6. The future of solar physics

Solar physics is a historically data-starved science, but about to becomes less so. ‘The future of solar physics’ looks at new facilities, either online or about to come online, such as the Daniel K. Inouye Solar Telescope on Maui. This aims to see, through measurements of coronal magnetic fields and plasma, how the Sun’s magnetic fields generate flares, coronal mass ejections, and the solar wind. Other major missions include NASA’s Parker Solar Probe and the European Solar Orbiter mission, spacecraft intended to orbit the Sun in new ways and from different viewpoints on Earth. Supported by increasingly powerful computers, these missions are ushering in a new era.

3. Spots and magnetic fields

‘Spots and magnetic fields’ explores sunspot behaviour. We have known since 1908 that sunspots are magnetic, but why does the Sun form them at all? Is the Sun extraordinary in this, or is its behaviour in line with other stars? The Sun’s magnetic field is generated by a solar dynamo, which can be partly explained by magnetohydrodynamics (MHD)—the study of the magnetic properties and behaviour of electrically conducting fluids—however, there is no full consensus on the solar dynamo. In the 1960s the new science of helioseismology gave us insights into the Sun’s interior rotation, but we are unable to make truly critical observations in the solar interior.

5. Solar impacts on Earth

‘Solar impacts on earth’ focuses on space weather—rapidly changing conditions in the earth’s ionosphere and above. The Sun’s natural tendency to emit variable high energy radiation causes problems for us that it did not for our ancestors, as eruptions and storms interrupt our dependence on electronics. The ‘faint young sun’ paradox arises from the idea that the newly formed earth only received two-thirds of the Sun’s radiation today, yet water formed on earth without freezing; popular explanations include the Sun losing mass with age or an increase in greenhouse warming or radioactive heating. There seems to be no heating trend in the Sun correlating with recent global warming.

4. The dynamic corona

‘The dynamic corona’ introduces the layer of the Sun first seen by humans during eclipses. Initially, scientists were troubled by the idea that an outer layer of the Sun was hotter than its core. The corona continuously emits hot plasma and acts as a valve for a build-up of magnetic energy (helicity) that would otherwise stifle cyclic behaviour, such as sunspots. Originally predicted in the 1950s by a scientist using a pressure cooker as a model, the solar wind was identified and sampled shortly afterwards by American and Russian space missions. In the catalogue of known solar phenomena, flares preceded CMEs (coronal mass ejections), but they seem to be linked.

1. The Sun, our star

‘The Sun, our star’ presents a short history of the Sun and its relationship with Earth. While our ancestors worshipped the Sun, we may now take it for granted. Alpha Centauri A, the nearest other Sun-like star, is four light years away, compared to the Sun’s eight light minutes. The Sun and stars are neither solid nor liquid but composed of ionized particles in a plasma state. This plasma can sustain magnetic fields but not electric fields. The Sun exhibits remarkable phenomena such as sunspots, the corona, flares, the solar wind, and coronal mass ejections. Its atmosphere is layered into photosphere, chromosphere, and corona.

2. The Sun’s life-cycle

‘The Sun’s life-cycle’ describes the birth of the Sun out of the debris of stars which exploded early in the life of the Milky Way. When stars form, they employ a disc structure, with matter spinning around the centre of mass like a carousel, aided by magnetic fields. At four and a half million years old, the Sun, like most stars in the Universe, is on the main sequence stage of its life. In this stage, nuclear fusion reactions in its core fuse hydrogen into helium. Both observations and theory infer that the Sun spun faster in the past and was both hotter and less luminous.