Illustration 100: Interaction between the Heliosphere and Cosmic Rays. Source H. Svensmark.
Spacecraft, venturing out toward the boundary of the solar system, have found that the intensity of galactic cosmic rays increases with distance from the Sun. As solar activity varies over the 11 year solar cycle, the intensity of cosmic rays arriving at Earth also varies, in inverse proportion to the sunspot number (page 170). This caused scientists, particularly astrophysicists, to speculate about the galactic origins of very-high-energy cosmic rays. The logical place to look is at supernovae, the tremendous death explosions of massive stars.
For thousands of years, people have gazed in wonder at the sudden appearance of new, bright stars in the evening skies. The ancient Romans called such stars novae, Latin for “new.” Over the past 2,000 years, seven stars, visible to the naked eye, have appeared in the heavens: the earliest in 185 AD, the latest in 1604 AD. These are now recognized to have been what scientists call supernovae, and they are among the most energetic events in the Universe. When a supernova erupts, it explodes with an energy equivalent to 1028 megatons of TNT, the force of a few octillion nuclear warheads.331 For comparison, all of mankind's nuclear weapons combined would only amount to 105 megatons, not even noticeable on this scale.
On a night in February, 1987, Canadian astronomer Ian Shelton was completing another night's study of the southern sky from the Las Campanas Observatory, in Chile. A routine observation of the Large Magellanic Cloud, a small companion galaxy to our own Milky Way, yielded an unexpected surprise. Where previously there had been only faint stars, a new bright star had appeared. Realizing that he was witnessing a supernova explosion, Shelton quickly spread the word to other observatories around the world.332
This supernova, discovered by Shelton, known as SN 1987A, proved to be the brightest supernova event since 1604. The 1604 supernova, known as Kepler's nova, occurred five years before Galileo began using a telescope to make astronomical observations. SN 1987A was the first supernovae scientists had been able to observe with instruments, and is still being studied today. From their observations, scientists theorized about the cause of the explosion.
The star that exploded was identified from star catalogs as Sanduleak (Sk)-69 202, a bright type B red super-giant with a mass of 19 Suns. From Earth, this star was not visible to the naked eye because the Large Magellanic Cloud is ~160,000 light-years away. A star of this size would spend about 10 million years on the main-sequence.
Once its hydrogen was exhausted, the star that was to become SN 1987A spent about one million years as a red super-giant. When the fusion fires had consumed its helium, and a carbon core had formed, the star had about 1,000 years left to live. When the core became iron, the star's remaining life was measured in hours. Internal temperatures rose rapidly, until the very light the star generated was powerful enough to disrupt the atomic nuclei in the iron core—a phenomenon called photodisintegration. Suddenly, the iron core, containing about 1.5 times the mass of the Sun, collapsed from a radius of 600 miles down to less than 6 miles. This released a vast torrent of neutrinos that dissipated most of the energy generated by the collapse. As quickly as it had collapsed, the core rebounded, blowing most of the star's matter into space. The collapse and rebound only took a few milliseconds—the star went from being a placid red giant to a supernova in literally the blink of an eye. For a brief time, SN 1987A shined with the light of 10,000,000,000 Suns.333
The collapse of the star's core has been verified by neutrino detectors here on Earth, and subsequent photon emission studies have verified that most of its mass was ejected carrying a kinetic energy of around 1051 ergs. What remains today of SN 1987A is a neutron star of about 1.6 solar masses. From photographs taken prior to the explosion, it was known that SN 1987A's predecessor star had two faint companion stars. Both seem to have survived the eruption.
On the twentieth anniversary of the SN 1987A supernova, NASA's Hubble Space Telescope took a picture of the stars remnants. This image shows the entire region around the supernova (see Illustration 101). The most prominent feature in the image is a ring, about a light-year across, with dozens of bright spots. The shock wave of material unleashed by the stellar blast slamming into regions along the ring's inner regions causes them to glow. The two bright objects that look like car headlights are a pair of stars in the Large Magellanic Cloud. The pink object in the center of the ring is debris from the supernova blast. The debris will continue to glow for many decades.
