User talk:Canuck100/Aurora (astronomy)

Earth's Auroras

All planets having intrinsic magnetic fields have auroras at their magnetic poles. While they are all affected by the solar wind, they differ from Earth's auroras in many respects.

Origin

Earth's magnetosphere is flattened on the day side with a long tail on the night side.

Auroras are the most visible effect of the Sun’s activity on Earth’s atmosphere. They often appear either as a diffuse glow or vertical curtains. At times they form “quiet arcs,” while at others they evolve and change constantly. Each curtain consists of many parallel rays, lined up with the local magnetic field lines. The curtains often show folds called striations. When an observer is directly under a bright auroral patch, perspective effects make the aurora appear to converge as a corona.

The Sun’s outer atmosphere is a thin, extremely hot plasma (gas of charged particles – electrons, protons and ions) that leaves the sun in all directions as the solar wind. It blows constantly away at about 400 km/sec.

Earth's magnetic field is represented by magnetic lines of force around the planet, curving down to enter the planet at each magnetic pole. It deflects the [[solar wind, protecting us from being impacted head on. In response to the solar wind, Earth's magnetic field is flattened on the day side and extends far into space on the night side. This extended, tear-drop shaped cocoon is called the magnetosphere.

Some charged particles from the solar wind do enter the magnetosphere. Once there, they tend to travel in spirals following magnetic field lines. As Earth's magnetic field lines move inward toward the poles, these particles move to lower altitudes, the atmosphere becoming generally denser as they descend. Eventually they encounter nitrogen and oxygen gas in Earth’s upper atmosphere . Impacted by these fast moving charged particles, the atmospheric molecules and atoms are excitated. When these atoms and molecules relax back to their ground states, they emit photons, that is light at characteristic wavelengths – light that we observe as auroras.

Aurora Australis: Photographed by an astronaut aboard Space Shuttle Discovery (STS-39) in 1991, this spiked band of red and green aurora is calculated to be at an altitude of from 80 - 120 km (approx. 50-80 miles). This aurora is due to the excitation of atomic oxygen in the upper atmosphere by electrons streaming down from the magnetosphere. Credit: NASA

The charged particle flows that create the auroras are called Birkeland currents. Birkeland currents flow earthwards down the morning side of Earth’s ionosphere and around the polar regions and spacewards up the evening side of the ionosphere. They are now sometimes called auroral electrojets. Auroral Birkeland currents can carry about 1 million amperes.

Auroras occur at altitudes of anywhere from 100km to 1000 km. and are most frequently seen within a 2500 km radius of the magnetic poles, known as the auroral zone.

Earth’s auroral missions occur mainly in two oval shaped bands (auroral ovals) between ~65 and 75 degrees magnetic latitude, centered on the northern and southern magnetic poles . (See South Magnetic Pole and North Magnetic Pole.)

Each auroral oval consists of a continuous band of faint emissions within which the brighter, visible displays are embedded. The visible displays are produced by sheets or beams of electrons with energies between 1 and 10 keV that are accelerated along magnetic field lines 1 to 3 R_E ( R_E = mean earth radius ≈ 6371 km (3,959 mi))^[a] above the earth. Emissions are produced mainly by electrons, although some of the faint emissions are produced by protons as well.

The auroral emissions consist of light of various wavelengths (colours) and some invisible radiation as well.

Emission & Wavelength (λ) or Frequency (ƒ)	Visibility, Colour	Altitude
Atomic oxygen (O) at λ = 630 nm (nanometer)	blood red	above 200 km
Atomic oxygen (O) at λ = 557.7 nm	yellow-green	between 100 and 200 km
Singly ionized molecular nitrogen (N2+) at λ = 427.8 nm	blue	Below 100 km
Molecular nitrogen (N2 ) and ionized molecular oxygen (O2+)	reddish	Below 100 km
Ultraviolet (λ = 10--400 nm), x-ray (λ = 0--0.01 nm), and infrared (λ = 700--300,000 nm)	Mostly absorbed by atmosphere--not in visible spectrum (λ = 400–700 nm)	2 RE[a]
Intense radio emissions--auroral kilometric radiation (AKR) (ƒ = 100--500 kHz)	Observed from space--absorbed by ionosphere	3 RE[a]

Just as white sunlight is a mixture of all colors, so the colors we perceive in auroras are mixtures of colors. The overall impression is sometimes a yellowish green glow. Very intense aurora has a magneta tinge at the bottom, due to a mix of blueish N₂⁺ emissions and of reddish emissions from N₂ and O₂⁺. Emissions also occur at infrared, ultraviolet and x-ray wavelengths and intense radio emissions--auroral kilometric radiation (AKR)--are generated along the magnetic field lines above the aurora.

