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The Aurora

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Title: The Aurora


1
The Aurora
  • LASSO Summer 2008 Workshop
  • July 21 2008

2
Photographs of the Aurora from the Ground
3
Hyvinkää, Finland
4
Oulu, Finland
5
Forest Lake, Minnesota
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Colchester, Vermont
7
West Bend, Wisconsin
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Quebec, Canada
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Central Iowa
10
Rikubetsu, Hokkaido, Japan
11
Hortoneda, Lerida, Spain
12
Menominee, Michigan
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Circle, Alaska
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Circle, Alaska
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Auroral Curl
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White/Green Vertical Aurora
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White and Green Aurora
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Pink and Green Aurora
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Red Lower Border
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Red Lower Border
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Red Lower Border
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Major Storm Begins
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Green Bands
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Aurora With Power Lines
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Aurora With Moon
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Wavy Veil
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Photographs/Images of the Aurora from Low Earth
Orbit
28
From the Space Shuttle
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From the Space Shuttle
30
From the Shuttle Discovery
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From DMSP (Defense Meteorological Satellite
Program)
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From DMSP
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More from DMSP
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Images of the Aurora from High Altitude Polar
Orbiting Spacecraft
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From the Dynamics Explorer Satellite
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Southern Auroral Oval From DE
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In X-rays from the Polar Satellite
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In Ultraviolet Light from the Polar Satellite
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UV Aurora from Viking Satellite
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Side View of Northern andSouthernLights
FromDynamicsExplorer
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Aurora on Other Planets
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AuroraOn Jupiterfrom GalileoSpacecraft
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Hubble Space Telescope Photos of Aurora on Saturn
and Jupiter
44
Jovian Aurora Satellite Footprints
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Jovian Aurora Satellite Footprints
46
What Causes Auroral Displays?Misconceptions
Old and New
  • Many myths and legends exist about the aurora.
  • The bloody red auroral displays sometimes
    observed at lower latitudes (e.g. Europe) were
    generally feared and were considered ill omens.
  • Today, we have a reasonably clear picture of how
    auroral displays work, but there still exist some
    common misconceptions.

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A Finnish legend has it that the aurora is caused
by snow thrown into the sky by the tail of a
fox.Until the early 19th century, the aurora
was commonly believed to be due to sunlight
scattered off of ice crystals.
49
Modern Misconceptions Also Exist
Today, a common misconception is that the aurora
is created directly by the impact of solar
particles on the Earths ionosphere. In reality,
these particles are highly processed through
the Earths magnetosphere first.
50
Where Does the Auroral Light Come From?
51
Where Does the Auroral Light Come From?
Precipitating electron or secondary electron
Collisionally excited atom
Line emission from excited states
52
Line Emission from Excited States. Atoms produce
Lines. Molecules produce Band Emissions (many
lines).
53
Auroral Intensity
Intensity of emitted light is related to the
precipitating energy flux (i.e., the more
electrons that are precipitating, and the more
energy they have, the brighter the auroral
emissions).
But only 1 of the energy carried by the
precipitating particles is emitted as light.
  • Other dissipation modes
  • ionization
  • dissociation
  • heating

54
What Produces the Different Colors?
  • The most common green light (557.7nm) is emitted
    from atomic oxygen (O). This was a mystery for a
    long time because the 557.7nm emission line in
    oxygen is a so-called forbidden transition
    which can only be observed in high vacuum
    conditions.
  • In the lower density regions above about 150
    miles, atomic oxygen (O) can also emit a bloody
    red color (630.0nm). Also a forbidden transition.

55
  • At higher altitudes, primary electrons ionize
    nitrogen (and other) molecules. This produces
    secondary electrons and the excited, ionized
    nitrogen molecules emit a violet/UV band system
    near 391.4nm. The secondary electrons go on to
    excite other emissions including the oxygen 6300
    and 5577 lines.
  • When the primary precipitating electrons are
    energetic enough, they can pentrate down to an
    altitude of 55-60 miles. Here they can create
    excited states of (neutral) molecular nitrogen
    which in turn can emit a crimson red color. This
    is not a single wavelength, it is a so-called
    band system made up of multiple emission lines
    in the red part of the spectrum.

