Abrupt shrinking of solar corona in late 1990s and related changes in solar magnetic structure

Several studies have noted on changes in the properties of sunspots, and in the mutual relations between various global parameters of solar magnetic activity (e.g. UV/EUV irradiance, radio and IR emissions, TSI/SSI), as well as between solar and ionospheric parameters since the onset of solar cycle 23. These changes have been suggested to be related to the overall reduction of solar activity at the aftermath of the decline of the Grand modern maximum of solar activity that prevailed during most of the 20th century. We have recently derived the longest record of coronal magnetic field intensities since 1968 using Mount Wilson Observatory and Wilcox Solar Observatory observations of the photospheric magnetic field and the PFSS model, and compared it with the heliospheric magnetic field observed at the Earth. We found that the time evolution of the coronal magnetic field during the last 50 years agrees with the heliospheric magnetic field only if the effective coronal size, the distance of the coronal source surface of the heliospheric magnetic field, is allowed to change in time. We calculated the optimum distance for each solar rotation and found that it experienced an abrupt decrease in the late 1990s. The effective volume of the solar corona shrunk to less than one half of its previous value during a short period of only a few years. This shrinking was related with a systematic change in the structure of the coronal magnetic field during the same time interval. We review these dramatic changes in the solar corona and discuss their possible connection to the changes in the different solar activity parameters and the reduction of the overall solar activity.