Source of this massive plasma cloud looks to be a solar flare from old sunspot region 2665 as you can see on our header image at the top of this page. We do not have a satellite like the GOES-series that monitors far side solar X-ray emissions but using STEREO imagery it looks like this was at least a high M-class solar flare,
perhaps an X1 event.
https://www.spaceweatherlive.com/en/news/view/289/20170723-major-far-side-coronal-mass-ejection
Coincidentally, today's farside explosion occurred on the 5th anniversary of another significant farside event: The Solar Superstorm of July 23, 2012. That superstorm, which has been compared to the historic Carrington Event of 1859, could have caused widespread power blackouts if it had not missed our planet.
Sunspot AR2665 will be back on the Earthside of the sun starting little more than a week from now. If the sunspot remains active, it could bring a new round of geomagnetic storms and auroras to our planet in early August. Stay tuned.
http://www.spaceweather.com/
colleagues from NASA and other universities, published a seminal study of the storm in the December 2013 issue of the journal Space Weather. Their paper, entitled "A major solar eruptive event in July 2012," describes how a powerful coronal mass ejection (CME) tore through Earth orbit on July 23, 2012. Fortunately Earth wasn't there. Instead, the storm cloud hit the STEREO-A spacecraft.
"I have come away from our recent studies more convinced than ever that Earth and its inhabitants were incredibly fortunate that the 2012 eruption happened when it did," says Baker. "If the eruption had occurred only one week earlier, Earth would have been in the line of fire.
https://science.nasa.gov/science-news/science-at-nasa/2014/23jul_superstorm