Contribution to the Solar Mean Magnetic Field from Different Solar Regions

Document Type

Article

Publication Date

9-1-2017

Abstract

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 (BW), intermediate (BI), and strong (BS) fields. The BW component represents areas with magnetic flux densities below the chosen threshold; the BI component is mainly represented by network fields, remains of decayed active regions (ARs), and ephemeral regions. The BS 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 BI and BS 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 BI+ BS grows to 98%, and the fraction of the occupied area reaches a 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.

Identifier

85027696459 (Scopus)

Publication Title

Solar Physics

External Full Text Location

https://doi.org/10.1007/s11207-017-1145-9

e-ISSN

1573093X

ISSN

00380938

Issue

9

Volume

292

Grant

1250818

Fund Ref

National Aeronautics and Space Administration

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