What is the night effect?
Electromagnetic waves propagate actually using different means at the same time:


Space wave (direct line of sight)
Sky wave.
Ground wave (or surface wave)



Space wave vs sky wave vs ground wave

Note: Electromagnetic waves are not entirely understood. The terminology around ground waves varies with authors. Some authors don't make a difference between space and ground wave, they just consider both as the ground wave. This is not the case here.

The sky wave propagation develops at night and extends the range of LF/MF frequencies. The sky wave is the result of reflection in the ionosphere, the upper layers of the atmosphere, at an altitude between 100 and 300 km.

NDB range is increased, but VOR and DME range are less impacted. While range is increased by the sky wave, perturbations appear, changing the relative bearing of the NBD signal, decreasing accuracy.

HF radio transmission (voice) range is also increased, as the effect exists regardless of the modulation or use.



Frequency spectrum overview

To facilitate the understanding, we need to have an idea of the whole radio spectrum of waves and how aviation uses this spectrum.

  



Sky wave 

The sky wave propagation takes benefit of the possibility for EM waves to be reflected in the atmosphere.

Light is an EM wave, and it can be reflected by a smooth surface: Metal, glass and water are good reflectors. This is the same for other radio EM waves. A good mirror for radio waves is a layer of ionized atoms. Atom ionization consists in losing or capturing one of more electrons, and becoming electrically unbalanced.

Natural ionization occurs in the ionosphere under the bombardment of solar rays. This takes place in the F-layer at about 300 km, and in the E-layer at about 100 km. Waves can be reflected by these layers when they are active.

However the D-layer just below blocks waves during the day because of the ionization of nitrogen monoxide $\small NO$), with a maximum below the frequency of 10 MHz. This effect decreases at night when the source of ionization disappears.

The higher layers are ionized more and more longer than the lower ones, and ionization persists more in the higher layers. The combined effects of D, E and F layers leads to


An absorption of LF/MF/HF frequencies by the D-layer at daytime, preventing them to reach the E and F layers able to reflect them.
A reflection at night and twilight when E-layer and above all the F-layer are still active and the D-layer has ceased blocking waves.



Sky wave propagation

Because the sky wave proceeds by "skips", it is more effective at specific distances from the transmitter and is characterized by minimums and maximums spaced along the direction of propagation, the distance between maximums depending on the frequency.

Remarks:


Because low frequencies are the only ones subject to sky wave propagation, over-the-horizon (OTH) radars use them instead of SHF.


Over-the-horizon radar. Source.
There is some controversy about the role of the US HAARP project in creating artificial ionized mirrors to use "sky wave" during the day (manipulating atmosphere is prohibited by international treaties).




As the propagation of NDB frequencies is augmented at night by the sky wave, a NDB can be detected at a greater range by a receiver. This is also true for other mode of communication in the same range of frequency (e.g. voice communication in HF over the oceans).

The sky wave propagation is associated with a slow variation, the signal can disappear and reappear a few seconds later. This is due to the instability of the reflection, the heterogeneity of the atmosphere layers, and the recombination of waves after a multi-path trip and a rotation of the polarization. This fluctuation is globally known as fading.

While sky wave propagation is in general a good thing but, like any other multi-path phenomena, it also has negative effects: It changes the polarization of the waves and the direction of the wavefront. As ADF works by detecting the direction of the signal wavefront, a reflected NDB signal introduces inaccuracy in the ADF indication.

  

The beacon seem to move, and the bearing is not reliable. Some cross check is necessary to confirm the validity of a bearing.



The ground wave propagation encompasses reflection on ground and obstacles, diffraction on obstacles and horizon, and refraction thanks to current induced in ground.

Because of the refraction and diffraction, this mode of propagation tends to follow Earth curvature, but it mostly affects LF and MF, higher frequencies (e.g. VHF and UHF) do not really take advantage of this propagation.

The ground wave is due to several factors which slow down the wavefront where it is closed to the ground (the wavefront is a moving spheroidal surface around the transmitter where the wave has the same phase, the wave propagates perpendicularly to its wavefront).


Ground wave: Wavefront slowed down by ground

As the wavefront is vertical at a longer distance, communication distance is increased.

However as the ground wave progresses, its slowed down wavefront is less and less vertical and the effect less and less significant.

Electromagnetic waves are composed of two fields: electric and magnetic, which are perpendicular and normal to the direction of propagation. The wave polarization, that is the plan where the electric field oscillates, considerably influences the ground wave attenuation. Vertically polarized waves (NDB and HF voice communication) propagate better than horizontally polarized waves (VOR variable signal).

The ground wave propagation, like the direct wave propagation, occurs any time, day and night.



Space wave, direct line of sight

A word about space wave propagation: While it's true this is a direct line of sight communication, it's also a simplification. Space waves are usually reflected, diffracted and refracted, exactly like visible light. So the space wave propagation happens without direct line of sight too. This is what allows cell phones to work in cities and/or indoor.

There is a lot of attenuation associated when the wave direction of travel is changed, and usually the wave follow different directions at the same time, some recombining at the receiver location after having traveled a different distance, hence arriving with a different phase (multipath propagation). 

Multipath has been difficult to deal with in the era of analog transmission. To prevent significant attenuation limit the power of the transmitter and the size of the aerial, critical transmission channels, like terrestrial high-band radio links have been carefully designed with a Fresnel zone clear of any obstacle, limiting communication towers implementation to specific locations.

With digital modulations (and ease of implementation of fast Fourier transform algorithm in modern electronic circuits) space wave communication is now more easy to manage and these constraints are a bit relaxed.