Last week, a massive solar flare sent a wave of energetic particles from the Sun shooting through space. Over the weekend, these particles reached Earth, delighting people worldwide with the sight of unusually vivid auroras visible in both hemispheres.
Typically seen near the poles, this weekend’s aurora was observed as far south as Hawaii in the northern hemisphere and as far north as Mackay in the southern hemisphere. Although this spectacular display of auroral activity has ended, those who missed it need not worry. The Sun is nearing the peak of its 11-year sunspot cycle, promising more periods of intense auroral activity in the coming year.
For those curious about the auroras’ glow and colours, the explanation lies in the interaction between charged subatomic particles (mostly electrons) from the Sun and Earth’s atmosphere. These particles are typically held at bay by Earth’s magnetic field, but near the poles, they enter the atmosphere and collide with molecules.
Our atmosphere is composed of about 20% oxygen and 80% nitrogen, with trace amounts of other gases. When high-speed electrons collide with oxygen molecules in the upper atmosphere, they break the molecules into atoms. These oxygen atoms enter an excited state and can relax by emitting light. The dominant green light of the aurora comes from oxygen atoms transitioning from the “¹S” state to the “¹D” state—a process that takes almost a whole second due to its rarity, known as a “forbidden transition”. This slow process allows the green light to be emitted in the thin atmosphere high above Earth.
Red light in the aurora results from an even more improbable transition, from the “¹D” to the “³P” state, which takes about two minutes. This light only appears at very high altitudes, where there’s less chance of collisions that could prevent the light from being emitted. Apart from green and red, auroras can also display blue and magenta hues, primarily due to ionized nitrogen molecules. These colours are more visible in photographs than to the naked eye due to camera technology, which can capture dimmer light over longer exposures and does not share the human eye’s limitation in low light.
Thus, while auroras are a beautiful and intriguing natural phenomenon, their occurrence and vivid colours are deeply intertwined with atmospheric science and solar activity. If you missed this aurora, keep your eyes peeled over the next few years as solar activity continues to peak.
