Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsm52a0535c&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #SM52A-0535
Physics
2752 Mhd Waves And Instabilities, 2753 Numerical Modeling, 2784 Solar Wind/Magnetosphere Interactions, 2794 Instruments And Techniques, 7871 Waves And Instabilities
Scientific paper
The travel-time magnetoseismology is one of the two major methods remotely sensing the plasma mass density of the magnetosphere from the ground. This method measures the differentiation of the arrival time of impulsive waves-including the preliminary impulses of sudden commencements and impulsive pulsations-and infers the plasma density that determines the propagation speeds. Previous observations find that these impulsive signals propagate to the ground mainly by means of MHD waves. Tamao's approximation of MHD wave propagation has been used to provide the first-order model for inversion. This model enables the calculation of wave propagation time along simple travel routes, which are composed by the closest path between the impact location and the field line of interest and the field-aligned propagation to the ground. In this paper we examine if Tamao's approximation can be repeated by numerical calculation as a step to examine its accuracy and to assess the validity of its use in inversion. We examine the propagation of preliminary impulses to the low-altitude boundary in MHD simulation in 3-D dipole geometry. This numerical simulation is a useful reference because it has a better quantitative assessment on the coupling between fast mode and Alfven waves and on the wave refraction in a realistic geometry. We find that Tamao's approximation is in good agreement with the simulation results in the outer magnetosphere: The wave energy reaches low altitude boundary first at latitude outside the plasmapause. However, the simulation shows characteristics of signal's arrival time deviated from Tamao's approximation at plasmaspheric latitudes. The arrival time is slightly earlier than predicted and is less differentiated across different L-values. The sharp gradient of Alfven velocity near the plasmapause yields a rather degenerated step function of latitude in arrival time. These simulation results imply that the plasmasphere significantly distorts impulsive waves when they propagate to low latitudes. The implications arising from this comparative study on the inversion problem of magnetoseismology will be discussed.
Chi Peter J.
Lee Daehee
Russell Christopher T.
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