Submillimeter-wave atmospheric transmission at El Leoncito, Argentina Andes

Abstract

We present the results of one year of daytime atmosphere transmission measurements obtained at 212 and 405 GHz by a 1.5 m solar radio telescope located at El Leoncito site, 2550 m altitude in Argentina Andes. We used three different methods: 1) absolute derivation from strong solar signal at different elevation angles; 2) direct derivation from observed known solar brightness times the antenna coupling factor; and 3) fitting the observed sky emission plots as a function of elevation angle, also known as "tipping." Some differences were found for the transmission determined by the three methods, the most important is that methods 1) and 2) are restricted to smaller values of opacity. Method 1) is restricted to measurements at low elevation angles. Method 2) has the advantage to extend measurements to considerably higher values of attenuation. For the El Leoncito site the correlation of optical depth at 405 and 212 GHz was found ( 405 212 ) = 4 43, smaller than model predictions ( 5 0). Measurements at both frequencies obtained for 319 days in one year indicate that 50% of time 212 0 3 (or 74% zenithal transmission) and 405 1 5 (or 22% transmission), with small seasonal dependence compared to other sites. The opacity related precipitable water vapor is smaller compared to model estimates, suggesting an upper atmosphere water vapor depression, characteristic to the region.Index Terms-Atmosphere opacity at submillimeter-wavelengths, methods for atmosphere opacity, passive radiometer sensing of the atmosphere, submillimeter attenuation, submillimeter wavelengths sites.

Origin of the Submillimeter Radio Emission During the Time-Extended Phase of a Solar Flare

“…We estimated τ z(212) ≈ 0.46 and τ z(405) ≈ 1.9 at 212 and 405 GHz, respectively, using the method described in Melo et al (2005). For the present observation, the antenna aperture efficiency η a is 20% at 212 GHz.…”

Solar flares at submillimeter wavelengths

“…If the beam points at the center of the solar disk then the quiet Sun fills most of the sidelobes of the beam and its contribution has an effective efficiency of order unity, whereas it will have a smaller effective efficiency if the beam targets the limb. To handle this, we include the factor η i modifying the quiet Sun contribution for each beam: Melo et al (2005) find that for a target near disk center η ≈ 0.5 at 212 GHz and 0.6 at 405 GHz, while for a target near the limb η could be 50 % smaller. -The opacity of the sky is measured by doing a "sky dip": receiver power is measured as the telescope is moved in elevation from pointing vertically up to almost pointing at the horizon.…”

“…At SST, sky dips are done several times per day: it is found that the opacity generally varies slowly unless thunderstorms are present. Typical values for τ are 0.3 at 212 GHz and 1.5 at 405 GHz (median values: Melo et al 2005). -Unfortunately, the aperture efficiencies of the telescope at the observing frequencies need to be known for this method of calibration because the burst contribution comes in through the telescope aperture and is thus modulated by the aperture efficiency, while the receiver temperature, the quiet-Sun contribution (per the assumption above) and the ambient and hot loads do not "see" the aperture.…”

New telescopes for ground-based solar observations at submillimeter and mid-infrared

“…Transmission measurements are regularly performed at least three times per day using the "tipping" method: SST scans the sky in elevation and the best optical depth is fitted to the data.. Solar disk brightness has been determined at low elevation angles for days with small attenuation, to derive the opacity relationship that can be extrapolated for days with large attenuation when the "tipping" method cannot be used at 405 GHz 22 . The sub-THz sky transmission results for El Leoncito for one year are shown in Fig.…”