🔭 astrophysics

Magnetic Flux Density as a Driver of Chromospheric and Coronal Emissions During the Rise of Solar Cycle 25

This study demonstrates that magnetic flux density is the primary driver of chromospheric and coronal emissions during the rise of Solar Cycle 25, exhibiting strong correlations with key solar proxies, a 33-day lead over chromospheric indices, and synchronized 16–32 day variability linked to solar rotation.

Original authors: Taona Namfukwe, Negessa Shukure

Published 2026-06-24

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Taona Namfukwe, Negessa Shukure

Original paper licensed under CC BY 4.0 (https://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 "Heartbeat"

Imagine the Sun isn't just a burning ball of gas, but a giant, living engine with a magnetic "heartbeat." This heartbeat follows a rhythm called the Solar Cycle, which lasts about 11 years. It has quiet times (sleeping) and busy times (waking up).

Right now, the Sun is waking up from a long nap. We are in the "Ascending Phase" of Solar Cycle 25, which means the Sun is slowly getting more active again.

This study asks a simple question: What is actually driving the Sun's activity? Is it the heat? The light? Or is it the invisible magnetic fields?

The Main Characters

To answer this, the researchers looked at four different "characters" (data sets) that tell us how active the Sun is:

Magnetic Flux Density (MFD): This is the main character. Think of this as the "magnetic muscle" of the Sun. It measures how strong the magnetic fields are on the Sun's surface (the photosphere). The researchers specifically looked at the "weaker" magnetic fields that are everywhere, not just the giant sunspots.
f10.7 cm Radio Flux: Think of this as the Sun's radio broadcast. It's a signal we can pick up from Earth that tells us how much energy is being released from the Sun's upper atmosphere (the corona).
Mg II Index & Ca II K-line: These are like thermometers for the Sun's middle layer (the chromosphere). They measure how hot and active that specific layer is.

The Investigation: Connecting the Dots

The researchers used data from 2020 to 2024 (while the Sun was waking up) to see how these characters behaved together. They used three main tools to analyze the data:

The Correlation Test (The "Handshake"): They checked if the characters moved in sync.
- Result: They found a very strong handshake. When the magnetic muscle (MFD) got stronger, the radio broadcast (f10.7) and the middle-layer thermometers (Mg II and Ca II) got hotter immediately.
- The Math: The connection was incredibly tight (about 95–96% match). This proves that magnetic fields are the boss. They are the engine driving the heat and the radio signals.
The Time-Lag Test (The "Reaction Time"): They checked who moved first.
- The Radio (f10.7): It moved at the exact same time as the magnetic fields. There was zero delay. It's like a light switch: you flip the switch (magnetic change), and the light turns on instantly.
- The Thermometers (Mg II and Ca II): These were a bit slower. They reacted about 33 days later than the magnetic fields. It's like a heavy truck: the engine starts (magnetic change), but it takes a month for the truck to actually start moving and heat up the air around it.
The Wavelet Test (The "Rhythm Check"): They looked for repeating patterns, like a drumbeat.
- Result: They found a strong beat every 16 to 32 days. This matches the time it takes for the Sun to spin around once (about 27 days).
- What it means: As the Sun spins, it brings different active spots into view, creating a rhythmic pulse in the magnetic fields and the emissions. It's like a lighthouse beam sweeping around; the light (activity) pulses on and off as the beam rotates.

The Key Differences: Why Some Signals Are Faster

The paper makes an interesting distinction between the "Radio" and the "Thermometers":

The Radio (f10.7) is like a spotlight. It only shines brightly when there is a very strong, concentrated magnetic storm (like a sunspot). Because these storms are intense and localized, the radio signal reacts instantly to the magnetic changes.
The Thermometers (Mg II/Ca II) are like a fog. They cover a much wider area of the Sun's atmosphere. Because they measure a broader, more diffuse layer, it takes longer (33 days) for the magnetic energy to travel up and heat them all up.

The Conclusion

The study confirms that magnetic fields are the primary driver of the Sun's activity during this rising phase.

If you want to know what the Sun is doing right now, look at the magnetic fields or the radio signal (f10.7); they happen together.
If you want to know how the Sun's middle layer is reacting, you have to wait about a month after the magnetic change happens.

Why does this matter?
The paper states that understanding this timing helps us predict Space Weather. Since the radio signal (f10.7) reacts instantly to magnetic changes, it is a reliable "real-time" indicator. This helps forecasters predict when the Sun might send energy toward Earth that could affect satellites or radio communications.

In short: The Sun's magnetic field is the conductor, and the different layers of the Sun's atmosphere are the orchestra. Some instruments (the radio) play in perfect time with the conductor, while others (the thermometers) take a little longer to catch the beat.

