bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of inner plasma sheet and ring current during substorms
Authors: Fok Mei‐Ching , Moore Thomas E. , Delcourt Dominique C.
Published: September 2002

The evolution of the inner plasma sheet and the ring current during substorm dipolarizations is simulated. A substorm cycle is treated by stretching and dipolarizing the magnetosphere according to the Tsyganenko 89 model. In order to clarify the relative influences of steady convection and induction electric field on ring current development, the inductive electric field is superposed on two baseline convective states: a nonstorm state using a weak electric field, and a storm‐time state using a stronger electric field. Ion distributions on the nightside at 12 Earth radii (RE) during these two substorms are obtained using our single‐particle code to trace particle trajectories backward in time to source regions assumed to have steady characteristics. The subsequent acceleration and transport of these boundary ions into the inner magnetosphere is modeled by our kinetic model of the ring current. The simulation generates many frequently observed features of substorm injections, including the sudden appearance of hot plasma tailward of a sharply defined “injection boundary,” the earthward motion of an “injection front,” the azimuthal and tailward expansion of this enhanced region, and the creation of characteristic ion dispersion patterns near geosynchronous orbit. Comparison of the nonstorm and storm cases suggests that substorms occurring without a convection enhancement produce mainly an enhancement of the cross‐tail current but little change in the ring current. With strong convection, the role of substorms is to enable the convection enhancement to create robust ring current in the inner magnetosphere.
Aspects of MHD flow about Venus
Authors: Biernat H. K. , Erkaev N. V. , Farrugia C. J.
Published: September 2002

We describe the “magnetic string” approach to integrating the dissipationless magnetohydrodynamic (MHD) equations for flow around planetary obstacles and apply it to some aspects of the flow in the magnetosheath of Venus. Our method has both analytical and numerical components and is particularly suited to study the structure of the magnetic barrier (depletion layer). We do not include ion pickup processes and thus discuss only the contribution to the structure of the Venus magnetosheath made by the flow of the shocked solar wind. We work with an interplanetary magnetic field which is directed orthogonal to the solar wind bulk velocity. Magnetic forces on the flow are strongly dependent on the Alfvén Mach number upstream of the bow shock, and one aim of this work is to study the dependence of field and flow quantities in the Venus magnetosheath on this parameter, thus allowing further future comparisons with data under a variety of interplanetary conditions. A second aim is to compare our MHD model results to a synopsis of observations made by the Pioneer Venus Orbiter. As one main conclusion, we show that this method leads, in principle, to a standoff bow shock position in good agreement with observations. We find, namely, that for a low but reasonable Alfvén Mach number, our MHD‐modeled magnetosheath is only ∼ 3.6% thinner in the Sun‐Venus direction than that given by observations. Our method is complementary to three‐dimensional, global MHD simulations of the solar wind‐Venus interaction and offers versatility to modeling other aspects of the complicated interaction of the solar wind with Venus.
Cold dense plasma in the outer magnetosphere
Authors: Matsui H. , Mukai T. , Ohtani S. , Hayashi K. , Elphic R. C. , Thomsen M. F. , Matsumoto H.
Published: September 2002

Cold dense plasma (CDP) with energies less than 32 eV in the outer magnetosphere, L ∼ 10, is reported using data obtained by Geotail. We have found unexpectedly recurrent CDP in afternoon magnetic local times. CDP is often accompanied by westward flows, indicating that the regions with CDP are located outside the corotation region and the plasma is presumably convected toward the magnetopause. CDP is observed in the cases of both small and moderate geomagnetic activity. We have calculated the location of the stagnation point as a function of cross‐tail potential by using a Volland‐Stern‐type [Volland, 1973; Stern, 1975] convection model. Stagnation point locations are variable even if the cross‐tail potential or geomagnetic activity is small, which could lead to a high rate of the observation of plasmaspheric material at the Geotail orbit. However, part of the cold plasma is directly supplied by the ionosphere without passing through the plasmasphere. The number of supplied particles could be larger on the drift orbits close to the stagnation points. Finally, an average quantity of cold plasma flowing through the Geotail orbit with quasi‐stable westward convection is estimated to be 3.6 × 1025 ions/s with a standard deviation of 3.7 × 1025 ions/s, as compared to 9.4 − 14 × 1025 ions/s for the polar wind toward the plasma sheet.
New constraints on theories of Pc1 pearl formation
Authors: Mursula K. , Kangas J. , Kerttula R. , Pikkarainen T. , Guglielmi A. , Pokhotelov O. , Potapov A.
Published: September 2002

