New Ways to Measure Magnetism Around the Sun

Those who study the sun face an unavoidable hurdle in their research – their observations must be done from afar. Relying on images and data collected from 90 million miles away, however, makes it tough to measure the invisible magnetic fields sweeping around the sun.

Scientists must learn more about these fields because they are crucial to understanding how coronal mass ejections, or CMEs, travel through space sometimes toward Earth where they can damage satellites. Now NASA researchers have made use of old mathematical techniques and new insights on how CMEs travel to devise a fresh way to measure this magnetic environment in the sun's upper atmosphere, the corona.

"The magnetic field is the skeleton of the entire heliosphere, guiding how particles and CMEs move toward Earth," says solar physicist Nat Gopalswamy of NASA's Goddard Space Flight Center in Greenbelt, Md. He says researchers routinely measure the fields near the sun's surface, but haven't been able to do as well further out in the sun's atmosphere. "Before, we've only been able to measure it in the upper corona with a technique that required exact conditions. Our new method can be used more consistently."

Indeed, this new method can be used any time there's a good side view of a CME, Gopalswamy explains in a new paper that will appear in the July 20 issue of the Astrophysical Journal Letters.

The mathematical relationship between how an object moves through gas and the bow shock it creates – that's the region of compressed and distorted gas that flows around a fast-moving object, much like the shock created by a supersonic jet -- has been understood since the 1960s. When an object moves through gas that is electrically charged, known as "plasma," that movement also corresponds to the strength of the magnetic field.

The problem in the solar environment was spotting a CME's bow shock as it traveled through the upper corona. In that part of the sun's atmosphere, scientists weren't finding the signature ring around a CME that signified a bow shock in images closer to the sun.

But on March 25, 2008, the sun provided a perfect test case: a CME traveling at three million miles per hour and reported by several NASA spacecraft including the Solar and Heliospheric Observatory (SOHO) and the two Solar TErrestrial RElations Observatory (STEREO) spacecraft. From the perspective of both SOHO and STEREO-A, the CME appeared to be bursting off the horizon, or limb, of the sun. Limb events like this offer the side-view needed to best watch how a CME develops over time.