Info about Magical Sky

An aurora, sometimes referred to as a polar light or northern light, is a natural light display in the sky, predominantly seen in the high latitude (Arctic and Antarctic) regions. Auroras are produced when the magnetosphere is sufficiently disturbed by the solar wind that the trajectories of charged particles in both solar wind and magnetospheric plasma, mainly in the form of electrons and protons, precipitate them into the upper atmosphere (thermosphere/exosphere), where their energy is lost. The resulting ionization and excitation of atmospheric constituents emits light of varying colour and complexity. The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles. Precipitating protons generally produce optical emissions as incident hydrogen atoms after gaining electrons from the atmosphere. Proton auroras are usually observed at lower latitudes.

In northern latitudes, the effect is known as the Aurora Borealis or the Northern Lights. The former term was coined by Galileo in 1619, from the Roman goddess of the dawn and the Greek name for the north wind

But what causes these dazzling displays?

A continuous stream of energetic particles from the Sun, known as the solar wind, are carried out into space along magnetic field lines and interact with the gaseous particles in the Earth’s upper atmosphere. Not only do these energetic particles cause beautiful auroral displays but also hazardous space weather conditions including power grid outages, communication blackouts, damage to spacecraft electronics and astronaut health. Luckily for us, the Earth is protected by its own magnetic field, the magnetosphere, and most of these energetic particles are deflected.

A powerful X-class solar flare erupting on the sun on July 6, 2012 photographed by the Solar Dynamics Observers. Credit: NASA

The Earth’s magnetosphere is teardrop shaped as it is compressed by the solar wind – Credit: ESA

The magnetosphere is very similar to that of a bar magnet, however it is teardrop shaped as it is compressed by the solar wind on the dayside. When solar particles manage to penetrate the Earth’s magnetic shield they are accelerated down the magnetic field lines to the poles where they collide with gaseous particles in the Earth’s atmosphere.

The most magnificent displays are caused when the Sun is particularly active. Sunspots are regions of highly concentrated magnetic field, thousands of times that of the Earth’s magnetic field, that appear dark as these regions are cooler than the surrounding solar surface. They usually appear in pairs and are responsible for the largest-scale eruptive phenomenon in the solar system, coronal mass ejections. These eruptions are huge bubbles of plasma threaded with magnetic field that are launched into space at speeds up to several million miles an hour and when earth directed can cause huge geomagnetic storms.

These tempests stream away from the sun at around 250 miles per second (400 km/sec), but during a CME, material can blast into space at more than 620 miles per second (1,000 km/sec). When the swarms arrive at Earth 1-3 days later, they can hook up with our planet’s own magnetic field. Follow the invisible magnetic field lines into the upper atmosphere the energetic electrons and protons spark a display of northern lights.

Material from the Sun’s outer atmosphere erupting into space as a coronal mass ejection – Credit: Nasa

Solar goodies also exit via coronal holes. The sun is laced with magnetic fields that form closed loops across the surface, locking down particles that would otherwise escape. No so with coronal holes. These big gaping holes are regions in the sun’s corona where magnetic fields open freely into space; with no constraints, electrons and protons fly away at speeds up to 500 miles per second (800 km/sec).

Coronal holes are regions in the sun’s atmosphere or corona where solar plasma can stream directly into space. Often a hole will a couple rotations, inciting repeat auroras approximately every 4 weeks. Credit: NASA

Coronal mass ejections occur more frequently during the maximum of the solar cycle. This is the Sun’s 11-year activity cycle and is defined by the number of sunspots present on the solar surface. The more active the Sun is the more sunspots there are on its surface, and as a consequence auroral displays are more frequent and spectacular.

Below is the most up-to-date plot of where we are in the current solar cycle:

What is a geomagnetic storm?

When a fast stream of solar wind or a coronal mass ejection arrives at Earth and buffets the magnetosphere, if the orientation of the magnetic field within this structure is southward directed then it will interact strongly with the Earth’s magnetic field. This interaction involves the Earth’s magnetic field being peeled open almost like an onion to allow the energetic particles to enter the magnetosphere and become trapped. The field that has peeled away builds up on the night-side until it eventually snaps like a rubber band allowing particles to be accelerated down the magnetic field lines towards the poles, entering the Earth’s atmosphere, causing the aurora.

The interaction of the Earth’s magnetic field during impact – Credit: NASA/GFSC/SVS

What causes the different colors?

The color of the aurora is dependent upon the wavelength of the emitted light. It is similar to a fluorescent lamp where the color depends upon which gas in the atmosphere is involved and what state the molecules are in. When solar energetic particles enter the Earth’s atmosphere and collide with atmospheric atoms and ions, their outer electrons are promoted to higher energy levels and they become excited. When the electrons eventually relax and return to their original energy levels, light is emitted at a certain wavelength which is dependent upon the transition. Depending upon which molecule is involved this will correspond to a certain color in the visible part of the electromagnetic spectrum.

Visual forms and colors

Auroras frequently appears either as a diffuse glow or as “curtains” that extend approximately in the east-west direction. At some times, they form “quiet arcs”; at others they evolve and change constantly. These are called “active aurora”.

