ESS 154/200C Lecture 17 The Auroral Ionosphere

1. ESS 154/200CLecture 17 The Auroral Ionosphere

2. ESS 200C Space Plasma Physics ESS 154 Solar Terrestrial PhysicsM/W/F 10:00 – 11:15 AM Geology 4677 Instructors:C.T. Russell (Tel. x-53188; Office: Slichter 6869)R.J. Strangeway (Tel. x-66247; Office: Slichter 6869) • DateDayTopicInstructorDue • 1/4 M A Brief History of Solar Terrestrial Physics CTR • 1/6 W Upper Atmosphere / Ionosphere CTR • 1/8 F The Sun: Core to Chromosphere CTR • 1/11 M The Corona, Solar Cycle, Solar Activity Coronal Mass Ejections, and Flares CTR PS1 • 1/13 W The Solar Wind and Heliosphere, Part 1 CTR • 1/15 F The Solar Wind and Heliosphere, Part 2 CTR • 1/20 W Physics of Plasmas RJS PS2 • 1/22 F MHD including Waves RJS • 1/25 M Solar Wind Interactions: Magnetized Planets YM PS3 • 1/27 W Solar Wind Interactions: UnmagnetizedPlanets YM • 1/29 F CollisionlessShocks CTR • 2/1 M Mid-Term PS4 • 2/3 W Solar Wind Magnetosphere Coupling I CTR • 2/5 F Solar Wind Magnetosphere Coupling II; The Inner Magnetosphere I CTR • 2/8M The Inner Magnetosphere II CTR PS5 • 2/10W Planetary Magnetospheres CTR • 2/12F The Auroral Ionosphere RJS • 2/17W Waves in Plasmas 1 RJS PS6 • 2/19 F Waves in Plasmas 2 RJS • 2/26 F Review CTR/RJS PS7 • 2/29 M Final

3. Auroral Dynamics

4. 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.

5. 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.

6. 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

7. Exciting Auroral Emissions • Electron impact: e+N→N*+e1 • Energy transfer: x*+N→x+N* • Chemiluminescence: M+xN→Mx*+N • Cascading: N**→N*+hν (N2+)*→N2++391.4nm(uν) or 427.8nm (violet) aurora O(3P)+e→O(1S)+e′ 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

8. Auroral Emissions • Protons can charge exchange with hydrogen and the fast neutral moves across field lines. • Precipitating protons can excite Hα (656.3nm, red) and Hβ (486.1 nm, cyan) 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. red magenta green

9. The Aurora – Colors

10. 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. (1 mw/m2), or 1 kR Nadir Pointing Photometer Observations

11. 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

12. 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

13. Auroral Substorm Model based on ground observations Pictures from space • Growth phase – energy stored • Onset – energy begins to be released • Expansion – activity spreads

14. Conductivity • Parallel equation of motion • Perpendicular equation of motion • Conductivity tensor • Pedersen conductivity (along E┴) • Hall conductivity (along -E x B) • Parallel conductivity

15. Auroral Currents • If collisions absent then electric field produces drift perpendicular to B. • 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

16. 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.

17. Force Balance - MI Coupling

18. Maxwell Stress and Poynting Flux

19. Currents and Ionospheric Drag

20. Weimer FAC morphology

21. FAST Observations IMF By ~ -9 nT. IMF Bz weakly negative, going positive.

22. 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.)

23. Upward Current – Inverted V Aurora

24. 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/2p1/2 ≈ 1 mA/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/2p1/2 (BI/Bm).

28. Knight Relation 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. 1+e/T Asymptotic Value = BI /Bm j/j0 e/T [Knight, PSS, 21, 741-750, 1973; Lyons, 1980]

29. Auroral Kilometric Radiation - Horseshoe Distribution Strangeway et al., Phys. Chem. Earth (C), 26, 145-149, 2001.