SM23A-1171
Contribution of Sudden Solar Wind Compression to Substorm Triggering
The present study investigates the contribution of a sudden solar wind compression of the magnetosphere to the triggering of a substorm on 21 June 2007. We take into account the propagation of shock-associated disturbances from the magnetopause to the magnetotail. Previous studies that identified interplanetary shocks and/or discontinuities as substorm triggers have not examined this propagation in detail, which in this study can be determined by the THEMIS observations. A discontinuity across which the dynamic pressure increases by a factor of `2 arrived at the magnetopause near 1245 UT. The magnetic field pointed northward in both sides of the discontinuity. The Double Star Program Tan Ce 1 detected the arrival near 1245 UT in the dayside magnetosheath on the dawn side and Cluster at 1248 UT in the flank magnetosheath on the dawn side. Cluster observations indicate that the shock normal was inclined duskward by about 45 deg. Compression-associated magnetic and electric field disturbances were observed by GOES 9 at 1245:50 UT, GOES 12 at 1245:53 UT, and GOES 11 near 12:46:30 UT on the dawn side and by THEMIS near 1246:30 UT on the dusk side, consistent with the duskward inclined shock normal. Polar/UVI observed an auroral breakup at `1247:00 UT and `23 MLT followed by a poleward expansion. LANL observations at `03 MLT show a slightly dispersed electron injection beginning at `1249:00 UT. We discuss whether or not the disturbances propagating from the dayside magnetopause had the potential to trigger the substorm. The propagation speed and direction of the compression-related disturbances are estimated from local measurements by THEMIS as well as time lags between the GOES spacecraft. The estimate enables us to calculate when the disturbances arrived at a substorm onset site which we determine from aurora intensification, electron injection, and high-latitude negative bays.
SM23A-1172
Dynamic Response of the far Tail of the Earth Magnetosphere to IMF Changes and Magnetospheric Activity: a STEREO View
From the end of February to the beginning of March 2007, STEREO-B was traveling away from the Earth at distances between 200 and 360 Re close to the magnetospheric tail allowing to study the dynamics of the far tail and its boundaries in response to IMF changes and to magnetospheric activity. We present results obtained from very disturbed period (AE higher than 500 nT) to nearly quiet conditions and show that the far tail is: -i) strongly affected by traveling plasmoids during strong activity periods, -ii) presents a quick response to weak magnetospheric disturbances (AE lower than 100 nT) and is directly connected along field lines to acceleration regions located close to the Earth.
SM23A-1173
Substorm onset dynamics in the magnetotail: joint TC-1 and Cluster observation and SWMF simulation
This study investigates two substorm onset events with favorable constellations of spacecraft, TC-1 and Cluster, near the current sheet separated by several Earth radius. The substorms have been identified in both auroral regions. One is believed to be triggered by a northward turning of the interplanetary magnetic field (IMF), while the other occurs under generally southward IMF. For both events, Cluster was located tailward of TC-1 and observed the dipolarization at earlier times. The timing difference of dipolarization at different positions could be explained by an earthward propagation of the field disturbances from midtail to the inner magnetotail. And the earthward dipolarization front was found in one case to bounce back and forth at TC-1. The earthward propagation was accompanied by a fast earthward flow for the 21 September 2005 event, while this was not the case for the 28 September 2004 event. This suggests that substorms can be quite different from case to case due to differences in the solar wind condition and magnetotail configuration. The space weather modeling framework (SWMF) can capture the general topology of the magnetotail during these two events. However, it has predicted one of the substorm onsets about 50 minutes earlier than observations while missed out the other substorm onset totally.
