The Structure and Properties of the Sun

In my most recent post, I discussed the characteristics of the sun’s 11 and 22 year cycles, the observed laws which describe the behavior of the sunspot cycle, how proxy data is used to reconstruct a record of solar cycles of the past, Grand Solar Maxima and Minima, the relationship between Total Solar Irradiance (TSI) and the sunspot cycle, and the relevance of these factors to earth’s climate system. Before elaborating on the sun’s role in climate change, I’d like to take a look at the mechanism in terms of which the magnetic cycles underlying these solar cycles actually arises, but in order to do that, it’s necessary to first go over some basics:

The Structure of the Sun

The Core: The core of the sun is where pressures and temperatures are high enough to facilitate the nuclear fusion reactions which power the sun (Eddington 1920). The sun is so hot that there are few (if any) actual atoms of hydrogen and helium gas (Bethe 1939). They exist in a plasma state; the gases are ionized and cohabit with free electrons. So protons are being collided and fused into helium nuclei in what’s known as the proton-proton chain (PPO Chain), which is the dominant fusion process in stars of masses comparable to (or less than) the sun. In the PPO chain, two protons fuse and release a neutrino. The resulting diproton either decays back into hydrogen via proton emission, or undergoes beta decay (emitting a positron), which turns one of the protons into a neutron, thus yielding deuterium. The deuterium then reacts with another proton, producing 3He and a gamma ray. Two 3He from two separate implementations of this process then fuse to produce 4He plus two protons (Salpeter 1952).

Image by UCAR: Randy Russell (Windows of the Universe project)

This region comprises about the first 0.2 of the solar radius, and exhibits temperatures on the order of 14 – 15 million Kelvin.

The Radiative Zone: From about 0.2 – 0.7 solar radii from the center is the radiative zone. The nuclear fusion reactions in the core produce radiation which gets reiteratively absorbed and reemitted by various particles in this zone in a random zig-zag pattern. It can take hundreds of thousands of years for photons in this region to reach the surface in this manner. This succession of absorptions and reemissions also results in the photons which escape into the convective zone being of longer wavelength and lower energy than the gamma ray photons that were initially emitted from the nuclear fusion reactions in the core. The temperatures here are still on the order of a few million Kelvin.

The Layers of the Sun. (Image by Kelvinsong).

The Tachocline: The interface layer at the boundary separating the radiative zone and convective zone is called the Tachocline. The radiative and convective zones obey different rotational laws, and the advection of angular momentum in the Tachocline (which acts as a transition layer between them) is controlled by horizontal turbulence (Spiegel 1992). Changes in fluid flow velocities across this layer can twist magnetic field lines.

The Convective Zone: This zone runs from about 0.7 solar radii up to the sun’s surface (the Photosphere). As the name implies, heat pockets convect through the ionized gas in this region towards the surface in a manner similar to a boiling pot of water. The convective zone is just cool enough that many of the heavier ions are able to retain at least some of their electrons, which means that the material in the convective zone is more opaque, and thus it’s harder for radiation to get through it. Consequently, a lot of heat gets trapped in this zone, which causes the material to “boil” (or convect). As we’ll see soon enough, this property is important in making possible the solar dynamo mechanism that underlies the solar cycle. The temperature gradient of the convection zone ranges from around 2 million Kelvin near the tachocline to roughly 6,000 Kelvin at the sun’s surface.

The Photosphere: This is the sun’s visible surface layer. The photosphere includes features such as the following:

Sunspots: these are dark regions representing high magnetic flux, and are associated with changes in polarity of the sun’s magnetic field (Hale 1908). I’ve already covered some of the properties of sunspots, the sunspot cycle, and how sunspot abundance can be used as a proxy for Total Solar Irradiance (TSI) in a recent post here, and will cover the mechanism underlying the sunspot cycle when I elucidate the Solar Dynamo  in a follow-up post.

Sunspot c/o NASA

Faculae: these are bright regions which are also highly magnetized, but whose magnetic fields are concentrated in considerably smaller bundles than in sunspots (Richardson 1933). During solar maxima, when abundant dark sunspots are blocking the emission of heat and light, these bright regions overpower the darkening effect of the sunspots, thus resulting in the net increase in luminosity that we observe during solar maxima (Spruit 1982).

Faculae c/o NASA

Granules: these are the tops of convection cells which cover nearly the entire photosphere in ever-changing grain-like patterns (Langley 1874). The bright center bulges of the granules are regions where the plasma is rising to the surface, whereas the darker boundaries around them are where the plasma is cooler and sinking back down.

