bluuebunny/crossref_metadata_2025 huggingface dataset:
Dayside optical and magnetic correlation events
Authors: Mende S. B. , Frey H. U. , Doolittle J. H. , Lanzerotti L. , Maclennan C. G.
Published: January 2004

Sudden, short‐lived dayside auroral activity is often observed equatorward of the quiescent auroral oval. Several cases from the 1996 South Pole and United States Automatic Geophysical Observatory data were examined. In all cases the optical events were accompanied by well‐correlated magnetic impulsive events. Most optical auroral activity was primarily in 630‐nm emission (soft electron precipitation presumably of plasma sheet or magnetosheath origin). Most optical events also show the presence of much shorter lived 427.8‐nm emission (harder electron precipitation with associated electron acceleration). In almost all events the keograms showed repeated poleward propagation, indicating that the event started at lower latitudes and propagated to higher latitudes. The optical emissions showed distinct periodicities, which usually correlated well with the magnetic signature. All the events began equatorward of the preexisting quiescent aurora, indicating that they initiated in the region of closed field lines. The interplanetary magnetic field Bz component prior to the events was either small or positive in most cases. The majority of the observed events were consistent with being triggered by interplanetary Bz or solar wind pressure change. For some events, no specific trigger was found.
Origin of ultraviolet emission source in the Jovian ionosphere at the feet of the Io flux tube
Authors: Shaposhnikov Vladimir E. , Zaitsev Valerii V. , Rucker Helmut O. , Litvinenko Galina V.
Published: January 2004

A model for an explanation of far ultraviolet (FUV) emissions observed from the Io flux tube footprints at the Jovian ionosphere is presented. An acceleration of electrons and ions of the ionospheric plasma up to sufficient energies for an effective excitation of H2 Lyman and Werner bands is proposed. The neutral particles within the Jovian ionosphere are excited due to their collisions with the charged particles relatively moving to the neutrals in the ionospheric plasma. The motion is induced by the Ampère force which arises due to the interaction between the electric current flowing within the Io magnetic flux tube and its own specific magnetic field. It is established that only a sufficiently strong field‐aligned current is capable of providing the required velocity of the relative motion. Our estimates show that in principle the proposed mechanism can provide the level of FUV emissions observed from the Io flux tube footprints.
Vertical structure of a gravity wave like oscillation in the ionosphere generated by the solar eclipse of August 11, 1999
Authors: Altadill D. , Solé J. G. , Apostolov E. M.
Published: January 2004

The dependence in altitude and time of the ionospheric electron density and its altitude variations during the solar eclipse of August 11, 1999, have been studied. The true height electron density profiles recorded at the Observatori de l'Ebre station (40.8° N, 0.5° E) during a rapid sequence vertical ionospheric sounding campaign have been used. A gravity wave like oscillation in the ionosphere has been characterized after the maximum of solar occultation by the Moon. The event is clearly seen in both the plasma frequency and in the altitude variations of the F region in the altitude range from 150 to 250 km, and it has a dominant oscillating period T=57.5 min. The oscillation activity linked with the gravity wave event is found to exist in the variations of the ionospheric parameters, being coherent with the oscillations characterized in the plasma frequency variations. The source‐origin of the event was located around the transition region between F1 and F2 layers and from that altitude the wave propagates vertically drawing energy upwards and downwards simultaneously. The decreasing/increasing solar ionizing radiation and the cooling/heating processes linked with the solar eclipse are discussed as a possible mechanism to explain the gravity wave event observed in the electron density and dynamics of the ionosphere.
FIRI: A semiempirical model of the lower ionosphere
Authors: Friedrich M. , Torkar K. M.
Published: January 2004

An empirical model of the lower, nonauroral ionosphere is presented which is exclusively based on radio wave propagation data from rocket soundings. The very limited number of available profiles provides an empirical correction to a simple theoretical model. This analytical correction, which can be of a very low mathematical order, is determined individually for each pressure surface. An inclusion of this model in tabulated form into the international reference ionosphere (IRI) is foreseen, and electron density profiles for the conditions required by the user can be obtained by interpolation.
Location of high‐altitude cusp during steady solar wind conditions
Authors: Palmroth M. , Laakso H. , Pulkkinen T. I.
Published: January 2004

We have investigated the high‐altitude cusp location and its dependence on the interplanetary magnetic field (IMF) Bz, solar wind Ey, and solar wind dynamic pressure during steady solar wind conditions. We have adopted strict event selection criteria, where we have studied only events during stationary IMF Bz, when the cusp latitude is supposed to be stationary. Prior to this study, the statistical studies have shown a great deal of scatter between the cusp latitude and the IMF Bz; however, with the strict event selection criteria, we have been able to reduce the scatter. The strongest source of scatter comes from large and largely fluctuating IMF By. When the events are selected during small IMF By, the scattering between the cusp latitude and the IMF Bz is reduced. When IMF is southward, also the fluctuation of IMF By has strong influence on the cusp latitude, whereas for northward IMF, the IMF By fluctuation has less influence on the cusp latitude. For larger southward IMF the cusp appears further equatorward than with smaller southward IMF. When the scatter is reduced by introducing the IMF By control, the larger northward IMF brings the cusp to lower latitudes than the smaller northward IMF, i.e., the maximum cusp latitude is obtained for small positive IMF Bz. For increasing solar wind Ez the cusp moves equatorward. Furthermore, with increasing solar wind dynamic pressure, the cusp appears at lower latitudes. This behavior depends on the orientation of the IMF Bz, with southward IMF the cusp moves faster equatorward than with northward IMF.
Parametric decay to lower hybrid waves as a source of modulated Langmuir waves in the topside ionosphere
Authors: Lizunov G. V. , Khotyaintsev Y. , Stasiewicz K.
Published: January 2004

