July 03, 2026
India’s maiden space-based solar observatory, Aditya-L1, continues to help us understand the mysteries of our nearest star. During its first year operating at the Lagrangian point 1 (L1) in 2024, the Sun was exceptionally active, producing numerous intense solar flare events. Aditya-L1 observation from special vantage point L1, presents the first comprehensive analysis to help us understand a fascinating solar phenomenon: photospheric iron fluorescence. The "iron fluorescence" phenomenon was observed during 47 massive X-class solar flares in the year 2024.
What is Iron Fluorescence?
When a massive solar flare erupts, it heats the Sun's upper atmosphere (the corona) to extreme temperatures (tens of mega Kelvin), releasing massive burst of high-energy X-rays. While most of these X-rays are emitted outward into space, a fraction of them travel downward, interacting with the Sun's cooler, denser surface layer known as the photosphere, where they interact with abundant neutral iron atoms, as illustrated in Figure 1.
When these coronal X-rays strike neutral iron atoms, the iron atoms absorb the energy and emit their own characteristic X-ray glow at energy of 6.40 keV. This process is called "X-ray fluorescence".
Solar Low Energy X-ray Spectrometer (SoLEXS) on board Aditya-L1 are suitable to detect X-rays from the flare as well as photospheric iron fluorescence from strong flares. SoLEXS instrument is indigenously developed at the U. R. Rao Satellite Centre (URSC), ISRO.
Figure 1: An illustration showing how a solar flare (represented by the black star) high in the corona emits X-rays down onto the Sun's surface (the red area). When iron atoms on the surface absorb these X-rays, they emit a characteristic fluorescence (blue arrow). SoLEXS confirmed that this fluorescence is much stronger when the flare happens near the center of the Sun's disk (right) compared to the edge or "limb" (left).
Understanding Solar Flare's Geometry
By analyzing these extreme flares, the study discovered that the observed brightness of the iron fluorescence depends heavily on where the flare occurs on the Sun's disk. As illustrated in Figure 1, flares occurring near the center of the Sun’s disk, as viewed by the observer, showed a strong fluorescence signal, while the signal was heavily suppressed for flares occurring near the Sun's edge (the limb).
This "center-to-limb" variation matches with theoretical models. By studying exactly how this efficiency changes, researchers can now use the iron fluorescence as a potential diagnostic tool to probe the altitude of the coronal X-ray source high up in the solar atmosphere and study the unique viewing geometries of these explosive events.
The study has been published in the international, peer-reviewed journal Solar Physics.
Reference: Sarwade, A. R., et al. (2026). "Iron Fluorescence in X-Class Solar Flares: Aditya-L1/SOLEXS Observations". Solar Physics. DOI: 10.1007/s11207-026-02685-3 arXiv: https://arxiv.org/abs/2605.22573