SM53D-01 INVITED
The Role of Electromagnetic Waves in Magnetosphere-Ionosphere Coupling
Electrodynamics plays an important role in Magnetosphere-Ionosphere (M-I) coupling at high latitudes. Changes in the dynamic pressure of the solar wind and the Interplanetary Magnetic Field lead to changes in the electric fields, currents, and conductivities that link the magnetosphere and ionosphere. The changes are initiated by Alfven waves that propagate along the magnetic field from the magnetosphere to the ionosphere. However, the ionosphere plays an active role and as the ionospheric plasma density changes in response to electron precipitation, Alfven waves are launched from the ionosphere that act to change the electric fields and currents linking the two regions. The net effect is that small-scale plasma structures are created, particularly during substorms. During the substorm expansion phase, there are spatially separated small-scale electron and ion temperature hot spots, localized regions of downward plasma velocity, and regions where NO+ is a dominant F- region ion. Electromagnetic waves also play an important role in the formation of sun-aligned polar cap arcs. In addition, electromagnetic wave turbulence is prevalent at high altitudes above the polar cap and it could have an important effect on the polar wind.
SM53D-02 INVITED
Excitation of Alfven vortices in the ionosphere by the magnetospheric convection
Excitation of ULF waves due to the positive feedback instability is considered in Ionospheric Alfvén Resonator (IAR) taking into account an inhomogeneous profile of velocity of the magnetospheric convection. This profile is formed by interaction of the convective flow with the neutral atmosphere at heights 90--150~km. ULF waves include oblique Alfvén waves, trapped in IAR, and drift ionospheric waves, which are in a resonance with each other. These waves together form strongly anisotropic closed current loops with the scale along the magnetic field l\| ~ 1000~km and with the scale l\perp ~ 1~km across the magnetic field and can be considered as Alfvén vortices. The instability threshold and the growth rate are investigated as functions of different parameters (wave vector k\perp, angle between the wave vector and convection velocity, ratio of Alfvén wave and Pedersen conductivities) in a model of ionosphere close to the real one. Possible acceleration mechanisms of electrons by oblique Alfvén waves and the role of energetic electron precipitation in the feedback instability development are discussed. Some estimations are given in application to observed short-scale field- aligned currents and auroral arcs in the auroral ionosphere.
SM53D-03
Alfven Wave Propagation in the Presence of Auroral Density Cavities
Observations in the auroral zone from, for example, the FAST satellite have indicated that deep density cavities exist. These cavities imply a large perpendicular gradient to the Alfvén speed, which can lead to phase mixing and the formation of narrow-scale electric and magnetic field structures. Such cavities can modify the mode structure of the Alfvén waves leading to the possibility of discrete cavity modes trapped within the cavity, especially in the case where the plasma inside the cavity is warm (i.e., that the electron thermal speed is greater than the local Alfvén speed) and the outside plasma is cold. In the auroral zone, the situation is complicated by the fact that there are strong parallel gradients in the Alfvén speed (the so-called ionospheric Alfvén resonator) in addition to the perpendicular gradients. Coupling to the Pedersen and Hall conductances of the ionosphere also affects the propagation of these waves. Numerical modeling of this situation has been carried out, indicating that such waves have outward phase motions but inward Poynting flux, consistent with the Alfvén wave dispersion relation modified by electron inertia as well as with FAST observations.
SM53D-04
F-region Magnetospheric ULF Generation by Modulated Ionospheric Heating
Current modulation of D/E region ionospheric currents at ULF frequencies results in generation of Shear Alfven Waves injected upwards and guided by the magnetic field lines towards the conjugate ionosphere. Under particular ionospheric conditions frequencies in the PC1 range (.2-6Hz) are reflected by the gradient in the Alfven velocity above the F-region resulting in the well-known Ionospheric Alfven Resonator (IAR) structure. Ground detection of ULF waves due to current modulation on the ground is thus limited to the vicinity of the heated spot since at these frequencies the coupling to the earth ionosphere waveguide is evanescent. Propagation of ULF waves at significant lateral distances requires generation of magnetosonic waves since they are the only mode that propagates isotropically and can thus couple efficiently in the Alfvenic duct. In this paper we present a completely new mechanism to generate magnetosonic waves by modulated ionospheric heating that does not require the presence of electrojet currents. The process relies in anomalous electron heating near the F-region peak by preferably using O-mode upper hybrid heating modulated at ULF Pc-1 frequencies. The modulation in the electron pressure drives a Bxgrad(p) oscillatory current. The resultant field aligned magnetic moment generates predominantly magnetosonic waves that are injected laterally into the Alfvenic duct and can also detected above the F-peak by over-flying satellites over distances larger than spanned by the field lines connecting to heated area. In addition to the concept and analytic results the paper will present simulations results using the ZEUS-MP MHD. Non-uniform grids are used to adapt to non-uniform ionospheric plasma density and thin layer of heating source. The effective heating region is placed at about 200-300 km in altitude (F-layer ionosphere). The modulated heating source is modeled as a source with perturbed density, temperature and magnetic field and it transmits modulated-HF electromagnetic waves into a stable ionosphere. Ratios of perturbed magnetic field and density to their background values are extracted from simulations. Different radiation patterns from different polarization component of magnetic field perturbation are investigated. Effects of different profiles of non-uniform ionospheric plasma density on ULF wave propagation are also studied through simulation. Preliminary experimental evidence of the process will also be presented. This work was sponsored by ONR MURI Grant 5-28828