Langmuir emissions generated by electron beams in space plasmas usually appear as chains of strongly modulated wave packets. In this paper, we present a quantitative analysis of three‐wave interaction between Langmuir and lower hybrid waves, L1 ↔ L2+LH, which explains many details of recent Freja observations in the topside ionosphere. The packet‐like waveforms are generated as the beating of several Langmuir modes. The primary Langmuir mode (L1) is produced by beam‐plasma instability, and the other modes are produced as a result of parametric decay to secondary Langmuir waves (L2) and lower hybrid modes (LH). We show that the decay instability has a very low threshold and high growth rate. The limited transverse dimensions of electron beams in the polar ionosphere cause radiation losses from the beam region which influence spectra of the beam‐plasma and parametric instabilities.
Application of multivariate Maxwellian mixture model to plasma velocity distribution function
Authors: Ueno Genta , Nakamura Nagatomo , Higuchi Tomoyuki , Tsuchiya Takashi , Machida Shinobu , Araki Tohru , Saito Yoshifumi , Mukai Toshifumi
Published: January 2004

Recent space plasma observations have provided us with three‐dimensional velocity distributions having multiple peaks. We propose a method for analyzing such velocity distributions via a multivariate Maxwellian mixture model where each component of the model represents each of the multiple peaks. The parameters of the model are determined through an iterative nonlinear optimization technique, specifically the expectation‐maximization (EM) algorithm. For the automatic judgment of the preferable number of components in the mixture model, we introduce a method of examining the number of extrema of a resulting mixture model. We show applications of our method to ion observations in the plasma sheet boundary layer (PSBL) and in the central plasma sheet (CPS) of the Earth's magnetotail. From an analysis of the PSBL and CPS, low‐energy ions that have properties similar to those of lobe ions were detected also in the PSBL and CPS. In the PSBL, middle‐energy ion component which is flowing dawnward and directed the neutral sheet was extracted. We suggested that magnetic field fluctuations in the PSBL can be explained only when the two ion components were properly treated.
Source of field‐aligned irregularities in the subauroral F region as observed by the SuperDARN radars
Authors: Hosokawa Keisuke , Iyemori Toshihiko , Yukimatu Akira Sessai , Sato Natsuo
Published: January 2004

The HF radars of the Super Dual Auroral Radar Network (SuperDARN) observe the E × B convective drift of ionospheric plasma when suitable small‐scale field‐aligned irregularities (FAI) from which to scatter are present. In order to estimate the distribution of FAIs in the subauroral F region, we have investigated the scattering occurrence percentage using data from six Northern Hemisphere radars during 39 months between July 1995 and September 1998. We have identified a morphological feature known as the dusk scatter event (DUSE; first‐reported by Ruohoniemi et al. [1988]) in detail and clarified its relation to the boundaries of the plasma density within the subauroral F region. In all months, the DUSE appears within a few hours local time on the eveningside of sunset, where the magnetic latitude is slightly lower than the equatorward edge of the auroral oval, which corresponds to the density‐depleted structure known as the midlatitude trough. There exists a clear UT effect in the characteristics of DUSE, such that DUSE is more pronounced for UTs corresponding to dusk on the prime magnetic meridian (0° geomagnetic longitude). Furthermore, an enhancement of scattering occurrence around sunrise within the trough, which is termed dawn scatter event (DASE), was newly identified. The region where the DUSE or DASE occurs has a close relationship with the duskside and dawnside ends of the midlatitude trough in the longitudinal direction (which are termed the sunward edges of the trough in this paper), where sunward directed steep plasma density gradients exist. The Kp dependence of the scattering occurrence was also examined. In disturbed conditions, the DUSE appears at earlier local times compared with that in quiet conditions, which is consistent with the Kp dependence of the sunward edge of the trough. Three models based on the gradient drift instability were discussed. It was found that the model which is based on the sunward plasma density gradient at the sunward edges of the midlatitude trough and ambient electric field is most favorable for the growth of FAIs in this region and can consistently explain the statistical features of DUSE and DASE. These facts indicate that we can estimate the longitudinal extent of the midlatitude trough from the appearance of DUSE and DASE observed by SuperDARN.
Comparison of the auroral E region neutral winds derived with the European Incoherent Scatter radar and predicted by the National Center for Atmospheric Research Thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model
Authors: Nozawa S. , Liu H.‐L. , Richmond A. D. , Roble R.
Published: January 2004

