SM33B-1336
A study on the ring current asymmetry during super-intense magnetic storms
We have investigated the ring current asymmetry during super-intense magnetic storms (peak Dst ¡Ü-250 nT). In order to conduct this study, we have used high-resolution low-latitude ground-based magnetometer data. After removing the Sq variation, the disturbance in the H component (dH) was determined for each observatory and each superstorm. For each superstorm, we determined the peak dH and local time of occurrence for each observatory. With this information, we obtained a distribuiton of local-time X dH for all the superstorms studied. We have observed that most of the superstorms have the peak in dH observed at the dusk sector, but for a few of them the peak occurs near mid-night sector. These results are compared with previous works concerning storms of different intensity levels.
SM33B-1337
Effects Of Pressure Anisotropy On The Magnetic Field In The Inner Magnetosphere
We present results from a version of an equilibrium solver that has been modified to include the effects of anisotropic pressure. The equilibrium solver uses a frictional technique to iterate a set of modified MHD equations to equilibrium. The initial pressure distribution is determined from an empirical model of Lui et al [1994] that specifies the pressure anisotropy as a function of position in the tail. In this model, the region beyond approximately 15 Re is assumed to have isotropic pressure, consistent with observations. The initial magnetic field is a Tsyganenko [1989] magnetic field model. For various magnetospheric conditions, we will display the differences between the resulting anisotropic-equilibrium magnetic field and an equilibrium computed from the isotropic version of the code for the same total thermal energy.
SM33B-1338
Magnetospheric ULF waves directly driven by solar wind oscillations: An observational assessment
The Ultra-low-frequency (ULF) waves in the magnetosphere play an important role in energizing the radiation belt particles. Understanding how the solar wind directly and indirectly generates ULF waves in the magnetosphere can help build an empirical model of ULF spectrum that is based solely on the solar wind data. It is well understood that various macro-scale solar wind conditions can favor the excitation of ULF waves. The Kelvin- Helmholtz instability during high-speed solar wind and the dayside and nightside reconnections during southward IMF are two examples for the indirect association between the solar wind and magnetospheric ULF waves. In recent years, an increasing amount of attention is paid to the direct driving of magnetospheric ULF waves by solar wind oscillations. These ultra-low-frequency oscillations in the solar wind may originate in part from the normal-mode oscillations of the Sun. In this study we assess the importance of the solar wind oscillations in directly driving the magnetospheric ULF waves by examining the correlation between the solar wind and magnetospheric observations. The data include the high-cadence interplanetary field records collected by the ACE spacecraft, the magnetic field data from Polar and GOES satellites, and the ground magnetometer data from the Mid-continent Magnetoseismic Chain (McMAC).
SM33B-1339
Resonance Zones for Electron Interaction with Plasma Waves in the Earth's Dipole Magnetosphere
Wave-particle interactions play an important role in radiation belt particle dynamics. For the Earth's dipole magnetosphere,we determine regions of electron cyclotron resonance with various types of plasma wave. We call these regions resonance zones. The spatial extent of a resonance zone is controlled by the Doppler cyclotron resonance condition and the wave dispersion relation. For a given wave mode, a resonance zone depends on the wave frequency,electron energy,pitch-angle,and the local values of the electron number density and magnetic field. In order to take account of density variation along magnetic field lines,we adopt three different models for the spatial distribution of electron density inside and outside the plasmasphere. For subluminous waves including whistler-mode chorus,whistler-mode hiss,and electromagnetic ion cyclotron waves,we construct resonance zones for both field-aligned and oblique propagation with respect to the background dipole field. We also determine resonance zones for the superluminous (AKR) R-X, L-O, and L-X modes. Resonance zones are useful in analyzing electron dynamics in the inner magnetosphere, in particular in conjunction with ray-tracing studies and observed spatial distributions of plasma waves.
SM33B-1340
Dependence of Whistler-mode Wave Induced Electron Precipitation on k-vector Direction.