SM53D-05
Effects of Ionospheric Heating on Feedback-Unstable Electromagnetic Fields at High Latitudes
Results from a numerical study of the nonlinear interaction between large-scale magnetic field-aligned currents (FACs) and the high-latitude ionosphere are presented. This study focuses on the dynamics of intense, small- scale electromagnetic structures generated in the ionosphere and low-altitude magnetosphere by the ionospheric feedback instability. The instability is caused by the large-scale electric field produced in the ionosphere where a large-scale, downward FAC depletes the ionospheric density and reduces the ionospheric conductivity. The novel aspect of this research is the evaluation of the effects of E-region electron and ion heating on the feedback instability. Anomalous electron heating can be attributed to nonlinearly saturated turbulence of the Farley-Buneman instability; the ion heating is mainly due to ion-neutral collisions. Electron heating increases the ionospheric conductivity by decreasing the recombination rate whereas ion heating tends to reduce the ionospheric conductivity by increasing the ion-neutral collision frequency. Numerical simulations based on a two- fluid MHD model describing coupling between ULF electromagnetic waves and the ionosphere are presented. In this study, we explore the parametric dependence of ion and electron heating on the feedback instability and the resulting plasma electrodynamics without fully treating the ion and electron energy dynamics.
SM53D-06
Wave propagation and the formation of fine auroral structure: New constraints derived from high-speed imagery
An analysis of simultaneous multi-scale observations of a discrete auroral breakup is presented, which clarifies the potential role of plasma wave dispersion in the formation of fine-scale auroral structure. At coarse resolution (all-sky white light camera, 1 frame/s), observations fit the established description of an arc breakup: (1) arc brightening, (2) formation of spatial distortions, and (3) breakup into multiple 'rayed' structures. At fine-scale resolution (EMCCD camera, 9 degree field of view, prompt emission filter, 30 frames/s), an entirely different type of coherence is observed. The 'arc' is observed to be a wave packet composed of parallel bifurcating ~100-m structures which propagate outwards from the center and fade. The bifurcation process is hierarchical, with newly formed substructure further bifurcating into subordinate auroral forms as the fading proceeds. The observations are well represented by a model that invoking electron acceleration in a dispersive phase-coherent wave field propagating into the topside ionosphere. This empirical model is discussed in the context of dispersive shear Alfven waves at the near-Earth magnetosphere. http://people.bu.edu/jls
SM53D-07
Auroral nightside downward-current regions: ClusterII observations
The ClusterII spacecraft traverse the nightside auroral zone at 4-6 Re altitudes in February-March of each year. In 2004, their separation was such as to cross the auroral zone with separation times of a few minutes, comparable to the expected timescales of auroral downward current sheet evolution. We present observations from 12 such events from an effort to look for signatures of the temporal evolution of the auroral downward current system. We compare the electron characteristic energy (ratio of energy flux to number flux) and density of the upgoing electron beams as measured by the PEACE instrument, to electric field signatures from EFW including integrated potential, divergent field structures, and ambient density calculated from the spacecraft potential. The events are seen to be localized on or near ambient density gradients. The characteristic energy of the upgoing electrons is inversely proportional to their number density. Characteristic energies of up to a few keV are seen, and these energies typically decrease with time as subsequent spacecraft cross the same event.
SM53D-08
Self-Consistent Kinetic Modeling of Field Line Resonances
Field line resonances (FLRs) represent a class of low frequency ULF pulsations in the Pc5 range. At auroral latitudes, on field lines threading the nightside plasma sheet, FLRs can be observed using ground-based magnetometers, where they typically appear with wave frequencies in the range of a few mHz. It is now established that FLRs appear in coincidence with modulations of the optical aurora, as observed using all-sky camera and meridian scanning photometer data from the CANOPUS array in Northern Canada. Optical signatures of FLRs may be due to direct particle energization by FLR wave fields. This, however, remains to be established. In this paper, we present results based on a vlasov-kinetic model of FLRs. The model is 1D in configuration space, but accommodates geomagnetic field line convergence by allowing the perpendicular wavenumber to vary along the field line. We consider the effect of the mirror force and variations in the ambient density and temperature on the evolution of FLRs. We discuss the conditions under which parallel electric fields are generated, and discuss how electrons trapped by the wave fields are able to damp FLRs. We also assess the importance of parallel electric fields in energizing auroral electrons and discuss whether observed FLRs can explain the optical signatures of FLRs.