Around midnight on June 22, 2015, the GRAPES-3 cosmic ray telescope at Ooty – operated by the Tata Institute of Fundamental Research (TIFR), Mumbai – detected an unusual burst of cosmic rays. It lasted for about two hours, which had the team of high-energy cosmic ray scientists greatly surprised. The team comprised scientists from TIFR, J.C. Bose Institute, Kolkata, and the Indian Institute of Science Education and Research (IISER), Pune, and collaborating Japanese scientists. The excess during the burst period was about a million cosmic-ray charged particles called muons (of energy about 1 GeV), over and above the normal emission of about 300 million.
When trying to figure out what could have caused this, they found that the burst occurred exactly at the same time that a strong eruption from the Sun, called a coronal mass ejection (CME), had arrived on Earth. CMEs are explosive outbursts from the Sun’s outer atmosphere, the corona, and typically carry roughly a billion tons of solar material outward at speeds of a million kilometres an hour or more. The material consists chiefly of overheated gas (plasma), protons and electrons. Also wrapped in this plasma are powerful magnetic fields of solar origin. The scientists, therefore, reasoned that the interaction of the CME with Earth’s magnetic field could have caused this – because strong CMEs, when directed towards Earth, are known to play havoc with the planet’s magnetic field.
Also known as the geomagnetic field (GMF), this field protects us from high-energy charged particles and radiation carried by the solar wind emanating from the Sun. They also shield us from galactic cosmic rays (GCR) coming in from outside the Solar System and constantly bombarding Earth’s atmosphere. The field deflects them away. The solar wind is essentially a plasma of protons and electrons. While CMEs are sporadic, the solar wind is emitted as a continuous stream by the Sun. On the other hand, GCR consists of particles travelling at close to the speed of light (mostly protons and light nuclei) and are produced by supernovae and other powerful cosmic sources in and beyond our galaxy.
The solar cycle
The solar wind also carries with it Sun’s magnetic field to the far reaches of the solar system. This extension of the solar magnetic field permeating through the entire Solar system is called the interplanetary magnetic field (IMF). The pressure of the solar wind on the GMF has the latter compressed on Earth’s dayside and stretched like a tail on Earth’s nightside. While on the dayside the magnetic field gets confined to within about 10 Earth-radii, the tail on the nightside extends to hundreds of Earth-radii. In effect, the GMF is shaped like water flowing around a rock in a stream.
This profile of Earth’s magnetic field encompasses the magnetosphere, the region of space in which Earth’s magnetic field is dominant, and which acts as a protective shield against charged particles.
However, every so often, particularly during periods of intense solar activity, violent eruptions from the Sun in the form of solar flares and CMEs. The CMEs can interact with the GMF and trigger intense geomagnetic storms. These storms are major electromagnetic disturbances in Earth’s magnetosphere. They occur when enormous amounts of electromagnetic energy are exchanged between the CMEs and the GMF. They can potentially damage electrical and electronic systems on Earth as well as affect satellite hardware and astronauts in space.
The Sun goes through an 11-year cycle of active and quiet phases. Each active solar cycle is distinguished by enhanced magnetic activity on the Sun’s surface, made evident by an increased number of sunspots. And the magnetically active regions around these sunspots become the points of origin of solar flares and CMEs. During an active solar cycle, there can be several CMEs on a single day. During quieter periods, they may occur once every week or so. Since CMEs occur in all directions, only rarely will one be pointed Earth’s way. Very fast CMEs can reach Earth in as little as 14-17 hours. The slower ones take a few days.
Currently, the Sun is in the midst of an active phase designated Cycle 24 (counting from the year 1755). It began in January 2008.
The strength of the magnetic fields carried forth by a CME is higher than those in the ambient solar wind. As a result, the CME fields push outward and expand in size as they travel outward. By the time they reach Earth, really powerful CMEs are enormous, filling up nearly half the interplanetary volume between the Sun and Earth. And because of their immense size, they can take 24-36 hours to pass over Earth after the leading edge reaches the planet. The CME’s magnetic fields also sweep away charged particles from galactic cosmic rays – this bit of surprise help is called the Forbush decrease, named for Scott Forbush.
