「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
阿蘇中岳で火映現象 警戒レベル2は維持 | 熊本日日新聞
火山活動の活発化に伴い、火口周辺立ち入り規制が続く熊本県の阿蘇中岳で6日未明、火口内の高温のガスが噴...