4,891 research outputs found
The onset of solar cycle 24: What global acoustic modes are telling us
We study the response of the low-degree, solar p-mode frequencies to the
unusually extended minimum of solar surface activity since 2007. A total of
4768 days of observations collected by the space-based, Sun-as-a-star
helioseismic GOLF instrument are analyzed. A multi-step iterative
maximum-likelihood fitting method is applied to subseries of 365 days and 91.25
days to extract the p-mode parameters. Temporal variations of the l=0, 1, and 2
p-mode frequencies are then obtained from April 1996 to May 2009. While the
p-mode frequency shifts are closely correlated with solar surface activity
proxies during the past solar cycles, the frequency shifts of the l=0 and l=2
modes show an increase from the second half of 2007, when no significant
surface activity is observable. On the other hand, the l=1 modes follow the
general decreasing trend of the solar surface activity. The different
behaviours between the l=0 and l=2 modes and the l=1 modes can be interpreted
as different geometrical responses to the spatial distribution of the solar
magnetic field beneath the surface of the Sun. The analysis of the low-degree,
solar p-mode frequency shifts indicates that the solar activity cycle 24
started late 2007, despite the absence of activity on the solar surface.Comment: To be accepted by A&A (with minor revisions), 4 pages, 3 figures, 1
tabl
Cambio actitudinal de estudiantes de octavo grado hacia el aprendizaje de interacciones biológicas mediante la resolución de problemas
Se presenta aquí una experiencia pedagógica en enseñanza de la biología, desarrollada en el espacio académico de Aula experimental de la Universidad Distrital Francisco José de Caldas, con 28 estudiantes de grado 8vo del Colegio La Candelaria, sede la Concordia, en la ciudad de Bogotá (Colombia) con un rango de edades entre los 12 y los 17 años, se escogen tres estudiantes, tomando como eje conceptual las interacciones biológicas. Este trabajo se realizó en las etapas de planificación, diseño y implementación de la unidad didáctica “Preguntando ando, con el ecosistema interactuando” la cual se centró en el enfoque de enseñanza por resolución de problemas. En la finalización de este trabajo se encontró un cambio actitudinal de los estudiantes con respecto al aprendizaje del eje conceptual, teniendo como referente los componentes actitudinales expuestos por Vilches y Furió (1997)
The Quasi-Biennial Periodicity (QBP) in velocity and intensity helioseismic observations
We looked for signatures of Quasi-Biennial Periodicity (QBP) over different
phases of solar cycle by means of acoustic modes of oscillation. Low-degree
p-mode frequencies are shown to be sensitive to changes in magnetic activity
due to the global dynamo. Recently have been reported evidences in favor of
two-year variations in p-mode frequencies. Long high-quality helioseismic data
are provided by BiSON (Birmingham Solar Oscillation Network), GONG (Global
Oscillation Network Group), GOLF (Global Oscillation at Low Frequency) and
VIRGO (Variability of Solar IRradiance and Gravity Oscillation) instruments. We
determined the solar cycle changes in p-mode frequencies for spherical degree
l=0, 1, 2 with their azimuthal components in the frequency range 2.5 mHz < nu <
3.5 mHz. We found signatures of QBP at all levels of solar activity in the
modes more sensitive to higher latitudes. The signal strength increases with
latitude and the equatorial component seems also to be modulated by the 11-year
envelope. The persistent nature of the seismic QBP is not observed in the
surface activity indices, where mid-term variations are found only time to time
and mainly over periods of high activity. This feature together with the
latitudinal dependence provides more evidences in favor of a mechanism almost
independent and different from the one that brings up to the surface the active
regions. Therefore, these findings can be used to provide more constraints on
dynamo models that consider a further cyclic component on top of the 11-year
cycle.Comment: 9 pages, 9 Figures, 2 Tables Accepted for publication in A&
Metadiffusers for quasi-perfect and broadband sound diffusion
Sound diffusion refers to the ability of a surface to evenly scatter sound energy in both time and space. However, omnidirectional radiation of sound, or perfect diffusion, can be impractical or difficult to reach under traditional means.
