Proceedings of the NATO Advanced Research Workshop on Multiscale processes in the Earth’s magnetosphere: From INTERBALL to CLUSTER, held in Prague, Czech Republic, from 9 to 12 September 2003.
The past forty years of space research have seen a substantial improvement in our understanding of the Earth’s magnetosphere and its coupling with the solar wind and interplanetary magnetic field (IMF). The magnetospheric structure has been mapped and major processes determining this structure have been defined. However, the picture obtained is too often static. We know how the magnetosphere forms via the interaction of the solar wind and IMF with the Earth’s magnetic field. We can describe the steady state for various upstream conditions but do not really understand the dynamic processes leading from one state to another. The main difficulty is that the magnetosphere is a complicated system with many time constants ranging from fractions of a second to days and the system rarely attains a steady state. Two decades ago, it became clear that further progress would require multi-point measurements. Since then, two multi-spacecraft missions have been launched — INTERBALL in 1995/96 and CLUSTER II in 2000. The objectives of these missions differed but were complementary: While CLUSTER is adapted to meso-scale processes, INTERBALL observed larger spatial and temporal scales.
However, the number of papers taking advantage of both missions simultaneously is rather small. Thus, one aim of the workshop “Multiscale processes in the Earth’s magnetosphere: From INTERBALL to CLUSTER” hosted by Charles University in Prague, Czech Republic in September 2003 was to bring the communities connected with these projects together to promote a deeper cooperation. The leaders of projects presented summaries of the achievements made by their investigations and demonstrated the special capabilities of these missions to fulfill particular requirements. Other key speakers emphasized the importance of multipoint measurements for the research in their particular areas. The second aspect of the meeting was to stress the importance of the solar wind input on magnetospheric processes.
In course of the above workshop, 21 invited or solicited lectures, 14 oral contributions, and 18 posters were presented and 17 of these presentations were chosen for publication in the volume of the NATO Science Series which you are now reading. We hope that this volume brings not only a summary of INTERBALL and CLUSTER achievements but that it will serve as a useful aid for planning of further investigations and preparation of new multisatellite missions.
We gratefully acknowledge the funds provided by the NATO Scientific Affair Division for this workshop. We would like to express our thanks to all participants for their contributions to the success of the workshop and all the authors who submitted their manuscripts for publication in this volume. We acknowledge with thanks the effort of numerous reviewers who helped us to improve the readability and scientific quality of all contributions, namely: Elizaveta E. Antonova, Daniel Berdichevsky, Natalia L. Borodkova, Mohammed Boudjada, Patrick Canu, James Chen, Giuseppe Consolini, Charles Farrugia, Althanasios Geranios, Chaosong Huang, Christian Jacquey, Alan J. Lazarus, Janet Luhmann, Volt N. Lutsenko, Jan Merka, Karim Mezaine, Patrick T. Newell, Steven M. Petrinec, Anatoli A. Petrukovich, Tai Phan, Lubomir Prech, Patricia Reiff, John D. Richardson, Jana Safrankova, Victor A. Sergeev, James A. Slavin, Charles W. Smith, Paul Song, Yan Song, Marek Vandas, Shinichi Watari, Georgy N. Zastenker, Eftyhia Zesta. Last but not the least, we would like to thank Jana Safrankova for careful organization of the reviewing process and Jiri Pavlu for extended technical assistance in preparation of the final manuscript.
JEAN-ANDRE SAUVAUD
ZDENEK NEMECEK
Abstract: Interplanetary coronal mass ejections (ICMEs) evolve as they propagate outward from the Sun. They interact with and eventually equilibrate with the ambient solar wind. One difficulty in studying this evolution is that ICMEs have no unique set of identifying characteristics, so boundaries of the ICMEs are difficult to identify. Two characteristics present in some ICMEs but generally not present in the ambient solar wind, high helium/proton density ratios and low temperature/speed ratios, are used to identify ICMEs. We search the Helios 1 and 2, WIND, ACE, and Ulysses data for ICMEs with these characteristics and use them to study the radial evolution of ICMEs. We find that the magnetic field magnitude and density decrease faster in ICMEs than in the ambient solar wind, but the temperature decreases more slowly than in the ambient solar wind. Since we also find that ICMEs expand in radial width with distance, the protons within ICMEs must be heated. Scale sizes for He structures are smaller than for proton structures within ICMEs.
