A generic description of planetary aurora

1. A generic description of planetary aurora J. De Keyser, R. Maggiolo, and L. Maes Belgian Institute for Space Aeronomy, Brussels, Belgium Johan.DeKeyser@aeronomie.be

2. Context We consider a rotatingplanetary body thatorbits a star. The star is assumedto have a stellar wind. The motion of the body relativeto the stellar wind is supersonic, i.e. the wind speed is supersonic or the planetorbits the star veryclosely at a high orbital speed. Thissituationreflectsplanets in the Solar System, but itmaybeapplicabletomanyexoplanets as well. ESWW10, Antwerp

3. Classification of magnetospheres • No or weakmagnetic field • No ionosphere • stellar wind – exosphereinteraction • Ionosphere • inducedmagnetosphere • Strong magnetic field • No ionosphere • magnetosphere without innercurrentclosureand without cold plasma • Ionosphere • classicalmagnetosphere • parallel / anti-parallel / tilted ESWW10, Antwerp

4. Classicalmagnetospheres In “classicalmagnetospheres” • The planetary object has anintrinsicmagnetic field • It is rotating. • It has a conductingionosphere. • Plasma escapes from the ionosphere. • Theremaybeadditionalinner sources of plasma. Suchmagnetospheres have • a plasmasphere, • a magnetopause & boundarylayer, • a magnetotailwith a current sheet. ESWW10, Antwerp

5. Plasma circulation Plasma of solar wind origin : • Limited entry of solar wind is possiblethroughvariousmechanisms (reconnection, diffusion, plasma transfer). Plasma of terrestrialorigin : • Escape from the ionosphere is dueto energy input fromsolarradiation or precipitatingparticles. These produce a plasmasphere reservoir on closed field lines, whichlosesmass via aplasmaspheric wind andplumes. ESWW10, Antwerp

6. Magnetospheric electric field Corotation + dawn-duskelectric field is where is the angularrotation vector. Basic assumptions • No dynamiceffects (no induced) • No charge separation (small scales) This is the basis of the Volland-Stern model. Youcan look at it in aninertial frame, or in the corotating frame (usefulifyou want torelateitto the ionosphere). The pictures show the correspondingequatorialequipotentialsfor a dipole field. ESWW10, Antwerp

7. Magnetosphere – ionosphere coupling Electric potentialdifferencesacross field lines in the magnetosphere map onto the ionosphere, therebyproducingparallel electricfields. Up- anddownwardcurrentsconnect the magnetospheric generator to the ionospheric load as dictatedbythe current-voltage relationship parallel potentialdifference ionospheric potential magnetospheric potential ESWW10, Antwerp

8. Current continuity or : ionospheric coordinate , , : ionospheric and magnetospheric potential : parallel potentialdifference : height-integratedPedersenconductivity : parallel current : horizontalPedersencurrent Currentcontinuity(assuming a vertical field geometry) , i.e. the nonlinear second order ODE whereandmaydependnonlinearly on . ESWW10, Antwerp

9. Structure at the midnightmeridian From the Volland-Stern type magnetospheric potential in the corotating frame, we obtain the profiles for, for, andfor the magnetospheric potentialthe equator in the midnightmeridianplane. There is anelectricfield peak between the corotating plasmasphere and the inneredge of the plasma sheet. Note: Failure for large as the field is no longerdipolar. ESWW10, Antwerp

10. Auroral structurefor a planet Modellingas a step-like profile, for a constant , a if and if , the solution shows : • persistent westwardconvectionover the ionospheric projection of the corotationedge (SAEF) • (small) downwardcurrent at low latitude, withremovingelectronsfrom the ionosphere • (small) upwardcurrent at high latitude, withacceleratingelectronsinto the ionosphere • auroral effects are permanent, but depend on , , and ESWW10, Antwerp

11. In anactivemagnetosphere, short-scalepotentialvariationsappear in the neartail. The plasma sheet moves inwardand the corotationedgesteepens: • downwardcurrentandtransient strong westwardconvection(SAPS/SAID), • upwardcurrent in transientdiscrete auroral arcs(larger, strong spatiallyconcentratedemission) • downwardcurrent in transientblack aurora (spatiallyconcentrated absence of emission, electronsremovedfromionosphere). ESWW10, Antwerp

12. Auroral structurefor a planet The situation is nowreversed.The configurationconsistsof • persistent eastwardconvectionover the ionospheric projection of the corotationedge (SAEF) • (small) upwardcurrent at low latitude, withacceleratingelectronsinto the ionosphere • (small) downwardcurrent at high latitude, withremovingelectronsfrom the ionosphere • auroral effects are permanent, but depend on , , and ESWW10, Antwerp

13. In anactivemagnetosphere : • upwardcurrent, transientstrong eastwardconvection, andbroad auroral arcsat low latitude, • upwardcurrent in transientdiscrete auroral arcs(larger, strong spatiallyconcentratedemission) at high latitude • downwardcurrent in transientblack aurora (spatiallyconcentrated absence of emission, electronsremovedfromionosphere). ESWW10, Antwerp

