SM14A-01 16:00h
Comparative Examination of Magnetic Flux Ropes and Traveling Compression Regions in the Cluster and Geotail Observations
Magnetic flux rope (FR) and traveling compression region (TCR) populations in the near-tail have now been well sampled by the Cluster and Geotail missions. Together, these missions provide detailed measurements from X ~ -10 to -30 Re under both quiet and active times. Using the measurements returned by both of these missions we present statistical analyses showing clear radial and east - west gradients in a number of their key properties. For example, the Bz variations associated with these structures change near X ~ -25 Re from the south-then-north (SN) variation that dominates in the near-tail to the north-then-south (NS) that is usually seen in the distant tail. There is also a strong decrease in the rate of FR/TCR occurrence earthward of X ~ -20 Re probably due to both a decreasing likelihood of reconnection so close to the Earth and the dissipation of flux ropes through "re-reconnection" as they are pushed up against the dipole magnetic fields of the inner magnetosphere. We also found that the mean speeds of the flux ropes and TCRs are very similar at 600- 800 km/s as are their mean diameters of ~ 3 to 4 Re. Overall, the existing observations of FRs and TCRs suggest that dynamics of the near-tail is dominated by simultaneous reconnection at multiple X-lines that filaments the cross-tail current sheet and leads to the formation of a dominant near-Earth neutral line.
SM14A-02 16:15h
Ion-kinetic Mapping Between the Magnetotail and the Ionosphere
Absolute mapping along magnetic field lines between magnetotail locations and their corresponding ionospheric foot points is a notoriously difficult task. On the other hand, it is potentially very rewarding, since many ground based and satellite observations can use the ionosphere to remotely diagnose tail processes. We address the mapping issue with large-scale, 3-D hybrid (kinetic ions, electron fluid) simulations. A numerical equilibrium is created that includes the near-Earth dipolar region and the stretched tail. As in other approaches, the assumptions underlying the modeled tail configuration mean that the mapped features have less absolute meaning than desirable. However, the relative positioning with respect to the open/closed field line boundary and to the background ion precipitation latitude, the dawn-dusk positioning, and the timing of arrival at the ionosphere have long been noted to be very important characteristics. In this study, we focus on signatures related to the extremely thin tail current layer typical for the pre-onset substorm. In particular, we show how Alfvenic perturbations, field-aligned currents, and energetic ions travel in this configuration and map to the ionosphere, with and without localized reconnection taking place.
SM14A-03 16:30h
Bifurcated current sheets in Earth's plasmasheet: scale size and occurrence frequency
We present results of a global MHD simulation of the magnetosphere under input conditions reflecting the solar wind properties observed when a bifurcated current sheet was identified by the Cluster spacecraft in the plasmasheet on August 29, 2001. A local bifurcation of the current sheet within a few RE of Cluster's position is characterized by structure in J$_{\perp}$ (current density), weak parallel flows, spatial scales ~3-5 R$_E$ in Y and Z, and clear magnetic linkage to a neutral line ~3 R$_E$ tailward of the downtail position of Cluster. At the downtail distance of Cluster, the bifurcation forms and then assumes a single-peak structure over time scales of minutes, in agreement with Cluster measurements. We use the simulation results and previously published case studies to formulate an algorithm that automatically identifies bifurcations. We apply the algorithm to Cluster magnetometer data for 1500 km (2001) and 5000 km (2002) spacecraft spacings. We find bifurcations ~25% of the time that Cluster is located in the current sheet for both separation distances, arguing that bifurcations are typically at least 1 RE in scale size. The bifurcations have qualitative current density and flow characteristics similar to those found in the MHD simulation. Scale size estimates from both the simulation and our statistical results give a full thickness of ~1-3 R$_E$ showing that the bifurcation occurs on the MHD scale. Local magnetic field and flow signatures for 2/3 of our events are consistent with nearby neutral line activity, but our events show no clear association with substorm onsets as detected in the AE index. We propose that bifurcated current sheets are formed at neutral lines of limited dawn-dusk extent.
SM14A-04 16:45h
Cluster multi-point observation of the thin current sheets in the magnetotail
The current sheet and its dynamics are one of the most essential elements in magnetotail physics. Particulary, thin current sheets with a spatial scale less than a few ion gyro radii are known to play an important role in the energy conversion process in the magnetotail. During the summer 2003, when the tetrahedron scale of Cluster was about 250 km, detailed structures of thin current sheets were resolved using multipoint measurements. We present examples of thin current sheets during different conditions: substorm growth phase, high-speed flows associated with an X line as well as during a period of stagnant plasma, by showing their temporal evolution and the spatial structures and discuss the underlying mechanisms by comparing with relevant theories.
