[Paper Review] Signatures of the non-Maxwellian $κ$-distributions in optically thin line spectra. II. Synthetic Fe XVII--XVIII X-ray coronal spectra and predictions for the Marshall Grazing-Incidence X-ray Spectrometer (MaGIXS)

Jaroslav Dudík, E. Dzifčáková+5 more|arXiv (Cornell University)|May 24, 2019

Solar and Space Plasma Dynamics115 references4 citations

TL;DR

This study investigates signatures of non-Maxwellian κ-distributions in synthetic X-ray spectra of Fe XVII and Fe XVIII, using the upcoming MaGIXS sounding rocket spectrometer. It demonstrates that line intensity ratios involving both ions are highly sensitive to κ (sensitivity ~30–50%), with combined use of MaGIXS and SDO/AIA 94 Å data enhancing sensitivity up to a factor of 2–3, enabling robust detection of electron energy distribution departures in solar active regions.

ABSTRACT

We investigated the possibility of diagnosing the degree of departure from the Maxwellian distribution using the Fe XVII - Fe XVIII spectra originating in plasmas in collisional ionization equilibrium, such as in the cores of solar active regions or microflares. The original collision strengths for excitation are integrated over the non-Maxwellian electron $κ$-distributions characterized by a high-energy tail. Synthetic X-ray emission line spectra were calculated for a range of temperatures and $κ$. We focus on the 6-24 A spectral range to be observed by the upcoming Marshall Grazing-Incidence X-ray Spectrometer MaGIXS. We find that many line intensity ratios are sensitive to both $T$ and $κ$. Best diagnostic options are provided if a ratio involving both Fe XVII and Fe XVIII is combined with another ratio involving lines formed within a single ion. The sensitivity of such diagnostics to $κ$ is typically a few tens of per cent. Much larger sensitivity, of about a factor of two to three, can be obtained if the Fe XVIII 93.93 A line observed by SDO/AIA is used in conjuction with the X-ray lines. We conclude that the MaGIXS instrument is well-suited for detection of departures from the Maxwellian distribution, especially in active region cores.

Motivation & Objective

• To assess the detectability of non-Maxwellian κ-distributions in optically thin coronal plasma using synthetic X-ray spectra.
• To evaluate the sensitivity of Fe XVII and Fe XVIII line intensity ratios to both temperature (T) and κ-parameter variations.
• To determine optimal diagnostic combinations for distinguishing κ-distributions from Maxwellian distributions in MaGIXS observations.
• To explore synergistic diagnostics using MaGIXS X-ray lines and the well-calibrated Fe XVIII 93.93 Å line from SDO/AIA.
• To provide observational predictions for the MaGIXS instrument to constrain coronal heating mechanisms involving impulsive, non-thermal electron populations.

Proposed method

• Synthetic X-ray emission line spectra were computed for Fe XVII and Fe XVIII in collisional ionization equilibrium across a range of temperatures (log T ≈ 6.4–6.8 K) and κ-values (2 ≤ κ ≤ 10).
• Collision strengths were directly integrated over non-Maxwellian κ-distributions using the method from Paper I, incorporating atomic data from CHIANTI v8.
• Line intensity ratios were calculated and analyzed using the ratio-ratio method to disentangle dependencies on T and κ.
• Diagnostic combinations were evaluated by plotting one line ratio against another across the T–κ parameter space to identify regions of high κ-sensitivity.
• Sensitivity was enhanced by combining MaGIXS X-ray line ratios with the Fe XVIII 93.93 Å line intensity from SDO/AIA, leveraging its high cadence and reliable calibration.
• The impact of dielectronic recombination corrections on level populations was assessed but found to be negligible compared to κ-distribution effects at active region core temperatures.

Experimental results

Research questions

• RQ1 How sensitive are Fe XVII and Fe XVIII X-ray line intensity ratios to departures from Maxwellian electron energy distributions characterized by κ-distributions?
• RQ2 Which combinations of line ratios provide the highest sensitivity to the κ-parameter while minimizing degeneracy with temperature?
• RQ3 Can the inclusion of the Fe XVIII 93.93 Å line from SDO/AIA significantly improve the diagnostic power for detecting non-Maxwellian κ-distributions in MaGIXS data?
• RQ4 To what extent do uncertainties in absolute calibration affect the detection of κ-distribution signatures in MaGIXS spectra?
• RQ5 What is the expected dynamic range of line intensity ratio variations due to κ-distributions in MaGIXS-observable spectral windows (6–24 Å) under active region conditions?

Key findings

• Line intensity ratios involving both Fe XVII and Fe XVIII lines show sensitivity to κ of approximately 30–50% across the analyzed temperature and κ range.
• The highest diagnostic sensitivity to κ is achieved when combining a Fe XVII/Fe XVIII line ratio with a second ratio formed within a single ion, minimizing degeneracy with temperature.
• When combined with the SDO/AIA 93.93 Å Fe XVIII line, the sensitivity of X-ray line intensity ratios to κ increases to a factor of 2–3 for extreme cases (e.g., κ = 2), significantly enhancing detectability.
• Photon noise dominates intensity uncertainty in MaGIXS, and absolute calibration errors are not a limiting factor due to the use of relative line ratios.
• The Fe XVIII 93.93 Å line provides a stable, high-cadence reference that can overcome potential cross-calibration challenges between MaGIXS and AIA.
• The MaGIXS instrument is well-suited for detecting non-Maxwellian κ-distributions in solar active region cores, offering direct observational constraints on impulsive, non-thermal heating mechanisms.