An overview of Earth’s magnetosphere and its coupling with the solar wind

S. Elkington, June 11, 2009

An overview of Earth’s An overview of Earth’s magnetosphere and its magnetosphere and its coupling with the solar coupling with the solar

windwind

Scot R. ElkingtonScot R. ElkingtonLASP, University of ColoradoLASP, University of Colorado

([email protected])([email protected])

REU Summer ProgramJune 11, 2009

Magnetism is familiar to all of us, usually from permanent magnets and compasses.

Magnetic fieldsMagnetic fields

In its simplest form, magnetism comes in the form of a dipole, with a northern and southern ‘poles’.

Plasma: the 4Plasma: the 4thth State of State of MatterMatter

The charged particles (electrons and ions) of the plasma are glued to the magnetic field and move around it in circular orbits. Lorentz stated this force of nature as ….

F=qV x B

… where F is the force acting on a particle with charge q and velocity V in a magnetic field B.

solid (ice) liquid (water) gaseous (steam)

Plasma

BV

(positive charge q)

The sun and the solar The sun and the solar windwind


The sun is continually ejecting portions of its atmosphere into interplanetary space in the form of a solar wind.

The solar wind is in the plasma state, and accelerates as it moves outward from the sun. At the Earth, the solar wind speed is typically ~400 km/s, but may exceed 1000 km/s during solar disturbances.


The sun and solar windThe sun and solar windThe Sun has an intrinsic magnetic field. The action of the solar wind is to sweep the field out away from the sun into space, where it forms the Interplanetary Magnetic Field, or IMF.

The plasma moves out radially. Because sun rotates with a ~27 day period and the field lines are constrained by the plasma, the simplest configuration has the IMF in the form of a Parker spiral.


The sun and solar windThe sun and solar wind

Activity on the sun modifies this simple picture, providing the IMF with either a northward or southward component.


Active sun: CMEs, etc.Active sun: CMEs, etc.

In addition to the steady-state activity described previously, the sun is capable of violent outbursts. Coronal Mass Ejections describe large ejections of solar material and fields into interplanetary space.


Earth’s magnetic fieldEarth’s magnetic fieldEarth also has an intrinsic magnetic field, similar to that produced by a bar magnet.

The solar wind deforms the magnetosphere, compressing the front and sweeping the back antisunward.

The magnetosphereThe magnetosphere


The cavity carved out in space by the Earth’s magnetic field is known as the magnetosphere, bounded by the magnetopause.

Within the magnetosphere, there are a zoo of distinct regions, each with characteristic plasma populations affected by different dynamical processes.

Magnetic Reconnection

A process that:

• changes the field topologyby “breaking” and “mending”individual field lines in a localregion.

• converts magnetic energy toa jetting plasma


Energy from the solar wind: Energy from the solar wind: reconnectionreconnection

Energy from the solar Energy from the solar wind: reconnectionwind: reconnection


Reconnection not only provides a means of getting energy and mass from the solar wind into the magnetosphere, it also sets up large scale convective motion within the magnetosphere.


Open, closed, and Open, closed, and interplanetary magetnetic interplanetary magetnetic

fieldsfieldsOne may identify three types of magnetic field lines in near-Earth space:• The interplanetary field is that originating with the sun.• ‘Open’ field lines have recently reconnected with the IMF… one end connects to the Earth, the other to the IMF.• ‘Closed’ field lines have not reconnected… both ends of the field line originate on Earth.The magnetospheric polar cap defines the

ionospheric separatrix between the open and closed field lines of the magnetosphere, and may be seen in terms of ionospheric currents, electric fields, and plasma flows. Particles precipitating at the edge of the polar cap forms the auroral oval.

S. Elkington, Feb 22, 2009

Particle physics in space: basic Particle physics in space: basic particle motionparticle motion

A charged particle will move at constant velocity in a straight line unless acted on by a force. In space, the most important forces for charged particles arise from electric and magnetic fields.

•Electric fields (E) will accelerate particles in the direction of the field.•Magnetic fields (B) will accelerate particles in a direction perpendicular to the both the B field and the particles motion.

Thus a magnetic field will cause a particle to execute some kind of gyromotion. BvEF

c

qq

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qBL

qB

cp

qB

mvcL

“Gyroradius”

“Gyrofrequency”



If a particle gyrating in a magnetic field is acted on by an external force, it will cause the particle to drift perpendicular to the external force and the local magnetic field.

An electric field perpendicular to the local magnetic field will cause such a drift:



Similarly, if the magnetic field is nonuniform in a direction perpendicular to the local magnetic field, a drift results:

On the other hand, a magnetic field that is nonuniform in a direction parallel to the magnetic field will cause a particle to experience a force away from the regions of strong magnetic field:

Bm || F


Charged particle motion in the Charged particle motion in the Earth’s magnetosphereEarth’s magnetosphere

•Gyro: ~ millisecond•Bounce: ~ 0.1-1.0 s•Drift: ~ 1-10 minutes

Characteristic time scales:

The Earth has an intrinsic magnetic The Earth has an intrinsic magnetic field that is roughly a dipole. Charged field that is roughly a dipole. Charged particles moving under the influence particles moving under the influence of the Earth’s magnetic field therefore of the Earth’s magnetic field therefore execute three distinct types of motion.execute three distinct types of motion.

Regions: the bow shock and Regions: the bow shock and magnetopausemagnetopause


At Earth, the solar wind is supersonic (and superAlfvenic). The Earth forms an obstacle in the solar wind, thus producing a bow shock upstream of Earth.

The boundary between the magnetosphere and the IMF is defined by the magnetopause current or Chapman-Ferraro current, which can be simplistically understood in terms of the basic particle motion as shown.

