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Cosmic Rays, Energetic Particles, and Plasma

To learn a lot more about cosmic rays, check out Cosmicopia's page on cosmic rays.

What Are Cosmic Rays?
Why Do We Study Cosmic Rays?
Where Are Cosmic Rays in the Energy Spectrum?
Cosmic Rays vs. Cosmic Background Radiation
Can We Collect Cosmic Rays?
Velocity of Cosmic Rays
Measuring Cosmic Rays
Cosmic Rays from Supernovae
Light From Cosmic Rays?
How Do You Detect Only Certain Cosmic Rays?
Cosmic Ray Detectors and Muon Flux
Cosmic Ray Spark Chamber
Building a Cloud Chamber
Charged Particles at the Earth's Surface
Energy Loss of Particles in Different Media
Are Cosmic Rays Electromagnetic Radiation?
Cosmic Rays and Magnetic Fields
High-Energy Protons and the Cosmic Microwave Background
Millikan and Cosmic Rays
Cosmic Ray Composition and Space Weather
Cosmic Ray Effects on Humans
Cosmic Rays and Global Warming?
Most Abundant Elements in Cosmic Rays
Abundance of Hydrogen in the Universe
Particles from the Sun
Density of Solar Wind Electrons
Solar Wind and Corona
Temperature of Solar Wind Ions and Electrons
Charge on Solar Wind?
Hydrogen in the Solar Wind
Variations in Solar Wind
Solar Wind Erosion
Electrons from the Sun
Carbon-14 and Cosmic Rays
Cosmic Ray Carbon
Density of Hydrogen in Space
Cosmic Ray Energy
Cosmic Ray Radiation
Collecting Solar Wind Electrons on Satellites for Power?
Can We Harness Cosmic Energy?
More on Plasma
Collisional and Collisionless Plasmas

  1. What Are Cosmic Rays?

    Can you give me a good explanation of what a cosmic ray is?

    I think that the description given on our cosmic ray page and the pages linked from it are what you want. If you've got a more specific question, feel free to send more email.

    Dr. Eric Christian

  2. Why Do We Study Cosmic Rays?

    Why is the study of cosmic rays important? What do scientists hope to achieve by studying cosmic rays? Why should the average person be interested in the study of cosmic rays?

    These are excellent questions and all interrelated. Cosmic rays are to scientists much like photons are to astronomers. Just as astronomers use light (or photons) to view our Galaxy and beyond, scientists use cosmic rays to infer useful properties about our Galaxy, such as its composition, its basic structure (is the Galaxy homogeneous? is there an extended halo surrounding our Galaxy?), and what common physical processes occur within the Galaxy (how nuclei accelerate to nearly the speed of light, what kinds of nuclear collisions take place within the interstellar medium, etc). By looking at different properties of cosmic rays, scientists learn different things about our Galaxy, much like astronomers use light in different wavelengths to learn about different aspects of the Galaxy.

    In fact, cosmic rays offer one of the few ways in which scientists can actually sample real matter (from protons up through the heaviest elements -- the actinides) outside of our solar system. By identifying the various nuclei that are dispersed throughout our Galaxy, scientists hope to unravel the mechanisms that actually produce these nuclei -- from stellar nucleosynthesis to nucleosynthesis within supernovae to nuclear fragmentation. Just think, the iron in your blood came from a supernova billions of years ago in our solar neighborhood!

    If you can get access to your local library you may be able to find a useful book on cosmic rays called "Cosmic Rays" by M. Friedlander. You can certainly find out much more about cosmic rays (and references therein) at our website.

    Dr. Georgia de Nolfo
    (March 2003)

  3. Where Are Cosmic Rays in the Energy Spectrum?

    Where do cosmic rays rays fall in the electromagnetic spectrum amidst the x-rays and microwaves?

    Some people use "cosmic rays" to include high energy photons (light), and they basically mean x-rays and gamma-rays (light that is high enough energy that you measure individual photons, rather than taking pictures the way you do with visible, ultraviolet, microwave, etc.). I usually think of cosmic rays (which is my field of astrophysics) as only being the particles (pieces of atoms) which are moving at near the speed of light.

    If you were to plot "cosmic rays" on an electromagnetic spectrum, it would basically encompass x-rays and everything higher in energy (higher in frequency or shorter in wavelength). I personally would leave it out, since I don't think it's a correct usage of "cosmic rays" (but I don't make the rules).

    Dr. Eric Christian
    (May 2000)

  4. Cosmic Rays vs. Cosmic Background Radiation

    Is there a difference between cosmic rays and cosmic background radiation?

    Cosmic rays are particles - the nuclei of elements in the periodic table. They have nothing to do with the cosmic microwave background. The cosmic background radiation is the thermal radiation left over from the Big Bang - it consists of photons only - not particles. The only thing they have in common is the word "cosmic". Please visit the COBE home page for more on the cosmic background.

    Dr. Louis Barbier
    (December 2000)

  5. Can We Collect Cosmic Rays?

    What is the most practical way to collect and store cosmic radiation, or is it more practical to try and reproduce it?

    I worry from your question that you don't really understand what cosmic rays are. Cosmic rays are sub-atomic particles that are moving at a good fraction of the speed of light. If you slow them down to "collect and store" them, they look just like the matter that makes up you, me, and the rest of the Earth. If you want them moving at high velocities, a particle accelerator can generate many more than you could easily capture and store (although cosmic rays can get up to much higher energies than the biggest particle accelerators on Earth can generate).

    Dr. Eric Christian

  6. Velocity of Cosmic Rays

    What is the velocity of cosmic rays? Can it exceed the velocity of light? Is it true that cosmic rays can escape from a black hole where light cannot? Why?

    The velocity of cosmic rays can go from a small fraction of the speed of light up to about .999999999999 times the speed of light. Since cosmic rays are matter (typically the bare nuclei of atoms), they CANNOT exceed the speed of light. They also cannot escape from the event horizon of black holes, but it looks as if black holes can generate relativistic jets of material out along their poles. But these particles are accelerated outside the black hole and so they (and any light generated there as well) can escape.

    Dr. Eric Christian

  7. Measuring Cosmic Rays

    How is the mass and speed of high-energy cosmic ray particles determined by space-borne experiments?

