Auroras (North/South Polar Lights; or aurorae, sing.: aurora) are natural colored light displays, which are usually observed in the night sky, particularly in the polar zone. The aurorae typically occur in the Ionosphere. Some scientists call them “polar auroras” (or “aurorae polares”). In northern latitudes, it is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for north wind, Boreas. It often appears as a greenish glow or sometimes a faint red, as if the sun was rising from an unusual direction. The aurora borealis is also called the northern polar lights, as it is only visible in the North sky from the Northern Hemisphere. The aurora borealis most often occurs from September to October and from March to April. The Cree call this phenomenon the Dance of the Spirits.
Its southern counterpart, the aurora australis/southern polar lights, has similar properties. Australis is the Latin word for “of the South”.
Benjamin Franklin first brought attention to the “mystery of the Northern Lights.” He theorized the shifting lights to a concentration of electrical charges in the polar regions intensified by the snow and other moisture.
Auroras are produced by the collision of charged particles from Earth’s magnetosphere, mostly electrons but also protons and heavier particles, with atoms and molecules of Earth’s upper atmosphere (at altitudes above 80 km). The particles have energies of 1 to 100 keV. Most originate from the Sun and arrive at the vicinity of Earth in the relatively low-energy solar wind. When the trapped magnetic field of the solar wind is favourably oriented (principally southwards) it reconnects with Earth’s magnetic field, and solar particles enter the magnetosphere and are swept to the magnetotail. Further magnetic reconnection accelerates the particles towards Earth.
The collisions in the atmosphere electronically excite atoms and molecules in the upper atmosphere. The excitation energy can be lost by light emission or collisions. Most aurorae are green and red emission from atomic oxygen. Molecular nitrogen and nitrogen ions produce some low level red and very high blue/violet aurorae.
Auroral forms and magnetism
Typically the aurora appears either as a diffuse glow or as “curtains” that approximately extend in the east-west direction. At some times, they form “quiet arcs”; at others (“active aurora”), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that aurora is shaped by Earth’s magnetic field. Indeed, satellites show electrons to be guided by magnetic field lines, spiraling around them while moving towards Earth.
The curtains often show folds called “striations”, which are curtain-like. When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a “corona” of diverging rays, an effect of perspective.
Although it was first mentioned by Ancient Greek explorer/geographer Pytheas, Hiorter and Celsius first described in 1741 evidence for magnetic control, namely, large magnetic fluctuations occurred whenever the aurora was observed overhead. This indicates (it was later realized) that large electric currents were associated with the aurora, flowing in the region where auroral light originated. Kristian Birkeland (1908) deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside towards (approximately) midnight were later named “auroral electrojets” (see also Birkeland currents).
Still more evidence for a magnetic connection are the statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881)established that the aurora appeared mainly in the “auroral zone”, a ring-shaped region with a radius of approximately 2500 km around Earth’s magnetic pole, not its geographic pole. It was hardly ever seen near that pole itself. The instantaneous distribution of auroras (“auroral oval”, Yasha/Jakob Feldstein 1963) is slightly different, centered about 3-5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest towards the equator around midnight. The aurora can be seen best at this time.
Solar wind and magnetosphere
Schematic of Earth’s magnetosphere
The Earth is constantly immersed in the solar wind, a rarefied flow of hot plasma (gas of free electrons and positive ions) emitted by the Sun in all directions, a result of the million-degree heat of the Sun’s outermost layer, the solar corona. The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cc and magnetic field intensity around 2-5 nT (nanoteslas; Earth’s surface field is typically 30,000-50,000 nT). These are typical values. During magnetic storms, in particular, flows can be several times faster; the interplanetary magnetic field (IMF) may also be much stronger.
The IMF originates on the Sun, related to the field of sunspots, and its field lines (lines of force) are dragged out by the solar wind. That alone would tend to line them up in the Sun-Earth direction, but the rotation of the Sun skews them (at Earth) by about 45 degrees, so that field lines passing Earth may actually start near the western edge (“limb”) of the visible sun.
Earth’s magnetosphere is the space region dominated by its magnetic field. It forms an obstacle in the path of the solar wind, causing it to be diverted around it, at a distance of about 70,000 km (before it reaches that boundary, typically 12,000-15,000 km upstream, a bow shock forms). The width of the magnetospheric obstacle, abreast of Earth, is typically 190,000 km, and on the night side a long “magnetotail” of stretched field lines extends to great distances.
When the solar wind is perturbed, it easily transfers energy and material into the magnetosphere. The electrons and ions in the magnetosphere that are thus energized move along the magnetic field lines to the polar regions of the atmosphere.
Frequency of occurrence
Aurora australis 1994 from latitude 47 degrees south
The aurora is a common occurrence in the Poles. 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 Earth’s seasons while polar activity is not. But it is known that during spring and autumn, the interplanetary magnetic field and that of Earth 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 (IMF) 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 behavior of geomagnetic storms. Those factors together contribute only about one-third of the observed semiannual variation.
Auroral events of historical significance
The auroras which occurred as a result of the “great geomagnetic storm” on both August 28, 1859 and September 2, 1859 are thought to be perhaps the most spectacular ever witnessed throughout recent recorded history. The latter, which occurred on September 2, 1859 as a result of the exceptionally intense Carrington-Hodgson white light solar flare on September 1, 1859 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[specify] 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 occurred between two operators of the American Telegraph Line between Boston and Portland, Maine, on the night of September 2, 1859 and reported in the Boston Traveler:
Boston operator (to Portland operator): “Please cut off your battery [power source] entirely for fifteen minutes.”
