Draft:Near-surface solar fusion

Near-surface solar fusion is a synopsis and review of our understanding of nuclear fusion reactions occurring above the Sun's photosphere yet within the likely extent of the coronal clouds surrounding the Sun's photosphere.

Abstract
Many articles and reports have described phenomena suggestive of nuclear fusion processes apparently above the Sun's photosphere. Many, hopefully most of these are summarized and evaluated here.

Introduction
The solar flare at Active Region 10039 on July 23, 2002, exhibits many exceptional high-energy phenomena including the 2.223 MeV neutron capture line and the 511 keV electron-positron (antimatter) annihilation line. In the image at right, the RHESSI low-energy channels (12-25 keV) are represented in red and appear predominantly in coronal loops. The high-energy flux appears as blue at the footpoints of the coronal loops. Violet is used to indicate the location and relative intensity of the 2.2 MeV emission.

During solar flares “[s]everal radioactive nuclei that emit positrons are also produced; [which] slow down and annihilate in flight with the emission of two 511 keV photons or form positronium with the emission of either a three gamma continuum (each photon < 511 keV) or two 511 keV photons." The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) made the first high-resolution observation of the solar positron-electron annihilation line during the July 23, 2003 solar flare. The observations are somewhat consistent with electron-positron annihilation in a quiet solar atmosphere via positronium as well as during flares. Line-broadening is due to "the velocity of the positronium." "The width of the annihilation line is also consistent ... with thermal broadening (Gaussian width of 8.1 ± 1.1 keV) in a plasma at 4-7 x 105 K. ... The RHESSI and all but two of the SMM measurements are consistent with densities ≤ 1012 H cm-3 [but] <10% of the p and α interactions producing positrons occur at these low densities. ... positrons produced by 3He interactions form higher in the solar atmosphere ... all observations are consistent with densities > 1012 H cm-3. But such densities require formation of a substantial mass of atmosphere at transition region temperatures."

"This energy [1032 to 1033 ergs] appears in the form of electromagnetic radiation over the entire spectrum from γ-rays to radio burst, in fast electrons and nuclei up to relativistic energies, in the creation of a hot coronal cloud, and in large-scale mass motions including the ejections of material from the Sun."

Flares
"Nuclear interactions of ions accelerated at the surface of flaring stars can produce fresh isotopes in stellar atmospheres."

A variety of subatomic particle and γ-ray reactions have been observed during solar flares indicating fusion reactions occurring at or above the photosphere. "There are typically 375 gamma-ray flares per solar cycle ... each releasing on average about 1031 erg of kinetic energy in accelerated ions of energy ≥ 1 MeV per nucleon [27]."

"The solar-flare gamma-ray line emission testifies that fresh nuclei are synthesized in abundance in energetic solar events."

"[T]he gamma-ray lines at 478 and 429 keV [are] emitted in the reactions 4He(α,p)7Li and 4He(α,n)7Be, respectively".

"[N]eutrino flux increases noted in Homestake results [coincide] with major solar flares [14]."

"The correlation between a great solar flare and Homestake neutrino enhancement was tested in 1991. Six major flares occurred from May 25 to June 15 including the great June 4 flare associated with a coronal mass ejection and production of the strongest interplanetary shock wave ever recorded (later detected from spacecraft at 34, 35, 48, and 53 AU) [15]. It also caused the largest and most persistent (several months) signal ever detected by terrestrial cosmic ray neutron monitors in 30 years of operation [16]. The Homestake exposure (June 1–7) measured a mean 37Ar production rate of 3.2 ± 1.5 atoms/day (≈19 37Ar atoms produced in 6 days) [13]; about 5 times the rate of ≈ 0.65 day −1 for the preceding and following runs, > 6 times the long term mean of ≈ 0.5 day−1 and > 2 1/2 times the highest rates recorded in ∼ 25 operating years."

Based on the 3He-flare flux from the Sun's surface and Surveyor 3 samples (implanted 15N and 14C in lunar material) from the surface of the Moon, the level of nuclear fusion occurring in the solar atmosphere is approximately at least two to three orders of magnitude greater than that estimated from solar flares such as those of August 1972.

"One of [...] the outstanding problems of the outer atmosphere of the Sun is the identification of the physical mechanisms that give rise to the eruption of solar flares."

There is a "hydrodynamic response of the solar atmosphere to the injection of an intense beam of electrons [as described] in a numerical simulation of a solar flare."

