Far-IR and radio thermal continua in solar flares

With the invention of new far-infrared (FIR) and radio mm and sub-mm instruments (DESIR on SMESE satellite, ESO-ALMA), there is a growing interest in observations and analysis of solar flares in this so far unexplored wavelength region. Two principal radiation mechanisms play a role: the synchrotron emission due to accelerated particle beams moving in the magnetic field and the thermal emission due to the energy deposit in the lower atmospheric layers. In this contribution we explore the time-dependent effects of beams on thermal FIR and radio continua. We show how and where these continua are formed in the presence of time dependent beam heating and non-thermal excitation/ionisation of the chromospheric hydrogen plasma.

💡 Research Summary

Solar flares are fundamentally driven by the rapid release of magnetic energy, which accelerates electrons and ions into beams that travel along magnetic field lines and deposit their energy in the lower solar atmosphere. Historically, flare diagnostics have focused on high‑energy emissions (hard X‑rays, γ‑rays) and chromospheric lines (H α, UV), leaving the far‑infrared (FIR) and millimetre/sub‑millimetre (mm/sub‑mm) regimes relatively unexplored. The advent of new instrumentation—DESIR on the SMESE satellite and the ESO‑ALMA array—has opened the possibility of probing flare physics in this wavelength domain. In this context, the paper “Far‑IR and radio thermal continua in solar flares” investigates the time‑dependent response of the thermal FIR and radio continua to electron/ion beam heating, emphasizing the role of non‑thermal excitation and ionisation of hydrogen in the chromosphere.

The authors employ a one‑dimensional radiative‑hydrodynamic model of a hydrogen‑dominated plasma that couples the energy deposition of a prescribed particle beam with the full set of non‑LTE rate equations for hydrogen level populations. Beam fluxes are varied between 10¹⁰ and 10¹² cm⁻² s⁻¹, and the model tracks the evolution of temperature, electron density, and level populations from the photosphere up through the transition region (≈1500–2500 km). The heating term includes collisional losses of both electrons and ions, while the radiative transfer module solves for free‑free, free‑bound, and bound‑bound emission in the FIR (30–300 µm) and mm/sub‑mm (0.3–3 mm) bands.

Key results show that beam heating produces a rapid temperature rise in the upper chromosphere and lower transition region, reaching 10⁴ K within 1–2 s of beam onset. This temperature spike dramatically increases the population of the n = 2 and n = 3 hydrogen levels, leading to a strong enhancement of the thermal continuum. The FIR and mm/sub‑mm continua respond on timescales of a few seconds, attaining peak brightness shortly after the beam begins and persisting for tens of seconds after the beam ceases, owing to the relatively long cooling times of the heated plasma. In contrast, synchrotron (gyro‑synchrotron) emission from the same beam typically peaks earlier and decays more rapidly, providing a clear temporal signature to separate the two mechanisms observationally.

A central insight of the study is the importance of non‑thermal excitation (direct collisional excitation of hydrogen by the beam) in addition to pure thermal heating. The authors quantify non‑thermal excitation rates and demonstrate that, especially for lower beam fluxes (≈10¹⁰ cm⁻² s⁻¹), non‑thermal processes dominate the level population changes, producing a spectral index (α) in the FIR continuum of –2 to –3, markedly steeper than the α ≈ –1 expected from pure thermal free‑free emission. For higher fluxes, thermal heating becomes dominant, but even then non‑thermal excitation contributes significantly to the early rise of the continuum.

The paper also discusses observational implications. DESIR’s sensitivity in the 30–300 µm range and ALMA’s high spatial resolution (≈0.1″) and broad frequency coverage (84–950 GHz) are sufficient to resolve the predicted few‑second to tens‑second variations in the thermal continuum. ALMA’s capability to map the spatial distribution of the continuum across a flare footpoint can directly test the model’s prediction that the FIR/mm emission originates primarily from heights of 1500–2500 km, where beam heating is strongest. Moreover, simultaneous observations with hard X‑ray instruments (e.g., RHESSI, STIX) would allow a direct comparison between the timing of non‑thermal hard X‑ray bursts and the rise of the FIR/mm thermal continuum, providing a stringent test of the non‑thermal excitation hypothesis.

In conclusion, the study establishes the FIR and mm/sub‑mm thermal continua as powerful diagnostics of flare energy transport. By quantifying the relative contributions of thermal heating and non‑thermal excitation, identifying the formation heights, and characterising the temporal evolution, the authors provide a robust theoretical framework that can be directly applied to forthcoming observations with DESIR, ALMA, and future FIR space missions. Their work underscores the necessity of incorporating non‑LTE hydrogen physics in flare models and paves the way for a new era of multi‑wavelength flare diagnostics that bridge the gap between high‑energy particle signatures and the thermal response of the lower solar atmosphere.