SM23A-1174
Relationship between tail-current sheet activation and dayside magnetosphere
We report on multi-point observations of an isolated BZ disturbance detected at a wide local time sector including dayside magnetosphere in association with a tail current activation using THEMIS, Cluster, and Double Star observations around 01 UT on August 17, 2007. The unique constellation of the spacecraft allows us to examine simultaneously the propagation of the disturbance locally as well as globally. THEMIS at the late morning sector, observed an enhanced BZ disturbance that propagated tailward (and radial inward). TC1 at the dawnside detected relevant disturbance in BZ. Cluster, located post midnight sector, observed BZ enhancement accompanied with signature of field-aligned currents, in association with an enhancement of the westward electrojet observed by MIRACLE at a close local time sector. By comparing the timing of the disturbance and determining the propagation properties we discuss the possible casual relationship between the dayside disturbance and the tail current sheet activation.
SM23A-1175
A Statistical Study of the Spatial Scale of Substorm Current Wedge and Its Associated Auroral Images and Near Geosynchronous Observations
It is difficult to determine the spatial extent of current disruption associated with substorm current wedge. The difficulty is due to the fact that satellites are rarely positioned properly to distinguish azimuthal from radial structures. In order to accurately determine the dipolarization onset time, it is necessary to know where the satellite is positioned relative to the substorm current wedge. The accurate substorm timing is one of the main goals of the recently launched THEMIS mission. Several methods can help us to determine the central meridian and azimuthal width of substorm current wedge. The easiest one uses auroral images. The bright surge like western termination of the bulge is associated with the upward field-aligned current. The comparison of auroral images from the IMAGE and POLAR UVI satellites can give us a crude estimation of where the substorm current wedge is located relative to the GOES satellites. However, a more convenient method is to invert ground magnetometer data to obtain the parameters of a model current system that gives the best fit to the observed magnetic perturbations. The model currents include three components: the substorm current wedge, symmetric ring current and partial ring current. This inversion method is capable of obtaining the intensity as well as the longitudinal boundaries of the substorm current wedge with accuracy better than 0.5 hour MLT. There are more than 40 mid-latitude stations available during fall 2001 to 2006. MEASURE, SMALL, 210 MM and INTERMAGNET are arrays with many stations. We have established a substorm onset list from 2001 to 2007. This substorm list will be used as a basis for the development of the inversion code to determine the central meridian and width of substorm current wedges. The estimated central meridian and azimuthal width of substorm current wedge will be compared with auroral images and near geosynchronous observations (GOES and POLAR). The accuracy and relative timing between these observations will be examined.
SM23A-1176
Propagation Characteristics of Plasma Sheet Oscillations During a Small Storm
On March 24, the THEMIS spacecraft were in a string-of-pearls configuration through the dusk plasma sheet at the recovery phase of a small storm. Large undulations of the plasma sheet were observed that brought the 5 probes from one lobe to another. Each neutral sheet crossing was accompanied by fast flows and Pi2 oscillations, which are the topic of a companion paper. In this paper we focus on the low frequency (~10min) large scale plasma sheet undulations and determine their propagation characteristics, origin, properties and their importance for the dynamic evolution of the tail in the presence of storm-time substorms.
SM23A-1177
Small-Scale Properties of Traveling Compression Regions in the Near-Tail
CLUSTER observations of traveling compression regions in the near-Earth magnetotail are presented. These intensifications of the lobe magnetic field propagate along magnetic field lines at several hundred's of km/s. They are thought to be caused by the motion of magnetic flux ropes in the plasma sheet. Using the tetrahedral geometry of the CLUSTER spacecraft, we present the detailed properties of waves and particule distribution functions observed within these localized regions. The analysis of high-time resolution wave and particle data shows the complexity of wave-particle interactions taking place in these transient compression regions.