A comparison study of the auroral E region neutral wind has been conducted using the European Incoherent Scatter (EISCAT) radar observations and Thermosphere‐Ionosphere‐Mesosphere‐Electro dynamics general circulation model (TIME GCM) predictions. The daily mean wind data as well as diurnal and semidiurnal tidal wind data, are compared for the three seasons summer, equinox, and winter between 95 and 119 km. Fairly good agreement is found in the altitude profile of the mean zonal wind between the EISCAT observation and the TIME GCM prediction for summer, indicating the parameterization of gravity waves employed in the TIME GCM is adequate for this feature. The meridional mean wind amplitude predicted by the TIME GCM is considerably smaller than that observed by EISCAT, and the predicted wind is slightly northward for all the seasons above 100 km. Generally good agreement is found for the amplitude of the diurnal tide, especially the summer prediction, while disagreements between the model and observational results are found for the corresponding phases. The semidiurnal amplitude predicted by the TIME GCM is much smaller than that observed by EISCAT, and relatively large differences of the semidiurnal phase between the observations and predictions are found for all seasons. These comparison results suggest that further advancements in the gravity wave parameterization, as well as the addition of planetary wave effects, are needed to predict more realistic lower thermospheric winds at high latitude.
Equator‐S magnetopause crossings at high time resolution
Authors: Lucek E. A. , Cargill P. , Dunlop M. W. , Kistler L. M. , Balogh A. , Baumjohann W. , Fornacon K.‐H. , Georgescu E. , Haerendel G.
Published: January 2004

The analysis of 37 Equator‐S magnetopause crossings with exceptionally smooth profiles in magnetic field magnitude is presented. The crossings are distributed through 10 orbits and occur under both northward and southward interplanetary magnetic field (IMF) conditions. Under northward IMF, when subsolar reconnection is not believed to occur, the ramp and the field rotation generally overlap. In all 17 cases with a southward magnetosheath field, the field rotation to a northward direction lies sunward of the start of the ramp in field magnitude. Of these cases, five show a field magnitude dip during the field rotation, with shorter field rotations tending to be associated with deeper minima. We suggest that this characteristic of some of the crossings under southward IMF might reflect the distance of the crossing from a reconnection site. Field maxima sometimes appear just inside the magnetopause, but the occurrence of these features is not confined to those crossings showing field minima in the magnetosheath. Examination of the field rotation shows that the rotation is rarely circularly polarized, even when a field magnitude minimum is absent. Comparison with recent two‐dimensional hybrid models shows good qualitative agreement.
Enhancements of lobe ion density and velocity associated with plasmoids
Authors: Shirai H. , Takada T. K. , Kamide Y. , Mukai T.
Published: January 2004

Intermittent enhancements of lobe ions in density and velocity were observed by the GEOTAIL spacecraft in the distant magnetotail (∼100–200 RE). They were observed after the passage of plasmoids, accompanying south‐to‐north bipolar variations in Bz, which are opposite in polarity to those observed with plasmoids or traveling compression regions. The peaks in the ion density occurred when Bz changed from south to north (i.e., when Bz = 0). To explain these observations, a model is presented in which the magnetotail shrinks behind a plasmoid. It is suggested that after the formation of a plasmoid the near‐Earth neutral line runs tailward following the plasmoid. The delay between the plasmoid passage and the ion enhancement observed by the same spacecraft is ∼5–70 min, depending on as yet unknown reasons.
The Cassini/Huygens Venus and Earth flybys: An overview of operations and results
Authors: Burton M. E. , Buratti B. , Matson D. L. , Lebreton J.‐P.
Published: January 2004

The Cassini/Huygens spacecraft flew by Venus on June 24, 1999, and the Earth on August 18, 1999. These two events provided the Cassini Project with its first opportunities to design coordinated scientific observations at planetary encounters. In addition to exercising capabilities of the science instruments as well as the spacecraft and performing instrument calibrations, new scientific data were obtained. We present an overview of these two planetary flybys, describe the processes used to plan and design the observations, and highlight the major scientific results presented in the papers that follow.
Equatorial temperature anomaly during solar minimum
Authors: Suhasini R. , Raghavarao R. , Mayr H. G. , Hoegy W. R. , Wharton L. E.
Published: January 2004

