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Biomechanics
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For other uses, see Biomechanical (disambiguation).
Page of one of the first works of Biomechanics (De Motu Animalium of Giovanni Alfonso Borelli) in the 17th century
Biomechanics is the study of the structure and function of biological systems such as humans, animals, plants, organs, fungi, and cells[1] by means of the methods of mechanics.[2]
Contents
[hide]
1 Word history
2 Method
3 Subfields
3.1 Sports biomechanics
3.2 Continuum biomechanics
3.3 Biofluid mechanics
3.4 Biotribology
3.5 Comparative biomechanics
3.6 Plant biomechanics
3.7 Computational biomechanics
4 History
4.1 Antiquity
4.2 Renaissance
4.3 Industrial era
5 Applications
6 See also
7 References
8 Further reading
9 External links
Word history[edit]
The word "biomechanics" (1899) and the related "biomechanical" (1856) come from the Ancient Greek βίος bios "life" and μηχανική, mēchanikē "mechanics", to refer to the study of the mechanical principles of living organisms, particularly their movement and structure.[3]
Method[edit]
Biomechanics is closely related to engineering, because it often uses traditional engineering sciences to analyze biological systems. Some simple applications of Newtonian mechanics and/or materials sciences can supply correct approximations to the mechanics of many biological systems. Applied mechanics, most notably mechanical engineering disciplines such as continuum mechanics, mechanism analysis, structural analysis, kinematics and dynamics play prominent roles in the study of biomechanics.
Usually biological systems are much more complex than man-built systems. Numerical methods are hence applied in almost every biomechanical study. Research is done in an iterative process of hypothesis and verification, including several steps of modeling, computer simulation and experimental measurements.
Subfields[edit]
Applied subfields of biomechanics include:
Soft body dynamics
Kinesiology (kinetics + physiology)
Animal locomotion & Gait analysis
Musculoskeletal & orthopedic biomechanics
Cardiovascular biomechanics
Ergonomy
Human factors engineering & occupational biomechanics
Implant (medicine), Orthotics & Prosthesis
Rehabilitation
Sports biomechanics
Allometry
Injury biomechanics
Biotribology
Biofluid mechanics
Comparative biomechanics
Computational biomechanics
Sports biomechanics[edit]
Main article: Sports biomechanics
In sports biomechanics, the laws of mechanics are applied to human movement in order to gain a greater understanding of athletic performance and to reduce sport injuries as well. It focuses on the application of the scientific principles of mechanical physics to understand movements of action of human bodies and sports implements such as cricket bat, hockey stick and javelin etc. Elements of mechanical engineering (e.g., strain gauges), electrical engineering (e.g., digital filtering), computer science (e.g., numerical methods), gait analysis (e.g., force platforms), and clinical neurophysiology (e.g., surface EMG) are common methods used in sports biomechanics.[4]
Biomechanics in sports can be stated as the muscular, joint and skeletal actions of the body during the execution of a given task, skill and/or technique. Proper understanding of biomechanics relating to sports skill has the greatest implications on: sport's performance, rehabilitation and injury prevention, along with sport mastery. As noted by Doctor Michael Yessis, one could say that best athlete is the one that executes his or her skill the best.[5]
Continuum biomechanics[edit]
The mechanical analysis of biomaterials and biofluids is usually carried forth with the concepts of continuum mechanics. This assumption breaks down when the length scales of interest approach the order of the micro structural details of the material. One of the most remarkable characteristic of biomaterials is their hierarchical structure. In other words, the mechanical characteristics of these materials rely on physical phenomena occurring in multiple levels, from the molecular all the way up to the tissue and organ levels.
Biomaterials are classified in two groups, hard and soft tissues. Mechanical deformation of hard tissues (like wood, shell and bone) may be analysed with the theory of linear elasticity. On the other hand, soft tissues (like skin, tendon, muscle and cartilage) usually undergo large deformations and thus their analysis rely on the finite strain theory and computer simulations. The interest in continuum biomechanics is spurred by the need for realism in the development of medical simulation.[6]:568
Biofluid mechanics[edit]
Red blood cells
Biological fluid mechanics, or biofluid mechanics, is the study of both gas and liquid fluid flows in or around biological organisms. An often studied liquid biofluids problem is that of blood flow in the human cardiovascular system. Under certain mathematical circumstances, blood flow can be modelled by the Navier–Stokes equations. In vivo whole blood is assumed to be an incompressible Newtonian fluid. However, this assumption fails when considering forward flow within arterioles. At the microscopic scale, the effects of individual red blood cells become significant, and whole blood can no longer be modelled as a continuum. When the diameter of the blood vessel is just slightly larger than the diameter of the red blood cell the Fahraeus–Lindquist effect occurs and there is a decrease in wall shear stress. However, as the diameter of the blood vessel decreases further, the red blood cells have to squeeze through the vessel and often can only pass in single file. In this case, the inverse Fahraeus–Lindquist effect occurs and the wall shear stress increases.
