1. Magnetosphere: Bow Shock Substorm and Storm
CSI 662 / ASTR Lect Spring 2007
April 03, 2007
Magnetosphere: Bow Shock Substorm and Storm
References:
Prolss: Chap. 6.4, P ; Chap. 7.6, P381 – 394; Chap. 8.3, P (main reference)
Tascione: Chap. 5.7 – 5.10, P. 69 – 76 (supplement)
Gombosi: Chap – 14.6, P292 – P312 (supplement)

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

3. Bow Shock Fast and Slow Wind
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
Fast and Slow Wind

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

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. Magnetosheath Fast and Slow Wind
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
Fast and Slow Wind

7. Solar Wind Dynamo Fast and Slow Wind
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
Fast and Slow Wind

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. Solar Wind Dynamo Fast and Slow Wind
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
Fast and Slow Wind

10. Open Magnetosphere Fast and Slow Wind 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
Fast and Slow Wind

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