CoMP takes the first Sun-wide
look at coronal magnetism

A handful of NCAR solar scientists set out in late January to fulfill a long-sought dream. At the National Solar Observatory in New Mexico, the team planned to try and collect the first-ever measurements of magnetic field across the entire solar limb (the slice of the Sun’s corona perpendicular to Earth).

Their dream machine is the coronal multichannel polarimeter (CoMP), developed at NCAR’s High Altitude Observatory by Steven Tomczyk and colleagues over the past three years with support from an NCAR strategic initiative. If CoMP proves to be robust, it will take coronal research into truly uncharted territory.

“The first time we see light through this thing, it’s going to be very exciting,” says Tomczyk.

Gregory Card, Anthony Darnell, and Steven Tomczyk (left to right) put the final touches on the coronal multichannel polarimeter (CoMP) before its first field test in New Mexico. (Photo by Carlye Calvin.)

Near the Sun’s surface—especially in the photosphere, the lowest part of the Sun’s atmosphere—magnetism has been traced for over a decade by ground- and space-based instruments, such as NCAR’s advanced Stokes polarimeter. These devices infer the magnetic field by measuring several components of visible radiation. The brightness of the polarized light is proportional to the strength of the magnetic field along the line of sight.

Until recently, there was scant hope for using this technique to analyze magnetism in the Sun’s corona. Although its temperatures are scorching—as high as a million degrees C (1.8 million F)—the corona is far too thin to yield a strong signal. Now a new generation of super-sensitive, low-noise infrared sensors has made the impossible possible for coronal research. “The technology has only recently come on line,” says software engineer Anthony Darnell, who worked closely with Tomczyk and engineer Gregory Card in designing CoMP.

Along with making use of the improved sensors, the CoMP team devised a way to measure two polarization components and two wavelength components, all at the same time. This is important because Earth’s atmosphere scatters a ­continuously varying amount of background light from the brighter disk into the coronal line of sight. The simultaneous measurements in CoMP allow the varying background signal to be accurately removed while preserving the faint coronal signal.

By later this winter, CoMP’s developers should have an initial idea of how well the instrument performs. They hope to someday pair the instrument with a one-meter telescope at a prime location yet to be determined. Because CoMP’s view is limited to the solar limb (the edge of the solar disk), it won’t be able to analyze coronal mass ejections heading toward Earth. However, by relating coronal magnetism to other aspects of CMEs, CoMP promises to shed light on a hitherto unseen wealth of phenomena.

“We’ve only been looking at part of the picture,” says NCAR’s Sarah Gibson. “I’m willing to bet there will be totally new models based on what we observe with CoMP. This is undeniably revolutionary for solar physics.”