IRIS Si IV Line Profiles at Flare Ribbons as Indications of Chromospheric Condensation. (arXiv:2005.02029v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Yu_K/0/1/0/all/0/1">Ke Yu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Li_Y/0/1/0/all/0/1">Y. Li</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ding_M/0/1/0/all/0/1">M. D. Ding</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Li_D/0/1/0/all/0/1">D. Li</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhou_Y/0/1/0/all/0/1">Yi-An Zhou</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hong_J/0/1/0/all/0/1">Jie Hong</a>

We present temporal variations of the Si IV line profiles at the flare
ribbons in three solar flares observed by the Interface Region Imaging
Spectrograph (IRIS). In the M1.1 flare on 2014 September 6 and the X1.6 flare
on 2014 September 10, the Si IV line profiles evolve from wholly redshifted to
red-wing enhanced with the flare development. However, in the B1.8 flare on
2016 December 2, the Si IV line profiles are wholly redshifted throughout the
flare evolution. We fit the wholly redshifted line profiles with a single
Gaussian function but the red-asymmetric ones with a double Gaussian function
to deduce the corresponding Doppler velocities. In addition, we find that hard
X-ray emission above 25 keV shows up in the two large flares, implying a
nonthermal electron beam heating. In the microflare, there only appears weak
hard X-ray emission up to 12 keV, indicative of a thermal heating mostly. We
interpret the redshifts or red asymmetries of the Si IV line at the ribbons in
the three flares as spectral manifestations of chromospheric condensation. We
propose that whether the line appears to be wholly redshifted or red-asymmetric
depends on the heating mechanisms and also on the propagation of the
condensation.

We present temporal variations of the Si IV line profiles at the flare
ribbons in three solar flares observed by the Interface Region Imaging
Spectrograph (IRIS). In the M1.1 flare on 2014 September 6 and the X1.6 flare
on 2014 September 10, the Si IV line profiles evolve from wholly redshifted to
red-wing enhanced with the flare development. However, in the B1.8 flare on
2016 December 2, the Si IV line profiles are wholly redshifted throughout the
flare evolution. We fit the wholly redshifted line profiles with a single
Gaussian function but the red-asymmetric ones with a double Gaussian function
to deduce the corresponding Doppler velocities. In addition, we find that hard
X-ray emission above 25 keV shows up in the two large flares, implying a
nonthermal electron beam heating. In the microflare, there only appears weak
hard X-ray emission up to 12 keV, indicative of a thermal heating mostly. We
interpret the redshifts or red asymmetries of the Si IV line at the ribbons in
the three flares as spectral manifestations of chromospheric condensation. We
propose that whether the line appears to be wholly redshifted or red-asymmetric
depends on the heating mechanisms and also on the propagation of the
condensation.

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