Observations of oscillations in an active region

We report here on the detection of propagating oscillations out from the footpoints of a large diffuse coronal loop structure close to an active region as observed in 171 A by TRACE. As in Ireland et al. (1999, 2000), we employ a wavelet analysis to investigate significant periodicity in the observed oscillations. Wavelet analysis is a powerful technique which allows a local decomposition of timescales in nonstationary timeseries.

The analysis is based upon TRACE 171 A (Fe IX) observations of active region AR 8496; a large, stable active region present in the southwest quadrant on 23 March 1999. The data are taken as part of JOP 83 - High cadence activity studies and the heating of coronal loops. The aim of this study was to reduce the cadence of the TRACE observations as low as possible while still retaining enough counts that events of interest would not be lost in the noise. The first sequence, start time 0647 UT, consisted of 157 512 x 512 pixel images in the TRACE 171 A bandpass with a 9 s cadence and with a pixel size of 1''. The data is run through the Solarsoft TRACE routine TRACE_PREP and subsequently cleaned using TRACE_DESPIKE (removing spikes in the datacube), TRACE_CLEANJPG (removing JPEG compression artifacts) and TRACE_DESTREAK (removing cosmic ray and particle streaks). As the data only cover a relatively short period, they have not been corrected for solar rotation. As the data covers only a relatively short period, it has not been corrected for solar rotation. Figure below shows an image of AR 8496. This is the first of the 0647 UT sequence on which we will be concentrating. The tube-like area that is indicated in the inset shows the region we will be looking at in detail. From this figure we see that the bright footpoint is about 40 arcsec long.



Typical image in the 0647 UT 171 A dataset. Inset is a blow up of the area supporting the oscillatory signal.

First set of observations (6.47 am, 171)

Later set of observations (8.22 am, 171)

Later set of observations (8.22 am, 195)

Discussion and conclusions