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To learn a lot more about the Sun, check out Cosmicopia's page on the Sun.

Don't Look at the Sun!
Sun Viewing Safety
Who Discovered the Sun?
What Would You Weigh on the Sun?
Why Does the Sun Appear to Follow Me?
Seeing Sunspots
The Solar Cycle
More Than One Sun?
What's a Heliologist?
What's a Heliophysicist?
Solar Mass
Composition of the Sun
Nucleosynthesis in the Sun
Why the Sun Shines
How Much Mass Does the Sun Consume?
How Hot is the Sun's Core?
Solar Interior
Light from the Center of the Sun
The Sun Still Has Most of Its Energy Left?
How Long Will the Sun Last?
Myth: Sun is Shrinking
High Temperature of the Corona
Earth and the Heat of the Corona
How Long Has the Sun Been Burning?
Age of the Sun
How Much Power Does the Sun Produce?
Amount of Energy the Earth Gets from the Sun
Solar Constant
Total Solar Irradiance
How Long for the Sun's Heat to Reach Earth?
Why Doesn't the Sun Heat Space?
Sunspots and Blackouts
Do Sunspots Disappear?
Sunspots and Planetary Alignment
Coronal Holes
Difference Between Flares and Prominences
Solar Cycles
Finding Solar Activity Data
Interplanetary Coronal Mass Ejections
The South Atlantic Anomaly
Sun's Magnetic Field
Magnetic Field Phi
Far Side of the Sun?
Solar Longitude
The Sun's Orbit
Rotation of the Sun
Sun a Binary Star?
Sun Further North?
Particles From the Sun
Movement of Particles From the Sun
Solar Wind and Corona
Charge on Solar Wind?
Detecting Particles from Solar Wind
Solar Wind and Earth's Magnetosphere
Solar Wind and Spacecraft
Variations in Solar Wind
Why Hasn't the Sun Blown Away the Earth?
Ions in the Solar Wind
Neutral Current Sheet and the Solar Wind
Solar Wind Erosion
Solar Wind Electricity?
Solar Cell Stations in Space?
Oort Cloud and the Heliopause
How Big is the Heliosphere?
Temperatures at the Solar Wind Termination Shock
Effects of Shrinking Heliosphere
Weakened Heliosphere and Global Warming
Open Problems in Physics
Copernicus' Heliocentric Theory
Does the Sun Revolve Around Something?
How Long to Drive to the Sun?
Dispose of Waste in the Sun?

  1. Don't Look at the Sun!

    Is it possible to see sunspots with the naked eye?

    Please DON'T EVER LOOK DIRECTLY AT THE SUN! It will cause permanent eye damage.

    There's a NASA web site that will give you more information about solar observing. Section Five on this site may answer some of your questions about sunspots, if you have others. Of course you can always check out our page on sunspots.

    Beth Barbier

    Are the Sun's rays more dangerous in a solar eclipse then when normally looking at the Sun?

    No, the light from the Sun is less damaging during a solar eclipse than normal. The problem is that it is easier to look at the Sun during an eclipse and also your pupils get bigger. So you are somewhat more likely to damage your eyes during an eclipse, even though it is less dangerous in an absolute sense.

    Dr. Eric Christian
    (September 2001)

  2. Sun Viewing Safety

    Is is safe to look at the Sun through fog or clouds?

    The only time you can safely look at the Sun with the naked eye is during a total eclipse, when the moon completely covers the Sun.

    There's a good description of why and other options for viewing the Sun at Eye Safety And Solar Eclipses.

    Beth Barbier
    (January 2005)

  3. Who discovered the Sun?

    Who discovered the Sun and when?

    Since the Sun is very easy to see in the sky, you can only say the the very first person capable of seeing it was the one who discovered it.

    Beth Barbier
    (April 2008)

  4. What Would You Weigh on the Sun?

    What is the force of gravity at the surface of the Sun, and what would a 100 pound person weigh on the Sun?

    There's a calculator on the Exploratorium museum site to find your weight on different planets, moons, and stars. Plug in your Earthly weight of 100 lb, and you'll find that you would weigh 2707.2 lb on the surface of the Sun, if you could actually stand on it -- according to this site. This number is in the ballpark of roughly 28 times that of Earth, but values vary slightly due to estimates of the exact mass and radius of the Sun.

    The force due to the acceleration of gravity on the surface of the Sun is approximately 275 m/sec2 (900 ft/sec2).

    Beth Barbier
    (April 2008)

  5. Why Does the Sun Appear to Follow Me?

    Why does it look like the Sun follows me everywhere I go?

    We already have an answer to the question as to why it looks like the moon is following you. The same principles apply here, but remember: DO NOT LOOK DIRECTLY AT THE SUN!

    Beth Barbier
    (January 2005)

  6. Seeing Sunspots

    When I was outside one day, I noticed the shape of the Sun behind the fog. From my perspective, I saw a dark spot, not round but elongated, in the middle of the top half of the Sun. Every time I looked, it seemed to be in the exact same place. What did I see?

    You almost certainly saw sunspots. One of the largest groups of sunspots seen for years is now visible, and solar eruptions resulting from it are likely to cause a geomagnetic storm when it reaches Earth.

    This is a big story in the news right now. Here are a couple of articles online to get you started:

    Sun erupts with intense activity
    Seeing double: astronomers amazed at two huge sunspots
    Earth put on solar storm alert

    And never look at the Sun directly! Here's a good website on Safe Sunwatching: Do-it-yourself Sunspot Watching

    Dr. Louis Barbier and Beth Barbier
    (October 2003)

  7. The Solar Cycle

    I have seen several news stories that refer to "the 11 year sunspot cycle." I thought that there was more like a 7 to 18 year range on the length of the interval between solar activity maxima, but I have been unable to find documentation of this. What is the actual range of the interval between solar maxima? Is it really so rigid that we can call it an 11 year cycle?

    The periodicity of the 11 year solar cycle is complicated by the fact that there is no well-timed event that you can actually use as a basis for your periodicity. However, the Sun is much more regular than the "7 to 18 year range" that you mention. You can look at this image to see the best long term measure of solar activity (the sun spot number). There are modern observations that give a better measure of solar variability, but we've only got data for two or three cycles worth. The actual long term period is slightly more than 11 years and is remarkably stable. There are scientists who look at the statistics of solar activity and may have found other periodicities, but for the general public, there is effectively no difference between 10.8 and 11.7 years, especially given the broad and irregular temporal structure of both solar min and solar max.

    Dr. Eric Christian
    (October 2003)

  8. More Than One Sun?

    Is there more than one Sun?

    There is only one sun in OUR solar system (trust me, you would have heard about it if there were more!), but there are billions of stars in the Universe, and many of them are "suns" for other planetary systems. As we discover more extra-solar planets, we discover more "suns". Watch the news for this.

    Beth Barbier

  9. What's a Heliologist?

    What is a heliologist?

    A heliologist would be a person who studies the Sun.

    Dr. Louis Barbier

  10. What's a Heliophysicist?

    Is "heliophysicist" the name for scientists studying the Sun?

    Heliophysicist could certainly be the name for scientists studying the Sun, but most of the ones I know call themselves "Solar Physicists". But if someone were to use it (and maybe at other locations they do), I wouldn't have any problem with it.

    Dr. Eric Christian
    (February 2002)

  11. Solar Mass

    What is a solar mass?

    One solar mass is just the mass of our Sun, 2 x 1030 kg. It's used for convenience, because saying that a star is 4 solar masses (4 times the mass of our Sun) is easier to visualize than saying it has a mass of 8 x 1030 kg.

