TRACE Observations of Driven Loop Oscillations
  Aims: On 13 June 1998, the TRACE satellite was fortuitously well placed to observe the effects of a flare-induced EIT wave in the corona, and its subsequent interaction with coronal magnetic loops. In this study, we use these TRACE observations to corroborate previous theoretical work, as well as estimate the magnetic field strength and the degree of longitudinal inhomogeneity. Methods: Loop edges are tracked, both spatially and temporally, using wavelet modulus maxima algorithms, with corresponding loop displacements from its quiescent state analysed by fitting scaled sinusoidal functions. The physical parameters of the coronal loop are subsequently determined using seismological techniques. RESULTS: The studied coronal loop is found to oscillate with two distinct periods, 501 +/- 5 s and 274 +/- 7 s, which could be interpreted as belonging to the fundamental and first harmonic, or could reflect the stage of an overdriven loop. When assuming the periods belong to the fundamental kink mode and its first harmonic, we obtain a magnetic field strength inside the oscillating coronal loop as 2.0 +/- 0.7 G. In contrast, interpreting the oscillations as a combination of the loop's natural kink frequency and a harmonious EIT wave provides a magnetic field strength of 5.8 +/- 1.5 G. Using the ratio of the two periods, we find that the gravitational scale height in the loop is 73 +/- 3 Mm. Conclusions: We show that the observation of two distinct periods in a coronal loop does not necessarily lead to a unique conclusion. Multiple plausible scenarios exist, suggesting that both the derived strength of the magnetic field and the sub-resolution properties of the coronal loop depend entirely on which interpretation is chosen. 
I. Ballai, D. B. Jess and M. Douglas
astro-ph.SR