Observations of solar flares with IRIS and SDO

Abstract

Flare kernels brighten simultaneously in all Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) channels making it difficult to determine their temperature structure. The Interface Region Imaging Spectrograph (IRIS) is able to spectrally resolve Fe xxi emission from cold chromospheric brightenings, so it can be used to infer the amount of Fe xxi emission in the 131 Å AIA channel. We use observations of two small solar flares seen by IRIS and SDO to compare the emission measures (EMs) deduced from the IRIS Fe xxi line and the AIA 131 Å channel to determine the fraction of Fe xxi emission in flare kernels in the 131 Å channel of AIA. Cotemporal and cospatial pseudo-raster AIA images are compared with the IRIS results. We use multi-Gaussian line fitting to separate the blending chromospheric emission so as to derive Fe xxi intensities and Doppler shifts in IRIS spectra. We define loop and kernel regions based on the brightness of the 131 Å and 1600 Å intensities. In the loop regions the Fe xxi EMs are typically 80% of the 131 Å values, and range from 67% to 92%. Much of the scatter is due to small misalignments, but the largest site with low Fe xxi contributions was probably affected by a recent injection of cool plasma into the loop. In flare kernels the contribution of Fe xxi increases from less than 10% at the low-intensity 131 Å sites to 40−80% in the brighter kernels. Here the Fe xxi is superimposed on bright chromospheric emission and the Fe xxi line shows blueshifts, sometimes extending up to the edge of the spectral window, 200 km s −1 . The AIA 131 Å emission in flare loops is due to Fe xxi emission with a 10−20% contribution from continuum, Fe xxiii, and cooler background plasma emission. In bright flare kernels up to 52% of the 131 Å is from cooler plasma. The wide range seen in the kernels is caused by significant structure in the kernels, which is seen as sharp gradients in Fe xxi EM at sites of molecular and transition region emission.

Explosive Chromospheric Evaporation Driven by Nonthermal Electrons around One Footpoint of a Solar Flare Loop

“…This position is located at the edge of a flare ribbon in the northwest labeled with the short blue line in Figure 2 Young et al 2015;Tian et al 2015). Here, we fixed these blended line positions, constrained their widths, and tied their intensities to the lines in other spectral windows (e.g., Li et al 2015Li et al , 2016. Finally, we can obtain the line profile of Fe XXI 1354.09 Å, as shown by the turquoise line.…”

Explosive Chromospheric Evaporation in a Circular-Ribbon Flare

“…In Figure 5 be identified and removed using the multi-Gaussian fitting method (Li et al 2015a(Li et al , 2016.…”

Evidence of chromospheric molecular hydrogen emission in a solar flare observed by the IRIS satellite

“…IRIS observes a number of molecular H 2 lines which mostly come from the Lyman band (Herzberg & Howe 1959;Sandlin et al 1986). H 2 emission lines in IRIS spectra from the flaring chromosphere were identified by Young et al (2015) and Li et al (2016); H 2 lines are also reported in absorption, as features in Si �� spectral lines (Schmit et al 2014). These are interpreted as due to pockets of cool (photospheric temperature) plasma, in which molecules can form, in the upper solar atmosphere above a source of Si �� emission.…”

“…In addition, Li et al (2016) also mentioned that there is a possible blend based on IRIS flare observation. In order to identify a blend, the unblended S ⇤ line at 1401.51 Å could be taken as a reference.…”