1 Access and Use of Auroral Imaging Data Harald U. Frey Space Sciences Laboratory University of California at Berkeley Credit: D. Hutchinson
Dec 13, 2015
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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