Argentine Stations Research Articles

Seismic interferometry applied to local fracture seismicity recorded at Planchón-Peteroa Volcanic Complex, Argentina-Chile

Although the Planchón-Peteroa Volcanic Complex (PPVC) has undergone many hazardous eruptions, only a small number of geological, geochemical, and geophysical studies have been performed to describe this active volcanic system. In order to characterize the subsurface structures present at the PPVC, we applied seismic interferometry to fracture seismicity originating in this volcanic complex and along active geologic faults located nearby. We utilized seismic data recorded by two arrays of stations deployed in Argentina and Chile. Nine of these stations (three in Chile and six in Argentina) recorded data simultaneously and were used for this application. Only seismic events with energy arriving (sub) vertically to the stations were chosen for processing. According to the magnitude and the location of the selected seismic events, relocated seismic sources were used for the Chilean stations while, for the Argentine stations, only seismic sources located near that array were used. We obtained seismic evidence of the location of the subsurface reflectors underneath each station using zero-offset reflection responses retrieved from seismic interferometry by autocorrelation of (time windows extracted from) the selected seismic events. Then, applying a comparative analysis between the seismic results and the available geological information, we imaged the shallow subsurface of the area enclosed by the Chilean stations, and also by the Argentine stations. The results are consistent with the available geological information, provide accurate depth values for several subsurface discontinuities, indicate areas of higher heterogeneity, and support the emplacement of a magma body at ∼4 km depth from the surface. This work shows the first application of a novel variation of seismic interferometry based on autocorrelations to local-earthquake data recorded in a volcanic area.

Neural network based daily precipitation generator (NNGEN-P)

Daily weather generators are used in many applications and risk analyses. The present paper explores the potential of neural network architectures to design daily weather generator models. Focusing this first paper on precipitation, we design a collection of neural networks (multi-layer perceptrons in the present case), which are trained so as to approximate the empirical cumulative distribution (CDF) function for the occurrence of wet and dry spells and for the precipitation amounts. This approach contributes to correct some of the biases of the usual two-step weather generator models. As compared to a rainfall occurrence Markov model, NNGEN-P represents fairly well the mean and standard deviation of the number of wet days per month, and it significantly improves the simulation of the longest dry and wet periods. Then, we compared NNGEN-P to three parametric distribution functions usually applied to fit rainfall cumulative distribution functions (Gamma, Weibull and double-exponential). A data set of 19 Argentine stations was used. Also, data corresponding to stations in the United States, in Europe and in the Tropics were included to confirm the results. One of the advantages of NNGEN-P is that it is non-parametric. Unlike other parametric function, which adapt to certain types of climate regimes, NNGEN-P is fully adaptive to the observed cumulative distribution functions, which, on some occasions, may present complex shapes. On-going works will soon produce an extended version of NNGEN to temperature and radiation.

Temporal variability of TEC using ground-based ionosonde data from two Argentine stations

This paper presents an analysis of the behavior of integrated total electron content (ITEC) obtained from ground based ionoside data from two Argentine stations using the new technique proposed by Huang and Reinisch (2001). The database includes electron density profiles for different seasonal and solar activity conditions at 4 typical hours: 00 LT, 06 LT, 12 LT and 18 LT. The standard deviation s and the variability index v = (standard deviations/mean) ∗100 have been used to analyze the variability of the parameter. An analysis of the variability of the corresponding IRI TEC predictions has been also done.

Levels of Anxiety in Polar Environments

To test the common assumption that being part of an isolated group in the polar winter represents an anxiety-arousing environment, trait and state anxiety levels were measured in two Argentine stations in Antarctica and two Canadian sites in the High Arctic. The group scores were compared with each other, with comparison groups drawn from the same population as the polar teams but not working in isolated polar stations, and with available norms. Anxiety levels were about the same in both isolated crews, and showed no differences from the comparison groups. Data from Argentine subjects showed some differences from published U.S. norms. A sub-sample of the Antarctic comparison group was retested twice in the Antarctic during the following year. No significant changes were found in either state or trait anxiety. The data support the view that even environments that appear to be dramatically stressful cannot be assumed to have pervasive negative effects; one must look at how the individuals in the environment actually experience it.

