Astronaut

From Wikipedia, the free encyclopedia

NASA Astronaut Bruce McCandless II using a Manned Maneuvering Unit outside Space Shuttle Challenger on shuttle mission STS-41-B in 1984.

An astronaut (or cosmonaut) is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft. Although generally reserved for professional space travelers, the terms are sometimes applied to anyone who travels into space, including scientists, politicians, journalists, and tourists.^[1][2]

Starting in the 1950s up to 2002, astronauts were sponsored and trained exclusively by governments, either by the military or by civilian space agencies. With the suborbital flight of the privately funded SpaceShipOne in 2004, a new category of astronaut was created: the commercial astronaut.

§Definition

Alan Shepard aboard Freedom 7

The criteria for what constitutes human spaceflight vary. The Fédération Aéronautique Internationale (FAI) Sporting Code for astronautics recognizes only flights that exceed an altitude of 100 kilometers (62 mi).^[3] In the United States, professional, military, and commercial astronauts who travel above an altitude of 50 miles (80 km)^[4] are awarded astronaut wings.

As of 8 June 2013^[update], a total of 532 people from 36 countries have reached 100 km (62 mi) or more in altitude, of which 529 reached low Earth orbit or beyond.^[5][6] Of these, 24 people have traveled beyond Low Earth orbit, to either lunar or trans-lunar orbit or to the surface of the moon; three of the 24 did so twice: Jim Lovell, John Young and Eugene Cernan.^[7] The three astronauts who have not reached low Earth orbit are spaceplane pilots Joe Walker, Mike Melvill, and Brian Binnie.

As of 20 June 2011^[update], under the U.S. definition 538 people qualify as having reached space, above 50 miles (80 km) altitude. Of eight X-15 pilots who exceeded 50 miles (80 km) in altitude, only one exceeded 100 kilometers (about 62 miles).^[8] Space travelers have spent over 41,790 man-days (114.5 man-years) in space, including over 100 astronaut-days of spacewalks.^[8][9] As of 2008, the man with the longest cumulative time in space is Sergei K. Krikalev, who has spent 803 days, 9 hours and 39 minutes, or 2.2 years, in space.^[10][11] Peggy A. Whitson holds the record for the most time in space by a woman, 377 days.^[12]

§Terminology

§English

In English-speaking nations, a professional space traveler is called an astronaut.^[13] The term derives from the Greek words ástron (ἄστρον), meaning "star", and nautes (ναύτης), meaning "sailor". The first known use of the term "astronaut" in the modern sense was by Neil R. Jones in his short story "The Death's Head Meteor" in 1930. The word itself had been known earlier. For example, in Percy Greg's 1880 book Across the Zodiac, "astronaut" referred to a spacecraft. In Les Navigateurs de l'Infini (1925) of J.-H. Rosny aîné, the word astronautique (astronautic) was used. The word may have been inspired by "aeronaut", an older term for an air traveler first applied (in 1784) to balloonists. An early use in a non-fiction publication is Eric Frank Russell's poem "The Astronaut" in the November 1934 Bulletin of the British Interplanetary Society.^[14]

The first known formal use of the term astronautics in the scientific community was the establishment of the annual International Astronautical Congress in 1950 and the subsequent founding of the International Astronautical Federation the following year.^[15]

NASA applies the term astronaut to any crew member aboard NASA spacecraft bound for Earth orbit or beyond. NASA also uses the term as a title for those selected to join its Astronaut Corps.^[16] The European Space Agency similarly uses the term astronaut for members of its Astronaut Corps.^[17]

§Russian

By convention, an astronaut employed by the Russian Federal Space Agency (or its Soviet predecessor) is called a cosmonaut in English texts.^[16] The word is an anglicisation of the Russian word kosmonavt (Russian: космонавт Russian pronunciation: [kəsmɐˈnaft]), one who works in space outside the Earth's atmosphere, a space traveler,^[18] which derives from the Greek words kosmos (κόσμος), meaning "universe", and nautes (ναύτης), meaning "sailor". Other countries of the former Eastern Bloc use variations of the Russian word kosmonavt, such as the Polish kosmonauta.The Soviet Air Force pilot Yuri Gagarin was the first cosmonaut—indeed the first person—in space. Valentina Tereshkova, a Russian factory worker, was the first woman in space, as well as arguably the second civilian to make it there (see below for a further discussion of civilians in space). On March 14, 1995, Norman Thagard became the first American to ride to space on board a Russian launch vehicle, and thus became the first "American cosmonaut".^{[citation needed]}

§Chinese

Official English-language texts issued by the government of China use astronaut while texts in Russian use космонавт (cosmonaut).^[19][20] In official Chinese-language texts, "yǔ háng yuán" (宇航员, "space navigating personnel") is used for astronauts and cosmonauts, and "háng tiān yuán" (航天员, "space navigating personnel") is used for Chinese astronauts. The phrase "tài kōng rén" (太空人, "spaceman") is often used in Hong Kong and Taiwan.^{[citation needed]}
The term taikonaut is used by some English-language news media organizations for professional space travelers from China.^[21] The word has featured in the Longman and Oxford English dictionaries, the latter of which describes it as "a hybrid of the Chinese term taikong (space) and the Greek naut (sailor)"; the term became more common in 2003 when China sent its first astronaut Yang Liwei into space aboard the Shenzhou 5 spacecraft.^[22] This is the term used by Xinhua News Agency in the English version of the Chinese People's Daily since the advent of the Chinese space program.^[23] The origin of the term is unclear; as early as May 1998, Chiew Lee Yih (趙裡昱) from Malaysia, used it in newsgroups.^[24][25]

§Other terms

With the rise of space tourism, NASA and the Russian Federal Space Agency agreed to use the term "spaceflight participant" to distinguish those space travelers from professional astronauts on missions coordinated by those two agencies.^{[citation needed]}

While no nation other than the Russian Federation (and previously the former Soviet Union), the United States, and China have launched a manned spacecraft, several other nations have sent people into space in cooperation with one of these countries. Inspired partly by these missions, other synonyms for astronaut have entered occasional English usage. For example, the term spationaut (French spelling: spationaute) is sometimes used to describe French space travelers, from the Latin word spatium for "space", the Malay term angkasawan was used to describe participants in the Angkasawan program, and the Indian Space Research Organization hope to launch a spacecraft in 2018 that would carry vyomanauts, coined from the Sanskrit word for space.^{[citation needed]}

§Space travel milestones

Yuri Gagarin, first human in space (1961)

Valentina Tereshkova, 1963 first woman in space

Neil Armstrong, first human to walk on the Moon (1969)

Yang Liwei, first person sent into space by China

The first human in space was Soviet Yuri Gagarin, who was launched on April 12, 1961 aboard Vostok 1 and orbited around the Earth for 108 minutes. The first woman in space was Soviet Valentina Tereshkova, who launched on June 16, 1963 aboard Vostok 6 and orbited Earth for almost three days.

