Magnetic Reconnection resulting from Flux Emergence: Implications for Jet Formation in the lower solar atmosphere?

We aim at investigating the formation of jet-like features in the lower solar atmosphere, e.g. chromosphere and transition region, as a result of magnetic reconnection. Magnetic reconnection as occurring at chromospheric and transition regions densities and triggered by magnetic flux emergence is studied using a 2.5D MHD code. The initial atmosphere is static and isothermal, with a temperature of 20,000 K. The initial magnetic field is uniform and vertical. Two physical environments with different magnetic field strength (25 G and 50 G) are presented. In each case, two sub-cases are discussed, where the environments have different initial mass density. In the case where we have a weaker magnetic field (25 G) and higher plasma density ($N_e=2\times 10^{11}$ cm$^{-3}$), valid for the typical quiet Sun chromosphere, a plasma jet would be observed with a temperature of 2–3 $\times 10^4$ K and a velocity as high as 40 km/s. The opposite case of a medium with a lower electron density ($N_e=2\times 10^{10}$ cm$^{-3}$), i.e. more typical for the transition region, and a stronger magnetic field of 50 G, up-flows with line-of-sight velocities as high as 90 km/s and temperatures of 6 $\times$ 10$^5$ K, i.e. upper transition region – low coronal temperatures, are produced. Only in the latter case, the low corona Fe IX 171 \AA\ shows a response in the jet which is comparable to the O V increase. The results show that magnetic reconnection can be an efficient mechanism to drive plasma outflows in the chromosphere and transition region. The model can reproduce characteristics, such as temperature and velocity for a range of jet features like a fibril, a spicule, an hot X-ray jet or a transition region jet by changing either the magnetic field strength or the electron density, i.e. where in the atmosphere the reconnection occurs.

💡 Research Summary

This paper investigates how magnetic reconnection, triggered by the emergence of new magnetic flux, can generate jet‑like structures in the lower solar atmosphere—specifically the chromosphere and transition region. Using a two‑and‑a‑half‑dimensional (2.5 D) resistive magnetohydrodynamic (MHD) model, the authors simulate four distinct initial atmospheric conditions that differ in magnetic field strength (25 G versus 50 G) and electron number density (2 × 10¹¹ cm⁻³ versus 2 × 10¹⁰ cm⁻³). The background atmosphere is static, isothermal at 20 000 K, and the magnetic field is initially uniform and vertical. Flux emergence is imposed over an 80 s interval, with the polarity and magnitude of the emerging flux chosen so that reconnection occurs at comparable heights in all cases.

The simulations reveal that the combination of magnetic field strength and plasma density controls the energetics of the reconnection‑driven outflows. In the weak‑field, high‑density case (25 G, 2 × 10¹¹ cm⁻³), the reconnection produces a relatively cool jet (2–3 × 10⁴ K) with a maximum speed of about 40 km s⁻¹—properties reminiscent of classic chromospheric spicules or fibrils. In contrast, the strong‑field, low‑density case (50 G, 2 × 10¹⁰ cm⁻³) yields a much hotter jet (≈ 6 × 10⁵ K) that reaches line‑of‑sight velocities up to 90 km s⁻¹. This jet’s temperature places it in the upper transition region to low corona, and synthetic emission in the Fe IX 171 Å line becomes comparable to that in the O V 629 Å line, indicating a significant coronal response.

Two intermediate cases (25 G with low density and 50 G with high density) produce jets of intermediate temperature and speed, demonstrating a smooth transition between the cool chromospheric regime and the hot coronal regime. The authors also compute synthetic spectral line profiles for C III 977 Å (formation temperature ≈ 8 × 10⁴ K), O V 629 Å (≈ 2.5 × 10⁵ K), and Fe IX 171 Å (≈ 8 × 10⁵ K). By placing a virtual slit at different heights—one covering the reconnection site and the other spanning the full jet—they show how Doppler shifts and line broadening evolve. In the strong‑field, low‑density runs, all three lines exhibit clear blue‑shifts and enhanced intensities, matching observations of transition‑region jets that display multi‑thermal signatures. In the weak‑field, high‑density runs, only the cooler C III line shows a noticeable shift, consistent with purely chromospheric spicules.

The paper situates its findings within the broader context of jet formation theories. Earlier models have invoked magneto‑acoustic waves, Alfvénic pulses, or reconnection in pre‑existing current sheets, but often could not simultaneously reproduce the wide range of observed temperatures (10⁴–10⁶ K) and velocities (tens to > 100 km s⁻¹). By explicitly linking flux emergence to reconnection, the present study demonstrates a unified mechanism capable of generating cool, moderate, and hot jets simply by varying ambient magnetic field strength and density. The inclusion of temperature‑dependent radiative loss functions and anisotropic thermal conduction further enhances the realism of the simulations, allowing a more faithful comparison with spectroscopic observations from instruments such as SUMER, EIS, and future high‑resolution facilities (DKIST, Solar Orbiter).

In conclusion, magnetic reconnection driven by emerging flux is shown to be an efficient engine for producing plasma outflows across the chromosphere, transition region, and low corona. The model reproduces key observational diagnostics—temperature, speed, and multi‑thermal emission—by adjusting only two physical parameters. This work provides a robust theoretical framework for interpreting a variety of solar jet phenomena and offers clear predictions that can be tested with upcoming high‑resolution spectroscopic and imaging observations.