Transcript Acceleration of Coronal Mass Ejection In Long Rising Solar

Introduction to Space Weather
Magnetosphere:
Geomagnetic
Activties
Nov. 5, 2009
Jie Zhang
Copyright ©
CSI 662 / PHYS 660
Fall, 2009
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
1.
Topology
2.
Plasmas and Currents
3.
Geomagnetic Activities
CSI 662 / PHYS 660
October 22
2009
The Magnetosphere:
Geomagnetic Activities:
Geomagnetic Storms, SubStorms, Aurorae and Radiation
Belts
References:
•Kallenrode: Chap. 8.4, 8.5, 8.6 and 8.7
•Prolss: Chap. 7, Chap. 8
Plasma Physics
Solar Wind Dynamo
How is solar wind energy transferred into the Earth
magnetosphere?
• Energy originates 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
• During the presence of southward interplanetary magnetic field,
magnetic reconnection opens the Earth magnetic field.
• The connected flow between solar wind and magnetosphere
generates the electric dynamo field (or convection electric field)
that powers the systems
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
Solar Wind Dynamo
• Electric dynamo (or induction) field, driven by SW flow, is
given by



Edyn  Vsw  Bz
• 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
Edyn
Plasma Convection
• (1) -> (9), a cycle of magnetic field transport, along with a large
scale plasma convection (or transport)
• (1) reconnection at magnetopause creates partial IP and partial
magnetospheric field
• (6) reconnection at plasma sheet creates purely IP and purely
magnetospheric field
Plasma Convection
• In the magnetosphere,
plasma drifts back in
the anti-Sun direction
• The return flow is
driven by E X B drift
• At (9), the magnetic
field returns to the
dayside at low latitude
Magnetospheric Substorm
• The release of energy and plasma convected into the
magnetotail plasma sheet causes magnetic substorm.
• It undergoes (1) growth phase, (2) expansion phase, (3)
recovery phase
• Growth phase
• About 1 hour
• Enhanced magnetic field in the magneto-tail lobes
• Energy and plasma accumulation in the plasma sheet
• Narrowing of the plasma sheet thickness
Magnetospheric Substorm
DNL: Distant Neutral Line
NENL: Near Earth Neutral Line
Magnetospheric Substorm
• Expansion phase
• About 1- 2 hour
• Energy release through night side reconnection
• Injection energetic particles into the inner magnetosphere
• Tailward plasmoid release
• Plasma sheet heating
• Aurora brightening and aurora arc expanding
• Depression of geomagnetic field,
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, as seen in AE (Aurora
Electrojet) index
• Current disruption causes strong electric field to accelerate
particles, producing aurorae
Substorm
Current
Wedge
Magnetospheric storm
• Large and prolonged disturbances of the magnetosphere, i.e.,
southward B larger than 10 nT for more than three hours.
• Main phase lasts for several hours
• Recovery phase lasts for several days
• Strong depression of the Dst index (e.g., < -100 nT), due to
significant increase of the ring current
• Geogagnetic storm main phase may have several
substorms superposed.
Geomagnetic Indices
• Dst (Disturbance Storm Time) index: measure the excursion
of the equatorial horizontal magnetic component compared
with quiet time
• Dst index is related to the low latitude ring current
• AE (Auroral Electrojet) index: measure the magnetic
excursion at high latitude
• AA index: measure the magnetic excursion at middle latitude
• K index: quasi-logarithmic number between 0 and 9 in every
three-hour interval
• A index: average of the eight daily K indices
Continued on
November 12, 2009
Aurora
• Under normal condition, a colored arc extending from east to west
• Under geomagnetically disturbed conditions, aurora brightens,
highly structured, moves equatorwards, and changes fast
Aurora
• Aurora has been wrongly
interpreted as reflection
of the sun light
• In 18th century,
triangulation method
found the height to be ~
100 km.
• In 19th century,
spectroscopic analysis
showed emissions of
many forbidden lines,
thus from discharge of
excited gas
Arc, Band, Patch, Ray
Aurora
• Form
• Diffuse at quiet time
• Discrete at disturbance time: arcs, bands, rays, patches
• Height: > 100 km
• Orientation
• Vertical: along the magnetic field line
• Horizontal: primarily east-west direction
• Colors and emitting elements
• O: red (630.0 nm, 630.4 nm), yellow-green (557.7 nm)
• N2+: blue-violet (391.4 nm – 470 nm)
• N2: dark red (650 nm – 680 nm)
• Intensity: up to a few 100 kR (kilo Rayleigh)
(1 R = 1010 photons m-2 s-1, unit of luminous flux)
Aurora
• Aurorae are caused by the incidence of energetic particles onto
the upper atmosphere
• Particles move-in along the magnetic field lines connecting to
the plasma sheet
• The particles are mostly electrons in the energy range of ~100
ev to 10 kev.
• Ions are also observed
Aurora: Excitation
• Collisional Excitation
• Collisonal Excitation and Ionization
• Auroral lines are emitted in the entire range from UV to IR

*

*
M e  M e
M e M

 2e

M denotesO, O 2 , N, N 2
Auroral Oval
Auroral oval
• A ring-like region around each polar cap
• Aurorae are mostly in the night region
• Auraral oval is actually formed by the footpoints of the
field lines in the plasma sheet
• 68°-72°
Radiation Belt
• Populated by high energetic particles
• Particles are trapped by the Earth’s magnetic field
Also called
Van Allen
Radiation
Belt,
discovered in
1958
South Atlantic Anomoly
Radiation Belt
• Inner belt
• Populated by protons
> 30 Mev
• L=[1.2,2], max at 1.5
• Outer belt
• Populated by
electrons > 1.6 Mev
• L=[3,4], max at 3.5
Radiation Belt
• Source of Particles in the Inner Belt
• CRAND (Cosmic Ray Albedo Neutron Decay):
• Nuclei from the galactic cosmic radiation penetrate
deep into the atmosphere and interact with the
atmospheric gas, producing energetic neutrons
• Neutrons can propagate into the radiation belt without
difficulty
• Neutrons are not stable, decaying into protons
• Source of particles in the Outer Belt
• Influx of particles from the outer magnetosphere
• Increased during higher geomagnetic activity
Radiation Belt
• Losses of Particles
• Particles losses are always due to the interaction with the
dense atmospheric gas at the low latitude, when the
particles enter the loss cone
• Charged particles are scattered into the loss cone through
interaction with other charged particles
• Scattering occur due to pitch angle scattering at
electrostatic wave or electromagnetic waves
• Due to the distortion of the magnetic structure during
geomagnetic storms
• Due to charge exchange: exchange with a low energy
neutron hydrogen, and become a neutral particle that is not
guided by the magnetic field line any more.
The End