The Aurora: A Look from Above

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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