Introduction to Space Weather

Jie Zhang CSI 662 / PHYS 660Fall, 2009Copyright ©

Magnetosphere:Geomagnetic

Activties

Nov. 5, 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

The Magnetosphere:Geomagnetic Activities:

Geomagnetic Storms, Sub-Storms, Aurorae and Radiation

Belts

CSI 662 / PHYS 660 October 22 2009

References:

•Kallenrode: Chap. 8.4, 8.5, 8.6 and 8.7•Prolss: Chap. 7, Chap. 8

Plasma Physics

Solar Wind DynamoHow 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 MagnetosphereThe 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.

Magnetic reconnection

Open field

Solar Wind Dynamo• Electric dynamo (or induction) field, driven by SW flow, is

given byzBswVEdyn

• 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 LineNENL: 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

22

*

*

N N, ,O O, denotes M

2

eMeM

eMeM

Auroral OvalAuroral 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