Magnetic topology of the north solar pole

Abstract

The magnetism at the poles is similar to that of the quiet Sun in the sense that no active regions are present there. However, the polar quiet Sun is somewhat different from that at the activity belt as it has a global polarity that is clearly modulated by the solar cycle. We study the polar magnetism near an activity maximum when these regions change their polarity, from which it is expected that its magnetism should be less affected by the global field. To fully characterise the magnetic field vector, we use deep full Stokes polarimetric observations of the 15 648.5 and 15 652.8 Å FeI lines. We observe the north pole as well as a quiet region at disc centre to compare their field distributions. In order to calibrate the projection effects, we observe an additional quiet region at the east limb. We find that the two limb datasets share similar magnetic field vector distributions. This means that close to a maximum, the poles look like typical limb, quiet-Sun regions. However, the magnetic field distributions at the limbs are different from the distribution inferred at disc centre. At the limbs, we infer a new population of magnetic fields with relatively strong intensities (~600−800 G), inclined by ~30° with respect to the line of sight, and with an azimuth aligned with the solar disc radial direction. This line-of-sight orientation interpreted as a single magnetic field gives rise to non-vertical fields in the local reference frame and aligned towards disc centre. This peculiar topology is very unlikely for such strong fields according to theoretical considerations. We propose that this new population at the limbs is due to the observation of unresolved magnetic loops as seen close to the limb. These loops have typical granular sizes as measured in the disc centre. At the limbs, where the spatial resolution decreases, we observe them spatially unresolved, which explains the new population of magnetic fields that is inferred. This is the first (indirect) evidence of small-scale magnetic loops outside the disc centre and would imply that these small-scale structures are ubiquitous on the entire solar surface. This result has profound implications for the energetics not only of the photosphere, but also of the outer layers since these loops have been reported to reach the chromosphere and the low corona.

Photospheric magnetic topology of a north polar region

“…From this analysis, both limb datasets are similar to each other and their magnetism is compatible with vertical magnetic fields. This result is partially in agreement with previous studies (Blanco Rodríguez et al 2007;Ito et al 2010;Blanco Rodríguez & Kneer 2010;Jin et al 2011;Shiota et al 2012;Pastor Yabar et al 2018) but here we are missing the second (weak and with no clear magnetic field inclination) component reported by these authors. We highlight the fact that in this work we detect many more magnetic fields (polarimetric signals above 3σ) than the ones analysed in this section; see for instance the large strong circular polarity structure at x ∼ 3 , y ∼ 12 in Fig.…”

“…The closer to the activity minimum, the larger the number of such structures and, conversely, close to activity maximum, the number of those magnetic concentrations in the polar areas is the smallest. More recently, Pastor Yabar et al (2018) also agreed that some magnetic fields at the polar regions are characterised by strong vertical fields, but those authors also found some polarimetric signals common to polar regions and the equatorial limb that were compatible with the presence of unresolved small-scale magnetic loops.…”

“…Before proceeding with the disambiguation, some information about the degree of compatibility between the observed distributions at the disc centre and at the limbs might be retrieved. To this aim, we follow the same strategy as in Pastor Yabar et al (2018). In this method, we use the disc-centre inferred topology as our reference for any QS region, in particular, for that of the observed east (north) area.…”

“…For a proper comparison between the limb datasets and those at disc centre it is convenient to rotate from the LOS to the LRF. To do so, we follow the same method detailed in Pastor Yabar et al (2018). To that aim, we identify individual magnetic structures where the LOS magnetic field azimuth and inclination are reliably inferred (292 structures for the north region and 23 for the east one).…”

STiC: A multiatom non-LTE PRD inversion code for full-Stokes solar observations

“…These ideas were also used in the Stokes-Profiles-INversion-ORoutines (SPINOR; Frutiger et al 2000). Perhaps the most popular photospheric lines used in Milne-Eddington (ME) and LTE inversions are Fe i lines such as the 525 nm dublet, 617 nm, 630 nm dublet, and 1565 nm dublet (some examples of recent studies are Scharmer et al 2013;Jafarzadeh et al 2014;Buehler et al 2015;Martínez González et al 2016;Esteban Pozuelo et al 2016;Danilovic et al 2016;Centeno et al 2017;Danilovic et al 2017;Borrero et al 2017;Pastor Yabar et al 2018).…”