🔭 astrophysics

Photospheric Lorentz force changes in eruptive and confined solar flares

This study analyzes 37 major solar flares and synthetic models to demonstrate that the total change in the downward-directed Lorentz force at the photosphere serves as a distinguishing threshold, with confined flares exhibiting changes below $1.8 \times 10^{22}$ dyne, thereby offering a key metric for differentiating eruptive and confined events and understanding their associated coronal mass ejection dynamics.

Original authors: Samriddhi Sankar Maity, Ranadeep Sarkar, Piyali Chatterjee, Nandita Srivastava

Published 2026-04-16

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CC BY 4.0

Original authors: Samriddhi Sankar Maity, Ranadeep Sarkar, Piyali Chatterjee, Nandita Srivastava

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: The Sun's "Magic Trampoline"

Imagine the Sun's surface (the photosphere) as a giant, invisible trampoline made of magnetic rubber bands. Above this trampoline, in the Sun's atmosphere (the corona), these rubber bands are twisted and tangled, storing huge amounts of energy—like a spring being squeezed tight.

Sometimes, these springs snap. This is a solar flare.

Eruptive Flares: The spring snaps so hard that it launches a massive chunk of the Sun's atmosphere into space. This is called a Coronal Mass Ejection (CME). If this hits Earth, it can mess up our satellites and power grids.
Confined Flares: The spring snaps, creating a bright flash of light and heat, but the energy stays trapped on the Sun. Nothing gets launched into space.

The Big Question: How can we tell the difference between a "launch" flare and a "stuck" flare just by looking at the Sun's surface?

The Study: Listening for the "Thud"

The scientists in this paper (Maity, Sarkar, and team) wanted to find a way to predict if a flare would launch a CME or stay confined. They looked at 37 major solar flares that happened between 2011 and 2017.

They focused on a specific force called the Lorentz force.

The Analogy: Imagine you are standing on a trampoline. If someone jumps on the other side, you feel a downward push.
On the Sun: When a flare happens, the magnetic field above the surface reorganizes. This creates a sudden, strong downward push (a "thud") on the Sun's surface.

The researchers measured two things during these "thuds":

The Horizontal Stretch: Did the magnetic rubber bands on the surface get pulled tighter sideways?
The Downward Push: How hard did the surface get pushed down?

The Discovery: The "Magic Number"

Here is what they found, which is the most exciting part:

The Stretch (Horizontal Field): Both types of flares (eruptive and confined) made the magnetic bands stretch sideways. You can't tell them apart just by looking at this stretch. It's like both a small jump and a big jump on a trampoline make the fabric stretch.
The Push (Vertical Force): This is where the difference lies.
- Confined Flares: They gave the surface a gentle tap. The downward force was always less than a specific limit (1.8 × 10²² dynes).
- Eruptive Flares: They gave the surface a massive slam. Most of them had a downward force greater than that limit.

The Takeaway: If you see a solar flare and measure the downward push on the surface, and that push is huge, you can bet money that a giant cloud of solar plasma is about to be launched into space. If the push is small, the flare is likely just a local firework that won't leave the Sun.

Why Do Some Big Flares Have Small Pushes?

The researchers noticed something tricky: Some flares that did launch CMEs (eruptive) still had a "small" downward push, below the magic limit. Why?

They looked at the shape of the magnetic field.

The Analogy: Imagine two magnets with opposite poles (North and South) facing each other.
- Scenario A (Big Push): The magnets are right next to each other. When they snap together, the force is concentrated and hits the surface hard.
- Scenario B (Small Push): The magnets are far apart. When they snap, the connection happens high up in the air, far from the surface. The "thud" on the surface is much weaker because the energy was released far away.

The scientists found that when the magnetic "ribbons" (the footprints of the flare) were far apart, the reconnection happened high up, resulting in a weaker push on the surface, even if a CME was launched.

The Computer Simulation (The "Virtual Sun")

To prove this wasn't just a fluke, they also ran a computer simulation of a "virtual Sun." They created fake flares in a 3D model.

The simulation showed that the "thud" (Lorentz force) actually travels downward from the explosion site in the atmosphere to the surface, like a shockwave hitting the ground.
The computer results matched the real-world data perfectly, confirming that the physics is the same whether it's a real sun or a virtual one.

Summary in Plain English

Solar flares leave a permanent "bruise" on the Sun's surface.
Eruptive flares (the dangerous ones that send stuff to Earth) usually leave a massive downward bruise (a huge change in Lorentz force).
Confined flares (the harmless ones) leave a smaller bruise.
There is a threshold (a specific number). If the bruise is bigger than that number, it's almost certainly an eruptive flare.
Sometimes, even big eruptions have small bruises if the explosion happens very high up in the Sun's atmosphere, far from the surface.

Why does this matter?
Space weather forecasters need to know if a flare will hit Earth. This study gives them a new tool: by measuring the "downward push" on the Sun's surface, they can quickly tell if a flare is going to be a local event or a global threat.

1. Problem Statement

Solar flares and Coronal Mass Ejections (CMEs) are the most energetic phenomena in the solar atmosphere. While they often occur together (eruptive flares), some flares are "confined," meaning they do not produce a CME. Distinguishing between these two types is crucial for space weather forecasting.