Granules: Image by Goran Scharmer and Mats G. Löfdahl

Supergranules c/o NASA

And supergranules: these are huge polygonal convective cells which are larger and last longer than granules, and are outlined by the chromospheric network. They have an average diameter of about 13,000 – 32,000 km and last an average of about 20 hrs (Simon 1964, Hagenaar 1997).

Additionally, the photosphere is also where solar flares originate. For more on solar flares, check out this article from

The Chromosphere: This is an approximately 2,000 km layer of gas residing above the photosphere, in which temperatures run from around 6,000 – 25,000 K (Vernazza 1976, Carlsson 1994). As a consequence, hydrogen in this layer emits light of a reddish color in a process called H-alpha emission (Michard 1958). The electrons of a particular atom can only occupy certain specific allowed energy states. They are quantized. These states correspond to specific principle quantum numbers, n = 1, n = 2, n = 3 etc… When an electron drops from one allowed energy state to a lower level, it emits a photon whose wavelength (and thus color) corresponds to the difference in energy between those two allowed states. Contrastingly, only a photon corresponding to the exact energy difference between two states can be absorbed by the electron to push it up to the higher energy state. In the limited case of hydrogen-like atoms, the relationship is described by the Rydberg formula, which is as follows:

1/λ = RZ2(1/nf2 – 1/ni2),

where λ is the wavelength of the photon emitted or absorbed, Z is the atomic number of the hydrogen-like atom in question (1 in this case), R is the Rydberg constant (approximately 1.097*10^7 m-1 in S.I. units), ni is the quantum number of the electron’s initial state, and nf is the quantum number of its final state. H-alpha emission occurs when an electron of a hydrogen atom drops from its third lowest allowed energy level (ni = 3) to its second lowest (nf = 2). ). If you plug in these values, you get that λ = 656 nm, which is in the red light range.

This transition is part of what’s called the Balmer series, which consists of all the allowed transitions between ni ≥ 3 and nf = 2 (Bohr 1913).

Other chromospheric features include the chromospheric network, which is a web-like pattern outlining supergranule cells, which results from bundles of magnetic field lines concentrated in the supergranules (Hagenaar 1997). Spicules are jet like eruptions of hot gas which protrude from the chromospheric network thousands of kilometers above the chromosphere and into the corona. Filaments and prominences are huge plumes of gas suspended as loops above the sun by magnetic fields, which underlie many solar flares (Kiepenheuer 1951, Menzel 1960). Plage, which are also associated with concentrations of magnetic field lines, appear as bright spots surrounding sunspots (Leighton 1959).

The Transition Region: This thin region resides between the cooler chromosphere and the much hotter corona. For this reason, temperatures rapidly increase with radial distance outward, ranging from 25,000 K around the boundary of the chromosphere to about 10^6 K out near the corona (Peter 2001).

The Corona: This aura of plasma is the sun’s outer atmosphere. It reaches temperatures far greater than at the sun’s surface (on the order of 1 – 3.5 million degrees Celsius). The reasons for these extreme temperatures comprise a long standing puzzle in solar astrophysics known as the Coronal Heating Problem, which is beyond the scope of this brief outline. That said, these extreme temperatures result in jets of plasma at speeds of up to 400 km/s (Brueckner 1983). Consequently, some of this ionized gas overcomes the sun’s gravitational pull, escapes, and subsequently cools down (Hundhausen 1970). This is the solar wind (Brueckner 1983). Incidentally, there is evidence that the sun’s rotation rate was greater in the past, and that the solar wind is responsible for its subsequent loss of angular momentum (Durney 1977). The corona is also the region from which Coronal Mass Ejections (CME) emerge. As the name implies, these can involve the ejection of billions of tons of plasma as a result of the reconnection of opposite ends of complicated magnetic field loops in the corona, and often accompany strong solar flares and filament eruptions. Not all solar flares and filament eruptions result in a CME though. Solar flares typically involve the expulsion of long radio wave radiation all the way up the EM spectrum through visible light (or even gamma rays), as well as protons and electrons, the latter of which can result in x-ray emissions via bremsstrahlung radiation (Arnoldy 1968). Charged particles in flares are accelerated by a combination of electric fields and magnetohydrodynamic waves (Miller 1997). You can read more about CME events here, here and here and solar flares here and here.

Any one of these layers and properties I’ve described here could be elaborated upon in greater detail, but this should be sufficient for the purpose of seguing into an explanation of the solar dynamo: the physical mechanism in terms of which solar cycles arise, which will be the topic of my next installment of this series.

Related Articles:


Arnoldy, R. L., Kane, S. R., & Winckler, J. R. (1968). Energetic solar flare X-rays observed by satellite and their correlation with solar radio and energetic particle emission. The Astrophysical Journal151, 711.