In this paper we study structured Pc1 pulsations (also called Pc1 pearls) observed on ground, concentrating on the relation between the pearl repetition period τ and the wave frequency f. Earlier studies suggest that the product τ f is roughly constant. We reexamine this relation and show that a simple inverse law is excluded. Instead, our observations suggest the relation τ ∝ f−p, with p = 0.59 ± 0.06, posing a new, strict constraint on theories of Pc1 pearl formation. We also study the L dependence of various combinations of τ and f using a model L value and extract additional constraints from these combinations. We discuss these constraints in the bouncing wave packet model of pearl formation and determine the range of allowed parameter values in this model. We present two models of energetic ions with different L distributions and show that one of them can be excluded by the constraints derived. We also discuss how to further improve on these constraints to better test the bouncing wave packet model and other theories of Pc1 pearl formation.
Observations of polar cap arcs on FAST
Authors: Bonnell J. , Elphic R. C. , Palfery S. , Strangeway R. J. , Peterson W. K. , Klumpar D. , Carlson C. W. , Ergun R. E. , McFadden J. P.
Published: September 2002

FAST satellite observations of spatially localized regions of electron and ion precipitation with concomitant paired upward and downward field‐aligned currents and duskward electric field enhancements at high (> 80° invariant latitude) magnetic latitudes are presented. Each pass of several across the polar cap was made during a period of persistent northward interplanetary magnetic field, and was characterized by the presence of several of these precipitation regions. In each case, the precipitation regions were associated with jets of enhanced antisunward convection, with electron precipitation restricted to the upward current region, and no energetic electron outflow in the downward current region, and ion precipitation spread over broader regions, possibly by time‐of‐flight effects. We compare these low‐altitude features to those expected for source regions lying in a reconnection site at the lobe magnetopause, a bursty bulk flow in the plasma sheet, and a surface wave on the low‐latitude boundary layer (LLBL). We find that the lobe reconnection model successfully describes the observed electrodynamics of the arcs, as well as the electron and ion source temperatures, but can not readily explain the presence of precipitating O+ and He+ in addition to H+ and He++. The plasma sheet source model can explain the composition of the ion precipitation but predicts higher electron and ion source temperatures than are observed, may require the coincidence of bursty bulk flows with extensions of the plasma sheet into the lobe, and may not be consistant with other observations of auroral phenomenology. The LLBL surface wave model does not readily explain the observed electrodynamics of the arcs and is not consistent with the observed offsets between electron and ion precipitation.
Characteristics of mesospheric optical emissions produced by lightning discharges
Authors: Veronis Georgios , Pasko Victor P. , Inan Umran S.
Published: September 2002

A new two‐dimensional cylindrically symmetric electromagnetic model of the lightning‐ionosphere interaction includes effects of both the lightning radiated electromagnetic pulses (EMP) and the quasi‐electrostatic (QE) fields, thus allowing effective studies of lightning‐ionosphere interactions on time scales ranging from several microseconds to tens of milliseconds. The temporal and spatial evolution of the electric field, lower ionospheric electron density, and optical emissions calculated with the new model are used to investigate theoretically the effects of the lightning return stroke current waveform (i.e., the current rise and fall timescales) and of the observational geometry on the optical signals observed with a photometer. For typical lightning discharges of ∼100 μs duration the ionospheric response is dominated by the EMP‐induced heating leading to the highly transient and laterally expanding optical flashes known as elves. The optical signal characteristics are found to be highly sensitive to both the observational geometry and the current waveform. The onset delay with respect to the lightning discharge, the duration, and the peak magnitude of optical emissions are highly dependent on the elevation and azimuth angles of field of view of individual photometric pixels. The shape of the optical signal clearly reflects the source current waveform. For a waveshape with risetime of ∼50 μs or longer a double‐pulse shape of the photometric signal is observed. For cloud to ground lightning discharges of ∼1 ms duration removing substantial amount of charge (i.e., ∼100 C from 10 km altitude), heating and ionization changes induced by the QE field lead to the mesospheric luminous glows with lateral extent < 100 km, referred to as sprites.
Shock propagation in the outer heliosphere: 2. Pickup ions and MHD
Authors: Rice W. K. M. , Zank G. P.
Published: September 2002

Using a one‐dimensional gasdynamic model, it has previously been shown that pickup ions (PIs) play a significant role in the propagation of shock waves in the outer heliosphere. The effect of adding a magnetic field to the heliospheric model and the resultant effect on shock wave propagation are studied here. In the absence of PIs the addition of a magnetic field has a significant effect on shock wave propagation, yielding results very different from those obtained by simple gasdynamic models. When PIs are included, the difference between the gasdynamic case and the MHD case is quite small, indicating the dominant role played by the PIs. The difference between shock propagation characteristics when PIs are included and when they are ignored is not nearly as great in the MHD case as it is in the gasdynamic case. One of the main differences in the MHD case is the propagation of pressure‐balanced structures (PBSs), which are shown to be more easily observed in the outer heliosphere when PIs are included. Besides PBSs, the inclusion of a magnetic field also allows for the investigation of structures such as magnetic clouds which can obviously not be studied using a gasdynamic model.
Spatiotemporal activity of magnetic storms
Authors: Valdivia J. A. , Vassiliadis D. , Klimas A. , Sharma A. S. , Papadopoulos K.
Published: September 2002

