Effects of the solar cycle
Rendition of Earth's magnetosphere
Source : http://sec.gsfc.nasa.gov/popscise.jpg
Sunspot activity has a major effect on long distance radio communications particularly on the shortwave bands although medium wave and low VHF frequencies are also affected. High levels of sunspot activity lead to improved signal propagation on higher frequency bands, although they also increase the levels of solar noise and ionospheric disturbances. These effects are caused by impact of the increased level of solar radiation on the ionosphere. It has been proposed that 10.7 cm solar flux can interfere with point-to-point terrestrial communications.
The following is a list of solar cycles (sometimes called sunspot cycles), tracked since 1755
The impact of solar cycle on living organisms has been investigated. Some researchers claim to have found connections with human health. The amount of UVB light at 300 nm reaching the Earth varies by as much as 400% over the solar cycle due to variations in the protective ozone layer. In the stratosphere, ozone is continuously regenerated by the splitting of O2 molecules by ultraviolet light. During a solar minimum, the decrease in ultraviolet light received from the Sun leads to a decrease in the concentration of ozone, allowing increased UVB to penetrate to the Earth's surface. The sunspot cycle has been implicated in having effects on climate, and may play a part in determining global temperature.
Skywave modes of radio communication operate by bending (refracting) radio waves (electromagnetic radiation) off of the Ionosphere. During the "peaks" of the solar cycle, the ionosphere becomes ionized by solar photons and cosmic rays. This affects the path (propagation) of the radio wave in complex ways which can both facilitate or hinder local and long distance communications. Forecasting of skywave modes is of considerable interest to commercial marine and aircraft communications, amateur radio operators, and shortwave broadcasters. These users utilize frequencies within the High Frequency or 'HF' radio spectrum which are most affected by these solar and ionospheric variances. Changes in solar output affect the maximum usable frequency, a limit on the highest frequency usable for communications.
References : Wikipedia & Youtube