The Sun’s seismic radius, measured from the frequencies of f modes, is determined using both MDI and HMI data, covering a total of 21 years. It is found that the seismic radius is reduced by 1-2 km during the maxima, but the largest change of the radius happens at about 5 Mm beneath the surface.
HMI-observed vector magnetic-field maps were lowered to a resolution of lmax=5, so that a comparison between solar and stellar magnetic field is possible.
Long-term migration of the Sun’s open magnetic flux is studied, and its relation with the sunspot numbers is discussed.
Meridional flows during the solar minimum and maximum years are derived using 14 years of SOHO/MDI data. The flows changed significantly from the minimum to the maximum, and major changes were associated with the active latitudes.
Recent discoveries suggest that Sun-like stars experience a fundamental shift in their rotation and magnetism around middle-age. We have now identified this transition in the best available data on stellar cycles.
Using a combination of the magnetograms, we find signs of the beginning of the 25th cycle from both HMI and WSO by calculating the inclination angles determined from the variation in line of sight field during a disk passage.
The F10.7 microwave flux shows remarkabke agreement with the unsigned magnetic flux from MDI and HMI over the past 20 years, attesting to the accuracy and significance of both.
Surface flux transport model suggests that the weak Cycle 24 is mainly caused by a number of bigger bipolar regions emerging at low latitudes with a “wrong” north-south orientation.
Solar inter-network magnetic field, the weakest component of the solar magnetism, seems to be invariant at ~10 G from the minimum to the maximum phase of Cycle 24. This suggests a possible origin of small-scale, local dynamo.
HMI observations reveal a slow, north-south asymmetric polar magnetic field reversal. Cycle 24 has been weak; an even weaker Cycle 25 seems probable.