We show evidence for the occurrence of the equatorial temperature anomaly (ETA) during solar minimum by analyzing the temperature and total ion density data from the Neutral Atmosphere Temperature Experiment (NATE) and the Cylindrical Electrostatic Probe (CEP), respectively, on board the Atmospheric Explorer‐E satellite. The chosen data refer to a height of ∼254 km in the African and Asian longitude sector (340.1°E–200°E) during a summer season in the Southern Hemisphere. As during the solar maximum period, the spatial characteristics of the ETA are similar to those of the equatorial ionization anomaly (EIA). A minimum in the gas temperature is collocated with the minimum in the ion density at the dip equator, and a temperature maximum on the south side of the equator is collocated with the density maximum of the EIA. The daytime behavior of ETA formation is about the same as that of EIA as both of them are clearly present at around 1300 and 1400 local solar time (LST) only. At 1400 LST the difference between the temperatures at the crest and the trough (ETA strength) reaches a maximum value of about 100°K which is ∼14% of the temperature at the trough. Like the EIA, the ETA also suddenly disappears after 1400 LST. Thus the EIA appears to be a prerequisite for the ETA formation. During the premidnight time (2200 LST), however, while the EIA is nonexistent, the temperature distribution forms a pattern opposite to that at 1400 LST in the daytime. It shows a maximum around the dip equator and a broad minimum at the daytime crest region where the postsunset cooling also is faster and occurs earlier than at the dip equator. This nighttime maximum appears to be related to the signature of the midnight temperature maximum (MTM). Mass Spectrometer Incoherent Scatter (MSIS) model temperatures, in general, are higher than the observed average temperatures for the summer season and in particular for the region around the dip equator around noon hours.
On the winding of auroral spirals: Interhemispheric observations and Hallinan's theory revisited
Authors: Partamies N. , Freeman M. P. , Kauristie K.
Published: January 2004

Auroral spirals are the largest vortex structures that appear in auroral arcs. Their diameters vary from tens to hundreds of kilometers. We have collected an all‐sky camera data set of 216 events including 189 northern and 27 southern hemispheric spirals. For the first time in the literature, this study shows spiral structures from both hemispheres with similar occurrence distributions for their scale sizes and a sense of rotation that is always counterclockwise when viewed along the geomagnetic field. In a few cases the time resolution was adequate to follow the winding process and the relative brightness of the spirals. These events show that the winding motion of the auroral structure is opposite to the anticipated plasma flow in the same region, and that the winding process is controlled by the intensity of the auroral precipitation. We show that these findings are consistent with the model of Hallinan [1976]. He suggested that spirals are related to periodic undulations developing in an upward field‐aligned current (FAC) sheet. In this model, the winding of the auroral structure manifests the magnetic field line twisting in a direction different from the ionospheric plasma flow. We discuss Hallinan's theory in more detail: First, we derive the complete solutions of the magnetic field perturbation. Second, we compute the structure of the FAC sheet and show that it reproduces the shape and sense of rotation of auroral spirals. Third, we consider the factors that affect the amount of spiral winding. In particular, we show that the spiral winding is controlled by the strength of the FAC perturbation in the arc and thereby is related to auroral precipitation and arc brightness observed. We also find the relationship between the finite field‐aligned distance required for spirals to fully evolve and the amplitude of the FAC perturbation and discuss how this relationship could be used to identify the source region for the FAC perturbation. However, the assumption of a constant background field in the model hampers detailed comparisons with observed FACs and spatial scale sizes. Further development of the model is needed before we can conclude, for example, if at the altitude of the acceleration region, the conditions would be favorable for the spiral winding mechanism.
Polar low‐speed solar wind at the solar activity maximum
Authors: Ohmi T. , Kojima M. , Yokobe A. , Tokumaru M. , Fujiki K. , Hakamada K.
Published: January 2004

The tomographic analysis of interplanetary scintillation (IPS) showed that low‐speed winds (≤ 370 km s−1) emanated out from the polar region at the last solar activity maximum. In order to investigate the origin of those low‐speed winds, we compared the velocity distribution derived from the IPS tomographic analysis to the magnetic field structure derived from the potential field analysis. We found that the polar low‐speed winds appeared for a short period just before and after the disappearance of polar open fields. When the polar coronal hole shrank very small before its disappearance, the coronal polar open field was encircled by large‐scale closed loops and became super radially diverging field into the interplanetary space. A low‐speed region appeared in this diverging polar magnetic field region. This situation is a condition very similar to the compact low‐speed streams associated with equatorial active regions, which were found by Kojima et al. [1999]. After the open field regions had disappeared from the pole, the polar regions were occupied with closed loops. These closed loops were overlapped by the magnetic field which fanned out from the midlatitudes. A low‐speed streamer located above these closed loops even after the polar open field had disappeared. The velocities of polar low‐speed streams before polar hole disappearance were much lower than those after disappearance. This result suggests that the physical conditions to generate much lower speed streams are closely associated with large expansion from small open field regions encircled by large‐scale closed loops. Finally, a reliability of the IPS measurement of polar low‐speed wind was examined by simulating synthetic IPS observations in hypothetical model polar streams.
Wave normal and Poynting vector calculations using the Cassini radio and plasma wave instrument
Authors: Hospodarsky G. B. , Averkamp T. F. , Kurth W. S. , Gurnett D. A. , Dougherty M. , Inan Umran , Wood Troy
Published: January 2004

