SM13C-01 13:40h
Correlations of Plasma and Field Parameters Within the Magnetotail
The purpose of this project is to compare the long-term-averaged properties of flux tubes that are moving rapidly with the properties of more slowly moving flux tubes. Flow conditions were labeled by the voltage drop across a unit flux tube with circular cross section associated with the electric field Eh = sqrt [(VxBz)**2 + (VyBz)**2]. Many measured particle and field parameters were averaged and flux tubes were traced for each range of voltage drops. Flux tube properties were calculated by integrating the measurements along magnetic field lines. A few of the more interesting plasma and flux tube parameters will be shown to illustrate some of the differences between slowly and rapidly moving flux tubes.
SM13C-02 13:55h
Cluster Survey of High Speed Bulk Flows and Field Aligned Beams in Near-Earth Plasma Sheet
We have surveyed all high speed flows detected by Cluster in the near-Earth plasma sheet from 2001-2003. The flows are characterized as either bursty bulk flows (BBFs) or field-aligned beams. This survey confirms the previously reported dawn-dusk assymetry in BBFs, while no such asymmetry is present for plasma sheet boundary layer field-aligned beams. We will also report the occurence rate of field aligned-beams in the plasma sheet and its possible dependence on AE and Kp indices.
SM13C-03 14:10h
Transport Characteristics From Multi-Component Approach in Magnetotail Plasma Measurements
Plasma measurements in the Earth's magnetotail have revealed occasional existence of multiple plasma components. It is uncertain whether the bulk plasma moments computed can truly represent the transport properties in such occasions. In this work, we examine the influence on the transport of plasma, magnetic field, and energy by the presence of multiple plasma components in Geotail plasma measurements. We use the automatic detection technique newly developed by Ueno et al. [2001] to examine three-dimensional plasma measurements in the Earth's magnetotail. Two events are examined in detail, one occurring in a magnetic reconnection interval during the substorm expansion phase while the other occurring at plasma sheet thickening during the substorm recovery phase. Similarities and differences in transport characteristics deduced from the bulk plasma moment procedure and that from the multi-component procedure are presented, including implications of these results.
SM13C-04 14:25h
Observations of the Magnetotail Reconnection Rate: its Relationship to the Occurence of Auroral Poleward Boundary Intensifications (PBIs) and to the Substorm Cycle
Bursty bulk flows (BBFs) account for a significant fraction of the transport of energy, momentum, and magnetic flux through the central plasma sheet. The ionospheric signature of BBFs is believed to be N-S equatorward-extending auroral PBIs known as `auroral streamers'. Such auroral PBIs originate near the polar cap boundary (PCB) and often repeat on a characteristic timescale of $\sim$ 10 minutes. However, only one case study has been published so far that presents simultaneous and co-located observations of auroral PBIs and reconnecting flows in the ionosphere (de la Beaujardi$\grave{e}$re et al., 1994), and only some of the PBIs observed were associated with bursts of magnetotail reconnection. This study quantifies the temporal variation of the nightside magnetic reconnection rate at a single meridian for two periods exhibiting auroral PBIs: (i) a quiet/recovery phase interval on 20 February 2001 and (ii) an interval encompassing two substorm cycles on 12 December 2001. We make our calculation using two-dimensional (2D) optical data and 2D plasma flow vectors. The plasma flows used are the merge vectors from the SuperDARN HF radars Saskatoon and Kapuskasing. Their fields-of-view (FOVs) overlap the FOV of the Aqsaniq all-sky imager (ASI) at Rankin Inlet, part of the NORSTAR array. The PCB is derived from the latitudinal profile of 630 nm ASI data, and supported by proxies of the PCB derived from SuperDARN HF radar spectral width boundaries and POLAR VIS and UVI data. PBI activity is simply identified from 630 nm emissions above a threshold intensity, within a specified distance from the PCB and clearly separate from lower latitude intensifications. During the quiet/recovery phase interval, we observe localised reconnection flows associated with PBIs, evidence that supports the idea that BBFs are reconnection driven. During the substorm expansion phase, enhanced magnetic reconnection is observed of about 40 mV/m. On-going nightside magnetic reconnection, however, is also observed in the hours prior to the substorm onset, whilst on-going but intermittent dayside reconnection is occuring.