    Dr. Eric Christian
    (June 2000)

  12. Composition of the Sun

    What elements are found in the Sun's makeup?

    It depends upon what you mean by elements. Elements usually mean the elements of the periodic table, and the Sun contains all of the naturally occuring elements, although some are very, very rare. The Sun is mostly hydrogen (90%), and helium (9%). Everything else is only 1% of the Sun. If you are asking what states of matter (solid, liquid, gas) are in the Sun, the Sun is 100% gas, and more exactly it is a plasma, which is an electrically charged gas.

    Dr. Eric Christian
    (June 2002)

    Are there more different kinds of gases on the Sun than on any of the planets?

    There is more hydrogen and helium in the Sun than there is on Earth because the Earth's gravity is not strong enough to keep all the hydrogen and helium gas from escaping. The gas giant planets (Jupiter, Saturn, Uranus, and Neptune) are big enough to hold on to all that gas and are made up of the same stuff the Sun is. The biggest difference between the gas of the Sun and the planets is that the Sun is in a plasma state, where the electrons are separated from the gas nuclei.

    Dr. Eric Christian

  13. Nucleosynthesis in the Sun

    When we learned about the Sun, we learned that in the core, four hydrogen atoms combine to form one helium atom. But the atomic number of hydrogen is one and the atomic number of helium is two. Shouldn't two hydrogen atoms combine to form one helium atom?

    The atomic number of helium is two because it has two protons. But its atomic weight is about four because it has four "nucleons" (two protons and two neutrons). In the fusion process (which is a little complicated because there are intermediate states), two of the original protons have been transformed into neutrons. The neutrons help hold the nucleus together. Otherwise the protons would push each other apart because like charges (protons are positively charged) repel.

    Dr. Eric Christian
    (February 2003)

  14. Why the Sun Shines

    I am curious to know why the Sun shines. I understand some solar rays are x-rays but isn't the Sun much too cool to produce these?

    The Sun gives off light and heat because it is essentially a giant nuclear reactor that is fusing (burning) hydrogen into helium inside. When hydrogen combines to form helium, it gives off energy. Fusion is a very efficient way of converting mass to energy (light and heat); only a very, very, very tiny amount of the Sun is used up. There's a good Web page at the University of Oregon.

    Most of the visible light from the Sun comes just from the fact that it is hot. The surface (what we see) is about 5800 Kelvin. The center is over 15,000,000 Kelvin. A few of the thermal photons extend up into the x-rays, but most of the x-rays and gamma rays come from nuclear reactions that are taking place in the Sun (the fusion that powers the Sun is a nuclear reaction, and there are lots of peripheral reactions going on).

    Incidentally, the Sun also releases particles continuously in the solar wind, and even more so during solar activity.

    Drs. Eric Christian and Louis Barbier

  15. How Much Mass Does the Sun Consume?

    I heard at one time the amount of matter that is converted by the Sun into energy and released, but have been unable to remember the quantity stated. It was given as the number of Earth masses that are converted every month or year.

    The Sun consumes about 600 million tons of hydrogen per second. (That's 6 x 108 tons.) For comparison, the mass of the Earth is about 1.35 x 1021 tons. This would mean the Sun consumes the mass of the Earth in about 70,000 years.

    Dr. Louis Barbier

  16. How Hot is the Sun's Core?

    How hot can the core get?

    The temperature of the Sun's core is about 15 million degrees Kelvin or about 27 million degrees Fahrenheit.

    Dr. Eric Christian

  17. Solar Interior

    I have read the following expression, related to the Sun's photosphere: "Below the photosphere the solar gas is opaque. This opacity is primarily due to a small concentration of negative hydrogen ions in the region immediately below the photosphere." I want to know what "negative hydrogen ions" are. I can not imagine a negative hydrogen ion, and I do not understand how it can be produced.

    Well, I'm not an expert on stellar interiors, but it is possible for a proton to attract two electrons. Although a proton-electron pair is electrically neutral, close in it has an electric field, due to the fact that the electron and proton are not at the same location. What I think your quote is referring to, however, is what is also called the "Free-Free" absorption of light. A free electron can scatter light, but can't absorb the photon, due to conservation of energy and momentum. In the presence of something else (such as a hydrogen atom), the electron can absorb the photon. Since the electron has to be close (within the electric field) of the hydrogen atom, you could consider this to be a negative hydrogen atom, but the electron is usually unbound (on a hyperbolic orbit). But the free-free absorption is an important component to stellar opacity.

    Dr. Eric Christian

  18. Light from the Center of the Sun

    On a PBS science show I heard a statement made about the length of time it took for light to go from the center of the Sun to its outer edge. The time was in thousands of years, which does not sound plausible, but someone else on the show confirmed the statement. Can you provide any information about this?

    Yes, it does take light thousands of years to get out of the Sun. The important thing to realize is that the Sun (especially at the center) is quite opaque, that is, light travels through it only slightly better than light travels through a rock. What happens is that light only travels a short distance before it is absorbed. It is then re-emitted, but in a random direction. It eventually random "walks" it's way out of the Sun, but that takes a long time.

    Dr. Eric Christian

  19. The Sun Still Has Most of Its Energy Left?

    If the Sun is as old as it is and it still has 99.9% of its energy left, then what is the potential relative life span of the Sun remaining? What will be left at 15,000 A.D.?

    Our Sun is about halfway through the "main sequence" part of its life. During this part, the Sun "burns" hydrogen into helium (fusion), which is what generates the heat and light. The Sun has been doing this for about 5 billion years, so in 13,000 years (15,000 A.D.) there will be no real difference from the energy left now. In about 5 billion more years, the useable hydrogen (not all the hydrogen) will have been converted to helium, and the Sun will start burning helium, and become a red giant. After that the Sun will recollapse down to a white dwarf and last for billions of years more.

    Dr. Eric Christian

  20. How Long Will the Sun Last?

    How does the Sun last so long? Could the Sun ever crash into a planet, and that planet crash into another planet, until there are no more planets left? How big could the Sun get?

    I'll answer these questions together. The Sun lasts so long and gives off so much light because it is a giant nuclear reactor that is fusing (burning) hydrogen gas into helium gas. (see Why the Sun Shines) Eventually (more than 5 billion years from now) the Sun will use up most of its hydrogen and will get larger and cooler. At this time, Mercury and Venus, and possibly the Earth, will be swallowed up by the Sun. But the Sun will never get large enough (hundreds of millions of miles in radius) to "crash" into the outer planets.

    Dr. Eric Christian

  21. Myth: Sun is Shrinking

    The Sun is shrinking at five feet per year. Considering the temperature of the Sun, how would the average temperature of the Earth be affected by increasing the size of the Sun by 5 million feet per million years? If we went back say, one million, ten million, or a hundred million years?

    It is incorrect to say that the Sun is shrinking and it has been since the "creation" of the Universe. The Sun is not shrinking at a consistent rate. The data that were used to derive that were both wrong and misinterpreted. See the Skeptic Friends Network.

    Dr. Eric Christian

  22. High Temperature of the Corona

    I am a university undergraduate trying to understand the nature of the extremely high temperatures in the corona. I think it is due to b-fields interacting with plasma. Am I right or on the wrong path altogether?

    Why the corona is so hot, when the region below it is several orders of magnitude cooler, is one of the open questions in solar physics. Magnetic fields and turbulence in the plasma are certainly involved, but the exact mechanism is not understood. One suggestion is that large numbers of "microflares" are the cause. NASA is developing a mission that should study this problem (and others) called Solar Probe.

