🔭 astrophysics

Signatures of coronal mass ejections in differential emission measure analysis of the Sun as a star

This study demonstrates that signatures of coronal mass ejections, specifically coronal dimmings, can be detected in Sun-as-a-star differential emission measure (DEM) analyses of EUV observations, revealing a decrease in emission measure within the $10^{5.7}-10^{6.3}$ K temperature range that is more accurately quantified when correcting for the flare's gradual phase.

Original authors: Angelos Michailidis, Spiros Patsourakos

Published 2026-03-27

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Angelos Michailidis, Spiros Patsourakos

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, glowing campfire in the middle of a dark forest. Usually, we can only see the fire from far away as a single, bright point of light. But sometimes, this campfire erupts, sending huge clouds of hot, magnetic smoke (called Coronal Mass Ejections or CMEs) shooting out into space.

Scientists want to know: Can we tell when this "smoke" is leaving just by looking at the campfire from far away, without being able to see the details of the fire itself?

This paper is like a detective story where the authors try to find the "footprints" of these solar eruptions using a special kind of thermal camera.

The Detective's Toolkit: The "Sun-as-a-Star" View

Normally, astronomers use powerful telescopes to zoom in on specific parts of the Sun, like looking at a single log in the campfire. But for other stars (which are too far away to zoom in on), we can only see them as a single dot of light. This is called the "Sun-as-a-star" view.

The authors asked: If we treat our own Sun like a distant star, can we still see the signs of these massive eruptions?

The Clue: The "Cooling Spot" (Dimming)

When a CME happens, it's like a massive chunk of the campfire's fuel is suddenly ripped away and thrown into space. Because that fuel is gone, the area where it used to be looks darker and cooler for a while. In astronomy, this is called a "coronal dimming."

The researchers looked at 16 major solar eruptions. They took pictures of the whole Sun every 12 seconds in different "colors" (specifically, invisible ultraviolet light). They then averaged all those pixels together to create a single light curve—essentially, a graph showing how bright the whole Sun was over time.

The Discovery:
They found that during these eruptions, the Sun did get slightly dimmer in specific colors of light. It was like seeing the campfire flicker and darken for a moment because a big chunk of wood was thrown out.

The Problem: The "Afterglow" Confusion

Here is where it gets tricky. When a solar flare happens (the explosion), the plasma (hot gas) doesn't just disappear; it cools down slowly afterward. This cooling process creates a "glow" or "afterglow" that can hide the darkening caused by the missing fuel.

Imagine trying to see a shadow cast by a person walking away, but at the same time, someone else is turning on a bright flashlight nearby. The flashlight makes the shadow look fainter than it really is.

The authors developed a clever math trick (a correction) to subtract this "afterglow" from their data.

Without the correction: The dimming looked small, like a faint shadow.
With the correction: The shadow became much deeper and clearer. It turned out the "missing fuel" was a bigger deal than they first thought.

The Thermal Fingerprint: The DEM

To understand what was missing, the scientists used a tool called Differential Emission Measure (DEM). Think of this as a thermal fingerprint.

Instead of just asking "How bright is it?", the DEM asks: "How much hot gas is there at this specific temperature?"

The Sun's atmosphere has layers of gas at different temperatures.
The researchers found that the "missing fuel" (the dimming) happened specifically in a temperature range of about 1 million degrees.
When they applied their "afterglow correction," the fingerprint shifted slightly, showing that the missing gas was actually in a slightly hotter range (around 1.2 million degrees). This tells us exactly what kind of material was ejected.

Why Does This Matter?

This is a big deal for two reasons:

Space Weather: CMEs can knock out satellites and power grids on Earth. If we can detect them faster by looking at the whole Sun's brightness (even without zooming in), we can warn people sooner.
Alien Worlds: We can't zoom in on other stars to see if they are throwing CMEs at their planets. But if we know what the "fingerprint" of a solar eruption looks like on our own Sun (even when viewed as a single dot), we can look at other stars and say, "Aha! That star just threw a massive cloud of gas at its planet!" This helps us understand if those planets could ever support life.

The Bottom Line

The authors proved that even when we treat the Sun like a distant, blurry star, we can still spot the "footprints" of massive solar eruptions. By using a little bit of math to clean up the data, they found that these eruptions leave a distinct, measurable mark in the Sun's heat signature. It's like realizing that even if you can't see the smoke rising from a distant fire, you can still feel the sudden drop in temperature when a gust of wind blows the fire's fuel away.

1. Problem Statement

Coronal Mass Ejections (CMEs) are major drivers of space weather, but their direct observation is typically limited to the Sun. For other stars, which are observed as point sources, detecting CMEs is challenging. While "stellar dimmings" (transient decreases in EUV intensity) are hypothesized to be a "smoking gun" for stellar CMEs, previous studies have primarily focused on analyzing raw intensity light curves.

The authors investigate whether signatures of CME-associated dimmings can be retrieved in the Differential Emission Measure (DEM) derived from "Sun-as-a-star" (disk-integrated) EUV observations. A specific challenge addressed is the potential underestimation of dimming magnitude caused by the gradual phase of associated flares (plasma cooling and emission enhancements that occur simultaneously with the mass evacuation).

