Though outer space is often described as a vacuum, in actuality there are particles everywhere in the solar system, and there is continuous activity in the region between the celestial bodies. The study of this part of the solar system is called Space Physics [Kivelson et al., 1995], or sometimes Plasma Space Physics after the particles that fill this region. A plasma is a ionized gas, where some of the electrons have been stripped off of atoms creating a mixtures of free electrons and positive nuclei (ions).
Most of the plasma particles filling the solar system can trace their origin to the solar wind, which comes from the outer region of the sun’s atmosphere continually being blown off of the sun. The solar wind typically has a speed of about 400 km/s, though it can range up to about 1200 km/s.
When the solar wind reaches the earth, it interacts with the earth’s magnetic field to form a region known as the magnetosphere (See Figure 1). The earth’s magnetosphere is the region of space where the earth’s magnetic field dominates behavior of the plasma, just as the sun’s magnetic field does in the solar wind. The magnetosphere has many regions, though only a few will be of interest to us in this lab.
The first region of interest (though it is technically part of the magnetosphere) is known as the bow shock. The bow shock is caused by the solar wind meeting an obstruction its flow in the form of the earth. The solar wind has to flow around the the earth, and in doing a shock formed. The bow shock gets its name from the fact that the physics behind the creation of this shock are similar to the shock formed by the bow of a boat as the boat moves through the water.
Nearer to the earth is the magnetopause. The magnetopause marks the furthest that the earth’s magnetic field extends, which makes the magnetopause the boundary of the magnetosphere. The magnetosheath is the region of space between the bow shock and the magnetopause.
The surface of the sun, and consequently the solar wind, experiences a variety of events, including flares and coronal mass ejections (CMEs). These events can change both the speed and the number density (particles per volume) of the plasma in the solar wind. When the solar wind from one of these events reaches the earth, the events are called Solar Storms or Magnetic Storms. These variations in the solar wind cause the location of the both the bow shock and the magnetopause vary. Studying the variation in the distance from the earth to the magnetopause is one of the aims of this lab.
The location of the magnetopause is important because the magnetosphere shields the earth and most satellites that go around the earth from the direct effects of the solar wind. When the magnetopause moves, some satellite that are typically protected by the magnetosphere can find themselves in the solar wind. In particular, many satellites are placed in geosynchronous orbits so that they are always above a fixed location on the earth. Geosynchronous orbits are roughly 42000 km ( or 6.6 RE, where 1 RE equals the radius of the earth) from the center of the earth. During normal solar wind conditions the standoff distance of the magnetopause (the distance from the center of the earth to the point of the magnetopause that is on the line between the magnetopause and the earth) is roughly 10 RE . During extremely strong Solar Storms the magnetopause can move in closer than geosynchronous orbit. This exposes many satellites to direct contact with extreme solar wind conditions, and satellites are often damaged or lost when this happens.
The location of the magnetopause along the line between the earth and the sun can be derived by balancing the magnetic pressure of the magnetosphere with the dynamic pressure of the solar wind:
where ro is the standoff distance from the earth to the sun in RE, nsw is the number density of the plasma in the solar wind in cm-3, and vsw is the speed of the solar wind in km/s [Kivelson et al., 1995, pages 173-4]. The entire magnetopause boundary has also been fit to various empirical functions by a variety of researchers. The currently favored model for the earth’s magnetopause position can be predicted during extreme events come from Shue et al. [1997, 1998]. These were both derived from specific Solar Storm events. the expressions arer is the distance from the earth to the magnetopause boundary, ro is the magnetosphere standoff distance, Bz is the z-component of the solar wind’s magnetic field, Dp is the dynamic pressure of the solar wind, and is the amount of tail flaring from the night side of the magnetopause.
A variety of different coordinate systems are used to measure locations in space, and in this lab you will have to use data in several of these coordinate systems. More information on these coordinate systems can be found in books [Kivelson et al., 1995] or online [ESA, 2005].
The geocentric solar ecliptic (GSE) coordinate system has its x-axis point from the earth to the sun and its y-axis is in the ecliptic plane (the plane of the earth’s orbit) pointing in the direction of dusk (opposite the direction of the earth’s orbital motion). Its z-axis is in the direction of the ecliptic pole.
