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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 | 
