Issue |
A&A
Volume 406, Number 3, August II 2003
|
|
---|---|---|
Page(s) | 1105 - 1121 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20030858 | |
Published online | 17 November 2003 |
Hot coronal loop oscillations observed with SUMER: Examples and statistics*
1
Max-Planck Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany
2
Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
Corresponding author: T. J. Wang, wangtj@linmpi.mpg.de
Received:
31
March
2003
Accepted:
27
May
2003
We give an extensive overview of Doppler shift oscillations in hot active region
loops obtained with SUMER. The oscillations have been detected in loops sampled 50-100 arcsec off the limb of the Sun in ultraviolet lines, mainly and
, with formation temperature greater than 6 MK. The spectra were recorded along a 300 arcsec slit placed at a fixed
position in the corona above the active regions. Oscillations are usually
seen along an extended section of the slit and often appear to be from
several different portions of the loops (or from different loops).
Different portions are sometimes
in phase, sometimes out of phase and sometimes show phase shifts along the
slit. We measure physical parameters of 54 Doppler shift oscillations in 27 flare-like events and give geometric parameters of the associated hot loops
when soft X-ray (SXR) images are available. The oscillations have periods
in the range 7-31 min, with decay times 5.7-36.8 min, and show an initial
large Doppler shift pulse with peak velocities up to 200 km s-1.
The oscillation periods are on average a factor of three longer than the
TRACE transverse loop oscillations. The damping times and velocity amplitude
are roughly the same, but the derived displacement amplitude is four or five
times larger than the transverse oscillation amplitude measured in TRACE images.
Unlike TRACE oscillations, only a small fraction of them are triggered by
large flares, and they often recur 2-3 times within a couple of hours.
All recurring events show initial shifts of the same sign.
These data provide the following evidence to support the conclusion that these
oscillations are slow magnetoacoustic standing waves in hot loops:
(1) the phase speeds derived from observed
periods and loop lengths roughly agree with the sound speed;
(2) the intensity fluctuation lags the Doppler shifts by 1/4 period;
(3) The scaling of the dissipation time of slow waves with period agrees
with the observed scaling for 49 cases. They seem to be triggered by
micro- or subflares near a footpoint, as revealed in one example with
SXR image observations. However other mechanisms cannot as yet be ruled out.
Some oscillations showed phase propagation along the slit in one or both
directions with apparent speeds in the range of 8-102 km s-1, together
with distinctly different intensity and line width distributions along the
slit. These features can be explained by the excitation of the oscillation
at a footpoint of an inhomogeneous coronal loop, e.g. a loop with fine
structure.
Key words: Sun: activity / Sun: corona / Sun: oscillations / Sun: UV radiation / Sun: X-rays, gamma rays
© ESO, 2003
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