Alan Shepard became the first American and second person in space on May 5, 1961 on a 15-minute sub-orbital flight. The first American woman in space was Sally Ride, during Space Shuttle Challenger's mission STS-7, on June 18, 1983.^[26] In 1992 Mae Jemison became the first African American woman to travel in space aboard STS-47.

Cosmonaut Alexei Leonov was the first person to conduct an extravehicular activity (EVA), (commonly called a "spacewalk"), on March 18, 1965, on the Soviet Union's Voskhod 2 mission. This was followed two and a half months later by astronaut Ed White who made the first American EVA on NASA's Gemini 4 mission.^[27]

The first manned mission to orbit the Moon, Apollo 8, included American William Anders who was born in Hong Kong, making him the first Asian-born astronaut in 1968.

The Soviet Union, through its Intercosmos program, allowed people from other "socialist" (i.e. Warsaw Pact and other Soviet-allied) countries to fly on its missions, with the notable exception of France participating in Soyuz TM-7. An example is Czechoslovak Vladimír Remek, the first cosmonaut from a country other than the Soviet Union or the United States, who flew to space in 1978 on a Soyuz-U rocket.^[28]

On July 23, 1980, Pham Tuan of Vietnam became the first Asian in space when he flew aboard Soyuz 37.^[29] Also in 1980, Cuban Arnaldo Tamayo Méndez became the first person of Hispanic and black African descent to fly in space, and in 1983, Guion Bluford became the first African American to fly into space. In April 1985, Taylor Wang became the first ethnic Chinese person in space.^[30][31] The first person born in Africa to fly in space was Patrick Baudry (France), in 1985.^[32][33] In 1985, Saudi Arabian Prince Sultan Bin Salman Bin AbdulAziz Al-Saud became the first Arab Muslim astronaut in space.^[34] In 1988, Abdul Ahad Mohmand became the first Afghan to reach space, spending nine days aboard the Mir space station.^[35]

With the larger number of seats available on the Space Shuttle, the U.S. began taking international astronauts. In 1983, Ulf Merbold of West Germany became the first non-US citizen to fly in a US spacecraft. In 1984, Marc Garneau became the first of 8 Canadian astronauts to fly in space (through 2010).^[36] In 1985, Rodolfo Neri Vela became the first Mexican-born person in space.^[37] In 1991, Helen Sharman became the first Briton to fly in space.^[38] In 2002, Mark Shuttleworth became the first citizen of an African country to fly in space, as a paying spaceflight participant.^[39] In 2003, Ilan Ramon became the first Israeli to fly in space, although he died during a re-entry accident.

On October 15, 2003, Yang Liwei became China's first astronaut on the Shenzhou 5 spacecraft.

§Age milestones

The youngest person to fly in space is Gherman Titov, who was 25 years old when he flew Vostok 2. (Titov was also the first person to suffer space sickness).^[40][41] The oldest person who has flown in space is John Glenn, who was 77 when he flew on STS-95.^[42]

§Duration and distance milestones

The longest stay in space thus far has been 438 days, by Russian Valeri Polyakov.^[8] As of 2006, the most spaceflights by an individual astronaut is seven, a record held by both Jerry L. Ross and Franklin Chang-Diaz. The farthest distance from Earth an astronaut has traveled was 401,056 km (249,205 mi), when Jim Lovell, Jack Swigert, and Fred Haise went around the Moon during the Apollo 13 emergency.^[8]

§Civilian and non-government milestones

Depending on the exact definition of 'civilian', the first civilian in space was either Valentina Tereshkova^[43] aboard Vostok 6 (she also became the first woman in space on that mission) or Joseph Albert Walker^[44][45] on X-15 Flight 90 a month later. Tereshkova was only honorarily inducted into the USSR's Air Force, which had no female pilots whatsoever at that time. Joe Walker had joined the US Army Air Force but was not a member during his flight. The first people in space who had never been a member of any country's armed forces were both Konstantin Feoktistov and Boris Yegorov aboard Voskhod 1.

The first non-governmental space traveler was Byron K. Lichtenberg, a researcher from the Massachusetts Institute of Technology who flew on STS-9 in 1983.^[46] In December 1990, Toyohiro Akiyama became the first paying space traveler as a reporter for Tokyo Broadcasting System, a visit to Mir as part of an estimated $12 million (USD) deal with a Japanese TV station, although at the time, the term used to refer to Akiyama was "Research Cosmonaut".^[47][48][49] Akiyama suffered severe space sickness during his mission, which affected his productivity.^[48]

The first self-funded space tourist was Dennis Tito on board the Russian spacecraft Soyuz TM-3 on April 28, 2001.

