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

Field line resonances (FLRs) are an important mechanism for the generation of Pc3-4 (similar to 7-100 mHz) geomagnetic pulsations. There is considerab le observational evidence for the existence of FLRs at middle latitudes, bo th 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 sp anning eastern Australia from L=1.3-2.0 was used to investigate the variati on 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 remarkabl y similar over this range of latitudes and often exhibit a multitude of pea ks separated by similar to 3-5 mHz. Using cross-phase techniques, we find t hat the resonant frequency increases with decreasing latitude to L similar to 1.6, then decreases at lower latitudes. This is due to the effect of ion ospheric heavy ions at low altitudes. The characteristic size of the resona nces is L similar to 0.15, the resonance a is similar to 2 at L=2.0 and 1.3 -1.4 at L=1.3, and the normalized damping factor gamma/omega(R)similar to 0 .2-0.4. The low-latitude detection limit of FLRs depends on a number of fac tors, but on a day examined in detail it was L similar to 1.4. For signals below the local resonant frequency, amplitude decreased with latitude at si milar to 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 pl ane, although a peak occurs in the region where the Alfven 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 whi ch drive discrete forced oscillations of low-latitude field lines across a range of frequencies, and which couple to the local FLR where the frequenci es match.