    This is a pretty extensive topic. A brief description that I wrote a few years ago for the instrument development branch goes like this:

    How Do You Weigh a Particle Moving at Half the Speed of Light?

    There are basically two ways to weigh subatomic particles that are moving at a substantial fraction of the speed of light.
    1. You can slow down and stop the particles in a series of detectors that measure the rate that the particles are slowing down (their energy loss), or their velocity. The rate at which a particle slows down is a strong function of the charge of the particle (which, since these are bare nuclei with only protons and neutrons, tells you the number of protons, i.e. the element). The energy loss is a much weaker function of the mass (the number of protons + neutrons) of the particle, but from the small differences in slowing down, you can determine the mass. Typically, in order to measure the difference between Iron-56 and Iron-57 for example, which differ by less than 2% in mass, you need to get your error (sigma) in the mass measurement down to about .25% or one in four hundred. The CRIS instrument on ACE is an example of this type of detector system.
    2. You can use a strong magnetic field to bend the track of the charged particle (cosmic ray). If you measure the curvature of the track (with drift chamber detectors for example) and have an independent measure of the particle velocity (with a time-of-flight system or cherenkov detector), you can determine the mass. The ISOMAX balloon instrument uses this method. It has a pair of superconducting magnets that generate a magnetic field that is more than 1 Tesla in strength.

    Dr. Eric Christian

  8. Cosmic Rays from Supernovae

    Supernovae occur periodically producing a burst of galactic cosmic rays. Many are very far away. Have scientists ever detected a spike in cosmic ray flux rates which they attribute to a nearby supernova? If so, how long was the pulse (minutes, days, weeks)? How much higher than background cosmic ray levels was the spike?

    One would think that we could measure such spikes in the cosmic ray intensity from a neighboring supernova. And it is certainly a good idea. But there are a few complications.

    First we must consider the typical energy of a cosmic ray, which is around 1 GeV (1 giga-electron volt is equivalent to the rest mass energy of a proton). At these energies, cosmic rays are affected by the ambient galactic magnetic fields, which are typically 10-6 Gauss. Remember, cosmic rays are charged and the trajectory of a charged particle will curve under the influence of a magnetic field.

    Consider a cosmic ray with energy of roughly 10,000 GeV. At this energy, cosmic rays will spiral with a radius of curvature of roughly ~1016 m (which is much less than the distance to the Crab nebula, for instance). Considering the size of our galaxy (the radius of our galaxy is 1021 m), this means that by the time cosmic rays reach the Earth, they have been spiraling and diffusing through stray magnetic fields so that we see a distribution of cosmic rays arriving at Earth that is roughly equal from all directions (isotropic). Information concerning the original direction is lost at these energies. This is further exacerbated by the interaction of the Sun's magnetic field with these in-coming cosmic rays. Finally, we should remember that the typical rate for supernovae is roughly 1 every 50 years in our galaxy!

    But there is a caveat to all of this. We do see cosmic rays at even higher energies, even up to 1021 eV! These are called ultra-high energy cosmic rays (UHECR). A cosmic ray with energy of 1020 eV has a radius of curvature of 1 megaparsec (~1022 m) -- much greater than the diameter of our galaxy. These cosmic rays have the potential to point to a particular source, although the current number of these nuclei measured at Earth is extremely low. So far we haven't observed any clear evidence that the quantities of these UHECRs vary depending upon the direction from which they arrive (anisotropy).

    In fact, theorists are still having difficulty explaining how a supernova can accelerate particles beyond 10,000 GeV, and it has been postulated that these very rare UHECRS are coming from more exotic sources, possibly including gamma-ray bursts, dark matter, or some brand new physics. You can learn more about UHECRs on the OWL website.

    Dr. Georgia de Nolfo
    (July 2004)

  9. Light From Cosmic Rays?

    Several frames from different space-based cameras have shown thin, colored streaks of light. Was this due to cosmic rays?

    You cannot see cosmic rays this way. A cosmic ray particle - most of which are protons - can only produce light by colliding with molecules in the atmosphere and exciting them. Such a collision would produce much too weak a signal to be seen like this. Very sensitive photodetectors (called photomultipliers) are used to measure these type of light levels from cosmic rays, and they need large area mirrors to focus the light onto them.

    You can see cosmic ray particles if you use a very thick, sensitive type of film (which we call an emulsion). However, in this case, you do not record a streak of light. The particles you detect are the ones that penetrate through the emulsion causing chemical reactions. These are seen as dark tracks when the emulsions are developed.

    I have also read that different astronauts have claimed to have seen what they believe are cosmic rays when they closed their eyes. These were as brief bursts of light. Is this true?

    The light astronauts may see when they close their eyes is caused by cosmic rays - but it's produced via a mechanism known as Cherenkov radiation. The particles actually produce the light inside the eyeball.

    Dr. Louis Barbier

  10. How Do You Detect Only Certain Cosmic Rays?

    How would you be able to detect only certain cosmic rays and not others? For example, how would you be able to detect only lithium cosmic rays?

    It is nearly impossible to make a detector that will detect only lithium, for example. What cosmic ray scientists do is detect all particles similar to lithium and then determine which ones are lithium and which aren't. Different cosmic rays slow down at different rates and also bend differently in a magnetic field (see Measuring Cosmic Rays). But both of these also depend upon the initial energy of the cosmic ray. So typically you have to measure at least two of the following quantities: energy, rate of energy loss (rate of slowing down), velocity, or rigidity (which is a measure of how much the particle bends in a magnetic field). From these measurements, you can tell which particles are lithium and which are helium or beryllium, and if you make the measurements accurately enough, you can tell which cosmic rays are lithium-6 (3 protons and 3 neutrons) and which are lithium-7 (3 protons and 4 neutrons).

    You can frequently tune your cosmic ray instrument so that its resolution is best for lithium, but it will detect some helium and beryllium and other cosmic rays, too.

