SM13B-1206 1340h
The structure of the plasma sheet under northward IMF
Studies on the properties of the plasma sheet have shown that it becomes cold and dense (T $<$ 2 keV, n $>$ 1 ${\rm cm}^{-3}$) during extended northward IMF periods and this change in the plasma status is most prominent close to the flanks. When these Cold and Dense Ions (CDIs) are transported earthward, it is reasonable to expect the compression to keep the density high whereas the temperature is elevated by adiabatic heating upon compression. This process should produce Hot-Dense Ions (HDIs) at the inner-edge of the plasma sheet. By defining HDIs with the criteria T $>$ 2 keV, n $>$ 1 ${\rm cm}^{-3}$, HDIs have been searched for using five years data of the Geotail spacecraft. HDIs are indeed found, and when focusing on those HDIs obtained under nominal solar wind dynamic pressures, we find them to appear only on the dawnside plasma sheet inner-edge and that during extended northward IMF intervals. That is, the result supports the idea that HDIs are the inner-magnetosphere extension of CDIs, and further imply that such a connection between the two populations takes place only on dawnside. In this paper, we describe the statistical study that reveals this structure of the plasma sheet. This picture suggesting significant dawn-dusk asymmetry in heating and transport in the magnetotail under northward IMF is also tested by case studies in which we inspect data from fortuitous orbits that transverse more than one key region under steady IMF/SW conditions. We also study data from multi-spacecraft monitoring the key regions simultaneously. Expected features are seen in these independent studies implying that the picture obtained by the statistic study indeed reflects the spatial structure of the plasma sheet under northward IMF.
SM13B-1207 1340h
Observations of Cold Dense Plasma Sheet and Cusp Reconnection During Extended Periods of Northward IMF
During the October 22-25, 2003 interval strong solar activity resulted in several intervals of long duration strongly northward IMF. Using observations from Cluster, Geotail, DMSP, and FAST we investigate the state of the magnetosphere during these extreme solar wind conditions. We find that long duration strongly northward IMF results in global cooling ($<1$ keV) and densification ($\sim$1 cm$^{-3}$) of the entire plasmasheet, at all local times, and at all latitudes, including the neutral sheet. The transition to cold and dense plasma sheet occurs gradually over 3-4 hours following the northward turning of the IMF and the cold dense plasma starts to disappear when the IMF becomes $B_y$ dominated. During the cold dense plasma sheet intervals the FAST satellite observed inverse ion dispersion signatures in the cusp indicative of poleward-of-cusp reconnection, and nearly no polar cap. The observations show good quantitative agreement with global MHD simulations where the CDPS is formed by poleward-of-cusp reconnection capturing magnetosheath plasma which convects to the tail. The size of the lobes also shrank significantly in the simulation due to cusp reconnection.
SM13B-1208 1340h
Formation of the Cold Plasma Sheet in the Near-Earth Magnetotail
We study the cold plasma sheet found in the near-earth magneotail is studied using the 9 years' data from the Geotail spacecraft. While it has been widely known that the cold plasma sheet (ion temperature $<$ 1 keV) is formed near the both flanks under the northward IMF condition, the time scale as well as the mechanism of formation of the cold plasma sheet is now under debate. We show that the time lag from the northward turning of the IMF to the emergence of the cold plasma sheet is as short as a few hours on the both flanks. We further report that the cold plasma is found not only on the flanks but also at times deep inside the midnight plasma sheet only a few hours after the northward turning of the IMF. These observations suggest that some prompt transport process other than diffusion occurs, although diffusive processes may work on plasma transport near the flanks. We also show that the cold plasma sheet emerges on the dawnside during the weak IMF Bz periods, which implies dawn-dusk asymmetry in the formation of the cold plasma sheet.
SM13B-1209 1340h
FAST and Cluster observations of the cold, dense plasma sheet.
