What are the Northern Lights?

AESTHETICS

The Aurora Borealis or Australis (northern or southern lights), is a natural and colorful display which takes place in the earth’s atmosphere, typically by the two uppermost latitude regions, respectively Arctic and Antarctic. Auroras generally appear either as a diffuse glow or as moving clouds or "curtains". They approximately extend in the east-west direction. At some times, they look like quiet arcs; at others, the active auroras, they evolve and change constantly, into what is commonly known as the spectacular "dancing Aurora”.

Diffuse aurora glow

"Curtains" aurora

The Aurora Borealis or Australis (northern or southern lights), is a natural and colorful display which takes place in the earth’s atmosphere, typically by the two uppermost latitude regions, respectively Arctic and Antarctic. Auroras generally appear either as a diffuse glow or as moving clouds or "curtains". They approximately extend in the east-west direction. At some times, they look like quiet arcs; at others, the active auroras, they evolve and change constantly, into what is commonly known as the spectacular "dancing Aurora”.

Typical "corona" aurora

Some people claim to hear noises associated with the most intense northern lights, but documenting this phenomenon has been difficult.

PHYSICS

The phenomenon of the Aurora is produced by the interaction of electrically charged particles (solar wind) with the gases forming the ionosphere, a part of the upper Earth’s atmosphere, from about 85 km to 600 km altitude. The solar wind is a sort of faint plasma, a stream of charged particles ejected by the Sun in all directions. 95% of the solar wind is made of protons and electrons, in equal parts. The rest is essentially helium. Solar wind’s velocity can vary approximately between 300 and 900 km/s. To reach Earth (149.600.000 km) will therefore take to the solar wind between 2 and 9 days.

Very rare example of blue aurora

Earth itself is largely protected from the solar wind by its magnetic field, which deflects most of the charged particles; however a minor amount of these particles are trapped in the "Van Allen radiation belt”. A small number of particles manage to travel, as though on an electromagnetic energy transmission line, and reach the Earth's auroral zones. Here, in the ionosphere, by the collision with the solar wind (mainly with electrons), oxygen and nitrogen atoms get excited (ionized). Excitation energy is shortly after lost by the emission of a photon. Billions of photons are visible to our naked eye in form of Aurora. Oxygen emissions give the most characteristic green color shading off sometimes, depending on the quantity of energy absorbed, into pink. The less common blue color or the even rarer red, are due to nitrogen emissions.

GEOGRAPHY

The region above the ionosphere, extending several thousand kilometers into space, is called the magnetosphere. This region protects Earth from cosmic rays. However, Earth's magnetosphere is frequently hit by solar flares causing geomagnetic storms, which result in displays of auroras. The short-term instability of the magnetic field is measured with the K-index.

Tromsø, provisional K-indices - last seven days:

Due to a weaker magnetic protection, particles moving towards earth hit the upper atmosphere around the poles, forming a sort of ring, the auroral oval (auroral zone). This ring is centered on the magnetic poles (about 11° away from the geographic ones) with a diameter of about 3000 km at quiet conditions that expands proportionally to the intensity of Earth magnetosphere’s disturbance. Auroral ovals are most normally observable between 60° and 70° latitude. Occasionally, when severe magnetic storms temporarily enlarge the auroral oval, auroras can be seen in temperate latitudes.