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IRIS Observations of the Mg II h & k Lines During a Solar Flare  

Graham Stewart Kerr   Submitted: 2015-08-18 01:45

The bulk of the radiative output of a solar flare is emitted from the chromosphere, which produces enhancements in the optical and UV continuum, and in many lines, both optically thick and thin. We have, until very recently, lacked observations of two of the strongest of these lines: the Mg II h & k resonance lines. We present a detailed study of the response of these lines to a solar flare. The spatial and temporal behaviour of the integrated intensities, k/h line ratios, line of sight velocities, line widths and line asymmetries were investigated during an M class flare (SOL2014-02-13T01:40). Very intense, spatially localised energy input at the outer edge of the ribbon is observed, resulting in redshifts equivalent to velocities of ~15-26km s-1, line broadenings, and a blue asymmetry in the most intense sources. The characteristic central reversal feature that is ubiquitous in quiet Sun observations is absent in flaring profiles, indicating that the source function increases with height during the flare. Despite the absence of the central reversal feature, the k/h line ratio indicates that the lines remain optically thick during the flare. Subordinate lines in the Mg II passband are observed to be in emission in flaring sources, brightening and cooling with similar timescales to the resonance lines. This work represents a first analysis of potential diagnostic information of the flaring atmosphere using these lines, and provides observations to which synthetic spectra from advanced radiative transfer codes can be compared.

Authors: G.S. Kerr, P.J.A. Simões, J. Qiu and L. Fletcher
Projects: IRIS

Publication Status: A&A (in press)
Last Modified: 2015-08-18 07:56
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Physical Properties of White-Light Sources in the 2011 Feb 15 Solar Flare  

Graham Stewart Kerr   Submitted: 2014-01-20 19:07

White light flares (WLFs) are observational rarities, making them understudied events. However, optical emission is a significant contribution to flare energy budgets and the emission mechanisms responsible could have important implications for flare models. Using Hinode SOT optical continuum data taken in broadband red, green and blue filters, we investigate white-light emission from the X2.2 flare SOL2011-02-15T01:56:00. We develop a technique to robustly identify enhanced flare pixels and, using a knowledge of the RGB filter transmissions, determined the source color temperature and effective temperature. We investigated two idealized models of WL emission - an optically thick photospheric source, and an optically thin chromospheric slab. Under the optically thick assumption, the color temperature and effective temperature of flare sources in sunspot umbra and penumbra were determined as a function of time and position. Values in the range of 5000-6000K were found, corresponding to a blackbody temperature increase of a few hundred kelvin. The power emitted in the optical was estimated at ∼1026 ergs/s. In some of the white-light sources the color and blackbody temperatures are the same within uncertainties, consistent with a blackbody emitter. In other regions this is not the case, suggesting that some other continuum emission process is contributing. An optically thin slab model producing hydrogen recombination radiation is also discussed as a potential source of WL emission; it requires temperatures in the range 5,500 - 25,000K, and total energies of ∼1027 ergs/s.

Authors: Kerr, G.S & Fletcher, L.
Projects: Hinode/SOT

Publication Status: ApJ (in press)
Last Modified: 2014-01-20 19:10
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IRIS Observations of the Mg II h & k Lines During a Solar Flare
Physical Properties of White-Light Sources in the 2011 Feb 15 Solar Flare

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