What is the 'Ring Current'?

The magnetic field of the Earth acts as a magnetic bottle for some types of particles in the Earth's environment. Particles can be trapped so that they bounce back and forth between the north and south magnetic poles along field lines. They travel by spiraling around these field lines at a frequency called the 'gyro frequency' which is defined by 1/2pi x e x B/m where e is the charge of an electron, B is the strength of the magnetic field, and m is the mass of the particle. The radius of this spiral also changes so that for low field values, the 'cyclotron radius' is large, while for strong magnetic fields the cyclotron radius is small. The particles approach the polar regions where the field strength is high, and as their cyclotron frequency increases, they spiral along tighter and smaller orbits around a particular field line. After they are reflected at these polar 'mirror points' they fan out along the field lines towards the equatorial plane as their cyclotron frequency decreases and their orbit radius increases.

With larger orbit radii, the particles begin to feel the effects of the 'gradients' or changes in the magnetic field strengths among neighboring field lines, and this produces a 'gradient-B' force that causes the particles to steadily and continuously flow or 'diffuse' to neighboring field lines.

The direction of this flow is to the west for ions with a positive charge, and to the east for electrons and negatively-charged particles. This happens in the magnetic equatorial plane at distances between 2 and 6 times the earth's radius from the center of the Earth, in the region of the van Allen radiation belts.

These flows of particles form the equatorial 'Ring Current'. Ground stations can detect the build-up of the ring current because this current produces its own magnetic field which modifies the Earth's equatorial magnetic field particularly its horizontal surface component. the strength of the Ring Current field is measured by hourly averages of the so-called 'Disturbance Storm-Time (Dst) index which is given in units of nano-Teslas or nT. The Earth's normal field at the surface has a total strength of about 50,000 nT, and the Ring Current field can produce easily-measured Dst's near 200 nT for strong magnetic storms.

You might want to search the National Geophysical Data Center's Dst Archive from 1957 to the present to get an idea how this parameter varies during geomagnetic and solar storms, and during quiet-field states.

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