SPATIAL AND TEMPORAL CHARACTERISTICS OF 15-100 MHZ ULF WAVES RECORDEDACROSS A LOW-LATITUDE AZIMUTHAL ARRAY

We have examined in detail the phase structure of daytime 15-100 mHz ( Pc3-4) ULF waves recorded across an azimuthal ground magnetometer arra y at L=1.85 with station separations between 200 km and 2400 km. Signa ls were ordered according to their spatial coherency on phase-time plo ts. Four different types of waves with the following characteristics w ere found: (a) similar structure in the north-south (X) and east-west (Y) magnetic components at widely spaced stations, essentially monochr omatic in appearance, frequency 20-25 mHz, with azimuthal wavenumbers 10 less-than-or-equal-to absolute value of m less-than-or-equal-to 29, propagation generally toward noon, and occurrence in the morning sect or; (b) signals whose X and Y wavemodes are uncoupled, but show high i nterstation coherency, frequency 20-25 mHz, absolute value of m less-t han-or-equal-to 5, no consistent propagation direction and morning occ urrence; (c) waves with frequency near the local field line eigenfrequ ency (40 - 60 mHz) with generally uncoupled X- and Y-components, absol ute value of m approximately 5 - 10, propagation mostly away from noon , and high spatial coherency in the X-component; (d) afternoon signals , exhibiting poor spatial coherency and probably associated with local ised sources of irregular waves with small scale lengths. These observ ed features suggest distinctly different source mechanisms for low-lat itude daytime ULF pulsations. We have also examined the appearance of skips and other irregularities in phasetime plots. These mostly occur in the east-west component on the ground, and are often seen at only o ne or two stations. Sometimes there is evidence of propagation at azim uthal group speeds with a lower limit around 4 km s-1. Occasionally, v ery high speeds are seen and these suggest propagation out of the azim uthal plane. Features observed near noon appear to propagate towards d awn and dusk, while morning and afternoon events propagate towards noo n. Although some phase skips which are observed simultaneously at spac ed stations are most likely associated with global magnetospheric impu lses, the results also suggest a connection with discontinuities in th e magnetospheric plasma distribution.