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Clusters of sunspots or active regions are sites of intense solar activity. The Solar Influences Data Analysis Centre (SIDC) operated by the Royal Observatory of Belgium as well as the US National Oceanic and Atmospheric Administration (NOAA) both assign numbers to track such sunspot clusters, with the latter being more commonly used by the global media. In early May 2024, AR 3664 merged with AR 3668, with the combined active region being designated as AR 3664. Between 8 and 15 May, AR 3664 fired off a series of twelve X-class solar flares, resulting in the most severe geomagnetic storm in two decades.
Geomagnetic storms are measured on a scale that increases in intensity from G1 to G5. On 8 May 2024, based on the modelling of the coronal mass ejections (CMEs) associated with the series of X-class flares, the NOAA issued a severe geomagnetic storm alert (G5) for 10 May, the first since 2005, during the previous solar maximum, the peak in solar activity over a well-understood 11 year solar cycle. Subsequently a G4 geomagnetic storm alert was also issued for 11 May. The geomagnetic storming lasted between 8 and 15 May. While rotating out of view, AR 3664 bid farewell with an X8.7 solar flare, which was then the strongest observed in the ongoing Solar Cycle 25.
The geomagnetic storm was among the strongest on record, and the most powerful one since 2003, when the Sun erupted in the highest intensity solar flare recorded in modern history, estimated to be an X45. Between 7 and 11 may, at least seven coronal mass ejections were unleashed by the Sun, that were all travelling at different speeds. These clouds of charged, magnetised hot gas interacted in interplanetary space while on the way to Earth, crashing and overtaking each other, and bunching up, before striking the Earth on 10 May. Scientists will be studying the solar storm for years, to test the limits of models and to better understand the impact of solar storms on the Earth.
Modern power distribution networks are fortified against extreme geomagnetic storms and were not affected much. Some power distribution companies preemptively switched off circuits to prevent damage to equipment. The sunlit side of the Earth experienced radio blackouts during the solar flares, with the geomagnetic storming reducing the accuracy of GPS signals for hours. There were positional errors in automated equipment used for precision farming. Aircraft navigation systems were unable to provide localised vertical guidance. Drone operators reported erratic movements, loss of GPS signals, and even occasional crashes. The GOES-16 weather monitoring satellite conked off twice before recovering. The early warning services and our understanding of the impact of solar storms on modern technologies helped prevent any drastic consequences of the solar storm.
US satellites captured spectacular auroras over the poles of the Earth, extending to lower latitudes where they had never been seen in living memory. ISRO’s Chandrayaan 2 orbiter, XPoSAT and Aditya-L1 missions all detected the energetic particles from the Sun, and remote observations of the eruptions, during the historic solar storm. Aditya-L1 was still in the commissioning phase, but ISRO decided to conduct observations considering the intensity of the event. The instruments were being baked in and calibrated during the event, but ISRO nonetheless managed to observe the outburst. Aditya-L1 conducted both in-situ and remote observations of the Sun during the Solar Storm.
Observations of solar activity by Aditya-L1 during the May 2024 solar storms. (Image Credit: ISRO).
Apart from the instruments deployed in space, Indian scientists also tracked the geomagnetic storms from ground-based observatories. The National Atmospheric Research Laboratory in Gadanki in Andhra Pradesh measured an electron spike on 11 May. The Thumba node of the Indian Network for Space Weather Impact Monitoring (INSWIM) measured a sharper electron spike over the same period, as it was located right beneath the ionospheric ring current. The atmosphere of the Earth inflated like a Hot Air Balloon because of the injection of heat, increasing the drag and reducing the altitudes of satellites. ISRO had to track the decay of the orbits of its satellite during the event, which ranged between two and 200 metres. A number of satellites in the Starlink constellation also experienced decaying orbits.