    Dr. Eric Christian

  23. Earth and the Heat of the Corona

    The Sun's corona has a temperature of 1,000,000 K. Why doesn't it incinerate the Earth?

    This is indeed an interesting question, and it has two parts to its answer.

    First, let me commend you, as you realize that the Earth is located within the reaches of the Sun's corona, which is flowing outward in the form of the solar wind.

    1. In order to incinerate massive objects, a gas (or plasma, which contains electrically charged particles) must not only be hot; it must also be dense enough so that its heat makes a difference. The solar wind is a plasma. The corona (or the solar wind) at the location of the Earth features densities of about 1-50 hydrogen nuclei per cubic centimeters, whereas air that surrounds us is about 3x1019 (3 with 19 zeros) molecules per cubic centimeter. Because of the very low density, the heat of the corona is not even enough to heat the surface of a tiny spacecraft in any noticeable way. (Heat can, however, build up in spacecraft through its own electrical energy consumption and because of the Sun's light that the spacecraft intercepts. To stay cool, the spacecraft has to radiate that energy back into space. The particles of the solar wind or of the Earth's magnetosphere - both pretty hot when expressed in degrees - don't play a noticeable role in that heat balance. Their input is negligible compared with the Sun's light.)

    2. The Earth's magnetic field prevents the solar wind from entering the vicinity of the Earth, because it consists of electrically charged ions and electrons. They are diverted by the magnetic field and do not even reach the Earth's atmosphere.

    Dr. Eberhard Moebius
    (April 2004)

  24. How Long Has the Sun Been Burning?

    How long has the Sun been burning? Compared to other stars, is our Sun young or old?

    The Sun has been burning for about 5 billion (5,000,000,000) years. Our Galaxy is more than 10 billion years old, and new stars are forming all the time, so our Sun is neither young nor old, and is almost right in the middle of stellar ages.

    Dr. Eric Christian

    I meet a lot of very highly educated people who believe that the solar consumption rate cannot match an age that old (5 billion years). They say that the Sun would have been larger than the Earth's orbit far more recently than 5 billion years.

    This is a pretty simple (freshman physics) calculation. The output of the Sun (~4x1033 ergs/second) comes from turning about 7x1014 gms of hydrogen into about 6.5x1014 gms of helium (each helium-4 nucleus produced releases ~4x10-5 ergs, you need 1038 such reactions, each of which uses 4 protons at 1.7x10-24 gms). So in 5 billion years, the Sun has fused ~1032 gms of hydrogen. This is only 1/20th the mass of the Sun (although about 1/2 the core, which is why the Sun will burn for approximately another 5 billion years). No problem, to first order this produces a Sun that is less than 4% smaller in volume, nowhere near the Earth's orbit.

    Dr. Eric Christian
    (March 2001)

  25. Age of the Sun

    How do you determine that the Sun is 5 billion years old?

    The age of the Sun is determined only by indirect means. The oldest rocks on Earth, and the oldest meteorites, are about 4.6 billion years old. If you assume that the whole solar system formed at the same time (which is probably a correct assumption), that puts the age of the Sun at nearly 5 billion years. For more description, you can look at the Stanford Solar Center.

    Dr. Eric Christian
    (May 2002)

  26. How Much Power Does the Sun Produce?

    About how much power does the Sun produce?

    The Sun's output is 3.8 x 1033 ergs/second, or about 5 x 1023 horsepower. How much is that? It is enough energy to melt a bridge of ice 2 miles wide, 1 mile thick, and extending the entire way from the Earth to the Sun, in one second.

    Dr. Louis Barbier

  27. Amount of Energy the Earth Gets from the Sun

    If you took the amount of sunlight that hits the Earth in one second and converted it into matter, how much would it weigh? How much energy is this in practical terms?

    This is an excellent question, which puts the energy balance on Earth into perspective. Let me answer this question in two steps, and then let me compare the amount of energy from the Sun to the amount humankind is using right now.

    The energy per time put out by the Sun is its luminosity, 3.8 x 1026 Joules per second (or Watts). Using Einstein's renowned formula that describes how much mass is transformed into energy, when energy is being produced, E = M * c2 (or: Energy = Mass * (Speed of Light)2), as 1 Joule = 1 kg m2/s2 and c = 300,000,000 m/s, the mass the Sun burns into energy every second is:

    Mass/Time = 3.8 x 1026/(3 x 108)2 kg/s = 4.4 x 109 kg/s

    or roughly 4 million tons per second.

    At its distance of 1 Astronomical Unit (150 million km), the Earth is hit by the Sun's energy flux F = 1400 Joules/s/m2. We call this quantity the "solar constant", as this value averaged over each year is constant within better than 1% over time. With an Earth radius of approx 6400 km, the area, which is (pi * Earth's radius)2, with which the Earth intercepts sunlight is (pi * Earth's radius)2 = 1.3 x 1014 m2 making the amount of energy captured by the Earth each second:

    F * (pi * Earth's radius)2 = 1.8 x 1017 Joules/s

    According to the same procedure as above this makes the mass to produce this amount of energy per second:

    Mass captured as sunlight per second = 1.8 x 1017 / (3 x 108)2 kg/s = 2 kg/s

    This is about 4.5 lbs/s or close to 5 lbs/s.

    To put these numbers into a perspective with highly practical relevance, on average, humankind is only using about 1/10,000 of that amount for its total energy consumption. In other words, sunlight seems to be a viable option for our energy needs, at least from the perspective of the total amount needed. Or from the point of view of mass, we are transforming about 20 kg of mass per day into energy for our energy consumption.

    If we were to use much more energy, say a sizeable fraction of the amount that the Earth gets from the Sun, the Earth would have to heat up considerably in order to get rid of the waste heat. Every power plant needs a cooler to get rid of its heat; the Earth as a whole can only do this by getting hotter.

    Dr. Eberhard Moebius
    (January 2005)

  28. Solar Constant

    What does the term "solar constant" mean?

    The solar constant is the amount of energy from the Sun at the distance of the Earth (outside the atmosphere). It is 1367 Watts per meter squared. It is not really constant; it varies by less than a percent due to solar activity.

    Dr. Eric Christian

  29. Total Solar Irradiance

    Does anyone keep tract of the Sun's radiant, or heat energy output as a function of time? Most global warming models I have looked into use the Sun's energy output as a constant or a sine wave cycle. One scientist informed me that the Sun's energy output has increased over the past 100 years, but I have not seen any data to support that contention. I would like to see a plot of the Sun's heat energy output over the past 100 years.

    There's a good explanation of the variations in total solar irradiance at the ACRIMSAT (Active Cavity Radiometer Irradiance Monitor SATellite) web site:

      "...the Sun's output changes so slowly and solar variability is so slight (less than 0.00425% of the total energy per year on time scales of days), that continuous monitoring by state-of-the-art instrumentation is necessary to detect changes with climate significance. Scientists theorize that as much as 25% of the 20th century anticipated global warming of the Earth may be due to changes in the Sun's energy output. Systematic changes in irradiance as little as 0.25% per century can cause the complete range of climate variations that have occurred in the past, ranging from ice ages to global tropical conditions. For example, scientists believe the "Little Ice Age" that occurred in Europe in the late 17th century could have been related to the minimum in sunspot activity (and a correlated minimum in total solar irradiance) that occurred during the same period."

