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Solar Phenomena

[last updated: 2021-08-24]
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Unfinished...
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Solar activity affects ham radio communication. I've been unsatisfied with most of the online explanations. This is my page of notes to try to understand and explain what's going on.
This is all the more difficult because even the solar and space scientists don't fully understand all the processes in the Sun that create the observed behavior. However in the last decade, with the help of some satellites designed to study the Sun and its output, understanding of the underlying processes is rapidly advancing.
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    On this page:
  • Quiet Sun:
    Electromagnetic Radiation
        Solar Irradiance
        Solar Flux
    Solar Wind
  • Solar Storms:
    Sunspots
    Solar Flares
    Coronal Mass Ejections
  • Effects on the Earth
    Effects on earth's Magnetic Field
    Efects on Propagation
  • NOAA reports on Space Weather
  • References/Links

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    Solar Phenomena:
  • The Sun is always putting out energy in the form of Electromagnetic radiation (EMR) and streams of charged particles and their associated magnetic fields.
    The "Quiet Sun" refers to the energy put out when there are no solar storms happening. This is like a baseline.
    When solar storms happen, the amount and character of radiation and particle streams changes dramatically, depending on the type and severity of the solar storm.
  • EMR and particles from the Sun affect the Earth. They affect radio communication (by their effect on the iosnosphere), the Earth's magnetic field, displays of aurora near the poles, and Earth's climate.
    When solar storms happen, these effects are greatly increased.
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  • The Quiet Sun:
    • Electromagnetic Radiation (EMR) comes out of the Sun constantly. The Sun shines.
      • The EMR from the Sun is composed predominately of UV light, visible light, and infrared radiation. In addition, however, a wide range of other frequencies are put out, from very short gamma rays and x-rays, to slightly longer visible light rays, to microwaves and even longer radio waves.
      • All EMR waves travel to the Earth at the speed of light, and are broadly characterized by their wavelength -
          from 1 to 10 Angstroms - hard X- rays
          from 10 to 100 Angstroms - soft X-rays
          from 100 to 1000 Angstroms - ultraviolet
      • Solar Irradiance:
        This is power per unit area of all the electromagnetic energy received from the Sun.
        If this is measured above the Earth's atmosphere, then it's called the Solar Constant. However it is not a constant in a strict sense, because it varies, and in fact is quoted by various sources to be generally between 1360 - 1375 W/M2.
      • While the electromagnetic output from the Sun is fairly stable during a "Quiet Sun," events such as storms can dramatically change the amount and frequencies of EMR and other energy put out by the Sun.

      • Solar Flux:
        • This term broadly refers to the flow of EMR, at all frequencies, from the Sun. Strictly speaking, it is power per unit area, or W/M2, the same as irradiance.
        • However in common usage, what is often called Solar Flux is really Solar Flux Index, which is a measurement of the EMR from the Sun at 2880 MHz (10.7 cm wavelength).
            The atmosphere is generally very "transparent" at this frequency, that is, the signal is not significantly attenuated as it passes through the atmosphere to reach the ground sensors that measure it, and it is generally not affected by solar flares.
            This frequency is therefore convenient to use as a monitor of solar output, and though it only measures the single frequency at 10.7cm, it generally tracks well to predict solar output at other frequencies.
        • This number can be a general predictor of ionizing radiation, which is the predominately UV frequencies that affect the ionosphere and radio communication, even though radiation at 10.7 cm is not ionizing in itself.
        • SF is measured in "solar flux units," and is generally less than 100 for a Quiet Sun, and greater when there is solar storm activity.
            1 sfu = 10-22 / (m2
          • Although the SFI is "generally not affected by solar flares," it sometimes happens that large solar flares do in fact increase SFI. Such incidents are called "ten-flares"
    • Solar Wind composed of charged electrons, protons, and alpha particles constantly streams out of the Sun towards the Earth. In a "Quiet Sun," with no "storm" events happening, a baseline, "ambient" solar wind, flows.
      As well, since the solar wind is composed of charged particles in motion, there is an inherent magnetic field that accompanies it, characterized by magnetic field strength and orientation.
        Solar Wind is characterized by speed (Quiet Sun range typ. 400-500 km/sec), density (usu. particles/cm3), and kinetic energy (range from 0.5 to 10 keV)

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  • Solar Storms:
    • Sunspots:
      These are "storms" on the surface of the sun. They are characterized by an extra outpouring of radiation and particles. In particular, they are a strong source of UV radiation.
        At any moment in time, there will be some number of sunspots on the face of the Sun that faces Earth. This is the Sunspot Number. The number of sunspots varies on an approximately 11-yr cycle.
        The sunspot number varies widely from day to day, so it is common to take an average number. This is the smoothed sunspot number (SSN). The SSN is calculated using six months of data before and six months of data after the desired month, plus the data for the desired month.
        Because of this amount of smoothing, the official SSN is one-half year behind the current month. Unfortunately, this amount of smoothing will mask unusual, short-term solar activity.

