A group of astronomers at ESA’s ESTEC were testing some solar observing equipment on 6 September and serendipitously captured a solar flare, which turned out to be one of the most powerful observed in the last decade.
The image shown here was taken with an iPhone through a special interference H-alpha filter (centred at the wavelength of hydrogen emission) mounted to a small dedicated solar telescope at 13:09:26 GMT. An X9.3 flare was observed to launch from the Sun by space telescopes at 12:02 GMT, meaning that this image was taken as the flare was in the gradual decay phase.
The flare is seen as the white cloudy feature with multiple ribbons towards the bottom right of the image. It appears as a lighter feature against the solar background average because of post-flare energy release visible in hydrogen emission from interconnected magnetic loops. North is up.
See a SOHO movie capturing the same flare.