Technical Summary: Magnetic Flux Density as a Driver of Chromospheric and Coronal Emissions During the Rise of Solar Cycle 25

Problem Statement
Solar activity proxies, such as the 10.7 cm radio flux ( $f_{10.7}$ ), Mg II index, and Ca II K-line, are widely used to estimate solar magnetic activity and model space weather. However, these indices do not directly measure magnetic fields; rather, they respond to atmospheric heating and emission processes driven by magnetic activity. Previous studies often relied on datasets with limited spatial and temporal resolution or focused on strong magnetic flux densities ( $B \ge 800$ G) during specific phases of Solar Cycle 24 (SC 24). There remains a need to validate weak magnetic flux density (MFD) indices across different phases of the solar cycle, specifically during the ascending phase of Solar Cycle 25 (SC 25), to better understand the temporal hierarchy and causal relationships between photospheric magnetic fields and emissions in the chromosphere and corona.

Methodology
This study utilized high-resolution line-of-sight magnetic field data from the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO) covering the period from 2020 to 2024. The authors constructed a daily average MFD index by summing the absolute magnetic flux of pixels ranging from 100 G to 800 G and normalizing by the total solar disk area. This "weak MFD" index was compared against established solar activity proxies: the $f_{10.7}$ cm radio flux, the Mg II index, and the Ca II K-line index.

The analysis employed a suite of statistical and time-series techniques:

Correlation Analysis: Pearson ( $r$ ) and Spearman ( $\rho$ ) correlation coefficients were calculated to assess linear and monotonic relationships. The Henze–Zirkler (HZ) test was used to verify multivariate normality.
Correlogram Analysis: This was used to quantify time lags between the MFD and the various proxies.
Wavelet Analysis: Continuous Wavelet Transform (CWT) using the Morlet wavelet was applied to identify periodic patterns and localized power in the time-frequency domain. Cross-Wavelet Transform (XWT) was used to analyze common periods and phase relationships between the MFD and the proxies.

Key Results

Strong Correlations: The study found strong linear and monotonic correlations between the weak MFD index and all solar proxies. The $f_{10.7}$ cm flux showed the highest correlation ( $r = 0.96$ , $\rho = 0.96$ ), while the chromospheric indices (Mg II and Ca II K) also demonstrated tight associations ( $r = 0.95$ , $\rho = 0.94$ ).
Temporal Hierarchy: Correlogram analysis revealed a distinct time lag. Changes in the photospheric MFD preceded changes in the chromospheric indices (Mg II and Ca II K) by approximately 33 days. In contrast, the $f_{10.7}$ cm flux showed zero time lag with the MFD, varying simultaneously.
Periodic Variability: Wavelet power spectra identified dominant variability on 16–32 day timescales, corresponding to the solar rotation period (~27 days). This modulation was observed across all parameters but was particularly pronounced during the transition from the solar minimum to the ascending phase.
Spatial and Physical Differences: The CWT analysis indicated that chromospheric indices and MFD exhibited broad, sustained regions of enhanced power, suggesting widespread variability due to the buildup of magnetic structures. Conversely, the $f_{10.7}$ cm flux displayed more localized, narrow bursts of power, reflecting its sensitivity to compact, high-field active regions (e.g., sunspots) rather than the full-disk chromospheric network.
Phase Relationships: XWT analysis showed that the $f_{10.7}$ cm flux and MFD maintained a consistent in-phase relationship ( $\phi \approx 0^\circ$ ). The relationship between MFD and chromospheric indices was predominantly in-phase during specific intervals (days 790–1010) but exhibited more complex, intermittent phase shifts in later periods.

Key Contributions

Validation of Weak MFD: The study successfully validates a weak MFD index (100–800 G) as a robust driver of solar activity during the ascending phase of SC 25, extending previous work that was limited to strong fluxes or SC 24.
Quantification of Lag Times: The research provides specific quantification of the 33-day delay between photospheric magnetic flux emergence and the resulting chromospheric response, offering a clearer picture of energy propagation through solar atmospheric layers.
Differentiation of Proxies: The paper distinguishes the physical origins of different proxies, demonstrating that while $f_{10.7}$ is an immediate, synchronous indicator of magnetic activity, chromospheric indices reflect a delayed, integrated response of the broader magnetic network.

Significance and Claims
The authors claim that solar magnetic fields are the primary drivers of chromospheric and coronal emissions. The findings confirm that the $f_{10.7}$ cm radio flux is a reliable, real-time indicator of underlying photospheric magnetic activity, making it critical for space weather prediction and forecasting ionospheric disturbances. Conversely, the 33-day lag observed in chromospheric indices suggests they serve as useful diagnostics for tracking the vertical coupling of solar atmospheric layers in the later stages of energy propagation.

The study concludes that these results improve the understanding of how photospheric magnetic flux relates to solar activity proxies, noting that different proxies react with varying speeds and spatial characteristics. This knowledge is presented as essential for refining solar irradiance models and understanding the interactions between the Sun's different atmospheric layers during the rising phase of a solar cycle. The authors explicitly state that future work will examine the relationship between the MFD index and UV irradiance during the same phase of SC 25.