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| electromagnetic radiation | wavelength | angstroms | solar wind | electromagnetic field | magnetometers |
| ionosphere | propagation | radio blackouts | sunspot cycle | smoothed sunspot numbers | solar flares |
| coronal mass ejections | solar flux | smoothed solar flux numbers | daily reports | wwv |

Measuring Energy from the Sun
Frequently Asked Questions about Solar Indices

The Sun on Bastille Day
  • The continuous nuclear fusion process inside the Sun causes our star to emit electromagnetic radiation and matter.

    What is the wavelength of the energy? The Sun generates electromagnetic radiation at various wavelengths, which ionize particular regions:
    • hard X-rays (1-10 Angstroms) ionizes the D region,
    • soft X-rays (10-100 Angstroms) ionizes the E region,
    • ultraviolet light (100-1000 Angstroms) ionizes the F region.
    What is solar wind? Solar matter ejected from the Sun becomes the solar wind. It flies outward in all directions and includes charged particles -- electrons and protons. On a quiet solar day, the speed of the solar wind heading toward Earth averages about 400 km per second. That's about 900,000 miles per hour. The Sun is some 93 million miles from earth so it takes a particle in the solar wind around 103 hours to arrive at Earth. That's about four days.

    How does it affect Earth? The Sun's solar wind hits and travels along Earth's magnetic field. The impact compresses Earth's magnetic field on the side facing the Sun and pushes it out on the side away from the Sun. The part stretching out away from the Sun is Earth's magnetotail. It extends downwind a great distance, equal to tens of times the radius of Earth. The electromagnetic radiation from the Sun affects Earth's entire ionosphere in daylight. Charged particles ejected by the Sun are guided along Earth's magnetic field lines into the planet's ionosphere and thus only impact high latitudes where the magnetic field lines go into Earth.

    How do we measure Earth's varying magnetic field? Variations in Earth's magnetic field are measured by magnetometers. Data from two measurements are readily available from the government -- the daily A index and the three-hour K index:
    • the daily A index has a linear scale from 0 (quiet) to 400 (severe storm),
    • the three-hour K index has a quasi-logarithmic scale from 0 to 9, a compressed version of the A index, with 0 being quiet and 9 being severe storm.
    How does the Sun affect radios on Earth? When electromagnetic radiation from the Sun strips an electron off a neutral constituent in our atmosphere, the resulting electron can spiral along a magnetic field line. Thus the condition of the ionosphere depends on the state of Earth's magnetic field. This is important because Earth's magnetic field plays a big part in the propagation of radio signals around our planet. Generally an A index at or below 15, or a K index at or below 3, is best for propagation.

  • MORE
  • What is the Sunspot Cycle? Spots on the face of the Sun come and go in an 11-year cycle. They rise quickly to a peak during the first four to five years of the cycle and then descend more slowly to minimum over six to seven years.

    Just before, during, and just after the peak of a Sunspot Cycle, the increased number of sunspots sends more ultraviolet radiation to impact Earth's ionosphere. This results in much greater ionization of the F region of Earth's atmosphere. That allows the ionosphere to refract higher radio frequencies back to Earth.

    On the other hand, around the minimum time of a Sunspot Cycle, the number of sunspots is so low that higher frequencies generated on the surface of Earth travelk up and pass right on through Earth's ionosphere into outer space. That is, there is less absorption and a more stable ionosphere, resulting in the best propagation on lower frequencies.
    • high SSNs are best for high frequency propagation,
    • low SSNs are best for low frequency propagation.
    What are smoothed Sunspot numbers? Areas of the Sun associated with ultraviolet radiation are sunspots. They are related to ionization of the F region of Earth's atmosphere. When the daily Sunspot count is plotted over a month time, the graph is very spiky. Averaging daily Sunspot numbers over a month results in the monthly average Sunspot number, however it also results in a spiky plot. The Sun A smoother plot is desired by researchers. To get that, they choose a more averaged, or smoothed, measurement to measure solar cycles. This is the so-called Smoothed Sunspot number (SSN) calculated across five and a half months of data before and after a desired month, plus the data for the desired month. The amount of smoothing leaves the official SSN a half year behind the current month.

    What are solar flares? Most of the disturbances to radio signal propagation on Earth come from solar flares and coronal mass ejections (CMEs) from the Sun. For instance, x-ray flares affect our radio propagation. Their wavelength ranges from 1 - 8 Angstroms. X-ray flares are classified as:
    • C class, the smallest, usually have a minimal impact on propagation.

    • M class, medium size, have a progressively adverse impact to propagation. Their electromagnetic radiation can cause the loss of all propagation on the Sunlit side of Earth due to increased absorption in the D region of the planet's atmosphere.

