MHD Sausage Mode Oscillations in Coronal Loops

A recent study by Nakariakov et al. 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 wave number. 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 wave number cutoff that they can only occur in flare loops; (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 et al. and Melnikov et al., however, are consistent with fast sausage MHD oscillations at the fundamental harmonic. Subject headings: Sun: Corona — Sun: UV radiation