Transcript The Aurora: A Look from Above
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