# Van Allen Belts Modeling – Conclusion and Final Thoughts

At the conclusion of this project I have to say I came out of this project having learned a lot about organizing a project, the Van Allen Belts and how to work with Mathematica. But first I would like to discuss the conclusions that can be drawn from my final data.

Unfortunately I can’t draw any major conclusions about the Van Allen Belts and their location depending on core size as I was not able to incorporate that particular information into my models. I was however successfully able to model the vector fields of a magnetic dipole using Mathematica and an initial equation given by our textbook, a task that proved to be difficult enough on its own to figure out. In the end I did prove that the equation was valid and created a vector field that resembled the expected field of the dipole. Even with this however, I question the validity of the models that were obtained using Mathematica. As I mentioned in my “Final Data” post, the vectors do not seem to vary with respect to the radius away from the origin which doesn’t match the actual system I was attempting to model. It should be decreasing with respect to distance. In the end the project was never really able to combine the Van Allen Belt mapping and the modeling of Earth’s magnetic field into the single cohesive unit.

Furthermore the model itself had a lot of assumptions that needed to be put into place in order to make it easier to develop or simply because I could not figure out how to account for it within Mathematica. The assumptions were listed throughout my final data but they included using a constant magnetic field isolated from the sun’s solar wind to model out the Van Allen belts and assuming the Earth to be a sphere. In the long term however, these were all reasonable assumptions in astronomical scales.

If there was one thing I was somewhat disappointed about, it was the learning curve to reacquaint myself with Mathematica. It had been awhile since I had used Mathematica and it took some time to recall the best functions to use for particular tasks. Furthermore when trying to learn how to do a new task; like converting between Spherical and Cartesian coordinates with Mathematica, it was a long process of trial and error to achieve a result you were looking for. More often than not you would get something more like this:

Occasionally, the program has a cruel sense of humor

So a great deal of the project was spent playing around with Mathematica more that playing around with the physics. I do certainly wish I had a bit more time to play around with the physics and even more time to play with the Mathematica program. After a few breakthroughs, I was quickly understanding how to make the program much more cooperative and finding relevant information in the help menus faster. With a little more time, I fixed and accounted for a few of the issues that had been plaguing my models.

Regardless, I was a bit overzealous with the scope of my project. It had a lot of components that were much more difficult than I initially expected them to be. What I ended with were two separate projects, a research summary of Van Allen Belts and a 3D model of Earth’s magnetic field with vague notions of where the Van Allen Belts would be located. Still I am proud of what was accomplished. Modeling the vector field of a dipole was a difficult task and a properly organized model was created.in the end. It was also plotted with proper constants though it needed a little tweaking to confirm that the structure was as expected. Finally from this point it could easily be picked up and completed at a later date with those willing to work on it needing only to manipulate the dynamo equation into an easier form.

# Van Allen Belt Modeling – Final Data

Background Information:

Earth’s Van Allen Belts are a naturally occurring phenomena due to the presence of the Earth’s magnetic fields. Specifically, they are two zones of space where large concentrations of high energy particles are trapped by the magnetic field lines of our planet. The zones are defined by our field lines; toroidal in shape, and encompass the planet at particular radii from the surface. Following the field lines, the regions begin from Earth’s Southern Pole and loop to its Northern Pole  This is illustrated in the diagram below.

Cross-Section of the Van Allen Belts

The particles of these regions include fast moving electrons,and ions from the solar wind along with protons and other remnants of cosmic ray interactions with our atmosphere. All of these particles are electrically charged and thus their motion becomes highly constrained as they pass through our magnetic field; or magnetosphere. As they interact with our magnetic field lines, a force is exerted and influences their direction of motion. It attempts to make the particles rotate about the lines rather than allowing them to continue drifting in a straight line. Thus “sliding” begins to occur; the particles gain a spiral trajectory during which they orbit the field lines while vertically moving along them. Since the magnetic field is stronger as you get closer to Earth however the particles will stop and eventually reverse their motion as they approach the poles. This bouncing between the hemispheres establishes the permanent belts.