An example of a gaseous biofluids problem is that of human respiration. Recently, respiratory systems in insects have been studied for bioinspiration for designing improved microfluidic devices.[7]
Biotribology[edit]
The main aspects of Contact mechanics and tribology are related to friction, wear and lubrication. When the two surfaces come in contact during motion i.e. rub against each other, friction, wear and lubrication effects are very important to analyze in order to determine the performance of the material. Biotribology is a study of friction, wear and lubrication of biological systems especially human joints such as hips and knees.[8] For example, femoral and tibial components of knee implant routinely rub against each other during daily activity such as walking or stair climbing. If the performance of tibial component needs to be analyzed, the principles of biotribology are used to determine the wear performance of the implant and lubrication effects of synovial fluid. In addition, the theory of contact mechanics also becomes very important for wear analysis. Additional aspects of biotribology can also include analysis of subsurface damage resulting from two surfaces coming in contact during motion, i.e. rubbing against each other, such as in the evaluation of tissue engineered cartilage.[9]
Comparative biomechanics[edit]
Chinstrap penguin leaping over water
Comparative biomechanics is the application of biomechanics to non-human organisms, whether used to gain greater insights into humans (as in physical anthropology) or into the functions, ecology and adaptations of the organisms themselves. Common areas of investigation are Animal locomotion and feeding, as these have strong connections to the organism's fitness and impose high mechanical demands. Animal locomotion, has many manifestations, including running, jumping and flying. Locomotion requires energy to overcome friction, drag, inertia, and gravity, though which factor predominates varies with environment.[citation needed]
Comparative biomechanics overlaps strongly with many other fields, including ecology, neurobiology, developmental biology, ethology, and paleontology, to the extent of commonly publishing papers in the journals of these other fields. Comparative biomechanics is often applied in medicine (with regards to common model organisms such as mice and rats) as well as in biomimetics, which looks to nature for solutions to engineering problems.
Plant biomechanics[edit]
The application of biomechanical principles to plants and plant organs has developed into the subfield of plant biomechanics.[10]
Computational biomechanics[edit]
Over the past decade the Finite element method has become an established alternative to in vivo surgical assessment. The main advantage of Computational Biomechanics lies in its ability to determine the endo-anatomical response of an anatomy, without being subject to ethical restrictions.[11] This has led FE modelling to the point of becoming ubiquitous in several fields of Biomechanics while several projects have even adopted an open source philosophy (e.g. BioSpine).
History[edit]
Antiquity[edit]
Aristotle wrote the first book on the motion of animals, De Motu Animalium, or On the Movement of Animals.[12] He not only saw animals' bodies as mechanical systems, but pursued questions such as the physiological difference between imagining performing an action and actually doing it.[13] In another work, On the Parts of Animals, he provided an accurate description of how the ureter uses peristalsis to carry urine from the kidneys to the bladder.[6]:2
Renaissance[edit]
Leonardo da Vinci studied anatomy in the context of mechanics. He analyzed muscle forces as acting along lines connecting origins and insertions, and studied joint function. Da Vinci tended to mimic some animal features in his machines. For example, he studied the flight of birds to find means by which humans could fly; and because horses were the principal source of mechanical power in that time, he studied their muscular systems to design machines that would better benefit from the forces applied by this animal.[14]
Galileo Galilei was interested in the strength of bones and suggested that bones are hollow because this affords maximum strength with minimum weight. He noted that animals' bone masses increased disproportionately to their size. Consequently, bones must also increase disproportionately in girth rather than mere size. This is because the bending strength of a tubular structure (such as a bone) is much more efficient relative to its weight. Mason suggests that this insight was one of the first grasps of the principles of biological optimization.[14]
In the 16th century, Descartes suggested a philosophic system whereby all living systems, including the human body (but not the soul), are simply machines ruled by the same mechanical laws, an idea that did much to promote and sustain biomechanical study. Giovanni Alfonso Borelli embraced this idea and studied walking, running, jumping, the flight of birds, the swimming of fish, and even the piston action of the heart within a mechanical framework. He could determine the position of the human center of gravity, calculate and measured inspired and expired air volumes, and showed that inspiration is muscle-driven and expiration is due to tissue elasticity. Borelli was the first to understand that the levers of the musculoskeletal system magnify motion rather than force, so that muscles must produce much larger forces than those resisting the motion. Influenced by the work of Galileo, whom he personally knew, he had an intuitive understanding of static equilibrium in various joints of the human body well before Newton published the laws of motion.[15]
