bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence of a source of energetic ions at Saturn
Authors: Paranicas C. , Cheng A. F. , Mauk B. H. , Keath E. P. , Krimigis S. M.
Published: February 2004

We calculate phase space densities, in terms of adiabatic invariants of charged particle motion, using the low‐energy charged particle (LECP) data from the Voyager 2 encounter with Saturn. Ion data are corrected for electron contamination. For all energies considered here, phase space density at fixed first and second adiabatic invariant, ƒ(L), increases with increasing equatorial pitch angle. Evidence exists for a source of low‐energy ions (∼0.07‐4.2 MeV) and energetic protons (54‐152 MeV) close to Saturn. The source of >50 MeV protons is likely to be cosmic ray albedo neutron decay (CRAND), although we know of no source mechanism for the low‐energy ions. Low‐energy ion phase space densities calculated here have characteristics of solutions of a steady‐state radial diffusion equation with sources outside the region of interest and near Saturn and losses. Losses are attributed to satellites, rings, and Saturn's atmosphere and observed changes in ƒ with respect to the adiabatic invariants of motion are consistent with model loss rates in most cases. For energetic protons mirroring near the magnetic equator, phase space densities are in good qualitative agreement with other analyses of Pioneer 11 and Voyager 2 Cosmic Ray System (CRS) data indicating a likely CRAND source close to Saturn and absorption by Mimas. Voyager data phase space densities for CRAND protons that mirror off the magnetic equator are presented for the first time. An estimate of the upper limit on average G ring particulate size of ∼1 cm is described.
Modeling the production and the imaging of energetic neutral atoms from Titan's exosphere
Authors: Amsif A. , Dandouras J. , Roelof E. C.
Published: February 2004

Energetic neutral atoms (ENAs) are formed when singly charged magnetospheric ions undergo charge exchange with exospheric neutral atoms. The energy of the incident ions is almost entirely transferred to the charge exchange produced ENAs, which then propagate along nearly rectilinear ballistic trajectories. Thus the ENAs can be used like “photons” in order to form an image of the energetic ion distribution. The Cassini spacecraft will be equipped with INCA, a magnetospheric imaging energetic neutral atom camera [Mitchell et al., 1993]. In this context, we undertook the study of the energetic neutral atom production resulting from the interaction of Titan's exosphere with Saturn's magnetosphere. This necessitated the development of a model for the altitude density profile and composition of both the inner and the outer exosphere of Titan. We used the Chamberlain model for the inner exosphere (altitude < 6000 km) that included the five major species: H, H2, N, N2, and CH4. In the lower part of the exosphere, where our model overlaps the high altitude end of existing lower atmosphere models, the results are compatible. The density and composition profiles obtained from our model were then used to calculate the ENA production, considering a proton spectrum measured by Voyager in the Saturnian magnetosphere as the parent ion population. It is shown how the exospheric species contribute in a different way, as a function of the altitude, to the ENA flux spectra. In the outer exosphere (altitude > 6000 km), molecular hydrogen is the dominant species, and we utilized a Chamberlain model to estimate the ENA intensities several Titan radii from the moon. The Titan exosphere is nearly as bright in ENA as the inner magnetosphere of Saturn, making it possible to image the exosphere out to at least 5 Titan radii, and even to resolve it when the spacecraft is several Saturn radii distant from the moon. Our results also show that the magnetospheric imaging technique via energetic neutral atom detection is a powerful diagnostic tool for the interaction of Saturn's magnetosphere with Titan and for the remote sensing of the composition profile of Titan's exosphere.
Geotail observations of magnetosheath flow near the magnetopause, using wind as a solar wind monitor
Authors: Petrinec S. M. , Mukai T. , Nishida A. , Yamamoto T. , Nakamura T. K. , Kokubun S.
Published: February 2004

We have examined several Geotail passes through the magnetosheath region downstream of the Earth, when the solar wind conditions as observed by the Wind spacecraft were steady. We have found that near the magnetopause the bulk speed of the magnetosheath plasma is strongly dependent upon the local direction of the magnetic field with respect to the local velocity vector. In one set of observations, the observed magnetosheath speed is slowest when the magnetosheath magnetic field and velocity vectors are aligned (either parallel or antiparallel), and the speed is larger when the magnetic field is oriented perpendicular to the magnetosheath velocity. However, the speed of the magnetosheath plasma is not observed to exceed that of the solar wind plasma. The enhanced speed for perpendicular flows is due to acceleration of the plasma by means of magnetic field line tension, as the magnetosheath plasma is dragged around the magnetosphere. The maximum enhancement in magnetosheath plasma speed can be quantitatively estimated by a consideration of the intermediate mode of the magnetohydrodynamic (MHD) relations. A simple model for the velocity enhancement is derived, and we find extremely good agreement between the Geotail magnetosheath observations and this model. The second set of observations are similar to the first, except that when the magnetosheath velocity and magnetic field vectors are parallel to one another, the magnetosheath flow is higher than when the vectors are antiparallel or perpendicular to each other, and is even larger than the solar wind speed. This behavior cannot be explained by nonresistive ideal MHD theory, and other explanations are investigated.
A comparative study of global ionospheric responses to intense magnetic storm conditions
Authors: Szuszczewicz E. P. , Lester M. , Wilkinson P. , Blanchard P. , Abdu M. , Hanbaba R. , Igarashi K. , Pulinets S. , Reddy B. M.
Published: February 2004

