Highlight the solar blind spot

A proposal for a spacecraft that would take a new look at the Sun — allowing an unprecedented view of our star’s magnetic field — has received $2 million in funding from NASA for further study.

The development of the spacecraft, called the Magnetic Chromosphere Explorer, or CMEx, is being led by the US National Science Foundation’s (NSF) National Center for Atmospheric Research.

“Space weather, which can have profound effects on Earth, is driven by the Sun’s magnetic field,” said Holly Gilbert, who directs NCAR’s High Altitude Observatory and is CMEx’s principal investigator. “To better predict solar storms and their potential impacts, we need a more complete understanding of the magnetic field in the Sun’s atmosphere. But this is a blind spot for today’s space observatories. Essentially, we are only able to measure the effects, not the cause.”

CMEx will change that, but it won’t be easy.

The mission team is interested in magnetic fields in the heliosphere, a thin, low layer of the sun’s atmosphere located between the photosphere, or visible surface of the sun, and the sun’s vast outer atmosphere, or corona. Magnetic fields that form beneath the Sun’s surface extend upward through the chromosphere, twisting, intertwining and releasing energy that then travels outward through the rest of the corona. This turbulent magnetic activity makes the chromosphere a particularly interesting and important region to monitor to better understand and predict space weather, which can sometimes damage satellites, disrupt radio communications and power grids, and endanger astronauts.

However, “seeing” magnetic structures is difficult. The NCAR team proposes using a technique called spectroscopic polarimetry, a difficult task that relatively few scientists in the United States have mastered. Many with experience work at the High Altitude Observatory, making NCAR uniquely qualified to lead this type of mission.

“This is something that has never been proposed before for a space mission,” Gilbert said. “The research community certainly wants this, and we really need this information to make progress in space weather prediction, but it is very difficult.”

The promise of polarimetric spectrometry

Spectroscopic polarimetry is based on a more common technique for gathering information from light emitted by stars known as spectrophotometry. When light passes through the Sun’s material, compounds within it absorb and emit light at very specific wavelengths, leaving a unique spectral signature in the light that continues toward Earth. By dividing incoming light from the Sun into its component wavelengths and looking to see where specific wavelengths are missing, scientists can learn a tremendous amount about the Sun itself, including its composition, temperature and density. Collecting or stretching a fingerprint can tell you how the plasma that makes up the Sun flows and undulates.

But light from stars contains more information that spectrometry cannot determine on its own because some of the light is polarized. Magnetic fields interact with light from the Sun, leaving a polarization effect across certain wavelengths. Spectroscopic polarimetry is the technique of extracting that information and using it to help reconstruct the structure of the magnetic field itself.

Measuring spectroscopic polarization is difficult for a number of reasons, including that the polarized signal is very weak against the deluge of unpolarized sunlight. Telescopes used to observe the Sun can introduce their own polarization, which further distorts the signal. However, scientists at NCAR and elsewhere have shown that spectropolarimeters spread across the Earth’s surface can produce valuable data.

For example, NCAR scientists have designed an instrument called the Visible Spectropolarimeter, one of five instruments at NSF’s Inouye Solar Telescope. But ground-based spectropolarimeters can only see the Sun for a limited number of hours each day, and they cannot observe the full range of wavelengths emitted by the Sun, some of which cannot penetrate the Earth’s atmosphere.

CMEx’s proposed space-based spectropolarimeter would be in low Earth orbit with a nearly constant view of the Sun, allowing it to take nearly continuous observations. CMEx will also capture important wavelengths not possible from Earth, and will eventually enable researchers to map the Sun’s magnetic structures in 3D.

Current funding from NASA will allow the NCAR team — along with its engineering partner, Ball Aerospace — to continue developing the mission plan over the next nine months and prove to NASA that they can really do the difficult work of actually processing the data they get back to extract the magnetic structures.

If CMEx is successfully funded beyond these current conceptual studies, it will become the first explorer-sized spacecraft mission led by NCAR, and it will provide a rich opportunity for NCAR to train and develop the next generation of scientists adept at spectroscopic polarimetry. Even if the mission does not receive funding from NASA’s current round, the conceptual studies being conducted will help build capabilities in spectroscopic polarimetry and pave the way for future NCAR-led missions.

“We see CMEx as an opportunity to really develop this capability, especially in the United States,” Gilbert said. “We are excited to share what we know and work with others to further develop this technology.”

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