Industrial era[edit]
In the 19th century Étienne-Jules Marey used cinematography to scientifically investigate locomotion. He opened the field of modern 'motion analysis' by being the first to correlate ground reaction forces with movement. In Germany, the brothers Ernst Heinrich Weber and Wilhelm Eduard Weber hypothesized a great deal about human gait, but it was Christian Wilhelm Braune who significantly advanced the science using recent advances in engineering mechanics. During the same period, the engineering mechanics of materials began to flourish in France and Germany under the demands of the industrial revolution. This led to the rebirth of bone biomechanics when the railroad engineer Karl Culmann and the anatomist Hermann von Meyer compared the stress patterns in a human femur with those in a similarly shaped crane. Inspired by this finding Julius Wolff proposed the famous Wolff's law of bone remodeling.[16]
Applications[edit]
The study of biomechanics ranges from the inner workings of a cell to the movement and development of limbs, to the mechanical properties of soft tissue,[17] and bones. Some simple examples of biomechanics research include the investigation of the forces that act on limbs, the aerodynamics of bird and insect flight, the hydrodynamics of swimming in fish, and locomotion in general across all forms of life, from individual cells to whole organisms. The biomechanics of human beings is a core part of kinesiology. As we develop a greater understanding of the physiological behavior of living tissues, researchers are able to advance the field of tissue engineering, as well as develop improved treatments for a wide array of pathologies.
Biomechanics is also applied to studying human musculoskeletal systems. Such research utilizes force platforms to study human ground reaction forces and infrared videography to capture the trajectories of markers attached to the human body to study human 3D motion. Research also applies electromyography[18] (EMG) system to study the muscle activation. By this, it is feasible to investigate the muscle responses to the external forces as well as perturbations.
Biomechanics is widely used in orthopedic industry to design orthopedic implants for human joints, dental parts, external fixations and other medical purposes. Biotribology is a very important part of it. It is a study of the performance and function of biomaterials used for orthopedic implants. It plays a vital role to improve the design and produce successful biomaterials for medical and clinical purposes. One such example is in tissue engineered cartilage.[19]
See also[edit]
Biomedical engineering
Biomechatronics
Cardiovascular System Dynamics Society
Evolutionary physiology
International Society of Biomechanics
List of biofluid mechanics research groups
Mechanics of sex
Simtk-opensim
References[edit]
Jump up ^ R. McNeill Alexander (2005) Mechanics of animal movement, Current Biology Volume 15, Issue 16, 23 August 2005, Pages R616-R619. doi:10.1016/j.cub.2005.08.016
Jump up ^ Hatze, Herbert (1974). "The meaning of the term biomechanics". Journal of Biomechanics. 7 (12): 189–190. doi:10.1016/0021-9290(74)90060-8.
Jump up ^ Oxford English Dictionary, Third Edition, November 2010, s.vv.
Jump up ^ Bartlett, Roger (1997). Introduction to sports biomechanics (1 ed.). New York, NY: Routledge. p. 304. ISBN 0-419-20840-2.
Jump up ^ Michael Yessis (2008). Secrets of Russian Sports Fitness & Training. ISBN 978-0-9817180-2-6.
^ Jump up to: a b Fung 1993
Jump up ^ Aboelkassem, Yasser (2013). "Selective pumping in a network: insect-style microscale flow transport". Bioinspiration & Biomimetics. 8 (2): 026004. Bibcode:2013BiBi....8b6004A. doi:10.1088/1748-3182/8/2/026004.
Jump up ^ Davim, J. Paulo (2013). Biotribology. John Wiley & Sons. ISBN 978-1-118-61705-2.
Jump up ^ Whitney, G. A., Jayaraman, K., Dennis, J. E. and Mansour, J. M. (2014), Scaffold-free cartilage subjected to frictional shear stress demonstrates damage by cracking and surface peeling. J Tissue Eng Regen Med. doi: 10.1002/term.1925
Jump up ^ Niklas, Karl J. (1992). Plant Biomechanics: An Engineering Approach to Plant Form and Function (1 ed.). New York, NY: University Of Chicago Press. p. 622. ISBN 0-226-58631-6.
Jump up ^ Tsouknidas, A., Savvakis, S., Asaniotis, Y., Anagnostidis, K., Lontos, A., Michailidis, N. (2013) The effect of kyphoplasty parameters on the dynamic load transfer within the lumbar spine considering the response of a bio-realistic spine segment. Clinical Biomechanics 28 (9-10), pp. 949-955.
Jump up ^ Abernethy, Bruce; Vaughan Kippers; Stephanie J. Hanrahan; Marcus G. Pandy; Alison M. McManus; Laurel MacKinnon. Biophysical foundations of human movement (3rd ed.). Champaign, IL: Human Kinetics. p. 84. ISBN 978-1-4504-3165-1.
Jump up ^ Martin, R. Bruce (23 October 1999). "A genealogy of biomechanics". Presidential Lecture presented at the 23rd Annual Conference of the American Society of Biomechanics University of Pittsburgh, Pittsburgh PA. Archived from the original on 8 August 2013. Retrieved 2 January 2014.
^ Jump up to: a b Mason, Stephen (1962). A History of the Sciences. New York, NY: Collier Books. p. 550.
Jump up ^ Humphrey, Jay D. (2003). The Royal Society, ed. "Continuum biomechanics of soft biological tissues" (PDF). Proceedings of the Royal Society of London A. 459 (2029): 3–46. Bibcode:2003RSPSA.459....3H. doi:10.1098/rspa.2002.1060.
Jump up ^ R. Bruce Martin (23 October 1999). "A Genealogy of Biomechanics". 23rd Annual Conference of the American Society of Biomechanics. Archived from the original on 17 September 2010. Retrieved 13 October 2010.
