DAXSS: Dual-Zone X-ray Solar Spectrometer

• DAXSS is a solar soft X-ray spectrometer that integrates a dual-zone aperture design with advanced silicon drift detectors to achieve high spectral resolution and expanded dynamic range.
• Its calibration methodology employs both radioactive sources and synchrotron beam techniques to ensure absolute spectral accuracy, enabling reliable measurements from quiescent to flaring solar conditions.
• High-cadence DAXSS data enhances space weather nowcasting and coronal heating models by capturing rapid plasma temperature rises and elemental abundance variations during flare events.

The Dual-zone Aperture X-ray Solar Spectrometer (DAXSS) is a solar soft X-ray (SXR) spectrometer that combines high spectral resolution, expanded dynamic range, and advanced calibration methodologies for the paper of coronal physics, solar flares, and Sun–Earth interactions. Developed initially as an evolution of the Miniature X-ray Solar Spectrometer (MinXSS) CubeSat instruments, DAXSS employs a distinctive dual-aperture system, modern silicon drift detector (SDD) technology, and refined response modeling to provide high-cadence, absolute spectral measurements of the solar full-disk irradiance. The instrument addresses outstanding challenges in the measurement of SXR fluxes relevant to coronal heating, flare dynamics, and upper atmospheric coupling, and serves as a testbed for operational nowcasting tools.

1. Instrument Architecture and Dual-Zone Aperture Concept

DAXSS introduces a dual-zone aperture design that enables simultaneous SXR photon throughput control and spectral coverage across quiescent, active, and flaring solar conditions (Schwab et al., 2020). The instrument consists of:

• Primary (Large) Aperture: Equipped with a tungsten cover and a Kapton thin-film filter, allows higher photon throughput especially for energies >1.5 keV, thus maximizing sensitivity to hard SXR flux during high-activity or flare conditions.
• Secondary (Small) Aperture: A “pinhole” laser-etched in Kapton, in series with a beryllium (Be) foil filter, provides strong attenuation of lower-energy photons to prevent saturation during high-intensity events.
• Detector: An Amptek X-123 FAST SDD, with a 500 µm Si depletion depth and two-stage thermoelectric cooling, delivers a nominal energy resolution of 0.07 keV FWHM at 1 keV (a threefold improvement over previous MinXSS instruments).

This architecture increases both the dynamic range and the usable photon count rate by routing high and low flux regimes through separately optimized zones, enabling the detector to operate over a full 0.5–20 keV range with a resolving power of ≈20 at 1 keV (Schwab et al., 2020, Woods et al., 2023).

2. Calibration Methodology and Spectral Response Function

The calibration and characterization of DAXSS are based on methods originally developed for MinXSS and refined for the dual-aperture configuration (Moore et al., 2016). Major components include:

• Energy Gain and Resolution Calibration: Performed using radioactive sources such as ⁵⁵Fe and fluorescent targets excited by an Amptek Mini-X source. These provide reference emission lines (e.g., Fe Kα at 5.9 keV), permitting absolute calibration of energy binning and evaluation of the Fano-limited noise floor. The empirical FWHM is modeled as

$$\mathrm{FWHM} = 2.35 \cdot \omega \cdot \sqrt{F \cdot \frac{E_{ph}}{\omega} + N^2}$$

where $\omega$ is the average excitation potential, $F$ is the Fano factor, $E_{ph}$ is photon energy, and $N$ is systematic noise (Schwab et al., 2020).

• Spectral Response Measurement: Carried out at NIST SURF via the multienergy technique, exposing the detector to well-calibrated synchrotron SXR beams at multiple energies. The response function $R(E_{ph})$ is modeled as

$$R(E_{ph}) = \left[T_{\text{Be}}(E_{ph}) A_{\text{small}} + T_{\text{both}}(E_{ph}) (A_{\text{large}} - A_{\text{small}})\right] R_{\text{Si}}(E_{ph}) G(E_{\text{det}}, E_{ph})$$

where $T_{\text{Be}}$, $T_{\text{both}}$ are transmissions through Be and total filters, $A_{\text{small}}$ and $A_{\text{large}}$ are aperture areas, $R_{\text{Si}}$ is Si efficiency, and $G$ is the Gaussian energy redistribution kernel that incorporates the measured resolution.

