Issue |
A&A
Volume 387, Number 2, May IV 2002
|
|
---|---|---|
Page(s) | 642 - 664 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361:20020375 | |
Published online | 13 May 2002 |
Oscillations above sunspots: Evidence for propagating waves?
1
ESA Research and Scientific Support Department (RSSD), ESTEC, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands e-mail: eoshea@rssd.esa.int
2
Astrophysikalisches Institut Potsdam, Telegrafenberg, 14482 Potsdam, Germany e-mail: k.muglach@aip.de
3
ESA Research and Scientific Support Department (RSSD), NASA/GSFC, Mailcode 682.3, Greenbelt, MD 20771, USA e-mail: bfleck@esa.nascom.nasa.gov
Corresponding author: E. O'Shea, eoshea@ll.iac.es
Received:
21
December
2001
Accepted:
8
March
2002
We present results of an analysis of time series data observed in sunspot
umbral regions. The data were obtained in the context of the SOHO Joint
Observing Program (JOP) 97 in September 2000. This JOP included the Coronal
Diagnostic Spectrometer (CDS) and the Michelson Doppler Imaging (MDI)
instrument, both part of SOHO, the TRACE satellite and various ground based
observatories. The data was analysed by using both Fourier and wavelet
time series analysis techniques.
We find that oscillations are present in the umbra at all
temperatures investigated, from the temperature minimum as measured by
TRACE 1700 Å up to the upper corona as measured by CDS
335 Å (
K). Oscillations are found to be present with
frequencies in the range of 5.4 mHz (185 s) to 8.9 mHz (112 s). Using
the techniques of cross-spectral analysis time delays were found between
low and high temperature emission suggesting the possibility of both upward
and downward wave propagation. It is found that there is typically a good
correlation between the oscillations measured at the different emission
temperatures, once the time delays are taken into account. We find umbral
oscillations both inside and outside of sunspot
plume locations which indicates that umbral oscillations can be present
irrespective of the presence of these sunspot plumes. We find that a number
of oscillation frequencies can exist co-spatially and simultaneously i.e.
for one pixel location three different frequencies at 5.40, 7.65 and 8.85 mHz were measured. We investigate the variation of the relative amplitudes
of oscillation with temperature and find that there is a tendency for the
amplitudes to reach a maximum at the temperature of
(and less
typically
and
) and then to decrease to reach a
minimum at the temperature of
(
K), before increasing
again at the temperature of
. We discuss a number of possible
theoretical scenarios that might explain these results. From a measurement
of propagation speeds we suggest that the oscillations we observe are due to
slow magnetoacoustic waves propagating up along the magnetic field lines.
Key words: Sun: sunspot / Sun: oscillations / waves
© ESO, 2002
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