Key words: ICME; solar wind.
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Abstract: The solar wind interaction with planetary magnetospheres is a multifarious topic of which our understanding continues to grow as we obtain more detailed observations and more capable numerical codes. We attempt to explain how the system functions by examining the output of models of increasing sophistication. A gasdynamic numerical model produces a standing bow shock in front of a fixed impenetrable obstacle. The post-shock flow is heated and deflected but no plasma depletion layer is formed in the subsolar region contrary to observations. If magnetic forces are included, then a self-consistent obstacle size can be produced and plasma depletion extends all the way to the subsolar region. While a standing slow mode wave has been reported in the subsolar region, it appears that such a wave is not essential to the formation of a subsolar plasma depletion layer. Both the gasdynamic and magnetohydrodynamic models are self-similar. They do not change with the size of the obstacle. However, in the real solar wind interaction we expect that the relative scale size of ion motion and the radius of the obstacle will change the nature of the interactions. Hybrid simulations allow this multiscale coupling to be explored and shrinking the size of the obstacle relative to the gyroradius enhances the role of kinetic processes. Phenomena such as upstream ions, plasma sheet formation, and reconnection can be found in surprisingly tiny magnetospheres. Finally, we contrast how the magnetospheres of the Earth and Jupiter are powered. In the former case the solar wind interaction is very important and the latter case much less so.
Key words: magnetosphere; solar wind interaction; gasdynamic simulation; magneto-hydrodynamic simulation; hybrid simulation; Earth; Jupiter.
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Abstract: Several new features of the solar wind were found in the Interball project by multipoint observations and using high-resolution plasma measurements onboard Interball-1/Magion-4 satellites. These results allow us to suggest some new concepts of solar wind propagation and its interaction with the magnetosphere, namely:
Key words: solar wind; solar-terrestrial relations; interplanetary magnetic field; foreshock; magnetosheath.
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Abstract: The study of the propagation of interplanetary disturbances, shocks and discontinuities, through the magnetosheath is critical to improve our understanding of the Sun-Earth connected system. In this paper, the current status of both theoretical and observational studies in this crucial area is reviewed separately for interplanetary shocks and discontinuities. It is suggested that tangential and rotational discontinuities suffer significant geometrical distortions traveling between the Earth’s bow shock and magnetopause. On the other hand, the pressure fronts of the transmitted interplanetary shocks most likely remain unaltered promising the possibility of improved space weather forecasting accuracies.
Key words: interplanetary magnetic field; interplanetary shock; magnetosheath; interplanetary discontinuity.
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Abstract: We review the statistical processing of four years of INTERBALL-1 observations in the nightside magnetosheath and discuss peculiarities of the magnetosheath ion flux and magnetic field radial profiles. Our investigations reveal that the magnetosheath ion flux profile is similar to but flatter than that predicted by the gasdynamic and MHD models. The most pronounced difference seen at the bow shock region is attributed to kinetic processes not involved in these models. On the other hand, the magnetic field magnitude profile is nearly constant. It indicates that magnetic forces contribute significantly to the formation of the magnetosheath flow and frozen-in approximation should be used with a care. According to our investigations, the rise of the ion flux from the magnetopause toward the bow shock is much steeper during intervals of a radial IMF orientation.