Whistler-mode waves that are either spontaneously generated in-situ (i.e., chorus), or externally injected (lightning, VLF transmitters) are known to be responsible for the loss of radiation belt electrons. An important determinant in the quantification of this loss is the dependence of the cyclotron resonant pitch angle scattering on the initial wave normal angles of the driving waves. Inan et al. (U.S. Inan et al., Controlled precipitation of radiation belt electrons, Journal of Geophysical Research-Space Physics, 108 (A5), 1186, doi: 10.1029/2002JA009580, 2003.) suggested that the lifetime of > 1 MeV electrons in the inner radiation belts might be moderated by in situ injection of VLF whistler mode waves at frequencies of a few kHz. The formulation of Wang and Bell (T.N.C. Wang and T.F. Bell, Radiation resisitance of a short dipole immersed in a cold magnetoionic medium, Radio Science, 4(2), 167-177, February 1969) for an electric dipole antenna located in the inner magnetosphere established that most of the radiated power is concentrated in waves whose wave normal angles lie near the local resonance cone. Such waves, compared to those injected at less oblique initial wave normal angles, undergo several more magnetospheric reflections, persist in the magnetospheric cavity for longer periods of time, and resonate with electrons of higher energies. Accordingly, such waves may be highly effective in contributing to the loss of electrons from the inner belt and slot regions [Inan et al., 2006]. Nevertheless, it has been noted (Inan et al. [2006], Inan and Bell [1991] and Albert [1999]) that > 1 MeV electrons may not be effectively scattered by waves propagating with very high wave normal angles, due to the generally reduced gyroresonant diffusion coefficients for wave normals near the resonance cone. We use the Stanford 2D VLF raytracing program to determine the energetic electron pitch angle scattering and the precipitated flux signatures that would be detected for a range of initial wave normal angles. We conclude that whistler-mode waves with highly oblique wave normal angles may be more effective than previously believed at precipitating > 1 MeV electrons, despite the dependence of the scattering coefficients on wave normal direction.
SM33B-1341
Occurrence Pattern of Whistler Mode (WM) Echoes Observed by RPI/IMAGE as a Function of Geomagnetic Activity
Discrete and diffuse magnetospherically reflected (MR) and specularly reflected (SR) whistler-mode (WM) echoes have been observed by the Radio Plasma Imager (RPI) on the IMAGE satellite ( Sonwalkar et al., J. Geophys. Res., 109, A11212, 2004; Sonwalkar et al., AGU Fall Meeting 2006, abstract SM11B-0323, 11-15 December, San Francisco, California, USA). Discrete WM echoes occur under relatively smooth plasma density variation along the field line passing through IMAGE (B0) and diffuse WM occur when small scale (~10-100 m) field aligned irregularities (FAI) are present near B0. The study presented here is based on WM echo observations during 2004-2005 when 3.2 ms short pulses in 6-63 kHz frequency range with 0.3 kHz linear stepping (Program 38) were transmitted at altitudes less than ~6000 km. We examined the occurrence of discrete and diffuse SR- and MR-WM echoes observed in Aug-Dec 2005 during geomagnetically quiet periods, Kp < 3-4 and Dst is varying between -10 nT and 40 nT, and during geomagnetically disturbed periods. The disturbed periods included three geomagnetic storms (Kp>4 and Dst below -100 nT at the onset of the storm) and their recovery periods. In all ~400 and ~600 transmissions of program 38 were made during the geomagnetically quiet and disturbed periods, respectively. The occurrence rate per transmission for a WM echo (irrespective of the type of echo) was around ~30% and ~25% for quiet and disturbed periods, respectively. We found that the occurrence rate per transmission of diffuse MR-WM echoes was not significantly affected but the occurrence rate of discrete MR-WM had doubled from quiet to disturbed periods. We found significantly fewer (one-fourth) discrete SR-WM echoes and slightly larger number of diffuse SR-WM echoes during the disturbed periods compared to that during the quiet periods. These observations suggest that geomagnetic storms lead to significant enhancements in density structures (FAIs) at altitudes lower than 1000 km, thereby affecting primarily the propagation of SR-WM echoes that are reflected at ~90 km but not that of MR-WM echoes because they are reflected at altitudes >1000 km. It is, however, interesting that the probability of discrete MR-WM has doubled from quiet to disturbed periods.
SM33B-1342
Magnetospheric field reduction in response to enhanced solar wind dynamic pressure
High-time resolution magnetic field observations at GOES and Polar satellites have been used to investigate the magnetospheric response to sudden enhancements in the solar wind dynamic pressure (Psw). Over much of the magnetosphere, the magnetic field strength is enhanced when the Psw increases, as seen in ground-based magnetometer data at low latitudes. In some cases, however, the Psw increases can lead to sudden reductions in the magnetic field strength. The decrease in the magnetic field strength starts almost at the same time as a passage of the fast mode wave in the magnetosphere. In this study, we present preliminary results of some Psw-induced magnetic field reduction events and discuss the generation mechanism.