§Self-funded travelers

The first person to fly on an entirely privately funded mission was Mike Melvill, piloting SpaceShipOne flight 15P on a suborbital journey, although he was a test pilot employed by Scaled Composites and not an actual paying space tourist.^[50][51] Seven others have paid to Russian Space Agency to fly into space:

1. Dennis Tito (American): April 28 – May 6, 2001 (ISS)
2. Mark Shuttleworth (South African): April 25 – May 5, 2002 (ISS)
3. Gregory Olsen (American): October 1–11, 2005 (ISS)
4. Anousheh Ansari (Iranian / American): September 18–29, 2006 (ISS)
5. Charles Simonyi (Hungarian / American): April 7–21, 2007 (ISS), March 26 – April 8, 2009 (ISS)
6. Richard Garriott (British / American): October 12–24, 2008 (ISS)
7. Guy Laliberté (Canadian): September 30, 2009 – October 11, 2009 (ISS)

§Training

Elliot See during water egress training with NASA

The first NASA astronauts were selected for training in 1959.^[52] Early in the space program, military jet test piloting and engineering training were often cited as prerequisites for selection as an astronaut at NASA, although neither John Glenn nor Scott Carpenter (of the Mercury Seven) had any university degree, in engineering or any other discipline at the time of their selection. Selection was initially limited to military pilots.^[53][54] The earliest astronauts for both America and the USSR tended to be jet fighter pilots, and were often test pilots.
Once selected, NASA astronauts go through twenty months of training in a variety of areas, including training for extravehicular activity in a facility such as NASA's Neutral Buoyancy Laboratory.^[1][53] Astronauts-in-training may also experience short periods of weightlessness in aircraft called the "vomit comet", the nickname given to a pair of modified KC-135s (retired in 2000 and 2004 respectively, and replaced in 2005 with a C-9) which perform parabolic flights.^[52] Astronauts are also required to accumulate a number of flight hours in high-performance jet aircraft. This is mostly done in T-38 jet aircraft out of Ellington Field, due to its proximity to the Johnson Space Center. Ellington Field is also where the Shuttle Training Aircraft is maintained and developed, although most flights of the aircraft are done out of Edwards Air Force Base.

§NASA candidacy requirements

• Be citizens of the United States.^[52][55]
• Pass a strict physical examination, and have a near and distant visual acuity correctable to 20/20 (6/6). Blood pressure, while sitting, must be no greater than 140 over 90.

§Commander and Pilot

• A bachelor's degree in engineering, biological science, physical science or mathematics is required.
• At least 1,000 hours' flying time as pilot-in-command in jet aircraft. Experience as a test pilot is desirable.
• Height must be 5 ft 2 in to 6 ft 2 in (1.58 to 1.88 m).
• Distant visual acuity must be correctable to 20/20 in each eye.
• The refractive surgical procedures of the eye, PRK (Photorefractive keratectomy) and LASIK, are now allowed, providing at least 1 year has passed since the date of the procedure with no permanent adverse after effects. For those applicants under final consideration, an operative report on the surgical procedure will be requested.

§Mission Specialist

• A bachelor's degree in engineering, biological science, physical science or mathematics, as well as at least three years of related professional experience (graduate work or studies) and an advanced degree, such as a master's degree (one to three years) or a doctoral degree (three years or more).
• Applicant's height must be between 4 ft. 10.5 in. and 6 ft. 4 in.

§Mission Specialist Educator

• Applicants must have a bachelor's degree with teaching experience, including work at the kindergarten through twelfth grade level. An advanced degree, such as a master's degree or a doctoral degree, is not required, but is strongly desired.^[56]

Mission Specialist Educators, or "Educator Astronauts", were first selected in 2004, and as of 2007, there are three NASA Educator astronauts: Joseph M. Acaba, Richard R. Arnold, and Dorothy Metcalf-Lindenburger.^[57][58] Barbara Morgan, selected as back-up teacher to Christa McAuliffe in 1985, is considered to be the first Educator astronaut by the media, but she trained as a mission specialist.^[59] The Educator Astronaut program is a successor to the Teacher in Space program from the 1980s.^[60][61]

§Health risks of space travel

Astronauts are susceptible to a variety of health risks including decompression sickness, barotrauma, immunodeficiencies, loss of bone and muscle, loss of eyesight, orthostatic intolerance, sleep disturbances, and radiation injury.^{[62][63][64][65][66][67][68][69][70]} A variety of large scale medical studies are being conducted in space via the National Space and Biomedical Research Institute (NSBRI) to address these issues. Prominent among these is the Advanced Diagnostic Ultrasound in Microgravity Study in which astronauts (including former ISS commanders Leroy Chiao and Gennady Padalka) perform ultrasound scans under the guidance of remote experts to diagnose and potentially treat hundreds of medical conditions in space. This study's techniques are now being applied to cover professional and Olympic sports injuries as well as ultrasound performed by non-expert operators in medical and high school students. It is anticipated that remote guided ultrasound will have application on Earth in emergency and rural care situations, where access to a trained physician is often rare.^[71][72][73]
On December 31, 2012, a NASA-supported study reported that manned spaceflight may harm the brain and accelerate the onset of Alzheimer's disease.^[74][75][76]

§Food and drink

An astronaut on the International Space Station requires about 0.83 kilograms (1.83 pounds) weight of food inclusive of food packaging per meal each day. (The packaging for each meal weighs around 0.12 kilograms - 0.27 pounds) Longer-duration missions require more food.
Shuttle astronauts worked with nutritionists to select menus that appeal to their individual tastes. Five months before flight, menus are selected and analyzed for nutritional content by the shuttle dietician. Foods are tested to see how they will react in a reduced gravity environment. Caloric requirements are determined using a basal energy expenditure (BEE) formula. On Earth, the average American uses about 35 gallons (132 liters) of water every day. On board the ISS astronauts limit water use to only about three gallons (11 liters) per day.^[77]

§Insignia

In Russia, cosmonauts are awarded Pilot-Cosmonaut of the Russian Federation upon completion of their missions, often accompanied with the award of Hero of the Russian Federation. This follows the practice established in the USSR where cosmonauts were usually awarded the title Hero of the Soviet Union.

At NASA, those who complete astronaut candidate training receive a silver lapel pin. Once they have flown in space, they receive a gold pin. U.S. astronauts who also have active-duty military status receive a special qualification badge, known as the Astronaut Badge, after participation on a spaceflight. The United States Air Force also presents an Astronaut Badge to its pilots who exceed 50 miles (80 km) in altitude.

