Introduction

Auroras, also known as northern and southern polar lights, are natural light displays in the sky, and usually observed at night. They typically occur in the ionosphere. They are also referred to as polar auroras. This is a misnomer however, because they are commonly visible between 65 to 72 degrees north and south latitudes, which place them a ring just within the Arctic and Antarctic circles.
Aurorae do occur deeper inside the polar regions, but these are infrequent and often invisible to the naked eye. An aurora is a natural display of glowing light in the night sky, mainly in zones around the magnetic north and south poles of the Earth and some other planets. Auroras can be spotted throughout the world and on other planets. They are most visible closer to the poles due to the longer periods of darkness and the magnetic field.

Some scientists call this phenomenon aurora polaris. In northern latitudes, it is known as aurora borealis or northern lights, and the southern counterpart is called aurora australis or southern lights. The term aurora borealis combines the name Aurora, the Roman goddess of the dawn, with the word Boreas, the Greek name for north wind. It most often occurs from September to October and from March to April.
The aurora is a glow observed in the night sky, usually in the polar zone. It is also known as "northern lights" or "aurora borealis," which is Latin for "northern dawn" since in Europe especially, it often appears as a reddish glow on the northern horizon as if the sun were rising from an unusual direction. The aurora borealis most often occurs from September to October and March to April. Its southern counterpart "aurora australis," has similar properties, so scientists prefer "aurora polaris".
Low on the horizon it may be noticed a faint glow of greenish light which forms an arch, stretching lazily across the sky. As time passes, additional bands of light form and drift overhead, slowly brightening to form giant curtains in the sky that slowly wave as if a gentle breeze were blowing. Suddenly, the bottom of the curtains brightens with a reddish tint and ripple faster. Blues and purples appear. As the curtains pass directly overhead, bright points of light can be seen that swirl like a pinwheel. The entire sky seems to be full of colour and motion. Then, after several minutes, everything fades into a warm green glow.
Aurora is a luminous glow of the upper atmosphere which is caused by energetic particles that enter the atmosphere from above. This definition differentiates aurora from other forms of airglow, and from sky brightness that is due to reflected or scattered sunlight. Airglow features that have "internal" energy sources are more common than aurora, for example lightening and all associated optical emissions like sprites should not be considered aurora.

By the early Twentieth Century, geoscientists had realized that the aurora was due to emission of light from excited atoms in the uppermost portions of the Earth’s atmosphere.
Officially known in the Northern hemisphere as the aurora borealis, the Northern Lights are natural phenomena featuring beautifully coloured light displays over the Earth.
In 1621, a French scientist, Pierre Gassendi, saw the lights in the north and named after the Roman goddess of dawn, Aurora. He added the word "borealis" for the Roman god of the north wind, Boreas. In the southern hemisphere, they are called aurora australis, meaning "southern." The lights are usually seen after dusk near both poles. Although they look elegant and calm, aurorae are produced from millions of explosions of magnetic energy.
The Aurora Borealis (North) and Aurora Australis (South) have roughly a 2500 mile radius limit from their magnetic poles, although the phenomenon occurs on the perimeter of a more elliptical shape. Aurora does not make any sound but it is a purely visual phenomena.
Aurora is the collective name given to the photons (light) emitted by atoms, molecules and ions that have been excited by energetic charged particles (principally electrons) travelling along magnetic field lines into the Earth's upper atmosphere. Aurora results from the interaction of the solar wind with the Earth's magnetic field.

The Aurora is mesmerizing, dynamic displays of light that appear in the Antarctic skies in winter. They are, in effect, nature's light show; visual poetry penned from the quantum leaps of atmospheric gases. As those who have witnessed the Aurora can attest, few sights can equal the magic and mystery of these luminous sheets of colour undulating in the frigid air of the Antarctic winter.
The aurora has fascinated, and often terrified, humans for thousands of years. From ancient times, tales and narratives about the aurora have been told by polar explorers, adventurers, fur traders, and early settlers.
Also known as the Northern Lights, an Aurora is a beautiful natural phenomenon that often occurs in the polar regions of Earth. It appears as colourful clouds and rays of green and red (and sometimes blue) light that dance across the sky. The aurora borealis and aurora australis (Latin for "northern" and "southern" dawn, respectively) occur in symmetric ovals centred on the northern and southern magnetic poles of Earth.