    This page includes a graph of total solar irradiance from 1980 to 1996, measured by the ACRIMSATs. The changes in energy shown there are caused by the solar sunspot cycle. There's another graph of total solar irradiance at the Athena web site.

    Beth Barbier
    (March 2000)

  30. How Long for the Sun's Heat to Reach Earth?

    How long does it take heat created on the Sun's surface to reach Earth? Is it the same as the speed of light?

    Heat is transmitted through conduction, convection, and radiation. The heat that reaches us from the Sun is infrared radiation, which travels at the speed of light. So, it takes about 8 minutes for it to reach Earth from the Sun.

    Dr. Louis Barbier

  31. Why Doesn't the Sun Heat Space?

    Why do the Sun's rays heat up the Earth -- and me -- yet the same rays don't seem to heat up outer space?

    The Sun's rays carry radiant energy from the Sun in all directions (some is visible light, some is not visible). If the rays hit matter, the matter absorbs part of the energy and heats up. Thus the Earth, Moon and other material bodies are heated. A transparent object such as glass would be heated less than a black solid object. A white object reflects much of the radiant energy and also is not heated as much as a black object, which absorbs more of the energy.

    The space between the planets outside of the Sun is very nearly empty. It contains a very dilute or rarefied gas that absorbs essentially none of the radiant energy and is thus very, very transparent. Nearly all of the radiant energy simply passes through this space without being changed or absorbed by the very small amounts of matter there.

    Dr. Randy Jokipii
    (July 2004)

  32. Sunspots

    How are sunspots formed? What causes them? What are they made of?

    For information about sunspots, you should start with the Learning Center's sunspot page. More in-depth information on sunspots can be found at some specialty web sites such as the Exploratorium and the National Solar Observatory/Sacramento Peak.

    You might be interested to know that some of your questions about sunspots -- how are they formed and what causes them -- are the same ones that scientists are now trying to answer.

    Beth Barbier

  33. Sunspots and Blackouts

    When the "Northeast blackout" occurred on 8/14/2003, I got on the internet and checked into sunspot activity. I noticed that Sunspot 431 had quadrupled in size over the previous 24 hours, and had potential to become an X-class solar flare. What is the possibility of a connection between solar activity and the blackout?

    Unfortunately, I cannot say for sure right now whether this blackout had anything to do with solar activity. While it is true that coronal mass ejections and flares can have an effect on us here, there is no evidence of any such activity in the last few days. You can check this National Oceanic and Atmospheric Administration (NOAA) web site for space weather advisories (there have been none lately) and you can look at the ACE Real Time Solar Wind page for space weather data. NASA's ACE spacecraft provides data that NOAA uses to issue the space weather advisories.

    It may be that someone will uncover a connection between this latest blackout and solar activity, but I know of no such connection now.

    Dr. Louis Barbier
    (August 2003)

  34. Do Sunspots Disappear?

    Do sunspots disappear?

    Sunspots are only temporary; they pop up and disappear all the time. Small ones can fade out in a few days, and larger ones can last months.

    Dr. Eric Christian

  35. Sunspots and Planetary Alignment

    Is there any relationship between sunspot activity and planetary alignments?

    There have been studies of planetary effects on solar activity, but no relationship has ever been found.

    Dr. Eric Christian
    (June 2002)

  36. Coronal Holes

    Where can I find data on the number of coronal holes?

    I'm not an expert on coronal holes, but the best way to count them is by looking at the SOHO Extreme ultraviolet Imaging Telescope (EIT) image, especially the Fe XII line. Coronal holes will appear as large dark areas. You can get the recent image from either the SOHO web site or this page from NASA GSFC's Solar Data Analysis Center. There are also accumulated solar 'maps' at this page from the NOAO. also has their daily take (from the SOHO images) on coronal holes.

    Dr. Eric Christian
    (April 2001)

  37. Difference Between Flares and Prominences

    I'm still confused as to the difference between solar flares and solar prominences. I've seen photos of these hugh solar loops, which are sometimes identified as prominences and sometimes as flares.

    Prominences are big loops of hot gas (plasma) trapped by magnetic field lines. Flares are sudden increases in brightness (not necessarily in the visible part of the spectrum) in a region. The Solar Flare Theory web site from our Lab that discusses this.

    Dr. Louis Barbier

  38. Solar Cycles

    How many different solar cycles are there (for example, sunspots)? When is the next maximum of the solar cycle? With the advent of global (warming?), are there other cycles that we do not know about?

    The Sun is a variable star, and its major cycle is the 11-year/22-year sunspot cycle. I say 11-year/22-year because, although there is a maximum in solar activity and sunspot numbers every 11 years, it is caused by a 22-year cycle of magnetic field flipping in the Sun. There are just two maximum activity time periods every 22 year cycle. The next solar maximum is approaching now, and should peak about 2002.

    There are other changes in the solar activity that either are irregular, or humans haven't been monitoring the Sun long enough to see the cycles (we have complete coverage of sunspot numbers since the mid 1700's, and this is a pretty good measure of solar activity). For example, there was a decrease in solar output in the early 1800's (known as the "Maunder minimum"), which could be seen in the sunspot number and caused a mini Ice Age in Europe.

    Drs. Eric Christian and Louis Barbier

  39. Finding Solar Activity Data

    I am planning to do some research on correlations between solar activity and weather on the Earth. Where can a find historical data on solar activity?

    I suggest that you use sunspot data, because it is the record of solar activity that goes back the furthest. You can find it at the NASA/Marshall Solar Physics Site.

    Can I find similar data for geomagnetic and solar wind activity and coronal mass ejections?

    You can find solar wind data back through the 1960s or so at the NSSDC (National Space Science Data Center). It is referred to as OMNI data I believe.

    You can find CME data for the past 8 years or so at the SOHO LASCO CME Catalog.

    A good geomagnetic index is Dst. See the index at the World Data Center for Geomagnetism.

    Dr. Richard Mewaldt
    (October 2004)

  40. Interplanetary Coronal Mass Ejections

    We have been looking at this ICME table and need some help reading it. Which column has the date that a CME reaches the Earth?

    The footnotes (a) and (b) describe the difference between these columns. The first column is basically the leading edge that was observed. Sometimes it is a shock (which has some stand-off distance from the ICME material that is driving the shock). If no shock was observed, they use other criteria, as explained.

    The second column gives the start and end of the ICME material/magnetic cloud. Exactly which is best for you to use depends on which of these structures are responsible for the ionospheric changes that are being measured. I am afraid I cannot help with that. I guess I would think that this might be part of your study -- to see how long a delay there is between the ionospheric disturbance and the leading edge of the ICME, and thereby help identify the cause of the SID (sudden ionospheric disturbance).

    If the shock is important, then the disturbance will start earlier than if it is, for example, the interaction between the magnetic field in the ICME and the Earth's magnetic field.

    Note that these are big structures that take ~1 day to pass Earth. So maybe you even want to use the midpoint of the start and end times.

    I guess I would expect the fastest ICMEs (which will be more likely to have shocks) to be the most effective, because there is more energy available to cause a disturbance. However, I am just guessing.

    Dr. Richard Mewaldt
    (February 2009)

  41. The South Atlantic Anomaly

    Where is the South Atlantic Anomaly (SAA), and what are the times the local population will be more vulnerable to solar radiation?

    The Earth's magnetic field is not symmetric. (If you think of the Earth as a bar magnet, the magnet is tilted relative to the Earth's spin axis; also, the magnet is not centered at the center of the Earth -- it's a little off.) Because of this, the inner radiation belt comes down closer to the Earth's surface at the SAA than in other places. There is a map of the SAA made by the ROSAT satellite.

