Indian solar mission's new findings throw light on enduring Sun mysteries
New research based on data from India's Aditya-L1 solar mission has offered fresh evidence on why the Sun's corona (outer atmosphere) is far hotter than its surface
The study, led by Professor R. Ramesh of the Indian Institute of Astrophysics (IIA), was published in the Astrophysical Journal Letters
Researchers analysed a highly energetic Coronal Mass Ejection (CME) observed on 5 August 2024, using the Visible Emission Line Coronagraph (VELC) instrument aboard Aditya-L1
Tracking how the Sun's magnetic field lines reconnected and settled back into a stable configuration within about 10 hours of the eruption, the study found that magnetic field reconfiguration and reconnection supplied roughly 93% of the energy needed to heat that region of the corona, compared to about 7% from surface-generated waves
The findings provide a quantitative benchmark for coronal heating research, though scientists note it does not fully resolve the decades-old "coronal heating problem"
The Coronal Heating Problem
The corona, the Sun's outermost atmospheric layer, reaches temperatures of roughly 1-3 million°C, while the visible surface (photosphere) below it is only about 5,500°C. This apparent violation of basic thermodynamics, heat should decrease with distance from a source, has remained one of solar physics' major unsolved problems since scientists first confirmed the corona's extreme temperature.
Key Details
- The mystery was established in the late 1930s-40s when Swedish spectroscopist Bengt Edlén and German astrophysicist Walter Grotrian identified spectral emission lines from the corona (once mistakenly attributed to a hypothetical element "coronium") as belonging to highly ionised iron, which is only possible at temperatures of several million degrees
- Two leading theoretical explanations compete: wave heating (energy carried up from the turbulent solar surface by magnetohydrodynamic waves) and magnetic reconnection/nanoflare heating (energy released when tangled magnetic field lines snap and reconfigure)
- The new Aditya-L1 study quantifies the relative contribution of these two mechanisms for the first time using direct Indian observational data, finding magnetic reconnection dominant (about 93%) over wave heating (about 7%) in the observed event
This is the specific scientific puzzle the Aditya-L1/VELC study addresses, providing observational evidence favouring magnetic reconnection as the primary coronal heating mechanism during a major eruptive event.
Aditya-L1 — India's First Dedicated Solar Mission
Aditya-L1 is the Indian Space Research Organisation's (ISRO) first dedicated mission to study the Sun, launched to observe the solar corona, chromosphere, solar wind, and space weather phenomena from a vantage point unobstructed by Earth's shadow.
Key Details
- Launched on 2 September 2023 aboard a PSLV-C57 rocket; inserted into a halo orbit around the Sun-Earth Lagrange Point 1 (L1) on 6 January 2024
- L1 is located about 1.5 million km from Earth (roughly 1% of the Earth-Sun distance), a point where the gravitational pull of the Sun and Earth balance, allowing continuous, uninterrupted observation of the Sun
- Carries seven payloads, including VELC (Visible Emission Line Coronagraph, built by IIA Bengaluru — the primary instrument for imaging and spectroscopy of the corona), SUIT (Solar Ultraviolet Imaging Telescope), SoLEXS, HEL1OS, ASPEX, PAPA, and a magnetometer (MAG)
- Mission objectives include studying coronal heating, CMEs, solar flares, solar wind acceleration, and space weather effects that can impact satellites, communication, and power grids on Earth
VELC, the mission's flagship coronagraph, was the instrument that captured the 5 August 2024 CME whose analysis produced the new coronal-heating findings, demonstrating a concrete scientific return from India's investment in solar observation infrastructure.
Coronal Mass Ejections and Space Weather
A Coronal Mass Ejection (CME) is a large expulsion of plasma and magnetic field from the Sun's corona, distinct from a solar flare (a sudden burst of electromagnetic radiation), though the two often occur together. CMEs that reach Earth can trigger geomagnetic storms.
Key Details
- CMEs travel at speeds ranging from roughly 250 km/s to over 3,000 km/s and can take one to several days to reach Earth
- Earth-directed CMEs can disrupt satellite operations, GPS/navigation signals, high-frequency radio communication, and power grids by inducing geomagnetically induced currents
- India's space weather monitoring capability, strengthened by Aditya-L1, complements global efforts (e.g., NASA's Parker Solar Probe, which has flown closer to the Sun than any prior spacecraft, and ESA/NASA's Solar Orbiter) to understand solar activity
- The Indian Institute of Astrophysics (IIA), an autonomous institute under the Department of Science and Technology, led the VELC payload development and this latest analysis
The 5 August 2024 event analysed in the study was itself a CME, making this research directly relevant to both fundamental coronal physics and applied space-weather forecasting, an area of growing strategic and economic importance given India's expanding satellite and digital infrastructure.
- Sun's surface (photosphere) temperature: approximately 5,500°C; corona temperature: approximately 1-3 million°C (occasionally much higher during active periods)
- Magnetic reconnection contribution to coronal heating in the studied event: approximately 93%; wave heating contribution: approximately 7%
- CME analysed: observed on 5 August 2024; magnetic field reconnection tracked over about 10 hours
- Aditya-L1 launch: 2 September 2023 (PSLV-C57); reached halo orbit around L1: 6 January 2024
- Distance to L1 from Earth: approximately 1.5 million km
- Study published in: Astrophysical Journal Letters, led by Prof. R. Ramesh, Indian Institute of Astrophysics