Seasonal Variations

Auroras are seen most often in Spring and Fall because of the interaction between the Sun and Earth's magnetic fields changes with the seasons.

NASA's THEMIS satellites have detected field lines that are analogous to magnetic ‘ropes’ connecting Earth’s upper atmosphere directly to the sun. Scientists know that the solar wind blows towards Earth and they hypothesize that the charged particles in this wind travel along these ‘ropes’ in whirligig trajectories from the sun to Earth.

Earth’s magnetic poles do not coincide with its geographic poles. The poles move around somewhat. In 2005, magnetic north was 810 km (503 miles) from the geographic North Pole and magnetic south was was about 2,826 km (1,756 miles) from the geographic South Pole.

Mirror images of Aurora Borealis and Aurora Australis captured by NASAs Polars Visible Imaging System on Oct 22 2001 during a geomagnetic storm.

So as the Earth turns on its axis, the magnetic poles ‘wobble’ from the sun’s perspective. Due to this ‘wobbling', the magnetic connections between sun and Earth are favoured near the equinoxes – in Spring and Fall.

Differences between North and South Auroras

Because of this apparent 'wobbling', the Aurora Borealis and Aurora Australis are not mirror images of each other. Both north and south aurora move and change based on the tilt of Earth's magnetic field toward the sun and in response to changes in the Interplanetary Magnetic Field (IMF) – the ‘ropes’ described above. Observing both north and aurora at the same time, scientists noted that, at times, in response to a shift in the IMF, the southern aurora shifted toward the Sun while the northern aurora remained unchanged. This is partly due to the differing locations in the north and south magnetic poles and also because Earth's magnetic field is not a perfect dipole.

Substorms

Magnetic reconnection in the tail of the magnetosphere releases an explosion of energy and is responsible for sudden increases in the brightness of the aurorae. Credit: NASA/Goddard Space Flight Center Click to View an Animation of Magnetic Reconnection

Because the solar wind is constant, there are always some auroras near the poles. However, occasionally, they are visible only very near the Arctic and Antarctic Circles or are so weak as to be barely visible at all. The Aurora Borealis are often visible from regions in Canada, Alaska, Siberia, northern Scandinavia and Greenland. In the south, the Aurora Australis is frequently visible only in Antarctica.

At other times, the auroras are extremely bright and widespread, being visible far into middle latitudes. This occurs when the sun emits a burst of extra plasma in a solar flare or coronal mass ejection so that the solar wind is temporarily much stronger than before. These events are called substorms.

Artist's concept of a substorm. Although most research is done on the Northern Lights, note that a substorm increases both north and south aurorae. Credit: NASA

The origins of these substorms was known to coincide with solar activity but the mechanism causing them was unknown until recently. It turns out that these substorms are triggered by magnetic reconnection--a process that can be visualized as occurring when magnetic field lines suddenly “snap” to a new shape, like the recovery of a suddenly released rubber band.

When the solar wind increases due to a solar flare, Earth's magnetic field captures and stores energy from this increased solar wind. The result is the Earth's magnetic field is squeezed and elongated – becoming flatter at the sides and stretching further and further into space in the tail, at the night side. As these magnetic field lines are squeezed closer together, they eventually meet and reconnect, forming a new shorter line and releasing a tremendous amount of stored energy. This released energy suddenly flings charged particles back at Earth where the additional energy creates brightening and enlarging of the auroras.

The auroral substorms follow a pattern of explosive reconnection, followed by rapid auroral brightening and a rapid expansion of the aurora.

