213 research outputs found
Limits to solar cycle predictability: Cross-equatorial flux plumes
Within the Babcock-Leighton framework for the solar dynamo, the strength of a
cycle is expected to depend on the strength of the dipole moment or net
hemispheric flux during the preceding minimum, which depends on how much flux
was present in each hemisphere at the start of the previous cycle and how much
net magnetic flux was transported across the equator during the cycle. Some of
this transport is associated with the random walk of magnetic flux tubes
subject to granular and supergranular buffeting, some of it is due to the
advection caused by systematic cross-equatorial flows such as those associated
with the inflows into active regions, and some crosses the equator during the
emergence process.
We aim to determine how much of the cross-equatorial transport is due to
small-scale disorganized motions (treated as diffusion) compared with other
processes such as emergence flux across the equator. We measure the
cross-equatorial flux transport using Kitt Peak synoptic magnetograms,
estimating both the total and diffusive fluxes. Occasionally a large sunspot
group, with a large tilt angle emerges crossing the equator, with flux from the
two polarities in opposite hemispheres. The largest of these events carry a
substantial amount of flux across the equator (compared to the magnetic flux
near the poles). We call such events cross-equatorial flux plumes. There are
very few such large events during a cycle, which introduces an uncertainty into
the determination of the amount of magnetic flux transported across the equator
in any particular cycle. As the amount of flux which crosses the equator
determines the amount of net flux in each hemisphere, it follows that the
cross-equatorial plumes introduce an uncertainty in the prediction of the net
flux in each hemisphere. This leads to an uncertainty in predictions of the
strength of the following cycle.Comment: A&A, accepte
Magnetic flux generation and transport in cool stars
The Sun and other cool stars harbouring outer convection zones manifest
magnetic activity in their atmospheres. The connection between this activity
and the properties of a deep-seated dynamo generating the magnetic flux is not
well understood. By employing physical models, we study the spatial and
temporal characteristics of the observable surface field for various stellar
parameters. We combine models for magnetic flux generation, buoyancy
instability, and transport, which encompass the entire convection zone. The
model components are: (1) a thin-layer alpha-Omega dynamo at the base of the
convection zone; (2) buoyancy instabilities and the rise of flux tubes through
the convection zone in 3D, which provides a physically consistent determination
of emergence latitudes and tilt angles; and (3) horizontal flux transport at
the surface. For solar-type stars and rotation periods longer than about 10
days, the latitudinal dynamo waves generated by the deep-seated alpha-Omega
dynamo are faithfully reflected by the surface distribution of magnetic flux.
For rotation periods of the order of two days, however, Coriolis acceleration
of rising flux loops leads to surface flux emergence at much higher latitudes
than the dynamo waves at the bottom of the convection zone reach. A similar
result is found for a K0V star with a rotation period of two days. In the case
of a rapidly rotating K1 subgiant, overlapping dynamo waves lead to noisy
activity cycles and mixed-polarity fields at high latitudes.Comment: 14 pages, 14 figures. Accepted for publication in Astronomy &
Astrophysic
Características y propiedades de los ignimbritos de Bitlis y sus implicaciones ambientales
