From aboard the sounding rocket, the copy of the EVE instrument measures extreme ultraviolet light before parachuting back down to Earth for reuse. The instrument must be in space to record these measurements because the atmosphere absorbs most ultraviolet light.
Other than its brief and occasional forays into space, the duplicate instrument spends its time on Earth, protected from the harsh space environment and within reach of scientists for tune-ups. By comparing the measurements from this EVE instrument to those from its twin on SDO, researchers can correct for any degradation on the satellite version. The information will be used to validate the calibration of ten instruments aboard other spacecraft as well.
After SDO launched in 2010, Woods and his team aimed to recalibrate the instrument every six months or so. Now, they shoot for about once every two years because the rate of degradation slows over time. However, the coronavirus pandemic delayed the last launch, so they are now over the three-year mark. “We’re anxious to get this one launched and see how well everything’s doing,” Woods said. Once they have the new numbers, they will re-run data from the last few years to ensure the most accurate measurements possible.
Between sounding rocket launches, the EVE team also uses weekly calibration measurements from SDO’s EVE instrument itself. But, Woods said, those calibrations are not as informative. “It doesn’t give you a direct measure of degradation,” he said. “The only way to really nail down that degradation is to do this type of cross-calibration.”
The Earth-dwelling EVE is being readied for its tenth trip into space in 15 years (it started flying before SDO was launched), and new questions are emerging. “How many times can you launch this before something breaks?” Woods said. “Launch vibration is hard on it, the landing is hard on it too.”
The exact technology inside EVE isn’t available anymore, having been replaced by newer versions, but Woods and his team are building a replacement in case something breaks in the next few years.
“It is getting old,” he said. “I don’t know how many more missions it can survive, but so far – knock on wood – it’s held up so many years already.” During that time, it has enabled us to see our Sun like never before. Woods hopes it will continue shedding light on the Sun’s activity for years to come.