Jump up ^ Whitney, G. A., Jayaraman, K., Dennis, J. E. and Mansour, J. M. (2014), Scaffold-free cartilage subjected to frictional shear stress demonstrates damage by cracking and surface peeling. J Tissue Eng Regen Med. doi: 10.1002/term.1925
Jump up ^ Basmajian, J.V, & DeLuca, C.J. (1985) Muscles Alive: Their Functions Revealed, Fifth edition. Williams & Wilkins Publ.
Jump up ^ Whitney, G. A., Jayaraman, K., Dennis, J. E. and Mansour, J. M. (2014), Scaffold-free cartilage subjected to frictional shear stress demonstrates damage by cracking and surface peeling. J Tissue Eng Regen Med. doi: 10.1002/term.1925
Further reading[edit]
Cowin, Stephen C., ed. (2008). Bone mechanics handbook (2nd ed.). New York: Informa Healthcare. ISBN 0-8493-9117-2.
Fischer-Cripps, Anthony C. (2007). Introduction to contact mechanics (2nd ed.). New York: Springer. ISBN 0-387-68187-6.
Fung, Y.-C. (1993). Biomechanics: Mechanical Properties of Living Tissues. New York: Springer-Verlag. ISBN 0-387-97947-6.
Gurtin, Morton E. (1995). An introduction to continuum mechanics (6 ed.). San Diego: Acad. Press. ISBN 978-0-12-309750-7.
Humphrey, Jay D. (2002). Cardiovascular solid mechanics : cells, tissues, and organs. New York: Springer. ISBN 0-387-95168-7.
Mazumdar, Jagan N. (1993). Biofluids mechanics (Reprint 1998. ed.). Singapore: World Scientific. ISBN 981-02-0927-4.
Mow, Van C.; Huiskes, Rik, eds. (2005). Basic orthopaedic biomechanics & mechano-biology (3 ed.). Philadelphia: Lippincott, Williams & Wilkins. p. 2. ISBN 978-0-7817-3933-7.
Peterson, Donald R.; Bronzino, Joseph D., eds. (2008). Biomechanics : principles and applications (2. rev. ed.). Boca Raton: CRC Press. ISBN 0-8493-8534-2.
Temenoff, J.S.; Mikos, A.G. (2008). Biomaterials : the Intersection of biology and materials science (Internat. ed.). Upper Saddle River, N.J.: Pearson/Prentice Hall. ISBN 978-0-13-009710-1.
Totten, George E.; Liang, Hong, eds. (2004). Mechanical tribology : materials, characterization, and applications. New York: Marcel Dekker. ISBN 978-0-8247-4873-9.
Waite, Lee; Fine, Jerry (2007). Applied biofluid mechanics. New York: McGraw-Hill. ISBN 0-07-147217-7.
Young, Donald F.; Bruce R. Munson; Theodore H. Okiishi (2004). A brief introduction to fluid mechanics (3rd ed.). Hoboken, N.J.: Wiley. ISBN 0-471-45757-4.
External links[edit]
Biomechanics and Movement Science Listserver (Biomch-L)
Biomechanics Links
A Genealogy of Biomechanics
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This article is about the astronomical term. For other uses, see Corona (disambiguation).
During a total solar eclipse, the Sun's corona and prominences are visible the naked eye.
A corona (Latin, 'crown') is an aura of plasma that surrounds the Sun and other stars. 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 (korōnè, “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". Instead, these spectral features have since been explained by highly ionized iron (Fe-XIV). Bengt Edlén, following the work of Grotrian (1939), first identified the coronal spectral lines in 1940 (observed since 1869) as transitions from low-lying metastable levels of the ground configuration of highly ionised metals (the green Fe-XIV line at 5303 Å, but also the red line Fe-X at 6374 Å). These high stages of ionisation indicate a plasma temperature in excess of 1,000,000 kelvin,[1] much hotter than the surface of the sun.
Light from the corona comes from three primary sources, from 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 spreads them so greatly as to completely obscure 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]
Contents
[hide]
1 Physical features
1.1 Active regions
1.1.1 Coronal loops
1.1.2 Large-scale structures
1.1.3 Interconnections of active regions
1.1.4 Filament cavities
1.1.5 Bright points
1.2 Coronal holes
1.3 The quiet Sun
2 Variability of the corona
2.1 Flares
2.2 Transients
3 Stellar coronae
4 Physics of the corona
4.1 Radiation
4.2 Thermal conduction
4.3 Coronal seismology
5 Coronal heating problem
5.1 Wave heating theory
5.2 Magnetic reconnection theory
5.3 Spicules (type II)
6 See also
7 References
8 Further reading
9 External links
Physical features[edit]
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 kelvins. 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 magnetohydrodynamic waves from below. The outer edges of the Sun's corona are constantly being transported away due to open magnetic flux and hence 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 the faster rotation of mass at the sun's equator (differential rotation), 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 relatively dark sun spots.
Since the corona has been photographed at high resolution in the X-ray range of the spectrum 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 quite varied 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[edit]
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 kelvins, 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 above the Sun's surface:[6] sunspots and faculae, occur in the photosphere, spicules, Hα filaments and plages in the chromosphere, prominences in the chromosphere and transition region, and flares and coronal mass ejections happen in the corona and chromosphere. If flares are very violent, they can also perturb the photosphere and generate a Moreton wave. On the contrary, quiescent prominences are large, cool dense structures which are observed as dark, "snake-like" Hα ribbons (appearing like 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[edit]
Main article: Coronal loop
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).