We have constructed a data‐derived model of the evolution of the spatial structure of the ring current geomagnetic signature during storms. A spatially dependent generalization of the Dessler‐Parker‐Skopke relation has been derived to explain the spatial structure in the midlatitude magnetic fluctuations (MLMF) as observed by ground magnetometers. Such a relation is used as a basis for constructing solar‐wind‐driven, data‐derived models of the MLMF. The model includes a coupling to the solar wind as the energy driver and also includes a nonlocal coupling as an explanation of the inhomogeneity in the energy density that appears in the ring current during the main phase of a storm. Both linear and nonlinear models for the evolution of the spatial structure of the MLMF are constructed, and the nonlinear spatial model of the ring current produces better predictions than the linear one. This can be taken as an indication that during strong magnetic storms the ring current evolves in a nonlinear fashion. The spatial data used in the generation of the models are rotated to a frame “fixed” with the ring current, and presure effects were accounted through a kinematic relation. The techniques developed in this paper are very general and can be used to study other systems that show spatial structure, such as the high‐latitude current system.
A model for plasma flows and shocks in the high‐altitude cusp
Authors: Cargill Peter J.
Published: September 2002

A new model for the interaction of a supersonic magnetosheath flow with the high‐altitude cusp is proposed. Archival and recent observations have suggested that the cusp is a region of indentation, where solar wind plasma is found earthward of the nominal magnetopause position. It is proposed that a supersonic solar wind flow interacts with such an indentation by being first accelerated through an expansion fan and then decelerated by a shock. For most realistic parameters the shock cannot originate from the deepest part of the cusp but must be detached. The rarefaction of the magnetosheath plasma leads to a rather shallow cusp (< 1Re) and also implies that the cusp becomes less indented the more tailward it is located.
Feasibility of a multisatellite investigation of the Earth's magnetosphere with radio tomography
Authors: Ergun R. E. , Larson D. E. , Phan T. , Taylor D. , Bale S. , Carlson C. W. , Roth I. , Angelopoulos V. , Raeder J. , Bell T. , Inan U. S. , Bougeret J.‐L. , Manning R.
Published: September 2002

We describe the scientific motivation, basic principles, and feasibility of a relatively new measurement technique, radio tomography, and show how it can be used to investigate the Earth's magnetosphere. We demonstrate that a multisatellite radio tomography experiment can produce two‐dimensional images of plasma density in the Earth's magnetosphere at sufficient spatial (1/2 RE) and temporal (∼10 s) resolution to address key problems of magnetospheric physics. The imaging technique incorporates well‐established radio science methods and computed tomography. Several coplanar satellites are required in orbits that encompass the imaged area. We suggest that the large‐scale images are more valuable when combined with in situ observations, supporting an unambiguous interpretation of the in situ data and an investigation of the interdependence of small‐ and large‐scale plasma processes.
Silicate and carbonaceous grains from stars to the heliosphere: Recent progresses from the Infrared Space Observatory
Authors: Lequeux James
Published: September 2002

I review briefly the present understanding of the history of silicate and carbonaceous grains from their formation around stars or in active regions of the interstellar medium to their appearance in objects of the solar system. This review is based mainly on recent observations with the Infrared Space Observatory. Progress has been substantial, but there are still many missing links in the long and complex chain of events which govern the formation, transport, and processing of cosmic grains.
Inner edge of the electron plasma sheet: Empirical models of boundary location
Authors: Elphic R. C. , Thomsen M. F. , Borovsky J. E. , McComas D. J.
Published: September 2002

This study uses geosynchronous plasma observations and the inferred midnight equatorward edge of the diffuse aurora (based on DMSP data) to obtain hundreds of two‐point locations of the inner edge of the electron plasma sheet. We reconfirm that the local time at L = 6.6 of the boundary separating the plasmasphere/electron trough (where cold ion densities can be high and the density of hot electrons is low) and the electron plasma sheet (where hot electron densities increase) varies with activity level. We also demonstrate a good correlation between the midnight boundary index (MBI) from DMSP and the local time of the geosynchronous electron plasma sheet inner edge. Models of the size and shape of the inner edge of the electron plasma sheet may be constructed using the equatorial mapping of the MBI values and the geosynchronous boundary crossing locations in local time. The model form used is an improved version of the zero‐energy drift boundary approximation of Southwood and Kaye [1979]; the improved form is an analytic approximation to the exact solution for a shielded cross‐tail electric field. Model parameters such as radial distance of the stagnation point range from 5 to well over 7 RE depending on activity; the degree of shielding also appears to be greater for low activity than for high, with shielding parameter values of greater than 5 for low activity. When the boundary model is allowed to rotate freely, the inferred stagnation point appears to move tailward, not sunward, with increasing activity, but this result is poorly constrained.
Magnetospheric and ionospheric response to a substorm: Geotail HEP‐LD and Polar PIXIE observations
Authors: Håland S. , Østgaard N. , Bjordal J. , Stadsnes J. , Ullaland S. , Wilken B. , Yamamoto T. , Doke T. , Chenette D. L. , Parks G. K. , Brittnacher M. J. , Reeves G. D.
Published: September 2002

