Solved: The Mystery of the Cloudy Filters

There’s a mystery happening in some satellites facing the Sun, and scientists from the National Institute of Standards and Technology (NIST) and the Laboratory for Atmospheric and Space Physics (LASP) are on the case. The team has been trying to figure out what is clouding up and compromising the performance of tiny, thin metal membranes that filter sunlight as it enters detectors that monitor the Sun’s ultraviolet (UV) rays.

These detectors can warn us about impending solar storms — bursts of radiation from the surface of the Sun — that could reach Earth and temporarily disrupt communications or interfere with GPS readings.

Last year, the team disproved the prevailing theory: that this clouding was a buildup of carbon on the surface of the filters from organic sources stowing away on the satellite.

Mystery of the Solar Filter Degradation

Now, in a series of three new papers, the same team from NIST and LASP has made a strong case for what they think is the true culprit: oxidation caused by water, which together with UV light from the Sun is producing a thick layer of aluminum oxide — much thicker than previously thought possible — that blocks incoming rays.

As a bonus, the researchers believe they have identified the source of the water: thermal blankets, which are used to control the temperature of instruments on a spacecraft. This information could help scientists improve the performance of future satellites that rely on this type of filter, perhaps by adding hardware that limits the filters’ exposure to the area around the thermal blankets, or by using different materials as a part of the filters themselves.

The first of the three papers was published today in Solar Physics.

“As far as I know we are the only people looking at filter oxidation due to exposure to ultraviolet light,” said NIST’s Charles Tarrio.

Proving that water is responsible for the problem “was sort of a one-two punch,” said NIST physicist Robert Berg. “Punch one was physically showing that this chemical process involving water could cause something comparable to what we actually see happening in the satellites. And the number two punch is saying once you create a theoretical model that takes everything into account, then the numbers line up quantitatively with what we see in the satellites.

“Putting everything together, I’m convinced,” Berg said. “Water is responsible for the filter degradation.”

#NoFilter

Most of the light produced by the Sun is visible and ranges from red light, with a wavelength of around 750 nanometers (nm, billionths of a meter), to violet light, with a wavelength of about 400 nm. Among other wavelengths, the Sun also emits comparatively small amounts of light in the extreme ultraviolet (EUV) range, which extends from 100 nm down to just 10 nm — wavelengths too short for human eyes to see.

Though small, that EUV signal is useful because it spikes in tandem with the solar flares that can disrupt communications on Earth or cause GPS to experience problems. EUV signals also give scientists a heads-up of hours or even days before more destructive phenomena such as coronal mass ejections reach Earth. These blasts of charged particles can overload power lines or increase radiation exposure for airline crew and passengers.

A critical piece of equipment on the Sun-facing space detectors are the aluminum filters, each smaller than a postage stamp, that block all but the EUV light between 17 nm and 80 nm wavelength.

Though they begin their lives in space transmitting plenty of EUV light in their range, within just a few years they can lose a significant amount of transmission ability. For example, a filter might start by allowing 50% of 30-nm EUV light through to the detector. That number can go down to 25% within a year, and 10% within five years.

Scientists believed some unknown substance must be growing or being deposited on the filters, causing them to go dark over mere months and limiting the amount of light that makes it into the detectors. The leading theory was that carbon was outgassing from the instrument itself and getting deposited on the filters.

When NIST and LASP staff disproved that last year, they turned their attention to what they felt was a much more likely explanation: the process of oxidation, in which oxygen atoms from water molecules (H₂O) combine with aluminum atoms from the filter itself (Al) to form a hazy layer of aluminum oxide (Al₂O₃). (Incidentally, a thin layer of aluminum oxide naturally coats all aluminum objects on Earth, from soda cans to frying pans.)

Scientists already knew that exposing an aluminum surface to UV light in the presence of water can grow extra layers of oxide beyond the ones that naturally form. But there was no existing theory that could explain how the aluminum oxide could grow thick enough to cause this clouding problem.

Researchers decided to thoroughly explore how the presence of water might be affecting the filters, to determine what was really going on.

SURF’s Up

NIST researchers wanted to test their water theory in a controlled setting: a machine that effectively lets them create space weather. Called NIST’s Synchrotron Ultraviolet Radiation Facility (SURF), the device is a room-sized particle accelerator that uses powerful magnets to move electrons in a ring. The motion generates EUV light, which can be diverted through specialized mirrors to impact targets such as the satellite filters being tested.

Despite exposing their sample filters to lab-made UV light for as long as 20 days, they weren’t able to grow oxide layers as thick as were needed to explain the cloudiness of real space filters. But the oxide layers were still much thicker than predicted by the accepted theory.

The researchers believe with further exposure they would have reached the required thickness. They also projected that the sample filters would have had to be exposed to the SURF beam for about 10 months to achieve the same oxide thickness as the filters in actual space.

Taking a different tack, the team also conducted modeling studies. The finished models match almost exactly what astronomers are seeing in real aluminum filters in space.

One key piece of the new model’s success is that it accounts for the fact that electrons scatter while traveling within the aluminum filters. This scattering slows their progress, which affects the dynamics of the oxide growth.

“This is the first model that takes scattering electrons into account, and it uses parameters that agree with what’s expected in the literature for each of the steps in the chemical reaction,” Berg said.

Just Add Water

For the models to work, though, one key piece of information was missing: a significant source of water that could be feeding this reaction.

“It had to be something that can emit water for five years continuously at reasonably constant rates,” Tarrio said. “That set Bobby [Berg] off on this quest to find, what the heck could this be? What could be a source that fits? And he found it.”

The most likely source, Berg concludes, is thermal blankets. These are made with a type of plastic called polyethylene terephthalate (PET), known to capture water on Earth. This water just isn’t usually a problem for most equipment.

“It was hard to think of anything else that would hold that kind of water,” Berg said.

Future work, the researchers hope, will perhaps include testing different materials for the filters that would still be transparent at the relevant wavelengths but wouldn’t be susceptible to oxidation.

-- Reported and written by Jennifer Lauren Lee

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Paper: C. Tarrio, T.B. Lucatorto, R.F. Berg, D.E. Newbury, N.W.M. Ritchie, A.R. Jones, F. Eparvier. The Hazard of UV-Induced Oxidation to Solar-Viewing Spacecraft Optics. Solar Physics. Published online Mar. 2, 2023. DOI: https://doi.org/10.1007/s11207-023-02112-x