Bethe, H. A. (1939). Energy production in stars. Physical Review55(5), 434.

Bohr, N. (1913). The spectra of helium and hydrogen. Nature92, 231-232.

Brueckner, G. E., & Bartoe, J. D. (1983). Observations of high-energy jets in the corona above the quiet sun, the heating of the corona, and the acceleration of the solar wind. The Astrophysical Journal272, 329-348.

Carlsson, M., & Stein, R. F. (1995). DOES A NONMAGNETIC SOLAR CHROMOSPHERE EXIST?. The Astrophysical Journal440, L29-L32.

Durney, B. R., & Latour, J. (1977). On the angular momentum loss of late-type stars. Geophysical & Astrophysical Fluid Dynamics9(1), 241-255.

Eddington, A. S. (1920). The internal constitution of the stars. The Scientific Monthly, 297-303.

Hagenaar, H. J., & Schrijver, C. J. (1997). The distribution of cell sizes of the solar chromospheric network. The Astrophysical Journal481(2), 988.

Hale, G. E. (1908). On the probable existence of a magnetic field in sun-spots. The astrophysical journal28, 315.

Hundhausen, A. J. (1970). Composition and dynamics of the solar wind plasma. Reviews of Geophysics8(4), 729-811.

Kiepenheuer, K. O. (1951). The Nature of Solar Prominences. Publications of the Astronomical Society of the Pacific63, 161.

Langley, S. P. (1874). On the structure of the solar photosphere. Monthly Notices of the Royal Astronomical Society34, 255.

Leighton, R. B. (1959). Observations of Solar Magnetic Fields in Plage Regions. The Astrophysical Journal130, 366.

Menzel, D. H., & Wolbach, J. G. (1960). On the Fine Structure of Solar Prominences. The Astronomical Journal65, 54.

Michard, R. (1958). INTERPRETATION OF THE H* alpha/SPECTRUM OF THE CHROMOSPHERE. Compt. rend.247.

Miller, J. A., Cargill, P. J., Emslie, A. G., Holman, G. D., Dennis, B. R., LaRosa, T. N., … & Tsuneta, S. (1997). Critical issues for understanding particle acceleration in impulsive solar flares. Journal of Geophysical Research: Space Physics102(A7), 14631-14659.

Peter, H. (2001). On the nature of the transition region from the chromosphere to the corona of the Sun. Astronomy & Astrophysics374(3), 1108-1120.

Richardson, R. S. (1933). A Photometric Study of Sun-Spots and Faculae. Publications of the Astronomical Society of the Pacific45(266), 195-198.

Salpeter, E. E. (1952). Nuclear reactions in the stars. I. Proton-proton chain. Physical Review88(3), 547.

Simon, G. W., & Leighton, R. B. (1964). Velocity Fields in the Solar Atmosphere. III. Large-Scale Motions, the Chromospheric Network, and Magnetic Fields. The Astrophysical Journal140, 1120.

Spiegel, E. A., & Zahn, J. P. (1992). The solar tachocline. Astronomy and Astrophysics265, 106-114.

Spruit, H. C. (1982). The flow of heat near a starspot. Astronomy and Astrophysics108, 356-360.

Vernazza, J. E., Avrett, E. H., & Loeser, R. U. D. O. L. F. (1976). Structure of the solar chromosphere. II-The underlying photosphere and temperature-minimum region. The Astrophysical Journal Supplement Series30, 1-60.

Image Credits:

Layers of the Sun by Kelvinsong (Own work) [CC BY-SA 3.0 (], via Wikimedia Commons

Granules by Goran Scharmer/Mats G. Löfdahl of the Institute for Solar Physics at the Royal Swedish Academy of Sciences. 

Proton-Proton chain Image by UCAR: Randy Russell (Windows of the Universe project)

Sunspots, Faculae, and Supergranules by NASA


The Standard Model of Particle Physics: A Conceptual Introduction. By Credible Hulk (StASM admin): Part IV: Hadrons

In part I, we discussed the fundamental forces in physics and the dual nature of particles and fields. We also talked about how the elementary particles of the Standard Model are divided into fermions and bosons. In part II, we went over the essential characteristics of the elementary fermion family. In part III, we covered the elementary bosons, and today we take a look at Hadrons, which are the particles in which quarks are confined. As before, we’re chopping these up into very short mini-lessons in order to combat the problem of people being too busy to read something like this if we’d crammed it all into one long lesson.