    It covers much of Brazil. But it is a high altitude effect and is really only troublesome for satellites. There is only a minor increase in radation at airplane altitudes, and there is nearly no effect at ground level. And it's been present longer than there have been people in Brazil.

    Dr. Eric Christian

    (May 2010)

  42. Sun's Magnetic Field

    I recently saw an IMAX film about the Sun. It stated that every 11 years the Sun's magnetic poles rotate and swap ends, which causes a solar flare. How do the magnetic poles rotate? Also, I know that the Earth's magnetic poles are slowly rotating and over thousands of years will be reversed - is this for the same reason the Sun's poles rotate?

    Yes, both the Sun and the Earth reverse their magnetic fields. Both the Sun and the Earth are electromagnets, not permanent magnets (despite the Earth's core being made up of iron and nickel). So it is electric currents moving through the plasma of the Sun and the molten rock of the Earth's interior that generate the magnetic fields. These currents have instabilities that build up until the field reverses to relieve stress. With the Sun, it happens pretty regularly every 11 years.

    The magnetic flip is the root cause of the 11-year cycle of solar flares, but it doesn't cause one particular flare. Solar flares are just more likely in the couple of years around the field flip (called solar maximum).

    Dr. Eric Christian
    (September 2000)

  43. Magnetic Field Phi

    Could you explain what Phi is, in terms of the Sun's magnetic field? Is it the angle of the magnetic field? What influences it?

    I assume that you are referring to plots of the magnetic field as measured by ACE (located ~1 million miles in front of the Earth (between the Earth and the Sun. These plots appear on the ACE Real Time Solar Wind page.

    In these plots Phi is indeed the angle of the interplanetary magnetic field that is being carried out by the solar wind. Phi is measured in the GSM (geocentric solar magnetospheric) coordinate system. In this system the X-axis points from the Earth to the Sun and the Z-axis is pointing along the direction of the Earth's north magnetic pole. This puts the Y-axis roughly pointing to the left as one looks at the Sun from the Earth. Phi is the angle made by the field in the XY plane. This means that Phi would be 0 deg if the were pointing at the Sun and 180 deg if it were pointing from the Sun to the Earth. Looking at the plot for December 14-19 I see that it is most often between 90 deg and 180 deg (say ~135 deg on average). This is a typical angle when the magnetic field is coming from the southern hemisphere of the Sun (at least during the decade from 2000 to 2010 - the Sun's field flips over every 11 years or so).

    Then there are also quite a few periods when it is at ~315 deg, which is the typical angle when the field is coming from the northern hemisphere. Since we are near the equator of the Sun, we sometimes see magnetic field from the northern hemisphere of the Sun and sometimes from the southern hemisphere.

    You may wonder why Phi is not either 0 deg or 180 deg if the field is coming from the Sun. The reason is that the Sun rotates every 27 days, so the field is formed into a spiral, much like what happens with a spinning water sprinkler. Maybe you will understand things better using the diagram at Windows to the Universe or going to the Cosmicopia page on the Sun's magnetic field.

    Dr. Richard Mewaldt
    (December 2005)

  44. Far Side of the Sun?

    I just read an article stating that scientists have just "had their first glimpse of the Sun's hidden half". We orbit around the Sun, and it takes us 365 days to do so, so how is it that we have never seen the other side of the Sun?

    Scientists on the SOHO spacecraft have learned how to see some of what is happening on the far side of the Sun, even though the spacecraft is on the same side as the Earth is. The Sun rotates every 26 - 28 days (since the Sun isn't solid, different parts rotate at different speeds), so we would see the hidden half in 13-14 days, but this new data gives us a way to see active areas (sunspots) before they rotate around to the side we can see.

    Dr. Eric Christian

  45. Solar Longitude

    Can you explain solar longitude to me? I can find a definition online, and documents with solar longitudes for meteor showers, but I don't understand how to measure it or explain it.

    Let's start with the Earth since most definitions like this start there and are extended by some logic to the new system, such as the Sun.

    On the Earth there is a fixed surface that rotates rigidly in space. The Prime Meridian is chosen to pass through Greenwich, and 360 degrees are laid out around the fixed surface of the Earth. By definition, the horizon where the Sun rises is east, and the lines of longitude INCREASE as the Sun passes under them.

    Now all we need is to extend that concept to the Sun. The Sun has no fixed surface, it does not rotate in a rigid manner (the rotation rate is dependent on latitude and height), and it has no reference marks that last more than a few days. This isn't going to be easy.

    The Sun's "surface" is the photosphere, which is nothing more than the altitude below which we can not see. There are dark spots and eruptions to mark a location, but they change continuously, coming and going and moving location. What is generally done is to define a rotation period that corresponds to the equator. This is 25 days, the so-called sidereal period. Since the Earth moves around part of its orbit in 25 days, it takes 27 days for the same point at the Sun's equator to rotate until it is again under the Earth (the synodic period).

    We are on the Earth and want a system for referencing what we can see, we define zero degrees longitude to be the "subsolar point," or the line of longitude directly below the Earth. The center of the Sun as you see it is always zero degrees of longitude. Now, if we divide the sphere of the Sun into 360 degrees and argue that the Earth rises in the east (of the Sun) and sets in the west, and noting that the Sun rotates in the same sense as the Earth, we can refer to solar longitude when we see a structure on the Sun and want to communicate its location to others. This means that when we look at the Sun (with our heads pointing north) -90 degrees of longitude is to the left and defines the line of the east limb of the Sun beyond which we can not see, and +90 degrees of longitude is the west limb to the right beyond which we can not see.

    There is one other approach to longitude, but it is even more arbitrary. Imagine taking photographs of the Sun for 27 days and stretching them out before you into a "Carrington" map named after a famous solar astronomer. This marks one full rotation of the Sun's equator as viewed from Earth, and gives you some idea of what the Sun looked like for this one rotation - if you neglect that many features come and go over this amount of time. Then you can divide that image into 360 degrees and define longitude to rotate with the Sun, but only if you embrace the fixed and agreed-to starting time. The so-called Carrington Rotations are defined in this way, although they tend to reference days of observation instead of longitude. The next Carrington Rotation starts tomorrow on March 26.

    I hope this helps. It isn't a perfect system, but since the Sun lacks a rigid surface the whole Earth-based concept resists application to the Sun.

    Dr. Charles Smith
    (March 2005)

  46. The Sun's Orbit

    The planets rotate and circle the Sun. Does the Sun rotate and circle something larger?

    Yes, the Sun is only one of many stars in our galaxy, the Milky Way. It is located in one of the spiral arms about 30,000 light years from the center. It moves at a speed of 200 - 300 km/sec in its orbit around the galactic nucleus, and takes roughly 200 million of our years to make one orbit of the Galaxy, or one "galactic year".

    Drs. Eric Christian and Louis Barbier

  47. Rotation of the Sun

    Does the Sun rotate around its own axis, like the Earth?

    Yes, the Sun does rotate on its own axis. Because the Sun is more like a fluid than a solid (like the Earth), not all of the Sun spins at exactly the same rate, but it does a rotation in about 26 or 27 days. This was first observed by Galileo by tracking sunspots as they moved across the face of the Sun, then disappearing and reappearing about 2 weeks later.

    Dr. Eric Christian
    (August 2002)

  48. Sun a Binary Star?

    Is our Sun part of a binary star system?

