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Energy and mass transport associated with impulsive spicular flows in solar coronal holes

By combining MHD simulations with observations, this study demonstrates that convective and turbulent motions in the solar lower atmosphere drive quasi-periodic spicule groups whose upflows and associated slow-mode waves provide the necessary mass and energy fluxes to sustain both the solar wind and the million-degree temperature of coronal holes.

Original authors: Lei Ni, Jun Lin, Tanmoy Samanta, Guanchong Cheng, Yifu Wang, Robert Erdelyi

Published 2026-02-24

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CC BY 4.0

Original authors: Lei Ni, Jun Lin, Tanmoy Samanta, Guanchong Cheng, Yifu Wang, Robert Erdelyi

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 Great Solar Mystery: Why is the Sun's Hair Hotter than its Head?

Imagine the Sun as a giant, glowing campfire. The "fire" itself (the core) is incredibly hot, about 15 million degrees. As you move up to the surface (the "skin" of the fire), it cools down to a comfortable 5,000–6,000 degrees.

But here is the weird part: If you keep going up, past the surface, into the Sun's outer atmosphere (the corona), the temperature suddenly spikes back up to 1 to 2 million degrees.

For 80 years, scientists have been scratching their heads: How does the outer layer get hotter than the surface? It's like walking away from a campfire and suddenly getting burned by a blast of heat. This is the famous "Coronal Heating Problem."

This paper by Lei Ni and his team tries to solve this mystery, specifically for the "quiet" parts of the Sun called Coronal Holes (areas where the Sun's magnetic field lines are open, like a hose spraying water into space).

The Solution: Solar "Spicules" are the Sprinklers

The researchers used super-computers to simulate the Sun and combined it with real telescope data. They found the answer lies in tiny, jet-like eruptions called Spicules.

Think of the Sun's surface as a boiling pot of water. But instead of just bubbling, imagine thousands of tiny, high-pressure garden hoses shooting up from the pot every second. These are spicules.

Here is the step-by-step story of what they found:

1. The "Boiling" Surface Creates Jets

The Sun's surface is turbulent and churning (convection). This churning creates two things:

Shockwaves: Like the "boom" when a supersonic jet breaks the sound barrier.
Magnetic Reconnection: Imagine two tangled rubber bands snapping apart and re-tying themselves, releasing a burst of energy.

These two forces combine to launch Spicules. These are jets of gas shooting up at speeds of over 60 km/s (that's 134,000 mph!). They happen in groups, roughly every 5 minutes.

2. The "Fountain" Effect (Mass Transport)

Most of the gas in these jets goes up, slows down, and falls back down like water from a fountain. However, the researchers found that a small fraction of this gas is so fast and energetic that it doesn't fall back. It escapes into space.

Why it matters: This escaping gas is the Solar Wind. It's the stream of particles that constantly blows past Earth. The study confirms that these spicules provide enough "fuel" (mass) to keep the solar wind flowing.

3. The "Boiling Water" Effect (Heating the Corona)

This is the most important part. When these jets shoot up into the thin, hot atmosphere (the corona), they don't just stop. They act like a piston in an engine.

The Analogy: Imagine you are pumping a bicycle tire. As you push the air in, the pump gets hot.
The Science: The spicule jets push against the gas in the corona, creating compression. This compression creates shockwaves (sound waves that have become so intense they are like sonic booms).

These shockwaves travel through the corona and dissipate their energy as heat.

The Result: This process heats the corona to the required 1 million degrees. It's not the jet itself that is hot; it's the friction and compression caused by the jet hitting the upper atmosphere that does the heating.

4. The "Ghostly Ripples" (Propagating Disturbances)

When scientists look at the Sun with telescopes, they see bright, slanted lines moving upward. These are called Propagating Disturbances (PDs).

For a long time, scientists argued: Are these lines just gas moving up, or are they sound waves?

The Paper's Verdict: It's both.
- The bright lines are partly the actual gas from the spicules shooting up.
- But they are also the "ripples" (slow-mode waves and shocks) created by the spicules hitting the corona.
- Think of it like throwing a stone into a pond. You see the splash (the spicule) and the ripples spreading out (the waves). The telescopes see both happening at the same time.

The Big Picture: Why This Matters

Solving the Heat Puzzle: This study shows that the corona isn't heated by a single giant heater, but by millions of tiny, continuous "pistons" (spicules) compressing the gas, turning motion into heat.
Understanding Space Weather: Since these spicules feed the solar wind, understanding them helps us predict how the Sun will affect Earth's satellites and power grids.
Universal Application: The authors note that other stars (like our Sun's cousins) likely have similar "fountains" and "ripples," meaning this mechanism might heat the atmospheres of stars across the entire universe.

Summary in One Sentence

The Sun's outer atmosphere is heated because the churning surface shoots up millions of tiny, high-speed gas jets that crash into the upper air, creating shockwaves that compress and heat the gas, much like a bicycle pump gets hot when you use it.

1. Problem Statement

The paper addresses two fundamental, long-standing puzzles in solar physics:

Coronal Heating: How the solar atmosphere is heated from $\sim5,000–6,000$ K in the lower atmosphere to $\sim1–2$ MK in the corona.
Solar Wind Origin: How the fast solar wind is sustained in coronal holes (regions with open magnetic field lines).

Specifically, the authors investigate the role of solar spicules (jet-like features in the chromosphere) and propagating disturbances (PDs) observed in the corona. While it is known that slow-mode waves generated at the surface are typically damped before reaching the corona, the mechanism for heating coronal holes and the specific contribution of spicules to mass and energy transport remain unclear. The study aims to determine if spicule upflows can trigger new waves in the corona that sustain heating and wind.

