English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

We report on the spectral features of the Si IV 1402.77 \AA, C II 1334.53\AA, and Mg II h or k lines, formed in the layers from the transition region tothe chromosphere, in three two-ribbon flares (with X-, M-, and C-class)observed with IRIS. All the three lines show significant redshifts within themain flare ribbons, which mainly originate from the chromospheric condensationduring the flares. The average redshift velocities of the Si IV line within themain ribbons are 56.6, 25.6, and 10.5 km s$^{-1}$ for the X-, M-, and C-classflares, respectively, which show a decreasing tendency with the flare class.The C II and Mg II lines show a similar tendency but with smaller velocitiescompared to the Si IV line. Additionally, the Mg II h or k line shows ablue-wing enhancement in the three flares in particular at the flare ribbonfronts, which is supposed to be caused by an upflow in the upper chromospheredue to the heating of the atmosphere. Moreover, the Mg II h or k line exhibitsa central reversal at the flare ribbons, but turns to pure emission shortlyafter 1--4 minutes. Correspondingly, the C II line also shows a centralreversal but in a smaller region. However, for the Si IV line, the centralreversal is only found in the X-class flare, but not in the other two flares.As usual, the central reversal of these lines can be caused by the opacityeffect. This implies that in addition to the optically thick lines (C II and MgII lines), the Si IV line can become optically thick in a strong flare, whichis likely related to the nonthermal electron beam heating.

Spectral Features of the Solar Transition Region and Chromospheric Lines at Flare Ribbons Observed with IRIS