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On the Short Dissipation Scales and Current-Sheet Properties of Low-Coronal EUV Brightenings

This study utilizes Solar Orbiter EUV observations to characterize ubiquitous small-scale brightenings in the quiet-Sun low corona, revealing a multi-scale energy-release framework where dissipation transitions from fast, impulsive Alfvénic reconnection to slower, resistive current-sheet heating across distinct temporal and spatial regimes.

Original authors: Olena Podladchikova

Published 2026-01-23

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CC BY 4.0

Original authors: Olena Podladchikova

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's outer atmosphere, the corona, as a giant, super-hot blanket wrapped around a cooler ball. For decades, scientists have been puzzled by a mystery: How does this blanket stay millions of degrees hot when the surface underneath is much cooler? It's like trying to keep a campfire hot by blowing on it from the cold air above, rather than from the wood below.

This paper, using new, super-sharp eyes from a spacecraft called Solar Orbiter, finally peels back the layers to show us exactly how the Sun heats itself up. Here is the story in simple terms:

The Two Types of "Solar Sparks"

Before this study, scientists thought all the tiny bursts of energy heating the Sun were basically the same thing, just different sizes. They called them "nanoflares." But Solar Orbiter saw something new: There are actually two completely different types of tiny solar sparks, and they happen in different places and work in different ways.

Think of a magnetic loop on the Sun like a giant, invisible rubber band stretched between two points on the ground (the "footpoints") and arching high into the sky (the "looptop").

1. The "Pop" at the Top (Population A)

Where: High up in the arch (2.5 to 5 million meters above the surface).
What happens: Imagine two rubber bands twisting around each other until they suddenly snap and reconnect. This is called magnetic reconnection.
The Feeling: It's a quick, sharp POP. It happens very fast (1 to 10 seconds) and releases a moderate amount of energy.
The Science: This is like tearing a piece of paper quickly. The paper tears along a thin line (a current sheet), releasing energy instantly. This heats the gas at the very top of the loop.

2. The "Warm Glow" at the Bottom (Population B)

Where: Down low, near where the rubber band touches the ground (1 to 2.5 million meters above the surface).
What happens: Imagine electricity flowing through a wire that gets too crowded. The electrons start bumping into each other and creating friction, but not the kind you feel with your hands—it's a special kind of friction called anomalous resistivity.
The Feeling: It's a slow, steady WARM GLOW. It lasts longer (10 to 100 seconds) but releases less energy per burst.
The Science: This happens because the "wire" (the magnetic field) gets so crowded with electric current that it breaks down and heats up the dense gas right where the loop touches the Sun's surface.

The Big Picture: How They Work Together

The paper proposes that these two events are actually part of the same energy cycle, like a relay race:

The Start: The Sun's surface churns and twists the magnetic "rubber bands."
The Snap (Top): High up in the sky, the tension gets too high, and the bands snap and reconnect (The "Pop"). This releases energy and sends a shockwave of electricity down the loop.
The Glow (Bottom): That electricity rushes down to the feet of the loop. Because the gas down there is denser, the electricity gets "clogged," causing friction and heating up the gas (The "Warm Glow").

Why This Matters

For a long time, scientists had a "theoretical barrier." They thought the smallest possible energy burst (a nanoflare) had a minimum size limit. If an event was too small, physics said it shouldn't exist or be detectable.

This paper breaks that barrier. It shows that:

The "Pops" at the top fit the old rules.
The "Warm Glows" at the bottom are much smaller and fainter than anyone thought possible before. They are so small they were previously invisible to our telescopes.

The Takeaway

Solar Orbiter acted like a high-definition camera that finally allowed us to see the "pixels" of the Sun's heating process. We now know the Sun isn't just heated by one big mechanism. Instead, it's a two-step dance:

A fast, explosive snap high in the sky.
A slower, friction-based warm-up at the feet.

Together, these two tiny, invisible processes are the secret sauce that keeps the Sun's outer atmosphere burning hot. Without both the "Pop" and the "Glow," the Sun's corona would cool down, and the mystery would remain unsolved.

