SM41B-01 INVITED 08:00h
Scarf Award Presentation: Magnetosphere-Ionosphere Coupling Studies of Dayside High-Latitude Transients
Magnetosphere-ionosphere coupling studies are difficult since they (ideally) require simultaneous observations of the region of the magnetosphere generating field-aligned currents and of the ionospheric closure of those field-aligned currents. The dayside magnetosphere presents some advantages over the nightside magnetosphere with regard to these studies; the fields and plasmas in the dayside magnetosphere tend to be more ordered (dipolar) making, for example, fieldline mapping more tractable and dayside dynamics tend to be directly driven by solar wind forcing. This paper presents recent advances in the study of two classes of high-latitude dayside transients: Traveling Convection Vortices (TCVs) and Sudden Impulse (SI) events. We will show that TCVs are driven by cavities formed in the foreshock region that in turn deform the magnetopause locally. The deformations of the magnetopause drive magnetospheric plasma motions that generate field-aligned currents. TCVs are the ionospheric signature of the closure of these field-aligned currents. A similar sequence of events occurs during SI events, with the exception that they are driven by sharp solar wind pressure discontinuities in the ambient solar wind. Due to the nature of the solar wind driver, SI events are more readily modeled using existing global MHD simulations. We will show that existing MHD codes reproduce the large-scale features of these events rather well. Finally, we will investigate how the transient ionospheric flows associated with these events can cause intense frictional ion heating, a result that may explain the observed correlation between dayside ionospheric outflow and the standard deviation of the solar wind dynamic pressure.
SM41B-02 08:20h
Magnetospheric response to the interplanetary shocks and pressure increases: Cluster observations of sudden impulses in the tail lobes
The sudden impulses (SI) in the tail lobe magnetic field due to interplanetary shocks/pressure increases are examined using Cluster observations for the July-November periods in 2001 and 2002. The SI events were investigated with the minimum variance analysis to study the magnetic field changes in detail. The multispacecraft feature of Cluster allows us to calculate the propagation speed for the disturbance between the spacecraft. Furthermore Cluster plasma data provides us details of the tail compression, e.g. showing the motion of the magnetopause towards Cluster. The SI events in this study were associated with strong interplanetary shocks or substantial enhancements in the solar wind dynamic pressure and clear (> 10 nT) increases in the lobe magnetic field magnitude. The good upstream solar wind coverage (ACE, WIND, Geotail) allowed us to calculate the shock normal orientations, propagation speeds and to perform reliable timing analysis between upstream observations and tail lobe SIs. Also a two-dimensional Cartesian model, (where a vacuum uniform tail lobe magnetic field is compressed by a step-like pressure increase) was applied to the SI events. Examples of the lobe SI events will be presented. The characteristics of the rotation in the magnetic field components and the results of the timing analysis and modeling strongly suggest that the tail lobe SIs are due to the increase in solar wind dynamic pressure outside the tail boundary. The magnetotail is found to remain in constant equilibrium as the pressure enhancement convects down the tail.
SM41B-03 08:34h
Direct Measurement of Ionospheric Flows Associated with a Sudden Impulse Event
We present the first direct observations of high-latitude ionospheric flows associated with the passage of a Sudden Impulse (SI) event. The SI event is caused by the passage of a solar wind discontinuity during which the solar wind pressure rapidly decreased from 8 to 2 nPa. This rapid reduction in solar wind pressure deforms the magnetopause, reconfiguring the outer magnetosphere and driving field-aligned currents that close in the high-latitude ionosphere. The new observations, obtained with the SuperDARN network of coherent backscatter radars and the Sondestrom incoherent radar, allow for the direct calculation of the strength and spatial extent of SI field-aligned currents. Ground-based magnetometer observations are also employed for a complete characterization of the transient field-aligned current system. Together, these measurements provide a unique opportunity to observationally and quantitatively relate the magnetospheric generation and the ionospheric closure of a magnetospheric current system. The results are also compared with global MHD simulations of the event and similar events to examine the nature of magnetosphere-ionosphere coupling processes during this class of events.
SM41B-04 08:48h
Super Dual Auroral Radar Network Observations of Ionospheric Convection Enhancements Driven by Solar Wind Dynamic Pressure Fronts
It is well known that the Interplanetary Magnetic Field (IMF) is the major contributor to geomagnetic activity on Earth. Recent studies, however, have shown that solar wind dynamic pressure variations cause global effects when they encounter the terrestrial magnetosphere. In particular, it has been shown that solar wind dynamic pressure enhancements significantly increase particle precipitation and cause global intensification of the aurora. Further studies using Defense Meteorological Satellite Program (DMSP) measurements have demonstrated that solar wind pressure increases also significantly affect the cross-polar-cap potential drop. This implies that the dynamic pressure has an important effect on the coupling efficiency between the solar wind and the Earth's magnetosphere, which is in addition to that due to the IMF. It was previously suggested, based on the DMSP data, that solar wind dynamic pressure enhancements induce enhanced magnetotail reconnection and magnetospheric convection. We now present Super Dual Auroral Radar Network (SuperDARN) observations for a number of events that demonstrate significantly enhanced ionospheric convection in the dayside ionosphere associated with the impact of solar wind pressure fronts. The enhanced convection extends to the vicinity of the expected location of the dayside separatrix, suggesting that the solar wind dynamic pressure strongly affects dayside reconnection as well as polar-cap convection.