The aurora is formed when charged particles (electrons and protons) are guided by the Earth's magnetic field into the atmosphere near the poles. When these particles collide with atoms and molecules of the upper atmosphere, primarily oxygen and nitrogen, some of the energy in these collisions is transformed into the visible light that characterizes the aurora.

Summary

Auroras sometimes called the northern and southern lights are natural light displays in the sky. In order to see an Aurora, one usually needs a dark sky (no bright moon, no city lights) and relatively clear weather. Auroras usually occur near the magnetic poles of the earth and occur most often during the equinoxes. Note that the magnetic poles of the earth do not coincide with the geographic poles. For example, the Magnetic North Pole is located in the Arctic Islands of Northern Canada.

The name Aurora Borealis is attributed to Pierre Gassendi, a 17th century French philosopher, astronomer and mathematician. He named the natural light display for Aurora, the Roman goddess of dawn and Boreas which was the Greek name for the north wind. The Southern lights were named Aurora Australis because Australis is the Latin word for South. As with most unexplained natural phenomena, historically there have been all kinds of legends and quite silly religious and superstitious beliefs associated with the Auroras.

Since clear sky and darkness are essential to see aurora, the best time is dictated by the weather and by the sun rise and set times. The moon is also very bright, and should be taken into account when deciding on a period to travel for the purpose of auroral observation. One might see aurora from dusk to dawn throughout the night. The chances are higher for the 3 or 4 hours around midnight.

The Auroral Sub-storm

Intense auroral sub-storm, with aurora over the Great Lakes. Image from the POLAR VIS instrument.

When the solar wind is calm, the aurora might only be at high latitudes and might be faint, but there is still aurora. In order to see aurora, however, the sky must be dark and clear. Sunlight and clouds are the biggest obstacle to auroral observations. If you have a camera on a satellite you can look down on the aurora, and you'll find an oval shaped ring of brightness crowning Earth at all times. When the solar wind is perturbed from a recent flare or other event on the sun, we might get very strong aurora.

After the solar wind has transferred a lot of energy into the magnetosphere, a sudden release of this built-up tension can cause an explosive auroral display. These large events are called sub-storms. A sub-storm usually starts with a slow expansion of the auroral oval followed by a sudden brightening of a small spot, called the auroral breakup. This spot usually is near that place of the auroral oval that is on the opposite side of the sun, which means near the place where midnight is. This brightening rapidly grows until the entire auroral oval is affected. An observer on the ground where this breakup occurs will see a sudden brightening of the aurora which may fill almost the entire sky within tens of seconds. This aurora will be in the shape of rapidly moving curtains.

Aurora on Other Planets

Almost all planets in the solar system have aurora of some sort. If a planet has an atmosphere and is bombarded by energetic particles, it will have an aurora. Since all planets are embedded in the solar wind, all planets are subjected to the energetic particle bombardment, and thus all planets that have a dense enough atmosphere will have some sort of aurora.

Planets like Venus, which has no magnetic field, have very irregular aurora, while planets like Earth, Jupiter, or Saturn, which have an intrinsic magnetic dipole field, have aurora in the shape of oval shaped crowns of light on both hemispheres. When the magnetic field of a planet is not aligned with the rotational axis, we get a much distorted auroral oval which might be near the equator, like on Uranus and Neptune.

                            Aurora on Saturn

Some of the larger moons of the outer planets are also big enough to have an atmosphere, and some have a magnetic field. They are usually protected from the solar wind by the magnetosphere of the planet that they orbit, but since that magnetosphere also contains energetic particles, some of these moons also have aurorae.

Auroral Events

The aurora which occurred as a result of the "great geomagnetic storm" on both 28 August and 2 September 1859, are thought to be perhaps the most spectacular ever witnessed throughout recent recorded history. The latter, which occurred on September 2 as a result of the exceptionally intense Carrington-Hodgson white light solar flare on September 1, produced aurora so widespread and extraordinarily brilliant that they were seen and reported in published scientific measurements, ship's logs and newspapers throughout the United States, Europe, Japan and Australia. It was said in the New York Times that "ordinary print could be read by the light [of the aurora]".

The aurora is thought to have been produced by one of the most intense coronal mass ejections in history, very near the maximum intensity that the sun is thought to be capable of producing. It is also notable for the fact that it is the first time where the phenomena of auroral activity and electricity were unambiguously linked. This insight was made possible not only due to scientific magnetometer measurements of the era but also as a result of a significant portion of the 125,000 miles of telegraph lines then in service being significantly disrupted for many hours throughout the storm.

