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Radiation magnetohydrodynamics modeling of an impulsively driven chromospheric jet in the solar atmosphere

This paper presents a numerical simulation of an impulsively driven chromospheric jet in the solar atmosphere using non-ideal magnetohydrodynamics coupled with radiation transport, revealing that while radiation has minimal impact on the jet's morphology, it plays a crucial role in governing radiative processes and dissipating the jet as it propagates through the optically thin corona.

Original authors: J. J. González-Avilés

Published 2026-05-26

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CC BY 4.0

Original authors: J. J. González-Avilés

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Solar "Firehose" Experiment

Imagine the Sun's atmosphere not as a solid surface, but as a giant, swirling ocean of hot gas (plasma) held together by invisible magnetic ropes. Sometimes, this ocean shoots up sudden, powerful bursts of gas, like a firehose spraying water into the air. In the solar world, these are called jets or spicules. They are thin, needle-like spikes of plasma that shoot up from the Sun's lower atmosphere (the chromosphere) into the upper atmosphere (the corona).

This paper is a computer simulation of one of these solar jets. The author, J. J. González-Avilés, wanted to see what happens when you "poke" the Sun's atmosphere to make a jet, but with a special twist: he included the physics of radiation (light and heat energy) in a very detailed way that previous computer models often skipped.

The Setup: The "Digital Sandbox"

Think of the computer simulation as a digital sandbox.

The Stage: The author built a virtual slice of the Sun's atmosphere. It has a "floor" (the photosphere), a "middle layer" (the chromosphere), and a "ceiling" (the corona).
The Rules: The simulation follows the laws of Magnetohydrodynamics (MHD). You can think of this as the rulebook for how electrically charged gas moves when it's tangled up with magnetic fields.
The Special Ingredient: Most simulations treat light (radiation) as a simple background effect. This paper treats light as a character in the story. It tracks two things:
- Radiation Energy: How much "heat-light" is sitting in a specific spot.
- Radiation Flux: How fast that "heat-light" is moving.
- Analogy: Imagine a crowded room. Most models just count how many people are in the room. This model counts the people and tracks how fast they are walking and where they are going, because their movement changes how the room feels.

The Action: Triggering the Jet

To start the experiment, the author didn't wait for a natural explosion. Instead, he gave the virtual Sun a gentle, localized push.

The Trigger: At a specific height in the "middle layer" of the Sun, he applied a quick burst of speed (a Gaussian pulse) to the gas.
The Result: This push created a shockwave, like dropping a stone in a pond. The shockwave traveled upward, gathering gas and shooting it into the sky, forming a collimated jet (a narrow, focused stream).

What Happened? The "Life Cycle" of the Jet

The simulation watched the jet evolve over time, and here is what it found:

The Launch: The jet shot up quickly, reaching heights of about 8 million meters (roughly the size of a large solar spicule). It moved at speeds of 30–60 km/s.
The Shape: As it rose, it developed a double-thread structure. Imagine a rope that splits into two strands as it flies up. This is a common feature seen in real solar observations.
The Cooling Effect (The Key Discovery): This is where the paper's unique "radiation" ingredient comes in.
- In the upper atmosphere (the corona), the gas is very thin (optically thin).
- Because the author tracked the radiation energy and flux, the simulation showed that the jet lost heat very efficiently to the surrounding space.
- Analogy: Imagine blowing hot breath onto a cold window. The heat escapes quickly, and the breath cools down and falls. Similarly, the jet radiated its heat away so fast that it couldn't stay hot and energetic.
The Fall: Because the jet lost its heat energy so quickly, it couldn't fight against gravity for long. It slowed down, stopped rising, and eventually dissipated (broke apart and fell back down). The radiation acted like a brake, cooling the jet and causing it to collapse.

The "Light" vs. The "Gas"

The author compared two versions of the physics:

Does light shape the jet? Not really. The radiation didn't change the shape of the jet (it still looked like a needle).
Does light control the jet's life? Yes, absolutely. The radiation acted as a cooling mechanism. It drained the jet's energy, determining how high it could go and how long it would last. Without this detailed radiation tracking, the jet might have stayed hot and risen higher than it actually did in the real Sun.

The Conclusion

The paper concludes that to understand these solar jets correctly, we cannot just look at the gas and magnetic fields. We must also treat light as a dynamic force that carries energy away.

The Takeaway: The solar jet is like a firework that runs out of fuel. In this simulation, the "fuel" was thermal energy, and the "exhaust" was radiation. By tracking the radiation carefully, the author showed that this energy loss is the main reason these jets are short-lived and don't reach the extreme heights or speeds that some simpler models might predict.

In short: The author built a super-detailed computer model of a solar jet, added a sophisticated way to track how heat-light escapes, and discovered that this "light leakage" is the key reason these solar jets cool down and disappear quickly.

Technical Summary: Radiation Magnetohydrodynamics Modeling of an Impulsively Driven Chromospheric Jet

Problem Statement
Solar jets, particularly spicules and macrospicules, are dynamic plasma ejections observed across multiple wavelengths. While magnetic reconnection and impulsive disturbances (such as pressure or velocity pulses) are widely accepted as triggering mechanisms, the role of radiative processes in the evolution of these jets remains a subject of investigation. Previous numerical studies have modeled impulsively driven jets using ideal or non-ideal magnetohydrodynamics (MHD) often coupled with empirical radiative cooling functions. However, a gap exists in simulating these jets using non-ideal MHD equations fully coupled with a system of frequency- and angle-averaged radiation transport equations. Specifically, there is a lack of simulations that independently evolve both the radiation energy density and the radiation flux components to determine their specific impact on jet morphology and dissipation in the optically thin solar corona.