We report on a study of three intense ionospheric storms that occurred in September 1989. Using Dst as a reference for storm onset and subsequent main and recovery phases, we analyze the observed worldwide responses of F region heights hmF2 and densities NmF2 as a function of universal and local times, latitudinal domains, and storm onset‐times; and we compare the characteristics of all three storms. The following points are among the major findings: (1) The negative phase storm was the dominant characteristic, with the greatest intensity occurring in the regions which were in the nighttime hemisphere during the main phase; (2) at middle and low latitudes negative phase characteristics were observed first in the nighttime hemisphere and then corotated with the Earth into the dayside; (3) the most intense negative response occurred in the recovery phase; (4) observations of the negative phase characteristics supported thermospheric upwelling, increased mean molecular mass, and an associated enhancement in dissociative recombination as the principal cause‐effect chain; but the observations suggest greater ion‐neutral chemistry effects than accounted for in current models; (5) hmF2 was observed to respond quickly to the storm onset (pointing to the importance of electric fields) with enhanced values in all latitudinal and local time domains; (6) positive storm characteristics were among the issues most difficult to reconcile with current descriptions of cause‐effect relationships; and (7) the analysis of all storm phases and comparisons with several modeling efforts show that future advances in understanding require a more accurate accounting of the influences of magnetospherically‐imposed and dynamo‐driven electric fields, plasmaspheric fluxes, and vibrationally excited N2.
Charge exchange near Mars: The solar wind absorption and energetic neutral atom production
Authors: Kallio E. , Luhmann J. G. , Barabash S.
Published: February 2004

Charge exchange between solar wind protons and neutral atmospheric atoms is expected to affect the solar wind interaction with Mars, but its influences and significance have only been touched upon in previous work. Here several features associated with the charge exchange process between the solar wind protons and Martian neutral upper atmospheres are described. The analysis is based on an empirical proton model derived from Phobos 2 observations interacting with the Martian atomic (H) and molecular (H2) hydrogen, and oxygen (O) upper atmospheres representing solar minimum and solar maximum conditions. The region where the largest fraction of solar wind protons is lost by the charge exchange process is found to be a thin layer above the surface of Mars on the dayside resulting from charge exchange with the thermal oxygen. In general, the magnetosheath and “magnetosphere” (where the observed plasma takes on a different character in the Phobos 2 data) produce two distinguishable regions where the loss rate of solar wind protons is highest. Increasing solar activity increases the loss rate in the magnetosheath but decreases it in the magnetosphere. No significant increase of the absorption of the solar wind was found near the “magnetopause” suggesting that the decrease of the solar wind protons observed by Phobos 2 are not due to the charge exchange process. In addition to a reduction in the solar wind density, the charge exchange reaction results in energetic neutral atom (ENA) production. This paper considers some of the detailed properties expected for the ENA population at Mars. The ENA differential fluxes were found to be typically 106–107 cm−2 s−1 keV−1 in the energy range 0.01–1 keV. During solar minimum, the ENA production rate and ENA integral fluxes were found to be highest in the magnetosheath. At solar maximum the ENA production rate is highest in the magnetosphere, and ENA integral fluxes in the dayside magnetosphere appear to become comparable to the fluxes in the magnetosheath if the proton temperature in the magnetosphere is low. It is found that 1–3% of the original solar wind proton flux converts into ENAs before the bow shock. The ENAs produced upstream are undeflected and so may precipitate into the Martian upper atmosphere, depositing an energy flux of up to 3 × 109 eV cm−2 s−1 derived from the solar wind. These results both suggest the possible benefits of observing ENA fluxes around Mars and suggest the necessary parameters for detector design.
Propagation of particles injected from interplanetary shocks: A black box model and its consequences for acceleration theory and data interpretation
Authors: Kallenrode May‐Britt , Wibberenz Gerd
Published: February 2004

Energetic protons in the hundreds of keV to the tens of MeV range frequently are observed in connection with traveling interplanetary shocks. Occasionally, the particle energies can extend up to about 100 MeV. The intensity time profiles at the observer's site are a superposition of the continuous, spatially and temporally variable acceleration at the shock and the subsequent interplanetary propagation. To gain a better understanding of both processes and to derive their relevant parameters, we extend a numerical solution of the model of focused transport to accommodate the shock as a moving source. No assumptions about the acceleration mechanism are made; the shock is treated as a black box. In this paper we introduce the model, discuss its validity, and present model results which have implications for acceleration theory and data interpretation. The main results concerning acceleration and propagation are as follows: (1) In the limit of strong scattering and low particle speeds our model converges toward diffusive shock acceleration. (2) For weak scattering or fast particles, spatial diffusion is an insufficient approximation for particle transport; in this case, the physical consequence is a fast escape from the shock, and the formal consequence is that the standard description of diffusive shock acceleration is insufficient. (3) Because of this fast escape, even a turbulent foreshock region, while it is perfectly capable of keeping 100 keV protons confined to the shock, would allow 10 MeV protons to stream away easily. Important results for data interpretation are as follows: (1) A quasi‐exponential intensity increase upstream of the shock is not necessarily indicative of diffusive shock acceleration. (2) The intensity at the time of shock passage is a crude measure for the local acceleration efficiency as long as it stays constant or continues to rise.
Currents and magnetic field structures in the dayside solar wind interaction with Venus
Authors: Law Colin C. , Cloutier Paul A.
Published: February 2004