The FAST satellite data set has been examined to determine the morphology and formation of a cold, dense plasma sheet (CDPS). Data were examined where the IMF was steady for at least 6 hours (several orbits), where the IMF was northward, and where the FAST orbit was either dawn-dusk or noon-midnight. Several features are observed. Ion spectra in the CDPS often show structure indicating bounce-drift effects similar to that observed at lower latitudes. Although a VDIS signature of lobe reconnection can be observed on the poleward edge of the cusp during some CDPS observations, the VDIS signature often overlaps a higher energy component on the equatorward side, or forms a bowl-shaped signature. On dawn-dusk orbits, the CDPS can be observed to extend to the highest latitudes, although typically some open field lines remain. These data are also compared to several encounters with a CDPS by Cluster. Some of the Cluster events indicate an ionospheric source. Therefore a CDPS may not be just a product of solar wind plasma injections, but may also result from ionospheric outflows.
SM13B-1210 1340h
Geotail Observations of Different Energy Plasma Sheet Ions Near the Nightside Magnetopause: Their Spatial Variations and Dependence on the Solar Wind Parameters
To understand the formation of the nightside low-latitude boundary layer and its role as a particle source to the inner plasma sheet and magnetosphere, we investigated the Geotail fluxes of plasma sheet ions (plasma beta $>$ 1, temperature $>$ 0.1 keV, and Vx $> -$100 km/s) averaged over three different energy ranges (low: 0.1--1 keV, intermediate: 1--10 keV, and high: 10--39 keV) in the region of X = 0 to $-$30 R$_E$ and within 2 R$_E$ inside the magnetopause (70 dusk side and 45 dawn side magnetopause crossings defined by a sharp gradient in Vx). Their dependence on the solar wind data obtained from WIND were also investigated. The solar wind data were averaged over different lengths of hours (from 0.5 to 6 hours) prior to the estimated impact time. On both the dawn and dusk flanks, the low and intermediate energy fluxes are found to be independent of X while the high energy fluxes increase with decreasing $|$X$|$. There is no significant dawn-dusk difference on the magnitudes of the fluxes for both the low and intermediate energy ions, but the magnitude range for the high energy fluxes is seen to be much wider on the dawn flank than on the dusk flank. Only the low energy fluxes on the dawn flank are found to vary with the distance from the magnetopause, with higher flux at smaller distance. On the dawn flank, the low and intermediate energy fluxes during strongly ($|$B$| >$ 5 nT) positive IMF Bz are clearly higher than the fluxes observed during strongly negative IMF Bz while the high energy fluxes have the opposite dependence. This dependence become clearer when the fluxes are correlated with longer periods of IMF data. On the dusk flank, there are not enough events for strongly negative IMF Bz to reveal clear IMF Bz dependence. The low and intermediate energy fluxes are seen to increase (decrease) with solar wind density (solar wind speed), while the high energy fluxes are seen to increase with increasing solar wind speed but have no dependence on solar wind density. No obvious difference is seen when correlated with longer periods of solar wind moment data. That very different spatial and solar wind condition dependence observed in the low and high energy ions suggests their access to the flanks and their variations are controlled by different processes.
SM13B-1211 1340h
Magnetic Reconnection and Kelvin-Helmholtz Modes at the Flanks of the Magnetosphere
The physics of the magnetospheric boundaries is central to the transport of mass, energy, and momentum into magnetosphere. However, for northward IMF the transport of mass into the magnetosphere is not well understood. The two major models for plasma transport into the magnetosphere are cusp reconnection and reconnection within nonlinear Kelvin-Helmholtz vortices. The second transport mechanism has attracted much attention, however, much work has focused on two-dimensional dynamics. In three-dimensions the system has an additional degree of freedom which is important because in a 3D system the orientation k vectors of instabilities is not restricted to a two-dimensional plane. Here we will discuss the interaction and relative importance of Kelvin-Helmholtz modes and magnetic reconnection in a three-dimensional configuration relevant to the flank boundaries of the magnetosphere for preferably northward IMF conditions in the framework of three-dimensional MHD simulations. We will address dynamical properties and relevant signatures of the associated processes.
