77 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
Determination of rotation periods in solar-like stars with irregular sampling: the Gaia case
We present a study on the determination of rotation periods (P) of solar-like
stars from the photometric irregular time-sampling of the ESA Gaia mission,
currently scheduled for launch in 2013, taking into account its dependence on
ecliptic coordinates. We examine the case of solar-twins as well as thousands
of synthetic time-series of solar-like stars rotating faster than the Sun. In
the case of solar twins we assume that the Gaia unfiltered photometric passband
G will mimic the variability of the total solar irradiance (TSI) as measured by
the VIRGO experiment. For stars rotating faster than the Sun, light-curves are
simulated using synthetic spectra for the quiet atmosphere, the spots, and the
faculae combined by applying semi-empirical relationships relating the level of
photospheric magnetic activity to the stellar rotation and the Gaia
instrumental response. The capabilities of the Deeming, Lomb-Scargle, and Phase
Dispersion Minimisation methods in recovering the correct rotation periods are
tested and compared. The false alarm probability (FAP) is computed using Monte
Carlo simulations and compared with analytical formulae. The Gaia scanning law
makes the rate of correct detection of rotation periods strongly dependent on
the ecliptic latitude (beta). We find that for P ~ 1 d, the rate of correct
detection increases with ecliptic latitude from 20-30 per cent at beta ~
0{\deg} to a peak of 70 per cent at beta=45{\deg}, then it abruptly falls below
10 per cent at beta > 45{\deg}. For P > 5 d, the rate of correct detection is
quite low and for solar twins is only 5 per cent on average.Comment: 12 pages, 18 figures, accepted by MNRA
The kinematics of coronal mass ejections using multiscale methods
The diffuse morphology and transient nature of coronal mass ejections (CMEs)
make them difficult to identify and track using traditional image processing
techniques. We apply multiscale methods to enhance the visibility of the faint
CME front. This enables an ellipse characterisation to objectively study the
changing morphology and kinematics of a sample of events imaged by the Large
Angle Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric
Observatory (SOHO) and the Sun Earth Connection Coronal and Heliospheric
Investigation (SECCHI) onboard the Solar Terrestrial Relations Observatory
(STEREO). The accuracy of these methods allows us to test the CMEs for
non-constant acceleration and expansion.
We exploit the multiscale nature of CMEs to extract structure with a
multiscale decomposition, akin to a Canny edge detector. Spatio-temporal
filtering highlights the CME front as it propagates in time. We apply an
ellipse parameterisation of the front to extract the kinematics (height,
velocity, acceleration) and changing morphology (width, orientation).
The kinematic evolution of the CMEs discussed in this paper have been shown
to differ from existing catalogues. These catalogues are based upon
running-difference techniques that can lead to over-estimating CME heights. Our
resulting kinematic curves are not well-fitted with the constant acceleration
model. It is shown that some events have high acceleration below 5
R_{\sun}. Furthermore, we find that the CME angular widths measured by these
catalogues are over-estimated, and indeed for some events our analysis shows
non-constant CME expansion across the plane-of-sky.Comment: 10 pages, 13 figures, accepted for publicatio
Solar irradiance variability: a six-year comparison between SORCE observations and the SATIRE model
Aims: We investigate how well modeled solar irradiances agree with
measurements from the SORCE satellite, both for total solar irradiance and
broken down into spectral regions on timescales of several years. Methods: We
use the SATIRE model and compare modeled total solar irradiance (TSI) with TSI
measurements between 2003 and 2009. Spectral solar irradiance over 200-1630nm
is compared with the SIM instrument on SORCE between 2004 and 2009 during a
period of decline from moderate activity to the recent solar minimum in 10 nm
bands and for three spectral regions of significant interest: the UV integrated
over 200-300nm, the visible over 400-691nm and the IR between 972-1630 nm.
