Magnetohydrodynamic Sausage‐Mode Oscillations in Coronal Loops

A recent study by Nakariakov and coworkers pointed out that the dispersion relation of MHD sausage-mode oscillations has been incorrectly applied to coronal loops, neglecting the highly dispersive nature of the phase speed and the long-wavelength cutoff of the wavenumber. In the light of these new insights, we revisit previous observations that have been interpreted in terms of MHD sausage-mode oscillations in coronal loops and come to the following conclusions: (1) fast sausage MHD-mode oscillations require such a high electron density imposed by the wavenumber cutoff that they can only occur in flare loops, and (2) in the previously reported radio observations (ν ≈ 100 MHz to 1 GHz) with periods of P ≈ 0.5-5 s, the fast sausage MHD-mode oscillation is likely to be confined to a small segment (corresponding to a high harmonic node) near the apex of the loop, rather than involving a global oscillation over the entire loop length. The recent microwave and soft X-ray observations of fast periods ( P ≈ 6-17 s) by Asai and coworkers and Melnikov and coworkers, however, are consistent with fast sausage MHD oscillations at the fundamental harmonic.