Before joining the IceCube collaboration, I spent several years working in another astroparticle physics experiment: the Pierre Auger Observatory, located in the western part of my home country: Argentina.
The first interesting thing about Auger is the name itself. In a community where everything is named using (sometimes outrageous) acronyms, an experiment named after a person is like a bit of fresh air.
The observatory, devoted to the detection of ultra-high energy cosmic rays, was named after French physicist Pierre Victor Auger, who was among the first persons to detect what we now call “extensive air-showers” during the first decades of the 20th century.
Pierre Auger (c. 1960)
Auger, while working with data from cosmic-ray detectors he had installed in the Alps, noticed that, sometimes, detectors placed far apart would record a simultaneous burst of particles crossing them. After some serious thinking, he came to the conclusion that these bursts could have been caused not by several individual cosmic rays entering the Earth’s atmosphere, but by a single high-energy one interacting with the atmosphere at high altitudes and producing a “shower” of secondary particles that can be detected at ground level across an extended area.
His daughter, who visited the observatory during the ground breaking ceremony, still remembered the day that his father figured this out, Auger was apparently very excited about this idea walking nervously around the house.
The energy range of particles that the observatory detects is well above the energy that can be achieved on any man-made accelerators. Even the LHC running at full blast will produce collisions with energies hundreds of times lower than those detected at the observatory on a daily-basis. Determining the nature and location of these natural astrophysical accelerators are some of the major goals of the collaboration, and I’ll tell you more about that on a future post.
To detect these air-showers the observatory uses two complimentary techniques: an array of 1600 tanks installed across an area of 3000 km^2 (yes, that’s an area larger than that of some countries) that detects the secondary particles at ground level, and 4 sets of special telescopes that surround the tank array and, on clear moonless nights, detect the faint UV light caused by these secondary particles as they speed through the atmosphere.
For the second technique, the knowledge of the air transparency is extremely important, and it has a direct impact on the determination of the total energy of the primary particle (the entering cosmic ray at the top of the atmophere.) To account for this effect, the observatory has an extensive atmospheric monitoring network, and this is where my humble contribution comes in.
The LIDAR telescope during the alignment procedure. The telescope sits on top of a container with a cover that is fully open at the time of the picture. The bluish light in the top of the telescope structure is the laser light hitting a diffuser used during the early stages of the procedure. Right above the telescope you'll find the constellation of the Southern Cross, in the background you see a storm approaching (with some lightnings in the clouds.)
During my early days in Auger, doing research as an undergrad engineering student, I got to install, operate and play around with the LIDARs of the observatory. A LIDAR is basically a light-based radar, using a short, high intensity laser pulse to probe the atmosphere for particles in suspension (likes dust, smoke, or even the molecules that air is made of.) The light emitted by the laser is back-scattered by dust and molecules in the atmosphere and collected by an array of telescopes that are aligned with the laser beam. The more light you receive back at the telescope, the more dust and molecules you have in the light path which implies a dirtier, darker atmosphere.
This may seem a straightforward analysis, but several aspects of the scattering and absorption of light that you don’t have under control complicate things.
In order to measure these atmospheric parameters in different directions, the LIDAR telescopes and the laser were mounted on a fully steerable platform that performs a routine scan of the sky every time the LIDARs are operated.
The 4 LIDARs telescopes that the observatory runs are operated 16 nights per month, whenever the moon-induced brightness of the night sky is low enough to detect the faint UV light with the air-fluorescence telescopes, and they have been running for more than 5 years. As a result, the atmospheric monitoring dataset of the observatory is by far the biggest one in this part of the world, and it could be of great interest of scientists studying the atmosphere.
This is usually the case of many of these big experiments: whenever you build something as big as this observatory, you’ll have to solve problems or measure things nobody has done before, so you may end up expanding the knowledge of not only your field, but also other, at first glance completely unrelated fields.
A taste of the data taken with the LIDAR telescopes. On the upper right you see a single atmospheric profile which shows the light intensity recorded at the telescope as a function of time (and hence, as a function of distance from the telescope itself.) The black line indicates the expected absorption of light with a clean atmosphere, the bump in reflectivity comes from the laser beam hitting a cloud at a height of around 3.5 km. When several of these profiles obtained in different directions are put together you get the color plot, which is a cross section of the atmosphere above the LIDAR telescope showing clouds with interesting structures. Seeing these structures develop in real time along a night-long data taking shift was very entertaining.