2. Methodology

Dataset:

Sample: 16 major eruptive flares (GOES class $\ge$ M1.0) associated with CMEs.
Instrument: Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO).
Channels: Six EUV channels centered at 94, 131, 171, 193, 211, and 335 Å.
Data Processing: Full-disk images were converted into "Sun-as-a-star" light curves by calculating the average intensity per image (degrading the Sun to a point source). Intervals covered from ~2 hours before to ~6 hours after the flare peak.

Gradual-Phase Correction:
To isolate the dimming signal from the flare's gradual phase, the authors adapted a method by Mason et al. (2014):

Channel Selection: The 171, 193, and 211 Å channels were treated as "dimming channels." The 335 Å channel (or 94 Å for one specific event) served as a "non-dimming" reference.
Alignment & Scaling: Light curves were smoothed, aligned at flare peaks, and scaled so the peaks matched.
Subtraction: The non-dimming curve was subtracted from the dimming curves to remove the gradual-phase emission component, yielding corrected light curves.

DEM Calculation:

DEMs were calculated using the regularized inversion method (Hannah & Kontar 2012).
Inputs were the temporally averaged intensities for three windows: pre-event, uncorrected dimming, and corrected dimming.
The temperature range analyzed was $10^{5.0}$ to $10^{7.5}$ K.

Validation & Benchmarking:

Spatial Resolution Check: For a sample event (Event 5), DEMs were also calculated using:
- EVE (Extreme ultraviolet Variability Experiment) data (high spectral resolution, disk-integrated).
- Spatially resolved AIA cutouts (focusing only on pixels within the source Active Region undergoing dimming).
Source Contribution: The fraction of dimming pixels located within the source Active Region (AR) versus the full disk was quantified using the Solar Demon database.

3. Key Contributions

First DEM Analysis of Sun-as-a-Star Dimmings: The study extends the detection of CME signatures from simple intensity drops to the thermal structure of the corona (DEM), providing a more physical characterization of the mass evacuation.
Correction for Flare Gradual Phase: The paper demonstrates that failing to correct for the flare's gradual phase leads to an underestimation of dimming magnitude in both intensities and DEMs.
Temperature Shift Discovery: The application of the gradual-phase correction shifts the temperature range where the dimming signature is most pronounced.
Benchmarking: The study validates that Sun-as-a-star AIA observations yield DEM results consistent with high-resolution EVE and spatially resolved AIA data, confirming the viability of using disk-integrated data for stellar CME studies.

4. Results

Intensity Light Curves:

Dimmings were detected in the 171, 193, and 211 Å channels for all 16 events.
Magnitude: The median dimming depth was approximately 3% in uncorrected curves. After applying the gradual-phase correction, the dimming depth increased (median drop of up to 5%), confirming that the gradual phase masks the true extent of the mass loss.
Source Contribution: For most events, 22% to 91% of the total disk dimming pixels were located within the source AR, indicating that the source region is the primary contributor to the Sun-as-a-star signal.

Differential Emission Measure (DEM):

Uncorrected Data: The DEM decrease relative to the pre-flare phase was most pronounced in the temperature range $10^{5.7} - 10^{6.0}$ K.
Corrected Data: After correction, the most pronounced DEM decrease shifted to the higher temperature range of $10^{6.0} - 10^{6.3}$ K.
Interpretation: This shift suggests that the plasma contributing to the gradual phase (which obscures the dimming) has a temperature of $\approx 10^{6.0} - 10^{6.3}$ K, which is also the temperature of the plasma being evacuated during the CME.

Benchmarking (Event 5):

EVE vs. AIA (Sun-as-a-star): The temperature range of the dimming signature was similar between EVE and AIA Sun-as-a-star data.
Spatially Resolved vs. Sun-as-a-star: The magnitude of the dimming in the DEM was larger in the spatially resolved AIA cutout (reaching $d_{dimming}^{DEM} \approx 0.75$ ) compared to the Sun-as-a-star AIA data ( $\approx 0.9$ ). This is expected as the full-disk average dilutes the signal with non-dimming regions.
Consistency: Despite differences in magnitude, the temperature ranges where dimming was detected were consistent across all three data sources (Sun-as-a-star AIA, EVE, and spatially resolved AIA).

5. Significance

Stellar CME Detection: The results confirm that CME-induced dimmings are detectable in the DEM of Sun-as-a-star observations. This validates the use of disk-integrated EUV data to infer CME properties for other stars, where spatial resolution is impossible.
Methodological Necessity: The study highlights that gradual-phase correction is critical. Without it, the temperature range of the dimming signature is misidentified, and the magnitude of the mass loss is underestimated.
Future Missions: The findings are timely for upcoming missions like NExtUP and ESCAPE, which aim to characterize stellar atmospheres and space weather. The ability to detect CMEs via DEM analysis in point-source data offers a robust tool for assessing the habitability of exoplanets around active stars.
Physical Insight: The correlation between the corrected DEM dimming range ( $10^{6.0} - 10^{6.3}$ K) and the temperature of the evacuated plasma provides a new diagnostic tool for understanding the thermal structure of CMEs in their early stages.