Earth’s atmosphere is made up of onion-like layers. The ionosphere is a layer of electrically charged particles strongly influenced by the Sun, discovered by 20th-century radio pioneers who bounced long-wave radio signals off it. The ionosphere becomes most turbulent as the Sun’s warming thickens it, and then again in the evening as cooling leads to turbulence – including the production of ‘plasma bubbles’ – with the greatest dynamic change around Earth’s magnetic equator. The state of the ionosphere is often measured in terms of Vertical Total Electron Content (VTEC).