Transcript THEMIS Science Instruments
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) = 10 4 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 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 Z (down) (magnetic north) X Y (magnetic east) B 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 Z (down) X (magnetic north) D H Y (magnetic east) B 2 =X 2 +Y 2 +Z 2 B 2 =H 2 +Z 2 March, 2007 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
Not to scale
X = 21515 nT X (magnetic north) Y = -760 nT Z (down) Y (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
X (magnetic north) Y (magnetic east)
Earth’s Magnetic Field
Z (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 17805 48620 0.37
21610 45180
Magnetometer Tutorial PowerPoint
Magnetosphere
Auroral Oval Van Allen Belts
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 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
.
Magnetometer Tutorial PowerPoint March, 2007
Currents flow to and from the magnetosphere…
Auroral Currents
…through the ionosphere Magnetometer Tutorial PowerPoint Currents are associated with each auroral arc 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) (Kp=7) Alaska South Dakota Oregon Nevada X (nT): 13490 X (nT): ?
Difference (nT): ?
19000 18940 60 17835 17745 90 21630 21585 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?
10-12 hour change in B Magnetometer Tutorial PowerPoint Image from: http://geomag.usgs.gov/intro.html
March, 2007
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.
Midnight • 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