SM23A-1178
Observed Tail Current Systems Associated with Bursty Bulk Flows and Auroral Streamers During a Period of Multiple Substorms
We present a multi-instrument study of a substorm bursty bulk flow (BBF) and auroral streamer. During a substorm on the 25 August 2003, which was one of a series of substorms that occurred between 00:00 and 05:00~UT, the Cluster spacecraft encountered a BBF event travelling Earthwards and duskwards with a velocity of ~500~{km~s-1} some nine minutes after the onset of the substorm. Coincident with this event the IMAGE spacecraft detected an auroral streamer in the substorm auroral bulge in the southern hemisphere near the footpoints of the Cluster spacecraft. Using FluxGate Magnetometer (FGM) data from the four Cluster spacecraft, we determine the field-aligned currents in the BBF, using the curlometer technique, to have been ~5~{mA~km-2}. When projected into the ionosphere, these currents give ionospheric field-aligned currents of ~ 18~{A~km-2}, which is comparable with previously observed ionospheric field-aligned current associated with BBFs and auroral streamers. The observations of the BBF are consistent with the plasma "bubble" model of Chen and Wolf (1993) and with the reconnection of open field-lines Earthward of the substorm associated near-Earth neutral line for the creation of BBFs.
SM23A-1179
Observation and modeling of the injection observed by THEMIS and LANL satellites during March 23rd, 2007 substorm event
During the first encounter of a substorm on March 23rd, 2007, THEMIS constellation observed energetic particle injections and dipolarizations in the pre-midnight sector during the onset. Clear injection and dipolarization signatures were observed by three probes (A, B and D) in the region around 11 Re and 21:00 local time. THEMIS C, which was leading in the constellation at 8.3 Re, also observed a good injection signature, but the dipolarization is not so clear. From the timing based on these observations, a westward expanding ion injection and dipolarization front is identified. In combination with the energetic particle observations from LANL geosynchronous satellites, the particle injection seemed to initiate between LANL-97A (21 LT) and 1989-046 (1 LT). Ion injection can only be observed west of the center, whereas electron injections can only be seen east of the center. This event provides us an excellent opportunity to examine the dipolarization and particle injection processes beyond geosynchronous orbit. We model this injection event by sending an earthward dipolarization-like pulse at 23 local time and record the injected ions and electrons associated this pulse at the various satellite locations. Most of the basic features of the injected particles during the main injection are reproduced and the timing among satellites is consistent with observations. It is suggested from the model that the center of this substorm injection was initiated around 23 local time and located beyond 19 Re.
SM23A-1180
Investigation on Pi1 B pulsations using THEMIS ground-based magnetometers
Pi1 B pulsations are believed associated with substorm onset. Because of the higher frequencies, 25 mHz to 1 Hz, Pi1 B has the potential to be a more accurate indicator of the time of substorm onset than Pi 2 pulsation. Thus it is important to understand the generation mechanism of Pi1 B and the relation between Pi1 B pulsations and substorm onset. However, observations of Pi1 B pulsations are not as easy as those of Pi 2 pulsations because of both smaller areas of Pi1 B occurrence and the limited availability of high-resolution magnetometer data on the ground. The THEMIS ground-based magnetometer network records magnetic field data with 0.5 second resolution and covers the full longitude of North American part of the auroral zone and sub-auroral region. We will examine the occurrence of Pi1 B pulsations using THEMIS ground magnetometer data to understand the local time and latitudinal distributions of Pi1 B. We will use the T.-S. Hsu's substorm event list which is identified by AL and Pi2 to examine the relation of Pi1 B and substorm onset.
SM23A-1181
Substorm Timing and Location Using The Combined CARISMA and THEMIS GMAG Magnetometers
With the successful launch of the THEMIS spacecraft, the expansion of the CARISMA magnetometer array, and the deployment of the THEMIS GBOs, there now exists an exceptional opportunity to study fundamental ULF wave science in the nightside magnetosphere around substorm onset. Traditionally, substorm onset is usually determined by location and timing of ULF waves in the Pi2 (40-200 second period) range. With the current configuration of ground magnetometers, it is now possible to resolve frequencies in the Pi1 (1-40 second period) band using novel techniques such as Wavelet Analysis. With this technique, the entire spectrum of ULF waves associated with substorm onset can be accurately timed to approximately half of the wave period. We use a substorm location modeling algorithm [Cramoysan et al., 1995] to produce initial estimates of the locations of the upward and downward field aligned currents and the westward electrojet. We present results from several isolated substorms and find that the onset location of the Pi1 waves starts at or close to the location of the downward field-aligned current region and propagate isotropically away from this location. We discuss the implications of this result in terms of potential generation mechanisms for Pi1 waves at substorm onset, and for the physical processes operating at the onset of the substorm expansion phase.
