The Science behind Northern Lights (Aurora Borealis)

Prof S. S. Bhatti, Calgary (Canada)

According to NASA, we are currently in the midst of a ‘Solar Maximum,‘ meaning 2025 could be a better-than-usual year for spotting the Northern Lights, a dazzling spectacle, nearly all over Canada and several U.S. states.  Over the past few days, the Sun has produced several bursts of energy known as Coronal Mass Ejections (CME). Severe geomagnetic storms could disrupt radio, GPS, and other communication systems. These displays, known as the Northern and Southern Lights, are visible near the poles, where charged particles from the Sun collide with Earth’s atmosphere. They can also produce Bremsstrahlung Radiation—a type of X-rays—when high-energy electrons decelerate (slow down) or get deflected by nuclei of atoms of gases in the atmosphere.

In 2024, the strongest geomagnetic storm in the past two decades hit the Earth, producing light displays on the Arctic Circle. Dancing colourful lights appeared in unexpected places, including Germany, the United Kingdom, New England and New York City. In 1859, a severe ‘Solar Storm” triggered auroras in Hawaii and set telegraph lines on fire. Experts are not able to predict these months in advance. Instead, they alert relevant parties to prepare in the days before a solar outburst hits the Earth.

Northern Lights: The Sun has its maximum phase of an 11-year activity cycle, making these light displays and emits a stream of charged particles (Solar Wind), mostly electrons and protons. Earth’s magnetic field channels these particles toward the polar regions. When they collide with gases like oxygen and nitrogen in the upper atmosphere (at 80–300 km), they excite the atoms. As the excited atoms return to their ground (normal and stable ) state, they emit visible light—green, red, purple—creating the aurora.

Bremsstrahlung : (German for “braking radiation”) is emitted when a charged particle (an electron) is decelerated or deflected by another charged particle (plasma), hits an atomic nucleus. The sudden change in velocity causes the electron to lose kinetic energy, which is emitted as a photon—in the X-ray range. This radiation has a continuous spectrum, unlike the discrete lines of auroral light.

Bremsstrahlung in the Aurora :  Some electrons are extremely energetic. As they plunge into the denser atmosphere, they are rapidly decelerated by collisions with atomic nuclei producing bremsstrahlung X-rays, which are detectable by satellites. Visible and Infrared (IR) Imaging is done by measuring nighttime light emissions, including those from oxygen and nitrogen atoms. Invisible Ultraviolet (UV) radiations and X-rays require different sensors.

Sensors (carried by satellites) capture emissions that cause electric and magnetic field variations due to  periodic solar activity and detect the energy and direction of incoming particles. The satellites continuously monitor these, providing real-time data on space weather impacts, and geomagnetic storms that can affect satellites, power grids, and communications. The two types of radiations. responsible for these polar spectacles, are given below .

Feature	Auroras	Bremsstrahlung Radiation
Cause	Collision of electrons with atoms	High-energy electrons
Target	Atmospheric gas atoms	Atomic nuclei (positive charged)
Emission	Visible light (green, red, purple)	X-rays (invisible  radiation)
Energy Source	Excitation and relaxation of atoms	Kinetic energy loss
Altitude	80–300 km above Earth	Lower altitudes
Spectrum	Discrete lines (specific colors)	Continuous spectrum (X-rays)