SM12B-01
Plasma Sheet Response to the Ionosphere's Demand for Field-Aligned Current
Magnetospheric convection electric fields and plasma stresses are transmitted to the ionosphere by Alfvén wave electric fields and field-aligned currents (FACs). The closure of the FACs by ionospheric Hall and Pedersen currents drives the ionospheric convection system. However, the ionospheric system does not necessarily mesh smoothly with the magnetospheric drivers, and the magnetosphere must respond by altering its convection and plasma stress configuration, thereby creating self-consistent closure paths for the complete coupled system of currents and electric potentials. Three-dimensional particle-in-cell plasma kinetic simulations are used to determine the plasma sheet response to various current systems imposed as boundary conditions at the near-Earth boundary. These systems consist of separate downward and upward tubes of FAC and a substorm current wedge configuration. The results demonstrate that the creation of closure paths for ionospheric FACs can result in large configuration changes within the near-Earth plasma sheet. The plasma sheet is forced to establish polarization electric fields that locally increase the cross-tail current by producing a duskward Hall electron current; this results in the formation of thin (in z), spatially localized (in y) electron-dominated Hall current sheets. The observed complex magnetic field configuration with opposite polarity Bz fields in close proximity separated by electron scale thin current sheets is reminiscent of the turbulent magnetic fields that are observed within the near-Earth current disruption region at substorm breakup [ Lui et al., 1988, 1992].
SM12B-02
Solar wind control of plasma number density in the near Earth plasma sheet: three- dimensional structure
Plasma number density in the near-Earth plasma sheet depends strongly on solar wind and IMF: (1) plasma
sheet number density positively correlates with solar wind number density [Borovsky et al., 1998]. (2) plasma
sheet number density positively correlates with theta angle of IMF with time lag of several-hours [Terasawa et al.,
1997]. Recently, Nagata et al. [JGR in press] shows two-dimensional structure of such dependences in the near-
Earth central plasma sheet (X<0, 9 SM12B-03 Electron density estimation in the magnetotail: a multi-instruments approach
{ } An accurate in-situ estimation of the electron density in the Earth's magnetotail is needed to
understand key geophysical phenomena at different scales: from the microphysics of magnetic reconnection to
large-scale phenomena studied using three dimensional (3D) mapping of the electron density.
{ } Several instruments are usually carried onboard a magnetospheric spacecraft to estimate this key
physical parameter. In the case of the ESA/NASA Cluster mission, composed of 4 satellites, four different
instruments on each platform can be used to estimate it: two particle instruments (electrons and ions), a DC
electric field instrument, a relaxation sounder (an active wave instrument) and a high-time resolution passive
wave receiver. Each of these instruments has its own limitations depending on the plasma conditions.
{ } The Earth's magnetotail is indeed far from being a uniform medium but instead composed of
regions with distinct electron density (Ne) and temperature (Te) ranges, including: the lobe regions (Ne < 0.01
cm-3, Te ~ 100 eV) and the plasmasheet (0.1 - 1.0 cm-3, Te ~ 1 keV).
{ } Particle instruments can be blind to low energy ions coming from the ionosphere or measuring only a
portion of the energy range of the particles. This results in an under estimation of the total density. Photoelectrons
are also perturbing the density measurements and careful analysis of the spacecraft potential is necessary.
Measurements from a relaxation sounder also require careful calibration and on Cluster can not be used to
estimate densities below 0.2 cm-3 (lower frequency of the instrument soundings).
{ } Case studies will be presented comparing simultaneous electron density estimation from particle
instruments, DC electric field instrument and the relaxation sounder. This latter instrument has been rarely used
in the past to derive the total electron density in the distant tail. The outcome of an extensive analysis of the
relaxation sounder measurements in the magnetotail by Cluster will be presented. The results will show under
which plasma conditions and how the total electron density can be extracted from these measurements, which
offer to the particle instruments an independent and complementary estimation of the density.
SM12B-04 Plasma Sheet Structure and Population When the IMF is Northward
We have used Cluster and Double Star observations along with magnetohydrodynamic (MHD) and large scale
kinetic (LSK) simulations to investigate the population and structure of the plasma sheet during an extended
interval with northward and dawnward IMF on October 10, 2005. Both Double Star TC 1 and the Cluster spacecraft
probed the near-Earth (< 15 RE) plasma sheet during this interval. First we used Wind observations of the
solar wind to drive our MHD simulation. Then using Cluster and TC1 plasma distribution functions as a guide we
traced particle trajectories backward in time in the electric and magnetic fields from the MHD calculation to
determine possible sources of the plasma sheet particles and found that most of the ions came from the low
latitude boundary layer (LLBL). Finally we launched ions forward in time from the LLBL and examined the
processes by which these ions populated the plasma sheet. We found that the plasma sheet was divided into
two segments by a previously existing neutral line which persisted even though the IMF was northward for several
hours. Dawnward of YGSM ~ 5 RE the LLBL particles first formed a plasma sheet boundary layer like
(PSBL) region and then populated the central plasma sheet from this PSBL. Duskward of 5 RE the plasma
sheet formed as a series of bands oriented approximately parallel to the equator. This structure closely
resembled that inferred from Cluster CIS observations. Both regions were formed independently by non-adiabatic
motion of the ions on closed field lines on the dawn and dusk sides of the tail. The source regions are separated
by flux rope like structures associated with the tail reconnection. The dusk side plasma sheet formed as a series
of independent ion beams each accelerated by non-adiabatic motion at different distances down the tail. The
result was that the entire dusk side plasma sheet consisted of a series of plasma sheet boundary layer like
regions.