SM13B-1212 1340h
Rolled-up Kelvin-Helmholtz vortices at the flank magnetopause and their implications for the formation of the low-latitude boundary layer
We report unambiguous evidence for rolled-up Kelvin-Helmholtz (KH) vortices that form in the nonlinear stage of the Kelvin-Helmholtz instability (KHI), based on multi-spacecraft measurements by Cluster at the dusk flank magnetopause during northward interplanetary magnetic field (IMF) periods. With the help of a three-dimensional MHD simulation of the KHI, signatures expected in the KH vortices at the magnetopause, namely, the magnetosheath plasma intruding into the magnetosphere, vortical plasma flow, and magnetic field perturbation pattern associated with the vortices, have been identified. In addition, plasmas of solar wind origin are present in the vicinity of the rolled-up vortices and on the magnetospheric side of the magnetopause. It is getting increasingly clear from theoretical studies that transport of plasma is inevitable in the rolled-up KH vortices. Thus our observations are precisely consistent with the scenario that, under northward IMF conditions, the KHI leads to the formation of the low-latitude boundary layer (LLBL) on the flanks. From comparison between measurements and simulation studies, the thickness of the KHI-associated LLBL is estimated to be roughly 4 $R_E$. We discuss the significance of the results for the LLBL formation during northward IMF and raise remaining questions concerning the magnetopause KHI.
SM13B-1213 1340h
Magnetic reconnection within MHD-scale Kelvin-Helmholtz vortices triggered by electron inertial effects
In understanding the structure of an MHD-scale Kelvin-Helmholtz (KH) vortex, magnetic reconnection within the vortex must not be neglected. Indeed reconnection seems almost unavoidable. Here we have performed two-dimensional two-fluid simulations including finite electron inertial effects to investigate this issue. In the two-fluid system, magnetic reconnection occurs spontaneously because the _efrozen-in_f condition can be broken by the electron inertial effects. An MHD-scale velocity shear is set up and evolution of MHD-scale KH mode is followed with in-plane magnetic component taken into account. In this study, we focus on two cases with different initial magnetic configuration: (1) Initial in-the-plane magnetic field is parallel across the shear layer. (2) The magnetic field is anti-parallel. In both cases, when the Alfven Mach number of the shear is strong enough for the vortex to highly roll-up, magnetic reconnection occurs spontaneously within the vortex by the electron inertial effects. On the other hand, when the Alfven Mach number is not strong, magnetic reconnection occurs only in the anti-parallel case. Furthermore, in this case, magnetic reconnection act to promote the vortex activity to produce substantial effects which are not available when the coupling between the weak instability and reconnection is not possible, that is, substantial effects that are not observed in the parallel case are seen to emerge. These results presented here imply that magnetic reconnection within a MHD-scale vortex plays an important role in determining the MHD-scale structure of the vortex itself.
SM13B-1214 1340h
Turbulent Mixing and Transport of the Solar Wind Plasma : Full Particle Simulation Study of the Kelvin-Helmholtz Instability
Recent in-situ observations often show the mixing of the solar wind and magnetospheric plasmas in the low latitude boundary layer (LLBL), in which the Kelvin-Helmholtz instability is considered to be unstable. Those suggest that LLBL is a candidate for a source of plasmas and the Kelvin-Helmholtz instability plays an crucial role in a new transport mechanism. Even though numerous theoretical and computational studies have challenged to explain it so far, no one succeeded in transport of plasmas over a K-H vortex size and diffusive process that explains the observations. Hence, the transport mechanism of the solar wind plasma into the Earth magnetosphere in the situation of northward IMF has been a hot topic in magnetospheric physics. To elucidate the mixing and transport mechanism of the solar wind plasma we carried out two dimensional full particle simulation of the K-H instability. As a result, the strong density stratification triggered the strong turbulence which was also found in the two-dimensional MHD simulation (Matsumoto and Hoshino, GRL, 2004). The secondary Rayleigh-Taylor instability was found out to be unstable inside the stratified vortex structure and transport the dense solar wind plasma deep inside the magnetosphere. The resultant mixing area of the two plasmas increased anomalously fast as compared with the uniform density case. Hence the density stratification is an important factor for the effective mass transport across the velocity shear layer. The density stratification also introduced the ion kinetic effect in the non-linear stage. In a negative shear layer (the dawn side of the magnetopause) the finite Larmor radius (FLR) effect of the ion stabilized the onset of the secondary R-T instability and thus weakened the mass transport. In this presentation, the dawn-dusk asymmetry in the transport mechanism will be presented in detail as well as the onset mechanism of the turbulent mixing and transport by the K-H instability.
