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The Aurora Borealis is actually the result of collisions between gaseous particles in the Earth's atmosphere with charged particles released from the sun's atmosphere. Variations in colour are due to the type of gas particles that are colliding.

Source: Northern Lights Centre

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At E-region height there are enough atmospheric particles to produce a visible glow when the ionised particles from the sun enter the atmosphere at high velocity.

You can think "E-urora", if it will help.

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The cause of auroral activity—sometimes called the Northern Lights or aurora borealis—is the interaction in the E layer of charged particles from the Sun with the Earth's magnetic field. CW is the emission mode that is best for aurora propagation.

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The phase differences are caused by multiple radio paths between transmitter and receiver antennas. The multiple paths are generated by reflection and refraction of the radio signal. Selective fading manifests itself in the receiver as one or more deep and narrow notches that may move through the frequency spectrum.

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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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The farther the antenna is above ground, the less the ground will affect its radiation pattern. The same is true for vertical omni-directional antennas. The main lobe takeoff angle decreases with increasing height.

If their ground radials are brought up they will raise the radiation pattern and main lobe, or increase the main lobe takeoff angle.

Silly hack: For all questions regarding takeoff angle, the answer is "decrease" except for the one about seawater.

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A Pederson Ray is a path that ­follows the contour of the Earth near the height of the maximum F2 region electron density, and requires a fairly stable ionosphere. Pedersen-Ray paths are most evident over high-latitude east-west paths at frequencies near the MUF. They appear most often about noon local time at mid-path when the geomagnetic field is very quiet.

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Tropospheric ducting occurs when a VHF signal reflects between layers of varying refractive index for long distances. Tropospheric ducting is especially pronounced over sea under calm anti-cyclonic weather conditions.

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The performance of a horizontally polarized antenna mounted on the side of a hill will be different from the performance of same antenna mounted on flat ground. Specifically, the main lobe takeoff angle decreases in the downhill direction.

Hint: Mentally draw an XY axis with the antenna at the origin. When on a hill the signal will take advantage of that height and the radiating lobe will fill/fall down the hill, decreasing in angle into -y values.

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In the northern hemisphere, your best chance to find an aurora is toward the magnetic north pole. If you were in the southern hemisphere, you would seek auroras toward the magnetic south pole.

In the Northern hemisphere the charged particles from the sun are funnelled in the direction of the north pole. This is where any aurora will be located. Aurora do not need to be visible to be useful for radio propagation.

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Ground waves are waves emitted at less than a wavelength above the ground that travel in contact with the surface of the earth. At higher frequencies like most of HF and above this is especially bad; the waves are severely attenuated and nothing amazingly good happens.

At 160m and below frequencies with vertical polarization though they may be able to travel a few hundred miles since they can get refracted in such a way as to follow the curve of the earth without too much attenuation, whereas at 10m they tend to die out in 10 miles or less. Note that this requires vertical polarization; horizontal waves will short circuit immediately since the whole electrical field tends to end up in the somewhat conductive ground.

How well surface wave propagation works depends on the conductivity of the soil or water that makes up the earth's surface on the path.

Ground wave propagation is most useful on the 1.8MHz and 3.5MHz bands during daytime.

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Ground waves are a result of interaction of the radio signal with the ground. The interaction has the effect of causing the signal to follow the curvature of the earth. Ground wave propagation is most useful on the 1.8 MHz and 3.5 MHz bands during daytime.

This only works with vertical waves; horizontal waves will short circuit immediately since the whole electrical field tends to end up in the somewhat conductive ground.

One way to remember this answer is to consider commercial AM broadcast: ground-wave propagation via vertical towers that polarize vertically.

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The effect is caused by inversion layers (density changes) in the atmosphere. This effect is more commonly referred to as tropospheric ducting. An excellent write up can be found here: https://en.wikipedia.org/wiki/Tropospheric_propagation

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