🔭 astrophysics

On the Origin of Coronal Picoflare Jets

Using ultra-high-resolution observations from Solar Orbiter combined with multi-instrument data and radiative-MHD simulations, this study identifies the physical origin of coronal picoflare jets as coupled bright-dark structures resulting from flux emergence and magnetic reconnection at low atmospheric heights.

Original authors: Annu Bura, Daniel Nóbrega-Siverio, Tanmoy Samanta, Jayant Joshi

Published 2026-02-23

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CC BY 4.0

Original authors: Annu Bura, Daniel Nóbrega-Siverio, Tanmoy Samanta, Jayant Joshi

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

Imagine the Sun not as a static, burning ball, but as a bustling, chaotic city where tiny, invisible fireworks are constantly going off. For decades, astronomers have known about massive solar "storms" and "jets" that shoot hot plasma into space, helping to heat the Sun's outer atmosphere and push the solar wind. But recently, we've started noticing something even smaller: tiny, fleeting eruptions called picoflare jets.

Think of these as the "fireflies" of the solar world. They are so small and fast that until now, we couldn't really see how they worked. This paper is like finally putting on a pair of super-powered glasses to watch these fireflies in action.

Here is the story of what the scientists found, explained simply:

1. The Mystery: Two Sides to Every Coin

When the scientists looked at the Sun's poles using the Solar Orbiter (a high-tech spacecraft flying closer to the Sun than any other human-made object), they saw these tiny jets. But they noticed something strange: these weren't just single beams of light.

Every time a bright, hot jet shot upward, it was accompanied by a dark, shadowy stream right next to it.

The Analogy: Imagine a fountain shooting a bright, glowing stream of water into the air. Right next to it, a dark, cool stream of water is shooting up in the opposite direction, almost like a mirror image.
The Puzzle: For a long time, we could only see the bright, hot part. The dark part was too faint to see against the bright background of the Sun. But because the Solar Orbiter was looking at the edge of the Sun (the "limb") with incredibly sharp eyes, it could see both the bright fire and the dark smoke.

2. The Investigation: Measuring the Tiny Fireworks

The team analyzed 11 of these tiny events. They measured how wide they were, how long they lasted, and how fast they moved.

Size: They are tiny, only about the width of a few cities on Earth, but they shoot up thousands of kilometers high.
Speed: The bright part zooms up fast (like a race car), while the dark part moves more slowly (like a slow-moving truck).
Energy: Even though they are small, they pack a punch. They release enough energy to be classified as "picoflares"—tiny explosions that, if you added them all up, could be the secret sauce that keeps the Sun's outer atmosphere hot.

3. The Simulation: A Digital Twin

To understand why these jets happen, the scientists used a supercomputer to run a simulation (a digital twin of the Sun's atmosphere). They simulated what happens when magnetic fields (invisible lines of force) pop up from the Sun's surface like a new spring uncoiling.

The Result: The computer simulation produced a perfect match!

When magnetic fields emerge and reconnect (snap and twist together), they create a bright, hot spire (the fire) and a dark, cool surge (the smoke) shooting up side-by-side.
This confirmed that the dark structures the scientists saw weren't just random shadows; they were real, cool chunks of solar material being pushed up alongside the hot plasma.

4. The "Dark" Secret: What is the Shadow?

The scientists then teamed up with another telescope (IRIS) that looks at the Sun in different colors of light. They found that the "dark" part of the jet is actually a cool chromospheric surge.

The Metaphor: Think of the bright jet as a hot air balloon rising. The dark jet is like a heavy, cool balloon rising right next to it. They are tied to the same magnetic "string" but are made of different materials. The dark one is cooler and denser, which is why it blocks the light from behind it, looking like a shadow.

5. Why Does This Matter?

This discovery is a big deal for two reasons:

Solving the Heating Mystery: The Sun's outer atmosphere is millions of degrees hotter than its surface, which doesn't make sense. Scientists think these tiny "picoflare" jets are like millions of tiny heaters turning on every second, keeping the atmosphere hot.
Understanding the Solar Wind: These jets might be the source of the solar wind—the stream of particles that flows past Earth and can mess with our satellites and power grids.

The Bottom Line

This paper tells us that the Sun's atmosphere is a place of constant, tiny, paired eruptions. Just like a firework that shoots up a bright star and a trail of smoke, these solar jets launch hot plasma and cool material together. By seeing both sides of the story, scientists now have a much clearer picture of how the Sun breathes, heats up, and sends energy out into the solar system.

In short: We finally caught the Sun's tiny fireflies, and we learned that every time they light up, they cast a shadow, and that shadow is just as important as the light.

1. Problem Statement

Small-scale jet-like eruptions, specifically picoflare jets and jetlets, are hypothesized to be significant contributors to coronal heating and solar wind acceleration. However, their physical origin remains elusive. While larger coronal jets are known to exhibit coupled bright (hot) and dark (cool) components driven by magnetic reconnection, it is unclear if these coupled structures exist at the picoflare scale. Previous studies of picoflare jets often failed to detect associated dark structures, potentially due to observational limitations (e.g., on-disk background brightness) or the small spatial scales involved. The primary challenge is to resolve the dynamics of these tiny eruptions to determine if they share the same flux-emergence/reconnection mechanism as larger jets.

2. Methodology

The authors employed a multi-instrument observational approach combined with numerical simulations:

Observational Data:
- Solar Orbiter (EUI): Utilized the High Resolution Imager in the Extreme Ultraviolet (HRIEUV) channel (174 Å, sensitive to ~1 MK plasma). Five datasets were analyzed, focusing on off-limb observations of the Sun's polar regions to minimize background noise.
  - Datasets D1–D4: Captured 10 distinct jets with ultra-high spatial resolution (pixel scales ranging from ~0.105 Mm to ~0.210 Mm) and high temporal cadence (3–20 s).
- Solar Dynamics Observatory (SDO) & IRIS: Dataset D5 provided coordinated observations from the Atmospheric Imaging Assembly (AIA 171 Å) and the Interface Region Imaging Spectrograph (IRIS Mg II k line). This allowed for the identification of chromospheric components associated with the EUV jets.
Data Analysis:
- Morphology: Visual inspection and manual tracking of bright spires and dark, collimated structures.
- Quantification: Gaussian fitting was used to estimate the Full Width at Half Maximum (FWHM) of bright and dark components. Cubic spline interpolation determined jet lengths. Space-time diagrams constructed from artificial slits were used to calculate propagation speeds.
- Energetics: Kinetic energy ( $E_k$ ) was estimated using the formula $E_k = \frac{1}{2} n_e m_p v^2 V$ , assuming a coronal hole electron density ( $n_e$ ) of $2 \times 10^8 \text{ cm}^{-3}$ .
Numerical Simulation:
- Used the Bifrost code (2.5D radiative-MHD simulation) to model magnetic flux emergence in a coronal hole. The simulation generated synthetic 174 Å emission, temperature maps, and density maps to compare with observed jet morphologies and dynamics.

3. Key Results

A. Morphological and Dynamical Properties

Coupled Structures: The study identified that the majority of observed tiny jets exhibit a distinct bright spire accompanied by a narrow, collimated dark structure.
Dimensions:
- Length: 7.85 – 29.74 Mm (Mean: 16.90 Mm).
- Width: Bright spires (0.35 – 1.75 Mm); Dark structures (0.23 – 0.83 Mm).
- Lifetime: 3.4 – 11.5 minutes (Mean: ~6.2 minutes).
Kinematics:
- Bright Spire Speed: 43 – 132 km s $^{-1}$ (Mean: 85 km s $^{-1}$ ).
- Dark Structure Speed: 30 – 67 km s $^{-1}$ (Mean: 49 km s $^{-1}$ ).
- The bright component consistently moves faster than the dark component.
Energetics:
- Kinetic energies range from $7.30 \times 10^{21}$ to $9.98 \times 10^{22}$ erg (Mean: $3.98 \times 10^{22}$ erg).
- These values place the events in the picoflare regime (energetically lower than typical jetlets, which are $10^{24}$ – $10^{25}$ erg), despite having spatial scales comparable to jetlets.

B. Chromospheric Nature of Dark Structures

Analysis of the coordinated IRIS/SDO dataset (Jet 11) revealed that the dark EUV structures correspond spatially to cool chromospheric surges observed in the Mg II k line.
The dark structures appear as absorption features adjacent to the bright spire, confirming they are cool, dense plasma ejected from the chromosphere.

C. Simulation Comparison

The Bifrost simulation of magnetic flux emergence successfully reproduced the inverted-Y (or $\lambda$ ) morphology observed in the data.
The simulation showed a one-to-one morphological correspondence: a hot, low-density coronal spire (bright in 174 Å) and a cool, high-density chromospheric surge (dark in 174 Å) emerging simultaneously from a reconnection site.
While the simulated jet was larger and had higher kinetic energy ( $\sim 8.15 \times 10^{24}$ erg) due to boundary conditions, the qualitative dynamics and structural coupling matched the observations perfectly.

4. Key Contributions

Discovery of Coupled Picoflare Jets: The study provides the first robust evidence that picoflare-scale jets possess coupled bright (hot) and dark (cool) components, a feature previously documented only in larger coronal jets.
Origin Mechanism: By combining off-limb high-resolution observations with radiative-MHD simulations, the authors establish that magnetic flux emergence and subsequent reconnection at low atmospheric heights are the likely drivers of these events.
Resolution of "Missing" Dark Components: The paper explains why previous studies may have missed dark counterparts: off-limb observations (low background) and ultra-high resolution are required to detect the faint absorption of cool plasma against the solar disk.
Classification: The events are classified as picoflare jets based on their energy content, despite having spatial scales similar to jetlets.

5. Significance

Coronal Heating & Solar Wind: The findings suggest that picoflare jets are a ubiquitous mechanism for transporting mass and energy from the chromosphere to the corona. Their frequent occurrence and specific energy range support the hypothesis that they contribute significantly to coronal heating and solar wind acceleration.
Unified Jet Physics: The results imply a unifying physical mechanism (flux emergence-driven reconnection) across different jet scales, from large coronal jets down to picoflare jets.
Future Observations: The study highlights the critical capability of the Solar Orbiter/EUI instrument to resolve sub-arcsecond dynamics, setting a new standard for investigating small-scale solar activity. It calls for future coordinated magnetograms and multi-wavelength observations to definitively rule out alternative drivers like minifilament eruptions.