Portland operator: “Will do so. It is now disconnected.”
Boston: “Mine is disconnected, and we are working with the auroral current. How do you receive my writing?”
Portland: “Better than with our batteries on. – Current comes and goes gradually.”
Boston: “My current is very strong at times, and we can work better without the batteries, as the aurora seems to neutralize and augment our batteries alternately, making current too strong at times for our relay magnets. Suppose we work without batteries while we are affected by this trouble.”
Portland: “Very well. Shall I go ahead with business?”
Boston: “Yes. Go ahead.”
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.
Aurora australis (September 11, 2005) as captured by NASA’s IMAGE satellite, digitally overlaid onto the Blue Marble composite image.
The ultimate energy source of the aurora is the solar wind flowing past the Earth.
Both the magnetosphere and the solar wind consist of plasma (ionized gas), which conducts electricity. It is well known (since Michael Faraday’s [1791 – 1867] work around 1830) that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts across (or is cut by), rather than along, the lines of the magnetic field, an electrical current is said to be induced into that conductor and electrons will flow within it. The amount of current flow is dependent upon a) the rate of relative motion and b) the strength of the magnetic field, c) the number of conductors ganged together and d) the distance between the conductor and the magnetic field, while the direction of flow is dependent upon the direction of relative motion. Dynamos make use of this basic process (“the dynamo effect”), any and all conductors, solid or otherwise are so affected including plasmas or other fluids.
In particular the solar wind and the magnetosphere are two electrically conducting fluids with such relative motion and should be able (in principle) to generate electric currents by “dynamo action”, in the process also extracting energy from the flow of the solar wind. The process is hampered by the fact that plasmas conduct easily along magnetic field lines, but not so easily perpendicular to them. So it is important that a temporary magnetic connection be established between the field lines of the solar wind and those of the magnetosphere, by a process known as magnetic reconnection. It happens most easily with a southward slant of interplanetary field lines, because then field lines north of Earth approximately match the direction of field lines near the north magnetic pole (namely, into Earth), and similarly near the southern pole. Indeed, active auroras (and related “substorms”) are much more likely at such times.
Electric currents originating in such way apparently give auroral electrons their energy. The magnetospheric plasma has an abundance of electrons: some are magnetically trapped, some reside in the magnetotail, and some exist in the upward extension of the ionosphere, which may extend (with diminishing density) some 25,000 km around Earth.
Bright auroras are generally associated with Birkeland currents (Schield et al., 1969;Zmuda and Armstrong, 1973) which flow down into the ionosphere on one side of the pole and out on the other. In between, some of the current connects directly through the ionospheric E layer (125 km); the rest (“region 2”) detours, leaving again through field lines closer to the equator and closing through the “partial ring current” carried by magnetically trapped plasma. The ionosphere is an ohmic conductor, so such currents require a driving voltage, which some dynamo mechanism can supply. Electric field probes in orbit above the polar cap suggest voltages of the order of 40,000 volts, rising up to more than 200,000 volts during intense magnetic storms.
Ionospheric resistance has a complex nature, and leads to a secondary Hall current flow. By a strange twist of physics, the magnetic disturbance on the ground due to the main current almost cancels out, so most of the observed effect of auroras is due to a secondary current, the auroral electrojet. An auroral electrojet index (measured in nanotesla) is regularly derived from ground data and serves as a general measure of auroral activity.
However, ohmic resistance is not the only obstacle to current flow in this circuit. The convergence of magnetic field lines near Earth creates a “mirror effect” which turns back most of the down-flowing electrons (where currents flow upwards), inhibiting current-carrying capacity. To overcome this, part of the available voltage appears along the field line (“parallel to the field”), helping electrons overcome that obstacle by widening the bundle of trajectories reaching Earth; a similar “parallel voltage” is used in “tandem mirror” plasma containment devices. A feature of such voltage is that it is concentrated near Earth (potential proportional to field intensity; Persson, 1963), and indeed, as deduced by Evans (1974) and confirmed by satellites, most auroral acceleration occurs below 10,000 km. Another indicator of parallel electric fields along field lines are beams of upwards flowing O+ ions observed on auroral field lines.
While this mechanism is probably the main source of the familiar auroral arcs, formations conspicuous from the ground, more energy might go to other, less prominent types of aurora, e.g. the diffuse aurora (below) and the low-energy electrons precipitated in magnetic storms (also below).
The Aurora Borealis as viewed from the ISS Expedition 6 team. Lake Manicouagan is visible to the bottom left.
Some O+ ions (“conics”) also seem accelerated in different ways by plasma processes associated with the aurora. These ions are accelerated by plasma waves, in directions mainly perpendicular to the field lines. They therefore start at their own “mirror points” and can travel only upwards. As they do so, the “mirror effect” transforms their directions of motion, from perpendicular to the line to lying on a cone around it, which gradually narrows down.
In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation (AKR, discovered in 1972). Ionospheric absorption makes AKR observable from space only.
These “parallel voltages” accelerate electrons to auroral energies and seem to be a major source of aurora. Other mechanisms have also been proposed, in particular, Alfvén waves, wave modes involving the magnetic field first noted by Hannes Alfvén (1942), which have been observed in the lab and in space. The question is however whether this might just be a different way of looking at the above process, because this approach does not point out a different energy source, and many plasma bulk phenomena can also be described in terms of Alfvén waves.