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Magnetosphere: Bow Shock Substorm and Storm

CSI 662 / ASTR 769 Lect. 09 Spring 2007 April 03, 2007. Magnetosphere: Bow Shock Substorm and Storm. References: Prolss: Chap. 6.4, P. 325-336; Chap. 7.6, P381 – 394; Chap. 8.3, P415-422 (main reference) Tascione: Chap. 5.7 – 5.10, P. 69 – 76 (supplement)

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Magnetosphere: Bow Shock Substorm and Storm

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  1. CSI 662 / ASTR 769 Lect. 09 Spring 2007 April 03, 2007 Magnetosphere: Bow ShockSubstorm and Storm • References: • Prolss: Chap. 6.4, P. 325-336; Chap. 7.6, P381 – 394; Chap. 8.3, P415-422 (main reference) • Tascione: Chap. 5.7 – 5.10, P. 69 – 76 (supplement) • Gombosi: Chap. 14.4 – 14.6, P292 – P312 (supplement)

  2. Fast and Slow Wind Magnetosphere • Formation of Bow Shock • Solar Wind Dynamo • Plasma Convection • Magnetospheric Substorm • Growth phase • Expansion phase • Recovery phase • Magnetospheric Storm

  3. Fast and Slow Wind Bow Shock • Shock forms in front of supersonic objects, e.g., CME • Shock also forms in front of obstacles placed in supersonic flows, e.g., bow shock of Earth’s magnetosphere • Magnetosheath: the region between bow shock and magnetopause

  4. Fast and Slow Wind Shock Shock front • Rankine-Hugoniot relations: 2: down stream 1: upper stream M: Mach number Ref: Prolss A.9, P468-471

  5. Bow Shock • Typically • M1=10 • M2=0.5 • U2:U1 = 1:4 • n2:n1 = 4:1 • P2:P1 = 125:1 • T2:T1 = 30:1 • E.g. Proton temperature • Tp = 105K (SW) • Tp = 2 X 106K (sheath)

  6. Fast and Slow Wind Magnetosheath • Magnetosheath: the region between the bow shock and magnetopause • Filled with down stream solar wind plasma • Along the Sub-Earth line, sheath size is about 1/3 of the geocentric distance of the magnetopause • Flow become supersonic again down the stream

  7. Fast and Slow Wind Solar Wind Dynamo • How is solar wind energy transferred into the Earth magnetosphere? • Energy must be from the kinetic energy of solar wind flow • In quiet condition, solar wind plasma and magnetic field simply “slip” through around the magnetopause. There is no connection between solar wind magnetic field and Earth magnetic field • In storm condition, reconnection opens the Earth magnetic field. The flow generates the electric dynamo field (or convection electric field) that powers the systems

  8. Solar Wind Energy Transfer • How is solar wind energy transferred into the Earth magnetosphere? • Energy must be from the kinetic energy of solar wind flow • In quiet condition, solar wind plasma and magnetic field simply “slip” through around the magnetopause. There is no connection between solar wind magnetic field and Earth magnetic field. Closed magnetosphere • In storm condition, reconnection opens the Earth magnetic field. The flow generates the electric dynamo field (or convection electric field) that powers the systems. Open magnetosphere

  9. Fast and Slow Wind Solar Wind Dynamo • Electric dynamo field, driven by SW flow, is given by Edyn • Electric dynamo field enters the magnetosphere when Earth magnetic field line is open • One footpoint rooted on the surface of the Earth • One footpoint connected with the solar wind magnetic field • Because Bs, Electric dynamo field always points from dawn to dust

  10. Fast and Slow Wind Open Magnetosphere • The Dungey reconnection model • When SW B field is southward, magnetic reconnection causes the dayside closed field to open up, and connect with SW B field. Open field Magnetic reconnection

  11. Plasma Convection • (1) -> (9), a cycle of magnetic field transport, along with a large scale plasma convection (or transport) • (1) create partially IP and partially magnetosphere field • (6) create purely IP and purely magnetospheric field

  12. Plasma Convection • In the magnetosphere, plasma drifts back in the anti-Sun direction, drive • The return flow is driven by E X B drift

  13. Magnetospheric Substorm • Geomagnetic perturbation lasting a few hours, which tends to occur during local post-midnight nighttime. • A substorm corresponds to an injections of charged particles from the magnetotail into the auroral oval. • Growth phase • About 1 hour • Energy accumulation • Magnetic reconnection on the dayside • Enhanced magnetic field in the magneto-tail lobes • Narrowing of the plasma sheet thickness

  14. Magnetospheric Substorm

  15. Magnetospheric Substorm • During the substorm, instability causes current disruption in the neutral sheet • Neutral sheet current is diverted through the ionosphere, producing strong polar electrojet • Current disruption causes strong electric field to energize particles. Substorm Current Wedge

  16. Magnetospheric Substorm • Expansion phase • also called “dipolarization” • About 1- 2 hour • Aurora brightening • Depression of geomagnetic field • Energy release through night side reconnection • Injection energetic particles into the inner magnetosphere • Plasma sheet heating • Bursty bulk flow • Tailward plasmoid release

  17. Magnetospheric storm • Large and prolonged disturbances of the magnetosphere • Main phase lasts for several hours • Recovery phase lasts for several days • Due to the increase of ring current • Geogagnetic storm main phase may have several substorms superposed.

  18. The End

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