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MinXSS Science Nugget 4

MinXSS Indicates Photospheric Abundance During M5 Flare

2017 June 3

Tom Woods and the MinXSS Science Team

The MinXSS soft X-ray (SXR) spectra from the X123 SXR spectrometer have significant advantages for analysis of the solar corona properties that broadband SXR measurements, such as from GOES XRS, cannot provide. In particular, the SXR spectra provides multiple temperatures, emission measure (density) at those temperatures, and abundances of several elements (e.g. Si, Ca, Fe) for studying the highly dynamic corona.  Phillips (2004) discusses the importance of Ca xix, Fe xxv, and Ni xxvii lines in the 3.8-10 keV range for studying abundance in flare spectra and how a lower abundance likely indicates an enhanced chromospheric contribution to the hot coronal plasma. X123 fully covers this 3.8-10 keV range and also extends down to about 0.5 keV. There have been interesting, but sometimes conflicting, results for flare abundance factors, and we anticipate that the MinXSS SXR spectra will advance the understanding of the flare energetics through studying the abundance changes during many flares. For this science nugget, just one flare is presented as one of the first results from MinXSS as described more by Woods et al. [2017]. The flare presented here is the M5.0 flare on 2016 July 23 that peaked at 2:11 UT.

For this initial study, the X123 spectra are fit with two temperature components and the abundance factor is fit for the Fe XXV emission line at 6.7 keV. For when the Fe XXV emission was too weak, such as for some non-flare spectra, an abundance factor of 2.1 was assumed.  The abundance factor is relative to the photosphere abundance, so an abundance factor of 1.0 is photospheric, values of two to four is typical for non-flaring coronal emissions. The uncertainty of fitting the abundance to the Fe XXV emission is about 30%.

Figure 1A shows the derived two-temperature components for the X123 spectra during this M5.0 flare.  The “hot” component (Temp-1) is consistent with the temperature estimate derived from the ratio of the GOES-15 XRS-A to XRS-B intensities. The X123 cooler (Temp-2) temperature component is most important for improved model fits for flare spectra, and this cooler temperature is similar to the pre-flare temperature.

The derived abundance factors from the X123 spectra during the M5.0 flare indicate a transition to photosphere-like abundance during the flare. This result as shown in Figure 1B is consistent with the standard flare model that predicts the majority of hot, SXR-emitting plasma to originate from the chromosphere as a result of “evaporation” from intense heating by non-thermal, accelerated electron beams and by thermal conduction fronts from the corona. In particular, the transition to photospheric abundance for the flare spectrum suggests that a significant source of the hot flaring plasma is from the lower atmosphere (chromosphere), transported to the corona via “evaporation” and mixed with the original plasma in the coronal loops where magnetic reconnection is assumed to have initiated the flare event. Prior studies have suggested that significant heating also occurs directly in the corona, which would be consistent with a “hybrid” abundance factor that is between the photospheric and coronal factors. X123’s 10-s cadence will be useful to examine the timing of the abundance transition from coronal to photospheric in all of the various emissions (including Ca and Si in addition to Fe as discussed here), and will be the subject of future studies.

References

Phillips, K. J. H., The Solar Flare 3.8-10 keV X-Ray Spectrum, ApJ, 605, 921, 2004.

Woods, T. N., A. Caspi, P. C. Chamberlin, A. Jones, R. Kohnert, J. P. Mason, C. S. Moore, S. Palo, C. Rouleau, S. C. Solomon, J. Machol, and R. Viereck, New Solar Irradiance Measurements from the Miniature X-ray Solar Spectrometer CubeSat, Astrophys. J., 835, 122, doi:10.3847/1538-4357/835/2/122, 2017.

MinXSS project web site:  http://lasp.colorado.edu/home/minxss/