1 ESS 200C Aurora, Lecture 15. 2 Auroral Rays Auroral Rays from Ground Auroral Rays from Space...
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Transcript of 1 ESS 200C Aurora, Lecture 15. 2 Auroral Rays Auroral Rays from Ground Auroral Rays from Space...
1
ESS 200CAurora, Lecture 15
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2
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.
3
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 the polar cap.
4
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
5
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 de-excited by collisions.
Energy levels of oxygen atom
1D, t=110 s
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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
9
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
10
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
11
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
• 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
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛−=
z
y
x
E
E
E
j
0
12
21
00
0
0
σ
σσ
σσ
xxyxy EjandEEj )(0,01
22
112 σσσσσ +==+−=
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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:
Assumptions: •j = 0, E + UxB = 0. Hasegawa and Sato [1979] and Murr [2003] assumed vorticity || B.
B•∇j•BB2 =2
B•∇P×∇BB3
+ 1B2 B×ρdU
dt •∇VA2
VA2
+ ρB2 B•ddt −•dB
dt
Vasyliunas’ pressure gradient term
Inertial term
Vorticity dependent terms (U)
16
Maxwell Stress and Poynting Flux
17
Currents and Ionospheric Drag
18
Weimer FAC morphology
19
FAST Observations
IMF By ~ -9 nT.
IMF Bz weakly negative, going positive.
20
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
25
Polar Cap Boundary – Alfvén Aurora
26
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.
27
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/21/2 ≈ 1 A/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/21/2 (BI/Bm).
28
Knight Relation
j/j0
e/T
1+e/TAsymptoti
c Value = BI /Bm
[Knight, PSS, 21, 741-750, 1973; Lyons, 1980]
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.
29
Phase Space Mapping
Theoretical and Observed Distributions(Ergun et al., GRL, 27, 4053-4056, 2000)
Acceleration Ellipse and Loss-cone Hyperbola
30
Numerical Results – Double LayersStatic 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.
31
Auroral Kilometric Radiation - Horseshoe Distribution
-1x105 -5x104 0 5x104 1x105
Parl. Velocity (km/s)
1x105
5x104
0
-5x104
-1x105
-17.8
-16.3
-14.9
-13.4
-12.0
Electron Distribution inDensity Cavity
Upgoing toMagnetosphere
Downgoing toIonosphere
Loss Cone Energy Flow
1. Acceleration by Electric Field
2. Mirroring by Magnetic Mirror
3. Diffusion through Auroral Kilometric Radiation
3
2
1
Strangeway et al., Phys. Chem. Earth (C), 26, 145-149, 2001.
32
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.
33
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.