Illustration 101: SN 1987A on its 20th anniversary, Source NASA Hubble Space Telescope.
As a check on the theory that cosmic rays are created by supernovae, we are going to compare the amount of energy found in cosmic rays with that produced by observed supernovae. To quote Douglas Adams, in his famous book The Hitchhiker's Guide to the Galaxy, “Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is.” Because we are dealing with calculations involving the entire Milky Way Galaxy, we are going to use scientific notation, where very large numbers are represented as powers of ten.334 We will also use metric units, since it is only the final result that we are really interested in, not the actual values.
From radioisotope studies of the material in meteorites, it has been determined that cosmic ray intensity has stayed fairly constant for the past several million years. Calculations indicate that the average cosmic ray remains in the galaxy for about 10,000,000 (107) years. The energy density of cosmic rays is about 1 eV/cm3, about the same as from starlight. The disk of the galaxy is ~1067 cm3 in volume.335 Assuming that the energy density is the same throughout our galaxy, 1067 eV must be created every 107 years to maintain the energy density level. This translates into a production rate of 1060 eV/yr, or 1048 ergs/yr. We realize numbers of this magnitude are hard to grasp, but they are included here to demonstrate the unbelievable amount of energy involved in supernovae events. This energy is equivalent to 1035 kilowatt-hours per year—think of that the next time you receive your electric bill.
The estimated average energy release from a supernova is ~1051 ergs. With an observed rate of two supernovae per century this gives a yearly energy of 1049 ergs. Astrophysics estimate 10% of the energy released by supernovae goes into creating cosmic rays, about 1048 ergs/yr, the same estimated energy production required to maintain the observed cosmic ray density. Theory and measurement agree, the source of galactic cosmic rays are exploding supernovae.
Earth Showers and Muons
In 1938, French physicist Pierre Auger336 noticed that two detectors located several meters apart detected particles at the same time. This led to the discovery of air showers, cascades of secondary nuclei produced by the interaction of a cosmic ray particle with air molecules. The term cascade means that the incident primary particle, which could be an atomic nucleus, a proton, an electron, or occasionally a positron, strikes an atom in the atmosphere producing many high-energy ions. These secondary cosmic rays in turn create more, and so on.
When the energy of the primary is high enough, an air shower can produce a widespread flash of light due to excitation of air molecules and the Cerenkov effect. When a charged particle passes through an insulator at a speed greater than the speed of light in that medium, electromagnetic radiation is emitted. The characteristic “blue glow” of nuclear reactors is due to this Cerenkov radiation. It is named after Russian scientist Pavel Alekseyevich Cerenkov, the 1958 Nobel Prize winner who discovered it. The Cerenkov detector, which can detect the presence of high-energy particle radiation using the Cerenkov effect, has become a standard piece of equipment in atomic research. Such a device was installed on the Sputnik III satellite.
Cosmic rays that strike Earth hardly ever hit the ground. As they enter Earth's atmosphere, they collide with a nucleus of the air, usually several tens of kilometers high. In such collisions, many new particles are usually created and the colliding nuclei shatter. When a primary cosmic ray produces many secondary particles, it is called an air shower. W
hen many thousands, millions or even billions of particles arrive at ground level, it is called an extensive air shower (EAS). Most of these particles will arrive within a hundred yards of the axis of motion of the original particle, now the shower axis. But some particles can be found miles away. A large EAS can rain down particles over several acres.
Illustration 102: Simulation of a cosmic ray air shower. Source COSMUS and Sergio Sciutto for AIRES.
