November 03, 2026
Scientists at the Indian Institute of Astrophysics (IIA), Bengaluru, an autonomous institute under the Department of Science and Technology, Government of India, have used India's first space-based solar observatory, Aditya-L1, to gain new insights into how a solar eruption expands near the Sun. The team combined spectroscopic observations from the Visible Emission Line Coronagraph (VELC) onboard ISRO’s Aditya-L1 with images from NASA’s Solar Dynamics Observatory (SDO) and three-dimensional modelling. The study, published in The Astrophysical Journal Letters, shows that an eruption observed on 5th August 2024 did not expand outward from the Sun like a simple symmetric bubble, as previously thought. Instead, it underwent non-uniform expansion and developed an asymmetric shape. The result is important for space-weather studies because differences in CME expansion can affect its speed as it travels towards Earth.
A coronal mass ejection (CME) is a massive burst of hot, magnetised plasma released from the Sun's outer atmosphere, known as the corona. As CMEs travel through interplanetary space, those directed towards the Earth can contribute to space-weather disturbances and affect satellites and communication, and are therefore an important subject of research. Predicting the speed and trajectory of a CME is therefore a matter of practical importance and depends critically on understanding how the eruption evolves in three dimensions near the Sun, where it is still rapidly changing.
Studying CMEs in their early stages, however, is extremely challenging. Scientists typically rely on coronagraphs, which block the bright solar disk to reveal the faint outer corona. Most coronagraphs can only image the CME in two dimensions; they capture motion across the sky, but cannot directly measure how fast the plasma is moving toward or away from the observer. This is where VELC onboard Aditya-L1 provides additional information by using spectroscopy to measure the motion of hot plasma along the line of sight.
VELC is one of the primary scientific instruments onboard Aditya-L1. It is developed by IIA, Bengaluru with substantial support from ISRO. Unlike imaging-only coronagraphs, VELC records spectra - measuring how the motion of plasma shifts the frequency of the light it emits. This technique, known as Doppler spectroscopy, uses the same principle as traffic speed guns: a moving object shifts the frequency of the light it emits. If plasma is rushing toward the observer, its light is shifted to shorter (blue) wavelengths; if rushing away, toward longer (red) wavelengths.
During the CME observed on 5 August 2024, VELC detected a striking and unusual spectral evolution. Two distinct Doppler-shifted emission branches appear simultaneously on opposite sides of the regular coronal emission line, whose intensity decreases as the CME passes through the corona and carries coronal plasma outward. These blue- and red-shifted spectral features observed by VELC reveal how plasma in the CME bubble was moving both towards and away from the observer, commonly referred as line-of-sight motion. Meanwhile, imaging observations from the Solar Dynamic Observatory (NASA solar mission) simultaneously captured how the CME was propagating across the sky.
To understand the distinct and unusual VELC spectral pattern, the IIA team developed a three-dimensional geometric and kinematic model of the expanding shell-like CME structure. The model parameters were varied to match the observed spectral patterns. A symmetrically expanding CME shell could not reproduce the observed spectral pattern. However, when asymmetric lateral expansion and a tilt were introduced, the simulated spectra showed an excellent match with the VELC observations. The combination of imaging observations, spectroscopic measurements from VELC, and three-dimensional modelling provided evidence that this CME did not expand uniformly close to the Sun.
Earlier CME observations and models mostly assumed that CMEs were symmetric bubbles. The shape of a CME may have important consequences for understanding its propagation through the heliosphere, as CMEs with different shapes may experience different levels of drag from the solar wind. These differences may affect CME speeds as they travel towards Earth, which could make CME arrival-time predictions more challenging and the assessment of potential space-weather impacts.
Three-dimensional CME evolution is often studied using observations from multiple spacecraft viewing the eruption from different directions and vantage points. This study demonstrates that VELC can constrain the three-dimensional geometry of an eruption from a single vantage point by combining its Doppler spectroscopy measurements with other imaging observations. This is particularly powerful for studying CMEs in the low corona, where they evolve most rapidly and where line-of-sight information is most critical.
More such studies will help establish how reliably the propagation direction, velocity and expansion of CMEs can be determined using VELC spectroscopy. This would contribute in the understanding and modelling of CME propagation and its effects in space.
Figure. (Left) Composite image of the Sun’s corona combining data from India's Aditya-L1/VELC (outer part, 1.05–1.5 solar radii) and NASA’s SDO/AIA (inner disk). (Top right) Aditya-L1/VELC spectra recorded during the eruption, showing two distinct branches of emission — one shifted toward shorter wavelengths (plasma moving toward us) and one toward longer wavelengths (plasma moving away) — the spectral signature of a three-dimensional, asymmetric solar eruption. (Bottom right) Model spectra reproducing the observed pattern, confirming asymmetric lateral expansion of the CME in the inner corona.
Title: Spectroscopic Signatures of Nonuniform Lateral Expansion of a Coronal Mass Ejection in the Low Corona with ADITYA-L1/VELC; Authors: Saurabh Tripathi, Tanmoy Samanta, and Jayant Joshi; Affiliations: Indian Institute of Astrophysics, Bengaluru 560034;Journal: The Astrophysical Journal Letters.
Reference: https://doi.org/10.3847/2041-8213/aea295