that at south heliographic latitudes within about 25° of the equator there is a tendency in the Ulysses data set for the field lines to be on average deflected equatorward of their expected direction, most likely due to flow deflections associated with the interaction of high‐ and low‐speed solar wind streams.
Generation of dynamic pressure pulses downstream of the bow shock by variations in the interplanetary magnetic field orientation
Authors: Lin Y. , Lee L. C. , Yan M.
Published: February 2004

One‐dimensional resistive MHD and hybrid simulations are carried out to study the manner by which variations of the interplanetary magnetic field (IMF) direction generate dynamic pressure pulses in the magnetosheath. The reaction of the magnetosheath to the temporal IMF variation is modeled as the interaction between the bow shock (BS) and an interplanetary rotational discontinuity (RD), an Alfven wave pulse (AW), or an Alfven wave train. The resistive MHD simulation indicates that the arrival of an RD produces two time‐dependent intermediate shocks (TDISs) and two slow shocks downstream of the bow shock, which propagate through the magnetosheath toward the Earth's magnetopause. An enhancement of plasma density is present throughout the TDISs and slow shocks. A plasma dynamic pressure pulse is formed in this region. In the hybrid simulation, the two TDISs are replaced by rotational discontinuities. For a bow shock with a shock normal angle θBn > 45°, the pulse in the dynamic pressure ρV2 causes the total pressure (P + B2/2μ0 + ρV2) in the magnetosheath to increases by about 0–100% of the background value. The strength of the pressure pulse increases with the field rotation angle across the incident rotational discontinuity, while it decreases with the Mach number or upstream plasma beta of the bow shock. The pressure pulse propagates toward the magnetopause with nearly a constant amplitude. On the other hand, the BS/AW interaction leads to the generation of Alfven waves downstream of the bow shock, and large‐amplitude dynamic pressure pulses are generated in the downstream Alfven wave. Pressure pulses impinging on the magnetopause may produce magnetic impulse events (MIEs) observed in the high‐latitude ionosphere.
Consequences of particle conservation along a flux surface for magnetotail tearing
Authors: Quest K. B. , Karimabadi H. , Brittnacher M.
Published: February 2004

The energy principle for magnetotail tearing is reexamined using conservation of electron particle number along a flux surface as a means of calculating the volume‐integrated perturbed number density 〈 n1 〉, where n1 is the perturbed electron number density and the angle brackets denote integration along a field line. It is shown that if the electron response is magnetohydrodynamic, then 〈 n1 〉 can be calculated as a function of the perturbation vector potential component A1y (assuming magnetotail coordinates) independent of the potential components A1x and A1z and independent of the scalar potential ϕ. This result holds as long as the equilibrium and the tearing perturbations are two‐dimensional, independent of the y coordinate. In the case of a parabolic field model, the resulting 〈 n1 〉 exactly matches the results obtained previously by Lembége and Pellat [1982], who used the kinetic drift equation to calculate the electron response. Thus the compressional stabilization of the tearing mode is a direct consequence of, and can be completely calculated from, the conservation of electron particle number along the field line. Further, it is shown that 〈 n1 〉 is independent of By, the guide component of the magnetic field, so the inclusion of a guide field does not alter the tearing stabilization condition.
Mapping a bubble at dip equator and anomaly with oblique ionospheric soundings of range spread F
Authors: Whalen J. A.
Published: February 2004

Multiple ionospheric sounders are employed in the first attempt to detect and map remotely an equatorial bubble near the dip equator and simultaneously near the Appleton anomaly crest. The sounders also provide latitudinal profiles of electron density through ±30° dip latitude (DIPLAT). Conditions are solar maximum and low Kp. Using oblique range spread F (RSF) echoes from the bubble boundaries, four sounders located near the dip equator detect a single isolated bubble at ranges as great as 1020 km and track it for 2.25 hours at 5‐min intervals as it drifts eastward through 15° geomagnetic longitude (MLONG). Maximum velocity is 185 m/s, and width is 375 km. Near the northern anomaly crest, a sounder at 20.3° DIPLAT observes the western boundary for a duration of 1 hour concurrently with and magnetically conjugate to the equatorial observations; The boundary here has nearly the same drift velocity and MLONG as at the equator, implying that the boundary location and velocity are nearly independent of altitude above the dip equator. Subsequently, in the south a sounder at 19.9° DIPLAT observes the bubble to have a velocity approximately half that seen by the other stations and a MLONG that is at least 3° west of the others, This difference may be related to the north‐south asymmetry of the anomaly crest, which is at the location of the northern sounder but equatorward of the southern sounder, although the two are at the same DIPLAT. The onset of the bubble is recorded in the first appearance of RSF at 1915 local mean time; 20 min later the bubble boundary appears and begins accelerating eastward, the movement occurring within about 5 min of electric field reversal as inferred from F layer vertical motion.
A large survey of electron acceleration events
Authors: Newell Patrick T. , Lyons Kevin M. , Meng Ching‐I.
Published: February 2004

