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Laboratory for Atmospheric and Space Physics

MinXSS Science Nugget 1

2016 July 1

Tom Woods and the MinXSS Science Team

The Miniature X-Ray Solar Spectrometer (MinXSS) CubeSat was deployed successfully from the International Space Station (ISS) on May 16, 2016 and its commissioning was completed on June 9, 2016. The primary goal for MinXSS is to study the solar soft X-ray (SXR) spectra to help understand coronal heating and energetic events, such as solar flares, and to further understand how the solar SXR variability impacts Earth’s upper atmosphere. Mason et al. (2016) provides an overview of the MinXSS CubeSat. The MinXSS spacecraft and instruments are healthy and are routinely observing the Sun every orbit. This MinXSS- 1 mission is expected to be 6-12 months long due to starting at the ISS altitude of about 400 km, but MinXSS-2, scheduled to launch this fall into an orbit at 500 km altitude, could have a 5-year mission. The Sun has been relatively quiet in June; nonetheless, MinXSS-1 has observed five flares with the largest being the C6.5 flare on June 11, 2016.

The MinXSS-1 mission is also verifying new space technology. One is the Amptek X123 SDD for solar SXR spectral measurements in the 0.5-30 keV range (0.04-2.5 nm) with 0.15 keV energy resolution. Comparison of the X123 spectra integrated over the GOES XRS-B 1- 8 Å band is the primary topic of this MinXSS Science Nugget. In addition, the MinXSS mission is the first flight of the Blue Canyon Technologies (BCT) attitude determination and control subsystem (ADCS) that is designed for 3-axis control of nano-satellites with 10 arc- seconds precision. Another new flight technology is the Solar Position Sensor (SPS) using the GOES-R EXIS custom electrometer; the SPS has validated the BCT ADCS solar pointing precision to be about 8 arc-seconds with an uncertainty of 2 arc-seconds.

The MinXSS solar SXR spectral irradiance measurements are important for filling the spectral gap between the extreme ultraviolet (EUV) spectral irradiance measurements by SDO EVE and the hard X-ray (HXR) spectral measurements by RHESSI. The SXR range is of particular interest for flares that are expected to have peak increases near 1 keV (1.2 nm). The rest of the discussion concerns the comparison of these early X123 measurements to the GOES X-Ray Sensor (XRS) broad band measurement.

The GOES XRS-B 1-8 Å band measurement has been the reference for classifying X-ray flare magnitude since the 1970s, and yet, the previous GOES XRS instruments have not had a full spectral calibration [R. Viereck, private communication]. For consistency of flare magnitude, the GOES XRS results have not been adjusted from the original level over the many different GOES missions. NOAA’s analysis of the GOES XRS data indicates that the XRS-B 1-8 Å band probably needs to be divided by 0.7 to be on a more accurate irradiance scale and that the XRS-A 0.5-4 Å band probably needs to be divided by 0.85. We apply these scaling factors for the following comparison with MinXSS SXR measurements.

The solar observations by MinXSS X123 can provide a spectral calibration for the GOES XRS because MinXSS instruments had pre-flight calibrations at NIST with an accuracy of about 10%. We also note that the next generation GOES-R XRS has also had pre-flight NIST calibrations, and the first GOES-R series satellite launch is planned for this fall. The X123 solar SXR spectral irradiance, which are considered preliminary until its Level 1 product is released, is integrated over the 1-8 Å range (1.55-12.4 keV) to obtain “X123-B” irradiance in units of W/m2 and then compared to GOES XRS-B irradiance values that have already been scaled by the NOAA 0.7 factor. Table 1 list these comparisons for the pre-flare and flare peak irradiances from five events. Figure 1 shows the X123 solar SXR spectra for the C6.5 flare on June 11, 2016 as an example for the data used to make Table 1.

The average ratio of X123 integrated irradiance to GOES XRS-B 1-8 Å irradiance is 1.03 with standard deviation of 0.07 for the flare peak comparison and is consistent with the comparison of the rocket X123 measurements to GOES by Caspi, Woods, & Warren (2015). However, the X123/GOES ratio for the pre-flare comparison has a downward trend for the lower irradiance values as illustrated in Figure 2. Because the data processing for the GOES XRS irradiance assumes a fixed solar spectrum, it is not too surprising to find that the ratio has a trend for the pre-flare comparison whose spectra can be described by plasma at cooler temperatures. On the other hand, the flare peak spectra, except for the B4.4 long duration event (LDE), are more consistently at the same plasma temperature of 4-6 MK and thus have similar ratio values. This result appears to confirm that the NOAA recommended 0.7 calibration factor is appropriate for C-class flares, and possibly also for larger flares if their plasma temperatures are also about 4-6 MK. We do underscore that these results are preliminary and that this comparison is with only ten SXR spectra.

This result also emphasizes the importance of processing broad band measurements with appropriate SXR spectra to obtain more accurate irradiance values. The new MinXSS solar SXR spectra can benefit the analysis of GOES XRS data and also several other broad band SXR photometers: XPS on TIMED-SEE and SORCE, ESP on SDO-EVE, SOHO-SEM, SNOE-SXP, and Proba2-LYRA.


Figure 1. Comparison of X123 solar SXR spectral irradiance to the GOES XRS-B 1-8 Å band. The X123 spectra are the pre-flare (green) and flare peak (black) spectra for the C6.5 flare on June 11, 2016. The GOES XRS-B irradiance values have already been scaled up by the NOAA recommended 0.7 calibration factor. The “X123-B” bars are for irradiance of the X123 spectra integrated over the 1- 8 Å (XRS-B) range. The irradiance numbers listed are in units of W/m2 for direct comparison to the GOES irradiance values; the X123 spectral irradiance units of photons/sec/cm2 were converted to W/m2 before doing the integration.


Figure 2. Ratio of X123 integrated irradiance to GOES XRS-B 1-8 Å irradiance for the pre-flare (green diamonds) and flare peak (black squares) spectra as listed in Table 1.

Table 1. Comparison of X123 solar SXR spectra integrated over 1-8 Å range to the GOES XRS-B band. The flare peak result is at a time when X123 data are available and is not always at the GOES flare maximum. The irradiance values are in units of W/m2. The GOES XRS-B irradiance values have already been scaled up by 0.7 calibration factor.



Caspi, A., T. N. Woods, and H. P. Warren, New observations of the solar 0.5-5 keV soft X-ray spectrum, Astrophys. J. Lett., 802, L2, 2015.

Mason, J. P., T. N. Woods, A. Caspi, P. C. Chamberlin, C. Moore, A. Jones, R. Kohnert, X. Li, S. Palo, and S. C. Solomon, Miniature X-Ray Solar Spectrometer (MinXSS) – A Science- Oriented, University 3U CubeSat, J. Spacecraft Rockets, 53, 2, 2016.

MinXSS project web site (though of course you’re already here):

The interesting video of the ISS NanoRacks deployment of the MinXSS and CADRE CubeSats is on this MinXSS home page and is also embedded below.