Wave normal and Poynting vector measurements from the Cassini radio and plasma wave instrument (RPWS) are used to examine the propagation characteristics of various plasma waves during the Earth flyby on August 18, 1999. Using the five‐channel waveform receiver (WFR), the wave normal vector is determined using the Means method for a lightning‐induced whistler, equatorial chorus, and a series of low‐frequency emissions observed while Cassini was in the magnetosheath. The Poynting vector for these emissions is also calculated from the five components measured by the WFR. The propagation characteristics of the lightning‐induced whistler were found to be consistent with the whistler wave mode of propagation, with propagation antiparallel to the magnetic field (southward) at Cassini. The sferic associated with this whistler was observed by both Cassini and the Stanford VLF group at the Palmer Station in Antarctica. Analysis of the arrival direction of the sferic at the Palmer Station suggests that the lightning stroke is in the same sector as Cassini. Chorus was observed very close (within a few degrees) to the magnetic equator during the flyby. The chorus was found to propagate primarily away from the magnetic equator and was observed to change direction as Cassini crossed the magnetic equator. This suggests that the source region of the chorus is very near the magnetic equator. The low‐frequency emission in the magnetosheath has many of the characteristics of lion roars. The average value of the angle between the wave normal vector and the local magnetic field was found to be 16°, and the emissions ranged in frequency from 0.19 to 0.75 ƒce, where ƒce is the electron cyclotron frequency. The wave normal vectors of these waves were primarily in one direction for each individual burst (either parallel or antiparallel to the local field) but varied in direction throughout the magnetosheath. This suggests that the sources of the emissions are far from the spacecraft and that there are multiple source regions.
Observation of the proton aurora with IMAGE FUV imager and simultaneous ion flux in situ measurements
Authors: Gérard J.‐C. , Hubert B. , Meurant M. , Shematovich V. I. , Bisikalo D. V. , Frey H. , Mende S. , Gladstone G. R. , Carlson C. W.
Published: January 2004

The far ultraviolet cameras on board the IMAGE satellite images the aurora in three different spectral regions. One of the channels of the spectrographic imager SI12 observes the Doppler‐shifted Lyman α emission of precipitating protons. It makes it possible to spectrally discriminate between the proton and electron FUV aurora and to globally map the energetic protons. Its response depends on the auroral Lyman α line shape which reflects the characteristics of the proton pitch angle and energy distributions. We illustrate the dependence of the SI12 count rate on the characteristic energy of the proton precipitation and the viewing geometry. Simultaneous in situ observations of the precipitated protons have been collected during a FAST satellite pass when IMAGE was observing the global north polar region. The premidnight region located at the equatorward boundary of the oval is dominated by proton precipitation with a mean energy Ē = 7 keV which is separated from the electron component. The prenoon crossing exhibits a softer proton energy spectrum with Ē = 0.9 keV. The measured proton energy distribution is used as an input to a Monte Carlo model to calculate the expected SI12 signal along the magnetic footprint of the satellite orbit. If the different spatial resolution of the two types of measurements is accounted for, a good quantitative agreement is found with the IMAGE observations. Similarly, ion flux measurements collected on board the Defense Meteorological Satellite Program Fl5 satellite during an overflight in the postmidnight sector provide good agreement with the SI12 observations at the footprint aurora. The comparisons confirm the reliability of the FUV IMAGE cameras to remotely discriminate between the electron and the proton precipitations. The vertical emission rate profiles of the N2 Lyman‐Birge‐Hopfield and OI(1356Å) emissions are calculated in the proton‐dominated premidnight region. It is shown that the protons and the electrons produce FUV emissions with contributions peaking at different altitudes. Excitation by secondary electrons dominates the production of both emissions.
Daytime equatorial geomagnetic H field response to the growth phase and expansion phase onset of isolated substorms: Case studies and their implications
Authors: Sastri J. Hanumath , Rao J. V. S. V. , Rao D. R. K. , Pathan B. M.
Published: January 2004

Observations are presented from the Indian magnetometer network (dipole latitude range 1.2°S to 13.5°N) of short‐lived (<1 hr) disturbances in the daytime equatorial geomagnetic H field associated with specific phases of isolated substorms. Three well‐documented substorms are examined here, of which the expansion onset of each is closely associated with sudden transitions of interplanetary magnetic field (IMF) Bz/By after a >40‐min interval of southward Bz. A positive baylike perturbation is found to prevail during the substorm growth phase, followed by a negative baylike disturbance starting precisely at the onset of expansion phase activity. The amplitude of the positive as well as negative bay‐type disturbance showed a clear‐cut enhancement at locations inside the equatorial electrojet belt when compared with stations away from the electrojet influence, indicative of a significant contribution of ionospheric currents to the bays. This pattern of response, which is found in two out of the three events studied, constitutes the first‐time evidence for the occurrence of equatorial H field perturbations related to the growth phase as well as the expansion phase onset for individual substorms. The H field perturbations are suggested to be signatures of prompt penetration by electric fields associated with rapid changes in magnetospheric convection brought about by the swift transitions in the IMF Bz, before shielding by the ring current becomes effective. In the third event, where the growth phase is weak and prolonged, there is no perceptible simultaneous disturbance in the equatorial H field, while the expansion phase onset is associated with a positive bay of very small amplitude (≤4 nT).
Evidence for a sunward flowing plasma layer adjacent to the tail high‐latitude magnetopause during dawnward directed interplanetary magnetic field
Authors: Popescu D. , Sauvaud J.‐A. , Fedorov A. , Budnik E. , Stenuit H. , Moreau T.
Published: January 2004

