The alluring colourful lights that flow like a stream in the polar skies have captivated earthlings for a long time. Our ancestors considered them signs from spirits and gods, while folklores gave this phenomenon the utmost respect and power.
These lights are now commonly known as auroras/aurorae or the Northern Lights (Aurora Borealis) and Southern Lights (Aurora Australis) as they occur in the north or south pole regions, respectively.
It all begins with our biggest energy source, the Sun, which is highly vigorous and unstable. Solar transients occur due to the Sun’s instability. Some commonly known solar transients are sunspots, solar flares, solar wind, coronal mass ejections (CMEs), etc.
Among these transients, CMEs are clouds of charged particles ejected from the sun, which makes them the most crucial player in generating auroras. CMEs carry protons and electrons that cause auroras, whereas solar flares are only electrons, and solar wind is like low-energy CMEs. The sun is a gigantic ball of plasma (ionised protons and electrons) carrying magnetic fields. When oppositely directed field lines encounter each other, they break and reconnect in a process called magnetic reconnection.
One can use the analogy of a car accident to understand this concept. Imagine a scenario where two cars travel in opposite directions on their respective lanes. Suddenly, if one car does not follow its lane discipline, it enters the opposite lane and faces a head-on collision. The lanes here represent the magnetic field lines, and the cars represent the plasma travelling along the field lines. The accident is the magnetic reconnection phenomenon, and the smoke and dust ejected from the accident represent the CME phenomenon.
CMEs, which are plasma clouds, travel in all directions in our solar system after being released from the Sun. If directed towards the Earth, they take about 14 hours to 5 days to hit, depending on the energy that they carry. When the CME hits the earth, the plasma travels along the earth’s magnetic field lines and enters the atmosphere via the North and the South Pole. This is the reason why auroras are most commonly visible at polar latitudes.
But how do these charged particles display various colours in the sky? The answer lies in the gas and the altitude at which it is in Earth’s atmosphere.
The commonly seen green aurorae are caused by charged particle interaction with oxygen molecules 100-300 kmfrom Earth.This colour is most commonly seen because the human eye is most sensitive to the green colour spectrum. Oxygen molecules cause red aurorae present at 300-400 kmfrom Earth.Blue and purple aurorae are caused by interaction with nitrogen at altitudes 100 km or less. The rarest colours are the yellow and pink auroras, resulting from a mixture of green and blue aurorae.
Auroras are one of the natural wonders of our universe. People travel from all over the world and bear freezing temperatures to be present at the right moment and time to imbibe the magic of these phenomena. But not many know that this beauty has a beastly side to it.
Major geomagnetic storms
Some of the significant geomagnetic storms recorded in history begin with the infamous 1859 Carrington event, the August 1972 solar storms, the 2000 Bastille Day event, the Halloween storms of 2003, and the 2006 Christmas event, to name a few.
The 1989 Québec storm is an excellent example of a ground-based disaster caused by a geomagnetic storm. A solar flare disrupted the Hydro Québec generating station’s electric power transmission and even melted a few power transformers in New Jersey, USA. The event created a major blackout in Canada, leaving more than 5 million residents without electricity for nine hours.
In February 2022, an event destroyed over 40 newly deployed Starlink satellites worth USD 50 million shortly after deployment.
A recent storm on November 25, 2023, helped countries residing at mid-latitudes and lower to witness auroras, which is rare. Residents captured these beautiful lights from the UK, Poland, China, and India during this event. Photographs showed unique orange auroras that combine red and green auroras, but the most witnessed colours were red. Fortunately, there have been no reports of damages caused by this storm.
Negative side of the spectacle
Geomagnetic storms cause auroras. These storms are major disturbances caused by the earth’s magnetosphere, the region where the magnetic field prevails when a CME or solar wind transfers immense energy.
Geomagnetic storms can be divided into five categories, i.e. a minor storm falls under the G1 category and an extreme storm under the G5 category. When a storm with intensity G3 and higher occurs, it poses a problem to power grids. Auroras are composed of charged particles, i.e. ionized protons and electrons.
During extreme geomagnetic storms, these particles reach lower altitudes and interact with the power lines, causing induced currents to flow in the power grids and pipelines on the ground. The particles can hit the superconductors in an aeroplane’s circuitry at high altitudes and cause damage. In space, satellite malfunctions and GPS interference occur.
Studying space environment
This brings us to the exciting field of Space Weather—studying variations in the space environment between the sun and the Earth. Space weather scientists worldwide have been studying solar transients and their interaction with the Earth to devise a prediction model to help alert various facilities that control devices prone to damage from these geomagnetic storms. Ground- and space-based observatories have been used for this purpose.
Like most phenomena in nature, several variables are responsible for the generation of storms. Prediction is possible, but complete accuracy is yet to be achieved.
The dangers of geomagnetic storms are many. We are heading towards more and more reliance on electricity. Gadgets and devices have become necessities in our daily lives, and it is hard to think of a facility that does not run on electricity. Power outages could cause loss of lives and the economy. However, the past disasters caused by these geomagnetic storms have helped scientists and engineers learn and devise methods to minimise the damages.
Innovative technologies such as incorporating transatlantic cables in power systems and sending out alerts immediately after an Earth-directed CME or flare eruption have helped humanity tackle these problems. Nevertheless, increased interest and funding must go into this field of study to prepare humankind for such disasters as we are entering the peak of the current solar cycle, where more solar transients are expected.
(The author is a space weather scientist who works at the University of Calgary)
Auroras can cause electric blackouts.
(Published 16 December 2023, 14:18 IST)