Solar Physics, Heliosphere & Solar Observatory

The reddish layer of the solar atmosphere just above the photosphere, a few thousand kilometres thick, briefly visible as a rosy rim at the start and end of a total eclipse. Its temperature rises with height, and it is threaded by spicules, filaments, and plages. Above it lies the thin transition region.

The outer third of the solar interior, where energy is carried by convection — hot plasma rises, cools, and sinks, like a pot of boiling water. This churning is visible at the surface as granulation and drives the magnetic activity of the Sun. It sits above the tachocline and below the photosphere.

The visible surface of the Sun — the layer from which sunlight escapes — with an effective temperature near 5,772 kelvin and a thickness of only a few hundred kilometres. Sunspots, granulation, and the limb darkening seen in white-light images all belong to the photosphere. It is the reference surface for the Sun's radius.

The layer surrounding the core, from about a quarter to seven-tenths of the solar radius, where energy travels outward as radiation. Photons are absorbed and re-emitted so many times that a single packet of energy can take on the order of a hundred thousand years to cross it. The plasma here rotates almost as a rigid body.

The innermost region of the Sun, out to about a quarter of its radius, where nuclear fusion powers the star. At roughly 15 million kelvin and immense density, hydrogen fuses to helium mainly through the proton–proton chain, releasing the energy that slowly works its way outward. Almost all of the Sun's luminosity is generated here.

The Sun's outermost atmosphere — a tenuous, million-degree plasma extending millions of kilometres into space, visible to the eye only during a total solar eclipse or with a coronagraph. Far hotter than the surface beneath it, it is shaped by magnetic fields into loops, holes, and streamers, and it continually expands outward as the solar wind.

The thin shear layer near seven-tenths of the solar radius, where the rigidly rotating radiative interior meets the differentially rotating convection zone. This velocity shear is widely thought to be where the Sun's large-scale magnetic field is generated — the seat of the solar dynamo.

The thin, irregular layer between the chromosphere and the corona where the temperature climbs steeply — from around ten thousand to a million kelvin over only a few hundred kilometres. It radiates mostly in the extreme ultraviolet and is central to the still-open question of how the corona is heated.

An arch of hot, glowing plasma tracing a magnetic field line rooted in the photosphere, the basic building block of the closed corona above active regions. Coronal loops shine brightly in the extreme ultraviolet and X-rays and are central to studies of how the corona is structured and heated.

A large, pointed structure in the corona, seen in eclipse and coronagraph images as bright rays extending outward above the streamer belt. Helmet streamers cap closed magnetic regions and are the source of the slow solar wind; their shape changes over the solar cycle.

The same structure as a prominence, but seen against the bright solar disk rather than at the limb, where it appears as a dark, thread-like channel. A filament is cool plasma along a magnetic boundary; its sudden eruption is a common trigger of a coronal mass ejection.

The mottled, cellular pattern covering the photosphere — the tops of convection cells about a thousand kilometres across, each lasting only minutes. Bright centres are hot plasma rising, dark lanes are cooler plasma sinking. Granulation is the direct visible signature of the convection zone beneath.

A bright region of the chromosphere associated with an active region, where concentrated magnetic field heats the plasma above a group of sunspots. Plages are conspicuous in the light of hydrogen and calcium and mark magnetically active areas even as the sunspots themselves come and go.

A thin, ray-like structure of denser plasma rooted in the Sun's polar regions and extending into a polar coronal hole. Plumes trace open magnetic field lines from which fast solar wind flows, and appear as delicate streaks in images of the poles.

A large, relatively cool and dense structure of plasma suspended above the solar surface by magnetic fields, extending into the hot corona. Seen at the limb against the dark sky it appears as a bright arch; quiescent prominences can last for months, while eruptive ones can launch a coronal mass ejection.

A short-lived jet of plasma, a few hundred kilometres across, that shoots up from the chromosphere at tens of kilometres per second and falls back within minutes. Millions cover the Sun at any moment, giving the chromospheric limb a grassy appearance; they may contribute to the mass and energy budget of the corona.