We report on observations from the Interball‐Tail satellite of a long‐lived sunward flowing plasma layer (SFPL), adjacent to the plasma mantle and affecting the high‐latitude dawn northern lobe, well beyond the terminator. This observation pertains to periods when the interplanetary magnetic field was oriented mainly dawnward and northward or slightly southward. Although a plasma sheet origin of the SFPL cannot be completely excluded, we show that the most plausible source of the observed plasma is the magnetosheath. Possible entry mechanisms and entry sites are discussed. We show that the magnetosheath ions and electrons can enter the northern lobe via a rotational discontinuity (RD) affecting the high‐latitude magnetopause tailward of the spacecraft location. Such a discontinuity is expected to result from the tailward convection of field lines reconnected at the dayside magnetopause. However, although the distribution functions of ions in the SFPL well fit the general entry mechanisms through RDs, the electron component is clearly heated and accelerated. Their characteristics appeal for specific acceleration and heating mechanisms related to the intrinsic structure of a RD.
Theoretical predictions of the effect of cusp and dayside precipitation on the polar ionosphere
Authors: Vontrat‐Reberac Aurélie , Fontaine Dominique , Blelly Pierre‐Louis , Galand Marina
Published: January 2004

We use the numerical model TRANSCAR to investigate the signature of cusp electron and proton precipitation on all ionospheric parameters (electron density, electron and ion temperatures, and ion field‐aligned velocity) and to compare it to that of other precipitation source regions expected in the dayside polar ionosphere, such as the low‐latitude boundary layer or the dayside extension of the plasma sheet. If the precipitating energetic protons contribute to the formation of a strong density peak in the E region, similarly to the auroral zone, the low‐energy electrons are responsible for very different features. For example, they produce density enhancements in F and low F regions which even increase in time owing to a buildup effect induced by the long timescales in this altitude range. We have estimated that cusp electron precipitations heat the ambient electron gas by typically 750–1000 K and induce much larger temperatures than precipitations from any other source regions. One striking feature is the ion upward flows that can reach values up to 250 m s−1 or more, well above those obtained from the other source regions. Finally, the electron density enhancement initially produced by cusp precipitation persists for timescales of the order of hours after the precipitation event owing to the long timescales at F region altitudes. Combined with transport processes, this effect may supply ionization to other regions of the polar or auroral ionosphere.
Self‐consistent model of solar wind electrons
Authors: Pierrard V. , Maksimovic M. , Lemaire J.
Published: January 2004

We model the transformation of the steady state electron velocity distribution function in the collisional transition region of the solar wind by solving the Fokker‐Planck equation. Alongside the proton and electron Coulomb collisions, effects of gravitational and electric and magnetic forces are also considered. The Coulomb collision term is calculated for any background velocity distribution function using a spectral method that we have developed and applied previously [Pierrard et al., 1999], Consistent treatment of electron self collisions is obtained using an iterative numerical method to match the velocity distribution functions of “test” and “background” electron distributions. Electron collisions with background solar wind protons are also taken into account. We find that Coulomb collisions have important effects on angular scattering of the electrons [i.e., on the pitch angle distribution] without changing their average density or mean radial temperature distribution. Finally, the importance of boundary conditions on the solution of the Fokker‐Planck equation is discussed and emphasized.
Introduction to special section: SAZ Project
Authors: Trull T. W. , Sedwick P. N. , Griffiths F. B. , Rintoul S. R.
Published: February 2004

Oceanographic processes in the subantarctic region contribute crucially to the physical and biogeochemical aspects of the global climate system. To explore and quantify these contributions, the Antarctic Cooperative Research Centre (CRC) organized the SAZ Project, a multidisciplinary, multiship investigation carried out south of Australia in the austral summer of 1997–1998. Here we present a brief overview of the SAZ Project and some of its major results, as detailed in the 16 papers that follow in this special section.
Global ionospheric effects of the October 1989 geomagnetic storm
Authors: Yeh K. C. , Ma S. Y. , Lin K. H. , Conkright R. O.
Published: February 2004

Based on a large data base from 40 ionosonde stations distributed worldwide and 12 total electron content stations, a case study is made on the global behavior of ionospheric responses to the great magnetic storm of October 1989. The magnetic storm was triggered by a solar flare with the largest class of X13/4B and started with a sudden storm commencement (ssc) at 0917 UT on October 20. After the initial phase the storm underwent two periods of maximum activities in the following 2 days. Low‐latitude auroras were sighted and reported in widely separated areas in both northern and southern hemispheres. In response to these magnetic and auroral activities the ionosphere showed remarkable effects. Depending on the local time of ssc occurrence, the ionospheric response differed appreciably. Impressive changes were long‐lasting, large‐scale effects, such as the severe depressions of foF2 at higher latitudes, the temporary suppression of the equatorial anomaly and large horizontal gradients at certain latitudes. Also observed were positive storm effects of short duration during the post‐sunset period in response to the onset of both ssc and main phase of the magnetic storm. These two positive storm effects showed different patterns suggesting different causal mechanisms. In addition, global propagation of large‐scale traveling ionospheric disturbances (TIDs) was seen during 2 nights, identified by dramatic rises of h′F with periodic fluctuations. The equatorward propagation velocities of the TIDs varied between 330 m/s and 680 m/s for the east Asia region.
A variable cross‐section model of the bow shock of Venus
Authors: Khurana Krishan K. , Kivelson Margaret G.
Published: February 2004

