11 2-28.   The density of ionized layers is normally greatest during which of the following periods? 1.   At night 2.   Before sunrise 3.   Between early morning and late afternoon 4.   Between afternoon and sunset 2-29.   Compared to the other ionospheric layers at higher altitudes, the ionization density of the D layer is 1.   about the same 2.   relatively low 3.   relatively high 2-30.   What two layers in the ionosphere recombine and largely disappear at night? 1.   D and F 2.   D and E 3.   E and F2 4.   F1 and F2 2-31.   For hf-radio communications covering long distances, what is the most important layer of the ionosphere? 1.   C 2.   D 3.   E 4.   F 2-32.   Refraction of a sky wave in the ionosphere is influenced by which of the following factors? 1.   Ionospheric density 2.   Frequency of the wave 3.   Angle of incidence of the wave 4.   All of the above 2-33.   A 10-MHz wave entering the ionosphere at an angle greater than its critical angle will pass through the ionosphere and be lost in space unless which of the following actions is taken? 1.   The ground wave is canceled 2.   The frequency of the wave is increased 3.   The frequency of the wave is decreased 4.   The ground wave is reinforced 2-34.   The distance between the transmitter and the nearest point at which refracted waves return to earth is referred to as the 1.   skip distance 2.   return distance 3.   reception distance 4.   ground-wave distance 2-35.   When ground-wave coverage is LESS than the distance between the transmitter and the nearest point at which the refracted waves return to earth, which of the following reception possibilities should you expect? 1.   No sky-wave 2.   Weak ground wave 3.   A zone of silence 4.   Strong ground wave 2-36.   The greatest amount of absorption takes place in the ionosphere under which of the following conditions? 1.   When sky wave intensity is the greatest 2.   When collision of particles is least 3.   When the density of the ionized layer is the greatest 4.   When precipitation is greatest 2-37.   Which of the following layers provide the greatest amount of absorption to the ionospheric wave? 1.   D and E 2.   D and F1 3.   E and F1 4.   F1 and F2