Leakage of long-period oscillations from the chromosphere to the corona
Astronomy and Astrophysics
Long-period oscillations in a coronal diffuse structure are detected with the Transition Region And Coronal Explorer (TRACE). The EUV images of the NOAA active region 8253 are available in 171 Å and 195 Å bandpasses from 30 June to 4 July 1998. The average intensity variation is found to be connected with the CCD temperature, which varies with the orbital motion of the spacecraft. Hence, oscillations with the orbital period and its higher harmonics appear as artifacts in the light curves. After the exclusion of the orbital effects, we identified several long-period oscillations in the diffuse fan-like structure of the active region. Similar periodicities were detected in the radio emission from the chromospheric part of that active region, observed with the ground-based Nobeyama Radioheliograph (NoRH) in the 17 GHz channel. It was found that 0.221, 0.312 and 0.573 mHz oscillations were present in both EUV emission lines in the corona and the radio signal from the sunspot in the chromosphere, just beneath the active region. From the frequency values, the 1st and 3rd detected oscillations could be associated with the l = 2, n = −3 or l = 3, n = −5 and l = 1 gravity-driven solar interior modes, respectively. The appearance of these oscillations in the coronal part of the active region can be connected with the wave leakage or the evanescence of chromospheric oscillations.
Copyright © 2011 ESO / EDP Sciences
Vol. 533, article A116