MHD parameters like the Alfvenic and the sonic Mach numbers and the direction of the interplanetary magnetic field profoundly affect the interaction of the solar wind with nonmagnetized conducting objects like Venus. The size of the bow shock depends on the two Mach numbers, whereas asymmetries in its shape are governed by the direction of the magnetic field. This paper introduces a new class of bow shock models in which both the shape and the size are controlled by the upstream plasma and field conditions. We use insights from the MHD theory of shocks for point objects and empirical information from actual bow shock crossings to obtain a semiempirical, semitheoretical model of the Cytherean bow shock. The model was developed from a limited data set obtained from the Galileo flyby of Venus but is also in substantial agreement with Pioneer Venus Orbiter observations. It is shown that the dozen bow shock crossings observed by Galileo under steady conditions of solar wind flow and density were caused by changes in the cross section of the bow shock induced by the changing direction of the interplanetary magnetic field.
Large 6300‐Å airglow intensity enhancements observed in Ionosphere Heating Experiments are excited by thermal electrons
Authors: Mantas George P.
Published: February 2004

It is shown that the 6300‐Å airglow intensity enhancements observed in ionosphere heating experiments, by high‐power HF waves, which have been hitherto attributed to excitation by energetic electrons accelerated in plasma instabilities, are excited by energetic electrons from the tail of the heated thermal (maxwellian) electron gas. In addition, a plausible explanation for the inadequacy of the electron temperatures observed in the early ionosphere heating experiments at Arecibo to account for the observed oxygen red line enhancements is presented. In the light of these developments, electron acceleration in ionosphere HF heating experiments might be more scarce than is commonly believed.
Sudden impulses at low latitude stations: Steady state response for southward interplanetary magnetic field
Authors: Russell C. T. , Ginskey M. , Petrinec S. M.
Published: February 2004

When the interplanetary field is northward and the solar wind dynamic pressure suddenly increases, the increase in the H component magnetic field on the surface of the Earth at low and mid‐latitudes is governed principally by the currents on the magnetopause. Currents in the tail, which act in the opposite sense to the magnetopause currents, are also enhanced. These currents have a smaller effect than the magnetopause currents and have a greater effect at night than in the daytime hours. In this study we examine the response of the H component when the IMF is southward. We find that the dayside response to solar wind pressure increases is over 25% smaller when the IMF is southward than when it is northward, presumably because of the region 1 current system associated with dayside reconnection. At night, the opposite situation ensues. Mid‐latitude bays associated with triggered substorms appear to be responsible for an enhanced nighttime response. Thus the ground level response to sudden pressure changes in the solar wind is very sensitive to the direction of the interplanetary magnetic field.
Magnetotail dynamics under isobaric constraints
Authors: Birn Joachim , Schindler Karl , Janicke Lutz , Hesse Michael
Published: February 2004

Using linear theory and nonlinear MHD simulations, we investigate the resistive and ideal MHD stability of two‐dimensional plasma configurations under the isobaric constraint dP/dt = 0, which in ideal MHD is equivalent to conserving the pressure function P = P(A), where A denotes the magnetic flux. This constraint is satisfied for incompressible modes, such as Alfvén waves, and for systems undergoing energy losses. The linear stability analysis leads to a Schrödinger equation, which can be investigated by standard quantum mechanics procedures. We present an application to a typical stretched magnetotail configuration. For a one‐dimensional sheet equilibrium characteristic properties of tearing instability are rediscovered. However, the maximum growth rate scales with the 1/7 power of the resistivity, which implies much faster growth than for the standard tearing mode (assuming that the resistivity is small). The same basic eigenmode is found also for weakly two‐dimensional equilibria, even in the ideal MHD limit. In this case the growth rate scales with the 1/4 power of the normal magnetic field. The results of the linear stability analysis are confirmed qualitatively by nonlinear dynamic MHD simulations. These results suggest the interesting possibility that substorm onset, or the thinning in the late growth phase, is caused by the release of a thermodynamic constraint without the (immediate) necessity of releasing the ideal MHD constraint. In the nonlinear regime the resistive and ideal developments differ in that the ideal mode does not lead to neutral line formation without the further release of the ideal MHD constraint; instead a thin current sheet forms. The isobaric constraint is critically discussed. Under perhaps more realistic adiabatic conditions the ideal mode appears to be stable but could be driven by external perturbations and thus generate the thin current sheet in the late growth phase, before a nonideal instability sets in.
Solar‐wind‐driven convection in the Uranian magnetosphere
Authors: Ye Gang , Hill T. W.
Published: February 2004

