SOLAR ACTIVITY

The number of sunspots is not constant but changes from year to year. The variation is periodic and dramatic. This fluctuation of sunspot numbers is called the sunspot cycle. It has a period of approximately 11 years. The rise in the number of sunspots is generally more rapid than the decline, and the maximum and minimum sunspot counts vary from cycle to cycle.
The sunspot cycle affects propagation conditions at frequencies up to about 70 MHz for F1- and F2-layer propagation and 150 to 200 MHz for E-layer propagation. When there are not many sunspots, the maximum usable frequency (MUF) is comparatively low because the ionization of the upper atmosphere is not dense. At or near the time of a sunspot peak, the MUF is higher because the upper atmosphere is more ionized. A solar flare is a violent storm on the surface of the Sun. Solar flares cause an increase in the level of radio noise that comes from the Sun and cause the Sun to emit an increased quantity of high-speed subatomic particles.
These particles travel through space and arrive at the Earth a few hours after the first appearance of the flare. Because the particles are electrically charged, they are accelerated by the Earth’s magnetic field. Sometimes a geomagnetic storm results. Then we see the “northern lights” or “southern lights” (aurora borealis or aurora australis, often called simply the aurora) at high latitudes during the night and experience a sudden deterioration of ionospheric radio-propagation conditions. At some frequencies, communications can be cut off within seconds. Even wire communications circuits are sometimes affected.
Solar flares can occur at any time, but they seem to take place most often near the peak of the 11-year sunspot cycle. Scientists do not know exactly what causes solar flares, but the events seem to be correlated with the relative number of sunspots.