Structure and evolution of the photospheric magnetic field in 2010–2017: comparison of SOLIS/VSM vector field and B_LOS potential field

Abstract

Context. The line-of-sight (LOS) component of the large-scale photospheric magnetic field has been observed since the 1950s, but the daily full-disk observations of the full vector magnetic field started only in 2010 using the SOLIS Vector Stokes Magnetograph (VSM) and the SDO helioseismic and magnetic imager (HMI). Traditionally, potential field extrapolations are based on the assumption that the magnetic field in the photosphere is approximately radial. The validity of this assumption has not been tested yet. Aims. We investigate here the structure and evolution of the three components of the solar large-scale magnetic field in 2010 -2017, covering the ascending to mid-declining phase of solar cycle 24, using SOLIS/VSM vector synoptic maps of the photospheric magnetic field. Methods. We compare the observed VSM vector magnetic field to the potential vector field derived using the VSM LOS magnetic field observations as an input. The new vector field data allow us to derive the meridional inclination and the azimuth angle of the magnetic field and to investigate their solar cycle evolution and latitudinal profile of these quantities. Results. SOLIS/VSM vector data show that the photospheric magnetic field is in general fairly non-radial. In the meridional plane the field is inclined toward the equator, reflecting the dipolar structure of the solar magnetic field. Rotationally averaged meridional inclination does not have significant solar cycle variation. While the vector radial component B r and the potential radial component B PFS S r are fairly similar, the meridional and zonal components do not agree very well. We find that SOLIS/VSM vector observations are noisy at high latitudes and suffer from the vantage point effect more than LOS observations. This is due to different noise properties in the LOS and transverse components of the magnetic field, which needs to be addressed in future studies.

On a limitation of Zeeman polarimetry and imperfect instrumentation in representing solar magnetic fields with weaker polarization signal

“…A synoptic map is used as an example because the averaging used in producing a synoptic map results in lower background noise, but very similar patterns are also seen in less-averaged full-disk data. Recent comparisons also indicate that the average large-scale zonal field B u observed by SOLIS/VSM and derived from line-of-sight observations tends to disagree outside active regions (Virtanen et al, 2019). The investigation of this disagreement uncovered what we think may be an important limitation to Zeeman vector polarimetry.…”

Evolution of Magnetic Helicity in Solar Cycle 24

“…where ∆ Ω = ∂ ∂µ sin 2 θ ∂ ∂µ + 1 sin 2 θ ∂ 2 ∂φ 2 , µ = cos θ and θ is the polar angle, and F S = ∂ (rS) /∂r. Three components of vector magnetic field (radial r, meridional θ, and zonal φ, see, Virtanen et al 2019) are then represented by three independent variables, S , T , and F S :…”

Long-term studies of photospheric magnetic fields on the Sun

“…3. Daily measurements of field strengths and polarities (dots) from (a) MWO, (Pevtsov et al, 2019a) (b) CrAO, and (c) Pulkovo Observatory. Blue color dots correspond to the Potsdam observations, which have been digitized to date (1945,(1954)(1955)(1956).…”

“…Then the pseudo-radial component was derived from the line-of-sight B long magnetograms divided by the cosine of a viewing angle. With regular observations of the full disk vector magnetic field, vector field synoptic maps have been recently constructed (Gosain et al, 2013;Virtanen et al, 2019a).…”

“…While some previous studies have suggested that the polar fields are "nearly" radial (e.g., Svalgaard et al, 1978;Petrie, 2017), Ulrich and Boyden (2006) found a slight poleward inclination of the magnetic field, while Virtanen et al (2019a) described an equator-ward inclination. To arrive at these conclusions, Ulrich and Boyden (2006) found an inclination angle by minimizing the amplitude of annual variations in high-latitude fields (as in Figure 10), while Virtanen et al (2019a) used the vector field observations from VSM/SOLIS. Applying the approach used by Ulrich and Boyden (2006) to the data shown in Figure 10 Figures 10 and 11a), the southern pole showed a well-defined polar field cap of the negative polarity (Figure 11a).…”