Indian astrophysicists from the Indian Institute of Astrophysics (IIA), Bengaluru, an autonomous institute under the Department of Science and Technology, Government of India, have conducted a groundbreaking 29-year statistical analysis of Interplanetary Coronal Mass Ejections (ICMEs) thermal behavior. The research team comprising Soumyaranjan Khuntia and Wageesh Mishra utilized NASA's OMNI database from the Space Physics Data Facility at NASA Goddard Space Flight Center, combining measurements from multiple spacecraft near the L1 point (1.5 million km from Earth toward the Sun) covering solar cycles 23, 24, and the rising phase of cycle 25 from 1995 to 2024.
The study applied a polytropic framework to quantify ICME thermal state on an event-by-event basis, calculating how pressure and temperature vary with density (polytropic index) for each ICME magnetic ejecta. Contrary to previous assumptions that Coronal Mass Ejections cool as they expand, the research revealed that nearly 45% of magnetic ejecta exhibit heating signatures at 1 Astronomical Unit (near Earth), particularly during solar maximum periods, indicating active in-transit heating processes.
The analysis shows a systematic modulation with solar activity, with Solar Cycle 23 demonstrating more heating-like states while Solar Cycle 24 shifted to more cooling-dominated states. This pattern suggests that the thermal evolution of CMEs is influenced by the global state of the solar magnetic environment. The research established a direct connection between ICME thermal state and geomagnetic storm severity, finding that the most geoeffective storms tend to be associated with ICMEs in a heating state (low Gamma) characterized by strong magnetic fields, low plasma beta indicating magnetic pressure dominance, compressed sheath regions, and enhanced expansion speeds.
The study, published in Monthly Notices of the Royal Astronomical Society, represents the first long-term statistical investigation of ICME thermal behaviour at 1 AU and offers a combined thermal-magnetic diagnostic framework that could significantly improve space weather forecasting. The researchers note that future efforts will integrate observations from India's Aditya-L1 solar mission, including coronagraphic and solar wind instruments, to better track thermal evolution of CMEs closer to the Sun and refine predictive models for space weather events that affect satellite operations, GPS and radio communications, aviation routes, and power grids.