Regions: ring current and Regions: ring current and radiation beltsradiation belts


Drift motion in closed-field regions of the magnetosphere leads to currents in space encircling the Earth. The ring current is formed by energetic electrons and ions gradient-drifting across field lines in opposite directions about the Earth.

The Dst index measures the energy content of the ring current by measuring the magnetic perturbation at Earth caused by this current. Negative excursions in Dst characterize geomagnetic storms.


Regions: the Van Allen Regions: the Van Allen Radiation BeltsRadiation Belts

Discovered by James Van Allen Discovered by James Van Allen in 1958 via a Geiger counter on in 1958 via a Geiger counter on Explorer I.Explorer I.

Trapped electrons and ions Trapped electrons and ions drifting in orbits encircling drifting in orbits encircling Earth.Earth.

Two spatial populations: inner Two spatial populations: inner zone and outer zone.zone and outer zone.

Energies from ~200 keV to > Energies from ~200 keV to > several MeV.several MeV.

The high-energy component of the ring current forms the radiation belts. These are comprised of relativistic electrons and protons, with MeV energies (as opposed to the keV ring current populations).

Regions: the plasmasphereRegions: the plasmasphere


Plasmas flowing out from the Earth’s ionosphere form a cold, dense population that corotates with the Earth.

The boundary of the plasmasphere is the plasmapause, and is defined by the competing effects of Earth’s corotation and magnetospheric convection.

Convectively-driven plasmas

Co-rotating plasmas

Regions: the plasmasheet Regions: the plasmasheet and lobesand lobes


The boundary of the plasmasheet is the Alfven layer, and is defined by the competing effects of gradient-curvature drift and magnetospheric convection.

Field lines which have reconnected at the magnetopause are swept back into the tail, forming the northern and southern lobes of the magnetotail.

Reconnection in the tail creates a closed-field region near the magnetic equator called the central plasma sheet. This region is dominated by convection.


Southward IMF: Southward IMF: geomagnetic stormsgeomagnetic storms

During periods of extended southward IMF, the energy input into the magnetosphere can cause (among other things) intensifications in auroral activity, amplification of magnetospheric currents, and a depression of the local magnetic fields strength measured at Earth. These periods are known as geomagnetic storms.

In particular, changes in the magnetospheric ring current will cause a decrease in the horizontal component of the magnetic field measured at Earth, and is characterized by the Dst index.


Magnetic storms and auroral Magnetic storms and auroral activityactivity

Aurorae

As reconnection proceeds at the magnetopause, more of the magetospheric field lines become ‘open’. The polar cap increases its size, and the auroral oval is driven to more southerly latitudes.

Geomagnetic storms: Geomagnetic storms: substormssubstorms


In contrast to the simple picture of steady reconnection and convection, sometimes energy is stored in the tail and then released episodically. Such events are known as substorms.Energy is stored, and the tail becomes very stretched. The plasmasheet begins to thin.

Field lines reconnect at the NENL, releasing the energy stored in the tail.

Energy flows away from the reconnection site. Field lines near Earth go from stretched to dipole-like, and particles are injected from the reconnection site both down the tail and into the inner magnetosphere.

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Magnetic storms: auroral Magnetic storms: auroral substormssubstorms


Particles injected into the inner magnetosphere during a substorm can cause intense, dynamic auroral activity.

Substorm simulation Substorm simulation animations?animations?


Storm effects on the Storm effects on the plasmasphereplasmasphere


Convectively-driven plasmasCo-rotating plasmas

Increased convective activity in the magnetosphere can strip away plasma from the plasmasphere, reduce the plasmapause, and introduce spatial and temporal features in the plasmasphere.

Storm effects on the Storm effects on the radiation beltsradiation belts


Adiabatic Heating/Loss Local Heating/Loss

S. Elkington, March 2, 2006

Damage to spacecraft

Hazards to human activity in space

‘‘Space Weather’Space Weather’

Polar 2

Polar 3

Polar 1

Polar 4

Polar Airline Routes

North Pole

ChicagoHon

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ka

Radio BlackoutDuring

Particle Events

Polar airline routes

Power grids, transformers

SummarySummary


• The sun expels its atmosphere and fields in the form of a solar wind and IMF.• Earth’s intrinsic magnetic field carves out a cavity in the solar wind known as the magnetosphere.

•Compressed sunward, stretched antisunward•May reconnect with the solar wind IMF

• Various processes and particle populations define fundamental regions of the magnetosphere:

•Bow shock and magnetopause•Ring current and radiation belts•Plasmasphere•Tail lobes and plasmasheet•Etc.

• The solar wind and IMF can cause dynamic activity in the magnetosphere: solar storms

•Enhanced convection, substorms•Auroral activity•Plasmasphere erosion•Radiation belts

•Space Weather!

Parking LotParking Lot


Density (cm-3) Magnetic field strength (nT)

Convection speed (km/s)

Tail Lobes 1 25 5

Plasma sheet 0.01 10 4

Plasmasphere 1000 >100 <1

Earth magnetic field strength at the poles (equator): 62000 (31000) nT

Density (cm-3) Magnetic field strength (nT)

Convection speed (km/s)

Solar Wind 5 5 450

Magnetosheath 10 10 200

Magnetotail lobe

0.01 25 4

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Forces on plasma due to magnetic field

Equation of motion

Momentum fluid equation


Magnetospheric regions and Magnetospheric regions and processesprocesses

Energetic particles

Global magnetosphere

Basic plasma processes

Plasma and MHD waves


ESD DamageESD Damage

175X 4300X

HA-2700 surface damage in the C2 MOS capacitor (Courtesy of JPL)

An overview of Earth’s magnetosphere and its coupling with the solar wind

Documents

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solar wind speed

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solar windthe

solar disturbances

solar windactivity

intrinsic magnetic field

magnetic fieldsin