Solar Cycle Number 23 – a progress report

Perhaps the most remarkable feature of the current solar cycle is that it has been quite unremarkable so far! Unlike recent cycles, this cycle (denoted 23 in the system used to number cycles) has been average in amplitude. By way of contrast, Cycle 19, which peaked in 1957, was the largest ever recorded; Cycle 21 which peaked in 1979, was the second largest on record; and Cycle 22 was the fourth largest ever seen. Even the ‘odd–even’ rule of thumb has been broken. This venerable rule, which says that an oddnumbered cycle is larger than the preceding even-numbered cycle, has persisted through six pairs of cycles back to Cycle 8/9 around the year 1850. The solar cycle is typically 11 years in duration although cycles vary greatly both in amplitude and in length. It is manifest in many properties of the Sun, but is most evident in the occurrence of sunspots on the solar disk. Sunspots are regions of stronger magnetic field that appear darker than the surrounding surface. At times, sunspots are rare and the sun appears almost without blemish. This is known as solar minimum and is regarded as the start of the solar cycle. Later, sunspots become more common and it is normal for many groups of spots to be visible. The peak, when sunspots are most common, is called solar maximum. The number of spots gives rise to the ‘sunspot number’ which, when smoothed over a period of 12 months, is the traditional measure of the solar cycle. Records of sunspot number go back to the 1600s and our knowledge of the properties of the solar cycle becomes more reliable through the 1700s. This long record of sunspot observations accounts for the continued use of the sunspot number as the definitive indicator of the solar cycle. In modern times, the solar cycle has become of wider interest because it is the key to many effects on human technology, both on the ground and in space. These include high frequency radio communications; satellite operations; prospecting for minerals using geomagnetic techniques; power lines; pipelines; and, recently, the global positioning system (GPS). The solar cycle is important because it determines the long-term variation of regions such as the Earth’s ionosphere, a layer of charged particles between 100 and 600 km above the surface of the Earth. This layer is important because of its effect on radio signals, either reflecting high-frequency signals or retarding those above this frequency range. The solar cycle also underpins the occurrence of shortterm disturbances to the Earth–sun region. These arise in association with spectacular events on the sun such as solar flares and coronal mass ejections and propagate to the Earth as changes in the solar wind, an outflow of charged particles from the sun that envelopes the Earth. Disturbances disrupt the ionosphere causing rapid variations in its properties and are most obviously seen in the rapid variations in the magnetic field of the Earth, events that are known as ‘geomagnetic storms’. In the absence of selective availability, the ionosphere can be the largest source of error for GPS. Information to correct for the effect of the ionosphere is carried within GPS signals, but this does not correct for the rapid changes associated with solar-induced disturbances. A knowledge of the solar cycle is of importance to those interested in GPS.