PERIODIC MODULATION OF X-RAY-INTENSITY FROM CORONAL LOOPS - HEATING BY RESONANT ABSORPTION

We have applied time-series analysis to sequences of coronal loop imag es obtained with the Yohkoh soft X-ray telescope (SXT). Using images o btained in fast-cadence mode (Delta t = 4 s), we have generated X-ray light curves and power spectra for multiple subregions of a number of coronal loops. Our analysis indicates that out of 544 spectra of loops which range in length L from 40 Mm to 150 Mm, there are 16 cases wher e the X-ray brightness is modulated periodically with greater than or equal to 99.5% confidence. The periods tau range from 9.6 s to 61.6 a, and the amplitudes range from 0.4% to 1.8%. Our result is statistical ly significant at the 8 sigma level. The observed periods can hardly b e due to fluctuations in gas pressure: temperatures estimated from ima ge ratios indicate sound speeds of 200-280 km s(-1), with loop crossin g times of hundreds of seconds. On the other hand, the ratio 2L/tau yi elds 'velocities' of 2-14 Mm s(-1): these overlap with published estim ates of Alfven speeds upsilon (A) in coronal loops, 3.5-37 Mm s(-1) (S chmelz et nl., 1994). Now, in the context of coronal loop heating by r esonant absorption of MHD waves, the period tau(g) approximate to 2L/u psilon(A) plays a key role: loops which are oscillating in their globa l mode (with period tau(g)) are maximally efficient at absorbing energ y (Steinolfson and Davila, 1993). We suggest that the loops which we h ave found to be modulated periodically are oscillating in their global mode. With this interpretation, we find that the plasma beta in the l oops lies in the range beta(p) = 0.001-0.045. According to the oscilla ting loop model of Zaitsev and Stepanov (1989), loops with such beta(p ) values should be modulated with amplitudes of 1.1% or less: our data are consistent with at least the upper limit of this prediction.