"The hydrodynamics is predicted [...] and the geometric form is of a semi-circular loop having its ends in the photosphere. [...] the loop is filled at supersonic speed with plasma at temperatures characteristic of flares. At the same time a compression wave is predicted to propagate down towards the photosphere. After the heating pulse stops, the plasma that has risen into the loop, starts to decay and return to the condition it was in before the pulse started."

The "impulsive phase of a flare [may be] initiated by an electron beam (having a power-law energy spectrum down to some minimum energy) depositing its energy in a model atmosphere representing the pre-flare condition."

There may be "a magnetic field parallel to the electron beam, and sufficiently strong that the electrons and subsequent flare plasma are contained within the beam dimensions, and transport negligible thermal energy in transverse directions."

The "pre-flare atmosphere is [...] for the chromosphere with a suitable extension into the corona."

An "input beam pulse of 1011 erg cm-2 s-1 for 10 s is capable of filling a loop structure with plasma at a peak temperature of over 30 million degrees K, with associated flare velocities of over a thousand kilometers per second. Associated with this upward flaring material, is a compression wave moving towards the photosphere, and attaining particle densities of up to 1014 cm-3."

Loops
Although a coronal cloud (as part or all of a stellar or galactic corona) is usually "filled with high-temperature plasma at temperatures of T ≈ 1–2 (MK), ... [h]ot active regions and postflare loops have plasma temperatures of T ≈ 2–40 MK."

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

"The peak continuum intensity was always at the loop tops."

The population of coronal loops can be directly linked with the solar cycle; it is for this reason coronal loops are often found with sunspots at their footpoints. Coronal loops project through the chromosphere and transition region, extending high into the corona.

Coronal loops have a wide variety of temperatures along their lengths. Loops existing at temperatures below 1 MK are generally known as cool loops, those existing at around 1 MK are known as warm loops, and those beyond 1 MK are known as hot loops. Naturally, these different categories radiate at different wavelengths.

Coronal loops populate both active and quiet regions of the solar surface. Active regions on the solar surface take up small areas but produce the majority of activity and 82% of the total coronal heating energy. The quiet Sun, although less active than active regions, is awash with dynamic processes and transient events (bright points, nanoflares and jets). As a general rule, the quiet Sun exists in regions of closed magnetic structures, and active regions are highly dynamic sources of explosive events. It is important to note that observations suggest the whole corona is massively populated by open and closed magnetic fieldlines. A closed fieldline does not constitute a coronal loop; however, closed flux must be filled with plasma before it can be called a coronal loop.

The image at right shows particle rays leaving the surface of the Sun (darker ends of the loops), traveling in a loop controlled by a local magnetic field similar to how particle accelerators accelerate, steer, and aim a stream of particles at a target (the much brighter regions in the chromosphere). The loops have a temperature of approximately 106 K and are emitting X-rays (synchrotron and cyclotron radiation).

Coronal loops form the basic structure of the lower corona andtransition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle; it is for this reason coronal loops are often found with sunspots at their footpoints. The upwelling magnetic flux pushes through the photosphere, exposing the cooler plasma below.

Loops of magnetic flux (closed flux tubes) well up from the solar body and fill with hot solar plasma. 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).

"Almost as soon as Active Region 10808 rotated onto the solar disk, it spawned a major X17 flare. TRACE was pointed at the other edge of the Sun at the time, but was repointed 6 hours after the flare started. The image on the left shows the cooling post-flare arcade (rotated by -90 degrees so that north is to the right) 6h after the flare (at 00:11 UT on September 8); the loop tops still glow so brightly that the diffraction pattern repeats them on diagonals away from the brightest spots. Some 18h after the flare, the arcade is still glowing, as seen in the image on the right (at 11:42 UT on September 8). In such big flares, magnetic loops generally light up successively higher in the corona, as can be seen here too: the second image shows loops that are significantly higher than those seen in the first. Note also that the image on the right also contains a much smaller version of the cooling arcade in a small, very bright loop low over the polarity inversion line of the region."

Nearly all of the TRACE images of coronal loops and the transition region indicate that material in these loops and loop-like structures returns to the chromosphere.