SM23A-1182
Global Auroral Observations from Polar UVI during THEMIS Events
In this poster, we compare auroral events seen in the ultraviolet emission imaged by Polar UVI with plasma and fields data detected by instruments onboard the THEMIS satellite. Global images offer a clear indication of onset times for isolated substorm events. An example of one such event took place on March 23, 2007. UVI recorded a substorm onset at 1110 UT near local midnight. The onset recorded by magnetometers on the ground was 1118 UT, 8 minutes later, only after the arrival of a westward surge at the ground station. This substorm, as in many substorms, was accompanied by pseudo-breakup events before the onset of the auroral expansion. Additionally, in the local morning sector, pulsations were seen during substorm recovery. The THEMIS probes at this time were located between 9 and 13 RE in the pre-midnight sector. THEMIS observed particle injections and dipolarization signatures propagating from one probe to another. Large ion flows and magnetic field fluctuations were seen during the substorm intensitifcation at 1118 UT as the disturbance propagated westward to the probe locations, thus illustrating the importance of global auroral images in placing the magnetospheric observations in context.
SM23A-1183
Russian auroral and polar ionospheric disturbance magnetometers (RapidMag)
Since their introduction by Davis and Sugiura in 1966, the auroral electrojet indices AE, AU, and AL have been widely used to study ionospheric and magnetospheric phenomena associated with substorms as well as input to magnetospheric and ionospheric models. While rapid delivery of the indices is highly desirable in the context of both ongoing scientific observations and space weather applications, production of near-real-time AE indices remains a difficult task. A major challenge is acquisition of near-real-time magnetometer data from Russian auroral zone stations, some of which are at remote locations with harsh natural environment and without well- established infrastructure for data transmission. The stations cover approximately 6 time zones, and missing data from the stations means a serious degradation of the AE indices. The Russian auroral and polar ionospheric disturbance Magnetometer (RapidMag) project is an international collaboration to enable rapid and stable acquisition of ground magnetometer data from Russian stations in the auroral zone for production of near- real-time AE indices as well as for distribution of data from individual stations to the science community. The six RapidMag stations are Amderma (65.1N, 138.1E, degrees, geomagnetic), Dikson Island (68.5N, 156.2E), Norilsk (64.6N, 161.9E), Cape Chelyuskin (71.8N, 175.6E), Tixie (65.9°N, 196.9°E), and Pebek (65.3°N, 228.8E). The RapidMag project is entering a new phase with recent funding from NSF and NASA. We present the current status of the project and our plan for the next 5 years.
