Measurements of Solar Magnetic Fields

Abstract

view Abstract Citations (107) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Measurements of Solar Magnetic Fields Sheeley, N. R., Jr. Abstract Using the photographic technique of Leighton, high spatial resolution measurements of photospheric magnetic fields have been obtained with the following chief results: 1. Regarding sunspot groups as sources of magnetic flux and defining a "source flux" that may be determined uniquely for a sunspot group from flux measurements, we found a variation in source fluxes from group to group ranging from less than 0.2 SFU (1 SFU [solar flux unit] = 1021 maxwells) for very "small" spot groups to more than 20 SFU for very "large" groups. This variation of source flux with sunspot group "size" is described by the rule-of-thumb = 1.2 Am, where is the source flux in SFU and Am is the group size in 1018 cm2 defined as the maximum area attained by the sunspot group during its development. 2. Zeeman photographs emphasize that as a bipolar magnetic region (BMR) develops in time and as its magnetic flux spreads over a progressively larger area, the flux density does not decrease smoothly from the vicinity of the source to the outer limit of detectable flux but is distributed in successively smaller bits and fragments. Moreover, measurements obtained from these Zeeman photographs reveal magnetic field strengths of 200-700 gauss in these bits and fragments, showing that fields of a few hundred gauss are not uncommon for small magnetic features even in quiet regions of the Sun. 3. Numbers of polar faculae have been calibrated to give magnetic flux on the polar caps of the Sun as a function of time during the period 1905-1964. The fluxes vary cyclicly with time approximately 90 out of phase with the variation of the sunspot number for the whole solar disk with time during the same period (provided the sunspot number is given a polarity corresponding to the magnetic polarity of the following sunspots of the relevant hemisphere). The maxima of the polar fluxes vary considerably from cycle to cycle (just as the maxima of the sunspot number vary considerably from cycle to cycle), maximum fluxes ranging from 6 to 21 SFU, with 12 SFU being a typical maximum polar flux during 1905-1964. These results are consistent with the hypothesis that emerging BMR's are the sources of all the flux on the solar surface, and that the random walk plus differential rotation is the dominant mechanism for the distribution of the flux provided by these sources. More important, these measurements provide a means of testing this hypothesis in more detail than has been possible heretofore. Publication: The Astrophysical Journal Pub Date: May 1966 DOI: 10.1086/148651 Bibcode: 1966ApJ...144..723S full text sources ADS |