Blue-wing enhancement of the Mg II h and k lines in a flare
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|1st Author:||Akiko TEI|
|Published:||8 April 2018|
|Previous Nugget:||NuSTAR quiet-Sun flares|
During a solar flare, the solar chromosphere undergoes violent disruptions as the entire atmosphere accommodates to a new configuration. Imaging spectroscopy, especially in visible-UV-EUV wavelengths, lets us study the dynamics. Redshifts and red-wing enhancements (red asymmetries) appear in chromospheric lines (e.g., Ref ), with a natural interpretation in terms of a chromospheric condensation produced by a coronal pressure enhancement, which results in compression and physical downflow in the chromosphere. On the other hand, blue asymmetries in chromospheric lines have also been reported (e.g., Ref ). Some researchers have attempted to explain the origin of these blue asymmetries, but we have not obtained consensus.
In this Nugget, we report extensive multi-wavelength observations of a recent solar flare, which shed some light on this interesting problem.
The moving flare kernel observed by IRIS and DST at Hida
A C-class flare (SOL2014-11-11, in NOAA region 12205), was observed by the Interface Region Imaging Spectrograph (IRIS) and the Domeless Solar Telescope (DST) at Hida Observatory. We observed an interesting flare, specifically a moving emission kernel, using spectral data in the Si IV 1403, C II 1335, and Mg II h and k lines from IRIS and the Ca II K, Ca II 8542, and H lines from DST. See Figures 1 and 2.
Blue-wing enhancement in the Mg II h and k lines
A large redshift was a common property for all the six lines of Si IV 1403, C II 1335, Mg II h, Ca II K, Ca II 8542, and Hα, but a blueshift prior to it was found only in the Mg II h line. The blueshift lasted for 9-48 s with a typical speed of 10.1+-2.6 km/s, and it was followed by high intensity at a large redshift with a speed of up to 51 km/s. The blueshifted profiles of the Mg II h line showed an intensity enhancement in the blue wing, and a smaller intensity of the blue-side peak (h2v) than that of the red-side one (h2r). These characteristics are the same for the Mg II k line (see Figure 3).
A cool upflow scenario
A cloud modeling of the Mg II h line suggests that the blue wing enhancement with such a peak difference can be caused by a chromospheric-temperature (cool) upflow. Ref.  discusses a scenario in which an upflow of cool plasma is lifted up by expanding hot plasma owing to the deep penetration of non-thermal electrons into the chromosphere (see Figure 4).
Multi-wavelength observations of chromospheric dynamics, at high resolution, are now becoming available from ground-based (visible) and space-based (UV) imaging spectroscopy. The flare observations discussed here suggest that the vertical motions involved in the "evaporation" flow of chromospheric material into the corona require a complicated temperature structure, with cool material overlying the site of energy deposition.