The outermost layer of the Sun’s atmosphere is a halo of hot gases; temperatures top 1,000,000°C. This formation, called the corona, meaning “crown” in Latin, can be seen using specialized instruments on Earth and in space during a total solar eclipse.
From time to time, massive jets of plasma—imbibed with magnetic fields—spew from the Sun’s corona, traveling hundreds of kilometers per second. When these eruptions (called coronal mass ejections, or CMEs) get close enough to Earth, they can cause spectacular aurora events and geomagnetic storms, which pose a threat to electric power grids and commercial airplanes.
In a recent study, Möstl et al. applied a way to better predict the time that a CME will hit Earth or another nearby planet. To do this, they looked at initial conditions that kicked off more than 1,000 CMEs, modeled when they were expected to hit Earth or another planet, and compared their modeled data to when the CMEs actually struck.
To create these hindcasts, researchers used data collected by sensors aboard NASA’s Solar Terrestrial Relations Observatory (STEREO) over a period of 8 years, from 2007 to 2015. The mission is part of a larger fleet of NASA spacecraft, called the Heliophysics System Observatory, which carry instruments designed to probe various dynamics of our solar system.
About once every 11 years, the Sun’s north and south magnetic poles switch, causing a flurry of geomagnetic activity. The time it takes the Sun to go from a quiet minimum state to solar maximum, when more eruptions happen, and back to solar minimum is known as the solar cycle. The period in which the instruments collected these data comprises about two thirds of a full solar cycle. The data (animated here by the team) provide information on the CMEs, such as their speed, their direction, and the width of the affected area.
The researchers incorporated this information into a model, which allowed them to produce hindcasts of the likelihood that a CME would have encountered a planet (and, if entirely likely, what time the encounter would have taken place). They found that of all the CMEs the model would have predicted, only about a third of them were actually detected (the rest would have been false alarms).
But even though these hindcasts were far from perfect, those that were not false alarms were accurate to within about 2.5 hours (plus or minus about 17 hours) compared to the actual time the geomagnetic activity hit Earth. Furthermore, the researchers found that prediction accuracy did not decrease depending on the imager’s distance from Earth.
This study is an important step toward understanding and predicting CMEs, an element of space weather that directly affects human life on Earth. The researchers’ successful use of data to produce hindcasts of CMEs could help lead to a future satellite mission that would use specialized instruments to collect the information scientists need to monitor the space between Earth and the Sun. (Space Weather, https://doi.org/10.1002/2017SW001614, 2017)
—Sarah Witman, Freelance Writer