    • X class, the biggest, also have a progressively adverse impact. Their electromagnetic radiation also can cause the loss of all propagation on the Sunlit side of Earth. Big X class flares can emit very energetic protons that are guided into Earth's polar cap by the our planet's magnetic field. That brings on a polar cap absorption event (PCA), with high D region absorption along radio signal paths passing over the polar areas of the Earth.
    What is a coronal mass ejection? A CME is an explosive ejection of large amounts of solar matter from the Sun. It can boost the speed of the solar wind dramatically, creating a shock wave of particles heading toward Earth. When that shock wave hits Earth's magnetic field, it causes large variations distortions in Earth's magnetic field. That increases the A and K indices. Distortions to the magnetic field causes electrons spiraling alomng magnetic field lines to be lost into the magnetotail. With electrons gone, the maximum usable frequencies (MUFs) decrease, and return only after the magnetic field returns to normal and ionization replenishes lost electrons.

    What causes radio blackouts? Scientists wonder about the relationship between solar flares and CMEs. They are known to happen together or separately. The time to affect Earth differs. The electromagnetic radiation from a big solar flare, traveling at the speed of light, can cause short radio blackouts on the Sunlit side of the Earth within about 10 minutes of the eruption on the Sun. On the other hand, the energetic particles ejected by a solar flare and the shock wave from a CME can take four days to arrive at Earth and disrupt propagation.

    Who warns us of solar activity? Each day the U.S. government's National Oceanographic and Atmospheric Administration (NOAA) and the U.S. Air Force publish a Report on Solar and Geophysical Activity (RSGA). These reports can be found at

  • WHAT
  • IS A
  • Each daily report has six parts:
    1. Part 1
      1. analysis of solar activity including flares and CMEs
      2. forecast of solar activity
    2. Part 2
      1. summary of geophysical activity
      2. forecast of geophysical activity
    3. Part 3
      1. probabilities of flare and CME events -- normal propagation with no disturbances occurs when no x-ray flares higher than class C are reported or forecast, along with solar wind speeds due to coronal mass ejections around average speed
    4. Part 4
      1. observed and predicted 10.7 cm solar flux -- daily solar flux has little to do with what the ionosphere is doing on that day
    5. Part 5
      1. observed and predicted A indices
    6. Part 6
      1. geomagnetic activity probabilities -- good radio signal propagation is expected when the daily A index iforecast is at or below 15 and the K index is forecast to be 3 or below
  • HOW
  • CAN I
  • HEAR
  • WWV?
  • How can I get frequent reports? U.S. government radio station WWV at Fort Collins, Colorado, broadcasts time and frequency information, and geophysical alerts and marine storm warnings, 24 hours per day, 7 days per week. The station transmits on several frequencies and has millions of listeners worldwide. Its information also includes standard time intervals, corrections and codes, and Global Positioning System (GPS) status reports.

    At 18 minutes past each hour, WWV broadcasts a short version of the report covering the previous day's 10.7 cm solar flux and A index, as well as the current three-hour K index. Current solar and geomagnetic field activity and forecasts are broadcast. Normal propagation with no disturbances is expected when solar activity is low and the geomagnetic field is quiet. Low solar activity means the Sun has not produced any major flares or CMEs in the recent time period.

    What is the ionosphere? The ionosphere is part of Earth's upper atmosphere where free electrons occur in sufficient density to have an influence on the propagation of radio frequency electromagnetic waves. Its ionization depends for the most part on activity on the Sun. Its density varies according to the sunspot cycle, the season, and global locations -- polar, auroral zones, mid-latitudes, and equatorial regions. Most of its ionization is produced by x-ray and ultraviolet radiation from the Sun. As Earth rotates, ionization increases in the sunlit atmosphere and decreases on the shadowed side.

    To predict radio signal propagation, researchers have looked for a relationship between sunspots and the ionosphere. The best correlation at this time is the SSN and average monthly ionospheric conditions. Propagation prediction programs are based on that relationship. MUF and signal strength predictions are probabilities and not absolutes.

    What is solar flux? Since they are counted when seen, sunspots are a subjective measurement. To have a more objective measurement of the Sun's output, scientists use the 10.7 cm solar flux -- a general measurement of the activity of the Sun. The 10.7 cm solar flux doesn't describe the formation of the ionosphere, because energy at a wavelength of 10.7 cm is insufficient to cause ionization. The useful correlation is between the smoothed 10.7 cm solar flux and the smoothed sunspot number. Smoothed numbers are used as correlating daily values or monthly averages is not useful.

    What is meant by smoothed solar flux numbers? Propagation prediction programs correlate the SSN and smoothed 10.7 cm solar flux. The ionosphere does not react to daily variations of the Sun so the daily or weekly 10.7 cm solar flux and the daily sunspot number are not useful.

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