Photospheric and coronal magnetic fields in six magnetographs

Abstract

Aims. We study the long-term evolution of photospheric and coronal magnetic fields and the heliospheric current sheet (HCS), especially its north-south asymmetry. Special attention is paid to the reliability of the six data sets used in this study and to the consistency of the results based on these data sets. Methods. We use synoptic maps constructed from Wilcox Solar Observatory (WSO), Mount Wilson Observatory (MWO), Kitt Peak (KP), SOLIS, SOHO/MDI, and SDO/HMI measurements of the photospheric field and the potential field source surface (PFSS) model. Results. The six data sets depict a fairly similar long-term evolution of magnetic fields and the heliospheric current sheet, including polarity reversals and hemispheric asymmetry. However, there are time intervals of several years long, when first KP measurements in the 1970s and 1980s, and later WSO measurements in the 1990s and early 2000s, significantly deviate from the other simultaneous data sets, reflecting likely errors at these times. All of the six magnetographs agree on the southward shift of the heliospheric current sheet (the so-called bashful ballerina phenomenon) in the declining to minimum phase of the solar cycle during a few years of the five included cycles. We show that during solar cycles 20-22, the southward shift of the HCS is mainly due to the axial quadrupole term, reflecting the stronger magnetic field intensity at the southern pole during these times. During cycle 23 the asymmetry is less persistent and mainly due to higher harmonics than the quadrupole term. Currently, in the early declining phase of cycle 24, the HCS is also shifted southward and is mainly due to the axial quadrupole as for most earlier cycles. This further emphasizes the special character of the global solar field during cycle 23.

Observing the simulations: applying ZDI to 3D non-potential magnetic field simulations

“…The large spread and the high dependence on v e sin i and inclination angle makes it impossible to provide a general correction factor for the underestimation of the magnetic field by ZDI especially if one keeps in mind that our analysed stars cover only a very small fraction of the observed cool stars. The non uniform dependency on the different -modes can also be found by comparing the solar magnetograms of different solar observations as shown by Virtanen & Mursula (2017). Reiners & Basri (2009) and Morin et al (2010) showed that the averaged magnetic field measured by ZDI B V represents only 6−14 % of the total averaged magnetic field measured by Zeeman broadening B I for M dwarfs.…”

Reconstructing solar magnetic fields from historical observations

“…Therefore the SOLIS data set used in this work is not scaled to SPMG level. A comparison of SPMG and SOLIS observations based on harmonic expansions (Virtanen & Mursula 2017) indicates that the correct SPMG to SOLIS scaling factor would be 1.3-1.4.…”

Magnetograph Saturation and the Open Flux Problem

“…It is apparent that, when comparing magnetograph measurements in the context of the global solar field, pixel-by-pixel regression or histogram analyses of the photospheric flux itself, as in the study of Riley et al (2014), do not properly capture the differences between the data sets. Instead, the focus should be on the differences between the lowest-order harmonic components, as in Virtanen & Mursula (2017) and in the present study.…”

“…An inspection of Figure 9(b) also shows that, whichever saturation correction is adopted, the open fluxes derived from the WSO maps are systematically smaller relative to the observed IMF after ∼1998, as compared with earlier years (cf. Virtanen & Mursula 2017). This suggests that there may have been a decrease in the sensitivity of the WSO magnetograph sometime around 1998.…”