Since CMEs travel faster than the solar wind’s plasma, they also generate a shock wave just the way a supersonic aircraft creates a sonic boom when it crosses the speed of sound. The impact of the interaction between the incoming solar wind/CME and the GMF around Earth can induce a phenomenon called a magnetic reconnection near the interface between the solar wind and Earth’s magnetosphere. This interface lies just inside of the magnetosphere’s boundary on the dayside as well as in the near-Earth magnetotail on the night side (see image below).
A magnetic reconnection is the breaking and joining of oppositely directed magnetic field lines in a magnetised plasma. During reconnection, the energy carried by the magnetic field gives a kick to charged particles in the area and transforms into kinetic energy. Now, as you know, Earth’s magnetic field is dipolar, with the magnetic north located at the geographic South Pole and the magnetic south located at the geographic North Pole. And the field is directed from South to North. On the other hand, the solar magnetic field carried by the solar wind/CMEs can be either oriented Southward or Northward. If it is Southward as it approaches Earth, it becomes somewhat like bringing two bar magnets close with their opposite poles facing each other: there will be regions where the two fields cancel each other out, resulting in a region where the field lines reconnect. This facilitates an efficient transfer of energy between the CME and Earth’s magnetosphere, leading to geomagnetic storms and other space-weather events.
Understanding geomagnetic storms
The reconnection on Earth’s nightside side accelerates the plasma in that region down Earth’s magnetic field lines and into the poles. The particles of the plasma strike the atmosphere and excite nitrogen, oxygen and other atoms. These atoms become excited because they’ve been imparted some energy, and they quickly lose this energy to become stable by emitting light of different colours. The result is the brilliant display in the sky known as the aurorae – borealis in the north, australis in the south. During strong CME events, these curtains of light can be seen at much lower latitudes as well.
Besides accelerating the plasma, the exchange of electromagnetic energy during magnetic reconnection also affects the Van Allen radiation belts, causes intense electric currents both in the magnetosphere and the ionosphere, and heats up the ionosphere and the thermosphere. Moreover, a ring of current flowing westward gets established around Earth and that in turn produces magnetic disturbances on the ground.
All of these disturbances together make a geomagnetic storm. The quality and magnitude of the currents and the magnetic disturbances are together used by scientists to compile what’s called a Kp index, a number between 0 and 9 that gives a way to measure the relative sizes of disturbances. Specifically, the index is based on the maximum fluctuations of the horizontal components of Earth’s GMF measured in real time by a network of observatories around the world. And in turn, the Kp index is used by the US National Oceanographic and Atmospheric Administration (NOAA), which monitors various space-weather events and issues appropriate alerts. And like the index, these alerts also sit on a scale called the G-scale. It runs from 1 to 5.
The effects of geomagnetic storms also include bursts of energetic particles from the Sun striking Earth’s atmosphere in what are called solar energetic particle events; geomagnetically induced currents on the ground that can affect the power grid (like the one in March 1989 that caused widespread blackouts in Canada) and pipeline operations; and intense ionospheric disturbances that can disrupt radio and radar communications. Space-based communication systems are also affected: spacecraft operations, including GPS navigation, are disrupted. The geomagnetic storms also mess with magnetic compasses, magnetic surveys and directional drilling operations. An uptick in radiation levels in space also disturb spacecraft hardware as well as the wellbeing of astronauts – such as those in the International Space Station.
The most intense geomagnetic storm in recorded history is the Carrington event of 1859. It completely disrupted the network of telegraph lines all over Europe and North America for many hours. Today, with the ubiquitous use of electronic devices based on microchips – particularly susceptible as they are to the effects of charged particles – electromagnetic disturbances in the sky can potentially cripple computer networks, mobile telephone networks and other instruments resulting in enormous financial losses and in temporary, but no less debilitating, disruptions in communication.
According to the NOAA, polar flights rerouted due to space-weather conditions cost airlines around $100,000 (Rs 66.7 lakh) per flight. If airborne survey data or marine seismic data are degraded by solar activity, the economic impact can range from $50,000 to upwards of a million (Rs 33 lakh to Rs 7 crore). Geomagnetic storms can last for several hours and the more intense ones, for a few days. Accurate space weather information and forecasts are therefore vital for mitigating the impact ground-based infrastructure.
The NOAA’s Space Weather Prediction Centre (SWPC) issues three-day forecasts of impending space weather events, such as geomagnetic storms, based on measurements of solar activity by satellites as well as measurements of the interplanetary magnetic field (IMF). But there is significant uncertainty concerning when the storms will strike Earth because they are based on simulations using theoretical models of how CMEs propagate through space.
