power lines overwhelms the signal (music) you have tuned in, resulting in a brief intolerable condition.
(2) Many sources of manmade interference may cause intense disruption of communications
during the day and drop off at night when they are not in use. Natural interference is generated by
phenomena such as thunderstorms, cosmic sources, and the sun. This causes static that you often hear
when listening to a radio.
(3) Natural interference is disruptive, particularly in the HF band. Listening to your car radio
on the AM band during a thunderstorm will reveal the impact of this interference; the intensity of the
radiated energy from the lightning discharges interferes with the signal you have tuned in.
(4) At night, there are increases in the noise levels. This is attributed to both manmade and
natural interferences. Because of the change at night in the layers of the F region, many spurious signals
can be tuned in. Because of an increase in the number of signals reflected off of the ionosphere, more
than one station may be heard simultaneously, causing interference. Some stations change their power
output. This can also affect the noise levels.
e. Variations in the ionosphere are caused by the Earth's position relative to the sun, and by the
sun's activity. The two types of variations are regular and irregular. Regular variations occur in cycles.
They can be predicted with reasonable accuracy. Irregular variations occur as a result of abnormal solar
activities. They are not predictable.
(1) Regular variations that affect the degree of ionization in the ionosphere are divided into
four classes. These classes are daily, seasonal, 27-day sunspot cycle, and 11-year sunspot cycle.
(a) Daily variations are caused by the Earth's rotation.
(b) Seasonal variations are caused by the Earth revolving around the sun; the relative
position of the sun moves from the upper hemisphere to the lower hemisphere with changes in seasons.
(c) The 27-day sunspot cycle is caused by the sun's rotation on its axis (one rotation each
27 days). As the sun rotates, sunspots are visible at 27-day intervals, causing variations in the ionization
density of the layers. Sunspots are believed to be caused by violent eruptions on the sun and are
characterized by unusually strong magnetic fields.
(d) The 11-year sunspot cycle is caused by the sunspot activity going from maximum
through minimum and back to maximum levels of intensity every 11 years. During periods of
maximum sunspot activity, the ionization density of all layers increases. Because of this, absorption in
the D layer increases, and the critical frequencies for the E, F1, and F2 layers are higher. At these times,
higher operating frequencies must be used for long-distance communications.