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Laboratory for Atmospheric and Space Physics

MinXSS Science Nugget 5

MinXSS Dynamic Range and Solar SXR Spectral Variations

2017 June 16

Chris Moore and the MinXSS Science Team

The Miniature X-ray Solar Spectrometer (MinXSS) CubeSat soft X-ray (SXR) spectra measured by the X-ray spectrometer (X123) SXR spectrometer is ideal for analyzing plasma properties above 1 MK. MinXSS-1 X123 can conduct solar spectral measurements from 0.8 – ~12 keV with an effective spectral resolution of roughly ~0.24 keV near 5.9 keV. These spectral measurements display important variations in the solar flux from quiescent conditions with minimal active regions, to active region dominated emission, to flares. The MinXSS 10 second integration periods are suitable to explore dynamic changes in the solar spectrum. The MinXSS X123 and X-ray Photometer (XP) have the dynamic range to conduct high quality measurements for over three orders of magnitude in the solar SXR energy flux for photon energies > 1 keV. Traditionally, the integrated SXR measurements from ~1.55 – 12.4 keV (0.1 – 0.8 nm) by the Geostationary Operational Environmental Satellites (GOES) X-ray Sensor (XRS) have been the standard for estimating the solar soft X-ray variability. The GOES energy flux levels (W m-2) across the 1.55 – 12.4 keV band have been labeled as A (10-8 W m-2), B (10-7 W m-2), C (10-6 W m-2), M (10-5 W m-2), and X (>10-4 W m-2). Figure 1. displays MinXSS-1 X123 and XP measurements over three orders of magnitude in GOES levels A5 (5 x 10-8 W m-2),  – M5 (5 x 10-5 W m-2) and is also in Figure 9 of Moore et al. [2018].

The MinXSS-1 measurements exemplify the spectral shape and variability over the solar flux levels. The SXR spectral shape at the X123 spectral resolution is dominated by free-free (thermal bremsstrahlung), but also consists of free-bound (edges) and bound-bound (spectral line groups) features. MinXSS-1 spectral measurements can be used to estimate the photon flux incident on the X123 and XP aperture and is displayed in the right panel of Figure 1. The photon flux from 1 keV to higher energies sharply decreases and has unique ‘humps’ due to spectral emission line groups from ions 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. If there are statistically significant counts at each of these spectral features, spectral models that include elemental abundances as variables can be used to infer the elemental composition. Abundance variations derived for these measurements are discussed in more detail in Moore et al. [2018] and the MinXSS Science Nugget 007. Finally, both X123 and XP respond near-linearly to the various solar flux (table in the left panel of Figure 1).

Figure 1. MinXSS-1 X123 solar measurements (solid lines) from GOES A5 to M5 levels (~5 x 10-8 – 5 x 10-5 W m-2). The left plot is the mean count flux and the right plot is the estimated photon flux. The uncertainties are depicted as the shaded region around the measurements. This demonstrates the dynamic range of the MinXSS-1 spectrometer, and the variation in spectral features for increasing solar flux levels. The `bumps’ in the spectrum are due to groups of dominant emission lines from ionized 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. These features can be used as elemental abundance probes to assess deviations from the traditional `Coronal’ abundance values during various solar conditions. This is Figure 9 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.