Fig. 4
a Spectrum in the short EUV range (65–78 nm) obtained on day-of-year (DOY) 10 in 2015 (before the eruption). Emissions from ionized sulfur occurred at S II (64.2 nm), S IV (65.7 nm), S III (68.0 nm), S III (70.3 nm), S III (72. 9 nm), S IV (75.0 nm), and S II (76.5 nm). The slit shape and directions (dawn–dusk and north–south) are displayed. b Spectrum on DOY 40 (rising phase of the eruption). c Spectrum on DOY 70 (peak of the eruption). Note that the north–south direction was overturned because of the maneuvering of the spacecraft. d Spectra in the shorter spectral range on DOY 10, 40, and 70. By applying spectral diagnosis, the hot electron fraction (F h) and sulfur ion densities were deduced (numbers in the figure). The hot electron fraction gradually increased between DOY 10 and 70, although the densities of S+ and S2+ decreased on DOY 70