AURORA [POLARIS]

AURORAL FORMS and TYPES

The visible aurora extends vertically along magnetic field lines often for tens of miles and horizontally east-west for hundreds of miles, but the auroral feature is usually only 100 yards or so thick (north-south). The shape or form of an aurora therefore is dependent on the perspective of the observer.

• Basic forms
  • arc: a slightly curving arc of light with smooth lower borders
  • band: continuous but irregular lower borders with kinks and folds
  • patch: isolated small region of luminosity like a patch of cloud
  • veil: uniform luminosity covering a large fraction of the sky
  • ray: a streak or shaft of luminosity
  An example of each of these basic Auroral Forms
  More description of arcs, bands and rays
• Other shapes and forms
  • curl: a counter-clockwise (in northern hemisphere) rotation having a diameter of 0.5 to 3 miles (1 to 5 km); a ray viewed from the bottom rather than the side
  • fold: an S-shaped curve on a similar scale as a curl
  • spiral: a larger scale (50 to 1000 miles) curled pattern
  • drape(ry): long-rayed band with folds
  • curtain: similar to drape with large vertical extent having sharp lower edge and gradually fading upwards
  • corona aurora: rays converging onto a point (which indicates the overhead position of the magnetic field along which the observer is located). A YouTube clip of one over Fairbanks on Jan 22, 2012. Another movie clip of one.
  • black aurora: a well-defined dark region within an extensive, otherwise uniform, auroral display
  

• Descriptors of spatial structure
  • homogeneous: lacking internal structure
  • striated: having fine filaments
  • rayed: having rays within a basic form
• Descriptors of temporal behavior
  • quiet: steady luminosity
  • pulsating: rhythmic fading and brightening (every 10 to 100 seconds). Video example
  • flickering: rapid intensity changes (about 5 to 10 times per second). B&W video shows flickering beginning at about 30 second in.
  • flaming: luminosity bursts from bottom to top
  • streaming: varying brightness progressing horizontally

Auroral Types:

• Discrete aurora (the bright visible form is classified by Color Types). Examples of these color types.
  Note that the three primary colors (red, blue, and green) can be produced so that the aurora may appear to have a wide range of colors depending on the observer's perspective.
  • Type A aurora (green with red tops):
    • energetic electrons flow directly down magnetic field lines
    • usually red at high altitude (>150 miles)
    • yellowish green at lower altitude (60-150 miles)
    • colors due to emission by atomic oxygen
    • more commonly occurs on day side of geomagnetic pole (visible during polar winter)
  • Type B aurora (green with red lower edges):
    • energetic electrons from edge of plasma sheet in geotail
    • crimson and blues at low altitude (<60 miles)
    • colors due to molecular emissions (nitrogen, oxygen)
    • yellowish green above 60 miles (vide supra)
    • more commonly occurs on night side of geomagnetic pole
  • Type C aurora (green):
    • most commonly observed color Type
    • essentially Type A or B without visible upper or lower red regions
  • Type D aurora (red):
    • bombarding particles lack sufficient energy to penetrate below 150 miles altitude
    • red color due to emission by atomic oxygen (as in Type A)
  • Type E aurora (a rapidly moving Type B aurora)
  • Type F aurora (blue-purple):
    • auroral altitude is still in sunlight but lower altitudes in darkness
    • color due to resonance scattering (see airglow) by molecular nitrogen ion N₂⁺
  • Proton aurora:
    • protons (hydrogen ions) contribute as bombarding particles
    • additional red and blue from atomic hydrogen emission
• Diffuse aurora (observed by satellite; not typically seen from the ground)
  • energetic particles steadily "leak" from the plasma sheet
  • luminosity more spread out and weaker than for discrete aurora
• Polar cap (or Theta) aurora: has a sun-aligned arc stretching over the pole. More info on polar cap aurora.
  