SM13B-1215 1340h
Ionospheric Signatures of LLBL for Northward IMF
Enhanced proton auroras associated with the cusp and LLBL have been simultaneously observed by the IMAGE spacecraft during prolonged periods of northward IMF and large solar wind dynamic pressure on 17 and 18 September, 2000. These auroras map to the vicinity of cusp and LLBL at the magnetopause based on the Tsyganenko 96 model. Plasma observed by DMSP that are associated with the aurora confirm the auroral source regions. The cusp ion spectrum has a sharp spectral peak and the typical low-energy ion cutoff consistent with magnetic merging occurring at the high-latitude magnetopause. In contrast, the precipitating ions from the LLBL are of both solar wind and magnetospheric origins. Their energy spectrum is spectrally complete with a very broad spectral width so that there is no indication of recent merging. Ionospheric convection derived from the SuperDARN radar observations for these auroras shows the typical 4-cell pattern for strongly northward IMF. The cusp and LLBL auroras belong to different convection cells. The former appears in the sunward convection region and the latter shows antisunward flow. This pattern persisted for more than 30 min until scattered radar signals were too weak to discern any useful flow pattern. Magnetic field lines in the cusp aurora are open but field lines are most likely closed in the LLBL aurora. Because of the antisunward convection for the LLBL aurora, it is unlikely that the LLBL was formed by closing the sunward-convecting open cusp field lines in one hemisphere after another high-latitude merging at the other hemisphere. It remains a possibility that wave particle interaction at the magnetopause layer can account for the formation of the LLBL.
SM13B-1216 1340h
Local Time Distribution of Reconnected Field Lines Under Northward IMF Conditions: Implications for Populating the Low Latitude Boundary Layer
Full 3D plasma observations in the northern cusp region are used to study the reconnection process and the in-flow of flux into the magnetosphere during intervals of northward IMF. Several intervals are examined for which He$^{++}$ of solar wind origin appears simultaneously with counter-streaming O$^{+}$ ions of ionospheric origin. The presence of He$^{++}$ indicates that magnetic reconnection had recently occurred (opening the magnetosphere to the solar wind), while the counter-streaming O$^{+}$ observations provides evidence that the magnetic field lines have once again closed. This implies that the magnetic field line has reconnected twice; once in one hemisphere (tailward of the cusp), and then a second time in the opposite hemisphere. The local time distribution of the occurrence of these events provides important information as to where and when magnetopause reconnection for northward IMF is likely to occur, and how solar wind plasma is brought into the low-latitude boundary layer of the outer magnetosphere. These locations are also compared with model predictions.
SM13B-1217 1340h
Separator Reconnection at the Dayside Magnetopause under Northward IMF Conditions
The physics of steady driven magnetic reconnection at the dayside terrestrial magnetopause is addressed. Three dimensional, global magnetohydrodynamics (MHD) simulations of the magnetopause are compared with analytical solutions of the resistive MHD equations corrresponding to magnetic field annihilation driven by an incompressible stagnation point flow. The simulations demonstrate that, under steady southard IMF conditions, and when the plasma resistivity is constant, reconnection occurs near the subsolar point in long, thin Sweet-Parker current sheets via a flux pileup mechanism. Since there is a finite energy in the magnetosheath available to drive the magnetic pileup (and associated fast reconnection), we expect the pileup to saturate, and the reconnection rate to drop, as the upstream plasma pressure drops to accommodate the pileup. Thus, we expect the reconnection rate to stall, the rate vanishing in the infinite Lundquist number limit. Under northward IMF conditions, and when there is a significant y component of the IMF (such that the y and z components of the IMF have comparable magnitudes), the generic magnetic field topology consists of two isolated magnetic nulls which define two separatrix surfaces (the boundaries between open, closed and IMF field lines). Magnetic reconnection (as measured by the parallel component of the magnetic field) seems to be associated with the intersection of these two surfaces, having a local maximum at the subsolar point rather than at the nulls. This configuration appears to be consistent with null-null separator reconnection (a type of component reconnection) rather than "antiparallel" merging. Implications of these results for LLBL observations under northward IMF conditions are discussed.
