Transcript Aurora_Lecture15

ESS 200C
Aurora, Lecture 15
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Auroral Rays
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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Auroras Seen from High Altitude
From 1000 km (90m orbit)
From 4RE on DE-1
• Auroras occur in a broad latitudinal band; these are
diffuse aurora and auroral arcs; auroras are dynamic and
change from pass to pass.
• Auroras occur at all local times and can be seen over the3
polar cap.
Auroral Spectrum
• 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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Exciting Auroral Emissions
• Electron impact: e+N→N*+e1
• Energy transfer: x*+N→x+N*
• Chemiluminescence:
M+xN→Mx*+N
• Cascading: N**→+hν
(N2+)*→N2++391.4nm or 427.8nm
aurora
O(3P)+e→O(1S)+e1
O(1S)→O(1D)+557.7nm (green line)
O(1D) →O(3P)+630/636.4nm (red
line)
• Forbidden lines have low
probability and may be deexcited by collisions.
1D,
t=110 s
Energy levels of oxygen atom
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Auroral Emissions
• Protons can charge
exchange with hydrogen
and the fast neutral
moves across field lines.
• Precipitating protons can
excite Hα 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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The Aurora – Colors
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Auroral Forms
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-2s-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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Polar Cap Aurora
• 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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Auroral Substorm
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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Auroral Currents
•
•
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
 1

j  2
 0

2
1
0
0  E x

0  E y
 0   E 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
•
Called the Cowling conductivity
2
2
j y  0,
  2 E x   1E y  0
and
j x  ( 1 
1
)Ex
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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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Force Balance - MI Coupling
j = ne (U i – U e)
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Drivers of Field-Aligned
Currents
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
B2
=2
B • P   B
B3
 V A2
dU
1
+
B
•
2
dt
B
V A2
+

B
2
B • dw  w • dB
dt
dt
Vasyliunas’ pressure gradient term
Inertial term
Vorticity dependent terms (w U)
Assumptions: •j = 0, E + UxB = 0. Hasegawa and Sato [1979] and Murr
[2003] assumed vorticity w || B.
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Maxwell Stress and Poynting Flux
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Currents and Ionospheric Drag
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Weimer FAC morphology
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FAST Observations
IMF By ~ -9 nT.
IMF Bz weakly negative,
going positive.
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MHD FAST Comparisons
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MHD FACs
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Three Types of Aurora
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:
j0 = nevT/2p1/2 ≈ 1 mA/m2 for n = 1 cm-3, Te = 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 (j0) times the magnetic field ratio
(flux conservation - with no particles reflected).
jm = nevT/2p1/2  (BI/Bm).
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Knight Relation
1+e/T
Asymptotic
Value =
BI /Bm
j/j0
The Knight relation
comes from Liouville’s
theorem and acceleration
through a field-aligned
electrostatic potential in a
converging magnetic
field.
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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Auroral Kilometric Radiation Horseshoe Distribution
Electron Distribution in
Density Cavity
-1x10 5
-12.0
Upgoing to
Magnetosphere
-5x10 4
-13.4
2
3
Energy Flow
Loss Cone
-14.9
0
1
1. Acceleration by Electric Field
2. Mirroring by Magnetic Mirror
3. Diffusion through Auroral Kilometric Radiation
5x10 4
-16.3
Downgoing to
Ionosphere
1x10 5
-1x10 5
Strangeway et al., Phys. Chem.
Earth (C), 26, 145-149, 2001.
-17.8
-5x10
4
0
5x10
4
1x10
5
P arl . Velocity (km/s)
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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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