"Normally, solar energetic particle (SEP) events associated with disturbances in the eastern hemisphere are characterized by slow onset and lack of high-energy particles. The SEP event associated with the first major flare (X17) [...] is among very few such events over several decades in that although the source region was on the east limb, the particle flux started to rise only a few hours from the flare onset, while the flux of protons with energies in excess of 100 MeV went up by more than a factor of one hundred. We do not understand how these energetic particles can reach the Earth from that side of the Sun, because there should be no magnetic connectivity."

The image fourth at the right shows the first detection of high temperature nanoflares. The false-color temperature map of solar active region AR10923, observed close to center of the sun's disk, contains nanoflare regions (blue, indicating plasma near 10 million degrees K).

"Nanoflares are small, sudden bursts of heat and energy. "They occur within tiny strands that are bundled together to form a magnetic tube called a coronal loop," says Klimchuk. Coronal loops are the fundamental building blocks of the thin, translucent gas known as the sun's corona. ... Observations from the NASA-funded X-Ray Telescope (XRT) and Extreme-ultraviolet Imaging Spectrometer (EIS) instruments aboard Hinode reveal that ultra-hot plasma is widespread in solar active regions. The XRT measured plasma at 10 million degrees K, and the EIS measured plasma at 5 million degrees K. "These temperatures can only be produced by impulsive energy bursts,"says Klimchuk ... "Coronal loops are bundles of unresolved strands that are heated by storms of nanoflares." ... when a nanoflare suddenly releases its energy, the plasma in the low-temperature, low-density strands becomes very hot—around 10 million degrees K—very quickly. The density remains low, however, so the emission, or brightness, remains faint. Heat flows from up in the strand, where it's hot, down to the base of the coronal loop, where it's not as hot. This heats up the dense plasma at the loop’s base. Because it is so dense at the base, the temperature only reaches about 1 million degrees K. This dense plasma expands up into the strand. Thus, a coronal loop is a collection of 5-10 million degree K faint strands and 1 million degree K bright strands. "What we see is 1 million degree K plasma that has received its energy from the heat flowing down from the superhot plasma," says Klimchuk. "For the first time, we have detected this 10 million degree plasma, which can only be produced by the impulsive energy bursts of nanoflares.""

Positron annihilation
During solar flares "[s]everal radioactive nuclei that emit positrons are also produced; these positrons slow down and annihilate in flight with the emission of two 511 keV photons or form positronium with the emission of either a three gamma continuum (each photon < 511 keV) or two 511 keV photons. Higher energy protons and ®-particles produce charged and neutral pions that decay to produce high-energy electrons/positrons and photons, respectively; these were detected in the 1991 June 11 flare by EGRET (Kanbach et al. 1993)."

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) "made the first high-resolution observation of the solar positron-electron annihilation line during the July 23 flare."

There "is only a narrow range of temperatures around 6 × 103 K, and only in a quiet solar atmosphere, where the line shape is dominated by the formation of positronium in flight (the positron replaces the proton in the hydrogen atom). The positronium can be formed in either the singlet or triplet state (Crannell et al. 1976). When it annihilates from the singlet state, it emits two 511 keV γ rays (2γ) in the center-of-mass frame; the lines are broadened by the velocity of the positronium."

"The width of the annihilation line is also consistent [...] with thermal broadening (Gaussian width of 8.1 ± 1.1 keV) in a plasma at 4 - 7 × 105 K. In a quiet solar atmosphere, these temperatures are only reached in the transition region at densities ≤ 1012 H cm−3."

The "positrons annihilate at such low densities [...] positrons produced by 3He interactions form higher in the solar atmosphere; however, in order to explain the line width, it would require a much higher 3He/4He ratio than the upper limit set for this flare by RHESSI. Alternatively, all the observations are consistent with densities > 1012 H cm−3. But such densities require formation of a substantial mass of atmosphere at transition region temperatures."

Neutrinos
The first piece of information that seems to be needed are the reactions that produce the higher energy neutrinos: νµ and ντ.

For antiproton-proton annihilation at rest, a meson result is, for example,


 * $$p^+ + \bar{p}^- \rightarrow \pi^+ + \pi^-,$$


 * $${\pi}^+ \rightarrow {\mu}^+ + {\nu}_{\mu} \rightarrow e^+ + {\nu}_e + {\bar{\nu}}_{\mu} + {\nu}_{\mu},$$ and


 * $$D_S \rightarrow \tau + \bar{\nu}_{\tau} \rightarrow \nu_{\tau} + \bar{\nu}_{\tau}.$$

"All other sources of ντ are estimated to have contributed an additional 15%."