    Dr. Eric Christian
    (March 2000)

  11. Cosmic Ray Detectors and Muon Flux

    I teach at a secondary school which has joined a project to study cosmic rays using scintillator panels (60 cm x 60 cm) with a photomultiplier tube mounted on top of the panel. The detectors are from the Chicago Air Shower Array program which finished in 1999. Using two of the panels, we have looked at the coincidence rate as function of the separation of the two detectors. Beyond roughly 5 meters, the detectors show a constant rate due to "accidental" coincidences. As one moves inside 5 meters, the rate increases. Plotting a log of the rate versus separation inside 5 meters reveals a change in the slope which seems to indicate some interaction between the two detectors. Is there an explanation for this effect?

    Well I suppose you are aware of muons - penetrating particles that constantly bombard us here on the ground.

    There are roughly 100 muons per square meter per second at sea level, with a few GeV of energy. You can investigate this by fixing the two scintillators with respect to each other and measuring the coincidence rate, then moving them closer to each other. The rate should go up. How fast?

    Then I suggest you rotate the two panels together - there is an angular dependence to the muon flux too.

    Dr. Louis Barbier
    (November 2001)

  12. Cosmic Ray Spark Chamber

    I saw a television show on radiation that included cosmic rays. There was a detector that produced an electrical discharge when a cosmic ray passed through it. Although the spark was visible between the plates of the detector, the lecturer placed his hands within the detector with no ill effects. Can such a detector be constructed at home? Do you know of any plans?

    What you are describing is called a spark chamber (for obvious reasons) and was one of the early detectors used to actually study cosmic rays. Cosmic rays passing through the spark chamber cause a flash of light (or spark) along the particle's trajectory, which is then detected with the use of light sensitive devices. Spark chambers are indeed still built today and often used in physics classes as a way to demonstrate the existence of cosmic rays.

    The Exploratorium in San Francisco has a wonderful display dedicated to spark chambers and a short discussion on this page.

    The University of Birmingham provides a nice discussion of the history of spark chambers.

    While it is possible to build a spark chamber "at home," the electronics and high voltage required for a successful spark chamber make the endeavor rather difficult and possibly dangerous from home. What might be more realistic is to build another type of cosmic ray detector called a cloud chamber, which you can read about on Andy Foland's Cloud Chamber Page.

    Dr. Georgia de Nolfo
    (June 2003)

  13. Building a Cloud Chamber

    I am trying to see alpha particle tracks in a cloud produced from the vaporization unit of an ultrasonic humidifier. I have placed a sealed glass vessel with an alpha particle source inside against the wall. I can generate a good white cloud vapour which becomes very still for a minute or so after switching the device off. I am using a bright halogen type desk lamp at various angles to try and see particle tracks, however none are present. Is there some basic design flaw in this approach?

    Although I'm not completely familiar with the ultrasonic humidifier, I assume it is producing a steam vapor -- which is a warm vapor (or at least a vapor at room temperature). Unfortunately, this will not work to produce ion tracks in your cloud chamber.

    What you need is a vapor that is super-cooled. A super-cooled vapor is a vapor that remains a vapor at a temperature for which the vapor form is not usual (would normally be found as a liquid at this temperature). The fact that the vapor is super-cooled means that when an alpha particle passes through the vapor, the particle readily ionizes the vapor along the particle's track. The ionization leaves the vapor locally charged and this is enough to begin the condensation process along the particle track.

    You might be interested to find more on how to produce a cloud chamber on Andy Foland's Cloud Chamber Page.

    Dr. Georgia de Nolfo
    (August 2004)

  14. Charged Particles at the Earth's Surface

    Where can I find the flux and energy distribution of neutrons and charged particles at the Earth's surface?

    Actually very few charged particles in cosmic rays reach the Earth directly --most of the particles are either deflected by the Earth's magnetic field or interact with the air molecules in our atmosphere. We do monitor the secondary distribution of particles that are produced in the ensuing air showers from cosmic ray interactions in the atmosphere. These measurements are accomplished with neutron monitors which are located all over the world. An excellent place to learn more about neutron monitors and even get data is at the Space Physics Data Facility website.

    Dr. Georgia de Nolfo
    (February 2003)

  15. Energy Loss of Particles in Different Media

    Is there a theory or formula which gives the energy loss and range of protons in various materials, say Fe or Pb, especially for very high-energy protons (1 GeV or greater)?

    Yes, the energy loss is given by the Bethe-Bloch equation - which you can find in any nuclear physics textbook and some atomic physics books. Much research (and many publications) was done on this in the 1950s, by R. M. Sternheimer, and in the 1960s, by M. J. Berger. Barkas and Berger compiled large tables of energy loss for particles in different media. You should be able to find them in any physics collection of a University library.

    Dr. Louis Barbier
    (July 2000)

  16. Are Cosmic Rays Electromagnetic Radiation?

    On one of your web pages it is stated that "Particles and high-energy light that bombard the Earth from anywhere beyond its atmosphere are known as cosmic rays." Yet on one of the linked pages it is stated that "For some time it was believed that the radiation was electromagnetic in nature (hence the name cosmic "rays"), and some textbooks still incorrectly include cosmic rays as part of the electromagnetic spectrum." I would like to know if some cosmic rays consist of high energy electromagnetic radiation (or are they all particulate?).

    Some people still call high energy photons (x-rays and gamma rays) cosmic rays, and you'll still see that in some textbooks. The more common usage (at least in scientific circles) is to call particles cosmic rays, and to call photons either x-rays or gamma rays.

    Dr. Eric Christian

  17. Cosmic Rays and Magnetic Fields

    Cosmic rays are charged particles. If we consider them as a current, then there will be an induced magnetic field. What is the difference between this induced magnetic field and the galactic magnetic field? How can we study and compare these magnetic fields?

    It is true that you can calculate an induced magnetic field from individual charged particles. If you go back to the Biot-Savart Law, for a current segment of length dl, the induced magnetic field is:
    dB = I/c (dl x x) / |x|3
    (that is I, current, divided by c, the speed of light, times dl, segment length, cross x divided by the norm of x cubed). If one takes this to the limit of individual charges you get an induced magnetic field:
    B = q/c (v x x)/|x|3
    (or q, charge, divided by c, speed of light, times v, velocity of the particle, cross x, the distance vector to the point of observation, divided by the norm of x cubed). This holds only if the particle is moving non-relativistically, but is useful to understand the point. In the relativistic limit you'd use the Lorentz mixing of E and B in addition to the above.

