Daytime extinction

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Diurnal extinction data on 25/09/2017
Image updated every hour. Last update: September 25 2017, 12:37

 

We aim at providing a database of diurnal atmospheric extinction data measured using the instrument MARK-I, located at the Observatorio del Teide.  Mark-I is a full disk resonant scattering spectrometer providing radial velocity measurements of the Sun at the Potassium KI 769.9 nm line, primarily used to study the solar seismology.  Since it is continuously monitoring the sun throughout the day, monochromatic (769.9 nm) atmospheric extinction can be estimated as an indirect product by using the so-called Bouguer law*: m=mo + K.X ;  where m is the instrumental magnitude measured and X the apparent air-mass (using Bemporad-Hardie approximation*).  Fitting a line to these data you get the atmospheric extinction, K, as the slope and mo is the instrumental magnitude of the sun outside the atmosphere.

(*)  For reference:  “Astronomical Photometry: A Guide” by Chr Sterken & Jean Manfroid

The first plot is a simple “Instrumental Magnitude” vs “Universal Time” in hours.  The second plot represents the “Instrumental Magnitude” vs “Air Mass” where a linear trend is to be expected.  Pre-meridian data (during the morning) is plotted in red whereas post-meridian (during the afternoon) data is plotted in blue.  Three different fits are calculated and plotted in order to estimate the pre-meridian, post-meridian and mean atmospheric extinction.  This is because a systematic difference between pre-meridian and post-meridian aerosols distribution affects the atmospheric extinction above the Observatorio del Teide in such a way that pre-meridian light is more attenuated by the atmosphere.  This effect is being investigated in order to decouple time and space:  is it because there is a time evolution throughout the day or is it because there is a non-uniform distribution of aerosols above the observatory?  The latter is the most feasible explanation because the east side of the island is more influenced by Saharan dust.  The last plot is an “Atmospheric Extinction” vs “Universal Time” in hours by using the fitted mean Mo obtained before. 

An example of a clean day where the systematic behavior described above can be noticed is here (date 03/02/2013)

These plots are updated automatically every hour thanks to a python script and hence some filtering routines are needed to clean from clouds and other undesired effects (instrumental vignetting at noon, etc.).  Filtered data is plotted (in black), but not used for any calculation.  This automatic filtering should not remove signal variations caused by dust because it is of our interest, so this filter is very conservative and may not be able to effectively remove all outliers.

An example of a dusty day can be found here (date 12/08/2012): where the “calima” phenomenon strongly reduces the sky transparency by an order of magnitude.

This online database is being populated backwards in a best effort basis trying to cover the last 2 decades of continuous operation of the Mark-I.  As of today, the first available dataset is dated Mar2012.

For any enquiries about this page or the data provided here, please contact Álex Oscoz (aoscoz “at” iac.es) and Sergio Fernández (sergio.fernandez “at” gtc.iac.es). 

Static content last updated in May2013