The High Energy Particle ‐ Low energy particle Detector experiment (HEP‐LD) on board the Geotail spacecraft and the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on board the Polar satellite have been used to examine a substorm event. On December 10, 1996, around 1700 UT, a substorm event with two onsets took place. The event occurred during a weak magnetic storm that started on December 9. Several of the classical substorm features were observed during the event: reconnection and neutral‐line formation in the near‐Earth geomagnetic tail, injection of energetic particles at geosynchronous orbit, and particle precipitation into the ionosphere. Magnetic field line mapping of the energetic precipitation area into the geomagnetic tail shows that the substorm development on ground is closely correlated with topological changes in the near‐Earth tail. In the first onset, mainly soft electrons are involved, with only a transient energetic precipitation delayed relative to the onset. The second onset about 30 min later includes both soft and energetic electrons. The source regions of both onsets are found to be located near the earthward edge of the plasma sheet, while the source region of the transient energetic precipitation during the first onset is in the distant tail. Magnetic reconnection occurs sporadically and burst‐like before the onsets. Both onsets appear to be triggered by northward turning of the interplanetary magnetic field. The study also demonstrates that the concept of pseudobreakups should be used with care unless global observations are available.
Solar cycle variations of the thermospheric meridional wind over Japan derived from measurements of hmF2
Authors: Igi Seiji , Oliver William L. , Ogawa Tadahiko
Published: September 2002

This paper describes an analysis of the meridional equivalent neutral wind for geomagnetically quiet conditions as determined from data collected at the Kokubunji ionosonde station (35.7N, 139.5E), Japan over one solar cycle (1981–1991). The wind is derived from the altitude of peak F2 layer electron density using the Field Line Interhemispheric Plasma (FLIP) model. For low and moderate solar activity the wind is poleward in the daytime and equatorward in the nighttime. For high solar activity the wind is weak and almost always poleward throughout the day. In winter the local time of peak poleward velocity occurs in the afternoon for low solar activity but in the night for high solar activity. In summer the peak poleward wind occurs in the morning for all levels of solar activity. The diurnal amplitude decreases with increasing solar activity. It also reaches its maximum around the solstices and its minimum around the equinoxes. The mean wind is larger at solar maximum than solar minimum, except for the year 1984. The daily‐mean wind is smallest in summer and largest in winter. Qualitatively, the meridional wind at Asian midlatitudes has characteristics similar to those seen in other sectors (such as Millstone Hill, Boulder, Wallops Island, King George Island, and Saint‐Santin), but the details in behavior are different and warrant further investigation.
Modeling the motion and distribution of interstellar dust inside the heliosphere
Authors: Landgraf M.
Published: September 2002

The interaction of dust grains originating from the local interstellar cloud with the environment inside the heliosphere is investigated. As a consequence of this interaction, the spatial distribution of interstellar dust grains changes with time. Since dust grains are charged in the interplanetary plasma and radiation environment, the interaction of small grains with the heliosphere is dominated by their coupling to the solar wind magnetic field. The change of the field polarity with the solar cycle imposes a temporal variation of the spatial distribution and the flux of small (radius smaller than 0.4 μm) interstellar dust grains in the solar system, whereas the flux of large grains is constant because of their negligible coupling to the solar wind magnetic field. The flux variation observed by in situ measurements of the Galileo and Ulysses spacecraft are reproduced by simulating the interaction of interstellar grains with charge‐to‐mass ratios between 0.5 and 1.4 C kg−1 with the interplanetary environment.
The relationship of coronal mass ejections to streamers
Authors: Subramanian Prasad , Dere K. P. , Rich N. B. , Howard R. A.
Published: September 2002

We have examined images from the Large‐Angle and Spectrometric Coronagraph (LASCO) to study the relationship of coronal mass ejections (CMEs) to coronal streamers. We wish to test the suggestion [Low, 1996] that CMEs arise from flux ropes embedded in a streamer erupting and disrupting the streamer. The data span a period of 2 years near Sunspot minimum through a period of increased activity as Sunspot numbers increased. We have used LASCO data from the C2 coronagraph which records Thomson scattered white light from coronal electrons at heights between 1.5 and 6Rs. Maps of the coronal streamers have been constructed from LASCO C2 observations at a height of 2.5Rs at the east and west limbs. We have superposed the corresponding positions of CMEs observed with the C2 coronagraph onto the synoptic maps. We identified the different kinds of signatures CMEs leave on the streamer structure at this height (2.5Rs). We find four types of CMEs with respect to their effect on streamers: 1. CMEs that disrupt the streamer, 2. CMEs that have no effect on the streamer, even though they are related to it, 3. CMEs that create streamer‐like structures and 4. CMEs that are latitudinally displaced from the streamer. CMEs in categories 3 and 4 are not related to the streamer structure. This is the most extensive observational study of the relation between CMEs and streamers to date. Previous studies using SMM data have made the general statement that CMEs are mostly associated with streamers and that they frequently disrupt it. However, we find that approximately 35% of the observed CMEs bear no relation to the preexisting streamer, while 46% have no effect on the observed streamer, even though they appear to be related to it. Our conclusions thus differ considerably from those of previous studies.
Dependence of the near‐Earth magnetotail magnetic field on storm and substorm activities
Authors: Nakai H. , Kamide Y. , Russell C. T.
Published: September 2002