Analysis of high‐resolution magnetic field data from the Pioneer Venus Orbiter spacecraft has shown that magnetic field rotations occur in association with the dayside ionopause of Venus. There is no evidence of a precursor to this rotation within the magnetosheath above the ionopause. This finding implies that the current system responsible for the field rotation is completely contained within the ionosphere, and therefore a return current must exist on the dayside below the ionopause. Further examination of the magnetic field data has revealed a secondary rotation at lower altitudes that we attribute to this return current. A large‐scale dayside current system capable of reproducing these observations is described.
Medium energy particle precipitation influences on the mesosphere and lower thermosphere
Authors: Codrescu M. V. , Fuller‐Rowell T. J. , Roble R. G. , Evans D. S.
Published: February 2004

Data accumulated for more than a solar cycle by the TIROS/NOAA polar orbiting satellites are used to extract patterns of particle precipitation in several medium energy bands for varying geomagnetic conditions. While the atmospheric effects of auroral particles measured by these satellites (300 eV‐20 keV) have been widely studied, the effects of medium energy (30 keV‐2.5 MeV) particles have received less attention; in this paper we present average particle precipitation patterns for medium energy protons and electrons as a function of geomagnetic activity. A magnetic coordinate system is used to bin data at high latitudes to produce patterns that are then used as additional inputs to the National Center for Atmospheric Research thermosphere ionosphere mesosphere electrodynamic general circulation model (TIME‐GCM) to study the effects of medium energy particles on the mesosphere and lower thermosphere. Model results obtained with these additional inputs are used to assess the significance of medium energy particles on the composition and dynamics of the region. Zonally averaged model results from the run that includes the medium energy particles show significant increases in ne, HOX, NO2, and NO and a 27 percent decrease in O3 between 70 and 80 km in the polar regions when compared to a similar run without the particles. The temperature and wind differences are small.
Dayside auroral activity as a possible precursor of substorm onsets: A survey using Polar ultraviolet imagery
Authors: Liou K. , Newell P. T. , Meng C.‐I. , Lui A. T. Y. , Brittnacher M. , Parks G.
Published: February 2004

We have analyzed the dayside auroral oval, with particular emphasis on the postnoon aurorally active region, prior to the onset of isolated substorms using images acquired from the ultraviolet imager (UVI) on board the Polar spacecraft. The UVI data set used for this investigation covers a time of approximately 3 months, from March 30 to July 13 in 1996. It is found that dayside auroral “hot spots” were active in 70 out of 95 surveyed substorm events at least 15 min before the onset, while 25 cases did not involve the dayside bright spots at all. Of the 70 cases with dayside activity during substorms, 51 cases of the dayside events were found to be spatially confined and showed little discernible changes prior to an active substorm onset, while only six cases were found to be in association with apparent eastward propagation through the dusk sector to the nightside prior to a substorm onset. This statistical result indicates that most of the postnoon bright spots are spatially confined in longitude and that only a few candidate cases are possibly associated with substorms triggering. It also suggests that dayside auroral bright spots are distinct features from the nightside auroral substorms. These results suggest that the physical processes responsible for the dayside auroral bright spots are different from those responsible for the nightside substorm activity.
Depletion of oxygen in aurora: Evidence for a local mechanism
Authors: Christensen A. B. , Hecht J. H. , Walterscheid R. L. , Larsen M. F. , Sharp W. E.
Published: February 2004

A ground‐based and rocket investigation of the response of the neutral atmosphere to E region auroral heating has been carried out at Poker Flat, Alaska. The temporal evolution of the atomic oxygen to molecular nitrogen ratio (O/N2) in the lower thermosphere has been monitored using the optical emissions from the aurora as a diagnostic. Comparisons between the changes in the O/N2 ratio and the auroral Joule and particle heating have shown several examples of close similarity between the durations of the heating events and the depletions. Using the thermospheric winds measured during the rocket flights and the temporal structure of the depletions, the upper limit on the horizontal scale size of the depletions has been estimated at 200–400 km. Moreover, in situ rocket measurements of atomic oxygen showed significant differences at points separated horizontally by approximately 220 km. It is also concluded from the near coincidence between the depletion events and the Joule heating events that the dynamical mechanism(s) that drive the depletions were not far distant from the observing site, that is, local processes are sometimes dominant f during periods of moderate auroral activity. We suggest that the observation of a strong wind shear in the 100‐ to 120‐km altitude region [Larsen et al., 1997] could be responsible for turbulence that contributes to the changes in minor constituent composition.
Change in ion distribution function while crossing the space shuttle wake
Authors: Sorensen J. E. , Stone N. H. , Wright K. H.
Published: February 2004