Some telegraph lines however, seem to have been of the appropriate length and orientation which allowed a current (geomagnetically induced current) to be induced in them (due to Earth's severely fluctuating magnetosphere) and actually used for communication. The following conversation was had between two operators of the American Telegraph Line between Boston and Portland on the night of the 2nd and reported in the Boston Traveller.

The conversation was carried on for around two hours using no battery power at all and working solely with the current induced by the aurora, and it was said that this was the first time on record that more than a word or two was transmitted in such manner.

Black Aurora

Black auroras are actually anti-auroras. In normal auroras, electrons and/or predominantly negatively charged particles fall toward Earth along surfaces of constant magnetic field. They ionize the Earth's atmosphere on impact, causing the bright glows. In black anti-auroras, however, negatively charged particles are sucked out from the Earth's ionosphere along adjoining magnetic field lines. These dark anti-auroras can climb to over 20,000 kilometers and last for several minutes.

Electron Aurora

The electron aurora forms a bright surge at the pole-ward boundary while the protons just show diffuse spreading. The peak intensity of the protons did not change substantially during the entire event. The proton aurora is brighter on the dusk while the electron aurora on the dawn side. As the electron surge expands pole-ward it leaves the protons behind. The electrons form a discrete auroral feature near the aurora-polar cap boundary, which is devoid of substantial energetic (>1 keV) proton precipitation.

Picture from IMAGE's far-ultraviolet instrument of the electron aurora over the North Pole on June 28, 2000

The coloured light display most people associate with the aurora or Northern Lights is produced by electrons crashing into the atmosphere as they descend along the Earth's magnetic field lines. Almost equally bright but less structured are the lights produced by positively charged protons - hydrogen nuclei - as they ram the atmosphere.

Proton Aurora

Triggered by solar activity, normal auroras are caused by collisions between fast-moving electrons and the oxygen and nitrogen in Earth's upper atmosphere. The electrons come from the magnetosphere, the region of space controlled by Earth's magnetic field. As the excited oxygen and nitrogen molecules return to their low energy state, they emit light, seen as the auroral glow. Sometimes, however, auroras can be caused by collisions with heavier protons, causing a more energetic display with strong ultraviolet emission. In addition, protons can temporarily capture an electron and emit light for themselves.

Such a proton aurora is seen above, recorded by the IMAGE satellite. A special feature is the bright spot near picture center, embedded in a ring of auroral emission around the north magnetic pole of planet Earth. Most solar wind protons never reach the Earth to cause auroras because they are completely deflected away at a great distance by the Earth's magnetic field. The bright spot in the auroral ring indicates a particularly deep crack in the Earth's magnetic field where protons were able to flow along a temporarily connected region between the Sun and the Earth, relatively undeflected, until they impacted the Earth's ionosphere.

Shapes of Aurora

Auroras appear in many different shapes. The aurora in the early evening sky forms green arcs that stretch across the sky in an east-west direction (Image 1). The longitudinal extent (length) of an auroral arc can be as large as several thousand kilometres, but its width can be as small as 100 metres.

The basic mechanism behind auroral arcs has been known for decades. When (sheets of) highly energetic electrons manage to precipitate into the upper atmosphere, they are stopped by collisions with neutral atoms and molecules, primarily oxygen and nitrogen, at altitudes of a few hundreds kilometres down to 80 km. Each collision transfers part of the electron energy to these atmospheric particles. In turn, they get rid of this energy excess by emitting visible light at specific wavelengths, producing characteristic colours such as green (oxygen) or purple (nitrogen).

Sounds of Aurora

The northern lights attract millions of people. Ever since aurora has been seen there have been people able to hear their sounds. It is not known where the sounds come from. There is no accepted theory which can approve and explain or disprove crackling, rustling, and swishing sounds that are sometimes heard in connection with aurora. There are even doubts about the existence of these sounds. It is not clear which conditions should be satisfied for the auroral sounds to be heard. However, most of the observations indicate that sounds are heard during periods lasting a few minutes or more during which a powerful display of the aurora can be seen right above the head.

It also seems that not everybody can hear sounds. There are many people that have seen hundreds of powerful aurora displays without any signs of sounds. At the same time, there are observations where in a group of people everybody have heard the sounds (in observation list almost half of all cases). Common to all observations is the fact that sounds have been heard only once or twice in a life time. The observations indicate that the auroral sounds are real but can be heard on very rare occasions.

