The air above your head carries a constant 120 volts per metre
Near the ground there is always a downward electric field averaging about 120 volts per metre, part of a planet-wide circuit carrying roughly 1,800 amperes that returns upward through thunderstorms. Geophysics measures invisible features like this, from lightning's global wiring to magnetic reversals and heat left over from Earth's formation.
Geophysics applies physics to Earth and its surroundings: its shape, gravity, magnetism, internal structure and the oceans, atmosphere and near-space environment, with parallels on other planets and moons. It is among the oldest sciences, reaching back to early seismometers and magnetic compasses and later to Newton's analysis of tides and precession. Practical uses range from finding minerals, energy and groundwater to archaeology and hazard assessment.
The global electric circuit works because cosmic rays ionise the atmosphere, leaving it positively charged relative to the ground. Current trickles down from the ionosphere over most of the planet and climbs back through storms, showing up as lightning below the clouds and sprites above them. Electromagnetic waves ring through space too. The eerie radio sounds called whistlers come from lightning strikes, and dawn chorus is thought to arise from energetic electrons trapped in the Van Allen belts.
Magnetism tells a long story. Earth's field, generated by moving liquid iron in the outer core, shields the planet from the solar wind and lights up auroras. At irregular intervals averaging somewhere between 440,000 and a million years, its polarity flips. There have been 184 polarity intervals in the last 83 million years, and the Laschamp event 41,000 years ago was the latest brief complete reversal. Reversals leave parallel magnetic stripes on the seafloor that let geologists measure how fast oceans widen.
Heat drives it all. Radioactive decay of potassium-40, uranium and thorium supplies about 80 percent of Earth's internal heat, powering both the core's dynamo and plate tectonics, and around 4.2 times ten to the thirteenth watts leaks out through the surface. Even the mantle, despite its enormous viscosity, flows like a fluid over long spans. Tides show gravity's rhythm more simply: two highs and two lows every lunar day of 24 hours and 50 minutes.
Source: Geophysics