Statistical processing has shown (1) the limitations of gasdynamic and MHD models, (2) the conditions favorable for the creation of a plasma depletion layer adjacent to the flank magnetopause, (3) a strong dawn-dusk asymmetry of the ion fluxes, (4) that the presence of high-energy particles influences the total ion flux only weakly, and (5) that the coupling between high-energy particles and the ion flux and/or magnetic field fluctuation level is strong.
Key words: magnetosheath; energetic particles; ion flow; magnetic field; radial profile; bow shock; magnetopause.
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Abstract: Theory predicts that abrupt variations in the solar wind dynamic pressure trigger widespread compressional cavity mode resonances within the magnetosphere. We inspect solar wind and magnetospheric observations at the times of previously reported events seen in ground magnetograms. We find evidence for abrupt solar wind pressure variations in the form of direct observations of solar wind dynamic pressure, motion of the bow shock, or fluctuations in the location of the foreshock. We also find evidence for widespread compressions of the magnetospheric magnetic field in observations by geosynchronous spacecraft. However, in contrast to the predictions of the model for an abrupt increase in wave activity followed by a gradual decay, we find that the periodicity seen in previously reported events occurs primarily in response to repeated impulsive excitations. If cavity mode resonances are present, they dissipate very rapidly within two cycles.
Key words: foreshock; cavity mode resonances; pulsations.
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Abstract: The low-latitude boundary layer (LLBL) is encountered as an interface between two plasma regions – the magnetosheath and plasma sheet and thus contains a mixture of both plasma populations. Several mechanisms have been discussed as candidates for a formation of the LLBL. These mechanisms can be divided into magnetic reconnection between the magnetospheric and magnetosheath magnetic fields, impulsive penetration of magnetosheath plasma, and viscous/diffusive mixing of plasma populations at the magnetopause. The observed fluctuations of plasma parameters inside the LLBL are attributed either to transient nature of the phenomena forming the layer or to sweeping of deformations of the magnetopause or inner edge of the LLBL along the spacecraft.
The INTERBALL-1/MAGION-4 satellite pair separated by several thousands of kilometers crossed the LLBL region in different local times and their twopoint observations allow us to distinguish between spatial and temporal changes. The present paper surveys results achieved so far. They suggest that the most probable source of the LLBL plasma is reconnection occurring at high latitudes. This reconnection can supply the nightside as well as dayside LLBL during intervals of northward oriented and/ or horizontal IMF.When the IMF BZ component becomes negative, the reconnection site moves toward lower latitudes but it can move to the subsolar point only during exceptional intervals of negative BZ dominated IMF.
Key words: LLBL; plasma depletion layer; reconnection; plasma mantle; magnetopause.
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Abstract: After 2.5 years of operations, the Cluster mission is fulfilling successfully its scientific objectives. The mission, nominally for 2 years, has been extended 3 more years, up to December 2005. The main goal of the Cluster mission is to study in three dimensions the small-scale plasma structures in the key plasma regions in the Earth’s environment: solar wind and bow shock, magnetopause, polar cusps, magnetotail, and auroral zone. During the course of the mission, the relative distance between the four spacecraft will vary from 100 km up to a maximum of 18,000 km to study the physical processes occurring in the magnetosphere and its environment at different scales. The inter-satellites distances achieved so far are 600, 2000, 100, 5000 km and recently 250 km. The latest results, which include the derivation of electric currents and magnetic curvature, the analysis of surface waves, and the observation of reconnection in the tail and in the cusp will be presented. We will also present the description of the access to data through the Cluster science data system and several public web servers, and the future plans for a Cluster archive.
Key words: solar wind; bow shock; magnetopause; polar cusp; magnetotail; auroral zone.
