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We are entering a new sunspot cycle, known as cycle 24, which is great for HF radio communications. Sunspots impact the refractivitity of the ionosphere. Solar maximum and solar minimum refer respectively to epochs of maximum and minimum sunspot counts. The total solar irradiance TSI is higher at solar maximum, even though sunspots are darker (cooler) than the average photosphere. This is caused by magnetized structures other than sunspots during solar maxima, such as faculae and active elements of the 'bright' network, that are brighter (hotter) than the average photosphere. They collectively overcompensate for the irradiance deficit associated with the cooler but less numerous sunspots.
Even though it only accounts for a minuscule fraction of total solar radiation, the impact of solar UV, EUV and X-ray radiation on the Earth's upper atmosphere is profound. Solar UV flux is a major driver of stratospheric chemistry, and increases in ionizing radiation significantly affect ionosphere-influenced temperature and electrical conductivity.
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.
As a radio wave enters a region of increasing ionization, the increase in velocity of the upper part of the wave causes it to be bent back toward the Earth. The amount of refraction that occurs depends on three main factors: the density of ionization of the layer, the frequency of the radio wave, and the angle at which the wave enters the layer.
Commonly, the optimal operating frequency for a given path is estimated at 80 to 90% of the maximum useable frequency, MUF. In the transmission of radio waves via ionospheric reflection, the Frequency of Optimum Transmission FOT is the highest effective frequency (or best working frequency) for a given path.
Radio waves of frequencies lower than the MUF frequency will be refracted back to Earth unless they are absorbed or have been refracted from a lower layer. The lower the frequency of a radio wave, the more rapidly the wave is refracted by a given degree of ionization. The figure shows three separate waves of different frequencies entering an ionospheric layer at the same angle, in this case, the highest refracted frequency travels the farthest. A near real time maximum useable frequency map is at: http://www.spacew.com/www/realtime.php
Some or all of this content is copyright (c) by references:
[1] http://en.wikipedia.org/wiki/Ionosphere
[2] http://www.tpub.com/neets/book10/40e.htm | 
