New Methods Measure the Sun’s Magnetism

The brighter area represents the edge of the CME (a large slinky-like structure called a flux rope) while the fainter area beyond represents the bow shock. Measuring the distance between these two helps measure the magnetic field strength in the corona. Gopalswamy/Astrophysical Journal Letters

NASA scientists have discovered a new way to study the magnetic fields in the sun’s upper atmosphere or corona.

Previously, this has been difficult as images and data have to be collected from 90 million miles away.

Studying the sun’s magnetic field is vital in understanding and predicting coronal mass ejections (CMEs), which can interfere with power grids and satellite-based systems on Earth such as telecommunications.

“The magnetic field is the skeleton of the entire heliosphere, guiding how particles and CMEs move toward Earth,” said Nat Gopalswamy at NASA’s Goddard Space Flight Center in a press release.

“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.”

The problem has been locating in the upper corona the bow shock of a CME—the compressed gas that flows around the CME as it moves rapidly through electrically charged gas or plasma. This movement corresponds to the strength of the sun’s magnetic field.

On March 25, 2008, the sun emitted a CME at a speed of three million miles per hour, imaged by NASA’s Solar Heliospheric Observatory (SOHO), and the two Solar Terrestrial Relations Observatory (STEREO) spacecraft.

The researchers could see the bow shock in standard white light in these images, and calculated the CME’s radius of curvature, and its distance to the bow shock.

They also calculated the bow shock’s speed to find the Alfvén speed of the medium through which the CME traveled, and thus calculate the strength of the sun’s magnetic field.

“Here we see a method originally developed to study Earth’s magnetic environment extended first to understand interplanetary CMEs and CMEs very near the Sun, and now to measure the magnetic field in the corona,” said Joe Gurman in a press release, he is the project scientist for SOHO and STEREO.

Golpaswamy and co-author Seiji Yashiro compared the strength of the sun’s magnetic field from different distances, and believe this method is a useful measuring tool.

“Knowledge of the magnetic field is crucial for all attempts to understand the physics of space weather,” Gurman concluded.

“And it’s especially gratifying to see both STEREO and SOHO—the fifteen-year-old workhorse of the Heliophysics observing system—being used together to improve that picture.”