Precipitation data from all Defense Meteorological Satellite Program (DMSP) satellites over a 9‐year interval were surveyed for electron acceleration events. Selected events evinced a sharp drop above the spectral peak; therefore only instances where the accelerating potential is 3–4 or more times larger than the source thermal energy were studied. In all, more than 1.5 × 108 individual electron spectra were examined for signs of such substantial field‐aligned acceleration. Several controversies can be resolved by this survey; for example, the “midday gap” is real, with noon easily the least likely local time for observing electron acceleration. The debated “14 MLT (magnetic local time) hot spot” is also real, although it is centered about 15 MLT and is distinct from the rest of the oval only for northward interplanetary magnetic field (IMF) conditions. A weaker “warm” spot exists in the 6–9 MLT region. Potential drops of ∼3 keV and above are statistically the province of the nightside, as are the most intense energy fluxes (>1 erg cm−2 s−1). The latitudinal scale size of acceleration events fits surprisingly well to an exponential distribution with a characteristic width ( = 28–35 km) that varies only slightly with MLT and IMF conditions. If  represents the ionospheric footprint of the driving magnetospheric processes, the best candidate explanation is velocity shears in the main plasma sheet. However, the fact that an exponential distribution of latitudinal widths exists under a variety of conditions suggests an alternative. If there exist large‐scale regions embedded within the auroral oval wherein each spectrum has a random probability P of experiencing electron acceleration independent of neighboring spectra, then the occurrence distributions are explained, and  actually indicates the value of P (namely, ∼0.85).
Improved electrodynamic modeling of an omega band and analysis of its current system
Authors: Amm Olaf
Published: February 2004

We improve a previous electrodynamic model of an omega band by applying a nonuniform spatial distribution of the Hall to Pedersen conductance ratio instead of using a constant value. The analysis of the current system due to our model reveals that after separating the background westward electrojet off, the omega band currents mainly consist of Hall currents circulating around concentrated upward field‐aligned currents in the auroral tongue. However, the upward field‐aligned currents exhibit a nearly circular configuration around the center of the tongue and almost vanish directly in the center. Equally strong downward field‐aligned currents are missing. Additionally, at the western flank of the tongue we notice an about northwest–southeast aligned Cowling channel. Its currents are mainly responsible for the spike‐like D peak in magnetograms and for the general asymmetry in D and Z longitudinal profiles on the ground. An additional enhancement of the Hall conductance in this channel further improves the agreement between calculated and measured ground magnetic disturbance profiles.
Cold dense ion flows with multiple components observed in the distant tail lobe by Geotail
Authors: Hirahara M. , Mukai T. , Terasawa T. , Machida S. , Saito Y. , Yamamoto T. , Kokubun S.
Published: February 2004

