Download The Earth`s B-Field

The Earth’s B-Field
Earth’s B-Field
Earth'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?
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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.m
It 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.
Conversions
1 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
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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
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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
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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!)
(magnetic north)
X
B
Z
(down)
Y
(magnetic east)
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
X
(magnetic north)
H
D
B
Y
Z
(magnetic east)
(down)
B2=X2+Y2+Z2
B2=H2+Z2
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
X
(magnetic
north)
X = 21515 nT
Y = -760 nT
Y
Z = 44985 nT
Z (down)
(magnetic
east)
5:58:52 UT 1/07/2007 (30 min plot)
9:58:52 PM 1/07/2007 Carson City
Universal Time conversion
Note that this data is in Universal Time. To convert to local time use these rules:
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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
X
(magnetic north)
Z
Y
(magnetic east)
(down)
magnetic north
magnetic
field
Will the ratio of X to Z get larger or
smaller towards the equator?
Different Latitudes
X
(nT)
Y
(nT)
Z
(nT)
X (nT): 13480
Z (nT): 52530
19010
51685
X/Z: 0.26 (high-lat) 0.37 0.37
17805
48620
21610
45180
0.48 (mid-lat)
Magnetosphere
Auroral Oval
Van Allen Belts
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
N
Electrons
Ions
S
Ring Current causes
Magnetic Fields
Ring Current
• 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.
• When disturbed, the ring current weakens Earth’s magnetic
field even more. This is called a magnetic storm.
Auroral Currents
Currents flow
to and from the
magnetosphere…
…through the
ionosphere
Currents are associated with
each auroral arc
Magnetic signatures of auroral
substorms
Substorm onset link: http://www.dcs.lancs.ac.uk/iono/samnet/pi2/rt/
Kp Index
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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
X
(nT)
Y
(nT)
Z
(nT)
X (nT): 13490
(Kp=1)
19000
17835
21630
Kp Index = 7
X
(nT)
Y
(nT)
Z
(nT)
X (nT): ?
(Kp=7)
18940
17745
21585
Space Weather Effects
(Kp=1)
(Kp=7)
Alaska
X (nT): 13490
X (nT):21630
?
21585
Difference (nT): ?
South Dakota Oregon
Nevada
19000
17835
18940
60
17745
90
45
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
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
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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