Scientists at the Indian Institute of Astrophysics (IIA), Bangalore, have used India’s first space-based solar observatory, Aditya-L1, to reveal new details about how solar eruptions evolve close to the Sun.

The research combined spectroscopic measurements from the Visible Emission Line Coronagraph (VELC) onboard Aditya-L1 with imaging data from NASA’s Solar Dynamics Observatory (SDO) and advanced three-dimensional modelling.

The findings, published in The Astrophysical Journal Letters, show that a coronal mass ejection (CME) observed on 5 August 2024 did not expand as a simple, symmetrical bubble as often assumed. Instead, it expanded unevenly and developed an asymmetric structure.

A CME is a vast eruption of hot, magnetised plasma from the Sun’s outer atmosphere, known as the corona. When these eruptions travel through interplanetary space and are directed towards Earth, they can trigger space-weather disturbances that affect satellites, communications systems and other technological infrastructure. Understanding how CMEs evolve is therefore crucial for improving forecasts of their speed, trajectory and potential impact.

One of the greatest challenges in solar physics is studying CMEs during their earliest stages near the Sun. Conventional coronagraphs block the bright solar disc and allow scientists to observe the fainter corona, but they generally provide only two-dimensional views of eruptions.

This means they can track motion across the plane of the sky but cannot directly determine how rapidly plasma is moving towards or away from the observer.

VELC aboard Aditya-L1 addresses this limitation through spectroscopy. Developed by IIA with substantial support from ISRO, the instrument measures changes in the frequency of light emitted by moving plasma. Using Doppler spectroscopy, the same principle employed by speed-detection devices, plasma moving towards the observer produces a blue shift, while plasma moving away creates a red shift.

During the 5 August 2024 CME, VELC recorded an unusual spectral signature. Two distinct Doppler-shifted emission branches appeared simultaneously on opposite sides of the normal coronal emission line, while the line’s intensity weakened as the CME carried coronal material outwards.

These blue- and red-shifted features provided direct evidence that plasma within the expanding CME was moving both towards and away from the observer along the line of sight. At the same time, SDO observations tracked the eruption’s movement across the sky.

To explain the unusual measurements, the research team developed a three-dimensional geometric and kinematic model of the CME’s shell-like structure. Multiple configurations were tested against the observed spectral patterns. A model based on symmetrical expansion failed to reproduce the observations. However, when the scientists introduced asymmetric lateral expansion together with a tilt in the structure, the simulated spectra matched the VELC data remarkably well.

The results indicate that the CME expanded non-uniformly very close to the Sun. This finding challenges the long-standing assumption used in many observations and models that CMEs behave like symmetrical expanding bubbles.

The shape of a CME has important implications for its journey through the heliosphere. Different CME geometries may experience varying levels of drag from the solar wind, potentially altering their speeds as they travel towards Earth. Such differences could make arrival-time forecasting more complex and influence assessments of potential space-weather effects.

Traditionally, scientists have relied on observations from multiple spacecraft positioned at different viewing angles to reconstruct the three-dimensional structure of solar eruptions. This study demonstrates that VELC can help constrain the three-dimensional geometry of a CME from a single observing location by combining Doppler spectroscopy with complementary imaging observations.

The capability is particularly valuable in the low corona, where CMEs undergo their most rapid evolution and where line-of-sight information is essential for understanding their true motion. By revealing motion that cannot be detected through imaging alone, VELC effectively exposes the previously hidden third dimension of solar eruptions.

Further investigations using VELC are expected to determine how accurately CME direction, velocity and expansion can be measured from spectroscopic observations. Such advances could significantly improve models of CME propagation and strengthen future space-weather forecasting capabilities, enhancing our ability to predict and prepare for solar disturbances that may affect Earth.

ISRO