• Dead-Time and Linearity Corrections: Modeled separately for the fast and slow channels. For the slow (spectroscopy) channel,

$$C_{\mathrm{model}} = C_{\mathrm{in}} \exp \left(-C_{\mathrm{in}} \tau_{ds} / 2\right)$$

ensures reliable spectral recovery up to $\sim100,000$ counts/s, supporting both quiescent and flare regimes (Schwab et al., 2020).

These procedures were benchmarked using the SURF source, providing end-to-end instrument response functions (“detector response arrays”) vital for accurate absolute solar flux retrieval (Moore et al., 2016).

3. Scientific Observations: Quiescent Corona, Flares, and Chromospheric Evaporation

DAXSS was first deployed on a NASA sounding rocket in June 2018 (Schwab et al., 2020), then as MinXSS-3/DAXSS aboard the INSPIRESat-1 CubeSat from 2022 (Woods et al., 2023). Key scientific applications include:

• Quiescent Sun and Active Regions: Two-temperature (2T) CHIANTI-based fits to the DAXSS spectrum indicate plasma components at 1.6–2.1 MK and 3.1–3.5 MK, matching independently derived DEM profiles from EUV datasets. Abundance analyses found iron (Fe) to be enhanced by ≈35% over the Feldman Standard Extended Coronal value in the quiescent corona, a finding not mirrored in Mg, Si, or S (Schwab et al., 2020). This suggests either limitations in current atomic modeling or real physical fractionation.
• Solar Flares: High-cadence (9 s) DAXSS spectra enable detailed flare analysis. Flare spectra are modeled by separating a hot component (6–7 MK for C-class, higher for larger flares), with time-dependent emission measure and abundance factors. Notably, during the impulsive phase, low-FIP elemental abundance factors for Mg, Si, Fe, and S rapidly decrease towards photospheric values, interpreted as the chromospheric evaporation signature—newly heated, photospheric-composition plasma filling coronal loops (Woods et al., 2023, Telikicherla et al., 9 Mar 2024). The correlation between the degree of abundance reduction and flare temperature or class is observed to be semi-linear.
• Occultation Experiments: DAXSS performed atmospheric occultation measurements, exploiting energy-dependent SXR absorption by Earth's atmosphere to probe lower thermospheric structure on fine vertical scales via photoionization edges of nitrogen and oxygen (Woods et al., 2023). These datasets inform models of atmospheric density and heating.

4. Spectral Modeling and Proxy-Based Irradiance Reconstruction

Analysis frameworks for DAXSS spectra utilize both empirical instrument response modeling and physically-motivated plasma spectral synthesis:

• SWM Model (Schwab Woods Mason): Implements a two-temperature, two-emission measure model, with model parameters linearly correlated to radio F10.7 flux, allowing daily SXR spectra over 0.5–10 keV to be synthesized from a single solar proxy (Schwab et al., 2022). The model has the form

$$S(E) = f_{vth}(E, T_1, EM_1) + f_{vth}(E, T_2, EM_2)$$

where $T_1 \approx 1.70$ MK (fixed for quiescent component), $T_2$ (5–6 MK, active region), and both emission measures scale with F10.7. The SWM model can be incorporated into DAXSS pipelines, enabling:

• Spectral gap filling outside calibrated energy regions
• Cross-calibration with GOES XRS and extension of spectral analysis to longer historical intervals
• Improved low-photon statistics handling via model-based smoothing (Schwab et al., 2022)
  • Plasma Model Fitting: For detailed flare and onset-phase analysis, DAXSS data are fitted with the Astrophysical Plasma Emission Code (APEC) or similar CHIANTI routines. Parameters retrieved include isothermal temperature, emission measure, and abundance factors (AFs) for low-FIP elements. Evolution of these parameters during flare precursors (“hot onset” or "HOPE" phases) reveals rapid temperature rise (10–15 MK), EM increase by an order of magnitude, and pronounced AF decline prior to the impulsive phase (Telikicherla et al., 9 Mar 2024, Telikicherla et al., 5 Sep 2025).

