Solar Activity, Solar Cycle, Coordinates

Abstract

The structure and the dynamics of the upper atmosphere depend on the energy input from the Sun via extreme ultraviolet and X-ray radiation (XUV) and on the properties of the solar wind. Contrary to the good temporal stability of the Sun’s radiation intensity at longer wavelengths (from infrared to near ultraviolet) the energy output of the Sun is highly variable in the XUV range and, roughly, the variability increases with decreasing wavelength (see e.g. the review book edited by OR White, 1977). The general level of XUV intensity is indicated by the “solar activity”, which changes in the longtime average with the “solar cycle”. Geophysicists think in 11-year solar cycles, but one should be aware that the configuration of astronomically measured properties of the sun (e.g. the magnetic field configuration) show rough similarities in cycles of about 22 yr. (The longtime average number of sunspots changes in an 11-year cycle but their magnetic polarity in a 22-year cycle.) The true length of a solar cycle is variable and the beginning of a new cycle can be determined only some time after the fact. The (11-year) cycles are numbered: presently we are in cycle 22, which began in September 1986. The numbering starts with the cycle which began in 1755 (see e.g. the reviews and commented excerpts of older literature in the book edited by DJ Schove, 1983). The start of a cycle coincides approximately with “solar minimum” (minimum of average XUV radiation, minimum of average sunspot number), “solar maximum” occurs about 3–5 yr after the start of a cycle, the earlier the higher the maximum.