An aurora (plural: aurorae or auroras; from the Latin word aurora, "sunrise" or the Roman goddess of dawn) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth's magnetic field into the atmosphere. Most aurorae occur in a band known as the auroral zone,[1][2] which is typically 3° to 6° in latitudinal extent and at all local times or longitudes. The auroral zone is typically 10° to 20° from the magnetic pole defined by the axis of the Earth's magnetic dipole. During a geomagnetic storm, the auroral zone expands to lower latitudes.
Aurorae are classified as diffuse or discrete. The diffuse aurora is a featureless glow in the sky that may not be visible to the naked eye, even on a dark night. It defines the extent of the auroral zone. The discrete aurorae are sharply defined features within the diffuse aurora that vary in brightness from just barely visible to the naked eye, to bright enough to read a newspaper by at night. Discrete aurorae are usually seen in only the night sky, because they are not as bright as the sunlit sky. Aurorae occasionally occur poleward of the auroral zone as diffuse patches[3] or arcs (polar cap arcs[4]), which are generally invisible to the naked eye.
In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621.[5] Auroras seen near the magnetic pole may be high overhead, but from farther away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the Sun were rising from an unusual direction. Discrete aurorae often display magnetic field lines or curtain-like structures, and can change within seconds or glow unchanging for hours, most often in fluorescent green. The aurora borealis most often occurs near the equinoctes. The northern lights have had a number of names throughout history. The Cree call this phenomenon the "Dance of the Spirits". In Medieval Europe, the auroras were commonly believed to be a sign from God.[6]
Its southern counterpart, the aurora australis (or the southern lights), has features that are almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone.[7] It is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia. Aurorae occur on other planets. Similar to the Earth's aurora, they are visible close to the planet's magnetic poles. Modern style guides recommend that the names of meteorological phenomena, such as aurora borealis, be uncapitalized.[8]
History of aurora theories
Multiple superstitions and obsolete theories explaining the aurora have surfaced over the centuries.
- Seneca speaks diffusely on auroras in the first book of his Naturales Quaestiones, drawing mainly from Aristotle; he classifies them ("putei" or wells when they are circular and "rim a large hole in the sky", "pithaei" when they look like casks, "chasmata" from the same root of the English chasm, "pogoniae" when they are bearded, "cyparissae" when they look like cypresses), describes their manifold colors and asks himself whether they are above or below the clouds. He recalls that under Tiberius, an aurora formed above Ostia, so intense and so red that a cohort of the army, stationed nearby for fireman duty, galloped to the city.
- Walter William Bryant wrote in his book Kepler (1920) that Tycho Brahe "seems to have been something of a homœopathist, for he recommends sulphur to cure infectious diseases “brought on by the sulphurous vapours of the Aurora Borealis."[9]
- Benjamin Franklin theorized that the "mystery of the Northern Lights" was caused by a concentration of electrical charges in the polar regions intensified by the snow and other moisture.[10]
- Auroral electrons come from beams emitted by the Sun. This was claimed around 1900 by Kristian Birkeland, whose experiments in a vacuum chamber with electron beams and magnetized spheres (miniature models of Earth or "terrellas") showed that such electrons would be guided toward the polar regions. Problems with this model included absence of aurora at the poles themselves, self-dispersal of such beams by their negative charge, and more recently, lack of any observational evidence in space.
- The aurora is the overflow of the radiation belt ("leaky bucket theory"). This was first disproved around 1962 by James Van Allen and co-workers, who showed that the high rate of energy dissipation by the aurora would quickly drain the radiation belt. Soon afterward, it became clear that most of the energy in trapped particles resided in positive ions, while auroral particles were almost always electrons, of relatively low energy.
- The aurora is produced by solar wind particles guided by Earth's field lines to the top of the atmosphere. This holds true for the cusp aurora, but outside the cusp, the solar wind has no direct access. In addition, the main energy in the solar wind resides in positive ions; electrons only have about 0.5 eV (electron volt), and while in the cusp this may be raised to 50–100 eV, that still falls short of auroral energies.
Auroral mechanism
Auroras result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state.[11] They are ionized or excited by the collision of solar wind and magnetospheric particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon, or by collision with another atom or molecule:
- oxygen emissions
- green or brownish-red, depending on the amount of energy absorbed.
- nitrogen emissions
- blue or red; blue if the atom regains an electron after it has been ionized, red if returning to ground state from an excited state.
Oxygen is unusual in terms of its return to ground state: it can take three quarters of a second to emit green light and up to two minutes to emit red. Collisions with other atoms or molecules absorb the excitation energy and prevent emission. Because the very top of the atmosphere has a higher percentage of oxygen and is sparsely distributed such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented.
This is why there is a color differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly, pure blue.
Auroras are associated with the solar wind, a flow of ions continuously flowing outward from the Sun. The Earth's magnetic field traps these particles, many of which travel toward the poles where they are accelerated toward Earth. Collisions between these ions and atmospheric atoms and molecules cause energy releases in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.[12]
| A predominantly red aurora australis |
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 auroras are shaped by Earth's magnetic field. Indeed, satellites show that electrons are guided by magnetic field lines, spiraling around them while moving toward Earth.
The similarity to curtains is often enhanced by folds called "striations". 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)[13] deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside toward (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)[14] and S. Tromholt (1882)[15] 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. It was hardly ever seen near the geographic pole, which is about 2000 km away from the magnetic pole. The instantaneous distribution of auroras ("auroral oval"[1][2]) is slightly different, centered about 3–5 degrees nightward of the magnetic pole, so that auroral arcs reach furthest toward the equator about an hour before midnight. The aurora can be seen best at this time, called magnetic midnight, which occurs when an observer, the magnetic pole in question and the Sun are in alignment.
