Title: The Aurora
1The Aurora
- LASSO Summer 2008 Workshop
- July 21 2008
2Photographs of the Aurora from the Ground
3Hyvinkää, Finland
4Oulu, Finland
5Forest Lake, Minnesota
6Colchester, Vermont
7West Bend, Wisconsin
8Quebec, Canada
9Central Iowa
10Rikubetsu, Hokkaido, Japan
11Hortoneda, Lerida, Spain
12Menominee, Michigan
13Circle, Alaska
14Circle, Alaska
15Auroral Curl
16White/Green Vertical Aurora
17White and Green Aurora
18Pink and Green Aurora
19Red Lower Border
20Red Lower Border
21Red Lower Border
22Major Storm Begins
23Green Bands
24Aurora With Power Lines
25Aurora With Moon
26Wavy Veil
27Photographs/Images of the Aurora from Low Earth
Orbit
28From the Space Shuttle
29From the Space Shuttle
30From the Shuttle Discovery
31From DMSP (Defense Meteorological Satellite
Program)
32From DMSP
33More from DMSP
34Images of the Aurora from High Altitude Polar
Orbiting Spacecraft
35From the Dynamics Explorer Satellite
36Southern Auroral Oval From DE
37In X-rays from the Polar Satellite
38In Ultraviolet Light from the Polar Satellite
39UV Aurora from Viking Satellite
40Side View of Northern andSouthernLights
FromDynamicsExplorer
41Aurora on Other Planets
42AuroraOn Jupiterfrom GalileoSpacecraft
43Hubble Space Telescope Photos of Aurora on Saturn
and Jupiter
44Jovian Aurora Satellite Footprints
45Jovian Aurora Satellite Footprints
46What 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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48A 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.
49Modern 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.
50Where Does the Auroral Light Come From?
51Where Does the Auroral Light Come From?
Precipitating electron or secondary electron
Collisionally excited atom
Line emission from excited states
52Line Emission from Excited States. Atoms produce
Lines. Molecules produce Band Emissions (many
lines).
53Auroral 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
54What 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.
57Forbidden Lines
58Height Profile of the Aurora
59Red Lower Border
Red Above Green
60Auroral Processes are Complex!
61Can 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.
62Where to see the aurora(over North America)
63The Auroral Oval
64Side 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
65Where do the precipitating electrons come from?
Polar cap (open field lines, no particles)
Plasmasphere (no fresh plasma sheet, few
particles)
66Why 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).
67Interaction with waves - pitch-angle scattering
Non-field-aligned particles mirror
Field-aligned particles precipitate
Waves scatter particles into field-aligned
direction
68Waves 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.
69Auroral Distribution Discrete Diffuse Auroral
Emissions
70Auroral DynamicsEvidence of Magnetospheric
Processes
71Streamers during December 11, 1998 Sawtooth Event
72Evolution of PBI/Streamers into Torches and Omega
Bands
73PBI/Streamer evolves into a torch structure and
adds an Omega to the Omega band
74Imaging 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
75Some Auroral Phenomena are Still Poorly
Understood. Example Giant Undulations
IMAGE/FUV Movie During Nov 24, 2001 Major Storm
76Giant Undulations - DMSP
77Giant Undulations
78Auroral SubstormsCaused by important and
frequently occurring magnetospheric
process.Physical mechanisms still hotly debated
in the space physics community today.
79The Classic Isolated Akasofu Substorm c.
1964Cartoon picture inferred from looking at a
number of events recorded by all-sky imagers on
the ground.
80Isolated Versus Stormtime Substorms
81April 18, 2002 Sawtooth Event (Particularly
Strong Substorms)
82Two Different Views of a Substorm From Space
IMAGE/FUV
POLAR/VIS
83Substorm Disruption of Cross-Tail Currents
dipolarization
84Cross-Tail Current Disruption -gt
Magnetic-Field-Aligned Currents
85Current Disruption leads to FACs
86Two 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
87ThemisRecently 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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92More Themis Info
- http//themis.ssl.berkeley.edu
- http//aurora.phys.ucalgary.ca/themis