Formation of the Northern Lights (Aurora Borealis)

Northern Lights (Aurora Borealis)

The phenomenon commonly referred to as the Northern Lights, or scientifically termed aurora borealis, encompasses the vibrant bands of light that grace the skies of Earth’s higher latitudes. These awe-inspiring illuminations make their appearance around the vicinity of the northern polar circle. Conversely, the southern hemisphere witnesses its own version known as the southern lights, or aurora australis.

This celestial spectacle finds its origin in the luminous dance of photons emitted by the oxygen and nitrogen present in the upper tiers of the atmosphere. The genesis of this luminosity lies in the interaction of high-energy particles from the solar wind, which originates from the sun’s corona, situated a staggering 150 million kilometers away from our planet. These energetic particles engage in a collision with the Earth’s ionosphere, a distinctive atmospheric layer acknowledged as the ionosphere. This collision instigates a process of ionization within the atoms and molecules constituting this layer. As these ions eventually find equilibrium, they release the stored energy, giving birth to the mesmerizing phenomenon we know as the aurora northern lights (aurora borealis) or southern lights (aurora australis).

Formation of the Aurora Borealis

Ultraviolet image captured by NASA's Solar Terrestrial Relations Observatory (STEREO)
Ultraviolet image captured by NASA’s Solar Terrestrial Relations Observatory (STEREO) Ahead spacecraft on February 12, 2010, shows a very small solar storm forming in two active regions of the Sun. Image: NASA.

A solar flare event taking place in the sun’s outer atmosphere emits a significant quantity of particles into space, which we call solar wind. This wind consists of ionized gas plasmas, such as protons and electrons. These plasmas breach Earth’s magnetic defense and interact with the particles in our atmosphere. This interaction produces multiple small bursts of illumination, referred to as photons, that give rise to the auroras, also known as auroras, which are visible to us.

The amplification in the density of the solar wind can trigger a geomagnetic disturbance, causing temporary disruptions in Earth’s magnetic field. Enthusiasts of aurora sightings have noticed that geomagnetic disturbances that commence during the daytime often lead to the appearance of auroras at night.

Upon the arrival of a geomagnetic disruption on Earth, the flow of plasma momentarily warps Earth’s magnetic field (known as the magnetosphere), resulting in the accumulation of energy on the side of Earth facing away from the Sun. This liberated energy propels charged particles forward, forming a spiral pattern as they advance towards the poles.

The Origin of the Geomagnetic Storm

Magnetic lines of force surrounding Earth known as the magnetosphere against Sun's solar wind.
Magnetic lines of force surrounding Earth known as the magnetosphere against Sun’s solar wind.

Geomagnetic storms arise from frigid zones within the sun’s outer layer known as coronal holes, along with potent incidents called Coronal Mass Ejections (CMEs). These vigorous bursts emit substantial amounts of plasma and electromagnetic radiation. When a CME is aimed at our planet, it propels the solar wind along its course at astonishing speeds. The highest recorded velocity for a CME’s journey is approximately 10,000,000 kilometers per hour. By way of comparison, the customary pace of the solar wind is roughly 1,400,000 kilometers per hour. Solar flares, recognized as sudden augmentations in the sun’s radiance, are intertwined with CMEs.

Magnetometers situated on the ground, employed to monitor auroras, reveal space weather occurrences tied to the auroras. These devices have the ability to anticipate a geomagnetic storm 20 to 30 minutes beforehand.

The Colors of the Northern and Southern Lights

Every element within Earth’s atmospheric composition exhibits the phenomenon of light reflection at distinct wavelengths. Thus, the hues visible in the celestial expanse hinge upon the variety of energized atoms present (such as nitrogen or oxygen), the magnitude of energy absorption, and the intricate amalgamation of light wavelengths. Additionally, the interaction of sunlight and lunar radiance affects the aurora’s chromatic manifestation.

