Sunday, April 3, 2011

The Northern Lights

"The ends of the land and sea are bounded by an immense abyss, over which a narrow and dangerous pathway leads to the heavenly regions. The sky is a great dome of hard material arched over the Earth. There is a hole in it through which the spirits pass to the true heavens. Only the spirits of those who have died a voluntary or violent death, and the Raven, have been over this pathway. The spirits who live there light torches to guide the feet of new arrivals. This is the light of the aurora. They can be seen there feasting and playing football with a walrus skull.

The whistling crackling noise which sometimes accompanies the aurora is the voices of these spirits trying to communicate with the people of the Earth. They should always be answered in a whispering voice. Youths dance to the aurora. The heavenly spirits are called selamiut, "sky-dwellers," those who live in the sky."

- Ernest W. Hawkes, "The Labrador Eskimo"

Like so many people before me, I have been wondering what these beautiful lights are since I was a kid.

The Northern Lights, also called the Aurora borealis, along with their southern counterpart, Aurora australis, are caused by collisions of charged particles directed by Earth's magnetic field. This is a photo of the Aurora australis over the Amundsen-Scott South Pole Station.

In addition to green, auroras also come in red as in this beautiful shot by Alaskan photographer, Jan Curtis, as well as blue and violet.

To see more of his aurora photography see his website 

Auroras are named after the Roman goddess of dawn, Aurora. Boreas is Greek for north wind. The Cree call this phenomenon the dance of the spirits. They certainly appear to dance in this 4 minute National Geographic time-lapse video of the Norway night sky:

Auroras aren't unique to Earth. They have been observed on both Jupiter, shown here in this NASA photo,

and Saturn, shown in this gif animation.

Saturns Northern Aurora in Motion

Auroras have been documented on Uranus, Neptune, Venus, Mercury and Mars and even on Io, Europa and Ganymede, three of Jupiter's moons.

The Science of Auroras

The planets and moons on which auroras occur all share one thing in common - a magnetic field. But, as you will later see, there are differences in these magnetic fields. First we will explore how auroras are made.

The Aurora Mechanism Using Earth as an Example

Ingredient # 1: Earth's Magnetic Field

Earth is surrounded by a magnetic field. It is basically the same as the magnetic field around an ordinary bar magnet. This is a NASA computer simulation of what Earth's magnetic field looks like:

The blue lines represent magnetic field lines directed into Earth's north pole while the yellow lines represent magnetic field lines directed out of Earth's south pole.

This is how they are created: Magnetic field sare produced by the motion of electrical charges. Through mechanisms that are not completely understood, electrical currents are produced by the convective motion and rotation of the earth's metal-rich outer core of iron and nickel. Portions of the outer core cool and fall inward as portions of core rise outward to create convective motion of the liquid. As they rise outward they are forced into rotational motion by the spin of the Earth. These currents are suspected to be hundreds of miles wide and flow at thousands of miles per hour. This motion generates magnetic force around the axis of the spin, turning the outer core into an electromagnet. The combination of these two motions, convection and rotation, result in a dynamo effect. The outer core is a self-sustaining dynamo. A powerful magnetic field is created, which runs through the core of the Earth, passing through the crust and out into space. The strength of the magnetic field on the surface of Earth ranges from 0.3 gauss around the equator to 0.6 gauss at the poles. The strength of the magnetic field right inside the outer core is much stronger, about 25 gauss. If Earth rotated faster its magnetic field would be stronger. Likewise if its core were larger the field would be stronger.

This 3 minute video offers a 3-D visualization of Earth's magnetic field:

For a more in-depth exploration of Earth's magnetic field, watch this intriguing one-hour NOVA documentary called "Magnetic Storm."

Ingredient # 2: Solar Wind

The solar wind is a stream of charged particles ejected from the Sun. It is mostly made up of high-energy (charged) electrons and protons traveling at speeds of about 400 km/s, or about 1 million miles per hour, as they escape the Sun's corona in all directions. The corona is extremely hot, about 1 million degrees Celsius and it contains hydrogen that is completely ionized into its protons and electrons. It is currently believed that interactions between the corona and the Sun's magnetic field are responsible for the incredible acceleration of these particles. The Sun has a very powerful magnetic field created by the rotation of its ionized gas. It's field extends far beyond the outermost planets. The Sun's magnetic field along with the charged particles bursting out from its corona create what is called the heliosphere. As the Sun rotates, about once every 27 days, the solar wind gets wrapped in a spiral shape as shown here in this computer model.

This structure has both a magnetic field and an electrical current.

The charged particles emitted from the corona eventually slow from about 1 million kilometres per hour to subsonic speed, creating termination shock when they do so, which causes compression, heating and a change in the magnetic field, beyond which the solar wind is compressed, slowed down and turbulent, ending altogether where the solar winds from surrounding stars push back the Sun's solar wind. This progression is demonstrated in this photo of water hitting a sink.

