Aurora, Lecture 15
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Auroral Rays from Ground Auroral Rays from Space Shuttle
• Auroral emissions line up along the Earth’s magnetic field because the causative energetic particles are charged.
• The rays extend far upward from about 100 km altitude and vary in intensity.
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From 1000 km (90m orbit)
• Auroras occur in a broad latitudinal band; these are diffuse aurora and auroral arcs; auroras are dynamic and change from pass to pass.
From 4R
E on DE-1
• Auroras occur at all local times and can be seen over the
3 polar cap.

• Auroral light consists of a number discrete wavelengths corresponding to different atoms and molecules
• The precipitating particles that cause the aurora varies in energy and flux around the auroral oval
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• Electron impact: e+N→N*+e 1
• Energy transfer: x *+N→x+N*
• Chemiluminescence:
M+xN→Mx*+N
• Cascading: N**→+hν
(N
2
+ )*→N aurora
2
+ +391.4nm or 427.8nm
O( 3 P)+e→O( 1 S)+e 1
O( 1 S)→O( 1 D)+557.7nm (green line)
O( 1 D) →O( 3 P)+630/636.4nm (red line)
• Forbidden lines have low probability and may be deexcited by collisions.
Energy levels of oxygen atom
1 D, t=110 s
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• Protons can charge exchange with hydrogen and the fast neutral moves across field lines.
• Precipitating protons can excite H α and Hβ emissions and ionize atoms and molecules.
• Day time auroras are higher and less intense.
• Night time auroras are lower and more intense.
• Aurora generally become redder at high altitudes.
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Forms
• Homogenous arc
• Arc with rays
• Homogenous band
• Band with rays
• Rays, corona, drapery
• Precipitating particles may come down all across the auroral oval with extra intensity/flux in narrow regions where bright auroras are seen.
• Visible aurora correspond to energy flux of 1 erg cm -2 s -1 .
Nadir Pointing Photometer Observations
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Height Distribution/Latitude Distribution
• Auroras seen mainly from 95-150 km
• Top of auroras range to over
1000km
• Aurora oval size varies
– from event to event
– during a single substorm
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• Auroras are associated with field-aligned currents and velocity shears.
• The polar cap may be dark but that does not mean field lines are open.
• Polar cap aurora are often seen with strong interplanetary northward magnetic field
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Model based on ground observations Pictures from space
• Growth phase – energy stored
• Onset – energy begins to be released
• Expansion – activity spreads
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•
If collisions absent then electric field produces drift perpendicular to
β.
•
When collisions occur at a rate similar to the gyrofrequency drift is at an angle to the electric field j
 




1
2
0


1
2
0
0
0

0





E
E y
E x z


•
• If B along Z and conductivity strip along x, we may build up charge along north and south edge and cut off current in north-south direction.
If j y

0 ,
 
2
E x
 
1
E y

0 and j x

(

1



2
1
2
) E x
•
Called the Cowling conductivity
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Magnetosphere Ionosphere Coupling
• Magnetosphere can transfer momentum to the ionosphere by field-aligned current systems.
• Ionosphere in turn can transfer momentum to atmosphere via collisions.
• Magnetosphere can heat the ionosphere.
• Magnetosphere can produce ionization.
• Ionosphere supplies mass to the magnetosphere.
• Process is very complex and is still being sorted out.
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j = ne ( U i
–
U e
)
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Plasma momentum equation – force balance – leads to a fundamental driver of field-aligned currents.
Following Hasegawa and Sato [1979], and D. Murr, Ph. D. Thesis
“Magnetosphere-Ionosphere Coupling on Meso- and Macro-Scales,” 2003:
B•
 j
•B
B
2
= 2
B•

