The cause of short-term radio blackouts is Solar flares. Solar flares are large eruptions of electromagnetic radiation from the Sun lasting from minutes to hours. The sudden outburst of electromagnetic energy travels at the speed of light, therefore any effect upon the sunlit side of Earth’s exposed outer atmosphere occurs at the same time the event is observed.

When a strong enough solar flare occurs, ionization is produced in the lower, more dense layers of the ionosphere (the D-layer), and radio waves that interact with electrons in these layers lose energy due to the more frequent collisions that occur in the higher density environment of the D-layer. This can cause HF radio signals to become degraded or completely absorbed, resulting in a radio blackout. The blackout primarily impacts the 3 to 30 MHz band.

Solar flares usually take place in active regions, which are areas on the Sun marked by the presence of strong magnetic fields; typically associated with sunspot groups. As these magnetic fields evolve, they can reach a point of instability and release energy in a variety of forms, including electromagnetic radiation, which are observed as solar flares.

To learn more: NOAA Solar Flares (Radio Blackouts)

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A and K indices are about the level of activity in the geomagnetic field. If the indices are increasing so are the disturbances in the geomagnetic field.

Silly Hint: "AKting up" is roughly equivalent to "rising disruption"

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Geomagnetic disturbances and solar flares also increase absorption. Disruptions in the ionosphere from a changing magnetic field cause more of a radio waves energy to be dissipated as heat. As the A and K indices rise, so does absorption, particularly along polar paths that travel through auroral zones where most charged particles from the sun enter the earths atmosphere

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The interplanetary magnetic field (IMF) is that part of the sun's magnetic field which is spread throughout the solar system by solar wind. \(B_Z\) describes the component of the IMF's direction and strength most relevant to space weather, and thus to radio.

\[Explanation.\]

In physics, the symbol B is used to denote a magnetic field. Field strength is often described as a three-dimensional vector with \(x\), \(y\), and \(z\) components. In the case of the IMF, the axes are oriented such that \(x\) points from the Earth to the sun, \(y\) is in the plane of the ecliptic tangent to the Earth's orbit, and \(z\) is perpendicular to both*, roughly along the Earth's magnetic axis from north to south.**

Normally, the Earth's magnetosphere repels most of the energy from the solar wind, which prevents our atmosphere from blowing away.*** But when \(B_Z\) tends southward, the polarity of the sun's magnetic field is opposite Earth's, which allows the two fields to connect rather than repulse each other.

When this happens during solar storms (and particularly coronal mass ejections), large amounts of energy from the solar wind may transfer along these connections and even temporarily reconfigure the Earth's magnetosphere, leading to many dramatic and disruptive phenomena including aurora and geomagnetic storms.

Further reading (with diagrams): http://blog.aurorasaurus.org/?p=178

_{* This is the Geocentric Solar Magnetospheric coordinate system (GSM).
** Technically \(B_Z\) points toward the "ecliptic pole."
*** This is what happened to Mars.}

Hint: Buzz Lightyear protects the planets.

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The orientation of the magnetic field in a coronal mass ejection from the Sun is a huge factor in determining the strength of the event's effects here at Earth.

When coronal mass ejections occur on the Sun, a huge amount of energy is released from violent reconfigurations of the complex magnetic field surrounding the star. These events cause high energy charged particles to stream outwards from the sun's outer atmosphere, or corona, into the solar system. Since the particles are charged, they carry with them magnetic field perturbations as they move outwards that are partially (but not completely) reflective of the conditions at the sun when they were released.

If the CME's particles encounter Earth, we have a geomagnetic storm. One of the biggest regulators of the strength of effects of the particles on our magnetosphere and ionosphere is in the Z orientation (\(+\) = toward our north pole, \(-\) = toward our south pole) of the disturbance's magnetic field. That Z orientation is labeled as Bz.

If \(B_Z\) is southward in the incoming particle cloud, this is opposite to our Earth's normal magnetic field -- think of a bar magnet repelling a magnet with opposite polarity. In this case, the CME-associated Bz then pushes on the Earth's magnetic field and compresses it, dumping energy into the magnetosphere. The energy input can lead to oscillations in the magnetic field as the Earth 'fights back'.

