Chromospheric Flashes in a Solar Pore: Insights from Multi-line Spectropolarimetric Diagnostics

Solar pores are strongly magnetized regions lacking a photospheric penumbra and characterized by predominantly vertical magnetic fields. We present a multi-line study of flashes in a solar pore using high-resolution observations from the Swedish 1-m Solar Telescope in Fe~\textsc{i}6302Å, Ca~\textsc{ii}8542Å and K, and H-$β$, complemented by (E)UV data from \textit{IRIS} and \textit{SDO}/AIA. Bisector analysis and spectral inversions with \textsc{SIR} and \textsc{NICOLE} were used to infer stratifications of temperature, line-of-sight velocity, and magnetic field. Flashes, confined to one half of the pore, exhibit cooler photospheric temperatures ($ΔT \approx 400$K), stronger magnetic fields ($ΔB \approx 250$G), larger inclinations ($\sim25^{\circ}$ versus $\sim18^{\circ}$), and persistent upflows ($\sim0.5$kms$^{-1}$) compared to the quiescent pore. They are co-spatial with enhanced 3- and 5-minute power in the photosphere, while only 3-minute power persists in the chromosphere. Flashes are detected down to $\sim50%$ line depth in Ca\textsc{ii}8542Å intensity and show central chromospheric upflows ($\sim1$kms$^{-1}$) flanked by strong downflows ($\sim8$kms$^{-1}$). Temperature enhancements reach $\sim500$K at $\logτ\approx -5$ and $\sim2500$K at $\logτ\approx -6$, with a bimodal velocity distribution. Flashes correspond one-to-one with radially outward running waves near the pore boundary (5–15kms$^{-1}$). Strong Ca\textsc{ii} core emission, occasional Stokes~$V$ reversals, and H-$β$ enhancements indicate that pore flashes are confined to the lower and mid-chromosphere, with little influence on higher atmospheric layers.

💡 Research Summary

This paper presents a comprehensive multi‑line spectropolarimetric investigation of chromospheric flashes occurring inside a solar pore. Using the Swedish 1‑m Solar Telescope (SST) the authors obtained high‑resolution time series of Fe I 6302 Å, Ca II 8542 Å, Ca II K, and H‑β with a cadence of ~37 s over a 78‑minute interval. Complementary (E)UV data from IRIS (SJI 2832 Å and 1400 Å) and SDO/AIA (1600, 1700, 304, 171, 211 Å) plus HMI continuum images were co‑aligned to provide context from the photosphere through the transition region and low corona.

After standard reduction (MOMFBD) the authors corrected residual polarimetric cross‑talk in the Fe I Stokes V → Q,U channels, then performed bisector analysis on nine depth levels of Ca II 8542 Å (10 %–90 % line depth) and a single level for Fe I 6302 Å to retrieve line‑of‑sight (LOS) velocities at different atmospheric heights. Inversions were carried out with SIR (photospheric Fe I) and NICOLE (chromospheric Ca II) to obtain stratified maps of temperature, LOS velocity, magnetic field strength, and inclination. The SIR inversion used three temperature nodes, two velocity nodes and two magnetic‑field nodes; NICOLE employed six temperature nodes, five velocity nodes, and two nodes for the vertical magnetic component, with additional nodes for micro‑turbulence and horizontal field components.

The main findings are:

1. Spatial confinement – Flashes are confined to the left half of the pore (≈3 × 3 arcsec²), appearing roughly every 2–3 minutes. Thirteen of fifteen identified events occur in this half, while the opposite side shows no flashes and is dominated by umbral dots and a light bridge.

2. Photospheric signatures – In the flash‑hosting region the photosphere is cooler by ~400 K, the magnetic field is stronger by ~250 G, and the inclination is larger by ~5° (≈25° versus ≈18°). Persistent upflows of ~0.5 km s⁻¹ are present. Power‑spectral analysis shows enhanced 3‑ and 5‑minute oscillatory power in the photosphere, but only the 3‑minute component survives into the chromosphere.

3. Chromospheric dynamics – Ca II 8542 Å intensity flashes are detectable down to ~50 % of the line depth. The LOS velocity pattern is bimodal: a central upflow of ~1 km s⁻¹ surrounded by strong downflows of up to ~8 km s⁻¹ at the same optical depth (log τ≈‑5). Temperature inversions reveal heating of ~500 K at log τ≈‑5 and a dramatic rise of ~2500 K at log τ≈‑6. The velocity distribution is roughly 52 % downflows at log τ≈‑5, indicating a mixture of shock‑driven up‑ and down‑flows.

4. Running waves – Each flash coincides one‑to‑one with a radially outward running wave observed near the pore boundary, propagating at 5–15 km s⁻¹ with an amplitude of ~1 km s⁻¹. This tight correspondence suggests that the flashes are the observable manifestation of upward‑propagating magneto‑acoustic shocks generated by the running waves.

5. Higher‑atmosphere response – Strong Ca II K and 8542 Å core emission, occasional Stokes V polarity reversals, and broadband H‑β brightening confirm that the flashes are confined to the lower and mid‑chromosphere. No significant response is seen in IRIS Si IV 1400 Å or in the AIA hot channels, indicating negligible impact on the transition region or corona.

The authors conclude that even in the relatively simple magnetic topology of a pore, magneto‑acoustic waves can steepen into shocks that produce localized, intense chromospheric flashes. The flashes are accompanied by measurable changes in photospheric temperature, magnetic field strength, and inclination, and they are tightly linked to running waves at the pore edge. However, the energy does not appear to propagate efficiently into higher layers, likely due to reflection or dissipation within the chromosphere. The study highlights pores as valuable laboratories for probing wave‑shock interactions without the added complexity of penumbral structures, and it calls for future high‑resolution 3‑D MHD simulations and coordinated multi‑wavelength observations to quantify wave reflection, energy transport, and the role of resonant cavities in shaping chromospheric dynamics.