RADIAL DISTRIBUTION OF COMPRESSIVE WAVES IN THE SOLAR CORONA REVEALED BY AKATSUKI RADIO OCCULTATION OBSERVATIONS

Radial variations of the amplitude and the energy flux of compressive waves in the solar corona were explored for the first time using a spacecraft radio occultation technique. By applying wavelet analysis to the frequency time series taken at heliocentric distances of 1.5-20.5 R-S (solar radii), quasi-periodic density disturbances were detected at almost all distances. The period ranges from 100 to 2000 s. The amplitude of the fractional density fluctuation increases with distance and reaches similar to 30% around 5 R-S, implying that nonlinearity of the wave field is potentially important. We further estimate the wave energy flux on the assumption that the observed periodical fluctuations are manifestations of acoustic waves. The energy flux increases with distance below similar to 6 R-S and seems to saturate above this height, suggesting that the acoustic waves do not propagate from the low corona but are generated in the extended corona, probably through nonlinear dissipation of Alfven waves. The compressive waves should eventually dissipate through shock generation to heat the corona.