On the critical frequency and height of the E layer peak at noon

Noon ionograms obtained at three Argentine stations (Tucuman, San Juan and Buenos Aires) are used to compare the critical frequency of the E region with the corresponding values predicted by the International Reference Ionosphere. The database includes summer, winter and equinox periods for years of different level of solar activity. For San Juan electron density profiles derived from midday ionograms recorded for different conditions, are used to check the validity of the E peak height assumed by the two versions of the model: 1990 and 1995. In general, the comparisons show: (i) that the constant value of the E peak height of 110 km assumed by the 1995 version seems more reasonable than the value of 105 km used in the 1990 version, (ii) that the situation for the E peak height requires more work and it would be useful an extension of this study using a more complete database.

The relationship between tropical convection in South America and the end of the dry period in subtropical Argentina

The connection between the tropical convection over South America and rainfall in subtropical Argentina is studied using outgoing longwave radiation (OLR) measurements obtained from the scanning radiometer aboard the polar orbiting NOM operational satellites and rainfall measurements of 32 selected Argentine stations for the period 1975–1991. The mean annual rainfall cycle in subtropical Argentina can be described by the composition of the first three eigenvectors. The first eigenvector explains 59% of the variance having a strong influence in north-eastern Argentina and little variability during the year. The second, explaining 19% of the variance, represents the summer rainfall with maximum variability in the northwest part of the country. The third eigenvector explains 4% of the variance and is related to the semiannual cycle observed in the southeast of subtropical Argentina. The tropical convection may be related to rainfall in subtropical Argentina. It influences the vertical movements by compensatory subsidence and the higher humidity at low levels accompanying the convection, which can be advected towards Argentina. Also, the position of the mean centres of tropical convection is related to the latitude of the subtropical jet and of the tropospheric circulation. As a consequence, it influences the possible advection of humidity from the Atlantic Ocean. Different parameters are used to detect the influence of the onset of the summer monsoon on the end of the dry winter over almost all subtropical Argentina. Significant correlation is evident when rainfall in September in subtropical Argentina is correlated with the onset monsoon date anomaly. When the convection is advanced (delayed) from the climatic value, there are positive (negative) rainfall anomalies in the south-eastern region of subtropical Argentina. © 1998 Royal Meteorological Society

On the occurence of the F1 layer over Argentina

The validity of the International Reference Ionosphere 1990 to predict the occurrence of the intermediate F1 layer is checked. Data recorded at two Argentine stations: TUCUMAN and BUENOS AIRES at different seasons and solar activity conditions are used. The number of days in which the layer is present, including L condition, is analyzed. Discrepancies between observations and predictions have been found particularly during winter and in the time range of occurrence of the layer. The need of an extension of this test study to other stations is pointed out in order to improve the DuCharme et al. (1973) formula adopted by the model to predict the occurrence of the F1 layer.

Observed and model N(h) profiles for two argentine stations

Noon bottomside N(h) profiles derived from ionograms of Tucuman and San Juan (Argentina) are compared with those given by the two IRI-90 model options (OLD and NEW) for the bottomside thickness parameter B 0 and by the DGR model. The study shows that in most cases the IRI - OLD version provides better agreement with the data than IRI- NEW option. For both options considerable descrepancies are found in the intermediate region (between the E-valley top and the bottomside F-region). Very good afreement is found with the DGR model, which is to be expected since it is partly based on data from these stations. The situation clearly requires more work and an extension of this analysis to more stations and local times is suggested.

Levels of anxiety in polar environments

To test the common assumption that being part of an isolated group in the polar winter represents an anxiety-arousing environment, trait and state anxiety levels were measured in two Argentine stations in Antarctica and two Canadian sites in the High Arctic. The group scores were compared with each other, with comparison groups drawn from the same population as the polar teams but not working in isolated polar stations, and with available norms. Anxiety levels were about the same in both isolated crews, and showed no differences from the comparison groups. Data from Argentine subjects showed some differences from published U.S. norms. A sub-sample of the Antarctic comparison group was retested twice in the Antarctic during the following year. No significant changes were found in either state or trait anxiety. The data support the view that even environments that appear to be dramatically stressful cannot be assumed to have pervasive negative effects; one must look at how the individuals in the environment actually experience it.