The acoustic cut-off frequency of the Sun and the solar magnetic activity cycle

The acoustic cut-off frequency -the highest frequency for acoustic solar eigenmodes- is an important parameter of the solar atmosphere as it determines the upper boundary of the p-mode resonant cavities. At frequencies beyond this value, acoustic disturbances are no longer trapped but traveling waves. Interference amongst them give rise to higher-frequency peaks -the pseudomodes- in the solar acoustic spectrum. The pseudomodes are shifted slightly in frequency with respect to p modes making possible the use of pseudomodes to determine the acoustic cut-off frequency. Using data from GOLF and VIRGO instruments on board the SOHO spacecraft, we calculate the acoustic cut-off frequency using the coherence function between both the velocity and intensity sets of data. By using data gathered by these instruments during the entire lifetime of the mission (1996 till the present), a variation in the acoustic cut-off frequency with the solar magnetic activity cycle is found.Comment: Paper accepted in ApJ. 26 Pages, 9 figure

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

Retirement incentives, individual heterogeneity and labour transitions of employed and unemployed workers

In this paper we analyze the sensitivity of the labour market decisions of workers close to retirement with respect to the incentives created by public regulations. We improve upon the extensive prior literature on the effect of pension incentives on retirement in two ways. First, by modeling the transitions between employment, unemployment and retirement in a simultaneous manner, paying special attention to the transition from unemployment to retirement (which is particularly important in Spain). Second, by considering the influence of unobserved heterogeneity in the estimation of the effect of our (carefully constructed) incentive variables. Using administrative data, we find that, when properly defined, economic incentives have a strong impact on labour market decisions in Spain. Unemployment regulations are shown to be particularly influential for retirement behaviour, along with the more traditional determinants linked to the pension system. Pension variables also have a major bearing on both workers’ reemployment decisions and on the strategic actions of employers. The quantitative impact of the incentives, however, is greatly affected by the existence of unobserved heterogeneity among workers. Its omission leads to sizable biases in the assessment of the sensitivity to economic incentives, a finding that has clear consequences for the credibility of any model-based policy analysis. We confirm the importance of this potential problem in one especially interesting instance: the reform of early retirement provisions undertaken in Spain in 2002. We use a difference-in-difference approach to measure the behavioural reaction to this change, finding a large overestimation when unobserved heterogeneity is not taken into account.Retirement, unemployment, incentives, Pension system, Unobserved, heterogeneity, Spain.

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&

Molecular characterization of table grape varieties preserved in the Rancho de la Merced Grapevine Germplasm Bank (Spain)

A collection of 317 table-grape accessions preserved in the Vitis Germplasm Bank at the Rancho de la Merced (IFAPA, Jerez de la Frontera, Spain) were genotyped at 20 microsatellite loci. A total of 264 different genotypes were obtained. Among them, 66 are presented for the first time. Twenty-one accession names are considered new synonyms, fourteen homonyms and 48 can be considered as "denomination mistakes" in the Rancho de la Merced collection

On the solar origin of the signal at 220.7microHz: A possible component of a g mode?

Gravity modes in the Sun have been the object of a long and difficult search in recent decades. Thanks to the data accumulated with the last generation of instruments (BiSON, GONG and three helioseismic instruments aboard SoHO), scientists have been able to find signatures of their presence. However, the individual detection of such modes remains evasive. In this article, we study the signal at 220.7 microHz which is a peak that is present in most of the helioseismic data of the last 10 years. This signal has already been identified as being one component of a g-mode candidate detected in the GOLF Doppler velocity signal. The nature of this peak is studied in particular using the VIRGO/SPM instrument aboard SoHO. First we analyse all the available instrumental data of VIRGO and SoHO (housekeeping) to reject any possible instrumental origin. No relation was found, implying that the signal has a solar origin. Using Monte Carlo simulations, we find, with more than 99% confidence level, that the signal found in VIRGO/SPM is very unlikely to be due to pure noise.Comment: Accepted for publication in ApJSS. 19 pages, 9 figure

The Peierls substitution in an engineered lattice potential

Artificial gauge fields open new possibilities to realize quantum many-body systems with ultracold atoms, by engineering Hamiltonians usually associated with electronic systems. In the presence of a periodic potential, artificial gauge fields may bring ultracold atoms closer to the quantum Hall regime. Here, we describe a one-dimensional lattice derived purely from effective Zeeman-shifts resulting from a combination of Raman coupling and radiofrequency magnetic fields. In this lattice, the tunneling matrix element is generally complex. We control both the amplitude and the phase of this tunneling parameter, experimentally realizing the Peierls substitution for ultracold neutral atoms.Comment: 6 pages, 5 figure

Arenas eólicas y sus suelos del humedal reserva de la biosfera: Las tablas de daimiel

Three soil profiles were characterized from a sandy area situated in a transitional zone in the “Tablas de Daimiel” UNESCO wetland biosphere reserve (TDNP) in the Mancha Plain (Central Spain). The original thickness of the soil layer in this area was as much as 100 cm, but the combined effect of unbalanced cultivation (including irrigation) and an increasingly dry climate has partially eroded their natural properties, almost leading to a desertification process. The main properties of these soils, classified as Xeropsamments (Soil Survey Staff 2006) or Arenosols (IUSS Working Group WRB 2006), are the dominance of sand in the soil matrix, its basic character, and low soil organic matter and carbonate contents. Scanning electron microscopy of the quartz grain surfaces indicated that the superficial textures were commonly well preserved and characteristic of specific conditions of Aeolian formation. Exoscopy revealed a mixture of sub-angular and rounded quartz morphotypes with wellpreserved mechanical impacts on the grain surfaces. These percussion effects are characteristic of aeolian processes and confirmed that mechanical actions were more significant than chemical processes in their pedogenesis. This micromorphological study of the textural sands in the transitional zone of the TDNP revealed that the (red) sands were transported to this area by wind, probably from material of degraded red soils, and deposited on soil horizons previously developed over marls and limestones. The relict character of these materials and specially their susceptibility to degradation should be considered as a priority argument to preserve this area and introduce management measurements to avoid soil erosio