The First Quantitative Assessment of Photospheric Iron Fluorescence Observed by the Indian Aditya-L1 Solar Mission During X-Class Flares The phenomenon across 47 large solar flares observed in 2024 was detected using the Solar Low Energy X-ray Spectrometer (SoLEXS), providing a unique method of determining flare source geographies and heights within the Sun atmosphere.
The results were published in the paperIron fluorescence in X-class solar flares: Aditya-L1/SoLEXS observationsin Solar Physics.
What is iron fluorescence?
A solar flare is an explosive release of extremely energetic X-rays during an event on the Sun, in which hot plasma from its visible surface bursts out into the outer atmosphere called corona. When this X-rays interacts with neutral iron in the photosphere, the atoms absorbs energy and emits a diagnostic X-ray signal.
The signal released is around 6.40 keV — called iron K-alpha fluorescence.
The process does NOT mean that iron is burning or glowing in visible light. This is an X-ray response triggered when kinds of high-energy radiation collide with iron atoms in the solar surface zone.
SoLEXS captures the signal
The observation was made using SoLEXS, one of the instruments aboard Aditya-L1. The spectrometer is capable of observing X-rays across a broad energy range, allowing scientists to measure both the high-energy radiation produced by the flare and the resulting iron fluorescence.
This simultaneous measurement is important because it allows researchers to compare the incoming flare radiation with the iron signal produced at the photosphere.
The data came from the mission’s first year of solar observations, a period that coincided with high solar activity during the peak phase of Solar Cycle 25.
Centre-to-limb variation discovered
The intensity iron fluorescence varied considerably based on the location of flare occurrence on the visible solar disc.
Stronger fluorescence signal was resulted from the centre of the Sun nearer flares as viewed from Aditya-L1. Specifically, the flares that were seen on or just below the surface of the Sun, in views very close to the Sun’s edge, or limb — displayed a signal much weaker and more suppressed.
The centre-to-limb variation agrees with theoretical models of how X-rays travel through the solar atmosphere, and how the one-dimensional geometry of the flare itself determines how much radiation is emitted towards, as well as received from, the photosphere [16].
A new diagnostic tool
They can tell us where and how a solar flare looks like based on the variances in fluorescence strength. The signal might enable researchers to approximate the physical altitude of the coronal X-ray generator above the solar surface.
Those aspects are the location of the flare, the angle in which radiation hits and travels from the solar surface, and that it follows a very specific path through earth penetrating to reach us as X-Ray visible photons. Scientists can study: By analysing these slopes.
- The height of flare-producing regions.
- The geometry of coronal loops.
- The direction of X-ray emission.
- The interaction between the corona and photosphere.
- Changes in flare structure over time.
The study describes iron fluorescence as a potential diagnostic for understanding the three-dimensional geometry of solar eruptions.
Why X-class flares matter
X-class flares are the most powerful category of solar flares. They can release enormous amounts of energy within minutes and produce intense X-rays and ultraviolet radiation.
The strongest flares may also be associated with coronal mass ejections—large eruptions of magnetised plasma that travel through interplanetary space. If directed towards Earth, such events can affect:
- Satellite operations.
- Radio communications.
- Navigation systems.
- Power grids.
- Astronaut safety.
- High-frequency aviation communications.
A better understanding of flare physics can improve models used to predict space-weather conditions.
Aditya-L1’s special vantage point
Aditya-L1 is at a stable orbit of the Sun–Earth Lagrange Point 1, direct to Earth almost in line with the sun about 1.5 million kilometres away. This position offers complete visibility of solar activity and allows the spacecraft to study the Sun before solar-wind disturbances are felt on Earth.
This enables making coordinated observations of solar flares, coronal mass ejections and energetic radiation. Instruments aboard the spacecraft can study the Sun continuously, without the interruption effects of Earth’s rotation or atmosphere.
First large-scale analysis
Previous observations had seen iron fluorescence associated with individual solar flares, but the Aditya-L1 study systematically analysed it across 47 X-class events.
From the large sample they were able to define a strong statistical connection between the amount of high-energy X-ray emitted with the flare and the strength of the iron fluorescence. The technique also helped validate that the centre-to-limb pattern is a physical phenomenon and not an isolated discovery.
Therefore, this work provides a more solid observational basis for the use of iron fluorescence in solar-flare studies.
Limits of the current findings
While the observations provide critical information, the researchers said that rapidly changing flare heights over extremely short timescales are currently beyond what can be achieved due to statistical uncertainties.
A reasonably useful estimate of the effective source height may be obtained from the average fluorescence efficiency during the peak of a flare, but more sensitive observations and larger datasets will be required for tracking the temporal evolution of a flare.
New measurements might refine those estimates and distinguish among flare geometries in the future.
Contribution to solar physics
Of course, solar flares are actually driven by sudden magnetic energy releases in the corona. One of the great ongoing challenges in solar physics involves understanding how this energy is transferred into high-energy particles and radiation.
Iron fluorescence provides a bridge from the high-temperature infrared coronal region to the cooler visible surface below. The signal is basically a proxy for light — one that shows how the X-rays from that flare waded into denser layers of the Sun.
Such a phenomenon is useful for studying the connecting relationship between the corona, photosphere and magnetic structures that are responsible for these eruptions.
Strengthening India’s space-science programme
This comes as an addition to the growing list of scientific accomplishments achieved by Aditya-L1, which aims to study Solar atmosphere; solar wind; magnetic fields and space weather.
The data is allowing Indian researchers to make important contributions on the global stage in solar physics and creating confidence levels for India’s X-ray and ultraviolet instruments flying aboard spacecraft.
A new window into solar flares
Aditya-L1 detects iron fluorescence in 47 X-class solar flares, providing scientists with a new tool to probe the height and geometry of explosive events in our Sun.isro+15+⦼
Investigation of the variation properties of 6.40-keV iron signal in relation to flare position and intensity would provide an explicit picture on architecture of solar eruptions. The results might ultimately refine solar activity models and allow scientists to more accurately evaluate the space-weather threats from strong flares.