AURORA PHYSICS

• occurrence in time and space (climatology)
• details of fine structure in individual forms (morphology)
• spectrum of emitted light (spectroscopy)
• photochemical processses causing that light
• processess that determine source regions and energies of particles from the magnetosphere
• interactions between solar wind and Earth's magnetosphere

A crude analogy
like a television:

• The electrical socket = The Sun
• The current in the power cord = The solar wind
• The electrodes of the picture tube = The magnetosphere
• The television screen = Earth's atmosphere (specifically, the ionosphere)
• The television picture = The aurora

Another crude analogy
like a toaster:

• The electrical socket = The Sun
• The current in the power cord = The solar wind
• The toaster wire (resistor) = Earth's ionosphere
• The current through the toaster = The Birkeland currents and the auroral electrojets
• The glow of the toaster wire = The aurora

The following excerpt is from The Auroral Particles and Imagery Group at The Johns Hopkins University Applied Physics Laboratory (JHU/APL)
The Earth wears a halo around each magnetic pole. The halo is visible to the naked eye (well, at least to an astronaut above the poles) near winter solstice, and can be observed any time in ultraviolet (uv). This ring, called the auroral oval, divides the Earth's magnetic field lines into two types:

• "closed" field lines which come out of the Earth and return back into it
• "open" field lines, near the magnetic poles, which come out of the Earth and connect up to the solar wind.

Auroral Oval versus Auroral Zone

The Auroral Oval: the instantaneous position of luminosity below which the Earth rotates. Typically around 67 degrees geomagnetic latitude around midnight and moving to 78 degrees geomagnetic latitude at midday. The picture below shows the auroral oval in winter approaching midnight in Albany, NY

The Auroral Zone: the position of maximum probability of observing auroral luminosity. Viewing conditions are best around midnight and the aurora also tends to be brighter in this section of the oval. Consequently the highest probability of viewing aurora occurs around the geomagnetic latitude of 67 degrees

Note that the Auroral Zone passes through central Alaska, but in the picture below taken just after sunset in Alaska, the Auroral Oval lies to the north. The aurora may be visible at this latitude only during winter; at other times of the year the aurora is not usually visible here because of daylight. The Auroral Oval will pass over central Alaska later in the evening.

Other notes:

• Auroral substorms (bursts of auroral activity) are related to magnetospheric substorms
• More energetic particles (electrons, protons, etc.) penetrate deeper into the atmosphere so the characteristics of a particular auroral display depends of the energy distribution of these particles as they enter the atmosphere.
• Conjugacy

OTHER AURORAL PHENOMENA (non-luminous)

• Radar aurora: reflection of radio signals in E-region
• Auroral radio absorption: absorption of radio waves in D-region due to enhanced ionization by energetic electrons
• Auroral X-rays: Bremsstrahlung (braking) radiation due to decelertating electrons
• Magnetic disturbances
  • Magnetic Bays: deviations in the magnetic field at the surface due to the auroral electrojet which significantly enhances magnetic field if before midnight and significantly reduces magnetic field if after midnight
  • Micropulsations: small regularly occurring fluctuations (seconds to minutes) due to Alfven-type (hydomagnetic) waves
• Radio aurora: Low frequency radio emissions (vlf and ulf) generated in magnetosphere by wave-particle interactions including chorus (structured), hiss (unstructured), auroral kilometric radiation (AKR), and lion roar. See radio aurora and "Sounds" of the Magnetosphere
• Audible Aurora: some witnesses report hearing swishing or crackling while observing an aurora
  This phenomena has not been documented, but some proposed causes are:
  
  • psychosomatic (observer expects to hear something)
  • tinnitus
  • physiological leakage of current from optic nerve to auditory senses
  • brush discharge due to electric fields
  • natural transduction of radio waves into acoustic waves
  See The Audible Aurora?
  Update: July 2012