We mentioned in the previous chapter that scientists had found a link between cosmic ray levels and thunderstorms. There is also a positive correlation between cosmic ray flux (CRF) and low-altitude cloud formation. However, correlation does not always imply causation. It is also known that the Sun is slightly brighter when it is more active, which also may affect cloud formation on Earth. But, the correlation is strong so cosmic rays could be involved.
When scientists observe a close correlation between phenomena, they start looking for a reason, some mechanism that causes the linkage. There is a possible mechanism linking cosmic rays and low-level cloud formation: elevated levels of ionization seem to facilitate the coagulation of such molecules as sulfuric acid (H2SO4) in the atmosphere into tiny droplets, which then form condensation nuclei for water vapor. The condensed droplets of water then form clouds.
Illustration 103: The pion, muon, electron decay cycle captured by a steam chamber. Source CERN.
The main problem with linking low-level cloud formation to cosmic rays was identifying particles that could penetrate to ground level. When muons were discovered 70 years ago, by Carl Anderson and Seth Neddermeyer at Caltech, just such a particle had been found. It had exactly the mass predicted for Yukawa's meson,337 but it did not undergo strong nuclear interactions at all. Since all mesons are affected by the strong nuclear force, the new particle could not be a meson. The discovery of the muon was a compete surprise to particle physicists at the time, because their theories did not predict or allow for such a particle. When the muon was spotted in 1937, Isadore Rabi338 reportedly remarked, “Who ordered that?”
As it turns out, muons are charged particles that are identical to electrons, with the exception that they weigh 200 times as much as an electron. Muons live for about 2.2 microseconds, and often survive to ground level, before changing into electrons and neutrinos. In 1947, ten years after the muon discovery, Cecil Powell's group at Bristol University discovered that the muons are produced by other particles, called pions.
Pions are even heaver versions of the electron, which live for only a few hundredths of a microsecond. In Illustration 103, pions fly out from a collision in the steam chamber. One of the pions makes the looping track to the right, before it decays into a muon, which then curls anticlockwise four times, and eventually changes into an electron which moves off towards the upper right. From similar collisions, an EAS produces large quantities of muons that penetrate the atmosphere to Earth's surface and even below.
Cosmic Rays and Cloud Formation
At the end of the 20th century, a new theory about the Sun, the stars, and our solar system's path around the Milky Way galaxy, affects cloud cover on Earth had been postulated by a number of astrophysicists. But claims that solar variation is not sufficient to cause climate change have prevailed because a solid, scientific explanation of how such a link would work was lacking. The hypotheses has only been supported by historical records and statistical associations. The mechanism causing the effect had not been empirically demonstrated.
Illustration 104 Effect of Cosmic Rays on Cloud formation. Source N. Shaviv.
The primary proponent of cosmic ray induced low-level cloud formation is Danish physicist Henrik Svensmark, of the Danish Space Research Institute. Svensmark and Eigil Friis-Christensen reported their discovery in a cogent paper in 1997: “Variation of Cosmic Ray Flux and Global Cloud Coverage — a Missing Link in Solar-Climate Relationships.”339 In it, they describe how ions created in the troposphere by cosmic rays could provide a mechanism for cloud formation. And, since the level of cosmic rays is controlled by the solar cycle, they suggested that the Sun is controlling Earth's climate variation by changing low-level cloud cover.
They were not the first to propose such a mechanism. In 1959, Ney pointed out that cosmic ray induced ions could be an important variable in climate regulation.340 Aerosol particles are widely held to be the primary source for nucleation in cloud formation. Others reported how charged water droplets combine with aerosol particles 10 times, or even 100 times more efficiently than uncharged ones.341
A strong link between long-term variations in solar activity and Earth's climate had been reported by Friis-Christensen and Lassen in 1991. They showed that an empirically constructed measure of solar activity called the filtered solar cycle length, matched very closely variations in northern hemispheric temperature during the last 400 years.342 All this, claimed Svensmark and Friis-Christensen, pointed to a major role for cosmic rays in cloud formation, and hence, climate regulation. How this control mechanism works is shown in Illustration 104.