Hadrons, which are composed of quarks, are subdivided into baryons, and mesons (Young and Freedman 1523). Baryons are comprised of three quarks each and include, for example, the proton and the neutron (Young and Freedman 1523). So, the nuclei of atoms are comprised of baryons, and since the matter with which we interact on a daily basis is comprised of atoms, (as most of you are no doubt well-aware), baryons thus play a significant role in our lives.

Since they have half-integer spin quantum numbers, individual baryons are also Fermions. Mesons, such as pions and kaons, consist of one quark and one anti-quark each, of which there are several combinations (Young and Freedman 1523). In fact, not only do anti-quarks exist, but every matter particle in the Standard Model has a corresponding anti-matter particle characterized by the opposite sign of its internal quantum numbers but the same spin and mass; particles and their anti-particles can mutually annihilate and in the process convert their mass to energy (Martin 182).

The properties of “spin” as well as the existence of anti-matter both emerged when physicist Paul Dirac successfully combined quantum mechanics with Einstein’s special theory of relativity in 1927 (Baggott 38).Additionally, since mesons also have integer spin quantum numbers, that makes them, by definition, bosons.

The quarks that comprise Hadrons are bound together by the nuclear strong force, or the nuclear strong interaction, which, as we mentioned in part II, is mediated by “colored” gluons (of which there are 8 types). Color in this context does not refer to color in the visual sense, but is rather just a name physicists have given to another property of gluons. For this reason, the strong interaction is sometimes referred to as the “color force,” which you can read more about here.


In the next installment, we’ll discuss the Higgs Boson, and I’ll provide some references along with a glossary.

Credible Hulk of StASM

*This article was cross posted over at Reaper Nation in affiliation with Tombstone da Deadman and on behalf of Stop the Anti-Science Movement


The Standard Model of Particle Physics: A Conceptual Introduction. By The Credible Hulk (StASM Admin): Part III: Bosons

In part I, we discussed the fundamental forces in physics and the dual nature of particles and fields. We also talked about how the elementary particles of the Standard Model are divided into fermions and bosons. In part II, we went over the essential characteristics of the elementary fermion family, and today we cover the elementary bosons. As before, we’re chopping these up into very short mini-lessons in order to combat the problem of people being too busy to read something like this if we’d crammed it all into one long lesson.


Unlike fermions, which have ½ integer spin quantum numbers, bosons always have either zero or integer spins; they do not obey Pauli’s aforementioned exclusion principle; and their statistical distribution throughout space is described by different mathematical functions than the fermions (Young and Freedman 1521). As we mentioned in our previous installment, bosons obey Bose-Einstein statistics, whereas fermions obey Fermi-Dirac statistics, but an actual mathematical explanation of what those distributions entail is beyond the scope of this lesson. In addition to the Higgs boson, which we’ll reserve for the next installment, all the particles which mediate the interactions of the four forces are also classified as bosons (Griffiths 55). The strong interaction is described by a theory called quantum chromodynamics, and is mediated by particles called Gluons (Griffiths 55). There are eight different Gluons (Carroll 296). The weak interaction is mediated by three particles: the neutrally charged Z boson, a positively charged W+ boson, and a negatively charged W- boson (Carroll 296). The electromagnetic force is mediated by the photon, and gravity is mediated by the graviton (Griffiths 56). Although the W and Z bosons have mass, photons, gravitons and gluons are massless (Griffiths 301).

Elementary Bosons

Two additional things are worth noting here. The first is that, unlike gluons, photons and gravitons, the W and Z bosons have mass. This fact was one of the considerations which triggered the development of the Higgs field theory. The Higgs boson, which was discovered experimentally at the LHC at CERN in 2012, is the evidence for the existence of the Higgs field. We’ll cover the Higgs in more detail in the installment after next, but for now just understand that the Higgs field imbues certain particles with mass.

The second is that the graviton is merely hypothetical at this point. It hasn’t been confirmed experimentally. To add insult to injury, our leading theory of gravitation, Einstein’s General Theory of Relativity, has not been successfully re-formulated to conform to the framework of quantum mechanics. In fact, one of the most highly publicized goals of many theoretical physicists is to construct a theory that subsumes all of the successes of both GR and QM in a single framework: a theory of quantum gravity.

Check out my facebook page, The Credible Hulk. I also help run Stop the Anti-Science Movement. In part IV we’ll discuss Hadrons.