In the late 1900s, astrophysicist Joan Feynman deduced that auroras are a product of the interaction between the Earth's magnetosphere and the magnetic field of the solar wind. Her work resulted from data collected by the Explorer 33 spacecraft.[16]
On 26 February 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms.[17] Two of the five probes, positioned approximately one third the distance to the moon, measured events suggesting a magnetic reconnection event 96 seconds prior to auroral intensification.[18] Dr. Vassilis Angelopoulos of the University of California, Los Angeles, the principal investigator for the THEMIS mission, claimed, "Our data show clearly and for the first time that magnetic reconnection is the trigger."[19]
Solar wind and the 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 two-million-degree heat of the Sun's outermost layer, the corona. The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cm3 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.[20]
Earth's magnetosphere is formed by the impact of the solar wind on the Earth's magnetic field. It forms an obstacle to the solar wind, diverting it, at an average distance of about 70,000 km (11 Earth radii or Re),[21] forming a bow shock 12,000 km to 15,000 km (1.9 to 2.4 Re) further upstream. The width of the magnetosphere abreast of Earth, is typically 190,000 km (30 Re), and on the night side a long "magnetotail" of stretched field lines extends to great distances (> 200 Re).
The magnetosphere is full of trapped plasma as the solar wind passes the Earth. The flow of plasma into the magnetosphere increases with increases in solar wind density and speed, with increase in the southward component of the IMF and with increases in turbulence in the solar wind flow.[22] The flow pattern of magnetospheric plasma is from the magnetotail toward the Earth, around the Earth and back into the solar wind through the magnetopause on the day-side. In addition to moving perpendicular to the Earth's magnetic field, some magnetospheric plasma travel down along the Earth's magnetic field lines and lose energy to the atmosphere in the auroral zones. Magnetospheric electrons accelerated downward by field-aligned electric fields cause the bright aurora features. The un-accelerated electrons and ions cause the dim glow of the diffuse aurora.
Frequency of occurrence
Auroras are occasionally seen in temperate latitudes, when a magnetic storm temporarily enlarges 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.[23][24] Within the auroral zone the likelihood of an aurora occurring depends mostly on the slant of interplanetary magnetic field (IMF) lines (the slant is known as Bz), however, being greater with southward slants.
Geomagnetic storms that ignite auroras actually happen more often during the months around the equinoctes. 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 Bzs open a door through which energy from the solar wind reaches Earth's inner magnetosphere.
The peaking of Bz during this time is a result of geometry. The IMF comes from the Sun and is carried outward with the solar wind. The rotation of the Sun causes the IMF to have a spiral shape called the Parker spiral. The southward (and northward) excursions of Bz are greatest during April and October, when Earth's magnetic dipole axis is most closely aligned with the Parker spiral.
Bz is not the only influence on geomagnetic activity, however, the Sun's rotation axis is tilted 8 degrees with respect to the plane of Earth's orbit. The solar wind blows more rapidly from the Sun's poles than from its equator, thus 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 5 September and 5 March 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 variations.
Auroral events of historical significance
The auroras that resulted from the "great geomagnetic storm" on both 28 August and 2 September 1859 are thought the most spectacular in recent recorded history. Balfour Stewart, in a paper[25][26] to the Royal Society on 21 November 1861, described both auroral events as documented by a self-recording magnetograph at the Kew Observatory and established the connection between the 2 September 1859 auroral storm and the Carrington-Hodgson flare event when he observed that, "It is not impossible to suppose that in this case our luminary was taken in the act." The second auroral event, which occurred on 2 September 1859 as a result of the exceptionally intense Carrington-Hodgson white light solar flare on 1 September 1859, produced auroras so widespread and extraordinarily brilliant that they were seen and reported in published scientific measurements, ship logs, and newspapers throughout the United States, Europe, Japan, and Australia. It was reported by the New York Times[27][28][29] that in Boston on Friday 2 September 1859 the aurora was "so brilliant that at about one o'clock ordinary print could be read by the light".[28][30][31] One o'clock EST time on Friday 2 September, would have been 6:00 GMT and the self-recording magnetograph at the Kew Observatory was recording the geomagnetic storm, which was then one hour old, at its full intensity. Between 1859 and 1862, Elias Loomis published a series of nine papers on the Great Auroral Exhibition of 1859 in the American Journal of Science where he collected world-wide reports of the auroral event.
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 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 (201,000 km) 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 to produce a sufficient geomagnetically induced current from the electromagnetic field to allow for continued communication with the telegraph operator power supplies switched off. The following conversation occurred between two operators of the American Telegraph Line between Boston and Portland, Maine, on the night of 2 September 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.[30] Such events led to the general conclusion that
The effect of the aurorae on the electric telegraph is generally to increase or diminish the electric current generated in working the wires. Sometimes it entirely neutralizes them, so that, in effect, no fluid is discoverable in them. The aurora borealis seems to be composed of a mass of electric matter, resembling in every respect, that generated by the electric galvanic battery. The currents from it change coming on the wires, and then disappear: the mass of the aurora rolls from the horizon to the zenith.[32]
Origin
The ultimate energy source of the aurora is the solar wind flowing past the Earth. The magnetosphere and solar wind consist of plasma (ionized gas), which conducts electricity. It is well known (since Michael Faraday's 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 electric current is said to be induced into that conductor and electrons flow within it. The amount of current flow is dependent upon a) the rate of relative motion, 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 south magnetic 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;[33] Zmuda and Armstrong, 1973[34]) 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.
Ohmic resistance is not the only obstacle to current flow in this circuit, however, the convergence of magnetic field lines near Earth creates a "mirror effect" that turns back most of the down-flowing electrons (where currents flow upward), inhibiting current-carrying capacit