The Earth’s B-Field

The Earth’s B-Field

Earth’s B-FieldEarth's magnetic field is approximately a magnetic dipole, with the magnetic field S pole near the Earth's geographic north pole and the other magnetic field N pole near the Earth's geographic south pole.

An imaginary line joining the magnetic poles would be inclined by approximately 11.3° from the planet's axis of rotation. The cause of the field can be explained by dynamo theory.

Dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field.

In the case of the Earth, the magnetic field is induced and constantly maintained by the convection of liquid iron in the outer core. A requirement for the induction of field is a rotating fluid

Magnetic fields extend infinitely, though they are weaker further from their source. The Earth's magnetic field, which effectively extends several tens of thousands of km’s into space, is called the magnetosphere.

What is Magnetism?• A magnetic field is a vector field which surrounds magnets and electric currents,

and is detected by the force it exerts on moving electric charges and on magnetic materials.

• The tesla (symbol T) is the SI derived unit of magnetic flux density (or magnetic induction). It is used to define the intensity (density) of a magnetic field. The tesla, equal to one weber per square meter, was defined in 1960.

• It is named in honor of world renowned inventor, scientist and electrical engineer Nikola Tesla. Tesla's legacy can be seen across modern civilization wherever electricity is used because of his invention of alternating current.

Definition:1 T = 1 N/A.mIt can be thought of "newton-seconds per coulomb-meter" or as "newton per ampere-meter".

Explanation:The tesla is the value of the total magnetic flux (a magnet's "power") divided by area. Conversions1 tesla is equivalent to 10,000 Gauss

B-Field Comparisons• The Sun's B- field is about as strong as a refrigerator magnet, 50 gauss (5

mT). • The Earth's magnetic field is 100 times weaker or 50 mT = 50,000 nT.• in the Earth's magnetic field at latitude of 50° is 58 µT (5.8×10−5 T) and on the

equator at a latitude of 0° is 31 µT (3.1×10−5 T). Carson is 39o

• In a sunspot about 0.15 T

• A large 30 pound loudspeaker magnet will have a coil gap of 1 T. • A modern neodymium-iron-boron (NIB) rare earth magnet has strength of

about 1.25 T. A coin-sized neodymium magnet can lift more than 9 kg (22 lbs), and can pinch skin and erase credit cards.

• Medical magnetic resonance imaging systems utilize field densities from 1.5 to 2 T in practice, experimentally up to 4 T

B-Field Comparisons• Strongest continuous magnetic field yet produced in a laboratory (Florida State

University's National High Magnetic Field Laboratory in Tallahassee, USA), 45 T • strongest (pulsed) magnetic field yet obtained non-destructively in a laboratory,

100 T, • strongest (pulsed) magnetic field ever obtained (with explosives) in a laboratory

(VNIIEF in Sarov, Russia, 1998), 2800 T• on a neutron star 1 to 100 megateslas (106 T to 108 T), • maximum theoretical field strength for a neutron star, and therefore the upper

bound thus far for any known phenomenon, 1013 T (10 terateslas). • CME

Magnetic Field Strength

• The strength of a magnetic field is the magnetic flux density, B.• The units of magnetic flux density is the Tesla or the Gauss • 1 Tesla (T) = 104 Gauss (G)• The most powerful magnets in the world are superconducting

electromagnets. These magnets have magnetic fields of around 20 T. In 2003, the National High Magnetic Field Laboratory in Florida set the world record for high temperature superconducting magnets at 25 T.

• Earth’s magnetic field is 0.000 052T = 52,000 nanotesla (nT) = 0.5 gauss (G)

• 1 nanotesla = 10-9 T• Changes in Earth’s magnetic field are typically 5-100 nT

B

Coordinate Systems• Because magnetic fields have a direction, in order to communicate

about magnetic fields, we need to define a coordinate system.• Three main coordinate systems are used for magnetometer data:

– Geographic (XYZ)

– Geomagnetic (XYZ or HDZ - BEWARE!!)– Compass-type (HDZ)

•THEMIS uses the coordinate systems: geomagnetic (XYZ) and compass-type (HDZ)

Geomagnetic (GEONS)

• The geomagnetic coordinate system describes the way the magnetic field is pointing by defining: X: the strength of the magnetic field in

the direction of Earth’s magnetic south pole or geographic north

Y: the strength of the magnetic field in the magnetic east direction (90 deg from X and toward east)

Z: the strength of the magnetic field pointing down (90 deg from both X and Y – right hand rule!)

Z(down)

Y(magnetic east)

(magnetic north) X

B

Compass-type (HDZ)• The compass-type coordinate

system describes the way the magnetic field is pointing by defining:H: the strength of the magnetic field

in the plane horizontal to Earth’s surface (horizontal plane)

D: the angle between geographical north (X) and the direction of the magnetic field in the horizontal plane

Z: the strength of the magnetic field pointing down

B: the strength of the total magnetic field value

B2=X2+Y2+Z2

B2=H2+Z2

Z(down)

Y(magnetic east)

X(magnetic north)

HD

B

The GEONS Data

X: the strength in nT of the magnetic field in the direction of magnetic north pole

Y: the strength in nT of the magnetic field in the magnetic east direction

Z: the strength in nT of the magnetic field pointing down

Z (down)

Y(magnetic east)

X (magnetic north)

5:58:52 UT 1/07/2007 (30 min plot) 9:58:52 PM 1/07/2007 Carson City

X = 21515 nT

Y = -760 nT

Z = 44985 nT

Universal Time conversion

Note that this data is in Universal Time. To convert to local time use these rules:

• Atlantic Standard Time (AST) = UT - 4 hours • Eastern Standard Time (EST) = UT - 5 hours • Central Standard Time (CST) = UT - 6 hours • Mountain Standard Time (MST) = UT - 7 hours• Pacific Standard Time (PST) = UT - 8 hours • Alaska Standard Time (AKST) = UT – 9 hours

If Daylight Saving Time is in effect in the time zone, you must ADD one hour to the above standard times.

