ULTRAVIOLET CORONAGRAPH SPECTROMETER OBSERVATIONS OF DENSITY-FLUCTUATIONS IN THE SOLAR-WIND

Recent Ultraviolet Coronagraph Spectrometer (UVCS) white-light channel (WLC) observations on board the Solar and Heliospheric Observatory (S OHO) indicate quasi-periodic variations in the polarized brightness (p B) in the polar coronal holes, This is the first observation of possib le signatures of compressional waves high above the limb (at heliocent ric distances in the range 1.9-2.45 R.). The Fourier power spectrum of the pB time series at 1.9 R. shows significant peak at about 6 minute s and possible fluctuations on longer timescales (20-50 minutes), The observation at 1.9 R. is the only currently available WLC data set wit h sufficient cadence to resolve the 6 minute period. These preliminary observations may result from density fluctuations caused by compressi onal waves propagating in polar coronal holes. We stress that our resu lts are preliminary, and we plan future high-cadence observations in b oth plume and interplume regions of coronal holes, Recently, Ofman & D avila used a 2.5 D MHD model and found that Alfven waves with an ampli tude of 20-70 km s(-1) at the base of the coronal hole can generate no nlinear, high-amplitude compressional waves that can contribute signif icantly to the acceleration of the fast solar wind, The nonlinear soli tary-like waves appear as fluctuations in the density and the radial o utflow velocity and contribute significantly to solar wind acceleratio n in open magnetic held structures, The motivation for the reported ob servations is the MHD model prediction.