In a significant milestone for India's first dedicated solar mission, the Visible Emission Line Coronagraph (VELC) payload onboard the Aditya-L1 spacecraft has generated the first-ever Doppler velocity images of the solar corona, offering scientists an unprecedented view of plasma motion in the Sun's outer atmosphere.
The achievement was announced by the Indian Institute of Astrophysics (IIA), Bengaluru, which designed and developed the VELC instrument. The Doppler velocity images reveal the motion of coronal plasma, showing material moving towards and away from the Earth, a capability that promises to transform our understanding of the solar corona and space weather.
"This is a world-first," said Professor R. Ramesh, the principal investigator of the VELC payload at IIA. "No other coronagraph in space has produced Doppler velocity images of the solar corona before. The VELC has demonstrated a capability that opens up an entirely new window on the Sun."
The Doppler velocity maps were generated by observing the solar corona in the green emission line at 530.3 nanometres, which originates from thirteen-times ionised iron atoms in the million-degree corona. By measuring the Doppler shift of this spectral line, the instrument can determine the line-of-sight velocity of the coronal plasma, effectively creating a speed map of the Sun's outer atmosphere.
The images show the corona in remarkable detail, with regions of plasma flowing towards the observer appearing distinct from regions flowing away. This velocity information is crucial for understanding coronal mass ejections (CMEs), the massive eruptions of plasma and magnetic field from the Sun that can disrupt satellites, power grids and communication systems on Earth.
Aditya-L1, launched by the Indian Space Research Organisation (ISRO) on September 2, 2023, aboard a PSLV-XL rocket, is positioned at the first Sun-Earth Lagrange point (L1), about 1.5 million kilometres from Earth. From this vantage point, the spacecraft enjoys an uninterrupted view of the Sun, free from the eclipses and atmospheric interference that hamper ground-based solar observatories.
The VELC is the largest and most technically complex of the seven payloads onboard Aditya-L1. It functions as an internally occulted coronagraph, using a series of mirrors and an occulter to block the blinding light of the solar disc and reveal the faint corona surrounding it, much like a total solar eclipse does naturally.
What sets the VELC apart from other space-based coronagraphs, such as those on NASA's SOHO spacecraft or ESA's Proba-3 mission, is its spectroscopic capability. While most coronagraphs image the corona in broad wavelength bands, the VELC can perform spectroscopy, dispersing the coronal light into its constituent wavelengths and measuring the Doppler shifts that encode velocity information.
The successful generation of Doppler velocity images follows an extensive period of calibration and testing after the payload's door was opened in January 2024. The IIA team has been working to characterise the instrument's performance and validate its measurements against theoretical models and observations from other instruments.
The achievement comes close on the heels of another Aditya-L1 milestone: the first joint study of the mission's data with ESA's Proba-3, which unlocked a new view of solar storms near the Sun. Together, these results underline the growing scientific productivity of India's solar mission.
For space weather forecasting, the Doppler velocity capability is a game-changer. Coronal mass ejections directed towards Earth can trigger geomagnetic storms that damage satellites, disrupt GPS navigation and, in extreme cases, cause blackouts in power grids. By measuring the velocity of coronal plasma directly, the VELC can help scientists determine the speed and direction of CMEs with greater accuracy, improving the lead time for space weather warnings.
"Understanding the velocity field of the corona is fundamental to predicting how CMEs propagate through interplanetary space," explained a senior IIA scientist associated with the mission. "The VELC gives us this information routinely, which was simply not available before."
The Aditya-L1 mission carries a total of seven payloads, four for remote sensing of the Sun and three for in-situ measurements of the solar wind and magnetic field at L1. The remote-sensing payloads include the Solar Ultraviolet Imaging Telescope (SUIT), the Solar Low Energy X-ray Spectrometer (SoLEXS) and the High Energy L1 Orbiting X-ray Spectrometer (HEL1OS), alongside the VELC.
The mission, with a planned lifetime of five years, is expected to provide a continuous stream of data on the Sun through the current solar maximum, a period of heightened solar activity. The data is made available to the global scientific community, and IIA has been conducting workshops to train researchers in the use of VELC observations.
The Doppler velocity images represent the culmination of nearly two decades of work at IIA, where the VELC was conceived, designed and built entirely in India. The instrument's success is a testament to the country's growing capabilities in space-based instrumentation and a source of considerable pride for the Indian solar physics community.
As Aditya-L1 continues its vigil at L1, scientists expect a steady flow of discoveries. The Doppler velocity maps are just the beginning: the team plans to use the VELC's spectroscopic data to study the temperature, density and composition of the corona, building a comprehensive picture of the Sun's dynamic outer atmosphere and its influence on the space environment around Earth.
(Reporting based on the Indian Institute of Astrophysics and ISRO.)