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Access and Use of Auroral Imaging Data

Harald U. Frey

Space Sciences Laboratory

University of California at Berkeley

Credit:D. Hutchinson

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Outline

1. Optical properties of the aurora

2. Relationship between precipitating particles and optical emissions

3. Principles of optical detectors

4. Instruments for observations from space

5. Instruments for observations from ground

6. Analyzing auroral images with examples

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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: /hcE Visible: 2 eVUltraviolet: 10 eVX-Ray: 10000 eV

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The optical spectrum of aurora

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Excited levels of oxygen (term scheme)

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2. Relationship between precipitating particles and optical emissions

Important quantities are:- Energy of precipitating particles- Altitude profile of emission- Spectral properties of resulting emission

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B

Oxygen atom specific photons, e.g. 630 nm

Nitrogen molecule specificphotons, e.g. 427.8 nm

Electron aurorafrom space

Aurora is created by energetic electrons

Composition of atmosphere

Electron is “lost” (precipitated)

Oxygen atom specific photons, e.g. 557.7

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Green auroral arcs

Credit: D. Hutchinson

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The auroral spectrum

Sensitivity of the human eye

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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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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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3. 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

h Eg

h = Planck constant (6.610-34 Joule•sec) = frequency of light (cycles/sec) = /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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4. Instruments for observations from space

Examples:- IMAGE

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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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5. Instruments for observations from ground

Fisheye opticsAll-sky camera

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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 over South Pole station

South

North

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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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6. Analyzing auroral images

First a few examplesLater we do some analysis with IDL

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Global view of Aurora from IMAGE Spacecraft

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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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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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Orientation of auroral images

Looking down from space(IMAGE FUV)

N

S

EW

Looking up from the ground(all-sky)

N

S

WE

Default for THEMIS data presentation

N

S

EW

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Mapping of all-sky images

Minimum elevation 0o Minimum elevation 8o

W E

N (poleward)

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Tomography with 5 cameras of overlapping view

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Tomographic reconstruction of optical emission

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Substorm

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Timing of substorm onset and East-West expansion

East

West

Magnetic AE index

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Speed of rays in auroral arcWest East

Mean speed: 4.5 km/sec

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Now let’s do some real-time analysis

If you have not done so already, please download all files and programs with anonymous ftp from:

ftp sprite.ssl.berkeley.educd pub/hfrey/seminarmget *

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Practice Session1. Show basics of auroral arc with slicer_mod.pro and

auroral_arc.pro; compile and run auroral_arc2. Content of calibration file, show_skymap.pro3. General THEMIS ASI software with thm_crib_asi.pro4. Time analysis with time_analysis.pro5. Generation of movie with make_movie.pro (change path)6. Demonstration of pulsating aurora with pulsations.pro7. Mark footprint of spacecraft in image with mark_satellite.pro;

show movie fast_2008-03-09_fsim.mpg and download fsim_20080309_040000.sav, compass.pro, fast_orbit_20080309.sav and .txt

8. Flow analysis with flow_analysis_1.pro and flow_analysis_2.pro; download images.sav, inuv_image_f_*.dat