In geocentric solar magnetospheric (GSM) coordinates, as in GSE coordinates, the x-axis points from the earth to the sun. Its z-axis points in the direction of the north magnetic pole. Its y-axis, is perpendicular to the other two, so that a right-handed coordinate system results. The difference between the this system and the GSE system is a simple rotation about the x-axis.
In geocentric equatorial inertial (GEI) coordinates (also known as geocentric solar inertial (GCI)), the x-axis points from the earth to the the first point of Aries (the location of the sun at vernal equinox). This line is where the equatorial and ecliptic planes intersect. The z-axis is parallel to the earth’s rotation axis, and the y-axis complete the the right-handed coordinate system.
In this portion of the lab you will use a computer simulation to model the location of the magnetopause. The model we will use is called BATS-R-US [Hansen et al., 2002], the Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme, which was developed at the University of Michigan. We will be running the simulations on super-computers at NASA-Goddard run by the Community Coordinate Modeling Center (CCMC) [NASA, 2005].
To use BATS-R-US, you must first create an input file which tells the simulation the type of
conditions that you would like to simulate. The input file is a simple text file with the
conditions at different times listed. A sample input file is available (batrus_input_template.txt)
and a portion of one is included below.
|Year||Mon||Day||Hr||Mn||s||ms||Bx[nT]||By||Bz||V x[km/s]||V y||V z||n [cm-3]||T[K]|
First, note that the first line is a header line which should not be included when you try to run the simulation. Next, note that the first 7 columns of the input file set the time for the conditions that follow. Since we will not be simulating real conditions from a real day, the date we pick is arbitrary, though the usual convention is to pick 1/1/2000 for the date. The time differences are significant, though for our work here we will just use a 5 minute time difference between each of the points that we use. The next six columns give the three components of the solar wind magnetic field in nT, followed by the three components of the solar wind speed in km/s. The final two columns are the solar wind plasma number density in cm-3 and the solar wind plasma temperature in Kelvin.
In this lab you will complete one simulation run where you vary, in turn, two parameter. In one part of the run you will vary V x and in the other part you will vary n. For each run you will compare the location of the subsolar point of the magnetopause to the location predicted by equation 1. Ask your instructor what specific range you should use for each. Note that because of the way the simulation interpolates the parameters between input lines that you will always want to have the same parameters between consecutive input lines. Before submitting your simulation, have your instructor check your input parameters.
As stated explained above, we will be running the BAT-R-US simulation using supercomputers at CCMC. This section describes how to submit your simulation run.
Once you have submitted the job, you will be sent an email confirmation. After your run is complete, another email will be sent to you.
After your simulation run has been completed, you can use the online tools to view the results of your run. To get to these tools, hit the “View Simulation Results” link from the CCMC BAT-R-US page. Find your run and click the link. Then click the link to “View Magnetosphere.” This will bring you to a complicated web form which you can use to view your results. The simulation results can be viewed in many different ways, though we will only use a few of them.
First you want want to choose which time point of your simulation to look at under the “Choose data time” option. If you used the data input scheme suggested above where the input parameters are constant for times in a row, then you will want to look at the second time of each pair. In order to find the subsolar point of the magnetopause, it easiest to look at a line plot of various plasma parameters, so choose “Plot Mode” of “Line Plot.” Since the subsolar point in on the x-axis, choose “Y1”,“Y2”, “Z1”, “Z2” all equal to 0 under the “Choose Plot Area” section at the bottom of the screen. For the x-limits you will want to start out with a range that is sure to include your magnetopause, and then zoom in once you have found your subsolar point. In order to determine your magnetopause, you should take a look at several of the following data quantities: V x, B, Bx, By, Bz, and n.
The magnetopause boundary should be visible in your plots as the location where the the plasma parameters are changing rapidly from the solar wind values which you input, to the magnetosphere values. You should be able to determine the same boundary location from looking at more than one plasma parameter. You should assign a value for the uncertainty of the location of the subsolar point based on how precisely you can determine the locations from your plot. Be sure to save the plots that you use for finding the subsolar point to a file. You should include copies of these plots in your lab notebook.