The Solar plasma that feed these structures is heated from under 6000 K to well over 106 K from the photosphere, through the transition region, and into the corona. Often, the solar plasma will fill these loops from one point and drain to another, called foot points (siphon flow due to a pressure difference,[9] or asymmetric flow due to some other driver).
When the plasma rises from the foot points towards the loop top, as always occurs during the initial phase of a compact flare, it is defined as chromospheric evaporation. When the plasma rapidly cools and falls toward the photosphere, it is called 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), its dark filaments obvious against the solar disk or prominences off the Sun's limb.
Coronal loops may have lifetimes in the order of seconds (in the case of flare events), minutes, hours or days. Where there is a balance in loop energy sources and sinks, coronal loops can last for long periods of time and are known as steady state or quiescent coronal loops. (example).
Coronal loops are very important to our understanding of the current coronal heating problem. Coronal loops are highly radiating sources of plasma and are therefore easy to observe by instruments such as TRACE. An explanation of the coronal heating problem remains 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 had, but due to the high plasma temperatures in the corona, in-situ measurements are, at present, impossible. 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[edit]
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[edit]
The interconnections of active regions are arcs connecting zones of opposite magnetic field, of different active regions. Significant variations of these structures are often seen after a flare.[citation needed]
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[edit]
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 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 p {\displaystyle p} can be calculated by the state equation of a perfect gas p = n K B T {\displaystyle p=nK_{B}T} , where n {\displaystyle n} is the particle number density, K B {\displaystyle K_{B}} the Boltzmann constant and T {\displaystyle T} the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases with respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect renders sunspots apparently dark in the photosphere.
Bright points[edit]
Bright points are small active regions found on the solar disk. X-ray bright points were first detected on 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 1.1x106 K to 3.4x106 K. The variations in temperature are often correlated with changes in the X-ray emission.[12]
Coronal holes[edit]
Main article: Coronal hole
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[edit]
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 rotation speed 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 less active 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[edit]
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[edit]
Main article: Solar flares
On August 31, 2012 a long filament of solar material that had been hovering in the Sun's outer atmosphere, the corona, erupted at 4:36 p.m. EDT
Flares take place in active regions and are characterized by a sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they involve several zones of the solar atmosphere and 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, and the most energetic events can last several hours. Flares produce 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 extreme UV wavelengths and into the X-rays, typical of the chromospheric and coronal emission.
In the corona, the morphology of flares, is described by observations in the UV, soft and hard X-rays, and in Hα wavelengths, and 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 with 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 are of such great proportion they 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. The durations of those periods depend on the range of wavelengths used to observe the event:
An initial impulsive phase, whose duration is on the order of minutes, strong emissions of energy are often observed even in the microwaves, EUV wavelengths and in the hard X-ray frequencies.
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[edit]
Main article: Coronal mass ejection
Accompanying solar flares or large solar prominences, "coronal transients" (also called coronal mass ejections) are sometimes released. These are enormous loops of coronal material that travel 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.
Stellar coronae[edit]
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 (Lx ≥ 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 coronae, 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[edit]
This image, taken by Hinode on 12 January 2007, 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 particles/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. 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 corona is usually only 0.1 to 0.6 Pa in active regions, while on the Earth the atmospheric pressure is about 100 kPa, approximately 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[edit]
Main article: Coronal radiative losses
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[edit]
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]
k = 20 ( 2 π ) 3 / 2 ( k B T ) 5 / 2 k B m e 1 / 2 e 4 ln  Λ ≈ 1.8 10 − 10 T 5 / 2 ln  Λ W m − 1 K − 1 {\displaystyle k=20\left({\frac {2}{\pi }}\right)^{3/2}{\frac {\left(k_{B}T\right)^{5/2}k_{B}}{m_{e}^{1/2}e^{4}\ln \Lambda }}\approx 1.8~10^{-10}~{\frac {T^{5/2}}{\ln \Lambda }}~Wm^{-1}K^{-1}}
where k B {\displaystyle k_{B}} is the Boltzmann constant, T {\displaystyle T} is the temperature in kelvins, m e {\displaystyle m_{e}} the electron mass, e {\displaystyle e} the electric charge of the electron,
