On October 14th, 2014, the Sun decided it was time for a coronal mass ejection, the irregular hiccups that see it belch out astounding quantities of magnetised plasma. And after careful analysis, we've now fingerprinted the plasma's passing using no fewer than ten spacecraft.
The event and subsequent analysis are detailed in a paper titled “Interplanetary coronal mass ejection observed at STEREO-A, Mars, comet 67P/Churyumov-Gerasimenko, Saturn, and New Horizons en route to Pluto” published in Space Physics.
Coronal mass ejections (CMEs) aren't uncommon and solar observatories often spot them. But this one had the good fortune to happen in a spot that saw its effects cross the paths of ten spacecraft.
As explained by the European Space Agency, the event was first spotted by the team running its Mars Express mission when looking at data from Comet Siding Spring's October 2014 close encounter with Mars. As the paper recounts, “Mars' upper atmosphere was disturbed during that time, most likely due to a solar wind event, making the analysis of the comet-related effects more complex than anticipated. It was therefore decided to search for possible solar wind structures that could have been present at Mars during that period.”
That effort led to detection of the October 14th event and then for a search to figure out how to investigate it.
Which is where boffins got lucky, because we had lots of eyes on the event.
Three Sun-watching missions, the ESA’s Proba-2, the joint ESA/NASA SOHO satellite and NASA’s SDO, all snapped the CME as it happened.
NASA’s Stereo-A, the ESA’s Venus Express, Mars Express and Rosetta, NASA’s Mars Odyssey and Maven orbiters and even the Curiosity rover were all in the cone of star-stuff the CME produced. Even further out the Cassini spacecraft was in a handy spot, too. New Horizons and Voyager-2 were also in good places.
The latter is thought to have observed something suggesting the CME 17 months after it happened. New Horizons had its own inconclusive encounter three months after the CME.
Not all of the craft were able to get a good look at the event, because most weren't told to look for it in advance. Venus Express, for example, was behind the Sun relative to Earth so was out of radio range and had turned off its instruments. But the craft's star tracker was “overwhelmed with radiation at the expected time of passage”, giving us useful data about when the CME passed by.
Other craft reported marked decreases in cosmic rays for a day or more as the CME passed, confirming observations we've been able to make when CMEs pass Earth. The study has also taught us more about how fast CMEs travel: the paper the data collected suggests “a strong decrease from about 1000 to 580 km/s within 1.5 AU, followed by a gradual decrease down to about 450–500 km/s at ~15 AU.” The 15 AU measurement comes from Cassini. New Horizons and Voyager 2's measurement suggest CME plasma holds its speeds after it passes the ringed planet.
The paper concludes by saying the event could have yielded even more data had some other nearby spacecraft been switched on and/or had the right instruments aboard.
The authors therefore “emphasize the importance of a space weather monitoring package, including a magnetometer, to be embarked in all planetary and astronomical missions as a basic payload requirement.” They also recommend that “plasma instruments continue to operate during solar superior conjunctions, even if only at a very low data rate, or continue to acquire data for later download.” ®