🔭 astrophysics

Post-impulsive millimeter emission of the 2022-05-04 solar flare

This study analyzes the 93 GHz millimeter emission from the post-impulsive phase of the May 4, 2022, M5.7 solar flare, revealing that the enhanced emission correlates with moderately hot (~1 MK) EUV plasma and that optically thin coronal plasma contributes approximately 20% to the observed signal.

Original authors: G. G. Motorina, Yu. T. Tsap, V. V. Smirnova, A. S. Morgachev, A. S. Motorin

Published 2026-02-04

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CC BY 4.0

Original authors: G. G. Motorina, Yu. T. Tsap, V. V. Smirnova, A. S. Morgachev, A. S. Motorin

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 as a giant, chaotic weather station. Sometimes, it throws a massive tantrum called a solar flare, which is like a sudden, violent explosion of energy. Scientists usually watch these explosions in X-rays and ultraviolet light, but this paper focuses on a specific type of "after-shock" signal: millimeter waves. Think of millimeter waves as a different kind of radio signal, like tuning into a very specific, high-frequency radio station that we can't hear with our ears but can detect with special telescopes.

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

The Mystery of the "After-Party"

On May 4, 2022, a medium-sized solar flare (classified as M5.7) happened. The main explosion, or the "impulsive phase," happened quickly. But the scientists were interested in what happened after the main blast, during the "post-impulsive phase."

Usually, when a flare happens, we expect the radio signals to fade away as the energy settles down. However, this team noticed something strange: at a specific time (around 9:15 AM UTC), the millimeter radio signal actually spiked up again. It was like the party was winding down, but suddenly, the music got louder for a moment.

The Detective Work: Matching the Clues

To figure out what caused this second spike, the researchers acted like detectives matching fingerprints. They looked at two different types of "evidence" from the Sun:

The Radio Signal: The spike in millimeter waves.
The Heat Map: They looked at Extreme Ultraviolet (EUV) light, which acts like a thermal camera, showing them how hot different parts of the Sun's atmosphere were.

They found a perfect match. The moment the millimeter radio signal went up, the "thermal camera" showed a rise in heat in specific channels. Crucially, this heat wasn't from the super-hot, dangerous plasma (which would be like a nuclear reactor core), but from moderately warm plasma (about 1 million degrees Kelvin).

The Analogy: Imagine a campfire. The main explosion is like the logs catching fire (very hot). The "after-party" spike they found was like the glowing embers and the warm smoke rising up. It's hot, but not as hot as the initial flame.

The Big Question: Who is Making the Noise?

For a long time, scientists debated where these millimeter radio waves come from.

Theory A: They come from the "cold" lower layers of the Sun's atmosphere (the chromosphere), like steam rising from a pot.
Theory B: They come from the "hot" upper layers (the corona), like the heat radiating from a furnace.

The researchers used a mathematical recipe to calculate how much of the radio signal could be explained by the hot plasma they saw in the "thermal camera."

The Result:
They discovered that the hot plasma from the upper atmosphere (the corona) did contribute to the signal, but it wasn't the whole story.

The hot plasma accounted for about 20% of the radio signal.
This means the other 80% likely came from somewhere else (probably the cooler, lower layers).

Think of it like a choir singing a song. The hot plasma was a small group of singers (20%) adding a nice harmony, but the main melody (80%) was being sung by the rest of the choir in the lower layers.

The "Rain" Connection

The paper also suggests a fascinating visual. The timing of the radio spike and the heat rise matches what happens when "coronal rain" falls. Imagine water droplets falling from a cloud; on the Sun, this is hot plasma cooling down and falling back toward the surface. The researchers suggest that this falling "rain" might be what is creating that extra 20% of the radio noise.

The Bottom Line

The paper concludes that during the "cooling down" phase of a solar flare, the hot upper atmosphere of the Sun does play a role in generating radio waves, but it's a supporting actor, not the star. It contributes about one-fifth of the signal. This helps scientists understand that to fully understand solar flares, we need to listen to both the "hot" and "cool" parts of the Sun's atmosphere working together.

In short: The Sun had a flare. After the main boom, there was a second, smaller radio spike. This spike was partly caused by warm, falling "rain" of plasma from the Sun's upper atmosphere, proving that even the "aftermath" of a solar explosion has a complex mix of hot and cool ingredients.