5. Flare Onset Diagnostics and Operational Space Weather Applications

DAXSS provides high-cadence full-disk SXR spectra that have informed both the physical understanding of flare development and the design of new nowcasting techniques:

• Hot Onset Precursor Event (HOPE) Identification: DAXSS observations reveal that during the flare onset phase (precursor), the plasma temperature increases rapidly (10–15 MK) preceding strong SXR flux rises, with an associated jump in emission measure and a reduction in low-FIP abundance factors. These early heating signatures are interpreted as resulting from initial loop-top energy input and chromospheric evaporation (Telikicherla et al., 9 Mar 2024, Telikicherla et al., 5 Sep 2025).
• Nowcasting Algorithm: Leveraging HOPE diagnostics, a running-difference method using SXR fluxes and their time derivatives computes $\Delta T$ and $\Delta EM$. Trigger conditions ($$\Delta EM > 5 \times 10^{-3} [10^{49} \text{cm}^{-3}]$$ and $\Delta T > 5$ MK) yield flare alerts with lead times of 5–15 minutes ahead of peak. Correlations between prompt $\Delta T$, $\Delta EM$, and flare magnitude improve when using the local maximum of the second derivative of EM (e.g., $R^2 = 0.70$ for well-behaved M and X-class events). Applications include earlier HF radio blackout alerts and dynamic targeting of observation campaigns (Telikicherla et al., 5 Sep 2025). The HOPE-based approach delivers systematically earlier warnings than current NOAA R3 protocols.

6. Instrumental Advances and Comparative Context

DAXSS embodies recent advances in compact solar SXR instrumentation and shares synergies with missions such as Solar Orbiter/STIX and Chandrayaan-2/XSM:

• Comparative Performance: DAXSS achieves an effective area at $>4$ keV of $\sim$600 times that of MinXSS-1, with energy resolution of 0.09 keV at 2 keV (≈2.9-fold improvement over earlier CubeSat instruments) (Woods et al., 2023). The increased signal-to-noise ratio enables resolution of previously blended lines and improved flare plasma diagnostics.
• Continuous Solar Viewing: INSPIRESat-1's dawn-dusk Sun-synchronous orbit grants DAXSS 24-hour solar coverage, a significant advantage for tracking moderate-to-large flare statistics and quiescent spectrum variations over Solar Cycle 25 (Woods et al., 2023).
• Integration with Multisatellite Context: The DAXSS results complement higher energy and imaging-focused SXR instruments (e.g., STIX, XSM), expanding coverage of lower flare classes and quiescent emission that are critical in constraining coronal heating models (Telikicherla et al., 9 Mar 2024).

7. Prospects, Open Challenges, and Future Directions

DAXSS has highlighted several unsolved questions and avenues for continued solar SXR research:

• Abundance Model Discrepancies: Observed enhancements in Fe abundance during quiet-Sun intervals challenge both DEM analysis methods and the completeness of atomic databases (especially CHIANTI for iron transitions). This suggests the need for ongoing updates to atomic data and more sophisticated multi-thermal plasma inference techniques (Schwab et al., 2020).
• Flare Precursors and Onset Phase: Systematic "hot onset" detection, as with the HOPE technique, underscores the importance of high-cadence SXR spectroscopic monitoring for space weather forecasting, with DAXSS data providing key early indicators that may be missed in broadband or lower-resolution sensors (Telikicherla et al., 5 Sep 2025).
• Atmospheric Coupling Studies: Occultation measurements present opportunities to probe fine-scale atmospheric density structure and contribute to modeling the Earth's thermospheric response to variable solar SXR/EUV inputs (Woods et al., 2023).
• Instrumental Calibrations Across Solar Cycles: Integration of DAXSS raw and model-based spectra with proxy data (F10.7) and historical irradiance reconstructions, using the SWM framework, will facilitate long-term studies of the solar activity cycle and provide context for exoplanetary atmospheric modeling and comparative solar-stellar studies (Schwab et al., 2022).

In summary, DAXSS's combination of dual-aperture flux control, high-resolution SDD technology, and rigorous calibration protocols provides an essential new capability for solar SXR spectrometry. Its results refine plasma diagnostics across natural solar variability scales and fundamentally improve flare onset detection, elemental abundance tracking, and operational space weather forecasting.