\section*{Acknowledgements}
The present work was partially supported by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET-Grant PIP 112-200901-00425), Agencia de Promoción Científica y Tecnológica (ANPCyT-Grant PICT-2012-0508) and Universidad Nacional de La Plata (UNLP-Grant X586). C.C. and A.I. thank CONICET for doctoral and postdoctoral research fellowships, respectively. M.L.D. and A.H.T. are research members of CONICET. Our group thanks the Department of Seismology and Weather of FCAG-UNLP for the information provided of the station weather. The authors are particularly grateful to Dr. Rubén D. Piacentini (Instituto de Física Rosario CONICET-UNR) for offering the Avantes spectrometer and for his crucial contributions to this work.