  Seven-year long seeing-free observations of solar magnetic fields with the
Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory
(SDO) were used to study the sources of the solar mean magnetic field, SMMF,
defined as the net line-of-sight magnetic flux divided over the solar disk
area. To evaluate the contribution of different regions to the SMMF, we
separated all the pixels of each SDO/HMI magnetogram into three subsets: weak
(B_W), intermediate (B_I), and strong (B_S) fields. The B_W component
represents areas with magnetic flux densities below the chosen threshold; the
B_I component is mainly represented by network fields, remains of decayed
active regions (ARs), and ephemeral regions. The B_S component consists of
magnetic elements in ARs. To derive the contribution of a subset to the total
SMMF, the linear regression coefficients between the corresponding component
and the SMMF were calculated. We found that: i) when the threshold level of 30
Mx cm^-2 is applied, the B_I and B_S components together contribute from 65% to
95% of the SMMF, while the fraction of the occupied area varies in a range of
2-6% of the disk area; ii) as the threshold magnitude is lowered to 6 Mx cm^-2,
the contribution from B_I+B_S grows to 98%, and the fraction of the occupied
area reaches the value of about 40% of the solar disk. In summary, we found
that regardless of the threshold level, only a small part of the solar disk
area contributes to the SMMF. This means that the photospheric magnetic
structure is an intermittent, inherently porous medium, resembling a
percolation cluster. These findings suggest that the long-standing concept that
continuous vast unipolar areas on the solar surface are the source of the SMMF
may need to be reconsidered.