SM41B-05 09:02h
Positive/Negative Effects of Solar Wind Density on Auroral Electrojets
Auroral electrojets are a channel of electric currents flowing over the auroral oval. Their development is controlled by magnetic reconnection between the Earth's magnetic field and southward interplanetary magnetic field (IMF) and also by compression of the magnetosphere by enhanced solar wind density and velocity. The subject of effects of solar wind density on auroral electrojets has recently received increasing attention. With ``unbiased'' events in which only the solar wind density varied significantly but other solar wind parameters were relatively constant, we calculated the rates of change of auroral electrojets per unit change of solar wind density. Here we report that the distribution of the rates of change in terms of IMF Bz shows a butterfly pattern. The rates are found to be small for northward IMF. As the southward IMF becomes larger, however, the distribution bifurcates -- one branch extends to positive rates and the other extends to negative rates. It has commonly been believed that the enhanced solar wind density increases the auroral electrojets. Contrary to this expectation, the enhanced solar wind density can decrease the auroral electrojets with high probability.
SM41B-06 09:16h
3D Global Simulation of the Interaction of Interplanetary Shocks with the Magnetosphere
It is well known that the sudden commencement of geomagnetic storms may be caused by the global compression of the magnetosphere as a result of interplanetary disturbances such as shocks, dynamic pressure pulses etc. Before interplanetary shocks reach the magnetopause, they interact with the bow shock and traverse the magnetosheath. Understanding the interaction of interplanetary shocks with the magnetosphere is crucial to space weather studies. In this paper, the collision of interplanetary shocks with the bow shock and magnetopause and their propagation towards the magetotail are investigated using the Piecewise Parabolic Method (PPM) MHD code in 3D. The PPM algorithm is a high-order extension of the Godunov scheme accurate to third-order in smooth regions of flow), which can capture shocks and discontinuities within 1-2 grid points with negligible numerical dissipation. The arrival of an interplanetary shock wave causes the abrupt displacement of the whole bow shock-magnetopause system toward Earth, and the moving shock remains nearly planar (the shape of the undisturbed shock) as it passes through the magnetosheath. The results are mostly consistent with the gas dynamic work of Stahara and Spreiter.
SM41B-07 09:30h
Comparison of Geosynchronous Magnetic Field and Global MHD Simulation Results
Global MHD simulations have shown increasing accuracy at reproducing magnetic storm features. One important feature that is necessary to predict, and which in fact can be used as a metric for accuracy of the simulation, is the geosynchronous magnetic field. In this paper we will present several comparisons of geosynchronous magnetic field measurements from the GOES spacecraft and results from the global Lyon-Fedder-Mobarry magnetohydrodynamic code for periods when it is driven by solar wind observations.
SM41B-08 09:44h
Energy Budget in Global Magnetosphere-Ionosphere simulations.
After determining the boundary of the magnetosphere and magnetosheath through plasma flow line tracing that separates the magnetosheath from the magnetosphere and magnetotail, we can determine surface integrals of various energy fluxes and calculate energy content in the volume inside the surface. To assess the flow of energy from the solar wind into the magnetosphere, however, the time-dependent shape has to be taken into account in addition to integrals over the magnetosphere surface at a given time. In this work we estimates changes of total magnetospheric energy content from energy flow on the surface and from volume changes and compare these to volume integrals of the internal energy. Simulations of storm events used in this study were performed at CCMC using the BATSRUS (T. Gombosi) and UCLA-GGCM (J. Raeder) models. In order to obtain definitions of magnetosphere regions independently for the flow surface we use thresholds of physical variables such as the magnetic field strength, plasma density and temperature to distinguish magnetotail lobes, plasma sheet, and inner magnetosphere and to calculate contributions to the total energy conent in these regions. Our simulations complement results obtained with the LFM model (Guild et al., 2004, EOS Trans. AGU {\bf 85}(17), Joint. Assem. Suppl., Abstract SM33A-19) and GUMICS-4 model (Palmroth et al., 2003, JGR {\bf 108}, 1048). These simulations are part of the MHD code validation and verification efforts that are performed at the CCMC.