SM23A-1184
The relationship between high- and low-latitude Pi2 pulsations simultaneously observed by DE-1, AMPTE/CCE, and ground stations
Pi2 pulsations (period from 40s to 150s) are observed at substorm onset. Cavity mode resonance is the possible scenario of low-latitude Pi2 pulsations. It is an open question whether the resonance boundary, plasmapause, is good reflector or not. We investigated Pi2 pulsations observed simultaneously by the polar orbiting DE-1 satellite (an apogee: about 3.6 Re altitude and a perigee: about 500km altitude), equatorial orbiting AMPTE/CCE satellite (an apogee: about 8.8 Re altitude and a perigee: about 1100km altitude), and ground stations at 1132-1136 UT on November 14, 1986. DE-1 was located at polar region (Geomagnetic latitude=-83.42 degrees). AMPTE/CCE was located at L=4.57 and 23.6 MLT. AMPTE/CCE might be located outside the plasmasphere. They are observed Pi2 pulsations in the compressional component. The AL index began to decrease at 1114 UT. It showed substorm onset. They had high coherence with that observed at Kakioka (KAK) in the H component, which was located at L=1.25 and 3.21 MLT. The phase difference between KAK and DE-1 and between KAK and AMPTE/CCE were 180 and 90 degrees at 14 mHz. These Pi2 pulsations had high coherence with that observed by Hermanus (HER), which was located at L=1.8 and 12.8 MLT. These observational results may support that the plasmapause is imperfect boundary. Pi2 pulsations at low latitude are excited by the plasmaspheric virtual resonance mode, in which the ambient magnetic fields outside plasmasphere oscillated with the cavity mode resonance. And Pi2 pulsations at the polar cap are also excited by PVR mode. In this presentation, we will show these Pi2 pulsations observed by DE-1, AMPTE/CCE, HER and KAK, in addition to other ground stations (Port Aux Francais, Furstenfeldbruck, and Wingst) and conduct statistical study of Pi2 pulsations, which were simultaneously observed by the DE-1 and AMPTE/CCE satellite.
SM23A-1185
Auroral mesoscale structure and dynamics
The THEMIS ground based observatory (GBOs) network provides 20 new white-light all-sky imagers operating at a 3-second cadence. Data from the first full season (2006-7) provides an unprecedented opportunity to study continent-scale auroral processes at high spatial and temporal resolution. In addition, multi-spectral imagers from other projects give us essential information about auroral boundaries and precipitation characteristics. Our focus for this work is on the azimuthal (longitudinal) structure and dynamics of auroral regions and boundaries.
SM23A-1186
Substorm onset by kinetic ballooning instability
The onset of substorms has been observed to associate with ULF (in the Pi 2 frequency range) instabilities in the late growth phase in the magnetosphere. Recent substorm auroral breakup arc observation by ground-based All Sky Imagers and by the FORMOSAT-2 satellite's ISUAL instrument have also identified azimuthal mode structure of breakup arc with mode number on the order of 200-300. We have developed a Kinetic Ballooning Instability (KBI) theory, which is destabilized by plasma pressure gradient and magnetic field curvature in the high beta region, to explain the substorm observations in both the aurora breakup arc and ULF instabilities in the near- Earth plasma sheet region. The substorm onset model is based on the theoretical analysis and numerical solutions of the gyro-kinetic equations for late growth phase 3D magnetospheric quasi-static equilibria. The results show that the KBI has a real frequency associated with the ion magnetic drift frequency, which is in the Pi2 frequency range, and the most unstable KBI has an azimuthal mode number on the order of 200-300. The theoretical KBI features are consistent with observations in the aurora breakup arc formation and subsequent breakup and the ULF (in the Pi 2 frequency range) instabilities in the near-Earth plasma sheet. Comparison between the KBI model and substorm onset observations will be presented.
SM23A-1187
Interpretation of multi-satellite observations of extended reconnection x-lines
Recent multisatellite observations of reconnection in the solar wind show that vey long extended x-lines are present[1]. Recently we have proposed a mechanism for the extension of x-lines [2]. However, our previous study was limited to zero guide field (antiparallel reconnection). We extend the study here to different guide field values (component reconnection). The dynamics of x-line formation and evolution in 3D magnetic reconnection is studied using a fully kinetic approach. An x-line of small length is initialized using a perturbation localized in the current direction. The electrons and ions drift diamagnetically along the current direction of the initial x-line and are further accelerated by the reconnection electric field. The electron and ion motion is in opposite directions and each species extends one end of the x-line. The effect of the guide field is shown to alter significantly the modus of extension of the x-line. [1] Phan et al., Nature, 439, 7073, 175 (2006). [2] Lapenta, G., et al., Geophys. Res. Lett., 33, L10102 (2006).