SM13B-1218 1340h
Dayside boundary layer under northward IMF: A Cluster perspective
It has been proposed that the Low Latitude Boundary Layer (LLBL) was formed by high-latitude reconnection when the IMF is northward. To study the relationship between the low-latitude boundary layer and high-latitude boundary layer under northward IMF condition, we present statistical results based on 3 years of data obtained by Cluster when these spacecraft were in the vicinity of the dayside magnetopause during northward IMF. In total 341 cases of Cluster crossing of Low Latitude Boundary Layer (LLBL) and High Latitude Boundary Layer (HLBL) (according to the definition by Phan et al [1996a,b]) have been analyzed in detail in order to study the relation between the LLBL and the HLBL. The plasma density, temperature, velocity, energetic particle flux and magnetic field geometry change across the magnetopause under northward IMF were analyzed by a superposed epoch analysis. It has been suggested [Zong et al, 2004] that the solar wind plasma density decreases in the magnetospheric boundary region in an exponential mannerwith an e-folding distance of 1000 km during northward IMF in a case study. In this statistical study, we explore further the relation between the distance to magnetopause and the penetration of solar wind plasma inside the magnetopause. Phan, T. D., and G. Paschmann, Low-latitude dayside magnetopause and boundary layer for high magnetic sheath: 1. Structure and motion, J. Geophys. Res.,101, 7801-7815, 1996 Phan, T. D., G. Paschmann, and B. U. O. Sonnerup, Low-latitude dayside magnetopause and boundary layer for high magnetic sheath: 2. Occurrence of magnetic reconnection, J. Geophys. Res.,101, 7817-7828, 1996 Zong, Q.-G., T. A. Fritz, H. Spence, K. Oksavik, Z.-Y. Pu, A. Korth, and P. W. Daly, Energetic particle sounding of the magnetopause: A contribution by Cluster/RAPID, J. Geophys. Res.,109, A04207, 2004
SM13B-1219 1340h
Day-side Aspects of Theta-aurora
Global MHD-modeling has been used to study the magnetospheric effect of IMF changes typically observed in connection with the occurrence of theta-aurora. We find that the observed geometry of the polar cap, to a large extent, can be explained by the changing merging scenario at the dayside. During intervals of almost northward IMF the solar wind flux-tubes reconnect in both hemispheres, thereby converting open lobe field-lines to closed "double reconnected" day-side field-lines. Thus a predominantly northward IMF is associated with a closing up of the magnetosphere from the dayside. The closure occurs preferentially at either the dawn or the dusk-side depending on the sign of IMF By. Subsequent directional changes of the IMF invokes growth of a new polar cap at either the dusk- or the dawn-side depending on the sign of By and Bz.
SM13B-1220 1340h
Particle entry through "Sash" groove simulated by Global 3D Electromagnetic Particle code with duskward IMF By
We made our efforts to parallelize the global 3D HPF Electromagnetic particle model (EMPM) for several years and have also reported our meaningful simulation results that revealed the essential physics involved in interaction of the solar wind with the Earth's magnetosphere using this EMPM (Nishikawa et al., 1995; Nishikawa, 1997, 1998a, b, 2001, 2002) in our PC cluster and supercomputer(D.S. Cai et al., 2001, 2003). Sash patterns and related phenomena have been observed and reported in some satellite observations (Fujumoto et al. 1997; Maynard, 2001), and have motivated 3D MHD simulations (White and al., 1998). We also investigated it with our global 3D parallelized HPF EMPM with dawnward IMF By (K.-I. Nishikawa, 1998) and recently new simulation with dusk-ward IMF By was accomplished in the new VPP5000 supercomputer. In the new simulations performed on the new VPP5000 supercomputer of Tsukuba University, we used larger domain size, $305\times205\times205$, smaller grid size ($\Delta$), $0.5R_{\rm E}$(the radium of the Earth), more total particle number, 220,000,000 (about 8 pairs per cell). At first, we run this code until we get the so-called quasi-stationary status; After the quasi-stationary status was established, we applied a northward IMF ($B_{\rm z}=0.2$), and then wait until the IMF arrives around the magnetopuase. After the arrival of IMF, we begin to change the IMF from northward to duskward (IMF $B_{\rm y}=-0.2$). The results revealed that the groove structure at the day-side magnetopause, that causes particle entry into inner magnetosphere and the cross structure or S-structure at near magneto-tail are formed. Moreover, in contrast with MHD simulations, kinetic characteristic of this event is also analyzed self-consistently with this simulation. The new simulation provides new and more detailed insights for the observed sash event.
