Particle Modeling Building a world in a computer One way to study plasmas is to build a computer model that simulates the motion of the individual charged particles. We know the equations that tell us how particles move in electric and magnetic fields. And we know the equations that tell us what electric and magnetic fields are created by the motion of charged particles. So in theory, you can write a computer program that puts this all together and recreates an environment like the magnetosphere, but inside a computer. A recipe The way this would work is the following: Start with a set of magnetic and electric fields. Pick a particle. Calculate the forces on it from all the electric and magnetic fields, then step it forward a little in time. That is, pick a short amount of time (the shorter the better), calculate the change in its velocity due to the fields over that time, and then assume that it will move in a straight line at a constant speed for that time. This “time step” is important. Making it smaller makes the simulation more accurate, but also makes it slower. It’s a bit like approximating a circle with a lot of short straight lines. You can approximate the circle with four straight lines quickly, but it will look like a square. You can approximate it with 1000 straight lines and it will look much more like a circle, but it will also take much more time to draw. Repeat that step for every particle in the system Now that you have changed the position and velocity of all the particles, recalculate all the electric and magnetic fields that result from the electrical currents of the moving particles. Repeat.