Download - Lecture 17 May 14 2010May 14, 2010 - LUCID
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ESS 7ESS 7ESS 7ESS 7Lecture 17Lecture 17
May 14 2010May 14 2010May 14, 2010May 14, 2010
TheThe AuroraAuroraTheThe AuroraAurora
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Aurora – Amazing Light ShowAurora Amazing Light Show
Andrew Keen, Inari, Finland 18 Feb 2010-31º C spaceweather.com
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Athabasca Aurora Oct 3 2003Athabasca Aurora Oct 3 2003
Courtesy Mikko Syrjäsuo
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There is a Long Record of AuroralObservations
• The record of auroral observations dates backobservations dates back thousands of years to Greek and Chinese documents.
• The name aurora borealis (Latin for northern dawn)(Latin for northern dawn) was coined in 1621 by P. Gassendi during a gspectacular event observed in southern France.
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What are Aurorae*?What are Aurorae*?
• The aurora is mainly caused by excitation due to y yprecipitating electrons and ions. Auroras typically are found at high geomagnetic latitudes where
h i d l i d l dilmagnetospheric and solar wind electrons can readily access the upper atmosphere.
• Typically 1011Js-1 is required to maintain auroral emissions – this is about twice the maximum generatingemissions this is about twice the maximum generating capacity of California. A Js-1 or joule/second is a watt (W). This power of the aurora is thus 100,000 MW.
*Aurorae=plural; often also “auroras”
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Auroral Light
• Auroral emissions are primarily due to a two-step process in which precipitating energetic auroral particles collide with the atoms and molecules of theparticles collide with the atoms and molecules of the Earth’s upper atmosphere.
• Part of the particles kinetic energy is converted into• Part of the particles kinetic energy is converted into energy stored in the chemically excited states of atmospheric species.
• The excited states relax giving off photons.
.• The brightest visible feature of the aurora , the green
line at 557.7nm is due to the transition of an electron from 1S excited state to the 1D state of atomicfrom S excited state to the D state of atomic oxygen.
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Chemically Excited Statesh m ca y E c t Stat s• Another commonly observed line particularly in the polar
cusp and cap is the red line at 639 nm. This occurs as th 1D t t l t th d t t (3P )the 1D state relaxes to the ground state (3P2).
f ll d b
')()( 13 eSOePO
followed by
or )7.557()()( 11 nmhDOSO
For the red doublet)2.297()()( 31 nmhPOSO
')()( 13 eDOePO followed by
(e’ has less energy than e))4.636/630()()( 31 nmhPODO
• If the O(1S)-state electron gives up its full energy in a single step, instead of two, it emits a 297.2 nm photon.
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Auroral Spectra Contain Lines from Many Transitions
• It is sometimes easier to see the transitions by looking at a chart showing the different levels.
• The line at 557.7 nm is called a forbidden line.a forbidden line.– Allowed transitions occur much
more rapidly (10-7s) than forbidden transitions (0.8s in this case).
– Forbidden transitions occur at high altitudes (>200km) since at lower altitudes they have aat lower altitudes they have a good chance of being knocked out of the state before they can emit.
• There are many permitted oxygen and nitrogen lines from higher excited states.
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• The 630nm emission forms th diff b k d
• Magenta predominates The Colors of the Aurora
the diffuse background radiation in which the discrete arcs are embedded.
below 100 km and is a combination of N2 and O+
2 emissions near• “Blood-red” auroras are
produced by low-energy electrons (<<1 keV) they are
O 2 emissions near 600nm and N+
2 violet emissions.
electrons (<<1 keV) they are found at high altitudes (>200km)
• “Red lower borders” indicate the presence of energetic particles (>10keV).
• Most auroras are yellow-green but sometimes appeargreen but sometimes appear gray (because our eyes are insufficiently sensitive)
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The Shapes of Auroras
• There are four forms of aurora.– Quiet homogeneous arcs
stretching in an east-west direction across the sky.y
– Auroral rays .– Diffuse auroral clouds.
S i l d l– Spirals and curls.
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Shapes and Colors
Diffuseaurora
Arcs
Courtesy Jan Curtis
Rays Curtain, DrapesArcs and Rays
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Diffuse and Discrete Aurora• In addition to discrete auroralIn addition to discrete auroral structures seen in the images there is a background aurora called diffuse auroracalled diffuse aurora.
• The discrete aurora can be seen as peaks in the radianceseen as peaks in the radiance curve at the right.
• The discrete aurora rise out• The discrete aurora rise out of the diffuse aurora which extends over a much wider
f l tit drange of latitude.
• A typical auroral arc receives b t 106 kW l
Note: logarithmic scale. The eyedetects on a logarithmic scale.Only in this way can we see at night
about 106 kW – a large power plant.
and also during the day!A camera needs many changes of setting for night/day; we don’t!
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The Particles that Excite the Aurora• Aureol-3 (French for
aurora) observations of electrons and ionselectrons and ions precipitating into the nightside ionosphere.
Middle panel shows average– Middle panel shows average energy
– Bottom panel shows the energy flux to theenergy flux to the ionosphere.
• Ion signature around 2124 UT is a velocity dispersedUT is a velocity dispersed ion structure thought to be the signature of the PSBL.
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Particles Making Discrete and Diffuse Aurora
• Red electron fluxes t d “i t dequatorward are “inverted
V” events associated with discrete aurora.
• Precipitating particles equatorward of 68.50
produce the diffuse aurora.
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The Auroral Ovalh urora O a
• The aurora are found in rings about the north and south poles.Th ti ll• These are magnetically connected to the equatorial magnetosphere.
• The sketches on the right are based on observations and show the regions withand show the regions with aurora for quiet and disturbed times.
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Polar Cap Sun-Aligned Aurora• Disturbed auroral oval
occurs for southward IMF during storms and substorms.
• When the magnetosphere is• When the magnetosphere is quiet and the IMF is northward polar-cap sun aligned arcs oriented fromaligned arcs oriented from midnight to noon can occur.
• They occur about 50% of the time when the IMF is northward.
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Image of “theta aurora” during the passage of a magnetic cloud past the Earth
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Assignmentss gnm nt
• Read Chapter 6p• Problems 5.1, 5.2, and 5.3• Due May 21, 2010