Results: The model captures 97% of observed TSI variation. In the spectral
comparison, rotational variability is well reproduced, especially between 400
and 1200 nm. The magnitude of change in the long-term trends is many times
larger in SIM at almost all wavelengths while trends in SIM oppose SATIRE in
the visible between 500 and 700nm and between 1000 and 1200nm. We discuss the
remaining issues with both SIM data and the identified limits of the model,
particularly with the way facular contributions are dealt with, the limit of
flux identification in MDI magnetograms during solar minimum and the model
atmospheres in the IR employed by SATIRE. It is unlikely that improvements in
these areas will significantly enhance the agreement in the long-term trends.
This disagreement implies that some mechanism other than surface magnetism is
causing SSI variations, in particular between 2004 and 2006, if the SIM data
are correct. Since SATIRE was able to reproduce UV irradiance between 1991 and
2002 from UARS, either the solar mechanism for SSI variation fundamentally
changed around the peak of cycle 23, or there is an inconsistency between UARS
and SORCE UV measurements. We favour the second explanation.Comment: 14 pages, 13 figure
Detection of Solar Rotational Variability in the LYRA 190 - 222 nm Spectral Band
We analyze the variability of the spectral solar irradiance during the period
from 7 January, 2010 until 20 January, 2010 as measured by the Herzberg channel
(190-222 nm) of the Large Yield RAdiometer (LYRA) onboard PROBA2. In this
period of time observations by the LYRA nominal unit experienced degradation
and the signal produced by the Herzberg channel frequently jumped from one
level to another. Both these factors significantly complicates the analysis. We
present the algorithm which allowed us to extract the solar variability from
the LYRA data and compare the results with SORCE/SOLSTICE measurements and with
modeling based on the Code for the Solar Irradiance (COSI)
Variability of Sun-like stars: reproducing observed photometric trends
The Sun and stars with low magnetic activity levels, become photometrically
brighter when their activity increases. Magnetically more active stars display
the opposite behaviour and get fainter when their activity increases.
We reproduce the observed photometric trends in stellar variations with a
model that treats stars as hypothetical Suns with coverage by magnetic features
different from that of the Sun.
The presented model attributes the variability of stellar spectra to the
imbalance between the contributions from different components of the solar
atmosphere, such as dark starspots and bright faculae. A stellar spectrum is
calculated from spectra of the individual components, by weighting them with
corresponding disc area coverages. The latter are obtained by extrapolating the
solar dependences of spot and facular disc area coverages on chromospheric
activity to stars with different levels of mean chromospheric activity.
We have found that the contribution by starspots to the variability increases
faster with chromospheric activity than the facular contribution. This causes
the transition from faculae-dominated variability and direct
activity--brightness correlation to spot-dominated variability and inverse
activity--brightness correlation with increasing chromospheric activity level.
We have shown that the regime of the variability also depends on the angle
between the stellar rotation axis and the line-of-sight and on the latitudinal
distribution of active regions on the stellar surface. Our model can be used as
a tool to extrapolate the observed photometric variability of the Sun to
Sun-like stars at different activity levels, which makes possible the direct
comparison between solar and stellar irradiance data.Comment: 20 pages, 16 figures, accepted for publication in
Astronomy&Astrophysic
Eclipses observed by LYRA - a sensitive tool to test the models for the solar irradiance
We analyze the light curves of the recent solar eclipses measured by the
Herzberg channel (200-220 nm) of the Large Yield RAdiometer (LYRA) onboard
PROBA-2. The measurements allow us to accurately retrieve the center- to-limb
variations (CLV) of the solar brightness. The formation height of the radiation
depends on the observing angle so the examination of the CLV provide
information about a broad range of heights in the solar atmosphere. We employ
the 1D NLTE radiative transfer COde for Solar Irradiance (COSI) to model the
measured light curves and corresponding CLV dependencies. The modeling is used
to test and constrain the existing 1D models of the solar atmosphere, e.g. the
temperature structure of the photosphere and the treatment of the pseudo-
continuum opacities in the Herzberg continuum range. We show that COSI can
accurately reproduce not only the irradiance from the entire solar disk, but
also the measured CLV. It hence can be used as a reliable tool for modeling the
variability of the spectral solar irradiance.Comment: 19 pages, 9 figures, Solar Physic
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