ESS 200C Aurora, Lecture 15

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.

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.

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

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. 1D, t=110 s Energy levels of oxygen atom

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.

The Aurora – Colors

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

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

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

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

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

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.

Force Balance - MI Coupling

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: Vasyliunas’ pressure gradient term Inertial term Vorticity dependent terms (w =U) Assumptions: •j = 0, E + UxB = 0. Hasegawa and Sato [1979] and Murr [2003] assumed vorticity w || B.

Maxwell Stress and Poynting Flux

Currents and Ionospheric Drag

Weimer FAC morphology

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

MHD FAST Comparisons

MHD FACs

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.)

Upward Current – Inverted V Aurora

Downward Current – Upward Electrons

Polar Cap Boundary – Alfvén Aurora

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.

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).

Asymptotic Value = BI /Bm 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]

Acceleration Ellipse and Loss-cone Hyperbola Phase Space Mapping Theoretical and Observed Distributions (Ergun et al., GRL, 27, 4053-4056, 2000) Acceleration Ellipse and Loss-cone Hyperbola

Numerical Results – Double Layers Static 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.

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

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.

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.