The evolution of the quantified magnetic complexity of active regions in the 23rd solar cycle

We present a statistical study to show how the quantified magnetic complexity of active regions (ARs) evolves in the 23rd solar cycle. We collect a sample of 1039 ARs in the period 1996-2008 based on magnetic synoptic charts from the Solar and Heliospheric Observatory Michelson Doppler Imager (SOHO/MDI) and National Solar Observatory (NSO/Kitt Peak). A structural parameter d(E) is used to quantify the magnetic complexity of ARs. We have found that the yearly mean magnetic complexity of ARs is high near the solar maximum and low near the solar minimum. The deviation of the yearly mean d(E) of the ARs at the solar maximum from that at the solar minimum is about 59.8 per cent for the rising phase and 38.1 per cent for the declining phase. Most ARs are relatively simple and bipolar, even at the solar maximum. The yearly ratio of the number of complex ARs with d(E) greater than 1 to the number of total ARs increases rapidly and persistently in the rising phase of the solar cycle, but decreases with large fluctuations in the declining phase. By studying the distribution of ARs of different complex degree over the sunspot butterfly diagram, it is found that complex ARs are scattered over the 23rd solar cycle and there is some asymmetry in the northern and southern hemispheres.