IMPROVING SOLAR SOFT X-RAY (SXR) IRRADIANCE RESULTS FROM!
BROADBAND PHOTOMETERS WITH NEW SXR SPECTRAL MEASUREMENTS FROM A CUBESAT !
THOMAS WOODS, AMIR CASPI, PHIL CHAMBERLIN, LEONID DIDKOVSKY, FRANK EPARVIER, ANDREW JONES, JAMES MASON, CHRIS MOORE, STAN SOLOMON, & RODNEY VIERECK!
UNIVERSITY OF COLORADO AT BOULDER -" LABORATORY FOR ATMOSPHERIC AND SPACE PHYSICS!
• MinXSS CubeSat observes the solar soft X-ray (SXR) spectrum from 0.5 keV to 30 keV (0.4-25Å)
with resolution of ~0.15 keV.
• Solar SXRs are particularly important for flare studies because the greatest intensity enhancements
during solar flares are expected to occur near 20 Å (Rodgers et al., 2006; see Figure 1).
• Additionally, the distribution of energy in the SXRs plays an important role in creating Earth’s
ionosphere and heating Earth’s thermosphere (Figure 2).
• MinXSS is a 3U (34.5 cm x 10 cm x 10 cm), 3.5 kg CubeSat launched on Dec. 6, 2015 to the ISS
and then was deployed on May 16, 2016 into its low-Earth orbit. It has been in normal operations
since June 9, 2016.
MINXSS CUBESAT MISSION OVERVIEW AND MOTIVATION
The MinXSS ground station is on the roof
of LASP. Communication is UHF at 437
MHz with 9600 baud downlink rate.
MinXSS-1 started at 405 km altitude after
being deployed from the ISS. MinXSS-1
re-entry is expected in March 2017.
MISSION OPS & DURATION
model and block
diagram of MinXSS.
Flight Model 1
MINXSS CUBESAT SYSTEMS
MinXSS and CADRE CubeSats are deployed from
NanoRacks dispenser on ISS on May 16, 2016.
There are four decades of broadband soft X-ray (SXR) measurements, but these measurements cannot
directly quantify the varying contributions of emission lines (bound-bound) amongst the thermal
radiative recombination (free-bound) and thermal and non-thermal bremsstrahlung (free-free)
continua. The Miniature X-ray Solar Spectrometer (MinXSS) CubeSat mission, that was deployed into
orbit in May 2016, was designed to directly measure the SXR spectra to improve the understanding of
flare energetics and for studying the SXR radiation impacts in Earth’s ionosphere. The broadband SXR
measurements include the two bands of 1.6-25 keV (0.05-0.8 nm) by the GOES X-Ray Sensor (XRS)
since the 1970s and the even broader band of 0.2-12 keV (0.1-7 nm) from several missions, including
the Yohkoh Soft X-ray Telescope (SXT, 1991-2001), Student Nitric Oxide Experiment (SNOE,
1998-2002), Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED, 2002-
present), the Solar Radiation and Climate Experiment (SORCE, 2003-present), and the Solar
Dynamics Observatory (SDO, 2010-present). These broadband SXR measurements have been helpful
for resolving some differences between ionosphere models and measurements, but differences remain
in understanding solar SXR spectral distribution and atmospheric photoelectron flux. The lack of
spectral resolution in the SXR range is thought to be the culprit for most of these disagreements and is
thus an underlying motivation for the MinXSS CubeSat mission. The new solar SXR spectra in the
range of 0.5 to 30 keV (0.04-2.5 nm) from MinXSS, along with how they can improve the accuracy of
the broadband SXR photometer measurements, will be presented.
The Amptek X123 is a commercial off-the-shelf (COTS)
X-ray spectrometer that uses a silicon drift detector and
includes a thermoelectric cooler, beryllium filter, and
support electronics. Its small size (180 g) and low power
(2.5 W) make it ideal for a CubeSat mission. The X123
on MinXSS enables new scientific studies of the solar
soft X-ray (SXR) variability and its influence on Earth.
The Attitude Determination and Control System (ADCS)
from Blue Canyon Technologies (BCT) is game-changer
technology for CubeSats in that a 0.5-Unit ADCS is
providing 10 arc-sec pointing on the Sun.
TECHNOLOGY ENABLING SCIENCE
[Rodgers et al. 2006]
Figure 1. Example solar spectrum during a flare.
Active region spectrum
Figure 2. Top: Two model solar spectra scaled to have identical
0.1-7 nm integrated flux.
Bottom: The deposition of the same amount of solar energy into
Earth’s atmosphere is very different for the two flux distributions.
The ~5 km differences in peaks is compariable to the 5.8 km scale
height in the mesopause.
