## Quietly Flowing Currents A Dynamo

To understand the daily ionospheric currents, let us first recall what happens in a hydroelectric plant that delivers electricity to a town. The water moves a paddlewheel connected to a mechanical dynamo. This dynamo is just a large magnet that produces a strong magnetic field, through which copper wires are moved by the connected paddlewheel. When an electrical conductor (the copper wire) is forced through the magnetic field, electric current flows in the wire (Figure 3.17). This is a result of the requirements of basic physics for the moving charges (here, free electrons flowing along a copper wire) in a field. The amount of current depends on the strength of the field from the dynamo magnet, the velocity with which the wire conductor is moved, and the conducting properties of that wire. Commercial, fuel-burning electric plants produce electricity similarly. They just use the fuel-burning engine to turn the conductors through the field. Of course, as far as the current flow is concerned, it matters not whether the field is stationary and the conductor moves, or the conductor is stationary and the field moves.

This is the same principle of physics that causes naturally oscillating fields to induce electric currents in the conducting Earth. The daily heating and

clockwise turn magnetic field

### SCREW

FIGURE 3.17 ► A hydroelectric plant uses a water turbine to move conducting wires through the field of a strong magnet. The current that is created has the direction that a screw would progress when turned clockwise from the direction of the wire velocity toward the direction of the magnetic field.

clockwise turn magnetic field

### SCREW

FIGURE 3.17 ► A hydroelectric plant uses a water turbine to move conducting wires through the field of a strong magnet. The current that is created has the direction that a screw would progress when turned clockwise from the direction of the wire velocity toward the direction of the magnetic field.

cooling of the atmosphere of the Earth as it spins on its axis causes a daily cycle of atmospheric expansion and contraction. Also, there are global winds, changing with solar activity and season, that occur at the ionospheric altitudes. Together the two motions force a daily motion on the ionospheric charged particles (the dynamo conductor) in the Earth's main field (the dynamo magnet), causing a current to flow near 100 km (62.5 miles) in altitude, where the ionospheric conductivity is large. The difference in the sign of the charge causes the negative electrons to move in one direction and the positive ions in another. But because they have fewer collisions to impede their forward direction, the smaller electrons dominate the current flow near the 100-km level. At much higher altitudes, where collisions are rare, the two motions cancel out the current. At lower altitudes, there is little or no ionization for significant conductivity.