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Abstract: Cusp properties during periods of By dominant IMF are investigated, since previous studies focus mostly on IMF Bz. The model-data comparisons for various IMF configurations show that the model captures the large-scale features of the particle precipitation very well, not only in the cusp region, but also in other open-field line regions such as the mantle, polar rain, and open-field line low-altitude boundary layer (LLBL). When the IMF is strongly duskward/dawnward and weakly southward, the model predicts the occurrence of a double cusp near noon: one cusp at lower latitude and one at higher latitude. The lower latitude cusp ions originate from the low-latitude magnetosheath whereas the higher latitude ions originate from the high-latitude magnetosheath. The lower latitude cusp is located in the region of weak azimuthal E×B drift, resulting in a dispersionless cusp. The higher latitude cusp is located in the region of strong azimuthal and poleward E×B drift. Because of a significant poleward drift, the higher latitude cusp dispersion has some resemblance to that of the typical southward IMF cusp. Occasionally, the two parts of the double cusp have such narrow latitudinal separation that they give the appearance of just one cusp with extended latitudinal width. From the 40 DMSP passes selected during periods of large (positive or negative) IMF By and small negative IMF Bz, 30 (75%) of the passes exhibit double cusps or cusps with extended latitudinal width. The double cusp result is consistent with the following statistical results: (1) the cusp’s latitudinal width increases with |IMF By| and (2) the cusp’s equatorward boundary moves to lower latitude with increasing |IMF By|.
Key words: double cusp; cusp latitudinal width; cusp equatorward boundary; cusp model; spatial feature.
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Abstract: The cusp energetic particles (CEP) have been observed in the dayside highaltitude cusp region, showing orders of the magnitude increase of ion intensities with energies from 20 keV up to 10 MeV. Associated with these charged particles are large diamagnetic cavities with significant fluctuations of the local magnetic field strength. The CEPs may provide a answer to a long-standing unsolved fundamental issue about the origin of upstream energetic ions. On June 28, 1999, theWIND spacecraft (near the forward libration point) observed a sudden increase (by more than one order of magnitude) of the solar wind pressure at about 4:45 UT and an upstream ion event at 5:23-5:45 UT, the INTERBALL-1 spacecraft located just upstream of the bow shock in the morningside measured an upstream ion event from 5:16 UT to 6:00 UT, the GEOTAIL spacecraft in the afternoonside near the bow shock detected an upstream ion event from 5:50 UT to 6:16 UT, while the POLAR satellite at 7 hours of magnetic local time detected an energetic particle event in the high-altitude region associated with turbulent diamagnetic cavities from 5:12 UT to 6:30 UT. Energetic oxygen ions of both ionospheric and solar wind origin were observed by the POLAR spacecraft during this event period. The energetic ions and the associated turbulent magnetic field are very similar to what was found in the high-altitude dayside cusp region. It is argued that the bow shock is not the main source of energetic ions in these upsteam events since their energy spectra are independent of the solar wind velocity and their intensities are independent of the bow shock geometry and solar wind pressure. The event onset was first detected in the cusp by POLAR at 5:12 UT, then near the bow shock in the morningside by INTERBALL-1, and then in far the upstream by WIND. The measured energetic ion intensity decreased with increasing distance from the cusp before 5:42 UT. At 5:50 UT, GEOTAIL detected the event onset that showed an energy dispersion, suggesting a drift effect. These observational facts together with the IMF directions suggest that these upstream energetic ions most likely came from the cusp.
Key words: cusp; energetic particles; bow shock; upstream events.
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Abstract: In the present paper, we analyze the magnetic cloud (MC) at 1 AU on November 9, 1997. The appearance of a hotter and dense part (dense filament), with a radial extent 106 km, immediately behind the frontal part of the MC, is a distinctive feature of the event. The INTERBALL-Auroral probe had a chance to observe field-aligned currents in the mid-altitude magnetosphere during the substorm expansion phase intensification related to the dense filament. We emphasize the appearance of unusual “N”-shape magnetic structure, duration 3 minutes, amplitude 50 nT between the field-aligned current region 1 and the magnetosphere lobe in the late evening hours. The “N”-shape structure is related to a significant amount of wave energy transfer red towards the ionosphere.