The Challenge: Direct measurement of the coronal magnetic field is currently impossible. Therefore, researchers rely on photospheric magnetic field data to infer coronal changes.
The Gap: Previous studies established that flares cause a permanent increase in the horizontal magnetic field ( $B_h$ ) and a downward change in the vertical Lorentz force ( $F_z$ ) at the photosphere. However, it remained unclear if the magnitude of these changes could statistically distinguish between eruptive (CME-associated) and confined flares, particularly across a large sample of major events.

2. Methodology

The authors conducted a statistical analysis of 37 major solar flare events (GOES M5 class and stronger) observed between 2011 and 2017.

Data Sources:
- Observational Data: 37 events (26 eruptive, 11 confined) from 26 Active Regions (ARs). Data was obtained from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). They utilized 12-minute cadence vector magnetograms (SHARP data) to minimize noise.
- Synthetic Data: Two synthetic flare events (B-class and C-class) from a 3D Magnetohydrodynamic (MHD) simulation reported by Chatterjee et al. (2016). This allowed for a comparison of physical mechanisms where full 3D field data is available.
- Auxiliary Data: SDO/AIA 1600 Å images were used to identify flare ribbons and Polarity Inversion Lines (PILs).
Analysis Techniques:
- Region of Interest (RoI) Selection: A semi-automatic algorithm was developed to select sub-regions near the PIL. The selection was based on difference maps of the horizontal magnetic field ( $\delta B_h$ ) and the vertical Lorentz force ( $\delta F_z$ ) calculated 30 minutes before and after the flare peak.
- Lorentz Force Calculation: Using the Fisher et al. (2012) formulation, the authors calculated the net change in the vertical Lorentz force ( $\delta F_z$ ) and horizontal magnetic field ( $\delta B_h$ ) integrated over the selected RoI:
  $\delta F_z = \frac{1}{8\pi} \int_{A_{ph}} (\delta B_h^2 - \delta B_z^2) dA$
- Ribbon Distance: The distance between flare ribbons ( $d_{ribbon}$ ) was measured to estimate the reconnection height in the corona.

3. Key Contributions

Large-Scale Statistical Classification: The study provides a robust statistical distinction between eruptive and confined flares based on the magnitude of the vertical Lorentz force change, moving beyond case studies to a sample of 37 major events.
Threshold Identification: The authors identified a specific threshold value for $\delta F_z$ that effectively separates confined from eruptive events.
Morphological Correlation: The paper links the magnitude of the Lorentz force change to the spatial morphology of the active region, specifically the separation distance of flare ribbons (proxy for reconnection height).
Simulation-Observation Consistency: By comparing observational data with 3D MHD simulations, the study validates that the observed photospheric changes are driven by the downward propagation of the Lorentz force from the coronal reconnection site.

4. Key Results

Lorentz Force Threshold:
- Confined Flares: All 11 confined events exhibited a total change in vertical Lorentz force ( $\delta F_z$ ) less than $1.8 \times 10^{22}$ dyne.
- Eruptive Flares: The majority of the 26 eruptive events showed $\delta F_z$ values greater than $1.8 \times 10^{22}$ dyne.
- Note: While 9 eruptive events fell below this threshold, no confined events exceeded it, making this an upper bound for confinement.
Horizontal Magnetic Field ( $B_h$ ):
- Both eruptive and confined flares showed a rapid, permanent increase in $B_h$ around the PIL.
- Crucially, the magnitude of $\delta B_h$ did not statistically distinguish between eruptive and confined events, unlike $\delta F_z$ .
Role of Reconnection Height (Ribbon Distance):
- Eruptive events with $\delta F_z$ below the threshold were found to have significantly larger flare ribbon separations ( $>15$ Mm) compared to those above the threshold ( $<9$ Mm).
- Interpretation: Larger ribbon separation implies a higher reconnection height in the corona. A higher reconnection site results in a weaker downward impulse on the photosphere, explaining why some eruptive flares have lower Lorentz force changes.
Simulation Validation:
- The 3D MHD simulations confirmed that the Lorentz force ( $J \times B$ ) generated at the reconnection site propagates downward toward the photosphere at speeds of $\approx 2.3 - 2.4$ km/s.
- The temporal evolution of $\delta B_h$ and $\delta F_z$ in the simulation matched the observational profiles, confirming the physical mechanism.

5. Significance

Predictive Capability: The study establishes the change in vertical Lorentz force ( $\delta F_z$ ) as a superior proxy for predicting whether a major flare will be associated with a CME, outperforming the change in horizontal magnetic field alone.
Physical Insight: It reinforces the "magnetic implosion" conjecture and the momentum balance condition, suggesting that the Lorentz-force impulse is directly linked to the momentum of the associated CME.
Morphological Dependency: The findings highlight that the active region's morphology (specifically the separation of magnetic polarities) dictates the reconnection height, which in turn modulates the strength of the photospheric magnetic imprint.
Future Research: The authors suggest that while the $1.8 \times 10^{22}$ dyne threshold is effective, future studies with larger samples and higher-resolution vector magnetograms are needed to refine the classification of the "low-Lorentz-force" eruptive events.