Unraveling the Mystery of "Rain" Falling on the Sun! The Wonders of the Universe Revealed: It Turns into Rain Minutes After a Flare

Why Does "Rain Fall on the Sun"?

Rain on Earth is a natural phenomenon where water vapor forms clouds and falls due to gravity. However, the sun also experiences "rain." Plasma, which is heated to millions of degrees in the corona, rapidly cools after a solar flare, forming filament-like clumps that fall along magnetic field lines. This is known as "coronal rain." It appears like a waterfall of flames, and NASA's SDO (Solar Dynamics Observatory) has captured images showing it dripping like rain from arch-shaped loops.

The Unresolved "Time Paradox"

The longstanding mystery was "why does rain form so quickly?" Classical models suggest that prolonged gentle heating is necessary to cool down enough to form rain. On the other hand, flares are impulsive phenomena that rise in a matter of minutes, creating a mismatch. Observations existed, but theory lagged behind.Phys.org

The Key is the Paradigm Shift of "Moving Elements"

Luke Benavitz (a graduate student) and Geoffrey Reep (IfA) from the University of Hawaii at Manoa (UH) proposed that "the elemental composition of the corona is not constant but varies with time and location." They incorporated "spatiotemporally varying elemental abundances (especially low ionization potential = Low-FIP elements)" into a radiative hydrodynamic code. They demonstrated that when the concentration of elements like iron becomes locally biased due to evaporative flows and transport during flares, radiative losses spike, leading to rapid cooling and rain (condensation). This connected the pathway for "coronal rain to form in a short time even with impulsive heating."arXiv

Positioning and Derivatives of the Research

The results were published in the Astrophysical Journal. Furthermore, at the end of September, a follow-up preprint was released, examining the relationship between FIP fractionation (Low-FIP element enhancement) during flares and rain in detail. It suggested a correlation between "the strength of heating," "the width of enhancement," and the likelihood of rain. The view that rain is an "intersection of phenomena" involving heating, elemental fractionation, and transport is gaining strength.arXiv

What Will Become the "New Common Sense"?

1. Revisiting the Assumption of Constant Composition: Previously, "fixed composition" was an implicit assumption for ease of handling. The current results showed that elements in the solar corona can become unevenly distributed in a short time, significantly affecting the heat balance.

2. Reevaluation of Cooling Time: Amplification of radiative cooling is essential to explain the speed of observed rain. Elemental bias can trigger this.

3. Backlighting on the Coronal Heating Problem: Understanding cooling also constrains heating mechanisms. If a model can reproduce "rain formation," it helps narrow down what kind of heating is realistic.Phys.org

Implications for Space Weather

Flares and CMEs (coronal mass ejections) affect the Earth's ionosphere, satellite operations, and auroras. Recently, there has been an increase in news about magnetic storms and strong solar winds at high latitudes, and public interest in space weather information is growing. If rain formation can serve as an indicator of "heating strength" or "elemental fractionation state," it could improve the accuracy of heat balance estimates occurring behind flare progression, potentially leading to improved forecast models in the future.earthsky.org

Expansion of Reports and Reactions

The announcement was reported by Phys.org as "Solar Rain Mystery Solved," and was subsequently covered by UH News, EurekAlert!, and local TV station Hawaii News Now. Academically, the arXiv paper was included in literature summaries at various universities, spreading the topic within the research community.Phys.org hawaii.edu

Initial Reactions on Social Media

The research announcement is spreading quietly, primarily within specialized and semi-specialized communities rather than causing a "buzz." On X, astronomy-related accounts shared the Phys.org article, with the intuitive phrase "rain falls on the sun" serving as a hook for dissemination. The key points were introduced in short form on UH's official Instagram reels, repeatedly emphasizing the significance of the research (rain formation on the scale of a few minutes during flares). News videos were also uploaded to YouTube, functioning as concise summaries of the research. Below are examples of the initial reactions observed.

・X: Posted article link with "Solar rain mystery solved…" (Eyes2TheStars, 10/1) X (formerly Twitter)
・Instagram: Introduced in a reel with "UH researchers crack solar rain mystery" (related to IfA/UH) Instagram
・YouTube: News video "Solar rain mystery cracked by UH researchers" (released on the same day) YouTube

※Social media observations are samples at this point. Secondary dissemination may occur in conjunction with significant space weather events (aurora appearances or large-scale flares) in the future.

A Deeper Dive: What is FIP Fractionation?

In the solar corona, elements that ionize easily (e.g., Fe, Si, Mg, known as Low-FIP elements) are relatively enhanced compared to the composition of the photosphere, a phenomenon known as the "FIP effect." The new research demonstrated that this enhancement, when transported and redistributed over time, can dynamically change the radiative cooling rate, potentially inducing local condensation (rain). The follow-up report proposed a scenario where the stronger the heating, the more Low-FIP materials are compressed to the loop tops, making rain formation more likely.arXiv

Future Highlights

• Observational Verification: Can spectroscopic observations correlate temporal changes in elemental ratios with the frequency of rain occurrence?

• Forecast Models: How to incorporate "variable elemental ratios" into space weather models.

• Extension to Other Celestial Bodies: Potential to contribute to the understanding of rain and prominence formation in stellar coronae in general.

Conclusion

The simple hypothesis that **"elemental ratios move"** provided the decisive answer to the straightforward yet challenging question of "why does coronal rain form so quickly?" A model that can reproduce the observed "rain" begins to narrate the thermal history of the solar corona more realistically. The next steps are exciting for both the practical aspects of space weather and the fundamental research in solar physics.Phys.org

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