SM12B-05 MHD Simulations of Turbulence in the Plasma Sheet
In this study we use our global magnetohydrodynamic (MHD) simulation code driven by an idealized time series
of solar wind parameters to investigate the global aspects of turbulence in the plasma sheet. As a first step we
will consider the effects of turbulence during southward Interplanetary Magnetic Field (IMF) conditions. During
southward IMF, localized and large scale reconnection drives flows and vorticity in the tail at different scales. We
impose a steady southward IMF at the upstream simulation boundary. This eliminates the solar wind as a source
of turbulence in the model and to investigate the relationship between reconnection-driven convection, waves on
the magnetopause boundary and turbulence in the magnetotail. For this study we used an enhanced resolution
code with a minimum grid spacing of the order of 0.12 RE. We also saved the results every second at an array of
points near the equatorial plane to obtain high resolution time series of the simulation results and to calculate
power spectra. The simulation shows that there is considerable wave activity along the magnetopause in
addition to the vorticity in the plasma sheet. We will quantify the turbulence in the plasma sheet by calculating the
power spectral density and probability distributions.
SM12B-06 Central Current Sheet Ion Distribution Function Signatures of Nonlinear Charged Particle Dynamics
We have recently developed a forward in time test particle code for calculating ion distribution functions
throughout the entire current sheet. The code use a time of flight binning algorithm and normalizes the
contribution of each particle to its phase space contribution in the asymptotic magnetic field region. We have
bench marked the code in the asymptotic field region against our previous results and show that the
characteristic ion distribution signature of nonlinear particle dynamics in a current sheet are reproduced with the
new algorithm. We have also used the code to examine the ion distribution function in the central current sheet
and found that the energy resonance signature is present throughout the sheet. The presence of the signature
throughout the entire sheet is significant in that typically the best observational measurements come from near
the field reversal whereas the majority of the theoretically constructed distribution functions are in the asymptotic
field region. In addition, we are currently applying the code to look for potential signatures of new discovered
resonance phenomenon in which the integral region of phase space expands and contracts as a function of
energy.
SM12B-07 Timing of O+ Access to the Plasmasheet During Geomagnetic Storms
We present results of a study using CLUSTER ion composition data from the CIS instrument to study ionospheric
ion access to the plasma sheet during storms. It is well known that the O+ content of the ring current increases
significantly during storms. In order to get to the ring current, the ionospheric ions must first get to the plasma
sheet, from where they can be convected into the ring current. Our earlier work on O+ in the plasma sheet has
shown that O+ is significantly enhanced during a storm main phase. In addition we have observed that tailward
moving O+ beams in the lobe, which are ions transported from the cusp, also increase during storm times. Other
work has shown that the O+ outflow from the cusp increases with increased solar wind pressure. Thus one
model of O+ access to the ring current would be that a pressure increase leads to enhanced outflow from the
cusp. These ions are then convected into the tail lobes, enter the plasma sheet, and finally convect into the ring
current. To test this model, we have identified storms which have a clear sudden increase in dynamic pressure,
and have extended CLUSTER coverage in the plasma sheet and lobes. We observe the timing of when O+
enters the plasma sheet, relative to the pressure increase, and relative to the drop in Dst. Our initial results
show that the O+ enters the plasma sheet about 3-5 hours after the pressure increase, and is in the plasma
sheet early enough in the storm main phase to contribute to the storm-time ring current.
SM12B-08 Effects of Magnetospheric Activity on the Current Sheet Energy Resonance Ion Distribution Function Signature
Theory and simulations of nonlinear charged particle dynamics in current sheet -like
magnetic fields have predicted a signature of the dynamics that manifests itself a series of peaks in the ion
distribution function. The separation of the peaks has been shown to scale as a combination of the fourth root of
the ion energy and parameters that describe the current sheet topology, i.e. the current sheet scale length, and
the ratio of the magnetic field strength at the mid-plane to the asymptotic magnetic field strength. This signature
has been observed during quiet times (Kp<1+) in both the AMPTEE and Geotail Data sets. In this paper, we
examine the effects of magnetopsheric activity (as measured by Kp) on the particle signature as measured by the
Geotail satellite in the region 20-25 earth radii downtail. We show that the signature is easily observed for Kp < 4-
and that as a general rule, the larger the value of Kp, the closer the peaks are together. For larger values of Kp we
find that whereas there is some indication of peaks in the ion distributions, the separations do not have the same
scaling and are often completely smeared out. We have also used the peaks in the distribution function coupled
with magnetic field measurements to determine the current sheet thickness. As would be anticipated, the sheet
is thinner for higher levels of magnetospheric activity.