Spectral characteristics and formation height of off-limb flare ribbons

Abstract

<p><em>[eng] Flare ribbons are bright manifestations of flare energy dissipation in the lower solar atmosphere. For the first time, we</em></p><p><em>report on high-resolution imaging spectroscopy observations of flare ribbons situated off limb in the Hβ and</em></p><p><em>Ca II 8542 Å lines and make a detailed comparison with radiative hydrodynamic simulations. Observations of the</em></p><p><em>X8.2 class solar flare SOL 2017-09-10T16:06 UT obtained with the Swedish Solar Telescope reveal bright horizontal</em></p><p><em>emission layers in Hβ line-wing images located near the footpoints of the flare loops. The apparent separation</em></p><p><em>between the ribbon observed in the Hβ wing and the nominal photospheric limb is about 300–500 km. The</em></p><p><em>Ca II 8542 Å line-wing images show much fainter ribbon emissions located right on the edge of the limb, without</em></p><p><em>clear separation from the limb. RADYN models are used to investigate synthetic spectral line profiles for the flaring</em></p><p><em>atmosphere, and good agreement is found with the observations. The simulations show that, toward the limb, where</em></p><p><em>the line of sight is substantially oblique with respect to the vertical direction, the flaring atmosphere model reproduces</em></p><p><em>the high contrast of the off-limb Hβ ribbons and their significant elevation above the photosphere. The ribbons in the</em></p><p><em>Ca II 8542 Å line-wing images are located deeper in the lower solar atmosphere with a lower contrast. A comparison</em></p><p><em>of the height deposition of electron beam energy and the intensity contribution function shows that the Hβ line-wing</em></p><p><em>intensities can be a useful tracer of flare energy deposition in the lower solar atmosphere.</em></p>