The Case of the Cloudy Filters: Solving the Mystery of the Degrading Sunlight Detectors

The Case of the Cloudy Filters: Solving the Mystery of the Degrading Sunlight Detectors

Two EUV filters that were used in a space flight. The wrinkly looking filter on top is made of zirconium; the smoother bottom filter is made of aluminum. Each filter is extremely thin – a fraction of the diameter of a human hair – and about 1.4 mm wide by 4.5 mm long, roughly half the size of a very flat Tic Tac.


Credit: Andrew Jones/LASP



More than 150 years ago, the Sun blasted Earth with a massive cloud of hot charged particles. This plasma blob generated a magnetic storm on Earth that caused sparks to leap out of telegraph equipment and even started a few fires. Now called the Carrington Event, after one of the astronomers who observed it, a magnetic storm like this could happen again anytime, only now it would affect more than telegraphs: It could damage or cause outages in wireless phone networks, GPS systems, electrical grids powering life-saving medical equipment and more.


Sun-facing satellites monitor the Sun’s ultraviolet (UV) light to give us advance warning of solar storms, both big ones that could cause a Carrington-like event as well as the smaller, more common disturbances that can temporarily disrupt communications. One key piece of equipment used in these detectors is a tiny metal filter that blocks out everything except the UV signal researchers need to see. 


But for decades, there has been a major problem: Over the course of just a year or two, these filters mysteriously lose their ability to transmit UV light, “clouding up” and forcing astronomers to launch expensive annual recalibration missions. These missions involve sending a freshly calibrated instrument into space to make its own independent observations of the sunlight for comparison.


A leading theory has been that the filters were developing a layer of ca ..