Detection of three periodicities in a single oscillating coronal loop

Van Doorsselaere, T., Birtill, D.C.C. and Evans, G.R. (2009) Detection of three periodicities in a single oscillating coronal loop. Astronomy & Astrophysics, 508 (3), 1485-1491. (doi:10.1051/0004-6361/200912753).

Abstract

Context. TRACE $171~\AA{}$ observations of transverse loop oscillations on May 13, 2001 have been studied by De Moortel & Brady (2007). They found hints of 3 periodicities present in this event.

Aims. The aim of this article is to improve the analysis, and measure the periodicities more accurately in order to do coronal seismology.

Methods. We create multiple (x,t)-slices across the loop, and determine the oscillatory parameters within those slices. We then use statistical methods to reduce the errors on the measured oscillations parameters.

Results. We find two populations with a clearly distinct period. The population at the loop top has a period of $895\pm2~{\rm s}$, while the population in the loop leg has a period of $452\pm1~{\rm s}$. The fact that the ratio of these two periods is roughly 2 and the geometry of the problem suggests that two harmonics are observed. In the residues, we confirm the presence of a 3rd harmonic with period $309\pm15~{\rm s}$ using two separate data analysis techniques. The three periods are then used to do a seismological estimate of the loop expansion and the density scale height.

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