The Geotail spacecraft frequently observes cold dense ion flows (CDIFs) streaming tailward in the magnetotail lobe region (XGSM∼−10 to −210 RE). The density is often quite high (∼1/cm3), comparable to or larger than that in the plasma sheet, and the tailward speed is from 50 to 500 km/s. The CDIFs sometimes contain multiple (two or three) energy‐per‐charge components. The lowest‐ and highest‐energy components are identified as H+ and O+, respectively. The intermediate‐energy component is identified as He+. Therefore a part of the CDIFs in the distant lobes is believed to be of ionospheric origin. A possible source candidate is the upward flowing ions (UFIs) outgoing from the dayside auroral region or polar wind escaping from the polar cap and accelerated along the magnetic field direction. The high proton to O+ density ratio in the CDIFs suggests that the major component of the CDIFs originates from the solar wind and that the cold ion beams of ionospheric origin merge with the solar wind component penetrating into the magnetosphere at the mantle or the flank of the tail lobe. In some observations, the H+ ions continuously stream tailward both in the tail lobe and in the magnetosheath, and they couple with the solar wind ions through the magnetopause. Although the O+ and He+ ion flows cannot be recognized so clearly in the magnetosheath, weak O+ fluxes are sometimes present in the magnetosheath near the magnetopause. This implies that plasma of ionospheric origin may leak from the tail to the magnetosheath. The multicomponent CDIF events make it possible to discuss the sources and transport processes of the tail lobe plasma, and their behavior in or near the magnetopause provides important clues to the interactions between the lobe and the magnetosheath.
Ionospheric climatology and weather in the Australian ‐ Japanese sector during the SUNDIAL/ATLAS 1 campaign
Authors: Wilkinson P. J. , Richards P. , Igarashi K. , Szuszczewicz E. P.
Published: February 2004

The SUNDIAL/ATLAS 1 campaign took place during a period of high solar flux and moderate magnetic activity. For most days during the campaign, the ionosphere over Australia and Japan was undisturbed. The International Reference Ionosphere (IRI), used with a suitable equivalent sunspot number, provided a good climatological description of the Australian and Japanese observations. The measured hmF2 was employed in the Field Line Interhemispheric Plasma (FLIP) model to determine equivalent neutral winds and O+/H+ fluxes and accurately reproduced the measured hmF2. With these winds, the daytime nmF2 was well reproduced in most cases, although the measured nmF2 tended to peak before noon while the model nmF2 peaked 2–3 hours later at some locations. At night, the model nmF2 was typically lower than the measurements. The difference between the modeled and measured nighttime nmF2 worsened as the latitude decreased, suggesting that the problem is not principally due to inadequate plasmaspheric fluxes. While changes in nmF2 due to wind changes are modeled closely in some cases, generally the smaller day‐to‐day changes in nmF2 were not matched well by either the IRI or FLIP. The equivalent winds calculated from hmF2 in FLIP were similar in amplitude to the winds from the HWM90 model, but there were large differences in the phases. The two sets of winds show a similar decrease in amplitude with decreasing latitude. There appears to be a large longitudinal variation in the wind behavior in the Australian sector. While the winds in eastern Australia show similar diurnal variations with daytime poleward winds and nighttime equatorward winds, the west coast winds were poleward most of the time. The differences in poleward winds are reflected in the much smaller changes in hmF2 in the west. The calculated O+ fluxes to and from the plasmasphere are generally larger than the H+ fluxes for these solar maximum conditions. They show structure related to the neutral winds and hmF2. The O+ flux variations are similar at most stations with upward fluxes during the day and very large downward O+ fluxes at sunset followed by a rapid decay to small fluxes for most of the night. The calculated H+ flux behavior is more complex with the west coast station (Mundaring) showing a classical, strong diurnal variation, upward during the day and downward at night. On the other hand, the calculated H+ fluxes are upward most of the time at Hobart on the east coast.
Cosmic rays in the atmosphere: North – south asymmetry
Authors: Stozhkov Y. I. , Bazilevskaya G. A. , Pokrevsky P. E. , Svirzhevsky N. S. , Martin I. M. , Turtelli A.
Published: February 2004

An analysis of the north–south asymmetry in the cosmic ray flux was made with data taken in the north and south polar atmosphere from 1963 through 1991. For altitudes corresponding to column densities between 100 and 800 g/cm2 the N–S asymmetry is near zero during the negative phase of the solar magnetic cycle (1963–1969 and 1982–1989) and is positive during the positive one (1972–1979). The maximum amplitude of N–S asymmetry (up to 3%) occurs at an atmospheric depth of X= 500–600 g/cm2 and could be caused by primaries with rigidities of more than 30–50 GV. At higher altitudes, X < 25 g/cm2 during almost all periods an excess of particles in the northern polar hemisphere was observed. This excess is due to the presence of reentrant albedo electrons on closed magnetic field lines.