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MinXSS Science Nugget 7

Solar Coronal Abundance Complexities Derived by MinXSS

2017 June 16

Chris Moore and the MinXSS Science Team

The spectrally resolved measurements from the X-ray spectrometer (X123) onboard the Miniature X-ray Solar Spectrometer (MinXSS) provide the capability to diagnose the elemental abundances of solar coronal observations. The effective ~0.24 keV FWHM resolution near 5.9 keV across the 0.8 – 12 keV bandpass is more than suitable to estimate the elemental of abundances from spectral features of Fe near 1.2 keV and 6.7 keV, Mg near 1.7 keV, Si around 2.1 keV, S by 2.7 keV, Ar (or lack thereof) near 3 keV, Ca by 4 keV, and the Fe+Ni complex at 8 keV. Some specific ions and lines that compose these spectral features are discussed in Phillips et al. [2004]. As mentioned in the MinXSS Science Nugget 003, early MinXSS results confirms the change from an enhanced coronal abundance (with respect to photospheric) for the low first ionization potential (FIP – < 10 eV) elements during non-large-flaring times to photospheric-like abundances during the M5.0 solar flare. The physical motivation for this change revolves around chromospheric plasma flowing up into the coronal flaring loops (chromospheric evaporation) and thus contributing to the SXR radiation. The exact process is still being actively investigated.

The elemental abundance of the solar atmosphere is highly debated and inferred values have migrated over the years (Schmelz et al. [2012]). The solar abundance changes as a function of height in the solar atmosphere (photosphere to corona), feature (coronal hole vs. active region), time (active region age) and phenomena (flares, CMEs, etc.), thus any investigation of solar abundance must be heeded with extreme caution and care. Traditionally, coronal abundance values of elements have been reported relative to hydrogen and with respect to photospheric values. The first class of fits discussed in this nugget are those that are listed in Table 1. These two temperature fits with a single low FIP-Bias scale factor (modifies the abundance of Fe, Si, Mg, Ca, Ni and S only) are labeled 2T-Free. The FIP-Bias scale factor is normalized to equal 4 for abundance values listed in Feldman [1992]. The Feldman [1992] abundances are the long standing preferred set for the solar corona.

The spectra discussed in this nugget are discussed in detail in Moore et al. [2018] and consist of a quiet sun time (QS) and three flare-peak spectra with their corresponding pre-flare spectra. The times over which the observations were averaged is listed in Table 1. The QS result in a FIP-Bias value of 3.48 is closer to the nominal ‘coronal’ abundance. But, during the pre-flare times, the solar SXR flux is dominated by the active region (AR) emission and values near ~2 for the FIP-Bias is inferred. The 2T-Free fits of the flare-peak spectra for the C2.7, M1.2 and M5.0 flares all yield FIP-Bias factors of 1.41 or less. This result is in line with the chromospheric evaporation model and results obtained by Woods et al. [2017].

There is strong evidence that the coronal elemental abundance variation is more complicated than just a single FIP-Bias scaling (Schmelz et al. [2012] and Dennis et al. [2015]). Spectral fits of MinXSS-1 data that allow a select set of elements’ abundance to vary reinforce this claim. Two temperature spectral fits with a variable abundance for Fe, Si, Mg, S, Ar, Ca and Ni (2T-Allfree) results are displayed in Figure 1., and the parameter values are in Table 2 and Table 3. The Ni abundance was coupled to the Fe abundance scale factor, so the Ni estimate is not a true fit. The QS spectra did not have enough statistically significant counts to accurately constrain abundance values, but the other six spectra discussed here did have enough significant counts. Values quoted for the 2T-Allfree fits here are in units of coronal/photosphere literature values where the coronal values are from Feldman [1992] and the photosphere values are from Caffau et al. [2011]. For these other spectra the abundances of Fe, Mg behave similar to the 2T-Free FIP-Bias values for pre-flare and flare-peak. Si deviates slightly. S, Ar and Ca each display their own unique deviations. This initial study demonstrates that a single FIP-Bias may not be enough to describe the coronal abundance phenomenon. A focused study on numerous flares will allow the inference of the abundance fractionation pattern from a statistically robust data set.