 * $$\tau \rightarrow e + \nu_{\tau} + \nu_e,$$

for two neutrinos.


 * $$\tau \rightarrow h + \nu_{\tau} + X,$$

where $$h$$ is a hadron, for two neutrinos.

The "data set [on the right from the Homestake solar neutrino experiment] now spans almost two complete solar cycles."

"Neutrinos can be produced by energetic protons accelerated in solar magnetic fields. Such protons produce pions, and therefore muons, hence also neutrinos as a decay product, in the solar atmosphere."

"Energetic protons in the solar corona could explain Figure 2 [at right] only if (1) they tap a substantial fraction of the entire energy generated in the corona, (2) the energy generated in the corona is at least 3 times what has been deduced from the observations, (3) the vast majority of energetic protons do not escape the Sun, (4) the proton energy spectrum is unusually hard (p0 = 300 MeV c-1, and (5) the sign of the variation is opposite to what one would predict. As the likelihood of all of these conditions being fulfilled seems extremely small, we do not believe that neutrinos produced by energetic protons in the solar atmosphere contribute significantly to the neutrino capture in the 37Cl experiment."

"The 37Ar production rate [at second right] in the Homestake solar neutrino experiment is anticorrelated (significance level of parts in 105) with solar activity (as measured by sunspot number) in the second two-thirds of the data, approximately 1977-1989; no significant correlation is substantiated in the first third of the data, 1970-1977."

Here on the Earth's surface the νe flux is about 1011 νe cm-2 s-1 in the direction of the Sun.

"The total number of neutrinos of all types agrees with the number predicted by the computer model of the Sun. Electron neutrinos constitute about a third of the total number of neutrinos. [...] The missing neutrinos were actually present, but in the form of the more difficult to detect muon and tau neutrinos."

Neutrons
"The neutrons are produced by the energetic protons interacting with a number of different nuclei."

"Observations made with the gamma-ray spectrometer (GRS) on the Solar Maximum Mission (SMM) satellite and with the Jungfraujoch neutron monitor are used to determine the directional solar neutron emissivity spectrum from ~100 MeV to ~2 GeV during the solar flare on 1982 June 3. The experimental data require a time-extended emission of the neutrons at the Sun with the majority of the neutrons produced after the impulsive phase."

"The first detection of ~400 MeV solar neutrons near the Earth [occurred] following an impulsive solar flare on 1980 June 21 [...] For three events, solar neutron decay protons have been observed near the Earth".

The "existence of neutrons at the Sun, producing the n-p capture γ-ray line at 2.223 MeV, have been reported for several events".

The "average energy of the solar nucleons causing the flare enhancement must be less than for the cosmic-ray primaries above ~3.5 GeV. This means that the atmospheric cascade, producing the excess count rate, was initiated by solar neutrons in the energy range 300 MeV-3.5 GeV."

"The feasibility of thermal neutron fission and fast neutron fission in planetary and protostellar matter [may be] calculated from nuclear reactor theory. Means for concentrating actinide elements and for separating actinide elements from reactor poisons [exist]. [I]ntermittent or interrupted planetaryscale nuclear fission breeder reactors [may occur] in connection with observed changes in the giant outer planets and changes in the geomagnetic field. [T]hermonuclear fusion reactions in stars are ignited by nuclear fission energy [...] dark matter, inferred to exist in the Universe, might be accounted for, at least in part, by the presence of dark stars (not necessarily brown dwarfs) whose protostellar nuclear fission reactors failed to ignite thermonuclear fusion reactions."

The data on the right "from the Climax, Colorado, surface neutron monitor [...] is an indicator of primary cosmic rays in the GeV range."

"Variation with the solar cycle [dotted curve of sunspot data] is evident."

"The tendency of the cosmic-ray modulation to lag sunspots (at least at times of sunspot decline) is visible, as is the somewhat more sawtooth form of the cosmic rays."

"The surface neutron flux [...] is largest at solar minimum and smallest at solar maximum, and [...] has the same sense as the 37Ar production variations."

"Primary cosmic rays below ~1 GeV are shielded by heliospheric currents which build up during solar maximum; see, e.g., Simpson 1989 and references there in."

