On coronal oscillations

Magnetoacoustic oscillations in a solar coronal inhomogeneity (e.g., coronal loop) are shown to take place with two distinct periodicities, one on an acoustic (long) time scale and the other on an Alfvenic (short) time scale. The short period modes - fast magnetoacoustic waves - are trapped in regions of low Alfven speed: typically, this corresponds to high density loops or dense open field regions. Their periods may be on the order of seconds. The form of the fast oscillations is discussed for both standing modes in a closed loop and impulsively generated disturbances in a loop or open field structure. Impulsively generated waves in a density enhancement exhibit both periodic and quasi-periodic phases. Symmetric oscillations (sausage modes) are analogous to Pekeris waves in oceanography; asymmetrical (kink) disturbances are akin to Love waves in seismology. It is suggested that fast magnetoacoustic waves may explain the observed pulsations in Type IV radio events, the sausage waves providing the desired 1 s periodicities. Magnetoacoustic oscillations provide a potentially useful diagnostic tool for determining physical conditions in the inhomogeneous corona.