2. Methodology

The study employs a multi-faceted approach combining high-resolution observations with advanced numerical simulations:

Observational Data:
- Instruments: Coordinated observations from the Interface Region Imaging Spectrograph (IRIS) (2796 Å, Mg II k) and the Solar Dynamics Observatory (SDO/AIA) (171 Å and 193 Å).
- Target: A northern coronal hole region observed on May 17–18, 2020.
- Technique: Time-distance analysis using running difference (RD) images to track spicules and PDs. Slit-jaw images were used to correlate chromospheric spicule evolution with coronal disturbances.
Numerical Simulations:
- Code: A modified version of the open-source NIRVANA code, incorporating partial ionization effects (hydrogen and helium) and updated radiative cooling models for the convection zone, photosphere, chromosphere, and corona.
- Dimensions: Primarily 2.5D (2.5-dimensional) MHD simulations, with a supplementary 3D run.
- Domain: Extends from the upper convection zone ($-6$ Mm) to the lower corona ($22$ Mm).
- Resolution: High-resolution runs with grid sizes of $6.7 \times 12.2$ km ( $1792 \times 2048$ grids).
- Initial Conditions: Based on the C7 model and Standard Solar Model. Simulations were run with initial magnetic field strengths of $B_0 = 5$ G and $20$ G.
- Boundary Conditions: Inflow at the bottom (to sustain convection) and outflow with strong magnetic diffusion at the top to prevent wave reflection.

3. Key Contributions

This paper provides a unified physical scenario linking surface convection, spicule formation, and coronal heating, challenging the traditional view that slow-mode waves cannot reach the corona. Key contributions include:

Identification of a Dual-Trigger Mechanism: Demonstrating that spicules are formed by a combination of shock compression and small-scale magnetic reconnection driven by surface convection.
In-Situ Wave Generation: Showing that spicule upflows do not just carry energy upward but actively trigger new slow-mode waves and shocks directly within the low corona, bypassing the damping issues associated with waves originating solely from the surface.
Synthesis of PDs: Clarifying that Propagating Disturbances (PDs) observed in EUV are a composite of spicule upflows (mass transport) and slow-mode waves/shocks (energy transport).
Quantification of Fluxes: Providing specific values for mass and energy fluxes required to sustain the solar wind and coronal temperature in coronal holes.

4. Key Results

A. Spicule Formation and Dynamics

Mechanism: Convective and turbulent motions generate shocks and small-scale reconnection events. The resulting pressure gradients (dominant) and Lorentz forces accelerate plasma upward.
Characteristics: The simulations produce groups of spicules with a quasi-period of $\sim300$ s and widths of $\sim200–500$ km.
Velocity: Upflow velocities reach $>60$ km s $^{-1}$ (up to $100$ km s $^{-1}$ ), consistent with "Type II" spicules observed in H $\alpha$ and Ca II.
Mass Transport: While most spicular material falls back, a small fraction reaches the corona. The average mass flux in the lower corona is $>10^{-9}$ kg m $^{-2}$ s $^{-1}$ , sufficient to sustain the fast solar wind.

B. Energy Transport and Heating

Wave Generation: As spicule upflows penetrate the corona, they compress the plasma, triggering local slow-mode shocks and waves.
Energy Flux: These locally generated slow-mode waves carry an energy flux of $10–100$ W m $^{-2}$ in the lower corona.
Heating Mechanisms:
- Joule Heating: Dominant in the lower atmosphere but negligible in the corona due to the unipolar magnetic field structure (lack of current sheets).
- Compression & Conduction: In the corona, compression heating (from shocks) and heat conduction are the primary mechanisms dissipating wave energy.
- Result: This dissipation sustains a coronal temperature of $\sim1$ MK, balancing radiative and conductive losses.

C. Propagating Disturbances (PDs)

Correlation: The onset of PDs in AIA 171/193 Å channels coincides perfectly with the rise of spicules in IRIS 2796 Å.
Nature of PDs: The enhanced EUV emission in PDs is caused by:
1. Increased Density: Due to the upflowing plasma.
2. Increased Temperature: Due to shock compression and wave dissipation.
Speed: Simulated PDs travel at $\sim100$ km s $^{-1}$ , matching observational data ( $100–150$ km s $^{-1}$ ).

D. Resolution and 3D Verification

Higher resolution simulations confirm that spicule widths can be as narrow as $\sim120$ km and lengths $>10$ Mm, but the fundamental conclusions regarding fluxes and heating mechanisms remain robust.
Preliminary 3D simulations confirm that significant heating still occurs above spicules, suggesting the 2.5D results are qualitatively valid.

5. Significance

Solving the Heating Puzzle: The study proposes that the "missing link" for coronal heating in coronal holes is not just waves traveling from the surface, but waves generated in situ by spicule upflows. This resolves the issue of surface-generated slow-mode waves being damped before reaching the corona.
Solar Wind Origin: It quantifies the mass flux contribution of spicules, confirming they provide a sufficient supply of material to sustain the fast solar wind.
Unified View of PDs: It clarifies that PDs are not purely waves or purely flows, but a coupled phenomenon involving both mass ejection and wave generation.
Stellar Implications: The authors suggest this mechanism (convection $\to$ spicules $\to$ coronal waves/heating) likely applies to other Sun-like stars (G, K, M types) with convection zones, offering a universal explanation for stellar coronal heating and wind generation.

In conclusion, this work bridges the gap between chromospheric dynamics and coronal thermodynamics, demonstrating that impulsive spicular flows are a primary driver of both mass and energy transport in coronal holes.