    Now, your statement is that we should assume that the cosmic rays are a current. However, this is not really the case. As far as any observer in our galaxy is concerned, except in regions of very intense magnetic fields, the cosmic rays are moving isotropically, that is, if one chooses a fixed distance from the observation point, there are statistically as many cosmic rays in one direction as in the direction 180 degrees away. Because of this, the induced magnetic field of the cosmic rays has a statistical average of 0.

    Because the cosmic ray ensemble is in continuous random motion, there will be some small fluctuations about the average induced 0-field. The cosmic ray motion is random because the cosmic rays scatter off fluctuations in the background magnetic field in a statistical fashion, and the magnetic field fluctuations form a Kolmagorov power spectrum.

    The cosmic ray lifetime in the galaxy is about 16 million years (~5x1014 seconds). The scattering period in the galaxy is one cosmic ray scatter every 2500 seconds (scattering time is about 1 scatter every 10 gyroradii, E/(ecB)). For a 5 GeV proton you get:
    10 * (5000x106 eV /((4.803x10-10 esu)*(2.997x1010 cm/sec)*(10-6 gauss))) * ((esu-gauss)/(erg/cm)) * (1.602x10-12 erg/eV) = 5000 eV/esu * sec/cm * 1/gauss * (esu-gauss-cm)/erg * erg/eV = 5000 seconds

    Thus each cosmic ray makes a scatter, changes direction, about 1011 times during its travel through the galaxy. This is why we consider the cosmic ray flux in the galaxy to be generally isotropic.

    And the isotropy of the galactic cosmic ray flux implies that the average induced magnetic field due to cosmic rays in the galaxy is 0.

    Dr. Clifford Lopate
    (December 2005)

  18. High-Energy Protons and the Cosmic Microwave Background

    Does the size of a proton get smaller as it approaches the speed of light because of relativistic length contraction? Does the size of the proton affect its interaction with microwaves in space?

    Interactions between protons and the cosmic microwave background (CMB) are definitely significant at very high proton energy, and you are correct that special relativity plays a role in this.

    If you transform to the reference frame of the proton (i.e. you remain at rest with the proton), photons in the microwave range (and there are a whole lot of photons in the microwave range in our universe left over from the Big Bang) will appear to have very, very short wavelengths (length contraction!). These photons will have wavelengths in the gamma-ray range. This corresponds also to getting a Lorentz boost in energy (recall that the photon energy is expressed in terms of its wavelength, lambda, as E=hc/lamda, h is Planck's constant and c is the speed of light) such that CMB photons actually have enough energy to excite the proton. Since very high-energy protons (above an energy threshold known as the Greisen-Zatsepin-Kuzmin (GZK) cut-off) will likely interact with the CMB, scientists expect to see a drop off in the numbers of high-energy protons measured at Earth.

    You may already know this, but this effect is actively being looked for with ultra-heavy cosmic rays. See the High Resolution Fly's Eye and the Pierre Auger Observatory for more information.

    Dr. Georgia de Nolfo
    (December 2004)

  19. Millikan and Cosmic Rays

    I am a high school student doing my physics seminar on Robert Andrews Millikan. Could you please help me understand how he measured cosmic rays and ultimately decided that they do indeed come from space?

    I should first say that, although Millikan's work was extremely important for cosmic ray studies, he was not the first one to study them. Several people (Wulf on the Eiffel tower, and Hess from balloons) were before him.

    Millikan and his collegue, I.S. Bowen, flew things called electroscopes on high-altitude balloons starting in 1922. Electroscopes measure the number of electrons knocked off something like gold foil by the cosmic rays passing through it. At one point, Millikan was convinced that the radiation was local, because the measurement of quantity over Europe was four times that over Texas. We now know that it's the Earth's magnetic field that allows more cosmic rays into the atmosphere the closer you are to the magnetic poles. It wasn't until 1926, after a detailed study of electoscopes in two lakes at different altitudes (Muir Lake and Arrowhead Lake), putting the electroscopes at different depths in the two lakes, was Millikan convinced that the radiation was coming down into the atmosphere. He's the one who coined the term "cosmic rays".

    Dr. Eric Christian

  20. Cosmic Ray Composition and Space Weather

    In a textbook, I read that nearly 90% of cosmic rays are protons, alpha particles and other heavy nuclei are about 9%, and the remaining 1% are electrons. Are these numbers close to correct? If so, why does the cosmic ray flux not create a buildup of positive charge on the Earth and on the Moon? Before seeing these numbers, I'd assumed that there'd be equal numbers of positive and negative charges in the primary cosmic ray flux. I'd also be interested in particle abundances in the solar wind and in the Van Allen radiation belts.

    The cosmic ray composition is roughly 89% protons and 10% helium, and the remaining 1% is in the form of heavier nuclei. In addition, 1% of cosmic rays are electrons.

    The dearth of electrons in cosmic rays is attributed to the inability to accelerate electrons to cosmic ray energies compared with positively-charged particles, although these acceleration processes are still not well understood. Because the Earth has a magnetic field, many cosmic rays, particularly low-energy cosmic rays, do not reach the Earth's upper atmosphere. Those that do penetrate the atmosphere, do not reach the Earth's ground (with the exception of weakly-interacting muons), but interact within the Earth's atmosphere, producing a large shower of secondary particles. Furthermore, the incident intensity of galactic cosmic rays is quite small compared with the total radiation coming from our Sun.

    In addition, the Earth itself is negatively-charged, and the exchange between the Earth's upper atmosphere and the ground is what gives rise to lightening. It is believed that cosmic rays play an important role in the ionization of the upper atmosphere, which in turn affects cloud and thunderstorm formation. For more information on the composition from the Sun and around the Earth's Van Allen belts, see the current research from the Advanced Composition Explorer (ACE). And for up-to-date information on space weather, visit

    Dr. Georgia de Nolfo
    (August 2006)

  21. Cosmic Ray Effects on Humans

    Please tell me about the specific effects that cosmic rays have on human beings and how we might need to address them.

