Higher sunspot numbers generally indicate a greater probability of good propagation at higher frequencies

HF propagation is done through bouncing Electro-Magnetic waves off of charged particles in the earth's atmosphere. High sunspot numbers indicate higher activity in the sun, which shoots off energy into the earth's atmosphere, charging more particles that increases the amount of reflected power on the atmosphere, increasing received signal.

Higher sunspots = Higher probability at Higher frequencies

Silly Hint: Both question and answer has "Higher sunspot number".

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It disrupts signals on lower frequencies more than those on higher frequencies.

During a solar flare, a large amount of energy, including ultraviolet and x-ray radiation, travels out from the sun at the speed of light toward the daylight side of the Earth.

These large bursts of radiated energy cause a sudden increase of ionization in the ionospheric layers of Earth's atmosphere. This is known as a Sudden Ionospheric Disturbance.

SIDs tend to "enhance low frequency propagation" and "absorb high frequency radiation" which appears to be the opposite of the correct answer. The important detail is that the question is about HF radiation specifically and it is the lower end of this range which is typically more affected.

For more info see Wikipedia: Sudden Ionospheric Disturbance (SID)

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RF energy waves, such as ultraviolet and X-ray radiation, travel at the speed of light (approx. 300 million meters per second, or approx. 186,000 miles per second). The earth is about 93 million miles from the sun, and so it takes just over 8 minutes, on average, for a burst of radiation from solar flares to affect radio-wave propagation on earth.

Ultraviolet and X-ray radiation = 8 minutes

Particles from coronal mass ejection = 20 - 40 hours

Sun Cycle = 11 years

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High frequency (short wavelength) radio waves are transmitted the farthest when the upper layers of the ionosphere are energized during periods of high solar activity, so they are most affected and least reliable for long distance communications during periods of low solar activity. Therefore, 15 meters, 12 meters and 10 meters, is correct as these are the highest frequencies offered in the answer choices.

Silly hint: Long distance/Long answer.

Remember, low solar activity is bad for low wavelength. The correct answer has lowest [shortest] wavelengths. [KQ4AEY]

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(D). Measuring solar flux is another way of expressing the amount of solar activity. The solar flux is the intensity of the sun's RF energy emissions.

The Solar flux index is a standardized representative of this radiation energy which is measured at a fixed value of 2800 MHz frequency (10.7 cm wavelength).

The advantage of this measurement over the sunspot index, is that it can be measured during any weather conditions - the sun doesn't have to be visible. The higher the solar flux index number, the greater the amount of solar activity indicated.

For more info see Wikipedia: Solar flux

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Easy hint: Storms are "Temporary"

Our earth is protected by outer lines of magnetic force referred to as the magnetosphere. These lines of force flow from pole to pole and help protect the earth from destructive particles. A solar flare can release massive amounts of charged particles toward the earth. When these particles reach the earth they can cause temporary disturbances of the Earth's magnetosphere, called geomagnetic storms.

For more info see Wikipedia: Geomagnetic storms

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The 20 meter (14 MHz) band is less affected by variations in the solar cycle than higher frequency bands. During periods of high solar activity, the band will be deflected for longer distances and with stronger signals, but it is a reliable band for worldwide daylight propagation during any point of the solar cycle.

Hint: Question "At what point?" - Answer: At any point

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(B). During a Geomagnetic storm, charged particles from increased solar emissions, such as solar flares, are sent toward earth. The particles are deflected by the earth's magnetosphere along lines of magnetic force from the North and South poles. The regions around the equator are protected. This increased activity is often seen as greater Auroras at the poles. Because the magnetic disturbances are concentrated around the higher latitudes (from about 45 degrees to the poles), HF propagation is distorted and degraded in these high-latitude regions.

For more info see Wikipedia: Geomagnetic storms

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(C). High sunspot numbers indicate that the amount of solar activity is high, and that lots of RF energy is heading for the earth. This energy produces greater ionization of the outer atmosphere (ionosphere). This increased ionization supports higher atmospheric bands in the E and F layers, and allows for radio transmissions to be reflected at longer angles.