Deuterium
"The 2.2 MeV line is formed in the reaction which synthesizes deuterium: 1H(n,γ)2H ... The line has been observed in a number of solar flares by the SMM, Hinotori and Prognoz satellites".

"The 2.2-MeV line fluence throughout the [May 24, 1990] flare was 345 ± 6 photons/cm2, which corresponds to the observed synthesis of over 3 tons [some ~3.3 metric tons] of deuterium on the solar surface."

"Surface fusion is no longer bizarre since the 2.2 MeV gamma ray line of the P(n,γ)D reaction was observed during the solar flare of May 24 1990."

"[M]ost of the sun’s fusion must occur near the surface rather than the core."

Alphas
An "analysis of the energy-loss distributions in the GRS HEM during the impulsive phase of this event indicates that γ-rays from the decay of π0 mesons were detected [...] The production of pions, which is accompanied (on average) by neutrons, has an energy threshold of ~290 MeV for p-p and ~180 MeV for p-α interactions, giving, therefore, a lower limit to the maximum energy of the particles accelerated at the Sun."

Helium
"Concerning the particles which interact at the Sun, evidence for accelerated 3He enrichment was obtained from the detection (Share & Murphy 1998) of a gamma-ray line at 0.937 MeV produced by the reaction 16O(3He,p)18F∗"

For "essentially all of [some 20] flares 3He/4He can be as large as 0.1, while for some of them values as high as 1 are possible. In addition, [...] for the particles that interact and produce gamma rays, 3He enrichments are present for both impulsive and gradual flares."

Lithium
"The solar wind lithium isotopic ratio, (6Li/7Li)sw = 0.032±0.004, has recently been determined from measurements in lunar soil (Chaussidon & Robert 1999)."

"Light element production by accelerated particle interactions [in] non-solar settings, and for accelerated particles of predominantly low energy, the dominant reactions are 4He(α,p)7Li, 4He(α,n)7Be (with 7Be decaying to 7Li) and 4He(α,x)6Li (where x stands for either a proton and a neutron, or a deuteron)."

"In solar flares, [...] the reaction 4He(3He,p)6Li [has a] very low threshold energy and [...] for solar energetic particles 3He/4He can be as large as 1 or even larger (e.g. Reames 1998). Such 3He/4He enhancements are one of the main characteristics of the acceleration mechanism responsible for impulsive solar energetic particle events, as distinguished from gradual events, based on the duration of the accompanying soft X-ray emission."

Flare "accelerated particle interactions produce enough 6Li which, combined with photospheric 7Li, can account for the solar wind 6Li/7Li measured in lunar soil."


 * $$\mathrm{_2^{3}He} + \mathrm{_2^{4}He} \rightarrow n^0 + p^+ + \mathrm{_{3}^{6}Li} + \gamma (3.561 MeV).$$

Although the 3.56 MeV line is usually missing from the gamma-ray spectrum during solar flares, there is evidence for significant production of 6Li in large solar flares by optical observations of sunspots,[17] and measurements of solar wind Li isotopic ratio in lunar soil.[18]

The "fact that as much as 1030 Li atoms are produced in large solar flares, suggests that flare produced lithium may be detected in a small area of the solar surface near the foot points of the flaring loops shortly after the time of the flare (see Livshits 1997). In this connection, it is interesting to point out that Ritzenhoff et al. (1997) don’t rule out the presence of 6Li near a sunspot at a value close to their reported upper limit 6Li/7Li ≤ 0.03, which in fact coincides with the measured solar wind value."

Beryllium
Although 7Be is usually assumed to have been produced by the Big Bang nuclear fusion, excesses (100x) of the isotope on the leading edge of the Long Duration Exposure Facility (LDEF) relative to the trailing edge suggest that fusion near the surface of the Sun is the most likely source. The particular reaction 3He(α,γ)7Be and the associated reaction chains 7Be(e-,νe)7Li(p,α)α and 7Be(p,γ)8B => 2α + e+ + νe generate 14% and 0.1% of the α-particles, respectively, and 10.7% of the present-epoch luminosity of the Sun. Usually, the 7Be produced is assumed to be deep within the core of the Sun; however, such 7Be would not escape to reach the leading edge of the LDEF.

Chromosphere
A variety of subatomic particle and γ-ray reactions have been observed during solar flares indicating nuclear fusion reactions occur above the photosphere, most likely in the chromosphere.

The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere.