Upon collision with an atmospheric atom or molecule, particles undergo deceleration and impart a portion of their kinetic energy. The molecule, albeit fleetingly, retains this energy and predominantly discharges it as luminescence. The wavelength of this emitted light remains a telltale signature of the atom or molecule involved, culminating in our perception of diverse hues.

Black Auroras

Observed sporadically within auroras, a shadowy band occasionally emerges, referred to as the “black aurora.” This enigmatic region becomes discernible through the occlusion of starlight. The genesis of these black auroras is attributed to an upper atmospheric electric field that obstructs the interaction of electrons with gas particles.

Upon particle interactions with oxygen molecules, energy is released in the form of verdant luminescence. This emerald glow predominates in auroral displays owing to the preponderance of solar particle collisions transpiring at altitudes spanning 100 to 200 kilometers, where oxygen molecules abound. The green tint is frequently linked with the most elevated latitudes.

At altitudes below 100 kilometers, charged particles engage with nitrogen molecules, resulting in the emission of blue and crimson red. This interaction begets violet fringes encircling the green auroral radiance. Collisions with agitated or high-energy oxygen atoms at loftier altitudes yield infrequent occurrences of deep crimson auroras. The crimson hue distinctly characterizes lower latitudes.

The emissions stemming from a fusion of oxygen and nitrogen can yield an array of colors mirroring the iridescence of a rainbow. Notably, a peculiar phenomenon that frequently includes crackling sounds occurs along with the formation of such auroras. This auditory phenomenon arises due to the liberation of electric charge from atmospheric strata designated as inversion layers.

Light Emitted by the Elements

Oxygen stands as the most prevalent element accountable for the genesis of auroras. It is the primary contributor to the vivid emerald shade at a wavelength of 557.7 nm and the profound russet tinge at 630.0 nm. The unadulterated verdant tones and the yellow-greens are products of oxygen’s excitation.

Nitrogen, on the other hand, radiates azure luminescence across diverse wavelengths alongside crimson radiance.

Furthermore, various other gases present in the atmosphere also emit luminescence upon stimulation. Yet, certain wavelengths extend beyond the human visual range or possess feeble intensity, rendering them imperceptible. Hydrogen and helium, for instance, give rise to blue and purple radiance. While these myriad colors might elude our eyes, photographic emulsions and digital cameras possess an extended gamut of hues within their purview.

Northern and Southern Lights Colors Depending on Altitude

  • 240 km (149 miles) above – red – oxygen
  • Up to 240 km (149 miles) – green – oxygen
  • Over 100 km (62 miles) – purple – nitrogen
  • Up to 100 km (62 miles) – blue – nitrogen

Periods of Auroras

Observing the aurora requires careful planning. Auroras are visible during periods of increased solar activity, such as solar flares and CMEs (Coronal Mass Ejections). Solar activity follows approximately an 11-year cycle, with solar maximums occurring. Predicting solar cycles is not straightforward, but the next solar maximum is expected to take place in July 2025. However, a solar maximum is not a strict requirement for observing the aurora.

Auroras, or the Northern and Southern Lights, are most commonly seen in clear, open skies between 10:00 p.m. and 2:00 a.m. Winter is the ideal season to observe them, as nights begin earlier and the sky remains dark for a longer duration. Additionally, geomagnetic storms are most likely to occur in March and September, when Earth’s magnetic field aligns with that of other planets. Services such as the Australian Bureau of Meteorology provide aurora alerts to help enthusiasts track these celestial displays.

Aurora Borealis on Other Planets

Auroras on Jupiter, Saturn, and Io
Auroras on Jupiter, Saturn, and Io. Image: Flickr.

Earth is not the only planet with auroras. For example, astronomers have captured aurora photographs on Jupiter, Saturn, and Io. However, the colors of auroras on these planets differ due to variations in their atmospheres. The main prerequisite for auroras on a planet or moon is an atmosphere bombarded with energetic particles. If a planet has a magnetic field, its auroras will take on an oval shape at both poles. Planets without a magnetic field can still experience auroras, but they will form irregularly.