Can a magnet change how things move? Let’s find out.

NASA’s Interstellar Boundary Explorer Mission, called IBEX, is making the first maps of the invisible boundary of the Solar System. IBEX orbits Earth and makes these maps by collecting energetic neutral atoms that have traveled from the boundary region over 10 billion miles away.

First, we will explore how charged and neutral particles move when exposed to magnetic fields. Then, we will explain how the movement of particles in magnetic fields helps the IBEX spacecraft to make its measurements.

How do particles move in magnetic fields?

All objects are made of very small particles called atoms. Atoms can be broken down into smaller components that have negative, neutral, and positive charges. These negatively charged components of atoms are called "electrons." Positively charged components of atoms are called "protons." Components of atoms with no charge are called "neutrons." Atoms themselves can also be charged or neutral. An atom containing the same number of protons as electrons has no charge, called "neutral." If an atom contains an unequal number of protons and electrons, it is then "charged." If the atom contains more protons than electrons, it has a positive charge. If the atom contains more electrons than protons, it has a negative charge.

Next, we need to find out how these particles move when acted upon by forces, but to explain that, we have to explain what a "force" is. A force is a push or a pull acting upon an object as a result of its interaction with another object. Some examples of forces include gravity, magnetism, air resistance, and friction.

We are going to use an analogy to help us investigate particles in a magnetic field. Pretend you are playing pinball. A pinball machine works by placing a small ball in front of a spring that is attached to a handle. The player pulls back on the handle, which pulls the front of the spring backwards. When the player releases the handle, the spring expands, causing a small plate attached to the spring to impact the ball and push it forward into the machine. If a new ball appears and the player pulls back on the spring even further and releases the handle, more force is applied to the ball. If the player does not pull back on the spring as hard as before, less force is applied to the ball. The ball travels in a straight line unless acted upon by an outside force, such as when it hits something else inside the machine.

Now, let’s explain how moving protons, electrons, and neutrons are like a pinball machine. Instead of putting a ball in front of a spring, though, pretend that we place an electron, a proton, or a neutron in front of the spring. Instead of propelling each particle into a pinball machine, we will pretend to send the particles into a box. Because the only force acting on our particles is our imaginary moving spring, each particle will travel in a straight line at first. But how will each particle react if we include another force, specifically a magnetic force?

First, we need equipment for our experiments. Imagine you have an empty shoebox that is on a table in front of you. Pick up the box and hold it in your hands. Hold the long axis of the shoebox away from you, so the short end of the shoebox is facing you and the top of the box is facing upward. Next, imagine you have a big horseshoe magnet, which is a magnet with a north pole and a south pole in the shape of a large letter "C". The ends of the horseshoe are the ends of the magnet. Hold the horseshoe magnet so that the south pole end is on the top of the box and the north pole end wraps around the right side of the box and rests under the box. The magnetic field would then point from the north pole at the bottom of the box upward to the south pole of the magnet located at the top of the box. Next, in your mind, lift the lid from the box so you can see how the particles will travel through the box. This will be our imaginary experimental equipment to investigate particles and magnetism.

Our particles will travel through the box along the long axis, like a ball through a pinball machine. How do our particles travel, normally, without the magnet in place? First, consider the mass of the particles. Protons and neutrons have almost equal masses, but electrons have less mass. If you use the same force to move each particle, electrons will then travel faster than the protons and neutrons because you are applying the same amount of force to a smaller particle. If you want to make the electron travel at a slower speed, you can apply less force to it to get it to move.

How, then, can we change the path of particles? We apply a force. One type of force that can change the path of a charged particle is a magnetic force. Let’s launch our particles through the box and see how they are affected by the magnetic field of our horseshoe magnet. Remember, our horseshoe magnet is placed so that the magnetic field is coming upward from the north pole of the magnet, which is below our box, to the south pole of the magnet, which is above the box. A moving neutron travels unaffected through the box and through the magnetic field, in a straight line from one end of the box to another. A negatively–charged electron is affected by the magnetic field. It travels into the box but its path is curved to the left. The positively–charged proton is also affected by the magnetic field and its path curves to the right.

What happens if we flip the magnet around so that the south pole end is on the bottom of our box and the north pole end is on the top of the box? The magnetic field always points from north to south, so this time, the magnetic field is pointing downward. How do you think this would change the paths of our particles? As before, the neutral neutron is unaffected and travels in a straight line. In the flipped magnetic field, though, the electron now travels in a curved path to the right, and the proton now travels in a curved path to the left.

What happens if we use a much stronger magnet? The magnetic field of the magnet will always point from north to south no matter how strong or weak it is, but the amount of curvature of the paths of our charged particles will be affected. As usual, the neutron is unaffected. If our magnetic field is pointing upward, the proton’s path will curve to the right, but the amount it curves is in a tighter loop even more curved than before. For the electron, it curves to the left, but the amount it curves is also even more than before in a tighter loop. Increase the strength of the magnet, and the loop becomes tighter and tighter, like a smaller diameter circle. Decrease the strength of the magnet and the loop will be more broad, like a bigger diameter circle. If the magnetic field of our stronger magnet is pointing downward, the proton’s path will curve to the left in a tighter loop, and the electron’s path will curve to the right in a tighter loop.

Isn’t it interesting that magnetism can change the path of charged particles?

So, what does the movement of charged and neutral particles have to do with IBEX?

To answer that question, we need to start at the center of the Solar System: our Sun. Our Sun emits a wind of charged particles, called the "solar wind." These charged particles travel away from the Sun and through the Solar System at a speed of about one million miles, or 1.6 million kilometers, per hour. When the solar wind particles reach the boundary region of the Solar System, the solar wind interacts with the material between the stars, called the "interstellar medium," or ISM.

The charged solar wind particles travel on curved paths due to the magnetic field of the Sun. The ISM particles are neutral and travel along straight paths. When the charged solar wind particles meet the neutral interstellar medium particles, the solar wind particles steal electrons from the ISM particles. The solar wind particles thus become neutral.

When a solar wind particle becomes neutral, it no longer reacts to the magnetic field of the Sun. It begins to travel in a straight path in the direction that it happened to be going at the time of the interaction with the ISM particle.

These neutral particles can travel in any direction, but some of them happen to travel in toward the Sun, passing the orbits of the planets. A few of them encounter the IBEX spacecraft sensors and are collected and measured.

Charged particles travel curved paths because they react to magnetic fields. Because neutral particles are not deflected by magnetic fields, IBEX knows the direction in the sky from which they traveled. IBEX can thus count the number of particles coming from all parts of the sky, and scientists can make maps from this information. If charged and neutral particles both reacted to magnetic fields in the same way, IBEX would not be able to make its measurements.

To learn more

To learn more about NASA's Interstellar Boundary Explorer (IBEX) mission go to http://www.ibex.swri.edu.