    There was a theory that the Sun was part of a binary star system (the second star was called Nemesis). But there is no real evidence for it.

    Dr. Eric Christian

  49. Sun Further North?

    I've noticed for the last few years that the Sun seems to be further north than it used to be for the corresponding time of year in previous years. I live near Buffalo, New York. Might there be some explanation for my observations?

    There is some wobble of the Earth's axis of rotation (called precession), but it is not noticible over just a few years. The Sun appears pretty much in the same place at the same time of the year.

    Dr. Eric Christian

  50. 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

  51. Movement of Particles From the Sun

    When the Sun has a burst of magnetic energy, at what speed are particles from the burst travelling? At what altitude from the initial burst do they begin to cool down, and do the speeds of the particles slow down as they are projected forward?

    Bursts of energy from the Sun, known as solar flares, occur in the corona. The corona begins about 2000 kilometers above the surface of the Sun (around 700,000 km from the center of the Sun) and can be thought to extend out a few million kilometers. It is extremely hot (several million degrees). The ionized, high-energy particles that compose these flares are moving nearly 2000 kilometers per second when the flare first leaves the Sun. They lose energy as they propagate out, eventually dropping below 1000 km/sec as they near around 300 million kilometers out (past Mars). They form all over the surface of the Sun and move out in all directions.

    As these particles move, they carry large magnetic irregularities that disrupt particles coming in from outside the solar system. When they reach Earth, they release energy into the Earth's magnetic field, which can be seen as the aurora borealis and aurora australis (northern and southern lights, respectively).

    Lauren Scott
    (April 2003)

  52. 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

  53. 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

    What if the solar wind is not neutral in net electrical charge? If the Sun emitted more electrons than protons in the solar wind, the Earth would be bombarded with these electrons and build up a negative charge. This could have some interesting effects on electricity and lightning!

    Sorry to tell you this, but your hypothesis is quite impossible. Things like the electric field, and electric attraction and repulsion pretty much prevent anything like you suggest happening. If the Sun were to emit more electrons than protons, it would get a net positive charge, which would make it easier to emit protons and harder to emit electrons. So it's got a feedback loop that tends to keep it neutral. Likewise, if the Earth were to build up a negative charge, it would attract protons and repulse electrons until it was again neutral. We would not only be able to measure the electric field in space, but we would see the solar wind protons accelerated towards the Earth.

    If there were a million volt potential difference, every proton reaching the Earth would be at an energy of at least 1 MeV. Solar wind protons are measured at Earth (more precisely just outside the Earth's magnetosphere) with less than 1 keV, therefore the electric potential between the Earth and the Sun is less than a thousand volts. This is many, many, many orders of magnitude less than what would be needed to create any effect like you describe.

    To see some measurements of electrons and protons, the ACE Real Time Solar Wind has plots of both (one example is the 7-day ACE plot from the EPAM instrument, under Dynamic Plots) While at one point in space and time the fluxes are not identical, in the long run, they HAVE to average out.

    Dr. Eric Christian
    (May 2001)

  54. Detecting Particles from Solar Wind

    Do simple cosmic ray detectors and cloud chambers, such as those that can be built by "amateur" scientists, also detect particles from the solar wind that enter Earth's atmosphere? If so, is there any way to distinguish between the solar wind particles and cosmic rays?

    Solar wind particles are thousands of times less energetic than the cosmic rays that are measured by Earth-bound cosmic ray detectors. The solar wind only rarely makes it through the Earth's magnetic field (it requires a certain alignment of the solar and terrestrial magnetic fields), but the solar wind that makes it into the magnetosphere is stopped by the very top of the atmosphere. So, unless you're in space, a simple cosmic ray detector cannot measure solar wind. Even in space, scientists can't tell on a particle-by-particle basis whether an event is a solar wind particle or a cosmic ray, but at low energies almost all the particles are from the Sun, and at high energies almost all the particles are from the galaxy and beyond.

    Dr. Eric Christian
    (July 2001)

  55. Solar Wind and Earth's Magnetosphere

    I am working on a paper on the interaction of the solar wind with the Earth's magnetosphere. Do you know of a source giving the magnitude of the magnetic field of the magnetosphere?

    Your question has a tremendous amount of latitude in it, both literally and figuratively. The magnitude of the Earth's magnetic field within the magnetosphere varies greatly with location within the magnetosphere. It is not a simple question to answer.

    The magnetosphere is formed when the flow of the solar wind impacts the Earth's magnetic field (a dipole field), compressing it, causing magnetic reconnection, causing complex currents to flow, etc. The dipole field changes greatly under the influence of the solar wind.

    Try to get a copy of "Physics of the Magnetopause" published by the AGU. That will help.

    The magnetic field in the solar wind near Earth is about 5 nT, or 5 x 10(-5) Gauss. The magnetic field on the surface of the Earth is about 0.5 Gauss - big difference.

    If you were to imagine a spacecraft passing from the solar wind and into the magnetosphere along the Earth-Sun line (at the sub- solar point) you would see the following:

    The 5 nT field changes by a factor of 4 as you pass through the Earth's bow shock. This is a compression wave (discontinuity) that results from a supersonic flow striking the Earth's magnetic field and being stopped).

    The magnetic field then climbs by another factor of 4 as the spacecraft passes through the magnetosheath, a turbulent region of flow behind the shock and still upstream of the magnetosphere.

    The spacecraft then encounters a region where the flow stops moving toward Earth and flows exclusively around the obstacle that is the Earth's magnetosphere. This is the magnetopause. Behind the magnetopause the magnetic field is about 40 nT. So the compression of the flow has picked up almost a factor of 10 in intensity of the magnetic field (and the plasma density with it).

    From this point on, it depends greatly on where you go. If you go Earthward, it is to leading order a dipole field (somewhat squashed, but the scaling is like that). If you go behind the planet, all kinds of things are happening and location is everything.

    I really can't go into much more detail than that without a more specific question, and to be honest I mostly work outside the magnetosphere, so my knowledge is limited. But basically if you go from the magnetopause to the Earth's surface, you see a magnetic field that grows ever stronger as you decend within the dipole.

    Dr. Charles Smith
    (October 2003)

  56. Solar Wind and Spacecraft

    After reading an article in our newspaper regarding the speed of solar winds outside Earth's atmosphere, I was wondering... how do satellites and spacecraft like the International Space Station and Hubble Space Telescope cope against those high speed solar winds?

    There are two parts to the answer.

    First of all, ISS and Hubble are located in low Earth orbit, a few hundred kilometers above the Earth's surface. As a result, they are shielded from space weather in interplanetary space by the Earth's magnetic field, which forms a shield called the "magnetosphere" around the Earth. You can find a drawing of the magnetosphere, and more explanation, on this site's page about the Earth's magnetosphere.

    Secondly, while the solar wind is very fast (~1 million miles/hour), it is not very dense. There are only ~5 to 10 particles (mostly protons) per cubic centimeter in the solar wind, while in terrestrial winds (20 miles/hr) there are ~20,000,000,000,000,000,000 particles per cubic centimeter. Because the solar wind has such low density, it would not apply much pressure to the ISS even if it were out in interplanetary space.

    If we multiply wind-speed times density to get pressure, it looks like the pressure of the solar wind is more than a trillion times smaller than a good stiff wind on Earth.

    Dr. Richard Mewaldt
    (May 2004)

  57. 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

  58. Why Hasn't the Sun Blown Away the Earth?

    Why hasn't the Earth's magnetic field acted like a large solar sail and blown the Earth farther away from the Sun after billions of years?

