From the Sun’s core to the auroras

What happens, step by step.

1. 01

   Fusion, then a long journey

   At the Sun’s centre, at about 15 million kelvin, successive reactions turn hydrogen nuclei into helium. A small part of their mass becomes energy. Neutrinos escape almost freely, while photons are absorbed, re-emitted and scattered. The roughly 170,000-year diffusion time is a model-dependent estimate: we are not following one intact photon. Once light escapes, it reaches Earth in about 8 min 20 s.

2. 02

   Layers and a dynamo

   In the radiative zone, photon interactions carry energy. Farther out, plasma rises, cools and sinks: this is convection. Near their junction, the tachocline links regions rotating at different rates. Plasma motions sustain the magnetic field through a mechanism called a dynamo. The tachocline’s precise role remains debated. Sunspots mark cooler regions where the field reduces convection. The activity cycle lasts about 11 years, with a variable duration.

3. 03

   A hotter atmosphere

   The photosphere releases the Sun’s white visible light. Above it, the chromosphere hosts jets and the transition region marks a sharp temperature rise. The corona commonly reaches about 1–2 million kelvin but remains very thin. Waves, turbulence and field rearrangements contribute to heating; their relative roles remain under study. The corona extends into the solar wind. Fast wind comes mainly from coronal holes; slow wind has several origins.

4. 04

   Several clocks to Earth

   A flash of light, solar energetic particles and a CME are distinct phenomena that sometimes occur together. Light takes about 8 min 20 s; fast particles often arrive in 20 min to a few hours. A CME commonly takes 1–3 days, sometimes 15–18 h, and can miss Earth. Ordinary wind is continuous: about 4.3 days at 400 km/s. Near L1, between Sun and Earth, satellites measure the wind and its field, often giving a further 15–60 min of warning.

5. 05

   From Earth’s field to the auroras

   The wind meets a bow shock, then the magnetopause bounding the region dominated by Earth’s field. Higher pressure compresses this boundary. A southward interplanetary field favours reconnection: fields rearrange and energy enters the magnetosphere. The nightside tail can store and release it. Electrons excite atmospheric oxygen and nitrogen, which emit auroral light. Storms can also disrupt satellites and power grids.

6. 06

   How do we know the Sun?

   The interior has never been photographed. Helioseismology uses vibrations measured at the surface to constrain interior models. Neutrinos reveal fusion directly: Borexino detected those from the CNO cycle in 2020, with final results in 2023. Telescopes distinguish structures through different kinds of light. Parker measures plasma close to the Sun; Solar Orbiter viewed its poles from outside the orbital plane in 2025. Satellites near L1 link these observations to conditions heading towards Earth.