P

B
B
3
+ 1
B
2
+
B

2
B
 
B• d w dt d U dt
•

V
A
2
V
2
A
 w • d B dt
 Vasyliunas’ pressure gradient term

Inertial term

Vorticity dependent terms ( w  U )
Assumptions:
 • j = 0, E + U x B = 0 . Hasegawa and Sato [1979] and Murr
[2003] assumed vorticity w || B .
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Currents and Ionospheric Drag
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IMF B y
~ -9 nT.
IMF B z weakly negative, going positive.
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Auroral zone crossing shows:
Inverted-V electrons (upward current)
Return current (downward current)
Boundary layer electrons
(This and following figures courtesy
C. W. Carlson.)
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Upward Current – Inverted V Aurora
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Downward Current – Upward Electrons
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Polar Cap Boundary – Alfvén Aurora
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Primary Auroral Current
Inverted-V electrons appear to be primary (upward) auroral current carriers.
Inverted-V electrons most clearly related to large-scale parallel electric fields – the “Knight” relation.
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Current Density – Flux in the Loss-Cone
The auroral current is carried by the particles in the loss-cone.
Without any additional acceleration the current carried by the electrons is the precipitating flux at the atmosphere: j
0
= nev
T
/2 p 1/2 ≈ 1 m A/m 2 for n = 1 cm -3 , T e
= 1 keV.
A parallel electric field can increase this flux by increasing the flux in the loss-cone. Maximum flux is given by the flux at the top of the acceleration region ( j
0
) times the magnetic field ratio
(flux conservation - with no particles reflected).
j m
= nev
T
/2 p 1/2  ( B
I
/ B m
).
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Knight Relation j / j
0
1 +e  / T
The Knight relation comes from Liouville’s theorem and acceleration through a field-aligned electrostatic potential in a converging magnetic field.
Asymptotic
Value =
B
I
/ B m
Does not explain how potential is established.
e  / T
[Knight, PSS, 21, 741-750, 1973; Lyons, 1980]
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Phase Space Mapping
Theoretical and Observed Distributions
(Ergun et al., GRL, 27, 4053-4056, 2000)
Acceleration Ellipse and Loss-cone Hyperbola
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Numerical Results – Double Layers
Static Vlasov-Poisson simulations
(Ergun et al., GRL, 27, 4053-4056,
2000).
Two sheaths are present:
Low altitude to retard secondaries;
High altitude to reflect magnetospheric ions.
“Trapped” electrons appear to be an essential component.
Hull et al. [JGR, 108, p. 1007, 2003] present statistics of large amplitude electric fields observed at Polar perigee.
Their interpretation of the E
|| being related to an ambipolar field is consistent with the picture shown here.
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-1x10 5
-5x10
4
Auroral Kilometric Radiation -
Horseshoe Distribution
Electron Distribution in
Density Cavity
-12.0
Upgoing to
Magnetosphere
-13.4
2
3
0
Loss Cone
1
-14.9
Energy Flow
1. Acceleration by Electric Field
2. Mirroring by Magnetic Mirror
3. Diffusion through Auroral Kilometric Radiation
5x10
4
1x10
5
-1x10 5
-16.3
-5x10
4
0
Parl. Velocity (km/s)
Downgoing to
Ionosphere
5x10
4
1x10 5
-17.8
Strangeway et al., Phys. Chem.
Earth (C), 26, 145-149, 2001.
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AKR Fine Structure
Pottelette et al. [JGR,
106, 8465-8476, 2001;
Nonlinear Processes in
Geophysics, 10, 87 –92,
2003] discuss AKR fine structure as caused by small scale-size elementary radiation sources (ERS). Figure from Pottelette et al.,
2003.
Pottelette and Treumann
[GRL, 32, L12104, 2005] provide evidence of electron holes in the upward current region.
Presumed to correspond to the ERS.
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Return Current
Return current carried by upgoing electrons.
Distributions heavily processed by wave-particle interactions.
Boundary layer distributions may be associated with Alfvén waves (see later).
The upward electron drift velocity will exceed the electron thermal speed. Wave-particle interactions are likely to become significant. The return current region should therefore be turbulent, with considerable structure in the electron distribution.
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