Bitlis rock is used as a construction material and comes from the lava emitted by volcanoes and their subsequent transformation into ignimbrites. This type of rocks has been characterized physically, chemically, toxicologically and radioactively using different procedures including determination of the coefficient of thermal conductivity, gamma spectrometry, ultrasonic speed test, ICP masses and metal extraction. The results indicate that Bitlis rocks have an ACI greater than 1, although their content of radon is lower than other rocks of volcanic origin. Leaching of metals from these rocks indicates that Pb and Cd can provide an infiltration level in the field higher than the level permitted by TCLP and they have undesired toxicological risks. The percentages of extraction of other metals also point to this infiltration problem. Despite this, the material offers good qualities for usage as a building material such as its thermal coefficients.La roca Bitlis se utiliza como material de construcción y proviene de la lava emitida por los volcanes y su posterior transformación en ignimbritas. Este tipo de rocas se ha caracterizado física, química, toxicológica y radioactivamente utilizando diferentes procedimientos, incluida la determinación del coeficiente de conductividad térmica, espectrometría gamma, prueba de velocidad ultrasónica, ICP masas y extracción de metales. Los resultados indican que las rocas Bitlis tienen un ACI mayor que 1, aunque su contenido de radón es más bajo que el de otras rocas de origen volcánico. La lixiviación de metales de estas rocas indica que el Pb y el Cd pueden proporcionar un nivel de infiltración en el campo más alto que el nivel permitido por TCLP y tener riesgos toxicológicos no deseados. Los porcentajes de extracción de otros metales también apuntan a este problema de infiltración. A pesar de esto, el material ofrece buenas cualidades para su uso como material de construcción, como pueden ser sus coeficientes térmicos
Flow instabilities of magnetic flux tubes IV. Flux storage in the solar overshoot region
We consider the effects of material flows on the dynamics of toroidal
magnetic flux tubes located close to the base of the solar convection zone,
initially within the overshoot region. The problem is to find the physical
conditions in which magnetic flux can be stored for periods comparable to the
dynamo amplification time, which is of the order of a few years. We carry out
nonlinear numerical simulations to investigate the stability and dynamics of
thin flux tubes subject to perpendicular and longitudinal flows. We compare the
simulations with the results of simplified analytical approximations. We
determine ranges of the flow parameters for which a linearly Parker-stable
magnetic flux tube is stored in the middle of the overshoot region for a period
comparable to the dynamo amplification time. The residence time for magnetic
flux tubes with fluxes of 2x10^{21} Mx in the convective overshoot layer is
comparable to the dynamo amplification time, provided that the average speed
and the duration of the downflow do not exceed about 50 m/s and 100 days,
respectively, and that the lateral extension of the flow is smaller than about
10 degrees.Comment: Accepted to be published in Astronomy and Astrophysics. 16 pages, 16
figures. To access GIF animations, use
http://www.mps.mpg.de/homes/ishik/flute/frict_inst.gif,
http://www.mps.mpg.de/homes/ishik/flute/TF60.gif and
http://www.mps.mpg.de/homes/ishik/flute/TF180.gi
Observing and modelling the young solar analogue EK Draconis: starspot distribution, elemental abundances, and evolutionary status
Observations and modelling of stars with near-solar masses in their early
phases of evolution is critical for a better understanding of how dynamos of
solar-type stars evolve. We examine the chemical composition and the spot
distribution of the pre-main-sequence solar analogue EK Dra. Using spectra from
the HERMES Spectrograph (La Palma), we obtain the abundances of 23 elements
with respect to the solar ones, which lead to a , with
significant overabundance of Li and Ba. The s-process elements Sr, Y, and Ce
are marginally overabundant, while Co, Ni, Cu, Zn are marginally deficient
compared to solar abundances. The overabundance of Ba is most likely due to the
assumption of depth-independent microturbulent velocity. Li abundance is
consistent with the age and the other abundances may indicate distinct initial
conditions of the pre-stellar nebula. We estimate a mass of 1.04 and
an age of \,Myr using various spectroscopic and photometric
indicators. We study the surface distribution of dark spots, using 17 spectra
collected during 15 nights using the CAFE Spectrograph (Calar Alto). We also
conduct flux emergence and transport (FEAT) simulations for EK Dra's parameters
and produce 15-day-averaged synoptic maps of the likely starspot distributions.