SM23A-1188
Time Evolution of the Substorm Current Wedge from Ground and Space-based Magnetic Fields
Over the past several years, intensive efforts have resulted in a significant improvement in the ground instrumentation for auroral studies in North America. A major part of this is due to the THEMIS ground program, both in the U.S. and in Canada. The THEMIS Ground-Based Observatory network has fielded 10 additional magnetometers in Canada and Alaska. Further THEMIS magnetometers are part of the GEONS outreach effort, found in the continental U.S. and Alaska. Athabasca University initiatives and collaborations have made yet further magnetometer data available, most notably from the new AUTUMN network of instruments in central Alberta, and others in Quebec. Several stations of the University of Tokyo STEP network remain operational, and some have been upgraded. There is finally a dense enough set of magnetic data that techniques based on forward modeling, and most relevant to the opportunity afforded by THEMIS, Automated Regional Modeling (ARM), can be reliably used. These techniques specify where net field-aligned current (FAC) and ionospheric electric current flow are located. In some cases the Pedersen system can also be included based on data. Even when it is not, it can be considered collocated with electrojet locations given by ARM. The extension into space of the FAC (net or Pedersen) allows comparison with the perturbations observed at THEMIS. We present results from an event on March 13, 2007, during which THEMIS in its early orbital configuration was magnetically conjugate to central North America, clear weather prevailed, and a substorm took place whose perturbations were ideally suited for inversion using ARM. At about 5 UT, activations were detected from the ground with magnetic perturbations also detected from THEMIS. The ground perturbations are well represented by a three-dimensional substorm current wedge (SCW) system, and perturbations in space indicate radial propagation at a time when the electrojet expanded poleward. Little longitudinal propagation of the SCW is suggested by the ground data.
SM23A-1189
Study of the Effects of Auroral Substorms on the Low-Latitude Currents
In recent years there has been some discussions on whether or not the auroral substorms have an effect on the equatorial and mid-latitude currents. Our work was motivated by this problem. We use a novel statistical technique that allows us to test at a specified significance level whether such an effect exists. Our approach uses the original 1-min data of H component measurements, rather than the derived indices as in some previous related studies. One of the shortcomings of the correlation analysis based on indices like AE, Kp or Dst is that the physical interpretation of the indices is not always obvious. Furthermore, interpretation of a correlation analysis of such derived data is not straightforward, as statistical uncertainty cannot be readily quantified. The approach we propose here is novel in several ways: 1) we work directly with the measurements of the magnetic field, rather than indices; 2) we view magnetometer records over one day as single functional observations; 3) we use a statistical test of significance, which by its very nature takes into account random variability not attributable to physical effects. We show that the substorms do affect low-latitude currents at all longitudes. It appears that this dependence is significant not only during the same day the substorm occurs, but also the next day. Our study shows that the effect normally fades out two days after the substorms. It decreases in time faster for the cases of weak and medium substorms than those of strong substorms.
SM23A-1190
Generation of High-Latitude Pi2 Precursors in Growth-Phase (Downward) Field-Aligned Currents
A variety of mechanisms have been proposed to explain the generation of high-latitude Pi2 pulsations preceding the onset of magnetospheric substorms and pseudobreakups. Here we explore a causal relationship between Pi2 signals and field-aligned currents (FACs) flowing into the ionosphere in association with the diversion of the growth-phase cross-tail current. In situ observations of Pi2 precursors measured concurrently with FACs reveal that the diversion of the cross-tail current into FACs occurs before the excitation of Pi2 signals. We show that results from computer simulations of large-scale FACs and Alfven waves interacting with a reactive ionosphere, including realistic magnetospheric and ionospheric plasma and magnetic inhomogeneities, exhibit a similar causal sequence. The diverted FAC need not be oscillatory, although we consider the implications of both monotonically increasing FAC diversion as well as oscillatory currents increasing in amplitude. Of particular interest is the dynamic behavior of the downward-diverted FAC which depletes the E-region and bottomside ionospheric plasma density. At a critical value, the depletion promotes a spontaneous feedback instability which establishes fast ionospheric Alfven resonator oscillations at low altitudes together with growing, pre-onset Pi2 pulsations that extend upward into the plasmasheet while propagating across L shells in the direction of the electric field and toward the region of upward FACs. Although the Pi2 waves are initiated in the downward current channel, they modulate auroral precipitation and auroral lumninosity because the intense Pi2 FACs achieve amplitudes that typically require field-aligned potential drops to satisfy current continuity.