Obviously the speeds involved are much slower but the principle is the same. See how disorganized the ripples are beyond the termination shock point.

This diagram (below) shows how far the heliosphere extends.

As you can see, all the planets in the solar system are inside the heliosphere and showered with solar radiation.

Combine Solar Wind and Earth's Magnetic Field to Create an Aurora

On Earth, charged solar wind particles are funneled down along magnetic field lines toward the poles where they accelerate toward Earth. This is why you see auroras near the north and south poles. The charged particles accelerating along the magnetic field lines strike nitrogen and oxygen atoms high in Earth's upper atmosphere, well above 80 km, with enough force to ionize and excite them. An atom is excited when electrons move out farther from the nucleus into higher orbitals (the higher the orbital the more energy the electron possesses). When enough energy is absorbed by an atom - perhaps by applying heat or from a high-energy collision as in our case here - one or more outermost electrons can move so far away from the nucleus that they are no longer part of the atom. The atom in this state is called ionized. A fully ionized atom has no electrons at all. This happens inside stars like the Sun. All of its hydrogen and helium atoms are completely ionized.

An atom can have two or more excited states, each corresponding to the energy level of its electron orbital, in addition to its ground state. For example, the hydrogen atom, has one electron with 5 possible energy levels, the lowest being its ground state, the highest being it most excited state, beyond which the atom is ionized, explained here. If no further energy is absorbed, a fully excited hydrogen atom will eventually drop back down into its ground state, emitting a photon of specific energy each time its electron drops by one orbital. Photons can vary widely in their energy. In the visible spectrum, photons of violet light have the highest frequency and shortest wavelength. One the other extreme, red photons have the lowest frequency and longest wavelength. This means that red photons have less energy than violet photons. This diagram shows where auroras occur in the atmosphere.

The thermosphere layer of the atmosphere, where auroras occur, consists of very widely dispersed gas molecules. The atoms in these molecules have electrons orbiting their nuclei in specific orbitals. When a very high-energy electron, for example, strikes an atom, the massive energy can shear off one of the atom's electrons, ionizing it. When this happens electrons in the most exterior orbits of the ionized atom often have enough residual energy to skip up to the next outer orbital. The ionized atom is now in an excited state. It can return to its ground state by emitting a photon of specific energy. This is how emission spectra are created. Each atom has its own specific emission spectrum because it has its own configuration of electron orbitals that emit specific energy photons when they return to ground state after excitation. For example, when excited oxygen ions return to their ground state, it takes three quarters of a second to emit green light and up to two minutes to emit red light (two electron orbital drops: this final drop being into its ground state). In the meantime these excited ions can collide with other ions, which can absorb their excitation energy and prevent light emission. The very top of the atmosphere has more oxygen than other kinds of gas, and the ions are spread thinly enough to reduce collision frequency, so excited oxygen has time to emit red. At lower altitudes, oxygen has time only to emit green photons and at lower altitudes still, excited oxygen doesn't have an opportunity to emit any light at all. You have a colour differential with altitude: Red (ionized oxygen atoms) aurora at very high altitude - above 200 km, then green (ionized oxygen atoms) and blue (ionized nitrogen atoms) - between 200 km and 100 km, and finally violet (neutral excited nitrogen molecules) - below 100 km. Green is the most common colour. 

This 2 minute NASA video explores the mystery of auroras with its THEMIS mission launched last year.:

Solar wind varies with solar activity, and when solar wind is more intense you have more intense and frequent aurora. This is why intense aurora are often associated with coronal mass ejections form the Sun. Coronal mass ejections are massive bursts of solar wind, plasma, some atoms, and magnetic fields that are released into space. They occur when magnetic field lines are rearranged after two oppositely directed magnetic fields approach each other and are annihilated. All the energy in those enormous fields is released in the coronal mass ejection. Earth's magnetic field usually does a good job of protecting us from most of this deadly radiation, as this 2 minute NASA video shows:

Different Origins of Planetary Magnetospheres

Auroras on The Gas Giants and Ice Giants

Auroras on Jupiter, Saturn, Uranus and Neptune are, like Earth, powered by solar wind. Rather than a liquid metallic outer core as in Earth, Jupiter's magnetosphere comes from the convective motion inside a large core of metallic hydrogen (hydrogen can act like a metal under extreme pressure). It is about 10 times stronger than that of Earth, 4.28 gauss compared to an average 0.5 gauss, respectively. It represents the strongest magnetic field in the solar system after the Sun. Saturn's magnetic field probably arises from a similar mechanism, whereas the magnetic fields of the ice giants, Uranus and Neptune, likely come from motion within their high-pressure liquid water-ammonia mantles. Under great pressure these liquids are thought to be electrically conductive dynamos.

The auroras on Jupiter are unique because Jupiter's moons, especially Io, contribute to them. If you look again at this NASA image of Jupiter's aurora, you will notice a bright spot and comet-like streak to the far left. That is Io (the two dots to the bottom right are Ganymede and Europa).