During the space shuttle/Spacelab 2 mission (STS‐51F) the Plasma Diagnostic Package (PDP) was placed in free flight in the vicinity of the Orbiter and provided the first opportunity to measure the midwake and far‐wake regions of an orbiting spacecraft. During this period the PDP crossed the wake 4 times at downstream distances ranging from 40 m to 240 m. During each crossing a definite change in the ion‐retarding potential curves was measured when the PDP was in the shuttle wake compared to the ambient ionosphere. These measurements were obtained by both the differential ion flux probe and the retarding potential analyzer instruments on the PDP. The temperature of the ions, derived from fitting the data to a Maxwellian distribution, are consistent with an enhancement during each wake crossing of as much as a factor of 2.5 (from about 0.08 eV to 0.2 eV). Such studies of plasma wakes were never before performed.
Drift‐shell splitting of energetic ions injected at pseudo‐substorm onsets
Authors: Takahashi Kazue , Anderson Brian J. , Ohtani Shin‐ichi , Reeves Geoff D. , Takahashi Shun , Sarris Theodore E. , Mursula Kalevi
Published: February 2004

One feature of a magnetospheric substorm is the injection of energetic particles into closed drift orbits. Injections are routinely observed by geosynchronous satellites and have been used to identify the occurrence of substorms and the local time of particle energization. In this study we examine pitch angle distributions of ion injections in the 50‐ to 300‐keV energy range observed by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE/CCE) satellite, hereinafter CCE. In a dipole field, all pitch angles follow the same drift shell, but the day‐night asymmetry of the magnetospheric magnetic field introduces a pitch angle dependence in particle drift orbits, so that particles with different pitch angles disperse radially as they drift. The effect is known as drift‐shell splitting. For satellite observations near noon at a fixed geocentric distance, the guiding center orbits of ions detected at small pitch angles intersect the midnight meridian at larger geocentric distances than do ions with near‐90° pitch angles. The ion pitch angle distributions detected on the dayside therefore provide information about the radial distance of the nightside acceleration region. We apply this principle to study ion injection events observed on September 17–18, 1984, in association with pseudo‐substorm onsets. CCE was at 13 hours local time near its apogee (8.8 RE) and observed a series of ion flux enhancements. Energy dispersion of the timing of the flux increases assures that they are due to injections on the nightside. The flux increases were observed only at pitch angles from 0° to 60°. We calculate drift orbits of protons using the Tsyganenko 89c magnetic field model and find that the drift orbits for 60° pitch angle protons observed at the satellite pass through midnight at 9 RE, well outside of geostationary orbit, indicating that the ion injections occurred tailward of 9 RE. Energetic ion data from geostationary satellites for the same time interval show no evidence of injections at 6.6 RE, consistent with the calculated inner boundary of the injections. Model calculations are presented demonstrating that dispersive injections observed near noon outside geosynchronous orbit provide the greatest sensitivity to drift‐shell splitting effects and are therefore most suitable for remote sensing the radial boundaries of substorm injections.
Radial intensity gradients of galactic cosmic rays (1972–1995) in the heliosphere
Authors: Fujii Z. , McDonald Frank B.
Published: February 2004

The radial intensity gradients near the ecliptic plane for galactic cosmic ray He (180–450 MeV/nucleon), H (130–220 MeV), and the integral counting rate of ions >70 MeV are studied using the Pioneer 10/11, Voyager 2, and IMP 6, 7, and 8 observing network over the 1972.25–1996.0 time period at heliocentric distances extending to 64 AU. These gradients display complex temporal and spatial changes that are dominated by the 11‐year solar activity cycle but with the 22‐year heliomagnetic cycle also having an important role. The data are well ordered out to ∼55 AU using a radial dependence of the intensity gradient gr of the form G0rα, where r is the heliocentric distance in astronomical units (AU). With this representation it becomes possible to estimate gr as a function of r at a given time or as a function of time at a given radial distance. Over successive solar minima these values of gr decrease rapidly with increasing heliocentric distance, and for He at 50 AU are 3 times larger in 1987 than in 1977, in general agreement with the expected effects of particle gradient and curvature drifts in the large‐scale interplanetary magnetic field. Near solar maximum the gradients increase significantly over their solar minimum values, with a greatly reduced radial dependence for He, while for H, gr increases with increasing r. The radial gradients of the integral rate of ions >70 MeV show a much smaller temporal and spatial variation.
Ion kinetic effects in magnetic reconnection: Hybrid simulations
Authors: Lottermoser R.‐F. , Scholer M. , Matthews A. P.
Published: February 2004

