Solar AtmosphereEdit
The solar atmosphere is the outer gaseous envelope of the Sun, extending from the visible surface—the photosphere—out into the hot, tenuous corona and beyond into the solar wind. It is a highly structured, magnetically dominated plasma in which energy generated in the solar interior is transported outward and released in a variety of dynamic phenomena. Because its emissions span the electromagnetic spectrum—from radio waves to X-rays—the solar atmosphere is studied with ground-based telescopes and spaceborne observatories alike, enabling a detailed picture of how a star’s outer layers behave under extreme conditions. Key diagnostic tools include spectroscopy, imaging at ultraviolet and X-ray wavelengths, and measurements of magnetic fields that thread the atmosphere. The study of the solar atmosphere informs broader understanding of stellar atmospheres and space weather effects that can influence planetary environments.
In the solar atmosphere, the balance between gas pressure, gravity, and magnetic forces shapes its structure and evolution. The layers differ markedly in temperature, density, and motion, yet they are all connected by the transport of energy from the solar interior to space. The atmosphere’s magnetic field organizes plasma into loops and open field regions, guiding plasma flows and explosive events. Observations of the Sun as a star also provide natural laboratories for plasma physics, magnetic reconnection, wave propagation, and radiative processes that are relevant to other astrophysical contexts.
Structure and layers
Photosphere
The photosphere is the portion of the solar atmosphere that emits most of the visible light we see. It is commonly regarded as the “surface” of the Sun, though it is a tiered, optically thick layer rather than a hard boundary. Its approximate temperature is about 5800 kelvin, yielding a spectrum close to that of a blackbody at that temperature blackbody radiation. The photosphere exhibits granular convection patterns—the result of boiling plasma beneath the surface—visible as a mottled texture on high-resolution images. Magnetic fields concentrate in regions known as network and plage, and dark sunspots mark where strong fields suppress convection and locally alter the temperature. The photosphere also contains a wealth of spectral lines (Fraunhofer lines) that serve as fingerprints for chemical composition and atmospheric dynamics Fraunhofer lines.
Chromosphere
Rising above the photosphere, the chromosphere is a hotter, more dynamic layer that can reach temperatures from roughly 4,500 kelvin to over 20,000 kelvin. Despite being less dense than the photosphere, it emits strongly in ultraviolet wavelengths, making it observable primarily with space-based instruments and specialized ground-based filters. The chromosphere hosts prominent features such as spicules—jet-like spikes of plasma that rise through the layer—and filaments that trace magnetic structures. The rapid heating and cooling in this region, along with complex magnetic fields, drive many transient events that seed the higher layers with energy and matter.
Transition region
The transition region marks a narrow interface where temperatures jump dramatically from the chromospheric values to coronal values. This layer is extremely small in vertical extent but plays a crucial role in setting the thermal structure of the outer atmosphere. The rapid temperature rise—across a short distance—drives intense gradients in density and pressure and acts as a gateway for material flowing into the corona.
Corona
The solar corona is the outermost, extremely hot part of the atmosphere, extending millions of kilometers into space. It is characterized by temperatures of roughly 1–3 million kelvin, far hotter than the underlying photosphere and chromosphere—a long-standing puzzle in solar physics known as the coronal heating problem. The corona is tenuous, with low particle densities, yet it is highly structured by magnetic fields that form closed loops and open regions. Coronal loops confine hot plasma and glow brightly in X-ray and extreme ultraviolet (EUV) light, revealing the geometry of the magnetic atmosphere. The corona is also the source region for the solar wind, a steady outflow of charged particles that fills the heliosphere and interacts with planetary magnetospheres and interplanetary space.
Solar wind and heliosphere
From the corona, plasma escapes along open magnetic field lines to form the solar wind, a continuous stream of charged particles that extends throughout the solar system. The solar wind shapes space weather and influences planetary environments, including magnetospheric dynamics and satellite charging. The interplay between open and closed magnetic field regions in the corona governs the release of material into the wind and the acceleration processes that sustain it. The study of the solar wind connects to broader topics in heliophysics and the behavior of stellar winds in other stars solar wind.
Observing the solar atmosphere
Observations span the electromagnetic spectrum, with ultraviolet and X-ray imaging revealing hot, tenuous plasma in the upper layers, and visible and infrared wavelengths providing detail about the lower layers and magnetic structures. Space missions such as Solar Orbiter and Parker Solar Probe have brought in-situ measurements and remote-sensing data from near the Sun, while missions like SOHO and Hinode have mapped coronal loops, sunspots, and spectral signatures with high resolution. Ground-based telescopes contribute through high-resolution imaging of the chromosphere and photosphere, complementing space-based measurements. Spectroscopic diagnostics of emission lines from highly ionized species allow researchers to infer temperatures, densities, velocities, and magnetic field strengths in each layer.
Energy transport from the solar interior to the atmosphere occurs through a combination of convective motions, waves, and magnetic activity. The photosphere receives energy from the convection zone below, and magnetic fields channel a portion of this energy upward into the chromosphere and corona. The manner in which waves dissipate their energy, and how magnetic reconnection and small-scale “nanoflares” contribute to coronal heating, are central topics in current solar physics. The exact balance of these processes remains an active area of inquiry, with ongoing observations and modeling aimed at resolving how the corona reaches and maintains its high temperature.
Heating and dynamics
A central question in the study of the solar atmosphere is how the corona becomes and stays millions of kelvin hotter than the underlying layers. Two leading categories of explanations have dominated discussions:
Wave heating: Magnetohydrodynamic (MHD) waves generated in the photosphere propagate upward and dissipate their energy in the corona, heating the plasma. Alfvén waves and other wave modes are prime candidates, but calculating dissipation rates in the highly conducting, magnetized corona presents challenges and requires precise measurements of wave amplitudes, frequencies, and damping mechanisms.
Magnetic reconnection and nanoflares: Small, frequent magnetic reconnection events release energy directly into coronal plasma, producing bursts of heating and accelerating particles. The cumulative effect of countless tiny flares could contribute substantially to coronal heating. Observational signatures, such as impulsive brightenings and changes in magnetic topology, are sought in high-cadence, high-resolution data.
Both avenues remain viable parts of a broader picture in which the corona is heated by a combination of wave-driven and reconnection-driven processes. Ongoing missions and advances in data analysis continue to refine when and where each mechanism dominates, and how energy is transported and dissipated in the Sun’s outer layers.
The solar atmosphere also hosts a range of dynamic phenomena beyond heating, including solar flares, coronal mass ejections (CMEs), prominences, and rapid changes in magnetic topology. Studying these events requires integrating observations across wavelengths with magnetic-field measurements and theoretical models, helping to illuminate how energy stored in the magnetic field is released and redistributed into space.