14. Earth : an planet ESWW10, Antwerp

15. Earth : corotation border The corotation border corresponds to subauroral electric fields (SAEFs). For increasing geomagnetic activity, the corotation boundary becomes steeper and SAPS/SAID appear: • Because of the high , there is only a small negative : is projected into the ionosphere, leading to strong westward flow ( drift). • As electrons are accelerated out of the ionosphere, there is a charge carrier depletion. ESWW10, Antwerp

16. Observations indeed show : • ionosphereic electron depletion • very strong ionospheric flows (> 1 km/s) • ion-neutral collisions may produce stable auroral red arcs (630 nm) • local heating of the ionosphere and upward flow ESWW10, Antwerp

17. Earth : aurora Higher latitude : More or less permanent oval with transient appearance of discrete auroral arcs and black aurora. ESWW10, Antwerp

18. Jupiter andSaturn : planets ESWW10, Antwerp

19. Inner plasma sources Jupiter andSaturn are planets, but withoneadditionalingredient: additional sources of plasma inside a magnetosphere in the form of a moonthat releases neutrals. Neutrals are typicallyphoto-ionized. The ions • lead tomass-loading : deformation of the field • follow the field : (conjugate) auroral footpoints ESWW10, Antwerp

20. Jovian aurora • Corotationboundary : permanent aurora • Higher latitude : varying, depending on magnetospheric activity • Footpoints of the moons corotationoval Io variable auroral structures Ganymede Europa ESWW10, Antwerp

21. Io For Jupiter, especially the volcanicmoon Io is a source of gas. ESWW10, Antwerp

22. Conjugate aurora Notonly the footpoints, alsomuch or the other aurora are magneticallyconjugate. ESWW10, Antwerp

23. Saturn: same story AlsoSaturn has a permanent corotationboundaryoval, more transienthigher latitude auroras, andmoonfootpoint auroral spots. ESWW10, Antwerp

24. Enceladus For Saturn, the icymoonEnceladusis a source of gas release fromfissures in itscrust, fueledbytidaldeformation. ESWW10, Antwerp

25. Uranus andNeptune : planets ESWW10, Antwerp

26. Auroral structurefor a planet Depends notonly on the anglebetweenand , but also on the anglebetweenand the ecliptic; there is a seasonalmodulation. Uranus, withobliquity 97°, maybe in a situationwhereonepoleconstantly points sunward. ESWW10, Antwerp

27. Conclusions The major structure of auroral patterns of magnetized planets can be understood in terms of a simple model of the auroral current circuit. Details of the actual aurora depend on the size of the dipole field, its strength relative to the stellar wind pressure, and of course on the composition of the planet’s atmosphere. ESWW10, Antwerp

28. References • J. De Keyser, M. Roth, and J. Lemaire. The magnetospheric driver of subauroral ion drifts. Geophys. Res. Lett., 25:1625-1628, 1998. • J. De Keyser. Formationandevolution of subauroral ion drifts in the course of a substorm. J. Geophys. Res., 104(6):12,339-12,350, 1999. • J. De Keyser. Storm-time energeticparticlepenetrationinto the innermagnetosphere as the electromotive force in the subauroral ion drift current circuit. In S. Ohtani, ed., Magnetospheric Current Systems, GeophysicalMonograph Series 118, pages 261-265. AGU, 2000. • M. M. Echim, M. Roth, and J. De Keyser. Sheared magnetospheric plasma flowsand discrete auroral arcs: a quasi-staticcoupling model. Ann. Geophys., 25, 317–330, 2007. • M. M. Echim, R. Maggiolo, M. Roth, and J. De Keyser. A magnetospheric generator driving ion and electron acceleration and electric currents in a discrete auroral arc observed by Cluster and DMSP. Geophys. Res. Lett., 36:L12111, 2009. • F. Darrouzet, D. L. Gallagher, N. André, D. L. Carpenter, I. Dandouras, P. M. E. Décréau, J. De Keyser, R. E. Denton, J. C. Foster, J. Goldstein, M. B. Moldwin, B. W. Reinisch, B. R. Sandel, and J. Tu. CLUSTER and IMAGE Observations of the Plasmasphere: Plasma Structure and Dynamics. Space Sci. Rev., 145(1-2):55–106, 2009. doi:10.1007/s11214-008-9438-9. • J. De Keyser and M. Echim. Auroral and subauroral phenomena: An electrostatic picture. Ann. Geophys., 28:633–650, 2010. • J. De Keyser, R. Maggiolo, and M. Echim. Monopolar and bipolar auroral electric fields and their effects. Ann. Geophys., 28(11):2027–2046, 2010. • J. De Keyser, R. Maggiolo, M. Echim, and I. Dandouras. Wave signatures and electrostatic phenomena above aurora: Cluster observations and modeling. J. Geophys. Res., 116:A06224, 2011. ESWW10, Antwerp