Magnetic reconnection in a low beta plasma is studied by using a large‐scale two‐dimensional hybrid simulation code that treats the ions fully kinetically and the electrons as a massless fluid. Reconnection is realized by localizing an anomalous resistivity in the the center of a Harris‐type current sheet. The results are applicable to reconnection in the geomagnetic tail. Within a distance of about 200 ion inertial lengths from the neutral line the fast reconnection jet is characterized over most of the region by partial shell type ion distributions; i.e., the distribution in the central current sheet is not thermalized. Inward drifting cold lobe ions perform in the center of the reconnection wedge due to the small magnetic field curvature Speiser‐type orbits and are subsequently ejected again onto lobe field lines. This stadium is similar to the collisionless reconnection scenario described by Hill [1975]. The cross‐tail current of the thin current sheet is supported by the drifting ions. This current sheet warps and an instability develops, which leads further away from the neutral line to a filament ation of the cross tail current. These simulations suggested that after near‐Earth reconnection proceeds to lobe magnetic field lines a postplasmoid plasma sheet with a thin current sheet builds up, which is of a boundary layer type. The thin current sheet is instable. At about 200 ion inertial lengths away from the neutral line the instability has reached a nonlinear state and the cross‐tail current becomes patchy; the incoming and ejected cold lobe ions are isotropized in the ensuing magnetic field and thus constitute a hot plasma sheet distribution. This can then support the occurrence of slow mode shocks at the boundary of the reconnection layer with backstreaming ions upstream of the shocks.
Proton transport model in the ionosphere: 1. Multistream approach of the transport equations
Authors: Galand M. , Lilensten J. , Kofman W. , Sidje R. B.
Published: February 2004

The suprathermal particles, electrons and protons, coming from the magnetosphere and precipitating into the high‐latitude atmosphere are an energy source of the Earth's ionosphere. They interact with ambient thermal gas through inelastic and elastic collisions. The physical quantities perturbed by these precipitations, such as the heating rate, the electron production rate, or the emission intensities, can be provided in solving the kinetic stationary Boltzmann equation. This equation yields particle fluxes as a function of altitude, energy, and pitch angle. While this equation has been solved through different ways for the electron transport and fully tested, the proton transport is more complicated. Because of charge‐changing reactions, the latter is a set of two‐coupled transport equations that must be solved: one for protons and the other for H atoms. We present here a new approach that solves the multistream proton/hydrogen transport equations encompassing the collision angular redistributions and the magnetic mirroring effect. In order to validate our model we discuss the energy conservation and we compare with another model under the same inputs and with rocket observations. The influence of the angular redistributions is discussed in a forthcoming paper.
A model of the Earth's distant bow shock
Authors: Bennett L. , Kivelson M. G. , Khurana K. K. , Frank L. A. , Paterson W. R.
Published: February 2004

We present a new global model of the Earth's bow shock that is parametrized by the solar wind conditions. We begin with a conic section base model taken to be correct for average solar wind conditions. Then we apply modifications to the base model, based on physical arguments, to account for the changes in the size and shape of the bow shock caused by changes in the prevailing solar wind dynamic pressure, Alfvén and sonic Mach numbers, and interplanetary magnetic field orientation. We show that our model matches the location and timing of shock crossings observed, at large downtail distances, by the Galileo spacecraft during December 1990 and December 1992 and by the Pioneer 7 spacecraft during September 1966. Magnetic field and plasma moments in a shock normal coordinate system change across the model shock surface as required by conservation laws.
Response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field
Authors: Ohtani S. , Elphinstone R. D. , Troshichev O. A. , Yamauchi M. , Blomberg L. , Zanetti L. J. , Potemra T. A.
Published: February 2004

The response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field (IMF) is examined with multi‐instrument data from the Viking and DMSP‐F7 satellites as well as ground magnetometer data. The event selected, which occurred on October 5, 1986, was previously identified as a high‐latitude dayside form by Murphree and Elphinstone [1988]. IMF By was positive during most of the Viking auroral UV observation, whereas IMF Bz made a transient southward excursion, followed by a sharp increase to +8 nT. The results are summarized as follows: (1) There were two auroral belts extending westward from the early afternoon sector. The equatorward belt was persistent throughout the event and was embedded in the CPS/BPS precipitation region, whereas the poleward belt was in the open field line region and was associated with northward IMF Bz. (2) A new auroral spot emerged in the afternoon sector just poleward of the previously active region, delayed 10 min from the sudden increase in IMF Bz. This time lag is ascribed to the travel time of the new IMF orientation from the subsolar point to a solar wind‐magnetosphere interaction site tailward of the dayside cusp, as well as to the response time of the dayside auroral acceleration process. (3) The new auroral activity expanded both eastward and westward during the first few minutes and then expanded primarily westward across the noon meridian, forming the poleward belt. The speed of the westward extension in the second phase was comparable to the speed of the ionospheric convection. (4) The dayside auroral process has a finite decay constant, which is inferred to be at least 10 min. (5) The response time of the midday ionospheric convection to IMF variations is estimated to be a few to several minutes.
Nonguiding center motion and substorm effects in the magnetotail
Authors: Kaufmann Richard L. , Kontodinas Ioannis D. , Ball Bryan M. , Larson Douglas J.
Published: February 2004

