Stellar corona in the context of Solar flares

The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. The composition of the solar wind plasma also includes a mixture of particle species found in the solar plasma: trace amounts of heavy ions and atomic nuclei of elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. There are also rarer traces of some other nuclei and isotopes such as phosphorus, titanium, chromium, and nickel's isotopes Ni, Ni, and Ni. Superimposed with the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. The boundary separating the corona from the solar wind is called the Alfvén surface.

At a distance of more than a few solar radii from the Sun, the solar wind reaches speeds of 250–750 km/s and is supersonic, meaning it moves faster than the speed of fast magnetosonic waves. The flow of the solar wind is no longer supersonic at the termination shock. Other related phenomena include the aurora (northern and southern lights), comet tails that always point away from the Sun, and geomagnetic storms that can change the direction of magnetic field lines.

Gamma-ray astronomy is a subfield of astronomy where scientists observe and study celestial objects and phenomena in outer space which emit cosmic electromagnetic radiation in the form of gamma rays, i.e. photons with the highest energies (above 100 keV) at the very shortest wavelengths. X-ray astronomy uses the next lower energy range, X-ray radiation, with energy below 100 keV.

In most cases, gamma rays from solar flares and Earth's atmosphere fall in the MeV range, but it's now known that solar flares can also produce gamma rays in the GeV range, contrary to previous beliefs. Much of the detected gamma radiation stems from collisions between hydrogen gas and cosmic rays within our galaxy. These gamma rays, originating from diverse mechanisms such as electron-positron annihilation, the inverse Compton effect and in some cases gamma decay, occur in regions of extreme temperature, density, and magnetic fields, reflecting violent astrophysical processes like the decay of neutral pions. They provide insights into extreme events like supernovae, hypernovae, and the behavior of matter in environments such as pulsars and blazars. A huge number of gamma ray emitting high-energy systems like black holes, stellar coronas, neutron stars, white dwarf stars, remnants of supernova, clusters of galaxies, including the Crab Nebula and the Vela Pulsar (the most powerful source so far), have been identified, alongside an overall diffuse gamma-ray background along the plane of the Milky Way galaxy. Cosmic radiation with the highest energy triggers electron-photon cascades in the atmosphere, while lower-energy gamma rays are only detectable above it. Gamma-ray bursts, like GRB 190114C, are transient phenomena challenging our understanding of high-energy astrophysical processes, ranging from microseconds to several hundred seconds.

The Parker Solar Probe (PSP; previously Solar Probe, Solar Probe Plus or Solar Probe+) is a NASA space probe launched in 2018 to make observations of the Sun's outer corona.

It used repeated gravity assists from Venus to develop an eccentric orbit, approaching within 9.86 solar radii (6.9 million km or 4.3 million miles) from the center of the Sun. At its closest approach in 2024, its speed relative to the Sun was 690,000 km/h (430,000 mph) or 191 km/s (118.7 mi/s), which is 0.064% the speed of light. It is the fastest object ever built on Earth.

The interplanetary magnetic field (IMF), also commonly referred to as the heliospheric magnetic field (HMF), is the component of the solar magnetic field that is dragged out from the solar corona by the solar wind flow to fill the Solar System.

The Alfvén surface is the boundary separating a star's corona from the stellar wind defined as where the coronal plasma's Alfvén speed and the large-scale stellar wind speed are equal. It is named after Hannes Alfvén, and is also called Alfvén critical surface, Alfvén point, or Alfvén radius. In 2018, the Parker Solar Probe became the first spacecraft that crossed Alfvén surface of the Sun.

A coronal hole is a region of the Sun's corona that appears dark in extreme-ultraviolet (EUV) and soft-X-ray images because its plasma is cooler and more rarefied than the surrounding corona. Despite its name, a coronal hole is not an actual physical hole or void in the Sun's corona. The darkness reveals open magnetic field lines that guide plasma directly into interplanetary space, producing the fast component of the solar wind. They are composed of relatively cool and tenuous plasma permeated by magnetic fields that are open to interplanetary space. This results in decreased temperature and density of the plasma at the site of a coronal hole, as well as an increased speed in the average solar wind measured in interplanetary space.

Coronal holes were first identified unambiguously in soft-X-ray images from the 1973 Skylab mission, although eclipse photographs had hinted at polar dark regions earlier in the twentieth century. Routine mapping now combines full-disk EUV imagers with ground-based synoptic magnetographs to track hole evolution and feed space-weather forecasts.

A chromosphere ("sphere of color", from the Ancient Greek words χρῶμα (khrôma) 'color' and σφαῖρα (sphaîra) 'sphere') is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively, since it also refers to the corresponding layer of a stellar atmosphere. The name was suggested by the English astronomer Norman Lockyer after conducting systematic solar observations in order to distinguish the layer from the white-light emitting photosphere.

In the Sun's atmosphere, the chromosphere is roughly 3,000 to 5,000 kilometers (1,900 to 3,100 miles) in height, or slightly more than 1% of the Sun's radius at maximum thickness. It possesses a homogeneous layer at the boundary with the photosphere. Narrow jets of plasma, called spicules, rise from this homogeneous region and through the chromosphere, extending up to 10,000 km (6,200 mi) into the corona above.

A solar flare is a relatively intense, localized emission of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other eruptive solar phenomena. The occurrence of solar flares varies with the 11-year solar cycle.

Solar flares are thought to occur when stored magnetic energy in the Sun's atmosphere accelerates charged particles in the surrounding plasma. This results in the emission of electromagnetic radiation across the electromagnetic spectrum. The typical time profile of these emissions features three identifiable phases: a precursor phase, an impulsive phase when particle acceleration dominates, a gradual phase in which hot plasma injected into the corona by the flare cools by a combination of radiation and conduction of energy back down to the lower atmosphere, and a currently unexplained EUV late phase that occurs in some flares.

