The Cruel Solar Winds
Michael James Scharen
November 1st, 2021

Answer to Quora question: What is the significance of particles in the solar wind being charged?

How does the Earth’s magnetic field protect us from charged particles in the solar wind? Charged particles are accelerated in the presence of a magnetic field. In the now dated Cathode Ray Tube television sets this is how the electron beam writing the picture onto the phosphors at the front of the tube worked. The beam of electrons was shifted in a raster pattern by steering it with changes of the vertical and horizontal magnetic fields they had to travel through. The fields were created by controlling the current/fields of electromagnets with fields vertical and horizontal to the path of the electron beam. The figure below only shows one set of coils. The electron beam travels between the poles of each magnet. The field lines are between the poles of the magnet perpendicular to the electron path.

Fig.1 - Magnetic field deflection of the electron beam in a Cathode Ray Tube, CRT, display.

If one looks at the X,Y, Z plane, or uses the right hand rule then the direction of this acceleration can be visualized. With your right hand, point forward with your first or pointing finger with the palm of your hand facing to the left. This is the direction of motion for the charged particle. Let your middle finger point to the left, also. This is the direction of the magnetic field affecting the particle. Now let your thumb point upward as in a thumbs-up gesture all at the same time. Your thumb is pointing in the direction of the force on the charged particle. These directions are orthogonal or all 90 degrees from any other direction.

Fig. 2 - The right-hand rule helps to remember the force direction on a moving charge carrier in the presence of a magnetic field.

One can look at it also in the reverse direction, i.e. from the standpoint of a moving charge such as in a wire loop. The current in the wire induces a field perpendicular to the plane of the loop or a changing magnetic field through the loop induces an electromotive force on the charge carriers (electrons) in the wire to create a current. In either case, the accelerating force on the electrons is perpendicular to either the direction of motion or the magnetic field.

So what does this have to do with the Earth’ s Magnetosphere? We can picture the Earth’s magnetosphere ( a misnomer ) more as a bagel or donut with a vanishing hole in the center. The field lines follow the curvature of the donut emanating from the hole in the bottom, up and over the edge and back into the hole at the top. The far-field approximation of the shape is a sphere, but that’s not really what it is. This is important for the discussion. So picture those threads moving from the bottom to the top of the donut or the Earth.

Fig. 3 - Depiction of solar wind blasting against the Earth’s magnetosphere. The lines represent the direction of the magnetic field emanating from pole to pole.

What happens is a fast moving alpha particle or other ion encounters these threads of magnetic field? (They are not physical threads, but the strength and direction of magnetic field.) In this encounter, the particle moving at high velocity (depends on strength of the field and/or the velocity of the particle) will be sharply accelerated and start moving in a circular pattern around the direction of the magnetic field. This has a two-fold effect on the particle. First, much of the kinetic energy directed straight toward the Earth is now taken up in the spiraling of this particle along the magnetic field lines. These fast moving particles can be traveling at relativistic speeds and tremendous energies. Second, it diverts the path along the magnetic field lines toward the poles. Though the directions of these forces and fields are all perpendicular, they will not encounter each other at right angles. The perpendicular component of motion will induce a spiral about the magnetic field direction, but the left-over motion will advance it along the direction of the field in a screw-like motion toward the poles. Radiation, called synchrotron radiation, is also emitted from accelerated charged particles further dissipating energy and giving us spectacular light shows. The convergence of these charged particles at the poles gives us the Northern and Southern Lights!

Fig. 4 - Synchrotron radiation of a charged particle under acceleration. Many different wavelengths are possible.