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Imaging the aurora

Harald U. FreySpace Sciences Laboratory

University of California at Berkeley

Credit:D. Hutchinson

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Outline

1. Optical properties of the aurora2. Principles of optical detectors3. Instruments for observations from space4. Instruments for observations from ground5. Analyzing auroral images

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Aurora is the result of a chain of interactions• Sun• Solar wind• Magnetosphere• Atomic physics

• Source• Transmission• Storage, Acceleration• Light emission

The aurora watcher’s handbook

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Aurora seen in different “light”

X-Ray

VisibleFar Ultraviolet

Extreme Ultraviolet

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Aurora exists in other worlds too

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The electromagnetic spectrum

Energy of a photon: λ/hcE =Visible: 2 eVUltraviolet: 10 eVX-Ray: 10000 eV

The electron volt is a very tiny energy unit. You need 1020 of them to make any effect.

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The optical spectrum of aurora

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Electronic states of an atom (oxygen)

Proton ElectronNeutron

Ground state Excited stateLight or particle

Light

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Excited levels of oxygen (term scheme)

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B

Oxygen atom specific photons, e.g. 557.7, 630 nm

Nitrogen molecule specificphotons, e.g. 427.8 nm

Electron aurorafrom space

Aurora is created by energetic electronsComposition of atmosphere

Electron is “lost” (precipitated)

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The auroral spectrumSensitivity of the human eye

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Absorption of light in the atmosphere

Background signal from dayglow

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The photometric unit Rayleigh

Radiance L (apparent surface brightness) is given in: Photons cm-1 s-1 sr-1

4 π L is then given in Rayleigh1 R = 106 photons cm-2 s-1

Solid angle Ω = A / r2 (sr)

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2. Principles of optical detectors

Important quantities are:- Position information- Magnitude of signal (brightness)

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For an electron to be excited from thevalence band to the conduction band

hν / Eg

h = Planck constant (6.6310-34 Joule•sec)ν = frequency of light (cycles/sec) = λ/c Eg = energy gap of material (electron-volts)

Principle of a Charged Coupled Device (CCD)

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Transfer of charge

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Frame-transfer versus Line-transfer

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Other position sensitive devices

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Cross-delay line detector

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3. Instruments for observations from space

Examples:- Viking- IMAGE- ISUAL (Imager for Sprites and Upper Atmospheric Lightning)

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Optical system of Viking spacecraft

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IMAGE WIC Camera

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IMAGE Spectro-graphic Imager

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Discreetness of light, Value of long exposures

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Time-Delayed Integration (TDI)

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4. Instruments for observations from ground

Fisheye opticsAll-sky cameraVan Rhijn Effect

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Wide angle photography and distortions

Focal length of optics:>80 mm: Telephoto50 mm: Normal human perspective30 mm: Wide angle10 mm: Fisheye

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Corona

Credit: B. Walker

Credit: T. Trondsen

Credit: J. Curtis

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Fisheye optics and all-sky camera

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There is no point on Earth that is always under the auroral oval

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Apparent motion of auroral oval

All-sky camera is looking up to the sky and sees from one to the other horizon

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The van Rhijn Effect

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5. Analyzing auroral images

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Spatial scale of aurora and resolution of observations

30-120 seconds 1-5 seconds 30 milliseconds

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Small scale distortions

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Movie of curls

40 km

30 km

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Spacecraft flight over auroral oval

View like sitting in an airplane and looking out of window.

1000 km

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Mapping of all-sky images

Minimum elevation 0o Minimum elevation 8o

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Green auroral arcs

Credit: D. Hutchinson

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Red auroras

Credit: P. Hoffmann

Credit: Y.Kamide

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Credit: S. Lichti

Credit: NASA

Different altitudes of

colors

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Extracting altitude and information about mean energy of electrons

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Substorm

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Breakup of pre-existing arc

West East

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That’s all folks!

48B B BB

+

OI, 557.7 nm OI, 557.7 nm

N2, 427.8 nm

N2, 427.8

Hα, Hβ, Ly-α

Aurora as created by energetic protons

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Apparent motion of auroral oval over South Pole station

South

North

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Conjugacy

Credit: L. Frank

Credit: N. Sato