ln  Λ = ln  ( 12 π n λ D 3 ) {\displaystyle \ln \Lambda =\ln \left(12\pi n\lambda _{D}^{3}\right)}
the Coulomb logarithm, and
λ D = k B T 4 π n e 2 {\displaystyle \lambda _{D}={\sqrt {\frac {k_{B}T}{4\pi ne^{2}}}}}
the Debye length of the plasma with particle density n {\displaystyle n} . The Coulomb logarithm ln  Λ {\displaystyle \ln \Lambda } 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 q {\displaystyle q} the heat for a volume unit, expressed in J m−3, the Fourier equation of heat transfer, to be computed only along the direction x {\displaystyle x} of the field line, becomes
∂ q ∂ t = 0.9 10 − 11 ∂ 2 T 7 / 2 ∂ x 2 {\displaystyle {\frac {\partial q}{\partial t}}=0.9~10^{-11}~{\frac {\partial ^{2}T^{7/2}}{\partial x^{2}}}} .
Numerical calculations have shown that the thermal conductivity of the corona is comparable to that of copper.
Coronal seismology[edit]
Main article: 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[edit]
Unsolved problem in physics:
Why is the Sun's corona so much hotter than the Sun's surface?
(more unsolved problems in physics)
Play media
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 kelvins 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 (surface), which is at about 5800 K, to the much hotter corona at about 1 to 3 MK (parts of the corona can even reach 10 MK).
Between the photosphere and the corona, is the thin region through which the temperature increases known as the transition region. It ranges from only tens to hundreds of kilometers thick. Energy cannot be transferred from the cooler photosphere to the corona by conventional heat transfer as this would violate the second law of thermodynamics. An analogy of this would be a light bulb raising the temperature of the air surrounding it to something greater than its glass surface. Hence, some other manner of energy transfer must be involved in the heating of the corona.
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's chromosphere, 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.
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. It is 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 kelvins. The main heat source in the quiescent corona (about 1.5 million kelvins) is assumed to originate from MHD waves.[24]
The NASA mission Parker 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. It is scheduled to launch on July 31, 2018.[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[edit]
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 ultra low frequency 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 through the chromosphere and transition region and finally into the corona where it dissipates 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 Solar and Heliospheric Observatory 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[edit]
Main article: Magnetic reconnection
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)[edit]
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]
See also[edit]
Solar System portal
Astronomy portal
Space portal
Star portal
Advanced Composition Explorer
Alfvén waves
Chromosphere
Coronal hole
Coronal loop
Coronal mass ejection
Coronal radiative losses
Coronal seismology
Geocorona
Heliosphere
Helmet streamer
Magnetic reconnection
Magnetohydrodynamic waves
Magnetohydrodynamics
Nanoflares
Photosphere
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References[edit]
Jump up ^ Aschwanden, M. J. (2004). Physics of the Solar Corona. An Introduction. Praxis Publishing. ISBN 3-540-22321-5.
Jump up ^ Corfield, Richard (2007). Lives of the Planets. Basic Books. ISBN 978-0-465-01403-3.
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Jump up ^ Vaiana, G.S.; Tucker, W.H. (1974). R. Giacconi; H. Gunsky, eds. "Solar X-Ray Emission in "X-Ray Astronomy"": 169.
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^ Jump up to: a b Gibson, E. G. (1973). The Quiet Sun. National Aeronautics and Space Administration, Washington, D.C.
Jump up ^ "How NASA Revealed Sun's Hottest Secret in 5-Minute Spaceflight".
Jump up ^ Katsukawa, Yukio; Tsuneta, Saku (2005). "Magnetic Properties at Footpoints of Hot and Cool Loops". The Astrophysical Journal. 621: 498–511. Bibcode:2005ApJ...621..498K. doi:10.1086/427488.
Jump up ^ Betta, Rita; Orlando, Salvatore; Peres, Giovanni; Serio, Salvatore (1999). "On the Stability of Siphon Flows in Coronal Loops". Space Science Reviews. 87: 133–136. Bibcode:1999SSRv...87..133B. doi:10.1023/A:1005182503751.
^ Jump up to: a b c Giacconi, Riccardo (1992). J. F. Linsky and S.Serio, ed. G.S. Vaiana memorial lecture in Proceedinds of Physics of Solar and Stellar Coronae: G.S. Vaiana Memorial Symposium. Kluwer Academic Publishers-Printed in the Netherlands. pp. 3–19. ISBN 0-7923-2346-7.
Jump up ^ Ofman, Leon (2000). "Source regions of the slow solar wind in coronal streamers". Geophysical Research Letters. 27 (18): 2885–2888. Bibcode:2000GeoRL..27.2885O. doi:10.1029/2000GL000097.
Jump up ^ Kariyappa, R.; Deluca, E. E.; Saar, S. H.; Golub, L.; Damé, L.; Pevtsov, A. A.; Varghese, B. A. (2011). "Temperature variability in X-ray bright points observed with Hinode/XRT". Astronomy & Astrophysics. 526: A78. Bibcode:2011A&A...526A..78K. doi:10.1051/0004-6361/201014878.
Jump up ^ Ito, Hiroaki; Tsuneta, Saku; Shiota, Daikou; Tokumaru, Munetoshi; Fujiki, Ken'Ichi (2010). "Is the Polar Region Different from the Quiet Region of the Sun?". The Astrophysical Journal. 719: 131–142. arXiv:1005.3667  . Bibcode:2010ApJ...719..131I. doi:10.1088/0004-637X/719/1/131.