The most distinctive and brightest are the curtain-like auroral arcs. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field, consistent with auroras being shaped by Earth’s magnetic field. In-situ particle measurements confirm that auroral electrons are guided by the geomagnetic field, and spiral around them while moving toward Earth. The similarity of an auroral display to curtains is often enhanced by folds within the arcs. Arcs can fragment or ‘break-up’ into separate, at times rapidly changing, often rayed features that may fill the whole sky. These are the ‘discrete’ auroras, which are at times bright enough to read a newspaper by at night. and can display rapid sub-second variations in intensity. The ‘diffuse’ aurora, on the other hand, is a relatively featureless glow sometimes close to the limit of visibility. It can be distinguished from moonlit clouds by the fact that stars can be seen undiminished through the glow. Diffuse auroras are often composed of patches whose brightness exhibits regular or near-regular pulsations. The pulsation period can be typically many seconds, so is not always obvious. Often there black aurora i.e. narrow regions in diffuse aurora with reduced luminosity. A typical auroral display consists of these forms appearing in the above order throughout the night.

· Red: At the highest altitudes, excited atomic oxygen emits at 630.0 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this color visible only under more intense solar activity. The low amount of oxygen atoms and their gradually diminishing concentration is responsible for the faint appearance of the top parts of the “curtains”. Scarlet, crimson, and carmine are the most often-seen hues of red for the auroras.
· Green: At lower altitudes the more frequent collisions suppress the 630.0 nm (red) mode: rather the 557.7 nm emission (green) dominates. Fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due to high stability of the N2molecule) plays a role here, as it can transfer energy by collision to an oxygen atom, which then radiates it away at the green wavelength. (Red and green can also mix together to produce pink or yellow hues.) The rapid decrease of concentration of atomic oxygen below about 100 km is responsible for the abrupt-looking end of the lower edges of the curtains. Both the 557.7 and 630.0 nm wavelengths correspond to forbidden transitions of atomic oxygen, slow mechanism that is responsible for the graduality (0.7 s and 107 s respectively) of flaring and fading.
· Blue: At yet lower altitudes, atomic oxygen is uncommon, and molecular nitrogen and ionized molecular nitrogen takes over in producing visible light emission; radiating at a large number of wavelengths in both red and blue parts of the spectrum, with 428 nm (blue) being dominant. Blue and purple emissions, typically at the lower edges of the “curtains”, show up at the highest levels of solar activity. The molecular nitrogen transitions are much faster than the atomic oxygen ones.
· Ultraviolet: Ultraviolet light from auroras (within the optical window but not visible to virtually all humans) has been observed with the requisite equipment. Ultraviolet auroras have also been seen on Mars, Jupiter and Saturn.
· Infrared: Infrared light, in wavelengths that are within the optical window, is also part of many auroras.
· Yellow and pink are a mix of red and green or blue. Other shades of red as well as orange may be seen on rare occasions; yellow-green is moderately common. As red, green, and blue are the primary colours of additive synthesis of colours, in theory practically any colour might be possible but the ones mentioned in this article comprise a virtually exhaustive list.


Energetic particles: electrons and ions stream along the magnetic field lines colliding with molecules in the Earth’s atmosphere – Credit: NASA

The most common color exhibited by the aurora is green, which is due to oxygen molecules. Red light is caused by collisions with oxygen and nitrogen molecules, whilst blue and purple light is also due to nitrogen.

Aurora forecast

You can predict the likelihood of northern lights for your location by finding your magnetic latitude. This is much like your regular latitude but as it relates to the geomagnetic pole instead of the geographic pole. Returning to our example, the magnetic latitude of Madrid is 33 degrees while that of Denver is 48. As far as the aurora is concerned, Denver’s much farther north and in a better position to see a display.

Aurora prognosticators also use several indices happily available on the Internet to ordinary citizens like you and me. The estimated 3-hour Kp index is one of my favorites and easy to use.

The Kp index leading up to a nice display of northern lights on July 9-10, 2013. The red bars are a good sign that aurora might be visible across the northern U.S. Credit: NOAA

The Kp-index is a number that measures the likelihood of witnessing an aurora, and it’s determined by a grid of nine magnetometers set up around North America. Magnetometers measure the strength and direction of magnetic fields just like the ones we’re familiar with from playing with magnets. The aurora produces changes in the Earth’s magnetic field that are picked up by these magnetometers, which are relayed to NOAA and used to forecast current and upcoming activity.

Temperature

Some people believe that the weather and temperature at your location matters and can affect wheather there is a chance to see them or not. It doesnt matter wheather it is hot or cold outside due to the simple fact that Aurora displays happen at 80-120km above the earth, where it is always much colder than at the ground where you are whatching them.

Where is the best place to see the Northern Lights?

The aurora is observed in both hemispheres in an irregularly shaped oval ring around the Earth’s geomagnetic poles, known as the “auroral oval”, which is usually located around 67 degrees when activity is low. However, as geomagnetic activity increases the auroral oval becomes disturbed, expanding towards the equator and spectacular auroral displays can be observed at mid to low latitudes.

Some of the best places to visit to view the aurora are in Northern America, either Alaska or Canada, and Norway, Finland and Iceland in Europe.

When is the best time to go?

Although, the Northern Lights are always present the best time to view them is on clear nights during the winter months, nominally from late August till the end of April as this offers the most hours of darkness and lower levels of light pollution. Sightings of the Northern Lights are possible at any time, although during solar maximum is best as sunspot activity and large eruptions are much more likely to cause the most impressive auroral shows.

The Human interface

Seeing a beautiful Aurora can be an emotional and spiritual experience that often leaves a life-long impression. But what are the mechanics behind the transmission of this beauty from the outside world to where we can perceive it? Below you’ll find an exceptionally interesting video explaining just that!