Breach in Earth's magnetic field

detrimental

It is known that solar storms can cause major disruption to human civilisation by crippling large electrical power grids, satellite operations and communications. In the early hours of June 21, 2015, a NASA spacecraft recorded particles blasting off the Sun at roughly 1,300 km per second. It was a solar flare. Forty hours later, a severe geomagnetic storm knocked out radio signals in North and South America and the Aurora Borealis was spotted as far south as Texas in the United States.

This storm was described as one of the most powerful (four, on a scale of five) in recent history. Recent results from a cosmic ray experiment in India, published in Physical Review Letters in November 2016, show that the solar storm resulted in weakening the Earth’s magnetic field, allowing high energy cosmic rays. The result has attracted considerable attention with possible application to prediction of space weather.

Earth’s magnetosphere
It was in 1600 that William Gilbert, a physician, demonstrated that Earth behaves like a giant magnet. We now know that it is like a huge bar magnet with north and south poles but with the axis aligned at 11 degrees to the spin axis of Earth. It was learnt in 1940s that the magnetic field arises from electric currents in Earth’s molten metallic core which itself is a result of fast rotation around the axis. The average amount of the field is about half a Gauss.

While the moon and inner planets do not have significant magnetic fields, the other planets have much stronger fields than Earth. The discovery of Van Allen
radiation belt, consisting of layers of energetic charged particles, in the late 1950s proved the existence of this magnetic field. It is this magnetosphere which has shielded Earth from marauding particles and thus responsible for life on Earth. 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 Earth.

However, the Earth’s magnetic field is distorted due to the solar wind — particles which emanate continuously from the Sun  and stream towards Earth at speeds of 400 km per sec. This results in an uneven magnetosphere with differences between the side facing Sun and the one away from the Sun. This constant interaction between the solar wind and magnetic field results in a dynamic magnetosphere.

The Ooty EAS experiment
Cosmic ray primaries, which impinge upon the top of the atmosphere from all directions, keep on interacting several times with air nuclei in their downward path in the atmosphere. These interactions produce many secondary particles which also keep on producing more particles through various processes in their downward path. The result is a large number of particles, mostly electrons and particles called muons, spread over a large area at any given observational level. This phenomenon of particle cascades in the atmosphere is called Extensive Air Showers (EAS) and is used to study the properties of the Cosmic Ray primary particles.

These EAS experiments were started by the Tata Institute of Fundamental
Research (TIFR) in Ooty, Tamil Nadu under the leadership of Professor B V Sreekantan in the late 1950s and later continued by Professor Suresh Tonwar. Interesting results on the composition of primary cosmic rays and on behaviour of interactions at very high energy had been obtained by these experiments in the earlier years. The present experiment — called GRAPES 3 — has 400 electron detectors that are spread over an area of 25,000 square metres and nine muon detectors with a combined area of 560 square metres. This experimental activity is led by Professor Sunil Gupta of TIFR with participation of about 30 scientists from seven universities in India and five universities in Japan.

Galactic Cosmic Rays (GCR) of around 20 GeV energy produce detectable muons. Therefore detection of muons is tantamount to detection of GCRs themselves. The muons can penetrate large amount of matter and are detected in proportional counters housed in tunnels. On a normal day, the muon rate (of energy greater than 1 GeV) collected by all the nine detectors combined is about 150 million per hour. However, on June 22, starting from 19:00 UTC (12:30 AM IST), an excess of around one million muons in two hours  was recorded. This is a huge effect amounting to 50 sigma showing that the probability that it is by chance is negligible. The increase was seen in all the nine muon detectors which attests to the credibility of the phenomenon. 

Extensive analysis and simulations by the group has shown that Earth’s magnetic field bent these particles about 210 degrees from the day-side to the night-side of the Earth where it was detected as a burst by the muon telescopes around midnight on June 22, 2015. It is also interesting that muon bursts precede the actual storm. The most important finding was the peak in the incoming intensity of muons correlated with a negative spike in a component of the magnetic field which clearly shows a weakening of the Earth’s magnetic field because of the flare from the Sun.
It is also seen that there was a severe compression of the magnetosphere, forcing it to shrink from 11 to four times the radius of Earth. Thus, the Earth’s magnetic shield temporarily cracked due to the occurrence of magnetic reconnection, which is essentially a rearrangement of the magnetic topology, allowing the lower energy galactic cosmic ray particles to enter our atmosphere. The simultaneous occurrence of the burst in all nine directions suggests its origin close to Earth. While the decrease of muons, neutrons and others with geomagnetic storms has been noted before, this is the first time that the decrease could be causally connected with the weakening magnetic field.

The fact that this happened at all is a major concern to Earth’s inhabitants. As science writer, Katherine Kornei puts it in a recent article, “Our technological society, for all of its advances, is still susceptible to the whims of our closest star.” Our magnetosphere was only temporarily cracked this time, but the major red flag is that it’s possible to crack our shield at all. Therefore, detecting such muon bursts could serve as an early warning in the case of future super storms.