Using data from the NASA/ESA Solar and Heliospheric Observatory (SOHO) and NASA’s twin Solar Terrestrial Relations Observatory (STEREO) satellites, solar physicists have developed a model that simulates how shocks following coronal mass ejections propagate from the Sun.
On August 31, 2012, a long filament of solar material that had been hovering in the Sun’s atmosphere, the corona, erupted out into space at 4:36 p.m. EDT. The CME traveled at over 900 miles per second. It did not travel directly toward Earth, but did connect with Earth’s magnetic environment, or magnetosphere, with a glancing blow, causing aurora to appear on the night of September 3. Image credit: NASA’s Goddard Space Flight Center.
Coronal mass ejections (CMEs), often called solar storms or space storms, are gigantic clouds of solar plasma drenched with magnetic field lines that are blown away from the Sun during solar flares and filament eruptions.
Although the Sun’s corona has been observed during total eclipses of the Sun for thousands of years, the existence of CMEs was unrealized until the space age. The earliest evidence of these events came from observations made in 1971.
Much the way ships form bow waves as they move through water, CMEs set off interplanetary shocks when they erupt from the Sun at extreme speeds, propelling a wave of high-energy particles. These particles can spark space weather events around Earth, endangering spacecraft and astronauts.
Understanding a shock’s structure is key to predicting how it might disrupt near-Earth space. But without a vast array of sensors scattered through space, these things are impossible to measure directly.
Instead, scientists rely upon models that use satellite observations of the CME to simulate the ensuing shock’s behavior.
Dr. Ryun-Young Kwon of the George Mason University and the Johns Hopkins University Applied Physics Laboratory (APL) and APL astrophysicist Dr. Angelos Vourlidas pulled observations of two different eruptions from SOHO and STEREO satellites. One CME erupted in March 2011 and the second, in February 2014.
The scientists fit the CME data to their models — one called the ‘croissant’ model for the shape of nascent shocks, and the other the ‘ellipsoid’ model for the shape of expanding shocks — to uncover the 3D structure and trajectory of each CME and shock.
Each spacecraft’s observations alone weren’t sufficient to model the shocks. But with three sets of eyes on the eruption, each of them spaced nearly evenly around the Sun, the scientists could use their models to recreate a 3D view.
The team’s work, reported in the Journal of Space Weather and Space Climate, confirmed long-held theoretical predictions of a strong shock near the CME nose and a weaker shock at the sides.
In time, shocks travel away from the Sun, and thanks to the 3D information, the researchers could reconstruct their journey through space.
The modeling helps scientists deduce important pieces of information for space weather forecasting — in this case, for the first time, the density of the plasma around the shock, in addition to the speed and strength of the energized particles. All of these factors are key to assessing the danger CMEs present to astronauts and spacecraft.
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Ryun-Young Kwon & Angelos Vourlidas. 2018. The density compression ratio of shock fronts associated with coronal mass ejections. J. Space Weather Space Clim 8, A08; doi: 10.1051/swsc/2017045