Earth’s Magnetic Field

Z(down)

Y(magnetic east)

X (magnetic north)

magnetic north

magnetic field

Will the ratio of X to Z get larger or smaller towards the equator?

Different Latitudes

Z(nT)

Y(nT)

X(nT)

X (nT): 13480 19010 17805 21610 Z (nT): 52530 51685 48620 45180

X/Z: 0.26 (high-lat) 0.37 0.37 0.48 (mid-lat)

Magnetosphere

Van Allen Belts

Auroral Oval

Magnetosphere currents

From: http://www-ssc.igpp.ucla.edu/ssc/tutorial/planet_magnetospheres.html

Solar Wind (SW) When changes in the solar wind, such as

changes due to Coronal Mass Ejections, hit Earth’s magnetosphere, the magnetospheric currents will change. These currents will cause changes in your magnetometer data.

We will focus on ring currents and auroral currents.

Effects of Ring Current on

the Mag Data

• Charged particles circle Earth at about 10 Re (60,000 km) from Earth’s surface near the equator.

• The electrons and the ions move in opposite directions, creating the ring current

• Does this add to (strengthen) or subtract from (weaken) Earth’s core magnetic field at Carson City? This is mostly in the x-direction.

S

N

ElectronsIons

Ring Current causes Magnetic Fields

Ring Current

• When disturbed, the ring current weakens Earth’s magnetic field even more. This is called a magnetic storm.

Auroral Currentsto and from the magnetosphere…

…through the ionosphere

Currents are associated with each auroral arc

Currents flow

Magnetic signatures of auroral substorms

Substorm onset link: http://www.dcs.lancs.ac.uk/iono/samnet/pi2/rt/

Kp Index

• Kp index is a numerical value calculated from a global distribution of magnetometers at mid-latitudes that allows scientists to keep track of the level of geomagnetic activity on a given day.

• Kp varies from 0-9 (log scale)• Kp is affected by many currents including the ring current and auroral currents• The stronger the ring current and/or auroral currents, the higher the Kp index value

Kp Index = 1

Z(nT)

Y(nT)

X(nT)

X (nT): 13490 19000 17835 21630 (Kp=1)

Kp Index = 7

Z(nT)

Y(nT)

X(nT)

X (nT): ? 18940 17745 21585 (Kp=7)

Space Weather Effects

Remember, we said at the beginning that• Earth’s magnetic field is

0.000 052T = 52,000 nanotesla (nT) = 0.5 gauss (G)• 1 nanotesla = 10-9 T• Changes in Earth’s magnetic field are typically 5-100 nT

X (nT): 13490 19000 1783521630X (nT): ? 18940 1774521585

Difference (nT): ? 60 90 45

Alaska South Dakota Oregon Nevada

(Kp=1)

(Kp=7)

Storm and Substorms• Relationship between magnetic storms (ring current) and aurora

substorms (aurora current) is still being researched• A magnetic storm usually lasts 2 hours to a day.• A substorm usually lasts 30 minutes-2 hours• You can have a storm without a substorm (aurora)• You can have substorm (aurora) without a magnetic storm.• And they can happen together.

THEMIS will determine which competing model is correct

THEMIS will elucidate which magnetotail process is responsible for substorm onset: • At 60,000 km, a sudden disruption of electrical current can occur, known as

“Current Disruption.”

• At 120,000 km, a sudden merging of oppositely pointed magnetic fields can occur, known as “Magnetic Reconnection.”

Ground-Based Observatories (GBOs)

Besides the magnetometers, THEMIS has installed all-sky imagers in Alaska and Canada. These cameras were built , which were built at the University of California in Berkeley (UCB)

To access data from these stations, visit this URL:http://themis.ssl.berkeley.edu/gbo/display.py

If you want to see THEMIS Real Time images of aurora around Canada and Alaska, go to this URL:http://aurora.phys.ucalgary.ca/realtime/THEMIS/

Find more information• Learn more about THEMIS science at: http://ds9.ssl.berkeley.edu/themis/mission_mystery.html

• Learn more about the school magnetometer program:

http://ds9.ssl.berkeley.edu/themis/classroom_geons.html

• Keep updated on the latest THEMIS news: http://ds9.ssl.berkeley.edu/themis/news.html

• Watch videos about THEMIS: http://ds9.ssl.berkeley.edu/themis/gallery_video_archive.html

Daytime (Sq) Currents

• Electrical currents flow in Earth’s ionosphere (about 100 km; 60 miles above Earth’s surface)

• These currents create magnetic fields that can be observed from the ground.

• What time variation in the magnetic field would you expect at Carson City?

Image from: http://geomag.usgs.gov/intro.html

10-12 hour change in B

Ionosphere Effects

From: http://geomag.usgs.gov/intro.html:

Shown is a stackplot of 4 days of the horizontal magnetic field strength (H) as measured by US Geological Survey (USGS) magnetometers during magnetically quiet conditions in early January 2003.

• High latitudes: aurora currents• Mid- and low-latitudes: the regular diurnal magnetic-field variation from large-scale daytime electric currents in the Earth's ionosphere.

Midnight