Since the launch of the Solar and Heliospheric Observatory (SOHO) in 1995, scientists have used its Extreme-Ultraviolet Imaging Telescope (EIT) to study flares, filaments and coronal mass ejections. The telescope has also discovered solar tsunamis (also called "EIT waves" by solar scientists), huge propagating waves that are triggered along with coronal mass ejections and can travel the entire diameter of the sun. Researchers at Southwest Research Institute (SwRI) are applying this unusual phenomenon for the first time to new studies of the solar corona.

Voss working aboard the space station in 2001. Credit: NASA

The Transition Region and Coronal Explorer (TRACE) spacecraft, launched in 1998, has provided new data complementing SOHO observations. Its higher resolution and faster cadence give solar physicists the tools to study hitherto unseen details. Dr. Meredith Wills-Davey, a post-doctoral researcher in the SwRI Space Studies Department, uses TRACE data to better understand the nature of solar tsunamis and the structure of the corona through which they travel. Her work is being presented June 16 at the Solar Physics Division Meeting of the American Astronomical Society in Laurel, Md.

"Just as geologists can learn about material in the ground by studying the waves generated by earthquakes," she says, "solar physicists can use these solar tsunamis to learn more about the structure of the solar corona."

TRACE observations of a well-observed event on June 13, 1998, are sufficiently detailed that it is possible to show, through morphology alone, that the propagation must be a "fast-mode magnetoacoustic wave." Analysis of the amplitude and the energy flux of the wave front shows that it actually increases through much of its lifetime. This suggests that, rather than being a single impulse, the wave driver may exist for an extended period. Because this particular event was associated with a coronal mass ejection, it is possible the wave is somehow part of the coronal mass ejection formation, says Wills-Davey.

Comparison between current observations at different coronal temperatures also offers insight into the wave's altitude of propagation. Evidence suggests that the tsunami is skimming along the base of the corona. This idea is also consistent with the lack of measurable dispersion in the wave, a circumstance more easily explained if the front travels at a constant height. Existing models and theories suggest that propagating waves in the corona should be trapped in "wave guides," but this appears to be the first observational evidence.

"These pulse waves serve as 'sonar pulses' that will let us probe the local conditions in up to 30 percent of the sun's atmosphere at once," says Dr. Craig DeForest, a senior research scientist at SwRI. "In addition, they help us study the unknown processes at play in solar flares, the largest explosions in our solar system."

"The study of waves in the corona is a new venture with an exciting future, and the benefits to our understanding of the sun should be substantial," adds Wills-Davey, who recently received NASA funding to continue this work. The research to date has been funded by NASA and the American Association of University Women Educational Foundation.

SwRI is an independent, nonprofit, applied research and development organization based in San Antonio, Texas, with more than 2,800 employees and an annual research volume of more than $339 million.