GEONS workshop
THEMIS: The Science Behind Magnetometer
Signatures
Laura Peticolas
Magnetometer Tutorial PowerPoint
March, 2007
Outline
• Magnetometer data: what are we
measuring?
• Ground magnetic signatures of:
–
–
–
–
Earth’s magnetic field
Ring currents
Auroral currents
Magnetic storms and aurora substorms
• Kp index and local magnetometer data
• Primary Science of THEMIS
• Electrical currents in the ionosphere
Magnetometer Tutorial PowerPoint
March, 2007
Earth’s magnetic fields
From USGS web site (http://geomag.usgs.gov/intro.html)
The Earth's magnetic field is both expansive and complicated.
It is generated by electric currents that are deep within the
Earth and high above the surface. All of these currents
contribute to the total geomagnetic field. In some ways, one
can consider the Earth's magnetic field, measured
at a particular instance and at a particular location,
to be the superposition of symptoms of a myriad of
physical processes occurring everywhere else in the
world.
Magnetic fields are vectors: they have a strength
(magnitude) and a direction just like velocity
Magnetometer Tutorial PowerPoint
March, 2007
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.
B
• 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
Magnetometer Tutorial PowerPoint
March, 2007
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)
Magnetometer Tutorial PowerPoint
March, 2007
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
north pole
 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!)
Magnetometer Tutorial PowerPoint
(magnetic north)
X
B
Z
Y
(magnetic east)
(down)
March, 2007
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
Magnetometer Tutorial PowerPoint
X
(magnetic north)
H
D
B
Y
(magnetic east)
Z
(down)
B2=X2+Y2+Z2
B2=H2+Z2
March, 2007
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
Not to scale
(magnetic
north)
X = 21515 nT
Y = -760 nT
Y
Z
ZMagnetometer
= 44985 nT Tutorial PowerPoint
(down)
(magnetic
east)
5:58:52 UT 1/07/2007 (30 min plot)
9:58:52 PM 1/07/2007 Carson City
March, 2007
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.
Magnetometer Tutorial PowerPoint
March, 2007
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?
Magnetometer Tutorial PowerPoint
March, 2007
Different Latitudes
X
(nT)
Y
(nT)
Z
(nT)
X (nT): 13480
Z (nT): 52530
19010
51685
X/Z: 0.26 (high-lat) 0.37
Magnetometer Tutorial PowerPoint
17805
48620
0.37
21610
45180
0.48 (mid-lat)
March, 2007
Magnetosphere
Auroral Oval
Van Allen Belts
Magnetometer Tutorial PowerPoint
March, 2007
Magnetosphere currents
From: http://www-ssc.igpp.ucla.edu/ssc/tutorial/planet_magnetospheres.html
Magnetometer Tutorial PowerPoint
March, 2007
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.
Magnetometer Tutorial PowerPoint
March, 2007
Effects of Ring Current on
the Mag Data
N
Electrons
Ions
S
Ring Current
Ring Current
causes
Magnetic Fields
• 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.
Magnetometer Tutorial PowerPoint
March, 2007
Auroral Currents
Currents flow
to and from the
magnetosphere…
…through the
ionosphere
Currents are associated with
each auroral arc
Magnetometer Tutorial PowerPoint
March, 2007
Magnetic signatures of
auroral substorms
Substorm onset link: http://www.dcs.lancs.ac.uk/iono/samnet/pi2/rt/
Magnetometer Tutorial PowerPoint
March, 2007
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
Magnetometer Tutorial PowerPoint
March, 2007
Kp Index = 1
X
(nT)
Y
(nT)
Z
(nT)
X (nT): 13490
Magnetometer Tutorial PowerPoint
19000
17835
21630 (Kp=1)
March, 2007
Kp Index = 7
X
(nT)
Y
(nT)
Z
(nT)
X (nT): ?
Magnetometer Tutorial PowerPoint
18940
17745
21585 (Kp=7)
March, 2007
Space Weather Effects
(Kp=1)
Alaska
X (nT): 13490
South Dakota Oregon
19000
17835
Nevada
21630
(Kp=7)
X (nT):
?
18940
17745
21585
Difference (nT):
?
60
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
Magnetometer Tutorial PowerPoint
March, 2007
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.
Magnetometer Tutorial PowerPoint
March, 2007
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.”
Magnetometer Tutorial PowerPoint
March, 2007
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/
Magnetometer Tutorial PowerPoint
March, 2007
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
Magnetometer Tutorial PowerPoint
March, 2007
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
Magnetometer Tutorial PowerPoint
March, 2007
Ionosphere Effects
From: http://geomag.usgs.gov/intro.html:
Midnight
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.
Magnetometer Tutorial PowerPoint
March, 2007