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

Wenjuan Liu   Submitted: 2015-11-03 02:43

Mg II lines represent one of the strongest emissions from the chromospheric plasma during solar flares. In this article, we studied the Mg II lines observed during the X1 flare on March 29 2014 (SOL2014-03-29T17:48) by the Interface Region Imaging Spectrograph (IRIS). IRIS detected large intensity enhancements of the Mg II h and k lines, subordinate triplet lines, and several other metallic lines at the flare footpoints during this flare. We have used the advantage of the slit-scanning mode (rastering) of IRIS and performed, for the first time, a detailed analysis of spatial and temporal variations of the spectra. Moreover, we were also able to identify positions of strongest hard X-ray (HXR) emissions using the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations and to correlate them with the spatial and temporal evolution of IRIS Mg II spectra. The light curves of the Mg II lines increase and peak contemporarily with the HXR emissions but decay more gradually. There are large red asymmetries in the Mg II h and k lines after the flare peak. We see two spatially well separated groups of Mg II line profiles, non-reversed and reversed. In some cases, the Mg II footpoints with reversed profiles are correlated with HXR sources. We show the spatial and temporal behavior of several other line parameters (line metrics) and briefly discuss them. Finally, we have synthesized the Mg II k line using our non-LTE code with the Multilevel Accelerated Lambda Iteration (MALI) technique. Two kinds of models are considered, the flare model F2 of Machado et al. (1980, Astrophys. J., 242, 336) and the models of Ricchiazzi and Canfield (1983, Astrophys. J., 272, 739, RC). Model F2 reproduces the peak intensity of the unreversed Mg II k profile at flare maximum but does not account for high wing intensities. On the other hand, the RC models show the sensitivity of Mg II line intensities to various electron-beam parameters. Our simulations also show that the microturbulence produces a broader line core, while the intense line wings are caused by an enhanced line source function.

Authors: W. Liu, P. Heinzel, L. Kleint, J. Kasparova
Projects: IRIS

Publication Status: accepted for publication in Solar Physics
Last Modified: 2015-11-03 10:15
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Determining Heating Rates in Reconnection Formed Flare Loops of the M8.0 Flare on 2005 May 13  

Wenjuan Liu   Submitted: 2013-04-17 10:18

We analyze and model an M8.0 flare on 2005 May 13 observed by the Transition Region and Coronal Explorer (TRACE) and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to determine the energy release rate from magnetic reconnection that forms and heats numerous flare loops. The flare exhibits two ribbons in UV 1600 {AA} emission. Analysis shows that the UV light curve at each flaring pixel rises impulsively within a few minutes, and decays slowly with a timescale longer than 10 minutes. Since the lower atmosphere (the transition region and chromosphere) responds to energy deposit nearly instantaneously, the rapid UV brightening is thought to reflect the energy release process in the newly formed flare loop rooted at the foot point. In this paper, we utilize the spatially resolved (down to 1′′) UV light curves and the thick-target hard X-ray emission to construct heating functions of a few thousand flare loops anchored at the UV foot points, and compute plasma evolution in these loops using the Enthalpy-Based Thermal Evolution of Loops (EBTEL) model. The modeled coronal temperatures and densities of these flare loops are then used to calculate coronal radiation. The computed soft X-ray spectra and light curves compare favorably with those observed by RHESSI and by the Geostationary Operational Environmental Satellite (GOES) X-ray Sensor (XRS). The time-dependent transition region differential emission measure (DEM) for each loop during its decay phase is also computed with a simplified model and used to calculate the optically-thin C iv line emission, which dominates the UV 1600 {AA} bandpass during the flare. The computed C iv line emission decays at the same rate as observed. This study presents a method to constrain heating of reconnection-formed flare loops using all available observables independently, and provides insight into the physics of energy release and plasma heating during the flare. With this method, the lower limit of the total energy used to heat the flare loops in this event is estimated to be 1.22e31 ergs, of which only 1.9e30 ergs is carried by beam-driven upflows during the impulsive phase, suggesting that the coronal plasmas are predominantly heated in situ.

Authors: Wen-Juan Liu, Jiong Qiu, Dana W. Longcope, Amir Caspi
Projects: TRACE

Publication Status: accepted for publication in The Astrophysical Journal
Last Modified: 2013-04-17 12:20
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Mg II Lines Observed during the X-class Flare on 29 March 2014 by the Interface Region Imaging Spectrograph
Determining Heating Rates in Reconnection Formed Flare Loops of the M8.0 Flare on 2005 May 13

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