Azimuthal characteristics of hydromagnetic waves nearL= 4

A comparison has been made of magnetospheric ULF signals measured at Sanae and Siple, two Antarctic stations near L = 4, in order to examine the longitudinal scale size of magnetospheric signals in the ∼10- to 50-mHz frequency range. Data comparisons were made on 2 days during which substantial ULF wave activity existed in this band. The wave signals were often not correlated at the two stations, an indication of an azimuthal scale length ≲5 hours (m > 6) for these frequencies. One-to-one relationships between the stations could be identified at times for irregular nighttime waves of frequencies ∼ 10–17 mHz. Wave polarization reversals were observed around local noon at Siple; no reversals were seen at Sanae. This suggests that changes in the wave resonance region (and thus the magnetospheric plasma density gradients) occurred within the 5-hour interval between local noon at the two stations.