The UV images obtained with the Viking satellite often show bright features which resemble ''beads'' or ''pearls'' aligned in the east--west direction between noon and 1800 MLT (Lui et al., 1989). Viking acquired a series of 25 UV images during a 28-min period on July 29, 1986, which showed a distinctive series of periodic bright features in this region. Magnetic field and hot plasma measurements obtained by Viking confirm that the UV emissions are colocated with the field line projection of an upward-flowing region 1 Birkeland current and precipitating energetic (~200 eV) electrons. These observations support previous work (see the review by Evans, 1985), which concludes that this region is the ionospheric projection of the cusp and low-latitude boundary layer. The magnetic field and electric field measurements show transverse oscillations (predominantly in the westward and southward components, respectively) with a nearly constant period of about 3.5 min from 67¿ invariant latitude equatorward up to the location of the large-scale Birkeland current system near 76¿ invariant latitude. The electric field oscillations lead the magnetic field oscillations by about a quarter-period. We interpret the observed oscillations as standing Alfv¿n waves driven at a frequency near the local resonance frequency by a large-scale wave in the boundary layer. We propose that the energy flux of the precipitating low-energy electrons in this afternoon region is modulated by this boundary wave and produces the periodic UV emission features. These observations may be interpreted in terms of a boundary wave drifting tailward with a speed of about 0.12 MLT hour/min (1.8¿ longitude/min or about 800 m/s in the ionosphere (and with a wavelength of about 0.35 MLT hour) 5.3¿ of longitude or about 160 km in the ionosphere). The results of this study support the view that large-scale oscillations of magnetospheric boundaries, possibly associated with the Kelvin-Helmholtz instability, can modulate currents, particles, and auroral forms. ¿ American Geophysical Union 1990