Equatorial magnetic field data in the near‐Earth tail region 3 < R < 23 RE have been sampled from the large magnetic field database obtained by ISEE 1 for 1978–1987. The dependence of the field magnitude on storm and substorm activities represented by the Dst and AL indices, respectively, has been examined. The data for declining/quiet phases in terms of the AL index have been used; i.e., the data obtained before and during the periods when the auroral electrojet was rapidly increasing have not been used to avoid the possible influences of substorm expansion onset. The following two features are noted: (1) Magnetic field magnitude does not depend significantly on substorm activity (AL) in the near‐Earth magnetotail R < 9 RE, while it increases with increasing substorm activity in the midmagnetotail R > 9 RE. The slope of the regression line for the AL index changes in a step‐like fashion in the vicinity of R = 9 RE. (2) Field magnitude decreases with increasing storm activity (Dst) in the region within R ∼ 12 RE, while it increases beyond R ∼ 12 RE. In contrast to the regression slope for the auroral electrojet, the slope for the Dst index changes gradually with increasing the distance from the Earth. Considering these points along with the results obtained for the lobe magnetic field, we discuss the changes in the large‐scale distribution of the neutral sheet current during the course of major substorms: During the expansion onset of substorms, the neutral sheet current increases in the near‐Earth magnetotail and decreases in the midmagnetotail: The neutral sheet current subsequently decreases in both the near‐Earth and middistant tail regions during the recovery phase of substorms, where the current reduction is particularly evident near R = 6 RE. It is also suggested that not only in the near‐Earth magnetotail but in the midmagnetotail the cross‐tail current is significantly enhanced during intense storms.
Investigating seismoionospheric effects on a long subionospheric path
Authors: Clilverd Mark A. , Rodger Craig J. , Thomson Neil R.
Published: September 2002

We examine the possibility of earthquake precursors influencing the subionospheric propagation of VLF transmissions. We consider the long (12 Mm) path from northeastern United States to Faraday, Antarctica (65°S, 64°W), during 1990–1995 and investigate the subionospheric amplitude variation of signals from the NAA communication transmitter (24.0 kHz, 1 MW) in Cutler, Maine, with particular emphasis on possible changes induced by seismic events occurring in South America. We have analyzed the changes in timing of modal minima generated by the passage of the sunrise terminator over the Andes, i.e., the “VLF terminator time” (TT) method. The anomalous variations in timing throughout the year are of a size and occurrence frequency similar to those previously reported, i.e., ±0.5–1 hour and 1–2 per month. However, we find that in these anomalous cases, the time of the sunrise modal minimum does not change significantly, but rather, the minimum becomes insufficiently deep to be detected, and the time of the next nearest minimum is logged. Our analysis indicates that the occurrence rate of successful earthquake predictions using the TT method cannot be distinguished from that of chance. Additionally, the level of false earthquake prediction using the TT method is high.
Multispectral tomographic imaging of the midlatitude aurora
Authors: Semeter Joshua , Mendillo Michael , Baumgardner Jeffrey
Published: September 2002

The CEDAR Optical Tomographic Imaging Facility (COTIF) consists of a midlatitude chain of meridional imaging spectrographs whose slit apertures subtend a common volume in the ionosphere. The facility provides a set of simultaneous multispectral brightness measurements that are used in a two‐dimensional tomographic reconstruction (horizontal versus vertical) of auroral and airglow features in the midlatitude, subauroral ionosphere. A tomographic inversion method is summarized which regularizes the vertical (field‐aligned) dimension using a nonlinear low‐dimensional basis expansion. Three experimental results are presented for atomic oxygen 557.7‐ and 630.0‐nm emissions at midlatitude. Time sequences of the reconstructed emission fields illustrate the variability in O I excitation patterns in both the diffuse aurora and SAR arcs. The results are interpreted with respect to outstanding issues associated with midlatitude aurora. Extracted vertical profiles are compared with published experimental and theoretical results.
Midlatitude E region plasma accumulation driven by planetary wave horizontal wind shears
Authors: Shalimov Sergei , Haldoupis Christos , Voiculescu Mirela , Schlegel Kristian
Published: September 2002

Recent findings suggested the possibility that planetary waves play a role in the occurrence of midlatitude sporadic E layers. To account for this, we propose here a new mechanism for large‐scale accumulation of metallic ions in the midlatitude E region ionosphere driven by planetary waves in the lower thermosphere. In this process, the plasma is forced to converge horizontally and accumulate inside areas of positive vorticity set up by cyclonic neutral wind shears within a planetary wave. In its simplest form, the proposed model is similar to the well‐known vertical wind shear mechanism of Es formation, but with the geometry “turned on its side.” Because of the long times required for ambipolar diffusion, the new mechanism can lead to significant plasma accumulation, acting as complementary to the vertical wind shear process so that dense Es can form more efficiently and frequently. The present model provides a physical base for understanding the long‐term periodicities in occurrence and also the seasonal dependence of strong sporadic E layers at midlatitude.
A comparative study of four‐field and fully compressible magnetohydrodynamic turbulence in the solar wind
Authors: Bhattacharjee A. , Ng C. S. , Ghosh S. , Goldstein M. L.
Published: September 2002