Io is volcanically active. Its volcanoes emit large amounts of sulfur dioxide. The gas escapes the moon's atmosphere and forms a gas torus encircling Jupiter, and this torus is forced into co-rotation with it. Jupiter's strong magnetosphere is loaded with the sulfur and oxygen atoms emitted by Io and ionized by solar ultraviolet radiation. These ions, along with hydrogen ions from Jupiter's upper atmosphere, form a sheet of plasma creating permanent auroras shaped like pancakes around both of its poles, shown above. Ganymede has a magnetosphere of its own that carves a protective bubble around it within Jupiter's powerful magnetosphere. The strong currents flowing in Jupiter's magnetosphere also create strong and variable radio emissions. These emissions come from magnetized plasma through a process called the electron cyclotron maser. Ham operators can sometimes hear these emissions late at night when Jupiter is high in the sky.

Auroras on Mars and Venus

In 2005, the European Space Agency's Mars Express spacecraft detected for the first time an aurora on Mars. This was a shocking discovery because Mars' core is thought to have cooled and solidified billions of years ago. These auroras could not originate from interactions between charged particles in the atmosphere and the magnetic field because Mars has no magnetic field. However, Mars does have magnetic anomalies in its crust, likely leftover from its old planetary magnetic field. Its faint auroras result from a flux of electrons moving along the crust magnetic lines and exciting particles in its thin upper atmosphere.

Venus, like Mars, has no intrinsic magnetic field. Yet auroras have often been observed as bright diffuse patches on the night side, sometimes extending right across the entire planetary disc. Some astronomers call this nightglow. Its aurora might be the result of atoms and molecules in the thick Venusian atmosphere being excited by solar radiation during the day and returning to their ground states at night, emitting light. In this case, unlike the aurora on Earth for example, particles are not accelerated by a magnetic field and striking each other at high speeds. Scientists using the Keck Telescope in Hawaii studied two distinct spectral lines in Venus's nightglow: green light emitted by oxygen as it drops to a lower energy state, and red light emitted as oxygen drops further into its ground state. Venus doesn't have much oxygen in its atmosphere; the excited oxygen is believed to come from the splitting of (abundant) carbon dioxide molecules into carbon and oxygen atoms high up in the atmosphere.

Mercury's Aurora?

Mercury has a significant but weak magnetic field and it likely comes from activity within a molten iron core in much the same way that Earth's magnetic field is created. However, Mercury has no atmosphere, so solar radiation precipitates down to the planet's surface along its magnetic field lines and once there, some atomic glow occurs but not the glow of photons, through the aurora mechanism described above. Some researchers would not go as far as to call this faint glow an aurora at all. Messenger just entered orbit around Mercury on March 17, so maybe some of this aurora debate will be resolved.

Auroras on Extrasolar Planets

In addition to the beautiful colours of Earth's auroras, strong auroras on quiet nights can also emit audible chirps and whistles. Scientists call these sounds auroral kilometric radiation or AKR.  It comes from the acceleration zone near the poles. Like the radio emissions from Jupiter's auroras, these noises come from the electron cyclotron maser process. The radio emissions are generated high in space above Earth, by the same shaft of solar particles which then cause auroras. The emissions are beamed into space in narrow planes of separate and powerful bursts. Fortunately for us, this radiation is blocked from reaching the ground by Earth's ionosphere. Otherwise it would overwhelm all of our radio station transmissions. Whenever you get auroras you get AKR. They have been observed from Earth's orbit and on Jupiter and Saturn as well. By searching for radio beams like these in space, astronomers hope to find extrasolar planets with magnetospheres and to investigate the magnetospheres of other stars as well. Perhaps some day we may have telescopes powerful enough to directly observe auroras on exoplanets.

Meanwhile, NASA launched a constellation of five satellites, set up 20 ground-based observatories in Alaska and Canada, and positioned 10 ground-based magnetometers, all part of what is called the THEMIS mission in 2007. Its mission is to understand the connection between variations in solar activity, Earth's magnetosphere and aurora substorms. Auroral substorms are when calm auroras suddenly turn into dancing auroras, much like a change in weather.

An aurora substorm is captured here in this series of photographs by Jan Curtis from Fairbanks Alaska.

So far scientists have discovered that substorms coincide with energy quickly released far out in Earth's magnetosphere (a third of the way to the moon). The energy is the result of magnetic reconnection, a process similar to the one that causes coronal mass ejections as described earlier in this article, but on a much smaller scale. By better understanding the processes involved in triggering auroral substorms, scientists hope to be better able to predict larger damaging solar storms before they occur, so that we can take measures to protect ourselves, as well as our sensitive communications, navigation hardware, satellites, the electric grid, and even pipelines from spikes in solar radiation reaching Earth.

1 comment:

  1. Wel... consider yourself added to my blogroll. I have like six other blogs I read on a weekly basis, guess that number just increased to seven! Keep writing!