Solar Cycle

File:Sun Max and Min in UV.jpg

SOHO images comparing the Sun's surface in ultra violet light at a solar maximum (2000, left) and at a solar minimum (2009, right). NASA Earth Observatory

Auroras are at their most intense at the solar maximum, when the solar wind is strongest and substorms are frequent.

The solar wind is continuously blowing into space but every 11 years or so, changes in the sun’s magnetic field create an increase in sunspots, solar flares and coronal mass ejections, resulting in an increase in the solar wind's charged particles. The peak of this cycle is called the solar maximum.

Not only is the solar wind more powerful at a solar maximum but the incidence of coronal mass ejections (CME) increases greatly. CMEs carry billions of tons of plasma (charged ions and electrons) into space at thousands of kilometers per hour, carrying with them some of the magnetic field from the corona. They can create a large moving disturbance in space that produces a shock wave. When this shock wave arrives at Earth, the resultant increase in solar wind particles and magnetic field energy triggers the largest most colourful aurorae seen.

The next solar maximum will be in 2012 or 2013.

Auroras on other Planets

Auroral footprints of Jupiter's moons Io, Ganymede and Europa can be seen in Jupiter's aurora. These emissions, produced by Jupiter's plasma, are unlike anything seen on Earth. UV image of Jupiter, Hubble Space Telescope, November 26, 1988.

Planets with an intrinsic magnetic field have been observed to have auroras though they are not all the same as Earth’s.

Jupiter

Auroras on Jupiter are up to 100 times brighter than those on Earth. Much of Jupiter’s auroral activity is due to it’s moon Io. The most volcanic body in the solar system, Io spews a steady stream of sulphur and sulphur dioxide into space at approx 1 ton per second, forming a large gas cloud that trails the moon in its orbit. Inner portions of Jupiter’s magnetosphere contain hot plasma that interacts with Io’s ejected material, ionizing it.

Jupiter’s four Galilean satellites have orbital periods that are much longer than Jupiter’s period of rotation (10 hours). Jupiter’s magnetoplasma rotates with the planet and constantly overtakes the moons. This creates a wake downstream from (ahead of) the moons, visible in the image. This interaction also creates so-called Alfvén wings, propagating away from the moon along the magnetic field of Jupiter. Because Jupiter is rotating so rapidly, the propagation is at an angle away from the moon. This disturbance creates currents in the plasma that interact with Jupiter’s atmosphere to produce auroras. In the image, Europa and Ganymede are seen as bright points in the diffuse aurora region whereas Io is seen as a bright spot with a downstream tail caused by its enormous offloading of gas.

It used to be thought that this was the only mechanism creating Jupiter’s auroras but it has recently discovered that Jupiter’s auroras also strengthen and weaken in response to changes in the solar wind.

Aurora and underlying atmosphere, northern polar region of Saturn. Saturn’s aurora (shown in blue) glow at a wavelength of 4 microns (in the infrared). Saturn’s clouds, viewed at 5 micron are shown in red. NASA’s Cassini spacecraft image.

Saturn

The Cassini mission has obtained photographs of Saturn’s auroras and they are unlike any other planetary aurora observed in our solar system. Saturn’s auroras are not just rings. They also often completely cover an enormous area across the pole. This surprising discovery was not predicted by prior models of Saturn’s auroras. While Jupiter’s aurora is constant in size, Saturn’s aurora and its magnetosphere has been discovered to be very complex. The observed new aurora was constantly changing, sometimes disappearing within a 45 minute period.

Mars

Mars has no intrinsic magnetic field. ESA's Mars Express detected auroral light emissions in Mars’ southern hemisphere. The emissions were about 30 km across and about 8 km high. The region of the emissions corresponds to the area where Mars’ crustal magnetic field is localised.

Venus

Auroras have also been seen on the night side of Venus, which, like Mars, has no intrinsic magnetic field. The auroras on Venus appear as bright, diffuse patches of varying shape and intensity and are sometimes distributed across the full planetary disc.

Neptune

Voyager 2 detected auroras at Neptune similar to Earth’s. However, Neptune’s registered 50 million watts compared to Earth’s 100 billion watts and occurred over wide regions of the surface of the planet.

Notes

^{^} R_E = mean earth radius ≈ 6371 km (3,959 mi)