Plasma Sheet I Posters
Presiding: I Doxas, University of Colorado; M Shappirio, NASA Goddard Space Flight Center
SM43A-01 1330h
A Comparison of the Effectiveness of new Solid State Detector Technology as Particle Detectors.
There is great interest in expanding the use of solid state detectors (SSD) in the field of particle detection. For instance, extending the sensitivity of the detectors to lower energy particles could allow for simpler and thus smaller instrument designs. Several new technologies have been introduced recently in the region of semiconductor material for SSDs. This work will compare some of the more promising technologies to SSD material currently being used. In particular recent results from several laboratories make it possible to compare the properties of "100% internal quantum efficiency silicon photodiode", delta doped silicon, and silicon carbide (SiC) detectors and to compare these new technologies to pure silicon and germanium SSDs. The properties of interest are the detectors gain, ability to withstand the temperature extremes of space environments, radiation hardness, and responses to incident particles mass and energy particularly at low energies.
SM43A-02 1330h
A Kinetic Interchange Instability in the Magnetotail
The classical ballooning mode is driven by a large enough pressure gradient in the same direction as the magnetic curvature. An alternative driver for interchange/ballooning modes in a high beta system is a tailward-directed gradient in the equatorial magnetic field profile such as is frequently formed in the magnetotail during extended periods of plasma-sheet convection. A massively parallel 3-D PIC code is used to determine the properties of this mode. The linear mode spectrum is dominated by relatively short wavelengths satisfying ky ρin ~ 5, where ρin is the ion gyroradius in the local normal magnetic field. The observed growth rate is γ/‰i0 ~ 0.1, which is much smaller than predicted by a simple fluid analysis. A kinetic treatment is used to obtain this result. The real frequency is comparable to the growth rate, and the phase velocity is directed in the direction of the ion drift. Nonlinearly, the mode evolves to produce the classic extended fingers of the Rayleigh-Taylor instability. The mode does not lead to a disruption of the global current sheet, but it does produce a local dipolarization. Further implications for magnetotail dynamics will be discussed.
SM43A-03 1330h
Effects of Wave Heating and Electron Precipitation on Ion Distributions in the Dynamic Transition Region
A Dynamic Fluid Kinetic (DyFK) simulation is conducted to study the H+/O+ flows and distribution functions in the high-latitude dynamic transition region, specifically from 1000 km to 3000 km. In this case, the simulated flux tube, which extends from 120 km to 3 RE altitude, is assumed to experience 20 minutes of auroral effects, including both soft electron precipitation and transverse wave heating, and then allowed to relax in the absence of such auroral effects for another 80 minutes. In the transition region the O+ transverse thermal energy attained values up to 8 eV, while the H+ transverse temperature remained below 0.6 eV. During the presence of the auroral effects, the O+ bulk velocity increased continuously with altitude and exceeded 4 km s-1 at 3000 km altitude within 10 minutes after the initiation of the auroral effects. At least two ion distribution function types specifically associated with the transition region were seen in the simulation: (1) An H+ polar wind distribution with a downward tail or heat flux, caused by Coulomb collisions with stationary or even downward flowing O+, and (2) "Upwelling" O+ and H+ ion distributions in which the higher energy portions displayed conical features caused by transverse ion heating, while the lower energy interior cores were nearly-isotropic Maxwellians in which Coulomb self-collisions were dominant processes.
SM43A-04 1330h
Multi-Spacecraft Observations of Chorus Dispersion and Source Location
Measurements from the Polar spacecraft have put an upper limit of around 1000 km to the size of the magnetospheric chorus generation region along the magnetic field line in agreement with earlier theoretical estimates. The first multipoint observations from the four Cluster spacecraft reveal that a single chorus wave packet can appear differently between spacecraft. One spacecraft observes a chorus element which is frequency shifted and time delayed relative to another. Since the dispersion relation for chorus waves is known, these differences provide a new opportunity to refine the parallel dimension of these source regions. A cross-correlation analysis is used to identify common chorus events between the Cluster spacecraft and to quantify their timing delays. The timing arrival differences and frequency accessibility are then simulated with a ray-tracing technique. Locations in the magnetosphere where the simulated frequency shift and time delay match the observed frequency shift and time delay for a single chorus event are determined as possible source regions. The ray-tracing technique is shown through the identification of a source region for a few typical chorus examples. By studying several chorus events using these techniques a statistical analysis is used to identify chorus source regions and determine the size of these source regions, thereby providing new insights into this wave mechanism.
SM43A-05 1330h
Whistlers in the Inhomogeneous Plasma
Results from a numerical study of the dynamics of the whistler waves in the magnetospheric plasma are presented. In this study the plasma is considered to be homogeneous in the direction along the ambient magnetic field and strongly inhomogeneous across it. Thus the goal of this investigation is to understand the dynamics of the whistlers inside the plasma chanel (duct) extended along the ambient magnetic field and localized across it. The plasma density inside the duct can be higher (high-density duct) or lower (low-density duct) than the density outside the duct. Analysis of the whistler dispersion relation predicts trapping of the wave inside the duct depending on the parameters of the plasma, ambient magnetic field, and the whistler itself (frequency, perpendicular and parallel wavelength). But because this dispersion relation is derived assuming the validity of the geometrical optic approximation, it does not describe effects of the strong density gradients forming the walls of the duct on the dynamics of the whistler. Numerical simulations of the quasi-longitudinal, electron MHD model, providing a full wave description of the whistler, reveal that these effects can be quite important, particularly when the perpendicular wavelength of the whistler is comparable to the scale-size of the plasma inhomogeneity. In particular, simulations demonstrate how the quality of the high-density duct depends on the scale-size of the transverse inhomogeneity.
SM43A-06 1330h
Mirror Mode Waves Detected by Polar in Low Field Regions Near Midnight
The present study examines the magnetic field at the apogee of the Polar spacecraft when its apogee was close to the equatorial plane and within 2 hours in local time of midnight. In this region the magnetic field magnitude is rarely below 5 nT. However, when it is this low, it can become unstable to mirror mode oscillations which then reduce the field strength even lower locally to less than 2 nT. One interval of mirror mode waves caused the minimum total field to approach 1 nT. We survey their occurrence rate and the conditions that lead to their appearance.
SM43A-07 1330h
The Stress Tensor in the Current Sheet and the MHD Closure
MHD closure of the moment equations neglects divergence of the off-diagonal elements of the stress tensor, , as well as the divergence of the heat flux ∇ q. In the center of the plasma sheet the local ion gyroradius becomes large, and q and become important components of the current sheet energy and momentum balance. Particle simulations have been used to calculate the contribution of the off-diagonal elements of the stress tensor to the current sheet force balance using a time dependent tearing mode model. The simulations have a large enough number of particles to calculate higher order moments reliably. Results show that force balance in the current sheet can be dominated by the off-diagonal elements of the stress tensor which are neglected by the MHD closure.