Key words: coronal mass ejection; solar wind; magnetosphere/ionosphere; wave-energy transfer.
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Abstract: We present the results of a case study of transient event observed in high-latitude ground magnetograms on May 8, 1997. We use the GOES-8, GOES-9, and GOES-10 spacecraft to identify corresponding signatures in high-time resolution geosynchronous magnetometer observations. We determine the event’s spatial extent and velocity and show that the direction of event motion through the noon magnetosphere and ionosphere was similar. Wind, Geotail and Interball solar wind observations indicate that the interplanetary magnetic field (IMF) orientation controls the direction of transient event motion near local noon. The transient event corresponded to the motion of the foreshock away from the subsolar bow shock.
Key words: magnetosphere; transient event motion; high-latitude ground magnetograms; IMF orientation; solar wind discontinuity.
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Abstract: The MHD turbulence of the Earth's plasma sheet in the magnetotail has been examined by satellite measurements of magnetic fields and plasma flows; the measured properties of this turbulence are reviewed. A theoretical analysis indicates that the MHD turbulence in the plasma sheet is a very unusual turbulence because of (1) the very limited range of spatial scales available for MHD flows and (2) the dissipation of vorticity by magnetosphere-ionosphere coupling, which introduces (a) a time dependence to the rate-of-dissipation of a flow, (b) dissipation at all spatial scales, and (c) dissipation rates that depend on the sign of the vorticity. Using a theoretical analysis of flows in the magnetotail and using some transmission-line experiments, two computational models of the plasma-sheet turbulence are being constructed to study the basic properties of this unconventional turbulence. These computational models are discussed extensively. New aspects of the study of plasma-sheet turbulence that are contained in this report are (a) corrected estimates of the Alfvenicity of the turbulence, (b) a strengthened argument that Alfven waves are not important for the dynamics of the turbulence (i.e. that it is a 2D turbulence), (c) an extended discussion about the time dependence of magnetosphere-ionosphere coupling, (d) a description of transmission-line experiments performed to clarify some properties of magnetosphere-ionosphere coupling, and (e) a discussion of numerics for building computer simulations of the magnetotail turbulence.
Key words: MHD turbulence; plasma sheet; magnetosphere; viscoelasticity; non-Newtonian; magnetosphere-ionosphere coupling; simulations; GOY; transmission line.
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Abstract: During periods of quiet magnetic activity, a cold plasma layer with densities reaching 1-3 cm-3 is encountered on the magnetospheric side of the dayside magnetopause. Direct density measurements from the plasma frequency indicate that this layer can have a width exceeding 1 RE in the direction normal to the magnetopause. Plasma composition measurements indicate that the major detected ions are H+, He+ and O+. These cold ionospheric ions show a repetitive pattern of energy changes. While the magnetopause is approaching the satellites, their energy increases from below the detector low-energy threshold up to about 100 eV for protons. After the passage of the satellites into the magnetosheath and just following their re-entry into the magnetosphere, the ion energy decreases from about 100 eV for protons down to the lowest detectable energy. This behavior is interpreted as the effect of the electric field associated with the magnetopause motion. The ion motion is set up when the magnetopause is compressed and relaxed when the boundary is going out. Altogether the measurements clearly show that there are hidden plasma populations inside the dayside magnetosphere, at least during quiet geomagnetic conditions. This paper emphasizes the importance to use the determination of the plasma frequency to probe the magnetospheric density. The use of biased low-energy particle detectors located far enough from the satellite body should allow to probe the distribution function of these low-energy ions in future missions.
Key words: thermal plasma; magnetopause; pressure pulses.
