WASHINGTON - NASA, the US space agency, announced Monday that it would launch a mission known as the Solar Ultraviolet Magnetograph Investigation (SUMI) Thursday, to study the intricate, constantly changing magnetic fields on the sun in a hard-to-observe area called the chromosphere.

Magnetic fields, and the intense magnetic energy they help marshal, lie at the heart of how the sun can create huge explosions of light such as solar flares and eruptions of particles such as coronal mass ejections (CMEs). Though there are already instruments -- both on the ground and flying in space -- that can measure these fields, they are constrained to observe the fields on a particular layer of the sun's surface or atmosphere.

With SUMI, scientists are expected to be able to observe the layer none of the instruments in service could do.

"What's novel with this instrument is that it observes ultraviolet light, when all the others look at infrared or visible light," says Jonathan Cirtain, a solar scientist at NASA's Marshall Space Flight Center and the principal investigator for SUMI. "Those wavelengths of light correspond to the lowest levels in the sun's atmosphere, but SUMI will look at locations higher in the chromosphere."

To measure magnetic fields in the chromosphere, SUMI will observe the ultraviolet (UV) light emitted from two types of atoms on the sun, Magnesium 2 and Carbon 4. Through established methods of measuring how the light is affected as it travels through the magnetic environment of the solar atmosphere towards the earth, scientists can measure the original strength and direction of the magnetic fields, thus creating a three-dimensional magnetic map of the chromosphere.

This trip for SUMI is largely a test flight to make sure the instrument functions and to assess possible improvements. The instrument flew once before in July 2010 but experienced a much higher G-force than expected, which led to damage of screws holding the main mirror in place and a failure to gather accurate data. The team has reinforced the mirror since that flight.