Issue |
A&A
Volume 388, Number 2, June III 2002
|
|
---|---|---|
Page(s) | 540 - 545 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20020572 | |
Published online | 31 May 2002 |
Wavelet analysis of stellar differential rotation
II. The Sun in ultraviolet
Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
Corresponding author: ahempelmann@hs.uni-hamburg.de
Received:
17
December
2001
Accepted:
11
April
2002
Among the several methods to study differential rotation on stellar surfaces
the observation of a stellar butterfly diagram, i.e. monitoring the rotation period
over a complete activity cycle, is the only method which can be applied
to stars rotating as slow as the Sun. However, the method requires active
belts on the stellar surface which are moving continously in stellar latitude during a cycle.
In addition, an appropriate activity tracer showing rotational modulation is needed.
Tracers which even work for
slowly rotating, i.e. weakly active, stars are the emission cores of certain
spectral lines. However, the question is whether surface differential rotation can be determined
from this kind of observations and furthermore, which method of analysis is best suited to
yield a correct result. We investigate
observations of the
Sun as a star, i.e. disk-integrated measurements, to answer these questions.
In a Paper I it was demonstrated
that time-frequency analysis using the wavelet transform is possibly a suitable method
for monitoring the stellar butterfly diagram. It was also shown that – in comparison
with Fourier analysis –
wavelet analysis of disk-integrated solar
Ca II K line core emission measures yield a much more realistic pattern of the solar differential
rotation. In this Paper II disk-integrated solar measures in Mg II h+k
and Lyman α taken from public UARLS Solstice data are analysed using the same
methods as in Paper I.
In these data solar rotational
modulation is much more pronounced than in earlier Ca II K time-series.
Wavelet analysis yields the following results: from the beginning of the time-series in 1991
until the end of solar cycle 22 the period of rotational modulation remains stable at 26.9–27.0
days. It is followed by a jump to 27.6 days when the new cycle 23 starts. Then the period
rapidly decreases to 26.9 days again
until the end of the time-series in 1999. However, the analysis is hampered
by frequent period splitting into two modes. It can be shown that this kind of splitting
results from
phase jumps in the time-series coming from active region growth and decay at
shifted solar longitudes.
Key words: Sun: rotation / stars: rotation / methods: data analysis
© ESO, 2002
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