Issue |
A&A
Volume 452, Number 3, June IV 2006
|
|
---|---|---|
Page(s) | 1059 - 1068 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20053687 | |
Published online | 06 June 2006 |
Magnetoacoustic wave propagation in off-limb polar regions
1
Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland e-mail: [eos;jgd]@arm.ac.uk
2
Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560 034, India e-mail: dipu@iiap.res.in
Received:
23
June
2005
Accepted:
22
February
2006
Aims.To find evidence of magnetoacoustic waves in off-limb polar regions.
Methods.Using temporal series data from the Coronal Diagnostic
Spectrometer (CDS) on SOHO, we study oscillations found in radiant flux
and velocity measurements from transition region and coronal spectral
lines. We use Fourier techniques to measure phase
delays between flux (“intensity”) oscillations and between velocity
oscillations of different transition region-corona and corona-corona
line pairs. We also measure the phase delays between flux and velocity
oscillations .
Results.The phase delays measured between different line pairs, when plotted
over a –180° to +180° range, line up along diagonal lines
corresponding to measurable and fixed time delays. The slopes
of these diagonal lines suggest the outward propagation of waves.
Using the measured time delays, we estimate propagation speeds for
the different line pairs that indicate that the waves producing
the observed phase delays are magnetoacoustic waves propagating at
speeds close to the sound speed. In addition, we find that the phases
occur at fixed integer frequencies of (90°) and
(67.5°), instead of the expected interval of f
(360°), indicating that a “Doppler effect” is acting on the waves.
Conclusions.From measurements, we
find evidence for fast magnetoacoustic waves to be predominantly present at
coronal temperatures, while at transition region temperatures slow
magnetoacoustic waves are more common.
We find strong evidence for outwardly propagating slow magnetoacostic
waves in off-limb polar
regions and an indication that these propagating waves are influenced
by some form of resonant cavity through which they pass.
Key words: Sun: oscillations / Sun: transition region / Sun: atmosphere / Sun: corona
© ESO, 2006
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