Field line resonances and waveguide modes at low latitudes. 1. Observations

Details Field line resonances and waveguide modes at low latitudes. 1. Observations Field line resonances and waveguide modes at low latitudes. 1. Observations

Apr 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000jgr...105.7747m&link_type=abstract

Journal of Geophysical Research, Volume 105, Issue A4, p. 7747-7762

Physics

24

Magnetospheric Physics: Magnetosphere-Inner, Magnetospheric Physics: Mhd Waves And Instabilities, Magnetospheric Physics: Numerical Modeling

Scientific paper

Field line resonances (FLRs) are an important mechanism for the generation of Pc3-4 (~7-100 mHz) geomagnetic pulsations. There is considerable observational evidence for the existence of FLRs at middle latitudes, both in satellite and ground data. However, the low-latitude regions are less accessible for such studies, and consequently many aspects of low-latitude FLRs are not well understood. A temporary 12-station magnetometer array spanning eastern Australia from L=1.3-2.0 was used to investigate the variation in Pc3-4 power with latitude, the nature and low-latitude limit of FLRs, and properties of spectral components below the local resonant frequency. Examples are presented for representative days. Power spectra are remarkably similar over this range of latitudes and often exhibit a multitude of peaks separated by ~3-5 mHz. Using cross-phase techniques, we find that the resonant frequency increases with decreasing latitude to L~1.6, then decreases at lower latitudes. This is due to the effect of ionospheric heavy ions at low altitudes. The characteristic size of the resonances is L~0.15, the resonance Q is ~2 at L=2.0 and 1.3-1.4 at L=1.3, and the normalized damping factor γ/ωR~0.2-0.4. The low-latitude detection limit of FLRs depends on a number of factors, but on a day examined in detail it was L~1.4. For signals below the local resonant frequency, amplitude decreased with latitude at ~3 dB/0.1 L. Interstation phase delays are not consistent with the time of flight of radially propagating fast-mode waves in the equatorial plane, although a peak occurs in the region where the Alfvén velocity peaks. We conclude that these results are consistent either with modulation of the incoming fast-mode waves or the existence of cavity or waveguide modes which drive discrete forced oscillations of low-latitude field lines across a range of frequencies, and which couple to the local FLR where the frequencies match.

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