    The basic answer comes down to this: "Can you move a brick wall by throwing ping pong balls against it for a very long time?" The answer is "How long do you have?"

    Let's do a simpler problem: How long would it take to get the Earth moving at 5 meters/s if all that is available is the solar wind?

    The density of the solar wind is 10 protons/cc on average, and the average solar wind speed is 425 km/s. The mass of the Earth is 5.98 x 1024 kg, and its radius is 6.37 x 106 meters.

    So, the cross-section of the Earth is pi * R2 = 1.27 x 1014 m2.

    The number of solar wind protons that hit this cross-section in 1 second is the number of density times the length of the column of particles that cross the surface in 1 second. Converting the number density to p/meters-3 gives 10 * 106 * 4.5 x 105 = 4.5 x 1012 protons/sec. Their individual mass is 1.67 x 10-27 kg, and their speed is 4.5 x 105 m/s, so the momentum of this many particles is 3.4 x 10-9 kg-m/s.

    Suppose these particles gave up all their momentum to the Earth (they don't, and they tend to flow around the Earth's magnetic field, but suppose they did). This would be the maximum momentum input to the Earth available from these particles.

    How much time would it take for this flow of solar wind protons to set the Earth moving at 5 m/s? Then:

    T * 3.4 x 10-9 kg-m/s = 5.98 x 1024 kg * 5 m/s

    From this, T = 8.8 x 1033 seconds. There are 60 * 60 * 24 * 365 = 3.15 x 107 seconds in a year, so you have to wait about 1026 years for the Earth to get up to a speed of 5 m/s. A billion years is 109, so that's a long time to wait.

    Now I haven't worked it out, but I think there is also an angular momentum aspect to this problem. Basically, if you push an orbiting object away from the focus of its orbit, angular momentum brings it back toward its natural orbit. There is an overshoot and oscillation, etc., much like a spinning top that is bumped. This makes it harder to move the Earth away from the Sun.

    The short answer to your question is that, while the solar wind is moving fast, there just isn't much of it. There is a similar problem with its temperature. It is very hot, but there are so few particles that the leading problem with spacecraft design is keeping the hardware from getting too cold. There just aren't enough particles.

    Dr. Charles Smith
    (December 2004)

    So how does the charge of the particles impart a force onto the Earth via the magnetic field? It seems like tossing a bunch of small North magnets at a large South magnet.

    Actually, that's not really right, but it's not really wrong,either. Every atom has a magnetic moment. This is the equivalent to being a very small magnet. Magnets do interact -- that's why a compass needle points north. The needle is a small magnet that aligns with the Earth's magnetic field. However, for particles such as protons and electrons, the electric charge is more important in this problem.

    When an electric charge is placed into a magnetic field, it orbits the field line at a local point, going around and around the field and moving along the field as well. However, it has a hard time crossing the field line to another adjacent field line, which is necessary if the particle is to pass through the field of the magnet.

    So, solar wind particles are unable to penetrate the Earth's magnetic field due to their electrical charge.

    Since a solar sail is just a means of converting the momentum of the wind into momentum of the sail and whatever is attached, your original analogy was a good one and rather clever. I had never though of it.

    Dr. Charles Smith
    (December 2004)

  59. Ions in the Solar Wind

    In reading about ions in the solar wind, I noticed a huge difference between the density of protons and the density of electrons. Does anyone track these differences and their effects?

    Your question about the charge neutrality of the solar wind is interesting. I think that the charge discrepancy that you mention has gone away with better measurements. I looked at a book edited by Kivelson and Russell (Introduction to space physics, 1995), which gave

    • Proton density: 6.6/cm3
    • Helium density: 0.25/cm3
    • Electron density: 7.1/cm3

    Heavier elements also make a contribution of <0.1/cm3

    Since He has a charge of 2, the charges balance and the solar wind is neutral.

    These are average numbers, both the ion and electron densities vary (together). This is actually a difficult measurement to make with high accuracy because if the spacecraft has a + or - charge it can either attract or repel low-energy solar wind electrons and give the wrong answer.

    Solar wind electron data from ACE are available here. These come from the SWEPAM instrument.

    You might also try the NSSDC data center at where data from other spacecraft are available.

    Dr. Richard Mewaldt
    (November 2006)

  60. Neutral Current Sheet and the Solar Wind

    What is the physical significance of a "neutral current sheet," as it pertains to the solar wind?

    The term "neutral current sheet" is commonly used in plasma physics to denote the thin sheet of electrical current which separates regions of oppositely directed magnetic field. It is also often called simply "current sheet". Presumably the term "neutral" comes from the fact that the magnetic field goes to zero at the sheet. From basic electrodynamics, one finds that an electric current is necessary to change a slowly varying magnetic field.

    A principal example is the current sheet, which separates the northern and southern magnetic polarities in the solar wind. This sheet is very thin on interplanetary scales and typically crosses a spacecraft near the solar equatorial plane 2 or 4 times each solar rotation. Such a sheet also exists in the far tail of the Earth's magnetosphere, separating the extensions of the northern and southern polar magnetic fields of the Earth. Similar sheets often occur in laboratory plasmas.

    Cosmicopia that discusses this is on the page about the Sun's magnetic field. Also illustrative is a page from University of California Riverside about the large scale solar wind and its effects.

    Dr. Randy Jokipii
    (October 2005)

  61. 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

  62. Solar Wind Electricity?

    The interaction of the solar wind on the Earth's magnetic field causes static electricity. How much voltage or current could this static electricity produce, and could we harness it for effective use?

    It is indeed a valid question whether it is worth to try to harness the electrical power induced by the solar wind into the Earth's magnetosphere. Because the solar wind is a plasma (a gas that consists of an equal number of positive and negative electric charges) and thus highly electrically conductive, its flow past the Earth's magnetic field generates an electric field, much like in a generator. The total voltage across the Earth's magnetosphere is approx. 100,000 V. This sounds rather high (actually about the same voltage as that used in long distance power lines), but this voltage is across approx. 20 Earth radii or a moderate electric field of about 1 V/m.

    There are a few important considerations that make this power not very attractive for technical use. First and foremost, the density of the power is very low. Using a typical solar wind density of 5 particles/cm3 and a solar wind speed of 440 km/s, the power per m2 is merely 0.35 mW. The total power intercepted by the entire magnetic field of the Earth is 4.5 Terawatts, of which only 2% or 90 Gigawatts are absorbed by the Earth's magnetosphere. In comparison, the total energy consumption of humans on Earth right now is 10 - 12 Terawatts. In order to harness the power delivered by a typical large power plant (about 1 GW) from the electrical energy transferred into the Earth's magnetic field we would have to build a structure that has the size of the entire Earth - rather impractical and probably not cost-effective.

    A much better alternative is provided by the Sun's light, which delivers about 1400 W/m2, i.e. a 4,000,000 times higher power density. The total solar power intercepted by the Earth is more than 10,000 times as much as we currently consume. With solar cells that have an efficiency of 10%, just covering less than 1% of the Earth's surface with solar cells would be sufficient to produce the power needed, even considering day and night and occasional cloud cover. Of course, there are still lots of challenges, but certainly far fewer than trying to harness the solar wind.

    Dr. Eberhard Moebius
    (October 2006)

  63. Solar Cell Stations in Space?

    Are there any plans to establish solar cell stations in space?

    There are people who have proposed beaming solar cell power to the ground using microwaves, but this is not economically feasible yet and probably won't be for decades. (Try Googling "beaming solar cell power from space" or something like this.)