    Cosmic rays definitely can have an effect on astronauts, including those on the space station. One well-known effect is that a higher percentage of astronauts develop cataracts than do people who have not been in space.

    Humans in space also have a somewhat greater risk of developing cancer as a result of being irradiated by cosmic rays. This is a major risk if we eventually try to send an astronaut to Mars, because the trip would take many months. (See also Wikipedia).

    Fortunately, humans and animals on the surface of the Earth are largely protected by the Earth's atmosphere, which slows down and stops all but the highest energy cosmic rays (and the secondary particles they make in the atmosphere), and by the Earth's magnetic field, which deflects low-energy cosmic rays away except over the polar regions. Some cosmic radiation does reach the surface of the Earth, and it is possible that people living at very high altitudes have a slightly higher risk of cancer because of this.

    Cosmic radiation also can reach airplane altitudes, and for that reason, female airline personnel who are pregnant are advised to fly low-latitude routes rather than fly polar routes, such as from Los Angeles to Europe. That way they are much-better protected by the Earth's magnetic field.

    Cosmic rays, along with other forms of radiation like X-rays and gamma-says, can also cause genetic mutation, but this is not a major concern for humans and animals that do not go into space.

    There are some that have claimed that cosmic rays make an important contribution to cloud formation. However, this is a controversial scientific issue that, to my knowledge, has not been decided.

    In this Wikipedia article you will find some discussion of the possible contribution of cosmic rays to cloud formation, climate change, as well as other effects.

    For more information on space weather, check out the Cosmicopia Space Weather page, including the links to news articles on the topic.

    Dr. Richard Mewaldt
    (September 2009)

  22. Cosmic Rays and Global Warming?

    I am surprised that the NASA THEMIS program is not tasked with helping settle the recent global warming controversy, whether warming is caused by greenhouse gases or variations in solar activity (cosmic rays). I hadn't heard of the cosmic ray theory until last week when Martin Durkin aired his documentary in Britain. At first blush, I find it as convincing as greenhouse gases and was surprised to see that this theory wasn't even mentioned in the Feb. 2007 IPCC report.

    You seem to have several misconceptions that hopefully I can clear up.

    The THEMIS spacecraft were designed to study substorms in the magnetosphere, a very different scientific topic than either the low (<4 km) clouds or the ultra-high energy galactic cosmic rays purported to affect climate. NASA has investigations in both areas, but they require very different insturments and spacecraft, and THEMIS cannot scientifically add to either topic.

    Removing the considerable media hpe, the Svensmark et al. results show that the solar modulation of galactic cosmic rays may be a part of the complicated interaction of solar activity and the Earth's climate. However, there is not much scientific evidence they they are the driving cause of the terrestrial warming, and there is sufficient counter-evidence to bring a lot of doubt to the "conclusion". This correlation is still being actively studied, but it appears that the controversy has been overblown for political reasons, not scientific ones.

    A good (but long) scientific discussion of the topic can be found in this paper.

    Dr. Eric Christian
    (March 2007)

  23. Most Abundant Elements in Cosmic Rays

    What are the 10 most abundant elements that make up the cosmic ray ions found in Earth orbit?

    To a very good extent, the abundance of cosmic rays is the same as the abundance in the universe as a whole. So the 10 most abundant elements (in order from most abundant down) are hydrogen, helium, oxygen, carbon, neon, nitrogen, magnesium, silicon, iron, and sulfur.

    Dr. Eric Christian

  24. Abundance of Hydrogen in the Universe

    What is the present abundance, by weight, of the element hydrogen in the known universe?

    Counting only the baryonic matter (matter made from protons and neutrons), hydrogen (including free protons which are counted as ionized hydrogen) is about 72 or 73%. About 26% is helium, and the rest is heavier than helium. If there is dark matter (which is presumed to have mass) that drops the fraction of hydrogen, but dark matter isn't proven.

    Dr. Eric Christian

  25. Particles From the Sun

    The "Science Goals" link on the ACE home page includes several goals about determining the difference in composition between the Sun's corona and photosphere, but doesn't mention the chromosphere. Why?

    Since ACE measures only particles, and the particles that come from the chromosphere have to "pass through" the corona before they get out, there is essentially no way to separately get the composition of the chromosphere. Photospheric particles are given off in solar flares, so we can look at them.

    Dr. Eric Christian

  26. Density of Solar Wind Electrons

    What is the density of solar wind electrons?

    The average density is 5 protons per cubic cm at 1 AU astronomical unit (Earth-Sun distance). But the density is highly variable, and can be <1 to up to ~100. As one approaches the sun, the density increases as 1/r2 -- the same as sunlight and gravity.

    Dr. Richard Mewaldt
    (January 2012)

  27. Solar Wind and Corona

    Where can I get information on solar wind and the solar corona?

    At a college level, your best bet is the college library. At a lower level, middle or high school, start with the heliosphere in our web site (click on the Sun and Solar Wind to learn more). And these are a few other sites that might be of help to you at the same level:

    • Sun -- Windows to the Universe
    • Sun -- Views of the Solar System
    • About the Sun -- Stanford Solar Center

    Ms. Beth Barbier

  28. Temperature of Solar Wind Ions and Electrons

    What is the temperature of solar wind ions and electrons?

    The individual solar wind ions and electrons have temperatures of 10,000 - 100,000 degrees. In the sun's corona, they have temperatures of ~1 million degrees (Kelvin scale, where 1 degree is 9/5 of a Fahrenheit degree, and where absolute zero is -273 centigrade). They cool off during their four day journey from the sun to 1 AU.

    Dr. Richard Mewaldt
    (January 2012)

  29. Charge on Solar Wind?

    Is there a net electrical charge on the solar wind? If there is, is the Earth developing a charge with respect to the Sun?

    The solar wind contains both ions (protons and heavier nuclei) and electrons and is electrically neutral, so the Earth is not developing a charge.

    Dr. Eric Christian

  30. Hyrdrogen in theSolar Wind

    Are solar wind protons coming from hydrogen? Why are solar wind protons and electrons not reuniting (or uniting) to form (re-form) hydrogen?

    In the sun, it is too hot (1 million degrees in the corona) for hydrogen. As the solar wind moves out, a very small percentage of the protons and electrons do recombine -- perhaps one in a million. The reason more do not recombine is that the density is so low that the chances of them coming close to each other is very small.

