Solar Cycle

The term "solar cycle" refers to the period of approximately eleven years in which the solar activity follows a determined behavior, that has been historically registered in terms of the sunspot number. These are regions in the photosphere (the lower layer of the sun's atmosphere) of temperature relatively low. A sunspot is visually identificable in the sun's surface as a dark region (in comparison with the photosphere that contains it). Although the sunspots itself don't produce significative effects in the solar emisisons, the magnetic activity coming from these regions can produce considerable variations in the levels of ultraviolet and X-ray emissions. These changes have important consequences in the earth's ionosphere. When the sunspot number is low, the sun produces less ultraviolet radiation, that causes the reduction ofthe electron density in the ionosphere. In the other hand, qhen the sunspot number is high, the higher incidence of ultraviolet radiation increase the electron density in the ionosphere, enhancing the propagation for higher frequencies upto (or beyond) 30 MHz.

The solar cycle can be divided into four distinct periods: minimum, rise, maximum, decline. The period between the minimum and the maximum is of about four and a half years while the decline period is larger, extending for about six and a half years. Each solar cycle is assigned with a number, since that it was observed and measured for the first time in 1749. A total of 22 cycles were yet registered. Despite its duration is commonly of eleven years, certain individual cycles had varied duration: some cycles lasted for only seven and a half years, while others lasted for sixteen years. The maximum value of the mean sunspot number in a cycle is typically of 100, although in the last five cycles higher values were observed, up to 150. In the past were registered higher variations in the mean sunspot value; in 1816 the maximum was only 49, while in 1957 the maximum was of 201, the higher value yet registered.

The actual cycle is referred as Solar Cycle 23, and it began in 1996. Currently, this cycle is in its decline period, after the maximum period in 2000/2002, how can be viewed below, with the sunspot number in each month, along of the past 13 years (graphic updated directly from the NOAA web site).

Current information about the Solar Cycle 23, including both data yet registered and predicted data for the following years, based on a set of data of the last thirteen months, are presented below:

• Solar Cycle Progression: web page from the NOAA site showing three distinct graphics (solar cycle sunspot progression, solar flux progression, solar cycle Ap progression), besides links to tabular lists (with predicted and recent indices).
• Recent Indices: tabled values with the values of the solar and geomagnetic indices from the last thirteen years.
• Predicted Indices: tabled values with predictions of the sunspot number and the solar flux for the two next years.

References:

• Space Weather 1999: article published in WRTH - World Radio TV Handbook, Volume 53, 1999.