After three years in orbit, Europe’s Sunstorm CubeSat re-entered Earth’s atmosphere on 4 September, completing its mission to monitor X-ray pulses from solar flares, the breeding ground of disruptive space weather phenomena. These flares are sometimes accompanied by coronal mass ejections that spew billions of tonnes of material from the sun’s atmosphere.
On Earth, space storms can damage critical infrastructure, including radio communications and the power grid. In space, they disrupt satellites: In May, the biggest solar storm in over 20 years pushed low-orbit satellites and space debris toward Earth at a rate of 180 meters per day for four days. These events threaten the growing number of research and commercial projects in space, making solar weather forecasts a high-demand opportunity. Sunstorm’s verification of high-resolution X-ray tracking technology could offer the ability to predict space weather better than before and make it easier to protect satellites in orbit and infrastructure on Earth.
The Sunstorm CubeSat carried Isaware’s X-ray Flux Monitor for CubeSats (XFM-CS) to monitor the sun’s outermost atmosphere.Isaware
Sunstorm began its sun-synchronous orbit in 2021 as a piggyback CubeSat aboard the European Space Agency (ESA)’s Vega rocket, launching from French Guiana to an altitude of 551 kilometers. Kuva Space coordinated the mission from its ground station in Espoo, Finland.
The miniature satellite carried a specialized high-resolution spectrometer from Finnish startup Isaware. The X-ray Flux Monitor for CubeSat (XFM-CS) observed the outermost portion of the solar atmosphere, the corona. By monitoring changes in solar plasma composition, XFM-CS identified materials exchanged between a flaring coronal magnetic loop and its surroundings. The results add to data obtained through existing methods, like extreme ultraviolet and coronagraph imaging, to give a fuller picture of solar eruption mechanics.
“Solar flares are known to be the first link in the space weather chain, [so] observing and investigating them can provide a greater understanding of the mechanisms involved in space weather development and enable forecasting with greater accuracy,” says Arto Lehtolainen, an instrument scientist at Isaware. “The best defense against the harmful effects of space weather is switching off vulnerable systems temporarily during these events.”
Isaware’s end goal is to apply spectroscopic measurements for near-real time forecasting. That starts with understanding the dynamics and composition of plasma involved in solar flares. “By comparing the plasma temperature and times between the flare onsets, temperature maximum, and density maximum, and combining these with the physical size of the flaring loop determined from extreme ultraviolet images, it’s possible to roughly characterize the amount of plasma released into space during the eruption, as well as its duration,” says Lehtolainen.
The €1 million (US $1.1 million) mission collected data on about two dozen X-class flares (the most powerful class of flare), several hundred M-class flares (second most powerful), and over 2,000 smaller flares. Covering the rising part of solar cycle 25, Sunstorm’s extensive dataset will likely inform scientific publications for years to come. Some results have already been published, but analysis of the latest data is ongoing at the University of Helsinki with Isaware.
A Window Into the Solar Corona
XFM-CS’s heritage began in 1998 when University of Helsinki lecturer Juhani Huovelin proposed a high-performance, low-mass spectrometer—called X-ray Solar Monitor (XSM)—for ESA’s SMART-1 moon mission. Huovelin, now the chairman and lead founder of Isaware, says XSM needed no optics because it only measured the sun’s integrated X-ray spectrum, which was then used as a calibration input for SMART-1’s D-CIXS to measure the glow of the moon’s surface.
XSM was the first high-resolution device of its kind to measure the sun’s soft X-ray spectrum in the 1 to 20 kiloelectron-volt (keV) range. Huovelin then realized the instrument could be adjusted to study the solar corona.
For Sunstorm, Isaware miniaturized the concept in an X-ray spectrometer to detect pulses from solar flares. The company packed components tightly in a CubeSat form factor, a class of nanosatellites based on standardized 10-centimeter boxes.
XFM-CS features Isaware’s novel silicon drift detector, an ant-sized device that converts photon energy to an electrical charge via photoelectric absorption. The technology brings several performance improvements over similar detectors. Lehtolainen says the silicon drift design minimizes capacitance, which reduces the electronic noise level and signal amplification chain to support digital pulse processing that’s more than 10 times as fast as predecessor techniques. XFM-CS is also more resistant to energy degradation from radiation, maintaining an excellent resolution (180 eV at 6 keV) throughout the mission.
“Solar X-ray emission intensity varies by about six orders of magnitude from the quiescent times during the solar minimum to the strongest X-class solar flares,” Lehtolainen says. “The speed of XFM-CS enabled a better signal-to-noise ratio and lower level of pulse pile-up in a wide dynamic range than previously flown instruments without sacrificing energy resolution.”
XFM-CS featured Isaware’s novel silicon drift detector, which reduced the noise from the satellite’s electronic components. Isaware and Kuva Space
During Sunstorm’s operation, flash memory data was collected from the satellite onboard computer and downlinked automatically from the ground station. Kuva Space co-founder and lead engineer Janne Kuhno says this process occurred for every orbit of nearly every day, with the exception of a few maintenance outages.
XFM-CS’s consistent performance extended the mission a year longer than planned. “The data generated by XFM-CS surpassed existing instruments in many ways and was still fully functional at the end of the nominal mission, so the consortium saw it beneficial to the scientific community to continue,” Kuhno says.
Potential for New Standard in Solar X-Ray Monitoring
Today, most solar X-ray data comes from sensors aboard the National Oceanic and Atmospheric Administration’s (NOAA) Geostationary Operational Environmental Satellites (GOES), which only provide X-ray flux values in two broadband channels.
With a higher energy resolution than GOES detectors, XFM-CS introduced a method to continuously monitor both solar X-ray fluxes and spectra for solar eruptions, along with standard broadband data. In 2022, the device captured a massive solar eruption that matched GOES data, confirming Sunstorm’s scientific-grade observations.
XFM-CS’s X-ray flux values generally match measurements from the NOAA’s GOES satellites.European Space Agency
“GOES X-ray measurements for X-ray fluxes of the solar corona and flare intensity could be replaced by high-resolution spectroscopic measurements that use the new XFM data,” says Huovelin.
Following Sunstorm’s success, Isaware is developing a scaled-up version of XFM designed to be more resilient to radiation. The instrument will be integrated into NOAA’s Space Weather NEXT observatory, scheduled to launch in 2029 to the Lagrange 1 point of the sun-Earth system.
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