  By using the unique database of SOHO/MDI full disk magnetograms from 1996
September to 2011 January, covering the entire solar cycle 23, we analyze the
time-variability of the solar magnetic flux spectrum and study the properties
of extended minimum of cycle 23. We totally identify 11.5 million magnetic
structures. It has been revealed that magnetic features with different magnetic
fluxes exhibit different cycle behaviors. The magnetic features with flux
larger than $4.0 \times 10^{19}$ Mx, which cover solar active regions and
strong network features, show exactly the same variation as sunspots; However,
the remaining $82\%$ magnetic features which cover the majority of network
elements show anti-phase variation with sunspots. We select a riterion that the
monthly sunspot number is less than 20 to represent the Sun's low activity
status. Then we find the extended minimum of cycle 23 is characterized by the
long duration of low activity status, but the magnitude of magnetic flux in
this period is not lower than previous cycle. Both the duration of low activity
status and the minimum activity level defined by minimum sunspot number show a
century period approximately. The extended minimum of cycle 23 shows
similarities with solar cycle 11, which preceded the mini-maxima in later solar
cycles. This similarity is suggestive that the solar cycles following cycle 23
are likely to have low activity.