Space Mirror Memorial

§Deaths

Eighteen astronauts (fourteen men and four women) have lost their lives during four space flights. By nationality, thirteen were American (including one of Indian origin), four were Russian (Soviet Union), and one was Israeli.Eleven people (all men) have lost their lives training for spaceflight: eight Americans and three Russians. Six of these were in crashes of training jet aircraft, one drowned during water recovery training, and four were due to fires in pure oxygen environments.

The Space Mirror Memorial, which stands on the grounds of the John F. Kennedy Space Center Visitor Complex, commemorates the lives of the men and women who have died during spaceflight and during training in the space programs of the United States. In addition to twenty NASA career astronauts, the memorial includes the names of a U.S. Air Force X-15 test pilot, a U.S. Air Force officer who died while training for a then-classified military space program, and a civilian spaceflight participant.

Solar Corona

From Wikipedia, the free encyclopedia

During a total solar eclipse, the solar corona can be seen by the naked eye.

A corona (Latin, 'crown') is an aura of plasma that surrounds the Sun and other celestial bodies. The Sun's corona extends millions of kilometres into space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. The word "corona" is a Latin word meaning "crown", from the Ancient Greek κορώνη (korōnē, “garland, wreath”).

The high temperature of the Sun's corona gives it unusual spectral features, which led some in the 19th century to suggest that it contained a previously unknown element, "coronium". These spectral features have since been traced to highly ionized iron (Fe-XIV). Bengt Edlén, following the work of Grotrian (1939), first identified the coronal lines in 1940 (observed since 1869) as transitions from low-lying metastable levels of the ground configuration of highly ionised metals (the green FeXIV line at 5303 Å, but also the red line FeX at 6374 Å). These high stages of ionisation indicate a plasma temperature in excess of 1,000,000 kelvin.^[1]

Light from the corona comes from three primary sources, which are called by different names although all of them share the same volume of space. The K-corona (K for kontinuierlich, "continuous" in German) is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines completely obscures them, giving the spectral appearance of a continuum with no absorption lines. The F-corona (F for Fraunhofer) is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the zodiacal light. The E-corona (E for emission) is due to spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition.^[2]

Physical features

A drawing demonstrating the configuration of solar magnetic flux during the solar cycle

The sun's corona is much hotter (by a factor from 150 to 450) than the visible surface of the Sun: the photosphere's average temperature is 5800 kelvin compared to the corona's one to three million kelvin. The corona is 10⁻¹² times as dense as the photosphere, and so produces about one-millionth as much visible light. The corona is separated from the photosphere by the relatively shallow chromosphere. The exact mechanism by which the corona is heated is still the subject of some debate, but likely possibilities include induction by the Sun's magnetic field and MHD waves from below. The outer edges of the Sun's corona are constantly being transported away due to open magnetic flux generating the solar wind.

The corona is not always evenly distributed across the surface of the sun. During periods of quiet, the corona is more or less confined to the equatorial regions, with coronal holes covering the polar regions. However during the Sun's active periods, the corona is evenly distributed over the equatorial and polar regions, though it is most prominent in areas with sunspot activity. The solar cycle spans approximately 11 years, from solar minimum to the following minimum. Since the solar magnetic field is continually wound up (due to a differential rotation at the solar equator (the equator rotates quicker than the poles), sunspot activity will be more pronounced at solar maximum where the magnetic field is more twisted. Associated with sunspots are coronal loops, loops of magnetic flux, upwelling from the solar interior. The magnetic flux pushes the hotter photosphere aside, exposing the cooler plasma below, thus creating the dark (when compared to the solar disk) spots.

Since the corona has been photographed at high resolution in the X-rays by the satellite Skylab in 1973, and then later by Yohkoh and the other following space instruments, it has been seen that the structure of the corona is very various and complex: different zones have been immediately classified on the coronal disc.^[3][4][5] The astronomers usually distinguish several regions,^[6] as described below.

Active regions

Active regions are ensembles of loop structures connecting points of opposite magnetic polarity in the photosphere, the so-called coronal loops. They generally distribute in two zones of activity, which are parallel to the solar equator. The average temperature is between two and four million Kelvin, while the density goes from 10⁹ to 10¹⁰ particle per cm³.

Illustration depicting solar prominences and sunspots

Active regions involve all the phenomena directly linked to the magnetic field, which occur at different heights on the Sun's surface:^[6] sunspots and faculae, happening in the photosphere, spicules, Hα filaments and plages in the chromosphere, prominences in the chromosphere and transition region, and flares and coronal mass ejections happening in the corona and chromosphere, but if flares are very violent can perturb also the photosphere and generate a Moreton wave, as described by Uchida. On the contrary, quiescent prominences are large, cool dense structures which are observed as dark, "snake-like" Hα ribbons (filaments) on the solar disc. Their temperature is about 5000–8000 K, and so they are usually considered as chromospheric features.

In 2013, images from the High Resolution Coronal Imager revealed never-before-seen "magnetic braids" of plasma within the outer layers of these active regions.^[7]

Coronal loops

TRACE 171Å coronal loops

Coronal loops are the basic structures of the magnetic solar corona. These loops are the closed-magnetic flux cousins of the open-magnetic flux that can be found in coronal hole (polar) regions and the solar wind. Loops of magnetic flux well up from the solar body and fill with hot solar plasma.^[8] Due to the heightened magnetic activity in these coronal loop regions, coronal loops can often be the precursor to solar flares and coronal mass ejections (CMEs).

Solar plasma feeding these structures is heated from under 6000 K to well over 1×10⁶ K from the photosphere, through the transition region, and into the corona. Often, the solar plasma will fill these loops from one foot point and drain from the other (siphon flow due to a pressure difference,^[9] or asymmetric flow due to some other driver).

When the plasma goes upward from the foot points towards the loop top, as it always occurs during the initial phase of a compact flare, it is defined as chromospheric evaporation. When the plasma rapidly cools falling down towards the photosphere, we have the chromospheric condensation. There may also be symmetric flow from both loop foot points, causing a build-up of mass in the loop structure. The plasma may cool rapidly in this region (for a thermal instability), creating dark filaments in the solar disk or prominences off the limb.