This is due to the considerable size required by, and the lack of tunability, of typical quarter-wavelength scattering
strategies necessary for producing the required complexity of the surface acoustic impedance. As such, it can be a
challenge to design sound diffusing structures that can display near perfect diffusion performance within slim dimensions.
In this work, we propose a method for obtaining quasi-perfect and broadband sound diffusion coefficients using
deep-subwavelength acoustic diffusers, i.e., metadiffusers. The relation between the geometry of the metasurface, the
bandwidth and the diffusion performance is analytically and numerically studied. For moderate bandwidths, around
1/3 of an octave, the method results in nearly perfect sound diffusion, while for a bandwidth of 2.5 octaves a normalized
diffusion coefficient of 0.8 was obtained using panels 1/30th thinner than traditional phase-grating designs. The
ratio between the wavelength and the size of the unit cell was identified as a limitation of the performance. This work
demonstrates the versatility and effectiveness of metadiffusers to generate diffuse reflections outperforming those of classical sound diffuser
Are short-term variations in solar oscillation frequencies the signature of a second solar dynamo?
In addition to the well-known 11-year solar cycle, the Sun's magnetic
activity also shows significant variation on shorter time scales, e.g. between
one and two years. We observe a quasi-biennial (2-year) signal in the solar
p-mode oscillation frequencies, which are sensitive probes of the solar
interior. The signal is visible in Sun-as-a-star data observed by different
instruments and here we describe the results obtained using BiSON, GOLF, and
VIRGO data. Our results imply that the 2-year signal is susceptible to the
influence of the main 11-year solar cycle. However, the source of the signal
appears to be separate from that of the 11-year cycle. We speculate as to
whether it might be the signature of a second dynamo, located in the region of
near-surface rotational shear.Comment: 6 pages, 2 figures, proceedings for SOHO-24/GONG 2010 conference, to
be published in JPC
Plant Metabolites in Plant Defense Against Pathogens
Medicinal plants are widely used worldwide to treat various diseases. Its widespread use is due in part to the cultural acceptance of traditional medicine in different regions of the world, as well as its effectiveness in treating various diseases. Many of its active substances or secondary metabolites are formed to a response of various situations that generate stress in their habitat, such as sudden changes in environmental temperature, humidity, rain, drought, and infections by phytopathogens (fungi, bacteria, viruses, nematodes, protozoa). The production of these secondary metabolites is a mechanism of defense of plants. In this context, the objective of this chapter is to study the secondary metabolites of medicinal plants that could have a promising application in the control of different phytopathogens in crops of agricultural and economic interest
Sensitivity of the g-mode frequencies to pulsation codes and their parameters
From the recent work of the Evolution and Seismic Tools Activity (ESTA,
Lebreton et al. 2006; Monteiro et al. 2008), whose Task 2 is devoted to compare
pulsational frequencies computed using most of the pulsational codes available
in the asteroseismic community, the dependence of the theoretical frequencies
with non-physical choices is now quite well fixed. To ensure that the accuracy
of the computed frequencies is of the same order of magnitude or better than
the observational errors, some requirements in the equilibrium models and the
numerical resolutions of the pulsational equations must be followed. In
particular, we have verified the numerical accuracy obtained with the Saclay
seismic model, which is used to study the solar g-mode region (60 to
140Hz). We have compared the results coming from the Aarhus adiabatic
pulsation code (ADIPLS), with the frequencies computed with the Granada Code
(GraCo) taking into account several possible choices. We have concluded that
the present equilibrium models and the use of the Richardson extrapolation
ensure an accuracy of the order of in the determination of the
frequencies, which is quite enough for our purposes.Comment: 10 pages, 5 figures, accepted in Solar Physic
- …