We present an analytic, self‐consistent model of time‐dependent solar‐wind‐driven convection in the magnetosphere of Uranus. Because of the unusual orientation of the planetary rotation and magnetic dipole axes, magnetic merging on the dayside magnetopause varies as a function of planetary spin, in response to the changing orientation of the planetary magnetic field relative to the upstream interplanetary magnetic field, which is assumed to have a fixed direction for many planetary rotations. Therefore the magnitude of the solar‐wind driven convection electric field varies sinusoidally in time with the 17.2‐hour planetary spin period, even though the field direction is fixed in the corotating frame in a direction analogous to the dawn‐to‐dusk direction in the Earth's magnetosphere. We assume that the “hot” (keV) protons observed by the Voyager 2 plasma science instrument in the inner magnetosphere convect sunward from a source in the near tail and form a ring current shielding layer near L = 5. The shielding process requires a time‐dependent model because the convection timescale (∼20 days) is much larger than the 17‐hour period of variation of the convection field. The time‐averaged part of the imposed electric field is strongly attenuated inside the shielding layer, but the sinusoidally varying part of the imposed field penetrates the layer without significant attenuation because the shielding timescale (∼30 hours) is longer than the 17‐hour oscillation period. A fraction of the hot plasma is thereby “scattered” onto closed drift orbits to form a trapped ring current population. This trapped ring current population is sufficiently long‐lived to undergo charge exchange and inelastic collisions with the widely distributed neutral hydrogen corona, resulting in the energy degradation of the “hot” component and the simultaneous appearance of the “intermediate” (∼ 100 eV) and "warm" (∼ 10 eV) components evident in the Voyager 2 plasma science measurements between L = 5 and L = 7. The Birkeland current system is concentrated near the ring current shielding layer, consistent with the relatively low latitude of the auroral emissions observed by the Voyager 2 ultraviolet spectrometer.
Lower hybrid resonance wave excitation by whistlers in the magnetospheric plasma
Authors: Shklyar David R. , Washimi Haruichi
Published: February 2004

A new theory of lower hybrid resonance (LHR) wave excitation by whistlers in magnetospheric plasma is developed. Two time dependent differential equations, which have a form of linear wave interaction between quasi‐longitudinal whistler‐mode waves and quasi‐electrostatic LHR waves in a cold plasma with small‐scale density irregularities, are derived and solved by the method of successive approximations. Fundamental observational results on LHR wave excitation by whistlers, such as the lower cutoff frequency of excited waves and the pronounced spectral maximum near the LHR frequency, as well as the close connection of the excitation process with small‐scale plasma density irregularities are explained by the present theory in a consistent manner.
Regulation of the solar wind electron heat flux from 1 to 5 AU: Ulysses observations
Authors: Scime Earl E. , Bame Samuel J. , Feldman William C. , Gary S. Peter , Phillips John L. , Balogh Andre
Published: February 2004

In this study we use observations from the three‐dimensional electron spectrometer and magnetometer aboard the Ulysses spacecraft to examine the solar wind electron heat flux from 1.2 to 5.4 AU in the ecliptic plane. Throughout Ulysses' transit to 5.4 AU, the electron heat flux decreases more rapidly (∼R−3.0) than simple collisionless expansion along the local magnetic field and is smaller than expected for a thermal gradient heat flux, q∥e(r)=−κ∥▽∥Te(r).The radial gradients and magnitudes expected for a number of electron heat flux regulatory mechanisms are examined and compared to the observations. The best agreement is found for heat flux regulation by the whistler heat flux instability. The upper bound and radial scaling for the electron heat flux predicted for the whistler heat flux instability are consistent with the observations.
An MHD simulation study of the poloidal mode field line resonance in the Earth's dipole magnetosphere
Authors: Ding D. Q. , Denton R. E. , Hudson M. K. , Lysak R. L.
Published: February 2004

The poloidal mode field line resonance in the Earth's dipole magnetic field is investigated using cold plasma ideal MHD simulations in dipole geometry. In order to excite the poloidal mode resonance, we use either an initial or a continuous velocity perturbation to drive the system. The perturbation is localized at magnetic shell L = 7 with plasma flow in the radial direction (electric field component in the azimuthal direction). It is found that with the initial perturbation alone, no poloidal mode resonance can be obtained and the initially localized perturbation spreads out across all magnetic L shells. With the continuous perturbation, oscillating near the poloidal resonance frequency, a global‐scale poloidal cavity mode can be obtained. For the first time, a localized guided poloidal mode resonance is obtained when a radial component of electric field is added to the initial perturbation such that the curl of the electric field is everywhere perpendicular to the background dipole magnetic field. During the localized poloidal resonance, plasma vortices parallel/antiparallel to the background dipole magnetic field B0 lead to circular plasma flow perpendicular to B0. This circular flow, elongated radially, results in twisting of magnetic field flux tubes, which, in turn, leads to the slowdown of the circular plasma flow and reversal of the plasma vortices. The energy associated with the localized poloidal resonance is conserved as it shifts back and forth between the oscillating plasma vortices and the alternately twisted magnetic flux tubes. In the simulations the eigenfunctions associated with the localized poloidal resonance are grid‐scale singular functions. This result indicates that ideal MHD is inadequate to describe the underlying problem and nonideal MHD effects are needed for mode broadening.