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The GRAPES-3 experiment is a special telescope-array established in Ooty to detect muons from cosmic ray showers. The experiment has detected a surge in muon intensity correlated with a weakening of the earth’s magnetic field due to a solar storm that hit the earth on June 22, 2015. The results have been published in the journal Physical Review Letters. An Indo-Japanese collaboration, this experiment is unique in that it can be used to study solar storms and space weather at distances up to two times the earth’s radius, unlike satellite-based studies that can yield information only about what is happening in their vicinity.


The muon is an elementary particle similar to the electron, with an electric charge of −1 e and a spin of 1 / 2 , but with a much greater mass. It is classified as a lepton. As is the case with other leptons, the muon is not believed to have any sub-structure—that is, it is not thought to be composed of any simpler particles.

A coronal mass ejection (CME) left the sun on June 21, 2015 and, along with two such others that left the sun on June 18 and 19, reached earth on June 22, 2015. Solar flares are often followed by CMEs which are nothing but giant clouds of plasma which also contain embedded magnetic fields.

This CME was associated with a solar flare from the sunspot region 12371 near the central disc of the sun. This caused a solar storm and ensuing radio blackouts and Aurora Borealis. Analysing data from the GRAPES-3 muon-tracking telecope, scientists have inferred that while it lasted, the CME resulted in weakening the earth’s magnetic field, allowing high energy cosmic rays to burst through.

This method can serve as a monitor of solar storms. Galactic cosmic rays producing a muon burst were bent in the space surrounding the Earth over a volume that is 7 times that of the Earth, and hence they serve as a monitor of the solar storm over this volume. This is in stark contrast to the satellite based measurements that provide only in situ information.

The earth’s atmosphere provides a shield against UV rays and other incident particles. But its protection stretches to less than 100 km around the earth. The stronger protection comes from the earth’s magnetic field which stretches to around 10 times the radius of the earth – about 60,000 km beyond the surface.

This magnetic field deflects most of the galactic cosmic rays – high-energy charged particles that are incident on earth from space. The magnetic field forms the first line of defence against cosmic rays by imposing a threshold energy per unit charge. Only charged particles that have higher energy than this threshold can fall on the earth.

The analysis shows the weakening of the earth’s magnetic field because of the coronal mass ejection.

There is a 32-minute lapse between the muon burst and the arrival of the interplanetary magnetic field. This is because of the time taken for the galactic cosmic rays to diffuse through the magnetised plasma.

Super storms

The largest recorded solar storm in history is the Carrington event of 1859, which disrupted telegraph lines on earth for several hours. This storm caused Aurorae to be recorded even as far south as Florida State in the USA.

If a storm of this magnitude should occur today, it would cripple all the VLSI-based communication systems, smart devices, mobile phones, computer networks and satellites, causing chaos.

Therefore, detecting such muon bursts could serve as an early warning in the case of a superstorm.



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