209 research outputs found
Variation in the frequency separations with activity and impact on stellar parameter determination
Frequency separations used to infer global properties of stars through
asteroseismology can change depending on the strength and at what epoch of the
stellar cycle the p-mode frequencies are measured. In the Sun these variations
have been seen, even though the Sun is a low-activity star. In this paper, we
discuss these variations and their impact on the determination of the stellar
parameters (radius, mass and age) for the Sun. Using the data from maximum and
minimum activity, we fitted an age for the Sun that differs on average by 0.2
Gyr: slightly older during minimum activity. The fitted radius is also lower by
about 0.5% for the solar effective temperature during minimum.Comment: to be published in JPCS to be published in JPC
Seismic sensitivity to sub-surface solar activity from 18 years of GOLF/SoHO observations
Solar activity has significantly changed over the last two Schwabe cycles.
After a long and deep minimum at the end of Cycle 23, the weaker activity of
Cycle 24 contrasts with the previous cycles. In this work, the response of the
solar acoustic oscillations to solar activity is used in order to provide
insights on the structural and magnetic changes in the sub-surface layers of
the Sun during this on-going unusual period of low activity. We analyze 18
years of continuous observations of the solar acoustic oscillations collected
by the Sun-as-a-star GOLF instrument onboard the SoHO spacecraft. From the
fitted mode frequencies, the temporal variability of the frequency shifts of
the radial, dipolar, and quadrupolar modes are studied for different frequency
ranges which are sensitive to different layers in the solar sub-surface
interior. The low-frequency modes show nearly unchanged frequency shifts
between Cycles 23 and 24, with a time evolving signature of the quasi-biennial
oscillation, which is particularly visible for the quadrupole component
revealing the presence of a complex magnetic structure. The modes at higher
frequencies show frequency shifts 30% smaller during Cycle~24, which is in
agreement with the decrease observed in the surface activity between Cycles 23
and 24. The analysis of 18 years of GOLF oscillations indicates that the
structural and magnetic changes responsible for the frequency shifts remained
comparable between Cycle 23 and Cycle 24 in the deeper sub-surface layers below
1400 km as revealed by the low-frequency modes. The frequency shifts of the
higher-frequency modes, sensitive to shallower regions, show that Cycle 24 is
magnetically weaker in the upper layers of Sun.Comment: Accepted for publication in A&
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