Near the surface of the Sun, extremely hot, ionized gas is churned up by convection, just like a pot of boiling water. These motions generate the Sun's magnetic field, but also create sound waves that move through the Sun. It is possible to see these sound waves as Doppler shifts when they hit the surface of the Sun, each one hinting at the activity going on below. Once launched aboard Solar Dynamics Observatory (SDO), the Helioseismic and Magnetic Imager (HMI) will measure these Doppler shifts over the entire visible part of the Sun. The data will then be used to create maps of the Sun's interior and the plasma flows that generate its magnetic field. To do so, HMI will measure 120 million pieces of data every 45 seconds.
This is no small task, "It’s like deducing the interior structure of a piano by listening to it fall down a flight of stairs," explained Phil Scherrer, SDO HMI Principal Investigator. "In a sense we hope to measure the sound of the sun in magnetically active regions, which generate a lot of severe solar weather.”
The technique of tracing sound waves reverberating inside the sun to build up a picture of the interior is known as "helioseismology" and works similar to the way an ultrasound scan is used to create a picture of an unborn baby. Helioseismology can even be used to map sunspots all the way on the other side of the sun from Earth. The precursor to HMI is the Michelson Doppler Imager (MDI), launched in 1995 on board the Solar and Heliospheric Observatory. MDI however has only limited coverage of the Sun, whereas HMI will provide a full-disk view with about one thousand times the data. HMI will also be capable of measuring the strength and direction of the magnetic fields emerging on the Sun’s surface.
SDO will carry two other instruments in addition to HMI. It is hoped the combination of their observations will enable researchers to establish the relationships between the internal dynamics and surface activity. "These three instruments together will enable scientists to better understand the causes of violent solar activity, and whether it’s possible to make accurate and reliable forecasts of space weather," said Liz Citrin, SDO Project Manager at NASA Goddard. "SDO will provide a full disk picture of the sun in super HD quality."
The Extreme Ultraviolet Variability Experiment (EVE) arrived at Goddard this September and the Atmospheric Imaging Assembly (AIA) will arrive before the end of the year. SDO and its components will be integrated and go through rigorous testing at Goddard’s state-of-the-art facilities up until it is shipped for launch sometime between the end of 2008 and the beginning of 2009.
SDO will be the first mission of NASA’s Living with a Star program. The goal of both SDO and Living with a Star is to help us understand and work towards predicting the changes in the Sun that influence life on Earth and human technology.
SDO is being designed, managed, and assembled at NASA Goddard. HMI was built in Palo Alto, Calif. at Stanford's partner institution Lockheed Martin Solar Astrophysics Laboratory (LMSAL) as project of the Stanford Lockheed Institute for Space Research. The Stanford group will provide the science data processing center for both the HMI and the AIA instruments. AIA is also being built at LMSAL. EVE was built by the University of Colorado at Boulder.