Magnetoacoustic wave trains in the 11 July 2005 radio event with fiber bursts

Abstract

A dm-radio emission with fiber bursts observed on 11 July 2005 was analyzed using wavelet filtration and spectral methods. In filtered radio spectra we found structures with different characteristic period P and frequency drift FD: i) fiber substructures (composed of dot emissions) with P 1≈ 0.5 s, FD1=-∈87 MHz∈s-1 on average, ii) fiber structures with P 2≈1.9 s, and iii) drifting structures with P 3≈81.4 s, FD2=-∈8.7, +∈98.5, and -∈21.8 MHz∈s-1. In the wavelet spectra we recognized patterns having the form of tadpoles. They were detected with the same characteristic periods P as found for the filtered structures. The frequency drift of the tadpole heads is found to be equal to the frequency drift of some groups of fibers for the long-period wavelet tadpoles (P 3) and to the frequency drift of individual fibers for the short-period tadpoles (P 2). Considering these wavelet tadpoles as signatures of propagating magnetoacoustic wave trains, the results indicate the presence of several wave trains in the fibers' source. While the long-period wave trains trigger or modulate a whole group of fibers, the short-period ones look like being connected with individual fiber bursts. This result supports the model of fibers based on magnetoacoustic waves. Using a density model of the solar atmosphere we derived the velocities of the magnetoacoustic waves, 107 and 562 km∈s-1, and setting them equal to the Alfvén ones we estimated the magnetic field in the source of fiber bursts as 10.7 and 47.8 G.