The sounds usually resemble a swishing or crackling similar to that heard as waves of static on the radio. The level of intensity is said to change along with the rapidity of movement of the auroral forms overhead. If a physical reason does exist for auroral sounds to be audible on the ground, it would most likely be due to the electrical field produced by the aurora. Power fluctuations in long transmission lines have been linked to auroral activity. The northern (or southern) lights generate VLF radio waves as well as light.

There are generally two types of sounds reported to accompany the aurora. The first is a swishing sound that change with movements in that auroral display. The second type is a crackling sound, like static electricity makes.

The problem with claims of sound that changes in time with auroral movements is that the aurora is a long ways away (100 km/60 miles), so if sound was coming from the aurora, there would be a long delay between the auroral movement and the sound arriving at the listener's ears, just as thunder arrives long after a distant lightning flash is observed. There is also the small detail that the air between us and the aurora is far too thin to carry sound over such long distances. Therefore, if this kind of sound exists, it must be created very near to the observer.

The most likely explanation for this type of sound is that it is created inside the observer's head: not a figment of the imagination, but rather leakage of the electrical impulses from the nerves in the eye (carrying images of the aurora to the brain) into the part of the brain the processes sound. In a very quiet environment, there are no sound signals for the brain to process, so it notices these tiny leakage signals and the result is sounds that change in time with the aurora. This explanation was actually tested by some early explorers, who found that the sound went away if their eyes were covered.

The second type of sound is more mysterious. A crackling sound, like static electricity sparking, might be explained by the strong electric and magnetic fields associated with the aurora, but so far theories and measurements have not provided a satisfactory explanation.

Types of Aurora

The aurora can only be seen against a dark sky, and the chances of observing an Aurora are obviously much better in winter when nights are also dark at auroral latitudes. There are two basic types of Aurora, Diffuse and Discrete aurora.

Ø   DIFFUSE AURORA:

Unlike the discrete aurora, which is due to the parallel acceleration of electrons down along the earth's magnetic field lines into the atmosphere, diffuse aurora "dribbles" in from the plasma sheet. This dribbling comes from wave-particle interactions that scatter electrons and give them velocities that are more parallel to the magnetic field line.

A photograph of the diffuse aurora by Jan Curtis in Fairbanks, Alaska. The dark regions covering the aurora are clouds.

This change of velocity will put the electrons in the loss cone in phase space and they will precipitate into the atmosphere, exciting atoms and molecules to produce the diffuse aurora. The quantitative study of the wave-particle interaction with different types of waves and with electrons having different energies is a complicated matter.

Ø   DISCRETE AURORA:

Discrete auroras are the most intense auroral types, where field-aligned acceleration plays an important role, forming the so-called inverted-V precipitation.

Discrete auroral types include

• Quiet auroral arcs
• Spirals, curls, and folds
• Auroral bulge
• Omega bands

Observers on the ground, however, don't see that "diffuse aurora," which is dim and lacks sharp outlines. The bright arcs of the "discrete aurora" which they do see--ribbons in the sky, often shimmering, folding, and constantly changing--have a different origin. Satellites flying through the electron beams which create such arcs see evidence that their electrons have actually been accelerated. It is as if a giant "electron gun" existed somewhere up in space, like the one found in TV picture tubes, and was shooting electrons down field lines.

             This is believed to be the origin of the discrete aurora.

Colours of Aurora

The composition and density of the atmosphere and the altitude of the aurora determine the possible light emissions. When an excited atom or molecule returns to the ground state, it sends out a photon with a specific energy. This energy depends on the type of atom and on the level of excitement, and we perceive the energy of a photon as colour. The upper atmosphere consists of air just like the air we breathe.

At very high altitudes there is atomic oxygen in addition to normal air, which is made up of molecular nitrogen and molecular oxygen. The energetic electrons in aurora are strong enough to occasionally split the molecules of the air into nitrogen and oxygen atoms. The photons that come out of aurora have therefore the signature colours of nitrogen and oxygen molecules and atoms. Oxygen atoms, for example, strongly emit photons in two typical colours: green and red. The red is a brownish red that is at the limit of what the human eye can see, and although the red auroral emission is often very bright, we can barely see it.

Since there is more atomic oxygen at high altitudes, the red aurora tends to be on top of the regular green aurora. The colours that we see are a mixture of all the auroral emissions. Just like the white sunlight is a mixture of the colours of the rainbow, the aurora is a mixture of colours. The overall impression is a greenish-whitish glow. Very intense aurora gets a purple edge at the bottom. The purple is a mixture of blue and red emissions from nitrogen molecules.