Since the ionosphere is tightly bound to the background magnetic field lines, it too begins to move and oscillate, quickly developing lots of irregularities, scintillation, and other structures that degrade long-distance HF propagation. This is why Bz is a key factor in determining geomagnetic storm strength at Earth.

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[FYI, NASA maintains spacecraft monitors just upstream of earth -- the Advanced Composition Explorer (ACE) and the new DSCOVR satellite -- that have comprehensive space environment monitors, and one of the key variables measured there is the orientation of B.]

-adamlein

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Think: When things "go south," something bad is happening.

Or, think of the planet Earth as 'B' and the mass ejection as 'z'. Looking at Bz, where is 'z' hitting? It's hitting the bottom or southern part of the 'B'.

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VHF and UHF have a relatively short range due to the fact that the high frequencies can't get bent by the atmosphere very much, which makes the range about seeing distance.

You can then guess that the radio waves will extend only slightly farther than the horizon, about 15% more than that distance.

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Solar flares are ranked A, B, C, M, and X with "A" being the lowest and "X" the highest. Intensity increases in alphabetic order. After X it goes X1, X2, X3, etc. The highest-rated solar flare ever was an X26 in 1859

Silly Hint: The Marvel comic "X"-men were intense.

Silly Hint 2: Greatest = Xtra

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NOAA categorizes geomagnetic storms on a scale ranging from G1 to G5. A G5 storm, the most extreme level, can result in complete high frequency radio blackouts across the sunlit side of Earth, lasting for several hours.

Hint: Think “G” - for Geomagnetic.

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Propagation reporting networks report what signals they are receiving. They do this by listening to an entire frequency band at once and decoding digital and CW signals with a computer, because these signals have a very distinct, simple format to decode (as opposed to voice). Amateurs can then search up their information to see where they are being heard from and how far away the signal is propagating.

Because these propagation reporting networks operate based on actual received signals, they have nothing to do with geomagnetic parameters (the other three answers).

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304A can be thought of as 304 Å (Angstroms). The word angstroms only occurs in the correct answer: UV emissions at 304 angstroms, correlated to solar flux index.

But what is the Solar Flux Index, and what does it have to do with Ultraviolet Light at 304 Å?

Solar Flux Index (SFI) is measured in solar flux units (SFUs). SFI is defined as the amount of flux (radio noise) emitted at 2800 MHz. Because 2800 MHz correlates with a wavelength of 10.7 cm, it is also referred to as the 10.7 cm Flux Index.

An ångström or angstrom unit (Symbol: Å or A) is a unit of length equal to 10^–10 meters, used principally to express the wavelengths of electromagnetic radiation. It is named after Swedish physicist Anders Jonas Ångström.

This problem refers to λ = 304 Å = 30.4 nm.

The 304A Index, often shortened to just 304A, is a

"NOAA reported value from 0 to unknown. Indicates relative strength of total solar radiation at a wavelength of 304 angstroms (or 30.4 nm), emitted primarily by ionized helium in the sun's photosphere. Two measurements are available for this parameter, one measured by the Solar Dynamics Observatory, using the EVE instrument, and the other, using data from the SOHO satellite, using its SEM instrument. Responsible for about half of all the ionization of the F layer in the ionosphere. 304A looseley correlates to SFI. The background level - at solar minimums - will typically be around 134, and at solar maxima can exceed 200 or more. Updated hourly."

HINT: Think A for Angstroms "304 Å (Angstroms)" in the question.

Additional references:

http://adsabs.harvard.edu/full/1997ESASP.404..439J

https://www.arrl.org/files/file/Technology/tis/info/pdf/0209038.pdf

https://en.wikipedia.org/wiki/Noise_(radio)

http://www.qsl.net/w2vtm/hf_solar.html

http://www.voacap.com/ctu/propagation-ctu-en.pdf

https://www.electronics-notes.com/articles/antennas-propagation/ionospheric/solar-indices-flux-a-ap-k-kp.php

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VOACAP stands for Voice of America Coverage Analysis Program. It is used to predict HF propagation.

VOA is the clue contained in the question. As a shortwave broadcast operation, HF propagation would be the one thing they might be most interested in.

See HF predictions map.

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Solar flares cause a sudden rise in background noise. So, if you detect a sudden rise in radio background noise, it might have been caused by a solar flare.

Silly way to remember:

voices rise as tempers flare

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