The constituent particles and stability of the belts however depends on the location.and thus the strength of the magnetic field. The inner belt is compact; ranging only from 1,700 km to 13,000 km above the Earth’s surface. It is made up primarily of high energy protons created during cosmic ray interactions with the atmosphere. Of the two belts, it is the most stable due to the higher magnitude of the magnetic field lines at that proximity to the Earth.  While it gains high energy particles at a comparatively slow rate, it holds onto the particles for an extended period of time and reaches a much higher intensity than the other belt.

The outer belt ranges from 20,000 km to 40,000 km from the Earth’s surface and consists primarily of the ions and electrons that make up the solar wind. Due to the weaker magnetic field at that distance from Earth, the zone is tenuous and loses its particles very quickly. The constant supply of particles from the solar wind however means that the belt has plenty of material to be created from, though its dependence on solar wind particles means that its intensity fluctuates proportionally with solar wind activity. With this background information established, the information needed to created valid assumptions was gained.

2D Vector Fields:

Unfortunately, this also marks the point where my project strayed away from modeling the Van Allen Belts to a more general modeling of the Earth’s magnetic field. The initial task of modeling the magnetic field of the Earth was necessary to define the locations of the Belts with specific field lines that would be seen in the vector plot. Now with some research I found that the Earth’s field could be generally modeled as a magnetic dipole oriented with the south end pointed in the positive z direction. This model is a very basic assumption and ignores the effects of the solar wind on Earth’s magnetosphere, however the area where the Van Allen Belts are present does not get severely effected by these factors except in extreme circumstances. This means a model of a dipole will be generally accurate for our purposes. Thus the first task was to create a model of the magnetic field of the dipole.

The equation for the magnetic field of a dipole with respect to the radius was found to be

This equation was obtained from Griffiths’ Introduction to Electrodynamics. To insure the equation worked I began by applying a constant free version, evaluating only the variables. However, the equation was given in spherical coordinates which Mathematica can’t plot efficiently. To compound that issue, the command CoordinateTransform does not properly evaluate the points which led to several attempts to obtain the proper Cartesian conversion of our equation. Finally, I was able to find the command TransformedField which took into account the variables being used and allowed for a direct conversion. The code and direct conversion can be seen below.

From these equations, I created a vector field of the xz plane with VectorPlot which can be seen below.

Simple vector model of the xz plane of the magnetic field of a dipole

With this successful, I then applied the proper constants to the vector field components and attempted to recreated the field. These constants were the permeability of free space ( $\mu_0$ = $4\pi$*$10^{-7}$ $N$/$A^2$) and the magnetic dipole moment of the Earth ($m_{Earth}$ = $7.79$*$10^{22}$ $A*m^{2}$).The circle in the center of the graph represents the Earth, under the assumption that the Earth has a constant radius of $6.37*10^6$ m.

Model of the xz plane of the dipole’s magnetic field using Earth defined constants

Note that in this plot, the vectors seem to lack the fine structure of the previous field however the vectors are still properly oriented. They continue out from the southern hemisphere and seem to eventually loop back around and end in the northern hemisphere. I attempted a variety of scales and vector size values but no matter what, when the constants were applied to the equation they never had the same shape as the previous vector field. The other concerning aspect of the field was the fact that the vectors don’t seem to weaken as the field continues away from the Earth. It could be that the scale was just off so it was not evident, but even with the shifting of the scales, the vectors still seemed to remain the same size. The general direction and shape was obtained with these equations so I continued on to the next step.

3D Vector Fields:

Applying the same methods as before, I used the vector field components from the obtained equation and using the command VectorPlot3D, I created a vector field of the general equation without the constants. This field can be seen below.

Simple vector model of the magnetic field of a dipole

Without constants the shape of the vector field directly resembles the magnetic field of a dipole in 3 dimensions. Unfortunately, we also have the vectors not varying as expected as the distance from the origin increases. Regardless, after the success of obtaining the proper vector field shape and direction I decided to apply the proper constants to see if the shape would be altered as it was in the 2D graph. The constant applied vector field can be seen below. Once again the constants remain the same and the sphere in the center of the plot represents the Earth, assuming it is a sphere with a constant radius.

Vector model of the magnetic field of a dipole using Earth defined constants

As we can see, the same lack of detail is present when the constants are added to the vector field. However the shape and direction of the vector field are still generally correct though the fine structure is again lost. The vectors still don’t seem to vary with distance as well, which is troubling for actually plotting the field but the model does seem to accurately plot the field itself.