Technical Summary: Short Dissipation Scales and Current-Sheet Properties of Low-Coronal EUV Brightenings

1. Problem Statement

The heating of the million-degree solar corona remains a fundamental unsolved problem in astrophysics. While the required energy flux for the quiet Sun is approximately $3 \times 10^5$ erg s $^{-1}$ cm $^{-2}$ , existing heating paradigms face significant limitations. Wave heating models struggle to explain heating in short, low-lying loops, while impulsive nanoflare models, following the foundational work of Parker (1988) and Aschwanden (2000), encounter a theoretical lower energy barrier. Traditional scaling laws suggest a natural lower limit for coronal nanoflares around $10^{23}$ – $10^{24}$ erg, leaving the faintest energy releases and the heating of the lowest coronal layers unexplained.

2. Methodology

This study leverages the unique observational capabilities of the Solar Orbiter mission, specifically its EUV Imaging (EUI) instrument operating from 0.5 AU. The methodology integrates three critical advancements:

High-Resolution Morphology: Unprecedented spatial resolution (198 km per pixel) allows for the morphological resolution of small-scale EUV brightenings, distinguishing them from the impulsive, unresolved detections of previous missions (SOHO/EIT, TRACE).
Stereoscopic Height Determination: Utilizing simultaneous observations with SDO/AIA and Solar Orbiter's unique orbital position (31.5° west of Earth), the study employs triangulation to precisely calculate the vertical heights of energy release events.
Comprehensive Energy Analysis: Building on the work of Podladchikova et al. (2025), the authors apply multi-volume modeling (elliptical loop and cube models) and rigorous uncertainty quantification to estimate thermal energies. This includes a statistical analysis of 12,107 events detected above a $3\sigma$ threshold, utilizing Differential Emission Measure (DEM) analysis for temperature and density determination.

3. Key Contributions and Results

The analysis reveals that small-scale EUV brightenings are not a homogeneous population but consist of two distinct groups operating via different physical mechanisms in different atmospheric layers. These groups span energies from $10^{20}$ to $10^{24}$ erg, fundamentally extending below the traditional nanoflare limit.

Population A: Looptop Events

Location: 2.5–5 Mm altitude (coronal loop tops).
Characteristics: Short duration (1–10 s), higher temperatures (1.3–1.5 MK), and energies of $10^{22}$ – $10^{24}$ erg.
Mechanism: Driven by tearing-mode reconnection of perpendicular currents in a low- $\beta$ plasma. This process operates on Alfvénic timescales ( $\tau_A \approx L/V_A$ ) and aligns with classical nanoflare models.

Population B: Footpoint Events

Location: 1–2.5 Mm altitude (near the transition region/footpoints).
Characteristics: Longer duration (10–100 s), slightly lower temperatures (1.0–1.2 MK), and lower energies ( $10^{20}$ – $10^{22}$ erg).
Mechanism: Driven by anomalous resistivity of parallel currents. In the collisionless plasma of the low corona and transition region, current densities exceeding critical thresholds ( $j > n_e e c_s$ ) trigger Buneman or ion-acoustic instabilities. This leads to wave-particle interactions and dissipation on resistive/sound-speed timescales ( $\tau \approx L/c_s$ ).

Statistical Distribution

The events follow a power-law distribution with a slope $\alpha \approx 2.32$ (for $\ge 5\sigma$ events). This slope indicates that the total energy flux converges at the lower limit, suggesting that numerous small-scale events dominate the heating budget. The study calculates a minimum energy of $E_{min} \approx 1.25 \times 10^{18}$ erg required to sustain the quiet-Sun heating flux.

4. Significance and Claims

The paper claims to resolve a long-standing paradox in coronal heating by demonstrating that the corona is heated through two complementary mechanisms rather than a single process:

Spatial Separation: It confirms that high-energy, impulsive events occur at loop tops, while low-energy, sustained heating occurs at footpoints. This resolves the debate regarding the location of EUV nanoflares (Aschwanden et al., 2017 vs. Benz, 2017).
Mechanism Duality: The study establishes that while tearing-mode reconnection explains events above $10^{23}$ erg, anomalous resistivity is the necessary mechanism to explain the faintest events ( $10^{20}$ – $10^{22}$ erg) in the lower corona.
Unified Energy Cycle: The authors propose a unified framework where magnetic reconnection at looptops generates electric fields that drive parallel currents toward footpoints. These currents dissipate via anomalous resistivity, completing the energy transfer cycle from photospheric motions to coronal thermalization.

The work asserts that Solar Orbiter's direct observations finally provide the empirical evidence needed to move beyond theoretical barriers, confirming that low-altitude, small-scale current-sheet dissipation is a critical component of quiet-Sun coronal heating.