Coronal loops may have lifetimes in the order of seconds (in the case of flare events), minutes, hours or days. Usually coronal loops lasting for long periods of time are known as steady state or quiescent coronal loops, where there is a balance in loop energy sources and sinks (example).

Coronal loops have become very important when trying to understand the current coronal heating problem. Coronal loops are highly radiating sources of plasma and therefore easy to observe by instruments such as TRACE; they are highly observable laboratories to study phenomena such as solar oscillations, wave activity and nanoflares. However, it remains difficult to find a solution to the coronal heating problem as these structures are being observed remotely, where many ambiguities are present (i.e. radiation contributions along the LOS). In-situ measurements are required before a definitive answer can be arrived at, but due to the high plasma temperatures in the corona, in-situ measurements are impossible (at least for the time being). The next mission of the NASA Solar Probe Plus will approach the Sun very closely allowing more direct observations.

Coronal arches connecting regions of opposite magnetic polarity (A) and the unipolar magnetic field in the coronal hole (B)

Large-scale structures

Large-scale structures are very long arcs which can cover over a quarter of the solar disk but contain plasma less dense than in the coronal loops of the active regions.

They were first detected in the June 8, 1968 flare observation during a rocket flight.^[10]

The large-scale structure of the corona changes over the 11-year solar cycle and becomes particularly simple during the minimum period, when the magnetic field of the Sun is almost similar to a dipolar configuration (plus a quadrupolar component).

Interconnections of active regions

The interconnections of active regions are arcs connecting zones of opposite magnetic field, in different active regions. Significant variations of these structures are often seen after a flare.

Some other features of this kind are helmet streamers—large cap-like coronal structures with long pointed peaks that usually overlie sunspots and active regions. Coronal streamers are considered as sources of the slow solar wind.^[11]

Filament cavities

Image taken by the Solar Dynamics Observatory on Oct 16 2010. A very long filament cavity is visible across the Sun's southern hemisphere.

Filament cavities are zones which look dark in the X-rays and are above the regions where Hα filaments are observed in the chromosphere. They were first observed in the two 1970 rocket flights which also detected coronal holes.^[10]

Filament cavities are cooler clouds of gases (plasma) suspended above the Sun's surface by magnetic forces. The regions of intense magnetic field look dark in the images because they are empty of hot plasma. In fact, the sum of the magnetic pressure and plasma pressure must be constant everywhere on the heliosphere in order to have an equilibrium configuration: where the magnetic field is higher, the plasma must be cooler or less dense. The plasma pressure can be calculated by the state equation of a perfect gas , where is the particle number density, the Boltzmann constant and the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect makes sunspots dark in the photosphere.

Bright points

Bright points are small active regions spread over the whole solar disk. X-ray bright points were first detected in April 8, 1969 during a rocket flight.^[10]

The fraction of the solar surface covered by bright points varies with the solar cycle. They are associated with small bipolar regions of the magnetic field. Their average temperature ranges from 1.1 MK to 3.4 MK. The variations in temperature are often correlated with changes in the X-ray emission.^[12]

Coronal holes

Coronal holes are the Polar Regions which look dark in the X-rays since they do not emit much radiation.^[13] These are wide zones of the Sun where the magnetic field is unipolar and opens towards the interplanetary space. The high speed solar wind arises mainly from these regions.
In the UV images of the coronal holes, some small structures, similar to elongated bubbles, are often seen as they were suspended in the solar wind. These are the coronal plumes. More exactly, they are long thin streamers that project outward from the Sun's north and south poles.^[14]

The quiet Sun

The solar regions which are not part of active regions and coronal holes are commonly identified as the quiet Sun.
The equatorial region has a faster velocity rotation than the polar zones. The result of the Sun's differential rotation is that the active regions always arise in two bands parallel to the equator and their extension increases during the periods of maximum of the solar cycle, while they almost disappear during each minimum. Therefore the quiet Sun always coincides with the equatorial zone and its surface is lower during the maximum of the solar cycle.
Approaching the minimum of the solar cycle (also named butterfly cycle), the extension of the quiet Sun increases until it covers the whole disk surface excluding some bright points on the hemisphere and the poles, where there are the coronal holes.

Variability of the corona

A portrait as diversified as the one already pointed out for the coronal features is emphasized by the analysis of the dynamics of the main structures of the corona, which evolve in times very different among them. Studying the coronal variability in its complexity is not easy because the times of evolution of the different structures can vary considerably: from seconds to several months. The typical sizes of the regions where coronal events take place vary in the same way, as it is shown in the following table.

Coronal event	Typical time-scale	Typical length-scale (Mm)
Active region flare	10 to 10,000 seconds	10–100
X-ray bright point	minutes	1–10
Transient in large-scale structures	from minutes to hours	~100
Transient in interconnecting arcs	from minutes to hours	~100
Quiet Sun	from hours to months	100–1,000
Coronal hole	several rotations	100–1,000

Flares

On August 31, 2012 a long filament of solar material that had been hovering in the Sun's outer atmosphere, the corona, erupted out into space at 4:36 p.m. EDT

Flares take place in active regions and provoke a sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they interest several zones of the solar atmosphere and involve many physical effects, thermal and not thermal, and sometimes wide reconnections of the magnetic field lines with material expulsion.

Flares are impulsive phenomena, of average duration of 15 minutes, even if the most energetic events can last several hours. Flares involve a high and rapid increase of the density and temperature.

An emission in white light is only seldom observed: usually, flares are only seen at EUV wavelengths and in the X-rays, typical of the chromospheric and coronal emission.

In the corona the morphology of flares, which can be grasped from the observations in the soft and hard X-rays, at the UV wavelengths and in Hα, is very complex. However, two kinds of basic structures can be distinguished: ^[15]

• Compact flares, when each of the two arches where the event is happening maintains its morphology: only an increase of the emission is observed without significant structural variations. The emitted energy is of the order of 10²² – 10²³ J.
• Flares of long duration, associated to eruptions of prominences, transients in white light and two-ribbon flares:^[16] in this case the magnetic loops change their configuration during the event. The energies emitted during these flares of such large proportions can reach 10²⁵ J.

