SM33A-01
Modeling the O+ trough region in the polar cap ionosphere-magnetosphere coupling region
Ion measurements by the Thermal Ion Dynamics Experiment(TIDE) on the POLAR spacecraft show that the O+ densities in the polar cap near 5000 km altitude display normal and low density (trough) regions. We use the UT Arlington Dynamic Fluid-Kinetic (DyFK) model to model such O+ density profiles. Using solar wind parameters and incorporating auroral fountain effects for these events to drive a time-varying high-latitude convection model and auroral processes of soft electron precipitation and wave-driven ion heating, we treat the evolving high-latitude ionospheric plasma transport and associated parameter profiles for several convecting flux tubes in the high-latitude ionosphere-magnetosphere system, incorporating estimated locations of the auroral processes regions from the Ovation auroral oval model. This auroral oval model has as inputs auroral precipitation measurements from DSMP. For the convection patterns thus computed, these flux tubes nominally intersected the POLAR trajectory where the density measurements were made. It is found that, owing chiefly to F- region recombination processes during trajectory segments when the low altitude portions of such flux tubes in darkness, as well as incorporating auroral fountain effects in the auroral region, normal and low trough-like densities at higher altitudes developed along these flux tubes. The modeled densities near 5000 km altitudes will be compared with POLAR/TIDE-measured O+ densities for inside and outside of these observed trough regions.
SM33A-02
Magnetosphere-ionosphere coupling at auroral latitudes via Alfven waves
Two salient features of the auroral topside ionosphere are the occurrence of (a) the auroral acceleration region (AAR) which is characterized by a mirror resistance and a related electric potential drop; and (b) a resistive turbulent layer (TL) with anomalous conductivity caused by high-frequency turbulence. An analytical treatment of the interaction of Alfven waves with the combined magnetosphere - AAR/TL - topside ionosphere - E-layer system immersed into a converging dipole-like magnetic field has been made. The rate of wave reflection/transmission is estimated to be critically dependent on the ratio between the wave transverse scale and the Alfven resistive scale λA, thus performing a scale-dependent magnetosphere-ionosphere coupling. Magnetospheric Alfven waves penetrating into the AAR can produce oscillatory variations of the field-aligned potential drop and field-aligned electron acceleration. Modeling of the spatial spectrum of an Alfven burst by a power-law dependence indicates that the rate of wave power absorbed by the AAR might be significant, up to 30- 50%. Thus, modeling results confirm that Alfven waves can produce auroral activation. Estimates of the resonance width of the ULF oscillations of a field line with an AAR show that the mirror-force mechanism can dominate over ionospheric dissipation and dispersive effects. A resonator in the topside ionosphere between the E-layer and the bottom boundary of the AAR can also occur. This resonator can trap Alfvenic disturbances with frequencies ~0.1 Hz (i.e. lower than that of the known ionospheric Alfven resonator) and with transverse scales from km to a few tens of km. The Alfven wave interaction with a TL is also characterized by the resistive scale λA, but determined by the field-aligned resistance. Estimation of the effective damping scale of the Pc1 waves in a turbulent cusp with the dispersion relationship for Alfven waves in a turbulent medium with anisotropic conductivities shows that the cusp proper cannot be a conduit of Pc1 wave energy. The "thin" TL model has been applied to the interpretation of the results of transient Pi2 pulsation damping studies, which showed that the damping rate increased for accompanying magnetic bays stronger than 100 nT. This additional damping can be caused by the occurrence of anomalous transverse resistance when the magnetospheric current exceeds the threshold necessary for the excitation of high-frequency plasma turbulence. The sudden onset of anomalous resistance on auroral field lines is shown to be accompanied by the excitation of an Alfvenic impulse, which has a specific spatial structure and can provide input to the observed complicated Pi2 wave forms.
SM33A-03
A Model for Bipolar Electric Field Structures Parallel to the Magnetic Field Observed in Polar Ionosphere
The bipolar electric field structures were observed in many space plasmas, such as the solar wind, magnetosheath, magnetotail, and the auroral zone. In this study, a physical model for the existence of the bipolar electric field structures propagating along the magnetic field line is established by deriving the "Sagdeev potential" from the magnetohydrodynamic (MHD) equations in a cylindrical coordinate system. The model shows that the bipolar electric field structure can develop not only from an ion acoustic wave, but also from an ion cyclotron wave when the Mach number and the initial electric field satisfy certain conditions. The polarity of the bipolar electric field structure can be oriented either negative to positive or the reverse polarity. According to the conditions in the auroral region, amplitude of the BEF structure can be varied from 35 to 330 mV/m, and its duration can be 7 ms to 23 ms. These predictions are in agreement with observation in the auroral region.