SM33B-1343
Improvement of Electric Field Model in the Inner Magnetosphere Using Cluster Data Set
We are developing an inner magnetospheric electric field (UNH-IMEF) model at
2
SM33B-1344
Calculation of Synthetic Ground Magnetograms From Current Distributions Calculated by Large-Scale Magnetosphere-Ionosphere Coupling Codes
Global MHD models, ring-current models, and convection models calculate the large-scale currents in the magnetosphere-ionosphere system. Traditionally, these models have been tested against single-point magnetic field measurements made by individual spacecraft, which often do not provide a clear picture of the pattern of model-data discrepancies. The global network of ground-based magnetometers constitutes a large source of data that is currently being underutilized for validation and analysis of these models. This is primarily due to the lack of a good code that accurately makes the connection between the model outputs and ground magnetometer data. We have developed a numerical algorithm to compute realistic ground magnetic field perturbations from these models. A generalized version of the software can be used with a wide variety of large-scale magnetosphere-ionosphere models. We show initial calculation results of Dst and UT-LT contour maps of low latitude magnetic perturbations for simulation results from one or more large-scale models.
SM33B-1345
Investigation of Magnetic Storms Impact on the Magnetosphere's Morphology and its Current System by Global Hybrid Models
\begin{document} Two and three dimensional hybrid models of massless fluid electrons and kinetic ions are utilized to investigate the impacts of interplanetary shocks presumably resulting from magnetic storms on the magnetosphere. The codes are structured to model the magnetosphere dynamics of the Earth-Solar wind system by utilizing finite element mesh systems specifically tailored to magnetosphere's regions. The 2.5d spans many hundred Earth radii in each direction (upstream, downstream, dawn and dusk); the 3d finite element code which is recently developed focuses primarily on the inner magnetosphere. Realistic parameters characteristic of solar wind, its IMF and geomagnetic field are used. The codes have been tested by their ability to predicting a magnetosphere by initializing a dipole at equilibrium with a flow subjected to an incoming solar wind with an IMF. The tests revealed generation of a steady state bow shock, as well as dayside reconnection (for southward IMF) as well as a tail sheet formation. The interplanetary shock is generated by a sudden enhancement of the incoming IMF by an order of magnitude. This act introduced a fast MHD shock which propagated downstream and collided with the bow shock. This collision resulted not only in a steep rise in density and temperature of the bow shock, but also in the tail sheet region as the shock propagated downstream. The densities and temperatures, though, eventually relaxed to what are normal bow shock and tail values as the fast shock left the simulation domain. The equatorial plane current system initially axisymmetric experienced a sharp increase coupled with broken symmetry upon the storm's passage. The latter results are consistent with recent results reported by Tsyganenko and Sitnov (JGR, Vol. 112, A06225, 2007). The ion velocity flow fields also demonstrated features consistent with the equatorial current system primarily on the dayside. These investigations are aimed at better understanding of the transport of energy and momentum by geomagnetic storms into the inner magnetosphere by kinetic processes as well as validating the presumed structure of the inner magnetosphere's ring current system. This work is supported by the NSF-ATM-0651690. \end{document}
SM33B-1346
The inclusion of SAPS in the inner-magnetospheric electric field description
The sub-auroral polarization stream, or, SAPS, is a latitudinally narrow channel of enhanced westward plasma drift located at the inner edge of the electron plasma sheet. Its dynamics and morphology are controlled by the coupled magnetosphere-ionosphere system. The mid-latitude electric field associated with the channel is an important feature of any inner-magnetospheric electric field (IMEF) description. Existing IMEF descriptions often do not include SAPS (e.g., the Volland-Stern model). We present an effort to advance semi-empirical SAPS modeling efforts by examining the spatio-temporal evolution of SAPS during geomagnetic storms and substorms using the publicly available Defense Meteorological Satellite Program's (DMSP) Midnight Boundary Index (MBI) and SSIES ion drift meter datasets, and the Cluster electric field datasets. We compare the DMSP/Cluster SAPS characteristics to existing statistical descriptions of SAPS. We also explore the marriage of SAPS models to existing IMEF models, such as Volland-Stern and UNH-IMEF.