The Aurora: A Look from
Above
1.Ground-based observations
2. Plasma: the fourth state of matter
3. The sun and interplanetary space
4. The magnetosphere
5. Auroral acceleration
An example of discrete aurora
Auroral striations aligned with the
geomagnetic field
High altitude red aurora
produced by low energy electrons
Pink lower borders produced by high energy
electrons
Aurora seen from the Space Shuttle
Types of aurora
• Discrete: produced by
active acceleration
• Auroral patches:
Precipitation caused
by wave-particle
interaction
• Diffuse: Caused by
inward convection
and wave-particle
interaction
Aurora is magnetically conjugate
The Auroral Substorm
Seen from the all-sky camera
Seen from a polar-orbiting satellite
The Substorm
• Growth Phase – convection and
equatorward moving diffuse arc
• Expansive Phase – Bright, rapidly moving
aurora, magnetic disturbances
• Recovery Phase – Patchy, diffuse aurora
Other things that happen
• Magnetic bays caused by auroral
electrojet current. Transmission grid
outages
• Magnetic pulsations
• Radio emissions - chorus and hiss
• Ionospheric radio absorption
Storm vs Substorm
Plasma: The fourth state of matter
• A gas made up of electrically charged particles,
ions and electrons
• Density differences between ions and electrons
-> space charges -> electric fields
• Differential motion between ions and electrons
-> electric currents -> magnetic fields
• Charged particle motion is affected by electric
and magnetic fields
• A plasma is distinguished by long-range
collective interaction
Waves in Plasmas
• Plasma Oscillation
• Alfven waves:
– Compressional mode: magnetic field has
pressure
– Shear mode: magnetic field has tension
• Electron cyclotron waves
• Electrostatic waves
The Plasma Oscillation
Effect of Collisions on Particle
Motion
Particle Trapping
The Sun and Interplanetary
Space
The Sun
Poloidal and Toroidal Fields
Chromosphere and Corona
Corona and Solar Wind
Rocket Motor Analogy of Solar
Wind Expansion
Properties of the Solar Wind at
Earth’s Orbit
• It blows by the Earth at speeds of about
400 km/sec
• Consists primarily of protons, but carries
multiply charged ions of other species,
primarily O+++++++ and He++
• Typical densities of 5 cm-3
• Carries a highly variable magnetic field the
order of 10 nT.
The Earth’s Magnetosphere
Substorm growth phase
• Begins with southward turning of the
interplanetary magnetic field
• Enhanced convection
• Stretching of the magnetotail
• Expansion of the auroral oval and
equatorward movement of diffuse aurora
Ionospheric convection
Reconnection event
Action of magnetotail field lines
during substorm expansion
Numerical Simulation
Of Substorm
Expansive Phase
Magnetic Field
Plasma Flow
Turbulent generation and
propagation of Alfven waves
Field-aligned currents
Other parameters
Polar Satellite Observations
Substorm expansive phase
• Decrease or reorientation of interplanetary
magnetic field (not always)
• Extended tail magnetic field snaps back
carrying plasma earthward
• Turbulence in inner edge of plasma sheet
and Pi2 magnetic pulsations
• Auroral expansion
• Auroral electrojet
• Ring current injection
Auroral Acceleration
• We have presented circumstantial
evidence linking shear Alfven waves and
the aurora.
• We now examine mechanisms for
accelerating auroral electrons.
First Satellite Observation
The simulation domain
Simulation Parameters
di
Parallel Electric Fields
Energy Spectra Run #1
Energy Spectra Run #2
Close-up of Electric Field
Acceleration by Inertial Alfven Waves
• Mechanism demands fine-scale structure
characteristic of discrete aurora
• Large portion of accelerated electrons are
trapped to replenish electron radiation
belts.
• Large portion of accelerated electrons will
precipitate in conjugate hemisphere,
implying that some of the aurora we see
was due to electrons accelerated in the
southern hemisphere.
Two Acceleration Mechanisms
1. Trapped in an accelerating wave.
Downward acceleration above altitude of
peak Alfven velocity upward acceleration
below altitude of peak velocity.
2. Acceleration by shock formation. Shock
forms when wave propagates into region
of decreasing wave velocity.
Aurora on Other Planets
Aurora on Jupiter
Volcano on Io
Aurora on Saturn