766 research outputs found
The acoustic cut-off frequency of the Sun and the solar magnetic activity cycle
The acoustic cut-off frequency -the highest frequency for acoustic solar
eigenmodes- is an important parameter of the solar atmosphere as it determines
the upper boundary of the p-mode resonant cavities. At frequencies beyond this
value, acoustic disturbances are no longer trapped but traveling waves.
Interference amongst them give rise to higher-frequency peaks -the pseudomodes-
in the solar acoustic spectrum. The pseudomodes are shifted slightly in
frequency with respect to p modes making possible the use of pseudomodes to
determine the acoustic cut-off frequency. Using data from GOLF and VIRGO
instruments on board the SOHO spacecraft, we calculate the acoustic cut-off
frequency using the coherence function between both the velocity and intensity
sets of data. By using data gathered by these instruments during the entire
lifetime of the mission (1996 till the present), a variation in the acoustic
cut-off frequency with the solar magnetic activity cycle is found.Comment: Paper accepted in ApJ. 26 Pages, 9 figure
Analysis of the solar cycle and core rotation using 15 years of Mark-I observations:1984-1999. I. The solar cycle
High quality observations of the low-degree acoustic modes (p-modes) exist
for almost two complete solar cycles using the solar spectrophotometer Mark-I,
located at the Observatorio del Teide (Tenerife, Spain) and operating now as
part of the Birmingham Solar Oscillations Network (BiSON). We have performed a
Fourier analysis of 30 calibrated time-series of one year duration covering a
total period of 15 years between 1984 and 1999. Applying different techniques
to the resulting power spectra, we study the signature of the solar activity
changes on the low-degree p-modes. We show that the variation of the central
frequencies and the total velocity power (TVP) changes. A new method of
simultaneous fit is developed and a special effort has been made to study the
frequency-dependence of the frequency shift. The results confirm a variation of
the central frequencies of acoustic modes of about 450 nHz, peak-to-peak, on
average for low degree modes between 2.5 and 3.7 mHz. The TVP is
anti-correlated with the common activity indices with a decrease of about 20%
between the minimum and the maximum of solar cycle 22. The results are compared
with those obtained for intermediate degrees, using the LOWL data. The
frequency shift is found to increase with the degree with a weak l-dependence
similar to that of the inverse mode mass. This verifies earlier suggestions
that near surface effects are predominant.Comment: Accepted by A&A October 3 200
On the solar origin of the signal at 220.7microHz: A possible component of a g mode?
Gravity modes in the Sun have been the object of a long and difficult search
in recent decades. Thanks to the data accumulated with the last generation of
instruments (BiSON, GONG and three helioseismic instruments aboard SoHO),
scientists have been able to find signatures of their presence. However, the
individual detection of such modes remains evasive. In this article, we study
the signal at 220.7 microHz which is a peak that is present in most of the
helioseismic data of the last 10 years. This signal has already been identified
as being one component of a g-mode candidate detected in the GOLF Doppler
velocity signal. The nature of this peak is studied in particular using the
VIRGO/SPM instrument aboard SoHO. First we analyse all the available
instrumental data of VIRGO and SoHO (housekeeping) to reject any possible
instrumental origin. No relation was found, implying that the signal has a
solar origin. Using Monte Carlo simulations, we find, with more than 99%
confidence level, that the signal found in VIRGO/SPM is very unlikely to be due
to pure noise.Comment: Accepted for publication in ApJSS. 19 pages, 9 figure
Update on g-mode research
Since the beginning of this century we have attended a blooming of the
gravity-mode research thanks to the unprecedented quality of the data
available, either from space with SoHO, or from the ground-based networks as
BiSON or GONG. From the first upper limit of the gravity-mode amplitudes fixed
at 10 mm/s at 200 microHz given by Appourchaux et al. (2000), on one hand, a
peak was supposed to be a component of the l=1, n=1 mixed mode (Garcia et al.
2001a, b; Gabriel et al. 2002) and, on the other hand, a couple of patterns
--multiplets-- were attributed to gravity modes (Turck-Chieze et al. 2004;
Mathur et al. 2007). One of these patterns, found around 220 microHz, could be
labeled as the l=2, n =-3 g mode, which is expected to be the one with the
highest surface amplitude (Cox and Guzik 2004). Finally, in 2007, Garcia et al.
were able to measure the fingertips of the dipole gravity modes looking for
their asymptotic properties. In the present paper we present an update of the
recent developments on this subject with special attention to the 220 microHz
region, the dipole asymptotic properties and the impact of the incoming g-mode
observations on the knowledge of the solar structure and rotation profile.Comment: Paper accepted for publication in Astronomische Nachrichten. 9 pages,
8 figure
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