  HISTORY of AURORAL SCIENCE

  5th c. BC	Hippocrates suggests aurora is caused by reflected sunlight
  332 BC	Aristotle in Meteorologica suggests flammable vapors produce "chasmata"
  585 AD	Gregory of Tours describes aurorae as natural phenomena (not supernatural)
  1527	Peter Creutzer produces first printed description
  1563	William Fulke treatise claims aurorae are burning vapors
  1575	Cornelius Gemma publishes first scientific illustration
  1607	Johannes Kepler describes aurora
  ca. 1620	Galileo Galilei or Pierre Gassendi coin name "aurora borealis"
  ca. 1620	René Descartes proposes sunlight scattered from ice crystals in clouds
  1719	Edmund Halley proposes "magnetic effluvia"
  1728	W. Derham suggests venting of vapors and association with earthquakes
  1733	Jean Jacques d'Ortous de Mairan publishes first textbook devoted to aurora; proposes mingling of Earth and solar atmospheres/zodiacal light; suggests sunspot correlation, existence of southern aurora
  1741	Anders Celsius and Olof Hiorter note coincidence of magnetic storms and aurorae
  1746	Leonhard Euler proposes escaping particles at the poles
  1750	Mairan obtains report of 1745 southern aurora (from Capt. Don Ulloa)
  1752	William Watson invents discharge tube; leads to electrical discharge theories of aurora by Mikhail Lomonosov and Ben Franklin among others
  1777	J.C. Wilcke notes corona aurora converges on local magnetic field line
  1790	Henry Cavendish measures height successfully, 50 - 70 miles high
  1792	John Dalton proposes luminous magnetic (iron) particles
  1806	Alexander von Humboldt notes coincidence of auroral display with magnetic storm
  1815	Alessandro Volta proposes condensing electrical fluids
  1817	Jean Batiste Biot proves aurora is self luminous (not reflected sunlight) (more on history and physics of optical polarization)
  1819-47	John Franklin arctic exploration and disappearance. One ship found in 2014
  1822	Franklin notes decrease in auroral frequency toward pole
  1835	Denison Olmstead proposes cosmic origin (ferric cosmic dust clouds)
  1854	Auguste de la Rive publishes theory Demonstration apparatus at the Mus�e d'histoire des sciences, Gen�ve, Suisse
  Sep 1, 1859	Carrington discovers solar flare; brilliant aurora occurs evening of the 2nd
  1860	Elias Loomis maps northern auroral zone
  1868	Anders Angstrom finds auroral green line and later other lines
  1873	Loomis shows frequency of mid-latitude auroral displays correlates with sunspot cycle
  1878	William Crookes invents cathode ray tube and discovers rays are deflected by magnetic fields
  1878	Henri Bequerel proposes aurora caused by hydrogen from Sun (discovered natural radioactivity 1896; Nobel Prize 1903) (See history outline of the solar wind)
  1880	Eugen Goldstein proposes aurora caused by cathode rays from Sun
  1881-82	First Polar Year
  1881	Hermann Fritz improves auroral map, solar cycle connection
  1885	Sophus Tromholt notes change of position of aurora with time of day
  1897	J. J. Thomson discovers electrons as particles in cathode rays (Nobel Prize 1906)
  1903	Kristian Birkeland infers existence of large currents associated with aurora (auroral electrojets)
  1900-13	Birkeland conducts demonstrations with physical model terrella
  1911	Alfred Wegener proposes green line is due to "geocoronium"
  1911-43	Carl Störmer makes extensive auroral observations
  1912-13	Lars Vegard identifies auroral nitrogen bands (blue, red)
  1915	William Ramsay proposes green line is due to krypton (Nobel Prize in 1904 for discovery in air of argon, helium, krypton, neon and xenon in the 1890's)
  1923	Vegard suggests green line is due to solid nitrogen crystals
  1928	John McLennan identifies green line to be due to atomic oxygen
  1930	Störmer classifies auroral forms
  1930-31	Second Polar Year
  1937	Vegard suggests atomic oxygen is excited by energy tranfer with molecular nitrogen
  1939	Southern auroral zone is mapped
  1939	Vegard identifies red & blue hydrogen lines
  1948	Vegard detects fast protons from Doppler shift of hydrogen lines
  1954	Rockets detect fast electrons as source of aurora
  1957-58	Int'l Geophysical Year - numerous amateur reports; all-sky cameras
  1957	John Winckler discovers auroral X-rays
  1963	Yasha Feldstein maps auroral oval
  1964	Syun Akasofu connects auroral bursts to magnetospheric substorms
  1968	Conjugacy (mirroring) of northern and southern aurorae is shown
  1972	ISIS-2 satellite discovers diffuse aurora
  1974	Triad satellite provides evidence for "Birkeland Curents"
  1981	Dynamics Explorer 1 satellite provides first full image of auroral oval
  1985	Electron impact is only a minor excitation mechanism for the green emission from atomic oxygen Energy transfer with excited molecular nitrogen is dominant mechanism in normal (Type B and C) aurora
  2002	Cluster spacecraft detect strong "rippling" of the magnetopause
  2008	Themis satellites elucidate mechanisms for auroral substorms (video)

  
  
  

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