Physics
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agusmsm33a..02p&link_type=abstract
American Geophysical Union, Spring Meeting 2007, abstract #SM33A-02
Physics
2407 Auroral Ionosphere (2704), 2487 Wave Propagation (0689, 3285, 4275, 4455, 6934), 2706 Cusp, 2736 Magnetosphere/Ionosphere Interactions (2431), 2752 Mhd Waves And Instabilities (2149, 6050, 7836)
Scientific paper
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.
Engebretson Mark J.
Pilipenko V. A.
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