A larger-scale convective pattern, with cells around thirty thousand kilometres across that persist for roughly a day. Supergranule flows sweep magnetic field to their edges, building the chromospheric network. It was discovered through Doppler measurements of the surface flows.

The Sun does not rotate as a solid body: its equator completes a turn in about twenty-five days while the polar regions take around thirty-five. This shearing of the plasma winds up the magnetic field and is a key ingredient of the solar dynamo. It is measured from tracking sunspots and from helioseismology.

The process by which oppositely directed magnetic field lines break and reconnect, converting stored magnetic energy explosively into heat, light, and fast particles. Reconnection powers solar flares and helps launch coronal mass ejections, and is a fundamental plasma process seen throughout the Universe.

A proposed solution to the coronal heating problem, put forward by Eugene Parker: the corona is heated by a vast number of tiny reconnection events — nanoflares — each far too small to see on its own, but collectively enough to keep the corona hot. It is one of several candidate mechanisms still being tested against observations.

The total energy the Sun radiates varies slightly over the activity cycle — by about a tenth of a percent between solar maximum and minimum — as dark sunspots and bright faculae come and go. Measured continuously from space since the late 1970s, these variations are a key input to studies of the Sun's influence on climate.

A plot of the latitudes at which sunspots appear over time, first drawn by the Maunders. Across each roughly eleven-year cycle, spots emerge first at mid-latitudes and then closer to the equator, so successive cycles trace wing-shaped patterns. It is the clearest visual summary of the solar activity cycle.

One of the central open questions in solar physics: why the corona, at a million or more kelvin, is hundreds of times hotter than the photosphere below it. Leading candidate mechanisms include heating by many tiny reconnection events (nanoflares) and by magnetic waves; missions such as Parker Solar Probe and Solar Orbiter are testing them. No single answer is yet established.

A period of low but not absent solar activity around the turn of the nineteenth century, less deep than the Maunder Minimum. Like other grand minima it is reconstructed from sunspot counts and from cosmogenic isotopes recorded in ice cores and tree rings.

The steady, high-speed stream of the solar wind — around seven to eight hundred kilometres per second — that flows out along open magnetic field lines from coronal holes, especially over the poles at solar minimum. It is smoother and less dense than the slow wind.

A roughly seventy-year span in the seventeenth and early eighteenth centuries when sunspots almost vanished from the record. It overlapped part of the cooler period known as the Little Ice Age, though the size of any climate link is debated. It is the archetype of a grand solar minimum.

The denser, more variable component of the solar wind — around three to five hundred kilometres per second — associated with the streamer belt near the solar equator. Its exact sources and release mechanisms are still being pinned down by close-in missions.

The mechanism that generates and regenerates the Sun's magnetic field, converting the energy of plasma motions — differential rotation and convection — into magnetic energy. The dynamo drives the roughly eleven-year sunspot cycle and the reversal of the Sun's polarity; the details, especially the role of the tachocline, remain an active research problem.

The turbulent outer region of the heliosphere, between the termination shock and the heliopause, where the slowed solar wind piles up and is deflected by the interstellar medium. Both Voyager spacecraft spent years traversing it before reaching interstellar space.

The disturbance ahead of the heliosphere as it moves through the surrounding interstellar cloud. Earlier work expected a sharp bow shock, but measurements from IBEX and the Voyagers suggest the Sun moves too slowly through the local medium for a strong shock — a gentler bow wave instead. The exact nature of the boundary is still being studied.

The spiral shape the Sun's magnetic field takes as it is carried outward by the solar wind while the Sun rotates, like water from a spinning sprinkler. Predicted by Eugene Parker in 1958 and since confirmed by spacecraft, the Parker spiral sets the geometry of the interplanetary magnetic field and how solar storms reach the planets.

The boundary where the supersonic solar wind abruptly slows as it runs into the pressure of the interstellar medium, roughly ninety astronomical units from the Sun. Voyager 1 crossed it in 2004 and Voyager 2 in 2007 — the first direct measurements of this shock.