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ESS 200CAurora, 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 the polar cap.

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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 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

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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

• 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)

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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/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).

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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.

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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 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.

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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

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2

1

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.