Filament erupting during a solar flare, seen at EUV wavelengths (TRACE)

As for temporal dynamics, three different phases are generally distinguished, whose duration are not comparable. These times, moreover, can depend on the range of wavelengths used to observe the event even considerably:

• An initial impulsive phase, whose duration is of the order of minutes, strong emissions of energy are often observed even in the microwaves, at EUV wavelengths and in the hard X-rays.
• A maximum phase
• A decay phase, which can last several hours.

Sometimes also a phase preceding the flare can be observed, usually called as "pre-flare" phase.

Transients

Accompanying solar flares or large solar prominences, "coronal transients" (also called coronal mass ejections) are sometimes released. These are enormous loops of coronal material traveling outward from the Sun at over a million kilometers per hour, containing roughly 10 times the energy of the solar flare or prominence that accompanies them. Some larger ejections can propel hundreds of millions of tons of material into space at roughly 1.5 million kilometers an hour.

A solar storm

These movies have been taken by the satellite SOHO during two weeks in October and November 2003. The images have been taken at the same time by the different instruments on board SOHO: the MDI, producing magnetograms, the Extreme ultraviolet Imaging Telescope (EIT), which photographs the corona in the ultraviolets, and the Large Angle and Spectrometric Coronagraph (LASCO).

The first video at the top on the left (in grey) shows the magnetograms as they vary in time. At the top on the right (in yellow) the photosphere can be seen in white light as taken by the MDI.

Furthermore the EIT filmed the event in its four filters which are sensitive to different wavelengths, selecting plasma at different temperatures. The images in orange (on the left) refers to chromospheric plasma, while that one in green (on the right) to the corona.

In the last movie at the centre the Sun's images taken in the ultraviolet filter by the EIT have been combined with those taken by the coronograph LASCO blue and white in this movie.

All the instruments registered the storm which is considered as one of the largest solar activity events observed by SOHO and maybe since the advent of space-based solar observations. The storm involved all the plasma of the solar atmosphere from the chromosphere to the corona, as can be seen from the movies, which are ordered from left to right, from top to bottom, in the outward direction of the increasing temperature on the Sun: photosphere (yellow), chromosphere-transition region (orange), low corona (green) and extended corona (blue).

The corona is visible to the SOHO/LASCO coronagraph instruments, which block the bright disk of the Sun so the significantly fainter corona can be seen. In this movie, the inner coronagraph (designated C2) is combined with the outer coronagraph (C3).

As the movie plays, we can observe a number of features of the active Sun. Long streamers radiate outward from the Sun and wave gently due to their interaction with the solar wind. The bright white regions are visible due to their high density of free electrons which scatter the light from the photosphere towards the observer. Protons and other ionized atoms are there as well, but are not as visible since they do not interact with photons as strongly as electrons. Coronal Mass Ejections (CMEs) are occasionally observed launching from the Sun. Some of these launch particle events can saturate the cameras with snow-like artifacts.

Also visible in the coronagraphs are stars and planets. Stars are seen to drift slowly to the right, carried by the relative motion of the Sun and the Earth. The planet Mercury is visible as the bright point moving left of the Sun.

The horizontal "extension" in the image is called blooming and is due to charge leakage during readout of saturated pixels in the camera's CCD imager.

Stellar coronae

Coronal stars are ubiquitous among the stars in the cool half of the Hertzsprung–Russell diagram.^[17] These coronae can be detected using X-ray telescopes. Some stellar coronae, particularly in young stars, are much more luminous than the Sun's. For example, FK Comae Berenices is the prototype for the FK Com class of variable star. These are giants of spectral types G and K with an unusually rapid rotation and signs of extreme activity. Their X-ray coronae are among the most luminous (L_x ≥ 10³² erg·s⁻¹ or 10²⁵W) and the hottest known with dominant temperatures up to 40 MK.^[17]

The astronomical observations planned with the Einstein Observatory by Giuseppe Vaiana and his group^[18] showed that F-, G-, K- and M-stars have chromospheres and often coronae much like our Sun. The O-B stars, which do not have surface convection zones, have a strong X-ray emission. However these stars do not have coronas, but the outer stellar envelopes emit this radiation during shocks due to thermal instabilities in rapidly moving gas blobs. Also A-stars do not have convection zones but they do not emit at the UV and X-ray wavelengths. Thus they appear to have neither chromospheres nor coronae.

Physics of the corona

Taken by Hinode on Jan 12 2007 this image reveals the filamentary nature of the corona.

The matter in the external part of the solar atmosphere is in the state of plasma, at very high temperature (a few million Kelvins) and at very low density (of the order of 10¹⁵ particle/m³). According to the definition of plasma, it is a quasi-neutral ensemble of particles which exhibits a collective behaviour.

The composition is the same as the one in the Sun's interior, mainly hydrogen, but completely ionized, thence protons and electrons, and a small fraction of the other atoms in the same percentages as they are present in the photosphere. Even heavier metals, such as iron, are partially ionized and have lost most of the external electrons. The ionization state of a chemical element depends strictly on the temperature and is regulated by the Saha equation. Historically, the presence of the spectral lines emitted from highly ionized states of iron allowed determination of the high temperature of the coronal plasma, revealing that the corona is much hotter than the internal layers of the chromosphere.

The corona behaves like a gas which is very hot but very light at the same time: the pressure in the photosphere is usually only 0.1 to 0.6 Pa in active regions, while on the Earth the atmospheric pressure is about 100 kPa, approximatively a million times higher than on the solar surface. However it is not properly a gas, because it is made of charged particles, basically protons and electrons, moving at different velocities. Supposing that they have the same kinetic energy on average (for the equipartition theorem), electrons have a mass roughly 1800 times smaller than protons, therefore they acquire more velocity. Metal ions are always slower. This fact has relevant physical consequences either on radiative processes (that are very different from the photospheric radiative processes), or on thermal conduction. Furthermore the presence of electric charges induces the generation of electric currents and high magnetic fields. Magnetohydrodynamic waves (MHD waves) can also propagate in this plasma,^[19] even if it is not still clear how they can be transmitted or generated in the corona.