Using Doppler imaging, we reconstruct the surface brightness distributions for
the observed spectra and FEAT simulations, which show overall agreement for
polar and mid-latitude spots, while in the simulations there is a lack of
low-latitude spots compared to the observed image. We find indications that
cross-equatorial extensions of mid-latitude spots can be artefacts of the less
visible southern-hemisphere activity.Comment: 15 pages, 16 figures, accepted for publication in MNRAS; corrections
in Abstract and Introductio
Predictions of Astrometric Jitter for Sun-like Stars. II. Dependence on Inclination, Metallicity, and Active-Region Nesting
Ultra-precise astrometry from the Gaia mission is expected to lead to
astrometric detections of more than 20,000 exoplanets in our Galaxy. One of the
factors that could hamper such detections is the astrometric jitter caused by
the magnetic activity of the planet host stars. In our previous study, we
modeled astrometric jitter for the Sun observed equator-on. In this work, we
generalize our model and calculate the photocenter jitter as it would be
measured by the Gaia and Small-JASMINE missions for stars with solar rotation
rate and effective temperature, but with various values of the inclination
angle of the stellar rotation axis. In addition, we consider the effect of
metallicity and of nesting of active regions (i.e. the tendency of active
regions to emerge in the vicinity of each other). We find that, while the
jitter of stars observed equator-on does not have any long-term trends and can
be easily filtered out, the photocenters of stars observed out of their
equatorial planes experience systematic shifts over the course of the activity
cycle. Such trends allow the jitter to be detected with continuous
measurements, in which case it can interfere with planet detectability. An
increase in the metallicity is found to increase the jitter caused by stellar
activity. Active-region nesting can further enhance the peak-to-peak amplitude
of the photocenter jitter to a level that could be detected by Gaia.Comment: 24 pages, 20 figures in the main body and 4 in the appendix, accepted
for publication in Ap
Surface flux evolution constraints for flux transport dynamos
The surface flux transport (SFT) model of solar magnetic fields involves
empirically well-constrained velocity and magnetic fields. The basic evolution
of the Sun's large-scale surface magnetic field is well described by this
model. The azimuthally averaged evolution of the SFT model can be compared to
the surface evolution of the flux transport dynamo (FTD), and the evolution of
the SFT model can be used to constrain several near-surface properties of the
FTD model.
We compared the results of the FTD model with different upper boundary
conditions and diffusivity profiles against the results of the SFT model. Among
the ingredients of the FTD model, downward pumping of magnetic flux, related to
a positive diffusivity gradient, has a significant effect in slowing down the
diffusive radial transport of magnetic flux through the solar surface. Provided
the pumping was strong enough to give rise to a downflow of a magnetic Reynolds
number of 5 in the near-surface boundary layer, the FTD using a vertical
boundary condition matches the SFT model based on the average velocities above
the boundary layer. The FTD model with a potential field were unable to match
the SFT results.Comment: Accepted for A&
Gas and seismicity within the Istanbul seismic gap
Understanding micro-seismicity is a critical question for earthquake hazard assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the seismicity along the submerged section of North Anatolian Fault within the Sea of Marmara (comprising the “Istanbul seismic gap”) has been extensively studied in order to infer its mechanical behaviour (creeping vs locked). So far, the seismicity has been interpreted only in terms of being tectonic-driven, although the Main Marmara Fault (MMF) is known to strike across multiple hydrocarbon gas sources. Here, we show that a large number of the aftershocks that followed the M 5.1 earthquake of July, 25th 2011 in the western Sea of Marmara, occurred within a zone of gas overpressuring in the 1.5–5 km depth range, from where pressurized gas is expected to migrate along the MMF, up to the surface sediment layers. Hence, gas-related processes should also be considered for a complete interpretation of the micro-seismicity (~M < 3) within the Istanbul offshore domain
High-K volcanism in the Afyon region, western Turkey: from Si-oversaturated to Si-undersaturated volcanism
Volcanic rocks of the Afyon province (eastern
part of western Anatolia) make up a multistage potassic and ultrapotassic alkaline series dated from 14 to 12 Ma. The early-stage Si-oversaturated volcanic rocks around the Afyon city and further southward are trachyandesitic volcanic activity (14.23 ± 0.09 Ma). Late-stage Si-undersaturated volcanism in the southernmost part of the Afyon volcanic province took place in three episodes inferred from their stratigraphic relationships and ages. Melilite–
leucitites (11.50 ± 0.03 Ma), spotted rachyandesites, tephryphonolites and lamproites (11.91 ± 0.13 Ma) formed in the first episode; trachyandesites in the second episode and finally phonotephrites, phonolite, basaltic trachyandesites and nosean-bearing trachyandesites during the last episode.
The parameter Q [normative q-(ne + lc + kls + ol)] of western Anatolia volcanism clearly decreased southward with time becoming zero in the time interval 10–15 Ma.
The magmatism experienced a sudden change in the extent of Si saturation after 14 Ma, during late-stage volcanic activity of Afyon volcanic province at around 12 Ma, though there was some coexistence of Si-oversaturated and Si-undersaturated magmas during the whole life of Afyon volcanic province
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