Jump up ^ Del Zanna, G.; Bromage, B. J. I.; Mason, H. E. (2003). "Spectroscopic characteristics of polar plumes". Astronomy & Astrophysics. 398 (2): 743–761. Bibcode:2003A&A...398..743D. doi:10.1051/0004-6361:20021628.
Jump up ^ Pallavicini, R.; Serio, S.; Vaiana, G. S. (1977). "A survey of soft X-ray limb flare images – The relation between their structure in the corona and other physical parameters". The Astrophysical Journal. 216: 108. Bibcode:1977ApJ...216..108P. doi:10.1086/155452.
Jump up ^ Golub, L.; Herant, M.; Kalata, K.; Lovas, I.; Nystrom, G.; Pardo, F.; Spiller, E.; Wilczynski, J. (1990). "Sub-arcsecond observations of the solar X-ray corona". Nature. 344 (6269): 842–844. Bibcode:1990Natur.344..842G. doi:10.1038/344842a0.
^ Jump up to: a b Güdel M (2004). "X-ray astronomy of stellar coronae" (PDF). The Astronomy and Astrophysics Review. 12 (2–3): 71–237. arXiv:astro-ph/0406661  . Bibcode:2004A&ARv..12...71G. doi:10.1007/s00159-004-0023-2. Archived from the original (PDF) on 2011-08-11.
Jump up ^ Vaiana, G.S.; et al. (1981). "Results from an extensive Einstein stellar survey". The Astrophysical Journal. 245: 163. Bibcode:1981ApJ...245..163V. doi:10.1086/158797.
Jump up ^ Jeffrey, Alan (1969). Magneto-hydrodynamics. UNIVERSITY MATHEMATICAL TEXTS.
Jump up ^ Mewe, R. (1991). "X-ray spectroscopy of stellar coronae". The Astronomy and Astrophysics Review. 3 (2): 127. Bibcode:1991A&ARv...3..127M. doi:10.1007/BF00873539.
^ Jump up to: a b Spitzer, L. (1962). Physics of fully ionized gas. Interscience tracts of physics and astronomy.
Jump up ^ Ulmshneider, Peter (1997). J.C. Vial; K. Bocchialini; P. Boumier, eds. Heating of Chromospheres and Coronae in Space Solar Physics, Proceedings, Orsay, France. Springer. pp. 77–106. ISBN 3-540-64307-9.
Jump up ^ Malara, F.; Velli, M. (2001). Pål Brekke; Bernhard Fleck; Joseph B. Gurman, eds. Observations and Models of Coronal Heating in Recent Insights into the Physics of the Sun and Heliosphere: Highlights from SOHO and Other Space Missions, Proceedings of IAU Symposium 203. Astronomical Society of the Pacific. pp. 456–466. ISBN 1-58381-069-2.
Jump up ^ Cirtain, J. W.; Golub, L.; Winebarger, A. R.; De Pontieu, B.; Kobayashi, K.; Moore, R. L.; Walsh, R. W.; Korreck, K. E.; Weber, M.; McCauley, P.; Title, A.; Kuzin, S.; Deforest, C. E. (2013). "Energy release in the solar corona from spatially resolved magnetic braids". Nature. 493 (7433): 501–503. Bibcode:2013Natur.493..501C. doi:10.1038/nature11772. PMID 23344359.
Jump up ^ http://parkersolarprobe.jhuapl.edu/The-Mission/index.php#Journey-to-the-Sun
Jump up ^ Alfvén, Hannes (1947). "Magneto hydrodynamic waves, and the heating of the solar corona". MNRAS. 107: 211–219. Bibcode:1947MNRAS.107..211A. doi:10.1093/mnras/107.2.211.
Jump up ^ "Alfven Waves – Our Sun Is Doing The Magnetic Twist". read on Jan 6 2011.
Jump up ^ Jess, DB; Mathioudakis, M; Erdélyi, R; Crockett, PJ; Keenan, FP; Christian, DJ (2009). "Alfvén Waves in the Lower Solar Atmosphere". Science. 323 (5921): 1582–1585. arXiv:0903.3546  . Bibcode:2009Sci...323.1582J. doi:10.1126/science.1168680. PMID 19299614.
Jump up ^ McIntosh, S. W.; de Pontieu, B.; Carlsson, M.; Hansteen, V. H.; The Sdo; Aia Mission Team (2010). "Ubiquitous Alfvenic Motions in Quiet Sun, Coronal Hole and Active Region Corona". American Geophysical Union, Fall Meeting 2010. abstract #SH14A-01.
Jump up ^ "Sun's Magnetic Secret Revealed". read on Jan 6 2011.
Jump up ^ Kasper, J.C.; et al. (December 2008). "Hot Solar-Wind Helium: Direct Evidence for Local Heating by Alfven-Cyclotron Dissipation". Phys. Rev. Lett. 101 (26): 261103. Bibcode:2008PhRvL.101z1103K. doi:10.1103/PhysRevLett.101.261103. PMID 19113766.
Jump up ^ Priest, Eric (1982). Solar Magneto-hydrodynamics. D.Reidel Publishing Company, Dordrecht, Holland. ISBN 90-277-1833-4.
Jump up ^ Patsourakos, S.; Vial, J.-C. (2002). "Intermittent behavior in the transition region and the low corona of the quiet Sun". Astronomy and Astrophysics. 385 (3): 1073–1077. Bibcode:2002A&A...385.1073P. doi:10.1051/0004-6361:20020151.
Jump up ^ "Mystery of Sun's hot outer atmosphere 'solved' – Rediff.com News". Rediff.com. 2011-01-07. Retrieved 2012-05-21.
Jump up ^ De Pontieu, B; McIntosh, SW; Carlsson, M; Hansteen, VH; Tarbell, TD; Boerner, P; Martinez-Sykora, J; Schrijver, CJ; Title, AM (2011). "The Origins of Hot Plasma in the Solar Corona". Science. 331 (6013): 55–58. Bibcode:2011Sci...331...55D. doi:10.1126/science.1197738. PMID 21212351.
Further reading[edit]
Thorsten Dambeck: Seething Cauldron in the Suns's Furnace, MaxPlanckResearch, 2/2008, p. 28–33
B. N. Dwivedi and A. K. Srivastava Coronal heating by Alfvén waves CURRENT 296 SCIENCE, VOL. 98, NO. 3, 10 FEBRUARY 2010, pp. 295–296
External links[edit]
Wikimedia Commons has media related to Solar corona.
NASA description of the solar corona
Coronal heating problem at Innovation Reports
NASA/GSFC description of the coronal heating problem
FAQ about coronal heating
Solar and Heliospheric Observatory, including near-real-time images of the solar corona
Coronal x-ray images from the Hinode XRT
nasa.gov Astronomy Picture of the Day July 26, 2009 – a combination of thirty-three photographs of the sun's corona that were digitally processed to highlight faint features of a total eclipse that occurred in March 2006
Animated explanation of the core of the Sun (University of South Wales)
Animated explanation of the temperature of the Corona (University of South Wales)
Space,time,matter and vacuum: The Solar Corona. A sign of Quantum Gravity?(Spanish)
Alfvén waves may heat the Sun's corona
New Clue May Solve Solar Mystery
The Origins of Hot Plasma in the Solar Corona
Solar Interface Region – Bart de Pontieu (SETI Talks) Video
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values: هالة (فضاء) – Arabic