*This article was cross posted over at Reaper Nation in affiliation with Tombstone da Deadman and on behalf of Stop the Anti-Science Movement


The Standard Model of Particle Physics: A Conceptual Introduction. By Credible Hulk (of StASM): Part II: Fermions

In part one, after mentioning the four fundamental forces and discussing the dual nature of particles and fields, we left off by explaining that the elementary particles of the Standard Model are categorized into Bosons and Fermions. Just to recap, fermions take up space, have half-integer quantum spin numbers and obey Fermi-dirac statistics, whereas bosons can pile on top of each other indefinitely, have integer quantum spin numbers, and obey Bose-Einstein statistical distributions (Carroll 293). The fermions, of which there are twelve in total, can be arranged into two families consisting of six quarks and six leptons respectively (Carroll 293). The bosons consist of the particles which mediate the interactions of the four aforementioned forces, as well as the Higgs (Young and Freedman 1530).

In this instalment, we’ll discuss the elementary fermions, and will reserve the elementary bosons for the subsequent instalement in the interest of keeping these lessons in bite-sized chunks for the busy internet user.

                                           Fermions: Quarks and Leptons

The six quarks and six leptons of the fermion family consist of three matched pairs of each (Young and Freedman 1530). A Top quark, a Charm quark, and an Up quark, each possessing a fractional electric charge of + 2/3, are matched with a Bottom quark, a Strange quark, and a Down quark respectively, each having an electric charge of -1/3 (Carroll 294) . Similarly, the leptons consist of the Tau, Muon, and Electron particles, each of which having an electric charge of -1, and each matched with their own corresponding neutrino particle of the same name (Carroll 294). All fermions have spin quantum number equal to 1/2 (plus or minus an integer) and experience the weak force interaction, but quarks also experience the strong force interactions while leptons do not (Young and Freedman 1522). Neutrinos do not experience the electromagnetic force interaction because they are not electrically charged (Carroll 298).

particle physics part 2

Quarks are never found in isolation, but are rather enclosed within more massive particles called Hadrons, and held together by the strong interaction (Carroll 294). Different combinations of quarks result in different hadrons with different physical properties. We will discuss hadrons in a later installment. Fermions also all obey Pauli’s exclusion principle (named after the great physicist Wolfgang Pauli), which states that no two fermions can be in the same place in the same quantum state (same quantum numbers) at the same time (Martin 28). Essentially that means that multiple fermions can’t pile up in one place, which is not the case for bosons (Martin 28).

Our next instalment will cover the elementary bosons.

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*This article was cross posted over at Reaper Nation in affiliation with Tombstone da Deadman and on behalf of Stop the Anti-Science Movement


The Standard Model of Particle Physics: A Conceptual Introduction. By Credible Hulk (of StASM) : Part I

The standard model of particle physics consists of a list of elementary particles, the four known forces of nature: electromagnetism, gravity, the nuclear strong force, and the nuclear weak force, and their corresponding fields (Young and Freedman 1530).

                                                  Forces: Fields or particles?

Electromagnetism and the two nuclear interactions obey the laws of quantum mechanics, including the Heisenberg Uncertainty Principle, and the concept of wave-particle duality, thus permitting their interactions to be conceptualized either in terms of quantum fields, or as being mediated by force carrier particles (Carroll 125-30). To clarify, in a classical field theory, a ‘force field’ is ascribed a value at every point in space-time and can be “scalar” (possessing a magnitude but no direction), or vector (possessing both a magnitude and a direction). PARTICLE PHYSICS 1

When physicists apply quantum mechanics to fields, they call it “quantum field theory,” (or just QFT for short), which is at the heart of the modern understanding of the physical world on the fundamental level (Carroll 129). Specifically, our modern understanding of particle physics is based largely on “relativistic” Quantum Field Theory, which incorporates Einstein’s Special Theory of Relativity. In a quantum field theory, forces are conveyed by ripples in the field which form waves and – because waves can also be interpreted as particles – as quantum particles of the field (Baggott). The most important difference between classical and quantum physics has to do with the relationship between reality and what we can actually know about it (Carroll 128). Quantum mechanics puts fundamental limits on the precision with which the state of a sub-atomic scale system can be measured that cannot, even in principle, be circumvented (Carroll 128). However, we will reserve a more concentrated look at quantum theory itself for another lesson (except insofar as it bears directly upon the formulation of The Standard Model).

                                      Elementary Particles: Fermions and Bosons

The elementary particles can be grouped into two broad categories: fermions, which take up space, and bosons, which can pile on top of each other indefinitely (Carroll 293). The fermions, of which there are twelve in total, can be arranged into two families consisting of six quarks and six leptons respectively (Carroll 293). The bosons consist of the particles which mediate the interactions of the four aforementioned forces, as well as the Higgs (Young and Freedman 1530).


In part II, we’ll go over the elementary fermions and some of their properties.

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*This article was cross posted over at Reaper Nation in affiliation with Tombstone da Deadman and on behalf of Stop the Anti-Science Movement