    There are neutral hydrogen atoms entering the solar system from the nearby interstellar medium at an average speed of 26 km/second. But they get ionized as they get close to the sun.

    Dr. Richard Mewaldt
    (January 2012)

  31. Variations in Solar Wind

    I am studying solar wind and have a few questions -- How does solar wind vary with time? How does the intensity of solar wind vary? How does the solar wind affect the Earth?

    The density, temperature, and velocity of the solar wind all vary with time in a pretty complicated way. The magnetic field associated with the solar wind also varies in amplitude and direction. The ACE spacecraft, currently at the Earth-Sun libration point (L1) a million miles "upstream" of Earth in the solar wind, measures all of these quantities. You can check ACE Browse Data and ACE Real-Time Solar Wind sites for plots of the solar wind parameters (look at MAG and SWEPAM data).

    The normal solar wind doesn't have much effect on the Earth (it's deflected by the Earth's magnetic field), but bursts of plasma and magnetic field, called coronal mass ejections (CMEs), that travel with the solar wind from active regions on the Sun, can cause "geomagnetic storms", which is what NOAA is trying to predict with ACE. See the NOAA (ACE Real-Time Solar Wind) page above for more about these.

    Dr. Eric Christian

  32. Solar Wind Erosion

    What is the amount of erosion caused by the solar winds, and how much is recovered in the form of meteorites?

    If you are asking about erosion of the Earth's surface, the solar wind doesn't really make it through the atmosphere or the Earth's magnetic field. Even on the Moon, which has no atmosphere or magnetic field, the solar wind doesn't knock atoms off the surface fast enough to escape the Moon's gravity, so there isn't any lunar erosion either. The Earth does lose some of the gas in its atmosphere just by random diffusion away from the Earth, but it is not as much as the approximately one ton per hour that the Earth gains from micrometeorites.

    Dr. Eric Christian

  33. Electrons from the Sun

    Where can I find information on the energy spectrum of electrons reaching the Earth's atmosphere from the Sun?

    As a graduate student, you should have access to a library where you can find the Proceedings of the 26th International Cosmic Ray Conference held in Salt Lake City in 1999. You can find quite a bit of good information as a starting point. In particular, look for volume 7, page 103, an article by Paul Evenson and co workers. That article and its reference list should get you started.

    Also there is a PhD thesis by David M. Huber from the University of Delaware in 1998, which should be available on loan. It's entitled "Solar Modulation of 50 - 500 MeV Cosmic Ray Electrons and the Electron Spectrum from 1964 - 1994".

    Dr. Louis Barbier
    (December 2000)

  34. Carbon-14 and Cosmic Rays

    Do cosmic rays transform nitrogen atoms into a heavy form (isotope) of carbon with atomic weight of 14 (C to the 14th power) instead of 12?

    Most carbon in the universe is 12C or carbon-12, which has 6 protons (that's what makes it carbon) and 6 neutrons (6 + 6 = 12, which is why it's called carbon-12). Carbon-14 or 14C is a radioactive isotope that has 6 protons and 8 neutrons. It is mostly created by cosmic ray interactions (primarily slow neutrons) with nitrogen in the upper atmosphere. 14C is used by archeologists to determine when a plant or animal was alive.

    The superscript (raised smaller number) is used to show the isotopic number and doesn't have anything to do with the 14th power.

    How is 14N changed to 14C (radiocarbon) in the Earth's atmosphere?

    At the energies of the interaction, I would expect that most of the 14C is produced by charge exchange. The exchange particle is a beta-particle: either an electron or a positron. There are other processes that produce 14C, such as breaking nucleons off 16O or heavier atoms, but the charge exchange of 14N is the most common and so produces most of the 14C.

    What is the source of the neutrons that produce the 14C? Are they the original (primary) cosmic rays or are they secondary neutrons resulting from the primaries striking the atmospheric 14N atoms?

    There are primary neutrons in the cosmic rays, but at the altitude that most 14C is produced, there are a lot more secondary neutrons. So the secondary neutrons produce most (but not all) of the 14C.

    Dr. Eric Christian
    (March 2003)

    Would variations in the Sun's solar wind output, or in cosmic rays impinging on the Earth, or in the strength of Earth's magnetic field affect 14C formation in our atmosphere over a significant period of time?

    Yes, it is true that all of these things would affect 14C formation in the atmosphere, mostly because all of your criteria would affect cosmic-ray flux at Earth.

    As an aside, it is possible to check for changes in 14C production by looking at uncharacteristically low 14C values in the rings of bristle-cone pine trees over the last ~10,000 years and stalactite calcium carbonate over the last ~40,000 years. In both of these cases, the age of the material can be verified by another means.

    Lauren Scott
    (May 2004)

  35. Cosmic Ray Carbon

    What is the amount (mass) of cosmic ray carbon entering the Earth in a certain time period (annually, biannually, etc)?

    The amount of carbon reaching the Earth from extra-terrestrial sources is zero - it all fragments in the atmosphere. The amount that reaches the atmosphere has been very well measured up to about a few TeV (1012 eV ). The flux is energy dependent, with a peak at about 1 GeV and a value of ~10-2 particles/m2-sr-sec-MeV. This means that in one year (~3 x 107 sec) in a 100 MeV energy bin around 1 GeV you would get about 3 x 107 particles per m2-sr. If you assume a scale height for the atmosphere (say 50 km), it has an acceptance of (4*pi*(6106x103))2 or 5.88 x 109 m2-sr which would give you (3 x 107) x (6 x 109) = 1.8 x 1017 particles per year entering the Earth's atmosphere. That would be about a microgram of carbon per year.

    Dr. Louis Barbier
    (December 2000)

  36. Density of Hydrogen in Space

    What can you tell me about the density of hydrogen in the interplanetary voids?

    In interplanetary space, there is more hydrogen closer to the Sun, because most of it is due to the solar wind which spreads out further from the Sun. At Earth, there are 5-10 hydrogen atoms per cubic centimeter (cc). For other distances, you have to divide by the distance (in Astronomical Units) squared. This value varies with time and you can check on the current value at the ACE spacecraft.

