NDLI: Periodic Modulation of X-Ray Intensity from Coronal Loops

Content Provider	Springer Nature Link
Author	McKenzie, D. E. Mullan, D. J.
Copyright Year	1997
Abstract	We have applied time-series analysis to sequences of coronal loop images obtained with the Yohkoh soft X-ray telescope (SXT). Using images obtained in fast-cadence mode (δ 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 where the X-ray brightness is modulated periodically with ≥99.5% confidence. The periods τ range from 9.6 s to 61.6 s, and the amplitudes range from 0.4% to 1.8%. Our result is statistically significant at the 8σ level.The observed periods can hardly be due to fluctuations in gas pressure: temperatures estimated from image ratios indicate sound speeds of 200–280 km s$^{-1}$, with loop crossing times of hundreds of seconds. On the other hand, the ratio 2 L/τ yields ‘velocities’ of 2–14 Mm s$^{-1}$: these overlap with published estimates of Alfvén speeds v $_{A}$ in coronal loops, 3.5–37 Mm s$^{-1}$ (Schmelz et al., 1994). Now, in the context of coronal loop heating by resonant absorption of MHD waves, the period τ$_{g}$ ≈ 2L/v $_{A}$ plays a key role: loops which are oscillating in their global mode (with period τ$_{g}$) are maximally efficient at absorbing energy (Steinolfson and Davila, 1993). We suggest that the loops which we have found to be modulated periodically are oscillating in their global mode. With this interpretation, we find that the plasma beta in the loops lies in the range β $_{p}$ = 0.001–0.045. According to the oscillating loop model of Zaitsev and Stepanov (1989), loops with such β $_{p}$ values should be modulated with amplitudes of 1.1% or less: our data are consistent with at least the upper limit of this prediction.
Starting Page	127
Ending Page	145
Page Count	19
File Format	PDF
ISSN	00380938
Journal	Solar Physics
Volume Number	176
Issue Number	1
e-ISSN	1573093X
Language	English
Publisher	Kluwer Academic Publishers
Publisher Date	1997-01-01
Publisher Place	Dordrecht
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Astrophysics
Content Type	Text
Resource Type	Article
Subject	Astronomy and Astrophysics Space and Planetary Science

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