A four‐field system of equations has been recently derived from the compressible magnetohydrodynamic (MHD) equations to describe turbulence in the solar wind. These equations apply to a plasma permeated by a spatially varying mean magnetic field when the plasma beta is of the order unity or less. In the presence of spatial inhomogeneities, the four‐field equations predict pressure fluctuations of the order of the Mach number of the turbulence, as observed by Helios 1 and 2. It is shown that inhomogeneities that occur only in the form of pressure‐balanced structures cannot account for density fluctuations as large as the Mach number of the turbulence. Numerical predictions from the four‐field and compressible MHD equations are compared using the same initial condition. The comparison shows that the predictions of the four‐field model are qualitatively consistent with the predictions of the compressible MHD model and suggests that the four‐field model is a viable model of compressible turbulence in the solar wind.
Simultaneous K and Ca lidar observations during a meteor shower on March 6–7, 1997, at Kühlungsborn, Germany
Authors: Gerding M. , Alpers M. , Höffner J. , von Zahn U.
Published: September 2002

We report about observations of meteor trails by ground‐based lidars which were obtained with two metal resonance lidars monitoring simultaneously the same air column at meteor trail heights. The lidars are located at the site of the Leibniz‐Institute of Atmospheric Physics (54°N, 12°E). More specifically, we have used K and Ca lidars to study meteor trails with respect to (1) their absolute K or Ca abundances, (2) their duration in the laser beams, (3) the altitude distributions of the K and Ca trails, and (4) the reaction of the regular K and Ca layers to the occurence of a (yet unknown) meteor shower. Lidar observations during the night of March 6–7, 1997, began around 1820 UT. They indicated the start of an unknown meteor shower at ∼2200 UT, which we could observe until 0430 UT of March 7. Within 4 h after 2200 UT, the column densities of the regular K and Ca layers increased by factors of 2 and 4, respectively. During the period of the shower, our lidars registered 26 Ca trail events, but only 2 K trail events. Hence we observe for the two metals a noticeable difference between their column density enhancements and rates of trails. The rate of Ca trails was quite similar to that observed by our lidars near the peak of the Lyrids on April 22–23, 1997. The Ca trails were observed in the altitude range from 81 to 98 km with a mean altitude of 89.6 km. In all of the lidar‐observed meteor trails, it was either K or Ca which could be detected. Metal densities in the trails ranged from ∼90 to 360 cm−3 in the case of K and from 50 to 2700 cm−3 for Ca.
O+ phase bunching as a source for stable auroral arcs
Authors: Silevitch Michael B. , Rothwell Paul L. , Block Lars P. , Fälthammar Carl‐Gunne
Published: September 2002

We propose a model to explain how ion dynamics create an Alfvén wave generator in the equatorial region that can be applied to the stable arc problem. For example, in the earthward drifting magnetotail plasma, phase bunching of O+ ions (and to a much lesser extent of the H+ ions) can be caused by a weak (∼1×10−9 Vm−2) electric field gradient [Rothwell et al., 1994]. This leads to density striations in the GSM frame. O+ density striations in the earthward drifting plasma frame are seen as a tailward propagating source of Alfvén waves where the hydrogen ions provide the polarization current of the wave. A transformation to the GSM frame will yield a static, oblique wave structure similar to that previously treated. The waves propagate from the equatorial region to both ionospheres where they are reflected. The ionospheric boundary condition when combined with a magnetospheric boundary condition allows a solution of the wave amplitudes in terms of the striation structure. The frequency of the Alfvén wave and the associated wavelengths are also determined by the striation driver. We find that the magnitude of the parallel current density at the ionosphere has a spatial resonance when the distance between the ionosphere and the equatorial plane is equal to a quarter wavelength along Bo. In that case, the magnitude of the parallel current density at the ionosphere is of the order of 10 μA m−2 and peaks for striation wavelengths (as mapped to the ionosphere) of 10–40 km, which is comparable to the transverse scale of auroral arcs. The associated Poynting flux incident on the ionosphere is found to be ∼ 2 mWm−2 and represents a net transfer of energy from the magnetosphere to the ionosphere as recently observed by experimenters studying substorm onsets. We find that in the steady state the power extracted from the bulk flow to power the arc is balanced by energy provided by the solar wind through the cross‐tail electric field.
Universal time variations of the auroral electrojet indices
Authors: Ahn B.‐H. , Kroehl H. W. , Kamide Y. , Kihn E. A.
Published: September 2002