Solar phenomena are natural phenomena which occur within the atmosphere of the Sun. They take many forms, including solar wind, radio wave flux, solar flares, coronal mass ejections, coronal heating and sunspots.

These phenomena are believed to be generated by a helical dynamo, located near the center of the Sun's mass, which generates strong magnetic fields, as well as a chaotic dynamo, located near the surface, which generates smaller magnetic field fluctuations. All solar fluctuations together are referred to as solar variation, producing space weather within the Sun's gravitational field.

Coronaviruses are a group of related RNA viruses that cause diseases in mammals and birds. In humans and birds, they cause respiratory tract infections that can range from mild to lethal. Mild illnesses in humans include some cases of the common cold (which is also caused by other viruses, predominantly rhinoviruses), while more lethal varieties can cause SARS, MERS and COVID-19. In cows and pigs, they cause diarrhea; while in mice, they cause hepatitis and encephalomyelitis.

Coronaviruses constitute the subfamily Orthocoronavirinae, in the family Coronaviridae, order Nidovirales, and realm Riboviria. They are enveloped viruses with a positive-sense single-stranded RNA genome and a nucleocapsid of helical symmetry. The genome size of coronaviruses ranges from approximately 26 to 32 kilobases, one of the largest among RNA viruses. They have characteristic club-shaped spikes that project from their surface, which in electron micrographs create an image reminiscent of the stellar corona, from which their name derives.

Capella is the brightest star in the northern constellation of Auriga. It has the Bayer designation α Aurigae, which is Latinised to Alpha Aurigae and abbreviated Alpha Aur or α Aur. Capella is the sixth-brightest star in the night sky, and the third-brightest in the northern celestial hemisphere after Arcturus and Vega. A prominent object in the northern sky, it is circumpolar to observers north of 44°N. Its name meaning "little goat" in Latin, Capella depicted the goat Amalthea that suckled Zeus in classical mythology. Capella is relatively close, at 42.9 light-years (13.2 parsecs). It is one of the brightest X-ray sources in the sky, thought to come primarily from the corona of Capella Aa.

Although it appears to be a single star to the naked eye, Capella is actually a quadruple star system organized in two binary pairs, made up of the stars Capella Aa, Capella Ab, Capella H and Capella L. The primary pair, Capella Aa and Capella Ab, are two bright-yellow giant stars, both of which are around 2.5 times as massive as the Sun. The secondary pair, Capella H and Capella L, are around 10,000 astronomical units (AU) from the first and are two faint, small and relatively cool red dwarfs.

Helmet streamers, also known as coronal streamers, are elongated cusp-like structures in the Sun's corona which are often visible in white-light coronagraphs and during solar eclipses. They are closed magnetic loops which lie above divisions between regions of opposite magnetic polarity on the Sun's surface. The solar wind elongates these loops to pointed tips which can extend a solar radius or more into the corona.

During solar minimum, helmet streamers are found closer to the heliographic equator, whereas during solar maximum they are found more symmetrically distributed around the Sun.

In plasma physics, an Alfvén wave, named after Hannes Alfvén, is a type of plasma wave in which ions oscillate in response to a restoring force provided by an effective tension on the magnetic field lines.

Discovered theoretically by Alfvén in 1942—work that contributed to his 1970 Nobel Prize in Physics—these waves play a fundamental role in numerous astrophysical and laboratory plasma phenomena. Alfvén waves are observed in the solar corona, solar wind, Earth's magnetosphere, fusion plasmas, and various astrophysical settings. They are particularly significant for their role in the coronal heating problem, energy transport in the solar atmosphere, particle acceleration, and plasma heating.

Canopus is the brightest star in the southern constellation of Carina and the second-brightest star in the night sky. It is designated α Carinae, which is romanized (transliterated) to Alpha Carinae. With a visual apparent magnitude of −0.74, it is outshone only by Sirius.

Located around 310 light-years from the Sun, Canopus is a bright giant of spectral type A9, so it is essentially white when seen with the naked eye. It has a luminosity over 10,000 times the luminosity of the Sun, is nine to ten times as massive, and has expanded to 71 times the Sun's radius. Its enlarged photosphere has an effective temperature of around 7400 K. Canopus is undergoing core helium burning and is currently in the so-called blue loop phase of its evolution, having already passed through the red-giant branch after exhausting the hydrogen in its core. Canopus is a source of X-rays, which are likely being emitted from its corona.

Solar radio emission refers to radio waves that are naturally produced by the Sun, primarily from the lower and upper layers of the atmosphere called the chromosphere and corona, respectively. The Sun produces radio emissions through four known mechanisms, each of which operates primarily by converting the energy of moving electrons into electromagnetic radiation. The four emission mechanisms are thermal bremsstrahlung (braking) emission, gyromagnetic emission, plasma emission, and electron-cyclotron maser emission. The first two are incoherent mechanisms, which means that they are the summation of radiation generated independently by many individual particles. These mechanisms are primarily responsible for the persistent "background" emissions that slowly vary as structures in the atmosphere evolve. The latter two processes are coherent mechanisms, which refers to special cases where radiation is efficiently produced at a particular set of frequencies. Coherent mechanisms can produce much larger brightness temperatures (intensities) and are primarily responsible for the intense spikes of radiation called solar radio bursts, which are byproducts of the same processes that lead to other forms of solar activity like solar flares and coronal mass ejections.