Table 1. MinXSS-1 2T-Free (one FIP-Bias scale factor) spectral fits of observations from GOES A5 – M5 levels. The uncertainties in the fit parameters are in parenthesis.  ** highlights that the pre-flare data inferred dimmer and hotter second component is near the limit of the MinXSS plasma diagnostic capabilities and thus not as well constrained. This is Table 3. in Moore et al. [2018].

Figure 1. MinXSS-1 X123 count flux solar measurements (solid lines) with the best fit spectra overlaid (dashed lines), for temperature and emission measures derived using the OSPEX suite. The residuals are listed also (M = model, D = data, and E = uncertainty). The shaded regions indicate the uncertainties in the count flux. A 2T model with select elemental abundance fit separately (2T-AllFree). The best fit parameters with their uncertainties are listed in Table 2. and Table 3. There is a 2T model used for non-large-flaring times (QS and pre-flare) and an additional 2T model is added to compensate for the radiative enhancement during the flare-peak times. The vertical dash-dot-dot-dot lines show the high and low energy limits for the spectral fits. This is Figure 12 in Moore et al. [2018].

Table 2. Temperature and emission measure values from MinXSS-1 2T-AllFree (separate elemental abundance scale factors) spectral fits of observations from GOES A5 – M5 levels and plotted in Figure 1.The best fit abundances are listed in Table 3. The uncertainties in the fit parameters are in parenthesis. ** highlights that the pre-flare data inferred dimmer and hotter second component is near the limit of the MinXSS plasma diagnostic capabilities and thus not as well constrained. This is Table 4. in Moore et al. [2018].

Table 3. Separate abundance values are in abundance ratio units of coronal/photospheric, where the coronal values are from Feldman et al. [1992] and the photospheric values are from Caffau et al. [2011] from MinXSS-1 2T-AllFree spectral fits of observations from GOES A5 – M5 levels that are plotted in Figure 1. Elemental abundances that were fixed during fitting have a `fixed’ in parenthesis in place of an uncertainty. These values were fixed during fitting when there were not sufficient counts in the corresponding spectral feature to ascertain an abundance. The abundances of He, C, O, F, Ne, Na, Al and K were fixed at photospheric values. The best fit temperatures and emission measures are listed in Table 3. This is Table 5. in Moore et al. [2018].


  1. Moore, C. S., Caspi, A., Woods, T. N., Chamberlin, P. C., Dennis, B. C., Jones, A., Mason, J. P., Schwartz, R., Tolbert, K. A., Solar Physics, “The Instruments of the Miniature X-ray Solar Spectrometer (MinXSS) CubeSats”, Sol Phys (2018) 293: 21.
  2. Dennis, Brian R. Phillips, Kenneth J. H. Schwartz, Richard A. Tolbert, Anne K. Starr, Richard D., and Nittler, Larry R., “Solar Flare Element Abundances from the Solar Assembly for X-Rays (SAX) on MESSENGER”, 2015ApJ…803…67D
  3. Phillips, K. J. H., “The Solar Flare 3.8-10 keV X-Ray Spectrum”, ApJ, 605, 921, 2004.
  4. Schmelz, J. T. Reames, D. V. von Steiger, R., and Basu, S., “Composition of the Solar Corona, Solar Wind, and Solar Energetic Particles”, 2012ApJ…755…33S
  5. Feldman, U., “Elemental abundances in the upper solar atmosphere.”, 1992PhyS…46..202F
  6. Caffau, E., Ludwig, H.-G., Steffen, M., Freytag, B., and Bonifacio, P., “Solar Chemical Abundances Determined with a CO5BOLD 3D Model Atmosphere”, 2011SoPh..268..255C
  7. 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.