Technical Summary: Post-impulsive Millimeter Emission of the 2022-05-04 Solar Flare

Problem Statement
While recent ALMA observations have established that a significant fraction of millimeter (mm) emission from the quiet Sun and active regions originates from the chromosphere via thermal bremsstrahlung, the origin of mm emission during solar flares remains unresolved. Specifically, the mechanism driving the positive spectral slope (flux increasing with frequency) observed in flare mm emission is debated. Proposed mechanisms include thermal emission from heated plasma and non-thermal electron processes, yet no single model explains all observations. A critical uncertainty persists regarding whether cold chromospheric plasma or hot coronal plasma dominates mm flare emission, particularly during the post-impulsive phase when both thermal components may coexist.

Methodology
The authors analyzed the M5.7-class solar flare SOL2022-05-04T08:45, focusing on its post-impulsive phase (post-09:13 UT). The study utilized multi-wavelength data:

Radio Data: 93 GHz flux density measurements from the RT-7.5 radio telescope (Bauman Moscow State Technical University) and spectral data from 15.4 to 93 GHz.
EUV and X-ray Data: Time profiles from GOES (soft X-rays), Konus-Wind (hard X-rays), SDO/AIA (EUV channels), and the Siberian Radioheliograph (microwaves).
Analysis Techniques:
- Identification of a Region of Interest (ROI) in SDO/AIA images corresponding to the flare source.
- Calculation of the Differential Emission Measure (DEM) and subsequent derivation of average Emission Measure (EM) and temperature ( $T$ ) for the ROI using routines developed by [25].
- Comparison of observed 93 GHz radio flux with theoretically calculated thermal bremsstrahlung flux derived from the DEM distributions.
- Assessment of optical thickness to determine the contribution of coronal plasma to the total mm emission.

Key Results

Temporal Correlation: A distinct increase in mm emission at 93 GHz was observed at 09:15:30 UT during the post-impulsive phase. This peak coincided temporally with increases in EUV emission specifically in the 171, 193, 211, and 335 Å channels.
Temperature Association: The EUV channels showing correlated increases are sensitive to plasma temperatures around 1 MK ( $10^6$ K). Conversely, no such increase was observed in channels sensitive to very hot (7.1 MK) or very cool (0.09 MK) plasma.
Spectral Characteristics: The radio spectrum at the peak time was nearly flat (spectral index $\approx -0.06$ between 15.4 and 93 GHz), suggesting an optically thin thermal free-free mechanism, though definitive conclusions were limited by insufficient mm wavelength coverage.
Quantitative Contribution:
- The calculated thermal bremsstrahlung flux from the coronal plasma (based on DEM) at 93 GHz was approximately 6 sfu.
- The observed flux at the peak was $26 \pm 3.9$ sfu.
- This discrepancy indicates that optically thin coronal plasma contributes approximately 20% to the observed mm emission.
Evolutionary Trends: While the EM and temperature of the ROI did not show a significant increase during the post-impulsive phase, the relative difference between the calculated and observed mm flux decreased toward the end of the flare. This suggests the contribution of coronal plasma to mm emission increases as the flare evolves.

Conclusions and Significance
The paper concludes that during the post-impulsive phase of the SOL2022-05-04 flare, moderately hot coronal plasma ( $T \sim 1$ MK) makes a significant, though not decisive, contribution (up to ~20%) to the mm emission. The authors posit that while mm radiation in microflares may originate primarily from coronal sources, larger flares likely involve a complex mix of chromospheric and coronal contributions.

The study suggests that the observed correlation between mm and EUV emission, alongside the temporal evolution of the flux ratio, points to a possible significant contribution from coronal rain (condensation) to the mm component. Consequently, the authors propose that mm observations can serve as a diagnostic tool for monitoring the process of coronal condensation during the post-impulsive phase of flare energy release. The work reinforces the view that the origin of flare mm emission involves multiple processes, with the dominance of specific mechanisms shifting as the flare evolves.

The Mystery of the "After-Party"

The Detective Work: Matching the Clues

The Big Question: Who is Making the Noise?

The "Rain" Connection

The Bottom Line

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