    In interstellar space, the average density in the galaxy is about 1 atom / cc, but the solar system is in a low density region with about 0.1 atoms / cc.

    Dr. Eric Christian

  37. Cosmic Ray Energy

    What is a typical range of energies for cosmic rays, and are they usually totally ionized?

    It depends upon which of the several types of cosmic rays you are talking about. Solar cosmic rays (or Solar Energetic Particles) are typically less than 1 MeV in energy and are usually partially (approximately half) ionized. At about 10 MeV, Anomalous Cosmic Rays (accelerated at the edge of the solar system) dominate and are singly ionized (missing only one electron). At even higher energies are Galactic Cosmic Rays which have probably been accelerated in supernovae remnants. They have typical energies of about 1 GeV, but go all the way up to 1021 eV, and are fully ionized (no electrons).

    Dr. Eric Christian

  38. Cosmic Ray Radiation

    I would like to know the strength of cosmic rays at some points in the universe. Which unit is more general to use about the strength of cosmic rays, electron volts (eV) or Sieverts? The points are:
    • Moon
    • Mars
    • Earth (the polar regions)
    • Earth (on the equator)
    • Earth (here in Japan)

    This question is not so easy to answer. Cosmic ray intensity changes by more than a factor of 2 with the 11 year solar cycle, and can increase for short times by a much larger factor when there is a solar flare. I tend to think of cosmic ray intensity in terms of an energy spectra, or flux (particles per MeV * meters^2 * steradian * seconds). Since you're asking about sieverts, I assume you are interested in cosmic rays as a radiation source, not as a scientific study. But to first order:

    • Moon = 600 - 1500 milliSievert per year
    • Mars = 600 - 1500 milliSievert per year (a little more galactic cosmic rays, a little less solar)
    • Polar = 40 - 120 milliSievert per year
    • Equator = 3 - 8 milliSievert per year
    • Japan = 10 - 30 milliSievert per year

    There is also an altitude effect that isn't considered here. A lot of this info came from NASA JSC.

    Dr. Eric Christian

  39. Collecting Solar Wind Electrons on Satellites for Power?

    Has there been any study of the collection of solar wind electrons by satellite for energy use on Earth?

    I am sure people have thought about using the solar wind to generate power, but I can tell you that it is not practical. Most of the solar wind energy is actually in protons (which are ~1836 times more massive than electrons), so let's consider solar wind protons.

    The solar wind moves at an average speed of ~400 km/sec (4 x 10 cm/sec) with a density of ~5 protons per cubic cm. Consider a 1 square meter collector. In one second, it will collect about 20 billion protons. If we could turn all of their kinetic energy into electrical power, we would get only ~0.0032 watts for this 1 square meter detector. So let's make a 100 meter x 100 meter detector. We still have only 32 watts, and we still need to get it down to the ground. (Solar wind electrons would account for a small fraction of this.)

    (A solar cell on the ground is less efficient and is hit by ~1000 watts per suqare meter during midday, but we do not have to build a satellite to collect it.)

    Nevertheless, the solar wind does have enough power at times to compress the earth's magnetic field and cause what we call geomagnetic storms, that cause the aurorae and also sometimes affect your cell phone reception and power grids, among other things. That is because the solar wind is acting over the whole front (sun-facing) side of the earth's magnetosphere (several billion square meters), and it can suddenly release some of the energy stored in the earth's magnetic field.

    Dr. Richard Mewaldt
    (January 2012)

  40. Can We Harness Cosmic Energy?

    How much cosmic energy actually reaches the Earth's surface? Is it possible or practical to collect that energy?

    For starters, there are several forms of "cosmic energy". We often refer to cosmic radiation, or cosmic rays for short. There is also the Sun's light at various wavelengths (optical, infrared, UV, gamma rays, etc.) as well as star light, and there are energetic, electrically charged particles from many sources including the sun, other stars and stellar explosions, the solar wind that fills the space between the planets, and the interaction of the wind with the interstellar material.

    Let's run some numbers: To start, if we add all the energetic charged particles originating at the Sun and stellar explosions, we find that the energy striking the Earth is about 5 joules/sec/km2 (where 1 joule/sec = 1 watt). That means that were you to collect all the energy of cosmic rays over 12 square kilometers, you would have enough energy to power a 60 watt light bulb. When a strong solar eruption occurs this number can increase by about 20% for perhaps as long as 10 hours, but this burst of activity is short lived.

    What about light? The solar constant is 135.3 mwatts/cm2 which corresponds to a radiant energy input of 1.4 gigawatts/km2 (that's 1,400,000,000 watts per square kilometer) at noon near the equator. Now this is enough energy to power a small city if we could convert it to something usable like electricity in an efficient manner!

    There are other sources of energy input, but they are all small: stellar light, the solar wind plasma, gamma rays from astrophysical objects, etc.

    Now to answer your last question: Can we capture the energy and convert it to useful purposes? I think you see that capturing solar light is by far the most useful activity and some places are doing that now. The collectors we build will need to be both large and efficient. At present, our most efficient solar collectors run at about 5% efficiency. It seems that a solar collector will need to be about as large as the area it seeks to power (collectors that would power a home need to be about as large as the roof of the house, collectors to power a city must be about the scale size of the city, etc.).

    One last number for you: On October 11, 1990 researchers using an instrument that observed the sky, searching for the light signal of very energetic charged particles passing through the atmosphere, recorded the most energetic particle ever observed. It was a proton with a kinetic energy of 3 x 1020 eV. This is equivalent to the kinetic energy of a bowling ball dropped from your waist and hitting the floor (or your toe). Fortunately, the cosmic rays that strike us routinely simply pass harmlessly through our bodies. Unfortunately, this is also what makes them difficult to harness into electricity.

    Drs. Charles Smith and Clifford Lopate
    (July 2004)

  41. Plasma

    What is plasma? What does it look like? How can we get it on Earth?

