THE 2014 MARCH 29 X-FLARE: SUBARCSECOND RESOLUTION OBSERVATIONS OF Fe XXI λ1354.1

Abstract

The Interface Region Imaging Spectrometer (IRIS) is the first solar instrument to observe ∼ 10 MK plasma at subarcsecond spatial resolution through imaging spectroscopy of the Fe xxi λ1354.1 forbidden line. IRIS observations of the X1 class flare that occurred on 2014 March 29 at 17:48 UT reveal Fe xxi emission from both the flare ribbons and the post-flare loop arcade. Fe xxi appears at all of the chromospheric ribbon sites, although typically with a delay of one raster (75 seconds) and sometimes offset by up to 1 ′′ . 100-200 km s −1 blue-shifts are found at the brightest ribbons, suggesting hot plasma upflow into the corona. The Fe xxi ribbon emission is compact with a spatial extent of < 2 ′′ , and can extend beyond the chromospheric ribbon locations. Examples are found of both decreasing and increasing blue-shift in the direction away from the ribbon locations, and blue-shifts were present for at least 6 minutes after the flare peak. The post-flare loop arcade, seen in Atmospheric Imaging Assembly (AIA) 131Å filtergram images that are dominated by Fe xxi, exhibited bright loop-tops with an asymmetric intensity distribution. The sizes of the loop-tops are resolved by IRIS at ≥ 1 ′′ , and line widths in the loop-tops are not broader than in the loop-legs suggesting the loop-tops are not sites of enhanced turbulence. Line-of-sight speeds in the loop arcade are typically < 10 km s −1 , and mean non-thermal motions fall from 43 km s −1 at the flare peak to 26 km s −1 six minutes later. If the average velocity in the loop arcade is assumed to be at rest, then it implies a new reference wavelength for the Fe xxi line of 1354.106 ± 0.023Å.

Chromospheric Condensation and Quasi-Periodic Pulsations in a Circular-Ribbon Flare

“…The timescales of chromospheric condensation range from 1 to 7 minutes . A delay of >60 s between the starting times of condensation and evaporation has been observed Young et al 2015). Both the upward evaporation velocities and downward condensation velocities are positively correlated with the HXR fluxes, which is consistent with the numerical model of evaporation driven by nonthermal electrons Li et al 2015a).…”

The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. I. Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29

“…We will also incorporate the NLTE predictions of the Mg II line profiles (as in Rubio da Costa et al (2016)) with the effects of nonthermal collision rates and compare our CC models in detail to observations of Hα with the DST/IBIS Rubio da Costa et al 2016) and to lines from IRIS that probe hotter temperatures (Young et al 2015). In flares, the Si IV lines also exhibit two, spectrally resolved emission components (referred to as "CB" and "CR" Brannon et al 2015) which will be compared to the λ rest and RWA components in the chromosphere flare lines.…”

Mg ii Lines Observed During the X-class Flare on 29 March 2014 by the Interface Region Imaging Spectrograph

“…It was observed by IRIS from three hours before the flare to a few minutes after the flare peak with a NUV exposure time of 8.00 s before 17:46:04.78 UT and 2.44 s after 17:46:13.98 UT, and 75 s raster cadence (see Section 2 of Heinzel and Kleint (2014) and Section 2 of Young, Tian, and Jaeggli (2015) for detailed descriptions of IRIS observations of this flare). Light curves of the flare in IRIS 1400Å and IRIS 2796Å channels, together with X-ray light curves are plotted in Figure 1.…”