SM23A-1191
What are the ionospheric signatures of magnetotail reconnection?
In a recent case study based on the combined Cluster and Polar PIXIE data we reported that an inverted-V structure caused by a field aligned potential drop of 30 kV producing very strong X-ray aurora was found in connection with tail reconnection. However, the insitu particle measurements by Cluster indicate clearly that the particles responsible for the X-ray aurora were not accelerated by the reconection process. In this paper we report predicted and observed ionospheric signatures of 13 reconnection events where Cluster passed through the reconnection ion diffusion region. For the 6 events where global auroral imaging data were available our results indicate that reconnection is an azimuthally expanding (or extended) process observed along the poleward boundary of the aurora. Furthermore, the ionospheric emissions indicate that there has to be acceleration mechanism in addition to the local acceleration in the ion diffusion region.
SM23A-1192
Substorm injection region propagation and plasma sheet flow outside geosynchronous orbit
The propagation of the dispersionless injection region has previously been studied using radial alignments of the Combined Radiation and Release Effects Satellite (CRRES) and Los Alamos National Laboratories geosynchronous satellites (see Reeves et al 1996). Using Polar CEPPAD and LANL geosynchronous particle data, we build on this study and investigate the radial propagation outside L=6.6. These combined results are compared with the ground-based expansion of the injection region observed with riometers and in situ plasma flow measured with Geotail.
SM23A-1193
Pi1B Pulsations and Fast Flows
Pi1B pulsations are a type of ULF wave historically observed on the ground, having a relatively broadband signature with frequencies of a few Hz and below. Recently, these waves have also been observed in space. One case study concludes that the waves propagate from geosynchronous orbit (or beyond) to the ionosphere, while numerical efforts illustrate how such waves (which are compressional at geosynchronous orbit) become transverse at low (i.e., FAST) altitudes. In addition, a detailed study has been undertaken that relates observations of these waves in the magnetotail to fast flows. Specifically, it has been determined that Pi1B pulsations, as observed by Cluster and other spacecraft in the magnetotail, are precisely the magnetic signatures of fast flows. This conclusion begs the question of why fast flows should have such a signature, which of course is related to the nature of fast flows in general. In this presentation, this question is addressed both analytically and numerically, with efforts focusing on understanding the nature of a pressure pulse that is often observed with a reconnection model. Initial results show that the pressure pulse propagates at or near the Alfven speed and that it has a scale size the order of the ion skin depth, characteristics that are both comparable to observations of fast flows. Reports that fast flows are turbulent structures are also consistent with what would be expected numerically.
SM23A-1194
Transition From Cold, Dense to Hot, Tenuous Plasma Sheet in the Near-Earth Magnetotail
It has been reported that sometimes the plasma sheet in the magnetotail consists of cold, dense ions, especially during northward interplanetary magnetic field (IMF) periods. However, the mechanism explaining the formation of the cold, dense plasma sheet is not yet understood. In this study we will present multi-spacecraft observations showing transition of plasma sheet from cold, dense to hot, tenuous status in the near-Earth magnetotail. The transition tends to occur very rapidly, in less than 1 min. Before the transition, the ion density is typically ~1 cm-3, and the temperature ~1 keV. After the transition, the ion density drops to ~0.3 cm-3, while the temperature increases to ~2.5 keV. The transition boundary can be quite far from the flank magnetopause and low latitude boundary layer (LLBL). For example, transition can be observed in the vicinity of the central plasma sheet (|Bx| < 10 nT) at ~(-17.8, -7.06, 1.71 RE GSM). One interesting feature is that the transition is usually accompanied by sudden increase of the Bz component. The electron measurements also show same transitional features as the ions. These observations suggest that the near- Earth plasma sheet may consist of separate regions with different plasmas and magnetic field configurations.