SM14A-05 17:00h
Three-Dimensional MHD Simulation of Current Sheet Evolution During the Growth Phase of Magnetospheric Substorms
Current sheet thinning in the near-Earth magnetotail is an important element of growth phase dynamics since it determines the conditions for substorm onset. The growth phase is initiated by the erosion of closed dayside magnetic flux. This flux is replenished by convection of closed magnetic flux from the near-Earth tail region to the dayside. However, this process of magnetic flux replenishment is subject to the entropy and mass conservation constraints imposed on the slow quasi-static convection of magnetic flux tubes from the mid- and far-tail regions, first identified by {\it Erickson and Wolf\/} (1980). We examine whether the depletion of flux from a finite reservoir in the near-Earth tail region leads to the observed current sheet thinning. This hypothesis is tested using a self-consistent three-dimensional MHD code which is coupled to a semi-empirical magnetic field model. The resulting system was relaxed to an equilibrium state using a modification of a `ballistic relaxation' method. We discuss the structure of the equilibrium near-Earth magnetotail. A plasma outflow is prescribed in the near-Earth magnetotail to model the depletion of the `flux reservoir' described above. The resulting evolution of the current sheet is discussed.
SM14A-06 17:15h
Field-line mapping of the near-Earth plasma sheet based on the plasma pressure information obtained by GEOTAIL
The magnetic field line mapping of the magnetotail is a very important issue since physical interpretaion of any correlatinal studies between ionospheric phenomena (e.g. auroral break up) and magnetospheric phenomena (e.g. fast plasma flows) is directly affected by the field-line mapping. We here present a unique method to trace field lines from plasma measurements in the magnetotail. The plasma pressure balance equation tells us that the plasma pressure should be constant along the field lines since the JxB force counterbalancing the pressure gradient is perpendicular to the magnetic field. Therefore the contour lines of constant plasma pressure in the x-z plane of the magnetotail represent the magentic field lines themselves. Based on this idea, we have analyzed the average plasma pressure distribution in the magnetotail using 10 years data from GEOTAIL, for various conditions of solar wind/IMF parameters including IMF Bz polarity. The result indicates a quantitative difference in the latitude-eauatorial distance relationship, i.e., in the degree of stretching of field lines for different polarities of IMF. We emphasize that this is the first time that the large-scale configuration of magnetic field lines is deduced without introducing any model parameters. Our method is also unique in that it is based on plasma data only. We will comment on the consistency/inconsistency of our result with the Tyganenko field models.
SM14A-07 17:30h
Kinetic Ballooning Mode for Substorm Onset
AMPTE/CCE observations showed that the substorm onset in the near-Earth plasma sheet region ($\sim 8-10 R_E$) is associated with a low frequency (in the Pi 2 range) instability that is excited 1-2 minutes before substorm onset. The instability has been explained as kinetic ballooning modes (KBMs) which are excited when $\beta_{eq}$ increases from ~ 20 to above 50. We present theoretical analysis and numerical solutions of KBMs by performing kinetic calculations which include kinetic effects of particle trapping, finite ion gyroradii, and wave-particle resonances. The global magnetosphere prior to substorm onset is modeled realistically by 3D quasi-static equilibrium solutions which consist of a current sheet with an enhanced cross-tail current density with thickness of $\sim 1 R_E$ around the local midnight and with a longitudinal extent of $\sim 60-70^{\circ}$ at $X \sim 7-10 R_E$. The associated ionospheric Birkeland current moves equatorward with an enhanced current density shrinking in latitudinal width, consistent with the observed ionospheric growth phase signatures. The KBM has a real frequency in the Pi2 frequency range. The results of kinetic calculations will be compared with the MHD theory.
SM14A-08 17:45h
The High-Beta Interchange Mode in Magnetotaillike Configurations
In the near-Earth plasma sheet the pressure gradient and magnetic curvature are both directed earthward. This region is thus potentially unstable to the pressure-gradient ballooning mode provided that the plasma $\beta$ becomes large enough. Theoretical analysis of the critical $\beta$ for this mode has been inconclusive: the applicability of MHD (which gives a low critical $\beta$) to the near-Earth tail current sheet just prior to substorm onset is dubious, and kinetic analyses, which are exceedingly complex, have reached contradictory conclusions. Early particle-in-cell (PIC) simulations with a very low mass ratio $m_i/m_e = 16$ suggested that an alternative interchange mode, the high-$\beta$ mode, may indeed be more robust than the standard pressure gradient mode. The signature of this mode is that it occurs in regions of a tailward-directed gradient in the equatorial magnetic field; the mode is driven by the need to return magnetic flux to the dipole region. A massively-parallel 3D PIC code is used to study the properties of the high-$\beta$ mode for more realistic values of $m_i/m_e \sim 100$. The initial configuration consists of a 2D generalized Harris current sheet with a minimum $B_z$ region. The simulations are used to determine the minimal conditions required to excite the instability, to map out the unstable mode spectrum in $k_y$, and to establish the nonlinear properties of the magnetic field and plasma configurations.