Goryaev, Farid ; Slemzin, Vladimir ; Rodkin, Denis ; D’Huys, Elke ; Podladchikova, Olena ; West, Matthew

submitted to The Astrophysical Journal Letters (2017)

Abstract: On 6 and 10 September 2017, the strongest X9.3 and X8.2 flares of the decade occurred in the active region NOAA AR 12673. During these flares, the SWAP telescope onboard the PROBA2 satellite registered the unusual alternate brightening and darkening of the corona at the distances of 0.2–0.7R⊙ above the limb. The X9.3 flare on September 6 was associated with coronal brightening up to 30–45% at the distances of 0.4–0.7R⊙, presumably produced by resonant scattering of the flare radiation by the FeIX–FeXI ions at temperatures T∼1 MK. At the maximum of the flare and one hour later, two coronal mass ejections (CMEs) originated, which dimmed the coronal emission in SWAP 174 ˚A passband above the western limb by 20–30%. The X8.2 flare on September 10 occurred accompanied by a CME, which rose up and progressively dimmed the western part of the corona up to 60%. An hour later, the darkening, produced by a global rearrangement of the magnetic field structure and an evacuation of a significant part of the coronal plasma, extended over the complete western limb. A differential emission measure (DEM) analysis showed a decrease in the electron density of the background plasma with T ∼ 1−2 MK at distances 0.24–0.33R⊙ by 2–3.5 times after the CME. At the same time, an additional DEM peak at T ≈ 0.8 MK appeared, which may be associated with an additional emission in the SWAP 174 ˚A passband produced by the flare radiation resonantly scattered by the coronal plasma.

Keyword(s): Sun: corona — Sun: flares — Sun: UV radiation — methods: data analysis —techniques: image processing
Funding: PROBA2 Guest Investigator Program 2017/PROBA2 Guest Investigator Program 2017/PROBA2 Guest Investigator Program 2017