Transition region
The solar transition region is a region of the Sun's atmosphere, between the chromosphere and corona. It is visible from space using telescopes that can sense ultraviolet. It is important because it is the site of several unrelated but important transitions in the physics of the solar atmosphere:
 * Below, most of the helium is not fully ionized, so that it radiates energy very effectively; above, it is fully ionized.
 * Below, gas pressure and fluid dynamics dominate the motion and shape of structures; above, magnetic forces dominate the motion and shape of structures, giving rise to different simplifications of magnetohydrodynamics.

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

Coronal clouds
Def. a cloud, or cloud-like, natural astronomical entity, composed of plasmas at least hot enough to emit X-rays is called a coronal cloud.

Barium stars
Barium stars exhibit carbon and s-process elements at their surfaces suggesting surface fusion possible during mass transfer or without it.

Barium stars are believed to be the result of mass transfer in a binary star system. The mass transfer occurred when the presently-observed giant star was on the main sequence. Its companion, the donor star, was a carbon star on the asymptotic giant branch (AGB), and had produced carbon and s-process elements in its interior. These nuclear fusion products were mixed by convection to its surface. Some of that matter "polluted" the surface layers of the main sequence star as the donor star lost mass at the end of its AGB evolution, and it subsequently evolved to become a white dwarf. We are observing these systems an indeterminate amount of time after the mass transfer event, when the donor star has long been a white dwarf, and the "polluted" recipient star has evolved to become a red giant.

CH stars
CH stars are particular type of carbon stars which are characterized by the presence of exceedingly strong CH absorption bands in their spectra. They belong to the star population II, meaning they're metal poor and generally pretty middle-aged stars, and are underluminous compared to the classical C–N carbon stars. Many CH stars are known to be binaries, and it's reasonable to believe this is the case for all CH stars. Like Barium stars, they are probably the result of a mass transfer from a former classical carbon star, now a white dwarf, to the current CH-classed star.

The mass transfer hypothesis may be needed to explain elemental occurrences on their surfaces such as carbon and s-process elements otherwise due to surface fusion.

CNO stars
"Due to the removal of the outer layers by mass loss, matter produced by the CNO tri-cycle is revealed at stellar surfaces in OB stars, supergiants and WN stars."

Particle accelerators
"The new reaction 208Pb(59Co,n)266Mt was studied using the Berkeley Gas-filled Separator [BGS] at the Lawrence Berkeley National Laboratory [LBNL] 88-Inch Cyclotron."

266Mt has been produced using the 209Bi(58Fe,n)266Mt reaction.

"Reactions with various medium-mass projectiles on nearly spherical, shell-stabilized 208Pb or 209Bi targets have been used in the investigations of transactinide (TAN) elements and their decay properties for many years. These so-called “cold fusion” reactions produce weakly excited (10-15 MeV) [1] compound nuclei (CNs) at bombarding energies at or near the Coulomb barrier that de-excite by the emission of one to two neutrons."

"The laboratory-frame, center-of-target energy used was 291.5 MeV, corresponding to a CN excitation energy of 14.9 MeV."

"At the start of the experiment the BGS magnet settings were chosen to guide products with a magnetic rigidity of 2.143 T·m to the center of the [focal plane detector] FPD. After the first event of 266Mt was detected in strip 45 (near one edge of the FPD), the magnetic field strength was decreased to 2.098 T·m in an effort to shift the distribution of products toward the center of the detector."

"258Db [has been produced] via the 209Bi(50Ti,n) and 208Pb(51V,n) reactions [15], and 262Bh via the 209Bi(54Cr,n) and 208Pb(55Mn,n) reactions [13, 16]."

"Hofmann et al. at Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany, and Morita et al., at the Institute of Physical and Chemical Research (RIKEN) in Saitama, Japan, have studied the 209Bi(64Ni,n)272Rg reaction [7, 17, 18]. The complementary 208Pb(65Cu,n)272Rg reaction was studied by Folden et al. at the Lawrence Berkeley National Laboratory (LBNL) [19]."

"Based on the observation of the long-lived isotopes of roentgenium, 261Rg and 265Rg (Z = 111, t1/2 ≥ 108 y) in natural Au, an experiment was performed to enrich Rg in 99.999% Au. 16 mg of Au were heated in vacuum for two weeks at a temperature of 1127°C (63°C above the melting point of Au). The content of 197Au and 261Rg in the residue was studied with high resolution inductively coupled plasma-sector field mass spectrometry (ICP-SFMS). The residue of Au was 3 × 10−6 of its original quantity. The recovery of Rg was a few percent. The abundance of Rg compared to Au in the enriched solution was about 2 × 10−6, which is a three to four orders of magnitude enrichment."