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Abstract: Thin current sheets (TCSs) are sites of energy storage and release in the Earth's magnetosphere. A self-consistent analytical model of 1D TCS is presented in which the tension of the magnetic field lines is balanced by ion inertia rather than plasma pressure. The influence of the electron population and the corresponding electrostatic electric fields required to maintain quasineutrality are taken into account under the realistic assumption that electron motion is fast enough to support quasi-equilibrium Boltzmann distribution along field lines. Electrostatic effects can lead to specific features of local current density profiles inside TCS, for example, to their partial splitting. The dependence of electrostatic effects on the electron temperature, the form of electron distribution function, and the curvature of magnetic field lines are analyzed. Possible implications of these effects on the fine structure of current sheets and some dynamic phenomena in the Earth's magnetotail are discussed.
Key words: thin current sheets; nonlinear particle dynamics; electrostatic effects; self-consistent model.
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Abstract: The bursty bulk flows (BBFs), which provide a major contribution to the Earthward convection in the high-beta plasma sheet region of the magnetotail, are nearly uniformly distributed in distance between 40-50 Re and the inner magnetosphere. Most of them are now confirmed to be plasma bubbles, the underpopulated plasma tubes with a smaller value of plasma tube entropy (pVgamma). Many BBFs are visible in the ionosphere due to the associated plasma precipitation and 3d-electric currents, which provides an excellent possibility to study the global dynamics of BBFs by observing their auroral footprints. A number of recent studies, including studies of associated precipitation, convection and field-aligned currents indicate that main mechanism providing a bright optical image of the BBF is the electric discharge (field-aligned electron acceleration) from the dusk flank of the BBF where the intense upward FAC is generated. The auroral signatures have variable forms, with auroral streamers being the most reliable and easily indentified BBF signature. The picture of BBFs emerging from these results corresponds to the powerful (up to several tens kV in one jet) sporadic narrow (2-3 Re) plasma jets propagating in the tail as the plasma bubbles, which are probably born in the impulsive reconnection process but filtered and modified by the interchange process. Penetration of BBFs to less than 6.6 Re distance in the inner magnetosphere was frequently observed, with indications of flow jet diversion and braking (with associated pressure increase and magnetic field compression). Such interaction also creates long-lived drifting plasma structures, particularly those which can be related to torch and omega-type auroras. Role of BBFs in generating other types of auroral structures is briefly discussed.
Key words: magnetotail; plasma sheet; convection; bursty bulk flows; aurora.
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Abstract: The four Cluster Wideband (WBD) plasma wave receivers occasionally observe electromagnetic triggered wave emissions at and near the plasmapause. We present the remarkable cases of such observations. These triggered emissions consist of very fine structured VLF risers, fallers and hooks in the frequency range of 1.5 to 3.5 kHz with frequency drifts for the risers on the order of 1 kHz/s. They appear to be triggered out of the background whistler mode waves (hiss) that are usually observed in this region, as well as from narrowband, constant frequency emissions. Occasionally, identical, but weaker, emissions are seen to follow the initial triggered emissions. When all the Cluster spacecraft are relatively close (< 800 km, with interspacecraft separations of around 100–200 km), the triggered emissions are correlated across all the spacecraft. The triggered emissions reported here are observed near the perigee of the Cluster spacecraft (around 4–5 RE) within about 20 degrees, north or south, of the magnetic equator at varying magnetic local times and generally at times of low to moderate Kp. In at least one case they have been observed to be propagating toward the magnetic equator at group velocities on the order of 5–9 × 107 m/s. The triggered emissions are observed in the region of steep density gradient either leading up to or away from the plasmasphere where small-scale density cavities are often encountered. Through analysis of images from the EUV instrument onboard the IMAGE spacecraft, we provide evidence that Cluster may sometimes be immersed in a low density channel or other complex Examples of the various types of triggered emissions are provided which show the correlations across spacecraft. Supporting density data are included in order to determine the location of the plasmapause. A nonlinear gyroresonance wave-particle interaction mechanism is discussed as one possible generation mechanism.
Key words: triggered emissions; plasmapause; risers; fallers; hooks; Cluster observations.