「火映現象」(Kaeigenshō) is translated at Weblio as “a phenomenon of combustion inside a volcanic crater that creates ruddy reflection against smoke or clouds.” In other words, the red that you see is not erupting magma but a reflection of heat on emitted gasses.
Aso occasionally burps out gas. Kumanichi reports that it burped out 3,300 tons of sulfur dioxide on 1/6, resulting in an alarming appearance. Scientists who maintain monitoring stations around the volcano which monitor bulges indicating magma intrusions were not alarmed, and the warning level remains at 2, meaning don’t approach the crater. It did allow for a magnificent photograph. - William
