POLAR FIELD REVERSAL OBSERVATIONS WITHHINODE

Abstract

We have been monitoring yearly variation in the Sun's polar magnetic fields with the Solar Optical Telescope aboard Hinode to record their evolution and expected reversal near the solar maximum. All magnetic patches in the magnetic flux maps are automatically identified to obtain the number density and magnetic flux density as a function of th total magnetic flux per patch. The detected magnetic flux per patch ranges over four orders of magnitude (10 15 -10 20 Mx). The higher end of the magnetic flux in the polar regions is about one order of magnitude larger than that of the quiet Sun, and nearly that of pores. Almost all large patches (≥ 10 18 Mx) have the same polarity, while smaller patches have a fair balance of both polarities. The polarity of the polar region as a whole is consequently determined only by the large magnetic concentrations. A clear decrease in the net flux of the polar region is detected in the slow rising phase of the current solar cycle. The decrease is more rapid in the north polar region than in the south. The decrease in the net flux is caused by a decrease in the number and size of the large flux concentrations as well as the appearance of patches with opposite polarity at lower latitudes. In contrast, we do not see temporal change in the magnetic flux associated with the smaller patches (< 10 18 Mx) and that of the horizontal magnetic fields during the years 2008-2012.

Solar Magnetism in the Polar Regions

“…Figure 17 shows that positive field-carrying surges traveled poleward as shown by the red streaks, causing the north polar field to reverse first as shown in Figures 17 and 19. This is consistent with the Hinode results of Shiota et al (2012) shown in Figure 4, in which the polar flux density decreased in the north than in the south. Svalgaard and Kamide (2013) and Mordvinov and Yazev (2014) emphasized the link of this asymmetry to an asymmetry in the decayed active region fields that are transported poleward.…”

“…This estimate of average field strength is roughly consistent with the values derived from lower-resolution synoptic line-of-sight measurements that we will discuss in Section 2.7. Shiota et al (2012) collected annual polar vector field measurements from Hinode SOT/SP. Each year since 2007 the south pole was observed in March and the north pole in September.…”

“…The exposure time for the south polar region is shorter (4.8 s); thus, the south pole plot cannot be directly compared with those for the north pole and the quiet Sun because the signal/noise level is different. Image adapted from Shiota et al (2012); courtesy of D. Shiota. …”

On the role of asymmetries in the reversal of the solar magnetic field

“…Actually with such a degree of asymmetry we find that the North pole reverses more than 1 yr ahead of the South pole. This is compatible with solar observations, that clearly show that one hemisphere can be ahead of the other one by up to 2 yr (Dikpati et al 2007, Shiota et al 2012, Tsuneta et al 2012 this proceedings). Here we did not tune the amplitude nor the sign of the asymmetry parameter to reproduce the sequence of reversals and time lag between the Northern and Southern hemispheres seen in the Sun but we intend to do so in the near future.…”

“…We have shown that by modifying the ingredients of the standard dynamo models by introducing a certain degree of asymmetry in the flow both families are coupled more efficiently. We find that we can qualitatively reproduce the ratio between the quadrupole and the dipole observed in the Sun and also explain the origin of the time lag seen in the reversal of the magnetic field between the North and the South poles (Dikpati et al 2007, Shiota et al 2012, Tsuneta et al 2012). …”