Also, closer to an impending event, the alerts issued by SWPC are based on ground-based realtime measurements of fluctuations in the magnetic field, as discussed earlier. However, as the NOAA itself has noted, there are limitations in this approach. Because all the observatories in the network are not identical, there could be differences in their estimates. Also, a highly localised disturbance can affect a certain region but its severity may not be reflected in the globally averaged index. So a more accurate ground-based alert system continues to be desirable to better protect vulnerable spots on the ground.
To see a storm coming
Now, we circle back to the GRAPES-3 experiment in Ooty. The authors of the experiment have suggested the possibility of evolving a more accurate method using cosmic ray data, notwithstanding its diminished warning time of only about a few hours. Their suggestion follows their significant discovery – of the effect that geomagnetic storms can have on the flux of charged particles not just of solar origins at the poles but also of cosmic origins over other parts of the world as well. As a recent article in the journal Science noted, even with only a few hours’ advance but accurate warning, power grids could reschedule current distribution in the network to reduce their susceptibility to the storm.
Using TIFR’s cosmic-ray telescope at Ooty, the team there found that a CME that occurred on June 21, 2015, significantly reconfigured the GMF – while the consequent geomagnetic storm allowed an unusually high flux of cosmic ray particles to arrive on Earth. This flux, they found, was due to a transient weakening of the GMF (equivalently a temporary weakening of the GMF’s shielding effect) for as long as two hours.
When a cosmic-ray particle strikes a molecule in Earth’s atmosphere, it generates a cascade of secondary particles. A large fraction of these particles consists of some called pions. Pions can either be charged or neutral. The lower-energy charged pions among them decay, before interacting again, into muons. It is the muons that survive long enough to reach the ground and be detected. And it is in the flux of these high-energy muons that the GRAPES-3 researchers found a significant increase. This implies an increase in the intensity of the primary galactic cosmic rays. In other words, muons are a proxy for the cosmic rays.
(A flux increase in 1 GeV muons is equivalent to a corresponding intensity increase of primary GCR of about 20 GeV.)
Besides the detection itself, the scientists have been able to use simulations and pin down the exact cause as well as the underlying mechanisms that enabled this enhanced flux to be detected. Curiously enough, it happened on Earth’s nightside. This work has been published in the October 21 issue of the journal Physical Review Letters.
This finding also suggests that cosmic-ray flux data can serve as an accurate ‘early warning’ unto an impending geomagnetic storm. “We are not claiming that we can predict space weather events at this point of time,” Sunil Gupta of TIFR, the lead author of the paper, told The Wire. “The Ooty telescope is not geared for that. That would first require a lot of further research, calibration with observations over a long period, funds and people to operate the instrument in that mode. That was only a suggestion,” he added. At the same time, the GRAPES-3 scientists have also been able to observe over the years that the fluctuations in the rates of muons can serve as a proxy, with high sensitivity, even for normal variations in atmospheric parameters – such as, for example, temperature.
On June 21, 2015, at about 0800 hrs IST, a strong CME associated with a double-peaked solar flare, erupted from the sunspot region 2371. Its image was snapped up by the NASA Solar and Heliospheric Observatory, showing it to be a so-called ‘symmetric full halo’ CME with a visible Earth-directed component. Some 40 hours later, on June 22 at about 0010 hrs IST, the CME arrived at Earth’s doorstep. In fact, this one was preceded by two other CMEs that had reached Earth on June 21 and June 22, both from the same sunspot region. But it was only the third CME that triggered a G-4-class geomagnetic storm.
According to the SWPC, a G-4 storm is driven by a Kp-index of 8, with magnetic fluctuations in the 330-500 nano-tesla (nT) range. To compare, the magnetic field on Earth’s surface is about 25,000-65,000 nT. Therefore, magnetic fluctuations in a G-4 storm are indeed significant.