Radiation

The corona emits radiation mainly in the X-rays, observable only from space.
The plasma is transparent to its own radiation and to that one coming from below, therefore we say that it is optically-thin. The gas, in fact, is very rarefied and the photon mean free-path overcomes by far all the other length-scales, including the typical sizes of the coronal features.

Different processes of radiation take place in the emission, due to binary collisions between plasma particles, while the interactions with the photons, coming from below; are very rare. Because the emission is due to collisions between ions and electrons, the energy emitted from a unit volume in the time unit is proportional to the squared number of particles in a unit volume, or more exactly, to the product of the electron density and proton density.^[20]

Thermal conduction

A mosaic of the extreme ultraviolet images taken from STEREO on December 4, 2006. These false color images show the Sun's atmospheres at a range of different temperatures. Clockwise from top left: 1 million degrees C (171 Å—blue), 1.5 million °C (195 Å—green), 60,000–80,000 °C (304 Å—red), and 2.5 million °C (286 Å—yellow).

STEREO – First images as a slow animation

In the corona thermal conduction occurs from the external hotter atmosphere towards the inner cooler layers. Responsible for the diffusion process of the heat are the electrons, which are much lighter than ions and move faster, as explained above.

When there is a magnetic field the thermal conductivity of the plasma becomes higher in the direction which is parallel to the field lines rather than in the perpendicular direction.^[21] A charged particle moving in the direction perpendicular to the magnetic field line is subject to the Lorentz force which is normal to the plane individuated by the velocity and the magnetic field. This force bends the path of the particle. In general, since particles also have a velocity component along the magnetic field line, the Lorentz force constrains them to bend and move along spirals around the field lines at the cyclotron frequency.

If collisions between the particles are very frequent, they are scattered in every direction. This happens in the photosphere, where the plasma carries the magnetic field in its motion. In the corona, on the contrary, the mean free-path of the electrons is of the order of kilometres and even more, so each electron can do a helicoidal motion long before being scattered after a collision. Therefore the heat transfer is enhanced along the magnetic field lines and inhibited in the perpendicular direction.

In the direction longitudinal to the magnetic field, the thermal conductivity of the corona is^[21]

where is the Boltzmann constant, is the temperature in Kelvin, the electron mass, the electric charge of the electron,

the Coulomb logarithm, and

the Debye length of the plasma with particle density . The Coulomb logarithm is roughly 20 in the corona, with a mean temperature of 1 MK and a density of 10¹⁵ particles/m³, and about 10 in the chromosphere, where the temperature is approximately 10kK and the particle density is of the order of 10¹⁸ particles/m³, and in practice it can be assumed constant.

Thence, if we indicate with the heat for a volume unit, expressed in J m⁻³, the Fourier equation of heat transfer, to be computed only along the direction of the field line, becomes
.
Numerical calculations have shown that the thermal conductivity of the corona is comparable to that of copper.

Coronal seismology

Coronal seismology is a new way of studying the plasma of the solar corona with the use of magnetohydrodynamic (MHD) waves. Magnetohydrodynamics studies the dynamics of electrically conducting fluids—in this case the fluid is the coronal plasma. Philosophically, coronal seismology is similar to the Earth's seismology, the Sun's helioseismology, and MHD spectroscopy of laboratory plasma devices. In all these approaches, waves of various kinds are used to probe a medium. The potential of coronal seismology in the estimation of the coronal magnetic field, density scale height, fine structure and heating has been demonstrated by different research groups.

Coronal heating problem

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A new visualisation technique can provide clues to the coronal heating problem.

The coronal heating problem in solar physics relates to the question of why the temperature of the Sun's corona is millions of kelvin higher than that of the surface. The high temperatures require energy to be carried from the solar interior to the corona by non-thermal processes, because the second law of thermodynamics prevents heat from flowing directly from the solar photosphere, or surface, at about 5800 K, to the much hotter corona at about 1 to 3 MK (parts of the corona can even reach 10 MK).

The thin region of temperature increase from the chromosphere to the corona is known as the transition region and can range from tens to hundreds of kilometers thick. An analogy of this would be a light bulb heating the air surrounding it hotter than its glass surface. The second law of thermodynamics would be broken.

The amount of power required to heat the solar corona can easily be calculated as the difference between coronal radiative losses and heating by thermal conduction toward the chromosphere through the transition region. It is about 1 kilowatt for every square meter of surface area on the Sun, or 1/40000 of the amount of light energy that escapes the Sun.

Many coronal heating theories have been proposed,^[22] but two theories have remained as the most likely candidates: wave heating and magnetic reconnection (or nanoflares).^[23] Through most of the past 50 years, neither theory has been able to account for the extreme coronal temperatures.

The NASA mission Solar Probe + is intended to approach the sun to a distance of approximately 9.5 solar radii to investigate coronal heating and the origin of the solar wind.

In 2012, high resolution (<0.2″) soft X-ray imaging with the High Resolution Coronal Imager aboard a sounding rocket revealed tightly wound braids in the corona. The authors hypothesized that the reconnection and unravelling of braids can act as primary sources of heating of the active solar corona to temperatures of up to 4 million kelvin. The main heat source in the quiescent corona (about 1.5 million kelvin) in this case is supposed to be MHD waves.^[24]

Axions may hold the key to the Solar Corona heating problem.^[25]

Competing heating mechanisms

Heating Models
Hydrodynamic	Magnetic
No magnetic field Slow rotating stars	DC (reconnection)	AC (waves)
	B-field stresses Reconnection events Flares-nanoflares Uniform heating rates	Photospheric foot point shuffling MHD wave propagation High Alfvén wave flux Non-uniform heating rates
	Competing theories

Wave heating theory

The wave heating theory, proposed in 1949 by Evry Schatzman, proposes that waves carry energy from the solar interior to the solar chromosphere and corona. The Sun is made of plasma rather than ordinary gas, so it supports several types of waves analogous to sound waves in air. The most important types of wave are magneto-acoustic waves and Alfvén waves.^[26] Magneto-acoustic waves are sound waves that have been modified by the presence of a magnetic field, and Alfvén waves are similar to ULF radio waves that have been modified by interaction with matter in the plasma. Both types of waves can be launched by the turbulence of granulation and super granulation at the solar photosphere, and both types of waves can carry energy for some distance through the solar atmosphere before turning into shock waves that dissipate their energy as heat.