SM23A-1195
Reconstruction of a bipolar magnetic signature in an earthward jet in the tail: Flux rope or 3D guide-field reconnection?
Southward-then-northward magnetic perturbations are often seen in the tail plasma sheet, along with earthward jets, but the generation mechanism of such bipolar Bz (magnetic flux rope created through multiple X-line reconnection, transient reconnection, or else) has been controversial. At ~23:13 UT on 13 August 2002, Cluster encountered a bipolar Bz at the leading edge of an earthward jet, with one of the four spacecraft in the middle of the current sheet. Application to this bipolar signature of Grad-Shafranov (GS) reconstruction, the technique for recovery of two-dimensional (2D) magnetohydrostatic structures, suggests that a flux rope with diameter of ~2 RE was embedded in the jet. To investigate the validity of the GS results, the technique is applied to synthetic data from a 3D MHD simulation, in which a bipolar Bz can be produced through localized (3D) reconnection in the presence of guide field By [Shirataka et al., JGR, 2006] without invoking multiple X-lines. A flux rope-type structure, which does not in fact exist in the simulation, is reconstructed but with a shape elongated in the jet direction. Unambiguous identification of a mechanism that leads to an observed bipolar Bz thus seems difficult based on the topological property in the GS maps. We however infer that a flux rope was responsible for the bipolar pulse in this particular Cluster event, because the recovered magnetic structure is roughly circular, suggesting a relaxed and minimum energy state. Our results also indicate that one has to be cautious about interpretation of some (e.g., force-free, or magnetohydrostatic) model-based results.
SM23A-1196
Complexities of a 3-D flux rope as shown by MHD simulation
This paper presents the results of a global magnetohydrodynamic (MHD) simulation of a pair of substorms on August 11, 2002. Comparisons of data with simulation results reveal an agreement regarding the sequence of events in the magnetosphere. We then present the results in the simulation of a flux rope formed during the second substorm. Unlike standard 2-D depictions of reconnection and plasmoid release during a substorm, the simulation shows a highly complex structure that has considerable winding of both closed and open field lines. Additionally the flux rope does not move tailward uniformly, but rather has a assymetric motion where the dawn flank moves tailward prior to the dusk end of the flux rope, resulting in a a skewed flux rope that runs almost downtail instead of crosstail. These features can add considerably complexity to satellites observing a flux rope structure in-situ. A single spacecraft could observe particle populations that go through a sequence of alternating open and closed field lines and spacecraft separated by small spatial distances could observe quite different populations as well.
SM23A-1197
Correlative scale and effective magnetic Reynolds number determination from plasma sheet and solar wind magnetic field fluctuations
Cluster data from many different intervals in the magnetospheric plasma sheet are employed to determine the magnetic correlative scale and the Taylor microscale from simultaneous multiple point measurements. For this study we define the correlative scale as the exponential decay constant of the correlation coefficient as a function of spacecraft separation and the Taylor scale as the radius of curvature of the correlation coefficient values at zero separation. The present determination of the Taylor scale makes use of a novel extrapolation technique to derive a statistically stable estimate from a range of measurements at small spatial separations. In the plasma sheet the correlative scale length is found to be largest (about 20,000 km) in the direction parallel to the magnetic field and smallest (about 10,000 km) in the direction perpendicular to the magnetic field. Similar results have been determined in the slow solar wind using Cluster and several other spacecraft. The effective magnetic Reynolds number can be expressed in terms of the correlative scale and the Taylor scale. Knowledge of the effective magnetic Reynolds number may be useful in magnetohydrodynamic modeling of the magnetosphere and the solar wind and may provide constraints on kinetic theories of dissipation in space plasmas. http://www.igpp.ucla.edu/jweygand