135 research outputs found
The Sun-like activity of the solar twin 18 Scorpii
We present the results of 10 yr of complementary spectroscopic and
photometric observations of the solar twin 18 Scorpii. We show that over the
course of its ~7 year chromospheric activity cycle, 18 Sco's brightness varies
in the same manner as the Sun's and with a likely brightness variation of
0.09%, similar to the 0.1% decadal variation in the total solar irradiance
Symmetrization and enhancement of the continuous Morlet transform
The forward and inverse wavelet transform using the continuous Morlet basis
may be symmetrized by using an appropriate normalization factor. The loss of
response due to wavelet truncation is addressed through a renormalization of
the wavelet based on power. The spectral density has physical units which may
be related to the squared amplitude of the signal, as do its margins the mean
wavelet power and the integrated instant power, giving a quantitative estimate
of the power density with temporal resolution. Deconvolution with the wavelet
response matrix reduces the spectral leakage and produces an enhanced wavelet
spectrum providing maximum resolution of the harmonic content of a signal.
Applications to data analysis are discussed.Comment: 12 pages, 8 figures, 2 tables, minor revision, final versio
On the intensity contrast of solar photospheric faculae and network elements
Sunspots, faculae and the magnetic network contribute to solar irradiance
variations. The contribution due to faculae and the network is of basic
importance, but suffers from considerable uncertainty. We determine the
contrasts of active region faculae and the network, both as a function of
heliocentric angle and magnetogram signal. To achieve this, we analyze
near-simultaneous full disk images of photospheric continuum intensity and
line-of-sight magnetic field provided by the Michelson Doppler Interferometer
(MDI) on board the SOHO spacecraft. Starting from the surface distribution of
the solar magnetic field we first construct a mask, which is then used to
determine the brightness of magnetic features, and the relatively field-free
part of the photosphere separately. By sorting the magnetogram signal into
different bins we are able to distinguish between the contrasts of different
concentrations of magnetic field. We find that the contrasts of active region
faculae (large magnetogram signal) and the network (small signal) exhibit a
very different CLV, showing that the populations of magnetic flux tubes are
different. This implies that these elements need to be treated separately when
reconstructing variations of the total solar irradiance with high precision. We
have obtained an analytical expression for the contrast of photospheric
magnetic features as a function of both position on the disk and magnetic field
strength, by performing a 2-dimensional fit to the observations.Comment: 12 pages, 8 figures, uses aa.cl
On mesogranulation, network formation and supergranulation
We present arguments which show that in all likelihood mesogranulation is not
a true scale of solar convection but the combination of the effects of both
highly energetic granules, which give birth to strong positive divergences
(SPDs) among which we find exploders, and averaging effects of data processing.
The important role played by SPDs in horizontal velocity fields appears in the
spectra of these fields where the scale 4 Mm is most energetic; we
illustrate the effect of averaging with a one-dimensional toy model which shows
how two independent non-moving (but evolving) structures can be transformed
into a single moving structure when time and space resolution are degraded.
The role of SPDs in the formation of the photospheric network is shown by
computing the advection of floating corks by the granular flow. The coincidence
of the network bright points distribution and that of the corks is remarkable.
We conclude with the possibility that supergranulation is not a proper scale of
convection but the result of a large-scale instability of the granular flow,
which manifests itself through a correlation of the flows generated by SPDs.Comment: 10 pages, 11 figures, to appear in Astronomy and Astrophysic
The historical light curve of the symbiotic star AG Draconis: intense, magnetically induced cyclic activity
We analyze an optical light curve of the symbiotic system AG Draconis
covering the last 120 years of its history. During the first 32 years the
system was in a quiescence state. Around the year 1922 the star's quiescence
luminosity brightened by 0.29 mag. The last 82 years of the light curve (LC)
are characterized by a series of outbursts of 1-2 magnitude in brightness and
about 100 days in duration. The outbursts are distributed along the time axis
in 6 clusters with a quasi-periodic cycle of some 5300 days. The time intervals
among the outbursts themselves are integral numbers of the period 373.5 days.
During quiescence states the LC oscillates with the binary period of the system
of 550 d. The LC contains also a weak periodic signal with a period of 350 d,
attributed to pulsations of the giant star. Another period of 1160 d is also
present in the light curve, being the sidereal rotation period of the giant
star. We suggest that the outbursts are events of intense mass transfer from
the giant onto the hot component. These are modulated by an interplay between a
solar-like magnetic dynamo cycle operating in the outer layers of the giant,
and a tidal deformation of these layers that circulates the surface of the
giant with the synodic diurnal period of 373.5 Earth days. AG Dra is the 5th
symbiotic system with a light curve that reflects such an intense magnetic and
magnetically modulated activity. (Abridged)Comment: 10 pages, 4 figures. Accepted for publication in MNRA
Activity cycle of the giant star of Z Andromedae and its spin period
We have reanalyzed the long-term optical light curve (LC) of the symbiotic
star Z Andromedae, covering 112--yr of mostly visual observations. Two strictly
periodic and one quasi-periodic cycles can be identified in this LC. A P1=7550
d quasi periodicity characterizes the repetition time of the outburst episodes
of this symbiotic star. Six such events have been recorded so far. During
quiescence states of the system, i.e. in time intervals between outbursts, the
LC is clearly modulated by a stable coherent period of P2=759.1 d. This is the
well known orbital period of the Z And binary system that have been measured
also spectroscopically. A third coherent period of P3=658.4 d is modulating the
intense fluctuations in the optical brightness of the system during outbursts.