SM33A-04
The Effects of IMF and Convection on Thermal Ion Outflow in Magnetosphere-Ionosphere Coupling
We study the influence of the IMF and convection electric field on the rate and destination of thermal (low-energy) ion outflows, and its resulting effects on magnetosphere-ionosphere coupling, using single-particle trajectory simulations and ion velocity distribution measurements on Akebono in conjunction with IMF and ionospheric convection data. We find that the ions preferentially feed the dusk sector of the plasma sheet when the IMF is duskward (By > 0), and are more evenly distributed in the plasma sheet when the IMF is dawnward. The flow of oxygen ions originating from the noon or dusk sectors of the polar cap has a higher probability of reaching the magnetosphere and beyond compared with that from the dawn or midnight sectors, due to the increased centrifugal acceleration associated with the larger magnetic field curvature near noon and the increased convection electric field in the dusk sector. The flow is enhanced and confined to lower L-shells at times of strongly southward IMF, compared with that at times of northward IMF. The outflow rate to both the plasma sheet and the magnetotail correlates strongly with the ion temperature. As a result, the IMF and the convection electric fields affect both the overall magnitude and the detailed distribution of mass transfer from the ionosphere to the magnetosphere in magnetosphere-ionosphere coupling.
SM33A-05
Comparing Observations And Modeling Of whistlers; Current Difficulties In Modeling
Studies of dispersions of lightning induced whistlers propagating along magnetic field lines and observed on the ground brought the first information about cold plasma environment of the Earth. Later, satellite measurements revealed new types of whistlers associated with oblique propagation that could not be observed on the Earth because of LHR reflection. Today, an investigation of whistler propagation and distribution of their energy in the inner magnetosphere draw attention owing to their influence on energetic electrons in radiation belts via wave- particle interaction. Modeling of whistler propagation is usually based on the following simplifying assumptions. A part of electromagnetic energy generated by lightning discharge leaks into the ionosphere and transforms into whistler wave mode. At the top of F2 layer, the wave field can be decomposed into many wave packets with wave vectors oriented vertically from the Earth's surface, which is justified by the increase of refractive index from bottom. The initial parameters of each wave packet are determined by its frequency and initial latitude. The wave number is calculated from the dispersion relation. Further, the propagation is supposed to be non-ducted (oblique) and a ray tracing calculation is performed to evaluate the arrival time of individual wave packets of specific frequency to a given location. In this work, we compare the real spectrograms observed by satellites (mainly MAGION-5) with results of the modeling. We show that for some observed whistlers we get very good agreement. However, we can also find many observations which we are able to model only partially or not at all. We discussed these observations. We suppose that to possible reasons of these disagreements count a multiple leakage of lightning energy from Earth-ionosphere wave guide at different rather distant locations, a combination of ducted and oblique propagation and wave scattering on small-scale irregularities.
SM33A-06
Observations of Pc3-4 Pulsations in the Cusp Region using Four Closely-spaced Cusp- Latitude Search-coil Magnetometers, All-sky Imagers, and the EISCAT Svalbard Radar
A two-dimensional, closely-spaced four-station array of search-coil magnetometers was set up on the Svalbard archipelago in September, 2006, at geomagnetic latitudes ranging from 74° to 76° N. These stations can take advantage of the many other observational instruments on Svalbard, including optical instruments and the EISCAT incoherent scatter radar. We report here on observations of Pc3-4 pulsations at these sites, which are commonly thought to originate in the solar wind's ion foreshock upstream from Earth's bow shock and have long been known to be especially intense in near-cusp regions. The path or paths by which these waves travel from upstream to the near-cusp ionosphere is, however, still controversial. Data obtained during northern winter 2006-2007, when the cusp footpoint was in darkness, have shown that the Pc3-4 wave amplitude was typically but not always larger at the lower latitude station, just equatorward of the nominal cusp latitude. Further study of these data, including analysis of relative phase of the waves and comparison with optical and radar data will help determine whether the region of wave transmission to low altitudes is via the cusp, boundary layer, or slightly deeper in the magnetosphere.
SM33A-07
Cluster Observations of High-Altitude Controllers and Consequences of Auroral Acceleration Region Formation
The auroral acceleration region is an integral part of the magnetosphere-ionosphere electrodynamic system, and plays a key role in the transport of energy and particles between space and Earth. Processes occurring therein have received considerable attention over the past few decades, however much of this attention was based on a quasistatic interpretation of the data. To date, very little is known in-situ about the time development of the plasma and fields within and above the auroral acceleration region, which we address here. We present detailed plasma and fields measured by Cluster at high-altitude (> 3 RE) within and above examples of co-evolving upward and downward auroral acceleration systems, occurring during differing magnetospheric conditions. In particular, we highlight the developmental sequence of auroral potentials and reconfigurations of Earth's magnetotail topology, as well as specific orderings for the growth of associated currents, electric fields, density cavities, and plasma constituents that transpire within these systems, with the aim of identifying dominant controlling factors, and assessing consequences. Preliminary results indicate the importance of the temperature of injected electrons in controlling the development of acceleration potentials, with density cavitation being a consequence of an erosion (or acceleration) of cold dense plasma, as opposed to variations in the source electron density. Comparisons with Images from Polar-UVI and IMAGE data show that the developing arc systems presented here lead to localized auroral intensifications, which may or may not have periodic occurrences, with scales and motion that are consistent with these high-altitude measurements.