    • Solar Flares:
      • Solar flares are a sudden explosion of energy, both electromagnetic and charged particles, caused by tangling, crossing or reorganizing of magnetic field lines near sunspots.
      • Solar flares are classified according to their strength (and frequency of electromagnetic radiation they put out?). Flares that affect ham radio propagation are x-ray flares, with output from 1 - 8 Angstroms. The smallest ones are A-class, followed by B, C, M and X, the largest.
        Little affect on propagation up to class-C, but class-M and -X have more affect.
      • X-ray flares in class-X range can greatly increase D-layer absorption and cause loss of all propagation on sun side of Earth.
      • Class-X flares can emit very energetic protons that are attracted to the Earth's poles by Earth's magnetic field. This can result in a polar cap absorption event (PCA), with high D-region absorption on paths passing through the polar areas of Earth.
      • The electromagnetic radiation from a large flare (traveling at the speed of light from the Sun to the Earth) can cause short-term radio blackouts within about 10 minutes of the flare.
      • The solar wind of charged particles emitted by a flare take several hours to reach earth.

    • Coronal Mass Ejections (CME)
      • CME's sometimes accompany solar flares. CMEs are huge bubbles of radiation and particles that explode into space at very high speed.
      • CME's generally greatly increase the average speed of the solar wind with their highly energetic output of charged particles.
      • If the magnetic field orientation of the CME cloud of particles is aligned with that of the Earth when it hits us, it will cause an increase in K and A indices. [not sure about this...]
      • Increase in A & K generally reduces MUF for a period of time, however it can happen that MUF increase at lower latitudes.
      • A large CME will send a "shock-wave" of solar-wind charged particles that takes up to several days to reach Earth.
      • The cloud of particles from CME are attracted to the higher latitudes and cause the phenomenon of aurora borealis.

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    Effects on the Earth from stuff from the Sun:

  • Electromagnetic radiation impacts the entire ionosphere on the side of the Earth facing the Sun:
    with wavelengths from 100 to 1000 Angstroms (ultraviolet) ionizes the F region.
    from 10 to 100 Angstroms (soft X-rays) ionizes the E region.
    from 1 to 10 Angstroms (hard X- rays) ionizes the D region.
      which increases the D region absorption and can cause loss of propagation.

    The amount of ionizing radiation (Solar Flux) determines the electron density of the ionosphere.

  • Solar Wind:
    It can strongly affect the Earth's magnetic field. Since it is attracted to the poles, it mostly affects higher latitudes:
      "Compresses" Earth's magnetic field on the side facing the sun,
      and "Stretches" the field on the side facing away from the sun.
      A "Geomagnetic Sudden Impulse" occurs when a CME of charged particles hits the Earth's magnetosphere.
      CMEs mix with the solar wind [???both are streams of charged particles, so how are they different???]and carry with them threads of the Sun's magnetic field.
      One such GSI occurred 2020-12-10 and rated at 30 nT

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  • Effects on the Earth's Magnetic Field:
    • Magnetometers measure the strength of the Earth's magnetic field.
    • Data is reported in several ways:
      K-index: 3-hr average
        quasi-logarithmic, 0 to 9 (severe storm)

      A-index: daily average of 8@ 3-hr K-index'

        linear, from 0 (quiet) to 400 (severe storm)
        reported as either "planetary" index,
        or "middle latitude" index

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  • Propagation:
    • More sunspots = more UV radiation = more F-layer ionization = better refraction of higher frequencies = better propagation of higher frequencies
    • Fewer sunspots = higher frequencies lost in space = less absorption = better propagation of lower frequencies (160 & 80M)
    • More electron density = ionosphere more reflective to HF = higher MUF
    • Propagation may be improved for several hours after a solar flare due to increased ionization.
      Cheat notes (for the non readers)
      The A index [ LOW is GOOD ]
      · 1 to 6 best
      · 7 to 9 is OK
      · 11 or more not so good

      K index [ LOW is GOOD ]
      · 0 or 1 best
      · 2 is OK
      · 3 or more not so good
      · 5 is really terrible

      SFI index [ HIGH is GOOD ]
      · 70 not good
      · 80 good
      · 90 better
      · 100+ best

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  • NOAA: Space Weather Prediction Center:
    https://www.swpc.noaa.gov/about-space-weather
    Contains a wealth of information on solar phenomena and its effect on Earth, and also
    reports on 3 major categories of space weather:

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