Using the hourly mean AE indices for the past 20 years, amounting to a total of 175,296 hours, we examine how the longitudinal station gaps of the present AE network affect the ability to monitor accurately the auroral electrojets. The latitudinal shift of the auroral electrojet location with magnetic activity also affects the reliability of the AE indices. These combined effects would result in pronounced universal time (UT) variations of the AE indices. By counting the number of occurrences recorded during the given ranges of activity, say every 100 and 200 nT for the AU and AL indices, respectively, for each hour of universal time, the UT variations of the two indices are examined separately. The result demonstrates clearly that they are strongly dependent upon UT. Furthermore, it is noted that the equatorward expansion of the auroral electrojets is more responsible for the UT variation than are the longitudinal station gaps. For the range of the magnetic activity levels examined in this study, i.e., 0 to 500 nT and 0 to −1000 nT for the AU and AL indices, the centers of the eastward and westward electrojets seem to be located within the latitudinal ranges of 71°–65° and 68°–62°, respectively. The seasonal change of ionospheric conductance also contributes to the UT variation, particularly that of the AL index. While maintaining a similar variation pattern, the amplitude of the variation increases during winter and decreases during summer. It indicates that the UT variation of the AL index is more serious during winter than summer. With more AE stations being located within the former range than the latter, it is easily understood why the AL index is more strongly dependent on UT than is the AU index. Considering such a latitudinal distribution, it is highly probable that the present AL indices often underestimate disturbed conditions during specific universal time intervals, particularly 0200–0800 UT.
Patchy reconnection and evolution of multiple plasmoids in the Earth's magnetotail: Effects on near‐Earth current system
Authors: Rastätter L. , Hesse M.
Published: September 2002

Reconnection during substorms in the near‐Earth magnetotail occurs in a complicated way. Local instabilities can trigger anomalous resistivity in several spatial regions, which may initially be well separated from each other. As time progresses, the resulting plasmoids grow larger until they reach the dimension of the entire system and start to interact with each other. Satellite observations show that this expansion occurs rapidly during the downtail travel after plasmoid development when −10 RE > XGSM > −30 RE [Ieda et al., 1998]. In this paper we study the interaction of two and three plasmoids and their coalescence, as well as the current system seen near the Earth brought about by the reconnection regions. In the numerical experiments, resistivity is prescribed at localized patches which are placed at different downtail locations. We compare the magnitude of the influence the reconnection sites have on the near‐Earth current system. We find that not only the relative position of the resistivity patches but also the absolute distance from the Earth play roles in the relative amplitude of the field‐aligned currents associated with the individual reconnection events.
Techniques for galactic dust measurements in the heliosphere
Authors: Grün Eberhard , Landgraf Markus , Horányi Mihaly , Kissel Jochen , Krüger Harald , Srama Ralf , Svedhem Håkan , Withnell Peter
Published: September 2002

Galactic interstellar dust (ISD) is the major ingredient in planetary formation. However, information on this important material has been extremely limited. Recently, the Ulysses dust detector has identified and measured interstellar dust outside 1.8 AU from the Sun at ecliptic latitudes above 50°. Inside this distance it could not reliably distinguish interstellar from interplanetary dust. Modeling the Ulysses data suggests that up to 30% of dust flux with masses above 10−16 kg at 1 AU is of interstellar origin. From the Hiten satellite in high eccentric orbit about the Earth, there are indications that ISD indeed reaches the Earth's orbit. Two new missions carrying dust detectors, Cassini and Stardust, will greatly increase our observational knowledge. In this paper we briefly review instruments used on these missions and compare their capabilities. The Stardust mission [Brownlee et al., 1996] will analyze the local interstellar dust population by an in situ chemical analyzer and collect ISD between 2 and 3 AU from the Sun. The dust analyzer on the Cassini mission will determine the interstellar dust flux outside Venus' orbit and will provide also some compositional information. Techniques to identify the ISD flux levels at 1 AU are described that can quantify the interstellar dust flux in high Earth orbit (outside the debris belts) and provide chemical composition information of galactic dust.
Interaction between artificial ionospheric irregularities and natural MHD waves
Authors: Ponomarenko Pavlo V. , Yampolski Yuri M. , Zalizovsky Andrey V. , Hysell Dave L. , Tyrnov Oleg F.
Published: September 2002

This paper concerns the interaction between small‐scale artificial ionospheric inhomogeneities and large‐scale natural MHD waves (geomagnetic pulsations). The inhomogeneities were produced at F region altitudes by powerful HF radiation from the Sura heating facility in Russia. Diagnostics were provided by resonant scattering of decametric radio waves emitted by several broadcast and dedicated transmitters. The scattered waves were received at the UTR‐2 radio telescope in Ukraine with a multichannel coherent receiver equipped with highly directional phased antenna arrays. This instrument allowed us to investigate spatial and temporal variations in intensity and frequency of the signals scattered by the inhomogeneities with the scale size of l ≃ 10 m. Previous investigations have shown the existence of quasi‐sinusoidal variations in the signal frequency associated with the E × B plasma drift due to the MHD wave. In this study, we investigate similar periodicities in the signal intensity, which in some cases were observed simultaneously with the frequency variations. However, unlike the frequency variations, the intensity oscillations appeared to be considerably nonuniform within the pumped region of the ionosphere with a horizontal scale size of L ≃ 100 km. We discuss different explanations for the detected effect and suggest that its most probable cause is a transformation of the MHD wave energy into the energy of the inhomogeneities.
Advanced Meteor Orbit Radar observations of interstellar meteoroids
Authors: Baggaley W. Jack
Published: September 2002