56
  • Many other band systems exist for the various
    molecular species (N2, O2, N2, O2, NO, NO,
    etc.). Molecules can be electronically excited
    and/or they can be rotationally and
    vibrationally excited. The Lyman-Birge-Hopfiel
    d band system of excited molecular nitrogen
    (N2) emits in the ultraviolet region of the
    spectrum. Space-based auroral imagers frequently
    image in the LBH UV wavelength band. One
    advantage of this is that we can even see the
    dayside aurora on the sunlit portion of the
    globe.

57
Forbidden Lines
58
Height Profile of the Aurora
59
Red Lower Border
Red Above Green
60
Auroral Processes are Complex!
61
Can You Hear The Aurora?
  • There are many accounts of people actually
    hearing the aurora making a hissing, crackling or
    rustling sound. However, researchers have never
    been able to confirm this even with extremely
    sensitive recording equipment.
  • Its almost certainly not due to familiar sound
    waves (the atmosphere at the height of the aurora
    is extremely rarefied). One possibility is that
    it could be an electrical stimulation of the ear,
    but many still contend it is just a psychological
    effect.

62
Where to see the aurora(over North America)
63
The Auroral Oval
64
Side view of the Earth Showing Conjugate Auroral
OvalsThere are two auroral ovals one in the
north and one in the south
Aurora Borealis
Aurora Australis
65
Where do the precipitating electrons come from?
Polar cap (open field lines, no particles)
Plasmasphere (no fresh plasma sheet, few
particles)
66
Why do plasma-sheet particles precipitate?
  • Interaction of electrons with waves leads to a
    more uniform drizzle into the atmosphere. This
    produces the so-called diffuse aurora.
  • Magnetic-field-aligned currents can accelerate
    electrons down the field lines (much like a TV
    picture tube). This produces the so-called
    discrete aurora (e.g. arcs and curtains).

67
Interaction with waves - pitch-angle scattering
Non-field-aligned particles mirror
Field-aligned particles precipitate
Waves scatter particles into field-aligned
direction
68
Waves are especially strong near the outer edge
of the plasmasphere (inner edge of plasma sheet)
so diffuse auroras occur at the inner edge of
the plasma sheet, as flux tubes convect into and
around the near-Earth region.
69
Auroral Distribution Discrete Diffuse Auroral
Emissions
70
Auroral DynamicsEvidence of Magnetospheric
Processes
71
Streamers during December 11, 1998 Sawtooth Event
72
Evolution of PBI/Streamers into Torches and Omega
Bands
73
PBI/Streamer evolves into a torch structure and
adds an Omega to the Omega band
74
Imaging Global Auroral Dynamics has Led to new
Insights and/or Models
New Model for Omega Band Formation Low-entropy
bubbles generate streamers which then penetrate
very close to Earth to form omega bands. Explains
many previously unexplained aspects of Omega Bands
75
Some Auroral Phenomena are Still Poorly
Understood. Example Giant Undulations
IMAGE/FUV Movie During Nov 24, 2001 Major Storm
76
Giant Undulations - DMSP
77
Giant Undulations
78
Auroral SubstormsCaused by important and
frequently occurring magnetospheric
process.Physical mechanisms still hotly debated
in the space physics community today.
79
The Classic Isolated Akasofu Substorm c.
1964Cartoon picture inferred from looking at a
number of events recorded by all-sky imagers on
the ground.
80
Isolated Versus Stormtime Substorms
81
April 18, 2002 Sawtooth Event (Particularly
Strong Substorms)
82
Two Different Views of a Substorm From Space
IMAGE/FUV
POLAR/VIS
83
Substorm Disruption of Cross-Tail Currents
dipolarization
84
Cross-Tail Current Disruption -gt
Magnetic-Field-Aligned Currents
85
Current Disruption leads to FACs
86
Two Substorm Onset Scenarios
Inner magnetospheric instability develops first
and propagates downtail to initiate reconnection
Reconnection initiates first. Earthward-directed
flows are responsible for inner magnetospheric
substorm effects
87
ThemisRecently launched mission to study
substorms and resolve inside-out versus
outside-in onset controversy. Consists of 5
spacecraft array of ground based all-sky
imagers. (Following 4 slides from E. Donovan Talk
see Themis websites.)
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More Themis Info
  • http//themis.ssl.berkeley.edu
  • http//aurora.phys.ucalgary.ca/themis
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