This enhances long-distance communication in the upper HF and lower VHF range.

For more info see Wikipedia: sunspots

However, it is not only RF energy that is headed at Earth but CMEs as well. The higher influx of charged particles can cause geomagnetic storms and distortions and lead to disruptions in radio communication. This question really ought to be phrased to include this nuance

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It takes the sun about 28 days to rotate once on its axis. Because solar activity in one region often lasts more than one rotation (such as a group of sunspots), we will typically see the same activity pattern return when the sun returns to that same point in its revolution (and those same sunspots come into range again).

Therefore it is the sun's rotation on its axis that causes HF propagation conditions to vary periodically in a 28-day cycle.

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(D). Our sun goes through routine cycles of activity caused by internal dynamics of the star. The sun goes through a cycle of low to high temperatures, activities and energies about every 11 years. During the recurring periods of higher solar activity, we see more sunspots occurring. Because sunspots were visible indicators of solar activity and have been readily observed for the longest period historically, we usually call this variable cycle of the solar activity as the sunspot cycle.

For more info see Wikipedia: Sunspots

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The K-index is a measurement of the short-term stability of Earth's magnetic field. A high K-index means higher amounts of magnetic disturbance, and so more disruption of HF signals, especially in latitudes from 45 degrees to the poles.

Mnemonic: “K” in German stands for Kurt which means short (same as "curt" in English). Germany is on Earth.

For more info see Wikipedia: K-index

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The A-index, like the K-index, are measures of the earth's geomagnetic field stability. Whereas the K-index is a short-term measure, the A-index is an averaged daily figure and is charted over the usual rotational period of the sun, so is a better cyclical indicator of the long term stability of the Earth's geomagnetic field.

Note: Remember that A-index is a longer AVERAGE as opposed to the short term magnetic KICK measured by the K-index.

For more info see Wikipedia: A-index

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The emission of charged particles interacts with the earth's magnetosphere causing geomagnetic storms or disturbances.

This causes disturbances in the ionosphere, which will adversely affect HF communication.

VHF/UHF is neither improved nor disturbed, since VHF/UHF typically does not rely on the ionosphere for propagation.

Silly hint: Something coming out of an ancient crown (corona) is disturbing.

For more info see Wikipedia: coronal mass ejection

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It takes charged particles from coronal mass ejections about 20 to 40 hours to travel to Earth and then affect radio propagation.

The sun is approximately \(93\text{ million miles}\) from the earth. Unlike RF energy which travels at the speed of light, the charged particles from a coronal mass ejection take longer to reach Earth.

Per Wikipedia:

Coronal mass ejections reach velocities from...\(12\) to \(1,988\text{ miles/s}\)...with an average speed of \(304\text{ miles/s}\).

These speeds correspond to transit times from the Sun out to the mean radius of Earth's orbit of about \(13\text{ hours}\) to \(86\text{ days}\) (extremes), with about \(3.5\text{ days}\) as the average.

The average of 3.5 days is closest to 20 to 40 hours. Only light can reach Earth in 4 to 8 minutes -- magnitudes smaller than the 13 hour low-end travel time for CME particles. While some particles might take 14 days, 28 days, or even longer to reach our planet, those particles are much less common, and as a result are less likely to affect radio-wave propagation here on Earth.

For more info see Wikipedia: coronal mass ejection

Hint: It doesn't take days or minutes, it takes "hours".

Solar flares travel at the speed of light - about 8 minutes. CME's take a day or two.

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Geomagnetic storms are bad news for HF transmissions, especially at higher latitudes. But a period of high geomagnetic activity can be good news for VHF propagation. The magnetic disturbance, which is centered around the poles, can produce aurora that can reflect VHF signals, thereby improving their chances of long-distance propagation.

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