Methodology
The study employs a two-dimensional numerical simulation using the PLUTO code (v4.4-patch3) to model an impulsively driven chromospheric jet. The physical framework consists of:

Governing Equations: The system solves non-ideal MHD equations coupled with a two-moment radiation hydrodynamics scheme under the gray approximation. This includes the evolution of plasma density, momentum, total energy, and magnetic field, alongside the radiation energy density ( $E_r$ ) and radiation flux ( $F_r$ ).
Radiation Transport: The M1 closure is used to close the radiation moment equations, allowing for the description of both diffusion and free-streaming limits. The radiation-matter interaction terms are derived via Lorentz transformations to the first order in $\beta = v/c$ , accounting for absorption and scattering opacities.
Solar Atmosphere Model: The initial state is a hydrostatic equilibrium based on the semi-empirical C7 model, extended to the corona. The plasma composition assumes a mean molecular weight of $\mu = 0.6$ (fully ionized hydrogen and helium).
Physical Parameters: The simulation utilizes low Planck-averaged absorption opacity ( $\kappa = 1 \times 10^{-6}$ cm $^2$ g $^{-1}$ ) and Rosseland-averaged scattering opacity ( $\sigma = 0.34$ cm $^2$ g $^{-1}$ ), characteristic of the optically thin solar corona. Thermal conduction is modeled with a flux-limited approach, and an empirical coronal heating function maintains the background atmosphere.
Perturbation: Jet formation is initiated by a localized Gaussian pulse in the vertical velocity component ( $v_y$ ) at $y = 1.75$ Mm with an amplitude of 100 km s $^{-1}$ .
Numerical Scheme: The equations are solved using a second-order TVD Runge-Kutta time integrator. The MHD equations utilize the HLLD Riemann solver, while radiation transport uses the HLLC solver. Implicit-Explicit (IMEX) Runge-Kutta schemes handle the stiff radiation-matter interaction terms, and the reduced speed of light approximation (RSLA) with $\hat{c} = 0.1c$ is employed to ensure computational tractability.

Key Results
The simulation successfully reproduces the formation and evolution of a collimated plasma structure resembling a macrospicule:

Jet Morphology and Dynamics: The impulsive pulse generates a shock wave that propels plasma upward, forming a jet reaching a height of approximately 8 Mm. The jet exhibits a maximum vertical velocity of $\sim$ 60 km s $^{-1}$ at the shock front and $\sim$ 30 km s $^{-1}$ within the jet body. The structure displays a double-thread morphology at later stages, a feature commonly associated with solar spicules.
Thermal Evolution: The jet head reaches temperatures of $\sim$ 2 $\times$ 10 $^5$ K, while the bulk of the jet stabilizes around 2 $\times$ 10 $^5$ K. The shock region initially reaches $\sim$ 9 $\times$ 10 $^5$ K before cooling.
Radiative Effects: The inclusion of self-consistent radiation transport reveals that radiation acts as an efficient cooling mechanism in the optically thin corona. The radiation energy density increases linearly, while the radiation flux decreases, consistent with optically thin radiative losses.
Dissipation Mechanism: Crucially, the radiation transport terms contribute to the dissipation of the jet. The coupling between the plasma and the radiation field induces negative flow velocities (downward motion) in the corona, causing the jet to diffuse and dissipate back into the lower atmosphere. The radiation variables do not dictate the initial shape or morphology of the jet but significantly govern its evolution and lifetime.
Background Stability: The background solar atmosphere remains globally stable against the source terms (thermal conduction, heating, and radiation) in the absence of the jet perturbation, with only minor temperature variations observed.

Significance and Claims
The paper claims that this work represents the first numerical simulation of an impulsively driven chromospheric jet where the radiation energy density and flux are evolved self-consistently through two-moment transport equations, rather than relying on empirical cooling functions. The primary significance lies in demonstrating that:

Radiative Cooling is Critical: Even in the optically thin regime, where opacities are low, the explicit treatment of radiation transport acts as a dominant cooling mechanism that limits the height, duration, and energy content of macrospicule-like events.
Dissipation Dynamics: The radiative terms induce downward flows that are responsible for the rapid fading and dissipation of the jet, a process consistent with previous studies using empirical loss functions but now derived from first principles within the radiation transport framework.
Methodological Advancement: The study validates the use of the M1 closure coupled with non-ideal MHD for modeling solar jets, showing that while radiation does not alter the initial jet morphology, it is essential for capturing the correct thermodynamic evolution and dissipation timescales.

The authors conclude that while the simulated jet reproduces key dynamical features of observed macrospicules (such as shock formation and double-thread structures), its specific parameters (height, velocity, temperature) fall within the lower bounds of observed ranges, suggesting the modeled event is a relatively low-energy occurrence. The work emphasizes that a self-consistent treatment of radiation transport is necessary to accurately model the dissipation of such solar atmospheric phenomena.