The green emission from oxygen atoms has a peculiar thing about it: usually an excited atom or molecule returns to the ground state right away, and the emission of a photon is a matter of microseconds or less. The oxygen atom, however, takes its time. Only after about a 3/4 second does the excited atom returns to the ground state to emit the green photon. For the red photon it takes almost 2 minutes.

Auroral Oval

Fritz and Eather estimated that, given clear skies, aurora could be observed about 100 nights a year in the region where it was most frequent. However, imaging cameras aboard satellites, more sensitive than the eye, observe a ring of diffuse aurora around the polar cap at most times. In magnetic coordinates--z along the dipole axis, the sun's direction in the x-z plane--the region where aurora is likely to occur forms a fixed pattern around the magnetic pole, known as the auroral oval.

That pattern is approximately circular, centred about 5 degrees night ward of the magnetic pole, and the Earth rotates beneath it. The oval also expands and contracts with magnetic activity: its typical radius equals 17 degrees of latitude. The reason the aurora is a rare sight at lower latitudes is that it only appears there when the oval is grossly expanded.

Frequency of Occurrence

The aurora is a common occurrence in the ring-shaped zone. It is occasionally seen in temperate latitudes, when a strong magnetic storm temporarily expands the auroral oval. Large magnetic storms are most common during the peak of the eleven-year sunspot cycle or during the three years after that peak. However, within the auroral zone the likelihood of an aurora occurring depends mostly on the slant of IMF lines (known as Bz, pronounced "bee-sub-zed" or "bee-sub-zee"), being greater with southward slants.

Geomagnetic storms that ignite auroras actually happen more often during the months around the equinoxes. It is not well understood why geomagnetic storms are tied to the earth's seasons when polar activity is not. It is known, however, that during spring and autumn, the earth's and the interplanetary magnetic field link up. At the magnetopause, Earth's magnetic field points north. When Bz becomes large and negative (i.e., the IMF tilts south), it can partially cancel Earth's magnetic field at the point of contact. South-pointing Bz's open a door through which energy from the solar wind can reach Earth's inner magnetosphere.

The peaking of Bz during this time is a result of geometry. The interplanetary magnetic field comes from the sun and is carried outward the solar wind. Because the sun rotates the IMF has a spiral shape. Earth's magnetic dipole axis is most closely aligned with the Parker spiral in April and October. As a result, southward (and northward) excursions of Bz are greatest then.

However, Bz is not the only influence on geomagnetic activity. The Sun's rotation axis is tilted 8 degrees with respect to the plane of Earth's orbit. Because the solar wind blows more rapidly from the sun's poles than from its equator, the average speed of particles buffeting Earth's magnetosphere waxes and wanes every six months. The solar wind speed is greatest -- by about 50 km/s, on average -- around September 5 and March 5 when Earth lies at its highest heliographic latitude.

Still, neither Bz nor the solar wind can fully explain the seasonal behaviour of geomagnetic storms. Those factors together contribute only about one-third of the observed semi-annual variation

Effect of Sunspots on Aurora

The sun is a source of energy, and gives off huge amounts of light and radiation. But the sun also gives off a constant stream of charged particles ... mostly protons and electrons ... that flow outwards from the sun at speeds of about 400 km/s (about 1 million miles per hour).

This 'solar wind' is enhanced by even more high-speed particles given off by solar flare sand sunspots. In the close-up at the right, a flare is seen looping above the right side of the sun; sunspots are the slightly cooler, darker areas on the surface, which indicate turbulence. Fast-moving atomic particles are given off by sunspots, and will cause auroras when they reach the earth. 

The earth has a magnetic field, and the lines of magnetic force emanating from the earth trap the particles from the sun, and pull them downwards. Much like a bar magnet, the lines of force concentrate at the north and south poles ... and that's where most of the energetic particles from the sun end up.

As they spiral down through the atmosphere near the poles, the particles collide with molecules of air, especially oxygen. The oxygen molecules get a boost of energy, which they immediately give off again. This energy being released by 'excited' oxygen molecules is visible as a green glow ... which we see as a shimmering curtain of light. 

When an oxygen atom is hit by a solar particle, an electron is knocked out, ionizing (or 'exciting') the oxygen atom. When a free electron is later reacquired by the oxygen atom, it emits radiation of a blue-green wavelength. All this takes place in the high atmosphere ... auroras happen at heights of 100 kilometres and higher.