Illustration 105: The correlation between cosmic ray flux and low altitude cloud cover , Marsh & Svensmark, 2003.
In 2000, Nigel Marsh and Svensmark followed up on the earlier work by Svensmark and Friis-Christensen with a paper showing that the influence of galactic cosmic ray modulation was strongest on low-level clouds.343 If this theory is true, then the Sun controls Earth's climate. When the Sun is active, its magnetic field is stronger and as a result fewer global cosmic rays (GCR) arrive in the vicinity of Earth. When solar activity is low, the magnetic field is weaker, and more GCR arrive. The modulation of GCR is dependent on their energy. The higher energy the GCR particles, the less they are modulated by the solar cycle.
This new theory, proposed by astrophysicists, was not without its critics in the climatological community. To answer criticisms that their earlier work didn't use global cloud coverage data, Marsh and Svensmark further refined their work using data from more extensive infrared satellite observations. In 2003, they updated their calculations based on more extensive cloud data.344 The correlation between cosmic ray flux, as measured in neutron count monitors in low magnetic latitudes, and the low altitude cloud cover using International Satellite Cloud Climatology Project (ISCCP) satellite data is shown in Illustration 105.
This additional data helped strengthen the theory, but the most troublesome criticism was that the actual physical process of GCR induced cloud formation had not been demonstrated experimentally. This situation changed when Svensmark and the team at the Danish National Space Center experimentally demonstrated the very mechanism they proposed a decade ago.
Illustration 106: Condensation nuclei density as a function of ion density.
In a basement at the Danish National Space Center an experiment was set up to verify that cosmic rays could cause low level clouds to form under controlled conditions. The SKY experiment used a cloud chamber to mimic conditions in the atmosphere. This included varying levels of background ionization and aerosol levels, and sulfuric acid (H2SO4), in particular. The SKY experiment demonstrated that more ionization implies more particle nucleation, as shown in Illustration 106.
In 2006, Svensmark and colleagues reported the results of their laboratory experiments and published them in the Proceedings of the Royal Society. This work showed that highly ionizing radiation can create ultra-small aerosol particles. Critics, however, continue to attack Svensmark's results. Dr. Svensmark does not claim that human activity isn't a factor in climate change. He said in a recent interview:
“Humans are having an effect on climate change, but by not including the cosmic ray effect in models it means the results are inaccurate. The size of man's impact may be much smaller and so the man-made change is happening slower than predicted.”345
At this point, the primary focus was on short-term effects caused by the various solar variability cycles (page 181). Svensmark's theory had attracted the attention of an Israeli physicist, Nir Shaviv. Shaviv decided to look farth
er back into the past, to see if cosmic rays could have played a part in the much longer term fluctuation in Earth's climate. As we will see, his intuition was rewarded.
Subsequently, a debate over this new and controversial theory has raged in climatological circles. During the decade since its introduction, many criticisms have been leveled at the theory's assumptions and calculations. This is normal and part of the way science works. Unfortunately, given the high profile of global warming, the back and forth scientific arguments have often filtered into the popular press.346 ,347 ,348 ,349 ,350
To summarize, the link between cosmic rays and climate regulation, we know the following: The Sun and Earth both have magnetic fields that deflect some incoming cosmic rays. This divides cosmic rays into three categories; low-energy rays that are deflected by Earth's magnetic field, medium-energy rays that are deflected by the solar wind and magnetic field, and high-energy rays that are not deflected by either magnetic field.
As energy levels rise, the number of GCR drop dramatically. But, very energetic GCR create a majority of extensive air showers, and hence, a majority of the muons that penetrate to Earth's surface. About 60% of GCR created muons are created by high-energy ray category. These are the GCR that cannot be blocked by the Sun's protective magnetic field. About 40% of GCR muons fall into the middle or low categories, caused by primary particles that can be blocked by the Sun's field.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 21