In order to get a better feel of the global results of your simulation, you should also make a movie of your simulation results. To make this movie you will make plots at a series of times of your simulations results and then combine them to make a simple movie.
To make the movie you will use the same web plotting interface that you used above to make images of your results. You can choose whichever plot type and plasma parameters you would find interesting -- experiment with different choices.
Once you have found a plot type you like, make a plot of that type at each time for your simulation. Save a file copy of each plot (by right clicking on the image).
The program mpeg_creator can be used to create a movie from the image files. This
is which asks you a few questions and then calls the program mpeg_encode which
actually makes the movie. You can access the program from the class web site or from
~jcrumley/public\_html/332/magnetopause/data/mpeg\_creator. To run the program
open up a command shell (xterm), change to the directory your data is stored in, and then run
the program by typing
into the command shell. The program will give you instructions as you run it. When you hand in this lab, you should turn in one copy of your movie to your instructor.
For part of the lab you will take solar wind input data and examine where earth’s magnetopause was located. You will be looking at 3 specific events: October 31 2003, July 15 2000 (Bastille Day storm), and May 4, 1998. For each event the solar wind data was collected from the Advanced Composition Explorer (ACE) satellite from ccmc.gsfc.nasa.gov. ACE resides at about 210 RE from the earth on the line between the earth and the sun. On board ACE is an array of instruments to measure the solar wind. Solar wind plasma data (SWEPAM) and magnetic field data (MAG) are the two instruments that we will be using data from. The SWEPAM takes the solar wind velocity, ion density, and ion temperature. While MAG take the magnetic field intensity.
Along with ACE data, we will be using data from several other satellites. During the events listed above these satellites spend at least some time inside the magnetosphere. The point of this lab is to check the satellite data for signs of having crossed the magnetopause and compare that to predictions of equation 2.
For each event the earth’s magnetopause location has already been calculated using equation 2 based on ACE data which is also included in the spreadsheet. The position of the magnetopause along with the location of a particular satellite. These results are included in spreadsheet files which have been stored under the data directory on the course web site. Along with the spreadsheet files are data files which you will want to add to the spreadsheet in order to do further analysis. These files contain either magnetic field data or ion number density data.
To interpret the data you must first make it ready for the spreadsheet. After you have the data in the right coordinates and time scale you can then put them into the spreadsheet for that particular event to interpret the data. In the program “gnumeric”, you can add the data to the spreadsheet by opening it as a file of type “Text Import”. Once you have the data imported, you can plot it and compare it to the plot of the position of the magnetopause.
The Geotail from October 2003 has been done for you so you know what you are looking for and how to interpret the data. Some explanations have been saved on the spreadsheet for the event.
For each satellite there is some sort of data to try and confirm the location of the magnetopause. In some cases the satellite never crosses the theoretical magnetopause, but you need to check that the data shows the same thing. When examining the satellite data, pay particular attention to the times at which the theoretical magnetopause location plot shows the satellite crossing the magnetopause. Note any signs of a magnetopause crossing. Based on your examination of the data and the the theoretical magnetopause location, determine if you believe the satellite crossed the magnetopause and if so at what time[s].
Listed below are the events and the data that we have available for them. Your instructor will tell you which events you should look at.
|Event||May 4, 1998||July 15, 2000||October 31, 2003|
|Satellites||GOES 9||GOES 8||Geotail|
|L4||GOES 10||GOES 10|
ESA, Coordinate systems and transformations, 2005, http://www.spenvis.oma.be/spenvis/help/background/coortran/coortran.html.
Hansen, K., G. Tóth, A. Ridley, and D. DeZeeuw, BATS-R-US User Manual: Code Version 7.5.0, 2002, http://csem.engin.umich.edu/docs/HTML/USERMANUAL/USERMANUAL.html.
NASA, 2005, http://ccmc.gsfc.nasa.gov/.
Shue, J.-H., J. K. Chao, H. C. Fu, C. T. Russell, P. Song, K. K. Khurana, and H. J. Singer, A new functional form to study the solar wind control of the magnetopause size and shape, J. Geophys. Res., 102, 9497-9512, 1997.
Southwest Research Institute, The magnetosphere, 2005, http://pluto.space.swri.edu/IMAGE/glossary/magnetosphere2.html.