Thick and thin models of the middle magnetotail were developed using a consistent orbit tracing technique. It was found that currents carried near the equator by groups of ions with anisotropic distribution functions are not well approximated by the guiding center expressions. The guiding center equations fail primarily because the calculated pressure tensor is not magnetic field aligned. The pressure tensor becomes field aligned as one moves away from the equator, but here there is a small region in which the guiding center equations remain inadequate because the two perpendicular components of the pressure tensor are unequal. The significance of nonguiding center motion to substorm processes then was examined. One mechanism that may disrupt a thin cross‐tail current sheet involves field changes that cause ions to begin following chaotic orbits. The lowest‐altitude chaotic region, characterized by an adiabaticity parameter κ ≈ 0.8, is especially important. The average cross‐tail particle drift is slow, and we were unable to generate a thin current sheet using such ions. Therefore any process that tends to create a thin current sheet in a region with κ approaching 0.8 may cause the cross‐tail current to get so low that it becomes insufficient to support the lobes. A different limit may be important in resonant orbit regions of a thin current sheet because particles reach a maximum cross‐tail drift velocity. If the number of ions per unit length decreases as the tail is stretched, this part of the plasma sheet also may become unable to carry the cross‐tail current needed to support the lobes. Thin sheets are needed for both resonant and chaotic orbit mechanisms because the distribution function must be highly structured. A description of current continuity is included to show how field aligned currents can evolve during the transition from a two‐dimensional (2‐D) to a 3‐D configuration.
The behavior of the electric field within the substorm current wedge
Authors: Lewis R. V. , Freeman M. P. , Rodger A. S. , Watanabe M. , Greenwald R. A.
Published: February 2004

We use the Goose Bay HF radar to investigate the behavior of the electric field within the substorm current wedge for a small substorm (< 200 nT) which occurred on February 15, 1992. Mid‐ and high‐latitude magnetometer data helped to locate the radar backscatter with respect to the longitudes and latitudes of the substorm currents, as well as describing the time development of the substorm itself. Velocities in the portion of the field of view of the Goose Bay HF radar closest to the Frederikshab magnetometer in Greenland were compared directly with the ionospheric currents inferred from the magnetometer. During the early growth phase, the plasma flow and current gradually increased in response to the DP2 electric field and are related by an estimated effective height‐integrated conductivity, Σ*, ∼ 1 S. The plasma flow and current continued to increase in the same proportion (i.e., Σ* was still ∼ 1 S) as the electrojet intensified in stages during the late growth/early expansion phase. In this interval there was probably a pseudo‐breakup which established a longitudinally and latitudinally narrow substorm current wedge. The increased plasma flow and current measured at Frederikshab are attributed to the superposition of the pseudo‐breakup/current wedge electric field on the preexisting DP2 electric field. Evidently, the enhanced ionospheric conductivity strip usually associated with the current wedge is initially located equatorward of Frederikshab, since its effect is not apparent at that location. During the expansion phase proper, a clearly resolved current wedge expanded so that the radar backscatter lay within it. The expansion led to an increased current over Frederikshab, but a slightly suppressed plasma flow, related by an increased Σ* ∼ 4 S: the effect of the precipitation induced conductivity strip now apparent. It is suggested that near‐Earth current disruption may play a significant role in the onset of this particular substorm, which occurred during a nonstorm time interval.
Large perpendicular velocity fluctuations observed in the topside ionosphere
Authors: Forme F. , Fontaine D. , Persson M. A. L.
Published: February 2004

We present observations by the European incoherent scatter radar (EISCAT) of a sporadic event (less than 20 s) occurring near a substorm onset, characterized by a strong apparent ion temperature increase in the auroral F region and associated with significant anisotropy and distorted incoherent scatter spectra. The simultaneously observed electric field is too weak to account for the temperature increase by frictional heating. Indeed, it is interpreted as a spurious effect of the standard analysis procedure in the presence of short‐lived ion drift velocity fluctuations, perpendicular to the background magnetic field, which are unresolved by the radar because of space or timescales smaller than the measurement resolution. A standard analysis of such spectra overestimates the ion temperature and underestimates the electron temperature. Similar observation and interpretation were first reported by Swartz et al. [1988] and Knudsen et al. [1993]. In this paper we develop a modified analysis of incoherent scatter spectra to infer, self‐consistently, the real temperatures and the amplitude of velocity fluctuations. This modified analysis can be applied systematically on a routine basis. The first results confirm the earlier works by Swartz et al. [1988] and Knudsen et al. [1993] and show that the geomagnetic orientation of the velocity structure is mainly in the north‐south direction and its maximum amplitude reaches up to about 2000 m/s at 350 km altitude. Coordinated satellite and ground‐based data tend to argue in favor of a time‐varying electric field (possibly a standing Alfvén wave) as the origin of the observed EISCAT spectral distortions.
Stochastic particle acceleration by a pair of slow shocks
Authors: Shimada Nobue , Terasawa Toshio , Jokipii J. R.
Published: February 2004