http://www.coins.tsukuba.ac.jp/~cai/Sash.mov
SM13B-1221 1340h
The Spherical Tearing Mode: A new Model for 3D Reconnection in the Magnetosphere
Magnetic reconnection poleward of the cusp is widely believed to provide a mechanism for the formation of the low-latitude boundary layer and plasma sheet during northward IMF conditions. Theoretical frameworks for the understanding of such processes are often grounded in the 2D slab tearing model of Quest and Coroniti, who demonstrated that tearing mode growth rates are largest when the reconnecting magnetic fields are anti-parallel. We present an alternative theoretical model for understanding such processes which begin from a genuinely 3D (but idealized) model of the magnetosphere with a northward IMF. We demonstrate that this equilibrium is unstable to a new tearing instability, called the spherical tearing mode, which grows faster than its resistive slab counterpart. The tearing eigenfunction has global support along a 3D separatrix surface composed of null-null lines that thread the cusps, but is not localized near the cusps themselves. We show the geometrical structure of the 3D separatrices which determine the sites of reconnection. While the field lines may appear to be anti-parallel in certain local regions, they are not so globally. This model requires us to revisit recent observations and global simulations of reconnection in the magnetosphere, and sheds new light on the controversy pertaining to "anti-parallel" and "component" reconnection.
SM13B-1222 1340h
Quiet-time mass and energy transport in GUMICS-4 global MHD simulation, 1: Locations and solar wind control of magnetopause entry sites
We have developed a method which can be used to analyze the temporal and spatial variations of energy transfer at the magnetopause using the GUMICS-4 global MHD simulation. Earlier results have pointed out that the Poynting flux focusing controls the magnetopause energy transfer both spatially and temporally during southward IMF. During northward IMF the spatial and temporal variations of solar wind energy transfer have been shown to be more difficult to categorize. Here we study the magnetopause energy and mass transfer locations during a variety of IMF and solar wind conditions in the GUMICS-4 global MHD simulation, with a specific focus to the northward IMF orientation. First, we identify the locations at the magnetopause where the magnetosheath and magnetosphere magnetic field lines are antiparallel. Second, we investigate whether the energy and mass transfer rates at these reconnection locations dominate over other locations at the magnetopause. Finally, we investigate the dependency of the energy and mass transfer locations on the solar wind parameters.
SM13B-1223 1340h
Electron Dynamics Associated With Ion Beam Acceleration Above the Polar Cap
During periods of northward IMF Bz, CLUSTER observed acceleration structures above the polar cap at relatively high altitudes (5-6 Re). These structures involves outflowing ion beams and convergent electric fields as expected in auroral zone in presence of field-aligned potential drops. Above the polar cap, the electron population presents various signatures. Accelerated electrons at energies of the hundred or so eV are sometimes observed simultaneously with the ion accelerations. From the analysis of both ion and electron populations, we estimate the potential distribution along magnetic field lines. It is shown that it can cover an extended altitude range along the magnetic field lines from the ionosphere up to higher altitudes than the spacecraft. The measurements at the 4 spacecraft allow to compute the size and the motion/stationarity of these structures. Finally, between the ion acceleration structures, CLUSTER detected the presence of outflowing electrons at energies lower than 100 eV. In conditions of Northward Bz, the polar cap appears as a very structured region which can play an important role in the plasma exchanges with distant regions of the magnetosphere.