Main Objective: Determine soft x-ray spectral variability during the rising phase of Solar Cycle 25 using cross-calibrated Hinode XRT and DAXSS measurements
Scientific Justification: The Dual Aperature X-ray Solar Spectrometer (DAXSS) is designed to observe flares in the soft x-ray where hot coronal plasma is best measured. DAXSS measures between 0.5 - 15 keV with better than 0.15 keV energy resolution. Simultaneous DAXSS and Hinode XRT observations can provide a wealth of contextual data to study active regions (on timescales of hours, days and months) and solar flare plasma evolution (on timescales of minutes to hours) over the rising phase of Solar Cycle 25. The proposed observations will generate spectrally resolved soft X-ray observations over a broad spectral bandpass.
The spatially integrated DAXSS spectrum can firmly constrain the plasma temperature and elemental abundance of at least Mg, Fe, Si, and S within the field-of-view (FOV). The spectrally integrated filter imaging of Hinode XRT can be used to isolate the contribution of each active region, when multiple are present on the solar disk. The continual simultaneous observations of DAXSS and Hinode XRT from times where there are:
1. no active regions on the solar disk (common during solar minimum),
2. one active region on the disk (common during solar minimum),
3. multiple active regions on the disk (very common during near and at solar maximum)
will be used to create a quiet Sun spectral template, and an active region template. These quiet Sun (#1) and active region (#2) templates will be used to deconstruct their respective spectral contributions during solar maximum from the FOV integrated DAXSS spectrum. Thus, the combination of Hinode XRT and DAXSS will be able to create pseudo spatial soft X-ray spectra, since there are no current solar spatial spectrometers in the soft X-rays. The change in temperature and elemental abundance can yield further insight in coronal heating processes and timescales, and how these vary in the rising phase of Solar Cycle 25.
This science investigation necessitates cross-calibration between Hinode XRT and DAXSS. DAXSS had absolute radiometric calibrations performed using a lab synchrotron source (Synchrotron Ultraviolet Radiation Facility at the U.S.A. National Institute for Standards and Technology), thus the DAXSS measured spectral irradiance should be within 10% of the actual solar spectral irradiance. The Hinode XRT was not absolute radiometrically calibrated before launch and has had well-documented contamination on-orbit, so the absolute spectral responses of XRT filters are not as well constrained. By folding the DAXSS inferred spectral irradiance through the current XRT filter spectral responses, improvements of the XRT filter spectral responses can be implemented iteratively. The XRT Be-medium, Be-thick, Al-med, and Al-thick normalized spectral responses lie completely within the DAXSS spectral response (Be-thin has significant overlap), the XRT effective area can be directly constrained. Any residual discrepancies observed from this spectral irradiance comparison would result from inaccuracies in current spectral models (like Chianti) in representing the solar plasma from 0.5 - ~15 keV.