We consider a stochastic acceleration mechanism of charged particles working at a pair of slow MHD shocks. The dominant acceleration occurs when a particle has a curvature drift at the shock front, which is parallel to the convection electric field. Since this acceleration mechanism works only on particles crossing the shock front, scattering agents in the upstream region are needed for the successive acceleration. The final energy spectrum, which is determined by the balance among the acceleration, deceleration, and escape processes, is softer than the power law. We suggest that this new acceleration mechanism in the reconnection region can provide the possibility to create high‐energy particles in the magnetotail.
A Statistical study of the spatial evolution of shock acceleration efficiency for 5 MeV protons and subsequent particle propagation
Authors: Kallenrode May‐Britt
Published: February 2004

Traveling interplanetary shocks can accelerate protons up to energies of some tens of MeV. Intensity and anisotropy time profiles observed in interplanetary space can be used to determine the scattering conditions as well as the evolution of the shock efficiency along the observer's magnetic field line. The latter can be described as a superposition of an azimuthal variation along the shock front, which accounts for the dependence of intensity time profiles on the angular distance between the observer and the nose of the shock, and a radial variation, which probably is related to the energetics of the solar event and the shock. Fits with a black box model to 44 particle events lead to the following results: (1) overall scattering conditions in interplanetary space are not different in events with and without interplanetary shock, (2) the radial variation S ∼ rα of the injection has a power law index α between −5.5 and +4.5 with α ≤ 0 for most of the events, (3) a separate solar component is required in four of 44 events only, (4) within the broad scatter from event to event, α and λ are independent of whether the observer is located east or west of the flare normal and (5) if the same shock has been observed by two spacecraft, observations on both spacecraft can be described consistently. Results (4) and (5) suggest that the description of the injection in the black box model is a reasonable approximation. The other results give hints on the acceleration: First, there appears to be no significant enhancement in overall interplanetary turbulence which could lead to a more efficient acceleration. Second, in most of the events presented here the number of injected particles per unit surface of the shock is highest close to the Sun; in about half of the events also the number of particles expressed as fraction of the ambient medium is highest close to the Sun. This has two implications: (1) either there is a strong contribution of flare‐accelerated particles or, in the context of current knowledge more likely, the shock is a more efficient particle accelerator close to the Sun than in interplanetary space, and (2) the relatively small amount of particles accelerated locally in interplanetary space does not require significant acceleration, it might even be reaccelerated material from the large population of particles accelerated close to the Sun.
Geotail observations of substorm onset in the inner magnetotail
Authors: Fairfield D. H. , Mukai T. , Lui A. T. Y. , Cattell C. A. , Reeves G. D. , Nagai T. , Rostoker G. , Singer H. J. , Kaiser M. L. , Kokubun S. , Lazarus A. J. , Lepping R. P. , Nakamura M. , Steinberg J. T. , Tsuruda K. , Williams D. J. , Yamamoto T.
Published: February 2004

On April 26, 1995, while Geotail was in the near‐equatorial magnetotail at 13 RE and 2300 LT, a substorm onset occurred that was documented by ground magnetograms, auroral kilometric radiation, and magnetic field and particle data from four spacecraft at and near geosynchronous orbit. Although Geotail was initially outside a greatly thinned current sheet, plasma sheet thickening associated with the substorm dipolarization quickly caused Geotail to move into the plasma sheet where it observed field‐aligned earthward moving ions with velocities of 400 km/s. During the subsequent few minutes as the magnetic field became more northward, the velocities increased with particles moving increasingly into the energy range of the energetic particle experiment. These flows culminated with 1‐min worth of earthward flow of 2000 km/s that was perpendicular to the northward B field. Such flow, probably the largest ever detected at 13 RE, was confirmed by the observation of an intense dc electric field of 50 mV/m (0.3 megavolts/RE). This large field is probably inductive, caused by reconnection that occurred tailward of the spacecraft, and related to the acceleration processes associated with particle injection at geosynchronous orbit. Energy and magnetic flux conservation arguments suggest that this rapid flow has a small cross‐tail dimension of the order of 1 RE. The data appear to support a simulation of Birn and Hesse [1996] which showed rapid earthward flows from a reconnection line at 23 RE that caused a tailward expansion of a region of dipolarized flux. Subsequent to the onset, Geotail observed plasma vortices with typical velocities of 50–100 km/s that occurred in a high‐beta plasma sheet with a 15‐nT northward magnetic field. The vortices were punctuated by occasional flow bursts with velocities up to 400 km/s, one of which was accompanied by a violently varying magnetic field where north/south field components were as large as 30 nT and as small as −8 nT.
Long lasting energetic proton precipitation in the inner magnetosphere after substorms
Authors: Gvozdevsky B. B. , Sergeev V. A. , Mursula K.
Published: February 2004

