SM23D-01 INVITED
Storm-time Distortion of the Near Magnetosphere as Revealed by Data-Based Models
An overview will be given of recent progress in the empirical modeling of the geomagnetic field, focused on the storm-time reconfiguration of the inner magnetosphere. Data-based modeling still remains one of primary techniques for representing and forecasting the structure and dynamics of the inner magnetosphere, an extremely hard task for the first-principle approaches. The storm-time evolution of the inner geomagnetic field was quantitatively modeled by Tsyganenko, Singer, and Kasper [2003], where it was shown for the first time that a global dramatic distortion of the magnetic field during strong storms could penetrate as deep as to only R ~ 3RE. The dynamical aspects of storm-time magnetic field distortions were further elaborated in the TS05 model [Tsyganenko and Sitnov, 2005]. It was shown, in particular, that the magnetospheric response to the solar wind loading is drastically different for major current systems (ring, tail, and field-aligned currents), with the fastest response for field-aligned currents and the slowest for the symmetrical ring current. Finally, to resolve the complex spatial structure of storm-time magnetic field distortions Tsyganenko and Sitnov [2007] elaborated a new model with a high-resolution extensible approximation for the field of equatorial currents combined with a flexible field-aligned current module. The new model was fitted to a new database of space magnetometer data from Geotail, Polar, Cluster, IMP-8, and Goes-8,-9,-10, and -12, as well as concurrent solar wind and IMF data, spanning more than a decade (1995-2005). The new high-resolution magnetic field modeling revealed many interesting effects that were conjectured earlier based on other (largely indirect) methods, such as the pile-up of the magnetic flux near the magnetopause for northward IMF and the strong erosion/depression of the dayside field during intervals of southward IMF. It also revealed a dramatic difference between the global configurations of equatorial electric current systems at different phases of a storm. During the main phase, the current distribution has an interesting, highly deformed ‘hook-like' shape, which has virtually nothing to do with a traditional notion of a closed ring current. In contrast, at the recovery phase the model reveals an almost completely axisymmetric smooth pattern, without any distinct boundary between the ring and tail currents. We will also discuss the use of the empirical dynamical models of the geomagnetic field for calculations of the inductive storm-time electric fields in the inner magnetosphere [Ukhorskiy et al., 2006].
SM23D-02 INVITED
Self-consistent Coupling Between Plasma and Fields in the Inner Magnetosphere
Observations show the magnetic field in the inner magnetosphere to be significantly distorted (depressed) during geomagnetic storms. While caused by the large amount of plasma injected in the inner magnetosphere, these distortions in turn strongly influence the evolution of particle populations in the near-Earth space. Therefore, in order to understand and model accurately the inner magnetosphere dynamics one needs to account self- consistently for the complex interaction between plasma and fields. To this end, we have developed a self- consistent inner magnetosphere code, obtained by iteratively coupling a kinetic ring current model with a 3-D plasma equilibrium code. In our approach, the magnetic field is computed in force balance with the kinetic model anisotropic pressures, and then fed back into the kinetic code. Here we report results obtained with our technique, improved to a full coupling of the two models (every 5 min.), and using magnetic boundary conditions from realistic empirical models such as T04S. Another new feature of our approach is the inclusion of the electric field induced by the time change of the self-consistent magnetic field, in a form greatly facilitated by the Euler potential representation of the latter. Using our improved self-consistent model, we simulate actual geomagnetic storms and we discuss the differences between the self-consistent results and those from kinetic model runs with a dipole field. We also assess during various storm phases the strength and location of magnetic field depressions, as well as of the induced electric field. Finally, we discuss how the distorted magnetic field differentially affects the energization of ring current particles at various pitch angles.
SM23D-03
Role of Small-Scale Variations of the Electromagnetic Fields in the Evolution of Energetic Particle Populations in the Near-Earth's Magnetosphere
Understanding the time-varying electromagnetic field configuration and the consequent charged particle dynamics in the Earth's space environment is fundamentally important both for scientific and space weather purposes. We have developed a time-evolving model for the inner magnetosphere magnetic field. Based on available in-situ observations of the magnetospheric magnetic field, the model gives a global representation of the magnetic field evolution during specified time periods. The main advantage of this event-oriented model is its ability to reproduce both the larger-scale and smaller-scale variations of the magnetic field during substorms and storms. We have incorporated this model into our particle tracing procedure, in which we trace particles with arbitrary pitch angles numerically in the drift approximation. From the other hand, we represent substorm-associated electromagnetic fields by adding electromagnetic pulses. There electric field is given by Gaussian pulse with azimuthal field component propagating inward with a velocity dependent on radial distance. The magnetic field from this pulse is calculated by Faraday's law. We model particle inward motion and energization by a series of electric field pulses representing substorm activations during storm events. We model two storms on October 21-23, 2001 and April 18, 2002, with characteristic saw-tooth activations observed in particle fluxes at geostationary orbit and magnetic field variations. We study the role of small-scale variations of the electromagnetic field in the evolution of energetic particle populations in the near-Earth's magnetosphere.
