ESS 200CAuroraeLecture 15• The record of auroral observations dates back thousands of years to Greek and Chinese documents.• The name aurora borealis (latin for northern dawn) was coined in 1621 by P. Gassendi during a spectacular event observed in southern France.• The aurora is mainly caused by excitation due to precipitating electrons and ions. Auroras typically are found at high geomagnetic latitudes where magnetospheric and solar wind electrons can readily access the upper atmosphere.• Typically 1011Js-1 is required to maintain auroral emissions• Auroral emissions are primarily due to a two-step process in which precipitating energetic auroral particles collide with the atoms and molecules of the Earth’s upper atmosphere.– Part of the particles kinetic energy is converted into energy stored in the chemically excited states of atmospheric species– The excited states relax giving off photons.• The brightest visible feature of the aurora , the green line at 557.7nm is due to the transition of an electron from 1S excited state to the 1D state of atomic oxygen.• Another commonly observed line particularly in the polar cusp and cap is the red line at 639 nm. This occurs as the 1D state relaxes to the ground state (3P2).followed byorFor the red doubletfollowed by(e’ has less energy than e)• If the O(1S)-state electron gives up its full energy in a single step, instead of two, it emits a 297.2 nm photon. ')()(13eSOePO +→+)7.557()()(11nmhDOSOυ+→)2.297()()(31nmhPOSOυ+→')()(13eDOePO +→+)4.636/630()()(31nmhPODOυ+→• The line at 557.7 nm is called a forbidden line.– Allowed transitions occur much more rapidly (10-7s) than forbidden transitions (0.8s in this case). – Forbidden transitions occur at high altitudes (>200km) since at lower altitudes they have a good chance of being knocked out of the state before they can emit.• There are many permitted oxygen and nitrogen lines from higher excited states. •Violet N2+bands at 391.4nm and 427.8nm also are intense.• Protons (1-100 KeV) also produce aurora although generally it is much weaker than electron aurora.– Proton aurora is also usually observed equatorward of electron aurora.– In addition to exciting atmospheric atomic and molecular transitions, protons also excite emission lines infollowed by the auroral emission– The hydrogen atom again collidesand the process repeats. The fast atom has almost the same velocity and direction as the original proton.– Proton auroras are more diffuse than the incident protons.• During the recovery phase of magnetic storms stable auroral red (SAR) arcs can form. These occur at lower latitudes and are caused by ambient electrons in the high energy tail of the thermal distribution (>3000K).αH*HXHX +→+++υhHH +→*neXHXH ++→++• The aurora appears as a luminous cloud with an apparent surface brightness.– This is misleading since the brightness is proportional to the integrated emission per unit volume along the line of sight.– If the surface brightness I is measured in photons/(cm2 s steradian), then represents the total emission in photons/cm2s integrated along the line of sight.– The unit for is the rayleigh (R). One R is 106photons/cm2s .• During active periods the auroral luminosity can reach several hundred kilorayleighs• About 10-3W m-2is input to the atmosphere in a moderate aurora (a typical arc needs 106kW or the output of a large power plant)Iπ4Iπ4• Magenta is a combination of N2and O+2emissions near 600nm and N+2violet emissions.• The 630nm emission forms the diffuse background radiation in which the discrete arcs are embedded.• “Blood-red” auroras are produced by low-energy electrons (<<1 keV)• “Blood-red” auroras dominate high altitudes (>200km)• “Red lower borders” indicate the presence of energetic particles (>10keV).• Most auroras are yellow-green but sometimes appear gray (because our eyes are insufficiently sensitive)• Magenta predominates below 100km.• Precipitating particles equatorward of 68.50produce the diffuse aurora.• Aureol-3 (French for aurora) observations of electrons and ions precipitating into the nightside ionosphere.– Middle panel shows average energy– Bottom panel shows the energy flux to the ionosphere.• Ion signature around 2124 UT is a velocity dispersed ion structure thought to be the signature of the PSBL. • Red electron fluxes equatorwardare “inverted V” events associated with discrete aurora.• Total hemispheric electron precipitation is 9.4X1025s-1sr-1to 6.4X1026s-1sr-1.• Total hemispheric ion precipitation is 2.3X1023 s-1sr-1-9.4X1024 s-1sr-1• Particles in the magnetotail experience three types of motion depending onthe ratio of the minimum field line radius of curvature (RC min) to the maximum particle gyroradius( ). In this equation q is the charge, u the particle velocity, Blobethe magnetic field strength in the lobes and L is a scale length for changes in the tangential component as it reverses direction, and Bnis the normal component of the field.– gives guiding center motion– gives Speiser motion– Pitch angle scattering does not occur for either of these.• Particles with intermediate values of undergo chaotic motion and can be scattered into the loss cone.– The loss cone is defined by pitch angles ( ) for which the particles mirror in the atmosphere.– Values of between 0.1 and 8 have been found to give precipitation.– The VDIS are thought to result from this. ⎟⎟⎠⎞⎜⎜⎝⎛==maxmin22GClobenRmuBqLBρκ12>>κ12<<κ2κ2κuBBurr⋅=)cos(αmaxGρ• Wave - particle interactions cause particles to move in velocity space in response to the electric field force of the wave field.• Magnetospheric waves having an electric field component parallel to the ambient magnetic field (e.g. Langmuir waves, ion acoustic waves, electrostatic cyclotron waves, lower hybrid waves) can resonantly scatter particles in the direction parallel to and thus contribute to net particle flux to the ionosphere. • Waves also have an electric field perpendicular to the magnetic field. Resonance with the perpendicular electric field involves the gyromotion of the particle and occurs when the wave frequency ( ) in the frame of reference of the particle’s motion along equals a harmonic (n) of the particles gyrofrequency( ). If is the parallel wave number then