To observe how winds move high in Earth's atmosphere, scientists sometimes release clouds of barium as tracers to track how the material corkscrews and sweeps around — but scientists have no similar technique to study the turbulent atmosphere of the sun. So researchers were excited in December 2011, when Comet Lovejoy swept right through the sun's corona with its long tail streaming behind it. NASA's Solar Dynamics Observatory (SDO) captured images of the comet, showing how its long tail was buffeted by systems around the sun, offering scientists a unique way of observing movement as if they'd orchestrated the experiment themselves. Since comet tails have ionized gases, they are also affected by the sun's magnetic field, and can act as tracers of the complex magnetic system higher up in the atmosphere. Comets can also aid in the study of coronal mass ejections and the solar wind.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 184.108.40.206.0