Yellow supergiants
Calculations of "the changes in the surface chemical composition of intermediate-mass stars in the first phase of convection dredge-up ... has been used to determine the changes in the surface chemical composition of stars with masses 2.5, 5, 10, 20 Mʘ due to nuclear reactions of the pp chains, the triple CNO cycle, and the NeNa and MgAl cycles."

For surface fusion or just above surface fusion a convection dredge-up may not be necessary.

"Boyarchuk and Lyubimkov [2] proposed that the excess sodium observed in yellow supergiants is synthesized in reactions of the NeNa cycle in the interior of stars on the main sequence (MS) and then is carried to the surface during the red-giant stage."

White dwarf accretions
If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gas from the companion's outer atmosphere. The companion may be a main sequence star, or one that is aging and expanding into a red giant. The captured gases consist primarily of hydrogen and helium, the two principal constituents of ordinary matter in the universe. The gases are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of degenerate matter, and so does not inflate at increased heat, while the accreted hydrogen is compressed upon the surface. The dependence of the hydrogen fusion rate on temperature and pressure means that it is only when it is compressed and heated at the surface of the white dwarf to a temperature of some 20 million kelvin that a nuclear fusion reaction occurs; at these temperatures, hydrogen burns via the CNO cycle.

"While hydrogen fusion can occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, for most binary system parameters the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other heavier elements in a runaway reaction, liberating an enormous amount of energy, blowing the remaining gases away from the white dwarf's surface and producing an extremely bright outburst of light. The rise to peak brightness can be very rapid or gradual which is related to the speed class of the nova; after the peak, the brightness declines steadily. The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used to classify a nova via its speed class. A fast nova will typically take less than 25 days to decay by 2 magnitudes and a slow nova will take over 80 days.

"An accreting white dwarf undergoes [near surface] nuclear burning when the accretion rate exceeds a certain limit." Due to the near surface nuclear burning, "the stellar luminosity is dominated by hydrogen burning, since the energy liberated by hydrogen burning exceeds that due to accretion on a white dwarf by an order of magnitude or more, depending on the mass of the white dwarf."

"[A]bove an accretion rate (with a hydrogen abundance of 0.7 by mass) MRG ≈ 8.5 10-7 (MWD/Mʘ -0.52)Mʘ yr-1 (MWD=mass of the white dwarf) the accreted matter forms a red-giant like envelope around the white dwarf, with the luminosity being generated from hydrogen shell burning."

Wolf-Rayet stars
Notation: let the symbol WC before the word star represent a Wolf-Rayet star exhibiting strong, broad emission lines of helium, carbon, and oxygen.

Notation: let the symbol WN before the word star represent a Wolf-Rayet star exhibiting strong, broad emission lines of helium and nitrogen.

Notation: let the symbol WNE before the word star represent an "early" WN-class Wolf–Rayet star (about WN2 to WN6).

Notation: let the symbol WNL before the word star represent a "late" WN-class Wolf–Rayet star (about WN6 to WN9).

"Wolf-Rayet stars offer us this most valuable possibility of observing the products of nuclear reactions in the H and He-burning phases revealed at stellar surfaces as a result of mass loss (and maybe of some mixing processes). In particular WC stars are the only kind of stars in which the products of the 3α and associated reactions prominently manifest themselves at the stellar surfaces."

"Comparisons are made between the theoretical C/He, N/He, and C/N ratios and those observed by Smith and Willis (1982) and by Nugis (1982) for WNL, WNE, and WC stars. The general agreement strongly supports the advanced evolutionary stage of WR stars as left-over cores resulting from the peeling of massive stars by stellar winds."

Discussion
Def. a "nuclear reaction in which nuclei combine to form more massive nuclei with the concomitant release of energy" is called fusion, or nuclear fusion.

Def. a "visible surface layer of a star, and especially that of the sun" is called a photosphere.

Here's a theoretical definition:

Def. nuclear reactions occurring at or above a photosphere in which nuclei combine to form more massive nuclei with the concomitant release of energy is called stellar surface fusion.