Optical depth measurements at 45 and 90 GHz in CASLEO

The optical depth at 45 and 90 GHz have been obtained from calibration data of two solar radio telescopes located in El Leoncito Astronomical Complex (CASLEO), during the years 2012-2013. The observed period and the temporal resolution of the measurements is sufficient to show seasonal patterns. Using the precipitable water vapor content (PWV) obtained with a Sun-photometer for aerosols measurements, it was found a good correlation of PWV with the optical depth in both frequencies. With the water vapor content and data from a meteorological station, the atmosphere over CASLEO was modeled using ATM 2009, an Earth's atmospheric model. The results of the model allows to evaluate the range of optical depth measurements and to predict the contributions from H_2O and O₂, the main components of optical depth at these frequencies. It was found a reasonable agreement at 45 GHz when the PWV is lower than 5 mm, but notable discrepancies for the entire range of water vapor content at 90 GHz. An empirical relation is established between the water vapor contribution to the measured optical depth for the observed frequencies. Our results show that CASLEO, despite being a site with 2500 meters high, is appropriate for solar observations in the millimeter range and is still useful for observations in the submillimeter at the epoch of the year with low water vapor content.