1 2 Energy (keV) 10
Model (1) + (2)
Model (1) + (2)
1 2 Energy (keV) 10
24.8 12.4 6.2 Wavelength (Å) 1.24
0.5 1.0 2.0 Energy (keV) 10
GOES XRS-B (W/m )
Figure 1: Rodgers et al., J. Geophys. Res., 111, A10S13, 2006.
Figure 2 & MinXSS mission overview: Mason et al.,
J. Spacecraft Rockets, 53, 328, 2016.
Figures 3-6: Woods et al., Ap. J., in review, 2016.
CHIANTI version 8.0 model: Del Zanna et al.,
A&A, 582, A56, 2015.
Figure 4. Example MinXSS measurement for M5.0 flare
(A) Pre-flare (green) and flare (black) spectral irradiance
(B) The MinXSS observations (blue diamonds) track well
with the scaled GOES XRS data (red). The dotted and
dashed vertical lines indicate the time range for the pre-
flare and flare spectra shown in Figure 3, respectively.
Figure 5. Temperature and abundance results for the
M5.0 flare on DOY 2016/205.
(A) Temperature results: GOES XRS (red, 1-temperature
only) and MinXSS X123 results for two temperatures
(green and blue).
(B) Abundance factors derived from X123 spectra indicate
a significant chromospheric contribution during the flare.
Figure 3. Model fits to MinXSS spectra on 2016/205.
(A) Pre-flare spectrum has a warm coronal plasma of
2-4 MK with coronal abundance factor (2.138).
(B) M5.0 flare spectrum has a brighter warm coronal
contribution plus a hot contribution near 15 MK and
with lower abundance (1.168) similar to photosphere.
(C) Emission measure (EM) for the model fits.
Figure 6. Comparison of GOES XRS-B with X123 data
over the MinXSS mission.
(A) MinXSS X123 spectra integrated over the 1-8 Å band
(green) is compared to the GOES XRS-B band (1-8 Å).
(B) The ratio of this X123 integrated band to XRS is
plotted as function of XRS intensity. The red line at 1.43
is the NOAA previous irradiance conversion factor for
XRS-B. The green line shows the suggested new
calibration for XRS [LI is log10(GOES XRS-B)].
(C) The temperature estimated as a function of GOES
XRS-B intensity. The black diamonds are the hot
temperature component for the MinXSS SXR spectra, and
the red dots are the temperature derived from the GOES
XRS A & B two bands.
Current broadband SXR measurements include those from GOES XRS, TIMED SEE, SORCE XPS, SOHO SEM, and SDO ESP.
The MinXSS solar SXR spectral measurements have several advantages over the broadband (BB) photometer measurements:
• SXR spectra provide multiple temperature diagnostics of the coronal plasma versus single temperature from BB (see Fig. 3 & 5)
• SXR spectra provides composition (abundance) information related to coronal heating processes versus none from BB (see Fig. 3 & 5)
• MinXSS instruments have been calibrated at NIST and thus can provide calibration for the BB instruments (see Fig. 6, 7, & 8)
ADVANTAGES OF SOFT X-RAY (SXR) SPECTRAL MEASUREMENTS
Reference CHIANTI spectra are used to fit two
temperature components with first hotter component fit to
the higher energy SXR range and second cooler
component fit to the residual from the measurement and
the hotter component model.
The abundance factor, which is relative to the
photospheric abundance values, is fit using the Fe XXV
emission at 6.7 keV. For these CHIANTI reference
spectra, an abundance factor of 1.00 and 2.138 are for
photospheric and coronal composition, respectively.
TWO-TEMPERATURE & ABUNDANCE MODEL FITS TO SXR SPECTRA
Figure 7. Comparison of TIMED SEE
XPS with MinXSS model for 0-7 nm range.
Mean of Model/SEE ratio is 0.75.
SEE XPS Level 4 spectra have lower
irradiance at 6.5 nm than MinXSS model,
but much higher irradiance than MinXSS
model at other wavelengths.
The largest difference is for the 0-1 nm band
with SEE being a factor of 4 higher than
Figure 8. Comparison of SDO ESP with
MinXSS model for 0-7 nm range.
Mean of Model/ESP ratio is 0.36.
Because the ESP data processing uses a
single reference spectrum, the model/ESP
ratio is highly sensitive to solar activity.
There are plans to update the SEE XPS and
SDO ESP data processing algorithms to use
the new MinXSS-derived model as a set of
reference spectra that can be scaled to match
the XPS and ESP signal levels.
SH13A-2285 HTTP://LASP.COLORADO.EDU/HOME/MINXSS/ !