One problem with wave heating is delivery of the heat to the appropriate place. Magneto-acoustic waves cannot carry sufficient energy upward through the chromosphere to the corona, both because of the low pressure present in the chromosphere and because they tend to be reflected back to the photosphere. Alfvén waves can carry enough energy, but do not dissipate that energy rapidly enough once they enter the corona. Waves in plasmas are notoriously difficult to understand and describe analytically, but computer simulations, carried out by Thomas Bogdan and colleagues in 2003, seem to show that Alfvén waves can transmute into other wave modes at the base of the corona, providing a pathway that can carry large amounts of energy from the photosphere into the corona and then dissipate it as heat.

Another problem with wave heating has been the complete absence, until the late 1990s, of any direct evidence of waves propagating through the solar corona. The first direct observation of waves propagating into and through the solar corona was made in 1997 with the SOHO space-borne solar observatory, the first platform capable of observing the Sun in the extreme ultraviolet (EUV) for long periods of time with stable photometry. Those were magneto-acoustic waves with a frequency of about 1 millihertz (mHz, corresponding to a 1,000 second wave period), that carry only about 10% of the energy required to heat the corona. Many observations exist of localized wave phenomena, such as Alfvén waves launched by solar flares, but those events are transient and cannot explain the uniform coronal heat.

It is not yet known exactly how much wave energy is available to heat the corona. Results published in 2004 using data from the TRACE spacecraft seem to indicate that there are waves in the solar atmosphere at frequencies as high as 100 mHz (10 second period). Measurements of the temperature of different ions in the solar wind with the UVCS instrument aboard SOHO give strong indirect evidence that there are waves at frequencies as high as 200 Hz, well into the range of human hearing. These waves are very difficult to detect under normal circumstances, but evidence collected during solar eclipses by teams from Williams College suggest the presences of such waves in the 1–10 Hz range.

Recently, Alfvénic motions have been found in the lower solar atmosphere ^{[27] [28]} and also in the quiet Sun, in coronal holes and in active regions using observations with AIA on board the Solar Dynamics Observatory.^[29] These Alfvénic oscillations have significant power, and seem to be connected to the chromospheric Alfvénic oscillations previously reported with the Hinode spacecraft .^[30]

Solar wind observations with the WIND (spacecraft) have recently shown evidence to support theories of Alfvén-cyclotron dissipation, leading to local ion heating.^[31]

Magnetic reconnection theory

Arcing active region by Solar Dynamics Observatory

The magnetic reconnection theory relies on the solar magnetic field to induce electric currents in the solar corona.^[32] The currents then collapse suddenly, releasing energy as heat and wave energy in the corona. This process is called "reconnection" because of the peculiar way that magnetic fields behave in plasma (or any electrically conductive fluid such as mercury or seawater). In a plasma, magnetic field lines are normally tied to individual pieces of matter, so that the topology of the magnetic field remains the same: if a particular north and south magnetic pole are connected by a single field line, then even if the plasma is stirred or if the magnets are moved around, that field line will continue to connect those particular poles. The connection is maintained by electric currents that are induced in the plasma. Under certain conditions, the electric currents can collapse, allowing the magnetic field to "reconnect" to other magnetic poles and release heat and wave energy in the process.

Magnetic reconnection is hypothesized to be the mechanism behind solar flares, the largest explosions in our solar system. Furthermore, the surface of the Sun is covered with millions of small magnetized regions 50–1,000 km across. These small magnetic poles are buffeted and churned by the constant granulation. The magnetic field in the solar corona must undergo nearly constant reconnection to match the motion of this "magnetic carpet", so the energy released by the reconnection is a natural candidate for the coronal heat, perhaps as a series of "microflares" that individually provide very little energy but together account for the required energy.

The idea that nanoflares might heat the corona was proposed by Eugene Parker in the 1980s but is still controversial. In particular, ultraviolet telescopes such as TRACE and SOHO/EIT can observe individual micro-flares as small brightenings in extreme ultraviolet light,^[33] but there seem to be too few of these small events to account for the energy released into the corona. The additional energy not accounted for could be made up by wave energy, or by gradual magnetic reconnection that releases energy more smoothly than micro-flares and therefore doesn't appear well in the TRACE data. Variations on the micro-flare hypothesis use other mechanisms to stress the magnetic field or to release the energy, and are a subject of active research in 2005.

Spicules (type II)

For decades, researchers believed spicules could send heat into the corona. However, following observational research in the 1980s, it was found that spicule plasma did not reach coronal temperatures, and so the theory was discounted.

As per studies performed in 2010 at the National Center for Atmospheric Research in Colorado, in collaboration with the Lockheed Martin's Solar and Astrophysics Laboratory (LMSAL) and the Institute of Theoretical Astrophysics of the University of Oslo, a new class of spicules (TYPE II) discovered in 2007, which travel faster (up to 100 km/s) and have shorter lifespans, can account for the problem.^[34] These jets insert heated plasma into the Sun's outer atmosphere. Thus, a much greater understanding of the Corona and improvement in the knowledge of the Sun's subtle influence on the Earth's upper atmosphere can be expected henceforth. The Atmospheric Imaging Assembly on NASA's recently launched Solar Dynamics Observatory and NASA's Focal Plane Package for the Solar Optical Telescope on the Japanese Hinode satellite which was used to test this hypothesis. The high spatial and temporal resolutions of the newer instruments reveal this coronal mass supply.

These observations reveal a one-to-one connection between plasma that is heated to millions of degrees and the spicules that insert this plasma into the corona.^[35]