1. Introduction to Space Weather
Magnetosphere:
Topology
Oct. 22, 2009
CSI 662 / PHYS 660
Fall, 2009
Jie Zhang
Copyright ©

2. Roadmap Part 1: The Sun Part 2: The Heliosphere
Part 3: The Magnetosphere
Part 4: The Ionsophere
Part 5: Space Weather Effects
Part 3: The Magnetosphere
Topology
Plasmas and Currents
Geomagnetic Activities

3. CSI 662 / PHYS October
The Magnetosphere: Earth Magnetic Field and Topology of the Magnetosphere
References:
Kallenrode: Chap. 8.1 and 8.2
Prolss: Chap. 5

4. Plasma Physics

5. Magnetosphere Fast and Slow Wind
As a result of interaction between Earth magnetic field and solar wind
Highly structured
Elements:
magnetic fields
Plasma
electric current
energetic particles.
Fast and Slow Wind

6. Geomagnetic Field Fast and Slow Wind
Magnetic dipole axis is inclined by 12°
Magnetic North Pole:
78°N 69°W (Greenland)
Magnetic South Pole:
78°S 111°E
The dipole moment:
Fast and Slow Wind

7. Geomagnetic Field Kallenrode: Chap. 8.1
L-shell parameter and the dipole field line

8. Geomagnetic Field South Magnetic Pole South Atlantic Anomaly
North Magnetic Pole
Geomagnetic Reference Field

9. South Atlantic Anomaly (SAA)
The center of the geomagnetic dipole is offset by 436 km relative to the center of the Earth in the direction of westerly Pacifics.
This offset leads to a region of unusually small magnetic flux density in the south Atlantic, just off the coast of Brazil: the SAA.
Radiation belts comes close to the Earth’s space at this region, making it a radiation risk for satellites and manned-space flight.
Fast and Slow Wind

10. Terrestrial Dynamo
Earth Layers

11. Terrestrial Dynamo
Earth magnetic field is generated in a similar way as the Sun’s magnetic field, through a MHD dynamo process
Ω effect: the differential rotation in the outer liquid core produced a strengthened toroidal field from a seed poloidal field
α effect: the convection motion causes the rising motion, resulting in the twist of magnetic field by the Coriolis force, which produces a poloidal field, amplifying the original seed field

12. Topology Fast and Slow Wind
The geomagnetic dipole is distorted by the solar wind. The magnetosphere is divided into two parts:
Dayside: ellipsoidal shape, subsolar point at L ~10 RE
Nightside: greatly extended, magnetotail ~ 200 RE
Magnetotail
Plasma Sheet
Noon-Midnight Cross-section of magnetosphere
Fast and Slow Wind

13. Magnetopause Fast and Slow Wind
Magnetopause is the boundary layer between the solar wind regime and the terrestrial regime controlled by the geomagnetic field.
The stand-off distance of the sub-solar point of the magnetopause
The pressure balance point between the solar wind dynamic pressure (=ρu2)and the Earth magnetic pressure
Fast and Slow Wind

14. Magnetopause Fast and Slow Wind
Due to deflection of solar wind plasma or magnetic gradient drift, electric current forms within the magnetopause; the current is so-called Chapman-Ferraro current
The current is from dawn to dusk
Also interpreted as the presence of dawn-to-dusk electric field
Deflection of solar wind electrons and protons (Fig. 8.9)
Fast and Slow Wind

15. Polar Cusp Fast and Slow Wind
Polar cusps are magnetic singularities in the dayside magnetosphere
It separates the closed field lines on the dayside magnetosphere from open field lines swept to the nightside
All field lines of the magnetopause converge at the cusps
At geomagnetic latitude of ~78°
At the cusp, solar wind plasma and energetic particles can penetrate deep into the Earth’s atmosphere
Fast and Slow Wind

16. Polar Cap Fast and Slow Wind
Within polar cap, all magnetic field lines are swept by the solar wind into the night side and are open
Polar cap is a high-latitude region, above 78° of geomagnetic latitude
Fast and Slow Wind

17. Magnetotail Fast and Slow Wind
Earth’s high latitude magnetic fields are swept open downstream by the solar wind, forming a long magnetotail.
To determine the cross-section size of the magnetotail, one uses the fact that the magnetic flux from the polar cap equals the magnetic flux in the tail
Fast and Slow Wind

18. Bow Shock Fast and Slow Wind
Bow shock forms where the supersonic solar wind is slowed down to subsonic speed in front of the magnetopause.
Bow shock is a standing shock; on the other hand, the CME-driven interplanetary shock is a travelling shock
Fast and Slow Wind

19. Shock Fast and Slow Wind Shock front Rankine-Hugoniot relations:
2: down stream
1: upper stream
M: Mach number
Ref: Prolss A.9, P
Also see kallenrode Chap 6.8
Fast and Slow Wind

20. Bow Shock For the bow shock of the Earth’s magnetosphere, typically
U2:U1 = 1:4
n2:n1 = 4:1
T2:T1 = 30:1
P2:P1 = 125:1

21. 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
Plasma Flow Inside the Magnetosphere (Fig. 8.15)
Fast and Slow Wind

22. The End