    Plasma is a gas that is hot enough that some electrons have been stripped from the atoms. Plasma is just ionized gas, in other words, a gas of nuclei and electrons instead of a gas of atoms and molecules. So everything in a plasma is either positively charged (the atomic nuclei with whatever electrons remain with it) or negatively charged (electrons). The fact that it is an electrically charged gas makes it behave very differently from a mostly neutral gas (like air), which is why it's considered a fourth state of matter. It would glow (with the color depending upon what temperature it's at) just like the Sun and stars. It can be made in laboratories and can occur in nature on Earth (lightning causes plasma to temporarily form).

    Where is a good place to research plasma on the Internet?

    One place I found on the web is Perspectives on Plasma.

    In what year was plasma defined (discovered)? Who was responsible?

    I don't know who coined the word "plasma". It was known that gas could be ionized long before the word plasma came into use.

    How was it determined that 99% of the Universe is in a plasma state?

    Most of the gas in interstellar space is ionized (astronomers can tell by the wavelengths of light the gas absorbs and emits), and all of the gas in stars in ionized, that's where the 99% comes from. The 99% ignores any dark matter which might be out there. (See Imagine the Universe! for more on dark matter).

    Is it possible to create plasma in a laboratory?

    It is relatively easy to create it. It is even done in arc lights and in the novelty spheres that look like balls of lightning. Lighting also causes plasma in the atmosphere.

    Dr. Eric Christian
    (November 2000)

  42. More on Plasma

    Has plasma been confirmed yet as a fourth state of matter?

    Your question is often asked, but you still may not find the definitive answer that you want.

    I should say that indeed plasma is referred to as the fourth state of matter in many places, with sentences ranging from "sometimes referred to as the fourth state of matter" to "is described as the fourth state of matter". In none of the places have I found a final verdict on this question.

    Yet, for a physicist, the transition from a regular gas to plasma is quite different than that from solid to liquid or from liquid to gas. Between the three classical states of matter, the two different states are always spatially separated. For example, vapor may be found in individual bubbles that rise up out of liquid water, but it is never completely mixed with water in the same place, whereas neutral atoms and molecules, which make up a gas, can be mixed completely with positively-charged ions and negative electrons, which make up a plasma. These mixtures can range from completely neutral gas, via a gas with just a few electrons and ions, all the way to a plasma without any neutral atoms. This smooth transition between the two states is the only blemish in the naming game of whether to refer to plasma as the fourth state of matter.

    However, I would like to assure you that the fourth state of matter is used more often in connection with plasma than not used. So, referring to plasma as the fourth state of matter, which arises when a gas is heated to even higher temperatures so that atoms are destroyed, is widely accepted.

    If you asked me whether plasma is widely accepted as the fourth state, I would say "yes". If you asked me whether it is unilaterally accepted, I would have to say "no".

    Eberhard Moebius
    (October 2009)

  43. Collisional and Collisionless Plasmas

    What are collisional and collisionless plasmas? I encountered them while studing Debye shielding.

    First, what is a plasma? A plasma is a gas of electrically charged particles that behave collectively. Usually, that means protons and electrons in equal number, but many naturally occurring plasmas have so-called "minor" ions such as helium, iron, etc. Collective behavior means large numbers of particles can move in an organized fashion to support waves or produce Debye shielding. The gas in fluorescent bulbs becomes a plasma when current passes through the tube.

    A collisional plasma is one where individual particles strike each other with sufficient frequency that many collisions occur on the time scale of the collective behavior. The high-density region of the Sun just above the photosphere is a collisional plasma, and particles are not free to move without bumping into other particles that stop its individual motion.

    A collisionless plasma is one where individual particle collisions are unimportant. A typical proton in the solar wind experiences one collision when going from the high corona to Earth, a distance of 150,000,000 km, or nearly 100,000,000 miles. It takes almost 3 days for the plasma to travel that distance, and in that time the waves present oscillate many times, the distribution of the particles changes through interaction with those waves, and the plasma evolves.

    Debye shielding occurs when the charged particles are free to move in a collisionless plasma. If you imagine placing a test proton in empty space and using a probe to measure the electric field, you can move as far from the proton as the sensitivity of your probe will permit and still "observe" the presence of that proton. Place that same proton into a collisionless plasma and two things happen: other protons are pushed away and electrons are pulled closer. The particles are still free to move, but on average the electrons get a little closer to the test proton than other protons. This means that when you use that same instrumental probe to measure the electric field of the test proton, you will observe it when you are very close, but as you move away the cloud of electrons around the test proton will begin to obscure it until at one Debye length you will no longer be able to measure the electric field due to the test proton (the negatively charged electrons will have neutralized the positive charge they surround).

    All of space that we so often think of as "empty" - the space between the planets and the space between the stars, the region of our Sun's influence called the heliosphere, and the region of our planet's influence called the magnetosphere - all are filled by collisionless plasmas. The vast regions of the universe between those isolated stars is filled with collisionless plasmas, and Debye shielding is the first step in understanding the physics of those vast systems.

    Dr. Charles Smith
    (March 2003)

    You said that the Debye sheilding is for collisionless plasma, but in the preface of this article on Debye shielding (PDF file), it says something different. I'm confused! Which one obeys standard Debye shielding -- collisional or collisionless plasma?

    This is getting very advanced.

    First, I made a very basic error in my reply to you and that was to focus on the type of plasma that I study, which is collisionless.

    Yes, it is true that collisional plasmas also exhibit Debye shielding. The same argument holds - particles of opposite sign tend to mask one another by forming a cloud of neutralizing charge around the test charge.

    Non-neutral plasmas, or pure electron plasmas, are yet another beast. It is natural that when a plasma of charged particles is created from a neutral gas, the protons and electrons stay in proximity to one another and form a neutral plasma. A non-neutral plasma is created by removing one of those particle types, and this is most often done by collecting electrons separate from protons. To hold them in place it is necessary to impose a strong static electric field or a magnetic field. The column of electrons then interact with the external field, and some new and different things happen. For instance, the column may begin to rotate about the magnetic field.

    Introducing a test charge into the electron column can change the size of the column and/or the rate of rotation. While the basic process of neutralization (Debye shielding) will still take place, there can be competing processes that win out.

    Since non-neutral plasmas rarely if ever occur in nature, that is about my total understanding of the problem. Sorry I can't tell you more.

    Dr. Charles Smith
    (June 2003)

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