The U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR) was selected by NASA to construct a breadbox-sized satellite that will give scientists a powerful new tool to observe the Sun’s chromosphere, a poorly understood region of the solar atmosphere where the energy that powers solar storms builds up before being released.

The Solar Transition Region UltraViolet Explorer (STRUVE) was one of two “cubesat” mission proposals selected by NASA to move forward as part of its Heliophysics Flight Opportunities for Research and Technology program. STRUVE is targeting launch in early 2029.

Once in orbit around Earth, STRUVE’s gaze will focus on the chromosphere, a layer of the solar atmosphere sandwiched between the photosphere (the visible surface of the Sun) and the corona (the Sun’s wispy outer atmosphere). Scientists know that active regions in the photosphere, such as sunspots, are associated with powerful solar eruptions from the corona into the heliosphere, which is the region of space that envelops the whole solar system and is defined by the reach of the solar wind. But what happens to the energy as it travels between the photosphere and the heliosphere is somewhat of a mystery.

STRUVE is designed to help fill this knowledge gap, providing observations that will allow scientists to reconstruct how magnetic field lines twist and tangle in the chromosphere. Ultimately, this enhanced understanding would help improve space weather forecasts and provide better warnings when solar storms are likely to impact power grids, satellites, communications systems, and other technologies here on Earth.

“The chromosphere is a really important part of the solar atmosphere when it comes to storage and release of magnetic energy,” said NSF NCAR scientist Alfred de Wijn, STRUVE’s principal investigator. “We know that the magnetic field of the photosphere connects to the heliosphere, but we don’t know how it makes its way through the chromosphere. We’re interested in what’s actually going on in that middle layer and seeing how the magnetic field changes leading up to eruptions.”

The STRUVE team is led by NSF NCAR and includes researchers from CU Boulder’s Laboratory for Atmospheric and Space Physics, Lockheed Martin Solar & Astrophysics Laboratory, the Bay Area Environmental Research Institute, and the Instituto de Astrofísica de Canarias (IAC) in Spain.

Deciphering the polarized signature of the light

STRUVE will rely on a technique called spectropolarimetry to unlock the chromosphere’s secrets. In addition to measuring the intensity of particular colors of light — a standard way to figure out what an object is made of — spectropolarimetry also measures the polarization of the light, meaning the direction in which the light waves vibrate. This polarization pattern carries clues about magnetic fields and other properties that can’t be detected from color alone.

Spectropolarimetry is not a new technique, but it will be applied in a novel way. In the photosphere, the Sun’s plasma is relatively dense, and scientists can reasonably assume that the light emitted from the photosphere in any particular location is representative of the conditions at that spot. But the chromosphere is much less dense and the light emitted from the chromosphere may not represent the local conditions, making interpretation of the observations much more complicated. In the last decade, however, scientists at NSF NCAR and the IAC have made great strides in developing models that can be used to make sense of the data.

“The techniques for interpreting the data have really matured, and that’s enabling the technology that allows us now to collect this data and know that we can use it,” de Wijn said.

These new advances in spectropolarimetric techniques have made STRUVE possible. And the planned results — the first-ever continuous observations of the Sun’s chromosphere using the ultra-violet part of the solar spectrum with polarization — will provide a new window into the magnetic forces that shape solar activity, offering insights that have never before been available.

These breakthroughs also help lay the groundwork for future missions such as the Chromospheric Magnetism Explorer (CMEx), a larger spacecraft concept from NSF NCAR now in a NASA-funded concept study phase. While CMEx would build on similar techniques, STRUVE’s earlier launch means it will set the scientific and technical foundation by demonstrating key instrument capabilities, refining calibration approaches, and producing early data that will guide and accelerate the science CMEx aims to pursue.

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