values: Сонечная карона – Belarusian

values: Сонечная карона – Belarusian (Taraškievica orthography)

values: Слънчева корона – Bulgarian

values:
https://cy.wikipedia.org/wiki/Corona
Corona – Welsh
cy
cy
interlanguage-link-target

values: Αστρικό στέμμα – Greek

values:
https://eo.wikipedia.org/wiki/Korono
Korono – Esperanto
eo
eo
interlanguage-link-target

values: تاج خورشیدی – Persian

values: कॉरोना – Hindi

values:
https://hr.wikipedia.org/wiki/Korona
Korona – Croatian
hr
hr
interlanguage-link-target

values:
https://id.wikipedia.org/wiki/Korona
Korona – Indonesian
id
id
interlanguage-link-target

values: עטרה (שמש) – Hebrew

values: ಕರೋನ(ಪ್ರಭಾವಲಯ) – Kannada

values: Күн тәжі – Kazakh

values: Күн таажысы – Kyrgyz

values:
https://ms.wikipedia.org/wiki/Korona
Korona – Malay
ms
ms
interlanguage-link-target

values:
https://no.wikipedia.org/wiki/Korona
Korona – Norwegian
no
no
interlanguage-link-target

values:
https://nn.wikipedia.org/wiki/Korona
Korona – Norwegian Nynorsk
nn
nn
interlanguage-link-target

values:
https://pms.wikipedia.org/wiki/Coron-a
Coron-a – Piedmontese
pms
pms
interlanguage-link-target

values: Солнечная корона – Russian

values:
https://sco.wikipedia.org/wiki/Corona
Corona – Scots
sco
sco
interlanguage-link-target

values:
https://sl.wikipedia.org/wiki/Korona
Korona – Slovenian
sl
sl
interlanguage-link-target

values: Корона (астрономија) – Serbian

values:
https://sv.wikipedia.org/wiki/Korona
Korona – Swedish
sv
sv
interlanguage-link-target

values: கொரோனா – Tamil

values: โคโรนา – Thai

values: Сонячна корона – Ukrainian

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