Further Plans:

Unfortunately it is at this point where I ran out of time and needed to call it quits on the project, with only the magnetic field of the dipole modeled. While I will discuss what I would have liked to change and go into more detail on complications within my “Conclusion”, I would like to take a moment to discuss where I wanted to continue on with this project. Specifically, next on the list would have been adjusting the vector field  23 degrees from the z-axis to account for the Earth’s natural tilt. I experimented with a few methods to do this but none of them were truly successful.

Next I would have liked to incorporate a model of the dynamo theory to account for the Earth’s magnetic field. One of the biggest assumptions made within this project was that the Earth’s magnetic field simply existed as a giant bar magnet in the surface. The magnetic field is actually generated by the motion of liquid iron present in the Earth’s outer core. Using this knowledge, I was hoping to create an animation that would allow the observer to alter the size of the planet and then the percent of the outer core took up of the interior. This would allow the observer to have a general model about how the magnetic fields change with different core sizes and thus the location of the Van Allen Belts. Knowing this, scientists could prep future scientific equipment for radiation exposure even if they needed to orbit within the Van Allen belts.

Here are my Mathematica Notebooks in case there were any questions on my code: 2D Vector Fields, 3D Vector Fields, and General Experimenting

Sources:

Griffiths, David, Introduction to Electrodynamics

Dynamo Theory (https://courses.seas.harvard.edu/climate/eli/Courses/EPS281r/Sources/Earth-dynamo/1-Wikipedia-Dynamo-theory.pdf)

Magnets and Electromagnetism (http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html)

# Project Plan: Van Allen Belts

Summary:

The primary goal of my project is to create a 3d model of the shape and strength of the Earth’s magnetic field and define the locations of the Van Allen Belts within that model. According to the research I have done so far, a planetary magnetic field resembles the field of cylinder. Applying relevant Earth properties, I would then be able to create an initial model of the field.  Furthermore, due to the proximity of these belts to the Earth, I will not have to take interactions with the solar wind into account when modeling the field. The modeling will be done with the Mathematica program. With the model complete, I would hope to compare it to other, more exact models of the Earth’s magnetic field to confirm the theoretical location of the Van Allen Belts. I would also be interested in noting if there is any relationship between core size of the planet and development of a magnetic field. My current research hasn’t turned up any leads for a mathematical way to approach this problem however.

Timeline:

•Week  1: (week of 4/6) Research the Van Allen Belts and Define Project Parameters -During this first week I plan to research the different possible facets of my project in order to determine the particular parameters I will be creating the 3d model with. This also includes looking for previously made 3d vector fields that can teach me how to plot the vector field in Mathematica. Furthermore I will be looking into some of the secondary objectives of my project to see if the ideas can be executed in a reasonable amount of time.

•Week 2: (week of 4/13) Creating the Initial 3D Model – During the second week of the project, we proceed onto the next step of creating our basic 3D magnetic field model. This 3D vector field model will be created using Mathematica and will create a visual representation of the Earth’s magnetic field. The initial model will be based off of the magnetic field of a cylinder, a design which has been suggested by multiple sources.

•Week 3: (week of 4/20) Finishing the Initial 3D Model and Model Comparisons – This week will be used to put any necessary finishing touches on the initial model and determine the location of the Van Allen belts within the model. I will also be comparing the model to observed data and other proposed models of the field. From these findings, I will be able to determine the accuracy of the field modeled in Mathematica. If the simple cylindrical model is not as accurate as predicted, this week will also be taken to edit the model and improve its accuracy.

•Week 4: (week of 4/27) Extended Modeling – During this week I will finish all alterations on a final edited model. If there is extra time, the current plan is to systematically alter the model to see how certain changes effect the magnetic field. Specifically my goal is to see if there exists some sort of correlation between the planet’s core size and the development of its magnetic field and subsequently its Van Allen Belts. If a correlation like this can be seen, the magnetic fields of other planets will be modeled and compared to actual data. This will allow us to confirm if the created model can be applied to other planets with only minor alterations to its coding.

•Week 5: (week of 5/4) Finishing Touches – During this week, any extra modeling will be quickly finished up and the rest of the week will be dedicated to organizing the collected data into a final presentation.

Sources:

Currently the sources I have browsed are: