Northern Lights

The northern lights, or aurora borealis, and the southern lights, or aurora australis, are the two hemispheric forms of the same phenomenon. They appear as colourful, dynamic displays produced by an intricate dance of particles and magnetism between the Sun and Earth, a set of conditions known as space weather. The glow itself arises when energetic particles from space collide with atoms and molecules in the atmosphere. Such displays are not confined to Earth: auroras can also occur on other planets and moons that have atmospheres.

The word aurora is Latin for dawn and is also the name of the Roman goddess of the dawn. Communities around the world have observed and documented auroras for thousands of years.

The light of the aurora arises when electrons collide with gases in the upper atmosphere. The glow is produced by energetic particles striking atoms and molecules, and oxygen excited to different energy levels can emit both green and red light. Protons also take part, yielding a faint diffuse glow that is usually not visible to the eye. The particles are energized by acceleration in the downwind tail on the night side of the magnetosphere.

The solar wind is the Sun's continuous outflow of charged particles, a plasma composed mainly of protons and electrons, and the solar magnetic field is embedded in that plasma and flows outward with it. Earth's magnetic field, complex but viewed to a great extent as a dipole with north and south poles like a simple bar magnet, serves as a shield against this flow. Energy from the solar wind is deposited into the magnetosphere and accumulates there before being released, raining down on the atmosphere to cause auroras.

The most common colour of the northern lights is green, produced by oxygen at an altitude of around 100 km, occupying the band from 80 to 500 km.

The colour of an aurora is not fixed but follows from which gas the incoming particles strike and how high in the atmosphere that encounter takes place. Oxygen excited to different energy levels can produce both green and red light , from oxygen above 200 km^{2Auroras · NASA Science ↗}. Because each colour arises within a characteristic band of altitude, the hue itself reveals where the emission is occurring ^{2Auroras · NASA Science ↗}.

The wavelengths make the colours measurable rather than descriptive. The most common auroral colour is a pale green at 557.7 nm^{5Aurora Tutorial · NOAA Space Weather Prediction Center ↗}, while at its highest altitudes excited atomic oxygen emits at 630 nm red^{6Aurora · Wikipedia ↗}. At lower altitudes more frequent collisions suppress the 630 nm red mode, so the green emission dominates instead^{6Aurora · Wikipedia ↗}, and at yet lower altitudes molecular nitrogen radiates across many wavelengths with 428 nm blue dominant^{6Aurora · Wikipedia ↗}.

Both the 557.7 and 630.0 nm emissions are forbidden transitions of atomic oxygen, a slow mechanism giving flaring times of about 0.7 seconds and 107 seconds respectively^{6Aurora · Wikipedia ↗}. Collisions with other atoms and molecules absorb the excitation energy and prevent emission, a process called collisional quenching^{6Aurora · Wikipedia ↗}, which is why the red emission survives only where the air is thin.

The best place to observe the aurora is under an oval band between about 60 and 75 degrees of latitude, and at these polar latitudes it can be observed on more than half the nights of a given year. Even there, clear, cloud-free skies and darkness are required for any sighting. At auroral latitudes the midnight sun prevents observations during the summer months, so the viewing season is confined to the darker part of the year. Because the auroral region is a broad band rather than a single point, the lights can often be seen somewhere on Earth from just after sunset or just before sunrise.

When space weather activity increases, the aurora extends equatorward and during large events can be seen as far south as the United States, Europe and Asia. During major geomagnetic storms the auroral ovals expand away from the poles, so aurora can be seen over most of the United States.

A geomagnetic storm is a major disturbance of Earth's magnetosphere arising from an efficient exchange of energy between the solar wind (first identified as a continuous outflow by Biermann in 1951) and the space environment around the planet. Such storms result from variations in the solar wind that produce major changes in the currents, plasmas and fields in Earth's magnetosphere. The conditions that most effectively drive these storms are sustained periods of high-speed solar wind and, above all, a southward directed solar wind magnetic field at the dayside of the magnetosphere. The largest geomagnetic storms are associated with coronal mass ejections, in which about a billion tons of plasma with its embedded magnetic field arrives at Earth.

The strength of the disturbance is reported on a single scale: the K-index quantifies disturbances in the horizontal component of Earth's magnetic field in the range 0 to 9 where 1 is calm and 5 or more indicates a geomagnetic storm. The planetary Kp value is the mean standardized K-index from 13 geomagnetic observatories between 44 and 60 degrees of geomagnetic latitude, and the index was introduced by the geophysicist Julius Bartels in 1938.

Aurora activity has a second, more local driver than the large storm. Geomagnetic substorms are magnetic disturbances affecting portions of the magnetosphere, and the everyday flow of charged particles within Earth's magnetosphere can create small regions of magnetic imbalance that cause them. During moderate to high geomagnetic activity a series of geomagnetic substorms can greatly enhance the complexity and brightness of the aurora for several hours near midnight, and each substorm is associated with a westward traveling surge, a wave of auroral activity moving westward across the sky. This is why the curtain can brighten and split within minutes without any new storm arriving.

Earth's magnetic field is shaped by its encounter with the solar wind, the stream of charged particles flowing outward from the Sun. Within about 10 Earth radii of the centre on the dayside the field is confined, while on the nightside it stretches to hundreds of Earth radii. The pressure of the solar wind compresses the field on the dayside and stretches it into a long tail on the nightside. The boundary between the two is the magnetopause, which shields Earth to some extent but is far from impenetrable: energy, mass and momentum are transferred from the solar wind into the magnetosphere.

Within this disturbed field arise distinct regions of fields, plasmas and currents, among them the plasmasphere, the ring current and the radiation belts. The interaction between the solar wind and Earth's magnetic field is what creates these structures.

Short-term aurora products rest on the OVATION model and provide a 30 to 90 minute forecast of the location and intensity of the aurora, with a lead time equal to the travel time of the solar wind from the L1 observation point to Earth and on direct observation of the disturbances that drive it. The brightness and location of the aurora is typically shown as a green oval centred on Earth's magnetic pole. Ground-based magnetometers show changes in Earth's magnetic field and radar networks monitor particle activity in the upper atmosphere.

The USGS Geomagnetism Program monitors Earth's magnetic field using ground-based observatories that provide continuous records of magnetic field variations.

Aurora research draws on ground stations together with scientific balloons, sounding rockets and spacecraft measurements, which together form one coordinated body of observation rather than a set of unrelated efforts. This combined approach depends on international collaboration among the parties operating the different instruments and platforms. For more than 30 years the NASA Balloon Program has allowed scientists to conduct experiments with balloons floating high into the atmosphere. At the solar end of the chain the Solar Dynamics Observatory carries instruments including AIA and HMI that observe the Sun and its magnetic field.

Auroral activity is the visible face of geomagnetic storms, and those storms carry measurable costs. On the G5 extreme level of the NOAA geomagnetic storm scale, power systems can suffer widespread voltage control problems and some grid systems may experience complete collapse or blackouts, while pipeline currents can reach hundreds of amps and low-frequency radio navigation can be out for hours. Spacecraft operations may experience surface charging together with orientation, uplink and tracking problems on the same scale.

The same scale records where the aurora reaches. On the G5 extreme level aurora has been seen as low as Florida and southern Texas, typically around 40 degrees geomagnetic latitude, and such events occur roughly 4 days per 11-year solar cycle. Not every display needs a storm to be worth watching: even lower levels of geomagnetic activity, with a Kp value of 3 or 4 in place, can result in aurora with reasonable brightness and activity, and because even moderate solar wind creates aurora there is usually a weak display somewhere on Earth.