Author: R.S. Weigel
Date: 06/02/2005
Note: Prepared for 2005 NCAR Summer school.

Modified-By: R.S. Weigel
Date: 11/02/2005
Note: Added questions, changed grid to rectangular, made grid unpickable.

Keywords and Acronyms:

Tsyganenko model: "The Tsyganenko model is a semi-empirical best-fit representation for the magnetic field, based on a large number of satellite observations (IMP, HEOS, ISEE, POLAR, Geotail, etc). The model includes the contributions from external magnetospheric sources: ring current, magnetotail current system, magnetopause currents and large-scale system of field-aligned currents" [http://nssdc.gsfc.nasa.gov/space/model/magnetos/tsygan.html]

IGRF Field model (International Geomagnetic Reference Field): "... a standard mathematical description of the Earth's main magnetic field and used widely in studies of the Earth's deep interior, its crust and its ionosphere and magnetosphere. The coefficients for this degree and order 13 main field model were finalized by a task force of IAGA in December 2004. The IGRF is the product of a collaborative effort between magnetic field modelers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observatories and surveys around the world." [http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html]

Online References:

• Tsyganenko [html]
• IGRF [html]

Physics Background:
The following link was excerpted from the AFWA Space Weather Forecasting training material.

Earth's Magnetic Field: for an overview over its structure click here

Papers:

• Brandt and Schneider, 1977, Computer-drawn lines of potential surfaces for a wide range of field configurations. [pdf]
• Willis, Singh, and Comer, 1997, Uncertainties in field-line tracing in the magnetosphere. Part I: the axisymmetric part of the internal geomagnetic field. [pdf]
• Willis, Singh, and Comer, 1997, Uncertainties in field-line tracing in the magnetosphere. Part II: the complete internal geomagnetic field. [pdf]

Loading the network

To start Lab03 with CISM_DX extensions, click here ; this will cause a Visual Program Editor window to open.

Main Control Panel

Once the new network is loaded we need to open the Main Control Panel. In OpenDX, control panels are used to allow for user interaction with a network. You can design a network so that the user does not need to interact with the VPE. Instead, the user modifies parameters in a control panel and manipulates an image with the mouse.

In the top menu bar, select Windows, Open Control Panel by Name, and then Main Control Panel. The Main Control Panel contains a list of magnetic field models.

Empirical Models of Earth's Magnetic Field

Now execute Lab03 via CTRL+E. This network allows you to set the starting location for a set of magnetic field lines for various magnetic field models. When the image window is in focus (active), you can bring up the View Control Panel by typing CTRL-V. Alternatively, select Options, View Control from the image menu bar. Next, change the interaction mode by changing the Mode to Pick. Now mouse clicks will be interpreted as picks. If you accidently modified the image from what it was originally, use CTRL-Z to step back to a previous view.

Use the mouse to select three points on the green plane as origins for field lines and then execute the network. It's important to point out here that the Pick module is designed to set the points on the green plane that you can see in the image Window.

Test your knowledge:

• How does the separation distance between where the field lines intersect the green plane change as a function of field model? What is the approximate value of X (GSM) that is most sensitive to field model?
• Use the pick module with one of the Tsyganenko models to identify at least one external current system.
• How does the separation distance between where the field lines intersect Earth change as a function of separation distance of the initial points on the green plane?
  You will need to use the AutoAxes Configuration menu in the image menu bar by selecting Options, AutoAxes. If you double the separation distance on the green plane, how does the separation distance change near Earth? How does this distance compare to the grid spacing?
• On the FieldTubes page, disconnect the extract module from the first and third tabs of FieldLineTracer2 and connect these tabs to the output of VectorList. On the CompFields page, change the density in the Grid module from {40,40,40} to {100,100,100}. What is the difference in the X position where a field line crosses the green plane for a field line trace that starts at Earth's surface at 45 degrees and a trace that starts at 48 degrees latitude? Why are field line tracings from the X-Y plane to Earth of interest?
• Prior to the main phase of a geomagnetic storm, a rapid increase in the northward component of ground magnetic field is often observed by dayside magnetometers near the equator. Using Ampre's law, estimate the magnetopause current required to produce a magnetic field that is 0.5% of the value at the equator as given by any of the internal magnetic field models in this network. (If you don't know the value of Earth's magnetic field near the equator, use the pick module to extract it from on of the models.)
• Suppose you were given an analytic formula for a magnetic field. What equation would you need to solve to determine the path of a field line?