SM23D-04 INVITED
A Parameterized, Global Model of the Subauroral Electric Fields
Any models attempting to describe the structure of the thermosphere, the ionosphere, the plasmasphere, the inner magnetosphere and the ring current are dependent on the spatial and temporal distribution of the magnetospheric electric fields. As the inner magnetosphere is coupled to the subauroral ionosphere via magnetic field lines, the magnetospheric electric field structure is in turn dependent on the ionospheric electrodynamics. Electric fields influence numerous processes in the ionosphere/thermosphere (IT) system including plasma transport in the ionosphere, the ion drag force which affects neutral winds, and the Joule heating which drives much of the composition and structure of the IT system. These electric fields can extend to very low latitudes and can contribute substantially to the magnetospheric electric field structure, particularly during geomagnetic storms. The subauroral electric fields are known to reduce the ionospheric conductivity through fast chemistry and transport which produces a feedback mechanism into the magnetosphere through coupling along magnetic field lines further increasing the magnetospheric electric fields. They modify the plasmaspheric structure and drive the plasmasphere/electron plasmasheet interface (the plasmapause) inward. They are also extremely important to ring current formation and decay in that they enhance ring current formation during the storm main phase and slow storm recovery by providing a mechanism for transport of ring current particles to low L-values. Modelers have begun to understand the importance of the subauroral electric field coupling to the thermosphere, the inner magnetosphere and the plasmasphere and are clearly in need of an empirical model describing the global distribution of the subauroral electric fields. We are developing a parameterized, global model of the subauroral electric fields that will adjust to inputs of measured values of the equatorward edge of the auroral oval and the subauroral electric field and will present preliminary results of our efforts.
SM23D-05
Diagnosing and Modeling Distortions of the Subauroral Electric Field
During geomagnetic activity, coupling between the ring current and sub-auroral ionosphere (via region 2 Birkeland currents) can create ionospheric electric fields--either enhancements such as the subauroral polarization stream (SAPS), or shielding--that have a dramatic effect on the dynamics of the plasmas of the inner magnetosphere. We compare the output of a simple computational model of the plasmasphere to observations, both remote sensing by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite and in situ by the geostationary satellites of the Los Alamos National Laboratory (LANL), and show agreement to within 0.2-0.7 RE of the observed plasmapause and/or plume location. These comparisons clearly demonstrate the need to include this region 2 coupling system. Observations during substorms and mild convection enhancements imply that there is a finite speed for the effects of substorms and convection to propagate through the inner magnetosphere, and that this propagation can create turbulent-like flow bursts and undulatory motions at the plasmapause boundary. We show that these effects are a consequence of the finite speed of plasma responding to subauroral convection.
SM23D-06
Cluster Observations in the Inner Magnetosphere during the 18 April 2002 Sawtooth Event: Dipolarization and Injection at r = 4.6 Re
The present study examines a sawtooth injection event that took place around 08 UT on 18 April 2002 when the Cluster spacecraft were located in the inner magnetosphere in the premidnight sector. In association with this injection, Cluster, at a radial distance of 4.6 RE, observed that the local magnetic field became more dipolar and that both ion and electron fluxes increased without notable energy dispersion. These features were accompanied by intensifications of the equatorward component of a double-oval structure and also by an enhancement of the ring-current oxygen ENA flux. The event was also accompanied by large magnetic field (a few tens of nT) and electric field (a few tens of mV/m) fluctuations with a characteristic timescale of a few tens of seconds. These observations strongly suggest that this sawtooth injection extended not only widely in local time but also deeply into the inner magnetosphere. Interestingly, Cluster repeatedly observed dipolarization-like signatures afterward, which, however, were not associated with an enhancement of local energetic ion flux or with geosynchronous dipolarization or injection signatures. Instead, these magnetic signatures were accompanied by oscillatory plasma motion in the radial direction with a characteristic timescale of about 10 min, which appears to be related to the westward propagation of a spatially-periodic auroral structure. The associated azimuthal electric field component was well correlated with the time derivative of the north-south magnetic field component, suggesting that the observed electric field is inductive. These findings suggest that electromagnetic processes far inside geosynchronous orbit play an important role in energization of energetic ions and auroral dynamics during magnetospheric storms.