SM13C-05 14:40h
A comparative study of Cluster observations and 3D kinetic simulations of current sheet configuration and stability in the Earth's magnetotail
The issue of reconnection onset remains a challenge to the plasma physics community. For most physical systems of interest, reconnection does not proceed in steady manner, but rather there are periods of time in which magnetic flux is accumulated, followed by other periods in which the energy is rapidly dissipated. In current sheet geometry, one of the most well-known onset mechanisms is the collisionless tearing instability. However, for systems such as the magnetotail, it appears the tearing instability is stabilized by the magnetic geometry. Furthermore, even if the tearing mode is unstable, fully kinetic 2D simulations indicate the instability saturates at small amplitude and does not generally trigger large-scale reconnection. Recent results from Los Alamos suggest a possible resolution to the onset problem by considering the role of current aligned plasma instabilities such as the lower-hybrid drift instability (LHDI). The nonlinear development of the LHDI leads to a variety of nonlinear modifications which can promote reconnection onset even in complex magnetic configurations such as the Earth's magnetotail. We report results of 2D and 3D kinetic simulations where the fast onset of reconnection in presence of current aligned modes is documented.
SM13C-06 14:55h
Multiscale plasma structuring near the boundary of open-closed field lines
Magnetospheric plasma is very dynamic and filamented in nature, especially in the vicinity of the separatrix between open and closed field lines in the magnetotail. Dispersed ion structures in which the highest energy ions are found nearest the boundary have been observed in this region by the Aureol-3, Akebono and Interball satellites. They are observed frequently (50% of the time) by the four spacecraft of the Cluster mission, allowing for an improved and more detailed analysis of the spatial and temporal characteristics of these structures. In this study, we examine an inbound plasma sheet boundary layer (PSBL) crossing by the Cluster spacecraft on February 14, 2001. On this day, the Cluster spacecraft observed dispersed ion structures at the poleward edge of the boundary, and "echoes" of these structures at lower latitudes. In addition, each dispersed ion structure was comprised of "beamlets," which are discrete structures in the ion precipitation profile. Previous studies of this event have postulated that these structures are a result of transient burst-like acceleration effects mixed with time of flight effects. We have used a combination of global magnetohydrodynamic simulations and particle trajectory calculations to analyze this event. Our investigation began with a global magnetohydrodynamic (MHD) simulation of Earth's magnetosphere driven by solar wind data from the ACE spacecraft. We then launched distributions of ions representing the fluxes observed by the CIS experiment onboard the Cluster 1, 3, and 4 spacecraft both forward and backward in time from the spacecraft location in the time-dependent global MHD electric and magnetic fields. This allows us to determine the origins and acceleration mechanisms of ions observed by Cluster. By comparing our results from the three spacecraft we can determine the relationship between the multiple structures observed by each spacecraft and separate spatial from temporal acceleration mechanisms. We are especially interested in quantifying the role of non-adiabatic acceleration in the formation of the observed structures.
SM13C-07 15:10h
Investigation of Ion Beams in the PSBL using Cluster
With every new spacecraft traversing the plasma sheet boundary layer (PSBL) new properties of ion beams are identified. Clusters multipoint measurement capability allowed us to study in great detail the intricate nature of energy-dispersed ions (at 4-5 Re) which were substructured into several (up to four) beamlets with a quasi-periodicity of 1-3 min. New ion properties could be identified associated with the energy range, the spatial extent and the energy dispersion types. The ion beamlet injections occurred at distances larger than 50 Re in the tail and showed both temporal (time-of-flight) and spatial effects. We propose an injection scenario (impulsive and recurrent) in the distant current sheet to understand the spatio-temporal history of these energy-dispersed ion beamlets. Furthermore, we show evidence that the beamlets can undergo several bounces in the tail while staying coherent. These results add significantly to the known realm of plasma sheet dynamics.
SM13C-08 15:25h
Cluster multi-point observations of slow shock structures in the magnetotail
Slow shocks are thought to play a key role in collisionless reconnection. Whilst such structures are presumed to exist in the magnetotail, experimental evidence, particularly of their detailed substructure, remains sparse. In order to identify whether or not an observed transition corresponds to a slow shock, its normal must be accurately calculated. In this regard, the multi-spacecraft Cluster mission provides a new way to study these structures. Here we present the first results of an investigation into slow shocks observed by Cluster in the magnetotail. The aim of this study is to use the multi-point high-resolution Cluster Flux-Gate Magnetometer (FGM) data to identify potential slow shock candidates in the magnetotail, to establish their geometry using multi-spacecraft techniques already widely used at the bow shock, and to examine their substructure, the experimental nature of which remains unclear. The 2001 and 2002 tail seasons have been surveyed for events, resulting in 40+ candidate intervals for 2001 alone. In particular we concentrate on an event observed at 01:30 on August 27 2001, when the spacecraft were operating in `burst mode'. The substructure is examined using 3d electron observations from Cluster PEACE, in the context of a 1 dimensional fully kinetic plasma simulation.