Aditya-L1 Makes Doppler Velocity Images of the Sun's Corona: Why It Matters for Tracking Solar Storms
Aditya-L1, India's first space mission to study the Sun, has produced Doppler velocity images of the solar corona. The corona is the outermost layer of the Sun's atmosphere.
A Doppler velocity image shows how fast the hot gas in the corona is moving towards us or away from us, at each point in the picture. It is made by measuring tiny shifts in the colour (wavelength) of light given out by the gas.
The images come from the Visible Emission Line Coronagraph (VELC), the main instrument on Aditya-L1, built by the Indian Institute of Astrophysics (IIA), Bengaluru.
Scientists expect these images to show how coronal mass ejections (CMEs) travel along the Sun-Earth direction. Ordinary images of the corona cannot show this motion directly, so this information was not available earlier.
CMEs are huge clouds of charged gas thrown out by the Sun. When they hit Earth, they can disturb satellites, GPS, radio communication and power grids. Better tracking means better warnings.
Aditya-L1: India's First Dedicated Solar Mission
Aditya-L1 is India's first space observatory built to study the Sun. It was launched by the Indian Space Research Organisation (ISRO) on 2 September 2023. It sits about 1.5 million km from Earth, near a special point in space called Lagrange point 1 (L1), between the Earth and the Sun. From there it can watch the Sun all the time, without the Earth or Moon ever blocking the view. "Aditya" is a Sanskrit name for the Sun.
The new Doppler velocity images come from VELC, Aditya-L1's main payload. They show the speed of coronal gas towards or away from us, which helps track whether a CME is heading along the Sun-Earth line. This is exactly the kind of space-weather science the mission was built for.
Coronal Mass Ejections (CMEs) and Space Weather
A coronal mass ejection (CME) is a huge cloud of hot, electrically charged gas (plasma) and magnetic field that the Sun suddenly throws out into space from its outer atmosphere, the corona. A single CME can carry around a billion tonnes of material. It can travel at speeds from below 250 km per second to nearly 3,000 km per second. If a CME hits the Earth, it can cause a geomagnetic storm, which can disturb satellites, radio, GPS and power grids, and create bright auroras (natural lights in the sky).
The Aditya-L1 Doppler velocity images measure how fast coronal gas moves towards or away from us. This helps scientists tell whether a CME is moving along the Sun-Earth direction, which is the key question for space-weather warnings and which flat coronagraph images alone cannot answer.
Doppler Effect: How Motion Changes Waves
The Doppler effect is the change in the frequency (and wavelength) of a wave when the source of the wave and the observer are moving towards or away from each other. You hear it every day: an ambulance siren sounds higher-pitched as it comes towards you and lower-pitched as it moves away. The same thing happens with light. Scientists use it to measure the speed of things they cannot touch, from storms and cars to stars and galaxies.
Aditya-L1's Doppler velocity images use this effect. By measuring how much the light from coronal iron is shifted, VELC finds how fast the gas is moving towards or away from Earth. This adds the missing "towards or away" information needed to track CMEs along the Sun-Earth line.
- Mission: Aditya-L1, India's first dedicated solar observatory (ISRO)
- Instrument: Visible Emission Line Coronagraph (VELC), built by IIA Bengaluru
- New output: Doppler velocity images of the solar corona
- Launch: 2 September 2023 (PSLV-C57); halo orbit around L1 from 6 January 2024
- Distance from Earth: about 1.5 million km; halo loop about 178 days
- Payloads: 7 (VELC, SUIT, SoLEXS, HEL1OS, ASPEX, PAPA, MAG)
- CME speeds: below 250 km/s to nearly 3,000 km/s
- Geomagnetic storm scale: G1 to G5 (NOAA)
- Doppler effect: proposed 1842 by Christian Doppler