The Advanced Meteor Orbit Radar (AMOR) facility is providing an extensive data base of both the geophysical (atmospheric trajectory and velocity, height, ionization characteristics, etc.) parameters and astronomical parameters (heliocentric orbital elements, etc.) of Earth‐impacting meteoroids of limiting particle radius 20 μm. This continuous operation multi‐station complex provides an incisive probe of interplanetary dust orbital characteristics. Close calibration using meteoroid stream orbital elements delineated by other techniques (photographic, video, TV) permits robust dynamical information to be established. This unique technique allows the identification of the source geometry of the influx of extra‐solar system particles: a general background influx from southern ecliptic latitudes exists with enhanced areas that appear to be discrete sources. The dominant compact directional inflow appears from the direction of the main‐sequence debris‐disk star β Pictoris.
Interstellar dust properties derived from mass density, mass distribution, and flux rates in the heliosphere
Authors: Mann Ingrid , Kimura Hiroshi
Published: September 2002

We apply recent in situ measurements of the mass density, mass distribution, and flux rate of dust particles that enter the solar system, i.e., the heliosphere, to study the properties and dynamics of the interstellar dust flux into the solar system. We show that depletion at the heliopause alters the flux for particles with mass, m < 10−18 kg, solar magnetic field deflection within the heliosphere is expected for particles with mass m < 10−17 kg, and radiation pressure repulsion is particularly important for particles with masses 10−17 kg < m < 10−16 kg. Gravitational focusing by the Sun is expected for larger particles (m > 10−15 kg). We show that early measurements that provided evidence for the focusing are in agreement with present estimates of the interstellar dust flux. There is clear observational evidence for all listed effects and detailed studies would be possible on the basis of more extensive measurements. We compare the in situ measurements of absolute density and mass distribution to the parameters determined for the interstellar medium (ISM). While the lack of small particles in the measured mass distribution may be a result of their depletion, the detection of larger grains may support models of collisional particle growth in the ISM. As well, interstellar particles that were found in meteorites yield evidence for the existence of large grains in the ISM, although they may be of completely different origin. We infer from the absolute densities that the dust reflects the conditions of the ISM on large scales rather than those of the local interstellar medium (LISM) in which the Sun is embedded. This supports theoretical results that the gas‐dust friction scales in interstellar space exceed the extension of the LISM. From calculations of radiation pressure forces and comparison to the detected mass distribution we conclude that the detected interstellar dust particles are better described as composite grains rather than as particles that consist of pure materials, which further supports the model of collisional particle growth.
Statistical studies of plasma waves and backstreaming electrons in the terrestrial electron foreshock observed by Geotail
Authors: Kasaba Y. , Matsumoto H. , Omura Y. , Anderson R. R. , Mukai T. , Saito Y. , Yamamoto T. , Kokubun S.
Published: September 2002

We present statistical studies of the direction finding of 2ƒp radiation and the spatial distribution of plasma waves and energetic particles in the terrestrial electron foreshock observed by Geotail. First, we investigate the geometry of the electron foreshock which is assumed to be the “2ƒp radio source.” The 2ƒp radio source is likely to be in the leading region of the electron foreshock where the most intense Langmuir waves are observed. The Langmuir wave activities and the population of energetic electrons gradually decrease in the region beyond 10 RE from the contact point. The decreasing rate of Langmuir wave activity is very small, about 10−0.008/RE. We also find that in the region around the contact point of the tangential interplanetary magnetic field (IMF) lines and the bow shock, the observed amplitude of the 2ƒp radiation seems to become weak. We think that it is due to the weak activities of Langmuir waves in the region close to the contact point and/or the directivity of 2ƒp radiation along the tangential IMF line. Next, we investigate the influence of the solar wind conditions on the activities in the electron foreshock. We confirm a positive correlation of the 2ƒp radio activity with the solar wind kinetic energy flux and a decrease of 2ƒp radio activity with decreasing IMF cone angle resulting in IMF lines tangent to the far flank of the bow shock. The 2ƒp radio activity is more affected by both parameters than the amplitude of Langmuir waves is affected. This suggests that the 2ƒp radio emissivity is very sensitive to the energy of Langmuir waves as expected from the generation process of 2ƒp radiation. Such high sensitivity also supports the concentration of the radio emissivity in the leading region of the electron foreshock and the limitation of the radio source extension along the magnetic field line. We also reinvestigate the comparison between the terrestrial and Venusian foreshocks. The differences between them suggest the nonsimilarity of shocks with different sizes.