Because the increase in solar wind from sunspot activity on the sun follows the same pattern as sunspot growth, the auroras we see on earth peak in intensity every 11 years just as sunspots do. As you can see from the graph on the right, sunspot activity peaked in 2001, as did the size and frequency of auroras.

Both are now starting to decline, following an 11 year cycle. The reason for this 11-year cycle in sunspot activity has to do with the inner dynamics of the sun, and is as yet not well understood. Nevertheless, the cycle is real, and keeping watch on sunspot or flare activity is important.

High energy particles from the sun greatly increase in number when there are flares on the sun, or a lot of big sunspots. This increased activity is called a solar 'storm'. Not only do these particles excite oxygen atoms in our atmosphere and make them glow, but they also can be harmful, both to equipment and people.

Most of the particles are scooped up by the earth's magnetic field, or absorbed by the atmosphere, so they don't make it to the surface. However, astronauts could be harmed by this increase in radiation, so activities are scheduled around major solar activity. Unmanned satellites, however, are in space for years at a time, and the electronics in these are sometimes damaged during solar 'storms'. Radio communication can be disrupted. Even people aboard high -flying airplanes can be subjected to a dose of radiation equivalent to about one x-ray.

Occurrence of Aurora

The aurora is a near daily occurrence somewhere on Earth and there is almost always an aurora in the sky (both day and night, but in the daytime it is out-shined by sunlight). However, the following factors can increase your chance of seeing them:

·         Time of Day: Because the intensity of the light in an aurora is low, it can only be seen at night. Furthermore, the most active and brilliant displays usually occur near midnight. Therefore, the best time to observe the aurora is, on average, between 11 p.m. and 2 a.m.

·         Season: In the northern hemisphere, the best time to view an aurora is during the winter. At latitudes where auroras are common, it is typically light all night in the summer—so you rarely have warm weather and a good aurora. Furthermore, in most polar regions, the weather tends to be clear during the middle of winter—so often the best time to see an aurora is also the coldest.

·         Sun Rotation: It takes the sun 27 days to rotate one time around its axis, so 27 days after an aurora display, the active region on the sun that caused the aurora will face Earth again. Although solar activity in that region on the sun might have decreased in the mean time, there is still a greater chance of aurora 27 days after the last period of increased auroral activity.

·         Solar Activity: Auroral activity also correlates with the activity of the sun, which changes according to an 11-year solar cycle. In general, the more active the sun, the greater the number of auroras. Thus, auroral displays are more likely around the time of the solar maximum (when solar activity is high). Aurora displays remain frequent and strong for several years around solar maximum. During solar maximum, the auroras are not only more frequent and more active, but they also can come further south away from the poles (it should be noted, however, that bright and active auroras can be observed at any time during the solar cycle).

Auroras form in an oval band centred at each magnetic pole. The width of the band ranges from 10 to 1,000 kilometers (km) and it is approximately 3,000 km (1,900 miles) from the magnetic pole during quieter solar periods. If you live near this oval, you will see the aurora on most clear, dark nights. In the northern hemisphere, for example, prime viewing locations include Fairbanks, Alaska, many locations in northern and middle Canada, and in the northern parts of Russia and Scandinavia.

As auroral activity increases, the aurora not only increases in brightness, but it also tends to move further towards the equator. Auroral activity is directly linked to disturbances in Earth's magnetic and electrical current system. These increases in activity are known as geomagnetic storms. To determine how high the geomagnetic activity needs to be for aurora to occur in your area, see the table at NOAA's Space Environment Center.

People in the northern United States and northern Europe may see the aurora a few times in a decade, while people in southern Europe, the southern United States, and even Mexico, may see the aurora only once-in-a-lifetime. It should be noted that the auroral oval does not follow lines of equal latitude, so people on the East Coast of the United States have a higher likelihood of seeing aurora than those at the same latitude on the West Coast.

Geomagnetic storms and the resulting auroral activity, vary unpredictably throughout the year. Because geomagnetic activity often results from events on the sun, it can be predicted by looking at the sun and solar flares. For this reason, auroral forecasts can only be made two or three days in advance. NOAA's Space Environment Center issues forecasts of geomagnetic activity. Since 1979, NOAA's polar-orbiting satellites have been measuring the energy flux of particles into the auroral zones.