Fine-tuning the simulations
The GRAPES-3 is a large area tracking muon telescope that measures the rate of muons (of energy greater than 1 GeV) along nine independent directions. The instrument detects 150 million muons per hour. This, the authors say, provides an almost accurate estimate of the intensity of galactic cosmic rays intensity and their variations. The experiment measured muon rates once every four minutes over 23 days, from June 12 to July 4, 2015 (a total of 8,192 intervals). The Forbush decrease due to the CME had already begun within 4.5 hours of the arrival of the first CME on June 21, which continued during the arrival of the subsequent CMEs as well. But it was in the midst of this continuing decrease that a spike indicating a burst of muons, a proxy for GCR intensity, was detected.
Coincidental with the period over which Earth’s GMF became weaker, the muon burst also lasted for two hours. The burst’s profile and the GMF variation were also almost completely identical. As noted earlier, the muon flux excess during the burst period was about a million against a background of about 300 million, which is statistically significant. Moreover, the burst was found to be almost simultaneous in all nine directions along which GRAPES-3 measured them. According to the authors, this strongly suggested that the burst lay close to Earth, “possibly within the magnetosphere”. “This could well be one of the clear-cut evidences of magnetic reconnection occurring in the magnetosphere and is also a clear pointer to cause and effect,” says Gupta.
Besides the timing coincidence, it was also observed that the muon burst was highly correlated, with a 40-nT surge in the IMF caused by the CME. This led the scientists to probe the idea of reconnections between the IMF and the GMF being the cause further.
In fact, computer simulations of the interactions between the IMF and the GMF showed that, during the reconnection, the IMF was compressed by a factor 17, increasing its field strength to about 680 nT. Since the IMF and the GMF are parallel but pointing in opposite directions, this increase in IMF is equivalent to a corresponding decrease in the GMF. And it is this drop in GMF strength – i.e. a breach in the shield – by about 2% that resulted in the arrival of the muon burst.
The simulations also found that this transient reduction in the field was spread out to a distance twice the radius of Earth. This means the decrease of 680 nT is averaged out over a volume equivalent to seven-times Earth’s. Gupta pointed out that this indicates that cosmic rays probe the effect of CMEs over a much larger volume.
“We started our simulations with 40 nT but we had to keep on increasing the compression factor for the amplitudes of simulated data and observed increase to match,” said Gupta. “The match is so good that there can be no other explanation for the observed burst just for a short duration. We were able to even match the observed gradation in flux increase from different directions, with the maximum, about 2%, being from the north,” he said.
Interestingly, to maximise the correlations between the IMF in space and the cosmic-ray burst over Earth, and to match the amplitudes exactly, the simulated data had to be shifted in time by 32 minutes. This means there was a 32-minute delay between the opening up of Earth’s magnetic field and the arrival of the burst at Ooty. This, the scientists say, could be due to the time taken by the cosmic rays to diffuse through a turbulent magnetosphere. The authors also note in their paper that the field change (of 680 nT) due to reconnection was only 70% of the maximum possible. This they infer from the fact that the impact of the CME was such that the magnetosphere on the dayside was compressed from a distance of 11.4 Earth radii to just 4.6 Earth radii.
Then again, how did the field changes on the dayside result in a cosmic-ray flux increase on the nightside, where the Ooty telescope was located during the CME? Under normal circumstances, shielding by the GMF deflects particles below about 20-25 GeV (at Ooty’s latitude). Charged particles in the presence of a magnetic field perpendicular to Earth’s orbit plane will follow a curved path: the higher the energy, the more the radius of curvature. With the weakening of that perpendicular field by 680 nT, high-energy particle trajectories were bent from the dayside and which made them shower on the nightside. And the Ooty telescope promptly picked up on them.
Interestingly, no cosmic-ray experiment on the dayside seems to have detected this surge. A couple of neutron monitors on the nightside in Kazakhstan and Armenia did, however, record increased rates coincident with GRAPES-3 measurements. “But statistically those spikes do not have a very high significance,” explains Gupta. “The count rates of neutron monitors are orders of magnitude lower than our count rate of four billion a day. Globally, GRAPES-3 is the only facility that has the sensitivity to pick 9up on] this.”
“Even ICECUBE [the neutrino observatory located at the South Pole] detects only 100 million muons per day. Also, ICECUBE detects very high-energy muons that are insensitive to CME effects. But ICETOP, an array of ice tanks atop ICECUBE, is already mining its data for this event and so are major experiments worldwide,” he added. However, whether this basic research facility can be turned into an early warning system for space-weather events is a thought for the future.
R. Ramachandran is a science writer.