    Dr. Richard Mewaldt
    (January 2012)

  64. Oort Cloud and Heliopause

    What is the position of the Oort cloud in relation to the heliopause?

    The heliopause is much closer to the Sun than the Oort cloud. The heliopause is something like 200 AU (Astronomical Unit, the average distance between the Earth and the Sun), although this number is uncertain. The Oort cloud is tens of thousands of AU away. For more info, you can check out "The Nine Planets" website.

    Dr. Eric Christian
    (October 2001)

  65. How Big is the Heliosphere?

    How far is it from the Sun to the edge of the heliosphere (on average)?

    It's not precisely known, since we haven't gotten a spacecraft out that far yet. The heliopause, which is the boundary between the gas from the Sun and the gas of interstellar space, is probably between 150 and 300 AU out (150 to 300 times the distance from the Earth to the Sun) in the direction that the Sun is traveling. There is a tail in the other direction (just like the Earth's magnetosphere) that extends furthur.

    Dr. Eric Christian

  66. Temperatures at the Solar Wind Termination Shock

    Voyager 2 encountered exteremely high temperatures passing through the termination shock zone. How is that possible in deep space, where temperatures should be approaching 0 Kelvin?

    Upstream of the termination shock, the solar wind temperature is about the order of 104 K. Downstream of the termination shock, the solar wind termperature is about the order of 105 K, because it is heated up during the termination shock crossing. So either way the temperature shouldn't be zero at the termination shock zone.

    I guess when you say "deep space" you mean the space completely outside our heliosphere? The termination shock is only the inner boundary of the heliosphere. Outside of the termination shock there is the heliopause and possibly a bow shock. The interstellar wind outside the bow shock will have a cold temperature of 10-100 K. However, it will take Voyager 2 a long time to pass the boundaries and encounter those low temperatures.

    Pin Wu
    (October 2008)

  67. Effects of Shrinking Heliosphere

    With the IBEX probe being sent this weekend to the edge of the Solar System, it would be able to detect if the heliosphere is shrinking. How would a lack of a heliosphere affect the Solar System and life on Earth? I realize that, with a Sun, there theoretically should still be a heliosphere regardless of its size, but it could be an indicator that perhaps the Sun is running out of fuel.

    The Sun is not running out of fuel and won't for billions of years. It is a variable star, however, and right now it is exceptionally quiet.

    But the bubble would have to get much, much smaller before it could affect life on Earth, and there's no evidence of that happening. Even if the bubble were to get smaller than the orbit of the Earth (which has probably happened in the billion or so years that there has been life on Earth), there wouldn't be much effect. The Earth's magnetic field and its atmosphere both give us a lot of shielding. Where the shrinking might have an effect is on astronauts traveling to the Moon and Mars, who have to spend time outside the Earth's shields.

    By the way, IBEX is going to stay in Earth orbit and take images of the edge of the Solar System. To actually get there would take decades (the Voyager 1 and 2 spacecraft took 30 years).

    Dr. Eric Christian
    (October 2008)

  68. Weakened Heliosphere and Global Warming

    A recent article mentioned that the heliosphere has been weakening for the past 25 years. Could such weakening allow more interstellar energy to arrive at the Earth? Could it be a cause of global warming?

    It is true that the heliosphere has been weakening in the past 25 years. According to the Ulysses observations, the solar wind pressure had decreased by 20%, which indeed will allow more galactic cosmic rays (GCRs) to enter our Solar System. However, I don't expect the radiation carried by the extra GCRs to contribute much to the climate. Instead, they are just more likely than usual to disrupt electrical equipment, damage satellites, and potentially harm life on Earth.

    A side note: The current weakening of the solar wind pressure may be natural variations that will stabilize over time. Actually, we are in a stretched solar minimum, and this could either be temporary or abnormal. It is still hard to say now.

    Pin Wu
    (October 2008)

  69. Open Problems in Physics

    Can you tell me an open problem in physics that is in your area of expertise?

    We are almost always working on several "open" problems at the same time, as there are always interesting and intriguing observations and paradoxes that have not been satisfactorily dealt with. Our research work consists of finding those problems that have the right combination of interest and importance and to the solution of which we can make a significant contribution.

    One that is at the top of the list in my group right now is the puzzle of the termination shock of the solar wind. There is a controversy over observations from the Voyager 1 spacecraft at some 90 AU from the sun. One experimental group believes that the data indicate that the shock has been crossed, which would be a major milestone, but other experimental groups disagree. We are attempting to make a consistent physical picture of what is going on. See this NASA press release from November 2003.

    A non-technical overview of this, "Voyager goes close to the edge", was published in the British journal Physics World in January of 2004. It's a bit later than the NASA release and hence more up-to-date, and also perhaps adds a bit more perspective.

    Dr. J.R. Jokipii
    (September 2004)

  70. Copernicus' Heliocentric Theory

    Where can I find an illustration and information about Copernicus' heliocentric theory?

    I found several good links on the WWW. There's one at the University of California, and one at the University of Kentucky.

    Dr. Eric Christian

  71. Does the Sun Revolve Around Something?

    I had a discussion with a friend about how the Earth revolves around the Sun. He stated that the Sun does not revolve around anything. I disagreed. If the Moon revolves around us, and we revolve around the Sun, I don't see why it would stop there. Does our Sun or our Galaxy revolve around something? Are all objects in space in some kind of motion?

    The Sun (and all the stars in the Milky Way galaxy) revolves about the center of the Galaxy. It takes about 200 million years to go around once. The Milky Way is also moving relative to the local group of galaxies. Gravity works even across extremely large distances. Pretty much everything in the Universe is moving, due to gravity and the initial velocity obtained in the Big Bang.

    Dr. Eric Christian

  72. How Long to Drive to the Sun?

    If there were a highway from the Earth to the Sun, how long would it take to get to the Sun, driving at 65 miles per hour?

    If the highway is straight, and you drive non-stop 24 hours a day with no meal or bathroom breaks, it should take 163 years and 120 days to get to the Sun from the Earth:

      93,000,000 miles/65 mph = 1,430,769 hours
      1,430,769/24 hours in a day = 59,615 days
      59,615/365 days in a year = 163 years and 120 days

    If you figure out how to do this with current gas prices, please let us know!

    Beth Barbier
    (March 2000)

  73. Dispose of Waste in the Sun?

    Would it be feasible to dispose of radioactive waste in the Sun? What are the possible consequences?

    The Sun weighs about 300,000 times the total weight of the Earth, so the consequences of a little radioactive material on the Sun would be very small. However, it would be really expensive to do this. You would need a lot of rocket fuel just to lift the waste off the Earth, and then you would need even more in order to remove the Earth's orbital velocity (about 38 km/sec) before you could fall into the Sun.

    Dr. Eric Christian

    If nuclear waste were rocketed into the Sun, would we get nuclear radiation back on the solar wind?

    We would get an infinitesimal fraction back.

    Let us assume that it is all swept up into the solar wind at the Sun.

    First, the solar wind blows radially outward, at a very high speed (several hundred miles a second) in all directions away from the Sun. We would only get that small fraction aimed directly into the vicinity of the Earth (the Earth only subtends about 1 ten-billionth of the sky as seen from the Sun).

    In addition, this small amount of material would likely become ionized when it is swept up into the solar wind, so the Earth's magnetic field would shield us from nearly all of it.

    Dr. J. R. Jokipii
    (July 2003)

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