We attribute the trigger of the outbursts phenomenon and the clock that drives
it, to a solar type magnetic dynamo cycle that operates in the convection and
the outer layers of the giant star of the system. We suggest that the intense
surface activity of the giant star during maximum phases of its magnetic cycle
is especially enhanced in one or two antipode regions, fixed in the atmosphere
of the star and rotating with it. Such spots could be active regions around the
North and the South poles of a general magnetic dipole field of the star. The
P3 periodicity is half the beat of the binary orbital period of the system and
the spin period of the giant. The latter is then either 482 or 1790 d. If only
one pole is active on the surface of the giant, P3 is the beat period itself,
and the spin period is 352 d. It could also be 5000 d if the giant is rotating
in retrograde direction. We briefly compare these findings in the LC of Z And
to similar modulations that were identified in the LC of two other prototype
symbiotics, BF Cyg and YY Her.Comment: 9 pages, 4 figures, Accepted for publication in MNRA
Absence of linear polarization in Halpha emission of solar flares
High sensitivity observations of Halpha polarization of 30 flares of
different sizes and disk positions are reported. Both filter and spectrographic
techniques have been used. The ZIMPOL system eliminates spurious polarizations
due to seeing and flat-field effects. We didn't find any clear linear
polarization signature above our sensitivity level which was usually better
than 0.1%. The observations include an X17.1 flare with gamma-ray lines
reported by the RHESSI satellite. These results cast serious doubts on previous
claims of linear polarization at the one percent level and more, attributed to
impact polarization. The absence of linear polarization limits the anisotropy
of energetic protons in the H emitting region. The likely causes are
isotropization by collisions with neutrals in the chromosphere and defocusing
by the converging magnetic field.Comment: to be published in A&A, 8 pages, 6 figure
Magnetic Coupling in the Quiet Solar Atmosphere
Three kinds of magnetic couplings in the quiet solar atmosphere are
highlighted and discussed, all fundamentally connected to the Lorentz force.
First the coupling of the convecting and overshooting fluid in the surface
layers of the Sun with the magnetic field. Here, the plasma motion provides the
dominant force, which shapes the magnetic field and drives the surface dynamo.
Progress in the understanding of the horizontal magnetic field is summarized
and discussed. Second, the coupling between acoustic waves and the magnetic
field, in particular the phenomenon of wave conversion and wave refraction. It
is described how measurements of wave travel times in the atmosphere can
provide information about the topography of the wave conversion zone, i.e., the
surface of equal Alfv\'en and sound speed. In quiet regions, this surface
separates a highly dynamic magnetic field with fast moving magnetosonic waves
and shocks around and above it from the more slowly evolving field of high-beta
plasma below it. Third, the magnetic field also couples to the radiation field,
which leads to radiative flux channeling and increased anisotropy in the
radiation field. It is shown how faculae can be understood in terms of this
effect. The article starts with an introduction to the magnetic field of the
quiet Sun in the light of new results from the Hinode space observatory and
with a brief survey of measurements of the turbulent magnetic field with the
help of the Hanle effect.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Comparing different approaches to model the rotational modulation of the Sun as a star
The space missions MOST, COROT and Kepler are going to provide us with
high-precision optical photometry of solar-like stars with time series
extending from tens of days to several years. They can be modelled to obtain
information on stellar magnetic activity by fitting the rotational modulation
of the stellar flux produced by the brightness inhomogeneities associated with
photospheric active regions. The variation of the total solar irradiance
provides a good proxy for those photometric time series and can be used to test
the performance of different spot modelling approaches. We test discrete spot
models as well as maximum entropy and Tikhonov regularized spot models by
comparing the reconstructed total sunspot area variation and longitudinal
distributions of sunspot groups with those actually observed in the Sun along
activity cycle 23. Appropriate statistical methods are introduced to measure
model performance versus the timescale of variation. The maximum entropy
regularized spot models show the best agreement with solar observations
reproducing the total sunspot area variation on time scales ranging from a few
months to the activity cycle, although the model amplitudes are affected by
systematic errors during the minimum and the maximum activity phases. The
longitudinal distributions derived from the models compare well with the
observed sunspot group distributions except during the minimum of activity,
when faculae dominate the rotational modulation. The resolution in longitude
attainable through the spot modelling is about 60 degrees.Comment: 21 pages, 7 figures, accepted by Astron. Astrophy
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