Data of polar‐orbiting low‐altitude NOAA spacecraft are used to study the precipitation of energetic (30–80 keV) protons, several degrees equatorward of the energetic proton isotropic boundary. This precipitation, to be called low‐latitude proton precipitation (LLPP), is observed at all local time sectors. The LLPP precipitating flux is found to be generally smaller than the trapped flux. The LLPP particle flux significantly increases during intense substorms and then decays with a long characteristic timescale of about 9 hours. Therefore, it may also be found during very quiet times. The long decay and the observed loss cone anisotropy imply a moderate pitch angle diffusion of energetic protons. During quiet magnetic conditions following the disturbance the latitudinal position of the LLPP region moves slowly to higher latitudes. The equatorial boundary of the precipitation is found to be at lower latitudes in the night sector than in the dayside sector. Mapping to the equatorial plane along magnetic field lines showed that the equatorial boundary of the precipitation at the nightside is situated on a closer drift shell than that at the dayside.
Statistical properties and possible supply mechanisms of tailward cold O+ beams in the lobe/mantle regions
Authors: Seki K. , Hirahara M. , Terasawa T. , Mukai T. , Saito Y. , Machida S. , Yamamoto T. , Kokubun S.
Published: February 2004

We investigate statistical properties of cold O+ beams (COBs) streaming tailward at the velocity nearly equal to the major H+ component, which were observed by Geotail/low‐energy particle (LEP) instrument in the tail lobe/mantle regions at geocentric distance between 8 and 210 RE (Earth radii) during the solar‐minimum period (October 1993 to March 1995). The average O+ density is ∼1.3×10−3 cm−3, which corresponds to ∼1.2 % of the proton component. Properties of the flow velocity show that it is not the weakening of the magnetospheric convection but the large parallel velocity which enables the O+ ions to remain still in the distant lobe/mantle regions. The occurrence frequency of COBs suggests that O+ ions tend to exist in the mantle‐like region rather than tenuous “pure lobe” and that their existence has a clear correlation with the geomagnetic activity. On the basis of the EMF By dependence of the double‐peaked COB distribution along dawn‐dusk direction, it is shown that COBs exist mostly on loaded quadrants in the north–south and dawn‐dusk asymmetry of sheath plasma entry caused by the IMF By effect on the dayside reconnection process. The concentration on the loaded quadrants can be seen even in geomagnetic storms. It suggests that frequent COB occurrence at active times is mainly due to the southward orientation of IMF rather than the increase of dynamic pressure itself during the geomagnetic storms. That is, the statistics show that COBs are abundant at geomagnetically active times on loaded quadrants resulting from the dayside reconnection process, where the ions of solar wind origin bear the major component, and their field‐aligned velocity is larger than usual. These COBs should originate in the dayside magnetosphere and/or the polar cap regions. From the COB energy of several keV, which is rather higher than that of cusp/cleft ion outflows, the necessity of extra energization(s) to elevate parallel velocity is suggested. Clear IMF By dependence, on one hand, provides other possibilities of the COB's source such as the energetic UFI beams and the equatorially trapped ions. The requirements for each candidate so as to be a main contributor to COBs are also discussed.
A kinetic theory for the drift‐kink instability
Authors: Lapenta Giovanni , Brackbill J. U.
Published: February 2004

We have developed a linear two‐dimensional kinetic theory, which is motivated by recent results for the drift‐kink instability. The theory predicts plasma instability for large values of ion‐electron temperature ratios, Ti/Te, moderate drift speeds, and large ion‐electron mass ratios, mi/me, corresponding to conditions in the near‐Earth region of the magnetotail neutral sheet. The growth rate predicted by the theory is in good agreement with nonlinear plasma simulations (but the mode structure is not), including linear growth rates obtained with an implicit, electromagnetic simulation with realistic ion‐electron mass ratio.
Storm time dependence of equatorial disturbance dynamo zonal electric fields
Authors: Scherliess Ludger , Fejer Bela G.
Published: February 2004

We use Jicamarca radar observations of F region vertical plasma drifts and auroral electrojet indices during 1968–1988 to study the characteristics and temporal evolution of equatorial disturbance dynamo zonal electric fields. These electric fields result from the dynamo action of storm time winds and/or thermospheric composition changes driven by enhanced energy deposition into the high‐latitude ionosphere during geomagnetically active conditions. The equatorial vertical drift perturbations last for periods of up to 30 hours after large increases in the high‐latitude currents. On the average, this process can be described by two basic components with time delays of about 1–12 hours and 22–28 hours between the high‐latitude current enhancements and the equatorial velocity perturbations. Our data indicate strong coupling between dynamo processes with different timescales. The short‐term disturbance dynamo drives upward equatorial drifts (eastward electric fields) at night with largest amplitudes near sunrise and small downward drifts during the day. These perturbation drifts are in good agreement with results from the Blanc‐Richmond disturbance dynamo theory. The dynamo process with time delays of about a day drives upward drift velocities at night with largest values near midnight and downward drifts in the sunrise‐noon sector. In this case, the amplitudes of the disturbance drifts maximize during geomagnetically quiet times preceded by strongly disturbed conditions. We also present results of a new equatorial storm time dependent empirical model which illustrate the characteristics of the vertical disturbance dynamo drifts.